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A MECHANISTIC STUDY OF SORPTION OF IONIC ORGANIC COMPOUNDS ON PHYLLOSILICATES
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
the Degree Doctor of Philosophy in the Graduate
School of The Ohio State University
By Sandip Chattopadhyay
*****
The Ohio State University
1997
Dissertation Committee: Approved by
Dr. Samuel J. Traina (Adviser)
Dr. Jerry M. Bigham
Dr. Yu-Ping Chin Adviser
Dr. Cliff T. Johnston Environmental Science Graduate Program UMI Number: 9731599
Copyright 1997 by Chattopadhyay, Sandip
All rights reserved.
UMI Microform 9731599 Copyright 1997, by UMI Company. All rights reserved.
This microform edition is protected against unauthorized copying under Title 17, United States Code.
UMI 300 North Zeeb Road Ann Arbor, MI 48103 Copyright by
Sandip Chattopadhyay
1997 ABSTRACT
Nitrogen heterocyclic compounds (NHCs) are present in many wastes generated by the use of everyday consumer products. These ionic organic contaminants are adsorbed by the clay fraction of the soil, indicating that adsorption by clays controls their fate in the environment. In order to determine the fate of these organic contaminants in the environment, effort was made to study the sorption characteristics of two model sorbates, which were acridine and acridine-9-carboxylic acid. Well-characterized phyllosilicates (hectorite, saponite, and muscovite) were used as sorbents. The effect of the presence of a cationic surfactant (hexadecyltrimethylammonium bromide) on the sorption behavior of the chosen sorbates was also investigated. Results obtained indicated that clay particles act as templates for the formation of stable aggregates of sorbed molecules. The cationic forms of the NHC molecules were attracted preferentially by the negatively charged clay surfaces over the neutral, zwitterionic and the anionic forms of the organic molecules. Organic molecules were sorbed not only on the external surface but also on the interlayer space of the swelling-type clays. Thermodynamics of the sorption process was also investigated to distinguish between the contributions made by the hydrophobic and the electrostatic forces. It was found that approximately half of the total change in free energy occurring during the process of sorption is due to hydrophobic interactions. Hydrophobic interactions are significant at high concentrations when sorbed molecules form aggregates. The presence of HDTMA dictates the mobility of NHCS in the environment. Results showed that sorbed surfactant molecules increased the amount of organic sorbed, whereas micelles of HDTMA, present at high concentrations, increase the mobility of organic molecules. Finally, the degree of sorption was found to be dependent on the nature of the participating sorbates and sorbents, and also on the prevailing chemical conditions. A comprehensive understanding of the sorption process can be useful to model the fate and transport of ionic organic compounds in soil and aquatic environments.
Ill ACKNOWLEDGMENTS
I would like to take this opportunity to express my sincere gratitude to my advisor.
Dr. Samuel J. Traina for his valuable advice, guidance and continued support during the course of this study. I am deeply grateful to Dr. Jerry M. Bigham for his guidance and free access to his lab and facilities. His advice on mineralogy aspect of my project was invaluable. I am also obliged to Dr. Cliff T. Johnston who allowed me to conduct microcalorimetric experiments in his lab. His feedback as an examination committee member and as an adviser to this project proved extremely useful. I would like to thank
Dr. Yu-Ping Chin for suggestions and comments. I also like to thank Dr. Terry J. Logan and Dr. Olli Tuovinen for their help and advice. I like to acknowledge the instrument grant received from US EPA and the financial grants received from Sigma Xi, The Clay
Minerals Society, and The Ohio State University Graduate Student Alumni Research awards.
I am thankful to Ubaldo Soto for helping me in XRD measurements and BET analyses and Foon-Yee Chong from Purdue University for her help in conducting the microcalorimetric experiments. During my tenure at the Ohio State University, I have met and made a lot of friends who have not directly impacted my research, but have helped me and commiserated with me, and made my life bearable and sometimes, even fun. I cannot possibly mention every one of them, but, I like to mention a few of them. I thank Dr. Valérie Laperche, Dr. Satish Myneni, Jagat Adhiya, Alex Stone, Marion Brill,
Ed O’Loughlin, Doug Beak, Stephanie Zamzow, Brenda Swank for their friendship and encouragement.
iv I also express my sincere gratitude to my parents and in-laws and my brother
(Parthasarathi), sister (Dalia), and brother-in-law (Anupam) for their love, support and understanding. I appreciate all the support and love of my wife, Devamita. Finally, my inspiration is my beautiful daughter, Shreya. VITA
1984 B.E., Chemical Engineering
Jadavpur University, Calcutta, India
1984-1990 Design and Process Engineer
Development Consultants' Limited, Calcutta, India
1986-1989 MBA, Operations Research
Institute of Business Management, Jadavpur University
1990-1992 MS, Chemical Engineering
Ohio University, Athens
1992-1993 Research Assistant
NSF-Industry-University Center for Corrosion and
Multiphase Fluid Flow, Ohio University, Athens
1993-present Teaching/Research Assistant
The Ohio State University, Columbus, OH
FIELDS OF STUDY
Major Field: Environmental Science
Chemical Engineering
VI TABLE OF CONTENTS
Page
ABSTRACT...... ii
ACKNOWLEDGMENTS...... iv
VTTA...... vi
TABLE OF CONTENTS...... vil
LIST OF TABLES...... xii
LIST OF FIGURES...... xiü
CHAPTERS
1. Introduction ...... 1
1.1 Introduction ...... l
1.2 Assumptions ...... 4
1.3 Objectives...... 5
2. Materials and methods ...... 8
2.1 Introduction ...... 8
2.2 Sorbates ...... 9
2.2.1 Acridine...... 9
2.2.2 Acridine-9-carboxylic acid ...... 14
vii 2.2.3 Hexadecyltrimethylammonium bromide ...... 16
2.3 Sorbents ...... 18
2.4 UV-visible and fluorescence spectroscopic measurements ...... 23
2.4.1 Overview...... 23
2.4.2 Procedure ...... 26
2.4.3 Applications ...... 29
2.4.3.1 pH effects ...... 30
2.4.3.2 Molecular aggregation and conformation ...... 32
2.4.3.3 Polarity of the mineral surface ...... 35
2.4.3.4 Fluorescence quenching ...... 37
2.5 X-ray diffraction measurements ...... 38
2.6 Thermal analysis ...... 40
2.7 Microcalorimetric analyses ...... 41
2.8 Measurement of interfacial tensions ...... 42
2.8.1 Measurement of surface tensions ...... 43
2.8.2 Measurement of contact angles ...... 46
2.9 Adsorption Isotherms ...... 52
3. Sorption of acridine and acridine-9-carboxylic acid on smectites ...... 64
3.1 Abstract...... 64
3.2 Introduction ...... 65
3.3 Materials and methods ...... 68
viii 3.3.1 Preparation of clay samples ...... 69
3.3.2 Experimental apparatus ...... 70
3.3.3 Effect of pH on acridine sorption ...... 72
3.3.4 Sorption Isotherms ...... 72
3.3.5 UV-visible and fluorescence spectroscopic measurements 72
3.3.6 X-ray diffraction (XRD) measurements ...... 73
3.3.7 Thermogravimetric analyses ...... 74
3.4 Results and discussion ...... 74
3.4.1 Effect of pH ...... 74
3.4.2 Sorption isotherms ...... 79
3.4.3 UV-visible spectra ...... 86
3.4.4 Fluorescence spectra ...... 89
3.4.5 X-ray diffraction measurements ...... 101
3.4.6 Thermogravimetric analyses ...... 106
3.5 Possible mechanism of clay-organic interactions ...... 107
4. Thermodynamic study of sorption of nitrogen heterocyclic compounds on
phyllosilicates ...... 110
4.1 Abstract...... 110
4.2 Introduction ...... 111
4.3 Theoretical Background ...... 114
4.3.1 Forces participating during sorption ...... 114
ix 4.3.2 Determination of K and AG ...... 117
4.3.3 Determination of interfacial tensions and AG" ...... 122
4.4 Materials and methods ...... 124
4.4.1 Materials...... 124
4.4.2 Methods ...... 125
4.4.2.1 Sorption isotherms ...... 125
4.4.2.2 Surface area measurements...... 126
4.4.2.3 Microcalorimetric measurements ...... 126
4.4.2.4 Measurement of contact angles and interfacial tensions 127
4.5 Results And Discussion ...... 129
4.5.1 Sorption isotherms ...... 129
4.5.2 K from sorption isotherms ...... 131
4.5.3 Microcalorimetric measurements ...... 136
4.5.4 Contact angles and interfacial tensions ...... 141
4.5.5 Comparison of AG and AG" ...... 146
4.5.6 Determination of entropy ...... 148
4.6 Conclusions ...... 151
5. Sorptive characeristics of acridine-surfactant-phyllosilicate systems ...... 153
5.1 Abstract...... 153
5.2 Introduction ...... 154 5.3 Materials and methods...... 158
5.4 Results and discussion ...... 162
5.4.1 Effect of HDTMA on sorbed NHCs ...... 162
5.4.2 Sorption isotherms ...... 166
5.4.3 Absorption and emission spectra...... 173
5.4.4 X-ray diffraction (XRD) measurements ...... 183
5.5 Conclusions ...... ! 88
6. Summary ...... 189
Bibliography ...... 193
XI LIST OF TABLES
TABLES Page
2.1 Physico-chemical properties of AcN ...... 12
2.2 Properties of clay minerals ...... 20
3.1 Amount of NHC sorbed and shifts in absorbance spectra obtained in
presence of clays ...... 82
3.2 Thermogravimetric analyses of AcN-clay suspensions ...... 106
4.1 Equilibrium constants and change in free energy for sorption of NHCs on
clay minerals: 1 ...... 132
4.2 Equilibrium Constants and Change in Free Energy for Sorption of NHCs on
Clay minerals: H ...... 133
4.3 Contact angles and interfacial tensions of phyllosilicate-AcN systems 142
4.4 Contact angles and interfacial tensions of phyllosilicate-AcNCOOH systems.. 143
5.1 Equilibrium constants (K) for sorption of AcN on clays pretreated with
HDTMA...... 172
XU LIST OF FIGURES
FIGURE Page
2.1 Structural formulae of AcN and AcNH^ ion ...... 10
2.2 Tautomeric forms of acridine 9-carboxylic acid ...... 15
2.3 Molecular formula of hexadecyltrimethylammonium bromide (HDTMA) 17
2.4 A schematic diagram of Sensadyne 6000 Surface Tensiometer ...... 44
2.5 Procedure for obtaining the contact angle values using the ADSA-CD
technique ...... 49
2.6 Schematic representation of the experimental apparatus used for contact
angle measurements ...... 52
3.1 Schematic representation of experiments conducted ...... 71
3.2 Effect of pH on sorption of AcN on Na-hectorite @ 115 mg L'* ...... 76
3.3 Effect of pH on the emission spectra of AcN solution ...... 77
3.4 Effect of pH on the emission spectra of AcNCOOH ...... 78
3.5 Sorption isotherms for AcN-clay suspensions ...... 80
3.6 Sorption isotherms for AcNCOOH-clay suspensions ...... 81
3.7 Absorbance spectra of AcN in the presence of Na-hectorite at pH 4.5 ...... 87
3.8 Emission spectra of AcN-Na-saponite suspensions at pH 4.5 ...... 90
xiii 3.9 Emission spectra of AcN-Na-saponite suspensions at pH 8.5 ...... 91
3.10 Emission spectra of AcN-Na-hectorite suspensions at pH 4.5 ...... 93
3.11 Emission spectra of AcN-Na-hectorite suspensions at pH 8.5 ...... 94
3.12 Emission spectra of AcNCOOH-Na-hectorite suspensions at pH 4.5 ...... 99
3.13 Change in d-spacings with increase in concentrations of AcN ...... 102
3.14 Change in d-spacings with increase in concentrations of AcNCOOH ...... 104
4.1 Schematic representation of sorption of AcN on clays ...... 117
4.2 Sorption isotherms of AcN on Na-hectorite and Muscovite ...... 129
4.3 Change in enthalpy as a function of amount of AcN sorbed on Na-hectorite.... 137
4.4 Change in enthalpy as a function of amount of AcN sorbed on muscovite 138
4.5 Change in entropy as a function of amount of AcN sorbed on Na-hectorite 149
4.6 Change in entropy as a function of amount of AcN sorbed on muscovite 150
5.1 Change in AcN concnetration in solution with increase in HDTMA
concentration ...... 162
5.2 Change in AcNCOOH concnetration in solution with increase in HDTMA
concentration ...... 165
5.3 Sorption isotherms of AcN on Na-hectorite ...... 166
5.4 Sorption isotherms of AcN on muscovite ...... 167
XIV 5.5 Emission spectra of AcN-muscovite-HDTMA suspension at pH 4.5 and
HDTMA concentration of 15 and 50 pmol L ' ...... 175
5.6 Emission spectra of AcN-muscovite-HDTMA suspension at pH 8.5 and
HDTMA concentration of 15 and 50 p.moi L"' ...... 176
5.7 Emission spectra of AcN-Na(hectorite)-HDTMA suspension at pH 4.5 and
HDTMA concentration of 15 and 50 jxmol L‘* ...... 177
5.8 Emission spectra of AcN- Na(hectorite)-HDTMA suspension at pH 8.5 and
HDTMA concentration of 15 and 50 pmol L'* ...... 178
5.9a Emission spectra of (Na)-hectorite-HDTMA-AcN suspension at [AcN] = 195
pmol L ‘ and [HDTMA] = 50 pmol L*' ...... 181
5.9b Emission spectra of muscovite-HDTMA-AcN suspension at [AcN] = 195
pmol L*' and [HDTMA] = 50 pmol L'‘ ...... 182
5.10 Change in basal spacings of Na-hectorite with increase in HDTMA
concentration ...... 184
5.11 Change in d-spacings for muscovite with increase in AcN concentration 185
5.12 Change in d-spacings for Na-hectorite with increase in AcN concentration 186
XV CHAPTER 1
INTRODUCTION
1.1 Introduction
Ionic organic compounds (lOCs) are present in wastes generated from the use of
many consumer products, ranging from pesticides and herbicides used for agricultural
purposes to dyes, detergents, emulsifying agents, drugs, coal and petroleum products, and others. Significant fractions of lOCs are nitrogen heterocyclic compounds (NHCs).
These compounds, featuring a wide variety of physical, chemical, and biological properties, are contaminants in the soil and aquatic environments. Some of the properties of these organic compounds that might be affected by their interactions with soils are their rate of dissolution, volatilization, transfer to sediments, biological uptake and bioaccumulation, and chemical degradation. Therefore, interaction with soil determines the behavior and fate of NHCs in the environment. Attention is focused on the clay component of soil, as clays are effective sorbents due to: (i) high surface area, (ii) associated electrical charge due to isomorphic substitution, defects and broken edges, and
(iii) presence of amorphous oxides and humic substances. Sorption is probably the most important mode of interaction between NHCs and clay minerals. Chemical migration in soil depends on whether sorption occurs on insoluble, immobile matter or on dissolved or suspended, mobile fractions. Evidence exists that NHCs readily sorb on clay mineral surfaces and subsoil materials containing little or no organic matter (DellaGuardia and Thomas, 1983; Cohen and Yariv, 1984;
Zachara et al., 1986, 1987; Schoonheydt et al., 1986; Cenens and Schoonheydt, 1988;
Traina and Onken, 1991; Tapia Estevez et al., 1993; Lopez Arbeloa, et al., 1995). The presence of N in the molecular structure of organic heterocyclic compounds creates an uneven distribution of electrons, with a higher electron density near N. This enables the molecule (R-N) to attract a proton (H ^ under acidic conditions to form a cation (R-NH^), which can actively interact with the negatively charged clay minerals.
Studies on the sorption mechanisms of neutral organic compounds and ionic inorganic compounds indicate that the partitioning of neutral organic compounds between water and natural sorbents is guided by hydrophobic interactions, whereas the partitioning of ionic inorganic compounds is primarily by electrostatic interactions. However, very few studies have been conducted to resolve ambiguities regarding the behavior of ionizable organic compounds in soil and aquatic environments. The partitioning of ionizable organic compounds is believed to be more complex than the partitioning of either ionic inorganic compounds or neutral organic compounds, as the presence of a charged group in an organic molecule enables it to sorb from aqueous solutions on clays by both hydrophobic and electrostatic interactions, and the complexity arises due to the coexistence of these two forces (Jafvert et al., 1990). Domenico and Schwartz (1990) have also indicated that both electrostatic and hydrophobic interactions play an important role in the sorption of organic cations to natural sorbents. The relative importance of these two forces should depend on the prevailing chemical conditions. Cohen and Yariv
(1984), Grauer et al. (1987), and Cenens and Schoonheydt (1988) have indicated that sorption of cationic dyes, such as acridine orange, pyronin Y, and methylene blue, on clays takes place by the mechanism of cation exchange when the dye molecules exist as organic cations. Ainsworth et al. (1987) have also shown that sorption results from cation exchange of the protonated species of the organic heterocyclic compound and the sorption is pH-dependent, with sorption increasing with decreasing pH (below pK).
Therefore, some ambiguity exists regarding the behavior of these compounds in soil and ground water environments. The intention of the present study was to probe the exchange and molecular sorption behavior of these compounds on well-characterized sorbents to understand the mechanism of sorption of NHCs. Both macroscopic and microscopic techniques have been used for the study. Such an understanding will eventually help in predicting the fate and transport of NHCs, and lOCs on a broader scale, in the environment. The information obtained from the macroscopic and microscopic studies of clay-organic systems can be applied to understand the behavior of NHCs in different areas of research, such as agriculture (stabilization of pesticides and immobilization of fertilizers), soil science, environmental science (wastewater decontamination), pharmaceutical and paint industries, petroleum refineries (oil processing and cleaning), heterogeneous catalysis, colloid science, and others. Furthermore, a comprehensive understanding of the sorption process can be useful in studies in different research areas. as organo-clay systems have numerous applications that include uses in thickeners, viscosity builders in numerous organic fluids, oil-based drilling muds, additives to oil to form non-soap greases for the lubrication of bearings, binders for water-free foundry sands, additives in tar, asphalts and bituminous emulsions, putties, waxes, paints and cosmetics, and others.
1.2 Assumptions
The present study was based on the following assumptions;
(i) Clay particles act as templates for the formation of stable NHC aggregates.
(ii) The amount of NHC sorbed on clays and the stability of the aggregates are
dictated by the nature of the participating sorbent and sorbate, and the prevailing
chemical conditions.
(iii) The sorption behavior of NHCs is influenced by the presence of surfactants, such
as hexadecyltrimethylammonium bromide.
(iv) Surfactant molecules compete with NHCs for surface sites on sorbates and/or
change the nature of interactions between NHCs and clay surfaces.
(v) Both hydrophobic and electrostatic forces dictate the sorption of NHCs on clays,
and their relative importance depends on the prevailing chemical conditions.
Aggregation of the sorbate molecules in the presence of clays, and micellar solubilization or immobilization of NHCs in the presence of surfactants, depending on the surfactant concentration, are our primary focus as it controls the mobility of NHCs in soil and aqueous systems. Any reduction in mobility is directly proportional to reduction in bioavailability of these compounds, and the resulting decrease in biological uptake and bioaccumulation might mitigate the biological impact of these contaminants. Also, understanding the nature of the forces during sorption will help in designing suitable remediation processes.
1.3 Objectives
The objective of the present study was to evaluate the surface-chemical interactions between NHCs and phyllosilicates that might lead to the formation of soil- organic complexes. The present study will ultimately lead to the prediction of the fate of ionic organic contaminants in soil. Due to the complexity of humic substances present in soil and insufficient knowledge about their chemical properties, it was realized that a better understanding of the interactions of organic materials with soil could be obtained through the study of systems consisting of pure clay minerals and simple well-defined organic compounds. The NHCs selected for the study were acridine (AcN), an organic base, and acridine-9-carboxylic acid (AcNCOOH), which can exist as a zwitterion between pH 3 and 5. Organic bases ionize to some extent, producing cations, depending on the pH of the soil solution. Attention is focused on either basic or zwitterionic compounds as organic cations have a higher selectivity for negatively-charged clay particles than do organic anions. The sorption behavior of NHCs on clays was also studied in the presence of a cationic surfactant (hexadecyltrimethylammonium bromide).
Interests in surfactants with respect to their behavior with contaminants in soil stem from the ability of these chemicals to partition hydrophobic organic compounds into their micellar core at concentrations greater than their critical micelle concentration (cmc.) and
onto the sorbed surfactant molecules at concentration less than their cmc., thus
influencing the aqueous solubility of these compounds.
Variations in sorption have been studied with respect to the following parameters:
(i) nature of sorbents (such as type of isomorphous substitution, surface charge density and charge distribution, exchangeable inorganic cations, and swelling characteristics), (ii) nature of sorbates (such as shape and size, ionic state, and concentration of NHC molecules), and (iii) pH of the clay-organic systems. The methods used to characterize the properties of the sorbents and the sorbates were ultraviolet-visible (UV-Visible) spectroscopy, fluorescence spectroscopy, x-ray diffraction (XRD) measurements, thermogravimetric (TG) analyses, microcalorimetric measurements, and interfacial tension measurements. Identification along with quantification of the sorbate molecules in dilute aqueous clay suspensions were accomplished with UV-visible and fluorescence spectroscopy. Adsorption isotherms were also constructed from the measurements made by UV-visible spectroscopy indicating the amount of NHC sorbed on a particular clay at a particular pH. XRD measurements detected the location, type of packing, and orientations of the organic molecules on the clay surfaces. The TG analyses were used to determine the competition between the interlayer water molecules and the incoming NHC molecules. The nature of the interactions occurring during the sorption of NHCs on clays was distinguished with microcalorimetric measurements, while the interfacial tension measurements determined the surface hydrophobicities of the clay surfaces. Information obtained from these experiments were used to identify the nature of the surface reactions occurring during sorption of NHCs on clays. Different thermodynamic quantities for the sorption of NHCs on clays were calculated. The change in free energy (AG) was determined from the sorption isotherms, while the change in enthalpy (AH) was measured with microcalorimetry. The change in entropy (AS) of the process was calculated from the difference between AG and AH. Determination of these thermodynamic quantities helped us identify the different forces participating during the sorption of NHCs on clays, and enabled us to predict the mechanisms of interaction between the different components of the system.
A detailed description of the materials and methods used is provided in Chapter 2.
Chapters 3 and 4 present the experimental results obtained from the study of sorption of
AcN and AcNCOOH on different phyllosilicates by using various techniques. The thermodynamic interpretation of the process is also provided in Chapter 4. The effect of surfactants on the sorption behavior of NHCs on smectite and mica is discussed in
Chapter 5. Finally, Chapter 6 summarizes the complete study. CHAPTER 2
MATERIALS AND METHODS
2.1 Introduction
The sorption studies were conducted with two nitrogen heterocyclic compounds
(NHCs), acridine (AcN), and acridine-9-carboxylic acid (AcNCOOH). The effect of the presence of a surfactant on sorption of NHCs on phyllosilicates was also investigated.
The surfactant used was hexadecyltrimethylammonium bromide (HDTMA). AcNs were selected as dilute solutions of these compounds are fluorescent, and they serve as useful probe molecules in the study of the behavior of microheterogeneous systems. AcN is an organic base with a pK of 5.6 (Albert, 1966). AcN molecules exist as cationic acridinium
(AcNH^ ions at pH < pK and as neutral molecules (AcN) at pH > pK. AcNCOOH was included in the study as this molecule can exist as a zwitterion (AcNH^COO ) between pH 3 and 5, which are its pK-values. AcNCOOH molecules exist as cations
(AcNH^COOH) below pH 3, and as anions (AcNCOO ) above pH 5. Therefore, the study of the sorption characteristics of AcN and AcNCOOH at different pHs was representative of the behavior of organic cations, anions, and neutral molecules. Both macroscopic and microscopic methods were used to characterize the physical and chemical properties of the sorbents and the sorbates that can influence the sorption and transport of NHCs. In the subsequent sections of this chapter, the techniques used for the study are discussed. The selected methods are of great utility in studies of sorbed organic molecules on clay mineral surfaces, as they have the distinct ability to detect very low concentrations of sorbed molecules (< 1 cmol kg’* clay) in hydrated clay suspensions. Based on the information obtained, it was possible to predict the nature of the surface reactions occurring during the sorption of NHCs on clay particles.
2.2 Sorbates
2.2.1 Acridine
AcN (C13H9N) is the N-heterocyclic analog of anthracene. AcN and its derivatives form one of the oldest groups of synthetic dyes. AcNs are also used for pharmaceutical purposes. Several members of the group structurally resemble protein moieties and are therefore useful models for pharmaceutical drug binding and solubility studies that can provide an understanding of drug complexation kinetics and biological transport properties (Skypinski et al, 1984). The biological interest in AcNs results primarily from their staining properties. Furthermore, study of the sorption behavior of the AcN molecule is more relevant than that of its amino-derivatives (such as, mono-, di-, and tri-aminoacridines) because of the higher solubility of AcN in aqueous solution. This property of AcN makes it more biologically available than its amino-derivatives. Albert
(1966) had reported that the mono-aminoacridines are two to six times less soluble than AcN, and the solubilities of the di- and tri-aminoacridines are even less. The lower solubilities of the aminoacridines are due to the relative strength of the N-H...N bonds among aminoacridines over the strength of the 0-H...N bonds (hydrogen bonds) between
AcN and water molecules. The strong N-H...N bonds among aminoacridines also leads to agglomeration of these molecules in aqueous solutions, and this further reduces their solubilities. Therefore, the potential impact of the presence of AcN on the environment is more significant.
The AcN ring consists of three benzene rings, with the central ring containing a
N-atom as shown in Figure 2.1.
AcN AcNfT" ion
Figure 2.1 Structural formulae of AcN and AcNH^ ion.
10 The dimensions of an AcN molecule are calculated with HyperChem™, as shown
in Table 2.1. Reported values of the dimensions of the AcN molecules are also similar
(Schuette et al., 1991). The presence of N in the middle ring creates a concentration of
electrons at the N position (position 10) and a deficiency of electrons at the 9 position
(Acheson, 1956). When AcN is predominantly in the neutral form, position 10 has a 6(-)
charge, while 9 position has a 6(+) charge, and the position 9 has the capacity to interact
with negatively charged particles. Under acidic conditions (pH < pK of AcN), the ô(-)
charge of the N enables AcN to attract a proton (H*^ to form the AcNH^ ion. During
investigation of the adsorption and surface reaction of AcN on silver sols. Oh et al.
(1991) reported that AcN interacted with the surface either via its tt-ring system or via N
lone pair electrons. Levi et al. (1993) have also found that adsorption takes place via the protonable N atom of AcN with its plane perpendicular to the surface when they studied the adsorption of AcN on silver sols.
11 Properties Parametric values
Dimensions: length x width 9.51 À X 4.97 À X 3.34 À
Molecular weight 179.22
pK in water at 20°C 5.6 (Albert, 1966)
Density 1.005 g mL"' (CRC Handbook, 1975-76)
Solubility in water 38.4 p.g mL"' or 214 pM (Zachara et ai, 1987)
Octanol/water partition coefficient 42(X) (Banwart and Hasett, 1982)
Boiling point 346 °C
Melting point 111 °C
Heat of combustion (in vapor state) 1596.6 kcal mol '
Heat of dehydrogenation 84.0+3.0 kcal mol'* (Albert, 1966);
Dipole moment -1.94 D (Albert, 1966).
Table 2.1 Physico-chemical properties of AcN.
12 A negative moment indicates that many electrons are considerably displaced
(delocalized) from the tc-layer in the direction of the ring N atom. The value of the calculated electron density of AcN at position 9 is 0.695, and at position 10 is 1.706, based on the molecular orbital method (Acheson, 1956). The molecule is essentially planer, but there is a small bend of 2° across the central (C®N) line. The departure from planarity indicates that the states of hybridization of both the (C atom at position 9) and N atoms are not of the ideal trigonal type. This is consistent with laboratory experience that nucleophilic reagents readily add to these positions, indicating that the state of hybridization can fairly easily be changed (Albert, 1966).
AcNs do not display surface activity as seen from their surface tension values in aqueous solutions. Presence of AcN at a concentration of 250 |iM brings about a surface tension depression of 2 dyne/cm (Albert, 1966) at 15°C and pH 7. Low surface activity can also be due to the compact structure of the AcN molecule. AcN molecules do not form significant amounts of aggregates in aqueous solution (Albert, 1966), as observed from the low deviations from Beer’s law. Albert (1966) reported that an aqueous AcN solution shows a 10% deviation from Beer’s law at a concentration of 200 |iM at pH 4.
Also, in the presence of sunlight AcN gave a yellow dimer that decomposed to the monomeric form on melting.
The fluorescence emission properties of AcN are pH-dependent (Traina and
Onken, 1991), and the presence of the AcNH^ ion causes a shift in emission peak intensity to longer wavelengths. At pH > 7.0, A.max (the emission wavelength corresponding to the maximum fluorescence emission intensity) for aqueous AcN was
13 found to be at 420 nm; however, the spectrum red-shifted to a A,max of 475 nm in solutions of pH < 4. The value of 475 nm corresponds to the AcNff^ ion while the A.max at 420 nm is due to the presence of the neutral AcN molecule. Medeiros et al. (1993) have shown that at low pH-values, excitation either at 356 nm or 400 nm gives a fluorescence spectrum with a maximum at 479 nm, due to AcNH^ ion emission, with a quantum yield
((])f) equal to 0.43. At higher pH-values, the emission spectra shifted towards shorter wavelengths (blue shifts), and the fluorescence quantum yields dropped considerably
(X«m"^ = 430 nm,(j)f=0.23).
Traina and Onken (1991) also studied the fluorescence spectra of sorbed AcN on hectorite and found that emission spectra obtained at pH-values of 4.56 and 7.89 indicated a predominance of AcNH^ ion in the sorbed state. Analysis of the fluorescence emission spectra of AcN molecules adsorbed on silica surfaces also indicated the presence of the protonized form of AcN on the surface (Smirnova et al., 1992).
Comparisons of the excitation and emission spectra of AcN indicate photoprotolytic reaction of AcN molecules taking place with the hydroxyl silica groups of the adsorption sites.
2.2.2 Acridine 9-carboxyIic acid
AcNCOOH is the carboxylic derivative of AcN. The AcNCOOH exists in a state of tautomeric equilibrium, as shown in Figure 2.2, where the carboxylic acid group can exist as either -COOH or -COO, and the nitrogen can exist as N or NH^, respectively
(Medeiros et al., 1993). The insertion of the acid group into the AcN nucleus gives rise
14 to a zwitterion. In the zwitterion, the strength of the basic group is usually little changed,
but the acid group tends to be slightly stronger than it would be in a benzene ring (Albert,
1966).
Figure 2.2 Tautomeric forms of acridine 9-carboxylic acid (Medeiros et al., 1993).
At 20°C, the pK-values are 3.0 (acidic pK) and 5.0 (of first proton added to ring N atom)
(Albert, 1966). The molecular weight of AcNCOOH is 223.23 and the melting point is
290°C. Like AcN, a solution of AcNCOOH is fluorescent in nature. Medeiros et al.
(1993) found that the for AcNCOOH was at 482 nm with a 15 2.2.3 Hexadecyltrimethylammonium bromide (HDTMA) Surfactants are one group of organic pollutants which may be introduced into the soils and although the level of contamination is frequently low, their effect on interfacial and colloid structural properties of soils may be profound. Surfactants are amphipathic molecules, and hence they are fairly soluble in water and are capable of interacting with hydrophobic organic compounds (HOCs). The presence of surfactants in the system will cause the HOCs to partition into them, resulting in either solubilization or immobilization depending on the concentration of the surfactants in the solution phase or sorbed state. Boyd et al. (1988) have shown that while smectite has no measurable affinity for sorption of nonpolar, nonionic organic solutes from aqueous solution, HDTMA-exchanged smectite was an effective sorbent for a number of common nonpolar, nonionic aqueous solutes. The critical micelle concentration (cmc) for HDTMA, which is a cationic surfactant, at 0.1 M ionic strength and 25°C is 24 (iM (Kibbey and Hayes, 1993). The aggregation of the surfactant molecules in dilute surfactant solutions at surfactant concentration > cmc is due to hydrophobic bonding, which tends to minimize contact between the hydrocarbon chains and water. While the headgroups remain exposed to water, the extent of exposure of the hydrocarbon chains to water is reduced to the residual contact between water and the hydrocarbon core of the aggregates. The cationic HDTMA molecule can actively compete for sorption sites on clay mineral with the AcNH^ ions. The molecular weight of HDTMA is 364.5, and the molecular formula is shown in Figure 2.3. 16 CH3 c C»3-( H2L-N^15 I -CH^Br CH 3 Figure 2.3 Molecular formula of hexadecyltrimethylammonium bromide (HDTMA). Non-linear isotherms were obtained by Scheider et al. (1994) for the sorption of HTAC (hexadecyltrimethylammonium chloride), which is similar to HDTMA, on oxide minerals, such as SiO? and ot-AliOs. These isotherms can be considered as typical examples of sorption of surfactants on clays and other oxide mineral surfaces. They observed three distinct regions in the sorption isotherms. At low surfactant concentrations, only a weak increase in surfactant adsorption with increasing equilibrium concentration was observed, and adsorption occurred primarily by electrostatic forces. In the second region, sorption occurred also by hydrophobic interactions between the organic chains of the sorbed molecules and those that were in solution. Formation of hemimicelles were also observed in this region. Finally, the isotherm reached a plateau when the surfactant concentration reached its cmc, as the surfactants molecules formed micelles at this concentration. 17 2.3 Sorbents Clays are layered silicates (phyllosilicates), where the atoms are arranged in sets of parallel planes. Their atomic arrangement makes them crystalline in nature. The layered silicates are made up of sheets of Si(0,0H)4 tetrahedra linked with M 2-3(OH)6 (M = Al, Mg) octahedra. A 2:1 clay lattice is formed by combination of an octahedral sheet with two tetrahedral sheets. Isomorphous substitutions in the tetrahedral and octahedral sheets give rise to different clay structures. Al^"^ and, less frequently, Fe^^ can replace Si'*'^ in the tetrahedral sheet. M g"\ Fe"^, Fe^\ and occasionally ions such as Zn~* and Cr""^ replace Al^"^ in the octahedral sheet. Replacement of the resident atom by one of lower valence gives rise to negative charge in the lattice. In addition to the charge arising from isomorphous substitution, charges can also arise at the broken edges of the clay lattices, where the primary bonds are broken and the valences of the exposed lattice atoms are not completely compensated. Clay minerals are very effective sorbents for polar or ionic organic contaminants and, hence, act as contaminant carriers. Three well-characterized 2:1 minerals were chosen as model sorbents for the study. The minerals selected had low iron content, so that the fluorescence properties of the probe molecules would not be quenched. Two of these minerals belonged to the smectite group, while the third was a muscovite. The clays were hectorite (SHCa-1) and saponite (SapCa-1), obtained from the Repository of the Clay Mineral Society. The size fraction < 2pm of hectorite and saponite was used for the experiments. The muscovite sample was obtained in the form of macroscopic sheets, which were later ground into fine particles with a disk mill. However, these sheets did 18 not disintegrate to a size which is typical for the clay minerals. In muscovite and other similar mica-type clays, cleavage occurs parallel to the silicate layers causing these minerals to exist in the form of flakes (van Olphen, 1977). These flakes resist grinding to the finer sizes common to the clay minerals (such as smectites). Both Na and Ca-saturated clays were used for the study, as the degree of sorption of organic cations on the clay minerals was believed to be dependent on the cation that initially occupied the exchange sites. The layered structures of the clay minerals have permanent negative charge that is compensated by hydrated inorganic cations (Na"^, Ca^, and others) that are exchangeable. Cohen and Yariv (1984) have studied the effect of the exchangeable cations (H, Na, Mg, Al, Cu) on the sorption of acridine orange on montmorillonite. They found that maximum amounts of the dye adsorbed on Na- exchanged clays. The order in which the exchangeable cations were replaced was Na > H > Cu > Mg > Al. The relevant properties of the mineral samples are presented in Table 2.2 below. 19 Properties Hectorite (SHCa-1) Saponite (SapCa-1) Muscovite (PELCO # 54)** Chemical (Si7 54A lo 46)(Fe'’ 0.12^10.3^85,22) (Sio.OoAl 1.94)(Al3.6lPG^ 0.26Tio. 04Mno,o 2M go.\ 3) formula Ozo(OH)Xo 66* Ozo (OH)4X" o .74* 02o(OH)4X 186 Source San Bernardino Ballarat, Ted Pella Co, County, California* California* California Charge tetrahedral: 0* tetrahedral: -0.46* tetrahedral: -1,94 distribution octahedral: -0.62* octahedral: -0,30* octahedral: +0,08 (mol/unit cell) total: -0,62* total: -0,76* total: -1.86 CEC 89,2* 80,4* ND (cmol/kg) Surface area 93,4 34,6 16,23 (Nz-BET), m^/g (Note: * = Jaynes and Bigham, 1987; CEC = cation exchange capacity; ** = The chemical analyses of the samples were conducted by XRAL Laboratories, Canada, The chemical formula and charge distribution were calculated based on the chemical analyses.) Table 2,2 Properties of clay minerals. Penetration of water molecules and/or organic compounds in the interlayer space of the clay particles (such as smectites) lead to swelling and an increase in the basal spacings. Incoming organic molecules may displace resident water molecules from the interlayer if the adsorption energy of these organic molecules is comparable to that of the water molecules (van Olphen, 1977). The organic compounds can also form clay-organic complexes, and the basal spacings depend on the size and the packing of the organic molecules. When the chains are too long to lie flat in the available space, they may tilt or, in swelling clays, two or more layers of organic cations may become superimposed. The adsorption of organic compounds is not limited by the cation exchange capacity (CEC) of the clay. Aggregation of adsorbed dye cations has been observed by Ghosal and Mukheijee (1972). Yariv (1988) has indicated that the amount of dye molecules sorbed on montmorillonite is well in excess of the clay CEC, and this might be due to aggregation of the sorbed dye molecules. Clay surfaces, even when saturated with basic cations, act as weak to moderately weak acids (Swoboda and Kunze, 1968). The ability of hydrated clay surfaces to donate protons to relatively weak bases provides a means of studying the surface acidity of clays. Organic bases may be protonated on the surfaces of clays as the negatively charged clay surface and the exchangeable cations polarize adsorbed water molecules. The nature and extent of protonation depend on the type of clay minerals, nature of the exchangeable cation, and the hydration of the system. Mortland and Raman (1968) demonstrated that the extent of Br0nsted acidity associated with charged 2:1 layer silicates was influenced strongly by the nature of the exchangeable cations, water content and pKa value of the 21 organic compounds. Zachara et al. (1986) have shown that protonated species dominate the sorbent surface complex, even when the solution pH exceeds the pK by 2 pH units. This suggests that the surface pH is lower than that of the solution pH. The interactions between organic cations and clays are primarily electrostatic, but physical, non-coulombic forces, are also present, van der Waals’ attraction between the organic molecules and the clay surface, and also between adjacent organic molecules, adds to the adsorption forces. Hectorite and saponite are trioctahedral smectites. Hectorite is a naturally occurring clay, and it is found in the Mojave desert near Hector, California. It is white in color, and it has been used in paints and cosmetics. Saponite occurs as a sedimentary rock and is formed as a product of the hydrothermal alteration and weathering of basalts and ultramafic rocks. The low structural Fe content in both hectorite and saponite makes them suitable for studies of the fluorescence properties of sorbed species. The negative charge on hectorite is on its octahedral sheet, leaving the tetrahedral sheet uncharged. In contrast to hectorite, the negative charge on saponite is primarily on the tetrahedral layer. The location of the negative charge in saponite causes the exchangeable cations to electrostatically pull the layers together more energetically and limit the extent of layer separation. Therefore, interlayer swelling of saponite is less than that in hectorite. Silicate clays with charge localized in the tetrahedral sheet hydrate more strongly than those with charge localized mainly in the octahedral sheet, hence, organic cations adsorb less energetically on saponite surfaces because of the energy required to displace water from the adsorption sites (McBride, 1994). 22 Muscovite is a dioctahedral mineral, with two Al atoms in the octahedral sheet (van Olphen, 1977). One of the four Si atoms in the tetrahedral sheet is substituted by an Al atom, and the resultant positive charge is compensated by K located in the interlayer. The total net negative layer charge which results from the substitutions and the amount of compensating K ions are larger than for smectites - often one and one-half times larger (van Olphen, 1977). Though the charge per unit cell is high, the CEC is low as cation exchange is limited to external surfaces due to the non-expanding behavior of muscovite. Since the layers do not part upon addition of water, the K ions between the layers are not available for exchange - they are fixed. Only K ions on external surfaces can be exchanged for other cations. The basal spacing of muscovite is about 10 Â, which is the same as that of montmorillonite with K ions as exchange ions in the dry state. 2.4 UV-visible and fluorescence spectroscopic measurements 2.4.1 Overview Direct physical methods such as, optical spectroscopy, can provide a wealth of information on the chemical interactions of organic sorbates with mineral surfaces. Probing an organic sorbate-clay complex with electromagnetic radiation can elucidate information on surface spéciation, configuration of sorbed molecules, nature of the local chemical environment at the mineral/water interface, and surface mobilities. In a typical spectroscopic measurement, a sample is placed in the path of an impinging source of electromagnetic radiation. Reaction of the sample with the incident radiation results in excitation of the analyte from its ground state. So, to an excited state Si, concomitant with 23 absorption of some fraction of the electromagnetic radiation. The analyst monitors the quantity of radiation transmitted through the sample as it is excited to Si, or emitted by the sample when it relaxes back to So, across a range of frequency and time domains. Electromagnetic radiation varies across a wide range of frequencies with useful spectra commonly available from 10^ to lO'® Hz. Transitions between the ground state and excited state energy levels of valence electrons can be observed in the region of 3 x lO'"* to 3 X lO’^ Hz with visible and ultraviolet spectroscopies, and the following paragraphs provides a review of visible and ultraviolet spectroscopies in the context of organic contaminant-clay mineral interactions. Transfer of energy from visible or ultraviolet light to valence electrons in the molecule of interest results in an excitation of electrons from an occupied to an unoccupied molecular orbital. The most important transitions are generally from the highest occupied molecular orbital to the lowest unoccupied molecular orbital. The electronic transition states are defined by the quantum mechanical state of the molecule. In organic molecules, all the electrons are paired in the ground state. That is, for every electron with a spin of +V2 there is one with a spin of -V2. The spin state of a molecule is defined as [S] = {2 (spin of electron 1 + spin of electron 2 + ...spin of electron n)} + 1. In the case of an organic molecule when all the electrons are paired this becomes 2(0 ) + 1 = 1, and the molecule is in a singlet state. If the excited electron becomes unpaired there will be two unpaired electrons in the molecule and both will have the same sign. Thus the molecule in a triplet excited state has T = 2 [V2 + ¥2} +1=3. A typical transition is the promotion of K electrons to it* orbitals. This is a transition from a singlet ground 24 State (So) to an excited-singlet state (Si, Si, S 3, etc.). The transition of a ground-state singlet to an excited-triplet state (So —> TO is a spin-forbidden transition and has a low probability of occurrence. Transition from the So to the S; state, where “i” > 2 , occurs on a time scale of 10'** sec. This time is generally less than that required for a single molecular vibration. Thus, the initial excited state molecule typically maintains the same structure as the ground state species. Since the most stable geometry of the excited state molecule is often different than its unexcited counterpart, the S, state is typically both an excited electron state and an excited vibrational state. The vertical transition from Sq —> Si, without change in the vibrational state, is known as a Franck-Condon transition. For real molecules the presence of multiple bonds and two or more vibrational configurations in So results in complex multidimensional patterns of energy levels. Within 10 '^ sec after excitation the first excited state relaxes through molecular vibrations to the lowest vibration level possible without a change in the electronic state. The excited molecule then releases energy to higher vibrational modes or through collisions with solvent molecules arriving at the lowest vibrational level of the lowest electronic excited state Si. This occurs approximately within 10'" sec after excitation. This relaxation from Si to Si is referred to as internal conversion. Typically, a molecule then remains in Si for approximately 10'^ sec. Subsequent relaxation of Si can occur through: 1) nonradiative internal vibrations or collisions with solute or solvent molecules; 2) conversion to the So state of a different molecule resulting in nonradiative phototransformations; 3) conversion to So through emission of a photon (fluorescence); or 4) intersystem crossing to the triplet state, Ti, which can itself relax through the same 25 nonradiative relaxations as S, or through release of a photon on the time scale of 10'^ sec (phosphorescence). The analyst can thus measure electronic absorption or emission spectra. The former provide information on the excitation of a molecule from the ground state, and the latter, relaxation from the excited state. The spectral profiles expressed by a given molecule as it is excited to Sj and as it relaxes to So are sensitive to the surrounding chemical environment. Thus, absorption and emission data can be used to probe the chemical nature of the cl ay-water interface. 2.4.2 Procedure Typically, samples are introduced into a UV-visible absorption spectrophotometer as a solution. Suspensions offer the benefit of simple sample preparation, and maintenance of the clays in a fully solvated state. These can be poured into a quartz cuvette before analyzing in the spectrophotometer. In the case of absorbance spectra, the light typically passes directly through the sample in a linear configuration. This requires the use of a cuvette with only two opposing polished windows. In contrast, luminescence spectra are usually collected with a 90° optical path, thus one must use a cuvette polished on all four sides. Stirred cell holders were used to collect spectra from unstable suspensions (which might otherwise settle out, below the light path). The principal problem in direct analysis of clay suspensions is excessive light scattering and insufficient throughput, both resulting from high optical densities of the suspension. Dual monochromator, double-beam, UV-visible absorption spectrometers were used to collect 26 high quality spectra from samples with absorbance values in excess of 3.0. A “control” clay sample (without the light absorbing organic molecule) was placed into the “reference beam” of the spectrophotometer and a cuvette containing the clay plus analyte was placed into the “sample beam”. This allows much of the light scattering and total light absorbance to be “ratioed out”. In the case of luminescence measurements, “front- surface” illumination techniques allowed collection of spectra from the front of the cuvette, reducing light scattering and light attenuation by optically dense suspensions. Once a sample was prepared for examination, UV-visible absorbance spectra were obtained by recording the quantity of light absorbed by the sample, across a range of energy levels or wavelengths (K). This was presented in a plot of “absorbance” versus X in nm. In fluorescence spectra, one has both an excitation and an emission monochromator. An excitation scan is measured by varying the excitation wavelength (Xex) while holding the emission wavelength (kem ) constant; an emission scan employs a fixed value of X«x while varying the value of : and a synchronous scan can be obtained by varying both the excitation and emission monochromators while maintaining a fixed offset between Xex and Luminescence spectra were generally presented as plots of “luminescence” in arbitrary units versus Xex or Xem in nm. Excessive light scattering by dense colloidal suspensions and structural quenching can present limitations to the use of UV-visible absorption and luminescence spectroscopies. Nevertheless, a careful choice of experimental conditions allows one to utilize these spectroscopies in studies of the reactions of organic molecules with hydrated clays. 27 UV-visible spectroscopy has been used to quantify and identify the sorbed molecules from the absorbance spectra. The amount of NHC sorbed on clay was measured by comparing the spectral peak heights of the NHC solution and the supernatants obtained from the clay-organic mixtures. The UV-visible spectra of AcN and AcNCOOH were examined at room temperature at different pH-values and different concentrations, and the variation in the spectra were correlated to the structures of the various forms of the organic molecules, as the spectra of aqueous solutions of NHCs are dependent on the concentration of the dye and pH of the clay-organic suspension. A study of the fluorescence spectrum of a substance yields information concerning its molecular structure of essentially the same nature as that derived from its absorbance spectrum. The shape of the fluorescence spectrum depends on the concentration of the organic in solution, and the nature of the sorbed species can be ascertained from whether there is a shift in the emission band to shorter or longer wavelengths (metachromasy) and whether fluorescence was quenched or remained relatively strong. For the more common sandwich or stacked type dimer, e.g. acridine orange, fluorescence of the dimer will be weaker than for the monomer, whereas for end-on type dimers, e.g. isocycnone dyes, fluorescence will be strong and approximately in the position of the longer wavelength absorption band. In addition to predicting aggregation of sorbed organic molecules, metachromic behavior of the emission spectra can also be used to predict sorption of the organic in the interlayer space of the clay. Tapia Estevez et al. (1994) and Ldpez Arbeloa et al. (1995) demonstrated that sorption of organic molecules (rhodamine 6G) in the interlayer space of 28 swelling clays (laponite) resulted in shifts in the emission spectra. They corroborated their inference from XRD measurements. Quenching of the intensity of the emission spectra is also an indicator of the formation of molecular aggregates. The fluorescence intensity is dependent on a large number of factors, such as: (i) pH of the sample solution measured, (ii) temperature, (iii) viscosity of the solution, which can be dependent on the concentration of the solute, (iv) type of solvent used, and (v) presence of quenchers, such as clays. In the present study, the quenching effect were studied by measuring the fluorescence intensity at different concentrations of the organic. 2.4.3 Applications The application of UV-visible and fluorescence spectroscopies of organic dyes adsorbed on clays has provided important information on the dispersion of clay particles in dilute colloidal suspension. Proper care was taken during selection of clays for these spectroscopic studies because octahedral Fe^^ and Fe"^ in 2:1 layered alumino-silicates quenches the adsorbed fluorophores and can influence the luminescence properties of the adsorbed dye (Guilbault, 1973; Traina, 1990). The selected probe molecules had a high absorption coefficient and/or a high fluorescence quantum yield. A suitable probe molecule can characterize heterogeneous systems since their photo-physical characteristics depend on environmental factors, such as pH, and degree of molecular aggregation. 29 2.4.3.1 pH eHects The UV-visible spectra of aromatic organic acids and bases are typically sensitive to the solution pH. Substituents of aromatic rings possessing nonbonding electrons can conjugate with the n. electron system of the ring. Since the energy of the n* state is lowered by delocalization over the entire conjugated system, the n—>ït* absorption occurs at longer wavelengths (red shift) than in the corresponding unconjugated chromophoric substituent. A typical example of such a n—> 7C* absorption is the conversion of phenol to the phenolate anion, where an additional pair of nonbonding electrons are made available to the conjugated system, resulting in a lowering of energy and shifting of the spectra to longer wavelengths. Correspondingly, when aniline is converted to the anilinium cation, a proton is attached to the nonbonding electron pair, thus removing it from conjugation with the K electrons of the ring. The increase in the energy level results in a blue shift of the spectrum from 280 nm to 254 nm. A similar but smaller shift is seen in the aqueous spectra of AcN and AcNH^ (354 nm for AcN and 353 nm for AcNH^) is observed due to the more extensive k electron system of the AcN molecules, and also due to the substitution of the amino group directly into the ring. Evidence of surface protonation can also be obtained from analyses of absorption spectra. Cenens and Schoonheydt (1988) have examined the sorption and spectral properties of methylene blue on barasym, hectorite and laponite. Surface-promoted protonation of the methylene blue molecule was evidenced by the appearance of an absorption band at 763 nm in spectra collected from clay suspensions. Therefore, visible spectroscopy allowed for direct quantification 30 of protonated surface species, providing information on both the Brpnsted acidity of the sorption site, and on the surface spéciation of the sorbate. The fluorescence emission spectra of organic bases are also pH dependent. Typically this dependence is greater than that observed in absorbance spectroscopy. The Xmax shifts towards shorter wavelengths with increase in pH. Two important aspects of luminescence spectroscopy are: (i) the magnitude of the pH-induced spectral shift is much greater than that which occurs in UV-visible absorption measurements; (ii) these spectral shifts can be used to distinguish between two or more possible species for a given organic solute. Fluorescence spectroscopy can also be used to study the protonation of organic molecules on clay surfaces. When sorbed to hectorite surfaces, the emission spectra of quinoline shows the clear presence of the quinolinium ion in the pH range 2.97 to 8.82, even though the pK for quinoline is 4.92 (Traina, 1990). The protonation of these organic bases by clay surfaces can be due to specific Brpnsted acid sites at the mineral- water interface, but it may also be due to strong ion exchange selectivity for the protonated organic base. The latter mechanism is supported by greater fluorescence of quinolinium in the presence of Na-acetate relative to NaCl. Apparently some quinolinium is stabilized in solution by the formation of quinolinium-acetate ion pairs, resulting in greater emission intensity and an effective increase in the value of the pK (Traina, 1990). In some cases, fluorescence emission spectra can be used to show the presence of both protonated and unprotonated surface species. Emission spectra from aminopyrene sorbed by colloidal silica show that both the protonated and unprotonated species are present at the silica-water interface (Hite et aL, 1986). 31 Further evidence for the existence of more than one AcN species on the surface of hectorite can be found in an examination of the dependence of the emission spectra on the excitation wavelength. If only one species of fluorophore is present in a given environment, then changes in the excitation wavelength will change the total fluorescence quantum yield throughout the entire emission spectrum. However, the relative intensities of each band in the spectrum will not change. If more than one species of fluorophore is present in a given system, then the relative intensities of the emission bands will be dependent on the excitation wavelength (Lakowicz, 1986). 2A.3.2 Molecular aggregation and conformation Molecular aggregation, or the formation of clusters of chromophores or fluorophores is readily observed in electronic spectroscopy. Aggregation of dye molecules in solution, results in the enhancement of the absorption spectral bands at higher energies (lower wavelengths). The aqueous absorption spectra of rhodamine 6G, provide a good example of this phenomena. At a solution concentration of 2x10'® mol L ' the absorption spectrum of rhodamine 6 G is dominated by a band at approximately 525 nm, designated as the a band. Increases in aqueous dye concentrations result in the growth of a band centered at about 498 nm, at the expense of the a band. The shifting of the primary absorption band to shorter wavelengths is metachromasy. The higher energy, or P band, is typically attributed to the formation of molecular aggregates or dimers (Lopez Arbeloa et al., 1982). The presence of even higher energy bands, with increased 32 solution concentrations can indicate the formation of trimers or even larger molecular aggregates. Molecular aggregation can also be observed in fluorescence emission spectra of dissolved organic molecules. The combination of an excited state molecule with a ground state molecule can produce a fluorescent dimer. If this dimer is comprised of two identical molecules, it is known as an excimer. If it consists of two different solute molecules, it is an exciplex. The spectral properties of these fluorescent aggregates are generally different than those of monomers. Energy transfer from the excited state molecule to the adjacent ground state molecule results in a shift in the emission maximum to longer wavelengths. This can be seen in the emission spectra of pyrene. As the total solution concentration increases form 10"^ to 10"^ mol L'% a broad diffuse band appears at approximately 475 nm, indicative of pyrene excimer emission (Traina, 1990). Metachromatic shifts and the presence of excimer or exciplex emissions, analogous to those described above, can be used to study molecular aggregation at the mineral-water interface. Cenens and Schoonheydt (1988) observed extensive metachromasy in methylene blue sorbed onto hectorite, laponite and barasym, indicating extensive aggregation of the chromophore molecules on both external and interlayer sites. Tapia Estevez et al. (1993), exploited the metachromasy of rhodamine 6 G, to ascertain its location on Wyoming montmorillonite surfaces (SWy-1) as a function of time. With 20% of the CEC satisfied by the organic cation, increased reaction time resulted in a decrease in the a band and an increase in absorbance at 470 nm. This was attributed to time-dependent conversion of externally adsorbed rhodamine monomers to 33 external surface aggregates of rhodamine. At a surface concentration of dye molecule corresponding to 0.4% of the total CEC, a very different time-dependent behavior was observed. Initially, rhodamine exhibited an absorbance spectrum that was red-shifted with respect to that of the aqueous monomer. Continued reaction time resulted in a significant red shift in the spectrum. Tapia Estevez et al (1993) attributed this to the migration of dye molecules from external sites to interlayer sites over time and suggested that the red shift of the internal monomer reflected differences in the local polarity of internal and external sites. Luminescence methods have also been used to study molecular aggregation and time-dependent reorganization at the clay-water interface. Adsorbed [4-(l-pyrenyl)butyl] trimethylammmonium (PN4), [3-(l-phyrenyl)propyl] trimethylammonium (PN3), and (8 (l-pyrenyl)octyl) ammonium (PN 8 ) cations generally cluster on the surfaces of hydrated clay minerals and exhibit excimer fluorescence at total concentrations less than those at which excimer fluorescence occurs in homogeneous aqueous solutions. Thus all excimer emission in these systems must result from adsorbed species (DellaGuardia and Thomas; 1983, Nakamura and Thomas, 1986; Viaene et ai, 1987, 1988). Viaene et al. (1987) observed a time-dependent decrease in excimer emission from PN3, subsequent to the addition of the fluorophore to aqueous suspensions of hectorite, laponite and montmorillonite. These were attributed to initial adsorption of the organic cation to exterior sites, resulting in aggregation and excimer fluorescence, followed by migration of the fluorophores to interlayer sites. The final distribution on the mineral surface 34 increased the average distance between individual PN3 molecules, reducing the total quantity of aggregates. 2.4.3 3 Polarity of the mineral surface The emission spectra of many fluorophores is highly sensitive to the apparent solvent polarity of the surrounding chemical environment. Perhaps the most classic example is provided by pyrene dissolved in different organic solvents. Changes in solvent polarity can cause a dramatic change relative to the emission intensities of the first and third vibronic bands. This behavior can be used to probe the polarity of the clay mineral surface. Upon adsorption to an air-dried Na-saturated laponite, the ratio of the peak intensities of the first and third vibronic bands (I/HI) was 0.90, significantly lower than the value of 1.74 reported for aqueous pyrene solutions (Labbé and Reverdy, 1988). Evidently, at low moisture contents, pyrene can sorb to low polarity surfaces on the basal planes. Insertion of the clay samples to a vacuum caused further dehydration and an increase in the I/m ratio to a value of 1.80. Apparently, dehydration resulted in a greater ordering of the remaining water molecules on the surface of laponite creating an interfacial region that was more polar than bulk water (Labbé and Reverdy, 1988). Peak intensity ratios of the vibronic bands of pyrene, PN3, and PN 8 adsorbed onto laponite from water and methanol solutions, indicated that the charge regions of the interlamellar surfaces of clay particles were indeed highly polar environments (Nakamura and Thomas, 1986; Viaene et al., 1987, 1988). In contrast, very low values were observed for the ratios of the I/m fluorescence vibrations of the molecules when they 35 were sorbed onto clays coated with alkylammonium surfactants. This is direct spectroscopic proof that the surfactant cations lower the polarity of the interfacial environment (Nakamura and Thomas, 1986; Viaene e/a/., 1987, 1988). Organic dye molecules can also alter the apparent polarity of clay mineral surfaces. Traina and Onken (1991) found that the presence of adsorbed quinolinium or AcNiT’ ions on hectorite facilitated the sorption of pyrene. This was attributed to the sorption of pyrene onto the surfaces of the adsorbed quinolinium and AcNîT’, producing heterogeneous molecular aggregates. Support for this contention was found in the fluorescence emission spectra of aqueous pyrene and pyrene sorbed by quinolinium hectorite. The emission spectrum of pyrene in water was typical for pyrene dissolved in polar solvents, with the ratio of emission intensities at X, = 372 and 391 nm (I372/I391 or 1/ni) being > 1. In contrast, the emission spectrum of pyrene sorbed by quinolinium- hectorite is very different in that the value of I372/I391 < 1. Clearly the pyrene on the quinolinium-hectorite surface is residing in a region that has a lower polarity than bulk water. One could argue that the blocking of the ion exchange sites by quinolinium ions could result in a displacement of interlayer water, facilitating the direct adsorption of pyrene to the mineral surface. However, the authors also showed that additions of quinoline to aqueous solutions of pyrene (in the absence of clay particles) produced a decrease in the I372/I391, indicating a general decline in the solution polarity below that of pure water. Thus, it seems likely that the spectral differences are due to the direct sorption of pyrene to adsorbed quinoline, indicating the formation of molecular clusters. 36 2.4.3 AFluorescence quenching Fluorescence quenching is a generic term which describes the non-radiative transfer of energy from an excited-state fluorophore to adjacent molecules in the system. Static quenching results from the formation of a ground-state complex between the potential fluorophore and the energy accepting molecule. Upon excitation, the fluorophore transfers energy to the other molecule in the complex. Relaxation of the quencher molecule occurs through a nonradiant process. Static quenching processes are important in studies of fluorophores at the mineral/water interface. Quenching of sorbed organic fluorophores by octahedral Fe(III) in 2:1 layered aluminosilicates has been reported by a number of investigators (Krenske et al., 1980; DellaGuardia and Thomas, 1983; Schoonheydt et ai, 1986). Structural quenchers (Fe(III), Cr(III) and Cu(II) in octahedral positions) are actually more efficient at quenching the fluorescence of adsorbed organic fluorophores than quenching cations (Fe^\ and Cu"^ ) residing on the interlayer exchange sites. This is thought to be due to the close association of fluorescent organic cations, such as tris(2,2'-bypridine) or PN-4, with the siloxane di- trigonal cavity (Krenske et al., 1980; Wheeler and Thomas, 1988). This results in a shorter distance between the excited state fluorophore and the quencher, facilitating more effective energy transfer. However, it is not known if this energy transfer is due to static or dynamic quenching. The occurrence of structural quenching represents one of the primary limitations on the utility of fluorescence spectroscopy in studies of clay suspensions. In some instances, the magnitude of this quenching can be extensive enough to completely 37 eliminate all luminescence. Even if some luminescence does occur, partial quenching will “hide” some of the probe molecules on the mineral surface. So, only limited information can be obtained from clays that contain structural quenchers. In practice this generally restricts the use of fluorescence methods to suspensions of “Fe-free” clays such as hectorite. Fluorescence quenching can be used to determine the location of a fluorescent molecule at the mineral-water interface, and/or in surfactant micelles (Kalyanasundaram, 1987). Typically in these studies, a fluorescent probe molecule is removed from bulk aqueous solution by partitioning into a micelle. This system is then reacted with different ionic or molecular quenchers of varying polarity. If the fluorophore resides in the nonpolar interior of the micelle, then it will not be quenched by a polar quencher such as Cu^^, but it could be readily quenched by a nonpolar quencher which could also be dissolved within the interior of the micelle. If the fluorophore resides at the micelle-water interface, then it would be readily quenched by both polar and nonpolar quenchers. This same experiment, when conducted on surfactant modified clays, could demonstrate the solubilization of a nonpolar fluorophore, such as pyrene, into hemimicellular surfactant moieties on the clay surface. 2.5 X-ray diffraction measurements Adsorption of organic compounds onto swelling clay may not be restricted to the exterior surfaces of the clay. Evidence exists that interlamellar adsorption takes place to form clay-organic complexes. This has been detected by an increase in the basal spacing 38 of the clay, and XRD methods play an important role in this detection. The phenomenon of diffraction involves the scattering of x-rays by atoms of a crystal and the reinforcement of scattered rays in definite directions away from the crystal. Reinforcement of the scattered rays is quantitatively related to the distance of separation of atomic planes as defined by Bragg’s law. Diffraction from a succession of equally spaced lattice planes result in a diffraction maximum which has sufficient intensity to be recorded. The regularly spaced atoms in the clay particles act as centers of scattering for x-rays. XRD measurements provide information on the amount of organic molecules sorbed, their orientation, and also their distribution in the interlayer space of the clays. Hermosin et al. (1993) observed broad diffraction peaks and nonintegral 001 reflections from monobutyltin (MBT)-montmorillonite complexes, which indicated that the distribution of the MET species was not homogeneous in all interlayers. In clay minerals, the topmost plane is composed of oxygen molecules. However, the plane formed by these oxygen molecules is not perfect on a molecular scale. When organic molecules are sorbed to this layer, there exists a possibility that these molecules will enter into the cavities and indentations of the plane. Yamagishi and Soma (1981) pointed out that under moist conditions, the spacing is determined solely by the intercalation of water molecules. When the clay is dispersed in water, the spacing may become much larger, and each clay sheet behaves independently in water, being little affected by the adsorption of the dye cations. XRD studies were conducted with oriented samples of the dye-treated clay samples. The clay samples were shaken with dye solutions of different 39 concentrations and pH to determine the change in basal spacings with increase In dye concentration. 2.6 Thermal analysis Thermal analysis is a term covering a group of analyses that determine some physical parameter, such as energy, weight, dimension, or evolved volatile, as a dynamic function of temperature. Differential thermal analysis (DTA) is an important tool in the characterization of clay minerals and for investigating the sorption of organics on clays. During thermal analyses, the water loss and phase changes occurring with change in temperature is detected. Water is driven off by heating a clay. This water is present in the clay in two distinct forms: OH ions and H?0 molecules. Customarily, the OH ions are referred to as crystal-lattice water, and their removal from the mineral is termed dehydroxylation. The H 2O molecules are referred to as water of hydration and water of adsorption, and their removal is termed as desorption or dehydration. The OH' ions are present in fixed positions, either as individual ions among oxygens of the tetrahedra or octahedra, or as continuous sheets. Their removal from the clay is irreversible and is accompanied by an irreversible change or a complete destruction of the clay mineral structure. The water molecules, on the other hand, are present either on external or internal surfaces of the clay particles. Their removal from the clay particle may or may not cause an irreversible change in the crystal lattice, depending on the clay mineral. Thermal analyses can also be used to provide information on the presence of possible tc interactions during sorption of NHC molecules on clays (Yariv et al., 1989). Samples 40 used for the thermogravimetric experiments were similar to those used for XRD measurements. The loss of water upon heating was measured by loss in weight of the clay mineral. 2.7 Microcalorimetric analyses Microcalorimetric measurements were used to distinguish between hydrophobic and electrostatic interactions under defined conditions by studying the change in enthalpy (AH) in conjunction with sorption isotherms. Microcalorimetric experiments allow the direct measurement of enthalpies of displacement of the solvent in which the sorbent has previously been immersed or of any presorbed species by a new one. The heat exchanges represent the heats of interaction between the clay and the organic molecules. These measurements can also estimate the homogeneity of the solid surface and the fraction of macromolecular units adsorbed on the surface. The heat of immersion at 25°C of the solid - liquid interactions was determined with a Calvet differential microcalorimeter of the Tian-Calvet type using the method described by Oliphant and Low (1982). The Tian-Calvet type differential calorimeter is capable of measuring quantities of heat from 0.01 to 2 cal, with a precision of 1% (Attree et al., 1958). The differential calorimeter consists essentially of a large block of metal, held at constant temperature, in which are mounted two reaction vessels. Thermopiles, with “hot” junctions mounted on the reaction vessels and “cold” junctions on the block, serve to measure the temperature differences between the block and the reaction vessels. In operation, the reaction to be measured is carried out in one reaction vessel, and a 41 suitable blank reaction in the other, first allowing the reagents to reach thermal equilibrium with the block. The emf output from the thermopiles is recorded as a function of time. The total area under the emf - time curve, from the initiation of reaction to the time when thermal equilibrium is established, is proportional to the heat released in the reaction vessel, i.e. AH = aJVdt (2.1). 0 The total heat released was calculated by multiplying the area under the emf - time curve by a calorimetric constant. Therefore, the heat released in kJ is the product of the area under the curve in mV and the calibration constant. 2.8 Measurement of interfacial tensions Clay-organic interactions are surface reactions, and surface properties dictate the extent of the reactions. The interfacial property that has been studied is the interfacial tension. During the sorption of NHC on clays, typically three phases (clay, liquid, and vapor) are involved, giving rise to three corresponding interfacial tensions ( 7 ,^ - interfacial tension between liquid and vapor phases, commonly termed as the surface tension of the liquid medium, 7^^ - interfacial tension between clay and vapor phases, and 7^, - interfacial tension between clay and liquid phases). Out of the above three, only 7 ,^ can be determined experimentally. The values of Ycv &nd Y;, can be calculated from semi- empirical equations with experimentally determined values of 7 ,^ and the contact angle 42 (0) that the liquid makes on the clay surface. With the available values of interfacial tensions, the change in free energy (AG) for the process of sorption of NHC on clay surfaces can be determined. Several techniques have been developed to obtain the values of and 0 for various clays. For the present study, the Maximum Bubble Pressure (MB?) was used for determining Yw The value of 8 was determined by the Axisymmetric Drop Shape Analysis - Contact Diameter (ADSA-CD) method. 2.8.1 Measurement of surface tensions The Maximum Bubble Pressure (MBP) method determines the dynamic surface tension of the liquid. In the case of the MBP method (Adamson, 1990), bubbles of an inert gas (nitrogen) are slowly blown into the test liquid through tubes projecting below the surface of the liquid. Considering a hemispherical shape of the bubble, the radius of the bubble will be equal to the radius of the tube and, since the radius is at its minimum, AP is at its maximum. Equating the work done against this pressure difference to the decrease in surface free energy, one gets: AP4ttr^dr = STcrydr (2.2), and the above equation simplifies to AP = ^ (2.3). r Therefore, y can be determined from the knowledge of the pressure difference and the radius of the tube. This method is good to a few tenths percent accuracy. The values 43 obtained by this method do not depend on the contact angle and this method will be able to measure only the surface tension value of the liquid. The influence of impurities on the surface tension values is also minimized in this method. The instrument used for measuring the 7 ,^ was a Sensadyne 6000 Surface Tensiometer from ChemDyne Research Corp. (Milwaukee, Wisconsin). A schematic diagram is shown in Figure 2.4. Gas Flow AP- Sensadyne 6000 Figure 2.4 A schematic diagram of Sensadyne 6000 Surface Tensiometer. 44 In this case, nitrogen is blown slowly through two probes of different radii (R, and Ri) that are immersed in a test fluid. The bubbling of the gas through the probes produces a differential pressure difference (AP) which is sensed by a differential pressure transducer. The transducer output is conditioned and sent through an analog interface board to the computer where it is scaled and offset in relation to a previously computer-calculated calibration curve. The value of surface tension is measured each time a new surface is formed and the bubble is released from the orifice. Keeping the two probes at approximately the same immersion depth cancels the effects of liquid level. Prior to data collection, the instrument has to be calibrated with two liquids of known surface tension (water and ethanol). This method can be termed dynamic because freshly formed liquid-air interfaces are involved. Dynamic surface tension is defined as any non-equilibrium value of surface tension that arises when the surface of a solution is extended or contracted. As the surface moves towards equilibrium, either by adsorption or desorption of solute, the surface tension changes toward its static value. Therefore, the surface tension of the liquid is not established immediately on creation of the surface. To attain equilibrium, the molecules have to rearrange themselves in a preferred orientation at the created surface, with an increase in their intermolecular distance. An equilibrium surface tension can also be determined with the MBP method by using a minimum bubble flow rate such that the bubble surface and the liquid comes to an equilibrium in the interval between two bubbles. A usual bubble rate of one per second is used, however, the bubble rate can be adjusted to a lower value. In the case of a pure liquid, equilibrium values are easily 45 reached (a few milliseconds) within a bubble rate of one bubble per second. However, when surface-active components are present in the liquid, more time is required to attain equilibrium, as it depends on the rate of diffusion of solute molecules to the surface. The rate of diffusion is dependent on the size of the molecule and its concentration (Ross and Morrison, 1988). As the instrument can attain only a certain minimum bubble flow rate, it is incapable of measuring the equilibrium surface tension value in a number of cases where surface-active agents are present. 2.8.2 Measurement of contact angles The Axisymmetric Drop Shape Analysis (ADSA) method is based on the shape of a static drop. Small drops tend to be spherical because surface forces depend on the area, which changes as the square of the linear dimension, whereas distortions due to gravitational effects depend on the volume, which changes as the cube of the linear dimension. The shape of a sessile drop is a result of a balance between gravitational effects and surface tension effects. In the absence of gravity, the drop will be spherical, since this geometry encloses the maximum volume within a minimum surface area. Gravitational forces acting on the drop will tend to lower its center of mass, thus, flattening it and increasing the surface area. In the present case, the drop is formed and measurements are made under conditions such that the drop is not subjected to disturbances. The contact angle is measured by using the sessile drop method. The main advantage of this method is that a small quantity of liquid can be used to obtain the data, and the accuracy is usually several tenths of a percent. 46 The ADSA method currently used was initiated by Rotenberg et al. (1982). The software, ADSA-P, determines both the surface tension and the contact angle from the profile of the drop. It involves a numerical scheme that is a combination of a Least Squares Fit and the Newton-Raphson method, in conjunction with the Incremental Loading method (Cheng et al., 1990). ADSA-P is suitable for both pendant drops and sessile drops. A modification of the ADSA-P program (ADSA-CD program) was used by Duncan-Hewitt et al. (1989) to measure the contact angles on cell surfaces. In the present study the ADSA-CD program has been used to measure the contact angle made by a liquid on clay films. The ADSA-CD program determines the contact angle made by a sessile drop on a solid surface. The ADSA-CD uses the average contact diameter, obtained from the top view of the system, to calculate the contact angle. The other inputs required for this software are the drop volume, liquid surface tension (y,y), and the difference in densities between the liquid and the vapor phases. The ADSA-CD program relies on numerical integration of the Laplace equation of capillarity, which is (Hiemenz, 1986): AP = Y (2.4), R, R where y is the interfacial tension, R| and Ri represent the two principal radii of curvature, and AP is the pressure difference across the interface. When a sessile drop is placed on the solid surface, the liquid volume displaces the volume occupied by the gas at that location. The weight of the displaced gas volume exerts a buoyant force on the meniscus of the liquid drop, and so the density difference (Ap) between the gas and the liquid is 47 required. The hydrostatic pressure exerted by the liquid is equal to Apgz, where g is the acceleration due to gravity and z is the vertical height of the drop measured from the reference plane. The value of "g" is required for the particular place in which the measurement is being made. The hydrostatic pressure (Apgz) can be equated to the pressure due to surface tension to provide the value of 7 . The general form of the equation is (Hiemenz, 1986): 7 sin—+ — 1 = 2 ^ + Apgz (2.5), I X R J b where b is the radius of curvature at the top of the drop, (p is the angle made with the axis of symmetry by R? and x is equal to R 2sintp. R, and R? are perpendicular to each other. In this method an objective function is developed to obtain the deviation between the physically observed curve from a theoretical Laplacian curve, and then this deviation is minimized by using numerical techniques. Figure 2.5 provides a scheme of the procedure followed for obtaining the contact angle values using the ADSA-CD technique. 48 o Top View of the sessile drop Sessile drop on clay layer I Digitized image o f drop Identify the profile Determine contact diameter r ® ADSA-CD - ® Contact angle Figure 2.5 Procedure for obtaining the contact angle values using the ADSA-CD technique. 49 The profile of the drop was obtained with the use of a video camera, which can be used directly to obtain an analog video signal of the sessile drop. This signal is transmitted to an image processor where the analog signal is converted to a digital signal containing the image data as digital picture elements, or pixels. The digital pixel data are then stored in frame memory one frame at a time. A computer is used to acquire the image from the image processor and to perform the image analysis and computation. Each set of the image of the drop was calibrated with an accurate grid that can also correct for any optical distortion. On comparing the pixel coordinates of the drop with that of the calibration grid, the pixel coordinates of the profile of the drop can be converted to Cartesian coordinates. In order to acquire the drop profile coordinates, digital image processing has been employed. The camera used is a CCD video camera. The Pulnix (model No. TM-640) CCD camera allows filming at a maximum speed of 120 Hz when operated in a non-interlaced mode. The camera also has a variable shutter which can be controlled from l/63s to l/15750s. It has a pixel resolution of 649H x 491V. A SVHS recorder was available for recording the event when the video camera was used. Care was taken regarding the proper alignment of the camera and the sample. This was done with a plumb line. The accuracy of the experimental data was also dependent on several experimental conditions, such as focusing of the camera, lighting, the effect of optical distortion, and the errors in the measurement of local gravitational acceleration, volume of the drop, and the density difference. Precautions were taken to minimize evaporation and to isolate the system from any external disturbances, such as air vibration. 50 Digitizing is the process of translation of information from visual images (continuous) to computer images (discrete digits). The digitization process was performed by scanning over the visual image. The digitization process assigns an integer value, called the gray value, usually between 0 and 255, to every pixel, based on the light intensity of the image at each and every point. Depending on the recording media, the image is either directly, or through one extra step, grabbed using a Dipix P360 frame grabber, which allows users to perform image processing operations on a personal computer (PC). Image processing is a process of enhancement of digitized images to improve the images so that the final image analysis steps can be handled more easily. Many of the image processing operations can be performed by using library functions available from the Dipix library written in Turbo C. In the case of the ADSA-CD technique, the pixel coordinates can be picked manually, and these points can be used to calculate the average diameter of the drop. A precisely manufactured grid has been used for calibration of the image. Figure 2.6 gives a schematic diagram of the experimental apparatus used for the study. The technique used to prepare clay films for contact angle measurements were similar to that used for XRD measurements. 51 ■fTO VTOEO CAMERA LENS — J SLIDE WITH ^ CLAY+DROP î UGHI Figure 2.6. Schematic representation of the experimental apparatus used for contact angle measurements. 2.9 Adsorption Isotherms Chapter 4 provides a detailed thermodynamic study of the process of sorption of NHCs on clays. The overall change in free energy for the process (AG) was determined for each NHC-clay system. In order to determine AG, the overall equilibrium constant (K) for sorption of NHCs on clays was calculated, as: AG = -R T lnK (2.6). 52 The overall K for various clay-NHC systems was calculated from the amount of NHC (or organic) sorbed. Sorption isotherms are plots of amounts of NHCs sorbed per unit weight of clay as a function of the equilibrium concentrations of NHC in solution, and the K determined from these plots will not be dependent on the assumption made regarding the mechanism of sorption. Several researchers have developed models to determine K from the sorption isotherms, and some of these models are discussed in this section. The results obtained with four of these models will be presented in Chapter 4. Traditionally, sorption of a solute from a solution has been described by Langmuir and Freundlich equations. These empirical equations were developed for physical adsorption processes (Kipling, 1965), and are usually unsatisfactory when applied to systems where ion exchange is involved. Langmuir equation: x = j (2.7), 1/ Freundlich equation: x = KC^" (2.8), where x is the amount of organic sorbed, Xmax is the maximum amount of organic sorbed, C is the equilibrium concentration of organic, and n and K are constants. Some of the other sorption isotherm equations, as discussed by Bums et al. (1973), are as follows: Wiegner-Jenny: Inx = —-—InC (2.9), n +1 n +1 1 K 1 and Vagelar: — = h — (2.10), X BCo B where Co is the initial solution concentrations of the organic, and B and n are constants. 53 Rothmund and Komfeld (1918, 1919) have proposed a simple ion-exchange sorption equation to describe the sorption of a cation on H^-adsorbent as: Amount of cation sorbed _ ^ ^Amount of free cation in solution^ % 7 — K. (2. 11). Amount of H on clay Amount of replaced The values of [H^ can be obtained from pH measurements on the equilibrium solutions, and values of [H^ on the sorbent can be obtained from the differences between CEC and the equilibrium amounts of organic adsorbed. Similarly, the amount of cation sorbed can be obtained from the difference between the amount of cations originally present and the amount of cations in equilibrium solutions. However, sorption can take place by mechanisms other than ion exchange. Reichenberg (1956) has shown that deviations from an ion-exchange mechanism occur when Donnan potentials are low. Under these conditions both counter-ions and co-ions enter the matrix and adsorption by processes other than ion exchange can take place. These processes could include charge transfer, hydrogen bonding, and van der Waals forces. Biggar and Cheung (1973) have suggested that K for sorption of solutes on solid surfaces is the ratio of the amount of solute sorbed per unit volume of solvent in contact with the adsorbent surface (Cs) and equilibrium concentration of solute (Q): K = ^ (2.12). The value of Cs can be calculated as: ------(2.13), N(x/m) Mg X10° 54 MxlO^'^ 1 ^ where A = 1.091 x 10 (2.14), Np pi is the density of the solvent (g mL''), Mi and M 2 are the molecular weights (g mL'') of the solvent and the solute, Ai and Ai are the cross-sectional areas (cm^ molecule ') of the solvent and the solute molecules, s is the surface area of the adsorbent (cm" g ‘), N is Avogadro’s number, and (x/m) is the specific adsorption (ng g '). This equation was also used by Moreale and van Bladel (1979) to calculate K for sorption of aniline and p- chloroaniline on different soils, and by Zachara et al. (1986) to calculate K for the sorption of quinoline on different types of soils. Though the Langmuir equation is widely used, it does not account for multilayer adsorption or the heterogeneity of clay surfaces. A modified Langmuir-type equation introduces a “n”-factor to account for the heterogeneity of the surface, and equation (2.7) can be rewritten as: ^ KC" ^ ^ ^max (2.15). 1 + KC" The “n”-factor can be termed as the degree of cooperativity of the process, or the aggregation number, which is the average number of molecules found in each aggregate formed by the sorbed solutes. The linearized form of equation (2.15) is: In = ln(K) + nln(C) (2.16). V^max Cotton and Wilkinson (1980) have used the modified Langmuir-type equation to describe the cooperative binding of molecular O 2 by myoglobin. 55 Klimenko (1978, 1979) has proposed a three-stage adsorption model based on the orientations of the solutes on the sorbent surfaces. In the first stage, the sorbate lies flat on the surface. At this point, sorption is guided by sorbate-sorbent interactions, and the sorbate-sorbate interaction is assumed to be negligible. As the concentration of the solute increases, more and more molecules are sorbed on the solid surface. Due to the increasing density of molecules, the solute molecules undergo sorbate-sorbate interactions, and there is a possibility that the position of the sorbed molecules might change from a horizontal to a nearly vertical position. Also, the initiation of the formation of multilayers occurs in the second stage. In the third stage, aggregates have already been formed, and the incoming molecules sorb on these aggregates, rather than on any sorption sites. Study of the sorption of AcN and AcNCOOH have shown that these molecules might change orientation with increase in the amount of AcNs sorbed, as seen from XRD measurements, and/or form aggregates with increase in concentration of the NHCs in solution, as seen from spectroscopic measurements (results presented in Chapter 3). For the first stage, the sorption equation is; C K ,= fa' (2.17), T*-X 1 - ^ V ^2 j and fa'= exp (2.18), x * -x where Ka is a constant for adsorbate-adsorbent interaction, K? is a constant for adsorbate- adsorbate interaction, ai and a? are the cross-sectional areas of the solvent and sorbate 56 molecules, T is the amount of organic sorbed, and C is the equilibrium concentration of the organic in solution. The has been considered to be either the cation exchange capacity (CEC) of the sorbent (clay) where sorption is in excess of CEC, or the maximum amount sorbed where sorption is less than the CEC. The sorption equation for the second stage is: X' x - x ' C K ,= fa" (2.19), ^2 x'a2-(T-T')a2Cosa x* -x' 1 - ^ V * 2; and ln(fa") = ln(fa%. +[ln(fa'% , - ln ( f a % .] - ^ ^ (2.20), where / is the amount of organic sorbed at the beginning of the second stage, and a is the angle made by the sorbate molecule with the solid surface. As the NHC molecules can change from a horizontal position to a nearly vertical position, the calculations were done considering a to be 0° and 90°. The final stage, when the sorbed molecules are in the aggregated state, can be represented as: C K ,= (2.21). ^max ^ The overall K is the product of the K-components from all three stages. Zhu and Gu (1989) have proposed a more elaborate form of the Langmuir-type equation, which was originally developed for sorption of surfactants molecules on solid surfaces. They have assumed that sorption takes place in two stages. In the first stage, individual ions or molecules are sorbed on the active sites through electrostatic attraction 57 and/or specific attraction. In the second stage, sorption proceeds by attachment of molecules from the solution to the sorbed molecules on the sorption sites, and this leads to the formation of aggregates, primarily by hydrophobic interactions. This equation explicitly accounts for aggregate formation among sorbed molecules, and is expressed as: T„,,k,C(-+kjC"-') X = ------^------r - (2.22), l + k,C(l + k2C"~') where ki and k? are the equilibrium constants for the different stages of sorption, and the overall equilibrium constant (K) is the product of k| and k?. Though AcN and AcNCOOH are not surfactants, experimental results (Chapter 3) have shown that these molecules can form aggregates on the clay surfaces, and therefore, the two-stage adsorption model is applicable for sorption of AcNs on clays. Brownawell et al. (1990) have developed an equation to account for non-linearity in the sorption isotherms, dependence of adsorption on electrolyte and pH, and the effect of properties of sorbent, solvent, and solute on the extent of adsorption. According to them, non-linearity in sorption isotherms are due to: (i) non-uniformity of energies of adsorption sites, with saturation of high-energy sites, (ii) sorbate-sorbate interactions such as repulsive electrostatic interactions or cooperative chain-chain interactions, (iii) experimental artifacts that stem from covariation of experimental conditions, such as changes of pH, major ion concentrations, or aggregation of particles, that accompany changes in organic cation concentrations. The adsorption model used by them was based on the following reactions: 58 - Kr X • R'^ + NaX = RX + Na+ (2.23), -K y ------R-"+Cr+Y = RClY (2.24), •74. . Ca^+ + 2NaX = CaX2 + 2Na+ (2.25), where the overbar represents a species associated with the sorbent, X is a negatively charged cation-exchange site, Y is an uncharged site (that could be thought of as a site for hydrophobic adsorption), and R^ represents the organic radical. The ion-exchange sites X and the adsorption sites Y are subject to the following material balance conditions: Tx = { x } + {NaX} + { ^ } + Z^CaXz} (2.26), and Ty = {Ÿ} 4- {r CIY} (2.27), where (q) represents the activity of species q in solution (mol L''), {q} represents concentrations of species q on the sorbent (mol kg '), and T is the total number of sites (mol kg '). Finally, the heterogeneity of the sorbent surface was taken into account by considering a set of X- and Y-sites, as (X|, X?, ...., X„) and (Y|, Yi, ...., Y„). Thus the model could be described as a multisite competitive Langmuir model. The parameters were determined with FITHQL (Westall, 1982), which is a weighted nonlinear least- squares adjustment procedure for multicomponent chemical equilibrium problems. Solution activity coefficients were calculated from the Davies equation and surface activity coefficients were set equal to unity. This model was applied to the sorption of dodecylpyridinium (DP) bromide on Lula aquifer materials. 59 Mehrian et al. (1993) have proposed a model to depict bilayer adsorption of surfactants on clays. They start with a homogeneous surface on which surfactant molecules adsorb, neglecting the different conformations of the surfactant molecules at the surface. Each adsorbed molecule forms an adsorption site for the formation of a second layer, and the affinity for the second layer is different from that for the first. According to them, owing to the amphiphilic nature of the surfactants, formation of more than two layers does not occur. The assumptions made are: (i) homogeneous (plate) surface, (ii) inert sorbent, and (iii) no difference in chain conformation. In this model, the fraction 0 of sites which are occupied by the surfactant molecules consists of two parts: (a) the fractions 0i of surfactant molecules in the first layer, (b) the fraction 02 in the second layer, and 0? < 0|. The mole-fraction of the surfactant in solution is x. The equilibrium equations are based on: (i) the rate of desorption of molecules from the first layer is proportional to 0| - 02, (ii) the rate of adsorption on the bare surface is proportional to (l-0|)x. Thus, taking into account lateral interactions: 0] —0% (a G ?+ 0,A G p) —----- = x.exp (2.28). 1 — 0 | RT Analogously, adsorption in the second layer is proportional to (0, - 02)x and desorption from the second layer to 02. Thus, e (a g 5 +02AG!;” )' -— = x.exp (2.29). 0, -02 RT 60 In the above two equations, AGj® and AGz° are the standard adsorption Gibbs energies for the formation of the first and second layer respectively, AGi'“' and AG?''" are the lateral interaction Gibbs energies for the first and second layer formation respectively. In 1994, Gu et al. proposed a model to describe the adsorption and desorption of natural organic matter (NOM) on iron oxide. The process of NOM adsorption on iron oxide was expressed as: C + S < - ! ^ q (2.30), where C is the solution NOM concentration, q is the amount of NOM sorbed, and S = (qmax - q) is the available surface sites on the adsorbent for NOM adsorption, where qma% is the maximum quantity of NOM adsorbed, K is the equilibrium constant and is equal to kf/kb, which are the forward and the backward rate constants. At equilibrium. max c where, (2.32). K Homenauth and McBride (1994) have also developed another equation to describe the sorption of aniline on clays, and have highlighted the cooperative adsorption occurring between sorbed species. In the case of aniline, monomers (predominantly anilinium) and clusters (anilinium-aniline complexes) may adsorb on clays simultaneously as: 61 Kl S + A<->S-A (2.33), Kz S + 2A <-> S — A 2 (2.34), where Ki and K2 are equilibrium constants. Considering that each adsorbed dimer removes two sorbate molecules from solution, the quantity adsorbed (Qa) can be expressed as: where N is the number of adsorption sites and n is the number of molecules in adsorbing clusters. Xu and Boyd (1995) have proposed another model for adsorption of surfactant molecules on clays. According to them, cationic surfactants may be adsorbed by two mechanisms, viz., cation exchange and hydrophobic bonding. Therefore, 9% - QcE +9HB (2.36), where qr, qcE. and qas denote the total surfactant adsorbed (mol kg '), surfactant adsorbed by cation exchange, and surfactant adsorbed by hydrophobic bonding, respectively. Binary exchange reactions between inorganic cations (A'''^ and cationic surfactant molecules (Q ^ on clay exchange sites (X ) can be described as AXy + vQ^ vQX + A''"'' (2.37). 6 2 They have derived an expression for qcE, which was found to be a function of Nqx (cation mole fraction of QX on the exchange sites), and the CEC of the clay, as . , NoxCEC The amount of surfactant adsorbed by hydrophobic bonding (qna ) was obtained by an empirical equation based on experimental data for surfactant adsorption on layer silicates and soils; 9HB _ Cm (2.39), QHB. oo and qHB.oc is the adsorption plateau, and Cm and C« are equilibrium aqueous phase monomeric surfactant concentrations corresponding to qHs and qHs,-. respectively. Based on a study of the various available models, four models were selected to obtain the most probable value of K and also to verify the authenticity of the calculated values. The selection criteria will be discussed in Chapter 4. The selected models were: (i) the Biggar equation (Biggar and Cheung, 1973); (ii) the modified Langmuir-type equation (Cotton and Wilkinson, 1980; Zhu and Gu, 1989); (iii) the two-stage adsorption model (Zhu and Gu, 1989); and (iv) the three-stage adsorption model (Klimenko, 1978, 1979; Clunie and Ingram, 1983). Detailed discussion and results obtained will be presented in Chapter 4. 63 CHAPTERS SORPTION OF ACRIDINE AND ACRTOINE-P-CARBOXYLIC ACID ON SMECTITES 3.1 Abstract The use of many everyday consumer products generates wastes containing organic compounds, such as nitrogen heterocycles. When these pollutants are released to the environment, they sorb on clay minerals in soils and sediments, and the present study is focused on understanding the sorption characteristics of certain nitrogen heterocyclic compounds (NHCs) on standard clay minerals. Acridine (AcN) and acridine-9-carboxylic acid (AcNCOOH) were used as model sorbates, while well characterized swelling clays from the smectite class of clay minerals were selected as model sorbents. Results obtained show that sorption of the selected NHCs was pH-dependent, with maximum sorption occurring at low pH-conditions, as the negatively charged clays prefer the protonated form of the sorbate molecules. Though sorption of cations on clays was preferred, neutral, zwitterionic, and anionic species of NHCs also sorbed on the clay surfaces. Furthermore, sorption of NHC molecules were not restricted to the external surfaces of the clays. X-ray diffraction (XRD) measurements and thermogravimetric 64 (TG) analyses showed that the NHC molecules penetrated the interlayer space of the clay particles, where they displaced water molecules. Spectroscopic studies have shown that the sorbed NHC molecules formed aggregates on the clay surfaces, which acted as templates for aggregate formation. It was also observed that at low concentrations, neutral AcN molecules were protonated at the clay surfaces. Finally, the degree of sorption was found to be dependent on the nature of the participating sorbates and sorbents, and also on the prevailing chemical conditions. 3.2 Introduction The presence of NHCs in the environment has drawn significant attention in recent years because of their potential threat to human health (Stuermer et al, 1982; Payne and Phillips, 1985; Raupach and Janik, 1988; Katritzky et al., 1992). Groundwater is particularly susceptible to contamination from NHCs (Leenheer and Stuber, 1981). Attention is focused on clays, a major inorganic component of soils, because the high surface area and reactivity of clay minerals make them effective sorbents for polar organic contaminants. In an acidic environment, the NHC molecules can exist in cationic form, which readily interacts with the negatively charged clay minerals. Under non- acidic conditions, NHCs can exist as either neutral molecules or anions. Neutral molecules can, presumably, sorb on clays by nonpolar forces, while anions may attach to edges of the silicate clay minerals (van Olphen, 1977). Despite the different forms of NHC molecules, evidence exists that acridine orange, quinoline, proflavine, methylene blue, rhodamine 6G, and others can readily sorb on clay minerals (Cohen and Yariv, 65 1984; Zachara et al., 1986, 1987; Schoonheydt et al, 1986; Cenens and Schoonheydt, 1988; Traîna and Onken, 1991; Tapia Estévez et al, 1993; Lopez Arbeloa, et al, 1995). However, very few studies have been conducted to provide insights into the sorption mechanisms. The objective of the present study is to probe the sorption characteristics of simple NHCs on clays by investigating the physical and chemical properties of the sorbates and sorbents, as sorption dictates the fate of these compounds in the environment. The following parameters were investigated; (i) type of isomorphous substitution in clay minerals, (ii) clay mineral surface charge density and charge distribution, (iii) exchangeable inorganic cations on the clay mineral surface, (iv) swelling characteristics of the clays, (v) shape and size of NHC molecules, (vi) charge on the NHC molecules, (vii) concentration of NHCs, and (viii) pH of the clay-organic systems. AcN and its carboxylic derivative, AcNCOOH were selected as model sorbates. AcN and its derivatives form one of the oldest groups of synthetic dyes, and are also used for pharmaceutical purposes. Several members of this group structurally resemble protein moieties and are, therefore, used as models for pharmaceutical drug binding studies (Aleman et al, 1996; Kim et al, 1996). AcNs have also been found to be toxic to mammals (Albert, 1966). Large doses of AcN to dogs caused death by respiratory paralysis. In smaller doses, AcNs can affect the central nervous system (Albert, 1966). Finally, dilute solutions of AcNs exhibit fluorescence, which make them useful probe molecules in the study of microheterogeneous systems. 6 6 UV-visible and fluorescence spectroscopy were used for identifying and quantifying the nature and state of the sorbates, as these techniques are most suitable for dilute organic-clay suspensions. The NHC molecules can sorb not only on the external surface, which is the clay-water interface, but also on the internal surface, which is the interlayer space of the smectite clay minerals. XRD and TG measurements were used to detect the packing and orientation of organic molecules in the interlayer space of clay particles, along with the competition between incoming NHC molecules and the sorbed water molecules. The advantages of using these methods are their sensitivity, and their suitability for very small loadings. Our study was designed to test the hypotheses that: (i) clay particles act as templates in the formation of stable NHC clusters or aggregates, and (ii) the stability of these aggregates and the degree of sorption are dictated by the nature of the participating sorbents and sorbates. Aggregation of sorbed NHC molecules reduces the mobility of NHCs in soil and aqueous systems, and consequently their effect on groundwater. Also, a reduction in mobility is directly proportional to a reduction in bioavailability of these compounds, with a corresponding mitigation of their biological impact. O’Loughlin (1991) observed that microbial degradation of 2-methylpyridine was attenuated due to sorption on smectites. Smith et al. (1992) found that surface-bound quinoline was not susceptible to microbial degradation, unlike quinoline in aqueous solution. Information obtained from such an understanding will eventually help in predicting the fate and transport of ionizable organic compounds in the environment, and in controlling their mobility and reactivity during remediation processes. 67 3.3 Materials and methods AcN (Aldrich Chemical) and AcNCOOH (Aldrich Chemical) of purity 99.9% and 99.7%, respectively, were used for the study. Both chemicals were used as received from the vendor. AcN is an organic base and exists as an organic cation at pH < pK (pK = 5.6) (Albert, 1966). AcNCOOH has two pK-values at 3.0 and 5.0 (Albert, 1966), and AcNCOOH molecules exist as zwitterions between pH 3.0 and 5.0. The dimensions of an AcN molecule, as calculated with HyperChem™ , are as follows: length = 9.51 Â, width = 4.97 Â. The negative dipole moment of the AcN molecule is due to the displacement of electrons from the tt-layer in the direction of the ring N atom (Acheson, 1956). The molecule is substantially planer, but there is a small bend of 2° across the central C-N line (Albert, 1966). The dimensions of an AcNCOOH molecule are similar, except for the protruding carboxylate group attached to the molecule. The aqueous concentrations of AcN for all experiments ranged from 6 to 195 p,mol L ‘, while the concentrations of AcNCOOH ranged from 0.5 to 112 jimol L *. The upper limits of the concentration range were dictated by the aqueous solubilities of the selected NHCs. All AcN and AcNCOOH solutions were freshly prepared before the experiments. Experiments were conducted at both high and low pH conditions, so as to study the sorption of different ionic forms of the NHC molecules on clay minerals. Well-characterized, smectite-type, 2:1 clays with low or no iron content were used as sorbents. Hectorite (SHCa-1) and saponite (SapCa-1) were obtained from the Source and Special Clays Repository of the Clay Minerals Society (MI). The minerals selected had low Fe content, so that the fluorescence properties of the probe molecules would not 6 8 be quenched. The cation exchange capacities (CEC) of the clays were 89.2 cmol kg ' for hectorite, and 80.4 cmol kg ' for saponite (Jaynes and Bigham, 1987). The negative charge on hectorite is due to substitution in its octahedral layer (charge: total = -0.62; on tetrahedral layer = 0; on octahedral layer = -0.62), while the primary charge on saponite is due to substitution in its tetrahedral layer (charge: total = -0.76; on tetrahedral layer = - 0.46; on octahedral layer = -0.30) (Jaynes and Bigham, 1987). 3.3.1 Preparation of day samples Both Na- and Ca-exchanged forms of the clays were used for this study. Na- exchanged clays were prepared by equilibrating the clay samples in 1 M NaCl solution thrice. The clay-NaCl suspensions were stirred for 24 hours, followed by centrifugation and resuspension. The size fraction < 2 pM was collected and repeatedly centrifuged, resuspended in double distilled deionized water, and dialyzed until a negative chloride test was obtained with 0.1 M AgNOs. The stable clay suspensions were stored at 277 K in the dark to minimize bacterial growth. The Ca-exchanged forms were prepared similarly by mixing the Na-exchanged clays in 1 M and 0.1 M CaC^ solutions. Clay- organic suspensions were prepared by adding measured amounts of clay suspensions to the desired concentrations of NHC (AcN or AcNCOOH) solutions at a particular pH with continuous stirring. The clay concentration for each set of experiments was determined by evaporating both an aliquot of the stock clay suspension and clay-organic suspension after the reaction, and drying at 100°C for 24 hours. 69 3.3.2 Experimental apparatus The stirred tank reactor (STR), as shown in Figure 3.1, was a glass-jacketed vessel with facilities to measure and control pH and temperature. The pH and the temperature of the clay-organic suspensions were monitored with a pH electrode (Orion Research, model 701 A) and a thermometer, respectively. An Auto Piston Buret (Kyoto Electronics, model APB-410-20B) was used to adjust the pH of the suspension. The temperature was maintained at 25° ± 0.2°C. The temperature control system was equipped with a controller, a recirculating pump (Polyscience Corporation, model 80T), and a cooling system (Forma Scientific). The clay-organic suspension volume in the STR was maintained at 250 mL, and a stirring plate (Fisher Thermix, model 120M) was used to maintain the clay-organic system in suspension.. Batch experiments in Teflon™ centrifuge tubes (suspension volume = 25 mL) were conducted to study the effect of pH on sorption of NHCs on clays. The clay-organic suspensions in these tubes were equilibrated on a reciprocating shaker. For sorption experiments, the clay-organic suspensions were equilibrated for 24 hours before samples were withdrawn. 70 Recirculating pump and Cooling temperature system contijoller Piston T,,,= 25+2<'C Stirring plate Fluorescence Supernatant spectroscopy UV-Vis spectroscopy UV-Vis spectroscopy Fluorescence — ^ spectroscopy s-^ acu u m I hermo- X-ray diffraction gravimetry Filtration apparatus Film on glass substrate Figure 3.1 Schematic representation of experiments conducted. 3.3.3 Effect of pH on AcN sorption The amounts of AcN sorbed on Na-hectorite were measured as a function of pH of the suspensions. The concentration of Na-hectorite in the clay-organic suspensions was maintained at 115 mg L'*. The samples, after equilibration, were centrifuged (Eppendorf microcentrifuge, model: 5415C) at 15,400 ref for 30 minutes. After centrifugation, aliquots of the supernatant were used for absorbance measurements in a Cary 3 UV-visible spectrophotometer (Varian, Australia) to determine the amounts of AcN sorbed. The pH of the suspensions and solutions were determined with an Orion pH meter, and necessary pH adjustments were made with either HCl or NaOH. 3.3.4 Sorption isotherms The amounts of AcN and AcNCOOH sorbed on different clays (Na-hectorite, Ca- hectorite, Na-saponite, and Ca-saponite) were measured as a function of the equilibrium concentration of NHC in solution. After equilibrating the clay-organic suspensions, the samples were centrifuged, and the absorbance of the supernatants was measured by UV- visible absorbance. The concentration of clay in the suspensions was maintained at 50 mg L''. The amount of NHC sorbed was measured at both pH 4.5 and 8.5 so as to investigate the sorption characteristics of cations, anions, and zwitterions on clays. 3.3.5 UV-Visible and fluorescence spectroscopic measurements Background absorbance spectra from each clay mineral suspension was collected in stirred 1 cm quartz cuvettes using a Varian/Cary 3 spectrophotometer. In each case, a 72 photometric blank containing only the clay suspension in background electrolyte was placed in the reference beam and the clay - organic suspension was placed in the sample beam. Absorbance scans were measured from 200 nm to 650 nm at a scan rate of 900 nm/min with a slit width of 2 nm. Fluorescence emission spectra of clay suspensions were measured in stirred 1 cm quartz cuvettes with a SLM-Aminco Bowman Series 2 luminescence spectrometer (model FA 256) interfaced to an OS-2 computer. Background spectra were collected from the clay suspension in the absence of the NHC (AcN or AcNCOOH). Clay suspensions containing various amounts of AcN or AcNCOOH were excited at a wavelength (X^x) of 390 nm, and emission spectra were collected from 400 to 600 nm at a scan rate of 1 nm/s with excitation and emission bandpasses of 2 nm. Three scans were made for each intensity measurement. 3.3.6 X-Ray Diffraction (XRD) measurements Clay films of the NHC-treated clay samples were prepared by using the Millipore® filter transfer method (Moore and Reynolds, 1989). The clay-organic suspensions were passed through a Millipore® filter (0.2 |iM size) using a vacuum pump. The clay films formed on the top of the filter membrane were then mounted on glass slides and dried in a desiccater containing P 2O5 for 15 days. The XRD patterns were collected using Cu-Ka radiation at 35 kV and 20 mA. Measurements were made using a step-scanning technique with a fixed time of 4 s per 0.05° 20. The XRD patterns were obtained from 2 to 55° 20. 73 3.3.7 Thermogravimetric analyses Thermogravimetric experiments were conducted using a Seiko SSC5020 instrument that provided simultaneous thermogravimetric (TGA) and differential thermal analysis (DTA) measurements under a continuous flow of N% (200 mL min '). Samples were placed in a platinum boat and held at 95°C until a constant weight was recorded. The temperature was then increased at a rate of 20°C min‘* until a final sample temperature of 850°C was attained. A temperature calibration was performed using the melting points of In and Sn. The thermal balance calibration was achieved using a reference weight provided by the instrument manufacturer. A reference calcite was analyzed after every 20 samples to assure instrument standardization. 3.4 Results and discussion 3.4.1 Effect of pH Sorption of AcN on Na-hectorite (cmol AcN sorbed per kg of clay) was found to be pH-dependent (Figure 3.2). Maximum sorption was observed at low pH, especially when pH < pK of AcN. The negatively charged clay surfaces readily attracted the positively charged acridinium ions (AcNH^, which were dominant at pH < pK. Similar results were also obtained by Zachara et al. (1986) for the sorption of quinoline (pK = 4.94) on subsurface materials and by Traina and Onken (1991) on specimen clays. The decrease in sorption of AcN with increase in pH was due to the reduced attraction for neutral AcN molecules, present in solution at high pH (pH > pK) and high concentrations, by the negatively charged clay surfaces. Moreover, at high pH-conditions, the neutral 74 organic molecules compete with water molecules for sorption sites on the clay. Helmy et al. (1983) have indicated that water molecules can be preferentially sorbed on clay surfaces over neutral organic molecules. It was also observed in the current study that the change in sorption with pH was more significant at high concentrations of AcN. At low concentrations, a sufficient number of sorption sites were available for both AcNH^ ions and neutral AcN molecules. At high concentrations, when the number of available AcN molecules were comparable to the number of available active sites, more AcNH"^ ions were sorbed by electrostatic interactions and the amount of neutral AcN molecules sorbed by weaker hydrophobic interactions. Based on these results, two pH-values (pH 4.5 and 8.5) were chosen for subsequent experiments. 75 160 Initial Acridine Concentration ® 195jimolL'^ 140 ° 139nmolL‘‘ ^ 84 jimol L ! 120 ^ 56 jimol L O 6 nmol L'^ ÿ 100 CEC 80 60 40 20 0 5 6 74 8 9 10 Equilibium pH Figure 3.2 Effect of pH on sorption of AcN on Na-hectorite @115 mg L' 76 The pH-dependency of sorption of AcNs on Na-hectorite can be attributed to the different ionic forms of the molecules at high and low pH-conditions. This is illustrated by the change in the position of of the fluorescence spectra of AcN and AcNCOOH solutions when the pH was lowered from 8.5 to 4.5 (Figures 3.3 and 3.4). 7 pH = 8.5 pH = 4.5 478 [Acridine] = 65 pmol L 6 437 C 5 3 I I 4 I 3 3o 2 tu 0 450 500 550400 600 Emission wavelength (nm) Figure 3.3 Effect of pH on the emission spectra of AcN solution. 77 8 pH = 8.5 436 [Acridine 9 carboxylic] = 112 pmol L 7 I 6 I* I 4 8 3 oI 2 461 478 0 400 450 500 550 600 Emission wavelength (nm) Figure 3.4 Effect of pH on the emission spectra of AcNCOOH. The X,max of the emission spectra of AcN solution shifted from 437 nm at pH 8.5 to 478 nm at pH 4.5. This spectral shift towards longer wavelengths with decrease in pH was due to a change in the dominant species in solution from neutral AcN molecules to AcNH^ ions. The spectral peak for AcNCOOH in solution at pH 8.5 was at 436 nm. As the pH was reduced to 4.5, double peaks in the emission spectra of AcNCOOH appeared at 461 nm and 478 nm, and the double peak might be due to the presence of AcNCOOH as a zwitterion at this pH. Other researchers have also observed pH-dependency of 78 sorption of various ionizable organic molecules on clays, such as pyrimidines and purines on montmorillonite (Lailach et al., 1968); quinoline on hectorite and montmorillonite (Traina and Onken, 1991); aniline on hectorite, montmorillonite and vermiculite (Homenauth and McBride, 1994); and 5-methyl-2-nitrophenol on kaolinite, illite and montmorillonite (Haderlein et al., 1996). 3.4.2 Sorption isotherms Sorption isotherms (Figures 3.5 and 3.6) define the amount of organic sorbed (cmol of NHC sorbed per kg of clay) as a function of the equilibrium concentrations of NHCs in suspension with clay. As discussed earlier, the amount of NHC (AcN and AcNCOOH) sorbed on any particular clay was greater at pH 4.5 than at pH 8.5. This behavior indicates that the absence of positive charge on the NHC molecules at high pH reduced the sorption. It was also found that sorption of AcN was higher than that of AcNCOOH on a particular clay at any pH. On comparing the maximum amounts of AcN and AcNCOOH (as a percentage of the cation exchange capacities (CECs) of clays) sorbed on any clay at both high and low pH-conditions (Table 3.1), it was found that the amounts of AcN sorbed > the CECs of the clays, whereas this was not the case for AcNCOOH. 79 Na-Hectonte, pH 4.5 Na-Saponite, pH 4.5 Ca-Hectorite, pH 4.5 Ca-Saponite, pH 4.5 Na-Hectonte, pH 8.5 Na-Saponite, pH 8.5 Ca-Hectorite, pH 8.5 Ca-Saponite, pH 8.5 5 100 Equilibrium AcN Concentration (pmol L ) Figure 3.5 Sorption isotherms for AcN-cIay suspensions. 80 Na-Hectonte, pH 4.5 Na-Saponite, pH 4.5 Ca-Hectorite, pH 4.5 Na-Hectorite, pH 8.5 Na-Saponite, pH 8.5 Ca-Hectorite, pH 8.5 Equilibrium AcNCOOH Concentration (ptmol L‘ ) Figure 3.6 Sorption isotherms for AcNCOOH-cIay suspensions. 81 NHC Clay Exchangeable pH Amount sorbed Spectral ion as percent of shift (nm) CEC (%) AcN Hectorite Na 4.5 184 7.5 8.5 96 <2 Ca 4.5 140 7.8 8.5 92 <2 Saponite Na 4.5 183 3.9 8.5 95 <2 Ca 4.5 141 3.9 8.5 91 <2 AcNCOOH Hectorite Na 4.5 52 <2 8.5 8 <2 Ca 4.5 31 <2 8.5 6 <2 Saponite Na 4.5 26 <2 8.5 3 <2 Ca 4.5 ND <2 8.5 ND <2 (Note: ND = Not determined). Table 3.1 Amount of NHC sorbed and shifts in absorbance spectra obtained in presence of clays. 82 This difference in sorption is due to the presence of negative charge on the AcNCOOH molecules at both pH 4.5 and 8.5. At pH 4.5, AcNCOOH existed as a zwitterion, and at pH 8.5, AcNCOOH existed as anionic acridine-carboxylate ions. The negative charge on the AcNCOOH molecules at the pH-conditions used in this study caused the negatively charged clay surfaces to repel these molecules more than the AcN molecules. Therefore, the charge of the sorbate molecules dictated their degree of sorption on clays. Additionally, the projecting carboxylate groups on the AcNCOOH molecules might have restricted the sorption of these molecules on clays by causing steric hindrance. The presence of the carboxylate group on the AcNCOOH molecules also reduced the flexibility of the molecules and thereby reduced their chances of attaching to sorption sites on the clay surfaces. The nature of clay-organic interactions on clay surfaces and the state of the sorbed molecules can be evaluated from the shapes of the sorption isotherms. Based on the classification scheme outlined by Giles etal. (1960), all the sorption isotherms, except for those of AcN at low pH, are L-shaped. The sorption isotherms obtained for AcN at low pH are S-shaped. The difference between S-type and L-type isotherms lies in the nature of the initial slopes of the isotherms. The initial slopes for S-type isotherms are concave upwards, indicating that sorption increases with increase in amount sorbed. The initial slopes of L-type isotherms are convex upwards, indicating that as more solute molecules are sorbed, there is less chance for a new molecule to find a sorption site (Giles et al., I960). 83 The S-type isotherms obtained for AcN at low pH suggests cooperative interactions among the sorbed organic species, which increases the affinity of the surface for other sorbate molecules (Giles et al., 1960; Sposito, 1984). S-type isotherms also indicate clustering of sorbates rather than random surface mixing (Sposito, 1984). Similar types of isotherms obtained by Ainsworth et al. (1987) for sorption of quinoline on montmorillonite were attributed to sorbate-assisted interactions on the montmorillonite surfaces, leading to molecular aggregation. Homenauth and McBride (1994) also found that sorption of aniline, another NHC, on clays produced S-type isotherms. In our case, at low pH, AcNH^ ions were sorbed readily on the negatively charged clay surfaces. The uncharged end of the sorbed AcN molecules provided another surface for the sorption of neutral molecules and/or more AcNH^ ions, and this resulted in cooperative interactions between sorbed molecules, van Olphen (1977) illustrated that the uncharged end of a second organic cation can physically attach to the uncharged end of the sorbed molecule by van der Waals forces. Irrespective of the type, all the isotherms reached a plateau with increase in the amounts of NHC sorbed. Such a plateau is indicative of monolayer formation (Giles et al., 1960). However, it should be pointed out that formation of a “monolayer” does not necessarily indicate complete coverage of a clay surface (Sposito, 1984). Further increase in sorption with increasing concentration may indicate the formation of aggregates of sorbed molecules. Additional evidence for the formation of multilayers by AcN molecules on the clay surfaces at low pH was sorption in excess of the CEC of the clays 84 (Table 3.1). However, sorption of NHCs at surface concentrations < the CEC does not preclude the formation of aggregates. Considering the maximum amounts of NHCs sorbed on any clay (Figures 3.5 and 3.6), it was found that more NHCs were sorbed on hectorite than on saponite at all pH- conditions, and this is probably due to the difference in isomorphic substitution in hectorite and saponite. The tetrahedral charge in saponite makes the surface more hydrophilic due to the increased hydrogen bonding between water molecules and oxygens of the silicate layers, and the hydrophilicity of the clay surface decreases the chances of replacement of sorbed water by incoming organic molecules. Sposito and Prost (1982) have pointed out that hydrogen bonding between adsorbed water molecules and the silicate layer is stronger when the primary negative charge is located on the tetrahedral sheet (saponite) rather than the octahedral sheet (hectorite). Though the maximum amounts of NHCs sorbed on hectorite were higher than on saponite, sorption isotherms of AcN-clay suspensions indicated that more AcN molecules were required to form a monolayer on saponite. This might be due to the presence of higher negative charge density on the saponite surface, and that allowed more AcN molecules to be sorbed directly on the clay surface. Finally, on comparing the amounts of NHCs sorbed on Na- and Ca-exchanged clays, it was found that, in general, more organic molecules were sorbed on Na- exchanged clays than on Ca-exchanged clays. This is because the electrostatic bond strength in the case of Na^ is two-thirds of that of Ca^*, though the ionic radii of Na"^ and Ca"^ ions are similar (McBride, 1994). Therefore, Na"^ ions are more readily displaced by 85 organic cations than are ions. Moreover, the hydration shell around Na"^ is more easily disrupted than the shell around Ca^^ by the organic cations. Xu and Boyd (1995) have also observed higher sorption of hexadecyltrimethylammonium bromide (HDTMA), a cationic surfactant, on Na-exchanged subsoil samples than on Ca-exchanged subsoils. They attributed this phenomenon to a higher degree of dispersion of Na-exchanged subsoils, thus offering more exchange sites to the sorbate molecules. 3.4.3 UV-visible spectra The UV-visible spectra for aqueous solutions of AcN and AcNCOOH were studied at both high and low pH conditions, and also at different concentrations. The absorption maximum (Xmax) of aqueous solutions of AcN (pH 4.5) was found to be at 353 nm, while Xmax of the aqueous solutions of AcNCOOH (pH 4.5) was at 354 nm. In both cases, other peaks of lesser intensities were also observed. Though the intensities of the absorbance spectra of both AcN and AcNCOOH increased with increases in concentration of the chromophores in aqueous solution, there was no change in the position of kmax- This indicates the absence of large quantities of aggregates of these molecules in solution. The UV-visible spectra of AcN and AcNCOOH were also studied in the presence of different clays. Though increases in the concentrations of AcN and AcNCOOH did not produce any shifts in the positions of Xmax in aqueous solutions, the Xmax shifted towards shorter wavelengths (hypsochromic shift or blue shift) in the presence of clays. 8 6 Figure 3.7 displays the shifts occurring in the UV-visible spectra of AcN in the presence of Na-hectorite at pH 4.5 with increase in concentration of AcN from 6 to 195 pmol L *. 2.5 [Acridine] ^max (pmol L*) (nm) a: 6 356.9 b: 28 353.9 2.0 - c; 56 350.6 d: 84 350.6 e: 140 350.3 f: 195 349.4 1.5 - I JS X) < 0.5 - 300 325 350 375 400 425 450 Wavelength (nm) Figure 3.7 Absorbance spectra of AcN in the presence of Na-hectorite at pH 4.5. 87 Such metachromic shifts occur when the principal absorption band is replaced by another band of higher energy. This effect can be due to interactions between the lone electron pair of oxygen atoms in the silicate layer with the tc-electrons of the organic molecules (Yariv and Lurie, 1971; Cohen and Yariv, 1984) and/or to aggregation of the organic molecules when sorbed on the clay surfaces (Grauer et al., 1984; Cenens and Schoonheydt, 1988; Tapia-Estévez etal., 1993; Tapia-Estévez etal., 1994). Yariv (1988) has also indicated that metachromic behavior due to Tt-interactions between organic molecules and silicate layers occurs primarily with tetrahedrally-substituted clays rather than with octahedrally-substituted clays, as tetrahedral substitution increases the basic strength and the donating capability of the lone pair of electrons of the oxygen atoms in the silicate layer. In the present case, spectral shifts were observed with both octahedral 1 y and tetrahedrally-substituted clays (hectorite and saponite), indicating that the primary cause of the metachromic behavior of AcN-clay suspensions was the formation of aggregates of AcNs on the clays. This conclusion is also supported by previous observations, which are: (i) nonlinear shape of the sorption isotherms; and (ii) sorption in excess of the CEC of the clays considered. The spectral shifts of magnitude > 2 nm were observed only with AcN-clay suspensions at low pH-conditions (Table 3.1). The amounts of AcN sorbed at high pH and the amounts of AcNCOOH sorbed on clays at all pH-values were low, and so there were, probably, insufficient amounts of sorbed molecules available to form surface aggregates, which could contribute to appreciable spectral shifts. Spectral shifts for sorbed AcN at low pH on hectorite were almost twice as great as those observed for 8 8 saponite (Table 3.1). This was presumably due to more aggregation among sorbed molecules in the case of hectorite, as more neutral molecules can sorb on hectorite than on saponite at low pH. Further discussion on this is provided in the next section. We have also seen from the sorption isotherms that sorption on hectorite was greater than on saponite. Again it is apparent that any metachromasy due to 7t-interactions is not significant with the clay-organic systems considered in the present study. No significant difference was observed between the Na-exchanged and the Ca-exchanged clays, indicating that the location of isomorphic substitution in the clays was more important in the formation of aggregates than the type of exchangeable cations. 3.4.4 Fluorescence spectra Even though cationic forms of NHC molecules were preferentially sorbed on clay surfaces, neutral molecules and anions also participated in the sorption process (Figures 3.5 and 3.6). Therefore, it is expected that more than one species of these compounds should be present on the clay surfaces, and this is demonstrated by the characteristic emission spectra of different clay-organic suspensions. On studying the emission spectra of different concentrations of AcN in suspension with Na-saponite at pH 4.5 and 8.5 (Figures 3.8 and 3.9), it was found that though other peaks of lesser importance existed, the most prominent peak at pH 4.5 occurred at 478 nm, while at pH 8.5 the X^ax was at 437 nm. 89 7 pH = 4.5 [Acridine] (p,mol L ) 6 140 5 195 § s 4 uI 3 § tu 2 1 0 = 400 450 475 500 525 550575 600425 Emission wavelength (nm) Figure 3.8 Emission spectra of AcN-(Na)-saponite suspensions at pH 4. 90 3.0 pH = 8.5 [Acridine] (|imoI L ) 2.5 140 195 2.0 .?c 3 5 1.5 o s o 3 tu 0.5 0.0 400 425 450 500475 525 550 575 600 Emission wavelength (nm) Figure 3.9 Emission spectra of AcN-(Na)-saponite suspensions at pH 8.5. 91 The position of X,max indicated that at low pH the primary species in the system, including both the bulk solution phase and the clay surfaces, was AcNH^. The negatively-charged tetrahedral sheet of saponite preferentially attracted the cationic species at low pH. At low pH, the Xmax of the supernatant also indicated the presence of AcNH^ ions. At high pH (Figure 3.9), the position of the Xmax at 437 nm indicates that the primary species was the neutral AcN molecule, which was also the primary species in solution at pH 8.5. Sorption of nonionic molecules occur primarily on the vicinal water layer, which is the layer of oriented water molecules on the mineral surface, and the thickness of this layer is -100 nm(Drost-Hansen, 1969). The transfer of the organic molecules from the bulk water phase to the vicinal water layer is favored (Schwarzenbach et al. 1993). The partitioning of the organic molecules into the “structurally oriented” water layer is entropically favorable as this releases the water molecules that were forming a “cage like” structure around the organic molecule in the bulk phase. The emission spectra of AcN in the presence of Na-hectorite (Figures 3.10 and 3.11) were significantly different from those obtained in the presence of Na-saponite. In the presence of Na-hectorite, a composite peak at 450 nm was obtained at both low and high pH. 92 pH = 4.5 [Acridine] (pmol L ) 7 a = 42 b = 65 c = 83 6 d = 99 f= 122 5 3 oi 3 2 0 >- 400 425 450 475 500 525 550 Emission wavelength (nm) Figure 3.10 Emission spectra of AcN-(Na)-hectorite suspensions at pH 4.5. 93 pH = 8.5 [Acridine] (jimol L ) 7 6 d= 122 5 3 I i 4 8 8 3 i 3 2 0 400 425 450 475 500 525 Emission wavelength (nm) Figure 3.11 Emission spectra of AcN-(Na)-hectorite suspensions at pH 8.5. 94 The X,max at 450 nm was due to the coexistence of both ionic and neutral species on the clay surface, and suggests that the absence of negative charge from the tetrahedral sheet in hectorite was conducive to the sorption of both neutral AcN molecules and AcNH* ions on the clay surface. Sorption of neutral AcN molecules on Na-hectorite surfaces can be due to: (i) direct sorption of neutral molecules; (ii) sorption of neutral molecules on molecules that are already sorbed; and (iii) sorption of neutral molecules after becoming protonated at the clay surface by a cation exchange mechanism. Evidence of surface protonation of the organic molecules will be presented in the subsequent paragraphs. Sorption of neutral molecules either on sorbed molecules or directly on clay surfaces is due to hydrophobic interactions (Zachara et al., 1986; Xu and Boyd, 1995), as the organic molecules and the clay surfaces are relatively nonpolar when compared to water molecules. Researchers (Mortland, 1970; Farmer, 1971) have indicated that uncharged organic molecules can sorb on clay surfaces by directly coordinating with the exchangeable cation and/or by indirectly attaching to the exchangeable cations by means of water molecules. We have also seen that neutral AcN molecules have sorbed on clay surfaces at high pH and high concentrations of AcN. This sorption can not fully be explained by surface protonation of AcN molecules and their subsequent sorption by a cation exchange mechanism because surface protonation is evident primarily at low concentrations. Also, attachment of a neutral molecule to the uncharged end of a sorbed molecule can lead to aggregation of sorbed molecules. An appreciable spectral shifts were not observed at high pH- conditions that would have accounted for aggregation of neutral species on sorbed 95 molecules. Therefore, it can be concluded that a significant portion of the sorption of these neutral molecules must have occurred directly on the clay surfaces. At low concentrations of AcN in the presence of Na-hectorite (Figures 3.10 and 3.11), spectral peaks at both 478 nm and 450 nm were prominent at pH 4.5 and 8.5. At low pH, AcNH*" ions were the primary species in the bulk phase, and it is expected that more AcNH^ ions would sorb on the clay surfaces. However, even at this low pH, neutral molecules were present in the system (92.6% AcNHT and 7.4% AcN at pH 4.5) which could have sorbed on the clay surface, and this is evident from the peak at 450 nm. With increase in concentration, the peak height at 450 nm increased due to an increase in the neutral molecules in solution, and a corresponding increase in the amount of neutral molecules in the sorbed state. At pH 8.5 and at low concentrations, the peak at 478 nm is a definite indication of the presence of AcNFT ions. This is in spite of the fact that AcNH^ ions were virtually absent in solution at pH 8.5 (99.8% AcN and 0.2% AcNH"^). Clearly, reaction with the clay surfaces have promoted the protonation of AcN to AcNH"^ ions. The protonation of organic bases by clay surfaces can be due to specific Br0nsted acid sites at the surfaces, where hydrated clay surfaces donate protons to weak bases forming a positively charged ion. Traina and Onken (1991) have shown that quinoline molecules were protonated when sorbed on hectorite surfaces in the pH range of 2.97 to 8.82. Evidence of surface protonation of organic molecules was also provided by Cenens and Schoonheydt (1988) when they studied the sorption of methylene blue on hectorite, laponite, and barasym. The difference between the emission spectra of AcN in the presence of Na-hectorite and Na-saponite can be explained by the fact that a more 96 negative octahedral sheet (as in hectorite) has a greater tendency to protonate (Mortland and Raman, 1968; Yariv and Heller, 1970; Yariv and Lurie, 1971). Opposing forces due to hydrogen bonds between the hydrogens of the water molecules and the oxygens of the clay silicate layers, and the bonds between the oxygens of the water molecules and adsorbed cations release hydrogen ions from water molecules, followed by protonation of the organic molecules. The weaker hydrogen bonds between hydrogens of water molecules and oxygens of the silicate layers in hectorite, where the negative charge resides on the octahedral sheet, facilitates the breaking of bonds and the release of hydrogen ions. Therefore, octahedrally-substituted hectorite has a higher protonating capability than tetrahedrally-substituted saponite, and this was responsible for the more prominent at 450 nm in the presence of Na-hectorite at pH 8.5. Surface protonation is evident primarily at low concentrations (Figure 3.11). The intensity of the peak at 478 nm decreased drastically with increase in concentration of AcN as the protonated species is not the favored form at pH 8.5. Also, due to the predominance of neutral AcN molecules at this pH, the intensity of the peak at 437 nm increased with an increase in the concentration of AcN in solution. Tapia Estevez et al. (1994) and Lopez Arbeloa et al. (1995) have reported blue shifts in the emission spectra of rhodamine 6G in the presence of saponite with increase in dye / clay concentrations. They found that the spectra shifted from a A.max of 560 nm for intermediate loadings to a of 544 nm for high loadings of rhodamine 6G. This shift towards shorter wavelengths was attributed to the presence of the monomer on both external and' internal surfaces of the clays, and to the possibility that dye molecules 97 located on different surfaces emit at different wavelengths. Similar spectral shifts were evident in the present study (Figures 3.10). In our case, XRD measurements for hectorite and saponite (results will be presented later) saturated with AcN indicate that more molecules have penetrated the interlayer space of hectorite than saponite, and this could have resulted in the appreciable blue shift in AcN-hectorite suspensions compared to that in AcN-saponite suspensions. Lakowicz (1986) had also indicated that a blue shift in the emission spectra of a fluorophore bound to a macromolecule can be caused by emission from unrelaxed fluorophores, typically due to a strong interaction between the fluorophore and its surrounding environment. Therefore, emission from unrelaxed AcN molecules, sorbed on clay surfaces, can result in blue shifts in the emission spectra. Thus the blue shift in the present data can be due to: (i) sorption of different ionic forms of AcN on Na-hectorite; (ii) sorption of AcN at both external and internal surfaces of Na- hectorite; and (iii) emission from unrelaxed fluorophores. The specific cause can not be conclusively determined at this time. Quenching of the fluorescence emission spectra of AcN-hectorite suspensions was observed at low pH (Figure 3.10). The intensity of the emission spectra increased with increase in the concentration of AcN, until it reached 99 pmol L''. At a concentration between 99 p.mol L'* and 111 jimol L'*, the trend reversed, and the intensity decreased with further increase in concentrations of AcN. This quenching of the emission spectra, occurring at higher concentrations of AcN, might be due to the formation of aggregates of sorbed AcN molecules. No quenching was observed for AcN-hectorite suspensions at 98 high pH, nor for AcN-saponite suspensions at either pH. Similar observations were made with Ca-exchanged clays. The Xanax of the emission spectra of AcNCOOH-(Na)-hectorite suspension at pH 4.5 (Figure 3.12) was at 436 nm. 4.5 436 [AcNCOOH] (pmol L'‘) 4.0 a: 22 b: 45 c: 67 3.5 pH = 4.5 i 3.0 C3 2.5 w 2 o 3 u. l.O 0.5 400 425 450 475 500 525 550 Emission wavelength (nm) Figure 3.12 Emission spectra of AcNCOOH-(Na)-hectorite suspensions at pH 4.5. 99 This contrasts with the solution spectra of AcNCOOH which exhibits strong emission peaks at 461 nm and 478 nm when the pH was 4.5, and a single prominent peak at 436 nm at pH 8.5 (Figure 3.4). At pH 4.5, the spectra of the AcNCOOH-(Na)-hectorite suspensions has shifted towards shorter wavelengths, coincident with the Xmax of AcNCOOH solution at pH 8.5 (Figure 3.12). Such a shift can be explained by analogy with the blue shifts discussed by Lakowicz (1986) for sorption of proteins on different substrates. Emission spectra of melittin, which is a small amphipathic peptide and which self-associates in solution, experiences a blue shift with increase in concentration, and also in contact with lipid bilayers. Such a blue shift might be due to the shielding of the fluorophore in the sorption site from contact with water due to self-association and/or sorption on lipid bilayers. By analogy, as the zwitterionic AcNCOOH attached to the clay surface, the positive end of the molecule could have been shielded from water. Moreover, Figure 3.4 has already shown that the relative intensity of the double peaks of zwitterions, obtained at pH 4.5, is significantly less than the peak due to the presence of the anionic species at pH 8.5. Therefore, even a small degree of quenching of the fluorophore due to its sorption on clays at pH 4.5, would make the peak due to the presence of zwitterion in solution insignificant when compared to that due to the anion. Since a much larger percent of the zwitterions were sorbed on the clay surfaces, the resultant peak at 436 nm might be due to the anions left in the system. There was no change in the emission spectra of AcNCOOH at pH 8.4, with or without clay. The obtained at high pH, with and without clay, was at 436 nm, which indicates the presence of AcNCOO' in both cases. Sorption of AcNCOO' could have occurred on the edges of 100 the clay particles which may bear a positive charge due to the presence of uncoordinated metal ions (Si% Fe^^ at the broken edges, kinks or holes on the surface of clay minerals (van Olphen, 1977). The uncoordinated metal ions may react with water molecules to form surface OH groups that can form inner-sphere complexes (Johnston, 1996). Similar behavior was also observed with AcNCOOH-saponite suspensions at both pH-conditions. 3.4.5 X-Ray Diffraction measurements XRD measurements were used to detect a change in d-spacings (Ad) due to sorption of organic compounds in the interlayer space of the reference clay minerals. The Ad depends on the size of the organic molecules, and also on their orientation and packing geometry in the interlayer space of the clays (Figures 3.13 and 3.14). In the absence of organic cations, the interlayer space is occupied by exchangeable cations (Na"^ or Ca"^ and their accompanying solvation shells. Sposito and Prost (1982) indicated that two or more layers of adsorbed water can occupy the interlayer space of smectites along with exchangeable cations. Suquet et al. (1975) have also reported that Na- and Ca- exchanged saponite form two-layer hydrates, and the thickness of the interlayer water for hectorite is greater than that of saponite. They have also shown that when the exchangeable cation in Ca-saponite was associated with two water molecules, the thickness of the interlayer space occupied by them was 2.3 Â. These water molecules were displaced by incoming organic molecules. At low surface concentrations, the AcN molecules were, probably, parallel to the clay surface, after displacing the exchangeable 101 cations and their associated water molecules, because of the planarity of the AcN molecules. The parallel position of the AcN molecules could also be due to the absence of negative charge, which would have promoted tt-interactions between AcN and the oxygen plane of the silicate sheet (Yariv, 1992). As a result of this horizontal position, a dip occurred in the plot of d-spacings versus AcN concentrations (Figure 3.13). 17 • 16 o ■ ■ a A O □ 15 □ • ▲ § 14 0 a. «0 1 É □ • 13 8 A d (Â ) s • Na-hectorite (pH 4.5) 4.42 ▲ o ▲ Na-hectorite (pH 7) 3.66 ■ Ca-hectorite (pH 4.5) 2.22 # • 0 Na-saponite (pH 4.5) 2.98 • □ Ca-saponite (pH 4.5) 2.32 1 1 1 1 11 1 0 50 100 150 200 [A cridine] (nm ol L" ) Figure 3.13 Change in d-spacings with increase in concentrations of AcN. 102 The maximum Ad at an AcN concentration of 195 iimol L'‘ for different clays ranged between 2.22 Â and 4.42 Â (Figure 3.13). It is possible that the AcN molecules could have formed bilayers in the interlayer space or could have also shifted from a horizontal to a diagonal position with increase in packing density in the interlayer space. Earlier, spectroscopic studies have also shown that AcN molecules formed aggregates on the clay surfaces. Similar results were also obtained by Cohen and Yariv (1984) and Grauer et al. (1987) when they studied the sorption of xanthene dye, pyronin Y, on clays. The Ad obtained for Na-hectorite at pH 7 was rather high (3.66 Â) though spectroscopic studies did not show a high degree of sorption. This high value of Ad might be due to the existence of AcN molecules in a solvated state (Helmy et al, 1983) which occupied more space. The change in Ad was greater for Na-hectorite (4.42 Â) than for Na-saponite (2.98 Â) at pH 4.5 (Figure 3.13). Earlier, it was also found from the sorption isotherms that sorption on hectorite was higher, and this could have resulted in more sorbed molecules in the interlayer space. The larger values of Ad in hectorite are consistent with this observation. Moreover, the lower Ad-values in saponite are indicative of their lower swelling potential. Tetrahedral charge dominates in saponite, and researchers (Newmann, 1987; Lopez Arbeloa et al., 1995) have shown that clays with dominantly tetrahedral charge (saponite) have a lower swelling capacity than octahedrally substituted clays (hectorite) as the tetrahedral substitution in clays favors the hydrogen bonding between water molecules and the oxygens of the clay (Yariv, 1988; 1992). Also, the Ad-values for Na-exchanged clays were found to be greater than those of Ca- 103 exchanged clays. Correspondingly, it was seen earlier that the amounts of AcN sorbed on Na-exchanged clays were higher. The d-spacings of Na-hectorite with AcNCOOH at both high and low pH- conditions were also measured (Figure 3.14). 15.0 p H = 4 .5 14.8 € « •0 14.6 -e # # e g. # 14.4 1 1 1 1 1 1 1 1 1 , 14.2 1 0 5 10 15 20 25 30 35 40 45 50 55 15 p H = 8.5 14 € o o O ■oÎ" o O 12 O 11 1. . . 1 1 I I 1 1 1 0 1 2 3 4 5 6 7 Amount of acridine-9-carboxylic acid sorbed (cmol/kg) Figure 3.14 Change in d-spacings with increase in concentrations of AcNCOOH. 104 At pH 4.5, the d-spacings remained almost constant until the amount of sorbed AcNCOOH exceeded 25 cmol kg ', and then the d-spacings decreased with increase in the amount of AcNCOOH sorbed. At pH 4.5, AcNCOOH existed as a zwitterion, and even though the zwitterions entered the interlayer space of the clay minerals at low concentrations, the molecules were likely to be far apart from each other. The high Ad at low concentrations can be due to the repulsive forces between the negative end of the sorbed AcNCOOH molecule and the negatively charged clay surface. Also, Tt- interactions between the AcNCOOH molecules and the silicate layers were unlikely because of the presence of negative charge on the AcNCOOH molecule, and this might force the AcNCOOH molecules to orient in a vertically tilted position, rather than in a horizontal position as was observed with AcN molecules. As the concentrations of AcNCOOH increased, more and more molecules entered the interlayer space. With increased packing density, the negative charge of one AcNCOOH molecule could become shielded by the positive charge of an adjacent AcNCOOH molecule. This could have reduced the repulsive force, which in turn could have reduced the d-spacings with increase in the surface concentration of AcNCOOH. At pH 8.5, AcNCOOH existed as an anion, which resulted in low sorption and small Ad. The decrease in d-spacings with increase in concentration of AcNCOOH, after an initial increase, was probably due to a reduction in the repulsive force between the anions and the negatively-charged clay surface, and this could have been due to shielding of the negative charge on the molecules with increase in packing density. 105 3.4.6 Thermogravimetric analyses The primary purpose for conducting the thermogravimetric analyses was to obtain corroboration for the results obtained with XRD measurements. Three regions were identified in the thermograms, and the weight loss in these three regions are indicated as the percent change in weight with increase in temperature during the analyses (Table 3.2). Clay pH Cone. AcN Percent weight loss in the tem perature range (ppm) 90-200°C 200-585°C 585-850°C Na-hectorite 4.5 0 5.11 4.09 3.54 5 0.47 2.25 1.44 10 0.36 3.33 1.62 25 0.47 6.13 1.70 35 0.41 8.86 7.72 Ca-hectorite 4.5 0 3.58 1.48 4.58 1 1.10 2.08 6.04 10 0.45 2.93 16.67 8.5 1 1.28 1.50 5.17 5 0.67 4.65 - 15 0.34 8.83 5.21 25 0.30 14.69 35 0.56 17.20 % wt. loss for a particular phase = (Initial wt. - Final wt.)/ (Starting wt of the sample) Table 3.2 Thermogravimetric Analyses of AcN-Clay Suspensions. 106 Water molecules, present on both external and internal surfaces of the clay particles, were removed when the samples were heated between 95°C to 200°C. The amount of water lost due to heating decreased with increased saturation of hectorite surfaces with AcN (Table 3.2). Clearly, the water molecules were displaced by AcN and this resulted in a change in d-spacings during XRD measurements and change in weight due to heating during thermogravimetric analyses. The second region (200° to 585°C) and the third region (585° to 850°C) might be due to oxidation of the organic cation and dehydroxylation of the clay respectively (Yariv et al. 1989). 3.5 Possible mechanism of clay organic interactions Both macroscopic and microscopic methods have been used to characterize the sorption of AcN and AcNCOOH at different ionic states on hectorite and saponite. Sorption of AcN was much higher than that of AcNCOOH. Hectorite was found to be a better sorbent than saponite, and sorption on Na-exchanged clays was higher than on Ca- exchanged clays. Therefore, the nature of both sorbent and sorbate influenced the amount of organic sorbed on clays. Sorption was also found to be pH-dependent, with maximum sorption occurring when the organic molecules existed in the protonated form. However, results have also shown that neutral molecules sorb on the clay surfaces or onto sorbed AcNH^ ions by hydrophobic interactions. Therefore, sorption of these NHC molecules takes place by more than one mechanism. Sorption by a cation exchange mechanism is the primary form of interaction at low pH and is responsible for most of the sorption of AcNs on clays. Evidence also exists that neutral AcN molecules can be protonated at the 107 clay surfaces, and the protonated AcN then sorbed by cation exchange. Anions can also sorb on clay edges by their short-range interactions with exchangeable cations. It was also found that the formation of aggregates is primarily due to hydrophobic interactions, as a neutral molecule can attach to the uncharged end of the sorbed molecule, and/or the uncharged end of another cation can attach to the uncharged end of the sorbed molecule. Sorbate molecules may also reside on the interlayer space of the expanding clay minerals. As the organic molecules enter the interlayer space, they replace the interlayer water molecules. These conclusions corroborate the inferences made by Helmy et al. (1983) and Cenens and Schoonheydt (1988) who have identified the steps leading to sorption of ionic organic compounds on clay surfaces as ion exchange, protonation, and aggregation. Considering the results obtained, the probable steps involved in sorption of AcN and AcNCOOH on clays at different pH have been postulated as follows: At low pH, the excess H^ ions in solution protonate the AcN molecules, whereas at high pH, the exchangeable cations along with the negative charge on the clay surface polarize the sorbed water molecules and the protons derived from this source protonate the AcN molecule (Eqs. 3.1 and 3.2). H2O H" + OH" (3.1), and, AcN + H^ AcNH^ (3.2). During sorption, the exchangeable cations of the clay mineral are replaced by the organic cations (AcNH^, as AcNH^ + Na-clay AcNH-clay + Na"^ (3.3). As explained earlier, neutral AcN molecules can also sorb on clay surfaces, as 108 AcN + AcNH-clay <-4 AcN-AcNH-clay (3.4), and AcN + Na-cIay AcN-[Na-clay] (3.5). Equation (3.4) also accounts for the formation of aggregates of AcN molecules on the clay surfaces due to sorption of neutral AcN molecules on sorbed molecules. Sorption of AcNCOOH at pH 4.5 takes place primarily by interaction between the positive charge of the zwitterion to the negatively charged clay surfaces, and the reactions are similar to equations (3.1) through (3.5). From the results obtained, it can be inferred that AcNs sorb on clays by both cation exchange and hydrophobic interaction mechanisms, and In the subsequent chapter, it is intended to quantify the contributions made by these two forces. 109 CHAPTER 4 THERMODYNAMIC STUDY OF SORPTION OF NITROGEN HETEROCYCLIC COMPOUNDS ON PHYLLOSILICATES 4.1 Abstract The presence of nitrogen heterocyclic compounds (NHCs) in the environment has drawn significant attention in recent years. The primary aim of this study is to gain an insight into the different forces that lead to sorption of NHCs on clays, as sorption dictates the fate of NHCs in the environment. For this purpose, the thermodynamics of sorption of acridine (AcN) and acridine-9-carboxylic acid (AcNCOOH) on clay minerals (hectorite, saponite, and muscovite) were studied. We have assumed that the total force, which leads to sorption of NHCs on clays, can be divided into two groups: electrostatic (EL) and hydrophobic (H). In order to determine the contributions made by these two groups, we have determined the AG (total change in free energy) and AG" (change in free energy due to hydrophobic interactions) for sorption of NHCs on clays, with the consideration that A G ^ (change in free energy due to electrostatic interactions) is the difference between AG and AG". AG was calculated from the overall sorption 110 equilibrium constant (K), which was determined from the sorption isotherms of the various clay-organic systems by using different sorption models. A thermodynamic equation was developed to determine the AG" from the different interfacial tensions involved in the system. The liquid-vapor interfacial tensions were determined experimentally by using the Maximum Bubble Pressure method, while solid-liquid and sol id-vapor interfacial tensions were determined by using two sets of semi-empirical equations. Contact angles between the liquid and the clay surfaces were measured by using the Axisymmetric Drop Shape Analysis method. Finally, microcalorimetric measurements were used to determine the changes in enthalpy (AH) occurring during the sorption of AcN on clays. Results obtained showed that there is considerable difference between the calculated values of K obtained with the different models. Based on the three-stage adsorption model, which was found to be most suitable, AO" is approximately half of AG. It was also found that electrostatic interactions are more dominant at low concentrations, while sorption due to hydrophobic interactions became significant with increase in concentration of the NHC in solution. 4.2 Introduction Knowledge of the behavior of clay-organic systems has wide applications, such as in the areas of colloid science, soil science, environmental science, heterogeneous catalysis, photochemical systems, petroleum technology, and others. The present study was undertaken to probe the sorption behavior of NHCs on clays. Significant portions of 111 the organic pollutants are NHCs, as these compounds are present in many everyday consumer products. The fate of these organic pollutants in the environment is dictated by their sorption on soil and subsurface materials, and therefore, to accurately predict solute migration in aqueous and soil environments, information on the mechanism of sorption of these compounds is necessary. NHCs are ionizable organic compounds. The partitioning of these compounds between water and soil is more complex than the partitioning of either neutral organic compounds or ionic inorganic compounds. Sorption of neutral, hydrophobic, organic compounds on soil is generally dominated by hydrophobic interactions (Jafvert et al., 1990), while sorption of inorganic cations on natural sorbents is driven primarily by electrostatic interactions. The complexity of the sorption of ionic organic compounds arises due to the presence of both polar and nonpolar forces. In this context, Domenico and Schwartz (1990) have indicated that hydrophobic sorption represents only part of the total sorption of ionic organic compounds. Electrostatic properties, such as surface charge, pH, and ionic medium, also significantly influence the sorption of organic cations and anions on soil. Though considerable amounts of studies have been conducted with inorganic compounds as well as nonpolar organic compounds, very few studies have been conducted on the sorption of ionic organic compounds on soil and subsurface materials. In the present study, we examine the complex nature of the sorption of two ionic, organic, NHCs (AcN and AcNCOOH) on well-characterized clays. Earlier studies (DellaGuardia and Thomas, 1983; Cohen and Yariv, 1984; Zachara et al., 1986, 1987; Cenens and Schoonheydt, 1988; Traina and Onken, 1991; Tapia 112 Estevez et al., 1993) have shown that NHCs, such as pyrene, acridine orange, quinoline, methylene blue, rhodamine 60, and others, are readily sorbed on clay mineral surfaces and subsoil materials containing little or no organic matter. Some preliminary postulates have also been made regarding the mechanism of sorption of some NHCs on clays and soils. According to Ainsworth et al. (1987), sorption of quinoline on montmorillonite results from ion exchange of the protonated species of the heterocyclic compound. Other studies (Cohen and Yariv, 1984; Cenens and Schoonheydt, 1988) have also shown that cationic dyes, such as acridine orange and methylene blue, are sorbed on clays by the mechanism of cation exchange. Earlier (Chapter 3), we have found that AcN and AcNCOOH have considerable affinities for clays (hectorite and saponite). Sorption of these compounds on clays was pH-dependent, and was also dependent on the ionic state of the sorbate molecules. Among the different forms of the NHC molecules that could sorb on the clay surfaces, the cationic form of the sorbate molecules was preferred by the negatively-charged clay surfaces. Spectroscopic studies have also indicated that sorbed NHCs form aggregates on clay. Aggregation can occur when a neutral molecule attaches to a sorbed molecule and/or when the uncharged end of an organic cation attaches to the uncharged end of a sorbed molecule (van Olphen, 1977). Though sorption of the cationic NHCs occur by electrostatic interactions between the sorbate and the sorbent, formation of aggregates definitely indicates the existence of forces other than electrostatic interactions, even at low pH-conditions. The primary cause of sorption of neutral molecules have been identified as hydrophobic interactions (van Oss, 1993). Therefore, it was found that both hydrophobic and electrostatic forces were present during sorption of 113 AcNs on clays at all pH-conditions. In order to provide a complete explanation of our earlier results, it is required to quantify the contributions made by hydrophobic and electrostatic interactions during the sorption of NHCs on clays. Such an understanding will eventually help in predicting the fate of NHCs in the environment. 4.3 Theoretical Background 4.3.1 Forces participating during sorption Studies (van Oss et al., 1988; Costanzo et ai, 1990; van Oss et al., 1990; Israelachvili, 1991; van Oss, 1993) have indicated that sorption of solutes on solid surfaces is dependent on various forces, such as hydrogen bridges, steric or orientation effects, hydration effects, hydrophobic effects, attractive London dispersion forces, repulsive Lennard-Jones forces, and van der Waals forces. Summarizing all the forces, Costanzo et al. (1990) and van Oss (1993) have divided the total force leading to sorption of solute on sorbents into three major groups: (i) electrostatic (EL) interactions, (ii) Lifshitz-van der Waals electrodynamic forces (LW), which comprises of van-der Waals- Keesom or orientation forces, van der Waals-Debye or induction forces, and van der Waals-London or dispersion forces, and (iii) polar forces or acid-base interactions (AB). Expressing the total force in terms of change in free energy, the overall AG is: AG = AG AG + AG^® (4.1 ). This is in contrast to the classical DLVO theory, which predicts that the total force is the summation of the repulsive electrostatic forces and the attractive van der Waals forces. 114 van Oss et al. (1988, 1990) have indicated the existence of additional forces, which were grouped together under the name of Lewis acid-base (AB) interactions or electron- acceptor-electron-donor interactions. Hydrogen-bonding interactions and interactions of TC-electrons are subsets of AB forces (van Oss, 1993). Both and AG'^® are dependent on the interfacial tensions (y), as AG''^ is a function of and is a function of y^, where is the apolar component and y^® is the polar component of interfacial tension. It should be noted that y, the surface tension or the interfacial tension, is the summation of and y ^ . Based on this consideration, we have grouped AG^''^ and AG'^® as AG", where AG" is the AG due to hydrophobic interactions. The applicability of the nomenclature comes from the fact that AG" is the fraction of the total AG that is due to the surface hydrophobicities, which are determined by the interfacial tensions between any two phases. Similar conclusions were also obtained by van Oss (1993). Therefore, equation (4.1) can be re-written as: F L H AG = AG + AG (4.2). Tanford (1980) have defined hydrophobic interaction or hydrophobic effect as the tendency of the nonpolar particles to form aggregates in water. This effect is due to the fact that the attraction between two or more water molecules is more than the attraction between the water molecules and the nonpolar organic molecules, and this causes the nonpolar molecules to move towards each other. During the sorption of NHC molecules on clays, the NHC molecules, which are of low polarity, moves from the bulk aqueous phase towards the surface of the clay, and hydrophobic interactions is definitely 115 responsible for this phenomena. Pashley and Israelachvili (1981) have also suggested that besides the double layer and van der Waals forces present during sorption of organic compounds on mica surfaces, an additional attractive force is present between the hydrophobic surfaces in the solution, and this additional forces is due to the hydrophobic effect. Similar type of distinction was also made by Hato (1996), who indicated that the long-ranged attractive forces acting between two macroscopic bodies in an aqueous medium can be divided into two distinct components: electrostatic and hydrophobic. Presently, we intend to quantify the contributions made by the electrostatic and hydrophobic interactions during sorption of AcNs on clays. The relative importance of the electrostatic and hydrophobic interactions during any sorption process, however, depend on the physical and chemical properties of the surfaces of the sorbents and the sorbates (such as, presence of specific chemical groups, degree of hydrophobicity, electrostatic potential, charge density) along with prevailing chemical conditions. When both the clay surface and the NHC are hydrophobic in nature, sorption will be guided primarily by nonpolar forces. Increase in surface charge, which is accompanied by an increase in polarity of the participating components, will increase the contributions of the electrostatic forces in sorption. A schematic representation of the forces present during sorption of AcN on clays is shown in Figure 4.1 116 AG^2 AG"2 'A G “ l Figure 4.1 Schematic representation of sorption of AcN on clays. 4.3.2 Determination of K and AG In order to determine the contributions made by hydrophobic and electrostatic forces during sorption of NHCs on clays, we have calculated AG and AC". The AG were calculated from the overall equilibrium constant (K) for sorption of NHCs on clays, as: AG = -R TlnK (4.3). The shape of the sorption isotherms have been used to calculate the overall K for various clay-NHC systems. Sorption isotherms are plots of amounts of NHC sorbed per unit weight of clay as a function of the equilibrium concentrations of NHC in solution, and the 117 K determined from these plots will not be dependent on any assumption made regarding the mechanism of sorption. Among the different models available, four models have been compared, to obtain the most probable value of K. These selected models do not presume the mechanism of sorption while calculating the values of K. Also, mathematical simplicity of these models, and capability of these models to determine K with minimum error were the guiding factors for their selection. The selected models are: (i) Biggar equation (Biggar and Cheung, 1973); (ii) modified Langmuir-type equation (Cotton and Wilkinson, 1980; Zhu and Gu, 1989); (iii) two-stage adsorption model (Zhu and Gu, 1989); and (iv) three-stage adsorption model (Klimenko, 1978, 1979; Clunie and Ingram, 1983). Following are the brief descriptions of the above models. Biggar and Cheung (1973) suggested that the K for sorption of solutes on solid surfaces is the ratio of the amount of solute sorbed per unit volume of solvent in contact with the adsorbent surface (Cs) and equilibrium concentration of solute (Ce): K = - ^ (4.4). Ce The value of Cs can be calculated as: (p,/M|)A |_ (4,3) s A2 N(x / m) M2 X10^ where pi is the density of solvent (g mL"'), Mi and M 2 are the molecular weights of the solvent and the solute, Ai and A 2 are the cross-sectional areas (cm^ molecule ') of the solvent and the solute molecules, s is the surface area of the adsorbent (cm" g '), N is the 118 Avogadro’s number, and (x/m) is the specific adsorption (p.g g*'). This equation was also used by Moreale and van Blade! (1979) to calculate the equilibrium constants for sorption of aniline and p-chloroaniline on different soils, and by Zachara et al. (1986) to calculate the K for the sorption of quinoline on different types of soils. The modified Langmuir-type equation can account for multilayer adsorption and surface heterogeneity by introducing a “n”-factor, and can be written as: KC" “ '^max (4.6). ^l + K C"/ where t is the amount of solute sorbed at any concentration (C), T^ax is the maximum amount of solute sorbed, and K is the equilibrium constant or the affinity parameter from which the AG for the process can be determined. The “n”-factor can be termed as the degree of cooperativity of the process, or the aggregation number, which is the average number of molecules found in each aggregate formed by the sorbed solutes. Thus, “n” accounts for multilayer coverage. Cotton and Wilkinson (1980) have used the modified Langmuir-type equation to describe the cooperative binding of molecular O? by myoglobin. The two-stage adsorption model proposed by Zhu and Gu (1989) assumes that sorption takes place in two stages: (i) sorption of individual ions or molecules on sorption sites; (ii) attachment of molecules from the solution to the sorbed molecules on the sorption sites. Therefore, this model accounts for formation of aggregates of sorbed molecules, and the assumption that sorption proceeds by both electrostatic and hydrophobic forces is inherent in the model, which can be expressed as: 119 '^max^iCC—+ k2C" ^ T = ■ (4.7), I + kiC(l + k2C"“ b where k| and ka are the equilibrium constants for two-stages of sorption, and the overall equilibrium constant (K) is the product of ki and ki. Besides considering the aggregation of the sorbed molecules, the three-stage adsorption model also accounts for the different orientations of the solutes on the sorbent surfaces (Klimenko, 1978, 1979). The orientation of the sorbate molecules changes with surface concentration. At low surface concentrations, the sorbate molecules lie flat on the sorbent surfaces. As the packing density increases, sorbate-sorbate interactions increases causing the molecules to become vertically tilted and/or form multilayers. Study of the sorption of AcN and AcNCOOH have shown that these molecules might change orientation with increase in the amount of AcNs sorbed and/or form aggregates with increase in concentration of the NHCs in solution (Chapter 3). For the first stage, the sorption equation is: %1 CKa = fa' (48% x * -x 1 - ^ . ^2. K2T and fa'= exp (4.9), x * -x t ’ where Ka is a constant for adsorbate-adsorbent interaction, K% is a constant for adsorbaie- adsorbate interaction, a\ and a2 are the cross-sectional areas of the solvent and sorbate 120 molecules, T is the amount of organic sorbed, and C is the equilibrium concentration of the organic in solution. The t * has been considered to be either the cation exchange capacity (CEC) of the sorbent (clay) where sorption is in excess of CEC, or the maximum amount sorbed where sorption is less than the CEC. The sorption equation for the second stage is: X-T' CK,= r ' ; 7 r ai fa" (4.10), a, I a2 X a2 - (x - x jaj cosa X*-T' 1 - and ln(fa") = ln(fa%. + [ln(fa"%. - In(fa')^.]^^^ (4.11), where x' is the amount of organic sorbed at the beginning of the second stage, and a is the angle made by the sorbate molecule with the solid surface. As the NHC molecules can change from a horizontal position to a vertical position, the calculations were done considering a to be 0° and 90°. The final stage, when the sorbed molecules are in the aggregated state, can be represented as: C K a = (4.12). 'C m a x - 'C The overall K is the product of all the K-components from all three stages. 121 4.3.3 Determination of Interfacial Tensions and AG" At a constant temperature (T) and constant pressure (P), the change in free energy can be expressed as a function of y between any two phases (Hiemenz, 1986) as: d G T .P = Y-dA (4.13), where dA is the increase in surface area. As indicated earlier, the change in free energy determined from the interfacial tensions is AG". Based on equation (4.13), AG" for the sorption process can be determined from the difference between the interfacial tensions existing before and after the NHC molecules sorb on the clay surface. When a clay-water suspension and a NHC solution are brought in contact with each other, the interfacial tension between clay particles and water (yew) and that between the NHC solution and the vapor phase (yiv) are replaced by the interfacial tension between the clay particles and the NHC solution (yd). This is because, after the mixing of the clay-water suspension and the NHC solution, the liquid phase surrounding the clay particles is no longer just water, but NHC solution. Therefore, from an energy balance: AG" = AG?fter - AGg-fore = Yd ~ Ylv " Yew (4.14). Similar types of equations were used by Absolom et al. (1983) and Chattopadhyay et al. (1995) to determine the AG for bacterial adhesion to polymeric substrates and cell adhesion to gas-liquid interface, respectively. The only quantity in equation (4.13) that can be determined experimentally is yiv- The other two interfacial tensions can, however, be determined with available semi- empirical equations (Owen and Wendt, 1969; Neumann et al., 1974; Gerson, 1982). In 122 our study, semi-empirical equations by Neumann et al. (1974) and Gerson (1982) have been used, as data obtained by one test liquid is sufficient for the necessary calculations. These equations were used in conjunction with the experimental data of the contact angle (0) between the test liquid and the clay surface and the liquid-vapor interfacial tension of the test liquid. Both water and NHC solutions were used as test liquids. The water contact angle (8*) and Ywv. surface tension of water, were used to determine Ycv, interfacial tension between clay and vapor phase, and Yew- Next, G, (contact angle made by the NHC solution on clay) and Yiv were used to determine Yci- A review of the calculations of the different interfacial tensions by the semi-empirical equations were outlined by Chattopadhyay et al. (1995). According to Neumann et al. (1974), the value of Ycv can be determined from the experimentally determined values of Ywv and 0 by using the following equation: 0 .0 15t ^w vV - 2 . o ,) ^ . T ,',T w ,v '^ Y ' WV cos0 = ------7 ------\ ----- (4.15) YwvH'5>cv1'wv -■ With the calculated value of Ycv, Yew and Yci can be determined with the following equations: ^cw “ , . 1/ (4.16); l-0015(Y„Ywvr 123 ,-0.0,XY.Yj^ A FORTRAN program written by Neumann et al. (1974) was used for the calculation of the different interfacial tensions by equations (4.15a), (4.15b), and (4.15c). The set of equations developed by Gerson (1982) are; Y«,v(cos0+1) I ------®’^p{(Y cv - Ywv coseX'^Ycv + b)} = 0 (4.18), V ' wv^cv (4.19), V * WV cv and^^^*^^^^-=xp{(Yd)(aYcv'^b)} = 0 (4.20). The values of “a” and “b” have been provided by Gerson (1982) as: a = 6.5 ± 1.0 x 10'^ and b = -0.010 ± 0.001. These parameters were obtained from testing a variety of systems (solid, liquid, and vapor components) from published literature. 4.4 Materials And Methods 4.4.1 Materials The NHCs used for the study were AcN (Aldrich Chemical) and AcNCOOH (Aldrich Chemical) of purity 99.9% and 99.7% respectively. Both these chemicals were used as received. AcN is an organic base with a pK of 5.6 (Albert, 1966). AcNCOOH 124 has two pK-values at 3.0 and 5.0 (Albert, 1966), and AcNCOOH molecules exist as zwitterions at pH between 3.0 and 5.0. Attention is focused on either basic or zwitterionic compounds as the organic cations have a higher selectivity for the negatively-charged clay particles. Smectite-type clays with low iron content were used as sorbents. The clay minerals used in this study were Na-exchanged hectorite (SHCa-1), Na-exchanged saponite (SapCa-1), and Muscovite (PELCO Mica Sheets #54, PELCO Electron Supplies). The SHCa-1 and SapCa-1 clay samples were obtained from the Source Clays Repository of the Clay Minerals Society (MI). The procedure for preparation of clay suspension has been outlined in Chapter 3. The hectorite and saponite samples were available in the powdered form. The muscovite sample were obtained in the form of sheets, and these sheets were used directly for contact angle measurements. However, the mica sheets were ground in a disk mill (Sieb Technik, G.M.B.H., Miilhein- Ruhr) for the sorption studies, and the microcalorimetric measurements. 4.4.2 Methods 4.4.2.1 Sorption isotherms Isotherms for sorption of AcN on Na-hectorite and muscovite at both high and low pH-conditions (pH 4.5 and 8.5) were constructed from spectroscopic measurements with a Cary 3 UV-visible spectrophotometer (Varian, Australia). The procedure has been outlined in Chapter 3. The concentration of hectorite and muscovite were maintained at 150 m gL‘‘. 125 4.4.2.2 Surface area measurements Single-point determination of specific surface area were performed using Ni adsorption by the continuous flow method (ASTM D4567) with a Micromeritics Flowsorb n 2300 instrument. The instrument was calibrated by injecting a known volume of analytical grade N? gas as described in ASTM D4567. At the beginning of each experiment, two standard reference materials (NIST 8570 and 8571) were analyzed. The quantities of both standards and samples were adjusted to yield surface areas in the range of 0.5 to 25 m^, as per instrument manufacturer instruction. All samples were analyzed in triplicate or until individual analyses were within ±10% of the mean values following removal of any outlying data points. 4.4.2.3 Microcalorimetric measurements The enthalpy changes (AH) which accompanied the interactions between clay (Na- hectorite and muscovite) suspensions and AcN solutions of different concentrations were measured by a microcalorimetric technique at both high and low pH-conditions. The heat of immersion of the solid - liquid interactions at 25°C was determined with a Calvet differential microcalorimeter of the Tian-Calvet type using the method described by Oliphant and Low (1982). The microcalorimetric elements were surrounded by sensitive thermopile and housed in a large, insulated aluminum block. Aqueous solution of the AcN (volume 4 mL) in the lower compartment of the sample cell in one calorimetric element was separated from the clay sample (volume 1 mL) in the upper compartment by 126 a Teflon™ plug on a Teflon disk. The heat of immersion was measured after the plugs in the Teflon™ disks of both the sample and reference cell were pulled simultaneously. Sufficient time was allowed for the system to attain thermal equilibrium. The differential voltage generated by the microcalorimetric elements was measured by a nanovoltmeter and the thermogram data were collected in a computer. The total area under the emf versus time curve, from the initiation of reaction to the time when thermal equilibrium is established, is proportional to the heat released in the reaction vessel. The total heat released was calculated by multiplying the area under the emf - time curve, with the calorimetric constant. 4.4.2.4 Measurement of Contact Angles and Interfacial Tensions The surface tension of water, AcN and AcNCOOH solutions of different concentrations were measured by using the Maximum Bubble Pressure method with a Sensadyne 6000 surface tensiometer from Chem-Dyne Research Corp. (Milwaukee, WI). As indicated earlier, determination of all other interfacial tensions requires the values of 6 made by the test liquids on the solid surfaces. Advancing contact angles were measured, and as indicated by Pashley and Israelachvili (1981) advancing contact angles on clays give the thermodynamically correct results, whereas receding angles do not. The contact angles made by a liquid drop on solid surface were determined by a modified version of the Axisymmetric Drop Shape Analysis - Contact Diameter (ADSA-CD) method (Rotenberg er a/., 1982; Duncan-Hewitt et a/., 1984). 127 In the ADSA-CD method, the average contact diameter of a sessile drop on a surface, as viewed from above, was measured to determine the contact angle that the drop makes with the surface. About 1 to 3^iL of liquid was used to form the sessile drops, and the image of this drop was recorded for approximately 30 seconds. The image of the sessile drop was recorded on a VCR using a charge-coupled device (CCD) video camera (Pulnix, model no. TM-640). Digital image processing was used to acquire the coordinates of the drop. Dipix P360 frame grabbing board in a PC was used to convert the analog video signal to digital signal, and then ADSA-CD was used to calculate the contact angles. The experimental apparatus used for contact angle measurements was as shown by Chattopadhyay et al. (1995). Besides the average contact diameter of the sessile drops, other inputs were also required for the program. These inputs are Ap (difference in density between the liquid and the vapor phase), volume of the drop of the test liquid, yiv of test liquid, and g (acceleration due to gravity). As indicated earlier, contact angles were measured directly on the mica sheets. Contact angles on hectorite and saponite were measured by preparing self-supporting films of the clay samples. Procedure for preparation of self-supporting clay films was similar to the procedure followed by us (Chapter 3) for preparation of clay films for XRD measurements by using the Millipore filter transfer method (Moore et al., 1989). 128 4.5 Results And Discussion 4.5.1 Sorption Isotherms The sorption isotherms of AcN in the presence of Na-hectorite and muscovite at a clay concentration of 150 mg L"' (Figure 4.2) are similar in nature to those obtained for AcN and AcNCOOH in the presence of Na-hectorite, Ca-hectorite, Na-saponite, and Ca- saponite at a clay concentration of 50mg L*’ (Chapter 3). 100 Na-hectorite, pH 4.5 Na-hectorite, pH 8.5 Muscovite, pH 4.5 ? Muscovite, pH 8.5 o I I E I I 1 i 1 1 o c 3 I < 0 0 5 0 100 150 200 Equilibrium concentration of AcN (junol L"') Figure 4.2 Sorption isotherms of AcN on Na-hectorite and Muscovite. 129 Due to a three-fold increase in the clay concentration, the amount of AcN sorbed (as cmol of AcN sorbed per unit weight of clay) on Na-hectorite is lower than that observed earlier, as increase in the weight of the clay added is considerably higher than the amount of NHC sorbed. However, irrespective of the amount of AcN sorbed at different clay concentrations, it was observed that the sorption isotherms obtained are all non-linear in nature, and these non-linearity can be attributed primarily to: (i) interactions between the sorbed molecules, (ii) saturation of active sorption sites on clays by the incoming sorbate molecules, and (iii) heterogeneity of the clay surfaces. Also, based on the classification scheme outlined by Giles et al. (1960), the sorption isotherms obtained for AcN with Na- hectorite at low pH was identified as S-type, while all other isotherms were L-type. As discussed earlier, S-type isotherms are indicative of cooperative sorption of molecules. As expected, more AcN sorbed on clays at low pH than at pH 8.5. This was due to the preference for the positively charged acridinium (AcNH^ ions by the negatively-charged clay surfaces. Though the cations are favored by the clays, neutral AcN molecules were also sorbed. It was also observed that the amounts of AcN sorbed on Na-hectorite at all pH-conditions were greater than the amounts of AcN sorbed on muscovite. This difference can be explained by comparing the clay structures. Though both hectorite and muscovite are 2:1 type clay minerals, muscovite is non-expanding type unlike hectorite (van Olphen, 1977; Fanning et ai, 1987). With hectorite, AcN molecules can sorb on both external and internal surfaces of the clay. The interlayer space of hectorite increases with increase in the amount of the organic sorbed, and thus, the sorption capability of hectorite is high. Due to the absence of interlayer swelling in muscovite, less organic 130 cations can sorb on these clays. The exchangeable cations located in the interlayer space of muscovite are not available for exchange, and therefore, sorption is exclusively on the external tetrahedral layer of the clay particles. Furthermore, the effective surface area of the clay particles available for sorption is higher in the case of hectorite, as the particle size of the hectorite sample was less than the average particle size of the muscovite. This is because cleavage occurs parallel to the silicate layers in muscovite and other similar mica-type clays, causing these minerals to exist as flakes, which resists grinding of these minerals to a finer sizes common to other clay minerals (such as smectites) (van Olphen, 1977). These sorption isotherms, along with the sorption isotherms presented earlier (Chapter 3) were used for the calculation of K for sorption of AcNs on clays. 4.5.2 K from Sorption Isotherms Equilibrium constants for sorption of NHCs on clays were determined from the sorption isotherms (Tables 4.1 and 4.2). The values of the BET surface areas of the clay minerals were required for the calculation of K by the Biggar equation. They were determined as: 93.4 m" g ' for hectorite, 34.6 m" g‘‘ for saponite, and 16.2 m" g ' for muscovite. As the absolute values of K are not known, authenticity of the calculated values of K were determined by comparing the values obtained by different models. The order of magnitudes of the K-values obtained by the four models were, in general, Biggar equation < modified Langmuir equation < three-stage adsorption model < two-stage adsorption model. 131 Clays NHCs pH By Biggar equation By modified Langmuir equation K (-)AG n K (-)AG (kJ/mol) (kJ/mol) Na-Hectorite AcN 4.5 1.7xl0®±9.6xl0^ 35.610.1 1.610.2 3.3x10^18.0x10^ 42.916.1 8.5 7.7xl0*±4.2xl0'‘ 33.610.1 1.310.1 8.3x10*16.1x10* 33.811.8 AcNCOOH 4.5 6.7x10*11.7x10'* 33.210.1 1.610.1 1.7xl0’ll.2xl0’ 41.311.7 8.5 1.1x10*14.9x10* 28.610.1 1.310.1 2.7x10*11.4x10* 31.011.3 Ca-Hectorite AcN 4.5 1.5x10*12.0x10* 35.310.3 1.710.1 5.6x10^12.4x10’ 44.211.1 8.5 5.3x10*15.6x10* 32.710.1 1.110.1 1.3x10*11.1x10* 29.212.2 AcNCOOH 4.5 5.0x10*11.9x10“ 32.510.1 1.510.1 7.0x10*15.3x10* 39.011.9 8.5 7.3xlO“l3.1xlO* 27.710.1 1.010.1 4.0xl0“l3.8xl0“ 26.312.4 Na-Saponite AcN 4.5 5.1x10*19.9x10“ 38.310.1 1.810.1 6.5x10*13.1x10* 50.311.2 8.5 3.2x10*11.9x10* 37.110.1 1.310.1 1.7x10*11.6x10* 35.612.5 AcNCOOH 4.5 8.2x10*18.0x10* 33.710.1 1.610.1 1.6x10*16.5x10* 41.111.0 w ro 8.5 1.6x10*11.6x10* 29.810.1 1.510.1 3.3x10*13.1x10® 37.212.4 Ca-Saponite AcN 4.5 7.1x10*13.9x10* 39.110.1 1.210.1 1.5x10*11.3x10* 35.212.1 8.5 3.2x10*12.2x10* 37.110.2 1.010.1 6.9xlO“l3.5xlO“ 27.611.3 Muscovite AcN 4.5 3.3x10*13.2x10“ 31.510.3 1.010.1 7.6xl0“l4.4xl0“ 27.811.4 8.5 1.8x10*19.7x10* 30.010.1 0.910.1 I.lxl0“l5.3xl0* 23.011.2 Na-hectorite-1 AcN 4.5 9.0x10*16.1x10“ 34.010.2 1.210.1 1.7x10*11.1x10* 29.811.6 8.5 6.3x10*16.9x10“ 33.110.3 0.910.1 1.5xlO“l9.5xlO* 23.811.6 Table 4.1 Equilibrium Constants and Change in Free Energy for Sorption of NHCs on Clay minerals. Clays NHCs pH By three-stage adsorption model By Gu equation K (-)AG n K (-)AG (kJ/mol) (kJ/mol) Na-Hectorite AcN 4.5 2.4xlO'z±7.5xlO': 70.710.8 3.710.8 1.3xlO”ll.lxlO'® 97.7120.7 8.5 1.3xlO®±1.3xiO’ 46.310.3 3.410.5 I.lxl0''’l4.8xl0'“ 80.1110.9 AcNCOOH 4.5 3.7xlO®±2.3xlO^ 37.510.2 3.110.4 3.9xlO'"ll.7xlO'* 83.2111.1 8.5 2.7xl0*±I.0xI0^ 31.010.9 2.410.6 1.9xl0''ll.2xl0” 64.3115.7 Ca-Hectorite AcN 4.5 2.0xl0'^±3.7xl0" 70.110.5 3.210.6 1.1x10**17.2x10'* 85.8116.2 8.5 5.2xlO’±4.8xlO® 44.010.2 2.510.4 3.3xlO"ll.2xlO'’ 65.719.1 AcNCOOH 4.5 2.2x10^13.4x10'^ 41.910.4 2.510.7 3.8xlO"l2.5xlO'’ 66.1116.2 8.5 2.8xlO’±2.4xlO' 31.012.2 2.110.8 7.1xlO’l5.9xlO"' 56.20120.5 Na-Saponite AcN 4.5 1.9xlO'^±3.2xlO" 70.010.4 3.510.7 2.1xl0'’ll.5 x l0 ‘* 98.8118.5 8.5 1.7x10*12.0x10’ 47.010.3 3.310.3 3.0xl0'*19.4xl0'* 76.917.7 w w AcNCOOH 4.5 1.3xlO'*16.6xlO’ 35.011.2 3.110.5 7.7xlO"*13.8xlO'* 84.9112.2 8.5 9.4x10^12.4x10^ 22.710.6 1.810.7 6.3x10*15.4x10’ 50.18121.4 Ca-Saponite AcN 4.5 6.4xlO”l7.9xlO" 73.110.3 3.110.6 3.7xlO'*12.3xlO"* 88.8115.7 8.5 1.0xl0’l3.9xl0* 51.410.9 2.610.7 1.2xlO'*18.8xlO” 74.7117.6 Muscovite AcN 4.5 7.0xl0‘’l3.3xl0“ 27.611.2 2.910.6 5.4xlO'*13.1xlO'^ 78.3114.1 8.5 8.4x10^15.5x10^ 22.411.6 2.410.5 5.5xlO'"l2.5xlO" 61.3111.4 Na-hectorite-1 AcN 4.5 2.1xl0’l2.3xl0'^ 41.710.3 3.110.7 4.1xl0'^13.0xl0'* 83.3118.4 8.5 7.4x10*13.3x10* 33.511.1 2.810.3 1.9xlO'*15.5xlO'* 75.817.2 Table 4.2 Equilibrium Constants and Change in Free Energy for Sorption of NHCs on Clay minerals. The standard errors (confidence level 95%) have also been calculated for all K-values. These errors were calculated by considering three replicates of each set of sorption isotherms. For the same experimental data, minimum error was observed by using Biggar equation, while maximum error was obtained by using the two-stage adsorption model. In general, the standard errors with Biggar equation and three-stage adsorption model were one order of magnitude less than the K-values, while in the case of two-stage adsorption model the standard errors were one order of magnitude higher than the K- values. The errors obtained with the modified Langmuir equation were, in general, less than the mean values. Linear regressions were used to determine the unknown parameters with the Biggar and the modified Langmuir equations, and the three-stage adsorption model. However, a curve-fitting program was used to fit the data to equation (4.7), which contains three unknown parameters (n, ki, and k^). Wittrock et al. (1996) have also used the two-stage adsorption model to determine the ki and ki-values for the sorption of a cationic surfactant, cetylpyridinium chloride (CpCl), on silica gel particles (size > 50 nm). They found that the equilibrium constants were of the order of I O'* and I0'° at 20°C. Seidel et al. (1996) found the k: and kz-values for the sorption of CpCl on Fractosil to be of the order of I O'* and lO’^ at 25°C. Though Wittrock et al. (1996) and Seidel et al. (1996) have reported agreement between the fit and the experimental points by using the two-stage adsorption model, they did not report the standard errors obtained for k|, and kz while fitting the data to the generated curve. High standard errors for K are not uncommon. Homenauth and McBride (1994) used an equation similar to equation 134 (4.7) to calculate K for sorption and cluster formation of aniline on clays (montmorillonite, vermiculite, and kaolinite) and have reported standard errors for Ko ranging from 97% to 289% of the mean values. Irrespective of the magnitudes of the standard errors, certain trends were observed with all values of K. These trends are similar to the conclusions obtained from the sorption isotherms. In general, it was observed that: (i) K-values obtained at low pH > K- values obtained at high pH; (ii) K-values obtained with AcN > K-values obtained with AcNCOOH; (iii) values of K for AcN and Na-hectorite at a clay concentration of 50 mg L ' > values of K for AcN and Na-hectorite at a clay concentration of 150 mg L''; (iv) values of K for AcN with Na-hectorite > values of K for AcN with muscovite, at the same clay concentration; (v) K-values for Na-exchanged clays > K-values for Ca-exchanged clays. There were some anomalies to these general trends of results, which were probably due to experimental error, and the error generated during the calculation of the values of K from these experimental data. Based on the magnitude of the standard errors, the Biggar equation and the three- stage adsorption models are more suitable for the present clay-organic systems than the modified Langmuir-type equation and the two-stage adsorption model. The two-stage and three-stage adsorption models consider sorption to occur in different stages, and the overall K is the product of the equilibrium constants of each stage. However, Biggar equation and modified Langmuir equation do not consider such stage-wise sorption, and thus, they are mathematically simpler. It was also found that the results obtained by the simpler models were similar. Therefore, the simplicity and generation of low standard 135 error makes the Biggar equation attractive. However, this equation is suitable primarily for very dilute solutions, and low surface coverage, and it does not account for any aggregate formation of the sorbate molecules. Therefore, at best, the Biggar equation is suitable for indicating a general trend. The three-stage adsorption model accounts for the change in the orientation of the sorbate molecules with increase in surface coverage. Earlier studies (Chapter 3) have shown that AcN molecules can form aggregates on clay surfaces, and/or change from a horizontal position to a vertically tilted position with increase in surface coverage. Therefore, for the present study, the three-stage adsorption model should be a better indicator of the sorption process. 4.5.3 Microcalorimetric measurements Microcalorimetry allows the measurement of enthalpies of displacement of the solvent by the solute, and also provides direct information on the nature of interactions involved during the process of sorption. Also, this method is suitable for complex systems involving heterogeneous surfaces. Heat is released or sorbed when NHCs are brought in contact with clays. Zhang et al. (1990) have listed the causes of enthalpy changes as due to: (i) heat consumed because of the bending and breaking of hydrogen bonds between water molecules to accommodate the solute molecules; and (ii) heat released due to the formation of new bonds between the solute molecules and the water molecules. The changes in enthalpy (AH), as kJ per mole of AcN sorbed, are plotted as a function of the amount of AcN sorbed on clays (cmol/kg clay) in Figures 4.3 and 4.4. 136 -250 Sorption on Na-hectorite -200 -150 ■o -100 pH 4.5 -50 pH 8.5 100 150 200 0 20 40 60 80 100 Amount of AcN sorbed (cmol/kg hectorite) Figure 4.3 Change in enthalpy as a function of amount of AcN sorbed on Na-hectorite. 137 -200 Sorption on Muscovite -150 -100 1 -50 pH 8.5 pH 4.5 g o I X < 100 150 200 250 0 2 4 6 8 10 Amount of AcN sorbed (cmol/kg muscovite) Figure 4.4 Change in enthalpy as a function of amount of AcN sorbed on muscovite. In all the cases, except for AcN in the presence of Na-hectorite at pH 4.5, the sorption of AcN on clays changes from an endothermie process to an exothermic process. The sorption of AcN on Na-hectorite at pH 4.5 lies entirely in the exothermic region (AH is negative) for the concentrations studied. The high degree of exothermicity seen in the case of AcN-hectorite suspension at low pH might be due to the fact that sorption of AcN on hectorite is considerably large, and the exothermicity indicates a favorable process. 138 The increased exothermicity might also be due to additional stabilization contributed by charge transfer interactions (Hayes et al., 1978) between the positively charged AcNH^ ions and the electron-rich clay surfaces, and this indicates that the sorption occurs by an ion exchange mechanism, as large (-)AH values are characteristic of ion exchange mechanism. Electrostatic interactions between the clay surfaces and the polar species are due to bonds of higher energy than hydrogen bonds (16-21 kJ/mol) or van der Waals bonds (2-8 kJ/mol) (Barshed, 1952). Eventually, in all the cases, the process became exothermic, and this exothermic effect might be due to the collapse of the clay lattice, which took place when the resident metal cations were replaced by the organic molecules. The collapse of the clay lattice produced a large exothermic effect because of the increased electrostatic attraction from the closer proximity of charges (Hayes et al., 1975). Sorption by electrostatic interactions can also occur at high pH conditions, even when AcN exists as neutral molecules. Previously (Chapter 3), we have shown that at low concentrations, neutral AcN molecules can become protonated at the clay surface, and then sorb on the clays by ion exchange mechanism, which can be responsible for the increase in (-)AH (Figures 4.3 and 4.4). After reaching an exothermic peak, the (-)AH decreased with further increase in the amount of AcN sorbed on clays. The decrease in (-)AH at higher concentrations might be due to the increase in hydrophobic interactions during the sorption process. Earlier from the sorption isotherms and spectroscopic studies, it was shown that as the amount of AcN sorbed on clays increased, the possibility of cooperative interactions 139 between the sorbates also increased. Such cooperative interaction led to aggregate formation, which was primarily due to hydrophobic interactions. Similar plots were obtained by Dekany (1992) when the displacement enthalpies for sorption of methanol- benzene mixture on different clay surfaces were plotted as a function of the mole fraction of methanol in the solution mixture. An exothermic peak was obtained at low concentrations of methanol, and with further increase in concentration of methanol, the exothermicity decreased. Dekany (1992) attributed the increase in exothermicity with increase in amount sorbed at low concentrations to be due to sorption by ion exchange mechanism, and the decrease in exothermicity at higher surface concentrations to increasing hydrophobization. Similar conclusions were also obtained by Seidel et al. (1996), who indicated that the exothermic heat effects at low concentrations was due to the sorption by electrostatic interactions, and aggregate formation at higher concentrations resulted in endothermie heats of adsorption. Sorption of AcN on hectorite was more exothermic than the sorption of AcN on muscovite (Figures 4.3 and 4.4). This corroborates with the observations made from the sorption isotherms, where the amount of AcN sorbed on muscovite was lower than the amount of AcN sorbed on hectorite. Also, a higher degree of exothermicity was observed at low pH-conditions than at high pH. This was because at low pH, AcN exists at AcNH^ ions and these ions can sorb on clays by electrostatic interactions. Therefore, the difference in AH between different organo-clay systems shows that different forces were involved in the sorption process, and the nature of these forces were dependent on the characteristics of the participating sorbent and sorbate. 140 4.5.4 Contact Angles and Interfaciai Tensions Determination of AG" requires the values of interfacial tensions between the different phases involved. The experimentally determined values of yiv and Y wv at 25°C, obtained by using the Maximum Bubble Pressure method, are: 71.8±0.4 mJ/m^ for water; 71.510.3 mJ/m^ for AcN at a concentration of 195 pimol L''; 71.710.2 mJ/m^ for AcN at a concentration of 28 |Xmol L''; 71.210.4 mJ/m^ for AcNCOOH at a concentration of 112 p.mol L'*; and 71.710.4 mJ/m^ for AcNCOOH at a concentration of 16 |imol L‘*. It was found that the lowering of surface tension by the addition of AcN or AcNCOOH was not very significant. Similar observations were also made by Giles (1983) for solutions of different aqueous solutions of cationic and anionic dyes with no alkyl substitution. The contact angles made by water (8*) and AcN / AcNCOOH solutions (8,) on different clay surfaces were required to determine yew and yd. The 8w and 8| made by water, AcN, and AcNCOOH on different clay surfaces are indicated in Tables 4.3 and 4.4. 141 Clay Equ. NHC used 0w e, Ycv Yew -AG" -AG" Sample (deg) (deg) (mJ/m^) (mJ/m^) (mJ/m ) (mJ/m^) (kJ/mole) Hectorite Neumann AcN @ 195 59.0+2.6 55.613.5 47.111.6 10.111.3 8.511.5 73.110.4 21.410.1 Gerson AcN @ 195 59.0±2.6 55.613.5 47.911.5 10.911.3 9.211.6 73.210.4 21.410.1 Neumann AcN @ 28 59.0±2.6 58.113.4 47.111.6 10.111.3 9.611.6 72.010.4 21.010.1 Gerson AcN @ 28 59.012.6 58.113.4 47.911.5 10.911.3 10.411.6 72.010.5 21.010.1 Saponite Neumann AcN 25.712.7 26.413.2 65.311.3 0.710.3 0.810.3 71.410.3 20.810.1 Gerson AcN 25.712.7 26.413.2 65.411.3 0.710.3 0.810.4 71.410.3 20.810.1 Muscovite Neumann AcN 30.513.0 35.615.9 63.111.5 1.310.4 2.211.2 70.510.8 20.610.2 Gerson AcN 30.513.0 35.615.9 63.211.5 1.410.5 2.411.3 70.410.9 20.610.3 Muscovite 1 Neumann AcN 57.213.6 51.813.3 48.112.2 9.311.7 6.911.3 73.910.5 21.610.1 Gerson AcN 57.213.6 51.813.3 48.912.1 10.111.7 7.511.4 74.010.5 21.610.1 (Note: Ywv = 71.8± 0.4 mJ/m ; Y v = 7 1.5± 0.3 mJ/m^ at 195 (imoi/L and 71.7± 0.2 mJ/m^ at 28 pmol/L; and Muscovite 1 = Muscovite presorbed in AcN solution.) Table 4.3 Contact angles and interfacial tensions of phyllosilicate-AcN systems. Clay Equ. NHC used 6w h 0. Ycv Yew 7c , -AG -AG h Sample (deg) (deg) (mJ/m^) (mJ/m^) (mJ/m ) (mJ/m^) (kJ/moIe) Hectorite Neumann AcNCOOH 59.0±2.6 54.114.3 47.111.6 10.111.3 7.811.8 73.510.6 21.510.2 @ 112 Gerson AcNCOOH 59.0±2.6 54.114.3 47.911.5 10.911.3 8.511.9 73.610.7 21.510.2 @ 112 Neumann AcNCOOH 59.0±2.6 58.512.6 47.111.6 10.111.3 9.711.2 71.610.4 20.910.1 @ 16 Gerson AcNCOOH 59.0±2.6 58.512.6 47.911.5 10.911.3 10.511.3 71.610.4 20.910.1 @ 16 Muscovite Neumann AcNCOOH 30.5±3.0 32.213.6 63.111.5 1.310.4 1.510.6 70.910.4 20.710.1 Gerson AcNCOOH 30.5±3.0 32.213.6 63.211.5 1.410.5 1.610.6 70.710.4 20.710.1 Muscovite 1 Neumann AcNCOOH 49.113.1 56.213.6 52.911.8 5.911.1 8.711.6 68.410.6 20.010.2 è Gerson AcNCOOH 49.113.1 56.213.6 53.511.7 6.511.2 9.411.7 68.210.6 19.910.2 (Note: Ywv = 71.8± 0.4 mJ/m ; Yiv = 71.2± 0.4 mJ/m^ at 112 (xmol/L and 71.7± 0.4 mJ/m^ at 16 |xmol/L; and Muscovite 1 = Muscovite presorbed in AcN solution.) Table 4.4 Contact angles and interfacial tensions of phyllosilicate-AcNCOOH systems. The 0w on different clay surfaces measures the wettability characteristics of the clay surfaces. Lower 0w indicates a more hydrophilic surface. Experiments showed that 0w on saponite and muscovite were lower (25.7°±2.73° and 30.5°±2.97°) than that on hectorite (59.0°±2.61°). Therefore, saponite and muscovite are more hydrophilic than hectorite. van Oss et al. (1990) have also reported a similar high value of 0w (63°) on hectorite samples. The relative hydrophilicity of saponite and muscovite can be due to substitution in the tetrahedral layer, and this results in a negatively-charged external surface of clay. The charged clay surface promotes attraction of water molecules by formation of hydrogen bonds with the water molecules. The external tetrahedral layer of hectorite is uncharged and so less water molecules are attracted to the clay surface, resulting in a fairly hydrophobic clay surface. Similar observations were also made by Schrader and Yariv (1990) when they measured the 0w on vermiculite and talc. The 0* on vermiculite were much lower than on talc, as the negative charge of vermiculite resides primarily on the tetrahedral layer, while talc is uncharged. Though 0w -values of saponite and muscovite indicate the presence of hydrophilic surfaces due to tetrahedral charge, the values of 0w on these clay samples are different. This difference can be due to the difference in surface roughness of the samples used for 0-measurements (Heimenz, 1986). Researchers have indicated that surface roughness gives a false indication of hydrophilicity by producing a lower contact angle. The muscovite used for contact angle measurement was obtained in the form of a sheet with a 144 smooth surface, thus the roughness of such a surface was considerably less than that of saponite, where the clay film was formed by depositing the clay particles on a glass slide. Contact angles were also measured on muscovite which had been pre-soaked in AcN and AcNCOOH solutions. The 8* on muscovite presoaked in AcN was 57.2°±3.63°, while 0w on muscovite presoaked in AcNCOOH was 49.1°±3.06°. The 0* on presoaked muscovite was considerably larger than the 0w on fresh muscovite, indicating that sorption of NHC molecules on muscovite increased the hydrophobicity of the surface, apparently due to the nonpolar nature of the sorbate molecules. The difference between the values for AcN and AcNCOOH systems was clearly due to the nature of these adsorbed molecules. The presence of the carboxylic group on the NHC molecule makes it relatively easier for the water molecules to form hydrogen bonds with the oxygens of the carboxylate group, thus reducing the 0w Sorption of NHC molecules on clays considerably modifies the surface properties, and the surface on which sorption is taking place is a dynamically changing entity. This surface is heterogeneous and is composed of empty surface sites and of adsorbed molecules. Contact angles were also measured with AcN and AcNCOOH solutions (0|) on all clay samples. It was found that the 0| < 0w for hectorite, while 0, > 0^ for saponite and muscovite. However, the differences between 0; and 0* were not large, as the NHC solutes were dilute aqueous solutions. It should be noted that change in pH or change in the type of exchangeable cation on clay did not affect the values of liquid surface tension 145 or the values of contact angles on clay surfaces. Similar results were also reported by Schrader and Yariv (1990) and van Oss et al. (1990). Two sets of semi-empirical equations were used to calculate the values of the different interfacial tensions (Tables 4.3 and 4.4), as it is not possible to determine the relative accuracy of each set of semi-empirical equations. No significant differences were obtained between Neumann’s and Gerson’s equations. The hydrophobicity of the hectorite surface is further indicated by a lower Ycv and a higher Yew and Y ci compared to that of saponite and muscovite. The calculated values of these interfacial tensions were used in the determination of AO". 4.5.5 Comparison of AG and AG” The overall AG for the sorption of AcN and AcNCOOH on different clays are indicated in Tables 4.1 and 4.2, while AG" -values are shown in Tables 4.3 and 4.4. Similar values were also obtained by Abraham John and Ramaraj (1996) who calculated the AG" for sorption of dyes on Nafion film. They found that the AG" for thionine, azure-A, and methylene blue were 21.0+0.04,-21.3±0.04, and -21.9±0.04 kJ/mol. Furthermore, according to Israelachvili and Pashley (1982) the hydrophobic free energy for dimerization (closely related to AG") of small solute molecules is approximately equal to -20a kJ mol ', where a ( as nm) is the diameter of the solute molecule. Based on this, the hydrophobic free energy for dimerization of AcN molecules should be approximately 19.02 kJ m of’, which is close to the value obtained by us. In the present study, 146 significant differences among the AG" -values of different clay organic systems were not evident. It should be noted that the calculation of AO"-values was based on empirical equations. The values of AG" for saponite and muscovite were lower than that for hectorite and modified muscovite. Furthermore, for all systems AG" increased with increase in concentration of NHCs. This corroborates earlier results that hydrophobic interactions are significant at higher concentrations of NHCs in solution. The AH-values obtained at low concentrations were also used as limits for the AG. From thermodynamics, it is known that as the solute concentration tends to zero, AS 0, and AG becomes equal to AH. As the concentration increased, more and more molecules were sorbed on the clay surfaces, and this led to the formation of aggregates. As indicated earlier, hydrophobic interactions became more predominant at high concentrations. Therefore, at high concentrations, AG —> AG". Therefore, one of the limits of AG is equal to AG", and the other limit is equal to AH at low concentrations. Based on these considerations, the limits of AG for AcN with various clay suspensions at a clay concentration of 150 mg L‘‘ are as follows: -21 to -197 kJ mol ' for Na-hectorite at pH 4.5; -21 to -158 kJ mol ' for Na-hectorite at pH 8.5; -20 to -177 kJ mol ' for muscovite at pH 4.5; -20 to -70 kJ mol ' for muscovite at pH 8.5. All the K-values calculated by the different models were within this limit, except for the K obtained by the two-stage adsorption model for muscovite at low pH. Finally, on comparing the values of AG, determined with the three-stage adsorption model, and AG", it was found that AG" is approximately half of AG. Similar conclusions were also reached by Abraham John and 147 Ramaraj (1996) who claimed that the contributions due to electrostatic and hydrophobic interactions were almost similar during sorption of dyes on Nafion films. 4.5.6 Determination of entropy The change in entropy (AS) for the sorption of AcN on Na-hectorite and muscovite are shown in Figures 4.5 and 4.6. These values were calculated by using the following equation: AS = (4.21), with AG obtained with the three-stage adsorption model, and AH obtained from the microcalorimetric experiments. 148 1.0 Sorption on Na-Hectorite 0.5 0 C/3 1 pH 8.5 pH 4.5 < -0.5 - 1.0 0 20 40 60 80 100 Amount of AcN sorbed (cmol/kg hectorite) Figure 4.5 Change in entropy as a function of amount of AcN sorbed on Na-hectorite. 149 Sorption on Muscovite 0.5 ■g ■S _pH 8.5 CO < pH 4.5 -0.5 -I.O 0 2 4 6 8 10 Amount of AcN sorbed (cmol/kg muscovite) Figure 4.6 Change in entropy as a function of amount of AcN sorbed on muscovite. The sorption of AcN on Na-hectorite and muscovite were enthalpy-driven, as the magnitudes of AH were considerably higher than the magnitudes of AS. In all the cases, except for AcN with Na-hectorite at pH 4.5, the initial AS was positive, which might be due to the release of water molecules from the so-called “ice structure”, and this was the 150 main driving force for sorption process. As the AcN molecules were sorbed on the clays, the degrees of freedom of these molecules decreased, resulting in a decrease in the entropy. The negative AS resulted from a decrease in flexibility due to the attachment of sorbate molecules to the sorption sites on the clay surfaces. Biggar and Cheung (1973) have also obtained negative AS for sorption of picloram on different soil systems, and have indicated that the negative AS indicated the stability of the complexes formed by the sorbate molecules on the sorbent. Zachara et al (1990) have indicated that quinoline form stable complexes when sorbed on clay surfaces, and the increased surface stability was due to its tr-electron ring structure allowing delocalization of charge over the total molecular surface. Hayes et al. (1975) also observed a decrease in entropy when they studied the sorption of paraquat and diquat by montmorillonite and vermiculite. 4.6 Conclusions The preferential sorption of organic molecules on clays with different surface hydrophobicities can be described by the thermodynamic data of the adsorbed layer. Three thermodynamic parameters have been evaluated: AG, AH, and AS. The AG, calculated from the shape of the sorption isotherms, indicated that the sorption of AcNs was favorable, as AG was negative. The AH of sorption was also, in general, negative, indicating a favorable, exothermic process, though in some cases, the sorption was found to be initially endothermie. The plot of AH versus amount of AcN sorbed indicated that at low surface concentrations, sorption was primarily due to electrostatic interactions. 151 while as the surface concentration increased, contributions due to hydrophobic interactions also increased. Also, in most of the cases, AS was found to be negative, which indicated that the complexes formed due to sorption of AcNs on clay were stable. Furthermore, negative values of AH and AS for sorption of AcN on hectorite and muscovite even at high pH, when AcN existed as neutral molecules, suggest that AcN was adsorbing as a protonated species, and AcNH^ ions were formed due to surface protonation. The higher sorption of AcN at low pH conditions are indicated by higher values of K, AG, and AH. Microcalorimetric data in conjunction with the sorption isotherms showed that at low concentrations, sorption of AcN took place via coulombic attraction. However, hydrophobic forces became prominent at higher concentrations and increase in surface coverage. 152 CHAPTERS SORPTIVE CHARACTERISTICS OF ACRIDINE-SURFACTANT-PHYLLOSILICATE SYSTEMS 5.1 Abstract Experiments conducted with acridine (AcN)-clay systems have shown that AcN, a typical nitrogen heterocyclic compound (NHC), has a considerable affinity for clays. However, various compounds are present in the natural environment that might affect the sorption characteristics of AcN. The present study will illustrate the influence of the presence of a cationic surfactant on the sorption characteristics of AcN on swelling and non-swelling clays. The amount of AcN sorbed increased when the clays were pretreated with HDTMA, as the sorbed surfactant molecules act as additional sorption sites for the AcN molecules. However, spectroscopic studies showed that when HDTMA was added to a clay-AcN suspension, some of the sorbed AcN molecules were displaced from the sorption sites by the surfactant molecules, especially at high concentrations of the surfactant. Such solubilization of sorbed AcN molecules was most evident at high concentrations of HDTMA when the sorbed organic molecules underwent solubilization in the micellar core. 153 5.2 Introduction Ionic organic nitrogen heterocyclic compounds (NHCs) are present in many everyday consumer products, such as detergents, dyes, fabric softeners, emulsifying agents, drugs, and coal and petroleum products, and they are often used in combination with surfactants, which might be present as dispersing or wetting agents. The wastes generated from the use of these household and industrial products interact with the soil and subsurface materials. Researchers have shown that NHCs sorb readily on soils and clays (DellaGuardia and Thomas, 1983; Cohen and Yariv, 1984; Zachara et al, 1986, 1987; Schoonheydt et al., 1986; Cenens and Schoonheydt, 1988; Traina and Onken, 1991; Tapia Estevez et al, 1993; Lopez Arbeloa et al, 1995). In previous chapters, the high affinity of acridine (AcN) and its carboxylic derivative, acridine-9-carboxylic acid (AcNCOOH) for clays was shown. Correspondingly, other researchers (Lee et al, 1989; Lyklema, 1994; Wittrock et al, 1996) have demonstrated the high selectivity of surfactants for soil and subsurface materials. The surfactant molecules can readily displace the naturally occurring inorganic exchangeable cations, and once bound, are held essentially irreversibly on the clay surface (Lee et al, 1989). Sorption of surfactants on clays proceeds by both cation exchange and hydrophobic bonding, as suggested by Xu and Boyd (1995) when they studied the sorption of hexadecyltrimethylammonium bromide (HDTMA), a cationic surfactant, on both swelling (montmorillonite) and non swelling (kaolinite) layer silicates. They found that the surfactant molecules initially sorbed on clays by cation exchange, while formation of aggregates at higher concentrations occurred by hydrophobic bonding. According to them, the interlayer 154 space were occupied during the initial stage of sorption. Aggregation proceeded only on the external surface of clays, where charges were balanced by anions bound directly to or swarming around the headgroups. They have also shown that sorption by hydrophobic bonding might result in positive charge development on surfaces and ultimately clay dispersion. Sorption of HDTMA beyond its critical micelle concentration (cmc) was not significant. Though the level of contamination by surfactants is frequently low, their effect on interfacial and colloid structural properties of soils may be profound. Ionic surfactants, especially cationic surfactants, have been reported to enhance the sorptive capacity of the clays and soil for hydrophobic organic contaminants (HOCs). Sorption of cationic surfactants on negatively-charged clay particles occurs by strong electrostatic forces. The hydrophobic tails of these sorbed surfactant molecules provide favorable sites for the sorption of more surfactant molecules or other hydrophobic organic molecules. Lee et al. (1989) have shown that the sorption of toluene, ethylbenzene, dichlorobenzene, and tetrachloroethylene, which are nonionic organic contaminants, from water on HDTMA- treated soil increased by approximately 200 times. Rheinlander et al. (1992) have also shown that sorption of another cationic surfactant, dodecyl trimethylammonium bromide (DTAB), on bentonite enhanced the amount of pesticide (biphenyl) sorbed, as the sorption of the surfactant makes the bentonite surface more hydrophobic. Klumpp et al. (1993) have shown that the sorption of cationic surfactants and 2-naphthol were influenced by the sequence of their addition on the clays, concentration of the surfactant in solution, extent of surface coverage, the surfactant chain length, the number of alkyl 155 chains per surfactant molecule, and the charge density of the clay. Kibbey and Hayes (1993) found that partitioning of phenanthrene, which is a hydrophobic three-ring polycyclic aromatic hydrocarbon, on surfactant-coated silica increased with increase in surface concentration of the surfactant. They indicated that a coating of HDTMA immobilized approximately 7-14 times more phenanthrene than would be immobilized by an equivalent coating, by weight, of natural organic matter. Xu and Boyd (1995) have also found that HDTMA can effectively immobilize organic contaminants dissolved in water. Besides enhancing the sorptive capacity for hydrophobic compounds, researchers have also shown that sorbed hydrophobic compounds can be solubilized by the addition of a surfactant to a clay-organic system. This phenomenon is evident primarily when the concentration of the surfactant exceeds its cmc (Klumpp et al., 1993). Nakamura and Thomas (1986) showed that the presence of HDTMA markedly decreased the quenching efficiency of 4-( 1 -pyrenyl)butyltrimethylammonium ion (PN^ on laponite. Hayakawa et al. (1987) found that the addition of DTAB (dodecyltrimethylammonium bromide) dissociated the dimers of proflavin and rhodamine 6G, formed in the presence of sodium salts of dextran sulfate and polyvinylsulfate, into the monomeric form. Micellar solubilization of NHCs by surfactants have been investigated by Kubota et a/. (1991) and Yeom et al. (1995). Kubota et a/. (1991) suggested that the first step in the solubilization process of acridine orange and its 10-alkyl derivatives is their adsorption on micelles formed by surfactants, followed by isomerization of the complexes. They also concluded that electrostatic forces play an important role during the adsorption process. Yeom et al. 156 (1995) have shown that up to 25% of sorbed polynuclear aromatic hydrocarbons could be solubilized by the addition of surfactants (such as Tween 80, Triton X-100, Brij 35). This indicates that surfactants can increase the mobilities of the dye molecules. Therefore, the fates of hydrophobic organic compounds and surfactants in the soil and aquatic environments are inter-dependent and are of great relevance. The objective of the present paper is to investigate the clay-NHC-surfactant system microscopically such that one can understand the interactions between these compounds and clays on a molecular level. Such an understanding will eventually help in predicting the fate of NHCs in the environment, where a number of different types of compounds are present simultaneously. In this study, the effect of the cationic surfactant (HDTMA) on the sorption characteristics of AcN has been investigated. The following parameters were considered: (i) clay mineral surface charge density and charge distribution, (ii) swelling characteristics of clays, (iii) charge on the NHC molecules, (iv) concentration of NHCs, (v) concentration of HDTMA in solution, and (vi) pH of the clay- organic systems. The amount of AcN sorbed in each case was determined from the absorbance of AcN in solution. UV-visible and fluorescence spectroscopy were used to identify the nature and state of the sorbates, while XRD measurements were used to detect the presence of AcN and HDTMA in the interlayer space of the clay minerals. Finally, the equilibrium constants (K) for the sorption of AcN on clays treated with surfactants were calculated to determine the total change in free energy (AG) for the process. 157 5.3 Materials And Methods AcN (Aldrich Chemical), AcNCOOH (Aldrich Chemical), and HDTMA (Sigma Chemical Co.) of purity 99.9%, 99.7%, and 99% respectively were used for the study. All chemicals were used as received from the vendor. Experiments were conducted at two pH-conditions: 4.5 and 8.5. The pK of AcN is 5.6, and the pK-values of AcNCOOH are at 3.0 and 5.0 (Albert, 1966). HDTMA ((C6H33N)[CH3]3Br) is a cationic surfactant, with a formula weight of 365.5 and its cmc in 0.1 M ionic strength solution was found to be 24 |XM (Kibbey and Hayes, 1993). Attention was focused on cationic compounds as these compounds have a higher selectivity for negatively-charged clay particles. Solutions of AcN / AcNCOOH and HDTMA were prepared by adding the desired amount of sorbate in deionized, reverse osmosis processed, UV-sterilized water, and all NHC solutions were freshly prepared before the experiments. The aqueous concentration of AcN for all experiments ranged from 6 to 195 p.mol L‘‘, and the concentration of the HDTMA in solution ranged from 1.5 to 170 p.mol L''. Experiments were conducted at both high and low pH conditions, so as to study the sorption of both the cationic and the neutral form of AcN on clay minerals in the presence of a cationic surfactant. The ionic strength for all experiments was maintained at 0.1 M. Well-characterized 2:1 clays were used for the study. Both swelling (hectorite) and non-swelling (muscovite) 2:1-type phyllosilicates with low iron content were chosen as sorbents. The hectorite (SHCa-1) was obtained from the Source Clays Repository of the Clay Minerals Society (MI), and muscovite (PELCO Mica Sheets #54) was procured from PELCO Electron Microscope Supplies. Though both minerals are formed by the 158 condensation of an octahedral sheet of AI 2O3 between two tetrahedral SiOi sheets, the sorptive capacities are significantly different as the inorganic exchangeable cations in the interlayer space of hectorite can be easily replaced by organic molecules whereas the interlayer K in muscovite can not. The negative charge on hectorite is located in its octahedral sheet (charge: total = -0.62; on tetrahedral sheet = 0 ; on octahedral sheet = - 0.62), while muscovite is a tetrahedrally-substituted mineral (charge: total: - 1.86 , tetrahedral: -1.94, octahedral: +0.08). The BET surface area of hectorite is 93.4 m" g ', and that of ground muscovite is 16.23 m^ g"' (Chapter 4). Hectorite was available in powdered form, while the muscovite samples were obtained in the form of sheets (4 cm x 1 cm), which were ground in a disk mill (Sieb Technik, G.M.B.H., Miilhein-Ruhr) for the sorption studies. The Na-exchanged form of hectorite was used for the present study, and the procedure followed for preparation of clay suspensions was as outlined in Chapter 3. The clay concentration was maintained at 150 mg L'' for all sorption experiments, and at 200 mg L'‘ for experiments conducted in the stirred tank reactor (STR) (Chapter 3). The latter experiments, conducted with a total suspension volume of 250 mL, were initiated to examine the effect of addition of HDTMA on sorbed NHCs. The suspension volume for all sorption experiments, conducted in Teflon centrifuge tubes, was maintained at approximately 25 mL. The clay concentration during each set of experiments was determined separately by evaporating an aliquot of the suspension and drying at 100°C for 24 hrs. The experiments were divided into two groups: (i) effect of an increase in HDTMA concentration on sorbed AcN; (ii) sorption characteristics of AcN on clay 159 surfaces pretreated with HDTMA. The first group of experiments was conducted with Na-hectorite only, which was presoaked in either an AcN solution with an initial concentration of 140 p.mol L"', or an AcNCOOH solution with an initial concentration of 112 pmol L‘‘ for 24 hours at pH 4.5. The solubilization of sorbed AcN / AcNCOOH by HDTMA was investigated by increasing the concentration of HDTMA added to the NHC-hectorite suspension. Aliquots of the hectorite-HDTMA-NHC suspension were withdrawn to determine the amount of NHC in the supernatant. The pH of the suspensions and solutions was determined with an Orion pH meter, and necessary pH adjustments were made with either HCl or NaOH. The second group of experiments was conducted with both muscovite and Na- hectorite. The samples were equilibrated with HDTMA solutions of concentrations of 15 |xmol L"' and 50 p,mol L '‘. Clay-HDTMA-AcN suspensions were prepared in Teflon© centrifuge tubes by adding measured amounts of HDTMA-clay suspension to the desired AcN solutions at a particular pH with continuous stirring. After equilibrating the clay- organic suspensions, the samples were centrifuged, and the absorbance of the supernatants was measured to determine the amount of AcN sorbed. Sorption of NHC by the clay suspensions was measured at pH 4.5 and 8.5. Subsamples from all sorption experiments were centrifuged (Eppendorf microcentrifuge, model: 5415C) at 15,400 ref for 30 minutes, and aliquots of the supernatants were used for absorbance measurements in a Cary 3 UV-visible spectrophotometer (Varian, Australia) to determine the amount of HDTMA and AcN sorbed. Absorbance and emission spectra of the suspensions were also determined by 160 with a Varian/Cary 3 spectrophotometer and a SLM-Aminco Bowman Series 2 luminescence spectrometer (model FA 256) interfaced to an OS-2 computer, respectively. Absorbance scans were measured from 190 nm to 550 nm at a scan rate of 900 nm/min with a slit width of 2 nm. The excitation wavelength (Xex) was 390 nm, and emission spectra of the clay suspensions were collected from 400 to 600 nm at a scan rate of 1 nm/s with excitation and emission bandpasses of 2 nm. Three scans were made for each measurement. The procedure used to obtain the absorbance and emission spectra of clay- HDTMA-AcN systems was similar to that described in Chapter 3. XRD measurements were made on clay films of the samples prepared by the Millipore® filter transfer method (Moore and Reynolds, 1989), as described in Chapter 3. The nature of the sorbed molecules was also investigated. The samples withdrawn from the second group of experiments were centrifuged, and the fluorescence spectra of the supernatant was obtained to detect the predominant species in solution. The precipitate was resuspended in HPLC-grade water, and the fluorescence spectra were then collected as described above. Sorption isotherms were constructed to illustrate the amount of AcN sorbed as a function of the equilibrium concentration of AcN in suspension. These isotherms were used to determine the equilibrium constant (K) and the change in free energy (AG) for sorption of AcN on clays in the presence of HDTMA. Three models: (i) Biggar equation (Biggar and Cheung, 1973); (ii) modified Langmuir-type equation (Cotton and Wilkinson, 1980; Zhu and Gu, 1989); and (iii) three-stage adsorption model (Klimenko, 1978, 1979; Clunie and Ingram, 1983) were used to estimate K. 161 5.4 Results And Discussion 5.4.1 Effect of HDTMA on sorbed NHCs HDTMA is strongly attracted by the negatively-charged clay surfaces, and has the capability of displacing sorbed NHC molecules. The concentration of AcN in the supernatant has been expressed in terms of absorbance (Figure 5.1). 1.10 pH = 4.5 [AcN] = 140 pmol L 1.05 1.00 0.95 8 Io 0.90 . £ 3 < 0.85 0.80 0.75 0.70 0 20 4 0 6080 100 120 140 160 180 [HDTMA] (pmol L‘ ‘) Figure 5.1 Change in AcN concentration in solution with increase in HDTMA concentration. 162 The plot of absorbance versus the dissolved concentration of HDTMA can be divided into three distinct regions. Initially, the amount of AcN added to the clay suspension was distributed between the sorbed phase and the solution phase. As HDTMA was added to the clay-AcN suspension, the absorbance increased (Region I), indicating that the concentration of AcN in solution increased, and this is probably due to the transfer of some of the AcN molecules from the sorbed state to the solution. The decrease in absorbance with further increase in HDTMA, as observed in Region U, can be due to re adsorption of the AcN molecules. In this region, the AcN molecules sorbed not only on available clay surfaces, but also on sorbed HDTMA molecules. The hydrophobic tail of the surfactant molecules provide an additional set of sorption sites for the NHC molecules. Klumpp et al. (1993) have also shown that the hydrophobic segment of the cationic surfactants, such as HDTMA, DTAB, and DDDAB (didodecyldimethylammonium bromide), enhances the sorption of 2-naphthol on clays (Ca-bentonite and illite). They observed that when the concentrations of the surfactants were below their respective cmc-values, the surface coverage and the density of the adsorbed surfactant layer were entirely responsible for the enhancement of 2-naphthol adsorption. At high concentrations of HDTMA (Region HI), the surfactant molecules were in the micellar form in solution. The increase in absorbance was probably due to AcN molecules becoming trapped in the micellar cores, and remaining in solution. Therefore, the surfactant micelles in solution competed for AcN along with the sorbed surfactant. Similaf results have been reported by Klumpp et at. (1993). Based on these 163 results the remainder of the experiments were conducted with HDTMA concentrations of 15 fxmol L'' and 50 p.mol L The initial absorbance of the supernatant from the AcNCOOH-(Na)hectorite-clay suspension was high (Figure 5.2), as sorption of AcNCOOH on Na-hectorite was low when compared to the sorption of AcN, and a considerable amount of AcNCOOH added to the clay suspension remained in solution. As HDTMA was added to the AcNCOOH- clay suspension, the surfactant molecules readily sorbed on the clay surfaces, and these sorbed surfactant molecules caused the AcNCOOH molecules to migrate from the solution to the clay surfaces. The increase in absorbance of the solution at high concentrations of HDTMA was due to the solubilization of the sorbed AcNCOOH molecules in the micellar cores of the surfactant molecules. Further experimentation was conducted with only AcN. 164 0.95 pH = 4.5 [AcNCOOH] = 112 pmol L 0.90 0.85 I 0.80 0.75 0.70 0 20 40 60 80 100 120 140 160 180 [HDTMA] (pmol L"') Figure 5.2 Change in AcNCOOH concentration in solution with increase in HDTMA concentration. 165 5.4.2 Sorption isotherms The amount of AcN sorbed by HDTMA-cIays was dependent on the type of the mineral, pH of the mineral-organic suspension, and the concentration of HDTMA (Figures 5.3 and 5.4). 120 pH 4.5 & [CTAB] = 15 pmol L pH 8.5 & [CTAB] = 15 pmol L pH 4.5 & [CTAB] = 50 pmol L 100 pH 8.5 & [CTAB] = 50 pmol L I a 73 s 2 < 'o c3 I < 0 5 10 1520 2530 35 40 Equilibrium AcN concentration (pmol L‘ ) Figure 5.3 Sorption isotherms of AcN on Na-hectorite. 166 30 pH 4.5 & [CTAB] = 15 p.moI L pH 8.5 & [CTAB] = 15 pmol L pH 4.5 & [CTAB] = 50 pmol L 25 pH 8.5 & [CTAB] = 50 pmol L 20 If I 15 c 3 I 10 < 5 0 20 40 60 80 100 120 140 160 180 Equilibrium AcN Concentration (pmol L' ) Figure 5.4 Sorption isotherms of AcN on muscovite. 167 The samples used for the sorption experiments were pretreated with HDTMA solutions. The amounts of sorbed HDTMA were; (i) 8.74 cmol per kg of Na-hectorite (10% of CEC) at [HDTMA] = 15 pmol L'‘, (ii) 5.54 cmol per kg of muscovite at [HDTMA] = 15 pmol L % (iii) 15.66 cmol per kg of Na-hectorite (18% of CEC) at [HDTMA] = 50 pmol L‘‘, and (iv) 12.11 cmol per kg of muscovite at [HDTMA] = 50 pmol L*'. More surfactant molecules were sorbed on Na-hectorite than on muscovite, and the amount of HDTMA sorbed on both clays increased with increases in HDTMA concentration. The presence of HDTMA on the clay surfaces greatly influenced the sorption characteristics of AcN. The presence of HDTMA promoted sorption of AcN on clays. This was because sorbed surfactant molecules provided additional sorption sites for the AcN molecules. The presence of HDTMA at a concentration of 15 pmol L'* increased the maximum sorption of AcN in comparison to HDTMA-free samples (Chapter 4): (i) from 87.7 to 106.1 cmol kg ' on Na-hectorite at pH 4.5, (ii) from 41.3 to 104.2 cmol kg ' on Na-hectorite at pH 8.5, and (iii) from 3.81 to 28.30 cmol kg ' on muscovite at pH 8.5. In the presence of HDTMA at a concentration of 50 pmol L ', the maximum sorption of AcN increased: (i) from 87.7 to 108.7 cmol kg ' on Na-hectorite at pH 4.5, (ii) from 41.3 to 109.8 cmol kg ' on Na-hectorite at pH 8.5, (iii) from 3.81 to 21.4 cmol kg ' on muscovite at pH 8.5. These results do not contradict the observations made earlier (Figure 5.1) where the presence of surfactants solubilized the sorbed NHC in their micellar core, as solubilization is evident at high concentrations of surfactant, whereas immobilization of NHC from the solution is evident at low concentrations of surfactants. 168 Also, the sequence of addition of surfactants to the clay-organic system dictates the degree of solubilization / immobilization of NHCs. The maximum amount of AcN sorbed on HDTMA-muscovite ([HDTMA]initiai = 50 ^imol L‘*) was less than the maximum amount sorbed when the HDTMA concentration was 15 ^imol L ' (Figure 5.4). However, the maximum amount of AcN sorbed on hectorite was higher when the HDTMA concentration was 50 jxmol L"‘ (Figure 5.3). This contrasting behavior was due to the difference in distribution of the HDTMA molecules on the clay surfaces. Both the external and the internal surfaces of hectorite are available as sites for sorption of AcN and HDTMA. Due to the non-swelling nature of muscovite, only the external surfaces were available to the sorbate molecules. Sorption being a surface reaction, the amount of organic sorbed is dependent on the available surface area, and the BET-surface area of hectorite is greater than that of muscovite, as indicated earlier. Therefore, the number of sorption sites available on hectorite was considerably larger than on muscovite, and more HDTMA molecules were adsorbed by hectorite. In contrast, more HDTMA molecules were left in solution to form micelles in the case of muscovite. As was seen in the earlier section, the presence of micelles in solution solubilizes the organic molecules in the micellar core, resulting in more organic molecules in solution than on the clay surface. The amount of AcN sorbed on hectorite increased with increase in HDTMA concentration, as more HDTMA molecules were present on the clay surface, and these sorbed surfactant molecules provided favorable sorption sites for additional AcN. 169 Though there was no significant difference between the amounts of AcN sorbed on Na-hectorite at pH 8.5 and 4.5 (Figure 5.3), results obtained showed that the amount of AcN sorbed at high pH on muscovite was considerably greater than at low pH. Due to the abundance of sorption sites on hectorite, AcNH^ ions can sorb on sites which were left unoccupied by the HDTMA molecules, and this can compensate for the preference of the neutral molecules by the sorbed surfactant molecules. However, the high degree of sorption of AcN on muscovite at high pH indicates that sorption of neutral AcN molecules were preferred instead of the acridinium (AcNH^ ions. At the surfactant concentrations considered in this study, the surface coverage on muscovite was high, and the hydrophobic tail of these sorbed surfactant molecules prefer the nonpolar organic molecules over the polar molecules. Moreover, partitioning of cations to the hydrophobic sorption sites is less favorable than partitioning of the neutral organic molecules because hydrogen bonds between water and the organic cations are strong. Another reason for lower sorption of the organic cations on muscovite is that these cations encounter electrostatic repulsion from the positively charged HDTMA molecules. Furthermore, on comparing the extent of increase in the maximum amount of AcN sorbed on clays pretreated with HDTMA, it was also found that sorption of the neutral molecules increased by a greater extent than the sorption of the cations (AcNH^. The shape of the sorption isotherms can also be used to deduce the nature of sorbed molecules on clay surfaces. All the sorption isotherms are non-linear in nature, and such non-linearity can be interpreted as due to heterogeneity of the sorbent surfaces, and interactions between the sorbates (Chapter 4). In all cases, the plots are characterized 170 by a steeper initial slope followed by a shallower slope at a higher concentration. This indicates that as more substrate sites were filled, it was increasingly difficult for the incoming AcN molecules to find vacant sites. The first plateau in the isotherms represent saturation of the surface sites or formation of a monolayer (Giles et al., 1960), though this does not necessarily indicate complete coverage of the clay surface (Sposito, 1984). Further increase in sorption with increase in concentrations of AcN was likely due to the formation of additional layers or aggregation of molecules. The shapes of these sorption isotherms also suggest association between the sorbed AcN molecules (Giles et al, 1960, Sposito, 1989). The sharp increase in slope of the sorption isotherms at low surface coverage can be due to the formation of hemimicelles. Hemimicelles form at surfaces when the attractive hydrophobic interaction between sorbed molecules becomes significant compared to the interaction between sorbed molecules and the surface. Equilibrium constants (K) for sorption of AcN on clays pretreated with HDTMA were determined from the shapes of the sorption isotherms (Table 5.1). Three different models were used to test the authenticity of the calculated values of K. The order of magnitudes of the K-values obtained by these models were, in general, Biggar equation < three-stage adsorption model < modified Langmuir equation. The standard errors, obtained from considering three replicates of each set of the sorption isotherms, were highest with the modified Langmuir equation. For the same experimental data, minimum error was observed by using the Biggar equation. Similar standard errors were also obtained for K of the sorption of AcN and AcNCOOH on different clays (Chapter 4). In general, the standard errors with the Biggar equation and the three-stage adsorption model 171 were one order of magnitude less than the K-values, while in the case of the modified Langmuir equation the standard errors were one order of magnitude higher than the K- values. Clay pH [HDTMA] Equation K -AG UM kJ mol“ Hectorite 4.5 15 Biggar I.lx l0 ’±2.32xl0^ 40.1±0.05 Langmuir 1.8xl0"^±2.20xI0'^ 81.3±30.72 Three I.lxl0'^±9.40xl0‘° 68.7±0.21 50 Biggar 8.3xl0®±4.40xl0^ 39.5±0.13 Langmuir 2.4xl0‘’±2.87xl0‘® 99.1±29.57 Three 2.0xl0‘^±2.15x10“ 70.2±0.26 8.5 15 Biggar 2.8xlO’±2.23xlO^ 42.5±0.20 Langmuir 4.0xl0“±3.42xl0‘® 76.6±26.67 Three 5 .6 x I0 "± l.13x10'^ 78.4+0.50 50 Biggar 1.0xl0®±3.13x10’ 45.7±0.76 Langmuir 9.2xl0’^±2.16xl0’^ 131.0±5.79 Three 3.9xlO'^±1.79xlO“ 77.5±1.13 Muscovite 4.5 15 Biggar 4.8xlO^±8.26xlO'^ 32.4±0.43 Langmuir 1.3xl0’±3.44xl0’ 40.6±6.48 Three 8.7xl0’±9.64xl0* 56.7+0.27 50 Biggar 2.0x10^+1.36x10'^ 30.2+0.17 Langmuir 1.4xl0*±2.78xl0® 46.4+5.04 Three 8.5xl0‘°±5.22xl0'° 62.3±1.53 8.5 15 Biggar 3.9xlO^±3.72xlO'‘ 31.910.24 Langmuir 3.2xl0'^±1.60xl0“ 100.4121.07 Three 2.7xl0“± l.11x10“ 65.2011.03 50 Biggar 1.2xlO^±3.87xlO^ 34.610.81 Langmuir 3.1xlO‘^±l.12x10*^ 88.418.92 Three 2.4xl0“±7.95xl0‘° 65.010.81 Table 5.1 Equilibrium constants (K) for sorption of AcN on clays pretreated with HDTMA. 172 Irrespective of the magnitudes of the standard errors, certain trends were observed, which were: (i) K - values in the presence of HDTMA > K-values in the absence of HDTMA, at a clay concentration of 150 mg L*'; (ii) K-values obtained at low pH < K-values obtained at high pH; (iii) K-values obtained with Na-hectorite > K-values obtained with muscovite. There are some exception to this general trend which may be attributed to experimental errors and to the error generated in calculation of K-values from these data. Values of AG were calculated from the K-values (Table 5.1). As was shown in Chapter 4, the Biggar equation and the three-stage adsorption models are more suitable for the present clay-organic systems. As the three-stage adsorption model accounts for stage-wise sorption as is found from the sorption isotherms, it will be prudent to consider this model to be the most suitable. 5.4.3 Absorption and Emission spectra The absorption maximum (X,max) of aqueous solutions of AcN was at 353 nm (Chapter 3), while the kmax of the HDTMA solution was at 190 nm. The X„,ax of the AcN- HDTMA-clay solutions at different pH-conditions was at 356 nm with Na-hectorite at pH 4.5, and at 352 nm with Na-hectorite at pH 8.5, and also with muscovite at both pH 4.5 and 8.5. The absorption spectra of AcN-HDTMA-clay suspensions were also studied with different concentrations of HDTMA, and no shift in the position of kmax was observed. This was likely due to the absence of dimerization of AcN molecules in the presence of HDTMA molecules. DellaGuardia and Thomas (1983) found that addition of 173 HDTMA to a PN^-montmorillonite suspension resulted in the disappearance of the excimer, which was present in the PhT-montmorillonite system. This indicated that the addition of the surfactants dispersed the PhT into monomers. The fluorescence emission spectra of AcN-muscovite-HDTMA and AcN- (Na)hectorite-HDTMA suspensions at different pH conditions are shown in Figures 5.5, 5.6, 5.7, and 5.8. 174 6 pH4.5 [Acridine] (pmol L'') [HDTMA] = 15 pmol L 5 c 3 h 4 112 2 195 'ë 3 8 2 I3 u. 0 400 450 500 550 600 Emission wavelength (nm) 5 pH 4.5 [Acridine] (pmol L' ) [HDTMA] = 50 pmol L' 4 c3 « 3 112 195 I0 1 2 § 0 400 450 500 550 600 Emission wavelength (nm) Figure 5.5 Emission spectra of AcN-Muscovite-HDTMA suspension at pH 4.5 and HDTMA concentration of 15 and 50 pmol L '. 175 3.5 pH 8.5 [Acridine] (nmol U') 3.0 [HDTMA] = 15 |J.moI L â 28 G S 2.5 56 112 I 195 1 2.0 1.5 § I 1.0 0.5 0.0 400 450 500 550 600 Emission wavelength (nm) 3.5 pH 8.5 [Acridine] (|imol L'*) [HDTMA] = 50 nmol L 3.0 28 e 56 3 2.5 112 2 195 € 2.0 S § 1.5 I 1.0 3 E 0.5 0.0 400 450 500 550 600 Emission wavelength (nm) Figure 5.6 Emission spectra of AcN-Muscovite-HDTMA suspension at pH 8.5 and HDTMA concentration of 15 and 50 pmol L’'. 176 0.6 pH 4.5 [Acridine] (jxmol L‘‘) [HDTMA] = 15 iimol L 0.4 112 195 0.3 0.2 0.0 400 450 500 550 600 Emission wavelength (nm) 0.3 pH 4.5 [Acridine] (pmol L"') [HDTMA] = 50 p.mol L C /a sC 0.2 2 112 195 S u 5 ^ 0.1 2 o a U. 0.0 400 500450 550 600 Emission wavelength (nm) Figure 5.7 Emission spectra of AcN-Na(hectorite)-HDTMA suspension at pH 4.5 and HDTMA concentration of 15 and 50 pmol L"'. 177 _ pH 8.5 [Acridine] (nmol L"’) 0.6 [HDTMA] = 15 nmol L 6 28 1 0.5 3 112 2 0.4 195 ' ë 8 ai 3o E 0.0 400 500 550 600450 Emission wavelength (nm) _ pH 8.5 [Acridine] (nmol L'') 0.5 [HDTMA] = 50 nmol L C 3 0.4 56 112 5 195 i 0.3 c8 8 0.2 S J u. 0.0 400 450 500 550 600 Emission wavelength (nm) Figure 5.8 Emission spectra of AcN-Na(hectorite)-HDTMA suspension at pH 8.5 and HDTMA concentration of 15 and 50 pmol L‘‘. 178 There was no shift in the position of Xmw of the emission spectra in the presence of muscovite with increase in concentration of either AcN or HDTMA (Figures 5.5 and 5.6). At pH 4.5, the Xmax occurred at 478 nm for all AcN and HDTMA concentrations, while at pH 8.5, the Xmax was at 431 nm. These spectral positions coincide with the X-^ax - values of the AcNH^ ions and neutral AcN molecules. Also, no quenching of the emission spectra was observed with increase in AcN concentration. However, the intensities of the emission spectra decreased with increase in pH. Sorption isotherms (Figure 5.4) have shown that the amount of AcN sorbed on pretreated muscovite was greater at pH 8.5, therefore, more sorption of AcN on clays resulted in decreased intensities or quenching of X.max at high pH. Increases in AcN concentration in the AcN-HDTMA-hectorite system resulted in shifts towards shorter wavelengths (blue shift) in the emission spectra. At pH 4.5 and HDTMA concentration of 15 jxmol L'\ the X,max shifted from 487 nm to 481 nm when the AcN concentration changed from 6 pmol L’’ to 195 pmol L"'. For the same range of concentration of AcN and same pH, the X-max shifted from 489 nm to 479 nm when the concentration of HDTMA was 50 pmol L'*. At low pH conditions, the primary peak was always at the higher wavelength, unlike that at high pH conditions, when with increase in AcN concentration the peak at lower wavelength became more prominent than the peak at higher wavelength (Figure 5.8). Moreover, a blue shift was also observed in the position of both peaks. The peak at higher wavelength shifted from 486 nm to 477 nm at HDTMA concentration of 15 pmol L '\ while the same shifted from 488 nm to 475 nm at 179 HDTMA concentration of 50 jimol L''. The peak at lower wavelength shifted from 462 nm to 450 nm at HDTMA concentration of 15 iimol L'‘, while the same shifted from 460 nm to 445 nm at HDTMA concentration of 50 (imol L '\ The blue shifts in spectra can be due to the change in the predominant species of the AcN molecules on the clay surface (Chapter 3), and the peak at lower wavelength is due to the presence of both the cation and the neutral forms of AcN. The prominence of the higher wavelength peak at low pH is due to the predominance of the AcNH^ ions. At high pH, spectral peaks at 486 nm (AcN concentration = 6 p.mol L"') can only explained by the presence of AcNH^ ions in the system due to the surface protonation of the neutral molecules, which are predominant at pH 8.5 (Chapter 3). The blue shift in the emission spectra might be due to the sorption of AcN in the interlayer space with increase in AcN concentrations. Similar behavior has also been noted during the sorption of AcN on Na-hectorite (Chapter 3), and also by Tapia Estevez et al. (1994) and Lopez Arbeloa et al. (1995). Sorption at the external and internal surfaces results in emission at two wavelengths. The blue shift can also occur due to the emission from unrelaxed fluorophores when a fluorophore binds to a macromolecule (Lakowicz, 1986), and such a condition can be expected in the present study. Further information on the state of sorbed AcN molecules was obtained by washing and then resuspending the clays in water. In the case of hectorite, the Xmax indicated the predominance of AcNH^ ions (Figure 5.9a). However, the fluorescence spectra of the muscovite-HDTMA-AcN system indicated the presence of neutral AcN molecules, except at low concentrations of AcN and HDTMA, where AcNH^ was the 180 predominant species (Figure 5.9b). The presence of AcNH^ ions on the muscovite surfaces can be due to the protonation of neutral molecules at the mineral surfaces. The supernatant of the muscovite-HDTMA-AcN system showed that the predominant species in solution was AcNH^ ions. Therefore, modification of clays with surfactant molecules results in the preferential sorption of neutral organic molecules. 0.09 0.08 0.07 S' 1 0.06 b :g 0.05 ? 0.04 § 0.03 E 0.02 0.01 0.00 400 450 500 550 600 Emission wavelength (nm) (contd.) Figure 5.9 Emission spectra of a) (Na)hectorite-HDTMA-AcN suspension and b) muscovite-HDTMA-AcN suspension at [AcN] = 195 pmol L"' and [HDTMA] = 50 pmol L ‘. 181 Figure 5.9 contd. 0.14 0.12 0.10 C 3 0.08 -S S 8 0.06 3 uu 0.04 0.02 0.00 400 450 500 550 600 Emission wavelength (nm) 182 5.4.4 X-Ray Diffraction (XRD) measurements Adsorption of organic compounds in the interlayers of hectorite, or other similar swelling-type clays, leads to an increase in the basal spacings, which has been detected by XRD measurements. The factors responsible for an increase in basal spacings are the amount of organic molecules present in the interlayer space and their orientation and packing geometry. The initial d-spacing (Figure 5.10) of 13.6 Â indicates that pretreating the clay (Na-hectorite) with AcN solution of concentration 140 pmol L'' resulted in sorption of AcN molecules in the interlayer space of hectorite. Further increase in d- spacings with addition of HDTMA indicates that the surfactant molecules have entered the interlayer space also. A small dip in the plot was also observed when the concentration of HDTMA was at 25 pmol L'% and this can be due to the displacement of the AcN molecules by the HDTMA molecules. However, this dip is not significant as the size of the surfactant molecule is considerably larger than the AcN molecule. The orientation of the HDTMA and the AcN molecules in the interlayers were dictated by the competition between the interlayer water molecules, the existing AcN molecules, and the HDTMA molecules. 183 16.0 120 140 160 180 [HDTMA] (nmol L ‘) Figure 5.10 Change in basal spacings of Na-hectorite with increase in HDTMA concentration. 184 10.10 10.05 10.00 I 9.95 e e e î 9.90 I 9.85 _ pH 4.5 [HDTMA]= 15 nmol L"' [HDTMA] = 50 nmol L 9.80 I I I 0 50 100 150 200 50 100 150 200 [AcN] |imoI L‘* [AcN] nmol L 10.10 10.10 10.05 - 10.05 - 10.00 • • ' ,< 10.00 " e & & c 9.95 — .5 9.95 ’i # e e « CL. s- 9.90 - -6 9.90 - # e • 9.85 -pH 8.5 9.85 - pH 8.5 [HDTMA] = 15 nmol L'' [HDTMA] = 50 nmol L'‘ 9.80 1 1 1 1 9.80 1 1 L____1. 50 100 150 200 0 50 100 150 200 [AcN] nmol L’’ [AcN] nmol L'* Figure 5.11 Change in d-spacings for muscovite with increase in AcN concentration. 185 15.0 • pH 4.5, [HDTMA] = 15 pmol L*' o pH 4.5, [HDTMA] = 50 pmol L‘‘ ■ pH 8.5, [HDTMA] = 15 pmol L’’ o □ pH 8.5, [HDTMA] = 50 pmol L*' 14.5 -e □ □ o O # o □ ■ □ 60 •S 14.0 S* □ e ■ • 13.5 ■ ■ 13.0 _ 1. 1 1 50 100 150 200 [AcN] nmol L"' Figure 5.12 Change in d-spacings for Na-hectorite with increase in AcN concentration. 186 The basal spacings of the muscovite remained approximately consistent, muscovite being a non-swelling clay. Any variation observed in Figure 5.11 was likely due to experimental error. The basal spacings of Na-hectorite increased with increase in AcN concentration (Figure 5.12), indicating that more AcN molecules occupied the interlayer space with increase in the concentration of AcN. When the basal spacings of Na- hectorite shown in Figure 5.12 were compared to those shown in Chapter 3 for clay without HDTMA, it was found that the initial basal spacings ([AcN] = 0) was higher, and this is due to the presence of the surfactant molecules in the interlayer space. However, the extent of increase in basal spacing when the AcN concentration varied from 6 jimol L'‘ to 195 nmol L‘* for Na-hectorite pretreated with HDTMA was less than that without HDTMA. The surfactant molecules are considerably larger than the AcN molecules, and they occupy more space. As the AcN molecules enter the interlayer space, the surfactant molecules might be displaced or may undergo compaction. This would lead to a decrease in the d-spacings. With further increase in AcN concentration, the basal spacings increase, but the extent of increase is less, as the d-spacing measured is a balance between the increase due to the AcN molecules in the interlayer space, and the decrease due to the compaction or reorientation of the surfactant molecules. When Dékâny et al. (1986) studied the sorption of methanol-benzene mixtures on montmorillonites modified by presorbing the clays with a surfactant (hexadecylpyridinium), they found that the basal spacings of the clays decreased with increase in concentration of the methanol in the interlayer space. Moreover, considering the height of the AcN molecule, and its 187 planarity, it is possible that the AcN molecules could have formed aggregates with the surfactant molecules in the interlayer space. Though more AcN molecules were sorbed on pretreated clays at high pH- conditions, the XRD measurements indicate that more AcNlT’ ions, present at pH 4.5, enter the interlayer space than the neutral AcN molecules (Figure 5.12). Also, the basal spacings were higher when the concentration of HDTMA was 50 pmol L‘‘ as more HDTMA molecules are available at this concentration. 5.5 Conclusions The clay surface modified with surfactants can be a model for the soil system containing natural organic matter. Results obtained show that the presence of surfactants in the clay-organic system modifies the sorption behavior of the organic molecules on clays, thus influencing the mobility of AcN and other similar NHC molecules in the environment. The cationic surfactant molecules have high affinities for clay surfaces, and sorbed surfactant molecules provide additional sites for sorption of neutral molecules. Therefore, sorbed surfactant molecules increases the degree of sorption of organic molecules on clays. When the concentration of surfactant is considerably high, solubilization of the sorbed organic can occur as the surfactant micelles compete with the clay surface for the organic molecules. 188 CHAPTER 6 SUMMARY Nitrogen heterocyclic compounds (NHCs) are present in both industrial and domestic wastes. As these NHCs from the wastes sorb readily on soil and subsurface materials, sorption dictates the fate of these compounds in the environment. The present study was conducted to investigate the sorptive characteristics of acridine (AcN) and acridine-9-carboxylic acid (AcNCOOH) on different clay minerals. The effect of the presence of surfactant on the sorption characteristics of the above NHCs were also investigated. The results obtained from our study are as follows: 1. Sorption is dependent on the ionic state of the sorbate. Maximum sorption on negatively charged clay minerals occurs when the sorbate is positively charged. Therefore, the amount of AcN sorbed at low pH-conditions (when AcN exists as AcNH^ ions) is the largest. 2. Organic cations are preferred by the negatively charged clays. 3. Presence of negative charge on the sorbate molecule reduces the degree of sorption. AcNCOOH exists as a zwitterion at low pH-conditions (between pH 3 and 5), and 189 due to the coexistence of the negative charge with the positive charge on the molecule, the amount of AcNCOOH sorbed on clays is less than AcN at the same pH- condition. 4. The affinity of AcN molecules for hectorite and saponite clay minerals is high as indicated by the large amounts of AcN sorbed, even in excess of the CEC of the clays. 5. Clay particles act as templates for aggregate formation of the sorbed molecules. 6. The degree of aggregation of the sorbed NHC molecules increases with increase in the amount sorbed. 7. Evidence of aggregation of sorbed molecules was obtained from the shape of the sorption isotherms, sorption in excess of CEC-values of the clays, and metachromic behavior of AcN. 8. Quenching of fluorescence spectra with increase in AcN concentration in the clay- organic suspension confirmed aggregation of organic molecules on clay surfaces at high surface concentrations. 9. Sorption was dependent on the clay-type, and results showed that more sorption occurred on hectorite than on saponite. Sorption on muscovite was minimum. 10. The low degree of sorption on muscovite was due to the non-swelling nature of the clay, which reduced the available surface area for sorption. 11. The difference in sorption on hectorite and saponite is due to the location of the negative layer charge. The negative charge is positioned completely on the octahedral sheet of hectorite, while in the case of saponite, over 60% of the total negative charge is located on the tetrahedral sheet. 190 12. The surface of hectorite was found to be more hydrophobic than saponite and muscovite. The hydrophobicity of the hectorite surface was responsible for the larger degree of sorption of NHCs on hectorite. 13. The hydrophilicity of saponite and muscovite is due to the negatively charged tetrahedral sheet. 14. Neutral AcN molecules can become protonated at the clay surface, and the degree of protonation is higher on hectorite than on saponite. 15. Sorption on Na-exchanged clays is larger than on Ca-exchanged clays. 16. The organic molecules sorb not only on the external surface of the clay particles, but also on the internal surface, as was indicated from an increase in the basal spacings of the clay. 17. In the interlayer space, the incoming organic molecules displace the interlayer water molecules. 18. At low concentrations, sorption of the organic molecules proceeds primarily by electrostatic interactions. As the surface concentration increases, hydrophobic interactions become important and more neutral molecules sorb on clay. 19. Electrostatic forces dictate sorption at low pH, while at high pH, hydrophobic interactions are also significant. 20. Formation of aggregates is primarily due to hydrophobic interactions. 21. The change in free energy due to hydrophobic interactions (AG") is approximately half of the total change in free energy (AG). 191 22. The surface complexes formed due to the sorption of AcN on clays are stable, as indicated by the negative AS (change in entropy) with increase in surface concentration. 23. Affinity of the surfactant molecules for the sorption sites on clays is comparable to that of the AcN molecules. 24. Presence of surfactants influences the degree of sorption of the NHCs. At low concentrations of surfactants, sorbed surfactant molecules enhance the amount of organic molecules sorbed. 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