Mechanisms of Bioleaching and the Visualization of these by Combined AFM & EFM

Abstract

The understanding of mechanisms, by which microorganisms are effecting the dissolution of metal sulfides, what in general is called bioleaching, has progressed largely during the last 15 years. Whereas in the beginning of the nineties of the last century the direct and the indirect mechanisms were state of the knowledge, nowadays it is generally accepted that contact and non-contact mechanisms besides ferric iron, perhaps with some organic sulfhydryl groups, are the relevant players [1]. Consequently, since the introduction of these ideas/hypotheses, research in this field has been revived considerably, as is obvious from the literature[2]. Also molecular biology has started to contribute to the understanding of these processes due to the clarification of the basic reactions [3, 4, 5]. Summarizing the main elements of the contact and non-contact mechanisms, we know now that in case of the contact mechanism bacteria need to attach to metal sulfide surfaces by means of their extracellular, polymeric substances (EPS) due to a primary electrostatic interaction between iron(II)ions complexed by uronic acid residues. This first interaction is followed by hydrophobic interactions (more than half of the EPS of A. ferrooxidans are lipids) and perhaps covalent bonding (speculative). These forces are active in the Angstrom range (Å). Consequently, the space between bacterial outer membrane and the surface of a metal sulfide, which is filled by EPS, ranges between 10 and 50 nm only. In this space the dissolution reactions occur.