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Superhydrophobic and superoleophilic carbon nanofiber grafted polyurethane for oil-water separation
Process Safety and Environmental Protection 123 (2019) 327–334
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Superhydrophobic and superoleophilic carbon nanofiber grafted polyurethane for oil-water separation Nadeem Baig a , Fahd I. Alghunaimi b , Hind S. Dossary b , Tawfik A. Saleh a,∗,1 a b
Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Saudi Aramco, EXPEC Advanced Research Center, Production Technology Division, Dhahran 31311, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 15 October 2018 Received in revised form 30 December 2018 Accepted 13 January 2019 Available online 15 January 2019 Keywords: Oil cleaner production Sustainable technical processes Hydrophobic Oleophilic Nanofiber
a b s t r a c t The oil contamination in water and industrial wastewater is a concern for the environment. The role of carbon nanofiber (CNF) grafted onto polyurethane (PU) for oil absorption was investigated under ambient conditions. Superhydrophobic and superoleophilic materials based on CNF grafted PU were synthesized by a dip coating for oil/water separation. The synthesized materials were characterized by SEM, FTIR, BET and by a dynamic oil separation setup. Surface grafting with CNFs substantially increased the surface area of PU by nearly 31X (9 m2 /g to 276 m2 /g), which also decreased the average pore size in the PU matrix from 2567 Å to 36 Å. The decrease in pore size significantly improved the capillary action of the synthesized CNF grafted PU and provided almost no chance for water to pass. CNF grafted PU has successfully separated oil from different oil/water models and displayed absorption capacity up to 50 times to its own weight. The absorbed oil can be recovered by squeezing CNF grafted PU, which retains its original shape after releasing the pressure, and thus it was successfully used multiple times. The flexibility and the mechanical stability of the material allows it to be used for the continuous separation of oil from oil-contaminated water. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Oil-water separation is considered to be worldwide challenge especially for produced water treatment and oil spillage (Hu et al., 2014; Xue et al., 2011). As a result, materials are being introduced to achieve the refined separation of oil and water for environmental and industrial applications. Materials were established for oil and water separation using their specific wettability (Baig and Saleh, 2018; Wen et al., 2013; Yoon et al., 2014). A few materials were synthesized which contained the properties of hydrophobicity and oleophilicity for the refined removal of oil and water (Calcagnile et al., 2012; Shang et al., 2012; Yu et al., 2014). These hydrophobic materials include porous ceramic membranes (Su et al., 2012), hydrophobic coated meshes (Lu et al., 2014), modified foam and sponges (An et al., 2015; Chen et al., 2017; Wu et al., 2014), carbon-based materials (Lee et al., 2015), and modified fabrics (Gu et al., 2017). Similarly, various methods were applied to attain hydrophobic surfaces such as electrospinning techniques (Liao et al., 2014), chemical vapor deposition (Hsieh et al., 2008), sol-gel methods (Venkateswara
∗ Corresponding author. E-mail addresses: tawfi[email protected], tawfi[email protected] (T.A. Saleh). 1 Home Page: http://faculty.kfupm.edu.sa/CHEM/tawfik/publications.html.
Rao et al., 2009), hydrothermal methods (Li et al., 2012), and solution immersion methods (Li et al., 2011). TiO2 -coated copper meshes have displayed ultrafast oil-water separation and possessed self-cleaning capability due to the light-sensitive behavior of the TiO2 (Liu et al., 2016). Similarly, Switchable self-cleaning TiO2 nanostructured mesh membrane was achieved by the electrochemical anodization and the heating process (Kang et al., 2018). In another example, the suspension polymerization method was used to develop a ZnO nanowires/acrylic ester composite resin (Yan et al., 2018). The combination of anodization, fluorination, and solvent exchange method was also used to develop a slippery liquid-infused porous hydrophobic surface. The melt extrusion and the water leaching method were used to construct a hydrophobic tridimensional interconnected porous high-density polyethylene bundles. The hydrophobicity was further improved by skin-peeling (Zhang et al., 2018). Apart from these, other methods are also being applied to the development of hydrophobic surfaces (Li et al., 2015a). The surface roughness and modification of the surface with low surface energy materials are playing a great role to achieve ultimate superhydrophobic surface (Latthe et al., 2015). The polymer nanocomposite and the polymer-based nanostructured material are getting significant attention for the separation of the oil and water. The superhydrophobic-superoleophilic polystyrene nanofibers were deposited on the stainless steel mesh by electrospinning has displayed excellent superhydrophobic-
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superoleophilic behavior (Lee et al., 2013). The micro/nano hierarchical structure of the shish-kebab was modified to develop a superhydrophobic material (Sun et al., 2018). The concentration controlled interaction of the nanocellulose and sodium dodecylsulfate has produced the super light 3D hierarchical nanocellulose aerogel foam for efficient oil absorption capacity (Zhang et al., 2019). For oil and water separation, the nanomaterial is generally introduced into the support surface which has the capability to take the oil or water due to its well organized or porous random network. There are numerous examples in which nanomaterial is utilized to improve the hydrophobicity of the material. The reduced graphene oxide was coated on the cotton fibers to impart the hydrophobic behavior to the cotton (Dashairya et al., 2018). A flexible polymer-based graphene foam was obtained by self-assembling the graphene sheets on the 3D skeleton of the polymer (Wu et al., 2013). Carbon nanotubes due to their hydrophobic nature were explored for the oil and water separation. Hydrophobic carbon nanotube (CNT) sponges were developed by chemical vapor deposition using ferrocene in dichlorobenzene as a precursor (Gui et al., 2010). A porous monolith of the polycarbonate/carboxyl-functionalized multiwalled carbon nanotubes has displayed the tendency to selectively absorb oil and organic solvents from the contaminated water (Li et al., 2018). Overall, continuous development in the fabrication of nanocomposite polymers and the nanostructured material has been observed for oil and water separation. Carbon nanofiber is another member of the carbon nanomaterial family. Carbon nanofiber due to its elongated shape and consisting of carbon atoms has displayed excellent hydrophobic characteristics. Zhen-Yu Wu et al. synthesized carbon nanofiber aerogels by a freeze-drying method with multiple steps. The prepared pure CNF aerogel demonstrated good ability to take oil (Wu et al., 2015). The preparation of the CNF aerogel is somewhat complex and requires extreme freezing and heating conditions, for example, heating up to 1450 ◦ C. The CNF is not much explored for the oil and water separation application. The CNF combination with porous support can make it a valuable material for the oil/water separation. Polyurethane sponge is porous in nature and provides an opportunity to use for the liquid absorption. Its specificity in wettability is improved by modifying its porous network with low surface energy hydrophobic material (Liu et al., 2015). Combination of the CNF and the polyurethane is a good choice to produce a hydrophobic sponge for cleaning and absorption of the oil from the contaminated water. Here, superhydrophobic carbon nanofiber grafted polyurethane (CNF grafted PU) was synthesized via dip coating for fast separation of oil from water. The porous network of the polyurethane provided enough space for the absorption of oil from water. CNF grafting performed two functions in the polyurethane. First of all, it increased the hydrophobicity of the polyurethane sponge and secondly, it increased the number of pores which might be responsible for capillary action to efficiently remove oil and other organic contaminants from water. The synthesized CNF grafted PU showed excellent performance in separating non-polar organic contaminants and oil from water in both static and dynamic modes. 2. Materials and methods 2.1. Materials Toluene and acetone were purchased from Merck (Germany). Hexane, heptane, and o-Xylene were obtained from Sigma-Aldrich (Germany). Ethanol was attained from Baker Analyzed ® Reagent. The polyurethane was purchased from the local market and cleaned thoroughly prior to use. The chemicals and the reagents were analytical grade and were used without any further purification. Distilled water was produced in-house using a Labstrong FiSTREEM II 2S Glass Still distiller.
2.2. Instrumentation A Fourier-transform infrared spectroscopy (FTIR) study of the samples was done using a Thermo Scientific Nicolet iS10. The BET Surface area and pore size of the different CNF grafted materials were measured using a MicromeriticsTriStar II Plus instrument. A thermo-scientific magnetic stirrer was used for the stirring of CNF dispersed ethanol during the CNF grafting process. The distilled water was collected from a homemade distillation unit. A Blue M oven was used for curing and drying the different materi® ® als. A Masterflex Easy-Load peristaltic pump was used to achieve the dynamic separation of oil from water. The surface inspection of the materials was done with the help of a Field Emission Scanning Electron Microscope (SEM) (JSM-6610LV, Scanning Electron Microscope, JEOL) by applying a 20 kV acceleration voltage. 2.3. Synthesis of g-CNF grafted polyurethane Polyurethane (PU) was sonicated for 30 min in acetone for activation to remove any particles inside and then it was dried in an oven. CNF was dispersed and prepared in ethanol (1 mg/mL) by sonication for 30 min. The polyurethane foam was kept in the CNF dispersed ethanol solution for five minutes. After 5 min the obtained g1 -CNF grafted PU was dried in an oven at 60 ◦ C. During the curing process, the CNF anchors onto the PU surface and pores. In a similar way, another PU was dipped in the dispersed CNF and dried, then dipped again and dried to obtain g2 -CNF grafted PU. Similarly, g3 -CNFgrafted PU was prepared by being dipped three times. The obtained materials were labeled as PU, g1 -CNF grafted PU, g2 CNF grafted PU, and g3 -CNF grafted PU. Scheme 1 demonstrates the synthesis route of CNF grafted PU. 3. Results and discussion 3.1. FTIR characterization FTIR was used as one of the tools to observe the CNF and PU interaction after grafting. Peak shift, the appearance or disappearance of peaks, is considered a criterion to observe changes. Some differences in the FTIR spectra of the PU and the g-CNF grafted PU were observed. The FTIR spectra of the pure polyurethane can be seen in Fig. 1A. The prominent peaks at 3368 cm−1 and 1563 cm−1 are due to the stretching and deformation vibration of the N H (Rangel-Vazquez et al., 2014). In the FTIR spectra, no characteristic asymmetric stretching vibration of NCO was observed at 2270 cm−1 , which is an indication that there is no free NCO functionality due to polymerization (Peng et al., 2013). The asymmetric and symmetric stretching frequency bands of the C H bond of the CH2 groups appear at 2975 and 2850 cm−1 , respectively. The carbonyl stretch peak was assigned to vibration at 1683 cm−1 (Jian et al., 2009). The FTIR spectra of the g-CNF grafted PU were slightly different from the pure PU. The CNF grafting into PU clearly shows a peak shift. Only a few peaks were shifted which is attributed to the functional groups. Moreover, the shift was regular as the grafted CNF concentration was increased by dip coating. The N H stretching, CO, and N H deformation vibration peaks were shifted from 3367 to 3351 cm−1 , 1682 to 1674 cm−1 , and 1563 to 1536 cm−1 , respectively. In all FTIR spectra, the basic structure of the polyurethane was retained and, after CNF grafting, some peak shift appeared. It is indicated that CNF is successfully grafted into the polyurethane. 3.2. Morphological study of the CNF grafted polyurethane The surface of the pristine polyurethane and the CNF grafted polyurethane was evaluated using scanning electron
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Scheme 1. Scheme illustrating the synthesis route of CNF grafted PU for oil/water separation.
Fig. 1. FTIR spectra of (A) Pure PU, (B) g1 -CNF grafted PU, (C) g2 -CNF grafted PU and (D) g3 -CNF grafted PU.
microscopy (SEM). The pristine polyurethane demonstrated a three-dimensional porous morphology. The different size of various diameters could be observed in the SEM images (Fig. 2Aa) which
were in the range of 150–900 m. The porous morphology of the polyurethane actually provides the channels for the retention or passage of water and oil. A high-resolution SEM image (Fig. 2Ba) of
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Fig. 2. SEM images at two magnification (A) 500 m, (B) 5 m, (C) 1 m of (a) PU, (b) g1- CNF grafted PU, (c) g2 -CNFgrafted PU, and (d) g3 -CNFgrafted PU.
the pristine polyurethane surface displayed a plane surface without any extended portion. Prior to CNF grafting the surface of the PU is entirely plane and no roughness on the surface can be seen. The SEM images of the g-CNF grafted PU demonstrated a porous threedimensional morphology like pristine polyurethane (Fig. 2Ab, Ac & Ad). This morphology explained that the grafting of CNF has no effect on the shape of the material porosity. However, the SEM images under high resolution, 1 and 5 m) showed fibers on the surface of the PU providing sub-channels. The presence of the CNF provided both the roughness and the additional channels with a small degree of porosity which enhanced the hydrophobicity of the large holes. The generation of a small degree of porosity with additional channels is evident from the magnified SEM image (Fig. 2Cd) of the g3 -CNF grafted PU. The generation of additional channels along with the surface roughness is effective for the separation of the oil from the water. The CNF grafting on the surface of the PU was substantially enhanced as the grafting process moved from the first to the third dip coating. Therefore, further grafting provided no significant enhancement. The ultimate g3 -CNF grafted PU was selected because it showed sufficient grafting of the CNF on the
surface of the polyurethane which is responsible for imparting the highly hydrophobic character to the polyurethane (Fig. 2Bd & Cd). The PU, the prepared CNF grafted PU surface area, and the porosity was investigated using nitrogen adsorption and desorption isotherms. Through this adsorption and desorption of nitrogen, the BET surface area of the modified and unmodified material was calculated. The BET surface area results revealed that the grafting of CNF has a substantial effect on the surface area of the PU, as is evident from Fig. 3. A regular increase in the surface area was found as the grafting of the CNF on the PU surface increased. The pure PU has a very low surface area of 9 m2 /g, although it was increased significantly after the first grafting of CNF to 103 m2 /g. It was continuously improved and the g3 -CNF grafted PU surface area was 276 m2 /g. The surface area after the CNF grafting was increased almost 31 times. The adsorption is directly related to the surface area and the porosity of the material. The CNF grafting had a significant effect on the pore size of the PU. The pure PU showed a large adsorption and desorption pore size of 2567 Å and 2569 Å, respectively. The CNF grafting considerably reduced the pore size and a regular trend was observed as the grafting moved from g1 to g3 (Table 1). These results showed that the CNF reduced the average pore size from
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Table 1 Data of the PU and different CNF grafted PU by Brunauer Emmett Teller Model. Name
Surface area m2 /g
Adsorption Pore Size (Å)
Desorption pore size (Å)
Qm (mmole/g)
Molecular Cross-Sectional Area (nm2 )
R2
PU g1 -CNF grafted PU g2 -CNF grafted PU g3 -CNF grafted PU
8.9 102.9 152.0 276.0
2567.5 403.7 81.6 36.4
2,568.6 404.5 81.8 36.5
0.091 1.05 1.56 2.8
0.162 0.162 0.162 0.162
0.997 0.901 1.00 0.869
Fig. 3. BET surface area comparisons of the PU and the different CNF grafted PU and the prepared CNF grated PU with different amount of CNF.
2567 to 36 Å. The porous arrangement of the CNF on the PU walls definitely played a role in enhancing the overall surface area of the material. The pure PU has a small surface area due to the existence of only the large pore size. The CNF grafted PU has a large surface area and at the same time an average small pore size. The average small pore size is evidence that the CNF grafting has generated a CNF porous network on the PU surface. That porous network actually contributed to reducing the pore size significantly. It is clear from the SEM images that large channels remain the same in the PU before and after the grafting with CNF. The significant decrease in the average pore size is due to the generation of new pores by the CNF which also contributed to a huge increase in the surface area. The CNF provided additional pores and new channels which significantly improved oil adsorption. Due to the large surface area and the small pore size of the hydrophobic surface, the CNF grafted PU reduced the slippage of the water from it. It also enhanced the capillary effect of the material for the intake of the oil. 3.3. Separation evaluation 3.3.1. Static removal of oil The water and oil taking capabilities of the variously modified sponges were investigated using a mixture of hexane and water, which were colored with methylene blue to improve the physical visualization of the phases. The behavior of the polyurethane was sharply changed before and after CNF grafting. The polyurethane before CNF grafting took both water and oil. The commercially available PU is hydrophilic in nature (Wang et al., 2015) and shows the capability to take both water and oil. This was also evident when the commercial PU was dipped into the water and hexane. It was almost half in the water and a half in the n-hexane. The hydrophobicity of the polyurethane foam was controlled by dip coating with CNF. The overnight dried material was also investigated in the mixture of hexane and water. It was observed that the g1 -CNF grafted PU still took water with n-hexane, although the water taking capability was significantly reduced compared to the PU (Fig. 4). The hydrophobicity of the g2 -CNF grafted PU was further improved by a second time dipping into CNF ethanol. Fig. 4 demonstrates that the hydrophobicity was significantly enhanced. Also this time the PU only took a few drops of water while the remainder was hexane. The superhydrophobicity of the g3 -CNF grafted PU was attained after the 3rd time dipping. This time the CNF grafted PU did not absorb any water and only hexane was taken by the PU. The water
Fig. 4. (A) Behavior of sponge after immersing and squeezing two times in Hexane and water mixture.
drops can be clearly seen in the 2nd dipped CNF-sponge, while these water drops are entirely absent in the 3rd dipped CNF-PU (Fig. 4). This could be used as one parameter to control the hydrophobicity of the surface. The CNF grafting process can continue until it does not take any water. Moreover, the hydrophobic behavior of the PU and the differently modified PU can also be seen in the mixture of the n-hexane and the water. Most of the PU was in water while, with the 1st dipped CNF grafted PU, some part of it was in the water. The 2nd dipped CNF grafted PU was slightly in water with the remainder in the n-hexane. Interestingly, the 3rd dipped CNF grafted PU just floats on the n-hexane and no part of it was in the water. The further coating showed no significant difference. Therefore, the g3 -CNF grafted PU was used as the optimum graft. These measurements were made by twice pressing on the bottom of the beaker in the n-hexane and water mixture (Fig. 4). It observed that the pure PU with hexane has taken a lot of water and its surface turns blue due to the methylene blue in the water, whereas this color was absent in the CNF grafted PU. This shows that CNF grafting on the surface extensively improved the hydrophobicity and prevented water penetration. The surface hydrophobicity of the various non-modified and modified PU was furthermore evaluated with the help of the water contact angle. The lowest water contact angle was observed with the PU 109 ± 6◦ which was substantially increased to 146 ± 4◦ after grafting with the CNF (g3 -CNF grafted PU). The water contact angle of the g1 -CNF grafted PU and g2 -CNF grafted PU was found 115◦ ±
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Fig. 6. Separation efficiency for various non-polar organic solvents. Fig. 5. Water contact angle on the various surfaces of the materials (A) PU, (B) g1- CNF grafted PU, (C) g2 -CNFgrafted PU, and (D) g3 -CNFgrafted PU.
2◦ , 133◦ ± 1◦ , respectively (Fig. 5). The hexane contact angle was found almost zero with all surfaces. Hexane was readily absorbed by all PU surfaces without staying on the surface. From the water contact angle study, it has become evident that the surface is highly oleophilic and enough hydrophobic to take oil from the contaminated water. The oleophilic and the hydrophobic behavior of the modified PU can be elucidated by the presence of the carbon nanofibers on the surface of the PU. The separation mechanism of the oil from the water with the g3 -CNF grafted PU is explained by the non-polar or polar nature of the materials and the liquids. The CNF is a non-polar in nature and allowed the oil to readily spread on its surface which rapidly moved into the porous network of the polyurethane. On the other hand, the water polarity is prevented it to pass or absorb by the non-polar surface of the CNF grafted PU.
3.4. Separation efficiency, Reproducibility, and intake of different petrol components The separation efficiency of the g3 -CNF grafted PU was evaluated by separating the various non-polar solvents such as hexane, heptane, octane and nonane from the water. The g3 -CNF grafted PU has displayed a good capability to separate the non-polar organic components from the water. The separation efficiency for hexane, heptane, octane, and nonane was found 97.8, 99.8, 95.0 and 96.3%, respectively (Fig.6). The separation capacity and the reproducibility of the CNF grafted PU was investigated with different oils and non-polar solvents and it was also applied to the real sample of petrol mixed with water. The reproducibility of the grafted PU was studied by using a hexane solvent. For this purpose, the weight of the dry g3 -CNF grafted PU was measured and then it was dipped into the hexane solvent. After that, its weight was again measured to find out the intake of the n-hexane. The weight gain ratio was calculated by applying the following formula: Weight gain ratio(%) = (weight of the solvent absorbed g3
3.3.2. Dynamic removal of oil For the oil-water mixture, it is preferable to use a dynamic system for separation. Therefore, the capacity of the prepared material was tested using the dynamic system. A highly hydrophobic porous network of g3 -CNF grafted PU can be applied to the separation of bulk oil from the water, which usually happens during oil spillages where a large quantity of oil is added to the body of water. The bulk of the hexane and the water mixture were used to evaluate the separation efficiency of the prepared materials. This was achieved by fitting the sponge into a pipe. This was accomplished very easily due to its mechanical stability and flexibility. The fitted sponge was inserted into the mixture of hexane and the methylene blue colored water in such a way that some of it was in the water while rest of the part in the hexane. A peristaltic pump was used to create pressure for the removal of hexane from the mixture of water and oil. As the peristaltic pump was started, it was observed that the hexane moved very fast from the g3 -CNF grafted PU without taking any water. After some time, in the beaker, there was only methylene blue colored water and hexane was separated from the mixture to the collector container. Moreover, after the removal of the hexane, the peristaltic pump continued to take water i.e. the pump was ON. Due to the pressure of the pump, only bubbles of air were passed through the pipe, while no methylene blue colored water was passed. This is an indication that the g3 -CNF grafted PU is highly hydrophobic and oleophilic. Due to its ease of preparation, its cost-effectiveness and its unique hydrophobic and oleophilic properties, the g3 -CNFgrafted PU may prove to be a valuable material for the removal of oil spillage from water.
-CNF grafted PU- the weight of the dry g3 -CNF grafted PU)/ (weight of the dry g3 -CNF grafted PU) ∗ 100 The CNF grafted PU demonstrated sober reproducibility for the intake of hexane after multiple cycles (Fig. 7A). The weight gain ratio for hexane was found to be in the range of 2650 to 2750% as calculated by the above-mentioned formula. The relative standard deviation (RSD) was found to be 1–2%, with number of measurements of 15. The small variation in the multiple cycles demonstrated that the developed hydrophobic material showed good reproducibility and can be used multiple times for the separation of oil or its contaminants. The behavior of the g3 -CNF grafted PU was also observed for other organic solvents (Fig. 7B). The percentage of weight gain ratio demonstrated that weight gain is also dependent on the density of the organic solvent. The organic solvent with a high density showed an increased weight ratio. However, it has demonstrated good separation capability for the hexane, heptane, toluene, and o-xylene. The weight gain ratio was in the range of 2600 to 5000% calculated by the abovementioned formula. This was also applied to the real sample of petrol mixed with water and the weight gain was found to be 3258%. This huge increase in the weight gain ratio of the g3 -CNFgrafted PU demonstrated that it has an excellent capability for the absorption of various nonpolar organic solvents, including petrol. The absorbed nonpolar components can be released very easily by simply compressing the CNF grafted PU with good reproducibility. Under pressure, its structure did not collapse and it easily regained
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Fig. 7. (A) Adsorption recyclability of the g3 -CNF grafted PU for the Hexane. (B) Adsorption capability of the g3 -CNF grafted PU for oil and different nonpolar solvents. Table 2 Comparison of the absorption capacity of the g3 -CNF grafted PU with other reported materials. Sr#
Material
Surface area (m2 /g)
Hexane (g/g)
Toluene (g/g)
Regeneration
Ref.
1
MnO2 /poly(nbutylacrylate-cobutyl methacrylate-co-methyl methacrylate) resin composite Fluorinated polydopamine/chitosan/reduced graphene oxide composite aerogel CNT/PDMS-coated PU sponge PU–CNT–PDA–ODA sponge Porous BNNS/PVDF composite material Graphene Foam Monolithic superhydrophobic silica aerogel Magnetic graphene foam g3-CNF grafted PU
74.38
2.07
–
Drying
Zhang et al. (2017)
51.76
12
8
Heating, Squeezing
Cao et al. (2017)
–
15
–
–
Wang and Lin (2013)
– –
34.9 3.8
5
Squeezing Ethanol washed and dried at 60 C◦ in air
Wang et al. (2015) Yu et al. (2015)
– –
15 7
20 9
Ethanol washing and drying in an oven Distillation and vacuum filtration
Yang et al. (2017) Wang and Wang (2018)
– 276
15 26.6
19 49.2
Hexane immersion Squeezing
Yang et al. (2014) This work
2
3 4 5 6 7
8 9
its shape as the applied pressure was released. The comparison of the g3 -CNF-grafted PU with other hydrophobic materials revealed that its absorption capability is much better (Table 2). The increased absorption capacity can be explained due to the exceptionally high surface area which substantially increased due to the CNF grafting. Moreover, the CNF also facilitated achieving the numerous porosities with small diameters which improved the selectivity of the material. The g3 -CNF-grafted PU has become selective for non-polar components in the contaminated water. This behavior of the g3 CNF-grafted PU makes it possible to remove emulsified oil from water. It not only displayed great capability for the adsorption of oil but it can also fit into the tube to perform the function of the continuous removal of oil contamination from the water. It also provides an opportunity to reuse the collected non-polar organic components and reduce the burden of organic pollution on the environment.
4. Conclusion This work has demonstrated that loading polyurethane (PU) with carbon nanofiber (CNF) could enhance the separation of oil from water. The prepared g3 -CNF-grafted PU showed higher efficiency in oil separation from water compared with either the g1 -and g2 -CNF-grafted PU or the pure PU. The CNF improved the hydrophobic character of the PU and the surface area was significantly improved from 9 to 276 m2 /g. It also substantially reduced the average pore size from 2567 to 36 Å which might be responsible for the huge surface area and the strong capillary action for
oil uptake. The contact angle of the water was found to be 146 ± 4◦ which demonstrated the high hydrophobicity of the surface. The curing process provides stability to the surface and it can be used multiple times for absorption processes. The enhanced absorption capacity can be explained by the high surface area. The absorption capacity for oil and other organic solvents was found comparable or better than others. Moreover, it can be used as a filter for bulk oil spillage. The developed g3 -CNF grafted PU provides an opportunity for use in the large-scale bulk removal of oil due to its continuous removal capability, mechanical stability, high absorption capacity, chemical inertness, large surface area, and small pore size. References An, Q., Zhang, Y., Lv, K., Luan, X., Zhang, Q., Shi, F., 2015. A facile method to fabricate functionally integrated devices for oil/water separation. Nanoscale 7, 4553–4558, http://dx.doi.org/10.1039/c5nr00026b. Baig, N., Saleh, T.A., 2018. Initiator-free natural light-driven vapor phase synthesis of a porous network of 3D polystyrene branched carbon nanofiber grafted polyurethane for hexane /Water separation. ChemistrySelect 3, 8312–8318, http://dx.doi.org/10.1002/slct.201801549. Calcagnile, P., Fragouli, D., Bayer, I.S., Anyfantis, G.C., Martiradonna, L., Cozzoli, P.D., Cingolani, R., Athanassiou, A., 2012. Magnetically driven floating foams for the removal of oil contaminants from water. ACS Nano 6, 5413–5419, http://dx.doi. org/10.1021/nn3012948. Cao, N., Lyu, Q., Li, J., Wang, Y., Yang, B., Szunerits, S., Boukherroub, R., 2017. Facile synthesis of fluorinated polydopamine/chitosan/reduced graphene oxide composite aerogel for efficient oil/water separation. Chem. Eng. J. 326, 17–28, http:// dx.doi.org/10.1016/j.cej.2017.05.117. Chen, J., You, H., Xu, L., Li, T., Jiang, X., Li, C.M., 2017. Facile synthesis of a two-tier hierarchical structured superhydrophobic-superoleophilic melamine sponge for rapid and efficient oil/water separation. J. Colloid Interface Sci. 506, 659–668, http://dx.doi.org/10.1016/j.jcis.2017.07.066.
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Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Superhydrophobic and superoleophilic carbon nanofiber grafted polyurethane for oil-water separation Nadeem Baig a , Fahd I. Alghunaimi b , Hind S. Dossary b , Tawfik A. Saleh a,∗,1 a b
Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Saudi Aramco, EXPEC Advanced Research Center, Production Technology Division, Dhahran 31311, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 15 October 2018 Received in revised form 30 December 2018 Accepted 13 January 2019 Available online 15 January 2019 Keywords: Oil cleaner production Sustainable technical processes Hydrophobic Oleophilic Nanofiber
a b s t r a c t The oil contamination in water and industrial wastewater is a concern for the environment. The role of carbon nanofiber (CNF) grafted onto polyurethane (PU) for oil absorption was investigated under ambient conditions. Superhydrophobic and superoleophilic materials based on CNF grafted PU were synthesized by a dip coating for oil/water separation. The synthesized materials were characterized by SEM, FTIR, BET and by a dynamic oil separation setup. Surface grafting with CNFs substantially increased the surface area of PU by nearly 31X (9 m2 /g to 276 m2 /g), which also decreased the average pore size in the PU matrix from 2567 Å to 36 Å. The decrease in pore size significantly improved the capillary action of the synthesized CNF grafted PU and provided almost no chance for water to pass. CNF grafted PU has successfully separated oil from different oil/water models and displayed absorption capacity up to 50 times to its own weight. The absorbed oil can be recovered by squeezing CNF grafted PU, which retains its original shape after releasing the pressure, and thus it was successfully used multiple times. The flexibility and the mechanical stability of the material allows it to be used for the continuous separation of oil from oil-contaminated water. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Oil-water separation is considered to be worldwide challenge especially for produced water treatment and oil spillage (Hu et al., 2014; Xue et al., 2011). As a result, materials are being introduced to achieve the refined separation of oil and water for environmental and industrial applications. Materials were established for oil and water separation using their specific wettability (Baig and Saleh, 2018; Wen et al., 2013; Yoon et al., 2014). A few materials were synthesized which contained the properties of hydrophobicity and oleophilicity for the refined removal of oil and water (Calcagnile et al., 2012; Shang et al., 2012; Yu et al., 2014). These hydrophobic materials include porous ceramic membranes (Su et al., 2012), hydrophobic coated meshes (Lu et al., 2014), modified foam and sponges (An et al., 2015; Chen et al., 2017; Wu et al., 2014), carbon-based materials (Lee et al., 2015), and modified fabrics (Gu et al., 2017). Similarly, various methods were applied to attain hydrophobic surfaces such as electrospinning techniques (Liao et al., 2014), chemical vapor deposition (Hsieh et al., 2008), sol-gel methods (Venkateswara
∗ Corresponding author. E-mail addresses: tawfi[email protected], tawfi[email protected] (T.A. Saleh). 1 Home Page: http://faculty.kfupm.edu.sa/CHEM/tawfik/publications.html.
Rao et al., 2009), hydrothermal methods (Li et al., 2012), and solution immersion methods (Li et al., 2011). TiO2 -coated copper meshes have displayed ultrafast oil-water separation and possessed self-cleaning capability due to the light-sensitive behavior of the TiO2 (Liu et al., 2016). Similarly, Switchable self-cleaning TiO2 nanostructured mesh membrane was achieved by the electrochemical anodization and the heating process (Kang et al., 2018). In another example, the suspension polymerization method was used to develop a ZnO nanowires/acrylic ester composite resin (Yan et al., 2018). The combination of anodization, fluorination, and solvent exchange method was also used to develop a slippery liquid-infused porous hydrophobic surface. The melt extrusion and the water leaching method were used to construct a hydrophobic tridimensional interconnected porous high-density polyethylene bundles. The hydrophobicity was further improved by skin-peeling (Zhang et al., 2018). Apart from these, other methods are also being applied to the development of hydrophobic surfaces (Li et al., 2015a). The surface roughness and modification of the surface with low surface energy materials are playing a great role to achieve ultimate superhydrophobic surface (Latthe et al., 2015). The polymer nanocomposite and the polymer-based nanostructured material are getting significant attention for the separation of the oil and water. The superhydrophobic-superoleophilic polystyrene nanofibers were deposited on the stainless steel mesh by electrospinning has displayed excellent superhydrophobic-
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superoleophilic behavior (Lee et al., 2013). The micro/nano hierarchical structure of the shish-kebab was modified to develop a superhydrophobic material (Sun et al., 2018). The concentration controlled interaction of the nanocellulose and sodium dodecylsulfate has produced the super light 3D hierarchical nanocellulose aerogel foam for efficient oil absorption capacity (Zhang et al., 2019). For oil and water separation, the nanomaterial is generally introduced into the support surface which has the capability to take the oil or water due to its well organized or porous random network. There are numerous examples in which nanomaterial is utilized to improve the hydrophobicity of the material. The reduced graphene oxide was coated on the cotton fibers to impart the hydrophobic behavior to the cotton (Dashairya et al., 2018). A flexible polymer-based graphene foam was obtained by self-assembling the graphene sheets on the 3D skeleton of the polymer (Wu et al., 2013). Carbon nanotubes due to their hydrophobic nature were explored for the oil and water separation. Hydrophobic carbon nanotube (CNT) sponges were developed by chemical vapor deposition using ferrocene in dichlorobenzene as a precursor (Gui et al., 2010). A porous monolith of the polycarbonate/carboxyl-functionalized multiwalled carbon nanotubes has displayed the tendency to selectively absorb oil and organic solvents from the contaminated water (Li et al., 2018). Overall, continuous development in the fabrication of nanocomposite polymers and the nanostructured material has been observed for oil and water separation. Carbon nanofiber is another member of the carbon nanomaterial family. Carbon nanofiber due to its elongated shape and consisting of carbon atoms has displayed excellent hydrophobic characteristics. Zhen-Yu Wu et al. synthesized carbon nanofiber aerogels by a freeze-drying method with multiple steps. The prepared pure CNF aerogel demonstrated good ability to take oil (Wu et al., 2015). The preparation of the CNF aerogel is somewhat complex and requires extreme freezing and heating conditions, for example, heating up to 1450 ◦ C. The CNF is not much explored for the oil and water separation application. The CNF combination with porous support can make it a valuable material for the oil/water separation. Polyurethane sponge is porous in nature and provides an opportunity to use for the liquid absorption. Its specificity in wettability is improved by modifying its porous network with low surface energy hydrophobic material (Liu et al., 2015). Combination of the CNF and the polyurethane is a good choice to produce a hydrophobic sponge for cleaning and absorption of the oil from the contaminated water. Here, superhydrophobic carbon nanofiber grafted polyurethane (CNF grafted PU) was synthesized via dip coating for fast separation of oil from water. The porous network of the polyurethane provided enough space for the absorption of oil from water. CNF grafting performed two functions in the polyurethane. First of all, it increased the hydrophobicity of the polyurethane sponge and secondly, it increased the number of pores which might be responsible for capillary action to efficiently remove oil and other organic contaminants from water. The synthesized CNF grafted PU showed excellent performance in separating non-polar organic contaminants and oil from water in both static and dynamic modes. 2. Materials and methods 2.1. Materials Toluene and acetone were purchased from Merck (Germany). Hexane, heptane, and o-Xylene were obtained from Sigma-Aldrich (Germany). Ethanol was attained from Baker Analyzed ® Reagent. The polyurethane was purchased from the local market and cleaned thoroughly prior to use. The chemicals and the reagents were analytical grade and were used without any further purification. Distilled water was produced in-house using a Labstrong FiSTREEM II 2S Glass Still distiller.
2.2. Instrumentation A Fourier-transform infrared spectroscopy (FTIR) study of the samples was done using a Thermo Scientific Nicolet iS10. The BET Surface area and pore size of the different CNF grafted materials were measured using a MicromeriticsTriStar II Plus instrument. A thermo-scientific magnetic stirrer was used for the stirring of CNF dispersed ethanol during the CNF grafting process. The distilled water was collected from a homemade distillation unit. A Blue M oven was used for curing and drying the different materi® ® als. A Masterflex Easy-Load peristaltic pump was used to achieve the dynamic separation of oil from water. The surface inspection of the materials was done with the help of a Field Emission Scanning Electron Microscope (SEM) (JSM-6610LV, Scanning Electron Microscope, JEOL) by applying a 20 kV acceleration voltage. 2.3. Synthesis of g-CNF grafted polyurethane Polyurethane (PU) was sonicated for 30 min in acetone for activation to remove any particles inside and then it was dried in an oven. CNF was dispersed and prepared in ethanol (1 mg/mL) by sonication for 30 min. The polyurethane foam was kept in the CNF dispersed ethanol solution for five minutes. After 5 min the obtained g1 -CNF grafted PU was dried in an oven at 60 ◦ C. During the curing process, the CNF anchors onto the PU surface and pores. In a similar way, another PU was dipped in the dispersed CNF and dried, then dipped again and dried to obtain g2 -CNF grafted PU. Similarly, g3 -CNFgrafted PU was prepared by being dipped three times. The obtained materials were labeled as PU, g1 -CNF grafted PU, g2 CNF grafted PU, and g3 -CNF grafted PU. Scheme 1 demonstrates the synthesis route of CNF grafted PU. 3. Results and discussion 3.1. FTIR characterization FTIR was used as one of the tools to observe the CNF and PU interaction after grafting. Peak shift, the appearance or disappearance of peaks, is considered a criterion to observe changes. Some differences in the FTIR spectra of the PU and the g-CNF grafted PU were observed. The FTIR spectra of the pure polyurethane can be seen in Fig. 1A. The prominent peaks at 3368 cm−1 and 1563 cm−1 are due to the stretching and deformation vibration of the N H (Rangel-Vazquez et al., 2014). In the FTIR spectra, no characteristic asymmetric stretching vibration of NCO was observed at 2270 cm−1 , which is an indication that there is no free NCO functionality due to polymerization (Peng et al., 2013). The asymmetric and symmetric stretching frequency bands of the C H bond of the CH2 groups appear at 2975 and 2850 cm−1 , respectively. The carbonyl stretch peak was assigned to vibration at 1683 cm−1 (Jian et al., 2009). The FTIR spectra of the g-CNF grafted PU were slightly different from the pure PU. The CNF grafting into PU clearly shows a peak shift. Only a few peaks were shifted which is attributed to the functional groups. Moreover, the shift was regular as the grafted CNF concentration was increased by dip coating. The N H stretching, CO, and N H deformation vibration peaks were shifted from 3367 to 3351 cm−1 , 1682 to 1674 cm−1 , and 1563 to 1536 cm−1 , respectively. In all FTIR spectra, the basic structure of the polyurethane was retained and, after CNF grafting, some peak shift appeared. It is indicated that CNF is successfully grafted into the polyurethane. 3.2. Morphological study of the CNF grafted polyurethane The surface of the pristine polyurethane and the CNF grafted polyurethane was evaluated using scanning electron
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Scheme 1. Scheme illustrating the synthesis route of CNF grafted PU for oil/water separation.
Fig. 1. FTIR spectra of (A) Pure PU, (B) g1 -CNF grafted PU, (C) g2 -CNF grafted PU and (D) g3 -CNF grafted PU.
microscopy (SEM). The pristine polyurethane demonstrated a three-dimensional porous morphology. The different size of various diameters could be observed in the SEM images (Fig. 2Aa) which
were in the range of 150–900 m. The porous morphology of the polyurethane actually provides the channels for the retention or passage of water and oil. A high-resolution SEM image (Fig. 2Ba) of
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Fig. 2. SEM images at two magnification (A) 500 m, (B) 5 m, (C) 1 m of (a) PU, (b) g1- CNF grafted PU, (c) g2 -CNFgrafted PU, and (d) g3 -CNFgrafted PU.
the pristine polyurethane surface displayed a plane surface without any extended portion. Prior to CNF grafting the surface of the PU is entirely plane and no roughness on the surface can be seen. The SEM images of the g-CNF grafted PU demonstrated a porous threedimensional morphology like pristine polyurethane (Fig. 2Ab, Ac & Ad). This morphology explained that the grafting of CNF has no effect on the shape of the material porosity. However, the SEM images under high resolution, 1 and 5 m) showed fibers on the surface of the PU providing sub-channels. The presence of the CNF provided both the roughness and the additional channels with a small degree of porosity which enhanced the hydrophobicity of the large holes. The generation of a small degree of porosity with additional channels is evident from the magnified SEM image (Fig. 2Cd) of the g3 -CNF grafted PU. The generation of additional channels along with the surface roughness is effective for the separation of the oil from the water. The CNF grafting on the surface of the PU was substantially enhanced as the grafting process moved from the first to the third dip coating. Therefore, further grafting provided no significant enhancement. The ultimate g3 -CNF grafted PU was selected because it showed sufficient grafting of the CNF on the
surface of the polyurethane which is responsible for imparting the highly hydrophobic character to the polyurethane (Fig. 2Bd & Cd). The PU, the prepared CNF grafted PU surface area, and the porosity was investigated using nitrogen adsorption and desorption isotherms. Through this adsorption and desorption of nitrogen, the BET surface area of the modified and unmodified material was calculated. The BET surface area results revealed that the grafting of CNF has a substantial effect on the surface area of the PU, as is evident from Fig. 3. A regular increase in the surface area was found as the grafting of the CNF on the PU surface increased. The pure PU has a very low surface area of 9 m2 /g, although it was increased significantly after the first grafting of CNF to 103 m2 /g. It was continuously improved and the g3 -CNF grafted PU surface area was 276 m2 /g. The surface area after the CNF grafting was increased almost 31 times. The adsorption is directly related to the surface area and the porosity of the material. The CNF grafting had a significant effect on the pore size of the PU. The pure PU showed a large adsorption and desorption pore size of 2567 Å and 2569 Å, respectively. The CNF grafting considerably reduced the pore size and a regular trend was observed as the grafting moved from g1 to g3 (Table 1). These results showed that the CNF reduced the average pore size from
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Table 1 Data of the PU and different CNF grafted PU by Brunauer Emmett Teller Model. Name
Surface area m2 /g
Adsorption Pore Size (Å)
Desorption pore size (Å)
Qm (mmole/g)
Molecular Cross-Sectional Area (nm2 )
R2
PU g1 -CNF grafted PU g2 -CNF grafted PU g3 -CNF grafted PU
8.9 102.9 152.0 276.0
2567.5 403.7 81.6 36.4
2,568.6 404.5 81.8 36.5
0.091 1.05 1.56 2.8
0.162 0.162 0.162 0.162
0.997 0.901 1.00 0.869
Fig. 3. BET surface area comparisons of the PU and the different CNF grafted PU and the prepared CNF grated PU with different amount of CNF.
2567 to 36 Å. The porous arrangement of the CNF on the PU walls definitely played a role in enhancing the overall surface area of the material. The pure PU has a small surface area due to the existence of only the large pore size. The CNF grafted PU has a large surface area and at the same time an average small pore size. The average small pore size is evidence that the CNF grafting has generated a CNF porous network on the PU surface. That porous network actually contributed to reducing the pore size significantly. It is clear from the SEM images that large channels remain the same in the PU before and after the grafting with CNF. The significant decrease in the average pore size is due to the generation of new pores by the CNF which also contributed to a huge increase in the surface area. The CNF provided additional pores and new channels which significantly improved oil adsorption. Due to the large surface area and the small pore size of the hydrophobic surface, the CNF grafted PU reduced the slippage of the water from it. It also enhanced the capillary effect of the material for the intake of the oil. 3.3. Separation evaluation 3.3.1. Static removal of oil The water and oil taking capabilities of the variously modified sponges were investigated using a mixture of hexane and water, which were colored with methylene blue to improve the physical visualization of the phases. The behavior of the polyurethane was sharply changed before and after CNF grafting. The polyurethane before CNF grafting took both water and oil. The commercially available PU is hydrophilic in nature (Wang et al., 2015) and shows the capability to take both water and oil. This was also evident when the commercial PU was dipped into the water and hexane. It was almost half in the water and a half in the n-hexane. The hydrophobicity of the polyurethane foam was controlled by dip coating with CNF. The overnight dried material was also investigated in the mixture of hexane and water. It was observed that the g1 -CNF grafted PU still took water with n-hexane, although the water taking capability was significantly reduced compared to the PU (Fig. 4). The hydrophobicity of the g2 -CNF grafted PU was further improved by a second time dipping into CNF ethanol. Fig. 4 demonstrates that the hydrophobicity was significantly enhanced. Also this time the PU only took a few drops of water while the remainder was hexane. The superhydrophobicity of the g3 -CNF grafted PU was attained after the 3rd time dipping. This time the CNF grafted PU did not absorb any water and only hexane was taken by the PU. The water
Fig. 4. (A) Behavior of sponge after immersing and squeezing two times in Hexane and water mixture.
drops can be clearly seen in the 2nd dipped CNF-sponge, while these water drops are entirely absent in the 3rd dipped CNF-PU (Fig. 4). This could be used as one parameter to control the hydrophobicity of the surface. The CNF grafting process can continue until it does not take any water. Moreover, the hydrophobic behavior of the PU and the differently modified PU can also be seen in the mixture of the n-hexane and the water. Most of the PU was in water while, with the 1st dipped CNF grafted PU, some part of it was in the water. The 2nd dipped CNF grafted PU was slightly in water with the remainder in the n-hexane. Interestingly, the 3rd dipped CNF grafted PU just floats on the n-hexane and no part of it was in the water. The further coating showed no significant difference. Therefore, the g3 -CNF grafted PU was used as the optimum graft. These measurements were made by twice pressing on the bottom of the beaker in the n-hexane and water mixture (Fig. 4). It observed that the pure PU with hexane has taken a lot of water and its surface turns blue due to the methylene blue in the water, whereas this color was absent in the CNF grafted PU. This shows that CNF grafting on the surface extensively improved the hydrophobicity and prevented water penetration. The surface hydrophobicity of the various non-modified and modified PU was furthermore evaluated with the help of the water contact angle. The lowest water contact angle was observed with the PU 109 ± 6◦ which was substantially increased to 146 ± 4◦ after grafting with the CNF (g3 -CNF grafted PU). The water contact angle of the g1 -CNF grafted PU and g2 -CNF grafted PU was found 115◦ ±
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Fig. 6. Separation efficiency for various non-polar organic solvents. Fig. 5. Water contact angle on the various surfaces of the materials (A) PU, (B) g1- CNF grafted PU, (C) g2 -CNFgrafted PU, and (D) g3 -CNFgrafted PU.
2◦ , 133◦ ± 1◦ , respectively (Fig. 5). The hexane contact angle was found almost zero with all surfaces. Hexane was readily absorbed by all PU surfaces without staying on the surface. From the water contact angle study, it has become evident that the surface is highly oleophilic and enough hydrophobic to take oil from the contaminated water. The oleophilic and the hydrophobic behavior of the modified PU can be elucidated by the presence of the carbon nanofibers on the surface of the PU. The separation mechanism of the oil from the water with the g3 -CNF grafted PU is explained by the non-polar or polar nature of the materials and the liquids. The CNF is a non-polar in nature and allowed the oil to readily spread on its surface which rapidly moved into the porous network of the polyurethane. On the other hand, the water polarity is prevented it to pass or absorb by the non-polar surface of the CNF grafted PU.
3.4. Separation efficiency, Reproducibility, and intake of different petrol components The separation efficiency of the g3 -CNF grafted PU was evaluated by separating the various non-polar solvents such as hexane, heptane, octane and nonane from the water. The g3 -CNF grafted PU has displayed a good capability to separate the non-polar organic components from the water. The separation efficiency for hexane, heptane, octane, and nonane was found 97.8, 99.8, 95.0 and 96.3%, respectively (Fig.6). The separation capacity and the reproducibility of the CNF grafted PU was investigated with different oils and non-polar solvents and it was also applied to the real sample of petrol mixed with water. The reproducibility of the grafted PU was studied by using a hexane solvent. For this purpose, the weight of the dry g3 -CNF grafted PU was measured and then it was dipped into the hexane solvent. After that, its weight was again measured to find out the intake of the n-hexane. The weight gain ratio was calculated by applying the following formula: Weight gain ratio(%) = (weight of the solvent absorbed g3
3.3.2. Dynamic removal of oil For the oil-water mixture, it is preferable to use a dynamic system for separation. Therefore, the capacity of the prepared material was tested using the dynamic system. A highly hydrophobic porous network of g3 -CNF grafted PU can be applied to the separation of bulk oil from the water, which usually happens during oil spillages where a large quantity of oil is added to the body of water. The bulk of the hexane and the water mixture were used to evaluate the separation efficiency of the prepared materials. This was achieved by fitting the sponge into a pipe. This was accomplished very easily due to its mechanical stability and flexibility. The fitted sponge was inserted into the mixture of hexane and the methylene blue colored water in such a way that some of it was in the water while rest of the part in the hexane. A peristaltic pump was used to create pressure for the removal of hexane from the mixture of water and oil. As the peristaltic pump was started, it was observed that the hexane moved very fast from the g3 -CNF grafted PU without taking any water. After some time, in the beaker, there was only methylene blue colored water and hexane was separated from the mixture to the collector container. Moreover, after the removal of the hexane, the peristaltic pump continued to take water i.e. the pump was ON. Due to the pressure of the pump, only bubbles of air were passed through the pipe, while no methylene blue colored water was passed. This is an indication that the g3 -CNF grafted PU is highly hydrophobic and oleophilic. Due to its ease of preparation, its cost-effectiveness and its unique hydrophobic and oleophilic properties, the g3 -CNFgrafted PU may prove to be a valuable material for the removal of oil spillage from water.
-CNF grafted PU- the weight of the dry g3 -CNF grafted PU)/ (weight of the dry g3 -CNF grafted PU) ∗ 100 The CNF grafted PU demonstrated sober reproducibility for the intake of hexane after multiple cycles (Fig. 7A). The weight gain ratio for hexane was found to be in the range of 2650 to 2750% as calculated by the above-mentioned formula. The relative standard deviation (RSD) was found to be 1–2%, with number of measurements of 15. The small variation in the multiple cycles demonstrated that the developed hydrophobic material showed good reproducibility and can be used multiple times for the separation of oil or its contaminants. The behavior of the g3 -CNF grafted PU was also observed for other organic solvents (Fig. 7B). The percentage of weight gain ratio demonstrated that weight gain is also dependent on the density of the organic solvent. The organic solvent with a high density showed an increased weight ratio. However, it has demonstrated good separation capability for the hexane, heptane, toluene, and o-xylene. The weight gain ratio was in the range of 2600 to 5000% calculated by the abovementioned formula. This was also applied to the real sample of petrol mixed with water and the weight gain was found to be 3258%. This huge increase in the weight gain ratio of the g3 -CNFgrafted PU demonstrated that it has an excellent capability for the absorption of various nonpolar organic solvents, including petrol. The absorbed nonpolar components can be released very easily by simply compressing the CNF grafted PU with good reproducibility. Under pressure, its structure did not collapse and it easily regained
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333
Fig. 7. (A) Adsorption recyclability of the g3 -CNF grafted PU for the Hexane. (B) Adsorption capability of the g3 -CNF grafted PU for oil and different nonpolar solvents. Table 2 Comparison of the absorption capacity of the g3 -CNF grafted PU with other reported materials. Sr#
Material
Surface area (m2 /g)
Hexane (g/g)
Toluene (g/g)
Regeneration
Ref.
1
MnO2 /poly(nbutylacrylate-cobutyl methacrylate-co-methyl methacrylate) resin composite Fluorinated polydopamine/chitosan/reduced graphene oxide composite aerogel CNT/PDMS-coated PU sponge PU–CNT–PDA–ODA sponge Porous BNNS/PVDF composite material Graphene Foam Monolithic superhydrophobic silica aerogel Magnetic graphene foam g3-CNF grafted PU
74.38
2.07
–
Drying
Zhang et al. (2017)
51.76
12
8
Heating, Squeezing
Cao et al. (2017)
–
15
–
–
Wang and Lin (2013)
– –
34.9 3.8
5
Squeezing Ethanol washed and dried at 60 C◦ in air
Wang et al. (2015) Yu et al. (2015)
– –
15 7
20 9
Ethanol washing and drying in an oven Distillation and vacuum filtration
Yang et al. (2017) Wang and Wang (2018)
– 276
15 26.6
19 49.2
Hexane immersion Squeezing
Yang et al. (2014) This work
2
3 4 5 6 7
8 9
its shape as the applied pressure was released. The comparison of the g3 -CNF-grafted PU with other hydrophobic materials revealed that its absorption capability is much better (Table 2). The increased absorption capacity can be explained due to the exceptionally high surface area which substantially increased due to the CNF grafting. Moreover, the CNF also facilitated achieving the numerous porosities with small diameters which improved the selectivity of the material. The g3 -CNF-grafted PU has become selective for non-polar components in the contaminated water. This behavior of the g3 CNF-grafted PU makes it possible to remove emulsified oil from water. It not only displayed great capability for the adsorption of oil but it can also fit into the tube to perform the function of the continuous removal of oil contamination from the water. It also provides an opportunity to reuse the collected non-polar organic components and reduce the burden of organic pollution on the environment.
4. Conclusion This work has demonstrated that loading polyurethane (PU) with carbon nanofiber (CNF) could enhance the separation of oil from water. The prepared g3 -CNF-grafted PU showed higher efficiency in oil separation from water compared with either the g1 -and g2 -CNF-grafted PU or the pure PU. The CNF improved the hydrophobic character of the PU and the surface area was significantly improved from 9 to 276 m2 /g. It also substantially reduced the average pore size from 2567 to 36 Å which might be responsible for the huge surface area and the strong capillary action for
oil uptake. The contact angle of the water was found to be 146 ± 4◦ which demonstrated the high hydrophobicity of the surface. The curing process provides stability to the surface and it can be used multiple times for absorption processes. The enhanced absorption capacity can be explained by the high surface area. The absorption capacity for oil and other organic solvents was found comparable or better than others. Moreover, it can be used as a filter for bulk oil spillage. The developed g3 -CNF grafted PU provides an opportunity for use in the large-scale bulk removal of oil due to its continuous removal capability, mechanical stability, high absorption capacity, chemical inertness, large surface area, and small pore size. References An, Q., Zhang, Y., Lv, K., Luan, X., Zhang, Q., Shi, F., 2015. A facile method to fabricate functionally integrated devices for oil/water separation. Nanoscale 7, 4553–4558, http://dx.doi.org/10.1039/c5nr00026b. Baig, N., Saleh, T.A., 2018. Initiator-free natural light-driven vapor phase synthesis of a porous network of 3D polystyrene branched carbon nanofiber grafted polyurethane for hexane /Water separation. ChemistrySelect 3, 8312–8318, http://dx.doi.org/10.1002/slct.201801549. Calcagnile, P., Fragouli, D., Bayer, I.S., Anyfantis, G.C., Martiradonna, L., Cozzoli, P.D., Cingolani, R., Athanassiou, A., 2012. Magnetically driven floating foams for the removal of oil contaminants from water. ACS Nano 6, 5413–5419, http://dx.doi. org/10.1021/nn3012948. Cao, N., Lyu, Q., Li, J., Wang, Y., Yang, B., Szunerits, S., Boukherroub, R., 2017. Facile synthesis of fluorinated polydopamine/chitosan/reduced graphene oxide composite aerogel for efficient oil/water separation. Chem. Eng. J. 326, 17–28, http:// dx.doi.org/10.1016/j.cej.2017.05.117. Chen, J., You, H., Xu, L., Li, T., Jiang, X., Li, C.M., 2017. Facile synthesis of a two-tier hierarchical structured superhydrophobic-superoleophilic melamine sponge for rapid and efficient oil/water separation. J. Colloid Interface Sci. 506, 659–668, http://dx.doi.org/10.1016/j.jcis.2017.07.066.
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