Impact of plant-derived biochars on biohydrogen production from sugar beet molasses in a continuous system: insights into the roles of microbial communities and chemical elements

Biochar shows great potential to enhance biohydrogen production via dark fermentation, but most studies have focused on short-term batch tests. This study investigates the impact of pine bark, coconut copra and cherry pit biochars in continuous bioreactors, integrating reactor performance, microbial community data, and elemental analysis. Being richest in several biologically important elements, the coconut-derived biochar produces the most significant improvement in biohydrogen yield, from 2-3 to 45 dm 3 H 2 /kg COD molasses. This correlates with an increased abundance of biohydrogen-producing microbial taxa and a 100-fold rise in hydA gene copies. Threefold-elevated butyrate and 33 %-reduced lactate levels suggest stimulation of butyrate synthesis. However, this enhancement is temporary, requiring periodic biochar replacement and bioreactor reinoculation. Although biochar promotes biofilm formation, excessive growth may inhibit activity. Furthermore, it has little influence on pH buffering but effectively adsorbs toxic metals, e.g., chromium. Overall, coconut-derived biochar is a promising but short-lived enhancer of hydrogen fermentation.