Study On The Construction Of Composite Microbial Agent And Its Remediation Effect On Pb And Cd Contaminated Soil From Iron Tailings

With the rapid development of the mining and metallurgical industries,heavy metal pollution caused by industrial activities such as mining,tailings accumulation,and mining abandonment has been widely concerned.Heavy metal pollution has caused many environmental problems like the decline of soil quality,the decrease of microbial diversity,and the reduction of vegetation coverage in the mining area and its surroundings.There have been many reports on the remediation methods of heavy metal contaminated soil in mining areas,among which microbial remediation has received widespread attention.At present,many studies focused on heavy metal tolerant bacteria and their remediation effect of artificial addition of pollutants on soil,but there are fewer studies on the remediation of truly heavy metal polluted tailings soil.The study was aiming at the remediation of heavy metal contaminated iron mine tailings soil in Pingchuan Town,Zhangye City,Gansu Province.The strains with Pb and Cd tolerance were isolated from tailings soil,and choosing dominant strains to construct the compound bacterial agent,and the removal rate of Pb and Cd was optimized.Then,the biosorption capacity and mechanism of Pb and Cd by strains were preliminarily explored.Finally,the effect of tailings soil remediation on a laboratory scale was studied.The main research results were as follows:Four Cd-tolerant strains and three Pb-tolerant strains were isolated from tailings soil.They were Klebsiella pneumoniae C1-Z,Bacillus wiedmannii C2-Z,Pseudomonas putida C3-Z,Bacillus sp.C4-Z,Enterobacter xiangfangensis P1-Z,Klebsiella pneumoniae P2-Z and Enterobacter xiangfangensis P3-Z,its tolerance to Cd and Pb can reach 500 mg/L and 4000mg/L,respectively.Among them,strain C2-Z has the highest removal rate of Cd,reaching66.71%.Strain P2-Z has the highest removal rate of Pb,reaching 99%.Combined with the results of the tolerance and antagonism test,strains C2-Z and P2-Z were selected as the strains to construct the compound bacterial agent,and the optimal removal rate of Pb and Cd was optimized when the concentration of Pb and Cd were 3000 mg/L and500 mg/L.The results showed that the optimal conditions were as follows:32?,p H 8.0,and Na Cl concentration 4%.The maximum tolerance concentrations of CP to Cu²⁺,Zn²⁺and Mn²⁺were 500 mg/L,3000 mg/L and 5000 mg/L,respectively.The maximum biosorption capacity of strain C2-Z to Cd²⁺and strain P2-Z to Pb²⁺were51.90 mg/g and 109.73 mg/g,respectively.The biosorption isotherms of Cd²⁺and Pb²⁺by the two strains could be fitted by the Freundlich model and Langmuir model.And the biosorption kinetic equations could be simulated by the pseudo-second-order kinetic equations.The biosorption of Cd²⁺by strain C2-Z and Pb²⁺by strain P2-Z were determined as biological precipitation(48.6%,47.07%),extracellular adsorption(24.5%,46.98%)and intracellular accumulation(1.4%,1.95%).Biological precipitation and extracellular biosorption are the main mechanisms.SEM-EDS confirmed that Cd²⁺and Pb²⁺precipitated on the surface of bacterial.FTIR spectrogram proved that the extracellular biosorption of C2-Z to Cd²⁺was mainly due to the ion exchange and complexation reaction between-OH,-C=O,COOH,-C-O-N-H groups and Cd²⁺.The extracellular biosorption of Pb²⁺by strain P2-Z was mainly through ion exchange and complex reaction with-OH,-CHO,and-NO₂ groups on the surface of the strains.The addition of compound bacteria agent can reduce the content of acid-soluble Pb(4.26%-5.72%)and acid-soluble Cd(2.98%%-17.32%)in the tailing soil,and the contents of residual Pb and Cd were also increased,which were 35.49%-44.31%and 7.47%%-17.33%,respectively.The activities of soil urease,acid phosphatase,and sucrase have all increased.At the same time,the number of soil bacteria(90.65%),fungi(72%),and actinomycetes(346.55%)were significantly increased under the treatment of compound bacteria agent.Moreover,the remediation effect of the experimental group with the addition of yellow-brown soil was better than that of the single use of total tailings soil.