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Title: Electrochemical and spectroscopic evidence on the one-electron reduction of U(VI) to U(V) on magnetite

Abstract

Reduction of U(VI) to U(IV) on mineral surfaces has been considered as a one-step two electron process. However, stabilized U(V), with no evidence of U(IV), found in recent studies indicates U(VI) can undergo a one electron reduction to U(V) without further progression to U(IV). We investigated the mechanisms of uranium reduction by reducing U(VI) electrochemically on a magnetite electrode at pH 3.4 . The one electron reduction of U(VI) was first confirmed using the cyclic voltammetry method. Formation of nano-size uranium precipitates on the surface of magnetite at reducing potentials and dissolution of the solids at oxidizing potentials were observed by in situ electrochemical AFM. XPS analysis of the magnetite electrodes polarized in uranium solutions at voltages from 0.1 ~ 0.9 V (vs. Ag/AgCl) showed the presence of only U(V) and U(VI). The highest amount of U(V) relative to U(VI) was prepared at 0.7 V, where the longest average U–Oaxial distance of 2.05 ± 0.01 Å was evident in the same sample revealed by EXAFS analysis. The results demonstrate that the electrochemical reduction of U(VI) on magnetite only yields U(V), even at a potential of 0.9 V, which favors the one-electron reduction mechanism. U(V) did not disproportionate but stabilized onmore » magnetite through precipitation of mixed-valence state U(VI)/U(V) solids.« less

Authors:
 [1];  [2];  [3];  [1];  [1];  [1]
  1. Univ. of Michigan, Ann Arbor, MI (United States)
  2. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
  3. Argonne National Lab. (ANL), Argonne, IL (United States)
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1203902
Report Number(s):
PNNL-SA-108945
Journal ID: ISSN 0013-936X; KC0302060
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Environmental Science and Technology; Journal Volume: 49; Journal Issue: 10
Country of Publication:
United States
Language:
English
Subject:
uranium; magnetite; reduction; electrochemstry

Citation Formats

Yuan, Ke, Ilton, Eugene S., Antonio, Mark R., Li, Zhongrui, Cook, Peter J., and Becker, Udo. Electrochemical and spectroscopic evidence on the one-electron reduction of U(VI) to U(V) on magnetite. United States: N. p., 2015. Web. doi:10.1021/acs.est.5b00025.
Yuan, Ke, Ilton, Eugene S., Antonio, Mark R., Li, Zhongrui, Cook, Peter J., & Becker, Udo. Electrochemical and spectroscopic evidence on the one-electron reduction of U(VI) to U(V) on magnetite. United States. doi:10.1021/acs.est.5b00025.
Yuan, Ke, Ilton, Eugene S., Antonio, Mark R., Li, Zhongrui, Cook, Peter J., and Becker, Udo. 2015. "Electrochemical and spectroscopic evidence on the one-electron reduction of U(VI) to U(V) on magnetite". United States. doi:10.1021/acs.est.5b00025.
@article{osti_1203902,
title = {Electrochemical and spectroscopic evidence on the one-electron reduction of U(VI) to U(V) on magnetite},
author = {Yuan, Ke and Ilton, Eugene S. and Antonio, Mark R. and Li, Zhongrui and Cook, Peter J. and Becker, Udo},
abstractNote = {Reduction of U(VI) to U(IV) on mineral surfaces has been considered as a one-step two electron process. However, stabilized U(V), with no evidence of U(IV), found in recent studies indicates U(VI) can undergo a one electron reduction to U(V) without further progression to U(IV). We investigated the mechanisms of uranium reduction by reducing U(VI) electrochemically on a magnetite electrode at pH 3.4 . The one electron reduction of U(VI) was first confirmed using the cyclic voltammetry method. Formation of nano-size uranium precipitates on the surface of magnetite at reducing potentials and dissolution of the solids at oxidizing potentials were observed by in situ electrochemical AFM. XPS analysis of the magnetite electrodes polarized in uranium solutions at voltages from 0.1 ~ 0.9 V (vs. Ag/AgCl) showed the presence of only U(V) and U(VI). The highest amount of U(V) relative to U(VI) was prepared at 0.7 V, where the longest average U–Oaxial distance of 2.05 ± 0.01 Å was evident in the same sample revealed by EXAFS analysis. The results demonstrate that the electrochemical reduction of U(VI) on magnetite only yields U(V), even at a potential of 0.9 V, which favors the one-electron reduction mechanism. U(V) did not disproportionate but stabilized on magnetite through precipitation of mixed-valence state U(VI)/U(V) solids.},
doi = {10.1021/acs.est.5b00025},
journal = {Environmental Science and Technology},
number = 10,
volume = 49,
place = {United States},
year = 2015,
month = 5
}
  • Reduction of U(VI) to U(VI) on mineral surfaces is often considered a one-step two-electron process. However, stabilized U(V), with no evidence of U(IV), found in recent studies Indicates U(VI) can undergo a one-electron reduction to U(V) without further progression to U(VI),. We investigated reduction pathways of uranium by reducing U(VI) electrochemically on a, magnetite electrode at,pH 3.4. Cyclic voltammetry confirms the one-electron reduction of U(VI) . Formation of nanosize uranium precipitates on the magnetite surface at reducing potentials and dissolution of the solids at oxidizing potentials are observed by in situ electrochemical atomic force microscopy. XPS, analysis Of the magnetitemore » electrodes polarized in uranium solutions at voltages - from -0.1 to -0.9 V (E-U(VI)/U(V)(0)= -0.135 V vs Ag/AgCl) show the presence of, only U(V) and U(VI). The sample with the highest U(V)/U(VI) ratio was prepared at -0.7 V, where the longest average U-O-axial distance of 2.05 + 0.01 A was evident in the same sample revealed by extended X-ray absorption fine structure analysis. The results demonstrate that the electrochemical reduction of U(VI) On magnetite only yields,U(V), even at a potential of -0.9 V, which favors the one-electron reduction mechanism, U(V) does not disproportionate but stabilizes on magnetite through precipitation Of mixed-valence state -U(V)/U(VI) solids.« less
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  • The interaction of aqueous U(VI) with galena and pyrite surfaces under anoxic conditions has been studied by solution analysis and by spectroscopic methods. The solution data indicate that uranyl uptake is strongly dependent on pH; maximum uptake (>98%) occurs above a pH range of between 4.8 and 5.5, depending on experimental conditions. Increasing the sorbate/sorbent ratio results in a relative decrease in uptake of uranyl and in slower sorption kinetics. Auger electron spectroscopy analysis indicates an inhomogeneous distribution of sorbed uranium at the surface. In the case of galena, formation of small precipitates ([approximately] 40 nm wide needles) of amore » uranium oxide compound are found. Pyrite shows a patchy distribution of uranium, mainly associated with oxidized surface species of sulfur and iron. X-ray photoelectron spectroscopy yields insight into possible redox processes indicating, for both sulfides, the concomitant formation of polysulfides and a uranium oxide compound with a mixed oxidation state at a U(VI)/U(IV) ratio of [approximately] 2. Furthermore, in the case of pyrite, at pH above 6 increased oxidation of sulfur and iron and higher relative amounts of unreduced surface-uranyl are observed. Fourier Transformed Infrared analysis of surface-bound uranyl shows a significant shift of the asymmetric stretching frequency to lower wavenumbers which is consistent with the formation of a U[sub 3]O[sub 8]-type compound and thus, independently, confirms the partial reduction of uranyl at the sulfide surface. The combination of AES, XPS, and FTIR provides a powerful approach for identifying mechanisms that govern the interaction of redox sensitive compounds in aqueous systems. The overall results indicate that sulfide minerals are efficient scavengers of soluble uranyl. Comparing the results with recent field observations, the authors suggest that thermodynamically metastable U[sub 3]O[sub 8] controls uranium concentrations in many anoxic groundwaters.« less
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