Adsorption of Phenol from Aqueous Solution by Hydroxyapatite Nanopowders. Part II: Kinetic, Equilibrium and Thermodynamic Studies

In this study, the adsorption kinetic, equilibrium and thermodynamic parameters of phenol from aqueous solutions by nano-size hydroxyapatite (n-HAp) powders were investigated at various temperatures and concentrations. The first-order kinetic model, pseudo-second-order kinetic model and intraparticle diffusion model were used to describe the kinetic data, and the data constants were evaluated. The results showed that the pseudo-second-order model was the best choice among the three kinetic models to describe the adsorption behavior of phenol onto n-HAp powders, suggesting that the adsorption mechanism might be a physisorption process. Adsorption isotherms and equilibrium adsorption capacities were determined by the fittings of the experimental data to the well-known Freundlich and Langmuir adsorption models. It was found that the Freundlich isotherm model provided the better correlation for phenol adsorption onto HAp nanopowders. The equilibrium constants were used to calculate the thermodynamic parameters, such as the change of Gibbs free energy (DeltadegGdeg), enthalpy (DeltaHdeg) and entropy (DeltaSdeg). The thermodynamic parameters suggested that the adsorption of phenol onto n-HAp was physisorption, spontaneous and endothermic in nature.