DIET-Intensified Dark Fermentation: Magnetite and Nickel–Iron-Doped Activated Carbon for Biohydrogen Production from Food Waste

Background: Dark fermentation (DF) of organic solid waste (OSW) is a promising route for sustainable biohydrogen (bioH2) production, but it is often limited by slow interspecies electron transfer and by acidification of the medium. Promoting direct interspecies electron transfer (DIET) with conductive additives is a candidate strategy to intensify the process. Objective and methods: This study compares the effect of magnetite (Fe3O4), granular activated carbon (AC), and laboratory-synthesized nickel–iron-doped activated carbon (DC) on the DF of genuine post-consumer food waste. Batch assays were conducted in 2.1 L anaerobic reactors under mesophilic conditions (35 °C), in triplicate, using food waste as the substrate and raw UASB sewage sludge as a mixed inoculum, without pH control. Biohydrogen was quantified with a portable biogas analyzer and expressed as mL H2/g COD removed; the additives were characterized by SEM–EDS, and yields were compared by one-way ANOVA with Tukey post hoc test (α = 0.05). Main findings: Additive type had a highly significant effect on biohydrogen yield (F(6,14) = 48.7; p < 0.001; η2 = 0.954). DC at 4 g achieved the best performance, reaching 0.271 mL H2/g COD removed (≈4.5-fold higher than the additive-free control) and COD removals of 65.8–72.4%, whereas magnetite produced only sporadic, non-reproducible gains and undoped AC gave a modest, consistent improvement. Excessive acidification (final pH 2.5–3.1), attributed to the accumulation of volatile fatty acids under the deliberately unbuffered conditions, was the main operational limitation and is interpreted as a conservative ceiling on the yields reported here. Prospects: Metal-doped carbonaceous materials emerged as the most robust and reproducible strategy for intensifying DF of food waste; future work should couple this route with pH buffering and with electrochemical and microbial-community analyses to confirm the DIET contribution and to support scale-up in integrated biorefineries.