Facile Preparation of Smart Sponge Based on a Zeolitic Imidazolate Framework for the Efficient Separation of Oily Wastewater

Abstract

The fabrication of durable materials with excellent oil-adsorption capacity and separation performance for the treatment of oily wastewater is meaningful based on the special property of smart responsiveness. Herein, a solvent-responsive melamine sponge (MS) was developed via silanization and the in situ growth of a zeolitic imidazolate framework-8 (ZIF-8). Detailed characterization of the resultant composite MS was conducted using scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), and X-ray diffraction (XRD). The multiscale hierarchical MS substrate exhibited highly hydrophobic properties in the pH range of 1–11, along with a satisfactory adsorption capacity in the range of 65.4–134.2 g/g for different oils. The modified surface transformed from superhydrophobic/superlipophilic to superhydrophilic/underwater superoleophobic upon ethanol wetting, reverting to its original superhydrophobic state upon drying. The separation flux of the MS substrate was above 1.5 × 10⁴ L/m²h for both oil and water removal, and the separation efficiency was greater than 98.7%. The absence of obvious changes in separation performance after 50 successive immiscible oil−water separations indicated the excellent durability and robustness of the anchored ZIF-8 nanoparticles on the surface of the modified MS substrate. More importantly, oil-in-water emulsion separation was successfully carried out via the ZIF-8 MS composite, showing high separation efficiency (over 99.1%). The developed smart sponge, which had high oil-adsorption capacity, excellent chemical stability, and fire resistance, has a wide range of potential practical applications in the convenient treatment of oily wastewater.

1. Introduction

Oily water pollution and frequent oil-spill events usually have large impacts on both human beings and the natural environment [1,2,3]. On the one hand, the illegal discharge of oily wastewater domestically and industrially has led to an apparent increase in water pollution; on the other hand, offshore oil spills have brought about large-scale marine oil pollution [4,5]. The rapid removal of oil and the prevention of the spread of oil spills are thus essential and urgently needed to minimize risks to the local environment and ecology. In general, oil−water separation usually involves several aspects, such as the separation of immiscible light oil−water, of immiscible heavy oil−water, of oil-in-water (O/W), of water-in-oil (W/O) emulsion in the absence of surfactants, of surfactant-stabilized emulsions, etc. So far, many novel strategies for oil−water separation have been attempted and many advanced materials have been tested to address these issues [6,7,8].

Two typical separation methods are usually used for oil−water separation [9,10]. Among the methods using separation materials with superhydrophobic and superoleophilic properties, one is the “oil-removing” approach. Options using separation media with opposing surface wettability (superhydrophilicity and superoleophobicity underwater) are known as “water-removing” approaches. Because of their inherent oleophilicity towards the former materials, oil can easily clog or foul these materials. Additionally, water, being denser than light oil, typically forms a barrier layer above the separating medium, obstructing the penetration of oil. As a result, these materials are limited in their practical applications for separating immiscible oil−water mixtures or surfactant-stabilized O/W emulsions. Water-removing materials, as opposed to the former materials, protect themselves from oil fouling due to their unique underwater properties of superhydrophilicity and superoleophobicity, making them more desirable for gravitational oil−water separation. Furthermore, they are better suited to specific applications such as high-viscosity-oil separation, wastewater purification, and use as a fence for offshore crude-oil leaks, among others.

Fascinating organic−inorganic hybrid materials called metal−organic frameworks (MOFs) are increasingly being developed as adsorbents or separation materials due to their high tunability, large surface area, and rich structure [11]. In the same way, MOFs with special superwetting and superantiwetting properties are generally selected as “water-removing” and “oil-removing” materials for the treatment of oily water. Li et al. prepared a filter paper through the in situ layer-by-layer growth of Cu-MOFs in combination with subsequent polydimethylsiloxane (PDMS) treatment. Highly efficient separation of immiscible oil−water mixtures and W/O emulsion was achieved using the resultant paper with a superhydrophobic and superoleophilic coating [12]. Dong et al. reported a novel copper mesh with underwater superoleophobic and underoil superhydrophobic properties, and HKUST-1 MOFs with Cu₂(OH)₂CO₃ particles were used as the precursors during the preparation process. It was effectively applied for oil−water separation, with a separation efficiency of 97% [13]. A stainless-steel mesh with a micro–nano hierarchical structure was coated with UiO-66 nanocrystals via a facile solution-immersion method. The mesh membrane, which exhibited hydrophilicity and underwater superoleophobicity, showed high separation efficiency and excellent water-permeation flux for the mixture of oil and water [14].

In general, two-dimensional (2D) materials (membrane, mesh, and fabric) present some disadvantages, such as their poor adsorption capacity, low separation efficiency, and weak stability of wettability in the treatment of oily water [15]. In contrast, 3D materials such as sponges, aerogels, and metallic foams are believed to have superior advantages [16]. Azam et al. [17] prepared a 3D separation material based on a porous polyurethane sponge, which was functionalized with ZIF-8 and stearic acid. High hydrophobicity (WCA = 140.8°), high oil-absorbing capacity (30.26–115.35 g/g), and good reusability were obtained for the oil−water separation. Tamsilian et al. [18] developed a superhydrophobic nanocomposite polyurethane (PU) sponge with ZIF-8 and polydimethylsiloxane (PDMS) as synergistic coatings. This sponge had a maximum oil-absorption capacity of 58 g/g and resulted in excellent oil−water separation. Zhang et al. [19] directly used an in situ growth method to encapsulate ZIF-67 on the melamine sponge (MS) materials and efficiently constructed an oil−water separation system. The successful self-assembly of ZIF-67 macroparticles on the surface of the 3D skeleton resulted in a highly hydrophobic modified MS with a WCA of more than 140°. Zhang et al. successfully encapsulated ZIF-8 on the MS substrate with the help of the adhesive attraction of dopamine, and the as-prepared ZIF-8-PDA@MS displayed superhydrophobicity with a WCA of 162°, high adsorption capacities over 85.45 g/g, and reusability over 40 times [20]. Recently, our group used a facile method of “double birds with one stone” to modify Ni foams via the initial solvothermal synthesis of Fe/Ni MOF and subsequent solution immersion of stearic acid. Both processes led to final foams with opposite surface wettability. The former Ni foam with the growth of superwetting MOF was superhydrophilic/underwater superoleophobic, making it suitable for the separation of light oil/water; the latter, with further chemical modification, was superhydrophobic/superlipophilic, which was beneficial for the collection of oil spills [21].

Motivated by the above studies with limited applications for oily water treatment, we prepared a smart composite by combining ZIF-8 with MS using a facile two-step strategy [18,22]. Prior to in situ ZIF-8 growth, the pristine MS composite lacked growth sites because of its smooth surface. Therefore, initial modification of the MS surface with economical (3-aminopropyl)triethoxysilane (APTES) was helpful to promote the coordination of free metal ions on its surface. The final APTES/ZIF-8/MS composites were systematically characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The as-prepared APTES/ZIF-8/MS composite’s oil-adsorption capacity, separation performance for immiscible oil/water and oil in water emulsion, antifouling, recyclability, flame resistance, etc., were investigated in detail.

2. Experimental Section

2.1. Materials

The melamine sponge was purchased from Zhengzhou Fengtai Nanomaterials Co., Ltd., Zhengzhou, China. Zinc nitrate hexahydrate (AR) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd., Tianjin, China. Methanol (HPLC grade), ethanol (≥99.8%), methylene blue trihydrate (indicator grade), Sudan Ⅲ (AR), n-hexane, petroleum ether, iso-octane, n-octane, Span80, sodium hydroxide (96%), and hydrochloric acid (AR) were from Aladdin Reagent Co., Ltd. (Shanghai, China). N,N-dimethylformamide (DMF, ≥99.9%) and toluene (AR) were purchased from Shanghai Energy Chemical Co., Shanghai, China. 2-Methylimidazole (98%) and (3-aminopropyl) triethoxysilane (APTES, 98%) were from Shanghai Titan Technology Co., Shanghai, China.

2.2. Preparation of ZIF-8/APTES/MS

Firstly, the MS material was cut into 2 cm × 2 cm × 2 cm pieces and washed by ultrasonication with deionized water and ethanol, respectively, for 10 min, and then dried in an oven at 80 °C. Secondly, the cleaned MS was immersed in 10 mL of toluene with the addition of 30 μL of APTES in a water bath at 100 °C for 90 min [20]. At room temperature, the cooled APTES/MS composite was rinsed with ethanol, so unreacted compounds such as APTES, toluene, etc., were removed. The resulting APTES/MS was then dried at 80 °C. Moreover, both zinc nitrate hexahydrate (12.5 mmol) and 2-methylimidazole (25 mmol) dissolved in 50 mL of methanol were quickly mixed and stirred thoroughly until fully dissolved [17,22]. Then, the as-prepared APTES/MS was dipped into the mixed solution at room temperature for 6 h. Finally, the ZIF-8/APTES/MS composites were obtained through thorough washing with methanol and drying at 80 °C for 12 h. The whole preparation process of this composite is illustrated in Scheme 1a, which included the initial silanization of APTES, the subsequent growth of Zif-8, and the switching of superwettability triggered by ethanol. Moreover, multiple applications of the as-prepared APTES/ZIF-8/MS are shown in Scheme 1b, which include oil removal, the efficient adsorption of floating light oil above the water surface and heavy oil underwater, the separation of O/W emulsion using a drying MS, and water removal using an MS wetted by ethanol.

2.3. Adsorption Experiments for Oil and Organic Solvent

Considering that oil and some organic reagents are common pollutants in water, we chose different oils (n-hexane, petroleum ether, toluene, CCl₄, and soybean oil) and organic solvents (methanol, ethanol, and DMF) for the investigation of the adsorption capacity of ZIF-8/APTES/MS composites. The oil-absorbing capacity of the sponges in different compositions and states was tested by immersing the sponges in various organic solvents and oils for 1 min at room temperature and weighing the mass of the sponges before and after oil adsorption. All experiments were repeated at least three times. The oil adsorption Q (g/g) was calculated according to Equation (1) [23,24]:

$$\mathrm{Q}={\displaystyle \frac{{\mathrm{M}}_1-{\mathrm{M}}_0}{{\mathrm{M}}_0}}$$

(1)

where Q is the adsorption capacity of the composite for oil, M₀ is the mass of the original ZIF-8/APTES/MS, and M₁ is the mass of ZIF-8/APTES/MS after saturated adsorption by the oil or organic solvent.

2.4. Recyclability and Reusability Tests

The as-prepared ZIF-8/APTES/MS materials were immersed into chloroform solution until saturated. Then, they were manually squeezed to recover the adsorbed oil, weighed, and reused for the next cycle. This test was conducted to examine the effect of the composite's adsorption capacity and residual capacity for 10 consecutive cycles.

2.5. Oil/Water Separation

The separation of immiscible oil/water mixtures was carried out using a glass funnel. Sudan red and methylene were used to dye the oil and water phases, respectively. A small piece of ZIF-8/APTES/MS substrate was put at the funnel neck as an intermediate filtration layer. The separation flux (F) was calculated according to Equation (2) [24]:

F = V/AΔt

(2)

where V (L) stands for the volume of filtrate, A (m²) stands for the effective area of the filter layer, and Δt (h) stands for the separation time.

The separation efficiency (η) was calculated as the ratio of the initial water mass (m₀) to the separated water mass (m₁) when no more droplets eluted from the glass funnel, and is expressed in Equation (3) [24]:

η = (m₁/m₀) × 100%

(3)

2.6. Emulsion Separation

To further investigate the adsorption capacity of ZIF-8/APTES/MS composites, surfactant-stabilized O/W emulsions were prepared. A total of 1 mL of n-hexane was added to 99 mL of water with 0.1 g of surfactant (Span 80), and the homogenizer (5000 r/min) was stirred for 5 min to obtain surfactant-stabilized O/W emulsions.

The separation efficiency η (%) of the O/W emulsions was calculated according to the methods in the literature. Specifically, the absorbance of the emulsion and filtrate was determined by a UV-Vis spectrophotometer and calculated using Equation (4) [24,25]:

$$\mathrm{Separation}\mathrm{efficiency}=(1-{\displaystyle \frac{I_1}{I_0}})\times 100\%$$

(4)

where I₀ and I₁ are the absorbance of the emulsion and filtrate, respectively.

2.7. Characterization

The microstructure and elements of the pristine sponges and ZIF-8/APTES/MS composites were investigated using a field emission SEM (Ultra Plus, Carl Zeiss, Oberkochen, Germany) and an EDS (Oxford X-MAX, Oxford, UK). The surface chemistry of the samples was analyzed by XPS (Thermo Scientific ESCALAB Xi+, Waltham, MA, USA). The absorbance of the emulsions and their filtrates was determined using a UV-Vis Spectrophotometer (Lambda 750 S, PerkinElmer, Inc., Shelton, CT, USA), and the distribution of oil droplets in the filtrates and emulsions was observed by fluorescence microscopy (Nexcope NE620LED, Chongqing Liuhui Technology Co., Ltd., Chongqing, China). Surface wettability was assessed using an optical contact angle meter (TST-300H, Shenzhen, China). The contact angles (CAs) of oil and water droplets (6 μL) were measured in the air and underwater conditions at room temperature. The static CA was calculated based on the average values from more than three different locations on the same substrate.

3. Results and Discussion

3.1. Characterization of ZIF-8/APTES/MS Composites

3.1.1. SEM Analysis

To investigate the morphology of ZIF-8 crystals on sponges, the microstructures of pristine sponges, ZIF-8/MS and ZIF-8/APTES/MS, were comparatively characterized by SEM. The pristine sponge had a micron-sized (50–100 μm) porous cross-linked structure (Figure 1a) and a relatively clean and smooth backbone (Figure 1b). If APTES was not used to modify the sponge before ZIF-8 crystallization, the network structure and porosity of the obtained ZIF-8/MS were similar to those of the pristine sponges, and ZIF-8 crystalline particles were hardly observed (Figure 1c,d), which demonstrated that chemical modification was indispensable for the successful growth of ZIF-8 nanoparticles. Therefore, APTES molecules as the bridges on the sponge surface provided more NH₂ groups and low-surface-energy chains for the immobilization of ZIF-8 nanoparticles in order to improve its hydrophobicity [26]. In addition, the 3-aminopropylsilyl group of APTES cooperated with the free Zn²⁺ center to directly bind the grown nanocrystals, so the resultant dense and rough ZIF 8 particles (Figure 1f and its inset) were the key to constructing a hydrophobic surface [27,28].

In order to calculate the distribution of the elements, the corresponding elemental compositions and energy spectra were further plotted. In Figure 2a,b, it is demonstrated that the pristine sponge contained the elements C, N, and O with contents of 37.64%, 50.59%, and 11.77%, respectively. However, in addition to the above three elements, the elements Si and Zn were also present in the ZIF-8/APTES/MF composites with contents of 1.45% and 3.11%, respectively. The increase in the Si content resulted from the presence of the APTES molecules, while the significant increase in the Zn element suggested that the ZIF-8 nanoparticles were successfully adhered to the MS substrate. The EDS pattern of ZIF-8/APTES/MF clearly displayed a uniform distribution of the elements C, N, O, Si, and Zn on the sponge skeleton (Figure 2c).

3.1.2. XPS Analysis

XPS was used to analyze the changes in the chemical composition of the sponge surface before and after modification. As shown in Figure 3a, the primitive sponge contained mainly C, N, and O elements. The XPS gross spectrum of the modified sponge exhibited an increase in the peak intensities of C 1s and O 1s, and a slight decrease in the peak intensity of N 1s, which could be attributed to the incorporation of APTES. Note that it was rich in elemental C and O on the surface of APTES. More importantly, two new peaks were found at 102.02 and 1022.11 eV for elemental Si and Zn, which further proved the successful modification of APTES and the successful growth of ZIF-8 nanoparticles. The above results were consistent with the EDS patterns. In addition, the fitting of the Zn 2p and Si 2p peaks were analyzed individually. In Figure 3b, asymmetric peak shapes were observed at 101.86 eV for overlapping Si 2p_3/2 and Si 2p_1/2, favorably demonstrating the successful modification of the MS by APTES molecules; and in Figure 3c, Zn 2p_3/2 and 2p_1/2 occurred at 1022.08 and 1045.14 eV, demonstrating the presence of Zn²⁺.

3.2. Surface Wettability Analysis and Antifouling Properties

As shown in Figure 4a,b, a water droplet (dyed blue by methylene blue) and an oil (CCl₄) droplet (dyed red with Sudan Ⅲ) were dropped on the surface of the pristine sponge, and both liquid droplets were immediately spread and adsorbed, indicating that the pristine sponge is superamphiphilic [19]. In contrast, the water droplet on the surface of the ZIF-8/APTES/MS was spherical, with a WCA of up to 158.1°, while the oil droplet was adsorbed, suggesting that it was remarkably superhydrophobic/superoleophilic. When the pristine sponge and ZIF-8/APTES/MS were put into water, a significant difference was observed. The pristine sponge quickly sank to the water bottom due to its superhydrophilicity, whereas ZIF-8/APTES/MS floated on the water surface because of its superhydrophobicity (Figure 4c). However, when ZIF-8/APTES/MS was pressed below the surface of the water under an external force, a shiny, reflective interface appeared, like a silver mirror. This was attributed to the fact that the special structure of the ZIF-8/APTES/MS surface formed an air layer on the surface and the trapped air bubbles around the surface created a "silver mirror" phenomenon on the underwater surface (Figure 4d). Interestingly, when the MS composite wetted by methanol was in contact with oil, underwater superoleophobicity was observed with a underwater oil contact angle (UWOCA) of 154.2° (Figure 4e) [29]. Even after a period of time, the oil droplet remained spherical on the ZIF-8/APTES/MS surface (Figure 4f). It should be noted that it regained its original superhydrophobicity after being dried. In addition, identical wetting effects on the ZIF-8/APTES/MS material were achieved using other solvents such as methanol, acetonitrile, and DMF, and the obtained UWOCAs were all more than 150°. The polarity, surface tension, and UWOCA required for each solvent to trigger responsiveness are listed in

Table 1. UWOCA values of ZIF-8/APTES/MS composites wetted by different organic solvents.

Solvent	Polarity	Surface Tension (mN/m)	UWOCA
Methanol	6.6	23.6	151.7°
Ethanol	4.3	22.39	154.2°
Acetonitrile	6.2	22.75	153.1°
DMF	7.87	25.7	150.6°

.

The mechanism of switchable superwettability from superhydrophobic/superoleophilic to superhydrophilic/superoleophobic underwater is explained as follows [30]: When a drop of water sits on the fabricated superhydrophobic surface, the trapped air in the micro/nanostructures of the surface can keep the water droplet spherical in the air in the Cassie state (Figure 5a). At the same time, the air layer on the superhydrophobic surface underwater reduces water penetration. Meanwhile, if one oil droplet contacts the surface underwater, it will infiltrate the air layer. As the MS composite is wetted by ethanol or other low-surface-energy liquid, as shown in Table 1, the voids in the micro/nanostructures will be filled with ethanol instead of the trapped air. Since ethanol molecules are highly soluble in water, they will be quickly diffused into the water layer and displaced by water molecules when MS composites are immersed into water. As a result, the micro/nanostructures on the wetted surface will be filled with water, and the Wenzel state (Figure 5b) is thus formed. Then, the MS composite will exhibit superhydrophilicity/superoleophobicity underwater before the MS material is dried again.

We systematically used a dynamic compression–separation process to further demonstrate the resistance of the prepared composite sponges to water or oil adhesion under different conditions. As shown in Figure 6a, a drop of water was used to slowly apply pressure to the dry-state ZIF-8/APTES/MS composite in air. Although a large portion of the water droplet contacted the surface of the composite, it did not detach from the tip, indicating that the resultant surface had significant resistance to water adhesion. Similarly, a drop of oil was slowly pressed onto the ethanol-wetted ZIF-8/APTES/MS composite in water. Although the oil droplet touched the surface of the sponge composite over a large area, it was lifted up without any trace (Figure 6b), and did not detach from the needle in the water, which indicated its excellent resistance to oil adhesion.

3.3. Adsorption of Oil and Organic Solvents and Oil/Water Separation Properties

Due to its superhydrophobicity and high porosity, oil and organic reagents in water can be selectively adsorbed and collected by the ZIF-8/APTES/MS composite. Herein, the composites were compared in terms of their ability to separate light oil (n-hexane) and heavy oil (CCl₄) from water. As shown in Figure 7a,b, when the ZIF-8/APTES/MS composites were in contact with n-hexane or CCl₄, the oil was completely adsorbed by the composites within a few seconds, and especially when the heavy oil was adsorbed, the formation of a layer of bubbles at the solid–liquid interface could be clearly seen, which implied that spatial exchanges between the CCl₄ and the air in the sponge structure took place, preventing the water from infiltrating into the sponge. It should be noted that both mixtures of light/heavy oil and water were completely adsorbed by the ZIF-8/APTES/MS, and there were no residual n-hexane and CCl₄ droplets in the beaker after the adsorbed sponge was removed. In addition, the adsorbed oil or organic solvent can be discharged by simple extrusion, thus enabling the reuse of ZIF-8/APTES/MS.

Organic reagents and oils discharged from laboratories and daily life usually lead to some degree of water pollution. Therefore, the adsorption capacity of the ZIF-8/APTES/MS composite on different typical solvents and oils was further investigated, including soybean oil, petroleum ether (PE), N,N-Dimethylformamide (DMF), etc. The results showed that good adsorption capacity was obtained for eight kinds of oils and organic reagents, even up to 65.4 times (for n-hexane) to 134.2 times (for CCl₄) its own weight (see Figure 7c). In addition, the adsorption capacities of previously reported ZIF-based/sponge composites were compared, and ZIF-8/APTES/MS had comparable or higher hydrophobicity and adsorption capacities, as shown in

Table 2. Comparison of adsorption properties of ZIF-8/APTES/MS composites with different ZIF-based/sponge composites.

Adsorbents	WCA	Adsorption Capacity (g/g)	Ref.
ZIF-8/PDMS/PU	156°	42–58	[18]
ZIF-8/APTES/MS	130°	76–164	[20]
ZIF-8-PDA@MS	162°	85.45–168.95	[22]
ZIF-8/SA/PU	143.2°	30.28–115.35	[32]
ZIF-8/PU	129.2°	28–79	[33]
ZIF-8/PE	-	17.6–55.8	[34]
ZIF-90/MS	154°	24–63	[34]
ZIF-8/APTES/MS	158.1°	65.4–134.2	This work

. Based on the fact that the adsorption capacity of sponges was highly related to the density of the adsorbed reagent (Figure 7d) [31], the higher the density of the solvent or oil, the higher the adsorption capacity of the composite sponge.

In addition, the reusability in practical applications for the ZIF-8/APTES/MS composites was investigated. Based on its superhydrophobicity and good absorbance of ZIF-8/APTES/MS, CCl₄ was chosen as the experimental solvent, and the adsorption capacity and residual capacity were calculated for each cycle (see Figure 8). No significant decrease in adsorption capacity and no obvious increase in residual capacity were found after 10 cyclic experiments. This was probably attributed to the good coverage and adhesion of the coating materials. The slight decrease in adsorption capacity was probably attributed to the following reasons [31,33]: insufficient squeezing of the residual adsorbed oil, structural damage to the sponge skeleton during the adsorption-squeezing cycle, and the shedding of a small amount of ZIF-8 particles from the sponge.

The prepared ZIF-8/APTES/MS was immobilized in a funnel, as shown in Figure 9a (marked with a red circle). Under normal drying conditions, ZIF-8/APTES/MS was superhydrophobic, so it was clearly observed that the red-dyed CCl₄ rapidly penetrated the sponge and flowed into the beaker due to its own gravity. Water was completely blocked due to the superhydrophobic ZIF-8/APTES/MS and sat in the upper funnel. On the contrary, the sponge after being wetted by ethanol was superhydrophilic/superoleophobic underwater (see Figure 9b), the blue-dyed water rapidly passed the sponge gravitationally, and n-hexane was thus blocked. The ZIF-8/APTES/MS composite remained solvent-responsive after five ethanol wetting and drying cycles (Figure 9c). This property makes it suitable for effectively separating water and removing oil using both modes. The separation efficiencies and fluxes of various types of oil/water mixtures (CCl₄, n-hexane, n-octane, iso-octane, and petroleum ether) are comparably exhibited in Figure 9d. Regardless of whether water or oil was passed through the composite, excellent separation flux in the range of 1.5 × 10⁴ to 1.8 × 10⁴ L/m²h was thus obtained, along with high separation efficiency of more than 98.7%. More meaningfully, only a slight decrease in the separation fluxes and stable separation efficiencies were found even after 50 consecutive cycles, taking immiscible CCl₄/water and n-hexane/water mixtures as examples (Figure 9e). These results indicated that the ZIF-8/APTES/MS composite was durable and robust for the treatment of oily water.

3.4. Emulsion Separation

The treatment of emulsified oil/water mixtures is more challenging and meaningful in practical application. Herein, we placed the composites in the oil (n-hexane)-in-water emulsion and moved them up and down rapidly for continuous adsorption and compression, and the emulsion was clarified after a few minutes, as shown in Figure 10a. Additionally, the emulsion appeared milky white before separation, as shown in Figure 10b (inlet), and a great number of oil droplets were obviously found with the help of an optical microscope. Comparatively, the emulsion after separation was transparent and colorless, as shown in Figure 10c (inlet), and almost no oil droplets were detected. The separation efficiency was determined to be as high as 99.1%. It was more likely for the superhydrophobic composite to adsorb oil rather than water when it contacted the emulsion, and its water-repelling ability would be further improved if oil wetted the composite. The encapsulated ZIF-8 nanoparticles resulted in a rougher surface with a stronger capillary force and a bigger surface area, which was beneficial for the adsorption of oil droplets and enabled the efficient separation of oil-in-water emulsions.

3.5. Durability and Chemical Stability

The chemical stability of the as-prepared ZIF-8/APTES/MS composite is desirable for long-term oily water treatment. The initial WCA was 158.1° for the superhydrophobic composite (see Figure 11a), and stable spheroids were maintained for a long time (120 min). However, in practice, the oil-containing wastewater may be acidic and alkaline. Therefore, it is necessary to investigate the stability of its surface wettability under different pH conditions. As shown in Figure 11b, the WCAs were 136.4° and 148.7° under the extreme conditions of pH = 1 and pH = 11, respectively. Moreover, the composite was superhydrophobic in near-neutral pH conditions (the WCA was more than 150° at pH = 7). This indicated that the as-prepared composite exhibited high hydrophobicity in the pH range of 1–11.

3.6. Flame Retardancy

The fabricated ZIF-8/APTES/MS composite possessed excellent flame retardancy. Vigorous combustion did not occur for the pristine sponge when it was in contact with an ignition source, and the ignited sponge extinguished instantly without a heat source, as shown in Figure 12a. This was probably attributed to the fact that partial conversion from the N element in the pristine sponge to N₂, NO, and NO₂ took place during the combustion process and the produced gasses inhibited the material from continuous burning [35]. Regarding the ZIF-8/APTES/MS composite, it demonstrated more excellent flame-retardant properties in the combustion test, as illustrated in Figure 12b. The composites inherited the natural flame retardancy of the pristine sponge, and on the one hand, the attachment of APTES containing the N element increased the whole N content in the final as-prepared sponge, and more gasses of N₂, NO, and NO₂ were produced during combustion, which was helpful for the prevention of further combustion; on the other hand, the zinc from the formed MOF also consumed oxygen during the combustion process, which was also advantageous for its flame retardancy. Therefore, compared with the morphology of the pristine sponge, ZIF-8/APTES/MS still retained its original shape after combustion with less volume loss.

4. Conclusions

In summary, a type of novel coating material was developed with switchable superwettability, taking a commercial MS as a basic substrate. Two-step approaches were carried out through the chemical modification of APTES and in situ growth of ZIF-8. In combination of the exceptional porous and superhydrophobic capability of the ZIF-8 coatings and the ordered MS backbones, the final composite exhibited a high adsorption capacity of 65.4–134.2 g/g for various oils, high-flux oil/water separation efficiency, and facile treatment for oil-in-water emulsion separation. Drying treatment and ethanol immersion yielded a rapid smart response between superhydrophobicity in the air and superoleophobicity underwater, with the resulting WCA as high as 158.1° and a UWOCA of more than 154.2°. Impressively, due to its compressibility and chemical stability, the fabricated MS sponge could be reused over 50 cyclic oil/water separations without a significant change in its separation performance. This novel composite is expected to be a promising candidate for oil spill cleaning and conventional oily wastewater treatment. Further research to prepare novel materials with reversibly switchable superwettability for oil/water separation should be conducted in the future.