Mechanism of Congo red adsorption on new sol-gel-derived hydroxyapatite nano-particle

In this study, an effective adsorbent calcium hydroxyapatite (CaHA) having high adsorption capacity about aqueous solution of Congo red dye in ambient temperature is presented. Nano particles of CaHA were prepared through a new easy alkoxide-based sol-gel technique. The structural characterization of synthesized CaHA were performed by X-ray diffraction (XRD), Fourier transform infrared (FIR) analysis, thermal behavior (DTA), morphology and elemental analysis via scanning electron microscopy (SEM). The nano particle size and micro-strain of CaHA were measured using modified Shcerrer equation as Williamson-Hall method and transmission electron microscopy (TEM). XRD pattern shows hexagonal structure of the nano-particle CaHA with P6(3)/m space group that the size measured through XRD and TEM methods are in good agreement as 25 nm. The equilibrium adsorption capacity (q(m)) for 100 ppm CR was measured to be 487.80 mg g(-1) under optimal conditions. The adsorption isotherms and kinetic data well fitted with the Langmuir model and the pseudo-second order equation with correlation coefficients of 0.9979 and 0.9902, respectively. The calculated structures of CR and its complex including Ca2+ and PO43- ions show the electrostatic interactions of anionic CR dye and Ca2+ site and hydrogen bonding of amine group and phosphate of CaHA surface. Thermodynamic parameters (Delta G degrees < 0 and Delta H degrees < 0) imply the exothermic and spontaneous adsorption process. Finally, the experimental results suggest that this new synthesized CaHA can be good candidate for application in the management of environmental. (C) 2017 Elsevier B.V. All rights reserved.