Selective Extraction of Second-Generation Sugars by Sequential Acid and Alkali Pretreatment of Arundo donax for Xylitol and Ethanol Production by Candida tropicalis

Arundo donax is a suitable lignocellulosic feedstock for the production of bio-based chemicals due to its low economic value, rapid growth in low-nutrient soils, and rich structural composition. The present study aimed to selectively extract hemicellulosic and cellulosic sugars from A. donax biomass by using dilute acid and alkali pretreatment as well as utilize those sugars for biological production of xylitol and ethanol by Candida tropicalis. Dilute acid pretreatment with 1.5% sulphuric acid effectively released 22.14 g/L of xylose in the hydrolysate, while, 1 M sodium hydroxide treatment removed 77.9% of the total lignin present in the biomass. The crystallinity index of A. donax biomass was reduced from 65.89 to 61.39% after alkali pretreatment. Dilute acid hydrolysate was detoxified by activated charcoal treatment that completely removed 5-HMF, whereas, 92.5% and 18.99% removal of furfural and acetic acid was achieved. The maximum xylitol yield and volumetric productivity in A. donax hydrolysate were 0.54 g/g and 0.274 g/L/h, respectively after 96 h of incubation. In the parallel process, enzymatic hydrolysis of cellulose-rich fraction of A. donax resulted in a glucan conversion of 85.66%. Subsequent ethanol fermentation in A. donax saccharified broth by C. tropicalis produced a yield and productivity of 0.47 g/g and 0.63 g/L/h, respectively. The present study concludes that sequential dilute acid and alkali pretreatments of A. donax were efficient in extracting most of the xylose and glucose from the biomass. These pretreatment methods can be successfully employed for maximum utilization of this low-value biomass for biological production of value-added chemicals including ethanol and xylitol by C. tropicalis.