A Sers Characterization of the Stability of Thiosulfate and Related Electrolytes at the Gold-Electrolyte Interface

Recent studies of the Au leaching process have employed thiosulfate as the alternative complexing ligand to cyanide for gold extraction from ores containing carbonaceous components which preferentially absorbs gold and gold-cyanide complexes ( the so called preg-robbing). Thiosulfate, in the presence of an appropriate oxidant such as dissolved oxygen, is well known to complex Au(I) ions and form a soluble [Au(S₂O₃)₂]^3- complex. However, the kinetics of this process are inhibited by the formation of a passivating layer of species on the Au surface that block the lixiviant reagents from reaching the underlying Au atoms, thereby preventing full recovery of Au from the ore. To determine the composition of this passive layer, the interfacial behaviour of tetrathionate, trithionate, the [Au(S₂O₃)₂]^3- complex and thiosulfate upon immersion of a Au nanorods electrode into each of the electrolyte solutions was investigated using surface enhanced Raman spectroscopy (SERS). Raman bands indicative of adsorbed tetrathionate, trithionate, and [Au(S₂O₃)₂]^3- were identified at early immersion times. However, bands assigned to decomposition products such as elemental sulfur, were observed in all SERS spectra at longer immersion times. The SERS spectra for tetrathionate, trithionate, the [Au(S₂O₃)₂]^3- complex were compared to SERS spectra collected in a thiosulfate electrolyte in an attempt to better identify the passivating species that accumulate on the Au surface in typical gold leaching conditions. In thiosulfate solution, Au surface passivation by elemental sulfur did not occur to the same extent relative as in the three other electrolytes in the time period studied. This behavior indicates that complexation of the oxidized Au atoms by thiosulfate and its subsequent mass transportation into the bulk electrolyte is the predominantly favoured pathway at early immersion times. However, the disappearance of the Raman bands characteristic of the [Au(S₂O₃)₂]^3- complex and the appearance of bands indicative of elemental sulfur on the Au surface at longer immersion times confirms the growth of a passivating film of elemental sulfur on the gold surface in the thiosulfate electrolyte, albeit at much longer immersion times.