Research On The Stabilization Effect And Influencing Factors Of Biochar Immobilized Microorganisms On Cadmium Contaminated Soil

The soil pollution survey report by the Ministry of Ecology and Environment shows that the rate of exceeding the standard for heavy metal sites in farmland soil in China has reached 19.4%,with heavy metals and similar heavy metals reaching as high as 82.4%.Therefore,strengthening research on cadmium contaminated soil remediation technology is of great significance for the safe utilization of land resources.Based on the growth characteristics and cadmium adsorption rate of five bacteria（Bacillus subtilis,Enterobacter cloacae,Hafnia alvei,Bacillus cereus,Enterobacter ludwigii）extracted from aconite putrefaction in a medical base in Hanzhong,this paper selects the dominant strains.Different biochar were prepared by pyrolysis of wheat straw,corn cob,and coconut shell at different temperatures.SEM,FTIR,BET,etc.were used to combine their cadmium adsorption efficiency,and the optimal biochar carrier was selected to construct immobilized microorganisms with microbial agents.And using immobilized microorganisms for soil cadmium pollution remediation experiments,by studying changes in soil cadmium concentration,soil cadmium form,and soil physicochemical properties,the remediation effect and possible mechanism of immobilized microorganisms on soil cadmium pollution are revealed.The following main conclusions are drawn:（1）All four bacteria have strong tolerance to cadmium,and their OD₆₀₀ is greater than 0.4 at a Cd concentration of 100 mg/L,and their cadmium adsorption effect is significant.At the same time,the optimal adsorption conditions were determined.When p H=6,temperature 35℃,and dosage 25mg/L,the adsorption rate of cadmium by the four strains reached the maximum.Among them,the adsorption rate of Bacillus subtilis was 77.32±4.16%,the adsorption rate of Enterobacter cloaceae was 81.27±4.53%,the adsorption rate of Hafnia alvei was 88.21±1.21%,and the adsorption rate of Bacillus cereus was 83.83±3.08%.The adsorption rate of cadmium by Hafnia alvei is the highest.（2）Nine types of biochar were prepared by pyrolysis of coconut shell,corn cob,and wheat straw at different temperatures.The maximum specific surface area of CH700（coconut shell biochar）was 358.82 m²/g,followed by SW700（straw biochar）with 243.21 m²/g.Both types of biochar have well-developed pore structures.According to FTIR,CH700 and 500 exhibit characteristic peaks of NH⁴⁺,CO2,-COO,C-O-C,and aromatic C-H,while SW700 exhibits the same fatty acids and aromatic substances.SEM shows that SW700 has a more developed macroporous and microporous structure.Through the adsorption kinetics model and isothermal adsorption model,it can be concluded that SW700 and CH700 have better cadmium adsorption effects.Based on the isothermal adsorption curve,SW700 has the highest adsorption efficiency in a40 mg/L heavy metal solution,adsorbing 38.82 mg/L heavy metal solution,with an adsorption efficiency of 97.05%.SW700 has a higher fixation rate for microorganisms,reaching 67.86%.Under simulated actual conditions,the adsorption rate of SW700 for cadmium also reached93.66±1.85%,with an adsorption capacity of 7.49±0.14 mg/g.Combining microbial immobilization rate and cadmium adsorption rate,SW700 was selected for immobilization experiments.（3）Immobilized microorganisms had good remediation effect on cadmium contaminated soil,and the maximum removal rate of cadmium was 43.42±0.18%,18.63%and 19.43%higher than that of the microbial group and the Biochar group,respectively.The removal rate of immobilized microorganisms in the high concentration heavy metal environment（50-100 mg/kg）is still better than the other two groups,indicating that Biochar provides a good growth environment for microorganisms and indirectly improves the resistance of microorganisms to heavy metals.After adding immobilized microorganisms for 50 days,the proportion of weakly acid soluble and reducible forms of cadmium in the soil significantly decreased（P<0.05）,and the residual content increased.Heavy metals shifted from highly bioavailable forms to lower forms.Spearman correlation analysis shows that in soils with immobilized microorganisms,the higher the concentration of heavy metals,the faster the decrease in organic carbon.The rate of decrease in soil organic carbon content in the immobilized microbial group and microbial agent group is the fastest,indicating that the rate of decrease in organic carbon content is related to microbial biomass.The change of soil p H showed that the addition of Biochar made the soil alkaline.After the addition of immobilized microorganisms,the p H first increased to 8.24,and then decreased to 7.92.Compared with only adding microbial agents,the p H decline rate is slower after adding immobilized microorganisms.This is because Biochar neutralizes acidic substances such as fatty acids produced by microbial decomposition of organic matter,indicating that the immobilized microorganisms have good acid-base buffer capacity.The addition of immobilized microorganisms also endows the soil with good water retention and fertilizer fixation capabilities.After the addition of immobilized microorganisms,the rate of decrease in soil moisture content slows down.