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 (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 Na2S2O4effectively reduced Cr(VI) to Cr(III), catalyzed by Fe(III). The subsequent addition of Na3PO4further immobilized Cr(III) by the formation of crystalline CrPO4·6H2O. However, addition of Na3PO4simultaneously with Na2S2O4(via a one-step procedure, OSP) impeded Cr(VI) reduction due to the competitive reaction of Na3PO4and Na2S2O4with 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 (Na2S2O4at a dosage of 12× the stoichiometric requirement for 15 days, and then Na3PO4in 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).
College of Environmental Sciences and Engineering, Peking University, Beijing Key Laboratory for Solid Waste Utilization and Management, 100871, PR China; School of Ocean and Earth Science, University of Southampton, Southampton SO14 3ZH, UK.
Yunyi
Y
Li
School of Ocean and Earth Science, University of Southampton, Southampton SO14 3ZH, UK. Electronic address: A.Cundy@soton.ac.uk.
Andrew B
AB
Cundy
Center for Environmental Education and Communications of Ministry of Environmental Protection, 100029, PR China.
Jingxuan
J
Feng
College of Environmental Sciences and Engineering, Peking University, Beijing Key Laboratory for Solid Waste Utilization and Management, 100871, PR China.
Hang
H
Fu
College of Environmental Sciences and Engineering, Peking University, Beijing Key Laboratory for Solid Waste Utilization and Management, 100871, PR China.
Xiaojing
X
Wang
College of Environmental Sciences and Engineering, Peking University, Beijing Key Laboratory for Solid Waste Utilization and Management, 100871, PR China. Electronic address: liuyangsheng@iee.pku.edu.cn.
Yangsheng
Y
Liu
D002857:Chromium / Q000737:chemistry*; D004227:Dithionite / Q000737:chemistry*; D005290:Ferric Compounds / Q000737:chemistry; D007220:Industrial Waste; D010710:Phosphates / Q000737:chemistry*; D056951:Photoelectron Spectroscopy; D014961:X-Ray Diffraction
D005290:Ferric Compounds; D007220:Industrial Waste; D010710:Phosphates; D002857:Chromium; D004227:Dithionite; C074702:chromium hexavalent ion; C018279:sodium phosphate
COPR; Cr(VI); Na(2)S(2)O(4); Na(3)PO(4); Reductive precipitation; Remediation