Chemical changes in residual lignin structure during peroxyacetic acid delignification of softwood kraft pulp

Peroxyacetic acid delignification of kraft pulp activates the residual lign in which makes it easily removable in the following bleaching stages. In th e present study, an oxygen-delignified softwood kraft pulp was treated with peroxyacetic acid at pH 3.3 and 5.7 to obtain 30% delignification. The res idual lignins of the initial pulp and the two peroxyacetic acid-treated pul ps were isolated (yield 52-59%) and characterized in order to explain the i ncreased lignin reactivity. Neither the methoxyl and phenolic hydroxyl group contents nor the molar mas ses of the residual lignins were affected by peroxyacetic acid treatment an d cannot therefore explain the enhanced lignin solubility and reactivity. O n the other hand, some cleavage of lignin-carbohydrate complexes probably o ccurred during the peroxyacetic acid treatments. The content of conjugated carbonyl groups increased, especially when the peroxyacetic acid treatment was performed at pH 3.3. Furthermore, peroxyacetic acid treatment increased the free and esterified carboxylic acid group contents. These modification s in the chemical structure of residual lignin increased the lignin's solub ility and enhanced the pulp's bleachability with alkaline peroxide.