Bioleaching is considered to be a low-cost, eco-friendly technique for leaching valuable metals from a variety of matrixes. However, the inherent slow dissolution kinetics and low metal leaching yields have restricted its wider commercial applicability. Recent advancements in bio-hydrometallurgy have suggested that these critical issues can be successfully alleviated through the addition of a catalyst. The catalyzing properties of a variety of metals ions (Ag+, Hg++, Bi+++, Cu++, Co++ etc.) during bioleaching have been successfully demonstrated. In this article, the role and mechanisms of these metal species in catalyzing bioleaching from different minerals (chalcopyrite, complex sulfides, etc.) and waste materials (spent batteries) are reviewed, techno-economic and environmental challenges associated with the use of metals ions as catalysts are identified, and future prospectives are discussed. Based on the analysis, it is suggested that metal ion-catalyzed bioleaching will play a key role in the development of future industrial bio-hydrometallurgical processes.

Petroleum refineries are known to generate huge quantities of spent hydroprocessing catalysts as solid waste every year. Spent hydroprocessing catalysts contain significant quantities of different toxic metals, and are designated as hazardous. Besides environmental concerns, the metals (Ni, V, Mo, Co, etc.) present in the spent hydroprocessing catalysts are also a valuable commodity and thus effective and eco-friendly methods are required to recover them. In recent years, different hydrometallurgical processes have been developed to leach these metals from spent hydroprocessing catalysts in an effective manner. Among various methods, the use of different organic acids is gaining attention as a promising green leaching method for spent catalyst due to their effectiveness, selectivity, and biodegradable nature. In this article, the properties and mechanism of different organic acids in leaching of metals from the refinery-spent hydroprocessing catalysts are reviewed. The advantages of organic acids as compared to other hydrometallurgical processes are emphasized. The potential of each organic acid in the leaching of metals from the spent hydroprocessing catalyst is elaborated. The key techno-economic challenges (efficiency, process economy, and safety) associated with the use of organic acids as leaching agents are identified, and future perspective are elaborated. Based on the analysis, it can be concluded that organic acid-based leaching might play a key role in the development of eco-friendly hydrometallurgical processes for the treatment of petroleum refinery spent hydroprocessing catalysts.