Enhanced ethanol production by Saccharomyces cerevisiae fermentation post acidic and alkali chemical pretreatments of cotton stalk lignocellulose

Abstract

Direct conversion of lignocellulose to biofuel without pretreatment always results in a low-ethanol yield owing to its highly rigid structure. The current study was performed to improve the effectiveness of two different chemical pretreatments, alkaline and acidic, prior to the enzymatic hydrolysis of cotton stalk, and the yeast fermentation process for ethanol production. Alkaline pretreatments used alkaline hydrogen peroxide (AHP) and sodium hydroxide (NaOH), while acidic pretreatments used sulfuric acid (H2SO4) and phosphoric acid (H3PO4) at concentrations of 1.0%, 3.0%, 5.0%, and 7.0%. The highest bioethanol production (3.956 g/L) was observed in the 1.0% AHP pretreated sample, while the reducing sugar yield after enzymatic hydrolysis was 178 mg/g. The highest reducing sugar yield after enzymatic hydrolysis was obtained from the samples pretreated with 7.0% NaOH and 7.0% AHP, which yielded 241 mg/g and 238 mg/g, respectively, and provided 3.798 g/L and 3.739 g/L of ethanol, respectively. Scanning electron microscopy and Fourier-transform infrared analyses of the biomass showed that the structure of the alkali-treated cotton stalk was more disrupted and distorted than the acid-treated cotton stalk. Therefore, alkaline chemical pretreatment is more effective for breaking down lignocellulose and enhancing the yield of reducing sugars and bioethanol production from cotton stalk.