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Title: Tetra- and hexavalent uranium forms bidentate-mononuclear complexes with particulate organic matter in a naturally uranium-enriched peatland

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, E h = –127 to 463 mV) using microfocused X-ray fluorescence spectrometry and bulk and microfocused U L 3-edge 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), $$\bar{x}$$ ± 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 ($$\bar{x}$$ = 50%, n = 15). Shell-fit analyses of bulk U L 3-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 ($$\bar{x}$$ ± 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. As a result, 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).
Authors:
 [1] ;  [2] ;  [3] ;  [2]
  1. ETH Zurich, Zurich (Switzerland); Univ. of Copenhagen (Denmark)
  2. ETH Zurich, Zurich (Switzerland)
  3. SLAC National Accelerator Lab., Menlo Park, CA (United States)
Publication Date:
Grant/Contract Number:
AC02-76SF00515
Type:
Accepted Manuscript
Journal Name:
Environmental Science and Technology
Additional Journal Information:
Journal Volume: 50; Journal Issue: 19; Journal ID: ISSN 0013-936X
Publisher:
American Chemical Society (ACS)
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES
OSTI Identifier:
1348983

Mikutta, Christian, Langner, Peggy, Bargar, John R., and Kretzschmar, Ruben. Tetra- and hexavalent uranium forms bidentate-mononuclear complexes with particulate organic matter in a naturally uranium-enriched peatland. United States: N. p., Web. doi:10.1021/acs.est.6b03688.
Mikutta, Christian, Langner, Peggy, Bargar, John R., & Kretzschmar, Ruben. Tetra- and hexavalent uranium forms bidentate-mononuclear complexes with particulate organic matter in a naturally uranium-enriched peatland. United States. doi:10.1021/acs.est.6b03688.
Mikutta, Christian, Langner, Peggy, Bargar, John R., and Kretzschmar, Ruben. 2016. "Tetra- and hexavalent uranium forms bidentate-mononuclear complexes with particulate organic matter in a naturally uranium-enriched peatland". United States. doi:10.1021/acs.est.6b03688. https://www.osti.gov/servlets/purl/1348983.
@article{osti_1348983,
title = {Tetra- and hexavalent uranium forms bidentate-mononuclear complexes with particulate organic matter in a naturally uranium-enriched peatland},
author = {Mikutta, Christian and Langner, Peggy and Bargar, John R. and Kretzschmar, Ruben},
abstractNote = {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 L3-edge 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), $\bar{x}$ ± 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 ($\bar{x}$ = 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 ($\bar{x}$ ± 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. As a result, 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).},
doi = {10.1021/acs.est.6b03688},
journal = {Environmental Science and Technology},
number = 19,
volume = 50,
place = {United States},
year = {2016},
month = {9}
}