Magnetic Walnut Shell Biochar Enhances Direct Interspecies Electron Transfer and Methane Yield from Fruit and Vegetable Waste’s Anaerobic Digestion

An exorbitant amount of organic fractions of the municipal solid waste, i.e., fruit and vegetable waste (FVW), generated from farm to fork are being treated through anaerobic digestion (AD). Anaerobic digestion (AD) of FVW only achieves <60% methane potential due to methanogen loss and indirect electron transfer. Hence, the technology necessitates further improvements in performance to maximise the methane gas yield by stabilising the methanogens using a potential additive. Magnetic biochar is a budding and promising additive in anaerobic digestion that amplifies biomethanation performance. This study focuses on the role of magnetic biochar in enhancing the viability of the AD system in biogas production from organic waste fractions. Herein, the magnetic biochar was produced using a FeCl3-impregnated walnut shell and then characterized. The derived magnetite was identified as the major crystalline phase in biochar with the presence of several oxygenated functional groups. The specific surface area, pore volume, and pore diameter were found to be 360.99 m2 g−1, 0.089 cm3 g−1, and 0.98 nm, respectively. The SEM and TEM images illustrated a good dispersion of the material, with size ranging between 18.2 and 46.6 nm, thus indicating the porous nature of the magnetic biochar. The incorporation of magnetic biochar in the CN ratio modified the AD system with enhanced methane production and the highest volume (1523.4 mL) reported in treatment, with a CN ratio of 25:1 and 0.5% magnetic biochar. The resulted gas yield is 35% more than the control (1125 ML) with reduced lag phase (4 vs. 12 days). It concludes that walnut shell MBC uniquely combines DIET conduits and biofilm support and enhances methane production from FVW. However, 16S rRNA confirmations of syntrophs, continuous reactor validation, and magnetic biochar recovery and reuse potential studies are essential for further scaleup.