Adsorption/desorption of Direct Yellow 28 on apatitic phosphate: Mechanism, kinetic and thermodynamic studies

In this study, the adsorption potential of apatitic tricalcium phosphate for the removal of Direct Yellow 28 (DY28) from aqueous solution has been investigated by using batch mode experiments. The effects of different parameters such as pH, adsorbent dosage, initial dye concentration, contact time, addition of ions and temperature have been studied to understand the adsorption behavior of the adsorbent under various conditions. The adsorbent has been characterized by pHzpc measurement, chemical analyses, FTIR, XRD and TEM. The Langmuir and Freundlich models are found to be the best to describe the equilibrium isotherm data, with a maximum monolayer adsorption capacity of 67.02 mg g−1. Thermodynamic parameters including the Gibbs free energy ΔG, enthalpy ΔH, and entropy ΔS have revealed that the adsorption of DY28 on the apatitic tricalcium phosphate is feasible, spontaneous and endothermic. Among the kinetic models tested for apatitic tricalcium phosphate, the pseudo-second-order model fits the kinetic data well. The introduction of orthophosphate ions in the medium causes a decrease of adsorption. The addition of Ca2+ ions favors the adsorption. The results of this study have demonstrated the effectiveness and feasibility of the apatitic tricalcium phosphate for the removal of DY28 from aqueous solution.