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

Abstract

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 μM) 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 238 U in the mobilized fraction (δ 238 U = 0.4−0.7 ‰ for EDTA; 0.3 ‰ for citrate; 0.2−0.3 ‰ for bicarbonate). For mobilization with bicarbonate, a Rayleigh approach was the most suitable isotope fractionation model, yielding a fractionation factor α of 1.00026− 1.00036. Mobilization with EDTA could be modeled with equilibrium isotope fractionation (α: 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.

Ligand-Induced U Mobilization from Chemogenic Uraninite and Biogenic Noncrystalline U(IV) under Anoxic Conditions

“… 38 Due to the larger lability of noncrystalline U(IV), a lower HCO 3 – concentration was used as in previous studies. 32 , 33 , 38 …”

“…Several studies including noncrystalline U(IV) with organic ligands have previously used the method. 33 , 50 …”

“…Prior to the start of each experiment, the U material was washed in a solution of anoxic NaHCO 3 (1 M for 10 h for UO 2 , 50 mM for 10 h for noncrystalline U­(IV)) to extract any initially present U­(VI) from the material surfaces . Due to the larger lability of noncrystalline U­(IV), a lower HCO 3 – concentration was used as in previous studies. ,, …”

“…An ion-exchange chromatography method for resolving U oxidation state , (method detailed Text S4) was used on the end-point samples for both ligand concentrations with noncrystalline U­(IV) (not carried out for UO 2 due to limit of quantification limitations). Several studies including noncrystalline U­(IV) with organic ligands have previously used the method. , …”

“…Noncrystalline U­(IV) has been found to be substantially more labile than UO 2 due to its propensity to be readily mobilized by bicarbonate and oxidized by O 2 and persulfate. ,, In previous studies with noncrystalline U­(IV), some U­(VI) was found to be associated with the noncrystalline U­(IV) even after bicarbonate extraction . These recent findings regarding the susceptibility of U­(IV) solubilization raise questions pertaining to the mobility of reduced U species in the environment.…”