Analysis Of Microbial Diversity In Lead-contaminated Soil And Study On The Characteristics Of Pb Mineralizing Bacteri

Soil plays an important role in food security,water security,and biodiversity.With the rapid development of the Chinese economy,it is urgent to solve the problem of soil heavy metal pollution caused by the exploitation and smelting of non-ferrous metal deposits.Microbial induced carbonate precipitation（MICP）has attracted extensive attention from experts and scholars due to its economic and ecological benefits.As the main body of this technology,microorganisms play an important role in controlling heavy metal pollution.Therefore,obtaining excellent heavy metal resistant strains from the environment are the key to developing this technology.In this study,the polluted soil around the historical zinc smelting area in northwest Guizhou Province was taken as the object to investigate the soil pollution degree and soil microbial community diversity in the study area.A strain of Klebsiella grimontii with high tolerance to lead was screened from contaminated soil.The growth of the strain and its stable mineralization characteristics to lead were studied,and the stable mineralization mechanism of the strain to lead was initially explored.The main research results are as follows:（1）The average contents of Pb,Zn,Cu,and Cd were 773.99,1491.62,60.47,and 5.64 mg·kg^-1,which were 22.0,15.0,1.9,and 8.6 times as much as the background values of Guizhou Province(Pb:35.2 mg·kg^-1,Zn:99.5mg·kg^-1,Cu:32 mg·kg^-1,Cd:0.659 mg·kg^-1),respectively,and the over-standard rate was 100%,100%,82%,and 100%.Nemerow composite pollution index showed that 90.9%of the sites in the study area showed heavy pollution（PN>3）.（2）Heavy metals（Pb,Zn,Cu,Cd）significantly affected the microbial community structure and diversity in the study area.The microbial community is mainly composed of bacteria.Proteobacteria is the main bacterial group in the soil of mining areas.At the genus level,the relative abundance of Acidobacteria and Betaproteobacteria is high.Metagenomic sequencing annotated multiple heavy metal resistance genes（czc A,cus A,cnr A,czc C,cus C,cnr C,czc B,cus B,cnr B,atm1,pex A）,among which,Acidobacteria and Betaproteobacteria had the highest contribution to heavy metal resistance genes,belonging to Acidobacteria and Proteobacteria phylum.（3）A lead resistant strain with high urease yield was selected from contaminated soil in the study area,named WH15.The strain is a Gram-negative and short rod-shaped bacteria,it was identified by 16S r DNA as Klebsiella grimontii,belonging to the dominant phylum Proteobacteria in the study area.The tolerance of strain WH15 to Pb was up to 2000 mg·kg^-1,and the removal rate of 500 mg·kg^-1Pb（Ⅱ）was above 98%at different pH and temperature.The addition of 100 mg·kg^-1Ca Cl₂could promote bacterial growth.The removal rates of 2000 mg·kg^-1Pb（Ⅱ）increased from 82%to99.7%,indicating that WH15 can effectively mineralize Pb（Ⅱ）,and the addition of an appropriate concentration of Ca²⁺can improve the mineralization effect.（4）SEM-EDS of the biomineralized sediments showed that some Pb（Ⅱ）was deposited around the bacteria,and the sediments mainly consisted of C,O,and Pb.FTIR spectra showed that the precipitation process was related to hydroxyl,alkyl,amide,phosphoric acid,and carbonate groups on the surface of bacteria.The XRD pattern shows that the mineral precipitates changed from lead phosphate to Ca CO₃and Pb CO₃after calcium addition,and the main components are Calcite,Vaterite,and Cerussite.The results show that WH15can transform Pb（Ⅱ）into insoluble carbonate through MICP to achieve stable mineralization of Pb.（5）Strain WH15 has a stable curing effect on the heavy metal Pb in the original contaminated soil.After 15 days of strain repair,soil EC,TN,TP,and OM increased significantly,the contents of exchangeable Pb in soil decreased by 19.75%and 74.54%,respectively,and the contents of carbonate bound Pb increased by 26.48%and 28.81%,respectively.SEM observed that there were many flake crystals of different shapes on the surface of soil particles repaired by MICP.Many irregular aggregate particles are formed.The content of Pb（Ⅱ）in the soil leaching solution was 5.22 mg·kg^-1and 11.90 mg·kg^-1lower than that in the CK group.Under the existing experimental conditions,the repaired strain WH15 could reduce the leaching concentration of heavy metals to a certain extent.（6）Acid rain at different pH values（3.5,4.5,5.6,7.2）was simulated to leach the repaired soil.The pH value of the leached solution was stable between 6.85 and 8.26.The cumulative leaching amount of Pb（Ⅱ）was the highest in samples with pH of 3.5.They were 5.6 mg·kg^-1,5.5 mg·kg^-1,2.1mg·kg^-1and 1.1 mg·kg^-1,respectively.The leaching amount of Pb（Ⅱ）increased with the decrease in pH of the leaching solution.It is difficult to observe the original clear flake mineral crystals in the SEM image,but the XRD pattern shows the formation of lead-bearing mineral Pb₂（SO₄）O,and the repaired soil has a certain acid stability.