Based On Microbiological And Chemical Repair Of Chromium Slag Yard Soil Quality And Ecological Risk Evaluation

The heavy metal-contaminated soil is increasingly severe and results in threaten to the groundwater safety and human health. It is a particular concerned environmental problem for the soil chromium-contamination. In recent years, the remediation of heavy metal contaminated soil has been paid extensive attentions around the world. Several kinds of remediation technology have been progressed significantly and some of them have already been carried in the field. However, the environmental risk assessment of the remedied soil is lack of research, which affects the further development of the soil remediation theory and technology.In the studies, chromium-contaminated soils were remedied by using the cyclic leaching of high efficiency Cr (â…¥) reduction bacteria. Compared with the remedied soils by FeSO4 leaching remediation, fertility quality, the risk of groundwater safety and plant growth of the remedied soils with microbial method were investigated. The ecological safety of microbial remediation was evaluated. The main results were showed as follows:1) The variation of the contents of organic matter, total N and available N in soils after microbial remediation were much smaller as compared with the soils without remediaton, but the contents of total P and available P were increased significantly. After chemical remediation, the organic matter and available P in soils decreased to some extent. However, the soils total N, available N and total P were little changed. The activities of polyphenol oxidase, dehydrogenase, urease and alkaline phosphatase distinctly increased in the soils with microbial remediation.2) The fuzzy mathematics and correlative analysis were used to comprehensively evaluate chemical and biological fertility qualities of chromium contamination soils before and after remediation. The content of water soluble Cr (VI) in the contaminated soil was 1276 mg/kg and the soil chemical fertility index was 0.260 in the soil before remediation. Little effect of microbial remediation on the fertility index was observed. However, chemical remediation had a lower quality index of soil chemical fertility as compared the soils before remediation, Furthermore, the soil biological fertility index was 0.082 in the soil before remediation,0.062 in the soil after chemical remediation which reduced by 25%,0.704 in the soil after microbial remediation which increased by 9 times, respectively. The results indicated that the soil biological fertility was obviously increased after microbial remediation.3) Under the leaching with simulated acid rain, the release of Cr in the remedied soil increased with increasing volume of simulated acid rain and reached to equilibrium after the sixth year. The release of Cr in the microbal remedied soil was more slowly than that in the chemical remedied soil. The accumulated total Cr release amounts in soils after microbial and chemical remediation were 29.8 mg/kg and 80.87 mg/kg in the sixth year, the contents of Cr (VI) in the leachate of soils with microbial and chemical remediation were 0.69 mg/L and 1.35 mg/L, respectively, and there was no Cr (VI) dectection after leaching for three years. In addition, Cr release was contributed to the derease in carbonate bounded chromium and organic bounded chromium. The Carbonate bounded chromium were reduced by 65.7% and 59.58%, organic bounded chromium were decreased by 22.7% and 10.89%, in the microbal remedied soil and the chemical remedied soil, respectively.4) The fuzzy comprehensive evaluate was used to evaluate the groundwater safety. The risk evaluation grade of the chormium contamination soil before remediation would be Class 11, The evaluation grade for the soil after microbial remediation would be Class 5, But the evaluation grade for the soil after chemical remediation would be Class 5, The results indicated that after the soil remediation, the safety to groundwater increased and effectively reduced the risk, but the efficiency of the microbial remediation is better than the chemical remediation.5) The pot experiment was used to evaluate the the growth and chromium content of plant. The average biomass of ryegrass and bermuda were 3.981 g/pot and 1.064 g/pot in the heavy-polluted soil, 4.951 g/pot and 4.358 g/pot in the microbal remedied soil,17.273 g/pot and 10.909 g/pot in the chemical remedied soil, respectively. The biomass of ryegrass and bermuda were increased significantly in remedied soils. The contents of chromium in aerial parts of the ryegrass and bermuda were 135.34mg/kg and 156.86mg/kg in the heavy-polluted soil,88.173 mg/kg and 85.25 mg/kg in the microbal remedied soil,81.28 mg/kg and 90.36 mg/kg in the chemical remedied soil, respectively. The chromium content of plant was decresed obviously in remedied soils. The result implied that the microbal remedied soil was more helpful for the ecological recovery and could be used for planting forage grass and the crops for water and soil conservation.6) The fuzzy comprehensive evaluate was used to evaluate the risk to plant growth. The results indicated that the risk evaluation grade for the chromium-contaminated soil before remediation would be Class 11, which was seriously harmful to plant growth. The risk to plant growth was reduced to Class 6 after microbial and chemical remediation. The safety grade of plant growth was increased by 5 level. The risks of plant growth and the ambient enviroment were effectively reduced.