Comparative Study on the Adsorption of Cyanide Gold Complexes onto Different Carbonaceous Samples: Measurement of the Reversibility of the Process and Assessment of the Active Surface Inferred by Flow Microcalorimetry

In this study, we investigate the adsorption of potassium gold cyanide from water onto various carbonaceous materials, including three activated carbon samples, four types of graphite, and two carbon blacks (Graphon and Vulcan). We employed flow microcalorimetry to assess the polar and accessible apolar graphitic basal plane surface areas of these adsorbents while evaluating their capacity for gold complex adsorption. We found that the degree of adsorption irreversibility varies significantly, with Graphon displaying 25% irreversibility and G212 exhibiting 54%. The measured enthalpic contributions to the adsorption process reveal negative values of approximately -50 kJ mol-1 for irreversible and -25 kJ mol-1 for reversible adsorption. Our analysis suggests that a significant proportion of the microporous structure in activated carbons is inaccessible for gold complex adsorption. Furthermore, it appears that aurocyanide can irreversibly attach as an unpaired anion, primarily through electrostatic interactions at polar active surface sites, while the less active sites facilitate reversible adsorption via van der Waals forces. This study contributes to understanding the mechanisms of gold cyanide adsorption on different carbonaceous materials, which has significant industrial implications.