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Enzymatic deinking of various types of waste paper: Efficiency and characteristics
Process Biochemistry 48 (2013) 299–305
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Enzymatic deinking of various types of waste paper: Efficiency and characteristics Chee Keong Lee a,∗ , Darah Ibrahim a , Ibrahim Che Omar b a b
Industrial Biotechnology Research Laboratory, School of Biological Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia Faculty of Agroindustry and Natural Resource, Universiti Malaysia Kelantan, Karung Berkunci 36, 16100 Pengkalan Chepa, Kelantan, Malaysia
a r t i c l e
i n f o
Article history: Received 17 August 2012 Received in revised form 18 December 2012 Accepted 22 December 2012 Available online 3 January 2013 Keywords: Enzymatic deinking Brightness Tensile Tear Burst Drainage rate
a b s t r a c t The aim of the present study was to characterize the enzymatic deinking of various types of waste paper. Studies on the optimization of enzymatic deinking have been performed previously using commercially available enzyme preparations containing cellulase and hemicellulase. The enzymatic deinking of different types of waste paper demonstrated a high efficiency of 86.6% on laser-printed paper, but a low deinking efficiency of 12.9% was obtained with newspaper. All enzymatic treatments significantly improved the drainage rate of the deinked waste paper. Enzymatic deinking increased the tensile index of magazine paper but reduced the tensile index of bubble jet-printed paper, photocopy paper and newspaper. Enzymatic hydrolysis caused a 21.1% reduction in the tear index for bubble jet-printed paper, but a 3.1% increase in the tear index was obtained for laser-printed paper relative to respective blank. In addition, enzymatic hydrolysis increased the burst index by 4.7% relative to blank for laser-printed paper. However, photocopy paper showed the highest reduction (8.3%) in the burst index relative to blank. Taken together, these results suggest that enzymatic hydrolysis is both advantageous and detrimental to the mechanical properties of deinked paper. Thus, the proper regulation of enzymatic hydrolysis is crucial to improve the quality of recycled paper. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Paper, as one of the largest solid waste materials, is relatively easy and inexpensive to recycle. The recycling of waste papers diminishes environmental pollution by reducing the flow of waste papers to landfills. Recycled and reused waste papers are important low-cost fiber resources (raw materials) for the pulp and paper industry. With increasing amounts of recycled and reused resources, waste papers constitute 95% of the raw materials in Malaysian paper mills [1]. In addition, the pulp and paper industry in Malaysia is heavily dependent on imported fiber, particularly virgin pulp, due to the lack of domestic sources of fresh fiber. Although Malaysia has extensive forest resources, with forests covering close to 60% of its land area, this country remains a major importer of paper. The Malaysian pulp and paper industry, including pulp/paper mills, paper converters and producers of paper containers/corrugated carton boxes, comprises more than 300 crowded and highly fragmented companies. In 2010 (Food and Agriculture Organization, 2011), the local production was estimated at a total capacity of more than 1.6 million metric tons per annum. However, this amount of production did not meet
∗ Corresponding author at: Bioprocess Technology Division, School of Industrial Technology, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia. Tel.: +60 4 6532224; fax: +60 4 6573678. E-mail addresses: [email protected], [email protected] (C.K. Lee). 1359-5113/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.procbio.2012.12.015
the total consumption of more than 4.2 million metric tons per annum. Thus, Malaysia imports pulp, newsprint, printing/writing papers and industrial papers, importing an estimated 0.98 million metric tons of printing/writing paper per year. Paper importing is inevitable, as only one mill in Malaysia (Sabah Forest) produces printing/writing paper, with a capacity of 165,000 metric tons per year [2]. Mixed office waste paper is a fast growing source of materials for recycling and the most difficult raw material for deinking [3,4]. A major problem with recycled fiber is the removal of ink. The degree of difficulty in ink removal depends primarily on the ink and fiber types and printing process. Currently, Malaysian paper mills use a chemical approach to de-ink the waste paper, which is generally more efficient with respect to ink removal. However, this method requires the use of a large amount of chemical agents, which are not environmentally friendly. In addition, deinking under high pH conditions causes a smeared pulp or low brightness, resulting in a significant increase in the level and concentration of environmentally damaging emissions and wastes that are expensive to contain within environmental regulations [5]. Due to the significant number of disadvantages of conventional chemical methods, the development of an alternative method for deinking is required. The use of enzymes has been reported as a potentially efficient solution to overcome the problems encountered with traditional deinking techniques [5,6]. The potential of enzymatic deinking has been evaluated and successfully demonstrated using different types of enzymes [3]. One of the benefits of
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using enzymes in deinking is the minimum treatment of effluent produced due to lower in COD content compared with chemical deinking process [6]. In addition, the used of cellulases and hemicellulase mixtures has been described to be able to deink high quality waste paper such as mixed office waste papers (MOW), which is the most difficult raw materials to be deinked by conventional deinking process. Thus, better brightness and cleanliness of the pulp can be obtained [4,7]. Besides the ink removal, enzymatic deinking may improve the strength properties of paper by removing fines content and improved the interfibrillar bonding of paper [7–9]. Additional benefits include the improved operation of thickeners due to better drainage. Improved drainage may results in faster machine speed, which yields significant savings in energy and thus the overall cost [10]. Currently, there are few studies on the effect of enzymes on deinking in Malaysia. Therefore, the use of enzymes for deinking must be given priority in the Malaysian paper industry as an environmentally friendly approach to recycling waste paper. Increasing the efficiency of waste paper recycling will lead to self-sufficiency, reduce imports and encourage foreign capital investment in the Malaysian pulp and paper industry. Therefore, the aim of this study was to characterize various types of enzymatically deinked waste paper, including laser-printed paper, photocopy paper, bubble jetprinted paper, newspaper and magazine paper, to provide further insight into the effects of enzymatic activity on various types of waste paper and develop an enzymatic deinking process that can improve the quality of recycled paper.
Table 1 Conditions used for enzymatic deinking of various types of waste papers. Pulping process Pulping consistency Pulping time Enzymatic hydrolysis process Temperature pH Pulp concentration Enzyme concentration Enzyme ratio (C:H) Hydrolysis time Flotation process pH Surfactant Surfactant concentration Airflow rate Flotation temperature Flotation time
2% 1 min 50 ◦ C 3.5 4% (w/w) 2.5 U/g of air dry pulp 1:1 60 min 6.0 Tween 80 0.5% (w/w) 10 L/min 45 ◦ C 15 min
flotation cell was constructed to collect the toner that lifted by the air bubbles during flotation process. The aeration was carried out by air sparging via a pump (GAST, USA) which was connected to a microporous fiber glass sparger [13]. After deinking, the pulp was rinsed with water (3X), and hand-sheets were generated to determine the efficiency of the deinking process. The conditions were previously optimized using laser-printed paper [13]. Blanks consisted of pulp slurry in the disintegrator that was not subjected to flotation. Control and sample pulps were processed in a similar manner, except heat inactivated enzymes and active enzymes, respectively, were used. The deinking efficiency (%) was determined by subtracting the blank brightness from the sample (or control) brightness, and then dividing by the blank brightness and multiplying by 100.
2. Methods
2.5. Evaluation of enzymatic deinking process
2.1. Source of enzymes and enzyme activities
The handsheets were prepared according to the methods of the Technical Association of the Pulp and Paper Industry (TAPPI), TAPPI Test Method T205; forming handsheet for physical test of pulp. The prepared handsheets were conditioned under controlled conditions as described in TAPPI test method TAPPI T402 (standard conditioning and testing atmosphere for paper, board, pulp handsheets and related products) before evaluation.
The commercially available enzymes, cellulase A “Amano”3 (C) and hemicellulase “Amano”90 (H) were obtained from Amano Pharmaceuticals Co. Ltd. (Nagoya, Japan). Both enzymes are produced by Aspergillus niger. The enzyme powders are water–soluble and were stored at 4 ◦ C prior to use. Cellulase and hemicellulase or xylanase activities were determined according to Gessesse and Gashaw [11] and Gessesse and Gashe [12], respectively.
2.6. Analysis of deinked paper properties 2.2. Selection and preparation of waste paper The waste paper used in this study was obtained from the Universiti Sains Malaysia (USM) campus. The waste papers were manually sorted to remove nonpaper objects. The sorted waste papers were stored in a room away from sunlight and high moisture prior to use. The waste papers were shredded into small pieces and pre-treated with 0.25 M HCl for 30 min prior to use in the enzymatic deinking process. 2.3. Characteristic of various waste papers For laser-printed paper, A4 size paper with a basis weight per area of 70–80 g/m2 was printed with toner on a Canon laser printer (Canon, China) covering approximately 40% of the paper area. One-month-old newspaper with an average weight per area of 50 g/m2 was used in this study. The newspaper was printed using a flexographic printing method, covering approximately 60% of the paper area. For photocopy-printed paper, photocopy toner (Xerox dry ink plus 5052/1050, Xerox Corporation) was used, and the photocopied paper was generated using a photocopier (Fuji Xerox, China). The average weight of the waste paper was 70–80 g/m2 , with an average photocopied area of 40%. For magazine paper, the glossy paper magazine (Cleo) was used, with ink covering the entire area of the paper (approximately 90%). The average weight per area of the waste paper was 70 g/m2 . Finally, bubble jet-printed waste paper, with an average weight of 70 g/m2 , was printed on a Canon bubble-jet printer using Canon BC 03 ink. Approximately 40% of the paper area of one side of each sheet was printed for use in the subsequent experiments.
After enzymatic deinking, the paper was examined for optical (brightness), mechanical (tensile, tear and burst) and pulp (drainage rate) properties. The brightness was measured as described in TAPPI T452 (Brightness of Pulp, Paper, and Paperboard; Directional Reflectance at 457 nm), using a brightness and opacity tester (Mode Micro S-5, Technidyne Corporation, USA). The tensile properties of the handsheets were examined using TAPPI test method T494 (Model LLOYD). The tear index was determined based on the internal resistance of the handsheet as described in the TAPPI T414 test method, using an electronic tearing tester (Model Protear, Thwing Albert, USA). The burst strength of the handsheet was measured according to the TAPPI T403 procedure (Bursting Strength of Paper), using a Model 3720 bursting strength tester (Lesson Industrial Corporation, Ltd., Taiwan). Finally, the drainage time of the handsheet was determined using TAPPI test T221 om-93 (Drainage time of pulp) procedures. This procedure was similar to the sheet making process, except the temperature of the diluted stock in the cylinder was adjusted within 20 ± 5 ◦ C. According to the protocol, a 400 ml sample volume (0.3% consistency) was used to produce a 1.2 g moisture-free handsheet. This handsheet was used for further analysis.
2.7. Total reducing sugar produced from enzymatic hydrolysis After enzymatic hydrolysis, 10 ml of fiber-free pulp slurry obtained by centrifugation under ambient temperature was used to examine the reducing sugar production through enzymatic hydrolysis of the pulp. The total reducing sugar produced was determined using DNS reagent [15].
2.4. Enzymatic hydrolysis and flotation process 2.8. Statistical methods The deinking process was performed according to Lee et al. [13], as described in Table 1. The batch deinking process involved pulp preparation, enzymatic hydrolysis and flotation. The pulp was prepared by disintegration using high speed mixer (National Model: MX-T100GN) for 4 min to produce 2% (w/w) pulp consistency [14]. The flotation process was performed using 600 ml vertically tubular flotation cell of diameter 3.2 cm and height 44.5 cm with water jacketed. The top section of the
All the experiments were performed in triplicate. The results are presented as the means of the triplicate experiments. The significance of differences between test variables was determined using a one-way ANOVA and the Least Significant Difference test with SPSS version 11.5 software. The evaluation of statistical significance was performed with a confidence interval of 95%.
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Deinking efficiency (%)
100
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E
80 60
d
d
D C
40
b
20
c
B A
a
0
Laser-printed
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Fig. 1. Enzymatic deinking efficiency of different types of waste papers. Arrow bars indicate means with standard error and means with the same letter (a, b, c, d, A, B, C, D, and E) indicated no significantly difference at 5% level of probability by least significance test.
3. Results and discussion 3.1. Deinking efficiency of different types of printed waste papers
Brightness (%)
Fig. 1 shows a significant difference (P < 0.05) in the deinking efficiency obtained with laser-printed paper compared with the other papers. The highest enzymatic deinking efficiency of 86.6% was obtained using laser-printed paper, with a final brightness of 83.4% (Fig. 2). However, photocopy paper demonstrated a deinking efficiency of 63.5%. The differences in the deinking efficiency between these two types of papers might reflect the differences in the printing process and toner composition. Laser printing involves imaging using a computer-directed laser that ‘prints’ on the paper, and the ink is rapidly fixed, using a combination of heat and pressure [4]. In addition, the toner composition for photocopy toner consisted of 10–15% pigment (carbon black) and 85–90% styrene acrylate, while the laser toner consisted of 50–70% styrene acrylate and 30–40% pigment (iron oxide) [16]. The lowest deinking efficiency of 12.9% was obtained using newspaper, with a 50% final brightness (5 unit increase compared with blank paper) (Fig. 2). This result was consistent with Ibarra et al. [17], who obtained 3–4 units of increased newspaper brightness after enzymatic deinking using cellulase and hemicellulase. In addition, Morkbak and Zimmermann [18] and Lee et al. [19] obtained 7.3% and 2.5% increased in newspaper brightness, respectively, after enzymatic deinking. The inefficient enzymatic deinking of newspaper might reflect the fact that newspaper is commonly printed using a flexographic method with flexographic ink, which might not be effectively hydrolyzed by the enzymes used in the present study. The deinking efficiency obtained with bubble
jet-printed papers was only 23.6%. The water-based ink used in bubble jet printing remained soluble in water and was difficult to separate from the bulk flotation liquid in the water. Flexo inks, which are very small (0.2–1.0 m) due to their hydrophilic nature, remained strongly attached to the fiber and were difficult to separate during flotation. Ferguson [20] showed that the ink particles acted as water-repellent and bind to collector chemicals for removal through flotation. Extremely small ink particles are carried in the water loops, making it difficult to transport these compounds to the water surface for separation. The ink vehicle used for printing influences the simplicity of ink removal from printed-paper through enzymatic hydrolysis [18]. The brightness of the enzymatic deinked magazine paper was 70.1%, showing a 42.7% increase in deinking efficiency. The inks used in magazine printing are oil-based offset heat inks that are typically dried through evaporation of the volatile solvent at 60 ◦ C, leaving resins and other material to bind the pigments to the paper [15]. This cross-linking resulted in a poor deinking efficiency and recycling of the magazine papers. 3.2. Effect of enzymes on the hydrolysis of different types of waste papers The total amount of reducing sugar produced from the hydrolysis of waste paper differed for the different types of waste paper (Fig. 3). Magazine paper was the most susceptible (5.1 mg/gpulp ) to enzyme hydrolysis, while newspaper was the most resistant to be degraded, as only 1.9 mg/gpulp of total reducing sugar was produced. A significant difference (P < 0.05) in the total reducing sugar produced was detected for newspaper compared with the other
90 80 70 60 50 40 30 20 10 0 Laser-printed
Photocopy
Bubble jet printed
Newspaper
Magazine
Waste papers Blank
Control
Sample
Fig. 2. Brightness of the enzymatic deinking of different types of waste paper. Blank refers to the pulp slurry in the disintegrator. Control and sample used heat inactivated and active enzymes, respectively. Arrow bars indicate means with standard error of three replicates.
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Total reducing sugar (mg/g pulp)
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6.0
a
a 5.0 4.0
b d
3.0
c
2.0 1.0 0.0 Laser-printed
Photocopy
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Waste papers Fig. 3. Total reducing sugar produced from the hydrolysis of different types of waste papers. Arrow bars indicate means with standard error and means with the same letter (a, b, c, and d) indicated no significantly difference at 5% level of probability by least significance test.
four types of waste paper examined. No consistency in the total reducing sugar profile was obtained, which might reflect differences in the amount of ink present in the waste papers. However, increasing amounts of total reducing sugar were obtained with increasing brightness of the blank sample, as the amount of ink present in a particular waste paper was reduced, facilitating the increased exposure of cellulose fiber for enzymatic hydrolysis. Consequently, the increased enzymatic hydrolysis would increase the amount of total reducing sugar. Moreover, different types of paper might be coated with different types of sizing agent. The sizing agent might inhibit enzyme activity through increased fiber hydrophobicity, physically shielding fiber surfaces from enzyme attachment or preventing access to fibers via covalent bonds with cellulose [21]. Thus, the total amount of reducing sugar generated will be reduced. Different papers can vary in the composition and types of inorganic fillers used, such as kaolin, talc and titanium dioxide. The presence of these fillers and others additives might affect the enzymatic degradation of the cellulose fiber [16]. Furthermore, the properties of enzymatic hydrolysis, such as adsorption and reactivity, depend on the competition between the ink particles and the enzyme molecules on the fiber surface [22]. Higher enzyme attachment to the fiber surface will therefore increase the enzymatic production of reducing sugar from cellulose.
the rate of machine runnability, resulting in significant savings in energy and the overall cost [10]. The analysis of the drainage rate showed a significant improvement (P < 0.05) in the deinked waste papers compared with their respective blanks (Fig. 4). The laserprinted paper showed the highest drainage rate of 103.7 L/min, reflecting a 12.0% improvement. Rutledge-Cropsey et al. [23] obtained a drainage rate of 107.0 L/min after the enzymatic hydrolysis of office waste papers. In addition, Pathak et al. [24] reported an improved drainage rate of enzymatic deinked photocopier paper through an 11.5% reduction in the drainage time. Although newspaper exhibited the lowest drainage rate (64.9 L/min), the highest improvement (30.7%) in the overall drainage rate was obtained with newspaper compared with its blank. Most cellulase and hemicellulase preparations have shown the potential to remove microfibrils and fines, thus improving drainage rates [21]. Some studies have shown that enzymatic deinking enhances drainage rates with little to no loss in the sheet strength [7]. The enzymes act on the surface of the fibers, producing a peeling effect [25,26]. During peeling, the enzymes only remove small elements or components that have great affinity for water but do not contribute to the overall hydrogen bonding potential of the fibers. Notably, the determined brightness and calculated drainage times of the deinked paper alone did yield an adequate pulp quality. Therefore, the brightness does not represent the overall deinked paper quality. Other parameters of the paper, such as burst strength, tensile strength and tear resistance, are also important. Therefore, further experiments were conducted to evaluate the quality of the deinked paper in term of its physical strength.
3.3. The drainage rate of enzymatic deinked waste papers
Drainage rate ( L/min)
The drainage rate refers to the rate at which water is drained when a standard sheet weighing 60 g/m2 (moisture free) is formed at 20 ◦ C ± 5 ◦ C. Improvements in the pulp drainage rate will increase 120
+12%
+7.4%
100
+6.3%
+23.1%
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+30.7%
h hi
j
g
h
i
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f
fg a
20
b
c
c
d
e
0 Laser-printed
Photocopy
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Newspaper
magazine
Waste papers Blank
Control
Sample
Fig. 4. Drainage rate of enzymatic deinking of different types of waste papers. Arrow bars indicate means with standard error and means with the same letter (a, b, c, d, e, f, g, fg, h, i, hi and j) indicated no significantly difference at 5% level of probability by least significance test. Means with the symbol (+, −) are different in percentage relative to respective blank.
Tensile index (Nm/g)
C.K. Lee et al. / Process Biochemistry 48 (2013) 299–305
45 40 35 30 25 20 15 10 5 0
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+2.3% -3.0%
a
b
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d
bc b
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a
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bc b
e
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b
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Fig. 5. Tensile index of enzymatic deinking of different types of waste papers. Arrow bars indicate means with standard error and means with the same letter (a, b, c, bc, d and e) indicated no significantly difference at 5% level of probability by least significance test. Means with the symbol (+, −) are different in percentage relative to respective blank.
3.4. Tensile strength
[27] demonstrated a 7.5% improvement in the breaking length of paper after the enzymatic deinking process. Pala et al. [28] used 11 commercial enzymes, comprising cellulase and hemicellulase, to deink mixed office waste. The results obtained indicated that seven commercial enzymes had detrimental effects, three enzymes showed improvement, and one enzyme had no effect on the tensile index tested. Thus, it is reasonable to conclude that different enzyme preparations differently affect the tensile index of paper. Different types of waste paper exhibited different effects on tensile index after enzymatic hydrolysis, reflecting differences in the cellulose fiber composition in the paper and different sizing agents used for coating the paper.
Tear index (mN m2/g)
The tensile index refers to tensile strength in N/m divided by the grammage. The tensile strength is the maximum tensile stress developed in a test specimen before rupture. It is a measure of the force required to tear the paper when pulled at opposite ends and in opposite directions. Cellulase and hemicellulase enzymes improve the tensile index of magazine paper (Fig. 5), and the loss in tensile index was not significant (P > 0.05) for laser-printed paper. Magazine paper showed a tensile index of 34.65 N m/g after enzyme treatment, which was increased 2.30% relative to its blank. Ibarra et al. [17] reported similar results in which the tensile index of magazine paper was improved after deinking using cellulase and hemicellulase from Aspergillus oryzae. Photocopy and bubble jet-printed paper showed large reductions of 21.2% and 23.9%, respectively, in the tensile index. However, these results were not consistent with Pathak et al. [24], who showed a 2.7% increase in the tensile index of enzymatic deinked photocopy paper. In addition, an increased amount of total reducing sugar (Fig. 3) was also detected for both photocopy and bubble jet-printed papers, suggesting that increased enzymatic hydrolysis of the papers occurred, resulting in the reduced tensile strength of the paper. Magazine papers showed improved tensile strength, although the total reducing sugar produced was also higher, potentially reflecting variations in the fiber composition. Gubitz et al. [3] reported a slight reduction in the tensile index relative to the control when endoglucanase and hemicellulase were used to deink xerographic and laser-printed papers. However, Prasad [4] obtained a different result, showing a slight increase in the tensile index of these papers relative to their blanks. Singh et al.
10 9 8 7 6 5 4 3 2 1 0
3.5. Internal tearing resistance Tearing resistance/strength refers to the ability of the paper to withstand the application of tearing forces measured in mN (milliNewtons). The tear index is calculated as the tearing strength divided by the grammage. Enzymatic hydrolysis improved the tear strength of laser-printed (3.1%) and magazine paper (2.8%) but reduced the tear strength of bubble jet-printed paper, newspaper and photocopy paper (Fig. 6). In contrast, Ibarra et al. [17] reported no effect or a slight increase in the tear index of newspaper after deinking using cellulase and hemicellulase from Humicola insolens and A. oryzae. A numbers of studies have examined the effects of enzymes on the tearing strength of deinked paper. Whereas some studies have obtained positive effects, other studies have demonstrated deteriorating effects. Morkbak and Zimmermann [18] showed a slight increase in the tear index after the enzymatic hydrolysis of mixed
-3.7% +3.1%
+2.8%
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-21.1%
f b
a
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c
c
a
Photocopy
d
b
f
e c
a
Bubble jet printed
Newspaper
c
c
Magazine
Waste papers Blank
Control
Sample
Fig. 6. Tear index of enzymatic deinking of different types of waste papers. Arrow bars indicate means with standard error and means with the same letter (a, b, c, d, e and f) indicated no significantly difference at 5% level of probability by least significance test. Means with the symbol (+, −) are different in percentage relative to respective blank.
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Burst index (kPa m2/g)
6 5
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+1.8%
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-8.3%
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4 3 2
a
b
b
d
d
b
f
c
c
de ef
ef
g
g
g
1 0 Laser-printed
Photocopy
Bubble jet printed
Newspaper
Magazine
Waste papers Blank
Control
Sample
Fig. 7. Burst index of enzymatic deinking of different types of waste papers. Arrow bars indicate means with standard error and means with the same letter (a, b, c, d, e, de, f, ef and g) indicated no significantly difference at 5% level of probability by least significance test. Means with the symbol (+, −) are different in percentage relative to respective blank.
office paper and old newspaper. However, Lee et al. [29] reported a 3.9% reduction in the tear index after the enzymatic hydrolysis of old newspaper. In addition, Prasad [4] reported a 5.9% reduction in the tear index after the enzymatic hydrolysis of laser and xerographic waste papers. Moreover, Pala et al. [28] reported a 34% reduction in the tear index when Buzyme 2523 was used to deink mixed office waste, but a 5% improved was obtained when CCMI 84 was used. Therefore, different enzyme preparations and fiber compositions are likely responsible for the differences in the tear effect of deinked paper. The widely accepted mechanism for the enzymatic removal of ink suggests that the surface fiber layers are removed through enzymatic activity on cellulose and the shear forces of mixing. The use of hemicellulase and cellulase enzymes have improve the freeness of the pulp due to removal of microfibrils and fines that aids in released of ink particles during flotation process [4,7,10]. In addition, for mild enzymatic hydrolysis, the reduction in pulp water interactions improves the drainage rate of the pulp without affecting the mechanical properties of the paper [4,5]. However, in the present work, it was observed that a reduction in the tear index for photocopy, newspaper and bubble jet-printed waste papers was detected, although these papers showed improvements in the drainage rate. This result suggests that lower enzyme dosages and reductions in the reaction times are essential to diminish deleterious effects, as too much enzyme or long reaction times can damage fibers. The adverse effects of higher enzyme doses are not completely understood but could reflect increased fibrillation [30]. 3.6. Bursting strength The burst strength refers to the hydrostatic pressure, measured in kilopascals (kPa) or pounds per square inch (psi), that is required to rupture the material when the pressure is increased at a controlled constant rate through a rubber diaphragm into a circular area with a 30.5 mm diameter. The burst index is calculated as the burst strength divided by the grammage. Enzymatic hydrolysis improved the burst strength for laser-printed paper, newspaper and magazine paper compared with their respective blanks (Fig. 7). However, the improvement was only significant (P < 0.05) for laserprinted paper (4.7%). A slight improvement in the burst index (1.8%) was also detected after the enzymatic hydrolysis of newspaper. These results were consistent with Lee et al. [29], who reported a 3.8% increase in the burst index of newspaper. The improvement in the burst index might not reflect the sizing agent present in the paper, as the sizing agent potentially limits enzymatic activity by increasing the
fiber hydrophobicity, physically shielding the fiber surfaces from enzyme attachment, or by preventing access to the fibers via covalent bonds with cellulose [21]. These effects can be observed in the low deinking efficiency and the low total reducing sugar produced from newspaper, which were 12.9% and 1.9 mg/gpulp , respectively (Fig. 3). In contrast, a significant drop (P < 0.05) in the burst strength was obtained for photocopy and bubble jet-printed paper, which was 8.3% and 7.3%, respectively. However, Prasad [4] and Gubitz et al. [3] reported an increased burst index after enzymatic deinking of laser and xerographic office waste papers, whereas Pathak et al. [24] reported a 15.3% improvement in the burst index of enzymatic deinked photocopier paper. However, according to Pala et al. [28], four commercial enzymes showed improvement, but seven enzymes showed detrimental effects in the burst index. These findings suggest that different enzyme preparations might confer different effects on the paper burst strength. Similarly, the different fiber compositions in different types of paper are likely responsible for the differences observed in the burst index. Generally, improvements in the burst strength reflect the removal of fines, which improves interfibrillar bonding and further enhances hydrogen bonding. Several studies have shown that during enzymatic deinking, cellulases act preferentially on the microfibrils and fines projecting out from the fiber surfaces [3,7,9]. 4. Conclusion The highest deinking efficiency was observed with laser-printed paper, but the lowest efficiency was obtained with newspaper. The enzymatic deinking process significantly improved the drainage rate of the deinked paper. However, the enzymatic process resulted in both positive and deleterious effects on the mechanical properties of deinked paper. Different papers are produced using different raw materials with various chemical compositions. In addition, different printing techniques and inks are used during the printing process. These differences might directly affect the efficiency of deinking, thereby resulting in mixed effects on the mechanical properties of deinked papers. Acknowledgements This work was financially supported by a research grant from the Ministry of Science, Technology and Innovation of Malaysia (MOSTI). The author (Lee Chee Keong) would like to thank Universiti Sains Malaysia for financial support through an incentive and short-term grant (304/PTEKIND/6311130).
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References [1] Malaysian Newsprint Industries Sdn Bhd (2007). Online available from World Wide Web: http://www.newsprint.com.my [accessed 30.10.12]. [2] Food and Agriculture Organization. Online available from World Wide Web: http://http://faostat.fao.org/ [accessed 2.11.12]. [3] Gubitz GM, Mansfield SD, Bohm D, Saddler JN. Effect of endoglucanases and hemicellulases in magnetic and flotation deinking of xerographic and laserprinted papers. J Biotechnol 1998;65:209–15. [4] Prasad DY. Enzymatic deinking of laser and xerographic office wastes. Appita 1993;46(4):289–92. [5] Prasad DY, Heitmann JA, Joyce TW. Enzyme deinking of black and white letterpress printed newsprint waste. Prog Pap Recycl 1992;1(3):21–30. [6] Putz HJ, Renner K, Gottsching L, Jokinen O. Enzymatic deinking in comparison with conventional deinking offset news. In: Proc Tappi Pulp Conf. 1994. p. 877–84. [7] Jeffries TW, Klungness LH, Sykes MS, Rutledge CKR. Comparison of enzymeenhanced with conventional deinking of xerographic and laser-printed paper. Tappi J 1994;77(4):173–9. [8] Gubitz GM, Mansfield SD, Saddler JN. Effectiveness of two endoglucanases from Gloeophyllum species in deinking mixed office wastepaper. In: Int Conf Biotechnol Pulp Pap Ind C. 1998. p. 135–8. [9] Lee JM, Eom TJ. Enzymatic deinking of old newsprint with alkalophilic enzymes from Coprinus cinereus 2249. J Korea Technol Assoc Pulp Pap Ind 1999;31:12–7. [10] Heise OU, Unwin JP, Klungness JH, Fineran WG, Sykes JRM, Abubakr S. Industrial scale up of enzyme-enhanced deinking of non-impact printed toners. Tappi J 1996;79(3):207–12. [11] Gessesse A, Gashaw M. High-level xylanases production by an alkaliphilic Bacillus sp. by using solid-state fermentation. Enzyme Microb Technol 1999;25:68–72. [12] Gessesse A, Gashe BA. Production of alkaline xylanases by an alkaliphilic Bacillus sp. isolated from an alkaline soda lake. J Appl Microbiol 1997;83:402–6. [13] Lee CK, Darah I, Ibrahim CO. Enzymatic deinking of laser printed office waste papers: some governing parameters on deinking efficiency. Bioresour Technol 2007;98:1684–9. [14] Moon T, Nagarajan R. Deinking of xerographic and laser-printed paper using block co-polymers. Colloid Surface A 1998;132:275–88. [15] Miller GL. Dinitrosalicyclic acid reagent for determination of reducing sugar. J Anal Chem 1959;31:426–8.
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[16] Nie X, Miller JD, Yeboah YD. The effect of ink type and printing processes on flotation deinking efficiency of wastepaper recycling. Environ Eng Policy 1998;1:47–58. [17] Ibarra D, Concepción Monte M, Blanco A, Martínez AT, Martínez MJ. Enzymatic deinking of secondary fibers: cellulase/hemicellulases versus laccase-mediator system. J Ind Microbiol Biotechnol 2012;39:1–9. [18] Morkbak AL, Zimmermann W. Deinking of mixed office paper, old newspaper and vegetable oil based ink printed-paper using cellulase xylanase and lipases. Prog Pap Recycl 1998;7(2):14–27. [19] Lee CK, Darah, Ibrahim, Ibrahim Che Omar, Wan Daud Wan Rosli. Pilot scale enzymatic deinking of mixed office wastepaper and old newspaper. Bioresources 2011;6(4):3809–23. [20] Ferguson LD. Deinking chemistry: part 2. Tappi J 1992;75(8):49–58. [21] Welt T, Dinus R. Enzymatic deinking. Prog Pap Recycl 1995;4(2):36–47. [22] Marsques S, Pala H, Alves L, Amaral-Collaco MT, Gama FM, Girio FM. Characterization and application of glycanases secreted by Aspergillus terreus CCMI 498 and Trichoderma viride CCMI 84 for enzymatic deinking of mixed office wastepaper. J Biotechnol 2003;100:209–19. [23] Rutledge-Cropsey K, Klungness JH, Abubakr SM. Performance of enzymatically deinked recovered paper on paper machine rennability. Tappi J 1998;81(2):148–51. [24] Pathak P, Bhardwaj NK, Singh AK. Optimization of chemical and enzymatic deinking of photocopier waste paper. Bioresources 2011;6(1):447–63. [25] Chanzy H, Henrissat B. Unidirectionl degradation of Valonia cellulose microcrystal subjected to cellulase action. FEBS Lett 1985;184(2):285–8. [26] Lee SB, Kim KH, Ryu JD, Taguchi H. Structural properties of cellulose and cellulase reaction mechanism. Biotechnol Bioeng 1983;25:33–52. [27] Singh A, Dutt Yadav R, Kaur A, Mahajan R. An ecofriendly cost effective enzymatic methodology for deinking of school waste paper. Bioresour Technol 2012;120:322–7. [28] Pala H, Mota M, Gama FM. Enzymatic versus chemical deinking of non-impact ink printed paper. J Biotechnol 2004;108:79–89. [29] Lee CK, Darah Ibrahim, Ibrahim Che Omar, Wan Daud Wan Rosli. Enzymatic and chemical deinking of mixed office wastepaper and old newspaper: paper quality and effluent characteristics. Bioresources 2011;6(4):3859–75. [30] Jeffries TW, Sykes MS, Rutlege-cropsey K, Klungness JH, Abubakr S. Enhanced removal of toners from office waste paper by microbial cellulases. In: Srebotnik E, Messner K, editors. Biotechno Pulp Pap Ind Proc 6th Int Conf Biotechnol Pulp Pap Ind. 1996. p. 141–4.
Contents lists available at SciVerse ScienceDirect
Process Biochemistry journal homepage: www.elsevier.com/locate/procbio
Enzymatic deinking of various types of waste paper: Efficiency and characteristics Chee Keong Lee a,∗ , Darah Ibrahim a , Ibrahim Che Omar b a b
Industrial Biotechnology Research Laboratory, School of Biological Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia Faculty of Agroindustry and Natural Resource, Universiti Malaysia Kelantan, Karung Berkunci 36, 16100 Pengkalan Chepa, Kelantan, Malaysia
a r t i c l e
i n f o
Article history: Received 17 August 2012 Received in revised form 18 December 2012 Accepted 22 December 2012 Available online 3 January 2013 Keywords: Enzymatic deinking Brightness Tensile Tear Burst Drainage rate
a b s t r a c t The aim of the present study was to characterize the enzymatic deinking of various types of waste paper. Studies on the optimization of enzymatic deinking have been performed previously using commercially available enzyme preparations containing cellulase and hemicellulase. The enzymatic deinking of different types of waste paper demonstrated a high efficiency of 86.6% on laser-printed paper, but a low deinking efficiency of 12.9% was obtained with newspaper. All enzymatic treatments significantly improved the drainage rate of the deinked waste paper. Enzymatic deinking increased the tensile index of magazine paper but reduced the tensile index of bubble jet-printed paper, photocopy paper and newspaper. Enzymatic hydrolysis caused a 21.1% reduction in the tear index for bubble jet-printed paper, but a 3.1% increase in the tear index was obtained for laser-printed paper relative to respective blank. In addition, enzymatic hydrolysis increased the burst index by 4.7% relative to blank for laser-printed paper. However, photocopy paper showed the highest reduction (8.3%) in the burst index relative to blank. Taken together, these results suggest that enzymatic hydrolysis is both advantageous and detrimental to the mechanical properties of deinked paper. Thus, the proper regulation of enzymatic hydrolysis is crucial to improve the quality of recycled paper. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Paper, as one of the largest solid waste materials, is relatively easy and inexpensive to recycle. The recycling of waste papers diminishes environmental pollution by reducing the flow of waste papers to landfills. Recycled and reused waste papers are important low-cost fiber resources (raw materials) for the pulp and paper industry. With increasing amounts of recycled and reused resources, waste papers constitute 95% of the raw materials in Malaysian paper mills [1]. In addition, the pulp and paper industry in Malaysia is heavily dependent on imported fiber, particularly virgin pulp, due to the lack of domestic sources of fresh fiber. Although Malaysia has extensive forest resources, with forests covering close to 60% of its land area, this country remains a major importer of paper. The Malaysian pulp and paper industry, including pulp/paper mills, paper converters and producers of paper containers/corrugated carton boxes, comprises more than 300 crowded and highly fragmented companies. In 2010 (Food and Agriculture Organization, 2011), the local production was estimated at a total capacity of more than 1.6 million metric tons per annum. However, this amount of production did not meet
∗ Corresponding author at: Bioprocess Technology Division, School of Industrial Technology, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia. Tel.: +60 4 6532224; fax: +60 4 6573678. E-mail addresses: [email protected], [email protected] (C.K. Lee). 1359-5113/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.procbio.2012.12.015
the total consumption of more than 4.2 million metric tons per annum. Thus, Malaysia imports pulp, newsprint, printing/writing papers and industrial papers, importing an estimated 0.98 million metric tons of printing/writing paper per year. Paper importing is inevitable, as only one mill in Malaysia (Sabah Forest) produces printing/writing paper, with a capacity of 165,000 metric tons per year [2]. Mixed office waste paper is a fast growing source of materials for recycling and the most difficult raw material for deinking [3,4]. A major problem with recycled fiber is the removal of ink. The degree of difficulty in ink removal depends primarily on the ink and fiber types and printing process. Currently, Malaysian paper mills use a chemical approach to de-ink the waste paper, which is generally more efficient with respect to ink removal. However, this method requires the use of a large amount of chemical agents, which are not environmentally friendly. In addition, deinking under high pH conditions causes a smeared pulp or low brightness, resulting in a significant increase in the level and concentration of environmentally damaging emissions and wastes that are expensive to contain within environmental regulations [5]. Due to the significant number of disadvantages of conventional chemical methods, the development of an alternative method for deinking is required. The use of enzymes has been reported as a potentially efficient solution to overcome the problems encountered with traditional deinking techniques [5,6]. The potential of enzymatic deinking has been evaluated and successfully demonstrated using different types of enzymes [3]. One of the benefits of
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using enzymes in deinking is the minimum treatment of effluent produced due to lower in COD content compared with chemical deinking process [6]. In addition, the used of cellulases and hemicellulase mixtures has been described to be able to deink high quality waste paper such as mixed office waste papers (MOW), which is the most difficult raw materials to be deinked by conventional deinking process. Thus, better brightness and cleanliness of the pulp can be obtained [4,7]. Besides the ink removal, enzymatic deinking may improve the strength properties of paper by removing fines content and improved the interfibrillar bonding of paper [7–9]. Additional benefits include the improved operation of thickeners due to better drainage. Improved drainage may results in faster machine speed, which yields significant savings in energy and thus the overall cost [10]. Currently, there are few studies on the effect of enzymes on deinking in Malaysia. Therefore, the use of enzymes for deinking must be given priority in the Malaysian paper industry as an environmentally friendly approach to recycling waste paper. Increasing the efficiency of waste paper recycling will lead to self-sufficiency, reduce imports and encourage foreign capital investment in the Malaysian pulp and paper industry. Therefore, the aim of this study was to characterize various types of enzymatically deinked waste paper, including laser-printed paper, photocopy paper, bubble jetprinted paper, newspaper and magazine paper, to provide further insight into the effects of enzymatic activity on various types of waste paper and develop an enzymatic deinking process that can improve the quality of recycled paper.
Table 1 Conditions used for enzymatic deinking of various types of waste papers. Pulping process Pulping consistency Pulping time Enzymatic hydrolysis process Temperature pH Pulp concentration Enzyme concentration Enzyme ratio (C:H) Hydrolysis time Flotation process pH Surfactant Surfactant concentration Airflow rate Flotation temperature Flotation time
2% 1 min 50 ◦ C 3.5 4% (w/w) 2.5 U/g of air dry pulp 1:1 60 min 6.0 Tween 80 0.5% (w/w) 10 L/min 45 ◦ C 15 min
flotation cell was constructed to collect the toner that lifted by the air bubbles during flotation process. The aeration was carried out by air sparging via a pump (GAST, USA) which was connected to a microporous fiber glass sparger [13]. After deinking, the pulp was rinsed with water (3X), and hand-sheets were generated to determine the efficiency of the deinking process. The conditions were previously optimized using laser-printed paper [13]. Blanks consisted of pulp slurry in the disintegrator that was not subjected to flotation. Control and sample pulps were processed in a similar manner, except heat inactivated enzymes and active enzymes, respectively, were used. The deinking efficiency (%) was determined by subtracting the blank brightness from the sample (or control) brightness, and then dividing by the blank brightness and multiplying by 100.
2. Methods
2.5. Evaluation of enzymatic deinking process
2.1. Source of enzymes and enzyme activities
The handsheets were prepared according to the methods of the Technical Association of the Pulp and Paper Industry (TAPPI), TAPPI Test Method T205; forming handsheet for physical test of pulp. The prepared handsheets were conditioned under controlled conditions as described in TAPPI test method TAPPI T402 (standard conditioning and testing atmosphere for paper, board, pulp handsheets and related products) before evaluation.
The commercially available enzymes, cellulase A “Amano”3 (C) and hemicellulase “Amano”90 (H) were obtained from Amano Pharmaceuticals Co. Ltd. (Nagoya, Japan). Both enzymes are produced by Aspergillus niger. The enzyme powders are water–soluble and were stored at 4 ◦ C prior to use. Cellulase and hemicellulase or xylanase activities were determined according to Gessesse and Gashaw [11] and Gessesse and Gashe [12], respectively.
2.6. Analysis of deinked paper properties 2.2. Selection and preparation of waste paper The waste paper used in this study was obtained from the Universiti Sains Malaysia (USM) campus. The waste papers were manually sorted to remove nonpaper objects. The sorted waste papers were stored in a room away from sunlight and high moisture prior to use. The waste papers were shredded into small pieces and pre-treated with 0.25 M HCl for 30 min prior to use in the enzymatic deinking process. 2.3. Characteristic of various waste papers For laser-printed paper, A4 size paper with a basis weight per area of 70–80 g/m2 was printed with toner on a Canon laser printer (Canon, China) covering approximately 40% of the paper area. One-month-old newspaper with an average weight per area of 50 g/m2 was used in this study. The newspaper was printed using a flexographic printing method, covering approximately 60% of the paper area. For photocopy-printed paper, photocopy toner (Xerox dry ink plus 5052/1050, Xerox Corporation) was used, and the photocopied paper was generated using a photocopier (Fuji Xerox, China). The average weight of the waste paper was 70–80 g/m2 , with an average photocopied area of 40%. For magazine paper, the glossy paper magazine (Cleo) was used, with ink covering the entire area of the paper (approximately 90%). The average weight per area of the waste paper was 70 g/m2 . Finally, bubble jet-printed waste paper, with an average weight of 70 g/m2 , was printed on a Canon bubble-jet printer using Canon BC 03 ink. Approximately 40% of the paper area of one side of each sheet was printed for use in the subsequent experiments.
After enzymatic deinking, the paper was examined for optical (brightness), mechanical (tensile, tear and burst) and pulp (drainage rate) properties. The brightness was measured as described in TAPPI T452 (Brightness of Pulp, Paper, and Paperboard; Directional Reflectance at 457 nm), using a brightness and opacity tester (Mode Micro S-5, Technidyne Corporation, USA). The tensile properties of the handsheets were examined using TAPPI test method T494 (Model LLOYD). The tear index was determined based on the internal resistance of the handsheet as described in the TAPPI T414 test method, using an electronic tearing tester (Model Protear, Thwing Albert, USA). The burst strength of the handsheet was measured according to the TAPPI T403 procedure (Bursting Strength of Paper), using a Model 3720 bursting strength tester (Lesson Industrial Corporation, Ltd., Taiwan). Finally, the drainage time of the handsheet was determined using TAPPI test T221 om-93 (Drainage time of pulp) procedures. This procedure was similar to the sheet making process, except the temperature of the diluted stock in the cylinder was adjusted within 20 ± 5 ◦ C. According to the protocol, a 400 ml sample volume (0.3% consistency) was used to produce a 1.2 g moisture-free handsheet. This handsheet was used for further analysis.
2.7. Total reducing sugar produced from enzymatic hydrolysis After enzymatic hydrolysis, 10 ml of fiber-free pulp slurry obtained by centrifugation under ambient temperature was used to examine the reducing sugar production through enzymatic hydrolysis of the pulp. The total reducing sugar produced was determined using DNS reagent [15].
2.4. Enzymatic hydrolysis and flotation process 2.8. Statistical methods The deinking process was performed according to Lee et al. [13], as described in Table 1. The batch deinking process involved pulp preparation, enzymatic hydrolysis and flotation. The pulp was prepared by disintegration using high speed mixer (National Model: MX-T100GN) for 4 min to produce 2% (w/w) pulp consistency [14]. The flotation process was performed using 600 ml vertically tubular flotation cell of diameter 3.2 cm and height 44.5 cm with water jacketed. The top section of the
All the experiments were performed in triplicate. The results are presented as the means of the triplicate experiments. The significance of differences between test variables was determined using a one-way ANOVA and the Least Significant Difference test with SPSS version 11.5 software. The evaluation of statistical significance was performed with a confidence interval of 95%.
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Deinking efficiency (%)
100
301
E
80 60
d
d
D C
40
b
20
c
B A
a
0
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Newspaper
Magazine
Waste papers Control
Sample
Fig. 1. Enzymatic deinking efficiency of different types of waste papers. Arrow bars indicate means with standard error and means with the same letter (a, b, c, d, A, B, C, D, and E) indicated no significantly difference at 5% level of probability by least significance test.
3. Results and discussion 3.1. Deinking efficiency of different types of printed waste papers
Brightness (%)
Fig. 1 shows a significant difference (P < 0.05) in the deinking efficiency obtained with laser-printed paper compared with the other papers. The highest enzymatic deinking efficiency of 86.6% was obtained using laser-printed paper, with a final brightness of 83.4% (Fig. 2). However, photocopy paper demonstrated a deinking efficiency of 63.5%. The differences in the deinking efficiency between these two types of papers might reflect the differences in the printing process and toner composition. Laser printing involves imaging using a computer-directed laser that ‘prints’ on the paper, and the ink is rapidly fixed, using a combination of heat and pressure [4]. In addition, the toner composition for photocopy toner consisted of 10–15% pigment (carbon black) and 85–90% styrene acrylate, while the laser toner consisted of 50–70% styrene acrylate and 30–40% pigment (iron oxide) [16]. The lowest deinking efficiency of 12.9% was obtained using newspaper, with a 50% final brightness (5 unit increase compared with blank paper) (Fig. 2). This result was consistent with Ibarra et al. [17], who obtained 3–4 units of increased newspaper brightness after enzymatic deinking using cellulase and hemicellulase. In addition, Morkbak and Zimmermann [18] and Lee et al. [19] obtained 7.3% and 2.5% increased in newspaper brightness, respectively, after enzymatic deinking. The inefficient enzymatic deinking of newspaper might reflect the fact that newspaper is commonly printed using a flexographic method with flexographic ink, which might not be effectively hydrolyzed by the enzymes used in the present study. The deinking efficiency obtained with bubble
jet-printed papers was only 23.6%. The water-based ink used in bubble jet printing remained soluble in water and was difficult to separate from the bulk flotation liquid in the water. Flexo inks, which are very small (0.2–1.0 m) due to their hydrophilic nature, remained strongly attached to the fiber and were difficult to separate during flotation. Ferguson [20] showed that the ink particles acted as water-repellent and bind to collector chemicals for removal through flotation. Extremely small ink particles are carried in the water loops, making it difficult to transport these compounds to the water surface for separation. The ink vehicle used for printing influences the simplicity of ink removal from printed-paper through enzymatic hydrolysis [18]. The brightness of the enzymatic deinked magazine paper was 70.1%, showing a 42.7% increase in deinking efficiency. The inks used in magazine printing are oil-based offset heat inks that are typically dried through evaporation of the volatile solvent at 60 ◦ C, leaving resins and other material to bind the pigments to the paper [15]. This cross-linking resulted in a poor deinking efficiency and recycling of the magazine papers. 3.2. Effect of enzymes on the hydrolysis of different types of waste papers The total amount of reducing sugar produced from the hydrolysis of waste paper differed for the different types of waste paper (Fig. 3). Magazine paper was the most susceptible (5.1 mg/gpulp ) to enzyme hydrolysis, while newspaper was the most resistant to be degraded, as only 1.9 mg/gpulp of total reducing sugar was produced. A significant difference (P < 0.05) in the total reducing sugar produced was detected for newspaper compared with the other
90 80 70 60 50 40 30 20 10 0 Laser-printed
Photocopy
Bubble jet printed
Newspaper
Magazine
Waste papers Blank
Control
Sample
Fig. 2. Brightness of the enzymatic deinking of different types of waste paper. Blank refers to the pulp slurry in the disintegrator. Control and sample used heat inactivated and active enzymes, respectively. Arrow bars indicate means with standard error of three replicates.
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Total reducing sugar (mg/g pulp)
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6.0
a
a 5.0 4.0
b d
3.0
c
2.0 1.0 0.0 Laser-printed
Photocopy
Bubble jetprinted
Newspaper
Magazine
Waste papers Fig. 3. Total reducing sugar produced from the hydrolysis of different types of waste papers. Arrow bars indicate means with standard error and means with the same letter (a, b, c, and d) indicated no significantly difference at 5% level of probability by least significance test.
four types of waste paper examined. No consistency in the total reducing sugar profile was obtained, which might reflect differences in the amount of ink present in the waste papers. However, increasing amounts of total reducing sugar were obtained with increasing brightness of the blank sample, as the amount of ink present in a particular waste paper was reduced, facilitating the increased exposure of cellulose fiber for enzymatic hydrolysis. Consequently, the increased enzymatic hydrolysis would increase the amount of total reducing sugar. Moreover, different types of paper might be coated with different types of sizing agent. The sizing agent might inhibit enzyme activity through increased fiber hydrophobicity, physically shielding fiber surfaces from enzyme attachment or preventing access to fibers via covalent bonds with cellulose [21]. Thus, the total amount of reducing sugar generated will be reduced. Different papers can vary in the composition and types of inorganic fillers used, such as kaolin, talc and titanium dioxide. The presence of these fillers and others additives might affect the enzymatic degradation of the cellulose fiber [16]. Furthermore, the properties of enzymatic hydrolysis, such as adsorption and reactivity, depend on the competition between the ink particles and the enzyme molecules on the fiber surface [22]. Higher enzyme attachment to the fiber surface will therefore increase the enzymatic production of reducing sugar from cellulose.
the rate of machine runnability, resulting in significant savings in energy and the overall cost [10]. The analysis of the drainage rate showed a significant improvement (P < 0.05) in the deinked waste papers compared with their respective blanks (Fig. 4). The laserprinted paper showed the highest drainage rate of 103.7 L/min, reflecting a 12.0% improvement. Rutledge-Cropsey et al. [23] obtained a drainage rate of 107.0 L/min after the enzymatic hydrolysis of office waste papers. In addition, Pathak et al. [24] reported an improved drainage rate of enzymatic deinked photocopier paper through an 11.5% reduction in the drainage time. Although newspaper exhibited the lowest drainage rate (64.9 L/min), the highest improvement (30.7%) in the overall drainage rate was obtained with newspaper compared with its blank. Most cellulase and hemicellulase preparations have shown the potential to remove microfibrils and fines, thus improving drainage rates [21]. Some studies have shown that enzymatic deinking enhances drainage rates with little to no loss in the sheet strength [7]. The enzymes act on the surface of the fibers, producing a peeling effect [25,26]. During peeling, the enzymes only remove small elements or components that have great affinity for water but do not contribute to the overall hydrogen bonding potential of the fibers. Notably, the determined brightness and calculated drainage times of the deinked paper alone did yield an adequate pulp quality. Therefore, the brightness does not represent the overall deinked paper quality. Other parameters of the paper, such as burst strength, tensile strength and tear resistance, are also important. Therefore, further experiments were conducted to evaluate the quality of the deinked paper in term of its physical strength.
3.3. The drainage rate of enzymatic deinked waste papers
Drainage rate ( L/min)
The drainage rate refers to the rate at which water is drained when a standard sheet weighing 60 g/m2 (moisture free) is formed at 20 ◦ C ± 5 ◦ C. Improvements in the pulp drainage rate will increase 120
+12%
+7.4%
100
+6.3%
+23.1%
80 60
+30.7%
h hi
j
g
h
i
40
e
f
fg a
20
b
c
c
d
e
0 Laser-printed
Photocopy
bubble jet printed
Newspaper
magazine
Waste papers Blank
Control
Sample
Fig. 4. Drainage rate of enzymatic deinking of different types of waste papers. Arrow bars indicate means with standard error and means with the same letter (a, b, c, d, e, f, g, fg, h, i, hi and j) indicated no significantly difference at 5% level of probability by least significance test. Means with the symbol (+, −) are different in percentage relative to respective blank.
Tensile index (Nm/g)
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45 40 35 30 25 20 15 10 5 0
303
+2.3% -3.0%
a
b
a
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b
a
-7.0%
-23.9%
-21.2%
d
bc b
c
Photocopy
a
Bubble jet printed
bc b
e
d
b
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Magazine
Waste papers Blank
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Sample
Fig. 5. Tensile index of enzymatic deinking of different types of waste papers. Arrow bars indicate means with standard error and means with the same letter (a, b, c, bc, d and e) indicated no significantly difference at 5% level of probability by least significance test. Means with the symbol (+, −) are different in percentage relative to respective blank.
3.4. Tensile strength
[27] demonstrated a 7.5% improvement in the breaking length of paper after the enzymatic deinking process. Pala et al. [28] used 11 commercial enzymes, comprising cellulase and hemicellulase, to deink mixed office waste. The results obtained indicated that seven commercial enzymes had detrimental effects, three enzymes showed improvement, and one enzyme had no effect on the tensile index tested. Thus, it is reasonable to conclude that different enzyme preparations differently affect the tensile index of paper. Different types of waste paper exhibited different effects on tensile index after enzymatic hydrolysis, reflecting differences in the cellulose fiber composition in the paper and different sizing agents used for coating the paper.
Tear index (mN m2/g)
The tensile index refers to tensile strength in N/m divided by the grammage. The tensile strength is the maximum tensile stress developed in a test specimen before rupture. It is a measure of the force required to tear the paper when pulled at opposite ends and in opposite directions. Cellulase and hemicellulase enzymes improve the tensile index of magazine paper (Fig. 5), and the loss in tensile index was not significant (P > 0.05) for laser-printed paper. Magazine paper showed a tensile index of 34.65 N m/g after enzyme treatment, which was increased 2.30% relative to its blank. Ibarra et al. [17] reported similar results in which the tensile index of magazine paper was improved after deinking using cellulase and hemicellulase from Aspergillus oryzae. Photocopy and bubble jet-printed paper showed large reductions of 21.2% and 23.9%, respectively, in the tensile index. However, these results were not consistent with Pathak et al. [24], who showed a 2.7% increase in the tensile index of enzymatic deinked photocopy paper. In addition, an increased amount of total reducing sugar (Fig. 3) was also detected for both photocopy and bubble jet-printed papers, suggesting that increased enzymatic hydrolysis of the papers occurred, resulting in the reduced tensile strength of the paper. Magazine papers showed improved tensile strength, although the total reducing sugar produced was also higher, potentially reflecting variations in the fiber composition. Gubitz et al. [3] reported a slight reduction in the tensile index relative to the control when endoglucanase and hemicellulase were used to deink xerographic and laser-printed papers. However, Prasad [4] obtained a different result, showing a slight increase in the tensile index of these papers relative to their blanks. Singh et al.
10 9 8 7 6 5 4 3 2 1 0
3.5. Internal tearing resistance Tearing resistance/strength refers to the ability of the paper to withstand the application of tearing forces measured in mN (milliNewtons). The tear index is calculated as the tearing strength divided by the grammage. Enzymatic hydrolysis improved the tear strength of laser-printed (3.1%) and magazine paper (2.8%) but reduced the tear strength of bubble jet-printed paper, newspaper and photocopy paper (Fig. 6). In contrast, Ibarra et al. [17] reported no effect or a slight increase in the tear index of newspaper after deinking using cellulase and hemicellulase from Humicola insolens and A. oryzae. A numbers of studies have examined the effects of enzymes on the tearing strength of deinked paper. Whereas some studies have obtained positive effects, other studies have demonstrated deteriorating effects. Morkbak and Zimmermann [18] showed a slight increase in the tear index after the enzymatic hydrolysis of mixed
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Fig. 6. Tear index of enzymatic deinking of different types of waste papers. Arrow bars indicate means with standard error and means with the same letter (a, b, c, d, e and f) indicated no significantly difference at 5% level of probability by least significance test. Means with the symbol (+, −) are different in percentage relative to respective blank.
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Burst index (kPa m2/g)
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Fig. 7. Burst index of enzymatic deinking of different types of waste papers. Arrow bars indicate means with standard error and means with the same letter (a, b, c, d, e, de, f, ef and g) indicated no significantly difference at 5% level of probability by least significance test. Means with the symbol (+, −) are different in percentage relative to respective blank.
office paper and old newspaper. However, Lee et al. [29] reported a 3.9% reduction in the tear index after the enzymatic hydrolysis of old newspaper. In addition, Prasad [4] reported a 5.9% reduction in the tear index after the enzymatic hydrolysis of laser and xerographic waste papers. Moreover, Pala et al. [28] reported a 34% reduction in the tear index when Buzyme 2523 was used to deink mixed office waste, but a 5% improved was obtained when CCMI 84 was used. Therefore, different enzyme preparations and fiber compositions are likely responsible for the differences in the tear effect of deinked paper. The widely accepted mechanism for the enzymatic removal of ink suggests that the surface fiber layers are removed through enzymatic activity on cellulose and the shear forces of mixing. The use of hemicellulase and cellulase enzymes have improve the freeness of the pulp due to removal of microfibrils and fines that aids in released of ink particles during flotation process [4,7,10]. In addition, for mild enzymatic hydrolysis, the reduction in pulp water interactions improves the drainage rate of the pulp without affecting the mechanical properties of the paper [4,5]. However, in the present work, it was observed that a reduction in the tear index for photocopy, newspaper and bubble jet-printed waste papers was detected, although these papers showed improvements in the drainage rate. This result suggests that lower enzyme dosages and reductions in the reaction times are essential to diminish deleterious effects, as too much enzyme or long reaction times can damage fibers. The adverse effects of higher enzyme doses are not completely understood but could reflect increased fibrillation [30]. 3.6. Bursting strength The burst strength refers to the hydrostatic pressure, measured in kilopascals (kPa) or pounds per square inch (psi), that is required to rupture the material when the pressure is increased at a controlled constant rate through a rubber diaphragm into a circular area with a 30.5 mm diameter. The burst index is calculated as the burst strength divided by the grammage. Enzymatic hydrolysis improved the burst strength for laser-printed paper, newspaper and magazine paper compared with their respective blanks (Fig. 7). However, the improvement was only significant (P < 0.05) for laserprinted paper (4.7%). A slight improvement in the burst index (1.8%) was also detected after the enzymatic hydrolysis of newspaper. These results were consistent with Lee et al. [29], who reported a 3.8% increase in the burst index of newspaper. The improvement in the burst index might not reflect the sizing agent present in the paper, as the sizing agent potentially limits enzymatic activity by increasing the
fiber hydrophobicity, physically shielding the fiber surfaces from enzyme attachment, or by preventing access to the fibers via covalent bonds with cellulose [21]. These effects can be observed in the low deinking efficiency and the low total reducing sugar produced from newspaper, which were 12.9% and 1.9 mg/gpulp , respectively (Fig. 3). In contrast, a significant drop (P < 0.05) in the burst strength was obtained for photocopy and bubble jet-printed paper, which was 8.3% and 7.3%, respectively. However, Prasad [4] and Gubitz et al. [3] reported an increased burst index after enzymatic deinking of laser and xerographic office waste papers, whereas Pathak et al. [24] reported a 15.3% improvement in the burst index of enzymatic deinked photocopier paper. However, according to Pala et al. [28], four commercial enzymes showed improvement, but seven enzymes showed detrimental effects in the burst index. These findings suggest that different enzyme preparations might confer different effects on the paper burst strength. Similarly, the different fiber compositions in different types of paper are likely responsible for the differences observed in the burst index. Generally, improvements in the burst strength reflect the removal of fines, which improves interfibrillar bonding and further enhances hydrogen bonding. Several studies have shown that during enzymatic deinking, cellulases act preferentially on the microfibrils and fines projecting out from the fiber surfaces [3,7,9]. 4. Conclusion The highest deinking efficiency was observed with laser-printed paper, but the lowest efficiency was obtained with newspaper. The enzymatic deinking process significantly improved the drainage rate of the deinked paper. However, the enzymatic process resulted in both positive and deleterious effects on the mechanical properties of deinked paper. Different papers are produced using different raw materials with various chemical compositions. In addition, different printing techniques and inks are used during the printing process. These differences might directly affect the efficiency of deinking, thereby resulting in mixed effects on the mechanical properties of deinked papers. Acknowledgements This work was financially supported by a research grant from the Ministry of Science, Technology and Innovation of Malaysia (MOSTI). The author (Lee Chee Keong) would like to thank Universiti Sains Malaysia for financial support through an incentive and short-term grant (304/PTEKIND/6311130).
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