1 Introduction

Dyes are widely used in modern industries, such as paper industry, textile industry, leather industry, rubber industry, plastic industry, cosmetic industry, and so on (Ahmed et al., 2013; Gao et al., 2013). Dyes in the waste water may cause damage to aquat-ic or even human health. Thus, the removal of dyes from the effluent attracted great attention of re-searchers. In the past several decades, large amounts of methods have been used to treat dyes in effluent like the coagulation method (Liang et al., 2014; Zhao et al., 2014), chemical oxidation method (Palma-Goyes et al., 2014), catalytic degradation method (Tang et al., 2014; Debnath et al., 2015; Luo et al., 2015), biodegradation method (Adnan et al., 2014; Almeida and Corso, 2014), and adsorption method (Cheng et al., 2015; Filice et al., 2015; Zolgharnein et al., 2015). Among the above-mentioned methods, adsorption is one of the most widely used ones due to low cost, ease of operation, high efficiency, and insensitivity to toxic materials. Till now, various adsorbents such as activated carbon (Namasivayam and Kavitha, 2002; Hameed et al., 2007), zeolite (Meshko et al., 2001; Wang et al., 2006), clay materials (Vimonses et al., 2009; Garrido-Ramirez et al., 2010), and natural and syn-thesized polymeric materials (Crini, 2005; Jarvis and Majewski, 2014; Fu et al., 2015) have been used to remove organic dyes from waste water. Although these adsorbents have shown good results in the ad-sorption of dyes from their waste water, there are limitations such as complexity and high cost of the synthesis procedures, sometimes low adsorption ca-pacities, and most importantly, lack of reusability. Thus, it is a major challenge to develop new types of efficient and reusable adsorbents.

In recent years, porous magnetic materials, es-pecially the Fe3O4-based porous materials, have at-tracted intense attraction in the research of dye ad-sorption area due to their good reusable property. A lot of effort has been spent in the design and synthe-sis of magnetic porous materials composed of Fe3O4 and inorganic or polymeric porous materials (Yan and Wang, 2014; Khoobi et al., 2015; Wang et al., 2015; Zolgharnein et al., 2015). The Fe3O4-based magnetic porous particles are now a promising can-didate in the adsorption of dyes because they com-bine the good porosity of polymers that can be syn-thesized according to the design and the reusability because of the magnetism of Fe3O4. Mak and Chen (2004) synthesized polyacrylic acid (PAA)-coated Fe3O4 particles and found out that this material showed efficient adsorption/desorption ability of methylene blue (MB) from its aqueous solution using PAA as the ionic exchange group. In the above-mentioned methods, the adsorption is specific because of the affinity of functional group of polymers with different dyes. Liu et al. (2010) synthesized porous magnetic microparticles using sulfonated macroporous polydivinylbenzene as a template fol-lowing ferrous ion exchange and oxidation procedures. Although the synthesized particles had combined characteristics of porosity and affinity of the cationic dyes, the synthesis procedure is considered to be difficult. Thus, a simply synthesized and widely applicable magnetic polymeric adsorbent of dye is needed.

We report a simple approach for synthesizing porous magnetic Fe3O4/poly(methylmethacrylate (MMA)-co-divinylbenzene (DVB)) supermagnetic microspheres with the existence of oleic acid-coated Fe3O4 via modified suspension polymerization of MMA and DVB in this study. A series of experi-ments were conducted to investigate the controlling factors of the porous structure of polymer. Further-more, the synthesized microspheres were applied in the adsorption of Rhodamine B from its aqueous solution.

2 Materials and methods

2.1 Materials

MMA was obtained from Sinopharm Chemical Reagent Co. Ltd., China. It was then pretreated as follows: first, it was washed with 10% aqueous solution of sodium hydroxide, then it was dried by anhydrous magnesium sulfate, and finally, it was distilled under a reduced pressure. DVB (80% grade) was obtained from Sigma-Aldrich Chemie GmbH, Germany and treated by passing through the alkaline alumina column to remove inhibitors. Poly(vinyl alcohol) (PVA) (88% hydrolyzed, Mn~88 000) was obtained from Aldrich Chemical Co. and used without pretreatment. Benzoyl peroxide (BPO) was obtained from Sinopharm Chemical Reagent Co., Ltd., China and refined by recrystalization from water. Rhodamine B was obtained from Aladdin Chemistry Co., Ltd., China and used without further purification. Ferric chloride hexahydrate (FeCl3·6H2O), ferrous chloride tetrahydrate (FeCl2·4H2O), aqueous ammonia (25%), sodium chloride, toluene, cyclohexane, and oleic acid were obtained from Sinopharm Chemical Reagent Co., Ltd., China and used without further purification.

2.2 Preparation of oleic acid-coated Fe3O4 mag-netic fluid

Oleic acid-coated Fe3O4 magnetic fluid was prepared by a conventional co-precipitation method (Chen et al., 2009). A three-necked, round-bottom flask reactor was installed with a mechanical stirrer and a reflux condenser with an argon gas inlet tube; then FeCl3 6H2O (24 g) and FeCl2 4H2O (9.8 g) were dissolved in 100 ml of deionized water in the flask under nitrogen gas protection with vigorous stirring at 80 °C for 30 min. Then 50 ml of 25% NH3 H2O was rapidly added to the previously dissolved solution. After 30 min, 5 ml of oleic acid was added dropwise to the suspension within 10 min. Then the suspension was heated for 1.5 h at 80 °C. After the reaction finished, the black precipitate was separated by a magnet, and then it was washed with deionized water and ethanol for several times to remove the remaining oleic acid. Finally, the magnetic precipi-tate was redispersed in octane to form magnetic fluid for the next step.

2.3 Preparation of Fe3O4/poly(MMA-co-DVB) magnetic porous microspheres

Fe3O4/poly(MMA-co-DVB) magnetic porous microspheres (MPMS) were synthesized by a modi-fied suspension polymerization (Tang et al., 2007). In a typical experiment, a mixture of 10.6 ml of MMA, 1.40 ml of DVB, 3 ml of toluene, and 2.0 g of as-synthesized magnetic fluid were mixed and used as the oil phase, and 2.5 g of PVA and 3 g of NaCl were dissolved in 100 ml of deionized water and used as the water phase. Then, the two phases were mixed in a 250 ml three-necked, round-bottomed flask reactor installed with a mechanical stirrer and a flux condenser with an argon gas inlet tube. Then the mixture was stirred for 30 min under argon gas protection at the room temperature to form a stable oil/water emulsion solution. Afterward, the temperature was increased to 70 °C, and then 0.3 g of BPO was added to initiate the polymerization. The reaction was carried out at 70 °C for 5 h, and a brown magnetic emulsion was obtained finally. After the reaction finished, the obtained black precipitate, MPMS, was separated by magnetic separation with a magnet and then was washed with deionized water and ethanol several times separately to remove the unreacted impurities.

2.4 Characterization

The chemical composite, which is to say the existence of oleic acid, MMA, and DVB in the as-synthesized MPMS, was examined by a Fourier transform infrared (FT-IR) spectrophotometer (Ni-colet 5700) using pressed pellets method. The mor-phology of the MPMS was observed by scanning electron microscope (SEM) (Hitachi S-4800). The SEM samples were coated with gold before testing. The internal structure was observed by transmission electron microscopy (TEM) (JEOL 1200EX). The TEM samples were obtained by dropping the ethanol solution of the samples on carbon-coated copper grids. The porosity of the samples was quantified by Hg intrusion method (Micromeritics' AutoPore IV 9500). The magnetization curves of the MPMS were measured with a superconducting quantum interfer-ence device (SQUID) at the room temperature. The crystallinity of the MPMS was examined by X-ray powder diffraction (XRD) analysis. It was carried out on a D/max-rA diffractometer using Cu-Kα radiation with a scan rate of 4 °/min from 10° to 90°. The amount of the polymer incorporated into the MPMS was examined by thermogravimetric analysis (TGA) (Perkin-Elmer TGA-7). The heating rate was 15 °C/min, and the temperature range was from 35 °C to 600 °C.

2.5 Adsorption and desorption tests toward Rho-damine B

The adsorption property of the synthesized par-ticles to Rhodamine B was examined as is described in the following. First, 0.05 g of the as-synthesized MPMS was washed with 1.0 ml of methanol and deionized water three times to make them wet. Then the wet MPMS was added to 5 ml of aqueous solution of a certain concentration of Rhodamine B in a glass bottle. Afterwards the mixture system was dis-persed by ultrasonic treatment for 5 min for suffi-cient interaction between MPMS and Rhodamine B particles. The MPMS saturated with Rhodamine B solution was then separated with a magnet and the concentration of Rhodamine B solution was deter-mined using UV-visual spectroscopy at a wavelength of 554 nm. The desorption test was done by dispers-ing the MPMS containing Rhodamine B into the ethanol solution by ultrasonic treatment for 5 min, and the concentration of the Rhodamine B ethanol solution was determined once again with UV-visual spectroscopy at a wavelength of 554 nm.

3 Results and discussion

3.1 Fabrication of Fe3O4/poly(MMA-co-DVB) magnetic porous microspheres

Magnetic porous poly(MMA-co-DVB) micro-spheres were synthesized by modified suspension polymerization method as mentioned in Section 2 (Tang et al., 2007). The illustration of the synthesis procedure is speculated and shown in Fig. 1. The synthesis conditions of the MPMS are shown in Table 1.

Fig. 1
figure 1

Schematic drawing of the preparation of porous Fe3O4/poly(MMA-co-DVB) microsphere and its pore structure

• MMA and DVB; • porogen; nuclei of poly(MMA-co-DVB)

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figure Tab1

3.2 Properties of the synthesized microspheres

3.2.1 Morphology of the synthesized microspheres

The SEM images of samples 1–9 are shown in Fig. 2. Samples 1–3 are the blank samples without Fe3O4 inside. As shown in Fig. 2, with the decrease in the amount of crosslinking agent DVB, the mor-phology of the particles turned from microspheres to amorphous structures (compare A1, B1, and C1). As illustrated in Fig. 1, two reasons were speculated to be the cause of porosity. First, during the polymeri-zation of DVB and MMA, a crosslinked structure was formed, thus helping in forming a macroscopic porous structure; second, the organic solution such as toluene or cyclohexane reacted as a porogen and caused phase separation during polymerization, finally leading to the formation of porous structure (Macintyre and Sherrington, 2004).

Fig. 2
figure 2

SEM images of sample 1 (A1-A3), sample 2 (B1-B3), sample 3 (C1-C3), sample 4 (D1-D3), sample 5 (E1-E3), sample 6 (F1-F3), sample 7 (G1-G3), sample 8 (H1-H3), and sample 9 (I1-I3)

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Comparing A1 and D1, we can obtain the con-clusion that with the addition of Fe3O4 fluent, the surface of the microspheres became more porous. This phenomenon is because the magnetic fluent reacted as the inhibition agent during the formation of the microspheres. Comparing D3, E3, and F3, we find that with the decreasing of the crosslinking agent DVB, the synthesized microspheres turned from porous microspheres to amorphous composites.

To illustrate the effect of the solvent on the synthesized microspheres, we choose the condition containing the smallest amount of the crosslinking agent DVB. The solvent in the system acted as a porogen that creates pores when it is extracted from the system. Normally, the types of porogen are divided into two groups. In the first category, porogens show good thermodynamic compatibility with the incipient polymer network, which can dissolve both monomers and polymers, and in the secondary category, porogens show bad thermodynamic compatibility with the incipient polymer network, which can only dissolve the monomers (Macintyre and Sherrington, 2004). It is believed that a good solvent for the polymer tends to produce expanded networks, resulting in micropores that will collapse when the solvent is removed. The poor solvent for the polymer produces large fixed pores (Rabelo and Coutinho, 1993). In our experiment, we choose toluene as an example of a good solvent and cyclohexane as a poor solvent. Comparing F1, H1, and I1 (the SEM images for samples 6, 8, and 9, respectively) in Fig. 2, it can be found that the synthesized composite turned from amorphous structures to microspheres with little porous surface. This result showed that the solvent played an important role in the formation of poly(MMA-co-DVB). With the increase in the amount of the solvent, the tension of the polymer system tends to be more stable to form microspheres.

3.2.2 Chemical composition and crystalline struc-ture of the synthesized microspheres

The chemical composition and crystalline struc-ture of the synthesized microspheres were demon-strated by FT-IR (Fig. 3), EDX result (Fig. 4), and XRD pattern (Fig. 5), respectively. In Fig. 3a, the strong peaks in 600 cm−1−1000 cm−1 region are as-signed to the C=C groups of DVB and the peak shown at 1770 cm−1 is the characteristic adsorption peak of the C=O groups of MMA monomer (Ning, 2000). As can be seen in Fig. 3b, the absorption peak at around 580 cm−1 is the characteristic peak for Fe-O which shows the existence of Fe3O4 in the microspheres (Mao et al., 2014). Compared with the bare Fe3O4 spheres, four new bands at 1457 cm−1, 1523 cm−1, 2852 cm−1, and 2922 cm−1 appeared in the FT-IR spectrum of Fe3O4/oleic acid spheres. The bands at 2852 cm−1 and 2922 cm−1 are attributed to the asymmetric -CH2 stretch and the symmetric -CH2 stretch in oleic acid, respectively, and the two bands at 1457 cm−1 and 1523 cm−1 are attributed to the asymmetric (−COO−) stretch vibration and sym-metric (−COO−) stretch vibration, respectively (Li et al., 2010).

Fig. 3
figure 3

FT-IR spectra of MMA, DVB, and poly(MMA-co-DVB) (sample 1) (a) and pure Fe3O4, Fe3O4/oleic acid, and Fe3O4/poly(MMA-co-DVB) (sample 4) (b)

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In Fig. 4, the EDX images of sample 8 and sample 4 show very clearly the existence of elements Fe, O, and C, which demonstrates the composition of the Fe3O4 core and the polymer shell.

Fig. 4
figure 4

EDX spectra of sample 8 (a) and sample 4 (b) (insertions are the SEM images)

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Fig. 5 shows the XRD pattern of sample 6. In Fig. 5, all the diffraction peaks can be indexed to face-centered cubic structure of magnetite according to JCPDS card No. 19-0629, which indicates that the coating did not influence the crystal structure of the Fe3O4 core.

Fig. 5
figure 5

Powder X-ray diffraction pattern of sample 6

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The five characteristic diffraction peaks of Fe3O4 located at 30.2, 35.8, 43.3, 57.2, and 62.6 are assigned to the planes of (220), (311), (400), (511), and (440), respectively (Li et al., 2010).

3.2.3 Coating amount of the polymer shell of the synthesized microspheres

The amount of the polymer shell coated on the Fe3O4 core is determined by TGA and its derivative curves (DTG) (Fig. 6).

Fig. 6
figure 6

TGA and DTG curves of sample 6 (a), sample 7 (b), sample 8 (c), and sample 9 (d)

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The weight loss from 100 °C to 400 °C is due to decomposition of poly(MMA-co-DVB) (Tai et al., 2011). The different weight losses of the particles showed different amounts of the polymer shell. We can see from Fig. 6 that the amount of polymer is about 90%.

3.2.4 Magnetism of the synthesized microspheres

The saturation magnetization curves of the syn-thesized MPMS were obtained by a SQUID under a magnetic field ranging from –10 000 Oe to 10 000 Oe at room temperature as shown in Fig. 7.

Fig. 7
figure 7

Magnetic curves obtained by SQUID of sample 6 (a), sample 7 (b), sample 8 (c), and sample 9 (d) at room temperature

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From Fig. 7 we can see that saturation magneti-zation of samples 6, 7, 8, and 9 are 9.8 emu/g, 0.75 emu/g, 0.78 emu/g, and 0.9 emu/g, respectively. The lower saturation magnetization of samples 7, 8, and 9 can be ascribed to the lower magnetite content of the magnetic microspheres, which is consistent with the TGA result. This result showed that both the amount and porogen type affect a lot of polymer structure. Besides, the typical characteristics of su-perparamagnetic behavior which means the mag-netism only shows on the condition of an external magnetic field (Mu and Wang, 2015) were observed because from the magnetic curve the coercivity and remanence at room temperature are almost unable to be observed.

3.2.5 Porosity and surface structure of the synthe-sized microspheres

Pore distribution and structures of the MPMS were determined by Hg intrusion and extrusion methods (Fig. 8).

Fig. 8
figure 8

Hg intrusion-extrusion curves of sample 6 (a), sample 7 (b), sample 8 (c), and sample 9 (d)

Note: 1 psia=6.896 kPa

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From the Hg ascending and descending branch-es, we can get not only pore distribution but also pore structures (Svata, 1972). At the same cumula-tive intrusion amount (the Y-axis value), if the pressure of the intrusion segment is the same as that of the extrusion segment, then the pores are most prob-ably in cylindrical shape or V shape; if the pressure of the intrusion segment is higher than that of the extrusion segment, then the pore shape is similar to that of an ink bottle with smaller neck and bigger cavity (Svata, 1972). From Fig. 8, we can see that the particles synthesized with a good solvent, e.g., toluene, tend to form pores with cylindrical shape or V shape, while a poor solvent helps to form pores with ink bottle shape.

3.2.6 Adsorption capacity to Rhodamine B of the synthesized microspheres

The standard curve of UV adsorption of Rho-damine B solution is calculated from the adsorption curve of Rhodamine B solution with different con-centrations (Fig. 9).

Fig. 9
figure 9

adsorption lines and related photograph of the solution (a) and the standard line of the concentration of Rhodamine B at the adsorption of λ=554 nm (b)

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Fig. 10 shows the adsorption of MPMS to Rho-damine B and the desorption effect of ethanol. The insertion is the photograph of the adsorption and desorption phenomena from which we can see clearly the strong adsorption of the synthesized particles to Rhodamine B and the desorption of ethanol from it.

Fig. 10
figure 10

UV adsorption spectra of the Rhodamine B solution after the treatment with porous spheres (sample 4) (a) and its desorption of ethanol (b)

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From Fig. 10, we can see that after 4 times using of 5 mg MPMS, the adsorption ability is still very high, which demonstrates the reusability of MPMS.

At last, the quantitative treatment ability of MPMS to Rhodamine B was also examined by the UV absorption method (Fig. 11).

Fig. 11
figure 11

UV adsorption spectra of the Rhodamine B solution after the treatment with different amounts of MPMS (a) and the adsorption ability of the microspheres to Rhodamine B solution calculated from the UV spectra (b)

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The concentration of the original solution is 2.04×10 5 mol/L, and as shown in Fig. 11, with the increase in the amount of MPMS, the concentration droped and quantitatively, the adsorption ability of MPMS to Rhodamine B is 0.81 mg/g (mg Rhoda-mine B/g MPMS).

4 Conclusions

Fe3O4/poly(MMA-co-DVB) MPMS was successfully synthesized. It was found that the crystallinity of Fe3O4 was stable during the synthesis of MPMS. The morphology of MPMS, pore distribution, and type were found to be affected by the ratio of MMA to DVB, the porogen type, and the ratio of the porogen to monomer. The microspheres exhibited relatively high adsorption capacity to Rhodamine B due to their pore structure and the polar groups of the surface and also as it has good reusability because of the magnetic core. Thus, a simple ap-proach was developed to synthesize MPMSs with porous and polar surface and magnetic core. These MPMS can have extensive potential applications in many fields, for example, dye and heavy metal con-taminant adsorption treatment, immobilization of enzymes, drug-targeted delivery, and biological separation.