A study of the gold colloid dissolution kinetics in oxygenated ammoniacal thiosulfate solutions

Colloidal gold has a characteristic absorption peak at 530 nm that can be used to monitor the extent of gold dissolution in different lixiviant systems using an ultraviolet–visible spectrophotometer. This paper reports the kinetics of the dissolution of gold colloid in ammoniacal thiosulfate solutions using oxygen as a oxidant at pH 9.3–9.5 and temperature 25–48 °C. The relative rates of gold dissolution in different lixiviant systems are in descending order: oxygen–cyanide > copper(II)–ammonia–thiosulfate > oxygen–ammonia–thiosulfate > oxygen–ammonia > copper(II)–ammonia. The predominant gold(I) species in oxygenated thiosulfate solution based on the measured potentials on gold electrode is gold–thiosulfate complex (Au(S2O3)23−). However, the measured potentials in the absence of thiosulfate are close to the reported values for the Au(NH3)43+/Au(NH3)2+ couple, which indicates that disproportionation or oxidation of Au(NH3)2+ occurs. The initial rates of gold dissolution show reaction orders of ≈0.3 with respect to oxygen and ammonia, and are independent of the concentration of thiosulfate and chloride. The dissolution kinetics follow a shrinking core model with an activation energy of 25–37 kJ mol−1, indicating a mixed chemical-diffusion-controlled reaction.