Efficient Conversion of Rice Straw to Fermentable Sugars and Bioethanol by Sodium Carbonate Pretreatment
Increasing demand for energy in addition to various environmental concerns intensify the significance of using renewable resources for energy production. Among different renewable resources, biomasses, especially lignocellulosic materials, are the most promising alternative for liquid biofuel production. Bioethanol should be considered as the most convenient biofuel to replace fossil fuels. Lignocellulosic materials, mostly include waste materials, are found in the cell wall of plants and composed of three major polymers: cellulose, hemicelluloses, and lignin. These polymers have strong interaction; event by chemical bonds, and make lignocellulosic materials stiff and inaccessible to various organisms. Cellulose, polymer of glucose monomers, could be enzymatically hydrolyzed to sugars and then microbially fermented to bioethanol. Unfortunately, these materials are recalcitrant to both acid and enzymatic hydrolysis leading to low hydrolysis yield. Because of that, a pretreatment step is necessary prior to hydrolysis to unlock packed and complex structure of these materials and reduce interaction of these polymers and therefore, enhance hydrolysis yield of them. Among various pretreatment methods, chemical pretreatments are more convenient in term of energy consumption and efficiency. In this study, rice straw, as one of the most produced lignocellulose, was treated with sodium carbonate prior to enzymatic hydrolysis and fermentation. All pretreatments were done in a high pressure reactor at 90, 120, 150, and 180°C with sodium carbonate concentration of 0.25, 0.5, and 1 Molar. To eliminate any preheating effect, the initial straw was injected to the reactor at desired temperature and at different time a 50 ml sample was taken. Enzymatic hydrolysis of untreated and all treated straws was conducted at 45°C for 72 h with enzyme loading of 20 FPU/g cellulase and 30 IU/g β-glucosidase. The hydrolysis was done in a shaker incubator with the speed of 100 rpm at 45ºC. Straw treated with Na2CO3 0.5 Molar at 180°C for 120 min was the best sample in term of sugar released. This pretreatment has improved glucose released from 1.8 g/l for untreated straw to 8.7 g/l for treated one. Untreated and some treated straws were subjected to simultaneous saccharification and fermentation to ethanol using s. cerevisiae as fermenting organism. The fermentation performed in a shaker incubator with the speed of 80 rpm at 38ºC. Ethanol production was enhanced from 90.2 g/l for untreated straw to 351.4 g/l for treated one. Lignin removal effect of pretreatment step was investigated by determination of lignin content of all treated samples. Lignin was reduced from 19.2% in untreated straw to 9.4% for treated straw. At low temperatures, lignin removal was the dominant phenomenon during pretreatment. However, at higher temperatures xylan removal was the prominent cause leads to observed improvements. FTIR analysis and SEM image confirm crystallinity reduction and morphological modification effect of pretreatment step as one of the main reason of improved hydrolysis and fermentation. Furthermore, bond absorbance obtained by FTIR also certifies lignin and xylan removal effect of pretreatment step.
Key Words:
Alkaline, delignification, lignocellulose, pretreatment, rice straw, sodium carbonate