Effects of Microbial Community Structure, Terminal Electron Accepting Conditions, and Molybdate on the Extent of U(VI) Reduction in Landfill Aquifer Sediments

Abstract

Addition of an electron donor, such as ethanol, glucose, or acetate, to the subsurface in order to stimulate biological reduction of soluble U(VI) to insoluble U(IV) is an important strategy for uranium immobilization in contaminated aquifers. Electron donor addition typically results in anaerobic conditions and the respiratory process (sulfate-reducing, iron-reducing, and/or methanogenic) will depend on site geochemistry. Although previous studies have found that U(VI) reduction can occur under any of these conditions, the goal of this study was to examine the relative extent of U(VI) reduction under different geochemical conditions and the influence of the different microbial populations on the reduction process. Sulfate-reducing (SR), iron-reducing (FeR), and methanogenic (Meth) conditions were stimulated by electron donor addition in sediment batch microcosms, and 100 μM U(VI) was added upon depletion of alternate electron acceptors. Within seven days, 89, 96, and 66% of soluble U(VI) was immobilized by sorption and/or precipitation from sulfate-reducing, iron-reducing, and methanogenic bottles, respectively. After 26 days, however, bicarbonate and nitric acid extractions of solid-associated U(VI) and total U showed that (i) there was no significant difference in the amount of U(VI) reduced between the different terminal electron accepting conditions stimulated in pre-incubations (0.204 ± 0.052, 0.263 ± 0.023, and 0.247 μmol total reduced U(IV) per g dry sediment in SR, FeR, and Meth bottles, respectively); (ii) geochemical conditions formed in FeR bottles contributed most to abiotic U(VI) reduction, with 0.0123 ± 0.0105, 0.0737 ± 0.0111, and 0.0464 ± 0.0002 μmol total reduced U(IV) per g dry sediment in formaldehyde-killed SR, FeR, and Meth bottles, respectively; and, (iii) molybdate inhibited biotic U(VI) reduction to some degree under each terminal electron accepting condition stimulated, with an average of 33% less U(IV) per g dry sediment in live bottles containing molybdate compared to controls. PLFA analysis of sediments showed that different major PLFAs groups were associated with the amount of U(IV) per g dry sediment with each terminal electron accepting condition stimulated during pre-incubations, with positive correlations between monounsaturates and U(IV) in SR bottles (r = 0.926), branched monounsaturates and U(IV) in FeR bottles (r = 0.886), and terminal-branched saturates and U(IV) in Meth bottles (r = 0.999). Collectively, these data suggesting that different populations may have been involved in U(VI) reduction under each condition and that the extent of U(VI) reduction does not differ whether sediments were sulfate-reducing, iron-reducing, or methanogenic prior to U(VI) addition.

Keywords:

• bioremediation
• uranium reduction
• molybdate
• nicrobial communities
• polar lipid fatty acid (PLFA) analysis