Biochemistry of sulfur extraction in bio-corrosion of pyrite by Thiobacillus ferrooxidans

Abstract

By adding a small amount of the amino acid cysteine to an acidic solution containing Thiobacillus ferroxidans cells and subsequent incubation with synthetic pyrite layers, the duration of the lag phase in the growth of T. ferrooxidans is minimized and the leaching rate of this sulfide is increased three times compared to the normal process without this biochemical additive. In the presence of cysteine, pyrite can be oxidized in the absence of bacteria with a leaching rate comparable with that attained by bacteria under normal leaching conditions. It seems that the sulfhydryl group of cysteine participates in a binding process with pyrite. Free-SH groups from the pyrite surface would be the counterpart for the formation of the corresponding disulfide. This thiol-disulfide reaction means that cysteine is consumed by the pyrite surface with the subsequent release of iron-sulfur species. This result is accounted for by the fact that pyrite without bacteria can be completely oxidized. Bacteria would take advantage of this biochemical corrosion process by uptake and oxidation of the released species which are continuously supplied to the acidic-biotope. The reactivity of cysteine with the pyrite surface, which improves the bacterial leaching rate, suggested the investigation of other mono-thiol molecules with the aim to mask the pyrite surface against bacterial attack. The latter can be relevant for protecting steel against microbiologically induced corrosion (MIC). S- tert-butyl mercaptan ((CH 3) 3C–SH, TBM)) was found to stop the bacterial corrosion by T. ferrooxidans. Permanent disulfide bonds between pyrite and the blocking SH-agent are presumably the cause for this protection.