Spectroscopic studies of the effects of selenate and selenite on cobalt sorption to gamma-Al2O3.

The fate and transport of toxic metal ions and radionuclides in the environment is often controlled by sorption reactions. The extent of sorption of divalent metal cations is controlled by a number of factors including the presence of cosorbing or complexing ligands. To study the impact of anion cosorption on metal cation sorption behavior, Co(II) sorption to gamma-Al2O3 in the presence of selenium oxyanions was investigated. To aid in the interpretation of macroscopic sorption results, X-ray absorption spectroscopy (XAS) experiments were conducted on single and bisorbate samples where the Co(II) surface coverage of the bisorbate sample was greater than or equal to the single sorbate sample. XAS data for single sorbate Co(II) samples were consistent with reported spectra of Co(II)-Al(III) layered double hydroxides (LDHs), indicating coprecipitates are forming in these samples. Comparison of data from single and bisorbate samples showed a decrease in the number of nearest neighbor cobalt atoms in the presence of Se(IV), irrespective of the order of Se(IV) addition and no change in Co coordination in the presence of Se(VI). The extent of the decrease in cobalt second shell features between single and bisorbate samples with equal Co(II) coverage increased with an increase in the Se:Co surface coverage ratio. This trend suggests that the effect of Se(IV) on Co(II) sorption is a function of the Se(IV) surface coverage. At low ratio values, Co(II)-Al(III) LDH precipitates dominate. Increasing the Se:Co surface coverage ratio results in a progressive conversion of coprecipitate to an unknown, disordered Co(ll)/Se(IV) phase. Based on macroscopic data, this new phase could be an LDH with Se(IV) in the interlayer, an alternative precipitate such as a mixed metal Se(IV) hydrate, or a ternary complex.