Objective The pith-ring zone of bamboo (Phyllostachys edulis) is often treated as processing waste due to its high susceptibility to shrinkage and deformation during drying. However, its high porosity and unique cell wall architecture also endow it with considerable potential as a natural functional material. This study systematically investigated the effects of three typical dehydration treatments (air drying, oven drying, and freeze drying) on the moisture sorption properties of bamboo pith rings, and further elucidated the underlying mechanisms in relation to microstructural evolution, aiming to provide a theoretical basis for upgrading bamboo pith rings from low-value residues into controllable hygroscopic materials.

Method Bamboo pith rings were subjected to air drying (20 ℃, 65%RH), freeze drying (–56 ℃, 36 h), and oven drying (60 ℃, 12 h). Dynamic vapor sorption (DVS) was employed to characterize the moisture adsorption behavior. The Hailwood-Horrobin (H-H) and Guggenheim-Anderson-de Boer (GAB) models were applied to fit the adsorption parameters and clarify the evolution of water states within the pith rings. Furthermore, small-angle neutron scattering (SANS) and small-/wide-angle X-ray scattering (SAXS/WAXS) were combined to reveal the relationship between the nanostructure of the pith rings and water adsorption.

Result DVS analysis indicated that the three dehydration methods had no significant effect on the monolayer water content of the cell wall but significantly altered the moisture transport rates. Oven-dried samples exhibited the fastest moisture sorption rate and the highest equilibrium moisture content under high-humidity conditions, whereas freeze-dried samples showed the slowest sorption rate and the most pronounced hysteresis. Air-dried samples exhibited intermediate behavior. WAXS results confirmed that none of the drying treatments altered the cellulose I crystal structure, suggesting that crystallinity was not the primary factor governing the sorption differences. SANS/SAXS analysis revealed that dehydration primarily modified the water transport pathways by regulating the aggregation state of cellulose microfibrils and the connectivity of nanopores. Oven drying induced the largest microfibril correlation length (approximately 48.4 Å), forming the loosest nano-network structure, while freeze drying led to local microfibril agglomeration, hindering water diffusion. Air drying resulted in the most compact structure, with the smallest correlation length (approximately 35.9 Å).

Conclusion Dehydration treatments predominantly influence the moisture sorption behavior of bamboo pith rings by modulating the aggregation state of cellulose microfibrils and the connectivity of nanopores, rather than altering the basic crystal structure. Oven drying induces a relatively loose arrangement of microfibrils, thereby enhancing the moisture sorption capacity. Although freeze drying increases porosity, its improvement on sorption kinetics is limited. Air drying forms a relatively dense cell wall layer, resulting in an intermediate sorption rate. This study clarifies the regulatory mechanism of dehydration on the microstructure and water interaction in bamboo, providing theoretical guidance for the design of bio-based functional materials and the high-value utilization of bamboo.