Effect of silicate on U(VI) sorption to γ-Al2O3: Batch and EXAFS studies

• Effects of U(VI) and silicate on their interaction with γ-Al2O3 is investigated.
• Investigate the microstructure of the ternary complexes by EXAFS spectroscopy.
• Obtain the sorption mechanism of U(VI) and silicate on γ-Al2O3.

The effect of soluble silicate on the sorption of U(VI) to γ-Al2O3 was investigated by batch experiments and extended X-ray absorption fine structure (EXAFS) method. The presence of silicate enhanced the sorption of U(VI) on γ-Al2O3 surface and the sorption was attributed to inner-sphere surface complexation. The structure of the adsorbed U(VI) and silicate on γ-Al2O3 was investigated in the analysis of EXAFS spectra. The fitting of the experimental EXAFS data was obtained by including two uranium coordination shells with 2 axial (Oax) and 5 equatorial (Oeq) oxygen atoms at 1.79 ± 0.02 and 2.43 ± 0.02 Å, respectively, and the third coordination shells with Al atom at ∼3.35 Å. Silicate contributed to the formation of ternary inner-sphere surface complexes, acting as “bridge” between U(VI) and γ-Al2O3 and enhanced the sorption of U(VI). The observations suggested that the interactions between U(VI) and silicate were important in controlling U(VI) retention.