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Positions in the cellulose chain possible to participate in oxidation reactions (partly adapted from Norimoto 2001)
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A fully bleached birch kraft pulp was treated with acidic hydrogen peroxide in the presence of ferrous ions (Fenton’s reagent) and thereafter treated mechanically in a colloid mill to produce a product containing microfibrillated cellulose (MFC). The produced MFC products were chemically and morphologically characterized and compared with MFC produ...
Citations
... The Fenton-type reaction is a promising oxidation method for cellulose because of its environmental friendliness and relatively low cost (Hellström et al. 2014). The Fenton process used hydrogen peroxide (H 2 O 2 ) as the oxidant and iron (II) ions as the catalyst, which are all available and relatively cheap. ...
... The oxidation rates (carbonyl contents) were considerably higher than the values reported for oxidation with ozone (Ruan et al. 2017), hydrogen peroxide without addition swelling salts (Duan et al. 2020;Hellström et al. 2014;Li et al. 2018), and periodate (Potthast et al. 2007) for the oxidizing agents' loadings in the same range and close to values reported for TEMPO ( (Calderón-Vergara et al. 2020;Praskalo et al. 2009) and Periodate (Sirvio et al. 2011) with higher charge of oxidizing agents (TEMPO and hypochlorite, or periodate). According to the methods (periodate, TEMPO-or Fenton-like, the oxidation rate of cellulose depends on several parameters such as the oxidizing agents, loading rate, catalyst concentration, temperature and even the quality of the raw fiber. ...
... The loading of the oxidation agent per unit mass of cellulose treated is an important parameter which may explain the difference. The results presented by Duan et al. (2020), Hellström et al. (2014) and Li et al. (2018) were obtained with a hydrogen peroxide loading of 80-150, 10-50 and 15-120 kg/ ton of cellulose, respectively, without the addition of swelling salts. Most TEMPO oxidations are carried out with the addition of a salt (NaBr) which could act as swelling agents such as LiCl or NaCl. ...
Cellulose from corn straw was oxidized by Fenton-type reagents (FeSO4. 7H2O or CuCl2. 2H2O) using alkaline metal salts (LiCl; NaCl). Cellulose pre-treatment using alkali metal salts (LiCl; NaCl) coupled with a high H2O2 loading (up to 500 kg per ton of pulp) are used as a novel approach to improve the oxidation rate of oxidized celluloses. The oxidation rate was determined by measuring the carboxylic acid and total carbonyl contents of the oxidized cellulose. The oxidized celluloses were characterized by Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR), X-ray diffraction (XRD) and thermogravimetric analyses (TGA). The results show that the oxidation efficiency was improved by using the metal salts and increasing the H2O2 loading for both catalytic systems (Fe²⁺ or Cu²⁺). The oxidization rate increased gradually with H2O2 loading up to a maximum 500 kg/t of pulp tested without a considerable loss of cellulose structure. The FTIR results revealed that oxidized celluloses exhibit almost similar predominant hydroxy-based structure as native cellulose with the presence of bands in the regions of 1640–1742 cm⁻¹ characteristic of aldehyde and carboxylic acid groups. The XRD results showed that the crystallinity index slightly decreased from 69.4 to 67–68 and 62–64% after oxidation of the cellulose. The equilibrium moisture contents of oxidized celluloses increased compared to the raw cellulose pulp. The moisture curves fit the page model and exhibits a sub-diffusion process.
Graphical Abstract
... Effect of hydrogen peroxide treatment is visible on all bands originating from O-H banding vibrations of cellulose and hemicellulose (bands at 1315, 1335, 1420 and 1456 cm -1 ). This effect can be ascribed to the fact that H2O2 oxidizes secondary hydroxyl groups to ketone [22], which is in accordance with the results obtained from XRD findings: such a reaction would lead to the breaking of hydrogen bond networks in cellulose and lowering of the crystallinity index. Interestingly, hydroxymethyl groups of cellulose (C(6)…O(6)H) related bands centered at 1020 and 985 cm -1 , are not affected, although they could be oxidized to aldehyde or carboxylic acid upon reaction with hydrogen peroxide [22]. ...
... This effect can be ascribed to the fact that H2O2 oxidizes secondary hydroxyl groups to ketone [22], which is in accordance with the results obtained from XRD findings: such a reaction would lead to the breaking of hydrogen bond networks in cellulose and lowering of the crystallinity index. Interestingly, hydroxymethyl groups of cellulose (C(6)…O(6)H) related bands centered at 1020 and 985 cm -1 , are not affected, although they could be oxidized to aldehyde or carboxylic acid upon reaction with hydrogen peroxide [22]. Effect of the NaOH treatment is, on the other hand, easily observable: two strong bands placed at 1727 cm -1 , associated with C=O stretching vibration in acetyl group of hemicellulose and at 1236 7 cm -1 , originating from C-O stretching vibration of carboxylic group of hemicellulose are almost completely disappeared confirming that alkali treatment of jute fibers even in low concentrations and relatively short times (3%, 30 min) leads to the dissolution of the amorphous hemicellulose. ...
... The entire region between 1700 and 1100 cm -1 is hidden by carbon nitride bands, preventing detailed analysis of the possible interaction between amino groups of carbon nitride and OH groups of jute samples. However, some conditional conclusions can be drawn: both, oxidative and alkali treatment "neutralize" secondary hydroxyl groups of cellulose (first by oxidizing them to ketone [22], and the later by ionization to alkoxide [19]), hydrogen bond network of the cellulose is disrupted in both chemically treated samples, Jo and Ja, leaving hydroxymethyl group of the cellulose available for the interaction with the carbon nitride. These groups could be the main anchoring spots of the carbon nitride on the chemically treated jute surface. ...
Non-woven jute (NWJ) produced from the carpet industry waste was oxidized by H2O2 or alkali treated by NaOH and compared with water-washed samples. Changes in the structure of the NWJ, tracked by XRD, showing that both chemical treatments disrupt hydrogen bond networks between cellulose Iβ chains of the NWЈ fibers. Thereafter, nano carbon nitride (nCN) was impregnated, using layer-by-layer technique, onto water washed jute sample (nCN-Jw), NaOH treated sample (nCN-Ja) and H2O2 treated sample (nCN-Jo). Analysis of the FTIR spectra of the impregnated samples, revealed that nCN anchors to the water washed NWЈ surface through hemicellulose and secondary hydroxyl groups of the cellulose. In the case of chemically treated samples nCN is preferentially bonded to the hydroxymethyl groups of cellulose. The stability and reusability of nCN-J samples were assessed by tracking the photocatalytic degradation of Acid Orange 7 (AO7) dye under simulated solar light irradiation. Results from up to ten consecutive photocatalytic cycles demonstrated varying degrees of effectiveness across different samples. nCN-Jo and nCN-Ja samples exhibited declining effectiveness over cycles, attributed to bond instability between CN and jute. In contrast, the nCN-Jw sample consistently maintained high degradation rates over ten cycles, with a dye removal percentage constantly above 90%.
... The Fenton-type reaction is a promising oxidation method for cellulose because of its environmental friendliness and relatively low cost (Hellström et al. 2014). The Fenton process used hydrogen peroxide (H 2 O 2 ) as the oxidant and iron (II) ions as catalyst, which are all available and relatively cheap. ...
... Prior, the HO • radicals attack C 2 , C 3 , and C 6 to form hydroxyalkyl radicals at these positions, and then an alcohol-ketone structure is generated by reaction with oxygen (reaction 1). This ketone structure can then be oxidized to a carboxylic acid by HOO • nucleophile (Hellström al. 2014). The COOH groups can dissociate into anionic groups, which increase the electrostatic repulsion of the cellulose bres according to the mechanism presented in (Fig. 5). ...
Cellulose from corn straw was oxidized by Fenton-type reagents (FeSO 4 . 7H 2 O or CuCl 2 . 2H 2 O) using alkaline metal salts (LiCl; NaCl). Cellulose pre-treatment using alkali metal salts (LiCl; NaCl) coupled with a high H 2 O 2 loading (up to 500 kg per ton of pulp) are used as a novel approach to improve the oxidation rate of oxidized celluloses. The oxidation rate was determined by measuring the aldehyde and carboxylic acid contents of the oxidized cellulose. The oxidized celluloses were characterized by Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR), X-ray diffraction (XRD) and thermogravimetric analyses (TGA). The results show that the oxidation efficiency was improved by using the metal salts and increasing the H 2 O 2 loading for both catalytic systems (Fe ²⁺ or Cu ²⁺ ). The oxidization rate increased gradually with H 2 O 2 loading up to a maximum 500 kg/t of pulp tested without a considerable loss of cellulose structure. The FTIR results revealed that oxidized celluloses exhibit almost similar predominant hydroxyl-based structure as native cellulose with the presence of bands in the regions of 1640-1742 cm ⁻¹ characteristic of aldehyde and carboxylic acid groups. The XRD results showed that the crystallinity index slightly decreased from 69.4 to 67-68 and 62-64% after oxidation of the cellulose. The equilibrium moisture contents of oxidized celluloses increased compared to the raw cellulose pulp. The moisture curves fit the page model and exhibits a sub-diffusion process.
... In thermomechanical pulping (TMP), an interstage treatment between primary and secondary refiners with Fenton's reagent has been found to decrease the refining energy consumption by up to 35 % (Walter, 2013). Fenton chemistry has also been studied as a pre-treatment method to enhance the production of microfibrillated cellulose (Hellström et al., 2014) and cellulose nanofibrils (Duan et al., 2020;Q. Li et al., 2019) from bleached kraft pulp, i.e. with delignified pulp. ...
Sawdust is an abundant high-quality residue from sawmills, representing 20–30 % of sawn products by volume. In this study, the chemical pre-treatment of pine sawdust with Fenton’s reagent, formed from hydrogen peroxide and iron catalyst under moderately acidic conditions, was found to intensify the microfibrillation process in terms of energy consumption and improve the grade of the high-yield lignin-containing microfibrillated cellulose (LMFC) produced.
With a minor yield loss of 5.5 wt.%, Fenton pre-treatment increased the microfibrillation rate and bonding potential of LMFC, indicating that the ultrastructure of the lignocellulose cell walls had been modified. Linear dependency between the growth of specific surface area and energy consumption was seen, i.e. microfibrillation followed Rittinger’s law of comminution. In comparison with the reference without any pretreatment,the total grinding energy consumption to a particle size of 14 µm was about 30 % lower (10.7 vs. 15 MWh/t) while the tensile strength and stiffness of LMFC films were 50 % (100 vs. 66 MPa) and 35 % higher (6.6 vs. 4.9 GPa), respectively. The advantageous effects of Fenton chemistry were assumed to originate from the cleavage of lignin-carbohydrate bonds, mainly between lignin and hemicelluloses. This phenomenon was supported by the substantially increased solubility of polysaccharides in dilute alkali.
The calculated manufacturing costs of LMFCs (using the above-mentioned specifications) was € 850/t, of which the raw material, chemical and electricity costs accounted for 10 %, 2 % and 88 %, respectively. Without any chemical pre-treatment, manufacturing costs were € 1100/t of which raw material accounted for 7 % and electricity 93 %.
... During the homogenization, cellulose microfibrils are delaminated by high shearing forces having both crystalline and amorphous regions with high aspect ratio. However, several drawbacks restricted the commercialization of NFC such as high energy consumption (30,000-70,000 kWh/ton) and clogging during the operation (Hellström, Heijnesson-Hultén, Paulsson, Håkansson, & Germgård, 2014). In the past few decades, many researchers studied on different extraction methods to overcome high energy demand and clogging issues associated with homogenizers such as grinding , microfluidizing (Tozluoğlu, Poyraz, Candan, Yavuz, & Arslan, 2017), ball milling (Baheti, Abbasi, & Militky, 2012), ultrasonication (Chen et al., 2011), cryocrushing. ...
Cellulose is one of the most ubiquitous and abundant natural biopolymer in the world. Nanocellulose are nanoscale cellulose-based materials isolated from trees, annual plants, agricultural residues, and algae or generated by bacteria or tunicates. Among many other sustainable nanomaterials, nanocellulose is drawing increasing interest for use in environmental remediation technologies due to its attractive properties such as excellent mechanical properties, high surface area, rich hydroxyl groups for modification, and natural properties with 100% environmental friendliness. Nanocellulose can be classified as micro/nanofibrillated cellulose (MFC/NFC), micro/nanocellulose crystals (MCC/NCC), and bacterial cellulose (BC). Nanocellulose has an existing potential as reinforcements in polymers, composite materials, and nanocomposites. Nanocellulose can be used in a variety of products such as automobile, aircraft, electronics, medical, textiles, food, optics, packaging materials, gels, composites, pharmaceuticals, bone replacement, dental products, tissue engineering, construction, coatings, wood-based composite materials, paints, papermaking, and cosmetics.
... A slight increase in SSA can be attributed mainly to the removal of hemicelluloses and the associated increased porosity. The SSA of all analyzed fibers (Fig. 4), was about 1-1.5 m2/g, which is consistent with that of birch kraft pulp (Håkansson et al., 2005;Hellström et al. , 2014). ...
This study presents an environmentally friendly process to produce high-purity cellulose (dissolving pulp) from birch wood by combining γ-valerolactone (GVL)/water fractionation and ionic liquid treatment of pulp, IONCELL-P (IP). A paper grade pulp was produced from optimal GVL cook with a similar composition to birch kraft pulp and was bleached with ECF sequence before the hemicelluloses were removed using the IONCELL-P process. The purity of the GVL-IP pulp significantly exceeded that of commercial prehydrolysis kraft (PHK) and prehydrolysis soda-anthraquinone (PH-Soda-AQ) pulps. IONCELL-P extraction removed more than 90% of the hemicelluloses, resulting in a dissolving pulp with a purity of 96% and a high molecular mass fraction, 2.3 times higher than that of a conventional PHK pulp. GVL-IP pulps are suitable not only for regenerated cellulose fibers or films, but also for high-purity, high-viscosity cellulose acetate and ethers, which cannot be produced in an environmentally friendly way by conventional processes.
... In addition, Fenton pretreatment eventuated in slightly lower energy after 2-5 passes through the HPH. In fact, the energy required for five passes through HPH was reported to be 10 kWh/kg for Fenton pretreatment and 12 kWh/kg for endoglucanase enzymatic pretreatment [156]. Therefore, this eco-friendly pretreatment is considered as a promising alternative to enzymatic pretreatment for the preparation of MFC. ...
Cellulose nanofibers (CNFs) and their applications have recently gained significant attention due to the attractive and unique combination of their properties including excellent mechanical properties, surface chemistry, biocompatibility, and most importantly, their abundance from sustainable and renewable resources. Although there are some commercial production plants, mostly in developed countries, the optimum CNF production is still restricted due to the expensive initial investment, high mechanical energy demand, and high relevant production cost. This paper discusses the development of the current trend and most applied methods to introduce energy-efficient approaches for the preparation of CNFs. The production of cost-effective CNFs represents a critical step for introducing bio-based materials to industrial markets and provides a platform for the development of novel high value applications. The key factor remains within the process and feedstock optimization of the production conditions to achieve high yields and quality with consistent production aimed at cost effective CNFs from different feedstock.
... In addition, Fenton pretreatment eventuated in slightly lower energy after 2-5 passes through the HPH. In fact, the energy required for five passes through HPH was reported to be 10 kWh/kg for Fenton pretreatment and 12 kWh/kg for endoglucanase enzymatic pretreatment [156]. Therefore, this eco-friendly pretreatment is considered as a promising alternative to enzymatic pretreatment for the preparation of MFC. ...
Cellulose nanofibers (CNFs) and their applications have recently gained significant attention due to the attractive and unique combination of their properties including excellent mechanical properties, surface chemistry, biocompatibility, and most importantly, their abundance from sustainable and renewable resources. Although there are some commercial production plants, mostly in developed countries, the optimum CNF production is still restricted due to the expensive initial investment, high mechanical energy demand, and high relevant production cost. This paper discusses the development of the current trend and most applied methods to introduce energy-efficient approaches for the preparation of CNFs. The production of cost-effective CNFs represents a critical step for introducing bio-based materials to industrial markets and provides a platform for the development of novel high value applications. The key factor remains within the process and feedstock optimization of the production conditions to achieve high yields and quality with consistent production aimed at cost effective CNFs from different feedstock.
... In addition, Fenton pretreatment eventuated in slightly lower energy after 2-5 passes through the HPH. In fact, the energy required for five passes through HPH was reported to be 10 kWh/kg for Fenton pretreatment and 12 kWh/kg for endoglucanase enzymatic pretreatment [156]. Therefore, this eco-friendly pretreatment is considered as a promising alternative to enzymatic pretreatment for the preparation of MFC. ...
jats:p>Cellulose nanofibers (CNFs) and their applications have recently gained significant attention due to the attractive and unique combination of their properties including excellent mechanical properties, surface chemistry, biocompatibility, and most importantly, their abundance from sustainable and renewable resources. Although there are some commercial production plants, mostly in developed countries, the optimum CNF production is still restricted due to the expensive initial investment, high mechanical energy demand, and high relevant production cost. This paper discusses the development of the current trend and most applied methods to introduce energy-efficient approaches for the preparation of CNFs. The production of cost-effective CNFs represents a critical step for introducing bio-based materials to industrial markets and provides a platform for the development of novel high value applications. The key factor remains within the process and feedstock optimization of the production conditions to achieve high yields and quality with consistent production aimed at cost effective CNFs from different feedstock.</jats:p
... 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. ...
With rapid developments in science and technology, mankind is faced with the dual severe challenges of obtaining needed resources and protecting the environment. The 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 Fe²⁺ 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. Then, the cotton was treated with NaOH solution to obtain cotton cellulose and oxidized using Fenton oxidation to obtain Fenton-oxidized cotton cellulose. The carboxylic acid content of the oxidized cotton cellulose was 126.87 μmol/g, and the zeta potential was −43.42 mV. Then, 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 Fe²⁺ catalyst-preloading Fenton oxidation. The 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). The use of catalyst-preloading Fenton oxidation technology, based on the principles of microreactors, along with high-pressure homogenization, was a promising technique to prepare MFCs from discarded cotton.
1. 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]. The 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. The 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]. The discarded cotton becomes trash and accumulates in waste dumps. The massive accumulation of discarded cotton is a huge waste of valuable resources, and, more importantly, discarded cotton causes severe environmental pollution. The 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-Ag2O) have been developed by the inclusion of silver oxide in polyethersulfone and cellulose acetate polymers [8]. Cellulose/ZrO2 nanohybrids have been synthesized by simple growth of ZrO2 on a cellulose matrix [9]. Lastly, chitosan-coated cellulose filter paper has been used as a support for cobalt nanoparticle preparation [10]. These 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 (Fe²⁺/H2O2) 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. The “microreactor” consists of a large number of cellulose macromolecules. FeSO4·7H2O is used as a catalyst for preloading, Fe²⁺ is adsorbed by fibers when infiltration occurs, and a complex catalytic system of “Fe²⁺ + cellulose” is formed. Then, the free Fe²⁺ 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. Thus, 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 H2O2 dose, FeSO4·7H2O 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. The 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.
2. Materials and Methods
2.1. 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. The 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]. Then, 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), FeSO4·7H2O, and sodium chloride were provided by Tianjin Tianli Chemical Co., Ltd. A solution of H2O2 (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.
2.2. Preparation of MFC
MFC was obtained from dry discarded cotton cellulose using the Fenton (Fe²⁺/H2O2) oxidation described by Duan and coworkers [17]. First, a certain amount of FeSO4·7H2O was dissolved in 300 ml of deionized water. Then, 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 Fe²⁺ was removed by vacuum filtration and pressing, and the complex catalytic system of “Fe²⁺ + 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. Then, a certain volume of H2O2 was added to the flask. The flask was sealed and shaken at a certain temperature in a shaker at 110 r/min. Then, the reaction was stopped, and the product was washed with deionized water until neutral pH to obtain Fenton-oxidized cotton cellulose.
The cellulose concentration was controlled at 1.0 wt.%. The 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 high-pressure homogenization cycle at 60.0 MPa. The MFC suspension was obtained and then stored in a refrigerator at 4.0°C.
2.3. Zeta Potential and Carboxyl Group Content of Oxidized Cotton Cellulose
The 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. The zeta potential of the oxidized cotton cellulose was measured by direct potentiometric titration.
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. Then, 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 CCOOH is the content of carboxyl groups (μmol/g), 50 is the volume of NaOH solution (ml), C1 is the concentration of NaOH (mol/L), V is the volume of consumed HCl solution (ml), C2 is the concentration of HCl (mol/L), and m is the weight of the dry sample of oxidized cotton cellulose (). The final results were calculated from the average of three parallel measurements for error analysis.
2.4. Yield of MFC
The 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. The weight of the dried sample was recorded as m1. The yield of MFC was calculated using the following equation:where yield was the yield of MFC, m1 was the weight of the dried MFC sample in 20.0 ml of MFC suspension, and V was the total volume of MFC suspension.
2.5. Characterization of MFC
2.5.1. 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. The samples were ground and mixed with dried potassium bromide (KBr) powder in an agate mortar at a 1 : 100 ratio. Then, 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.
2.6. Scanning Electron Microscope (SEM) Analyses
The surface morphologies of the cotton cellulose and the MFC were observed using a scanning electron microscope (S-4800, Hitachi, JPN). The 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.
2.7. X-Ray Diffractometer (XRD) Analyses
The 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. The diffracted radiation was scanned from 5° to 40° (2θ) with a scanning speed of 2.0°/min. The crystallinity index (CrI) was calculated from the ratio of the height of the 002 peaks (I002) to the height of the lowest intensity peak (Iam), as shown in the following equation [20]:where I002 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 Iam is the cellulose amorphous region of the lowest intensity at a diffraction angle of approximately 2θ = 19.2°.
2.8. Thermogravimetry Analyses (TGA)
The 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.
3. Results and Discussion
3.1. The Effect of FeSO4·7H2O Dosage on Fenton Oxidation Efficiency
In this study, the effect of FeSO4·7H2O dosage on the Fenton oxidation efficiency of cotton cellulose was evaluated under the following conditions: pH of 3.0, 25.0% cotton cellulose concentration, H2O2 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 FeSO4·7H2O dosage increased from 0.10 g to 0.90 g, the carboxylic acid content first increased and then decreased. With increasing doses of FeSO4·7H2O, Fe²⁺ acted as a catalyst, and some mass of ·OH was generated by H2O2. Meanwhile, C2, C3, and C6 of the cotton cellulose were oxidized to aldehyde groups, which are further oxidized into carboxyl groups [21, 22]. The accumulation of carboxylic acid reached a maximum of 126.75 μmol/g when the FeSO4·7H2O dosage was 0.70 g. However, when the FeSO4·7H2O dosage was increased further, excess Fe²⁺ was oxidized into Fe³⁺, the cotton cellulose quality was reduced, and its color changed to yellow. When the FeSO4·7H2O dose increased from 0.10 g to 0.90 g, the zeta potential showed a tendency to decrease initially before leveling off. When the FeSO4·7H2O 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, Cu²⁺ can also act as a catalyst, and ·OH will be generated by H2O2. However, compared with Cu²⁺, Fe²⁺ has a greater oxidizing ability and catalytic effect on hydrogen peroxide [24], and Fe²⁺ is more suitable for the Fenton oxidation system to process cotton and obtain MFCs.
