Abstract
BackgroundHydrothermal pretreatment of lignocellulosic biomass such as rice straw can dissolve part of the lignin and hemicellulose into a liquid fraction, thus facilitating enzyme accessibility to cellulose in bioethanol production process. Lignin is awaited to be recovered after hydrothermal pretreatment for utilization as value-added chemical, and lignin recovery also means removal of fermentation inhibitors. To recover lignin with high content from the liquid fraction, it is necessary to separate lignin and hemicellulose-derived polysaccharide. Therefore, the following processes were applied: membrane separation with nanofiltration (NF) and enzymatic hydrolysis by hemicellulase. To clarify lignin-concentrated fraction obtained during these processes, the fates of lignin and polysaccharide components were pursued by a solution NMR method and confirmed by compositional analysis of each fraction.ResultsAfter hydrothermal pretreatment of rice straw, the NF concentrate of the supernatant of liquid fraction was hydrolyzed by hemicellulase and the resulting black precipitate was recovered. In this black precipitate, the intensity of NMR spectra related to lignin aromatic regions increased and those related to polysaccharides decreased, compared to rice straw, the solid fraction after hydrothermal pretreatment, and the NF concentrate. The lignin content of the black precipitate was 65.8 %. Lignin in the black precipitate included 52.9 % of the acid-insoluble lignin and 19.4 % of the soluble lignin in the NF concentrate of supernatant of liquid fraction.ConclusionA precipitate with high lignin content was obtained from supernatants of the liquid fraction. These results suggested that precipitation of lignin was enhanced from concentrated mixtures of lignin and hemicellulosic polysaccharides by hydrolyzing the polysaccharides. Precipitation of lignin can contribute to lignin recovery from lignocellulosic biomass and, at the same time, allow more efficient ethanol production in the subsequent fermentation process.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0273-4) contains supplementary material, which is available to authorized users.
Highlights
Hydrothermal pretreatment of lignocellulosic biomass such as rice straw can dissolve part of the lignin and hemicellulose into a liquid fraction, facilitating enzyme accessibility to cellulose in bioethanol production process
The supernatant of the enzymatically hydrolyzed NF concentrate was filtered through UF to remove high molecular weight (≥150,000 Da) fermentation inhibitors
The components related to lignin and polysaccharides of these fractions were analyzed in detail following 2D nuclear magnetic resonance (NMR)
Summary
Hydrothermal pretreatment of lignocellulosic biomass such as rice straw can dissolve part of the lignin and hemicellulose into a liquid fraction, facilitating enzyme accessibility to cellulose in bioethanol production process. Technology to utilize lignocellulosic biomass, including pretreatment, enzymatic hydrolysis, and fermentation by specialized organisms, has been developed [9,10,11]. In this process, the remaining lignin in lignocellulosic biomass should be recovered [12], to allow utilization of lignin as a highvalue-added product in such forms as polymers and carbon fibers [13]. Recovery of lignin means removal of fermentation inhibitors, because high concentrations of lignin reduce bioethanol production [14]
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