Integrated Biogeochemical Modeling for Sustained Uranium Removal

Improper management or accidental leakage of hazardous wastes may lead to contaminated air, soil and water, which is not only hazardous to public health but also to the ecosystem. For example, uranium contamination is a serious concern at several sites in the U.S. The soluble hexavalent uranium [U(VI)] can migrate to groundwater in surrounding areas, posing a problem for the drinking water safety. While evidences have shown that the reduction of soluble U(VI) to insoluble tetravalent uranium [U(IV)] is mainly a biotic process (microbial dissimilatory metal reduction), some bench-scale experiments indicate that U(VI) is readily reduced by some abiotic process (e.g. natural reductants such as ferrous iron and hydrogen sulfide). Since experimental investigation of biotic and abiotic reduction was conducted under different environmental conditions, it is necessary to develop models that combine both biotic and abiotic factors for predicting establishment of sustained uranium removal at actual environmental interfaces.

In this work, we developed a comprehensive 3-D reactive-transport model for a recent column experiment (Moon et al., 2010) packed with the sediment from Old Rifle, CO. Groundwater from the Old Rifle site (which contains ca. 9 mM of sulfate but was amended with 20 µM U(VI)) was pumped upflow through the column at a rate of 0.035 ml/min. The biotic process such as genome-scale and kinetic models for microbial communities (Geobacter, Rhodoferax and Desulfovibrio) and electron capacitance for Geobacter (Zhao et al., 2010) has been coupled to the abiotic process in HYDROGEOCHEM (Yeh et al., 1998), where the finite element method was used to iteratively solve governing equations by discretizing the column with 9 elements and 36 nodes (Fig. 1A). The abiotic reduction of U(VI) is based on two published mechanisms -- uranium-hydroxyl species are the ones being reduced by sulfide (Hua et al., 2006) and the surface-adsorbed uranium species can be reduced by surface-bound Fe(II) (Rosso et al., 2006 and Liger et al., 1999).

The model predicts that the biotic process plays a major role in the U(VI) reduction under actual sediments (Fig.1B), which is consistent with field-experiments (Anderson et al., 2002). Due to the mechanism of electron capacitance through which the reduced cytochromes can transfer electrons directly to oxidized metal ions with high rates (Wang et al., 2008), the uranium bioremediation process is still efficient and effective even when the sediments has been dominated by sulfate-reducing bacteria after 40 days of acetate amendment. This simulation supports the recent findings (Williams et al., 2011) suggesting that following prolonged acetate addition, the small fraction of planktonic Geobacter species may still be largely responsible for the vast bulk of U(VI) immobilization. Further, the model provides an explanation for the insignificant effect exerted by the abiotic process on the U(VI) reduction. Due to the dominant uranium-carbonate species present in the carbonate-containing system, the calculated total concentrations of uranium-hydroxyl species are very low, leading to an extremely slow sulfide-driven U(VI) reduction. Similarly, the low concentration of the surface-bound U(VI) (Moon et al., 2007) results in the low rate of surface catalysis of U(VI) reduction by Fe(II).

The model results suggest that the microbially-generated hydrogen sulfide by sulfate-reducing bacterium may be helpful for the U(VI) reduction if the carbonate concentration can be reduced in the column amended with 20 µM U(VI). Fig. 2 indicates that by reducing the influent bicarbonate concentration from 8 mM to 3.5 mM, the total concentration of uranium-hydroxyl species is significantly increased, generating a more thorough removal of the soluble U(VI) for long-term uranium immobilization. It should be noted that such abiotic process is dependent upon certain combinations of experimental conditions, including the concentrations of U(VI), sulfide, bicarbonate and Ca(II). For example, if U(VI) concentration is low (e.g. typical values for Rifle groundwater), it might be possible that the bicarbonate amendment is beneficial to U(VI) reduction, since it will improve the U(VI) desorption process, and hence increase the probability of the biotic reactions in groundwater.

The comprehensive model may provide a useful tool for investigating mechanisms and dynamics of abiotic and biotic interactions under actual environmental conditions and subsequently for designing an optimal strategy for sustained uranium removal.

Reference

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(A)                                                   (B)

Fig. 1. Experimental 3-D domain for the flow-through column (A) and the effect of abiotic and biotic process on U(VI) reduction (B). EC 每 Electron Capacitance for Geobacter.

Fig. 2. Effect of carbonate concentration on total concentration of U(VI) species (A) and U(VI)-hydroxyl (B)  in effluent.