New insight into Eu(III) sorption mechanism at alumina/water interface by batch technique and EXAFS analysis

In this study, the interactions among Eu(III), humic acid (HA) and γ-Al2O3 as a function of pH and ionic strength were investigated by batch technique and extended X-ray absorption fine structure (EXAFS) analysis. For binary γ-Al2O3/Eu(III) systems, the pH-dependent and ionic strength-independent sorption behavior was indicative of an inner-sphere complexation mechanism. The EXAFS spectra analysis showed that Eu(III) sorption mechanisms at pH 5.0, 6.5 and 8.0 could be attributed to the formation of edge-shared and corner-shared surface complexes. In contrast, surface precipitation and/or surface polymerization tended to be the predominant sorption mechanism at pH 10.0. The spectra analysis results suggested no difference between the proportions of edge-shared and corner-shared complexes formed at different ionic strengths. For ternary γ-Al2O3/HA/Eu(III) systems, the sorption percentage of Eu(III) increased with increasing ionic strength. The EXAFS-derived structural parameters pointed to the formation of binary surface complexes (i.e., edge-shared mode and corner-shared mode) at low ionic strength and ligand-bridging ternary complexes at high ionic strength. In addition, the stoichiometry of ternary complexes showed a trend from 1:1 form (i.e., γ-Al2O3/L/Eu(III), L represents HA sites) to 1:2 form (i.e., γ-Al2O3/L2/Eu(III), L represents HA sites) with increasing ionic strength. The experimental findings in this study can provide an important theoretical basis for further predicting the fate of trivalent radionuclides in aquatic systems. (orig.)