SCANNING-TUNNELING-MICROSCOPY STUDIES OF GALENA - THE MECHANISMS OF OXIDATION IN AQUEOUS-SOLUTION

A scanning tunneling microscope (STM) was used to study the surfaces o f galena (PbS) in aqueous solution. The influences of pH and type of p urging gas on the dissolution and oxidation processes at galena surfac es were investigated. STM topographical imaging of the galena surfaces showed the development of sub-nanometer pits with increasing reaction time. Complementary atomic force microscopy (AFM) imaging confirmed t he behavior. The depths (z-dimensions) of these pits correspond direct ly to the unit cell dimensions of galena (0.3 or 0.6 nm) and suggest t hat the main surface process occurring is congruent dissolution; this has been confirmed by X-ray photoelectron spectroscopy studies. The x, y-dimensions of these pits and the rate of their formation depended st rongly on the pH and the type of purging gas used. Introduction of Pb2 + ions resulted in the formation of lead hydroxide colloids at the gal ena surface; distinct directionality is evident. Mechanisms for the in itial stages of the surface chemical reactions of galena in aqueous so lution are proposed and discussed in the context of flotation separati on.