Improvement of kraft pulp bleaching by treatments with laccase, urea, and refining

Unbleached Eucalyptus kraft pulps were treated by three methods and the bleaching effects have been evaluated. The treatments were performed by a laccase-mediator system (LMS), refining (R), and urea (U) alone and in combination of these methods. The bleached pulps were characterized in terms of kappa number, hexenuronic acid content, brightness, and viscosity. In addition, the lignin-carbohydrate complexes (LCCs) were fractionated and characterized with regard to the fractionation yield, lignin content, and carbohydrate composition. Moreover, the H-1 nuclear magnetic resonance spectra were recorded and the thioacidolysis-gas chromatography and thioacidolysis-size exclusion chromatography analyses were performed. The LMS treatment resulted mainly in a slightly better delignification as manifested by a kappa number (K no.) reduction of 0.8 units. The effects of the U treatment included delignification (1.4 kappa units), hexenuronic acid removal (3 mu mol g(-1)), and improved pulp brightness (3.4 ISO units). The R treatment darkened the pulp (3 ISO units), whereas the bleaching of the R-pulp by subsequent LMS or U treatment enhanced the brightness (0.5 unit more) or delignification (0.3 unit more). The residual lignin in the pulp samples was present mainly as xylan-lignin. The Klason lignin content from the xylan-lignin fraction decreased as the degree of delignification increased. The pulp darkening followed by the R treatment was interpreted as the result of the formation of condensed lignin structures, but these condensed lignins were substantially removed by the subsequent LMS or U treatment.