During the 1980S a Variety of Zeolite Type and Other Related New

Nanostructured Metal Oxides as Adsorbents and Photocatalysts

A thesis presented to

Queensland University of Technology

In fulfillment of the requirements for the degree of

Doctor of Philosophy

By BLAIN PAUL

Based on research conducted in the

Faculty of Science and Technology

Queensland University of Technology, Brisbane, December, 2010

Declaration of Original Authorship

The work contained in this thesis has not been previously submitted to meet requirements for an award at this or any other higher education institution. To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made.

Blain Paul

December, 2010

Abstract

This research underlines the extensive application of nanostructured metal oxides in environmental systems such as hazardous waste remediation and water purification. This study tries to forge a new understanding of the complexity of adsorption and photocatalysis in the process of water treatment. Sodium niobate doped with a different amount of tantalum, was prepared via a hydrothermal reaction and was observed to be able to adsorb highly hazardous bivalent radioactive isotopes such as Sr2+ and Ra2+ions. This study facilitates the preparation of Nb-based adsorbents for efficiently removing toxic radioactive ions from contaminated water and also identifies the importance of understanding the influence of heterovalent substitution in microporous frameworks.

Clay adsorbents were prepared via a two-step method to remove anionic and nonionic herbicides from water. Firstly, layered beidellite clay was treated with acid in a hydrothermal process; secondly, common silane coupling agents, 3-chloro-propyl trimethoxysilane or triethoxy silane, were grafted onto the acid treated samples to prepare the adsorption materials.

In order to isolate the effect of the clay surface, we compared the adsorption property of clay adsorbents with -Al2O3 nanofibres grafted with the same functional groups. Thin alumina (γ-Al2O3) nanofibres were modified by the grafting of two organosilane agents 3-chloropropyltriethoxysilane and octyl triethoxysilane onto the surface, for the adsorptive removal of alachlor and imazaquin herbicides from water. The formation of organic groups during the functionalisation process established super hydrophobic sites along the surfaces and those non-polar regions of the surfaces were able to make close contact with the organic pollutants.

A new structure of anatase crystals linked to clay fragments was synthesised by the reaction of TiOSO4 with laponite clay for the degradation of pesticides. Based on the Ti/clay ratio, these new catalysts showed a high degradation rate when compared with P25. Moreover, immobilized TiO2 on laponite clay fragments could be readily separated out from a slurry system after the photocatalytic reaction.

iii

Using a series of partial phase transition methods, an effective catalyst with fibril morphology was prepared for the degradation of different types of phenols and trace amount of herbicides from water. Both H-titanate and TiO2-(B) fibres coated with anatase nanocrystal were studied. When compared with a laponite clay photocatalyst, it was found that anatase dotted TiO2-(B) fibres prepared by a 45 h hydrothermal treatment followed by calcination were not only superior in performance in photocatalysis but could also be readily separated from a slurry system after photocatalytic reactions.

This study has laid the foundation for the development of the ability to fabricate highly efficient nanostructured solids for the removal of radioactive ions and organic pollutants from contaminated water. These results now seem set to contribute to the development of advanced water purification devices in the future. These modified nanostructured materials with unusual properties have broadened their application range beyond their traditional use as adsorbents, to also encompass the storage of nuclear waste after concentrating from contaminated water.

Table of Contents

DECLARATION OF ORIGINAL AUTHORSHIP ------II

ABSTRACT ------III

TABLE OF CONTENTS ------V

LIST OF FIGURES ------IX

LIST OF TABLES ------XVI

LIST OF SCHEMES ------XVII

LIST OF ABBREVATIONS ------XVIII

ACKNOWLEDGEMENTS ------XX

PUBLICATIONS ARISING FROM THIS WORK ------XXII

ORGANISATION OF RESEARCH CONTENTS ------XXIII

CHAPTER 1 ------1 -

INTRODUCTION AND RATIONALE FOR RESEARCH ------1 -

1.1 Literature Review ------2 - 1.1.1 Introduction ------2 - 1.1.2 Niobate nanostructures------8 - 1.1.3 Clays and clay modifications ------10 - 1.1.4 Alumina nanostructures------17 - 1.1.5 Photocatalysis ------19 - 1.2 Conclusions ------30 - 1.3 Description of Research Problems Investigated ------31 - 1.3.1 Overall Objectives of the study ------31 - 1.4 References ------33 - CHAPTER 2 ------50 -

EQUIPMENTS AND CHARACTERISATION TECHNIQUES ------50 -

2.1 Equipments ------51 - 2.1.1 Autoclaves ------51 - 2.1.2 High temperature oven ------52 - 2.1.3 Photochemical reactor ------52 - 2.2 Characterization Techniques ------53 - 2.2.1 Scanning Electron Microscopy ------53 - 2.2.2 Energy Dispersive X-ray Spectrometer (EDS) Analysis ------55 - 2.2.3 Transmission Electron Microscope ------56 - 2.2.4 Fourier Transform Infrared (FTIR) Spectroscopy ------57 - 2.2.5 Raman Spectroscopy ------59 - 2.2.6 X-Ray Diffraction Analysis (XRD) ------60 - 2.2.7 UV-Vis diffuse reflectance Spectrophotometer ------63 - 2.2.8 Solid State Nuclear Magnetic Resonance Spectroscopy ------64 - 2.2.9 Surface Area Analysis ------66 - 2.2.10 Thermo Gravimetric Analysis (TGA) ------67 - 2.2.11 Contact Angle Measurement ------67 - 2.2.12 Inductively Coupled Plasma Optical Emission Spectrometry ------69 - 2.2.13 High Pressure Liquid Chromatography (HPLC) ------70 - 2.2.14 UV-Visible Spectrophotometer ------71 - 2.3 References ------72 - CHAPTER 3 ------73 -

SODIUM NIOBATE ABSORBENTS DOPED WITH TANTALUM FOR THE

REMOVAL OF BIVALENT RADIOACTIVE IONS FROM WASTE WATER

------73 -

3.1 Introduction ------74 - 3.2 Materials and Equipment ------78 - 3.3 Discussion of Experimental Procedures ------79 - 3.3.1 Sample preparation ------79 - 3.3.2 Adsorption experiments ------80 - 3.4 Results and discussion ------82 - 3.4.1 Particle morphology ------82 - 3.4.2 XRD Patterns and Raman Spectra ------84 - 3.4.3 UV- Visible Spectra ------87 -

3.4.4 Sorption of bivalence radioactive ions ------89 - 3.5 Conclusions ------96 - 3.6 References ------97 - CHAPTER 4 ------101 -

NANOSTRUCTURED CLAY AND ALUMINA BASED FILTER

MEMBRANES FOR THE SEPARATION OF ORGANIC POLLUTANTS

FROM WATER ------101 -

4.1 Introduction ------102 - 4.2 Materials and Equipment ------111 - 4.3 Discussion of Experimental Procedures ------112 - 4.3.1 Acid treatment ------112 - 4.3.2 Modification of acid treated clay with silane groups ------113 -

4.3.3 Preparation of –Al2O3 nanofibres ------114 -

4.3.4 Modification of –Al2O3 nanofibres with silane groups ------115 - 4.3.5 Adsorption experiments ------116 - 4.4 Results and discussion ------118 - 4.4.1 Elemental Analysis ------118 - 4.4.2 TEM Images ------119 - 4.4.3 X-ray diffraction ------121 - 4.4.4 Nitrogen adsorption ------126 - 4.4.5 Thermogravimetric Analysis ------133 - 4.4.6 FTIR spectra ------139 - 4.4.7 Contact Angle ------144 - 4.4.8 TEM images ------147 - 4.4.9 Solid-state 29Si MAS NMR spectra ------148 - 4.4.10 Adsorption of pollutants from water ------150 - 4.4.11 Mechanism of adsorption ------163 - 4.5 Conclusions ------170 - 4.6 References ------172 - CHAPTER 5 ------180 -

vii

PHOTOCATALYTIC STUDIES OF IMMOBILISED TITANATE AND

MIXED-PHASE TITANIA BASED NANOFIBRES FOR DECOMPOSING

PESTICIDES AND PHENOLIC COMPOUNDS ------180 -

5.1 Introduction ------181 - 5.2 Materials and Methods ------186 - 5.3 Discussion of Experimental Procedures ------187 -

5.3.1 Preparation of immobilised TiO2 on laponite ------187 -

5.3.2 Synthesis of H-tianate and TiO2-(B) based photocatalysts ------188 - 5.3.3 Photocatalytic experiments ------189 - 5.4 Results and discussion ------190 - 5.4.1 XRD patterns ------190 - 5.4.2 BET Surface Area ------194 - 5.4.3 FTIR Spectra ------200 - 5.4.4 TEM and SEM Images------201 - 5.4.5 Raman spectroscopy ------205 - 5.4.6 UV-visible Spectroscopy ------208 - 5.4.7 Photocatalytic activity ------210 - 5.4.8 Mechanism ------224 - 5.5 Conclusions ------228 - 5.6 References ------229 - CHAPTER 6 ------235 -

CONCLUSIONS AND FUTURE WORK ------235 -

6.1 Conclusions ------236 - 6.2 Future work ------240 - 6.3 References ------242 -

viii

List of Figures

Figure 1-1 – Types of micro porous solid [21]: where, SAPO–silica aluminophosphate, ElAPO–elemental aluminophosphate, MeAPO–metallo aluminophosphate, MoPO–molybdenum phosphate and MeMoPO– metallomolybdenum phosphate. ------3 -

Figure 1-2 − Molecular sieve materials [23]: a) Zeolite (4-coordination); b) Aluminophosphate (4-coordination); c) Mixed Oxides (4, 5, 6-coordination); d) Germanates (4, 5, 6-coordination). ------4 -

Figure 1-3 − Pore characteristics in the aluminophosphates [25]: a) AlPO4-11; b) AlPO-5; c) VPI-5. ------5 -

Figure 1-4 − Shape and pore size of ETS-10 [33]: a) elliptical pore shape b) circular pore shape. ------6 -

Figure 1-5 − Ion exchange process with interlayer ions (e.g. exchange of sodium 3+ − ions by HO2C–R–NH Cl ). ------11 -

Figure 1-6 − Schematic structure of a hydrotalcite like clay such as layered double hydroxides [84]. ------12 -

Figure 1-7 − Structural representation of smectite. ------13 -

Figure 1-8 − Schematic drawing illustrating the incorporation of complex species into different positions of clay solids [116]. ------15 -

Figure 1-9 − The crystal structure of boehmite [144]. ------18 -

Figure 1-10 – Titanate nanostructures in different morphologies: a) circular- shaped rods; b) multi nanosheets c) spheroids; d) rectangular shaped fibres; e) multi-wall nanotubes [148]. ------20 -

Figure 1-11 – A history timeline explaining the development of TiO2 nanostructures [149]. ------21 -

Figure 1-12 – Atomic architecture of the nanosheet [161]. ------21 -

Figure 1-13 – A schematic model of the layered structure of a scrolled titania nanotube [166]. ------22 -

Figure 1-14 – Mechanism of trititanate nanotubes formation [153]. ------23 -

Figure 1-15 – Chemical and structural transformations of titanate nanotubes and nanofibres [149]. ------24 -

Figure 1-16  The crystal structure of TiO2 polymorphs: (a) anatase; (b) rutile; (c) brookite [176] ------26 -

Figure 1-17 − Schematic representation of surface electron and hole processes in the reduction of O2 at illuminated titania [190]. ------27 -

Figure 2-1  Photograph of hydrothermal reactor.------51 -

Figure 2-2 – A schematic illustration of specimen-electron beam interactions of SEM imaging. ------54 -

Figure 2-3 – A technical explanation of TEM. [Redrawn from "Botany online - The Internet Hypertextbook" Hamburg university]. ------57 -

Figure 2-4 − A technical explanation of FTIR. ------58 -

Figure 2-5 − Schematic overview of the basic experimental arrangement to perform XRD. G1, G2, G3 are the reflections of d-spacings d1, d2, d3, respectively ------61 -

Figure 2-6 – Diagram represents the possible electronic band structure within metals, semiconductors and insulators. ------63 -

Figure 2-7 – Diagram represents the electromagnetic spectrum of radiation. --- - 64 -

Figure 2-8 – Diagram represents the relationship between energy levels and magnetic field. ------65 -

Figure 2-9 – A schematic illustration of a droplet resting on the solid surface; the angle formed between the solid/liquid interface and the liquid/vapor interface, which has a vertex where the three interfaces intersect is referred to as contact angle (). ------68 -

Figure 2-10 – A diagram illustrates the basic arrangement to perform HPLC. -- - 70 -

Figure 2-11 – A diagram represents the working principle of UV-Vis Spectrophotometer. ------71 -

Figure 3-1 – SEM image of Na-4-mica [17]. ------75 -

Figure 3-2 − SEM image of titanate [23]. ------76 -

Figure 3-3 – The crystal structure of microporous niobate (Na2Nb2O6·H2O) phase [24]. ------77 -

Figure 3-4 – Structural transformations of nanostructures under hydrothermal conditions [25]: (b) Low crystalline niobate solids, (c) Highly crystalline niobate fibres. ------80 -

Figure 3-5 – Niobate solids illustrated by the SEM images of the samples: (a) 0% Ta, (b) 0.5% Ta, (c) 1% Ta, (d) 2% Ta, (e) 5% Ta, and (f) 10% Ta. ------82 -

Figure 3-6 – XRD patterns of TaV doped niobate specimen. ------85 -

Figure 3-7 – Raman spectra of TaV doped niobate specimen.------85 -

Figure 3-8 – The UV–Vis absorption spectra of TaV doped sodium niobates. -- - 88 -

Figure 3-9 – The isotherms of M2+ sorption by the Ta doped niobate samples: (a) Ba2+and (b) Sr 2+. ------90 -

Figure 3-10 – Ion exchange assisted absorption of radioactive ions on the framework of niobate: (a) sodium niobate crystals; (b) hydrated sodium niobate crystals; (c) hydrated radioactive ions; (d) sodium ions exchanged with a divalent cation.------92 -

Figure 3-11 – The XRD patterns of the TaV doped sodium niobates before and after sorption of Ba2+ and Sr2+ ions. ------94 -

Figure 3-12 – The Raman spectra of the TaV doped sodium niobates before and after sorption of Ba2+ and Sr2+ ions. ------95 -

Figure 4-1 – A schematic represention of beidellite clay. ------103 -

Figure 4-2 – Fibrous morphology attributes fibrillar interstices. ------109 -

Figure 4-3 – Schematic illustration of acid treatment and grafting of beidellite clay with silane groups. ------113 -

Figure 4-4 – High resolution TEM images. (a) Pure beidillite clay, (b), (c) and (d) are 0.05, 0.2 and 0.5 M acid treated samples, respectively. ------120 -

Figure 4-5 – XRD patterns of the pure beidillite clay (B1) and the acid treated samples (B2–0.05, B3–0.2 and B4–0.5 M). ------122 -

Figure 4-6 – XRD patterns of samples: B4 – 0.5 M acid treated clay; AC1, AC2 and AC3 – CPTES grafted samples; AO1and AO2 – OTES grafted samples. ------124 -

Figure 4-7 – XRD patterns of -Al2O3 samples: AF and AF (A) are as synthesized and acid washed clay respectively; AFC8 (50) and AFC8 (100) – CPTES grafted samples; AFC1 (50) and AFC1 (100) – OTES grafted samples. ------125 -

Figure 4-8 – Nitrogen adsorption/desorption isotherms of beidillite clay (B1) and the acid treated samples (B2–0.05, B3–0.2 and B4–0.5 M). ------127 -

Figure 4-9 – N2 adsorption/desorption isotherms: B4 – 0.5 M acid treated clay; AC1 (5%), AC2 (25%) and AC3 (50%) – CPTES grafted samples; AO1 (50%) and AO2 (100%) – OTES grafted samples; ------129 -

Figure 4-10 – N2 adsorption/desorption isotherms: AF and AF (A) as synthesized and acid washed -Al2O3 respectively; AFC8 (50) and AFC8 (100) – CPTES grafted samples; AFC1 (50) and AFC1 (100) – OTES grafted samples. ------131 -

Figure 4-11 – TGA and DTG curves of 0.5 M acid treated beidellite clay. ------134 -

Figure 4-12 – TGA and DTG curves of clay samples: a) AC3 – CPTES grafted samples; b) AO2 – OTES grafted samples, respectively. ------135 -

Figure 4-13 – TGA and DTG curves of acid washed -Al2O3 fibres – AF (A). - 137 -

Figure 4-14  TGA and DTG curves of modified -Al2O3 fibres: a) AFC8 (100) – OTES grafted samples, and (b) AFC1 (100) – CPTES grafted samples, respectively. ------138 -

Figure 4-15 – FTIR spectra of pure beidillite clay (B1) and the acid treated samples (B2-B4). ------140 -

Figure 4-16  FTIR spectra of samples: B4 – 0.5 M acid treated clay; AC1, AC2 and AC3 – CPTES grafted samples; AO1and AO2 – OTES grafted samples. - - 142 -

Figure 4-17  FTIR spectra of modified -Al2O3 fibres: AF(A) – acid washed; AFC1(50) and AFC1(100) – CPTES grafted samples; AFC8(50) and AFC8(100) – OTES grafted samples. ------143 -

Figure 4-18  Contact angle of non grafted and grafted clays: (a) non-grafted sample B4; (b) CPTES grafted sample AC3; (c) OTES grafted sample AO2. - - 145 -

Figure 4-19  The profile of water droplets on the surface of the pellets form of modified -Al2O3 fibres: (a) AF(A) – acid washed; (b) AFC1(100) – CPTES grafted samples; (c) AFC8(100) – OTES grafted samples. ------146 -

Figure 4-20  Transmission electron micrograph of modified -Al2O3 fibres, (a) AF(A) – acid washed; (b) AFC1(100) – CPTES grafted samples; (c) AFC8(100) – OTES grafted samples. ------147 -

29 Figure 4-21  Solid-state Si MAS NMR spectra of modified -Al2O3 fibres: (a) AFC8(50) and AFC8(100) – OTES grafted samples; (b) AFC1(50) and AFC1(100) – CPTES grafted samples. ------149 -

Figure 4-22  Molecular structures of pollutants.------151 -

Figure 4-23 – Sorption ability of pure beidillite clay (a) and the acid treated samples (b–0.05, c–0.2 and d–0.5 M). Dark shaded bars indicate the initial concentration and light shaded bars are for the concentration of simazine after sorption reaction. - 152 -

Figure 4-24 – FTIR spectra of 0.5 M acid treated clay (B4) before and after sorption. ------153 -

Figure 4-25 – Schematic illustration of the acid treatment of beidillite clay and simazine sorption. ------154 -

Figure 4-26 – XRD pattern of 0.5 M acid treated clay (B4) before and after sorption. ------155 -

Figure 4-27  Adsorption isotherms of various grafted clays: Adsorbed amount versus equilibrium concentration. ------158 -

Figure 4-28 Adsorption performance of modified clay samples: Adsorbed amount versus amount added in solutions.------159 -

xii

Figure 4-29  XRD patterns of samples: (a) AO2A and AO2I – sample AO2 after adsorption of alachlor and imazaquin, respectively; (b) AC3A and AC3I – sample AC3 after adsorption of alachlor and imazaquin, respectively. ------162 -

Figure 4-30  Schematic illustrations of the possible orientation of silane groups and pollutants: I. After adsorption of pollutants by long chain grafted clay: II. Pollutants molecule adsorbed at short chain grafted clay. ------163 -

Figure 4-31 – The schematic diagrams of -Al2O3 fibres before (a) and after grafting (b). ------165 -

Figure 4-32  Adsorption isotherms of -Al2O3 fibres for alachlor (a) and Imazaquin (b): Adsorbed amount versus equilibrium concentration. ------166 -

Figure 4-33  The amount of pollutants adsorbed plotted against the square root of time.------168 -

Figure 4-34  A schematic view of grafted surface and possible interaction with pollutants. ------168 -

Figure 5-1 – Thermal transformation of H-titanate nanofibres. ------182 -

Figure 5-2 – General view of the crystal structure and phase transformations of TiO2 [21, 23]: (a) Na2Ti3O7 ; (b) H2Ti3O7 ; (c) TiO2-B. ------183 -

Figure 5-3  XRD patterns of immobilised TiO2 on laponite clay fragments. - - 191 -

Figure 5-4  The XRD patterns of the mixed-phase TiO2 photocatalysts: (a) H- titanate fibres with anatase nanoparticles and (b) TiO2-(B) nanofibres with anatase nanoparticles. ------193 -

Figure 5-5  N2 adsorption and desorption isotherms of photocatalytically modified laponite clay. ------195 -

Figure 5-6 – The N2 adsorption isotherms of the mixed-phase TiO2 photocatalysts: (a) H-titanate core with anatase shell and (b) After calcination, H-titanate converted to TiO2-(B) but anatase remains same. ------198 -

Figure 5-7  IR-spectra immobilised TiO2 on laponite clay fragments as photocatalysts (Dashed line is eye guide for peak shifts). ------200 -

Figure 5-8  TEM images of immobilised TiO2 on laponite clay fragments: images ae are for L-Ti (5)100, L-Ti(10)100, L-Ti(15)100, L-Ti(15)150 and L-Ti(15)200 samples respectively. ------202 -

Figure 5-9  SEM images of H-titanate fibres with different amount of anatase particles on the surface: (a) H  pure H-titanate; (b) HA2  anatase particles coated on the surface of H-titanate by 30 h of HT; (c) HA4  45 h of HT; (d) HA5  4 days of HT. ------203 -

xiii

Figure 5-10  SEM images of TiO2-(B) related nanofibres: (a) CA2  TiO2-B fibres with anatase particles on the surface prepared by 30 h of HT; (b) CA3  40 h of HT; (c) CA4  45 h of HT; (d) CA5  4 days of HT. ------204 -

Figure 5-11  Raman spectra of immobilised anatase crystals on laponite clay fragments. ------206 -

Figure 5-12  Raman spectra of mixed phase fibre photocatalysts: (a) as prepared hydrogen titanate nanofibres and with anatase phase; (b) mixed phase of anatase and TiO2-(B). ------207 -

Figure 5-13  UV-visible adsorption spectra of mixed phase fibre photocatalysts: (a) H-titanate core with anatase shell; (b) TiO2-(B) core with anatase shell. ------209 -

Figure 5-14  Molecular structure of herbicides. ------211 -

Figure 5-15  Photocatalytic degradation of alachlor and bromacil using immobilised TiO2 on laponite clay fragments and P25. ------213 -

Figure 5-16  Photocatalytic degradation of chlorotoluran and imazaquin using immobilised TiO2 on laponite clay fragments and P25. ------214 -

Figure 5-17  Photocatalytic degradation of sulfosulturon using immobilised TiO2 on laponite clay fragments and P25. ------215 -

Figure 5-18  Molecular structures of organic pollutants. ------217 -

Figure 5-19  Photocatalytic degradation of phenol by mixed phase fibre catalysts: (a) anatase nanoparticles dotted on titanate nanofibres; (b) anatase nanoparticles dotted on TiO2-(B) fibres. H and T represent the pure titanate and TiO2-(B) fibres, respectively. ------218 -

Figure 5-20  Photocatalytic degradation of diphenol by mixed phase fibre catalysts: (a) anatase nanoparticles dotted on titanate nanofibres; (b) anatase nanoparticles dotted on TiO2-(B) fibres. H and T represent the pure titanate and TiO2-(B) fibres, respectively. ------219 -

Figure 5-21  Photocatalytic degradation of triphenol by mixed phase fibre catalysts: (a) anatase nanoparticles dotted on titanate nanofibres; (b) anatase nanoparticles dotted on TiO2-(B) fibres. H and T represent the pure titanate and TiO2-(B) fibres, respectively. ------220 -

Figure 5-22  Photocatalytic degradation of alachlor by mixed phase fibre catalysts: (a) anatase nanoparticles dotted on titanate nanofibres; (b) anatase nanoparticles dotted on TiO2-(B) fibres. H and T represent the pure titanate and TiO2-(B) fibres, respectively. ------221 -

Figure 5-23  Photocatalytic degradation of imazaquin by mixed phase fibre catalysts: (a) anatase nanoparticles dotted on titanate nanofibres; (b) anatase nanoparticles dotted on TiO2-(B) fibres. H and T represent the pure titanate and TiO2-(B) fibres, respectively. ------222 -

xiv

Figure 5-24  Photocatalytic degradation of simazine by mixed phase fibre catalysts: (a) anatase nanoparticles dotted on titanate nanofibres; (b) anatase nanoparticles dotted on TiO2-(B) fibres. H and T represent the pure titanate and TiO2-(B) fibres, respectively. ------223 -

Figure 5-25  Schematic picture of photoelectrochemical mechanism of single phase catalyst [62]. ------225 -

Figure 5-26  Illustrating the mechanism of mixed phase of anatase dotted TiO2-B fibres. ------225 -

List of Tables

2+ V Table 3-1– The Kd values for M ion sorption by Ta doped sodium niobate -- - 91 -

Table 4-1 – Element analysis ratio, specific surface area (BET) and pore volume (Vp) of the pure clay and acid treated samples. ------119 -

Table 4-2 – Specific surface area, pore volume and mean pore diameter of samples: A0 – 0.5 M acid treated clay; AC1, AC2 and AC3 – CPTES grafted samples; AO1and AO2 – OTES grafted samples.------130 -

Table 4-3 – Specific surface area, pore volume and mean pore diameter of samples: AF and AF (A) as synthesized and acid washed -Al2O3 respectively; AFC8 (50) and AFC8 (100) – CPTES grafted samples; AFC1 (50) and AFC1 (100) – OTES grafted samples. ------132 -

Table 5-1 –Specific surface area, pore volume and mean cystal size of anatase of laponite based photocatalysts. ------196 -

Table 5-2 – Effects of hydrothermal treatment and calcination on the BET specific surface area (SBET) and crystal size of mixed phase fibres. ------199 -

xvi

List of Schemes

Scheme 4-1 – Silica surface modified with organic functional groups for the chemical selectivity [66]...... - 106 -

Scheme 4-2 – Schematic overview of the 3-mercaptopropyltrimethoxysilane (MTS) grafted onto silica [66]...... - 107 -

Scheme 4-3 – Schematic representation of the selective grafting reactions on the external surface of silica [76]...... - 107 -

xvii

List of Abbrevations

AC1 Beidellite clay modified with 5% 3- chlotopropyltriethoxysilane

AC2 Beidellite clay modified with 25% 3- chlotopropyltriethoxysilane

AC3 Beidellite clay modified with 50% 3- chlotopropyltriethoxysilane

AF Pure thin -Al2O3 fibres

AF (A) -Al2O3 fibres treated with 0.2 M HCl

AFC8(50) -Al2O3 fibres grafted with 50% octyltriethoxysilane

AFC8(100) -Al2O3 fibres grafted with 100% octyltriethoxysilane

AFC1(50) -Al2O3 fibres grafted with 50% 3- chloropropyltriethoxysilane

AFC1(100) -Al2O3 fibres grafted with 100% 3- chloropropyltriethoxysilane

AO1 Beidellite clay modified with 50% octyltriethoxysilane

AO2 Beidellite clay modified with 100% octyltriethoxysilane

B1 Pure beidellite clay

B2 Beidellite clay treated with 0.05 M hydrochloric acid

B3 Beidellite clay treated with 0.02 M hydrochloric acid

B4 Beidellite clay treated with 0.5 M hydrochloric acid

BET Porosity and surface area analysis

CA1 Anatase particles coated on TiO2-B fibres by 15 h of HT and 450 oC calcination

CA2 Anatase particles coated on TiO2-B fibres by 30 h HT and 450 oC calcination

CA3 Anatase particles coated on TiO2-B fibres by 40 h HT and 450 oC calcination

CA4 Anatase particles coated on TiO2-B fibres by 45 h HT and 450 oC calcination

xviii

CA5 Anatase particles prepared by 5 days of HT and 450 oC calcination

EDX Energy dispersive X-ray analysis

FTIR Fourier transform infrared spectroscopy

HPLC High performance liquid chromatography

H Pure H-titanate fibres

HT Hydrothermal treatment

HA1 Anatase particles coated on titanate fibres by 15 h of hydrothermal treatment

HA2 Anatase particles coated on titanate fibres by 30 h of hydrothermal treatment

HA3 Anatase particles coated on titanate fibres by 40 h of hydrothermal treatment

HA4 Anatase particles coated on titanate fibres by 45 h of hydrothermal treatment

HA5 Anatase particles prepared by 5 days of hydrothermal treatment

LDHs Layered double hydrotalcites

L-Ti(5)100 Ti/Laponite clay ratio of 5 mmol/g and hydrothermal temperature of 100 oC

L-Ti(10)100 Ti/Laponite clay ratio of 10 mmol/g and hydrothermal temperature of 100 oC

L-Ti(15)100 Ti/Laponite clay ratio of 15 mmol/g and hydrothermal temperature of 100 oC

L-Ti(15)150 Ti/Laponite clay ratio of 15 mmol/g and hydrothermal temperature of 150 oC

L-Ti(15)200 Ti/Laponite clay ratio of 15 mmol/g and hydrothermal temperature of 200 oC

NMR Nuclear magnetic resonance spectroscopy

T Pure TiO2-B fibres

TGA Thermogravimertic analysis

XRD X-ray diffraction

xix

Acknowledgements

This thesis would not have been possible without the following individuals who spent their time and shared their knowledge in order to enable me to achieve the best possible outcome:

I acknowledge and express my gratitude to my supervisor Prof. Ray L. Frost, and also to my associate supervisor Dr. Wayde N. Martens for their continuous guidance and support. It is with great pleasure that I thank Prof. Huai Yong Zhu for sharing some of his academic experience and knowledge with me as he guided my research project in a successful direction.

I gratefully acknowledge the funding for the years of research through a grant from the Australian Research Council. Three years of research for this thesis were undertaken while I was in the receipt of a Discovery and Fee Waiver Scholarships. I would also like to thank the Faculty of Science for an extra few more months of scholarship to finish my thesis writing.

Special thanks go to our Discipline Head, A/Prof. Godwin Ayoko, whose support and understanding helped me to overcome many difficulties during my studies. I have also benefited from his efforts to arrange financial support in the last few months of my thesis completion.

I would like to extend my special thanks to Dr. Wayde N. Martens for his support, and his reassuring smile. He spent lot of his precious time reading my thesis chapters and sharing his critical comments about them with me. It would be no exaggeration to say that his words were always a sparkling light of motivation to me. I am indebted to him more than he knows.

I am grateful to Dr. Dongjiang Yang for his encouragement as well as his concrete guidance. Through our long discussions, the design for this research project grew coupled with our curiosities.

I would also like to acknowledge the Director of Research Studies Prof. Terry Walsh for his support and his leadership qualities.

I appreciate the help of the IT department and Library staff who responded promptly to all the urgent requests I made.

I must also thank Dr. Chris Carvalho and Mr. Patrick Stevens for their availability and kindness with regard to instrumental techniques. I appreciate Dr. Chris for his knowledge and rational suggestions, mixed with his sense of humour.

I am grateful to Dr. Liew Rintoul to whom I always looked for his expertise and assistance with the details of the various spectroscopic techniques.

I would also like to extend a big thanks to the AEMF staff and particularly to Dr. Thor E. Bostrom, Dr. Loc Duong, Ms. Christina Theodoropoulos and Mr. Lambert Bekessy for assisting me in my research.

Thanks also go to Mr. Anthony Raftery from XRD facilities for his assistance and availability throughout the year and his understanding of the fact that a material scientist can‟t survive without XRD analysis.

I also give gratitude to Mr. Bill Kwiecien and Mr. Shane Russell who were extremely helpful in assisting in the operation of the ICP analytical facility. I appreciate Bill‟s sharp sense of humour. Special thanks go to Dr. Robert M. Wellard from the NMR facility for his willingness and dedication to support research.

I would also like to take this opportunity to thank Ms. Vera Combeer for her tireless responses to my requests for chemicals and all other necessary items. Enormous thanks go to Mr. Nick Ryan for his ability to apply lab safety strictness accompanied by a broad smile.

I am indeed grateful to Ms. Lois McLaughlin from the Student Research Centre for her kind and friendly words of encouragement. I would also like to thank Ms. Kerry Kurger for her excellent assistance. Her big welcoming smile always made me forgetting my stress.

Last but not least, with some guilty feelings, I wish to acknowledge the patience of my parents who encouraged me to follow my heart and to choose my career in research. My heart felt gratitude and thanks to my brother Mr. Blits Paul with whom my childhood was spent happily in a loving home. His calm and encouraging words always showered my stress. I would also like to extend my thanks to my friends who were to some extent neglected during the period of my research.

Blain Paul, June 2010

xxi

Publications Arising From This Work

Blain Paul, Dongjiang Yang, Xuzhuang Yang, Xuebin Ke, Ray Frost, Huaiyong Zhu “Adsorption of the Herbicide Simazine on Moderately Acid-Activated Beidellite” Applied Clay Science, 2010, 49, (1-2), 80-83

Dongjiang Yang, Blain Paul, Huaiyong Zhu, Wujun Xu, Yong Yuan, Erming Liu, Xuebin Ke, Mark Wellard, Xueping Gao, Yao Xu, Yuhan Sun “Alumina nanofibres Grafted with Functional Groups: A new design in Efficient Sorbents for Removal of Toxic Contaminants from water” Water Research, 2010, 44, 7 4 1-7 5 0

Blain Paul, Dongjiang Yang, Wayde Martens, Ray Frost “Sodium Niobate Adsorbents Doped with Tantalum (TaV) for the Removal of Bivalence Radioactive Ions from Waste Water” Journal of colloidal and interface science, 2010, Manuscript Accepted

Blain Paul, Wayde Martens, Ray Frost “Modification of Activated Alumina Fibres for the Removal of Non-Ionic Alachlor and Anionic Imazaquin” Water Research, 2010, Manuscript Submitted

Blain Paul, Wayde Martens, Ray Frost “Use of Organosilane in the Modification of Activated Beidellite Clay for the removal of Non-Ionic Alachlor and Anionic Imazaquin” Applied Clay Science, 2010, Manuscript Submitted

Blain Paul, Wayde Martens, Ray Frost “Photocatalytic Studies of Mixed-Phase Titania Nanofibres for the Degradation of Trace Pesticides and Phenols” Journal of Photochemistry and Photobiology, 2010, Manuscript Submitted

Blain Paul, Wayde Martens, Ray Frost

“Photocatalytic Degradation of Trace Pesticides and Phenols Using TiO2 Immobilised on Laponite Clay” Applied Clay Science, 2010, Manuscript Submitted

xxii

Organisation of Research Contents

xxiii Chapter One

CHAPTER 1

Introduction and Rationale for Research

- 1 - Chapter One

1.1 Literature Review

1.1.1 Introduction

Although many new nanostructured materials are superior in performance to conventional materials, it is important to search for an effective approach in order to create new nanostructures of high efficiency for advanced application in water purification. Nanosized materials such as nanowires, nanorods, nanofibres and nanotubes, are of scientific and technological interest because of their high performance in various applications compared to bulk materials [1-9]. The various kinds of applications of nano porous solids involve ion-exchange, industrial adsorbent, catalysis and catalyst support [10-19].

The surface atoms in the porous solids create an environment that determines the way materials interact with the surroundings. More precisely, in the case of high surface area materials, surface properties can dominate the overall material behaviour. This allows one to construct host materials that are no longer determined by the bulk material, but by their nanoscale architectures. The design of different sizes, shapes and volumes of the pores in porous solids directly influences their ability to perform in unique applications.

For instance, porous silica can adsorb organic components from water because of the hydrophobic nature of the surface whereas aluminosilicates are hydrophilic and thus can adsorb water from organic solvents. Here we cover several examples ranging from silicates to nonsilicates and also demonstrate the tuneability of porous materials for various applications such as water purification and photocatalysis. Based on pore size, these are classified by International Union of Pure and Applied Chemistry (IUPAC), those below 2 nm are micropores, and those in the range of 2 nm to 50 nm are mesopores, those above 50 nm belong to the category of macropores.

- 2 - Chapter One

The term „molecular sieve‟, first used by McBain in 1932, refers to a material having a fixed, open network structure capable of reversibly desorbing an adsorbed phase. They may be used to separate hydrocarbons or other mixtures by selective occlusion of one or more of the constituents [20]. A useful classification of crystalline oxide molecular sieves is shown in Figure 1-1.

Only the crystalline oxide materials are considered in the diagram, so amorphous or partially crystalline materials such as amorphous silica, alumina, pillared clays, porous carbons, membranes and organic systems are not covered. Molecular sieves of the crystalline aluminosilicate type, usually referred to as zeolites (Figure 1-2 a), 4– 5– are formed by corner sharing [SiO4] and [AlO4] tetraheda. These crystalline zeolites have significant ion-exchange capacity [22]. Silicon-oxygen tetrahedra are electrically neutral when connected together in a three dimensional network of 4+ 3+ quartz (SiO2). The replacement of Si by Al creates an electrical imbalance which is countered by exchangeable cations held electrostatically within the zeolites.

- 3 - Chapter One

The zeolites, of which there are many naturally occurring and synthetic examples, are the best known and most studied type of microporous solid. Although many types of metal cations can be easily exchanged into the pores of the zeolites, it is difficult to incorporate large, stoichometric amounts of metal cations into the framework as they often substitute into tetrahedral sites in an apparently random fashion while maintaining the original structure of the zeolite framework.

Figure 1-2 − Molecular sieve materials [23]: a) Zeolite (4-coordination); b) Aluminophosphate (4-coordination); c) Mixed Oxides (4, 5, 6-coordination); d) Germanates (4, 5, 6-coordination).

The zeolites and aluminophosphate materials account for the bulk of the known microporous solids [20]. (For the purpose of clarity, in this particular section, I will first discuss the synthetic microporous materials, and later will address natural clay and alumina fibres).

A variety of zeolite type and other related new materials with novel micorporous framework structures have been synthesised. In recent years, crystalline aluminophosphate compositions were synthesized. These are microporous

- 4 - Chapter One framework oxide molecular sieves (Figure 1-2 b) [21, 24]. This is a vast field which now contains, with all the framework substitutional variants, more species than the 5- 3- zeolites. These materials are formed from [AlO4] and [PO4] corner shared tetrahedral groups, overall the framework has no net charge and consequently, no cation-exchange properties. It therefore exhibits few catalytic capabilities (Figure 1- 3) [20, 22].

Other classes of porous tetrahedral framework solids related to the AlPO phases are the metalloalumino-phosphates (MeAPO) [24]. This class of materials includes, in addition to the MeAPO (Me=Co, Fe, Zn, Mg, Mn, etc.) solids, the silicoaluminophosphates (SAPO) and quaternary systems containing metals, Si, P and Al in an oxide matrix [21]. Most of these tetrahedral solids are less acidic than aluminosilicates and therefore are catalytically less reactive in a wide variety of reactions. Other tetrahedral solids include phases in the Ge–Al–O and Ga–Ge–O systems, and most are isostructural with known zeolites or minerals [21].

Figure 1-3 − Pore characteristics in the aluminophosphates [25]: a) AlPO4-11; b) AlPO-5; c) VPI-5.

In a recent study, a new class of materials with inter-linked octahedral and tetrahedral framework microporous structures based on titanium silicate system has been reported [26]. These were discovered by Kuznicki and co-workers in 1989 at Englehard and were referred to as ETS-10 (Englehard titanium silicates) (Figure 1- 4). A detailed study has proven the structure of highly crystalline ETS-10 [27]. Das et al. reported similar results for the synthesis of ETS using TiCl4 precursor, another titanium source [28]. Liu and Thomas have shown that under appropriate conditions

- 5 - Chapter One

the thermodynamically stable TiO2 phases of anatase and rutile can be used as suitable titanium sources for the synthesis of porous titanosilicates [29].

Chapman and Roe used organotitanium compounds in the preparation of GTS-1 (Grace titanium silicate) while Clearfield and co-workers synthesized a novel porous titaniosilicate using titanium isopropoxide [26, 30]. The synthesis of ETS-10 usually requires the presence of both Na+ and K+ ions in the parent gel but some of the materials are produced in a pure state only when a single type of cation is present [31]. Another detailed study of pure and more crystalline ETS-10 prepared from TiCl3 and anatase has been reported [32].

Figure 1-4 − Shape and pore size of ETS-10 [33]: a) elliptical pore shape b) circular pore shape.

Detailed studies on framework insertion of aluminium, gallium, boron and niobium in ETS-10 have been reported [34-37]. The following sources of aluminium, gallium, niobium and boron for element insertion in the ETS-10 frame work have been reported: NaAlO2, GaCl3, Nb(HC2O4)5 and Na2B2O4 [32, 34-37]. ZrCl4 and

Zr(OC3H7)4 have been used as the zirconium sources for the synthesis of microporous zirconosilicates [38-41]. The vanadosilicate AM-6, a structural analogue of ETS-10, has been prepared from VOSO4.5H2O [42].

Another class of material discussed here is built up from MoO6 octahedra and PO4 tetrahedra. The PO4 terahedra only share corners, while the MoO6 octahedra

- 6 - Chapter One predominantly share corners but also occasionally share their edges and faces as well. The octahedral-tetrahedral molybdenum phosphate system has very rich crystal chemistry and contains a large number of new structure types such as tunnels and cages with diverse polyhedral connectivities [43].

In all the discussed materials the framework metals are in tetrahedral or inter-linked octahedral and tetrahedral coordination, although under certain circumstances (for example, various hydration conditions) the coordination might change to octahedral [44]. Porous structures comprised entirely of octahedral sites have also been generated. For example, the manganese oxides known as OMS materials contain only octahedral framework atoms [45]. A series of manganese octahedral molecular sieves (OMS) have been synthesized with MnO6 octahedra sharing corners and edges to form tunnel structures of varying sizes [46-50].

Synthetic todorokite which has a tunnel structure having a pore size of about 6.9 Å is referred to as OMS-1. Synthetic cryptomelane which is a related structure of the mineral hollandite in the K+ form, having a tunnel pore size of about 4.6 Å is referred to as OMS-2. Natural todorokite contains Ni, Co, Cu Zn, Al, and Mg in an oxide matrix. Futhermore, some of these cations have been found to be mobile and stabilize the structure of the nodule. Such cations may even be incorporated into the framework of todorokite or probably by isomorphously substituting the Mn2+ cations found in the framework of todorokite [51, 52]. One basis for such accommodation of inorganic cations is the relative size of the Mg2+, Co2+, Ni2+, Cu2+ and Zn2+ cations coordinated with six water molecules which are similar to the tunnel size of todorokite. In view of this, ion-exchanged OMS-1 materials having divalent ions substituting for Mn2+ have been prepared. Shen et al. have successfully used five hydrated inorganic divalent cations, Mg2+, Co2+, Ni2+, Cu2+ and Zn2+, as templates for the synthesis of OMS-1 [44, 45, 47, 53]. Among different tunnel cations, Cu2+ was also reported in a different study [54].

- 7 - Chapter One

1.1.2 Niobate nanostructures

In recent years, Nyman et al. have synthesized a new class of octahedral microporous phases by the substitution of Ti or Zr in a NaNbO3 octahedral framework with the compositions Na2Nb2-x Mx O6-x (OH)x. H2O (M=Ti, Zr; and 0< x) [55, 56]. These phases, named Sandia Octahedral Molecular Sieves (SOMS), possess a framework structure composed of [NbO6], [MO6], and [NaO6] octahedral linked by corners or edge-sharing. In the structure, Nb and Ti/Zr occupy the same framework positions and the remaining Na resides in the channels. This structure is unusual in the sense that Na, which is typically an extra framework cation, also participates in the framework.

A new microporous phase Na2Nb2O6.H2O, which transforms to NaNbO3 perovskite on heating, has been synthesized by hydrothermal methods [57]. This phase belongs to the recently synthesized Sandia octahedral molecular sieves (SOMS) family [55,

56]. The Na2Nb2-xMxO6-x(OH)xH2O (M=Ti, Zr; and 0< x) phases can be considered as derivatives of the nominal Na2Nb2O6.H2O structure. Zhu et al. have investigated the structural evolution during the reaction between Nb2O5 powder and concentrated aqueous NaOH solution under hydrothermal conditions which provides an opportunity for selecting the niobate products with the desired morphologies and structures through the kinetic control of the reaction [58]. During the hydrothermal synthesis, Nb2O5 powder aggregates first to irregular bars, then to niobate fibres with a formula of Na2Nb2O6.2/3H2O and finally the fibres convert to cubes. These fibres are found to be microporous niobate molecular sieves and are quite different to cubes of NaNbO3 in terms of the composition, shape, crystal phase, and ion exchange properties. In the microporous fibres, the NbO6 octahedra are mainly in an edge sharing configuration but in NaNbO3 crystals the NbO6 octahedra are in a corner sharing mode. NaNbO3 cubes are thermodynamically more stable than the crystal of the fibres.

With the examples of various microporous structures, we begin to find new applications in their traditional areas of use, such as catalysis, separation and ion exchange. In addition, non-traditional applications exploiting the ion exchange

- 8 - Chapter One property of the material itself for various environmental applications. Ion exchange is an effective separation technique used in industry for the removal of metal ions especially in water and wastewater treatments [59-63]. Although adsorption is also used for the removal of metal ions from waste water [64-66].

However, ion exchange processes are selective, very effective and able to remove very low levels of metal ions from water [61]. In fact, ion-exchange property is important for industrial applications because industrial wastewaters often contain more than one metal ion. Ion exchange is applicable only if an exchanger has a high selectivity for the metals to be removed. Many zeolites are able to remove cations from aqueous solutions by ion exchange [60, 62, 63]. Although, in non-traditional applications, using crystalline porous solids, separation of radioactive ions from water and their safe disposal have not yet been attempted in large scale [59, 61]. It was found that bentonite can adsorb a maximum capacity of 32.94 mg of Sr2+/g at a pH of 8.5 [67]. Uranium and thorium uptake by phyllosilicate mineral is well documented in the literature [68].

There are a few reports available on the effective removal of radioactive cesium and strontium from water [61, 69-71]. Organic materials undergo radiolysis in the presence of large amount of radioactive cations. Due to high selectivity and high radiation stability, inorganic ion exchange materials are preferred to organic resins in the treatment of nuclear waste effluents. There are, however, some serious problems in using inorganic materials for the removal of nuclear wastes. Zeolites and metal salts decompose in highly alkaline solutions and clay is not selective at high sodium concentrations. Development of chemically stable materials which can selectively uptake radioactive cations from water is still a great challenge. Several efficient inorganic exchangers, e.g. silicates [72-74], titanates [75-77], silicotitanates [30, 78] and hexacyanoferrate [30, 79, 80] compounds have been prepared. These materials can remove radioactive cations efficiently even from concentrated salt solutions.

The main conclusion drawn from this discussion is that the ion-exchange rates of many of these above mentioned crystalline structures are low. With the examples of these crystalline solid structures as encouragement, we started to investigate the

- 9 - Chapter One hydrothermal synthesis of niobates to try to incorporate larger cations to modify the framework structures as well as to improve the ion exchange properties for the selective removal of radioactive cations from water. One of the necessary advantages of hydrothermal synthesis of solids is the ability to attain a reaction between two solid-state starting materials at much lower temperatures than those required for the inter-diffusion of the solids without a liquid phase. More important however in the present study would be the possibility of incorporating large inorganic cations into the framework of niobate solids during the hydrothermal synthesis.

1.1.3 Clays and clay modifications

Under International Association for the study of clays (AIPEA) nomenclature, only natural materials can be denoted as clays. Synthetic clay is more properly distinguished as a clay mineral. As mentioned in the beginning, this section mainly discusses natural clay minerals. Clays are abundant and inexpensive aluminosilicates. In general, layered clays can be of two types: firstly clays of a 1:1 type and secondly clays of a 2:1 type. The 1:1 group shows less reactivity compared to the 2:1 group which is associated with an expanding lattice [81].

These materials can be modified by different processes such as ion exchange with interlayer ions, ion exchange at crystal edges, the deposition of metal and oxide particles, the grafting of organic species, and the chemical bonding of organic ligands. One important consequence of the charged nature of the clays is they are generally highly hydrophilic and therefore naturally incompatible for applications as adsorbents. A necessary prerequisite for such applications is therefore the alternation of the clay polarity to make the clay organophilic. The most often used modification method is the ion-exchange reaction with an organic cation such as an alkyl ammonium ion, which can increase the clay interlayer spacing and create a more favourable organophilic environment (Figure 1-5) and is also called intercalation.

- 10 - Chapter One

In the case of an intercalate, the organic component is inserted between the layers of the clay such that the interlayer space is expanded, but the layers still bear a well defined spatial relationship to each other. Another modification approach involves a direct grafting reaction to form covalent bonds with clays [82]. All modification methods will be discussed briefly at the end of this section. Depending on the layer charge, layered clays can be divided into two groups such as anionic clays and cationic clays. Layered double hydroxides (LDHs) are a family of compounds which belong to anionic clays whereas the smectites group belongs to cationic clays.

Figure 1-5 − Ion exchange process with interlayer ions (e.g. exchange of sodium 3+ − ions by HO2C–R–NH Cl ).

Layered double hydroxides (LDHs) (Figure 1-6), also know as anionic clays, are a family of Mg–Al compounds consisting of positively brucite-like layers 2+ 2– [Mg6Al2(OH)16] with exchangeable compensating anions, such as CO3 in the interlayer regions [83]. The electric charge of the layers and the charge of the ions in the interlayer space are opposite to that found in silicate clays (cationic clays).

- 11 - Chapter One

Figure 1-6 − Schematic structure of a hydrotalcite like clay such as layered double hydroxides [84].

As in silicate clays, the ions present in the interlayers of hydrotalcite can easily be exchanged [85-87]. The cations present in the layer can also be substituted and studies have been reported in literature based on the systems with M2+/M3+, M2+/M4+ and M+/M3+cations in the layers [88-90]. The intercalation of metal- containing anions in the interlayer space of hydrotalcites provides several advantages. LDHs intercalated with halo-complexes have been investigated as catalysts for chloride exchange reactions and also as modified electrodes [91-93]. Layered double hydroxides intercalated with cyano-complexes of iron, cobalt and molybdenum, and their properties for hydrocarbon adsorption and electrochemical behaviour, have been studied [94-96]. Mo–complexes in the interlayer space of LDH and their role as catalysts have also been reported [97, 98].

Layered silicates of the smectite clays such as montmorillonite, beidellite, hectorite and saponite, are a class of layered aluminosilicate minerals with a unique combination of swelling, intercalation and ion-exchange properties which make

- 12 - Chapter One these nanostructures valuable in various fields of technology [99-101]. The basal sheet of phylosilicates is 2:1 layered aluminosilicate; each sheet consists of two tetrahedral silicate layers and one octahedral alumino layer with cations such as Na+, K+, Ca2+ or Mg2+ located in the interlayer space (Figure 1-7). The general formula of 2:1 dioctahedral smectite is Nax(SiaAl8-a)AlbMg4-bO20(OH)4, where x = (12–a–b) is the layer charge and Na+ is the balancing interlayer cation. Montmorillonite and beidellite structure have been generated from the well-defined crystal structure of dioctahedral pyrophyllite having formula Si8Al4O20(OH)4. The subsequent formulas of idealized montmoorillonite and beidellite are

NaSi8All3MgO20(OH)4 and NaAlSi7Al4O20 (OH)4, respectively.

Figure 1-7 − Structural representation of smectite.

The cation exchange capacity of these clays, which depends on the substitution of low-valent atoms, e.g., Mg2+ for Al3+ in the octahedral sheet and Al3+ for Si4+ in the tetrahedral sites. As a result, the layers have a fixed negative charge and neutrality is achieved by the exchangeable cations present within the intralamellar region between two clay layers. Therefore the intercalation process in these systems is equivalent to ion exchange. Moreover, the exchangeable cations of these materials

- 13 - Chapter One can be replaced with other cationic species or with organic species. The negative charge on the layers affect many fundamental properties of the clays such as swelling ability, cation exchange capacity, water holding and specific surface areas.

The distance between the layers can be measured by using X-ray diffraction and is mainly depends on the number of intercalated water and cation molecules within the interlamellar spaces [102]. Furthermore, the clay layers are coupled through relatively weak dipolar and Van der Waals forces. However, smectites have some inherent drawbacks such as low surface area, and high hydrophilicity. To overcome such drawbacks, the controlled chemical modification of the clay structure is necessary. Acid activation and the introduction of functional groups into the layers through chemical modification such as intercalation and grafting are the key ways of alleviating this problem (Figure 1-8). These functional groups can confer new properties without destroying the many desirable intrinsic properties.

Intercalation of porphyrins and metallophthalocyanines has been extensively used in clay modifications. In this method, the fixation process of the complexes is carried out based on the ion-exchange properties of the clay host. Intercalation of water soluble tetracationic metalloporphyrins can be achieved by ion-exchange in the interlayer space of the montmorillonite by hydrothermal processes [103]. Kosiur modified montmorillonite clay by intercalating hemin, protoporphyrin and hematoporphyrin in aqueous solutions [104]. Cady and Pinnavaia applied this intercalation technique for metal complexes of meso-tetraphenylporphyrin into montmorillonite [105].

Successful synthesis of meso-tetra-alkylporphyrins within the montmorillonite clay using aliphatic aldehydes and pyrrole has been achieved by Onaka et al. [106, 107]. The adsorption of two long aliphatic chains in interstack galleries of the aluminosilicates Na+ montmorillonite has been studied by Gerasin et al. [108]. Similarly, Jin et al. prepared protein-silicate hybrids from polyamine intercalation of layered montmorillonite [109]. In a new approach, Hu and Rusling [110] reported the use of a surfactant–intercalated clay films to incorporate neutral molecules such as metallophthalocyanine. Further studies have also been published on the excellent stability of this system [111].

- 14 - Chapter One

Recently, the effect of the molecular structure of a cationic azo dye on the photo induced intercalation of phenol into the azo dye-montmorillonites has also been investigated [112]. In a recent publication, Guerra et al. reported the use of 1,4- bis(3-aminopropyl)piperazine functionalised with montmorillonite and synthetic kanemite which then reacted with methylacrylate to yield new inorganic-organic chelating materials [113]. Intercalation of triethylene glycol monodecyl ether (C10E3) non-ionic surfactant into Ca-montmorillonite has also been investigated [114]. In a recent publication, Li et al. reported that anionic surfactant modified Fe- pillared montmorillonites prepared using Fe-hydrate solution and sodium dodecyl sulfate (SDS) solution [115].

Figure 1-8 − Schematic drawing illustrating the incorporation of complex species into different positions of clay solids [116].

The industrial history of the acid modification of clay to impart new properties such as high surface area support for catalysts and as selective adsorbents can be tracked back to 1950‟s [117-124]. Acid activated clays can be considered as an active compound due to the presence of the hydroxyl groups in each clay residue [125]. Acid treatments can enhance the accessibility and the reactivity of clay for subsequent grafting reactions by opening internal pores and cavities, thereby disturbing the crystalline order and increasing the availability of more reactive hydroxyls [126].

- 15 - Chapter One

Among the methods of modification of clay, direct covalent grafting offers an attractive and versatile means of imparting a variety of functional groups to clay [127, 128]. In fact, clay based materials with valuable properties can be achieved via a grafting process by changing parameters such as the types of grafting agents and the grafting density [129]. According to the end use or specific needs, clay can be grafted using specific grafting agents [130]. Recently, this modification method has been used by it combining with the ion-exchange modification in preparation of nanocomposites [131]. In recent publications it is suggested that the end group of silanes could react with monomers followed by polymerization, resulting in the covalently bonded polymers on the clay surfaces [128, 131]. Many studies have been carried out to characterise the adsorption and the activities of organic pollutants on the smectite family [132-140]. To our knowledge, there are not many reports concerning the mechanism of immobilisation of organic pollutants on organically modified clay. Therefore, a more detailed study needs to be carried out in order to have a more complete understanding of all of the factors influencing this mechanism.

- 16 - Chapter One

1.1.4 Alumina nanostructures

As discussed in Section 1.1.3, a key requirement for removing pollutants from water is the capacity of the sorbents to withstand high interactions of microstructures with pollutants. The use of porous silicates for pollutant filtration is often limited because of its tendency to fill the open or continuous porosity during the process of modification. Although many of the new adsorbents discussed in the last sections are superior in performance, these functionalised matrixes also remain relatively less efficient because most of the interacting sites are located within the pores and resulting is the weak interaction with the target pollutants. It is important to search for an effective approach to develop fibre (boehmite fibres) based filtration techniques, which can provide high flow rate during the filtration process.

In the last decade, preparation of stable ultra thin oxide nanofibres has been reported [141, 142]. It is generally accepted that boehmite (AlO(OH)) nanofibres are a precursor for various nanostructured materials [143]. Boehmite has a layered structure [144]. As seen in Figure 1-9, the basic layer is composed of two sheets of edge sharing AlO6-octahedra. The aluminum atom in the octahedron is coordinated by four O(1) and two O(2) atoms, the O(1) atoms are inside the layer and are shared by adjacent AlO6-octahedra. O(2) is connected to the hydrogen atoms.

The phase transition of boehmite fibres into -alumina at higher temperature gives thermodynamical stability to the fibres [143, 145]. When the texture and morphology of the -alumina are considered, it is easy to guess that grafting of alumina surface with large number of organic functional groups can make them possible to use as a filter membrane for the removal of the pollutants from water.

- 17 - Chapter One

Figure 1-9 − The crystal structure of boehmite [144].

The fibrillar interstices between thin nanofibres are much larger than the pores in conventional mesoporous materials, allowing not only a higher flow rate but also giving easy access to the adsorption sites. Surface modification can attribute excellent performance to the adsorption process, which can be specifically tuned to adsorb various pollutants. Wettability of the fibres can also be controlled during the grafting process by using various organic functional groups. This would add significantly more valuable properties to enhance the uptake of pollutants. In addition, covalent grafting on the surface should be permanent and not changed at high water streams.

- 18 - Chapter One

1.1.5 Photocatalysis

Large scale titanium production was initiated after the Second World War due to the high demand from the aerospace technologies for the light and high melting temperature metal. In the 1950s, the traditional Kroll molten salt extraction process for the industrial production of titanium had been improved drastically to meet the demands in various industries.

In 1972 Fujishima and Honda discovered that water could be split upon photo excitation of TiO2 single-crystal electrode after applying a small electrochemical bias [146]. Following this finding, extensive research has been underway to develop an ideal method for the production of hydrogen (as a combustible fuel) from water as a means of solar energy conversion. This has also prompted the interest of novel redox reactions of organic and other inorganic substrates by variety of semiconductor particles (TiO2, CdS, ZnO, Fe2O3, and ZnS) with sizes ranging from clusters and colloids to powders and large single crystals [147].

Over the last two decades, improvements in the synthesis of nanostructured titanium dioxide and titanate materials have been observed in a wide range of applications such as photocatalysis, dye sensitized photovoltaic cells and sensors [150]. Initially, the synthesis of nanostructured TiO2 materials was based on sol-gel techniques, in which the spheroidal particles of various sizes range down to few nanometers. In 1998, synthesis of titanium oxide nanotubes in an alkaline hydrothermal route was achieved by Kasuga et al. [151]. Figure 1-10 represents the different types of morphological forms that will be discussed in this section [148]. The basic synthesis method for the preparation of titanate nanotubes includes the use of TiO2 raw material in 10 M NaOH at temperatures in the range 110−150 oC for 24 h. The average length of the nanotubes can be adjusted either by using ultrasonic treatment of TiO2 raw material [152] or by improving the transport conditions during the alkaline hydrothermal treatment [153]. Using different synthesis temperatures the average diameter of the nanotubes can be adjusted [154]. The shape of the tubular agglomerates can also be controlled by using hydrogen peroxide [155].

- 19 - Chapter One

Figure 1-10 – Titanate nanostructures in different morphologies: a) circular- shaped rods; b) multi nanosheets c) spheroids; d) rectangular shaped fibres; e) multi-wall nanotubes [148].

Several methods have been developed to reduce the synthesis time such as using microwave heating [156, 157] or the improvement of mixing using a resolving autoclave [158]. Many attempts have been made to avoid the use of pressurized reactors resulting in the inventions of nanosheets instead of nanotubes [159] (Figure 1-11). In one of the low temperature routes, the spontaneous formation of titanate nanotubes was achieved by the addition of NaOH to a colloidal solution of titanate nanosheets [160]. Another approach involved the calcination of TiO2 with Cs2CO3 followed by the ion exchange of Cs+ with H+ and then by the exfoliation of titanate nanosheets in the presence of tetrabutylammonium hydroxide at room temperature [161] (Figure 1-12). A recently reported approach used a mixture of NaOH and KOH to achieve a low temperature route [162].

- 20 - Chapter One

Figure 1-11 – A history timeline explaining the development of TiO2 nanostructures [149].

Figure 1-12 – Atomic architecture of the nanosheet [161].

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Titanate nanofibres were synthesized using the hydrothermal treatment of TiO2 raw materials with 10 M NaOH solution at temperatures higher than 170 oC [154]. Some other researchers used KOH instead of NaOH, which also resulted in the formation of nanofibres [163, 164]. It was also found that lower synthesis temperatures lead to the formation of a mixture of nanofibres and nanotubes [165].

Crystal structures of the tubular products prepared from the alkaline hydrothermal treatment of TiO2 have been studied by several researchers and have been found to correspond more closely to sodium titanates rather than anatase or TiO2-(B) [166, 167] (Figure 1-13). The layers in the nanotube wall consist of edge and corner- sharing TiO6 octahedrons which build up into zigzag, corrugated structures [149].

Figure 1-13 – A schematic model of the layered structure of a scrolled titania nanotube [166].

The following steps describe the formation mechanism of nanotubes [149]: (1) formation of layered nanosheets of sodium titanates by the dissolution of the raw

TiO2 (2) exfoliation of the nanosheets (3) folding of the nanosheets into tubular structures (4) growth of the nanotubes along the axis (5) exchange of Na+ ions by H+ ions during washing and separation of nanotubes. The driving force for folding the nanotubes has been studied by several groups. Zheng et al. have proposed that the imbalance of the hydrogen or sodium ion concentration on two different sides of a nanosheet provide excess surface energy for the bending of nanosheets [168]. As can be seen in Figure 1-14, Kukovecz et al. have suggested that the titania nanotubes were formed by oriented crystal growth instead of individual layers which roll up into nanotubes [153]. The alkaline treatment of titanate nanotubes at temperatures higher than 170 oC results in the formation of titante nanofibres [169].

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In this instance the crystallisation rate is high and nanosheets become too thick for any kind of bending. This leads to the formation of nanofibres rather than nanotubes. However no evidence has been presented for the transformation of nanofibres back to nanotubes at lower temperatures. At higher temperatures, the bending of nanosheets doesn‟t occur and the resulting nanofibres are perceived to be growing in a crystallographic direction c. It is interesting to observe that the axis of nanotubes does not always coincide with the axis of nanofibres. Bending of nanosheets will occur around the b-axis and there will only be two directions for the growth of nanotubes namely, along the crystallographic a-axis and along the crystallographic b-axis.

Figure 1-14 – Mechanism of trititanate nanotubes formation [153].

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In this section the phase transformation of titanate nanostructures at different temperatures is discussed. It is important to understand the range of stable operational conditions of titanate nanostructures for many applications.

Transformations of TiO2 occurring under the influence of various conditions are shown in Figure 1-15.

Figure 1-15 – Chemical and structural transformations of titanate nanotubes and nanofibres [149].

In the following section our intention is to give an overview of some of heterogeneous semiconductor photocatalysis and potential applications as an environmental remediation technology. Heterogeneously dispersed semiconductors can act as a sensitizer for light-induced redox reactions due to their electronic structure. The illumination of semiconductors with a photon of energy leads to simultaneous oxidation and reduction.

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This process often accomplishes the oxidation or reduction of an organic or inorganic substrate. In most experiments and applications with semiconductor photocatalysis, molecular oxygen is present to act as an oxidizing agent. The organic degradation per active site on catalysts can be attained without any significant changes in the catalytic activity of the catalyst.

Over the last 20 years the scientific interest in these chemical redox reactions has grown exponentially because of the use of photoexcited semiconductor dispersions in a set of environmental problems related to the remediation of hazardous wastes, contaminated groundwater and the control of air pollution. The main advantages of photocatalytic degradation of pollutants over other processes are [170]: (a) oxidation of pollutants in ppb range (b) complete mineralization of pollutants within few hours (c) inexpensive (d) no need of control for proper temperature and pH (e) no sludge disposal problem. There are a wide range of semiconducting materials which are readily available, but the following points need to be considered when choosing a suitable semiconductor photocatalyst for the mineralization of organic pollutants [170]: (1) it should be photoactive (2) ability to use visible and/or near UV light (3) inexpensive (4) biologically and chemically inactive (5) photostable

(6) nontoxic. Semiconductor photo catalysis with wide applicability on TiO2 as a nontoxic durable photocatalyst has been applied to a variety of problems of environmental interest such as the treatment of toxic and biologically stable compounds [171-175].

The term photodegradation is usually used to refer to complete oxidative mineralization. Photodegradation reactions utilize the oxidizing power of highly active intermediates directly or indirectly for the transformation of organic – compounds to Cl , CO2, H2O, or other oxides. The degradation processes mainly start with photooxygenation, photooxidative cleavages, or other oxidative conversions through functional group interconversions. A tremendous level of work in semiconductor photochemistry has shown that organic contaminants can be completely mineralized on illuminated TiO2 suspended in water [177-185]. Semiconductor photocatalysis appears to be a promising technology because semiconductors are cheap, nontoxic and possible for repeated use without any loss of activity after having been recovered by filtration or centrifugation.

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There are four titania polymorphs (rutile, anatase, brookite, and TiO2-(B) found in nature. The crystal structure of anatase, rutile and brookite are shown in Figure 1- 16. Each titania polymorph is described by representative octahedra made of oxygen atoms at its vertices and a titanium atom near the center and is the building block of all the polymorphs [176, 186, 187]. TiO2-(B) and anatase contain chains of edge- sharing octahedra in one orientation but rutile and brookite have chains in two orientations. TiO2 in the anatase form suspended in aerated water has proven to be the most active photocatalyst and most practical of the semiconductors for environmental applications such as water purification and hazardous waste treatment [188]. The photocatalytic activities of semiconductors are due to their special electronic structure with a filled valence band and an empty conduction band [189].

Figure 1-16  The crystal structure of TiO2 polymorphs: (a) anatase; (b) rutile; (c) brookite [176]

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When a photon with energy of hν matches or exceeds the band gap energy, Eg, of the semiconductor, an electron is promoted from the valence band into the conduction band leaving a hole behind (Figure 1-17). These electrons and holes react with electron donors and electron acceptors adsorbed on the surface of the semiconductor particles. Under steady conditions, the rate of formation of electrons and holes can be represented by the following equations.

+ + − + d[h ]/dt = 0 = 0 − k1 [h ]k3[e ][h ] (3)

+ − − + d[e ]/dt = 0 = 0 − k2 [e ][O2] i − k3[e ][h ] (4)

 0 is the intensity of light absorbed by TiO2 which initiate the formation of electrons and holes. The [h+] and [e–] represent the concentration of hole and electron respectively. The [O2]i denotes the oxygen concentration at the TiO2/water interface. Here k1 and k2 are the rate constants for the removal of holes and electrons by reacting with electron donors and electron acceptors. The k3 is the rate constant for the recombination of electrons and holes.

Figure 1-17 − Schematic representation of surface electron and hole processes in the reduction of O2 at illuminated titania [190].

These valence-band holes and conduction-band electrons are powerful oxidants and reductants, respectively [191]. Most organic reactions utilize this oxidizing power.

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In the absence of suitable electron and hole scavengers, the stored energy is released as heat within a few nanoseconds by recombination. If a suitable electron acceptor or electron donor is available to trap the electron or hole, recombination is prevented and subsequent redox reactions may occur. Water exclusively involved as a hole acceptor produces hydroxyl radicals as an intermediate and the electrons then interact with the molecular oxygen present in the oxygenated aqueous solutions to •– produce a superoxide intermediate(O2 ). In the presence of any oxidizable species such as organic pollutants, hydroxyl radicals undergo reduction resulting in the mineralization of organic pollutants [192].

In recent years, Degussa P25 TiO2 has set the standard for photo reactivity in environmental applications. This is a nonporous 70:30% anatase to rutile mixture with a BET surface area of 55 m2g–1 and a crystallite size of 30 nm in 0.1 μm diameter aggregates [170]. The high photoreactivity of P25 is due to the slow recombination rate of the conduction band electrons and valence band holes [193]. Different recombination and interfacial charge transfer rates could be achieved by different preparation methods of the samples but which lead to different morphologies and defect structures [194]. For many reactions the rutile phase particle is photocatalytically less active than the anatase phase [170, 195]. CdS is also an efficient catalyst and is a suitable alternative to TiO2 but is unstable during irradiation [170].

Considerable information is available on the photocatalytic mineralization process in terms of reactions and degradation mechanism of individual compounds. David Ollis and his co-workers used semiconductor photocatalysis first time as a method of water purification in 1983 for the degradation of trichloroethylene in water by using TiO2 [196]. Besides this, Hsiao et al. have shown the photocatalytic degradation of dilute solutions of dichloromethane (CH2Cl2), chloroform (CHCl3), and carbon tetrachloride (CCl4) with an illuminated TiO2 photocatalyst [197]. In the case of photocatalytic degradation of aromatic compounds, Matthews demonstrated the photocatalytic oxidation of chlorobenzene in aqueous suspensions of titanium dioxide [198]. Subsequently, many researchers have extensively studied the photocatalytic degradation of phenol due to its high toxicity and ubiquity [199-201]. UV illumination of fluoroalkenes or aromatics in an aerated aqueous suspension

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results in the formation of CO2 and HF [202]. Common contaminants of municipal water supplies such as chloroform, carbon tetrachloride and aromatic halogenated substrates were studied intensively [203]. Atrazine is degraded in very short time of irradiation using particulate TiO2 as photocatalyst under simulated solar light whereas chlorodioxin takes many hours for complete mineralization [204]. DDT can be transformed into harmless products by sunlight illumination over TiO2 [205]. Long chain surfactants with aromatic rings could be more easily decomposed than those surfactants containing only alkyl and/or alkoxylate groups under solar exposure using TiO2 suspended particles [206].

Both the theory and some of the photocatalytic degradation reactions were discussed in this chapter however other existing reviews will be covered appropriately elsewhere in Chapter 5. Having completed this brief survey of the typical photocatalytic degradation processes there is significant evidence that this is an area that requires further research. We are now in a position to assume that a detailed study under carefully controlled conditions is required in order to improve the efficiency and time of the photocatalytic degradation of organic pollutants using titanate nanostructures.

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1.2 Conclusions

A review of the current literature has shown that nanocrystalline metal oxides such as aluminosilicates, niobates, silicates, alumina fibres and titanates appeared to be promising materials that have a number of potential applications in environmental systems such as hazardous waste remediation and water purification. The modifications of these metal oxides have greatly advanced in water detoxification technology. More pathways to produce new nanostructured adsorbent and photocatalyst materials are now available than ever before. Important challenges for the further development of new efficient nanostructured adsorbents and photocatalysts include: (1) lack of understanding the relationship between synthesis conditions and the morphologies (2) complexity of describing structure-property relationships (3) difficulty of understanding the mechanism for various applications such as water treatment and heavy metal recovery. However, the demand for such materials could flourish by finding new applications.

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1.3 Description of Research Problems Investigated

Research Aims: To investigate the possibility of developing efficient nanostructured adsorbents for the removal of radioactive cations and organic pollutants from an aqueous environment. To develop a new generation of photocatalysts for the elimination of toxic and hazardous chemical substances such as organic hydrocarbons from waste effluents. The following are the specific objectives:

1.3.1 Overall Objectives of the study

 To optimize the overall efficiency of the ion-exchange property of sodium niobate by substituting various transition metals in the structural framework and also to study the feasibility of removing radioactive ions such Sr2+ and Ra2+ from an aqueous environment. Furthermore, the scope of the project was also extended to explore the possibility of using sodium niobate as an absorbent which can trap radioactive ions irreversibly inside the framework for safe disposal. In addition, it was also planned to quantify the effect of competing ions for the selective removal of Sr2+ and Ra2+ from water using sodium niobate as adsorbent.

 To develop new adsorbents for the removal of pesticides from water by the grafting of beidellite clay with various silane groups in the external and internal surface and also to optimise the removal capacity of the adsorbents. This study has also aimed to gain an understanding of the mechanism of such an adsorption process.

 To develop an alumina based filter membrane for the continuous mode separation of pollutants such as pesticides and heavy metal ions. Furthermore to investigate the possibility of improving the efficiency of the separation of pollutants by optimizing the conditions such as the grafting density and the distribution of grafting groups.

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 To investigate the use of immobilised titania on laponite clay fragments for the photocatalytic degradation of herbicides from water and also to compare the photocatalytic effect of various amounts of immobilised titania for optimising the modification conditions.

 To prepare and study the photocatalytic activity of H-titanate nanofibres coated with different amounts of anatase particles. It is also within the scope

of this study to compare the efficiency of these catalysts with that of TiO2- (B) nanofibres coated with anatase crystals for the photocatalytic degradation of the organic pollutants.

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198. R. W. Matthews, Photocatalytic oxidation of chlorobenzene in aqueous suspensions of titanium dioxide, Journal of Catalysis. 97 (1986) 565-568.

199. G. Al-Sayyed, J. C. D'Oliveira, P. Pichat, Semiconductor-sensitized photodegradation of 4-chlorophenol in water, Journal of Photochemistry and Photobiology A: Chemistry. 58 (1991) 99-114.

200. A. Sclafani, L. Palmisano, M. Schiavello, Influence of the preparation methods of titanium dioxide on the photocatalytic degradation of phenol in aqueous dispersion, Journal of Physical Chemistry. 94 (2002) 829-832.

201. G. Sivalingam, M. H. Priya, G. Madras, Kinetics of the photodegradation of substituted phenols by solution combustion synthesized TiO2, Applied Catalysis B-Environmental. 51 (2004) 67-76.

202. B. Ohtani, Y. Ueda, S. Nishimoto, T. Kagiya, H. Hachisuka, Photocatalytic oxidative decomposition of fluoroalkenes by titanium dioxide, Journal of Photochemistry and Photobiology A: Chemistry. (1990) 1955.

203. D. W. Bahnemann, J. Moenig, R. Chapman, Efficient photocatalysis of the irreversible one-electron and two-electron reduction of halothane on

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platinized colloidal titanium dioxide in aqueous suspension, Journal of Physical Chemistry. 91 (1987) 3782-3788.

204. E. Pelizzetti, V. Maurino, C. Minero, V. Carlin, M. L. Tosato, E. Pramauro, O. Zerbinati, Photocatalytic degradation of atrazine and other s-triazine herbicides, Environmental Science & Technology. 24 (1990) 1559-1565.

205. C. S. Lin, T. C. Chang, Photosensitized reduction of DDT using visible light: The intermediates and pathways of dechlorination, Chemosphere. 66 (2007) 1003-1011.

206. H. Hidaka, J. Zhao, E. Pelizzetti, N. Serpone, Photodegradation of surfactants. 8. Comparison of photocatalytic processes between anionic DBS and cationic BDDAC on the titania surface, The Journal of Physical Chemistry. 96 (1992) 2226-2230.

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CHAPTER 2

Equipments and Characterisation Techniques

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2.1 Equipments

2.1.1 Autoclaves

Autoclaves are specially designed reactors that have been created to handle very high pressure and temperature for hydrothermal reactions (Figure 2-1). They are generally known as bomb reactors. An autoclave consists of two different parts: Teflon-lined inner part and an outer stainless steel case. The inner part can easily fit into the outer part which can resist high pressure during a reaction. These reactors were used for the hydrothermal synthesis of solids for this research and were purchased from Parr Instrument Company. One of the major advantages of the hydrothermal synthesis of solids is that it is easy to achieve the reaction of two solid-phase starting materials at much lower temperatures than those required for the inter-diffusion of the solids without the presence of a liquid phase [1].

Figure 2-1  Photograph of hydrothermal reactor.

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2.1.2 High temperature oven

The ovens used for the experiments were conventional ovens, which are designed to maintain a very stable temperature upto 200 oC throughout the reaction. Autoclaves were kept inside the oven after carefully sealing the lid.

2.1.3 Photochemical reactor

The following provides an illustration of the „batch‟ photoreactor used in the study of water purification by semiconductor photocatalysis. The photocatalysts were often used as a dispersion to sensitize photomineralization reactions. In a „batch‟ reactor system the catalyst will not be fixed and therefore it needs to be separated out by filtration or settling/resuspension at the end of the reaction. In the reactor system the illumination was carried out by using a UV light source with a six tubular 38W Hg lamp (NEC, FL20BL T8). The maximum intensity was at about

350 nm. The catalyst concentration was 50 mg in 50 mL initial concentration (C0) of pollutants. A powder form of the catalysts and herbicide solutions were mixed together in the presence of air in Pyrex glass vessels for 30 min in order to achieve a complete adsorption on the surface of the photocatalyst prior to UV illumination. Several researchers have indicated the relationship between adsorbability on the surface of the photocatalyst and the degradation of pollutants [2-4]. These vessels were placed on a magnetic stirring plate with a fixed distance of 25 cm from the liquid suspension surface to the tubular lamps. This UV light source was attached horizontally to the internal top of a wooden box (100 cm × 35 cm × 35 cm) with mirror lined walls. One fan was positioned in order to minimize the heat effect generated by the lamp. During the reaction, the liquid of the reaction system was collected every 15 min and the samples were filtered through a Millipore filter prior to the analysis in order to remove the catalyst particles. However, in many commercial „flow‟ reactor systems, the photocatalyst will be fixed and contaminated water will flow over it. Such a flow system would eliminate the need for the separation of the photocatalyst [5].

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2.2 Characterization Techniques

Various instrumental techniques have been used to characterize the materials throughout this research. Niobates, modified clays, alumina fibres and titanate nanofibres were characterized by exploiting the following analytical techniques:

2.2.1 Scanning Electron Microscopy

The scanning electron microscope [6] was used to obtain information about the external morphology (texture) of the sample. This morphology significantly influences the performance of the sample for various applications. The SEM images depend on electron interactions at the surface rather than on transmissions so it can produce images that are a good representation of the 3D structure of the sample. Therefore, SEM images are considered to provide similar information in terms of topography, morphology, composition and crystallographic information about the sample surface. In topography, the surface skin of the sample and its texture can be imaged. In morphology, the shape, size and arrangement of the particles that constitute the surface of the sample, or that have been exposed by grinding or coating can be examined. However, all these detectable features are limited to 1 to 5 nm in size.

A detailed explanation of a typical SEM function [7] is given in Figure 2-2. During SEM analysis, a beam of electrons is focused on a selected area of the sample and there is therefore an exchange of energy from the electron beam to the sample surface. The electrons produced from the electron gun at the top are referred to as primary electrons. The electrons that originated from the specimen are known as secondary electrons. These electrons are then amplified, analysed and translated into images. The intensity of the secondary electrons from the specimen increases as the energy of the primary electrons increases. However at a certain point, emissions of the secondary electrons diminish when higher energy primary electrons start to interact deep below the surface of the sample. The secondary electrons that originated from such depths start to recombine before reaching the surface. Therefore, it is common to use low kV to prevent beam penetration into the sample,

- 53 - Chapter Two resulting is the generation of secondary electrons from the true surface structure of a sample.

In addition to the secondary electrons, there are reflected electrons that are referred to as backscattered electrons. Backscattered electrons carry more energy than secondary electrons and also travel in a specific direction. All electrons that possess an energy value of more than 50 eV are considered as reflected electrons. They can‟t be collected by a secondary electron detector, unless the detector is adjusted to its path of emission.

Figure 2-2 – A schematic illustration of specimen-electron beam interactions of SEM imaging.

Scanning electron microscopy was conducted using a FEI Quanta 200 SEM/ESEM operating in a standard high vacuum mode. The filament used was a standard tungsten cathode and the images were taken at 5–20 kV depending on the sensitivity of the material to the electron beam. Samples were placed on a specimen stub which

- 54 - Chapter Two was lined with double sided adhesive, conducting tape and was then coated with a thin layer of gold in order to reduce sample charging.

2.2.2 Energy Dispersive X-ray Spectrometer (EDS) Analysis

This technique was used to study and qualitatively or semi-quantitatively measure the chemical compositions of the selected points of the sample. It was also useful in identifying materials and contaminants and even in distinguishing their relative concentrations on the surface of the specimen. This technique is used as an attachment on SEM and utilizes the high-energy electrons that are ejected by an elastic collision of an incident electron, typically with a sample atom‟s nucleus and are referred to as back scattered electrons (Figure 2-2). Backscattered electron yield is proportional to the atomic number of the elements in the sample. Therefore, backscattered electron imaging provides image contrast as a function of elemental composition, as well as surface topography. These high-energy electrons are produced from much deeper than secondary electrons, so surface topography is not as accurately resolved as secondary electron imaging. However, for sample composition, the elements and compounds, and their relative ratios in areas of one micrometer in diameter were determined using this technique. Backscatter imaging systems can recognize elements with atomic number differences of at least 3. EPMA analysis was carried out using an Electron Probe Micro analyzer JEOL JXA 480A. At least five analyses at different spots were performed for each sample in order to obtain an average value.

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2.2.3 Transmission Electron Microscope

TEM has been used to study the finest structural details of the samples down to near atomic levels. The possibility for high magnification has made the TEM a valuable tool to study the morphological and crystallographic structural details of the samples in this research. TEM image resolution for an ideal sample is approximately in the range of few angstroms (10–10 m). TEM can provide much higher spatial resolution than SEM, using electron beam energies in the range of 60 to 350 kV.

A more technical explanation of a typical TEM‟s working [8] is shown in Figure 2- 3. Unlike SEM which depends on the electrons that originated from the specimen forming an image, TEM works with the electrons that are transmitted through the specimen. Like the SEM, a TEM operates with an electron gun at the top, producing a beam of monochromatic electrons. This beam is focused to a small, thin, coherent beam by the use of condenser lenses. This beam is then restricted by a condenser aperture to strike the specimen. Parts of it are transmitted to the other side of the specimen and the transmitted portion is focused by the objective lens. This is very similar to a light microscope, however, the shorter wavelength of the generated electrons allows for higher magnification or better resolutions. Objective aperture enhancing contrasts by blocking out high-angle diffracted electrons, the selected area aperture enables the user to observe the diffraction of the incident electron beam through the ordered arrangements of atoms in the specimen. The beam transmitted through the specimen is passed through the intermediate and projector lenses. Finally, the image strikes the phosphor image screen and light is generated thereby enabling the user to see the image. The darker areas of the image represent the areas of the specimen where fewer electrons were passed through because they are thicker. The lighter areas of the image represent the areas of the sample where more electrons were passed through because they are thinner.

A Philips CM 200 transmission electron microscope at 200 kV was used to investigate the morphology. For the TEM studies all of the samples were dispersed

- 56 - Chapter Two in an absolute ethanol solution and then dropped onto copper grids coated with carbon film and dried in an oven at 60 oC for 10 min.

Figure 2-3 – A technical explanation of TEM. [Redrawn from "Botany online - The Internet Hypertextbook" Hamburg university].

2.2.4 Fourier Transform Infrared (FTIR) Spectroscopy

FTIR spectroscopy was used to identify the various types of chemical bonds (functional groups) within the samples. The wavelength of the light adsorbed is a characteristic of the chemical bonding or molecular structure of the materials. The chemical bonds in the samples were determined by interpreting the infrared adsorption spectrum. FTIR was also used for some quantitative analyses of the samples because the strength of the adsorption is proportional to the concentration. Absorption bands in the range of 4000–400 cm–1 are due to the functional groups and 1500-400 cm–1 are known as the fingerprint region.

The working principle [9] of the instrument is based on the fact that bonds and groups of bonds vibrate at characteristic frequencies. When a material is exposed with polychromatic infrared radiation, absorbed infrared energy excites molecules into a higher vibrational state, which is characteristic of that specific molecule.

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Moreover, the frequency of light absorbed by a particular molecule is a function of the energy difference between the at-rest and excited vibrational states. The energy adsorbed by the sample is characteristic of its molecular structure. IR stretching vibrations are only possible with assymetrical molecules.

A technical explanation of the instrument is presented in Figure 2-4. The FTIR spectrometer consists of an interferometer to modulate the radiation from the source. A detector measures the intensity of transmitted or reflected light as a function of its wavelength. The signal from the detector is an interferogram, the resulting signal is then analyzed with a computer using Fourier transform to get a single beam infrared spectrum. The FTIR spectra are usually plotted as intensity versus wavenumber (cm–1). The intensity is plotted as a percentage of light transmittance or absorbance.

Figure 2-4 − A technical explanation of FTIR.

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In our study FTIR spectra were acquired using Nicolet 380 spectrometer equipped with a Ge/KBr beamsplitter and a DGTS/KBr detector. The collection time was about 1 minute (64 scans) for the background and the sample. The spectrometer was purged with dry air. To prepare KBr pellets, about 2 mg of sample were taken, ground for 1-2 minutes together with about 200 mg of KBr (FT-IR grade, Fluka, dried). The pellets were then pressed under a vacuum for 4-6 minutes at an 8 tonne weight to produce transparent disks about 1 mm thick and 13 mm in diameter. The samples were dried before preparation. A KBr pellet was used as a reference to background and its spectrum was subtracted from the sample spectrum to reduce the spectral artifacts caused by KBr and water.

2.2.5 Raman Spectroscopy

The importance of Raman spectroscopy in this research is its ability to provide structural information where no IR absorption takes place. IR stretching vibrations are only possible with assymetrical molecules. Similar to IR spectroscopy, Raman spectroscopy measures the vibrational frequencies of the different components of a molecule. Raman spectra provide information about the molecular nature of the sample, which includes information about the chemical composition as well as the intra and intermolecular phenomena. Although the IR and Raman technique measure the vibrational frequencies of a molecule, the fundamental physical process involved in the effects of these techniques is however different. For a molecule to be Raman active there must be a change in the polarity during the vibration. On the other hand, molecule must undergo dipole moment change during the vibration to be observed any IR absorption lines. For instance, a water molecule gives a very strong IR absorption spectrum whereas the Raman signal from water is very weak, making the Raman spectra suitable for the measurements of wet samples. Furthermore, molecules with strong dipole moment are typically hard to polarize. Molecules that cannot be detected with the one method can be easily detected with the other, and that leads to use them both in a complementary way.

In IR spectroscopy, the sample is illuminated by a broad range of IR frequencies and the absorbed frequencies are then measured; whereas in the Raman

- 59 - Chapter Two spectroscopy [10], the sample is illuminated by a single wavelength of light from a laser and scattered frequencies are then measured. During the measurement, a large amount of light which is scattered at the wavelength of the incident light, is referred to as Rayleigh scattered light and is then filtered out and the Raman scattered light at a different wavelength is allowed to pass through to the detector.

In this research Raman spectra were measured by the presentation of the dehydrated sample on a stainless steel slide and the spectra were collected on Renishaw 1000 Olympus BHSM microscope system equipped with 10× and 50× objectives, which was fitted with a monochromator, a filter system and a charge coupled device (CCD). The excitation source was a Renishaw doubled laser diode-pumped Nd- YAG emitting at a wavelength of 633 nm at a resolution of 4 cm−1 in the range between 100 and 4000 cm−1. Repeated acquisition using the highest magnification was accumulated to improve the signal-to-noise ratio. Spectra were calibrated using the 520.5 cm−1 line of a silion wafer.

2.2.6 X-Ray Diffraction Analysis (XRD)

Powder X-ray diffraction was a primary tool for investigating the structure of crystalline samples. This technique was used throughout the research to study the phase composition, structure variations and crystallinity. Crystallite sizes between 2 and 100 nm can be calculated from the half widths according to the Scherrer‟s equation D = k   cos, where D is the crystal size; k is a constant;  is the X-ray wavelength;  is the full width at half maximum of the diffraction line and  is the diffraction angle.

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Figure 2-5 − Schematic overview of the basic experimental arrangement to perform XRD. G1, G2, G3 are the reflections of d-spacings d1, d2, d3, respectively. [Diagram with the courtesy of Dr. Paul Schroeder (lecture note), University of Georgia].

During XRD measurement [11], monochromatic X-rays hit a crystalline sample, resulting interaction produces constructive interference with the same frequency as the incoming beam. If the atoms are arranged in a regular pattern, scattered rays leaving the sample reinforce each other when conditions satisfy Bragg's Law and are referred to as a diffracted beam. The arrangement of atoms in a crystal lattice form a series of parallel planes separated from one another by a distance (d), which varies according to the crystalline nature of the material. Crystal dimensions are defined by the three axes „a‟, „b‟, „c‟ and the angle between them „‟, „,‟ „.‟ The orientation and interplanar spacings of these planes are defined by three integers „h‟, „k‟, and „l‟ which are called indices. A given set of planes with indices h, k , l cut the a-axis of the unit cell in h sections, the b axis in k sections and the c axis in l sections.

The working principle of a goniometer depends on the fact that the sample is always equidistant to the source and the detector (Figure 2-5). At lower angles the sample receives only part of the beam resulting in reduced beam intensity per unit area of sample surface and increases the possibility of background scatter from the sample

- 61 - Chapter Two holder. At higher 2 deeper. For any crystal, planes exist in a number of different orientations, each with its own specific d-spacing. A diffraction pattern consists of a plot of reflected intensities versus the detector angle (2). The value of d-spacing depends only on the shape of the unit cell. From the Bragg‟s equation (n = 2dsin), d-spacing can be calculated as a function of 2. It is therefore possible to calculate the dimension of the unit cell from the d-spacing and from the corresponding indices h, k, l. A full data set may consist of many separate images taken at different orientations of the crystal. The first step of data processing is to identify which peaks appear in two or more images and to scale the relative images so that they have a consistent intensity scale. Optimizing the intensity scale is critical because the relative intensity of the peaks is the key information from which the structure is determined. The position of each diffraction spot is controlled by the size and shape of the unit cell inherent within the crystal. If a related structure is known, it can be used as a search model to determine the orientation and position of the molecules within the unit cell.

XRD analyses were performed on a PANalytica X‟ pert PRO X-ray diffractometer (radius: 240.0 mm) with the incident X-ray tube operating at 45 kV and 35 mA. The powdered sample was pressed into a sample holder with a smooth plane surface. Normally the sample was ground down to particles of about 0.002 mm to 0.005 mm cross section. The incident beam passed through a 0.04 rad Soller slit, a 1/2o divergence slit, a 15 mm fixed mask and a 1o fixed antiscatter slit, a 15 mm fixed mask and a 1o fixed antiscatter slit. After interaction with the sample, the diffracted beam was detected by an X‟ Celerator RTMS detector fitted to a graphite post diffraction momochromator. The detector was set in scanning mode, with an active length of 2.022 mm. Samples were analysed using BraggBrentano geometry over a range of 375o 2 with a step size of 0.02o 2, with each step measured for 200 s. Peak fitting analysis was carried out using peakfit software (Jandel Scientific, Postfach 4107, D-40688 Erkrach, Germany). Pearson functions were used and peakfit analysis was undertaken until squared correlation coefficients with R2 greater than 0.995 were obtained.

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2.2.7 UV-Vis diffuse reflectance Spectrophotometer

This technique was used to understand the band gap between the valance band and the conduction band of powder samples. The band gap is affected by the coordination chemistry of the metal atoms in the solid samples. The band gap can be calculated for practical purposes by the equation, Eg = 12400   , where „‟ is the absorbance wavelength (nm) [12]. The adsorption shoulder can be found by extrapolating a straight line from the spectrum to the x-axis. The working principle of the instrument is that when sample molecules are exposed to light having energy that suit a possible electronic transition within the molecule, some of the energy is adsorbed as the electron is promoted to a higher energy orbital. As shown in Figure 2-6, there is an energy gap separating a conduction band from a valence band within the molecular orbitals. In UV-vis spectroscopy the adsorption is carried out in the visible region as well as near the ultraviolet region of the spectrum (Figure 2-7).

Figure 2-6 – Diagram represents the possible electronic band structure within metals, semiconductors and insulators.

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Figure 2-7 – Diagram represents the electromagnetic spectrum of radiation.

The diffuse reflectance UV-visible (DR-UV-Vis) spectra of the samples were acquired with the Varian Cary 5E UV-Vis-NIR spectrophotometer. The powdered samples were prepared in a sample holder cavity and the powder was then firmly pressed against the quartz window by incorporating an aluminum insert backed with a spring. A second cell holder was used for reference to collect a baseline. The spectrometer was equipped with three detectors: a photomultiplier tube for the UV- Vis region, and InGaAs and PbS detectors for the NIR region. The three detectors ensured high sensitivity during transmittance and reflectance, even in the switchover range, and also significantly reduced noise in order to enable the precise measurement of low reflection samples. Liquid samples were analysed by using Varian Cary 100 UV-Visible spectrophotometer with quartz 1 cm cuvettes.

2.2.8 Solid State Nuclear Magnetic Resonance Spectroscopy

The solid–state silicon NMR technique was used to identify 29Si present in the sample. This technique provided information about the composition, the nearest neighbouring atoms, the local symmetry and the coordination numbers in which the

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29Si atom is located. This method is also used for quantitative analysis based on the fact that the amplitude of a nuclear magnetic resonance (29Si) signal is proportional to the number of 29Si nuclei contained in the molecule.

The working principle behind this technique is based on the phenomenon of magnetic resonance which occurs when the nuclei of certain atoms are immersed in a magnetic field [13]. Some nuclei experience this phenomenon (such as 1H, 13C and 29Si) and others do not such as 12C. This is dependent upon whether they possess a property called spin (I). For instance, the nucleus of the 12C atom has no overall spin, whereas the 29Si nucleus does and corresponds to a spin value of +1/2. Due to this spin the atomic nucleus possesses a magnetic moment (μ). This moment has two spinning states, +1/2 and −1/2 in the presence of an external magnetic field

(B0). The difference in energy (E = μ B0/ I) between the two spin states depends on the strength of the magnetic field (B0). This small energy difference (ΔE) ranges from 20 to 900 MHz, depending on the magnetic field strength and on the specific nucleus being studied. The illumination of a sample with a radio frequency energy corresponding to the energy difference between the spin states of a particular set of nuclei will cause excitation of those nuclei from +1/2 state to the higher −1/2 spin state (Figure 2-8).

Figure 2-8 – Diagram represents the relationship between energy levels and magnetic field.

Silicon magic angle spinning (MAS) NMR spectra were acquired at a magnetic field of 9.4 T with a Varian 400 MHz spectrometer, operating at an excitation

- 65 - Chapter Two frequency of 79.4 MHz for 29Si. Finely ground powders of the specimens were filled in silicon nitride pencil rotors. In silica NMR measurements employed 5 mm rotors undergoing magic angle spinning (MAS) at 10 kHz, where the sample is rapidly rotated about an axis at the magic angle (≈54.74o) with respect to the static

Si magnetic field. Typical nutation frequencies were nut / 2  45 kHz and

N 29 nut / 2  50 kHz. The Si chemical shifts are quoted relative to neat tetramethylsilane (TMS).

2.2.9 Surface Area Analysis

BrunauerEmmettTeller (BET) surface area analysis was performed in order to measure the surface areas and porosities of the solids used in this research. Porosity is the ratio of the volume of openings (voids) to the total volume of the solid. Porosity is one of the factors which controls the performance of the materials used in this research. The analysis of the porous texture was carried out by N2 adsorption/desorption techniques at 77 K. Adsorption isotherms were determined using a Micrometrics (Norcross, GA, USA) Tristar 3000 automated gas adsorption o analyser. The samples were degassed at 110 C under the flow of N2. Surface areas were obtained by using the Brunauer–Emmet–Teller (BET) equation, whereas the pore volumes and the external surface areas were obtained by the t–plot method [14]. Gas adsorption is used in this method for the evaluation of monolayer capacity, Vm (in volume units). The relation between the specific surface area, 2 (m /g), and Vm are shown below.

23 –20  = Vm × 6.03 × 10 × 10 ×  m / 22400 (1)

 = 0.269 Vm  m

2 Where m = area (Å ) that one adsorbed molecule would occupy in a completed monolayer.

1/2 m = 3.646[M / 4.2 N] (3)

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Where M = molecular weight, N = Avogadro‟s number,  = density of the condensed phase solid or liquid at the temperature of the isotherm

BET equation can be written in the linear form:

P / V (Po– P) = 1 / Vm C = (C1 / Vm C. P/Po) (4)

where P and P0 are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption, V = molar volume of adsorbed gas, C = BET constant.

2.2.10 Thermo Gravimetric Analysis (TGA)

Thermo gravimetric analysis is the quantitative measurement of weight loss (weight gain) of a material as a function of temperature and time, in an inert environment. It was very useful in this research to investigate the thermal stability of the materials in a controlled atmosphere such as nitrogen. The working principle of the instrument is based on the fact that when materials are heated, they loose weight from simple processes such as drying, structural water release, structural decomposition, carbonate decomposition and gas evolution. To understand the thermal stability of the material, knowledge of magnitude and temperature range of those reactions is necessary. The thermal decomposition was carried out in a TA Instruments high- resolution thermogravimetric analyser (series Q500) in a flowing nitrogen atmosphere (60 cm3min1). Approximately 35 mg of the sample underwent thermal analysis with a heating rate of 5 oC/min, and a temperature programming range from 25 to 1000 oC. With the quasi-isothermal, quasi-isobaric heating programme of the instrument, the furnace temperature was regulated precisely to provide a uniform rate of decomposition in the main decomposition stage.

2.2.11 Contact Angle Measurement

In this research, the contact angle was used to measure the surface properties of the materials such as wettability, the effects of surface treatment and repellency, etc.

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The contact angle was an effective tool to characterize the wettability of the solid‟s surface and to understand the efficiency of surface modifications. As seen in Figure 2-9, a droplet with a large contact angle is hydrophobic and this condition reflects the poor wetting of a solid surface with low free energy whereas, a droplet with a small contact angle is hydrophilic. This condition reflects better wetting, and a higher surface energy. The FTA200 was a flexible video system consisting of a measurement platform and a video capture system running within a personal computer. Sample position can easily be adjusted with the help of a movable sample table. A dosing system can deposit a drop on the surface of the sample in a fraction of a second. The adjustable illumination and image sharpness together with a camera with an auto zoom of 360 frames per second ensures optimal drop presentation. The camera records the digital image and allows a perfect drop shape analysis. The contact angle can be calculated using software.

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2.2.12 Inductively Coupled Plasma Optical Emission Spectrometry

This technique was used for the qualitative and quantitative determination of metal ions in extremely low concentration. Concentrations from parts per million to parts per billion in solutions were measured. The concentration of barium and strontium were recorded on a Varian Liberty 2000 ICPOES. Quantitative analysis was conducted by calibrating the instrument with a series of appropriate solution standards and then measuring signals from the samples of interest. The concentration of each element in the solution was obtained using an integration time of 3-seconds with 3 replications. The first step in the working principle of ICP-OES is the conversion of a liquid sample into plasma. The sample is pumped using a peristaltic pump to a nebulizer, where it is converted to a fine spray and mixed with argon in a spray chamber. Most of the sample is carried into the plasma, where the temperature is sufficiently high to break chemical bonds, liberate elements present and transform them into a gaseous atomic state. A number of the atoms pass into the excited state and emit radiation. The frequency of this radiation is characteristic of the element that emitted it. The intensity of the radiation is proportional to the concentration of that element within the solution and so can be used for quantitative purposes.

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2.2.13 High Pressure Liquid Chromatography (HPLC)

High performance liquid chromatography (HPLC) was a powerful tool that was employed to separate organic pollutants in the supernatant solutions. The working principle is based on the time needed for the analyte molecule to interact with the mobile and stationary phases. If the analyte molecules interact weakly with the stationary phase, it will flow down the column easily with the mobile phase. The molecules, which have a strong interaction at the stationary phase, are left behind. As seen in Figure 2-10, HPLC utilizes three different main components such as a pump that moves the mobile phase and analyte through the column, and a detector that provides a characteristic retention time for the analyte.

In this study the analytical separations were carried out by using an HP 1100 HPLC instrument equipped with a stainless steel column. The separation was carried out with a Waters symmetry 5 micron C18 (150 mm length × 4.5 mm) column.

Figure 2-10 – A diagram illustrates the basic arrangement to perform HPLC.

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2.2.14 UV-Visible Spectrophotometer

UV-Visible spectrophotometer was used in this research to analyse the pollutant molecules in water. It detects the response of a sample to ultraviolet and visible range of electromagnetic radiation. Molecules and atoms have electronic transitions while most of the solids have interband transitions in the UV and Visible range. The instrument operates by passing a beam of light (0) through a sample and measuring the intensity of light () reaching a detector. The instrumental data is used to calculate two quantities: the transmittance (T) and absorbance (A) and are shown below in the equations.

T = I / Io ------(1)

A= – log T ------(2)

The instrument used for sample analysis was Varian Cary 100 spectrophotometer with quartz 1 cm cuvettes.

Figure 2-11 – A diagram represents the working principle of UV-Vis Spectrophotometer.

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2.3 References

1. R. Albrecht, The Role of Hydrothermal Synthesis in Preparative Chemistry, Angewandte Chemie International Edition in English. 24 (1985) 1026-1040.

2. V. Subramanian, V. G. Pangarkar, A. A. C. M. Beenackers, Photocatalytic degradation of PHBA: relationship between substrate adsorption and photocatalytic degradation, Clean Products and Process. 2 (2000) 149-156.

3. K. Tanaka, K. Padermpole, T. Hisanaga, Photocatalytic degradation of commercial azo dyes, Water Research. 34 (2000) 327-333.

4. E. Pelizzetti, C. Minero, E. Borgarello, L. Tinucci, N. Serpone, Photocatalytic activity and selectivity of titania colloids and particles prepared by the sol-gel technique-photooxidation of phenol and atrazine, Langmuir. 9 (1993) 2995-3001.

5. R. W. Matthews, Solar-electric water purification using photocatalytic oxidation with TiO2 as a stationary phase, Solar Energy. 38 (1987) 405-413.

6. S. Lanfredi, L. Dessemond, A. C. M. Rodrigues, Dense ceramics of NaNbO3 produced from powders prepared by a new chemical route, Journal of the European Ceramic Society. 20 (2000) 983-990.

7. W. Zhou, R. Apkarian, Z. L. Wang, D. Joy, Scanning Electron Microscopy, Springer, New York, US, 2007.

8. D. B. Williams, C. B. Carter, Transmission Electron Micropscopy, Springer, New York, US, 2009.

9. P. Atkins, J. D. Paula, Physical Chemistry, Oxford University Press, Oxford, UK, 2006.

10. J. R. Ferraro, K. Nakamoto, Introductory Raman Spectroscopy, Academic Press, USA, 1994.

11. C. Suryanarayana, G. M. Norton, X-Ray Diffraction, A Practical Approach, Plenum Press, New York, 1998.

12. T. Lopez, R. Gomez, E. Sanchez, F. Tzompantzi, L. Vera, Photocatalytic activity in the 2,4-dinitroaniline decomposition over TiO2 sol-gel derived catalysts, Journal of Sol-Gel Science and Technology. 22 (2001) 99-107.

13. A. Abragam, Principles of Nuclear Magnetic Resonance, Cambridge University Press, Cambridge, UK, 1968.

14. S. Brunauer, P. H. Emmett, E. Teller, Adsorption of Gases in Multimolecular Layers, Journal of the American Chemical Society. 60 (1938) 309-319.

- 72 - Chapter Three

CHAPTER 3

Sodium Niobate Absorbents Doped with Tantalum for the Removal of Bivalent Radioactive Ions from Waste Water

- 73 - Chapter Three

3.1 Introduction

This chapter starts out by extending earlier discussions from Chapter 1 (Section 1.1.2) on the possibility of research into the modification of sodium niobate nanostructures, thereby aiming to increase the scope of using in the safe disposal of nuclear waste from water. There are several important aspects that have to be addressed when dealing with nuclear waste disposal:

1) The stability of the absorbent for long term storage.

2) The ability to remove nuclear waste from very low concentration solutions.

3) The selective adsorption from a mixture of ions.

4) The easy separation of an absorbent from water.

5) The ability to retain the radioactive ions inside the framework without any leaching.

Furthermore, as was already addressed in Chapter 1, we wish to investigate to what extent these questions have been addressed in recent technology. While investigating the contamination of the environment by radioactive ions from the heap-leach residues of uranium mining industry (such as 226Ra2+) and addressing the by-product of nuclear fission reaction (such as 90Sr2+) as well as the leakage of the nuclear reactors; one should not be daunted, extensive research is underway to develop advanced technologies for the elimination of hazardous radioactive ions from water [1-7].

In fact, different methods are used to remove radioactive metal ions from waste water such as ion exchange and adsorption [8-11]. Ion exchange processes are selective, very effective and are able to remove very low levels of metal ions from solutions [12-16]. However, the development of materials with high level of efficiency and selectivity is still in its infancy. In fact, the ever more sophisticated versions of scavenger solids also demands a framework capable of binding guests

- 74 - Chapter Three such as radioactive ions inside irreversibly without any leaching as a safe storage material [1].

Environmental remediation efforts have been somewhat successful after the discovery of the following materials. Organic resins are unlikely to compete with zeolites and other clay absorbents due to their limited long-term stability under high radiation conditions and their high cost [4]. Moreover, the inability of organic resins to maintain permanent thermal, chemical, mechanical and radiation stabilities and also avoid structural rearrangements upon guest exchange has been an obvious drawback [18, 19]. But despite the promise of selectivity and high radiation stability, zeolites and clays are high pH responsive materials [20]. Hence, synthetic inorganic cation exchangers may provide the best examples of practical successes. One of the first such solids was synthetic mica which has an efficiency larger than any natural inorganic exchangers [3, 5] (Figure 3-1). Another example is a - zirconium phosphate framework [2]. Very recently developed titanate [1, 21, 22] and niobate [6, 7] solids exhibit a high storage capacity as well as high selectivity (Figure 3-2).

Figure 3-1 – SEM image of Na-4-mica [17].

- 75 - Chapter Three

Moreover, synthetic inorganic cation exchangers seemed particularly suitable for immobilizing radioactive ions and thus mitigate toxicity from contaminated water [3, 5]. It is worth noting that these reinforced solids favour the uptake of radioactive ions not only at distinctly lower concentrations, but also with considerably high selectivity [2-7, 22]. New types of synthetic cation exchangers that can act as host material for the immobilization of nuclear waste are being developed; however, commercial viability of the resultant solid is often limited because of high production cost. Clearly, suitable compounds have not yet been found and numerous challenges remain. The synthesis of an efficient material raises other challenges such as low yield, low speed and reproducibility, which is not new, but remains unsolved. We have recently begun to fabricate structures with nanometric precision to incorporate several of these desirable features into one solid-stage material.

Figure 3-2 − SEM image of titanate [23].

Recently, sodium niobate molecular sieves doped with Ti/Zr -IV were synthesized by hydrothermal treatment [6, 7]. It was found that the sodium niobate molecular sieves (Figure 3-3) can selectively adsorb Sr2+ ions from highly concentrated Na+ aqueous solutions [24]. A structural evolution during the reaction between Nb2O5 powder and concentrated aqueous NaOH solution under hydrothermal conditions was also reported by our group [25], which provides a marvelous route to the further growth of niobate community.

- 76 - Chapter Three

Figure 3-3 – The crystal structure of microporous niobate (Na2Nb2O6·H2O) phase [24].

Herein, a new family of TaV doped sodium niobates were synthesised using chloride precursors via the reported hydrothermal reaction at 165 ºC. The Ta:Nb ratio in the resultant product is directly correlated with the precursor molar ratio for the range of 0–10% Ta:Nb. Within this composition range, bar-like solids with poorly crystalline and well-crystallized fibres (2%) are formed, as shown by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The bar-like solids possess excellent abilities to exchange with radioactive Sr2+ and Ra2+ ions in water. More importantly, by monitoring the structure change before and after adsorption with XRD and Raman spectroscopy, we are able to determine the structure collapse of bar-like solids due to the toxic bivalent ions exchanged with Na+ ions in the solids. The structure collapse induced by the ion-exchange process entraps the radioactive ions in the adsorbents permanently, so that the hazardous radioactive contaminants can be safely disposed of. This significant research can provide exciting developments for the removal of radioactive ions from water.

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3.2 Materials and Equipment

V Ta doped sodium niobates were synthesised from NbCl5 and TaCl5 precursors. All the chemicals used were reagent grade and were provided by the Sigma Chemical

Cooperation. BaCl2 and SrCl2 were purchased from Aldrich. The water used in all the experiments was purified with a milli-Q-plus system. NaOH pellets used in the experiments were purchased from Aldrich.

The structural properties of niobate materials were examined by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), Raman spectroscopy and diffuse reflectance UV-visible (DR-UV-Vis) spectroscopy techniques. The structure rearrangements after adsorption were also examined using XRD and Raman spectroscopy. The concentration of soultions before and after absorption test were analysed by inductively coupled plasma (ICP) technique using a Varian Liberty 200 ICO emission spectrometer (Section 2.2.12). All the characterization techniques were conducted in a manner outlined in Chapter 2.

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3.3 Discussion of Experimental Procedures

3.3.1 Sample preparation

Incorporation of tantalum into niobates was carried out by dissolving 0.5 g of NbCl5 in 20 mL of dry propan-2-ol mixed with different weight ratio of tantalum in a range from 0, 0.5, 1, 2, 5 to 10% under nitrogen flow. When NbCl5 dissolved in propan-2-ol was used as Nb precursor, a yellow precipitate initially appeared after propan-2-ol addition, which disappeared again after stirring for 20 min. A transparent solution was obtained after 24 h of stirring, to which 10 mL of distilled water was carefully added in a dropwise manner and finally a transparent gel was formed after 24 h. The precipitates were separated by filtration and washed with deionised water several times in order to remove chlorides. The resulting mixture of oxides was then reacted with 10 M NaOH under hydrothermal conditions. The structural evolutions are shown in Figure 3-4. The syntheses of Ta incorporated niobates were prepared in specially designed autoclaves (Section 2.1.1) at 165 ºC for 2 h. The solid products in the reaction mixture were recovered by filtration, washed with deionized water several times, then with ethanol once, and finally dried at 100 ºC for 24 h. The syntheses from chloride precursors were perfectly reproducible compared to synthesis directly from oxide precursors.

- 79 - Chapter Three

Figure 3-4 – Structural transformations of nanostructures under hydrothermal conditions [25]: (b) Low crystalline niobate solids, (c) Highly crystalline niobate fibres.

3.3.2 Adsorption experiments

It should be noted that radioactive strontium and radium ions are highly hazards in nature therefore due to safety reasons all studies were carried out in a non- radioactive environment. Ba2+ ions (1.36 Å) have similar ionic radius to the radioactive 226Ra2+ ions (1.43 Å) as well as similar ion exchange properties [26]. Therefore, there is a significant incentive to use barium as a representative of radium to mimic the properties except radioactivity. We also used non radioactive strontium. The sorption isotherms of M2+ ions (M2+ = Sr2+ and Ba2+) were determined by equilibrating 10 mg of adsorbent in 50 mL of SrCl2 or BaCl2 aqueous solutions at concentrations between 0 to 200 ppm, for 24 h at room temperature. In fact, the deposition of carbonate as SrCO3 or BaCO3 is often accompanied on the surface of the sorbent at high pH, in order to avoid that

- 80 - Chapter Three tendency the pH of the solution was adjusted to the range between 6 and 7 using diluted HCl solution during the adsorption process.

Distribution coefficients (Kd) were determined by the analysis of the filtered solution [3, 6, 27] and were calculated from the expression:

where Co is the initial concentration (mg/mL), Ceq is the concentration after equilibrating (mg /mL), V is the solution volume (mL) and m is the mass of exchanger (g). The Kd represents the ratio of the amount of barium or strontium adsorbed per gram of solid to the amount of strontium or barium remaining per milliliter of the solution, and thus has units mL/g.

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3.4 Results and discussion

3.4.1 Particle morphology

Figure 3-5 – Niobate solids illustrated by the SEM images of the samples: (a) 0% Ta, (b) 0.5% Ta, (c) 1% Ta, (d) 2% Ta, (e) 5% Ta, and (f) 10% Ta.

- 82 - Chapter Three

The SEM images of the samples in Figure 3-5 illustrate the interesting morphological changes of the solids by incorporating TaV in various percentages into the frame work structures. Without any doping, the niobate product displays grain morphology of several micrometers in size (Figure 3-5a), which is similar to our reported early-stage niobate samples prepared at 180 ºC by the use of Nb2O5 as reagent [28]. With the introduction of 0.5% TaV, long bar-like particles with a length of tens of micrometers were obtained (Figure 3-5b).

A few fibres with a length of tens of micrometers and a width of several micrometers were formed at the expense of the bars were observed in 1% TaV doped sample (Figure 3-5 c). As the doping amount of TaV increased to 2%, highly uniform fibril morphology was obtained (Figure 3.5d). It has also been observed that a further increase in TaV amount leads to the disappearance of fibre structure. For example, when the doped TaV amount had reached a level of 5% and 10%, the fibre phase had almost totally disappeared and only long bar-like particles were retained (Figure 3-5e and 3-5f). This result suggests that a change in the amount of TaV, was found to cause a change in the morphology as well as the overall crystalline nature of the products.

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3.4.2 XRD Patterns and Raman Spectra

The XRD patterns of the solids are presented in Figure 3-6. For the pure fibre phase (2% TaV), the diffraction peaks in the XRD pattern are narrow and have a high intensity, indicating well-crystallized fibres were formed. The crystal parameters are the space group C2/c, a = 16.984 Ǻ; b = 5.0245 Å; c = 16.432 Ǻ; β= 113.88º. They fit well with recent reported work [24]. However, the diffraction peaks in the XRD patterns of other samples have a low intensity, which is indicative of less crystallinity in these bar-like solids (Figure 3-5). XRD patterns of 0.5 and 10% TaV doped samples contain additional unidentified narrow peaks which is probably correspounding to the formation of mixtures of different phases.

The Raman spectra of samples have features typical of niobate phases [29-34]. Similar to the XRD patterns of the samples, the Raman spectrum of the pure fibre niobate is different from those of the bar-like solids (Figure 3-7). For the bar-like solids, the strong peak at 897 cm1 is assigned to the short Nb=O stretching mode 1 (A1g), and an intrinsic band at 851 cm is also due to the vibration of the short Nb=O bond (~1.8 Å) [25]. There is a weak band at 567cm1, this peak corresponding to the Eg mode arising from stretching vibration of Nb-O-Nb. The 1 Raman lines at 290 and 220 cm can be assigned as the A1g and T2g modes of bar- 1 like niobate phase, respectively. The A1g mode at 290 cm arising from the 1 breathing vibration of the long NbO bond and T2g mode at 220 cm reveals the bending vibration of NbONb (bridging O atom) [32]. For the 2% Ta doped sample, new bands at 461 and 641 cm-1 were observed. The former corresponds to the bridge NbO stretching mode with T2g symmetry and the latter is also probably associated with the edge-sharing of the NbO6 octahedra [25]. The other obvious 1 change is that the band at 570 cm has disappeared. Furthermore, the A1g band observed at 288 cm1 in the spectra of poor-crystallized solids split into two weak bands at 270 and 305 cm1, respectively. The bands at 897 and 851 cm1 shifted to 883 and 842 cm1, respectively. These spectra are basically similar to those of our previously reported sodium niobates [25]. However, no Raman bands of crystalline 1 Ta2O5 were observed at 105, 253 and 627 cm in any of the solids [35, 36].

- 84 - Chapter Three

10%

1% 0.5%

Intensity,counts/sec. 0%

10 20 30 40 50 60 2 , deg

Figure 3-6 – XRD patterns of TaV doped niobate specimen.

897

851 220 567 288 10% 5%

842 641 461 305 270 883 2% 216

1% Raman Intensity 0.5% 0%

900 800 700 600 500 400 300 200 100 -1 Wavenumber/cm

Figure 3-7 – Raman spectra of TaV doped niobate specimen.

- 85 - Chapter Three

According to our previous work, the bar-like solids with less a crystalline phase are 8− mainly composed of intermediates with edge-sharing Nb6O19 units. The well- crystallized fibril solids are composed of edge-sharing NbO6 octahedral units [25]. Note that the central Nb atom is octahedrally coordinated by six oxygen atoms in each unit. Clearly, these octahedra are the primary building block of different morphologies by sharing corners as well as occasionally sharing edges and faces. In our tantalum doped hydrothermal syntheses of niobates, the doped element takes part in the coordination entities, as a result those metal cations influence the build up of polyhedra which connected in sequence. All of the hydrothermal preparations have the Nb:Ta ratio within a relatively narrow range. Too much or too little tantalum in these reactions gives rise to bar like-solids.

Although the cationic radii of Nb5+ and Ta5+ are similar the ordering of cells are governed largely by the atomic ratios of Ta5+. Substitution of Nb5+ by Ta5+ should slightly alter the volume of NbO6 octahedra, and hence the cell volume of Ta-doped niobates. The pure monoclinic phase crystallyses as fibres when solution containing 2% Ta. Soutions with tantalum slightly outside these 2% ratios result in the crystallization of additional phases together with monoclinic phase. Some confindence may be derived from the fact that an understanding of the phase relationships and detailed compositions are critically important to prove the mechanism herein. However, the true stoichiometry is difficult to recognize because of the wide range of substitutions that occur. It would seem that the bonding of Ta5+ in its asymmetric site is so energetic that self-organise the growth of additional phases. At 2% Ta ratio the cell retain its monoclinic symmetry whereas in other ratios it is difficult to crystallize without some distortion of the octahedral coordination. It is also evident that distortions introduced in these structures are appeared to be equally stable. It is also probable that disordered monoclinic structures which may be metastable with respect to some other ordered structures or to some mixture of structures.

- 86 - Chapter Three

3.4.3 UV- Visible Spectra

Figure 3-8 presents the UV-vis spectra of various niobate products. According to the early reports [25, 37-39], oxygen to metal charge transfer bands can be influenced by the transition metal ion and the number of oxygen atoms surrounding it. Therefore, the changes in UVVis absorbance upon doping are consistent with the types of transition metal oxide configurations present in the sample. Pure Ta2O5 exhibits a main absorption band at 270 nm which is from the octahedral configuration of Ta5+ ion [40]. As anticipated, 2% TaV doped sample with pure fibril morphology and the well-crystallized phase has a weak but distinguishable charge-transfer transitions at above 300 nm because the fibril sample is mainly composed of edge-sharing NbO6 octahedra, which concurs with the XRD and Raman results. For the 1% TaV doped one, the intensity of the absorption at above 300 nm is the second highest because it was composed of fibre and a bar-like solid. Other samples, especially the undoped one, displayed weak absorption. These are mainly composed of intermediates with edge-sharing octahedra, and their absorption bands were observed at only ~250 nm. Furthermore, the spectra obtained for 10% Ta mixed Nb oxides showed a different evolution when compared with the spectra of pure niobate (0% Ta). It can be seen that for various concentration of Ta in the niobate matrix results in an obvious shift of absorption bands. The significant shift means that the presence of Ta within the niobate matrix not only influences the morphology of the products, but also has an effect both on the lattice structure and on the electronic properties.

- 87 - Chapter Three

1.5

1.0 0.5% Ta

5% Ta 2% Ta 0.5 1% Ta 10% Ta

Absorbance (a.u.) 0% Ta 0.0 200 250 300 350 400 450 Wavelength/nm

Figure 3-8 – The UV–Vis absorption spectra of TaV doped sodium niobates.

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3.4.4 Sorption of bivalence radioactive ions

Figure 3-9 illustrates the adsorption isotherms of Ba2+ and Sr2+ by various amount of tantalum doped niobate samples. For the removal of Ba2+ ions (Figure 3-9 a), the bar-like solids exhibited a high sorption ability. For instance, the uptake capacities of Ba2+ ions by 0, 0.5, 5, and 10% TaV doped samples are quite similar and are ~ 470 mg/g. However, 2% of the Ta doped fibre phase sample displays weak sorption ability and its capacity is only ~ 220 mg/g. The uptake capacity of 1% TaV doped sodium niobate composed of bar-like solids and few fibres are ~ 400 mg/g. For the removal of Sr2+ ions, bar-like solids also displayed higher exchange capacity than the fibril one (Figure 3-9 b), which is similar to that of the sorption of Ba2+ ions. For example, the uptake capacities for Sr2+ in bar-like solids doped with 0.5, 5, and 10% TaV are 250, 260, and 270 mg/g, respectively. The value of the pure sodium niobate solid is slightly less than 250 mg/g. This means that the availability of exchangeable Na+ ions in the poorly-crystallized bar-like solids is more than that of the well-crystallized fibres. The results presented above highlights the impact of tantalum in the framework design and resulting morphological forms obtained after hydrothermal synthesis which performed differently in the ion-exchange processes. Too much (10%) or too little (0% and 0.5%) tantalum in the reactions gives rise to solids which don‟t crystallize well. These less crystalline solids possess loosely held frameworks and therefore more ions are easily available for exchange reactions. Furthermore, exchange of radioactive ions in the framework of weakly crystalline solids often imparts a high degree of rearrangements in the framework in an apparently random fashion which favors the encapsulation of radioactive ions irreversibly inside the framework. The sorption data leaves no doubt that incorporation of TaV affecting the formation of framework which can easily control the availability of exchangeable Na+ ions. Furthermore, the semi crystalline solids contribute more active adsorption sites for the exchange of radioactive sites.

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500 0% Ta a 0.5% Ta 400 1% Ta 2% Ta 5% Ta 300 10% Ta

200

exchange (mg/g)

Ba 100

0 0 30 60 90 120 Ba2+in equilibrium solution (p.p.m)

300 b 250

) 0% (Ta) 200 0.5% (Ta) mg/g 1% (Ta)

( 2% (Ta) 150 5% (Ta) 10% (Ta) 100

exchange

Sr 50

0 0 30 60 90 120 Sr2+in equilibrium solution (p.p.m)

Figure 3-9 – The isotherms of M2+ sorption by the Ta doped niobate samples: (a) Ba2+and (b) Sr 2+.

- 90 - Chapter Three

2+ V Table 3-1– The Kd values for M ion sorption by Ta doped sodium niobate

v v v v v v *CNa 0%.Ta 0.5%.Ta 1%.Ta 2%.Ta 5%.Ta 10%.Ta Ba2+ Sr2+ Ba2+ Sr2+ Ba2+ Sr2+ Ba2+ Sr2+ Ba2+ Sr2+ Ba2+ Sr2+ 0 45000 4800 39000 4700 18000 3900 1200 680 49000 5100 54000 5300 0.1M 16800 3700 17000 3500 6000 3100 950 500 23400 3200 23000 3100 * 2+ CNa is the concentration of NaCl in the solution of M ions.

2+ The distribution coefficient, Kd, represents the ratio of concentration of M ions absorbed on one gram of various niobate versus the equalibirium concentration of M2+ left in solution (per milliliter) and is listed in Table 3-1. It can be observed that V the bar-like solids doped with 0.5, 5, and 10% Ta exhibit higher Kd than those of the fibril one; this is due to their higher sorption capacities of Ba2+ and Sr2+.

Furthermore, it is also noted that even in the presence of sodium ions Kd remains significantly high for semi-crystalline solids. The result is illustrated that semi- crystalline solids have achieved a requisite crystallinity for the removal of Sr2+ and 2+ 2+ 2+ Ba (Ra ). The large drop of Kd for Ba in NaCl solution is explained by the mode packing of adsorbed ions in the crystal lattices. The uptake of Ba2+ and Sr2+ must have different types of regularities in the crystal lattice [1].

As seen in Figure 3-10, the exchange of Sr2+ and Ba2+ (Ra2+). was carried out in two different steps 1) hydration of sodium ions present in the niobate crystals 2) hydrated Sr2+ and Ba2+ (Ra2+) exchanged with sodium ions due to the ion‟s tendency to form less soluble products, creates products with less free energy. The property of permanently trapping the radioactive cations allows us to isolate the radioactive ions from contaminated water, so that the tantalum doped sodium niobate and the used adsorbents can be disposed of safely without the risk of release of the adsorbed cations from the adsorbents that may cause secondary contamination. To ensure the binding of radioactive ions inside the niobate solids, we carried out desorption tests. The solid contents were separated out from aqueous solutions by using a centrifuge. The recovered solids were kept at 80 oC for 24 h. The solid samples were then added to water and the dispersions were equilibrated in a shaker at room temperature for 48 h. The salt released into the water was analyzed by the ICP

- 91 - Chapter Three technique. This experiment has revealed that bar-like solids show high level of leaching resistance from the framework. It was found that only ~ 5% of the adsorbed M2+ ions were detected in solution for the bar-like solids, indicating that ~ 95% of the radioactive ions had been locked in the adsorbents. However, in the sample with fibril morphology, 90% of the adsorbed M2+ ions were detected in solution. This means that it is more feasible to use the bar-like solids as adsorbents in the removal of radioactive ions from water. In particular, this method is suitable for concentrating most of the radionnuclides from contaminated water and would allow the bulk of the waste to be disposed of.

To account for the large difference in adsorption properties between bar-like solids and fibre adsorbents, we compared the XRD patterns and Raman spectra of the TaV doped sodium niobates before and after sorption. The XRD patterns are shown in Figure 3-11. For the bar-like solids, the XRD patterns changed greatly after sorption (Figure a, b, e, and f). The most obvious variation is that the peaks between 2 = 20 and 60 disappeared or decreased significantly after the sorption of Ba2+ and Sr2+ ions. It is also noticeable that the peaks at 2 = 10 to 15 shifted to low angle after sorption of Ba2+ ions. Although the structure change is too complicated to be

- 92 - Chapter Three analyzed in detail, we can determine that the higher sorption capacity and leaching resistance are due to the easily changed structure of the bar-like solids. The adsorbed barium ions are positioned in a regular order whereas strontium ions are distributed randomly in the crystal lattice. XRD data (Figure 3-12) is an important piece of evidence which supports the above interpretation.

Because the bar-like solids are poor-crystallized, the sodium ions in the solids can more easily be exchanged by the radioactive ions. After sorption, the structure of the metastable solids deformed or collapsed, which resulted in the divalent cations being locked into the solids and not being released again. However, for the fibril solid, no structural change is observed after the sorption process (Figure 3-11d). As discussed in Figure 3.6, the fibre adsorbent is a well-crystallized solid. Therefore the sodium ions available for the exchange of Ba2+ or Sr2+ are relatively less compared to the bar like solids. XRD signals obtained from the sample after adsorption of radioactive ions were essentially identical to the corresponding fresh fibres. Furthermore, the crystal structures of fibres were almost insensitive to the adsorption process, indicating that radioactive ions added were not involved in the framework rearrangement. Therefore the recovered radioactive ions were easily leached out from the framework. For the solid composed of bars and fibers, only the bar-like solids really adsorb the radioactive ions and the fibril solids are not strongly involved in this part of the absorption process. Therefore, the peaks that correspond to the poorly-crystallized bar solids are changed after adsorption the peaks assigned to the fibril phase still remain unchanged (Figure 3-11c). The Raman results are in good agreement with XRD data (Figure 3-12). The results show a large shift of peaks before and after adsorption, emphasising the effective involvement of adsorbed ions. Some obvious shift of Raman peaks observed at 897 cm-1 is evidenced the main distortion of octahedra. These distortions are off-center displacements of the metal cation with respect to the regular octahedron upon the involvement of adsorbed ions to the O6 octahedron.

- 93 - Chapter Three

a b

0%Ta_Ba 0.5%Ta_Ba

0%Ta_Sr 0.5%Ta_Sr

Intensity, a.u. Intensity, Intensity, a.u. Intensity,

0%Ta 0.5%Ta

10 20 30 40 50 60 10 20 30 40 50 60

2 , deg 2 , deg c d

1%Ta_Ba 2%Ta_Ba

2%Ta_Sr

1%Ta_Sr

Intensity, a.u. Intensity,

Intensity, a.u. Intensity, 1%Ta 2%Ta

10 20 30 40 50 60 10 20 30 40 50 60

2 , deg 2 , deg d f

10%Ta_Ba

5%Ta_Ba

10%Ta_Sr

5%Ta_Sr

Intensity, a.u. Intensity, Intensity, a.u. Intensity, 10%Ta 5%Ta

10 20 30 40 50 60 10 20 30 40 50 60 2 , deg 2 , deg Figure 3-11 – The XRD patterns of the TaV doped sodium niobates before and after sorption of Ba2+ and Sr2+ ions.

- 94 - Chapter Three

a b

0%Ta_Ba 0.5%Ta_Ba

0%Ta_Sr

0.5%Ta_Sr

Intensity (a. u.) (a. Intensity Intensity (a. u.) Intensity 0%Ta 0.5%Ta

100 200 300 400 500 600 700 800 900 100 200 300 400 500 600 700 800 900 -1 -1 Raman shift / cm Raman shift / cm c d 1%Ta_Ba

2%Ta_Ba

1%Ta_Sr

2%Ta_Sr

Intensity (a. u.) (a. Intensity Intensity (a. u.) (a. Intensity 1%Ta 2%Ta

100 200 300 400 500 600 700 800 900 100 200 300 400 500 600 700 800 900

-1 -1 Raman shift / cm Raman shift / cm e f

10%Ta_Ba 5%Ta_Ba

10%Ta_Sr

5%Ta_Sr

Intensity (a. u.) (a. Intensity Intensity (a. u.) (a. Intensity 0.5%Ta 10%Ta

100 200 300 400 500 600 700 800 900 100 200 300 400 500 600 700 800 900

-1 -1 Raman shift / cm Raman shift / cm Figure 3-12 – The Raman spectra of the TaV doped sodium niobates before and after sorption of Ba2+ and Sr2+ ions.

- 95 - Chapter Three

3.5 Conclusions

Sodium niobates doped with different amounts of TaV (0, 0.5, 1, 2, 5, and 10%) was prepared via a hydrothermal reaction process. Pure nanofibril solids were obtained when 2% TaV was introduced into the reaction system. These doped fibres are well- crystallized and are mainly composed of edge-sharing NbO6 octahedra. Bar-like solids can be obtained under 5% and 10% TaV doped conditions. These bar-like solids are poorly-crystallized and are mainly composed of intermediates with edge 8− sharing Nb6O19 units. It was observed that the bar-like solids possess high sorption capacities for bivalent radioactive ions, such as Sr2+ and Ba2+ (Ra2+) ion. Even in the presence of 0.1 M of Na+ ions, the solids are able to selectively adsorb the bivalent radioactive ions. More importantly, considerable structural deformation or collapse occurs during the sorption process, resulting in semi-permanent entrapment of the dangerous bivalent cations in the solids. In particular, it is possible that a metastable framework rather than a rigid structure, which consequently enabled the solids to be suitable for concentrating most of the radionnuclides from contaminated water and would allow the bulk of the waste to be disposed of. These doped niobate adsorbents may provide a new pathway to remove toxic radioactive ions from contaminated water.

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3.6 References

1. D. J. Yang, Z. F. Zheng, H. Y. Zhu, H. W. Liu, X. P. Gao, Titanate Nanofibers as Intelligent Absorbents for the Removal of Radioactive Ions from Water, Advanced Materials. 20 (2008) 2777-2781.

2. S. Komarneni, N. Kozai, W. J. Paulus, Superselective clay for radium uptake, Nature. 410 (2001) 771-771.

3. W. J. Paulus, S. Komarneni, R. Roy, Bulk synthesis and selective exchange of strontium ions in Na4Mg6Al4Si4O20F4 mica, Nature. 357 (1992) 571-573.

4. S. Komarneni, R. Roy, Use of γ-zirconium phosphate for Cs removal from radioactive waste, Nature. 299 (1982) 707-708.

5. S. Komarneni, R. Roy, A Cesium-Selective Ion Sieve Made by Topotactic Leaching of Phlogopite Mica Science. 239 (1988) 1286.

6. M. Nyman, A. Tripathi, J. B. Parise, R. S. Maxwell, W. T. A. Harrison, T. M. Nenoff, A new family of octahedral molecular sieves: Sodium Ti/Zr-IV niobates, Journal of the American Chemical Society. 123 (2001) 1529-1530.

7. M. Nyman, A. Tripathi, J. B. Parise, R. S. Maxwell, T. M. Nenoff, Sandia octahedral molecular sieves (SOMS): Structural and property effects of charge-balancing the M-IV-substituted (M = Ti, Zr) niobate framework, Journal of the American Chemical Society. 124 (2002) 1704-1713.

8. P. K. Mohapatra, D. S. Lakshmi, A. Bhattacharyya, V. K. Manchanda, Evaluation of polymer inclusion membranes containing crown ethers for selective cesium separation from nuclear waste solution, Journal of Hazardous Materials. 169 (2009) 472-479.

9. T. P. Valsala, S. C. Roy, J. G. Shah, J. Gabriel, K. Raj, V. Venugopal, Removal of radioactive caesium from low level radioactive waste (LLW) streams using cobalt ferrocyanide impregnated organic anion exchanger, Journal of Hazardous Materials. 166 (2009) 1148-1153.

10. F. Belloni, C. Kuetahyali, V. V. Rondinella, P. Carbol, T. Wiss, A. Mangione, Can carbon nanotubes play a role in the field of nuclear waste management, Environmental Science & Technology. 43 (2009) 1250-1255.

11. P. Sharma, R. Tomar, Synthesis and application of an analogue of mesolite for the removal of uranium(VI), thorium(IV), and europium(III) from aqueous waste, Microporous and Mesoporous Materials. 116 (2008) 641- 652.

12. A. Dyer, J. Newton, L. O'Brien, S. Owens, Studies on a synthetic sitinakite- type silicotitanate cation exchanger. Part 2. Effect of alkaline earth and alkali metals on the uptake of Cs and Sr radioisotopes, Microporous and Mesoporous Materials. 120 (2009) 272-277.

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13. M. J. Manos, N. Ding, M. G. Kanatzidis, Layered metal sulfides: Exceptionally selective agents for radioactive strontium removal, Proceedings of the National Academy of Sciences of the United States of America. 105 (2008) 3696-3699.

14. P. K. Mohapatra, S. A. Ansari, A. Sarkar, A. Bhattacharyya, V. K. Manchanda, Evaluation of calix-crown ionophores for selective separation of radio-cesium from acidic nuclear waste solution, Analytica Chimica Acta. 571 (2006) 308-314.

15. N. Rawat, P. K. Mohapatra, D. S. Lakshmi, A. Bhattacharyya, V. Manchanda, Evaluation of a supported liquid membrane containing a macrocyclic ionophore for selective removal of strontium from nuclear waste solution, Journal of Membrane Science. 275 (2006) 82-88.

16. D. L. Guerra, A. A. Pinto, R. R. Viana, C. Airoldi, Layer silicates modified with 1,4-bis(3-aminopropyl)piperazine for the removal of Th(IV), U(VI) and Eu(III) from aqueous media, Journal of Hazardous Materials. 171 (2009) 514-523.

17. T. Kodama, S. Nagai, K. Hasegawa, K. Shimizu, S. Komarneni, Synthesis of novel Na-rich mica and selective strontium ion exchange and fixation, separation science and technology. 37 (2002) 1927-1942.

18. B. W. Mercer, L. L. Ames, P. W. Smith, Cesium purification by zeolite ion- exchange, Nuclear Applications and Technology. 8 (1970) 62-69.

19. A. Dyer, A. M. Yusof, The mechanism of ion-exchange in some crystalline sodium and caesium-I zirconium phosphates, Journal of Inorganic and Nuclear Chemistry. 41 (1979) 1479-1481.

20. C. B. Amphlett, L. A. McDonald, M. J. Redman, Synthetic inorganic ion- exchange materials zirconium phosphate, Journal of Inorganic and Nuclear Chemistry. 6 (1958) 220-235.

21. G. M. Bancroft, J. B. Metson, S. M. Kanetkar, J. D. Brown, Surface studies on a leached sphene glass, Nature. 299 (1982) 708-710.

22. E. A. Behrens, P. Sylvester, A. Clearfield, Assessment of a sodium nonatitanate and pharmacosiderite-type ion exchangers for strontium and cesium removal from DOE waste simulants, Environmental Science & Technology. 32 (1998) 101-107.

23. Y. V. Kolen'ko, K. A. Kovnir, A. I. Gavrilov, A. V. Garshev, J. Frantti, O. I. Lebedev, B. R. Churagulov, G. Van Tendeloo, M. Yoshimura, Hydrothermal Synthesis and Characterization of Nanorods of Various Titanates and Titanium Dioxide, Journal of Physical Chemistry B. 110 (2006) 4030-4038.

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24. H. W. Xu, M. Nyman, T. M. Nenoff, A. Navrotsky, Prototype sandia octahedral molecular sieve (SOMS) Na2Nb2O6H2O: Synthesis, structure and thermodynamic stability, Chemistry of Materials. 16 (2004) 2034-2040.

25. H. Y. Zhu, Z. F. Zheng, X. P. Gao, Y. N. Huang, Z. M. Yan, J. Zou, H. M. Yin, Q. D. Zou, S. H. Kable, J. C. Zhao, Y. F. Xi, W. N. Martens, R. L. Frost, Structural evolution in a hydrothermal reaction between Nb2O5 and NaOH solution: From Nb2O5 grains to microporous Na2Nb2O6 .2/3H2O fibers and NaNbO3 cubes, Journal of the American Chemical Society. 128 (2006) 2373-2384.

26. M. Jurado-Vargas, M. Oliguín, E. Erdonez-Regil, M. Mimenez-Reyes, Ion exchange of radium and barium in zeolites, Journal of Radioanalytical and Nuclear Chemistry. 218 (1997) 153-156.

27. R. G. Anthony, R. G. Dosch, D. Gu, C. V. Philip, Use of silicotitanates for removing cesium and strontium from defence waste, Industrial & Engineering Chemistry Research. 33 (1994) 2702-2705.

28. V. J. Inglezakis, M. D. Loizidou, H. P. Grigoropoulou, Equilibrium and kinetic ion exchange studies of Pb2+, Cr3+, Fe3+ and Cu2+ on natural clinoptilolite, Water Research. 36 (2002) 2784-2792.

29. B. Morosin, P. S. Peercy, Structural studies on the hydrolysis products of Ti, Nb and Zr alkoxides, Chemical Physics Letters. 40 (1976) 263-266.

30. J.-M. Jehng, I. E. Wachs, The molecular structures and reactivity of supported niobium oxide catalysts, Catalysis Today. 8 (1990) 37-55.

31. S. Lanfredi, L. Dessemond, A. C. M. Rodrigues, Dense ceramics of NaNbO3 produced from powders prepared by a new chemical route, Journal of the European Ceramic Society. 20 (2000) 983-990.

8- 32. F. J. Farrell, V. A. Maroni, T. G. Spiro, Vibrational analysis for Nb6O19 8- and Ta6O19 and the the Raman intensity criterion for metal-metal interaction, Inorganic Chemistry. 8 (2002) 2638-2642.

33. J. M. Jehng, I. E. Wachs, Molecular structures of supported niobium oxide catalysts under in situ conditions, Journal of Physical Chemistry. 95 (2002) 7373-7379.

34. M. N. Iliev, M. L. F. Phillips, J. K. Meen, T. M. Nenoff, Raman spectroscopy of Na2M2O6. H2O and Na2Nb2-xMxO6-xOHx. H2O (M = Ti, Hf) ion exchangers, Journal of Physical Chemistry B. 107 (2003) 14261-14264.

35. Y. Chen, J. L. G. Fierro, T. Tanaka, I. E. Wachs, Supported Tantalum Oxide Catalysts: Synthesis, Physical Characterization, and Methanol Oxidation Chemical Probe Reaction, Journal of Physical Chemistry B. 107 (2003) 5243-5250.

36. J. Huuhtanen, M. Sanati, A. Andersson, S. Lars T. Andersson, Catalytic and spectroscopic studies of vanadium oxide supported on group IVb and Vb

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metal oxides for oxidation of toluene, Applied Catalysis A: General. 97 (1993) 197-221.

37. G. Blasse, G. P. M. van den Heuvel, Vibrational and electronic spectra and crystal structure of cubic Na3NbO4, Materials Research Bulletin. 7 (1972) 1041-1043.

38. G. Blasse, L. G. J. De Haart, The nature of the luminescence of niobates MNbO3 (M = Li, Na, K), Materials Chemistry and Physics. 14 (1986) 481- 484.

39. M. Wiegel, M. Hamoumi, G. Blasse, Luminescence and nonlinear optical properties of perovskite-like niobates and titanates, Materials Chemistry and Physics. 36 (1994) 289-293.

40. W.-J. Chun, A. Ishikawa, H. Fujisawa, T. Takata, J. N. Kondo, M. Hara, M. Kawai, Y. Matsumoto, K. Domen, Conduction and Valence Band Positions of Ta2O5, TaON, and Ta3N5 by UPS and Electrochemical Methods, The Journal of Physical Chemistry B. 107 (2003) 1798-1803.

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CHAPTER 4

Nanostructured Clay and Alumina Based Filter Membranes for the Separation of Organic Pollutants from Water

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4.1 Introduction

As discussed in Chapter 1 (Section 1.1.3), a variety of treatments can be implemented to improve the access of pollutants to the surface of adsorbents. These can be performed in different ways such as acid treatment, intercalation and grafting. Before we discuss the surface activation of clays, it is convenient to recall some of the disadvantages of other solid matrices like polymeric resins, cellulose fibres and mesoporous silica. These disadvantages include: a) the attachment of the functional groups being difficult due to the large number of cross linking bonds b) the unstable composition which suppresses the easy analysis and interpretation of the results c) the lack of stability in many chemical environments d) the loss of mechanical stability in continuous operations e) the high cost of production [1-3]. Directly bonding to inorganic clay surfaces is rather difficult due to the inertness of the surface in its native state. In the following discussion we will try to pursue three main aims. The first aim is to provide an overview of acid activation and its possible environmental applications. The second aim is to qualitatively determine how introducing functional groups may influence the adsorption of given organic pollutants. Our third aim is to compare the adsorption properties with different substrates (alumina fibres) but the same functional groups in an attempt to understand further of the sorption process.

The aim of this section is to demonstrate how the acid treatment of beidellite clay (Figure 4-1), using hydrothermal conditions, is a pathway to control the pore size distribution and its resulting chemical selectivity as an absorbent. From a practical point of view, these modification methods are important in environmental engineering, due to their potential applicability in removing pollutants from contaminated water. The morphological build-up of the native clay has an important influence on its physical and chemical properties. The accessibility of pollutants to the active sites of clay may be restricted due to its complex physical structure. If the surfaces of the clay platelets are very close to one another, the access to the surface by the pollutants may be completely restricted without a prior swelling operation. Furthermore, treated clay is activated due to the increased presence of hydroxyl groups within the clay residue, which are mainly responsible for the reactions and

- 102 - Chapter Four the absorptive properties of clay residues [4-7]. Thus, mild acid treatment can be used to depopulate the octahedral sheet. However the fastest process to achieve this is to remove the exchangeable cations and to substitute them with protons [8]. The dissolution rate of the octahedral sheet increases with the amount of Mg and/or Fe that is substituted for Al in the octahedral layer [5, 7].

Let us now turn to some of the recent investigations on the sorption of organic species on smectites and other clays [9-23]. Out of the various layered clays, smectites are probably the best clays to fulfill the requirements of swelling ability and high surface areas for access of the guest molecule. Smectites include beidellite, hectorite, fluorhectorite, saponite, sauconite, montmorillonite and nontronite [24].

Figure 4-1 – A schematic represention of beidellite clay.

Earlier studies have shown that smectite clay can adsorb many major types of pesticides, such as carbamates, nitrophenols and triazines [16, 25, 26]. Aggarwal et al. proved triazine sorption by saponite and beidellite clay minerals [27]. Celis et al. reported that montmorillonite is the main mineral soil colloid contributing to simazine sorption [28]. Another study has reported the sorption of atrazine on smectites [29]. The sorption of simazine on mineral rich Brazilian soils was researched by Oliveira et al. [30].

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Obviously, in these reports, the adsorption behaviour of clay is decisively determined by the layers within the clay and in particular, the physical and chemical properties of the system of voids and channels. It is known from previous studies, that the important factors which affect the sorption capacity on acid activated clays include surface acidity, surface area and porosity [31-33]. Based on our knowledge of clay minerals we deduce that the accessibility of pollutants can be controlled by means of the pore dimensions. Acid treatment largely destroys the integrity of clay layers and this process has an effect on weakening the intermolecular hydrogen bonds between the layers therefore loosening the packing. As a result, the accessibility of pollutants to the external and interlayer surfaces of clay can be improved.

The first part of the present study aims to systematically investigate how the extent of acid treatment leads to the decomposition of the parent structure of beidellite. It also aims to investigate how this structural change contributes to the sorption ability of pollutants from water. The treatment time and temperature of the acid treatment were specifically considered, together with the acid concentration so that the structural decomposition of beidellite could be controlled.

So far in this chapter we have discussed the acid activation of native clay to improve its chemical and physical properties toward the sorption of organic species. In the following sections we will consider a treatment involving the grafting of silane groups onto the surface of the clay. In this manner the new functional groups can confer new properties to the acid activated beidellite clay. The insertion of specific functional groups onto the surface of the inorganic matrix makes them capable of interacting with the pollutant species in a more favourable manner. In recent years, many materials have been developed by modifying the solid surfaces of materials with organic functional groups in a controlled fashion. The intracrystalline assembly of nanostructured organic-inorganic functionalised materials was initially achieved in smectite clay minerals through the exchange reactions of organic cations [34, 35]. However, large hydrophobic guest molecules have a significantly lower likelihood of approaching a hydrophilic reaction site. There are extensive reports on how the interaction between a hydrophobic molecule and an inorganic surface could be achieved by the simple grafting of hydrophobic

- 104 - Chapter Four groups onto the layer surface [36, 37]. More recently, the interaction between silane and swelling clay minerals laponite, kaolinite and magadiite has also been reported in literature [38-42]. High surface area is an advantage of montmorillonite and other related clays when using them as sorbents. In order to use them as adsorbents for trace organic pollutants in water, a conversion of the clay surface to a more hydrophobic surface is essential [43-46]. Studies on the interaction between hydrophobic molecules and montmorillonite clay have been reported [47-51]. In other studies, organic silane functional groups were grafted on the internal surface of the montmorillonite by condensations which have become widely used for environmental applications [52-54]. Furthermore, the adsorption of phenol, p- nitrophenol and aniline by bentonite and an organic herbicide on modified montmorillonite has been studied [55, 56]. Additionally, the uses of montmorillonite clay complexes for the removal of neutral and anionic organic pollutants have been implemented for the purification of water [48, 57]. The hydrolysis and kinetics of the adsorption of different pesticides on beidellite type minerals have also been investigated [58]. Apart from naturals clays, the interlayer surfaces of synthetic smectite clays have also been grafted using an organotrialkoxysilane [59] and even the grafting of a halloysite clay nanotube with aminopropyltriethoxysilane has been recently reported [60]. Organic modification of a mineral‟s surface can decrease its hydrophilic nature and increase its organophilic nature thereby increasing its ability to absorb organic solutes from water [61-65].

Other types of adsorbent materials, such as alkylsilane modified silica (Scheme 4- 1), carbon black and polymeric resins, may also be used for the removal of organic pollutants from water [66-70]. Once the surface has been silanized other species, such as ion-chelating organic molecules, may be immobilised onto the silica surface. For instance, recent reports highlight the hydrophobic interaction of methyl groups of the dichlorodimethyl silane or chlorotripropyl silane immobilized on silica and the carbon chain of Alamine 336 or LIX 84 or Cyanex 272 [71, 72]. In another heterogeneous process, 3-mercaptopropyltrimethoxysilane (MTS) was grafted onto silica and then condensed to a one molecular equivalent of ethylenimine (ETN). In a homogeneous route, MTS was treated with ETN to yield

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3-trimethoxysilylpropylthioethylamine (MPTT), which was then covalently anchored onto a silica gel surface (Scheme 4-2) [73].

Scheme 4-1 – Silica surface modified with organic functional groups for the chemical selectivity [66].

We provide here only representative examples of some typical modifications of silica in an attempt to represent the versatility of these methods. More complex sequences are sometimes encountered for modification; a silica surface was modified with -diketoamine after reaction with 3-bromopentanedione on a silanized surface was achieved by Gambero et al. [74]. It is also known that a silica acid surface was prepared by the reaction of 2,4-diclorophenoxyacetic acid with 3- trimethoxysilyl propylamine functionalised silica in an inert medium [75]. In a more complicated sequence, immobilized formylsalicylic acid on a silica surface was prepared by Mahmoud et al. (Scheme 4-3) [76].

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Scheme 4-2 – Schematic overview of the 3-mercaptopropyltrimethoxysilane (MTS) grafted onto silica [66].

Scheme 4-3 – Schematic representation of the selective grafting reactions on the external surface of silica [76].

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Obviously, from these reports, it soon became apparent, that the insertion of suitable specific functional groups into the inorganic surfaces makes them capable of interacting with guest molecules. In addition, we found that researchers paid great attention to the effect of different functional groups whereas little attention was paid when comparing the effect of various substrates for environmental applications. In the second part of the present study, acid activated beidellite clay grafted with 3- chloropropyltriethoxysilane (CPTES) and octyltriethoxysilane (OTES) was synthesized to produce adsorbents for removing pollutants from water. The surface of these adsorbents becomes hydrophobic when the interlayers are modified with long-chain organic molecules and the hydrophobic nature of clay surfaces changes depending on the nature of the grafting agents. The adsorption properties of the adsorbents critically depend on the arrangement, orientation, and conformation of the long organic chains of the graft molecules. It is then the interaction of these long graft chains with pollutant molecules that drives the sorption of pollutants from water.

Most adsorbents are comprised of ultrafine powders and are therefore difficult to recover after the adsorption process. In this instance, fine powders of clay adsorbents offer a superior sorption performance but are extremely difficult to separate from the solutions. Therefore this study will also investigate different morphologies, such as nanofibres and nanorods, to use in continuous mode separation methods. Different types of metal oxide nanofibres have been studied extensively by several investigators [77-81]. The fibrous morphology, high surface area and high stability of –Al2O3 nanofibres permit them to be used for surface modified sorbents. As seen in Figure 4-2, the fibrous morphology enables fibrillar interstices, which provide the necessary flow rate for slurry system and also provides better interaction of the pollutants with the adsorbents.

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Figure 4-2 – Fibrous morphology attributes fibrillar interstices.

Porous solids such as activated carbon, natural clays and mesoporous silica possess a very high surface area but lack a surface favourable for interaction with the pollutants. In other words, it is inadequate to explain the adsorption behaviour only by using the surface area of the substrate and ignoring the nature of the interaction with the surface. The presence of specific functional groups on the surface of the substrate contributes certain characteristics that enhance the adsorption at extremely low concentrations. However, for such modified materials, it is difficult to maintain the stability of the internal pore structures in order to get steady flows through the internal channels. Many of the pores possess molecular dimensions which will accept certain pollutant molecules and reject others based on size. Once the sorbing molecule reaches a critical dimension larger than the pore, the molecule will be rejected from the pore. On the other hand, the abundance of hydrophobic and protruding adsorption sites enhances the adsorption due to the ease of access that the sorbate has to the surface. In general, the formation of hydrophobic groups on the surface of the substrates during the functionalisation process affects the adsorptive capacity for many pollutants [61-65]. This hydrophobic nature can also cause the avoidance of the preferential interaction of water during the sorption process; hence this can prevent the blockage of part of the surface and helps the surface to directly interact with the pollutant molecules.

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The current investigation concentrates on the adsorption properties of modified beidellite clay for a wide range of organic pollutants and compares these results with the adsorption properties of modified –Al2O3 nanofibres. Modifications were carried out by using two organosilane grafting agents, 3-chloropropyltriethoxysilane (CPTES) and octyltriethoxysilane (OTES).

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4.2 Materials and Equipment

Beidellite, commercially known as Cloisite Na+, obtained from Southern Clay Products Inc. was used as starting material. The organosilane-grafting agents 3- chloropropyltriethoxysilane (CPTES, 99%) and octyltriethoxysilane (OTES, 99%) were purchased from Sigma-Aldrich and were used as received. All other reagents unless otherwise stated, were used as received from Sigma-Aldrich except the toluene (99.8%), which was obtained from Merck and was used after drying (freshly distilled after keeping for 24 h on Na2CO3).

The starting reagents for the preparation of –Al2O3 nanofibres were polyethylene oxide (PEO) surfactant (Tergitol 15S-7) and NaAlO2 were purchased from Sigma- Aldrich which were both used as received. Acetic acid and hydrochloric acid were obtained from Merck.

X-ray Diffraction (XRD) patterns, FTIR spectra, Scanning Electron Microscopy (SEM), Transmissions Electron Microscopy (TEM), Energy-dispersive X-ray (EDX) and Nitrogen adsorption-desorption techniques were used to characterize the samples. All characterization techniques were performed in accordance with the procedure outlined in Chapter 2.

The HPLC analysis of phenol, aniline, 4-tert-amyl phenols, 4-nonylphenol and simazine in the solutions were conducted in a manner outlined in Section 2.2.13. The mobile phase was acetonitrile:water (30:70) and the flow rate was 1 mL min−1. The absorbance was monitored in the 225 nm range.

Alachlor, imazaquin, bromacil, chlorotoluran and sulfosulfuron solutions were analysed in quartz 1 cm cuvettes with a Cary 100 UV-Visible spectrophotometer with. The absorbance of alachlor was monitored at 196 nm and imazaquin at 242 nm respectively. Bromacil, chlorotoluran and sulfosulturon were monitored at 210, 211 and 214 nm respectively. The analyses were carried out based on the description in Section 2.2.14.

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4.3 Discussion of Experimental Procedures

4.3.1 Acid treatment

Each 20 g of air dried raw clay was hydrothermally treated with a 400 mL aliquot of hydrochloric acid with concentrations of 0.05 M, 0.2 M and 0.5 M, respectively, for two days at 100 ºC in 1000 mL Schott special glass bottles. The samples were subsequently denoted as B1 (pure clay), B2 (0.05 M), B3 (0.2 M) and B4 (0.5 M). After acid treatments, the samples were washed repeatedly with ultra pure water under vacuum filtration repeatedly until the pH of the suspension reached a value ~7. The samples thus obtained were dried at 80 oC for 24 h and ground for the sorption tests.

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4.3.2 Modification of acid treated clay with silane groups

0.5 M acid treated beidellite clay (B4) was chosen for modification because of its high surface area and was grafted with organosilane following the general procedure reported in the literature [82]. Approximately 2 g of the acid activated clay was dispersed ultrasonically in 60 mL of dried toluene in a reflux flask for 30 min. To this suspension, CPTES was added in order that a silane/clay (mass ratio) of 5% (sample labeled as AC1), 25% (sample labeled as AC2) and 50% (sample labeled as AC3) were achieved. The same procedure was followed for OTES except the OTES/clay mass ratios, were 50% (AO1) and 100% (AO2). The suspension was then refluxed at 110 oC for 24 h under constant stirring. The solid in the resultant mixture was filtered and washed six times with fresh toluene to remove the excess organosilane, then rinsed with ethanol twice and finally oven dried overnight at 80 oC. The process of modification is schematically illustrated in Figure 4-3.

Figure 4-3 – Schematic illustration of acid treatment and grafting of beidellite clay with silane groups.

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4.3.3 Preparation of –Al2O3 nanofibres

Thin -Al2O3 fibres were synthesised by a hydrothermal method and are denoted as AF. These fibres are 5-7 nm thick and 40-60 nm long with a specific surface area of 290 m2g1. They were prepared by treating an aluminium hydroxide precipitate with a polyethylene oxide (PEO) surfactant (Tergitol 15S-7 from Aldrich) at 100 oC based on the reported work [79, 80]. The reactions were performed in the following manner: 18.8 g of NaAlO2 (0.2 mol of Al) was mixed with 50 mL of 5 M acetic acid solution with continual stirring. The obtained white precipitate was filtered and washed four times in order to remove residual Na ions (pH = 4~5). Aluminium hydrate cake obtained after washing was combined with 40 g PEO surfactant

(chemical formula C12-14H25-29O (CH2CH2O)7H and an average molecular of weight 508) and stirred for 4 h to homogenise the slurry thoroughly. The sticky paste was transferred into a Teflon-lined stainless steel autoclave and heated in an oven at 100 o C. The molar ratio of Al(OH)3/PEO/H2O was 1:4:16. After two days, fresh aluminium hydrate cake was added to the heating mixture and was stirred for 30 min. The processes of adding aluminium hydrate cake continued two more times during a period of two days. The final ratio of Al(OH)3/PEO/H2O was 5:1:8, 7.5:1:12 and 10:1:16, respectively for each time. After 8 days of extensive heating in the oven, the reaction mixture led to the conversion of boehmite fibres. The phase transformation occurred during the calcination of boehmite fibres at 500 oC to -

Al2O3 nanofibres.

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4.3.4 Modification of –Al2O3 nanofibres with silane groups

As discussed in the case of clay adsorbents, the same organosilane agents, 3- chloropropyltriethoxysilane (CPTES) and octyltriethoxysilane (OTES) were chosen to modify the surface after refluxing the -Al2O3 nanofibres in 0.2 M HCl for 6 h. The acid refluxed fibres are denoted as AF(A). The procedure used for the grafting of -Al2O3 fibres is described as follows: 1 g of acid treated fibres was placed in a 500 mL flask containing 60 mL dried toluene under stirring, 0.5 mL or 1 mL of CPTES (mass ratio, silane/alumina) was slowly added by means of a syringe. The mixture was refluxed at 120 oC for 48 h. After cooling, the product was filtered and washed several times with anhydrous enthanol to remove unreacted CPTES and then dried in a vacuum at 110 oC for 10 h. The resulting material was ground in a mortar and kept in a plastic tube for further characterization and utilization. The modified products were denoted as AFC1(50) (0.5 mL CPTES) and AFC1(100) (1 mL CPTES), respectively. Identical procedures were followed in the case of OTES except that the amount of OTES was 0.57 mL or 1.15 mL. The final materials were denoted as AFC8(50) (0.57 mL of OTES) and AFC8(100) (1.15 mL of OTES), respectively.

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4.3.5 Adsorption experiments

Adsorption experiments were performed in three different stages. The first stage, studied the adsorption ability of acid treated clay over a range of organic pollutants, in the second stage, the adsorption experiments employed a modified clay which were compared to results with modified –Al2O3 nanofibres. Preliminary studies showed that neither phenol nor aniline have any affinity for acid treated nor for modified clay adsorbents whereas, 4-tert-amyl phenols, 4-nonylphenol, bromacil, chlorotoluran and sulfosulturon showed less than 40% adsorption on the modified clay adsorbant. Simazine however shows a strong affinity for the surface of the acid activated clay. It was also found that alachlor and imazaquin were adsorbed strongly on the modified clays. In the following sections we studied only the highly adsorbed pollutants such as simazine, alachlor and imazaquin.

The sorption testing of simazine by various clay samples was carried out using the batch equilibration method. Standard solutions (0.5, 1, 2, 3, 5 ppm) of simazine were prepared and stored at room temperature in the dark. For each determination, 10 mg of air-dried clay was dispersed into 20 mL of a stock solution. The aqueous solubility of simazine is 5 ppm at 20-22 oC [83]. The suspensions were equilibrated for 24 h at room temperature (23 oC) on a linear shaker in 50 mL polypropylene centrifuge tubes. The suspensions were then centrifuged at a speed of 4000 rpm, and the amounts of simazine in the supernatant solutions were determined by HPLC in the manner outlined in Section.2.2.13. Separated clay samples were dried at 80 oC for 24 h and were further used for XRD and FTIR analyses.

Sorption experiments for alachlor and imazaquin were carried out in a manner identical to the simazine experiment except for the concentrations of the pesticides. These concerntrations were 2.5, 5, 7.5 and 10 ppm for alachlor, and 1, 2, 3, 4 and 5 ppm for imazaquin. Following equilibration, the suspensions were centrifuged, and the amounts of the pollutants in the supernatant solutions were determined with a Varian UV-vis spectrophotometer. The absorbances of alachlor at 196 nm and imazaquin at 242 nm were monitored respectively.

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For modified –Al2O3 nanofibres, 20 mg of an air-dried sample was used instead of 10 mg for each determination. However, all other procedures were the same.

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4.4 Results and discussion

4.4.1 Elemental Analysis

To investigate the change in the clay layers caused by the acid treatment, elemental analysis was carried out by SEM-EDX. Table 4.1 shows the atomic ratio of various elements to that of the silica which should remain unchanged by acid treatment. The atomic ratio of Si and Al in the raw beidillite clay (B1) was found to be 0.210. The ratios of Na, Mg and Fe with silica present in the raw clay were 0.061, 0.05, and 0.056, respectively. It was found that after the acid treatment with 0.05 M HCl at 100 ºC for 2 days, that almost 80% of the sodium present in the clay was exchanged. However, the other elementals ratios remained unchanged. The acid treatment with 0.2 M HCl slightly attacked the octahedral layers. This was detected by the elemental ratios changing slightly. However it was found that acid treatment with 0.5 M HCl caused substantial changes in the elemental ratios. Sodium present in the clay was almost completely exchanged with hydrogen and the Al/Si ratio dropped to 0.157. This means that after the acid treatment, part of the elements in the octahedral and inters layers spaces are leached out but the tetrahedral silica layer remains unchanged without the total collapse of the frameworks. The ratios of Al, Fe and Mg shown in Table 4-1 illustrate the gradual dissolution of the octahedral sheet. This dissolution increases by increasing the acid concentration.

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Table 4-1 – Element analysis ratio, specific surface area (BET) and pore volume (Vp) of the pure clay and acid treated samples.

Sample HCl(M) Na/Si Al/Si Mg/Si Fe/Si BET S.A. Vp (m2g-1) (cm-3g-1) B1 0 0.06 0.21 0.05 0.057 18.4 0.075 B2 0.05 0.01 0.21 0.047 0.059 61.9 0.088 B3 0.2 0.004 0.20 0.043 0.055 93.19 0.142 B4 0.5 0.003 0.16 0.033 0.046 236.4 0.407

4.4.2 TEM Images

The TEM images of various acid treated bedillite clay samples are displayed in Figure 4-4. The measurements were done by using the Photoshop ruler tool. It can be observed that the basal spacing of pure bedillite clay, the d001 spacing, is about

1.125 nm (Figure 4-4a). After acid treatment, the d001 spacing increased to 1.31 nm (Figure 4-4b-d). It can be observed that the layered structure was damaged when the acid concentration was high, but it remained even after the treatment with 0.5 M HCl (4-4d).

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Figure 4-4 – High resolution TEM images. (a) Pure beidillite clay, (b), (c) and (d) are 0.05, 0.2 and 0.5 M acid treated samples, respectively.

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4.4.3 X-ray diffraction

XRD patterns of the pure beidillite clay and the acid treated samples are shown in Figure 4-5. For pure beidillite clay, three strong peaks are observed at 7.3o, 19.9o, and 28.5o 2. According to the PDF card 043-0688 (Sodium Aluminum Silicate Hydroxide Hydrate), these peaks are from diffraction by the (001), (100), and (004) planes, respectively. Upon acid treatment, the diffraction peaks from (001) and (004) planes changed due to the dissolution of alumina layers, but no obvious change in the peak from the (100) plane is observed. For example, the interlayer spacing (d001 spacing) calculated using X'Pert HighScore software showed an increase in d-spacing from 1.19 to 1.36 nm, and the diffraction intensity of (001) and (004) planes gradually decreases by increasing the acid concentration. These results are in well agreement with TEM data. It is known that beidellite is a 2:1 layered aluminosilicate, in which a clay layer consists of two tetrahedral silicate sheets, one octahedral alumino sheet sandwiched between the silicate sheets and cations such as Na, K, Ca, or Mg located in the space between the clay layers [84]. This indicates that the clay layers are destroyed and that part of non-silicon elements in the octahedral alumina layer was leached out from the clay layers by the acid treatment.

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a B1 d=1.19 nm B2 (001) B3 B4

d=1.36 nm

Intensity,counts/sec.

4 5 6 7 8 9 10

2(degree) b (004) (100) (002) (110) B1

Intensity,counts/sec. B4

10 15 20 25 30 35 40 45 2(degree)

Figure 4-5 – XRD patterns of the pure beidillite clay (B1) and the acid treated samples (B2–0.05, B3–0.2 and B4–0.5 M).

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Figures 4-6a and 4-6b show the XRD patterns after modification of the acid treated clay. As seen in the XRD patterns, a significant amount of peak (001) shifting towards lower 2 values is observed for the grafted samples, which is consistent with literature [47, 85], indicating an increase of the d (001) spacing. For all of the grafted samples, the d-spacing increases by increasing the ratio of organosilane to clay, from 1.289 nm of the non-grafted sample to 1.407 nm of the 50% CPTES grafted one, or to 1.43 nm of the OTES grafted one. This change suggests that the molecules of organosilane are on the inner-surface of the clay interlayer and that the higher the amount of the organosilane molecules in the interlayer, the larger the d- spacing change observed.

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d=1.407 nm a (001)

(100) AC3

AC2

d=1.289 nm AC1 B4

Intensity,counts/sec.

5 10 15 20 25 30 2(degree)

d=1.43 nm b (001)

(100) AO2

1.289 nm AO1

Intensity,counts/sec.

5 10 15 20 25 30 2(degree)

Figure 4-6 – XRD patterns of samples: B4 – 0.5 M acid treated clay; AC1, AC2 and AC3 – CPTES grafted samples; AO1and AO2 – OTES grafted samples.

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Figure 4-7 shows the XRD patterns before and after modification of -Al2O3 fibres. It can be seen from the XRD patterns that a only slight decrease in peak sharpness was observed for the samples after grafting, indicating that there was little change in crystallinity after the grafting process. The standard XRD pattern of -Al2O3 from JCPDS cards (4-007-2479, 1-74-4629) were used in order to identify the diffraction lines. The calculation of unit cell dimensions indicated that unit cell parameters were not affected in the grafting process. Furthermore, the overall crystal structures were identified as similar before and after grafting.

(511)

(440)

(222)

(220)

(311) (400) AFC1(100) AFC1(50)

AFC8(100)

AFC8(50) AF(A)

Intensity,counts/sec. AF

20 30 40 50 60 70

2, deg

- 125 - Chapter Four

4.4.4 Nitrogen adsorption

N2 sorption data was used to estimate the specific surface areas and porosities of the clay samples which had received various acid treatments. N2 adsorption/desorption isotherms of the samples are illustrated in Figure 4-8. The samples B1, B2, B3 and pure beidellite exhibit a typical H3 hysteresis loop that reflects the presence of slit- shaped pores [86]. The shape of the isotherm of B4 samples is remarkably different from the other samples. The adsorption is much larger and the hysteresis loop deviates substantially from shape of H3 type loop. This suggests that the acid treatment with high concentration of 0.5 M yielded a large amount of pores, and the clay layer surface was damaged seriously during the treatment (the pores in the treated sample are not slit-shaped). The BET surface area of pure beidellite derived from nitrogen adsorption data is about 18 m2/g whereas the BET specific surface areas of the acid treated samples increased to 63 to 236 m2/g. The pore volumes were approximately 0.088, 0.142, and 0.407 cm3/g compare with 0.075 cm3/g of pure untreated clay. These results suggest the formation of large pores after acid treatment.

- 126 - Chapter Four

250 B4

STP)

-1 B3

g 3 200 B2 B1 150

100

Volume Adsorbed (cm Adsorbed Volume 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure(P/P ) 0

Figure 4-8 – Nitrogen adsorption/desorption isotherms of beidillite clay (B1) and the acid treated samples (B2–0.05, B3–0.2 and B4–0.5 M).

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The nitrogen adsorption isotherms of grafted samples are shown in Figure 4-9. The isotherms of all the samples exhibit a type IV feature, corresponding to mesoporous materials with capillary condensation. The hysteresis of these samples exhibits the

H3 hysteresis loop, which does not exhibit any limiting adsorption at high P/Po, and is often observed with aggregates of plate-like particles giving rise to slit-shaped pores. As seen in Figure 4-9a and 4-9b, the isotherm morphology of the grafted samples is similar to that of the non-grafted sample, suggesting the grafting process does not change the feature of the pore structure. However, with the increase of the ratio of organosilane to clay, the specific surface area and porous volume decrease, and the pore diameter slightly increases (Table 4-2). Apparently, the organic grafting reduces the surface area and the pore volume, which probably resulted from the grafting molecules occupying or blocking the interlayer spacing.

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300

-1

g B4

3 a 250 AC1 AC2 200 AC3

150

100

adsorption in STP, cm STP, in adsorption

N 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure(p/p ) 0 300

-1

3 B4 b 250 AO1 200 AO2

150

100

adsorption in STP, cm STP, in adsorption

N 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure(p/p ) 0

Figure 4-9 – N2 adsorption/desorption isotherms: B4 – 0.5 M acid treated clay; AC1 (5%), AC2 (25%) and AC3 (50%) – CPTES grafted samples; AO1 (50%) and AO2 (100%) – OTES grafted samples;

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Table 4-2 – Specific surface area, pore volume and mean pore diameter of samples: A0 – 0.5 M acid treated clay; AC1, AC2 and AC3 – CPTES grafted samples; AO1and AO2 – OTES grafted samples.

2 -1 a 3 -1 b Samples SBET(m .g ) Vp (cm .g ) mean BET D (nm)

B4 241 0.41 6.73

AC1 221 0.36 6.44

AC2 150 0.26 6.86

AC3 114 0.20 6.91

AO1 214 0.37 6.89

AO2 175 0.34 7.68

a b Single point adsorption total pore volume of pores at P/P0 0.99. Adsorption average pore diameter(4V/A by BET).

The isotherm plot for nitrogen adsorption of parent and modified -Al2O3 fibres are shown in Figure 4-10. The isotherms of these solids exhibit a type H3 hysteresis loop [86]. As the extent of modification is increased, the specific surface area and porous volume decreases (Table 4-3). The grafting process reduces the surface area of the -Al2O3 fibres to a greater extent. As a consequence of the presence of grafting molecules, the empty space available for nitrogen adsorption is reduced, resulting in the lower surface area values of the grafted samples.

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AF(F) AFC8(50)

-1 AFC8(100)

g 800 3 AFC1(50) AFC1(100) 600

400

adsorption in STP, cm 200

0 0.0 0.2 0.4 0.6 0.8 1.0

Relative pressure(p/po)

- 131 - Chapter Four

mean D (nm)

2 -1 a 3 -1 b c Samples SBET(m .g ) Vp (cm .g ) BET BJH

AF(F) 292 1.45 19.9 14.0

AFC1(50) 266 1 14.9 12.5

AFC1(100) 232 1 15.7 11

AFC8(50) 252 1 16.5 11.4

AFC8(100) 217 0.84 15.3 10.7 a b Single point adsorption total pore volume of pores at P/P0 0.99. Adsorption average pore diameter (4V/A by BET). C Barrett─Joyner─Halenda (BJH) desorption average pore diameter (4V/A).

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4.4.5 Thermogravimetric Analysis

Figures 4-11 and 4-12 show the mass loss (TG) and derivative mass loss (DTG) curves of beidellite samples before and after modification. For the acid treated beidellite clay sample ( Figure 4-11), the mass loss from 35–140 oC is assigned to physically adsorbed water [23], and another major mass loss in the range of 456– 691 oC, corresponding to the DTG peak at 585 oC, is assigned to the dehydroxylation of structural AlOH group of the beidellite. The mass loss from 140 oC to 456 oC accounts for 2.31% of the total mass of this sample, which is assigned to the water molecules that are hydrogen-bonded with hydroxyls on the inner surface of clay layers. The final mass loss occurs at DTG peak of 914 oC which can be attributed to the loss of structural water.

For the CPTES grafted sample AC3 (Figure 4-12a), the mass loss from 36.1 oC to 117.9 oC, corresponding to a DTG peak at 64 oC, is 2.21%, which is assigned to the physical absorbed water on the external surface or shallow part of clay lumen. From 117.9 oC to 776.7 oC, a great amount of mass loss, accounting for 14.5% of the total mass, is observed, which is a complicated mass loss procedure and can be resolved into four steps, corresponding to DTG peaks at 244.3 oC, 430.2 oC, 595.7 oC and 692.7 oC. The mass loss that corresponds to a DTG peak of 244.3 oC is assigned to the CPTES molecules physically absorbed on the external surface of clay, and the mass loss at DTG peaks of 430.2 oC, 595.7 oC and 692.7 oC are attributed to the decomposition of CPTES species grafted onto SiOH and AlOH groups on the external and internal surface of clay or oligomerized CPTES species [23]. This multistep mass loss also involves the dehydration of AlOH and is partially overlapped with the decomposition of the grafted species. Apparently, the mass loss, which resulted from hydroxyls of SiOH and AlOH groups in sample AC3, is not as remarkable as that of sample B4. This is because most hydroxyl groups on the clay surface are bonded with CPTES. The loss of the last portion of the hydroxyl water correspounds to DTG peaks at 894.7 oC.

A similar type of mass loss was observed in the sample AO2 (Figure 4-12b). The mass loss from 30.3 oC to 154.2 oC, accounting for 2.24% of the total mass, is assigned to physical absorbed water, and that from 154.2 oC to 793.1 oC, accounting

- 133 - Chapter Four for 12.69% of the total mass, involves several stages of mass loss which can be attributed to the desorption of the physical absorbed OTES species and the decomposition of the OTES species that are grafted onto the SiOH and AlOH groups on the external and internal surface of clay. When temperature is increasing above 800 oC another mass loss is accompanied by loss of last portion of structure water.

98 B4 0.08

2.46% C 94 o 2.31% 0.05 69.75 oC 90 585.25 oC 3.33%

Mass Loss, % 914 oC 0.02 86

Deriv. Mass,% /

82 -0.01 0 200 400 600 800 1000 o Temperature, C Figure 4-11 – TGA and DTG curves of 0.5 M acid treated beidellite clay.

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98 (a) AC3 0.09 2.21% 93

0.07 o 64.3 oC 88 244.3 oC 14.49% 0.05 595.8 oC 83 430.5 oC 0.03 o Mass Loss, % 78 692.8 C

o 0.01 Deriv. Mass,% / 73 894.7 C

68 -0.01 0 200 400 600 800 1000 Temperature,oC

99 (b) AO2 0.09 2.24%

94 C 0.07 o 12.69% 89 0.05 o 71.2 C 702.7 oC 600 oC 84 0.03

Mass Loss, % o 913.9 C Deriv. Mass, % / 79 0.01

74 -0.01 0 200 400 600 800 1000 Temperature, oC

Figure 4-12 – TGA and DTG curves of clay samples: a) AC3 – CPTES grafted samples; b) AO2 – OTES grafted samples, respectively.

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Figures 4-13 and 4-14 show the mass loss (TG) and the derivative mass loss (DTG) curves of -Al2O3 samples before and after modification. For the sample AF(A), the mass loss from 36–319 oC is assigned to the removal of physically adsorbed as well as the loose of structural water [80]. But in the case of the grafted samples the stages of dehydration are partially overlapped with the decomposition of the organic species. The major mass loss is in the range of 211–704 oC for AFC8(100) corresponding to the DTG peak at 451 oC (Figure 4-15a). This mass loss is assigned to the degradation of the organic groups. In addition, it should be mentioned that the small sharp derivative peak around 530 oC may be ascribed to the decomposition of grafted groups from the inner pore walls. Sample AF(A) illustrated a slightly higher mass loss during the first stage due to the high amount of physically adsorbed water with less hydrophobic surfaces. For the OTES grafted sample AFC8(100), the first stage mass loss is 2% which corresponds to physically adsorbed water (Figure 4- 15a). The second stage of mass loss is associated with a very distinct peak and consists of the thermal decomposition of the organic species, which is about 14%. Almost similar mass loss was observed in the sample AFC1(100).

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99 0.09

96 C

0.07 o 7.62% 91 oC 93 0.05

263.4 oC 90 0.03

Mass Loss, %

87 0.01 Deriv. Mass, % /

84 -0.01 0 200 400 600 800 1000 Temperature, oC

Figure 4-13 – TGA and DTG curves of acid washed -Al2O3 fibres – AF (A).

- 137 - Chapter Four

98 2% (a) 0.09 o 92 451 C

14% C 0.07 o 86

80 0.05

o 74 68.37 C 0.03

Mass Loss, % 68

0.01 Deriv. Mass, % / 62

56 -0.01 0 200 400 600 800 1000 Temperature, oC

98 1.8% (b) 0.09 92 285 oC

14% C

0.07 o 86

80 0.05 70 oC 74 530 oC 0.03

Mass Loss, % 68

0.01 Deriv.Mass, % / 62

56 -0.01 0 200 400 600 800 1000 o Temperature, C

Figure 4-14  TGA and DTG curves of modified -Al2O3 fibres: a) AFC8 (100) – OTES grafted samples, and (b) AFC1 (100) – CPTES grafted samples, respectively.

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4.4.6 FTIR spectra

For the FTIR spectra of beidellite shown in Figure 4-15, the strongest band near 1040 cm1 is attributed to the Si─O stretching vibrations of the tetrahedral layers. A shift of this vibration band from 1043 to 1051 cm1 was observed after acid treatment. The shift is ascribed to the alteration in the amount of amorphous silica, which resulted from decomposition of the octahedral sheet during acid treatment [4, 5, 87, 88]. The bands at 523 and 468 cm1 correspond to the SiO bending vibrations of beidellite [4, 88]. The absorption bands between 840 and 930 cm1 are assigned to the bending vibrations of the OH groups coordinated to the octahedral cations. For instance, the bands at 917, 883, and 845 cm1 correspond to the vibration of AlAlOH, AlFeOH, and AlMgOH, respectively [4, 88]. With 0.5 M acid treatment, the intensity of these bands was largely reduced because part of the octahedral cations had been leaching out by the acid.

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1043

1051

917 468 523 845 625 B1

883 B2 797 B3

Absorbance B4

1200 1050 900 750 600 450

-1 Wavenumber/cm Figure 4-15 – FTIR spectra of pure beidillite clay (B1) and the acid treated samples (B2-B4).

- 140 - Chapter Four

Figure 4-16 displays the FTIR spectra of samples after modification. The band at 1053 cm1 is assigned to the in-plane Si-O stretching and the band at 1114 cm1 is assigned to the perpendicular Si-O stretching. The bands at 535 cm-1 and 470 cm1 are attributed to the bending vibration of Al-O-Si and Si-O-Si, respectively. The bands at 1630 and 3425 cm1 are the OH bending vibration and stretching vibration of water molecules in the interlayer space and the band at 3640 cm1 is due to the OH stretching of the inner surface hydroxyl groups. The band centred at 2930 cm1 is a distinct characteristic of the grafted samples, which is attributed to the aliphatic CH stretching [60, 89, 90]. It can be observed from Figure 4-16 that the Si-O vibrations, centred at 1053 and 1114 cm1, and the CH stretching vibrations centred at 2930 cm1, intensify with increasing the organosilane to clay ratio, suggesting the increase of grafted organosilane in the samples.

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3640 1053

3425 1114 470 2930 535 1630 AO2 AO1

AC3

Absorbance AC2 AC1 B4

3600 3000 2400 1800 1200 600 Wavenumber/cm-1

Figure 4-16  FTIR spectra of samples: B4 – 0.5 M acid treated clay; AC1, AC2 and AC3 – CPTES grafted samples; AO1and AO2 – OTES grafted samples.

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Since all grafted -Al2O3 fibres showed similar features, only representative spectra have been shown in Figure 4-17. Samples AFC1 and AFC8, show bands in the 3000–2700 cm–1 region are found to be aliphatic C–H stretching vibrations [91]. –1 Aliphatic CH2 groups give rise to a doublet at 2938 and 2853 cm in the AFC8 spectra, which is assigned to anti-symmetric and symmetric stretching, respectively.

(CH2)3Cl AFC1(100)

AFC1(50)

CH2(CH2)6CH3 AFC8(100)

Absorbance AFC8(50) AF(A)

3500 3000 2500 2000 1500 1000 Wavenumber/cm-1

Figure 4-17  FTIR spectra of modified -Al2O3 fibres: AF(A) – acid washed; AFC1(50) and AFC1(100) – CPTES grafted samples; AFC8(50) and AFC8(100) – OTES grafted samples.

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4.4.7 Contact Angle

Due to the hydrophobic nature of the grafted organic groups, it can be anticipated that the surface free energy of the clay samples will be altered after grafting. The hydrophobic nature of the clay surface was studied by measuring the contact angle (CA) of water on the surface of the samples. Figure 4-18 shows the shape of a water droplet on the surface of a pellet formed from the samples. For the acid treated samples, the water CA is as low as 16±2º (Figure 4-18a), displaying a hydrophilic surface. However, as shown in Figure 4-19b and c, the water CA of the chloropropyl group grafted clay (AC3) increased to 69±2º; the value of the octyl group grafted clay (AO2) is 106±2º. Clearly, the surface modification of clay leads to a significant change in the surface groups from hydrophilic to hydrophobic, resulting in an increase in the contact angle of water. This suggests that the adsorption performance for molecules filled with hydrophobic groups will be enhanced.

- 144 - Chapter Four

Figure 4-18  Contact angle of non grafted and grafted clays: (a) non-grafted sample B4; (b) CPTES grafted sample AC3; (c) OTES grafted sample AO2.

- 145 - Chapter Four

Figure 4-19 shows the shape of a water droplet on the surface of the tablet form of

-Al2O3 fibre samples. For the acid washed fibres  AF(A), water CA is as low as 18±2º (Figure 4-20a), displaying a hydrophilic surface. However, as shown in Figure 4-20b and c, the water CA of the chloropropyl group grafted fibres  AFC1(100) increased to 63±2º and the value of octyl group grafted fibres  AFC8(100) is 146±2º. Clearly, the surface modification of fibres leads to a significant change in the polar components therefore resulting in an increase in the contact angle.

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4.4.8 TEM images

Figure 4-20 shows the morphology of -Al2O3 fibres before and after modification. As seen in the micrograph, functionalisation causes the aggregation of fibres which attributes fibrillar interstices for the effectiveness of contact time. The inter-linked fibrillar morphology enables the easy flow of the slurry system and also provides better interaction of pollutants, suitable for continuous mode water purification methods.

Figure 4-20  Transmission electron micrograph of modified -Al2O3 fibres, (a) AF(A) – acid washed; (b) AFC1(100) – CPTES grafted samples; (c) AFC8(100) – OTES grafted samples.

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4.4.9 Solid-state 29Si MAS NMR spectra

Silica-29 NMR spectra of clay samples were not obtainable due to the presence of paramagnetic elements in the clay layers. The following discussion is based on the solid state silicon-29 NMR spectra of modified -Al2O3 fibres. The formation of a different type of silica environment after grafting was confirmed by the solid-state 29 Si MAS NMR spectra (Figures 4-21). The presence of T1, T2 and T3 signals observed at –76, –67 and –58 ppm corresponding to three different environments of silica atoms in all four samples [92]. T1, T2 and T3 correspond to * * [(CH3CH2O)2(AlO)Si (CH2)3Cl], [CH3CH2O(AlO)2Si (CH2)3Cl] and * [AlO)3Si (CH2)3Cl], respectively for the CPTES grafted -Al2O3 fibres. The OTES grafted -Al2O3 fibres, T1, T2 and T3 correspond to * * [(CH3CH2O)2(AlO)Si (CH2)7CH3], [CH3CH2O(AlO)2Si (CH2)7CH3] and * [AlO)3Si (CH2)7CH3], respectively. Unlike that of OTES grafted samples, there is little change in the spectra of CPTES samples with increasing the amount of grafting from 50 to 100%, which is likely due to the saturation of grafting with 50% and less chance for further loading.

- 148 - Chapter Four

(a)

AFC8(50)

T2 T3 Intensity(a.u.) T1 AFC8(100)

-20 -40 -60 -80 -100 -120 Chemical shift(ppm) (b) T2 T3

T1 AFC1(50)

Intensity(a.u.)

AFC1(100)

-20 -40 -60 -80 -100 -120 Chemical shift (ppm)

29 Figure 4-21  Solid-state Si MAS NMR spectra of modified -Al2O3 fibres: (a) AFC8(50) and AFC8(100) – OTES grafted samples; (b) AFC1(50) and AFC1(100) – CPTES grafted samples.

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4.4.10 Adsorption of pollutants from water

The adsorption properties of a wide range of organic pollutants have been tested in this study. The molecular structures of all of the tested pollutants are shown in Figure 4-22. It was observed that either phenol or aniline were not effectively adsorbed by acid treated clays nor were by their silane modified counter parts. Less than 40% of 4-tert-amylphenol, 4-nonylphenol, bromacil, chlorotoluran and sulfosulturon were removed from solutions using modified clay adsorbents. The herbicide simazine could be removed efficiently using clay sorbents while alachlor and imazaquin were adsorbed efficiently on the surface of the modified clay. All organic pollutants are attracted to the surfaces of the adsorbents by Van der Waals forces. This attraction is complementary if functional groups are attached to the surfaces that are capable of hydrogen bonding with organic pollutants. The surface attraction energy is very strong for H-donor and H-acceptor sorbates such as water. Such surfaces prefer to bind with water over small ionic or nonionic organic pollutants. Thus, the overall energy change resulting from the adsorption of organic pollutants directly to these adsorbents would have to reflect the high cost of desorption of water from the same surface. In the following sections the strongly adsorbed simazine, alachlor and imazaquin are discussed in detail.

Simazine adsorbtion studies were conducted for the adsorption of trace amounts (<= 5 ppm) of simazine onto the surface of acid treated beidellite clay from water. For comparison, the sorption ability of pure beidellite clay was also investigated. In Figure 4-23 is the sorption of simazine on various clays products is shown, the dark shaded bars indicate five different initial concentrations of simazine and light shaded bars illustrate the final concentration after sorption. Clearly, the pure beidillite clay could not adsorb the toxic organic simazine from water (Figure 4- 23a). However, the acid treated samples displayed superior sorption ability (Figure 4-23b-d). The sorption capacity of the samples increases with an increase in the acid treatment concentration. For instance, whereas sample B2 can only fully remove the simazine solution less than 1 ppm; sample B4 can almost decontaminate the simazine solution at 5 ppm.

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Figure 4-22  Molecular structures of pollutants.

- 151 - Chapter Four

The dried samples were assessed with FTIR. As shown in Figure 4-25, typical adsorption peaks were observed at 2920 and 2850 cm1 in B4S, which is the sample B4 after simazine adsorption, indicating simazine has been adsorbed in the clay samples. Comparing the FTIR spectrum of B4S with those of simazine [93], it is strongly indicated that simazine was adsorbed on B4S.

- 152 - Chapter Four

B4S

Absorbance 2920 2850

4000 3000 2000 1000 -1 Wavenumber/cm Figure 4-24 – FTIR spectra of 0.5 M acid treated clay (B4) before and after sorption.

A scheme to explain the acid treatment process and sorption of simazine on acid treated clay is presented in Figure 4-25. Before acid treatment, the beidellite clay displays a typical 2:1 layered structure, two tetrahedral silicate surface layers and one octahedral alumino sheet (Figure 4-25a). After acid treatment, part of the octahedral alumino sheet was leached out. As mentioned in the elemental analysis result (Table 4-1), part of non-silicon cations, such as Na, Al, Fe, and Mg, have been gradually dissolved out from the alumino sheet, which directly results in an

- 153 - Chapter Four obvious increase in interlayer spacing, specific surface area, and pore volume. When the acid treated samples were dispersed into the simazine solution, simazine molecules can directly enter into the destroyed alumino sheet and be adsorbed there (Figure 4-25c).

Figure 4-25 – Schematic illustration of the acid treatment of beidillite clay and simazine sorption.

The adsorption of simazine could be carried out by different types of binding mechanisms such as Van der Waals interactions, hydrogen bonding, and ion-dipole interactions. The clay surface has different kinds of anchoring sites such as reactive OH groups, hydrated interlayer cations and charged sites resulting from isomorphous substitutions. The terminal OH groups consist of SiOH from the tetrahedral layers and AlOH from octahedral layers. At pH 7-8 most of silanol groups dissociate and the alumino groups also acquire an amphoteric character. On the other hand, negatively charged surfaces are produced on the edges, and are considered to be the most active sites to which charged species can bind.

Acid treatment favours the enrichment of hydroxyl species on the edges and faces which are readily accessible to the approaching simazine molecules. Many such sites should be located at the external surfaces and at the openings of the interlayer spaces because the layers are partially degraded during acid treatment. Acid- activated clays are the source of protons as most of the interlayer cations are replaced with monovalent hydrogen ions [94]. Like many other organic pesticides simazine is a weak base and belongs to the s-triazines family. The adsorption mechanism of simazine by beidellite could be due to the sorption of the protonated species produced by the acidic clay surface. Positively charged protonated species

- 154 - Chapter Four are then adsorbed on the negatively charged surfaces. The heterocyclic ring nitrogen of simazine can also be protonated, which will also cause binding to the negative sites. The process of protonation of simazine may occur mainly by surface and interlayer hydrated protons. It can be assumed that simazine can be anchored on the external surfaces and edges of the interlayer sheets through hydrogen bonding, Van der Waals and electrostatic forces.

d=1.3231 nm d=1.2894 nm

( 001) B4S

Intensity,counts/sec. B4 3 4 5 6 7 8 9 2 (degree)

Figure 4-26 – XRD pattern of 0.5 M acid treated clay (B4) before and after sorption.

The basal surface of a clay mineral carries a constant negative charge from isomorphous substitution [95]. The X-ray diffraction patterns of the clay after simazine uptake shows a considerable amount of expansion of the d-spacing from 1.29 to 1.32 nm and thus indicates the existence of a large amount of simazine in the interlamellar spaces of beidellite after adsorption (Figure 4-26) [85]. This suggests that uptake is mainly due to the intercalation of simazine molecules in the interlamellar spaces. The adsorption of simazine onto these basal surfaces occurs mainly by hydrogen bonding to the aluminol and silanol groups. Simazine can be

- 155 - Chapter Four protonated readily by the interlayer hydronium ions existing in the interlayer space after acid treatment. After the first sorption of simazine, the hydrophobic nature of the interlayer spaces is increased due to the presence of simazine, which promotes further adsorption from the solution [29].

The exchangeable cations such as Na+ and Mg2+ that remain in the interlayer region undergo hydration. The process of hydration enhances the hydrophilic nature of interlayer space. Furthermore, the Na+ has higher capacity for hydration than Mg2+. Therefore the presence of more Mg2+ in turn causes the interlayer space to have less of a hydrophilic nature and therefore a higher capacity to adsorb organic pollutants [84].

Interlayer cations can also interact via electrostatic forces and this can be explained by the formation of a coordinate bond with simazine molecules and cations, which are located in the interlamellar spaces via water molecules as bridging ligands [96]. Another possible mechanism which may contribute to the adsorption process is the aggregation of clay particles leading to the entrapment of the herbicide between the coagulated clay particles [97]. The results of the adsorption studies of alachlor and imazaquin on silane grafted clays are shown in Figure 4-27.

The adsorption performance of the grafted samples for alachlor and imazaquin is superior to that of the acid treated and the raw clay, while the adsorption performance of the acid treated clay B4, is slightly better than that of the raw sample. The OTES grafted samples, AO1 and AO2, show the highest adsorption ability for alachlor, but a lower adsorption ability for imazaquin than the CPTES grafted samples, AC2 and AC3, implying a different affinity between the pollutant molecules and the different grafted molecules, which has a strong impact on the adsorption performance of the grafted samples. For samples with the same organosilane grafted species, the adsorption performance increases with an increase in the amount of organosilane. The order of the adsorption ability, with respect to the same organosilane graft, is AO2>AO1 and AC3>AC2>AC1 for both pollutants. This suggests that the nature and quantity of the organosilane groups is responsible for the adsorption performance of the grafted samples. The adsorption performances of grafted samples are shown in Figure 4-28.

- 156 - Chapter Four

Sample AO2 can absorb alachlor 5 mg/g of adsorbent at an initial lower amount in solution of 5 mg/g and about 16 mg/g can be adsorbed at an initial higher amount of 20 mg/g and sample AC3 can absorb imazaquin at about 1.8 mg/g at an initial lower amount of 2 mg/g and about 8.2 mg/g at an initial higher amount of 10 mg/g.

- 157 - Chapter Four

16 AO2 a AO1 AC3 12

AC2

8 AC1 B4 4

Adsorbed amount, mg/g B1

0 0 2 4 6 8 10 Solution concentration(Alachlor), mg/L

AC3 8 b

6 AC2 AO2 4 AO1 AC1 2 B4

Adsorbed amount,mg/g B1

0 0 1 2 3 4 5 Solution concentraion(Imazaquin), mg/L Figure 4-27  Adsorption isotherms of various grafted clays: Adsorbed amount versus equilibrium concentration.

- 158 - Chapter Four

16 a AO2 AO1 14 AC3 12

10 AC2

8 AC1 6 A0 4

Amount adsorbed, mg/g 2 B1 0 0 5 10 15 20 25 Amount added in solution (Alachlor), mg/g of clay 8 b AC3 7 6 5 AC2 AO2 4 AO1 3 AC1 2 A0

Amount adsorbed,mg/g 1 B1 0 0 2 4 6 8 10 12 Amount added in solution (Imazaquin), mg/g of clay

Figure 4-28 Adsorption performance of modified clay samples: Adsorbed amount versus amount added in solutions.

- 159 - Chapter Four

The acid treatment of raw clay results in the partial removal of the Al3+ from the octahedral layer and results is more Si-OH and Al-OH groups in the interlayer. The increase in the hydroxyl groups on the surface of the treated clay is favourable to the organosilane grafting process as well as to the subsequent adsorption process. During the grafting process, the organosilane molecules condense with hydroxyl groups in the interlayer region of the acid treated clay, giving rise to the organosilane grafted clay (Figure 4-3). The organosilane molecules in the clay interlayer result in the increase of the d(001) spacing, as shown in Figure 4-7. The organosilane molecules also react with the AlOH in the residual octahedral layer of clay, which will also aid the adsorption process. The acid treating and grafting process are schematically shown in Figure 4-3.

As acid treated clay has an abundance of polar hydroxyl groups, they exhibit a strong affinity for water molecules. This contrasts with the grafted samples which display a hydrophobic nature due to the abundant hydrophobic groups of the organosilane molecules, which are beneficial to the adsorption of hydrophobic pollutant molecules, such as alachlor and imazaquin. After the adsorption of alachlor or imazaquin, the basal spacing of OTES grafted samples remains unchanged, while that of the CPTES grafted increases, as seen in Figure 4-29a and b. This phenomenon might be related to the reasoning behind the d(001) spacing of sample AO2 being larger than that of sample AC3, which is related to the chain length of the grafted silane. A schematic representation of how the adorbate are packing during the adsorption processes is illustrated in Figure 4-30. It is proposed that the main interaction between the organosilane grafted clay and the pollutant molecule is the interaction between the organic chain of the silane and the organic back bone of the pollutant. Further to this the polar parts of the pollutant may interact directly with the unreacted hydroxyl groups on the surface. This means the pollutant will occupy the space between two adjacent silane groups on the surface. If the pollutant molecule is larger than the silane group or interacts in such a way that it protudes from the silane layer, it will interact with the species sitting on the opposite layer in the interlayer. This interaction of the protruding pollutant will lead to an increase in the d(001) spacing as portrayed in Figure 4-29. If the pollutant molecule is able to fit between the silane species on the surface no such swelling will be observed in the d(001). From Figure 4-29a and 4-29b, it can be observed

- 160 - Chapter Four that the 2θ value of sample AC3A or AC3I is still larger than that of sample AO2 although it is smaller than that of sample AC3, which supports the above argument from another angle. Since the size of alachlor or imazaquin is larger than that of the CPTES grafted on the interlayer of clay, the d(001) spacing of the this clay enlarges after the adsorption of alachlor or imazaquin.

- 161 - Chapter Four

(001) a AO2 AO2A

AO2I

Intensity,counts/sec.

4 5 6 7 8 9 2(degree)

(001) b AC3A AC3I

AC3

Intensity,counts/sec.

4 5 6 7 8 9 2(degree)

- 162 - Chapter Four

4.4.11 Mechanism of adsorption

These selective adsorption phenomena can be explained by the cooperative effect of the chemical nature of the pollutants and the geometry of the immobilized functional units in the interlayer space of beidellite clay. These two differently modified beidellites exhibited different adsorption behaviour. Furthermore, the increased basal spacing after silylation is effectively utilised for the adsorption process. The basal spacing of the long chain grafted clay didn‟t change upon the adsorption process. On the other hand, the adsorbed solute formed densely packed aggregates in the interlayer space with octylsilyl groups as seen in Figure 4-30(I). The adsorbed pollutants simply pack between the long chain groups without giving additional pressure for the layers to expand further. According to Giles and Smith‟s classification [98], the adsorption isotherm of alachlor for AO2 is type H in which the solute has a high affinity. This is shown by the complete absorption of the pollutant molecule at low concentrations. The amount of uptake increased with the

- 163 - Chapter Four increase in concentration until it reached above 10 ppm. After this concentration, no further adsorption occurred with the maximum amount adsorbed being 16 mg/g from 10 ppm solution. It is difficult to quantitatively determine the amount of uptake due to the washing and drying which can destroy pollutants from the intercrystal spaces.

The adsorption isotherm of imazaquin for AC3 is linear, following the typical type C adsorption [98].The linearity of type C absorption shows that the number of sites available for adsorption remains constant. Such a situation could arise when the solute penetrates into the hydrophobic region of the substrate. An increase in the basal spacing of AC3 was observed after this adsorption. The adsorbed pollutants are oriented by the hydrogen bonding interactions between polar heads, especially, between the silane groups. A change in the d-spacing can be explained by the arrangement of the imazaquin in the interlayer space as seen in Figure 4-30(II). The size of the sorbed pollutants is bigger than the short chain silane groups. This size differential facilitates the additional expansion of layers under the pressure induced by the adsorbed pollutants from both sides of the interlayer space.

In order to isolate the effect of the clay surface, we compare the above adsorption results with -Al2O3 fibres grafted with the same functional groups (Figure 4-31). The contact angle measurements show that the majority of the fibre surface was converted to a hydrophobic surface after the grafting process. As a result, the non- polar region of the surface can make close proximity with the organic pollutants due to the non-polar nature of the surface. The main advantage of using fibre as compared to clays is their easy separation from solutions after the adsorption process. The adsorption capacity of fibres was slightly less than that of the clay adsorbents for both pollutants. Sample AFC8(100) can absorb 1.6 mg/g of alachlor at an initial amount of 2 mg/g of adsorbent and sample AFC1(100) can absorb imazaquin about 1 mg/g at an initial amount of 1 mg/g. It was also recognized that the rate of adsorption of imazaquin by CPTES was very high. Sorption isotherms were shown in Figure 4-32.

- 164 - Chapter Four

Figure 4-31 – The schematic diagrams of -Al2O3 fibres before (a) and after grafting (b).

The linear part of the isotherms reflects the situation at low concentrations where the strongest adsorption sites are far from being saturated. The second part of the isotherms shows those situations in which, at higher pollutant concentrations, it becomes more difficult to adsorb additional molecules. This occurs in situations where the adsorption sites start to be filled. The grafted nanofibres exhibit a very high adsorption ability when compared to non-grafted -Al2O3 fibres. As seen in Figure 4-32, the adsorption isotherm of alchlor for AFC8 (100) is also type H and the linearity of type C adsorption isotherm of imazaquin for AFC1(100) and AFC1(50) shows that the availability of more sites depend on the adsorption proceeds.

- 165 - Chapter Four

6 (a) AF AF(A) AFC8(50) AFC8(100) AFC1(50) 4 AFC1(100)

Adsorbed amount, mg/g Adsorbed

0 0 2 4 6 8 10 Solution concentration (Alachlor), mg/L

(b) AF 4 AF(A) AFC1(50) AFC1(100) 3 AFC8(50) AFC8(100)

Adsorbedamount,mg/g

0 0 1 2 3 4 5 Solution concentration(Imazaquin) mg/L

Figure 4-32  Adsorption isotherms of -Al2O3 fibres for alachlor (a) and Imazaquin (b): Adsorbed amount versus equilibrium concentration.

- 166 - Chapter Four

The kinetics of the uptake of the two pesticides was measured to evaluate the time needed to reach adsorption equilibrium. The rate of adsorption was measured by determining the change in concentration of the pollutants in contact with the adsorbent as a function of time. The sorbed amount of pollutant was then plotted against the square root of time (Figure 4-33). Imazaquin was adsorbed readily by AFC1(100), reaching the adsorption equilibrium within 30 min whereas, a slow rate of adsorption was measured for alachlor by AFC8(100) with complete equilibrium being achieved only after 24 h. The adsorption mechanism is assumed to be proceeded in a multi-steps process. In the first step, the pollutant molecules transport from solution to the hydrophobic surfaces of alumina fibres. Secondly, the solute molecules diffuse into hydrophobic nanospaces and finally the adsorption process takes place. The small size of alachlor compared with imazaquin, allows its easy adsorption on to the OTES grafted substrate which is mainly due to the hydrophobic interactions of the methyl groups present in the grafted species as well as in the alachlor molecules. Furthermore, molecules with larger number of hydrophobic alkyl groups are preferentially adsorbed. It is believed that the enhanced adsorption rate of imazaquin was determined by the hydrophobicity of alkyl groups as well as due to the large number of  bonding electrons in the imazaquin molecules. Hydrogen bonding plays a prominent role in the mechanism of the last step adsorption process (Figure 4-34). It involves the interactions of aromatic  electron ring and the chlorine groups in a donor-acceptor mechanism take place through chlorine as an electron donor and an aromatic ring as the acceptor [1].

- 167 - Chapter Four

2.0

1.6

1.2

AFC1(100)-Imazaquin 0.8 AFC8(100)-Alachlor

0.4

Amount adsorbed(mg/g) 0.0 0 1 2 3 4 5 6 7 1/2 Square root of time hours) (

Figure 4-33  The amount of pollutants adsorbed plotted against the square root of time.

Figure 4-34  A schematic view of grafted surface and possible interaction with pollutants.

- 168 - Chapter Four

Intermolecular hydrogen bonds exist between the fibres providing cohesion of the fibrillar units. Indeed, these cohesive forces in the fibrillar interstices have a pronounced effect on the accessibility and the interactions of pollutants with the fibre. The aggregation of fibrils attributes the easy and high speed of the downstream separation.

- 169 - Chapter Four

4.5 Conclusions

The acid treatment process partially destroys the octahedral alumina layer in beidellite clays. Non-silicon atoms such as Al, Mg, and Fe can be partially leached from the octahedral alumino sheet. This leaching process increases the interlayer spacing, specific surface area, and pore volume. These enhanced properties make it possible for the acid treated beidellite to be effectively used as a sorbent. These acid treated samples have displayed excellent sorption ability for simazine herbicides.

Organosilane molecules such as OTES and CPTES grafted onto acid treated beidellite clay exhibit a superior ability for adsorbing the herbicides, alachlor and imazaquin. The efficiency of the pollutant removal from water by the grafted clays is significantly higher than that of non-grafted clays. The grafting process provides a relatively large number of highly hydrophobic sites, which yields an efficient and extremely large uptake capacity for neutral and anionic pollutants. The removal of alachlor by clay grafted with OTES was much higher than its removal by other clays. A higher adsorption capacity of imazaquin was observed for CPTES grafted clay in comparison with OTES. Clay modified with OTES can absorb alachlor 5 mg/g of adsorbent at initial lower amount of 5 mg/g and 16 mg/g at an initial higher anount of 20 mg/g and CPTES grafted clay can absorb imazaquin at about 1.8 mg/g of adsorbent at an initial lower amount of 2 mg/g and at about 8.2 mg/g at an initial higher amount of 10 mg/g. Organically modified products were analyzed by FTIR which disclose the existence of organic groups, and N2 adsorption results show a significant decrease in the surface area and an increase in the mean pore diameter after grafting. Swelling of the d(001) after the grafting process was also revealed by XRD measurements.

The adsorption characteristics of functionalised thin -Al2O3 fibres were examined and compared to the adsorption properties of the modified clays. The degree of linearity enabled the fibres to interweave together. Grafted -Al2O3 fibres were found to have a high cohesive energy that is enhanced by the fact that the grafted functional groups are capable of forming hydrogen bond networks between the fibres. Transport of the pollutants to the adsorption sites occurred within the

- 170 - Chapter Four interfibrillar interstices. The efficiency of alachlor and imazaquin removal from water using grafted -Al2O3 fibres are significantly higher than that of non grafted fibres. It was seen that these modified fibres exhibit different adsorption characteristics, i.e., some are strongly adsorbed whereas others are weakly adsorbed. This depends on the functionalities grafted on the surface. A higher selectivity of imazaquin was observed for CPTES in comparison with OTES and an opposite effect was associated with alachlor uptake. -Al2O3 fibres modified with OTES can absorb alachlor 1.6 mg/g of adsorbent at an initial lower amount of 2 mg/g and

CPTES grafted -Al2O3 fibres can absorb imazaquin about 1 mg/g of adsorbent at an initial amount of 1 mg/g. This adsorption capacity is slightly less than that of clays for both pollutants. After evaluating the kinetics of uptake, it was observed that CPTES grafted fibres could remove imazaquin much quicker than alachlor by the OTES modified substrate. The surface modification provided a relatively large number of highly hydrophobic sites and a superhydrophobicity was observed in the OTES system. Organically modified products were analyzed by FTIR which disclosed the existence of organic groups which was accompanied by a significant decrease in the surface area as detected by BET. XRD measurements indicated that grafting did not alter the crystal structure of the substrate.

- 171 - Chapter Four

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61. S. A. Boyd, M. M. Mortland, C. T. Chiou, Sorption Characteristics of Organic Compounds on hexadecyltrimethylammonium-Smectite, Soil Science Society of America Journal 52 (1988) 652-657.

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62. J. A. Smith, A. Galan, Sorption of Nonionic Organic Contaminants to Single and Dual Organic Cation Bentonites from Water, Environmental Science & Technology. 29 (1995) 685-692.

63. J. A. Smith, P. R. Jaffe, Benzene Transport through Landfill Liners Containing Organophilic Bentonite, Journal of Environmental Engineering- Asce. 120 (1994) 1559-1577.

64. J. A. Smith, P. R. Jaffe, Adsorptive Selectivity of Organic-Cation-Modified Bentonite for Nonionic Organic Contaminants, Water Air and Soil Pollution. 72 (1994) 205-211.

65. J. A. Smith, P. R. Jaffe, C. T. Chiou, Effect of ten quaternary ammonium cations on tetrachloromethane sorption to clay from water, Environmental Science & Technology. 24 (1990) 1167-1172.

66. P. K. Jal, S. Patel, B. Mishra, Chemical modification of silica surface by immobilization of functional groups for extractive concentration of metal ions, Talanta. 62 (2004) 1005-1028.

67. H. Bagheri, A. Mohammadi, Pyrrole-based conductive polymer as the solid- phase extraction medium for the preconcentration of environmental pollutants in water samples followed by gas chromatography with flame ionization and mass spectrometry detection, Journal of Chromatography A. 1015 (2003) 23-30.

68. L. Groisman, C. Rav-Acha, Z. Gerstl, U. Mingelgrin, Sorption of organic compounds of varying hydrophobicities from water and industrial wastewater by long- and short-chain organoclays, Applied Clay Science. 24 (2004) 159-166.

69. N. Masque, M. Galia, R. M. Marce, F. Borrull, New chemically modified polymeric resin for solid-phase extraction of pesticides and phenolic compounds from water, Journal of Chromatography A. 803 (1998) 147-155.

70. N. Masque, R. M. Marce, F. Borrull, Comparison of different sorbents for on-line solid-phase extraction of pesticides and phenolic compounds from natural water followed by liquid chromatography, Journal of Chromatography A. 793 (1998) 257-263.

71. J. S. Kim, S. Chah, J. Yi, Preparation of modified silica for heavy metal removal, Korean Journal of Chemical Engineering. 17 (2000) 118-121.

72. S. Chah, J. S. Kim, J. H. Yi, Separation of zinc ions from aqueous solutions using modified silica impregnated with CYANEX 272, Separation Science and Technology. 37 (2002) 701-716.

73. L. N. H. Arakaki, C. Airoldi, Ethylenimine in the synthetic routes of a new silylating agent: chelating ability of nitrogen and sulfur donor atoms after anchoring onto the surface of silica gel, Polyhedron. 19 (2000) 367-373.

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74. A. Gambero, L. T. Kubota, Y. Gushikem, C. Airoldi, J. M. Granjeiro, E. M. Taga, E. F. C. Alcantara, Use of chemically modified silica with beta- diketoamine groups for separation of alpha-lactoalbumin from bovine milk whey by affinity chromatography, Journal of Colloid and Interface Science. 185 (1997) 313-316.

75. A. G. S. Prado, C. Airoldi, Immobilization of the pesticide 2, 4- dichlorophenoxyacetic acid on a silica gel surface, Pest Management Science. 56 (2000) 419-424.

76. M. E. Mahmoud, E. M. Soliman, Silica-immobilized formylsalicylic acid as a selective phase for the extraction of iron(III), Talanta. 44 (1997) 15-22.

77. H. Y. Zhu, X. P. Gao, Y. Lan, D. Y. Song, Y. X. Xi, J. Zhao, Hydrogen Titanate Nanofibers Covered with Anatase Nanocrystals: A Delicate Structure Achieved by the Wet Chemistry Reaction of the Titanate Nanofibers, Journal of the American Chemical Society. 126 (2004) 8380- 8381.

78. S. C. Shen, Q. Chen, P. S. Chow, G. H. Tan, X. T. Zeng, Z. Wang, R. B. H. Tan, Steam-Assisted Solid Wet-Gel Synthesis of High-Quality Nanorods of Boehmite and Alumina, Journal of Physical Chemistry C. 111 (2006) 700- 707.

79. H. Y. Zhu, X. P. Gao, D. Y. Song, Y. Q. Bai, S. P. Ringer, Z. Gao, Y. X. Xi, W. Martens, J. D. Riches, R. L. Frost, Growth of Boehmite Nanofibers by Assembling Nanoparticles with Surfactant Micelles, Journal of Physical Chemistry B. 108 (2004) 4245-4247.

80. H. Y. Zhu, J. D. Riches, J. C. Barry, -Alumina Nanofibers Prepared from Aluminum Hydrate with Poly(ethylene oxide) Surfactant, Chemistry of Materials. 14 (2002) 2086-2093.

81. Y. Xia, P. Yang, Guest Editorial: Chemistry and Physics of Nanowires, Advanced Materials. 15 (2003) 351-352.

82. E. F. Vansant, P. Van Der Voort, K. C. Vranchen, Characterization and chemical modification of the silica surface, Elsevier, New York, 1995.

83. W. J. Hayes, E. R. Laws (eds), Handbook of pesticide Toxicology, Vol. 3, Classes of Pesticides, Academic Press Inc., New York, 1990.

84. F. Secundo, J. Miehe-Brendle, C. Chelaru, E. E. Ferrandi, E. Dumitriu, Adsorption and activities of lipases on synthetic beidellite clays with variable composition, Microporous and Mesoporous Materials. 109 (2008) 350-361.

85. N. Greesh, P. C. Hartmann, V. Cloete, R. D. Sanderson, Adsorption of 2- acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and related compounds onto montmorillonite clay, Journal of Colloid and Interface Science. 319 (2008) 2-11.

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86. S. J. Gregg, K. S. W. Sing, Adsorption, Surface Area and porosity, Academic Press, New York, 1982.

87. J. Madejova, P. Komadel, B. Cicel, Infrared Study of Octahedral Site Populations in Smectites, Clay Minerals. 29 (1994) 319-326.

88. J. Madejova, FTIR techniques in clay mineral studies, Vibrational Spectroscopy. 31 (2003) 1-10.

89. J. C. Dai, J. T. Huang, Surface modification of clays and clay-rubber composite, Applied Clay Science. 15 (1999) 51-65.

90. J. T. Kloprogge, Spectroscopic studies of synthetic and natural beidellites: A review, Applied Clay Science. 31 (2006) 165-179.

91. J. A. Gadsden, The Infrared Spectra of Minerals and Related Inorganic Compounds, Butterworth, London, 1975.

92. S. Atsushi, K. Kazuyuki, Direct Formation of Mesostructured Silica-Based Hybrids from Novel Siloxane Oligomers with Long Alkyl Chains13, Angewandte Chemie International Edition. 42 (2003) 4057-4060.

93. M. Cruz-Guzman, R. Celis, M. C. Hermosin, J. Cornejo, Adsorption of the Herbicide Simazine by Montmorillonite Modified with Natural Organic Cations, Environmental Science & Technology. 38 (2003) 180-186.

94. P. Komadel, Chemically modified smectites, Clay Minerals. 38 (2003) 127- 138.

95. G. Lagaly, Characterization of clays by organic compounds, Clay Minerals. 16 (1981) 1-21.

96. V. C. Farmer, M. M. Mortland, An Infrared Study of the Co-ordination of Pyridine and Water to Exchangeable Cations in Montmorillonite and Saponite, Journal of the Chemical Society A. (1966) 344 - 351.

97. A. Nennemann, S. Kulbach, G. Lagaly, Entrapping pesticides by coagulating smectites, Applied Clay Science. 18 (2001) 285-298.

98. C. H. Giles, T. H. MacEwan, S. N. Nakhwa, Studies in adsorption. Part XI. A system of classification of solution adsorption isotherms, and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids, Journal of Chemical Society. (1960) 3973 - 3993.

99. J. S. Mattson, H. B. Mark, Jr. Activated Carbon-Surface Chemistry and Adsorption from Solution, Marcel Dekker Inc., New York, 1971.

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CHAPTER 5

Photocatalytic Studies of Immobilised Titanate and Mixed-Phase Titania Based Nanofibres for Decomposing Pesticides and Phenolic compounds

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5.1 Introduction

Chapter 1 (section 1.1.5) focused in the recent advances in the synthesis, characterisation and the environmental applications of TiO2. In this chapter the discussions will continue by extending the discussion about the synthesis and photocatalytic application of titanate nanostructures to the decomposition of organic pollutants. The powder form of the most common commercially available photocatalyst, P25, shows significant photocatalytic activity, however due to its nanometer size, its use requires an additional and somewhat difficult operation to separate it out from the solution [1]. In addition to photocatalytic activity, four important factors are considered when developing new titania based structures for environmental applications: (1) the ease of separation from the solution for routine use, (2) the stability towards various chemical environments, (3) the flexibility for various photo reactors, and (4) the non-toxicity. Based on these factors, we designed and studied two different types of titania based photocatalysts for the decomposition of organic pollutants. One is based on the immobilisation of anatase crystals on laponite clay fragments and the other is hinged on mixed phase titanate nanofibres. Several techniques were proposed to alleviate the above mentioned difficulties. The use of binders is a method that can be used to solve problems with separation from the solution. Extensive investigations have been reported for the immobilisation of

TiO2 on a photochemically stable substrate [2-8]. Cordierite monolith, stainless steel plates and beta-SiC foamare have been used as a support for titania fine powder [9].

TiO2 can be laminated using various binders such as silca or alumina gels [9]. The degradation of relatively stable alachlor has been examined by using TiO2 film immobilised on a glass tube [10]. The fabrication of TiO2 with a porous structure has also attracted enormous interest due to it strong oxidation potential and non- toxicity [8, 11-14]. The most common substrates available for use are clays, zeolites and silica based materials due to their large surface area and chemical stability [3,

15-18]. For example, the preparation of TiO2 cross-linked montmorillonite has demonstrated the usefulness of metal oxide pillars on a clay substrate [15]. Using a related method, Yang et al. prepared anatase crystals immobilised on mesoporous laponite clay fragments [3].

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In this chapter, the focus will be more on nanofibres and nanotubes. It is particularly important to be aware of the acidic, mechanical and thermal stabilities of titanate nanostructures when one wants to derive different phases for environmental application. Most of the procedures for the modification of structure and morphology belong to the thermal evolution of Na-free titanate structures. Different thermal conditions were studied in order to evaluate their effect on the final product morphology. The nanofibres are relatively more stable and maintain a fibrous o texture up to 1000 C. Recent thermal studies of sodium-free titanate (H2Ti3O7) nanofibres have suggested that by heating the nanofibres were transformed into o TiO2-(B) while maintaining fibrous morphology at 300-500 C, then further transformed into an anatase structure at 700 oC while retaining the fibrous shape and then finally transformed into rode-shaped rutile at 1000 oC (Figure 5-1) [19, 20].

TiO2-(B) is a metastable polymorph formed by the dehydration of layered hydrogen titanate and is also known as monoclinic TiO2 [20]. The layered crystal structures of hydrogen tri-titanate (H2Ti3O7), sodium tri-titanate (Na2Ti3O7) and TiO2-(B) phases are shown in Figure 5-2.

Figure 5-1 – Thermal transformation of H-titanate nanofibres.

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A conversion of titanate nanofibres into anatase nanofibres is possible under hydrothermal treatment in water at 150 oC [21, 22]. In the presence of hydrogen o flow, Na2Ti3O7 can be converted into Na2Ti6O13 nanofibres at 500 C [23]. Exchangeable sodium present in the titanate nanotubes can influence their thermal stability [24-27].

Figure 5-2 – General view of the crystal structure and phase transformations of

TiO2 [21, 23]: (a) Na2Ti3O7 ; (b) H2Ti3O7 ; (c) TiO2-B.

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Several studies report the importance of the pH in solution with regard to the conversion of H-titanate and the final morphology of the products. Hydrothermal treatment under acid conditions transforms H-titante nanofibres into anatase aggregates at 120 oC in the presence of 0.05 M nitric acid [28]. Titanate nanofibres convert to anatase nanofibres when placed under acidic hydrothermal conditions at a temperature of 175 oC [29]. At pH 2-7, the pure anatase phase was formed whereas a mixture of rutile and the brookite phases were obtained with the hydrothermal o post-treatment of H-titanate fibres at 180 C for 24 h in HNO3 solution at pH 0.

Moreover, with the samples obtained by the hydrothermal reaction in the HNO3 solution at pH 2, the H-titanate nanofibres appeared to have a smooth surface whereas at a pH in the range of 4-7, the samples seemed to have more of a fibrous morphology and the surfaces of the products were rough. At pH 7 a porous fibre structure with aggregated nanocrystals was obtained [30].

Studies on the influence of morphology on the charge separation and recombination processes of photo generated charge carriers have revealed that the lifetime of trapped electrons in nanotubes is prolonged when compared to that of TiO2 nanoparticles [31]. This would suggest that these structures would be effective photocatalysts however other factors such as sodium content also affect their photoactivity. It was observed that sodium contents reduce the photocatalytic activity of titanate nanotubes during the oxidation of dyes [27, 32]. In contrast, Riss et al. have found that protonation of sodium titanate nanotubes suppresses the photocatalytic activity [33]. However, this negative effect could be recovered by an exchange with alkali ions again. Photocatalytic oxidation of NH3 as well as dyes using as-synthesised titanate nanotubes is slower than that of a standard P25 [34, 35]. H-titanate nanotubes can be transformed into anatase nanoparticles or nanorods at 400 oC and the resulting products showed a better photocatalytic activity in the oxidation of various organic pollutants [36-38]. However any further increase in calcination temperature leads to a reduction in the OH groups and a decrease in the photocatalytic activity. The hydrothermal treatment of H-titanate nanotubes with 0.1 o M HNO3 at 180 C forms anatase fibres with better photocatalytic activity for dye degradation [30].

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Further improvement of the photocatalytic activity can be achieved by using a binary system, which is composed of two crystal forms of TiO2 with a small difference in band gaps [39]. Improved photocatalytic activity, over that of P25 for the hydrogen production from ethanol, was observed for the mixed-phase nanocomposites which consisted of 33% TiO2-(B) nanotubes and 67% anatase nanoparticles which were prepared from H-titanate nanotubes [40]. Anatase particles could be coated on the surface of the titanate nanotubes [41] and the nanofibre [42] by the hydrolysis of TiF4 in the presence of H3BO4.

This Chapter begins by looking at the photocatalytic efficiencies of anatase crystals immobilised on leached laponite clay fragments for the decomposition of herbicides. The investigation will be extended to the photocatalytic activities of H- titanate and TiO2-(B) nanofibres, in combination with the deposition of anatase nanoparticles onto the surface of the fibres. The electrical contact between the anatase nanoparticles deposited on the nanofibres and the fibres themselves can provide an efficient charge-transfer region, which makes this binary system suitable for photocatalysis.

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5.2 Materials and Methods

Laponite, synthetic layered clay, was obtained from Fernz Specialty Chemicals, Australia. It is a 2:1 layered hydrous magnesium lithium silicate consisting of two tetrahedral silica sheets sandwiching a central octahedral magnesia sheet, with the formula reported as Na0.35[Mg2.75Li0.15]Si4O10(OH)2nH2O [43]. Laponite has a BET specific surface area of 367 m2/g and a cation exchange capacity of 55 meq per 100 g of clay. TiOSO4.xH2O (98%) and hydrochloric acid (36%) were obtained from Fluka and were used without further purification. Tetraisopropoxy titanium (IV)

(Ti[O-CH(CH3)2]4, TPT) was purchased from Aldrich. The water used in all the experiments was purified with a milli-Q-plus system. Herbicides used in the experiments were HPLC grade and purchased from Aldrich. The TiO2 source used for the preparation of titanate nanofibres was commercial grade TiO2 powder (P25, Degussa AG, Germany).

X-ray Diffraction (XRD) patterns, FTIR spectra, Scanning Electron Microscopy images, Transmissions Electron Microscopy (TEM) images, Raman spectra and Nitrogen adsorption-desorption technique were used to characterize the samples. All characterization techniques were conducted in accordance with the procedure outlined in Chapter 2.

Phenol, dichlorophenol and trichlorophenol in the solutions were analyzed using a high-performance liquid chromatography (HP 1100 HPLC).The HPLC analysis was conducted in a manner outlined in Chapter 2 (Section 2.2.13)

Alachlor, imazaquin, bromacil, chlorotoluran sulfosulfuron and simazine solutions were analysed with a UV-Vis spectrometer. The absorbance of alachlor was monitored at 196 nm and imazaquin at 242 nm respectively. Bromacil, chlorotoluran and sulfosulturon were monitored at 210, 211 and 214 nm respectively. The analyses were carried out in a manner discussed in Chapter 2 (Section 2.2.14).

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5.3 Discussion of Experimental Procedures

5.3.1 Preparation of immobilised TiO2 on laponite

The synthesis procedure of modified laponite is based on previously reported work [3]. An aqueous suspension was prepared by dispersing 4.0 g of laponite into 200 mL water, and stirred until the suspension become homogeneous. An initial aqueous solution of TiOSO4 was prepared by dissolving 128 g of TiOSO4.xH2O into 1 L of deionised water. The required amount of the TiOSO4 solution was introduced into clay suspension and the reaction mixture was agitated for at least 3 h. The mixture was transferred to an autoclave and hydrothermally treated for 24 h under autogeneous water pressure at 100, 150, 200 oC, respectively. The catalysts with different ratios of titanium to clay were prepared using different quantities (25, 50 and 75 mL) of the stock TiOSO4 solutions. The products were then separated by filtration, washed thoroughly with distilled water and dried at 80 oC for 24h in air. The samples were then ground to a fine power and calcined at 500 oC for 20 h at a rate of 2 oC min1. For the purpose of comparison five different photocatalysts were prepared with different hydrothermal temperatures and ratios. In the sample names, the numbers in the brackets indicate the Ti/clay ratio with the numbers at the end indicating the hydrothermal temperature.

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5.3.2 Synthesis of H-tianate and TiO2-(B) based photocatalysts

In a typical synthesis, 6 g of TiO2 powder was mixed with 80 mL of 10 M NaOH solution [21, 28, 44]. The suspensions were agitated in an ultrasonic bath for 30 min which was followed by a hydrothermal treatment of the mixture in a Teflon-lined stainless steel autoclave at 180 oC for 48 h. After the hydrothermal treatment the precipitate (sodium titanate nanofibres) was separated by filtration and washed with distilled water (to remove excess NaOH). The washed precipitate was then + exchanged with H by washing with a 0.1 M HCl solution to produce H2Ti3O7 nanofibres, and washed again with distilled water until the pH value of the rinsing solution reached ~ 7. The washed samples were dried at 80 oC for 12 h. The mixed phase photocatalysts were prepared by successive modification of H2Ti3O7 nanofibres. The coating of anatase nanoparticles on the surface of H2Ti3O7 nanofibres were carried out in a hydrothermal process [45]. In a typical procedure,

0.4 g of H2Ti3O7 nanofibres and 40 mL of 0.05 M HNO3 acid solution were put into a Teflon-lined stainless autoclave. The autoclave was heated at 110 oC for 15, 30, 40, 45 h and 5 days, respectively. The amount of coating was substantially affected by the duration of the hydrothermal treatment. The TiO2-(B) photocatalysts with anatase particles on the surface were prepared by using the following procedure. o The H2Ti3O7 nanofibres dotted with anatase nanocrystals were calcined at 450 C to convert the H2Ti3O7 into the TiO2-(B). It is noteworthy that the anatase crystals on the surface remain unchanged during the heating process. After calcination TiO2-(B) fibres with varying amounts of anatase crystals on the surface were obtained. HA1, HA2, HA3, HA4 and HA5 represent titanate fibres with anatase shells based on the increasing order of the hydrothermal treatment time. For comparison, pure H2Ti3O7 fibres labelled as „H‟ and pure TiO2-(B) fibres as „T‟ were synthesized by heating the H2Ti3O7 nanofibres at 723 K. For instance, HA1 is the sample obtained after 15 h of hydrothermal treatments. CA1, CA2, CA3, CA4 and CA5 represent the TiO2- (B) core with the anatase shell.

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5.3.3 Photocatalytic experiments

The illumination of the photocatalyst was carried out by using a UV light source which consisted of six tubular 20 W Hg lamps (NEC, FL20BL T8), which emit a peak wavelength of c.a. 365 nm. A catalyst loading of 50 mg per 50 mL of solution was used. The initial concentration (C0) of pollutant (phenol or dichlorophenol or Trichlorophenol) in this solution was 25 ppm. Due to the different solubility of pollutants, when bromacil and alachlor were used as the target pollutants, the initial concentration was 10 ppm, whereas when chlorotoluron, sulfosulfuron, imazaquin and simazine were pollutants, the initial concentration was 5 ppm. The catalyst powder and herbicide solution were mixed in the dark in an open Pyrex evaporation dish for 30 min. This mixing ensured that the surface concentration of the catalyst reached equilibrium with the pollutant prior to irradiation. The working principle of the set up was based on description in Chapter 2 (Section 2.1.3). The reaction mixture was sampled at 15 min time intervals and filtered through 0.45 μm Millipore syringe filter. The filtrates of phenol simazine, alachlor, imazaquin, bromacil, sulfosulfuron and chlorotoluron were analysed by using an UV spectrophotometer. Dichlorophenol and trichlorophenol were determined by HPLC. The analyses were carried out based on the description in Chapter 2 (Section 2.2.13 and 2.2.14)

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5.4 Results and discussion

5.4.1 XRD patterns

Samples of TiO2 immobilised on laponite clay fragments were analysed by XRD and are shown in Figure 5-3. The XRD pattern of anatase is clearly distinct from that of clay. The results of XRD give an indication of the extent of TiO2 crystallisation onto the clay lattice. Two different phases of TiO2 were observed, in addition to the anatase, a small amount of an additional phase was also observed at 12.8o which are indicated by stars and squares respectively. The average crystallite size calculated using the Scherrer equation is shown in Table 5-1. Sample (L-Ti

(5)100) prepared with the smallest amount of TiO2 and the lowest hydrothermal temperature, still shows the diffraction peaks of laponite at 19.5, 34.8, 60.8o. In other samples new reflection peaks appeared due to the high dose of anatase crystallisation after the high degree of hydrothermal treatment temperature. These reactions include layer dissolution and crystallisation; as a result the fragments of host materials achieve different fabrication through TiO–Si bonds [46]. This crystallisation of TiO2 coincided with an observed loss of opalescence in the corresponding clay structures which presumably results from layer degradation and framework collapse. The crystalline anatase peaks appeared in the samples L- Ti(5)100 and were sharpened in the L-Ti(15)200 sample.

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L-Ti(15)200 anatase phase L-Ti(15)150 * L-Ti(15)100 L-Ti(10)100 * L-Ti(5)100 L . * * * *

Intensity, counts/sec. (001) (110,020) (130)

10 20 30 40 50 60 70 2,deg

Figure 5-3  XRD patterns of immobilised TiO2 on laponite clay fragments.

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The fibre photocatalysts of the mixed phases were also examined by powder XRD measurements (Figure 5-4). A corresponding XRD pattern (Fig. 5-4a) exhibited obvious diffraction peaks of anatase in addition to the sharp peaks of titanate, suggesting that nanocrystals coated on the surface of the titanate nanofibres were anatase. The pure hydrogen titanate nanostructures have peaks at 2θ values of 9.6o, 24.7o, 28.02o, 48.22o and 62o, which can be attributed to the 020, 110, 130, 200, and 002 peaks respectively [47, 48]. It is clear that samples HA1, HA2, HA3, and HA4 contained mixed phases of hydrogen titanate and anatase which are indicated in Figure 5-4a. The peak intensity corresponding to the anatase increases from HA1 to HA5 whereas in HA5 the hydrogen titanate disappeared completely and was converted to anatase after 5 days of hydrothermal treatment. As can be seen in the Figure 5-4b, upon calcination at 450 oC the titanate characteristic peaks have disappeared and new peaks of TiO2-(B) appeared. After calcination, peaks intensities of anatase slightly increased due to improved crystallinity and the hydrogen titanate phase was converted into metastable TiO2-(B). The diffraction peaks of samples CA1, CA2, CA3, and CA4 in Figure 5-4b could be indexed to the anatase and TiO2-(B) phases. The average crystallite size of all the samples calculated using the Scherrer equation is presented in Table 5-2. With an increasing hydrothermal treatment time from 15 h to 4 days, the crystallite size of the anatase increased from 10.16 to 16.15 nm. Calcination to 450 °C also increased the crystallites size with maximum crystallite sizes reaching 17.9 nm.

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a  TiO (H) 2 * anatase phase H      HA1 HA2 * * * HA3 * * *

Intensity, counts/sec. HA4 HA5

10 20 30 40 50 60 70 2,deg b * anatase phase TiO (B) # 2 T CA1 # *# * * CA2 # # * # * * CA3

CA4

Intensity, counts/sec. CA5

10 20 30 40 50 60 70 2 deg

Figure 5-4  The XRD patterns of the mixed-phase TiO2 photocatalysts: (a) H- titanate fibres with anatase nanoparticles and (b) TiO2-(B) nanofibres with anatase nanoparticles.

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5.4.2 BET Surface Area

The specific surface area of all the laponite catalysts are in the range of 450-230 m2/g and were found to decrease upon increasing the anatase content and the hydrothermal temperature (Figure 5-5). It can be further assumed that that anatase has a lower surface area than SiO2. The shape of the N2 adsorptiondesorption curves for the samples are quite different depending on the amount of anatase and the hydrothermal temperature, which reflects the different mode of fabrication of the structures in each of the samples. Hysteresis effects were observed in all of the samples due the strong interactions of the absorbed gas with the sample surface and results in a delay in desorption [49]. The samples prepared at 150 and 200 oC show a steep increase in the adsorption from a P/P0 of about 0.5 which indicates the existence of a very large pore volume of mesopores in the samples [50]. The large pore volume is mainly from the intercrystallite voids of anatase nanocrystals and collapsed clay residues.

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/g 400 L-Ti(15)200 3 L-Ti(15)150 350 L-Ti(5)100 300 L-Ti(10)100 L-Ti(15)100 250 200 150 100 50

adsorption volume, cm

N 0 0.0 0.2 0.4 0.6 0.8 1.0 p/p 0

Figure 5-5  N2 adsorption and desorption isotherms of photocatalytically modified laponite clay.

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Table 5-1 –Specific surface area, pore volume and mean cystal size of anatase of laponite based photocatalysts.

DA (nm)

2 -1 a 3 -1 Samples SBET(m .g ) Vp (cm .g )

L-Ti(5)100 420 0.38 4.3

L-Ti(10)100 398 0.31 5.9

L-Ti(15)100 340 0.29 7.6

L-Ti(15)150 315 0.43 7.7

L-Ti(15)200 232 0.55 8.5

P25 49.9 0.092 8.3 a Single point adsorption total pore volume of pores at P/P0 0.99, Mean anatase crystal size (DA) was calculated by using the Scherrer equation.

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The N2 sorption isotherms for the mixed phase fibre catalysts are presented in Figure 5-6. It was noted that all samples have very low surface areas. It can be seen that all of the isotherms are typically of a type H3 hysteresis loop according to the BDDT classification [51]. However an almost negligible amount of the hysteresis effect was observed in all of the tested samples, indicating only a small amount of or the complete absence of mesopores (2-50 nm). A large uptake is observed close to the saturation pressure, where the capillary condensation of the aggregates of titanate nanofibres starts. Moreover, in all of the tested samples the hysteresis loops approach P/P0 = 1, suggesting the presence of macropores (>50 nm) [52]. On the other hand, the areas of the hysteresis loops gradually increase upon increasing the hydrothermal treatment time. A slight increase in the area of the hysteresis loops was also observed after calcination. A remarkably small adsorption at low P/P0 was noted for all the samples, which indicates a very low microporosity. A steep increase in P/P0 position for the samples with long hours of hydrothermal treatment was observed, which indicated changes in the texture of the crystals after the prolonged treatment [53].

The isotherms shown in Figure 5-6a suggests that the pore volume increases as the content of anatase nanoparticles increase from H1 to H5 depending on the hydrothermal treatment time. As seen in Table 5-1, there was also a significant 2 increase for SBET from 28.4 to 64 m /g and an increase in pore volume from 0.08 to 0.33 cm3/g, along with the formation of the anatase nanoparticles on the surface of H-titanate nanofibres,. Sample HA5 shows a slight decrease in surface area. This can be attributed to the complete phase transition from H-titanate to anatase, which results in changes in the fibrillar morphology. It is worth noting that the average pore diameter increases after calcination at 450 oC. This might be caused by the decrease in the wall thickness of the anatase crystals.

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250 H a HA1 200

/g STP) /g HA2 3 HA3 150 HA4 HA5 100

Volumeadsorbed (cm

0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure,P/P 0 250 CA1 b CA2 200 CA3

/g STP) /g 3 CA4 150 CA5

100

Volumeadsorbed (cm

0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure,P/P 0

Figure 5-6 – The N2 adsorption isotherms of the mixed-phase TiO2 photocatalysts: (a) H-titanate core with anatase shell and (b) After calcination,

H-titanate converted to TiO2-(B) but anatase remains same.

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Table 5-2 – Effects of hydrothermal treatment and calcination on the BET specific surface area (SBET) and crystal size of mixed phase fibres.

Mean D (nm)

a DA SBET Vp (nm) Samples BETb BJHc (m2.g-1) (cm3.g-1)

H 28.4 0.08 11.8 12.4 0

HA1 39.4 0.12 12.2 12.0 10.16

HA2 66.5 0.22 13.1 12.0 11.73

HA3 57.3 0.24 17 13.9 12.35

HA4 64 0.26 16.4 13.9 13.46

HA5 62 0.33 22 18.1 16.15

CA1 42.1 0.15 14.6 13.2 11.1

CA2 58 0.26 18 15 12.9

CA3 59 0.31 21.4 18 14.98

CA4 57 0.32 22.6 19 16.99

CA5 49 0.33 27 22 17.9 a b Single point adsorption total pore volume of pores at P/P0 0.99. Adsorption average pore diameter C (4V/A by BET). Barrett─Joyner─Halenda (BJH) desorption average pore diameter (4V/A), DA denotes the average crystallite size of anatase particles calculated by using the Scherrer equation.

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5.4.3 FTIR Spectra

Figure 5-7 provides additional evidence for the degree of heterogeneous bonding within the laponite matrix and there is also evidence that the chemical stability is maintained in the fabrication. Absorption bands of the silica and titania species in the samples are particularly useful for characterising the bonding in the materials. The pure clay sample exhibit a strong adsorption peak at 969 cm1 corresponding to 4– the symmetric vibration of the (SiO4) species. The adsorption band at around 1087 cm1 displays a shift to a higher frequency as the Si:Ti ratio increases. This behaviour can be attributed to an increase in the heterogeneous bonding of Ti─O─Si in the framework, which results in an average decrease in bond strength of Si─O bonds [54].

L L-Ti(5)100 L-Ti(10)100 L-Ti(15)100 1087 L-Ti(15)150 L-Ti(15)200 969 2359 1635

Absorbance

2500 2000 1500 1000 Wavenumber, cm-1

Figure 5-7  IR-spectra immobilised TiO2 on laponite clay fragments as photocatalysts (Dashed line is eye guide for peak shifts).

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5.4.4 TEM and SEM Images

Figure 5-8 presents the TEM micrographs of the immobilised TiO2 on laponite clay in different ratios. An interesting feature in the micrographs is the presence of small openings formed by the anatase crystals and the delamination of the clay layers. Thermal shock and shrinkage in the framework during hydrothermal treatment and subsequent calcination result in the irregular ordering of the porosity. Furthermore, most of the anatase particles are poorly crystallised on to the degraded clay network.

The morphology and microstructural details of anatase particles coated on the surface of H-titanate and TiO2-(B) fibres are shown in Figures 5-9 and 5-10 respectively. SEM images (Figure 5-9) show the different degree of morphological changes experienced after the hydrothermal treatment for the coating of the anatase particles on the surface of H-titanate nanofibres. We can observe in Figure 5-9a that the H-titanate fibres are smooth with a length larger than several micrometers. As seen in Figure 5-9d and 5-10d, prolonged hydrothermal treatment slightly destroyed the fibrous morphology and changed more into particle. These particle morphologies have a strong tendency to agglomerate and are difficult to be separated out completely from a slurry system after the photocatalytic reaction.

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Figure 5-8  TEM images of immobilised TiO2 on laponite clay fragments: images ae are for L-Ti (5)100, L-Ti(10)100, L-Ti(15)100, L-Ti(15)150 and L- Ti(15)200 samples respectively.

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5.4.5 Raman spectroscopy

Figure 5-11 illustrates the Raman spectra of TiO2 nanocrystals immobilised on the laponite clay fragments. The intensity of the main bands at 145, 399, 518 and 640 cm1 increases as the hydrothermal temperature and Ti/clay ratio increase. This result supports the assumption that anatase TiO2 nanocrystals were attached to the degraded clay residues. This can be supported with the result obtained from the XRD.

Phase transformation of mixed phase fibre photocatalysts can be confirmed by the analysis of Raman spectra. Figure 5-12 displays the spectra of hydrogen titanate consisting of very broad bands near 195, 280, 450 and 680 cm1 respectively. The 1 bands observed at 145, 197, 399, 518 and 640 cm can be assigned as the Eg, B1g,

A1g or B1g and Eg modes of anatase phase [55]. The lowest frequency Eg, mode at 145 cm1 indicates the existence of the long-range order of the anatase phase, whereas the weak broader peaks in the high-frequency region indicate the lack of short range order of the anatase phase [56]. Higher-frequency well resolved Raman bands of HA5 and CA5 indicated that samples were highly pure with few defects and were likely to be pure anatase in phase. The lowest-frequency Eg mode is known to be closely related to the particle size of the anatase phase [57]. As the hydrothermal treatment time increases, the intensity of the lowest frequency Eg mode increases. These observations demonstrate the hydrothermal-dependent evolution of the anatase phase. The spectrum observed for sample CA1 (Figure 5-

12b) shows very similar peak positions and profiles to that of bulk TiO2-(B) [58]. It is evident that, the abundance of sharp peaks gradually decreases in intensity as the amount of TiO2-(B) phase decreases. These become completely absent in the spectrum of CA5.

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145 640 518 399 L-Ti(15)200

L-Ti(15)150

L-Ti(15)100

L-Ti(10)100

L-Ti(5)100

Raman Intensity

800 700 600 500 400 300 200 100 -1 Wavenumber/cm

Figure 5-11  Raman spectra of immobilised anatase crystals on laponite clay fragments.

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a 145

640 518 399

HA5 HA4 HA3 HA2

Raman Intensity HA1 H 680 450 280 195 800 700 600 500 400 300 200 100 Wavenumber/cm-1 b 145

CA1 640 399 518 247 200 CA2

CA3

CA4

Raman Intensity CA5

800 700 600 500 400 300 200 100 Wavenumber/cm-1

Figure 5-12  Raman spectra of mixed phase fibre photocatalysts: (a) as prepared hydrogen titanate nanofibres and with anatase phase; (b) mixed phase of anatase and TiO2-(B).

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5.4.6 UV-visible Spectroscopy

UV-visible spectra of mixed phase fibre photocatalysts are shown in Figure 5-13. All the tested samples exhibited an absorption band in the UV region. The adsorption edge of all calcined samples (Figure 5-13b) shifted to the blue regime.

The band gap (energy gap, Eg) can be calculated for practical purposes by using the following equation (1) [59]: The absorbance wavelength () can be obtained by extrapolating the linear part of the corresponding curves to the abscissa axis.

Eg = 12400/ (1)

where  is the absorbance wavelength.

Anatase nanoparticles dispersed on a large-bandgap matrix are expected to be different to those of the bulk materials. The slight blue shift in the absorption edge after calcination is probably due to an increase in crystallinity, a decrease in the concentration of the defects in the structure and the removal of the volatile impurities [60].

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a H HA5 HA4 HA3 HA2 HA1

Absorbance(a.u)

200 250 300 350 400 450 500 Wavelength (nm)

b CA5 CA4 CA3 CA2 CA1

Abosorbance(a.u.)

200 250 300 350 400 450 500 Wavelength (nm)

Figure 5-13  UV-visible adsorption spectra of mixed phase fibre photocatalysts: (a) H-titanate core with anatase shell; (b) TiO2-(B) core with anatase shell.

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5.4.7 Photocatalytic activity

The first section of the study investigated the photocatalytic degradation of a range of herbicides such as bromacil, chlorotoluran, sulfosulturon, alachlor and imazaquin using TiO2 immobilised on laponite clay fragments. The molecular structures of herbicides are shown in Figure 5-14. P25 powder was also used to compare the photocatalytic efficiencies of the above mentioned catalysts. In the last section of this study, the photocatalytic activities of two different types of catalysts such as anatase particles coated on the surface of titanate nanofibres as well as on TiO2-(B) nanofibres will be investigated in order to provide an insight into the role of the immobilised anatase crystals on the laponite clay fragments as a photocatalyst.

Figures 5-15, 5-16 and 5-17 show the degradation rate using immobilised TiO2 as well as P25. The degradation rates increased with the TiO2 contents in the catalyst. It was found that L-Ti(15)200 and L-Ti(15)150 exhibited significant photocatalytic activity for the degradation of herbicides. Experimental observations indicated that the 80% of bromacil, chlorotoluran and sulfosulturon have been removed after UV irradiation for 1 h. Alachlor and imazaquin were removed 60% and 100% respectively after 1 h of UV irradiation. The most significant variables for the decomposition of herbicides are the crystallinity of the TiO2 crystals and the unique pore dimensions. If we compare the photocatalytic activity of P25 with respect to the Ti/clay ratio of the new fabricated structures, it is interesting to note that the photocatalytic activity was increased after modification. It is also important to understand that the significant differences in the overall performance of the catalysts are likely to be attributed to the physicochemical properties of TiO2. Upon irradiation of the aqueous solution of the herbicides in the presence of anatase, the crystals generate electron hole pairs. As seen in Figure 5-25 the free electrons move to the aqueous phase and positively charged holes are accumulated on the surface of

TiO2, which then lead to the formation of strong oxidants such as hydroxyl radicals.

Earlier studies show that the degradation rate increases with the increase in the TiO2 content [61]. The crystal size is also an important parameter to ensure the superior activity of a catalyst. The samples with highly crystallised anatase nanocrystals and high porosity not only allow large organic molecules but also light to access the reaction sites on the surface of the anatase crystals, which results in high degradation rates. For instance, sample L-Ti(15)200 which has the largest pore

- 210 - Chapter Five volume and crystal size, exhibits the highest photocatalytic activity although samples L-Ti(15)150 and L-Ti(15)100 contain the same dose of titania. The specific surface area of the sample is the sum of both the anatase crystals and the clay residues however the photocatalytic activity depends only on the surface area of the anatase crystals. For example, samples L-Ti(15)150 and L-Ti(15)200 (Table 5-1) have a lower surface area but a high degradation rate than L-Ti(15)100, which has the largest surface area. This observation can be rationalised in terms of crystallinity and the accessible surface area of the anatase nanocrystals, which contributes significantly to the degradation rate.

Figure 5-14  Molecular structure of herbicides.

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It should be noted that all the catalysts prepared from laponite clay exhibited a higher SBET and pore volume than those of P25. Further observation indicated that the average crystal size of the samples, L-Ti(15)200 and L-Ti(15)150 was almost the same as the crystal size of P25. According to the above, there are two factors resulting in the increased photocatalytic activity of L-Ti(15)200 and L-Ti(15)150. The first is the higher pore volume formed by the aggregation of clay residues and second is the TiO2 contents. Furthermore, the TiO2 contents attached to the clay fragments have an optimal crystal size to enhance the photocatalytic reactions.

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a 100

40 L-Ti(5)100 L-Ti(10)100 L-Ti(15)100 20 L-Ti(15)150

Concentraion of alachlor, L-Ti(15)200 P25 0 0 15 30 45 60 Time, min

b 100 80

% 40 L-Ti(5)100 L-Ti(10)100 20 L-Ti(15)100 L-Ti(15)150 L-Ti(15)200 Concentraion of bromacil, 0 P25 0 15 30 45 60 Time, min

Figure 5-15  Photocatalytic degradation of alachlor and bromacil using immobilised TiO2 on laponite clay fragments and P25.

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a 100

% 40 L-Ti(5)100 L-Ti(10)100 L-Ti(15)100 20 L-Ti(15)150 L-Ti(15)200 P25

Concentraion of chlorotoluron, 0 0 15 30 45 60 Time, min

b 100

L-Ti(5)100 60 L-Ti(10)100 L-Ti(15)100 40% L-Ti(15)150 L-Ti(15)200 P25 20

Concentraion of imazaquin, 0 0 15 30 45 60 Time, min

Figure 5-16  Photocatalytic degradation of chlorotoluran and imazaquin using immobilised TiO2 on laponite clay fragments and P25.

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100

% 40 L-Ti(5)100 L-Ti(10)100 20 L-Ti(15)100 L-Ti(15)150 L-Ti(15)200 0 P25

Concentraion of sulfosulfuron, 0 15 30 45 60 Time, min

Figure 5-17  Photocatalytic degradation of sulfosulturon using immobilised

TiO2 on laponite clay fragments and P25.

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In the next section of the study we compare the above discussed photocatalytic properties of immobilised TiO2 on laponite clay photocatalysts with uniformly dotted anatase nanoparticles on two different substrates, namely titanate nanofibres and TiO2-(B) fibres. In this case, to aid in the speed of the study, we chose alachlor and imazaquin as model pollutants to compare to the results obtained with those of the previous clay based catalysts. To ensure we do not limit the study we also tested the degradation of simazine and phenols due to the fact that most herbicides contain phenolic moieties.

Figures from 5-19 to 5-24 show the degradation profile of phenol, dichlorophenol, trichlorophenol, alachlor, imazaquin and simazine respectively. The molecular structures of simazine and phenols are presented in Figure 5-18. The photocatalytic activities of the H-titanate and TiO2-(B) fibres before and after the coating of the anatase nanoparticles were evaluated. It was found that without the coating of the anatase particles both titanate as well as TiO2-(B) fibres showed less photocatalytic activity. After the anatase nanoparticles were uniformly coated on the surface of the nanofibres, the samples showed a high photocatalytic activity, which was dependent on the amount deposited on the surface. Experimental observation indicated that 100% degradation of alachlor could be achieved within 30 min by CA3, CA4 and CA5, whereas, with anatase decorated H-titanate fibres the degradation rate was reduced. It was also found that the as synthesized TiO2-(B) fibres showed greater, although still small, photocatalytic activity compare to the H-titanate fibres. When immobilised TiO2 on laponite clay was used as a photocatalyst, the rate of degradation was much less than with the anatase dotted fibre catalysts. Similarly, degradation of imazaquin by anatase coated TiO2-(B) fibres showed superior performance over laponite clay catalysts. A complete removal of phenol was observed to occur in the presence of HA5, CA3 and CA4 after 120 min of UV illumination. Phenol is considered as one of the most difficult pollutants to degrade, which is attested by the degradation rates of trichlorophenol and dichlorophenol being much higher than the degradation rates of phenol. The data indicates that simazine is the most difficult pollutant to degrade (Figure 5-24). Only a small decrease in the concentration of phenol was observed with catalysts H and T. An almost identical pattern of degradation was observed with all other pollutants except

- 216 - Chapter Five for the overall degradation rate. Therefore it is reasonable to surmise that anatase decorated TiO2-(B) fibres are one of the most active novel photocatalysts for the application of pollutant degradation in water. As stated previously, the crystal size is an important parameter for achieving superior activity in a photocatalyst. The crystal size calculated for anatase particles for HA5 and CA4 are 16.1 and 16.9 nm (Table 5-2), respectively. The anatase nanocrystals obtained have achieved a fine tuning of crystal size and porosity which allows not only organic molecules but also light to access the reaction sites on the surface.

Figure 5-18  Molecular structures of organic pollutants.

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a 100 1

60 H HA1

inwater,% 40 HA2 HA3 20 HA4

Concentrationphenol of HA5 0 0 30 60 90 120 Time,min a 100 2

60 T 40 CA1

in water, % in water, CA2 CA3 20

Concentrationphenol of CA4 CA5 0 0 30 60 90 120 Time,min

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b 100 H 1 HA1 80 HA2 HA3 HA4 60 HA5

phenol% in water,

Concentrationdichloro of 0 0 30 60 90 120 150 180 210 240 Time,min b 100 T 2 CA1 80 CA2 CA3 CA4 60 CA5

phenol% in water,

Concentrationdichloro of 0 0 30 60 90 120 150 180 210 240 Time,min

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c 100 1 H HA1 80 HA2 HA3 HA4 60 HA5

phenol% in water,

Concentrationtrichloro of 0 0 30 60 90 120 150 180 210 240 Time,min

c 100 T 2 CA1 80 CA2 CA3 CA4 60 CA5

phenolinwater,%

Concentrationtrichloro of 0 0 30 60 90 120 150 180 210 240 Time,min

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d 100 1 80

40 H HA1 in water, % in water, HA2 20 HA3 HA4

Concentrationalachlor of 0 HA5 0 15 30 45 60 Time,min d 100 2 80

60 T 40 CA1 CA2 in water, % in water, CA3 20 CA4 CA5

Concentrationalachlor of 0 0 5 10 15 20 25 30 Time,min

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e 100 1

H 40 HA1 HA2 in water, % in water, 20 HA3 HA4 HA5 0

Concentrationimazaquin of 0 5 10 15 20 25 30 Time,min e 100 2 T CA1 80 CA2 CA3 CA4 60 CA5

in water, % in water, 20

Concentrationimazaquin of 0 0 15 30 45 60 Time,min

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f 100 1 80

60 H 40 HA1 HA2

inwater,% 20 HA3 HA4 HA5

Concentrationsimazine of 0 0 30 60 90 120 150 180 210 240 Time,min f 100 T 2 CA1 80 CA2 CA3 CA4 60 CA5

inwater,% 20

Concentrationsimazine of 0 0 30 60 90 120 150 180 210 240 Time,min

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5.4.8 Mechanism

The performance of immobilised TiO2 on laponite clay photocatalysts is related to the charge-carrier dynamics and the degradation of various pollutants, which proceed through a complicated multistep interfacial charge transfer process [62]. As in Figure 5-25, k1 and k2 are the charge transfer rate constants across the solid-liquid interface. The rate constant k3 represents the recombination of electron and hole pairs. The charge carriers either recombine within the bulk of the material or move to the surface of the particle where they can also recombine or can be trapped at defect sites. The holes can react with surface hydroxyl groups which produce strong oxidising hydroxyl radicals, which again react with the organic compounds leading to complete mineralisation. The electrons interact with molecular oxygen to form  the superoxide radical anion, O2 [63]. If the rate of recombination of electrons and holes increases, then the efficiency of the interfacial charge transfer decreases which leads to a decrease in the rate of photo reactions. The efficiency of a catalyst depends on the ability to slow down the recombination of electrons and holes. The recombination rate is dependent on the defects in the samples and may be due to the different sample preparation conditions that lead to materials with the morphology of a particular micro-shape and various different crystal defects. The high photocatalytic activities of CA3 and CA4 [TiO2-(B) core with anatase particles on the surface] can be explained by considering two factors. One is the light trapping ability of the surface layer anatase and other is the depth of the interaction of two different phases which are in physical contact each other [45, 64].

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Figure 5-25  Schematic picture of photoelectrochemical mechanism of single phase catalyst [62].

Figure 5-26  Illustrating the mechanism of mixed phase of anatase dotted

TiO2-B fibres.

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As seen in Figure 5-26, the chemical potential of electrons in the semiconductor represented by the position of the Fermi level and the chemical potential of the solution depends on the dissolved substance in the solution. Semiconductors always experience a flow of electrons when in contact with an electrolyte and vice versa. This flow is dependent on the chemical potentials of the electrolyte and the photocatalyst. Interestingly, in the case of the anatase photocatalyst (n-type) the excess electrons move to the electrolyte and leave a positive charge in the semiconductor and negative charge in the electrolyte. These charge separations are located at the solid-liquid interface and the established Schottky barrier [62]. The electric field formed in the Schottky barrier causes the bending of VB and CB at the solidliquid interface. Because of the charge separation in the solidliquid interface, holes in the valence band formed after the migration of electrons to the conduction band diffuse to the surface. At the same time, electrons in the conduction band move in the opposite direction, due to the lower energy of CB in the TiO2-(B)solid. These electrons can again migrate into the lowest energy of the conduction band in the TiO2-(B) phase induced by the space-charge layers [64]. This kind of inter-band migration suppresses the electron-hole pair recombination. This assures the electron-hole pair a lifetime which is sufficiently long to enable these species to participate in interfacial electron transfer.

Another important factor which determines the degradation rates of organic pollutants are the molecular structures and functional groups present in the molecule. From our previous discussion, it is obvious that degradation of organic compounds is carried out by the attack of hydroxyl free radicals on pollutants (Figure 5-25). The functional groups present in the pollutant molecule influence the nature of the attack. As a result some of the molecules degrade at a faster rate than the others during the photocatalytic reaction. Aromatic rings present in the pollutant molecule are the centres of attack, resulting is the formation of o-, m- and p-hydroxy intermediates [65]. Depending on the nature of the functional groups present on the benzene ring, the hydroxyl free radical attaches to different positions of the ring. There are two types of functional groups, some of them are electron donating and others are electron withdrawing groups. Electron donating groups contain unshared pair of electrons which are mainly o and p directing species. The electron donating

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effect of functional groups decreases in the following order: NR2  NHR  OH  OR  NHCOR  OCOR. The electron withdrawing tendency of functional groups is shown in the following order: NO2  CN  SO3H  CHO  COR  COOH  COOR

 CONH2  CCl3  NH3 [66]. Pollutant molecules with stronger electron donating or electron withdrawing groups are degraded at a faster rate. Our results indicate that the degradation of imazaquin is at a much faster rate than that of alachlor, which supports the above arguments. Complete degradation of organic pollutants into carbon dioxide and water does not occur when the molecule contains nitrogen atoms in the ring. There is some evidence to indicate that the final degradation product of atrazine is cyanuric acid [67]. It is therefore logical to expect that pollutants like simazine would not decompose to carbon dioxide and water after photocatalytic degradation reactions.

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5.5 Conclusions

The photocatalytic degradation of bromacil, chlorotoluran, sulfosulturon, alachlor and imazaquin were studied with a new structure of anatase crystals linked to laponite clay fragments. The degradation rate was increased by increasing the anatase crystal contents in the clay fragments. Based on the Ti/clay ratio, these new structure catalysts facilitate the degradation at a faster rate than the P25 catalyst. This can be attributed to the fact that the former maintained a smaller crystallite size and a higher pore volume. A mesoporous structure of clay fragments opens pathways to the interior of the crystal, through which pollutants could be transported to interact with the anatase surface.

It has been seen that the photocatalytic activities of anatase decorated titanate as well as the TiO2-(B) fibres prepared in a series of phase transition processes were superior in performance over immobilised the TiO2 laponite clay catalysts. The intimate contact between the two phases inhibits the recombination of excited state conduction band electrons and valence band holes by the possible migration of electrons into the lower energy conduction band of the TiO2-(B) fibril core. The multiphase structures achieved a phase charge layer between the two phases facilitating the passage of charges with each other. These novel multiphase photocatalysts have the potential to alleviate the disadvantages associated with powdered photocatalysts as well as thin film photocatalysts. Moreover, the obtained anatase decorated TiO2-(B) fibres prepared by a 45 h hydrothermal treatment followed by calcination is not only superior in performance as photocatalysts but they could also be readily separated out from the slurry system after photocatalytic reactions.

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5.6 References

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3. X. Z. Yang, Y. Dongjiang, H. Y. Zhu, J. W. Liu, W. N. Martins, R. Frost, L. Daniel, Y. N. Shen, Mesoporous Structure with Size Controllable Anatase Attached on Silicate Layers for Efficient Photocatalysis, Journal of Physical Chemistry C. 113 (2009) 8243-8248.

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8. G. Palmisano, V. Augugliaro, M. Pagliaro, L. Palmisano, Photocatalysis: a promising route for 21st century organic chemistry, Chemical Communications. (2007) 3425-3437.

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12. H. Yamashita, K. Mori, Applications of single-site photocatalysts implanted within the silica matrixes of zeolite and mesoporous silica, Chemistry Letters. 36 (2007) 348-353.

13. H. Yamashita, M. Anpo, Local structures and photocatalytic reactivities of the titanium oxide and chromium oxide species incorporated within micro- and mesoporous zeolite materials: XAFS and photoluminescence studies, Current Opinion in Solid State & Materials Science. 7 (2003) 471-481.

14. H. Yamashita, K. Yoshizawa, M. Ariyuki, S. Higashimoto, M. Che, M. Anpo, Photocatalytic reactions on chromium containing mesoporous silica molecular sieves (Cr-HMS) under visible light irradiation: decomposition of NO and partial oxidation of propane, Chemical Communications. (2001) 435-436.

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17. H. Chen, A. Matsumoto, N. Nishimiya, K. Tsutsumi, Preparation and characterization of TiO2 incorporated Y-zeolite, Colloids and Surfaces a- Physicochemical and Engineering Aspects. 157 (1999) 295-305.

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20. R. Yoshida, Y. Suzuki, S. Yoshikawa, Syntheses of TiO2-(B) nanowires and TiO2 anatase nanowires by hydrothermal and post-heat treatments, Journal of Solid State Chemistry. 178 (2005) 2179-2185.

21. H. Y. Zhu, X. P. Gao, Y. Lan, D. Y. Song, Y. X. Xi, J. C. Zhao, Hydrogen titanate nanofibers covered with anatase nanocrystals: A delicate structure achieved by the wet chemistry reaction of the titanate nanofibers, Journal of the American Chemical Society. 126 (2004) 8380-8381.

22. H. Yu, J. Yu, B. Cheng, M. Zhou, Effects of hydrothermal post-treatment on microstructures and morphology of titanate nanoribbons, Journal of Solid State Chemistry. 179 (2006) 349-354.

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24. M. Qamar, C. R. Yoon, H. J. Oh, D. H. Kim, J. H. Jho, K. S. Lee, W. J. Lee, H. G. Lee, S. J. Kim, Effect of post treatments on the structure and thermal stability of titanate nanotubes, Nanotechnology. 17 (2006) 5922-5929.

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26. E. Morgado, M. A. S. de Abreu, G. T. Moure, B. A. Marinkovic, P. M. Jardim, A. S. Araujo, Effects of thermal treatment of nanostructured trititanates on their crystallographic and textural properties, Materials Research Bulletin. 42 (2007) 1748-1760.

27. C. K. Lee, C. C. Wang, M. D. Lyu, L. C. Juang, S. S. Liu, S. H. Hung, Effects of sodium content and calcination temperature on the morphology, structure and photocatalytic activity of nanotubular titanates, Journal of Colloid and Interface Science. 316 (2007) 562-569.

28. H. Y. Zhu, Y. Lan, X. P. Gao, S. P. Ringer, Z. F. Zheng, D. Y. Song, J. C. Zhao, Phase transition between nanostructures of titanate and titanium dioxides via simple wet-chemical reactions, Journal of the American Chemical Society. 127 (2005) 6730-6736.

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31. T. Tachikawa, S. Tojo, M. Fujitsuka, T. Sekino, T. Majima, Photoinduced charge separation in titania nanotubes, Journal of Physical Chemistry B. 110 (2006) 14055-14059.

32. M. Qamar, C. R. Yoon, H. J. Oh, N. H. Lee, K. Park, D. H. Kim, K. S. Lee, W. J. Lee, S. J. Kim, Preparation and photocatalytic activity of nanotubes obtained from titanium dioxide, Catalysis Today. 131 (2008) 3-14.

33. A. Riss, T. Berger, H. Grothe, J. Bernardi, O. Diwald, E. Knozinger, Chemical control of photoexcited states in titanate nanostructures, Nano Letters. 7 (2007) 433-438.

34. H. H. Ou, C. H. Liao, Y. H. Liou, J. H. Hong, S.-L. Lo, Photocatalytic Oxidation of Aqueous Ammonia over Microwave-Induced Titanate Nanotubes, Environmental Science & Technology. 42 (2008) 4507-4512.

35. G.-S. Guo, C. N. He, Z. H. Wang, F. B. Gu, D.-M. Han, Synthesis of titania and titanate nanomaterials and their application in environmental analytical chemistry, Talanta. 72 (2007) 1687-1692.

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36. M. Zhang, Z. Jin, J. Zhang, X. Guo, J. Yang, W. Li, X. Wang, Z. Zhang, Effect of annealing temperature on morphology, structure and photocatalytic behavior of nanotubed H2Ti2O4(OH)2, Journal of Molecular Catalysis A: Chemical. 217 (2004) 203-210.

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39. G. Li, S. Ciston, Z. V. Saponjic, L. Chen, N. M. Dimitrijevic, T. Rajh, K. A. Gray, Synthesizing mixed-phase TiO2 nanocomposites using a hydrothermal method for photo-oxidation and photoreduction applications, Journal of Catalysis. 253 (2008) 105-110.

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41. H. G. Yu, J. G. Yu, B. Cheng, J. Lin, Synthesis, characterization and photocatalytic activity of mesoporous titania nanorod/titanate nanotube composites, Journal of Hazardous Materials. 147 (2007) 581-587.

42. H. Yu, J. Yu, B. Cheng, Preparation, characterization and photocatalytic activity of novel TiO2 nanoparticle-coated titanate nanorods, Journal of Molecular Catalysis A: Chemical. 253 (2006) 99-106.

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45. D. Yang, H. Liu, Z. Zheng, Y. Yuan, J. Zhao, E. R. Waclawik, X. Ke, H. Zhu, An Efficient Photocatalyst Structure: TiO2(B) Nanofibers with a Shell of Anatase Nanocrystals, Journal of the American Chemical Society. 131 (2009) 17885-17893.

46. V. M. Gun'ko, V. I. Zarko, V. V. Turov, R. Leboda, E. Chibowski, L. Holysz, E. M. Pakhlov, E. F. Voronin, V. V. Dudnik, Y. I. Gornikov, CVD- Titania on Fumed Silica Substrate, Journal of Colloid and Interface Science. 198 (1998) 141-156.

47. Y. Mao, M. Kanungo, T. Hemraj-Benny, S. S. Wong, Synthesis and Growth Mechanism of Titanate and Titania One-Dimensional Nanostructures Self-

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Assembled into Hollow Micrometer-Scale Spherical Aggregates, The Journal of Physical Chemistry B. 110 (2005) 702-710.

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60. M. A. Debeila, M. C. Raphulu, E. Mokoena, M. Avalos, V. Petranovskii, N. J. Coville, M. S. Scurrell, The influence of gold on the optical properties of sol-gel derived titania, Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing. 396 (2005) 70-76.

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CHAPTER 6

Conclusions and Future Work

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6.1 Conclusions

 The scope of this study was to investigate the preparation of a novel class of nanostructured ion-exchangers, adsorbent and photocatalyst materials capable of performing water remediation.

 The modification of sodium niobate nanostructures was identified as being an effective approach to improve the ion-exchange property, since the incorporation of tantalum in a hydrothermal synthesis, favoured the formation of a framework, which would therefore be suitable for the easy exchange of sodium ions due to the low crystalline nature of the framework.

 The formation of less crystalline bar-like niobate solids was facilitated by the incorporation of 5 to 10% tantalum as a vital framework constituent. These solids were found to be very suitable for the selective removal of strontium, barium and potentially radium ions from water due to their high ion-exchange capacity.

 Bar-like niobate solids have shown a substantial framework deformation after the uptake of Sr2+ and Ba2+. It is possible that a metastable framework rather than a rigid structure, which consequently enabled the solids to be suitable for concentrating most of the radionnuclides from contaminated water and would allow the bulk of the waste to be disposed of.

 Highly crystalline fibres were formed at 2% tantalum, however this demonstrated a low uptake of large divalent cations due to its weak ability to exchange sodium ions. Furthermore, the fibre phase is less suitable for storage due to high leaching into water.

 Modifications of beidellite clay under heterogeneous conditions were achieved in a two step processes such as acid treatment and the covalent grafting of functional groups.

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 The unexceptional uptake capacity of simazine from water has been demonstrated by the treatment of beidellite clay with 0.5 M hydrochloric acid. The acid treatment was found to leach out octahedral alumina layers, resulted in an increase of surface area and adsorption sites.

 The introduction of functional groups such as 3-chloropropyltriethoxysilane (CPTES) and octyltriethoxysilane (OTES) into the clay system has imparted a high adsorption capacity towards certain organic pollutants such as alachlor and imazaquin.

 Grafting agents performed differently towards a variety of pollutants depending on the chain lengths and the interactions of chain end groups. Octyltriethoxysilane (OTES) grafted clays have shown a high affinity towards alachlor whereas 3-chloropropyltriethoxysilane (CPTES) exhibited a stronger affinity towards imazaquin. Clays modified with OTES can absorb alachlor 5 mg/g of adsorbent at initial lower concentration of 5 mg and about 13.8 mg/g at an initial higher concentration up to 20 mg and CPTES grafted clays can absorb imazaquin at about 1.8 mg/g of adsorbent at an initial concentration of 2 mg and at about 8.2 mg/g at an initial concentration of 10 mg.

 A short chain of 3-chloropropyltriethoxysilane (CPTES) grafted clay adsorbent could attract pollutants more deeply into the interlayer space which caused the expansion of layers.

 It was found that grafting with octyltriethoxysilane (OTES) which has a long chain, could induce more pressure between the interlayer spaces that impart a higher expansion of layers. The long chain groups can provide more easy access towards the pollutants, however no expansion of layer after adsorption was observed.

 Short chain grafted clay that has a chlorine head attracts imazaquin more strongly than alachlor. This can be attributed to the donor- acceptor mechanism between aromatic  electrons and the chlorine groups.

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 In order to isolate the adsorption behaviour of clay surface, the adsorption

results was compared with -Al2O3 fibres grafted with the same functional groups. In alumina fibres the adsorption of pollutants is mainly carried out

within the fibrillar interstices. The -Al2O3 fibres modified with OTES can absorb alachlor 1.6 mg/g of adsorbent at an initial lower concentration of 2

mg and CPTES grafted -Al2O3 fibres can absorb imazaquin about 1 mg/g of adsorbent at an initial concentration of 1 mg. This uptake capacity is slightly less than that of clay adsorbents for both pollutants.

 The long chain grafted silane group has a special affinity towards alachlor uptake whereas imazaquin adsorbed significantly on the short chain grafted alumina fibres. Imazaquin uptake is faster than alachlor uptake.

 The easy separations of alumina based adsorbents from solutions, that are suitable for a „flow reactor‟ system, were observed.

 Based on the Ti/clay ratio, the immobilised TiO2 on laponite clay fragments has shown high photocatalytic activity compared with P25 for the degradation of herbicides.

 Compared with nanosized powder catalysts such as P25, the immobilised

TiO2 on laponite clay catalysts could readily be separated out from the slurry system and could easily be re-used.

 TiO2-(B) fibres decorated with anatase particles on the surface, were prepared by a 45 h hydrothermal treatment of H-titanate fibres followed by calcination. This catalyst showed excellent degradation ability in

comparison with TiO2 immobilised laponite photocatalysts for the degradation of herbicides and phenolic pollutants.

 The fibrous morphology facilitates the easy separation of catalysts from the slurry system.

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 The pure anatase phase with particle morphology that was obtained after 5 days of hydrothermal treatment of H-titanate fibres, showed a degradation

ability that is the same as that of TiO2-(B) fibres decorated with anatase crystals which was prepared by 45 h of hydrothermal treatment followed by calcination, however it is difficult to separate the former out from solutions.

 This work offers a promising example of the potential developments that could lead to new efficient devices for water purification. Furthermore, these new types of nanostructured materials will also be able to improve the commercial viability of decontaminating water. Clearly, numerous challenges still lie ahead, however the rate of advance in this area promises to deliver exciting new developments in the very near future.

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6.2 Future work

Further research needs to be undertaken in order to fully understand the amazingly complex structures of niobate and the correlations of the structures obtained within the reaction conditions.

Niobate fibres can be modified for many applications including the removal of organic pollutants and heavy metals. One of the modifications involves the exchange of sodium ions with organic cations such as dioctodecyldimethylammonium (DDDA), hexadecyltrimethylammonium (HDTA), hexadecylpyridinum (HDPC), trimethylphenylammonium (TMPA), tetramethylammonium (TMA), 4-mercaptopyridinium (4-MP) and ammonium. The size and nature of the organoammonium ions significantly affect the properties of the resulting hybrid materials. Immobilized organic cations can act as active sites for the removal of the organic pollutants. Niobate bar like solids contain large amount of exchangeable sodium ions, which are readily available for exchange reactions with organic cations.

The possibility of doping titanium or some other transition metals into the framework of niobate using different precursors should also be addressed. Consequently there is much potential for the evolution of niobate nanotubes at temperatures below a fibre forming temperature. Nanotube formation is also possible under the influence of some dopants.

Niobate nanofibres can be used for the development of catalysts. Ruthenium (III) hydrated oxide is a promising new catalyst, which is used for the selective oxidation of alcohols in liquid molecular oxygen [1]. In order to successfully apply this catalyst in industry, it needs to be immobilised on a substrate. So far, the most active catalyst reported has been ruthenium

deposited on -Al2O3 [2]. The ruthenium (III) hydrated oxide could be deposited onto niobate nanofibres by the consecutive exchange of Na+ with H+, followed by deposition of Ru3+ ions. It is probable that some level of

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Even organically modified clay adsorbents are difficult to separate out from the solution; this drawback can be alleviated by the partial grafting of a polymer with a reactive end group onto the clay surface in order to make a polymer-clay composite. A second step in this process is the chemical bonding of the active sites of clay layers with monomeric type ligands such as trimethylsilyl, butyldimethylsilyl and octyldimethylsilyl, as well as polymeric-type 3-aminopropylsilyl and octyldimethylsilyl ligands.

There is definite scope for a future study in the possibility of developing photocatalyst sensitive to visible light. Anatase particles deposited on H- titanate fibres can be modified for visible light application. The first potential application is the ion-exchange of ammonium ions with protons in the H-titanate followed by the calcination of the sample in air at 400 oC, resulting is the formation of N-doped TiO2-(B) nanofibres. Furthermore doping of nitrogen atoms can form additional levels in the forbidden zone of the wide-band gap TiO2 thus enabling them to be used in visible light [3].

Further work needs to be done to evaluate the performance of Cr3+ doped mixed phase nanofibres for the splitting of water in the presence of visible light. This can be achieved by the doping of Cr3+ in the lattice of anatase crystals and therefore forming electron levels in the forbidden zone [4].

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6.3 References

1. D. V. Bavykin, A. A. Lapkin, P. K. Plucinski, J. M. Friedrich, F. C. Walsh, TiO2 nanotube-supported ruthenium(III) hydrated oxide: A highly active catalyst for selective oxidation of alcohols by oxygen, Journal of Catalysis. 235 (2005) 10-17.

2. Y. Kazuya, M. Noritaka, Supported Ruthenium Catalyst for the Heterogeneous Oxidation of Alcohols with Molecular Oxygen, Angewandte Chemie International Edition. 41 (2002) 4538-4542.

3. H. Tokudome, M. Miyauchi, N-doped TiO2 nanotube with visible light activity, Chemistry Letters. 33 (2004) 1108-1109.

4. C. C. Tsai, H. Teng, Chromium-doped titanium dioxide thin-film photoanodes in visible-light-induced water cleavage, Applied Surface Science. 254 (2008) 4912-4918.

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1. Mattson, J.S., H.B. Mark, and Jr., Activated Carbon-Surface Chemistry and Adsorption from Solution, 1971, Marcel Dekker, Inc.: New York.

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