MgCuZnAl-Layered Double Hydroxides for Cr(VI) Adsorption: Effect of Interlayer Anions on Uptake Behavior and pH-Dependent Stability

Layered double hydroxides (LDHs) are functional materials with tunable structural, surface, and adsorption properties. In this study, multicationic LDHs were synthesised and evaluated as adsorbents for chromate(VI). Among the investigated materials, the tetracationic Mg2Cu0.5Zn0.5Al1 composition exhibited consistently high mass-normalised Cr(VI) uptake in the screening experiments and was therefore selected for detailed investigation. Therefore, its chloride and carbonate forms were selected as model materials to examine the role of interlayer anions in Cr(VI) uptake and pH-dependent stability. The chloride form exhibited a higher adsorption capacity (70 mg/g) than the carbonate form (34 mg/g), indicating that more weakly bound interlayer anions may increase the accessibility of adsorption sites and promote anion exchange during adsorption. In contrast, the carbonate form showed greater structural stability, as confirmed by reduced cation leaching over a wide pH range. The behaviour of Cr(VI) in equilibrium solution was analysed as a function of its initial concentration, which allowed a simple description of Cr(VI) sorption over a wide range of initial pH values. XRD, FTIR, and XPS analyses supported partial interlayer anion exchange and changes in the coordination environment of surface metal centres after Cr(VI) sorption. Adsorption energies of 13–16.4 kJ/mol further suggested that Cr(VI) uptake cannot be assigned to a single interaction type but involves electrostatic interactions, anion exchange, and surface complexation. These results demonstrate that the interlayer anion is a key factor governing both the Cr(VI) adsorption efficiency and aqueous stability of LDH-based adsorbents.