Quantitative determination of chromium valence in environmental samples using XAFS spectroscopy

Conference · Fri Aug 01 04:00:00 EDT 1997

OSTI ID:500991

Peterson, M L; Brown, Jr, G E; Parks, G A ^[1]

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1. 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
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
54 ENVIRONMENTAL SCIENCES
ABSORPTION SPECTROSCOPY
CHROMIUM
CHROMIUM OXIDES
GEOCHEMISTRY
IRON OXIDES
REDUCTION
SANITARY LANDFILLS
SEDIMENTS
SITE CHARACTERIZATION
SOILS
X-RAY SPECTROSCOPY