Synthesis and characterization of porous fibers/polyurethane foam composites for selective removal of oils and organic solvents from water

Abstract

In the field of oil/water separation, functional oil-absorbing materials with both controllable porous structures and swelling properties are highly desirable. In the present study, we report a novel strategy for fabricating Mg–Al porous fiber (Mg–Al PF)/polyurethane (PU) foam composites using a combined biotemplate method and foaming technology, and discuss their application in the absorption of oils and organic solvents. The Mg–Al PF composites with a hierarchical porous structure are fabricated based on nanoplatelets on the surfaces of microscale inorganic fibers. The PU foam composites with excellent oil swelling properties are synthesized by addition of Mg–Al PF composites to PU foams. In order to enhance the hydrophobic and oleophilic properties, the surfaces of Mg–Al PF composites are chemically modified using silane coupling agent (KH 570). The surface modified Mg–Al PF composites show high repellency towards water with a water contact angle of 146.6°. Owing to their unique pore structures and superhydrophobic and swelling properties, the foam composites can remove oils and organic solvents from water with high selectivity and absorption capacity, and can absorb not only floating oil but also heavy organic solvents underwater. In general, the absorption capacities of the PU foam composites for oils and organic solvents are 5.06–44.81 times their own weight, partly depending on the density and viscosity of the absorbate. The PU foam composites still maintain relatively consistent absorption properties for oil and organic solvent absorption after 10 cycles. These outstanding properties potentially make the as-prepared PU foam composites promising candidates for practical oil absorption and oil/water separation.

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1. Introduction

Oil/water separation is a worldwide challenge owing to the frequent occurrence of oil spill accidents and the increasing quantities of industrial and domestic oily wastewater.¹ Considerable efforts have been made to separate the oils and organic solvents from water using various methods, including absorption,^2,3 solvent extraction,⁴ biodegradation,^5,6 in situ burning,⁷ solidification⁸ and floating boom techniques.^9,10 However, the oil removal efficiencies of solvent extraction and floating boom techniques are not very high, especially for complex oil/water mixture systems. Bioremediation, which makes use of microorganisms to break down the oils, is time-consuming, and requires appropriate amounts of oxygen to be present, appropriate temperature, and the presence of organic species. The methods of in situ burning and solidification are not environmentally recommended since the processes are hazardous to the environment. Among the separation techniques mentioned, the absorption method is considered to be one of the most effective approaches because it is readily available, environmentally friendly, inexpensive and offers good recyclability.^11,12

There has been an increasing amount of research on the synthesis of oil-absorbing materials for the separation of oils and organic solvents from water. However, traditional oil-absorbing materials have low oil absorption capacity and a slow oil absorption rate. New oil-absorbing materials with high absorption capacity and good reusability for highly efficient separation of oils from the water surface are urgently required to be developed.

Currently, three strategies have been used for efficient separation of oils and organic solvents from water using oil-absorbing materials. One of the most employed strategies is to use porous materials to absorb oils and organic solvents because of their large specific surface area and oil storage space. The macroporous structures of these materials may provide an oil storage space for oil absorption. Considerable research efforts have been invested in developing different macroporous oil-absorbing materials for oil separation, such as sponges,^13–15 carbon aerogels^16–18 and graphene foams.^19–22 Li et al.¹⁴ fabricated superhydrophobic/superoleophilic sponges for selective oil absorption from oily water. By utilizing the macroporous structures, Bai et al.¹⁹ fabricated ultra-light and elastic graphene foams for oil absorption.

The second strategy is to use fibrous materials to absorb oils and organic solvents by utilizing their staggered structures. Following this strategy, Cortese et al.²³ fabricated superhydrophobic/oleophilic cotton textiles that can efficiently separate oil and water mixtures using gravity. Yuan et al.²⁴ used superwetting nanowire membranes for selective absorption and reported a maximum oil absorption of organic solvents and oil reaching 20 g g⁻¹.

The third strategy is to use non-porous oil-absorbing polymer materials by utilizing the swelling properties of the polymer. Studies on non-porous oil-absorbing materials primarily focus on oil-absorbing resins and oil-absorbing resin composites. Wang et al.¹² prepared acrylic ester resins by suspension polymerization. The as-prepared oil-absorbing resins exhibit excellent stability over five cycles of use and can be regenerated without a significant decrease in oil absorption. Nevertheless, low oil absorption capacity, poor mechanical properties, high cost and complicated fabrication procedures for oil-absorbing materials seriously hinder their practical application in oil separation. Oil-absorbing materials with the desirable combination of high oil absorption capacity, high oil retention ability, good mechanical and chemical stability remain to be developed.

Polyurethane (PU) foam is a porous polymer with the characteristics of low cost, good elasticity and easy large-scale fabrication that could make it a promising candidate for oily waste water treatment.^25–27 Recently, the introduction of functional nanomaterials into PU materials has received great attention in applications for selective oil absorption. For example, Lü et al.²⁸ fabricated rough hydrophobic PU sponge surfaces by coating PU sponges with SiO₂/graphene oxide nanohybrids. The surface modified PU sponge can serve as an effective and rapid sorbent of organic solvents, and can be recycled with imperceptible loss of sorption capability and hydrophobicity. Wu et al.³ prepared oil absorbents by modifying PU sponges with TiO₂ sol. Li et al.²⁹ prepared superhydrophobic and superoleophilic SiO₂ coated PU sponges through a simple solution-immersion process. In addition, surface modification of PU materials, which involves the introduction of magnetic nanoparticles into the PU sponge, has been reported in the literature for oily water separation. Wu et al.³⁰ fabricated magnetic, durable, and superhydrophobic PU sponges by chemical vapor deposition of tetraethoxysilane to bind the Fe₃O₄ nanoparticles tightly on the sponge and then dip-coating in a fluoropolymer aqueous solution. The absorbency of the modified sponge for industrial fuels, food oils and organic solvents is in the range of 13.26–44.50 g g⁻¹. The oil absorption properties of PU materials are closely related to their surface properties and pore structure. However, most studies focus on the surface modification of PU sponges, whereas the pore structure design of oil-absorbing PU sponges has rarely been discussed.

Hierarchically structured Mg–Al porous fiber (PF) with both macroporous structure and fibrous morphologies has potential application in oil and organic solvent absorption. Here we report a facile approach for preparing PF/PU foam composites by combining the oil absorption properties of PF composites and PU foams. The performances of the PF/PU foam composites are studied in the whole scope of oil absorption, including selectively absorbing floating oils on the water surface and heavy organic solvents underwater. The as-prepared PU foam composites with both controllable porous structures and swelling properties exhibit excellent oil absorbency, high oil/water separation efficiency as well as high mechanical and chemical stabilities, and have promising applications in oily waste water treatment.

2. Materials and methods

2.1. Materials

The absorbent cotton was chosen as the biotemplate in this study. After washing several times with absolute ethanol, it was cleaned with distilled water and dried in air at 80 °C for 12 h. Other reagents (analytical grade) were used as received from commercial suppliers without any further purification. Aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O), hexamethylenetetramine (HMT, C₆H₁₂N₄), magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O), silicone oil ([–Si(CH₃)₂O–]_n), silane coupling agent (KH 570), and sodium bicarbonate (NaHCO₃) were obtained from Sinopharm Chemical Reagent Co., Ltd. Polyether polyol (NJ-330, M = 3000 g mol⁻¹) was supplied by Ningwu Chemical Co. (Jurong, Jiangsu, China). Isophoronediisocyanate (IPDI, NCO content ≥ 37.5%) was obtained from Rongrong Chemical Co. (Shanghai, China). Double-distilled water was used in all experiments.

2.2. Preparation of hierarchically structured Mg–Al PF composites

The Al₂O₃ fibers were prepared based on structure replication of cellulose fibers as described in our previous work.^31–33 The preparation of hierarchically porous Mg–Al PF composites was based on Mg–Al layered double hydroxide (LDH) nanoplatelet growth on the surface of Al₂O₃ fibers, followed by calcination; 0.84 g of hexamethylenetetramine and 1.026 g of Mg(NO₃)₂·6H₂O were dissolved in 60 mL of distilled water in an 80 mL autoclave Teflon vessel. Then, 0.204 g of as-prepared Al₂O₃ fibers was immersed in the above mixed solution. The vessel was further treated under hydrothermal conditions at 120 °C for 8 h. After that, the as-prepared products were taken out of the vessel, washed with distilled water several times, and dried in air at 80 °C. Finally, the obtained product was calcined at 500 °C for 4 h to obtain hierarchically porous Mg–Al PF composites.

2.3. Surface hydrophobic modification of Mg–Al PF composites

The hydrophobic modification of Mg–Al PF composites was performed by grafting the hydrophobic group on the surfaces of PF composites, and the modification process of hierarchical PF is illustrated in Scheme 1. Firstly, 0.5 g of KH 570, 2.5 g of porous Mg–Al PF composite, and 80 mL of distilled water were added to a 250 mL three-necked round-bottomed flask with a mechanical stirrer for 30 min. Then the mixture was sealed and placed in a microwave reaction system (XH-100A, Beijing XiangHu Science and Technology Development Co., Ltd.) with a frequency of 2.45 GHz. The reaction system was rapidly heated to 85 °C at a power of 700 W, and maintained at 85 °C for 60 min with rotation and magnetic stirring. Finally, the microwave treated products were filtered, washed with distilled water, and then dried at 60 °C in air for 12 h before characterization.

Scheme 1 Schematic illustration for the preparation of Mg–Al PF/PU foam composites.

2.4. Preparation of PF/PU foam composites

Polyether polyol-based PU foams were synthesized by polyaddition of NJ-330 and IPDI and the schematic illustration for the preparation of Mg–Al PF/PU foam composites is shown in Scheme 1. In a typical synthesis, a mixture containing 10 g of NJ-330, 2.22 g of IPDI, 1 g of NaHCO₃, 0.65 g of silicone oil and 0.4 g of hydrophobic Mg–Al PF composites was prereacted with vigorous mechanical stirring until the appearance of small bubbles. Then the mixture was placed in a ventilated drying oven at 100 °C for 3 h. In order to optimize the oil absorbency of the PF composites/PU foam, the content of PF composites was investigated in detail.

2.5. Sample characterization

The morphologies of the PF, PU foam and PU foam composite surfaces were characterized with a JEOL JSM-PLUS/LA scanning electron microscope. The SEM specimens were prepared by sputter coating a thin gold layer approximately 3 nm thick. X-Ray diffraction (XRD) data were recorded using a Shimadzu XRD-6100 instrument with Cu Kα radiation at 40 kV and 30 mA, a scanning rate of 4° min⁻¹, and a 2θ angle ranging from 10° to 80°. The functional groups and chemical structures were confirmed using a Nicolet FTIR spectrometer in the range of 4000–450 cm⁻¹ via potassium bromide (KBr, optical grade) pellet. The contact angle of hydrophobic Mg–Al PF composites was tested using a commercial CAM200 optical system by the sessile drop method. The volumes of the droplets used in the experiments were about 5 μL and at least three different spots were taken on the same sample surface for contact angle measurements to obtain a mean value.

2.6. Oil absorption

Oil absorption was performed by dipping PF/PU foam composite samples into oil (or organic solvents)/water mixtures. The absorbed oils PF/PU foam composites were determined by a weighing method. In a typical absorption process, 0.2 g of oil absorbent samples was immersed in oil (including edible oils, organic solvents and fuels) at room temperature for 24 h. After that, the samples were taken out of the oil, drained for 3 min to remove residual oil, and then weighed immediately. The oil absorption properties of absorbent samples were calculated by deduction of the weight of the saturated filter bag, and calculated with the following formula:

Q (g g⁻¹) = (m_t − m₀)/m₀ (1)

where m_t and m₀ are the weight of the oil absorbents dispersed in oil for time t and the dry weight of the oil absorbent, respectively.

2.7. Recycling of used absorbent

To test the regeneration capacity of the PF/PU foam composite samples, the saturated oils and organic solvents were first squeezed from the foams by a simple mechanical compression method. Then the samples with absorbed organic solvents were directly dried in an oven and then heat treated at 120 °C in an ambient atmosphere for 12 h, while the samples with absorbed oils were immersed in 50 mL of anhydrous ethanol to release the absorbed oil, followed by drying at 120 °C for 12 hours. The process was repeated 10 times to confirm the reusability of PF/PU foam composites. For each cycle, the PU foam composites were weighed before and after oil absorption.

3. Results and discussion

As a green and inexhaustible resource in nature, cellulose has properties of high absorption capacity, high water or oil retention ability and good recyclability due to its unique tubular structure, and it can be converted into inorganic materials with fibrous structures for application in oil–water separation. Fig. 1 shows typical SEM images of Al₂O₃ fibers and Mg–Al PF composites prepared after in situ growth of the nanoplatelets. As shown in Fig. 1A, the Al₂O₃ fibers retain the original fibrous cotton morphology by removal of biotemplates. However, the SEM results (Fig. 1B–D) revealed the presence of a micro/nano-hierarchical structure in the samples of PF composites, which mimics the microscale fiber structures of cotton, and has a nanoscale sheet-like structure. As seen in Fig. 1B, the length of the Mg–Al PF composites ranges from tens to several hundred micrometers and the outer diameter varies from several to tens of micrometers. It should be noted in Fig. 1C that the surfaces of inorganic fibers are covered by a layer of nanosheets. The nanosheets are vertically aligned on the surface of inorganic fibers, indicating that the mechanism of the hydrothermal process was governed by in situ crystallization. It should be noted in Fig. 1D that most of the nanosheets exhibit nonuniform shapes with the majority of crystallites ranging from 600 nm to 1.5 μm in size. In addition, the macroporous structures, ranging from nanometer to micrometer scales, are formed by interleaved nanosheets, which can produce interesting surface effects for oil absorption and storage.

Fig. 1 SEM images of Al₂O₃ fibers (A) and Mg–Al PF composites (B–D).

The hydrothermal and calcination treatment not only influence the morphology of the materials obtained, but also determine the composition and structure of PF composites. The crystal structures of as-synthesized Al₂O₃ fibers and Mg–Al PF composites were analyzed by XRD, as shown in Fig. 2A. A single broad diffraction peak appeared in the 2θ range of 20–30° (Fig. 2Aa), implying that the Al₂O₃ fibers are amorphous. Fig. 2Ab depicts the XRD pattern of the Mg–Al PF composites obtained by hydrothermal and calcination treatment. All diffraction peaks can be readily indexed to γ-Al₂O₃, which can be confirmed by the broad peaks centered at 37.44°, 39.67°, 45.78° and 67.31°. The reflections of the PF composites show broad and asymmetric peaks, indicating the poor crystallinity of γ-Al₂O₃. The results indicated that hydrothermal and calcination treatments led to the change of structure of inorganic fibers and the transformation of the phase from amorphous to γ-Al₂O₃. No diffraction peaks of magnesium-containing compounds are observed in the XRD pattern, which may be ascribed to the magnesium being uniformly dispersed in the aluminum oxide. The XRD patterns obtained from the hydrothermally treated inorganic fibers are shown in Fig. 2B. The hydrothermally treated samples showed better crystallinity as seen in their distinctive XRD peaks compared with those of Al₂O₃ fibers and Mg–Al PF composites. The samples contained well-resolved peaks at 11.7°, 23.6°, 35.0°, 35.6°, 39.6°, 47.1°, 53.8°, 56.7°, 62.3°, 64.0° and 66.3° characteristic of the peaks in Mg–Al LDH (JCPDS no. 35-0964), and indicating their monophasic nature.

Fig. 2 (A) XRD patterns for Al₂O₃ fibers (a) and Mg–Al PF composites (b); (B) XRD patterns for the standard card (JCPDS no. 35-0964) (a) and hydrothermally treated inorganic fibers (b).

Wettability of PF composites to water is one of the most important considerations when selecting an absorbent suitable for oil absorption. The surfaces of PF composites were chemically modified using silane coupling agent (KH 570) to form the hydrophobic/oleophilic surfaces. To evaluate the hydrophobicity of the Mg–Al PF composites, the contact angles were measured. Fig. 3 shows the water contact angle profiles of the substrates coated with the raw Mg–Al PF composites and KH 570 modified PF composites. As shown in Fig. 3A and B, the raw Mg–Al PF composites exhibit a hydrophilic character with the water drop spreading almost instantly. However, the results on the measurement of contact angles show that the KH 570 modified PF composites display a hydrophobic surface with a stable water contact angle of 146.6°, implying that the hydrophobic groups were successfully grafted on the surfaces of PF composites, as evidenced by the FT-IR results (see the ESI in Fig. S1†).

Fig. 3 Water contact angle images for Mg–Al PF composites before (A and B) and after (C and D) KH 570 modification.

In the present system, the PF composites may play a critical role in the process of foaming reaction and further determine the structure of PU foams. The macroscopic and microscopic structures of PU foams and PF/PU foam composites are illustrated in Fig. 4. As shown in Fig. 4A, the macroscopic structures of PU foams reveal a typical porous network-like structure. However, due to the soft PU matrix, pure PU foams exhibit a slightly collapsed structure. Fig. 4B and C display the microscopic structures of PU foams, which clearly indicate that the PU foams are mainly composed of hollow microspheres with a diameter of approximately 600 μm. The hollow structures of the microspheres can provide an oil storage space that can enhance the oil absorption properties of the PU foams, while the holes in the surfaces of microbubbles can effectively reduce the mass transfer resistance of foams and accelerate the oil absorption rate. It is clear that the surfaces of microbubbles are relatively smooth and apparent holes are observed.

Fig. 4 Photographs and SEM images of PU foams (A–C) and PF/PU foam composites (D–F).

The macroscopic shape of PF/PU foam composites is shown in Fig. 4D. Effective porous foams are obtained because of the critical supporting role played by PF composites in the internal PU foam. The volume of PF/PU foam composites is far higher than that of pure PU foams. The microscopic structures of PU foam composites are shown in Fig. 4E and D. Similar to the pure PU foams, the PU foam composites exhibit spherical morphology and have sizes between 500 and 800 μm. Compared with the hollow pure PU bubbles with thick and densely packed shells, the porous-shell hollow structures, formed by hierarchically structured Mg–Al PF in the bubble shells, are able to provide an extra space for oil diffusion and increase the oil storage space. The hollow structure and swelling properties of PU foam composites may result in the excellent oil absorbency. As shown in Fig. 4E, slight breakage of the spherical bubbles can be found in the PU foam composites. In the formation processes of foams, the air phase volumes are dramatically increased with decomposition of NaHCO₃, which results in breakage of the bubbles and an increase in oil storage space. The Mg–Al PF composites are mainly located on the inside of the bubble walls, and only a small amount of inorganic fibers are exposed on the surfaces of hollow bubbles. Compared with the Mg–Al PF composites, the surfaces of inorganic fibers are covered with a layer of PU and display a smooth surface.

In the process of foam formation, the content of PF in PU foam composites may play a critical role in the process of foaming reaction and further determines the internal structures of foams, and the sizes and shapes of bubbles, thus affecting the oil absorption properties. Fig. 5 illustrates the effect of PF content on the oil absorption properties of PU foam composites. As can be seen in Fig. 5, the chloroform absorption properties of PU foam composites increase with increasing PF content, and reach a maximum absorption capacity of 43.2 mg L⁻¹ at a PF content of 0.6 g, whereas there was a decline in absorption properties when further increasing the PF content. A similar trend is observed for soybean oil absorption. Optimized oil absorption of PU foam composites was achieved with inclusion of 0.6 g of PF. This can absorb up to 43.2 times and 12.9 times its weight equivalent of chloroform and soybean oil, respectively, higher than that of the previously reported results.^34–36 The enhanced absorption properties of PU foam composites may be attributed to the increased oil storage space brought about by introducing the PF. Note that the chloroform absorption properties of PU foam composites are higher than that of soybean oil, mainly due to the self-swelling properties of PU foams in small molecule solvents (see the ESI in Fig. S2†).

Fig. 5 Oil absorption properties of PU foam composites affected by the content of PF.

One important application of foam materials is in the absorption of oils and organic solvents. The PU foam composites exhibit excellent structural flexibility, high porosity, and large surface area and contain interconnected open pores; the as-synthesized foam composites should be ideal candidates for the absorption of oil pollutants. To demonstrate the feasibility of PU foam composites in the absorption of oils and organic solvents, soybean oils and chloroform dyed with Sudan III were selected as representative absorbates on behalf of organic solvents with different densities in the performance of the removal of oils and organic solvents from water. As illustrated in Fig. S3,† the dyed soybean oil floating on the surface of water can be quickly absorbed by a small piece of PU foam composite, indicating the high oil-absorbing ability of the foam composites. No dripping of the absorbed oil was observed in the handling process indicating excellent absorption by the PU foam composites. In addition, PU foam composites could also be used for the absorption of heavy organic pollutants underwater and which were removed from the water without leaving a trace (see the ESI in Fig. S3†). Once it comes into contact with the oil underwater, the heterogeneous sponge/water interface with entrapped air automatically converts to a homogeneous sponge/oil interface owing to the low surface tension of oils. This commendable property indicates the possibility for environmental applications in absorbing floating pollutants and those underwater.

In order to measure the absorption properties of the PU foam composites, samples were immersed in oils or solvents without water and the maximum absorption capacities were determined by mass change. Six kinds of frequently encountered organic solvents in chemistry laboratories, namely toluene, acetone, n-hexane, dimethylformamide (DMF), carbon tetrachloride (CTC) and chloroform, were used to evaluate the absorption properties of the PU foam composites, and the results are shown in Fig. 6A. The as-prepared PU foam composites show high absorbency for these organic solvents. In general, the absorption capacities of the PU foam composites for organic solvents were 7.73–44.81 times their own weight, partly depending on the density and viscosity of the solvents. For example, the Q value of PU foam composites towards CTC is 37.04 g g⁻¹, which is much higher than the absorption of toluene and DMF. The maximum absorption capacity could reach up to 44.81 g g⁻¹ for chloroform absorption, higher than the values reported in the literature for commercial PU sponge absorption. In order to compare the absorption capacity of pure PU foams and PU foam composites, the absorption capacities of the pure PU foams were investigated under the same conditions as shown in Fig. 6A. However, the absorption capacities for a wide range of these organics ranged from 3.38 to 22.68 times that of the pure PU foams. The enhanced absorption capacities may originate from the high porosity and excellent swelling properties of PU foam composites.

Fig. 6 Comparison of absorption capacities of pure PU foams and PU foam composites towards different organic compounds (A), edible oils and fuels (B).

In addition to analyzing the organic solvent absorption properties of the PU foam composites, the absorption of PU foam composites toward various types of oils and fuels was also examined, and the results are shown in Fig. 6B. The absorption of the PU foam composites for these oils and fuels is in the range of 5.06–11.57 g g⁻¹, which is much lower than the absorption of organic solvents. It is known that the absorption properties of an oil-absorption material are closely related to the macroporous structures and swelling properties of the absorbents. In addition, the density, viscosity and molecular weight of the absorbates as well as the physical and chemical properties of the absorbents strongly affect the absorption properties of oil-absorption materials. In this case, the density of absorbates and swelling properties of absorbents are the two main factors determining the oil and organic solvent absorption. Heavy organic solvents may result in a higher Q value. In addition, the PU foam composites can be swelled in organic solvents, resulting in higher absorption properties (see the ESI in Fig. S2†). Hence, the variation of the absorption properties of PU foam composites is caused by the density and swelling properties of oils and organic compounds. It should be noted that the oil absorption capacities of pure PU foams are enhanced by introducing the PF composites, which may originate from the changed pore structures of PU microbubbles. A comparison of the maximum sorption capacity of PF/PU foam composites used in this study with other previously reported absorbents is presented in Table 1. The PF/PU foam composites show a high oil absorption capacity, reaching up to 44.8 g g⁻¹.

Table 1 Comparison of several reported absorbents for oil absorption

Absorbents	Maximum sorption capacity, qmax (g g^−1)	References
Molecularly imprinted polymers	35.0	2
Multiwall carbon nanotubes/PUF	20.4	7
Mn2O3/resin composites	34.0	11
Modified acrylic ester resin	38.5	12
Carbon nanofiber sponges	75.0	13
Superhydrophobic/superoleophilic sponge	60.0	14
Superwetting nanowire membranes	20.0	24
PUF modified with nanoclay	18.0–22.0	26
PF/PU foam composites	44.8	This work

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Cooking-generated oil contaminants have been considered to be one of the major indoor pollutants and can be a significant harm to health. The removal of waste cooking oils remains one of the most intractable problems. By using commercial sponge as the cleaning material, drawbacks that existed in the removal of waste cooking oils, such as the low absorption capacity, poor decontamination results and difficulties in recycling owing to the hydrophilic surface of sponge, cannot be overcome. Detergents containing surfactants are generally used to remove waste cooking oils on the surface of tableware and tables. The as-prepared PU foam composites exhibit hydrophobic and lipophilic properties and excellent oil absorbency, which have promising applications in the removal of waste cooking oils in public and residential kitchens. Fig. 7 shows the process of removing dyed cooking oil from a lunch box and kitchen table. As soon as the PU foam composites came in contact with the dyed cooking oils on the surfaces of the lunch box and table, the oil was quickly and completely absorbed by the PU foam composites, indicating their potential use in the kitchen as a selective absorbent for removal cooking oils. In addition, the dyed organic solvents on the surfaces of glassware can be completely removed by as-prepared PU foam composites, indicating the potential application in the laboratory (see the ESI in Fig. S4†).

Fig. 7 Photographs of the removal of cooking oil from lunch (A–C) box and table (D–F) in kitchen.

Desorption of the absorbed PU foam composites is essential as the absorbents can then be reused to absorb further oils and organic solvents. In order to evaluate the reusability of PU foam composites, the as-synthesized PU foam composites were first immersed in oils and organic solvents to reach saturated absorption. Then the absorbed oils could be removed by washing with anhydrous ethanol followed by drying at 120 °C, while the absorbed organic solvents could be simply removed by drying at 120 °C. Fig. 8 shows the experimental results for the amount of oil and organic solvent absorption in 10 absorption–regeneration cycles. It can be seen that the PU foam composites still maintain relatively consistent absorption properties after 10 cycles of oil and organic solvent absorption. Thus, as-synthesized PU foam composites can work as an efficient and durable oil-absorbing material for the separation of oils and organic solvents.

Fig. 8 Comparison of oil absorption capacities of PU foam composites in 10 absorption–regeneration cycles.

4. Conclusions

We report the synthesis of PF/PU foam composites with both porous structures and oil swelling properties for the application of oil and organic solvent absorption. In this strategy, the hierarchically structured Mg–Al PF composites comprised of nanoplatelets and inorganic fibers are fabricated by the biotemplate method and in situ techniques. The PU foam composites with excellent oil absorption properties are synthesized by addition of PF composites to PU foams. Results from the measurement of contact angles show that the KH 570 modified Mg–Al PF composites display a hydrophobic surface with a stable water contact angle of 146.6°, implying that the hydrophobic groups are successfully grafted on the surfaces of PF composites. The PU foam composites show high selectivity towards oil and organic solvent absorption from oil/water mixtures. The absorbency of the PU foam composites for these oils and fuels is in the range of 5.06–44.81 g g⁻¹, partly depending on the density and viscosity of the absorbate. They can absorb floating oils on the water surface and heavy organic solvents underwater. In addition, they can be used to remove waste cooking oils on the surface of tableware and tables. More importantly, the regenerated PU foam composites can be reused without significant loss of oil and organic solvent absorption capacity after 10 cycles of oil and organic solvent absorption. It is believed that the synthesis of PU foam composites can be extended for the fabrication of other porous oil-absorbing materials for practical oil absorption and oil/water separation.

Acknowledgements

The authors would like to thank Jiasheng Fang and anonymous reviewers for their suggestions and comments which have significantly improved the quality of this manuscript. The National Natural Science Foundation of China (U1507115 and 21576120), Natural Science Foundation of Jiangsu Province (BK20160500, BK20161362 and BK20160491) and Scientific Research Foundation for Advanced Talents, Jiangsu University (15JDG142) are thanked for their financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10916k

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