Spectroscopic study of galena surface oxidation in aqueous solutions I. Identification of surface species by XPS and ATR/FTIR spectroscopy

The measurements by ATR/FTIR and X-ray photoelectron spectroscopy were performed in order to identify galena surface compounds formed in air-saturated solutions under different experimental conditions. Wet polishing under distilled water was found to produce lead- or sulphur-rich galena surface depending on the semiconducting nature of the sample. The probable mechanisms of this phenomenon are discussed. After cathodic polarization at −0.5 V (SHE) in borate buffer (pH 9.2), the surface appears to be lead-rich regardless of the origin of galena. Bulk lead hydroxide Pb(OH)2 was established to be the major product of the galena oxidation at +0.4 V in the air-saturated buffer. The assignment of IR bands and XPS peaks of bulk lead hydroxide was made. It was shown with the ATR/FTIR spectroscopy that galena, when being oxidized with hydrogen peroxide, is covered by lead hydroxide, lead thiosulphate and adsorbed oxygen in addition to elemental sulphur and basic lead sulphate (detected earlier by XPS). The relative quantity of each compound as well as the outermost layer composition was found to depend strongly on the oxidation kinetics.