Effect And Mechanism Of Bacteria Loaded Biochar On Remediation Of Arsenic,Lead,and Cadmium Complex Contaminated Soil

Arsenic(As),lead(Pb),and cadmium(Cd)were the most important heavy metal contaminants at lead-zinc smelting sites.They were highly bioaccumulative,persistent,and toxic,which posed a serious threat to human health.Bacteria loaded biochar was a safe and environmentally friendly remediation measure.Therefore,in this study,four functional strains were screened and identified from 3 heavy metal contaminated site soils.Then the strains were combined with biochar to remediate lead-zinc smelting site soils,in order to provided a theoretical basis for the application of bacteria loaded biochar in the remediation of 3 heavy metal complex contaminated soils.The main conclusions of this paper were as follows.(1)Four functional strains,HN2,GS6,HN8,and HN11,were screened from the soil at the site soil for the following capacities:heavy metal tolerance,As oxidation,and phosphorus solubilization.The 4 functional strains were identified by 16S rDNA as Bacillus amyloliquefaciens,Bacillus cereus,Bacillus velezensis,and Bacillus velezensis,respectively.The maximum tolerance concentrations of of the 4 functional strains were 0.5-50,15-50,and 15-50 mmol/L,respectively.The growth curves of the 4 functional strains showed that all of them could reach the logarithmic phase at around 8-12 h.(2)After optimizing the culture conditions of the strains,the biomass of HN2,GS6,HN8,and HN11 could reach 5.9×109,1.0×109,1.1×109,and 1.2×109 CFU/mL,respectively.The adsorption capacity of the mixed bacteria for simultaneous remediation of multiple heavy metals were 67.03,10.39,and 1.65 μg/g for As,Pb,and Cd,respectively.XRD,FTIR,and XPS analyses showed that the mixed bacteria adsorbed As,Pb,and Cd through extracellular polymers,co-precipitated Pb and Cd through biomineralization,reduced the toxicity of As through oxidation of As(Ⅲ).(3)Five materials were prepared:biochar(BC),calcium modified biochar(CBC),mixed bacteria(M),biochar and calcium modified biochar physisorbed bacteria(BCM),and sodium alginate embedded biochar and bacteria(BCB).BCM and BCB at 1%was chosen as the dosage based on the remediation effect and cost.XRD,FTIR and XPS were used to investigate the mechanisms of BCM and BCB in remediation of heavy metal contaminated soil,the results showed that these 2 materials reduced the toxicity of As through the formation of Ca2AS2O7,functional group complexation,and oxidation of As(Ⅲ);remediated Pb contamination through the formation of Pb5(PO4)3OH and functional group complexation;remediated Cd contamination through ion exchange.And co-precipitation also occurred between As and Pb/Cd.(4)A 28-day soil incubation experiment was conducted to verify the effectiveness of the material in the remediation of heavy metals in soil and to evaluate the safety of the material.The results showed that the addition of the materials transformed the heavy metals in the soil from the exchangeable state to the organic bound states and residue states;the pH of the soil increased steadily by 0.2-0.4 and the EC decreased steadily by 400-500 μS/cm;BCM increased the dehydrogenase activity by 47.75%without significantly affecting the urease activity(p=0.080>0.05);CBC and BCB increased microbial species richness,M and BCM increased the evenness of the dominant bacterial population.