Renewable wood resources in our Nation’s forests are poised to play a major role in the future bioeconomy, both as a feedstock for biorefineries producing energy, chemicals, and fuels, as well as, continuing to be the basis for wood-based construction materials. However, achieving the full potential of wood resources is hindered by an incomplete understanding of many fundamental properties, including how chemicals are transported through wood cell walls. For example, diffusion constant measurements of inorganic ions moving through wood cell walls at moisture levels below water-saturation are completely lacking.
To provide this needed information, we developed synchrotron-based X-ray fluorescence microscopy (XFM) as an experimental tool to measure time-dependent concentration profiles of inorganic ions diffusing through individual loblolly pine (Pinus taeda) cell wall layers. Ions were locally applied using either KCl or CuCl2 aqueous salt solution microdroplets and experiments were performed under 70%, 75%, or 80% relative humidity (RH). An analytical model based on Fick’s second law for diffusion was used to calculate diffusion constants.
Results revealed that diffusion rates increased with RH, the larger Cu2+ diffused more slowly than the K+, and the Cl- diffusion constant was the same as that for the counter cation, indicating cations and anions diffused together to maintain charge neutrality. This more complete, quantitative description of diffusion in wood cell walls will enable optimization of cell wall diffusion processes for specific end uses and accelerate utilization of wood resources to meet our societal needs.