OXIDATION OF GALENA IN ACETATE BUFFER INVESTIGATED BY ATOMIC-FORCE MICROSCOPY AND PHOTOELECTRON-SPECTROSCOPY

Galena oxidation was investigated by AFM in acetate buffer under poten tiostatic control and by photoelectron spectroscopy on potentiostatica lly pretreated specimens. At +236 mV (SHE) formation of sulfur protrus ions could be observed with AFM. XPS showed the formation of elemental sulfur to start at potentials more anodic than +161 mV (SHE). Element al sulfur could only be retained on the galena surface if sample cooli ng was started before the beginning of the evacuation in the spectrome ter entry chamber. Sulfur-oxygen species could not be detected on gale na samples oxidized in acetate buffer even when investigated with sync hrotron-excited X-ray photoelectron spectroscopy. AFM images showed tw o important features: Oxidation starts with a roughening of the sample surface. At slightly more anodic potentials oxidation products are pr esent on the samples as protrusions of 10-200 nm in height and with mu tual distances of several hundred nanometers. Two types of sulfur depo sits are formed differing in the emergence potential, size, and mutual distance. The formation of such protrusions can only be understood if the reactants for the depositions reach the growing protrusion by dif fusion in the liquid phase. Therefore, it is proposed that the process causing the surface roughening is a dissolution of PbS to lead(II) io ns and hydrosulfide ions while the deposition reaction is the electroc hemical oxidation of hydrosulfide ions to elemental sulfur. By removal of the hydrosulfide ion from the aqueous solution, further dissolutio n becomes possible at other sample regions. The sulfur formation occur s at distinct points which are not preferentially located at steps. It is likely that the sulfur formation starts at impurity locations. Dif ferent impurities may be responsible for different rates of deposit fo rmation, leading to protrusions of different size which however cannot be distinguished by XPS.