The adsorption of gold thiourea complex onto activated carbon

The equilibrium and kinetics for the adsorption of gold onto activated carbon from acidic thiourea solutions have been studied. The equilibrium gold loading decreases with increasing thiourea concentration, pH and temperature. The gold loading is not affected by small amounts of Fe(2+) or Fe(3+), but is greatly lowered by Cu(2+) presumably through competitive adsorption. The rate of gold adsorption at the initial stage, in terms of the decrease of gold concentration in solution, can well be described with first-order kinetics. The rate increases with increased initial gold concentration, agitation and temperature, but is not greatly affected by thiourea concentration or the presence of Fe(2+), Ag(+) and Cu(2+) ions significantly reduce the rate since they strongly compete for the surface sites. A high concentration of Fe(3+) (5 g/L) also substantially reduces the rate, probably due to a change in solution chemistry. Gold is normally adsorbed on activated carbon as the gold thiourea complex. However, partial decomposition of the complex to metallic gold has been observed under certain conditions. The surface of the activated carbon loaded with gold has been studied using SEM, EDS and XPS techniques. (C) 2004 Elsevier B.V. All rights reserved.