A study of galena (PbS) surfaces under controlled potential in pH 4.6 solution by synchrotron radiation excited photoelectron spectroscopy

The sulfur and lead species formed on galena (PbS) surface at different electrochemical potentials in acetate buffer solution (pH 4.6) were investigated using surface sensitive synchrotron radiation excited photoelectron spectroscopy (SR-XPS). At +50 to +150 mV versus SHE a new, very weak, S 2p component was observed at binding energy of about 2.8 eV higher than that from PbS, indicating that restructuring of the surface, due to oxidation, starts in this potential region. At +250 mV metal polysulfides were observed on the surface. In addition to polysulfides, enrichment of copper, present as a minor element in bulk of the natural galena sample, was detected. Since the quantity of polysulfides on the surface was not sufficient enough to explain the observed increase in S:Pb ratio with potential, it is proposed that copper diffuses from the bulk of the sample filling the vacant sites formed in anodic dissolution of lead. Formation of a metal deficient surface, where the sulfur state remains unchanged, is also possible. Indication of metallic lead, formed in reduction of PbS at -550 mV, was found. The better surface sensitivity gained by using synchrotron excitation instead of a conventional X-ray tube with Mg or Al anode turned out to be essential when studying submonolayer coverages of sulfur species on a sulfide mineral.