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Evaluation of different methods for extraction of nanocellulose from yerba mate residues
Carbohydrate Polymers 218 (2019) 78–86
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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Evaluation of different methods for extraction of nanocellulose from yerba mate residues
T
Marcos Aurélio Dahlem Júnior , Cleide Borsoi, Betina Hansen, André Luís Catto ⁎
Centro de Ciências Exatas e Tecnológicas, Universidade do Vale do Taquari – Univates, Lajeado, RS, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Steam explosion Chemical treatment Characterization Cellulose nanofiber
In this work, we evaluate the production of nanocellulose from yerba mate sticks (YMS) using soft chemical and steam explosion treatments. The nanocellulose is characterized by chemical characterization, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The main results showed that after the chemical treatment and steam explosion, the YMS fiber reached diameters of 11–15 nm and aspect ratios (L/D) of 12–24. The XRD results showed that there is an increase in the index of crystallinity of up to 35% when compared to raw YMS. We discover that it is possible to obtain cellulose nanofibers (CNF) from the YMS without the use of alkaline treatment, which reduces the generation of liquid waste. Thus, the production of CNF by means of acid hydrolysis, bleaching and steam explosion represents an alternative route.
1. Introduction Growing concern regarding the preservation of the environment and the advent of the sustainability debate has made the indiscriminate use of resources from natural sources an important topic for discussion. The integration of social, economic and environmental debates is necessary and leads to studies aimed at the conservation of resources and the use of renewable sources that reduce the impacts caused to the environment and the economy (Abdul Khalil et al., 2016). Using resources from the environment, there are opportunities for studies related to the use of various agroindustrial residues, such as sugarcane bagasse, grape stalks, tobacco rods, wheat straw, rice hulls and yerba mate sticks (YMS), and these are becoming more prominent in the academic community (Phanthong et al., 2018). These new materials, which can be obtained from low-cost biomasses, provide sustainable and economically correct alternatives, since, in addition to their high availability in nature, they represent a viable option for the production of materials with higher added value (Kunaver, Anžlovar, & Žagar, 2016; Mondal, 2017). Yerba mate (Ilex paraguariensis, St. Hill) is a subtropical plant, commonly found in South America, especially in countries like Paraguay, Argentina and Brazil. It is used due to its antioxidant, diuretic, anti-inflammatory and medicinal properties, in addition to being widely used in these regions through a drink popularly known in Brazil as "chimarrão", consumed from the extract of dry leaves
immersed in hot water (Arrieta, Peponi, López, & Fernández-García, 2018; Deladino et al., 2013). According to the Brazilian Institute of Geography and Statistics (IBGE, 2018), the production of yerba mate in Brazil in 2016 was ˜346,000 tons. During the processing of the leaves of the plant, the residue of yerba mate, commonly used as fertilizer, can be burned in boilers for energy generation or also in the production of coal, being generated in considerable quantities, representing up to 2% mass production (Gonçalves, Guerreiro, & Bianchi, 2007). In general, agroindustrial residues are composed of 40–50% cellulose, 20–30% hemicellulose and 10–25% lignin (Fortunati, Luzi, Puglia, & Torre, 2016). As one of the most abundant natural polymers, cellulose is renewable and biodegradable, and is obtained through diversified sources (wood, cotton, hemp, straw, sugarcane bagasse, agroindustrial waste and other plant materials). Cellulose is used in the textile, pharmaceutical, food and paper industries (Li et al., 2012). Composed of remarkable physical and mechanical properties, cellulose is characterized by crystalline and amorphous structures. The amorphous region contributes to fiber flexibility, while the crystalline region ensures rigidity and material resistance. When extracted from cellulose microfibrils, the crystalline and amorphous region may result in nanocellulose (Mishra, Sabu, & Tiwari, 2018; Phanthong, Guan, Ma, Hao, & Abudula, 2016, 2018). Nanocellulose consists of cellulose particles that have at least one of
Corresponding author. E-mail addresses: [email protected] (M.A. Dahlem), [email protected] (C. Borsoi), [email protected] (B. Hansen), [email protected] (A.L. Catto). ⁎
https://doi.org/10.1016/j.carbpol.2019.04.064 Received 28 March 2019; Received in revised form 17 April 2019; Accepted 17 April 2019 Available online 25 April 2019 0144-8617/ © 2019 Elsevier Ltd. All rights reserved.
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Abraham et al. (2011), where we chose to use only one fiber evaluating the behavior of the YMS fiber in different types of treatments. It was aimed to use the steam explosion at the beginning of the process to solubilize the hemicellulose and defibrillate the fiber, so that in the following stages there would be better interaction with the chemical processes. The YMS-ASBHS and YMS-ABH samples were treated with 3% (w/v) NaOH (ratio 1:5) at 30 °C for 4 h under constant stirring. Then, the samples were washed in distilled water until neutral pH. The SE was applied only to the samples YMS-ASBHS and YMS-SBHS using an autoclave (PRISMATEC, Mod. CS75, Brazil) maintained at 127 °C and 1.5 bar pressure for 1 h, releasing all the pressure at once immediately and repeating this process three times. The samples were removed from the autoclave and washed in water. All the samples were bleached using 100 mL of hydrogen peroxide 10% (w/v) at 50 °C for 2 h under constant stirring. After that, the samples were again washed in distilled water until neutral pH. The hydrolysis acid treatment was divided in two methods, treating the YMS-ASBHS and YMS-SBHS samples with 5% (w/v) oxalic acid in an autoclave until it attained 127 °C and 1.5 bar pressure for 1 h, which was immediately released and the process repeated three times. The YMS-ABH sample was treated with 10% (w/w) oxalic acid at 60 °C for 6 h. For this sample, a higher concentration and time were used since the SE step was not applied in conjunction with AH. After this process, all samples were washed in distilled water until they returned to pH neutrality. The collected nanofibrils were suspended in water and kept stirring with a mechanical stirrer of type (HEIDOLPH, Mod. DIAX 900, Germany) at 26,000 rpm for ˜5 min until the fibers are uniformly dispersed.
their dimensions smaller than 100 nm, generating a great interest in several applications, due to its excellent mechanical, chemical and optical properties (Abitbol et al., 2016; Niu et al., 2017; Niu et al., 2018). These particles can be divided into three groups, from the way they are obtained, presenting different characteristics and properties. Cellulose nanofibers (CNF) are usually obtained through mechanical, chemical or chemical-mechanical treatments. Cellulose nanocrystals are derived from chemical hydrolysis and bacterial nanocellulose is acquired by bacterial synthesis through specific microorganisms (Klemm et al., 2018; Nechyporchuk, Belgacem, & Bras, 2016; Nie et al., 2018; Tao, Zhang, Wu, Liao, & Nie, 2019). Various treatments can be used to obtain nanocellulose, whether chemical or mechanical. Mechanical grinding, steam explosion, cryogenic grinding and homogenization by high pressure are classified as mechanical processes in the preparation of nanocellulose, such as alkali or acid treatments, ionic liquid and enzymatic hydrolysis, which represent the common chemical processes used (Abraham et al., 2011; Kumar, Ha, Verma, & Tiwari, 2018; Song et al., 2018). The combined use of mechanical and chemical methods aims to reduce the amount of energy used in mechanical processes, making the production of nanocellulose more economically viable (Klemm et al., 2018; Mishra et al., 2018). Thus, this work aims to evaluate different methods of obtaining CNF from an agroindustrial residue, using as a raw material YMS, especially for its wide availability in the southern region of Brazil. The extraction of CNF will be evaluated by chemical processes, using alkaline treatment (AT), bleaching (BT) and acid hydrolysis (AH), and mechanical processes, such as steam explosion (SE).
2.3. Characterization
2. Experimental
2.3.1. Chemical composition The YMS sample was characterized in relation to its natural components, and the samples were prepared in triplicate according to ABNT (NBR 6923). The extractive (TAPPI T 204 cm-97), ashes (TAPPI T 211 om-02), lignin (TAPPI T 222 om-02) and α-cellulose (TAPPI T 203 cm99) contents were carried out and adapted according to the standards of the Technical Association of Pulp and Paper Industry (TAPPI), and the hemicellulose content was obtained by calculating the mass difference.
2.1. Materials The YMS used in this study undergo the production process and were previously grinded by the company Elacy, located in Venâncio Aires - RS, Brazil. The chemicals used in fiber extraction and in the preparation of the nanocellulose were acetone, sulfuric acid, hydrogen peroxide, sodium hydroxide and oxalic acid from Labsynth in São Paulo. It is noteworthy that all the reagents used were analytical grade.
2.3.2. Fourier transform infrared spectroscopy The FTIR analysis was recorded by a spectrophotometer (PERKIN ELMER, Mod. Frontier, USA) with an attenuated total reflectance (ATR) accessory. The wavelength range was 4000–500 cm−1 and each spectrum was obtained using 32 scans.
2.2. Isolation of cellulose nanofibrils The fibers obtained from YMS were classified in uniform size using a sieve system with 250 mesh Tyler (63 μm) and divided into four different samples, according to the sequence of treatments performed: YMS-RAW (untreated), YMS-ASBHS (AT, SE, BT and AH with SE), YMSSBHS (SE, BT and AH with SE) and YMS-ABH (AT, BT and AH). Table 1 shows the samples and the treatments performed for the extraction of the nanocellulose. The methodology of nanocellulose extraction was adapted from
2.3.3. X-ray diffraction XRD measurements were performed using a diffractometer (Phillips, Mod. X’Pert MDP, Netherlands) at 40 kV and 30 mA with monochromatic CuK α radiation of λ = 0.1542 nm. Intensities were measured in the 5° < 2θ < 40° (0.05°/2 s) range. According to Canevarolo (2003), the concentration of crystalline cellulose can be estimated by the method of Ruland (1961), which considers a relation of the areas in the crystalline region and the total area. The peaks in regions 110, -110 and 002 were separated and obtained through the Peak Analyzer from the OriginLab® software given in Eq. (1):
Table 1 Methods of treatments used to obtain nanocellulose from YMS samples. Treatments Steps
Sample YMSRAW
Untreated Alkaline treatment Steam explosion Bleaching Acid hydrolysis Acid hydrolysis + Steam explosion
X – – – – –
YMSASBHS – X X X – X
YMSSBHS – – X X – X
YMS-ABH
CI% =
– X – X X –
Ac Atotal
(1)
where CI is crystallinity index, ƩAc is the area of the diffraction peak related to the crystalline plane and ƩAtotal refers to the total area. 2.3.4. Dynamic light scattering The particle diameter of the YMS samples was measured at 20 °C using a dynamic light scattering (DLS) analyzer (MALVERN, Mod. Zetasizer Nano ZS, UK) provided with a He-Ne laser beam at 658 nm and a detection angle of 173°. The definition of particle size was given
Meaning of each sample letter: (A) alkaline - (S) steam - (B) bleaching – (H) hydrolysis. 79
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by the average value of at least three consecutive runs. 2.3.5. Transmission electron microscopy The morphological characterization of YMS samples was obtained using a transmission electron microscope (TECNAI, Mod. G2 T20. FEI, Netherlands) with an acceleration voltage of 75 kV. The nanofibril solution was sonicated for 1 h before being deposited in a microgrid covered with a thin film of carbon (∼200 nm). The deposited fibers were subsequently stained with a 2% uranyl acetate solution to improve the microscopic resolution. 2.3.6. Atomic force microscopy Using an AFM (BRUKER, Mod. Dimension Icon PT, USA), the analysis was performed by 670 nm laser multifunctional scanning. Samples were prepared in an ultrasonic bath for 1 h and then collected for 2 μL of supernatant. Afterwards, it was deposited on a mica plate. Finally, the samples were dried at room temperature. 3. Results and discussion 3.1. Chemical characterization In the literature, there are few studies that characterize YMS fibers or even physico-chemical analyzes geared specifically to this type of residue. Table 2 shows the chemical composition of the untreated YMS, based on the TAPPI standards. The structure of the YMS cellulosic fibers is composed of cellulose, lignin and hemicellulose, as well as ash and extractives that present varying amounts depending on their origin (Siqueira, Bras, & Dufresne, 2010). Lignin and hemicellulose normally constitute 15–25% and 20–30% (by weight), respectively, of the total chemical composition of the cellulosic fibers (McKendry, 2002). However, a great variability in its chemical composition may occur depending on the type of lignocellulosic material. The lignocellulosic composition may vary depending on its derivation, such as hardwood or softwood. In addition, factors, such as plant species, climate, agricultural conditions and land, also have a significant impact on the composition (Frollini, Bartolucci, Sisti, & Celli, 2013). However, the YMS-RAW sample presented relatively low concentrations of alphacellulose when compared to other hardwoods, where some authors present concentrations close to 40% (Chin, Sung Ting, Ong, & Omar, 2018). Thus, this difference can be related to some of the factors, such as the age of the trees in the harvest, since with the yerba mate tree, the harvest can begin with trees of 4–6 years, but only when the trees reach 10–12 years, it will be considered stable for harvesting (Zanon, 1988). 3.2. Fourier transform infrared spectroscopy To study the changes in the chemical composition of untreated YMS and after AT, SE, BT and AH, FTIR-ATR spectroscopy was used to investigate the differences in the YMS-RAW, YMS-ASBHS, YMS-SBHS and YMS-ABH samples, which are shown in Fig. 1. The analysis of YMS fibers reveals that the bands at around 3350 and 2916 cm−1 exhibit stretching vibrations of hydrogen-bonded hydroxyl groups (OeH) and alkyl group vibration elongation, respectively (Li et al., 2012). YMS fibers also showed a characteristic vibration of hemicellulose (C]O) at ˜1737 cm-1 (Sun, Xu, Sun, Fowler, & Baird, 2005). The bands corresponding to 1604 and 1508 cm-1 refer possibly to the stretching (C]C)
Fig. 1. FTIR spectrum of YMS fibers with different treatments: (A) YMS-ASBHS, (B) YMS-SBHS and (C) YMS-ABH.
Table 2 Chemical composition of YMS-RAW fibers. Material
α-cellulose (%)
Hemicellulose (%)
Lignin (%)
Extractives (%)
Ashes (%)
YMS-RAW
34.85 ± 0.28
24.77 ± 0.16
25.78 ± 0.01
10.11 ± 0.34
4.49 ± 0.17
± Mean SD (n = 3). 80
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of the aromatic bonds of the lignin. The peaks at around 1457 and 1420 cm-1 (−CH2) correspond to lignin deformation and bending of the intermolecular hydrogen attraction at the C6 group (Morán, Alvarez, Cyras, & Vázquez, 2008). The peak at 1317 cm−1 is related to the polysaccharide groups (Phanthong et al., 2016), while peaks at ˜1250 cm−1 relate to the C–OeC stretching bonds of lignin or C–O of hemicellulose (Mondragon et al., 2014). The peak of 1157 cm−1 (C–H), related to the alt-of-plane may indicate that there is lignin remaining (Yang, Kenny, & Puglia, 2015). Regarding the cellulose, the peak detected at ˜897 cm−1 is related to the (C–H) rocking vibration of the βglycosidic bonds between the glucose units (Phanthong et al., 2016). It was possible to observe changes along the infrared spectra of the YMS-ASBHS, YMS-SBHS and YMS-ABH samples submitted to the treatments. There was an increase in the intensity of the peaks in the 3350 and 2950 cm−1 regions in all samples, mainly for the AT and AH, which may be related to an exposure of the hydroxyl groups of the cellulose molecules, enabling water absorption and moisture through the hydrogen bonds (Thomas et al., 2015). It is also observed in Fig. 1(A) that for the YMS-ASBHS sample, there is an increase in peak intensity in the region of 1050 cm−1, related to the presence of cellulose, thus indicating a relative increase in the amount of cellulose in the sample that went through the AT and SE, therefore being an indication of the efficiency of this method in the removal of hemicellulose, lignin and other fiber compounds. For the YMS-SBHS sample in Fig. 1(B), which has no AT and an initial SE step, this peak shows a lower intensity, already for the treatment of YMS-ABH sample, without SE but with AT and AH, shown in Fig. 1(C), there is an increase again in the cellulose peak in this region, although less intense than in the YMSASBHS sample. These results showed that there were components and residual binding materials removed, leading to an increase in the cellulose content of the fiber. The intensities of peaks at 3420, 2910 and 1640 cm−1 were also enhanced with increasing treatment processes, in agreement with previous reports (Zhao et al., 2013). Peaks in the 1750 and 1604 cm−1 regions showed slight decreases in intensities as they pass through chemical treatments, with the smallest peak in the BT and AH with SE, suggesting that there was a decrease in the amount of hemicellulose and lignin present in the samples after the treatments. This can be explained by hemicellulose solubilization and lignin depolymerization (Feng et al., 2018). There are other characteristic peaks of lignin and hemicellulose, such as those in the regions of 1508 and 1288 cm−1. The peak at 1508 cm−1 showed a weak decrease throughout the treatments, but the peak at 1288 cm−1 showed an absence of intensity after treatments, suggesting a total removal of the
C–OeC or C–O stretches attributed to lignin or hemicellulose (Feng et al., 2018). The peak in the 896 cm−1 region associated with cellulose shows an increasing tendency throughout the treatments and SE, indicating the removal of materials with strong bonds and a probable exposure of the pure cellulosic fibers (Feng et al., 2018). Similar results were observed by Thomas et al. (2015) when studying the preparation and characterization of jute nanofibers during the production of nanocomposites and by Abraham et al. (2011) when extracting nanofibers from lignocellulosic materials, demonstrating that the use of SE and chemical treatment can partially remove lignin, hemicellulose and other materials. 3.3. X-ray diffraction In order to analyze the crystalline behavior of the YMS fiber during chemical treatments and SE, XRD analysis was performed on untreated and treated fibers. By removing the non-cellulosic constituents of the fibers by chemical treatment, the crystallinity index (CI) will change. The lignocellulosic fibers are constituted of crystalline and amorphous regions, as can be observed in the diffractogram shown in Fig. 2. The YMS fiber shows an increase in the intensity of the crystalline peaks that undergo chemical treatments and SE when compared to the untreated fiber (YMS-RAW). The increased intensity in the crystalline region suggests a decrease in the concentration of components, such as hemicellulose, lignin and noncrystalline cellulose. This may be explained by the ability to solubilize the amorphous region with the AH treatment. In addition, the use of the SE with AH may improve the concentration of cellulose type I (crystalline). Thus, with increasing peaks in the crystalline region, the results indicate that the hydrolysis occurred preferentially in the amorphous region. Similar results were reported by several authors (Feng et al., 2018; Li et al., 2012; Thomas et al., 2015). The effect of the crystalline cellulose concentration on the treatments submitted to the samples can be verified in Fig. 2. It can be observed that the YMS-RAW sample already had a low intensity peaks in the regions of 22.1° (crystalline plane 002) and 14.8° (crystalline plane 110) 2θ, representing cellulose type I. After the samples were submitted to chemical treatments and SE, there was an increase of intensity in these peaks, mainly in the crystalline region (002) (Cherian et al., 2010). Initially, the YMS-RAW sample had ˜40.62% of crystalline cellulose. After the treatments, the YMS-ASBHS sample reached 75.99%, YMSSBHS reached 66.19% and YMS-ABH reached 64.36%. These results
Fig. 2. X-ray diffractometry of different treatments of YMS fibers. 81
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size distribution, and finally, the YMS-ABH sample showed peaks in the range of 24–50 nm with a 31% size distribution, and in the range of 142–295 nm with a 65.8% size distribution. Furthermore, this sample also showed a peak in the 5500 nm region with a 3.1% size distribution. According to Espino et al. (2014), nanocellulose has an easily agglomerated character when disposed in aqueous solution, so it is suggested that this localized agglomeration in the 5500 nm region may be due to this nanocellulose capacity. Similar results were observed by Arrieta et al. (2018) when producing bionanocomposite films with yerba mate residues, by Hernandez, Ferreira, and Rosa (2018) when analyzing nanocrystals obtained from corn straw by chemical treatments and by Kunaver et al. (2016) when isolating nanocellulose from cellulosic raw materials.
Table 3 Crystalline and amorphous contribution in different samples of YMS. Samples
CI (%)
Amorphous (%)a
YMS-RAW YMS-ASBHS YMS-SBHS YMS-ABH
40.62 75.99 66.19 64.36
59.38 24.01 33.81 35.64
a
The amorphous phase was estimated by the difference: 100% – CI (%).
suggest that the YMS-SBHS and YMS-ABH samples obtained lower concentrations of crystalline cellulose than the YMS-ASBHS, since they did not undergo AT and SE, respectively. The AT and SE are usually related to the total or partial removal of hemicellulose and lignin, which are amorphous polymers (Frollini et al., 2013). Thus, the YMSASBHS sample covered AT and SE, with better removal of the amorphous constituents, suggesting a better packaging of the cellulose chains of the fibers (Buson, Melo, Oliveira, Rangel, & Deus, 2018; Mondragon et al., 2014). In addition, the YMS-ASBHS sample shows two peaks in the 14.8° (110) and 16.4° (-110) 2θ regions, characteristic of materials with high cellulose concentrations, since the presence of materials such as lignin, hemicellulose and amorphous cellulose tends to unify this region and form only one peak (Spinacé, Lambert, Fermoselli, & De Paoli, 2009). Table 3 shows the results of the crystallinity index and the amorphous phase of the YMS samples.
3.5. Transmission electron microscopy In order to observe the morphology and to evaluate the size of the YMS samples after the different treatments, TEM analysis was performed. To estimate the sample size, it was considered as base some fibers isolated in the micrographs, which can be seen in Fig. 4. Fig. 4(A) showed that the YMS-ASBHS sample had a mean length and diameter of 228 and 15.2 nm, respectively. The YMS-SBHS sample (Fig. 4(B)) showed 279 and 11.4 nm and finally the YMS-ABH sample (Fig. 4(C)) had dimensions of 257 and 11.5 nm. On this basis, it is possible to observe that all samples obtained stems and needle-like nanocellulose, especially the YMS-ASBHS and YMS-SBHS samples, presenting a different aspect of the YMS-ABH sample. The TEM image confirmed the presence of nanocellulose in the form of CNF. The diameter of the CNF was in the range of 10–100 nm showing that this combination of the SE process in autoclave and chemical treatments effectively defibrillated the cellulose present in the nanoscale fiber. The diameter of the CNF in this study was similar to those previously reported for CNF produced from banana pseudo-stem and cotton, which was in the range of 5–60 nm (Faradilla et al., 2017; Ferreira, Pinheiro, Gouveia, Thim, & Lona, 2018; Klemm et al., 2018; Roohani et al., 2008). From these results, it is suggested that the SE process exhibits better defibrillation of the sample. Moreover, the YMS-SBHS sample showed similar results to the YMS-ASBHS sample, although it did not undergo the AT step, which can thus reduce a step in the nanocellulose production process. Since it is considered as one of the most important parameters for the applicability of nanofibers in composites, the aspect ratio between the fiber length and diameter (L/D) was estimated in this study. Aspect ratios of 12, 24 and 23 were determined for YMS-ASBHS, YMS-SBHS and YMS-ABH, respectively. The YMS-ASBHS sample presented results similar to those found by Teixeira et al. (2010), who obtained a L/D
3.4. Dynamic light scattering DLS analysis is commonly used to verify the size and stability of particles in the medium and is usually related to the Brownian motion of the nanoparticles (Morais et al., 2013). In addition, one of the advantages of using this technique is its fast, easy and reproducible results for spherical particles in suspensions (Braun, Dorgan, & Chandler, 2008; Chu & Liu, 2000). However, when used to analyze stem or fibril-like structures, there are some limitations in accurate size estimation, in addition to strongly depending on the orientation of the fibers in the fluid (Frone, Panaitescu, & Donescu, 2011). Some authors use correlations of data obtained with techniques such as TEM to verify the mean particle size (Kunaver et al., 2016). The DLS results of the YMS samples treated in solution are shown in Fig. 3, presenting peaks preferably in the regions below 300 nm. The YMS-ASBHS sample showed a peak of lower intensity in the range of 0.6–1.3 nm with 24% of size distribution and a peak of greater intensity in the range of 105–220 nm with a size distribution of 76%. The YMSSBHS sample showed only a peak in the range of 50–91 nm with 100%
Fig. 3. Particle size distribution of YMS samples by DLS. 82
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Fig. 4. TEM micrographs of YMS-ASBHS (A), YMS-SBHS (B) and YMS-ABH (C) CNF.
ratio of 10–14 for regular cotton fiber and Capadona, Shanmuganathan, Seidel, Rowan, and Weder (2009) and Shanmuganathan, Capadona, Rowan, and Weder (2010) who obtained a ratio from 11 to 13 for microcrystalline cellulose. The YMS-SBHS and YMS-ABH samples presented similar results to those of Morais et al. (2013), where a ratio of 20–24 for cotton linter was obtained. Despite the presence of agglomerated regions, which can be attributed to a low negative charge on the surface that repels nanocrystals to non-agglomeration (Lima, de, & Borsali, 2004), the process proved to be efficient for the isolation of nanocellulose from the YMS fibers. Similar results were observed by Morais et al. (2013) when extracting and characterizing nanocellulose from raw cotton linter.
of 29.5 nm, ranging from 11.5–60 nm. YMS-SBHS had a mean diameter of 42.6 nm, ranging from 24.8–50.4 nm and YMS-ABH had a mean diameter of 63.7 nm, with a variation of 54.7–75.6 nm. The YMSASBHS and YMS-SBHS samples, which undergo the AH treatment with the SE, showed smaller diameters, suggesting a better removal of components such as lignin and hemicellulose. However, it was not possible to determine the length of the samples due to the agglomeration of the nanofibers. According to Hernandez et al. (2018), this agglomeration problem can be fixed using a different methodology to maintain a stable colloidal suspension, for example, the use of different pH values and sulfates to obtain high ionic strengths, as used by Espinosa, Sánchez, Otero, Domínguez-Robles, and Rodríguez (2017). Another suggestion would be the use of a mica substrate for a better AFM image definition, as reported by Boluk, Lahiji, Zhao, and McDermott (2011). Similar results were observed in several types of materials, such as wheat straw fibers with diameters between 10 and 80 nm found by Rahimi and Behrooz (2011), soybean stock with diameters between 50 and 100 nm found by Gamelas, Pedrosa, Lourenço, and Ferreira (2015) and pineapple leaf with diameters between 5 and 60 nm observed by Cherian et al. (2010).
3.6. Atomic force microscopy AFM was performed with the intention of corroborating with the DLS and TEM analyzes, which aimed to demonstrate the size of the samples and their morphology. Fig. 5 shows the images obtained by AFM, where it can be seen that the treated YMS samples showed a fibrillar appearance. It was possible to determine the mean diameter of the samples, with the YMS-ASBHS sample having an average diameter
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Fig. 5. AFM images of YMS-ASBHS (A), YMS-SBHS (B) and YMS-ABH (C) CNF.
4. Conclusion
Acknowledgements
This study investigated the extraction of nanocellulose from YMS fibers from mild chemical treatments and SE. The FTIR spectra showed differences in the peaks after the treatment, where a decrease was observed in components such as lignin and hemicellulose. In addition, it was observed that even the YMS-SBHS sample without alkaline treatment was able to have considerable decreases when compared to the other samples. After the treatments, it was possible to notice an increase in the crystallinity index of the samples in relation to the raw fiber, with an increase of 35.37% for YMS-ASBHS, 25.57% for YMS-SBHS and 23.74% for YMS-ABH, demonstrating that the use of the AT and SE steps favor the isolation of the crystals. DLS analysis showed that all samples obtained a size distribution below 300 nm, with the YMS-SBHS sample having a better homogeneity in the distribution, with a size between 50 and 91 nm. The TEM and AFM assays corroborate the other characterizations, demonstrating the isolation of nanocellulose in all samples, but the YMS-ASBHS and YMS-SBHS samples obtained better aspects and results when compared to the YMS-ABH sample, again suggesting that the use of the SE assists in the removal of amorphous components. Finally, it was possible to obtain CNF in the samples that were used the SE stage, even in the YMS-SBHS that did not undergo the AT, suggesting a probable decrease of steps in obtaining nanocellulose for YMS fibers.
The authors would like to thank Universidade do Vale do Taquari (Univates) for financial support, Universidade Federal do Rio Grande do Sul (UFRGS) and Instituto Federal de Educação, Ciência e Tecnologia do Rio Grande do Sul (IFRS-Farroupilha) for their support in characterization analyzes and Elacy company manufacturer for donating the residues of yerba mate. References Abdul Khalil, H. P. S., Davoudpour, Y., Saurabh, C. K., Hossain, M. S., Adnan, A. S., Dungani, R., ... Haafiz, M. K. M. (2016). A review on nanocellulosic fibres as new material for sustainable packaging: Process and applications. Renewable and Sustainable Energy Reviews, 64, 823–836. https://doi.org/10.1016/j.rser.2016.06. 072. Abitbol, T., Rivkin, A., Cao, Y., Nevo, Y., Abraham, E., Ben-Shalom, T., ... Shoseyov, O. (2016). Nanocellulose, a tiny fiber with huge applications. Current Opinion in Biotechnology, 39(I), 76–88. https://doi.org/10.1016/j.copbio.2016.01.002. Abraham, E., Deepa, B., Pothan, L. A., Jacob, M., Thomas, S., Cvelbar, U., ... Anandjiwala, R. (2011). Extraction of nanocellulose fibrils from lignocellulosic fibres: A novel approach. Carbohydrate Polymers, 86(4), 1468–1475. https://doi.org/10.1016/j. carbpol.2011.06.034. Arrieta, M. P., Peponi, L., López, D., & Fernández-García, M. (2018). Recovery of yerba mate (Ilex paraguariensis) residue for the development of PLA-based bionanocomposite films. Industrial Crops and Products, 111(October 2017), 317–328. https:// doi.org/10.1016/j.indcrop.2017.10.042.
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Evaluation of different methods for extraction of nanocellulose from yerba mate residues
T
Marcos Aurélio Dahlem Júnior , Cleide Borsoi, Betina Hansen, André Luís Catto ⁎
Centro de Ciências Exatas e Tecnológicas, Universidade do Vale do Taquari – Univates, Lajeado, RS, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Steam explosion Chemical treatment Characterization Cellulose nanofiber
In this work, we evaluate the production of nanocellulose from yerba mate sticks (YMS) using soft chemical and steam explosion treatments. The nanocellulose is characterized by chemical characterization, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The main results showed that after the chemical treatment and steam explosion, the YMS fiber reached diameters of 11–15 nm and aspect ratios (L/D) of 12–24. The XRD results showed that there is an increase in the index of crystallinity of up to 35% when compared to raw YMS. We discover that it is possible to obtain cellulose nanofibers (CNF) from the YMS without the use of alkaline treatment, which reduces the generation of liquid waste. Thus, the production of CNF by means of acid hydrolysis, bleaching and steam explosion represents an alternative route.
1. Introduction Growing concern regarding the preservation of the environment and the advent of the sustainability debate has made the indiscriminate use of resources from natural sources an important topic for discussion. The integration of social, economic and environmental debates is necessary and leads to studies aimed at the conservation of resources and the use of renewable sources that reduce the impacts caused to the environment and the economy (Abdul Khalil et al., 2016). Using resources from the environment, there are opportunities for studies related to the use of various agroindustrial residues, such as sugarcane bagasse, grape stalks, tobacco rods, wheat straw, rice hulls and yerba mate sticks (YMS), and these are becoming more prominent in the academic community (Phanthong et al., 2018). These new materials, which can be obtained from low-cost biomasses, provide sustainable and economically correct alternatives, since, in addition to their high availability in nature, they represent a viable option for the production of materials with higher added value (Kunaver, Anžlovar, & Žagar, 2016; Mondal, 2017). Yerba mate (Ilex paraguariensis, St. Hill) is a subtropical plant, commonly found in South America, especially in countries like Paraguay, Argentina and Brazil. It is used due to its antioxidant, diuretic, anti-inflammatory and medicinal properties, in addition to being widely used in these regions through a drink popularly known in Brazil as "chimarrão", consumed from the extract of dry leaves
immersed in hot water (Arrieta, Peponi, López, & Fernández-García, 2018; Deladino et al., 2013). According to the Brazilian Institute of Geography and Statistics (IBGE, 2018), the production of yerba mate in Brazil in 2016 was ˜346,000 tons. During the processing of the leaves of the plant, the residue of yerba mate, commonly used as fertilizer, can be burned in boilers for energy generation or also in the production of coal, being generated in considerable quantities, representing up to 2% mass production (Gonçalves, Guerreiro, & Bianchi, 2007). In general, agroindustrial residues are composed of 40–50% cellulose, 20–30% hemicellulose and 10–25% lignin (Fortunati, Luzi, Puglia, & Torre, 2016). As one of the most abundant natural polymers, cellulose is renewable and biodegradable, and is obtained through diversified sources (wood, cotton, hemp, straw, sugarcane bagasse, agroindustrial waste and other plant materials). Cellulose is used in the textile, pharmaceutical, food and paper industries (Li et al., 2012). Composed of remarkable physical and mechanical properties, cellulose is characterized by crystalline and amorphous structures. The amorphous region contributes to fiber flexibility, while the crystalline region ensures rigidity and material resistance. When extracted from cellulose microfibrils, the crystalline and amorphous region may result in nanocellulose (Mishra, Sabu, & Tiwari, 2018; Phanthong, Guan, Ma, Hao, & Abudula, 2016, 2018). Nanocellulose consists of cellulose particles that have at least one of
Corresponding author. E-mail addresses: [email protected] (M.A. Dahlem), [email protected] (C. Borsoi), [email protected] (B. Hansen), [email protected] (A.L. Catto). ⁎
https://doi.org/10.1016/j.carbpol.2019.04.064 Received 28 March 2019; Received in revised form 17 April 2019; Accepted 17 April 2019 Available online 25 April 2019 0144-8617/ © 2019 Elsevier Ltd. All rights reserved.
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Abraham et al. (2011), where we chose to use only one fiber evaluating the behavior of the YMS fiber in different types of treatments. It was aimed to use the steam explosion at the beginning of the process to solubilize the hemicellulose and defibrillate the fiber, so that in the following stages there would be better interaction with the chemical processes. The YMS-ASBHS and YMS-ABH samples were treated with 3% (w/v) NaOH (ratio 1:5) at 30 °C for 4 h under constant stirring. Then, the samples were washed in distilled water until neutral pH. The SE was applied only to the samples YMS-ASBHS and YMS-SBHS using an autoclave (PRISMATEC, Mod. CS75, Brazil) maintained at 127 °C and 1.5 bar pressure for 1 h, releasing all the pressure at once immediately and repeating this process three times. The samples were removed from the autoclave and washed in water. All the samples were bleached using 100 mL of hydrogen peroxide 10% (w/v) at 50 °C for 2 h under constant stirring. After that, the samples were again washed in distilled water until neutral pH. The hydrolysis acid treatment was divided in two methods, treating the YMS-ASBHS and YMS-SBHS samples with 5% (w/v) oxalic acid in an autoclave until it attained 127 °C and 1.5 bar pressure for 1 h, which was immediately released and the process repeated three times. The YMS-ABH sample was treated with 10% (w/w) oxalic acid at 60 °C for 6 h. For this sample, a higher concentration and time were used since the SE step was not applied in conjunction with AH. After this process, all samples were washed in distilled water until they returned to pH neutrality. The collected nanofibrils were suspended in water and kept stirring with a mechanical stirrer of type (HEIDOLPH, Mod. DIAX 900, Germany) at 26,000 rpm for ˜5 min until the fibers are uniformly dispersed.
their dimensions smaller than 100 nm, generating a great interest in several applications, due to its excellent mechanical, chemical and optical properties (Abitbol et al., 2016; Niu et al., 2017; Niu et al., 2018). These particles can be divided into three groups, from the way they are obtained, presenting different characteristics and properties. Cellulose nanofibers (CNF) are usually obtained through mechanical, chemical or chemical-mechanical treatments. Cellulose nanocrystals are derived from chemical hydrolysis and bacterial nanocellulose is acquired by bacterial synthesis through specific microorganisms (Klemm et al., 2018; Nechyporchuk, Belgacem, & Bras, 2016; Nie et al., 2018; Tao, Zhang, Wu, Liao, & Nie, 2019). Various treatments can be used to obtain nanocellulose, whether chemical or mechanical. Mechanical grinding, steam explosion, cryogenic grinding and homogenization by high pressure are classified as mechanical processes in the preparation of nanocellulose, such as alkali or acid treatments, ionic liquid and enzymatic hydrolysis, which represent the common chemical processes used (Abraham et al., 2011; Kumar, Ha, Verma, & Tiwari, 2018; Song et al., 2018). The combined use of mechanical and chemical methods aims to reduce the amount of energy used in mechanical processes, making the production of nanocellulose more economically viable (Klemm et al., 2018; Mishra et al., 2018). Thus, this work aims to evaluate different methods of obtaining CNF from an agroindustrial residue, using as a raw material YMS, especially for its wide availability in the southern region of Brazil. The extraction of CNF will be evaluated by chemical processes, using alkaline treatment (AT), bleaching (BT) and acid hydrolysis (AH), and mechanical processes, such as steam explosion (SE).
2.3. Characterization
2. Experimental
2.3.1. Chemical composition The YMS sample was characterized in relation to its natural components, and the samples were prepared in triplicate according to ABNT (NBR 6923). The extractive (TAPPI T 204 cm-97), ashes (TAPPI T 211 om-02), lignin (TAPPI T 222 om-02) and α-cellulose (TAPPI T 203 cm99) contents were carried out and adapted according to the standards of the Technical Association of Pulp and Paper Industry (TAPPI), and the hemicellulose content was obtained by calculating the mass difference.
2.1. Materials The YMS used in this study undergo the production process and were previously grinded by the company Elacy, located in Venâncio Aires - RS, Brazil. The chemicals used in fiber extraction and in the preparation of the nanocellulose were acetone, sulfuric acid, hydrogen peroxide, sodium hydroxide and oxalic acid from Labsynth in São Paulo. It is noteworthy that all the reagents used were analytical grade.
2.3.2. Fourier transform infrared spectroscopy The FTIR analysis was recorded by a spectrophotometer (PERKIN ELMER, Mod. Frontier, USA) with an attenuated total reflectance (ATR) accessory. The wavelength range was 4000–500 cm−1 and each spectrum was obtained using 32 scans.
2.2. Isolation of cellulose nanofibrils The fibers obtained from YMS were classified in uniform size using a sieve system with 250 mesh Tyler (63 μm) and divided into four different samples, according to the sequence of treatments performed: YMS-RAW (untreated), YMS-ASBHS (AT, SE, BT and AH with SE), YMSSBHS (SE, BT and AH with SE) and YMS-ABH (AT, BT and AH). Table 1 shows the samples and the treatments performed for the extraction of the nanocellulose. The methodology of nanocellulose extraction was adapted from
2.3.3. X-ray diffraction XRD measurements were performed using a diffractometer (Phillips, Mod. X’Pert MDP, Netherlands) at 40 kV and 30 mA with monochromatic CuK α radiation of λ = 0.1542 nm. Intensities were measured in the 5° < 2θ < 40° (0.05°/2 s) range. According to Canevarolo (2003), the concentration of crystalline cellulose can be estimated by the method of Ruland (1961), which considers a relation of the areas in the crystalline region and the total area. The peaks in regions 110, -110 and 002 were separated and obtained through the Peak Analyzer from the OriginLab® software given in Eq. (1):
Table 1 Methods of treatments used to obtain nanocellulose from YMS samples. Treatments Steps
Sample YMSRAW
Untreated Alkaline treatment Steam explosion Bleaching Acid hydrolysis Acid hydrolysis + Steam explosion
X – – – – –
YMSASBHS – X X X – X
YMSSBHS – – X X – X
YMS-ABH
CI% =
– X – X X –
Ac Atotal
(1)
where CI is crystallinity index, ƩAc is the area of the diffraction peak related to the crystalline plane and ƩAtotal refers to the total area. 2.3.4. Dynamic light scattering The particle diameter of the YMS samples was measured at 20 °C using a dynamic light scattering (DLS) analyzer (MALVERN, Mod. Zetasizer Nano ZS, UK) provided with a He-Ne laser beam at 658 nm and a detection angle of 173°. The definition of particle size was given
Meaning of each sample letter: (A) alkaline - (S) steam - (B) bleaching – (H) hydrolysis. 79
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by the average value of at least three consecutive runs. 2.3.5. Transmission electron microscopy The morphological characterization of YMS samples was obtained using a transmission electron microscope (TECNAI, Mod. G2 T20. FEI, Netherlands) with an acceleration voltage of 75 kV. The nanofibril solution was sonicated for 1 h before being deposited in a microgrid covered with a thin film of carbon (∼200 nm). The deposited fibers were subsequently stained with a 2% uranyl acetate solution to improve the microscopic resolution. 2.3.6. Atomic force microscopy Using an AFM (BRUKER, Mod. Dimension Icon PT, USA), the analysis was performed by 670 nm laser multifunctional scanning. Samples were prepared in an ultrasonic bath for 1 h and then collected for 2 μL of supernatant. Afterwards, it was deposited on a mica plate. Finally, the samples were dried at room temperature. 3. Results and discussion 3.1. Chemical characterization In the literature, there are few studies that characterize YMS fibers or even physico-chemical analyzes geared specifically to this type of residue. Table 2 shows the chemical composition of the untreated YMS, based on the TAPPI standards. The structure of the YMS cellulosic fibers is composed of cellulose, lignin and hemicellulose, as well as ash and extractives that present varying amounts depending on their origin (Siqueira, Bras, & Dufresne, 2010). Lignin and hemicellulose normally constitute 15–25% and 20–30% (by weight), respectively, of the total chemical composition of the cellulosic fibers (McKendry, 2002). However, a great variability in its chemical composition may occur depending on the type of lignocellulosic material. The lignocellulosic composition may vary depending on its derivation, such as hardwood or softwood. In addition, factors, such as plant species, climate, agricultural conditions and land, also have a significant impact on the composition (Frollini, Bartolucci, Sisti, & Celli, 2013). However, the YMS-RAW sample presented relatively low concentrations of alphacellulose when compared to other hardwoods, where some authors present concentrations close to 40% (Chin, Sung Ting, Ong, & Omar, 2018). Thus, this difference can be related to some of the factors, such as the age of the trees in the harvest, since with the yerba mate tree, the harvest can begin with trees of 4–6 years, but only when the trees reach 10–12 years, it will be considered stable for harvesting (Zanon, 1988). 3.2. Fourier transform infrared spectroscopy To study the changes in the chemical composition of untreated YMS and after AT, SE, BT and AH, FTIR-ATR spectroscopy was used to investigate the differences in the YMS-RAW, YMS-ASBHS, YMS-SBHS and YMS-ABH samples, which are shown in Fig. 1. The analysis of YMS fibers reveals that the bands at around 3350 and 2916 cm−1 exhibit stretching vibrations of hydrogen-bonded hydroxyl groups (OeH) and alkyl group vibration elongation, respectively (Li et al., 2012). YMS fibers also showed a characteristic vibration of hemicellulose (C]O) at ˜1737 cm-1 (Sun, Xu, Sun, Fowler, & Baird, 2005). The bands corresponding to 1604 and 1508 cm-1 refer possibly to the stretching (C]C)
Fig. 1. FTIR spectrum of YMS fibers with different treatments: (A) YMS-ASBHS, (B) YMS-SBHS and (C) YMS-ABH.
Table 2 Chemical composition of YMS-RAW fibers. Material
α-cellulose (%)
Hemicellulose (%)
Lignin (%)
Extractives (%)
Ashes (%)
YMS-RAW
34.85 ± 0.28
24.77 ± 0.16
25.78 ± 0.01
10.11 ± 0.34
4.49 ± 0.17
± Mean SD (n = 3). 80
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of the aromatic bonds of the lignin. The peaks at around 1457 and 1420 cm-1 (−CH2) correspond to lignin deformation and bending of the intermolecular hydrogen attraction at the C6 group (Morán, Alvarez, Cyras, & Vázquez, 2008). The peak at 1317 cm−1 is related to the polysaccharide groups (Phanthong et al., 2016), while peaks at ˜1250 cm−1 relate to the C–OeC stretching bonds of lignin or C–O of hemicellulose (Mondragon et al., 2014). The peak of 1157 cm−1 (C–H), related to the alt-of-plane may indicate that there is lignin remaining (Yang, Kenny, & Puglia, 2015). Regarding the cellulose, the peak detected at ˜897 cm−1 is related to the (C–H) rocking vibration of the βglycosidic bonds between the glucose units (Phanthong et al., 2016). It was possible to observe changes along the infrared spectra of the YMS-ASBHS, YMS-SBHS and YMS-ABH samples submitted to the treatments. There was an increase in the intensity of the peaks in the 3350 and 2950 cm−1 regions in all samples, mainly for the AT and AH, which may be related to an exposure of the hydroxyl groups of the cellulose molecules, enabling water absorption and moisture through the hydrogen bonds (Thomas et al., 2015). It is also observed in Fig. 1(A) that for the YMS-ASBHS sample, there is an increase in peak intensity in the region of 1050 cm−1, related to the presence of cellulose, thus indicating a relative increase in the amount of cellulose in the sample that went through the AT and SE, therefore being an indication of the efficiency of this method in the removal of hemicellulose, lignin and other fiber compounds. For the YMS-SBHS sample in Fig. 1(B), which has no AT and an initial SE step, this peak shows a lower intensity, already for the treatment of YMS-ABH sample, without SE but with AT and AH, shown in Fig. 1(C), there is an increase again in the cellulose peak in this region, although less intense than in the YMSASBHS sample. These results showed that there were components and residual binding materials removed, leading to an increase in the cellulose content of the fiber. The intensities of peaks at 3420, 2910 and 1640 cm−1 were also enhanced with increasing treatment processes, in agreement with previous reports (Zhao et al., 2013). Peaks in the 1750 and 1604 cm−1 regions showed slight decreases in intensities as they pass through chemical treatments, with the smallest peak in the BT and AH with SE, suggesting that there was a decrease in the amount of hemicellulose and lignin present in the samples after the treatments. This can be explained by hemicellulose solubilization and lignin depolymerization (Feng et al., 2018). There are other characteristic peaks of lignin and hemicellulose, such as those in the regions of 1508 and 1288 cm−1. The peak at 1508 cm−1 showed a weak decrease throughout the treatments, but the peak at 1288 cm−1 showed an absence of intensity after treatments, suggesting a total removal of the
C–OeC or C–O stretches attributed to lignin or hemicellulose (Feng et al., 2018). The peak in the 896 cm−1 region associated with cellulose shows an increasing tendency throughout the treatments and SE, indicating the removal of materials with strong bonds and a probable exposure of the pure cellulosic fibers (Feng et al., 2018). Similar results were observed by Thomas et al. (2015) when studying the preparation and characterization of jute nanofibers during the production of nanocomposites and by Abraham et al. (2011) when extracting nanofibers from lignocellulosic materials, demonstrating that the use of SE and chemical treatment can partially remove lignin, hemicellulose and other materials. 3.3. X-ray diffraction In order to analyze the crystalline behavior of the YMS fiber during chemical treatments and SE, XRD analysis was performed on untreated and treated fibers. By removing the non-cellulosic constituents of the fibers by chemical treatment, the crystallinity index (CI) will change. The lignocellulosic fibers are constituted of crystalline and amorphous regions, as can be observed in the diffractogram shown in Fig. 2. The YMS fiber shows an increase in the intensity of the crystalline peaks that undergo chemical treatments and SE when compared to the untreated fiber (YMS-RAW). The increased intensity in the crystalline region suggests a decrease in the concentration of components, such as hemicellulose, lignin and noncrystalline cellulose. This may be explained by the ability to solubilize the amorphous region with the AH treatment. In addition, the use of the SE with AH may improve the concentration of cellulose type I (crystalline). Thus, with increasing peaks in the crystalline region, the results indicate that the hydrolysis occurred preferentially in the amorphous region. Similar results were reported by several authors (Feng et al., 2018; Li et al., 2012; Thomas et al., 2015). The effect of the crystalline cellulose concentration on the treatments submitted to the samples can be verified in Fig. 2. It can be observed that the YMS-RAW sample already had a low intensity peaks in the regions of 22.1° (crystalline plane 002) and 14.8° (crystalline plane 110) 2θ, representing cellulose type I. After the samples were submitted to chemical treatments and SE, there was an increase of intensity in these peaks, mainly in the crystalline region (002) (Cherian et al., 2010). Initially, the YMS-RAW sample had ˜40.62% of crystalline cellulose. After the treatments, the YMS-ASBHS sample reached 75.99%, YMSSBHS reached 66.19% and YMS-ABH reached 64.36%. These results
Fig. 2. X-ray diffractometry of different treatments of YMS fibers. 81
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size distribution, and finally, the YMS-ABH sample showed peaks in the range of 24–50 nm with a 31% size distribution, and in the range of 142–295 nm with a 65.8% size distribution. Furthermore, this sample also showed a peak in the 5500 nm region with a 3.1% size distribution. According to Espino et al. (2014), nanocellulose has an easily agglomerated character when disposed in aqueous solution, so it is suggested that this localized agglomeration in the 5500 nm region may be due to this nanocellulose capacity. Similar results were observed by Arrieta et al. (2018) when producing bionanocomposite films with yerba mate residues, by Hernandez, Ferreira, and Rosa (2018) when analyzing nanocrystals obtained from corn straw by chemical treatments and by Kunaver et al. (2016) when isolating nanocellulose from cellulosic raw materials.
Table 3 Crystalline and amorphous contribution in different samples of YMS. Samples
CI (%)
Amorphous (%)a
YMS-RAW YMS-ASBHS YMS-SBHS YMS-ABH
40.62 75.99 66.19 64.36
59.38 24.01 33.81 35.64
a
The amorphous phase was estimated by the difference: 100% – CI (%).
suggest that the YMS-SBHS and YMS-ABH samples obtained lower concentrations of crystalline cellulose than the YMS-ASBHS, since they did not undergo AT and SE, respectively. The AT and SE are usually related to the total or partial removal of hemicellulose and lignin, which are amorphous polymers (Frollini et al., 2013). Thus, the YMSASBHS sample covered AT and SE, with better removal of the amorphous constituents, suggesting a better packaging of the cellulose chains of the fibers (Buson, Melo, Oliveira, Rangel, & Deus, 2018; Mondragon et al., 2014). In addition, the YMS-ASBHS sample shows two peaks in the 14.8° (110) and 16.4° (-110) 2θ regions, characteristic of materials with high cellulose concentrations, since the presence of materials such as lignin, hemicellulose and amorphous cellulose tends to unify this region and form only one peak (Spinacé, Lambert, Fermoselli, & De Paoli, 2009). Table 3 shows the results of the crystallinity index and the amorphous phase of the YMS samples.
3.5. Transmission electron microscopy In order to observe the morphology and to evaluate the size of the YMS samples after the different treatments, TEM analysis was performed. To estimate the sample size, it was considered as base some fibers isolated in the micrographs, which can be seen in Fig. 4. Fig. 4(A) showed that the YMS-ASBHS sample had a mean length and diameter of 228 and 15.2 nm, respectively. The YMS-SBHS sample (Fig. 4(B)) showed 279 and 11.4 nm and finally the YMS-ABH sample (Fig. 4(C)) had dimensions of 257 and 11.5 nm. On this basis, it is possible to observe that all samples obtained stems and needle-like nanocellulose, especially the YMS-ASBHS and YMS-SBHS samples, presenting a different aspect of the YMS-ABH sample. The TEM image confirmed the presence of nanocellulose in the form of CNF. The diameter of the CNF was in the range of 10–100 nm showing that this combination of the SE process in autoclave and chemical treatments effectively defibrillated the cellulose present in the nanoscale fiber. The diameter of the CNF in this study was similar to those previously reported for CNF produced from banana pseudo-stem and cotton, which was in the range of 5–60 nm (Faradilla et al., 2017; Ferreira, Pinheiro, Gouveia, Thim, & Lona, 2018; Klemm et al., 2018; Roohani et al., 2008). From these results, it is suggested that the SE process exhibits better defibrillation of the sample. Moreover, the YMS-SBHS sample showed similar results to the YMS-ASBHS sample, although it did not undergo the AT step, which can thus reduce a step in the nanocellulose production process. Since it is considered as one of the most important parameters for the applicability of nanofibers in composites, the aspect ratio between the fiber length and diameter (L/D) was estimated in this study. Aspect ratios of 12, 24 and 23 were determined for YMS-ASBHS, YMS-SBHS and YMS-ABH, respectively. The YMS-ASBHS sample presented results similar to those found by Teixeira et al. (2010), who obtained a L/D
3.4. Dynamic light scattering DLS analysis is commonly used to verify the size and stability of particles in the medium and is usually related to the Brownian motion of the nanoparticles (Morais et al., 2013). In addition, one of the advantages of using this technique is its fast, easy and reproducible results for spherical particles in suspensions (Braun, Dorgan, & Chandler, 2008; Chu & Liu, 2000). However, when used to analyze stem or fibril-like structures, there are some limitations in accurate size estimation, in addition to strongly depending on the orientation of the fibers in the fluid (Frone, Panaitescu, & Donescu, 2011). Some authors use correlations of data obtained with techniques such as TEM to verify the mean particle size (Kunaver et al., 2016). The DLS results of the YMS samples treated in solution are shown in Fig. 3, presenting peaks preferably in the regions below 300 nm. The YMS-ASBHS sample showed a peak of lower intensity in the range of 0.6–1.3 nm with 24% of size distribution and a peak of greater intensity in the range of 105–220 nm with a size distribution of 76%. The YMSSBHS sample showed only a peak in the range of 50–91 nm with 100%
Fig. 3. Particle size distribution of YMS samples by DLS. 82
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Fig. 4. TEM micrographs of YMS-ASBHS (A), YMS-SBHS (B) and YMS-ABH (C) CNF.
ratio of 10–14 for regular cotton fiber and Capadona, Shanmuganathan, Seidel, Rowan, and Weder (2009) and Shanmuganathan, Capadona, Rowan, and Weder (2010) who obtained a ratio from 11 to 13 for microcrystalline cellulose. The YMS-SBHS and YMS-ABH samples presented similar results to those of Morais et al. (2013), where a ratio of 20–24 for cotton linter was obtained. Despite the presence of agglomerated regions, which can be attributed to a low negative charge on the surface that repels nanocrystals to non-agglomeration (Lima, de, & Borsali, 2004), the process proved to be efficient for the isolation of nanocellulose from the YMS fibers. Similar results were observed by Morais et al. (2013) when extracting and characterizing nanocellulose from raw cotton linter.
of 29.5 nm, ranging from 11.5–60 nm. YMS-SBHS had a mean diameter of 42.6 nm, ranging from 24.8–50.4 nm and YMS-ABH had a mean diameter of 63.7 nm, with a variation of 54.7–75.6 nm. The YMSASBHS and YMS-SBHS samples, which undergo the AH treatment with the SE, showed smaller diameters, suggesting a better removal of components such as lignin and hemicellulose. However, it was not possible to determine the length of the samples due to the agglomeration of the nanofibers. According to Hernandez et al. (2018), this agglomeration problem can be fixed using a different methodology to maintain a stable colloidal suspension, for example, the use of different pH values and sulfates to obtain high ionic strengths, as used by Espinosa, Sánchez, Otero, Domínguez-Robles, and Rodríguez (2017). Another suggestion would be the use of a mica substrate for a better AFM image definition, as reported by Boluk, Lahiji, Zhao, and McDermott (2011). Similar results were observed in several types of materials, such as wheat straw fibers with diameters between 10 and 80 nm found by Rahimi and Behrooz (2011), soybean stock with diameters between 50 and 100 nm found by Gamelas, Pedrosa, Lourenço, and Ferreira (2015) and pineapple leaf with diameters between 5 and 60 nm observed by Cherian et al. (2010).
3.6. Atomic force microscopy AFM was performed with the intention of corroborating with the DLS and TEM analyzes, which aimed to demonstrate the size of the samples and their morphology. Fig. 5 shows the images obtained by AFM, where it can be seen that the treated YMS samples showed a fibrillar appearance. It was possible to determine the mean diameter of the samples, with the YMS-ASBHS sample having an average diameter
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Fig. 5. AFM images of YMS-ASBHS (A), YMS-SBHS (B) and YMS-ABH (C) CNF.
4. Conclusion
Acknowledgements
This study investigated the extraction of nanocellulose from YMS fibers from mild chemical treatments and SE. The FTIR spectra showed differences in the peaks after the treatment, where a decrease was observed in components such as lignin and hemicellulose. In addition, it was observed that even the YMS-SBHS sample without alkaline treatment was able to have considerable decreases when compared to the other samples. After the treatments, it was possible to notice an increase in the crystallinity index of the samples in relation to the raw fiber, with an increase of 35.37% for YMS-ASBHS, 25.57% for YMS-SBHS and 23.74% for YMS-ABH, demonstrating that the use of the AT and SE steps favor the isolation of the crystals. DLS analysis showed that all samples obtained a size distribution below 300 nm, with the YMS-SBHS sample having a better homogeneity in the distribution, with a size between 50 and 91 nm. The TEM and AFM assays corroborate the other characterizations, demonstrating the isolation of nanocellulose in all samples, but the YMS-ASBHS and YMS-SBHS samples obtained better aspects and results when compared to the YMS-ABH sample, again suggesting that the use of the SE assists in the removal of amorphous components. Finally, it was possible to obtain CNF in the samples that were used the SE stage, even in the YMS-SBHS that did not undergo the AT, suggesting a probable decrease of steps in obtaining nanocellulose for YMS fibers.
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