Abstract
Sodium hydroxide is commonly used as the main alkali source in the oxidation degumming of ramie in the current studies for natural fiber extraction. However, due to the strong alkalinity of sodium hydroxide, the oxidation reaction speed of hydrogen peroxide is difficult to control and thus results in great damage to the treated fibers. In this paper, magnesium hydroxide was selected as an effective sustained-release alkali source to improve the tensile properties of degummed fibers and reduce the chemical oxygen demand (COD) values of degumming wastewater. This novel reagent can adjust and buffer pH values in the degumming solution. The chemical components and structure properties of degummed fibers were characterized by Fourier transform infrared spectroscopy and X-ray diffraction, respectively, and the as-developed oxidation degumming solution performance was monitored by an oxidation–reduction potentiometer. The results showed that the optimal substitution rate of magnesium hydroxide was 20% during the oxidation process. Compared with the degummed fibers without magnesium hydroxide, the tenacity, work of rupture and degumming yield of treated fibers increased by 39.82%, 46.15% and 5%, respectively. Moreover, the COD values of wastewater decreased by 20% at the same time.
Ramie is a perennial herb that originates from China. The production of ramie fiber in this country accounts for more than 90% of the total yields in the world. 1 In China, ramie is one of the main economic crops, reaching a production of 500,000 t of fibers per year. 2 Ramie fiber possesses many excellent properties, such as favorable hygroscopicity, coolness, antibacterial, excellent thermal conductivity, ventilation function, etc.3,4 Cellulose is the main component of ramie fibers, while the other components in ramie, such as pectin, lignin, water soluble substance, etc., are defined as gums. Degumming refers to the removal of heavily coated gummy material from the cellulosic part of plant fibers, and it is necessary prior to the further spinning process.5,6
Although bio-degumming is an eco-friendly method of ramie fiber extraction, it requires a long degumming time and the stability of degummed fiber quality is low.7,8 Therefore, further research on chemical degumming is still necessary. Compared with the traditional degumming process (sodium hydroxide is mainly used), oxidation degumming with H2O2 is effective, eco-friendly and of high fiber yield. 9 The free radicals (such as O–2·, OH·, OOH·, OH–, etc.) generated by alkali H2O2 have a strong oxidizing ability and can remove the gums in raw ramie quickly; however, they also produce significant cellulose damage that can affect subsequent processing of the fiber.10,11
Previous studies showed that the oxidizing ability of H2O2 was strongly affected by its decomposition speed, and the decomposition speed of H2O2 depends on the pH value of the degumming or bleaching solutions.12–14 Therefore, researchers tried to control the oxidizing properties of H2O2 solutions by adjusting their pH values. Li and Yu 15 controlled the decomposition speed of H2O2 by multiple feeding NaOH and H2O2 in degumming solution. This method reduced the cellulose damage and protected tensile properties of fiber. However, the degumming process was complicated and difficult to control in large industrial production. Wang 16 used an acid-based buffer system to control the pH value of H2O2 bleaching solution and successfully improved the bleaching effect of paper pulp; however, the pH values of acid-based buffer systems were not suitable for ramie degumming.
In order to control the oxidation reaction of H2O2, in this study, Mg(OH)2 was introduced to oxidation degumming. Mg(OH)2 was slightly soluble in deionized water, but its solubility increased greatly in degumming solution that contains degumming auxiliaries. Mg(OH)2 can dissolve gradually into the degumming solution and act as one kind of effective sustained-release alkali source; thus, it can control the pH value and oxidizing ability of degumming solution in a proper range. The pH value of degumming solution can be adjusted by changing the substitution rate. In addition, the substitution rate was defined as the mole proportion of NaOH replaced by Mg(OH)2 under the total alkali dosage of 10% (o.w.f.),
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that is, the weight of Mg(OH)2 can be calculated by equation (1). In addition, Mg2+ can reduce the ineffective decomposition of H2O2 and protect cellulose and hemicellulose from damage by over oxidation; therefore, the use of Mg(OH)2 can improve the tensile properties of fiber and increase the yield of degumming18,19
Experimental details
Materials
Chemical composition of raw ramie (before degumming)
Chemicals
The main chemicals used in this study were H2O2, Mg(OH)2, NaOH, Na5P3O10, H2O2 stabilizer FA001, HEDP, NaHSO3 and anthraquinone, which were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). All chemicals used in this study were analytical grade.
Process for the degumming of ramie
The degumming solution was composed of 6% (o.w.f.) H2O2, 10% (o.w.f.) alkali (a certain ratio of NaOH and Mg(OH)2), 4% (o.w.f.) Na5P3O10, and 1% (o.w.f.) anthraquinone, 2% (o.w.f.) 1-Hydroxyethylidene-1,1-diphosphonic acid (HEDP) and 1% (o.w.f.) H2O2 stabilizer FA001 (the concentration of H2O2 and HEDP was 30% and 50%, respectively), with a liquor ration of 1:10. In the first period of degumming, raw ramie was immersed in the degumming solution, then the temperature was raised to 85℃, and this temperature was kept for 60 minutes. In the second period of degumming, the temperature was raised to 125℃ (with pressure of 0.6 kg), and was kept at 125℃ for another 60 minutes. Subsequently, the treated fibers were immersed in solution of 4% (o.w.f.) NaHSO3 at the temperature of 90℃ for 60 minutes for reducing the reaction process. 9 Finally, the degummed ramie fibers were washed thoroughly with deionized water.
Solubility test of Mg(OH)2
The solubility of Mg(OH)2 in degumming solution can determine whether it can provide enough alkalinity for degumming. In order to test the solubility of Mg(OH)2 in degumming solution, 2 g Mg(OH)2 was added in 100 mL deionized water and 100 mL solution with completely soluble degumming additives (4% (o.w.f.) Na5P3O10, 2% (o.w.f.) HEDP, 1% (o.w.f.) H2O2 stabilizer FA001), respectively. The deionized water and degumming solution was raised to 85℃, and then the undissolved Mg(OH)2 was extracted with sintered discs. The solubility of Mg(OH)2 was calculated by equation (2) as follows
Yield of degumming and residual gums
The yield of degumming and residual gums was calculated using equations (3) and (4), respectively
The effect of Mg(OH)2 on pH value
The pH value of degumming solution has great effect on the reaction speed of H2O2. In this experiment, the pH value of the degumming solution was determined and monitored by MODEL 421 ORP meter (Dapu Instrument, Shanghai, China).
Residual H2O2 calibration
The residual H2O2 contents were tested according to Chinese standard GB 1616-2003 ‘Hydrogen peroxide for industrial use’.
Oxidation reduction potential value
Oxidation reduction potential (ORP)20,21 is an important water chemistry parameter and it provides a measurement tool for oxidizing or reducing the capacity of the ambient water. ORP was measured in volts (V) or millivolts (mV) with an oxidation-reduction potentiometer. The more positive the potential value, the greater the species’ affinity for electrons and tendency to oxidize.
The relationship between ORP and the concentrations of the oxidized and reduced forms of a substance is given by Nernst equation (5)
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In this experiment, the ORP value of the degumming solution was also determined and monitored by a MODEL 421 ORP meter (Dapu Instrument, Shanghai, China).
Degree of polymerization test
The degree of polymerization (DP) of ramie fibers was tested according to Chinese standard GB/T 5888-1986 ‘the measurement of degree of polymerization in ramie fiber’, which describes the method of determining their average viscometric values in ramie fibers. The determination is achieved by measuring the intrinsic viscosity of the ramie fibers solution in copper ethylene-diamine solvent.
The ramie fibers were firstly degreased with benzene and ethyl alcohol mixture in a 2:1 (v/v) ratio. The solvent was allowed to evaporate in air at room temperature. The samples were cut into short pieces (1–2 mm). Then the samples were kept in controlled-humidity atmosphere in a closed weighing container until it reached equilibrium of water content before removing the materials required for test purposes. By using the titration method, the copper and ethylene-diamine content in the solvent were determined to be 1.01 and 2.03 mol/L, respectively. An Ubbelohde viscometer was used to measure the intrinsic viscosity of ramie samples.
Mechanical property test
Fiber samples were conditioned in standard atmospheric condition (T = 20℃ ± 2℃, relative humidity (RH) = 65% ± 2%) for 24 h before the mechanical test. Tenacity, breaking elongation and work of rupture were tested using a XQ-2 fiber strength instrument under the condition of 20℃ and RH 65%. The pre-tension was 0.3 cN/dtex. The clamping distance was set at 20 mm, and the descending speed of the bottom clamp was 20 mm/min.
The linear density was tested by the gravimetric method. Linear density refers to the weight (g) of 1000-meter long fibers under the official regain of ramie (12%). Linear density (dtex) was calculated using equation (6) as follows
Chemical oxygen demand test
The COD (chemical oxygen demand) values of degumming wastewater were tested according to Chinese standard GB/T 15456-2008 ‘The COD value of industrial circulating cooling water - KMnO4 method’. Wastewater with a high COD value will cause severe pollution to the environment.
Infrared spectroscopy analysis
Fourier transform infrared spectroscopy (FT-IR) analysis was employed to determine the chemical functional groups in treated fibers. The fibers were analyzed using a Nicolet 6700 spectrometer (Thermo Fisher, USA). The spectra obtained were the result of 30 scans within the range of 4000–400 cm–1 at a resolution of 8 cm–1.
X-ray diffraction analysis
X-ray diffraction (XRD) analysis was used to test the crystallinity and crystal transition of fiber. The fibers were analyzed using a D/Max-2550 PC. XRD patterns were recorded from 2θ = 5–60° with a D/max-RB diffractometer equipped with a graphite monochromator and Cu Kα radiation at λ = 0.154 nm (40 kV, 200 mA).
Results and discussion
Solubility test of Mg(OH)2
NaOH can be completely dissolved in deionized water; therefore, the pH value of NaOH is high. However, Mg(OH)2 was slightly soluble in deionized water and the pH value of Mg(OH)2 was much lower than that of NaOH. In order to assess whether Mg(OH)2 can provide a high enough pH value for degumming, the solubility of Mg(OH)2 and the pH value of Mg(OH)2 solution in deionized water and degumming solution were tested. As demonstrated in Figure 1, Mg(OH)2 was slightly soluble in deionized water, but its solubility increased by 21.65% in degumming solution that contains degumming auxiliaries. There were two reasons for this observation. On the one hand, the acidic degumming auxiliaries HEDP can accelerate the dissolution of Mg(OH)2; on the other hand, the salt effect caused by the other degumming auxiliaries also improves the solubility of Mg(OH)2.23,24 Figure 1 also shows the pH value of the two solutions in this solubility test. It can be seen that the pH value provided by Mg(OH)2 (without any dosage of NaOH in this solution) was about 10.0–11.0, which was high enough for degumming.
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Solubility and pH values of Mg(OH)2 in deionized water and degumming solution. Left: Mg(OH)2 deionized water solution; right: degumming solution with 4% Na5P3O10, 2% HEDP, 1% H2O2 stabilizer FA001 and 2 g Mg(OH)2.
The effect of the substitution rate of Mg(OH)2 on residual gums and fiber yield
Figure 2 depicts the effect of the substitution rate of Mg(OH)2 on residual gums and yield. It is obvious from Figure 2 that the yield of degumming and residual gums of degummed fibers increased with the increasing of the substitution rate of Mg(OH)2. When the substitution rate of Mg(OH)2 was over 80%, the residual gums of degummed fibers exceeded 20%, which was too high for fibers to separate from each other. Figure 3 shows the topographies of ramie fibers degummed with various substitution rates of Mg(OH)2. It can be seen from Figure 3 that when the substitution rate of Mg(OH)2 was lower than 80%, the separation degree of fibers was sufficient and the color of fibers was white. However, when the substitution rate of Mg(OH)2 was over 80%, the separation degree of fibers was not sufficient and the color of fibers was yellow. Therefore, it can be concluded that Mg(OH)2 can be partly used as the alkali resource of degumming, but its substitution rate must be lower than 80%. Therefore, we only discuss the substitution rate from 0% to 80% in the following part of this study.
The effect of the substitution rate of Mg(OH)2 on residual gums and yield. Topographies of ramie fibers degummed with various substitution rates of Mg(OH)2. Substitution rates: (a) 0%; (b) 20%; (c) 40%; (d) 60%; (e) 80%; (f) 100%.
The effect of the substitution rate of Mg(OH)2 on the pH value of degumming solution
The pH value of degumming solution had a great effect on the type of free radicals generated by H2O2 and the oxidation reaction speed. When the initial pH value of degumming solution was higher than 12.0, the synergism effect of the free radical OH· and ·O–2 would cause massive cellulose damage through β-elimination reaction. However, when the initial pH value of degumming solution was lower than 12.0, the reaction speed between H2O2 and gums was five or six times faster than the reaction speed between H2O2 and cellulose. It can be seen from Figure 4 that when the substitution rate of Mg(OH)2 was 0%, the initial pH value of the degumming solution was above 12.0. In comparison, when Mg(OH)2 was used as an alkali source, the initial pH value of the degumming solution was below 12.0. Therefore, the utilization of Mg(OH)2 was helpful for adjusting and buffering the pH value of oxidation degumming.
pH values of degumming solution with various substitution rates of Mg(OH)2. Note: SR represents substitution rate.
When the substitution rate of Mg(OH)2 was 0%, the pH value of the degumming solution decreased by 13.2% in the degumming process. However, when the substitution rate of Mg(OH)2 was higher than 20%, the pH value of degumming solution showed a slight increase in the degumming process. It can be deduced that when Mg(OH)2 was employed as an alkali resource, the pH value of degumming solution became more stable. Therefore, the use of Mg(OH)2 can keep the proper reaction speed of H2O2 in a proper range. However, NaOH cannot be completely replaced by Mg(OH)2, otherwise the pH value of degumming solution will not be high enough for degumming. The most suitable pH value for degumming was achieved when the substitution rate of Mg(OH)2 was 20–40%.
The effect of the substitution rate of Mg(OH)2 on residual H2O2 in degumming solution
When the substitution rate of Mg(OH)2 was 0%, the initial pH value was above 12.0 (Figure 4). Comparatively, the consumption speed of H2O2 was higher than for the other situations when Mg(OH)2 was applied (Figure 5). It can be seen from Figure 5 that when Mg(OH)2 was used, the final residual H2O2 in degumming solution was about 3 g/L, which means the consumption rate of H2O2 was approximately 50% lower compared with the situation without Mg(OH)2. Previous studies showed that when H2O2 was used under high temperature (above 100℃), it would not cause much cellulose damage; in order to test whether this theory would work on ramie degumming, during the time period of 60–150 minutes of the degumming process, the temperature of degumming solution was raised to 100℃ and 125℃, respectively. The results (Table 2) show that the residual H2O2 in degumming solution would continue to play a positive role in the second period of degumming (60–150 minutes). This means that H2O2 can remove the gums without damaging cellulose fibers under high temperature (above 100℃). The reason for this phenomenon can be explained as follows. There were two parallel reactions
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in the oxidation degumming solution. One was the reaction between H2O2 and gums, which was beneficial for degumming (the reaction of H2O2 and gums); the other one (the reaction of H2O2 and cellulose) was ineffective decomposition of H2O2 (mainly catalyzed by metallic elements), which would cause damage to cellulose and thus decreased the tensile properties of degummed fibers. The rise of temperature would induce the acceleration of both reactions (the reaction speed increased by two or four times when the temperature raised per 10℃). The growth of reaction speed for H2O2 and gums was much higher than for H2O2 and cellulose. This was because its activation energy was higher, which made it more sensitive to temperature change. It can be deduced that the residual H2O2 in degumming solution was beneficial for the tensile properties of degummed fibers.
Residual H2O2 in degumming solution with various substitution rates of Mg(OH)2. Note: SR represents substitution rate. Tensile properties of degummed fiber scouring under different temperatures
The effect of the substitution rate of Mg(OH)2 on the ORP value of degumming solution
The ORP value reflected the comprehensive oxidation ability of degumming solution. More particularly, the ORP values of alkali and H2O2 were negative and positive, respectively. The initial ORP value of degumming solution was about –100 mV, and it decreased to between –600 and –700 mV in the degumming process. There were two reasons for this change. On the one hand, when the residual amount of H2O2 in the degumming solution dropped to a certain amount, the effect of H2O2 on the ORP value of degumming solution decreased gradually and eventually disappeared. On the other hand, Mg(OH)2 dissolved gradually in the degumming solution with the consumption of alkali in this degumming system. Figure 6 displays the ORP value of degumming solution with various substitution rates of Mg(OH)2. The increasing of the substitution rate can postpone the descending point of the ORP value and extend the reaction time of H2O2. Compared with the substitution rate of 20%, a higher substitution rate would induce the degumming solution to retain a comparatively higher ORP value, which was not good for the degumming efficiency.
Oxidation reduction potential (ORP) value of degumming solution with various substitution rates of Mg(OH)2. Note: SR represents substitution rate.
The effect of the substitution rate of Mg(OH)2 on the degree of polymerization
The DP of degummed fibers was the average polymerization degree of all the components in fiber. The DP of degummed fibers increased with the substitution rate of Mg(OH)2 from 0% to 20%, but decreased with further increase of the substitution rate (Figure 7). The reason for the upward trend was that cellulose was damaged less when Mg(OH)2 was used. The coprecipitation of Mg(OH)2 and metal elements Fe and Mn, which derived from raw ramie, can prevent cellulose from degradation.
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Degree of polymerization of fiber degummed with various substitution rates of Mg(OH)2.
When the substitution rate of Mg(OH)2 was over 40%, a large number of gums with low DP were retained in the degummed fiber and therefore the DP of the fiber began to decrease.
The effect of the substitution rate of Mg(OH)2 on the tensile property of fiber
Figure 8 reflects that the linear density of degummed fibers increased with an increase of substitution rate of Mg(OH)2. This was because the residual gums increased with the increasing substitution rate. In addition, to a certain extent, the more celluloses and gums existed, the coarser the degummed fibers were, and therefore the linear density was greater, according to equation (6).
Tensile property of degummed fibers with various substitution rates of Mg(OH)2.
The tenacity and elongation of ramie fibers increased with the substitution rate of Mg(OH)2 from 0% to 20%, but decreased with a further increase of the substitution rate. The upward trend was because the ORP value reached its most proper point when the substitution rate of Mg(OH)2 was 20%. When the substitution rate of Mg(OH)2 was over 40%, the time for the degumming solution to remain at high ORP value was too long, which caused cellulose damage and thus induced the decrease of tenacity and elongation of fibers. The work of rupture was the integrated indicator of tenacity and elongation of fiber; thus, the work of rupture showed the same trend with tenacity and elongation. (At the beginning of the degumming process, cellulose was covered by a great number of noncellulose components, that is, gums; degumming solution with a high ORP value benefited for removing these gums. However, at a later stage of degumming, few noncellulose components were retained in fiber and the cellulose was exposed directly to degumming solution, and degumming solution with a high ORP value would do damage to cellulose by over oxidation).
The effect of the substitution rate of Mg(OH)2 on the COD values of wastewater
It is obvious from Figure 9 that the COD values of degumming wastewater decreased with the increasing of substitution rate of Mg(OH)2. The COD values decreased by 20% when the substitution rate of Mg(OH)2 was 20%. It has been proved that COD derived from the lignin, acetic acid and other organic materials dissolved in the degumming wasterwater.25,26 The content of these materials in degumming wastewater was low when Mg(OH)2 was used, which was due to the low solubility and alkaline of Mg(OH)2. The less removal of gums, the lower the COD value was. Therefore, the use of Mg(OH)2 was helpful for reducing the cost of wastewater treatment.
Chemical oxygen demand (COD) values of wastewater with various substitution rates of Mg(OH)2. Note: the COD values in this figure do not include the reducing process; the COD value of the reducing process is about 2000 mg/L.
FT-IR analysis
FT-IR spectroscopy analysis of samples was performed in order to identify the change in chemical composition of degummed fibers.
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The spectra were exhibited by signals in the region of 3400–2800 cm–1 due to the stretching vibration of -CH and -OH, and the peak at 2900 cm–1. In particular, pure cellulose presents the same spectra characteristics in all fibers.
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The vibration at 2850 cm–1 was originating from C-H stretching in lignin and waxes,
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and it is obvious from Figure 10 that the introduction of Mg(OH)2 had little influence on removal of the lignin and waxes in degummed fibers.
Fourier transform infrared spectroscopy of fibers degummed with various substitution rates of Mg(OH)2. Note: SR represents substitution rate.
The carbonyl peak at 1730–1750 cm–1 was attributed to the C = O stretching of C-OH bending in hemicellulose. 29 Compared with the substitution rate below 20%, the relative intensities at 1730–1750 cm–1 were much higher when the substitution rate of Mg(OH)2 was over 40% (Figure 10). This observation indicated that the hemicellulose can be removed effectively when the substitution rate was only lower than 20%, which explained why the curve of tensile properties possessed the highest value at the substitution rate of 20%.
XRD analysis
The XRD pattern obtained for degummed fibers is shown in Figure 11, where all the curves presented major crystalline peaks for 2θ ranging between 22° and 23°, which corresponded to the (002) crystallographic plane family of cellulose I; the other peaks for 2θ presented between 14.8° and 16.7°, corresponding to the (101) crystallographic plane family of cellulose II.30,31 Therefore, the change of substitution rate did not cause any transformation of crystal forms. It can be seen from Table 3 that the crystallinity of fibers degummed with NaOH serving as the alkali resource was lower than that degummed with NaOH and Mg(OH)2. This was because cellulose fibers suffered less damage by over oxidation when Mg(OH)2 was employed. The crystallinity of degummed fibers declined with the substitution rate of Mg(OH)2, which was due to the amorphous residual gums increasing with the substitution rate.
X-ray diffraction of fibers degummed with various substitution rates of Mg(OH)2. Note: SR represents substitution rate. Crystallinity of fiber degummed with various substitution rates of Mg(OH)2
Conclusion
In this paper, Mg(OH)2 was used as an effective sustained-release alkali source for oxidation degumming of ramie. When NaOH was partly replaced by Mg(OH)2, the tensile properties of treated fibers and yield of degumming were improved and the COD values of wastewater were reduced during the oxidation degumming process. Mg(OH)2 can adjust and buffer pH values in degumming solution. Tensile properties and DP in degummed fibers increased gradually with the substitution rate of Mg(OH)2 increasing from 0% to 20%; however, these two properties decreased obviously when applying more Mg(OH)2, that is, the substitution rate exceeded over 20%. Moreover, the yield of degummed fibers and COD values of wastewater were found to increase and decrease with the substitution rate of Mg(OH)2 from 0% to 100%, respectively. With the optimal substitution rate of Mg(OH)2 for 20%, the tenacity, the work of rupture and the degumming yield of treated fibers increased by 39.82%, 46.15% and 5%, respectively. Moreover, the COD values of wastewater decreased by 20% at the same time. The sustained-release alkali source can serve as a significant reagent in the oxidation degumming process.
Footnotes
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the earmarked fund for China Agriculture Research System for Bast and Leaf Fiber Crops (grant number CARS-19), the China Academy of Agricultural Science and Technology Innovation Project (grant number ASTIP-IBFC07), the innovation fund for graduate students in Donghua University (grant number 16D310107) and the ‘Xiaoping science and technology innovation team’ (industrialization integrated R & D group of bast fiber biological degumming), China Scholarship Council.