Hollow Carbon Fibers Derived from Natural Cotton as Effective

Dec 4, 2013 - Carbonized cotton fibers (CCFs) with a hollow tubular structure were ...... Wei , Q. F.; Mather , R. R.; Fotheringham , A. F.; Yang , R...
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Hollow Carbon Fibers Derived from Natural Cotton as Effective Sorbents for Oil Spill Cleanup Bin Wang,*,† Rengasamy Karthikeyan,† Xiao-Ying Lu,† Jin Xuan,†,‡ and Michael K. H. Leung*,† †

Ability R&D Energy Research Center, School of Energy and Environment, City University of Hong Kong, Hong Kong, China State-Key Laboratory of Chemical Engineering, School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai, China

‡

S Supporting Information *

ABSTRACT: Because of increasing numbers of oil spill accidents, considerable attention has been paid to the development of effective and inexpensive oil sorbents. Carbonized cotton fibers (CCFs) with a hollow tubular structure were successfully prepared by treating natural cotton in a N2 atmosphere and used as high-capacity oil sorbents. The material properties of the asprepared CCFs were investigated by scanning electron microscopy, X-ray diffraction, contact-angle measurements, and N2 adsorption−desorption. Maximum oil sorption tests indicated that CCFs-400 showed the highest oil adsorption capacity and could absorb up to 32−77 times its own weight in pure oils and organic solvents, suggesting an increase of 27−126% compared with the capacity of cotton fibers. Also, repeatability, selectivity, and floating-ability tests suggested that CCFs-400 showed much better performance than cotton fibers in pure oil medium or water−oil mixtures. Owing to their multiscale porous structures, superhydrophobicity, and superoleophilicity, the CCFs demonstrated great potential as low-cost and effective sorbents in oil adsorption.

1. INTRODUCTION The effective removal of organic pollutants from contaminated water is of great importance because of frequent oil spill accidents occurring during extraction, transportation, and storage. There are many methods for treating oil spills, including mechanical,1 biological,2 and photochemical3 strategies. Mechanical collection with skimmers followed by separation of the water and oil phases is widely used in practical oil removal from water. However, relatively calm weather conditions and a reasonable thickness of the floating oil (e.g., a few millimeters or more) are required for skimmer technology.4 Recently, adsorption has become regarded as one of the most promising techniques for oil spill cleanup.5 So far, oil-sorbent materials can generally be classified into three major types, namely, inorganic, synthetic organic, and natural materials.5 Inorganic sorbents, such as Fe/C nanocomposites,6 carbon nanotubes,7 and graphene,8,9 have been investigated in recent studies for oil sorption applications. In addition, synthetic organic products, such as polypropylene,10,11 polyurethane,12 polystyrene,13 and polydimethylsiloxane,14 are the most commonly used commercial sorbents in oil spill cleanups because of their oleophilic and hydrophobic characteristics. However, the high cost and tedious fabrication procedures of these materials are the main obstacles to their practical application. Recently, much attention has been paid to the use of renewable resources, such as agricultural wastes or natural products, to produce effective oil sorbents. Natural oil-sorbent materials, such as straw, corn cob, wood fiber, cotton fiber, and kapok fiber, have been examined for the removal of oil from water.15−18 In addition, thermally treated rice husks were form to be an efficient sorbent for oil adsorption.19,20 Compared to inorganic and synthetic organic products, natural products are © 2013 American Chemical Society

abundant, renewable, biodegradable, and low in cost, suitable for large-scale production and practical applications. Although natural sorbents are more economical than most synthetic sorbents, one of the major drawbacks for these materials is their relatively low oil sorption capacity. So far, this shortcoming has been improved by surface modifications and pore-structure adjustments. On one hand, the surface properties of cellulosic materials can be modified by being coated with a resin or treated by a reaction between hydroxyl groups and fatty reagents.16 Hydrophobic surface properties can increase the oil sorption capacity. Recently, Payne et al. treated woodderived cellulosic fibers with the hydrophobic sizing agent alkenyl succinic anhydride to achieve a superior ability to sorb oil.21 However, the chemical treatment of natural materials can cause environmental pollution and increase the cost of the oil sorbents. On the other hand, natural materials with hollow structures have exhibited superior oil sorption capacities. Lim and Huang reported that the sorption capacity of kapok (Ceiba pentandra) was significantly higher than that of polypropylene, which was attributed to its waxy surface and hollow structure.22 To solve the above-mentioned problems with treating natural products to obtain high-capacity sorbents, we propose that directly converting natural cellulosic fibers with a hollow structure into carbon materials might be a promising approach to the preparation of high-capacity oil sorbents. This is because, after the carbonization process, the material surface becomes more hydrophobic as a result of the effective elimination of hydrophilic groups in the cellulose surface, without damaging Received: Revised: Accepted: Published: 18251

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2.3. Oil Types and Property Measurements. Peanut oil (Imperial-Banquet, 100%), ethylene glycol (Sigma-Aldrich, >99%), toluene (Sigma-Aldrich, >99.7%), tert-butanol (SigmaAldrich, >99%), ethanol (Sigma-Aldrich, >99.8%), silicon oil (Accuchem, >99%), vacuum pump oil (Edwards, >99%), and crude oil (Liaohe Oilfield, northeast China, >99%) were used as simulated oils for oil-sorption-capacity studies. Oil densities were measured at room temperature using a portable density meter (DMA 35, Anton Paar). The physical properties of the investigated oils and organic solvents are listed in Table S1 (Supporting Information). 2.4. Maximum Oil Sorption Capacity and Oil Sorption Rate. The maximum oil sorption capacities of dry CCF samples were evaluated at room temperature in pure oil medium without any water. In a typical measurement, a dry CCF sample of 0.2 g was placed on top of 100 mL of the above-mentioned oil samples in a glass bottle without any stirring. After 60 min of sorption, the oil was drained for 2 min, and the wet sample was weighed. The maximum oil sorption capacity of the sorbent was calculated as

the hollow structure. In addition, for practical applications, the fiber structure is easier for recycling than powdered sorbents (e.g., activated carbon). In this article, we report, for the first time, the successful preparation of carbon fibers with a hollow tubular structure by pyrolysis of natural cotton fibers in a nitrogen atmosphere. The hollow tubular structure was confirmed by field-emission scanning electron microscopy. The hydrophobic and oleophilic nature of the surface was confirmed by contact-angle measurements. Studies also indicated that the oil sorption capacities of the carbonized cotton fibers (CCFs) were much higher than that of natural cotton fibers, as a result of the combination of the multiscale porous structures (macropores and micropores), superhydrophobicity, and superoleophilicity of the obtained sorbents. Therefore, hollow carbon fibers derived from natural cotton are promising low-cost alternatives to conventional sorbents for use in the large-scale removal of oil spills or organic solvents from water surfaces.

2. EXPERIMENTAL SECTION 2.1. Fabrication of Hollow Carbon Fibers. In this study, a series of carbonized cotton fibers were prepared by the pyrolysis of cotton balls (100%, Banitore) in a nitrogen atmosphere at 100−1000 °C for 1 h at a heating rate of 5 °C/ min. The CCFs prepared at 100, 200, 300, 400, 600, 800, and 1000 °C are denoted as CCFs-100, CCFs-200, CCFs-300, CCFs-400, CCFs-600, CCFs-800, and CCFs-1000, respectively. In addition, CCFs-400 samples were also fabricated with different durations of pyrolysis (0.5, 2.0, and 4.0 h) at 400 °C to investigate the effect of pyrolysis time on the maximum oil sorption capacity. The as-prepared CCFs were used directly as sorbents for material characterization and oil-sorption-capacity studies without any further treatment. 2.2. Material Characterization. The morphologies of the CCFs were characterized by field-emission scanning electron microscopy (FE-SEM, Hitachi S4800). To directly measure the macropore size using the FE-SEM (Hitachi S3400) technique, the CCFs and cotton fibers were embedded in epoxy adhesive, and the epoxy was allowed to solidify. The solidified samples were fractured with pliers under liquid nitrogen to obtain smooth surfaces. Prior to FE-SEM characterization, the fractured surfaces were vacuum-coated with a thin layer of Pd−Au (1:1) using a sputter coater (BAL-TEC SCD 005). The crystal phases of the CCFs and cotton fibers were measured by X-ray diffraction (XRD) on a Bruker instrument equipped with Cu Kα12 X-radiation and a LynxEye detector. Differential thermogravimetric (DTG) analysis of the cotton fibers was carried out on an EXSTAR TG/DTA 6300 instrument (SII Nanotechnology, Tokyo, Japan) under a nitrogen atmosphere from room temperature to 1000 °C at a heating rate of 5 °C/ min. Elemental analysis of CCFs-400 was performed by X-ray photoelectron spectroscopy (XPS, model PHI5600). Contactangle measurements of dry CCFs and cotton fibers were made on a contact-angle meter (DM-CE1, Apex Instruments Co.) using drops of water and oil. The surface areas and pore size distributions of the CCFs were recorded on a Micrometics ASAP 2020 instrument using the N2 adsorption−desorption isotherm. The samples were first degassed at 350 °C in a N2 atmosphere for 4 h, and the measurement was performed at liquid-nitrogen temperature (−196 °C). The surface area and pore size distribution of the CCFs were calculated by the Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods, respectively.

q = (mf − m0)/m0

(1)

where q is the maximum sorption capacity (goil/gsorbent), mf is the final weight of the wet sample after 2 min of drainage (g), and m0 is the initial weight of the sorbent sample (here, m0 = 0.2 g). For comparison, natural cotton fibers (100%, Banitore) were used as a reference material in the oil sorption experiments. Dry CCFs-400 samples prepared with pyrolysis times of 0.5, 2, and 4 h were also investigated using peanut oil. All of the oil sorption measurements were performed in triplicate. Also, the rates of peanut-oil sorption of CCF samples and cotton fibers were compared in the time interval of 1−5 min. 2.5. Oil Retention Capacity. The oil retention capacity was investigated to evaluate the performance of dry CCF samples as sorbents for oil spill cleanup applications. Wet CCF samples were removed from the oil (peanut oil and ethylene glycol), and the oil was drained for 10 min. The dynamic oil retention capacity was calculated as η = (mf − m0 − mi)/(mf − m0)

(2)

where η is the percentage dynamic oil retention capacity and mi is the weight of oil drained from the sorbent at time i, in seconds (i = 20, 40, 60, 90, 120, ...). 2.6. Repeatability of CCFs. Repeatability tests using dry CCFs-400 and cotton fiber samples were compared in pure peanut-oil medium. Sorbent samples of 0.2 g were placed on 100 mL of peanut oil for 60 min. After the sorption was finished, the oil sample was squeezed from the sorbent using a 20 mL syringe, and then the weight was measured. The adsorption−desorption process was repeated for five cycles to evaluate the reusability of CCFs-400 and cotton fibers. 2.7. Selectivity in Oil Sorption. The practical use of sorbents for oil recovery might take place in the presence of water. In this work, CCFs-400 and cotton fibers were compared in terms of the selectivity of oil sorption from water−oil systems. First, 300−400 mL of water was poured into a 400 mL glass beaker, and then different amounts of peanut oil were poured on the water. Oil films with thicknesses of 2, 4, 6, 8, 10, and 12 mm were studied for oil/water sorption by dry sorbents. The dry sorbents were slowly placed on the oil surface for oil/ water sorption. After 60 min of sorption, the liquid was squeezed from the sorbents using a syringe. The amounts of 18252

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Figure 1. FE-SEM images (35× magnification) of CCFs synthesized by heat treatment in a nitrogen atmosphere at (a) 400, (b) 600, (c) 800, and (d) 1000 °C and of (e) cotton fibers. (f) XRD patterns of CCFs.

Figure 2. Cross-sectional FE-SEM images of cotton fibers and CCFs with hollow structures: (a,b) CCFs-400, (c) CCFs-600, (d) CCFs-800, (e) CCFs-1000, (f) cotton fibers.

2.8. Floating Ability. The ability of the sorbent to float after oil sorption was also experimentally investigated. For this

adsorbed oil and water were estimated on the basis of the liquid volumes and densities. 18253

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this unique hollow structure resulted directly from the natural cotton fibers. Similarly, the hollow structure was also observed in CCFs-600, CCFs-800, CCFs-1000, and cotton fibers, as shown in Figure 2c−f. Based on the FE-SEM image in Figure 2b, the macropore size and wall thickness of CCFs-400 were estimated to be 6.6 μm and 220 nm, respectively. Therefore, the porosity of the as-prepared carbon fibers was about 93%. A hollow tubular structure has also been found for other natural products such as kapok, and the external diameters of kapok from Malaysian and Thailand are 21.5 and 16.5 μm, respectively.22,27 It has also been found that white cotton can be completely converted into black carbon material. The XPS analysis in Figure S5 (Supporting Information) suggests that 91.4 wt % carbon and 8.6 wt % oxygen were present in the CCFs-400 sample. It should be noted that the major component of natural cotton fibers is cellulose.28 Previous work suggested that the main gaseous products from the pyrolysis of cellulose include CO2, CO, CH4, C2H4, C2H6, H2, and trace amounts of larger gaseous organics and water vapor.29 These gaseous products from the pyrolysis of natural cotton can also be used as fuel for energy supply. The macropores and high porosity of carbon fibers are beneficial for oil sorption applications. 3.2. DTG Analysis of Cotton Fibers. The thermal properties of cotton fibers pyrolyzed in a nitrogen atmosphere were investigated by differential thermogravimetric analysis at a heating rate of 5 °C/min. Figure 3 presents the DTG curve of

test, 30 mL of crude oil was poured into a 400 mL beaker containing 350 mL of water. A dry 0.4-g sample of as-prepared CCFs-400 was gently placed on top for the collection of crude oil from the water surface.

3. RESULTS AND DISCUSSION 3.1. Morphology and Crystal Phase Characterization. Figure 1 shows FE-SEM images of cotton fibers and CCFs derived from cotton by pyrolysis at different temperatures (400−1000 °C). As shown in Figure 1, the microstructures of CCFs-400, CCFs-600, CCFs-800, and CCFs-1000 exhibited fibrous structures, and the lengths of these carbon fibers were in the range of 4.6−10.9 mm (Figure S1, Supporting Information). The fibrous structure of the cotton used in this work was also observed (Figure 1e). The length of cotton fibers was about 27.4 ± 4 mm, comparable with the values reported in a previous study (ca. 12−95 mm).16 The reduction in the lengths of the fibers from the cotton to the CCFs was due to the pyrolysis process. In addition, with the increase of the pyrolysis temperature from 400 to 1000 °C, the lengths of the CCFs decreased further from 10.9 ± 3.1 to 4.6 ± 1.1 mm (Figure S1, Supporting Information). Figure 1f displays the XRD patterns of the as-prepared CCFs obtained at different temperatures. The diffraction peak at 22° corresponds to the (002) reflection of amorphous graphitic carbon, in good agreement with the XRD pattern of porous carbon derived from celtuce leaves.23 With the increase of the treatment temperature, a strong diffraction peak at 44° was observed for CCFs-800 and CCFs1000. This suggests that the graphitization degree of the carbon fibers increased upon heat treatment at higher temperatures. The XRD pattern of pristine cotton fibers (Figure S2, Supporting Information) was quite different from those of the CCFs. The strongest peak at 23° is due to the cellulose (002) crystalline plane.24 The sharp peaks at 39°, 45°, 65°, and 78° are consistent with cellulose fibers, indicating high degree of crystallinity in the structure of the cotton fibers.25 Previous work suggested the crystallinity of biomass can be affected by the contents of wax (high-molecular-mass hydrocarbons and fatty components) and cellulose and the complex nature of bonding among cellulose, hemicellulose, and lignin.26 The XRD measurements further confirmed that the as-prepared products were carbon materials. The enlarged FE-SEM images in Figure S3 (Supporting Information) show the typical hollow structures of the CCFs. To accurately measure the macropore sizes of the CCFs and cotton fibers, these hollow-structured samples were embedded in epoxy adhesive for FE-SEM characterization. Figure 2 displays cross-sectional FE-SEM images of CCFs and cotton fibers. The top and bottom layers were silicon wafers used in sample preparation. As shown in Figure 2, the presence of hollow CCFs and cotton fibers can be clearly seen on the fractured surfaces. The average macropore sizes of the CCFs and cotton fibers were measured based on the FE-SEM technique, and the results are presented in Figure S4 (Supporting Information). It was found that the pristine cotton fibers exhibited a macropore size of 12.5 ± 3.2 μm. When the cotton fibers were heat-treated in a nitrogen atmosphere from 400 to 1000 °C, the macropore sizes correspondingly decreased from 6.6 ± 1.1 to 2.6 ± 0.6 μm. Thus, the pyrolysis temperature had a significant influence on the fiber lengths and macropore sizes of the CCFs. During the pyrolysis of natural cotton in a nitrogen atmosphere, no structure-directing agents were added. Thus,

Figure 3. DTG curve of cotton fibers tested in a nitrogen atmosphere (heating rate = 5 °C/min).

cotton fibers tested from room temperature to 1000 °C. An obvious mass loss was found in the range of 300−400 °C, indicated by the DTG peak located at about 350 °C. This significant mass loss of cotton fibers should be attributed to the pyrolysis reaction, converting cellulose into carbon material. Yang et al. also reported that cellulose pyrolysis can be achieved at temperatures of 315−400 °C and that the maximum weight loss rate was obtained at 355 °C.29 Thus, the carbonization process of cotton fibers should be conducted in a nitrogen atmosphere at a pyrolysis temperature above 400 °C, which showed the end of the carbonization process. The graphitization process might occur at 400−1000 °C. In this work, CCFs prepared at 400−1000 °C were systemically investigated for oil sorption applications. 18254

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Figure 4. Contact-angle measurements of (a−e) a water drop and (f−j) an oil drop on the surface of (a,f) CCFs-400, (b,g) CCFs-600, (c,h) CCFs800, (d,i) CCFs-1000, and (e,j) cotton fibers.

3.3. Contact-Angle Measurements. Figure 4 shows contact-angle measurements for water and oil on dry CCF samples and cotton fibers. It was found that the CCF sorbents exhibited superhydrophobic and superoleophilic characteristics, as indicated by water contact angles of about 150° and oil contact angles of 0°. The superhydrophobicity and superoleophilicity of the CCFs were attributed to the graphite phase and the unique microstructures of the CCFs. Chu and Pan suggested that macroporous structures trap large amounts of air and that carbon layers on the outer surface reduce the surface free energy. As a result, the water droplet minimizes the surface tension by keeping a spherical shape.6 The oil droplets can immediately spread on the carbon fiber surface. The oil sorption by CCFs was ascribed to van der Waals forces, and the penetration of the oil into the pores of the CCFs was attributed to capillary action.6 On the contrary, the contact angles of both water and oil on the cotton fibers were close to 0° (Figure 4e,j), suggesting that cotton fibers are poor in the selectivity of removing oil from water. The fact that the contact angles of both water and oil on cotton fibers were close to 0° is probably due to the coexistence of hydrophobic and hydrophilic groups on the cotton surface. The waxy components (ca. 0.4−0.8%) and cellulose in cotton fibers are the sources of hydrophobic and hydrophilic groups (e.g., hydroxyl groups), respectively. It is suspected that, for cotton fibers, the space occupied by water can decrease the oil sorption capacity. Previous work also reported the superhydrophobicity and superoleophilicity characteristics of other carbon materials, such as nanotubes and graphene.30,31 Therefore, carbonization is a promising strategy for improving the oleophilic/hydrophobic properties of natural products, thus increasing the oil sorption rate and selectivity in oil sorption from water. 3.4. Specific Surface Area and Pore Size Distribution. The specific surface areas and pore size distributions of the CCFs were measured by N2 adsorption−desorption isotherms. As shown in Table 1, the specific surface areas of the CCF samples depended strongly on the pyrolysis temperature. Thus, the highest CCF surface area of (369.6 m2/g) was obtained at 600 °C. The high surface area of CCFs-600 can mainly be ascribed to the presence of micropores. The formation of micropores can possibly be attributed to the removal of hydrogen and oxygen atoms from cellulose molecules by the pyrolysis process. The obvious decrease in specific surface area with the further increase in temperature from 600 to 1000 °C is

Table 1. Specific Surface Areas and Pore Size Distributions of CCF Samples sample CCFs-400 CCFs-600 CCFs-800 CCFs1000

SBET (m2/g)

micropore area (m2/g)

pore volume (cm3/g)

pore size (nm)

28.2 369.6 321.4 79.8

7.5 250.1 249.6 60.3

0.004 0.23 0.19 0.06

2.5, 3.4 3.8 3.1 37.4

possibly due to the collapse of the microporous structure. As can be seen in Figure 5, the pore size distributions of CCFs-

Figure 5. Pore size distributions of carbon fibers prepared at 400, 600, 800, and 1000 °C.

400, CCFs-600, CCFs-800, and CCFs-1000 were 2.5−3.4, 3.8, 3.1, and 37.4 nm, respectively. The obvious change in pore size from a few nanometers to 37.4 nm further confirms that damage to the micropores occurred at high temperature (e.g., 1000 °C). It is suggested that the macropores confirmed by FESEM (Figure 2) and the micropores measured by the BET and BJH methods (Table 1 and Figure 5) have a significant influence on oil sorption capacity and oil sorption rate. Thus, the multiscale porous CCF sorbents can be regarded as ideal candidates for removing oil from water. 18255

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Table 2. Maximum Oil and Organic Solvent Sorption Capacities of Dry CCFs and Cotton Fibers sample cotton fibers CCFs-400 CCFs-600 CCFs-800 CCFs-1000

peanut oil 34.1 57.1 44.7 42.9 44.3

± ± ± ± ±

1.5 1.5 2.4 1.8 0.8

ethylene glycol 44.6 67.4 53.5 46.1 42.0

± ± ± ± ±

0.4 0.4 0.8 0.9 0.8

toluene 23.2 37.1 31.7 28.1 28.9

± ± ± ± ±

0.5 0.6 1.5 1.0 0.4

ethanol 18.5 41.7 35.4 27.3 26.4

± ± ± ± ±

0.3 0.3 1.3 0.6 0.3

tert-butanol 25.7 32.7 29.8 30.5 30.4

± ± ± ± ±

0.8 0.8 0.7 0.6 1.2

silicon oil 54.4 71.6 64.7 63.1 58.2

± ± ± ± ±

1.0 2.9 2.3 1.6 1.4

pump oil 30.3 44.6 41.7 40.4 39.3

± ± ± ± ±

1.5 0.9 0.8 0.7 1.0

crude oil 56.0 76.6 75.4 76.5 78.9

± ± ± ± ±

2.1 1.5 4.6 2.2 1.4

sponge were about 5 and 4 g/g, respectively.14 Recently, Lei et al. reported porous boron nitride nanosheets exhibiting an oil sorption capacity of 33 g/g.36 It is believed that synthetic adsorbent materials generally have relatively high oil sorption capacities. Poly(vinyl chloride)/polystyrene fibers showed sorption capacities of 119 and 81 g/g for peanut oil and ethylene glycol, respectively.37 Electrospun polystyrene fibers also exhibited a peanut-oil sorption capacity of 112.3 g/g.38 It should be noted that the crude-oil sorption capacity of CCFs400 in this work was much higher than those of butyl rubber (7.6 g/g) and recycled wool-based nonwoven material (∼12 g/ g) in previous studies.35,39 Recently, much higher oil sorption capacities were reported for carbon nanotube sponges.7,40,41 It was found that the ethanol, vegetable oil, and ethylene glycol sorption capacities were about 96, 124, and 170 g/g, respectively.40 However, although these synthetic materials showed higher oil sorption capacities, their main problems include high synthesis costs, complicated fabrication procedures, and difficulties in recovering the oil from nanoporous sorbents. It should be mentioned that oil sorption capacity has no strong correlation to specific surface area. CCFs-600, which had the highest surface area (i.e., 369.6 m2/g) in this work, did not provide the best performance in oil and solvent sorption. The larger surface area of CCFs-600 was mainly ascribed to its richness in micropores (i.e., 250.1 m2/g), which were found to provide no significant improvement in oil sorption capacity. Similarly, an activated carbon of high surface area (i.e., 1000 m2/g) was found to exhibit a very low oil sorption capacity (i.e., 1 g/g).42 As suggested by a previous study, a high sorption capacity can be attributed to the intertube space with largesized macropores (3−300 μm).43 Therefore, the bridging space and hollow tubes can play roles as the main reservoirs for the sorption of oils and organic solvents. 3.6. Effects of Pyrolysis Temperature and Time on Oil Sorption. Figure 6 shows the effects of pyrolysis temperature and time on peanut-oil sorption. It can be seen that minor treatment of cotton fibers (e.g., 100, 200, and 300 °C) can improve the oil sorption capacity compared to that of cotton fiber (34.1 ± 1.5 goil/gsorbent), probably through minor changes in surface character. When the pyrolysis temperature was increased from 100 to 400 °C, the peanut-oil sorption capacity gradually increased from 36.3 ± 0.6 to 57.0 ± 1.5 goil/gsorbent. This implies that the minor treatment of cotton fibers might be insufficient for the improvement of the oil sorption capacity even though the fiber length and strength of the fibers were well maintained. As mentioned, a further increase of the pyrolysis temperature from 400 to 1000 °C did not improve the oil sorption ability of the CCFs. To further verify the improvement of oil sorption by the carbonization process, untreated raw cotton fibers received from China were used for the preparation of CCFs at 400−1000 °C. The peanut-oil sorption capacities of the CCFs and untreated raw cotton fibers are presented in Figure S6 (Supporting Information). The

3.5. Oil Sorption Capacity. Table 2 presents the maximum oil and organic solvent sorption capacities of CCFs and cotton fibers. It is obvious that the sorption capacities of CCFs for oil and organic solvents were much higher than those of cotton fibers. The peanut-oil sorption capacity of the cotton fibers used in this work was found to be 34 ± 1.5 goil/gsorbent, comparable with that reported in a previous work.32 Compared with cotton fibers, the CCFs had maximum oil sorption capacities that were higher by an average of 27−126% for various oils and organic solvents. The higher oil sorption capacities can be ascribed to the superhydrophobicity and superoleophilicity of CCFs in comparison with cotton fibers. The oil sorption ability of cotton fibers was previously reported to be due to the hydrophobic waxy coating (ca. 0.4−0.8%) on the cotton surface.15 In addition, among these four CCF sorbents, CCFs-400 showed the highest oil sorption capacity. The sorption capacities of CCFs-400 for peanut oil, ethylene glycol, toluene, ethanol, tert-butanol, silicon oil, vacuum pump oil, and crude oil were 57.1 ± 1.5, 67.4 ± 0.4, 37.1 ± 0.6, 41.7 ± 0.3, 32.7 ± 0.8, 71.6 ± 2.9, 44.6 ± 0.9, and 76.6 ± 1.5 goil/ gsorbent, respectively. The sorption capacity difference in CCFs is probably due to the unique pore structure and length of the fibers. CCFs-400 exhibited a multiscale pore structure (ca. 2.5 nm, 3.4 nm, and 6.6 μm). Moreover, with increasing heattreatment temperature, the length of the carbon fibers was reduced upon thermal treatment, as indicated in Figure S1 (Supporting Information). It should be mentioned that the sorption capacities for various oils and organic solvents differed in each sorbent sample. The differences in the sorption capacities were due to the differences in the densities and viscosities of the oils and organic solvents. In this work, the crude-oil sorption capacities of CCFs-400, CCFs-600, CCFs-800, and CCFs-1000 were 76.6 ± 1.5, 75.4 ± 4.6, 76.5 ± 2.2, and 78.9 ± 1.4 goil/gsorbent, respectively. These higher sorption capacities were ascribed to the relatively high density and viscosity of crude oil.6 Because of the high viscosity of crude oil, the sorption capacities of the CCFs did not show significant differences. This is because the mesoporous structures (2−50 nm) in the CCF sorbents became less important than the external structures (e.g., macropores) and low packing densities, due to the fact that it might be difficult for the crude oil to enter the fine-scale internal pores. In this study, the maximum oil sorption capacities of CCFs400 were better than or comparable to those reported in previous works on natural products and synthetic organic materials. For example, the amounts of vegetable oil adsorbed by natural products, such as walnut shell, cotton, and silkworm cocoon, were reported to be 0.6−0.8, 20−30, and 37−60 g/g, respectively.16,33,34 In terms of synthetic organic materials, Ceylan et al. found the toluene sorption capacity of macroporous butyl rubber to be 11.5 g/g.35 In addition, Choi et al. reported that the maximum toluene and ethanol sorption capacities of sugar-templated polydimethylsiloxane (PDMS) 18256

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Figure 6. Effects of pyrolysis temperature and time on peanut-oil sorption (dry sorbents immersed in pure peanut-oil medium). Insets: Digital photographs of the as-prepared CCF sorbents under different conditions.

Figure 7. Oil retention percentages on CCFs-400 and cotton fibers.

peanut-oil sorption capacity of untreated raw cotton fibers was 19.9 ± 0.7 goil/gsorbent, which did not show any improvement in oil sorption. This is because oil sorption capacity can be affected by the properties of cotton fibers from different regions, such as fiber length, pore size, and percentage of waxy coating. Similarly, the oil sorption capacity of the CCFs was improved by 57−75%, because of the carbonization process at 400−1000 °C. Also, 400 °C was considered as the optimal pyrolysis temperature for CCF preparation. In addition, pyrolysis time also played a critical role in CCF formation. As shown in Figure 6, the maximum oil sorption capacity of CCFs-400 was achieved in a pyrolysis time of 1 h. When the pyrolysis of the cotton fibers was performed at 400 °C for 0.5 h, the product showed a brown color, indicating an insufficient carbonization process. Furthermore, after increased pyrolysis times (2−4 h), the oil sorption capacity of CCFs-400 was reduced from 57.0 ± 1.5 to 43.6 ± 1.6 goil/gsorbent, probably due to a reduction in fiber lengths. Thus, pyrolysis of cotton fibers at 400 °C for 1 h provided the optimal conditions for the synthesis of effective CCF sorbents. 3.7. Dynamic Oil Retention Capacity. Figure 7 shows the dynamic oil retention capacities of CCFs-400 and cotton fibers. It was found that approximately 90% ethylene glycol and peanut oil can be retained by CCFs-400 within 10 min. In addition, the oil retention capacity reached a stable value within 2 min. On the contrary, about 20% ethylene glycol and 30% peanut oil can drip from cotton fibers. Also, peanut oil stopped dripping from cotton fibers at t = 6 min, and ethylene glycol still gradually dripped even after 10 min. This implies that the affinity between oils and cotton fibers is not strong as that between oils and CCFs. The retention capacities for different organics were also related to their densities and viscosities. The higher oil retention capacity of CCFs-400 in this study was due to its superhydrophobic and superoleophilic characteristics. 3.8. Oil Sorption Rate. Figure 8 shows the oil sorption rates for CCF samples and cotton fibers obtained using peanut oil. For the first 1 min, CCFs-600, CCFs-800, CCFs-1000, and cotton fibers almost reached oil sorption saturation, indicating oil sorption rates of 38, 39, 32, and 29 g/min, respectively. The oil sorption behaviors of CCFs-600 and CCFs-800 were not significantly different. Compared with cotton fibers, CCF

Figure 8. Peanut-oil sorption rates on carbon fibers and cotton fibers as functions of time.

samples generally showed higher oil sorption rates, possibly because of their superhydrophobic and superoleophilic characteristics. The reduced oil sorption rate for CCFs-1000 was due to the structure change (e.g., fiber length) caused by its high treatment temperature. The high oil sorption rate in the first stage (t = 0−1 min) was based on the intertube space and voids in the hollow fibers. It should be noted that the oil sorption rate of CCFs-400 in the first stage was 40 g/min, and this sample could further adsorb oil until the saturation time of t = 10 min. The oil sorption rate in the second stage (t = 1−10 min) was much lower, equivalent to 1.4 g/min. The low oil sorption rate in the second stage was possibly due to the slow penetration of oil into the nanopores in the CCF samples. The equilibrium time for the oil sorption process was about 1−10 min, much faster than in a previous work (60−90 min).44 3.9. Repeatability of Oil Sorption. Figure 9 shows the repeatability of CCFs-400 and cotton fibers in oil sorption tests. It was found that, for the first cycle, 75% of the peanut oil could be extracted from the CCF sorbent by mechanical force using a syringe, whereas around 95% of the peanut oil could be squeezed from the cotton fibers. The incomplete recovery of oil 18257

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Figure 10 presents the water/oil sorption capacities of CCFs400 and cotton fibers in water−oil mixtures with different oil

Figure 9. Repeatability of CCFs-400 and cotton fibers in peanut-oil sorption tests. Figure 10. Water/oil sorption capacities of CCFs-400 and cotton fibers in water−oil mixtures with different oil film thicknesses.

from the sorbents was due to the fact that oil residuals trapped inside hollow tubular structures are more difficult to squeeze out than those trapped in interfiber spaces. Also, the strong interaction between peanut oil and CCFs can lead to the loss of oil extraction by mechanical force. As suggested by Zhu et al., directly burning off oil in air is also an effective approach for extracting oil from carbon nanotube−polytetrafluoroethylene bulk materials.45 It was also found that, after five adsorption− desorption cycles, the reduction percentages of oil extraction from CCFs and cotton fibers were 9% and 27%, respectively. This reduction was due to the deformation of the hollow tubular structures by mechanical force. Obviously, the excellent repeatable performance of CCFs in oil sorption was confirmed in this study. It should be noted that the highly hydrophobic sorbent materials used in oil recovery by the squeeze method should have good flexibility and an elastic nature. The typical procedures of oil sorption by CCFs-400 and release of oil from sorbents by syringe are shown in Figure S7 (Supporting Information). CCFs-400 was instantaneously wetted in peanut oil, which did not show a dependence on time (Figure S7c,d, Supporting Information). It can be seen that peanut oil could be successfully released from CCFs-400 by the squeeze method and that the sorbents could also be reused in oil sorption because of their good flexibility and elastic nature (Figure S7e− i, Supporting Information). CCFs-400 samples could also recover their original shape quickly when the force was released, indicating the good elastic nature of CCF sorbents (Figure S8, Supporting Information). 3.10. Selectivity in Oil Sorption. Figure S9 (Supporting Information) shows digital photographs of toluene removal from the surface of water by CCFs-400. It is obvious that the organic solvent could be completely removed from water using the CCF sorbents, indicating their good selectivity in oil cleanup from water. The oil adsorbed by the CCFs can be reused by the mechanical squeeze method. Although the oil sorption capacities of the CCFs were not as high as those of carbon nanotube sponges, CCFs still have obvious advantages, such as low cost, excellent repeatability and selectivity, and convenience in large-scale fabrication and practical oil recovery.

film thicknesses. When the oil film thickness was increased from 2 to 12 mm, the oil sorption capacity of CCFs-400 measured by the squeeze method increased slightly from 34.0 ± 1.5 to 40.3 ± 1.3 goil/gsorbent, consistent with the oil sorption capacity tested in pure peanut-oil medium (Figure 9). In addition, a small amount of water might have been present in CCFs-400 when the oil film thickness was 2 mm. However, this could be completely avoided when the oil film thickness was greater than 4 mm. These results suggest that the water−oil system did not show significant impacts on the oil sorption capacity of CCFs-400. In contrast, the water−oil system exhibited a great influence on the water/oil sorption capacities of cotton fibers. With increasing oil film thickness, the oil sorption capacity of the cotton fibers gradually increased, whereas the water sorption capacity was declined. When the oil film thickness was in the range of 2−8 mm, the adsorbed liquid was mainly composed of water rather than peanut oil. Moreover, when the oil film thickness was 12 mm, the adsorption of water by the cotton fibers was effectively prohibited. The low selectivity of cotton fibers in oil sorption is caused by the serious uptake of water, which is probably due to the large amounts of hydrophilic groups (e.g., −OH) on the cotton surface. It should be noted that sample CCFs-400 was predominantly in the oil phase, whereas the cotton fibers could enter the water phase (Figure S10, Supporting Information). The high selectivity of oil sorption by CCFs-400 in water−oil systems was demonstrated, compared to that of cotton fibers. 3.11. Floating Ability. In view of practical applications, it is important for CCF sorbents to remain floating after the collection of oil from the water surface. A high buoyancy of sorbents is helpful for oil sorption and the removal of sorbents from the spilled area.37 Figure 11 shows the results of the floating-ability test of CCFs-400 for the removal of crude oil from the surface of water. In the static test, when a sample of CCFs-400 was gently placed on the crude oil surface, the sorbent remained floating on the surface and started to adsorb the crude oil (Figure 11c). After 60 min of adsorption, a 0.4-g sample of CCFs-400 sorbent could completely adsorb 30 mL of 18258

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Figure 11. Floating-ability test of CCFs-400 after crude oil sorption: (a) crude oil on the water surface, (b) CCFs-400 sorbents placed on the spill area, (c) CCFs-400 starting to adsorb oil, (d) crude oil adsorbed by CCFs-400, (e) CCFs-400 floating on the surface after crude oil sorption, and (f) cleaned water surface.

crude oil and remained floating after oil collection (Figure.11e). This behavior can be attributed to the superhydrophobicity and superoleophilicity of CCFs-400. The oil-absorbed CCFs-400 can be easily removed by forceps, and clean water was obtained (Figure 11f). It should be noted that further investigations are needed to test the CCF materials under dynamic conditions. Nevertheless, CCFs-400 showed good buoyancy in mixtures of oil and water even after the sorption of crude oil from water.

sorbents, XPS survey spectrum of CCFs-400, peanut-oil sorption capacities of untreated raw cotton fibers and carbonized raw cotton fibers, typical procedures of peanut-oil sorption by CCFs and release of oil from sorbents using the squeeze method, elastic nature of CCFs-400, digital photographs of toluene removal from the surface of water by sample CCFs-400, digital photographs of the water/oil sorption capacities of CCFs-400 and cotton fibers in a water−oil system, and properties of organic compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSIONS In conclusion, hollow carbon fibers derived from natural cotton were successfully fabricated by nitrogen annealing at 400−1000 °C. The carbon fibers exhibited multiscale porous structures and superhydrophobicity and superoleophilicity characteristics, as confirmed by material characterizations. The results indicated that the oil sorption of CCFs was increased by an average of 27−126%, compared with that of natural cotton fibers. The excellent oil retention, repeatability, selectivity, and good floating ability of CCFs make them a promising alternative for the effective removal of oil and organic solvents from water surfaces. It was also found that pyrolysis of cotton fiber at 400 °C for 1 h provided the optimal conditions for the synthesis of effective CCF sorbents. The fabrication method used in this study can also be applied to other natural porous sorbents or cotton fiber wastes to produce high-capacity sorbents for oil spill cleanup.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected], [email protected]. Tel.: +(852)-3442-4626 Fax: +(852)-3442-0688. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work presented in this article was funded by the School of Energy and Environment and Ability R&D Energy Research Centre at the City University of Hong Kong.

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ASSOCIATED CONTENT

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Fiber lengths of cotton fibers and CCF sorbents, XRD pattern of pristine cotton fibers, FE-SEM images of CCFs with hollow structure, average macropore sizes of cotton fibers and CCF 18259

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