Leaching of copper, nickel and cobalt from Indian Ocean manganese nodules by Aspergillus niger

The leaching of copper, nickel and cobalt from polymetallic manganese nodules from the Indian Ocean was investigated using a fungus — Aspergillus niger. Parameters such as initial pH, pulp density, particle size and duration of leaching were optimized for the bio-recovery of metals. At an initial pH of 4.5, 35 ºC temperature and 5% (w/v) pulp density, about 97% Cu, 98% Ni, 86% Co, 91% Mn and 36% Fe were dissolved in 30 days time using adapted fungus — as against only 4.9% Cu, 8.2% Ni, 27% Co, 6.3% Mn and 7.1% Fe solubilized in control experiment. The results indicate that A. niger released organic acids such as oxalic and citric acids which in turn reduced the host metal oxides/hydroxides to their lower valence states, and thus dissolving the base metals following the indirect mechanism. A comparison of results obtained with chemical leaching of sea nodules using citric and oxalic acids and bio-leaching using A. niger show that the leaching of metals was more effective in presence of the fungus. The appearance of some lower oxide phases of manganese and iron in the leach residue identified by XRD phase analyses may account for unlocking of the host lattice leading to release and dissolution of metals during leaching.

Research Highlights
► Leaching of valuable metals such as Cu, Ni, and Co from polymetallic nodules of Indian Ocean was investigated using Aspergillus niger. At an initial pH of 4.5, 35 ºC temperature and 5% (w/v) pulp density, about 97% Cu, 98% Ni, 86% Co, 91% Mn and 36% Fe were dissolved in 30 days time using adapted fungus as against only 4.9% Cu, 8.2% Ni, 27% Co, 6.3% Mn and 7.1% Fe solubilized in control experiment.
► The results also indicate that A. niger released organic acids such as oxalic and citric acids which in turn reduced the host metal oxides/ hydroxides to their lower valence states, and thus dissolving the valuable metals following the indirect mechanism.
► A comparison of results obtained with chemical leaching of sea nodules using citric and oxalic acids and bio-leaching using A. niger show that the leaching of metals was more effective in presence of the fungus. Leach residue was characterized by XRD and SEM which show the presence of lower valence manganese and iron phases confirming the dissolution of metals with highly corroded surface morphology.