Exploring the enzymatic parameters for optimal delignification of eucalypt pulp by laccase-mediator

Pycnoporus cinnabarinus laccase was compared with commercial laccases from Trametes villosa and Myceliophthora thermophila in terms of stability and mediator oxidation rates. Because of its high thermal stability and efficiency oxidizing 1-hydroxybenzotriazole (HBT), the P. cinnabarinus laccase was selected for totally chlorine free (TCF) bleaching of paper pulp, using HBT as mediator. Inactivation of laccase by HBT (50% in 4 h) decreased 20% in the presence of eucalypt kraft pulp. Laccase–HBT delignified (four points-decrease of kappa number with respect to the control) and bleached eucalypt pulp (6% ISO brightness increase) and subsequent alkaline treatment with hydrogen peroxide enhanced pulp brightness in 16% ISO. Short enzymatic treatment applied between oxygen delignification and peroxide bleaching stages enabled to obtain a final pulp with 90.3% ISO brightness and kappa number 5. The most significant improvement of pulp properties was produced during the first 2 h of laccase–HBT treatment, and no differences between enzyme performances at 65 and 50 °C were seen. Determination of hexenuronic acids, which contributed up to 50% to kappa number value after laccase-mediator treatment of eucalypt kraft pulp, provided a more realistic estimation of the final delignification rate that attained 81%. FTIR analysis of residual lignins from the treated pulps revealed strong modifications during laccase–HBT treatment.