Biobleaching of Softwood and Hardwood Kraft Pulp with Manganese Peroxidase

Unbleached softwood kraft pulp (SWKP) and hardwood kraft pulp (HWKP) were biobleached with partially purified manganese peroxidase (MnP), and the bleaching performances were evaluated. No difference was observed between SWKPs and HWKP in the delignification rates by a single MnP treatment. Brightness of HWKP increased with increasing treatment time, whereas the brightness of SWKPs decreased during the first 3 or 6 h and increased more thereafter. Alkaline nitrobenzene oxidation analysis suggested that the oxidation of non-condensed lignin that resulted in the formation of chromophore and the condensation reaction by MnP may lead to greater brightness decreases of SWKPs than of HWKP. Repeated MnP treatments combined with alkaline extraction gave more significant delignification and brightening than did a single MnP treatment. The total effective chlorine required to obtain a pulp of 85% brightness by chlorine-based chemical bleaching after repeated MnP treatments was reduced to 51%, 66%, and 69% for normal-lignin-content SWKP, low-lignin-content SWKP, and HWKP, respectively, compared to the conventional bleaching process without introducing enzyme treatments.