Cell-wall chemistry governs anisotropic shrinkage during wood carbonization: roles of polysaccharide degradation and exchangeable cations

The anisotropic shrinkage of wood during carbonization critically affects biochar microstructure, but its underlying mechanisms remain unclear. We examined shrinkage behavior in the radial (R) and longitudinal (L) directions in relation to polysaccharide thermal degradation, exchangeable cation species, and cell structure, using ten wood species (five softwoods and five hardwoods) and their cation-exchanged derivatives (H + , Na + , K + , Mg 2+ , Ca 2+ ). Thermogravimetric and continuous dimensional analyses revealed that the onset of shrinkage in the R-direction is primarily associated with hemicellulose degradation, whereas shrinkage in the L-direction is linked to cellulose degradation and occurs rapidly at temperatures 10–20 °C higher than the derivative thermogravimetric peak, when most cellulose microfibrils have already decomposed. Na + and K + lowered shrinkage onset temperatures across most species, whereas in hardwoods, R-direction shrinkage magnitude increased with H + and Mg 2+ substitution. Anatomical differences, particularly in ray parenchyma morphology, explained why hardwoods generally shrank more in the R direction. These results demonstrate that anisotropic shrinkage during wood carbonization is governed by polysaccharide-specific thermal degradation, modulated by exchangeable cation species and anatomical features. Furthermore, directional shrinkage can be predictably tuned through cation control and species selection to optimize biochar microstructure.