Immobilization Mechanism And Soil Risk Assessment Of Pb²⁺ By Combination Of Penicillium Oxalicum And Calcium Phosphate

Pb immobilization by phosphorus（P） compounds are recognized as an effective chemical remediation method.However,soluble P compounds are susceptible to result in groundwater eutrophication and more expensive than insoluble P compounds.Thus,researchers tended to immobilize Pb by insoluble P compounds for long term ecological protection and cost benefit in recent years.Nevertheless,the low solubility of insoluble phosphorus compounds limits the immobilization efficiency of Pb.Penicillium oxalicum is considered to be a biological resource with wide distribution for promoting insoluble phosphorus compounds dissolution by low-molecular organic acid secretion.Constructing the combined system of P.oxalicum and insoluble calcium phosphate（TCP）can imrpove the solubility of insoluble P compounds,and the content of soluble P in soil,thus achieve the high immobilization efficiency of Pb.While,the study about the P.oxalicum+TCP mediated Pb immobilization mechanism is still limited.Besides,little information about the soil safety is reported after remediation.In this study,the combined system of P.oxalicum and TCP were employed.The Pb immobilization mechanism,and its effects on the soil quality,microbial community structure and Pb potential environmental risks of immobilized soil were discussed to clarify the feasibility of the combined remediation method applied to Pb contaminated soil,which help to find alternative eco-friendly and cost-effective technology for Pb contaminated soil.The main conclusion of this research are as follows:（1）The effects of different initial concentrations of Pb²⁺,TCP and different initial p H on the Pb²⁺removal efficiency by P.oxalicum+TCP were systematically investigated.The removal rates of P.oxalicum+TCP at different initial concentrations of Pb²⁺（100,500,1000 mg/L）were above 99%and showed a significant cost advantage.In addition,the P.oxalicum+TCP showed higher immobilization efficiency of more than 99%in both acidic and alkaline conditions..（2）The potential mechanism of Pb²⁺ immobilization with P.oxalicum+TCP was proposed.The negatively charged functional groups of strain in P.oxalicum+TCP adsorbed the positive charge Pb²⁺through electrostatic interaction and the tryptophan protein-like substances in EPS also participated in the biosorption of Pb²⁺.Otherwise,the P.oxalicum+TCP can convert Pb²⁺into extremely stable Pb₅（PO₄）₃(K_sp=10^-71.6)by the ability of dissolving TCP induced by their high oxalate-producing.Moreover,the toxicity characteristic leaching results showed the Pb-containing products in this combined system had the highest stabilities.（3）The effects of different remediation treatments on soil quality,heavy metal speciation and availability,and microbial community structure were comprehensively expounded.When P.oxalicum and TCP were added simultaneously,soil urease activities increased significantly and available P significantly increased by 2309.4%,,but acid phosphatase activities had little change.Compared with CK,the bioavailability of Pb in P.oxalicum+TCP was significantly decreased by 49.2%（p<0.05）and had the most significant effect on reducing soil Pb availability.Tessier extraction analysis showed that P.oxalicum+TCP effectively decreased the soil Pb migration.In addition,P.oxalicum+TCP promoted the diversity and richness of soil microbial community structure,and P.oxalicum survived only under the combined treatment.（4）The environmental risks of different remediation treatments to soil were assessed.Compared to CK,the leachability,migration and bioavailability of Pb in soil in TCP,P.oxalicum+TCP decreased notably（p<0.05）.Further comparison showed that the TCLP-Pb,SPLP-Pb and SBET-Pb in soil after P.oxalicum+TCP remediation decreased by 11.3%,78.5%and 8.1%,respectively in comparison to TCP remediation.The risk reduction rates and human health exposure risk of soil Pb were decreased.The biomass of lettuce increased considerably by 43.2%,55.9%（aboveground）,877.7%and 1205.6%（underground）（p<0.05）in TCP and P.oxalicum+TCP treatments,respectively.The various degrees of acute toxicity were observed clearly in TCP,P.oxalicum,P.oxalicum+TCP treatments,while in P.oxalicum+TCP treatment,the enrichment factor of Pb in root was decreased significantly by 88.1%（p<0.05）,which ensured the health and growth of plant root.Furthermore,the oxidation dense abilities in leaves had been ameliorated in P.oxalicum+TCP.In conclusion,P.oxalicum+TCP remediation is the most beneficial to decrease environmental risks of Pb.