Adsorption of Au(III) from Aqueous Solution by Calcined Gibbsite

Abstract

Gibbsite (GB) was prepared in this work by calcination between (200 and 1000) °C (GB200 and GB1000, respectively). Properties of the GBs prepared over this range of temperatures, such as their morphologies, crystallinities, specific surface areas, the number of hydroxyl groups, pore volumes, and mean pore diameters were investigated. The amount of Au(III) adsorbed onto the GBs was also evaluated. Our findings show that the amount of Au(III) adsorbed onto the GBs is related to the specific surface area and the number of hydroxyl groups. In this study, GB400, that is, GB calcined at 400 °C, had the largest specific surface area and number of hydroxyl groups, as well as the largest amount of adsorbed Au(III). Furthermore, the most suitable pH for the adsorption of Au(III) onto GB400 was approximately 6.0, and the gold chloro-hydroxy species [AuCl2(OH)2]− was selectively adsorbed at this temperature. The equilibrium for adsorption was reached within 24 h, and the experimental data were fit to the pseudo-second-order model. The adsorption isotherm data were better characterized by the Langmuir model than the Freundlich model. The presence of chloride ions (Cl–) affected the adsorption of Au(III) onto GB400. The increase of ΔG with temperature showed that the adsorption was endothermic and more favorable at higher temperatures. The positive ΔH values also indicated that Au(III) adsorption on GB400 was endothermic. The positive ΔS values suggest an increase in randomness at the solid–solution interface during the adsorption process. GB400 could be used for at least three Au(III) adsorption/desorption cycles. Collectively, these results suggest that GB400 would be useful for the adsorption of Au(III) from aqueous solutions.