U(VI) Bioreduction with Emulsified Vegetable Oil as the Electron Donor − Microcosm Tests and Model Development Guoping Tang, †, * Wei-Min Wu, ‡,§ David B. Watson, † Jack C. Parker, ∥ Christopher W. Schadt, ⊥ Xiaoqing Shi, #,▽ and Scott C. Brooks † † Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, MS-6038, Oak Ridge, Tennessee 37831-6038, United States ‡ Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020, United States § Center for Sustainable Development & Global Competitiveness, Stanford University, Stanford, California 94305-4020, United States ∥ Department of Civil and Environmental Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States ⊥ Biosciences Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, MS-6038, Oak Ridge, Tennessee 37831-6038, United States # Department of Scientific Computing, Florida State University, Tallahassee, Florida 32306, United States ▽ School of Earth Sciences and Engineering, Department of Hydrosciences, Nanjing University, Nanjing, 210093, China * S Supporting Information ABSTRACT: We conducted microcosm tests and biogeochemical modeling to study U(VI) reduction in contaminated sediments amended with emulsified vegetable oil (EVO). Indigenous microorganisms in the sediments degraded EVO and stimulated Fe(III), U(VI), and sulfate reduction, and methanogenesis. Acetate concentration peaked in 100−120 days in the EVO microcosms versus 10−20 days in the oleate microcosms, suggesting that triglyceride hydrolysis was a rate-limiting step in EVO degradation and subsequent reactions. Acetate persisted 50 days longer in oleate- and EVO- than in ethanol-amended microcosms, indicating that acetate-utilizing methanogenesis was slower in the oleate and EVO than ethanol microcosms. We developed a comprehensive biogeochemical model to couple EVO hydrolysis, production, and oxidation of long-chain fatty acids (LCFA), glycerol, acetate, and hydrogen, reduction of Fe(III), U(VI) and sulfate, and methanogenesis with growth and decay of multiple functional microbial groups. By estimating EVO, LCFA, and glycerol degradation rate coefficients, and introducing a 100 day lag time for acetoclastic methanogenesis for oleate and EVO microcosms, the model approximately matched observed sulfate, U(VI), and acetate concentrations. Our results confirmed that EVO could stimulate U(VI) bioreduction in sediments and the slow EVO hydrolysis and acetate-utilizing methanogens growth could contribute to longer term bioreduction than simple substrates (e.g., ethanol, acetate, etc.) in the subsurface. ■ INTRODUCTION Numerous electron donors such as hydrogen, acetate, lactate, ethanol, methanol, and glucose have been tested to stimulate indigenous microbial communities for U(VI) reduction and immobilization in contaminated aquifers. 1−8 Use of these rapidly consumed electron donors requires daily to weekly injection to maintain reducing conditions and prevent biogenic U(IV) from reoxidizing to more mobile U(VI) species. 8−13 Slow release electron donors have been considered to maintain long-term reducing conditions in the subsurface with less frequent injection. For example, perchlorate was degraded for over two years in downgradient wells after a single edible oil emulsion injection in a field test; 14 a one-time 2 h emulsified vegetable oil (EVO) injection at the DOE Oak Ridge Integrated Field Research Challenge (ORIFRC) site resulted in anaerobic conditions in a fast flowing aquifer for over a year. 15,16 While U(VI) bioreduction with rapidly consumed electron donors has been under extensive investigation for more than 10 years, 1−8 there have been few studies on the use of slowly degraded complex substrates. Based on the relative abundance of representative operational taxonomic units and known physiologies of closely allied species or genera, Gihring et al. 15 developed a conceptual model for EVO degradation and subsequent reactions during a field injection test (SI Figure S1). The first step in the degradation of vegetable oil involves triglyceride hydrolysis to glycerol and long chain fatty acids (LCFA, palmitic, oleic, and Received: November 13, 2012 Revised: February 5, 2013 Accepted: February 11, 2013 Published: February 11, 2013 Article pubs.acs.org/est © 2013 American Chemical Society 3209 dx.doi.org/10.1021/es304641b | Environ. Sci. Technol. 2013, 47, 3209−3217