Adsorption of toluidine blue on pulp fibers

How pulp fibers interact with salt is important for fiber swelling, water retention and drainage. The dye toluidine blue is a salt, consisting of a chloride anion and an aromatic cation, which can be easily detected by light absorbance measurements. In distilled water, toluidine blue adsorption on fibers proceeds up to charge stoichiometry, provided the accessible lignin content is below a critical level. Thus, toluidine blue adsorption provides a convenient alternative method to determine the charge of fibers. The adsorption of toluidine blue, on thermomechanical and kraft pulp fibers, was found to depend strongly on pH and ionic strength. The data for thermomechanical pulp (TMP) fibers can be explained by a model in which the dye can adsorb on fibers in two ways: physisorption on lignin and by ion-exchange with cations inside the fiber wall. For fibers with a low charge density, and for ionic strengths larger than about 0.1 mM, an adsorption plateau is reached which corresponds to maximum toluidine blue adsorption on lignin. At lower ionic strengths the adsorption is increased due to ion-exchange up to charge stoichiometry, i.e. the (positive) charge of the adsorbed dye equals the (negative) charge of the fiber wall. For highly charged fibers, more lignin becomes accessible due to fiber swelling, and toluidine adsorption on lignin can exceed stoichiometry, resulting in positively charged fibers. For kraft fibers, which contain very little lignin, adsorption is governed by ion-exchange only. Despite the fact that the fibers are subjected to shear, the rate of adsorption is determined by diffusion, due to the small size of the dye molecules.