Preparation of surfactant modified inorganic nanofibers for the removal of oily compounds from water
محورهای موضوعی : Nanotechnology Studies in Textiles
1 - Salamat Pardaz Hakim Laboratory
کلید واژه: Oil absorption, Superhydrophobic surface, [3-(2, 3- Epoxypropoxy)-propyl]-trimethoxysilane, Water/oil separation,
چکیده مقاله :
Oily compounds threat people’s health and aquatic life due to their toxic nature. In this project, surfactant grafted functionalized mesoporous silica nanofiber with the aim of separating the oil spillage was synthesized. Firstly, the electrospun mesoporous silica nanofiber was functionalized by [3-(2, 3- Epoxypropoxy)-propyl]-trimethoxysilane in an alkali condition. Secondly, the 4-nonylphenol as a surfactant was grafted onto the surface of functionalized mesoporous silica nanofiber. The grafting yield of surfactant on the surface of functionalized mesoporous silica nanofiber was evaluated. Then by using the BET analysis, the nanofibre surface area was investigated, and finally, optimum conditions with respect to the grafting yield of 4-nonylphenol and BET analysis were reported. In order to characterize the synthesized nanofibers, FTIR, SEM, and AFM analyses were used. The pore size distribution of synthesized nanofibers was investigated by the BJH method.The results showed that the [3-(2, 3- Epoxypropoxy)-propyl]-trimethoxysilane and surfactant were attached to the nanofiber surface through the covalent linkages. SEM images exhibited the deposition of a dense layer on the surface of grafted nanofibers. Moreover, AFM analysis revealed that the surface of the nanofiber became rough after the functionalization and grafting processes. In order to evaluate the superhydrophobic properties of nanofibers, contact angle analyses were investigated. The synthesized absorbent showed a high absorption capacity of 128.1 and 102.3 g/g for heavy motor oil and diesel fuel, respectively. The absorbed oil was easily removed by vacuum filtration and the nanofiber could be reused for several cycles while keeping high absorption capacity
Oily compounds threat people’s health and aquatic life due to their toxic nature. In this project, surfactant grafted functionalized mesoporous silica nanofiber with the aim of separating the oil spillage was synthesized. Firstly, the electrospun mesoporous silica nanofiber was functionalized by [3-(2, 3- Epoxypropoxy)-propyl]-trimethoxysilane in an alkali condition. Secondly, the 4-nonylphenol as a surfactant was grafted onto the surface of functionalized mesoporous silica nanofiber. The grafting yield of surfactant on the surface of functionalized mesoporous silica nanofiber was evaluated. Then by using the BET analysis, the nanofibre surface area was investigated, and finally, optimum conditions with respect to the grafting yield of 4-nonylphenol and BET analysis were reported. In order to characterize the synthesized nanofibers, FTIR, SEM, and AFM analyses were used. The pore size distribution of synthesized nanofibers was investigated by the BJH method.The results showed that the [3-(2, 3- Epoxypropoxy)-propyl]-trimethoxysilane and surfactant were attached to the nanofiber surface through the covalent linkages. SEM images exhibited the deposition of a dense layer on the surface of grafted nanofibers. Moreover, AFM analysis revealed that the surface of the nanofiber became rough after the functionalization and grafting processes. In order to evaluate the superhydrophobic properties of nanofibers, contact angle analyses were investigated. The synthesized absorbent showed a high absorption capacity of 128.1 and 102.3 g/g for heavy motor oil and diesel fuel, respectively. The absorbed oil was easily removed by vacuum filtration and the nanofiber could be reused for several cycles while keeping high absorption capacity
Preparation of surfactant-modified inorganic nanofibers for the removal of oily compounds from water
Abstract
Water pollution is a major issue of developing various industries around the world. Oily compounds threaten people’s health and aquatic life due to their toxic nature. In this study, surfactant-grafted functionalized mesoporous silica nanofiber with the aim of separating the oil spillage was synthesized. Firstly, the electrospun nanofiber was functionalized by [3-(2, 3- Epoxypropoxy)-propyl]-trimethoxysilane. Secondly, surfactant was grafted onto the surface of functionalized mesoporous silica nanofiber. The grafting yield of surfactant on the surface of functionalized mesoporous silica nanofiber was evaluated. Then by using the BET analysis, the nanofiber surface area was investigated, and finally, optimum conditions with respect to the grafting yield of 4-nonylphenol and BET analysis were reported. In order to characterize the synthesized nanofibers, FTIR and SEM analyses were used. The pore size distribution of synthesized nanofibers was investigated by the BJH method and in order to evaluate the superhydrophobic properties of nanofibers, contact angle analyses were investigated. The results showed that the [3-(2, 3- Epoxypropoxy)-propyl]-trimethoxysilane and surfactant were attached to the nanofiber surface through the covalent linkages. SEM images exhibited the deposition of a dense layer on the surface of grafted nanofibers. The synthesized absorbent showed a high absorption capacity of 128.1 and 102.3 g/g for heavy motor oil and diesel fuel, respectively. The absorbed oil was easily removed by vacuum filtration and the nanofiber could be reused for several cycles.
Keywords: Superhydrophobic surface, [3-(2, 3- Epoxypropoxy)-propyl]-trimethoxysilane, Oil absorption, Water/oil separation.
1. Introduction
Water pollution is a major issue of developing various industries around the world. Generally, water pollutants can be divided into insoluble and soluble compounds. Oily compounds as insoluble pollutants threaten people’s health and aquatic life due to their toxic nature. Many methods including mechanical extraction, chemical degradation, dispersants, air flotation, skimming, and combustion have been used for purifying the polluted water. However, these methods are limited because of low separation efficiency, energy cost, and complex separation instruments [1]. Moreover, the oil remaining in the water after poor separation still poses risks to human health and aquatic life. Consequently, efficient and eco-friendly materials and treatment techniques require immediate investigation by governments and researchers. With the increasing knowledge of the fabrication and processing of nanostructured materials, scientists are now investigating and developing various types of novel polymeric nanomaterials that have good physicochemical characteristics and can be used in different applications, using advanced techniques; in this way, new technologies have been established [2].
In the last decade, absorbent materials with superhydrophobic and superoleophilic nature have been attracted many researchers because of the economy and efficiency for the removal and collection of oily compounds. It is known that three factors including surface energy, surface roughness, and homogeneity control the wetting of a solid. Many absorbents such as zeolite, perlite, graphite, carbon nanotube, cellulose-based materials, and polyurethane are widely used in oil spill cleanup. Among them, synthetic organic products including polymeric materials are considered as the most effective oil sorbent because of their high absorption capacity and reusability. Despite the various absorbents with high absorption capacities have been made in different shapes and scales, some drawbacks such as low specific surface area and difficulty in the removal of absorbed oil still exist and need to be overcome. In this regard, producing sorbents with high reusability and high surface area is highly desired [1].
Recently, with the advent of nanotechnology, various nanostructured materials with different compositions and morphologies (e.g., nanoparticles, nanofibers, nanowires, and nanotubes) have been successfully developed to manipulate the surface wettability and construct hierarchical porous structures for oil-water separation materials. Among them, nanofibrous materials with well-designed selective wettability, a large specific surface area, high porosity, high aspect ratio, and multiporous structures with unique chemical, physical, and mechanical functions have been considered as the most promising candidates for highly efficient oil-water separation. Electrospinning, one of the most versatile and effective methods for the manufacture of nanofibrous materials, has gained increasing attention in the oil-water separation field due to its great capability in controlling the porous structures of the fibrous materials and manipulating the surface chemical properties of individual fibers via elaborate design. Consequently, various new types of oil sorbents and oil-water filters have been developed based on electrospinning technology, which exhibits obvious advantages in comparison with conventional fibrous membranes [3].
Electrospun nanofibers as a 1D material have great potential in water treatment because of their highly porous and interconnected pore structure, submicron pore sizes, and large surface area to volume ratio. Nanofiber mats can be easily removed from the solution, which reduces the operation cost. Furthermore, electrospinning is known as a powerful technique to provide sufficient surface roughness for superhydrophobicity [1].
Most soluble polymers with sufficiently high molecular weight can be electrospun. The most attractive electrospinning characteristic is the process’s ability to efficiently form a three-dimensional nanofibrous membrane with a large surface area and high mass flux. Thereby, fabricating a self-standing porous membrane, especially consisting of mesostructured nanofibers, is quite meaningful and highly desirable for adsorption and separation processes. During the past years, there have been many reports on the fabrication of electrospun silica nanofibers. However, polymers with high molecular weight, such as PVP, PVA, PMCM, or PMMA, are often required to maintain a spinnable viscosity in the precursor sol. Without the addition of polymers, fewer porous electrospun silica nanofibers can be prepared. Moreover, of the electrospun silica nanofibers reported before, fewer examples with large areas and self-standing structures seem to be available [4].
Polymers with low-surface-energy groups, such as -CH2, -CH3, -CF2, -CF2H, and -CF3, are the preferential materials in the fabrication of hydrophobic-oleophilic nanofibers by direct electrospinning. Hence, the polymers are tremendous and they can be categorized into two classes: non-fluorinated and fluorinated polymers.
Recently, some low-surface energy biodegradable polymers, including biodegradable polyesters (e.g., polylactide (PLA), poly (glycolic acid) (PGA), poly (ε-caprolactone) (PCL), poly(lactic acid-co-glycolic acid) (PLGA), and poly(hydroxybutyrate) (PHB)) and cellulose derivatives (e.g., cellulose acetate (CA) and ethylcellulose), have attracted a great deal of attention for fabricating hydrophobic-oleophilic surface because of their sustainability and environmental friendliness [5].
Organic−inorganic hybrids are important in a variety of fields as they combine the desirable properties of the inorganic phase (thermal stability, rigidity) with that of the organic phase (flexibility, processability, ductility). In recent years, polymer−silica hybrids with enhanced thermal and mechanical properties (because of the silica component), better flexibility (due to the polymer content), and various tailored properties have attracted a lot of attention for a variety of applications including catalysis, adsorption, pervaporation, sensors, and enzyme encapsulation. The scope and utility of these polymer−silica hybrids can be further broadened by transforming them into nanofibrous structures that exhibit high surface area to volume ratios.
Of the various approaches developed to synthesize polymer−silica hybrid nanofibers, the one-step electrospinning process has received a lot of attention due to its simplicity, cost-effectiveness, and speed [6].
In this study, [3-(2,3- Epoxypropoxy)-propyl]-trimethoxysilane was covalently bonded to the surface of mesoporous silica nanofiber then, 4-nonylphenol as a nonionic surfactant was grafted to the functionalized mesoporous silica nanofiber and its potential to remove oily compounds was investigated.
2. Experimental
2.1. Materials
Triblock copolymer Pluronic P123 and 4-nonylphenol were supplied from Sigma Aldrich, USA. Tetraethyl Orthosilicate (TEOS), [3-(2, 3- Epoxypropoxy)-propyl]-trimethoxysilane, Poly Vinyl Alcohol (PVA) (degree of polymerization: 600), 2-propanol and ethanol were purchased from Merck, Germany.
2.2. Preparation of mesoporous silica nanofiber
A PVA solution with a concentration of 10 % w/w was prepared by dissolving 1 g polymer in 10 mL distilled water at 85 °C under vigorous stirring for 1 h. 1.5 g of TEOS, 2 mL ethanol, 0.1 g 2 M HCl (as the catalyst of hydrolyzation of TEOS), and 0.5 g of P123 were added to the PVA solution and the stirring was continued for 1 h at the temperature of 80 °C under reflux condition. The solution was then electrospun under a fixed electrical field of 20 kV when the distance between the tip of the needle and the collector was 16 cm. The electrospinning apparatus was a Gamma High Voltage Research RR60 power supply and nanofibers were collected onto an aluminum (Al) sheet. The feeding rate of the polymer solution was 0.5 mL/h. In order to remove the polymeric part, the obtained nanofiber was placed in an electrical furnace at 550°C for 3 h with a heating rate of 5 °C/min.
2.3. Functionalize mesoporous silica nanofiber
The functionalization of silica nanofibers was performed by immersing the synthesized nanofibers in a batch containing TEOS (0.3 g), [3-(2, 3- Epoxypropoxy)-propyl]-trimethoxysilane (0.34 g), 2-propanol (30 mL) and distilled water (5 mL) at 30 °C for 1 h. Then, the temperature rose to 60 °C to evaporate solvents. The obtained nanofibers were placed in a vacuum oven at 80 °C for 2 h. Finally, the prepared nanofibers were washed with distilled water and 2-propanol with a ratio of 2:1 and dried.
2.4. Surface grafting of 4-nonylphenol onto functionalized mesoporous silica nanofiber
The functionalized nanofibers were placed in a mixture containing different amounts of 4-nonylphenol (0.22, 0.25, 0.32, 0.38, and 0.41 g), 30 mL of 2-propanol, distilled water (5 mL) for 2 h at 30 °C under vigorous stirring. The pH of the solution was adjusted to 8.5 by 0.1 M NaOH solution. Then, in order to evaporate the solvents, the temperature of the mixture increased to 60 °C. The obtained nanofibers were placed in a vacuum oven at 80 °C for 2 h. Finally, the prepared nanofibers were washed with distilled water and 2-propanol with a ratio of 2:1 and dried.
2.5. Characterization
The BET surface area (Micromeritics Gemini III 2375, USA) and pore-size distribution (AutoPore III, Micromeritics Instrument Co., USA) of synthesized nanofibers were evaluated using the nitrogen adsorption/desorption. Change in the chemical composition of the samples which occurred in different processes was examined by Fourier transform infrared (FTIR) spectroscopy, (Thermo Nicolet NEXUS 870 FTIR from Nicolet Instrument Corp., USA). The surface morphology and topography of nanofibers were characterized by a Scanning Electron microscope (SEM, LEO1455VP, ENGLAND). The contact angle between nanofiber mats and the liquid phase was measured using the static digital method described in the standard D5725-97 (ASTM, 2003). The nanofiber mats were placed on a lifting table mounted on an optical microscope (Olympus SZ e STU2) equipped with a digital camera (Olympus Camedia C-3040). The droplets of liquid (heavy motor oil, diesel fuel, and water) were placed onto the surface of the mats by a microliter syringe. Digital images of droplets on the surface of the mats were captured and the contact angle was calculated.
The grafting yield of 4-nonylphenol on the surface was calculated by Eq. (1) [1].
Where Wa and Wb are the weights of the PDA-PAN NF after and before the grafting process, respectively.
2.6. Oil absorption study
The oil absorption experiments were conducted in pure oils. The experiments were done five times and the standard deviation was <5 %.
The synthesized nanofibers (0.5 g) were immersed in pure oil (heavy motor oil and diesel fuel) at room temperature. After the saturation of absorbents, the absorption capacity Q was calculated by Eq. (2) [1].
Where Wx and Wy are the weight of saturated (after draining for 15 s) and initial nanofibers, respectively. The absorption capacity gives the amount of oil caught by the samples.
2.7. Reusability
The recovery of absorbent was performed by a sand core funnel and a vacuum air pump for 5 min. Then, the nanofibers were used again for absorbing the oil. This cycle was continued for 10 times to evaluate the reusability.
3. Results and discussion
3.1. Gravimetric Analysis
The grafting yield of 4-nonylphenol on the nanofiber surface was calculated and the results are given in Table 1. As shown, an increment in the amount of 4-nonylphenol in the grafting process caused the raising of the grafting yield up to a certain value. It could be related to the existence of a certain amount of reactive sites on the nanofiber surface for attaching the surfactant molecules. After filling these sites, a saturation point was reached. Furthermore, with an increase in the concentration of surfactant, the amount of grafted molecules raised because of the increase in the driving force of the concentration gradient [1].
Table 1. Grafting yield (%) of grafted nanofiber with various amounts of surfactant.
4-nonylphenol content (g) | 0.22 | 0.25 | 0.32 | 0.38 | 0.41 |
Grafting yield (%) | 9.1 | 11.12 | 14.85 | 17.01 | 17.11 |
3.2. BET Analysis
The Brunauer–Emmett–Teller (BET) analysis was performed to evaluate the surface area of various nanofibers and the results are given in Table 2. The result revealed that the silica nanofibers have a large surface area in comparison with other sorbents [2, 3]. The amount of surface area was decreased from 195.7 (for silica nanofibers) to 181.9, 170.1, 162, 157, 92.1, and 48 m2/g for functionalized NF, surfactant grafted NF (for 0.22, 0.25, 0.32, 0.38 and 0.41 g 4-nonylphenol) respectively. Decreasing the BET surface area values is due to raising the diameter of the nanofibers. Surface area is an important parameter in the absorption process. A larger surface area leads to a larger absorption capacity [1].
According to the BET results and also according to the final goal of the present study, which is the preparation of porous nanofibers, maintaining pores on the surface of nanofibers in all stages is of special importance. Therefore, the BET analysis shows that when the amount of surfactant increases to 0.32 g, the decrease in BET can be justified based on the increase in the diameter of nanofibers. The amount of surface area of the grafted sample with 0.32 g of surfactant indicates the presence of pores on the surface of the nanofibers. But as can be seen, with the increase of the active surface to 0.38 and 0.41 g, the BET value drops sharply, which indicates the closing of the pores on the surface of the nanofibers. As a result, due to the large drop in BET values, the optimal amount of surfactant was considered to be 0.32 g.
The nitrogen adsorption/desorption isotherms of membrane nanofiber as shown in Figure 1. The curves can be classified as type IV, typical for mesoporous materials [7]. The capillary condensation occurred at a relative pressure higher than 0.9, indicating the presence of mesopores on nanofibers. Mesoporous structure resulted in a high BET surface area value (195.7, 181.9, and 157 m2/g for silica NF, functionalized NF, and surfactant grafted NF, respectively) in comparison with other research [8].
Table 2. The result of BET analysis for different nanofibers.
Method | Based on nitrogen adsorption | ||
Sample | BET (m2/g) | Vm (cm3/g) | C |
Silica nanofibers | 195.7 | 22.342 | 79.224 |
Functionalized nanofibers | 181.9 | 20.501 | 69.174 |
Surfactant grafted nanofibers (0.22) | 170.1 | 19.314 | 78.248 |
Surfactant grafted nanofibers (0.25) | 162 | 18.478 | 82.654 |
Surfactant grafted nanofibers (0.32) | 157 | 17.639 | 70.364 |
Surfactant grafted nanofibers (0.38) | 92.1 | 10.145 | 69.472 |
Surfactant grafted nanofibers (0.41) | 48 | 5.257 | 72.146 |
Vm: volume of adsorbed nitrogen on the nanofiber surface
(A) |
Volume Adsorped (cm3/gr) |
Relative pressure (P/P0) |
(B) |
Volume Adsorped (cm3/gr)
|
Relative pressure (P/P0)
|
(C) |
Volume Adsorped (cm3/gr)
|
Relative pressure (P/P0)
|
Figure 1. Nitrogen adsorption-desorption isotherm of (A) Silica NF (B) Functionalized NF (C) surfactant grafted NF.
3.3. FTIR analysis
The FTIR spectra of Silica NF, Functionalized NF, and surfactant grafted NF are shown in Figure 2. In the silica nanofiber spectrum (curve A), the stretching and bending vibrations of released or absorbed water molecules were detected at 3395 and 1638 cm-1, respectively [9]. Also, the bending vibrations of Si-O-Si and O-Si-O bond presented at 400-550 cm-1. The asymmetric stretching vibrations of Si-O-Si were detected at 1082 cm-1 [10].
In the Functionalized nanofiber spectrum (curve B), the stretching vibrations of silanol (Si-O-H) were detected at 3799 cm-1 [11]. Absorption characteristic peaks at 3371 and 1630 cm-1, 2941 and 2874 cm-1, 1460 and 1401 cm-1 relating to the O–H stretching and bending vibration, stretching mode of CH2 group, and bending mode of CH2 group, respectively [12]. Also, the vibrations of C-O bond presented at 1204 cm-1.
In the surfactant grafted nanofiber spectrum (curve C), the bands at 3869 and 3741 cm-1 are related to the stretching mode of silanol (Si-O-H). The stretching and bending vibrations of OH groups, stretching vibrations of C-C and C-H bond in benzene rings exhibited the absorption peaks at 3302 and 1632 cm-1, 3052 and 1502 cm-1, respectively [1]. Also, other bands observed at 2928 and 2861 cm-1, 1462 and 1372 cm-1 that can be attributed to stretching and bending vibrations of CH2, respectively. The bands at 1252 and 1111 cm-1 are related to the vibrations of C-O bond and C-O-C bond, respectively [13]. Absorption characteristic peaks at 1103, 813, 603 and 475 cm-1 relating to stretching vibrations of silica. The possible reaction among the components can be proposed in Figure 3.
Figure 2. FTIR spectra of (A) Silica NF (B) Functionalized NF (C) surfactant grafted NF
Figure 3. The possible reaction among the components.
3.4. BJH analysis
The Barret–Joyner–Halenda (BJH) analysis was performed to evaluate the pore size distribution of synthesized nanofibers and the results are given in Table 3. The result revealed that the mean pore diameter for silica nanofiber, functionalized nanofiber, and surfactant grafted nanofiber were 6.47, 5.89, and 4.05 nm, respectively. Also, the amount of mean pore diameter was decreased after functionalizing and surfactant grafting onto nanofiber because of an increase in the nanofiber diameter. Similar results have been reported by other researchers [14].
Table 3. The results of BET and BJH methods.
Sample | BET (m2/g) | Vtotal (cm3/g) | ap (nm) |
Silica nanofibers | 195.7 | 0.666 | 6.47 |
Functionalized nanofibers | 181.9 | 0.589 | 5.89 |
Surfactant grafted nanofibers (0.32) | 157 | 0.515 | 4.05 |
ap: mean pore diameter
Vtotal: total pore volume
3.5. Microscopic characterization
Figure 4 shows the SEM images of silica nanofiber, functionalized nanofiber, and surfactant grafted nanofiber. As seen in this figure, the silica nanofiber had a rigid structure with an almost uniform diameter, and the average diameter of silica NF was 100 nm (fig.5 A). The SEM image of functionalized nanofiber showed less fragility in comparison to the silica NF. This is probably due to the presence of the molecules of [3-(2, 3- Epoxypropoxy)-propyl]-trimethoxysilane and the incorporation of silica with amorphous nature on the surface [15]. Also, the continuity and the length of nanofibers were increased. The average diameter of functionalized NF was 109 nm (fig.5 B). The SEM image of surfactant grafted nanofiber showed a uniform structure with an average diameter of 119.5 nm. The continuity of nanofibers was increased in comparison to the two previous ones. Also, the surface of the nanofiber showed less fragility (fig.5 C). Regarding the SEM images, it is inferred that the organic molecules and surfactant on the surface of nanofibers have been successfully grafted because the average diameter of the nanofibers has increased, while the fragility of nanofibers decreased.
Figure 4. SEM image of (A) Silica NF (B) Functionalized NF (C) surfactant grafted NF.
3.6. Oil absorption experiment
3.6.1. Determination of contact angle
In order to reflect the hydrophobic and oleophilic nature of synthesized nanofibers, surface wettability has been investigated. In this regard, oil and water droplets were dripped on the surface of nanofiber mats, and their contact angles were recorded. The results show that the heavy motor oil and diesel fuel droplets have a contact angle of 93 and 120° on the surface of silica nanofiber, respectively. This value decreased to 90 and 110° for functionalized silica nanofiber. This is related to the presence of long hydrocarbon chains on the surface of silica nanofibers. The water droplet on the surface of the silica nanofiber and functionalized silica nanofiber had a contact angle of 45 and 52°, respectively, confirming the hydrophobic nature of the functionalized nanofiber. For the surfactant grafted nanofiber, the contact angles for both heavy motor oil and diesel fuel were 0°. This indicates on the superhydrophobic property of the surface grafted nanofiber (the water contact angle was 157°) [16]. Also, the researchers stated that increasing the number of carbon atoms would increase hydrophobicity [17].
Figure 5. Contact angle different liquids test for Silica NF, Functionalized NF, and surfactant grafted NF.
3.6.2. Maximal oil absorption capacity
The maximum oil absorption capacity of silica nanofiber, functionalized nanofiber, and surfactant grafted nanofiber for heavy motor oil and diesel fuel are calculated and the results are presented in Figures 7 and 8. The results show that the absorption capacity for the grafted nanofiber is 128.1 and 102.3 times of its own weight for motor oil and diesel fuel, respectively. The higher sorption capacity of heavy motor oil than diesel fuel is a result of motor oil being heavier than diesel within the same unit volume [1]. Furthermore, the oil viscosity is the other reason for this difference. High viscosity can enhance the absorption capability by improving the adherence of oil onto the nanofiber surface [18]. The viscosity of the motor oil and diesel fuel was 0.31 and 0.06 Pa.S-1, respectively.
The Van der Waals forces play an important role in absorbing the oils, especially at the initial stage of absorption. After that, the presence of pores in the absorbent and high surface area values influences on the absorption efficiency. The high surface area provides large oil-contacting areas on the nanofiber.
Figure 6. The absorption capacity of Silica NF, Functionalized NF, and surfactant grafted NF for diesel fuel.
Figure 7. The absorption capacity of Silica NF, Functionalized NF, and surfactant grafted NF for heavy motor oil.
3.6.3. Reusability of nanofiber
Figure 8 shows the maximum oil absorption capacity of surfactant-grafted nanofibers at different cycles (1–10). As can be seen, the maximum absorption capacity gradually decreased during the cycles. It is related to the irreversible deformation of absorbent [19]. Also, the decrease in oil absorption capacity can be attributed to the residual oils in the voids of the nanofiber assembly [18]. The difference between the absorption capacity of cycles 1 and 2 was higher than the difference between cycles 2 and 10. It is because of the penetration of oils into some absorbent pores that full recovery is not possible. Such a result is reported by researchers [20]. However, from the second cycle until the tenth cycle, no significant changes in the sorption capacity were observed. This can be due to the conserved structure of nanofiber. The oil recovery in the first cycle was 78.49 and 80.79 % for motor oil and diesel fuel, respectively. These values increased to 93.22 and 94.92 % in the second cycle. The recovery value was slightly decreased during the next cycles. It was concluded that the surfactant grafted nanofiber can be a good candidate for the removal of oil spillage.
Figure 8. The maximum absorption capacity of absorbed oils by surfactant grafted nanofiber.
4. Conclusion
In this study, superhydrophobic and superoleophilic nanofiber for the removal of oil spills from the water was synthesized by grafting the 4-nonylphenol onto the surface of functionalized silica nanofiber. Functionalization of silica nanofiber was performed by [3-(2, 3- Epoxypropoxy)-propyl] - trimethoxysilane in an alkali condition. Optimum conditions with respect to the grafting yield of 4-nonylphenol and BET analysis were reported and the optimal amount of 4-nonylphenol was 0.23 gr (equal to 85.81% graft). The nitrogen adsorption/desorption isotherms of membrane nanofiber represent an isotherm of type IV that confirms the mesoporous structure. The total pore volume and BET surface area of the grafted nanofiber were 0.51 cm3/gr and 157 m2/gr, respectively caused to a high absorption capacity. SEM images confirmed a successful grafting of surfactant on the nanofiber surface through the deposition of a dense layer on the surface. Also, it was found that the grafted nanofiber has superhydrophobic and superoleophilic properties because of the oil contact angle of 0.° The absorption capacity of the synthesized nanofiber was 128.1 and 102.3 g/g for heavy motor oil and diesel fuel, respectively.
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