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A novel hierarchical hollow SiO2@MnO2 cubes reinforced elastic polyurethane foam for the highly efficient removal of oil from water
Chemical Engineering Journal 327 (2017) 539–547
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
A novel hierarchical hollow SiO2@MnO2 cubes reinforced elastic polyurethane foam for the highly efficient removal of oil from water Dengsen Yuan a, Tao Zhang a,b, Qing Guo c, Fengxian Qiu a,⇑, Dongya Yang a, Zhongping Ou a,⇑ a
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu Province, China Institute of Green Chemistry and Chemical Technology, Jiangsu University, Zhenjiang 212013, Jiangsu Province, China c School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The composite PU foam was
synthesized by a facile one-step foaming technology. Hierarchical hollow SiO2@MnO2 cubes enhanced adsorption capacity of foam. The composite PU foam selectively adsorb and continuously remove water from oil. The composite PU foam showed excellent elasticity and recyclability. The composite PU foam could be reused for at least ten cycles.
a r t i c l e
i n f o
Article history: Received 17 April 2017 Received in revised form 20 June 2017 Accepted 25 June 2017 Available online 26 June 2017 Keywords: Oil-adsorption Hollow SiO2@MnO2 cubes Hydrophobicity Reusability Composite foam
a b s t r a c t Oil-polluted water has become a worldwide problem due to ever-increasing industrial oily wastewater as well as the frequent oil spill accident. A superior oil adsorbing material capable of separating oil–water mixtures efficiently, especially with a high oil adsorption capacity and mechanical strength, is urgently desired. Here, we report a novel strategy of preparing composite foams with hierarchical porous structures for effective oil/water separation and selective oil adsorption. The composite polyurethane foam is synthetized by a facile one-step foaming technology, which uses hierarchical hollow SiO2@MnO2 cubes to modify the inner structures of composite polyurethane foam. The incorporation of hierarchical hollow SiO2@MnO2 cubes into the foam could not only improve the morphologies, specific surface areas and hydrophobic properties but also enhance the adsorption capacity and mechanical properties. The asprepared composite PU foam is put into the oil/water mixture to adsorb oils, and the adsorbed oils can be recovered simply by squeezing oil from composite PU foam. The resulting composite PU foam exhibits high oil adsorption capacity, high selectivity and oil adsorption capacity for carbon tetrachloride was measured to be 31.6 g/g. Also, the composite PU foam shows excellent elasticity and the height of the composite PU foam decreased from 100% to 94% after 30 compressing–releasing cycles. Furthermore, the as-prepared composite PU foam exhibits the high reusability and durability by being reused for oil–water separation for 10 cycles without losing its hydrophobicity, which makes it a good candidate for industrial oil-polluted water treatments and oil spill cleanup. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction ⇑ Corresponding authors. E-mail addresses: [email protected] (F. Qiu), [email protected] (Z. Ou). http://dx.doi.org/10.1016/j.cej.2017.06.144 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.
The rapid development of industrialization brings us economic growth and it also leads to environmental pollution problems. Fre-
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quent oil spill accidents have happened during the oil exploration and transportation processes, such as 2010 Gulf of Mexico oil spill accident, which usually brings severe effects to the marine ecosystem [1,2]. Besides oil spill accidents, an increasing amount of industrial oily wastewater and the leakage of water-insoluble organic solvents (such as carbon tetrachloride, cyclohexane, toluene, and so on) also threaten public health and the surrounding ecosystem [3]. In order to protect our surrounding environment, many processes, such as mechanical extraction, in situ burning, chemical dispersion and oil adsorption materials, have been utilized to eliminate oil spill pollution [4]. Among these methods, the application of adsorption materials is believed to be one of the most promising technologies for the removal of oil contaminants from water as it can effectively remove or recover oil from the water with no secondary pollution [5,6]. The conventional oil adsorption materials, including activated carbon, wool fibers, zeolites, straw, fly ash, and so on have been used to remove oil from water [7,8]. However, these traditional oil adsorption materials generally experience various problems, such as low oil adsorption, storage capacity, poor recyclability, low oil/water separation efficiency and high materials cost [9,10]. It is greatly urgent to develop oil adsorption materials with high adsorption capacity, high selectivity and mechanical properties in recent years. Polyurethane (PU) foam with high porosity, low density, high adsorption ability, low cost, large surface area and good elasticity is a kind of commercially available 3D porous material, which can be used in fields of adsorption and removal of oils and organic solvents from water surfaces [11]. Compared with membrane materials, porous foam with three-dimensional architecture can not only achieve oil/water separation but also increase the volume of oil adsorption [12,13]. Still, the pure PU foams exhibit low mechanical durability under harsh chemical conditions and the pure PU foams are naturally hydrophilic, which makes them inefficient to be used directly for selective removal of oils from water for their adsorbing water and oil simultaneously [14]. In the past decades, various low-surface-energy materials, roughening strategies have been applied for advanced oil sorbents, which has not only greatly improved the performance of oil sorbents but also decreases their cost [15]. When a rough surface comes into contact with water, air trapping in the pockets created by the rough area contributes greatly to the increase in hydrophobicity [16]. Barry et al. [17] reported the functionalized foams exhibited high sorption and selectivity, favorable reusability characteristics, and simple fabrication schemes using commercial starting materials. Therefore, some modifications are required to increase the surface roughness and decrease the surface energy to enhance the hydrophobicity of the PU foams for more efficient separating oil from water [4]. For the purpose of practical applications, it is extremely distinguished to find a facile strategy to fabricate durable foams separating oil from water effectively. In fact, the cleanup process of oil spills by using oil adsorbents is somewhat labor-consuming and time-consuming, and requires a large amount of adsorption materials. Thus, it is necessary to improve oil adsorption capacity, decrease the usage amount of adsorbents and simplify the oil spill cleanup processes. Recently, PU foams functionalized with different kinds of porous inorganic materials which possessed many excellent properties have gained tremendous attention [18]. The properties of PU foam are predominantly determined by the intrinsic properties of the material, such as its porosity and specific surface area. The PU foam with high surface roughness and low surface tension would be optimized to suit a broader range of applications if some suitable nanoparticles were introduced to PU foam. Shi et al. [19] reported CNP-PU foam (carbonaceous nanoparticles polyurethane foam) exhibited much rougher surface, excellent mechanical properties and good organic solvent adsorption. The PU sponge incorporated with Fe3O4 and
SiO2 nanoparticles exhibited not only fast magnetic response but also superior oil adsorption capacity and stability [20,21]. Therefore, durability of PU foams can be improved distinctly by taking advantage of stable chemical properties of nanomaterials. Herein, a kind of robust SiO2@MnO2@PU foam has been fabricated through a one-step method, the micro-scaled interconnected skeleton of the foam and nano-scaled hierarchical hollow SiO2@MnO2 cubes both contribute to the functionalized nano–micro network structure, which enhanced substantially hydrophobicity of composite PU foam. The inner hierarchical hollow SiO2@MnO2 cubes were introduced into pure PU foam to improve the surface roughness and reduce surface energy. More importantly, the composite foam exhibits excellent environmental stability, which is used for not only separating the mixtures of oil and various organic solvents well, but also adsorbing high viscosity crude oil efficiently. Therefore, this study provides a facile and inexpensive method to fabricate a robust PU foam that can be applied to clean up oil spills on the surface of water and protect our environment. 2. Material and methods 2.1. Material All chemicals in this experiment are of reagent grade and used without any further purification. Polyether polyol (NJ-330, M = 3000 g/mol) was supplied by Ningwu Chemical Co., in Jurong, Jiangsu, China., Toluene diisocyanate (TDI), Dibutyltin dilaurate (DBLT), Sodium Bicarbonate (NaHCO3), Polydimethylsiloxane (PDMS), Ferric chloride (FeCl36H2O), Ammonium Hydroxide (NH3H2O), Tetraethoxysilane (TEOS), Hydrochloric acid (HCl), Sodium hydrate (NaOH), Potassium Permanganate (KMnO4), Ethyl alcohol absolute were purchased from Sinopharm Chemical Reagent Co., in Shanghai, China. 2.2. Synthesis of polyurethane foam modified with hierarchical hollow SiO2@MnO2 cubes
2.2.1. Preparation of hierarchical hollow SiO2@MnO2 cubes In a typical synthesis of cube-shaped hematite colloidal particles, the mixture of 6.0 M NaOH solution (90 mL) and 2.0 M wellstirred FeCl36H2O solution (100 mL) were placed in a glass bottle for 5 min, and the agitation was continued for an additional 10 min. The suspension containing Fe(OH)3 gel was transferred to a Teflon-lined stainless-steel autoclave in a laboratory oven preheated to 100 °C and the gel was aged for 8 days. After the treatment, hematite colloidal (Fe2O3) cubes were collected by filtration, washed three times with deionized water and ethanol before dried at 50 °C overnight. The synthesis of hollow SiO2 cubes was achieved by a solution process using the pre-fabricated Fe2O3 cubes as sacrificial templates. For a typical silica coating, Fe2O3 cubes (0.2 g) were first dispersed in a mixture consisting of ethanol (320 mL) and deionized water (80 mL) by ultrasonication, followed by the addition of NH3H2O (12 mL, 28%). The mixture was poured into a glass bottle, which was then placed in an ultrasonic water bath under 50 °C. Then, TEOS (1.2 mL) was added in the bottle. After aged for 3 h, the products were collected by filtration, washed three times with deionized water and ethanol before vacuum-drying at 80 °C for 10 h. The hematite cores of the as-prepared hematite/silica (Fe2O3@SiO2) particles were almost etched by 4.0 M HCl solution (20 mL) at 100 °C for 24 h to achieve hollow SiO2 cubes. The synthesis of hierarchical hollow SiO2@MnO2 cubes was achieved by a hydrothermal process using the pre-fabricated hollow SiO2 cubes as sacrificial templates. To preparing hierarchical
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hollow SiO2@MnO2 cubes, hollow SiO2 cubes (0.2 g) were first dispersed by ultrasonication in water (30 mL), followed by the addition of KMnO4 (1.2 g). Then, the suspension was transferred to a Teflon-lined stainless-steel autoclave, which was then heated in an air flow electric oven at 150 °C for 48 h. After the autoclave cooled down to room temperature naturally, the hierarchical hollow SiO2@MnO2 cubes were collected by centrifugation, washed three times with ethanol and deionized water. 2.2.2. Hydrophobic treatment of hierarchical hollow SiO2@MnO2 cubes In order to improve the hydrophobicity of the hierarchical hollow SiO2@MnO2 cubes, the hydrophobic modifications were made on the surface of hierarchical hollow cubes. In accordance with the proportion of ethanol and water, a mixed quantity was taken into the three-mouth flask. Next, the hierarchical hollow SiO2@MnO2 cubes (0.1 g) and KH570 (1.0 g) were added in the flask and adjust pH to 3–4. Then, the composites were mixed with the ethanol solution followed by ultrasonic dispersing for 15 min and were kept in microwave by magnetic stirring at 80 °C for 2 h. The hydrophobic hierarchical hollow SiO2@MnO2 cubes were collected by filtration, washed three times with ethanol and deionized water. 2.2.3. Synthesis of polyurethane foam materials modified with hydrophobic hierarchical hollow SiO2@MnO2 cubes The polyurethane foam materials (SiO2@MnO2@PU foam) modified with SiO2@MnO2 were synthesized by one-step method. The typical procedure was as follows: firstly, a certain amount of NJ330, SiO2@MnO2, DBLT, silicone oil and NaHCO3 were premixed and stirred at a speed of about 1800 rpm for 4 min to obtain mixture. Then, TDI was added to the as-prepared mixture and stirred at the same speed for 20 s until bubbling in the mixture, then the mixture was transferred to an air-dry oven immediately for 3 h at 100 °C. In this study, the mole ration of -NCO: -OH was chosen to be 2.5:1 to guarantee that -OH group could be reacted completely. The SiO2@MnO2/NJ-330 mass ratio of 1:25 was chosen at last, which is based on the previous study [22]. The synthetic route of the composite PU foam modified with hierarchical hollow MnO2@SiO2 cubes is shown in Scheme 1.
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2.3. Characterization The surface morphologies of PU foams before and after modification were examined by a scanning electron microscopy (SEM) at accelerating voltage of 30 kV, the internal structures, and the size of MnO2 and SiO2 were examined by a transmission electron microscope (TEM). X-ray diffraction (XRD) patterns were recorded on a type XRD-6100Lab. Radial scans were recorded in the reflection scanning mode in the 2h range from 10° to 80° with a scanning rate of 4° min1. In order to reveal the compositions of the composite structures, Fourier transform infrared spectra (FT-IR) were recorded on a Fourier transform spectrophotometer (Nicolet Nexus 470) using KBr pellets in the range from 4000 to 400 cm1. The water contact angle measurement was carried out by applying 5 lL of deionized water and the WCA values were the average of at least four measurements of an ultrapure water droplet at different positions on each sample. 2.4. Oil adsorption capacity experiment Oil adsorption ability is a standard for evaluating the performance of an oil adsorption material. The adsorption capacity can be measured by following steps. Weighed amounts of the samples were put into different kinds of oils and organic solvents for adsorption tests. Adsorption tests were conducted on initially dry foams following the ASTM Standard F726. These composite PU foams were immersed in a pure liquid bath for 15 min and then removed from the oil and paused for 30 s to keep the excess oils from these composite PU foams. Then oil-adsorption composite PU foams were weighed again. The adsorption capacity (Q) was calculated using the following Eq. (1):
Q¼
Mt Mo Mo
ð1Þ
where M0 and Mt are the weights of the composite PU foams before and after adsorption, respectively. The weight measurements of the composite PU foams with adsorbed oil were done quickly to avoid evaporation of some certain oils or solvents.
Scheme 1. Schematic illustration of fabrication of the composite PU foam.
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2.5. Reusability experiment To study the reusability of composite PU foam, it was placed into the ethanol to remove the adsorbed oils or organic solvents and the solvent-saturated foam was removed from the solvent bath and dried to constant weight in a vacuum oven. The dry composite PU foam was cooled to ambient temperature and could be utilized in the next adsorption-drying process. This operation reused for oil/water separation for 10 cycles were tested for each experiment. 3. Results and discussion 3.1. Preparation of hierarchical hollow SiO2@MnO2 cubes The Fe2O3 cubes were prepared via a robust hydrothermal reaction based on the high temperature environment. As revealed by scanning electron microscopy (SEM), the Fe2O3 cubes possessed good dispersity in aqueous solution owing to hydroxyl groups on the surface, which were also favorable for the subsequent synthesis of Fe2O3@SiO2 cubes (Fig. S1). Through a sol–gel process by the hydrolysis and condensation of TEOS in the ethanol–ammonia mixture, a uniform silica layer could be formed on individual Fe2O3 particle seeds. The as-prepared Fe2O3@SiO2 cubes were almost etched by HCl solution to obtain hollow SiO2 cubes uniform with edge-length of about 1 lm (Fig. 1A). Compared with hollow SiO2 cubes, Fig. 1B shows MnO2 sheets were formed successfully in situ on the surfaces of hollow SiO2 cubes and uniform MnO2 nanosheets could be formed on individual SiO2 particle seeds, resulting in core–shell SiO2@MnO2 cubes. The SiO2@MnO2 particles were perfectly cube with a rough surface and show a clear core–
shell structure (Fig. 1C and D), which could confirm the hollow SiO2 cubes prepared successfully. In fact, the MnO2 nanoparticles are built from two-dimensional nanosheets and the nanosheets align with one another to form a cube with an interior cavity. Fig. 1D is an enlarged image of a single particle, which clearly shows each MnO2 particle is filled with two-dimensional nanosheets and possesses a rough surface. In order to further investigate the surface properties of the hierarchical hollow SiO2@MnO2 cubes, N2 adsorption-desorption analysis was performed to characterize the specific surface area of the hierarchical hollow cubes. As shown in Fig. S2, N2 adsorption–desorption isotherms show representative type-III curves with hysteresis loop, indicating cylindrical pores with a narrow pore size. The BET surface area of hierarchical hollow SiO2@MnO2 cubes was calculated to be 18.07 m2/g. The high specific surface area of the hierarchical hollow cubes facilitate the mass transfer from the liquid phase into solid phase. When the rough surface comes into contact with water, air trapping in the pockets created by the rough area contributes greatly to the increase in hydrophobicity. To further verify the formation of the hierarchical hollow SiO2@MnO2 cubes, Fig. 2 shows the XRD patterns of samples obtained different growth processes. Eight characteristic peaks (2h = 24.1°, 33.1°, 35.6°, 40.8°, 49.4°, 54.0°, 62.4° and 64.0°), related to their corresponding indices ((0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4) and (3 0 0)) were observed in the case of the Fe2O3 cubes in Fig. 2a. In contrast to XRD pattern of Fe2O3, Fig. 2b shows a broad peak at 2h = 12°–30° corresponded to the amorphous peak of SiO2 and no characteristic diffraction peaks related to Fe2O3 can be observed in the XRD patterns of SiO2, suggesting that the Fe2O3 cubes have been removed completely. Fig. 2c displays an XRD pattern of the hierarchical hollow SiO2@MnO2 cubes and a broad peak
Fig. 1. (A) SEM image of hollow SiO2; (B) SEM image of hierarchical hollow SiO2@MnO2; (C-D) TEM images of hierarchical hollow SiO2@MnO2 with low and high magnifications.
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Fig. 2. XRD patterns of (a) Fe2O3 cubes, (b) hollow SiO2 cubes and (c) hierarchical hollow SiO2@MnO2 cubes.
at about 2h = 26° corresponded to the amorphous peak of SiO2. The diffraction peaks can be indexed to the hollow MnO2 cubic structure based on the MnO2 (JCPDS No. 80-1098) and four characteristic peaks (2h = 12.5°, 25.2°, 36.2° and 65.6°) related to their corresponding indices ((0 0 1), (0 0 2), (1 1 0) and (0 2 0)) were observed in the XRD pattern of hierarchical hollow SiO2@MnO2 cubes. Due to amorphous nature of the hollow SiO2 cubes, the XRD pattern of hierarchical hollow SiO2@MnO2 cubes is close to the pattern of pure MnO2. 3.2. Wetting behaviors of hierarchical hollow SiO2@MnO2 cubes A thin layer of those organic monomers would endow the substrate with hydrophobicity and oleophilicity. By taking this advantage, rough surfaces can be hydrophobilized without affecting their original surface topographies. The properties of obtained hierarchical hollow SiO2@MnO2 cubes were characterized by FT-IR and WCA. Both the chemical composition and morphology of the surface were the influential factors of the hydrophobic properties. Due to the low-surface-energy alkyl chain of silane coupling agent
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(KH570), the resulting hierarchical hollow SiO2@MnO2 cubes exhibited hydrophobicity and superoleophilicity. The FT-IR spectra of SiO2@MnO2 cubes before and after modification with KH570 are shown in Fig. 3A. The absorption band at 550 cm1 is corresponded to the Mn-O group of the MnO2, and the absorption bands 960 cm1 and 784 cm1 are attributed to the symmetric stretching vibration of Si-OH of SiO2@MnO2 cubes. The new peaks are observed at 2957 cm1 (the asymmetric stretching vibration of CH3), 1719 cm1 (the characteristic bond of C@O), 1637 cm1 (the characteristic bond of C@C) and 1116 cm1 (the asymmetric stretching vibration of Si-O-Si) in the spectrum of modified SiO2@MnO2 cubes, indicating that the hierarchical hollow SiO2@MnO2 cube was successfully modified with the KH570. In addition, the chemical compositions of the hierarchical hollow SiO2@MnO2 cubes before and after the modification with KH570 were also analyzed by XRD. In comparison, XRD pattern of the original SiO2@MnO2 is close to the modified SiO2@MnO2 (Fig. S3). It may result from a typical characteristic peak for SiO2 overlapped with KH570 corresponding peak. Above all, the hierarchical hollow SiO2@MnO2 cubes could possess an extremely low surface energy after modification. Furthermore, it could improve the hydrophobic effect, which offers great potential in applying in the composite foam to separate oil from water. The wetting property of modified SiO2@MnO2 cubes was studied by contact angle measurement and the static WCA was measured at room temperature. By comparing the images of Fig. 3B and C, the contact angle measurements on the modified SiO2@MnO2 cubes, showing a high-water contact angle of near 150o, differs from the SiO2@MnO2 cubes before modification (WCA = 0°), which proves modified hierarchical hollow SiO2@MnO2 cubes have reached hydrophobic effect. The special wettability of the composite PU foams can be explained by the combination of the intrinsic hydrophobicity of the PU foam with the hierarchical morphology of modified SiO2@MnO2 powder. Thus, the hierarchical hollow hydrophobic SiO2@MnO2 cubes can improve hydrophobicity of composite PU foam efficiently. 3.3. Synthesis of hierarchical hollow SiO2@ MnO2 cubes reinforced PU foams A composite PU foam has been fabricated in a one-step method combined the advantages of interconnected PU foam skeleton and
Fig. 3. (A) FT-IR spectra for (a) the pristine hierarchical hollow SiO2@MnO2 cubes and (b) the modified hierarchical hollow SiO2@MnO2 cubes; (B) WCA of hierarchical hollow SiO2@MnO2 cubes before modification; (C) WCA of hierarchical hollow SiO2@MnO2 cubes after modification.
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hierarchical hollow SiO2@MnO2 nanoparticles. As shown in Fig. 4A, it can be seen that the pure PU foams possess a cylinder structure with diameter of approximately 50 mm. Fig. 4B shows the interior image of the pure PU foams, which demonstrated the 3D porous structure of pure PU foams. The schematic of the preparation process is shown in Fig. 4C and D, the composite PU foam were synthesized by one-step method. With the hierarchical hollow SiO2@MnO2 nanoparticles are introduced in the pure foam, the color of composite foam changes from white (Fig. 4B) to black (Fig. 4E). Compared with the pure foam, the composite PU foam (Fig. 4E) shows a rougher surface that contributes greatly to increase in hydrophobicity. As is shown in the inset of Fig. 4F, image of the surface and the cross section of the composite PU
foam with spherical water drops dyed with malachite green on it but completely wetted by a drop of carbon tetrachloride stained by Sudan Red III when being dropped onto the surface of foam. Therefore, the as-prepared composite PU foam displays both hydrophobicity and oleophilicity when it is exposed to water and oil respectively. To verify the successful attachment of SiO2@MnO2 onto PU foam and the rough surface structure of the composite foam, the microstructures of pure PU foams and composite PU foams were observed by SEM (Fig. 5). The SEM images of the foams taken with different magnification showed highly porous 3D interconnected networks and micro-porous structures. For the pristine PU foam without SiO2@MnO2 nanoparticles treatment, it is clearly seen that
Fig. 4. Formation schematic and macroscopic morphologies of composite foam. (A-B) optical images of the pure PU foam, (C-D) formation schematic of the composite PU foam, (E-F) optical images of the composite PU foam and water drops on the composite foam after being kept and a drop of carbon tetrachloride is readily adsorbed (F, inset).
Fig. 5. Microscopic analysis of the structure of PU foam. SEM images of the original PU foam (A-B) and the composite PU foam (C-D) with different magnifications.
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the foam surface was smooth and flat and numerous pores with pore sizes of 50–150 lm were randomly distributed on the surface and interior of PU foams (Fig. 5A and B). The 3D network structure was formed by the pore-forming agent (NaHCO3) heated to produce gas during the process of preparing the PU foam. The large macropore size would be a great benefit to the transfer of fluid in the porous monolith and the rough surface formed by the macroporous structure could improve hydrophobicity of compos-
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ite PU foam. As presented in Fig. 5C and D, compared with SEM diagrams of the pure PU foams, the pores of the composite PU foam become rougher than the pores of pure PU foam. The contact angle values (shown in Fig. S4) were found to be 97 ± 1° and 124 ± 1° respectively, showing the hierarchical hollow SiO2@MnO2 cubes can improve hydrophobicity of composite PU foam. With introduction of the hierarchical hollow SiO2@MnO2 cubes, the composite PU foams form a rough surface structure, which makes it possess a better oil storage capacity, thereby further enhancing its oil/ water separation performance. 3.4. Elasticity capacity of composite polyurethane foam
Fig. 6. Height recovery of composite PU foam as a function of recycle number at 65% compression strain. The insets illustrate the compressing–releasing processes of the composite PU foam.
The cyclic compressing tests of composite PU foam were conducted to evaluate the mechanical property. The good mechanical properties of composite PU foam when compressed in organic liquid is the key merit for our strategy of collection of adsorbed oils by simple squeezing and the composite foam with a better mechanical compressibility and hydrophobicity that was resulted from the synergetic effect of its components. The insets of Fig. 6 illustrate the compressing–releasing processes of composite PU foam and the foam could almost recover its initial height after being pressed, which suggested composite PU foam possessed excellent elasticity. In fact, the composite PU foam immediately recovered to their original shapes with no plastic deformation after the release of the pressure. As is shown in Fig. 6, the height of the composite PU foam decreased by 6% after 30 compressing–releasing cycles and 94% of their initial heights were maintained after being repeatedly pressed for 30 cycles. These results suggested that the composite PU foam possessed excellent flexibility, which could greatly facilitate the oil recovery and reuse of the foam.
Fig. 7. Oil adsorption tests of the composite PU foams. (A) Adsorption process of soybean oil (stained with Sudan III) on the surface of water by the composite PU foams; (B) Maximum adsorption capacities of the oil towards eight different organic compounds; (C) The oil adsorption capacity of composite PU foams during the repeated adsorption and release of several organic solvents for 10 cycles.
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3.5. Adsorption capacity of composite polyurethane foam
4. Conclusions
In order to investigate the practical application of the oil spill cleanup behavior of the composite PU foams, we imitated that the composite PU foam separates oil from mixture under natural conditions. As shown in Fig. 7A, employing soybean oil (stained with Sudan III) as the organic solvent in the selective adsorption experiment (the concentration ratio of soybean oil to water is 1%), the composite PU foam could selectively adsorb soybean oil on the surface of water. Once the composite PU foam contacted soybean oil on the surface of water, red soybean oil could be quickly sucked into the composite PU foam within a few seconds (Video. S1). A simple squeezing process can readily collect the adsorbed soybean oil in the foam and no any red pollutants were observed in the water, indicating high separation efficiency of the composite PU foams with no secondary pollution. As is shown in Fig. S5, according to the original added volume (7.6 mL) and collected volume (6.6 mL), we could calculate the collecting efficiency to be 87%. Furthermore, the composite PU foam could also easily adsorb carbon tetrachloride (stained with Sudan III) from the bottom of water (Fig. S6). Considering the growing number of oil-spill accidents and variable composition of oils, the composite PU foams could be used to collect multiple types of oils and organic solvents, which is a significant feature for practical applications. Therefore, we extended the collection experiment to several oils and organic solvents, including carbon tetrachloride, toluene, and soybean oil. Under static conditions, Fig. 7B demonstrated the adsorption capacities of the composite PU foam for several other oils and organic solvents were 8–32 times than its own weight, mainly depending on the density of oils and organic solvents and swelling property of composite foams. The adsorption capacities (Q) for carbon tetrachloride, toluene, THF, acetone, DMF, soybean oil, pump oil, diesel oil was measured to be 31.6, 11.2, 24.2, 14.5, 13.2, 8.9, 9.2 and 8.88 g/g respectively. The adsorbed solvents are stored in the abundant pores of the composite PU foams. No dripping of the adsorbed organics was observed in the handling process, indicating excellent adsorption capability of composite PU foam for various oils and organic solvents.
In summary, the porous composite PU foam with controllable inner porous structures was successfully fabricated through a one-step method. The hierarchical hollow SiO2@MnO2 cubes can be synthesized by in situ growth of MnO2 nanoplatelets on the surface of SiO2 cubes. With introduction of the hierarchical hollow SiO2@MnO2 cubes, hierarchical hollow structure and large surface area of the sample are beneficial to the improved oil adsorption capability of composite PU foam. The as-prepared composite PU foam exhibits the characteristics of superwetting properties, superior elasticity, highly efficient and selective adsorption of some sorts of oil or organic solvents from the mixture. The maximum oil adsorption capacities for carbon tetrachloride and THF were measured to be 31.6 and 24.2 g/g respectively. In addition, oil adsorbed in composite PU foam was easily collected by squeezing. What is more, it could be reused for oil–water separation for 10 cycles without affecting its oil adsorption performance and 94% of their initial heights were maintained after being repeatedly pressed for 30 cycles, demonstrating the great reusability and durability of composite PU foam. Due to simple fabrication, lowcost, environmental friendly, great reusability, as well as fine elastic stability under different conditions, the composite PU foam reported herein shows it has a great potential of application in the cleanup of oil spills in the ocean and it is also expected to be an ideal adsorbent in oil spillage cleanup.
3.6. Reusability of composite polyurethane foam
Appendix A. Supplementary data
Reusability is a significant property for oil-adsorbents in practical application. In this study, the impact of repeatedly wetting by the oils on the wettability of the composite PU foams was investigated to evaluate reusability. In the process of oil adsorption and collection for 10 cycles, the composite PU foams were washed with ethanol and the adsorption capacities (Q) were measured after each water–oil separation experiment. The change of the adsorption capacities with adsorption and collection cycles is shown in Fig. 7C. After this adsorption and regeneration process was repeated 4 times with different organic solvents–water as an example of a mixture system, the composite PU foam still maintains a stable adsorption capacity, exhibiting good repeatability. A slight decrease in the adsorption capacity after the 5th cycle was observed (especially in the case of THF), which may be ascribed to decrease of composite PU foam surface roughness by cleaning of ethanol. The excellent reusability of composite PU foam is believed to arise from the mechanical durability of SiO2@MnO2 nanoparticle powder. Rough porous morphologies still cover the composite PU foams after the 10th cycle. Consequently, the regenerated porous composite PU foam was still reused for the selective adsorption of oil from water. Above all, the excellent properties of high adsorption capacity and good reusability of the composite PU foam will guarantee its promising application for oil–water separation purposes.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2017.06.144.
Acknowledgements We thank the Natural Science Foundation of Jiangsu Province (BK20160500, BK20161362 and BK20161264) and National Nature Science Foundation of China (U1507115) for financial support. This study was also financially supported by the China Postdoctoral Science Foundation (2016M600373), High-Level Personnel Training Project of Jiangsu Province (BRA2016142), China Postdoctoral Science Foundation of Jiangsu Province (1601016A) and Scientific Research Foundation for Advanced Talents, Jiangsu University (15JDG142).
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Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
A novel hierarchical hollow SiO2@MnO2 cubes reinforced elastic polyurethane foam for the highly efficient removal of oil from water Dengsen Yuan a, Tao Zhang a,b, Qing Guo c, Fengxian Qiu a,⇑, Dongya Yang a, Zhongping Ou a,⇑ a
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu Province, China Institute of Green Chemistry and Chemical Technology, Jiangsu University, Zhenjiang 212013, Jiangsu Province, China c School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The composite PU foam was
synthesized by a facile one-step foaming technology. Hierarchical hollow SiO2@MnO2 cubes enhanced adsorption capacity of foam. The composite PU foam selectively adsorb and continuously remove water from oil. The composite PU foam showed excellent elasticity and recyclability. The composite PU foam could be reused for at least ten cycles.
a r t i c l e
i n f o
Article history: Received 17 April 2017 Received in revised form 20 June 2017 Accepted 25 June 2017 Available online 26 June 2017 Keywords: Oil-adsorption Hollow SiO2@MnO2 cubes Hydrophobicity Reusability Composite foam
a b s t r a c t Oil-polluted water has become a worldwide problem due to ever-increasing industrial oily wastewater as well as the frequent oil spill accident. A superior oil adsorbing material capable of separating oil–water mixtures efficiently, especially with a high oil adsorption capacity and mechanical strength, is urgently desired. Here, we report a novel strategy of preparing composite foams with hierarchical porous structures for effective oil/water separation and selective oil adsorption. The composite polyurethane foam is synthetized by a facile one-step foaming technology, which uses hierarchical hollow SiO2@MnO2 cubes to modify the inner structures of composite polyurethane foam. The incorporation of hierarchical hollow SiO2@MnO2 cubes into the foam could not only improve the morphologies, specific surface areas and hydrophobic properties but also enhance the adsorption capacity and mechanical properties. The asprepared composite PU foam is put into the oil/water mixture to adsorb oils, and the adsorbed oils can be recovered simply by squeezing oil from composite PU foam. The resulting composite PU foam exhibits high oil adsorption capacity, high selectivity and oil adsorption capacity for carbon tetrachloride was measured to be 31.6 g/g. Also, the composite PU foam shows excellent elasticity and the height of the composite PU foam decreased from 100% to 94% after 30 compressing–releasing cycles. Furthermore, the as-prepared composite PU foam exhibits the high reusability and durability by being reused for oil–water separation for 10 cycles without losing its hydrophobicity, which makes it a good candidate for industrial oil-polluted water treatments and oil spill cleanup. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction ⇑ Corresponding authors. E-mail addresses: [email protected] (F. Qiu), [email protected] (Z. Ou). http://dx.doi.org/10.1016/j.cej.2017.06.144 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.
The rapid development of industrialization brings us economic growth and it also leads to environmental pollution problems. Fre-
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quent oil spill accidents have happened during the oil exploration and transportation processes, such as 2010 Gulf of Mexico oil spill accident, which usually brings severe effects to the marine ecosystem [1,2]. Besides oil spill accidents, an increasing amount of industrial oily wastewater and the leakage of water-insoluble organic solvents (such as carbon tetrachloride, cyclohexane, toluene, and so on) also threaten public health and the surrounding ecosystem [3]. In order to protect our surrounding environment, many processes, such as mechanical extraction, in situ burning, chemical dispersion and oil adsorption materials, have been utilized to eliminate oil spill pollution [4]. Among these methods, the application of adsorption materials is believed to be one of the most promising technologies for the removal of oil contaminants from water as it can effectively remove or recover oil from the water with no secondary pollution [5,6]. The conventional oil adsorption materials, including activated carbon, wool fibers, zeolites, straw, fly ash, and so on have been used to remove oil from water [7,8]. However, these traditional oil adsorption materials generally experience various problems, such as low oil adsorption, storage capacity, poor recyclability, low oil/water separation efficiency and high materials cost [9,10]. It is greatly urgent to develop oil adsorption materials with high adsorption capacity, high selectivity and mechanical properties in recent years. Polyurethane (PU) foam with high porosity, low density, high adsorption ability, low cost, large surface area and good elasticity is a kind of commercially available 3D porous material, which can be used in fields of adsorption and removal of oils and organic solvents from water surfaces [11]. Compared with membrane materials, porous foam with three-dimensional architecture can not only achieve oil/water separation but also increase the volume of oil adsorption [12,13]. Still, the pure PU foams exhibit low mechanical durability under harsh chemical conditions and the pure PU foams are naturally hydrophilic, which makes them inefficient to be used directly for selective removal of oils from water for their adsorbing water and oil simultaneously [14]. In the past decades, various low-surface-energy materials, roughening strategies have been applied for advanced oil sorbents, which has not only greatly improved the performance of oil sorbents but also decreases their cost [15]. When a rough surface comes into contact with water, air trapping in the pockets created by the rough area contributes greatly to the increase in hydrophobicity [16]. Barry et al. [17] reported the functionalized foams exhibited high sorption and selectivity, favorable reusability characteristics, and simple fabrication schemes using commercial starting materials. Therefore, some modifications are required to increase the surface roughness and decrease the surface energy to enhance the hydrophobicity of the PU foams for more efficient separating oil from water [4]. For the purpose of practical applications, it is extremely distinguished to find a facile strategy to fabricate durable foams separating oil from water effectively. In fact, the cleanup process of oil spills by using oil adsorbents is somewhat labor-consuming and time-consuming, and requires a large amount of adsorption materials. Thus, it is necessary to improve oil adsorption capacity, decrease the usage amount of adsorbents and simplify the oil spill cleanup processes. Recently, PU foams functionalized with different kinds of porous inorganic materials which possessed many excellent properties have gained tremendous attention [18]. The properties of PU foam are predominantly determined by the intrinsic properties of the material, such as its porosity and specific surface area. The PU foam with high surface roughness and low surface tension would be optimized to suit a broader range of applications if some suitable nanoparticles were introduced to PU foam. Shi et al. [19] reported CNP-PU foam (carbonaceous nanoparticles polyurethane foam) exhibited much rougher surface, excellent mechanical properties and good organic solvent adsorption. The PU sponge incorporated with Fe3O4 and
SiO2 nanoparticles exhibited not only fast magnetic response but also superior oil adsorption capacity and stability [20,21]. Therefore, durability of PU foams can be improved distinctly by taking advantage of stable chemical properties of nanomaterials. Herein, a kind of robust SiO2@MnO2@PU foam has been fabricated through a one-step method, the micro-scaled interconnected skeleton of the foam and nano-scaled hierarchical hollow SiO2@MnO2 cubes both contribute to the functionalized nano–micro network structure, which enhanced substantially hydrophobicity of composite PU foam. The inner hierarchical hollow SiO2@MnO2 cubes were introduced into pure PU foam to improve the surface roughness and reduce surface energy. More importantly, the composite foam exhibits excellent environmental stability, which is used for not only separating the mixtures of oil and various organic solvents well, but also adsorbing high viscosity crude oil efficiently. Therefore, this study provides a facile and inexpensive method to fabricate a robust PU foam that can be applied to clean up oil spills on the surface of water and protect our environment. 2. Material and methods 2.1. Material All chemicals in this experiment are of reagent grade and used without any further purification. Polyether polyol (NJ-330, M = 3000 g/mol) was supplied by Ningwu Chemical Co., in Jurong, Jiangsu, China., Toluene diisocyanate (TDI), Dibutyltin dilaurate (DBLT), Sodium Bicarbonate (NaHCO3), Polydimethylsiloxane (PDMS), Ferric chloride (FeCl36H2O), Ammonium Hydroxide (NH3H2O), Tetraethoxysilane (TEOS), Hydrochloric acid (HCl), Sodium hydrate (NaOH), Potassium Permanganate (KMnO4), Ethyl alcohol absolute were purchased from Sinopharm Chemical Reagent Co., in Shanghai, China. 2.2. Synthesis of polyurethane foam modified with hierarchical hollow SiO2@MnO2 cubes
2.2.1. Preparation of hierarchical hollow SiO2@MnO2 cubes In a typical synthesis of cube-shaped hematite colloidal particles, the mixture of 6.0 M NaOH solution (90 mL) and 2.0 M wellstirred FeCl36H2O solution (100 mL) were placed in a glass bottle for 5 min, and the agitation was continued for an additional 10 min. The suspension containing Fe(OH)3 gel was transferred to a Teflon-lined stainless-steel autoclave in a laboratory oven preheated to 100 °C and the gel was aged for 8 days. After the treatment, hematite colloidal (Fe2O3) cubes were collected by filtration, washed three times with deionized water and ethanol before dried at 50 °C overnight. The synthesis of hollow SiO2 cubes was achieved by a solution process using the pre-fabricated Fe2O3 cubes as sacrificial templates. For a typical silica coating, Fe2O3 cubes (0.2 g) were first dispersed in a mixture consisting of ethanol (320 mL) and deionized water (80 mL) by ultrasonication, followed by the addition of NH3H2O (12 mL, 28%). The mixture was poured into a glass bottle, which was then placed in an ultrasonic water bath under 50 °C. Then, TEOS (1.2 mL) was added in the bottle. After aged for 3 h, the products were collected by filtration, washed three times with deionized water and ethanol before vacuum-drying at 80 °C for 10 h. The hematite cores of the as-prepared hematite/silica (Fe2O3@SiO2) particles were almost etched by 4.0 M HCl solution (20 mL) at 100 °C for 24 h to achieve hollow SiO2 cubes. The synthesis of hierarchical hollow SiO2@MnO2 cubes was achieved by a hydrothermal process using the pre-fabricated hollow SiO2 cubes as sacrificial templates. To preparing hierarchical
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hollow SiO2@MnO2 cubes, hollow SiO2 cubes (0.2 g) were first dispersed by ultrasonication in water (30 mL), followed by the addition of KMnO4 (1.2 g). Then, the suspension was transferred to a Teflon-lined stainless-steel autoclave, which was then heated in an air flow electric oven at 150 °C for 48 h. After the autoclave cooled down to room temperature naturally, the hierarchical hollow SiO2@MnO2 cubes were collected by centrifugation, washed three times with ethanol and deionized water. 2.2.2. Hydrophobic treatment of hierarchical hollow SiO2@MnO2 cubes In order to improve the hydrophobicity of the hierarchical hollow SiO2@MnO2 cubes, the hydrophobic modifications were made on the surface of hierarchical hollow cubes. In accordance with the proportion of ethanol and water, a mixed quantity was taken into the three-mouth flask. Next, the hierarchical hollow SiO2@MnO2 cubes (0.1 g) and KH570 (1.0 g) were added in the flask and adjust pH to 3–4. Then, the composites were mixed with the ethanol solution followed by ultrasonic dispersing for 15 min and were kept in microwave by magnetic stirring at 80 °C for 2 h. The hydrophobic hierarchical hollow SiO2@MnO2 cubes were collected by filtration, washed three times with ethanol and deionized water. 2.2.3. Synthesis of polyurethane foam materials modified with hydrophobic hierarchical hollow SiO2@MnO2 cubes The polyurethane foam materials (SiO2@MnO2@PU foam) modified with SiO2@MnO2 were synthesized by one-step method. The typical procedure was as follows: firstly, a certain amount of NJ330, SiO2@MnO2, DBLT, silicone oil and NaHCO3 were premixed and stirred at a speed of about 1800 rpm for 4 min to obtain mixture. Then, TDI was added to the as-prepared mixture and stirred at the same speed for 20 s until bubbling in the mixture, then the mixture was transferred to an air-dry oven immediately for 3 h at 100 °C. In this study, the mole ration of -NCO: -OH was chosen to be 2.5:1 to guarantee that -OH group could be reacted completely. The SiO2@MnO2/NJ-330 mass ratio of 1:25 was chosen at last, which is based on the previous study [22]. The synthetic route of the composite PU foam modified with hierarchical hollow MnO2@SiO2 cubes is shown in Scheme 1.
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2.3. Characterization The surface morphologies of PU foams before and after modification were examined by a scanning electron microscopy (SEM) at accelerating voltage of 30 kV, the internal structures, and the size of MnO2 and SiO2 were examined by a transmission electron microscope (TEM). X-ray diffraction (XRD) patterns were recorded on a type XRD-6100Lab. Radial scans were recorded in the reflection scanning mode in the 2h range from 10° to 80° with a scanning rate of 4° min1. In order to reveal the compositions of the composite structures, Fourier transform infrared spectra (FT-IR) were recorded on a Fourier transform spectrophotometer (Nicolet Nexus 470) using KBr pellets in the range from 4000 to 400 cm1. The water contact angle measurement was carried out by applying 5 lL of deionized water and the WCA values were the average of at least four measurements of an ultrapure water droplet at different positions on each sample. 2.4. Oil adsorption capacity experiment Oil adsorption ability is a standard for evaluating the performance of an oil adsorption material. The adsorption capacity can be measured by following steps. Weighed amounts of the samples were put into different kinds of oils and organic solvents for adsorption tests. Adsorption tests were conducted on initially dry foams following the ASTM Standard F726. These composite PU foams were immersed in a pure liquid bath for 15 min and then removed from the oil and paused for 30 s to keep the excess oils from these composite PU foams. Then oil-adsorption composite PU foams were weighed again. The adsorption capacity (Q) was calculated using the following Eq. (1):
Q¼
Mt Mo Mo
ð1Þ
where M0 and Mt are the weights of the composite PU foams before and after adsorption, respectively. The weight measurements of the composite PU foams with adsorbed oil were done quickly to avoid evaporation of some certain oils or solvents.
Scheme 1. Schematic illustration of fabrication of the composite PU foam.
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2.5. Reusability experiment To study the reusability of composite PU foam, it was placed into the ethanol to remove the adsorbed oils or organic solvents and the solvent-saturated foam was removed from the solvent bath and dried to constant weight in a vacuum oven. The dry composite PU foam was cooled to ambient temperature and could be utilized in the next adsorption-drying process. This operation reused for oil/water separation for 10 cycles were tested for each experiment. 3. Results and discussion 3.1. Preparation of hierarchical hollow SiO2@MnO2 cubes The Fe2O3 cubes were prepared via a robust hydrothermal reaction based on the high temperature environment. As revealed by scanning electron microscopy (SEM), the Fe2O3 cubes possessed good dispersity in aqueous solution owing to hydroxyl groups on the surface, which were also favorable for the subsequent synthesis of Fe2O3@SiO2 cubes (Fig. S1). Through a sol–gel process by the hydrolysis and condensation of TEOS in the ethanol–ammonia mixture, a uniform silica layer could be formed on individual Fe2O3 particle seeds. The as-prepared Fe2O3@SiO2 cubes were almost etched by HCl solution to obtain hollow SiO2 cubes uniform with edge-length of about 1 lm (Fig. 1A). Compared with hollow SiO2 cubes, Fig. 1B shows MnO2 sheets were formed successfully in situ on the surfaces of hollow SiO2 cubes and uniform MnO2 nanosheets could be formed on individual SiO2 particle seeds, resulting in core–shell SiO2@MnO2 cubes. The SiO2@MnO2 particles were perfectly cube with a rough surface and show a clear core–
shell structure (Fig. 1C and D), which could confirm the hollow SiO2 cubes prepared successfully. In fact, the MnO2 nanoparticles are built from two-dimensional nanosheets and the nanosheets align with one another to form a cube with an interior cavity. Fig. 1D is an enlarged image of a single particle, which clearly shows each MnO2 particle is filled with two-dimensional nanosheets and possesses a rough surface. In order to further investigate the surface properties of the hierarchical hollow SiO2@MnO2 cubes, N2 adsorption-desorption analysis was performed to characterize the specific surface area of the hierarchical hollow cubes. As shown in Fig. S2, N2 adsorption–desorption isotherms show representative type-III curves with hysteresis loop, indicating cylindrical pores with a narrow pore size. The BET surface area of hierarchical hollow SiO2@MnO2 cubes was calculated to be 18.07 m2/g. The high specific surface area of the hierarchical hollow cubes facilitate the mass transfer from the liquid phase into solid phase. When the rough surface comes into contact with water, air trapping in the pockets created by the rough area contributes greatly to the increase in hydrophobicity. To further verify the formation of the hierarchical hollow SiO2@MnO2 cubes, Fig. 2 shows the XRD patterns of samples obtained different growth processes. Eight characteristic peaks (2h = 24.1°, 33.1°, 35.6°, 40.8°, 49.4°, 54.0°, 62.4° and 64.0°), related to their corresponding indices ((0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4) and (3 0 0)) were observed in the case of the Fe2O3 cubes in Fig. 2a. In contrast to XRD pattern of Fe2O3, Fig. 2b shows a broad peak at 2h = 12°–30° corresponded to the amorphous peak of SiO2 and no characteristic diffraction peaks related to Fe2O3 can be observed in the XRD patterns of SiO2, suggesting that the Fe2O3 cubes have been removed completely. Fig. 2c displays an XRD pattern of the hierarchical hollow SiO2@MnO2 cubes and a broad peak
Fig. 1. (A) SEM image of hollow SiO2; (B) SEM image of hierarchical hollow SiO2@MnO2; (C-D) TEM images of hierarchical hollow SiO2@MnO2 with low and high magnifications.
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Fig. 2. XRD patterns of (a) Fe2O3 cubes, (b) hollow SiO2 cubes and (c) hierarchical hollow SiO2@MnO2 cubes.
at about 2h = 26° corresponded to the amorphous peak of SiO2. The diffraction peaks can be indexed to the hollow MnO2 cubic structure based on the MnO2 (JCPDS No. 80-1098) and four characteristic peaks (2h = 12.5°, 25.2°, 36.2° and 65.6°) related to their corresponding indices ((0 0 1), (0 0 2), (1 1 0) and (0 2 0)) were observed in the XRD pattern of hierarchical hollow SiO2@MnO2 cubes. Due to amorphous nature of the hollow SiO2 cubes, the XRD pattern of hierarchical hollow SiO2@MnO2 cubes is close to the pattern of pure MnO2. 3.2. Wetting behaviors of hierarchical hollow SiO2@MnO2 cubes A thin layer of those organic monomers would endow the substrate with hydrophobicity and oleophilicity. By taking this advantage, rough surfaces can be hydrophobilized without affecting their original surface topographies. The properties of obtained hierarchical hollow SiO2@MnO2 cubes were characterized by FT-IR and WCA. Both the chemical composition and morphology of the surface were the influential factors of the hydrophobic properties. Due to the low-surface-energy alkyl chain of silane coupling agent
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(KH570), the resulting hierarchical hollow SiO2@MnO2 cubes exhibited hydrophobicity and superoleophilicity. The FT-IR spectra of SiO2@MnO2 cubes before and after modification with KH570 are shown in Fig. 3A. The absorption band at 550 cm1 is corresponded to the Mn-O group of the MnO2, and the absorption bands 960 cm1 and 784 cm1 are attributed to the symmetric stretching vibration of Si-OH of SiO2@MnO2 cubes. The new peaks are observed at 2957 cm1 (the asymmetric stretching vibration of CH3), 1719 cm1 (the characteristic bond of C@O), 1637 cm1 (the characteristic bond of C@C) and 1116 cm1 (the asymmetric stretching vibration of Si-O-Si) in the spectrum of modified SiO2@MnO2 cubes, indicating that the hierarchical hollow SiO2@MnO2 cube was successfully modified with the KH570. In addition, the chemical compositions of the hierarchical hollow SiO2@MnO2 cubes before and after the modification with KH570 were also analyzed by XRD. In comparison, XRD pattern of the original SiO2@MnO2 is close to the modified SiO2@MnO2 (Fig. S3). It may result from a typical characteristic peak for SiO2 overlapped with KH570 corresponding peak. Above all, the hierarchical hollow SiO2@MnO2 cubes could possess an extremely low surface energy after modification. Furthermore, it could improve the hydrophobic effect, which offers great potential in applying in the composite foam to separate oil from water. The wetting property of modified SiO2@MnO2 cubes was studied by contact angle measurement and the static WCA was measured at room temperature. By comparing the images of Fig. 3B and C, the contact angle measurements on the modified SiO2@MnO2 cubes, showing a high-water contact angle of near 150o, differs from the SiO2@MnO2 cubes before modification (WCA = 0°), which proves modified hierarchical hollow SiO2@MnO2 cubes have reached hydrophobic effect. The special wettability of the composite PU foams can be explained by the combination of the intrinsic hydrophobicity of the PU foam with the hierarchical morphology of modified SiO2@MnO2 powder. Thus, the hierarchical hollow hydrophobic SiO2@MnO2 cubes can improve hydrophobicity of composite PU foam efficiently. 3.3. Synthesis of hierarchical hollow SiO2@ MnO2 cubes reinforced PU foams A composite PU foam has been fabricated in a one-step method combined the advantages of interconnected PU foam skeleton and
Fig. 3. (A) FT-IR spectra for (a) the pristine hierarchical hollow SiO2@MnO2 cubes and (b) the modified hierarchical hollow SiO2@MnO2 cubes; (B) WCA of hierarchical hollow SiO2@MnO2 cubes before modification; (C) WCA of hierarchical hollow SiO2@MnO2 cubes after modification.
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hierarchical hollow SiO2@MnO2 nanoparticles. As shown in Fig. 4A, it can be seen that the pure PU foams possess a cylinder structure with diameter of approximately 50 mm. Fig. 4B shows the interior image of the pure PU foams, which demonstrated the 3D porous structure of pure PU foams. The schematic of the preparation process is shown in Fig. 4C and D, the composite PU foam were synthesized by one-step method. With the hierarchical hollow SiO2@MnO2 nanoparticles are introduced in the pure foam, the color of composite foam changes from white (Fig. 4B) to black (Fig. 4E). Compared with the pure foam, the composite PU foam (Fig. 4E) shows a rougher surface that contributes greatly to increase in hydrophobicity. As is shown in the inset of Fig. 4F, image of the surface and the cross section of the composite PU
foam with spherical water drops dyed with malachite green on it but completely wetted by a drop of carbon tetrachloride stained by Sudan Red III when being dropped onto the surface of foam. Therefore, the as-prepared composite PU foam displays both hydrophobicity and oleophilicity when it is exposed to water and oil respectively. To verify the successful attachment of SiO2@MnO2 onto PU foam and the rough surface structure of the composite foam, the microstructures of pure PU foams and composite PU foams were observed by SEM (Fig. 5). The SEM images of the foams taken with different magnification showed highly porous 3D interconnected networks and micro-porous structures. For the pristine PU foam without SiO2@MnO2 nanoparticles treatment, it is clearly seen that
Fig. 4. Formation schematic and macroscopic morphologies of composite foam. (A-B) optical images of the pure PU foam, (C-D) formation schematic of the composite PU foam, (E-F) optical images of the composite PU foam and water drops on the composite foam after being kept and a drop of carbon tetrachloride is readily adsorbed (F, inset).
Fig. 5. Microscopic analysis of the structure of PU foam. SEM images of the original PU foam (A-B) and the composite PU foam (C-D) with different magnifications.
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the foam surface was smooth and flat and numerous pores with pore sizes of 50–150 lm were randomly distributed on the surface and interior of PU foams (Fig. 5A and B). The 3D network structure was formed by the pore-forming agent (NaHCO3) heated to produce gas during the process of preparing the PU foam. The large macropore size would be a great benefit to the transfer of fluid in the porous monolith and the rough surface formed by the macroporous structure could improve hydrophobicity of compos-
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ite PU foam. As presented in Fig. 5C and D, compared with SEM diagrams of the pure PU foams, the pores of the composite PU foam become rougher than the pores of pure PU foam. The contact angle values (shown in Fig. S4) were found to be 97 ± 1° and 124 ± 1° respectively, showing the hierarchical hollow SiO2@MnO2 cubes can improve hydrophobicity of composite PU foam. With introduction of the hierarchical hollow SiO2@MnO2 cubes, the composite PU foams form a rough surface structure, which makes it possess a better oil storage capacity, thereby further enhancing its oil/ water separation performance. 3.4. Elasticity capacity of composite polyurethane foam
Fig. 6. Height recovery of composite PU foam as a function of recycle number at 65% compression strain. The insets illustrate the compressing–releasing processes of the composite PU foam.
The cyclic compressing tests of composite PU foam were conducted to evaluate the mechanical property. The good mechanical properties of composite PU foam when compressed in organic liquid is the key merit for our strategy of collection of adsorbed oils by simple squeezing and the composite foam with a better mechanical compressibility and hydrophobicity that was resulted from the synergetic effect of its components. The insets of Fig. 6 illustrate the compressing–releasing processes of composite PU foam and the foam could almost recover its initial height after being pressed, which suggested composite PU foam possessed excellent elasticity. In fact, the composite PU foam immediately recovered to their original shapes with no plastic deformation after the release of the pressure. As is shown in Fig. 6, the height of the composite PU foam decreased by 6% after 30 compressing–releasing cycles and 94% of their initial heights were maintained after being repeatedly pressed for 30 cycles. These results suggested that the composite PU foam possessed excellent flexibility, which could greatly facilitate the oil recovery and reuse of the foam.
Fig. 7. Oil adsorption tests of the composite PU foams. (A) Adsorption process of soybean oil (stained with Sudan III) on the surface of water by the composite PU foams; (B) Maximum adsorption capacities of the oil towards eight different organic compounds; (C) The oil adsorption capacity of composite PU foams during the repeated adsorption and release of several organic solvents for 10 cycles.
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3.5. Adsorption capacity of composite polyurethane foam
4. Conclusions
In order to investigate the practical application of the oil spill cleanup behavior of the composite PU foams, we imitated that the composite PU foam separates oil from mixture under natural conditions. As shown in Fig. 7A, employing soybean oil (stained with Sudan III) as the organic solvent in the selective adsorption experiment (the concentration ratio of soybean oil to water is 1%), the composite PU foam could selectively adsorb soybean oil on the surface of water. Once the composite PU foam contacted soybean oil on the surface of water, red soybean oil could be quickly sucked into the composite PU foam within a few seconds (Video. S1). A simple squeezing process can readily collect the adsorbed soybean oil in the foam and no any red pollutants were observed in the water, indicating high separation efficiency of the composite PU foams with no secondary pollution. As is shown in Fig. S5, according to the original added volume (7.6 mL) and collected volume (6.6 mL), we could calculate the collecting efficiency to be 87%. Furthermore, the composite PU foam could also easily adsorb carbon tetrachloride (stained with Sudan III) from the bottom of water (Fig. S6). Considering the growing number of oil-spill accidents and variable composition of oils, the composite PU foams could be used to collect multiple types of oils and organic solvents, which is a significant feature for practical applications. Therefore, we extended the collection experiment to several oils and organic solvents, including carbon tetrachloride, toluene, and soybean oil. Under static conditions, Fig. 7B demonstrated the adsorption capacities of the composite PU foam for several other oils and organic solvents were 8–32 times than its own weight, mainly depending on the density of oils and organic solvents and swelling property of composite foams. The adsorption capacities (Q) for carbon tetrachloride, toluene, THF, acetone, DMF, soybean oil, pump oil, diesel oil was measured to be 31.6, 11.2, 24.2, 14.5, 13.2, 8.9, 9.2 and 8.88 g/g respectively. The adsorbed solvents are stored in the abundant pores of the composite PU foams. No dripping of the adsorbed organics was observed in the handling process, indicating excellent adsorption capability of composite PU foam for various oils and organic solvents.
In summary, the porous composite PU foam with controllable inner porous structures was successfully fabricated through a one-step method. The hierarchical hollow SiO2@MnO2 cubes can be synthesized by in situ growth of MnO2 nanoplatelets on the surface of SiO2 cubes. With introduction of the hierarchical hollow SiO2@MnO2 cubes, hierarchical hollow structure and large surface area of the sample are beneficial to the improved oil adsorption capability of composite PU foam. The as-prepared composite PU foam exhibits the characteristics of superwetting properties, superior elasticity, highly efficient and selective adsorption of some sorts of oil or organic solvents from the mixture. The maximum oil adsorption capacities for carbon tetrachloride and THF were measured to be 31.6 and 24.2 g/g respectively. In addition, oil adsorbed in composite PU foam was easily collected by squeezing. What is more, it could be reused for oil–water separation for 10 cycles without affecting its oil adsorption performance and 94% of their initial heights were maintained after being repeatedly pressed for 30 cycles, demonstrating the great reusability and durability of composite PU foam. Due to simple fabrication, lowcost, environmental friendly, great reusability, as well as fine elastic stability under different conditions, the composite PU foam reported herein shows it has a great potential of application in the cleanup of oil spills in the ocean and it is also expected to be an ideal adsorbent in oil spillage cleanup.
3.6. Reusability of composite polyurethane foam
Appendix A. Supplementary data
Reusability is a significant property for oil-adsorbents in practical application. In this study, the impact of repeatedly wetting by the oils on the wettability of the composite PU foams was investigated to evaluate reusability. In the process of oil adsorption and collection for 10 cycles, the composite PU foams were washed with ethanol and the adsorption capacities (Q) were measured after each water–oil separation experiment. The change of the adsorption capacities with adsorption and collection cycles is shown in Fig. 7C. After this adsorption and regeneration process was repeated 4 times with different organic solvents–water as an example of a mixture system, the composite PU foam still maintains a stable adsorption capacity, exhibiting good repeatability. A slight decrease in the adsorption capacity after the 5th cycle was observed (especially in the case of THF), which may be ascribed to decrease of composite PU foam surface roughness by cleaning of ethanol. The excellent reusability of composite PU foam is believed to arise from the mechanical durability of SiO2@MnO2 nanoparticle powder. Rough porous morphologies still cover the composite PU foams after the 10th cycle. Consequently, the regenerated porous composite PU foam was still reused for the selective adsorption of oil from water. Above all, the excellent properties of high adsorption capacity and good reusability of the composite PU foam will guarantee its promising application for oil–water separation purposes.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2017.06.144.
Acknowledgements We thank the Natural Science Foundation of Jiangsu Province (BK20160500, BK20161362 and BK20161264) and National Nature Science Foundation of China (U1507115) for financial support. This study was also financially supported by the China Postdoctoral Science Foundation (2016M600373), High-Level Personnel Training Project of Jiangsu Province (BRA2016142), China Postdoctoral Science Foundation of Jiangsu Province (1601016A) and Scientific Research Foundation for Advanced Talents, Jiangsu University (15JDG142).
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