Immobilization and ligand-induced remobilization of uranium in anoxic aquifer environments

Uranium (U) is a naturally occurring radionuclide that poses serious health risk owing to its chemical and radiological toxicity. The main route of U exposure towards humans is drinking (ground)water. The mobility of U is essentially driven by its speciation, where U(VI) is present as relatively mobile species and U(IV) as largely immobile in the environment. Hence, many U remediation strategies are targeted towards biotic or abiotic reduction of U(VI) to immobile U(IV) species. However, success of such remediation processes is dependent on biogeochemical conditions caused by interactions between dissolved species, mineral surfaces, and microbial activities. Protecting water quality thus requires better understanding of the geochemical factors that control U stability in the subsurface environment; one such factor is the presence of organic ligands. The role of naturally occurring organic ligands in affecting nutrient and contaminant mobility is well established in subsurface environments. We tested the hypothesis that these organic ligands have the potential to effectively remobilize the reduced uranium and bring it to a concentration that exceeds the WHO safety guideline for U in drinking water (30µg L-1). We utilized aquifer sediment from Retz, Austria in which uranium bioreduction is known to occur. Bioreduction and accumulation of dissolved U was observed in a flow through column experiment. These reduced U containing sediments were then treated with various organic ligands (e.g., citrate, oxalate, EDTA, DTPA, and DFO-B) in varying concentrations in batch and column experiments to elucidate the mechanism and extent of ligand-induced mobilization of U. We also used geochemical modelling with PhreeqC to generate a conceptual model for ligand-induced mobilization of U. The results of this study provide insights in the extent of ligand-induced mobilization of U in the environment. This information is essential in planning future aquifer management and U remediation strategies