Chromium Slag Yard Contaminated Soil Repair Process Research

Chromium (Cr) is a common heavy metal contaminant in soil and ground water. Cr pollution has received continuous attention due to carcinogenicity and mutagenicity of function and gene for human beings and animals. The traditional treatment methods such as chemical solidaficaton, chemical washing and electrokinetics for the remediating the contaminated soils by Cr were not practically used because of high cost and secondary pollution. In this thesis, the contaminated soils in a deposition site of chromium-containing slag at a ferroalloy manufactory in Hunan Province was taken as a research objective, the remediate effeciency by indigenous microorganism was investigated. The following results were achieved.The opitimal parameters for the Cr(VI) remediation by indigenous microorganism of the field soils contamined by chromium-containing slag and the simulatively contaminated soil were investigated by laboratory experiments under shaking condition. The results showed that water soluble Cr(VI) in soils was completely remediated when enough culture medium was added. The opitimal carbon source was glucose, and optimum amount of carbon and nitrogen source were 8g/L and 13g/L respectively. The indigenous microorganism isolated from field soil in the previous research can completely remediate Cr(VI) in the simulatively contaminated soils. The inoculation amount of microorganism, pH value of culture medium, the initial Cr(VI)concentration and aerobic condition obviously affected Cr(VI) remediation. High inoculation amount of microorganism, high pH value and low initial Cr(VI) concentration resulted in high Cr(VI) removal effeciency. Moreover, the Cr(VI) removal effeciency in aerobic condition was higher than that in anaerobic condition.In leaching experiment, the addition of culture medium could remediate water soluble Cr(VI) in soils contaminated by chromium-containing slag, exchangeable Cr(VI) and carbonate-bonded Cr(VI). The single carbon source could not completely remediate Cr(VI) in soils, while the combination of carbon source and nitrogen source could accomplish Cr(VI) remediation. The highest Cr(VI) remediation efficiency was achieved at supplying 4g/L of glucose and 5g/L nitrogen source. The optimal pH range was 7.5-8.5. The continous circulation leaching had better remediation effectiveness than intermittent leaching. Cr(VI) remediation in the contaminated soils with 345mg/kg of initial Cr(VI) content could be obtaineded in 3 days.A batch incubation experiment was explored to investigate the Cr(VI) remediation kinetics and its mechanism. The results revealed that Cr(VI) remediation rate was dependent on pH value of culture medium, the initial Cr(VI) concentration in soils, amount of culture medium and temperature. The process of remediation of Cr(VI) by indigenous microorganism can be described with zero order reaction. When the ratios of soil to solution were 2:1, 1:1, 1:1.5, 1:2 and 1:2.5, the rate constants for kinetic equation of Cr(VI) remediation in soils were C_t=C₀-3.659t, C_t=C₀-5.622t, C_t=C₀-5.983t, C_t=C₀-6.549t, C_t=C₀-6.61t, respectively. The results from the calculation of apparent active energy showed that Cr(VI) remediation in the contaminated soils influenced by temperature. The relationship between remediation rate and temperature was k(T)= exp(12.65-3261.26/T). Soil organic matter, iron oxides and magnesium oxides did not involve in Cr(VI) reduction during the process of Cr(VI) reduction by indigenous microorganism. However, Cr(VI) remediation in soils was contributed to Cr(VI) microbial reduction.