Durable and modified foam for cleanup of oil contamination and separation of oil–water mixtures

Abstract

With the continuous exploitation and utilization of oil resources, oil pollution has become increasingly serious, which not only causes economic loss, but also damages the living environment of human beings. Herein, oleic acid coated Fe₃O₄ particles and PS microspheres were introduced to the surface of pure foam through an inexpensive two-step immersion method; we obtained a durable and modified magnetic polystyrene foam (DMMPF) with high efficiency and selectivity. The as-obtained DMMPF exhibited superhydrophobicity, superoleophilicity, and fast magnetic response via the surface chemical modification. In addition, the as-prepared DMMPF could be used for continuous separation of various oils and organic solvents from the water surface. The absorption intake capacity of DMMPF was 40.1 times its own weight and the absorbed oils/organic solvents could be collected by simple mechanical extrusion. Furthermore, the as-prepared DMMPF retained an excellent absorption capacity and a large water contact angle after 60 separation cycles. It is recognized that DMMPF could be suitable for large-scale oil spill processing.

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Introduction

In the past half century, oil spills which occur on the ocean have become a serious global problem.¹ The adverse impacts on human health and the environment have raised great concern.^2–4 It is reported that a thin oil film on the water surface could cause serious damage to water organisms.^5–7 This is because that the oil film prevents oxygen from entering the water body. What's worse, the toxicity of oil results in a sharp deterioration of the ecological environment and the vast majority of unsettled oils will continue to remain in the ocean. Organic vapors generated by oil evaporation pollute the atmospheric environment and lead to chemical smog.^8–10 In order to address these environmental issues caused by oil spills, many approaches have been employed to clean up the oils, such as physical absorption,^11–14 enhanced bioremediation^15,16 and chemical reaction.¹⁷ Among the above-mentioned methods, physical absorption using an oil absorbing material with superhydrophobic and superoleophilic properties is considered as the most useful measure, as it has the advantages of low cost, convenience and no secondary pollution, etc.^18–22 However, most leaked chemicals such as crude oil, diesel oil, benzene, and dichloromethane can undergo an emulsification reaction in the oil–water mixture, making it difficult to clear up.^23,24 Considering this, it is necessary to develop a type of physical absorption material which should have the following functions. Firstly, the material should have a selective absorption capacity and be able to absorb various oils and organic solvents. Secondly, the material should have excellent hydrophobicity and oleophilicity. Last but not least, the material should still retain a high absorption capacity and water contact angle after many cycles.^25–28

In recent years, the functionalized foams and sponges with many superior performances have attracted extensive attention.^29–31 Due to the mutual connection of their 3D network structure, these materials have an excellent absorption capacity and good oil-holding capacity. However, their high energy consumption, low selective absorption capacity and low recycling rate limited their practical application.^32,33 As a type of high oil absorption material with a 3D structure, durable foam has a very large space for successive use.^34–38 In this article, a durable and modified magnetic polystyrene foam (DMMPF) with high efficiency and excellent absorption capacity was successfully synthesized by a simple and inexpensive two-step immersion method. Combining effective absorption, superhydrophobicity, superoleophilicity and good magnetic response, the as-prepared foam with a 3D network structure could be used for selective oil absorption and continuous oil–water separation. More importantly, the absorbed oils and organic solvents could be collected by mechanical squeezing before the next cyclic operation of DMMPF. The study may provide a facile and novel preparation method to prepare a durable and functionalized foam, which could be used to deal with large scale oil spills.

Experimental

1. Materials and chemicals

All chemicals were of analytic reagent grade and used without further treatment, and all solutions were prepared with deionized water. Polyethylene (PE) shock absorption foam and vinyltriethoxysilane (VTES) were obtained from Aladdin. Absolute ethanol (C₂H₅OH) was bought from the Nanjing Chemical Reagent Co. Ltd., Nanjing, China. Azobisisobutyronitrile (AIBN) and acetone (C₃H₆O) were gained from Shanghai No. 4 Reagent & HV Chemical Co. Ltd., Shanghai, China. Sodium hydroxide (NaOH) was received from Xilong Chemical Reagent Co. Ltd., Shantou, China. Hydrochloric acid (HCl) and n-hexane (C₆H₁₄) were purchased by Sinopharm Chemical Reagent Co. Ltd., Shanghai, China.

2. Preparation of durable and modified magnetic polystyrene foam

In a typical synthesis, shock absorption foam (3 cm × 1.5 cm × 1.5 cm) was purified in deionized water and acetone with mechanical stirring at 60 °C for 3 h successively. Initially, a piece of the pure foam was immersed in 15 mL of absolute ethanol containing 25 mg of the homogeneously dispersed oleic acid coated Fe₃O₄ particles which were prepared through our previously reported method.³⁹ Then, oleic acid coated Fe₃O₄ particles were attached to pure foam under ultrasonic oscillation along with mechanical vibration at room temperature. A durable magnetic polystyrene foam (DMPF) was obtained via introducing PS microspheres which were prepared by dispersion polymerization in ethanol onto the surface of the magnetic foam using 50 mg of AIBN as initiator at 50 °C for 3 h. After that, the durable and modified magnetic polystyrene foam (DMMPF) was fabricated by immersing an n-hexane solution containing VTES (7% v/v) for 1 h. Finally, the durable and modified magnetic polystyrene foam with superhydrophobicity and superoleophilicity was prepared after being dried at 60 °C for 3 h. The preparation procedures are exhibited in Fig. 1.

Fig. 1 Illustration for the synthesis of DMMPF.

3. Characterization of the durable and modified magnetic polystyrene foam

X-Ray diffraction (XRD) analysis was performed on a D8 Advance (Bruker D8 Super Speed) X-ray diffractometer with Kα radiation. The surface morphologies of the as-prepared foams were observed by field-emission scanning electron microscopy (FESEM, Model-S4800, Hitachi, Japan). All samples were fixed on aluminum stubs and coated with gold to obtain optimum efficiency. Surface chemical compositions were investigated using energy-dispersive X-ray spectroscopy (EDS, OXFORD INCA Energy Dispersive Spectrometer). The mechanical properties of the as-prepared samples were measured by a dynamic mechanical analyzer (DMAQ800, TA Instruments) at room temperature. Thermal gravimetric analysis (TGA) was performed with a Model TA2100 (TA Instruments, USA) in the range from 50 °C to 650 °C at a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere. Contact angles were detected by using a Drop Shape Analyzer SL200B (CAs, SL200B, Solon Tech. Co. Ltd., China). Water and lubricating oil droplets with a volume of 4.0 μL were dropped carefully onto the surface of the as-prepared samples. The magnetic properties of the samples were investigated in a fields with a vibrating sample magnetometer (VSM, Lake Shore 735).

4. Oil-absorption experiments

The oil-absorption capacities of the as-prepared samples were measured by weight measurements at ambient temperature. In a typical procedure, nine kinds of oils and organic solvents including lubricating oil, diesel oil, salad oil, dichloromethane, cyclohexane, benzene, DMF, ethylene glycol and tetrahydrofuran along with water were poured on the surface of water in a beaker, respectively. Then, a piece of sample was weighed and immersed in mixed solution. The oils and organic solvents were selectively and quickly absorbed by the as-prepared foams after several seconds, then drained and wiped to remove excess oils or organic solvents. Later, the absorbed oils and organic solvents together with the foam were collected by a magnetic bar or a simple external force. The absorption capacity was calculated using the following formula: S = (M_a − M_b)/M_b, where S is the absorption capacity of the as-prepared foams, and M_a and M_b are the weight of the foam after and before absorption, respectively.

5. Regeneration of absorbent

After the absorption process, the oils and organic solvents were collected by simple mechanical squeezing. After being dried at 60 °C for 3 h, the regenerated DMMPF was reused to remove oils and organic solvents from the water surface. The absorption–desorption procedure was repeated 60 times. For each cycle, the water contact angle and oil absorbency were measured.

Result and discussion

The as-prepared DMMPF was fabricated through a facile two-step immersion method. First, the composition of the sample was confirmed by XRD analysis (Fig. 2a), exhibiting the characteristic patterns of DMMPF. The intense peaks at 2θ = 30.1°, 35.7°, 43.1°, 57.2° and 62.6° represent the (220), (311), (400), (511) and (440) reflections of Fe₃O₄ (JCPDS file no. 19-0629), respectively. It was obviously seen that the curve showed no characteristic peak of any other crystallite, indicating that only oleic acid coated Fe₃O₄ particles were attached onto the surface of the pure foam. To confirm further the surface chemical composition of the as-prepared samples, we conducted EDS measurements to test the existing elements. The elements of C, Fe, and O could be seen clearly in the spectrum of the oleic acid coated Fe₃O₄, DMPF and DMMPF. Due to the process of the reaction, the percentage composition of C presents an increasing trend which was attributed to the PS layer attached to the surface of the magnetic foam (see the ESI, Fig. S1a and b and Table S1†). The additive element of Si was ascribed to hydrophobic surface modification by VTES (Fig. S1c†). These changes of chemical composition could improve the wettability of the as-prepared foam.

Fig. 2 (a) XRD pattern for DMMPF. SEM images of pure foam (b), magnetic foam (c and d), DMPF (e and f) at different magnifications.

In order to study the hydrophobicity of the as-prepared foams, the surface morphology, which is an important factor to evaluate surface wettability, was taken into account. As shown in Fig. 2b, the pure foam possessed a 3D hierarchical porous structure and the surface was very smooth without any distributed microscale protrusions. From Fig. 2c and d, it was obviously that the smooth foam skeleton was covered by many uniformly dispersed particles i.e. oleic acid coated Fe₃O₄ particles, with a size of about 300 nm. As the coated reaction continued, the surface of DMPF exhibited random roughness, which was very important to the hydrophobicity of the foam (Fig. 2e and f).

Interestingly, when DMMPF was compressed to 30% of its original shape under extra force, it could recover 98% of its original dimensions when the extra force was released, as shown in Fig. 3. Moreover, it was found that the as-prepared foam retained its elasticity after several cycles of compression tests. The cyclic compression tests of pure foam, DMPF and DMMPF were conducted to evaluate the mechanical properties and the results are summarized in Fig. S2.† Compared with Fig. S2a–c,† it was evidenced that the stress of DMMPF at 80% compression strain increased after modification. In particular, pure foam reached a stress of 8.5 kPa at 80% and the stress increased to 46 kPa at 80% strain for DMPF after coating by OA-Fe₃O₄ and PS. Among them, DMMPF exhibited superior structural stability and the stress for DMMPF could reach up to 48 kPa at 80% strain. It was noted that the compression stress of DMMPF decreased slightly even after 60 compression cycles, which confirmed the excellent mechanical properties. There were probably two reasons for the excellent mechanical properties. Firstly, the interconnected three-dimensional network structure of the as-prepared foam improved the structural stability. Secondly, chemical bonds and van der Waals forces were enhanced by introducing magnetic particles and PS layer, and grafting hydrophobic groups on the surface of pure foam. Therefore, when DMMPF was used for oil and organic solvent absorption, the absorbed oil and organic solvent could be easily extruded out of the DMMPF pores.

Fig. 3 Images showing compression and recovery for DMMPF.

The thermal decomposition behavior of the as-prepared samples was investigated by TG-DSC techniques. The TG and DSC curves of pure foam, DMPF and DMMPF are exhibited in Fig. 4a and b, respectively. It can be observed from the TG curves in Fig. 4a that the difference in residual content between curve (a) and curve (c) was about 26.1 wt%, indicating that 26.1 wt% of the PS layer was introduced onto the surface of DMPF. Obviously, the 10.9% weight loss which was obtained from curve (b) and curve (c) was attributed to alkyl hydrophobic groups on the surface of DMMPF. Owing to the high content of alkyl hydrophobic groups, DMMPF exhibited excellent hydrophobic properties. From Fig. 4b, we could observe that an endothermic process presented in all samples at about 449.1 °C could be ascribed to the thermal decomposition of pure foam, during which pure foam was decomposed to generate some gaseous products. The low temperature decomposition peak at 95.2 °C was attributed to the glass transition temperature of PS, as exhibited in curve (b) and curve (c). After modification by VTES, the high temperature decomposition peak appeared at 415.8 °C (curve c), which could be attributed to the grafting of alkyl hydrophobic groups. The TGA and DSC curves of the as-prepared samples exhibited the same information.

Fig. 4 (a) TGA and (b) DSC curves of pure foam, DMMPF and DMPF. (c) Magnetic curves of oleic acid coated Fe₃O₄, magnetic foam, DMPF and DMMPF.

Moreover, oleic acid coated Fe₃O₄ on the surface ensured magnetism, which allowed the oil-absorbed foam to be easily collected under the magnetic field (Movie S1†). The superparamagnetic propertis of oleic acid coated Fe₃O₄ had not changed after modification by PS and VTES. Representative hysteresis curves of oleic acid coated Fe₃O₄, magnetic foam, DMPF and DMMPF are illustrated in Fig. 4b. It could be obviously seen that all the samples showed superparamagnetic properties between −5 and +5 kOe at room temperature, as indicated by the absence of remanence and coercivity upon removing an external applied magnetic field. The saturation magnetization values of oleic acid coated Fe₃O₄, magnetic foam, DMPF and DMMPF were 53.7, 31.9, 21.1 and 18.7 emu g⁻¹, respectively. The results showed that the saturation magnetization decreased after modification. There are two reasons that account for this phenomenon. One reason was that the mass fraction of oleic acid coated Fe₃O₄ decreased after modification. The other reason was that coating layer increased after modification. When the coating layer became thicker, the saturation magnetization decreased. This result was in good agreement with the TGA results.

The superoleophilicity and superhydrophobicity of the as-prepared foams were estimated by the measurement of contact angles. When a lubricating oil droplet was placed on the surface of pure foam, DMPF and DMMPF, it immediately spread into the 3D structure of foams, as shown in Fig. 5a–c. It was noteworthy that the oil contact angle decreased in turn, indicating that the oleophilicity of the samples increased successively. What's more, the oil contact angle of DMMPF was almost 0°, exhibiting that the sample was superoleophilic. As shown in Fig. 5d and e, the water contact angles of pure foam and DMPF were about 114.5° and 131.5°, respectively. The increase in the contact angle was attributed to the PS layer on the surface of the magnetic foam. However, the water contact angle value of DMPF hardly satisfied the conditions of superhydrophobicity. In order to achieve these conditions, we utilized a silane coupling agent (VTES) as a hydrophobic surface chemical modification agent and the water contact angle could be increased to 150.5° (Fig. 5f). Interestingly, the pure foam sank beneath the surface of water. However, when DMMPF was immersed in water under an external force, and the foam surface was surrounded by air bubbles, Cassie–Baxter nonwetting behavior was observed owing to the signature of the composite solid–liquid–air interface⁴⁰ (Fig. S3†). DMMPF immediately floated on the water surface after release of external force, and no water uptake was observed (Movie S2†). Moreover, the as-prepared DMMPF displayed stable hydrophobicity, even when floating on 1.0 M HCl and NaOH solution for 72 h, respectively, as illustrated in Fig. S4.†

Fig. 5 Optical images of a lubricating oil droplet (a–c) and a water droplet (d–f) placed onto the surface of pure foam, DMPF and DMMPF, respectively.

The as-prepared DMMPF could be used for absorbing oils and organic solvents. The absorption capacities under different conditions were also measured by weight measurement. The lubricating oil absorption process (dyed by Sudan I) is illustrated in Fig. 6 and Movie S3.† When DMMPF was placed on the mixture, it selectively absorbed lubricating oil from the mixture in 60 s and still floated on the water surface, indicating the excellent hydrophobicity. After absorption, the oil-soaked DMMPF could be readily removed with a magnetic bar and collected before the next cyclic operation. Interestingly, DMMPF could also absorb organic solvents. A benzene absorption experiment was shown in Fig. 7 with photographs taken in intervals of 20 s. When a piece of DMMPF was placed on the surface of a benzene–water mixture, the benzene (labeled by Sudan Red for clear observation) was quickly absorbed by DMMPF. The organic-solvent-absorbed-DMMPF could be easily collected by a simple extra force. It only took 60 s for DMMPF to reach its maximum absorption capacity, demonstrating that DMMPF could be an oil absorbent with high efficiency. However, whether the water would be absorbed was studied, as exhibited in Fig. S5a–f.† When a piece of DMMPF was forced into deionized water (which was labeled by copper sulphate for clear observation), it exhibited a silver mirror-like surface, and water could not spread into the DMMPF. We put DMMPF immersed in water on a napkin and also tested the quantity. More interestingly, we found that there was no water droplet on the napkin (Fig. S5e and f†) and the mass of the napkin hardly increased, indicating the excellent hydrophobicity and selectivity of DMMPF.

Fig. 6 Lubricating oil removal from water surface with DMMPF under magnetic field.

Fig. 7 Benzene removal from water surface with DMMPF under extra force.

The absorption capacities of pure foam, DMPF and DMMPF were investigated by employing water, lubricating oil, diesel oil, salad oil, dichloromethane, cyclohexane, benzene, DMF, ethylene glycol and tetrahydrofuran as absorbing targets. As shown in Fig. 8a, pure foam gained more than 6 times of its own weight after absorbing water and it also exhibited a certain absorption capacity to oils and organic solvents. It was demonstrated that pure foam without modification was not suitable for oil absorption. Therefore, the pure foam should be modified, and we introduced oleic acid coated Fe₃O₄ and PS onto the surface of pure foam to obtain DMPF. The intake capacity of DMPF exhibited that the S values of DMPF could reach a maximum of 37.9 g g⁻¹ for dichloroform, as shown in Fig. 8b. It was obviously seen that the absorption capacity for various oils and organic solvents increased and the intake capacity of water decreased after modification. The above results displayed that the absorption capacity and selective ability of DMPF were both improved. However, it was difficult to meet the requirement that only oils and organic solvents could be absorbed. Hence, we modified DMPF, and the as-obtained DMMPF could satisfy the above-mentioned requirement. The absorption capacities of DMMPF for lubricating oil, diesel oil, salad oil, dichloromethane, cyclohexane, benzene, DMF, ethylene glycol and tetrahydrofuran were about 22.7, 17.2, 20.1, 40.1, 20.5, 22.9, 18.3, 20.9 and 25.1 g g⁻¹, respectively. It should be noted that the water absorption capacity of DMMPF decreased to less than 0.3 g g⁻¹, as shown in Fig. 8c. The absorption capacity depended on the density, the viscosity and surface tension of absorbing objects. For example, lubricating oil had a higher density (0.923 vs. 0.843 g cm⁻³), viscosity (61.2 vs. 6.2 cP) and surface tension (31.7 vs. 13 mN m⁻¹) than diesel oil; the absorption capacity of lubricating oil was accordingly larger than that of diesel oil.⁴¹ As expected, DMMPF had high absorption capacity, which was higher than magnetic grapheme foam^42,43 and grapheme/carbon cryogels.⁴⁴ The superior absorption property was attributed to the interconnected 3D network structure along with the attached PS layer in DMMPF. What's more, the absorption capacity of DMMPF was higher than DMMP, because DMMPF had fewer polar function groups.

Fig. 8 Absorption capacity of (a) pure foam, (b) DMPF, and (c) DMMPF for water, various oils and organic solvents.

In practical applications, the as-obtained material could be regenerated through a simple squeezing process. As could be seen in Fig. S6,† DMMPF still remained an excellent and stable mechanical property. The compression stress only exhibited 9% loss after 60 absorption–desorption cycles (Fig. S6a†) and the mechanical properties of DMMPF were enough to regenerate oils/organic solvents in an absorption–desorption process. Moreover, we could obviously see in Fig. S6b and c† that a three-dimensional hierarchical porous structure still existed after 60 absorption–desorption cycles, suggesting its stable mechanical properties. Therefore, the absorbed oils/organic solvents could be easily reclaimed through mechanical force for the next absorption operation, as shown in Fig. S7.† To our surprise, there were no droplets in the collected oil and organic solvent, implying outstanding properties of hydrophobicity. More importantly, DMMPF with a high absorption capacity and efficiency could be reused many times. The absorption capacities of the as-prepared samples after several cycles were also investigated to evaluate the reusability, the results are shown in Fig. 9. It was clearly seen in Fig. 9a and b that the water absorption capacity of pure foam and DMPF increased distinctly and the oils/organic solvent absorption capacity obviously decreased after 60 cycles. In contrast, the absorption capacities of water, benzene and lubricating oil and the contact angle of DMMPF weakened slightly after 60 cycles, indicating a stable absorption performance and the excellent reusability of DMMPF. Similar oil-absorbing foams/sponges and their performances are listed in Table S2.† Compared with these absorbents, DMMPF with low cost, excellent absorption capacity and wettability has been demonstrated to be a promising absorbent for the removal of oil spills and organic solvents.

Fig. 9 Absorption capacities of (a) pure foam, (b) DMPF, and (c) DMMPF under different cycles. (d) Contact angle after different water-oils/organic solvent separation cycles.

Conclusions

In summary, a durable magnetic polystyrene foam was fabricated through a facile immersion method by introducing Fe₃O₄ particles and a PS layer onto the surface of pure foam. As hydrophobic groups were grafted onto the magnetic polystyrene foam, DMMPF showed superhydrophobicity and superoleophilicity, which was beneficial for its application in oil–water separation. In addition, the as-obtained DMMPF not only exhibited high mechanical and chemical stability, but also quickly and selectively removed a variety of oils/organic solvents under a magnetic field. The maximum absorption capacity of DMMPF could reach up to 40.1 g g⁻¹ of its own weight. Furthermore, we collected the oils and organic solvents by simple squeezing before the next cyclic operation. Above all, DMMPF could still maintain an excellent absorption capacity and large water contact angle after 60 cycles. Due to its good properties, simple fabrication method and much lower cost than other absorption materials, we believe that this kind of durable and modified magnetic polystyrene foam is a promising candidate for the removal of contaminants from water surfaces.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Project No. 41101287), the Scientific and Technical Supporting Programs of Jiangsu province (BE2012758) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Information on EDS spectra; stress–strain curves; situation of immersion in a water bath; optical image after floating on HCl and NaOH solution; experiment of removing deionized water; collection of oil and organic solvent; weight percentage of the as-prepared samples; three videos showing the magnetic properties, unsinkable properties and oil absorption process of the sample. See DOI: 10.1039/c5ra27370f

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