| Phosphorus is one of the essential nutrients for all living organisms,and it is also a non-renewable resource.However,there are limited reserves of phosphate rock,which makes phosphorus a limiting factor for human development.At the same time,due to the extensive use of phosphorus-containing products such as fertilizer,a large amount of phosphorus is discharged into natural water bodies,resulting in water eutrophication.Recent studies have shown that bacterial mineralization of struvite from phosphorus-containing wastewaters may be an ideal response to the phosphorus crisis.Among them,Shewanella oneidensis MR-1 has been proved to be able to efficiently mineralize struvite and recover inorganic phosphorus and organic phosphorus in wastewaters.However,the heavy metal ions prevalent in wastewaters and dissolved oxygen during mineralization may affect the growth and metabolism of microorganisms,as well as the nucleation,growth,composition and morphology of struvite.Relevant studies have not been carried out yet.Therefore,in this dissertation,we focus on the influence and mechanism of heavy metal ions and dissolved oxygen on the mineralization of struvite by S.oneidensis MR-1,as well as the potential application of recovered struvite in the remediation of heavy metal contaminated soil.The main contents of this dissertation are as follows:1.Due to its wide use in industrial fields such as electroplating,dye and stainless steel welding,Cr(VI)is a common pollutant in sewage,highly soluble and highly toxic.When recovered Cr-containing struvite is used as fertilizer,Cr will accumulate in crops and soil.It can reach a high concentration after enrichment in the food chain,posing a threat to human health.Therefore,it is urgent to explore the interaction between recovered struvite and Cr(VI).In this part,X-ray diffraction(XRD),field-emission scanning electron microscopy(FESEM),X-ray photoelectron spectra(XPS),Fourier transform infrared(FT-IR),thermo gravimetric and differential thermal analyses(TG-DTA)and other characterization techniques were used to systematically study the effect and behavior of heavy metal Cr(VI)in microbial mineralization struvite system.By measuring the bacterial growth curve and the pH evolution curve,it was found that strain MR-1 was almost not affected at lower Cr(VI)concentrations.When the concentration was further increased to 0.8 mM,the growth of bacteria was inhibited at the initial stage,and the time required to reach the stable stage was prolonged,but the strain could still restore growth.The pH value of the system could also reach 9.0 after 4 d,indicating that the strain MR-1 had strong tolerance to Cr(Ⅵ)and could create a suitable alkaline environment for struvite mineralization.The XRD spectra showed that the products were all pure struvite after 5 d of mineralization in the initial Cr(Ⅵ)concentration range of 0-0.8 mM,and the transformation efficiency of Mg,the yield,structure and stability of struvite were almost not affected by Cr(Ⅵ).In addition,strain MR-1,as a dissimilatory metal reducing bacteria,can also efficiently reduce Cr(Ⅵ)to Cr(Ⅲ)under aerobic conditions,and the reduction product Cr(Ⅲ)was mainly present in the supernatant in the form of soluble organic complexes.Compared with the abiotic precipitation system,this effectively prevents the heterogeneous nucleation of Cr(OH)3 on struvite,reduces the content of Cr in struvite,and improves the utilization value of struvite.XPS analysis indicated that Cr in struvite mainly existed on the surface in the form of amorphous Cr(Ⅲ).Further FT-IR analysis showed that the antisymmetric stretching vibration peak of PO43-was not found in our biotic samples,indicating that distortion in the PO43-tetrahedron is minimal,that is,Cr(Ⅲ)has little effect on the struvite structure and substitution into the struvite lattice is limited.This part of the study can not only provide new insights for the struvite recovery from wastewaters,but also provide an effective strategy to repair the Cr(Ⅵ)-contaminated environment.2.In addition to heavy metal Cr(Ⅵ),Pb2+ is also one of the common heavy metals in phosphorus-rich wastewaters such as livestock wastewater.Its content is usually between 0.69-30.73 mg/L,which is much higher than the surface water quality standard in China.Previous studies have shown that the crystallization and precipitation process of struvite in abiotic system is significantly affected by Pb2+.The content of Pb2+in struvite is too high,which affects the recovery and utilization of struvite.How to improve the recycling value of struvite,avoid the high content of Pb2+in struvite,and realize the removal of Pb2+ in sewage is still a top priority.In this context,we still used S.oneidensis MR-1 as a model strain to mineralize struvite in simulated lead-containing wastewater,focusing on the effects and mechanisms of heavy metal Pb2+ during struvite mineralization.XRD and Rietveld refinement spectra showed that the bottom precipitates collected after 5 days of mineralization were all pure struvite at Pb2+ concentrations ranging from 1 to 50 mg/L,and no additional peaks related to Pb crystalline compounds were detected.In addition,the removal efficiency of Pb2+ in the supernatant of the system reached 97.5%at the concentration of 50 mg/L,indicating that Pb2+ mainly incorporated into the bacterial cell pellets.Further analysis of the cell pellets by XRD and transmission electron microscopy(TEM)indicated that Pb2+ was mineralized to pyromorphite(Pb5(PO4)3Cl)on the surface of bacterial cells.Biomimetic mineralization experiments showed that the functional groups such as phosphoryl,carboxyl and amino groups in the bound extracellular polymeric substances(BEPS)secreted by strain S.oneideLsis MR-1 provided adsorption sites for heavy metal Pb2+,which helped to immobilize Pb2+on the surface of bacterial cells and gradually transformed into stable pyromorphite,facilitating the effective separation of pyromorphite from struvite.In addition,the obtained struvite products were characterized by XPS,which showed that trace Pb2+hosted in struvite was mainly adsorbed on the surface of struvite through Pb-PO4 bonds.This indicates that the phosphate-mineralizing bacterium S.oneidensis MR-1 cal not only efficiently remove Pb2+ from wastewater,but also mineralize pure struvite,reducing the adverse effects of Pb2+ on struvite.This part of the study can promote the development of the simultaneous remediation of wastewaters polluted with P and the heavy metal Pb2+.3.In addition to heavy metal ions,phosphorus-rich wastewaters often have the problem of eutrophication.Algae and plankton proliferate rapidly,dissolved oxygen(DO)in water decreases,and most of the wastewater is in an anoxic state.The concentration of dissolved oxygen is closely related to the growth and metabolism of bacteria,so its change is bound to affect the process of microbial mineralization of struvite.Therefore,in this part of study,the facultative anaerobic bacterium S.oneidensis MR-1 was used as a model microorganism to systematically study the similarities and differences of struvite mineralization under aerobic and anoxic conditions.By measuring the bacterial growth curve,it was found that the growth of strain MR-1 in the log growth period was significantly obstructed under anoxic conditions,however,strain MR-1 achieved the similar growth profile to that of aerobic conditions when the culture time was extended to 36 h.In addition,the content of alkaline phosphatase secreted by strain MR-1 under both aerobic and anoxic conditions can reach more than 80 U/L,indicating that strain MR-1 can still efficiently mineralize organic phosphorus under anoxic condition.Therefore,dissolved oxygen has little effect on the yield of struvite and the transformation efficiency of Mg.In order to explore the effect of dissolved oxygen on the metabolites of bacterial MR-1,the supernatant components and EPS were characterized by three-dimensional fluorescence spectroscopy(EEM).The results showed that compared with aerobic conditions,the fulvic acid-like substances in the supernatant under anoxic conditions were significantly reduced and the tryptophan-like substances were slightly increased,while the tryptophan-like,tyrosine-like and humic-like substances of the EPS were reduced,and the content of fulvic acid-like substances did not change much.SEM observation showed that the morphology of struvite recovered under different oxygen concentrations was different.It was arrow-like under aerobic conditions,while under anoxic conditions,it was plate-like or short rod-like.Through EEM analysis of struvite,it was speculated that this may be due to the different content of tryptophan-like substances hosted in struvite obtained under aerobic and anoxic conditions.In addition,the outer membrane cytochrome C of bacteria was reduced under anoxic conditions.Compared with aerobic conditions,the Zeta potential value of bacteria was significantly reduced,and its adsorption capacity for heavy metals Cu2+ and Cd2+was enhanced.However,there is no significant difference in the content of heavy metals in struvite obtained under different oxygen concentrations.This part of the work reveals the effect and mechanism of oxygen concentration on microbial mineralization of struvite,indicating that it is still feasible to use facultative anaerobic bacteria to achieve struvite mineralization under anoxic conditions,which further promotes the development of struvite recovery technology in sewage treatment systems.4.In addition to the microbial mineralization process,the application research of the product struvite is also very important.Struvite can be directly mineralized and recovered from phosphorus-containing wastewaters.It is not only an ideal slow-release fertilizer,but also can fix a variety of heavy metals through surface adsorption,lattice substitution and other approaches.It is expected to be used as an efficient remediation agent to achieve the remediation of contaminated soil.The problem of heavy metal pollution in soil has become a worldwide environmental problem.The chemical remediation technology of adding passivating agent to soil to fix heavy metals has been widely used in recent years.Due to the high cost of some repair agents,chemical remediation still faces great challenges.Compared with other remediation agents,the preparation process of struvite is simple,low cost and no second pollution.Therefore,this part of the study used struvite as a remediation agent to systematically study its remediation efficiency for heavy metal cadmium and lead contaminated soil.Through the heavy metal toxicity leaching experiment and the soil available phosphorus content,it was found that the addition of struvite could not only reduce the bioavailability of heavy metal cadmium and lead in soil,but also increase the content of soil available phosphorus.In addition,the active state(exchangeable,bound to carbonates,bound to Fe-Mn oxides and bound to organic matter)and residual state of heavy metals in soil were extracted by Tessier sequential extraction method.The results showed that struvite could significantly reduce the content of active states of heavy metals Cd and Pb,and increase their proportion in residual state.XPS spectra showed that this was due to the complexation or precipitation of phosphate ions released by struvite decomposition with heavy metal ions in soil,which enhanced the stability of heavy metal ions.In order to further analyze the characteristics of bacterial community structure under struvite treatment,16 s rRNA gene sequencing and diversity analysis were carried out on soil samples,and it was found that the addition of struvite helped to improve the species abundance and diversity of soil bacteria.This shows that struvite has potential value in the remediation of heavy metal contaminated soil,and further broadens the application field of struvite. |
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