Hydrodynamic intensification of red soil microbial fuel cells: enhanced Acid Red 73 degradation and bioelectricity...

The environmental risk of azo dyes arises from their recalcitrant nature and potential carcinogenicity. Microbial fuel cells (MFCs) have emerged as a sustainable technology for concurrent wastewater treatment and renewable electricity generation, yet their efficiency is constrained by mass transfer limitations, low electron recovery, and the complex responses of microbial communities. This study evaluated the effects of free-fall influent (FF) mode on red soil MFCs treating the disazo dye Acid Red 73 (AR73). Compared with conventional operation, FF mode enhanced both pollutant removal and bioelectrochemical performance. The hydrodynamic impact of inflowing droplets increased cathodic dissolved oxygen by 44.7-45.8%, thereby promoting oxygen reduction. These physicochemical shifts mitigated cathodic polarization, resulting in a maximum power density of 2056 mW/m³ under dye-containing conditions. Coulombic efficiency also improved, reflecting more efficient electron recovery from organic substrates. GC-MS analysis identified the major degradation products in both FF and non-FF modes, revealing differences that clarified the AR73 degradation pathway. Microbial community analyses revealed that FF mode restructured both bacterial and fungal communities. Electroactive genera, including Anaeromyxobacter, Dechloromonas, Citrifermentans, and Caulobacter were enriched, together with organic degraders such as Xanthobacter, Methyloversatilis, Rhodoplanes, and Aquabacterium. Fungal communities, dominated by Ascomycota and Basidiomycota, also displayed functional shifts, with FF mode promoting the abundance of degradative taxa including Ganoderma, Nigrospora, and Sterigmatomyces. Overall, FF mode provides a hydrodynamic strategy that enhances both energy recovery and pollutant removal. These findings suggest that hydrodynamic intensification can improve the sustainability of wastewater treatment in bioelectrochemical systems.