Remediation Effect And Mechanism Of Manganese Modified Biochar On Soil Contaminated With Arsenic And Cadmium

With the rapid development of industry and agriculture in China,the combined pollution of arsenic（As）and cadmium（Cd）in farmland soil is a serious problem.Due to the different chemical properties of As and Cd,remediation of As-Cd composite contaminated soil is difficult.Biochar has been widely used in the remediation of heavy metal contaminated soil due to its relatively abundant functional groups and strong ion exchange capacity.However,there are relatively few adsorption sites for As on the surface of the original biochar,and its adsorption performance cannot fully meet the needs of removing As-Cd compound pollution at the same time.Therefore,modification of biochar is essential to enhance the remediation ability of biochar to As-Cd compound contaminated soil.In this study,rice straw was used as raw material and potassium permanganate（KMnO₄）was used as modifier to prepare manganese modified biochar（MBC）by high-temperature pyrolysis of rice straw,and the adsorption and fixation effects of MBC on As and Cd were investigated by batch adsorption experiments of As and Cd;the mechanism of As and Cd immobilization by MBC were characterized by Scanning electron microscopy-Energy spectrometry（SEM-EDS）,Fourier transform infrared spectroscopy（FTIR）,X-ray diffraction（XRD）,Raman spectrometer（RM）and X-ray photoelectron spectrometer（XPS）;the passivation effect of MBC on As and Cd in As-Cd contaminated soil was investigated by laboratory simulated soil culture experiment.The main results and conclusions are summarized as follows:（1）MnO₂ were loaded on the surface of MBC,and the zero charge point of MBC increased with the increase of Mn content,the cation exchange capacity increased.The zero charge point and Mn content increased from 3.42 and 0.13 mg/g of BC to 6.01 and235.4 mg/g of MBC,respectively.The vibration peak of Mn-O bond appeared in FT-IR spectra,indicated that the Mn were successfully loaded on the surface of MBC,SEM-EDS and XRD patterns showed that there were MnO₂ particles loaded on the surface of MBC,simultaneously.（2）The adsorption capacity of MBC was strong for As and Cd,and the coexistence of As and Cd ions promoted the synergistic adsorption of As and Cd by MBC,the oxidation of As（Ⅲ）existed in the adsorption process,and As（Ⅴ）was the main form adsorbed by MBC.The maximum adsorption capacities of MBC for As and Cd in As（Ⅲ）-Cd coexistence solution at pH 7 were 141.1 and 224.4 mg/g,which were significantly higher than those of MBC for As in a single As（Ⅲ）solution（75.0 mg/g）and for Cd in a single Cd solution（122.1 mg/g）.The kinetic adsorption process of As and Cd by MBC was more consistent with the pseudo-second-order kinetic adsorption model,which belonged to chemical adsorption.（3）As in solution were mainly immobilized by reacting with MnO₂ on the surface of MBC,and Cd were mainly immobilized through complexation reaction,electrostatic adsorption,and formation of precipitation with MBC.In the As（Ⅲ）-Cd coexistece solution,the mechanism of the synergistic effect of As and Cd was that the formation of Type A ternary complex（MBC-Cd-As）promoted the adsorption of As and type B ternary complex（MBC-As-Cd）promoted the adsorption of Cd（Cd concentration>150 mg/L）.（4）The ecological pollution risk and potential ecological risk of As and Cd in the soil were reduced after application of MBC into the soil,and the more MBC applied,the lower the risk of Cd.The results of soil incubation showed that MBC application to soil could clearly reduce the content of TCLP-extractable As and Cd in As-Cd contaminated soil compared with BC,the content of TCLP-extractable Cd decreased with the increase of MBC application dosage,and reduced the lowest at 4%,while the content of TCLP extractable As was lowest at 3%.In addition,As and Cd in soil were passivated by MBC mainly by reducing the contents of non-obligate adsorbed As,crystalline Fe-Al oxide bound As,acid-soluble Cd,reduced Cd,and increasing the contents of residual As,residual Cd.In conclusion,MBC made from rice straw is a promising material for the remediation of As-Cd composite contaminated soil,and the passivation of As and Cd in contaminated soil is of great significance for remediation of heavy metal soil in China.