Preparation and mechanism of carbon felt supported iron trioxide and zero-valent iron for enhancing anaerobic digestion performance

Promoting direct interspecies electron transfer (DIET) for methanogenesis via conductive materials is an effective strategy to enhance anaerobic digestion (AD) for methane production. In situ growth of iron trioxide (ITO) and zero-valent iron (ZVI) on carbon felts (CF) was first prepared as composite CF@ITO and CF@ZVI for AD application in this study via hydrothermal deposition and co-precipitation, respectively. The optimal preparation conditions of 2-fold concentration and 120℃ for CF@ITO and of 1-fold concentration for CF@ZVI were determined via batch AD tests adding different iron-carbon composites. Methane production was increased by 47.73% and 34.98%, respectively, for the optimally prepared CF@ITO and CF@ZVI compared to the control. The mechanisms behind were further explored in a more systematic and profound way. Cyclic voltammetry measurements showed that CF@ITO and CF@ZVI had around 10 times higher double-layer capacitance than CF, demonstrating the enhanced conductivity of iron-carbon composites. Fe and fluorescence analysis revealed that both composites increased Fe bioavailability and sludge conductivity potentially via enriching electrochemically active microorganisms and stimulating the secretion of protein-like substances, which enhanced microbial activity for methanogensis. Metagenomic sequencing analysis indicated that both composites enriched the functional genes associated with energy production and conversion, carbon and methane metabolism, and polysaccharide lyase activity. Iron reducing bacteria enhancing carbohydrate metabolic activity were more enriched in sludge after adding composites. Methanothrix capable of undertaking DIET for methanogenesis and electron carrier proteins associated genes were significantly enriched in biofilm on CF@ITO and CF@ZVI, implying the mediation of DIET via both composites and electron carrier proteins. Methanocorpusculum and Methanobacterium via interspecific hydrogen transfer (IHT) for methanogenesis were only significantly enriched in biofilm on CF@ZVI, demonstrating both enhanced DIET and IHT by CF@ZVI.