Title: Eco-friendly Preparation of Aerogels based Cellulose Nanofiber Isolated from Sugar Palm Frond as Effective Palm Oil Absorbent
Authors: Vini Vidi Vici Batubara, Diana Alemin Barus, Riski Titian Ginting, Timbangen Sembiring Affiliations: Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Sumatera Utara, Medan, Indonesia
Contact email: [email protected]
Abstract
In this work, the isolation of cellulose nanofiber (CNF) from sugar palm frond was prepared by high shear homogenization and aerogels was obtained by freeze-drying method with addition of polyvinyl alcohol (PVA). The hydrophobic modification of CNF:PVA aerogel was completed with via chemical vapor deposition (CVD) of methyltrimethoxysilane (MTMS). The prepared CNF:PVA aerogels were determine using scanning electron microscopy (SEM), x-ray diffraction (XRD) and fourier-transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA) and water contact angle (WCA) for better understanding of morphology, crystallinity, functional group, thermal property and surface wettability.
The CNF:PVA aerogel demonstrates effective oil absorption for palm oil of 13.97 g/g compared to canola oil, soybean oil, sunflower oil and vacuum pump oil of 6-12 g/g. These results confirmed that CNF isolated from sugar palm as aerogel prepared via high shear homogenizer is a promising ecofriendly adsorbent material for residual palm oil.
Graphical abstract
Keywords: cellulose nanofiber, sugar palm frond, aerogels, freeze drying, oil adsorbent
Specifications Table
Subject area Physical Chemistry
Compounds Cellulose nanofiber, sugar palm frond, PVA
Data category Synthesized
Data acquisition format SEM, FTIR, XRD, water contact angle
Data type Analyzed
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Procedure In this work, sugar palm frond was bleached and treated with high shear homogenizer to obtain cellulose nanofiber (CNF). The aerogel was formed by addition of polyvinyl alcohol (PVA), followed by freeze drying for 24 h and treated with methyltrimethoxysilane (MTMS) via chemical vapor deposition. Various ratio concentration between CNF and PVA was investigated using SEM, FTIR, XRD, TGA and oil absorption capacity.
Data accessibility All data was analyzed.
Rationale
The wastewaters containing spilled oil, organics or dye cause by rapid development of petroleum industry, oil mining and marine oil transportation could lead to negative impact on ecological environment and human health [1]. Besides, palm oil is the largest commodities and plantation in Indonesia, however palm oil mill effluent (POME) discharges an enormous amount of residual palm oil. Therefore, finding an alternative approach is essential to recover residual palm oil in POME and reused to increase the yield of palm oil, while simultaneously reduce environmental pollution. Most common method such as up-flow continuous adsorption system [2, 3], activated biochar [4, 5] and bio-adsorbent [6] for POME treatment.
Among other method, bio-adsorbent is favorable due to cost-effective, abundant, environmental friendly, and hydrophobic and oleophilic surface [7].
Different type of bio-adsorbent for palm oil residuals has been utilized has been proposed in several report namely activated carbon, rubber powders, clay, perlite and fly-ash boilers [8, 9]. However, these materials demonstrate poor biodegradability and low oil absorption. In addition, mixed sawdust and wood chip could absorb with relatively poor oil adsorption capacity of only 3-6 g/g [10]. Nanocellulose is promising materials commonly isolated from wood or agricultural byproducts as aerogel not only possesses the advantages of high porosity and low density but also has good biocompatibility, lightweight sponges and environmentally friendly adsorbent. Moreover, hydrophobic CNF-based aerogels can be prepared by adding polyvinyl alcohol (PVA) solution could lead to high-density hydrogen bonding with CNF and improve the mechanical properties of aerogel. Previously, it has been reported that hydrophilic CNF based aerogels can be modified with trimethylchlorosilane (TMCS) and methyltrimethoxysilane (MTMS) to obtain hydrophobic or olephilic aerogel [11]. Several studies have demonstrated that hydrophobically modified CNF based aerogels are promising for oil-absorbing materials, which are used to remove various type of oils and organic solvent [12-14].
In this study, CNF was isolated from sugar palm frond using high-shear homogenizer and used to prepare CNF:PVA aerogel sample via freeze drying method, followed by chemical vapor deposition (CVD) of MTMS to modified its surface to obtain hydrophobic/oleophilic aerogel. To confirm the properties of the resulting aerogel, several characterizations was used to identify the morphology, crystallinity, functional groups, thermal properties, hydrophobicity using scanning electron microscopy (SEM), fourier transform infrared (FTIR) spectroscopy, thermal gravimetric analysis (TGA) and water contact angle. The results show that the obtained CNF from sugar palm frond was highly crystalline, nanofiber formation, strong interaction between CNF and PVA, lower density for CNF:PVA ratio of 1:3 and good thermal
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properties. It also demonstrated good oil absorption capacities for palm oil and other vegetables oil including vacuum pump oil.
Procedure Materials
Sulfuric acid, NaOH, H2O2, sodium hypochlorite, acetic acid, methyltrimethoxysilane (MTMS) and PVA were purchased at Sigma Aldrich, Singapore. All chemicals were used without further purification.
Preparation of CNF
Sugar palm frond was obtained from local farmers was rinsed with tap water and dried overnight in laboratory oven. Subsequently, the dried frond was milled and fine powder pass through 200 mesh sieves, followed by diluted H2SO4 (10 %) was used to remove hemicellulose in the frond. After 4 h treatment at room temperature, then the frond was treated with 15% NaOH solution and heated at 90 oC for 2 h.
Afterwards, the frond was rinsed with tap water and placed in a solution containing 10% H2O2, followed by heating at 80 oC for 2 h and washed with acetic acid and distilled water. The cellulose was dried in oven at 60 oC for overnight. The cellulose with concentration of 0.5 wt% was added into distilled water and treated in high shear homogenizer at 12,000 rpm for 2 h with continuous flow of cold water (-5 oC). The solution containing CNF was kept in storage at 5 oC condition for further use.
Preparation of hydrophobic CNF/PVA aerogel
The composite aerogel was prepared with various concentration of CNF:PVA (0.5:3), CNF:PVA (0.5:5), CNF:PVA (1:3) and CNF:PVA (1:5). Firstly, PVA was dissolved in hot solution containing CNF with various concentration at 90 oC for 1 h. Each solution was cooled in freezer at -15 oC for 12 h, followed by freeze drying at pressure 8 Pa in cold trap of -65 oC for 24 h. Then, the aerogel was place in vacuum oven heated to 60 oC for 6 h along with 300 µL of MTMS via chemical vapor deposition (CVD).
Characterization
The morphology of aerogel with various concentration of CNF:PVA (0.5:3), CNF:PVA (0.5:5), CNF:PVA (1:3) and CNF:PVA (1:5) were determined using scanning electron microscope (SEM) model Zeiss EVO 15.
Fourier-transform infrared spectroscopy (FTIR) Thermo-Scientific model Nicolet iS50 to evaluate functional group of composite aerogels. X-ray diffraction (XRD) model Bruker D8 Advance to determine the crystallinity of CNF. Thermal gravimetric analysis (TGA) model NETZSCH STA 449 F1 Jupiter and contact angle goniometer to assess thermal property and surface wettability of composite aerogel.
Oil adsorption experiments
Soybean oil, palm oil, sunflower oil, vacuum pump oil and canola oil were used to determine the adsorption capacity. The aerogel sample size of 2 x 2 x 2 cm3, where the adsorption capacity, Qt (g/g) = (m2
− m1)/m1, where m1 and m2 are the weights of the aerogels before (g) and after oil absorption (g). All the oil absorption capacity value was based on average of absorption tests repeated four times for each sample.
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Data, value and validation (max 800 words)
Figure 1a shows the defibrillated CNF isolated from sugar palm frond after high-shear homogenizer for 2 h. It can be clearly seen that the rough surface of CNF indicates the nano-sized fibril bundles strongly attached due to hydroxyl bond breakdown, which is similar with previous report for CNF isolated from bamboo plant [15]. This can be explained due to the aggregated bundle nanofibers were form in larger structure that incorporate into hemicellulose with disorder orientation within fibers crystalline structure [16]. Figure 1b depicts the morphology of CNF:PVA (0.5:3) structure with larger porous randomly distributed on the surface, meanwhile for CNF:PVA (1:3) with well distributed pore uniformity and three dimensional structure. As can be seen in inset Fig. 1c, it shows the corresponding sample size of CNF:PVA (1:3) aerogel. In contrast, high concentration of PVA for CNF:PVA (1:5) sample shows compact layer without pore formation, implying the crosslinked between PVA and CNF. Consequently, the optimal concentration for three-dimensional structure is aerogel with CNF:PVA (1:3) sample.
Figure 1. SEM images of (a) CNF and PVA:CNF with different ratio of (b) 0.5:3, (c) 1:3 and (d) 1:5.
Fig. 2a present the XRD pattern of CNF with strong peak 2θ of 22.64o attributed to the polymorphs of cellulose I structure with typical peaks at 2θ of 15.1o and 22.6o [17], which correspond to crystallographic plane of (110) and (002), respectively. In addition, the peak at 2θ of 18.26o can be assign to amorphous region (Iam) of cellulose chains. According to the empirical method by Segal et al. [18], the crystallinity index (Xc) of cellulose can be calculated, as follows:
(1) 𝑋𝑐=𝐼002𝐼‒ 𝐼𝑎𝑚
002 𝑥 100
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Where I002 and Iam represents the peak intensity of crystalline and amorphous region, respectively. The defibrillated CNF shows crystallinity index of 75.6% was due to the shear forces of homogenizer break down the amorphous region of cellulose fibers, thus higher crystallinity of CNF was achieved [19]. However, this result is is slightly lower compared to previous report [17], but close to the value reported for nanofibrillated cellulose from water hyacinth and pineapple leaves [20]. In addition, the low crystallinity index was due to the high shear homogenizer process cause degradation of crystalline structure region between intermolecular hydrogen bonds of cellulose chains [21]. The broad crystalline peak located at 19.2o and the shoulder peak centered at 22.3o corresponding to the orthorhombic lattice structure of PVA [22]. Moreover, the diffraction peak of PVA was decrease for CNF:PVA (1:3) due to increasing amount of CNF in aerogel. To further understand the functional group of CNF and CNF:PVA aerogel, FTIR analysis was carried out as shown in Fig. 2b. The broad peak at 3317 cm-1 was attributed to the functional group of -OH stretching vibration. The sharp peak at 1028 cm-1 seen in CNF can be correspond to the C-O-C stretching vibration of cellulose β-1,4-glycosidic ring linkage between the D- glucose units and side group vibration, which is in accordance with previous report for CNF from pineapple residue [23]. Notably, there is no peak observed at 2852 cm−1, which indicates high purity of CNF isolated from sugar palm frond without any residual of wax on fibers surface [24]. For CNF:PVA (0.5:3) aerogel sample, the hydroxyl group was shifted to lower wavenumber of 3257 cm-1 implying strong interaction between CNF and PVA bonding. The distinct peak 2914 cm-1 and 1416 cm-1 can be ascribed to C-H tensile vibration and C-OH bending of PVA, respectively. The peak at 1088 cm-1 for CNF:PVA was shifted to higher wavenumber from 1028 cm-1 for pristine CNF, which could be due to the presence of C-O stretching vibration of PVA in the aerogels. In addition, the sharp peaks for CNF:PVA (1:3) sample at wavenumber 1267 cm-1 suggesting the Si-O-Si absorption band of siloxane compound originated from silanization process from MTMS. Meanwhile, for the (0.5:3) and (1:5), the absorption band of Si-O-Si was overlapping with broad absorption peak of C-O bond and indicates that poor attachment of Si on the surface of aerogels. Accordingly, the CNF:PVA (1:3) was promising as oil absorbent compared with other ratio concentration.
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Figure 2. (a) XRD pattern and (b) FTIR spectra of CNF and various ratio of CNF:PVA.
To further understand the surface wettability, the water contact angle measurement was performed for CNF:PVA with different ratio of 0.5:3, 1:3 and 1:5. As shown in Fig. 3, the unmodified aerogels demonstrate hydrophilicity due to the presence of enrich hydroxyl group of CNF with water contact angle of 22.71o, 24.02o and 70.08o for CNF:PVA ratio of 0.5:3, 1:3 and 1:5, respectively. Meanwhile, after silanization the water contact angle of aerogel was changed to 94.14o, 110.53o and 91.77o for CNF:PVA ratio of 0.5:3, 0.5:3 and 1:5, respectively, which suggesting the hydrophobic surface owing to the treated with MTMS. This can be explained by both high surface roughness and decreased of surface energy caused by polysiloxane particles formation during silanization.
Figure 3. Water contact angle of various ratio of CNF:PVA aerogel sample before and after silanization with MTMS.
Table 1. Contact angle before and after silanization of various sample ratio PVA:CNF.
Sample Density
(kg m-3) Before silanization (o) After silanization (o)
0.5:3 119.42 22.71 ± 0.42 94.14 ± 0.44
1:3 92.88 24.02 ± 0.40 110.53 ± 2.17
1:5 147.15 70.08 ± 1.20 91.77 ± 1.58
Fig. 4 depicts the TGA curves of CNF:PVA with concentration ratio of 0.5:3 and 1:3. All samples demonstrate similar profile with decreased in weight (about 8 %) from 25 to 225 oC due to the removal of moisture trapped in CNF:PVA aerogel. The second and third step at 330 oC and 550 oC were rapidly decrease the weight by 50 % and 80% and caused by decomposition of three-dimensional polymer
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network structure of PVA and CNF [25] and degradation of nanocellulose polymer chains and residuals [26].
Figure 4. TGA curves of CNF:PVA with ratio concentration of 0.5:3 and 1:3.
In order to understand the oil adsorption capacity of CNF:PVA of 0.5:3, 1:3 and 1:5, a series of oils including canola oil, soybean oil, sunflower oil, palm oil and vacuum pump oil were evaluated and the results are shown in Fig. 5. As shown in Fig. 5a-c, the absorption capacity as function of time for all CNF:PVA sample only needed 10 min for oil absorption and gradually reached absorption saturation for longer time, which suggests the porous structure of CNF:PVA aerogel facilitating oil absorption via large number of channels. The highest absorption capacity for CNF:PVA (0.5:3) was soybean oil of 7.39 g/g and the lowest capacity for vacuum pump oil. In contrast, the maximum and lowest absorption capacity for CNF:PVA (1:3) aerogel sample were palm oil of 13.97 g/g and canola oil of 6.67 g/g. Large improvement of oil absorption capacity of CNF:PVA (1:3) was correlated to the large number of porous active adsorption sites by interior surface channels of aerogel provides capillary channels for oil entry and oil storage [27]. In addition, the CNF:PVA (1:5) exhibit highest palm oil absorption of 7.64 g/g, however lower than that CNF:PVA (1:3) aerogel sample. In comparison, the oil absorption capacity of 11.5 g/g for wood pulp CNF aerogel [28], 7-11 g/g for bamboo fiber aerogel [29], 4-7 g/g for cellulose/montmorillonite aerogel [30], 8-14 g/g for bacterial cellulose/silica aerogel [31] and close to the 16.78 g/g for CNF aerogel derived from Eucalyptus sp. [32]. Additionally, the palm oil absorption capacity performance of CNF:PVA (1:3) was higher than previous report using polypropylene micro/nanofiber based adsorbent adsorption capacity of 10.93 g/g for refined palm oil [33]. The summarized absorption capacity of various oil based on several CNF:PVA ratio concentration was shown in Fig. 5d. The increase of oil absorption capacity of CNF:PVA (1:3) aerogel sample about 92.88 kg m-3 due to lower density of aerogel, meanwhile CNF:PVA (0.5:3 and 1:5) demonstrates higher density of 119.42 and 147.15 kg m-3, which indicates less porous structure as summarized in Table 1.
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Fig. 5 The absorption capacity of CNF:PVA with ratio of (a) 0.5:3, (b) 1:3 and (c) 1:5 as function of time.
(d) Oil absorption capacity comparison with various oils.
The MTMS treated CNF:PVA aerogel sample was ideal absorbent material for oil removal from water. The silane treatment of CNF:PVA aerogels affects the chemical composition with lipophilicity and hydrophobicity [7]. It is expected that CNF:PVA aerogels repelled water when immersed in water and floated as shown in Fig. 6a. Accordingly, the process of palm oil removal (stained with red congo dye) was contact and rapidly absorbed by CNF:PVA aerogel due to its porous structure and oil-water selectivity.
After removal of CNF:PVA aerogel, the remaining palm oil was fully absorbed and effective oil absorption became completely clean without any palm oil residual.
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Fig. 6 Digital photograph of (a) before and after dipping. (b) Removal of palm oil (dyed with congo red) from water surface using CNF:PVA (1:3) sample.
Conclusion
In summary, three-dimensional porous network of CNF isolated from sugar palm frond was successfully prepared with PVA aerogel via freeze drying, followed by surface modification using MTMS via CVD technique. The rough surface of CNF indicates the nanofiber formation and strong interaction between CNF and PVA bonding were confirm by SEM and FTIR analysis. The hydrophobic surface was high surface roughness and decreased of surface energy caused by polysiloxane particles during silanization process by MTMS. The highest oil absorption of CNF:PVA (1:3) aerogel sample was achieved of 13.97 g/g for palm oil as compared to canola oil, soybean oil, sunflower oil and vacuum pump oil.
Acknowledgement
The author would like to acknowledge Ministry of Education, Culture, Research, and Technology No.
059/E5/PG.0200/PT/2022 and 8/UN5.2.3.1/PPM/KP-DRTPM/L/2022. Authors would like to thank Nanomaterials for Renewable Energy (NRE) Laboratory, Medan, Indonesia for providing research facilities and data analysis.
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