Weak Iron Oxidation by Sulfobacillus thermosulfidooxidans Maintains a Favorable Redox Potential for Chalcopyrite Bioleaching

Abstract

Bioleaching is an emerging technology, describing the microbially assisted dissolution of sulfidic ores that provides a more environmentally friendly alternative to many traditional metal extraction methods, such as roasting or smelting. Industrial interest is steadily increasing and today, circa 15–20% of the world’s copper production can be traced back to this method. However, bioleaching of the world’s most abundant copper mineral chalcopyrite suffers from low dissolution rates, often attributed to passivating layers, which need to be overcome to use this technology to its full potential. To prevent these passivating layers from forming, leaching needs to occur at a low oxidation/reduction potential (ORP), but chemical redox control in bioleaching heaps is difficult and costly. As an alternative, selected weak iron-oxidizers could be employed that are incapable of scavenging exceedingly low concentrations of iron and therefore, raise the ORP just above the onset of bioleaching, but not high enough to allow for the occurrence of passivation. In this study, we report that microbial iron oxidation by Sulfobacillus thermosulfidooxidans meets these specifications. Chalcopyrite concentrate bioleaching experiments with S. thermosulfidooxidans as the sole iron oxidizer exhibited significantly lower redox potentials and higher release of copper compared to communities containing the strong iron oxidizer Leptospirillum ferriphilum. Transcriptomic response to single and co-culture of these two iron oxidizers was studied and revealed a greatly decreased number of mRNA transcripts ascribed to iron oxidation in S. thermosulfidooxidans when cultured in the presence of L. ferriphilum. This allowed for the identification of genes potentially responsible for S. thermosulfidooxidans’ weaker iron oxidation to be studied in the future, as well as underlined the need for new mechanisms to control the microbial population in bioleaching heaps.

Pyrite-Based Cr(VI) Reduction Driven by Chemoautotrophic Acidophilic Bacteria

“…In the chemical system, proton dependent Fe(III) or Cr(VI) chemical attack led to pyrite dissolution. It can be identified from Figure 1 that the Fe(II) concentration in the BI system was lower than that in the CI system, which was caused by the biological oxidization of Fe(II) by A. ferrooxidans (Zhu et al, 2013;Christel et al, 2018). This would have adverse effects on Fe(II)-based Cr(VI) reduction, but it might promote pyrite dissolution.…”

Systems biology of acidophile biofilms for efficient metal extraction

“…The problems associated with chalcopyrite bioleaching (i.e., when the target metal forms part of the mineral matrix) are slow rates and poor total recoveries often attributed to passivation of the mineral surface that has recently (2020) 7:215 | https://doi.org/10.1038/s41597-020-0519-2 www.nature.com/scientificdata www.nature.com/scientificdata/ been suggested to be by iron-oxyhydroxides 6 . One method to avoid chalcopyrite passivation is to carry out the bioleaching at low redox potentials and high temperatures 7,8 and several methods have been suggested to maintain the redox potential in the desired range including controlling the oxygen concentration 9 or utilizing a microbial community that maintains the potential in the desired range 10,11 . An additional critical factor for chalcopyrite bioleaching, especially in early stages of bioheap inoculation, is the attachment and biofilm formation on the mineral surface 12 .…”

“…The complete data in this descriptor have not been previously reported although parts have been included in published articles. These include RNA transcripts and protein concentrations of Leptospirillum ferriphilum T in axenic culture 13 , RNA transcripts and protein concentrations of simple defined mixed cultures 11 , microscopy images 14,15 , and on reverse engineering of omics data to generate gene regulatory networks 16 .…”

“…Three bacterial acidophile species were used: Leptospirillum ferriphilum T DSM 14647 20 , Sulfobacillus thermosulfidooxidans T DSM 9293 21 , and Acidithiobacillus caldus T DSM 8584 22 . Cells were maintained in the exponential growth phase at 38 °C in three separate axenic continuous cultures 11 until inoculation for further experiments. Continuous cultures (1 L working volume) containing Mackintosh basal salt (MAC) medium 23 and ferrous sulfate (100 mM) as electron donor were adjusted to pH 1.4 for L. ferriphilum, or with 5 mM potassium tetrathionate adjusted to pH 2.3 and pH 2.0 for S. thermosulfidooxidans and A. caldus, respectively.…”

“…Bioleaching experiments were carried out and analyzed as previously reported 11 . Quadruplets of 100 mL MAC medium were adjusted to pH 1.8 by addition of sulfuric acid and supplemented with 2% (wt/vol) chalcopyrite concentrate.…”