Adsorption Mechanism And Passivation Effect Of MgCl₂ Modified Pennisetum Giganteum Z.x.lin Biochar On Cd²⁺

According to the National Soil Pollution Survey Bulletin,cadmium（Cd）accounts for the highest proportion of inorganic pollution（7%）.Due to the strong mobility,high toxicity and difficult degradation of Cd,it is easy to be enriched by plants in water or soil,which will not only affect the growth of plants,but also enter organisms through the food chain,resulting in different degrees of damage to various organs.Therefore,it is necessary to develop efficient and economical materials for Cd²⁺adsorption or passivation.In this paper,raw biochar（BC）was prepared from Pennisetum giganteum z.x.lin,and then modified by anhydrous Mg Cl₂ to obtain Mg Cl₂ modified biochar（MBC）.The biochar materials before and after modification were characterized.The changes of biochar before and after modification and the adsorption performance of biochar for Cd²⁺in solution were studied.Firstly,the adsorption performance and mechanism（ion exchange,precipitation,cation-πinteraction,complexation）of biochar for Cd²⁺were investigated by solution adsorption experiments;secondly,the changes of soil pH,cation exchange capacity（CEC）,soil organic matter（SOM）and Cd forms were studied by adding biochar to the actual acidic Cd-contaminated soil.After applying biochar to the soil,conclusions are as follows:（1）Scanning electron microscope equipped with an energy dispersive spectrometer（SEM-EDS）,X-ray photoelectron spectroscopy（XPS）and X-ray diffractometer（XRD）analysis showed that the minerals loaded on the surface of MBC are mainly Mg O.And the specific surface area of MBC（202.75 m²/g）is 107 times that of BC.（2）The adsorption process of BC and MBC for Cd²⁺followed the pseudo second order kinetic model（PSOM）,indicating that the adsorption process of biochar for Cd²⁺was mainly chemical adsorption.Langmuir model is more suitable than Freundlich model,its adsorption characteristics are mainly monolayer adsorption,and there are homogeneous active sites of Cd²⁺on the surface.In addition,the maximum theoretical adsorption capacity of MBC for Cd²⁺is 763.12 mg/g,which is about 11.15 times that of BC.In the range of 25℃to 45℃,the increase of temperature is beneficial to the adsorption of Cd²⁺by BC and MBC.The removal rate of Cd²⁺in the solution increases with the increase of the pH value（2～9）.（3）After adsorption of Cd²⁺by BC and MBC,Cd-O was formed on the biochar;the main crystal on Cd-O;BC-Cd surface was Cd CO₃,and the main crystal on MBC-Cd surface was Cd（OH）₂.The mechanism of adsorption of Cd²⁺by BC or MBC is mainly the interaction of mineral precipitation(Q_cmp),ion exchange(Q_cme)and Cd²⁺-π(Q_cπ),which follows the order of Q_cmp（73.43%）>Q_cme（22.67%）>Q_cπ（3.88%）.（4）After MBC was applied to the soil,soil pH and CEC increased with the increase of biochar dosage（0.5%,1%,2%）,compared with the control group（CK）,the pH value increased by 0.52～1.53 pH units,and the CEC value of soil increased by 0.47 cmol/kg～0.93cmol/kg;the change trend of SOM content in MBC group with biochar dosage（0.5%,1%,2%）was not obvious,but the increase was higher than that of BC group,which was0.28%～2.99%compared with CK.（5）The available cadmium content in the soil of the MBC group decreased with the increase of the dosage（0.5%,1%,2%）,especially at 90 days,with the largest decrease of57.53%.The content of exchangeable cadmium in the MBC（2%,90 d）group decreased most significantly（24%）,which was mainly converted to iron-manganese oxide combined state（6%）and residual cadmium（17%）.