GEOC 37 |
| Amanda L. Mifflin, Kathrine A. Gerth, Brian M. Weiss, and Franz M. Geiger. Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 |
| Hexavalent chromium contamination in ground water is widespread and has received much attention in the past decades. Using the nonlinear optical laser spectroscopy surface second harmonic generation (SHG), we have studied chromate adsorption and desorption at the fused quartz/liquid water interface in real time, at room temperature and at chromate concentrations ranging from 1x10-6 to 2x10-4 M. A polarization analysis of the SHG signal shows a nonlinear optical response directed predominantly along the surface normal, consistent with the electric dipole of the surface-bound chromate being directed along the surface normal. Adsorption isotherms measured at pH values ranging from 4 to 11 indicate that chromate forms two to three hydrogen bonds when bound to the quartz-water interface; its free energy of adsorption is 38(1) kJ/mol. The pH dependence of the free energies of adsorption is discussed in the context of quartz-water interfacial structure and hydrogen bonding. Finally, a significant blue shift of the nonlinear electronic surface spectrum obtained from the surface-bound chromate compared to the bulk UV-VIS spectrum suggests a distortion of the ligand field energy levels, which could be due to either the formation of hydrogen bonds to the quartz/water interface, a geometric change in the ligand field structure, or both. Real-time kinetic measurements of chromate adsorption and desorption in the microM chromate concentration range show that chromate binding to the fused quartz/water interface is completely reversible, consistent with the high mobility of Cr(VI) in soils and the $B&$(BGads determined from our adsorption isotherm measurements. Kinetic studies carried out using varying flow rates at constant chromate concentration allow us to assess the importance of mass transfer through the hydrodynamic boundary layer in our laminar flow system. Kinetic measurements carried out with varying chromate concentrations at a constant flow rate allow us to determine the adsorption and desorption rate constants, which yield binding constants that are in good agreement with the thermodynamic results obtained from the adsorption isotherms. Results obtained from chromate binding studies to acid-terminated organic silanes on silica indicate that, in contrast to the silica/water interface, Cr(VI) adsorption is not fully reversible. A modified adsorption model consistent with the experimental data is presented. |
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Chemistry of Metals in Terrestrial and Aquatic Systems
1:30 PM-5:10 PM, Monday, March 29, 2004 Marriott -- Marquis NE, Oral
Division of Geochemistry |