Bioleaching of Gallium from Electronic Waste

Gallium (Ga) is indispensable for semiconductor chips in light-emitting diodes (LEDs). Due to its supplyscarcity and irreplaceable role, Ga is classified as "critical" raw material. Urban mining of End-of-Life(EoL) devices offers a secondary supply, yet recovery attempts for Ga hitherto relied on non-selectiveinorganic reagents (i.e., HCl, H2SO4 ), posing environmental burdens. This study investigates theuse of cell-free supernatants from Aspergillus niger (ATCC1015), rich in biogenic organic acids, assustainable and selective solvents for Ga recovery from LED waste. Comparative leaching experimentsdemonstrated that Ga extraction was hindered by the interference of metals, specifically with thoseof chemical similarities (i.e., Al3+, Fe3+). Notwithstanding, biogenic supernatant achieved 0.194% Gaextraction over 24 hours, doubling the efficiency of the abiotic control. Kinetic analysis revealed thatdissolution remained in a lag phase at 24 h, underscoring the necessity for prolonged exposure times incomplex waste matrices. Despite the extraction yields were hindered by the post-fermentation oxalatecrystallization (magnesium oxalate) and complex waste matrix, the present study established a roadmapfor sustainable recovery. It suggests, coupling waste pre-treatment and co-optimization of fermentationwith indirect bioleaching represents a promising strategy for enhancing Ga extraction.