Sorption of Se(VI) and Se(IV) on AFm phases

Selenate and selenite can sorb on AFm phases such as monosulfate, monocarbonate or hemicarbonate. Sorption and co-precipitation experiments were performed to determine their potential for the immobilization of two of the main aqueous species of selenium: Se^VIO₄²⁻ and Se^IVO₃²⁻. A preferential uptake by hemicarbonate over monosulfate, and a much weaker sorption on monocarbonate was observed for both Se(IV) and Se(VI). Increased sulfate and carbonate concentrations as present at higher pH lowered the uptake of Se(VI) and Se(IV).

Experimentally obtained sorption isotherms on hemicarbonate and monosulfate were reproduced with thermodynamic solid solution models, considering both surface sorption and interlayer bonding. For monocarbonate interlayer sorption was suppressed by the rigid structure and narrow interlayer distance. The observed limited uptake could be described by an anion exchange model on the outer surfaces only. This study shows that both anion exchange in the interlayer as well as sorption on outer surface sites can determine Se uptake by AFm phases.