Photofermentative biohydrogen production by Rhodopseudomonas palustris: Assessing agro-industrial residual carbon sources for circular biorefineries.

The conversion of agro-industrial residues into biohydrogen (BioH2) represents a promising strategy for waste valorization and renewable energy generation. This study evaluated cashew apple bagasse-derived streams, including enzymatic hydrolysate (CAB-EH) and fermentation vinasse (CABV), as feedstocks for photofermentative H2 production by Rhodopseudomonas palustris ATCC 17001. Cheese whey (CW) was included as a comparative substrate. R. palustris consumed 22.6 ± 0.2 % of the lactose present in CW and produced 1.4 ± 0.4 mmol H2⋅g-1 carbon. In comparison, CAB-EH resulted in the highest volumetric production, reaching 1654.2 ± 37.7 mL H2⋅L-1 medium, corresponding to a BioH2 yield of 10.6 ± 3.0 mmol H2 g-1 carbon and a carbon consumption of 81.2 ± 0.7 %. Furthermore, CABV produced 50.0 ± 1.1 mL H2⋅L-1 medium, demonstrating its potential as a feedstock for H2 production despite residual ethanol. Based on initial feedstock mass, CW provided the highest BioH2 yield based on the initial feedstock mass, reaching 8910.0 L H2⋅t-1. These findings demonstrate, for the first time, the feasibility of converting cashew apple bagasse-derived stream into BioH2 through photofermentation, highlighting their potential for integrated residue valorization.