Wet strength improvement of unbleached kraft pulp through laccase catalyzed oxidation

Previous investigations have shown that laccase catalyzed oxidation of lignin containing wood fibers can enhance the strength of medium density fiberboards. In the present work it was investigated if laccase treatment had any impact on the tensile strength of a high yield unbleached kraft pulp. Treatment with laccase alone had only a very little effect on the wet strength of the pulp, whereas addition of lignin rich extractives increased the wet strength after the enzyme treatment significantly. A mediated oxidation gave a similar improvement of the wet tensile strength although no lignin was added to the fiber suspension. Furthermore, it was found that a heat treatment combined with a mediated oxidation gave a higher improvement in wet tensile strength than could be accounted for by the individual treatments. No change in dry tensile strength from the laccase treatment was observed. It is suggested that the observed improvement in wet tensile strength is related to polymerization of lignin on fibers in the hand sheet and/or coupling of phenoxy radicals on lignin associated to adjacent fibers. For the different mediators studied, a correlation was found between oxygen consumption upon mediated oxidation and generation of wet strength in the pulp.     

Biobleaching of kraft pulp

This review reviews current research on the application of biotechnology to the bleaching of kraft pulp. Biobleaching research focuses on using white rot fungi, and ligninase and hemicellulase enzymes. Bleaching with white rot is generally very slow and the primary problems appear to be the control of cellulose degradation, the technical aspects of scale up and the avoidance of an initial lag phase. Ligninase enzymes depolymerize lignin and in combination with an extraction stage, partially bleach kraft pulp. Bleaching with hemicellulase enzymes looks promising. A reduced need for active chlorine has been obtained by boosting the bleachability of pulp through pretreatment with hemicellulose hydrolysing enzymes.     

Trichoderma reesei cellulases in the bleaching of kraft pulp

In this work the effects of individual purified cellulases of Trichoderma reesei were studied in the enzyme-aided bleaching of kraft pulps. The cellobiohydrolases I and II, when used alone, had no positive effect on the bleachability of kraft pulps. The endoglucanase I (EG I), however, acted on pulp similarly to xylanases and with an enzyme dosage of 0.1 mg/g a clear increase in pulp brightness could be observed. Due to the unspecificity of this enzyme, the viscosity of the pulp was simultaneously decreased. Of the cellulases, EG II was clearly most detrimental in reducing the pulp viscosity. Hence, the action of purified cellulases of T. reesei on pulp as a substrate differs profoundly, and all cellulases are not detrimental to the pulp properties. Correspondence to: J. Buchert     

Effects of surfactants on the enzymatic bleaching of kraft pulp by xylanase

A xylanase was purified from a commercial crude xylanase, Pulpzyme HC, and used for the bleaching of kraft pulp in the absence or in the presence of nonionic surfactants, Tween 20, Tween 80, and Igepal C930. The purified xylanase has a molecular weight of 23,500 as determined by a reducing SDS-PAGE. Tween 20 was most effective to enhance the efficiency of the enzymatic bleaching of kraft pulp by xylanase.     

Endoglucanase enzymatic modification of kraft pulp during recycling

Objective

To investigate the cellulose modification process on kraft pulp during recycling by mono-endoglucanase.

Results

Pichia pastoris expressing endoglucanase, EG1, was grown in a 10 l fermenter yielding a high carboxymethyl cellulase (CMCase) activity of 340 U mg^?1. EG1-mediated modification of kraft pulp resulted in a paper sheet with the tensile index and burst index increased by 10 and 6.5 %, respectively. The kink index (indicating abrupt bends in fibres) of the enzyme-treated group decreased sharply by 45 % after the first recycling, compared with a reduction of only 1 % in the control group. Furthermore, EG1 treatment decreased the growth of crystallinity from 73.5 to 73.2 % and crystal size from 7.45 to 7.21 nm, which alleviated paper aging.

Conclusion

Endoglucanase EG1 modifies the interfacial properties of fibers, which affects fibre morphology during the recycling process and improves the technical properties of the resulting pulp and paper.

     

Partial delignification of unbleached kraft pulp with ligninolytic fungi

Summary Unbleached kraft pulp is partially delignified on incubation under specified conditions with ligninolytic fungi, thereby decreasing requirements for bleaching chemicals. Studies withPhanerochaete chrysosporium demonstrated important effects of nutrient nitrogen and molecular oxygen concentrations. Possible approaches to rate enhancement are suggested.     

Changes in the structural composition of hardwood kraft pulp during bleaching

The possibility of using xylanase preparations for hydrolyzing hemicelluloses in a non-bleached kraft pulp in order to facilitate its bleaching was studied. The effects of enzymatic preparations of fungal and bacterial origins were examined, and the optimal conditions for xylanase activity were determined. UV spectroscopy demonstrated that the treatment of kraft pulp with enzymatic preparations containing xylanase facilitated the subsequent removal of lignin and increased the brightness by 5%. The effect of enzymatic treatment was retained in the case of peroxide bleaching. The enzymatic preparations studied are promising for the development of chlorine-free pulp bleaching technologies.     

Acetylation and characterization of xylan from hardwood kraft pulp

Alkaline treatment of eucalyptus hardwood kraft pulp with 10% NaOH yielded 6-8% xylan. The acetylation of the extracted xylan was carried in DMAC/LiCl/pyridine system to obtain a series of xylan acetates with different degrees of substitution (DS). Structure elucidation of xylan and xylan acetate was obtained by ¹H and ¹³C NMR spectroscopy and other homonuclear and heteronuclear 2D-NMR techniques. Inverse-gated ¹³C NMR was employed to determine the DS of xylan acetate. Furthermore, results also revealed equal reactivities at the C-2 and C-3 positions of xylan towards acetylation. Thermal stability, solubility behavior and nanofiber formation of xylan acetate were influenced by its DS values. The mechanical properties of xylan acetate propionate were also investigated.     

Laccase-HBT bleaching of eucalyptus kraft pulp: influence of the operating conditions

Different operating conditions (viz. pulp consistency, oxygen pressure and treatment time) in the biobleaching of eucalyptus kraft pulp with the laccase-HBT system was tested in order to describe their effect and normalize a biobleaching protocol. A high O(2) pressure (0.6MPa) was found to result in improved laccase-assisted delignification of the pulp. Also, a high pulp consistency (10%) and a short treatment time (2h) proved the best choices with a view to obtaining good pulp properties (kappa number and ISO brightness) under essentially mild conditions. The laccase-HBT treatment was found to result in slight delignification (in the form of a 20-27% decrease in kappa number); however, an alkaline extraction stage raised delignification to 41-45%, a much higher level than those obtained in the control tests (16-23%). Also, the use of hydrogen peroxide in the extraction stage resulted in improved brightness (14-19%), but in scarcely improved delignification (4-7%). Treating the pulp with the laccase-HBT system reduced the amount of hydrogen peroxide required for subsequent alkaline bleaching by a factor of 3-4 relative to control tests.     

Delignification of an unbleached hardwood kraft pulp by Phanerochaete chrysosporium

Summary Culture conditions affecting lignin degradation of an unbleached hardwood kraft pulp by Phanerochaete chrysosporium have been examined. Optimum pH and temperature for lignin degradation (about 33%) were 3.5 and 38°C, respectively. Optimum fungal growth was at a pH of 4.5 and a temperature of around 32°C. Addition of exogeneous glucose to the cultures lessened the degradation of pulp carbohydrates. Lignin degradation was stimulated by oxygen atmosphere and non-agitated cultures. Increased surface to volume ratio (decreased culture depth) enhanced lignin degradation (about 56% at a depth of 1.2 cm). Finally, the correlations: pulp yield vs. residual glucose, ligninase activity vs. mycelium, and extent of delignification vs. residual extracellular H₂O₂ were discussed in light of recent findings of ligninases responsible for ligninolysis.     

A comparative study of the effect of refining on organosolv pulp from olive trimmings and kraft pulp from eucalyptus wood

This paper examines the influence of the degree of refining of different pulps, produced from olive trimmings and eucalyptus wood, on various physical properties. Corresponding pulps were obtained by means of different cooking processes. Pulp from olive trimmings was obtained by means of an organosolv process and pulp from eucalyptus was obtained by means of a kraft process. Pulp from olive trimmings exhibited a lower specific surface area, water retention volume, breaking length, stretch and burst index, but a higher porosity, than eucalyptus pulp. On the other hand, the same degree of refining was achieved with less energy for olive pulp than for eucalyptus pulp. Mixed pulp from olive trimmings and eucalyptus provided paper sheets with acceptable physical properties but with reduced refining energy costs relative to eucalyptus pulp alone.     

Continuous anaerobic treatment of wastewater from a kraft pulp mill

A pilot-scale study of the thermophilic anaerobic digestion of high-strength wastewater (evaporator condensate, EC) discharged from a kraft pulp production process was performed. The system consisted of a microfiltration (MF) membrane module for oily substances removal, a stripping system using evolved gas from the digester for sulfur compounds removal, an anaerobic fixed-bed bioreactor for methane fermentation, and an ultrafiltration (UF) membrane module for retention of a high density of bacterial cells. The bioreactor had a fixed-bed with an effective volume of 5 m³ packed with pumice stone. In a continuous run with only the MF membrane module for oily substances removal, the digester efficiency declined because of methanogenic inhibition by sulfur compounds. After fitting of the stripping system which used evolved gas from the digester, the inhibitive sulfur compounds in the EC were removed more than 80%, and high-loading and high-efficiency operation could be attained. The BOD loading and BOD removal of 35.5 kg BOD/m³/d and 93%, respectively were attained. By anaerobic treatment of the evaporator condensate waste before the conventional aerobic activated sludge method, the total costs would be reduced to ¥3.31/m³ wastewater compared with ¥4.53/m³-wastewater by the aerobic activated sludge method only. The stability of digester performance against interruption by feed stoppage was also examined.     

Reutilization of effluents from laccase-mediator treatments of kraft pulp for biobleaching

Several effluents from laccase-mediator treatments of kraft pulp were recovered and subsequently reused with fresh pulp in order to simulate recirculation of effluents during biobleaching. The effluents were used as a new bleaching stage without any modification except enzyme addition. Pulp treated with effluents were afterwards chemically bleached by using the simple sequence LQPo, where L represents the treatment with effluent and laccase addition, Q is a chelating stage and Po is an alkaline peroxide stage. This system showed a promising potential on delignification, with kappa number ranging from 5.5 to 6.6 after LQPo sequence, depending on the type of effluent employed in L stage. Improvements on pulp brightness were also reported compared with control experiment.     

Preparation and characterization of hemicellulosic derivatives containing carbamoylethyl and carboxyethyl groups

A series of novel water-soluble hemicellulosic derivatives, containing carbamoylethyl and carboxyethyl groups, were heterogeneously synthesized from wheat-straw hemicelluloses with acrylamide (AA) under alkaline conditions. The factors such as reaction temperature, reaction time, molar ratio of catalysis to xylose unit in hemicelluloses and molar ratio of acrylamide to xylose unit in hemicelluloses, were investigated. The average degree of substitution (DS) was calculated by ¹H NMR spectroscopy. DS values up to 0.23 in a one-step synthesis of hemicelluloses derivatives were obtained. Under optimum conditions (60 °C, NaOH to xylose unit in hemicelluloses molar ratio of 0.8, AA to xylose unit in hemicelluloses molar ratio of 8.0, reaction time of 1 h) an expected ratio of carbamoylethyl group to carboxyethyl group of 4.8 in the hemicellulosic derivatives was obtained. The structural features of the hemicellulosic derivatives were characterized by FTIR, NMR spectroscopy, and by elemental analysis. The current work provides a facile method for the synthesis of hemicellulose derivatives with bifunctional groups, which could be used as wet-end additives in the papermaking industry.     

Modification of high-lignin softwood kraft pulp with laccase and amino acids

This study demonstrates the potential of laccase-facilitated grafting of amino acids to high-lignin content pulps to improve their physical properties in paper products. Research studies have recently reported that increases in anionic fiber charge can improve strength properties of paper. In an effort to increase carboxylic acid groups, we developed a unique two-stage laccase grafting protocol in which fibers were initially treated with laccase followed by grafting reactions with amino acids. The bulk acid group content was measured, and a variety of amino acids including glycine (Gly), phenylalanine (Phe), serine (Ser), arginine (Arg), histidine (His), alanine (Ala), and aspartic acid (Asp) were examined. The effects of optimizing laccase dose, and amino acid structures, on fiber modification chemistry were studied. Histidine provided the best yield of acid groups on pulp fiber, and was used for the preparation of handsheets for physical strength testing. Laccase-histidine treated pulp showed an increase in strength properties of the resulting paper.     

Effect of polydimethylsiloxane oxygen carriers on the biological bleaching of hardwood kraft pulp by Trametes versicolor

Summary Improving the availability of oxygen by adding polydimethylsiloxanes (PDMS) oxygen carriers to Trametes versicolor cultures increased pulp brightening. The presence of the oxygen carriers in cultures of T. versicolor with hardwood kraft pulp increased the growth rate of the fungus, but not the ultimate biomass yield. The PDMS also stimulated brightening of hardwood kraft pulp by it T. versicolor immobilized in polyurethane foam. A threefold increase in the oxygen uptake rate in T. versicolor cultures with PDMS was observed. This increase can be explained by elevated oxygen transfer rate and attributed to the surfactant properties of PDMS. Offprint requests to: E. ZiomekIssued as NRCC 32760     

Fiber modification of kraft pulp with laccase in presence of methyl syringate

An approach aiming at improving paper properties is to use laccase to copolymerize low-molecular weight phenols with the pulp before papermaking. The addition of methyl syringate (MS) gave a twice wet tensile index of unbleached kraft pulp with laccase treatment but had little effect on the dry tensile index and substantially decreased the brightness of pulp. The radical concentration in laccase-treated fibers increased up to 2.5 times of that in control sample after 60 min. The radical concentration in laccase/MS-treated fibers increased up to 20 times of that in control sample within 2 min, and a highest radical concentration (increased by near 65 times) was obtained after 40 min. A strong agglutination of large-area fibers in handsheet was observed after laccase/MS treatment. The surface lignin coverage of the laccase-treated fibers increased from 54.96% (control) to 59.36%, while that of the laccase/MS-treated fibers increased only up to 55.22%. It is suggested that the graft of MS on fiber and degradation of surface lignin occurred simultaneously. The addition of MS can enhance the activation of fibers and extend the enzymatic oxidation of lignin within the cell wall. An increased bonding area of fibers resulting from interaction of laccase, MS and fibers via radical-coupling reaction maybe account for the significantly improved wet strength of pulp.     

Xylanase prebleaching of fractions of Douglas-fir kraft pulp of different fibre length

A commercial xylanase from Trichoderma longibrachiatum was used to treat the fractions of Douglas-fir kraft pulp of different fibre length. Enzymatic prebleaching was followed by chelation and peroxide bleaching. An evaluation of both optical and physical properties of the distinct fractions was conducted. A difference in susceptibility of the fractions of different fibre length to xylanase prebleaching was observed. The bleach-boosting effect observed with all fractions appeared to be related to the high-molecular-mass UV-absorbing material solubilized during enzyme treatments. Xylanase treatments resulted in beneficial effects to handsheet density and roughness as well as to some of the strength properties. However, the response to the xylanase treatments exhibited by all fractions of different fibre length was not uniform, indicating that fiber composition played a key role in the effectiveness of xylanase treatments.     

The role of two Trichoderma reesei xylanases in the bleaching of pine kraft pulp

Summary The two major xylanases of Trichoderma reesei with different pI values and pH optima were compared for increasing the bleachability of pine kraft pulp. The efficiencies of the two enzymes acting on pulp substrate were very similar in hydrolysis yield, extraction kappa number or final brightness value. Only slight synergism between the two enzymes was observed in both hydrolysis and bleaching tests. The pH optimum of the pI 5.5 xylanase was similar in pulp treatment and in the hydrolysis of isolated substrates, and the bleaching result also correlated well with the hydrolysis of pulp xylan. By contrast, the pI 9.0 xylanase acted differently on pulp than on isolated xylans at different pH values and the pH optimum on pulp was increased. The bleachability of pulp by the pI 9.0 xylanase was improved more than expected at pH 7.0, although the hydrolysis of pulp xylan was substantially decreased. A similar phenomenon was also observed when the hydrolysis was performed in water instead of buffer. It thus appears that the degree of hydrolysis needed to obtain improved bleachability with pI 9.0 xylanase can be minimized by proper adjustment of the hydrolysis conditions. Correspondence to: J. Buchert     

Reevaluation of the manganese requirement for the biobleaching of kraft pulp by white rot fungi

Manganese dependent peroxidase (MnP) is the main enzyme implicated in the biobleaching of kraft pulps by white rot fungi. The goal of this study was to evaluate the Mn requirement for biobleaching of eucalyptus oxygen delignified kraft pulp (OKP) by various white rot fungi: Trametes versicolor, Phanerochaete sordida, Phlebia radiata, Stereum hirsutum and Bjerkandera sp. strain BOS55. All of the strains tested produced MnP and provided extensive bleaching of OKP when 33 μM Mn was included in the medium. Bjerkandera sp. strain BOS55 was the only strain that also displayed MnP production and biobleaching activity of EDTA-extracted OKP in the complete absence of Mn. However, MnP and biobleaching activity in the absence of Mn was dependent on the presence of organic acids in the medium. The fact the biobleaching was correlated to MnP activity irrespective of whether Mn was present or absent suggests that there may be roles for MnP in Bjerkandera under Mn-deficient conditions. Although manganese-independent peroxidase (MIP) and lignin peroxidase (LiP) were also detected, the titres were much smaller in comparison with those of MnP, so their relative role in biobleaching can be predicted to have a minor importance in comparison with MnP. Only in the case of Bjerkandera, was the expression of LiP stimulated in the presence of oxalate but final brightness was not substantially affected.