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 H-2 was examined with seven H-2-producing pure strains isolated from a high-rate H-2-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 H-2 fermentation shows that only Cl. butyricum and Klebsiella sp. strains could utilize xylose for H-2 production, while all of them can grow and produce H-2 on sucrose. Among all strains examined, Cl. butyricum CGS5 was the best H-2 producer on xylose with the highest H-2 production rate and yield of 212.5 ml/h/l and 0.73 mol H-2/mol xylose, respectively, taking place at 20 g COD/l of xylose. In contrast, Cl. pasteurianum CH4 was most efficient in converting sucrose to H-2; the highest H-2 production rate (569 ml/h/l) and yield (2.07 mol H-2/Mol hexose) were obtained at a sucrose concentration of 40g COD/l. The substrate preference of the H-2-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 H-2 productivity over static ones, indicating the importance of efficient mass transfer for H-2 production. The dependence of cell growth and H-2 production on carbon substrate concentration could be described by the proposed kinetic models with good agreements. (c) 2007 Elsevier Ltd. All rights reserved.