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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 (Na.sub.2S.sub.2O.sub.4) was used to reduce Cr(VI) to Cr(III) in COPR containing high available Fe, and then sodium phosphate (Na.sub.3PO.sub.4) 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 Na.sub.2S.sub.2O.sub.4 effectively reduced Cr(VI) to Cr(III), catalyzed by Fe(III). The subsequent addition of Na.sub.3PO.sub.4 further immobilized Cr(III) by the formation of crystalline CrPO.sub.4*6H.sub.2O. However, addition of Na.sub.3PO.sub.4 simultaneously with Na.sub.2S.sub.2O.sub.4 (via a one-step procedure, OSP) impeded Cr(VI) reduction due to the competitive reaction of Na.sub.3PO.sub.4 and Na.sub.2S.sub.2O.sub.4 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 (Na.sub.2S.sub.2O.sub.4 at a dosage of 12x the stoichiometric requirement for 15 days, and then Na.sub.3PO.sub.4 in a molar ratio (i.e. Na.sub.3PO.sub.4: 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).
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 (Na.sub.2S.sub.2O.sub.4) was used to reduce Cr(VI) to Cr(III) in COPR containing high available Fe, and then sodium phosphate (Na.sub.3PO.sub.4) 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 Na.sub.2S.sub.2O.sub.4 effectively reduced Cr(VI) to Cr(III), catalyzed by Fe(III). The subsequent addition of Na.sub.3PO.sub.4 further immobilized Cr(III) by the formation of crystalline CrPO.sub.4*6H.sub.2O. However, addition of Na.sub.3PO.sub.4 simultaneously with Na.sub.2S.sub.2O.sub.4 (via a one-step procedure, OSP) impeded Cr(VI) reduction due to the competitive reaction of Na.sub.3PO.sub.4 and Na.sub.2S.sub.2O.sub.4 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 (Na.sub.2S.sub.2O.sub.4 at a dosage of 12x the stoichiometric requirement for 15 days, and then Na.sub.3PO.sub.4 in a molar ratio (i.e. Na.sub.3PO.sub.4: 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).
Remediation of hexavalent chromium contamination in chromite ore processing residue by sodium dithionite and sodium phosphate addition and its mechanism
Li, Yunyi (author) / Cundy, Andrew B / Feng, Jingxuan / Fu, Hang / Wang, Xiaojing / Liu, Yangsheng
2017
Article (Journal)
English
BKL:
43.00
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