Enzymes for Pulp and Paper Processing - ACS Publications

Chapter 2

Enzyme-Aided Bleaching of Kraft Pulps: Fundamental Mechanisms and Practical Applications Downloaded by NORTH CAROLINA STATE UNIV on October 8, 2012 | http://pubs.acs.org Publication Date: November 21, 1996 | doi: 10.1021/bk-1996-0655.ch002

Liisa Viikari, A. Suurnäkki, and J. Buchert VTT Biotechnology and Food Research, P.O. Box 1501, FIN-02044 VTT, Finland Hemicellulase-aided bleaching is one of the few well established, ecomically feasible biotechnical applications in the pulp and paper industry. The enzymatic treatments, based on xylanases and mannanases, introduce modifications in the carbohydrate structures, leading to enhanced delignification. The mechanism is based on the partial depolymerization of hemicelluloses, which impede the chemical removal of residual lignin from pulp fibres. The enzymatic method can be combined to various types of kraft pulping processes and bleaching sequences. The benefits obtained by the enzymatic treatment depend on the type of raw material, pulping process and bleaching sequence. The enzymatic step leads to reductions in chemical consumption and costs and maintains product quality. The method is applied in mill scale in several countries. This review describes the present knowledge about the mechanisms of the method as well as the practical results obtained.

The pulp bleaching technologies entered a new era in late 80s due to growing concern about the formation and release of chlorinated compounds in the recipient. In this context, the hemicellulase-aided bleaching, first introduced in 1985, offered a new type of environmentally safe approach (7). The hemicellulase treatment is an indirect bleaching method, rendering the fibres more accessible to bleaching chemicals and leading to more efficient delignification. After the launching of commercial hemicellulases to the markets, the method was soon adopted at many mills. Since then, however, other bleaching technologies have also been developed. These include various chlorine-free, oxygen based bleaching sequences using chemicals, such as peroxide, oxygen and ozone. The location and structure of hemicelluloses in the fibres affect the delignification of pulps as well as the technical properties of the fibre products. The removal of residual lignin seems to be both physically and chemically restricted by

0097-6156/96/0655-0015$15.00/0 © 1996 American Chemical Society

In Enzymes for Pulp and Paper Processing; Jeffries, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

Downloaded by NORTH CAROLINA STATE UNIV on October 8, 2012 | http://pubs.acs.org Publication Date: November 21, 1996 | doi: 10.1021/bk-1996-0655.ch002

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ENZYMES FOR PULP AND PAPER PROCESSING

hemicelluloses in the fibre matrix. Lignin-carbohydrate linkages have been proposed to restrict the chemical removal of residual lignin from pulp fibres (7). Due to solubilization and relocation during pulping process, hemicelluloses and lignin may form physically or chemically interlinked matrices in the fibres. Different hypotheses for the mechanism have been presented. It has been suggested that xylanases attack xylan chemically bound to lignin (1-3) or degrade reprecipitated xylan, deposited on fibre surfaces during the cook (4). The type of xylan attacked has also been suggested to contain chromophores (5). Hitherto, all these assumptions have gained some support. However, it is obvious that both the type of raw material (hardwood or softwood) and the pulping conditions profoundly affect the amount and location of hemicelluloses and lignin in the fibres. Recent results in pulp chemistry have also murninated the fundamentals of hemicellulase-aided bleaching. In addition to xylanases, mannanases capable of attacking pulp glucomannans have been shown to offer great potential in improving the bleachability of new types of low kappa number pulps. The scientific interest in this method is reflected in the number of papers published during recent years describing numerous xylanases from new sources, as well as bleaching results obtained using various hemicellulases, pulps and bleaching sequences. Several reviews have been published (6-9). In this paper, current knowledge on the proposed mechanisms and practical results are reviewed.

Structure of Hemicelluloses in Kraft Pulps The native hemicellulose structure is heavily modified during pulping processes. In the beginning of conventional sulphate i.e. kraft cooking, xylan in wood is partly solubilized in the alkaline cooking liquid and many of the side groups and acetic acid residues are cleaved off (10, 77). It has recently been observed that the majority of the 4-O-methylglucuronic acid side groups in xylan are converted to hexenuronic acid already in the early phases of the kraft cook (72, 75). As the alkali concentration decreases towards the end of the kraft cook, dissolved xylan tends to readsorb on the surface of cellulose microfibrils (14, 15). It has recently been suggested that pine kraft xylan reprecipitates evenly to all accessible surfaces to the fibre wall (16). In addition to xylan chains, dissolved lignin and covalently bound lignin and xylan have been suggested to reprecipitate to fibre surfaces during cooking (77,18), resulting in relatively high amounts of lignin on the fibre surfaces (19-21). The amount of xylan readsorbed during cooking depends on the wood species used in pulping. High amounts of xylan have been found to locate on the surface of birch kraft fibres, probably partly due to readsorption, whereas in pine kraft fibres the concentration of xylan on the fibre surfaces has not been observed to be higher than in the whole fibres (22). A large part of wood glucomannan is also dissolved in the beginning of the kraft cook, but due to their instability in alkali, the solubilized polymers are completely degraded in the pulping liquor (75,23,24). As a result of the solubilization of hemicelluloses during cooking the distribution and content of xylan and glucomannan in kraft pulp fibres differ from that in the native wood fibres (25). In softwood kraft fibres the xylan concentration is generally higher in outer layers, and glucomannan is more concentrated in the middle layers of the fibre. However, due to different analysis methods variations in the



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distribution of polysaccharides in softwood kraft fibres have been reported (26-28), although there is general agreement that the outer surface layer of hardwood kraft fibres is rich in xylan. Recently, several modified kraft pulping methods as well as totally new sulphate pulping methods have been introduced (29). In the pulps produced by these methods no or little reprecipitation of xylan and lignin is expected to occur due to the relatively constant alkali concentration throughout the cooking process. Consequently, the composition of outer surfaces of pulp fibres is probably different from that of the conventional kraft pulp fibres. In sulphite cooking, hemicellulose is extensively solubilized to mono- and oligomeric compounds and no reprecipitation occurs (30). Thus, the distribution of hemicellulose is relatively constant across the pulp fibres.

Hemicellulases Several species of fungi and bacteria are known to produce the whole spectrum of hemicellulose-degrading enzymes; xylanases and mannanases (31-36). Most of the xylanases characterized are able to hydrolyze xylans from various origins, showing differences only in the spectrum of end products. The main products formed from the hydrolysis of xylans are xylobiose, xylotriose and substituted oligomers of two to five xylosyl residues. The chain length and the structure of the substituted products depend on the mode of action of the individual xylanase. Some xylanases, however, show rather strict substrate specificity. The three dimensional structures of several low molecular mass xylanases have recently been determined (37-40). The structure of the Trichoderma reesei pi 9 xylanase is ellipsoidal, having dimensions of about 30 to 40 Â (39). Unlike most cellulases, it does not contain any separate substrate binding domain. Some bacterial xylanases, however, have been found to contain either a cellulose binding domain (41-43) or a xylan binding domain (44, 45). Compared with xylanases, mannanases are a more heterogenous group of enzymes. The main hydrolysis products from galactomannans and glucomannans are mannobiose, mannotriose and various mixed oligosaccharides. The hydrolysis yield is dependent on the degree of substitution as well as on the distribution of the substituents (46). The hydrolysis of glucomannans is also affected by the glucose/mannose ratio. Recently, the mannanase of T. reesei was found to have a multidomain structure similar to that of several cellulolytic enzymes (47). The protein contains a catalytic core domain which is connected by a linker to a cellulose binding domain (48). Hitherto, no three dimensional structures of mannanases have been published. Most xylanases studied are active in slightly acidic conditions between pH 4 and 6 and at temperatures below 70°C. More thermophilic and alkalophilic xylanases are of great importance due to the prevailing conditions in pulp processing. Xylanases which are stable and function efficiently at high temperatures are produced by several thermophilic bacteria (36). The most thermophilic xylanases hitherto described are produced by the extremely thermophihc bacterium Thermotoga sp. (43,44). Several xylanase genes encoding proteins active at temperatures from 75°C up to 95°C (pH 6-8) have been isolated. Thermophilic mannanases have been purified e.g. from C. saccarolyticus and Thermotoga neapolitana. Xylanases and mannanases with alkali pH optimae have been detected in an alkalophilic Bacillus sp. (36).



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The main enzymes needed to enhance the delignification of both hardwood and softwood kraft pulp have been shown to be endo-p-xylanases (49, 50). A positive effect has been achieved with most xylanases studied, independently of the origin of the enzyme. Both fungal and bacterial xylanases have been shown to increase the bleachability (57) and several commercial xylanases are available, varying with respect to their pH and temperature optimae. Mannanases, on the other hand, appear to be more specific with respect to their substrate, and only few mannanases have been shown to hydrolyze glucomannans in softwood pulps. Recently, the effects of purified or partially purified endo-acting β-mannanases from Caldocellulosiruptor saccharolyticum, Aspergillus niger and Trichoderma reesei on pulp delignification were compared in bleaching (Suurnâkki, A . et al. Tappi J., in press). Of these enzymes, the T. reesei mannanase was shown to be most efficient The mannanase of B. subtilis has been shown to be able to solubilize wood mannan but was totally unable to solubilize mannan which was bound to kraft pulp (52). The first bleach-boosting mannanase product, produced by T. reesei, emerged on the market in 1995. As compared with xylanases and mannanases, the side-group cleaving enzymes alone or in combination with endoenzymes have had only minor effects on pulp bleachability (6). Other purified enzymes which have been studied for improving the bleachability of pulps include individual cellulolytic enzymes (53). Only the unspecific endoglucanase I from Trichoderma reesei, also exhibiting xylanase activity, was shown to increase the bleachability.

Mechanisms of Hemicellulase-Aided Bleaching The effect of hemicellulases in bleaching is based on the modification of pulp hemicelluloses, enhancing the removal of lignin in chemical bleaching. It has been proposed that the action of xylanases is due to the partial hydrolysis of reprecipitated xylan (4) or to removal of xylan from the Ugnin-carbohydrate (LC) complexes (3). However, these hypotheses are not mutually exclusive, i.e. relocated xylans may contain L C complexes and both mechanisms would allow the enhanced diffusion of entrapped lignin from the fibre wall. Limited removal of pulp xylan is known to increase the leachability of residual lignin from kraft pulps (54) and thus also to increase the pulp bleachability during subsequent bleaching stages. In addition, it has been suggested that the hemicellulase treatment removes chromophoric groups from the pulp (5, 57). The suggested mechanisms as well as their consequences are presented in Figure 1. The methods used for mechanistic studies have included modified pulping methods, production of model pulps, analysis of degradation products of enzymatic treatments, chemical extractions of lignin and xylans, mechanical peeling, surface analysis by ESCA and different delignification tests. Relocated Xylan. Both xylan and lignin are dissolved and partially readsorbed on the fibres during pulping. A rather high content of lignin has been observed both in the primary fines and in the surface material of pine kraft fibres (79, 20). Xylanases combined with ESCA have been used to determine the xylan content on the outer surfaces of cellulose fibres. In softwood kraft fibres, removal of xylan by xylanases was found to uncover lignin (27). The relatively low amount of xylan, observed on the


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Xylanases acting on:

Result:

Relocated xylan

Uncoverage of lignin

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Increased porosity Xylan linked to lignin

Increased solubility of lignin

Chromophore containing xylan

Release of chromophores

Figure 1. Suggested mechanisms of xylanase-aided bleaching. outer surfaces of fibres, can obviously be easily removed by xylanases (22). Thus, it can be expected that removal of xylan improves the extractability of lignin by exposing lignin surfaces. As dissolved xylan molecules can penetrate most of the pores in cellulose fibres, it can be postulated that xylanases, acting on all available surfaces, enhance the removal of lignin in the whole fibre. In birch kraft pulp, the primary fines and the fibre surface material were found to be considerably richer in both xylan and lignin than the whole pulp (22). Therefore, relatively more xylan should be enzymatically removed from the outer surfaces and expose the lignin. The role of reprecipitated xylan in the xylanase-aided bleaching of birch kraft pulps has been confirmed by comparing the effect of xylanase treatment on bleachability of kraft pulps cooked by a batch method and of pulps produced in a flow-through digester and therefore containing only traces of reprecipitated xylan (4). On the other hand, xylanase pretreatment has later also been reported to enhance the bleachability of softwood pulps produced by novel cooking methods with more stable alkali profiles (55, 56) and presumably thus containing less reprecipitated xylan than conventional softwood kraft pulps. In addition, the role of reprecipitated xylan in xylanase-aided bleaching has been studied by extraction with dimethyl sulfoxide (DMSO), a chemical which has been claimed to remove reprecipitated xylan selectively from pulps (57). This treatment did not improve the bleachability of kraft pulp, whereas xylanase treatment did. Recently, however, Allison et al. (58) reported that the removal of pulp xylan by DMSO is dependent on the degree of polymerization (DP) of xylan. As the DP of reprecipitated xylan is not known, the role of reprecipitated xylan in xylanase-aided bleaching cannot be conclusively determined by DMSO extractions. Despite the obvious analytical difficulties in determining relocated xylans, it can be expected that the mechanism of xylanase-aided bleaching is not based on hydrolysis of relocated xylan alone. Lignin Carbohydrate Complexes. Both softwood and hardwood kraft pulps have been reported to contain LC-complexes in which carbohydrates and lignin may be connected to each other by ether or glycosidic linkages (2, 59). However, no direct evidence for the type of linkage(s) existing between carbohydrates and lignin has yet



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been presented. Increased solubilization of xylan-lignin complexes both from model pulps (60) and from kraft pulps (3, 61) has been observed by xylanase treatment According to G P C analyses, part of the lignin released during the enzymatic treatment appears to be covalently bound to xylan, whereas most of the lignin may be physically interlinked with xylan in the fibre matrix. The action of xylanases on both reprecipitated and LC-xylan in enhancing bleachability suggests that it is probably not only the type but also the location of the xylan that is important in the mechanism of xylanase-aided bleaching. The xylanase of T. reesei has been observed to act rather uniformly in all accessible surfaces of kraft pulps (62, 63), indicating that the effect of xylanase on bleachability is not only an outer surface phenomenon. The composition of xylan solubilized in limited or extensive treatments has not revealed essential differences, indicating that the structure of xylan is rather similar in all parts of fibres, or that the enzymes are specific to a certain type of xylan (61). Chromophores. It has frequendy been observed that xylanase treatment has a slight decreasing effect on the kappa number. This has been explained to be due to removal of lignin fragments or chromophoric structures from pulp (61). However, the reduction in the kappa number as measured by permanganate oxidation may be partially due to an artefact. The recently discovered hexenuronic acid (12), containing a double bond, may give rise to the consumption of permanganate, increasing the apparent kappa number. Thus, enzymatic removal of xylan containing hexenuronic acid groups can lead to a lower kappa number. Mode of Action of Mannanases. Compared with xylanase-aided bleaching, the mechanism of mannanase-aided bleaching has attracted only minor interest, probably due to its rather limited effect in most pulp types. However, the mechanism of mannanase-aided bleaching has been assumed to differ from that of xylanase-aided bleaching, due to the different distribution of glucomannan and xylan in pulp fibres (63, 64). Unlike in the case of xylanases, no correlation between the amount and composition of enzymatically solubilized glucomannan and the effect on bleachability has been observed. However, the role of the composition and configuration of the outer surfaces of pulp fibres seems to be important in mannanase-aided bleaching. Mannanase treatment was found to enhance die bleachability of pulps produced by modified or continuous pulping methods (56, Suurnâkki, A. et ai, Tappi J., in press), which are generally considered to contain less reprecipitated xylan and lignin on the fibre surfaces. Furthermore, mannanase treatment was effective in enhancing the bleachability of conventional pine kraft fibres only when the outer surface material of fibres was mechanically removed prior to treatment (62). It is possible that underneath the outermost surface layer of kraft fibres and on the surface of modified pulps the glucomannan is more closely located to lignin and that the enzymatic removal of glucomannan therefore increases the teachability of lignin.

Practical Results The main goal in the enzymatic bleaching of kraft pulps has been to reduce the consumption of chlorine chemicals in the traditional and E C F bleaching processes. However, enzymes can also be used successfully for increasing the brightness of pulp,



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which is of key importance in the development of totally chlorine-free (TCF) bleaching sequences. Addition of an enzymatic step to any conventional chemical bleaching sequence results in a higher final brightness value of the pulp. During the past five years, the method has been continuously used in industrial scale together with low-chlorine or totally chlorine-free bleaching methods. The reasons for using enzymes vary depending on the conditions at mill sites. Today xylanases are used both in E C F and T C F sequences. In E C F sequences the enzymatic step is often adopted due to the limiting chlorine dioxide production capacity. The use of enzymes allows bleaching to higher brightness values when chlorine gas is not used. In TCF sequences, the advantage of the enzymatic step is due to improved brightness, maintenance of fibre strength and savings in bleaching costs. The enzymatic treatment is generally carried out with brown-stock or oxygen delignified pulp prior to the actual bleaching stage. As the first generation enzymes used in pulp treatments have generally been active in the pH range of 5-8 and at moderate temperatures of 40-60°C., adjustment of pH and temperature prior to the treatment has been necessary. Xylanase treatment has been studied to enhance pulp bleachability in traditional and various E C F and T C F bleaching sequences. The benefits attained by enzymatic pretreatment obviously vary between the different bleaching sequences (Suurnàkki, A . et ai, Tappi J., in press, 65, 66, 67). In conventional kraft softwood and hardwood pulps, xylanases have been reported to result in an increase of about 2-6 ISO units in brightness values after modern bleaching sequences and in a decrease in chlorine chemical consumption of about 10-20% in conventional bleaching sequences. In softwood pulps produced by modified pulping procedures, ie. by E M C C and M C C , the effect of xylanase treatment on bleachability has been reported to be less pronounced than in conventional kraft pulps after E C F bleaching (65, 66). Oxygen delignification carried out prior to the actual bleaching sequence has been found to enhance the susceptibility of pulp hemicellulose to enzymatic solubilization (4, 65, 68). However, when ECF bleaching sequences are used, oxygen delignification has not been found to increase the effect of xylanase treatment on the bleachability of softwood kraft pulps (69, 70). In 1992 more than ten mills worldwide were reported to use xylanases continuously for improved bleaching of kraft pulps. Almost one hundred mill trials have been carried out, about half of them in Europe. Most of the kraft pulp in Europe is produced in Scandinavia, where most of the mill trials have also been performed. Different paper products, including magazine papers (SC, LWC) and tissue papers, manufactured from enzymatically treated pulps have been successfully introduced to the markets. Conclusions Hemicellulases were the first group of specific enzymes used in large scale in the pulp and paper industry. The method is also an example of sustainable technology in the traditional chemical industry. The method has clear environmental benefits and is economically attractive. The hemicellulase treatment, together with a chemical extraction, leads to a significant reduction in the residual lignin content of the pulps. The partial hydrolysis of xylan facilitates the extraction of lignin from pulp in higher amounts and with higher molecular weights. However, due to the indirect mode of



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action, the effect of hemicellulase-aided bleaching is limited. The improved bleachability is mainly based on the action of endo^-xylanases, a group of enzymes which can be efficiently produced in industrial scale. In addition to lignin modifying enzymes, new commercial hemicellulases with higher pH and temperature optima should improve the applicability of enzymes. The two main mechanisms proposed, i.e. the action of enzyme in reprecipitated xylan or in LCC-xylan in enhancing the leachability of lignin, are not mutually exclusive and seem both to be valid in the xylanase-aided bleaching of kraft pulps. The relative importance of the two types of xylans in the mechanism seems to depend on the type of pulp. The bleachability of pine kraft pulp has been found to be most affected by the action of xylanase on xylan, physically or chemically linked to lignin, in all accessible pulp surfaces. In birch kraft pulp, the xylanases appear to act most efficiently on the outer surfaces of fibres, possibly mostiy on the reprecipitated xylan. In the case of mannanases, the structure of the outer surface of fibres seems to be most important in determining the efficiency of enzyme-aided bleaching. In addition to improvement of the enzymatic application, research on the action of enzymes in cellulose fibres has resulted in new analytical methods and improved knowledge of wood chemistry.

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