| In recent years,Chinese soil heavy metal Cd pollution is serious,threatening environmental safety and human health.How to overcome the deficiency of traditional soil passivation and phytoremediation technology in Cd contaminated soil remediation,and to develop convenient and efficient soil remediation technology is still the focus of current research.Sepiolite（Sep）has abundant reserves,low price and excellent structural properties.Functional magnetic sepiolite has both heavy metal adsorption capacity and magnetic separation capacity.In soil Cd pollution remediation,Cd can be first enriched and adsorbed,and then heavy metal Cd can be removed from the soil through magnetic separation.In this thesis,with sepiolite as the skeleton,magnetic modified sepiolite（MMSep）was prepared by magnetic modification.Ti O2 modified magnetic sepiolite（MMSep@Ti O2）was prepared by further Ti O2 surface modification.The repair effect of modified materials on Cd contaminated soil under different conditions was discussed.The repair mechanism of heavy metal Cd in soil by modified materials was studied.The main conclusions are as follows:（1）Modified materials MMSep and MMSep@Ti O2 were prepared by co-precipitation method and hydrolytic precipitation method,and the adsorption effect of the three materials on Cd2+in aqueous solution was first explored by batch adsorption experiment,so as to clarify the adsorption process and mechanism of the three materials on Cd2+.The results show that,compared with Sep and MMSep,the adsorption of Cd2+by MMSep@Ti O2 is less affected by p H value,and the removal efficiency is higher.The adsorption reaction reached equilibrium within 60min,and the removal rates of Cd2+of the three materials were 27.23%,45.56%and 98.02%,respectively.The adsorption of Cd2+by Sep fits the pseudo-first-order kinetic model,and the adsorption isothermal process conforms to the Freundlich model.The adsorption of Cd2+by MMSep and MMSep@Ti O2 fits the pseudo-second-order kinetic model,and the adsorption isothermal process conforms to the Langmuir model.The theoretical maximum adsorption capacities of Sep,MMSep and MMSep@Ti O2 were 10.69 mg/g,15.38 mg/g and 26.18 mg/g,respectively.MMSep@Ti O2 has good adsorption-desorption properties for Cd2+,and the removal efficiency of Cd2+still reaches 92.30%after 5 consecutive adsorption-desorption cycles.（2）SEM,XRD,BET,VSM,FTIR and XPS were used to investigate the morphology,structure and adsorption mechanism of the materials before and after modification.The results showed that Fe3O4 particles were successfully loaded onto the sepiolite surface through co-precipitation,and the particles were spherical with small particle size.Ti O2 was coated on the surface of Fe3O4 microspheres by hydrolysis condensation reaction of butyl titanate in the mixed solution of ethanol and concentrated ammonia water,which increased the specific surface area of the adsorbent.Cd2+adsorption and analysis did not damage the main structure of MMSep@Ti O2.The specific surface areas of MMSep and MMSep@Ti O2 are 29.54 m2/g and 41.48 m2/g,respectively,compared with Sep.MMSep and MMSep@Ti O2 have superparamagnetic properties with saturation magnetization of 16.35 emu/g and 13.39 emu/g,respectively.Under the action of external magnetic field,better separation effect can be achieved in a short time.The adsorption of Cd2+has little effect on the saturation magnetization of materials.The main reason why Cd2+can be adsorbed by MMSep@Ti O2 is the lattice defect in sepiolite framework and the covalent or hydrogen bond of-OH or-O,among which Si-O,Ti-O and Mg-O bonds play an important role in the adsorption process of Cd2+,respectively.（3）The remediation effect of three materials on Cd-contaminated soil under different conditions was studied through soil culture remediation experiment,and the remediation mechanism of modified materials on heavy metal Cd under soil environment was explored.The results showed that under different repair time conditions,the repair rate of MMSep@Ti O2 was fast and reached a balance at 14 days.The removal rate of Cd in soil was high,which was 1.78 times higher than that of MMSep.The DTPA-Cd content in the repaired soil decreased by 1.42 mg/g after 28days of MMSep@Ti O2.At different p H values,MMSep@Ti O2 has strong stability.When p H is 4,the sample recovery is 1.64 times of MMSep.At different dosage,MMSep@Ti O2 is less than MMSep when adsorption saturation is reached.Sep can transform the active extraction state of weak acid and reducible state of Cd into stable oxidizable state and residue state through lattice displacement and fixation.There are more surface active sites at MMSep@Ti O2,which has better adsorption effect on weak acid extraction state and reducible state of Cd in soil,and also has certain adsorption effect on oxidizable state and residual state.The removal rate of all forms is obviously higher than that of MMSep.Citric acid was used as activator to repair Cd-contaminated soil,and the influence of activator on the remediation effect of Cd-contaminated soil was explored.The results showed that the addition of citric acid was beneficial to the removal of Cd in soil,and the removal rates of MMSep and MMSep@Ti O2 were 32.12%and 54.54%,respectively.The addition of activator increased the mass loss of the material.The recoveries of MMSep and MMSep@Ti O2 were 63.22%and 90.92%,respectively.The activator can facilitate the transfer of Cd forms to the weak acid extraction state and promote the repair effect.The activator had little effect on the content of DTPA-Cd. |
