Biobleaching of Chemical Pulps by Laccase/Mediator Systems, 1997 Biological Sciences Symposium Proceedings

Three different chemical pulps, i.e. a pine kraft pulp, a two-stage oxygen-delignified pine kraft pulp and birch formic acid/peroxyformic acid (MILOX) pulps were used for de lignification studies with lactase in the presence of oxygen and a mediator, which was either HBT (l-hydroxy-benzotriazole) or ABTS [2,2’-azinobis(3-ethylbenzthiazoline-6- sulphonate)]. Neither lactase nor the mediator alone gave any delignification. The oxygen-delignitied pulp was ds lignified using the HBT-mediated lactase system followed by alkaline extraction to kappa number 3.6, which corresponds to a delignification of nearly 60%. Under the same reaction conditions, the delignification was 43% for pine kraft pulp and 24-42% for birch MILOX pulp, depending on the initial kappa number. Compared either on weight basis or on molar basis, HBT was more effective than ABTS in the presence of lactase. Little or no brightness improvement was achieved with the laccase/HBT system, and in some cases the pulp actually darkened. All the pulps darkened significantly during the laccase/ABTS treatments. All the laccase/HBT-treated pulps showed increased reactivity towards alkaline hydrogen peroxide, and as a consequence, oxygen-delignified kraft pulp and MILOX pulp reached full brightnesses (88-90%) with alkaline peroxide in one and two stages, respectively. Xylanase in the bleaching sequence, either before or together with the laccase/HBT stage, slightly improved the final pulp brightness after peroxide bleaching. The results show that different pulps respond differently to the laccase./mediator treatments. This may be due partly to the different amounts of residual lignin present in the pulps, and partly to the different chemical structures of the residual lignins.