Mechanisms of uranyl sorption

Abstract

Abstract Detailed knowledge of the reactions at the water/colloid/mineral interface is crucial to model accurately actinide behaviour in nature. In this paper, we review current knowledge of the sorption of actinides and of the mechanisms of sorption, with a particular focus on uranyl. Of major interest is the influence of the aqueous uranyl species (e.g., carbonate complexes, polynuclear species, colloids) on the uranyl sorption species. We present extended X-ray absorption fine structure (EXAFS) and X-ray photoelectron spectroscopy (XPS) studies on the coordination of uranyl onto an amorphous Al phase and onto quartz, respectively. Our XPS investigations show that two components having uranyl ions in very distinct coordination environments co-exist on quartz at high uranyl surface coverage, independently of the presence of uranyl carbonate complexes or uranyl colloids in solution. One component corresponds to polynuclear surface species and/or schoepite-like surface precipitates. In the case of similar uranyl concentrations and of high carbonate solution concentrations, polymeric uranyl species are formed on quartz, whereas no such surface species occurs on the Al phase. Uranyl is found on the Al phase as mononuclear uranyl carbonato surface complexes only. These results are of importance because they suggest that mineral surface characteristics strongly control the uranyl surface species in aquifers.