Abstract
Using acidophilic bacteria to catalyze the reductive dissolution of oxidized minerals is an innovative process that facilitates the extraction of valuable base metals (principally cobalt and nickel) from limonites, which are otherwise often regarded as waste products of laterite mining. The most appropriate conditions required to optimize reductive mineral dissolution are unresolved, and the current work has reassessed the roles of Acidithiobacillus spp. in this process and identified novel facets. Aerobic bio-oxidation of zero-valent sulfur (ZVS) can generate sufficient acidity to counterbalance that consumed by the dissolution of oxidized iron and manganese minerals but precludes the development of low redox potentials that accelerate the reductive process, and although anaerobic oxidation of sulfur by iron-reducing species can achieve this, less acid is generated. Limited reduction of soluble iron (III) occurs in pure cultures of Acidithiobacillus spp. (Acidithiobacillus thiooxidans and Acidithiobacillus caldus) that do not grow by iron respiration. This phenomenon (“latent iron reduction”) was observed in aerated cultures and bioreactors and was independent of electron donor used (ZVS or hydrogen). Sufficient ferrous iron was generated in the presence of sterilized hydrophilic sulfur (bio-ZVS) to promote the effective reductive dissolution of Mn (IV) minerals in limonite and the solubilization of cobalt in the absence of viable acidophiles.
Highlights
While exploiting microorganisms to extract and recover base and precious metals from mineral ores and wastes (“biomining”) is well established as a global biotechnology, it is currently limited, in commercial-scale operations, to reduced materials (Johnson, 2014)
We report a series of experiments in which the relative merits of bioleaching laterite samples under aerobic and anaerobic were assessed, and how ferric iron is reduced by Acidithiobacillus spp. that cannot respire on iron was investigated
In order to elucidate how ferrous iron may be generated and redox potentials lowered during aerobic bioleaching of limonite by Acidithiobacillus spp. that cannot grow by ferric iron reduction, as reported previously for both A. thiooxidans (Brock and Gustafson, 1976) and A. caldus (Johnson et al, 2017) a series of experiments was carried out using a strain of A. thiooxidans (DSM 103717, obtained from the DSMZ, Braunschweig, Germany) which was found, unlike the type strain and most others, to be able to use molecular hydrogen, as well as zero-valent sulfur (ZVS) and various sulfur oxy-anions, as an electron donor for aerobic growth (Falagán and Johnson, 2018)
Summary
While exploiting microorganisms to extract and recover base and precious metals from mineral ores and wastes (“biomining”) is well established as a global biotechnology, it is currently limited, in commercial-scale operations, to reduced (sulfidic) materials (Johnson, 2014). Elemental (zero-valent) sulfur (ZVS) has usually been provided as the electron donor for the microorganisms, which were initially grown aerobically to generate acidity and to build up biomass, before switching to anaerobic conditions by gassing bioreactors with oxygen-free nitrogen (OFN) While this accelerated the rate of mineral dissolution and release of base metals, anaerobic bioleaching required continuous inputs of sulfuric acid to maintain leachate liquors at low pH, as the reductive dissolution of iron (III) and manganese (IV) oxy-hydroxides consumes hydronium ions (Johnson and du Plessis, 2015). The data obtained have allowed the roles of acidophilic sulfur-oxidizing bacteria in bioprocessing limonite to be reassessed, and have suggested how conditions for bio-processing different limonitic samples could be varied to both maximize base metal extraction and minimize costs of consumables
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