| Chromium(Cr) is one of the most common heavy metal in industrial activity, and is mainly used in electroplating, wood preservation, dye, leather production and alloy. Because of the mining, the emission of industrial "three wastes", city life garbage dumping, etc. The toxicity of chromium is very strong. Agricultural soil chromium pollution is increasingly serious. Traditional method of remediating chromium pollution is with high processing costs, and is easy to cause secondary pollution. while bioremediation is safe, inexpensive, and can harness chromium pollution in situ. It is a kind of environmental friendly method.We used a strain that was identified as Bacillus cereus Cr4-1 that could reduce hexavalent chromium in contaminated soil and was isolated from the activated sludge in a chromate plant in the early stage of our research team to explore the impact of the carbon and nitrogen source, the initial concentration of hexavalent chromium and temperature to hexavalent chromium reduction, and determine the optimal reduction conditions of Bacillus cereus. We also evaluated the impact of Bacillus cereuson the micro-ecology of hexavalent chromium contaminated soil, provided a scientific basis for its application in the in-situ bioremediation.Part 1 The study of reduction efficiency of Bacillus cereus Cr4-1in chromium contaminated in soilObjective: The purpose of this study was exploring the effectiveness on Cr(VI) reduction and achieving the optimum reduction conditions in the contaminated soil by Bacillus cereus Cr4-1Methods: By means of 3 groups of microcosms tests, we investigated the reducing effectiveness at different doses of carbon and nitrogen source, initial Cr(VI) concentrations, temperatures. The dynamic equation of Cr(VI) under different initial Cr(VI) concentrations was fitted with some classic dynamic model.Results: For contaminated soil with an initial Cr(VI) concentration of 120mg/kg, the addition of carbon and nitrogen source could accelerate the reducing rate of Bacillus cereus Cr4-1. It was able to reduce soils contaminated with Cr(VI) of 60 mg/kg, 120 mg/kg and 240 mg/kg in 20 days. The remove rates could reach up to 95% after 20 days. When the initial concentration of Cr(VI) increased, the half-time of reduction reaction prolonged. With regard to the soil contaminated with Cr(VI) of 120 mg/kg, when the soil was incubated at temperatures of 20℃,25℃ and 30℃,the Bacillus cereus Cr4-1 could reduce the contaminated soil effectively. When the temperature rise, the reduction efficiency increased.The reduction efficiency was highest at 30℃. After 20 days, the remote rate could reach up to 97.1%. The first order kinetic model could describe the bio-reduction process of Cr(VI).Conclusion: The Bacillus cereus Cr4-1 could reduce Cr(VI) effectively. The he addition of carbon and nitrogen source could promote the reduction of Cr(VI).When the environmental temperature reach at 30℃,the reduction efficiency could be the highest.Part 2 The impact of Bacillus cereus Cr4-1 on the micro-ecologyof Cr(VI) contaminated soilObjective: To evaluate the impact of Bacillus cereus Cr4-1 on the micro-ecology of Cr(VI) contaminated soil.Methods: Soil microcosm was applied to simulate ecosystem. One group of microcosm was added Bacillus cereus Cr4-1 in Cr(VI) contaminated soil. Another group of microcosm which did not add Bacillus cereus Cr4-1 treated as control group. Then we gathered the soil samples at 0, 5, 10, 15, 20 days after the addition of Bacillus cereus Cr4-1 in Cr(VI) contaminated soil prepared for NGS(next generation sequencing).We also collect the unpolluted soil prepared for NGS. Then the OTU(operational taxonomic unit) analysis, the alpha diversity analysis, the Metastats analysis were analysed on these samples.Results: The microbial diversity of unpolluted soil was more than that in polluted soil whether inoculated Bacillus cereus Cr4-1 or not. The relative abundance of polluted soil which inoculated Bacillus cereus Cr4-1 and its control group in the 20 th day of the reduction period was compared with each other. It could be found that only nine genus’ relative abundance differ from the other group, which counts for 4.07% of the whole genus. And the difference between them was significant. The P value was less than 0.05. The polluted soil which inoculated Bacillus cereus Cr4-1 at 0th and 20 th in the reduction period was compared with each other. There were 72 genus’ relative abundance differentiate from the other group. It counts for 32.58% of the whole genus. And the difference between them was significant. The P value was less than 0.05.Conclusion: The chromium pollution could lead to the decline of soil microbial species and their relative abundance. Microbial diversity and relative abundance were not influenced by the application of Bacillus cereus Cr4-1 in the remediation of chromium contaminated soil. |
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