One-Step Versatile Fabrication of Superhydrophilic Filters for the Efficient Purification of Oily Water.

As industrial oily wastewater can seriously damage ecosystems, the use of filtration technology with functional filters has emerged as an effective approach for purifying oily wastewater and protecting the environment. Although several methods for preparing functional filters with specific wettability have been reported, most methods are complicated, expensive, and time-consuming. Furthermore, these methods are only applicable to specific substrates, which hinder their practical applications. Here, a simple and versatile method for the fabrication of a superhydrophilic filter on any substrate using a one-step dipping process is reported. The method is easily scaled-up to fabricate large-area superhydrophilic filters; moreover, mass production is possible using a roll-to-roll process. The resulting filter is durable, stable, and, due to its stable hydrophilic layer, shows no deterioration in wetting behavior; it also exhibits self-cleaning properties. Based on its selective wetting characteristics, oil/water mixtures and oil-in-water emulsions stabilized by surfactants can be purified in a highly efficient manner. Importantly, owing to its self-cleaning properties, the filter can be reused after simply immersing and washing in water. This easy, cost-effective, fast, and versatile method for fabricating superhydrophilic filters can be practically applied in industries that need to purify oily water.

Introduction

The amounts of wastewater from industrial manufacturing processes that contain oily pollutants as well as the number of accidental oil spills have been increasing, raising deep concerns about their enormous impact on the environment and human health.[1,2] As such, treating oily wastewater to protect the environment and to meet the stringent discharge standards of industrial effluents has become greatly significant. Several conventional techniques, such as gravity separation, coagulation, centrifugation, and flotation, have been used to purify oily water.[3] However, such commercial techniques have disadvantages, including secondary pollution, low separation efficiencies, the inability to handle emulsified wastewater, and their time-consuming nature, which hinder the efficient remediation of wastewater.[4,5] Meanwhile, micro/nanoengineered surfaces with special wetting properties such as superhydrophobic surfaces showing a high contact angle (CA) above 150° and superhydrophilic surfaces showing a low CA below 10° can improve functional properties.[6−8] Hence, these kinds of surfaces have attracted interests in various fields, for example, self-cleaning, energy harvesting, water harvesting, and bubble nucleation applications.[9−16] Particularly, techniques for the purification of oily water that use filters with super-wetting properties have attracted significant levels of interest because they do not produce secondary pollution, consume little energy, and separate oil and water efficiently.[17−22]

Generally, two types of filters (superhydrophobic and superhydrophilic filter) are used to treat oily water. Due to its selective permeability, a superhydrophobic filter can be used to purify oily water;[23−25] however, its oleophilic nature results in oil contamination and the severe deterioration of filtering performance, which hinders its effective use in the industry.[26,27] On the other hand, due to its hydration layer, a superhydrophilic filter does not directly contact with oil; hence, the filter surface does not become contaminated by oil.[28,29] Consequently, a superhydrophilic filter can be effectively used to treat oily water.[30]

Meanwhile, polymer-based filters, such as polyethylene (PE), polypropylene (PP), and polytetrafluoroethylene (PTFE) filters, and a metallic mesh, such as aluminum (Al), stainless steel (STS), and copper (Cu) mesh, are widely used as filter substrates in a variety of applications, in part because of their mechanical durability, chemical resistance, and flexibilities. However, the oil adsorption properties attributed to the hydrophobic/oleophilic nature of these substrates limit their applicability to the treatment of oily water.[31,32] Therefore, surface treatment is required to alter wetting behavior such that these types of substrates can be used to treat oily water. Even though several surface treatment approaches, such as blending, grafting, and coating, have been reported,[33−36] these methods have some disadvantages. For example, the fabrication process is complicated, time-consuming, costly, and hard to scale-up. Furthermore, previous methods are somewhat restricted in industrial settings that use a variety of substrates because they can only be applied to specific substrates.[37] To date, most research has focused mainly on filtration performance without giving much consideration to industrial applicability; consequently, these approaches are still far from being industrially useful. Therefore, the development of a simple method for the fabrication of superhydrophilic filters that can be applied to various kinds of substrates is highly desirable for practical application in the oily water purification field.

With the aim of applying the results of our oily water purification research to the industry, we now report a simple and versatile method for the fabrication of a superhydrophilic filter that uses a one-step dipping process with a mixed solution of cross-linking and oxidizing agents. Using the proposed method, we fabricated superhydrophilic filters on various kinds of substrates, such as polymeric, metallic, and even superhydrophobic surfaces. Large superhydrophilic filters are easily produced because the method is easy to scale-up, and a roll-to-roll process can be used to mass-produce the filter. We investigate the durability and wetting behavior of the fabricated superhydrophilic filter and further demonstrate the self-cleaning properties of an oil-contaminated filter. Notably, oil/water mixtures and even oil-in-water emulsions can be highly efficiently separated using the superhydrophilic filter. We expect that this simple and versatile method for the fabrication of a superhydrophilic filter with a diverse range of advantages will be practically used in a variety of industrial settings.

Results and Discussion

Fabricating the Superhydrophilic Filter

Figure a shows the one-step process for fabricating the superhydrophilic filter using a mixed solution of a cross-linker and a radical source. At 65 °C, ammonium persulfate (APS) acts as a radical initiator for the alkene moieties in N,N′-methylenebisacrylamide (BIS) to induce its radical polymerization (Figure S1).[38,39] During polymerization, hydrophilic polymer groups enwrap substrate fibers and are deposited on the target surface stably. Consequently, a hydrophilic layer is uniformly introduced onto the filter surface to produce a superhydrophilic filter, irrespective of its surface characteristics.

Figure 1

Fabrication of the superhydrophilic filter. (a) Schematic illustration of the one-step fabrication process. (b) FT-IR spectra of the filter before and after treatment. SEM images of the filter (c) before and (d) after treatment (insets: water droplet on each filter).

The substrate surface was investigated by Fourier transform infrared (FT-IR) spectroscopy before and after treatment of a commercial PE filter with a hydrophilic layer (Figure b). The spectrum of the pristine filter exhibits peaks at 1472, 2847, and 2914 cm–1 that correspond to typical PE functional groups.[40] After treatment with the developed method, new characteristic peaks at 1538 (C=O), 1652 (C=O), and 3296 cm–1 (N–H) were observed, which confirm that hydrophilic groups had been successfully deposited on the filter surface.[41] Furthermore, newly formed and increased peaks around 400 and 530 eV from X-ray photoelectron spectroscopy (XPS) spectra indicate formation of hydrophilic groups after treatment, demonstrating the successful deposition of the hydrophilic layer on the filter surface (Figure S2). This one-step treatment did not significantly change the surface structure. As a result, the roughness value (Ra) was not significantly higher after treatment; the Ra of the pristine filter was determined to be 21.1 nm while that of the treated filter was 27.3 nm (Figure S3). Hence, as shown in Figure c,d, while the wetting behavior had changed, the filter pore size was the same after treatment. These results demonstrate that a superhydrophilic filter was effectively produced by the one-step method developed herein.

Wetting Characteristics

The simple fabrication method can be applied to any substrate, regardless of its surface characteristics. To demonstrate this, we subjected polymeric, metallic, and superhydrophobic substrates to the newly developed method and measured water CAs on the various filters (Figure ). Hydrophobic polymer-based filters composed of PE, PP (nominal pore size: 10 μm), PP (nominal pore size: 0.1 μm), and PTFE, with CAs of 124.5, 120.5, 122, and 134.1°, respectively, were all transformed into superhydrophilic filters by the treatment method (Figure a–d). This superhydrophilicity is attributable to the hydrophilic layer formed by the cross-linker, along with the microscale fiber structures of the polymeric substrates. Furthermore, a metallic mesh composed of stainless steel, aluminum, or copper was also transformed into a superhydrophilic filter upon treatment; the CAs of the mesh before treatment were 124.7, 122.2, or 130.5° (Figure e–g, respectively). These wettability changes are also attributable to microscale wire structures and hydrophilic layers. Amazingly, a superhydrophobic substrate could also be coated using this method; a superhydrophobic metallic mesh with a CA of 159.9° was easily converted into a superhydrophilic filter with a CA of 0° upon treatment (Figure h). These results show that any type of substrate can readily be used to fabricate a superhydrophilic filter by simple treatment. Moreover, the developed one-step method is easy and simple to use; a large superhydrophilic filter with surface dimensions of about 400 mm × 1000 mm was readily fabricated from a hydrophobic stainless steel mesh (Figure i,j). Roll-to-roll manufacturing is a well-known inexpensive and novel mass production technique, which can be adopted to fabricate superhydrophilic filters because the fabrication step involves a simple one-step dipping process.[42,43] With this in mind, we expect that superhydrophilic filters can be mass-produced by roll-to-roll manufacturing, as shown in Figure S4. We believe that the proposed method can be usefully applied to industries that require superhydrophilic filters to be mass-produced in a simple process using any kind of substrate with a large surface area.

Figure 2

Modifying the wettability of various substrates. (a–h) Water droplet images on various substrates before and after treatment. (i, j) Modifying the wettability of a large-area substrate.

As is well known, the hydration layer associated with superhydrophilicity prevents oil droplets from adhering to the surface, which engenders the filter with underwater superoleophobicity;[22,44,45] hence, this selective wettability enables the filter to separate oil from water. Before examining the oil/water separation performance of treated filters, we investigated their wetting characteristics (Figure ). Due to its excellent water wettability, a water droplet was completely spread over the filter surface within 3.7 s (Figure a). On the other hand, an oil droplet hardly adhered to the filter in water; the underwater oil CA was determined to be 157.9° (Figure b). Water trapped at the filter surface, which is highly repulsive to oil, is responsible for the underwater anti-oil properties of the filter.[46] Therefore, an oil droplet forcibly adhered to the filter is easily detached from the filter surface and leaves no trace (Figure c).

Figure 3

Wetting characteristics of the superhydrophilic filter. (a) Images of water droplets on the superhydrophilic filter. Underwater oil repellency properties of the superhydrophilic filter under (b) static and (c) dynamic conditions. Wetting characteristics of the superhydrophilic filter after (d) ultrasonication and (e) abrasion. (f) Wetting characteristics of the superhydrophilic filter in solutions of varying pH.

Strong durability and stability are crucial factors for practical filter applications. To evaluate these, we investigated changes in wettability after several mechanical or chemical tests. First, the filter was impacted by a strong jet of water (∼100 kPa) for 60 s, which did not affect the superhydrophilicity of the filter; after testing, the filter exhibited the same area of spread when a 15 μL water droplet contacted its surface (Figure S5). Moreover, the hydrophilic layer remained firmly attached to the filter, with wettability retained even after ultrasonication for 300 min; the superhydrophilicity and underwater superoleophobicity of the filter was preserved, as evidenced by a water CA of 0° and an underwater oil CA of 159.8° (Figure d). Furthermore, the superhydrophilicity and underwater superoleophobicity of the filter were maintained even after abrasion testing; after abrading for 1500 mm, the filter exhibited a water CA of 0° and an underwater oil CA of 159.3° (Figure e). These mechanical testing results highlight the excellent durability of the filter, which is attributable to strong and stable bonding between the hydrophilic layer and the filter. Moreover, Figure f reveals that superoleophobicity was maintained when the filter was immersed in solutions of various acidic and alkaline water (pH 3–9); the water CA at each pH was 0°, and the oil CA in an acidic solution (pH 3) was 153.4°, while it was 158.0° in a mild alkaline solution (pH 9). Although the water CA for a strong alkaline solution (pH 11) was 0°, the oil CA could not be measured because the oil droplets formed a stable emulsion at pH 11. This observation is explainable by the high affinity of oil molecules for strong alkaline solutions.[47,48] Despite not being able to demonstrate chemical stability in a strong alkaline solution, the chemical durability of the filter under strong acidic and mild alkaline conditions was demonstrated. The mechanical durability and chemical stability results indicate that the filter can be used in harsh environments.

Along with its robust superhydrophilicity and underwater superoleophobicity, the fabricated superhydrophilic filter has self-cleaning properties, which endow the filter with resiliency against oil contamination. When pre-wetted with water, the superhydrophilic filter is unlikely to be contaminated by oil due to the hydration layer that prevents the surface from contacting the oil.[49,50] However, the superhydrophilic filter can be wetted by oil in the absence of a hydration layer on the filter. Nevertheless, the filter will self-clean when simply immersed in water. Figure shows the underwater self-cleaning ability of the superhydrophilic filter. The pristine filter, which is composed of hydrophobic PE fibers, is easily wetted by oil. When this oil-contaminated filter was immersed in water, the oil remains attached to the surface due to the oleophilicity of the filter; therefore, red oil is clearly observed on the filter surface (Figure a). The superhydrophilic filter was also wetted by oil in the absence of a hydration layer; however, when this oil-contaminated filter was immersed in water, the oil clearly became detached from its surface. No oil remained on the filter surface after the contaminated filter had been immersed in water for 10 s to afford a clean filter (Figure b). This self-cleaning ability of the superhydrophilic filter is attributable to strong interactions, such as hydrogen-bonding interactions between the hydrophilic groups of the filter surface and water.[51] Because water is more attracted to the surface than oil, the area wetted by oil becomes gradually wetted by water.[52] Accordingly, widespread oil is accumulated, and the accumulated oil droplets are subsequently detached from the surface, resulting in a clean surface. The mechanical durability, chemical stability, and self-cleaning properties of the fabricated superhydrophilic filter make it widely practically applicable to industrial settings.

Figure 4

Self-cleaning tests: (a) pristine filter and (b) superhydrophilic filter.

Oil/Water Separation

The selective wettability of the superhydrophilic filter provides selective permeability that enables clean water to be separated from an oil/water mixture. Figure a and Figure b show a mechanism and schematic of the oil/water separation process, respectively. Due to the affinity of water for the filter, water immediately spreads to form a hydration layer after which it passes through the filter. On the other hand, the highly oil-repulsive nature of the filter, which is attributable to the hydration layer, leads to a high underwater oil CA.[53] The pressure required to pass oil through an underwater filter with a superhydrophilic surface can be expressed by the Young–Laplace equationwhere γ is the surface tension, θo is the underwater oil CA, and rp is the pore radius.[54] Apparently, ΔP is a positive value because θo is larger than 90°, which indicates that additional pressure is required to pass oil through the filter (Figure a).[55] Therefore, only water in the mixture can pass through the filter under ambient conditions, while oil is rejected by the filter, which results in the production of clean water (Figure b). Figure c shows that oil and water were successfully separated using the prepared superhydrophilic filter, driven solely by gravity. To evaluate the oil/water separation performance of the filter, we examined separation efficiency and flux using various oil/water mixtures (Figure d). All types of oils were rejected by the filter during separation, while clean water passes through the filter. Separation efficiencies of 99.1, 99.0, 98.8, and 99.1% were observed for diesel, hexane, xylene, and benzene, respectively, with corresponding fluxes of 2898, 2855, 2718, and 2972 L m–2 h–1, respectively. These high separation efficiencies, regardless of oil type, are attributable to the superhydrophilic nature of the filter. The hydration layer on the filter surface is formed in a variety of oil/water mixtures, which engenders the filter with selective permeability and the ability to provide clean water from these mixtures. Furthermore, the affinity of water for the porous filter provides high fluxes, which highlights the suitability of the filter for oil/water separation applications. In addition, the treated superhydrophilic filter has self-cleaning properties; hence, any oil remaining on the filter is simply removed by immersion in water. Therefore, the filter can be reused to separate oil from water while maintaining high separation efficiencies and fluxes, even after several cycles. Using diesel as a sample oil, we separated a diesel/water mixture 10 times with one filter to evaluate its recyclability. The filter was simply washed between cycles by dipping it in water for 30 s. As shown in Figure e, the separation efficiency (99.2%) and flux (2902 L m–2 h–1) remained high even after 10 separation cycles. Furthermore, the filtrate obtained from each of the 10 separation cycles had a low total organic carbon (TOC) content (below 5 ppm), which highlights the outstanding oil/water separation performance and recyclability of the filter (Figure f). The filter is highly durable due to the stable hydrophilic layer, as shown in Figure . Therefore, various mixtures, including hot water/oil, HCl/oil, and NaCl/oil, were successfully separated with high filtration fluxes (3341, 3129, and 2021 L m–2 h–1, respectively) using the filter, as shown in Figure S6. Considering that our filter, which is fabricated in one simple step that is easily scaled-up, exhibits durability and excellent oil separation performance, we believe that this filter can be practically used in a variety of oily water treatment applications.

Figure 5

Oil/water separation performance using the superhydrophilic filter. (a) Mechanism and (b) schematic illustration of the oil/water separation process. (c) Photographic images showing the separation of an oil/water mixture using the superhydrophilic filter. (d) Separation efficiencies and filtration fluxes of various oil/water mixtures. (e) Oil/water separation efficiency and filtration flux as functions of the filter reuse cycle. (f) Oil content of the filtrate as a function of the filter reuse cycle.

Emulsion Separation

Tiny oil droplets form stable emulsions with surfactants in industrial wastewater, which are hard to separate using conventional techniques because extremely small droplets easily penetrate filters.[56,57] These tiny oil droplets also need to be separated from oily water to protect the environment and to meet environmental protection regulations.[58] A superhydrophilic filter with pores smaller than an oil droplet needs be prepared to purify such a stable emulsion. However, fabricating a superhydrophilic filter to separate emulsions is difficult because modifying such small filter pores is challenging, and consequently, wettability is difficult to be controlled. Here, we developed a one-step method for modifying the wettability of any kind of substrate. Even a polymeric filter with extremely small pores is simply modified to become superhydrophilic, as demonstrated above (Figure ). Using a superhydrophilic PP filter (nominal pore size: 0.1 μm) as an emulsion separation filter, we evaluated the emulsion separation performance under vacuum filtration conditions; Figure shows its stabilized feed emulsion and filtrate. Clean water, without any visible oil droplets, was obtained after filtering the milky emulsion. The size distribution of the oil droplets was further analyzed by dynamic light scattering (DLS) measurements.

Figure 6

Emulsion separation performance. (a) Optical and microscopic images of a surfactant-stabilized oil-in-water emulsion. (b) Particle size distribution of the surfactant-stabilized oil-in-water emulsion. (c) Optical and microscopic images of the filtrate. (d) Particle size distribution of the filtrate. Scale bars: 100 μm.

Compared to the feed emulsion (average droplet size: 157.98 nm), only tiny droplets about 10 nm in size are present in the filtrate (average droplet size: 10.43 nm); the filtering mechanism is schematically illustrated in Figure S7. Although these tiny oil droplets cannot aggregate in the presence of a surfactant to form an emulsion, oil droplets unfiltered by the porous structures can accumulate on the filter surface to form a filter cake; such a cake prevents oil droplets that are even smaller than the filter pores from passing through the filter, thereby providing high emulsion separation performance. However, filtration flux is lowered by the filter cake because the smaller effective pores affect the flow of the filtrate.[32,57,59] Nevertheless, the filter can be used in industries that require emulsified wastewater to be purified due to its high separation efficiency (99.7%) and relatively high flux (149 L m–2 h–1). Considering that the treated filter is able to self-clean in water, the filter can repeatedly be used to treat emulsions, with cleaning performed by briefly dipping in water between cycles. As expected, the filter can be reused to separate emulsions without any deterioration of separation efficiency and flux; high separation efficiency (99.7%) and flux (147 L m–2 h–1) were maintained after 10 separation cycles (Figure ).

Figure 7

Surfactant-stabilized emulsion separation efficiency and filtration flux as functions of the filter reuse cycle.

It should be noted that the main purpose of this work was the development of a novel method for fabricating superhydrophilic filters on various substrates and that the separation performance (i.e., separation efficiency and filtration flux) can be improved by adjusting the pore size. In addition, the filter can be easily scaled-up due to the simplicity of the fabrication process, enabling the treatment of large amounts of emulsified wastewater. These advantages reveal that the filter has great potential for practical use in a variety of industries that need to purify emulsified wastewater on a large scale.

Conclusions

We successfully fabricated a superhydrophilic filter using a one-step dipping process with a mixed cross-linker (BIS) and oxidizer (APS) solution. The method can be applied to various substrates such that hydrophobic polymeric substrates, hydrophobic metallic substrates, and even a superhydrophobic substrate can be used to prepare superhydrophilic filters. Moreover, we demonstrated the easy fabrication of a large superhydrophilic filter using this method and proposed a roll-to-roll process to mass-produce the filter. The treated filter is highly mechanically durable and chemically stable and maintains its wettability due to the hydrophilic layer that was stably introduced onto the substrate. Furthermore, the filter has self-cleaning abilities that are attributable to the stable hydrophilic layer. Importantly, the filter can be used to efficiently purify oily wastewater (i.e., an oil/water mixture and an oil-in-water emulsion stabilized with a surfactant). Notably, we confirmed that the filter can be repeatedly used to purify oily water after cleaning the used filter in water. We believe that this simple and mass-producible method, which is applicable to any large-area substrate, is promising for a variety of industries that need to purify industrial wastewater.

Experimental Section

Materials

PE and PP membrane filters (nominal pore size: 10 μm) were obtained from Pall Life Science (USA). PP (nominal pore size: 0.1 μm) and PTFE membrane (nominal pore size: 1 μm) filters were supplied by GVS Filter Technology (USA) and iNexus Inc. (Korea), respectively. An metallic mesh (aluminum, stainless steel, and copper) was supplied by TWP Inc. (USA). APS, n-hexane, and ethanol were obtained from Samchun Chemical (Korea). BIS, oil red O, octadecyltrichlorosilane (OTS), and sodium dodecyl sulfate (SDS) were purchased from Sigma-Aldrich. Diesel was supplied by SK Energy (Korea), while benzene and p-xylene were obtained from Junsei (Japan).

Fabricating the Superhydrophilic Filter

Substrates were pre-wetted with ethanol and immersed in a 65 °C coating solution (30 mM BIS and 45 mM APS) for 1 h. The treated filter was washed three times with deionized water and then dried under ambient conditions.

Oil/Water Separation Testing

Diesel was used as a general oil for oil/water separation tests. The prepared superhydrophilic filter was immersed in water for 10 s prior to testing to form a hydration layer on the filter surface. The filter was then fixed between a glass flask and cylinder of a filtration apparatus (Deschem, China), and the oil/water mixture or emulsion was poured onto the filter. The oil/water mixture was prepared by mixing water and diesel (1:1, v/v). The filtration flux (L m–2 h–1) and separation efficiency (SEm) of the mixture were calculated using the following equationwhere V (L) is the volume of the filtrate, A (m2) is the effective area of the filter, Δt (h) is the separation time, m0 is the initial water mass, and m1 is the collected water mass. The emulsion was prepared by adding a surfactant (SDS, 2 g L–1) to an oil-in-water (1:99, v/v) mixture followed by ultrasonication (5510E-DTH, BRANSON, USA) for 60 min. The separation efficiency of the emulsion (SEe) was calculated using the following equationwhere C0 is the measured oil concentration in the feed and C1 is the measured oil concentration in the filtrate. The used filter was cleaned between separation cycles by immersion in water for 30 s after which the filter was reused in the next testing cycle.

Characterization

For characterizations, samples were prepared by cutting the filter to 10 × 10 mm in size. Surface morphologies were examined, and chemical compositions were determined by field-emission scanning electron microscopy (SEM; SU6600, Hitachi, Japan) and Fourier transform infrared (FT-IR; Nicolet iS50, Thermo Fisher Scientific Co., USA) spectroscopy. SEM and FT-IR measurements were conducted at room temperature. Surface roughness values were obtained by atomic force microscopy (AFM; VEECO Dimension 3100, VEECO, USA). A contact angle analysis device (SmartDrop, Femtofab Co., Korea) was used to determine water CAs and underwater oil CAs. The CA measurement testing was performed at room temperature. The listed water and oil CAs were averages of values measured at five points. Deionized water and diesel droplets (5 μL each) were used to characterize the wetting properties of the filter. To create a superhydrophobic mesh, an aluminum mesh was treated according to a previously reported method for forming hierarchical structures,[60,61] and the treated mesh was coated with OTS to endow it with superhydrophobicity. To evaluate chemical resistance, oil CAs were measured while the filter was immersed in various pH buffer solutions (Samchun Chemical, Korea). To investigate self-cleaning properties, the filter was dipped in red-dyed oil for 10 s after which it was immersed in water. All experiments were conducted using a PE membrane filter (pore size: 10 μm) unless otherwise stated. Oil droplet sizes and oil concentration were determined using a zeta potential–particle size analyzer (ELSZ-2000, Otsuka, Japan) and by optical microscopy (OM; Olympus MX51, Olympus, Japan). The oil content was determined using a total organic carbon analyzer (TOC-L, Shimadzu, Japan).