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Influence of Iron Sulfides on Abiotic Oxidation of UO2 by Nitrite and Dissolved Oxygen in Natural Sediments

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journal contribution
posted on 20.01.2015, 00:00 by Julian Carpenter, Yuqiang Bi, Kim F. Hayes
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.