Significance of xylan and glucomannan in the brightness reversion of kraft pulps

The brightness reversion of kraft pulps has been reported to be caused by the presence of residual lignin, chlorinated extractives, or oxidized carbohydrates. In this work, the effect of hemicellulose content, i.e., xylan and glucomannan, on the thermal stability of modern bleached kraft pulps was studied. Different oxygen-delignified hardwood and softwood kraft pulps were bleached with different bleaching sequences containing chlorine dioxide, hydrogen peroxide, or ozone. Hemicellulose-degrading enzymes, i.e., xylanase or mannanase, were used for selective removal of the respective hemicellulose from the pulps, and the role of partially removed xylan and glucomannan on the brightness stability of these pulps was studied. Due to the structure of kraft xylan, enzymatic removal of xylan also resulted in a decreased carboxyl group content in the pulps, whereas glucomannan removal did not affect the carboxyl group content. By reducing the carboxyl groups in the pulps in conjunction with xylan removal, the thermal aging of the pulps was significantly reduced. The role of glucomannan was less significant. Thus, the uronic acids present in the pulp were found to participate in the brightness reversion of kraft pulps.