SERS and electrochemical studies of the gold–electrolyte interface under thiosulfate based leaching conditions

Changes at the gold–solution interface in an alkaline thiosulfate solution under leaching conditions were investigated using surface enhanced Raman spectroscopy (SERS). Oxidation product of thiosulfate, such as tetrathionate, adsorbed thiosulfate and products of thiosulfate decomposition such as adsorbed sulfide or elemental sulfur were identified as the main contributor to the film that forms on the gold surface as a result of direct contact with the thiosulfate solution. The roles of thiourea and ammonia additives on the enhanced leaching kinetics of thiosulfate-based electrolyte solutions were also investigated. The SERS spectra indicate that these two additives form surface complexes with Au(I) and their presence inhibits either the formation or adsorption of the oxygen-containing sulfur species responsible for passivation of the gold surface. Leaching currents were determined from the analysis of current–potential curves and mixed potential theory. The initial leaching rates were higher in the presence of the additives, suggesting that they inhibit the passivating species from interfering with the Au leaching reaction and assist this reaction by forming complexes with the released Au(I) ion.