Study On Sulfur Transformation Mediated Cd And Pb Immobilization By Sulfate Reducing Bacteria

Acid mine wastewater（AMD）is the main pollutant produced in the mining process.AMD has the characteristics of low p H value and high heavy metal toxicity,which is easy to cause serious environmental pollution,making animals and plants unable to survive.Sulfate reducing bacterium Sulfate Reducing Bacteria（SRB）is an anaerobic microorganism found throughout the nature,which can reduce SO₄^2-to S^2-combined with heavy metal ions to produce insoluble sulfide precipitation during growth and metabolism.The method of using SRB is efficient and promising to overcome the problems of secondary pollution and expensive treatment.In this study,by investigating and analyzing the characteristics of soil heavy metal pollution and soil microbial diversity in lead and zinc smelting sites,the sulfate-reducing bacteria with sulfate reduction ability and certain tolerance to Cd and Pb were selected.The characteristics of mineralization products of heavy metals were explored,and the mechanism of sulfate-reducing bacteria by settling heavy metal ions by reducing S^2-complexation was revealed.Further studies revealed that biochar,iron-based nanoparticles and gypsum favored the fixation of Cd and Pb by sulfate-reducing bacteria.The results show that the combined chemical material of sulfate reducing bacteria is a great method for Cd and Pb immobilization in mine pollution treatment.The main study contents and the results are as follows:（1）By collecting the soil around different lead and zinc smelting sites,the characteristics of soil around lead and zinc smelting sites were systematically studied.The Alpha diversity,genus classification,flora abundance difference and community gene function of different lead-zinc smelting sites were analyzed and compared.The study shows that the basic physical and chemical properties of soil such as effective potassium and effective phosphorus are quite different in different lead-zinc smelting areas.There are many heavy metals exceeding the standard in the soil of lead-zinc smelting site,among which the toxic Cd and Pb seriously exceed the standard.The analysis of microbial community diversity shows that the diversity of the communities in the soil of HY1 and GZ6 is small;in addition,the abundance of bacterial community in the soil of GZ 6 is large,and belongs to the dominant bacterial group.Among them,some bacteria in the soil of HY1（such as Bryobacter sp.H-NOV 22）were proved to have sulfuric acid reduction function genes,which laid a certain foundation for the subsequent screening of sulfate reducing bacteria from the soil of lead and zinc treatment site.（2）Isolation and screening from the soil surrounding the lead-zinc smelting area obtained a sulfate-reducing bacterium with sulfate reduction ability and certain tolerance to Cd and Pb,and performed physiological and biochemical tests and DNA identification.We also characterized the immobilization of Cd²⁺and Pb²⁺and immobilization products at different reaction time,temperature,p H,and solid-liquid ratio and heavy metal concentrations after combining them with biochar.Studies showed that the selected sulfate-reducing bacteria were Clostridium sulfidigenes HY-1.The selected C.sulfidigenes HY-1 was well tolerated for both Cd²⁺and Pb²⁺.The immobilization rate of C.sulfidigenes HY-1 combined biochar for Cd²⁺and Pb²⁺was significantly better than that of C.sulfidigenes HY-1 and biochar in a separate system.Furthermore,the optimal immobilization conditions for C.sulfidigenes HY-1 combined biochar were 3 d,p H 8.0,30℃,biochar addition of 0.6 g/L,and initial Cd²⁺and Pb²⁺concentrations of 5 and 30mg/L,respectively.Under these conditions,the immobilization rate of Cd²⁺and Pb²⁺was67.9%and 95.7%.Fourier transform infrared spectroscopy（FTIR）and scanning electron microscopy（SEM）,x-ray diffraction（XRD）and x-ray photoelectron spectroscopy（XPS）showed that the immobilized products were cadmium sulfide and lead sulfide.（3）Different iron-based nanoparticles combined iron-based material for Cd²⁺and Pb²⁺,and the ability of C.sulfidigenes HY-1 with nFe⁰to fix Cd²⁺and Pb²⁺was systematically studied by changing the amount of nano-zero Fe（nFe⁰）,reaction time,initial concentration of p H,Cd²⁺and Pb²⁺.The immobilization process of Cd²⁺and Pb²⁺by C.sulfidigenes HY-1 and nFe⁰was discussed.We showed that C.sulfidigenes HY-1 showed the best fixation of Cd²⁺and Pb²⁺when nFe⁰was used as a combined iron-based material.The reaction time was 5 d,the addition of nFe⁰was 30 mg,p H was 7.0,30℃,and the initial concentration of Cd²⁺and Pb²⁺were 5 mg/L and 20 mg/L,respectively;the fixation rate of C.sulfidigenes HY-1 combined with nFe⁰reached 89.3%and 97.8%for Cd²⁺and Pb²⁺,respectively.The FTIR,SEM,and energy dispersion spectroscopy（EDS）analysis indicated that the immobilization product of C.sulfidigenes HY-1 combined with nFe⁰may exist in the sulfide form.XRD and XPS spectroanalysis further confirmed cadmium sulfide,lead sulfide,and ferrous sulfide.Moreover,the system fixed Cd²⁺and Pb²⁺in both water and soil.（4）Three kinds of gypsum（phosphogypsum,titanium gypsum and desulfurized gypsum）were selected as sulfur sources to study the sulfate reduction ability of C.sulfidigenes HY-1,and then the fixed effect and mechanism of C.sulfidigenes HY-1 combined with desulfurized gypsum for Cd²⁺and Pb²⁺were explored.The study showed that C.sulfidigenes HY-1 has the most powerful reduction ability of sulfate in desulfurized gypsum.C.sulfidigenes HY-1 combined with desulphized gypsum fixed 80.5%of low concentrations of Cd²⁺and 90.4%of Pb²⁺,respectively.Analysis of Cd S and Pb S by combining the XRD and XPS results shows that C.sulfidigenes HY-1 uses the uniqueness of sulfate in gypsum to allow heavy metal ions to generate sulfide at low concentrations and thus the ability to fix metal well.Desulfurized gypsum as a sulfur source is characterized by the production of a small amount of metal sulfite precipitation,and because of the introduction of a large amount of Ca²⁺by gypsum,the formation of calcium carbonate biomineralization products.Moreover,the fixation rate of Cd²⁺and Pb²⁺in the soil reached over 90%.