Adsorption performance and mechanism analysis of hydrotalcite derived CaFe-bimetal materials for highly efficient Cd²⁺ removal process from kinetic and isotherm aspects

The removal of toxic and soluble Cd²⁺ is facing challenges in developing low-cost materials with retention stability and high capacity. To solve this problem, the CaFe-bimetal hydrotalcite (LDH) and its oxide (LDO) were adopted due to their integrated excellent ion exchange and isomorphic substitution property. The influencing factors, including Ca/Fe mole ratio, calcination temperature, the initial Cd²⁺ concentration and the adjustable ion kinds, were considered to obtain better adsorption performance. Similar ion size of Ca²⁺ and Cd²⁺ and the calcined LDOs with expanded channels promoted the Ca²⁺ dissolution and the mass transfer of Cd²⁺ entering into the laminate, and thus the isomorphic substitution sufficiently improved the high efficiency of Cd²⁺ removal of about 1079 mg/g in 20 min with Ca₄Fe₁-LDO-300. Based on the characterization and DFT results, LDOs exposed more active sites and enhanced the isomorphic substitution of Cd²⁺ for Ca²⁺ process. The thermodynamic and kinetic results revealed that the abundant –OH sites in hydrotalcite were beneficial for Cd²⁺ removal through surface adsorption, electrostatic attraction, and other interactions. The obtained adsorption mechanism offers guidance for the highly efficient Cd²⁺ removal in an aqueous solution system.