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Turkey Red oil - An effective alkaline extraction booster for enhanced hemicelluloses separation from bamboo kraft pulp and improved fock reactivity of resultant dissolving pulp
Industrial Crops & Products 145 (2020) 112127
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Turkey Red oil - An effective alkaline extraction booster for enhanced hemicelluloses separation from bamboo kraft pulp and improved fock reactivity of resultant dissolving pulp
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Qiuyan Chena,b,c,*, Xinping Wangb, Hai Huangb, Shilin Caob, Lihui Chenb, Liulian Huangb, Xiaojuan Mab,* a b c
College of Life Sciences, Fujian Agriculture and Forestry University, Fuzhou, 350002, China College of Materials Engineering, Fujian Agriculture and Forestry University, Fuzhou, 350002, China State key laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cold caustic extraction Turkey red oil Wettability Hemicelluloses separation Reactivity
The development of effective separation of hemicelluloses from fully bleached cellulosic pulp fibers is conducive to the development and utilization of high value-added cellulose products. In this study, Turkey Red Oil (sulfonated castor oil) (TRO), a renewable source, was proposed to facilitate hemicelluloses separation from chemical pulp in cold caustic extraction (CCE). As expected, TRO application can significantly promote hemicelluloses separation in the CCE process. By contrast to the traditional CCE process, TRO/CCE could decrease the hemicelluloses content from 9.1 to 6.1%, while the hemicelluloses removal selectivity increased from 72.5 to 82.3% and efficiency, increased from 52.4 to 68.1%. Moreover, without sacrificing hemicelluloses removal, the TRO utilization can reduce the alkaline consumption and therefore preserve the cellulose I crystal from conversion to cellulose II. Additionally, the TRO/CCE procedure made the cellulose fiber more flexible and more accessible to react with CS2 and therefore a high Fock reactivity.
1. Introduction The rational development and utilization for the renewable biological resources is the key way forward that is truly in harmony with nature. Lignocellulosic biomass, with cellulose and hemicelluloses as its main carbohydrates, has been considered as the most abundant natural resources of raw material on the earth, which is in rising demand. And there, cellulose is a leading component in cell wall and carries out many important physiological functions. Based on its fascinating structure and properties, greater progresses had been made on these researches on the preparing high-value products (such as viscose staple fibers, cellulose ester, cellulose ethers, etc.) with high-purity cellulose (Wu et al., 2018; Arnoul-Jarriault et al., 2015; Chen et al., 2016; Kumar and Christopher, 2017). Dissolving pulp, also called high-purity cellulosic fibers, are regarded as prototypes of the future forest biorefineries and are used extensively for production of a range of cellulose derivatives (Van, 2006; Kumar and Christopher, 2017). The growing demand for dissolving pulp has prompted the kraft mill to develop into dissolving pulp mill (Lundberg et al., 2014). Nevertheless, there were some issues to be exposed during the
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process of dissolving-grade pulps production including lower dissolving pulp yields (generally below 30%), higher chemical costs and higher inventories requirements as technical conditions for pulping and bleaching were more demanding (Köpcke, 2010), which make the production investments more than other chemical paper-grade pulps. In order to breakthrough the bottleneck in the development of dissolving pulp, many economical methods for upgrading paper pulps to dissolving pulps were proposed, where the removal of hemicelluloses and cellulose reactivity increase have to be taken into account (Duan et al., 2016; Kumar and Christopher, 2017). In the downstream application of dissolving pulp (viscose rayon manufacturing), an increased reactivity of dissolving pulp can reap more homogenous viscose dope, and at the same time, can avoid excessive consumption of CS2, since the highly toxic CS2 would bring the environmental contamination and health damage (Kvarnlöf et al., 2007). Deserved to be mentioned, the Fock reactivity which was the indicator of the term “reactivity” of dissolving pulp was defined originally by Fock (1959), and the repeatability and accuracy of the test method were improved by Tian et al. (2013). Hence, intensive investigations have been concentrated on increasing the cellulose purity by the selective removal of hemicelluloses as well as
Corresponding authors. E-mail addresses: [email protected] (Q. Chen), [email protected] (X. Ma).
https://doi.org/10.1016/j.indcrop.2020.112127 Received 17 November 2019; Received in revised form 6 January 2020; Accepted 13 January 2020 0926-6690/ © 2020 Published by Elsevier B.V.
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to develop new alternatives for the improvement of CCE process. Besides, no literatures were found to investigate the effects of TRO addition to CCE treatment for bamboo kraft pulp. The majority of conventional surfactants are produced from aliphatic hydrocarbons and aromatic hydrocarbons derived from petroleum (non-reproducible resources), while TRO is a derivative of abundant castor oil. The relatively low-cost production of TRO could enable its mass application. Bamboo bleached kraft pulp is taken for the attractive feedstock for purification due to the fact that the perennial bamboo grows fast and contains considerable amount of cellulose (Zhao et al., 2017; Li et al., 2012). However, hemicelluloses content in bamboo is higher than that of wood species, resulting in high hemicelluloses content in bleached pulp during the paper-making. It is challenging to use bamboo bleached kraft pulp for purification in CCE process. Therefore, an attempt was made to develop a method for the hemicelluloses extractions of bamboo bleached kraft pulps via TRO-assisted alkaline treatment. This is possible since optimization of alkali liquor-cellulose system (reaction occurs at liquid-solid phrase) is required to change interface properties. Moreover, TRO is an anionic surfactant with a short fat and a strong hydrophilicity, and has sufficient solubility in a concentrated alkali solution, which is a prerequisite for becoming an additive to the CCE system. Addition of TRO to the CCE process for effective/selective removal of hemicelluloses at a low NaOH concentration was employed, which was so as to allow the commercial applicability of the process. In addition, investigations were made to understand the changes in cellulose structure of the original sample and treated sample with and without TRO addition to the CCE process. And, the effects on fiber morphologies, including pore volume, pore size and specific surface area (SSA), and the degree of polymerization (DP) as well as Fock reactivity were determined.
cellulose reactivity, such as cold caustic extraction (CCE), nitren and cuen extraction, enzymatic hydrolysis, ionic liquids extraction, fractionation and mechanical pretreatment that are used individually, combined and/or sequentially (Arnoul-Jarriault et al., 2015; Ibarra et al., 2010; Puls et al., 2006; Ma et al., 2017; Li et al., 2015). One of the most common anionic surfactants, Turkey Red Oil (TRO) commonly known as sulfonated castor oil, was formed from the modification of castor oil. TRO with molecular formula C18H32Na2O6S is produced through washing and neutralizing with sodium hydroxide solution following adding concentrated sulphuric acid to castor oil under the condition of 25−30 °C for several hours (Ogunniyi, 2006). It is mainly used in textile, pesticide, metal processing and paper making due to its versatile dispersing and emulsifying properties (Nawaby et al., 1998). Especially, among many auxiliaries as the necessary chemicals for textile production, TRO remains one of the most commonly used and superior additives in the textile printing and dyeing (Ogunniyi, 2006). The popularity of this surfactant is due to its ease and low cost of production and availability of raw material. In addition, because of excellent functions like solubility, dispersion, wetting and permeability, TRO lends itself to co-accelerate reactions with cooking liquor for the removal of most noncellulosic constituents such as hemicelluloses, lignin, pectin, wax and ash (Xiao et al., 2013). Its high wetting and penetration make it applicable as additives of various physical and chemical treatment. The CCE process is commonly applied in mill practice, in which increasing the removal of residual hemicelluloses from pulp fiber in sodium hydroxide by resulting in initial fibers swelling and then diffusion of hemicelluloses from fiber wall to bulk phase (Sixta, 2006). For traditional CCE process that a very selective way for removing the hemicelluloses, the emphasis in this treatment includes three aspects: (1) It is well known that CCE treatment of chemical pulp is carried out in aqueous solution, and chemical pulps have numerous air-filled pore structures with considerable specific surface areas. Because of this particular structure, mutual exclusion interface formed by contact between alkali liquor and cellulosic fibers could slow wetting and limit permeation resulting in uneven fiber swelling, such that hemicelluloses removal are negatively impacted (Tan et al., 2015). In such a case, wettability and surface free energy of fibers are considered to be technological importance (Simončič and Rozman, 2007); (2) the pulp reactivity is related to the crystalline form of cellulose. High NaOH concentration (usually at NaOH concentration 9% or higher) usually stimulates the formation of cellulose Ⅱ. In the literature, the pulp with cellulose Ⅱcrystalline form would be more reactive in chemical reactions, if the water in the pulp was removed directly by solvent exchange without drying process (Roselli et al., 2014). However, the drying process is often accompanied by a phase completation of the pulp operation. Cellulose Ⅱ favors a dense network of hydrogen bonds during subsequent drying process, making pulp more resistant to re-wetting and much lower accessibility/reactivity towards chemicals (Kolpak and Blackwell, 1967). The interests of employing a low NaOH concentration in TRO-assisted cold caustic extraction process for the hemicelluloses removal in preparing high-grade dissolving pulp, is that the formation of cellulose II could be avoided and resulting products can maintain a good cellulose accessibility towards chemicals, such as carbon disulfide (Li et al., 2015); (3) the greatest challenges in higher chemical consumption and environmental pollution, because of complications in recirculation of alkaline process derived from operation of high NaOH concentration in the typical CCE treatment, drive us to use less NaOH in a more efficient CCE process (Gehmayr and Sixta, 2012). For these purposes, a CCE process added by poly (ethylene glycol) (PEG) used as wetting, diffusing, dispersing, and solubilizing, had been reported; and more effective hemicelluloses removal for PEG/CCE-treated sample at lower alkali concentration was found compared to the sample from the control trial without PEG (5.30% versus 8.06%) (Li et al., 2016). However, since the PEG addition could help more hemicelluloses remove from cellulose fibers during the CCE stage, it would be beneficial
2. Experimental 2.1. Materials Fully bleached bamboo kraft pulp was kindly provided by a local pulp mill (Fujian, China). The chemical pulp was of 80.8% cellulose and 19.1% of hemicelluloses. The TRO with molecular weight of 422.48 was purchased from Shanghai Aladdin Biochemical Technology CO. LTD., China. All the chemicals used in the experiment were of analytical-grade. 2.2. Cold caustic extraction and TRO-assisted cold caustic extraction Cold caustic extraction (CCE): A 30 g of pulp (oven-dry) was treated with 6% or 10% NaOH at 25 ℃ for 45 min, and the pulp consistency was kept at 5%. A hand kneading was needed for an even reaction between chemical pulp and NaOH. After the reaction, the chemical pulp was filtered and the filtrate was removed. The CCE-treated samples were signed as CCE-6 and CCE-10 corresponding to the alkali concentration. TRO-assisted cold caustic extraction (TRO/CCE): 0.2% TRO (based on the oven dry pulp) was added into the CCE-6 process; and the sample, after TRO/CCE treatment, was denoted as TRO/CCE-6. 2.3. Determination of cellulose and hemicelluloses content In the work, α-cellulose and pentosan contents were used to describe the cellulose and hemicelluloses content, respectively. The αcellulose content, calculated based on the dissolved carbohydrates in 17.5% NaOH solution, was determined by following Technical Association of the Pulp and Paper Industry (TAPPI) test method T 203 cm-99. The content of hemicelluloses was determined by following TAPPI test method T 223 cm-01. The specific method description can also be referred to the literature from Luo et al. (2013). All testing procedures were carried out in duplicate and the average results are 2
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Fig. 1. Schematic illustration of TRO/CCE treatment for enhancing hemicelluloses removal.
Images of typical representatives were selected to be shown. The analysis of specific surface area and pore structure was carried out at Belsorp-Max volumetric gas adsorption instrument (Bel Japan, Inc., Osaka, Japan). The specific surface area (SSA), pore volume and pore size for cellulosic fibers were determined in accordance with the Brunauer-Emmett-Teller (BET) method. Prior to the determination, the pulp samples were air-dried. All testing procedures were carried out in duplicate and the average results are given.
given. 2.4. Determination of pulp selectivity and efficiency of hemicelluloses separation The samples were collected and weighted after cold caustic extraction for the further data analysis. The calculations of pulp yield loss, hemicelluloses separation selectivity and efficiency were calculated according to Eq. (1), (2) and (3), respectively (Li et al., 2019).
Yieldl (%)
Wb-Wa × 100 Wb
2.8. X ray diffraction (1)
Selectively (%) =
C b-C a × 100 Y ieldl
(2)
Efficiency (%) =
C b-C a × 100 Cb
(3)
The X-ray diffraction (XRD) spectra of the samples were acquired on a powder X-ray diffraction apparatus (X’PERT PRO MPD, Philips, Holland) using pellets prepared by pressing 50 mg dried pulp. The X-ray diffractometer was operated at at the Cu Kα wavelength at 40 Kv 20 Ma. The spectra were recorded in the range of 5° -40 ° operating with a scanning speed of 5 °/min. All testing procedures were carried out in duplicate and the average results are given. The crystallinity index (CrI) of samples was calculated based on a method reported by Segal et al. (1959) as expressed in Eq. (4).
where Yieldl is the pulp yield loss, Mb is the pulp weight before cold caustic extraction, Ma is the pulp weight after cold caustic extraction, Cb is the hemicelluloses content before treatment, and Ca is the hemicelluloses content after treatment.
Crystallinityindex(%) = 2.5. Degree of polymerization and Fock reactivity
(I 002 − I am) × 100 I 002
(4)
where I002 is the intensity at 22.7°, and Iam is the intensity at 16.8°. The intrinsic viscosity [η] of all samples was measured by Chinese national standard FZ/T 50010.3 (2011). The average DP was calculated from the intrinsic viscosity (i.e., DP0.905 = 0.75[η]). The Fock reactivity of samples was measured by the modified Fock method (Fock, 1959; Tian et al., 2013). This method describes a procedure for using sodium hydroxide concentration of 9%, xanthation temperature of 19 °C, carbon disulfide dosage of 1.3 mL and xanthation time of 3 h. All testing procedures were carried out in duplicate and the average results are given.
3. Results and discussion 3.1. Concept of enhancing hemicelluloses removal via TRO/CCE treatment In the process of CCE, the alkali solution needs to wet the cellulosic fibers and gradually penetrate into the fibers. Since the fibers are swelled, the low-molecular weight (Mw) hemicelluloses will diffuse into the bulk phase from the fiber structure. The concept of TRO/CCE treatment for enhancing the hemicelluloses removal was illustrated in Fig. 1. For the CCE process, hemicelluloses removal is challenging because of the extremely compact fiber structure originated from powerful inter- and intra -molecular hydrogen bonds (Hu et al., 2014; Carrillo-Varela et al., 2019). In addition to compact structure, the pore presented the cellulosic fibers is filled with air, as described previously. It's even worse that the alkali liquor with a high surface tension has more difficulties in swelling fibers. Besides, the gradual loss of hemicelluloses in the CCE-treated pulp could promote cellulose fibril aggregation. Dou and Tang (2017) investigated the effect of CCE treatment on the fibril aggregate size of dissolving pulps, and the results indicated that the CCE treatment led to 70% increase in fibril aggregate size when the NaOH concentration was increased from 5 to 9%. TRO, as an anionic surfactant, has a series of excellent solubility and stability in alkali liquor. TRO can reduce the surface tension to a large
2.6. Surface tension The surface tension of NaOH solution (6% NaOH concentration) with and without TRO was evaluated by the platinum ring method with a precision of ± 0.1 mN/m. The surface tensiometer (Shanghai Fangrui QBZY-1 automated tensiometer, China) was used in a room at constant temperature (25 °C). All testing procedures were carried out in duplicate and the average results are given. 2.7. Fiber morphology The observation of fiber sample was carried out by a scanning electron microscopy (SEM, VEGA 3 S-4800, Hitachi, Japan). The collected wet samples were freezedried prior to the SEM observation. 3
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Fig. 4. Effect of the TRO addition to CCE treatment on the component percentage of bamboo kraft pulp. Fig. 2. Effect of the TRO dosage on the surface tension.
in Fig. 3. In comparison to the straight and compactness of untreated sample, the NaOH-treated sample and TRO/CCE-treated sample showed a more increase in stretch and swelling, especially the samples from TRO/CCE-6 and CCE-10 process. And intriguingly, the samples treated with TRO/CCE-6 and CCE-10 presented a particularly soft posture. Serebryakova and Tokareva (1996) concluded that addition of derivatives of castor oil to alkali solution in alkali-steeping process was beneficial for the fiber swelling and alkali penetration during the viscose rayon process. In addition, the treated samples were found to have a peeling effect with more and longer fluff on the fiber surface.
extend (to be shown in Fig. 2). More specifically, TRO can be oriented at the interface between cellulosic fibers and NaOH solution, thereby accelerating wetting of alkali solution on the surface of cellulosic fibers and permeation into the fiber structure. With full and uniform penetration, cellulosic fibers can absorb NaOH more effectively and evenly, thus increasing fiber swelling during the CCE process. The specific changes in fiber swelling include softer solid fibers and enlargement of volume, which may eventually allow more hemicelluloses to detach from the fiber walls. Besides, the TRO presented in the resulting pulp due to TRO addition can possess more surface area of pulp than that without TRO. Hence, it is feasible to use TRO for decreasing the surface tension of NaOH and therefore promoting the swelling and penetrating properties of NaOH. Fig. 2 shows the surface tension of NaOH solution at the different TRO addition. It was reported that TRO can penetrate or accelerate the penetration of liquid into the porous fibers by reducing the surface tension of the liquid and the interfacial tension between the solid and liquid (Hong, 2002). The addition of TRO to the NaOH solution could allow the alkaline to rapidly penetrate into the fiber structure for promoting fiber swelling, thus promoting hemicelluloses to release from fiber structure and then diffuse into bulk phase. Further, effects of fiber swelling on surface morphology were shown
3.2. CCE vs. TRO/CCE for hemicelluloses separation According to the previous discussion, the dissolution of hemicelluloses during the CCE process is closely related to the wetting and penetration of the liquid phase into the solid phase cellulosic fibers. The addition of the TRO is intended to promote the effects of wetting and penetration and therefore hemicelluloses removal. The weight percentage of hemicelluloses and cellulose in the resultant pulp after CCE process and TRO/CCE treatment was shown in Fig. 4. It was obvious that the hemicelluloses content decreased with a rise of NaOH concentration (from 9.1% at 6% NaOH to 5.7% at 10% NaOH); accompanied with an increase of cellulose content (from 90.9% in the CCE-6
Fig. 3. The SEM images. a starting fiber sample, b fiber sample from CCE-6 process, c fiber sample from TRO/CCE-6 process, d fiber sample from CCE-10 process. 4
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sample to 94.2% of the CCE-10). The conventional CCE process is carried out at a NaOH concentration of 8–10 %, since a high alkali concentration could contribute to excellent cellulose swelling, leading to prominent hemicelluloses removal (Sixta, 2006). A lower alkali concentration would lead to a decrease in cellulose swelling and depth of penetration into the cellulosic structure, resulting in poor accessibility of cellulosic fibers to the alkaline lye (Hutterer et al., 2016). To be surprise, it was observed that TRO addition can significantly improve the hemicelluloses removal with 6.1% of hemicelluloses content and 93.9% of cellulose content in the resultant pulp. The TRO/ CCE-6 procedure was comparable with the CCE-10 procedure in the hemicelluloses separation. Several similar reports are available on the positive impact of surfactants on hemicelluloses, lignin and non-cellulosic compounds removal during pulping process. It was found that the addition of surfactant (Tween 80) led to an increase in hemicelluloses extraction without increasing the acid concentration during sulfuric acid pretreatment of wood chips (Wei et al., 2011). Tween 80 is beneficial to enhance the penetration and diffusion of cooking liquor into the wood chips as well as accelerate cooking liquor penetration, which led to a better hemicelluloses extraction. Besides, the TRO can enhance the dispersibility of the nanomaterials in water (Yang and Liu, 2010). As a result, TRO could allow the dissolved hemicelluloses to diffuse and migrate from the fiber structure into the bulk phase. It can be concluded that the addition of TRO to the CCE process has good potential for production of high-grade dissolving pulp, showing similar degree of hemicelluloses removal when decreased the NaOH concentration.
Fig. 5. XRD patterns of bleached bamboo kraft pulp after diffferent treatment.
xylanase addition to 40 g/L NaOH solution with 5.5% pulp consistency at normal temperature for 120 min (Hakala et al., 2013). The hemicelluloses removal selectivity and efficiency are critical for the cellulose purification, since the serious loss of cellulose will cause pressure on production cost and economic benefit. 3.4. Crystal structure of cellulose The XRD patterns of original bleached bamboo kraft pulp, CCEtreated pulp and TRO/CCE-treated pulp were illustrated in Fig. 5. For the purified pulp, a typical diffractograms of cellulose I showed peaks at around 14.8°, 16.4° and 22.7°, and a small peak at 34.7°, which correspond to the (1¯10 ), (110), (002) and (004) crystal planes (Oh et al., 2005). The diffraction patterns observed for original bleached pulp and CCE-6 treated sample remained in cellulose I crystal structure; while the introduction of a 10% NaOH extraction partially converted cellulose I to cellulose II, since the peak broadened, obtaining two peaks with small reflection emerge at 2θ of 12° and 20°. There were several studies reported that kraft pulp treated with 10% of NaOH did not completely converted to cellulose II, indicating that cellulose I and cellulose II coexisted simultaneously (Carrillo-Varela et al., 2019, 2018). CCE/TRO treatment does not alter the supramolecular structure of the pulp, and the treated pulp was consisted with cellulose I crystal structure even with a lower hemicellulose content. In parallel to similar degree of hemicelluloses dissolution compared to CCE-10 pulp (shown in Fig. 4), crystal transition from cellulose I to cellulose II was not found in CCE/ TRO-6. Without considering other factors, the remained cellulose I was reported to be in favor of dissolving pulp reactivity (Krässig, 1993; Arnoul-Jarriault et al., 2015). The CrI results shown in Table 2 indicated that concentrated NaOH treatment was detrimental for crystal structure of cellulose, the CrI decreased with an increase of NaOH concentration from 6% to 10%. This observation was accordance with the earlier report by Carrillo-Varela et al. (2019) that CCE treatment by 10% NaOH resulted in CrI reduction. The CrI loss was attributed to the de-crystallizing effect of the inter- and intra- hydrogen bonds of cellulose when the high NaOH concentration was applied to an alkaline aqueous solution (Ding et al., 2012). The addition of TRO almost had no effect on CrI; the sample from CCE-6 (63.1%) has the similar CrI with the sample from TRO/CCE-6 (63.6%). As shown in Table 2, the DP of the treated samples was higher than that of the original sample (596); the hemicellulose (low DP) removal could be responsible for this increase. Specifically, with the increase of hemicelluloses removal, the DP of TRO/CCE-6 treated pulp was increased from 608 (CCE-6) to 616, and further to 652 for CCE-10. The results agree with those reported in the literature that a CCE step could isolate low molecular weight substances from cellulosic fibers (for example, hemicelluloses), leaving the high molecular weight cellulose (Dou and Tang, 2017).
3.3. CCE vs. TRO/CCE for hemicelluloses removal selectivity and efficiency As the hemicelluloses content is one of major challenges for production of the high quality dissolving pulp, CCE process is commonly used for high-purity cellulose production. In addition to the dissolution of hemicelluloses, it is known that the dissolution of short chain cellulose fractions also takes place, which leads to the overall yield loss (Li et al., 2015). As shown in Table 1, the yield of CCE-6 was 86.2%, which was higher than that from the CCE/TRO-6 and CCE-10 (84.2% and 83.8% in Table 1). These results were well correlated with subsequent results for degree of hemicelluloses removal (hemicelluloses removal selectivity and efficiency). In the CCE process, the inter- and intra- molecular hydrogen bonds of cellulose chains are cleaved by alkali ions, which leads the swelling of cellulose and promotes hemicelluloses dissolution (Kamide et al., 1990). In this study, as can be noted in Table 1, the addition of TRO to the CCE treatment increased the dissolution of hemicelluloses in liquor, the hemicelluloses removal selectivity increased to 82.3% from 72.5% for CCE-6. The hemicelluloses removal efficiency results are also included in Table 1. TRO addition to the CCE process increased fiber swelling and enhanced hemicelluloses removal efficiency from 52.4% for the CCE-6 to 68.1%. It was observed that hemicelluloses removal selectivity and efficiency of the TRO/CCE-6 were similar to the CCE-10 (82.3% and 68.1% vs. 82.7% and 70.2%). Similar behavior was observed in an earlier study with xylan removal for a xylanase-aided alkaline extraction process of a bleached hardwood kraft pulp, where the xylan removal efficiency increased from 49.16 to 52.11% and the selectivity negligibly increased from 68.23 to 68.33% after 20 nkat/g Table 1 Comparison of hemicelluloses removal efficiency and selectivity. Procedure
Yield (%)
Hemicelluloses removal selectivity (%)
Hemicelluloses removal efficiency (%)
CCE-6 TRO/CCE-6 CCE-10
86.2 84.2 83.8
72.5 82.3 82.7
52.4 68.1 70.2
5
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Table 2 Specifications of CrI, DP, fiber mophorlogies and Fock reactivity changes of various samples. Procedure
CrI (%)
Control CCE-6 CCE-10 TRO / CCE-6
60.7 63.1 58.4 63.6
± ± ± ±
DP 2.0 1.8 3.0 1.9
596 608 651 616
± ± ± ±
35 29 31 20
Specific surface area (m2/g)
Total pore volume (×10−2 cm3/g)
Average pore diameter (nm)
Fock reactivity (%)
1.72 0.73 0.32 0.98
1.89 1.02 0.56 1.51
5.16 2.20 0.71 2.92
72.5 61.6 51.1 66.2
± ± ± ±
0.011 0.009 0.010 0.012
± ± ± ±
0.03 0.01 0.03 0.03
± ± ± ±
0.08 0.06 0.07 0.10
± ± ± ±
2.2 1.8 1.9 2.0
studied CCE treatment of bleached kraft pulps of different Eucalyptus species, and found that Fock reactivity enhancement was not only due to a decrease of pulp viscosity and the overall CrI was not the decisive factor that affects the pulp reactivity. In another study, the reduced pulp reactivity was likely caused by the abundance of aggregates of cellulose when the CCE process decreased the hemicelluloses content below 10% (Dou and Tang, 2017). The TRO distributed in fiber structure could avoid the pulp fibers agglomeration due to dehydration process, and make the dry pulp fluffy, which contributed to improvement of reactivity. TRO can also play an important role in changes of interface properties and fiber swelling. It may be because TRO adsorbed on the mutually exclusive interface, changing the surface properties of cellulosic fibers, lye and carbon disulfide, so that the interface was activated, and the cellulosic fibers can be quickly and fully soaked by alkali liquor, which was more conducive to hemicelluloses removal and the reaction of more cellulose and reagents, additionally improving the reactivity of cellulose. Of course, it was known that surfactants can also remove metal ions, dust and other impurities from the pulp, which made a part of contributions to the improvement of reactivity. To summarize, the presence of TRO can obtain a more softer alkali cellulose during CCE process, additionally in the increase in fiber specific surface area, pore size, and pore volume. This could be advantageous not only for further promoting hemicelluloses removal in the alkali-steeping but for improving cellulose reactivity during the viscose process of dissolving pulp.
3.5. Fiber mophorlogies and Fock reactivity The morphologies of the fiber, including the pore volume, pore size and specific surface area (SSA), could affect alkali-induced swelling of fibers, the NaOH penetration/diffusion and further removal of hemicelluloses in the downstream viscose fiber production process, the reactivity of cellulose and the quality of the final products, normally being considered as the most important performance of dissolving pulp. Therefore, it appears that porosity of the pulp is the parameter affecting the pulp reactivity. The changes on fiber morphology under the setting conditions are shown in Table 2. For the original sample, the largest porosity was supported by the presence of increasing amounts of hemicelluloses in the pulp, which agrees with previous study (Gehmayr and Sixta, 2012), during the production of dissolving pulp using CCE treatment. The CCE treatment was to decrease in SSA of bamboo kraft pulp with increasing NaOH concentration, which was from 0.73 m2/g at 6% NaOH concentration to 0.32 m2/g at 10% NaOH concentration. Moreover, the total pore volume decreased from 1.02 × 10−2 cm3/g to 0.56 × 10−2 cm3/g and average pore diameter decreased from 2.20 nm to 0.71 nm, when the NaOH concentration increased from 6 to 10%. The changes on narrow pores were due to the fact that a higher NaOH concentration system for dissolving hemicelluloses was very easy to result in more cellulose fibril aggregation (caused by extensive intermolecule hydrogen bonding). Compared to CCE-6 and CCE-10, TRO/ CCE treatment further reduced the hemicelluloses content, but did not cause more serious fiber aggregation. TRO/CCE-6 had more accessible surface areas and more expanded pores than that of CCE-treated pulp. As shown, the TRO/CCE treatment improved the SSA (from 0.73 m2/g and 0.32 m2/g of the CCE-treated samples to 0.98 m2/g of TRO/CCE-6). In addition, the TRO/CCE treatment led to an enhancement in pore volume (from 1.02 and 0.56 to 1.51 × 10−2 cm3/g). The TRO/CCE-6 also, had a higher average pore diameter (from 2.20 and 0.71 to 2.92 nm). Making better in fiber morphologies by adding surface active agents has been well reported in the literature. For example, Uneback and Creutz (1985) studied the influence of use of a polyglycolderivative on wet pulp, and found that the presence of surfactant in chemical pulp made dry pulp softer and fluffier compared to untreated pulp, because of the reduction of the shredding energy considerably and hydrogen bonds between fibers when the additive is well distributed in the pulp. Further, the Fock reactivity was investigated and the results were shown in Table 2. The original sample showed the highest Fock reactivity (72.5%) as a result of the hemicelluloses that abounded in pulps, which agrees with previous study. Dou and Tang (2017) found that the presence of hemicelluloses as the amorphous material were highly susceptible to chemicals (CS2) during Fock reactivity test. However, considering the hemicelluloses content limitation in the preparation of dissolving pulp, such pulps were not on our list of options. Through CCE treatment for dissolving pulp production, the Fock reactivity of CCE/TRO-6 was 66.2%, which is higher than that from the CCE alone process (61.6% at 6% NaOH and 51.1% at 10 % NaOH). The results agree with that reported in the literature that the derivatives of castor oil as surfactant can make a significant contribution to increase the cellulose reactivity in the production of viscose (Serebryakova and Tokareva, 1996). Specifically, among the CCE-treated samples, addition of TRO to the CCE process with the highest Fock reactivity was not found to be with the lowest CrI and DP. Carrillo-Varela et al. (2019)
4. Conclusions A novel TRO-enriched alkaline extraction was proposed to separate hemicelluloses from chemical pulp to purify chemical pulp. The TRO introduction in the CCE process not only promoted hemicelluloses separation from cellulosic fibers, but also protected cellulose I from conversion to cellulose II. More importantly, the purified pulps from TRO/CCE were more flexible indicating as a higher specific surface area and total pore volume; therefore, making the cellulose more accessibility towards carbon disulfide. CRediT authorship contribution statement Qiuyan Chen: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Xinping Wang: Investigation, Validation. Hai Huang: Investigation, Software, Visualization. Shilin Cao: Supervision. Lihui Chen: Project administration. Liulian Huang: Project administration. Xiaojuan Ma: Supervision, Writing - review & editing. Declaration of competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors are grateful for the financial support from National Natural Science Foundation of China (Grant No. 3177063) and Open 6
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Foundation of State Key Laboratory of Pulp and Paper Engineering of China (201805). References Arnoul-Jarriault, B., Lachenal, D., Chirat, C., Heux, L., 2015. Upgrading softwood bleached kraft pulp to dissolving pulp by cold caustic treatment and acid-hot caustic treatment. Ind. Crops Pro. 65, 565–571. Carrillo-Varela, I., Pereira, M., Mendonça, R.T., 2018. Determination of polymorphic changes in cellulose from Eucalyptus spp. Fibres after alkalization. Cellulose 25, 6831–6845. Carrillo-Varela, I., Retamal, R., Pereira, M., Mendonça, R.T., 2019. Structure and reactivity of cellulose from bleached kraft pulps of different Eucalyptus species upgraded to dissolving pulp. Cellulose 26, 5731–5744. Chen, C., Duan, C., Li, J., Liu, Y., Ma, X., Zheng, L., Stavik, J., Ni, Y., 2016. Cellulose (dissolving pulp) manufacturing processes and properties: a mini-review. BioResources 11, 5553–5564. Ding, Z.D., Chi, Z., Gu, W.X., Gu, S.M., Liu, J.H., Wang, H.J., 2012. Theoretical and experimental investigation on dissolution and regeneration of cellulose in ionic liquid. Carbohydr. Polym. 89, 7–16. Dou, X., Tang, Y., 2017. The influence of cold caustic extraction on the purity, accessibility and reactivity of dissolving‐grade pulp. ChemistrySelect 2, 11462–11468. Duan, C., Verma, S.K., Li, J., Ma, X., Ni, Y., 2016. Combination of mechanical, alkaline and enzymatic treatments to upgrade paper-grade pulp to dissolving pulp with high reactivity. Bioresour. Technol. 200, 458–463. FZ/T 50010.3, 2011. Pulp Board for Viscose Fiber—determination of Viscosity. China National Standards Press, Beijing. Fock, W., 1959. A modified method for determining the reactivity of viscose-grade dissolving pulps. Papier 13, 92–95. Gehmayr, V., Sixta, H., 2012. Pulp properties and their influence on enzymatic degradability. Biomacromolecules 13, 645–651. Hakala, T.K., Liitiä, T., Suurnäkki, A., 2013. Enzyme-aided alkaline extraction of oligosaccharides and polymeric xylan from hardwood kraft pulp. Carbohydr. Polym. 93, 102–108. Hong, Z., 2002. Functions and applications of textile sizing assistants. Cotton Textile Technology 2, 13–16. Hu, H., Zhang, Y., Liu, X., Huang, Z., Chen, Y., Yang, M., Qin, X., Feng, Z., 2014. Structural changes and enhanced accessibility of natural cellulose pretreated by mechanical activation. Polym. Bull. 71, 453–464. Hutterer, C., Schild, G., Potthast, A., 2016. A precise study on effects that trigger alkaline hemicellulose extraction efficiency. Bioresour. Technol. 214, 460–467. Ibarra, D., Köpcke, V., Larsson, P.T., Jääskeläinen, A.S., Ek, M., 2010. Combination of alkaline and enzymatic treatments as a process for upgrading sisal paper-grade pulp to dissolving-grade pulp. Bioresour. Technol. 101, 7416–7423. Kamide, K., Yasuda, K., Matsui, T., Okajima, K., Yamashiki, T., 1990. Structural change in alkali-soluble cellulose solid during its dissolution into aqueous alkaline solution. Cellulose chem. technol. 24, 23–31. Kolpak, F.J., Blackwell, J., 1967. Determination of the structure of cellulose II. Macromolecules 9, 273–278. Köpcke, V., 2010. Conversion of Wood and Non-wood Paper-grade Pulps to Dissolvinggrade Pulps. Doctoral Thesis. KTH Royal Institute of Technology, Stockholm, Sweden. Krässig, H.A., 1993. Cellulose-StructureAccessibility and reactivity Gordon and breach science publishers yverdon. Switzerland 307, 308. Kumar, H., Christopher, L.P., 2017. Recent trends and developments in dissolving pulp production and application. Cellulose 24, 2347–2365. Kvarnlöf, N., Germgård, U., Jönsson, L.J., Söderlund, C.A., 2007. Optimization of the enzymatic activation of a dissolving pulp before viscose manufacture. Tappi journal 6, 14–19. Li, J., Liu, X., Zheng, Q., Chen, L., Huang, L., Ni, Y., Ouyang, X., 2019. Urea/NaOH system for enhancing the removal of hemicellulose from cellulosic fibers. Cellulose 1–8. Li, J., Liu, Y., Duan, C., Zhang, H., Ni, Y., 2015. Mechanical pretreatment improving
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Turkey Red oil - An effective alkaline extraction booster for enhanced hemicelluloses separation from bamboo kraft pulp and improved fock reactivity of resultant dissolving pulp
T
Qiuyan Chena,b,c,*, Xinping Wangb, Hai Huangb, Shilin Caob, Lihui Chenb, Liulian Huangb, Xiaojuan Mab,* a b c
College of Life Sciences, Fujian Agriculture and Forestry University, Fuzhou, 350002, China College of Materials Engineering, Fujian Agriculture and Forestry University, Fuzhou, 350002, China State key laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cold caustic extraction Turkey red oil Wettability Hemicelluloses separation Reactivity
The development of effective separation of hemicelluloses from fully bleached cellulosic pulp fibers is conducive to the development and utilization of high value-added cellulose products. In this study, Turkey Red Oil (sulfonated castor oil) (TRO), a renewable source, was proposed to facilitate hemicelluloses separation from chemical pulp in cold caustic extraction (CCE). As expected, TRO application can significantly promote hemicelluloses separation in the CCE process. By contrast to the traditional CCE process, TRO/CCE could decrease the hemicelluloses content from 9.1 to 6.1%, while the hemicelluloses removal selectivity increased from 72.5 to 82.3% and efficiency, increased from 52.4 to 68.1%. Moreover, without sacrificing hemicelluloses removal, the TRO utilization can reduce the alkaline consumption and therefore preserve the cellulose I crystal from conversion to cellulose II. Additionally, the TRO/CCE procedure made the cellulose fiber more flexible and more accessible to react with CS2 and therefore a high Fock reactivity.
1. Introduction The rational development and utilization for the renewable biological resources is the key way forward that is truly in harmony with nature. Lignocellulosic biomass, with cellulose and hemicelluloses as its main carbohydrates, has been considered as the most abundant natural resources of raw material on the earth, which is in rising demand. And there, cellulose is a leading component in cell wall and carries out many important physiological functions. Based on its fascinating structure and properties, greater progresses had been made on these researches on the preparing high-value products (such as viscose staple fibers, cellulose ester, cellulose ethers, etc.) with high-purity cellulose (Wu et al., 2018; Arnoul-Jarriault et al., 2015; Chen et al., 2016; Kumar and Christopher, 2017). Dissolving pulp, also called high-purity cellulosic fibers, are regarded as prototypes of the future forest biorefineries and are used extensively for production of a range of cellulose derivatives (Van, 2006; Kumar and Christopher, 2017). The growing demand for dissolving pulp has prompted the kraft mill to develop into dissolving pulp mill (Lundberg et al., 2014). Nevertheless, there were some issues to be exposed during the
⁎
process of dissolving-grade pulps production including lower dissolving pulp yields (generally below 30%), higher chemical costs and higher inventories requirements as technical conditions for pulping and bleaching were more demanding (Köpcke, 2010), which make the production investments more than other chemical paper-grade pulps. In order to breakthrough the bottleneck in the development of dissolving pulp, many economical methods for upgrading paper pulps to dissolving pulps were proposed, where the removal of hemicelluloses and cellulose reactivity increase have to be taken into account (Duan et al., 2016; Kumar and Christopher, 2017). In the downstream application of dissolving pulp (viscose rayon manufacturing), an increased reactivity of dissolving pulp can reap more homogenous viscose dope, and at the same time, can avoid excessive consumption of CS2, since the highly toxic CS2 would bring the environmental contamination and health damage (Kvarnlöf et al., 2007). Deserved to be mentioned, the Fock reactivity which was the indicator of the term “reactivity” of dissolving pulp was defined originally by Fock (1959), and the repeatability and accuracy of the test method were improved by Tian et al. (2013). Hence, intensive investigations have been concentrated on increasing the cellulose purity by the selective removal of hemicelluloses as well as
Corresponding authors. E-mail addresses: [email protected] (Q. Chen), [email protected] (X. Ma).
https://doi.org/10.1016/j.indcrop.2020.112127 Received 17 November 2019; Received in revised form 6 January 2020; Accepted 13 January 2020 0926-6690/ © 2020 Published by Elsevier B.V.
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to develop new alternatives for the improvement of CCE process. Besides, no literatures were found to investigate the effects of TRO addition to CCE treatment for bamboo kraft pulp. The majority of conventional surfactants are produced from aliphatic hydrocarbons and aromatic hydrocarbons derived from petroleum (non-reproducible resources), while TRO is a derivative of abundant castor oil. The relatively low-cost production of TRO could enable its mass application. Bamboo bleached kraft pulp is taken for the attractive feedstock for purification due to the fact that the perennial bamboo grows fast and contains considerable amount of cellulose (Zhao et al., 2017; Li et al., 2012). However, hemicelluloses content in bamboo is higher than that of wood species, resulting in high hemicelluloses content in bleached pulp during the paper-making. It is challenging to use bamboo bleached kraft pulp for purification in CCE process. Therefore, an attempt was made to develop a method for the hemicelluloses extractions of bamboo bleached kraft pulps via TRO-assisted alkaline treatment. This is possible since optimization of alkali liquor-cellulose system (reaction occurs at liquid-solid phrase) is required to change interface properties. Moreover, TRO is an anionic surfactant with a short fat and a strong hydrophilicity, and has sufficient solubility in a concentrated alkali solution, which is a prerequisite for becoming an additive to the CCE system. Addition of TRO to the CCE process for effective/selective removal of hemicelluloses at a low NaOH concentration was employed, which was so as to allow the commercial applicability of the process. In addition, investigations were made to understand the changes in cellulose structure of the original sample and treated sample with and without TRO addition to the CCE process. And, the effects on fiber morphologies, including pore volume, pore size and specific surface area (SSA), and the degree of polymerization (DP) as well as Fock reactivity were determined.
cellulose reactivity, such as cold caustic extraction (CCE), nitren and cuen extraction, enzymatic hydrolysis, ionic liquids extraction, fractionation and mechanical pretreatment that are used individually, combined and/or sequentially (Arnoul-Jarriault et al., 2015; Ibarra et al., 2010; Puls et al., 2006; Ma et al., 2017; Li et al., 2015). One of the most common anionic surfactants, Turkey Red Oil (TRO) commonly known as sulfonated castor oil, was formed from the modification of castor oil. TRO with molecular formula C18H32Na2O6S is produced through washing and neutralizing with sodium hydroxide solution following adding concentrated sulphuric acid to castor oil under the condition of 25−30 °C for several hours (Ogunniyi, 2006). It is mainly used in textile, pesticide, metal processing and paper making due to its versatile dispersing and emulsifying properties (Nawaby et al., 1998). Especially, among many auxiliaries as the necessary chemicals for textile production, TRO remains one of the most commonly used and superior additives in the textile printing and dyeing (Ogunniyi, 2006). The popularity of this surfactant is due to its ease and low cost of production and availability of raw material. In addition, because of excellent functions like solubility, dispersion, wetting and permeability, TRO lends itself to co-accelerate reactions with cooking liquor for the removal of most noncellulosic constituents such as hemicelluloses, lignin, pectin, wax and ash (Xiao et al., 2013). Its high wetting and penetration make it applicable as additives of various physical and chemical treatment. The CCE process is commonly applied in mill practice, in which increasing the removal of residual hemicelluloses from pulp fiber in sodium hydroxide by resulting in initial fibers swelling and then diffusion of hemicelluloses from fiber wall to bulk phase (Sixta, 2006). For traditional CCE process that a very selective way for removing the hemicelluloses, the emphasis in this treatment includes three aspects: (1) It is well known that CCE treatment of chemical pulp is carried out in aqueous solution, and chemical pulps have numerous air-filled pore structures with considerable specific surface areas. Because of this particular structure, mutual exclusion interface formed by contact between alkali liquor and cellulosic fibers could slow wetting and limit permeation resulting in uneven fiber swelling, such that hemicelluloses removal are negatively impacted (Tan et al., 2015). In such a case, wettability and surface free energy of fibers are considered to be technological importance (Simončič and Rozman, 2007); (2) the pulp reactivity is related to the crystalline form of cellulose. High NaOH concentration (usually at NaOH concentration 9% or higher) usually stimulates the formation of cellulose Ⅱ. In the literature, the pulp with cellulose Ⅱcrystalline form would be more reactive in chemical reactions, if the water in the pulp was removed directly by solvent exchange without drying process (Roselli et al., 2014). However, the drying process is often accompanied by a phase completation of the pulp operation. Cellulose Ⅱ favors a dense network of hydrogen bonds during subsequent drying process, making pulp more resistant to re-wetting and much lower accessibility/reactivity towards chemicals (Kolpak and Blackwell, 1967). The interests of employing a low NaOH concentration in TRO-assisted cold caustic extraction process for the hemicelluloses removal in preparing high-grade dissolving pulp, is that the formation of cellulose II could be avoided and resulting products can maintain a good cellulose accessibility towards chemicals, such as carbon disulfide (Li et al., 2015); (3) the greatest challenges in higher chemical consumption and environmental pollution, because of complications in recirculation of alkaline process derived from operation of high NaOH concentration in the typical CCE treatment, drive us to use less NaOH in a more efficient CCE process (Gehmayr and Sixta, 2012). For these purposes, a CCE process added by poly (ethylene glycol) (PEG) used as wetting, diffusing, dispersing, and solubilizing, had been reported; and more effective hemicelluloses removal for PEG/CCE-treated sample at lower alkali concentration was found compared to the sample from the control trial without PEG (5.30% versus 8.06%) (Li et al., 2016). However, since the PEG addition could help more hemicelluloses remove from cellulose fibers during the CCE stage, it would be beneficial
2. Experimental 2.1. Materials Fully bleached bamboo kraft pulp was kindly provided by a local pulp mill (Fujian, China). The chemical pulp was of 80.8% cellulose and 19.1% of hemicelluloses. The TRO with molecular weight of 422.48 was purchased from Shanghai Aladdin Biochemical Technology CO. LTD., China. All the chemicals used in the experiment were of analytical-grade. 2.2. Cold caustic extraction and TRO-assisted cold caustic extraction Cold caustic extraction (CCE): A 30 g of pulp (oven-dry) was treated with 6% or 10% NaOH at 25 ℃ for 45 min, and the pulp consistency was kept at 5%. A hand kneading was needed for an even reaction between chemical pulp and NaOH. After the reaction, the chemical pulp was filtered and the filtrate was removed. The CCE-treated samples were signed as CCE-6 and CCE-10 corresponding to the alkali concentration. TRO-assisted cold caustic extraction (TRO/CCE): 0.2% TRO (based on the oven dry pulp) was added into the CCE-6 process; and the sample, after TRO/CCE treatment, was denoted as TRO/CCE-6. 2.3. Determination of cellulose and hemicelluloses content In the work, α-cellulose and pentosan contents were used to describe the cellulose and hemicelluloses content, respectively. The αcellulose content, calculated based on the dissolved carbohydrates in 17.5% NaOH solution, was determined by following Technical Association of the Pulp and Paper Industry (TAPPI) test method T 203 cm-99. The content of hemicelluloses was determined by following TAPPI test method T 223 cm-01. The specific method description can also be referred to the literature from Luo et al. (2013). All testing procedures were carried out in duplicate and the average results are 2
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Fig. 1. Schematic illustration of TRO/CCE treatment for enhancing hemicelluloses removal.
Images of typical representatives were selected to be shown. The analysis of specific surface area and pore structure was carried out at Belsorp-Max volumetric gas adsorption instrument (Bel Japan, Inc., Osaka, Japan). The specific surface area (SSA), pore volume and pore size for cellulosic fibers were determined in accordance with the Brunauer-Emmett-Teller (BET) method. Prior to the determination, the pulp samples were air-dried. All testing procedures were carried out in duplicate and the average results are given.
given. 2.4. Determination of pulp selectivity and efficiency of hemicelluloses separation The samples were collected and weighted after cold caustic extraction for the further data analysis. The calculations of pulp yield loss, hemicelluloses separation selectivity and efficiency were calculated according to Eq. (1), (2) and (3), respectively (Li et al., 2019).
Yieldl (%)
Wb-Wa × 100 Wb
2.8. X ray diffraction (1)
Selectively (%) =
C b-C a × 100 Y ieldl
(2)
Efficiency (%) =
C b-C a × 100 Cb
(3)
The X-ray diffraction (XRD) spectra of the samples were acquired on a powder X-ray diffraction apparatus (X’PERT PRO MPD, Philips, Holland) using pellets prepared by pressing 50 mg dried pulp. The X-ray diffractometer was operated at at the Cu Kα wavelength at 40 Kv 20 Ma. The spectra were recorded in the range of 5° -40 ° operating with a scanning speed of 5 °/min. All testing procedures were carried out in duplicate and the average results are given. The crystallinity index (CrI) of samples was calculated based on a method reported by Segal et al. (1959) as expressed in Eq. (4).
where Yieldl is the pulp yield loss, Mb is the pulp weight before cold caustic extraction, Ma is the pulp weight after cold caustic extraction, Cb is the hemicelluloses content before treatment, and Ca is the hemicelluloses content after treatment.
Crystallinityindex(%) = 2.5. Degree of polymerization and Fock reactivity
(I 002 − I am) × 100 I 002
(4)
where I002 is the intensity at 22.7°, and Iam is the intensity at 16.8°. The intrinsic viscosity [η] of all samples was measured by Chinese national standard FZ/T 50010.3 (2011). The average DP was calculated from the intrinsic viscosity (i.e., DP0.905 = 0.75[η]). The Fock reactivity of samples was measured by the modified Fock method (Fock, 1959; Tian et al., 2013). This method describes a procedure for using sodium hydroxide concentration of 9%, xanthation temperature of 19 °C, carbon disulfide dosage of 1.3 mL and xanthation time of 3 h. All testing procedures were carried out in duplicate and the average results are given.
3. Results and discussion 3.1. Concept of enhancing hemicelluloses removal via TRO/CCE treatment In the process of CCE, the alkali solution needs to wet the cellulosic fibers and gradually penetrate into the fibers. Since the fibers are swelled, the low-molecular weight (Mw) hemicelluloses will diffuse into the bulk phase from the fiber structure. The concept of TRO/CCE treatment for enhancing the hemicelluloses removal was illustrated in Fig. 1. For the CCE process, hemicelluloses removal is challenging because of the extremely compact fiber structure originated from powerful inter- and intra -molecular hydrogen bonds (Hu et al., 2014; Carrillo-Varela et al., 2019). In addition to compact structure, the pore presented the cellulosic fibers is filled with air, as described previously. It's even worse that the alkali liquor with a high surface tension has more difficulties in swelling fibers. Besides, the gradual loss of hemicelluloses in the CCE-treated pulp could promote cellulose fibril aggregation. Dou and Tang (2017) investigated the effect of CCE treatment on the fibril aggregate size of dissolving pulps, and the results indicated that the CCE treatment led to 70% increase in fibril aggregate size when the NaOH concentration was increased from 5 to 9%. TRO, as an anionic surfactant, has a series of excellent solubility and stability in alkali liquor. TRO can reduce the surface tension to a large
2.6. Surface tension The surface tension of NaOH solution (6% NaOH concentration) with and without TRO was evaluated by the platinum ring method with a precision of ± 0.1 mN/m. The surface tensiometer (Shanghai Fangrui QBZY-1 automated tensiometer, China) was used in a room at constant temperature (25 °C). All testing procedures were carried out in duplicate and the average results are given. 2.7. Fiber morphology The observation of fiber sample was carried out by a scanning electron microscopy (SEM, VEGA 3 S-4800, Hitachi, Japan). The collected wet samples were freezedried prior to the SEM observation. 3
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Fig. 4. Effect of the TRO addition to CCE treatment on the component percentage of bamboo kraft pulp. Fig. 2. Effect of the TRO dosage on the surface tension.
in Fig. 3. In comparison to the straight and compactness of untreated sample, the NaOH-treated sample and TRO/CCE-treated sample showed a more increase in stretch and swelling, especially the samples from TRO/CCE-6 and CCE-10 process. And intriguingly, the samples treated with TRO/CCE-6 and CCE-10 presented a particularly soft posture. Serebryakova and Tokareva (1996) concluded that addition of derivatives of castor oil to alkali solution in alkali-steeping process was beneficial for the fiber swelling and alkali penetration during the viscose rayon process. In addition, the treated samples were found to have a peeling effect with more and longer fluff on the fiber surface.
extend (to be shown in Fig. 2). More specifically, TRO can be oriented at the interface between cellulosic fibers and NaOH solution, thereby accelerating wetting of alkali solution on the surface of cellulosic fibers and permeation into the fiber structure. With full and uniform penetration, cellulosic fibers can absorb NaOH more effectively and evenly, thus increasing fiber swelling during the CCE process. The specific changes in fiber swelling include softer solid fibers and enlargement of volume, which may eventually allow more hemicelluloses to detach from the fiber walls. Besides, the TRO presented in the resulting pulp due to TRO addition can possess more surface area of pulp than that without TRO. Hence, it is feasible to use TRO for decreasing the surface tension of NaOH and therefore promoting the swelling and penetrating properties of NaOH. Fig. 2 shows the surface tension of NaOH solution at the different TRO addition. It was reported that TRO can penetrate or accelerate the penetration of liquid into the porous fibers by reducing the surface tension of the liquid and the interfacial tension between the solid and liquid (Hong, 2002). The addition of TRO to the NaOH solution could allow the alkaline to rapidly penetrate into the fiber structure for promoting fiber swelling, thus promoting hemicelluloses to release from fiber structure and then diffuse into bulk phase. Further, effects of fiber swelling on surface morphology were shown
3.2. CCE vs. TRO/CCE for hemicelluloses separation According to the previous discussion, the dissolution of hemicelluloses during the CCE process is closely related to the wetting and penetration of the liquid phase into the solid phase cellulosic fibers. The addition of the TRO is intended to promote the effects of wetting and penetration and therefore hemicelluloses removal. The weight percentage of hemicelluloses and cellulose in the resultant pulp after CCE process and TRO/CCE treatment was shown in Fig. 4. It was obvious that the hemicelluloses content decreased with a rise of NaOH concentration (from 9.1% at 6% NaOH to 5.7% at 10% NaOH); accompanied with an increase of cellulose content (from 90.9% in the CCE-6
Fig. 3. The SEM images. a starting fiber sample, b fiber sample from CCE-6 process, c fiber sample from TRO/CCE-6 process, d fiber sample from CCE-10 process. 4
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sample to 94.2% of the CCE-10). The conventional CCE process is carried out at a NaOH concentration of 8–10 %, since a high alkali concentration could contribute to excellent cellulose swelling, leading to prominent hemicelluloses removal (Sixta, 2006). A lower alkali concentration would lead to a decrease in cellulose swelling and depth of penetration into the cellulosic structure, resulting in poor accessibility of cellulosic fibers to the alkaline lye (Hutterer et al., 2016). To be surprise, it was observed that TRO addition can significantly improve the hemicelluloses removal with 6.1% of hemicelluloses content and 93.9% of cellulose content in the resultant pulp. The TRO/ CCE-6 procedure was comparable with the CCE-10 procedure in the hemicelluloses separation. Several similar reports are available on the positive impact of surfactants on hemicelluloses, lignin and non-cellulosic compounds removal during pulping process. It was found that the addition of surfactant (Tween 80) led to an increase in hemicelluloses extraction without increasing the acid concentration during sulfuric acid pretreatment of wood chips (Wei et al., 2011). Tween 80 is beneficial to enhance the penetration and diffusion of cooking liquor into the wood chips as well as accelerate cooking liquor penetration, which led to a better hemicelluloses extraction. Besides, the TRO can enhance the dispersibility of the nanomaterials in water (Yang and Liu, 2010). As a result, TRO could allow the dissolved hemicelluloses to diffuse and migrate from the fiber structure into the bulk phase. It can be concluded that the addition of TRO to the CCE process has good potential for production of high-grade dissolving pulp, showing similar degree of hemicelluloses removal when decreased the NaOH concentration.
Fig. 5. XRD patterns of bleached bamboo kraft pulp after diffferent treatment.
xylanase addition to 40 g/L NaOH solution with 5.5% pulp consistency at normal temperature for 120 min (Hakala et al., 2013). The hemicelluloses removal selectivity and efficiency are critical for the cellulose purification, since the serious loss of cellulose will cause pressure on production cost and economic benefit. 3.4. Crystal structure of cellulose The XRD patterns of original bleached bamboo kraft pulp, CCEtreated pulp and TRO/CCE-treated pulp were illustrated in Fig. 5. For the purified pulp, a typical diffractograms of cellulose I showed peaks at around 14.8°, 16.4° and 22.7°, and a small peak at 34.7°, which correspond to the (1¯10 ), (110), (002) and (004) crystal planes (Oh et al., 2005). The diffraction patterns observed for original bleached pulp and CCE-6 treated sample remained in cellulose I crystal structure; while the introduction of a 10% NaOH extraction partially converted cellulose I to cellulose II, since the peak broadened, obtaining two peaks with small reflection emerge at 2θ of 12° and 20°. There were several studies reported that kraft pulp treated with 10% of NaOH did not completely converted to cellulose II, indicating that cellulose I and cellulose II coexisted simultaneously (Carrillo-Varela et al., 2019, 2018). CCE/TRO treatment does not alter the supramolecular structure of the pulp, and the treated pulp was consisted with cellulose I crystal structure even with a lower hemicellulose content. In parallel to similar degree of hemicelluloses dissolution compared to CCE-10 pulp (shown in Fig. 4), crystal transition from cellulose I to cellulose II was not found in CCE/ TRO-6. Without considering other factors, the remained cellulose I was reported to be in favor of dissolving pulp reactivity (Krässig, 1993; Arnoul-Jarriault et al., 2015). The CrI results shown in Table 2 indicated that concentrated NaOH treatment was detrimental for crystal structure of cellulose, the CrI decreased with an increase of NaOH concentration from 6% to 10%. This observation was accordance with the earlier report by Carrillo-Varela et al. (2019) that CCE treatment by 10% NaOH resulted in CrI reduction. The CrI loss was attributed to the de-crystallizing effect of the inter- and intra- hydrogen bonds of cellulose when the high NaOH concentration was applied to an alkaline aqueous solution (Ding et al., 2012). The addition of TRO almost had no effect on CrI; the sample from CCE-6 (63.1%) has the similar CrI with the sample from TRO/CCE-6 (63.6%). As shown in Table 2, the DP of the treated samples was higher than that of the original sample (596); the hemicellulose (low DP) removal could be responsible for this increase. Specifically, with the increase of hemicelluloses removal, the DP of TRO/CCE-6 treated pulp was increased from 608 (CCE-6) to 616, and further to 652 for CCE-10. The results agree with those reported in the literature that a CCE step could isolate low molecular weight substances from cellulosic fibers (for example, hemicelluloses), leaving the high molecular weight cellulose (Dou and Tang, 2017).
3.3. CCE vs. TRO/CCE for hemicelluloses removal selectivity and efficiency As the hemicelluloses content is one of major challenges for production of the high quality dissolving pulp, CCE process is commonly used for high-purity cellulose production. In addition to the dissolution of hemicelluloses, it is known that the dissolution of short chain cellulose fractions also takes place, which leads to the overall yield loss (Li et al., 2015). As shown in Table 1, the yield of CCE-6 was 86.2%, which was higher than that from the CCE/TRO-6 and CCE-10 (84.2% and 83.8% in Table 1). These results were well correlated with subsequent results for degree of hemicelluloses removal (hemicelluloses removal selectivity and efficiency). In the CCE process, the inter- and intra- molecular hydrogen bonds of cellulose chains are cleaved by alkali ions, which leads the swelling of cellulose and promotes hemicelluloses dissolution (Kamide et al., 1990). In this study, as can be noted in Table 1, the addition of TRO to the CCE treatment increased the dissolution of hemicelluloses in liquor, the hemicelluloses removal selectivity increased to 82.3% from 72.5% for CCE-6. The hemicelluloses removal efficiency results are also included in Table 1. TRO addition to the CCE process increased fiber swelling and enhanced hemicelluloses removal efficiency from 52.4% for the CCE-6 to 68.1%. It was observed that hemicelluloses removal selectivity and efficiency of the TRO/CCE-6 were similar to the CCE-10 (82.3% and 68.1% vs. 82.7% and 70.2%). Similar behavior was observed in an earlier study with xylan removal for a xylanase-aided alkaline extraction process of a bleached hardwood kraft pulp, where the xylan removal efficiency increased from 49.16 to 52.11% and the selectivity negligibly increased from 68.23 to 68.33% after 20 nkat/g Table 1 Comparison of hemicelluloses removal efficiency and selectivity. Procedure
Yield (%)
Hemicelluloses removal selectivity (%)
Hemicelluloses removal efficiency (%)
CCE-6 TRO/CCE-6 CCE-10
86.2 84.2 83.8
72.5 82.3 82.7
52.4 68.1 70.2
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Table 2 Specifications of CrI, DP, fiber mophorlogies and Fock reactivity changes of various samples. Procedure
CrI (%)
Control CCE-6 CCE-10 TRO / CCE-6
60.7 63.1 58.4 63.6
± ± ± ±
DP 2.0 1.8 3.0 1.9
596 608 651 616
± ± ± ±
35 29 31 20
Specific surface area (m2/g)
Total pore volume (×10−2 cm3/g)
Average pore diameter (nm)
Fock reactivity (%)
1.72 0.73 0.32 0.98
1.89 1.02 0.56 1.51
5.16 2.20 0.71 2.92
72.5 61.6 51.1 66.2
± ± ± ±
0.011 0.009 0.010 0.012
± ± ± ±
0.03 0.01 0.03 0.03
± ± ± ±
0.08 0.06 0.07 0.10
± ± ± ±
2.2 1.8 1.9 2.0
studied CCE treatment of bleached kraft pulps of different Eucalyptus species, and found that Fock reactivity enhancement was not only due to a decrease of pulp viscosity and the overall CrI was not the decisive factor that affects the pulp reactivity. In another study, the reduced pulp reactivity was likely caused by the abundance of aggregates of cellulose when the CCE process decreased the hemicelluloses content below 10% (Dou and Tang, 2017). The TRO distributed in fiber structure could avoid the pulp fibers agglomeration due to dehydration process, and make the dry pulp fluffy, which contributed to improvement of reactivity. TRO can also play an important role in changes of interface properties and fiber swelling. It may be because TRO adsorbed on the mutually exclusive interface, changing the surface properties of cellulosic fibers, lye and carbon disulfide, so that the interface was activated, and the cellulosic fibers can be quickly and fully soaked by alkali liquor, which was more conducive to hemicelluloses removal and the reaction of more cellulose and reagents, additionally improving the reactivity of cellulose. Of course, it was known that surfactants can also remove metal ions, dust and other impurities from the pulp, which made a part of contributions to the improvement of reactivity. To summarize, the presence of TRO can obtain a more softer alkali cellulose during CCE process, additionally in the increase in fiber specific surface area, pore size, and pore volume. This could be advantageous not only for further promoting hemicelluloses removal in the alkali-steeping but for improving cellulose reactivity during the viscose process of dissolving pulp.
3.5. Fiber mophorlogies and Fock reactivity The morphologies of the fiber, including the pore volume, pore size and specific surface area (SSA), could affect alkali-induced swelling of fibers, the NaOH penetration/diffusion and further removal of hemicelluloses in the downstream viscose fiber production process, the reactivity of cellulose and the quality of the final products, normally being considered as the most important performance of dissolving pulp. Therefore, it appears that porosity of the pulp is the parameter affecting the pulp reactivity. The changes on fiber morphology under the setting conditions are shown in Table 2. For the original sample, the largest porosity was supported by the presence of increasing amounts of hemicelluloses in the pulp, which agrees with previous study (Gehmayr and Sixta, 2012), during the production of dissolving pulp using CCE treatment. The CCE treatment was to decrease in SSA of bamboo kraft pulp with increasing NaOH concentration, which was from 0.73 m2/g at 6% NaOH concentration to 0.32 m2/g at 10% NaOH concentration. Moreover, the total pore volume decreased from 1.02 × 10−2 cm3/g to 0.56 × 10−2 cm3/g and average pore diameter decreased from 2.20 nm to 0.71 nm, when the NaOH concentration increased from 6 to 10%. The changes on narrow pores were due to the fact that a higher NaOH concentration system for dissolving hemicelluloses was very easy to result in more cellulose fibril aggregation (caused by extensive intermolecule hydrogen bonding). Compared to CCE-6 and CCE-10, TRO/ CCE treatment further reduced the hemicelluloses content, but did not cause more serious fiber aggregation. TRO/CCE-6 had more accessible surface areas and more expanded pores than that of CCE-treated pulp. As shown, the TRO/CCE treatment improved the SSA (from 0.73 m2/g and 0.32 m2/g of the CCE-treated samples to 0.98 m2/g of TRO/CCE-6). In addition, the TRO/CCE treatment led to an enhancement in pore volume (from 1.02 and 0.56 to 1.51 × 10−2 cm3/g). The TRO/CCE-6 also, had a higher average pore diameter (from 2.20 and 0.71 to 2.92 nm). Making better in fiber morphologies by adding surface active agents has been well reported in the literature. For example, Uneback and Creutz (1985) studied the influence of use of a polyglycolderivative on wet pulp, and found that the presence of surfactant in chemical pulp made dry pulp softer and fluffier compared to untreated pulp, because of the reduction of the shredding energy considerably and hydrogen bonds between fibers when the additive is well distributed in the pulp. Further, the Fock reactivity was investigated and the results were shown in Table 2. The original sample showed the highest Fock reactivity (72.5%) as a result of the hemicelluloses that abounded in pulps, which agrees with previous study. Dou and Tang (2017) found that the presence of hemicelluloses as the amorphous material were highly susceptible to chemicals (CS2) during Fock reactivity test. However, considering the hemicelluloses content limitation in the preparation of dissolving pulp, such pulps were not on our list of options. Through CCE treatment for dissolving pulp production, the Fock reactivity of CCE/TRO-6 was 66.2%, which is higher than that from the CCE alone process (61.6% at 6% NaOH and 51.1% at 10 % NaOH). The results agree with that reported in the literature that the derivatives of castor oil as surfactant can make a significant contribution to increase the cellulose reactivity in the production of viscose (Serebryakova and Tokareva, 1996). Specifically, among the CCE-treated samples, addition of TRO to the CCE process with the highest Fock reactivity was not found to be with the lowest CrI and DP. Carrillo-Varela et al. (2019)
4. Conclusions A novel TRO-enriched alkaline extraction was proposed to separate hemicelluloses from chemical pulp to purify chemical pulp. The TRO introduction in the CCE process not only promoted hemicelluloses separation from cellulosic fibers, but also protected cellulose I from conversion to cellulose II. More importantly, the purified pulps from TRO/CCE were more flexible indicating as a higher specific surface area and total pore volume; therefore, making the cellulose more accessibility towards carbon disulfide. CRediT authorship contribution statement Qiuyan Chen: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Xinping Wang: Investigation, Validation. Hai Huang: Investigation, Software, Visualization. Shilin Cao: Supervision. Lihui Chen: Project administration. Liulian Huang: Project administration. Xiaojuan Ma: Supervision, Writing - review & editing. Declaration of competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors are grateful for the financial support from National Natural Science Foundation of China (Grant No. 3177063) and Open 6
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