Thermodynamic Parameter Estimation for the Adsorption of Scandium onto Chelating Resin in Sulfuric Acid Solution

During the recovery of nickel from laterite ores, scandium is recovered from sulfuric acid leachates as a byproduct using an ion-exchange resin method. The efficient recovery of scandium requires the kinetic and thermodynamic behaviors of the scandium adsorption reaction to be elucidated. In general, the calculation of the standard Gibbs free energy change (ΔG°) for the adsorption of metal ions from solution onto ion exchange resins is complex. Therefore, the use of the Langmuir model to derive the equilibrium constant has been widely discussed. Calculation of the equilibrium constant, which represents the ratio of the activities of the reactants to those of the products at equilibrium, requires the determination of the activity coefficients. In this study, we performed a thermodynamic analysis of the adsorption isotherms to investigate in detail the adsorption reaction of Sc3+ from the sulfuric acid solution onto the chelating resin. As a portion of Sc3+ forms complexes with sulfate ions, we calculated the concentration and activity coefficient of uncomplexed Sc3+ using chemical equilibrium calculations and constructed adsorption isotherms. In this study, we fitted the Sc3+ adsorption isotherms using both the Langmuir model, which assumes a homogeneous surface, and several modified models that account for surface heterogeneity. Our analysis indicates that the Langmuir model accurately describes the adsorption behavior of Sc3+ under the experimental conditions. Based on the equilibrium constant obtained from the Langmuir adsorption isotherm and on the standard enthalpy change (ΔH°) value of approximately 30 kJ mol–1 determined by using the van’t Hoff equation, we concluded that the adsorption of Sc3+ onto the chelating resin is an endothermic process. Our findings are expected to contribute to improving the efficiency of the recovery of scandium from leachates.