Delignification of the Paddy Husk Using Binary and Ternary Deep Eutectic Solvents

Pretreatment of lignocellulosic biomass is crucial to make the biomass structure suitable for the hydrolysis step and its conversion into monomers. Pretreatment with deep eutectic solvents (DESs) is economical, nontoxic, and environmentally friendly, as DESs are biodegradable. In this study, pretreatment and delignification of paddy husk using binary and ternary DES were investigated. Two- and three-component DES systems consisting of citric acid monohydrate (CA), ethylene glycol (EG), choline-chloride (ChCl), and 1-butanol (1but) were prepared. Pretreatment experiments were carried out with EG/CA, EG/ChCl, CA/ChCl, EG/CA/ChCl, and EG/CA/1but DES systems at 110°C for 2 and 4 h. Cellulose, hemicellulose, and lignin percentages were determined by NREL/TP-510-42618 method. In addition, structural changes and morphological structures of pretreated samples were examined by Fourier-transform infrared spectroscopy (FT-IR), x-ray diffraction (XRD), and scanning electron microscopy (FE-SEM). Enzymatic hydrolysis process was applied to pretreated samples, and conversion percentages were calculated. The highest lignin removal was obtained from paddy husk pretreated with EG/CA/1but for 2 h, as 60.58%. It was observed that the lignin removal efficiencies of ternary DES systems were slightly higher than those of binary systems. Lignin removal in the range of 47.72%-60.58% was achieved with the DES systems used in this study. In addition, it was observed that the enzymatic hydrolysis efficiencies were proportional to the lignin removal percentages. The results showed that EG/CA/1but DES systems can be used for efficient delignification and enzymatic hydrolysis of paddy husks.