The Mechanisms Of Lead Immobilization By Phosphate Minerals And Phosphate Solubilizing Fungi

Lead(Pb)contamination is an emerging environmental issue in recent decades due to the rapid development of industry.Pb has known highly concealed,bioaccumulative,irreversible and long persistence in environment.Elevated concentration of Pb is highly toxic to biological population in water system,and the irrigation of Pb-containing waste water would create more farmland pollution.The inpute of Pb contaminants in soil could inhibit microbial activity and affect microbial populations.Meanwhile,Pb could also reduce the crop production and seriously restricts agricultural development.Then,Pb in soil would be uptaken by crops and accumicated through the food chain,further to influence human beings.The application of previous Pb remediation techniques has successfully reduced the biotoxicity via physical and chemistry techniques.However,these techniques have high cost and potential risk of secondary pollution.Therefore,it requires eco-friendly techniques for long-term Pb remedation.Apatite is the most abundant phosphate minerals on the Earth,mainly used in agricultural phosphate fertilizer production and chemical phosphorus(P)processing.Apatite is also an effective material for Pb remedation via the formation of insoluble Pb-minerals.However,its application is limited by apatite's relatively low solubility.Therefore,the key for Pb remedation by apatite is the ability of P released from apatite.Strong chemical acids can quickly dissolve apatite,but have high cost and potential risks of casuing secondary pollution to the environment.In addition,the released P within short term can be easily immobilized by soil organic groups and iron-aluminum coating,which reduces the effect of Pb remedation.Therefore,the long-term stable and mild acidic environment condition is a potentail way to effectively utilize apatite in Pb immobilization.Phosphate-solubilizing fungi(PSF),secreened from soil,were able to secrete large amounts of organic acids,which can decrease the solution pH and then promote the release of P from apatite.In addition,PSF also reduced the biological toxicity of Pb by both intracellular and extracellular processes.Compared with typical Pb remedation,the combination of PSF and phosphate could be an economic and sustainable option.The aim of this study is to evaluate the potential of utilizing apatite and Aspergillus niger and Penicillium oxalicum in remediation of Pb contamination.Especially,Pb tolerance mechanism confirmed that these two PSF are able to provide bright future for Pb remediation via microorganisms.In this study,the mechanisms of Pb remedation by phosphate minerals and fungi were studied from aqueous system to soil system.Firstly,Pb remediation was discussed based on biochar and the relatively high solubility bioapatite.Then,the mechanisms of Pb immobilization by FAp was investigated via Aspergillus niger and Penicillium oxalicum in aqueous solution.Moreover,the mechanisms of Pb tolerance between these two fungi were also analyzed.Finally,the feasibility of Aspergillus niger combined with BAp in Pb-contaminated soil was also discussed.This study provides a theoretical basis for the application of phosphate solubilizing fungi combined with phosphate minerals in the remedation of Pb.The specific research content and results of this study are as below:1.The mechanisms of Pb immobilization by bioapatite(BAp)in aqueous solutionAqueous solution is the direct and optimal observe system for the research of Pb remedation by phosphate minerals.We utilized bioapatite combined with biochar(as a reaction platform)to investigate the removal rate of Pb2+ in aqueous solution and process of Pb conversion.The results showed that wheat straw pellets biochar(WSP)and hardwood biochar(SB)combined with BAp can effectively remove Pb2+ in aqueous solution,and the removal rates reached 99.75%and 96.95%,respectively.The results of X-ray diffraction(XRD),attenuated total reflectance infrared spectroscopy(ATR-IR)and scanning electron microscopy(SEM)showed that Pb2+ in the solution was finally precipitated on the surface of biochar by the form of cerussite/hydrocerussite and hydroxypyromorphite.Biochar has high surface area,porous structure and high adsorption capacity,which can be used as an effective reaction platform for BAp and Pb2+ in aqueous solution.2.The immobilization of Pb2+ in aqueous solution by Penicillium oxalicum combined with geological fluorapatite(FAp)This reasearch explored the process of Pb2+ immobilization by FAp and P.oxalicum in aqueous solution.The quantity of organic acids secreted by P.oxalicum reached the maximum on the fourth day,which elevated soluble P concentrations from 0.4 to 108 mg/L in water.In addition,it was shown that?98%lead cations were removed while the fungus was cultured with Pb(?1700 mg/L)and FAp.The mechanism is that the released P from FAp(enhanced by organic acids)can react with Pb2+ to form the stable pyromorphite mineral.The precipitation of lead oxalate also contributed to Pb immobilization.This study sheds light on bright future of applying P.oxalicum in Pb remediation.3.The mechanisms of Pb tolerance based on a study of A.niger and P.oxalicumThis part investigated the stress response of the two typical PSF,Aspergillus niger and Penicillium oxalicum under different Pb levels,and the potential mechanisms of Pb tolerance between these two fungi were also explored.The secreted oxalic acid can react with Pb2+ to form insoluble Pb minerals,primarily lead oxalate.Then,the enhanced biosorption via forming new border of cell wall prevented the transportation of Pb2+ into hypha.In addition,the fungal activity could be maintained even at high Pb concentration due to the intracellular accumulation.It was confirmed that A.niger had the higher Pb tolerance compared with P.oxalicum.Meanwhile,Pb levels below 1000 mg/L partially stimulated the bioactivity of A.niger,which was confirmed by its elevated respiration.This subsequently enhanced microbial functions of A.niger to resist Pb toxicity.A better understanding of Pb tolerance of these two fungi sheds a bright future of applying them to remediate lead-contaminated environments.4.A contrast of Pb immobilization via bioapatite in different soil systemsThis part firstly investigated the feasibility of Pb immobilization via BAp and CO2 in the acidic red soils(RS)and saline-alkaline soils(SS).The elevated CO2 significantly promote the dissolution of BAp in water,i.e.,the P release was enhanced from 2 to 20 mg/L as pH was decreased from 6.9 to 5.6.In the RS soil,addition of BAp significantly increased the content of water-soluble P without CO2 and promoted the Pb adsorpt ion to iron(hydr)oxides.To the contrast,the elevated CO2 is essential for P release in SS.The elevated CO2 can promote the dissolution of BAp in SS,and the released P can react with Pb2+ to form the insoluble pyromorphite to reduce availability of Pb.It was shown that 30-40%available Pb can be removed from RS and SS soils with a combination of BAp and CO2 elevation after incubation.Consistent with the results of P release,the combination of BAp and CO2 are necessary for Pb immobilization in the saline-alkaline soil whereas adding BAp only is feasible for the acid soil.In addition,the feasibility of Pb remedation by Aspergillus niger and BAp in soil was also discussed.Aspergillus niger can survive in high Pb concentration soil(2000 mg/L),and the combination ofAspergillus niger with BAp can significantly reduce the soluble Pb concentration from?1%to?20%in the soil.This work investigated the potentials of applying BAp and microorganisms in Pb remediation for various soils.