Study On Reduction And Removal Of Cadmium From Paddy Soil By Magnetic Biochar

Among 1.826 billion mu of cultivated land in China,the over-standard rate of cadmium（Cd）is as high as 7%.Cd has potential hazards to food production safety,agricultural product quality and human health.Therefore,it is extremely crucial to carry out Cd contaminated farmland restoration.The reduction technology can effectively reduce the content and harm of Cd in the soil,thus having a favorable application foreground.In the thesis,magnetic recovery technology is used to reduce and repair Cd contaminated soil,and hydroxyapatite magnetic biochar（HMBC）is selected as the adsorption material of magnetic recovery technology to adsorb dissolved Cd²⁺and particulate Cd in soil.The effects of magnetic biochar（MBC）and original biochar（BC）on the removal of Cd in soil are compared.The mechanism of HMBC reduction and removal of Cd in soil is discussed,and the parameters in the subtractive removal process are optimized.The main research contents and results are presented as follows:（1）The proportion of Cd in paddy soil and its distribution in soils with different particle sizes are analyzed.The total amount of Cd in the soil was 0.74 mg kg^-1,and the dissolved and particulate content was 2.70%and 97.3%in the total amount of Cd.The ratio of clay,silt and sand in the soil was 9.6%,15.5%and 749%,and the corresponding Cd content was 1.83,1.62 and 0.41 mg kg^-1,respectively,accounting for 24.0%,34.4%and 41.6%of the total Cd.（2）The composition and structure of HMBC,MBC and BC are analyzed.Compared with MBC and BC,HMBC has the strongest polarity and hydrophilicity,the most oxygen-containing functional groups and the largest specific surface area,which is beneficial to the adsorption of Cd.The Hydroxyapatite（HAP）loading on HMBC is beneficial to the ion exchange of Cd.The supportedγ-Fe₂O₃ makes the magnetic saturation intensity reach 202 emu g^-1,which can effectively separate and recover HMBC by external magnetic field.（3）The adsorption characteristics and mechanism of HMBC,MBC and BC on dissolved Cd²⁺are studied.The saturated adsorption capacities of HMBC,MBC and BC to Cd²⁺were 35.2,5.58 and 22.2 mg g^-1,respectively;the adsorption rate was in accordance with the pseudo second-order kinetic model,indicating that the adsorption rate is mainly affected by chemical adsorption.The adsorption of Cd²⁺by HMBC is divided into rapid and slow stage.In the slow stage,the diffusion of Cd²⁺in the biochar particles is the main factor affecting the adsorption rate.The reaction equilibrium time is 12 h.The adsorption of Cd²⁺by HMBC and BC accords with the Freundlich equation,R2（0.961）,indicating that it’s mainly multi-layer adsorption.The DRK model calculates the adsorption free energy E of HMBC for Cd²⁺to be 18.3 KJ mol^-1,indicating that the adsorption is mainly chemical adsorption.Strong acid environment（pH3）inhibited the adsorption of Cd²⁺by HMBC,the adsorption amount is 13.8 mg g^-1,when pH increased to 4,the adsorption amount increased by 132%.While the adsorption amount of HMBC to Cd is not significantly different with pH6⁸,indicating that electrostatic adsorption is not the main controlling factor affecting the adsorption of Cd²⁺by HMBC.The main adsorption mechanism of HMBC to Cd²⁺is the ion exchange of Ca²⁺and Cd²⁺,which forms Cd-HAP precipitate,followed by the complexation of oxygen-containing functional groups-COOH and-OH to Cd²⁺.BC mainly adsorbs Cd²⁺through the complexation of oxygen-containing functional groups-COOH and-OH to Cd²⁺.（4）The reduction effect of HMBC on particulate Cd is studied,and the optimum process conditions are obtained,that is,the ratio of water to soil is 2:1,and allowed to stand for 24 h,the content of HMBC is 5%（6 g/200 g）.After magnetic recovery of HMBC,41.2%of the Cd in the soil is removed.Then,the removal rate of Cd in clay,silt and sand in magnetically recovered soil and remaining soil is analyzed.The Cd content of clay,silt and sand soil in recyclable soil was 3.60,3.10 and 1.95 mg kg^–1,respectively,which is much higher than that of the remaining soil(1.38,1.20 and 0.3 mg kg^–1).It is indicated that HMBC enriches Cd in different particle sizes.The adsorption effect of HMBC to clay is the best,which accounting 20%,but the removal rate of total Cd in sand is the highest,which accounting 44.4%.The contribution rates of clay,silt and sand to the total removal of Cd in the original soil are 30.1%,21.4%and 48.5%,respectively.（5）The effects of HMBC on soil properties are analyzed.The soil pH increased from5.1 to 5.9.The available phosphorus,available potassium and organic matter increased by29.6%,194%and 28.8%,respectively.The use of HMBC helps to improve the fertility of acidic soils.However the alkali nitrogen content has reduced from 182 mg kg^-1 to 125 mg kg^-1.It’s necessary to add nitrogenous fertilizers to maintain soil fertility.