Detection of Oxyanion Adsorption at the Silica-Aqueous Interface using Total Internal Reflection (TIR)-Raman Spectroscopy Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Brant M. Finzer, B.S. Graduate Program in Chemistry The Ohio State University 2014 Master's Examination Committee: Prof. Dr. Heather C. Allen, Advisor Prof. Dr. Claudia Turro Copyright by Brant M. Finzer 2014 ABSTRACT Total internal reflection (TIR)-Raman spectroscopy was used to investigate the adsorption of selenate and selenite oxyanions at the silica-aqueous interface over the pH range 5-7. If adsorption were to occur, the symmetry of the Td selenate oxyanion would be reduced to either that of C3v or C2v corresponding to a monodentate or bidentate binding geometry respectively. This would cause the symmetric stretching mode in the Raman spectrum to split into two or three separate peaks respectively. It was determined that the peak corresponding to the symmetric stretching mode did not split, therefore no adsorption occurred. This provides insight to the potential mobility of selenate in the environment as adsorption to silica would greatly impact mobility through the Earth’s crust. ii DEDICATION This thesis is dedicated to my parents. iii ACKNOWLEDGMENTS I would first like to thank my advisor Dr. Heather C. Allen for her guidance and support in my graduate studies. I would also like my colleagues in the Allen Lab. I would especially like to thank Dr. Dominique Verreault for the countless times I went to him for assistance with research or helping me to organize my thoughts in the writing of this thesis. iv VITA October 28th, 1989 .........................................................Born – Dover, Ohio 2008 – 2012 ...................................................................B.S. Chemistry, Shawnee State University Portsmouth, Ohio, USA 2012 – 2014 ...........................................................Graduate Teaching Associate/Graduate Research Associate, Department of Chemistry and Biochemistry, The Ohio State University – Columbus, Ohio, USA FIELD OF STUDY Major Field: Chemistry v TABLE OF CONTENTS ABSTRACT ....................................................................................................................... ii DEDICATION ................................................................................................................... iii ACKNOWLEDGMENTS ................................................................................................. iv VITA ....................................................................................................................................v FIELD OF STUDY ..............................................................................................................v TABLE OF CONTENTS .................................................................................................. vi LIST OF FIGURES ........................................................................................................... ix LIST OF TABLES ........................................................................................................... xiii LIST OF ABBREVIATIONS AND SYMBOLS ............................................................ xiv CHAPTER 1: INTRODUCTION ........................................................................................1 1.1 Environmental Contamination ...................................................................................1 1.2 Importance of Oxyanion Adsorption to Mineral Oxides ...........................................2 1.3 Previous Studies on the Sorption of Oxyanions to Minerals/Metal Oxides ...............3 1.4 Silica and Selenium ....................................................................................................5 1.4.1 The Silica Surface ...............................................................................................5 1.4.2 Previous Studies of Adsorption on Silica ............................................................7 vi 1.4.3 Selenium in the Environment ..............................................................................9 1.4.4 Previous Selenium Oxyanion Sorption Studies on Mineral/Metal Oxides .......11 1.5 Objectives of the Current Study ...............................................................................13 CHAPTER 2: THEORETICAL BACKGROUND ...........................................................15 2.1 Total Internal Reflection (TIR-) Raman Sectroscopy ..............................................15 2.1.1 Classical Theory of Raman Scattering ..............................................................15 2.1.2 Corrections to Classical Theory and a Quantum Approach ..............................18 2.1.3 Total Internal Reflection and the Critical Angle ...............................................20 2.1.4 Evanescent Wave and Penetration Depth ..........................................................22 2.2 Group Theory and the Reduction of Symmetry Upon Adsorption ..........................24 2.2.1 Basic Group Theory Background ......................................................................24 2.2.2 Symmetry of Bulk Oxyanions and their Active Modes ....................................27 2.2.3 Symmetry of Adsorbed Species and their Active Modes ..................................28 CHAPTER 3: MATERIALS AND METHODS ...............................................................31 3.1 Materials ...................................................................................................................31 3.2 Solution Preparation .................................................................................................32 3.3 Conventional Raman Set-up.....................................................................................34 3.4 TIR-Raman Setup .....................................................................................................36 3.5 Data Processing and Fitting .....................................................................................39 vii CHAPTER 4: RESULTS AND DISCUSSION ................................................................41 4.1 Selenate ....................................................................................................................41 4.1.1. Bulk Raman Spectra of Selenate Solutions ......................................................41 4.1.2. TIR-Raman Spectra of Selenate Solutions on Silica ........................................54 4.2 Selenite .....................................................................................................................64 4.2.1 Bulk Raman Spectra of Selenite Solutions ........................................................64 4.2.2 TIR-Raman of Selenite Solutions on Silica ......................................................67 CHAPTER 5: CONCLUSIONS AND FUTURE WORK.................................................71 REFERENCES ..................................................................................................................74 APPENDIX A Copyright Information for Figure 2.3 .......................................................77 viii LIST OF FIGURES Figure 1.1: Diagram of the various surface sites possible at the silica-aqueous interface. 6 Figure 1.2: Speciation curve for silica surface sites vs pH. Note that at pH<7, >95% of surface sites exist as Si-OH. ................................................................................................ 7 Figure 1.3: Diagram representing the geometries of monodentate and two different bidentate adsorption species. ............................................................................................ 13 Figure 2.1: Energy level diagram depicting Stokes, Rayleigh, and anti-Stokes scatterings. In Stokes scattering, molecules absorb a photon of energy hνex and emit a photon of energy h(νex-νv) = hνs. in contrast, anti-Stokes scattering emits a photon of energy h(νex+νv) = hνas. ................................................................................................................................... 16 Figure 2.2: Examples of light incident below, at, and above the critical angle of the silica- aqueous interface. The refractive indices of silica and water at 532 nm are 1.461 and 1.336, respectively. ...................................................................................................................... 22 Figure 2.3: Absolute values of the Fresnel factors 푡푃푥 (solid blue line), 푡푆푦 (dashed green line), and 푡푃푧 (dotted red line) as a function of incident angle at the silica-aqueous interface This figure is reproduced from Ref. 3 with permission from The Royal Society of Chemistry (see Appendix A for copyright information). .................................................. 24 Figure 2.4: Vibrational modes of water. .......................................................................... 25 Figure 2.5: Diagram of the splitting of the symmetric stretch (ν1) upon adsorption to a surface and subsequent lowering of symmetry from a Td geometry to either C3v or C2v. 30 ix Figure 3.1: Speciation curve for aqueous selenate and selenite. The green areas highlight 2- the range covered by the prepared solutions. Note that at pH > 5, [SeO4 ] > 99% [Se]tot. ........................................................................................................................................... 33 Figure 3.2: Overview of the setup used for conventional