UO 2 reoxidation in the presence of chelators and Fe(III) (hydr)oxides

Abstract

A proposed method of limiting uranium (U) migration is the reduction of soluble U(VI) to U(IV) with subsequent precipitation of uraninite, UO 2(S). 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 DFB (desferrioxamine B), citrate, EDTA (ethylenediaminetetraacetic acid), and NTA (nitrilotriacetic acid) on biogenic UO 2 reoxidation with ferrihydrite, goethite, or hematite. Experiments were run in anaerobic batch systems in PIPES buffer (pH 7) with bicarbonate for approximately 80 days. U(VI) concentrations were measured using a kinetic phosphorescence analyzer. Results showed EDTA accelerated UO 2 reoxidation the most at an initial rate of 9.5 micron day -¹ according to the non-zero-order rate equation R = k 1 [chelator], with a rate constant k 1 of 0.049, 0.044, and 0.046 day -¹ for ferrihydrite, goethite, and hematite, respectively. NTA accelerated UO 2 reoxidation with ferrihydrite at a rate of 4.8 micron day -¹ with k 1 = 0.026 day -¹; rates were less with goethite and hematite (0.66 and 0.71 micron day -¹ with k 1 = 0.0038 and 0.0004 day -¹, respectively). Citrate increased UO 2 reoxidation with ferrihydrite at a rate of 1.8 micron day -¹ with k 1 = 0.009 day -¹, but did not increase the extent of reaction, and no reoxidation occurred with goethite or hematite. DFB inhibited UO 2 reoxidation with ferrihydrite, and no reoxidation occurred with goethite or hematite. 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 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. The rate of UO 2 reoxidation was dependent on the stability constant between chelator and Fe(III), with DFB likely inhibiting reoxidation due to a very high stability constant (log K = 30.6). These results indicate that common chelators found in U contaminated sites can play a significant role in mobilizing U affecting efforts for bioremediation.