Understanding of the potential role of carbon dots in promoting interspecies electron transfer process in anaerobic co-digestion under magnetic field: Focusing on methane and hydrogen production

Anaerobic digestion shows great potential for biomass energy recovery, however, the low electron transfer efficiency between microorganisms limits overall anaerobic digestion performance. In this work, aloe peel-derived carbon dots (carbon quantum dots, CQDs) were designed and investigated as accelerants for anaerobic digestion systems under an applied magnetic field to promote the interspecific electron transfer (IET) efficiency in methane and hydrogen production processes. Compared with the control group (188.9 mL/g VS, 4.2 mL/g VS, 34.7 %, and 26.3 %), the applied magnetic field (5 mT) significantly increased methane yield (256.1 mL/g VS), hydrogen yield (7.0 mL/g VS), total chemical oxygen demand reduction (41.7 %), and energy conversion efficiency (35.7 %). The introduction of the CQDs (0.18–0.45 wt%) under a magnetic field (5 mT) has further positive effects on digestion performance. The experimental group incorporating 0.36 wt% CQDs under a 5 mT magnetic field obtained the highest methane yield (348.9 mL/g VS), hydrogen yield (9.9 mL/g VS), and total chemical oxygen demand reduction (58.4 %), and thus the energy recovery efficiency was increased by 85 % compared with control group. The digestate incorporating 0.36 wt% CQDs exhibited excellent thermal stability (59.71 %) and total nutrient content (38.17 g/kg), and has great potential as a main ingredient in compound fertilizer. The potential role of the magnetic field and CQDs in enhancing hydrogen interspecies electron transfer (HIET) and direct interspecies electron transfer (DIET) for methane and hydrogen production is further illustrated, providing a feasible solution for improving energy recovery from biomass.