Synthesis of hydroxyapatite/clay and hydroxyapatite/pumice composites for tetracycline removal from aqueous solutions

Synthesized hydroxyapatite/clay (HA-C) and hydroxyapatite/pumice (HA-P) composites were used for tetracycline (TC) uptake studies from aqueous solution and their uptake capacities were compared. HA-C and HA-P composites were synthesized by precipitation method and the structures of the synthesized composites were characterized by XRD, SEM and BET analyses. Cation exchange capacities of HA-C and HA-P were found to be 84 meq/100 g and 33 meq/100 g, respectively. The TC adsorption using HA-C and HA-P was studied on batch mode. Various parameters such as contact time, solution pH, initial TC concentration, composite dosage, salinity and temperature were optimized. Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherm models were applied to the equilibrium data. The maximum adsorption capacity onto HA-C was found to be 76.02 mg/g and about four times larger than the adsorption capacity of the HA-P (17.87 mg/g). The results indicated that the TC uptake onto HA-C and HA-P composites is mainly by a surface complexation and ion-exchange mechanism which depend on the solution pH. The calculated values of thermodynamic parameters indicated that the TC adsorption is favorable, physicochemical in nature. The sorption process follows pseudo-second-order and intraparticle diffusion kinetic models. The TC adsorption mechanism by HA-C and HA-P has been proposed.