Quantitative analysis of the mechanism of biochar in alleviating product inhibition in different fermentative hydrogen production processes

Abstract Dark fermentation is a biological process that converts organic molecules into molecular hydrogen and stands out as one of the most promising methods for extracting bioenergy from wastewater. The accumulation of end-products causes varying inhibitory effects on the process, posing a key challenge. This study explored the efficacy and mechanisms of biochar as a cost-effective solution to mitigating the inhibitory effects of end-products in major types of dark fermentative hydrogen production processes and provided for the first time a quantitative analysis of the relative contribution of each mechanistic pathway. Results showed that biochar was more effective in butyrate-type than ethanol-type fermentations. In butyrate-type fermentation facing endogenous and exogenous volatile acid inhibition, biochar increased hydrogen production by 145.74% and 64.95%, respectively. In ethanol-type fermentation, biochar increased hydrogen production by 10.53% and 18.09% under endogenous and exogenous inhibitions from ethanol, respectively. Mechanistic analyses revealed three primary pathways through which biochar mitigated product inhibition: pH buffering, cell colonization, and inhibitor adsorption. The relative contribution of each pathway varied by fermentation type. In butyrate-type fermentation, pH buffering was critical, accounting for 42.9% of the mitigation effect, while cell colonization was primary in ethanol-type fermentation, contributing 32.4%. This study demonstrated the different roles of biochar in mitigating production inhibition in diverse fermentation types, highlighting its potential to enhance hydrogen energy recovery in dark fermentation. Graphic Abstract