Chemical dissolution of the galena(001) surface observed using electrochemical scanning tunneling microscopy

The chemical dissolution of galena(001) surfaces immersed in acidic and neutral sulfate electrolyte solutions was investigated using Electrochemical Scanning Tunneling Microscopy (ECSTM). The nanometer-scale detail provided by the STM in this investigation provides new mechanistic information regarding surface dissolution processes at the mineral/water interface. In acidic solutions (pH = 2.7), we observe that dissolution occurs by selective removal of step edge species. This process is crystallographically anisotropic, resulting in steps with [110] orientations. Under neutral pH conditions (pH = 7), the dissolution rate is significantly lower, and no preferential step edge orientation is observed. The anisotropy observed in acidic solutions is attributed to different kinetic rates of dissolution along the [110] and [100] directions. Furthermore, we propose that the rate limiting step in the dissolution mechanism is the desorption of hydrosulfide (HS-) from step edges. The origin of the crystallographic anisotropy is discussed in terms of possible atomic structures of the step edges.