Synthesis of a Novel Highly Oleophilic and Highly Hydrophobic

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Synthesis of a Novel Highly Oleophilic and Highly Hydrophobic Sponge for Rapid Oil Spill Clean up Maryam Khosravi, and Saeid Azizian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07504 • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 26, 2015

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ACS Applied Materials & Interfaces

Synthesis of a Novel Highly Oleophilic and Highly Hydrophobic Sponge for Rapid Oil Spill Clean up Maryam. Khosravi† and Saeid. Azizian∗,† †

Department of Physical Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan,

Iran KEYWORDS: oil spill cleanup, hydrophobic sponge, high hydrophobicity, high oleophilicity, continuous separation

ABSTRACT: A highly hydrophobic and highly oleophilic sponge was synthesized by simple vapor-phase deposition followed by polymerization of polypyrrole followed by modification with palmitic acid. The prepared sponge shows high absorption capacity in the field of separation, removal of different oil spills from water surface, and was able to emulsify oil-water mixtures. The sponge can be compressed repeatedly without collapsing. Therefore, absorbed oils can be readily collected by simple mechanical squeezing of the sponge. The prepared hydrophobic sponge can collect oil from water in both static and turbulent conditions. The proposed method is simple and low-cost for the manufacture of highly oleophilic and highly hydrophobic sponges, which can be successfully used for effective oil spill clean-up and water filtration.

ACS Paragon Plus Environment

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1. INTRODUCTION With the development of marine oil exploitation and marine transportation of oils, frequent oilspill and chemical leakage have caused severe ecological problem.1-5 Spilled oil (and related organic contaminants) is a serious threat for the aquatic system and the environment as a whole.6,7 The conventional methods used to remove oil in water sources can be divided into the following categories: collection and separation of oil from water surface; physical absorption by oil-sorption materials; mixing of oil with water using dispersing factors to facilitate natural decay; in situ combustion; oil containment boom; physical diffusion; oil skimmer vessels; enhanced bioremediation; etc. The usage of efficient sorbent materials is considered to be one of the most effective methods for separation of oils and organic solvents from water.8 An ideal absorbent material should have attributes such as excellent selectivity, high absorption capacity, low cost, superior recyclability and be environmentally friendly.9 So far, fabricating sorbents with premier oil sorption performance has remained a major challenge. Recently the collection of oil using hydrophobic and oleophilic materials has attracted considerable interest because of their ability to efficiently separate oils/organic pollutants from the marine ecosystem.10,11 Widely used sorbent materials include: zeolites12; carbon nanotube sponges (CNT sponges)13; poly dimethyl siloxane (PDMS) sponges14; Fe/CNT composites15-17; carbon fiber aerogel18; steel mesh19; and poly dimethyl siloxane (PDMS) coated polyurethane (PU) sponge20; etc. Among them, the polyurethane sponge is a type of porous and hydrophilic polymer that has excellent sorption capacity, low density and easily scalable fabrication processes. However, it usually absorbs both water and organic chemicals, simultaneously, which limits its application for selective oil separation from water with high efficiency. Given the above, it is necessary to change the hydrophilicity of polyurethane sponges such that they become highly hydrophobic and highly


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oleophilic, allowing for continuous absorption and removal of oil contaminants from water with high separation capacity.21 Recently, Zhou et al. fabricated a PPy-PTES sponge by dip coating of polyurethane into an FeCl3 and 1H,1H,2H,2H-Perfluorooctyltriethoxysilane (PTES) solution and then coating with polypyrrole (PPy), and demonstrated its ability to remove an oil spill.22 In the present work, a polyurethane sponge was initially coated with polypyrrole (PPy) and subsequently modified to become highly hydrophobic and highly oleophilic by dip coating in a palmitic acid (PA) solution. As a result of the combination of high hydrophobicity, high porosity and strong mechanical stability, the obtained sponge exhibits full volume absorption capacity, high selectivity and superior recyclability. The oil sorption capacities our prepared sponge in various oil/water mixtures were investigated, and the long-term cycling efficiency was also studied. It was found that the obtained sponges exhibited good oil absorption capacities and could be utilized many times, exhibiting high recyclability. Importantly, the absorbed oil could be easily harvested from the sponge by a simple mechanical squeezing method. The PPy-PA sponge could be used in conjunction with the help of a vacuum pump for the continuous absorption of oil pollutant from water surface. This work provides a simple, facile and low-cost path toward the preparation of highly hydrophobic and highly oleophilic sponges, which have potential application in the large scale collection of spilled oil and other organic pollutants from the surface of seawater. 2. EXPERIMENTAL SECTION Material. The polyurethane (PU) sponge materialswhich was used as a basic material for further modifications, were obtained from a local furniture store. The pyrrole monomer and ferric chloride were purchased from Merck Co. Palmitic acid (PA) was obtained from BDH Chemicals Ltd.


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Sample Preparation. The original PU sponge was cleaned ultrasonically in acetone and distilled water. Different conditions were used to find the optimum highly hydrophobic/highly oleophilic sponge (Table S1). For optimal modification, the sponge was first successively immersed in a homogeneous ethanolic ferric chloride solution (50 g L-1) and withdrawn after 5 min. After drying under ambient conditions, the coated sponges were placed into a closed chamber on the bottom of which was a layer of pyrrole liquid. For the polymerization and deposition of pyrrole on the sponge surface the chamber was maintained at room temperature for 2 h. Following the polymerization reaction, the sponge was immersed into an ethanol solution of 0.1 M palmitic acid (PA) at 30 ᵒC for about 30 min. Afterward, the sponge was rinsed with ethanol several times and dried in an oven at 60 ᵒC for 30 min to obtain the dark highly hydrophobic and highly oleophilic PPy-PA sponges. Characterization. The morphologies of the original and prepared sponges were observed by scanning electron microscopy (SEM) (TSCAN–Czech Republic). FT-IR spectra of samples were recorded using a Tensor 65 spectrometer (Perkin Elmer). Pickering emulsions were observed with a microscope (Leica Microsystems-wetzlar GmbH). Raman spectra of samples were recorded using a SENTERRA (2009) BRUKER (Germany). Testing of Oil Sorption Performance. The sorption performance was evaluated by sorption capacity, sorption rate, micron-sized oil droplets removal and recyclability. The sorption capacity, S, was defined as:

S=

(m s − mi ) mi

(1)


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where ms represents the saturated sponge mass and mi is the initial mass of sponge. The sorption capacity was determined by the following procedure. The sponges were weighed to determine mi, then were placed in beakers containing oils or organic solvent. Due to the oleophilicity, the sponges can be fully immersed into the organic liquids. After 2 min, the sponges were lifted for few seconds allowing the oils or organic solvent on the exterior surface to drip away before recording the final mass ms. Then the sorption capacity can be calculated via equation (1). To investigate the kinetics of sorption, sponge samples were placed onto the surface of the oils or organic solvent, instead of being immersed, recording the mass of the sponge at different time intervals. The sorption capacity at any time, St, was calculated.

St =

(mt − mi ) mi

(2)

where, mt is the mass of sample after sorption at a given time. Micron-sized oil droplets in water (emulsions) were generated by ultrasonically treating the mixture of oil and deionized water at a volume ratio of 1:10, oil:water. The removal of oil droplets from emulsion test was carried out by agitation of the modified sponges in the emulsion for a certain time, during which the milky emulsion became colorless and transparent. Regeneration of PPy-PA Sponges. The oil-absorbed PPy-PA sponge was regenerated by a mechanical squeezing process, and then the PPy-PA sponges were used in the further cycles. The weights of PPy-PA sponges were recorded before and after each cycle to determine the absorption capacity.


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Measurement of pore volume and porosity of original PU sponge and PPy-PA sponge. The original PU sponge and PPy-PA sponge (having a cubic structure, (1×1×1 cm3), were completely immersed in a solvent for approximately 2 min and then removed for measurement. The pore volume and porosity were calculated as follows: (3)

mb = m s − mi vc =

mb

(4)

ρb

Porosity=

vc × 100 vt

(5)

where, mb is the mass of solvent, vc is the pore volume, ρb is the density of solvent and vt is the bulk volume of the sponge.

3. RESULTS AND DISCUSSION The PPy-PA sponges were synthesized by a simple and effective method. After dip coating in the ferric chloride solution and then dried under ambient conditions, the sponge became yellow in color. This was followed by the vapor-phase polymerization of pyrrole onto the ferric chloride containing sponge, after which the sponge became black. The small quantity of Fe3+ introduced into the PU sponge plays a significant role in the establishment of PPy. Fe3+ oxidizes the pyrrole monomer to obtain PPy, during which the Fe3+ ions are reduced to Fe2+ ions.22 Finally, PA attached to the sponge through simple adsorption process. A Schematic internal morphology of the prepared PPy-PA sponge is represented in Figure 1a. SEM images of the original sponge, PPy and PPy-PA coated sponges are also shown in Figure 1. The untreated sponge has porous structure (Figure. 1b) and smooth surface (Figure 1c). After modification with PPy, the smooth


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skeleton of the original sponge (Figure 1c) was covered with a thin coating of PPy and becomes slightly rough as shown in (Figure 1e). PA is adsorbed on the surface of PPy coated sponge, but as shown in (Figure 1g), nanoaggregates of PA with diameter of ca. 20 nm are formed. Comparison of (Figure 1b, 1d and 1f) shows that the mentioned treatments did not change the porous skeleton of original sponge. The FT-IR spectra of the original PU sponge, PPy sponge and PPy-PA sponge are provided in the supporting information (Figure S1). The band observed at approximately 1068 cm−1 (curve b) corresponds to the =C–H, indicating the presence of PPy. The peak at 2862 cm−1 (curves a and c) is due to the CH2 stretching vibration, which can be attributed to PA and polyurethane, respectively. The vibrating absorption peak at 3360 cm-1 (curves a and b) indicates the N–H vibration of polyurethane and pyrrole, respectively. Thus, the results show that the coating layer on the sponge structure was composed of PPy and PA. Figure S2 shows the Raman spectra of original sponge, PPy-sponge and PPy-PA sponge. In the Raman spectra of original sponge, the bands at 740, 900, 1063 and 1470 cm-1 correspond to the aromatic rings, the band at 1420 cm-1 corresponds to the CH2 bending, the 1700 cm-1 band is due to carbonyl groups and that at 1554 cm-1 is due to amide. The Raman spectra of PPy-sponge shows the vibrational band at 946 cm-1 and is assigned to C-H bond. Other peak at approximately 1357 cm-1 is attributed to C-N stretching vibration. The 1578 cm-1 band is assigned to the C=C stretching. The peak observed in the Raman spectra of PA-PPy sponge at 1308 cm-1 can be attributed to CH2 stretching, the 1592 cm-1 band is for O-H stretching vibration and the 1738 cm1

corresponds to C=O bond in acidic group of palmitic acid.

The pore volume and porosity for the original PU sponge and PPy-PA sponge are presented in Table 1.


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For the measurement pore size, the pore structure of the sponge was visualized using a digital microscope. Figure S3 illustrates the pore size distribution of samples. Results indicate that the average pore sizes of the original PU sponge and the PPy-PA sponge are centered at 0.5 and 1 mm, respectively (Table 1). The wettability of the sponges was investigated through contact angle measurements. Images of a water droplet on the surface sponge show a significant change in wettability. The original sponge exhibits surface hydrophilicity with a contact angle of 68ᵒ (Figure 2a). After treatment, the PPy coated sponge become hydrophobic with a contact angle of approximately 93ᵒ (Figure 2b) and the PA modified PPy coated sponge shows significant water repellency with a water contact angle of 140ᵒ (Figure 2c). As well as, Figure S4 (supporting information) shows the contact angles of ethylene glycol (EG), glycerol, poly ethylene glycol 400 (PEG) and water droplets placed on original sponge, PPy sponge and PPy-PA sponge. Figure S3 shows that for the original PU sponge the water contact angle is lower than those for EG, glycerol and PEG 400; confirming that the parent PU sponge is hydrophilic. After the coating with PPy the water contact slightly increases reaching the highest value of 140ᵒ for the PPy-PA sponge, indicating a switch from hydrophilic to highly hydrophobic. This figure also shows that for all of the investigated polar compounds PPy-PA similarly has a higher contact angle, consistent with a change in the material from hydrophilic to hydrophobic. Figure 2d shows a water droplet on a piece of the PPy-PA sponge which was cut. This Figure shows that not only the surface of the sponge, but the whole of the prepared PPy-PA sponge was made hydrophobic. In Figure 2e one can see that the original PU sponge (white) is fully immersed in the beaker without application of external force. However, the PPy-PA modified sponge (black) floated on the water surface, with no significant immersion; again demonstrating


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the change in character of the sponge from hydrophilic to hydrophobic. As depicted in the photographic image (also provided in the supplemental information as a movie, S1) a water droplet exhibits stable spherical shape onto the PPy-PA sponge surface. On the other hand the PPy-PA sponge can easily wetted by oil drops and as shown in this movie the petrol could penetrate into the sponge very rapidly. Based on the selective wettability, the PPy-PA sponge can collect oil micro droplets from surfactant free toluene, chloroform, n-heptane and carbon tetrachloride emulsified in water. A piece of PPy-PA sponge was forced into the emulsion and agitated. The emulsion turned transparent gradually within 2 minutes (Figures 3a and S5). Fig. 3b shows microscope images of the emulsion before and after insertion of sponge. It is clear that no droplets are found in the sponge-absorbed solution indicating the effectiveness of the PPy-PA sponge for separating toluene in water emulsions. Transmittance measurements were made to establish the time course of oil droplet uptake from the emulsion by the PPy-Pa sponge (Figure 3c). The results show that complete toluene removal (100%) from emulsion was within two minutes. The hydrophobic sponge provided a large contact area with the oil droplets, allowing the oil droplets to break wet the oleophilic sponge Therefore the PPy-PA sponge is an ideal absorbent for the removal of micron sized oil droplets from aqueous phases. The driving force for demulsification in the present system is intermolecular forces between highly hydrophobic sponge and non-polar oil-inwater droplets, which is London force.23 By absorption of oil droplet on the hydrophobic sponge, total surface energy of the system decreases, because the oil-water interfacial area decreases.24 In order to investigate the oil–water separation properties of the prepared PPy-PA sponge at the water surface a piece of the sponge was placed in contact with colored n-heptane dispersed on a water surface. After 1 min, no n-heptane could be observed on the water surface (movie S2).


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Similarly, the highly hydrophobic sponge was also applied to quickly absorb colored carbon tetrachloride (which due to its higher density sank to the bottom of the vessel), where the carbon tetrachloride droplet was rapidly absorbed by the sponge (Figure 4a). To study the oil spill cleanup behavior of the PPy-PA sponge, a piece of sponge was placed on a crude oil/water system. It was observed that the cured oil was absorbed within 2 min (Figure 4b).The absorption capacity of the PPy-PA sponge for different oils and organic pollutants was also evaluated and shown (Figure 4c). The PPy-PA sponge could separate large amounts of various kinds of oil in the range of 22-62 times the original weight of the sponge. The absorption capacity depends on the density, viscosity, and surface tension of the oils. Compared with inorganic graphene aerogel18, polydimethyl siloxane sponge14, and PPy-PTES sponges22 the prepared PPY-PA sponge has an excellent absorption capability for different oils and/or solvents. The dynamic sorption process of carbon tetrachloride, n-decane, n-heptane and sunflower oil are shown in Figure 4d. The quantity of sorbed liquid initially increases rapidly and then gradually reaches saturation, indicating that the oil absorption by the prepared PPy-PA sponge is very fast, and was complete within 30 s. The key criteria for oil/chemical cleanup application are the recyclability of the absorbent and the recoverability of oil.25-30 Figure S6 shows the stress-strain curves of PPy-sponge and PPy-PA sponge at a maximum strain of 70%. The PPy-PA sponge is structurally stabile during repeated compression tests. The cubic structure (1.5×1.5×1.5 cm) PPy-sponge and PPy-PA sponge can survive 70% compression and completely recover their original volume after the external force is removed. After 100 cycles of the compression/release cycle the PPy sponge and PPy-PA sponge still maintains their interconnected structure and macroscopic appearance. The compressive durability is immensely favorable for the application of the PPy-PA sponge for oil spill removal.


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The recyclability of the prepared PPy-PA sponge is exhibited in Figures 5a and b. The sorbed oils can be harvested by squeezing the sponge. As shown in Figure 5a the oil squeezes out from the sponge under compression. After compression, the PPy-PA sponge recovered its original form and could be reused for oil/water separation for many cycles (Figure 5b). The oil sorption capacities of the PPy-PA sponge decreases only slightly after 10 cycles, exhibiting good recyclability. The small decrease in oil absorption capacity was likely caused by oil that remained entrained in the pores of the sponges. We found that the PPy-PA sponge could be used in association with a vacuum system (vacuum pressure 30 cm Hg) for continuous absorption and removal of a large amount of organic solvent from water surface. A small piece of PPy-PA sponge was connected to a tube and used as filter that was immersed into mixture of oil (kerosene, labeled with a yellow dye) and water. As indicated, the kerosene could be absorbed within 1 min (repelling the water completely because of its high oleophilicity) shown in Figure 5c. The kerosene was removed completely from the water surface, leaving only clear water in the beaker (movie S3). In addition, no water droplets were observed to the naked eye in the collected filtrate kerosene. A small piece of PPy-PA sponge (2.0 × 2.0 × 3.0 cm3, ca. 0.5 g) could be used for the continuous absorption of at least 13.0 L of kerosene from water surfaces. Under actual environmental conditions, such as in a river orin the sea, the bulk water is always in motion, not stationary as in the last section (Figure 5c). Therefore, in order to emulate the real environmental conditions, this experiment was repeated under high turbulent conditions. To provide this turbulent effect, the kerosene-water system was stirred to induce the establishment of kerosene droplets in water before the vacuum pump was turned on (movie S4). The kerosene droplets were again continuously removed from the water under turbulence conditions as shown in (Figure 5d). The results show that the


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prepared PPy-PA sponge has great potential applicability for the continuous separation of oil from water.

4. CONCLUSIONS The novel prepared hydrophobic PPy-PA sponge has advantages of simple preparation, scalable fabrication, high porosity and flexibility, and showed excellent absorption performance for different types of oil/water mixtures. The sorbed oils can be easily harvested by exerting mechanical pressure; the oil contaminated sponge could be restored to their original form and recycled in oil/water separation many times. The prepared sponge demonstrated easy separation of a large amount of oil from water in the continuous absorption experiments, thus offering great potential for application of this sponge in the large scale removal of oil spills and organic pollutants in the marine and aquatic systems.


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PA monolayer PPy thin film FeCl3 thin film

(a)

Sponge

(c)

(b)

500 µm

200 nm

(e)

(d)

500 µm

(f)

200 nm

(g)

500 µm

200 nm

Figure 1. (a) Schematic internal morphology of prepared PPy-PA sponge. SEM images of the (b,c) original, (d,e) PPy coated and (f,g) PPy-PA coated sponges.


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(b)

(a)

(c)

(d)

(e)

Figure 2. Contact angle of a water droplet on a (a) original sponge, (b) PPy sponge and (c) PPy-

PA sponge. (d) Image of the cross section of the PPy-PA sponge with spherical water droplet (e). Photograph of the original (white) and as-prepared (black) sponges after being placed on water.


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(a)

(b)

(c)

Figure 3. (a) Photographs of toluene-in-water surfactant-free emulsion before and after

absorption with PPy-PA sponge. (b) The optical microscopic images of the emulsion before and after absorption with PPy-PA sponge (×400 magnification). (c) Transmittance of emulsion solution as a function of time.


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(a)

(b)

(c)

(d)


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Figure 4. (a) Photographs of the absorption process of carbon tetrachloride by using the highly

hydrophobic PPy-PA sponge. (b) Optical images of the removal of petroleum from the surface of water by the as-prepared sponge. (c) Absorption capacity of PPy-PA sponge for various organic liquids after 2 min. (d) Sorption kinetics of different oils by PPy-PA sponge.


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(a)

(b)

(c) Dipped sponge

Collected kerosene Transparent H2O

To vacuum Kerosene H2O

(d)

Agitated mixture of kerosene and H2O To vacuum

Transparent H2O Dipped sponge in mixture


Collected kerosene

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Figure 5. (a) The regeneration process of PPy-PA sponge by mechanical extrusion. (b)

Absorption recyclability of the as-prepared sponge for different oils. (c) Digital photographs illustrating the progress of the continuous removal of kerosene from a non-turbulent oil–water system. (d) Digital photographs illustrating the progress of the continuous removal of kerosene from a turbulent oil–water system using a magnetic stirring plate.


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Table 1. Obtained structural properties of the original PU sponge and PPy-PA- sponge. Pore volume Sample

% porosity

Pore size (mm)

(cm3/g) Original PU sponge

18.6

90

0.5

PPy-PA sponge

18.9

88

1


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ASSOCIATED CONTENT Supporting Information.

Optimization of prepared sponge conditions, FT-IR spectra, pore size, contact angles, demulsifying different oils and mechanical properties of sponges. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author

* E-mail address: [email protected] and [email protected] Tel: +988138282807; Fax: +988138380709 ACKNOWLEDGMENT The authors acknowledge the financial support of Bu-Ali Sina University (Grant number: 1276). The authors also acknowledge the assistance of Dr. Shawn Wettig (University of Waterloo) for reading the paper and improving its English. REFERENCES (1) Kwon, G.; Kota, A. K.; Li, Y.; Sohani, A.; Mabry, J. M.; Tuteja, A. On‐Demand Separation of Oil‐Water Mixtures. Adv. Mater. 2012, 24, 3666–3671. (2) Hu, H.; Zhao, Z.; Gogotsi, Y.; Qiu, J. Compressible Carbon Nanotube–Graphene Hybrid Aerogels with Superhydrophobicity and Superoleophilicity for Oil Sorption. Environ. Sci. Technol. Lett. 2014, 1, 214–220.


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(3) Wang, Z.; Wang, D.; Qian, Z.; Guo, J.; Dong, H.; Zhao, N.; Xu, J. Robust Superhydrophobic Bridged Silsesquioxane Aerogels with Tunable Performances and Their Applications ACS Appl. Mater. Interfaces. 2015, 7 (3), 2016–2024. (4) Xue, S. Wang, L. Lin, L. Chen, M. Liu, L. Feng, L. Jiang. A Novel Superhydrophilic and Underwater Superoleophobic Hydrogel-Coated Mesh for Oil/Water Separation. Adv. Mater. 2011, 23, 4270–4273.

(5) Yang, Y.; Deng, Y.; Tong, Z.; Wang, C. Renewable Lignin-Based Xerogels with SelfCleaning Properties and Superhydrophobicity. ACS Sustainable Chem. Eng. 2014, 2 (7), 1729– 1733. (6) Guia, X.; Lic, H.; Wanga, K.; Weia, J.; Jiaa, Y.; Lia, Z., Fana, L.; Caoc, A.; Zhua, H.; Wua, D. Recyclable Carbon Nanotube Sponges for Oil Absorption. Acta Mater. 2011, 59, 4798–4804. (7) Liang, H.W.; Guan, Q. F.; Chen, L.F.; Zhu, Z.; Zhang, W.J.; Yu. S. H. Macroscopic-Scale Template Synthesis of Robust Carbonaceous Nanofiber Hydrogels and Aerogels and Their Applications. Angew. Chem., Int. Ed. 2012, 124, 5191–5195. (8) Wang, F.; Lei, S.; Xue, M.; Ou, J.; Wen, L. In Situ Separation and Collection of Oil from Water Surface via a Novel Superoleophilic and Superhydrophobic Oil Containment Boom. Langmuir. 2014, 30 (5), 1281–1289. (9) Dong, X.; Chen, J.; Ma, Y.; Wang, J.; Chan-Park, M. B.; Liu, X.; Wang, L.; Huang, W.; Chen, P. Superhydrophobic and Superoleophilic Hybrid Foam of Graphene and Carbon Nanotube for Selective Removal of Oils or Organic Solvents from the Surface of Water. Chem. Commun. 2012, 48, 10660-10662.


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Graphical Abstract

Water

PA monolayer

Crude Oil

PPy thin film FeCl3 thin film

Sponge

Water Crude Oil


Synthesis of a Novel Highly Oleophilic and Highly Hydrophobic

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