Screening And Reduction Mechanism Study On Cr(Ⅵ) Reducing Bacteria In Chromium Contaminated Soil

Cr(Ⅵ)and its salt compounds are important industrial raw materials.As industrial by-products,Cr(Ⅵ)and its salt compounds are brought into the environment,which not only threaten human health,but also adversely affect the physical and chemical properties and biological characteristics of soil.In this study,two strains of indigenous Cr(Ⅵ)reducing bacteria were screened from actual chromium contaminated soil to study their reducing characteristics and mechanism.After adding microbial agents to Cr(Ⅵ)contaminated soil,pollutant indexes and bacterial diversity in soil were monitored,and metabolomics analysis was used to predict the KEGG(Kyoto Encyclopedia of Genes and Genomes)pathway of Cr(Ⅵ)reducing bacteria,providing theoretical basis and technical support for in-situ remediation of Cr(Ⅵ)contaminated soil.The main research results of this paper are as follows:(1)Two strains with strong reducing ability were screened from soil contaminated by chromium residue near a chromium salt plant in North China,and were named Agrobacterium sp Cr-1 and Lysinibacillus sp Cr-2 after 16 S r DNA sequencing.The optimum combination condition was selected by single factor experiment.Under the optimal reduction conditions,both strains could effectively reduce Cr(Ⅵ)at 50-500 mg/L.Agrobacterium sp Cr-1 has better tolerance to high concentration of Cr(Ⅵ)than Lysinibacillus sp Cr-2.(2)The effects of hexavalent chromium on the cell structure of Agrobacterium sp Cr-1 and Lysinibacillus sp Cr-2 were observed by SEM-EDS.It was found that the reduced bacteria cells mainly contained C,N,O and Na.The ruptured cell components have limited Cr(Ⅵ)reduction ability.The reduction rate of Cr(Ⅵ)in resting cells was much lower than that in intact cells.The first-order reaction equation can fit Agrobacterium sp Cr-1 and Lysinibacillus sp Cr-2 well in the process of removing Cr(Ⅵ)concentration of 50,200 and 500mg/L under the optimal combination conditions,and can better simulate the change trend of substrate decay with time.(3)After 42 days of reaction,the soil hexavalent chromium added to Agrobacterium sp Cr-1 decreased from 1100mg/kg to 33.49mg/kg,and the soil hexavalent chromium added to Lysinibacillus sp Cr-2 decreased to 92.29mg/kg.With the increase of reaction time,the water-soluble and exchangeable Cr in soil was transformed by microorganisms into iron and manganese oxidation state and residual Cr with lower toxicity and bioavailability,which reduced the biotoxicity and mobility of Cr in soil.(4)After adding two kinds of reducing bacteria for 42 days,the bacterial diversity in the soil was analyzed.It was found that adding Agrobacterium sp Cr-1reduced bacteria stimulated the growth of the indigenous reducing bacteria in the original soil and caused the growth competition between different bacteria.The results of metabonomic analysis showed that the two main metabolic pathways of Cr(Ⅵ)reducing bacteria were glycolysis pathway and pentose phosphate pathway,which provided theoretical basis for the regulation of Cr(Ⅵ)reduction process.