Effect of lead ions on gold dissolution in the Cu2+ - en - S2O32− system

Abstract

The effect of lead ions on the dissolution behaviour of gold, thiosulfate, and Cu(en)22+ complex consumption in the Cu2+ − ethylenediamine (en) - S2O32− system has been investigated. Leaching experiments combined with X-ray photoelectron spectroscopy (XPS) analyses showed that the addition of an appropriate concentration of lead ions to the Cu2+- ethylenediamine (en) - S2O32− system facilitates the formation of a part of spotted PbO2 on the surface of the gold foils, which promotes the dissolution of gold, and also prevents the additional consumption of the thiosulfate and Cu(en)22+ complex. The Eh-pH diagram analysis of the system showed that the E(PbO2 / Pb2+) in the system decreased with the increase in lead ion concentration, and that the higher concentration of the lead ions in the system favoured the formation of PbO2. The cyclic voltammetry curves measured using a gold foil which was leached in higher concentration lead ion system as the working electrode in the leaching solution, indicate a significant PbO2 reduction peak. Furthermore, a deposition peak indicating that a part of PbO2 was formed on the gold foil after leaching in the system with the addition of lead was also observed. The Pb2+ in the leaching solution can also form PbO2, and the PbO2 deposition peak and reduction peak of the gold foil electrode after the leaching of lead ions system for 24 h were the most significant. The results of leaching experiments on Au-PbO2 electrode prepared by electrochemical deposition showed that covering a part of PbO2 on the gold foil can promote the dissolution of gold and increase its rate of dissolution. The mechanism of the appropriate concentration lead ions and gold foils in the Cu2+ − en - S2O32− gold leaching system would form many PbO2/Au micro battery is proposed. These micro batteries further improve the dissolution of gold in this system.