Study On COPR Detoxification By Na₂CO₃ Solution And Chromium-contaminated Soil Remediation By Compost

Hexavalent chromium (Cr(VI)) present in the chromite ore processing residue (COPR) and chromium-contaminated soil causes serious damage to the environment and human health for its high toxicity. It is critical to reduce the content of Cr(VI) or to fix Cr(VI) in the process of COPR detoxification and chromium-contaminated soil remediation. We studied three kinds of COPR detoxification techniques using Na2CO3 solution as the leaching reagent and one kind of chromium-contaminated soil remediation technique by compost, evaluated the detoxification and remediation effect using toxicity characteristic leaching procedure (TCLP), and investigated the remediation mechanism of chromium-contaminated soil by compost. Main researches and conclusions are as follows:Through the experimental study on COPR detoxification by using Na2CO3 solution as heap-leaching reagent, the influences of factors, such as leaching reagents, Na2CO3 concentration, particle size of COPR and leaching duration time, on the leaching rate were investigated. Cr(VI) and total Cr concentrations in the leachate of TCLP of the detoxified COPR are 4.31 mg/L and 4.53 mg/L respectively under the optimal expermental conditions. Based on the experimental study on COPR detoxification by the combination process of heap-leaching using Na2CO3 solution as leaching reagent and Cr(VI) reduction by FeSO4, the effects of cycle times on Na2CO3 concentration, pH, Cr(VI) concentration in the leaching solution and the leaching rate were studied. The initial particle size of COPR has a great influence on the detoxification effect. Cr(VI) and total Cr concentration in the leachate of TCLP of the final detoxified COPR (Particle size<0.15mm) is 1.98 mg/L and 2.45 mg/L, respectively, which is below the regular limit of HJ/T301-2007. XRD analyses of the untreated and detoxificated COPR approximately are agreement with the thermodynamic calculation. According to the experimental study on COPR detoxification by the combination process of stirring-leaching using Na2CO3 solution as the leaching reagent and Cr(VI) reduction by FeSO4, the proportions of leaching and reduction of the total detoxification effect were researched under the expermental conditions. The Cr(VI) and total Cr concentrations in the leachate of TCLP of the final detoxificated COPR are 6.38 mg/L and 10.14 mg/L respectively, which is beyond the regular limit of HJ/T301-2007.Effects of remediation reagents, the content of H2O and compost, and stirring on soil remediation efficiency were identified, and the transformation of forms of Cr in the soil remediation process was studied by the experimental study on chromium-contaminated soil remediation by compost. The Cr(VI) and total Cr concentrations in the leachate of TCLP of the remediated chromium-contaminated soil are 0.28 mg/L and 1.13 mg/L respectively, which is a little bit lower than the regular limit of GB8978-1996, whats more the remediation effect is stable after 120 days.In the study on remediation mechanism of chromium-contaminated soil by compost, it is determined that the organic materials present in the compost are the key factors influencing the remediation effect, and the effective groups are Hydroxyl group, acidamide group and carboxylic acid group. Chemical and FTIR spectroscopy analyses show that hydroxyl group directly acts on Cr(VI) in soil by redox reaction, and acidamide group and carboxylic acid group indirectly act on Cr(VI) in soil by strengthening remediation effect of organic materials in soil.