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Coupled redox transformations of catechol and cerium at the surface of a cerium(III) phosphate mineral |
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| Authors: | Javiera Cervini-Silva Benjamin Gilbert Sirine Fakra Stephan Friedlich Jillian Banfield |
| Institution: | aUniversidad Nacional Autónoma de México, Instituto de Geograf?´a, Circuito Exterior, Ciudad Universitaria, Coyoacan, C.P. 04150, México City, Mexico bNASA Astrobiology Institute, University of California Berkeley, Berkeley, CA 94720-3110, USA cLawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA dLawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA eDepartment of Earth and Planetary Science, University of California Berkeley, 307 McCone Hall, Berkeley, CA 94720-4767, USA |
| Abstract: | Highly insoluble Ce-bearing phosphate minerals form by weathering of apatite Ca5(PO4)3.(OH,F,Cl)], and are important phosphorous repositories in soils. Although these phases can be dissolved via biologically-mediated pathways, the dissolution mechanisms are poorly understood. In this paper we report spectroscopic evidence to support coupling of redox transformations of organic carbon and cerium during the reaction of rhabdophane (CePO4·H2O) and catechol, a ubiquitous biogenic compound, at pH 5. Results show that the oxic–anoxic conditions influence the mineral dissolution behavior. Under anoxic conditions, the release of P and Ce occurs stoichiometrically. In contrast, under oxic conditions, the mineral dissolution behavior is incongruent, with dissolving Ce3+ ions oxidizing to CeO2. Reaction product analysis shows the formation of CO2, polymeric C, and oxalate and malate. The presence of more complex forms of organic carbon was also confirmed. Near edge X-ray absorption fine structure spectroscopy measurements at Ce-M4,5 and C-K absorption edges on reacted CePO4·H2O samples in the absence or presence of catechol and dissolved oxygen confirm that (1) the mineral surface converts to the oxide during this reaction, while full oxidation is limited to the near-surface region only; (2) the Ce valence remains unchanged when the reaction between CePO4·H2O and O2 but in the absence of catechol. Carbon K-edge spectra acquired from rhabdophane reacted with catechol under oxic conditions show spectral features before and after reaction that are considerably different from catechol, indicating the formation of more complex organic molecules. Decreases in intensity of characteristic catechol peaks are accompanied by the appearance of new π* resonances due to carbon in carboxyl (ca. 288.5 eV) and carbonyl (ca. 289.3 eV) groups, and the development of broad structure in the σ* region characteristic of aliphatic carbon. Evolution of the C K-edge spectra is consistent with aromatic-ring cleavage and polymerization. These results further substantiate that the presence of catechol, O2 (aq) causes both the oxidation of structural Ce3+ and the transformation of catechol to more complex organic molecules. Scanning Transmission X-Ray Microscopy measurements at the C K and Ce M4,5 edges indicate three dominant organic species, varying in complexity and association with the inorganic phase. Untransformed catechol is loosely associated with CeO2, whereas more complex organic molecules that exhibit lower aromaticity and stronger C |
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