Effects And Mechanisms Of Lead Contaminated Soil Remediation By Phosphate Solubilizing Bacteria Cooperated With Iron Based Materials

Lead is a kind of heavy metal with strong biological accumulation and lasting toxicity,which is a serious threat to human health.Phosphorus solubilizing microorganisms?PSMs?can immobilize Pbby releasing phosphate to form lead precipitation,secreting metabolites to chelate with lead,and changing the fractions of lead.However,due to low activity and remediation efficiency,it is difficult to directly remediate lead pollution in soil by using phosphate solubilizing microorganisms.The technology of microbial coupled with iron materials is considered to be an efficient and environmentally friendly remediation method.Based on this,the lead with the most serious pollution was taken as the main research object,phosphate solubilizing microorganisms combined with iron-based materials were utilized to strengthen the remediation of lead contaminated soil.The main research results are as follows:Firstly,phosphorous solubilizing microorganisms were isolated from heavy metal contaminated soil,and the phosphate solubilizing abilities and tolerance to heavy metals in different phosphate solubilizing microorganisms were studied.The results showed that 9 strains had relatively high lead resistant capabilities with minimum inhibitory concentrations?MICs?ranged from 0.5 to 8 mmol/L,and were identified as species of Leclercia adecarboxylata,Pseudomonas putida,Aspergillus niger and Penicillium granulatum.Among those strains,1 Leclercia adecarboxylata strain named L1-5 had high lead resistant capabilities with MICs 8mmol/L and had the stable phosphate solubilizing ability,was chosen as the main strain for further research.In addition,the organic acids secreted by phosphate solubilizing bacteria were mainly acetic acid,while the organic acids secreted by phosphate solubilizing fungi were mainly gluconic acid,and the secreted amount was much higher than that of phosphate solubilizing bacteria.In order to further clarify the response of phosphate solubilizing bacteria to lead stress and the mechanism of lead passivation,the effects of lead stress on the growth and metabolism of PSMs were investigated from the aspects of phosphate solubilizing ability,organic acid and phosphatase secretory abilities,and extracellular polymer substance contents.The role of different layers of extracellular polymeric substance?EPS?that were extracted from Leclercia adecarboxylata was also explored in Pb²⁺adsorption.Results showed that high concentration of Pb²⁺could inhibit the growth and metabolism of PSBs,resulting in a decrease in the soluble phoaphate content of the system.In the meantime,the species and contents of organic acids secreted by the strain were greatly affected.The secretion of succinic acid and citric acid was decreased,and the activity of acid phosphatase was enhanced.Under lead stress,the secretion of extracellular polymer substances was increased or decreased to some degree.After the bacteria reacted with the lead solution,more than 90%of the lead ions were mainly distributed in the soluble extracellular polymer,while in the bacteria isolated from the extracellular polymer substances,the passivation amount of Pb²⁺was only 5%.The results showed that extracellular precipitation,complexation and electrostatic adsorption are the main mechanisms of phosphate solubilizing bacteria and the extracellular polymeric substances on lead passivation.In order to sove the problem of limited actives in lead remediation process by PSMs bio-beads were studied to remediated lead contamination,which were prepared form the BC/nZVI embedded in the PVA-SA with high surface area and porous structure.The passivation efficiency and mechanisms of lead via bio-beads under different conditionswere also investigated.The results showed that the bio-beads prepared in this study have good phosphate solubilizing ability under lead stress,and when the dosage is 5%,the initial lead concentration is 1 m M,the p H is 5,and the temperature is 30°C,the removal rate of Pb²⁺can reach 93%.It was also concluded that surface functional groups carboxyl,hydroxyl and amido provide sites for the adsorption of Pb²⁺,and the lead could be transformed into stable crystal texture,such as Pb₅?PO₄?₃Cl and Pb₅?PO₄?₃OH.In addition,bio-beads were used to simulate soil remediation,indicating that niche competition made Pseudomonas,Sphingomonas and Pontibacter became the dominant bacteria groups,and reducible fraction lead was increased 64.85%and some acid soluble fraction of lead was also increased during the remediation process,probably because the phosphorus source released by the bio-bead was not enough in soil.Finally,in order to solve the problem of insufficient phosphorus source in soil when lead contaminated soil was remediated by bio-beads,the iron-based containing phosphate nanomaterials was prepared and the performance was optimized.In addition,the phosphate solubilizing bacteriacoordinated iron-based containing phosphate nanomaterials remediation system was constructed,and the lead contaminated soil remediation experiment was simulated and also set up to explore the transformation law of lead fractions and the mechanisms of lead passivation in soil.The preparation of iron-based containing phosphate nanomaterials?nZVI@C/P1?with core-shell structure was composed of nZVI,C,Fe₄?P₂O₇?₃ and Fe₃?PO₄?₂.This material associated with phosphate solubilizing microorganisms has good passivation efficiency in Pb²⁺under acid condition?p H=3?5?and high temperature?37°C?,which can reach 100%.Simulated soil remediation experiments also showed that Leclercia adecarboxylata became the dominant population during the inoculation time.Soil residue fraction lead was increased from 2%to 33%,and can promote the formation of Pb₁₀?PO₄?₆?OH?₂,Ca₂Pb₈?PO₄?₆?OH?₂ and Pb₅?PO₄?₃Cl via enhancing phosphate released from iron-based containing phosphate nanomaterials.In the toxicity leaching experiment,the lead leached from soil after the remediation by phosphate solubilizing bacteria and nZVI@C/P1 was reduced by 16.62%compared with the blank.Based on the results of all experiment,it can be seen that the system can effectively passivate lead and provide a new way for the remediation of lead contaminated soil.