Bioleaching of spent hydrotreating catalyst by acidophilic thermophile Acidianus brierleyi: Leaching mechanism and effect of decoking

► Decoking removed impurities from spent catalyst and affected bioleaching. ► Ni concentration in leachate was reduced to below regulated level after bioleaching. ► Spent medium leaching showed higher efficiency than two-step leaching. ► Bioleaching efficiency was higher than chemical leaching for most metals.