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.
Highlights
Biomining is a sustainable process for metal extraction from sulfidic ores that has been studied by researchers around the globe since its emergence in the early 1950s (Temple and Colmer, 1951; Bryner and Jameson, 1958)
Bioleaching of chalcopyrite was tested with single, binary, and tertiary combinations of the three model species (A. caldus, L. ferriphilum, and S. thermosulfidooxidans), plus uninoculated controls to investigate the effect of species composition on redox potential and copper release (Figure 1 and Supplementary Figure 1)
The same behavior was observed in the experiment inoculated exclusively with A. caldus, where the redox potential remained at circa 370 mV until day 12 when likely environmental bacteria that survived the sterilization process on the mineral became active and commenced iron oxidation
Summary
Biomining is a sustainable process for metal extraction from sulfidic ores that has been studied by researchers around the globe since its emergence in the early 1950s (Temple and Colmer, 1951; Bryner and Jameson, 1958). Ferrous iron (Fe2+)-oxidizing acidophilic microorganisms are responsible for the regeneration of the chemical oxidant ferric iron (Fe3+), which in turn attacks the sulfidic mineral, and breaks its covalent bonds. This releases ferrous iron plus any other contained metals and completes the catalytic cycle (Vera et al, 2013). Today, increasing amounts of metals are extracted or processed by biomining technologies in many countries that include Chile, Australia, and South Africa, with the bioleaching of secondary copper sulfides accounting for an estimated 15–20% of the world wide copper production (Brierley and Brierley, 2013)
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