Study on the Adsorption Mechanism of Cu²⁺ by ZnAl-LDH-Containing Exchangeable Interlayer Chloride Ions

Different Zn/Al ratios of Cl^- intercalated ZnAl-layered double hydroxide (ZnAl-LDH) were prepared using the coprecipitation method, and their adsorption performance for Cu²⁺ in aqueous solution was evaluated. The factors affecting adsorption properties, such as dosage, reaction time, and pH, were determined by adsorption experiments. Then, the adsorption kinetics and isotherm models were fitted to evaluate the adsorption mechanism. The results show that the Zn/Al ratio has a great influence on the adsorption effect, the best adsorption effect is obtained when the Zn/Al ratio is 4:1, and the maximum adsorption capacity of Cu²⁺ is 213 mg/g. The mechanism study shows that the adsorption of Cu²⁺ by ZnAl-LDH is mainly an isomorphic substitution. Additionally, during the adsorption of CuSO₄, the presence of SO₄^2- undergoes interlayer anion exchange with Cl^-, and the process of SO₄^2- entering the interlayer facilitates the isomorphic substitution of Cu²⁺ and Zn²⁺. X-ray diffraction (XRD) analysis shows that as the Zn/Al ratio increases, the interlayer spacing of ZnAl-LDH increases, and the crystallinity decreases. The adsorption process conforms to the pseudo-second-order kinetic process and the Langmuir isotherm adsorption model. Therefore, the adsorption type of ZnAl-LDH for Cu²⁺ is monolayer chemical adsorption. The adsorption thermodynamic results indicate that the adsorption of Cu²⁺ is a spontaneous endothermic process. The research results revealed the mechanism of ZnAl-LDH adsorbing Cu²⁺, providing ideas for removing and recovering copper-containing electroplating wastewater.