Selenium(IV) Sorption Onto γ-Al₂O₃: A Consistent Description of the Surface Speciation by Spectroscopy and...

The sorption processes of Se(IV) onto γ-Al₂O₃ were studied by in situ Infrared spectroscopy, batch sorption studies, zeta potential measurements and surface complexation modeling (SCM) in the pH range from 5 to 10. In situ attenuated total reflection fourier-transform infrared (ATR FT-IR) spectroscopy revealed the predominant formation of a single inner-sphere surface species at the alumina surface, supporting previously reported EXAFS results, irrespective of the presence or absence of atmospherically derived carbonate. The adsorption of Se(IV) decreased with increasing pH, and no impact of the ionic strength was observed in the range from 0.01 to 0.1 mol L^-1 NaCl. Inner-sphere surface complexation was also suggested from the shift of the isoelectric point of γ-Al₂O₃ observed during zeta potential measurements when Se(IV) concentration was 10^-4 mol L^-1. Based on these qualitative findings, the acid-base surface properties of γ-Al₂O₃ and the Se(IV) adsorption edges were successfully described using a 1-pK CD-MUSIC model, considering one bidentate surface complex based on previous EXAFS results. The results of competitive sorption experiments suggested that the surface affinity of Se(IV) toward γ-Al₂O₃ is higher than that of dissolved inorganic carbon (DIC). Nevertheless, from the in situ experiments, we suggest that the presence of DIC might transiently impact the migration of Se(IV) by reducing the number of available sorption sites on mineral surfaces. Consequently, this should be taken into account in predicting the environmental fate of Se(IV).