Direct versus indirect bioleaching

The dissolution of metal sulfides is controlled by their solubility product and thus the [H+] concentration of the solution and further enhanced by se veral chemical mechanisms which lead to a disruption of sulfide chemical bo nds. They include extraction of electrons and bond breaking by [Fe3+], extr action of sulfur by polysulfide and iron complexes forming reactants [Y+] a nd electrochemical dissolution by polarization of the sulfide [high Fe3+ co ncentration]. All these mechanisms have been exploited by sulfide and iron oxidizing bacteria. Basically, the bacterial action is a catalytic one duri ng which [H+], [Fe3+] and [Y+] are breaking chemical bonds and are recycled by the bacterial metabolism. While the cyclic bacterial oxidative action v ia [H+] and [Fe3+] can be called indirect, bacteria had difficulties harves ting chemical energy from an abundant sulfide such as FeS2, the electron ex change properties of which are governed by coordination chemical mechanisms (extraction of electrons does not lead to a disruption of chemical bonds b ut to an increase of the oxidation state of interfacial iron). Here bacteri a have evolved alternative strategies which require an extra cellular polym eric layer for appropriately conditioned contact with the sulfide. Thiobaci llus ferrooxidans cycles [Y+] across such a layer to disrupt FeS2 and Lepto spyrillum ferrooxidans accumulates [Fe3+] in it to depolarize FeS2 to a pot ential where electrochemical oxidation to sulfate occurs. Corrosion pits an d high resolution electron microscopy leave no doubt that these mechanisms are strictly localized and depend on specific conditions which bacteria cre ate. Nevertheless they can not be called 'direct' because the definition wo uld require an enzymatic interaction between the bacterial membrane and the cell. Therefore, the term 'contact' leaching is proposed for this situatio n. In practice, multiple patterns of bacterial leaching coexist, including indirect leaching, contact leaching and a recently discovered cooperative ( symbiotic) leaching where 'contact' leaching bacteria are feeding so wastef ully that soluble and particulate sulfide species are supplied to bacteria in the surrounding electrolyte.