Bioreduction and precipitation of uranium in ionic liquid aqueous solution by Clostridium sp.

Abstract The ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF 6 ], N -ethylpyridiniumtrifluoroacetate [EtPy][CF 3 COO] and N -ethylpyridiniumtetrafluoroborate [EtPy][BF 4 ], affected the reduction and precipitation of uranium by Clostridium sp. to a varying degree. Characterization of uranium association with the ionic liquids showed that uranium formed a monodentate complex with the anion BF 4 − and PF 6 − of [EtPy][BF 4 ] and [BMIM][PF 6 ], respectively; and a bidentate complex with carboxylate of [EtPy][CF 3 COO]. Bioreduction of U(VI) was influenced by the type of complex formed: monodentate complexes were readily reduced whereas the bidentate complex of U(VI) with [CF 3 COO] was recalcitrant. [EtPy][BF 4 ] affected the rate and extent of precipitation of the reduced uranium; at higher concentration the reduced U(IV) remained in the solution phase. The results suggest that by tuning the properties of ionic liquids they may be valuable candidates for uranium biotreatment.