Valorization of Rice Straw for Ethanol Production and Lignin Recovery Using Combined Acid-Alkali Pre-treatment

Rice straw was sequentially pre-treated with sulphuric acid (3% v/v) and sodium hydroxide (4% w/v) which resulted in the removal of ~ 90% hemicelluloses and ~ 55% lignin, respectively. The pre-treated rice straw was saccharified with cellulases to produce 787 mg/g reducing sugars, which were further fermented with Saccharomyces cerevisae HAU. The acid hydrolysate produced after acid treatment was also subjected to fermentation by Pichia stipitis NCIM 3499. The hexose and pentose fermentation leads to production of 26.9 g/L and 9.4 g/L of ethanol (79% and 93% fermentation efficiency, respectively). High purity (98%) lignin (146 mg/g dry substrate) was precipitated by direct acidification with HCl from the spent liquor obtained after alkali treatment. Extracted lignin was characterized by FTIR and TGA. Overall, the study aimed at effective utilization of all major polymers of rice straw into valuable products making the bioethanol production process more sustainable.