Eco-friendly design of alginate-coated hydroxyapatites for the effective removal of Rhodamine B dye from water: equilibrium, kinetics and thermodynamic study

Hydroxyapatite-alginate composites with different biopolymer contents (0%, 10%, 25% and 50%) were prepared according to an eco-friendly process using low-cost precursors. The prepared materials were firstly subjected to characterizations by Fourier-transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and physisorption of N2 techniques. The composites were then tested as sorbents for Rhodamine B dye under different sets of experimental conditions, including initial pH, contact time, initial dye concentration and temperature. The aim was to investigate the effects of these parameters on the extent of dye removal and to understand the adsorption behaviour of the composites under various conditions. The dye adsorption equilibrium is reached after 20 min of contact for the unmodified hydroxyapatite, being attained within a few minutes for the composite samples. Moreover, the pseudo-second-order kinetics model was found to be suitable to describe the adsorption of dye onto the uncoated product, contrarily to the modified compounds for which the kinetic data could be better predicted by the pseudo-first-order model. The equilibrium data were analysed by different isotherm models, with the Freundlich model providing the best fit. The maximum adsorption capacity increased from 225.2 to 289.5 mg/g by the grafting effect, suggestion the potential use of these materials in wastewater treatment.