Superhydrophobic P (St-DVB) foam prepared by the high internal phase emulsion technique for oil spill recovery
Introduction
Nowadays, common oil spillage accidents have polluted the environment, damaged the marine ecosystem and even caused huge economic losses [1], [2]. In order to protect environmental surroundings, oil collection machines, oleiphilus, oil dispersing agents and oil adsorption materials [3] have been performed to eliminate oil spill pollution. Among these methods, the application of adsorption agents was believed to be one of the most promising techniques because of the advantages of convenience, low cost, high efficiency and no secondary pollution [4].
In general, granular or powder materials tend to aggregate, resulting in the reduction of the active surface area [5], and thus affecting the adsorption efficiency and capacity. Moreover, it is difficult to separate the particle materials from the water and regenerate them for recycling [6]. For comparison, three-dimensional monolithic porous materials, such as aerogels, sponges and foams possessing open and interconnected pores, were regarded as more ideal oil adsorption materials because they were equipped with the features of high adsorption capacities toward oils and were easily to be recycled and reused [7].
Aerogels as oil adsorbents have aroused considerable research interests for the advantages of high porosity and low density [8]. Traditional inorganic aerogels, such as SiO2 aerogels, were not suitable for real oil removal because of the intrinsic brittle character [9]. Carbon-based aerogels, including carbon nanotubes, carbon nanofibers, pyrolytic carbon, graphite and graphene aerogels, exhibited good recyclability [10]. Among them, graphene aerogels displayed outstanding oil adsorption capacities. However, special equipment and complex operations were involved in the preparation of graphene aerogels. For instance, mechanically stable poly (acrylic acid)/reduced graphene oxide aerogels should be prepared by the method of freeze-drying in liquid nitrogen and vacuum, otherwise obvious shrinkage of the as-prepared aerogel can be observed [11]. The aerogel showed superior oil capacity of 120 times of its own weight. However, high energy consumption and high cost were inevitable.
Sponge was another research topic in the field of oil adsorption due to their pore structures and compressibility [12]. However, the oil and water could be adsorbed by the sponge simultaneously, which undoubtedly reduced the oil capacities and caused the subsequent problem of the separation of water from the recovered oil [13]. As a result, the surface of the original sponge should be modified to improve the hydrophobicity. Khosravi et al. [14] modified the polyurethane sponge by successive operations of vapor-phase deposition, polymerization of polypyrrole and modification with palmitic acid. After modification, the hydrophobicity of the sponge was greatly improved with the WCA reaching 140.234° from the original 68.525°. Obviously, these modification processes were complicated and various solvents/organic monomers were used in the experiments, which made it hard to expand these procedures from laboratory scale to the industrialization scale.
During the past two decades, HIPE technique has been investigated extensively for the fabrication of porous materials [15], [16]. In 2013, low-molecular mass gelator was used to stabilize the HIPE to fabricate the ultra-low density porous polystyrene monolith [17]. However, in this work the WCA of the material reported was 124.8° and the oil adsorption capacities for nine common organic solvents were only 2.51–20.21 g/g. In 2014, two kinds of spherical Fe3O4 nanoparticles with different sizes were employed to prepare the HIPE of styrene and divinylbenzene [18]. Finally, the obtained polyHIPEs were closed-cell polymer foams. This reference did not research the application of the material. However, the unconnected pore structures were not beneficial for the transportation of the liquid undoubtedly. In 2015, our research group prepared the open-cell P (St-DVB) foam with the carbonyl iron powders as solid stabilizer [19]. The polymer foam exhibited good oil adsorption capacity of 23 g/g toward diesel and hydrophobicity with WCA of 142°. More recently, we have achieved substantially improved results about the preparation of the superhydrophobic P (St-DVB) material with a WCA approaching 156°. Polymer foams with varied interconnected microstructures were prepared successfully with different types of Fe3O4 particles. Despite only low cost raw materials were used, they had excellent oil adsorption capacity, even better than that of graphene aerogel prepared by our research group in 2015 [20].
Herein, we described a facile and novel one-step method to fabricate superhydrophobic P (St-DVB) foams with interconnected porous structures for oil/water separation directly, without further surface modification. It was worth noting that the P (St-DVB) monolith was processed without complicated operations such as freeze or vacuum drying, which saved the energy and economic cost and reduced environmental pollution. Moreover, Fe3O4 particles were used to stabilize the emulsions as well as to improve the hydrophobicity of the material because of the formation of the hierarchical pore structures. Three types of Fe3O4 particles with different morphologies and sizes enabled varied microstructures of the PolyHIPEs. And the adsorption capacities towards oil/organic solvents and recyclability of the monolithic P (St-DVB) foams were also investigated in detail.
Section snippets
Chemicals and materials
All chemicals in this work were of reagent grade and used without any further purification. Ferric chloride hexahydrate (FeCl3·6H2O), sodium acetate (NaAc), cubic crystal Fe3O4 (about 0.1–2 μm in diameter), polyethylene glycol 4000 (PEG 4000), Span 80, stain Sultan IV and methylthionine chloride were purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai. Ethylene glycol was obtained from Shanghai No. 4 Reagent & HV Chemical Co. Ltd. Azobisisobutyronitrile (AIBN) and styrene (St) were
The morphology of the Fe3O4 particles and P (St-DVB) foam
Fig. 1a shows the spherical Fe3O4-1 particles prepared by the solvent thermal method. The sizes are remarkably uniform and the diameter is approximately 300 nm. Fe3O4-2 particles are cubic shape crystal with average size of about 200 nm (Fig. 1b). Fig. 1c shows irregular Fe3O4-3 particles, with the sizes ranging from 0.1 μm to 2 μm.
During the preparation of the HIPEs, a large amount of water was dispersed in the emulsions with the assistance of Span 80 and solid particles of Fe3O4, leading to the
Conclusions
Highly porous monolithic foams with superhydrophobicity and excellent oleophilicity were fabricated by one-step process successfully without any further modification. The introduction of different types of Fe3O4 particles that have varied morphologies and sizes enabled different microstructures of the PolyHIPEs. The hierarchical structures at the micro-nanometer scale contributed to the superhydrophobicity of the composite polymers, with the WCAs exceeding 150°. Ultra-low density of 0.024 g·cm−3
Acknowledgments
We appreciate the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Fundamental Research Funds for the Central Universities (30920130112003) and the Natural Science Foundation of China (50972060) for their financial supports.
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