Semiconductor‐electrochemical aspects of bacterial leaching. I. Oxidation of metal sulphides with large energy gaps

Abstract

AbstractThiobacillus ferrooxidans has been cultivated successfully on synthetic metal sulphides with large energy gaps (CdS, ZnS) as the only energy source for many culture generations during a period of 4 years. The results obtained, which were quantitatively evaluated by calculations of electron transfer probabilities, show that a direct electron transfer from the metal sulphide valence band to the bacterial metabolic system (hole injection into the valence band of the sulphide) has to be excluded as the cause for the enhanced oxidative dissolution for energetic reasons. A detailed analysis of the dynamics of the metal sulphide aqueous electrolyte interface reveals that protons are involved in the mechanism on which bacterial activity is based. By reacting chemically with the metal sulphide surface they break chemical bonds and shift electronic states energetically into the forbidden energy gap to produce surface states which can be chemically described as −SH∂ groups. These control the rate of dissolution of the metal sulphide and are removed by bacterial activity. In this way the proton is recycled and its action can be considered catalytic. For sulphides in which the valence band of the semiconductor is derived from metal orbitals instead of from sulphur orbitals, this mechanism is bound to fail. MoS2 and WS2 are discussed as examples of such metal sulphides which are not a suitable energy source for bacteria. Some kinetic aspects of the bacterial surface reaction are discussed.