Facile Fabrication of Magnetic, Durable and Superhydrophobic Cotton for Efficient Oil/Water Separation

In this paper, we present a facile and efficient strategy for the fabrication of magnetic, durable, and superhydrophobic cotton for oil/water separation. The superhydrophobic cotton functionalized with Fe3O4 magnetic nanoparticles was prepared via the in situ coprecipitation of Fe2+/Fe3+ ions under ammonia solution on cotton fabrics using polyvinylpyrrolidone (PVP) as a coupling agent and hydrophobic treatment with tridecafluorooctyl triethoxysilane (FAS) in sequence. The as-prepared cotton demonstrated excellent superhydrophobicity with a water contact angle of 155.6° ± 1.2° and good magnetic responsiveness. Under the control of the external magnetic field, the cotton fabrics could be easily controlled to absorb the oil from water as oil absorbents, showing high oil/water separation efficiency, even in hot water. Moreover, the cotton demonstrated remarkable mechanical durable properties, being strongly friction-resistant against sandpaper and finger wipe, while maintaining its water repellency. This study developed a novel and efficient strategy for the construction of magnetic, durable, and superhydrophobic biomass-based adsorbent for oil/water separation, which can be easily scaled up for practical oil absorption.


Introduction
Oil spills due to the release of marine oils or industrial wastewater have attracted worldwide attention for their potential pollution to the environment and impact on human health [1][2][3]. Tradition methods such as gravity separation [4], filtration [5], and centrifuge have been used for the separation of organic oils from polluted water. Nevertheless, the above methods are still insufficient in generality, usability and adaptability for oil/water separation owing to the low separation efficiency or complicated operation procedures.
Since Jiang et al. [6] innovatively proposed a straightforward, rapid, and economical method for the construction of steel mesh film with superhydrophobic/superhydrophilic coatings for oil/water separation, various superhydrophobic substrates have gained attention for their potential application in oil/water separation [7][8][9][10][11][12][13][14]. The common oil/water separation materials such as stainless steel [15][16][17], 0.80 g/0.30 g to 1.0 g/0.35 g) at room temperature. After magnetic stirring for 20 min under N 2 atmosphere, NH 3 ·H 2 O solution was added dropwise until the pH reached 8. Then, the mixture was stirred vigorously for 6 h. After being washed in sequence with EtOH and water, the Fe 3 O 4 -modified cotton was dried at 60 • C for 12 h.

Fabrication of Superhydrophobic Cotton
The solution containing 1.00 mL of tridecafluorooctyl triethoxysilane (FAS) and 99 g of absolute ethanol was mechanically stirred for 2 h. After pre-hydrolysis, the Fe 3 O 4 -modified cotton fabrics were placed in the above solution and mechanically stirred for at least 3 h. The stable water-repellent cotton fabrics were dried at 60 • C for 12 h.

Characterization
Fourier transform infrared spectrometer (FT-IR) was used to analyze structures of cotton before and after surface modification (Spectrum 2000, Perkin Elmer, Waltham, MA, USA). X-Ray photoelectron spectroscopy (XPS) was used to determine the elemental composition of samples (Thermo Electron Escalab 250, Thermo Fisher Scientific Waltham, MA, USA), with Al Ka radiation (20 eV) as the excitation source. A scanning electron microscope (FESEM: Quanta 400F, FEI, Hillsboro, OR, USA) was used to observe the morphological microstructures of samples. The crystallographic phase analysis of samples was performed on an X-ray diffractometer (PW3040/00 X'Pert MPD, Philips, Amsterdam, Netherlands) using Cu-Kα, ranging between 10 • and 90 • . Thermogravimetric analysis (TGA) was performed on a Q600 instrument with a heating rate of 20 • C/min from 40 to 600 • C under nitrogen atmosphere. A Dataphysics OCA20 contact angle system was used to measure water contact angles (WCAs) of samples with liquid droplets of 5 µL. The oil absorption capacity of samples was determined according to the previously reported method [42].

Results
Polyvinylpyrrolidone (PVP) is a non-ionic macromolecular compound, which is the most representative among the n-vinyl amide polymers and the most widely studied fine chemicals. PVP can adsorb onto different substrate surfaces, such as polymers, silica, plant fibers, metals and rare earths, attributed to polar -C-N-C=O groups in PVP [49]. This study shows how PVP plays a key role in cotton absorbing Fe 3+ /Fe 2+ ions and subsequent precipitation in situ forming Fe 3 O 4 -modified cotton. The green procedure for the synthesis of superhydrophobic/superoleophilic Fe 3 O 4 -modified cotton fabrics and following magnetic control of oil/water separation is depicted in Scheme 1. PVP was first grafted onto cotton fabrics, then Fe 3+ /Fe 2+ ions were adsorbed and subsequently coprecipitated onto cotton surfaces due to the strong static adsorption between Fe 3+ /Fe 2+ ions and highly polar -C-N-C=O groups in PVP. After hydrophobic treatment with FAS, the cotton fabrics containing Fe 3 O 4 nanoparticles could be magnetized under external magnetic field, allowing them to be flexibly controlled by a magnet during the oil/water separation process. coating thickness and a multilayer shell, creating a micro/nanostructured roughness surface ( Figure  1d). Scheme 1. Synthetic procedure for superhydrophobic/superoleophilic Fe3O4-modified cotton fabrics and subsequent magnetic control of oil/water separation. Surfaces of pristine cotton and cotton modified with different amounts of Fe 3 O 4 were illustrated via SEM analyses. In Figure 1a, raw cotton fabrics had a smooth surface and typical arrangement of fibrils in spiral fashion [50], which were beneficial for loading more Fe 3 O 4 nanoparticles. After coprecipitation with 0.54 g ferric chloride and 0.19 g ferrous chloride, a thin layer of Fe 3 O 4 nanoparticles was arranged well on cotton fabrics (Figure 1b). With the amount of ferric chloride and ferrous chloride increased, the amounts of Fe 3 O 4 nanoparticles on cotton surface increased accordingly. When the addition of ferric chloride and ferrous chloride were 1.0 g and 0.35 g respectively, the Fe 3 O 4 nanoparticles on the surface of cotton fabrics had a much more heterogeneous coating thickness and a multilayer shell, creating a micro/nanostructured roughness surface (Figure 1d).
FT-IR spectra of the cotton fabrics before and after surface modification in different stages are shown in Figure 2. In Figure 2b, compared with the spectra with raw cotton in Figure 2a, the new vibration peak at 1640 cm −1 corresponds to the -C=O group in amide carbonyl of PVP chains, which proved the successful graft of PVP onto the cotton surface. After surface in situ coprecipitation with Fe 2+ /Fe 3+ ions, the peak around 580 cm −1 became stronger due to the stretching vibration of the Fe-O bond in Fe 3 O 4 in Figure 2c [51]. After hydrophobization treatment of FAS in Figure 2d, new peaks at 1195 cm −1 and 1226 cm −1 were due to C-F stretching vibrations in FAS [48]. These results indicated that cotton@PVP@Fe 3 O 4 @FAS composites were successfully synthesized to construct magnetic, durable, and superhydrophobic cotton for oil/water separation. FT-IR spectra of the cotton fabrics before and after surface modification in different stages are shown in Figure 2. In Figure 2b, compared with the spectra with raw cotton in Figure 2a, the new vibration peak at 1640 cm −1 corresponds to the -C=O group in amide carbonyl of PVP chains, which proved the successful graft of PVP onto the cotton surface. After surface in situ coprecipitation with Fe 2+ /Fe 3+ ions, the peak around 580 cm −1 became stronger due to the stretching vibration of the Fe-O bond in Fe3O4 in Figure 2c [51]. After hydrophobization treatment of FAS in Figure 2d, new peaks at 1195 cm −1 and 1226 cm −1 were due to C-F stretching vibrations in FAS [48]. These results indicated that cotton@PVP@Fe3O4@FAS composites were successfully synthesized to construct magnetic, durable, and superhydrophobic cotton for oil/water separation. XPS spectra were further used to confirm the surface chemical composition of raw cotton fabrics, cotton@PVP@Fe3O4, and cotton@PVP@Fe3O4 composites after fluorination ( Figure 3). In Figure 3a1, the XPS survey spectra of pristine cotton showed the binding energy of 285.3 eV and 532.6 eV belonging to carbon (C 1s) and oxygen (O 1s) of cotton [52]. In Figure 3a2  Binding Energy (eV) Binding Energy (eV) Raw  XPS spectra were further used to confirm the surface chemical composition of raw cotton fabrics, cotton@PVP@Fe3O4, and cotton@PVP@Fe3O4 composites after fluorination ( Figure 3). In Figure 3a1, the XPS survey spectra of pristine cotton showed the binding energy of 285.3 eV and 532.6 eV belonging to carbon (C 1s) and oxygen (O 1s) of cotton [52]. In Figure 3a2  Binding Energy (eV) Binding Energy (eV) Raw  High-resolution spectra of C 1s of pristine cotton, cotton@PVP, and cotton@PVP@Fe 3 O 4 composites are displayed in Figure 3d-f. In Figure 3d, the C 1s peak was deconvoluted into three peaks of -C=O, C-O, and C-C located at 288.0, 286.5, and 284.8 eV, respectively [53]. After the surface was grafted with PVP (Figure 3e), the relative intensity of C-C/C=C at 284.8 eV and C=O at 288.0 eV increased due to the pyrrole ring in PVP. After surface precipitation of Fe 3 O 4 (Figure 3f), the intensity of the C=O peak decreased, probably owing to the elimination of the −C-N-C=O of PVP during the deposition of iron ions [54]. These results indicated that PVP acted as a bond bridge for the surface in situ deposition of Fe 3+ /Fe 2+ and cotton fabrics to synthesize cotton@PVP@Fe 3 O 4 composites successfully.
Besides, the surface elemental mapping analysis (EDS mapping) illustrated in Figure 4 demonstrates that each kind of element were uniformly attached to cotton fabrics, which further certified the successful fabrication of cotton@PVP@Fe 3 O 4 @FAS composites. High-resolution spectra of C 1s of pristine cotton, cotton@PVP, and cotton@PVP@Fe3O4 composites are displayed in Figure 3d-f. In Figure 3d, the C 1s peak was deconvoluted into three peaks of -C=O, C-O, and C-C located at 288.0, 286.5, and 284.8 eV, respectively [53]. After the surface was grafted with PVP (Figure 3e), the relative intensity of C-C/C=C at 284.8 eV and C=O at 288.0 eV increased due to the pyrrole ring in PVP. After surface precipitation of Fe3O4 (Figure 3f), the intensity of the C=O peak decreased, probably owing to the elimination of the −C-N-C=O of PVP during the deposition of iron ions [54]. These results indicated that PVP acted as a bond bridge for the surface in situ deposition of Fe 3+ /Fe 2+ and cotton fabrics to synthesize cotton@PVP@Fe3O4 composites successfully.
Besides, the surface elemental mapping analysis (EDS mapping) illustrated in Figure 4 demonstrates that each kind of element were uniformly attached to cotton fabrics, which further certified the successful fabrication of cotton@PVP@Fe3O4@FAS composites. Figure 5 shows the XRD patterns of pristine cotton fabrics, cotton@PVP@Fe3O4 composites, and FAS-modified cotton@PVP@Fe3O4 composites.   The thermal stability and Fe3O4 amount coated on cotton fabrics were presented in Figure 6. It can be seen that both the raw cotton fabrics and Fe3O4 modified cotton fabrics with different amount of ferric chloride and ferrous chloride began to decompose at temperatures above 300 °C. This observation showed that Fe3O4 surface modification did not destroy the chemical structure of raw cotton fabrics. In Figure 6a, the remaining inorganic component of raw cotton fabrics was 8.70 wt% [57]. After surface precipitation of Fe3O4 particles (Figure 6b), the residual solid slightly increased to 14.79 wt% (Figure 6b. With the addition of ferric chloride and ferrous chloride from 0.80 g/0.30 g to 1.0 g/0.35 g, the residual solid increased from 21.18 wt% to 31.13 wt%, illustrating more Fe3O4 were attached to cotton fabrics (Figure 6c,d). These TGA curves combined with the SEM analysis in Figure  3 proved that the composites were composed of cotton fabrics and Fe3O4, which constituted organicinorganic hybrid composites. The superhydrophobic properties of the Fe3O4-modified cotton fabrics were characterized by WCA measurement shown in Figure 7a,b. The as-prepared superhydrophobic cotton fabrics presented excellent superhydrophobic property, with water droplets dyed with methylene blue The thermal stability and Fe 3 O 4 amount coated on cotton fabrics were presented in Figure 6. It can be seen that both the raw cotton fabrics and Fe 3 O 4 modified cotton fabrics with different amount of ferric chloride and ferrous chloride began to decompose at temperatures above 300 • C. This observation showed that Fe 3 O 4 surface modification did not destroy the chemical structure of raw cotton fabrics. In Figure 6a, the remaining inorganic component of raw cotton fabrics was 8.70 wt% [57]. After surface precipitation of Fe 3 O 4 particles (Figure 6b), the residual solid slightly increased to 14.79 wt% (Figure 6b. With the addition of ferric chloride and ferrous chloride from 0.80 g/0.30 g to 1.0 g/0.35 g, the residual solid increased from 21.18 wt% to 31.13 wt%, illustrating more Fe 3 O 4 were attached to cotton fabrics (Figure 6c,d). These TGA curves combined with the SEM analysis in Figure 3 proved that the composites were composed of cotton fabrics and Fe 3 O 4 , which constituted organic-inorganic hybrid composites. The thermal stability and Fe3O4 amount coated on cotton fabrics were presented in Figure 6. It can be seen that both the raw cotton fabrics and Fe3O4 modified cotton fabrics with different amount of ferric chloride and ferrous chloride began to decompose at temperatures above 300 °C. This observation showed that Fe3O4 surface modification did not destroy the chemical structure of raw cotton fabrics. In Figure 6a, the remaining inorganic component of raw cotton fabrics was 8.70 wt% [57]. After surface precipitation of Fe3O4 particles (Figure 6b), the residual solid slightly increased to 14.79 wt% (Figure 6b. With the addition of ferric chloride and ferrous chloride from 0.80 g/0.30 g to 1.0 g/0.35 g, the residual solid increased from 21.18 wt% to 31.13 wt%, illustrating more Fe3O4 were attached to cotton fabrics (Figure 6c,d). These TGA curves combined with the SEM analysis in Figure  3 proved that the composites were composed of cotton fabrics and Fe3O4, which constituted organicinorganic hybrid composites. The superhydrophobic properties of the Fe3O4-modified cotton fabrics were characterized by WCA measurement shown in Figure 7a,b. The as-prepared superhydrophobic cotton fabrics presented excellent superhydrophobic property, with water droplets dyed with methylene blue The superhydrophobic properties of the Fe 3 O 4 -modified cotton fabrics were characterized by WCA measurement shown in Figure 7a,b. The as-prepared superhydrophobic cotton fabrics presented excellent superhydrophobic property, with water droplets dyed with methylene blue standing firmly on the cotton fabric's surface and maintaining spherical shapes with WCA of 155.6 • ± 1.2 • . After being placed on the water surface, the raw cotton fabrics were quickly wetted, absorbed, and sank into the water, while the modified cotton fabrics still floated on water surface owing to its superhydrophobicity (Figure 7c). Moreover, the silver mirror-like phenomenon could be seen by pressing the modified cotton fiber into the water by an external force (Figure 7d). This nonwetting phenomenon by water was due to the synergistic effect of micro-nano hierarchical structure and low surface energy on the cotton fabrics surface, which was in Cassie-Baxter state [58]. standing firmly on the cotton fabric's surface and maintaining spherical shapes with WCA of 155.6° ± 1.2°. After being placed on the water surface, the raw cotton fabrics were quickly wetted, absorbed, and sank into the water, while the modified cotton fabrics still floated on water surface owing to its superhydrophobicity (Figure 7c). Moreover, the silver mirror-like phenomenon could be seen by pressing the modified cotton fiber into the water by an external force (Figure 7d). This nonwetting phenomenon by water was due to the synergistic effect of micro-nano hierarchical structure and low surface energy on the cotton fabrics surface, which was in Cassie-Baxter state [58]. The superhydrophobic property of cotton@Fe3O4@FAS composites endowed them with highselectivity oil adsorption capacity. Besides, the cotton can be used for oil/water separation under the control of an external magnetic field. In Figure 8a and Video S1, the cotton quickly absorbed the red oil-dyed n-hexane spread on water and the red oil-dyed chloroform from the bottom of the water within several seconds controlled by the external magnetic field. The oil-loaded cotton remained floating on the water's surface, leaving a transparent region of water and could be taken out of water under magnetic control after oil/water separation, which depicted excellent oil/water separation capability of cotton fabrics. Moreover, the magnetic-responsive cotton also provided a new approach for magnetically controlling the oil/water separation process. The superhydrophobic property of cotton@Fe 3 O 4 @FAS composites endowed them with high-selectivity oil adsorption capacity. Besides, the cotton can be used for oil/water separation under the control of an external magnetic field. In Figure 8a and Video S1, the cotton quickly absorbed the red oil-dyed n-hexane spread on water and the red oil-dyed chloroform from the bottom of the water within several seconds controlled by the external magnetic field. The oil-loaded cotton remained floating on the water's surface, leaving a transparent region of water and could be taken out of water under magnetic control after oil/water separation, which depicted excellent oil/water separation capability of cotton fabrics. Moreover, the magnetic-responsive cotton also provided a new approach for magnetically controlling the oil/water separation process. The cotton fabrics also presented durable superhydrophobicity and excellent oil absorption capacity even in hot water at temperatures higher than 90 °C. In Figure 8b and Video S2, the cotton quickly selectively absorbed the n-hexane spread on water, after it was placed into the mixture of nhexane/chloroform and hot water (>90 °C). When the cotton was immersed into hot water by an external force, it absorbed the red oil-dyed chloroform from the bottom of water within a few seconds. The hot water solution became clean and transparent with the volume nearly unchanged, which showed excellent superhydrophobic stability under high temperature of cotton@Fe3O4@FAS composites.
Most reported superhydrophobic materials are inherently fragile to mechanical forces, and even cannot withstand a finger touch owing to their fragile micro/nano hierarchical structure. As a superhydrophobic/superoleophilic oil/water separation material, mechanical stability is of vital importance for the properties and applications of the final oil absorbent material. Therefore, a fingerwipe test was used to evaluate the mechanical durability of the superhydrophobic cotton (Figure 9a and Video S3). After several finger-wipes, rub and tear, the Fe3O4 nanoparticles remained on the cotton surface with no visible paint loss. Moreover, the water quickly fell off the surface of cotton fabrics, which proved that the cotton still maintained superhydrophobicity.
The friction-resistant experiment against sandpaper in terms of superhydrophobicity, simulating practical conditions, was also tested. As shown in Figure 9b and Video S4, the cotton fabrics were placed on a 400-mesh sandpaper under a weight of 50 g. After several back-and-forth friction cycles, though the sandpaper abrasion could remove the fabrics from the surface of the cotton, and the embedded fabrics were exposed, the cotton still maintained good water repellency. The effect of friction-resistance against sandpaper on the superhydrophobicity of cotton was negligible even after 50 cycles. The cotton fabrics also presented durable superhydrophobicity and excellent oil absorption capacity even in hot water at temperatures higher than 90 • C. In Figure 8b and Video S2, the cotton quickly selectively absorbed the n-hexane spread on water, after it was placed into the mixture of n-hexane/chloroform and hot water (>90 • C). When the cotton was immersed into hot water by an external force, it absorbed the red oil-dyed chloroform from the bottom of water within a few seconds. The hot water solution became clean and transparent with the volume nearly unchanged, which showed excellent superhydrophobic stability under high temperature of cotton@Fe 3 O 4 @FAS composites.
Most reported superhydrophobic materials are inherently fragile to mechanical forces, and even cannot withstand a finger touch owing to their fragile micro/nano hierarchical structure. As a superhydrophobic/superoleophilic oil/water separation material, mechanical stability is of vital importance for the properties and applications of the final oil absorbent material. Therefore, a finger-wipe test was used to evaluate the mechanical durability of the superhydrophobic cotton (Figure 9a and Video S3). After several finger-wipes, rub and tear, the Fe 3 O 4 nanoparticles remained on the cotton surface with no visible paint loss. Moreover, the water quickly fell off the surface of cotton fabrics, which proved that the cotton still maintained superhydrophobicity.
The friction-resistant experiment against sandpaper in terms of superhydrophobicity, simulating practical conditions, was also tested. As shown in Figure 9b and Video S4, the cotton fabrics were placed on a 400-mesh sandpaper under a weight of 50 g. After several back-and-forth friction cycles, though the sandpaper abrasion could remove the fabrics from the surface of the cotton, and the embedded fabrics were exposed, the cotton still maintained good water repellency. The effect of friction-resistance against sandpaper on the superhydrophobicity of cotton was negligible even after 50 cycles. As an oil/water separation absorbent, the oil absorption ability and durability of the prepared cotton are of great importance for practical applications. To evaluate the oil absorption ability, different kinds of oil were used for the absorption experiment, as shown in Figure 10a. The cotton showed good oil absorption capacity varying from 20.7 g/g to 51.2 g/g depending on different oil densities. Besides, as shown in Figure 10b, the as-prepared cotton also showed good reusability with no obvious decline in absorption capacity even after more than 15 cycles of the absorption process using dichloromethane, crude oil, DMF, and chloroform for study. The results further proved that the as-prepared cotton possesses excellent oil absorption ability for a wide range of oils and mechanical durability even after being reused for 15 cycles, which are quite important properties for actual applications.  As an oil/water separation absorbent, the oil absorption ability and durability of the prepared cotton are of great importance for practical applications. To evaluate the oil absorption ability, different kinds of oil were used for the absorption experiment, as shown in Figure 10a. The cotton showed good oil absorption capacity varying from 20.7 g/g to 51.2 g/g depending on different oil densities. Besides, as shown in Figure 10b, the as-prepared cotton also showed good reusability with no obvious decline in absorption capacity even after more than 15 cycles of the absorption process using dichloromethane, crude oil, DMF, and chloroform for study. The results further proved that the as-prepared cotton possesses excellent oil absorption ability for a wide range of oils and mechanical durability even after being reused for 15 cycles, which are quite important properties for actual applications. As an oil/water separation absorbent, the oil absorption ability and durability of the prepared cotton are of great importance for practical applications. To evaluate the oil absorption ability, different kinds of oil were used for the absorption experiment, as shown in Figure 10a. The cotton showed good oil absorption capacity varying from 20.7 g/g to 51.2 g/g depending on different oil densities. Besides, as shown in Figure 10b, the as-prepared cotton also showed good reusability with no obvious decline in absorption capacity even after more than 15 cycles of the absorption process using dichloromethane, crude oil, DMF, and chloroform for study. The results further proved that the as-prepared cotton possesses excellent oil absorption ability for a wide range of oils and mechanical durability even after being reused for 15 cycles, which are quite important properties for actual applications.

Conclusions
In summary, a facile approach for the fabrication of a magnetic, durable, and superhydrophobic cotton@Fe 3 O 4 @FAS oil absorbent was proposed by in-situ precipitation of Fe 2+ /Fe 3+ ions onto a cotton surface in the presence of PVP and subsequent hydrophobic modification with FAS. The pyrrolidone ring of PVP could adsorb Fe 2+ /Fe 3+ and subsequently immobilized the Fe 3 O 4 nanoparticles on cotton fabrics. The binding of Fe 3 O 4 nanoparticles on the surface of cotton not only increased the surface