An improved TCF sequence for biobleaching kenaf pulp: Influence of the hexenuronic acid content and the use of xylanase

•LMS system was incorporated to an industrial-type bleaching sequence, LmediatorQPo.•LHBTQPo sequence was very effective in bleaching kenaf pulp.•LHBTQPo-treated kenaf fibers retained high cellulose content.•Recalcitrant HexA had negative effects in bleaching kenaf pulp.•Xylanase applied after LMS system moderately removed HexA content. Enzymatic delignification with laccase from Trametes villosa used in combination with chemical mediators (acetosyringone, acetovanillone and 1-hydroxybenzotriazole) to improve the totally chlorine-free (TCF) bleaching of kenaf pulp was studied. The best final pulp properties were obtained by using an LHBTQPo sequence developed by incorporating a laccase-mediator stage into an industrial bleaching sequence involving chelation and peroxide stages. The new sequence resulted in increased kenaf pulp delignification (90.4%) and brightness (77.2%ISO) relative to a conventional TCF chemical sequence (74.5% delignification and 74.5% brightness). Also, the sequence provided bleached kenaf fibers with high cellulose content (pulp viscosity of 890g·mL−1 vs 660g·mL−1). Scanning electron micrographs revealed that xylanase altered fiber surfaces and facilitated reagent access as a result. However, the LHBTX (xylanase) stage removed 21% of hexenuronic acids in kenaf pulp. These recalcitrant compounds spent additional bleaching reagents and affected pulp properties after peroxide stage.