Isolation And Characterization Of Cold-resistant Sulfate-reducing Bacteria Applied In The Cd Contamination

With the rapid development of China’s social economy,the problem of soil heavy metal cadmium pollution caused by man-made activities such as mining,smelting,metallurgy,sewage irrigation,etc.is more prominent and has attracted widespread attention from the society.Because cadmium pollution is non-biodegradable and biotoxic,and it will gradually accumulate in the food chain,it may destroy the ecological balance in the polluted area and endanger human health and life safety.The methods of traditional heavy metal treatment cannot adapt to new pollution scenarios due to the complexity of the polluted environment.The exploration of new and effective treatment technologies is required to solve the current problem of soil heavy metal cadmium pollution.Although new methods of microbial treatment of heavy metal pollution have been studied,the growth activity of general microorganisms under low temperature conditions is hindered,the metabolism is slow,and the remediation of cadmium heavy metal pollution in low temperature environments is greatly restricted.The research on the remediation of cadmium-contaminated soil by sulfate-reducing bacteria at low temperature is extremely limited.In view of this,this paper mainly focuses on the scientific issue of the application of low-temperature sulfate-reducing bacteria in the treatment of cadmium-contaminated soil,and comprehensively utilizes microbial culture technology and scanning electron microscopy,X-ray diffraction and other modern analysis and testing techniques,identified the biological species of sulfate-reducing bacteria,studied the influence of different growth environmental conditions on their growth and reproduction,discussed the composition and shape of mineralized crystalline products,and finally explored the remediation effect of sulfate-reducing bacteria on cadmium-contaminated soil under low temperature conditions.The findings are as follows:（1）In this study,a low-temperature-tolerant sulfate-reducing strain（SRB）was obtained by screening and breeding from the soil.The SRB strain was identified as Klebsiella.The screened SRB strains showed good low temperature adaptability and cadmium tolerance,and the extreme value of OD₆₀₀ was greater than 0.40 at 15 degrees Celsius.（2）Through single factor and orthogonal experimental analysis,this study concluded that sodium citrate,sodium sulfate and ammonium chloride were the optimal carbon source,optimal sulfur source and optimal nitrogen source for the screened SRB,respectively;the most suitable p H value of the solution for growth was 6,and the optimal inoculum amount was controlled at 6%;Under the optimized culture conditions at 15°C,the sulfate reduction efficiency of SRB was up to 55%,the cadmium removal rate was over 95%,the final ORP value was reduced to-345m V,and the OD₆₀₀ maximum value was 0.455.（3）The extracellular polymers（EPS）produced by sulfate-reducing bacteria can provide crystallization sites for mineralization.The mineralized crystals are mostly columnar and spherical.XRD confirmed the presence of cadmium sulfide crystals in the mineralized product.Temperature has influence on the structure and morphology of mineralized products.With the increase of temperature,the crystallinity of mineralized products is getting better and better.The crystallinity at 20 degrees Celsius is good,the crystal is regular,the crystal plane is smooth,and the ridges are straight.The initial cadmium ion also has influence on the resultant morphology of the mineralized products.Too low concentration of cadmium ion produces less mineralized products,and the results are mostly in the form of fine particles.Too high concentration of cadmium ion forms agglomerates without obvious regular crystals,and the mineralized products crystallize well under the cadmium ion concentration of 30 mg/L.（4）The remediation period of cadmium-contaminated soil treated by sulfate-reducing bacteria is about 6 days.With the weakening of soil acidity,the soil remediation effect of sulfate-reducing bacteria gradually improved,and the remediation effect was the best when the p H value reached 6.At 15°C,sulfate-reducing bacteria had strong activity and could perform cadmium-contaminated soil remediation work.When the initial soil cadmium concentration was high,the sulfate-reducing bacteria could still survive and carry out mineralization remediation.As the soil cadmium concentration decreased,the soil cadmium remediation effect became better.In the actual remediation of cadmium-contaminated soil in Hunan,sulfate-reducing bacteria can effectively reduce the effective cadmium content in the soil which can remediate contaminated soil by cadmium and reduce the risk of ecological damage.