Abstract
This study aimed to determine the influence of prehydrolysis of wood chips on the kraft pulping of Populus x euramericana. Optimum cooking conditions were determined by evaluating different alkali concentrations and cooking periods. Xylanase was used to hydrolyze chips before cooking. Prehydrolysis process increased the yield and viscosity of pulp. Consequently, the mechanical properties of paper changed remarkably. In contrast, the optical properties of the unbleached kraft pulp declined due to lignin content.
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Effect of Xylanase Pretreatment on the Kraft Pulping of Poplar
Mehmet Akgül,a Mehmet Onurhan Gücüş,b,* Birol Üner,c and Celil Atik d
This study aimed to determine the influence of prehydrolysis of wood chips on the kraft pulping of Populus x euramericana. Optimum cooking conditions were determined by evaluating different alkali concentrations and cooking periods. Xylanase was used to hydrolyze chips before cooking. Prehydrolysis process increased the yield and viscosity of pulp. Consequently, the mechanical properties of paper changed remarkably. In contrast, the optical properties of the unbleached kraft pulp declined due to lignin content.
Keywords: Enzyme treatment; Kraft pulp; Prehydrolysis; Xylanase
Contact information: a: Karamanoglu Mehmetbey University, 70100 Karaman, Turkey; b: Department of Food Engineering, Karamanoglu Mehmetbey University, 70100 Karaman, Turkey; c: Department of Metallurgy and Materials Engineering, Karamanoglu Mehmetbey University, 70100 Karaman, Turkey; d: Department of Forest İndustry Engineering, İstanbul University, Beyazıt, 34452 Fatih İstanbul, Turkey; *Corresponding author: onurgucus@kmu.edu.tr
INTRODUCTION
Traditional chemical pulping processes are typically carried out under strong alkaline or acidic cooking conditions. Besides, some chemical and enzymes can be added to process pulp improve yield and properties. Xylanase pretreatment has been the most applied process in recent laboratory research (Parthasarathy 1987; Quick 1999; Zhao et al. 2002; Solár et al. 2009; Li et al. 2011; Salazar et al. 2012; Mohieldin 2014; Hassan et al. 2018; Zhou et al. 2019). The basic approach of this process is to treat wood with chemicals or enzymes before cooking. As a result, the chips are softened, and some content is removed. During the main cooking process, lesser amounts of chemicals and less energy are utilized, and better fiber quality can be obtained.
Xylanase pretreatment has been used to enhance the conventional soda pulping of wheat straw and to evaluate the effect of xylanase pretreatment prior to soda pulping. Zhao et al. (2002) found that the pulp obtained from wheat straw pretreated with xylanase had a longer fiber length and a lower fine grain content than the control. The effect of xylanase pretreatment on rice straw unbleached pulp used to obtain nanofibers was examined, with soda and neutral sulfite pulping process used for isolation (Hassan et al. 2018). Slightly lower energy consumption (approximately 8%) was recorded for xylanase-treated soda pulp, whereas higher energy consumption (approximately 21%) was recorded for xylanase-treated neutral sulfite pulp relative to the untreated pulps. Xylanase treatment facilitated initial-stage fibrillation to separate fibrils via the removal of easily accessible xylan located mainly among cellulose fibrils of micrometer size but had no substantial effect on nanoscale fibrillation due to difficulties in the removal of xylan located between nanoscale fibrils. However, cellulose nanofibril samples from different xylanase treatments showed large decrease in the degree of polymerization (DP) (Zhou et al. 2019).
The use of alkaline and xylanase-assisted pretreatments have been studied. Following alkaline-xylanase treatment of the fibers (AXB-fibers), the required number of bleaching sequences to achieve an 83.3% whiteness index was decreased from 6 to 4. Two-stage pretreatment, using a combination of alkaline and xylanase-assisted pretreatments, effectively reduced the number of bleaching sequences required, which reduced the amount of bleaching chemicals required and improved the whiteness of the fibers (Vanitjinda et al. 2019). When the pretreatment of xylanase enzyme to the mulberry branch fibers was applied, refining energy was reduced by 4%, and the fiber yield remained more than > 85%. Further, the fiber widths increased because of the improved swelling effect of the xylanase pretreatment. In some instances, the fine elements were reduced (Tao et al. 2019). Poplar wood is a fast-growing hardwood species. It has the lowest density and porous structure among the hardwood species. Consequently, pulping solution can easily penetrate and remove lignin from poplar wood (Akgül and Kırcı 2009). Therefore, this research aimed to investigate the physical and optical properties of paper produced from poplar wood pretreated with xylanase enzyme. The optimal cooking conditions were determined by evaluating prehydrolyzed pulp properties.
Materials
The 10- to 12-year-old poplar wood (Populus x euramericana (I-214)) used in this study was obtained from the Düzce-Sakarya-Kocaeli basin (Forest Regional Directorates, Turkey).
The chipping process and drying were performed in the Duzce University Forest Products Engineering Laboratory. Then, the chips were sieved, and the appropriate chip size (25 mm × 20 mm × 2 mm) was selected for the cooking process. Air dried chips were cleaned, and a homogenized size was obtained.
Cooking Process
The chips were cooked in a 15-L batch-rotating digester (4rpm) that was heated with electricity. Each cook was performed with 500 g of chips. The wood to liquid ratio was kept constant at 5:1. To determine optimum cooking conditions for the pretreated kraft pulp, a controlled cooking process was carried out. The cooking conditions were given in Table 1. After each cook, the mixture was cooled with water and rinsed with tap water for approximately 1 h. to remove alkali. The pulp was then disintegrated with a laboratory pulp disintegrator for 10 min. After washing, the pulp was stored in airtight polyethylene bags. The dimensions of polyethylene bags were 15 * 30 cm. The results were compared to determine the optimum treatment condition.
Modified Kraft Pulp
Modified kraft pulp was prepared by adding 0.1% anthraquinone (AQ) and 4% polysulphide (PS) into the optimized cooking process.
Enzyme Pretreatment
The optimal conditions were found by testing various proportions according to the xylanase enzyme activity determined for the enzyme pretreatments. The optimal conditions were found where the amount of sugar removed from the chip at pH 8 and at the end of 1.5 h. The application conditions of enzyme 25 IU/g and 50 IU/g, pH values of 8 and 9, and cooking period of 60 min, 90 min, and 120 min were tested.
Table 1. The Control Kraft Cooking Process Used to Determine Optimum Conditions
* AA: Active alkali; AQ: Anthraquinone; PS : Polysulphide
Pulpzyme HC enzyme with activity of 1000 IU/g was placed into a polyethylene bag that contained 100 g of wood chips oven dry (OD). The solvent to chip ratio was kept constant at 6:1. The polyethylene bags were placed in a preheated (60 °C) water bath. Reducing sugar analysis was performed on the solutions in the polyethylene bags according to the dinitrosalicylic acid (DNS) method (Miller 1959). After pretreatment, the chips were washed and prepared for the next step in the chemical cooking process.
Physical and Mechanical Properties
The kappa number, viscosity, and mechanical properties were measured for each pulp category. The standard procedures followed for tests were as follows: basis weight was determined according to ISO 536 (2012), density was determined according to ISO 534 (2011), moisture content was determined according to ISO 287 (2017), viscosity was determined according to SCANCM 15:88 (1998), kappa number was determined according to TS ISO 302 (2015), burst strength was determined according to TS EN ISO 2759(2014), tear strength was determined according to TS EN ISO 1974 (2012) thickness was determined according to TN ISO 534 (2012), tensile and elongation properties were determined according to TS EN ISO 1924-2 (2013), and opacity was determined according to TS ISO 2471 (2015).
RESULTS AND DISCUSSION
The kraft process is commonly utilized to obtain pulp. The optimum cooking conditions were identified as those of KK4 due to the high viscosity, high degrees of polymerization (DP), high screened yield, and low kappa number achieved. Pretreatment and chemical additions were carried out in the KK4 cooking process. Optimized conditions are shown in Table2. Polysulphide and AQ are the most common additives for improving pulp properties due to their selectivity. Adding PS and AQ into kraft pulping resulted in higher pulp viscosity, which indicated that the addition of these additives prevented cellulose degradation in pulping. The polysulphide pulping technique is used in kraft mills to increase pulp yield (Vaaler 2008). In general, the yield increase depends on the amount of sulphur added, pulping conditions, and the wood species. Multiple pre-treatment methods and techniques have been applied to increase the accessibility of the cellulose polymers.
Note: KKOPT = KK4; K10 = KK4 + AQ; K11 = KK4 + PS, KK4 = Kraft optimum; AQ = Anthraquinone; PS = Polisulphur; KSLNZ = Xylanase |
Table 2. Optimized Cooking Conditions
Pre-treated pulp production systems have become important in recent years due to environmental concerns, energy usage, and efforts to increase the efficiency of the pulping process (Martin et al. 2000). Pretreatments aim to make the fibrous raw materials more suitable for the pulping and papermaking process. Table 3 shows that lignin content was similar for each cooking process. However, the DP of cellulose and viscosity were higher when xylanase pretreatment was applied. Lignin amounts decreased or remained the same in kraft, modified kraft, and kraft with pre-treatment. In contrast, the DP of cellulose increased. Oxidative pretreatment strengthens carbohydrates against degradation and activates during the pulping process to remove lignin. Chemical consumption was reduced, and the final brightness was improved with the application of this enzyme. During the pulping process, xylan deposits onto fibers and protects them against alkali. Xylanase enzyme reacts with xylan in wood and breaks lignin-carbohydrate bonds. Therefore, lignin was easily reached and extracted using a solvent (Tarasov et al. 2018). During this process, some hemicelluloses were also removed from the pulp and the holocellulose content was reduced. Pretreatment of the chips with enzyme reduced the screened yield. In contrast, the total yield, viscosity, and DP increased. Enzyme treatments cause physical loosening of fiber walls due to the partial depolymerization of the hemicellulose chain. In addition, xylan precipitation onto cellulose fibers prevents lignin removal. Pretreatment reduces the xylan content and overcomes this protection, which aids lignin removal and changes the chemistry of the resulting fiber.
Pretreatment improved the physical and mechanical properties. Xylanase may have changed the crystalline region of fiber and improved fibrillation, which enhanced fiber-to-fiber bonding, increased breaking length, and increased the tensile index (Table 4). In contrast, Batalha et al. (2011) found different results when kraft pulp was treated with xylanase. In their study, fiber was treated after the kraft pulping process, and xylanase treatment reduced the tensile index. Fibrillation may improve inter-fiber bonding properties, and treatment may change the chemistry of fiber, cause carboxylic acid formation, and increase hydrogen bonding ability (Liu et al. 2012).
Table 3. Pulp Properties
Table 4. Pulp and Paper Properties (Mechanical and Optical)
The xylanase treatment caused higher porosity and fibrillation of pulp fibers, and the fibers became more flexible and released more fibrils. The hydrolysis of xylan and the removal of surface lignin eased the pulping process and resulted in better fiber characteristics. However, the lignin amount in pulp varies. This could have caused increased opacity, reduced brightness, and decreased whiteness (Table 4).
Paper optical properties can be expressed as brightness, opacity, and whiteness. These properties varied with the light absorption and reflectivity. Brightness is a feature associated with the ability to reflect light centered at 457 nm wavelength. Enzyme-treated pulp had the lowest brightness. This may have been due to rendered lignin cellulose linkages causing precipitation onto fibers, which could have prevented the penetration of chemicals. Consequently, increased opacity and the reduced removal of chemicals decrease the brightness. This is related to lignin carbohydrate complex. Lignin and cellulose are maybe linked each other in contrast to lignin hemicellulose cellulose complex.
CONCLUSIONS
The aim of the oxidative pretreatment was to protect the carbohydrates against degradation and oxidation and activate to the pulp for the delignification process. The pretreatment process resulted in higher yield and improved pulp quality. The pretreatment process employed in this study is an efficient method to remove lignin. Therefore, it can provide energy savings to factories that produce pulp and high-quality fibers and improve strength properties.
ACKNOWLEDGEMENTS
The authors thank The Scientific and Technological Research Council of Turkey (Grant No. TUBİTAK-COST110O558) for their support.
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Article submitted: June 13, 2020; Peer review completed: Sept. 27, 2020; Revised version received: November 2, 2020; Accepted: December 13, 2020; Published: December 16, 2020.
DOI: 10.15376/biores.16.1.979-986