Remediation mechanism of EK-PRB coupling remediation of Cd-contaminated soil

Electrokinetic permeable reaction barrier (EK-PRB) coupled remediation technology combines the advantages of both electrokinetic remediation and permeable reaction barrier, which can effectively remove heavy metal ions from contaminated soil in coal mine area. Taking the widely distributed and harmful Cd-contaminated soil as the object, the existence forms and evolution rules of the complexation reaction of acetic acid, lactic acid and citric acid with Cd²⁺ were clarified by means of molecular dynamics simulation. The difference of adsorption behavior of illite, montmorillonite and kaolinite for Cd²⁺and the effect of voltage gradient on their adsorption characteristics were clarified. The remediation mechanism of EK-PRB coupling remediation of heavy metal contaminated soil was revealed. The results show that acetic acid, lactic acid and citric acid react with Cd²⁺ to form different forms of complexes, but the existing forms and proportion of complexes after citric acid reaction are more abundant and diverse, which is more suitable to be used as EK-PRB coupling remediation electrolyte. illite and montmorillonite adsorb Cd²⁺, through the formation of outer complex and inner complex, while kaolinite adsorbs Cd²⁺ mainly by inner complex, and Cd²⁺ tends to be adsorbed on Si—O tetrahedron. The mean square displacement (MSD) curve and diffusion coefficient of Cd²⁺ on the surface of illite, montmorillonite and kaolinite are anisotropic. After the action of electric field, the adsorption site of Cd²⁺ on the surface of clay minerals changes, but the adsorption capacity is constant; Cd²⁺ migrates and accumulates close to the crystal plane of clay minerals, and the peak value of relative concentration increases with the increase of voltage gradient; the mass density of Cd²⁺ on the surface of three kinds of clay minerals is montmorillonite > illite > kaolinite; The coordination number of Cd²⁺ hydration crust on clay mineral surface decreases, the total diffusion coefficient of Cd²⁺ increases significantly, the migration rate increases obviously, but the hydration ability decreases. Compared with illite and kaolinite, montmorillonite is more suitable to be used as active filling material of EK-PRB coupling remediation.