Study On Effect Of Phosphorus Modified Biochar With Bacillus Subtilis Of Cadmium Immobilization

The present state of cadmium（Cadmium,Cd）pollution in agricultural soils in China is not encouraging,and efforts to improve harmless remediation and treatment are needed.Because of its excellent adsorption qualities,biochar is gaining popularity in the remediation of heavy metal contaminated soil.Phosphate-containing groups on the surface of phosphate have heavy metal adsorption and immobilization effects,and phosphate-rich phosphate can produce insoluble precipitates with heavy metals such as Pb²⁺and Cd²⁺.The use of phosphate-modified biochar as a slow-release phosphorus fertilizer has enhanced the original biochar’s adsorption properties.Bacillus subtilis,as a heavy metal tolerant bacteria,can transform and complex heavy metals via biochemical reactions,and its loading on the surface of biochar buffered the toxic impact of the external environment on microorganisms while also improving metabolic activity and mineralization precipitation.Heavy metal passivation caused by biochar-loaded microorganisms has recently become a study focus.The impact and mechanism of interaction between the carrier and functional microorganisms such as heavy metal-tolerant microorganisms,adsorbing microorganisms,and others.The interaction effect and mechanism between the carrier and functional microorganisms such as heavy metal-tolerant microorganisms,adsorbing microorganisms,and transforming microorganisms remain unknown,and the synergistic passivation mechanism between phosphate-modified biochar and Bacillus subtilis needs to be investigated further.In this research,potassium phosphate（K₃PO₄）was chosen as the modifier to create phosphate-modified biomass carbon,and adsorption kinetics and isotherm experiments were performed to investigate its adsorption behavior.To characterize the biochar before and after adsorption,scanning methods such as scanning electron microscopy（SEM）,Fourier transform infrared spectroscopy（FTIR）,and X-ray photoelectron spectroscopy（XPS）were used.The phosphorus-modified biomass charcoal-loaded Bacillus subtilis composites were made,the optimal bacterial inoculum and immobilization duration were determined,and the effects of pH,material dosing,and initial Cd concentration on adsorption performance were investigated.Soil incubation experiments were used to compare the passivation effects of five materials on soil Cd（biochar,phosphorus-modified biomass charcoal,single Bacillus subtilis,biochar-bacteria,and phosphorus-modified biomass charcoal-bacteria）,analyze trends in soil physicochemical properties（pH,total/effective phosphorus,and CEC）,and investigate their environmental ecological effects.（microbial taxonomic composition and soil enzyme activity）.The following were the major conclusions:（1）The pore structure of potassium phosphate modified biochar was rich,the number of oxygen-containing functional groups increased,and P-O groups and their compounds appeared.The phosphorus modified biochar was easier to combine with Cd,and the P-containing groups interacted with Cd during the adsorption process,forming chemical precipitates,which improved the adsorption efficiency and increased the adsorption amount by 10.28%～42.43%compared to before modification.（2）The best preparation circumstances for phosphorus-modified biochar loaded with Bacillus subtilis were a biochar:bacterial suspension（W:V）ratio of 1:20 and an immobilization time of 18 hours.The maximum adsorption of cadmium by the composites prepared under this condition was 37.54 and 40.15 mg·g^-1,respectively.The biochar materials after loading microorganisms were found to be intact and full of rods on the surface and in the pores The intact and full-bodied rod-shaped bacteria were observed in the surface and pores of the loaded biochar materials,which proved that the microorganisms were successfully loaded onto the biochar and their surface and internal structures were favorable for bacterial colonization and reproduction.（3）The optimal adsorption conditions of phosphorus-modified biochar-loaded bacteria on Cd were pH 7～8,0.2%material loading,initial Cd concentration of 150mg/L,and adsorption capacity of up to 63.12～63.95 mg·g^-1.The loading microorganisms had better passivation effect than biochar alone,and the overall performance was phosphorus-modified biochar-bacteria>biochar-bacteria>phosphorus-modified biochar>biochar.modified biochar>biochar,indicating that the loading microorganisms had a significant enhancement effect on the passivation ability of biochar alone.（4）Soil culture experiments showed that single addition of Bacillus subtilis significantly reduced soil pH,and the remaining treatments（biochar,phosphorus-modified biomass charcoal,biochar-bacteria,and phosphorus-modified biochar-bacteria）increased soil pH and CEC.the changes in soil fast-acting phosphorus content of biochar alone treatments（biochar,modified biochar）were not significant,while loaded microbial treatments（Bacillus subtilis,biochar-bacteria,and modified bio The microbial loadings（Bacillus subtilis,biochar-bacteria,and modified biochar-bacteria）significantly increased the fast-acting phosphorus content,indicating that the addition of microorganisms was beneficial for increasing the phosphorus availability.The addition of different remediation agents increased soil sucrase,urease,alkaline phosphatase and catalase activities,with modified biochar-bacteria being the most significant,with the four enzymes increasing by 51.25%,87.82%,196.43%and43.06%,respectively.（5）The results of soil bacterial and fungal community composition showed that the relative abundance of Bacillus spp.（Bacillus）increased to some extent in all treatment groups,with the relative abundance of Bacillus spp.（Bacillus）in the loaded microbial treatment groups（biochar-bacteria and phosphorus-modified biomass charcoal-bacteria）being 20.82%and 22.1%,respectively,indicating that Bacillus spp.Bacillus）exhibited good tolerance to the heavy metal Cd.The redundancy analysis showed that soil pH,AP and CEC contents were correlated with microbial abundance,with Bacillus spp.showing a significant positive correlation with AP,which was consistent with the significant increase in soil AP content after the addition of microorganisms alone in the previous paper.（6）The results of different forms of soil Cd showed that all treatment groups reduced soil Cd availability,and the soil exchangeable Cd content was significantly reduced by 0.26～0.63 mg/kg compared to CK,while the residual Cd content was increased by 3.6～6.67 times.Among them,the microbial loading treatment significantly reduced the exchangeable Cd and increased the residual Cd compared with biochar alone,indicating that the presence of microorganisms improved the passivation ability of biochar itself.The higher adsorption efficiency of the phosphorus-modified biochar compared with the original biochar indicated that the modification was more favorable for the conversion of Cd to the less available form.Overall,the phosphorus-modified biochar loaded with Bacillus subtilis showed the best passivation effect,resulting in the conversion of most Cd to the less available form.