X-RAY-ABSORPTION SPECTROSCOPIC STUDIES OF CADMIUM AND SELENITE ADSORPTION ON ALUMINUM-OXIDES

To enhance our understanding of trace element partitioning at oxide-wa ter interfaces, we studied the local coordination environment of cadmi um(II) and selenite complexes sorbed on aluminum oxides, in situ, usin g X-ray absorption spectroscopy (XAS). The adsorbents included porous, high surface area transition aluminas (ALCOA CP-5 and C-33) and the m inerals corundum (alpha-Al2O3) and gibbsite (gamma-Al(OH)(3)). Cadmium sorption densities ranged from 1.2 to 12.0 mu mol/m(2) and selenite s orption densities ranged from 1.2 to 4.6 mu mol/m(2). X-ray absorption fine structure (XAFS) analysis of the selenite spectra indicates that Se is coordinated to three O atoms at 1.69 Angstrom, regardless of su bstrate, and that the complexes are mononuclear. XAFS analysis of low sorption density cadmium complexes suggests that cadmium is coordinate d to six oxygens at 2.33 Angstrom and that the complexes are mononucle ar. Analysis of a high total cadmium concentration sample (10(-3) M) g ave O first neighbors at 2.35 Angstrom and Cd second neighbors at 3.84 Angstrom, suggesting the formation of a disordered cadmium hydroxide or cadmium hydroxocarbonate precipitate. Absence of Al second neighbor s In the selenite and the low sorption density cadmium samples is prob ably caused by the low backscattering amplitude of Al and thermal and static disorder effects. These results, in combination with sorption i sotherm data, suggest that, under the conditions studied, cadmium and selenite diffuse into the pores of the transition aluminas and sorb as mononuclear complexes. These results have significant implications fo r the fate of trace elements in subsurface environments and the remedi ation of waters and groundwaters.