Thus, the adsorption of Ca2+ ions on the activated carbon surface enhances copper adsorption, as experimentally observed for CN/Cu = 3 (Fig. 11). For the conditionwhere Cu(CN)4
3 − species represents the predominant complex, CN/Cu = 4 (Fig. 12), this beneficial effect of Ca2+ ions addition to the cyanide solution is minimized in function of adverse factors such as steric hindrance and more negative charges to be neutralized.