Immobilization Remediation Of Weakly Alkaline Cd-contaminated Soils Using Combined Treatments Of Biochar And Sepiolite

In recent years,the problem of soil pollution is becoming more serious with the development of industry and agriculture and the promotion of urbanization.According to the National Survey of Soil Pollution.Biochar is a soil modifier with large specific surface area and developed pore structure,and it has great potential for remediating of heavy metal polluted water and soil.Sepiolite(Si₁₂O₃₀Mg₈（OH）₄（H₂O）₄·8H₂O)is a kind of clay mineral material,whick has certain ability to adsorb heavy metals because of its porous surface and large specific surface area.Field trails and batch experiments were conducted to investigate the remediation effect and mechanism of single and combined treatments of biochar（rice husk biochar,corn bioc har,sawdust biochar）and sepiolite（rice husk biochar,corn biochar,sawdust biochar,sepiolite,rice husk biochar+sepiolite,corn biochar+sepiolite and sawdust biochar+sepiolite,which were referred as r BC、c BC、s BC、Sep、r BC+Sep、c BC+Sep and s BC+Sep respectively）.Therefore,it could provide theoretical basis and technical support for in-situ passivation remediation of heavy metal contaminated farmland.The main results of this study are as follows:（1）The results of adsorption kinetics and isothermal adsorption experiments showed that Cd²⁺adsorption onto biochar and sepiolite fitted well with the quasi-second-order kinetic equation and the Langmuir model.When the temperature was 298 K,the maximum adsorption of Cd fitted by the Langmuir model r BC、c BC、s BC、Sep、r BC+Sep、c BC+Sep and s BC+Sepwas in the decreasing order of 34.87,44.66,27.68,28.39,52.56,57.04 and 52.41 mg·g^-1,which was c BC+Sep>r BC+Sep>s BC+Sep>c BC>s BC>Sep>r BC.The behavior of Cd²⁺on biochar and sepiolite belonged to surface monolayer adsorption,and its adsorption process to Cd²⁺was the combined results of physical adsorption and chemical adsorption.Structural characterization showed that both biochar and sepiolite materials had large specific surface area and pore structure,which was beneficial to the adsorption of Cd²⁺.Three kinds of biochar materials exhibited similar absorption and characteristic peaks before adsorption Cd²⁺.The adsorption mechanisms for Cd²⁺included carbonate deposits or hydroxide deposits,and electrostatic interactions and ion exchange.Ca CO₃was the main component of Sep.Adsorption mechanisms for Cd²⁺were electrostatic interactions,ion exchange,coordination and precipitation of O-H groups with Cd²⁺complexes or hydroxyl complexes.The compound material had the characteristics both of biochar and sepiolite,which showed a better effect in adsorption of Cd²⁺.The possible adsorption mechanism included precipitation,electrostatic interaction and ion exchange,and O-H groups and Si-O-Si were also involved in the adsorption process.（2）Field experiments was conducted to investigate the effects of single and combined treatments of three kind of biochar and sepiolite on remediation of weakly alkaline Cd-contaminated soils.The content of DTPA-Cd in non-rhizosphere soil and rhizosphere soil decreased significantly,and the remediation effect was better with the increase of passivator.Moreover,the effect of combined treatment of biochar and sepiolite was better than those of single application.DTPA-Cd content in non-rhizosphere soil and rhizosphere soil decreased by 37.2%and 36.0%,respectively,when 0.2%rice husk biochar+0.5%sepiolite was applied to the soil.Cd in soils tended to transform from higher active exchangeable and carbonate bound form to mor e stable fraction of residue and iron-manganese oxide bound states.The effectiveness of passivation effect for Cd was corn biochar>rice husk biochar>sawdust biochar.Cd content in maize was significantly inhibited,and the risk of Cd accumulation in maize grains was reduced.The enrichment coefficient of Cd in maize grain decreased significantly,and the enrichment coefficient in low accumulation varieties of Liyu 16 and Sanbei 218 was lower than that of Zhengdan 958.The results above showed that the combined treatments of biochar and sepiolite has certain research prospect and feasibility.（3）Under treatments of single and combined treatments of biochar and sepiolitethe velues of soil pH and conductivity were increased.The soil pH and conductive was increased by 0.01-0.17and 6.6-66.8 units,respectively,under the compound treatment of rice hull biochar and sepiolite,0.23%-1.68%and 5.10%-34.97%,respectively for compound treatment of corn biochar and sepiolite,and 0.02-0.19 and 14-51.7 units,respectively,for the compound treatment of sawdust biochar and sepiolite.Meanwhile,the application of these passivation materials also increased soil organic carbon content.Under the compound treatment of huck biochar+sepiolite,cor biochar+sepiolite and sawdust biochar+sepiolite,soil soluble organic carbon content increased by6.45%-14.77%,8.23%-13.43%and 3.12%-32%,respectively.The enzymatic activity of S-CAT,S-UE and S-AKP/ALP were increased under the combined treatment of biochar and sepiolite.S-UE shows the trend of increasing first and then decreasing under the combined treatment of rice biochar and sepiolite.When the amount of sepiolite applied is 2%,S-UE was higher under the application of rice husk biochar 2%than 1%.However,when the amount of sepiolite was increased to 5%,the activity of S-UE decreased.After A2S1 treatment,soil urease activity increased by 46.86%compared with control group.When 0.2%maize biochar and 0.2%sepiolite were applied,soil urease and alkaline phosphatase reached maximum.The increases were 199.12μg·d^-1·g^-1 and 5.22μmol·d^-1·g^-1,respectively.This also indicated that the best proportion of soil enzyme activities to alleviate heavy metal toxicity.