Enhanced co-production of hydrogen and ethanol from brown algae fermentation by supplementing nano zero-valent iron (nZVI)

Nano zero-valent iron (nZVI) has been shown to facilitate biofuel production, notably in hydrogen generation. However, its effect on bioethanol production is rarely studied, and even less so on its impact on hydrogen and ethanol co-production. This study therefore conducted ethanol-type hydrogen-producing fermentation with brown algae to explore the effect of nZVI. With the supplementation of nZVI, fermentation was accelerated and the production of hydrogen and ethanol were both significantly enhanced. At the optimal nZVI concentration of 100 mg/L, the production of hydrogen and ethanol increased by 71.44% and 65.11%, respectively. A high hydrogen yield of 2.32 mol/mol-mannitol was achieved. The ethanol yield (1.86 mol/mol-mannitol) reached 93% of the theoretical maximal yield with a high selectivity of 91.09%. Electron distribution analysis showed that nZVI increased the total electron amounts of fermentation, and directed more electrons to ethanol than hydrogen. The reduced redox potential (ORP) and increased NADH level further confirmed the role of nZVI. Moreover, nZVI fostered Fe²⁺ availability to hydrogen-producing bacteria, enhancing the enzymes hydrogenase and alcohol dehydrogenase, as well as cell biomass. The bioenergy conversion efficiency (BioECE) improved from 19.08% to 31.74% with 100 mg/L nZVI treatment. This study provided a first demonstration regarding the role of nZVI in hydrogen and ethanol co-production, highlighting the great potential of multiple bioenergy recovery from marine biomass.