Uranium reoxidation in previously bioreduced sediment by dissolved oxygen and nitrate.

Flow-through sediment column experiments examined the reoxidation of microbially reduced uranium with either oxygen or nitrate supplied as the oxidant. The uranium was reduced and immobilized via long-term (70 days) acetate biostimulation resulting in 62-92% removal efficiency of the 20 microM influent uranium concentration. Uranium reduction occurred simultaneously with iron reduction as the dominant electron accepting process. The columns were reoxidized by discontinuing the supply of acetate and either replacing the anaerobic gas used to purge the influent media with a gas mixture containing 20% oxygen (resulting in a dissolved oxygen concentration of 0.27 mM) or adding 1.6 mM nitrate to the influent media. Both oxygen and nitrate resolubilized the majority (88 and 97%, respectively) of the uranium precipitated during bioreduction within 54 days. Although oxygen is more thermodynamically favorable an oxidant than nitrate, nitrate-dependent uranium oxidation occurred significantly faster than oxygen-dependent uranium oxidation at the beginning of our experiment due, in part, to oxygen reacting more strongly with other reduced compounds. Nitrate breakthrough at the effluent of the column occurred within 12 h, which was significantly earlier than when oxygen was detected at the effluent (26 days). Although, over time, the majority of uranium was reoxidized by either oxidant, these results indicate that the type of oxidant and its reactivity with other reduced compounds will influence the fate of reduced uranium during a short-term oxidation event that may occur during a uranium bioremediation scenario.