Selenium(IV) sorption onto γ-Al2O3: a consistent description of the surface speciation by spectroscopy and thermodynamic modeling

The sorption processes of selenium(IV) onto γ-Al2O3 were studied by in situ vibrational spectroscopy, batch sorption studies, zeta potential measurements and Surface Complexation Modeling (SCM). In the pD range from 5 to 9, 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 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. The formation of inner-sphere surface complexes was also suggested from the shift of pHIEP of γ-Al2O3 observed during zeta potential measurements at the highest Se(IV) concentration applied (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, using one inner-sphere bidentate surface complex. The results of competitive sorption experiments strongly suggested that the surface affinity of Se(IV) towards γ-Al2O3 is higher than that of dissolved inorganic carbon (DIC). Nevertheless, the competing effect might impact the migration of selenium(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 selenium(IV).