Speciation of solid-phase uranium in uranium-contaminated subsurface sediments undergoing uranium bioremediation demonstrated
that although microbial reduction of soluble U(VI) readily immobilized uranium as U(IV), a substantial portion of the U(VI)
in the aquifer was strongly associated with the sediments and was not microbially reducible. These results have important
implications for in situ uranium bioremediation strategies.
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"However, there are some disadvantages that contribute greatly to the overall effectiveness. For example, some studies have shown that acetate oxidizing sulfate reducing bacteria (SRB) are less efficient at mediating the redox reactions necessary for uranium immobilization (Yabusaki et al, 2010; Anderson et al, 2003; Ortiz-Bernad et al, 2004). Also, organic substrates will select for heterotrophic bacteria which have a high growth yield (Rittmann and McCarty, 2001), which could result in aquifer clogging, consequently, impeding further delivery into the aquifer. "
[Show abstract][Hide abstract]ABSTRACT: In situ recovery (ISR) of uranium alters the baseline groundwater geochemistry in order to mobilize and complex uranium to extract and concentrate it. ISR wellfield restoration is often challenging, and primarily relies on groundwater sweep, reinjection of treated permeate, and/or injection of reductants. Greater focus should be placed on injection-based approaches (using existing infrastructure) based on established engineering concepts used for in situ remediation of groundwater plumes. Such injection-based approaches could shorten restoration timeframes consequently reducing long-term operational costs. This paper addresses various types of injection-based approaches to wellfield restoration based on in-situ remediation success with other constituents that require similar treatment. This paper describes necessary design practices for injecting reactive chemicals (e.g., reductants or biostimulants), operations and maintenance (O&M) strategies to prevent fouling of the chemical delivery system and well network, and natural constraints to restoration. Best practices for successful biostimulation strategies that minimize residual uranium, radium, oxoanions, and other trace elements are as described. Advantages of such injection-based approaches could include significant reductions in restoration times, and consequently, lower restoration costs.
"Here, bioremediation is used to refer specifically to the biomineralization phase of DMRB bioremediation strategies. Bioremediation has been successfully demonstrated in laboratory systems and in a general sense at the field-scale (e.g., Holmes et al., 2002; Senko et al., 2002; Anderson et al., 2003; Ortiz-Bernard et al., 2004). However, wider application of the technique would benefit from knowledge of its effectiveness in dual-porosity porous media. "
[Show abstract][Hide abstract]ABSTRACT: Uranium contamination is a serious environmental concern worldwide. Recent attention has focused on the in situ immobilization of uranium by stimulation of dissimilatory metal-reducing bacteria (DMRB). The objective of this work was to investigate the effectiveness of this approach in heterogeneous and structured porous media, since such media may significantly affect the geochemical and microbial processes taking place in contaminated sites, impacting remediation efficiency during biostimulation. A biogeochemical reactive transport model was developed for uranium remediation by immobile-region-resident DMRB in two-region porous media. Simulations were used to investigate the parameter sensitivities of the system over wide-ranging geochemical, microbial and groundwater transport conditions. The results suggest that optimal biomineralization is generally likely to occur when the regional mass transfer timescale is less than one-thirtieth the value of the volumetric flux timescale, and/or the organic carbon fermentation timescale is less than one-thirtieth the value of the advective timescale, and/or the mobile region porosity ranges between equal to and four times the immobile region porosity. Simulations including U(VI) surface complexation to Fe oxides additionally suggest that, while systems exhibiting U(VI) surface complexation may be successfully remediated, they are likely to display different degrees of remediation efficiency over varying microbial efficiency, mobile-immobile mass transfer, and porosity ratios. Such information may aid experimental and field designs, allowing for optimized remediation in dual-porosity (two-region) biostimulated DMRB U(VI) remediation schemes.
Full-text · Article · May 2011 · Journal of Hydrology
"A key disconnect between the experimental data and the simulation results is that the simulation predicted complete reduction of U(VI) at the outset the Fe(III) reduction phase, whereas U(VI) reduction did not proceed to completion (Figure 4F). Other studies have documented incomplete reduction of solid-associated U(VI) in reduced subsurface sediments that contain excess electron donor and abundant Fe(II) as a potential chemical reductant for U(VI) (10, 42, 43). The persistence of substantial solid-associated U(VI) during active Fe(III) reduction provides an explanation for the increase in dissolved U(VI) that took place later on during the methanogenic phase of the experiment: complexation of residual U(VI) by DIC (>10 mM) produced during methanogenic oxidation of acetate could have easily shifted the balance between aqueous and surface-associated U(VI) (44). of the reduced slurries to assess possible metabolic (as opposed to geochemical) reasons for incomplete U(VI) reduction observed in the ethanol-amended slurries. "
[Show abstract][Hide abstract]ABSTRACT: A laboratory incubation experiment was conducted with uranium-contaminated subsurface sediment to assess the geochemical and microbial community response to ethanol amendment. A classical sequence of terminal electron-accepting processes (TEAPs) was observed in ethanol-amended slurries, with NO3- reduction, Fe(III) reduction, SO4(2-) reduction, and CH4 production proceeding in sequence until all of the added 13C-ethanol (9 mM) was consumed. Approximately 60% of the U(VI) content of the sediment was reduced during the period of Fe(III) reduction. No additional U(VI) reduction took place during the sulfate-reducing and methanogenic phases of the experiment Only gradual reduction of NO3-, and no reduction of U(VI), took place in ethanol-free slurries. Stimulation of additional Fe(III) or SO4(2-) reduction in the ethanol-amended slurries failed to promote further U(VI) reduction. Reverse transcribed 16S rRNA clone libraries revealed major increases in the abundance of organisms related to Dechloromonas, Geobacter, and Herbaspirillum in the ethanol-amended slurries. Phospholipid fatty acids (PLFAs) indicative of Geobacter showed a distinct increase in the amended slurries, and analysis of PLFA 13C/12C ratios confirmed the incorporation of ethanol into these PLFAs. A increase in the abundance of 13C-labeled PLFAs indicative of Desulfobacter, Desulfotomaculum, and Desulfovibrio took place during the brief period of sulfate reduction that followed the Fe(III) reduction phase. Our results show that major redox processes in ethanol-amended sediments can be reliably interpreted in terms of standard conceptual models of TEAPs in sediments. However, the redox speciation of uranium is complex and cannot be explained based on simplified thermodynamic considerations.