Preparation Of Modified Agricultural Waste-Based Biochar And Its Effect And Mechanism On Cadmium Adsorption And Passivation

Soil cadmium（Cd）pollution situation is a serious problem in China,the use of passivation and stabilization of soil cadmium is the most economical and effective safe utilization technology for cadmium contaminated soil.At the same time,the agricultural waste production of China is rich,agricultural waste resource utilization is one of the important contents of China’s ecological civilization construction.In this paper,agricultural waste was used as raw material to develop biochar materials with high efficiency of cadmium passivation,which provided material and theoretical support for the construction of technical system,utilization of agricultural waste and safe utilization of cadmium-contaminated soil.The main research contents and results are as follows:Six kinds of common agricultural wastes,chicken manure,pig manure and cow manure,were selected to prepare biochar at 300℃and 700℃,respectively,and biochar from rice straw,corn straw and wheat straw were prepared at 300℃and 500℃,respectively.The adsorption process and mechanism of biochar prepared at different raw materials and temperatures on Cd²⁺were analyzed in the solution system.The fitting results of Langmuir equation showed that the maximum adsorption capacity of cow manure,chicken manure and pig fecal-based biochar for Cd²⁺increased from 83.40,19.65 and 96.74 mg·g¹ to 106.54,268.89 and 164.53mg·g^-1 with the increase of pyrolysis temperature,respectively.However,there was no significant difference in the maximum adsorption amount of Cd²⁺among straw-based biochar prepared at different pyrolysis temperatures.Agricultural waste-based biochar was alkaline,and ash content increased with the increase of pyrolysis temperature except cow dung biochar.The pore structure of biochar became richer,oxygen-containing functional groups increased,and aromatic structure appeared with the increase of pyrolysis temperature.With the increase of pyrolysis temperature,the contribution rate of cation-πinteraction increased from 1%-13%to 8%-30%,but the contribution rate of chemical precipitation and ion exchange remained high（70%-93%）.The chicken manure biochar with poor adsorption capacity and rice straw biochar with pyrolysis temperature of 300℃,which is easy to be collected from raw materials,were selected.The initial biochar was added to the mixed solution of CS₂and Na OH solution to obtain sulfur-modified chicken manure biochar（SCMB）and sulfur-modified rice straw biochar（SRSB）.The original biochar was immersed in Ca Cl₂ and（NH₄）₂HPO₄ solution to obtain hydroxyapatite-modified chicken manure biochar（HCMB）and hydroxyapatite-modified rice straw biochar（HRSB）.The original biochar was immersed in KMn O₄ and Fe Cl₃·H₂Osolution to obtain Fe-Mn oxide-modified chicken manure biochar（FMCMB）and Fe-Mn oxide-modified rice straw biochar（FMRSB）.SCMB and SRSB were successfully loaded with sulfur functional groups,the adsorption capacity for Cd²⁺was 188.20 and 130.60 mg·g^-1,respectively,and the contribution rate of complexation increased to 22%and 48%,respectively.HCMB and HRSB were successfully loaded with hydroxyapatite（HAP）,the adsorption capacity of Cd²⁺was increased to 111.53 and 112.70 mg·g^-1,respectively,the contribution rate of HCMB precipitation increased to 82%,and the contribution rate of HRSB cation-πinteraction increased to 46%.FMCMB and FMRSB successfully loaded with iron and manganese The adsorption capacity of Cd²⁺was increased to 109.94 and 100.88 mg·g^-1,respectively,and the contribution rate of Cd²⁺adsorption by cation-πeffect increased to10%and 13%.Modified chicken manure biochar had better effect on Cd passivation in soil and Cd reduction in small green vegetables than modified rice straw biochar,and 5%SCMB had the best effect.It could significantly reduce the available Cd content in soil（33.01%）and the Cd content in small vegetable plants（20.78%）.Compared with the control group,SCMB increased soil p H value,while HRSB decreased soil p H value.Adding unmodified biochar increased the biomass of young cabbage while adding modified biochar reduced the biomass of Brassica campestris L.ssp.chinensis Makino（var.communis Tsen et Lee）.Compared with the control group,the application of SCMB significantly increased the soil p H,and the application of HRSB significantly decreased the soil p H.The application of unmodified biochar increased the biomass of cabbage,and the application of modified biochar decreased the biomass of cabbage.In addition to HRSB treatment,the application of modified chicken manure biochar reduced the content of exchangeable Cd in the soil and increased the carbonate-bound and organic-bound content.The application of FMRSB and FMCMB increased the iron and manganese oxides content of Cd in soil.In summary,this study provided a useful exploration for the modification of agricultural waste-based biochar to passivate cadmium-contaminated soil,and provided a new way for the resource utilization of agricultural waste.