The effect of thermal and acid treatment of kaolin on its ability for cyanide removal from aqueous solutions

The removal of free cyanide from aqueous solutions was investigated by kaolin, calcined kaolin (metakaolin), and acid-treated metakaolin. The kaolin in raw form had no ability for cyanide adsorption, but calcination and acid treatment significantly improve its adsorptive properties. The best calcination conditions were determined to be at temperature of 600 °C and calcination time of 1 h. The scanning electron microscope (SEM), wavelength-dispersive X-ray (WDX), X-ray fluorescence (XRF), and X-ray diffraction (XRD) analysis methods were used to explain the adsorptive properties of the adsorbents. The loading capacity of metakaolin increased from 0.66 to 5.32 mg/g, and the loading capacity of acid-treated metakaolin increased from 1.33 to 10.67 mg/g by increasing the initial cyanide concentration from 25 to 531 mg/L. The kinetics of cyanide adsorption onto both adsorbents increases by increasing the initial cyanide concentration (25–531 mg/L) and solution temperature (30–50 °C). The equilibrium and kinetic data on both adsorbents are best fitted by the Freundlich isotherm and pseudo-second-order kinetic models, respectively. The thermodynamic study showed that the cyanide adsorption onto both adsorbents is endothermic and spontaneous in nature. The activation energy for the cyanide adsorption onto metakaolin and acid-treated metakaolin was determined to be 16.68 and 5.79 kJ/mol which indicates that the adsorption of cyanide ions onto metakaolin is chemisorption and onto the acid-treated metakaolin is a physicochemical process.