Fermentation-based oxalic acid production aimed at sustainable critical metal recovery from electronic waste

This study systematically investigates the biogenic production of oxalic acid (OA, H₂C₂O₄) via submerged fermentation to establish a sustainable reagent for critical metal recovery applications. Preliminary shake-flask experiments evaluated Aspergillus niger strains (ATCC 1015 and CECT 2807), optimal carbon sources, and the influence of pH adjustment. Despite inherent biological variance across replicates, A. niger ATCC 1015 demonstrated a superior production trajectory, yielding 71.0 ± 27.7 mM OA in 5 days. Intermittent pH adjustment above 4.0 significantly enhanced secretion to 90.4 ± 5.8 mM, while glucose was identified as the optimal carbon source (Y_P/S $\approx$ 0.4 g/g). Building upon these preliminary findings, process intensification was conducted in a 10 L stirred-tank bioreactor. Transitioning to a fed-batch strategy with pulsed feeding and continuous pH control effectively contributed to substantially higher OA titers, achieving a peak OA titer of 260.1 ± 4.8 mM over 14 days. Co-production of gluconic acid and acidogenesis inhibition posed a challenge in attaining higher product yields. Collectively, these findings establish a robust, scalable bioprocess for sustainable OA generation, directly supporting its emerging application as a highly selective leaching agent for critical metal recovery from electronic waste streams. KEY POINTS: A.niger strain ATCC1015 was pinpointed as a suitable strain for biogenic OA production Fed-batch with pulsed feeding maximizes OA titer in scale-up. Gluconic acid and phosphate excess hinder selective OA production.