Ion exchange equilibria of Ni(II) and Co(II) on a layered double hydroxide-attapulgite composite

This study aims to evaluate the efficiency, mechanisms, and surface characteristics of LDH-attapulgite (LDH-ATP) for the removal of Ni(II) and Co(II) ions from aqueous solutions. The study focuses on determining sorption capacity, characterizing surface features using fractal analysis, and investigating the thermodynamic and mechanistic aspects of ion exchange. LDH-ATP was assessed as a sorbent through ion exchange experiments, and fractal analysis was employed to quantify surface characteristics, including the fractal dimension (Ds) and pre-exponential coefficient (C), which serve as unique descriptors of surface complexity. The Ds value of 2.05 indicates a slightly irregular and complex surface. Binary ion-exchange experiments (Ni(II)/Mg(II) and Co(II)/Mg(II)) were conducted in batch mode under varying temperatures (298, 313, 323 K), optimized pH, equilibrium time, and constant total ionic strength. Equilibrium data were fitted to Freundlich, Langmuir, and Redlich-Peterson isotherm models, with the Redlich-Peterson model providing the best fit, suggesting a hybrid sorption mechanism involving both monolayer and multilayer interactions. The extended Debye-Hückel relation was used to estimate activity coefficients in solution. Thermodynamic analysis revealed that sorption is endothermic and spontaneous, as indicated by positive enthalpy and entropy changes and negative Gibbs free energy values at elevated temperatures. The enhanced performance of LDH-ATP is attributed to the synergistic interaction between layered double hydroxides and attapulgite, resulting in improved structural integrity, higher sorption capacity, and selective metal ion uptake. Overall, the findings demonstrate that LDH-ATP is a promising material for the removal of Ni(II) and Co(II) from contaminated aqueous solutions.