Feasibility of Bioleaching in Removing Metals (Al, Ni, V and Mo) from as Received Raw Petroleum Spent Refinery Catalyst: A Comparative Study on Leaching Yields, Risk Assessment Code and Reduced Partition Index

This study investigates the effectiveness of bioleaching in recovery of metals (Al, Ni, V and Mo) from raw petroleum refinery spent catalyst using Acidithiobacillus ferrooxidans (At. ferrooxidans) and Acidithiobacillus thiooxidans (At. thiooxidans). It was found that bioleaching with At. ferroxidans or At. thiooxidans resulted in higher leaching yields of Ni (55.8-58.6%) and V(33.0-33.4%) as compared to Al (9.3-10%) and Mo (3.9-5.8%). After 168 h of bioleaching with either At. ferrooxidans or At. thiooxidans, the remaining metals in the bioleached spent catalyst samples were present in stable forms (oxidizable and residual fractions). Bioleaching also led to increase in the reduced partition index of all the metals in the bioleached residues (Ni: 0.63-0.65, Al: 0.98, V: 0.90-0.91, Mo: 0.80-0.83) as compared to feed spent catalyst (Ni: 0.14, Mo: 0.63, V: 0.70, Al: 0.94). The low 'risk assessment code' (RAC) values of the bioleached residues as compared to feed spent catalyst indicated that bioleached residues posed low or no risk to the environment. The results of the present study suggested that the bioleaching with either At. ferrooxidans or At. thiooxidans is effective in leaching of Ni and V, whereas leaching of Mo and Al requires further treatment.