The reduction of Cr(VI) by the thiol-containing compounds cysteine and glutathione and by reduced sulfur in humic substances was monitored with sulfur and chromium X-ray absorption near-edge structure (XANES) spectroscopy in chromium-contaminated soils. Reaction of humic acids with Cr(VI) resulted in a reduction of the peak area of thiols and an increase in the peak area of disulfides in the sulfur XANES spectra. Analysis of the sulfur XANES spectra in various systems indicates that the reduction of Cr(VI) by humic substances involves a thiol/disulfide redox couple analogous to that of the Cr(VI) reduction by the simple thiol-containing compounds cysteine and glutathione. A fraction of the hexavalent chromium present in industrially-contaminated soils was not reducible by thiols. Reduction of Cr(VI) to Cr(III) in soils by thiols has little effect on the pH of the system in contrast to the pH decrease resulting from reduction by Fe(II).
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