Isotope Fractionation of 238U/235U during Mobilization of Non-Crystalline U(IV) by Complexation with Organic Ligands

Uranium mining and processing left a legacy of contamination around the world. Strategies for remediation are commonly based on the reduction of the mobile U(VI) to more immobile U(IV), e.g., by stimulated microbial reduction. It has been demonstrated that the major product of U bioreduction is a non-crystalline U(IV) (NC-U(IV)) species associated with the microbial biomass. NC-U(IV) is not well characterized but is significantly more labile than uraninite (UO_2(s)) (Stylo et al., 2013). Oxidation or complexation by organic ligands can cause (re)mobilization of NC-U(IV). Organic ligands, for instance derived from humic acids, are present at many contamination sites and may form soluble complexes with U(IV), as previously shown for EDTA and citrate (Luo & Gu, 2011).

Here, we examine the effectiveness of NC-U(IV) mobilization by complexation and the associated U isotope fractionation with a variety of ligands.

First, U(VI) with an initial U concentration of 400 μM was reduced by Shewanella oneidensis MR-1 in a phosphate-containing medium (WLP) under anoxic conditions to produce NC-U(IV) (Stylo et al., 2013). Subsequently, U was mobilized with EDTA (100 μM, 400 μM and 1000 μM). Samples collected at various time points were analyzed for their U concentration and ²³⁸U/²³⁵U isotope ratios. The first results confirm that EDTA mobilizes NC-U(IV). A larger ligand to U ratio results in faster and more extensive U mobilization. The mobilized U(IV) fraction exhibit a minor increase of the δ²³⁸U values compared to the initial NC-U(IV) isotope composition. These initial findings indicate that non-redox-related mobilization of U(IV) may result in an opposite isotope fractionation relative to incomplete oxidative U mobilization (Wang et al., 2015). In the near future, experiments with different ligands (e.g., citrate) will be performed.

References:

Luo, W., & Gu, B. (2011). ES & T, 45(7), 2994-2999.

Stylo, M., Alessi, D. S., Shao, P. P., Lezama-Pacheco, J. S., Bargar, J. R., & Bernier-Latmani, R. (2013). ES & T, 47(21), 12351-12358.

Wang, X., Johnson, T. M., & Lundstrom, C. C. (2015). GCA, 150, 160-170.