Biogenic U(IV) oxidation by dissolved oxygen and nitrate in sediment after prolonged U(VI)/Fe(III)/SO₄^2- reduction

Sediment column experiments were performed to quantify the effect of biogenic iron sulfide precipitates on the stability of bioreduced uranium during and after a simulated bioremediation scenario. In particular, this study examined the effect of different oxidants (dissolved oxygen and nitrate) on biogenic U(IV) oxidation in sediment that experienced significant sulfate reduction in addition to Fe(III) and U(VI) reduction. The experimental set-up included five replicate columns (each 5 cm in diameter, 15 cm long and packed with background sediment from a site contaminated with uranium) that were bioreduced for 70 days by injecting a nutrient media containing 3 mM acetate and 6 mM sulfate prior to oxidation. Upon oxidation, iron sulfide precipitates formed during bioreduction acted as a buffer to partially prevent biogenic U(IV) oxidation. The iron sulfides were more effective at protecting biogenic U(IV) from oxidation when dissolved oxygen was the oxidant compared to nitrate. A constant supply of 0.25 mM and 1.6 mM nitrate over a 50 day period resulted in uranium resolubilization of 11% and 60%, respectively, while less than 1% of the uranium was resolubilized in the column supplied 0.27 mM dissolved oxygen during the same time period. Over time, oxidation increased pore water channeling, which was more pronounced during oxidation with nitrate. Increased channeling with time of oxidation could affect the transport of an oxidant through the previously reduced zone, and hence the oxidation dynamics of the reduced species.