Quantitative determination of chromium valence in environmental samples using XAFS spectroscopy
- Stanford Univ., CA (United States)
Synchrotron x-ray absorption fine structure (XAFS) spectroscopy provides quantitative information on the speciation of Cr in complex oxide samples, including the Cr(VI):Cr(III) ratio, without any sample pretreatment. The authors discuss several XAFS methods that can be used for this purpose, including: (1) measurement of the height of the pre-edge peak of Cr K-edge spectra which is caused by 1s {r_arrow} 3d electronic transitions occurring predominantly in the Cr(VI)O{sub 4} tetrahedron; (2) fitting observed XANES spectra using combinations of experimental spectra from Cr(III)- and Cr(VI)-containing model compounds; (3) fitting observed EXAFS spectra using combinations of experimental spectra from Cr(III)- and Cr(VI)-containing model compounds; and (4) ab-initio modeling of observed Cr K-EXAFS spectra using theoretical phase-shift and amplitude parameters for Cr(III)O{sub 6} and Cr(VI)O{sub 4} models. The first method is shown to be the simplest and most accurate. The samples examined using these methods include highly pure model compounds, Cr{sub 2}{sup 3+}O{sub 3} and Na{sub 2}Cr{sup 6+}O{sub 4}:4H{sub 2}O, as well as more complex soil samples taken from Cr-contaminated industrial sites and Cr waste disposal pits. The authors present XAFS evidence for the reduction of Cr(VI) to Cr(III) after reaction with the Fe(II)-bearing oxide magnetite, Fe{sub 3}O{sub 4}, and show that Cr associated with magnetite in soils is reduced, whereas that associated with non-Fe(II)-bearing silicate minerals is not. Also presented are ab-initio multiple-scattering calculations of XANES spectra of an Cr(VI)O{sub 4} tetrahedron, which help explains the XANES features of this Cr species in solids and aqueous solutions.
- Sponsoring Organization:
- National Science Foundation, Washington, DC (United States)
- OSTI ID:
- 500991
- Report Number(s):
- CONF-960401-; ISBN 1-55899-335-5; TRN: IM9732%%219
- Resource Relation:
- Conference: Spring meeting of the Materials Research Society (MRS), San Francisco, CA (United States), 8-12 Apr 1996; Other Information: PBD: 1997; Related Information: Is Part Of Aqueous chemistry and geochemistry of oxides, oxyhydroxides, and related materials; Voigt, J.A. [ed.] [Sandia National Labs., Albuquerque, NM (United States)]; Wood, T.E. [ed.] [3M Co., St. Paul, MN (United States)]; Bunker, B.C. [ed.] [Pacific Northwest Lab., Richland, WA (United States)]; Casey, W.H. [ed.] [Univ. of California, Davis, CA (United States)]; Crossey, L.J. [ed.] [Univ. of New Mexico, Albuquerque, NM (United States)]; PB: 384 p.; Materials Research Society symposium proceedings, Volume 432
- Country of Publication:
- United States
- Language:
- English
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