Chromate is a common groundwater contaminant as a result of extensive industrial use. The fate and transport of chromate in subsurface environments are largely governed by adsorption to the surfaces of aluminum and iron oxides. Characterization of adsorption mechanisms is needed to constrain thermodynamic models, better predict the fate and transport of environmental pollutants, and expand our fundamental knowledge of surface geochemistry. However, the mechanisms of chromate adsorption in soils have remained unclear. This dissertation clarifies the mechanisms by which chromate adsorbs to aluminum and iron oxide minerals on the basis of molecular spectroscopy and quantum mechanical calculations.

Chromate adsorption was first characterized on ferrihydrite using in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR--FTIR) and theoretical frequency calculations. The effects of pH, aqueous chromate concentration, and ionic strength were investigated. It was determined that chromate primarily forms inner--sphere surface complexes on ferrihydrite. Monodentate complexes were observed to form at pH $>$ 6 and at low surface coverage under acidic pH conditions. Bidentate complexes are dominant below pH 6 and at high surface coverage. Theoretical infrared frequencies are in close agreement with those observed experimentally.

Characterization of the chromate adsorption on hematite was performed using in situ ATR--FTIR spectroscopy and extended X--ray absorption fine structure (EXAFS) spectroscopy. The results were interpreted in the context of the optimized geometries and thermodynamics of the theoretical chromate--iron oxide cluster models. The results indicate that chromate binds to the surface of hematite in a similar manner as that of ferrihydrite; monodentate complexes dominate at high pH, and bidentate complexes dominate at low pH.

The last system investigated was chromate adsorption on the surface of the aluminum oxyhydroxide boehmite. Both in situ ATR--FTIR and EXAFS spectroscopies were used to characterize chromate adsorption as a function of pH, ionic strength, and chromate and sulfate concentrations. It was determined that chromate primarily forms outer--sphere complexes on boehmite over a broad range of pH and chromate concentrations. Results from EXAFS and ATR--FTIR spectroscopy indicate the presence of a small fraction of inner--sphere chromate under acidic conditions.