Extraction and characterization of lignin from lignocellulose biomass using citric acid and fructose-based NADES

Lignin, an important biopolymer in lignocellulosic biomass, is key to sustainable material applications and advances in biorefinery. Over the past decade, deep eutectic solvents (DES) have appeared as environmentally friendly solvents for lignin extraction, with choline chloride-based DES being widely used. Choline chloride, though biodegradable, is synthesized using hydrochloric acid, making it not fully organic. In this study, a 100 % organic, natural deep eutectic fruit-based solvent (NADES) composed of citric acid and fructose was used for the first time for lignin extraction. An acid-free lignin recovery protocol was also developed, eliminating the need for the traditional acid precipitation method and bringing the entire process in line with green chemistry principles. NADES were produced at different molar ratios and pH values to optimize the lignin dissolution. At pH 1.4, a maximum yield of 9.50 % by weight and a recovery rate of 44.10 % were achieved. Scanning electron microscopy and elemental analysis confirmed the removal of non-lignin components, whereas ultraviolet spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction confirmed the enrichment of lignin. The aromatic structure of lignin was preserved, ensuring high purity and no degradation of the aromatic structure. Thermogravimetric and differential scanning calorimetry analyses showed the thermal stability of the extracted lignin, while the activation energy (E_a) and elemental analysis indicated carbon enrichment. In this study, a novel lignin extraction method using 100 % organic fruit-based NADES combined with an acid-free recovery process is presented, which represents a significant advancement in this field.