Study On The Remediation And Mechanism Of Lead And Cadmium Co-Contaminated Soil By Sulphur-Iron-Modified Biochar

The fossil fuels combustion,mining and metallurgy in the process of industrial production often cause lead（Pb）and cadmium（Cd）pollution of soil,which further affects microbial activities and poses a serious threat to human beings.Hence,it is pressing to explore high-efficiency remediation technology of Pb and Cd contaminated soil.Chemical passivation technology has become a promising technology in the remediation of Pb and Cd contaminated soil,owing to its excellent restoration effect and no secondary pollution.Biochar has been widely used in the remediation of Pb and Cd contaminated soil due to its environmental friendliness,biocompatibility and abundant raw materials.However,the passivation capacities of Pb and Cd in soil by original biochar is limited due to few active sites,thus it is urgent to explore effective techniques to improve its passivation capacity.Therefore,in this study,sulfur-iron modified biochar（BC-Fe-S）was prepared by loading Fe₂O₃nanoparticles and grafting sulfur-containing functional groups onto biochar,and the passivation efficiencies and mechanisms of Pb and Cd in soil by BC-Fe-S were explored.The main results are:（1）The surface functional group and morphological features of BC-Fe-S were analyzed via Fourier infrared transform spectroscopy（FTIR）and scanning electron microscopy（SEM）.The results showed:Na and S elements were uniformly distributed on the BC-Fe-S surface,indicating that BC-Fe-S has abundant sulfur-containing functional groups.Furthermore,the distribution mapping of the Fe elements confirmed the presence of iron oxides on the surface of the BC-Fe-S.According to the specific surface area and pore size analysis,the specific surface area of BC-Fe-S was 41.30 m²/g,which was mainly composed of mesopores and some micropores.The FTIR results indicated that-OH,Fe-O,C=S,and C-O existed on the BC-Fe-S surface,demonstrating the successful loading of Fe₂O₃nanoparticles and grafting of sulfur-containing functional groups.Moreover,according to XRD patterns,BC-Fe-S has characteristic peaks consistent with the spindle crystal hematite（α-Fe₂O₃）,demonstrating the successful loading of Fe₂O₃nanoparticles.（2）The adsorption performance of Pb（II）and Cd（II）by BC-Fe-S was explored by water phase adsorption tests.The influences of initial p H,reaction time,Pb（II）and Cd（II）initial concentrations and reaction temperature on the adsorption effect were studied,and kinetic,isotherm model fitting and thermodynamic analysis were performed.The results showed:the initial p H with the optimal adsorption effect of Pb（II）and Cd（II）by BC-Fe-S was 5.0,and the adsorption reached equilibrium at 30 min.Furthermore,the pseudo-second-order kinetic model and Langmuir model were more suitable to describe the adsorption processes of Pb（II）and Cd（II）by BC-Fe-S,suggesting the adsorption of Pb（II）and Cd（II）by BC-Fe-S were monolayer chemical adsorption with the uniformly distributed active sites on the BC-Fe-S surface.According to Langmuir model,the theoretical maximum adsorption capacities of Pb（II）and Cd（II）by BC-Fe-S were 124.62 and 57.71 mg/g,separately.On the basis of the consequences of the thermodynamic analysis,the adsorption processes of Pb（II）and Cd（II）by BC-Fe-S were spontaneous and exothermic.（3）The passivation performance of BC-Fe-S on bioavailable-Pb and-Cd based on DTPA extraction in soil was explored.The influence of reaction time,material dosage,soil p H and soil moisture content on the passivation performance of different chemical forms Pb and Cd based on Tessier classification was explored.Moreover,the effects on the passivation performance of bioavailable-Pb and-Cd under freeze-thaw cycle and dry-wet cycle were investigated.The results showed:the BC-Fe-S had an efficient passivation performance of Pb and Cd in soil,and the passivation of bioavailable-Pb and-Cd in soil reached equilibrium within 5 days.After 20 days of BC-Fe-S treatment,the bioavailable-Pb and-Cd contents decreased by 59.22%and 70.28%.After BC-Fe-S treatment,the labile forms（exchangeable and carbonate bound speciation）decreased by12.22%and 16.21%and then converted into residual speciation.With increasing time and material dosage,the toxicities of Pb and Cd in soil decreased,transforming from labile speciation to stable speciation.Meanwhile,variations in soil p H and moisture could affect the chemical speciation of Pb and Cd in the soil.The pseudo-second-order model was better suitable to describe the passivation process of Pb and Cd in soil by BC-Fe-S,revealing the chemical binding between BC-Fe-S and Pb and Cd.The fitting results of the intra-particle diffusion showed multiple linear regression,indicating two or more passivation mechanisms co-existed in the passivation of Pb and Cd by BC-Fe-S.Moreover,the effect of freeze-thaw cycle and dry-wet cycle on bioavailable-Pb and-Cd in soil was slight,indicating the excellent stability of BC-Fe-S.（4）The effects of BC-Fe-S on soil physicochemical properties and the microbial communities were explored.The results showed:the addition of BC-Fe-S improved the soil p H and electrical conductivity,increased the content of soil organic matter and dissolved organic carbon,and significantly improved the basic physicochemical properties of the soil.After the treatment of BC-Fe-S,the significant increase in Shannon and Pielou＿e indices indicated that soil microbial communities diversity and uniformity increased,and communities structure became more stable.In the soil before and after BC-Fe-S treatment,Proteobacteria was dominant,and both ofα-Proteobacteria andγ-proteobacteria were dominant,indicating that BC-Fe-S did not affect the main structure of the bacterial communities.At the same time,the addition of BC-Fe-S improved the relative abundance of Actinobacteria and Chloricella by 4.59%and 8.13%,respectively,which might indirectly promote soil nitrogen and carbon cycle.Moreover,the addition of BC-Fe-S improved the relative abundance of heavy metal-resistant bacterium,such as ammonia-oxidized bacteria（Ellin6067）and Blastococcus,which were conducive to improving the diversity of microbial communities.（5）The interaction mechanisms of Pb and Cd with BC-Fe-S were studied by X-ray photoelectron energy（XPS）analysis.The results showed:-OH,-COOH,ferrooxide functional group and sulfur-containing functional group were involved in the binding of Pb and Cd.The interaction mechanism of Pb and Cd with BC-Fe-S mainly included cation-πinteraction,pore filling,surface chelation,ion exchange,and chemical precipitation.