Determination of Uranyl Incorporation into Biogenic Manganese Oxides Using X-ray Absorption Spectroscopy and Scattering S. M. WEBB,* ,† C. C. FULLER, ‡ B. M. TEBO, § AND J. R. BARGAR † Stanford Synchrotron Radiation Laboratory, Menlo Park, California 94025, United States Geological Survey, Menlo Park, California 94025, and Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093. Βiogenic manganese oxides are common and an important source of reactive mineral surfaces in the environment that may be potentially enhanced in bioremediation cases to improve natural attenuation. Experiments were performed in which the uranyl ion, UO 2 2+ (U(VI)), at various concentrations was present during manganese oxide biogenesis. At all concentrations, there was strong uptake of U onto the oxides. Synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray diffraction (XRD) studies were carried out to determine the molecular-scale mechanism by which uranyl is incorporated into the oxide and how this incorporation affects the resulting manganese oxide structure and mineralogy. The EXAFS experiments show that at low concentrations (<0.3 mol % U, <1 µM U(VI) in solution), U(VI) is present as a strong bidentate surface complex. At high concentrations (>2 mol % U, >4 µM U(VI) in solution), the presence of U(VI) affects the stability and structure of the Mn oxide to form poorly ordered Mn oxide tunnel structures, similar to todorokite. EXAFS modeling shows that uranyl is present in these oxides predominantly in the tunnels of the Mn oxide structure in a tridentate complex. Observations by XRD corroborate these results. Structural incorporation may lead to more stable U(VI) sequestration that may be suitable for remediation uses. These observations, combined with the very high uptake capacity of the Mn oxides, imply that Mn-oxidizing bacteria may significantly influence dissolved U(VI) concentrations in impacted waters via sorption and incorporation into Mn oxide biominerals. Introduction Uranium is one of the most toxic heavy metals. Contamina- tion from ore mining, processing, and manufacturing has contributed to the pollution of soil and groundwater in numerous locations in the United States (1). One of the most significant mechanisms for exposure to uranium contami- nation is through groundwater transport away from these sites (2). Assessment of the threat that this contamination poses depends on the ability to predict uranium transport. Uranium mobility in oxic groundwater aquifers is believed to be controlled by the adsorption of dissolved uranyl ion (UO2 2+ , denoted as U(VI) from hereon) onto mineral surfaces (2-4). Previous investigations have studied the affinity of uranium on iron oxide mineral components, including such mineral phases as hematite (5), goethite (6, 7), ferrihydrite (8, 9), and schwertmannite (10). Many of these studies utilized synchrotron-based X-ray absorption experiments to char- acterize the complexation of uranyl on these surfaces, and they have clearly shown the formation of inner-sphere, mononuclear, bidentate U(VI) surface complexes in the near- neutral pH regime. Additionally, strong evidence exists for the importance of ternary uranyl complexes of common anions, such as carbonate and sulfate, in the primary sorption mechanism under many environmental conditions (5, 11). Manganese oxides may also play an import role in uranium sorption and sequestration. Manganese oxide surfaces have been shown to have large capacities for heavy metal ion uptake (12-15) and are ubiquitous in the environ- ment (16, 17). In some groundwater environments, it has been shown that transuranic elements associate with man- ganese oxides preferentially over other mineral surfaces, including iron oxides (18). Mn III -bearing minerals, such as hausmannite and manganite, have been shown to adsorb transuranic elements, such as plutonium, as well as par- ticipate in redox reactions with the contaminant metals (19). Manganese oxides have also been examined as potential sorbents to extract uranium from water samples for X-ray fluorescence measurements (20) as well as molecular sieves for use in remediation applications (21). Many of the manganese oxides found in the natural environment are considered to be of biologic origin, as biogenic rates of oxidation have been shown to be up to 5 orders of magnitude greater than abiotic rates under the same conditions (22-24). Many diverse species of bacteria have been shown to rapidly catalyze the oxidation of Mn(II) to Mn(III,IV), and Mn-oxidizing bacteria are common in a wide range of settings, including soil, marine, and freshwater environments (16, 17). Because of their rapid biotic oxidation rates, ubiquitous nature, and their tendency to form grain coatings, Mn-oxidizing bacteria can exert significant control over metal transport (25). Thus, understanding both the extent and the molecular mechanisms that are involved in the binding of uranyl to biogenic manganese oxide surfaces is important for applications such as predicting uranium mobility through aquifers or the design of Mn-based perme- able reactive barriers and similar subsurface remediation technologies to enhance sequestration of uranium for remediation of U-contaminated groundwaters and waste- waters. While some studies have been performed showing the utility of manganese oxides as sorbents for actinide species and measurements of distribution coefficients (21, 26), little to no spectroscopic information exists on the nature of the sorbed uranium complex on these compounds. Significantly, no work has focused on complexation of U(VI) onto biogenic manganese oxides. In this work, we present the use of U LIII-edge and Mn K-edge X-ray absorption spectroscopy and X-ray diffraction to examine how uranyl is sorbed to or incorporated into biogenic manganese oxide that forms in the presence of the uranyl contamination, and how this incorporation affects the structure of the biogenic manganese oxide. † Stanford Synchotron Radiation Laboratory. ‡ US Geological Survey. § University of California, San Diego. Current address: Department of Environmental and Biomolecular Systems, Oregon Graduate Institute School of Science and Engineering, Oregon Health & Science University, Portland, Oregon 97239. * Corresponding author e-mail: samwebb@slac.stanford.edu. Environ. Sci. Technol. 2006, 40, 771-777 10.1021/es051679f CCC: $33.50  2006 American Chemical Society VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 771 Published on Web 12/30/2005