Study On Remediation Effects And Mechanisms Of Cadmium Pollution Using Modified Sepiolite

Recently,with the rapid development of urbanization and industrialization,cadmium（Cd）pollution incidents occur frequently and it’s imminent for Cd pollution remediation.Immobilization remediation is widely used because of its economy,simplicity and good effect.Sepiolite,as a porous magnesium silicate clay mineral,has a low price and a wide range of sources.However,it is limited for the remediation of heavy metal pollution due to cavity obstruction and limited specific surface area,it is necessary to strengthen the development and utilization of sepiolite.The purpose of this study was to explore the adsorption properties,emediation effects and r passivation mechanisms of different modified sepiolite on Cd,which could provide theoretical basis for the remediation of Cd pollution by sepiolite.The main contents are as follows:（1）The ABsep was obtained by Sepiolite（Sep）taken as a raw material using the method of combined acid-base modificating.The structural properties of Sep and ABsep were analyzed by adsorption-desorption isotherms of N₂,TEM,SEM-EDS,FTIR,and XRD.The effects of time,mass ratio of ABsep/Cd²⁺,temperature,adsorbent dosage,pH and co-existing ions on the adsorption of Cd²⁺by ABsep were evaluated.The research showed that the pore structure of ABsep was more developed.Compared with those of Sep,specific surface area,average pore diameter and pore volume of ABsep were increased by 66.1%,15.7%,and 34.8%,respectively.The main components of ABsep were SiO₂and Mg（OH）₂.The adsorption process of Cd²⁺by Sep and ABsep could be well fitted with pseudo-second-order kinetic equation and Langmuir isotherm,and both were spontaneous endothermic reactions,which were mainly chemical adsorption along with physical adsorption.The best mass ratio of ABsep/Cd²⁺was 3:1,and the maximum saturated adsorption capacity fitted by Sips model of ABsep on Cd²⁺at 298 K was 142.43 mg·g^-1,which was 3.55 times of Sep.With the increase of adsorbent dosage,the adsorption amounts of Cd²⁺increased firstly and then decreased,with the optimum dosage at 0.3 g·L^-1.The amounts of Cd²⁺adsorbed by ABsep increased with the initial pH in the solution,with the best pH at 7.The adsorption of Cd²⁺was inhibited by the presence of K⁺,Na⁺,Mg²⁺and Ca²⁺at different concentrations and the highest inhibition was by Mg²⁺.（2）The HyFe/ABsep was obtained by surface coating with NaOH and Fe（NO₃）₃·9H₂O on ABsep substrate.The structure and morphology of HyFe/ABsep were characterized through BET,TEM,SEM-EDS,FTIR,XRD,and XPS.The adsorption performance and mechanism of Cd by HyFe/ABsep were analyzed,and the repair mechanism of Cd in soil by HyFe/ABsep was discussed.The research showed that the hydroxyl irons were successfully loaded onto ABsep.After modification,HyFe/ABsep had more developed pores and a rougher surface morphology,and the salt-soluble ion contents was increased.The surface-loaded iron species were mainly composed of FeOOH.Adsorption process of Cd²⁺by HyFe/ABsep was conformed well by the preudo-second-order model and Redlich-Peterson models,respectively.The behavior over a whole range of adsorption was consistent with chemical adsorption being the rate-controlling step.And the theoretical maximum adsorption capacity（Q_m）obtained for the HyFe/ABsep was 220.9 mg·g^-11 at 298 K,which is 4.87 times of Sep.HyFe/ABsep also had better passivation effect on bioavailability of soil Cd than Sep.Moreover,the results indicated that the main mechanisms of Cd passivation in soil by HyFe/ABsep including precipitation,ion exchange and complexation of active silanol groups.（3）SDBS/ABsep was prepared by organic modification on ABsep substrate.The morphology and adsorption mechanism of SDBS/ABsep were investigated by BET,SEM-EDS,TEM,FTIR,XRD and XPS.The adsorption performance of SDBS/ABsep on Cd²⁺in solution was studied by static adsorption experiments.Soil culture experiments were conducted to analyze the remediation effect and mechanism of SDBS/ABsep in Cd polluted soil.The research showed that the surface morphology of SDBS/ABsep was rougher and looser,and the specific surface area was increased.SDBS was covered on the surface of ABsep without changing its crystal structure.In the static adsorption test,when the initial pH value was at 5.5,the adsorbent dosage was at 1 g·L^-1,the saturated adsorption capacity of SDBS/ABsep on Cd²⁺could reach 68.86 mg·g^-1,which is 4.28 times of Sep.The adsorption process of Cd²⁺by SDBS/ABsep conformed well to the pseudo-second-order reaction kinetics.The adsorption isotherm data conformed well to Langmuir and Redlich-Paterson equations.The whole adsorption reaction was a spontaneous endothermic reaction.The soil pH and electrical conductivity was increased by the addition of SDBS/ABsep with the contents of available Cd reduced.The acid extraction state and reducible state of Cd in soil were converted to residue state by the addition of SDBS/ABsep.The passivation effect of SDBS/ABsep on Cd in soil was better than Sep.The passivation mechanism included complexness of sulfonic acid group and hydroxyl group with Cd²⁺and cation exchange.