Acid-Modified Red Mud Enhances Anaerobic Digestion of Food Waste via Synergistic Adsorption and Electron Transfer: Performance and Mechanism
Anaerobic digestion (AD) is a viable route for energy recovery from food waste (FW), yet it often suffers from process instability due to volatile fatty acids (VFAs) accumulation and subsequent pH drop. This study modified red mud (RM) with hydrochloric acid to produce acid-modified red mud (AMRM), aiming to optimize its alkalinity and physicochemical properties. The efficacy of AMRM as an additive in FW anaerobic digestion was systematically evaluated. Results demonstrated that adding 3% AMRM significantly enhanced the buffering capacity and controlled VFAs accumulation, particularly propionic acid. This was attributed to the developed pore structure of AMRM, whose specific surface area increased by 347% compared to raw RM, facilitating rapid VFAs adsorption. Furthermore, AMRM enriched hematite (Fe2O3), which elevated electron transport system (ETS) activity and coenzyme F420 content, suggesting its role as an electron carrier promoting direct interspecies electron transfer (DIET) between syntrophic bacteria and methanogens. Consequently, the cumulative methane yield reached 633.9 mL/g VS, which was 175.1% and 55.2% higher than the control and raw RM groups, respectively. The VS removal efficiency was 75.1%. This study provides a dual-pathway mechanism—adsorption and electron transfer—for enhancing AD performance, offering a cost-effective and sustainable strategy for FW treatment and RM valorization.