The structure and morphology of CoFe2O4/SDB particles. As the FT-IR test results of CoFe2O4/SDB(a), SA- CoFe2O4 (b) and CoFe2O4 powder (c) shown in Fig.1. In the CoFe2O4 powder (c) of Fig.1, the peak at 581 cm-1 is derived from the Fe-O vibration peak, and 1624 cm-1 is the characteristic peak of -OH on the surface of CoFe2O4; In the SA-CoFe2O4 (b) of Fig.1, the stretching vibration peaks at 2924 cm-1 and 2821 cm-1 were owing to methyl and methylene groups respectively, the adsorption peak at 1710 cm-1 is the stretching vibration adsorption peak of the carboxylic acid carbonyl group in SA, and the characteristic peak at 1434 cm-1 is the adsorption peak of Fe-O, which shows that SA has modified the surface of CoFe2O4 nanoparticles and the CoFe2O4 were successfully prepared; In the CoFe2O4/SDB (a) of Fig.1, 1400 cm-1 ~2000 cm-1 are the characteristic peaks of stretching vibration of the benzene ring skeleton and the characteristic adsorption peak of CoFe2O4 can also be clearly seen in CoFe2O4/SDB (a).
In Fig. 2, XRD patterns demonstrated that the diffraction peaks at 2θ = 30.1° (220),35.5° (311),43.1° (400),57.0° (511) and 62.6°(440) belonging to CoFe2O4 powder are the same as that in S1-S3 samples (JCPDS file no. 22-1086). The average crystal diameter perpendicular to (220) planes of CoFe2O4 is about 39.71 nm calculated by the Scherrer equation. The results further prove that the CoFe2O4 nanoparticles were successfully introduced into the SDB microspheres without any transformation in the crystal forms.
The rough display of S0-S1 were taken in Fig.3(a), with the increase of CoFe2O4, the deeper color and rough surface were the S0-S1 displayed. Without the adding of CoFe2O4, S0 demonstrate a white spherical surface and increase the adding amount of CoFe2O4, getting the CoFe2O4 is unevenly distributed on the surface and inside of S1; Then the S2 were get a relatively uniform particle size distribution and a very bright surface. Due to the excessive content of CoFe2O4 in S3, the surface of the S3 particles is not shiny and particle agglomeration occurs. To further characterize the morphology and size of the CoFe2O4/SDB composites, SEM analysis of S2 were carried out and shown in Figure 3(b, c, d). The diameter of CoFe2O4/SDB is about 1~2 mm (Figure 3(b)) and the SA-CoFeO4 nanoparticles was distributed on the surface of the SDB obviously (Figure 3(c)). In addition, the CoFe2O4 particles are easily to be found from the fracture surface of S2 further elucidate the CoFe2O4/SDB microspheres successfully prepared (Figure 3(d)).
It is well-known that the super hydrophobicity of the solid surface depends on the layered roughness and lower surface energy [23-24]. Owing to the CoFe2O4 nanoparticles that anchored on the surface, the CoFe2O4/SDB particles possess a rough surface and present highly hydrophobic property. The contact angle of CoFe2O4/SDB surface was tested as shown in table 2, the water contact angles on the surface of the CoFe2O4/SDB are increased with the CoFe2O4, which are 78.94°,83.84°, 127.00°, and 154.70°, respectively for S0, S1, S2 and S3. 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.
Table 2. The contact angel and compressive strength of S0-S3 samples
Samples
|
Degree(°)
|
Compressive Strength(N)
|
S0
|
78.84
|
32.10
|
S1
|
83.84
|
181.36
|
S2
|
127.00
|
197.69
|
S3
|
154.70
|
221.91
|
The compressive strength tests were carried three times to get the average value and the results are shown in Table 2. CoFe2O4/SDB is prepared by in-situ polymerization, the SA-CoFe2O4 were embedded in a cross-linked structure composed of St and DVB to enhance the strength owing to the ultrahard characteristic of CoFe2O4. In Table 2, the compressive strength of S1 sample, S2 sample and S3 sample can reach 5.66 times, 6.16 times and 6.92 times of the compressive strength of S0 sample, respectively. The reinforcement of CoFe2O4/SDB make it can be used many times without deformation.
The magnetic properties of the CoFe2O4/SDB particles (S1~S3) were measured at room temperature and the results are shown in Figure 4. The saturation magnetization of the S1-S3 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 CoFe2O4/SDB have been successfully synthesized and the adding amounts of CoFe2O4 in the CoFe2O4/SDB particles can affect the magnetism property. It is well known that CoFe2O4/SDB could be easily separated from the mixture of water and kerosene with the external magnetic field. Owing to the magnetic property of CoFe2O4/SDB material could be positively correlated with handling the removal of different types water and kerosene.
N2 adsorption-desorption experiments were performed at 77 K to examine the specific surface area and the pore volume of CoFe2O4/SDB (S2-Figure 5). When the relative pressure of the system is low, the adsorption-desorption curve of S2 does not close, indicating that there are micropores in the structure of S2, which may be due to the chemical adsorption between the S2 itself and N2, resulting in the incomplete removal of N2. In terms of pore size distribution, S2 mainly has micropores that are about 2 nm. The small pore size makes N2 difficultly to be removed from the pore of CoFe2O4/SDB resulting in the retention of N2 in the pores. At the same time, the N2 adsorption-desorption experiments were applied to determine the pore structure of the S0, S1 and S3, the results show that the S2 possess the largest pore volume of 2.1948 cm3/g, as shown in Table 3, which suggest that the S2 is the best candidate for the oil apportion.
Table 3. The pore structure data of S0-S3 samples
Sample number
|
Pore size(nm)
|
Pore volune(cm3/g)
|
S0
|
1.326
|
1.214
|
S1
|
1.688
|
0.8175
|
S2
|
1.435
|
2.1948
|
S3
|
4.427
|
0.00478
|
We can clearly observe the initial decomposition temperatures of S0-S3 samples from Figure 7, which are 360 oC, 410 oC, 490 oC and 500 oC, respectively. The final decomposition temperature of S0~S3 gradually increases while the adding amount of CoFe2O4 were increased in the meantime. The main reason is that the inorganic material CoFe2O4 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 CoFe2O4/SDB particles and since CoFe2O4 itself occupies a certain steric hindrance, this action impedes the thermal motion of the molecular chain of CoFe2O4/SDB. With the increased adding amount of CoFe2O4, the stronger force of interaction between two phases and sterically hindered were demonstrated, thereby the thermal stability of the CoFe2O4/SDB is increased by improving the thermal decomposition energy.
The kerosene adsorption capacity of CoFe2O4/SDB. The CoFe2O4/SDB particles prepared in this study were used for handling the kerosene on the surface of water. The weight measurement methords were carried to study the kerosene adsorption efficiency of SDB and CoFe2O4/SDB (S0~S3). Results of weight measurement are obviously seen in Fig. 7 due to the highest pore volume, S2 demonstrate the best oil adsorption capability and the adsorption capacities (P) of S0, S1, S2 and S3 are 5.723, 6.356, 6.526 and 5.625, 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 CoFe2O4, the π-π interaction between benzenes and the kerosene can be improved [25]. Taking the pore volume and the effect of CoFe2O4, into consideration, the S2 should perform well when applied to absorb oil.
As we expected, the S2 adsorbed kerosene quickly in a few minutes when they were added into watchglass with a layer of kerosene on a water surface. Because of the presence of magnetic CoFe2O4 nanoparticles, the S2 could be taken to the assigned mixture region of water and kerosene by external magnetic field. Furthermore, the S2 can be separated easily from the oil-water system and regenerated by washing in ethanol with a help of ultrasonic which was flowed by drying steps. Moreover, as shown in Fig. 9 the regenerated S2 still show good performances in oil 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 CoFe2O4/SDB particles did not sink even when they were put on the surface of oil 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 CoFe2O4/SDB particles would be used widely for the treatment of the oil spillage.
RESULTS
In sum, CoFe2O4 nanoparticles with uniform particle size(about 39.71 nm) and strong magnetism was prepared via the low heat solid phase sol-gel method, which played an important role in construction of CoFe2O4/SDB particles. CoFe2O4/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 CoFe2O4/SDB in a few minutes and have a good adsorption capability which could be reused under the existing of external magnetic field and washing by absolute alcohol. Moreover, S2 sample showed the best oil-adsorbent capacity up to 6.526 times of their weight and we could also learn that the S2 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 CoFe2O4/SDB particles which shows a highly promising absorbent for oil and water separation.