Field‐scale uranium (VI) bioimmobilization monitored by lipid biomarkers and ¹³C‐acetate incorporation

Abstract

At the Old Rifle uranium mill-tailing site in eastern Colorado, a test of subsurface amendment with acetate to stimulate the reductive immobilization of uranium was monitored by using lipid biomarker analysis and incorporation of 13 C-labeled acetate into lipid biomarkers. Both sediment and groundwater samples were analyzed. Within 7 days of acetate addition, groundwater microbial biomass increased by a factor of 5, and remained higher than control values in most samples for the 28 days sampled. At 29 days after the beginning of acetate amendment, 4 of 12 sediment samples had microbial biomass greater than the 95 percent confidence interval of controls. The mole percents of the phospholipid fatty acids 16:1ω7c and 16:1ω5c increased over control values upon acetate amendment, and incorporated high levels of 13 C from labeled acetate in groundwater and sediment samples. 16:1ω7c is a biomarker for Geobacter, and evidence is provided that 16:1ω5crepresents an unidentified iron-reducing bacterium, probably a member of the Desulfobulbaceae.Biomarkers for organisms other than iron-reducing bacteria, iso-and anteiso-branched fatty acids and 18:1ω9c, decreased upon acetate amendment, and had their highest stable isotope incorporation at least 4 days after labeled acetate amendment ended, evidence for carbon-sharing between iron-reducers and other microorganisms. O c 2011 Wiley Periodicals, Inc. INTRODUCTIONGroundwater contamination by uranium is a localized but worldwide problem. While uranium contamination can derive from natural sources, most groundwaters are contaminated by uranium leaching from mining waste and mill tailings (Wall & Krumholtz, 2006). There is currently no proven cost-effective remediation strategy for uranium-contaminated aquifers. Strategies such as soil washing, solidification, chemical immobilization, chemical reduction, and phytoremediation are being explored (Abd El-Sabour, 2007). Uranium can be removed from potable waters by ion-exchange or reverse osmosis, but these technologies are too expensive to be scaled up to an aquifer. Bioimmobilization has emerged as an attractive approach to controlling uranium groundwater contamination. The soluble form of uranium is U(VI), usually complexed with carbonate (Wall & Krumholtz, 2006). Some metal-reducing bacteria, in particular Geobacter, can reduce U(VI) to U(IV), which forms insoluble uranite, UO 2 . There is also evidence that U(VI) is adsorbed by metal sulfides formed by sulfate-reducing bacteria. Acetate, ethanol, and glucose are the subsurface amendments most commonly used to drive bioimmobilization in situ. Issues to be resolved include the timing and amount of subsurface amendment to maximize bioimmobilization, and the long-term stability of bioimmobilized uranium. Bacterial reduction of iron (Lovley, 2000) and uranium (Wall & Krumholtz, 2006) have been reviewed. Phospholipid fatty acid (PLFA) analysis has been extensively applied to monitor subsurface bioremediation, including uranium bioimmobilization (Anderson et al., 2003;Chang et al., 20...

Arsenic geochemistry in a biostimulated aquifer: An aqueous speciation study

“…At the former uranium mill site in Rifle, Colorado, USA, multiple strategies are being investigated to remediate uranium groundwater contamination. For over a decade, this site has been used to investigate strategies of uranium remediation as part of the Department of Energy's Uranium Mill Tailings Remedial Action Project . Injection wells, installed several years prior to the experiment reported herein, were used to inject the remediation fluid into the contaminated area, as described for previous experiments .…”