Fermentation-Based Oxalic Acid Production for Sustainable Gallium Recovery from Electronic Waste

Oxalic acid (OA) is a versatile reagent ubiquitously used in pharmaceutical, agricultural, and chemical industries. Aligning with the European Union's 2050 circular economy agenda, its potential application in ore processing, and particularly metal extraction from electronic waste (e-waste) has garnered significant attention. Gallium (Ga) is a technologically strategic metal at high supply risk, driven by high demand from the electronics industry and limited primary production. Oxalic acid (OA), known for superior dissolution and complexation, is a highly selective metal recovery reagent that has been reported to outperform conventional reagents in gallium (Ga) recovery. However, current petrochemical OA production opposes global sustainability goals. To address this, submerged fermentation using filamentous fungi, notably Aspergillus niger offers a promising OA production route, that goes hand-in-hand with selective metal recovery. This study investigates OA generation through submerged fermentation, beginning with shake-flask experiments that identified ATCC1015 as the suitable candidate, achieving 71 \((\pm)\) 27.73 mM OA in 5 days, which later was enhanced to 90.37 \((\pm)\) 5.8 mM through intermittent pH adjustment above 4. Glucose was reported as the optimal carbon source, yielding Y\textsubscript{P/S} \((\approx)\) 0.4 (g/g). Scale-up in a 10 L bioreactor using fed-batch fermentation with pulsed feeding and pH control achieved 260.1 \((\pm)\) 4.8 mM OA over 14 days. Co-production of gluconic acid and acidogenesis inhibition posed a challenge in attaining higher product yields. Collectively, these findings establish fed-batch fermentation with robust pH control as a viable strategy for sustainable OA production, enabling targeted Ga recovery from waste streams.