Characteristics Of Adsorption And Passivation Cd By Phosphorus Based Biochar And Its Mitigation Effect On Cadmium Toxicity Of Soybean

Cd-contaminated soil and water threaten the quality of agricultural products and human health.Therefore,it is necessary to choose economic and effective remediation technology to alleviate the continuous deterioration of Cd pollution in farmland and water.In this study,phosphorus-based biochar was prepared by co-pyrolysis ammonium polyphosphate,ammonium dihydrogen phosphate,and phosphoric acid with peanut shell.The adsorption mechanism of P-based biochar was characterized by various characteristics and batch adsorption experiments,and the different mechanism of Cd passivation and remediation between three phosphorus-based biochar and original biochar was studied.The main results obtained are as follows:（1）The differences and mechanisms of physical and chemical properties and Cd adsorption between original biochar（PBC）and three phosphorus-based biochar were explored.The adsorption characteristics of ammonium polyphosphate modified biochar（PABC）,phosphoric acid modified biochar（PHBC）and ammonium dihydrogen phosphate modified biochar（PNBC）on Cd were studied by FTIR,SEM-EDS,XPS,and XRD.The results showed that the three phosphorus-based biochar significantly improved the specific surface area of biochar,and increased the oxygen-containing functional groups of biochar and the microporous structure in biochar;Isothermal adsorption and adsorption kinetics experiments showed that the Langmuir model and pseudo-second-order kinetic model could better simulate the adsorption process of Cd on biochar.The maximum adsorption capacity of Cd on PABC,PHBC,and PNBC were 155,13,8 and 99 mg g^-1,respectively,which were 4.84,4.32 and3.10 times higher than that of original biochar;³¹P-NMR analysis showed that the proportion of orthophosphate in total P of PABC,PHBC and PNBC before Cd adsorption was 82.1%,62.8%,and 54.5%respectively.After Cd adsorption,the proportion decreased to 28.24%,33.51%,and 29.34%respectively.It is speculated that orthophosphate on the surface of phosphorus-based biochar may be one of the important factors affecting the adsorption of Cd by biochar.（2）The different mechanism of soil Cd passivation by biochar and three phosphorus-based biochar was explored.Biochar was added with 1%,3%,and 5%to Cd contaminated soil for 15,30,45 and 60 days.The results showed that with the increase of biochar,the content of soil DTPA-Cd decreased.On the 60th day with 5%addition,PBC,PABC,PHBC and PNBC reduced DTPA-Cd by 14.11%,42.70%,27.12%and 24.65%respectively,which reduced the acid souble Cd by 41.41%,31.35%and 31.64%respectively,and the reducible Cd increased by 80.35%,122.00%and77.29%respectively.With the increase of culture time,the available Cd content of PHBC and PNBC treatment decreased first and then increased.However,available Cd content of PABC treatment gradually decreased and stabilized,showing stronger timeliness.Therefore,the order of Cd fixation efficiency of biochar is:PABC>PHBC>PNBC>PBC.According to the NMR,orthophosphate on the surface of biochar significantly affects the passivation effect of Cd.Orthophosphate effectively immobilized Cd in soil by forming Cd₃（PO₄）₂ precipitation,which is consistent with the results of adsorption experiment.Moreover,in addition to the direct adsorption of Cd by biochar,this study also showed that biochar could indirectly increase the passivation of Cd by increasing the content of soil p H,CEC,available phosphorus and soil organic matter.（3）The remediation effect of three phosphorus based biochar on Cd pollution soil were explored,and the mechanism of phosphorus based biochar to alleviate soybean Cd toxicity was clarified.3%biochar was fully mixed and applied to Cd contaminated soil in mining area.Compared with the control,PBC,PABC,PHBC and PNBC reduced the available Cd in rhizosphere soil by 20.18%,35.92%,24.70%and27.74%respectively,and the Cd content in grains decreased by 2.47,2.57,1.47 and1.63 times,respectively.The effect followed:PABC>PNBC>PHBC>PBC;PABC,PHBC and PNBC reduced the MDA content of shoot by 59.41%,49.81%and 38.99%respectively,and the H₂O₂ content by 36.32%,43.69%and 30.75%in soybean respectively.Phosphorus based biochar also alleviate the toxicity of Cd to soybean by enhancing the activity of antioxidant enzymes and altering the subcellular distribution of Cd.In addition,phosphorus based biochar significantly increased the activities of catalase,urease and acid phosphatase in soil.