Nanomaterials-based air-cathodes use in microbial desalination cells for drinking water production: Synthesis, performance and release assessment

Abstract

Global water desalination capacity is around 95 million m3 day−1, accounting for an enormous energy demand, thus low energy desalination technologies are needed. Microbial desalination cells (MDC), a bioelectrochemical reactor that allows water desalination while treating wastewater with low energy requirement represents a potential low energy solution. Air-cathode based MDC is a promising environmentally friendly reactor configuration as it reduces operational costs but has limited performance due to low cathode oxygen reduction reaction (ORR) kinetics. Air-cathodes with high ORR performance must be developed considering cost, performance and toxicologic characteristics. In the present work we developed two different carbon-based platinum free air-cathodes: iron doped carbon nanofibers (CNF-Fe) and gas diffusion electrodes based on MnO2 (MnO2-GDE). The air-cathodes demonstrated dissimilar ORR performances; CNF-Fe performed better at low overpotential while MnO2-GDE did better at high overpotentials. MDC reactors were tested employing both air-cathodes, achieving average output electrical current densities of 4.1 mA cm−2 and 1.3 mA cm−2 for MnO2-GDE and CNF-Fe, respectively. Also, the desalination performance of MDC employing MnO2-GDE was higher than CNF-Fe based MDC, reaching a salt removal rate of 4.8 g m−2 h−1 and 3.6 g m−2 h−1, respectively. Finally, the release of nanomaterials during both air-cathodes use in MDC operation was assessed, which represents the first study of this type to author’s knowledge. The results demonstrated some cathode catalyst release, but at a concentration below the threshold to represent a threat for the environment and humans.