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

Abstract

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 (Na₂S₂O₄) was used to reduce Cr(VI) to Cr(III) in COPR containing high available Fe, and then sodium phosphate (Na₃PO₄) 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 Na₂S₂O₄ effectively reduced Cr(VI) to Cr(III), catalyzed by Fe(III). The subsequent addition of Na₃PO₄ further immobilized Cr(III) by the formation of crystalline CrPO₄·6H₂O. However, addition of Na₃PO₄ simultaneously with Na₂S₂O₄ (via a one-step procedure, OSP) impeded Cr(VI) reduction due to the competitive reaction of Na₃PO₄ and Na₂S₂O₄ 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 (Na₂S₂O₄ at a dosage of 12× the stoichiometric requirement for 15 days, and then Na₃PO₄ in a molar ratio (i.e. Na₃PO₄: 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).

Keywords: COPR; Cr(VI); Na(2)S(2)O(4); Na(3)PO(4); Reductive precipitation; Remediation.