Study on the efficiency and recyclability of Fenton-MnFe2O4 for the degumming of hemp fibers

Preparation of MnFe₂O₄

The synthesis method of MnFe₂O₄ is as follows: first, NaAc (2.7 g) was completely dissolved in EG (30 mL) under ultrasound for 15 minutes, and the molar ratio of MnCl₂·4H₂O and FeCl₃·6H₂O was 1 : 2.⁶ The mixture was then transferred to an autoclave and placed in an oven at 180 °C for 10 h. After naturally cooling down to room temperature, the autoclave was opened, the solution was transferred to the beaker, and the prepared MnFe₂O₄ was sucked out with a magnet. Then, it was washed three times with water and ethanol, and dried at 50 °C. Finally, dry MnFe₂O₄ was stored at room temperature for further use.

Traditional alkali degumming hemp fibers

In this study, the traditional alkali degumming process was used as a control group.⁷ The specific experimental materials and reaction conditions are shown in Table 1. The experimental process is shown in Fig. 1. The weight of the raw hemp sample used in each degumming process was about 20.0 g, and the dosage of chemical agent was the weight percentage of the hemp fiber (w/w).⁵

Fenton degumming process of hemp

In this study, the Fenton degumming hemp process was used as a control group. The specific experimental reagents and reaction conditions are shown in Table 2. The weight of raw hemp sample used in each degumming process was about 20.0 g, and the dosage of chemical agent was the weight percentage of hemp fiber (w/w).

Fenton-MnFe₂O₄ degumming process

The effects of Fenton-MnFe₂O₄ process parameters on hemp fiber properties were studied. Different contents of MnFe₂O₄ (0%, 0.015%, 0.025%, 0.035%, and 0.045%, (w/w)), different contents of H₂O₂ (0%, 4%, 5%, 6%, and 7%, (w/w)) and different contents of FeSO₄·7H₂O (0%, 0.1%, 0.25%, 0.40%, and 0.55%) were added into the Fenton solution to form the degumming solution, and 20 g raw hemp was immersed in the solution at 40 °C for 40 min.⁷ The degummed fiber was washed with deionized water to remove the glial residues, impurities and chemical reagents on the surface of the fiber. Finally, it was dried in an oven to obtain a refined dry hemp.⁸

On the basis of the primary single-factor experiment, three important variables, namely, FeSO₄·7H₂O content, H₂O₂ content and MnFe₂O₄ content, were optimized. By using the Box–Behnken Design (BBD) called the response surface method (RSM), the correlation between the single factor effect of factors and indexes and the interaction between factors was studied. In this study, a three-factor three-level design was used, and 17 experiments were needed. The FeSO₄·7H₂O content (X₁), H₂O₂ content (X₂), MnFe₂O₄ content (X₃) and residual gum content (Y₁) were evaluated.

Determination of the residual gum content (RGC) of hemp fibers

The residual gum content and chemical composition content were analyzed according to GB/T5889-1986.⁹ The residual glue ratio was calculated as shown in eqn (1):

1

where W₀ is the dry weight of the industrial hemp fiber before treatment expressed in g and W₁ is the dry weight of the industrial hemp fiber after treatment expressed in g.

Fourier transform infrared (FT-IR) spectroscopic analysis

By Fourier transform infrared spectroscopy, a certain amount of industrial hemp fiber and KBr were mixed and ground, and then pressed into thin slices. The structure was characterized. The scanning range was 4000–500 cm⁻¹, the resolution was 4 cm⁻¹, and each wavelength was scanned 32 times.¹⁰

Thermogravimetric analysis

A STA449F3 synchronous thermal analyzer was used to analyze the thermal stability of the industrial hemp fiber before and after degumming.¹¹ The test was conducted in a nitrogen atmosphere at a gas flow rate of 10 mL min⁻¹. The test range was from 25 °C to 600 °C at a heating rate of 10 °C min⁻¹.

Scanning electron microscopic (SEM) analysis test

A scanning electron microscope (SEM) was used to observe the surface morphology of the industrial hemp fiber before and after degassing by adjusting different multiples of a S-3400 scanning electron microscope.¹²

Fracture strength test

The fracture strength test was analyzed according to GB/T5889-1986. The test was conducted using a LY-06E electronic single fiber strength meter, with a tensile interval of 10 mm and a tensile rate of 20 mm min⁻¹. The tensile times of each sample were 20 times, and the average value was taken.

Polymerization degree test

The polymerization degree of hemp fiber was tested using the Chinese standard GB/T5888-86.¹³ First, the hemp fiber was defatted in a fat extractor with a mixture of benzene and ethanol in a ratio of 2/1 (v/v), and the sample was chopped into fragments (≤1 mm). The hemp sample was then dissolved in a copper–ethylenediamine solvent.¹⁰ The contents of ethylenediamine and copper in a solvent were 2.03 and 0.99 mol L⁻¹, respectively. A viscosimeter was used to measure the viscosity of hemp samples.

X-ray diffraction analysis (XRD)

The crystal structure and crystallinity analysis of the hemp fiber was performed using a wide-angle X-ray diffractometer equipped with a CuKa radiation source (40 kV/180 mA).¹⁴ X-ray diffraction (XRD) patterns were recorded at a scan rate of 2° min⁻¹ over a 2 hour period, ranging from 5 to 50°.

Effect of the Fenton-MnFe₂O₄ system addition on degumming effects

FeSO₄·7H₂O content, H₂O₂ content and MnFe₂O₄ content in Fenton-MnFe₂O₄

In the above study, we analyzed the effects of various factors on the fiber properties in the Fenton-MnFe₂O₄ system. Therefore, through preliminary single-factor experiments, the range of significant changes in FeSO₄·7H₂O content (A), H₂O₂ content (B) and MnFe₂O₄ content (C) was determined to be 0.1–0.4%, 3–6%, 0.01–0.04%, respectively, as shown in Table 3. The Box–Behnken Design (BBD) of the software Design Expert 13 response surface method (RSM) was used to study their interaction and the optimization of degumming process conditions. As shown in Table 4, the two properties of residual gum rate (Y₁) and breaking strength (Y₂) of the hemp fiber directly determine the spinnability of subsequent processing of hemp fibers, so we use these two as indicators to evaluate the fiber properties.²⁹

Tables 5 and ​and66 are the test results of the response surface test design scheme. The results indicated that the experimental model designed according to Design Expert 13 is significant, indicating that the residual gum rate of the hemp fiber has a good fitting degree with the three factors studied. The regression effect is significant and statistically significant. According to the F value and P value, it can be concluded that in the single-factor experiment, the influence on the residual gum rate follows the order B > A > C, and the influence on the hemp fiber breaking strength follows the order B > A > C, both of which show the same trend. The effect of interaction between factors on the residual gum rate follows the order AC > AB > BC, and the effect of interaction between factors on breaking strength follows the order AB > AC > BC. The two do not show the same trend, which indicates that the effect of interaction between factors on the residual gum rate and breaking strength is no longer consistent, and there is interaction between factors. Based on the principle of least square method, the experimental data were regressed and fitted, and the quadratic function relationship Y₁ of the response surface of the residual gum rate and the quadratic function relationship Y₂ of the response surface of the breaking strength were obtained.

Fig. 6 shows the 3D response surface diagram of the interaction between the residual gum rate and the breaking strength factor. The interaction between the factors of residual gum rate is shown in Fig. 6(a)–(c). With the increase in reactant concentration, ignoring the influence of other variables, the RGC of the hemp fiber decreases rapidly to the lowest point, which indicates that the three substances of A, B and C can promote the degumming of the hemp fiber. After reaching the lowest point, there is an upward trend or it tends to be flat. This shows that it is not the higher the concentration of A, B and C, the smaller the residual gum rate of the fiber.²⁶ The possible reason is that with the increase in A, B and C concentration, a large number of non-cellulose components of the fiber are removed, which promotes the depolymerization of cellulose. However, when the concentration of A, B and C reaches a certain level, the reaction rate of the system becomes too fast. The substances A, B and C cannot react with the fiber colloid, but they react with other substances in the degumming solution. A, B and C cannot be used at the highest level, so that the fiber degumming is not complete, which leads to the rise after the lowest point in the response surface image. The interaction between the breaking strength factors is shown in Fig. 6(d)–(f). With the increase in the concentration of the reactant, ignoring the influence of other variables, the change in the breaking strength of the fiber decreases rapidly with the increase in the concentration of the reactant, reaching the lowest point. This shows that the three factors of A, B and C have a significant effect on the breaking strength of the fiber. After reaching the lowest point, there is a slight upward trend. This part of the upward trend may be affected by the change in the trend of fiber residual gum rate. The increase in fiber residual gum rate represents the increase of gum wrapped on the fiber surface. The increase of gum will increase the toughness of the fiber and affect the residual gum rate. High residual gum rate is often accompanied by high breaking strength. In addition, the optimal Fenton-MnFe₂O₄ conditions for hemp fiber RGC were obtained using the Design-Expert. 13 software, which are as follows: FeSO₄·7H₂O = 0.31%, H₂O₂ = 5.4%, and MnFe₂O₄ = 0.040%. Under the optimal process conditions, five parallel experiments were carried out (as shown in Table 7). The average RGC of the fiber was 5.92% ± 0.10, and the average breaking strength was 15.22 cN ± 0.10, which was close to the predicted residual gum rate of 5.79% using BBD. The breaking strength is 15.20 cN. This result indicates that the model is predictive and reliable for the RGC of the hemp fiber.

Surface morphology analysis (SEM)

Scanning electron microscopy (SEM) was used to analyze the surface morphology of untreated hemp fibers³⁰ (Fig. 7(a)), traditional alkali degumming-treated hemp fibers (Fig. 7(b)), traditional Fenton degumming-treated hemp fibers (Fig. 7(c)), and Fenton-MnFe₂O₄ degumming-treated hemp fibers (Fig. 7(d)).

The untreated hemp fiber is rough, and the surface of the fiber is wrapped with a large amount of gum. The single fiber is adhered together by the gum to form a bundle fiber, and the single fiber is not observed³¹ (Fig. 7(a)). Hemp fiber after traditional alkali degumming treatment, bundle fiber have been separated, a large number of gum on the surface of hemp fiber has been removed, but there are still some gums (Fig. 7(b)). The gum on the surface of hemp fibers treated by traditional Fenton degumming is almost removed, and the single fiber can be clearly observed. The surface of the hemp fiber is smooth. The hemp fiber treated under this condition has a small enough diameter, high dispersion and softness^26,32 (Fig. 7(c)). The raw hemp treated by the Fenton-MnFe₂O₄ degumming method (Fig. 7(d)) has been separated from the fiber that was originally stuck together by the gum. The single fiber is clearly visible, and the gum on the surface of the fiber has almost been removed, which is similar to the SEM image of Fig. 7(c), but the addition of H₂O₂ content is greatly reduced, which also proves the catalytic effect of MnFe₂O₄ on the Fenton system.

Fourier transform infrared (FT-IR) analysis

In order to verify the changes in the chemical structure of hemp fiber prepared by different methods,²⁵ we performed the Fourier transform infrared (FT-IR) analysis, as shown in Fig. 8(a). The –OH stretching vibration at about 3148 cm⁻¹, the C–H stretching vibration at about 2916 cm⁻¹, the cellulose β-1,4-glycosidic bond at 899 cm⁻¹, and the C–OH stretching vibration at 668 cm⁻¹ are the characteristic peaks of the infrared spectrum of cellulose. These peaks exist in all four groups of samples, indicating that the main component of hemp fiber is cellulose. After the treatment of hemp fiber, the intensity of these peaks increased, indicating that the non-cellulose components in the hemp fiber were removed and the cellulose components of hemp fiber were improved.²⁹ The CO stretching at about 1733 cm⁻¹ is derived from ketones or esters in hemicellulose. This peak only exists in raw hemp, and other treated hemp fibers do not have this peak, indicating that hemicellulose in hemp fiber gum is more easily oxidized and decomposed. The vibration of the acidic group (carboxyl or aldehyde group) of the hemp fiber at about 1640 cm⁻¹ can be observed.³³ The strength of the place is stronger than that of the raw hemp fiber, especially the hemp fiber after Fenton-MnFe₂O₄ degumming, which indicates that more hydroxyl groups are oxidized and form carboxyl or aldehyde groups on the cellulose. This is consistent with the results in Fig. 3(b).

X-ray diffraction analysis (XRD)

Fig. 8(b) shows the X-ray diffraction patterns treated by different methods. As shown in these four curves, three main crystal peaks can be seen at 2θ = 14.8°, 17.0°, and 22.7°, corresponding to the 101 crystal plane, 110 crystal plane and 200 crystal plane of cellulose I, respectively.⁹ This shows that for Fenton-MnFe₂O₄, like other degumming methods, the crystal structure and crystal type of cellulose have not changed. In addition, the peak intensity of the 200 crystal plane of cellulose I increased after Fenton-MnFe₂O₄ degumming treatment, indicating an increase in crystallinity.³⁴ The dissolution of some amorphous cellulose and hemicellulose in the hemp fiber gum will cause an increase in the crystallinity of the fiber. The traditional Fenton degumming method shows the same trend of crystallinity, which also shows this. However, the crystallinity of traditional alkali degumming is reduced, because under alkaline conditions, the cellulose chain on the crystal is easier to be removed, which will result in a decrease in the crystallinity of hemp fibers.

TG and DTG analysis

In order to verify the thermal stability of hemp fibers treated by different degumming methods, we carried out TG and DTG analyses, as shown in Fig. 8(c) and (d). It can be seen from the TG diagram that the hemp fiber treated by four different methods have three stages of mass loss. In the first stage, 50–120 °C, the fiber mass has a small range of loss. The reason is that in this temperature range, due to the endothermic precipitation of internal water and small molecules, the evaporation of water, the removal of crystal water caused by the loss of mass. It can be seen from the image that this part of the weight loss is relatively small. In the second stage, the weight loss rate of hemp is large, which is due to the decomposition of hemp fiber macromolecular chain between 300 °C and 600 °C. In the temperature range of 220 °C to 300 °C, it is due to the pyrolysis of cellulose and the cleavage of glycosidic bonds of cellulose structure.³⁵ As can be seen from the image, the mass loss of hemp fiber treated by different methods is very large in this temperature range, and the mass loss of hemp fiber treated by Fenton-MnFe₂O₄ is the largest. The degumming method was changed, and the degradation temperature was also different. The degradation temperature of hemp fiber treated by Fenton-MnFe₂O₄ was the highest. The increase in degradation temperature meant that the non-cellulose substances were removed and the crystallinity of the fiber was enhanced, which was consistent with the results obtained, as shown in Fig. 8(b).³⁶ After the third stage at 375 °C, the weight loss was due to the thermal decomposition of the cellulose component.^1,37

As shown in Fig. 8(d), the thermal stability of hemp fibers treated with Fenton-MnFe₂O₄ was greatly improved compared with that of the raw hemp and hemp fiber treated by traditional alkali degumming.^38,39 Compared with the traditional Fenton degumming method, the thermal stability of hemp fiber treated with Fenton-MnFe₂O₄ was not reduced, and there was a slight increase. This means that Fenton-MnFe₂O₄ is beneficial for improving the thermal stability of the hemp fiber.

This work was supported by the 2022 Heilongjiang Provincial Colleges and Universities Basic Scientific Research Business Fee Scientific Research Project (No. 145209510).