Dark H₂ fermentation from sucrose and xylose using H₂-producing indigenous bacteria: Feasibility and kinetic studies

Cellulosic materials are the major components in biomass feedstock used for bioenergy production. Hydrolytic products of cellulosic substances consist primarily of hexose (e.g., glucose) and pentose (e.g., xylose). In this study, the efficiency of fermentative conversion of sucrose (representing hexose) and xylose into H₂ was examined with seven H₂-producing pure strains isolated from a high-rate H₂-producing system in our recent work. The isolates were identified as Clostridium butyricum (strains CGS2 and CGS5), Clostridium pasteurianum (strains CH1, CH4, CH5, and CH7), and Klebsiella sp. Batch H₂ fermentation shows that only Cl. butyricum and Klebsiella sp. strains could utilize xylose for H₂ production, while all of them can grow and produce H₂ on sucrose. Among all strains examined, Cl. butyricum CGS5 was the best H₂ producer on xylose with the highest H₂ production rate and yield of 212.5 ml/h/l and 0.73 mol H₂/mol xylose, respectively, taking place at 20 g COD/l of xylose. In contrast, Cl. pasteurianum CH4 was most efficient in converting sucrose to H₂; the highest H₂ production rate (569 ml/h/l) and yield (2.07 mol H₂/mol hexose) were obtained at a sucrose concentration of 40 g COD/l. The substrate preference of the H₂-producing isolates was consistent with the bacterial community structure that existed in the bioreactor, showing that Cl. butyricum and Cl. pasteurianum were predominant in the cultures grown on xylose and sucrose, respectively. Irrespective of the carbon substrate used, butyrate and acetate were the predominant soluble metabolites. Shake-flask cultures displayed higher H₂ productivity over static ones, indicating the importance of efficient mass transfer for H₂ production. The dependence of cell growth and H₂ production on carbon substrate concentration could be described by the proposed kinetic models with good agreements.