Insights into sorption species of Eu(III) on γ-Al2O3 and bentonite under different pH: Studies at macro- and micro-scales

The sorption species of Eu(III) on gamma-Al2O3 and bentonite was investigated by batch, surface complexation model (SCM), and X-ray absorption spectroscopy (XAS). The results showed that sorption edges of Eu(III) on gamma-Al2O3 and bentonite were as expected shifted forward high pH with the increasing in Eu(III) concentration, and sorption of Eu(III) was strongly dependent on pH. In gamma-Al2O3 system, sorption of Eu(III) was decreased above pH 8.5 at low concentration of Eu(III) because of water soluble carbonate species of Eu(III), however the decline did not appear at high concentration of Eu(III) possibly due to a offset effect of surface precipitation. Actually, the sorption species of Eu(III) on bentonite mainly referred to at least four kinds of species including ion exchange (>X3Eu0) at low pH, inner-sphere complexes (>AlOEu2+ and >SiOEu2+) at neutral condition, and hydrolysis species (>SiOEu(OH)(2)(0)) at alkaline condition. Linear combination fitting (LCF) in k space testified that hydrolysis of Eu(OH)(3)(s) and oxide of Eu2O3 species were major for Eu(III) sorption on gamma-Al2O3, whereas Eu3+(aq) and hydrolysis species comprised sorption species on bentonite. Extended X-ray absorption fine structure (EXAFS) analysis further confirmed the prediction from SCM and LCF. In addition, the typical shells of Eu-Al in R range of 3.0-3.4 angstrom and Eu-Si at similar to 4.0 angstrom were found in radial structure functions, which was possibly identified to edge-shared bidentate of Eu(III) on Al2O3 and bentonite.