Bioleaching - a result of interfacial processes caused by extracellular polymeric substances (EPS)

Extracellular polymeric substances (EPS) seem to play a pivotal role in bioleaching-the winning of precious metals as well as acid rock drainage. For a better control of both processes structure and function of extracellular polymeric substances from leaching bacteria are of crucial importance. Our research focussed on Acidithibacillus ferrooxidans, the composition of its extracellular polymeric substances and further deduction of their function. The extracellular polymeric substances of Acidithiobacillus ferrooxidans consist mainly of neutral sugars and lipids. The functions of the extracellular polymeric substances of this leaching bacterium seem to be (i) to mediate attachment to a (metal) sulfide surface, (ii) to concentrate iron(III) ions by complexation through uronic acids or other residues at the mineral surface and, thus, allowing for an oxidative attack on the sulfide. Consequently, dissolution of the metal sulfide is enhanced, which may result in an acceleration of 20- to 100-fold over chemical leaching. Experiments were performed to elucidate the importance of the iron(Ill) ions complexed by extracellular polymeric substances for strain-specific differences in oxidative activity for pyrite. The preliminary data indicate that strains of Acidithiobacillus ferrooxidans, with a high amount of iron(III) ions in their extracellular polymeric substances, possess a higher oxidation activity than those with less iron(III) ions. These data provide new insight into the function of extracellular polymeric substances and the consequent advantage conferred to bacteria involved in bioleaching. (C) 2003 Elsevier B.V. All rights reserved.