Aylin Nur Erkmen

• Gallium (Ga) is a critical metal, indispensable for semiconductor industry, used in versatile products such as in light-emitting diodes (LEDs), liquid crystal displays (LCDs), back-lightning applications, integrated circuits (ICs), and so on. In light of technological advancements, demand for these aforementioned products is higher than ever, pressuring the electronic industry to meet the increasing demands. Current supply line for Ga is dependent on primary reserves, predominantly located in China. Ga is irreplaceable, owing to its unique features in semiconductors. Therefore, European Union (EU) commission enlisted Ga as a critical raw material (CRM) to urge the circularity for this particular metal. Urban mining, a secondary production route for metal extraction, involves the recycling of metals from end-of-life (EoL) devices, aiming to address the growing built-in obsolescence related electronic waste (e-waste) conundrum as well as the scarcity of technologically-critical metals such as Ga, rare earth elements (REEs) and so on. Since EoL devices contain a higher concentration of Ga in the waste matrix than known primary ore reserves, attempts have been made to develop processes for the targeted recovery of Ga. This Ga is present in a dispersed electronic waste matrix that also contains base metals, precious metals, and other critical metals. However, current recycling technologies are based on processes that require high energy input and involve the use of aggressive inorganic reagents such as hydrochloric acid (HCl), which poses a significant burden on the environment. Despite the promising results of preliminary studies, these methods achieve low selectivity, as other metals such as Cu, Fe, and Al are also extracted from the waste matrix. In this context, organic acids and microbial systems represent a promising alternative. Studies have shown that organic acids, due to their unique dissociation and ligand-forming properties, can selectively dissolve and complex certain metals within the same waste matrix. Another novel technology, bioleaching, is based on a microbial approach in which microorganisms secrete metabolites that serve as suitable extractants for targeted metal recovery. This work addresses the efficacy and solubility of gallium in environmentally-friendly, selective organic reagents as well as fungal-based bioleaching systems. The aim of this work is to propose a bioprocessing strategy as the basis for a bottom-up approach that is applicable not only to Ga but also to other critical elements in end-of-life (EoL) processing. Thus, not only is the aspect of the circular economy for critical metals taken into account, but the proposed recovery scheme also aligns with the EU’s sustainability goals for 2050.