Fungal biodegradation unravels potential low-tech pathway for paper electronics waste valorization

Abstract

Abstract Metallic traces are core component of simple electronic products such as printed circuit boards (PCB) and radio frequency identification (RFID) tags, which are central items of the Internet of Things (IoT). However, these systems come with an ecological footprint since metals, even if present at very low amounts, are non-renewable resources. Currently, more than 80% of electronic waste is still not properly recycled and the recent global semiconductor crisis demonstrates a risk of metal shortage in the upcoming decades. Paper electronics represent a sustainable alternative to standard FR4 PCBs since paper can be recycled, albeit the end-of-life treatment of the silver or copper printed metallization remains a major issue. Here, we investigated a microbial pathway for their biorecycling, where paper would be used as a carbon source for microbial growth while the metallic traces would be recovered through microbe-metal interactions. More specifically, we hypothesized that a bacterium and a fungus in co-cultures could use technical paper as a sole carbon source. In addition, they would cooperate to first solubilize, then translocate, and finally biomineralize Ag or Cu contained in metallic traces printed over the technical paper. We also tested whether an alternative carbon and nutrient source, spent coffee grounds, may enhance microbial growth and activity to eventually design a process fitting an industrial scale. Two fungal strains (Boeremia exigua and Neurospora sitophila) and two bacterial strains (Pseudomonas putida and Cupriavidus necator) were compared, alone and combined, under various nutritive conditions. Results: The presence of bacteria associated to fungi did not have any direct effect on metal-related processes. However, bacteria altered the architecture of the mycelial network, eventually modulating metal transformations. Hence, fungal activity only effectively led to metal mobilization and then immobilization through both extra- and intracellular precipitates. Conclusions: Although at this stage metal recovery was not actually achieved due to slow biodegradation, the results give a clear signal to the biotechnology communities that valorizing organic and electronic waste together may be envisioned.