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Effect of hydrogen peroxide bleaching on anionic groups and structures of sulfonated chemo-mechanical pulp fibers
Colloids and Surfaces A xxx (xxxx) xxxx
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Effect of hydrogen peroxide bleaching on anionic groups and structures of sulfonated chemo-mechanical pulp fibers Mingfu Lia,c,1, Juan Yina,b,1, Lingyu Hua,c, Siyuan Chenb, Douyong Mina,c, Shuangfei Wanga,c, ⁎ Lianxin Luoa,c, a
College of Light Industry and Food Engineering, Guangxi University, Nanning, Guangxi, 5300004, PR China School of Management Science and Engineering, Guangxi University of Finance and Economics, Nanning, Guangxi, 530003, PR China c Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Nanning 530004, PR China b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: SCMP Hydrogen peroxide bleaching Anionic group Carbohydrates XPS
The properties of the chemo-mechanical pulp were determined by the position and quantity of anionic groups in the pulp fibers. In this study, the effects of anion group content and structure in sulfonated mulberry stem chemomechanical pulp (SCMP) were investigated during the hydrogen peroxide (H2O2) bleaching process. Surface morphology and anion group structure of the pulp fibers were analyzed by scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and gas chromatography-mass spectrometry (GCeMS). The results indicate that the sulfonic group content in bleached pulp decreases with the concentration of H2O2 to a minimum content of 71.53 μmol g−1, which is reduced by 22.68% in comparison to the unbleached pulp content of 92.51 μmol g−1. The content of carboxyl groups in pulp first increases and then decreases with the concentration of H2O2. The maximum carboxyl group content is 164.79 μmol g−1, which is increased by 53.97% compared with unbleached pulp at 107.03 μmol g−1. The total anion groups content of SCMP first increases and then decreases with the concentration of H2O2. FTIR and XPS analysis show that the content of anion groups on the surface of the pulp fibers increases slightly after H2O2 bleaching. GCeMS analysis demonstrates that the content of uronic acid increases after H2O2 bleaching and the order of increase is 4-O-MeGlcA > GlcA > GalA. The contents of sulfonic acid group, total anion groups (TAGs) and surface anion groups (SAGs) of bleached pulp increase because of the hydroxyl group oxidation, terminal group reduction, methyl esterification of pectin and decomposition of polyxylose, resulting in the formation of new carboxyl groups and more uronic acid. This study reveals the mechanism of the effect of hydrogen peroxide bleaching on the anionic groups in the SCMP pulp, which is conducive to the development of bleaching technology for high yields of mulberry stem pulp.
Corresponding author at: College of Light Industry and Food Engineering, Guangxi University, Nanning, Guangxi, 5300004, PR China. E-mail address: [email protected] (L. Luo). 1 These authors contributed equally to this work and should be considered co-first authors. ⁎
https://doi.org/10.1016/j.colsurfa.2019.124068 Received 17 July 2019; Received in revised form 29 September 2019; Accepted 2 October 2019 Available online 03 October 2019 0927-7757/ © 2019 Published by Elsevier B.V.
Please cite this article as: Mingfu Li, et al., Colloids and Surfaces A, https://doi.org/10.1016/j.colsurfa.2019.124068
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1. Introduction
H2O2 bleached mulberry stem SCMP were investigated in this paper, and the changes in surface morphology and chemical groups of pulp fiber were also analyzed. In addition, the change mechanism of the anion groups on the surface of mulberry stem SCMP was clarified after H2O2 bleaching. It is beneficial to develop bleaching technology for the high yield pulp of mulberry stem SCMP and to provide a theoretical basis for mulberry branch pulping and papermaking.
Mulberry is a deciduous woody plant, which is widely distributed in many countries all over the world and which can grow naturally from the tropics to the temperature zones [1]. Mulberry stem is a potential non-wood material for pulping and papermaking; the phloem fiber of mulberry stem is long and the ratio of length to width is large, comparable to that of softwood fiber, while its rod fiber is similar to that of hardwood fiber [2]. Research on mulberry primarily focuses on the species and genus of mulberry, the extraction methods of substances, and the medicinal value and medical contribution of the extract [3–5], and there are few reports on research regarding mulberry branch pulping and papermaking. At present, either chemical pulping or mechanical pulping is mainly used for mulberry stem pulping, including mulberry branch phloem fiber pulping and mulberry branch whole rod pulping [6,7]. Hu et al. studied the effect of different pretreatment conditions on the bleaching performance of mulberry pulp, and the results showed that the bleaching effect could be obviously improved by adding a small amount of hydrogen peroxide [8]. Through caustic soda-anthraquinone pulping, and then CEpH (Chlorine bleach (C)-hydrogen peroxide enhanced alkali extraction bleach (Ep)-hypochlorite bleach (H)) three-stage bleaching, the middle and high-grade pulp products with good physical strength and high whiteness can be obtained [9]. Mulberry stem offers many advantages in pulping and papermaking, but the pulping and bleaching technology for full-rod mulberry branches is not mature enough [10]. The production of new anionic groups in the process of pulp grinding and bleaching affects the properties of the pulp. The amount of alkali, sulfides and pretreatment temperature will affect the degradation of wood raw material, the degree of degreasing and the degree of vulcanization of the pulp. Furthermore, the content of anionic groups was also affected. Functional groups are the atoms or groups that determine the chemical properties of pulp. Anionic groups (AGs) are the most important functional group, which can make the fiber swell, reduce the beating time, affect the binding force between fibers, and alter the interaction between fiber and papermaking chemicals. Besides, anionic groups also affect the strength of paper, paper sizing, and paper photochemical stability. AGs in the raw materials of paper-making fiber mainly include sulfonic group, carboxyl group, a phenol hydroxyl group, and an alcoholic hydroxyl group, and they are also produced in pulping, bleaching and paper-making processes. The anion groups in fiber cells affect the wettability of fiber and the softening of wood, thus affecting the cohesion of the cell wall, and then affect the pulping and paper strength. The interaction of anionic groups with metal ions can promote the swelling of fiber walls and significantly affect the entrance and exit of colloid particles [11]. Synergistic effects of carboxyl and carbonyl groups in anionic groups affect paper aging [12]. The content of carboxyl groups in pulp fiber is positively related to the tensile strength. Increasing the content of carboxyl groups in pulp fiber helps to moisten the fiber and increase its flexibility, thus improving the paper strength [13]. Konn et al. found that the formation of anionic groups would increase the energy consumption of grinding pulp [14]. The higher the content of total anion groups, the longer the average length of fibers and the fewer fiber fragments lead to the higher compressive properties. Compared with other pulps, the chemical thermomechanical pulp (CTMP) has a higher total anion group content, mainly because the process does not significantly destroy or remove glycolic acid from the pulp. During sulfonation, sulfonic acid groups and pectin are converted to uronic acid by a de-esterification reaction [15]. Alkaline pretreatment or bleaching generated hydrolysis of uronic acid residues in wood, resulting in the formation of new anionic groups [16]. Mulberry branch, as a non-wood pulping and papermaking material, has not been reported concerning the changes of anion groups in the bleaching process of high yield chemical mechanical pulp, and the change of anion groups will affect the physicochemical properties of paper. Therefore, the changes in fiber structure and anion group content in
2. Materials and methods 2.1. Materials Air-dried mulberry stems came from a sericulture base (Nanning, Guangxi) and were cut into 3–4 cm pieces (YQ-300 type press, Junling), placed into a sealed bag, and moisture was balanced for 24 h for further experimentation. Potassium bromide, hexamethyldisilazane (HMDS) and trimethylchlorosilane (TMCS) were purchased from Sigma-Aldrich (Sigma-Aldrich, Shanghai), and other analytical chemical reagents were purchased from Aladdin Chemical company (Aladdin, Shanghai). 2.2. Cooking, refining and bleaching The sulfonation of mulberry was performed by a cooking machine (Greenwood Instruments, LLC., USA). Mulberry was soaked in water for 24 h, then filtered from the water and placed in an electric cooker for cooking. The sulfonated condition of the concentration of Na2SO3 was 15% (based on oven-dry raw material, the same as below), the 4% NaOH and liquid ratio was 1:5, the temperature was increased from 25 °C to 130 °C at a rate of 2.5 °C·min−1, and the residence time was 120 min [17]. The sulfonated pulp was washed by water after cooking was completed. In addition, the sulfonated mulberry pulp was refined by a high-concentration refiner (ZSP300, Jilin), and refining performance was separated into three stages, with the distances between refiner discs being 0.45, 0.15 and 0.15 mm, respectively. After refined completely, the pulp was dehydrated and placed into a sealed bag to balance the moisture for 24 h before being used for bleaching. Taking 15 g SCMP into a polyethylene sealing bag, the pulp concentration was 10%, which was added into 0.3% EDTA and deionized water, the pH of the solution was adjusted to 3 with 0.2 mol/L NaOH and 0.1 mol/L HCl, and the pulp was placed into a water-bath at 60 °C for 60 min, twisting the pulp every 20 min. The pulp was washed to neutral with deionized water after chelation. SCMP pulp was bleached by different H2O2 concentrations of 2%, 4%, 6%, 8%, and 10%, respectively, the ratio of alkali was 0.7, the concentrations of Na2SiO3 and MgSO4 were 3% and 0.1%, respectively, the bleaching temperature and time were 80 °C and 105 min, respectively, and the concentration of pulp was 15%. After bleaching completely, the pulp was washed to neutral with deionized water, and then the pulp was freeze-dried to obtain bleached pulp. 2.3. Determination of carboxyl and sulfonic groups The content of carboxyl and sulfonic groups was determined by conductance titration [18,19]. Three g pulp (unbleached and bleached) was put into 100 mL of HCl (0.1 mol·L−1) solution for 45 min with electromagnetic stirring of 100 r/min, which was repeated two times for the conversion of the salt groups to acid reactive groups. Then, the pulp was washed by deionized water with no carbon dioxide to stabilize electric conductivity. The pulp was filtered and placed into 450 mL NaCl (0.001 mol·L−1) solution, 5 mL HCl (0.1 mol·L−1) was added into solution, and the solution was titrated by the standard solution of NaOH (0.1 mol· L−1) at the rate of 0.1 mL·min−1 under the environment of electromagnetic stirring at 50 r/min and with nitrogen. Meanwhile, the titration curve was recorded with the conductivity meter. Finally, the pulp samples were washed and baked to a constant weight. The content of sulfonic groups and carboxyl groups was calculated based on the formulas of (1-1) and (1–2) as follows: 2
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C(SO32
)=
C( COO
)=
C2 V2
C1 V1 m
C2 V3
C2 V2 m
× 1000
(1-1)
× 1000
(1-2)
In the formulas, C(SO32−) and C(COO-) represent the content of sulfonic and carboxyl groups (μmol∙g-1), respectively. C1 is the concentration of HCl (mol·L-1) standard solution, and V1 is the volume of HCl standard solution of 5 mL. C2 is the concentration of NaOH (mol·L1 ) standard solution. V2 is the volume of NaOH (mL) standard solution consumed at the first equivalence point. V3 is the volume of NaOH (mL) standard solution consumed at the second equivalence point, and m is the mass of the sample (g). 2.4. Determination of total anion groups (TAGs) content
Fig. 1. Effect of H2O2 bleaching on the sulfonic group and carboxyl content.
The unbleached pulp and hydrogen peroxide bleached pulp were washed by deionized water at 60 °C to remove dissolved and colloidal substances, and then the pulp was extracted by acetone solution (acetone/water, 9:1, v/v). Then, metal ions were removed by washing with 0.01 mol·L−1 EDTA and 0.01 mol·L-1 HCl solution. The pulp was washed with deionized water until the conductivity was close to the preferred value, and 2 mmol·L−1 NaHCO3 solution (1% pulp concentration) was added into the pulp and stirred for 30 min. After that, the pulp was filtered and cleaned with deionized water. 100 mg pulp was added into 50 mL methylene blue solution with different concentrations, the pH was 8, and the adsorption time and temperature were 30 min and 20 °C, respectively. After adsorption and filtration, the filtrate was determined by UV detection with the absorbance at 664 nm. The concentration of the filtrate was obtained according to the methylene blue standard curve, and the equilibrium concentration Ce of methylene blue and the equilibrium adsorption quantity Qe of the adsorbent were calculated, respectively, according to the standard curve equation (1–3):
Qe =
(C0
Ce ) V M
Fig. 2. Reaction process of introducing sulfonic acid groups into pulp.
2.7. Gas chromatography–mass spectrometry (GC–MS) The content of glucuronic acid in paper pulp was studied by methanol alcoholysis [22]. Ten mg unbleached and bleached SCMP pulp samples (accurate to 0.1 mg) were placed in a 10-ml pear-shaped flask, 2 mL anhydrous methanol solution containing 2 mol·L−1 HCl and internal standard was added, and the sample was placed into a 100 °C oven for 3 h for acidification methanol alcoholysis. When the sample was cooled to room temperature, 100 μL pyridine was added to neutralize the acidic solution. After adding 4 mL of methanol solution, the sample was dried in a vacuum dryer for 1 h at 40 °C. Then, 100 μL pyridine was added to dissolve samples. A total of 150 μL hexamethyldisilazane (HMDS) and 80 μL trichloromethylsilane (TMCS) were added for derivatization. After 4 h, each sample was filtered by an organic filtration membrane with an aperture of 0.22 μm for GC–MS analysis. Gas chromatography-mass spectrometry (GC–MS) (7890B, Agilent, USA) was used for the analysis of samples. Gas chromatographic conditions: the VF-5 ms quartz capillary chromatographic column (30 m × 0.25 mm × 0.25 μm) was used, the column temperature was 50 °C, which was held for 4 min, then increased at a rate of approximately 10 °C⋅min−1 to 160 °C, and at a rate of approximately 3 °C⋅min-1 up to 280 °C, where it was held for 5 min. The carrier gas was He, with a flow rate of 1 mL·min−1. The sample temperature was 250 °C, and the injection volume was 1 μL. The split ratio was 1:10, electron bombardment (EI) was 70 eV, multiplier voltage was 1250 V, and scanning mass range was from 45 to 500 μ.
(1-3)
In the formula, Qe is the adsorption capacity of anionic groups on methylene blue in pulp, μmol·g−1; C0 is the initial concentration of methylene blue solution (μmol·L−1); Ce is the concentration of methylene blue solution at adsorption equilibrium (μmol ·L−1); V is the volume of methylene blue solution (L); M is the mass of adsorbent (g). 2.5. Scanning electron microscopy (SEM) The surface morphology analysis of unbleached and bleached pulp was performed by scanning electron microscopy (SEM, Phenom Pro, Finland). The samples were fixed on a carrier platform with conductive adhesive, and the samples were sprayed with gold for 60 s with current of 8–10 mA and vacuum of 6–8 Pa/mmHg. Then, the samples were placed into a scanning electron microscope and scanned with a secondary electron detector. The scanning voltage was 5 kv, and the amplification factor was 3000 times. 2.6. X-ray diffraction photoelectron spectroscopy (XPS) The anionic groups on the surface of pulp fibers were analyzed by methylene blue adsorption combined with X-ray diffraction photoelectron spectroscopy (MB-XPS) [20,21]. Leybold Max 200 X-ray photoelectron spectroscopy was used to analyze the elements on the surface of pulp. The detection conditions were monochromatic Al Ka X-ray source, photoelectronic transfer of 150 eV, and 45° angle surface level, with the analysis area of 400 μm × 400 μm. To determine the analytical error, each sample was measured at least three different focal points.
3. Results and discussion 3.1. Sulfonic and carboxyl groups in SCMP pulp of mulberry stem Anionic groups in pulp include carboxyl, phenolic resin, and sulfonic groups. However, only carboxyl and sulfonic groups were ionized under neutral or weak acid pulping conditions. Sulfonic groups are 3
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Fig. 3. MB adsorption isotherms of unbleached SCMP and hydrogen peroxide bleached pulp.
Fig. 4. SEM-EDS spectra of unbleached SCMP. 4
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Fig. 5. SEM-EDS spectra of hydrogen peroxide (4%) bleached SCMP.
capacity between pulp fiber and ions. Most of the carboxyl groups in pulp fiber derived from the oxidative decomposition of hemicellulose and pectin. The oxidative degradation products can be carbonyl, aldehyde, and carboxyl, but under alkaline conditions, the main oxidative group is carboxyl [13]. Wei et al. showed that hexenyl uronic acid is one of the important reasons for yellowing of paper pulp, and 4-Omethyl glucuronic acid can be partially converted into hexenyl uronic acid [23]. The carboxyl group content of hexenyl uronic acid is altered by different types of oxidants. To study the effect of H2O2 bleaching on the content of sulfonic and carboxyl groups, conductance titration was used to detect the content of sulfonic and carboxyl groups in mulberry stem SCMP pulp and bleached pulp, and the results are shown in Fig. 1. As seen from Fig. 1, the content of sulfonic acid groups decreases with the increase of H2O2 concentration. For example, compared with the sulfonic group content of 92.51 μmol g−1 in the unbleached pulp, when the concentration of H2O2 was 10%, the minimum content of sulfonic group was 71.53 μmol g−1, which decreased by 22.68%. The carboxyl content first increased and then decreased with the increase of H2O2. For example, the content of carboxyl group increased with the increase of H2O2 from 2% to 6%, and when the concentration of H2O2 was 6%, the content of carboxyl group increased from 107.03 μmol g−1 in unbleached pulp to 164.79 μmol g−1, an increase of 53.97%. When the concentration of H2O2 was more than 6%, the carboxyl group content decreased with the increase of H2O2. When the concentration of H2O2 was 10%, the carboxyl group content decreased from 107.03 μmol g−1 in unbleached pulp to 91.36 μmol g−1, which was reduced by 14.64%. Sulfonic acid groups are produced through sulfonation in the cooking process, and the process of introducing sulfonic acid groups into the pulp is shown in Fig. 2. Lignin undergoes an electrophilic reaction under the action of NaOH and Na2SO3, and the introduction of sulfonic acid groups can increase the hydrophilicity of mulberry stem to produce permanent softening. When H2O2 is used in bleaching, the content of sulfonic acid groups in pulp changed. When the concentration of H2O2 is high, the sulfonic acid groups of the pulp are destroyed and reduced in content. Under alkaline conditions, the content of carboxyl in pulp fibers was increased by H2O2 oxidative bleaching, because the hydroxyl and reducing end groups were oxidized and glucuronic acid decomposition occurred and were linked with polyxylose to form new carboxyl groups. Besides, some pectin was methyl esterified by hydrogen peroxide bleaching to form galacturonic acid [24]. However, the carboxyl content dropped when the concentration of H2O2 was high; this result is consistent with previously reported findings [25]. When the concentration of H2O2 is low, the CD (cationic demand) value of the solution does not increase significantly. When the concentration of H2O2
Fig. 6. FTIR spectra of mulberry stem SCMP bleached pulp adsorbed in different MB concentrations. Table 1 Effect of H2O2 bleaching on surface anionic groups. Samples
SAG (μmol·g−1)
Mulberry stem Unbleached pulp 2% H2O2 4% H2O2 6% H2O2 8% H2O2 10% H2O2
1.4 2.0 2.3 2.2 2.3 2.0 2.0
Table 2 The atomic compositions of SCMP and hydrogen peroxide bleached pulp. Samples
N/%
S/%
C/%
O/%
S/C
O/C
Unbleached pulp 2% H2O2 6% H2O2 10% H2O2
1.84 2.01 2.14 1.86
0.54 0.63 0.68 0.53
68.61 67.79 66.68 66.55
29.01 29.57 30.50 30.06
0.00787 0.00929 0.01019 0.00796
0.42 0.43 0.45 0.45
produced in the sulfonation reaction. Therefore, they only exist in the chemo-mechanical pulp. Carboxyl groups are found in all commercial pulp and are the most important anionic groups. The content of carboxyl groups plays an important role in determining the exchange 5
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Fig. 7. C1s peak characteristic curves of unbleached and hydrogen peroxide bleached pulp.
Unlike chemical mechanical pulp, most of the anionic groups in sulfate pulp are derived from the saccharin acid which was formed by peeling reactions in cellulose and hemicellulose. The increase of anionic groups in bleached sulfate pulp is mainly derived from the chemical degradation of carbohydrates in the oxidation stage, and the content of anionic groups is much lower than that of chemical mechanical pulp [28]. With the increase of H2O2 concentration, the content of total anionic groups in mulberry stem SCMP increased first and then presented an irregular downward trend, which was similar to the changing trend of carboxyl content. The high concentration of H2O2 reduced the content of total anionic groups in pulp fiber, possibly due to the oxidation and dissolution of lignin.
Table 3 C1s peak area of unbleached and bleached pulp. Samples
Unbleached pulp 2% H2O2 6% H2O2 10% H2O2
Total C1s = 100% C1
C2
C3
43.76 38.19 35.20 36.31
47.46 50.63 51.35 52.67
8.78 11.18 13.45 11.02
is high, the value of CD increases significantly, because lignin is oxidized by H2O2 to produce lignin fragments, which dissolve into water and increase the value of CD, thereby reducing the carboxyl content of the fiber.
3.3. SEM The surface microstructure of materials was analyzed by SEM, and EDS can obtain micro areas of several hundred or even one thousand nanometers deep to detect element types and contents. Fiber morphology and the surface elemental composition of unbleached and bleached SCMP are shown in Figs. 4 and 5. A large number of substances were distributed on the surface of unbleached and bleached SCMP, according to the analysis of SED-EDS, and the deposited material of keratin might be lignin, and hemicellulose [30]. It is reported that a portion of the high yield pulp dissolved substances and lignin will be deposited on the surface of the fiber, forming the horn-like materialwrapped fibers. The surface of the cellulose of SCMP was split and fibrillated, and more microfibers were exposed after 4% H2O2 bleaching. The contents of N and S elements exhibited minor change, the content of the C element decreased, and the O/C ratio increased from 50.76% to 54.23%. The results of the investigation on the distribution of anionic groups in different pulp fibers showed that the anionic groups at the micron depth of the pulp fibers detected by EDS account for 60 to 97%
3.2. Total anion groups (TAGs) The isothermal adsorption saturation (SL) obtained after MB adsorption could represent the total anionic group content in pulp fiber [20,26]. The content of TAGs mainly depends on the TAGs of raw materials, chemical reactions in the pulping and bleaching processes, and the dissolution of chemical ingredients. MB isothermal adsorption curves of unbleached and bleached SCMP are shown in Fig. 3. Compared with unbleached pulp, the content of total anionic groups in the bleached pulp is significantly higher than the unbleached pulp, because significant methyl-esterification of pectin and oxidation of partial lignin increased the content of total anionic groups after H2O2 bleaching, which is consistent with a previous report [14]. For unbleached mulberry branch SCMP, 4-O-methyl glucuronic acid units of xylose were not completely transformed into hexenyl glucuronic acid in the pulping process, resulting in no increase of AG content [27]. 6
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smaller than its critical diameter, and the effective surface area of the adsorbent only exists within the micropore where the adsorbent molecule can enter. Because the fiber wall is a porous structure, pulp fiber is equivalent to an adsorbent with multiple micropores: the pore diameter range is approximately 5–50 nm [29], and the solid-liquid interface is negatively charged due to the effect of anionic groups. The MB cations are approximately 1.8 nm long and 0.9 nm wide [20]. Furthermore, these cations can penetrate the cell wall of pulp fibers and neutralize anionic groups with negative charge due to electrostatic interaction. 3.5. XPS analysis 3.5.1. Analysis of anionic groups on the surface of pulp fibers The papermaking process involves the addition of fibers and a large number of papermaking chemicals to a dilute aqueous suspension that adheres to the pulp fibers by interacting with AGs: for high-speed modern paper machines, the interaction time is very short, and therefore the number of AGs and their position and distribution uniformity in the pulp fibers are critical, and the role of surface anionic groups (SAGs) cannot be ignored. The number of anionic groups is different on the surfaces of different pulp fibers, and generally, the surface anionic groups account for 1–3% of the total anionic groups. The calculation method of surface anionic groups is as follows: SAG=[S(32.06)/[C(12.00)+O(15.99)+N(14.00)+S(32.06)]]×[(1/ 32.06) ×10,000] (2-1) XPS can detect atomic composition on the surface of a few nanometers with higher accuracy than SEM-EDS. The content of C, O, S, and N was measured by XPS after adsorption of MB. The anionic group content on the surface of SCMP pulp fiber was calculated according to formula (2-1). 1/32.06 and 10,000 were parameters for converting the unit of SAG to μmol·g−1. The anionic group content on the surface of pulp is shown in Table 1. It can be seen from Table 1 that the content of anionic groups on the surface of SCMP is higher than in mulberry stem raw material, and the content of surface anionic groups increases after partial pulp bleaching, which may be due to the formation of new AGs and SAGs by alkaline hydrogen peroxide bleaching, and more than 30% of the anionic groups are derived from lignin oxidation [21]. The anionic group content on the fiber surface affects the strength of the paper, and the pulp strength is also affected by other factors, such as the fact that the uneven distribution of anionic groups on the surface will affect the interaction between fiber and other chemicals, the surface roughness will hinder the contact between fiber and fiber, and the presence of lignin will affect the flexibility of the fiber cell wall [27].
Fig. 8. TIC of mulberry SCMP (a) and hydrogen peroxide bleached pulp (b). Table 4 The content of uronic acid in SCMP and hydrogen peroxide bleached SCMP. Retention time /min Unbleached pulp
H2O2 bleached pulp
20.63 21.77 23.99 —
20.62 21.76 23.98 —
Components
GalA 4-O-MeGlcA GlcA Total uronic acids
Relative content /% Unbleached pulp
H2O2 bleached pulp
1.22 0.63 0.82 2.67
1.37 1.35 1.18 3.90
3.5.2. Analysis of surface elements of pulp fibers XPS can detect all elements except H and He. The binding energy of the C element is 285–289.5 eV, and the binding energy of the N element is 397.0–402.2 eV. The O elemental binding energy is 529.4 to 531.1 eV, and the S element-binding energy is 161.2 to 168.5 eV. The element compositions on the surfaces of the unbleached pulp and bleached pulp produced by different H2O2 concentrations are shown in Table 2. The percentages of N and S elements before and after bleaching did not change significantly. The percentages of N and S elements were increased along with the concentration of H2O2; however, when the concentration of H2O2 was 10%, the content of both elements decreased. S/C reflects the degree of lignin sulfonation on the surface of pulp fibers, and sulfonated lignin has the advantages of improving the fiber bonding strength and reducing pulping energy consumption [31]. As seen from Table 2, the S/C value of the fiber surface after H2O2 bleaching is higher than that of the unbleached pulp, leading to improvement of the degree of sulfonation of the pulp fiber surface after H2O2 bleaching.
of the total anionic groups [27]. 3.4. FTIR Fardim et al. used FTIR-ATR to evaluate the adsorption effect of MB in a 1-μm surface layer [20]. FTIR spectra of mulberry stem SCMP bleached pulp adsorbed in different methylene blue (MB) concentrations are shown in Fig. 6. The characteristic absorption peak of MB molecules was 1632 cm−1, and when MB concentration increased from 50 to 250 μmol⋅L−1, the peak intensity showed similar trends to the MB adsorption isotherm, which was enhanced with the increase of MB adsorption; this showed that a large number of MB molecules had moved barrier-free into the pore of the fiber cell wall and that large amounts of anionic groups were present in the pulp fiber cell wall. This may be because an adsorbent molecule cannot enter a micropore 7
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The percentage of C decreased and the percentage of O increased with the concentration of H2O2, leading to an increase in the O/C value. The increase of oxygen-carbon ratio may be due to the small amount of extract or because the lignin on the surface of the fiber was removed. Meanwhile, oxygen content increases with the concentration of H2O2, indicating an increase in the number of carbohydrates, which indicates that H2O2 bleaching reduced the content of extract and lignin on the surface pulp fiber. Generally, C1s of fiber can be divided into four combined types of C1, C2, C3 and C3 [32]; among them, C1, C2, and C3 are the main binding modes: the binding energy of C1 at 285 eV is attributed to CCor C–H of extract and lignin, the binding energy of C2 at 286.5 eV is attributed to C–O of carbohydrate, the binding energy of C3 at 288–288.5 eV is attributed to C = O or O-C–O of lignin or cellulose, the binding energy of C4 at 289–289.5 eV is attributed to O = C–O of carboxylic acids, and the peak splitting diagram of C1s is shown in Fig. 7. C1, C2 and C3 peak areas of C1s are shown in Table 3: the content of C1 decreased, and the contents of C2 and C3 increased after hydrogen peroxide bleaching, indicating that the lignin content of the pulp is reduced and the cellulose and hemicellulose content are increased, which is due to the partial lignin dissolution and exposure of more carbohydrates on the surface of the fibers, thus leading to the increase of total anion groups on the surface of the fibers. The results are consistent with the XPS full spectrum analysis.
The content of uronic acid increased after H2O2 bleaching, and the order of increase of each component is as follows: 4-OMeGlcA > GlcA > GalA. H2O2 bleaching increases the content of sulfonic acid groups, TAGs, and SAGs in the pulp, and this is due to the oxidation of hydroxyl groups, reduction of terminal groups, methylation of pectin, as well as the fact that the decomposition of polyxylose produces both new carboxyl groups and more uronic acid. In summary, the content of total anionic groups in the pulp increased after H2O2 bleaching, which was beneficial for the improvement of the pulp chemistry and promoted the development of mulberry high yield pulp with hydrogen peroxide bleaching technology.
3.6. Analysis of uronic acid components by GC–MS
References
The content of uronic acid measured by gas chromatography after methanol dissolution generally accounts for 70%–100% of the total anionic groups of the mechanical pulp [32]. The total content of anionic groups is generally greater than or equal to the sum of the sulfonic acid group content and the uronic acid content. A total ion chromatogram of unbleached pulp and H2O2 bleached pulp is shown in Fig. 8. The calculation results of uronic acid content are shown in Table 4. The content of uronic acid increases after H2O2 bleaching: compared with the uronic acid in the unbleached pulp, the content of 4-O-methyl glucuronic acid (4-O-MeGlcA) increased by 2.14 times, and the content of galacturonic acid (GalA) increased by 12.30%. In addition, the content of glucuronic acid (GlcA) increased by 43.90%, and the order of increase of uronic acid in pulp after H2O2 bleaching is as follows: 4-OMeGlcA > GlcA > GalA, which may be due to the change of the total contents of glucuronic acid and hemicellulose in the pulp being impacted by H2O2 bleaching, and the pectin carboxylic acid or galacturonic acid produced by the deacetylation and bleaching of glucomannan in the pulp. Furthermore, approximately 15% of the pectin in the pulp fibers was oxidized by H2O2 bleaching in a previous report [14]. Besides, the uronic acid of the pulp is mainly derived from poly-4O-methyl glucuronic acid (decomposed into 4-O-methyl glucuronic acid) and pectin (decomposed into galacturonic acid) linked to lignin. H2O2 bleaching increases the content of sulfonic acid groups, TAGs, and SAGs due to the oxidation of hydroxyl groups, reduction of terminal groups, methylation of pectin, and decomposition of polyxylose to produce new carboxyl groups and more uronic acids.
[1] L. Liu, L. Zhang, W.G. Zhao, Y.L. Pang, J. Plant Genet. Resour. 3 (2004) 285–289, https://doi.org/10.3969/j.issn.1672-1810.2004.03.016. [2] S.H. Bae, H.J. Suh, LWT-Food Sci. Technol. 40 (2007) 955–962, https://doi.org/10. 1016/j.lwt.2006.06.007. [3] P.N. Chen, S.C. Chu, H.L. Chiou, W.H. Kuo, C.L. Chiang, Y.S. Hsieh, Cancer Lett. 235 (2006) 248–259, https://doi.org/10.1016/j.canlet.2005.04.033. [4] S. Ercisli, E. Orhan, Food Chem. 103 (2007) 1380–1384, https://doi.org/10.1016/j. foodchem.2006.10.054. [5] S. Arabshahi-Delouee, A. Urooj, Food Chem. 102 (2007) 1233–1240, https://doi. org/10.1016/j.foodchem.2006.07.013. [6] Y.F. Cao, Q.Q. Hong, X.L. Zhang, F. Hong, J.F. Tao, L. Shao, L.F. Shou, M.Q. Ding, R.Q. Kai, Forest Ind. 5 (1996) 22–26 doi: 10.19531/j.issn1001-5299. 1996. 05. 006. [7] W.X. Cai, China Pulp Paper 1 (2001) 67, https://doi.org/10.3969/j.issn.0254-508X. 2001.01.019. [8] J.M. Hu, K.Z. Shen, G.G. Fang, F.M. Liang, Y.J. Deng, P. Li, Pap. Sci. Technol. 28 (2009) 16–19, https://doi.org/10.3969/j.issn.1671-4571.2009.04.005. [9] Y.Y. Wu, B.Q. Kong, L.F. Wei, J. Northwest Inst. Light Ind. 2 (1999) 85–88, https:// doi.org/10.3969/j.issn.1000-5811.1999.02.017. [10] L.Y. Hu, F.L. Cui, H. Wang, X.L. Lian, R. Qin, X.M. Zhang, L.X. Luo, China Pulp Pap. 35 (2016) 17–23, https://doi.org/10.11980/j.issn.0254-508X.2016.01.004. [11] S.X. Xu, Huazhong University of Science and Technology, (2008) doi: 10.7666/d. D508564. [12] C.A.B. Luiz, C.R.A. Maltha, A.J. Demuner, C.M. Cazal, E.L. Reis, J.L. Colodette, BioResources 8 (2013) 1043–1054, https://doi.org/10.15376/biores.8.1.10431054. [13] X.H. Li, X.R. Qian, China Pulp Pap. 7 (2008) 51–57, https://doi.org/10.3969/j.issn. 0254-508X.2008.07.014. [14] J. Konn, A. Pranovich, P. Fardim, B. Holmbom, Colloids Surf. A Physicochem. Eng. Asp. 296 (2007) 1–7, https://doi.org/10.1016/j.colsurfa.2006.09.047. [15] K. Koljonen, M. Österberg, L.S. Johansson, Colloids Surf. A Physicochem. Eng. Asp. 228 (2003) 143–158, https://doi.org/10.1016/S0927-7757(03)00305-4. [16] K.E. Sundberga, B.R. Holmboma, A.V. Pranovicha, J. Wood Chem. Technol. 23 (2003) 89–112, https://doi.org/10.1081/WCT-120018617. [17] G.L. Zhao, Guangxi University, (2014) doi: 10.7666/d. D524768. [18] Shi S L, He F W. Beijing: China light industry press, 2010. [19] Y.W. Ma, H.L. Lv, Y. Wang, J.Q. Wang, Pap. Sci. Technol. 6 (2011) 1–6, https://doi. org/10.19696/j.issn1671-4571.2011.06.001. [20] P. Fardim, T. Moreno, B. Holmbom, J. Colloid Interface Sci. 290 (2005) 383–391, https://doi.org/10.1016/j.jcis.2005.04.067. [21] Z. Li, Y. Qin, M. Qin, N. Liu, Q. Xu, Y. Fu, Z. Yuan, Carbohydr. Polym. 88 (2012) 1041–1046, https://doi.org/10.1016/j.carbpol.2012.01.063. [22] A. Sundberg, K. Sundberg, C. Lillandt, B. Holmbom, Nord. Pulp Pap. Res. J. 11 (1996) 216–219, https://doi.org/10.3183/npprj-1996-11-04-p216-219. [23] X.L. Wei, H.C. Li, Pap. Biomater. 6 (2013) 48–52, https://doi.org/10.3969/j.issn. 1006-2599.2013.06.009. [24] J. Kennedy, G. Phillips, P. Williams, Great Abington Cambridge, Woodhead Publishing Limited Abington Hall, 1996, https://doi.org/10.1533/ 9781845698690.frontmatter. [25] H.Y. Mou, Shandong Institute of Light Industry Pulping and Papermaking, (2007)
Declaration of Competing Interest The authors declare no competing interests. Acknowledgments This project was funded by the Innovation Project of Guangxi Graduate Education (YCBZ2019017), the National Natural Science Foundation Project of China (No. 31660182), the Natural Science Foundation Project of Guangxi (2018GXNSFAA281336) and the National Natural Science Foundation of China (No.41461110). The authors gratefully acknowledge with thanks Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control for technical support.
4. Conclusion During the H2O2 bleaching process, the content of sulfonic groups in the pulp decreased with the concentration of H2O2, and the content of carboxyl groups increased first and then decreased with the increase of the concentration of H2O2. In addition, as the concentration of H2O2 increased, the total anionic group content of the SCMP first increased and then decreased. A large number of anionic groups were present in the cell wall of the pulp fiber, a slight increase in anionic groups on the surface of pulp after H2O2 bleaching, probably due to alkaline hydrogen peroxide bleaching and oxidation of lignin to form new AGs and SAGs. 8
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M. Li, et al. doi: 10.7666/d.y1401430. [26] S. Rohrsetzer, P. Kovacs, M. Kabai-Faix, Cellulose Chem. Technol. 29 (1995) 65–75, https://doi.org/10.35812/CelluloseChemTechnol.1995.29.65. [27] P. Fardim, B. Holmbom, Colloids Surf. A Physicochem. Eng. Asp. 252 (2005) 237–242, https://doi.org/10.1016/j.colsurfa.2004.10.117. [28] P. Fardim, B. Holmbom, A. Ivaska, J. Karhu, Nord. Pulp Pap. Res. J. 17 (2002) 346–351, https://doi.org/10.3183/npprj-2002-17-03-p346-351. [29] T. Maloney, H. Paulapuro, J. Pulp Pap. Sci. 25 (1999) 432, https://doi.org/10.
3168/jds.S0022-0302(07)71634-X. [30] M. Paulsson, A.H. Hultén, J. Wood Chem. Technol. 23 (2003) 31–46, https://doi. org/10.1081/WCT-120018614. [31] Q.F. Yang, H.Y. Zhan, S.F. Wang, C.X. Huang, Pap. Sci. Technol. 4 (2006) 22–24, https://doi.org/10.3969/j.issn.1671-4571.2006.04.005. [32] X.P. Wang, B.H. He, L.Y. Qian, Y.M. Deng, Pap. Sci. Technol. 1 (2009) 42–45, https://doi.org/10.3969/j.issn.1671-4571.2009.01.011.
9
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Effect of hydrogen peroxide bleaching on anionic groups and structures of sulfonated chemo-mechanical pulp fibers Mingfu Lia,c,1, Juan Yina,b,1, Lingyu Hua,c, Siyuan Chenb, Douyong Mina,c, Shuangfei Wanga,c, ⁎ Lianxin Luoa,c, a
College of Light Industry and Food Engineering, Guangxi University, Nanning, Guangxi, 5300004, PR China School of Management Science and Engineering, Guangxi University of Finance and Economics, Nanning, Guangxi, 530003, PR China c Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Nanning 530004, PR China b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: SCMP Hydrogen peroxide bleaching Anionic group Carbohydrates XPS
The properties of the chemo-mechanical pulp were determined by the position and quantity of anionic groups in the pulp fibers. In this study, the effects of anion group content and structure in sulfonated mulberry stem chemomechanical pulp (SCMP) were investigated during the hydrogen peroxide (H2O2) bleaching process. Surface morphology and anion group structure of the pulp fibers were analyzed by scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and gas chromatography-mass spectrometry (GCeMS). The results indicate that the sulfonic group content in bleached pulp decreases with the concentration of H2O2 to a minimum content of 71.53 μmol g−1, which is reduced by 22.68% in comparison to the unbleached pulp content of 92.51 μmol g−1. The content of carboxyl groups in pulp first increases and then decreases with the concentration of H2O2. The maximum carboxyl group content is 164.79 μmol g−1, which is increased by 53.97% compared with unbleached pulp at 107.03 μmol g−1. The total anion groups content of SCMP first increases and then decreases with the concentration of H2O2. FTIR and XPS analysis show that the content of anion groups on the surface of the pulp fibers increases slightly after H2O2 bleaching. GCeMS analysis demonstrates that the content of uronic acid increases after H2O2 bleaching and the order of increase is 4-O-MeGlcA > GlcA > GalA. The contents of sulfonic acid group, total anion groups (TAGs) and surface anion groups (SAGs) of bleached pulp increase because of the hydroxyl group oxidation, terminal group reduction, methyl esterification of pectin and decomposition of polyxylose, resulting in the formation of new carboxyl groups and more uronic acid. This study reveals the mechanism of the effect of hydrogen peroxide bleaching on the anionic groups in the SCMP pulp, which is conducive to the development of bleaching technology for high yields of mulberry stem pulp.
Corresponding author at: College of Light Industry and Food Engineering, Guangxi University, Nanning, Guangxi, 5300004, PR China. E-mail address: [email protected] (L. Luo). 1 These authors contributed equally to this work and should be considered co-first authors. ⁎
https://doi.org/10.1016/j.colsurfa.2019.124068 Received 17 July 2019; Received in revised form 29 September 2019; Accepted 2 October 2019 Available online 03 October 2019 0927-7757/ © 2019 Published by Elsevier B.V.
Please cite this article as: Mingfu Li, et al., Colloids and Surfaces A, https://doi.org/10.1016/j.colsurfa.2019.124068
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1. Introduction
H2O2 bleached mulberry stem SCMP were investigated in this paper, and the changes in surface morphology and chemical groups of pulp fiber were also analyzed. In addition, the change mechanism of the anion groups on the surface of mulberry stem SCMP was clarified after H2O2 bleaching. It is beneficial to develop bleaching technology for the high yield pulp of mulberry stem SCMP and to provide a theoretical basis for mulberry branch pulping and papermaking.
Mulberry is a deciduous woody plant, which is widely distributed in many countries all over the world and which can grow naturally from the tropics to the temperature zones [1]. Mulberry stem is a potential non-wood material for pulping and papermaking; the phloem fiber of mulberry stem is long and the ratio of length to width is large, comparable to that of softwood fiber, while its rod fiber is similar to that of hardwood fiber [2]. Research on mulberry primarily focuses on the species and genus of mulberry, the extraction methods of substances, and the medicinal value and medical contribution of the extract [3–5], and there are few reports on research regarding mulberry branch pulping and papermaking. At present, either chemical pulping or mechanical pulping is mainly used for mulberry stem pulping, including mulberry branch phloem fiber pulping and mulberry branch whole rod pulping [6,7]. Hu et al. studied the effect of different pretreatment conditions on the bleaching performance of mulberry pulp, and the results showed that the bleaching effect could be obviously improved by adding a small amount of hydrogen peroxide [8]. Through caustic soda-anthraquinone pulping, and then CEpH (Chlorine bleach (C)-hydrogen peroxide enhanced alkali extraction bleach (Ep)-hypochlorite bleach (H)) three-stage bleaching, the middle and high-grade pulp products with good physical strength and high whiteness can be obtained [9]. Mulberry stem offers many advantages in pulping and papermaking, but the pulping and bleaching technology for full-rod mulberry branches is not mature enough [10]. The production of new anionic groups in the process of pulp grinding and bleaching affects the properties of the pulp. The amount of alkali, sulfides and pretreatment temperature will affect the degradation of wood raw material, the degree of degreasing and the degree of vulcanization of the pulp. Furthermore, the content of anionic groups was also affected. Functional groups are the atoms or groups that determine the chemical properties of pulp. Anionic groups (AGs) are the most important functional group, which can make the fiber swell, reduce the beating time, affect the binding force between fibers, and alter the interaction between fiber and papermaking chemicals. Besides, anionic groups also affect the strength of paper, paper sizing, and paper photochemical stability. AGs in the raw materials of paper-making fiber mainly include sulfonic group, carboxyl group, a phenol hydroxyl group, and an alcoholic hydroxyl group, and they are also produced in pulping, bleaching and paper-making processes. The anion groups in fiber cells affect the wettability of fiber and the softening of wood, thus affecting the cohesion of the cell wall, and then affect the pulping and paper strength. The interaction of anionic groups with metal ions can promote the swelling of fiber walls and significantly affect the entrance and exit of colloid particles [11]. Synergistic effects of carboxyl and carbonyl groups in anionic groups affect paper aging [12]. The content of carboxyl groups in pulp fiber is positively related to the tensile strength. Increasing the content of carboxyl groups in pulp fiber helps to moisten the fiber and increase its flexibility, thus improving the paper strength [13]. Konn et al. found that the formation of anionic groups would increase the energy consumption of grinding pulp [14]. The higher the content of total anion groups, the longer the average length of fibers and the fewer fiber fragments lead to the higher compressive properties. Compared with other pulps, the chemical thermomechanical pulp (CTMP) has a higher total anion group content, mainly because the process does not significantly destroy or remove glycolic acid from the pulp. During sulfonation, sulfonic acid groups and pectin are converted to uronic acid by a de-esterification reaction [15]. Alkaline pretreatment or bleaching generated hydrolysis of uronic acid residues in wood, resulting in the formation of new anionic groups [16]. Mulberry branch, as a non-wood pulping and papermaking material, has not been reported concerning the changes of anion groups in the bleaching process of high yield chemical mechanical pulp, and the change of anion groups will affect the physicochemical properties of paper. Therefore, the changes in fiber structure and anion group content in
2. Materials and methods 2.1. Materials Air-dried mulberry stems came from a sericulture base (Nanning, Guangxi) and were cut into 3–4 cm pieces (YQ-300 type press, Junling), placed into a sealed bag, and moisture was balanced for 24 h for further experimentation. Potassium bromide, hexamethyldisilazane (HMDS) and trimethylchlorosilane (TMCS) were purchased from Sigma-Aldrich (Sigma-Aldrich, Shanghai), and other analytical chemical reagents were purchased from Aladdin Chemical company (Aladdin, Shanghai). 2.2. Cooking, refining and bleaching The sulfonation of mulberry was performed by a cooking machine (Greenwood Instruments, LLC., USA). Mulberry was soaked in water for 24 h, then filtered from the water and placed in an electric cooker for cooking. The sulfonated condition of the concentration of Na2SO3 was 15% (based on oven-dry raw material, the same as below), the 4% NaOH and liquid ratio was 1:5, the temperature was increased from 25 °C to 130 °C at a rate of 2.5 °C·min−1, and the residence time was 120 min [17]. The sulfonated pulp was washed by water after cooking was completed. In addition, the sulfonated mulberry pulp was refined by a high-concentration refiner (ZSP300, Jilin), and refining performance was separated into three stages, with the distances between refiner discs being 0.45, 0.15 and 0.15 mm, respectively. After refined completely, the pulp was dehydrated and placed into a sealed bag to balance the moisture for 24 h before being used for bleaching. Taking 15 g SCMP into a polyethylene sealing bag, the pulp concentration was 10%, which was added into 0.3% EDTA and deionized water, the pH of the solution was adjusted to 3 with 0.2 mol/L NaOH and 0.1 mol/L HCl, and the pulp was placed into a water-bath at 60 °C for 60 min, twisting the pulp every 20 min. The pulp was washed to neutral with deionized water after chelation. SCMP pulp was bleached by different H2O2 concentrations of 2%, 4%, 6%, 8%, and 10%, respectively, the ratio of alkali was 0.7, the concentrations of Na2SiO3 and MgSO4 were 3% and 0.1%, respectively, the bleaching temperature and time were 80 °C and 105 min, respectively, and the concentration of pulp was 15%. After bleaching completely, the pulp was washed to neutral with deionized water, and then the pulp was freeze-dried to obtain bleached pulp. 2.3. Determination of carboxyl and sulfonic groups The content of carboxyl and sulfonic groups was determined by conductance titration [18,19]. Three g pulp (unbleached and bleached) was put into 100 mL of HCl (0.1 mol·L−1) solution for 45 min with electromagnetic stirring of 100 r/min, which was repeated two times for the conversion of the salt groups to acid reactive groups. Then, the pulp was washed by deionized water with no carbon dioxide to stabilize electric conductivity. The pulp was filtered and placed into 450 mL NaCl (0.001 mol·L−1) solution, 5 mL HCl (0.1 mol·L−1) was added into solution, and the solution was titrated by the standard solution of NaOH (0.1 mol· L−1) at the rate of 0.1 mL·min−1 under the environment of electromagnetic stirring at 50 r/min and with nitrogen. Meanwhile, the titration curve was recorded with the conductivity meter. Finally, the pulp samples were washed and baked to a constant weight. The content of sulfonic groups and carboxyl groups was calculated based on the formulas of (1-1) and (1–2) as follows: 2
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C(SO32
)=
C( COO
)=
C2 V2
C1 V1 m
C2 V3
C2 V2 m
× 1000
(1-1)
× 1000
(1-2)
In the formulas, C(SO32−) and C(COO-) represent the content of sulfonic and carboxyl groups (μmol∙g-1), respectively. C1 is the concentration of HCl (mol·L-1) standard solution, and V1 is the volume of HCl standard solution of 5 mL. C2 is the concentration of NaOH (mol·L1 ) standard solution. V2 is the volume of NaOH (mL) standard solution consumed at the first equivalence point. V3 is the volume of NaOH (mL) standard solution consumed at the second equivalence point, and m is the mass of the sample (g). 2.4. Determination of total anion groups (TAGs) content
Fig. 1. Effect of H2O2 bleaching on the sulfonic group and carboxyl content.
The unbleached pulp and hydrogen peroxide bleached pulp were washed by deionized water at 60 °C to remove dissolved and colloidal substances, and then the pulp was extracted by acetone solution (acetone/water, 9:1, v/v). Then, metal ions were removed by washing with 0.01 mol·L−1 EDTA and 0.01 mol·L-1 HCl solution. The pulp was washed with deionized water until the conductivity was close to the preferred value, and 2 mmol·L−1 NaHCO3 solution (1% pulp concentration) was added into the pulp and stirred for 30 min. After that, the pulp was filtered and cleaned with deionized water. 100 mg pulp was added into 50 mL methylene blue solution with different concentrations, the pH was 8, and the adsorption time and temperature were 30 min and 20 °C, respectively. After adsorption and filtration, the filtrate was determined by UV detection with the absorbance at 664 nm. The concentration of the filtrate was obtained according to the methylene blue standard curve, and the equilibrium concentration Ce of methylene blue and the equilibrium adsorption quantity Qe of the adsorbent were calculated, respectively, according to the standard curve equation (1–3):
Qe =
(C0
Ce ) V M
Fig. 2. Reaction process of introducing sulfonic acid groups into pulp.
2.7. Gas chromatography–mass spectrometry (GC–MS) The content of glucuronic acid in paper pulp was studied by methanol alcoholysis [22]. Ten mg unbleached and bleached SCMP pulp samples (accurate to 0.1 mg) were placed in a 10-ml pear-shaped flask, 2 mL anhydrous methanol solution containing 2 mol·L−1 HCl and internal standard was added, and the sample was placed into a 100 °C oven for 3 h for acidification methanol alcoholysis. When the sample was cooled to room temperature, 100 μL pyridine was added to neutralize the acidic solution. After adding 4 mL of methanol solution, the sample was dried in a vacuum dryer for 1 h at 40 °C. Then, 100 μL pyridine was added to dissolve samples. A total of 150 μL hexamethyldisilazane (HMDS) and 80 μL trichloromethylsilane (TMCS) were added for derivatization. After 4 h, each sample was filtered by an organic filtration membrane with an aperture of 0.22 μm for GC–MS analysis. Gas chromatography-mass spectrometry (GC–MS) (7890B, Agilent, USA) was used for the analysis of samples. Gas chromatographic conditions: the VF-5 ms quartz capillary chromatographic column (30 m × 0.25 mm × 0.25 μm) was used, the column temperature was 50 °C, which was held for 4 min, then increased at a rate of approximately 10 °C⋅min−1 to 160 °C, and at a rate of approximately 3 °C⋅min-1 up to 280 °C, where it was held for 5 min. The carrier gas was He, with a flow rate of 1 mL·min−1. The sample temperature was 250 °C, and the injection volume was 1 μL. The split ratio was 1:10, electron bombardment (EI) was 70 eV, multiplier voltage was 1250 V, and scanning mass range was from 45 to 500 μ.
(1-3)
In the formula, Qe is the adsorption capacity of anionic groups on methylene blue in pulp, μmol·g−1; C0 is the initial concentration of methylene blue solution (μmol·L−1); Ce is the concentration of methylene blue solution at adsorption equilibrium (μmol ·L−1); V is the volume of methylene blue solution (L); M is the mass of adsorbent (g). 2.5. Scanning electron microscopy (SEM) The surface morphology analysis of unbleached and bleached pulp was performed by scanning electron microscopy (SEM, Phenom Pro, Finland). The samples were fixed on a carrier platform with conductive adhesive, and the samples were sprayed with gold for 60 s with current of 8–10 mA and vacuum of 6–8 Pa/mmHg. Then, the samples were placed into a scanning electron microscope and scanned with a secondary electron detector. The scanning voltage was 5 kv, and the amplification factor was 3000 times. 2.6. X-ray diffraction photoelectron spectroscopy (XPS) The anionic groups on the surface of pulp fibers were analyzed by methylene blue adsorption combined with X-ray diffraction photoelectron spectroscopy (MB-XPS) [20,21]. Leybold Max 200 X-ray photoelectron spectroscopy was used to analyze the elements on the surface of pulp. The detection conditions were monochromatic Al Ka X-ray source, photoelectronic transfer of 150 eV, and 45° angle surface level, with the analysis area of 400 μm × 400 μm. To determine the analytical error, each sample was measured at least three different focal points.
3. Results and discussion 3.1. Sulfonic and carboxyl groups in SCMP pulp of mulberry stem Anionic groups in pulp include carboxyl, phenolic resin, and sulfonic groups. However, only carboxyl and sulfonic groups were ionized under neutral or weak acid pulping conditions. Sulfonic groups are 3
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Fig. 3. MB adsorption isotherms of unbleached SCMP and hydrogen peroxide bleached pulp.
Fig. 4. SEM-EDS spectra of unbleached SCMP. 4
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Fig. 5. SEM-EDS spectra of hydrogen peroxide (4%) bleached SCMP.
capacity between pulp fiber and ions. Most of the carboxyl groups in pulp fiber derived from the oxidative decomposition of hemicellulose and pectin. The oxidative degradation products can be carbonyl, aldehyde, and carboxyl, but under alkaline conditions, the main oxidative group is carboxyl [13]. Wei et al. showed that hexenyl uronic acid is one of the important reasons for yellowing of paper pulp, and 4-Omethyl glucuronic acid can be partially converted into hexenyl uronic acid [23]. The carboxyl group content of hexenyl uronic acid is altered by different types of oxidants. To study the effect of H2O2 bleaching on the content of sulfonic and carboxyl groups, conductance titration was used to detect the content of sulfonic and carboxyl groups in mulberry stem SCMP pulp and bleached pulp, and the results are shown in Fig. 1. As seen from Fig. 1, the content of sulfonic acid groups decreases with the increase of H2O2 concentration. For example, compared with the sulfonic group content of 92.51 μmol g−1 in the unbleached pulp, when the concentration of H2O2 was 10%, the minimum content of sulfonic group was 71.53 μmol g−1, which decreased by 22.68%. The carboxyl content first increased and then decreased with the increase of H2O2. For example, the content of carboxyl group increased with the increase of H2O2 from 2% to 6%, and when the concentration of H2O2 was 6%, the content of carboxyl group increased from 107.03 μmol g−1 in unbleached pulp to 164.79 μmol g−1, an increase of 53.97%. When the concentration of H2O2 was more than 6%, the carboxyl group content decreased with the increase of H2O2. When the concentration of H2O2 was 10%, the carboxyl group content decreased from 107.03 μmol g−1 in unbleached pulp to 91.36 μmol g−1, which was reduced by 14.64%. Sulfonic acid groups are produced through sulfonation in the cooking process, and the process of introducing sulfonic acid groups into the pulp is shown in Fig. 2. Lignin undergoes an electrophilic reaction under the action of NaOH and Na2SO3, and the introduction of sulfonic acid groups can increase the hydrophilicity of mulberry stem to produce permanent softening. When H2O2 is used in bleaching, the content of sulfonic acid groups in pulp changed. When the concentration of H2O2 is high, the sulfonic acid groups of the pulp are destroyed and reduced in content. Under alkaline conditions, the content of carboxyl in pulp fibers was increased by H2O2 oxidative bleaching, because the hydroxyl and reducing end groups were oxidized and glucuronic acid decomposition occurred and were linked with polyxylose to form new carboxyl groups. Besides, some pectin was methyl esterified by hydrogen peroxide bleaching to form galacturonic acid [24]. However, the carboxyl content dropped when the concentration of H2O2 was high; this result is consistent with previously reported findings [25]. When the concentration of H2O2 is low, the CD (cationic demand) value of the solution does not increase significantly. When the concentration of H2O2
Fig. 6. FTIR spectra of mulberry stem SCMP bleached pulp adsorbed in different MB concentrations. Table 1 Effect of H2O2 bleaching on surface anionic groups. Samples
SAG (μmol·g−1)
Mulberry stem Unbleached pulp 2% H2O2 4% H2O2 6% H2O2 8% H2O2 10% H2O2
1.4 2.0 2.3 2.2 2.3 2.0 2.0
Table 2 The atomic compositions of SCMP and hydrogen peroxide bleached pulp. Samples
N/%
S/%
C/%
O/%
S/C
O/C
Unbleached pulp 2% H2O2 6% H2O2 10% H2O2
1.84 2.01 2.14 1.86
0.54 0.63 0.68 0.53
68.61 67.79 66.68 66.55
29.01 29.57 30.50 30.06
0.00787 0.00929 0.01019 0.00796
0.42 0.43 0.45 0.45
produced in the sulfonation reaction. Therefore, they only exist in the chemo-mechanical pulp. Carboxyl groups are found in all commercial pulp and are the most important anionic groups. The content of carboxyl groups plays an important role in determining the exchange 5
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Fig. 7. C1s peak characteristic curves of unbleached and hydrogen peroxide bleached pulp.
Unlike chemical mechanical pulp, most of the anionic groups in sulfate pulp are derived from the saccharin acid which was formed by peeling reactions in cellulose and hemicellulose. The increase of anionic groups in bleached sulfate pulp is mainly derived from the chemical degradation of carbohydrates in the oxidation stage, and the content of anionic groups is much lower than that of chemical mechanical pulp [28]. With the increase of H2O2 concentration, the content of total anionic groups in mulberry stem SCMP increased first and then presented an irregular downward trend, which was similar to the changing trend of carboxyl content. The high concentration of H2O2 reduced the content of total anionic groups in pulp fiber, possibly due to the oxidation and dissolution of lignin.
Table 3 C1s peak area of unbleached and bleached pulp. Samples
Unbleached pulp 2% H2O2 6% H2O2 10% H2O2
Total C1s = 100% C1
C2
C3
43.76 38.19 35.20 36.31
47.46 50.63 51.35 52.67
8.78 11.18 13.45 11.02
is high, the value of CD increases significantly, because lignin is oxidized by H2O2 to produce lignin fragments, which dissolve into water and increase the value of CD, thereby reducing the carboxyl content of the fiber.
3.3. SEM The surface microstructure of materials was analyzed by SEM, and EDS can obtain micro areas of several hundred or even one thousand nanometers deep to detect element types and contents. Fiber morphology and the surface elemental composition of unbleached and bleached SCMP are shown in Figs. 4 and 5. A large number of substances were distributed on the surface of unbleached and bleached SCMP, according to the analysis of SED-EDS, and the deposited material of keratin might be lignin, and hemicellulose [30]. It is reported that a portion of the high yield pulp dissolved substances and lignin will be deposited on the surface of the fiber, forming the horn-like materialwrapped fibers. The surface of the cellulose of SCMP was split and fibrillated, and more microfibers were exposed after 4% H2O2 bleaching. The contents of N and S elements exhibited minor change, the content of the C element decreased, and the O/C ratio increased from 50.76% to 54.23%. The results of the investigation on the distribution of anionic groups in different pulp fibers showed that the anionic groups at the micron depth of the pulp fibers detected by EDS account for 60 to 97%
3.2. Total anion groups (TAGs) The isothermal adsorption saturation (SL) obtained after MB adsorption could represent the total anionic group content in pulp fiber [20,26]. The content of TAGs mainly depends on the TAGs of raw materials, chemical reactions in the pulping and bleaching processes, and the dissolution of chemical ingredients. MB isothermal adsorption curves of unbleached and bleached SCMP are shown in Fig. 3. Compared with unbleached pulp, the content of total anionic groups in the bleached pulp is significantly higher than the unbleached pulp, because significant methyl-esterification of pectin and oxidation of partial lignin increased the content of total anionic groups after H2O2 bleaching, which is consistent with a previous report [14]. For unbleached mulberry branch SCMP, 4-O-methyl glucuronic acid units of xylose were not completely transformed into hexenyl glucuronic acid in the pulping process, resulting in no increase of AG content [27]. 6
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smaller than its critical diameter, and the effective surface area of the adsorbent only exists within the micropore where the adsorbent molecule can enter. Because the fiber wall is a porous structure, pulp fiber is equivalent to an adsorbent with multiple micropores: the pore diameter range is approximately 5–50 nm [29], and the solid-liquid interface is negatively charged due to the effect of anionic groups. The MB cations are approximately 1.8 nm long and 0.9 nm wide [20]. Furthermore, these cations can penetrate the cell wall of pulp fibers and neutralize anionic groups with negative charge due to electrostatic interaction. 3.5. XPS analysis 3.5.1. Analysis of anionic groups on the surface of pulp fibers The papermaking process involves the addition of fibers and a large number of papermaking chemicals to a dilute aqueous suspension that adheres to the pulp fibers by interacting with AGs: for high-speed modern paper machines, the interaction time is very short, and therefore the number of AGs and their position and distribution uniformity in the pulp fibers are critical, and the role of surface anionic groups (SAGs) cannot be ignored. The number of anionic groups is different on the surfaces of different pulp fibers, and generally, the surface anionic groups account for 1–3% of the total anionic groups. The calculation method of surface anionic groups is as follows: SAG=[S(32.06)/[C(12.00)+O(15.99)+N(14.00)+S(32.06)]]×[(1/ 32.06) ×10,000] (2-1) XPS can detect atomic composition on the surface of a few nanometers with higher accuracy than SEM-EDS. The content of C, O, S, and N was measured by XPS after adsorption of MB. The anionic group content on the surface of SCMP pulp fiber was calculated according to formula (2-1). 1/32.06 and 10,000 were parameters for converting the unit of SAG to μmol·g−1. The anionic group content on the surface of pulp is shown in Table 1. It can be seen from Table 1 that the content of anionic groups on the surface of SCMP is higher than in mulberry stem raw material, and the content of surface anionic groups increases after partial pulp bleaching, which may be due to the formation of new AGs and SAGs by alkaline hydrogen peroxide bleaching, and more than 30% of the anionic groups are derived from lignin oxidation [21]. The anionic group content on the fiber surface affects the strength of the paper, and the pulp strength is also affected by other factors, such as the fact that the uneven distribution of anionic groups on the surface will affect the interaction between fiber and other chemicals, the surface roughness will hinder the contact between fiber and fiber, and the presence of lignin will affect the flexibility of the fiber cell wall [27].
Fig. 8. TIC of mulberry SCMP (a) and hydrogen peroxide bleached pulp (b). Table 4 The content of uronic acid in SCMP and hydrogen peroxide bleached SCMP. Retention time /min Unbleached pulp
H2O2 bleached pulp
20.63 21.77 23.99 —
20.62 21.76 23.98 —
Components
GalA 4-O-MeGlcA GlcA Total uronic acids
Relative content /% Unbleached pulp
H2O2 bleached pulp
1.22 0.63 0.82 2.67
1.37 1.35 1.18 3.90
3.5.2. Analysis of surface elements of pulp fibers XPS can detect all elements except H and He. The binding energy of the C element is 285–289.5 eV, and the binding energy of the N element is 397.0–402.2 eV. The O elemental binding energy is 529.4 to 531.1 eV, and the S element-binding energy is 161.2 to 168.5 eV. The element compositions on the surfaces of the unbleached pulp and bleached pulp produced by different H2O2 concentrations are shown in Table 2. The percentages of N and S elements before and after bleaching did not change significantly. The percentages of N and S elements were increased along with the concentration of H2O2; however, when the concentration of H2O2 was 10%, the content of both elements decreased. S/C reflects the degree of lignin sulfonation on the surface of pulp fibers, and sulfonated lignin has the advantages of improving the fiber bonding strength and reducing pulping energy consumption [31]. As seen from Table 2, the S/C value of the fiber surface after H2O2 bleaching is higher than that of the unbleached pulp, leading to improvement of the degree of sulfonation of the pulp fiber surface after H2O2 bleaching.
of the total anionic groups [27]. 3.4. FTIR Fardim et al. used FTIR-ATR to evaluate the adsorption effect of MB in a 1-μm surface layer [20]. FTIR spectra of mulberry stem SCMP bleached pulp adsorbed in different methylene blue (MB) concentrations are shown in Fig. 6. The characteristic absorption peak of MB molecules was 1632 cm−1, and when MB concentration increased from 50 to 250 μmol⋅L−1, the peak intensity showed similar trends to the MB adsorption isotherm, which was enhanced with the increase of MB adsorption; this showed that a large number of MB molecules had moved barrier-free into the pore of the fiber cell wall and that large amounts of anionic groups were present in the pulp fiber cell wall. This may be because an adsorbent molecule cannot enter a micropore 7
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The percentage of C decreased and the percentage of O increased with the concentration of H2O2, leading to an increase in the O/C value. The increase of oxygen-carbon ratio may be due to the small amount of extract or because the lignin on the surface of the fiber was removed. Meanwhile, oxygen content increases with the concentration of H2O2, indicating an increase in the number of carbohydrates, which indicates that H2O2 bleaching reduced the content of extract and lignin on the surface pulp fiber. Generally, C1s of fiber can be divided into four combined types of C1, C2, C3 and C3 [32]; among them, C1, C2, and C3 are the main binding modes: the binding energy of C1 at 285 eV is attributed to CCor C–H of extract and lignin, the binding energy of C2 at 286.5 eV is attributed to C–O of carbohydrate, the binding energy of C3 at 288–288.5 eV is attributed to C = O or O-C–O of lignin or cellulose, the binding energy of C4 at 289–289.5 eV is attributed to O = C–O of carboxylic acids, and the peak splitting diagram of C1s is shown in Fig. 7. C1, C2 and C3 peak areas of C1s are shown in Table 3: the content of C1 decreased, and the contents of C2 and C3 increased after hydrogen peroxide bleaching, indicating that the lignin content of the pulp is reduced and the cellulose and hemicellulose content are increased, which is due to the partial lignin dissolution and exposure of more carbohydrates on the surface of the fibers, thus leading to the increase of total anion groups on the surface of the fibers. The results are consistent with the XPS full spectrum analysis.
The content of uronic acid increased after H2O2 bleaching, and the order of increase of each component is as follows: 4-OMeGlcA > GlcA > GalA. H2O2 bleaching increases the content of sulfonic acid groups, TAGs, and SAGs in the pulp, and this is due to the oxidation of hydroxyl groups, reduction of terminal groups, methylation of pectin, as well as the fact that the decomposition of polyxylose produces both new carboxyl groups and more uronic acid. In summary, the content of total anionic groups in the pulp increased after H2O2 bleaching, which was beneficial for the improvement of the pulp chemistry and promoted the development of mulberry high yield pulp with hydrogen peroxide bleaching technology.
3.6. Analysis of uronic acid components by GC–MS
References
The content of uronic acid measured by gas chromatography after methanol dissolution generally accounts for 70%–100% of the total anionic groups of the mechanical pulp [32]. The total content of anionic groups is generally greater than or equal to the sum of the sulfonic acid group content and the uronic acid content. A total ion chromatogram of unbleached pulp and H2O2 bleached pulp is shown in Fig. 8. The calculation results of uronic acid content are shown in Table 4. The content of uronic acid increases after H2O2 bleaching: compared with the uronic acid in the unbleached pulp, the content of 4-O-methyl glucuronic acid (4-O-MeGlcA) increased by 2.14 times, and the content of galacturonic acid (GalA) increased by 12.30%. In addition, the content of glucuronic acid (GlcA) increased by 43.90%, and the order of increase of uronic acid in pulp after H2O2 bleaching is as follows: 4-OMeGlcA > GlcA > GalA, which may be due to the change of the total contents of glucuronic acid and hemicellulose in the pulp being impacted by H2O2 bleaching, and the pectin carboxylic acid or galacturonic acid produced by the deacetylation and bleaching of glucomannan in the pulp. Furthermore, approximately 15% of the pectin in the pulp fibers was oxidized by H2O2 bleaching in a previous report [14]. Besides, the uronic acid of the pulp is mainly derived from poly-4O-methyl glucuronic acid (decomposed into 4-O-methyl glucuronic acid) and pectin (decomposed into galacturonic acid) linked to lignin. H2O2 bleaching increases the content of sulfonic acid groups, TAGs, and SAGs due to the oxidation of hydroxyl groups, reduction of terminal groups, methylation of pectin, and decomposition of polyxylose to produce new carboxyl groups and more uronic acids.
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Declaration of Competing Interest The authors declare no competing interests. Acknowledgments This project was funded by the Innovation Project of Guangxi Graduate Education (YCBZ2019017), the National Natural Science Foundation Project of China (No. 31660182), the Natural Science Foundation Project of Guangxi (2018GXNSFAA281336) and the National Natural Science Foundation of China (No.41461110). The authors gratefully acknowledge with thanks Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control for technical support.
4. Conclusion During the H2O2 bleaching process, the content of sulfonic groups in the pulp decreased with the concentration of H2O2, and the content of carboxyl groups increased first and then decreased with the increase of the concentration of H2O2. In addition, as the concentration of H2O2 increased, the total anionic group content of the SCMP first increased and then decreased. A large number of anionic groups were present in the cell wall of the pulp fiber, a slight increase in anionic groups on the surface of pulp after H2O2 bleaching, probably due to alkaline hydrogen peroxide bleaching and oxidation of lignin to form new AGs and SAGs. 8
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