Environ. Sci. Technol. 2007, 41, 4587-4592
Uranium Reoxidation in Previously Bioreduced Sediment by Dissolved Oxygen and Nitrate HEE SUN MOON, JOHN KOMLOS, AND P E T E R R . J A F F EÄ * Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey 08544
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 µM 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 oxygendependent 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.
Introduction Uranium is the most common radionuclide in soils, sediments, and groundwater at U.S. Department of Energy (DOE) sites and is of particular concern because of its carcinogenicity, long half-life (∼109 years), and potential mobility in the environment (1, 2). Uranium commonly exists as either U(VI) or U(IV) in the environment, and its fate and transport are governed by its oxidation state, with the oxidized form, U(VI), typically highly mobile and the reduced form, U(IV), much less mobile (3, 4). Simultaneous microbial reduction of U(VI) concurrently with iron and/or sulfate reduction has shown potential for the control of uranium migration in subsurface environments, and this process can be stimulated through the addition of an electron donor such as acetate or lactate (5-12). However, * Corresponding author phone: (609) 258-4653; fax: (609) 2582799; e-mail:
[email protected]. 10.1021/es063063b CCC: $37.00 Published on Web 06/06/2007
2007 American Chemical Society
very little has been done to investigate the stability of bioreduced uranium after a bioremediation scenario has been completed. Limited batch studies indicate that biogenic U(IV) can be oxidized and remobilized by oxygen (13-15). Nitrate also has been shown to oxidize and remobilize U(IV) (8, 1618) and is a common co-contaminant of U(VI) in uranium contaminated subsurface zones. Unlike oxygen, nitrate does not oxidize uranium abiotically (8, 16), but the intermediates of denitrification (NO2-, N2O, and NO) have been shown to oxidize U(IV) abiotically (16). Both nitrate and oxygen can be transported by groundwater into reduced zones where U(IV) has precipitated when biostimulation is terminated. Oxygen also has the potential to enter reduced zones through the infiltration of oxygenated rainwater. Nitrate concentrations typically vary in groundwater and have been found to be as high as 168 mM in uranium contaminated aquifers (9), which is much higher than the maximum oxygen concentration in groundwater (0.3 mM at 15 °C). Even under reducing conditions, the reoxidation of U(VI) has been reported to occur after prolonged periods of organic carbon oxidation and the formation of stable carbonate-U(VI) species (19). Therefore, research is needed to explore the effects of these oxidants on U(IV) stability under field relevant conditions. The purpose of this research was to investigate how fast and to what extent uranium reacts with oxygen or nitrate in previously bioreduced sediment under flowing conditions. The results of this study contribute to understanding the stability of bioreduced uranium after a uranium biostimulation scenario has been terminated.
Materials and Methods Sediment Description. Sediment used for all experiments was from the background area of a former uranium processing site at Old Rifle, CO, currently a Uranium Mill Tailings Remedial Action (UMTRA) site that is part of the U.S. DOE’s Environmental Remediation Sciences Program. A description of the field site and groundwater characteristics can be found in Anderson et al. (6). Nitrate was not detected in the groundwater at the site, and the average dissolved oxygen concentrations were low (
