Conductive Fe₃O₄ nanoparticles for enhanced methane production from algal biomass via anaerobic digestion

In the present study, the potential of algal biomass for renewable biogas production was evaluated through Fe₃O₄ nanoparticle-assisted anaerobic digestion under mesophilic conditions. Algal biomass was selected as a sustainable substrate because of its biodegradable organic matter content and potential for wastewater nutrient recovery. However, its anaerobic digestion can be limited by the recalcitrant nature of algal cell walls and inefficient electron-transfer processes. To address these limitations, Fe₃O₄ nanoparticles were supplemented at concentrations of 25, 50, and 75 mg L⁻¹ in batch anaerobic digesters. The reactor supplemented with 50 mg L⁻¹ Fe₃O₄ demonstrated optimal performance, achieving a 34.3% increase in cumulative biogas production (685 ± 15 mL g⁻¹ VS) and a 76.9% increase in specific methane yield compared to the unamended control. Furthermore, optimum nanoparticle supplementation significantly enhanced chemical oxygen demand (COD) removal to 82.6% and volatile solids reduction to 69.8%, while maintaining superior pH buffering stability. Electrochemical analyses (CV and EIS) and 16S rRNA microbial community profiling revealed substantial reductions in charge-transfer resistance and the selective enrichment of electroactive syntrophs and methanogens. The combined electrochemical and microbial results provide supporting evidence for altered extracellular electron-transfer characteristics and a possible contribution of DIET to enhanced digestion performance, offering a highly efficient and environmentally sustainable strategy for algal biomass valorisation and bioenergy recovery.