Enhanced Performance of Microbial Fuel Cells with Electron Mediators from Anthraquinone/Polyphenol-Abundant Herbal Plants

The limited amount of electron mediators (EMs) excreted by microorganisms has restricted electricity generation and the related pollutant removal in microbial fuel cells (MFCs). The polyphenolic-rich plants contain simple polyphenols or anthraquinones that have promising electron-shuttling potential, but this has not been well understood. Herein, four herbal plants Polygonum multiflorum (T. fallopia), rhubarb (B. rheum), radix rubiae (L. Rubia) and semen cassiae (Catsia tora Linn) were selected and then studied to produce EMs to stimulate electricity generation in MFCs. B. rheum had the highest redox activity and 2% acid pretreatment contributing the most to the release of electroactive substances. The highest power density (18.67 W/m³) and Coulombic efficiency (29.03%) and the lowest internal resistance (29.02 Ω) were achieved in MFCs with B. rheum addition compared to other herbal plants. The satisfactory COD (93.68%) and NH₄⁺-N (39.68%) removal were also obtained with a 0.01 g/L dosage. Confocal laser scanning microscopy and high throughput sequencing analysis showed that B. rheum had the least negative effects on biofilm microstructure and microbial species, corresponding to its outstanding performance. These findings first suggested that B. rheum can be regarded as a promising redox mediator to improve MFC performance. This study provided a new thinking to apply herbal wastewater or pharmaceutical waste in improving the bioelectrochemical system.