Chromium speciation in residues after sequential extraction of a thermally treated sludge analog.

XANES fitting indicates that non-Cr2O3 form of Cr(III), transformed from CrO3 by four-hour heat application at 1100 degrees C, is the key species sorbed onto or lattice-diffused into kaolin matrices that exhibits strong leaching resistance to sequential extraction using various combined solutions including HNO3, H2O2, and HF. The EXAFS spectra are all in agreement with the XANES results. It is suggested that some Cr2O3 might have reacted with kaolin to form new compounds such as Cr2O3 x Al2O3 x 2SiO2 to account for the slight dissimilarity between the EXAFS spectra of samples and that of Cr2O3.