X-ray absorption fine structure spectroscopy study of Eu(III) sorption products onto amorphous silica and γ-alumina: Effect of pH and substrate

摘要

A molecular level understanding of metal ions sorption onto solids is necessary for modelling the sorption process in a predictive manner and thus for planning of the safe disposal of nuclear wastes. In this study, we have used X-ray absorption fine structure spectroscopy (XAFS) in conjunction with batch sorption and solubility studies to investigate the effect of pH and substrate on the formation and structure of Eu(III) sorption products on two environmentally relevant mineral oxides, namely amorphous silica and γ-alumina. 0.1 mM Eu solution was contacted with silica and alumina over pH 4-8 in an ionic medium of 0.1 M NaClO _4. Batch sorption data indicates a stronger sorption capacity of alumina in comparison to silica. Silica solubility is orders of magnitude higher than that of alumina over pH 3-8. The pH (6-8) and metal ion concentration in the XAFS samples corresponds to the undersaturated to oversaturated state with respect to Eu(OH) _3 precipitation. Modelling of the EXAFS spectra indicates: (1) the formation of a Eu sorption product at pH ～6 in the form of a small atomic cluster wherein Eu binds to oxygen atoms on both solids in a monodentate corner sharing and edge sharing manner, (2) alumina prefers an edge sharing mode with increasing pH over silica, (3) both solids form a surface precipitate containing Eu at higher pH values, with the sorption product being richer in Eu content at the silica surface, and (4) the appearance of a Eu-Eu neighbour distance of ～3.5 and 4.0 for alumina and silica, respectively. The Eu-Eu distances for sorption samples, compares with the 3.67 and 4.09 Eu-Eu distances in Eu(OH) _3, and thus substantiate the edge and corner sharing preference for alumina and silica, respectively. Though Eu forms similar sorption products on silica and alumina over pH 6-8, there is a distinct difference in their formation pattern at the two surfaces.