Aerobic processes for bioleaching manganese and silver using microorganisms indigenous to mine tailings

Mining wastes containing appreciable concentrations of silver are considered alternative sources for metal extraction, although these wastes are often refractory due to the presence of manganese oxides. The high cost and/or environmental impact of the hydrometallurgical and pyrometallurgical extraction processes make it necessary to search for biotechnological processes for the solubilization of manganese compounds. This paper describes the characterization of chemoorganotrophic microorganisms indigenous to the tailings of a silver mine located in Coahuila, México, regarding their capability to remove manganese and silver present in these residues by lixiviation. The Bacterial and fungal strains isolated were identified by sequencing the rDNA 16S and ITS-1-ITS-2 genomic regions, respectively; the bacterial strains correspond to isolates of Roseospira sp. and Sphingomonas sp., whereas the fungal strains include isolates of Cladosporium sp. A, Cladosporium sp. B and Penicillium chrysogenum. These fungal strains show an effective capacity to lixiviate manganese and silver from solid mine residue when incubated in 9 k medium; it was found that under these conditions, leaching of metals occurs due to a mixed biotic-abiotic process, which yields manganese and silver leaching efficiencies in the ranges of 58-74% and 40-67%, respectively. The fungal strains grown in the LMM medium and the bacterial strains incubated in the PDB medium caused leaching of manganese with a lower efficiency in the range of 0.17-0.24% and 1.42-1.73%, respectively; under these conditions, silver leaching by fungal and bacterial strains appeared to be reduced (< 0.1%).Through in vitro cultures, it was determined that P. chrysogenum and Sphingomonas sp. showed the highest levels of silver resistance.