Boosting the effect of a laccase-mediator system by using a xylanase stage in pulp bleaching.

Using an enzyme-based stage involving a xylanase (X) or laccase (as part of a laccase-mediator system, L) in a bleaching process can help reduce reagent consumption and hence its environmental impact. In this work, both types of enzymes were applied to eucalypt pulp. The influence of process variables in the laccase-mediator treatment (viz. laccase dose, HBT dose and reaction time) was assessed by using a three-variable sequential statistical plan. The effect of a pretreatment with X on the previous variables was also assessed. Kappa number and brightness models for the L stage and XL sequence were found to perform disparately, which suggests the formation of lignin derivatives interfering with brightness measurements. The L system oxidized readily accessible lignin within the first hours of treatment and affected the contents in cellulose and hexenuronic acids (HexA) of the resulting pulp. Xylanase facilitated access of the laccase-HBT system to lignin and HexA in cellulose fibres. The L treatment increased effluent properties such as Microtox toxicity, COD and colour, and led to strong inactivation of the enzyme. The increased toxicity of the effluents was due to HBT; based on statistical data, however, the effect can be reduced by lowering the mediator dose.