Scaly bionic structures constructed on a polyester fabric with anti-fouling and anti-bacterial properties for highly efficient oil–water separation

Abstract

Oil–water separation has become a hot issue due to excessive amounts of oily waste as well as frequent oil leakage accidents that highly threaten biological safety. Inspired by fish scales, we designed a scaly structure on modified polyester fabric to give full play of its outstanding anti-pollution and water-capturing properties. As the substrate with micro/nanoscale structure, ZnO nanorods were assembled on a polyester fabric for further modification. Furthermore, a PEI coating was added to establish a scaly structure for superhydrophilicity. The two factors mentioned above contributed to both superhydrophilicity and anti-fouling properties for the modified polyester fabric. Further studies explored the as-prepared fabric with super-hydrophilicity and found that the underwater superoleophobicity was extremely suitable for oil–water separation with ultra-high separation efficiencies (>99.99%) under only natural gravity forces. Furthermore, the as-prepared fabric could be washed by flowing water and retained the ultra-high separation efficiency even after 10 circles. Surprisingly, the modified fabric had superb performance under high-salinity solution (10 wt% NaCl) and high temperature (40 °C, 60 °C and 80 °C) conditions. In addition, the as-prepared fabric had outstanding anti-bacterial ability, which could inhibit the growth of both Gram-negative and Gram-positive bacteria, greatly increasing the service life of the modified fabric and preventing the water from being secondly polluted. Moreover, the research provided an innovative way to develop effective oil–water separation methods and related devices.

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Introduction

With the expansion of the oil industry, including oil production, oil transportation and the generation of industrial oily water wastes, oil spill accidents, which severely damage ecological equilibrium and pose threats to biological safety, occur frequently.^1–3 Under such circumstances, the question of how to separate the oil–water mixture effectively becomes the main focus for primary research.^4–7 Various porous materials, such as sponges,⁸ metal meshes⁶ and fabrics,⁹ have been widely researched and applied for oil–water separation thanks to their remarkable selective absorption/separation capacity and chemical stability. Among them, fabrics have attracted much attention because of its low price and high mechanical stability.^10–12 In addition, polyester fabrics have flexible but robust mechanical properties, leading polyester fabrics to be an optimal candidate for oil/water separation.¹³ But unfortunately, these common porous materials cannot selectively absorb or separate oil and water. Therefore, much attention has been roused to modify these porous materials to absorb or separate oil and water selectively.

Very recently, materials with special wetting behaviors (superhydrophobicity, superoleophilicity, super-hydrophilicity and superoleophobicity)^14,15 have aroused much attention due to their attractive advantages in separating oil–water mixtures. It has been widely reported that many materials with super-hydrophobicity, superoleophilicity, super-hydrophilicity or superoleophobicity can separate oil and water only by gravity and retain high separation efficiency even after several cycles. Wang et al.¹⁶ reported a novel rough pDA coating film, which showed superhydrophobicity, and could be utilized for self-driven oil spill cleanup. Zhang et al.¹⁷ prepared superhydrophobic and superoleophilic polyester fabrics by growing silicone nano-filaments. The materials with both superhydrophobicity and superoleophilicity could effectively absorb oil from oil–water mixtures. However, it was worth noting that the “absorb-oil” materials tended to be fouled or even blocked by oil and were not easy to clean and reuse, which severely affected the oil–water separation efficiency. On this basis, materials with superhydrophilicity and underwater superoleophobicity were studied to overcome the above disadvantage. Zhu et al.¹⁸ reported a PVDF membrane grafted by zwitterionic polyelectrolyte with superhydrophilicity and underwater superoleophobicity. For this membrane, water could permeate the membrane while oil retained. This “water-permeate” film guided new directions towards oil–water separation, but it was regrettable that the separation processes could only be applied under 0.1 MPa of pressure. Zhang et al.¹⁹ fabricated a copper mesh film with hierarchical structure through electrodeposited Cu(OH)₂ coating. Zheng et al.²⁰ built self-cleaning scaly fabric membranes with high separation efficiency by coating titanium oxide to fabricate scaly nanostructures. Noticeably, the films containing micro/nanoscale hierarchical structures revealed superb advantages in separating oil–water mixtures. Besides this, the modified films retained high separation efficiency even after several cycles and could be reused under normal service conditions, which was seldom referred to in most papers.

In recent years, researchers realized that oil–water mixtures often contained high concentrations of salts, even microorganisms, such as Gram-negative bacteria (Escherichia coli, EC) and Gram-positive bacteria (Staphylococcus aureus, SA). The following reports cautiously considered the introduction of anti-bacterial agents into oil–water separation. Liu et al.²¹ synthesized graphene oxide–SiO₂ coated mesh film with anti-bacterial (EC) properties. Unfortunately, only a Gram-negative anti-bacterial property was reported, while the Gram-positive anti-bacterial property was not reported.

The bionic scaly structure had both the advantage of micro/nanoscale hierarchical structure and the capacity of anti-pollution and capturing water. In this paper, we chose polyester fabric as the main raw material and a scaly structure was intentionally built onto the surface of the fabric through the hydrothermal method. Polyethyleneimine (PEI) with high amino group density was coated onto the fabric to build a scaly structure that showed superhydrophilicity and underwater superoleophobicity. Unlike usual fabrics that are always being fouled by oil, leading to a sharp decrease of separation performance, our modified fabric retained nearly constant separation efficiency even after 10 cycles. Better yet, the modified fabric could be regenerated in several minutes only by flowing water and kept clean even after 10 cycles, which showed unexceptionable anti-fouling properties. Besides, the wetting properties as well as the separation effect showed no apparent variation after immersion in high-salinity solutions (10 wt% NaCl) and high temperature oil–water mixtures. Meanwhile, it could not be ignored that the modified fabric had excellent anti-bacterial abilities (including both EC and SA). In conclusion, the modified fabric could be used effectively for oil–water separation and has a promising future in various operating conditions due to its special environmental stability.

Experimental section

Materials

Polyethyleneimine (PEI, M_w = 600) was obtained from Aladdin (China). Anhydrous ethanol, sodium chloride (NaCl), sodium hydroxide (NaOH), 1,2-dichloroethane, zinc acetate dihydrate [Zn(CH₃COO)₂·2H₂O], zinc nitrate hexahydrate [Zn(NO)₃·6H₂O], hexamethylenetetramine (HMTA) and dimethyl silicone oil were obtained from Tianjin Kermel Chemical Reagent Co., Ltd. (China). Yeast extract (LP0021) and tryptone (LP0042) were obtained from Oxoid Ltd, (England). Agar was obtained from Japan. Crude oil, soybean oil, kerosene and polyester fabric were provided by a local store. All the reagents and solvents were used as received without further treatment. The fabric was cleaned with deionized water and anhydrous ethanol by sonication.

Preparation of the scaly structure onto polyester fabric

Having the property of superhydrophilicity and underwater superoleophobicity, fabrics with scaly structure were functionalized through three separate surface treatments. Firstly, the cleaned fabrics were submerged in a ZnO colloid suspension (used as quantum dot seeds). As ref. 22 showed, 0.0125 mol L⁻¹ solution of Zn(CH₃COO)₂·2H₂O in ethanol and 0.02 mol L⁻¹ solution of NaOH in ethanol were made separately. After that, 40 mL Zn(CH₃COO)₂ solution was mixed with 320 mL ethanol and 40 mL NaOH solution was mixed with 100 mL ethanol. Then, the above two solutions were preheated separately to 65 °C and mixed for 30 min under magnetic stirring. After cooling down in an ice-bath to room temperature, the cleaned fabric was dipped into a mixed solution for 15 min, and then annealed at 150 °C for 10 min. This step was repeated five times.

Then, the polyester/ZnO fabric was prepared as follows: 0.025 mol L⁻¹ Zn(CH₃COO)₂ and 0.025 mol L⁻¹ HMTA water solution was prepared, and the solution was heated to 90 °C under magnetic stirring. The fabric coated with ZnO seeds was immersed into the above mixed solution for 1, 2, 4 and 8 h, respectively. After that, the fabric was washed with deionized water under sonication for 5 min and dried at 100 °C.

At last, the polyester/ZnO/PEI fabric was fabricated by dipping polyester/ZnO fabric into 1 wt% PEI water solution at 60 °C for 15 min under magnetic stirring. Then, the fabric was washed thoroughly with deionized water by sonicating for 5 min and dried under vacuum.

Oil–water mixture separation process

For oil–water separation experiments, the fabric was fixed below an upper glass tube. The fabric was wetted by deionized water before separation. Then, the oil–water mixture (volume ratio, 1 : 3) was poured into the upper tube and followed by the collection of permeating liquid under the tube. The oil concentration of the collected liquid was analyzed using an Infrared Spectrometer Oil Content Analyzer (CY2000, China). Besides this, the separation efficiency was calculated by the following equation.while C₀ and C_p were oil concentrations of the original oil–water mixture and collected solution after the separation process, respectively.

Anti-bacterial evaluation

The anti-bacterial activity of the fabric was tested by the inhibition zone method.²³ In this method, EC and SA were taken as model bacteria, representing Gram-negative bacteria and Gram-positive bacteria, respectively. For this study, the fabrics were cut into small circular pieces (10 mm). At least two samples each were placed on an agar plate that was inoculated with 1 mL of bacterial suspension. Then, the plates were incubated at 37 °C for 16 to 24 h. After that, the diameter of zones around the fabric was measured and recorded as the standard criterion of inhibition against bacteria.

Instruments and characterization

The surface morphology of the fabric was observed by field-emission scanning electronic microscopy (SEM, NanoLab 600i, FEI Company, America). The crystal structure of the fabrics was characterized by X-ray diffractometry (XRD, X'pert, Philips Electronic Instruments, Mahwah, NJ, America) with Cu Kα radiation (λ = 0.15418 nm). The surface chemical analysis of the fabric was carried out by X-ray photoelectron spectrometry (XPS, PHI5700), using the monochromatized Mg Kα X-ray source (1486.6 eV photons) at a constant well time of 100 ms and a pass energy of 40 eV. Water contact angles (CAs) was examined by static water contact angle measurements (SL200KB, Shanghai Solon Company, China) at room temperature. A 3 μL water or oil droplet was used in water or oil for underwater CA measurements. For water CA measurements in air, the water droplet was dropped onto the fabric directly, while for oil CA measurements in water or high-salinity solution, 1,2-dichloroethane was chosen as the test oil due to its density being higher than water. The oil droplet was put onto the fabric underwater or high-salinity solution. The high-salinity solution was 10 wt% aqueous NaCl. Seven measurements were taken at different points of the fabric to ensure the repeatability of the results for CAs.

Results and discussion

In order to realize the scaly structure surface with superhydrophilicity and underwater superoleophobicity, nanomaterials were indispensable. In this paper, polyester fabric was the candidate for oil/water separation owing to its excellent flexible, but robust, mechanical properties. ZnO nanorods were assembled onto the fabric surface through the hydrothermal method, providing a coarse structure. Besides, PEI with plenty of hydrophilic amino groups was coated onto fabric to build a scaly structure, which further increased the superhydrophilicity and anti-fouling performance. Consequently, the as-prepared fabric achieved the ability of capturing water and anti-fouling. Fig. 1a shows the SEM image of the fabric surface being neat and smooth without any nanoparticles. The surface of the fabric assembled with ZnO seeds became rough and some nanoparticles appeared sporadically (Fig. S1†). However, for the polyester/ZnO fabric, uniform ZnO nanorods were assembled onto the surface of the fabric with an average diameter of 50 nm and length of 200–400 nm (Fig. 1b). The above structure contributed to enhance the surface roughness and increase specific surface area. Furthermore, the existence of ZnO nanorods induced the establishment of a scaly structure. The ZnO nanorods assembled onto the fabrics treated at different times are depicted in Fig. S2.† The amplified images of these fabrics also displayed the particle morphology.

Fig. 1 SEM images of (a) polyester fabric, (b) polyester/ZnO fabric (hydrothermal reaction time of 4 h) and (c) polyester/ZnO/PEI fabric.

The wetting ability of polyester/ZnO fabrics was compared with each other through water CA and oil in water CA measurements. In air, both water and oil could permeate through the polyester/ZnO fabrics rapidly, demonstrating superhydrophilicity and superoleophilicity. While in water, the oil contact angles were different from each other (Fig. S3†). The polyester/ZnO fabric after reacting for 4 h reached the maximum CA (both in 1,2-dichloroethane and crude oil). However, all the oil CAs for diverse modification conditions in water were higher than that of polyester, demonstrating great underwater superoleophobicity of polyester/ZnO fabrics. Hence, polyester/ZnO fabric after reacting for 4 h was chosen for further decoration due to its highest oil CAs in water.

Inspired by fish scales, polyester/ZnO/PEI fabric with a scaly structure, which had excellent superhydrophilicity and underwater superoleophobicity, was preferentially chosen to separate oil–water mixtures. Coated with PEI, the morphology of the polyester/ZnO/PEI fabric changed greatly. As Fig. 1c shows, ZnO nanorods could not be observed directly once the scaly structure had already formed. Compared with untreated polyester and polyester/ZnO fabrics, the polyester/ZnO/PEI fabric showed superior wetting abilities. In air, oil could permeate all the fabrics, including polyester, polyester/ZnO and polyester/ZnO/PEI fabrics. Meanwhile, water could permeate the polyester/ZnO/PEI fabric at a faster pace than through the polyester/ZnO fabric (Movies 1 and 2†). The water CAs of polyester was about 132° (Fig. S4†), manifesting that the modification with ZnO and PEI could transform the fabric from hydrophobic to superhydrophilic. However, after immersion into water, the polyester fabric showed underwater oleophobicity; whereas the polyester/ZnO/PEI fabric exhibited underwater superoleophobicity. On the other hand, the polyester/ZnO/PEI fabric exhibited the optimum underwater superoleophobicity due to the fact that the oil CA in water was higher than that of the polyester/ZnO fabric (Fig. 2).

Fig. 2 The oil contact angles of different fabrics in water: (a) 1,2-dichloroethane and (b) crude oil.

The XRD patterns of different fabrics are displayed in Fig. 3. The characteristic peaks of polyester were observed in all samples. For the polyester/ZnO fabric, characteristic peaks at 31.9 (100), 34.5 (002), 36.4 (101), 47.7 (102), 56.7 (110), 63.2 (103), 68.2 (112) and 69.2 (201) degrees could be observed. These peaks were consistent with the values in the standard card (JCPDS 36-1451) and could be indexed as the hexagonal wurtzite ZnO phase. The narrow full width at half-maximum (FWHM) of the ZnO peaks demonstrated that well crystallized ZnO were assembled on the surface of the fabric. As for the polyester/ZnO/PEI fabric, the characteristic peaks of ZnO remained after coating with PEI while the crystallization peak strength of ZnO weakened. From the above results, it could be concluded that the ZnO structure can be assembled onto polyester fabric through a hydrothermal reaction and still remained after coating with a PEI layer.

Fig. 3 XRD pattern of different fabrics indicating that the ZnO structure was assembled onto the fabric and retained its structure after coating with PEI.

In order to confirm the interaction between polyester, ZnO and PEI, the XPS spectra was obtained. Elemental content and distribution of different fabrics are shown in Fig. 4a and Table 1. As Fig. 4a shows, only C, O and N elements existed in the polyester fabric. The Zn element appeared in polyester/ZnO and polyester/ZnO/PEI, confirming the existence of ZnO. In addition, the N element content of polyester/ZnO/PEI was the highest, which was due to the existence of PEI. The above were in accordance with the XRD results. To further distinguish the different types of functional groups for polyester, polyester/ZnO and polyester/ZnO/PEI, the deconvolution of the C1s spectra of the different fabrics are shown in Fig. 4b and the results are listed in Table 2. For polyester and polyester/ZnO, four peaks were designated as peak 1, peak 3, peak 4 and peak 5, respectively. Compared with polyester, the content of C–OH and C=O increased while the content of C–C decreased because intermolecular hydrogen-bonds formed between polyester and ZnO. Due to the introduction of PEI, which interacted with ZnO through hydrogen-bonding, the C–N peak (peak 2) appeared in polyester/ZnO/PEI.

Fig. 4 (a) Full XPS spectra of different fabrics. (b) XPS C1s spectra of different fabrics.

Table 1 Surface element content of different fabrics

Sample	Elemental content (%)
	C	O	N	Zn
Polyester fabric	68.08	29.98	1.94	—
Polyester/ZnO fabric	51.08	34.30	1.32	13.30
Polyester/ZnO/PEI fabric	67.25	26.75	3.91	22.09

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Table 2 Surface functional group content of C1s from different fabrics

Fabric sample	Functional group content (%)
	Peak 1	Peak 2	Peak 3	Peak 4	Peak 5
Polyester fabric	44.27	—	23.75	19.78	12.40
Polyester/ZnO fabric	35.01	—	29.89	23.23	11.86
Polyester/ZnO/PEI fabric	38.65	8.99	21.65	20.43	10.28
Binding mode	–C–C– (%)	C–N (%)	–C–OH (%)	–C=O (%)	–COOR/–COOH (%)
	284.6 eV	287.5 eV	286.1 eV	286.9 eV	288.9 eV

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As previous reports mentioned, scaly hierarchical micro/nanostructure films with superhydrophilicity and underwater superoleophobicity have obvious advantages in separating oil–water mixtures. In order to test the oil–water separation capability of the polyester/ZnO/PEI fabrics, a series of experiments were carried out. The oil–water separation process is illustrated in Fig. 5. Before the separation process, the fabrics were wetted by deionized water. Then, the oil–water mixture was poured into the upper tube that was above the tested fabrics. It could be clearly observed that, for the polyester/ZnO/PEI fabric, water could permeate through the fabric under only the force of natural gravity owing to its superhydrophilicity. On the contrary, oil could not permeate and remained in the upper tube due to its underwater superoleophobicity (Fig. 5a and Movie 3†). With this method, an oil–water mixture could be separated effectively with the result that no oil drops would be observed in the filtered solution at the end of the permeation process. In addition, the separation processes of polyester fabric and polyester/ZnO fabric were studied (Movies 4 and 5†). It could be observed that not only water, but also oil could permeate the polyester fabric and oil could be obviously observed in the filtered solution (Fig. 5b). For the polyester/ZnO fabric, only water could permeate, resulting in effective oil–water mixture separation (Fig. 5c). The above paradoxical phenomenon was due to the fact that the polyester/ZnO fabric also exhibited superhydrophilicity and underwater superoleophobicity, although its wetting ability was weaker than that of the polyester/ZnO/PEI fabric, proving that superhydrophilicity and underwater superoleophobicity played an active role in separating the oil–water mixture.

Fig. 5 Oil–water mixture separation process: the fabric was fixed below the upper tube, before the separation process, the fabric was wetted by water, after that, the oil–water mixture was poured into the upper tube. (a) Separation process through the polyester/ZnO/PEI fabric, only water permeate through the fabric, the filtered solution was pure. (b) Separation process through polyester fabric, both oil and water permeated through the fabric, and the filtrated solution was unclear. (c) Separation process through the polyester/ZnO fabric, only water permeated through the fabric. All these facts proved that the oil–water mixture could be separated by fabrics with superhydrophilicity and underwater superoleophobicity.

In order to evaluate separation efficiency accurately, the oil contents of the filtrated solutions were measured (Fig. 6). Besides this, the separation efficiency was also calculated and illustrated in Fig. 6. As shown in Fig. 6, the oil content of the collected solution that permeated through the polyester/ZnO/PEI fabric was the lowest, which was below 15 ppm (according to effluent limitations made by the Environment Protection Agency of the US government for Offshore Oil and Gas Activities in the Eastern Gulf of Mexico, a daily maximum oil limitation of produced water discharges is 42 ppm and a monthly average limitation is 29 ppm for oil and grease¹⁸). At the same time, the separation efficiency of the polyester/ZnO/PEI fabric reached higher than 99.99%. Besides this, the oil–water mixture could also be separated by the polyester/ZnO fabric whose separation efficiency was more than 99.9%, showing high efficiency for oil–water separation as well.

Fig. 6 Oil content of filtered solutions and the separation efficiency of different fabrics for crude oil–water mixtures.

According to the earlier reference, the fabric with easy-cleaning abilities has obvious advantages in the oil–water separation process. The fabrics were prone to being fouled or even blocked by oil, severely affecting the oil–water separation efficiency, and thus could not be recycled. Therefore, anti-fouling and easy-cleaning performances of the fabrics were tested. As Movies 6 and 7† show, the fabric was first immersed in water and then the fabric was immersed in crude oil. Finally, the fabric was cleaned by flowing water. After this cycle, the polyester fabric was polluted by oil completely and no clean part could be observed (Fig. 7a and Movie 6†). The polyester/ZnO fabric was also polluted partly by oil with a few smudges observed on the fabric (Fig. 7b and Movie 7†). However, the polyester/ZnO/PEI fabric still kept as clean as the original after this cycle (Fig. 7c and Movie 8†), demonstrating that the polyester/ZnO/PEI fabric with scaly structure had excellent anti-fouling and easy-cleaning properties. Even if the polyester/ZnO/PEI fabric was immersed into crude oil directly, it regained a new status merely after cleaning with flowing water.

Fig. 7 The photographs of fabrics after the anti-fouling test. The fabrics were first immersed in water and then immersed in crude oil. Finally, they were cleaned by flowing water and were dried out. (a) Polyester fabric was polluted by oil completely. (b) Polyester/ZnO fabric was polluted by oil partly. (c) Polyester/ZnO/PEI fabric remained as clean as the original, indicating that the polyester/ZnO/PEI fabric had anti-pollution and easy-cleaning properties.

Other kinds of oils, including soybean oil, kerosene, dimethyl silicone oil and 1,2-dichloroethane were also used to investigate the separation ability of the polyester/ZnO/PEI fabric (Fig. 8). It could be clearly observed that all types of oil–water mixtures could be separated effectively: the oil contents of the filtered solution were lower than 30 ppm and the separation efficiency was higher than 99.99%. The phenomenon clearly indicated that the polyester/ZnO/PEI fabric had outstanding advantages in separating various oil–water mixtures.

Fig. 8 Oil contents of filtered solution and the separation efficiency for various oil–water mixtures.

From what has been mentioned above, the polyester/ZnO/PEI fabric displayed easy-cleaning, anti-fouling abilities and excellent recycling performance could be expected. A separation process cycle was set and after each cycle, the polyester/ZnO/PEI fabric was cleaned by flowing water. After 10 separation circles, the water CA in air and oil CA in water were tested, respectively (Fig. S5†). The water CA in air was still zero degrees, while the oil CA in water was 165.66°, which changed little, proving that the polyester/ZnO/PEI fabric had excellent recycling performance and could remain superhydrophilic and underwater superoleophobic even after 10 separation cycles.

For each cycle, the oil content of the filtered solution, together with the separation efficiency, were inspected (Fig. S6†). It could be seen that the oil content of the filtered solution increased a little with each ongoing separation cycle. The separation efficiency decreased little and remained higher than 99.99%. All the above indicated that the polyester/ZnO/PEI fabric could retain high separation efficiency even after 10 separation cycles, exhibiting great recycling performance. Besides this, it could also be observed that the polyester/ZnO/PEI fabric was easy to clean (Movie 9†). As long as water flowed on the fabric, crude oil on the fabric was washed off. The fabric could be regenerated in several minutes only with cleaning by flowing water and was still kept clean even after 10 cycles (Fig. S7†).

The environmental stability of the fabric for separation under different circumstances was quite necessary because fabric contacted with the oil–water mixture directly, so it was of great importance for the fabric to retain stable properties in separating oil–water mixtures under high-salinity and high-temperature circumstances. It was pleasing to see that the polyester/ZnO/PEI fabric had such abilities. The separation process for oil in a high-salinity mixture was carried out to evaluate the ability of the polyester/ZnO/PEI fabric under high-salinity conditions (the water for tests was replaced by a high-salinity solution). Firstly, the polyester/ZnO/PEI fabric was immersed into the high-salinity solution (10 wt% NaCl) for 24 h. Then, the wetting ability and separation efficiency were examined, as shown in Fig. 9. It could be seen that the polyester/ZnO/PEI fabric still retained underwater superoleophobicity (Fig. 9a) and high separation efficiency (Fig. 9b) after the high-salinity immersion. Besides this, a high temperature oil–water mixture separation process was also carried out (Fig. 9c), revealing that the fabric kept high separation efficiency under various temperatures and the separation efficiency was even enhanced with an increased separation temperature. All these facts above showed that the polyester/ZnO/PEI fabric had excellent environmental stability, including outstanding high-salinity and high-temperature performance.

Fig. 9 (a) Oil content of filtered solutions and the separation efficiency of polyester/ZnO/PEI fabric for oil-high-salinity solution mixtures. (b) Oil CAs of polyester/ZnO/PEI fabric in high-salinity solutions. (c) Oil content of filtered solution and the separation efficiency of polyester/ZnO/PEI fabric under various temperatures.

To avoid secondary contamination by bacteria, an anti-bacterial fabric was desired. As Fig. 10 shows, polyester had no anti-bacterial property, while polyester/ZnO only had anti-Gram-positive properties. On the other hand, it was desirable to see that the polyester/ZnO/PEI fabric had both anti-Gram-negative and anti-Gram-positive properties.

Fig. 10 Anti-bacteria test of fabrics: (a) polyester, polyester/ZnO and polyester/ZnO/PE against EC. An inhibition zone could be only found around the polyester/ZnO/PEI. (b) Polyester, polyester/ZnO and polyester/ZnO/PEI against SA. An inhibition zone could be found around both polyester/ZnO and polyester/ZnO/PEI, but not around the polyester.

It could be clearly seen that the scaly structure with excellent superhydrophilicity and underwater superoleophobicity were key elements that could greatly affect the separating ability. Due to superhydrophilicity, water could infiltrate the fabric completely, while oil remained in the upper tube because of the underwater superoleophobicity. To further understand the mechanism of the oil–water separation process, the wetting mechanism of polyester, polyester/ZnO and polyester/ZnO/PEI fabrics were modeled in Fig. 11. Before wetting with water, the pores of the fabrics were covered with an air layer. For polyester fabric, its hydrophobicity resulted in difficulty being totally wetted by water (Fig. S4†). After wetting by water, the air film was partly replaced by water with some air bubbles still existing at the interface, thus forming a fourth interface besides solid/water/oil interface that was unsuitable for oil–water separation. When the oil–water mixture contacted the fabric, water could penetrate the fabric through water channels and oil could penetrate the fabric through the air bubbles because of the superoleophilicity in air. Both oil and water could wet and penetrate the polyester fabric, so the fabric could not separate the oil–water mixture efficiently. For the polyester/ZnO fabric, water could spread very quickly and replaced most of the air film, thus little air bubbles remained. When the oil–water mixture flowed through the fabric, water could permeate through while the oil was prevented. However, oil droplets could adhere onto the fabric where little air bubbles remained, which resulted in the polyester/ZnO fabric not remaining as clean as the original starting fabric. Due to the PEI, with a high content of amino groups, having excellent wetting performance, the scaly structure fabricated through coating PEI on the substrate of the ZnO nanorods structure could greatly amplify the surface wetting behaviors. Therefore, for the polyester/ZnO/PEI fabric, water could spread very quickly because of its superhydrophilicity and no air bubbles remained. When the water–oil mixture was poured on the fabric, a solid/water/oil interface was formed where water could penetrate the fabric very quickly while oil remained above the fabric. As a result, the polyester/ZnO/PEI fabric could be washed as clean as the original with consistent high separation efficiency for oil–water mixtures.

Fig. 11 Schematic wetting mechanism diagrams of the oil–water mixture separation process based on different fabrics: (a) for polyester fabric, air bubbles formed the fourth interface besides the solid/water/oil interface. Both oil and water could wet and penetrate the polyester fabric. (b) For the polyester/ZnO fabric, a small amount of air bubbles remained. Water and a little oil could penetrate the fabric and oil could pollute the fabric where a little air bubble existed. (c) For the polyester/ZnO/PEI fabric, no air bubbles remained and only water could penetrate the fabric. No pollution happens.

The schematic mechanism could partly explain the reason why only the polyester/ZnO/PEI fabric had the properties of anti-pollution and easy-cleaning. To further illustrate the separation mechanism, the water absorbing capacities of the fabrics (polyester, polyester/ZnO, polyester/ZnO/PEI and polyester/PEI fabrics) were further measured with an extended response time (Fig. S8†). The water absorbing capacity–time curves showed that the water absorbing capacity of polyester was almost zero due to the fact that water could not infiltrate spontaneously because of the hydrophobicity. On the contrary, the other three fabrics could absorb water with different efficiencies. The water absorbing capacity of polyester/ZnO was lower than that of the polyester/PEI fabric, which confirmed that PEI contributed to achieve the character of capturing water. However, PEI was not the only factor. The generated polyester/ZnO/PEI fabric got the highest water absorbing capacity and water absorption rate. The scaly structure built through ZnO nanorods, together with the intrinsic hydrophilic property of PEI, was essential for water-capturing as well. With the high water-capturing property, water could infiltrate the polyester/ZnO/PEI fabric completely without any air bubbles remaining for oil infiltration. The excellent water infiltration ability could prevent the oil droplets from permeating the fabric. Therefore, once water contacted the polyester/ZnO/PEI fabric, it could infiltrate the fabric in a rapid pace to ensure that the fabric was surrounded with water completely. In the above separation process, oil could not possibly touch the fabric, resulting in the fabric being easily cleaned.

Except for the excellent water wetting ability, underwater superoleophobicity was also of great necessity for oil–water separation where scaly structure played a key role to achieving the underwater superoleophobic property. For the polyester/PEI fabric, oil droplets were found in the filtered solution after the separation process, thus the fabric could not be cleaned with flowing water (Fig. S9†). Besides this, the oil CA underwater of the polyester/PEI fabric (Fig. S10†) was lower than that of the polyester/ZnO/PEI fabric. It could be seen that, when oil droplets contacted with the fabric, the entire oil droplet could adhere to the polyester/PEI fabric (Movie 10†), while ultralow amounts of oil droplets remained on the polyester/ZnO/PEI fabric (Movie 11†). When the oil droplet was placed on the fabric underwater, the oil droplet could not infiltrate the polyester/ZnO/PEI fabric surface easily. Due to the existence of the scaly structure, both surface roughness and contact areas increased, which were conducive to capturing water and rejecting oil in water. For such a solid/water/oil interface, the Cassie eqn (2) could be used to explain the high oil CA underwater:

cos θ_c = f(cos θ + 1) − 1 (2)

where θ_c and θ represent the oil CAs of an oil droplet that contacts the scaly structure of the polyester/ZnO/PEI fabric and flat polyester/PEI fabric (ideally flat, which is very difficult to achieve), respectively. f is the area fraction of the fabric in contact with oil. It could be calculated that f was 0.18, representing that most of the contact area was the solid–water interface. In fact, the polyester/PEI fabric was nearly superoleophobic underwater and the surface of the fabric was not absolutely smooth. Therefore, in fact, f could be much smaller than 0.18. All these facts above could explain why the polyester/ZnO/PEI fabric had the property of easy-cleaning and anti-pollution. All in all, superhydrophilicity, underwater superoleophobicity and scaly micro/nanoscale hierarchical structures were all equally necessary to fabricate the anti-pollution and easy-cleaning fabric for efficient oil–water separation.

Conclusions

In conclusion, a novel polyester fabric with scaly structure has been prepared through a hydrothermal and coating method. The as-prepared fabric showed superhydrophilicity, underwater superoleophobicity and excellent separation efficiency for various oil–water mixtures. Besides this, the as-prepared fabric exhibited great anti-pollution properties and could be washed simply by flowing water, leading to ultrahigh separation efficiency of fabric even after 10 cycles. In addition, the as-prepared fabric had excellent high temperature resistance and retained high separation efficiency even after immersion in 10 wt% NaCl solution for 24 hours. Importantly, the fabric inhibited the growth of both Gram-negative and Gram-positive bacteria, which prevented secondly pollution by bacteria. As a consequence, the as-prepared fabric has a promising future for various oil–water separation processes under extreme conditions due to its special environmental stability.

Acknowledgements

The authors would like to thank Harbin Application Technology Research and Development Projects (Outstanding Subject Leaders Type A) (No. 2015RAXXJ028) and Heilongjiang Province Natural Science Foundation (No. ZD201311E031403) for financial supports.

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Footnote

† Electronic supplementary information (ESI) available: SEM images of fabric after different reacting conditions; oil CAs in water for fabric reacting for different times; water CA in air of polyester fabric; water CA in air and oil CA in water of polyester/ZnO/PEI fabric after 10 separation cycles; oil content of filtered solution and the separation efficiency of polyester/ZnO/PEI fabric for each cycle; photograph of the polyester/ZnO/PEI fabric after 10 separation cycles; the wettability of different fabrics by water; photos of polyester/PEI fabric for separation process; the oil CA underwater of polyester/PEI fabric. Movies 1–2 showed spreading wetting process of polyester/ZnO fabric and polyester/ZnO/PEI fabric, respectively. Movies 3–5 showed the separation process of polyester/ZnO/PEI, polyester and polyester/ZnO fabric, respectively. Movies 6–8 showed easy-cleaning and anti-fouling test of the polyester, polyester/ZnO and polyester/ZnO/PEI fabric, respectively. Movie 9 showed that polyester/ZnO/PEI fabric could be cleaned easily even after 10 separating cycles. Movie 10 showed that an oil droplet could adhere to the polyester/PEI fabric underwater. Movie 11 showed that ultralow oil droplet could remain on polyester/ZnO/PEI fabric underwater. See DOI: 10.1039/c6ra19993c

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