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Redox Chemistry of Chromate with Well-Defined Iron Oxide SurfacesP. Liu,(a) C. Doyle,(a) E. Nelson,(a) T. Kendelewicz,(a) G. E. Brown, Jr.,(a) and S. A. Chambers Supported by OBER/OBES EMSP and NSF-MRSEC. The fate and transport of redox-active contaminants in the subsurface environment depend critically on the heterogeneous chemistry of these species with minerals that are present. If the minerals themselves are redox active, such chemistry may produce changes in the oxidation state of the contaminant that will render it less mobile, less bioavailable, and/or less toxic. Such is the case with chromate, CrO4-2, interacting with magnetite, Fe3O4. Chromate is a highly carcinogenic agent that was used for years as a biocide and passivation coating in several technologies. Significant quantities of chromate have been discharged to the ground at many government and industrial sites, and chromate poses a health risk that is being seriously investigated. Fundamental studies of the Cr(VI) ® Cr(III) reaction that employ well-defined single-crystal surfaces of magnetite and partially reduced hematite, Fe2O3, have been carried out in our respective laboratories (Kendelewicz et al. 1999). The iron oxides are important secondary mineral coatings that are found in many geologic formations. The specimens we have used include both natural and synthetic magnetite and hematite crystals. The synthetic materials were grown by oxygen-plasma-assisted molecular beam epitaxy at PNNL (Kim et al. 1997; Gao et al. 1997, 1998; Gao and Chambers 1997) and transported to SSRL for chromate chemistry experiments under controlled atmosphere conditions. The iron oxide surfaces were first cleaned in ultrahigh vacuum (UHV) by annealing in a background pressure of oxygen. They were then brought back to atmospheric pressure within the glove box and immersed in chromate solutions of various concentrations for a range of exposure times. Surface spectroscopy experiments were then carried out either in UHV, in atmospheric pressure with a high humidity level to maintain several monolayers of water on the surface, or under a thin (£ 1 micron) layer of chromate solution. Measurements in UHV were preceded by blowing the specimens dry with nitrogen gas, and consisted of core-level and valence-band photoemission (PES). Experiments carried out in moist air and solution consisted of x-ray absorption spectroscopy (XAS) and glancing-incidence x-ray absorption fine structure (GI-EXAFS). The latter two environments have the distinct advantage that the surface remains in an aqueous environment, whereas in the former environment, waters of hydration must be removed in order to make the measurement. These experiments revealed that Cr reduction from Cr(VI) to Cr(III) occurs only on surfaces that contain at least some Fe(II), which is oxidized to Fe(III) in the process. Figure 2.7 shows Cr L edge XAS spectra for several iron oxides after chromate exposure, as well as spectra for reference compounds that contain Cr(III) and Cr(VI). The latter provide spectral fingerprints for Cr(III) and Cr(VI) that are clearly distinct. Hematite surfaces that have not been ion sputtered to cause partial reduction of surface Fe(III) to Fe(II) show no evidence of Cr reduction to Cr(III). Hematite that has been partially reduced and magnetite show clear evidence for Cr reduction.
We have shown that this redox reaction occurs over a time scale of several minutes at room temperature on magnetite, and then ceases. A mixed, amorphous, insulating Fe(III)/Cr(III) oxyhydroxide layer forms on the surface that eventually blocks electron transfer across the interface. Magnetite is a conductor at room temperature, so the electron transfer reaction can occur as long as the reaction product layer thickness is not too great. The thickness of this layer increases with Cr(VI) concentration and exposure time up to a maximum thickness, which is ~30 Å. The kinetics of the reaction have been measured, and they are amazingly similar to those measured on so-called "zero-valent iron," which is essentially iron filings with a thin native oxide skin. These similarities strongly suggest that the actual surface doing the Cr(VI) reduction with zero-valent iron is Fe3O4, rather than Fe0, and that the process is self-limiting for the same reason it is self-limiting on magnetite. This finding suggests that the effectiveness of zero-valent iron as a remediation agent for chromate will be limited to what can occur on the surface of the particles; high surface area material will be required. It also suggests that structural Fe(II) incorporated into a high-surface-area material such as a zeolite may be much more effective than iron filings. We have also determined the chemisorption of Cr(III) of Fe3O4(111) after reduction has taken place (Kendelewicz et al. unpublished). It forms a multi-nuclear, inner-sphere surface complex with Cr-O, Cr-Cr, and Cr-Fe bond lengths of 2.0 Å, 3.0 Å, and 3.5 Å, respectively. The fact that an inner-sphere complex is formed (meaning there are no intervening waters of hydration between the anion and the surface) suggests that the less toxic and bioavailable Cr(III) species will be strongly bound and perhaps effectively immobilized on magnetite mineral surfaces in the field. ReferencesGao, Y., and S. A. Chambers, J. Crystal Growth 174, 446 (1997). Gao, Y., Y. J. Kim, and S. A. Chambers, J. Mat. Res. 13, 2003 (1998). Gao, Y., Y. J. Kim, G. Bai, and S. A. Chambers, J. Vac. Sci. Technol. A 15, 332 (1997). Kendelewicz, T., P. Liu, C. S. Doyle, G. E. Brown, Jr., E. J. Nelson, and S. A. Chambers, Surf. Sci. 424, 219 (1999). Kendelewicz, T., P. Liu, C. S. Doyle, G. E. Brown, Jr., E. J. Nelson, and S. A. Chambers, unpublished. Kim, Y. J., Y. Gao, and S. A. Chambers, Surf. Sci. 371, 358 (1997).
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