Removal of Cr(VI) from aqueous solution by nano-carbonate hydroxylapatite of different Ca/P molar ratios

Abstract

Adsorption of Cr(VI) from aqueous solution by nano-carbonate hydroxylapatites (nano-CHAPs) of different Ca/P molar ratios (1.72, 1.74 and 1.78) was investigated. Nano-CHAPs were characterized by X-ray diffraction (XRD), Energy Dispersive Spectrometry (EDS) and Scanning Electron Microscopy (SEM). The effects of pH, contact time, and temperature were conducted to evaluate the adsorption performance in batch experiments. Increasing the Ca/P molar ratio increased the specific surface area which was propitious to adsorption process. Maximum Cr(VI) sorption occurred at initial pH 3.0, and the adsorption capacity enhanced with the increase of Ca/P molar ratio. Langmuir model fitted the experimental equilibrium data better than the Freundlich model. The adsorption kinetics data were well described by the pseudo-second order model. The thermodynamic parameters (ΔG0, ΔH0 and ΔS0) calculated at three different temperatures revealed that the adsorption of Cr(VI) onto Nano-CHAP was spontaneous, endothermic and physical in nature.