Can Sulfate Be the First Dominant Aqueous Sulfur Species Formed in the Oxidation of Pyrite by Acidithiobacillus ferrooxidans?

Sarka Borilova, Martin Mandl, Josef Zeman, Jiri Kucera, Eva Pakostova, Oldrich Janiczek, Olli H. Tuovinen

Research output: Contribution to journalArticle

Abstract

According to the literature, pyrite (FeS2) oxidation has been previously determined to involve thiosulfate as the first aqueous intermediate sulfur product, which is further oxidized to sulfate. In the present study, pyrite oxidation by Acidithiobacillus ferrooxidans was studied using electrochemical and metabolic approaches in an effort to extend existing knowledge on the oxidation mechanism. Due to the small surface area, the reaction rate of a compact pyrite electrode in the form of polycrystalline pyrite aggregate in A. ferrooxidans suspension was very slow at a spontaneously formed high redox potential. The slow rate made it possible to investigate the oxidation process in detail over a term of 100 days. Using electrochemical parameters from polarization curves and levels of released iron, the number of exchanged electrons per pyrite molecule was
estimated. The values close to 14 and 2 electrons were determined for the oxidation with and without bacteria, respectively. These results indicated that sulfate was the
dominant first aqueous sulfur species formed in the presence of bacteria and elemental sulfur was predominantly formed without bacteria. The stoichiometric calculations are consistent with high iron-oxidizing activities of bacteria that continually keep the released iron in the ferric form, resulting in a high redox potential. The sulfur entity of pyrite was oxidized to sulfate by Fe3C without intermediate thiosulfate under these conditions. Cell attachment on the corroded pyrite electrode surface was documented although pyrite surface corrosion by Fe3C was evident without bacterial participation. Attached cells may be important in initiating the oxidation of the pyrite surface to release iron from the mineral. During the active phase of oxidation of a pyrite concentrate sample, the ATP levels in attached and planktonic bacteria were consistent with previously established ATP content of iron-oxidizing cells. No significant upregulation of three essential genes involved in energy metabolism of sulfur compounds was observed in the planktonic cells, which represented the dominant biomass in the pyrite culture. The study demonstrated the formation of sulfate as the first dissolved sulfur species with iron-oxidizing bacteria under high redox potential conditions. Minor aqueous sulfur intermediates may be formed but as a result of side reactions.
Original languageEnglish
Article number3134
Number of pages13
JournalFrontiers in Microbiology
Volume9
DOIs
Publication statusPublished - 18 Dec 2018
Externally publishedYes

Bibliographical note

Copyright © 2018 Borilova, Mandl, Zeman, Kucera, Pakostova, Janiczek and Tuovinen. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

Keywords

  • Acidithiobacillus ferrooxidans
  • cellular ATP
  • pyrite electrode, pyrite oxidation
  • tetrathionate hydrolase
  • iron-oxidizing bacteria

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