(529a) 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 ¨C 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.