The debate on the mechanism of adsorption of gold and silver di-cyanide onto activated carbon could be divided into two schools of thought. On the one hand, it has been proposed that the combined effects of electrostatic and chemical interaction between the activated carbon surface and adsorbing gold di-cyanide anion results in the adsorption of the latter. On the other hand the `ion-pair' adsorption reaction mechanism has been vigorously advocated as the dominant mechanism responsible for the adsorption of gold di-cyanide onto activated carbon. It is maintained that gold is adsorbed onto the carbon surface as the neutral ion-pair [M"+][Au(CN)z ]". Adsorption is viewed as the result of mainly the chemical interaction while electrostatic interactions are neglected._x000D_
The validity of the `ion-pair' adsorption mechanism may be challenged on a number of counts. However, the alternative mechanism (based on aqueous surface chemistry principles) has not gained general acceptance yet, because it has not been applied widely enough to explain the different adsorption phenomena associated with gold di-cyanide on activated carbon. In the present study the case in favour of an adsorption mechanism based on general aqueous surface chemistry principles is presented. A model framework is developed for the adsorption reaction mechanism of gold di-cyanide onto activated carbon which incorporates the effects of chemical and electrostatic interactions between the dissolved species, the aqueous solution and the carbon surface. This model is then employed to explain a wide variety of reported adsorption phenomena for gold di-cyanide on activated carbon, such as the effects of pH, the adsorption of different metal cyano complexes, ionic strength, and type and concentration of cations in solution. In addition the effects of cyanide and dissolved oxygen concentration were investigated. No ready explanation, in terms of aqueous surface chemistry principles, is yet available for the switch in adsorption behaviour that agrees with ion exchange to ion-pair adsorption with increasing ionic strength.