Uranium(VI) sorption complexes on montmorillonite as a function of solution chemistry

We have investigated the effect of changes in solution chemistry on the nat ure of uranyl sorption complexes on montmorillonite (SAz-1) at different su rface coverages (1.43-53.6 mu mol/g). Uranyl uptake onto SAz-1 between pH 3 and 7 was determined in both titration and batch-mode experiments. These p H values result in solutions that contain a range of monomeric and oligomer ic aqueous uranyl species. Continuous-wave and time-resolved emission spect roscopies were used to investigate the nature of U(VI) sorbed to SAz-1. A d iscrete set of uranyl surface complexes has been identified over a wide ran ge of pH values at these low to moderate coverages. For ail samples, two su rface complexes are detected with spectral characteristics commensurate wit h an inner-sphere complex and an exchange-site complex; the relative abunda nce of these two species is similar over these pH values at low coverage (1 .43-2.00 mu mol/g). In addition, surface species having spectra consistent with polymeric hydroxide-like sorption complexes form at the moderate cover ages (similar to 34-54 mu mol/g), increasing in abundance as the capacity o f the amphoteric surface sites is exceeded. Furthermore, a species with spe ctral characteristics anticipated for an outer-sphere surface complex is ob served for wet paste samples at low pH (3.7-4.4) and both low (similar to2 mu mol/g) and moderate (similar to 40 mu mol/g) coverage. There are only su btle differences in the nature of sorption complexes formed at different pH values but similar coverages, despite markedly different uranyl speciation in solution. These results indicate that the speciation in the solution ha s minimal influence on the nature of the sorption complex under these exper imental conditions. The primary control on the nature and abundance of the different uranyl sorption complexes appears to be the relative abundance an d reactivity of the different sorption sites. (C) 2001 Academic Press.