Reduction of Cr(VI) by heating may be a useful detoxification mechanism for thermal immobilization. Using X-ray absorption spectroscopy, the change of speciation of chromium in 105 ℃ dried 3.7% Cr(VI)-sorbed kaolin further heated at 500, 900, or 1100 ℃ was studied. The 105 ℃ dried 3.7% Cr(VI)-sorbed kaolin sample was prepared by mixing 1.5 L of 0.257 M CrO{sub}3 solution (pH 0.71) with 0.5 kg of kaolin powder for 48 h, and then the slurry was heated (dried) at 105 ℃ until a constant weight was reached. The toxicity characteristic leaching procedure method was used to determine the percentage of leached chromium from all heated samples. In all 500-900 ℃ heated Cr(VI)-sorbed kaolin samples, Cr2O{sub}3 transformed from the hydrated Cr(VI) by a 4-h heat application was identified by the X-ray absorption near edge structure and extended X-ray absorption fine structure (EXAFS) spectroscopy as the key species that is leaching-resistant due to its low solubility. For the 1100 ℃ heated Cr(VI)sorbed kaolin sample, the Fourier transform of its EXAFS spectrum indicates that the intensity of the peaks at 2.45 (Cr-Cr shell of Cr{sub}2O{sub}3) and 5.00 A (Cr-Cr and Cr-O shells of Cr{sub}2O{sub}3) without phase shift correction is either relatively smaller or disappearing, compared with that of the 500-900 ℃ heated Cr(VI)-sorbed kaolin samples. It is suggested that chromium octahedra were bridged to silica tetrahedra and incorporated in minerals formed at 1100 ℃, such as mullite or sillimanite, since these phases were detected by XRD. Cr of this form is not easily leached.
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