Oil–water separation has recently become a worldwide challenge due to the frequent occurrence of oil spill accidents and increasing industrial oily wastewater. In this work, the multifunctional mesh films with underwater oleophobicity and certain bacteriostatic effects are prepared by layer-by-layer assembly of graphene oxide-silica coatings on stainless steel mesh. The mesh film exhibits excellent environmental stability under a series of harsh conditions. The new, facile and reusable separation system is proposed to achieve deep treatment of oily wastewater, and the oil collection rate can reach over 99%.

INTRODUCTION

Oil–water separation is an urgent environmental issue because of the increasing industrial oily wastewater, as well as the frequent oil spill accidents (Hu et al. 2014). Hence, materials with special wettability have caused broad attention in recent years, especially the novel interfacial materials with superhydrophobicity and superoleophobicity used as oil filtration or absorbent membrane (Teng et al. 2014), such as polyurethane foam (Zhang et al. 2013b), reduced graphene oxide (GO) foams (Zhang & Seegar 2011), metal/metal oxide nanocrystals with thiol modification coating fabric (Wang et al. 2013), and Polytetra fluoroethylene coating mesh (Feng et al. 2004). Although these methods can obtain a high rate of oil–water separation, those superoleophilic materials are plugged by the adhered oil and easily fouled, which limits their practical applications (Teng et al. 2014).

Inspired by the wetting behavior of fish scales, which present underwater superoleophobicity due to water-phase micro–nanohierarchical structures (Liu et al. 2009, 2015), artificial hydrophilic and underwater superoleophobic surfaces have been prepared in recent years. Zhang et al. (2013a) reported a self-cleaning underwater superoleophobic mesh that can be used for oil–water separation is prepared by the layer-by-layer (LbL) assembly of sodium silicate and titanium dioxide (TiO2) nanoparticles on the stainless steel mesh. Xue et al. (2011) fabricated a superhydrophilic and underwater superoleophobic polyacrylamide hydrogel-coated mesh at a three-phase oil–water–solid interface, which could selectively separate water from oil–water mixtures with a high separation efficiency and resistance to oil fouling.

Very recently, GO has attracted tremendous research interest in designing super-wetting materials for effective separation of various oil–water mixtures because it has many hydrophilic functional groups such as carboxyl and hydroxyl, and further chemical modification can change these functional groups and get different interface properties (Dong et al. 2013). Kou & Gao (2011) synthesized graphene oxide--silicon dioxide (GO–SiO2) nanohybrids in a water–alcohol mixture at room temperature and can be directly applied as a general kind of building blocks. In fact, oily wastewater often contains a high concentration of salts, different kinds of acid and alkali, even microorganisms such as Escherichia coli (E. coli). Although several oil–water separation materials stay stable in harsh conditions, to the best of our knowledge, there has been no report about multifunctional mesh films with a certain antibacterial effect and high stability.

In the current work, multifunctional mesh films with underwater oleophobicity and certain bacteriostatic effects were fabricated via LbL assembly of GO–SiO2 nanohybrids on stainless steel mesh. The property of the multifunctional mesh films on oil–water separation and antibacterial activity was investigated. The steel mesh was chosen as the base material because of its inherent porous structure and good mechanical and chemical stability, as well as its easy availability and low cost (Lu et al. 2014). GO–SiO2 were chosen as the coating due to their hierarchical micro–nano structures, hydrophilic nature, excellent dispersity and the ability to form thin film via solution-casting.

MATERIAL AND METHODS

Synthesis of GO–SiO2 nanohybrids

GO was derived from natural graphite flakes through the modified Hummers method (Hummers & Offeman 1958). Silica nanoparticles were deposited on GO by in situ hydrolysis of tetraethoxysilane (TEOS) (Kou & Gao 2011). In a typical procedure, GO powder was dispersed in alcohol–water (5:1, v/v) solution by sonication. After that, the pH was adjusted to 9.0 with ammonia solution and then TEOS was added to the solution. After being vigorously sonicated, the mixture was kept for 24 h at room temperature. Finally, the GO–SiO2 suspension was centrifuged and washed with alcohol and the resulting product was stored in alcohol.

LbL assembly of GO–SiO2 coatings on stainless steel mesh

A pre-cleaned stainless steel mesh was etched with copper(II) chloride (CuCl2) (0.2 mol/L) solution (Lu et al. 2014). In each cycle, the cleaned mesh was immersed in poly(dialkyl-dimethyl- ammonium chloride) (PDDA, Mw = 200,000–350,000, 20 wt %) solution (2.0 mg/mL) for 10 min to render surface positively charged, followed by rinsing with water and drying with nitrogen gas (N2) flow. Then the mesh were immersed in GO–SiO2 suspension for 10 min, followed by rinsing with water. Finally, the as-prepared coatings were dried under vacuum at 60 °C overnight. In the process of the experiment, pore sizes of the stainless steel mesh are 50, 80, 100, 150, and 200 meshes with different coating cycles of 10, 15, 20, 25, and 30, respectively. Eventually, the 100 mesh size with 20 coating cycles was selected for further experiment by comparing the contrast oil–water separation effect and membrane flux (data not shown).

Characterization of mesh film

Freshly fabricated coatings were examined by scanning electron microscope–energy dispersive X-ray spectrometer (SEM-EDS, S-4800, Hitachi, Japan). Water contact angle (CA) and oil CA of the meshes were measured at ambient temperature on a CA–interface system (DSA25, KRÜSS, Germany), where 4 μL liquid volume was used for proper observation if not otherwise indicated.

Oil–water separation test

The mesh film was pre-wetted by water and fixed between two filter funnels as the separation membrane. A mixture of water and oil was poured slowly into the filtering system and the permeated liquid was collected in a beaker. The oil phase was dyed with Sudan III for easy observation. No external pressure was applied to drive the liquid through the filter. Different mass compositions of oil–water mixtures were chosen to measure its efficiency for oil removal (Lu et al. 2014). The oil removal rate (R) was calculated as:
formula
1
where M1 and M2 are the mass of the oil before and after separation, respectively.

Antibacterial test

The antibacterial activities of mesh films loaded with GO–SiO2 were tested by an inhibition zone method (Ravindra et al. 2010). In this method, E. coli was taken as the model bacteria. For this study, the films were cut into small pieces, which had three samples each. The plates were examined for possible clear zone formation after incubation at 37 °C for 16 to 18 h. The presence of a clear zone around films on the plates was recorded as an inhibition against E. coli.

Stability and recyclability experiments

The stability of as-fabricated films was investigated by immersing the film into corrosive solution of 1 M sodium hydroxide (NaOH) and 1 M hydrogen chloride (HCl) for 48 h separately, and 1 M sodium chloride (NaCl) for 1 month. After that, the CAs of the immersed mesh film were measured. Recyclability was studied by the parameter that oil–water separation rate of different oil (soybean oil and diesel) after films being used for 5 or 10 cycles.

RESULTS AND DISCUSSION

Characterization of mesh film

The surface morphology of GO–SiO2 coated mesh is elucidated by SEM observations, shown in Figure 1. Figure 1(a) shows the image of the bare stainless steel mesh with an average pore diameter of 100 meshes, and the inset is the magnified image of the meshes. The SEM image of coated mesh films shows that the original mesh has been completely covered by GO–SiO2 (Figure 1(b)), which were dispersed uniformly on the stainless steel wires. The magnified view shows the micro–nanoscale hierarchical rough surface of the prepared mesh films (Figure 1(c)).
Figure 1

Characteristic images of stainless steel mesh. (a) Scanning electron microscopy (SEM) images of bare stainless steel mesh. (b) SEM images of pre-prepared stainless steel mesh. (c) SEM images of magnified view of the GO–SiO2.

Figure 1

Characteristic images of stainless steel mesh. (a) Scanning electron microscopy (SEM) images of bare stainless steel mesh. (b) SEM images of pre-prepared stainless steel mesh. (c) SEM images of magnified view of the GO–SiO2.

Wettability behavior of mesh film

The subsequent wettability measurement of the as-prepared mesh film shows that the surface shares the special wettability behavior. The surface of mesh film is superhydrophilic (CA = 4 ± 1.0°) and oleophilic (CA = 5 ± 1.0°) in air whereas it possesses underwater oleophobicity (2 μL, chloroform, CA = 143.5 ± 1.0° in water) and low oil adhesion force in pure water. Superhydrophilicity in air and underwater oleophobicity can be described on the basis of Wenzel (Wenzel 1936) and Cassie's theory (Cassie & Baxter 1944). According to the Wenzel equation:
formula
2
where r is roughness and refers to the ratio of the actual area of the solid–liquid interface contact and the projection area (r ≥ 1); θw and θ represent apparent CA and intrinsic CA, respectively. The existing research has proved that increasing the roughness of surface increases the surface wettability. Figure 2(a1) illustrates that the bare stainless steel mesh in air is a hydrophobic surface (CA ≈ 103°). Figure 2(a3) and 2(b3) show that CAs of water and oil droplets on films modified by chemical etching and coating with GO–SiO2 are both below 5°, indicating that the modified mesh films are amphipathic (superhydrophilicity and superoleophilicity) in air due to their hierarchical micro–nanostructures and coating's hydrophilic nature.
Figure 2

(a1) Photograph of water droplets on the surface of the uncoated mesh in air; (a2), (a3) the process of water droplet spread on the coated mesh surface, respectively; (b1) shapes of an oil droplet (dyed with Sudan III) staying on the coated mesh underwater; and (b2), (b3) photograph of an oil droplet with a contact angle of nearly zero in air, respectively.

Figure 2

(a1) Photograph of water droplets on the surface of the uncoated mesh in air; (a2), (a3) the process of water droplet spread on the coated mesh surface, respectively; (b1) shapes of an oil droplet (dyed with Sudan III) staying on the coated mesh underwater; and (b2), (b3) photograph of an oil droplet with a contact angle of nearly zero in air, respectively.

Underwater oleophobicity of modified mesh films can be explained with Cassie's theory (Cassie & Baxter 1944). The contact object was translated to oil droplets and modified mesh films underwater compared to the original Cassie's theory. According to the Cassie equation:
formula
3
where θ is CA of surface with structure and θ1 is CA of smooth surface, f1 is ratio of the area of the solid–liquid interface to the total projection area, f2 is the ratio of the area of the solid–gas interface to the total projection area, and ƒ1 + ƒ2 = 1. The Cassie model tells us the apparent contact angle will increase with decreasing f factor. The roughened surface makes f decrease. Thus, the surfaces with GO–SiO2 coatings that were prepared have high contact angle. The CA of the oil droplet on the mesh increases from 25° (on the bare stainless steel mesh underwater) to 143.5° (on the coated mesh films) (Figure 2(b1) and Figure 3), which proved the underwater oleophobicity.
Figure 3

(a) Oil droplets on the surface of a untreated bare stainless steel mesh and (b) mild steel mesh etched by CuCl2 solution and treated by modifying and coating.

Figure 3

(a) Oil droplets on the surface of a untreated bare stainless steel mesh and (b) mild steel mesh etched by CuCl2 solution and treated by modifying and coating.

Oil–water separation

There are three categories of oil–water separation systems using mesh or membrane such as tube-membrane–mesh-container (Li et al. 2012; Kwon et al. 2012), mesh–oil container–water container (Pan et al. 2008) and membrane (or mesh) sealed vessel–water container (Wang et al. 2009; Crick et al. 2013). The separation of oil and water was carried out by using the setup as illustrated in Figure 4(a). To further study the separation efficiency, the mass of the oil before and after the separation was measured and made a mathematical calculation according to the Equation (1). The oil collection rate can reach >99% (Supplementary Table S1, available with the online version of this paper), which reveals that the mixtures of water and oil could be separated effectively.
Figure 4

(a) Oil–water separation through the coated mesh. (b) Photograph showing collected water and oil after the separation.

Figure 4

(a) Oil–water separation through the coated mesh. (b) Photograph showing collected water and oil after the separation.

Stability and recyclability

Figure 5(a) shows the oil–water separation rate of different oils (soybean oil and diesel) after films were used for 5 or 10 cycles. Separation rates can remain >99% when films were used for 5 cycles, and there is a modest decline for 10 cycles, which proved that the films have good recyclability. As shown in Figure 5(b), there is little change on the contact angle of the oil droplet compared with the freshly prepared films, proving that the mesh films have good stability in harsh conditions.
Figure 5

(a) Separation rates of different oils after being used for different cycles. (b) Contact angles of oil droplets (chloroform) on the coated mesh in different corrosive solutions.

Figure 5

(a) Separation rates of different oils after being used for different cycles. (b) Contact angles of oil droplets (chloroform) on the coated mesh in different corrosive solutions.

Antibacterial activity

Figure 6(a) and 6(b) show the antibacterial effect of mesh film before and after GO–SiO2 coating, respectively. The sample in which E. coli was cultured with the bare stainless steel mesh has almost no inhibition zone (Figure 6(a)). In Figure 6(b), a clearly defined bacterial free zone around each sample can be observed, which confirms the growth inhibition effect induced by the coated mesh film. Antibacterial activity of the mesh film is mainly caused by direct cell contact with GO–SiO2 by inducing membrane damage, formation of reactive oxygen species, mediated by physical disruption, and extraction of lipid from the cell membrane (Perreault et al. 2014). Bacteriostatic effects of coated films not only avoid the membrane fouling effectively but also achieve deep treatment for oily wastewater.
Figure 6

(a) Escherichia coli (E. coli) were cultured with the bare stainless steel mesh. (b) Bacteriostatic rings of coated stainless steel mesh.

Figure 6

(a) Escherichia coli (E. coli) were cultured with the bare stainless steel mesh. (b) Bacteriostatic rings of coated stainless steel mesh.

CONCLUSIONS

In summary, multi-functional mesh films with oleophobicity underwater and certain bacteriostatic effects have been fabricated via LbL assembly method. The mesh films can separate various oils (soybean oil, diesel, chloroform) from water efficiently, effectively avoiding or reducing the possibility of membrane fouling and clogging. The as-fabricated mesh is highly stable in the harsh conditions, such as highly acidic, alkali, and salt conditions. The oleophobicity underwater and certain bacteriostatic films may open a new avenue for applications in industry and everyday life, for example, deep treatment of waste oil, oil fences for oil spill accidents, etc.

ACKNOWLEDGEMENTS

This research has been supported by Jiangsu Province Research Joint Innovation Fund-Prospective Joint Research Project (BY2014123-08), Scientific Research Innovation Project of Jiangsu University (KYXX-0028), the Scientific Research Foundation for Talented Scholars of Jiangsu University (No. 10JDG039), and the Open Fund of State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences (No. Y052010043).

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Supplementary data