Synthesis and characterization of highly efficient oil–water separation, recyclable, magnetic particles CoFe2O4/SDB

In the oil–water separation, the difficulty to recover and low hydrophobicity are key limitation factors for practical applications. In this paper, we design Cobalt ferrite hybrid Polystyrene divinylbenzene microspheres (CoFe2O4/SDB), which were conducted through in situ suspension copolymerization. The CoFe2O4 is prepared by low heat solid phase sol-gel method. It had been found that the CoFe2O4/SDB have a spherical structure, good adsorption behavior, highly hydrophobicity and even superhydrophobicity. The adsorption capacity of CoFe2O4/SDB composites could absorb kerosene up to 6 times of its own weight. Interestingly, kerosene can be easily separated from the surface of CoFe2O4/SDB particles with ultrasonic operation. CoFe2O4/SDB particles can still maintain good hydrophobicity and adsorption capacity of kerosene after 11 cycles after drying. With in situ polymerization of St:DVB and CoFe2O4, CoFe2O4/SDB as a promising absorbent of kerosene which has great potential in application of oil–water separation.


Introduction
With the development of the global economy, the demand for oil is also increasing and the risk of marine oil spill accidents continue to increase at the same time [1][2][3]. The oil spill accident and the discharge of a large amount of oily sewage have seriously damaged the ecological environment of lakes and oceans, which not only seriously disrupted people's lives, but also disrupted normal commercial production. Although oceans and lakes have the ability to self-purify oil pollution, this ability is limited [4]. Furthermore, oil pollution of water will directly affect the ecology and human health. Especially the oil leakage on the ocean will cause large-scale water pollution, removing oil spills from the water surface as quickly and effectively as possible should be the most important step in protecting the marine environment and life [5].
Recently, the desired oil-adsorption materials are getting much attention, which should have superhydrophobic and super oleophilic properties as oil absorbents. Many natural materials and man-made absorbents have been widely considered in the oil-water separation such as sponge [6][7][8][9], films [10], fiber [11][12][13], and graphene [14,15]. Calcagnile et al. [16] reported a PTFE powder and iron oxide nanoparticles modified polyurethane sponge, which is superhydrophobic, superoleophilic and shows magnetic response. The polyurethane sponge can be recovered by ultrasound in toluene, but with a low recovery rate of 80%. Sen Wang et al. [17] reported a 3D porous polyoxometalate (POM)-hybridized GA (POM-GA) as a highly efficient and recyclable absorbent for oil/water separation, its oil-adsorption capacity retention rate is about 90% after 10 absorbingsqueezing cycles. SiQi et al. [18] have prepared a 3D porous sponge based on amino-terminated polydimethylsiloxane (PDMS) and graphene oxide (GO), over 80% strain of this material will occur after 20 compression cycles, demonstrating its weak compressive strength. Generally, these natural and synthetic materials show low adsorption capacity, low compressive strength, weak recoverability, and big difficult in recycle.
To overcome these limitations, more efforts should be paid in recent years to the high performances oil-adsorbent materials which have high oil adsorption capacity, low density, high porosity, large specific surface, and are hydrophobicity and environmentally friendly. As far as we know, the polystyrene divinylbenzene (SDB) porous microspheres have all the advantages mentioned above, and the suspension polymerization is one of the important methods to prepare SDB microspheres [19]. Therefore, it is reasonable to fabricate high oil-absorbent materials of magnetic particles modified SDB through in situ suspension polymerization. We believe that with the simple polymerization method of magnetic SDB might be applied to the process of oil-water separation which could be recycled as the characteristics of magnetic materials that could response to external magnetic field. The CoFe 2 O 4 is noticed due to its spinel-type and strong magnetic response in a wide variety of magnetic materials [20][21][22].
In this work, we prepared a kind of high strength, environmentally friendly and recyclable oil-absorbent CoFe 2 O 4 /SDB material, which is able to remove oils from water surface based on porous structure. Firstly, CoFe 2 O 4 nanoparticles with uniform size and good magnetic property were prepared by the low heat solid phase sol-gel method. Then, magnetic CoFe 2 O 4 /SDB particles were successfully prepared by in situ suspension polymerization which have the high strength and superhydrophobic properties. In addition, the influence of different adding amounts of CoFe 2 O 4 for the preparing of CoFe 2 O 4 /SDB are discussed with the compressive stress, magnetic property, hydrophobicity and adsorption ability.

Characterization of the prepared particles
X-ray diffraction (XRD) analysis was carried by using a Bruker D8 Advanced diffractometer with Cu Kα radiation and the scanning angle ranged from 3 to 80° of 2θ at 40 kV and 40 mA to classify the crystal structure of CoFe 2 O 4 and CoFe 2 O 4 / SDB. Morphologies of CoFe 2 O 4 /SDB composites were investigated by using scanning electron microscope (SEM, UItra55, Carl zeissNTS GmbH, Germany). Contact angles (CAs) tests were carried by contact angle measurement instrument (DSA30, Kluss. Co. Ltd., German). Thermal gravimetric (TG) analysis was carried out by SDTQ600 under a nitrogen atmosphere. The magnetic properties of CoFe 2 O 4 and CoFe 2 O 4s /SDB were measured by a vibrating sample magnetometer (VSM, Lake-Shore 7404). The nitrogen adsorption analyze was carried by an automatic specific surface and pore size distribution analyzer (Autosorb-iQ, Quantachrome).

Preparation of CoFe 2 O 4 nanoparticles
CoFe 2 O 4 nanoparticles were synthesized through the low heat solid phase solgel method. Specific synthesis steps were as follows: firstly, Fe(NO 3 ) 3 ·9H 2 O (10.1 g) was grinded with polyethylene glycol (20 g) for 30 min, then marked as A. Co(NO 3 ) 2 ·6H 2 O (7.3 g) was grinded with oxalic acid (9.6 g) for 30 min, then marked as B. After that, the mixture marked as A was grinded with the one marked as B and the obtained mixture was heated at 393.15 K for 2 h in the oven, forming a pale-yellow homogeneous solution. Then, the mixed solution was transferred to a muffle furnace, with the temperature setting at 633.15 K and time setting at 1 h. After that, the temperature was settled at 973.15 K and the heating was carried out for 2 h. The prepared black products were washed three times by deionized water and ethanol. Then, get it to a vacuum drying oven in 333.15 K for 2 h. Finally, finally got the CoFe 2 O 4 nanoparticles. For a better copolymerizing with St and DVB, CoFe 2 O 4 nanoparticles were modified with SA.

Synthesis of CoFe 2 O 4 /SDB composites with different amounts
Briefly, St (16.67%, weight), DVB (16.67%, weight), toluene (26.33%, weight), 1,2-dichloroethane (20.67%, weight), n-heptane (19.33%, weight), and BPO (0.33%, weight) were added to a beaker with different amounts of CoFe 2 O 4 , and then, the solution was mixed in ultrasonic oscillation for 30 min to form the black solution, giving liquid A. Then LAS (0.1 g) and PVA-1788 (2.0 g) were added into the deionized water (200 mL), giving liquid B. The liquid B and liquid A were mixed in a three-neck flask and set the temperature at 45 °C in N 2 atmosphere with the mechanical stirring speed at 70 rad/min. Subsequently, the temperature was gradually raised to 70 °C as N 2 flow was stopped with the constant heating rate 5 °C/15 min is used through the polymerization. Finally, the temperature was turned to 88 °C for 6 h. After the polymerization finished, the obtained Consequently, the CoFe 2 O 4 /SDB were synthesized with different adding dosages of CoFe 2 O 4 by in situ polymerization which had the same crosslinking ratio of St: DVB = 1:1 and were named for S 0 , S 1 , S 2 and S 3 for the different CoFe 2 O 4 /SDB samples which are listed in Table 1.

Oil-adsorption experiments
The oil adsorption capacity of CoFe 2 O 4 /SDB particles was researched by weight measurement. Besides, water and kerosene were poured to a watch-glass in order to see the kerosene on the surface of water. Finally, CoFe 2 O 4 /SDB were added into the watchglass and it is obvious that the kerosene was quickly absorbed by the CoFe 2 O 4 /SDB in a few minutes. The adsorption capacity (P) of CoFe 2 O 4 /SDB particles were counted by this formula: P = (M 2 − M 1 )/M 1 . Besides, the added CoFe 2 O 4 /SDB particles were weighed and defined as M 1 ; M 2 is the weight of the adsorbed kerosene and CoFe 2 O 4 / SDB particles which were gathered through a magnet bar. After the adsorption process, CoFe 2 O 4 /SDB particles were washed by absolute alcohol three times and then dried in the oven at 45 °C for 4 h. All the kerosene adsorption experiments were carried and repeated for three times to get the average value. The performance of recycled CoFe 2 O 4 /SDB particles were examined to remove kerosene from water surface.

Results and discussion
The The rough display of S 0 -S 1 were taken in Fig. 3a, with the increase of CoFe 2 O 4 , the deeper color and rough surface were the S 0 -S 1 displayed. Without the adding of CoFe 2 O 4 , S 0 demonstrates a white spherical surface and increase the adding amount  morphology and size of the CoFe 2 O 4 /SDB composites, SEM analysis of S 2 was carried out and shown in Fig. 3b-d. The diameter of CoFe 2 O 4 /SDB is about 1-2 mm (Fig. 3b) and the SA-CoFe 2 O 4 nanoparticles was distributed on the surface of the SDB obviously (Fig. 3c). In addition, the CoFe 2 O 4 particles are easily to be found from the fracture surface of S 2 further elucidate the CoFe 2 O 4 /SDB microspheres successfully prepared (Fig. 3d).
It is well-known that the super hydrophobicity of the solid surface depends on the layered roughness and lower surface energy [24,25]. Owing to the CoFe 2 O 4 nanoparticles that anchored on the surface, the CoFe 2 O 4 /SDB particles possess a rough surface and present highly hydrophobic property. The contact angle of CoFe 2 O 4 / SDB surface was tested as shown in Table 2, and the water contact angles on the surface of the CoFe 2 O 4 /SDB are increased with the CoFe 2 O 4 , which are 78.94,83.84, 127.00 and 154.70°, respectively, for S 0 , S 1 , S 2 and S 3 . The intrinsic hydrophobicity of an absorbent is critical to improve oil adsorption property when used to oil-water separation due to a fact that the absorbent can selectively adsorb oil and repel water.
The compressive strength tests were carried three times to get the average value and the results are shown in Table 2. CoFe 2 O 4 /SDB is prepared by in situ polymerization, the SA-CoFe 2 O 4 were embedded in a cross-linked structure composed of St and DVB to enhance the strength owing to the ultrahard characteristic of CoFe 2 O 4 . In Table 2, the compressive strength of S 1 sample, S 2 sample and S 3 sample can reach 5.66 times, 6.16 times and 6.92 times of the compressive strength of S 0 sample, respectively. The reinforcement of CoFe 2 O 4 /SDB provides that it can be used many times without deformation.
The magnetic properties of the CoFe 2 O 4 /SDB particles (S 1 -S 3 ) were measured at room temperature and the results are shown in Fig. 4. The saturation magnetization of the S 1 -S 3 samples at an external field of 2 T are 0.6119 emu/g, 1.19538 emu/g and 1.73652 emu/g, respectively. This result states the magnetic CoFe 2 O 4 /SDB have been successfully synthesized and the adding amounts of CoFe 2 O 4 in the CoFe 2 O 4 / SDB particles can affect the magnetism property. It is well known that CoFe 2 O 4 / SDB could be easily separated from the mixture of water and kerosene with the external magnetic field. Owing to the magnetic property of CoFe 2 O 4 /SDB material could be positively correlated with handling the removal of different types water and kerosene. N 2 adsorption-desorption experiments were performed at 77 K to examine the specific surface area and the pore volume of CoFe 2 O 4 /SDB (S 2 - Fig. 5). When the relative pressure of the system is low, the adsorption-desorption curve of S 2 does not close, indicating that there are micropores in the structure of S 2 , which may be due to the chemical adsorption between the S 2 itself and N 2 , resulting in the incomplete removal of N 2 . In terms of pore size distribution, S 2 mainly has micropores which average pore size is about 1.435 nm with the other size of mesoporous which could be from1.32 to 33.28 nm which could be more suitable for the size of oil(about 1-20 nm) as shown in Fig. 5b. The small pore size makes N 2 difficultly to be removed from the pore of CoFe 2 O 4 /SDB resulting in the retention of N 2 in the pores. At the same time, the N 2 adsorption-desorption experiments were applied to determine the pore structure of the S 0 , S 1 and S 3 , the results show that the S 2 possess the largest pore volume of 2.1948 cm 3 /g, as shown in Table 3, which suggest that the S 2 is the best candidate for the oil apportion. (Fig. 6). We can clearly observe the initial decomposition temperatures of S 0 -S 3 samples from Fig. 7, which are 360, 410, 490 and 500 °C, respectively. The final decomposition temperature of S 0 -S 3 gradually increases while the adding amount of CoFe 2 O 4 were increased in the meantime. The main reason is that the inorganic material CoFe 2 O 4 were in situ polymerized with the long crosslinking chain of St and DVB which form the strong interaction between two phases of inorganic material and organic material to hampered bodily movement of CoFe 2 O 4 /SDB particles and since CoFe 2 O 4 itself occupies a certain steric hindrance, this action impedes the thermal motion of the molecular chain of CoFe 2 O 4 /SDB. With the increased adding amount of CoFe 2 O 4 , the stronger force of interaction between two phases and sterically hindered were demonstrated, thereby the thermal stability of the CoFe 2 O 4 /SDB is increased by improving the thermal decomposition energy.

The kerosene adsorption capacity of CoFe 2 O 4 /SDB
The CoFe 2 O 4 /SDB particles prepared in this study were used for handling the kerosene on the surface of water. The weight measurement methods were carried to study the kerosene adsorption efficiency of SDB and CoFe 2 O 4 /SDB (S 0 -S 3 ). Results of weight measurement are obviously seen in Fig. 6 due to the highest pore volume,  S 2 demonstrate the best kerosene adsorption capability and the adsorption capacities (P) of S 0 , S 1 , S 2 and S 3 are 5.723, 6.356, 6.526 and 1.625, respectively. Furthermore, the residual big π bond in SDB which could have reaction with the π bond in the kerosene is another drive for the adsorption process. And with the existence of CoFe 2 O 4, the π-π interaction between benzenes and the kerosene can be improved [26]. Taking the pore volume and the effect of CoFe 2 O 4, into consideration, the S 2 should perform well when applied to absorb kerosene. As we expected, the S 2 adsorbed kerosene quickly in a few minutes when they were added into watch-glass with a layer of kerosene on a water surface. Because Fig. 6 The TGA curves of S 0 -S 3 Fig. 7 The kerosene adsorption capacity curves of S 0 -S 3 of the presence of magnetic CoFe 2 O 4 nanoparticles, the S 2 could be taken to the assigned mixture region of water and kerosene by external magnetic field. Furthermore, the S 2 can be separated easily from the oil-water system and regenerated by washing in ethanol with a help of ultrasonic which was followed by drying steps. Moreover, as shown in Fig. 8, the regenerated S 2 still show good performances in kerosene adsorption capacity which reach 6.021 times even after the 11th cycle. The recoverability and durability of the absorbent as well as the efficiency of kerosene removal play an important role in controlling the extent of pollution and protecting the environment.
Interestingly, the magnetic CoFe 2 O 4 /SDB particles did not sink even when they were put on the surface of kerosene and water mixture under oscillation for one night. This is due to their low density, excellent hydrophobicity which can overcome their own weight and prevent them from immersing in water surface to increase the water displace volumes. We believe that CoFe 2 O 4 /SDB particles would be used widely for the treatment of the kerosene spillage.

Results
In sum, CoFe 2 O 4 nanoparticles with uniform particle size (about 39.71 nm) and strong magnetism were prepared via the low heat solid phase sol-gel method, which played an important role in construction of CoFe 2 O 4 /SDB particles. CoFe 2 O 4 /SDB particles were prepared through in situ polymerization, which had advantages of strong magnetism, high pore volume, unsinkability and low density. The kerosene was quickly adsorbed by the prepared CoFe 2 O 4 /SDB in a few minutes and has a good adsorption capability which could be reused under the existing of external magnetic field and washing by absolute alcohol. Moreover, S 2 sample showed the best kerosene-adsorbent capacity up to 6.526 times of their weight and we could Fig. 8 The kerosene adsorption capacity of S 2 recorded after 11th cycles also learn that the S 2 could be regenerated at least 11 times. All in all, the excellent hydrophobicity, large pore volume and pore diameter, superoleophilicity, recovery of magnetic medium and environmental-friend are the advantages of CoFe 2 O 4 / SDB particles which shows a highly promising absorbent for kerosene and water separation.