Comparing the efficiency of the laccase-NHA and laccase-HBT systems in eucalyptus pulp bleaching

Laccase-mediator systems have the disadvantage that the mediator is expensive and potentially toxic. In this work, we used N-hydroxyacetanilide (NHA) in combination with laccase for the first time to bleach eucalypt pulp and found it to be a very promising, advantageous alternative to 1-hydroxybenzotriazole (HBT) as mediator. Thus, NHA is efficiently oxidized by laccase to a radical that absorbs light at 350 nm. Also. NHA is a better substrate for laccase than is HBT. An innovative result is that the enzyme is inactivated to a similar extent by both mediators under the typical treatment conditions of the bleaching step (L). This adverse effect, however, is strongly reduced in the presence of pulp. Moreover, the laccase-NHA system is as efficient as the laccase-HBT system in reducing the kappa number of eucalyptus pulp. Using a xylanase pretreatment or unbleached pulp boosts kappa number reduction and bleaching with the laccase-mediator system. Based on the results of cyclic voltammetry tests, NHA has a slightly lower redox potential than HBT, which further supports use of the former; also, unlike HBT, NHA is oxidized in a reversible, pH-dependent manner. Interestingly, the laccase-NHA system provides more efficient bleaching of eucalyptus pulp at pH 5 than it does at pH 4. (C) 2010 Elsevier B.V. All rights reserved.