Use of adapted Aspergillus niger in the bioleaching of spent refinery processing catalyst

Spent refinery processing catalyst is listed as a hazardous waste; the toxicity characteristic leaching procedure (TCLP) extracts of the catalyst are found to contain heavy metals at concentrations exceeding the regulated levels. In the present investigation, Aspergillus niger was adapted to single metal ions Ni, Mo or Al (at 100-2,000 mg/L in steps of 100mg/L) and then to a mixture of Ni, Mo and Al (at a mass ratio of 1:2:6, as approximately present in the spent catalyst). Adaptation experiments with single metals showed that the fungus could tolerate up to 1,000 mg/L Ni, 1,200 mg/L Mo and 2,000 mg/L Al. In the presence of a mixture of these metals, the fungus was able to tolerate up to 100mg/L Ni, 200mg/L Mo and 600 mg/L Al. One-step bioleaching experiments with 1 wt% spent catalyst (of particle size <37 microm) were carried out using un-adapted and various adapted fungal strains. In contrast to the adapted strains, the un-adapted strain showed no growth in the presence of the catalyst. Ni:Mo:Al-adapted strain was the most efficient in the leaching of metals from the catalyst (at 78.5% Ni, 82.3% Mo and 65.2% Al) over 30 days due to its tolerance to the toxic elements at 1 wt%. More importantly, the Ni:Mo:Al-adapted strain was capable of bioleaching up to 3 wt% spent catalyst. The TCLP extracts of the spent catalyst after bioleaching using the Ni:Mo:Al-adapted strain showed the concentrations of Ni and Mo were well within the regulated levels.