- Top of page
- Materials and methods
Aims: Investigation of waste pulps and soda pulp bleaching with xylanase (X) and laccase mediator system (LMS) alone and in conjunction (one after the other) (XLMS).
Methods and Results: Soda and different grades of waste pulp fibres [used for making three-layered duplex sheets – top layer (TL), protective layer (PL) and bottom layer (BL)] when pretreated with either xylanase (40·0 IU g−1) or LMS (up to 200·0 U g−1) alone and in combination (one after the other) (XLMS) exhibited an increase in release of reducing sugars [up to 881·0% soda pulp; up to 736·6% (TL), up to 215·7% (PL) and up to 198·0% (BL) waste pulp], reduction in kappa number [up to 17·6% soda pulp; up to 14·0% (TL), up to 25·3% (PL) and up to 10·9% (BL), waste pulp], improvement in brightness [up to 20·4% soda pulp; up to 23·6% (TL), up to 8·6% (PL) and up to 5·0% (BL), waste pulp] when compared with the respective controls. The usage of XLMS along with 15% reduced level of hypochlorite at CEHHXLMS/EHHXLMS bleaching stage reduced kappa number [5·5% soda pulp; 11·4% (TL), 7·9% (PL), waste pulp] and improved brightness [1·0% soda pulp; 0·9% (TL), 1·4% (PL) waste pulp] when compared with the controls. Scanning electron microscopic studies revealed development of cracks, flakes, pores and peeling off the fibres in the enzyme-treated pulp samples. These modifications of the fibre surface during enzymatic bleaching in turn indicated the removal of lignin and derived compounds from the fibre cell wall.
Conclusions: The work describes synergistic action of xylanase with LMS for bleaching of waste and nonwood pulps for eco-friendly production of paper and thus reveals a new unexploited arena for enzyme-based pulp bleaching.
Significance and Impact of the Study: The drastic improvement in pulp properties obtained after xylanase and LMS treatment would improve the competitiveness of enzyme–based, environmentally benign processes over chemicals both economically and environmentally.
- Top of page
- Materials and methods
The ecological and environmental concerns, market and legislative pressures, wood supply issues, change in agricultural policies and forest land management practices have lead both developed and developing countries to recommend the use of nonwood fibres in paper manufacturing (Roncero et al. 2003; Camarero et al. 2004). The principal advantages of nonwood agricultural fibres (wheat straw, sunflower stalks, flax, hemp, kenaf and jute) are high growth rate, adaptability to various soil types and act as excellent fibres for high-value added paper (Roncero et al. 2003; Sigoillot et al. 2005). Recently developed totally chlorine-free chemical bleaching sequences (using H2O2, O2 and/or O3) often do not confer adequate brightness to these pulps, and Cl2 and ClO2 are being currently used for their bleaching thus alarming environmental concerns (Akin et al. 1996).
Waste pulp, a mixture of pulps obtained from various waste paper types (copy paper, office stationery, printed books, magazines, old news papers, etc.) is de-inked and bleached by chemicals (detergents, fatty acids, dispersants, dithionite, peroxide and chlorine) for making recycled paper and paper board (e.g. duplex sheet which is used for packaging, storage, etc.).
By-products (adsorbable organic halides, chlorinated lignin derivatives, chlorophenols and chlorobiphenyls) generated during conventional bleaching of pulps are toxic, mutagenic, bioaccumulating and are a serious threat to already dwindling eco-systems (Bajpai 1999). Therefore, efforts should be geared towards development of environment-friendly processes for paper manufacturing from nonwood and waste fibres.
The laccase (E.C 1·10·3·2) mediator system (LMS) enhances the delignification response of laccase during biobleaching of pulps by oxidation of lignin moieties, oxidative cleavage of side chains, such as cleavage of Cα–Cβ bonds in lignin and selective oxidation and dehydrogenative polymerization of free phenoxy groups in lignin (Sealey et al. 1997; Bajpai 2004).
Biobleaching and bioprocessing of pulps using a combination of xylanase and laccase further broadens the horizon of enzymes in pulp and paper industry. Once modified, hemicellulose(s) is removed by xylanase, the lignin layer is easily available for penetration and degradative action of laccase (Viikari et al. 1994; Bajpai 2004). The exposed lignin moiety thus requires less chlorine for its removal (Kuhad et al. 1997; Bajpai 2004).
In the present work, an attempt has been made to evaluate the differential and synergistic abilities of xylanase from Bacillus pumilus strain MK001 (Sharma et al. 2005) and LMS [laccase from Cyathus stercoreus in conjunction with 4-Hydroxy benzotriazole (HBT)] in prebleaching and poststage treatment of soda and waste pulps.