Fabrication of a silica gel coated quartz fiber mesh for oil–water separation under strong acidic and concentrated salt conditions

Abstract

A superhydrophilic and underwater superoleophobic mesh is prepared by compounding a quartz fiber with silica gel, and is further enhanced by adding 1,2-bis(triethoxysilyl)ethane and a high molecular weight polyacrylamide. Driven by gravity solely, the as-prepared mesh can separate oil–water mixtures with high efficiency in strong acidic and high salt conditions.

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Oil–water separation is becoming an important research direction related to the treatment of industrial oily wastewater and oil-spill pollution, in which the preparation of highly efficient and stable oil–water separation materials has always been the aim for scientists.^1–7 So far, oleophilic materials, so called “oil-absorbing” or “oil-removing” type of materials, have been widely used by introducing superhydrophobic and superoleophilic properties. These materials, such as polymer modified metal substrates,^8,9 carbon nanotubes,^10,11 polymer nanotubes,¹² silicone nanofilament coated textiles,¹³ poly(lactic acid),¹⁴ spongy graphene,¹⁵ functional polyurethane sponges,^16,17 and core–shell oil-absorbing particles,^18–20 can selectively separate oil from water. However, these materials can be easily contaminated by oil, which will decrease the separation efficiency and the life span, and inevitably introduce extra costs at the post-treatment of these polluted materials.²¹

In 2009, Jiang et al. first found that fish scales showed a wetting behavior of superoleophobicity in water as well as superhydrophilicity in air, which endowed the fish scales with a tolerance to adhesion or fouling by oil underwater.²² Inspired by this phenomenon, we presented a novel strategy to realize anti-contamination oil–water separation by introducing superhydrophilic and underwater superoleophobic wettability on a polyacrylamide (PAM) hydrogel coated mesh.²³ Water permeated the mesh driven by gravity directly, while oil was obstructed by a water barrier forming at the surface of the mesh. This mesh had a high separation efficiency without being blocked by oil. Based on “underwater superoleophobicity”, “water-removing” materials and a series of switchable wettability materials have been fabricated for oil–water separation in recent years.^24–28 However, as far as we are concerned, there was no mesh that could separate an oil–water mixture in a strong acidic environment or condition characterized by a high salt concentration, which is important in industrial oily wastewater treatment and petroleum refining.

In this work, a superhydrophilic and underwater superoleophobic mesh is prepared by compounding a quartz fiber with silica gel, and is further enhanced by adding 1,2-bis(triethoxysilyl)ethane (BTSE) and a high molecular weight PAM. The silica gel does not react with any acid or salt except hydrofluoric acid. Therefore, it is hydrophilic like a normal hydrogel but its water retaining capacity will not be influenced by an acid and salt. As a result, the as-prepared mesh can filter water from an oil–water mixture selectively with a high separation efficiency even in a strong acidic and high salt environment for a long life cycle. This work has potential as an industrial application for water treatment under harsh conditions.

The scanning electron microscopy (SEM) images of the quartz fiber and the substrate coated with silica gel are shown in Fig. 1a and b, respectively. The quartz fiber is chosen as the substrate since it is inert to acid and salt as well as silica gel. Fig. 1a is a typical image of a twilled quartz fiber textile which is knitted from bundles of quartz fibers with an average diameter of approximately 8 μm. The twill with a smooth surface is flexible. Originally, the silica gel was anticipated to coat the substrate without any further treatment. However, the mixed pre-gel solution is not thick enough, resulting in an uneven distribution of silica gel onto the quartz fiber. Besides, the weak interaction between the silica gel and the quartz fiber, which is attributed to physical adsorption, enables the silica gel to be easily stripped off the quartz fiber. Accordingly, the high molecular weight PAM and BTSE were added to solve these problems by both physical and chemical enhancement. The high molecular weight PAM was used as an adhesive agent to improve the viscosity of the mixed pre-gel solution; on the other hand, BTSE, a silane coupling agent with double triethoxysilyl groups, was used to improve the composite degree between the silica gel and the quartz fiber by forming covalent bonds.^29,30 As a result, the silica gel coated quartz fiber mesh was successfully fabricated with a wholly rough surface, which was evenly and firmly occupied by silica gel (see Fig. 1b).

Fig. 1 SEM images of the as-prepared mesh. (a) Original twilled quartz fiber as a substrate without treatment; the inset shows a low magnification SEM image with a scale bar of 180 μm. (b) The as-prepared mesh coated with silica gel, enhanced by adding BTSE and a high molecular weight PAM, shows a rough hierarchical structure; the inset shows a low magnification SEM image with a scale bar of 18 μm.

It is worth noting that the “hydrophilicity” of the mesh is due to the intrinsic properties of the silica gel, additionally, the roughness improves the “superhydrophilicity” of the mesh, which leads to the formation of an “underwater superoleophobic” surface.³¹ Because of the intrinsic properties of the silica gel and the quartz fiber, the superhydrophilic and superoleophobic properties of the as-prepared mesh are retained in a strong acidic and concentrated salt environment.

Fig. 2 shows the wettability of the as-prepared mesh characterized by underwater oil contact angle (OCA) measurements. A typical image of a hexane droplet on the surface of the silica gel coated mesh with a contact angle of 160.4° in a 10 mol L⁻¹ H₂SO₄ solution is shown in Fig. 2a. The performance of hexane droplets in a concentration gradient of sulfuric acid (H₂SO₄) and sodium chloride (NaCl) aqueous solutions is shown in Fig. 2b. The underwater OCAs under a series of harsh conditions are over 150°, proving outstanding underwater superoleophobicity and stability of the as-prepared meshes. The underwater OCAs of hexane in an acidic and salty environment are higher than in deionized water, probably because the nonpolar oil droplets are prone to present more regular spheres in an ionic environment. Fig. 2c and d show the wetting behavior of different oil droplets on the silica gel coated mesh surface in 2 mol L⁻¹ H₂SO₄ and 3.5% (about the salt concentration in seawater) NaCl aqueous solutions, respectively. The silica gel coated mesh shows underwater superoleophobicity for various organic solvents and oil, including hexane, petroleum ether, gasoline, diesel, and crude oil. Stable superhydrophilic and superoleophobic properties of the as-prepared meshes guarantee the capacity of oil–water separation under strong acidic and high salt concentration conditions.

Fig. 2 Underwater OCA measurements of the as-prepared mesh. (a) The typical image of a hexane droplet (2 μL) on the surface of the silica gel coated mesh with a contact angle of 160.4° in a 10 mol L⁻¹ H₂SO₄ solution. Since the density of hexane is lower than water, the measuring equipment is inverted. (b) Underwater OCA of meshes in a concentration gradient of H₂SO₄ and NaCl aqueous solutions. (c) Wetting behavior of different oil droplets on the silica gel coated mesh surface in a 2 mol L⁻¹ H₂SO₄ solution. (d) Wetting behavior of different oil droplets on the silica gel coated mesh surface in a 3.5% NaCl aqueous solution.

To test the oil–water separation capability and universality of the silica gel coated meshes, a series of studies were performed as shown in Fig. 3. Photos before and after the oil–water separation are shown in Fig. 3a. The as-prepared mesh was anchored between two PTFE fixtures, which were attached with two glass tubes. The diameter of each glass tube was 30 mm. The whole apparatus was placed vertically upon a beaker. Water and crude oil were mixed in a volume ratio of 1 : 1. Then the oil–water mixture was poured into the upper glass tube. Because of superhydrophilicity, water permeated the mesh, driven by gravity, within 1 minute, while oil was obstructed by a water barrier forming at the surface of the mesh, (see Video 1, ESI†) which resulted in underwater superoleophobicity of the mesh. The filtrate was collected in the beaker below.

Fig. 3 Studies of the oil–water separation capability and universality of silica gel coated meshes. (a) Photos before and after the oil–water separation. (b) Oil content in water measured after separation of mixtures consisting of different concentrations of H₂SO₄/NaCl aqueous solutions and n-hexane. (c) Oil content in water measured after separating gradient concentration H₂SO₄/NaCl aqueous solutions and gasoline. (d) Oil content in water measured after separation of mixtures consisting of 2 mol L⁻¹ H₂SO₄ and several kinds of oil.

The measurement of the oil–water separation capacity was carried out in a series of concentration gradient H₂SO₄ and NaCl aqueous solutions. The capacity was evaluated by measuring the oil content in the filtrate (see Fig. 3b–d). Hexane and gasoline were chosen as the oil phase and shown in Fig. 3b and c, respectively. It is known that crude oil is a complex natural mixture containing many ingredients which can react with concentrated sulfuric acid, and the mixture of crude oil and sulfuric acid flows like sludge. With the rise in acid concentration, the viscosity of the mixture increases gradually and the apparatus will finally be blocked by the semisolid mixture. Therefore, crude oil was not chosen as the oil phase because of the harsh environment. Similarly, when gasoline is mixed with H₂SO₄ aqueous solutions, especially at high concentrations of 8 mol L⁻¹ and 10 mol L⁻¹, some ingredients react with the acids, resulting in some water-soluble impurities with the color of the water phase changing to yellow or orange-red. With the rise in acid concentration, the content of impurities increases, and the value of the oil content in water collected increases simultaneously. It was proven that the values of the oil content in water collected from gasoline–aqueous solution mixtures (see Fig. 3c) were much higher than that collected from n-hexane–aqueous solution mixtures (see Fig. 3b). Furthermore, the values of the oil content in 8 mol L⁻¹ and 10 mol L⁻¹ H₂SO₄ are higher than the others collected from the gasoline–aqueous solution mixtures (see Fig. 3c). Fig. 3d shows measurements taken after the separation of mixtures consisting of 2 mol L⁻¹ H₂SO₄ and several kinds of oil.

According to the data shown in Fig. 3, the silica gel coated mesh possesses an universal oil–water separation capacity towards a common oil like n-hexane, gasoline, diesel, crude oil etc. The oil–water separation efficiency is calculated by the oil elimination coefficient (η) according to eqn (1):

where C₀ is the oil concentration of the initial oil–water mixtures, and C_P is the oil concentration of the filtrate. The separation efficiency of the silica gel coated mesh for a variety of oils is above 99.6% with all the initial oil–water mixtures in a volume ratio of 1 : 1. In addition, our meshes are easily cleaned by rinsing with water.

Conclusions

In summary, a superhydrophilic and underwater superoleophobic mesh with a tolerance to a strong acidic and high salt environment is fabricated for separating an oil–water mixture with the following method: coating a quartz fiber with silica gel by adding BTSE and a high molecular weight PAM for the further enhancement in both chemical and physical aspects. Driven by gravity solely, the as-prepared mesh can separate oil–water mixtures with high efficiency in strong acidic and high salt conditions as well as a mild environment. This work solves the problem that traditional materials are easily contaminated or destroyed in a harsh environment, and broadens the application in industry. Furthermore, this work provides a feasible approach to separate an oil–water emulsion, which demands further research.

Acknowledgements

Authors acknowledge financial support from the National High Technology Research and Development Program of China (2012AA030306), the National Natural Science Foundation (51173099) and the National Research Fund for Fundamental Key Projects (2011CB935700).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, discussion about the synthesis of the silica gel, and video of the oil–water separation. See DOI: 10.1039/c3ra46661b

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