Abstract
Spent petroleum catalyst as a repository of several toxic metals is recommended for metal removal before safe disposal. To evaluate an effective biotechnological approach for metal removal, a comparative study between sequential-aerobic and sequential-anaerobic bioleaching processes was conducted for the removal of metals from crushed-acetone-pretreated spent petroleum catalyst. The SEM-EDX and XPS analysis confirmed the presence of Ni, Al, Mo and V in their oxidic and sulphidic forms in spent catalyst. The bioleaching experiments were performed in stirred tank batch reactors (2.5 L), temperature 30 degrees C, pH 1.4 and stirring speed 250 rpm for the period of 160 h. Sulfuric acid acted as lechant for both sequential-aerobic (Acidithiobacillus ferrooxidans oxidised sulfur to sulfuric acid aerobically) and sequential-anaerobic (Acidithiobacillus ferrooxidans oxidised sulphur to sulfuric acid coupled with the ferric reduction to ferrous anaerobically) bioleaching studies. The higher Ni and V extractions compared to Al and Mo for all the studies were due to increased solubility of Ni and V, and supported by XPS which showed marginal signs of Ni and V peaks in leach residues compared to feed spent catalyst. At the end (320 h), sequential-aerobic bioleaching was resulted to 99% Ni, 65% Al, 90% Mo and 99% V extraction quite more effective than sequential-anaerobic bioleaching (88% Ni, 28% Al, 33% Mo and 77% V) and sequential-control leaching (94% Ni, 20% Al, 40% Mo and 57% V). Although anaerobic bioleaching a possible approach, aerobic condition was found to be more suitable for sulfuric acid generation by A. ferrooxidans and high yield. So aerobic bioleaching is recommended to be favourable approach compared to anaerobic counterpart for future study and extrapolation.