Unlocking Lignin from Date Fruit Pomace: Comparative Extraction Using Organosolv, Alkaline, and Deep Eutectic Solvents

Date fruit pomace (DFP) is a plant biomass that can be valorized through lignin extraction and is an underutilized polymer with significant potential. This study focused on the extraction of lignin from DFP using organosolv, alkaline, and deep eutectic solvent (DES; choline chloride–lactic acid). Response surface methodology was used to optimize extraction conditions. The physicochemical and thermal properties of lignin and solid residues were evaluated. Under optimal conditions organosolv produced lower lignin yield (32 %) than alkaline (44 %) or DES (40 %). However, lignin solubilized using organosolv had higher purity (78 %), contained more guaiacyl and syringyl units, was more amorphous, had low molecular weight and smaller fragments, and exhibited lower thermal stability compared to lignin from alkaline and DES extractions. Organosolv also preserved the cellulose chains in the residue better than alkaline and DES. Notably, the lignin solubilized using DES and residual lignin in the residue appeared to form a strong association with protein. Overall, in lignocellulose complexed with proteins, organosolv produces purer lignin and exerts less effect on the residual cellulose chains, providing a relatively pure lignin and energy-dense residues for thermochemical conversion, whereas alkaline and DES produce carbohydrate- and protein-rich fractions that are potential substrates for downstream bioenergy processes.