Metal sulfide semiconductor electrochemical mechanisms induced by bacterial activity

Abstract

Various bacterial species (e.g. Thiobacilli, Leptospirillum) have adapted to utilize solid inorganic semiconducting sulfides (e.g. FeS 2, ZnS, CuFeS 2) as their energy source for carbon dioxide fixation. The interfacial electrochemical mechanisms which they apply have practical relevance for bioleaching of minerals, acid mine pollution, for the biocorrosion of steel, and for solar powered chemosynthesis based on the bacterial energy cycle. The bacterial attack on the sulfide surface is based on the use of recyclable chemical species (H +, Fe 2+, thiol-compound) which disrupt chemical bonds in the sulfide interface and thereby induce disintegration. Model experiments performed with 100-nm thin synthetic FeS 2 layers allowed a detailed study of the interaction between bacterial cells and the pyrite interface. Thiobacillus ferrooxidans uses an organic polysaccharide layer to extract sulfur in the form of colloids via a cysteine-based carrier molecule. The mechanism which leads to corrosion pit formation of bacterial size could be analyzed in great detail. Addition of a surface active agent was found to induce increasingly localized leaching activity of bacterial cells. Addition of cysteine stimulates bacterial activity and acts as a sulfide dissolving agent even in the absence of bacteria. Mechanisms to block bacterial attack were identified on the basis of thiol chemistry. Leptospirillum ferrooxidans was found to apply a different strategy. It can only exist on Fe 2+ oxidation and dissolves FeS 2 by pushing — via a very positive Fe 2+/3+ redox potential, generated within the organic capsula — the semiconductor towards the electrochemical dissolution potential. The FeS 2 interface thus disintegrates into small fragments from which free energy of electrons for Fe 3+ reduction is utilized. Two isostructural materials with analogous electronic structure, FeS 2 which serves as energy source for T. ferrooxidans and RuS 2 which cannot at all be oxidized, are compared in detail to understand the molecular aspects of bacteria-induced semiconductor electrochemistry.