Removal of Cu(II) ions from aqueous solutions by low-cost natural hydroxyapatite/chitosan composite: Equilibrium, kinetic and thermodynamic studies

In this study, a simple method for providing natural hydroxyapatite/chitosan (N-HAP/chitosan) composite is introduced and adsorption suitability of this composite for treating the solutions containing Cu(II) ions has been studied. Kinetic and isotherm studies were carried out by studying the effects of parameters such as adsorbent particle size, temperature and pH. The equilibrium data at different temperatures were analyzed by Langmuir and Freundlich isotherms. Equilibrium data agreed well with Langmuir isotherm, based on this model the maximum adsorption capacity of N-HAP/chitosan composite for Cu(II) was found to be 1.776 mmol/g at pH 7.50 and 25 °C for 47.6 nm adsorbent particle size. A pseudo-second order kinetic model has been proposed to correlate the experimental data. The thermodynamic parameters such as standard Gibb's free energy (ΔG°), standard enthalpy (ΔH°), standard entropy (ΔS°) and activation energy (Ea) were evaluated by applying the Van't Hoff and Arrhenius equations. The results reveal Cu(II) adsorption onto N-HAP/chitosan is spontaneous, has endothermic nature and is followed by physisorption.