Article pubs.acs.org/est
Influence of Iron Sulfides on Abiotic Oxidation of UO2 by Nitrite and Dissolved Oxygen in Natural Sediments Julian Carpenter,† Yuqiang Bi,† and Kim F. Hayes*,† †
Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States S Supporting Information *
ABSTRACT: Iron sulfide precipitates formed under sulfate reducing conditions may buffer U(IV) insoluble solid phases from reoxidation after oxidants re-enter the reducing zone. In this study, sediment column experiments were performed to quantify the effect of biogenic mackinawite on U(IV) stability in the presence of nitrite or dissolved oxygen (DO). Two columns, packed with sediment from an abandoned U contaminated mill tailings site near Rifle, CO, were biostimulated for 62 days with an electron donor (3 mM acetate) in the presence (BRS+) and absence (BRS−) of 7 mM sulfate. The bioreduced sediment was supplemented with synthetic uraninite (UO2(s)), sterilized by gamma-irradiation, and then subjected to a sequential oxidation by nitrite and DO. Biogenic iron sulfides produced in the BRS+ column, mostly as mackinawite, inhibited U(IV) reoxidation and mobilization by both nitrite and oxygen. Most of the influent nitrite (0.53 mM) exited the columns without oxidizing UO2, while a small amount of nitrite was consumed by iron sulfides precipitates. An additional 10-day supply of 0.25 mM DO influent resulted in the release of about 10% and 49% of total U in BRS+ and BRS− columns, respectively. Influent DO was effectively consumed by biogenic iron sulfides in the BRS+ column, while DO and a large U spike were detected after only a brief period in the effluent in the BRS− column.
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INTRODUCTION Uranium (U) contamination of groundwater is prevalent worldwide at former U mill and mine tailing sites, a result of uranium ore extraction for nuclear energy and weapons production. To control soluble U migration, methods that convert U to species with low solubility in groundwater have been under consideration via either metabolic or abiotic processes.1−4 Column and field studies show that in situ stimulation of iron- and sulfate-reducing bacteria by electron donor addition is an effective way to reduce soluble U(VI) to sparingly soluble U(IV) solids.5−7 Uraninite, being more stable and resistant to oxidation compared to nonuraninite species,8 is the preferred U(IV) solid phase end point for bioremediation of U contaminated groundwater.9 After U immobilization in the subsurface, U(IV) reoxidation may occur when oxidants re-enter previously active bioreduction zones once electron donor additions cease or run out. Dissolved oxygen (DO) and nitrate have been identified as key oxidants of U(IV) in biostimulated subsurface sites for mobilizing reduced U.5,10−12 Thermodynamically, UO2 can be effectively oxidized and consequently dissolved in the presence of oxygen.13,14 Although nitrate does not directly react with reduced U species, denitrification intermediates such as nitrite may readily oxidize U(IV).11 Senko et al.15 proposed an Fe(III) catalyzed mechanism for indirect oxidation of U(IV) by nitrite, and noted that direct oxidation of U(IV) by nitrite was slow. In previous sediment experiments, however, nascent microorganisms were able to complicate the analysis of oxidation © 2014 American Chemical Society
mechanisms of U(IV) species, as both biological and chemical pathways may be occurring simultaneously.16 During U immobilization under sulfate reducing conditions, simultaneous bioreduction of other constituents in the subsurface, such as Fe(III) and sulfate, results in precipitation of sulfide minerals.17−19 Reduced iron sulfide phases may subsequently impact the rate of reoxidation of U by providing additional reducing capacity in the sediment. Mackinawite, typically the first and most reactive iron sulfide phase formed during simultaneous reduction of sulfate and Fe(III),18,20 can directly reduce U(VI) to U(IV),21,22 and can also inhibit U(IV) solid phase oxidation by scavenging DO.23−25 Although studied in continuously mixed batch25 and flow-through reactor systems,26 the role of biogenic mackinawite or other reduced iron phases in inhibiting uraninite oxidation and subsequent remobilization in abiotic sediment systems has not been extensively explored. Whether iron sulfides are effective in retarding nitrite-induced U(IV) oxidation also remains to be demonstrated. Likewise, the extent to which reduced sulfide species affect transport of oxidants through a previously bioreduced sediment zone needs further study. The current study investigated the potential for biogenic iron sulfides formed during the biostimulation of Rifle area Received: Revised: Accepted: Published: 1078
September 15, 2014 December 18, 2014 December 19, 2014 December 19, 2014 DOI: 10.1021/es504481n Environ. Sci. Technol. 2015, 49, 1078−1085
Article
Environmental Science & Technology
approximately 350 g of sediment, containing 80% dried RABS and 20% microbially active RABS from a recently drilled Rifle, CO well. Both columns, fitted with an adjustable depth cap and 20-μm bed supports, were coated with a thin (
