Uranium Reoxidation in Previously Bioreduced Sediment by Dissolved Oxygen and Nitrate

Abstract

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.

Structural Similarities between Biogenic Uraninites Produced by Phylogenetically and Metabolically Diverse Bacteria

“…Reduction from a valence of six to four can be mediated by a variety of dissimilatory metal-and sulfate-reducing bacteria (DMRB and DSRB) in liquid batch, including strains of the genera Anaeromyxobacter, Geobacter, Shewanella, and Desulfovibrio (1)(2)(3)(4)(5). Soil columns amended with DMRB, denitrifying bacteria, or DSRB have confirmed that reductive immobilization can occur in simplified soil systems (6,7), and analogous field-scale pilot biostimulation studies have successfully immobilized uranium in the subsurface (8,9).…”

Influence of Uranyl Speciation and Iron Oxides on Uranium Biogeochemical Redox Reactions

“…In fact, microbially mediated reductive immobilization of uranium has been studied as a potential form of stimulated natural attenuation in several contaminated sites (Anderson et al, 2003;Michalsen et al, 2006;Wu et al, 2006b). However, several factors including competing electron acceptors in the form of nitrate and Fe(III) (hydr)oxides (Abdelouas et al, 1998b;Wielinga et al, 2000;Wu et al, 2006a), for example, and the potential for re-oxidation of UO 2 by a variety of oxidants including molecular oxygen and Fe(III) Moon et al, 2007;Wu et al, 2007), may limit reductive immobilization and thus diminish the long term effectiveness of this remediation strategy. Additionally, several studies have focused on describing kinetic behavior and quantifying rates of uranium reduction by DMRB and SRB; both Spear et al (2000) and Lui et al (2002) reproduced U(VI) reduction trends with first order models, and Luo et al described concurrent U(VI) and sulfate reduction with a saturation kinetic model (2007).…”

Biogenic Uraninite Nanoparticles and Their Importance for Uranium Remediation

“…Laboratory and field studies have observed the oxidation of U(IV), likely representing different mixtures of uraninite and U(IV) adsorbed on biomass, on timescales from hours to several months (Moon et al 2007;Senko et al 2007;Wu et al 2007;Komlos et al 2008). Oxidation rates increase with decreasing particle size and decreasing degree of aggregation.…”