Bicarbonate impact on U(VI) bioreduction in a shallow alluvial aquifer

Abstract

Field-scale biostimulation and desorption tracer experiments conducted in a uranium (U) contaminated, shallow alluvial aquifer have provided insight into the coupling of microbiology, biogeochemistry, and hydrogeology that control U mobility in the subsurface. Initial experiments successfully tested the concept that Fe-reducing bacteria such as Geobacter sp. could enzymatically reduce soluble U(VI) to insoluble U(IV) during in situ electron donor amendment (Anderson et al. 2003, Williams et al. 2011). In parallel, in situ desorption tracer tests using bicarbonate amendment demonstrated ratelimited U(VI) desorption (Fox et al. 2012). These results and prior laboratory studies underscored the importance of enzymatic U(VI)-reduction and suggested the ability to combine desorption and bioreduction of U(VI). Here we report the results of a new field experiment in which bicarbonate-promoted uranium desorption and acetate amendment were combined and compared to an acetate amendment-only experiment in the same experimental plot. Results confirm that bicarbonate amendment to alluvial aquifer sediments desorbs U(VI) and increases the abundance of Ca-uranyl-carbonato complexes. At the same time, the rate of acetate-promoted enzymatic U(VI) reduction was greater in the presence of added bicarbonate in spite of the increased dominance of Ca-uranylcarbonato aqueous complexes. A model-simulated peak rate of U(VI) reduction was ~3.8 times higher during acetate-bicarbonate treatment than under acetate-only conditions. Lack of consistent differences in microbial community structure between acetatebicarbonate and acetate-only treatments suggest that a significantly higher rate of U(VI) reduction in the bicarbonate-impacted sediment may be due to a higher intrinsic rate of microbial reduction induced by elevated concentrations of the bicarbonate oxyanion. The findings indicate that bicarbonate amendment may be useful in improving the engineered bioremediation of uranium in aquifers. * Concentration/enrichment within the injection tank. ** Tank #2 injection was initially started on 9-Sept-10; however, a closed injection valve prevented flow from the tank; injection was restarted on 13-Sept-10, as indicated.

Mechanism of Uranium Reduction and Immobilization in Desulfovibrio vulgaris Biofilms

“…50,51 This is supported by our results (biotic system) and indicates the effectiveness of biological U reduction in the subsurface in comparison to abiotic pathways if U(VI) is in the aqueous phase. Moreover, microbial metabolic activity can contribute to an increase in the bicarbonate concentration as a result of CO 2 production.…”

“…Several studies demonstrated that the addition of 30−37 mM bicarbonate to the U-contaminated subsurface with a background concentration of bicarbonate of ∼6 mM 49 allows for enzymatic U reduction and enhanced U immobilization. 50,51 This is supported by our results (biotic system) and indicates the effectiveness of biological U reduction in the subsurface in comparison to abiotic pathways if U(VI) is in the aqueous phase.…”

Natural Uranium Contamination in Major U.S. Aquifers Linked to Nitrate

“…11 The U(VI)−Ca−CO 3 complexes are less likely to adsorb and are not as bioavailable to U-reducing bacteria for rereduction and, thus, will remain mobile. 19,29 Even if U(IV) is oxidized by nitrate, U mobility would be promoted and cocontrolled by alkalinity and calcium.…”

“…These parameters have been demonstrated to influence U mobility. 11,12,15,19,20 In the absence of ligands, filtered groundwater samples collected for analysis of iron and manganese represent aqueous reduced species as solid-phase oxide minerals will not be measured in groundwater. 21 Statistical Analyses.…”

Snowmelt controls on concentration‐discharge relationships and the balance of oxidative and acid‐base weathering fluxes in an alpine catchment, East River, Colorado

“…Samples were pumped via peristaltic pump into uncapped 1 L polyethylene bottles and retrieved at regular intervals. Bulk samples were filtered using 0.45 µm Hydrophilic Polyvinylidene Fluoride (PVDF) syringe filters into individual vials for subsequent anion, base cation, and dissolved organic and inorganic carbon analysis, as previously described [ Williams et al ., ; Long et al ., ]. Briefly, chloride, nitrate, and sulfate concentrations were quantified via ion chromatography, while base cation concentrations were determined using inductively coupled plasma mass spectrometry.…”