Modification of properties of old newspaper pulp with biological method

Bioresource Technology 101 (2010) 7041–7045

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Modification of properties of old newspaper pulp with biological method Yangmei Chen a,*, Jinquan Wan a,b,**, Yongwen Ma a,b, Huilin Lv b a b

College of Environmental Science and Engineering, South China University of Technology, Guangzhou 510006, China State Key Lab Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China

a r t i c l e

i n f o

Article history: Received 18 January 2010 Received in revised form 2 April 2010 Accepted 8 April 2010

Keywords: Laccase Histidine Tensile strength Carboxyl group content Fiber surface morphology

a b s t r a c t Modification of properties of old newspaper (ONP) deinked pulp with laccase and histidine was investigated. It was found that the optimum conditions for laccase–histidine treatment were: the concentration of laccase 0.9 U/g dry pulp, the concentration of histidine 1% relative to the dry pulp, room temperature, reaction time 1.5 h, pH 7, the pulp consistency 5% and O2 atmosphere. The results also showed that, in the optimum conditions, compared to the control pulp, the wet tensile strength, the carboxyl group content and water retention value of ONP treated with laccase and histidine were increased by 55.1%, 39.1% and 45.7%, respectively. Moreover, environmental scanning microscope images showed that more collapse and more fibrillation were observed on the laccase–histidine-treated fiber surface than the control samples, which led to form better bonding between fibers in handsheets resulting in the increase of the paper strength of laccase– histidine-treated ONP pulp. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction It was estimated that the using amount of recycled fibers had reached to 1700 million tons in China’s papermaking industry (Chen et al., 2009). Waste paper recycling is an effective measure to reduce deforestation and environmental pollution. Old newspaper (ONP) is one of main materials of reused papermaking of recycled fibers. However, fibers would undergo a decrease in quality during recycling. For example, with each subsequent recycling, the strength of paper could decrease, which makes the papermaking potential of recycled fiber decrease (Chen et al., 2009, 2010). In order to increase the papermaking potential and the using value of the recycled fibers, it is necessary to modify the recycled fibers through some methods. There are many methods to modify the recycled fibers at present. Such as, beating (Jiao et al., 1998), chemical pretreatment (Waterhouse and Liang, 1995), carboxymethyl treatment using the chloroacetic acid (Rácz and Borsa, 1997), adding cellulose derivatives (carboxymethyl cellulose) (Blomstedt et al., 2007) and enzyme treatment (Pala et al., 2001; Ruel et al., 2003; Choi and Jong, 2001; Kim, 1996), etc. But beating and chemical pretreatment are only the remedial measures, and the effect is not evident and is only limited to the first recycling. In addition it is well known that the swelling ability of cellulosic fibers is largely affected by the acid

* Corresponding author. Tel.: +86 159 1582 0693; fax: +86 20 39380560. ** Corresponding author. Tel.: +86 20 87114970; fax: +86 20 39380560. E-mail addresses: [email protected] (Y. Chen), [email protected] (J. Wan). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.04.015

group content of cellulose (Scallan, 1983). The carboxyl group content would greatly decrease when the fibers are once dried (Rácz and Borsa, 1997), which could lead to the decrease of the fiber swelling ability resulting in the decrease of fiber bonding ability and fiber strength. Therefore, in order to keep the fiber strength from changing, the cellulosic fiber must be hold the acid groups as much as possible. Recently, the two common chemical treating methods introducing the acid groups (for example carboxyl group) are carboxymethyl treatment using the chloroacetic acid and adding cellulose derivatives (carboxymethyl cellulose). However, chemical treatments involve harsh reaction conditions, loss of desirable components, and potential use of hazardous chemicals. But enzymatic treatment conditions are often milder, less damaging of the fiber, and are environmentally friendly. Laccase is a multi-copper-containing oxidoreductase, which can catalyze the oxidation of various substrates including phenols, diphenols, aminophenols, polyphenols, polyamines, and lignin-related molecules (Burton, 2003; Nicotra et al., 2004a,b; Zhang et al., 2002; Galli and Gentili, 2004; d’Acunzo et al., 2006). Laccase is called the ‘green’ catalyst, due to which works with air and produce water as the only by-product (Riva, 2006). Laccase oxidation of cellulosic fibers has been reported to increase acid group content and improve virgin pulp properties (Viikari et al., 1999). Carboxylic acids groups are beneficial in the bonding of pulp fibers in paper and can increase the strength of the paper (Zhang et al., 2007). Mohandass et al. have found that laccase treatment of recycled dyed pulp increased acid group content, tear index, tensile index, and color removal in a dose-dependent manner (Mohandass et al., 2008). Laccase treatment has been shown to improve the

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strength of mechanical pulp by enhancement of the tear–tensile relationships (Mansfield, 2002). The wet tensile index of unbleached kraft pulp treated with the laccase in presence of methyl syringate was the twice of only treated with the laccase (Liu et al., 2009). Laccase-meditor treatment of high-kappa SW kraft pulps with 2,20 -azino-bis(3-ehylbenzthiazoline-6-sulphonic acid (ABTS) or phenonthiazine-10-propionic acid (PPT) has been reported to beneficially improve wet tensile strength values (Lund and Felby, 2001). Recent study by Witayakran and Ragauskas (2009) has also demonstrated the potential of laccase-facilitated grafting of amino acids (phenylalanine, serine, aspartic acid, histidine, arginine and alanine) to high-lignin softwood kraft pulps thereby increasing fiber charge and sheet strength properties. Based on these results, it was apparent that laccase can be employed to generate reactive quinonoid structures in lignin-rich fibers that could then be reacted with histidine to generate enhanced carboxyl group content or fiber charge (Witayakran and Ragauskas, 2009). This study examines the optimal laccase–histidine treatment conditions with respect to ONP sheet dry and wet tensile indexes, and also investigates the impacts of laccase–histidine treatment of ONP pulp on the carboxyl group content of the pulp, water retention value, fiber length and coarseness, and fiber surface morphology.

2.5. Carboxylic acid content determination The carboxylic acid content of each pulp was determined according to TAPPI Standard T237. Pulp (1.5 g o.d.) was stirred in a solution of 0.1 M HCl (300.00 mL) for 1 h. The pulp was then filtered and rinsed in a buchner funnel with deionized water. The sample was transferred into 250.00 mL of 0.001 M NaCl solution which was the acidified with 1.50 mL of 0.1 M HCl. This solution was titrated conductometrically with 0.05 M NaOH at 0.2 mL increments, recording the conductivity at each increment. The titration data was plotted as conductivity vs. volume to determine the milli-equivalent of acid groups per kilogram of pulp. The reported results were the average of two measurements. 2.6. Water retention value (WRV) determination WRV was determined by a centrifugal method with 1.5 g samples (o.d.) at 3000 r/min for 15 min. The wet pulp weight and dry weight after drying are measured. The WRV is calculated as follows:

WRV ¼

m1 —m2  100% m2

ð1Þ

where m1 is the weight of wet pulp after centrifugation, m2 is the weight of dry pulp.

2. Methods

2.7. Environmental scanning electron microscope (ESEM) analysis

2.1. Materials

ESEM images of fiber surface morphology were obtained using a FEI Quanta 200 environmental scanning electron microscope, operated in gas secondary electron detector at an accelerating voltage of 15 kV. The sample was coated with gold prior to analysis. Images of fibers were obtained in magnification of 4000.

The old newspaper deinked pulp used in this study, 12 months old, was taken from a paper mill in Dongguan city, China. Laccase with an activity of 900 U/g was provided by Novozymes (China). Histidine was provided by Guangzhou Jingke Co. Ltd., China.

3. Results and discussion 2.2. Pulp treatment 0.9 U/g of laccase and 1% (w/w) of histidine relative to the dry pulp were added with stirring to a 5% consistency aqueous suspension of ONP pulp buffered to pH 7 with 0.1 M sodium bicarbonate solution. The pulp slurry was magnetically stirred at room temperature under O2 atmosphere (continuous bubbling) for 1.5 h. After treatment, the pulp sample was filtered, washed with deionized water until the filtrate was colorless and air-dried.

2.3. Paper testing Treated and control pulps were disintegrated for 30,000 revolutions and then were refined for 10,000 in a PFI mill according to TAPPI Standard T248. Handsheets were formed according to TAPPI Standard T205 and conditioned for at least 24 h at 23 °C, 50% relative humidity before physical testing. The tensile strength was measured with a tensile strength from Lorentzen and Wettre. The wet tensile strength was tested after immersing the test slip into distilled water for 1 h (Lund and Felby, 2001). Five handsheets were made from each pulp sample and 12 test strips were used for dry and wet tensile strength testing according to TAPPI Standard T494 and T456, respectively.

2.4. Average fiber length and coarseness determination Average fiber length and coarseness of ONP pulp were measured on a KAJAANI FS-300 fiber analyzer (Valmet Automation Kajaani Ltd.) according to TAPPI Standard T-pm-91.

3.1. Effect of laccase dose on the tensile strength of ONP pulp In this study, old newspaper pulp was treated with varying amounts of laccase which are 0, 0.45, 0.9, 1.35, 1.95, and 2.25 U/g dry pulp in the presence of 1% (w/w) histidine in sodium bicarbonate buffer pH 7 at room temperature for 1.5 h. The laccase dose response with respect to dry, wet tensile strength indexes is shown in Table 1. From Table 1, it can be seen that the dry, wet tensile indexes of ONP pulp handsheets increased when the amount of laccase increased. The dry, wet tensile strength indexes reached the highest values, and compared with the control sample, the values was increased by 33.1% and 55.1%, respectively. When the amount of laccase exceeded 0.9 U/g, the dry tensile index had little change and the wet tensile index decreased. Therefore, the optimal amount of laccase for laccase–histidine modification was 0.9 U/g. The dry, wet tensile strength of laccase–histidine-treated ONP pulp all increased, but the increasing percentage of wet tensile strength was much larger than dry tensile strength, which is in accord with the reported results (Wang et al., 2005; Mou et al., 2009). We think the reason why the increase in wet tensile strength is larger than dry tensile strength is probably because of the increased level of inter fiber bonding is low and may be of a nature that does not contribute to the dry tensile strength (Lund and Felby, 2001). Dry tensile strength in paper originates mainly from hydrogen between adjacent fibers, whereas wet tensile strength is related to bonds being resistant to water sorption like covalent- and ionic bonds or creation of a polymer matrix around fibers (Lund and Felby, 2001). Lund and Felby considered that the improvement in wet tensile strength of unbleached kraft pulp through laccase catalyzed oxidation is related to polymerization of lignin on fibers in the

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Y. Chen et al. / Bioresource Technology 101 (2010) 7041–7045 Table 1 Effect of laccase dose on tensile strength of ONP pulp. Laccase dose (U/g)

Con

0

0.45

0.90

1.35

1.95

2.25

Dry tensile index (Nm/g ± SD) Wet tensile index (Nm/g ± SD)

24.8 ± 1.1 0.583 ± 0.009

25.0 ± 0.2 0.592 ± 0.011

27.1 ± 0.4 0.801 ± 0.003

33.0 ± 0.6 0.904 ± 0.015

33.1 ± 0.2 0.858 ± 0.023

33.4 ± 0.3 0.889 ± 0.008

33.2 ± 0.6 0.865 ± 0.007

SD = Estimated standard deviation of the mean.

hand sheet and/or coupling of phenoxy radicals on lignin associated to adjacent fibers (Lund and Felby, 2001). 3.2. Effect of histidine dose on the tensile strength of ONP pulp Histidine dose is a very important influence factor for the laccase–histidine treatment process. In this study, different amounts of histidine (i.e. 0%, 0.5%, 1%, 1.5%, 2% and 2.5% relative the dry pulp) were examined to find the optimal amount of histidine for modifying fibers. Other reaction conditions were: laccase dose 0.9 U/g, pulp consistency 5%, room temperature (about 25 °C), pH 7, reaction time 1.5 h, O2 atmosphere. Table 2 shows the effect of different amounts of histidine on the dry, wet tensile strength of ONP pulp handsheets. As shown in Table 2, the greater the amount of histidine employed, the greater increases in dry, wet tensile indexes. The dry, wet tensile indexes reached the maximum values when the amount of histidine was 1%. When the amount of histidine exceeded 1%, the dry tensile index decreased and the wet tensile index had little change. Therefore, 1% was chosen as an optimal amount of histidine for this treatment system. In addition, compared with the control sample, the wet tensile index of laccasetreated pulp was only increased by 3.6%, however laccase–histidine-treated pulp (0.9 U/g and 0.5% histidine) were increased by 46.5%. This indicated that the strength properties of ONP deink pulp were significantly improved when the pulp was treated by laccase with histidine. These results are in accord with Witayakran and Ragauskas’ study (Witayakran and Ragauskas, 2009). They suggested that the study about the effects of the laccase–amino acid grafting treatment on softwood kraft pulp highlights a novel technologies approach for the modification of lignocellulosic fibers by laccase via oxidation-Michael addition.

histidine dose 1%, pulp consistency 5%, pH 7, reaction time 1.5 h, O2 atmosphere. The results of this treatment showed that the increase in temperature did not increase the dry, wet tensile indexes of the ONP pulp (Table 3). Therefore, the optimal condition of ONP pulp modification was the treatment at room temperature. 3.4. Effect of reaction time on the tensile strength of ONP pulp Reaction time is also not ignored for the laccase–histidine treatment process. Shorter reaction time can lead to the incomplete catalytic oxidation–reduction reaction of laccase, and higher reaction time can lead to the decrease efficiency and the waste of energy (Wang et al., 2005). The effect of different reaction time on tensile strength can be seen in Table 4. Other reaction conditions were: laccase dose 0.9 U/g, histidine dose 1%, pulp consistency 5%, pH 7, room temperature, O2 atmosphere. The dry, wet tensile indexes were increased by 3.7% and 13.4%, respectively, when the reaction time increased from 1 to 1.5 h. however when the reaction time exceeded 1.5 h, the dry, wet tensile indexes got little change. Therefore, the optimal reaction time for the laccase–histidine treatment was 1.5 h. This result is different from the data obtained by other authors, most likely due to fundamental differences in sample materials. For example, Lund and Felby reported the optimal reaction time of wet strength improvement of unbleached kraft pulp through the laccase catalyzed oxidation treatment time was 1 h (Lund and Felby, 2001), while Witayakran and Ragauskas reported the optimal reaction time of modification of high-lignin softwood kraft pulp (linerboard pulp) with laccase and amino acids was 24 h (Witayakran and Ragauskas, 2009). We suggested that the ONP deink pulp was more easily catalyzed by laccase with histidine through Michael addition than the softwood kraft pulp.

3.3. Effect of reaction temperature on the tensile strength of ONP pulp 3.5. Effect of pH on the tensile strength of ONP pulp Reaction temperature is also an important factor to consider because increasing temperature can potentially increase both the rate of the desired enzymatic reaction and the rate of enzyme inactivation (Peterson et al., 2007). Therefore, in this study, the effect of different reaction temperature (25, 40, 50 and 60 °C) on the strength properties of laccase–histidine-treated pulp was also examined. Other reaction conditions were: laccase dose 0.9 U/g,

To determine the effect of the treatment conditions on strength properties, the pH of the treatment was changed from 3.5 to 7.0 which is known to be the optimal pH for laccase (Chakar and Ragauskas, 2001). Other reaction conditions were: laccase dose 0.9 U/ g, histidine dose 1%, pulp consistency 5%, room temperature, O2 atmosphere.

Table 2 Effect of histidine dose on tensile strength of ONP pulp. Histidine dose (%)

Con

0

0.5

1

1.5

2

2.5

Dry tensile index (Nm/g ± SD) Wet tensile index (Nm/g ± SD)

24.8 ± 1.1 0.583 ± 0.009

25.7 ± 0.2 0.639 ± 0.011

26.5 ± 0.4 0.854 ± 0.027

33.8 ± 0.6 0.904 ± 0.015

33.5 ± 0.2 0.916 ± 0.013

33.0 ± 0.3 0.910 ± 0.005

33.2 ± 0.1 0.920 ± 0.001

SD = Estimated standard deviation of the mean.

Table 3 Effect of reaction temperature on tensile strength of ONP pulp. Temperature (°C)

Con

25

40

50

60

Dry tensile index (Nm/g ± SD) Wet tensile index (Nm/g ± SD)

24.8 ± 1.1 0.583 ± 0.009

33.0 ± 0.6 0.904 ± 0.015

33.4 ± 0.5 1.04 ± 0.037

33.1 ± 0.3 0.965 ± 0.016

33.6 ± 0.2 0.976 ± 0.025

SD = Estimated standard deviation of the mean.

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Table 4 Effect of reaction time on tensile strength of ONP pulp. Time (h)

Con

1

1.5

2

2.5

Dry tensile index (Nm/g ± SD) Wet tensile index (Nm/g ± SD)

24.8 ± 1.1 0.583 ± 0.009

31.8 ± 0.8 0.796 ± 0.037

33.0 ± 0.6 0.904 ± 0.015

33.2 ± 0.3 0.916 ± 0.008

32.2 ± 0.7 0.92 ± 0.019

1

30 0.8

25 20

0.6

15

0.4

10 0.2

5 0

3.5

4.5 pH Dry tensile index

7

0

Wet tensile index

Fig. 1. Effect of pH on tensile strength of ONP pulp.

The effect of pH on the dry tensile index was unobvious, but on the wet tensile index was great (Fig. 1). The wet tensile index of laccase–histidine-treated pulp greatly increased under different pH values, compared with the control sample. The wet tensile index was increased by 26.6% and 13.3% when the treatment pH was 7, compared with pH 3.5 and 4.5. The reason is that the laccase–histidine treatment at neutral pH range can obtain the more ideal strength property, which is beneficial to industrial application. The requirement of using higher pH (pH 7) for the Micheal addition catalyzed by laccase was also reported by previous researchers (Witayakran and Ragauskas, 2009; Witayakran et al., 2007).

3.6. The change of carboxyl group content and water retention value after laccase–histidine treatment The above experimental results showed that there was the highest tensile strength of laccase–histidine-treated pulp when the treatment conditions were laccase dose 0.9 U/g, histidine dose 1%, pulp consistency 5%, room temperature, reaction time 1.5 h, pH 7, O2 atmosphere. Therefore, these reaction conditions were chosen to investigate the change of carboxyl group content. The results of analysis of carboxyl group content for control group (Con), laccase-treated pulp (Lac), histidine-treated pulp (His) and laccase– histidine pulp (Lac–His) are shown in Fig. 2. These results demonstrate that laccase-treated pulp provided a higher yield of carboxyl content compared with the control pulp due to the oxidation or degradation of lignin by laccase. The potential of laccase to oxidize lignin has been reported by many studies (Konishi et al., 1974; Wei et al., 2006; Shleev et al., 2006). The formation of carboxyl groups by laccase oxidation of the terminal primary alcohols of lignin side was reported by Konishi et al. (1974). Wei et al. (2006) have confirmed the oxidation of lignin by laccase. They studied laccase-treated lignin structure, and found that the phenolic hydroxyl content of lignin decreased after lacccase treatment and C–C bond cleavage occurred on side chain of lignin. In addition, the formation of quinonoid structures in laccase-treated lignin was also observed by Shleev et al. (2006). Witayakran and Ragauskas (2009) have also concluded that the increase in carboxyl group for laccase-treated linerboard pulp. Therefore, the increase

90

180

80 70 60

160 140 120

50 40 30 20

100 80 60 40

10 0

20 0 Con

Lac

His

Carboxyl group content

WRV, %

Dry tensile index,Nm/g

35

Wet tensile index,Nm/g

1.2

40

Carboxyl group content, mmol/kg

SD = Estimated standard deviation of the mean.

Lac-His WRV

Fig. 2. Carboxyl group content and water retention value (WRV) of control pulp (Con), laccase-treated pulp (Lac), histidine-treated pulp (His) and laccase–histidinetreated pulp (Lac–His).

in carboxyl group for laccase-treated ONP pulp in this study agrees with those in literature. Fig. 2 shows that compared with control group, the histidinetreated pulp provided a 14.9% increase of carboxyl group content. The result indicates that histidine can react with pulp fibers presumably due to quinonoid structures present in ONP pulp (Dyer and Ragauskas, 2003). However, when the pulp was treated with laccase and histidine, the treated pulp gave the highest yield of carboxyl groups, which is in accord with Witayakran and Ragauskas’ report (Witayakran and Ragauskas, 2009). They also suggested that the increase of carboxyl group indicated that laccase-treated fibers facilitated the grafting of histidine onto the fiber lignin. Laccase– histidine-treated pulp gave a 45.7% increase of carboxyl group content compared with control group. The increase of carboxyl group content led to the increase of the bonding of pulp fibers resulting in the increase of the strength of the paper, which is in accord with the above experimental results. Water retention value (WRV) is a general measure of fiber swelling. The results of analysis of WRV for control group (Con), laccase-treated pulp (Lac), histidine-treated pulp (His) and laccase–histidine pulp (Lac–His) are also shown in Fig. 2. Compared with control pulp, WRV of laccase-treated pulp, histidine-treated pulp and laccase–histidine pulp was increased by 16.2%, 10.5% and 45.7%, respectively. The laccase–histidine-treated ONP pulp gave the highest WRV, which led to the highest bonding of pulp fibers resulting in the highest strength of the paper.

3.7. Average fiber length and coarseness Average fiber length and coarseness are important factors in ONP deinked pulp. The average fiber length and coarseness of the control, laccase-treated, histidine-treated and laccase–histidinetreated pulp fibers are listed in Table 5. Compared with the control pulp, the fiber average length of laccase-treated pulp, histidinetreated pulp and laccase–histidine-treated pulp changed little. This suggested that the fibers are not ruptured during these treatment processes. In addition, the coarseness also changed little. These results demonstrated that the treatment mainly acted on the fiber surface, and did not touch on fiber inner.

Y. Chen et al. / Bioresource Technology 101 (2010) 7041–7045 Table 5 Effect of laccase and histidine treatment on fiber average length and coarseness. Pulp

Ln mm ± SD

Lw mm ± SD

Lww mm ± SD

Coarseness mg/m ± SD

Control Laccase Histidine Laccase– histidine

0.53 ± 0.07 0.55 ± 0.09 0.55 ± 0.13 0.55 ± 0.06

1.09 ± 0.08 1.19 ± 0.06 1.15 ± 0.01 1.17 ± 0.07

1.87 ± 0.09 1.91 ± 0.05 1.89 ± 0.02 1.97 ± 0.04

0.178 ± 0.05 0.171 ± 0.07 0.175 ± 0.03 0.168 ± 0.08

SD = Estimated standard deviation of the mean.

3.8. ESEM observations of fiber surface morphology ESEM images of fiber surface morphology of the control pulp, laccase-treated pulp, histidine-treated pulp and laccase–histidine-treated pulp were investigated, which are not shown. The fiber surface of the control pulp was comparatively smooth. When the ONP pulp was treated with laccase, the fiber surface became rough, and fibrils can be seen on the fiber surface, which demonstrated it occurred delignification on the fiber surface, releasing fibrils. When the ONP pulp was treated with histidine, the fiber surface also became rough. The fiber surface became rougher after laccase–histidine treatment, and more fibrils can be seen and longitudinal tearing could also be observed. From the ESEM image of fiber surface morphology of laccase–histidine-treated pulp, it can also be seen that the laccase–histidine-treated pulp fibers were more collapse than the control and laccase-treated pulp fibers. These results are in accord with the previous reports (Witayakran and Ragauskas, 2009; Xu et al., 2007). Therefore, these changes of fiber surface morphology, such as cracks and fibrillation, led to form better bonding between fibers in handsheets resulting in the increase of the paper strength of laccase–histidine-treated ONP pulp. 4. Conclusions The tensile strength, carboxyl group content and WRV of the pulp obviously increased after laccase–histidine treatment, but the fiber average length and coarseness of laccase–histidine-treated ONP pulp changed little. In addition, the wet tensile strength under different reaction temperature (25, 40, 50 and 60 °C) changed little when the ONP pulp was treated by laccase and histidine. The wet tensile strength was the maximum when the ONP pulp was treated by laccase and histidine at pH 7. Therefore, the laccase–histidine treatment could be carried out at room temperature and pH 7, which is very beneficial to the practical application. Acknowledgements This work was supported by the National Technology Research and Development of China (863 Programme) (No. 2007AA03Z433), Science and Technology Key Project of Guangdong Province, China (No. 2008A0302008) and Science and Technology Plan Project of Guangdong Province, China (No. 2008B030302035). References Blomstedt, M., Mitikka-Eklund, M., Vuorinen, T., 2007. Simplified modification of bleached softwood pulp with carboxymethyl cellulose. Appita J. 60 (4), 309– 314. Burton, S.G., 2003. Laccases and phenol oxidases in organic synthesis – a review. Curr. Org. Chem. 7, 1317–1331. Chakar, F.S., Ragauskas, A.J., 2001. Formation of quinonoid structures in laccasemediator reactions. ACS series fundamentals and catalysis of oxidative

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