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°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 Al₂O₃ 2SiO₂ to account for the slight dissimilarity between the EXAFS spectra of samples and that of Cr₂O₃.