Electrochemical Behavior of the Oxidative Dissolution of Gold Catalyzed by Lead Ion in Thiourea Solution

Abstract

ABSTRACT Thiourea is a low-toxicity gold lixiviant that has promise to substitute highly toxic cyanide for gold extraction. The electrochemical behavior of lead ion in the dissolution of gold in thiourea solution was investigated by electrochemical and analytical technique. Electrochemical results suggest that the addition of lead ion (10 mg/L and 20 mg/L) to thiourea solution increased the open circuit potential from 0.31 V to 0.32 V and 0.34 V, respectively, due to the significant increase of cathodic current density. The presence of lead ion could significantly improve the oxidative dissolution of gold and thus resulted in higher gold dissolution in thiourea solution, particularly at higher anodic potentials. The scanning electron microscope (SEM) equipped with energy dispersive spectroscope (EDS) and X-ray photoelectrode spectroscopy (XPS) analyses indicate the improvement of gold dissolution is attributed to the formation of gold-lead (AuPb2) layer on gold surface, which acts as a cathode to promote galvanic corrosion of gold. The possible mechanism is proposed that gold dissolution in thiourea solution with lead ion follows a two-step process: (1) the formation of AuPb2 layer on gold surface and (2) the dissolution of AuPb2 layer under the applied anodic potential.