Article pubs.acs.org/est
Tetra- and Hexavalent Uranium Forms Bidentate-Mononuclear Complexes with Particulate Organic Matter in a Naturally UraniumEnriched Peatland Christian Mikutta,*,†,‡ Peggy Langner,† John R. Bargar,§ and Ruben Kretzschmar† †
Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science, CHN, ETH Zurich, 8092 Zurich, Switzerland ‡ Section for Environmental Chemistry and Physics, Department of Plant and Environmental Sciences, University of Copenhagen, DK-1871 Frederiksberg C, Denmark § Stanford Synchrotron Radiation Lightsource Directorate, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States S Supporting Information *
ABSTRACT: Peatlands frequently serve as efficient biogeochemical traps for U. Mechanisms of U immobilization in these organic matter-dominated environments may encompass the precipitation of U-bearing mineral(oid)s and the complexation of U by a vast range of (in)organic surfaces. The objective of this work was to investigate the spatial distribution and molecular binding mechanisms of U in soils of an alpine minerotrophic peatland (pH 4.7−6.6, Eh = −127 to 463 mV) using microfocused X-ray fluorescence spectrometry and bulk and microfocused U L3edge X-ray absorption spectroscopy. The soils contained 2.3−47.4 wt % organic C, 4.1−58.6 g/kg Fe, and up to 335 mg/kg geogenic U. Uranium was found to be heterogeneously distributed at the micrometer scale and enriched as both U(IV) and U(VI) on fibrous and woody plant debris (48 ± 10% U(IV), x̅ ± σ, n = 22). Bulk U X-ray absorption near edge structure (XANES) spectroscopy revealed that in all samples U(IV) comprised 35−68% of total U (x̅ = 50%, n = 15). Shell-fit analyses of bulk U L3-edge extended X-ray absorption fine structure (EXAFS) spectra showed that U was coordinated to 1.3 ± 0.2 C atoms at a distance of 2.91 ± 0.01 Å (x̅ ± σ), which implies the formation of bidentate-mononuclear U(IV/VI) complexes with carboxyl groups. We neither found evidence for U shells at ∼3.9 Å, indicative of mineral-associated U or multinuclear U(IV) species, nor for a substantial P/Fe coordination of U. Our data indicates that U(IV/VI) complexation by natural organic matter prevents the precipitation of U minerals as well as U complexation by Fe/ Mn phases at our field site, and suggests that organically complexed U(IV) is formed via reduction of organic matter-bound U(VI).
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INTRODUCTION Uranium is a natural radioactive trace element without any known essential physiological function in biota and an average upper crust abundance of 2.8 mg/kg.1 In soils, the content of U (typically 1−4 mg/kg)2 depends mainly on the parent rock composition. Uranium concentrations in crystalline rocks tend to increase with Si content, with granites having the highest (
