PES magnetic microspheres: preparation and performance for the removal of endocrine disruptor-BPA

In this paper, polyethersulfone (PES) magnetic microspheres were prepared via an in situ reaction method. Firstly, the PES microspheres were produced by an electrospray method and then the magnetic properties were delivered to the microspheres through in situ reaction. Scanning electron microscopy (SEM) coupled with image analysis software, transmission electron microscopy (TEM), and X-ray diffraction (XRD) were employed to investigate the structure and morphology of the magnetic microspheres. The results indicate that the majority of the magnetic nanoparticles are dispersed uniformly and embedded into the surface of the PES electrosprayed microspheres. A vibrating sample magnetometer was used to evaluate the magnetic properties of the PES magnetic microspheres and the saturation magnetization values were found to be up to 23.96 emu g . A separation test showed that the prepared magnetic microspheres can realize fast separation under an external magnetic field. Additionally, compared with the original microsphere counterparts, PES magnetic microspheres exhibited better adsorption capacity and reuse performance in adsorption tests. These results indicate that the PES in situ magnetic microspheres have the potential to be used in water treatment and environmental depuration.


Introduction
Bisphenol A (BPA) is an important chemical for the synthesis of polycarbonate and epoxy resins which are raw materials for bottles, cups and the inside coatings for food cans. 1,2 Through daily use and the ecological cycle, BPA has been universally found in water resources including rivers, lakes and oceans. Meanwhile, BPA is found to be a kind of endocrine disruptor, which has been widely reported to cause endocrine disorder, cancer and sexual precocity. 3,4 It has grown into a serious and urgent issue to eliminate BPA in waters. Conventional methods for the elimination of BPA include biodegradation, 5 molecular imprinting, 6 photocatalysis 7 and adsorption approaches. [8][9][10] Among these techniques, adsorption is the most extensively utilized method due to the low cost and high efficiency. 11 As a novel adsorbent, microspheres prepared by electrospraying, which is easily controllable and machinable with low environmental requirements, have attracted signicant attention in water treatment owing to their homogenous dispersion, porous structure and large specic surface area. [12][13][14][15] Our previous research shows that PES electrosprayed microspheres have a good performance in adsorption of BPA. 16,17 However, for the practical application in adsorption, the ability of directional separation is also an import factor besides the adsorption capacity. Otherwise, it will cause secondary pollution of water environment. Controlling the movement of the microspheres is difficult due to their micro-scale size. It is of great important to nd a convenient and efficient method to separate the microspheres from water environment. It is a promising method to prepare the magnetic microspheres to solve this issue. 18 Magnetic polymer microspheres possess great magnetic properties so that they can move directionally in the external magnetic eld. In this way they can be separated directly from the environment. Magnetic microspheres have attracted much attention in recent years and will be potentially used in many elds, such as wastewater treatment, 19,20 enzyme immobilization, 21 magnetic resonance imaging (MRI) 22 and cell separation, 23 etc. Conventionally, the preparation methods include monomer polymerization 24,25 and embedding method, 26,27 both involving the synthesis of magnetic particles as well as the microspheres and the following binding of particles and microspheres. However, the magnetic microspheres prepared in these ways have some drawbacks including low magnetic particles content and uneven distribution of particles on microspheres. For these shortcomings, in situ reaction is a promising solution. In this way, magnetic particles are generated in situ on synthesized microspheres. The prepared microspheres are soaked in the solution of ferric ion rst so that the ferric ions are adsorbed onto the microspheres and then the adsorbed ions are turned into magnetic particles in situ through adjusting the pH and temperature. [28][29][30][31] With this method, the magnetic microspheres are obtained.
Literatures concerning the in situ reaction are not much, and most of them are limited to several kinds of polymer, such as chitosan [28][29][30] or cellulose. 31 The conventional preparation methods include sol-gel transition and microemulsion approaches. Magnetic microspheres prepared via these ways are hindered by the following shortcomings such as long preparation cycles, application of surfactants and co-surfactants which do harm to the environment to assist the fabrication of microspheres and uneven microspheres diameters. Considering the advantages mentioned above, in this paper, magnetic microspheres based on PES, a special engineering polymer with good heat and chemical resistance and sprayability, were prepared by in situ reaction combined with the electrospraying, a novel route for the preparation of magnetic microspheres. The PES magnetic microspheres were found to have excellent magnetic and adsorption properties, having potential applications in water treatment and environment depuration especially for the adsorption of endocrine disruptor-BPA.

Materials
PES (Ultrason E 6020P) with an intrinsic viscosity of 0.34 dL g À1 in dimethyl sulfoxide (DMSO) at 303.2 K was purchased from BASF Chemical Co. (Germany) and the molecular weight of PES is about 50 000 g mol À1 . Bisphenol A (BPA) was purchased from Changlian Chemical Co. (China). Solvent used in this study, such as N,N-dimethylformamide (DMF), NaOH, H 2 O 2 (30%) and FeCl 2 $4H 2 O were purchased from Kelong Chemical Co. (China). All the chemicals are of analytical grade and were used without further purication unless described. Deionized water used throughout the study was lab homemade.

Preparation of in situ magnetic microspheres
PES was dissolved in DMF at 60 C for 4 h until the solution became homogeneous, and its concentration is 0.10 g mL À1 . For preparing microspheres, the solution was placed in a 50 mL syringe, to which a capillary tip of 0.5 mm inner diameter was attached. A direct current high-voltage generator (BGG, Beijing Electro-mechanical Research Institute Supervoltage Technique Company) was used to provide a voltage of 10 kV. The positive electrode of high-voltage power supply was connected to the capillary tip. The grounded electrode was connected to a metallic collector immersed in a water bath. The distance between the tip and the collector was maintained at 6 cm (ref. 17) and the ow rate was xed at 1 mL h À1 . The spraying stream is broken up aer moving a certain distance, and then electrosprayed microspheres were collected in the water bath. Then PES electrosprayed microspheres were immersed in deionized water and washed for 48 h at 80 C to remove the solvent, and then they were ltered and dried. 0.01 g PES electrosprayed microspheres (PEM) and different contents of 0.5, 1.0, 1.5, 2.0 g FeCl 2 $4H 2 O (named as M5, M10, M15, M20) were put into each of four tubes, and 20 mL deionized water was added under nitrogen atmosphere. Aerward, the tubes were dispersed in ultrasonic instrument (KQ-500, Kun Shan Ultrasonic Instruments Co., Ltd) at 40 C for 48 h to make the microspheres achieve the goal of Fe 2+ adsorption equilibrium. Subsequently, the materials were centrifuged at 5000 rpm for 10 min, and the supernatant liquid was poured out. And then, 20 mL NaOH aqueous solution (0.02 mol L À1 ) was added into the tubes which were put into a water bath (60 C for 4 h). Aer that, 10 mL H 2 O 2 (1 wt%) was added, and then put the tubes into a 30 C water bath for 4 h. Finally, the microspheres were isolated, and dried at 100 C for 24 h. According to the above steps, PES in situ magnetic microspheres (PMM) were obtained. The certain experimental parameters are shown in Table 1, and the preparation procedure is illustrated in Scheme 1.

Microscopy and structure of the microspheres
The morphology of PEM and PMM was observed by scanning electron microscopy (SEM Inspector F, FEI Company, all operation at an accelerating voltage of 20 kV). The samples were coated by an E-1045 ion sputter coater with Au/Pd to reduce charging. The diameters of PEM were measured by an image analysis soware of Image J based on the SEM results. The diameters of more than 200 PEM were randomly measured for statistic signicance. Besides, the size and morphology of magnetic nanoparticles were observed by a transmission electron microscopy (TEM) instrument (Hitachi, JEM-2100F), using an accelerating voltage of 200 kV.
The ferrite content of dried PMM was measured on a thermo gravimetric analysis (TGA) instrument (TA Instruments, Q500) and heated from room temperature to 700 C at a heating rate of 20 C min À1 under nitrogen atmosphere (ow rate 20 mL min À1 ).
The crystal structure of PMM was measured on a wide-angle X-ray diffraction (XRD) diffractometer (X'Pert pro-MPD, Philips). The XRD patterns with Cu Ka radiation at 40 kV and 35 mA were recorded in the region of 2q from 10 to 70 .

Magnetic characterization and separation experiment
The magnetic properties of the PMM were measured with a vibrating sample magnetometer (VSM) at 300 K and within the magnetic intensity range from À15 000 Oe to +15 000 Oe. Moreover, 4 mg PMM were put into a sample bottle, and 10 mL deionized water was also added. Aer stirring for 10 min, microspheres were evenly dispersed. Then permanent magnet was placed in one side of the bottle, and movement of magnetic microspheres was recorded in a certain period of time. The microspheres were separated from the liquid phase by the permanent magnet as they have almost reached the wall of the bottle. Then the liquid phase was poured, and PMM were dried.

Adsorption experiments
3 mg PEM and M10 microspheres were dispersed in 10 mL BPA solution (BPA was dissolved in ethanol and then diluted with deionized water.) with initial concentration of 100 mmol L À1 , 200 mmol L À1 and 300 mmol L À1 , respectively. And the mixtures were shaken at 20 C. Aer adsorption, a permanent magnet was used to separate the adsorbent and the supernatants were collected. The BPA concentrations in the supernatant were detected by a UV-vis spectrophotometer (UV-2310II, Shanghai TECHCOMP, Shanghai, China) at the wavelength of 276 nm. The adsorbed capacity of microspheres was calculated as following: where q t is the adsorbed BPA amount per gram of microspheres at time t (mmol g À1 ); C 0 is the initial concentration of BPA solutions (mmol L À1 ); C t is the concentration at time t (mmol L À1 ); V is the volume of BPA solution used in the experiment (L); and M is the weight of microspheres (g). The adsorption kinetic behaviors of M10 microspheres can be expressed by the pseudo-rst-order adsorption model 32 or the pseudo-second-order adsorption model 33 where q t is the adsorbed BPA amount per gram of microspheres at time t (mmol g À1 ), q e is the theoretical adsorption equilibrium amount (mmol g À1 ), K 1 is the rate constant of pseudo-rst-order kinetics equation (min À1 ), and K 2 is the rate constant of pseudosecond-order kinetics equation (g mmol À1 min À1 ).
The reuse performance of the M10 microspheres was also tested. 3 mg M10 was added to a BPA aqueous solution (10 mL, 300 mmol L À1 ) and shaken for 24 h (to ensure that equilibrium adsorption reached totally) at 20 C. The mixture was then separated with a permanent magnet and the concentration of BPA in aqueous solution was determined by a UV-vis spectrophotometer. While the desorption process was conducted in 10 mL ethanol with shaking at 20 C for 24 h to ensure complete removal of the residual BPA in M10. Aer desorption, separation of the adsorbent and the aqueous phase was achieved with the help of the same permanent magnet. The separated M10 was washed with deionized water three times, and they were not dried aer separated from deionized water, because the dried microspheres dispersed poorly in BPA aqueous solution, which would affect the adsorption performance of M10. Consequently, the regenerated M10 was added into a new BPA aqueous solution (10 mL, 300 mmol L À1 ) to start the next cycle of adsorption. Adsorption and desorption were repeated for ve times by using the same M10 magnetic microspheres.
All experiments were performed in triplicate.

The structure of microspheres
In order to study the surface morphology of the microspheres, the scanning electronic microscopy was conducted, and the images are shown in Fig. 1a. It is clearly visible that these microspheres exhibit spherical shape with porous structure Scheme 1 The preparation procedure of PMM. (mean diameter about 32 nm). The formation process of porous PEM can be divided into two steps. The rst step is electrospraying which was accompanied with rapid evaporation of solvent DMF while the second step is the guration of microspheres in coagulation bath with the liquid-liquid phase separation which occurred by rapid exchange of the solvent DMF and no-solvent water. 34 According to the forming mechanism of microspheres, the microspheres interior should be porous structure. In order to observe the internal structure of PEM, a facile method 35 reported earlier by our group was adapted. The insert in the right top corner of Fig. 1a shows the image of the cross section of PEM, and a uniform cellular structure can be seen. Meanwhile, the size distribution of microspheres is counted with Image J, and the result is shown in Fig. 1b. Their diameters ranged from 2.5 mm to 5.5 mm, presenting an average diameter of about 3.6 mm, and the size distribution tted the Gaussian distribution on the whole.
The porous PES electrosprayed microspheres were used as the solid template microreactor for co-precipitation reaction. This could be explained by the fact that Fe 2+ could be readily embedded into the PEM matrix, and were bound to the PEM via physical adhesion, rstly. Then with the increase of pH, Fe(OH) 2 were precipitated on the surface of PEM. Fe(OH) 2 were oxidized to ferrite by H 2 O 2 and the reaction systems were kept for a long time, which make ferrite obtain better magnetic properties. Thus the ferrite nanoparticles were synthesized in situ on PEM to form nanocomposite. The corresponding chemical reactions are shown in following equations.
TEM images of PMM are shown in Fig. 2. The results revealed that the ferrite nanoparticles which are proved by the following XRD results, were approximately spherical with diameter about 30 nm which is in good agreement with the value of the pore diameter obtained from Fig. 1a, and they dispersed uniformly in the PEM matrix. The size of the ferrite particles hardly changed, which indicates that the pores on the surface of the PEM acted as microreactors to limit the growth of the ferrite. And the homogenous distribution demonstrates that the ferrite nanoparticles were fabricated successfully by in situ reaction on PEM pores which were used as the microreactors to form ferrite nanoparticles. Fig. 3 shows the SEM images of four kinds of PMM. The images display the morphology of PMM, indicating their spherical shape. The rough surface of PMM is signicantly different from that of the raw PEM. This indicates that a large amount of in situ generated ferrite nanoparticles had accumulated on the surface of the PEM. And the pore volume of PMM was much smaller than that of PEM. This indicates that the ferrite nanoparticles have occupied of the pore space of the PEM. From the SEM images of M15 and M20, it can be found that some ferrite particles dispersed between the polymeric microspheres. It means that due to the increasing concentration of FeCl 2 $4H 2 O in solution, M15 and M20 microspheres may adsorb more Fe 2+ . But the attachment sites on the surface of each microsphere are limited, and part of ferrite nanoparticles in M15 or M20 cannot attach to the microspheres, and became freestanding instead. And the free ferrite nanoparticles tend to agglomerate as shown in Fig. 3c and d. So the ferrite contents (as shown in Table 2) of M10, M15 and M20 didn't change obviously with the increase of Fe 2+ content. This further indicates that the pores of the PEM act as micro-chambers to control the embedment of ferrite nanoparticles. Besides, according to Table 2, it could not be found appreciable change   in the average diameter of the raw PEM and PMM. This could be explained that the generation of the ferrite nanoparticles occurred in the pores of the PEM, and the ferrite nanoparticles occupied only the surface of PEM, keeping the shape and size of original microspheres, which also revealed that the pores on the surface of PEM were used successfully as reaction chambers.
In addition, 4 mg of each kind of PMM were dispersed in 10 mL deionized water by ultrasonic as shown in Fig. 4. Clearly, the color of each suspension liquid changed from light orange to light brown and then to dark brown with increasing the concentration of FeCl 2 $4H 2 O.
Therefore, XRD was employed to study the actual reason for why the color altered among the samples. Fig. 5 shows the XRD patterns of these four kinds of PMM and the original PEM. Since PES resin is a kind of amorphous polymers, there is no crystal structure can be detected in the original PEM. Conversely, all the magnetic microspheres exhibit crystallization peaks, which conrm the presence of the ferrite nanoparticles and imply the success of the in situ reaction. By comparing with the peaks, M5 display some distinct crystallization peaks which are quite different from those of M10, M15 and M20. M5 proles exhibit three groups of peaks at 2q ¼ 33. 15 In a word, in situ generated ferrite nanoparticles have been successfully accumulated in the pores on the surface of PEM, which obviously changes the properties of microspheres such as the following magnetic properties.

Magnetic properties
To investigate the magnetic property of the PMM, VSM test was carried out and the results are shown in Fig. 6. The magnetization of PMM increased with enhancing the intensity of magnetic eld. The magnetization of M5 is weak and shows a lack of saturation, while the others exhibit an extremely small hysteresis loop and low coercivity, which is a typically characteristic of superparamagnetic materials. And the saturation magnetization of M10, M15 and M20 obtained from the Fig. 4 The photograph of PMM dispersed in water.  hysteresis loop is 17.78, 19.38 and 23.96 emu g À1 , respectively. Compared with M5 whose maximum magnetization is about 1.24 emu g À1 , the values of M10, M15 and M20 are improved more than 10 times. The results reveal that the microspheres with Fe 3 O 4 nanoparticles possess better magnetic properties than those with Fe 2 O 3 . Furthermore, the saturation magnetization increased slightly with the increasing of Fe 3 O 4 content. It is obvious that the more Fe 2+ in the solution, the higher saturated magnetization of microspheres in magnetic hysteresis loop. To study the relationship between the concentration of Fe 2+ and the separating performance of the microspheres, the separation experiment was conducted and the results are shown in Fig. 7. Actually, separation performance does not present the same trend as the saturated magnetization results.
From Fig. 7, it can be observed that the PMM possessed a sensitive magnetic response so they could move directionally and align quickly by applying an external magnetic eld. It can also be observed that supernatant uid of M5 is still turbid aer 40 minutes, but for M10 microspheres, the supernatant uid became transparent and the absorbance detected by UV-vis spectrophotometer was 0.484 aer 5 min, indicating that M10 microspheres had been separated completely from the water environment. And M15 and M20 required more time to achieve the similar result in order. Besides, on account of the gradual increase of free Fe 3 O 4 nanoparticles in M10, M15 and M20, the Brownian motion of the particles was more erce than the motion alongside the magnetic eld, so the worst separation performance occurred in M20 whose saturated magnetization was highest. Furthermore, to research the mechanism, supernatant uid of M20 was extracted aer directional separation that lasted for 20 min in deionized water, dried and tested via SEM as shown in Fig. 8, which further conrmed the existence of unbounded nanoparticles having spherical shape and the diameter of 26-35 nm and the result is in good agreement with the values obtained from Fig. 2. It suggests that there are lots of free ferrite nanoparticles in M20 suspensoid which may offer no contribution to the separation performance of the microspheres.
In view of the above results, the M10 in situ magnetic microspheres show the best directional separation performance. So the M10 in situ magnetic microspheres will be mainly used for adsorption of BPA.

Adsorption properties
The dynamic adsorption curves of the M10 sample for BPA in its aqueous solution are shown in Fig. 9. It indicates that the M10 microspheres present a high rate of adsorption in the beginning 60 min when BPA entered some easy accessible surface pore sites and binding with the microspheres, and then the adsorption rate slowed down might be due to the BPA molecular diffusing into some deeper pores. 14 Finally, the adsorption equilibrium reached and the equilibrium adsorption amounts were 115, 165, 273 mmol g À1 , respectively, when the concentration of BPA aqueous solution was 100, 200 and 300 mmol L À1 . As seen in Fig. 9, the amount of adsorbed BPA on M10 sample increased with an increase of adsorption time and BPA solution concentration. Therefore, the adsorption capacity of BPA can be controlled by changing the adsorption time and BPA solution concentration. Fig. 7 The photographs of the magnetic separation of the PMM prepared with different Fe 2+ concentration. Fig. 8 The Low (a)-and high (b)-magnification SEM images of the M20 supernatant after the magnetical separation. Fig. 9 The binding amounts per unit mass of the M10.
In order to investigate adsorption mechanism of M10 microspheres for BPA, the kinetic model equations were employed and the results are shown in Fig. 10 and Table 3. The results showed that the t/q t versus t plots in pseudo-second-order adsorption model for BPA adsorption yielded a better linear relationship. From the slope of the straight line, the theoretical adsorption equilibrium amount with different concentration of BPA can be calculated (123, 178 and 277 mmol g À1 ), and the results were in good accordance with the experimental data as well as the high correlation coefficient R 2 (>0.99). Therefore, the adsorption of BPA onto PES magnetic microspheres followed to the pseudo-second-order model which considers that the rate of occupation of adsorption sites is proportional to the square of the number of unoccupied sites. Compared with PES electrosprayed microspheres (as shown in Fig. 11), the magnetic microspheres possessed much larger BPA adsorption capacity, and the equilibrium adsorption amounts of M10 had increased signicantly to 327.1%, 305.4% and 288.9%, respectively, when the concentration of BPA aqueous solution was 100, 200 and 300 mmol L À1 . Generally, the affinity to adsorbate molecule could affect the adsorption behavior. Since PES is a kind of hydrophobic polymer, the dispersion of the PES electrosprayed microspheres is limited in water, which limits the adsorption of BPA. While, Fe 3 O 4 nanoparticles can improve the hydrophilicity of PES microspheres, so the interaction between BPA molecules and microspheres was enhanced. Besides, aer the in situ reaction process, the surface of magnetic microspheres became much rougher, which may result in an increased specic surface area. And according to the kinetic model, the adsorption was a kind of physical adsorption process which mainly controlled by the specic surface area of the adsorbent. Thereby, adsorption capacity of the microspheres improved. These results demonstrated that both the hydrophilic properties and rough surface of microspheres would affect the adsorption behavior optimistically. Moreover, the microspheres were bathed in boiling water in order to eliminate the residual solvent, and the boiling water would provide anchorage force which enhanced the mechanical force of microspheres to prevent the microspheres from collapse in practical applications. 36 So the PES in situ magnetic microspheres can be applied to remove endocrine disruptors from their aqueous solutions, and can play an important role in the eld of wastewater treatment.
Meanwhile, the reuse performance of the M10 microspheres was also tested as shown in Fig. 12 since the regeneration and  Table 3 The kinetic parameters for the adsorption of BPA onto the M10 C 0 (mmol L À1 ) Experiment q e (mmol g À1 )

Pseudo-rst-order
Pseudo-second-order q e (mmol g À1 ) K 1 (Â10 À2 min À1 ) R 2 q e (mmol g À1 ) K 2 (Â10 À4 g mmol À1 min À1 ) R 2  recyclability of adsorbent is crucial in improving the processing economy. The results showed that the M10 sample had a good reuse performance. The adsorption capacity of BPA could be more than 80% of the initial adsorption capacity aer ve repeated adsorption-desorption cycles. It suggested that the PES in situ magnetic microspheres could be reused for many times without signicant loss of adsorption capacity for BPA and had the potential to be applied in industrial activities.

Conclusion
PES magnetic microspheres were prepared successfully by in situ generation of Fe 2 O 3 or Fe 3 O 4 nanoparticles in the pores on the surface of PES electrosprayed microspheres. It is a new and facile method for the preparation of PES magnetic microspheres which have excellent properties. The magnetic microspheres exhibited sensitive magnetic response, superparamagnetic properties and can be rapidly and directional separated from surroundings with applied magnetic eld. Moreover, compared with the pure PES electrosprayed microspheres, the prepared PES magnetic microspheres possessed more excellent adsorption capacity for BPA. And the adsorption data t the pseudo-second-order adsorption model well. Besides, the resultant microspheres also had a good reuse performance. Therefore, the electrosprayed PES in situ magnetic microspheres have signicant advantages and a great potential for applications in the eld of water treatment and environmental depuration.

Conflicts of interest
There are no conicts of interest to declare.