Optimization of a laccase-mediator stage for TCF bleaching of flax pulp

Flax pulp obtained by anthraquinone-soda pulping, resulting in a kappa number of 11.1, a viscosity of 950 ml/g and 36.7 % ISO brightness, was bleached in a totally chlorine-free sequence using the enzyme laccase from the fungus Pycnoporus cinnabarinus and 1-hydroxybenzotriazole (HBT) as redox mediator (stage L), followed by a hydrogen peroxide stage (P). The laccase treatment was optimized using a three-variable sequential statistical plan over the following ranges: 1-20 U/g o.d.p. (oven-dried pulp) laccase dose, 0.5-7.5 % o.d.p. HBT dose and 1-24 It reaction time. The influence of these variables on several pulp properties after the P stage of the LP sequence was examined. The models defined from the results obtained predicted variations in ISO brightness, viscosity and kappa number of 57.6-74.8 %, 590-955 ml/g and 0-6.2, respectively. The variables most strongly influencing these pulp properties were found to be the reaction time and the enzyme dose. A compromise was adopted as regards the operating conditions in order to ensure optimum results. The study was completed by conducting a biobleaching assay in a pressurized reactor (590 kPa) to assess the effect of oxygen pressure. The high pressure level resulted in improved pulp properties by the laccase-mediator system.