Combined Effects of Fe(III)-Bearing Nontronite and Organic Ligands on Biogenic U(IV) Oxidation.

Bioreduction of soluble U(VI) to sparingly soluble U(IV) solids was proposed as a remediation method for uranium contamination. Therefore, the stability and longevity of biogenic U(IV) are critical to the success of uranium remediation. However, co-occurrence of clay minerals and organic ligands could potentially reoxidize U(IV) to U(VI). Herein, we report a combined effect of Fe(III)-rich nontronite (NAu-2) and environmentally prevalent organic ligands on reoxidation of biogenic U(IV) at circumneutral pH. After 30 days of incubation, structural Fe(III) in NAu-2 oxidized 45.50% U(IV) with an initial rate of 2.7 × 10-3 mol m-2 d-1. Addition of citrate and ethylenediaminetetraacetic acid (EDTA) greatly promoted the oxidative dissolution of U(IV) by structural Fe(III) in NAu-2, primarily through the formation of aqueous ligand-U(IV) complexes. In contrast, a model siderophore, desferrioxamine B (DFOB), partially inhibited U(IV) oxidation due to the formation of stable DFOB-Fe3+ complexes. The resulting U(VI) species intercalated into an NAu-2 interlayer or adsorbed onto an NAu-2 surface. Our results highlight the importance of organic ligands in oxidative dissolution of U(IV) minerals by Fe(III)-bearing clay minerals and have important implications for the design of nuclear waste storage and remediation strategies, especially in clay- and organic-rich environments.

Microbial physiology-based model of ethanol metabolism in subsurface sediments.

A biogeochemical reaction model was developed based on microbial physiology to simulate ethanol metabolism and its influence on the chemistry of anoxic subsurface environments. The model accounts for potential microbial metabolisms that degrade ethanol, including those that oxidize ethanol directly or syntrophically by reducing different electron acceptors. Out of the potential metabolisms, those that are active in the environment can be inferred by fitting the model to experimental observations. This approach was applied to a batch sediment slurry experiment that examined ethanol metabolism in uranium-contaminated aquifer sediments from Area 2 at the U.S. Department of Energy Field Research Center in Oak Ridge, TN. According to the simulation results, complete ethanol oxidation by denitrification, incomplete ethanol oxidation by ferric iron reduction, ethanol fermentation to acetate and H(2), hydrogenotrophic sulfate reduction, and acetoclastic methanogenesis: all contributed significantly to the degradation of ethanol in the aquifer sediments. The assemblage of the active metabolisms provides a frame work to explore how ethanol amendment impacts the chemistry of the environment, including the occurrence and levels of uranium. The results can also be applied to explore how diverse microbial metabolisms impact the progress and efficacy of bioremediation strategies.