Low-Cost and Superhydrophobic Magnetic Foam as an Absorbent for

Aug 23, 2016 - National Special Superfine Powder Engineering Research Center of China, Nanjing University of Science and Technology, Nanjing 210094, P...
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Low-Cost and Superhydrophobic Magnetic Foam as an Absorbent for Oil and Organic Solvent Removal Liuhua Yu, Xiang Zhou, and Wei Jiang* National Special Superfine Powder Engineering Research Center of China, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China S Supporting Information *

ABSTRACT: A convenient self-assembly method was developed to prepared a highly efficient and recyclable magnetic titanium dioxide foam by using polyethylene ordinary shock absorption foam as the support. The magnetic titanium dioxide foam acquired superhydrophobicity after a facile surface chemical modification process, and it could easily float on the water surface after selective absorption, exhibiting its excellent unsinkable property. The uptake capacity of the superhydrophobic magnetic foam was about 10 times that of the original shock absorption foam, making the as-obtained foam to be a kind of promising absorption material for oil/organic solvent removal. In addition, the superhydrophobic magnetic titanium dioxide foam could be simply recycled under the magnetic field and still retained excellent absorption capacity even after 80 absorption−desorption cycles, indicating its excellent durability. Moreover, the absorbed oils and organic solvents could be reclaimed through mechanical squeezing, which could avoid wasting of resources. The results implied that the as-prepared foam had practical applications in the treatment of oil leakage and the removal of organic solvents from wastewater. solvents from the oil−water mixture.28 Liu et al. fabricated reduced graphene oxide (RGO) foams via a freezing method, and the modified foam had excellent absorption capacity and recyclability.29 Su obtained a highly hydrophobic and oleophilic foam through coating a superhydrophobic film for selective absorption.30 However, the above-mentioned fabrication methods were generally complicated, consuming expensive raw materials and were largely energy-demanding. Moreover, these modified foams presented low chemical stability, inferior hydrophobicity, and weak mechanical properties, which hinder their large-scale applications. The ability to selectively absorb oils/organic solvents while repelling water completely is very necessary for the remedy of large volumes of oil spills.31−35 In general, the ordinary shock foam absorbs not only oil/organic solvent but also water, which makes it impractical for the selective removal of oils/organic solvents from water surface. Actually, there are many different kinds of foams in real life, some with intrinsically high oil/water absorption capacities while others not. Therefore, it is appropriate to evaluate the efficiency of a modification method by studying the enhancement of the absorption capacity rather than the absolute absorption capacity itself.

1. INTRODUCTION The vast majority of the earth is covered by the sea. Once oil spills in the sea occur which are caused by unpredicted leakages and natural disasters, they cause disastrous consequences to the environment and wildlife.1,2 Hence, it is very important to prevent and deal with oil spill accidents in the sea.3−6 Generally speaking, thick oil layers can be treated with oil−water separator and thin oil layers should be disposed of by a physical absorption method or an enhanced bioremediation method after the occurrence of oil leakages.7−10 Due to the light weight of the thin oil, it will spread widely under the situation of strong sea breezes, which makes it very difficult to handle. The physical method of adding absorption material is generally considered one of the most effective methods to deal with the thin oil layers because of its fast, efficient absorption and no second pollution.11−16 Recently, researchers have developed a lot of materials as absorbents, such as polymer nanopowder,17,18 high hydrophobic fiber,19 anticompression aerogel,20,21 and three-dimensional porous foam.22−25 Compared with other materials, the modified foam with interconnected three-dimensional network structure exhibits a great advantage owing to its low cost, excellent absorption capacity, and environmental friendliness.26,27 In recent years, modified foams with three-dimensional (3D) network structure were prepared for oil absorption and oil− water separation. Arora and Balasubramanian used a solid− liquid phase separation technique to prepare porous PVDF/ nano-SiC foam which could be used to absorb oils and organic © XXXX American Chemical Society

Received: June 12, 2016 Revised: August 20, 2016 Accepted: August 23, 2016

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DOI: 10.1021/acs.iecr.6b02278 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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other byproducts and dried at 65 °C for 2.5 h to achieve the superhydrophobic magnetic titanium dioxide foam. 2.3. Characterization. The chemical compositions of the samples were confirmed by X-ray diffraction (XRD) analysis on a Bruker D8 Advanced diffractometer (Bruker D8 Super Speed) with Cu Kα radiation at 40 kV and 40 mA. Field emission scanning electron microscopy (FE-SEM)/energydispersive X-ray spectrometer (EDS) characterization was performed under an electron beam with an accelerating voltage of 15 kV equipped with an energy-dispersive X-ray spectrometer (EDS; OXFORD INCA). All of the samples were coated with a thin layer of gold prior to analysis. The functional groups in the samples were measured by Fourier transform infrared (FT-IR) spectroscopy (4000−500 cm−1) on a Bruker Vector 22 spectrometer. The chemical stability of the as-prepared samples was checked by thermal gravimetric analysis (TGA; TA Instruments, Model TA2100 USA) in the range from 35 to 700 °C at a heating rate of 10 °C/min under N2 atmosphere. The exact weights of samples were determined at room temperature and atmospheric pressure by Electronic Analytical Balance (ME204E, China), which has a precision of 0.0010 g. The magnetic properties of the samples were measured in fields between ±5 kOe at an ambient temperature by a vibrating sample magnetometer (VSM; LakeShore 735). 2.4. Wetting Properties and Selective Absorption Capacity. Contact angle measurements were recorded to investigate the hydrophobicity and oleophilicity of the asprepared samples through a drop shape analyzer SL200B (CAs, SL200B, Solon Tech. Co. Ltd., China) at an ambient temperature using a syringe (100 μL) with a needle with a length of 35 mm, a diameter of 0.8 mm, and dripped liquid volume of 6 μL at a time (instrument precision in 0.1°). Deionized water and lubricating oil were tested as indicatorsm and all the contact angles were determined by averaging the values measured at five different points on each sample surface. In addition, the maximum selective absorption capacities of the samples were investigated by placing a piece of as-obtained sample in contact with oil/organic solvent and taking it out after saturation by a magnetic bar or a tweezer. Four kinds of oils and organic solvents were used as targets along with water, including edible oil, lubricating oil, DMF, and tetrachloromethane. The absorption capacity, E, was calculated by the following equation:

In this work, a facile and environmentally friendly approach was reported to synthesize superhydrophobic magnetic titanium dioxide foam with a high absorption capacity. TiO2 is a cheap and commercially available raw material. It was introduced into the magnetic foam to modify the surface morphology and chemical composition, which could improve the absorption capacity targeted at the oil/organic solvent. The hydrophobic property of the as-prepared foam was improved by a simple chemical surface modification process, which could enhance the recyclability and decrease the water absorption capacity. The effect of the contact time and oil film thickness on the absorption capacity was also studied. Surprisingly, the superhydrophobic magnetic foam showed excellent absorption capacity and selectivity to both thin and thick oil films. Moreover, the absorbed oils and organic solvents could be reclaimed through mechanical squeezing and the as-obtained foam could be recycled many times. The simple preparation process to fabricate the environmentally friendly absorption material which is a promising absorbent to deal with industrial oil pollution possesses great development potential.

2. EXPERIMENTAL SECTION 2.1. Materials. Polyethylene (PE) ordinary shock absorption foam and methyltrimethoxysilane (MTMS) were received from Aladdin. TiO2 nanoparticles were supplied by Deco Daojin Technology Co. Ltd., Beijing, China. Acetone (C3H6O) and dimethylformamide (DMF; C3H7NO) were purchased from Shanghai No. 4 Reagent & HV Chemical Co. Ltd., Shanghai, China. Sodium chloride (NaCl) and n-hexane (C6H14) were gained from Xilong Chemical Reagent Co. Ltd., Shantou, China. Tetrachloromethane (CCl4) was bought from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. Absolute ethanol (C2H5OH) was purchased from Nanjing Chemical Reagent Co. Ltd., Nanjing, China. All chemical solvents were of analytical reagent grade and used without further treatment. Deionized water was used throughout the preparation process. 2.2. Preparation of Superhydrophobic Magnetic Titanium Dioxide Foam. First, ordinary shock absorption foam (30 × 15 × 1 mm3) was ultrasonically cleaned in deionized water and acetone to remove possible impurities under mechanical stirring at 65 °C for 2.5 h, successively. Then, 65 mg of oleic acid coated Fe3O4 sphere particles (OA-Fe3O4) with a diameter of about 200 nm (Figure S1), and which were prepared by an improved solvothermal method,36 were completely dispersed in 25 mL of absolute enthanol to form a black homogeneous precursor solution. The precursor dispersion solution (2.6 mg/mL) was applied to the cleaned foam (100 mg) under ultrasonic and mechanical oscillation at 40 °C for 1.5 h. (About 93% of OA-Fe3O4 particles were attached on the cleaned foam and magnetic particles in the magnetic foam accounted for about 38% w/w.) The asobtained foam substrate was directly immersed into the titanium dioxide solution with different concentrations, and then the products formed after the self-assembly process at an ambient temperature for 2 h. The prepared samples were denominated as MF6, MF30, MF63, MF94, MF125, and MF219 in accordance to the TiO2 mass fractions of 0.006, 0.030, 0.063, 0.094, 0.125, and 0.219%, respectively. After that, the prepared foam was immersed in absolute enthanol containing methyltrimethoxysilane (6% v/v) under ultrasonic oscillations at 35 °C for 50 min. Finally, the as-obtained foam was washed in n-hexane to remove the unreacted monomers or

E = [Msatd abs − M init]/M init

where E is the absorption capacity (g/g), Msatd abs is the weight of saturated absorption capacity (g), and Minit is the initial weight of the absorbent (g), respectively. 2.5. Absorption Capacity in Different Contact Times and Different Thicknesses of Oil Films. All experimental processes were performed at an ambient temperature and repeated five times to determine the average value. In a typical process, 70 mL of deionized water was added to a certain amount of lubricating oil in a 100 mL beaker to form a 0.5−6 mm oil film. After that, the sample was put into the beaker and drained for 1 min until no residual oil/organic solvent droplet was left from the surface. The absorption capacity and absorption rate of the samples to four kinds of oils/organic solvents were recorded in terms of different thicknesses of oil films by the above-mentioned equation. 2.6. Collection of Oils/Organic Solvents and Reusability Tests. After each separation process, the absorbed oils and organic solvents could be collected by pressing the B

DOI: 10.1021/acs.iecr.6b02278 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research absorbent. Then the sample was washed with n-hexane three times and dried in a vacuum oven at 70 °C for 2 h. The water contact angle and absorption capacity were measured after each cycle. 2.7. Error Analysis. Errors are the objective existence in the process of any test. According to the causes and properties, errors can be divided into inherent errors and random errors. Inherent error refers to the error of the measuring instrument itself and its value directly reflects the accuracy of the measurement instrument. Random error is the difference between a single measurement and the average of bulk duplicate measurements, and it is caused by many uncertain factors such as measuring method, external conditions, and so on. Uncertainty represents the reliable degree of experimental data and many uncertainties change with the value of the variable itself. Error analysis and uncertainty for our research were studied to ensure the experimental data scientifically. The measurement of contact angle and absorption capacity of the as-prepared samples was performed under room temperature, atmospheric pressure, and the same humidity. The inherent error of the contact angle tester and electronic analytical balance are 0.1° and 0.0010 g, respectively. As mentioned, the absorption capacity of the as-prepared samples was calculated by a weighting method. The propagation of errors from the individual uncertainties to the calculated value has been calculated through t-distribution under 95% confidence limit. The specific calculation process is as follows. First, the mean and standard deviation of Msatd abs and Minit were calculated. The population mean (μ), combined uncertainties (u), and the relative uncertainties (ur) were calculated by the following equations, respectively:

Ef = E1̅ ± u f

where E̅ 1 is the mean of the absorption capacity in five parallel tests.

3. RESULTS AND DISCUSSION The fabrication process of the superhydrophobic magnetic titanium dioxide foam is illustrated schematically in Figure 1.

Figure 1. Photographic and schematic illustrations of the fabrication process of the superhydrophobic magnetic titanium dioxide foam.

After being treated with oleic acid coated Fe3O4 particles, polyethylene ordinary shock absorption foam became black in color (Figure 1b). The foam finally became gray after attachment of TiO2 nanoparticles through a self-assembly method and a facile chemical surface modification by MTMS, as shown in Figure 1d. The crystal structure and chemical composition of the magnetic titanium dioxide foam were recorded by using XRD analysis in the scanning angle of 2θ range from 20 to 75° and EDS, as exhibited in Figure 2 and Figure S2, respectively. It was clearly seen that evidently five peaks appeared at 30.1, 35.6,

ts μ = x̅ ± n

u= ur =

⎛ ts ⎞2 ⎜ ⎟ + u12 ⎝ n⎠ u × 100% x̅

where x̅ is the sample mean, s is the standard deviation, n is the sample size, and u1 is the inherent error of the electronic analytical balance or the contact angle tester, respectively. The t could be found at t-distribution under the 95% confidence limit. The relative uncertainties of a single absorption capacity measurement (urf) were calculated by the equation urff =

urMsatd abs 2 + urM init 2

where urMsatd abs is the relative uncertainty of Msatd abs and urMinit is the relative uncertainty of Minitial, respectively. Every absorption capacity of uncertainty, uac, was calculated by the equation: uac = Eu ̅ rf

where E̅ is the mean of the absorption capacity. Hence, the result of a single absorption capacity test could expressed as E̅ ± uac. In our study, all the measurements were repeated five times and the final uncertainties (uf) of absorption capacity were calculated by the equation uf =

uac12 + uac2 2 + uac32 + uac4 2 + uac52

Figure 2. XRD patterns for (a) OA-Fe3O4, (b) TiO2 NPs, and (c) magnetic titanium dioxide foam.

The final experiment results (Ef) were recorded as C

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Figure 3. SEM images of (a, b) cleaned foam, (c, d) magnetic foam, (e, f) sample MF219, and (g, h) sample MF219 after modification at different magnifications.

Figure 4. (a) Water and oil contact angles of as-prepared samples. (b) FT-IR spectra of the cleaned foam, magnetic titanium dioxide foam, and SMF63. (c) TGA curves of magnetic titanium dioxide foam and SMF63. (d) Room-temperature magnetization curves of magnetic foam, SMF63, and oil absorbed SMF63.

cleaned foam, magnetic foam, sample MF219, and sample MF219 after modification are shown in Figure 3, and all the samples exhibited an interconnected 3D network structure with a diameter of about 500 μm. It is obviously seen in Figure 3a,b that the cleaned foam presented a relatively smooth surface and no impurities existed on the surface. In comparison, the magnetic foam surface consisting of many OA-Fe3O4 tiny spheres obtained a hierarchical roughness, as shown in Figure 3c,d. Compared with Figure 3a−d, Figure 3e−h shows that the surface became much rougher after the introduction of TiO2 nanoparticles and modification with MTMS, and the TiO2 nanoparticles formed a layer of thin film which could tightly lock the OA-Fe3O4 particles existing on the surface. The possible mechanism of anchoring OA-Fe3O4 particles and TiO2 nanoparticles onto the cleaned foam surface was proposed as follows: OA-Fe3O4 particles dispersed in absolute ethanol and a

43.1, 57.2, and 62.8° in Figure 2a, which were assigned to diffraction from the (220), (311), (400), (511), and (440) crystal planes of the cubic structure of OA-Fe3O4. The pure anatase phase of TiO2 (2θ = 25.3, 37.8, 48.1, and 53.9°) was ascribed to the crystal planes of (101), (004), (200), and (105), which were detected in the XRD pattern (Figure 2b), suggesting that the coating layer consisted of Fe3O4 particles and TiO2 nanoparticles (NPs). Moreover, the position of the characteristic diffraction peaks was unchanged and no other impurity peaks were newly generated, as shown in curve c of Figure 2. As could be observed, the newly added Fe, O, and Ti elements were detected on magnetic titanium dioxide foam rather than the C element existing in cleaned foam, which agreed well with XRD analysis and is shown in Figure S2. SEM was carried out to investigate the morphologies of the as-prepared samples. The representative SEM images of the D

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that surface modification did not interfere with the original chemical composition of the as-prepared material. The newly generated peaks at 1258.4 and 1003.9 cm−1 could be attributed to the typical symmetric vibrations of C−O−C and Si−O−Si, respectively. In addition, the peaks at 879.5 and 705.2 cm−1 corresponded to the typical bending vibration characteristic of −CH3, as exhibited in Figure 4b. Based on the coincided results of EDS and FT-IR, it was concluded that the functional hydrophobic groups were grafted onto the foam successfully. These functional groups enhanced the hydrophobicity of the as-prepared foam and thus weakened its capacity of water uptake. In order to investigate the mass ratio of the grafted hydrophobic groups and the thermal stability of SMF63, TGA was performed and the results are shown in Figure 4c. The weight loss could be easily calculated to be about 10.2%, which was ascribed to the decomposition of grafted hydrophobic groups of MTMS. Magnetic titanium dioxide foam started to decompose at 100 °C. In contrast, SMF63 did not show noticeable weight loss until 220 °C, implying that the material possessed high thermal stability. To sufficiently prove the excellent thermal stability of the as-prepared foam, SMF63 was tested by thermal gravimetric analysis in the range from 30 to 200 °C and maintained under the temperature of 200 °C for 3 h at a heating rate of 10 °C/min under N2 atmosphere, and the results are exhibited in Figure S4c. As shown in Figure S4c, we could see that no apparent weight loss of SMF63 was observed, proving the excellent thermal stability again. Wettability was determined by measuring the contact angle of SMF sample, as exhibited in Figure S5. Moreover, the dynamic absorbing process of water/lubricating oil on the surface of SMF63 was recorded and presented in Figure S6. As shown in Figure S6a, the water contact angle was up to 152.1 ± 1.2° and it was unchanged even after 30 min, indicating that SMF63 was superhydrophobic. In contrast, lubricating oil droplet spread and penetrated completely into SMF63 in less than 1.2 s, which illustrated that SMF63 displayed both superoleophilic and superhydrophobic properties. When SMF63 was immersed into water by an external force, air bubbles were entrapped at the interface forming a silver mirrorlike surface, which were attributed to a continuous air layer trapped between the superhydrophobic surface and water. After the external force was released, SMF63 floated instantaneously on the water surface. How to recycle the oil/ organic solvent absorbed materials was a big challenge, and a facile method called magnetic recovery technology was adopted by using coated OA-Fe3O4 particles. When our materials absorbed oil and organic solvent, they could be easily controlled under the magnetic field. As shown in Figure 4d, the saturation magnetization values of magnetic foam, SMF63, and oil absorbed SMF63 were 30.2, 21.8, and 10.0 emu/g, respectively. Because of the increasing coating thickness, the saturation magnetization decreased.36 Even though SMF63 reached its maximum absorption capacity, it still retained a certain magnetism which could make it recyclable by applying an external magnetic field. As mentioned previously, ordinary shock absorption foam could absorb water and oil/organic solvent without selectivity. In order to evaluate the oil selectivity and the absorption efficiency of the prepared materials (taking SMF63 as an example), a series of oils and organic solvents along with water were investigated, including edible oil, lubricating oil, DMF, and tetrachloromethane, which could represent the main oil

large amount of active bubbles formed on account of the enormous energy from ultrasonic and mechanical oscillation treatment. When active bubbles contacted the surface of the cleaned foam, they would take OA-Fe3O4 particles inserting into asymmetric shapes at a high speed. As time went on, the contact between the OA-Fe3O4 particles and the surface of cleaned foam was increasingly tight. After that, TiO2 nanoparticles were added into this reaction system and formed a filmlike coating on the surface of OA-Fe3O4 with the help of a huge impetus rising from the ultrasonic wave. The coating made OA-Fe3O4 more closely attach to the surface of cleaned foam and enhanced the surface roughness, which could improve its absorption capacity and hydrophobicity. Superhydrophobic magnetic titanium dioxide foam was finally fabricated by adding MTMS, which grafted functional hydrophobic groups onto the surface of magnetic titanium dioxide foam via an environmentally friendly chemical modification method. As a result, OA-Fe3O4 particles and TiO2 nanoparticles could be firmly anchored into the cleaned foam surface under the synergistic effect of the ultrasonic and mechanical oscillation. Interestingly, samples with different TiO2 mass fractions all exhibited a considerably enhanced surface roughness than that of the cleaned foam by forming a hierarchical structure (Figure S3 in the Supporting Information). A hierarchical structure was widely considered to play an important role in the hydrophobic surface, which could introduce air pockets trapped between the solid surface and the droplet to minimize the contact area and could improve the high intake capacity at the same time.37 Contact angle measurements were carried out to illustrate hydrophobicity of the as-obtained foam. It could be obviously observed from Figure S4a that the water contact angle of the magnetic foam was 110.0 ± 1.1°, indicating its hydrophobic property. As exhibited in Figure 4a, the water contact angle of MF increased inconspicuously and the oil contact angle decreased slightly as the mass fraction of TiO2 increased. In addition, the water contact angle and oil contact angle almost reached plateaus as the mass fraction of TiO2 further increased. The highly hydrophobic and oleophilic properties were attributed to the 3D interconnected network structure of the as-obtained foam and the hierarchical roughness provided by the OA-Fe3O4 particles and TiO2 NPs. However, the superhydrophobicity necessitated the adoption of a surface functional group with an even lower surface energy, and MTMS was chosen for this purpose. Superhydrophobic MF6, MF30, MF63, MF94, MF125, and MF219 were fabricated via grafting functional hydrophobic groups of MTMS, which were labeled as SMF6, SMF30, SMF63, SMF94, SMF125, and SMF219, respectively. Besides the surface structure, the effect of the surface chemistry on the water contact angle could not be ignored. EDS and FT-IR analyses were conducted to explore the chemical composition of SMF63 (taking SMF63 as an example). The EDS survey spectrum showed the appearance of the element Si, which could be originated from the siloxy that improved the hydrophobicity (Figure S4b). As exhibited in Figure 4b, three strong absorption peaks at 2901.1, 2831.4, and 1458.0 cm−1 appeared in the FT-IR spectrum of SMF63, which could be ascribed to the two characteristic stretching vibration peaks of and the bending vibration of the C−H bond, respectively.18 Furthermore, the bands observed around 582.6 cm−1 could be assigned to Fe−O bonds. After surface modification, all observed peaks remained in SMF63, proving E

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piece of SMF63 was placed to contact with tetrachloromethane droplets (dyed by Sudan I for clear observation), tetrachloromethane was absorbed quickly and completely. Finally, the solvent-soaked SMF63 was carried out to measure its absorption capacity and squeezed before the next cyclic operation (Figure 6e). As shown in Figure 7a, the absorption capacities of all asprepared SMFs targeted at different oils/organic solvents were prominently increased, which were approximately 10 times more than those of the cleaned foam, and it was a big improvement compared with many other modified foams or sponges.28,29,38,39 Furthermore, the absorption capacity increased first and then decreased with the increasing TiO2 mass fraction. The absorption capacities of SMF63 were 35.23 ± 1.30 g/g for edible oil, 45.92 ± 1.31 g/g for lubricating oil, 51.72 ± 0.85 g/g for DMF, and 64.31 ± 0.92 g/g for tetrachloromethane, respectively, which were the highest among the samples. The reason for this phenomenon was that TiO2 nanoparticles played an important role in the oil absorption process and more TiO2 led to enhanced absorption to some extent. However, further increase in mass fraction had almost no contribution to the absorption capacity and, on the other hand, the abundant TiO2 smoothed the surface of foam, which decreased the absorption capacity slightly.40 Compared with that of cleaned foam, the water intake capacities of SMFs were significantly reduced (from 6.01 ± 0.14 to 0.25 ± 0.03 g/ g), as exhibited in Figure 7b. Further study was conducted to investigate the absorption time of SMF63 to different solvents. To our surprise, the absorption time of SMF63 varied a lot with different solvents, as shown in Figure 8a. The absorption rates of SMF63 to the four kinds of solvents decreased in the following order: edible oil > lubricating oil > DMF > tetrachloromethane. This was related to the viscosity and density of the solvents. The viscosity and density of edible oil was the smallest among these four kinds of solvents, and hence it could be absorbed by SMF63 in 50 s (Table S1). In contrast, the viscosity of tetrachloromethane was the largest among the abovementioned solvents, and thus SMF63 must overcome the strong force between the molecules of tetrachloromethane to achieve the purpose of soakage. Therefore, higher viscosities and densities of solvents led to lower absorption rates. The effect of oil film thickness on the absorption capacity was also studied. Lubricating oil with a thickness of 0.5−6 mm was

pollution pollutants in our daily life and industry. In a typical process, a certain amount of lubricating oil was placed onto the water surface and a piece of SMF63 was dropped to contact the oil film. As shown in Figure 5, SMF63 could selectively absorb

Figure 5. Process of lubricating oil removal from an oil−water mixture. Lubricating oil was dyed by Sudan I for clear observation.

lubricating oil rather than water until saturation, and then SMF63 could be collected by a magnetic bar. In addition to the absorption of low-viscosity oils, SMF63 could also be used to absorb high-viscosity oils. As shown in Figure 6a−d, when a

Figure 6. (a−d) Optical images of the removal of tetrachloromethane by SMF63. (e) Collection of tetrachloromethane from the as-prepared foam by simple squeezing.

Figure 7. (a) Maximum absorption capacity of as-prepared samples for different kinds of oils/organic solvents. (b) Water absorption capacity of asprepared samples. F

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Figure 8. (a) Absorption capacity of SMF63 versus contact time for various oil pollutants. (b) Absorption capacity of SMF63 at different thicknesses of lubricating oil film.

Figure 9. (a) Water contact angle after different water−oil/organic solvent separation cycles. Recyclability of SMF63 for absorption of (b) water (errors about ±0.04 g/g), (c) edible oil (errors about ±1.23 g/g), (d) lubricating oil (errors about ±1.35 g/g), (e) DMF (errors about ±1.38 g/g), and (f) tetrachloromethane (errors about ±1.26 g/g).

placed on the water surface for testing, and the oil absorption capacity rose from 39.92 ± 0.88 to 45.92 ± 1.31 g/g and then stabilized as the oil film thickness increased, as shown in Figure 8b. The results indicated that SMF63 was suitable for the removal of both thin and thick oil films. A simple mechanical squeezing method was employed to reclaim oils/organic solvents and recovered pristine SMF63 owing to its excellent mechanical properties (Figure 6e). Unlike other recovery methods, this one was simple, low energy consuming, and environmentally friendly. Recyclability of SMF63 to four kinds of oils/organic solvents was investigated and the water contact angle was measured after each cycle, with the results shown in Figure 9. After absorption, each foam recovered to its original shape and could be reused for many cycles after being washed by n-hexane. The absorption and desorption process was repeated at least 80 times to investigate the recyclability of the as-prepared foam. As exhibited in Figure

9a, the water contact angle of SMF63 decreased slightly even after many cycles. After 20 absorption−desorption cycles, the absorption capacity of SMF63 could recover almost 95% because of the robust structure of SMF63, as shown in Figure 9b−f. The saturation absorption capacity of the four kinds of oils and organic solvents attenuated after even more cycles. However, the absorption values all remained above 25.0 g/g and the water uptake capacity increased inconspicuously, indicating the broad application prospect.

4. CONCLUSIONS In the effort described above, we successfully fabricated the low-cost and superhydrophobic magnetic foams by controlling the mass fraction of TiO2 using polyethylene ordinary shock absorption foam as support. The as-prepared foams had advantages in terms of easily scalable fabrication process, flexibility, and excellent selective absorption of oils/organic G

DOI: 10.1021/acs.iecr.6b02278 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

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solvents. The uptake capacity of superhydrophobic magnetic titanium dioxide foam could reach about 64.31 kg of oil by employing 1 kg of sorbent. After a facile mechanical squeezing and washing process, the absorbed foams could be restored to their original form and retain the excellent absorption capacity and high hydrophobicity even after 80 separation cycles. Polyethylene ordinary shock absorption foams are readily available and malleable compared with many other raw materials, making them better candidates for absorption applications. The economical, highly efficient, and recyclable absorption material developed in this study is quite suitable for the treatment of industrial wastewater and oil spills.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02278. SEM images, TEM images, TGA curves, and EDS spectra, images of contact angles, dynamic wetting behavior of SMF63 for water and lubricating oil, situation of SMF63 immersed in strong acid/alkali and NaCl aqueous solution, and tables (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 86-25-84315042. E-mail: superfi[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (Project No. 50972060), the Fundamental Research Funds for the Central Universities (No. 30920130112003), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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REFERENCES

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DOI: 10.1021/acs.iecr.6b02278 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.6b02278 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX