Influence of chelating agents on biogenic uraninite reoxidation by Fe(III) (Hydr)oxides

Microbially mediated reduction of soluble U(VI) to U(IV) with subsequent precipitation of uraninite, UO(2(S)), has been proposed as a method for limiting uranium (U) migration. However, microbially reduced UO(2) may be susceptible to reoxidation by environmental factors, with Fe(III) (hydr)oxides playing a significant role. Little is known about the role that organic compounds such as Fe(III) chelators play in the stability of reduced U. Here, we investigate the impact of citrate, DFB, EDTA, and NTA on biogenic UO(2) reoxidation with ferrihydrite, goethite, and hematite. Experiments were conducted in anaerobic batch systems in PIPES buffer (10 mM, pH 7) with bicarbonate for approximately 80 days. Results showed EDTA accelerated UO(2) reoxidation the most at an initial rate of 9.5 μM day(-1) with ferrihydrite, 8.6 μM day(-1) with goethite, and 8.8 μM day(-1) with hematite. NTA accelerated UO(2) reoxidation with ferrihydrite at a rate of 4.8 μM day(-1); rates were less with goethite and hematite (0.66 and 0.71 μM day(-1), respectively). Citrate increased UO(2) reoxidation with ferrihydrite at a rate of 1.8 μM day(-1), but did not increase the extent of reaction with goethite or hematite, with no reoxidation in this case. In all cases, bicarbonate increased the rate and extent of UO(2) reoxidation with ferrihydrite in the presence and absence of chelators. The highest rate of UO(2) reoxidation occurred when the chelator promoted both UO(2) and Fe(III) (hydr)oxide dissolution as demonstrated with EDTA. When UO(2) dissolution did not occur, UO(2) reoxidation likely proceeded through an aqueous Fe(III) intermediate with lower reoxidation rates observed. Reaction modeling suggests that strong Fe(II) chelators promote reoxidation whereas strong Fe(III) chelators impede it. These results indicate that chelators found in U contaminated sites may play a significant role in mobilizing U, potentially affecting bioremediation efforts.