Macroscopic and spectroscopic characterization of selenate, selenite, and chromate adsorption at the solid–water interface of γ-Al 2O 3

Abstract

The interaction of selenate, selenite, and chromate with the hydrated surface of γ-Al 2O 3 was studied using a combination of macroscopic pH edge data, electrophoretic mobility measurements, and X-ray absorption spectroscopic analyses. The pH edge data show generally increased oxyanion adsorption with decreasing pH, and indicate ionic strength-(in)dependent adsorption of chromate and selenate across the pH range 4–9, and ionic strength-(in)dependent adsorption of selenite in this pH range. The adsorption of chromate peaks at pH 5.0, whereas for selenate and selenite no pH adsorption maxima are observed. Electrophoretic mobility measurements show that all three oxyanions decrease the zeta potential of γ-Al 2O 3 upon adsorption; however, only selenite decreased the pH PZC of the γ-Al 2O 3 sorbent. EXAFS data indicate that selenite ions are coordinated in a bridging bidentate fashion to surface AlO 6 octahedra, whereas no second-neighbor Al scattering was observed for adsorbed selenate ions. Combined, the results presented here show that pH is a major factor in determining the extent of adsorption of selenate, selenite, and chromate on hydrated γ-Al 2O 3. The results point to substantial differences between these anions as to the mode of adsorption at the hydrated γ-Al 2O 3 surface, with selenate adsorbing as nonprotonated outer-sphere complexes, chromate forming a mixture of monoprotonated and nonprotonated outer-sphere adsorption complexes, and selenite coordinating as inner-sphere surface complexes in bridging configuration.