Superhydrophobic kapok fiber oil-absorbent: Preparation and high oil absorbency

Superhydrophobic kapok fiber oil-absorbent: Preparation and high oil absorbency

Chemical Engineering Journal 213 (2012) 1–7 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal journal homepage: www.el...

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Chemical Engineering Journal 213 (2012) 1–7

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Superhydrophobic kapok fiber oil-absorbent: Preparation and high oil absorbency Jintao Wang a,b, Yian Zheng a,b, Aiqin Wang a,⇑ a b

Center of Eco-Material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate University of the Chinese Academy of Sciences, Beijing 100049, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" Superhydrophobic kapok fiber was

prepared via sol–gel method and used for oil sorption. " The modified kapok fiber is covered by silica nanoparticles and surface become rough. " The modified kapok fiber have high oil sorption capacity and oil–water separation selectivity. " The modified kapok fiber have the capability of removing oil in oil/ water mixture.

a r t i c l e

i n f o

Article history: Received 9 August 2012 Received in revised form 20 September 2012 Accepted 21 September 2012 Available online 11 October 2012 Keywords: Superhydrophobic Silica nanoparticles Kapok fiber Oil sorption capacity Reusability

a b s t r a c t Superhydrophobic and oleophilic oil sorbent was successfully prepared by the incorporation of silica nanoparticles onto kapok fiber via sol–gel method and subsequent hydrophobic modification using hydrolyzed dodecyltrimethoxysilane (DTMS). The formation of silica nanoparticles was confirmed by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and investigation of the wetting behavior of water and oil on fiber surface. The coated fiber exhibited excellent oil/water selectivity in the cleanup of oil over water. The as-prepared fiber can quickly absorb diesel and soybean oil up to above 46.9 and 58.8 g/g, with the improvement in oil sorption capacity to be 46.6% and 20.2% compared with raw fiber, respectively. Owing to high oil sorption capacity, excellent hydrophobic property and reusability, and good environmental friendliness, the as-prepared oil sorbent can be considered as promising alternative for organic synthetic fiber to clean up the spilled oil. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In recent years, water pollution caused by the oil spillage has become increasingly serious with the acceleration of urbanization and industrialization process [1]. Commonly used methods of solving these oil-leakage problems include mechanical extraction, combustion and chemical degradation. Owing to the economy and efficiency for oil spill cleanup, mechanical extraction by sorption materials is regarded as one of the most desirable choices for the recovery of oil. Although many sorption materials such as inor⇑ Corresponding author. Tel.: +86 931 4968118; fax: +86 931 8277088. E-mail address: [email protected] (A. Wang). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.09.116

ganic mineral materials [2], synthetic materials, [3] and natural materials [4] have been widely studied for the removal of spilled oil, these materials still have some limitations such as low oil sorption capacity, inadequate buoyancy, high cost, and poor reusability. Especially, most of materials studied have poor hydrophobicity, resulting in low oil–water separation selectivity and efficiency [5,6]. Hence, the exploitation of new oil sorption materials with high sorption capacity, low cost, low water pickup, excellent environmental benefit and reusability is rather important for oil pollution treatment. Superhydrophobic surface with water contact angles higher than 150° has attracted extensive interest [7,8]. Previous studies have revealed that superhydrophobicity depends on not only the

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low surface energy of the substrate but also the hierarchical microand nanostructures of the surface. So far, various methods such as phase separation [9], laser etching [10], sol–gel method [11], and chemical vapor deposition [12] have been used to prepare superhydrophobic surface. Therein, nano-structured surface generated by sol–gel method followed by further hydrophobic modification, is taken as a simple and effective technique for fabricating superhydrophobic coatings onto the surface of materials [13], by which the resulting materials will exhibit excellent affinity to oil and water repellency in water–oil surroundings, and accordingly, these superhydrophobic materials can be applied in the field of oil spill clean-up. A variety of materials, such as carbon nanotube sponges [14], nanowire membranes [15], superhydrophobic and superoleophilic sponges [16], superhydrophobic, and oleophilic calcium carbonate powder [17] have been developed for separating oil from water. Even so, the drawbacks of these materials with complicated preparation process and high cost still limit their application scale in practice. Kapok is a kind of natural plant fiber that has low density, good buoyancy, huge hollowness and excellent hydrophobicity. These unique characteristics endow kapok fiber with higher oil sorption capability compared with common natural fibers and commercial oil absorbent [18,19]. However, the smooth fiber surface due to the coverage of a small amount of waxy coating makes it difficult to effectively retain oil to fiber assembly. If the oil sorption capacity and hydrophobicity of kapok fiber can be further enhanced by surface modification, it would be more valuable for the oil spill cleanup. The silica coating shows excellent adhesion on cellulosic materials by the condensation between the hydroxyl groups of the hydrolyzed silanes and those existing on the surface of cellulose [20]. Hence, it is feasible to turn the surface of kapok fiber from hydrophobicity into superhydrophobicity. To the best of our knowledge, there is no report on the use of superhydrophobic kapok fiber for oil sorption. Herein, the oil sorbent based on superhydrophobic kapok fiber was prepared through a facile sol–gel technique. The sorption capacity and hydrophobicity of modified fiber were evaluated. The results of this study provide ideas for the application of other superhydrophobic modified cellulosic materials in oil spill cleanup. 2. Experimental 2.1. Materials Kapok was purchased from Shanghai Pan-Da Co. Ltd., China. NaClO2 (chemically pure) was provided by Beijin Hua-Wei Chemical Reagent Co., China. Acetic acid (analytical grade), sodium dodecyl benzene sulfonate (SDBS, chemically pure), and CHCl3 (analytical grade) were received from Shanghai Chemical Reagent Factory, China. Tetraethylorthosilicate (TEOS, chemically pure) were supplied by Tianjin Chemical Reagent Factory, China. Dodecyltrimethoxysilane (DTMS, chemically pure) was provided by Sinopharm Chemical Reagent Co., Ltd., China. NH3H2O (analytical grade) was obtained Baiyin Chemical Reagent Factory, China. Ethanol, toluene, n-hexane (analytical grade) were supplied by Tianjin Li-An Chemical Reagent Co. Ltd., China. Gasoline, diesel and soybean oil came from the local market, Lanzhou, China. 2.2. Preparation process of superhydrophobic kapok fiber Raw fiber was treated with NaClO2 according to the reported process [21]. Raw fiber was placed into 400 mL of NaClO2 solution (0.5 wt.%), while a certain amount of acetic acid was added to adjust pH 4.5. Afterwards, the treatment was kept at 80 °C for 1 h at 700 rpm. The treated fiber was washed several times with distilled

water until the pH level of filtrate reached neutrality, then dried in an oven at 70 °C to constant weight and used in all the later modification. TEOS (4 wt.%) and SDBS (1.2 mmol/L) were added to a certain amount of distilled water, stirred at room temperature for 1 h, then NaClO2-treated kapok fiber was added into the mixture and stirred for 20 min followed by a gradual addition of NH3H2O (1.8 wt.%). The reaction was maintained at room temperature for 4 h. The resulting fiber was washed several times with methanol, and dried in vacuum oven at 60 °C to constant weight. Then, the coated fiber was added into ethanol solution of DTMS (2 wt.%) and hydrolyzed for 1 h. Finally, the obtained sample was filtered, dried at room temperature and cured at 120 °C for 1 h. 2.3. Measurements of oil sorption capacity In oil medium without water: The dried sample (0.1 g) was put into a stainless-steel mesh weighed beforehand and immersed in oil at room temperature. The sample and the mesh were taken out from the oil together after a certain time, drained for 10 s, and wiped with filter paper to remove excess oil from the bottom of the mesh. The oil absorbency of the sample was determined by weighing the samples before and after the absorption, and calculated by the following formula:

Q ¼ ðM t  M i  M w Þ=M i where Q is the oil sorption capacity of the sorbents calculated as grams of oil per gram of sample, Mt the weight of the wet sorbents after draining (g), Mi the initial weight of sorbents (g) and Mw is the weight of water absorbed in the sorbents (g). In pure oil medium without any water, Mw is equal to zero. In oil/water mixture: The diesel or soybean oil was mixed with 60 mL of artificial seawater (3.5 wt.% NaCl) in a 100 mL conical flask for 10 min at 150 rpm over an orbital shaker. The agitation can make the oil float to the surface of the artificial seawater and form the oil layer. Then, 0.1 g sorbent was added to the oil/water mixture. The concentrations of diesel and soybean oil used varied from 0.01 to 0.14 g/mL of water. The sorbent was left in the oil/ water mixture and shaken for 60 min at 30 °C. After that, the sample was removed from the flask using mesh screen, drained for 1 min, and weighed. Water content was determined by the method of extraction separation using n-hexane as the solvent. 2.4. Reusability Kapok fiber of absorbing oil from the water surface was removed with the aid of a mesh screen, which was then placed on a sand core funnel and drained under vacuum for 10 min before weighing. Oil will be recovered without severe disruption of the appearance of the fiber. The sorption/desorption cycle was repeated for eight cycles to evaluate the recyclability of the fiber. 2.5. Characterizations Fourier transform infrared (FTIR) spectra were recorded on a Nicolet NEXUS FTIR spectrometer using KBr pellets. The micrographs of samples were examined using SEM (JSM-5600LV, JEOL). Before SEM observation, all samples were fixed on aluminum stubs and coated with gold. The surface wettability of water and oil on the surface of kapok fiber was observed with a digital SLR camera after the water or oil dyed with coloring matter was dripped on the surface of fiber from a syringe (1 mL). Contact angle measurements were carried out using a Krüss DSA 100 (Krüss Company, Ltd., Germany) apparatus at ambient temperature, and the volumes of probing liquids in the measurements were approximately 5 lL. Prior to observation, kapok fiber assembly was flattened with tablet machine.

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3. Results and discussion 3.1. Fabrication of superhydrophobic and oleophilic surface The superhydrophobic modification process for raw kapok fiber is shown in Fig. 1. The hydroxyl groups on the cellulosic materials play a vital role for the formation of superhydrophobic surface [20,21]. The surface of raw kapok fiber is covered by natural plant wax, which makes silica nanoparticles more difficult to attach to the surface of the fiber. Therefore, it is necessary to remove the surface plant wax of kapok fiber. In this study, kapok fiber was treated with NaClO2, and the hollow structure of the fiber was still intact after the treatment [22]. Moreover, the treatment will result in the generation of interspaces and increase the hydroxyl concentration of the fiber surface, making the hydrolyzed silanol easily penetrate into the interspaces of the fiber to form silica nanoparticles [23]. Thereinto, partial silica nanoparticles present on the surface of the fiber by means of the hydrogen bond interaction between the Si–OH group of the hydrolyzed silane and the hydroxyl groups of the fiber, while some silica nanoparticles can firmly adhere to the fiber surface in the form of physical conglutination. In order to reveal the hydrophobic and lipophilic effect of the modified fiber surface, the surface wettability of water and oil on the surface of raw, treated, and modified fiber was observed, as shown in Fig. 2. The blue-colored water drop shows a large contact angle on raw kapok fiber (h = 116°), while the water drop sinks rapidly into the fiber treated with NaClO2 to form a large spreading radius on the surface. The kapok fiber coated by the silica hydrosol with subsequent hydrolyzation in DTMS has water contact angle of 151°, which is much higher than raw fiber, indicating that the treatment effectively improves the hydrophobic property of raw fiber. When the red-colored oil drop is applied on the surface of three kinds of fiber, the drop sinks completely into the fiber in a very short time, suggesting that the oil drop is a wetting liquid for all three fibers. Owning to its light density and superhydrophobic nature, the modified fiber may float on the surface of water more steadily before and after collecting all of oils from water, a predominant characteristic for practical use of oil spills cleanup. 3.2. FTIR spectra The FTIR spectra of raw, treated, and superhydrophobic kapok fibers are displayed in Fig. 3. Comparing the spectra of raw, treated and superhydrophobic fibers, the following findings will be obtained. In Fig. 3c, the intensity of the peak at 3411 cm1 (stretching

Fig. 1. Schematic representation of transition from raw kapok fiber to superhydrophobic kapok fiber.

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vibration peak of surface –OH) decreases obviously as compared with that of treated fiber; a absorption peak (symmetric stretching vibration of Si–O–Si) is observed at 467 cm1 [24]; strong absorption bands corresponding to asymmetric and symmetric stretching vibration of CH2 and CH3 in DTMS appear at 2923 and 2856 cm1 [25]. In addition, the characteristic absorption peaks around 1100–1000 cm1 attributed to Si–O–Si of silica nano-particles and DTMS seem to be overlapped by the C–O stretching vibration of cellulose in fiber. These changes indicate that the total numbers of –OH was reduced and hydrophobic silica nano-particles were formed on the surface of kapok fiber. 3.3. Morphology analyses The surface appearances of raw, treated and superhydrophobic kapok fibers are shown in Fig. 4. It is obvious that raw kapok fiber shows a smooth surface owning to the coverage of inherent plant wax (Fig. 4a), while the treatment makes the fiber surface become rough with subtle textures and wrinkles, implying that waxy coating has been removed from the fiber surface and the fiber changes from hydrophobic to hydrophilic due to the exposition of more cellulose hydroxyl groups [22]. Different from raw and treated fiber, the modified fiber is covered by dense silica nanoparticles without any interstices, which renders the surface rougher. In addition, even though the external surface of the fiber is covered by silica nanoparticles, the internal hollow lumen is not blocked by the silica nanoparticles, ensuring that the intrinsic oil sorption capacity is not reduced for raw kapok fiber as an oil sorbent. 3.4. Oil sorption capacity of raw and superhydrophobic fiber for various oils The superhydrophobic modification of kapok fiber not only fabricates a rough structure on the surface but also decreases the surface energy, which in turn affects its oil affinity. To study the maximum oil sorption capacity of superhydrophobic fiber for various common oils, the sorption experiment was carried out in pure oil without any water. For comparison, the oil sorption capability of raw kapok fiber was also investigated. As shown in Fig. 5, the oil sorption capacity of raw kapok fiber for n-hexane, toluene, chloroform, gasoline, diesel, and soybean oil is 22.8, 30.4, 41.9, 34.1, 38.1, and 49.1 g/g, respectively, while the oil sorption capacity of modified fiber for these oils can reach about 41.8, 56, 85.5, 50.5, 54.2, and 59.8 g/g, respectively. This means that superhydrophobic modification is very useful for preparing a kind of oil sorbent with excellent oil sorption performance. In addition, it can be observed that the modified fiber exhibits high oil sorption capacity for chloroform and vegetable oil. The high oil sorption capacity for chloroform is due to its high relative density [21], for vegetable oil, this is mainly attributed to its high viscosity [18,23]. The oil with high viscosity is easier to adhere to the fiber surface and be kept in the fiber assembly. Generally, for kapok fiber, the parameters such as the amount of surface wax, hollow lumen, surface roughness, twist, crimp and fineness etc. play an important role in the retention of oil [26,27]. The oil that is retained in the fiber assembly can be divided into two kinds of types: the oil stored in an internal lumen and the oil retained in the voids among fibers [18,22,28]. After the modification of kapok fiber, the oil absorbed in the voids of the fibers is difficult to escape from the surface of fiber during the dripping of the oil-loaded fiber assembly due to more stable capillary bridging between fiber bundles. According to Wenzel model and the Cassie– Baxter model [29,30], the fabrication of a proper microsturcture can make a smooth oleophilic surface to be more oleophilic or even superoleophilic due to the capillary effect. Consequently, the oil affinity of raw kapok fiber was markedly improved by roughening the surface and lowering the surface energy. By contrast, more oil

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Fig. 2. Pictures of water droplet (dyed with methylene blue) on (a) raw, (b) treated, and (c) superhydrophobic kapok fiber surface; oil droplet (dyed with oil red O) on (a1) raw, (b1) treated, and (c1) superhydrophobic kapok fiber surface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.5. Effect of amount of oil on water

Fig. 3. FTIR spectra of (a) raw, (b) treated, and (c) superhydrophobic kapok fiber.

can escape from raw kapok fiber assembly as a result of the smooth surface. The findings in this study imply that the surface roughness and surface energy of kapok fiber are two important factors that influence the oil sorption capability. Besides, the oil sorption capacities of modified fiber in this study and other recently reported oilabsorbing fiber also are compared and shown in Table 1, and the results are encouraging. Although the oil sorption capacity of polyvinyl chloride/polystyrene fiber is close to that of raw kapok fiber [32], this type of synthetic organic material as oil sorbent is difficult to be used on a large scale due to its high cost and nonbiodegradability. Compared with this type of synthetic sorbents, superhydrophobic kapok fiber is much easier to prepare for practical use, demonstrating its great potentials in the removal of toxic organic solvents or oil spills from water. The application of oil sorbent based on natural fiber may offer an opportunity to alleviate the current environmental crisis especially for the global scale of serious water contamination arising from oil spills and industrial organic contaminants.

It is very important to know exact sorption capacity of oil sorbent on water surface for effective use of one kind of sorbent in practical oil spill cleanup. The oil and water sorption capacities of raw and superhydrophobic fiber in the oil on the water containing various amount of oil (diesel and soybean oil) are shown in Fig. 6. The investigation of amounts of water pickup will reflect the hydrophobic characteristic of sorbent and its oil affinity. It is observed that the oil sorption capacities of both raw and modified fibers increase with increasing the initial concentration of oil until they reach a plateau. When the concentrations of diesel and soybean oil used are below the values of 3.6 and 4.8 g/mL respectively, both raw fiber and superhydrophobic fibers are capable of removing all of floating oils on water, giving an indication that similar oil sorption capacities for raw and superhydrophobic fibers can be obtained within this range of oil concentration. Afterwards, superhydrophobic fiber always exhibits higher sorption capacities compared with raw fiber. As such, a certain amount of oil floating on water can be completely picked up in a shorter time by the superhydrophobic fiber, which is useful for preventing the dispersion of spilled oil on water. Such high oil sorption capacity should be mainly attributed to the surface roughness within nanometer size magnitude. Therefore, as-prepared fiber can better hold the oil to the fiber assembly. In oil/water mixture systems, the maximum oil sorption capacities of raw fiber are about 32 and 48.9 g/g for diesel and soybean oil, while the oil capacities of modified fiber are 46.9 and 58.8 g/g for the two oils, respectively. This indicates that the oil sorption capacity of raw fiber can be improved significantly by the surface modification. In addition, the water pickup of superhydrophobic fiber is obviously lower than that of raw fiber, showing its good oil/water selectivity. Excellent selectivity for oil on water surface and high oil sorption capacity demonstrate that modified fiber is full of application potential in the large-scale removal of oils from water. 3.6. Removal of oil film by raw and superhydrophobic fiber In order to exhibit the oil sorption characteristics of modified fiber on artificial sea water, optical images for the cleanup of soy-

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Fig. 4. SEM micrographs of (a, a1) raw, (b, b1) treated, and (c, c1) superhydrophobic kapok fiber.

Fig. 5. Maximum sorption capacities of raw and superhydrophobic kapok fiber for different oils.

Table 1 Comparison of oil sorption capacities from this study and other oil-absorbing fibers. Absorbing fiber

Type of oil

Sorption capacity (g/g)

Reference

Raw cotton fiber

Vegetable oil Diesel

30

[31]

38

[32]

Polyvinyl chloride/ polystyrene fiber Commercial polypropylene Methacrylate–lauryl methacrylate fiber Wool fiber-based nonwoven

Raw kapok fiber Superhydrophobic kapok fiber

Diesel Toluene Chloroform Diesel Vegetable oil Diesel Soybean oil Diesel Soybean oil

8 15 34.7 10.6 14.5 38.1 49.1 54.2 59.8

[33] [34]

This work

bean oil from the water surface by raw and as-prepared fiber are displayed in Fig. 7. It can be seen that the superhydrophobic fiber is easier to separate oil from the water surface compared with raw

fiber. When the superhydrophobic fiber is immersed in oil/water mixture, the oil is quickly absorbed by the fiber within several seconds. After that, the oil can removed easily from the oil/water mixture by taking away the oil-loaded modified fiber from the water surface. Interestingly, almost no dripping of the oil appears in the process of moving the fiber from the water surface, and there is also no obvious residual oil can be observed in the container. On the contrary, for raw fiber, 6 g of oil is difficult to be completely cleanup from the oil/water mixture, and the dripping of oil is very serious when picking up the oil-loaded fiber. The sorption of oil by fiber is mainly regulated by surface sorption and capillary action, thus the factors such as surface wax, fineness, pore structure, and surface roughness will affect the oil sorption capacity of fiber assembly. For this study, the surface roughness plays an important role in the retention of oil, and the improvement of surface roughness after coating with silica nanoparticle hinders the escape of absorbed oil from the fiber assembly, while the absorbed oil is easier to escape from the assembly of raw fiber due to smooth lumen surface.

3.7. Reusability To recover absorbed oil and reuse the superhydrophobic fiber, oil-loaded fiber assembly is squeezed with the aid of vacuum pump, and the fiber of removing oil is used for the next sorption of oil on the water surface. The reusability change in oil sorption capacity of superhydrophobic kapok fiber for diesel and soybean oil after eight sorption and desorption cycles is shown in Fig. 8. It can be seen that the oil sorption capacities decrease slightly throughout the whole cycles and the decrease of oil sorption capacity does not exceed 20% after 8 cycles of sorption/desorption. The decreases of oil-absorption capacity are mainly attributed to the residual oils in the voids of fiber assembly. Importantly, about more than 90% volume of absorbed oil can be removed by the vacuum filtration. Once the vacuum is generated in the oil recovery system, the absorbed oil is immediately released and delivered into filter container from the funnel. The recovering of oil from fibers via strong mechanical pressure can cause the severe loss of intrinsic sorption capability of porous sorbent due to the occurrence of

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Fig. 6. Removal of oil as a function of oil concentration by raw and superhydrophobic kapok fiber.

Fig. 7. Pictures for the cleanup of soybean oil (colored with oil red O) from water. (a) Soybean oil was floating on water; (b) the oil was absorbed by superhydrophobic fiber; (c) the oil-loaded superhydrophobic fiber was picked up; (d) the oil was absorbed by raw fiber; and (e) the oil-loaded raw fiber was picked up. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

by the hydrophobization of DTMS. After the surface modification of kapok fiber, the existence of uniform nanoscale roughness protuberances with low surface energy imparted the fiber better oil/water selectivity and higher oil sorption capability. The absorbed oil can be easily recovered with the aid of simple mechanical squeezing, with no severe loss of sorption properties after reusing several times. The results also suggest that superhydrophobic modification for cellulosic materials is an effective method to improve the oil sorption capability. Owning to the advantages of high oil sorption capacity, low density, easily scalable fabrication, and excellent hydrophobic property and reusability, the as-prepared fiber is promising as a candidate for the replacement of organic oil sorbent and applied in the large-scale removal of spilled oil on water surface. Acknowledgments

Fig. 8. Reusability of raw and superhydrophobic kapok fiber.

irreversible deformation and the contraction [18,22]. As a result, the sorbents can only be utilized for limited times. In this study, oil is recovered by a large margin from oil-loaded fibers by milder procedure without severe disruption of the fiber hollow lumen. This also implies that removing oil from porous materials with vacuum pump may be an effective method in practical oil recovery. The excellent reusability makes superhydrophobic fiber be more attractive than traditional oil sorbents like polypropylene and active carbons in the cleanup of spilled oil. 4. Conclusions A superhydrophobic surface was fabricated on kapok fiber through the sol–gel method using TEOS as the precursor followed

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