(29q) Biochar Regulates Anaerobic Metabolism for Increased Production of Biohydrogen

Biomass, one of the most plentiful renewable resources on Earth, can be used as feedstock for biofuel production. A large portion of biomass is difficult to degrade or toxic to microorganisms, raising the demand for further exploration of their potential role in biofuel production. Recently, biochar, which is produced by biomass pyrolysis, has drawn extensive attention due to its fascinating features and functions in various biochemical scenarios. However, the effects, influencing factors, underlying mechanisms, and application potentials of biochar in biofuel production remain to be fully elucidated.

Firstly, this study investigated the effect of biochar in regulating anaerobic fermentative hydrogen production. Rice straw was pyrolyzed at an anoxic condition to prepare biochar. Different doses of biochar were supplemented to study their effects on two dominant fermentation types of biohydrogen production: ethanol-type and butyrate-type fermentations. Batch fermentation results showed that biochar promoted substrate consumption and hydrogen production. Complete substrate consumption was achieved at 3 g/L biochar for ethanol-type fermentation and 6 g/L biochar for butyrate-type fermentation. The addition of 3 g/L rice straw-derived biochar showed the best promoting effect for both types of fermentations. Hydrogen production increased by 118.4% and 79.6% in ethanol-type and butyrate-type fermentations, respectively. The maximal hydrogen yields of ethanol-type and butyrate-type fermentations were 1.34 and 2.36 mol/mol-glucose, respectively. Further increase of biochar dose did not show higher hydrogen yield.

This study then investigated the influences of two crucial factors, feedstock and pyrolysis temperature, on the promoting effect of biochar in ethanol-type fermentative hydrogen production. The physicochemical characteristics and promoting effects of biochars prepared with five biomass wastes (coffee ground, corn stalk, Ginkgo biloba leaf, mealworm frass, and sugarcane bagasse) were determined. Element compositions, structures, and surface features differed between biochars of different feedstocks. Among these biochars, sugarcane bagasse-derived biochar (SBBC) showed the best hydrogen production-promoting effect. The physicochemical properties of biochar, such as pH, and functional groups, were significantly affected by pyrolysis temperature, but the promoting effects were not significantly different. The hydrogen production-promoting effect of biochar in ethanol-type fermentation was mainly affected by feedstock instead of pyrolysis temperature.

Finally, the relationships between physicochemical characteristics and hydrogen promotion effect were investigated and a potential mechanism was given. Mechanism study showed that biochar boosted biohydrogen production with different pathways. Biochar buffered the broth pH with its mild alkalinity, released mineral nutrients, and immobilized cells due to its high porosity, which was named the Cell Promoting Pathway (P_C). Biochar showed redox activity by oxidizing/reducing its surface oxygen-containing functional groups. The addition of biochar lowered the redox potential of fermentation, indicating biochar could donate electrons to the hydrogen-producing bacteria, which was named the Electron Promoting Pathway (P_E). Biochar prepared at low temperatures boosted hydrogen production through P_E, while that at high temperatures achieved the promotion via P_C.

This study demonstrated the beneficial effect of biochar on anaerobic fermentative hydrogen production and revealed the underlying regulation pathways. This work provides a novel method that recalcitrant biomass can promote biofuel production as biochar instead of feedstock, which exhibits great application potential in bio-processes for the production of biofuel and value-added products.