Abstract:
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.