Pyrite surfaces after bio-leaching: a mechanism for bio-oxidation

A pyrite mineral surface was exposed to a mixed culture of the lithotrophic bacteria Thiobacillus-ferrooxidans (Tf). The surface structure of this sam ple was compared to surfaces exposed to uninoculated solutions of either su lphuric acid or ferric and ferrous sulfate dissolved in sulfuric acid solut ion at equivalent concentrations as the inoculated solution. All surfaces w ere examined using X-ray photoelectron spectroscopy (XPS) and scanning elec tron microscopy (SEM). A chemical profile of the surfaces, as a function of depth, was obtained by surface ablation. The colonised surface produced an oxidised phase extending to a depth of more than 4 mum A carbonaceous phas e was not observed within the oxide layer. No oxide layers of significant d epth were observed for samples exposed to uninoculated solutions. The impor tance of the sessile cells in bio-oxidation of pyrite is demonstrated and a qualitative model is proposed. (C) 2000 Elsevier Science B.V. All rights r eserved.