Extraction and Characterization of Microfibrillated Cellulose from Discarded Cotton Fibers through Catalyst Preloaded Fenton Oxidation

With rapid developments in science and technology, mankind is faced with the dual severe challenges of obtaining needed resources and protecting the environment.*e need for sustainable development strategies has become a global consensus. As the most abundant biological resource on Earth, cellulose is an inexhaustible, natural, and renewable polymer. Microfibrillated cellulose (MFC) offers the advantages of abundant raw materials, high strength, and good degradability. Simultaneously, MFC prepared from natural materials has high practical significance due to its potential application in nanocomposites. In this study, we reported the preparation of MFCs from discarded cotton with short fibers by a combination of Fe catalyst-preloading Fenton oxidation and a high-pressure homogenization cycle method. Lignin was removed from the discarded cotton with an acetic acid and sodium chlorite mixed solution. *en, the cotton was treated with NaOH solution to obtain cotton cellulose and oxidized using Fenton oxidation to obtain Fenton-oxidized cotton cellulose. *e carboxylic acid content of the oxidized cotton cellulose was 126.87 μmol/g, and the zeta potential was −43.42mV. *en, the Fenton-oxidized cotton cellulose was treated in a high-speed blender under a high-pressure homogenization cycle to obtain the MFC with a yield of 91.58%. Fourier transform infrared spectroscopy (FTIR) indicated that cotton cellulose was effectively oxidized by Fe catalyst-preloading Fenton oxidation. *e diameter of the MFC ranged from several nanometers to a few micrometers as determined by scanning electron microscopy (SEM), the crystallinity index (CrI) of the MFC was 83.52% according to X-ray diffraction (XRD), and the thermal stability of the MFC was slightly reduced compared to cotton cellulose, as seen through thermogravimetric analysis (TGA). *e use of catalystpreloading Fenton oxidation technology, based on the principles of microreactors, along with high-pressure homogenization, was a promising technique to prepare MFCs from discarded cotton.


Introduction
Cotton is an important cash crop that plays a key role in economic affairs worldwide [1]. As an excellent natural material, cotton provides the main supply of natural fibers for textile industries and other fields [2,3]. e production of cotton fabrics has constantly increased in the past few decades, and necessities made of cotton can be found practically everywhere consumer textile products are sold. e annual production of cotton fibers is approximately 25.43 million tons [2], and, with the rapid increase in the global population, the demand for, and consumption of, cotton is increasing year by year worldwide. In the process of combining cotton to produce fabrics, short fibers that lack the quality to form threads are discarded and piled up [4]. e discarded cotton becomes trash and accumulates in waste dumps. e massive accumulation of discarded cotton is a huge waste of valuable resources, and, more importantly, discarded cotton causes severe environmental pollution. e discarded cotton fiber is mainly comprised of cellulose (approximately 95%) [5], and cellulose is a homopolymer of β-1,4-d-glucose molecules linked in a linear chain [6]. Cellulose and cellulose derivatives have been widely used in chemical, biological, and medical industries [7]. Cellulose acetate is an important cellulose derivative, and researchers have prepared nanocomposites based on cellulose acetate and cellulose. For example, antibacterial nanocomposites (NC1-NC4) are produced by dispersing ZnO nanofillers in a cellulose acetate matrix [7]. Nanocomposite membranes (PES-CA-Ag 2 O) have been developed by the inclusion of silver oxide in polyethersulfone and cellulose acetate polymers [8]. Cellulose/ZrO 2 nanohybrids have been synthesized by simple growth of ZrO 2 on a cellulose matrix [9]. Lastly, chitosan-coated cellulose filter paper has been used as a support for cobalt nanoparticle preparation [10]. ese nanocomposites have a variety of desirable antibacterial, catalytic, redox, and toxic organic properties.
Currently, cellulose is also widely used as the source of microfibrillated cellulose (MFC) and nanocellulose. MFCs are a promising natural material because of their biocompatibility, high mechanical strength, large surface area, low density, and excellent optical and mechanical properties. MFCs have been widely used in paper-making, as catalyst carriers, and for biomedical applications and nanocomposites. MFCs have been prepared from cotton waste using hydrothermal reactions [11], phosphotungstic acid [12], alkaline/urea treatment [13], dilute sodium hydroxide, dilute inorganic acids, and so on. MFCs can also be prepared by chemical pretreatment (TEMPO system oxidation, periodate oxidation, carboxymethylation, Fenton oxidation, etc.) combined with a mechanical method, which can help realize the separation of filaments and reduce mechanical properties at the same time [14,15].
Fenton (Fe 2+ /H 2 O 2 ) oxidation technology has the advantages of environmental friendliness, high efficiency, and low cost, and it has recently been widely studied in the field of cellulose and MFC preparation [16]. However, in the traditional Fenton oxidation system, hydrogen peroxide has been left unreacted in the solution before effectively oxidizing the fiber, which leads to an increase in hydrogen peroxide concentration and a decrease in oxidation efficiency. Catalyst-preloading Fenton oxidation technology is the solution based on the principles of microreactors. e "microreactor" consists of a large number of cellulose macromolecules. FeSO 4 ·7H 2 O is used as a catalyst for preloading, Fe 2+ is adsorbed by fibers when infiltration occurs, and a complex catalytic system of "Fe 2+ + cellulose" is formed.
en, the free Fe 2+ is removed, and hydrogen peroxide is introduced, so the Fenton reaction is limited in the "microreactor" [17].
Catalyst-preloading Fenton oxidation technology based on the microreactor principle is an effective method of MFC preparation [17], as it can improve the oxidation efficiency and effectively reduce the amount of hydrogen peroxide.
us, catalyst-preloading Fenton oxidation was performed on discarded cotton, followed by high-pressure homogenization to prepare MFCs. When the pH value of the Fenton oxidation system was 3.0, the effects of various parameters, such as the H 2 O 2 dose, FeSO 4 ·7H 2 O dose, reaction temperature, and time, on the Fenton oxidation efficiency were discussed in the production process of MFCs. Furthermore, the MFC was characterized by SEM, FTIR, XRD, and TGA. e results indicated that the use of catalyst-preloading Fenton oxidation technology based on the microreactor principle along with high-pressure homogenization was a promising technique to prepare MFCs from discarded cotton.

Materials.
Cotton with short fibers discarded in industrial processes was used in this study. First, 50.0 ml acetic acid and 60.0 g sodium chlorite were dissolved in 500 ml of deionized water. e as-supplied discarded cotton (6.0 g) was uniformly mixed in the solution under continuous stirring for 1 h at 75.0°C to remove lignin, polyphenols, and proteins [18]. en, the sample was cleaned with deionized water to pH 7.0. Next, the sample was treated in a solution of 2.0% NaOH (500 ml) and stirred for 2 h at 80°C. At the end of the process, the reaction was stopped; deionized water was used to clean the sample and reach a pH of 7.0, and cotton cellulose was then obtained by drying under vacuum at 55°C for 12 h.
Sodium hydroxide pellets (NaOH), FeSO 4 ·7H 2 O, and sodium chloride were provided by Tianjin Tianli Chemical Co., Ltd. A solution of H 2 O 2 (30.0%) was received from Shanghai Aladdin Biochemical Technology Co., Ltd. All other chemical reagents used in the experiments were of analytical grade. Deionized water was used throughout the experiment.

Preparation of MFC.
MFC was obtained from dry discarded cotton cellulose using the Fenton (Fe 2+ /H 2 O 2 ) oxidation described by Duan and coworkers [17]. First, a certain amount of FeSO 4 ·7H 2 O was dissolved in 300 ml of deionized water. en, 3.0 g of dry cotton cellulose was reacted with the solution at a temperature of 30.0°C for 60 min under vigorous stirring. Next, the free Fe 2+ was removed by vacuum filtration and pressing, and the complex catalytic system of "Fe 2+ + cellulose" was formed. Preloaded cotton cellulose was put into a flask, and the concentration of cotton cellulose was adjusted to 25.0% with deionized water. en, a certain volume of H 2 O 2 was added to the flask. e flask was sealed and shaken at a certain temperature in a shaker at 110 r/min. en, the reaction was stopped, and the product was washed with deionized water until neutral pH to obtain Fenton-oxidized cotton cellulose. e cellulose concentration was controlled at 1.0 wt.%. e fibers were treated five times in a high-speed blender at 20000 r/min for 40 s, and a Fenton-oxidized cotton cellulose suspension was obtained. Lastly, the Fenton-oxidized cotton cellulose suspension was treated twenty times with a highpressure homogenization cycle at 60.0 MPa. e MFC suspension was obtained and then stored in a refrigerator at 4.0°C.

Zeta Potential and Carboxyl Group Content of Oxidized Cotton Cellulose.
e oxidized cotton cellulose samples were diluted to 0.5 wt.% and dispersed via a magnetic stirrer (120 r/min) for 30 min at room temperature (25.0°C) with deionized water as the dispersant. e zeta potential of the oxidized cotton cellulose was measured by direct potentiometric titration.

Advances in Materials Science and Engineering
To measure the carboxyl group content of the oxidized cotton cellulose, 20.0 ml of oxidized cotton cellulose suspension containing 1.0 wt.% of the oxidized cotton cellulose was mixed with 50.0 ml of 1 mM NaOH solution and stirred uniformly. en, the oxidized cotton cellulose solution was titrated with 2 mM HCl solution, and the carboxylic acid content was calculated using the following equation [19]: where C COOH is the content of carboxyl groups (μmol/g), 50 is the volume of NaOH solution (ml), C 1 is the concentration of NaOH (mol/L), V is the volume of consumed HCl solution (ml), C 2 is the concentration of HCl (mol/L), and m is the weight of the dry sample of oxidized cotton cellulose (g). e final results were calculated from the average of three parallel measurements for error analysis.

Yield of MFC.
e volume of the MFC suspension was brought to precisely 20.0 ml, and the dried MFC sample was obtained by freeze-drying the suspension for 48 h. e weight of the dried sample was recorded as m 1 . e yield of MFC was calculated using the following equation: where yield was the yield of MFC, m 1 was the weight of the dried MFC sample in 20.0 ml of MFC suspension, and V was the total volume of MFC suspension.

Fourier Transform Infrared Spectroscopy (FTIR)
Analyses. Changes in the chemical structures of the cotton cellulose and MFC samples were investigated by FTIR spectroscopy. Samples were freeze-dried separately and compressed into a thin film tablet before analysis. e samples were ground and mixed with dried potassium bromide (KBr) powder in an agate mortar at a 1 : 100 ratio. en, FTIR analysis was conducted using a Prestige-21 instrument (Shimadzu, JPN) in the range of 4000-400 cm −1 at a 4.0 cm −1 resolution.

Scanning Electron Microscope (SEM) Analyses.
e surface morphologies of the cotton cellulose and the MFC were observed using a scanning electron microscope (S-4800, Hitachi, JPN). e freeze-dried samples were fixed in the sample holder with double-sided, gold-plated, conductive adhesive tape and observed at an accelerating voltage of 5.0 kV.

X-Ray Diffractometer (XRD) Analyses.
e cotton cellulose and MFC were freeze-dried, and the X-ray diffraction patterns were measured using a D8 Advance X-ray diffractometer (Bruker, GER) with Cu Ka radiation at 40 kV and 40 mA. e diffracted radiation was scanned from 5°to 40°(2θ) with a scanning speed of 2.0°/min. e crystallinity index (CrI) was calculated from the ratio of the height of the 002 peaks (I 002 ) to the height of the lowest intensity peak (I am ), as shown in the following equation [20]: where I 002 is the maximum intensity peak of the 002 diffraction at a diffraction angle of approximately 2θ � 22.5°, corresponding to the cellulose crystalline region, and I am is the cellulose amorphous region of the lowest intensity at a diffraction angle of approximately 2θ � 19.2°.

ermogravimetry Analyses (TGA).
e cotton cellulose and the MFC were freeze-dried, and the thermal stability of the samples was tested by a DSC-60A thermogravimetric analyzer (Shimadzu, JPN) in the temperature range of 25.0°C to 600.0°C under a nitrogen steam with a flow rate of 25.0 ml/ min and a heating rate of 20.0°C/min. Approximately 4.0 mg of sample was used for the TGA test.
2.9. Statistical Analysis. All data were expressed as their mean ± SD. Analysis was performed using SAS software ver. 8.1, and the comparison of means was performed using Duncan's test.

e Effect of FeSO 4 ·7H 2 O Dosage on Fenton Oxidation
Efficiency. In this study, the effect of FeSO 4 ·7H 2 O dosage on the Fenton oxidation efficiency of cotton cellulose was evaluated under the following conditions: pH of 3.0, 25.0% cotton cellulose concentration, H 2 O 2 dosage of 0.20 g/g dry oxidized cotton cellulose, reaction temperature of 40.0°C, and reaction time of 150 min. As shown in Figure 1, when the FeSO 4 ·7H 2 O dosage increased from 0.10 g to 0.90 g, the carboxylic acid content first increased and then decreased. With increasing doses of FeSO 4 ·7H 2 O, Fe 2+ acted as a catalyst, and some mass of ·OH was generated by H 2 O 2 . Meanwhile, C2, C3, and C6 of the cotton cellulose were oxidized to aldehyde groups, which are further oxidized into carboxyl groups [21,22]. e accumulation of carboxylic acid reached a maximum of 126.75 μmol/g when the FeS-O 4 ·7H 2 O dosage was 0.70 g. However, when the FeS-O 4 ·7H 2 O dosage was increased further, excess Fe 2+ was oxidized into Fe 3+ , the cotton cellulose quality was reduced, and its color changed to yellow. When the FeSO 4 ·7H 2 O dose increased from 0.10 g to 0.90 g, the zeta potential showed a tendency to decrease initially before leveling off. When the FeSO 4 ·7H 2 O dose was 0.50 g, the absolute value of the zeta potential decreased to 42.89 mV. Copper-cotton cellulose and other metal-based cellulose materials have also gained considerable attention due to their high catalytic activity [23]; the hydroxyl groups on the cellulose structure interact electrostatically with copper nanoparticles to form nanocomposites. When hydrogen peroxide is introduced, Cu 2+ can also act as a catalyst, and ·OH will be generated by H 2 O 2. However, compared with Cu 2+ , Fe 2+ has a greater oxidizing ability and catalytic effect on hydrogen peroxide [24], and Advances in Materials Science and Engineering Fe 2+ is more suitable for the Fenton oxidation system to process cotton and obtain MFCs.

e Effect of H 2 O 2 Dosage on Fenton Oxidation Efficiency.
e effect of H 2 O 2 dosage on the Fenton oxidation efficiency of cotton cellulose was evaluated under the following conditions: pH of 3.0, 25.0% cotton cellulose concentration, FeSO 4 ·7H 2 O dosage of 0.70 g, reaction temperature of 40.0°C, and reaction time of 150 min. As shown in Figure 2, when the H 2 O 2 dosage increased from 0.05 g/g dry oxidized cotton cellulose to 0.25 g/g, the carboxylic acid content first increased and then decreased. Under the catalysis of Fe 2+ , ·OH with strong oxidizing properties will be produced by H 2 O 2 , effectively oxidizing the surface of cotton cellulose [25]. e accumulation of carboxylic acid reached a maximum of 124.52 μmol/g when the H 2 O 2 dosage was 0.20 g/g. Meanwhile, H 2 O 2 could release a mass of H + in an aqueous solution, and these H + ions were freely available to interact with the oxygen atoms in the ether linkages of cellulose [26,27]. e hydrolysis of cotton cellulose was more facile [12]. However, when the H 2 O 2 dosage was increased further, excess H 2 O 2 existed in the reaction system, and ·OOH with low oxidation potential was produced through a side reaction of H 2 O 2 and ·OH [28]. Excess H 2 O 2 also caused ineffective consumption of ·OH because of the reaction (·OH + ·OH ⟶ O 2 + H 2 O) [29], which led to a decrease in the carboxylic acid content of cotton cellulose. When the H 2 O 2 dosage increased from 0.05 g/g to 0.25 g/g, the zeta potential also showed the trend of decreasing first and then increasing. When the H 2 O 2 dosage was 0.20 g/g, the absolute value of the zeta potential decreased to its lowest value, 43.22 mV, with the most stable cotton cellulose suspension.

e Effect of Reaction Temperature on the Fenton Oxidation Efficiency.
e effect of reaction temperature on the Fenton oxidation efficiency of cotton cellulose was evaluated under the following conditions: pH of 3.0, 25.0% cotton cellulose concentration, FeSO 4 ·7H 2 O dosage of 0.7 g, H 2 O 2 dosage of 0.20 g/g dry oxidized cotton cellulose, and a reaction time of 150 min. As shown in Figure 3, with the increase in reaction temperature, the carboxylic acid content initially increased. When the temperature exceeded 40.0°C, the carboxylic acid content of cotton cellulose decreased; the maximum carboxylic acid content was 122.91 μmol/g. A reaction temperature above 40 [30]. When the reaction temperature increased from 20.0°C to 60.0°C, the zeta potential showed a tendency to first decrease and then increase. When the reaction temperature reached 40.0°C, the absolute value of the zeta potential was 41.66 mV.

e Effect of Reaction Time on the Fenton Oxidation
Efficiency.    [12,31]. e FTIR spectra of MFC and cotton cellulose are shown in Figure 5. e 2988-3800 cm −1 region was attributed to hydrogen bonds, which were associated with the -OH stretching vibrations of cellulose [32,33]. e band at 2908 cm −1 represented CH-symmetric stretching vibrations [34]. e band at 1438 cm −1 could be attributed to crystalline absorption [35]. e absorption band at 1356 cm −1 could be The carboxylic acid content (μmol/g) The reaction temperature (°C) The carboxylic acid Zeta potential  Advances in Materials Science and Engineering attributed to -OH bending vibrations [36].
e band at 1148 cm −1 was attributed to the C-O-C stretching vibrations of cellulose [34,37]. e band at 1046 cm −1 was also attributed to C-O-C stretching vibrations.
e absorption band at 872 cm −1 could be ascribed to the β-1,4-glycosidic bonds in cellulose [12]. e FTIR peaks of MFC and cotton cellulose were similar, so MFC had similar characteristics to cotton cellulose. Compared with cotton cellulose, there were some differences in the FTIR spectra of MFC. e band at 1627 cm −1 of MFC could be attributed to the absorption of carbonyl groups, which were oxidized by C-OH groups [38].

SEM.
e morphological and structural changes of cotton cellulose after being treated with Fe 2+ catalyst-preloading Fenton oxidation and a high-pressure homogenization cycle are shown in Figure 6. Compared to cotton cellulose, the microstructure of MFC changed significantly. e fabric structure of cotton cellulose was destroyed, transforming the cotton cellulose into irregular MFCs. It was found that cotton cellulose had a smooth surface and stiff structure (Figures 6(a) and 6(b)). When subjected to Fe 2+ catalyst-preloading Fenton oxidation, the cotton cellulose adsorbed the Fe 2+ , and a complex catalytic system of "Fe 2+ + cellulose" was formed.
en, a mass of ·OH was generated by H 2 O 2 , and the surface of the cotton cellulose was effectively oxidized. e C2, C3, and C6 of the cotton cellulose were oxidized to aldehyde groups. e aldehyde groups were further oxidized into carboxyl groups, and the physical structure of the fiber cell wall was seriously damaged [22,39,40]. en, mechanical treatment with a highpressure homogenization cycle effectively destroyed the firm structure of the cotton cellulose fabric by the disruptive forces of cavitation, turbulence, and shear stress [41]. In addition, the morphology of the fibers changed, and the fiber size decreased. At the same time, the cotton cellulose was sheared into smaller fragments with a decrease in the length and width; the diameter ranged from several nanometers to a few micrometers (Figures 6(c) and 6(d)). e length could reach tens or even hundreds of microns, indicating a high aspect ratio, which provided a performance guarantee for its application in adsorption carriers and reinforcement materials. However, aggregation of the MFC occurred after freeze-drying, and the formation of the aggregate structure was ascribed to growing ice crystals and sublimation during the process of freeze-drying ( Figure 6(c)) [42].

XRD.
e crystallinity of MFC and cotton cellulose was measured and determined by X-ray diffraction (XRD), the results of which are shown in Figure 7. e XRD patterns of MFC and cotton cellulose contained two obvious characteristic peaks at 2θ � 16.2°and 2θ � 22.5°, which were attributed to the 110 crystal face and 002 crystal face, respectively. ese peaks are characteristic of cellulose I [43], indicating that treatment with Fe 2+ catalyst-preloading Fenton oxidation and a high-pressure homogenization cycle did not change the characteristic XRD peaks of cellulose. As shown in Figure 7, the MFC samples had a higher crystallinity (CrI � 83.52%) than cotton cellulose (78.59%) due to the oxidation and shearing of cotton cellulose by treatment with Fe 2+ catalyst-preloading Fenton oxidation and a high-pressure homogenization cycle.

TGA.
e TGA and DTG spectra of MFC and cotton cellulose are shown in Figure 8. It was noted that MFC and cotton cellulose illustrated the same thermal degradation trend. MFC and cotton cellulose samples both displayed a small weight drop at temperatures below 100.0°C due to retained moisture loss [13,44]. In addition, the samples displayed a dramatic weight loss between 290.0°C and 370.0°C corresponding to cellulose thermal decomposition, including the depolymerization, dehydration, decarboxylation, and decomposition of glycosyl units [12]. We observed that the initial degradation temperature of MFC (222.0°C) was lower than that of cotton cellulose (264.0°C). e maximum mass loss rate of MFC also occurred at a lower temperature (342.0°C) than that of cotton cellulose (369.0°C). is was because, compared to cotton cellulose,   Advances in Materials Science and Engineering the particle size of MFC decreased, the specific surface area increased, and the thermal stability decreased [45]. When the temperature was greater than 360.0°C, ashing of MFC and cotton cellulose occurred; the char content of MFC was lower because of the higher crystallinity [46].

Principle of Catalyst-Preloading Fenton Oxidation Technology Based on Microreactors.
e principle of catalystpreloading Fenton oxidation technology based on a microreactor for MFC preparation is shown in Figure 9. e surface of cotton cellulose is electronegative, and Fe 2+ can be effectively adsorbed by it. e "microreactor" consists of a large number of cellulose macromolecules. FeSO 4 ·7H 2 O is used as a catalyst for preloading, Fe 2+ is adsorbed by fibers when preloading occurs, and a complex catalytic system of "Fe 2+ + cellulose" is formed. en, the free Fe 2+ is removed, and hydrogen peroxide is introduced, so the Fenton reaction is limited in the "microreactor." A mass of ·OH is generated by H 2 O 2 when H 2 O 2 is introduced, and the surface of cotton cellulose is effectively oxidized. C2, C3, and C6 of cotton cellulose are oxidized into aldehyde groups. e aldehyde groups are then further oxidized into carboxyl groups, and the physical structure of the fiber cell wall is seriously damaged. At the same time, the β-1,4-glycosidic bond of cellulose is cleaved by ·OH.
en, mechanical treatment with a high-pressure homogenization cycle can effectively destroy the firm structure of the cotton cellulose fabric by the disruptive forces of cavitation, turbulence, and shear effects. Cotton cellulose is sheared into smaller fragments with a decrease in length and width; the diameter ranges from several nanometers to a few micrometers, and the length can reach tens or even hundreds of microns. Finally, the MFC is obtained.

Conclusions
(1) An effective method was developed for the preparation of MFCs from discarded cotton fibers by a combination of Fe 2+ catalyst-preloading Fenton oxidation and a high-pressure homogenization cycle method. (2) An MFC was obtained from dry discarded cotton cellulose by Fenton (Fe 2+ /H 2 O 2 ) oxidation technology. In this process, C2, C3, and C6 of the cotton cellulose were oxidized into carboxyl groups, and the β-1,4glycosidic bond of the cotton cellulose was cleaved. (3) Preloading the Fenton oxidation system with the Fe 2+ catalyst could significantly increase the content of carboxyl groups and reduce the zeta potential of the cotton cellulose, which favored the preparation of MFCs.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.