Uranium Isotope Fractionation during the Anoxic Mobilization of Noncrystalline U(IV) by Ligand Complexation

Uranium (U) isotopes are suggested as a tool to trace U reduction. However, noncrystalline U(IV), formed predominantly in near-surface environments, may be complexed and remobilized using ligands under anoxic conditions. This may cause additional U isotope fractionation and alter the signatures generated by U reduction. Here, we investigate the efficacy of noncrystalline U(IV) mobilization by ligand complexation and the associated U isotope fractionation. Noncrystalline U(IV) was produced via the reduction of U(VI) (400 mu M) by Shewanella oneidensis MR-1 and was subsequently mobilized with EDTA (1 mM), citrate (1 mM), or bicarbonate (500 mM) in batch experiments. Complexation with all investigated ligands resulted in significant mobilization of U(IV) and led to an enrichment of U-238 in the mobilized fraction (delta U-238 = 0.4-0.7 parts per thousand for EDTA; 0.3 parts per thousand for citrate; 0.2-0.3 parts per thousand for bicarbonate). For mobilization with bicarbonate, a Rayleigh approach was the most suitable isotope fractionation model, yielding a fractionation factor a of 1.00026-1.00036. Mobilization with EDTA could be modeled with equilibrium isotope fractionation (alpha: 1.00039-1.00049). The results show that U isotope fractionation associated with U(IV) mobilization under anoxic conditions is significant and needs to be considered when applying U isotopes in remediation monitoring or as a paleo-redox proxy.

Automated summary

This study investigated the mobilization of noncrystalline U(IV) through ligand complexation and the associated U isotope fractionation. The results showed significant U isotope fractionation during U(IV) mobilization under anoxic conditions, which should be considered when using U isotopes for remediation monitoring or as a paleo-redox proxy. The different ligands used for U(IV) mobilization had varying effects on U isotope fractionation.