Robust superhydrophobic and oleophilic silk fibers for selective removal of oil from water surfaces

Abstract

The fabrication of efficient sorbents having high selectivity and sorption capacity from natural products has attracted considerable interest due to practical applications in oil spill clean-ups and recovery of spilled oil. Our work presents the preparation of a natural sorbent material with superhydrophobic character, excellent selectivity and high oil sorption capacity using silk fibroin fibers, where the fibers were surface modified with octadecylamine via a simple synthetic approach. The sorbent fibers were found to possess superhydrophobic character with a static water contact angle of 150 ± 3°. The results of oil sorption experiments on oil–water mixtures infer that the modified fibers possess excellent selectivity as well as oil sorption ability, where the oil sorption capacity (OSC) of the material was found to be 46.83 g g⁻¹ for crude oil and 84.14 g g⁻¹ for motor oil. Moreover, the oil sorption capacity of our fiber for motor oil is almost 8 times higher than natural wool, and twice higher than the silkworm cocoon waste. The modified fibers have significantly higher OSC values for crude oil (3.5 times higher) than any wool based sorbents. The suitability of the material over a wide pH range of 3–11 substantiates its advantage in oil sorption even in any corrosive environment. Further, the oil recovery and reusability of the fibers were tested to investigate their applicability for repeatable usage in oil spill clean-up application.

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1. Introduction

Considering the increasing global demand for petroleum fuels such as diesel, gasoline, kerosene, etc., their conservation, production and regeneration have become an important issue to the world community as petroleum is a non-renewable energy source. However, various oil utilization anthropogenic activities such as transportation through sea routes, loading-unloading of oil, tank washing,¹ oil rig damage, and accidents results in a huge loss of precious petroleum products every year. The oil that is released to the aquatic environment (commonly termed as oil spill) during these anthropogenic activities also causes severe damage to the entire aquatic ecosystem^2,3 and the environment.^4,5 In this context, numerous techniques have been proposed and utilized for cleaning and recovering the spilled-oil, which includes physisorption by sorbent materials,⁶ mechanical recovery by oil skimmers, in situ burning,⁷ and biodegradation.⁷ Among these techniques, the use of sorbent materials is considered to be the most effective method owing to its simplicity and ease of utilization at the affected site. Hence, much attention has been given to the fabrication of efficient and economical sorbent materials for the removal and recovery of the spilled-oil. An ideal sorbent for spilled-oil removal and recovery must possess superhydrophobic and oleophilic character in addition to exhibiting sufficient buoyancy, high selectivity and rate of uptake.^8,9 They should also be reusable or regenerable and recovering the oil from them is feasible and straight forward.⁶ Till date, various sorbent materials have been reported for oil spill clean-ups that include natural products, inorganic mineral products⁹ and organic synthetic products.⁸ Among these materials, the use of natural products has drawn much attention due to their biodegradable, economic and environment-friendly nature. Although, numerous natural products such as cotton,^10–14 wool,^15,16 rice straw,¹⁷ bagasse,^18–21 vegetable fibers,²² rubber,^23–27 etc. have already been used to develop sorbent materials with high oil-sorption capacities, their major drawback is their poor selectivity in aquatic environment. The poor selectivity not only reduces the efficiency of the sorbent, but recovery of the spilled oil also becomes difficult from them due to the presence of absorbed water. The present study, is therefore, focused on the fabrication of an efficient and economical sorbent material from silk fibers possessing superhydrophobic–oleophilic character, sufficient buoyancy, high selectivity, easy oil-recovery and good reusability. Silk is a natural protein fiber, mainly composed of fibroin (fibrous protein) and sericin (globular, gumming protein) and is usually obtained from the cocoons of the larvae of mulberry silkworms. In recent literature, Moriwaki et al.²⁸ have reported the utilization of silkworm cocoon, as a hydrophobic sorbent material (with no water contact angle measurements) for the removal of oil from water surface. However, the sorbent lacks superhydrophobic character that reduces its selectivity in water bodies. In the present work, the hydrophilic silk fibers (obtained after the removal of sticky sericin protein from the cocoons) were modified into an efficient superhydrophobic sorbent material (validated through water contact angle measurements) with better oil sorption capacity via a facile and simple route involving the treatment of the fibers with octadecylamine (C₁₈H₃₉N, ODA) under ambient condition. The wettability of the modified silk fibers was investigated through contact angle measurements and various laboratory experiments. The buoyancy, selectivity, oil sorption capacity, reusability and oil recovery efficiency of the fibers were also tested for determining their applicability for usage as sorbent material for efficient removal and recovery of spilled-oil from water surfaces.

2. Experimental

2.1. Chemicals and materials

Silk cocoons of bombyx mori were obtained from Debra Sericulture Farm, (West Bengal, India). Octadecylamine (ODA) was procured from SRL (India), while sodium carbonate was bought from Merck (India). Sodium chloride (NaCl) was purchased from RFCL (India). Congo red, methyl orange and methylene blue were purchased from Loba Chemie (India). Crude oil was kindly provided by M/S Haldia Refinery, Haldia (India), while motor oil was bought from Indian Oil Fuel Station in Kharagpur (West Bengal, India). Absolute ethanol and distilled water was used for all the experiments.

2.2. Pre-treatment of silk cocoons

The pre-treatment of silk cocoons involves the degumming process of removing the sticky sericin protein. The cocoons were cut into small pieces and boiled in 0.02 M aqueous solution of sodium carbonate for 20 min under constant stirring. The whole mass was then washed repeatedly with water to remove the sticky yellow-colored sericin protein. Finally, the white mass was dried at 50 °C for 12 h to get the dried silk fibroin fibers (SFF). The degumming process of the silk cocoons is presented in the schematic diagram in Fig. 1.

Fig. 1 Schematic representation showing the degumming process of silk cocoons to remove the sericin protein.

2.3. Preparation of superhydrophobic and oleophilic silk fibers

The dried degummed SFF were immersed in a saturated ethanolic solution of ODA and continuously stirred for 30 min at ambient temperature (25 ± 1 °C). After that, the fibers were taken out and washed three times with ethanol to remove excess ODA. Finally, the ODA treated silk fibroin fibers (SFF–ODA) were dried at 50 °C for 6 h and analyzed by various techniques and experiments to test their applicability for selective oil sorption application.

2.4. Oil sorption experiments

The oil sorption experiments of the sorbent fibers were performed quantitatively in oil–water mixtures, prepared by using distilled water and two different oil samples viz. crude oil (viscosity, η = 4.3 cP) and motor oil (η = 239.2 cP). Oil–water mixtures (in the ratio of 1 : 20 (w/w)) were used to mimic the oil spill condition. All the experiments were performed in triplicate under static condition.

2.4.1. Oil removal efficiency studies. The efficiency of SFF–ODA to remove oil from the water surface was determined through two different studies performed at 25 °C. Oil–water mixtures were prepared by adding 7.5 g of oil to 150 g distilled water followed by agitation for 10 min. In the first study, varying weight of SFF–ODA was added to the oil–water mixtures and kept undisturbed for 20 min for the sorption of oil. After sorbing the oil, the fibers were removed from the water surface using forceps and remaining oil (separated from the mixture using a separating funnel) was weighed. The oil removal efficiency of SFF–ODA was calculated using eqn (1).

Oil removal efficiency (%) = (W_I − W_F)/W_I × 100 (1)

where, W_I is the initial weight of oil in the oil–water mixture and W_F is the final weight of oil in the mixture.

In the second study, the effect of sorption time on the oil removal efficiency of SFF–ODA was investigated. For the study, 0.15 g SFF–ODA was added to the oil–water mixtures and kept undisturbed for varying time periods. After the specified time periods, the oil-sorbed fibers were removed and oil removal efficiency of the fibers was calculated using eqn (1).

2.4.2. Oil sorption capacity studies. The maximum oil sorption ability of SFF–ODA towards the two oil samples was determined through the oil sorption capacity (OSC) studies, performed at 25 °C. Two different studies were performed using oil–water mixtures, prepared by the above-mentioned procedure using 15 g oil and 300 g distilled water. In one study, the OSC of both SFF and SFF–ODA was investigated. For the study, 0.15 g of the two fibers was added to the oil–water mixtures and kept undisturbed for 10 min for the sorption of oil. After that, the oil-sorbed fibers were removed from the water surface and weighed. The amount of sorbed water in the oil-sorbed fibers was determined by washing the fibers in chloroform (which is immiscible with water) to extract the oil from the fibers and the remaining fiber that contains the sorbed water was weighed (W_fw). The amount of sorbed water (W_W) was calculated by subtracting the weight of dry fiber from W_fw. The oil sorption capacity (OSC, in g g⁻¹) of the fibers was calculated using eqn (2).

OSC = (W_T − W_S − W_W)/W_S (2)

where, W_T is the total weight of the oil-sorbed fiber (in g), W_S is the initial weight of the dry fiber (in g) and W_W is the weight of sorbed water in the oil-sorbed fibers (in g). The errors in the values of OSC were estimated as ±0.01–0.03 g g⁻¹.

In the next study, the effect of NaCl concentration and pH of water on the OSC of SFF–ODA fibers was investigated. For studying the effect of NaCl concentration on OSC, aqueous NaCl solutions (concentration varying from 0.0 to 0.5 M) were used to prepare the oil–water mixtures, while the amount of SFF–ODA was kept constant (0.15 g). On the contrary, the effect of pH (within the range of 3.0 to 11.0) on OSC was studied using fresh distilled water (0.0 M NaCl) for preparing the oil–water mixtures and constant amount of SFF–ODA (0.15 g). pH adjustments were achieved with dil. HCl and NaOH aqueous solutions. The fibers were added to the oil–water mixtures, and OSC was similarly calculated using eqn (2).

2.5. Reusability and oil recovery studies

Reusability tests of SFF–ODA were performed in crude oil–water mixtures, prepared by using 15 g crude oil and 300 g distilled water. Five cycles of sorption process were performed, where the fibers were added to the oil–water mixtures, left undisturbed for 10 min, removed using forceps, and squeezed to recover the sorbed oil. The OSC of the regenerated fibers after each sorption cycle was calculated using eqn (2), while the amount of recovered oil was calculated using eqn (3).

Oil recovered (%) = (W_OR/W_OS) × 100 (3)

where, W_OR is the weight of recovered oil (in g) and W_OS is the weight of sorbed oil (in g).

2.6. Characterization techniques

2.6.1. Fourier transform infra red (FT-IR) analysis. Functional group analyses of the silk fibers were performed by attenuated total reflection-Fourier transform infra red (ATR-FTIR) spectroscopy using Thermo Scientific Nicolet 6700 FT-IR instrument (model NEXUS-870) within the scan range 650–4000 cm⁻¹ and 32 scans per sample.

2.6.2. Scanning electron microscopic (SEM) analysis. The surface morphology of the fibers was determined by scanning electron microscopy (SEM) using ZEISS EVO 60 Scanning Electron Microscope at two magnifications (viz. 500× and 5k×). The samples were gold coated in a sputter coating unit for 10 min prior to the SEM analyses.

2.6.3. Contact angle measurements. The contact angle measurements were performed with distilled water at 25 °C using Ramé-Hart Automated Goniometer, model 290-G using the sessile drop method. For the analysis, the fibers were pasted on a glass slide using double adhesive tape and then placed in the analysis window of the contact angle analyzer.

3. Results and discussion

3.1. The formation of superhydrophobic fibers

The pristine silk fibers (i.e. SFF) are mainly composed of fibroin protein as the other component sericin was removed during the pre-treatment of silk cocoons. The primary structure of fibroin protein consists of the recurrent amino acid sequence (Gly–Ser–Gly–Ala–Gly–Ala)_n bonded via amide linkages. Upon treatment of SFF with ODA, hydrogen bonds are formed between the amine groups of ODA and hydroxyl as well as amide groups of fibroin, as presented in the schematic diagram in Fig. 2. The attachment of ODA to the fibroin protein of the silk fibers via hydrogen bonds resulted in the orientation of the ODA molecule with the hydrophilic head towards the fiber and the long hydrophobic tail outwards. This orientation of the ODA molecules resulted in the superhydrophobic character of the ODA treated silk fibroin fibers (SFF–ODA).

Fig. 2 Schematic diagram showing the formation of SFF–ODA from SFF and ODA.

3.2. Characterization of the fibers

The ATR-FTIR spectra of SFF as well as SFF–ODA are presented in Fig. 3.

Fig. 3 FT-IR spectra of SFF (a) and SFF–ODA (b).

The FT-IR spectrum of SFF (Fig. 3a) depicts a broad peak at 3333 cm⁻¹ that can be assigned to O–H stretching vibration of the adsorbed water molecules. The weak peaks at 2925 and 2859 cm⁻¹ correspond to C–H stretching vibrations, while the peaks at 1633, 1527, and 1230 cm⁻¹ corresponded to C=O, –NH and C–N bond vibrations respectively in the amide groups of the fibroin molecules.²⁹

After coating the pristine silk fibers with ODA (i.e. SFF–ODA), the intensity of the peak at 3332 cm⁻¹ corresponding to O–H stretching vibration is significantly reduced (Fig. 3b) which might be due to superhydrophobic character of the fibers. In contrast, the peaks at 2916 and 2849 cm⁻¹, corresponding to C–H stretching vibrations are significantly enhanced in SFF–ODA which is probably due to the presence of ODA coating on the fibers. These significant changes in the intensity of the peaks corresponding to O–H and C–H stretching vibrations in the FT-IR spectrum of SFF–ODA as compared to that for pristine silk fiber (i.e. SFF) confirms the presence of ODA in SFF–ODA.

Morphological analyses of the silk fibers by SEM reveal smooth surface for SFF with very sparse hairy features on the surfaces (Fig. 4a and b). However, the SEM image of SFF–ODA depicts fibers covered with flakes of varying thicknesses (Fig. 4c and d). These flakes in the SEM image of SFF–ODA represent the ODA moieties on the surface of the fibers, which arises after the ODA treatment of the pristine silk fibers. The surface area of a sorbent is an important parameter that determines its efficiency for oil sorption processes. In the SEM image of SFF–ODA, it was observed that the surface coverage of SFF by ODA results in a rough surface in SFF–ODA compared to SFF thereby increasing their surface area, vital for sorption processes.

Fig. 4 SEM images of SFF (a and b) and SFF–ODA (c and d).

3.3. Wettability tests

The optical images, shown in Fig. 5a and b, represent the wettability of SFF and SFF–ODA towards water and different oil samples (viz. crude oil and motor oil). Fig. 5a shows that SFF exhibits excellent wettability towards both water (blue colored) and oil samples (dark brown: crude oil and crimson: motor oil), as all the three liquid samples are readily sorbed by it, thereby indicating the hydrophilic and oleophilic character of the fibers. Although the oil samples are readily sorbed by SFF–ODA indicating its oleophilic character, the water drop is retained on its surface even after 10 min of applying the drop (Fig. 5b). The non-wettable behavior of SFF–ODA towards water reveals its hydrophobic character, which was further tested by contact angle measurements using water. The static water contact angle for SFF–ODA was observed to be 150 ± 3° (Fig. 5c), which demonstrates the superhydrophobic character crucial for an ideal sorbent. To examine the wettability of SFF–ODA in different corrosive liquids such as acidic, basic and neutral aqueous medium, water drops of varying pH (pH 3 to 11) were applied on the fibers. It was observed that the water drops were retained on the surface of SFF–ODA without being sorbed (Fig. 5d) even after 10 min. This non-wettable behavior indicates that the superhydrophobic character of the fibers is not affected by the pH of the aqueous medium, thereby substantiating the applicability of the fibers for oil sorption in all kinds of corrosive liquids.

Fig. 5 Optical image showing the drops of water (dyed with methylene blue), crude oil (dark brown coloured) and motor oil (crimson coloured) on SFF (a), SFF–ODA (b); water contact angle measurement on SFF–ODA (c); optical image showing water drops of different pH on SFF–ODA: pH 3 (water dyed with methylene blue), pH 5 (water dyed with methyl orange), pH 7 (water dyed with congo red), pH 9 (pure water), pH 11 (water dyed with methylene blue) (d) (water was dyed for clear visibility).

3.4. Buoyancy and selectivity tests

The buoyant natures of SFF and SFF–ODA were tested qualitatively and the demonstration is presented in Fig. 6.

Fig. 6 Qualitative demonstration showing the buoyancy of SFF (a and b) and SFF–ODA (c and d) in water.

For the study, the fibers were placed on water surface and kept undisturbed for 30 min. It was observed that SFF submerged in the water (Fig. 6a and b) owing to its hydrophilic character. However, SFF–ODA floats on the water surface even after 30 min, indicating its excellent buoyant nature (Fig. 6c and d). The buoyant nature of SFF–ODA can be attributed to its superhydrophobic character, which renders it to repel water and float on the water surface.

In an attempt to investigate the selective oil sorption behavior of SFF–ODA, the fibers were added to crude oil–water mixture. The qualitative demonstration in Fig. 7 reveals that the sorbent (SFF–ODA) floats on the water surface and selectively sorbs the oil (Fig. 7c and d). After the removal of oil sorbed sorbent from the water surface using forceps (Fig. 7e), clean water was left behind with no apparent residue of crude oil (Fig. 7f). The water content in the oil-sorbed fibers was found to be <1%, indicating the selective oil sorption behavior of the fibers from the water surface. The excellent selectivity of SFF–ODA can again be attributed to the superhydrophobic and oleophilic character of the fibers that persuades them to repel water and selectively sorb the oil from the water surface.

Fig. 7 Qualitative demonstration showing the selectivity of SFF–ODA fibers in crude oil–water mixture: pure water and fibers (a), crude oil–water mixture and fibers (b), fibers added to the mixture (c and d), oil-sorbed fibers removed from the mixture (e), separated oil-sorbed fibers and clean water (f).

3.5. Oil sorption experiments

3.5.1. Oil removal efficiency studies. The results of oil removal efficiency (ORE) studies of SFF–ODA with varying sorbent weight and sorption time are shown in Fig. 8.

Fig. 8 Oil removal efficiency (ORE) of SFF–ODA with varying sorbent weight (a) and varying sorption time (b).

The ORE of the fibers increases with an increase in the weight of the fibers for both the oil samples. It was observed that 0.15 g SFF–ODA could remove 93.3 and 99.5% of 7.5 g crude oil and motor oil respectively from water surface. The ORE studies with varying sorption time (thereby representing the sorption kinetics of the fibers) shows that ORE increases with an increase in the sorption time for both the oil samples and reaches saturation at the sorption time of 10 min after which no significant influence on sorption process was observed. Hence, sorption time of 10 min was chosen for the remaining investigations on sorption behavior of the fibers (viz. oil sorption capacity studies and reusability tests). The qualitative demonstration in Fig. 9 further substantiates the fact that the fibers can completely sorb the oil from water surface within 10 min.

Fig. 9 Qualitative demonstration showing the sorption of oil by SFF–ODA from water surface with progressing time: after 2 min (a), 5 min (b), 8 min (c), 10 min (d), and 15 min (e).

3.5.2. Oil sorption capacity studies. The oil sorption capacities (OSC) of SFF and SFF–ODA were studied in oil–water mixtures to investigate their maximum capability to sorb oil from water surfaces. The OSC of SFF–ODA was observed to be 46.83 g g⁻¹ for crude oil and 84.14 g g⁻¹ for motor oil, whereas the OSC of SFF was observed to be 6.6 g g⁻¹ for crude oil and 75.26 g g⁻¹ for motor oil (Fig. 10a). The higher OSC of SFF–ODA towards both the oil samples compared to SFF can be attributed to two reasons. First, the higher surface area of SFF–ODA, arising after the ODA treatment of SFF (previously observed in the SEM images in Fig. 4), provides a larger surface for the sorption process. Second, the superhydrophobic character of SFF–ODA enhances its selectivity towards oil compared to SFF, which is hydrophilic in nature.

Fig. 10 Oil sorption capacity (OSC) of SFF and SFF–ODA towards crude oil and motor oil (a); OSC of SFF–ODA towards crude oil and motor oil with varying NaCl concentration (b) and pH (c) of the aqueous medium.

Further, the higher OSC of SFF–ODA towards motor oil compared to crude oil can be related to the viscosity of the oil samples. Motor oil, being the most viscous, adheres more strongly to the sorbent fibers compared to crude oil and hence results in a higher OSC value. The OSC of SFF–ODA for some oil samples (kerosene, silicone oil and paraffin oil) and few organic solvents (toluene, hexane and xylene) are determined, which are in the range of 40.30–68.17 g g⁻¹ (Table S1†). The results infer that the OSC could be directly related with viscosity of the oil samples or solvents for the prepared sorbent. The effects of NaCl concentration as well as pH of the aqueous medium on the OSC of SFF–ODA are presented in Fig. 10b and c, respectively.

It was observed that both pH and NaCl concentration of the aqueous medium has no remarkable effect on the OSC of the fibers, which is probably due to their superhydrophobic character that promotes the fibers to repel water and hence are unaffected by the pH and NaCl concentration. Thus, it can be inferred that SFF–ODA is suitable for application as a sorbent material in both fresh as well as salt water bodies and in a wide range of corrosive liquids from acidic to basic aqueous solutions.

Table 1 presents a comparison of the OSC of SFF–ODA in the present study with some typical sorbents reported in the literature. It was observed that the OSC for SFF–ODA is superior to a number of typical sorbents reported in the literature, which is probably due to its superhydrophobic character and excellent selectivity in oil–water systems.

Table 1 The oil sorption capacity of some typical fibers reported in the literature

Sorbent	Oil	Oil sorption capacity (g g^−1)	Literature
Silkworm cocoon waste (fiber)	Motor oil	42–52	28
	Vegetable oil	37–60
Wool-based nonwoven fibers	Crude oil	∼14	15
	Diesel oil	∼12.5
	SN 150	∼16
Recycled-wool-based nonwoven material (fiber)		18.5
Recycled wool-based nonwoven material (fiber)	Diesel fuel	9.62	16
	Crude oil	11.06
	Base oil	12.98
	Vegetable oil	13.16
	Motor oil	15.80
Raw cotton fibers	Vegetable oil	30	14
Loose natural wool fibers	Motor oil	5.56	30
Recycled wool based nonwoven material (fiber)		5.48
Silk fibroin fibers modified with ODA	Crude oil	46.83	Present work
	Motor oil	84.14

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3.6. Reusability and oil recovery studies

Reusability tests of the SFF–ODA sorbent fibers were performed in crude oil–water mixtures (in the ratio of 1 : 20 (w/w)) to determine their efficacy for repeatable usage in oil spill clean-up application. After each sorption cycle, the oil-sorbed fibers were squeezed (as shown in Fig. 11) to collect the oil and regenerate the fibers for further use.

Fig. 11 Qualitative demonstration showing the recovery of sorbed oil from oil-sorbed SFF–ODA sorbent fibers by squeezing.

It was observed that the amount of recovered oil after the first sorption cycle is lower than the succeeding sorption cycles (Fig. 12a), which is probably due to the retention of oil in the fibers of the sorbent, even after the squeezing process.

Fig. 12 Oil recovered from the oil-sorbed SFF–ODA fibers (a) and oil sorption capacity (OSC) of the SFF–ODA fibers (b) for the five sorption cycles.

Similarly, owing to the retention of oil in the sorbent fibers, the OSC of the fibers slightly decreases with progressing sorption cycles (Fig. 12b). However, the OSC was observed to be >38 g g⁻¹ for crude oil and >76 g g⁻¹ for motor oil, even after five sorption cycles and the amount of recovered oil is >93% for both the oil samples. Both the results indicate that the SFF–ODA sorbent fibers are reusable for more than five times.

The optical image showing water drops on the regenerated fibers (Fig. 13a–e) validates the retention of hydrophobic character by the fibers even after fifth sorption cycle, thereby substantiating their applicability for repeated usage in oil removal application.

Fig. 13 Optical images showing water drops (pure water and water dyed with methylene blue) on the regenerated fibers: after first (a), second (b), third (c), fourth (d), and fifth (e) sorption cycle.

The water contact angle for the regenerated fibers were observed to be 150 ± 3°, 150 ± 3°, 149 ± 3°, 148 ± 3°, and 146 ± 3° for the first, second, third, fourth, and fifth sorption cycles, respectively (Fig. 14).

Fig. 14 Water contact angle for the regenerated SFF–ODA sorbent fibers: after first (a), second (b), third (c), fourth (d), and fifth (e) sorption cycle.

4. Conclusions

In summary, an excellent superhydrophobic natural sorbent was prepared from silk fibroin fibers via a simple synthesis involving surface modification of the fibers with octadecylamine under ambient condition. The sorbent fibers offer excellent selectivity and could sorb oil efficiently from water surface, while repelling the water. The repellence and non-wettable behavior of the fibers towards water arises from its superhydrophobic character (static water contact angle 150 ± 3°). The sorbed oil can be easily recovered from the oil sorbed fibers by squeezing and can be reused. The fibers also exhibits sufficient buoyancy and can be reused for more than five times with high oil sorption ability. The oil sorption capacity (OSC) of the fabricated material is 46.83 g g⁻¹ and 84.14 g g⁻¹ for crude oil and motor oil, respectively. Interestingly, the OSC values of the superhydrophobic fiber are almost 8 times superior than the natural wool fiber, and twice higher than the silkworm cocoon waste. Our modified fibers are also significantly sorbing the crude oil (with 3.5 times higher OSC value) than any wool based sorbent. Additionally, the fibers were found to be applicable in both salt as well as fresh water bodies and in a wide range of corrosive liquids from acidic to basic aqueous solutions in the pH range of 3–11. The simple fabrication approach, wide availability, excellent selectivity, high oil sorption capacity, and easily employable at affected site as bulk sorbent or as filler in sorbent booms promotes the fibers to find practical applications in future oil spill clean-ups.

Acknowledgements

The authors would like to acknowledge Prof. S. Dasgupta, Department of Chemical Engineering, IIT Kharagpur for helping with the contact angle measurements and MoES, New Delhi for the financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Supplementary information includes oil sorption capacity of the sorbent fibers for some more oil samples and organic solvents. See DOI: 10.1039/c6ra14723b

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