| Irregular handling of chromate-containing waste in chromate plants caused serious chromium contaminated site issues.In order to ensure the subsequent development and utilization of polluted soil,it is urgent to investigate the risk assessment and remediation technology of chromium contaminated site.This paper made a detailed investigation on the chromium contaminated site in Yima,Henan.Also,risk assessment for the chromium contaminated site was conducted and the site remediation target limit was calculated.Two hexavalent chromium reducing strains were isolated from chromium-contaminated soil,and their identification and phylogenetic tree analysis were employed.The influence of the environmental factors on hexavalent chromium reducing by the two strains were also explored.And then the remediation of chromium contaminated soil by chemical,microbial and chemical-microbial combined processes were studied.Comparing the monitoring results of the chromium-contaminated site with the soil screening value in Beijing City,it concluded that the trivalent chromium and the hexavalent chromium were the concerned pollutants.According to the site planning for industrial land,it was determined that exposure ways were soil ingestion,inhalation and skin contact.The cumulative risk were 7.045×1 0-5 and 7.045 respectively after calculating the cancer and non-cancer risk of the concerned pollutants.Taking into consideration of 95%confidence interval with the upper content-limit as the concentration of chromium,the target value of the site was calculated as 45 mg/kg.The strains of hexavalent chromium reducing bacteria isolated from chromium-contaminated soil were identified as Bacillus and Exiguobacterium respectively.The effects of environmental factors on hexavalent chromium reducing by two strains were investigated.As for Bacillus the optimum condition was pH of 7.0 and temperature of 35 ℃.For Exiguobacterium,the optimum pH and temperature were 8.0 and 40 ℃respectively.The initial hexavalent chromium concentration less than 75mg/l would be good for the reduction by the two strains.Glucose and sodium oxalate could promote the hexavalent chromium reduction by the Bacillus,on the contrary,citric acid acted as a disincentive.Sodium oxalate also promoted the hexavalent chromium reduction by by Exiguobacterium,yet glucose and sodium citrate inhibited the reduction.Heavy metals such as Cu2+and Mg2+ could promote the hexavalent chromium reduction by the two strains,however,zinc inhibited the reduction,and Ag+ even killed the two strains.Using ferrous sulfate to reduce four typical chromium-contaminated soils,the reduction rates were 74.01%,86.56%,66.05%and 97.02%,respectively.However,the residual hexavalent chromium concentration was more than 131.34 mg/kg,and the residual concentration of Yima chromium-contaminated soil reached 409.71 mg/kg,which could not meet the target value.It demonstrated that the two strains could not reduce hexavalent chromium due to high concentration when Bacillus and Exiguobacterium both were employed to remediate the Yima chromium-contaminated soil and the one with high concentration of hexavalent chromium.Chemical and biologiacl combined process could reduce hexavalent chromium in the chromium-contaminated soil with high concentration of chromium to less than 100 mg/kg.Although the concentration of hexavalent chromium in Yima chromium-contaminated soil remediated by chemical and microbiological combined process could not meet the target limit,it could be reduced from 407.9 mg/kg by chemical process,further to 150 mg/kg by biological process. |
