Oxidant Generation on Photolysis of Silver Chloride Suspensions: Implications to Organic Contaminant
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Oxidant generation on photolysis of silver chloride suspensions: implications to organic contaminant degradation Tian Ma Supervisor: Scientia Professor T. David Waite A thesis in fulfilment of the requirements for the degree of Doctor of Philosophy School of Civil and Environmental Engineering Faculty of Engineering March, 2014 A ORIGINALITY STATEMENT ‘I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged.’ Signed …………………………………………….............. Date ……………………….............................................. i COPYRIGHT STATEMENT ‘I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstract International (this is applicable to doctoral theses only). I have either used no substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied/will apply for a partial restriction of the digital copy of my thesis or dissertation.' Signed ……………………………………………........................... Date ………………………........................................................ AUTHENTICITY STATEMENT ‘I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis. No emendation of content has occurred and if there are any minor variations in formatting, they are the result of the conversion to digital format.’ Signed ……………………………………………........................... Date ………………………....................................................... ii Abstract The photochemistry of silver chloride and other silver halides has attracted intensive attention among researchers for the past twenty years, in part at least, because it is recognized as a promising technology for degradation of organic contaminants in waters and wastewaters. Silver chloride displays semiconducting properties and absorbs incoming photons in the UV- visible region with resultant formation of several highly reactive species and, in many instances, the degradation of organics, if organics are present. The fact that the actual reaction mechanism remains uncertain hinders its practical application. This thesis describes work investigating the photolysis of silver chloride under various solution conditions relevant to real life environmental water systems and describes the photocatalytic decay of formic acid and rhodamine B induced by silver chloride. The impact of solution matrix on the formation, size, aggregation behaviour and stability of silver chloride was also investigated by employing advanced analytical technologies including Dynamic Light Scattering (DLS), SEM (STEM) and X-ray Diffraction (XRD), as well as spectrophotometry and spectrofluorimetry. These techniques allow determination of the concentration of species at micro- or nano- molar levels and particle sizes down to the nanometer size range. Kinetic modelling of the mechanism on photolytic activity of silver chloride and its photocatalytic decomposition of formic acid was done with Matlab-Acuchem. The concentration change of formic acid was detected using a radioisotope-labelling technique coupled with scintillation counting. A Results show that reactive species superoxide, hydrogen peroxide and singlet oxygen are generated through irradiation of silver chloride colloid, as well as the generation of free chlorine and silver nanoparticles. The relative importance of these various species was influenced by the initial concentrations of Ag(I), Cl–,and bicarbonate as well as by the pH and the presence of oxygen. Simulation of laboratory results by kinetic modelling of the processes indicated that oxidation of Cl– by holes resulted in formation of Cl0 while scavenging of 0 •– photo-generated electrons by Ag(I) and O2 led to the formation of Ag and O2 respectively. Results obtained indicate that holes are responsible for the oxidation of formic acid while for rhodamine B, reaction with holes and chlorine atoms was found to be responsible for its decay. B Table of Contents Page Abstract .................................................................................................................................... A Table of Contents ....................................................................................................................... I ORIGINALITY STATEMENT .................................................................................................. i COPYRIGHT STATEMENT .................................................................................................... ii AUTHENTICITY STATEMENT ............................................................................................. ii Acknowledgements .................................................................................................................. iii List of Figures ........................................................................................................................... iv List of Tables ....................................................................................................................... xviii Chapter 1: Introduction .............................................................................................................. 1 1.1 Problem statement and thesis purpose ............................................................................ 1 1.2 Photochemistry of AgCl(s) ............................................................................................. 2 Properties of silver chloride ............................................................................................... 3 Synthesis and characterization of silver chloride ............................................................... 4 Mechanism of photocatalytic activities of Ag/AgX .......................................................... 5 Mechanism of degradation of organics .............................................................................. 5 1.3 Silver nanoparticle .......................................................................................................... 7 I Properties of Silver Nanoparticles ..................................................................................... 7 Optical Properties............................................................................................................... 7 1.4 Key objectives and outcomes .......................................................................................... 8 Chapter 2: Characterisation of AgCl(s) Particle ..................................................................... 9 2.1 Introduction ..................................................................................................................... 9 2.2 Experimental Details ..................................................................................................... 20 2.2.1 General ................................................................................................................... 20 2.2.2 Particle Size and Zeta-Potential Measurement ...................................................... 21 2.2.3 Sample Preparation for SEM/STEM Analysis ...................................................... 22 2.2.4 Electronic Spectra of AgCl(s) Colloid ................................................................... 23 2.2.5 XRD measurement ................................................................................................. 23 2.3 Results and discussion .................................................................................................. 23 2.3.1 Formation of AgCl(s) Colloid ................................................................................ 23 2.3.2 Aggregation of Silver Chloride in the Presence of Sodium Chloride .................... 25 2.3.3 Zeta Potential ......................................................................................................... 32 2.3.4 Silver Chloride Particle Surface Topography and Composition by SEM ............. 33 2.3.4.1 EDS Results from SEM Analysis ................................................................... 34 II 2.3.4.2 SEM of AgCl(s) particles kept in the dark...................................................... 36 2.3.4.3 SEM of AgCl(s) particles irradiated without organics ................................... 38 2.3.4.4 SEM of AgCl(s) particles irradiated in the presence of rhodamine B ............ 38 2.3.5 Silver Chloride Particle Surface