Green Fabrication of a Multifunctional Sponge as an Absorbent for Highly Efficient and Ultrafast Oil-Water Separation.

Oil leakage results in serious environmental pollution and severe waste of resources, which makes the development of low-cost, environmentally friendly, high-capacity, and durable oil absorbents an urgent task. In this paper, superhydrophobic coatings of activated carbon (AC)-TiO2-PDMS@PDMS were developed without using any fluorine-containing reagents. The TiO2 particles were grown on the AC surface to form AC-TiO2 powders. The hydrophilic AC-TiO2 powders were further grafted with polydimethylsiloxane (PDMS) molecules (AC-TiO2-PDMS) to achieve superhydrophobicity through covalent reaction between PDMS and TiO2 under UV light. The AC-TiO2-PDMS powder was mixed with a PDMS polymer to form a superhydrophobic coating solution, which made the commercial sponge obtain durable superhydrophobicity. It showed high liquid repellency and antifouling ability toward various liquids and drinks. Taking advantage of the large surface area and high absorption capacity of AC, the coated sponge showed superior high absorption capacity (up to 100-158 g/g) toward various oils and organic solvents with a high absorption speed. Besides, the sponge showed high reusability that could be repeatedly used to absorb various oils and organic solvents. Moreover, the sponge also presented photocatalytic capability, which could repeatedly photodegrade the oil contaminants without influencing the superhydrophobicity, therefore largely increasing the recyclability and lifetime of the sponge. It also could separate immiscible oil-water mixtures with high efficiency and continuously remove oils from water. It was chemically stable and mechanically durable and could resist various harsh conditions without losing its superhydrophobicity. This study developed a facile, cost-effective, and environmentally friendly method to fabricate very promising absorbents for large-scale oil and solvent cleanups and recovery.

Introduction

In recent years, bioinspired superhydrophobic surfaces with “lotus effect” have aroused tremendous interest in research and industrial fields, which present very promising applications in self-cleaning,[1−3] anti-icing,[3−5] drag reduction,[6−8] antifogging,[9,10] oil–water separation,[11,12] and so forth. Numerous techniques such as etching,[13,14] sol–gel,[15,16] hydrothermal,[17] electrospinning,[18] electrodeposition,[19] dip-/spin-/spray-coating,[20−22] and so forth have been developed to fabricate superhydrophobic surfaces. Among them, the coating fabrication strategy is demonstrated to be very effective and widely applied, which supplies a protective functional layer on the substrate, without considering the substrates’ shape, size, roughness, and so forth. For the coating method, how to improve the adhesion between the coating and the substrate is the critical problem in this field. A very popular strategy of “paint + adhesive” is proved to be very effective to fabricate chemically stable and mechanically durable superhydrophobic surfaces.[2,23,24] Although these reports are very promising and effective, they all have used fluorine-containing reagents, which are very expensive and environmentally harmful. Recently, Long et al[25] reported a “PDMS + ZnSn(OH)6” method to fabricate durable superhydrophobic surfaces. However, hydrophilic particles are used in the coatings, which increase the possibility of losing their superhydrophobicity. As a result, if both the “paint” and the “adhesive” are (super)hydrophobic, the coating would be even more durable.

The ever increasing scale of crude oil leakage or oily/organic solvent-polluted water poses severe ecological threats and arouses high cleanup costs because it is very challenging to clear away these oil spills and organic solvents. Various traditional methods such as skimming,[26] chemical dispersion,[27] and so forth have been used to clean up the waste oils. However, some problems such as high cost, secondary pollution, and limited absorption capacity have seriously restricted their real applications. In view of the above problems, superwetting sponge-based materials with selective wettability to oil and water have been identified as very promising oil cleanup absorbents owing to their superior advantages of unique three-dimensional skeleton architecture, high surface area and porosity, high mechanical property, low cost, excellent elasticity, and so forth. However, the original sponges are not suitable for oil absorption, considering their natural hydrophilicity. Therefore, many strategies have been developed to fabricate superhydrophobic and superoleophilic sponges.[28−32] For example, Wang et al[29] fabricated a fluorizated kaolin-modified melamine sponge to efficiently separate oil from water. Wu et al[33] prepared a superhydrophobic polyurethane@Fe3O4@SiO2@fluoropolymer sponge for selective oil absorption and oil–water separation. However, most of the reported techniques have used fluorine compounds. Besides, their recyclability and regeneration remain poor after adsorbing large amounts of oils, and these techniques do not mention how to remove the adsorbed oil contaminants from sponges. Therefore, it is very desirable to develop a low-cost, fluorine-free, and highly efficient method to fabricate superhydrophobic and superoleophilic sponges with good recyclability.

Photocatalytic capability has proved to be very effective to photodegrade oil pollutants, therefore causing regeneration of the materials , which hugely increases the materials’ recyclability. However, many photocatalysts would simultaneously degrade the modifiers when they degrade the oil pollutants, finally inducing loss of superhydrophobicity. Until now, the reported superhydrophobic sponges with photocatalysis capability are very scarce, not to mention the photocatalytically stable superhydrophobic sponge. Recently, Wooh et al[34] developed a polydimethylsiloxane (PDMS)-grafted method to fabricate a series of photocatalytically stable (super)hydrophobic metal-oxide surfaces and powders. Owing to the strong covalent binding between the metal oxides and PDMS, these (super)hydrophobic materials show very superior UV resistance without affecting the grafted PDMS brush.

Hence, in this paper, in view of the abovementioned problems and taking advantage of some techniques, we fabricated multifunctional coatings consisting of PDMS-grafted activated carbon@TiO2 and PDMS (marked as AC–TiO2–PDMS@PDMS). The TiO2 particles were first deposited onto the AC surface, followed by grafting with PDMS molecules to achieve superhydrophobicity. Because of the high surface area and high absorption ability of AC, the sponges deposited with the coatings not only showed superior high absorption capacity (up to 100–158 g/g) toward various oils and organic solvents with a high absorption speed but also quickly separated various immiscible oil–water mixtures. The coated sponge showed high recyclability and regeneration owing to its superior photocatalytically stable superhydrophobicity. Besides, continuous removal of oils or organic solvents from water is also realized. It was believed that the sponges used in this study will find very promising applications in oil absorption and water-purification fields.

Results and Discussion

Figure a–c shows the wettability toward water drops of various powders. As can be seen, water drops collapsed into the AC and AC–TiO2 powders, indicating their hydrophilicity; however, they stood spherically onto the AC–TiO2–PDMS powder, which proved that superhydrophobicity was achieved (Figure c). The AC powder provided its large surface area to deposit TiO2 particles, therefore achieving photocatalytic capability. When the AC–TiO2 was further grafted with PDMS molecules, superhydrophobicity was achieved. The scanning electron microscopy (SEM) images for various samples are presented in Figure d–g. The pure AC powder exhibited a very smooth structure (Figure d), while large amounts of TiO2 particles were homogeneously deposited onto the AC surface of the AC–TiO2 powder (Figure e,f). The size of the TiO2 particles was in nanoscale. The morphology of the AC–TiO2–PDMS powder did not show big change compared with the AC–TiO2 powder (Figure g).

Figure 1

(a–c) Pictures showing the water drops positioned on various powders of AC, AC–TiO2, and AC–TiO2–PDMS. Only the AC–TiO2–PDMS powders showed superhydrophobicity toward water drops. (d) SEM image of the AC powder. (e,f) Low- and high-magnification SEM images of the AC–TiO2 powder. (g) SEM image of the AC–TiO2–PDMS powder.

The energy-dispersive spectroscopy (EDS) spectra of various samples are presented in Figure a–c. As can be seen, the pure AC consisted of elements of C, N, O, and S (Figure a), while the new element Ti was present in the AC–TiO2 powder (Figure b). For AC–TiO2–PDMS powders, the Si element was obviously a newly occurred one (Figure c). The IR spectra for the AC, AC–TiO2, and AC–TiO2–PDMS particles are shown in Figure d. Compared with pure AC, the peaks positioned at 500–700 cm–1 were obviously observed, which belonged to the TiO2 particles. The characteristic peak for PDMS of 1259 cm–1 marked by a black arrow is clearly observed from the red line in Figure d. The XPS data of the O 1s for the deposited TiO2 particles are shown in Figure e. In addition to the original Ti–O–Ti (529.9 eV) bond, new bonds, namely, Ti–O–Si (531.1 eV) and Si–O–Si (532.6 eV), were formed. The XPS survey data for AC–TiO2 are presented in Figure S1. The composition includes C, Ti, O, and Si occurred in the spectra. All the above results strongly indicated the successful grafting of PDMS molecules onto the AC–TiO2 powder. PDMS reacted with TiO2 particles during the UV illumination process, which was proved by previous papers.[34] The grafting mechanism between TiO2 particles and PDMS can be mainly described as follows:[34,35] TiO2 particles produce numerous hydroxyl groups and water molecules under UV irradiation. These activated molecules partially split the siloxane bonds of PDMS molecules into segmented siloxane-based groups, which would further form covalent bonds with TiO2 by Ti–O–Si bonds, therefore generating surrounding grafted-PDMS brushes around the particles (Figure S2). Because the PDMS molecules were covalently bound with TiO2 particles, the superhydrophobicity of the particles was very stable. Besides, the TiO2 particles were homogeneously distributed onto the AC powder, therefore inducing homogeneous distribution of the grafted PDMS molecules, which finally resulted in uniform superhydrophobicity. The Brunauer–Emmett–Teller (BET) data for various samples including AC, AC–TiO2, and AC–TiO2–PDMS particles are summarized in Table S1. As can be seen, the pore volume and surface area of the AC–TiO2 and AC–TiO2–PDMS particles did not show big change in comparison with those of the pure AC, indicating that the coating of TiO2 and TiO2–PDMS did not affect the large surface area and high porosity of the AC.

Figure 2

EDS spectra of various powders of (a) pure AC, (b) AC–TiO2, and (c) AC–TiO2–PDMS. (d) IR spectra of various powders. (e) X-ray photoelectron spectroscopy (XPS) spectra of O 1s of the deposited TiO2 powders.

The original sponge showed superhydrophilicity with a water contact angle (WCA) of 0°; however, it achieved superhydrophobicity with a WCA of 163° (Figure a). The coated sponge showed very low adhesion with water drops, as demonstrated in Figure b. When the surface made a strong contact with the water drops, water drops still could not drop down even when it was seriously distorted. Water drops collapsed into the pure sponge, while they stood spherically on the coated sponge (Figure c). Figure d shows that the coated sponge floated on the water surface, and it exhibited mirror-like appearance when it was completely immersed in water (Figure e). The above pictures all strongly indicated that the coated sponge was highly water-repellent. Moreover, besides the air condition, the coated sponge also showed superior under-oil superhydrophobicity when positioned under various oil environments (Figure S3). The EDS mapping images also strongly indicated that elements of C, O, Ti, and Si were homogeneously distributed on the coated sponge (Figure S4).

Figure 3

(a) WCAs for pure sponge and coated sponge. The coated sponge showed superhydrophobicity, while the pure sponge was originally superhydrophilic. (b) Coated sponge showed very low adhesion with the water drop. (c) Pictures for water drops when deposited onto the coated sponge and pure sponge, respectively. (d) Coated sponge was floated on the water surface. (e) Coated sponge showed obvious mirror-like phenomena when it was immersed in water.

The morphology of the pure sponge and the coated sponge are presented in Figure . The low- and high-magnification SEM images of the pure sponge showed that the sponge had a smooth structure (Figure a,b). However, large amounts of coatings were bound onto the fiber structure of the sponge, and the coatings were filled with the whole sponge (Figure c,d). Besides, the PDMS binder was also clearly observed, which could strongly fasten the particles onto the sponge.

Figure 4

Low- and high-magnification SEM images of the pure sponge (a,b) and coated sponge (c,d).

The superhydrophobic sponge presented very superior absorption capability, as indicated in Figure . The chloroform and kerosene oil were dyed in red by Sudan. As can be seen, the coated sponge quickly absorbed a bulk of chloroform from water, which made the water clean (Figure a1–a4). For light oil, it also instantly adsorbed large volume of kerosene floated on the water surface (Figure b1–b4). The inset in Figure b4 was the collected kerosene oil obtained after squeezing the sponge each time. Videos S1 and S2 demonstrate that the absorption was very fast. Some dense oils such as diesel oil are commonly used in daily life, which were also very hard to clear away. Therefore, it is very significant to study the ability for absorbing such kind of commonly used dense oils by the coated sponge. As indicated in Figure c1–c4, the sponge could also absorb a large volume of the diesel oil floated on the water surface, which finally made the surface completely clean. The superior oil absorption capability of the coated sponge was ascribed to the high adsorption ability of the AC and the superoleophilicity of the sponge. The absorption abilities of sponges toward diverse kinds of organic solvents or oils (diesel oil, methanol, motor oil, vegetable oil, rapeseed oil, kerosene, hexane, chloroform, acetone, and ethanol) were investigated, and the results are shown in Figure d. The absorption capabilities for the selected organic solvents and oils by the coated sponges ranged from 100 to 158 g/g. The differences between these selected oils and organic solvents were their density and viscosity. The coated sponge was a porous material, and therefore, the increase in oil viscosity would lead to large amounts of oil being stuck in the void of the sponge, finally causing it to malfunction. Therefore, the adsorption capabilities for the organic solvents were much larger than those for the dense oils such as diesel oil/motor oil. The recyclability of the sponge was also examined, and the results are shown in Figure e. As can be seen, the sponge could be recycled and used to adsorb kerosene for as many as 120 times. The absorption capability for kerosene did not have large variation even after it was used for 120 cycles, indicating its high recyclability. These results all indicated that the sponge may serve as one of the most promising and potential sorbent materials for organic solvent/oil cleanup.

Figure 5

Absorption processes for various types of oils from water by the coated sponge. Heavy oil (chloroform) (a1–a4), light oil (kerosene) (b1–b4), and diesel oil (c1–c4). The coated sponge could quickly absorb these kinds of oils. The inset in (b4) was the collected kerosene oil obtained by squeezing the sponge each time. (d) Adsorption capacity of the coated sponge for various kinds of oils. (e) Relationship between adsorption capacity and the number of used cycles for the coated sponge. The used oil was kerosene each time.

Because the system contained TiO2 particles, the photocatalysis capability was investigated. As shown in Figure a1–a3, the coated sponge originally showed superhydrophobicity; however, it became superhydrophilic after it was polluted by diesel oil. Water drops stood spherically onto the originally coated sponge, while they quickly collapsed into the diesel oil-polluted sponge (Figure a2). However, the sponge recovered its superhydrophobicity after UV illumination treatment for around 3 h, which suggested that the coated sponge photodegraded the diesel oil under UV light, which thus recovered the surface superhydrophobicity. Similar phenomena occurred for the dodecane-polluted sponge, as shown in Figure b1–b3. The dodecane-polluted sponge also became hydrophilic; however, it also regained its superhydrophobicity after UV irradiation for about 1 h (Figure b3). The intensity of UV light for the above experiments was 10 mW/cm2. All these results proved the superior photocatalysis capability of the sponge, which was attributed to the existence of TiO2 particles. The Fourier transform infrared (FTIR) spectra of the coated sponge and the diesel oil-contaminated coated sponge before and after UV irradiation are presented in Figure S5. These results indicated that the diesel oil was successfully degraded after UV irradiation, suggesting the superior photocatalytic capability of the coatings. The sponge showed very high recyclability, as demonstrated in Figure c. Even when it repeatedly experienced dodecane pollution and was followed by UV illumination treatments for 40 cycles, it still could maintain its superhydrophobicity with a WCA larger than 150°. The sponge showed extremely stable superhydrophobicity even when it was exposed to UV for at least 80 h (Figure d). The FTIR spectra for the coated sponge before and after UV irradiation (10 h) presented no big difference, indicating its stability (Figure S6). PDMS was covalently grafted onto the TiO2 particles, forming strong combination through Si–O–Ti bonds, which thus showed very stable superhydrophobicity under serious conditions.

Figure 6

Photocatalysis experiments for the coated sponge. (a1–a3) Original superhydrophobic sponge turned to a superhydrophilic sponge after adsorbing large amounts of diesel oil; however, it recovered its superhydrophobicity after UV irradiation. (b1–b3) Similar phenomena also occurred for the dodecane-polluted sponge. Its superhydrophobicity was regained after UV irradiation. (c) WCA variations of the coated sponge through dodecane pollution and UV irradiation for at least 40 cycles. The superhydrophobicity always recovered even when it was polluted by dodecane for 40 times. (d) Coated sponge showed extremely stable superhydrophobicity even after it was exposed to UV illumination for as long as 80 h.

The coated sponge showed antifouling capabilities when immersed into various solutions, as indicated in Figure . When the sponge was completely immersed in tea solution, it was still completely clean when it was pulled out (Figure a1–a3). Similar phenomena occurred when it was immersed into juice (Figure b1–b3), milk (Figure c1–c3), and even muddy water (Figure d1–d3). These results all strongly indicated that the surface showed excellent liquid-repellent and strong antifouling capabilities. The coated sponge also presented superior self-cleaning property (Figure S7).

Figure 7

Antifouling tests of the coated sponges. The coated sponges were always clean even after they were completely immersed in (a1–a3) tea, (b1–b3) juice, (c1–c3) milk, and (d1–d3) muddy water.

The coated sponge also showed superior oil–water separation capabilities. As indicated in Figure a1,a2, the coated sponge was fastened between two glass tubes. When water–chloroform mixtures were poured into the top glass tubes, chloroform quickly flowed through the sponge and was further collected at the bottom beaker, while the water was blocked onto the top glass tube. No dyed water was observed in the bottom beaker, proving the high efficiency. The sponge also showed stability. As presented in Figure b1–d2, it also could quickly separate various corrosive liquids including HCl, NaOH, and NaCl solutions. All the water-based solutions were blocked on the top, while chloroform was collected at the bottom. These results all demonstrated the high separation efficiency and wide applicability of the sponge. Video S3 carefully records the separation process of NaOH–chloroform mixtures.

Figure 8

Coated sponge could efficiently separate various kinds of oil–water mixtures. (a1,a2) Water–chloroform mixtures, (b1,b2) NaCl–chloroform mixtures, (c1,c2) NaOH–chloroform mixtures, and (d1,d2) HCl–chloroform mixtures. (e) Separation efficiencies for various mixtures. (f) Relationship between separation efficiency and separation cycles when the coated sponge was recycled and used for separating water–chloroform mixtures.

The separation efficiencies for the four mixtures were all greater than 99% (Figure e). The recyclability of the coated sponge was very superior. As indicated in Figure f, the separation efficiency for the water–chloroform mixture was still greater than 98% even when it was used for 120 cycles. Besides immiscible oil–water mixtures, the coated sponge could also separate water-in-oil emulsions, as presented in Figure S8. The optical image of the water-in-dodecane emulsion indicates that large amounts of water drops were distributed in dodecane. However, the solution became very transparent, and almost no water drops were observed in dodecane. These results all strongly proved the successful separation of the water-in-dodecane emulsion.

In the view of practical application, a continuous oil absorption capacity from water for the absorbents is very significant. Therefore, in this paper, a continuous oil–water separation device was used to investigate the capacity of the sponge to continuously collect oil from water , and the results are presented in Figure and Video S4. The results indicated that kerosene (dyed by Sudan) could be continuously extracted from water by using this system. The whole process did not bring any water, indicating its high efficiency. Finally, the kerosene was completely collected in a beaker, while the water remained in the original one, which successfully realized separation. No water could be seen in the oil beaker, and the volume of the kerosene was almost the same as the original volume. The diesel oil with higher density and higher viscosity was also used to study the performance of the sponge. Besides, all the contaminated sponges could be regenerated after photocatalysis under UV light. These oil contaminants were photodegraded finally, therefore making the sponge reusable again. This could hugely improve the recyclability of the sponge. The superior selective absorption performance of the sponge was ascribed to its large porosity and its durable and stable superhydrophobicity and superoleophilicity.

Figure 9

Continuous removal process of kerosene (dyed with Sudan) from the water surface using the coated sponge. (a) Coated sponge connected with rubber tube was immersed into the oil–water mixture. (b,c) Continuous removal of kerosene from water after the pump was worked. (d) Kerosene was successfully separated from water.

In order to fulfill the requirements of real applications, stability for the coated sponges was evaluated by using various techniques, and the results are shown in Figure . As can be seen, the sponge always maintained stable superhydrophobicity toward liquids with various pH values (even for strong acid/strong alkali solutions) (Figure a). When the coated sponge was exposed outside for various number of days, the contact angles (CAs) were always larger than 150°, even after three months (Figure b). The coated sponge was demonstrated to resist repeated solvent immersion tests. The sponge was immersed in hexane solvent for 10 min each time (defined as one cycle). As indicated in Figure c, the superhydrophobicity was maintained without a big change even after 100 immersion cycles. All the above tests strongly indicated the superior superhydrophobic stability of the coatings. Moreover, the coated sponge showed superior self-healing capability, as shown in Figure d. The sponge immediately became superhydrophilic with a WCA around 0° after plasma treatment; however, it could quickly recover its superhydrophobicity after a simple heat treatment. Such cycles could be repeated at least 34 times. All the grafted PDMS molecules on the particle and the binder PDMS could self-migrate onto the top after the heat treatment, thus recovering the surface superhydrophobicity. This kind of self-healing ability hugely increases the lifetime of the coated sponge.

Figure 10

Stability tests and self-healing ability of the coated sponges. Relationship between the CA and pH values (a), exposed days (b), and immersion cycles in hexane solvent (c). (d) Variations in WCAs on the coated sponges by oxygen plasma treatment and heat treatment for at least 34 cycles.

Conclusions

In this paper, a nonfluorine, green, low-cost, and highly effective soaking method was used to successfully fabricate a multifunctional superhydrophobic sponge using AC–TiO2–PDMS@PDMS coatings. The sponge presented superior liquid repellency and antifouling performances. The sponge showed ultrafast absorption capacities (up to 100–158 g/g) for various kinds of oils/organic solvents. It could quickly absorb diverse heavy or light oils (also including diesel oil, motor oil, etc.) from water without any water uptake. The coated sponge also exhibited photocatalytically stable superhydrophobicity, which could repeatedly degrade various oils and dyes without affecting its superhydrophobicity. It had superior high recyclability owing to its photocatalysis and stable superhydrophobicity, and it could repeatedly absorb oils or organic solvents. Moreover, its properties also functioned well in serious conditions such as wide pH range, long-term outside exposure, solvent immersion, repeated plasma treatment, and so forth. Besides, it also could be used to separate immiscible oil–water mixtures with high efficiency. Therefore, this sponge was a very promising candidate to be used for oil cleanup.

Experimental Section

Materials

All the chemicals used in this paper were of analytical grade. Commercial polyurethane sponge was obtained from a local market, which was further cut into pieces having a size of 4 cm × 4 cm × 4 cm. AC was obtained from the Xinhua chemical plant in Shanxi Province, China. Titanyl sulfate, ammonia, nitrate acid, hydrochloric acid, sodium chloride, and all solvents used in this work were bought from Huaxin Company, Baoding, China. Sudan red and methyl blue were purchased from Aladdin. PDMS ((C2H6OSi), SYLGARD 184) was purchased from Dow Corning in the USA. In order to remove some impurities, the AC powders were treated in boiling water four times followed by drying at 80 °C for 12 h before use.

Methods

Fabrication of AC–TiO2

Four grams of titanyl sulfate (TiOSO4) was added to 100 mL of deionized (DI) water and stirred to form a homogeneous solution. Then, ammonia was added to regulate the above TiOSO4 solution at pH 7. The whole solution was stirred vigorously. Then, the obtained precipitates were washed by DI water several times to remove the impurities. Seven milliliters of nitrate acid was added followed by ultrasonication at 70 °C for 30 min. Then, 2 g of AC was added under vigorous stirring to form a stable and homogeneous mixture solution. The precipitates were next washed with DI water several times followed by drying at 80 °C. Then, the obtained powder was further calcined at 400 °C for 2 h, and AC–TiO2 was obtained.

Fabrication of AC–TiO2–PDMS

The grafting process was carried out as follows: the AC–TiO2 powder (50 mg) was first dispersed into 10 mL of hexane solvent by sonication for 30 min. Then, 5 g of the PDMS prepolymer was added, and the whole solution was vigorously stirred under room temperature until the solvent evaporated completely. Then, the mixture was placed under UV light for 2 h (intensity of 10 mW/cm2, wavelength of 200–400 nm). Because of the photoactivity of TiO2 particles, PDMS reacted with TiO2 particles, and finally PDMS was grafted onto the TiO2 surface. After that, the mixtures were repeatedly washed by hexane solvent more than 10 times until the unreacted PDMS was completely removed. Finally, the AC–TiO2–PDMS powder was obtained.

Fabrication of the Coated Sponge

One gram of AC–TiO2–PDMS particles was dispersed into 20 mL of hexane solvent, and then, 0.2 g of PDMS and 0.02 g of the curing agent were further added. The whole solution was sonicated for 20 min and then stirred for another 30 min to form a homogeneous solution. The sponge was cut into pieces and then further washed by ethanol and water several times. Then, the cleaned sponge was immersed in the coating solution for 5 s, raised to squeeze out the liquid, and then was reimpregnated into the solution several times until a homogeneous coating layer was formed on the sponge. Finally, the coated sponges were obtained after drying at 60 °C for 2 h.

Preparation of Water-in-Oil Emulsions

Span 80 (0.1 g) was added to dodecane; then, water was added to the mixture (the volume ratio of dodecane to water was 50:1), and then the solution was stirred for 6 h to obtain a homogeneous water-in-dodecane emulsion. Other kinds of water-in-oil emulsions were also fabricated in the same way.

Characterization

Surface morphologies of the samples were observed by SEM (S4800 Hitachi, Japan). The compositions of some samples were measured by an EDS appurtenance (HORIBA, 7593-H). The surface compositions of other samples were analyzed using an X-ray photoelectron spectrometer (250XI, Shimadzu, PHI). CAs of the samples in air or under oil were obtained using OCA35 (DataPhysics, Germany) using a high-speed video camera at room temperature (∼28–30 °C). Five diverse positions were measured to calculate the average CA value. The photographs were captured using a digital camera (Nikon D7200, Japan). Photodegradation experiments of oil pollutants were carried out under UV light (PLS SXE 300 with a xenon lamp, Perfectlight). The distance between the light source and the samples was regulated by the required intensity. FTIR spectroscopy (FTIR TENSOR 27, Bruker, Germany) was performed using KBr at room temperature in the range of 400–4000 cm–1. The plasma experiments were treated using an oxygen plasma instrument (DT-03, Suzhou OPS oxygen plasma technology) at a power of 60 W for 20 s. The pore properties of various powders were measured at −196 °C by N2 adsorption using a porosity analyzer (Nova 3200e, Quantachrome, USA). Specific surface area (SBET), micropore volume (Vm), and average pore diameter (D) were separately calculated using BET, Dubinin–Radushkevich, and Horvath–Kawazoe equations.

Adsorption Capacity Measurements for the Coated Sponge

The sponge was immersed into the selected oil or organic solvents to achieve absorption saturation. Then, the sponge was pulled out and then weighted only if no more oil droplets fell down. The absorption capacity Qm was calculated using the following equationwhere m0 and m1 are the weights of the original sponge and saturated sponge, respectively.

Recyclability Tests

The reusability of the coated sponge was tested by repeated absorption–squeezing processes. The sponges were dipped into the oil or organic solvents until saturation. Then, the saturated sponges were compressed by mechanical force until no oil drops fell down. Then, the sponges were used to absorb oils or organic solvents again. All the processes were conducted repeatedly. The weight of the saturated sponges and the sponges after squeezing them completely were respectively recorded for each cycle.