Non-steady alkali diffusion-component dissolution coupling during bamboo alkaline pretreatment: A stage-resolved diagnosis.

Alkaline pretreatment is an important process for bamboo component fractionation and biorefinery utilization. However, the radial pore-structure gradient of bamboo often causes non-uniform alkali impregnation and asynchronous component dissolution, thereby limiting delignification selectivity and carbohydrate retention. Scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), laser-induced breakdown spectroscopy (LIBS), and kinetic modeling were combined to investigate bamboo alkaline pretreatment. This approach elucidated the coupling among the radial pore-structure gradient, non-steady alkali diffusion, and cell-wall component dissolution. The results showed that bamboo exhibited a radial pore-size gradient from macropores in the inner region to micropores in the outer region, resulting in depth-dependent and time-decaying alkali diffusion. A time-dependent effective diffusion coefficient (Deff) model based on LIBS data showed that the diffusion activation energy increased from 13.72 to 33.00 kJ·mol-1, indicating that structural shrinkage and pore-channel evolution increased diffusion resistance. The apparent activation energies for lignin and carbohydrate dissolution were 40.03 and 24.88 kJ·mol-1, respectively, suggesting that lignin dissolution dominated cell-wall relaxation in the middle stage, whereas prolonged treatment increased the risks of carbohydrate loss and structural collapse. Accordingly, a three-stage regulation mechanism for bamboo alkaline pretreatment was proposed, consisting of dissolution-limited behavior in the initial stage, lignin dissolution-alkali diffusion coupling control in the middle stage, and diffusion-limited behavior in the later stage. This framework provides stage-specific criteria for pretreatment optimization and may help improve delignification selectivity and component fractionation efficiency in non-wood biomass.