Abstract
Xylanase, pectinase complex, and BL11 pectinase were employed for the pretreatment of gampi and paper mulberry bast fiber pulps prior to chlorine dioxide bleaching. The bleaching efficiencies of the pulps with different enzymatic pretreatments were investigated. Accelerated aging by heat-humidity treatment was also conducted to evaluate yellowing phenomena and to estimate the prevention of brightness reversion (brightness retention) by enzymatic pretreatment. The order of active chlorine required with respect to pretreatment was pectinase complex > xylanase > BL11 pectinase for soda and soda/oxalate gampi pulps and pectinase complex > BL11 pectinase > xylanase for soda and soda/oxalate paper mulberry pulps. Higher brightness retention values were observed for soda/oxalate pulps compared to soda pulps. The brightness retention levels for gampi pulps and mulberry pulps after ClO2 bleaching with enzymatic pretreatment were higher than the levels of ClO2 and NaClO bleaching pulps. Enzymatic treatments were thus able to reduce the usage of ClO2 and to assist in producing photo-stable paper materials for art and artifact-repairing applications. Thus, enzymatic pretreatment of the pulp has the potential to meet world trends and environmental sustainability for pulp and paper industries.
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Properties of Enzyme Pretreated Wikstroemia sikokiana and Broussonetia papyrifera Bast Fiber Pulps
Lang-Dong Lin,a Fang-Chih Chang,b,* Chun-Han Ko,b,c,* C. Y. Wang,c and Ya-Nan Wang c
Xylanase, pectinase complex, and BL11 pectinase were employed for the pretreatment of gampi and paper mulberry bast fiber pulps prior to chlorine dioxide bleaching. The bleaching efficiencies of the pulps with different enzymatic pretreatments were investigated. Accelerated aging by heat-humidity treatment was also conducted to evaluate yellowing phenomena and to estimate the prevention of brightness reversion (brightness retention) by enzymatic pretreatment. The order of active chlorine required with respect to pretreatment was pectinase complex > xylanase > BL11 pectinase for soda and soda/oxalate gampi pulps and pectinase complex > BL11 pectinase > xylanase for soda and soda/oxalate paper mulberry pulps. Higher brightness retention values were observed for soda/oxalate pulps compared to soda pulps. The brightness retention levels for gampi pulps and mulberry pulps after ClO2 bleaching with enzymatic pretreatment were higher than the levels of ClO2 and NaClO bleaching pulps. Enzymatic treatments were thus able to reduce the usage of ClO2 and to assist in producing photo-stable paper materials for art and artifact-repairing applications. Thus, enzymatic pretreatment of the pulp has the potential to meet world trends and environmental sustainability for pulp and paper industries.
Keywords: Bast fiber; Brightness; Broussonetia papyrifera (L.) Hert. ex Vent; Pectinase; Polygalacturonase; Wikstroemia sikokiana Franch. et Sav.; Xylanase
Contact information: a: Department of Cultural Heritage Conservation, National Yunlin University of Science and Technology, Yunlin 640, Taiwan; b: The Experimental Forest, National Taiwan University, No. 12, Sec. 1, Chien-Shan Rd., Chu-Shan, Nan-Tou 55750, Taiwan; c: School of Forestry and Resource Conservation, National Taiwan University, Taipei 10617, Taiwan;
* Corresponding author: d90541003@ntu.edu.tw; chunhank@ntu.edu.tw
INTRODUCTION
Pulping chemistry has been thoroughly studied for more than 100 years. The pulping and commercial processes have been refined and optimized to produce high-quality paper at low prices. Changes in wood resources have prompted researchers to explore whether fibers other than softwood and hardwood can be used for papermaking (Han and Rymsza 1999). Non-wood resources are important raw materials in many countries where wood is not available in sufficient quantities to meet the demand for pulping and papermaking (Salmela et al. 2008).
Fibers can be grouped into three categories: wood, non-wood (grass, bast, leaf, or fruit fibers), and non-plant (Saijonkari-Pahkala 2001). Bast fibers have been utilized to obtain high-quality paper (Marques et al. 2010; Shi et al. 2011; Tahir et al. 2011; Jin et al. 2012). Gampi (Wikstroemia sikokiana) and paper mulberry (Broussonetia papyrifera) are bast fibers that have a long history of use in hand papermaking (Hubbe and Bowden 2009). They are sustainable and reproducible raw materials and have been applied in the making of conventional oriental art paper. In comparison to wood pulps, non-wood pulps produced from specific plants are more heterogeneous, and the ability to process these materials into paper products would have the additional benefit of mitigating disposal costs and the associated environmental pollution (Spiridon 2007). It is also expected that non-wood fibers will play an increasingly important role in the pulp and paper industry due to diminishing forest resources, environmental issues, growing paper consumption, and progress in advanced technologies of non-wood utilization (Hammett et al. 2001; Salmela et al. 2008).
Enzymes including lipases, esterases, pectinases, hemicellulases, cellulases, and ligninolytic enzymes have been applied to improve the properties of woody and nonwoody fibers (Spiridon 2007; Fillat et al. 2010; Pathak et al. 2011; Maijala et al. 2012; Martín-Sampedro et al. 2012a,b; Virk et al. 2012). Biobleaching and bioprocessing of pulps using enzymes is the most suitable biological alternative to environmentally unfriendly chemicals (Ninawe and Kuhad 2006; Dhiman et al. 2008; Kumar and Satyanarayana 2012; Iqbal et al. 2013; Jegannathan and Nielsen 2013). Enzymes are green chemicals that can improve pulp and paper operations because they require less energy in refining, less consumption of bleaching agents, and lower toxic pollution loading (Spiridon 2007; Bajpai 2010; Lian et al. 2012). To meet environmental and legislative requirements in the market, the pulp and paper industry is modifying delignification, bleaching, and effluent technologies to reduce the environmental impact. Thus, elimination of chlorine in the bleaching process and the application of chlorine- free bleaching processes has become increasingly important (Spiridon 2007; Reinstaller 2008; Singh et al.2011).
The main aim of this study is to investigate the effects of enzymatic pretreatment (xylanase, pectinase complex, and BL11 pectinase) on the pulp properties of gampi and paper mulberry bast fibers, which are suitable for art paper. The bleaching agent efficiency on pulp after the enzymatic pretreatments was analyzed. Accelerated aging by heat-humidity treatment were also conducted to evaluate yellowing phenomena and brightness retention.
EXPERIMENTAL
Materials
Gampi (Wikstroemia sikokiana) is a protophyte found in the Philippines whose bark was imported for the experiments. The inner bark of a one-year-old paper mulberry (Broussonetia papyrifera) tree was imported from Thailand. Gampi and paper mulberry bark were cut into pieces approximately 5 to 7 cm in length. Certain bark was dried in an oven to measure moisture content. Both gampi and paper mulberry bark were treated with either 15% NaOH or 15% NaOH with 1.35% Na2C2O4 for 90 min before being transferred to the digester. The chemical compositions of gampi and paper mulberry pulp are shown in Table 1. A significant decrease in the xylose content was observed, which shows that much of the xylose was solubilized during the pulping processes. However, most of the lignin was still retained in the pulp. The high lignin content yielded pulps that are prone to yellowing during storage (Baty et al. 2010).
Table 1. Chemical Compositions of Gampi and Paper Mulberry Pulp
Data provided as the mean of triplicate ± the standard deviation
Xylanase used in this study was Pulpzyme HC, provided by Novozymes (Denmark). Pulpzyme HC is produced by a genetically modified Bacillus microorganism under submerged fermentation and was used in experiments to catalyze the hydrolysis of deacetylated xylan substrates. Pectinase used in this study was Novozym 863, provided by Novozymes. Novozym 863 is a highly active pectolytic enzyme preparation produced by a selected strain of Aspergillus aculeatus. The BL11 pectinase was produced by the selected strain BL11 Paenibacillus campinanesis. The bacterial strains were screened and isolated from thermo-alkaline black liquor (58 °C, pH 9.83) from Hsinying Mill of the Taiwan Pulp and Paper Company (Tainan, Taiwan).
Methods
Bleaching stages
Four types of pulp were placed in polyethylene bags and then treated with chlorine dioxide for bleaching. The control pulp was treated with ClO2 at Kappa factors of 0.05, 0.1, 0.15, 0.2, and 0.3. Applied chlorine dioxide charge is defined as active chlorine value divided 2.63. The chlorine dioxide dose of D0 is usually expressed using the Kappa factor, where a greater Kappa factor is used as more lignin is removed. DE (D: Chlorine dioxide, E: Alkaline extraction) bleaching materials were obtained from an industrial source, with bleaching stage conditions of pH 2 adjusted by HCl, 0.25% active chlorine as Cl2, and 5% consistency at 50 °C for 1 h. Alkaline extraction was carried out at 10% consistency, at 70 °C for 1 h. Pulp with a Kappa factor of 0.3 was then treated with a DD bleaching succession. Pulp brightening was carried out at 10% consistency with either 0.35% or 0.15% ClO2 at 70 °C for 3.5 h. After each bleaching stage, the pulp was washed with deionized water and dewatered using a 75-μm mesh. The NaClO bleaching was at 5% consistency, 70 °C, and pH 11 for 1 hr, then compared with chlorine dioxide bleaching.
For gampi soda pulp, gampi soda pulp with oxalate, paper mulberry soda pulp, and paper mulberry soda pulp with oxalate, enzymatic pretreatments were carried out in polyethylene bags with 60 g of air-dried pulp at 10% consistency in a water bath at 50 °C for xylanase and BL11 pectinase and at 35 °C for pectinase complex. Xylanase, pectinase complex, and BL11 pectinase dosages ranged from 0 to 3000 IU/g oven-dried pulp (g o. d. p.) for 3 h at pH 3.5, 7, and 10, respectively. To ensure complete mixing, the enzyme was mixed into the pulp by hand kneading. Prior to full DE bleaching, the enzymatic treated pulps were thoroughly washed and dewatered using a 75-μm mesh. After pretreatment with enzymes, pulps were bleached with the DE sequence and treated in parallel with the control group.
Analytical methods
All tests were performed according to standard methods as follows: forming handsheets for physical tests of pulp (TAPPI T205 2002); physical testing of pulp handsheets (TAPPI T220 2010); tensile properties (TAPPI T494 2006); tearing strength (TAPPI T414 2012); low bursting strength (CNS 1353 2008); and folding endurance (TAPPI T511 2013). The basis weight of each physically tested handsheet was 60 g/m2. Kappa number, a measure of lignin content, was determined by reaction of pulp samples with acidified potassium permanganate (TAPPI T236 2013). Brightness was determined by measuring the reflectance at 457 nm on handsheets using the Technidyne brightness meter (TAPPI T452 2008). Brightness retention was determined using the Technidyne brightness meter according to the TAPPI test method T452. Samples were treated at 85 °C and 87% RH for 16 h. The calculation of brightness retention was performed according to Chang et al. (1998). The surface structure of unbleached and bleached pulp were analyzed by scanning electron microscope (SEM) provided with Vega TC software.
RESULTS AND DISCUSSION
Pulp Properties of Enzymatic Pretreatment after Chlorine Dioxide Bleaching
Compared with the results found by Ko et al. (2010), xylanase pretreatment at 2.5 IU/g.o.d.p. increased brightness by 4.6% for hardwood kraft pulp using a Kappa factor of 0.1 in the D0 stage of DED bleaching, and xylanase pretreatment at 6 IU/g.o.d.p. increased the brightness of gampi soda pulp by 6.93%, oxalate gampi soda pulp by 6.00%, paper mulberry soda pulp by 5.33%, and oxalate paper mulberry soda pulp by 3.60%.
Increasing the dosage of xylanase treatment on pulp had the expected result of decreasing Kappa number and increasing brightness (Table 2). In terms of both gampi and paper mulberry Kappa number and brightness, adding oxalate increased Kappa number and decreased brightness. The low bleaching efficiency of soda pulp with oxalate addition may be attributed to lower xylose content compared to soda pulp with no addition. The xylanolytic enzyme system has less glycan to convert into its constituent sugars. From the experimental results, we suggest that adequate dosages of xylanase are 300 IU/g.o.d.p. for gampi soda pulp, 60 IU/g.o.d.p. for gampi soda pulp with oxalate, 1200 IU/g.o.d.p. for paper mulberry soda pulp, and 1200 IU/g.o.d.p. for paper mulberry soda pulp with oxalate.
Increasing the dosage of pectinase complex decreased the Kappa number and increased brightness obviously (Table 2). The bleaching efficiency was higher with pectinase complex pretreatment compared to xylanase pretreatment. In terms of both gampi and paper mulberry, adding oxalate increased the value of the Kappa number. Oxalate addition had a negative impact on the brightness of gampi pulp only for the 3000 IU/g.o.d.p. pectinase complex dosage. Oxalate addition had a negative impact on the brightness of paper mulberry at most pectinase complex dosages. Based on the experimental results, it is suggested that adequate dosages of pectinase complex are 600 IU/g.o.d.p. for gampi soda pulp, 3000 IU/g.o.d.p. for gampi soda pulp with oxalate, 600 IU/g.o.d.p. for paper mulberry soda pulp, and 240 IU/g.o.d.p. for paper mulberry soda pulp with oxalate.
Table 2. Impact of Enzymatic Addition on Brightness of 0.15 Kappa Factor Bleached Pulp
Data provided as the mean of triplicate ± the standard deviation
If the objective is to obtain a target brightness of 47.5% GE, then xylanase pretreatment can decrease the usage of chlorine dioxide. A xylanase dosage of 3000 IU/g.o.d.p. can decrease required active chlorine per g.o.d.p. for gampi soda pulp by 33.3%, and a xylanase dosage of 1200 IU/g.o.d.p. can decrease required active chlorine per g.o.d.p. for gampi soda pulp with oxalate by 14.29%. If the objective is to obtain a target brightness of 59% GE, a xylanase dosage of 1200 IU/g.o.d.p. can decrease required active chlorine per g.o.d.p. for paper mulberry soda pulp by 33.33% and a xylanase dosage of 3000 IU/g.o.d.p. can decrease required active chlorine per g.o.d.p. for paper mulberry soda pulp with oxalate by 25.00% (Fig. 1a). Pectinase complex can decrease the usage of chlorine dioxide for gampi and paper mulberry pulp. If the objective is to obtain a target brightness of 47.5% GE, then a pectinase complex dosage of 600 IU/g.o.d.p. can decrease required active chlorine per g.o.d.p. for gampi soda pulp with oxalate by 66.67% and a pectinase complex dosage of 60 IU/g.o.d.p. can decrease the required active chlorine per g.o.d.p. for gampi soda pulp with oxalate by 33.73%. If the objective is to obtain a target brightness of 59%, then a pectinase complex dosage of 600 IU/g.o.d.p. can decrease the required active chlorine per g.o.d.p. for paper mulberry soda pulp by 50.00% and a pectinase complex dosage of 240 IU/g.o.d.p. can decrease the required active chlorine per g.o.d.p. for paper mulberry soda pulp with oxalate by 60.11% (Fig. 1b).
Fig. 1. Impact of different dosages of (a) xylanase and (b) pectinase on the brightness of pulp. Data provided as the mean of triplicate ± the standard deviation
To obtain a target brightness of 47.5% GE, a pectinase complex dosage of 170 IU/g.o.d.p. can decrease the required active chlorine per g.o.d.p. for gampi soda pulp with oxalate by 14.29%. To obtain a target brightness of 59% GE, a pectinase complex dosage of 260 IU/g.o.d.p. can decrease the required active chlorine per g.o.d.p. for gampi soda pulp with oxalate by 33.33%. Even if brightness was not as high as in the case of a pectinase complex, they still exceeded the results for xylanase (Fig. 2).
Fig. 2. Effect of enzymes on the brightness of pulp. Data provided as the mean of triplicate ± the standard deviation
The surface structure of unbleached pulp is shown in Fig. 3a. Gampi and paper mulberry fibers are slender and remain intact after pulping, although gampi pulp has more impurities. The SEM micrographs of ClO2 bleached pulp are shown in Fig. 3b.
Fig. 3. SEM micrographs obtained for (A) gampi soda pulp and (B) gampi soda pulp with oxalate for (a) unbleached pulps, (b) 0.15 Kappa factor ClO2 bleached pulps, and (c) after 0.15 Kappa factor ClO2bleaching with pectinase BL11 260 IU/g.o.d.p pretreatment
Fig. 4. Effect of enzymes on handsheet properties of (a) gampi soda pulp with oxalate and (b) paper mulberry soda pulp with oxalate. Data provided as the mean of triplicate ± the standard deviation.
As the chemical dosage of the bleaching sequence D0ED1D2 was increased, the impurities of all pulps decreased. At a dosage of 0.15 Kappa factor on D0E, gampi and paper mulberry pulp had a lot of impurities (Fig. 3b). However, there were fewer pulp impurities with BL11 pectinase pretreatment (Fig. 3c). Only gampi soda pulp with oxalate had few impurities.
In this study, oxalate addition had no significant effect on the tear index, folding resistance, burst index, or tensile index of gampi pulp or paper mulberry pulp (at level of P < 0.05 according to the Scheffe test). Thus, oxalate addition did not improve the handsheet properties. The handsheet properties of pulps decreased as enzymatic dosage increased. The handsheet properties of pectinase complex pretreated pulp were lower than those of pulp pretreated with xylanase or BL11 pectinase. The BL11 pectinase pretreatment yielded better handsheet properties than pectinase complex pretreatment (Fig. 4).
Prevention of Brightness Reversion
The brightness retention (B.R.) of NaClO bleaching decreased to 60 to 70% after aging. Only paper mulberry soda pulp obtained high brightness retention. Oxalate addition had a positive effect on the brightness retention of 0.15 NaClO Kappa factor bleached gampi pulp. While the bleaching dosage increased, the brightness retention of gampi pulp decreased (Table 3). Compared with the NaClO bleaching process, ClO2 bleaching can improve brightness retention, but paper mulberry soda pulp had a lower initial brightness retention. Oxalate addition had positive effects on all ClO2 bleached pulps (Table 3).
Oxalate addition had a significant effect on the brightness retention of xylanase pretreated 0.15 Kappa factor bleached pulps. The use of xylanase in the post-treatment of bleached hardwood kraft pulp resulted in an obvious reduction of yellowing (Gangwar et al. 2014). The best xylanase dosages for increasing brightness retention are 60 IU/g.o.d.p. for gampi soda pulp, 6 IU/g.o.d.p. for gampi soda pulp with oxalate, 60 IU/g.o.d.p. for paper mulberry soda pulp, and 6 IU/g.o.d.p. for paper mulberry soda pulp with oxalate (Table 4). Paper mulberry soda pulp can give higher brightness retention than paper mulberry soda pulp with oxalate (Table 3). Oxalate addition had no obvious effect on pectinase complex pretreated 0.15 ClO2 Kappa factor bleached gampi pulp.
Table 3. Brightness Retention (%/%) of NaClO or ClO2 Bleached Gampi and Paper Mulberry Pulp
Data provided as the mean of triplicate ± the standard deviation
Table 4. Brightness Retention (%/%) of Gampi and Paper Mulberry at 0.15 Kappa Factor and ClO2Bleached Pulp Pretreated with Xylanase, Pectinase Complex, and BL11 Pectinase
Data provided as the mean of triplicate ± the standard deviation
It is suggested that pectinase complex dosage should be increased to 60 IU/g.o.d.p. of each pulp for the best brightness retention. From Tables 3 and 4 it can be seen that the enzyme pretreatment can improve brightness retention. Pulp has the least resistance to brightness reversion under the high temperature and humidity. The enzymes used in this study have a potential industrial use.
CONCLUSIONS
- The enzymatic treatments were able to reduce the active chlorine usage of chlorine dioxide on the achievement of the same Kappa value, where pectinase complex pretreatments saved the largest values of active chlorine. The BL11 pectinase was the most effective biological alternative to conventional bleaching chemicals.
- The most appropriate doses of enzymes were found to be 300 IU/g.o.d.p. of xylanase for gampi soda pulp, 60 IU/g.o.d.p. for gampi soda pulp with oxalate, 1200 IU/g.o.d.p. for paper mulberry soda pulp, and 1200 IU/g.o.d.p. for paper mulberry soda pulp with oxalate; 600 IU/g.o.d.p. of pectinase complex for gampi soda pulp, 3000 IU/g.o.d.p. for gampi soda pulp with oxalate, 600 IU/g.o.d.p. for paper mulberry soda pulp, and 240 IU/g.o.d.p. for paper mulberry soda pulp with oxalate; and 170 IU/g.o.d.p. of BL11 pectinase for both gampi and paper mulberry pulps.
- Xylanase assisted bleaching was achieved through the cleavage of xylan and its associated lignin-carbohydrate complexes. Hence, resulting pulp after enzyme pretreatment will have low residual xylan and galacturonic acid contents.
- Pretreatment by enzymes can decrease brightness reversion under high temperature and high humidity and assist in producing photo-stable paper materials for art and artifact-repairing applications.
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Article submitted: July 15, 2014; Peer review completed: February 5, 2015; Revisions received and accepted: April 6, 2015; Published: April 28, 2015.
DOI: 10.15376/biores.10.2.3625-3637