# Magnetic and Hydrophobic Composite Polyurethane Sponge for Oil-Water Separation
## 1 Introduction  
With the rapid development of the petroleum industry, more and more oilfields are exploited, including many offshore oilfields. However, problems such as oil spills can occur in the process of oil extraction, especially oil spills in offshore oil fields can pollute the marine environment [1; 2; 3]. It is especially important to recover the leaked oil from the surface of seawater. Many materials have been used to adsorb oil from water, but their efficiency and selectivity in oil/water separation are not high enough [4; 5; 6; 7; 8; 9]. Changing the wetting properties of the material surface changes the selective adsorption of water-oil. A variety of oil-adsorbing and hydrophobic materials for oil-water separation have been prepared and encouraging results were obtained [10; 11; 12]. In addition, there are many 3D porous materials with hydrophobic and oil adsorption capacity that have been successfully applied to oil-water separation, but their practical applications are still limited by complex preparation methods and high raw material costs [13; 14; 15; 16; 17].  
Polyurethane (PU) sponge is a commercially obtained low-cost 3D porous material with strong adsorption capacity and reusability. Many studies have reported the preparation of functional polyurethane sponges for oil-water separation, but the preparation cost was high and the amount of oil adsorption was insufficient [18; 19; 20; 21; 22; 23; 24; 25]. [PERSON] et al. [26] bonded the reduced graphene oxide (rGO) onto the surface of polyurethane (PU) sponge using (3-aminopropyl)triethoxysilane and titanium(IV (triethanolaminato)isopropoxide and found that pump oil adsorption capacities of sponges are around20-30 g/g. [PERSON] et al. [27] coated the robust superhydrophobic polysiloxane layer onto the surface of 3D porous polyurethane sponges and the crude oil and bean oil adsorption capacities of sponges are around 24 g/g and 23 g/g, respectively. From the above findings, it can be seen that the preparation process of the above-mentioned hydrophobic oil-adsorbing polyurethane sponge for oil-water separation is complicated and the preparation cost is high, and it is extremely inconvenient to recover the sponge from the surface of seawater.  
Therefore, it is necessary to prepare a PU sponge with low cost, simple preparation process, and magnetic properties, which can be applied to oil-water separation. Expanded graphite (EG) is a porous worm-shaped material formed by the expansion of natural graphite flakes at high temperatures, and it is extremely inexpensive and has a strong adsorption capacity [28; 29; 30; 31; 32]. Stearic acid is a water-insoluble fatty acid, which has good hydrophobic properties and is inexpensive [33; 34; 35; 36]. Fe\({}_{3}\)O\({}_{4}\) magnetic nanoparticles are a kind of spinel ferrite with stable magnetic permeability and superparamagnetism, and the preparation process of Fe\({}_{3}\)O\({}_{4}\) magnetic nanoparticles is simple and inexpensive [37; 38; 39; 40]. In this work, we attempted to disperse EG and Fe\({}_{3}\)O\({}_{4}\) magnetic nanoparticles into the polyurethane sponge, and apply a stearic acid coating on the skeleton surface of the polyurethane sponge to form a hydrophobic and magnetic sponge for oil-water separation. The EG attached to the skeleton surface of the polyurethane sponge can enhance the oil absorption ability, and the stearic acid coating on the skeleton surface can give the sponge better hydrophobicity. By adding Fe\({}_{3}\)O\({}_{4}\) magnetic nanoparticles to the PU sponge as a filler, a PU sponge with magnetism can be obtained, thereby facilitating the use of a magnet to retrieve the PU sponge from the surface of seawater. The magnetism, oil absorption capacity, hydrophobicity, and reusability of the composite PU sponge were tested.