Since conventional leaching of nickel and copper from limonitic laterites via pyro- and hydrometallurgical processes requires large amounts of energy and results in high CO2 emissions, environmentally friendly alternatives like bioleaching are becoming increasingly important. The reductive bioleaching of limonitic laterites showed promising results using bacteria of the genera Acidithiobacillus and Sulfobacillus [1]. Thermoacidophilic archaea of the genus Metallosphaera are known to oxidize sulfur and iron as well as reduce iron and they are suggested to have a high potential for reductive bioleaching of laterites [2].
The goal of this study was to investigate the influence of different substrates on the iron reduction of the thermoacidophilic archaeon Metallosphaera sedulaT and to evaluate its bioleaching potential in an initial leaching experiment.
M. sedula was incubated under anaerobic conditions at 70°C in Norris Medium with 50 mM Fe(III) and one of the following substrates: (i) 0.1 % complex organics (e.g. yeast extract, casamino acids), (ii) 10 mM amino acids (e.g. L-alanine, L-cysteine), (iii) 10 mM D-mannose, (iv) 1% elemental sulfur. In total 11 different substrates were tested. Bioleaching experiments were carried out anaerobically with 1% (w/v) goethite, hematite, and the laterites BaSt and BaC (from a site in Brazil). Iron reduction efficiency was monitored using the Ferrozine assay in all experiments. Fe, Ni and Co concentrations in the leach solution from laterite bioleaching were detected using ICP-OES.
The experiments revealed significant differences in iron reduction efficiency depending on the substrate used. Peptone showed the best results followed by S0 with a complete iron reduction after 5 and 7 days. The other substrates showed either a very slow reduction (L-alanine), incomplete reduction (yeast extract) and/or precipitation of iron(III) (yeast extract, glutamic acid). For the other amino acids and D-mannose, iron reduction was also observed in the negative controls. The bioleaching experiments revealed, that laterite and goethite were successfully leached by M. sedula, which will be explored for further application in reductive bioleaching.
[1] Stankovic et al. (2022). Effect of mineralogy on Co and Ni extraction from Brazilian limonitic laterites via bioleaching and chemical leaching. Miner. Eng., 184, 107604.
[2] Malik, L., & Hedrich, S. (2021). Ferric Iron Reduction in Extreme Acidophiles. Front. Microbiol., 12, 818414