Effects of surface properties of activated carbon on the adsorption mechanism of copper cyanocomplexes

Copper cyanocomplexes have been recognized as important competitors for the adsorption sites of activated carbons during gold adsorption. In the present paper, an adsorption mechanism of negatively charged Cu cyanocomplexes on available sites of different activated carbons is proposed. The mechanism considers the (i) solution chemistry (e.g., effects of pH, CN/Cu molar ratio, and ionic strength by adding Na+, K+, and Ca2 + cations); (ii) surface properties of the activated carbon samples (e.g., density of surface functional groups and point of zero charge — PZC); and (iii) copper speciation. The adsorption experiments demonstrated that the enhanced interaction of copper species at pH < PZC and low ionic strength can be ascribed to the net positive charge on the surface of the solid phase. When the basic surface groups are completely deprotonated (i.e., pH > PZC > pKa basic groups), electrostatic repulsion inhibits the adsorption of negatively charged Cu species. It is also proposed that the interaction of Ca2 + ions with acid groups generate an excess of locally positive charges on the surface of the activated carbon, and such positive charges enhance the adsorption of negatively charged copper cyanocomplexes, even when the net surface charge is negative. The results from thermodynamic and Raman analyses indicated that under typical, practical conditions of gold adsorption from cyanide solutions (pH > 9 and CN/Cu molar ratio > 4), the main competitor for the adsorption sites of the activated carbon is the Cu(CN)32 − species and not the Cu(CN)2− species.