Facile Removal and Collection of Oils from Water Surfaces through

ARTICLE pubs.acs.org/JPCC

Facile Removal and Collection of Oils from Water Surfaces through Superhydrophobic and Superoleophilic Sponges Qing Zhu, Qinmin Pan,* and Fatang Liu School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, 150001, P. R. China

bS Supporting Information ABSTRACT: The development of a convenient method for the removal (or collection) of oils and organic solvents from water surface is of great significance for water environmental protection, especially for the cleanup of oil spillage on seawater. A major challenge is the fabrication of an oil absorber with high absorption capacity, low cost, scalable fabrication, high selectivity, and excellent recyclability. In this paper, we present a simple method for the removal and collection of oils and organic solvents from the surfaces of water based on superhydrophobic and superoleophilic sponges that were fabricated by solution-immersion processes. The as-prepared sponges fast and selectively absorbed various kinds of oils up to above 13 times the sponges’ weight while completely repelling water through a combination of porous, superhydrophobic, and superoleophilic properties. More interesting, the absorbed oils were readily collected by a simple mechanical squeezing process, and the recovered sponges could be reused in oilwater separation for many cycles while still keeping high separation efficiency. The findings presented in this study might provide a fast and simple approach for the cleanup of oils and organic solvents on water surfaces.

’ INTRODUCTION Because of the frequent occurrence of water pollution caused by oil spillage and chemical leakage, removal or collection of the organic pollutants from water surfaces has attracted worldwide attention.14 The conventional methods used to solve these problems include combustion, oil containment booms, oil skimmer vessels, and absorbent materials.5 Owing to the possibility of removal and collection of oil, absorbent materials including zeolites,7,8 activated carbon,8 organoclays,6,7 straw,5,7 hair,8 wool fibers,810 etc., are considered a most desirable choice for the cleanup of oil spillage.510 Although widely applied in practical applications, these absorbent materials still have limitations such as environmental incompatibility, low absorption capacity, poor recyclability, and so on. In particular, these materials absorb not only oils but also water, which reduces the separation selectivity and efficiency.11 Therefore, novel absorbent materials combined with high absorption capacity, high selectivity and efficiency, low cost, excellent recyclability, and environmental friendliness are important for the development of advanced oilwater separation technology. Recently, surfaces with superhydrophobic and superoleophilic properties have attracted considerable interest in the field of oil water separation because they only absorb oil while repelling water completely, exhibiting high oilwater separation efficiency and selectivity.1229 Several materials based on this principle, including MnO2 nanowire membrane,15 mesh films,1619 filter paper,20 gelators derived from sugar,21 carbon nanotube sponges,22,23 conjugated polymer,2430 etc., have been used for separating oil from water. Nevertheless, these materials have limitations for practical applications because of high r 2011 American Chemical Society

cost, complex preparation processes, and difficulty in scalable fabrication. A polyurethane sponge is a kind of porous and hydrophilic polymer that has characteristics of high absorption ability, low density, good elasticity, and easily scalable fabrication. However, it usually absorbs both water and oils (or organic solvents), which makes it impractical for selective removal of oils from water with high efficiency. Therefore, it is essential to change the hydrophilicity of sponges to superhydrophobicity and superoleophilicity for practical application. To the best of our knowledge, few works have reported the sponges with superhydrophobic and superoleophilic properties for oilwater separation. Here, we present a simple method to fabricate high oil-absorption ability materials based on superhydrophobic and superoleophilic polyurethane sponges. The sponges were initially coated with a film of copper via electroless deposition and subsequently modified with superhydrophobic and superoleophilic coatings through a simple solution-immersion step. The as-prepared sponges selectively absorbed various kinds of oils up to above 13 times the materials’ weight while repelling water completely. More interesting, the removal of the absorbed oils was readily achieved by a simple mechanical squeezing process (Scheme 1). The sponges also showed good stability in corrosive solutions and excellent recyclability in oilwater separation. The findings of this study offer a facile method for the cleanup of oil spillage and chemical leakage on the water surface. Received: May 9, 2011 Revised: July 27, 2011 Published: August 01, 2011 17464

dx.doi.org/10.1021/jp2043027 | J. Phys. Chem. C 2011, 115, 17464–17470

The Journal of Physical Chemistry C

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Scheme 1. Illustration for the Removal and Collection of Oils from the Water Surface by Superhydrophobic and Superoleophilic Sponges

Scheme 2. Preparation Procedure of Superhydrophobic Sponges

Figure 1. (a) Optical image of the as-prepared sponges. (b) A piece of sponge deformed by tweezers. (c) Three-dimensional configuration of the sponges.

’ EXPERIMENTAL SECTION In a typical experiment, original polyurethane sponges were ultrasonically cleaned in acetone and distilled water for 3 h successively. Then, the sponges were etched, sensitized, and activated successively for pretreatment at ambient temperature. The obtained sponges were immersed in etching solution containing CrO3 (100 g L1) and H2SO4 (98 wt %) (100 g L1) for 1 min. After being washed with distilled water, the sponges were immersed in sensitizing solution containing SnCl2 (40 g L1) and HCl (37 wt %) (100 mL L1) for 20 min and activation solution containing AgNO3 (8 g L1) and NH3 3 H2O (25 wt %) (10 mL L1) for 10 min. After being washed with distilled water, the sponges were placed in a plating cell for electroless deposition of copper film for 5 min at 50 °C. The constituents of the plating solution are composed of CuSO4 3 5H2O (20 g L1), ethylenediamine tetraacetic disodium salt (EDTA 3 2Na) (20 g L1), C4H4O6KNa 3 4 H2O (14 g L1), and HCHO (15 mL L1). The pH value of the plating solution was adjusted to 1213 by NaOH. After being

washed with distilled water and dried in argon, the sponges coated with copper films were immersed in an ethanol solution of AgNO3 (5 mM) and n-dodecanoic acid (10 mM) for 30 s to obtain superhydrophobic surfaces. The preparation procedure of superhydrophobic sponges was illustrated in Scheme 2. The reactions involved in the above processes can be described as follows 2Agþ þ Sn2þ f 2Ag þ Sn4þ

ð1Þ

Cu2þ þ 2HCHO þ 4OH f Cu þ 2HCOO þ 2H2 O þ H2 v

ð2Þ

Cu þ 2Agþ f 2Ag þ Cu2þ

ð3Þ

Ag þ n-C11 H23 COOH f C11 H23 COOAg

ð4Þ

Contact angles (CA) were measured at ambient temperature (OCA20, Dataphysics Instruments GmbH, Filderstadt). 17465

dx.doi.org/10.1021/jp2043027 |J. Phys. Chem. C 2011, 115, 17464–17470

The Journal of Physical Chemistry C The morphologies of the original and as-prepared sponges were observed by a scanning electron microscope (SEM, FEI Quanta 200). The three-dimensional configuration of the sponges was recorded by Infinite Focus (Alicona). X-ray photoelectron spectroscopy (XPS) was recorded by PHI5700. The removal of oils from the water surface was carried out by dipping the as-prepared sponges into oilwater mixtures.

Figure 2. Low-magnification SEM images of the sponges before (a) and after (b) coating with copper film. High-magnification SEM images of the as-prepared sponges (c, d): (a) 48, (b) 73, (c) 2000, (d) 40 000.

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The four kinds of oils used in this study were N100 lubricating oil, octane, decane, and dodecane. The oil-absorption ability k of the sponges was calculated by weight measurements according to our previous report at room temperature.27 Then the absorbed oils were collected from the sponges by a simple mechanical squeezing process. Water content in the collected oils was measured by a Fourier transform infrared spectrometer (FT-IR, Spectrum One, Perkin-Elmer).

’ RESULTS AND DISCUSSION After being blow dried, the as-prepared sponges displayed a dark brown and porous appearance (Figure 1a). The soft and elastic sponges could be deformed readily by an external force, e.g, mechanical squeezing (Figure 1b). The three-dimensional configuration showed that the as-prepared sponges had a porous and interconnected framework, and the pore sizes of the sponges were in the range of 200 and 450 μm (Figure 1c). It is expected that the porous and interconnected framework provides a huge space for oil storage. The morphologies of the sponges before and after modification with copper films and superhydrophobic coatings were studied by SEM measurements (Figure 2). It was shown that the smooth skeletons of the original sponges were covered with copper crystalline after electroless deposition (Figure 2a, b). For the superhydrophobic sponges, the surfaces of the skeleton were coated with nanoparticles of copper-C11H23COOAg. The grain size of the nanoparticles varied from 100 to 200 nm, and the nanoparticles agglomerated into larger particles to form hierarchical

Figure 3. Survey spectrum (a) and C 1s (b), O 1s (c), and Ag 3d (d) spectra of C11H23COOAg on the surfaces of the as-prepared sponges. 17466



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Figure 4. (a) Optical image of a water droplet on the surface of the as-prepared sponge. (b) Video snapshots of a drop of N100 lubricating oil absorbed by the as-prepared sponges.

Figure 5. Pictures of a piece of the as-prepared (a) and original (b) sponge placed on a water bath. Water was labeled by methyl violet for clear observation. (c) and (d) The pictures of the as-prepared sponges immersed in the water bath by an external force.

Figure 6. (a) Optical image of a water droplet on the superhydrophobic sponges after floating on 0.1 M NaCl solution for 19 h. (b) Contact angles of the as-prepared sponges after floating on aqueous solutions with different pH values for 19 h.

structures that are necessary for superhydrophobicity.31 After immersion in an ethanol solution of AgNO3 and n-dodecanoic acid, the presence of C11H23COOAg was confirmed by XPS, as shown in Figure 3. In C 1s and O 1s spectra, the peaks at 284.6 and 531.4 eV were ascribed to CC and adsorbed oxygen, and those at 286.6 and 532 eV were attributed to CO; the signals at 288 and 532.9 eV were related to CdO, and that at 285.6 eV was

assigned to CCOOH, respectively. The Ag 3d peaks at 373.7, 374.2 and 367.7, 368.2 eV were consistent with Ag and univalent Ag(I). Contact angle measurements revealed that the as-prepared sponges displayed superhydrophobic and superoleophilic properties, as shown in Figure 4. The apparent water contact angle of the as-prepared sponges was higher than 170°. The time for 17467



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Figure 7. Optical images for the removal of lubricating oil from the water surface by the as-prepared sponges. Oil was labeled by oil red 24 dye for clear observation.

Figure 8. Oil-absorption capacities k of the as-prepared sponges for the four kinds of oils after different oilwater separation cycles.

absorbing a drop of lubricating oil (6 μL) added to the sponges was only 1.76 s, which was much shorter than that of the original sponges (more than 8 s), confirming that the as-prepared sponges absorbed oil quickly but repelled water completely. The original sponges sank beneath the surface of water, while the as-prepared sponges floated on the water surface when in contact with water. The as-prepared sponges appeared as silver mirrorlike surfaces when they were partly or totally immersed in water by an external force (Figure 5), which is the CassieBaxter nonwetting behavior owing to a continuous air layer between the superhydrophobic surface and water.32 They immediately floated on the water surface after release of the external force, and no water uptake was observed by subsequently weighing the sponges. These results demonstrate that the as-prepared sponges can be used on water surfaces. Moreover, the as-prepared sponges displayed a stable superhydrophobicity even floating on 0.1 M NaCl solution and aqueous solutions with pH values ranging from 1 to 13 for 19 h, respectively, as shown in Figure 6. A decrease in water contact angle was observed only for the solutions pH < 3.

Figure 9. FT-IR spectra of the four kinds of oils collected from the asprepared sponges after the ninth oilwater separation cycle.

As expected, the superhydrophobic and superoleophilic sponges separated oils from the water surface easily. By dipping the sponges into a mixture of water and oils, the oils were quickly absorbed by the sponges in a few seconds. Then, the oils were completely removed from the mixture when pulling the sponge out of the water surface, as illustrated in Figure 7 (see Supporting Information, Movie 1). The oil-absorption capacities k of the asprepared sponges for the four kinds of oils were all up to above 13 times the materials’ weight, as shown in Figure 8. More interesting, the absorbed oils in the sponges were readily collected by a simple squeezing process (see Supporting Information, Movie 2). The recovered sponges could be reused for oilwater separation for many cycles. The oil-absorption capacities k of the sponges for four kinds of oils all kept above 6 even after 9 cycles of oilwater separation, exhibiting good recyclability. The decreases of oilabsorption capacity were caused by the residual oils in the pores of the sponges. Then the oils after the ninth oilwater separation cycle were collected, and water content in the collected oils was investigated by FT-IR. As shown in Figure 9, all absorption 17468



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Figure 10. Collection of oils from the as-prepared (a) and original (b) sponges by mechanical squeezing.

bands at 28502960 cm1, ∼1460 cm1, ∼1377 cm1, and ∼720 cm1 were ascribed to the characteristic vibrations of alkanes, while no band assigned to water was observed, confirming the high purity of the collected oils. Indeed, no water droplet could be seen by the naked eye in the collected oils. In contrast, water droplets were clearly observed in the oils collected from the original sponges, as shown in Figure 10. These results indicate the high selectivity and efficiency of the superhydrophobic sponges for oilwater separation. Although the oil-absorption capacity of these sponges was lower compared to the carbon nanotube sponges,22 they still had the advantages of low cost, scalable fabrication, excellent recyclability, high separation efficiency, and selectivity. We believe that an elaborate design on the micro- and nanoscale structures of the sponges might further improve the oil-absorption capacity.

’ CONCLUSIONS In this study, we reported the facile removal and collection of oils (or organic solvents) from water surfaces via superhydrophobic and superoleophilic sponges. These novel sponges, which have advantages of low cost, ready availability, easily scalable fabrication, low density, porosity, and elasticity, were prepared through simple solution-immersion processes. The oils are quickly and selectively removed from the water surface by dipping the asprepared sponges in oilwater mixtures. The oil-absorption capacity of the sponges is up to above 13 times their weight. More interesting, the absorbed oils are collected readily by a simple mechanical squeezing process, while no water exists in the recycled oils. The recovered sponges can be reused in oilwater separation for many cycles. The excellent absorption characteristics of the asprepared sponges lie in the combination of superhydrophobic, superoleophilic, porous, and elastic properties. The as-prepared sponges also exhibited excellent superhydrophobicity even after floating on corrosive media for 19 h. Therefore, this kind of sponge might be a promising substitute for the conventional absorbent materials used in the large-scale removal of oil spills or organic solvents from water surfaces. ’ ASSOCIATED CONTENT

bS

Supporting Information. Movie 1. Removal of lubricating oil from the water surface by the superhydrophobic sponge. The lubricating oil was labeled by oil red 24 dye for clear

observation. Movie 2. Collection of oils from the superhydrophobic sponge by mechanical squeezing. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: 86-451-86413721. Fax: 86451-86418616.

’ ACKNOWLEDGMENT This work was financially supported by the Natural Science Foundation of China (NSFC, Grant No. 50803013) and selfplanned task of state key laboratory of robotics and system (No. SKLRS200901C), Harbin Institute of Technology. The authors express their gratitude to professor Yang Gan for the measurement of water contact angles. ’ REFERENCES (1) Chen, Y. C.; Li, C. F.; Xu, Q.; Garcia-Pineda, Oscar; Baltazar Andersen, Ole; Pichel, W. G. Mar. Pollut. Bull. 2011, 62, 350–363. (2) Schaum, J.; Cohen, M.; Perry, S.; Artz, R.; Draxler, R.; Frithsen, J. B.; Heist, D.; Lorber, M.; Phillips, L. Environ. Sci. Technol. 2010, 44, 9383–9389. (3) Dalton, T.; Jin, D. Mar. Pollut. Bull. 2010, 60, 1939–1945. (4) Aurell, J.; Gullet, B. K. Environ. Sci. Technol. 2010, 44, 9431–9437. (5) Chol, H. M.; Cloud, R. M. Environ. Sci. Technol. 1992, 26, 772–776. (6) Adebajo, M. O.; Frost, R. L.; Kloprogge, J. T.; Carmody, O.; Kokot, S. J. Porous Mater. 2003, 10, 159–170. (7) Radetic, M. M.; Jocic, D. M; Jovancic, P. M.; Petrovic, Z. L.; Thomas, H. F. Environ. Sci. Technol. 2003, 37, 1008–1012. (8) Bayat, A.; Aghamiri, S. F.; Moheb, A.; Vakili-Nezhaad, G. R. Chem. Eng. Technol. 2005, 28, 1525–1528. (9) Annunciado, T. R.; Sydenstricker, T. H. D.; Amico, S. C. Mar. Pollut. Bull. 2005, 50, 1340–1346. (10) Teas, Ch.; Kalligeros, S.; Zanikos, F.; Stournas, S.; Lois, E.; Anastopoulos, G. Desalination 2001, 140, 259–264. (11) Ceylan, D.; Dogu, S.; Karacik, B.; Yakan, S. D.; Okay, O. S.; Okay, O. Environ. Sci. Technol. 2009, 43, 3846–3852. (12) Guo, Z.; Liu, W. Plant Sci. 2007, 172, 1103–1112. (13) Burton, Z.; Bhushan, B. Ultramicroscopy 2006, 106, 709–719. (14) Yao, X.; Song, Y. L.; Jiang, L. Adv. Mater. 2011, 23, 719–734. (15) Yuan, J. K.; Liu, X. G.; Akbulut, O.; Hu, J. Q.; Suib, S. L.; Kong, J.; Stellacci, F. Nature Nanotechnol. 2008, 3, 332–336. 17469



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Facile Removal and Collection of Oils from Water Surfaces through

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