The effect of enzymatic pre-treatment of recycled pulp with the combined sequence of cellulase and laccase in hydrogen peroxide bleaching

Abstract
Background and Objectives:
The recycling of different paper and paperboard products can supply the major shortage of cellulosic raw materials needed for the production of pulp and paper, and it can be important from the economic and environmental point of view. Using recycled pulp in papermaking reduces the quality of the final paper. Therefore, the use of enzyme pre-treatment separately or combined sequences as well as pulp bleaching can be effective in improving the quality properties of the final paper obtained from recycled pulp. This research was conducted with the aim of investigating the effect of enzyme pre-treatment with cellulase and laccase sequence on the bleachability of the resulting pulp with hydrogen peroxide, and the quality of the final pulp after bleaching system were investigated.

Materials and Methods:
At first, the mixture of waste newspaper and magazine papers were deinked using enzyme pre-treatment (combined sequence of cellulase and laccase) as well as conventional chemical method. Enzymatic pre-treatment was done as experimental treatments C1L1 (cellulase usage: 0.1% and duration time: 10 minutes; laccase usage: 20u and duration time: 120 minutes), C1L2 (cellulase usage: 0.1% and duration time: 10 minutes; laccase usage: 40u and duration time: 120 minutes), C2L1 (cellulase usage: 0.1% and duration time: 20 minutes; laccase usage: 20u and duration time: 120 minutes) and C2L2 (cellulase usage: 0.1% and duration time: 20 minutes; laccase usage: 40u and duration time: 120 minutes) under specific process conditions. Also, control pulp (without adding chemicals or enzymes) and chemically deinked pulp samples were also prepared and then bleaching of deinked pulp was accomplished using hydrogen peroxide bleaching system including 2% hydrogen peroxide, 2% sodium hydroxide, 2% sodium silicate, 0.2% DTPA chelating agent and 0.1% magnesium sulfate in separate plastic bags under process conditions of 10% consistency, 70°C temperature and 2 hours in a water bath. At the end, the optical and strength characteristics of the bleached pulp were evaluated.
Results:
The results of the evaluation of bleached pulp with hydrogen peroxide after deinking with conventional method and also cellulase and laccase enzyme pre-treatment sequences showed that the use of enzyme pre-treatment in deinking improves the overall performance of the hydrogen peroxide bleaching process and resulted in the papers with better optical and strength characteristics after bleaching compared to pulp pre-deinked by conventional chemical method. C2L2 and C2L1 pre-treatments in combined enzymatic treatments after bleaching with hydrogen peroxide led to an improvement in brightness by 11.12% (increase of about 5 units) and 9.5% (increase of about 4 units), respectively. In these pretreatments, the whiteness of the paper increased by 31.92% and 24.54% (an increase of about 9 and 6 units), respectively. Also, the yellowness of the paper decreased by 40.19% and 44.25%, respectively, and the opacity of the paper reduced by 37.74% and 23.97%, respectively. Also, in experimental runs of C2L1 and C2L2, the paper tensile index increased 29.17% and 35.64%, respectively (40.64 Nm/g and 38.7 Nm/g, respectively), breaking length enhanced by 64.42% and 99%, respectively (5.92 km and 4.89 km), and the paper burst index also increased by 70.85% and 92.37% (3.81 mN.m2/g and 4.29 kPa.m2/g), respectively. The tear indices of paper in these runs were determined on average about 6.82 mN.m2/g and 6.62 mN.m2/g, which are a little lower compared to the chemically deinked pulp after bleaching, but in the pretreatment with the combined sequence C2L1, the tear index of paper was almost equivalent to that of chemically deinked pulp.