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 simultaneo us ly 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 remediatio n 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 stoichiome tr ic 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).

Impact of an in-situ Cr(VI)-contaminated site remediation on the groundwater

“…Analogous reactions can also be written for other sulfite species. The S 2 O 6 2− (dithionate), produced in reaction 3, is able to reduce Fe(III) to Fe(II), which provides a possible explanation of the supporting effect of present Fe for the reduction of Cr(VI) (Li et al 2017).…”

Approaches to the development of environmental standards for the content of petroleum hydrocarbons and Pb, Cr, Cu, Ni in soils of Greatest Caucasus

“…Apparently, this is since the first three elements are more cation-forming, and their mobility, and, consequently, toxicity for biota, is more pronounced in acidic soils (Bezuglova et al, 2016;Kabir et al, 2012). Chromium has shown high toxicity not only in acidic, but also in slightly alkaline soils, where a highly mobile chromate is formed from chromium oxide (Li et al, 2017;Qafoku et al, 2010).…”

A comprehensive review on chromium chemistry along with detection, speciation, extraction and remediation of hexavalent chromium in contemporary science and technology

“…In another study in decolorization of hexavalent chromium, Cundy and co-workers applied sodium dithionite as reducing agent. [162] The two-steps procedure (TSP) for the Cr(VI) reduction was catalyzed by chromite ore processing residue (COPR) containing high Fe. Results showed that Fe(III) can be catalyzed the reduction process by Na2S2O4.…”