Sorption of Eu3+ on Na-montmorillonite studied by time-resolved laser fluorescence spectroscopy and surface complexation modeling

Abstract

The influence of pH and concentrations of Eu3+ and NaNO3 on the sorption of Eu3+ to Na-montmorillonite were investigated through batch sorption measurements and time-resolved laser fluorescence spectroscopy. The pH had little effect on the distribution coefficients (Kd) in the range of pH 4-7 at 0.01 M NaNO3, which indicates that the cation exchange reaction is a dominant sorption process. Meanwhile, the Kd strongly depended on pH at 1 M NaNO3, suggesting the formation of inner-sphere surface complexes. A cation exchange model combined with a one-site non-electrostatic surface complexation model was successfully applied to the measured Kd data. Linear free-energy relationship was used to estimate the formation constants of the surface species from those of the corresponding aqueous hydrolyzed species. The TRLFS spectra of Eu3+ sorbed on Na-montmorillonite were processed by parallel factor analysis, which provided the fluorescence spectra, decay lifetimes, and relative intensity profiles of three Eu3+ surface species. These species corresponded to one outer-sphere (factor A) and two inner-sphere (factors B and C) complexes. It turned out that factors A and B correspond to Eu3+ sorbed by ion exchange to permanent charge sites of Na-montmorillonite and inner-sphere complexation with surface hydroxyl groups of the edge faces. Factor C became dominant at relatively high pH and ionic strength and likely corresponded to the precipitation of Eu(OH)3 on the surface of Na-montmorillonite.