Beneficial effect of in-situ citrate-grafting of hydroxyapatite surface for water treatment

Abstract

Hybrid materials based on hydroxyapatite (HAP) were synthesized by the co-precipitation method in the presence of citric acid (Cit) at different molar ratios Cit/HAP (wCit-HAP). By increasing the quantity of citric acid added, a loss of crystallinity of the grafted materials is observed and this grafting makes it possible to control the size of their nanoparticles and the surface characteristics. Their surface properties were tested by adsorption of methylene blue (MB) as a model dye. Several parameters influencing pollutant removal from aqueous solutions are discussed. The contact time to reach equilibrium for maximum adsorption does not exceed 60 min and the kinetic data are well fitted by a pseudo second order model. The best adsorption capacity was obtained for 0.2Cit-HAP. The calculated thermodynamic parameters showed that the adsorption was a spontaneous and endothermic process and that all ΔH° values are below 20 kJ mol−1. This indicates that there are weak electrostatic Van der Waals forces as well as the formation of hydrogen bonds. This approach was proven by quantum chemical calculations performed using the density function theory (DFT) method which provided new insights into the mechanism of adsorption.