Construction Of Microbial Compound Bacterial Agent And Its Combination With Plants For Iron Tailings Remediation

Zhangye City,Gansu Province,has rich reserves of iron ore.The tailings left over from overexploitation and smelting of these iron ores pose significant ecological ricks to the surrounding soil environment and local agricultural production.The research on the remediation of tailings pollution has always been valued by many scholars.The main research methods include physical remediation,chemical remediation,and bioremediation.The production process of physical remediation is expensive and difficult to implement,and chemical remediation is easy to cause secondary pollution.At present,bioremediation technology has become a hot topic in the remediation of heavy metals contaminated soil in recent years due to its economic and environmental protection,especially the plant-microorganism joint remediation technology has been favored by scholars.However,most studies only conduct indoor bioremediation experiments for a certain heavy metal pollution problem in tailings.There are rare reports on on-site remediation of tailings soil pollution.This study is mainly aimed at the pollution problem of Pb,Cd,Mn in the iron tailings soil of Pingchuan Town,Linze County,Zhangye City,Gansu Province.Based on the advantageous strains selected from the iron tailings soil in the laboratory in the early stage,which are resistant to heavy metals,select the strains that have plant growth promoting characteristics and can effectively remove Pb,Cd,Mn to construct microbial compound bacterial agent,and optimize its action conditions.Finally,the combination of microbial compound bacterial agent and Peganum harmala L.was applied to the outdoor site remediation experiment of iron tailings soil to conduct a comprehensive evaluation of the remediation effect,exploring the impact of the introduction of microbial compound bacterial agent on the diversity and composition of soil bacterial communities,the response of soil bacterial communities to environmental variables and their remediation effects on tailings soil.The main results are as follows:1.The selected seven dominant strains with heavy metals resistance were C1-h,C4-h,C8-h,M1-h,M3-h,P2-h,P3-h,of which C1-h（Klebsiella pneumoniae）,C4-h（Bacillus sp.）and C8-h（Pseudomonas fluorescens）were Cd resistant strains,M1-h（Bacillus altitudinis）and M3-h（Bacillus pumilus）were Mn resistant strains,P2-h（Klebsiella pneumoniae）and P3-h（Enterobacter sp.）were Pb resistant strains.The results of antagonistic experiment showed that there was no persistent resistance between the seven strains.2.The results of the study on the characteristics of growth promotion showed that all the seven strains had the ability of dissolving inorganic phosphorus and producing IAA.Among them,M1-h and P3-h had strong ability of dissolving inorganic phosphorus,with HD/CD reaching 2.13 and 1.89,respectively.The color of C8-h culture solution and Salkowski’colorimetric solution was the darkest,and the IAA yield was 161.15±0.19 mg/L.There were three strains with nitrogen-fixing capacity,namely C1-h,M1-h and M3-h,while the strains with iron-producing carrier capacity was only C8-h.3.The optimal ratio of the compound bacterial agent was 1:1:1:1:1:1,and the optimal culture time was 12h.Carry out the action conditions of the best proportion of the compound bacterial agent.The results showed that when the culture temperature was 28℃,the pH was 7,the inoculation amount was 3%,and the salt concentration was 3%,the removal rate of heavy metal ions(Mn²⁺,Cd²⁺,Pb²⁺)by the compound bacterial agent were the highest,reaching 60.72%,71.63%,90.48%.4.The results of outdoor site remediation test of iron tailings soil showed that with the increase of remediation time,the physical and chemical properties of the soil of the compound bacterial agent group and the compound bacterial agent-Peganum harmala group were improved,which showed that the soil pH gradually decreased and tend to be neutral,WHC showed an increase,POR gradually increased,and EC showed an overall downward trend.In addition,the activities of soil sucrase,alkaline phosphatase and urease were also significantly increased,and the soil quality was improved.Compared with the control groups,the inoculation of the compound bacterial agent-Peganum harmala significantly reduced the content of heavy metals in the tailings and reduced the soil toxicity.The removal rates of heavy metals Pb,Cd and Mn could reach 54.74%,57.46%and 24.97%respectively.5.The results of bacterial diversity analysis in the tailings soil showed that compared with the control group,the soil microbial richness and diversity of the compound bacterial agent group and the compound bacterial agent-Peganum harmala group also increased with the increase of remediation time,especially the effect of the compound bacterial agent-Peganum harmala group was significant（<0.05）.The composition and relative abundance of bacterial community showed that the introduction of compound bacterial agent and compound bacterial agent-Peganum harmala would change the composition of bacterial community in tailings soil,and the relative abundance of bacterial community composition in different remediation time periods was also different.RDA analysis and VPA analysis were conducted for the 8 environmental variables measured,and the results showed that 7 variables had significant impact on bacterial community（p<0.05）,they were pH,WHC,TEMP,POR,Pb,Cd,Mn,respectively.Similarly,Spearman correlation heat map also showed that bacterial populations of different treatment groups had different responses to environmental variables,and the effect of Pb and Cd on bacterial communities was significantly greater than that of Mn.To sum up,for the tailings soil contaminated by multiple heavy metals in the northwest region,the remediation effect of plant growth promoting compound bacterial flora combined with Peganum harmala was evaluated well,the soil quality was improved,the content of heavy metals in the soil decreased,the soil microbial diversity increased and the bacterial community structure changed.