Superhydrophilic and underwater superoleophobic mesh coating for efficient oil–water separation

Abstract

Metal meshes were conveniently coated with a nanoflake honeycomb network through NiOOH seeding and growth on a substrate during a chemical bath deposition process. The resultant meshes exhibited outstanding superhydrophilicity and underwater superoleophobicity, and supported high separation efficiency (>98%) and good reusability in oil–water separation.

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Oil–water separation has become a worldwide task due to increasing release of industrial oily wastewater as well as frequent oil leakage pollution.^1–3 More and more materials have been developed for oil–water separation: such as kapok fibers,⁴ carbonaceous materials,^5–7 hydrophobic aerogels,⁸ cross-linked polymer gels,^9,10 etc. These organic matrices are limited by low separation efficiency, poor recyclability, and secondary pollution. Separating tiny oil droplets from water is especially challenging for these traditional matrices, which makes further treatment necessary to achieve satisfying separation efficiency.¹¹ Therefore, membrane-based technologies are attractive for the separation of oil and water, and some superhydrophobic and superoleophilic membranes have been successfully fabricated as oil-removing separators. However, it is energy-intensive to use these membranes as oil–water separators, because the common oils float on water surface and the massive water acts as a barrier layer preventing the oil permeation through the oleophilic membrane. Energy is consumed continuously to drive oil to flow and go through the membrane pores.^12–14

Recently, a new concept taking advantage of high-surface-energy materials with water-favoring property was presented to construct underwater superoleophobic surfaces for a water-removing separator, which was inspired by the anti-wetting behavior of oil droplets on fish scales in water.^15–19 Following this strategy, superoleophobic surfaces could be easily achieved in water in a simple and fluoride-free way. Xue et al. took the first step in fabricating a hydrogel-coated mesh with underwater superoleophobicity for conceptually separating an immiscible oil–water mixture.²⁰ Later on, other polymer gels were also successfully manufactured on meshes for controlled oil–water separation.^21,22 However, such organic layers may be unstable when exposed to some harsh conditions, because polymer hydrogel tends to degrade at high temperature and is relatively weak in swelling state. Therefore, Zhang et al. designed stable inorganic films for more practical applications in oil–water separation, which is a nanowire-haired inorganic membrane with superhydrophilicity and underwater superoleophobicity.²³ This manufacturing approach is only suitable for copper substrates. Other metal oxides were also used to modify a copper mesh for the separation of oil–water emulsions.²⁴

Here, we report a novel method to fabricate highly efficient oil–water separators from metal meshes by producing a superhydrophilic and underwater superoleophobic coating with inorganic hierarchical structures. This novel water-removing separator has completely opposite wettability to traditional oil-removing separators and thus overcomes the easy fouling and hard recycling limitations in essence. It is a new attempt to use special wettability to design next-generation separators for oil–water mixtures, which supports potential applications in industrial waste water treatment and oil spill cleanup.

All chemicals were purchased from the Aldrich Co. and used without further purification. Metal meshes (300 mesh size) consisting of stainless steel or copper threads (diameter ∼ 50 μm) were purchased from local supermarkets. The metal meshes were modified by a chemical bath deposition (CBD) technique as follows: clean metal meshes with a size of 6 × 6 cm were placed vertically in the fresh CBD solution, a mixture of 1.0 M NiSO₄·6H₂O, 0.25 M K₂S₂O₈, and 25% ammonia aqueous solutions in a mass ratio of 5/4/1. The deposition process was completed in 15 min at 20 °C with mild stirring (150 rpm). Then the treated meshes were rinsed with DI water and dried at 80 °C.

The meshes were observed under a field-emission scanning electron microscope (FE-SEM, Jeol-2100, Japan). Liquid contact angles (CAs) on the mesh surface were measured with a contact angle system OCA 15 plus (Dataphysics Instrument Co., Germany) at ambient temperature. The oil droplets (1,2-dichloroethane, about 5 μL) were carefully injected into water bath and dropped onto the metal meshes at the bottom of water. The average CA value was adopted from five measurements at different positions on the same sample. To evaluate the performance of modified metal meshes as oil–water separators, the coated mesh was placed in between two glass tunnels with diameters ∼ 50 mm. The oil–water mixtures (30/70 v/v) were poured onto the mesh separator. The separation was achieved under the natural weight of the liquids. Separation efficiency was calculated by the equation: SE (%) = (1 − C₁/C₀) × 100, where C₀ and C₁ are the oil concentrations in the original mixture and the separated water, respectively. The oils in water were extracted with CCl₄ and measured with Fourier transform infrared spectrometry (FTIR, Perkin-Elmer System 2000). Based on the oil absorbance at 2960 cm⁻¹, oil contents were calculated.

The mesh surface was greatly modified by the CBD process. Fig. 1 shows the images of original and modified meshes, which demonstrate an obvious color change from lustrous grey into matt black. The CA of a water droplet on the original mesh surface is up to 144°, indicating its hydrophobic nature (Fig. 1a inset). The mesh surface modified by the CBD shows a superhydrophilic property, where a water droplet rapidly spreads into a film and gives a CA of 0°. While immersed in a water bath, the modified mesh gives a CA of 152° for an oil droplet (1,2-dichloroethane) in water, indicating an underwater superoleophobic property (Fig. 1b inset). Being different from the superoleophobicity in air which depends on low-surface-energy materials,^5,25 the underwater superoleophobicity is achieved on a high-surface-energy material, like hydrogel coatings.^20–22

Fig. 1 Photos of the original steel mesh (a) and modified steel mesh (b): the inset (a) is photograph of a water droplet (in air) on the original mesh; the insets (b) show the appearance of water (in air) and 1,2-dichloroethane (in water bath) droplets on the modified mesh surface (the CAs are 144°, 0°, and 152° respectively).

As we know, the surface wetting property is closely related to both the substrate material and its surface structure.^26–29 Here, the modified steel mesh was proved to be superhydrophilic and underwater superoleophobic, which would be contributed by a high-surface-energy material with outstanding surface roughness. Under SEM, the modified mesh presents rough surface with protruding particles, in contrast to the smooth surface of original mesh (Fig. 2a and b). Although the additional coating has made the mesh pores narrower, open pores still exist on the mesh, which ensures free passage of water through the modified mesh. At higher magnifications, the rough coating on modified mesh is revealed to be thoroughly distributing honeycomb structure constructed from interconnected nanoflakes (Fig. 2c and d). The random protruding particles (diameter 1–2 μm) are also formed by orientational nanoflakes (thickness < 50 nm). All these nanoflakes grow perpendicularly to the surface, resulting in many nano-craters (mouth size 50–300 nm) isolated by the nanoflakes as walls.

Fig. 2 SEM images of the stainless steel meshes: original mesh (a) and modified mesh in different magnifications (b–d).

How were these nanoflakes formed by the CBD? According to the reference investigation, the nanoflake honeycomb structure would result from two-step reactions: the first step is the homogeneous nucleation and precipitation of Ni(OH)₂; then Ni(OH)₂ is further oxidized by persulfate into higher oxidation state NiOOH that seeds on substrate and grows continuously.³⁰ Without the addition of persulfate, nucleation and growth on substrate surface were not observed. With the existence of persulfate, the growth of nanoflakes was also successful on a copper mesh, besides the above mentioned steel mesh and the glass slide reported in the reference. The possible chemical reactions are listed as follows:

[Ni(H₂O)_6−x(NH₃)_x]²⁺ + 2OH⁻ → Ni(OH)₂ + (6 − x)H₂O + xNH₃ (1)

2Ni(OH)₂ + S₂O₈²⁻ → 2NiOOH + 2SO₄²⁻ + 2H⁺ (2)

To clarify the contribution of surface roughness to the superhydrophilicity of NiOOH coating, we also deposited the NiOOH on a flat PMMA plate, then compressed (7.0 ton pressure under clean glass) one half of the area into a smooth surface. The two halves were tested with water droplets separately. As shown in Fig. 3a and b, the rough coating gives a CA 0° while the flattened coating gives a CA 87°. This result approved the essential role of surface roughness in obtaining superhydrophilicity.

Fig. 3 SEM images of the deposited NiOOH honeycomb structure (a) and the flattened NiOOH coating (b), which give water contact angles of 0° and 87° (the insets) respectively.

To test the oil–water separation capability of the NiOOH-coated mesh, a mixture of 1,2-dichloroethane (containing solvent red 26 at 0.1 wt%) and water (containing methylene blue dye at 0.1 wt%) was poured onto the mesh (Fig. 4a). Water quickly permeated through the modified mesh and flowed into the beaker below. Meanwhile, the oil was retained above the mesh because of its underwater superoleophobic property (Fig. 4b). The separation efficiency measurement reached a high value of 99.1%. Another three oils, hexane, petroleum ether (P.E.) and vacuum pump oil (AV46, Pro-lon), were also used in testing the same mesh and efficiently separated from water, as proved by the SE of 99.6%, 99.3% and 98.8% (Fig. 5a). These results are comparable to those reported for the hydrogel-coated mesh.²⁰ Using this water-removing device, no external force was involved during the separation process, which was driven only by the liquid weight. In addition, the modified mesh was easily cleaned by water rinse for reuse and presented stable performance in following 10 cycles (Fig. 5b). Considering its easy manufacturing, repeatable and widely applicable separation performance, this CBD-modified mesh with NiOOH nanoflake coating is a good candidate for industrial oil–water separation applications.

Fig. 4 Oil–water separation device with the modified mesh inserted between two glass tunnels: (a) pouring a mixture of water and oil into the upper glass tunnel; (b) the resultant two liquids (red clear oil staying in the upper glass tunnel, blue clear water residing in the receiving beaker).

Fig. 5 Separation efficiency for several types of oils from their mixtures with water (a) and the values for 11 cycles of repeating usage of the device for the dichloroethane/water separation (b).

Conclusions

The metal meshes constructed from steel and copper wires were successfully modified with the facile CBD method. The modified mesh surfaces exhibited outstanding superhydrophilic and underwater superoleophobic properties.

These properties were contributed from the NiOOH nanoflake honeycomb structure as observed under SEM. These uniformly coated meshes separated several oil–water mixtures effectively without consumption of external energy, and gave stable performances in following repeat cycles. These oil-fouling-resistant water-removing separators would have potential applications in the treatment of industrial oil–water wastes and the cleanup of oil spill accidents.

Acknowledgements

We gratefully acknowledge the Innovative Technology Fund ITS/237/12 from the Hong Kong SAR government and the Internal Funds 1-ZV7L, A-PL17 and A-PM04 from the Hong Kong Polytechnic University.

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