Electrophoretic studies of the adsorption of gold and silver from aqueous cyanide solutions onto activated carbon

The effect of the ions Au(CN) sub 2 exp - , Ag(CN) sub 2 exp - , or Ca exp 2+ present in a suspension solution after their adsorption on activated carbon was investigated by measuring the zeta potential of the activated carbon. Over a wide range of pH values, the polyvalent cations (Ca exp 2+ ) decrease the negative zeta potential of the activated carbon. The cations interact with the negative centres (COO exp - and CO exp - ) of the adsorbent. In contrast, Au(CN) sub 2 exp - and Ag(CN) sub 2 exp - , with the monovalent cation K exp + , exert no influence on the zeta potential in alkaline and weakly acidic pH ranges. These compounds are adsorbed by way of chemisorptive interactions, possibly on uncharged active centres. The co-adsorbed cations and anions may, therefore, be situated at adjacent positions on the surface of the activated carbon. Through the influence of protons, positive centres are also formed on activated carbon in the strongly acidic pH range. The Au(CN) sub 2 exp - and Ag(CN) sub 2 exp - may then be electrostatically adsorbed at such active centres of the carbon. In a system which contains Ca exp 2+ and Au(CN) sub 2 exp - ions, another positively charged species [Ca exp 2+ Au(CN) sub 2 exp - ] exp + may be adsorbed in addition to Ca exp 2+ . Copyright (c) 1997 Elsevier Science B.V. All rights reserved.