Multidimensional electroactive material for high-solids anaerobic digestion of agricultural waste: Synergistic enhancement of extracellular electron transport and metabolism

Abstract

High-solids anaerobic digestion (HSAD) of agricultural waste (TS>15 %) faces critical bottlenecks in volatile fatty acid (VFA) inhibition and syntrophic electron transfer inefficiency. This study presents a synthesis of a novel multidimensional electroactive (Ni-N-BC) composite, with an aim to assess the efficacy of the material in strengthening HSAD from straw and swine manure, and to elucidate the underlying mechanisms involved. Characterization revealed abundant redox-active moieties (pyrrolyl nitrogen, pyridine nitrogen, Ni²⁺/Ni⁰). Batch experiments demonstrated 74.4 % methane yield enhancement versus control, accompanied by 30 % faster VFA conversion. Enzymatic assays confirmed Ni-N-BC's dual stimulation: 2.3-fold increased hydrolytic/acidogenic enzyme activities and 1.8-fold elevated methanogenic coenzyme F₄₂₀ levels. Electrochemical analyses verified superior electron transfer capacity, enhancing the direct interspecies electron transfer (DIET) of syntrophic bacteria (Geobacter, Syntrophus) and methanogens (Methanobacterium, Methanosarcina). Metagenomic profiling revealed metabolic pathway restructuring, with hydrogenotrophic and acetoclastic methanogenesis genes upregulated 1.5–2.0 fold. Morever, the life cycle assessment (LCA) analysis demonstrated that Ni-N-BC contributes to both terrestrial and aquatic acidification, with respective values of 37 × 10⁻³ kg SO₂ eq and 46 × 10⁻³ kg MJ. However, it should be noted that Ni-N-BC achieves a net environmental benefit ratio (NEBR) of 1.8 and reduces Global warming potential (GWP) by 2.88 kg CO₂ eq/tonne through increased methane production and accelerated VFA conversion. This study provides a theoretical basis for the optimisation of HSAD mediated by electroactive materials and its sustainable development.