Investigation of pretreatment parameters in the delignification of paddy husks with deep eutectic solvents

Abstract In the present study, the delignification of paddy husks was carried out with ethylene glycol (EG)/citric acid (CA) deep eutectic systems (DES). The effects of the molar ratio of DES components, pretreatment temperature and pretreatment time on the lignin removal efficiency were investigated. The approximate quantitative analysis, ATR/FT-IR spectra and SEM-EDS analysis of untreated and pretreated samples were used to determine the lignin removal from paddy husks. The results obtained with ATR/FT-IR spectra and approximate analysis were determined to be consistent and supported the removal of lignin from the structure. The analysis results showed that the silica, which gives chemical strength to the structure, was also removed from the structure. Using DES systems with EG/CA molar ratios of 1:1, 1:2 and 2:1, lignin removal percentages at 90 °C were found to be 47.10%, 52.35% and 51.17%, respectively. The obtained results showed that two important parameters affecting the removal of lignin from the paddy husk are the pretreatment temperature and time. It was concluded that at high temperatures, intermolecular hydrogen bonds are weakened, the viscosity of the DES solution decreases, DES penetrates the lignocellulosic structure easily and thus the lignin removal efficiency is increased. The highest lignin removal percentage was found as 57.33% after 4 h of pretreatment time at 120 °C. The results showed that efficient delignification of paddy husks is possible using EG/CA DES systems.