Dissolution and Mobilization of Uranium in a Reduced Sediment by Natural Humic Substances under Anaerobic Conditions

Abstract

Biological reduction and precipitation of uranium (U) has been proposed as a remedial option for immobilizing uranium at contaminated sites, but the long-term stability and mobility of uranium remain a concern because the uranium is neither removed nor destroyed. In this study, the dissolution and mobilization of reduced and oxidized forms of uranium [U(IV) and U(VI)] by natural humic substances were investigated in batch and column-flow systems using a bioreduced sediment containing both U(IV) and U(VI). The addition of humic substances significantly increased the dissolution of U(IV) under anaerobic conditions. Humic acid (HA) was found to be more effective than fulvic acid (FA) in dissolving U(IV) in 1 mM KCl or KHCO3 background solution. However, more U(VI) was dissolved in 1 mM KHCO3 than in 1 mM KCl background electrolyte. HA also was found to be more effective than FA in mobilizing uranium under reducing and column-flow conditions, although the cumulative amount of eluted U(VI) and U(IV) was relatively low (<60 microg) after leaching with approximately 97 pore volumes of the humic solution in 1 mM KHCO3. These observations suggestthat natural humic substances could potentially influence the long-term stability of bioreduced U(IV) even under strongly reducing environments.

Electron shuttles in biotechnology

“…influence the long-term stability of the uranium treatment [7]. In such cases, HSs reduced by precipitated uranium can be electron donors for microbial respiration [8] and recycled.…”

“…In another case, HSs are shown to stimulate the reduction of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), a widely used explosive, thereby contributing to its effective clean-up strategies [9]. Similarly, many bioremediation processes are now thought to be influenced by HSs, for example, those for sites contaminated with aromatic compounds [10][11][12], methyl tertbutyl ether [13], halogenated organics (see below), arsenate [14,15], and metals [7,[15][16][17].…”

Microbial Community Changes in Response to Ethanol or Methanol Amendments for U(VI) Reduction

“…Natural humic material, extracted from uncontaminated topsoil obtained at the ORFRC, was added in a concentration of 30 mg liter Ϫ1 to one of the methanol treatments. Detailed extraction procedures and characteristics of isolated humics have been given elsewhere (23,40). Four replicate microcosms were prepared for each treatment.…”

In situ and ex situ bioremediation of radionuclide-contaminated soils at nuclear and norm sites

“…Naturally occurring organic ligands, such as siderophores, citrate, phthalates, Figure 9.4 XANES analyses of the sludge and sediment from the Y-12 Plant, Oak Ridge, Tennessee, at the Mv edge before and after bacterial activity. Remobilization of biologically immobilized U(IV) by naturally occurring organic ligands is a major concern in the long-term stability of uranium following in situ remediation Dodge & Francis, manuscript in preparation;Vazquez et al, 2009;Gu et al, 2005;Luo & Gu, 2009 9.7 Ex situ remediation of uranium-contaminated soils, sediments, and wastes After bacterial treatment there was a shift in the absorption maximum to 3550.5 eV, which is slightly higher than that of tetravalent uranium (3550.4 eV), but much less than that of U(VI), indicating reduction to U(IV).…”