Study on controlling of cadmium pollution with fly ash-bentonite blocking wall

A novel blocking wall was synthesized using fly ash and bentonite with strong adsorption capability. The optimized material ratio of the blocking wall was determined by penetration tests, shear tests and adsorption tests. The morphologies and stabilities of blocking wall samples with optimized material ratio were investigated by scanning electron microscopy (SEM), energy spectrum analysis (EDS), and erosion tests, the adsorption capability of blocking wall with optimized material ratio was investigated by adsorption kinetic and isotherm tests, the migration and penetration time of Cd pollutants in blocking wall with optimized material ratio were investigated by penetration tests and numerical simulations. Results indicated that the optimized fly ash/bentonite ratio (in mass) of blocking wall was 5:1. The permeability coefficient was 1.11 × 10^-8 m s^-1 and the maximum adsorption rate was 98.38%. Meanwhile, Cd²⁺ adsorbed on the blocking wall homogeneously; the alkali corrosion resistance of the blocking wall was higher than its acid corrosion resistance, and its resistance to organic pollutants and solutions with high Cd²⁺ concentrations were high. The adsorption process can be described by the Quasi-first-order kinetics and the Freundlich equation. Specifically, the overall adsorption efficiency was simultaneously affected by liquid membrane diffusion and particle diffusion and the adsorption process is an endothermic one dominated by physical adsorption. Additionally, Cd²⁺ penetration through the wall was dominated by molecular diffusion. The variation of permeability coefficient was inversely proportional to the initial concentration of Cd²⁺. The penetration of Cd²⁺ at initial concentration of 500 mg L^-1 through the blocking wall needs 40 years.