Exploring the varied effects of lignin modification during deep eutectic solvents pretreatment on enzymatic cellulose hydrolysis

While the mechanism for extracting lignin from biomass using deep eutectic solvent (DES) pretreatment has been widely investigated, few studies have specifically identified the grafting of hydrogen bond donors from DES onto the lignin surface during pretreatment and examined the effect of lignin modification on enzymatic cellulose hydrolysis. This study combines choline chloride (CC) with lactic acid (LA), oxalic acid (OA) and glycerol (Gly) to treat lignin. Structural characterization confirmed the incorporation of LA and OA via esterification, and Gly via etherification, onto lignin surface. Enzymatic hydrolysis results indicated that CC-OA-treated lignin strongly inhibited hydrolysis, with glucose yields ranging from similar to 30 % to similar to 46 %. In contrast, CC-LA- and CC-Gly-treated lignin exhibited weaker inhibition, with glucose yields varying from similar to 50 % to similar to 59 % for the former and similar to 47 %- similar to 60 % for the latter, compared to untreated lignin with glucose yields of 51.25 % and 38.95% at two loadings. Notably, a CC-Gly-treated lignin, with lower loading, resulted in glucose yields ranging from 57.02 % to 60.22 %, showing no inhibition on cellulose hydrolysis compared to the control without lignin addition (57.87 %). Physicochemical analyses revealed variations in water contact angle, surface charge, and total phenolic hydroxyl content among DES-treated lignin samples, affecting cellulase-lignin interactions-specifically hydrophobic, electrostatic, and hydrogen bonding. This study clarifies the relationship between DES composition, lignin modification, and enzymatic cellulose hydrolysis efficiency, providing valuable insights into the effects of DES- induced lignin modifications on cellulose saccharification and addressing a significant gap in the literature.