Isotherms, kinetics, and thermodynamics of methionine adsorption onto poorly crystalline hydroxyapatite with different Ca/P ratios

The adsorption properties of hydroxyapatite of low crystallinity towards methionine have been examined. The chemical composition of hydroxyapatite was taken as an experimental variable in order to have a point of view on the parameters of the adsorption process and the mechanisms established between adsorbent-adsorbate. The adsorption kinetics are relatively fast, and the high amounts adsorbed at saturation are obtained for non-stoichiometric hydroxyapatite, containing more HPO42- ions and having a high specific surface area. The good agreement of the experimental data with kinetic models confirms that the mechanism can be perfectly described by pseudo-second-order kinetics. Adsorption isotherm models show that Langmuir's model gives a better fit of experimental data compared to that of Freundlich, Temkin and Dubinin-Kaganer-Radushkevich. Fourier transform infrared spectroscopy confirmed the interaction between the -COO- ions of methionine and the Ca2+ ions of hydroxyapatite. The thermodynamic parameters, the isoelectric point of methionine and the point of zero charge of hydroxyapatite with different Ca/P ratios show that the adsorption process is considered spontaneous, exothermic and often controlled by physisorption with interactions electrostatic.