Diffusive Mass Transport of Ions in Wood

Doctoral thesis, 2016

Understanding the conditions that control the mass transfer of various chemicals/ions in wood is of great importance when it is subjected to various processes. Homogeneous impregnation with chemicals/ions, for example, increases the uniformity of the treatments to which the wood is subjected, reduces reaction times and may increase the yield of the final products. Wood material is porous and anisotropic and the constituents of the cell wall (solid) are composed of a variety of functional groups, so the properties of the cell wall may influence not only the mass transport of chemicals through the pores in the wood but also how these chemicals interact with the cell wall’s components. The mass transports involved when wood material is impregnated with chemicals are advective (penetration) and diffusive (diffusion). The latter is a complex mass transfer with several processes that may involve the diffusion of chemicals through the cell pores (lumen and pit pores)and through the cell walls at certain conditions, as well as sorption at solid surfaces. Although several concepts and theories currently exist, some aspects of this type of mass transfer remain unclear. The aim of this work is to develop a methodology to investigate the diffusive mass transport of ions in wood, combining experimental and modelling work. Experiments were performed on Norway spruce in the form of wood flour, pieces of wood and isolated components of wood. An experimental methodology was developed to measure the concentration profiles of cations through the porous structure of a piece of wood that considered the effect of sorption of the ions onto the matrix of the solid wood. In order to differentiate the amount of ions distributed between the solid wood and the external solution located in the wood pores (partition coefficient), additional experimental methodology was developed, using wood flour samples, to investigate the sorption of ions onto wood. The diffusion accessible porosity of the wood material was estimated from the intrusion pore volume measured by mercury (Hg) porosimetry and the solid volume of the cell wall was determined using a helium pycnometer. The impregnation of Norway spruce wood with lithium chloride (LiCl) was investigated in this study. Spectroscopic analysis (XPS, FTIR andAAS) was employed to investigate the potential interactions that occur on the surfaces of the wood material upon LiCl treatment. Using the concentration profiles, the partition coefficients and porosities measured, the effective diffusion coefficients along with tortuosity factors were estimated, using a transport model which was developed in COMSOL® multiphysics modelling software. The findings in this thesis showed that, for the experimental conditions chosen, the methods developed gave reasonable results. However, defects in the pieces of wood (micro-cracks remained and were detectable. It was observed that the sorption of Li+ ions in Norway spruce wood flour was a spontaneous process that probably involved several types of interaction/bonding between the various functional groups in the cell wall of the wood and the Li+/Cl- ions; interactions with functional groups containing oxygen, for example, are identified as Li-O interactions.