# Magnetic and Hydrophobic Composite Polyurethane Sponge for Oil-Water Separation
## 3 Results and Discussion
Compared with sponge sample 1, the surface of sponge sample 2 is rougher because of the coating of stearic acid. The oil-water separation experiment was performed using the sponge sample 2 and the process is shown in the Figure 7. The peanut oil was used for the experiment. The sponge was moved above the oil-water mixture and then put on the oil surface carefully. The sponge sample 2 adsorbed peanut oil completely from the oil-water mixture and suspended on the water. Finally, there was only water left in the beaker after removing the sponge. Additionally, the photographs in Figure 7a-f exhibited the process of the sponge sample 2 adsorbing a thick layer of peanut oil on the water surface. Due to its excellent mechanical flexibility, the adsorbed oils can be easily removed from the composite sponge by a simple mechanical extrusion method [48]. Due to the good hydrophobicity of stearic acid on the surface of sponge skeleton, sponges can quickly adsorb oil and block water out. Figure 5: FT-IR spectra of (**i**) expanded graphite, (**ii**) stearic acid, (**iii**) Fe\({}_{3}\)O\({}_{4}\) nanoparticles, (**iv**) original polyurethane sponge, and (v) composite sponge sample 2. Figure 6: Morphology of composite sponge sample 1 (a1–a4) and composite sponge sample 2 (b1–b4). In order to explore the adsorption capacity of sponge samples, we carried out more oil adsorption experiments. For comparison, the sponge unmodified by stearic acid was also tested. Sponge sample 1 and sample 2 were tested for the oil adsorption with peanut oil, lubricating oil, and silicone oil and the results are shown in Figure 8. Obviously, the two sponge samples could adsorb a wide range of oils, and exhibits high adsorption capacities ranging from around 32-40 times of its own weight as around 32-40 g/g. Moreover, the adsorption capacities of sponge samples in this work are comparable or even better than those of the other previously reported ones, which indicates the expanded graphite coated on the sponge samples skeleton surface increased the oils adsorption capacities. The hydrophobicity of sponge samples were also tested and the results are showed in Figure 8. Compared with sponge sample 1, the water contact angle of sponge sample 2 increases to around 45\({}^{\circ}\), which demonstrates that the hydrophobicity of the sponge enhanced after the modification of stearic acid. A stearic acid hydrophobic layer was successfully formed on the skeleton surface of sponge sample 2. From Table 2, the oil adsorption capacities of different adsorbent materials are lower than that in this work. The adsorption capacity of pure PU sponge for lubricating oil is only 28.8 g/g. Even after the modification to FGN/polyurethane sponge, the adsorption capacity of lubricating oil is only 34.2 g/g, which is still lower than that of sponge sample 1 and sample 2 in our work [49]. The times taken for the composite sponge samples to adsorb different types of oil to reach full saturation are listed in Table 3. It can been seen that the composite sponge samples adsorbed different types of oil to reach full saturation within around 20 s.  
The reusability is a crucial index to evaluate the property of composite sponge. In practical industrial application, the sponge will be used for adsorption over several cycles successively, so the sponge will be squeezed and for the next absorbing-squeezing cycle immediately. The recyclability of sponge sample 1 and 2 for oils adsorption for six absorbing-squeezing cycles is presented in Figure 2:0  
Figure 2:1 Digital photographs (**a–f**) show the oil-water separation process of composite sponge sample 2. As for the sponge sample 1, after an adsorbing-squeezing cycle, the oils adsorption capacities decreased, and with the increase of absorbing-squeezing cycles, the oils adsorption capacities turn to be stable.