Assembly of nano-sized hydroxyapatite onto graphene oxide sheets via in-situ fabrication method and its prospective application for defluoridation studies

A novel material, assembly of nano sized hydroxyapatite (nHAp) onto graphene oxide sheets (GOs) has been prepared through simple in-situ co-precipitation fabrication method. The synthesized GOs showed the characteristic band in UV-vis spectroscopy and the XRD peak confirms its crystalline nature. The surface morphological studies were carried out using TEM, SEM and EDAX before and after fluoride adsorption to describe the surface changes during adsorption. FTIR spectroscopy has been used to identify the changes before and after fluoride adsorption of bare-nHAp and GOs-nHAp. The synergistic performance of fluoride uptake by GOs-nHAp composite was observed with removal efficiencies of 96.3% than bare nHAp and GOs under batch equilibrium method. Batch sorption experiments were also performed to find the effect of various influencing parameters such as effect of pH, contact time, competitor co-anions, and temperature. The kinetic study of fluoride adsorption on GOs-nHAp could be well described by the pseudo-second-order than pseudo-first-order rate law. In addition, the thermodynamic parameters of enthalpy of adsorption indicate that the fluoride adsorption was classified as physical force of attraction and standard free energy values attributed the fluoride adsorption was spontaneous in nature. The Freundlich isotherm model fits well with the equilibrium adsorption isotherm data and calculated maximum monolayer adsorption capacity was 44.068 mg/g at 303 K. Herein, the mechanism of fluoride removal using GOs-nHAp was electrostatic interaction as well as ion-exchange at acidic and neutral conditions respectively. This research finding provides insights into the development of a model for fluoride removal using the synthesized material.