Biobleaching of loblolly pine kraft pulp with Trametes trogii culture fluids followed by a peroxide stage. Application of Doehlert experimental design to evaluate process parameters

Loblolly pine kraft pulp was bleached in a totally chlorine-free sequence that involved treatment with culture supernatants from the white-rot fungus Trametes trogii followed by a peroxide stage. The whole process was performed at 28 °C, and did not require mediator addition in the enzymatic stage. Different operating conditions in the peroxide stage (pH, peroxide concentration and treatment time), were tested by using response surface methodology based on a Doehlert experimental design, in order to describe their effects and normalize a biobleaching protocol. The results showed that all three independent variables had significant effect on the luminance (L*) and Chroma (C*) of the enzyme-treated pulp. Best results were obtained after 1 h of enzyme incubation (352 U laccase, 2 U Mn-peroxidase per g of oven-dry pulp), followed by 96 h treatment with 2.5% hydrogen peroxide in sodium succinate buffer pH 6 (5% consistency). We obtained a noteworthy increase in L* = 94.45 (compared with 94.5 of the white reference standard (titanium oxide), 69.94 of the initial pulp, and 83.11 of the peroxide-bleached control), a decrease in C* (9.85), with minor pulp yield loss (less than 5%), under essentially mild conditions, using a low-cost source of enzyme.