Delignified Kraft Pulp Research Articles

Quantification method for hydrogen peroxide formation during oxygen delignification of kraft pulp

Article Quantification method for hydrogen peroxide formation during oxygen delignification of kraft pulp was published on December 1, 2005 in the journal Nordic Pulp & Paper Research Journal (volume 20, issue 4).

Infrared spectroscopic study of alkaline oxygen treatment of lignin with ATR technique in aqueous state 1: method for determining quantitative spectra of oxygen-degraded lignin

For the fundamental study of oxygen delignification of kraft pulp, structural changes of kraft lignin during alkaline oxygen treatment were investigated with the use of infrared measurement with attenuated total reflectance (ATR) technique. In the neutralized reaction mixture of alkaline oxygen-treated kraft lignin, there is a significant amount of NaCl, so that the spectral changes of water due to the coexistence of NaCl was investigated, and how to remove the huge absorption of NaCl solution is discussed. Sodium vanillate–NaCl solutions were employed as model solutions for the reaction mixture. Partial least square (PLS) regression was applied for the prediction of NaCl concentration, and the spectrum of NaCl solution was subtracted from the spectrum of sodium vanillate–NaCl solution as background measurement. This allowed us to obtain the vanillate spectra free from the absorption of NaCl solution. In addition, the mathematical method for reconstructing the spectrum of NaCl solution is discussed. The spectrum of NaCl solution is reconstructed as the linear combination of basic spectra calculated by singular value decomposition (SVD), and it was subtracted from that of the sodium vanillate–NaCl solution. By this procedure, the vanillate spectra were also obtained quantitatively, as has been demonstrated in PLS regression study. It was also confirmed that the quantitative spectra of high molecular weight fraction of alkaline oxygen-treated kraft lignin were obtained by the use of this reconstruction technique.

Aryl substituted N-hydroxyphthalimides as mediators in the laccase-catalysed oxidation of lignin model compounds and delignification of wood pulp

Aryl substituted N-hydroxyphthalimides (NHPIs) have been tested as mediators in the laccase-promoted oxidation of non-phenolic monomeric and dimeric lignin model compounds and in the delignification of kraft pulp samples. In the oxidation of the model compounds a significant increase in the product yields was observed upon increasing the electron donating properties of the NHPI ring substituent. Product yields also increased, but to a smaller extent, by increasing the electron donating properties of the aromatic substituent in the lignin models. These results suggest the contribution to the overall reactivity of both the oxidation of the aryl substituted NHPI to the N-oxyl radical (X-PINO) by laccase and the hydrogen atom transfer step from the substrate to the X-PINO, with a major contribution of the former process. When applied to the delignification of kraft pulps again the mediation efficiency increased by increasing the electron donating properties of the NHPI aryl substituent, the best mediators being 4-Me-NHPI and 4-MeO-NHPI. Thus, the use of non-phenolic lignin model compounds in the oxidation promoted by the laccase/X-NHPI system is well suited to mimic the behaviour of the lignin polymer.

A novel approach for the oxidative catalysis employing polyoxometalate–laccase system: application to the oxygen bleaching of kraft pulp

A novel approach for the catalytic oxygen bleaching of kraft pulp using the heteropolyanion [SiW 11Mn III(H 2O)O 39] 5− (SiW 11Mn III) and laccase of Trametes versicolor is proposed. The oxidation of the residual lignin in pulp with the heteropolyanion was followed by the catalyst re-oxidation by laccase in a separate stage. The alterative treatment in a multi-stage process with SiW 11Mn III at 110 °C and with laccase at 45 °C allowed a sustainable eucalypt kraft pulp delignification with removal of about 50% of the residual lignin with minimal polysaccharides damage. Heteropolyanion maintained stable after redox turnovers during the treatments.

Surface Characterization of Unbleached and Oxygen Delignified Kraft Pulp Fibers

The chemical and morphological surface properties of different unbleached and oxygen delignified softwood kraft pulp fibers were evaluated by using different ESCA and SEM techniques. The results of the different techniques have revealed that the surface of unbleached and oxygen delignified kraft pulp fibers has a higher content of lignin and in some cases also extractives, in comparison with the bulk of the fibers. However, the amount of lignin obtained on the fiber surfaces differs substantially from one technique to another. The mercurization technique (mercurization followed by the determination of mercury by ESCA) gave about three to five times lower values on the surface coverage of lignin, in comparison with the normal ESCA procedure. Good correlation between the surface coverage of lignin and the total amount of lignin was obtained for all techniques, however, the best correlation was obtained for the mercurization technique. The results also show that the total and surface charges of the unbleached and oxygen delignified pulp fibers decreased with a decrease in kappa number and that the extent of fibrillation increased with a decrease in kappa number.

Electrochemical Oxidation of Transition Metal-Based Mediators for Pulp Delignification

In order to depolymerize and remove the lignin that remains in paper pulp following the pulping process, the pulp must be bleached. Environmental considerations and system closure requirements have created a need for replacement of the traditionally used chlorine bleaching technologies. Mediated laccase oxidation of residual lignin in pulp has gained in popularity during the last 10 years and could be considered as a good alternative bleaching method. The mechanisms leading to mediated lignin oxidation are still unclear. To clarify the role of the mediator during mediated kraft pulp delignification and to increase the efficiency of the system, we have studied the oxidation of (MoCN) mediator in an electrolysis cell prior to its reaction with pulp. The electrochemical oxidation of MoCN gave similar delignification levels to enzymatic oxidation under similar conditions, while the iron-based mediators gave better delignification than enzymatic oxidation. The electrodelignification setup allowed us to study the effect of pH and oxygen on the lignin degradation. We also report on the use and efficiency of high formal potential mediators that are suspected to be good lignin oxidants but which are not readily oxidized by the laccase. By studying the charge transferred to the solution to achieve mediator oxidation we were able to show that the mediator acts as catalyst, i.e., is being continuously regenerated throughout the process, to perform efficient pulp delignification. © 2001 The Electrochemical Society. All rights reserved.

Polyoxometalates as mediators in the laccase catalyzed delignification

The polyoxometalate (POM)-laccase catalytic system was applied for the first time to aerobic delignification of kraft pulps at moderate (40–60 °C) temperatures. Laccase was found to readily catalyze the re-oxidation of different kinds of polyoxometalates, including those, which cannot be re-oxidized by dioxygen even at high temperatures (PMo 11V 1, SiW 11V 1, etc.). This allows a sequence of catalytic redox cycles similar to that in the laccase-mediator system (LMS) where electrons are transferred from the substrate (lignin) via POM and laccase to oxygen. Results obtained showed that the POM-laccase system could decrease κ number of eucalypt kraft pulp from 13.7 to 8.5 though the reaction rate is relatively slow. Among different POM used, SiW 11V showed the best results. The effect of the process variables on the delignification was studied. The best results in delignification of eucalypt pulp were obtained at 60 °C, oxygen pressure of 5 bar, pH 6.3, SiW 11V concentration of 4.2 mM and laccase concentration of 0.65 U/ml. The reaction temperature appears to be one of the crucial factors in the achievement of a delignification rate acceptable for practical application.

Cellobiose dehydrogenase is essential for wood invasion and nonessential for kraft pulp delignification by Trametes versicolor

Cellobiose dehydrogenase (CDH)-deficient strains of the basidiomycete Trametes versicolor were produced by transforming protoplasts of strain 52J with a plasmid carrying the T. versicolor cdh gene disrupted with a phleomycin resistance cassette. Of 143 phleomycin-resistant colonies analyzed, 3 did not produce measurable CDH during two successive two-week culture periods. Two of these mutants were shown to lack functional CDH when grown in CDH induction medium. They biobleached and delignified industrial unbleached kraft pulp as efficiently as did wild-type T. versicolor, indicating that CDH is not required for the degradation and biobleaching of kraft lignin. The ability to degrade 14C-guaiacyl dehydrogenative polymerizate (synthetic lignin) also appeared to be unaffected. However, compared to the parent strain, all three mutants grew poorly on minimal agar with highly crystalline cellulose as the sole carbon source. This difference was not observed on non-crystalline carbohydrates. All three mutants had a greatly decreased ability to colonize and degrade both seasoned and fresh native white birch wood, a natural substrate of T. versicolor. The dramatic decrease in T. versicolor 52J’s ability to invade and grow on birch wood caused by the loss of its secreted CDH strongly suggests that this enzyme is essential to its wood invading and degrading niche in the forest ecosystem.

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.

Investigations into Laccase-Mediator Delignification of Kraft Pulps

SummaryThe structure activity effects of 1-hydroxy benzotriazole and phthalimide derivatives as mediators for laccase were studied. Using a softwood kraft pulp it was shown that the N-hydroxy unit is a key component of 1-hydroxybenzotriazole for efficient laccase mediator delignification to occur. It was also found that the 1-hydroxybenzotriazole structure was very sensitive to substituent effects with respect to laccase-mediator delignification. Computational results from PM3 indicate that the bond dissociation energy, and electronic factors of the radical may contribute to the efficiency of the mediator for LMS delignification.

Effects of the laccase-mediator system on the handsheet properties of two high kappa kraft pulps

It has been suggested that the laccase-mediator system (LMS), an oxidative enzyme system, could be used to delignify kraft pulp. A series of scoping experiments found that this enzyme system could have a range of effects on the physical properties of two high kappa kraft pulps, kappa 70 and kappa 94. The mediator itself (1-hydroxybenzotriazole) was found to modify pulp properties. At a low dose, the mediator alone increased handsheet densification. This effect was negated in both pulps when either a low LMS dose or a high mediator dose was applied. There was, however, an increase in the stretch of the handsheets at a given apparent density that may indicate a change in fiber wall structure. For the kappa 70 pulp, the low LMS dose also increased tensile strength at a given handsheet density. All these changes to the physical properties of pulp, along with changes to optical properties, indicated that a degree of lignin modification had occurred during LMS treatment. Most of the LMS-derived effects were masked by subsequent peroxide extraction. Future work should consider examining the effects of laccase without the mediator.

Investigation of Laccase/N-Hydroxybenzotriazole Delignification of Kraft Pulp

N-hydroxybenzotriazole, a mediator for laccase delignification of kraft pulps, was shown to be unstable under the biobleaching conditions. The treatment of N-hydroxybenzotriazole either with laccase alone or in the presence of kraft pulp yielded benzotriazole. The reductive conversion of N-hydroxybenzotriazole to benzotriazole was found to occur rapidly in the presence of pulp. Furthermore, benzotriazole was found to be inactive as a mediator for laccase catalyzed delignification of kraft pulps. Hence, the overall conversion of N-hydroxybenzotriazole to benzotriazole is detrimental towards the bio-delignification process.

Copper-dependent depolymerization of lignin in the presence of fungal metabolite, pyridine

Thus far, it has not been recognized that copper complexes are able to depolymerize lignin under physiological conditions of white rot decay. However, we have found that both phenolic and non-phenolic synthetic lignins were intensively depolymerized by Cu(II) and lipid hydroperoxide model compounds in the presence of a metabolite of ligninolytic fungi, pyridine at room temperature in aqueous media. Treatment of 14C-labeled oxygen-prebleached kraft pulp (OKP) by the copper-dependent reaction evidenced effectiveness of this reaction for the delignification of kraft pulps. In contrast to the organic peroxide system, Cu(II)/pyr/H 2O 2 system was much less effective for the lignin depolymerization. However, treatment of unbleached kraft pulp (UKP) by Cu(II)/H 2O 2 and Cu(II)/pyr/H 2O 2 systems demonstrated that the damage of cellulose was suppressed by the coordination of pyridine although high brightness gain was obtained independently of the presence of the coordinator. Spin trapping experiments demonstrated that not hydroxyl radical but superoxide anion is involved in the Cu(II)/pyr/H 2O 2 system. This finding not only introduces a new concept of non-enzymatic lignin biodegradation by wood-degrading fungi but also presents a new strategy for decomposing lignin and lignin-related compounds by copper complexes and peroxide-producing system.

Fate of residual lignin during delignification of kraft pulp by trametes versicolor

The fungus Trametes versicolor can delignify and brighten kraft pulps. To better understand the mechanism of this biological bleaching and the by-products formed, I traced the transformation of pulp lignin during treatment with the fungus. Hardwood and softwood kraft pulps containing 14C-labelled residual lignin were prepared by laboratory pulping of lignin-labelled aspen and spruce wood and then incubated with T. versicolor. After initially polymerizing the lignin, the fungus depolymerized it to alkali-extractable forms and then to soluble forms. Most of the labelled carbon accumulated in the water-soluble pool. The extractable and soluble products were oligomeric; single-ring aromatic products were not detected. The mineralization of the lignin carbon to CO2 varied between experiments, up to 22% in the most vigorous cultures. The activities of the known enzymes laccase and manganese peroxidase did not account for all of the lignin degradation that took place in the T. versicolor cultures. This fungus may produce additional enzymes that could be useful in enzyme bleaching systems.

Kraft Pulp Bleaching UsingIn-situDimethyldioxirane: Mechanism and Reactivity of the Oxidants

Dimethyldioxirane (DMD) is a cyclic peroxide made by oxidizing acetone with the peroxymonosulfate (PMS) anion in water buffered at pH 7.5 using sodium bicarbonate. It has been shown that DMD generation can be achieved in-situ within a pulp suspension allowing very selective TCF bleaching of kraft pulp. This process involves simultaneous generation of DMD, reaction of PMS and DMD with residual lignin, and spontaneous decomposition of both oxidants. In this work, we have studied the effect of pH and the nature of the buffering agent on the efficiency of delignification of kraft pulp. We have found a positive linear relationship between delignification and pH in the range from 2 to 11 when using buffers other than bicarbonate. However, using bicarbonate gives a significant enhancement of delignification between pH 7 and 8. From this work, we have proposed that the H 2 CO 3 /HCO 3 - buffer improves the rate of DMD formation during DMD in-situ bleaching and possibly during DMD synthesis. A possible mechanism accounting for this improvement is proposed.

Reactivities of various mediators and laccases with kraft pulp and lignin model compounds

Laccase-catalyzed oxygen delignification of kraft pulp offers some potential as a replacement for conventional chemical bleaching and has the advantage of requiring much lower pressure and temperature. However, chemical mediators are required for effective delignification by laccase, and their price is currently too high at the dosages required. To date, most studies have employed laccase from Trametes versicolor. We have found significant differences in reactivity between laccases from different fungi when they are tested for pulp delignification in the presence of the mediators 2,2(prm1)-azinobis(3-ethylbenzthiazoline-6-sulfonate) (ABTS) and 1-hydroxybenzotriazole (HBT). A more detailed study of T. versicolor laccase with ABTS and HBT showed that HBT gave the most extensive delignification over 2 h but deactivated the enzyme, and therefore a higher enzyme dosage was required. Other mediators, including 1-nitroso-2-naphthol-3,6-disulfonic acid, 4-hydroxy-3-nitroso-1-naphthalenesulfonic acid, promazine, chlorpromazine, and Remazol brilliant blue, were also tested for their ability to delignify kraft pulp. Studies with dimeric model compounds indicated that the mechanisms of oxidation by ABTS and HBT are different. In addition, oxygen uptake by laccase is much slower with HBT than with ABTS. It is proposed that the dication of ABTS and the 1-oxide radical of HBT, with redox potentials in the 0.8- to 0.9-V range, are required for pulp delignification.

Kraft pulp bleaching and delignification by Trametes versicolor

In 1989 we reported that the white-rot basidiomycete fungus Trametes versicolor could delignify and substantially brighten unbleached kraft pulps. Since that time, considerable effort has been dedicated to understanding the biological mechanisms of this efficient delignification system in the hope that part or all of the system can be applied industrially. Early work indicated that all components necessary for extensive kraft pulp bleaching and delignification are secreted by the fungus, but that one or more of these components must be constantly replaced or renewed. T. versicolor can bleach O 2-delignified and extended-cooking pulps as well as conventional hardwood and softwood kraft pulps. The fungus demethylates and solubilizes the residual lignin in the pulps, seemingly without depolymerization. Enzyme families secreted by T. versicolor which may be involved in delignification include lignin peroxidases (LiP), manganese peroxidases (MnP), laccases, and cellobiose dehydrogenases (CDH). Each of these families has been investigated for its interactions with kraft pulp lignin. Two laccase isozymes (laccase I and II) have been purified. The reactivities of the two isozymes on all substrates tested, including kraft lignin, were similar, with no evidence of lignin depolymerization by either enzyme. However the presence of the mediator 2,2′-azinobis (3-ethylbenzthiazoline-6-sulphonate) (ABTS) prevented and reversed the polymerization of kraft lignin by either laccase. Both laccases were able to delignify kraft pulp, but only in the presence of the mediator. Secreted MnP activity and the presence of substantial available Mn(II) ions appear necessary for delignification and pulp brightening by T. versicolor. Mutants unable to secrete MnP cannot delignify. Purified MnP substantially delignifies pulp when supplied with Mn (II), H 2O 2 and Mn (II)-complexing agents. Furthermore, the resulting pulp is made easier to bleach to high brightness with alkaline hydrogen peroxide. LiP, while produced under secondary metabolic conditions, appears to play no role in the delignification of kraft pulp by T. versicolor. White-rot fungi typically carry out net oxidation of the lignin and hydrolytic depolymerization of the carbohydrate in wood. Is there likely to be a significant role in delignification for enzymes which oxidize wood carbohydrates and reduce lignin? The family of carbohydrate-oxidizing, quinone-, metal ion- and organic free radical-reducing enzymes known as the cellobiose oxidases and cellobiose dehydrogenases (CBO, CDH, formerly CBQase) are reported to occur in a number of wood-degrading fungi. A CDH from T. versicolor has been purified, characterized and shown to decolorize and prevent laccase-mediated lignin polymerization. It can help supply MnP with chelator, hydrogen peroxide, and manganese. The rate of diffusion of enzymes or mediators in the kraft fiber wall may be a limiting factor for delignification. In order to visualize these diffusion bottlenecks we have built a model of the secondary wall and compared the pore sizes in the wall with the sizes of potential diffusible delignification agents.

Biobleaching of oxygen delignified kraft pulp by several white rot fungal strains

Twenty-five white rot fungal strains were tested for their ability to bleach Eucalyptus globulus oxygen delignified kraft pulp (OKP). Under nitrogen-limited culture conditions, eight outstanding biobleaching strains were identified that increased the brightness of OKP by more than 10 ISO units compared to pulp incubated in sterile control medium. The highest brightness gain of approximately 13 ISO units was obtained with Bjerkandera sp. strain BOS55, providing a high final brightness of 82% ISO. This strain also caused the greatest level of delignification, decreasing the kappa number of OKP by 29%. When the white rot fungal strains were tested in nitrogen-sufficient medium, the extracellular activities of laccase and peroxidases increased in many strains; nonetheless, the pulp handsheets were either destroyed or brightness gains were lower than those obtained under nitrogen-limitation. The titer of ligninolytic enzymes was not found to be indicative of biobleaching potential. However, the best biobleaching strains were generally characterized by a predominance of manganese dependent peroxidase (MnP) activity compared to other ligninolytic enzymes and by a high decolorizing activity towards the polyanthraquinone ligninolytic indicator dye, Poly R-478.

Do the non-catalytic polysaccharide-binding domains and linker regions enhance the biobleaching properties of modular xylanases?

Xylanase A (XylA) from Pseudomonas fluorescens subsp. cellulosa consists of an N-terminal non-catalytic cellulose-binding domain joined to a functionally independent C-terminal catalytic domain by a sequence rich in serine residues. Xylanase D (XylD) from Cellulomonas fimi also exhibits a modular structure comprising an N-terminal catalytic domain linked to an internal non-catalytic xylan-binding domain and a C-terminal cellulose-binding domain. To determine the importance of the non-catalytic polysaccharide-binding domains and linker sequences of XylA and XylD in relation to their capacity to hydrolyse pulp xylan and enhance bleachability, purified full-length and modified derivatives of both enzymes were incubated with a hardwood kraft pulp. Deletion of the cellulose-binding domain or linker region from XylA decreased the activity of the enzyme against pulp xylan, but had no significant effect on the capacity of the enzyme to facilitate delignification and reduce pulp kappa number. While full-length and truncated forms of XylD, lacking either the cellulose-binding or the cellulose- and xylan-binding domains, were equally effective in hydrolysing pulp xylan, enzyme derivatives containing a polysaccharide-binding domain were marginally more efficient in reducing pulp kappa number. The reduction in kappa number elicited by full-length and isolated catalytic domains of XylA and XylD was reflected in an increase in the brightness of paper handsheets derived from pretreated pulps. Thus, the polysaccharide-binding domains of XylA and XylD did not appear to confer any advantage in terms of the ability of the enzymes to improve pulp bleachability. However, XylA and XylD, which belong to different glycosyl hydrolase families, differed in their ability to hydrolyse pulp xylan and facilitate the delignification of kraft pulp.

Xylanase treatment for the peroxide bleaching of oxygen delignified kraft pulps derived from three softwood species

Three commercial enzymes were used to evaluate the use of xylanase treatment for enhancing the peroxide bleaching of three oxygen delignified kraft pulps derived from the softwoods red cedar, Douglas fir, and western hemlock. Two bleaching sequences were examined, where the enzyme stage was placed before or between the two peroxide stages. The brightness gain achieved was dependent on the enzyme preparation, the origin of the pulp and the position of the enzyme stage in the bleaching sequence. In general, xylanase was found to enhance the pulp brightness achieved with two peroxide stages by 1–1.5% ISO. It could directly brighten oxygen delignified pulps before and after the first peroxide stage. Direct brightening of the oxygen delignified pulps, before peroxide bleaching, appeared to be correlated with the amount of UV-absorbing material solubilized during the enzyme stage.