Innovative oxalic acid production improvement via mixed culture strategy and its application in eco-friendly hybrid leaching of spent printed circuit boards

Abstract

Recovering metals from spent printed circuit boards is crucial for sustainability and environmental protection. In a unique approach, this study aims first to enhance oxalic acid production in the Bacillus megaterium-Aspergillus niger mixed culture, which has yet to be explored, and then to apply it in a sequential extraction method followed by glycine. Oxalic acid dissolves some metals, but at high concentrations, some metals precipitate; hence, glycine was used in the second step, assuming it can dissolve metal oxalate precipitates. It has also been shown to dissolve metals, especially copper. In order to improve the oxalic acid production and modify the culture medium, components like yeast extract, buffered peptone water, manganese, calcium carbonate, and diammonium hydrogen phosphate were tested to assess their impact on acid yield. The carbon sources, bacterial inoculation percentage, and timing of fungal spore addition were also assessed. The optimal conditions resulted in oxalic acid overproduction of 11,201 mg/l. Within 3 h at 60 °C, Mn (100 %), Sn (100 %), Al (86.2 %), and Fe (42.15 %) were extracted, while Cu (11.5 %) and Au (7.6 %) were leached. The extraction time was extended to 48 h, allowing complete recovery of Al while Fe recovery reached 73.5 %. Next, the hybrid method's data provided valuable information on many metals in this high-metal content waste, showing that metal behavior depends on the leaching agent choice, the order in which agents are used, the type of target metal, and the progression toward selective extraction that can be effective for future downstream processes. Structural analyses helped to analyze the process further.