Study On The Mechanism Of Controlling Lead Polluted Environment By Sulfate Reducing Bacteria Based On Sugar By-Products Cultivation

With the deepening of industrialization and urbanization,heavy metal pollution by human activities in local environment is very severe.For this reason.Five major pollutants,lead,mercury,cadmium,chromium and metal-like arsenic,are explicitly required to be controlled in the 12th Five-Year Plan for the Comprehensive Prevention and Control of Heavy Metal Pollution.Sulfate reducing bacteria(SRB)is a kind of bacteria with high efficiency in removing heavy metals and has been widely used However,the metabolic process of SRB requires the additional carbon source,the development of low-cost carbon source has gradually attracted attention.As the main sucrose producing area in China,Guangxi has a huge output of sugar by-products such as molasses and filter mud in the process of sugarcane sugar production.The utilization of sugar by-products as resources has great practical significance for the environment sustainable development of Guangxi.The aim of this study is to cultivate sulfate-reducing bacteria with low-cost sugar by-products,and apply them to control lead-polluted environment,to study its effects and mechanism of controlling lead pollution The main research results are as follow(I1)The results of mixture design experiment show that the best culture effect of sulfate reducing bacteria could be obtained by mixing the filter mud and vinassse in a ratio of 1:1 to 3:1.The results of single factor experiment show that the optimum initial growth pH and temperature of SRB in sugar by-product culture is 7 and 35 C respectively.However,due to the influence of complex carbon source in sugar by-products,the time when SRB entered the logarithmic phase would be 5-10 hour later than in chemical reagent medium.Therefore,lovw-cost filter mud and vinassse mixture can be used as a medium for SRB instead of chemical reagent medium,providing a new way for the utilization of filter mud and vinassse.(2)Under the culture of sugar by-products,SRB has a lead removal rate of more than 96.97%in solution with lead concentration of is between 10-100 mg/L,and SRB has a higher tolerance to high concentration of lead solution due to factors such as varied organic substance composition of sugar by-products and different porosity of filter mud.The EPS extracted from SRB has excellent adsorption of Pb2+,and the thermodynamic and kinetic equilibrium experiments show that the adsorption process could be well fitted by Langmuir isotherm adsorption model and quasi-secondary dynamic equation.The FT-IR analysis indicate that the hydroxyl,carboxyl and polysaccharide C-O-C groups in EPS are the main groups binding to Pb2+.Scanning electron microscopy combined with energy dispersive spectrometry and X-ray diffraction analysis show that SRB mainly causes Pb2+ to form PbS precipitate through redox to remove lead from the solution.The process of removing lead ions in the sugar by-product cultivated SRB is firstly the combination of lead ions with the active site on the cell wall,followed by further formation of PbS precipitate under the influence of SRB.Meanwhile,Plackett-Burman experiment results show that SRB had high removal rate of lead,copper,zinc,cadmium and iron in solution under the culture of sugar byproduct,and the maximum removal rate of cadmium in the composite system is 99.05%,followed by copper 96.74%,zinc 96.32%,lead 96.17%,and iron 95.96%.(3)Under anaerobic static cultivation,66.23%?88.63%of lead in the soil exists in reducible form under the influence of SRB,and lead shell-soil core particles gradually appeared in the soil from the third day after mixed cultivation.FT-IR analysis show that the-CH2 and-OH functional groups on the surface of bacteria such as SRB,plays a key role in the formation of core-shell particles.The results of correlation analysis indicate that content of soil organic matter,NH4+-N,NO3+-N and available potassium play a key role in the form transformation of lead under the influence of SRB The high-throughput sequencing results reveal that the dominant bacteria in the soil are Firmicute,Actinobacteria,Proteobacteria,Acidobacteria,Chloroflexi and Gemmatirmonadetes.The SRB group mainly composed of Desulfurellaceae and Desufitobacterium plays a key role in the form transformation process of lead.Bacteria such as Bacteroide.s,Anaerobes,Clostridium and Veillonella are continuously providing carbon source and energy for SRB metabolism through organic mineralization and fermentation,which affects the conversion of lead in the soil.The results of redundancy analysis indicate that SRB inoculation into lead-contaminated soil mainly affects the bacterial community structure by changing the contents of organic matter,total phosphorus,total potassium,available potassium and oxidizable lead in soil.Under the apprearance of SRB.the influence of transformed lead in soil on the structure and function of bacterial community is weakened,which is manifested in less difference in LEfSe species and minished variation of bacterial community function in soil with different concentrations of lead.The addition of certain amount of sugar by-products to the lead polluted soil under the influence of SRB doesn't affect the transformation effect of lead and significantly improve soil physicochemistry properties and increase bacterial community diversity.