Green Biphasic Pretreatment Using Phenoxyethanol Coupled with Citric Acid: Molecular Mechanisms Driving Lignin Removal and Enhanced Cellulase Hydrolysis of Bamboo Residues

Green pretreatment strategies that enhance cellulose hydrolysis while maintaining material sustainability are essential for lignocellulosic biorefineries. Herein, bamboo residues were pretreated with a phenoxyethanol coupled citric acid biphasic system, and the mechanisms governing lignin removal and enzymatic hydrolysis were systematically elucidated. Results showed that increasing citric acid concentration produced a pronounced synergistic effect, achieving lignin removal of 67.3% and hemicellulose solubilization of 60.4%, while maintaining a high cellulose recovery of 95.5% at 160 °C, 10% citric acid concentration. However, the pretreated residues’ enzymatic hydrolysis yield initially increased and then declined. Kinetic adsorption analysis revealed that this nonmonotonic behavior was directly associated with changes in the nonproductive binding of residual lignin to cellulase. At a moderate citric acid concentration (10%), the lignin-cellulase binding rate constant decreased from 7.6 × 10 3 to 3.5 × 10 3 M –1 s –1, whereas further acid addition led to a rebound to 6.4 × 10 3 M –1 s –1 . MD and DFT calculations showed that phenoxyethanol interacted with lignin mainly via hydrogen bonds and van der Waals forces. The addition of citric acid increased system polarity and hydrogen-bond acceptor sites, enhancing phenoxyethanol-lignin interactions, and promoting lignin depolymerization and solubilization. This work provides mechanistic insights and a green strategy for efficient bamboo pretreatment.