Robust Superhydrophobic Sepiolite-Coated Polyurethane Sponge for

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Robust superhydrophobic sepiolite-coated polyurethane sponge for highly efficient and recyclable oil absorption Shengmeng Qiu, Yunfei Li, Gongrang Li, Zhaoyang Zhang, Yujiang Li, and Tao Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00098 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Sustainable Chemistry & Engineering

Robust superhydrophobic sepiolite-coated polyurethane sponge for highly efficient and recyclable oil absorption Shengmeng Qiu, † Yunfei Li, † Gongrang Li, § Zhaoyang Zhang, † Yujiang Li, *, † and Tao Wu*, ‡ † Shandong

Provincial Research Center for Water Pollution Control, School of

Environmental Science and Engineering, Shandong University, 27 Shanda South Road, Jinan, 250100, PR China. ‡ Key

Laboratory of Colloid and Interface Science of Education Ministry, Shandong

University, 27 Shanda South Road, Jinan, 250100, PR China. § Drilling

Technology Research Institute, Shengli Petroleum Engineering Corporation

Limited of SINOPEC, Dongying, 257017, China. Corresponding Authors *Yujiang Li: E-mail: [email protected]. Tel/Fax: +86-531-88363358; *Tao Wu: E-mail: [email protected]. Tel/Fax: +86-531-88365437; ABSTRACT: With the rapid increase of oil production and offshore transportation, the probability of accidental oil leakage is rising. Utilization of superhydrophobic materials constitutes an effective method to solve oil pollution on the water surface. In this study, a modified superhydrophobic sepiolite (SEP) layer was loaded onto the skeleton surface of three dimensional (3D) porous polyurethane (PU) sponges through a one-step ultrasonic dip-coating process. The as-prepared superhydrophobic sponges can rapidly and selectively absorb multiple oils and nonpolar solvents that are more than 29 times the weight of the sponge, while completely repelling water. In addition, the as-prepared 1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

composite material could be reused for oil-water separation for more than 10 times with a high separation efficiency of over 99.45%. The composite material also exhibited robust superhydrophobicity in corrosive liquids and hot water. The results of this research may provide a promising absorbent material that might be used to effectively remove oil spills from the surface of water. KEYWORDS: superhydrophobic; PU sponge; sepiolite; selectively; oil-water separation. INTRODUCTION With the rapid development of oil production and the ocean transportation industry, the possibility of marine oil spills is increasing. Consequently, methods for the removal or collection of oil pollutants from the water surface have attracted global attention.1-3 For example, the explosion of BP's Deepwater Horizon oil rig resulted in the release of large amounts of crude oil into the Gulf of Mexico in 2010.4 Various conventional approaches have been used in oil spill clean-up, such as in situ burning, mechanical collection, and skimmers.5 However, these methods constitute energy-consuming, low-efficiency, and complicated separation procedures.6 Moreover, traditional absorbent materials, such as zeolites, activated carbon, organic clay, straw, hair and wool fibers, possess limitations of low absorption capacity and poor recyclability.7-8 Especially, these traditional adsorbent materials absorb water and oil at the same time, and cannot selectively absorb oil, which greatly reduced the separation efficiency.9 Recently, membrane materials with superhydrophobic and superhydrophilic properties have attracted broad attention due to their effective separation ability of oil and other organic solvents.10 They cannot be utilized for oil spills, however, because they need to accumulate polluted water first and then filter 2 ACS Paragon Plus Environment

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it, which is cumbersome and energy-intensive for large-scale marine oil spill treatment.11 Therefore, the development of new absorbing materials with high selectivity, high adsorption capacity, high efficiency, excellent recyclability, and low cost is very important for

oil-water

separation.12

In

previous

reports,

3D

porous

materials

with

superhydrophobic/superoleophilic properties have been suggested to rapidly and selectively collect multiple oils and nonpolar solvents from the water surface, providing a facile method for the clean-up of oil spills.1, 4 Polyurethane (PU) sponge is a type of commercial 3D porous material with the characteristics of high resilience, low density, high absorption ability, and easily scalable fabrication, which have the promising application as oil-water separating materials and absorbents for other purposes.13-15 However, surface modification is required because the PU sponge is usually hydrophilic, because of its hydrophilic amino and carboxyl groups on the surface.9 An effective method to fabricate such absorbent materials is to use hydrophobic modifying agent to form a layer of superhydrophobic coating on the PU sponge surface.9,

16

Indeed, numerous superhydrophobic PU sponge absorbents for oil-

water separation have been prepared.17 For example, Zhu et al. fabricated a superhydrophobic Cu-C11H23COOAg-coated PU sponge for oil-spill clean-up.1 Wang et al. developed a superhydrophobic CNT/PDMS-coated sponge for oil/water separation.4 Li and co-workers reported a novel hydrophobic sponge coated by Cu2O for the treatment of oil/water

mixtures.18

More

recently,

graphene-coated

PU

sponges

with

superhydrophobicity have been fabricated and successful employed for the treatment of oils and nonpolar solvents.4, 19-21 Nevertheless, the aforementioned superhydrophobic PU sponges possess the disadvantages of complicated manufacturing processes and high 3 ACS Paragon Plus Environment


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cost.17, 22 In addition, oil spills normally occur in complex environmental systems, and thus fabrication

of

superhydrophobic/superoleophilic

materials

that

maintain

superhydrophobicity in corrosive solutions (e.g., strong acidic, alkaline, or salt aqueous solutions) and high temperature is more important.23-24 Therefore, fabrication of superhydrophobic sponges that possess the characteristics of high adsorption capacity, high selectivity, excellent mechanical durability, and low cost is urgent.19, 25 Superhydrophobic materials must possess two features:(1) a low surface energy substance; and (2) a micro-nano rough structure on the surface of the material.26 Therefore, two ways exist to prepare superhydrophobic materials:(1) modifying the low surface energy substance after constructing the micro-nano structure on the surface of the material; and (2) constructing a micro-nano structure on a surface having a low surface energy substance.27 In this study, we present a simple and inexpensive method to prepare superhydrophobic sepiolite-coated PU sponge by a one-step ultrasonic dip-coating method. Here, because of its robust superhydrophobicity to corrosive liquids and high temperatures, the superhydrophobic SEP powder modified by octadecyl trimethylammonium bromide (OTAB) and octadecyltrichlorosilane (OTS) was selected as modified material. The micronano rough structure formed by modified sepiolite and low surface energy material (OTS) make the PU sponge have a superhydrophobic surface. The superhydrophobic SEP-coated PU sponge possess the characteristics of high adsorption capacity, high selectivity and high separation efficiency when it is used as an adsorbent to collect oil and organic solvent from the water surface. More interestingly, the superhydrophobic sponge exhibits good reusability in oil-water separation without losing its superhydrophobicity, showing good mechanical durability. Therefore, the findings of this research offer a facile and 4 ACS Paragon Plus Environment

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inexpensive method to fabricate robust superhydrophobic 3D porous absorbent materials that might be used for cleaning oil spillage and organic solvent leakage on the water surface. EXPERIMENTAL SECTION Materials Polyurethane (PU) sponge was purchased from a commercial vendor. Pure sepiolite(SEP) with natural mineral content >95% was purchased from Vallecas-Vicalvaro Clay Deposits (Madrid, Spain), the cation exchange capacity (CEC) of the sepiolite is 30mmol/100 g. Octadecyltrichlorosilane (OTS) and Octadecyl trimethylammonium bromide (OTAB) having a purity of 98% was acquired from the Shanghai McLean Chemical Reagent Co., Ltd., (Shanghai, China). Crude oil with water content of < 0.5% was obtained from the Shengli oilfield in China. Various chemical reagents, including concentrated hydrochloric acid, acetone, anhydrous ethanol, toluene, NaCl, and CCl4 were all of analytical grade. Fabrication of superhydrophobic SEP The superhydrophobic SEP was fabricated by two steps. The first step was an organic modification process. Added 10g of natural SEP to a mixed solution of 400mL anhydrous ethanol/ultrapure water (1:1, v/v), followed by stirring for 30 min with a magnetic stirrer. Dispersed 1.0 CEC of OTAB in 100 mL of ultrapure water, and stirred for 30 min. Then, added the dissolved cationic surfactants to the mixing solution and stirred for 24 h at 60 °C. After processing, the products were filtered by a vacuum, and then washed with ultrapure water and absolute anhydrous ethanol several times until there was no bromide

5 ACS Paragon Plus Environment


ion in the filtrate (as determined by 0.1mol/L AgNO3). The products were then dried in a vacuum oven at 60 °C. The second process is the superhydrophobic modification process. Organic SEP is first transferred to a three-necked flask containing 200 ml of toluene solution equipped with a reflux condenser and a collecting thermostat heated magnetic stirrer. Excess OTS was then added. The superhydrophobic modification process continued at 80 °C for 5 h. After the modification procedure was completed, the mixed solution was filtered and rinsed thoroughly with anhydrous ethanol. The obtained superhydrophobic SEP was then dried in a vacuum oven at 80 °C. Fabrication of superhydrophobic sponges The original PU sponge was ultrasonically washed in acetone and anhydrous ethanol for 30 min successively in order to remove impurities on the skeleton surface, and then dried in a vacuum oven at 80 °C. Then, the washed PU sponge was immersed in ethanol solution with different masses of superhydrophobic SEP (wt% equal 0%, 1%, 2%, 3%, 4%, 5%, 6%, and 7%), ultrasonicated for 1 h, and then allowed to stand for 5 h. Finally, the superhydrophobic SEP-coated PU sponges were taken out and dried in a vacuum oven at 80 °C. Characterization Field emission scanning electron microscopy (FE-SEM) images of the pristine and as-prepared PU sponges were characterized by a JSM-6330F scanning electron microscope (JEOL Ltd., Japan). Fourier transform infrared spectroscopy (FT-IR) spectra of the pure and the modified SEP were characterized by the Nicolet Nexus 670 FT-IR system (Thermo Fisher Scientific, U.S.A.). The surface chemical composition of the SEP samples modified 6 ACS Paragon Plus Environment

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before and after were characterized by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI, U.S.A.). The static water contact angle (WCA) was measured by interfacial rheometer (Tracker, IT Concept, France). Specifically, WCA was measured at three different positions for each as-prepared sponge sample, and then calculating the average value. Experiments of oil/water separation The adsorption capacity k of the as-prepared sponges was obtained by the following process. The SEP-coated sponges were placed on the surface of oil and organic solvent until reaching adsorption saturation. The adsorption capacity k was calculated by the equation: k = (𝑚𝑠 ― 𝑚𝑝)/𝑚𝑝

(1)

where 𝑚𝑝 and 𝑚𝑠 are the quality of the as-prepared sponges before adsorption and after adsorption saturation, respectively. The oil and organic solvent were removed by dipping the SEP-coated sponge onto the surface of the immiscible oil/water mixtures. The oil in the treated wastewater was extracted by CCl4. The oil concentration in CCl4 was then analyzed by an infrared spectrometer oil content analyzer (Oil 460) to determine the oil content in the treated wastewater.



RESULTS AND DISCUSSION In this study, the fabrication of superhydrophobic SEP is crucial for the preparation of superhydrophobic PU sponges. Because natural sepiolite possess strong hydrophilicity, the coated sponge will not be able to obtain superhydrophobic property if the sepiolite was not modified. The superhydrophobic SEP was synthesized by modifying a low surface energy substance(OTS) on the organic SEP. As presented in Fig. 1a, FT-IR analysis was performed for the pristine and modified SEP. In the FT-IR spectra of the pristine SEP, the adsorption band at 3687 cm-1 corresponded to the stretching vibration of Mg-OH;28 peaks centered at 3563 cm-1 and 3393 cm-1 were ascribed to the O-H vibrations of coordinated water and zeolitic water, respectively;29 the O-H bending peak at 1662 cm-1 was attributed to water molecules in channels;30 the adsorption bands at 1211 and 1017 cm-1 were ascribed to Si-O vibrations of the Si-O-Si;31 the peak at 787 cm-1 was due to O-H bending vibration of Mg-Fe-OH;32 the bands at around 691 and 648 cm-1 were assigned to the Mg-OH bending vibrations;33 and the adsorption band observed at 473 cm-1 was attributed to Si-OSi bending vibration.34 After modification of SEP with OTAB, it was found that the FT-IR spectra of organic modified sepiolite (OSEP) exhibited some changes. Except for the -OH, Mg-OH and Si-O-Si groups, the new absorption peaks at 2930 cm-1 and 2855 cm-1 corresponded to the asymmetric and symmetric stretching of CH2, respectively, and 1468 cm-1 (the bending of CH2) in the spectra of the organic SEP, confirming the presence of OTAB on the surface of SEP. After modification of OSEP with OTS, the peak strength of


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the three above-mentioned peaks was significantly augmented, which indicates that a large amount of octadecyltrichlorosilane was successfully modified to the surface of SEP.

In addition, the chemical components of SEP modified prior to and after were determined by XPS. As presented in Fig. 1b, for both SEP modified before and after, the peaks observed at 103.2 eV ,284.8 eV, 532.4 eV and 1304.7 eV were attributed to Si2p, C1s, O1s and Mg1s, respectively.24 Moreover, compared with the XPS spectra of the original SEP, the intensity of the O1s adsorption peak decreased after modification with OTS, and this was ascribed to the grafting of OTS onto the -OH group of Mg-OH in the superhydrophobic modification process. As a result, the intensity of the C1s adsorption peak in the XPS spectrum was significantly enhanced. In addition, detailed XPS spectra of C1s and O1s of SEP powder prior to and after modification were also obtained (Fig. S1). From the above analysis, it can be concluded that the SEP were successfully modified with OTAB and OTS.

(b)

(a)

O 1s Mg 1s

SEP

3687

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3563

1662

3393

2930 2855

3682

OSEP

3389

787 691 648

1211

784 693 652

1660

3562

1207

1021

473

OSEP

OSEP/OTS 3681

3378

3564

4000

3500

3000

2500

786 691 651

1468 1658 1205

2920 2851

2000

Si 2p

SEP

473

1017 1468

C 1s

1500

1023

473

1000

500

OSEP/OTS 1400

-1

1200 1000

Wavenumber/cm

800

600

400

Binding Energy(eV)

Figure 1. FT-IR (a) and XPS spectra (b) of the SEP modified before and after.


200

0


Page 10 of 29

The preparation process of the as-prepared sponges mainly comprised two parts. Firstly, the natural SEP was modified with OTAB and OTS. Subsequently, the superhydrophobic SEP was loaded onto the skeleton surface of sponge by an ultrasonic dip-coating process. Fig. 2a shows the influence of different loading ratios of modified SEP on the water contact angle of the as-prepared sponges. The following equation is used to calculate the loading ratio: Loading ratio =

𝑚𝐿 ― 𝑚𝑝 𝑚𝑝

× 100%

(2)

where 𝑚𝐿 is the quality of the as-prepared sponges with different superhydrophobic SEP loading ratios; and 𝑚𝑝 is the quality of the pristine sponge. The wettability changes of sponge are related to the mass of the superhydrophobic SEP coating, as presented in Fig. 2a. Moreover, the water contact angle of the SEP-coated sponge increased with the increase of the loading ratio of the superhydrophobic SEP until the water contact angle reached 158°, which present that the modified SEP-coated sponge possessed superhydrophobic properties. Therefore, the ultrasonic dip-coating process was applied to the sponges with an ethanol solution with a mass fraction of 5% of superhydrophobic SEP, and the corresponding loading ratio was 19%. As presented in Fig. 2b and c, the water droplet (dyed with methylene blue) can maintain a spherical shape on the as-prepared sponge and rolled off easily, while the pristine sponge was wetted. Fig. S2 displays a 3-µL water droplet suspended on a syringe being squeezed from a sphere into an ellipsoidal shape on the surface of SEP-coated sponge to further investigate the superhydrophobic property of the as-prepared sponge. As is shown, the water droplets can be unaffected by adhesion force with the as-prepared sponge, and easily and completely departed from the surface. In addition, no residue of the water droplet existed 10 ACS Paragon Plus Environment

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on the sponge’s surface. Furthermore, although crude oil droplets were absorbed by both sponges, the time that it took for the oil droplet to be completely absorbed differed significantly. As presented in Fig. 3a and b, the absorption time of the crude oil droplets (3 μL) added to the as-prepared sponge was only 0.31 s, this time is much shorter than that of the pristine sponge (4.28 s). Moreover, both sponges were placed on the water surface, and it was found that the as-prepared sponge can float on the water, while the pristine sponge absorbed water and sunk below the water surface (Fig. 2d). Fig. 2e shows the appearance changes of the as-prepared PU sponge when it is pressed into the water surface by an external force. Air bubbles formed a silver mirror-like surface on the as-prepared sponge, suggesting that the as-prepared sponge exhibited Cassie-Baxter surfaces.35-36 The asprepared sponge also floated on the surface of water immediately when removing the external force. Subsequently, the sponge was weighed, and no water absorption was observed. The above findings confirmed that the superhydrophobic SEP-coated sponge can quickly absorb oil while completely repelling water.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Water contect angle()


Page 12 of 29

160 140 120 100 80 0

5

10 15 20 Loading ratio(%)

25

Figure 2. (a)The effect of the loading ratio of superhydrophobic SEP on water CAs of the coated PU sponges; Optical images of the wetting behavior of crude oil and water droplets drop onto the pristine(b) and coated (c) PU sponge surfaces;(d) Optical image of the coated PU sponge floating on the water surface and pristine PU sponge immersed into the water; (e)Optical image of the coated PU sponge immersed in a water bath under an external force. (a)

(b) 0s

0.86s

0s

0.07s

4.28s

2.44s

0.31s

0.15s

Figure 3. Video snapshots of a drop of crude oil absorbed by the pristine (a) and coated (b) sponges. 12 ACS Paragon Plus Environment

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The surface morphologies of the original and the as-prepared sponges were studied by FE-SEM. The different magnification images of the pristine and the as-prepared PU sponges was presented in Fig. 4. The original PU sponge (Fig. 4a) possessed a unique 3D structure and porous properties. Moreover, its pore size was between 200μm and 600μm, which was essential for obtaining a high adsorption capacity for the as-prepared sponge. Fig. 4b and c provide higher magnification pictures of the pristine PU sponge, it can be seen that the original PU sponge skeleton surface was smooth and flat. As presented in Fig. 4d, after the pristine PU sponge was coated with superhydrophobic SEP, its original 3D porous structure was retained, which indicates that the structure of the porous framework has not been damaged in the ultrasonic dip-coating process. The surface morphology and chemical composition of the material significantly affect its wetting behavior.37-38 Fig. 4e and f show that the surface of the PU sponge skeleton was completely coated with a layer of densely distributed fibrous SEP, and the surface roughness necessary for superhydrophobic properties was enhanced, resulting in the formation of a micro-nano rough structure. The micro-nano rough structure combined with the low surface energy substance (OTS) on the surface of the SEP was required for the PU sponge to possess superhydrophobic property.



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Figure 4. The FE-SEM images of the unmodified PU sponge (a-c) and the SEP coated PU sponge (d-f) with different magnification. As

anticipated,

the

as-prepared

PU

sponges

with

the

properties

of

superhydrophobicity and 3D porous structure could adsorb multiple oils and nonpolar solvents, such as crude oil, n-hexane, hexadecane, soybean oil, toluene, n-decane, and petroleum ether.38-40 As shown in Fig. 5, by dipping a piece of the as-prepared sponge on the oily wastewater surface, the oil was absorbed quickly by the as-prepared sponge within a few seconds. Due to the presence of a capillary force, the oil was absorbed into the inside of the as-prepared sponges through the open pores, while water was repelled completely by the superhydrophobic surface, and the oils were stored in the pores formed by the interconnected skeleton of the sponge, exhibiting a high oil absorption capacity.2, 41-42

Figure 5. Optical images for the removal of n-hexane from the water surface by a piece of coated sponges. n-hexane was labeled by oil red for clear observation.


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As presented in Fig. 6a, the absorption capacity k for the seven kinds of oils and organic solvents of the SEP-coated sponge ranged from 29 to 68 times the materials’ weight, which depended primarily on the viscosity and density of the oils and organic solvents. In addition, this material has a much higher adsorption capacity for oils and organic solvents than some other materials.1, 4, 15, 43 More interestingly, the oil absorbed in the as-prepared sponge can be easily recovered by a simple mechanical extrusion process, and the recycled sponge can be repeatedly used for oily wastewater treatment with multiple cycles. This requires the composites to have good mechanical stability. As shown in Fig. S3, the as-prepared sponge undergoes a certain deformation when the weight was placed on it. And the sponge was recovered after the weight was removed. It can be seen that the sponge only undergoes a slight irreversible deformation when the above process was repeated 10 times, possessing good mechanical stability. The relationship between the adsorption capacity of two nonpolar solvents (n-hexane, hexadecane) and the number of cycles is shown in Fig. 6b. The as-prepared sponges possessed different adsorption capacities for the two solvents with different viscosities and densities. In addition to the slight decrease in the first two cycles, the adsorption capacity remained basically unchanged in subsequent cycles. The small decrease of oil adsorption capacity of the asprepared sponges may be attributed to slight deformation of the sponge and residual oil adsorbed in pores after the extrusion process.44-45 Furthermore, the superhydrophobic asprepared sponges can easily remove oil from the water surface. The following equation is used to calculate the separation efficiency (E): E=

(

) × 100%

𝐶0 ― 𝐶 𝐶0


(3)


where 𝑐0 represents the oil content of the initial oily wastewater; and c represents the oil content of oily wastewater after treatment with the as-prepared sponges. The separation efficiencies of the superhydrophobic SEP-coated sponges for the four selected immiscible oily wastewater were all above 99.66%, and the separation efficiency remained higher than 99.45% even after 10 separation cycles. These results indicate that the sponge’s recycling process had no significant effect on separation efficiency. However, the surface morphology of the as-prepared sponge changed obviously after 10 cycles, as shown in Fig. 6d. After 10 cycles of treatment of the n-hexane/water mixture, the superhydrophobic SEP on the surface of the sponge skeleton is slightly off. Therefore, the as-prepared sponge lost its superhydrophobicity because the micro-nano rough structure was destroyed, and the water contact angle decreased to 145.6°. But we can immerse the composites in an ethanol dispersion of superhydrophobic modified sepiolite by ultrasonic dip-coating process to restore its original superhydrophobicity, continuing oil-spill clean-up.


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70

Adsorption capacity(g/g)

Petrolrum ether n-decane Toluene Soybean oil Hexadecane n-hexane Crude oil 0

10

100.0

Separation efficiency(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 30 40 50 60 70 Adsorption capacity(g/g) Croud oil Hexadecane

80

Hexadecane

60

n-hexane

50 40 30 20 10 0

1

2

3

4 5 6 7 8 Cycle number

9 10

n-hexane Soybean oil

99.8 99.6 99.4 99.2

1

4 7 Recycle numbers

10

Figure 6. (a)The adsorption capacity of the coated PU sponges for various kinds of organic solvents and oils; (b)The coated PU sponges show a different adsorption capacity for the two types of oil density and viscosity (hexadecane and n-hexane); (c) The separation efficiency of immiscible oil-water mixtures with the coated sponges in ten cycles; (d)SEM images of the coated sponges after oil-water separation for 10 cycles. Insets show optical images of a water droplet placed on the corresponding coated sponges. The superhydrophobic stability of the as-prepared sponges immersed in corrosive liquid and hot water is critical for the treatment of oily wastewater because oil leakage usually occurs in complex acid-base solutions and high temperature environments.46-47 As presented in Fig. 7a, after immersing the as-prepared PU sponges into pH=1, pH=7, pH =14 and 0.1M NaCl aqueous solutions for 24 h, although the contact angle of the asprepared sponge at strong acidic and alkaline pH was less than that at neutral pHs, it was still shown to be greater than 150°. In addition, the as-prepared sponges can also maintain 17 ACS Paragon Plus Environment


good superhydrophobicity at high temperatures. As indicated in Fig. 7b, the WCAs of the SEP-coated sponges remained essentially unchanged with the increase of temperature, and the WCA was still greater than 150° even if the water temperature reaches 80°C. In order to test the adsorption performance of the as-prepared in a corrosive solution and hot water, hexadecane (dyed with oil red O) was mixed with a corrosive solution and hot water (80 °C). As shown in Fig. S4, the hexadecane dispersed under the above conditions was quickly and completely adsorbed by the as-prepared sponge. This indicates that the as-prepared sponge was sufficiently stable to resist corrosive liquid and high temperatures. Indeed, this material was more adaptable to complex environments than other materials.1, 12 (b) 170

160

160

Water contect angle()

(a) 170 Water contect angle()

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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150 140 130 120 110

150 140 130 120 110 100

100 PH=1

PH=7

PH=14

0.1 M NaCl

30

40

50

60

Temperature/°C

70

80

Figure 7. (a)The water CAs of the coated PU sponges after immersion into pH =1, pH = 7, pH = 14 and 0.1 M NaCl aqueous solutions for 24 h.(b) Effect of temperature on the water contact angles of the coated sponges. Adsorption selectivity is another crucial property of adsorbent materials for oily wastewater treatment.48 As presented in Fig. 8, in the oil squeezed from the as-prepared sponge, no visible water droplets to the naked eye. Instead, water droplets can be clearly seen from the oil that was collected from the pristine sponge. The above results presented that the as-prepared sponge possessed high selectivity for oil-water separation.


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(a)

(b) coated sponge

pristine sponge

water droplet

Figure 8. Collection of oils from the coated (a) and pristine (b) sponges by mechanical squeezing. CONCLUSION In this study, a robust superhydrophobic PU sponge was prepared through a facile one-step ultrasonic dip-coating process. The as-prepared sponge possesses the characteristics of superhydrophobicity and superoleophilicity, high absorption capacity, and easy scalable fabrication, which makes it an excellent adsorbent material for the selective collection of oils and nonpolar solvents from the water surface. Besides, the oil absorption capacity of the as-prepared sponge is as high as 29-68 times its own weight, and the superhydrophobic SEP-coated sponge has a high separation efficiency of up to 99.66% when used to separate immiscible oil-water mixtures. Furthermore, the as-prepared sponge could be reused for oil-water separation with high efficiency for 10 times. The superhydrophobic SEP-coated sponge also exhibits excellent superhydrophobicity after immersion in corrosive solutions for 24 h, and its superhydrophobicity was not affected by the ambient temperature range of 30-80 °C. Therefore, the as-prepared sponge in this study might constitute a promising adsorbent material for large-scale marine oil spill treatment.



ASSOCIATED CONTENT Supporting information. Additional information including the fine XPS spectrum of O1s and C1s of SEP modified before and after, water adhesion test of composites, mechanical durability test, experiment on the removal of oil from corrosive solution and hot water by composites.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21677087) and the National Science and Technology Major Project of China (Grant No.2016ZX05040-005)


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TOC Graphic: OH

OH

OH

Mg,Si

OH

1.0CEC(OTAB)

Organic modification

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


OH

Si O N+

OH N+

Synopsis Selectively adsorption of oils and organic solvents from water surface with modified superhydrophobic sepiolite-coated polyurethane sponges that is recyclable and has a high adsorption capacity.


Robust Superhydrophobic Sepiolite-Coated Polyurethane Sponge for

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