Surface characterization of stepwise oxidized spruce thermomechanical pulp samples by different analytical methods

An investigation of the fiber samples with various amounts of lignin, hemicelluloses and cellulose was conducted by means of dye adsorption, spectrometric and chemical analyses. Five fiber samples were prepared by the oxidation of unbleached Norway spruce thermo-mechanical pulp with acidic potassium permanganate solution. Each sample was oxidized with defined amount of KMnO₄ and the degree of oxidation was determined as kappa number. Fibers were additionally characterized by conductometric titration, the determination of hemicelluloses and infrared spectrometry. Sorption of various types of dye, namely Methylene Blue (MB), Crystal Violet (CV) and Astra Blue (AB) on the fiber surface was studied and compared to the kappa number and other fiber characteristics. The adsorption of MB and CV is found to follow the amount of anionic (carboxylic) groups, which at first increase and later decrease toward the final stages of oxidation. The behavior of AB is nearly the opposite, its adsorption increases with the depletion of lignin and carboxylate groups, pointing to a different mechanism of AB binding on fibers.