Insights into Lignin Structural Divergence Induced by Alkali–Ethanol Pretreatment and Its Regulating Effect on Cellulase Nonproductive Adsorption

Alkali–ethanol pretreatment effectively removes lignin while preserving cellulose; however, the structural transformations of lignin during this process and their influence on the inhibition of enzymatic hydrolysis remain insufficiently understood. Mitigating the nonproductive adsorption of cellulase enzymes by lignin is of significant industrial importance, as it can reduce enzyme-related costs in large-scale biorefining operations. This study examined lignin fractions derived from alkali–ethanol pretreatment of poplar, specifically black liquor lignin (BL) and pretreatment material milled wood lignin (PMWL), using milled wood lignin (MWL) as a reference. The lignin samples were characterized in terms of molecular weight, functional group composition, surface properties, and cellulase adsorption behavior. BL was found to possess a lower molecular weight, an elevated phenolic hydroxyl content (5.01–5.37 mmol/g), and increased surface polarity. In contrast, PMWL exhibited a higher molecular weight, greater condensation, enhanced hydrophobicity, and a reduced phenolic hydroxyl content (2.56–2.83 mmol/g). Quartz crystal microbalance with dissipation monitoring (QCM-D) analysis demonstrated that BL exhibited the highest levels of nonproductive cellulase adsorption (450–621 ng/cm 2 ), which correlated strongly with phenolic hydroxyl density, suggesting that hydrogen bonding is the predominant interaction mechanism. Despite its hydrophobic nature, PMWL showed comparatively lower cellulase adsorption (391–421 ng/cm 2 ). These findings indicate distinct inhibitory mechanisms: PMWL restricts enzyme accessibility through aggregation and hydrophobic shielding, whereas BL facilitates nonproductive enzyme binding via abundant polar functional groups. Collectively, this study advances the molecular-level understanding of lignin–enzyme interactions and offers a theoretical framework for optimizing pretreatment methods and lignin modification strategies aimed at minimizing enzyme inhibition in lignocellulosic biorefining processes.