Synergistic Use of Thermostable Laccase and Xylanase in Optimizing the Pre-Bleaching of Kraft Pulp

Abstract

The continuous requirement for pre-bleaching processes on kraft pulp, employing a range of compatible enzymes, aims to mitigate the pollution caused by chemical bleaching agents. In the present study, the laccase-producing bacterium Bacillus licheniformis BK-1 was isolated from the Bakreshwar hot spring in India and tested for laccase production using different lignocellulosic substrates. The isolate was found to produce maximum laccase (8.25 IU/mL) in the presence of rice bran as a substrate, followed by 5.14 IU/mL using sawdust over a 48 h period. Laccase production doubled when medium parameters were optimized using a central composite design. The bleaching of rice straw pulp was accomplished using a laccase, xylanase (previously extracted from the same bacteria), and laccase–xylanase mixture. The mix-wood kraft pulp treated with the enzyme mixture at pH 7.0 and 50 °C temperature for up to 180 min reduced the chlorine amount by 50% compared to the control. The results also revealed that the enzyme mixture improved the pulp’s optical (brightness 10.39%) and physical (tear index 39.77%, burst index 22.82%, and tensile strength 14.28%) properties with 50% chlorine dose. These exceptional properties underscore the enzyme mixture’s suitability for pulp pre-bleaching in paper manufacturing, offering a safer and more environmentally friendly process.