Remediation of hexavalent chromium contamination in chromite ore processing residue by sodium dithionite and sodium phosphate addition and its mechanism.

Large amounts of chromite ore processing residue (COPR) wastes have been deposited in many countries worldwide, generating significant contamination issues from the highly mobile and toxic hexavalent chromium species (Cr(VI)). In this study, sodium dithionite (Na2 S2 O4 ) was used to reduce Cr(VI) to Cr(III) in COPR containing high available Fe, and then sodium phosphate (Na3 PO4 ) was utilized to further immobilize Cr(III), via a two-step procedure (TSP). Remediation and immobilization processes and mechanisms were systematically investigated using batch experiments, sequential extraction studies, X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). Results showed that Na2 S2 O4 effectively reduced Cr(VI) to Cr(III), catalyzed by Fe(III). The subsequent addition of Na3 PO4 further immobilized Cr(III) by the formation of crystalline CrPO4 ·6H2 O. However, addition of Na3 PO4 simultaneously with Na2 S2 O4 (via a one-step procedure, OSP) impeded Cr(VI) reduction due to the competitive reaction of Na3 PO4 and Na2 S2 O4 with Fe(III). Thus, the remediation efficiency of the TSP was much higher than the corresponding OSP. Using an optimal dosage in the two-step procedure (Na2 S2 O4 at a dosage of 12× the stoichiometric requirement for 15 days, and then Na3 PO4 in a molar ratio (i.e. Na3 PO4 : initial Cr(VI)) of 4:1 for another 15 days), the total dissolved Cr in the leachate determined via Toxicity Characteristic Leaching Procedure (TCLP Cr) testing of our samples was reduced to 3.8 mg/L (from an initial TCLP Cr of 112.2 mg/L, i.e. at >96% efficiency).