The nature of uptake and elution of gold from iodide solution on carbon

Abstract

The mechanism of gold uptake from iodide solutions by carbon has been investigated. A variety of samples of activated carbon were used. Adsorption isotherms for gold and triiodide were established and successive loading tests of gold and triiodide on carbon were also accomplished. About 300 mg gold and 1000 mg iodine were extracted onto 1 g of activated carbon at maximum loading. Examination of loaded activated carbon particles by scanning electron microscopy reveals isolated patches of gold deposited on the carbon surface. High-resolution transmission electron microscopy shows that very fine gold particles (1-5 nm diameter) are uniformly distributed within the carbon. Thermal analyses confirm the existence of gold-iodide complex inside the carbon structure. X-ray photoelectron spectrometry results demonstrate that the gold iodide complex is preferentially adsorbed only at the edge of highly-oriented pyrolytic graphite (HOPG). Iodine, on the other hand, is adsorbed at both the edge and basal plane of HOPG. The uptake of gold by carbon from iodide thus involves a combined reduction and adsorption process, where the gold iodide species is reduced to the metallic state superficially and also adsorbed in the porous structure of carbon. Moreover, elution data exhibit that gold-bearing carbon samples from iodide solution can be stripped by caustic cyanide solution at 96°C, and the desorption percentage, similar to that for gold cyanide system, could reach more than 90%. Thermal pressure elution is very effective for gold cyanide system even when the gold loading on carbon is very high, but not suitable for gold halide systems owing to the low decomposition temperature of gold halide complexes. A few experiments for gold cyanide and other halides were conducted for comparison in order to gain a better understanding of this gold-iodide system. A loading model for gold loading from iodide solution by carbon is proposed.