URANIUM BIOREDUCTION AND PRECIPITATION: STABILITY OF U(IV)-CITRATE COMPLEXES

Mining and processing of uranium has resulted in substantial groundwater contamination throughout the world. The use of bacterial reduction to precipitate uranium as U(IV) species is being considered as a viable remediation option at uranium-contaminated sites. Various bacteria have been shown to reduce U(VI) species. However, bioreduction of U(VI) by Clostridium sp. and Shewanella putrefaciens in the presence of citric acid, a metabolite of organic matter degradation, results in the formation of soluble U(IV)-citrate complexes. Formation of such U(IV)-citrate complexes may limit immobilization, thereby reducing the effectiveness of bioreduction as a remediation strategy. Determining the stability of U(IV)-organic ligand complexes, such the U(IV)-citrate species, will advance our understanding of the fate of uranium in the environment, and potentially increase the efficiency of uranium bioremediation approaches.

In our work, batch ion exchange experiments were conducted under an inert (nitrogen) atmosphere to ensure a reducing environment. Experiments were conducted under acidic pH conditions to mitigate the potential for U(IV) precipitation. Uranium concentrations were obtained by difference using ICP-MS. The resulting data was analyzed using FITEQL ver. 4.0 assuming 1:1 and / or 1:2 U(IV)-citrate complexes, combined with critically-evaluated equilibrium constants for the U(IV) hydrolysis and complexation reactions.