Role of Organic Ligands on the Stability of Tetravalent Uranium under Anoxic Conditions

Biogeochemistry

Anthropogenic activities associated with mining, ore processing, and nuclear materials production have left a legacy of uranium (U) contamination in aqueous systems, soils, and sediments. As toxic metals cannot be degraded, remediation efforts have focused on stabilization so as to decrease the mobility and bioavailability of the contamination. The stimulation of microbial reduction of the soluble hexavalent U (U(VI)) to sparingly soluble tetravalent U (U(IV)) by indigenous anaerobic metal-reducing bacterial communities has been utilized as an in-situ strategy for the immobilization of uranium in contaminated aquifers. Yet, the success of this strategy rests on the low solubility of U(IV) phases that are formed as the product of microbial reduction. Recent research efforts though have shown that these reduction products include not only the recalcitrant uraninite (UO2), but also labile biomass-bound noncrystalline U(IV). Furthermore, biogenic ligands and synthetic chelators have been shown to potentially influence the long-term stability of bioreduced U(IV) by mobilizing U even under strong reducing conditions.

 

In light of the drastically differing labilities of UO2 and noncrystalline U(IV) in addition to the ability of various organic ligands to mobilize U under anoxic conditions, we aim to study ligand-induced U mobilization under anoxic conditions. We will test the hypothesis that biogenic noncrystalline U(IV) is more labile towards ligand-induced U mobilization than UO2 in laboratory batch experiments. The role of various environmental processes will increasingly be incorporated into experiments, such as competing cation effects and metal exchange reactions with humic substances. Finally, extensive experiments will be carried out with field sediments aimed to mimic bioremediation practices by stimulating biologically-mediated reduction of U under anaerobic conditions in flow-through columns. The stability of bioreduced U sediments will then be probed for its propensity to remobilization by various organic ligands in batch and flow-through column experiments. Experimental work will be paired with geochemical equilibrium and transport modelling.

 

This project was funded by the Austrian Science Fund (FWF, Project I 2704-N34).

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