Fe₃O₄@P(DVB/MAA)/Pd composite microspheres: preparation and catalytic degradation performance

Abstract

Fe₃O₄@P(DVB/MAA)/Pd composite microspheres were synthesized with a combination of coating and loading processes using Fe₃O₄ nanoparticles as a core. A series of characterizations revealed that a combination of Fe₃O₄ coated with a thin polymer shell and deposited Pd nanoparticles was successful. The saturation magnetization of Fe₃O₄@P(DVB/MAA)/Pd was 55.2 emu g⁻¹ and the Pd content was 21.3%. Degradation activity studies confirmed that the as-obtained microspheres showed excellent degradation behavior towards RhB. The degradation efficiency of the Fe₃O₄@P(DVB/MAA)/Pd microspheres for rhodamine B (RhB) can reach 99.6% within 90 s. The regenerated catalyst still showed a catalytic degradation percentage of 98% after being used fifteen times. Meanwhile, the obtained catalyst also displayed degradation ability for other organic dyes. Using Fe₃O₄@P(DVB/MAA)/Pd microspheres is a very promising approach for the remediation of wastewater containing organic dyes due to the fast degradation rate, high degradation efficiency and excellent reusability.

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1. Introduction

Dyes have been widely used in the leather, food and apparel industries and some other areas. Their industrial mass production and application generate a large amount of sewage, which creates a serious burden on the environment. During the past few decades, various methods have been used for treatment of dyes in wastewater. They can be divided into two categories: adsorption and degradation. Porous materials with high specific surface area or high selectivity are often used as adsorbents, including mesoporous SiO₂,^1,2 activated carbon,^3,4 polymeric microspheres,^5–7 etc. Semiconductor materials exhibit high degradation activity and materials such as independent or load-type TiO₂,^8,9 ZnO,^10,11 cobalt oxide^12,13 and quantum dots^14,15 have been reported to decolorize wastewater.

In order to recycle treatment agents easily, magnetic functional materials have received great attention.^16–19 Compared with conventional separation techniques such as centrifugation, precipitation and filtration, magnetic separation is a promising technique because of its easy operation and fast separation. A large number of load-type magnetic adsorbents and degradation agents have been used for dye treatment in liquids, and they have achieved satisfactory effects. These include magnetic carbon nanotubes,²⁰ magnetic mesoporous silica,^1,2 molecularly imprinted magnetic particles,^21,22 Fe₃O₄@TiO₂,^23,24 Fe₃O₄@SiO₂@TiO₂,^25–27 etc. Magnetic degradation agents display obvious application advantages because of the complete treatment and high recycling rate.^23–27 In addition to photocatalytic degradation, in recent years, precious metal catalytic oxidation reduction degradation has become a research hotspot due to its fast degradation speed. This was demonstrated by Yang et al.,²⁸ who loaded Pd nanoparticles onto Fe₃O₄ nanochains for the degradation of methylene blue (MB). The results showed that the hydrogenation of MB occurred in microdroplets and was catalyzed by Pd. The blue microdroplets became colorless in 75 s. This study demonstrates an effective method for the rapid removal of dyes in microsystems. However, it is not suitable for the treatment of large-scale industrial wastewater containing dyes.

In our previous work, we reported three kinds of magnetic degradation agents composed of Fe₃O₄ and a metal oxide semiconductor for removing organic dyes.^13,24,29 The degradation efficiencies of the above materials are satisfactory, but the degradation speed needs to be improved. In this work, we report a rapid reductive degradation method for organic dyes based on the design and successful preparation of Fe₃O₄@P(DVB/MAA)/Pd magnetic composite microspheres. The advantages of the obtained catalysts are as follows: (1) compared with carbon materials and pure polymer microsphere carriers, magnetic materials as Pd carriers show excellent separation and recovery performance; (2) the polymer shell provides carboxy groups for the combining of Pd as well as improving the stability of the magnetic core; (3) the catalysts display the property of rapid degradation. The degradation performance of the as-prepared catalyst was investigated. The influences of initial concentration, reaction time and type of dye on the degradation efficiency were also studied.

2. Experimental sections

2.1 Materials

Divinylbenzene (DVB, 80% mixture of isomers) and palladium chloride (PdCl₂) were purchased from J&K Scientific Ltd. Methacrylic acid (MAA), ethylene glycol (EG), rhodamine B (RhB), methylene blue (MB), methyl orange (MO), bromocresol green (BG), sodium borohydride (NaBH₄) and azobisisobutyronitrile (AIBN) were bought from Sinopharm Chemical Reagent Co. Ltd. Sodium citrate, ferric chloride hexahydrate (FeCl₃·6H₂O), ethanol, hexamethylenediamine (HMD) and acetonitrile were purchased from Xi’an Sanpu Fine Chemical Plant. All of the above chemicals were analytical reagents. The water used throughout the work was deionized water produced by apparatus for pharmaceutical purified water (Aquapro Co. Ltd.).

2.2 Preparation of Fe₃O₄@P(DVB/MAA) microspheres

Fe₃O₄ nanoparticles were prepared through a hydrothermal method. 2.70 g of FeCl₃·6H₂O and 10.43 g of HMD were dissolved in 60 mL of EG under ultrasonic irradiation. The solution was transferred to a 100 mL Teflon-lined stainless-steel autoclave and heated at 200 °C for 12 h. The autoclave was then carefully cooled at room temperature. The black products of Fe₃O₄ nanoparticles were washed with ethanol and water using magnetic separation, and then dried by vacuum freeze-drying. Fe₃O₄@P(DVB/MAA) microspheres were synthesized by distillation precipitation emulsion polymerization, as in our previous work.³⁰ Typically, 0.10 g of Fe₃O₄ nanoparticles, 0.05 g of AIBN, 0.60 g of MAA and 1.40 g of DVB were dispersed and dissolved in 150 mL acetonitrile. The suspension liquid was treated with ultrasonic irradiation. The reaction was carried out under mechanical stirring at 80 °C. After 10 h, the products were separated and washed with ethanol and water using magnetic separation. The obtained materials were Fe₃O₄@P(DVB/MAA) microspheres.

2.3 Preparation of Fe₃O₄@P(DVB/MAA)/Pd composite microspheres

A typical experimental procedure for the fabrication of Fe₃O₄@P(DVB/MAA)/Pd composite microspheres was as follows.³¹ Typically, 0.20 g of Fe₃O₄@P(DVB/MAA) was dispersed into a mixture of ethanol and water under ultrasonic irradiation. The volume of the mixture was 60 mL and the volume ratio of ethanol and water was 2 : 1. 10 mg of PdCl₂ was added into the mixture under mechanical stirring at 45 °C for 12 h. Then, the products were separated and washed with deionized water three times. After vacuum freeze-drying, the Fe₃O₄@P(DVB/MAA)/Pd(II) precursor particles were obtained. 0.15 g of the as-prepared precursor particles was dispersed into 50 mL of a mixed solution of ethanol and water (the volume ratio was 3 : 2) under ultrasonic irradiation. 0.10 g of NaBH₄ was added into the system. The reaction was carried out at 45 °C for 24 h. The Fe₃O₄@P(DVB/MAA)/Pd composite microspheres were obtained after washing with water and vacuum freeze-drying.

2.4 Determination of degradation performance

300 μL of an aqueous solution of NaBH₄ (0.25 mol L⁻¹) was poured into a 1.5 mL EP tube containing 5 mg of the Fe₃O₄@P(DVB/MAA)/Pd composite microspheres. After dispersing well, 700 μL of the dye solution was added to the above mentioned mixture and the initial concentration was 15 mg L⁻¹. Then, the EP tube was put on a vertical mixer for the reaction. After a defined time, the Fe₃O₄@P(DVB/MAA)/Pd composite microspheres were magnetically separated from the suspension liquid. The supernatant was investigated by UV-vis for the calculation of the degradation efficiency. RhB, MO, MB and BG were chosen as model substrates. 5, 15, 20, 25, 30 and 35 mg L⁻¹ were selected for the investigation of the initial concentration. The reaction variables at 10, 15, 20, 25, 30, 35, 40, 50, 60 and 90 s were also detected.

2.5 Characterization

Fourier transform infrared (FTIR) spectra were acquired on a TENSOR27 FTIR spectrometer (Bruker). The morphology of the nanoflowers was observed using a Scanning Electron Microscope (SEM, JEOL JSM-6700F) and a Transmission Electron Microscope (TEM, JEOL JEM-3010). Specific surface areas and pore size distribution were computed from the results of N₂ physisorption (Tristar3020, Micromeritics) using the BET (Brunauer–Emmet–Teller) method. Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000 diffractometer using Cu Kα radiation. The concentration of the dye was analyzed using a UV-vis spectrophotometer (BlueStar, LabTech). The magnetic properties of the magnetic particles were assessed using a vibrating sample magnetometer (VSM, Lake Shore 7307). The magnetic content was measured according to the weight percentage of the residue after thermal analysis from room temperature to 550 °C with a heating rate of 10 °C min⁻¹ under an air atmosphere. The content of palladium was recorded on an inductive coupled plasma emission spectrometer (ICP, 6300).

3. Results and discussion

3.1 Morphology of the composite microspheres

The synthesis procedure, illustrated in Fig. 1, begins with the synthesis of Fe₃O₄ nanoparticles through a solvothermal method. Then, the Fe₃O₄ nanoparticles were initially coated with a thin polymer shell, using DVB and MAA as monomers, by precipitation polymerization, to protect the magnetic cores and introduce carboxy groups to the surface of the magnetic core. The product is denoted Fe₃O₄@P(DVB/MAA). Subsequently, Pd nanoparticles were deposited onto the surface of the as-prepared Fe₃O₄@P(DVB/MAA) using PdCl₂ as the precursor and NaBH₄ as the reducing agent. Then, the Fe₃O₄@P(DVB/MAA)/Pd composite microspheres were obtained.

Fig. 1 Schematic illustration of the preparation procedure of Fe₃O₄@P(DVB/MAA)/Pd composite microspheres.

Fe₃O₄ nanoparticles modified with amino groups were prepared using a solvothermal method. TEM and SEM images are shown in Fig. 2A and B. The average diameter of the Fe₃O₄ nanoparticles was estimated to be about 100 nm. It can be seen from the TEM image (Fig. 2A) that the mass thickness contrast of the particles was different. The SEM image in Fig. 2B confirms that the particles were constructed by Fe₃O₄ nanocrystals and many pores were observed between the nanocrystals. Therefore, the light gray spots on the nanoparticles in the TEM image were attributed to the above mentioned pores. In comparison with the pure Fe₃O₄ nanoparticles, a thin polymer layer, which was coated on the surface of the Fe₃O₄ nanoparticle, was clearly observed in Fig. 2C, and the surface roughness of the Fe₃O₄@P(DVB/MAA) composite microspheres (see Fig. 2D) was decreased. This suggested that the P(DVB/MAA) shell successfully covered the outer surface of the as-obtained Fe₃O₄ nanoparticles. Fig. 2E and F are TEM and SEM images of the Fe₃O₄@P(DVB/MAA)/Pd composite microspheres. As shown in Fig. 2D, small Pd nanoparticles are attached to the surface of the Fe₃O₄@P(DVB/MAA) microspheres. The increase in surface roughness seen in Fig. 2F further confirmed the existence of Pd nanoparticles. The diameter of the Pd nanoparticles was about 15 nm.

Fig. 2 TEM and SEM images of the as-prepared particles: Fe₃O₄ nanoparticles (A and B); Fe₃O₄@P(DVB/MAA) composite microspheres (C and D); Fe₃O₄@P(DVB/MAA)/Pd composite microspheres (E and F) (the scale bar is 100 nm).

3.2 Composition and performance of the composite microspheres

In order to validate the composition of the products, the samples were examined by powder X-ray diffraction. As demonstrated in the XRD pattern in Fig. 3A, in the 2θ range of 20–80°, a series of characteristic peaks of Fe₃O₄ at around 2θ = 30.2°, 35.6°, 43.3°, 53.7°, 57.3° and 62.8°, which were related to the reflections of the (220), (311), (400), (422), (511) and (440) planes, respectively, were observed for all samples and are well indexed to the typical cubic inverse spinel structure (JCPDS card no. 19-0629).³² These peaks are sharp and intense, indicating the well crystallized structure. As deduced from Debye–Scherrer’s formula, the average size of the Fe₃O₄ was about 24.6 nm. There were no additional peaks in the pattern of the Fe₃O₄@P(DVB/MAA) microspheres. The reason for this was that the polymer shell was amorphous. In addition, the deposition of the Pd nanoparticles onto the microspheres can be confirmed from the corresponding XRD data. In the blue curve in Fig. 3A, besides the obvious peaks of Fe₃O₄, there also exist three other diffraction peaks (labeled with the symbol ■). The three peaks positioned at 2θ values of 40.1°, 46.6° and 68.1°, can be attributed to the reflections of the (111), (200) and (220) crystalline planes of Pd (JCPDS card no. 65-2867), respectively. This result demonstrated the occurrence of the combination of Fe₃O₄@P(DVB/MAA) and Pd.

Fig. 3 X-ray diffraction patterns (A) and FTIR spectra (B) of Fe₃O₄ nanoparticles, Fe₃O₄@P(DVB/MAA) microspheres and Fe₃O₄@P(DVB/MAA)/Pd microspheres.

Fig. 3B shows the FTIR spectra of the as-prepared particles. The strong characteristic absorption peaks at 580 cm⁻¹, which were observed in the spectra of all the products, were attributed to the Fe–O vibration. Compared with Fe₃O₄ nanoparticles, the increased peaks at 846, 1448, 1552, 1602, 1701 and 2800–3000 cm⁻¹, which belong to absorption bands of benzene rings, carbonyl groups and saturated carbon–hydrogen bonds, were detected in the spectrum of Fe₃O₄@P(DVB/MAA). This indicated that the shell coated on the surface of the Fe₃O₄ nanoparticles was polymerized by DVB and MAA. The main absorption peaks of Fe₃O₄@P(DVB/MAA)/Pd were similar to those of the Fe₃O₄@P(DVB/MAA) composite microspheres.

The weight percentages of each component in the Fe₃O₄ nanoparticles, Fe₃O₄@P(DVB/MAA) and Fe₃O₄@P(DVB/MAA)/Pd composite microspheres were estimated based on mass data from TGA curves. As shown in Fig. 4A, it can be clearly observed that the weight loss of the Fe₃O₄ nanoparticles was 5.6%, which can be attributed to the thermal decomposition of small organic molecules and functional groups. The magnetic content of the Fe₃O₄@P(DVB/MAA) microspheres was 87.3%. It can be calculated that the ratio of the thin polymer shell was 7.8%. The weight loss of the Fe₃O₄@P(DVB/MAA)/Pd composite microspheres was 9.4%. The solid residues were Fe₃O₄ and Pd. Meanwhile, the content of Pd nanoparticles was measured by ICP, and the value was 21.3%.

Fig. 4 The thermal gravimetric curves (A) and magnetization curves (B) of the Fe₃O₄ nanoparticles, Fe₃O₄@P(DVB/MAA) microspheres and Fe₃O₄@P(DVB/MAA)/Pd microspheres.

The magnetic properties of the samples were determined by VSM at room temperature as illustrated in Fig. 4B. The values of saturation magnetization (M_s) of the Fe₃O₄ nanoparticles, Fe₃O₄@P(DVB/MAA) and Fe₃O₄@P(DVB/MAA)/Pd composite microspheres were 83.6, 74.7 and 55.2 emu g⁻¹, respectively. The results showed that the polymer shell and the deposited Pd nanoparticles weakened the saturation magnetization of the Fe₃O₄ core to some extent. However, the final catalyst still showed good magnetic properties. In addition, no pronounced hysteresis loop was found and the remanence of the particles was about zero. This indicated that the as-obtained magnetic materials were superparamagnetic.

Fig. 5 shows the N₂ adsorption–desorption isotherms and pore size distributions of the particles. The adsorption–desorption isotherms (Fig. 5A) of all samples belonged to the type IV isotherm (isotherms with a hysteresis loop) according to IUPAC, and the hysteresis loops were H3 type. Such a hysteresis loop type indicated that the samples contained stack-type pores. The specific surface areas of the Fe₃O₄ nanoparticles, Fe₃O₄@P(DVB/MAA) and Fe₃O₄@P(DVB/MAA)/Pd composite microspheres were 55.25, 13.05 and 23.84 m² g⁻¹, respectively, and the average pore sizes were 8.03, 21.24 and 18.23 nm, respectively.

Fig. 5 BET (A) and pore size distribution (B) curves of the Fe₃O₄ nanoparticles, Fe₃O₄@P(DVB/MAA) microspheres and Fe₃O₄@P(DVB/MAA)/Pd microspheres.

3.3 Degradation of dyes

RhB was chosen as the pollutant model to investigate the degradation properties of the Fe₃O₄@P(DVB/MAA)/Pd composite microspheres. To examine the influence of time on dye degradation, kinetics experiments were carried out at room temperature. The initial concentration of RhB was 15 mg L⁻¹. As shown in Fig. 6A, the degradation efficiency of the Fe₃O₄@P(DVB/MAA)/Pd microspheres was up to 98.9% within 60 s, whereas the removal efficiencies of Fe₃O₄, Fe₃O₄@P(DVB/MAA) and NaBH₄ were 13.1%, 7.5% and 1.9%, respectively, under the same conditions. When the reaction time was prolonged to 90 s, the degradation efficiency of the Fe₃O₄@P(DVB/MAA)/Pd microspheres reached 99.6%. Investigations into the effect of the initial concentration of RhB on the catalytic degradation efficiency were carried out by adjusting the initial concentration of RhB from 5 mg L⁻¹ to 35 mg L⁻¹ with a reaction time of 60 s, and the results are depicted in Fig. 6B. As the initial concentration of RhB changed from 5 mg L⁻¹ to 15 mg L⁻¹, the degradation efficiency showed no obvious change. At a higher initial concentration, the degradation efficiency decreased with the increase of the initial concentration. It is well known that the reaction is reactant-limited and that the high initial concentration and the long reaction time is needed.

Fig. 6 The time dependence of the removal efficiency (A); effect of initial concentration of RhB on the removal efficiency (B).

The reusability of the Fe₃O₄@P(DVB/MAA)/Pd microspheres was investigated in this study. The reuse process is schematically illustrated in Fig. 7. Deionized water was used as a cleaning agent for the regeneration of the Fe₃O₄@P(DVB/MAA)/Pd microspheres. The magnetic catalyst materials were collected by a permanent magnet each time and they can be re-dispersed again simply by shaking after removing the magnet. Consequently, the treatment is a green process with low energy consumption, which normally does not lead to any waste disposal problems. From the TEM images (inset of Fig. 7), it can be seen that a good morphology of Fe₃O₄@P(DVB/MAA)/Pd remained after being used fifteen times, indicating that the obtained catalysts had excellent stability. Repeated experiments were carried out to check the reusability of the Fe₃O₄@P(DVB/MAA)/Pd microspheres according to the above described process. As shown in Fig. 8, the degradation capacity of the as-prepared catalyst for RhB did not show any significant decrease even after fifteen cycles of successive reuse. After being used fifteen times, it still showed a catalytic degradation percentage of 98% under the same conditions (Fig. 9).

Fig. 7 Schematic diagram of the reuse process of the Fe₃O₄@P(DVB/MAA)/Pd composite microspheres.

Fig. 8 Reusability of the Fe₃O₄@P(DVB/MAA)/Pd composite microspheres for the degradation of RhB.

Fig. 9 The removal efficiency of different dyes by the Fe₃O₄@P(DVB/MAA)/Pd composite microspheres.

The catalytic degradation properties of the Fe₃O₄@P(DVB/MAA)/Pd microspheres were also evaluated for other organic dyes including MB, MO and BG. It was obvious that the obtained catalyst displayed degradation ability for the above dyes. At a reaction time of 60 s, the degradation efficiency showed a trend of MB > BG > MO, and the values were 92.6%, 71.8% and 23.6%, respectively, which is lower than that of RhB. The major reason for the difference in catalytic conversion of different dyes under the same conditions is the structure of the dye. RhB is anionic and it tended to adsorb on the Pd nanoparticles. The other three kinds of cationic dyes were not inclined to adsorb on Pd nanoparticles.³³ Therefore, the degradation efficiency of RhB was higher. During the degradation process, the reduction reaction happened on the surface of the Pd nanoparticles, which played the role of the catalysts. The reaction occurred between the dyes and NaBH₄, which were adsorbed on the Pd nanoparticles.

A comparison of the performance of Pd-based materials is given in Table 1. Comparing the as-prepared Fe₃O₄@P(DVB/MAA)/Pd microspheres with previously reported catalysts, it could be seen that the designed catalysts exhibited excellent degradation efficiency and a rapid degradation rate.

Table 1 Removal ability of Pd-based materials

Pd-based materials	Dyes	Concentration (mg L^−1)	Removal ability		References
			Time	Removal efficiency
a The data were read from the curves which are shown in the references.
Pd-activated carbon	Bromophenol red	20	19 min	97%	34
Agar@Fe/Pd	RhB	5	35 min	81%	35
Pd-activated carbon	MB	7	9.5 min	95%	36
Pd nanocubes/multiwalled carbon nanotubes	MO	20	60 min	99%	37
Pd/hydroxyapatite/Fe3O4	Methylene red	7	80 min	>90%^a	38
Pd-activated carbon	Congo red	25	24 min	>90%^a	39
Pd nanoparticles(6.6 nm)	RhB	3.2	2 min	>95%	40
Pd-activated carbon	Congo red	20	18 min	80%	41
Fe3O4@P(DVB/MAA)/Pd	RhB	15	1.5 min	99.6%	This work

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4. Conclusion

In summary, Fe₃O₄@P(DVB/MAA)/Pd microspheres, with a high catalytic degradation activity and a size of about 100 nm, were prepared in this paper. The initial concentration of RhB affected the degradation efficiency. The degradation efficiencies reached up to 98.9% within 60 s when the initial concentration of RhB was 15 mg L⁻¹. The as-prepared catalyst can be regenerated by washing with water and it exhibited excellent and stable reusability. The preliminary results suggest that this kind of magnetic catalyst has potential application value in the ultra-fast and highly efficient degradation of organic dyes. Because of its ease of recovery after liquid phase reaction, it will greatly facilitate the practical running of industrial pollutant cleanup.

Acknowledgements

The authors are grateful for the financial support provided by the State Key Program of National Natural Science of China (Grant No. 51433008), the Natural Science Foundation of Shaanxi Province (Grant No. 2015JQ2055, 2015JM2050), and the National Undergraduate Training Programs for Innovation.

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