The kinetics of adsorption of gold dicyanide onto activated carbon was intensively investigated over the last 2 decades. Yapu et al. [1] recently explored the possibility that the rate-determining mechanism for the adsorption of either gold or silver dicyanide is the sorption reaction at the external surface of the carbon. It is the aim of this discussion to present arguments to show that film and intraparticle mass transfer are indeed the more likely rate-determining mechanisms.The rate of gold adsorption is not strongly affected by temperature; activation energies between 8.4 to 13.8 kJ/mol were reported for the adsorption of gold and silver dicyanide onto activated carbon. [2][3][4][5][6][7] These low values are considered typical of mass-transfer rate-controlled processes and are normally associated with fast adsorption reaction kinetics.The rate of uptake of gold dicyanide increases with increasing stirring rate [4,6,[8][9][10] in stirred reactors. Van Deventer [8] and Yapu et al. [1] found with batch-stirred experiments that as soon as the carbon particles were fully fluidized, the effect of further increases in the stirring speed on the rate of uptake became negligible.Woollacott and Afewu [10] studied the effect of mixing on the adsorption kinetics of gold dicyanide onto activated carbon. They demonstrated that the rate of increase of the film mass-transfer coefficient with respect to stirring speed dropped significantly once the particles were fully suspended. Their findings are similar to those reported by Harriot, [11] who investigated the influence of mixing on the rate of sodium ion exchange on resin in stirred tanks. The negligible effect of increasing stirring speed on the rate of gold dicyanide uptake once the particles are fully suspended can thus be attributed to the hydrodynamic effects of mixing rather than to a change in the rate-determining mechanism from mass transfer to the sorption reaction.In contrast, McDougall et al. [12] and Cho and Pitt [7] reported that the rates of uptake of gold and silver dicyanide, respectively, in packed beds of activated carbon were unaffected by changes in flow rate. Van Deventer and Jansen van Rensburg [13] and Petersen and Van Deventer [14,15] also employed packed bed reactors for the adsorption of gold dicyanide onto activated carbon. They used film mass transfer and surface diffusion as the rate-determining steps in their kinetic model. Their estimated that film mass-transfer
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.