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 gamma-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 gamma-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 gamma-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., gamma-Al2O3/L/Eu(III), L represents HA sites) to 1 : 2 form (i.e., gamma-Al2O3/L-2/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.