Synthesis of Emulsion-Templated Magnetic Porous Hydrogel Beads

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Synthesis of emulsion-templated magnetic porous hydrogel beads and their application for catalyst of Fenton reaction Shengmiao Zhang, Xiaoxing Fan, Fangning Zhang, Yun Zhu, and Jianding Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00009 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018

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Synthesis of emulsion-templated magnetic porous hydrogel beads and their application for catalyst of Fenton reaction Shengmiao Zhang,* Xiaoxing Fan, Fangning Zhang, Yun Zhu, and Jianding Chen Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China.

ACS Paragon Plus Environment

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ABSTRACT: Pristine Fe3O4 nanoparticle (FeNP) is supposed to be a good catalyst of Fenton processes which have shown significant potential for water purification. Herein the magnetic macroporous hydrogel beads, having an open-cell structure, were synthesized by sedimentation polymerization of pristine FeNP stabilized oil-in-water high internal phase emulsions. The effects of the FeNP amount, internal phase fraction and the co-stabilizer Tween85 concentration on the structure, such as interconnecting degree, void size and its distribution of both the surface and inner of the beads, were investigated. With a methyl orange (MO) aqueous solution passing through a chromatography column that was filled with the FeNPs loaded hydrogel beads, the efficiency of these hydrogel beads as catalyst for Fenton reaction to decompose MO in water was tested. The MO was decomposed quickly at the first hour, followed by decomposed gradually in further 5 h. And the decomposition rate of MO could be up to 99.6% at the end of the test. Moreover, MO decomposition rate remained over 98.2% in 6 batches which were run in the same beads filled column. The results showed that these FeNPs loaded porous hydrogel beads were reusable and highly efficient supporter for catalysis of Fenton reaction for decomposing organic pollutants in water.

KEYWORDS: High internal phase emulsion, porous hydrogel, Fenton reaction, sedimentation polymerization, magnetic materials


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INTRODUCTION Porous hydrogels with high porosity and good interconnectivity attract increasing attentions due to its widely potential applications, such as drug delivery1, 2, tissue engineering3, support for catalyst4-6, adsorption7-11, and separation12. Oil-in-water (o/w) high internal phase emulsion (HIPE) template is one of the effective approaches to fabricate macroporous polymer hydrogels with extraordinary advantages in the inherent high porosity due to the high dispersed phase fraction, as well as the controllable morphology13. HIPE is often defined as a concentrated emulsion, where the volume fraction of the dispersed phase exceeds 74%13, which provides a very convenient route to synthesize macroporous polymers (so called polyHIPEs) by polymerizing the monomers in the continuous phase14. Conventional HIPEs are commonly stabilized by large amounts (5-50 vol %) of small molecular surfactants15, such as Span 8016, Triton X-40517, and amphiphilic polymer18,

19

. The sheer quantity of surfactant required to

stabilize HIPEs both limits properties and raises cost, despite conventional HIPE-templated polymers normally have a well-defined interconnected porous structure. Alternatively, particles have also been used to stabilize HIPEs (Pickering HIPEs) and produce macroporous materials (i.e. poly-Pickering-HIPEs)20. Utilizing particles as stabilizer provides a number of benefits compared with small molecular surfactants act as sole stabilizer. Firstly, the particles used as stabilizers in Pickering HIPEs are irreversibly adsorbed at the water-oil interface because of their high energy of attachment, which makes the emulsion extremely stable21. Secondly, the use of dispersed droplets in Pickering emulsions as templates can decorate the void walls of the resulting porous materials with a layer of solid particles, which could, for example, contain functional groups and lead to a variety of further applications.


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While considerable progress has been worked on the preparation of Pickering emulsions and their resulting macroporous materials, one important limitation has still remained that Pickering HIPE templated materials commonly have a closed-cell porous structure22, 23, which leads to very low permeability. Two approaches have been made to enhance the interconnectivity of those porous polymers. Firstly, some groups reported interconnected porous polymers based on the HIPE that was solely stabilized by surface-modified particles7,

12, 24-27

. In those work, the

interconnected pores were obtained by carefully controlling the shrinkage of continuous phase resulted from the consolidation of the continuous phase. Secondly, taking a cue from conventional HIPEs, some groups have got the open-cell structure by adding surfactants into Pickering HIPEs before consolidation of the continuous phase28-33. It has been reported that many solid particles, such as silica particles34, 35, titania particles36, 37, copolymer particles38,

39

, and graphene oxide flakes40, can be good stabilizer for HIPEs.

Recently, several groups have used surface modified Fe3O4 nanoparticles (FeNPs) to stabilize water-in-oil (w/o) HIPE and resulted in a magnetic hydrophobic polymeric monolith. The use of surfaced modified FeNPs in w/o HIPE systems was pioneered by Bismarck et al.41. Afterwards, Ghosh et al.42 and Li et al.43 reported the preparation of polystyrene monolith from surface modified magnetic nanoparticle stabilized w/o emulsion templates. Krajnc et al.44, Slugovc et al.45, Zhang et al46, and Mert’s group47 also reported nanocomposite foams based on the w/o HIPEs stabilized by FeNPs. The FeNPs used in those work were surface modified with humic acid or oleic acid, the HIPE stabilized by those particles were w/o type, and the investigated polyHIPE base material was hydrophobic polymer (i.e. poly(styrene-co-divinylbenzene) (P(St/DVB)) or poly(dicyclopentadiene) (PDCPD)), which make the resulting polyHIPEs unsuitable to be a catalyst support in aqueous mediate. In addition, the organic acid upon the


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surface of the FeNPs may reduce or even eliminate the natural function, such as catalyst of Fenton processes, of the particles by keeping the reaction substrates away from the particle surface. FeNP is supposed to be a good catalyst of Fenton processes which have shown significant potential for water purification48, 49. Fenton reaction was proposed in 189450. Since then, it has been explored tremendously and applied widely, especially, in the decomposition of aromatic organic pollutants in water51, 52. The organic pollutants are degraded by some reactive oxygen species such as hydroxyl radical (•OH) and hydroperoxyl radical (•OOH/O2•-) that are generated from H2O2-Fe3+/Fe2+ reactions51. In this work, uniform magnetic hydrogels beads, having a well-defined open-cell structure, were designed and prepared as catalyst for Fenton reaction. These beads were obtained via sedimentation polymerization of o/w HIPEs technique, which was proposed by Ruckenstein et al.53, 54 and developed by Cooper and co-workers.55, 56. The porous beads herein were synthesized from the HIPE stabilized by pristine FeNPs and a small amount of Tween85. Compared with solid beads, porous beads have the advantages of much higher specific surface area and lower bulk density, which provides the material opportunity to be a highly efficient catalyst support. And the efficiency of these FeNPs embedded beads as catalyst for Fenton reaction to decompose methyl orange (MO) in water was tested by passing the MO aqueous solution through a chromatographic column that was filled with the porous hydrogel beads. The results showed that the beads were a highly efficient and reusable catalyst for Fenton reaction, which provides the material opportunity to practical application in wastewater purification.


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EXPERIMENTAL SECTION Materials Acrylamide

(AM),

N,

N'-

methylene

bisacrylamide

(MBAM)

and

N,N,N',N'-

tetramethylethylenediamine (TMEDA) were purchased from Sigma-Aldrich and used as received. Paraffin, hydrochloric acid (37%), and hydrogen peroxide (H2O2, 38%) were provided from Admas (Shanghai, China) and used as received. Cyclohexane, Tween85 and methyl orange (MO) were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received. Fe3O4 nanoparticles (FeNPs, with diameter of 100~300 nm) was obtained from Aladdin. Ammonium persulfate (APS) was purchased from Lingfeng chemical reagents Co. Ltd (Shanghai, China) and was recrystallized twice before use. Deionized water was used. Table 1 Composition of the Pickering HIPE templates and the properties of the resulting porous polymer beads. FeNP a

Fib

Tween85

Ds c

ds d

Is e

Di f

di g

Ii h

Si

(wt%)

(%)

(wt%)

(µm)

(µm)

(%)

(µm)

(µm)

(%)

(m2/g)

B-1-80-3

1.0

80

3.0

50.2

18.4

24.2

60.4

19.2

25.2

6.9

B-2-80-3

2.0

80

3.0

42.3

16.4

23.6

50.6

18.8

24.7

8.2

B-4-80-3

4.0

80

3.0

36.6

10.4

16.1

42.8

12.5

17.0

8.4

B-2-80-1

2.0

80

1.0

-j

-j

-j

74.2

9.2

5.6

4.5

B-2-80-5

2.0

80

5.0

26.2

6.2

24.2

32.4

7.2

21.2

14.2

B-2-75-3

2.0

75

3.0

22.6

5.4

7.6

28.6

9.4

8.2

5.6

B-2-90-3

2.0

90

3.0

96.4

26.6

34.8

108.4

30.6

15.2

12.4

Sample

a

Particle concentration relative to the water used in aqueous phase. b Internal phase volume fraction. c Average void diameter on the surface of beads. d Average interconnected pore diameter on the surface of beads. e Interconnectivity on the surface of beads. f Average void diameter inside the beads. g Average interconnected pore diameter inside the beads. h Interconnectivity inside the beads. i Surface area. jBoth the void and interconnected pore structure could not be identified, and therefore it is difficult to calculate interconnectivity.


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Preparation of porous magnetic hydrogel beads The preparation of porous magnetic hydrogel beads is illustrated in Scheme 1. An aqueous dispersion (4.0 ml) containing 1.42 g AM, 0.31 g MBAM and 0.04 g APS as well as appropriate amount of FeNPs and Tween85 was prepared as the aqueous phase. The aqueous phase was poured into a 100 ml baker equipped with a mechanical stirrer. Then paraffin as oil phase was added dropwise to this aqueous phase that was stirred with a stirring rate of 500 rpm at 25 ºC. Once all of the oil phase had been added, TMEDA of 16 µl was added to the emulsion and keeping stirring for 1 minute. The composition of the HIPE is listed in Table 1. The o/w HIPE formed was then dropped (via a Longer Pump LSP01-1A syringe pump) to a glass reaction column filled with paraffin at 50 °C. The HIPE droplets were left in the sedimentation column at 50 °C for about 6 h to complete the polymerization. The polymerized beads were removed from the column, freeze-dried, and purified with cyclohexane and deionized water for 24 h separately, then the beads were dried under vacuum at 50 ºC to constant weight.

Scheme 1. Preparation of magnetic porous hydrogel beads via sedimentation polymerization of o/w HIPE, and decomposition of methyl orange in water using a chromatographic column filled with the porous hydrogel beads.


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Decomposition of methyl orange in water As shown in Scheme 1, the hydrogel beads (~1.0 g) were filled in a chromatographic column with its inner diameter and length of 1.0 and 30 cm, respectively. Then 15 ml of aqueous solution, containing MO of 20 ppm and H2O2 (38%) of 0.2 ml, was adjusted to pH = 2 with HCl. This aqueous solution circulated in the column under the action of a HL-2B constant flow pump at 30 rpm. The concentration of MO in water was detected with an ultraviolet-visible spectrophotometer (UV-2550PC). For the testing the reusability, these beads were washed with deionized water and dried under vacuum at 50 ºC to constant weight, before they were used for the next batch. Characterization of HIPEs and their resulting porous beads The stability of the Pickering HIPE was evaluated by detecting the backscattering data of monochromatic light (λ = 880 nm) from the HIPE with an optical analyzer (Turbiscan Lab Expert, Formulaction, France). Once the preparation of HIPE was completed, the sample was moved to a flat bottomed cylindrical glass tube with its height of 70 mm and external diameter of 27.5 mm. The tube was placed in the instrument, and the backscattering of light from the sample was then measured periodically along the height at 25 ºC. The results were presented as the curve of backscattering versus time. The emulsion viscosity was characterized by a stress-controlled rheometer (HAAKE, MARS III) with parallel-plate geometry spaced 1 mm apart. The shear rate ranges from 0.001 to 20 s−1. The photographs of the HIPEs and porous beads were taken on a Nikon SLR camera D90. The average bead size was obtained by measuring at least 50 beads for each sample with a Vernier caliper.


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The micro morphology of the hydrogel beads was detected by a Hitachi S-4800 SEM. The interconnectivity degree (I) of the foam was calculated by31

I% = × × × 100%

(1)

where n is the average number of interconnected pores per void, d is the average interconnected pore diameter, D is the average void diameter, and calculated from SEM images. At least 100 voids and interconnected pores were respectively calculated for each sample. The bulk density ρ of the resultant beads were determined by equation (2):

=

= / × 50

(2)

where M is the mass of 50 beads, and V is their volume. Pore volumes were recorded by mercury intrusion porosimeter using a Micromeritics Autopore IV 9500 porosimeter. Samples were subjected to a pressure cycle starting at approximately 1 psia, increasing to 33,000 psia in predefined steps to give pore volume information. Thermogravimetric analysis (TGA) was performed on a NETZSCH STA 449 F3 thermogravimetric analyzer by heating the samples from 50 to 800 °C at 5 °C/min with a N2 flow of 20 cm3/min. Six specimen were analyzed for each sample, and the Fe3O4 content of the sample was given as the average value of these six specimen. N2 adsorption/desorption measurement was carried out on a surface area and porosimetry analyzer ASAP2010N at 77 K and the surface area of the macroporous polymer was calculated by the Brunauer-Emmet-Teller(BET) method. Five measurements were performed for each


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sample, and the average value of these measurements was given as the surface area of the corresponding sample. Elemental mapping for Fe element of the surface and internal morphology of the porous beads were performed via a QUANTAX 400-30 EDS on a Hitachi S-4800 SEM. The magnetic properties (M-H curves) of samples were evaluated on a MPMS-XL-7 magnetometer (Quantum Design Corporation). The concentration change of MO was recorded by the optical absorption curve using an UV2550PC ultraviolet visible spectrophotometer at the light wavelength of 505 nm.

Backscattering / %

60

(b)

50 40 30 20 10 0

0

10

20

t/h 8

10 7 10 6 10

η / mPa·s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(c)

Measured as soon as it was prepared Measured 24 h after it was prepared

5

10 4 10 3

10 2 10 1 10 0 10 1E-3

0.01

0.1

-1

1

10

γ/S

Figure 1. The O/W HIPE with an internal phase volume fraction of 80% stabilized by FeNPs of 2.0 wt% relative to the aqueous phase. (a) photograph of the Pickering HIPE solely stabilized by pristine FeNPs; (b) Backscattering data of the HIPE at 25 ºC, and these data were represented as a function of time; (c) Viscosity of Pickering HIPEs (η) as a function of shear rate (γ).


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RESULTS AND DISCUSSION Closed-cell hydrogel beads from o/w HIPEs Firstly, HIPE having an internal phase of 80 vol% was prepared using FeNPs as a sole stabilizer. An aqueous dispersion (4.0 ml) containing AM of 1.42 g (20 mmol) and MBAM of 0.31g (2 mmol) as well as FeNPs of 0.08 g (2.0 wt% relative to water used in the aqueous solution) was prepared as the aqueous phase. Then 16 ml paraffin was added dropwise to this aqueous phase. After the paraffin was dropped over, a milky-black O/W HIPE was obtained (Figure 1a). Turbiscan analysis of this HIPE showed that the backscattering intensity of the emulsion decreased slightly at the beginning, then kept constant in further 24 h (Figure 1b), which indicated that this Pickering HIPE was stable. And the stability of the Pickering HIPE was also confirmed by viscosity analysis. The viscosity of the Pickering HIPE, measured at 24 h after the stirring was stopped, was similar to that measured as soon as the Pickering HIPE was prepared (Figure 1c). This phenomenon indicated that the HIPEs did not undergo obvious coalescence during 24 h after it was prepared12. All the results suggested that Pickering HIPE solely stabilized by pristine FeNPs could be obtained, and the HIPE could be an emulsion-template for the preparation of porous materials. This HIPE was dropped to a glass column that was filled with paraffin at 50 °C, and subsequently kept in the sedimentation column at 50 °C to complete the polymerization. After the polymerization, uniform polymer beads (Figure S1 a, Supporting Information) were obtained. While SEM analysis showed the resulting porous material had a cell-closed structure (Figure S1 b, Supporting Information), which is consistent with those reported porous monolith based on the


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HIPEs stabilized solely by nanoparticles22,

23, 33, 34

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. In attempt to fabricate open-cell porous

materials, Tween85 (3 wt%, relative to aqueous phase) was added to the aqueous phase before emulsification. As shown in Figure 2 B-2-80-3, both the surface and inside of the polymeric beads have well-defined open-cell structures. The open-cell structure was preferred in further application of the porous materials, e.g. as flow-through column catalyst for Fenton reaction.

Figure 2. SEM images of the surface and inner morphology of the emulsion-templated beads with different feeding amount of FeNPs. The Tween85 concentration in the continuous phase was 3.0 wt%, the internal phase volume ratio of the HIPE was 80 %. The feeding amount of FeNPs (relative to the aqueous phase) for B-1-80-3, B-2-80-3, and B-4-80-3 were 1, 2, and 4 wt%, respectively.

Open-cell hydrogel beads from o/w HIPEs As shown in Figure 2 and S2 (shown in Supporting Information), porous beads were obtained based on the HIPEs stabilized by the mixture of 3 wt% of Tween85 and varied contents (1, 2, and 4 wt%, respectively) of FeNPs. And the average sizes of the beads with 1, 2 and 4 wt%


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FeNPs were 1.94±0.15, 2.10±0.19 and 2.24±0.14 mm, respectively. Increasing the fed FeNPs content caused more nanoparticles and less surfactant Tween85 adsorbed at the surface of the HIPE droplet. Owing to the HIPE droplet surface tension is not modified by particle adsorption at its surface57, the higher FeNPs content, the higher tension of HIPE-air interface, and therefore the larger size of the HIPE droplet and their resulting polymeric beads.

Figure 3. SEM-EDS images for the surface (a) and inner (b) morphology of the bead B-2-80-3. SEM observation of these beads showed that all the three samples have an open-cell structure of both their surface and inner. The high interconnecting degree (~24.2% and ~25.2% for the bead surface and inner, respectively, Table 1) make liquid easier permeate through the polymer beads. Moreover, as shown in Figure 2 and Table 1, both the voids and interconnected pores in bead surface are smaller than those in the inner of bead, respectively. This may be due to the fact that the stabilizer (both FeNPs and Tween85) enriched in the outer layer of the HIPE droplets during these droplets dropped from the syringe to paraffin and subsequently suspended in paraffin in the column. It had been reported that the surfactant in central of a droplet would move


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to its outer layer when the droplet is in a multiphase system59. Herein, the FeNPs rich in the surface layer of the beads was confirmed by SEM-EDS analysis. As shown in Figure 3, the concentration of the red points, representing Fe element, in the bead surface was much higher than that in the bead inner. The high stabilizer concentration caused the smaller dispersed droplets in HIPEs, and consequently the smaller voids and interconnected pores59.

Figure 4. Void diameter distribution of the bead surface (A) and inner (B). a) Sample B-1-80-3; b) Sample B-2-80-3; and c) Sample B-4-80-3.

It was also found that both the void and interconnected pore sizes of the bead surface decreased with increasing of the FeNPs (Figure 2 and Table 1). Since the FeNPs have been confirmed as an effective stabilizer of the HIPEs, it can be expected that increasing FeNP content enhanced the stability of the HIPE, and therefore decreased the void size of the resulting beads. Smaller void size caused a higher surface area of the beads (Table 1). In term of interconnecting degrees (I) of the beads, both of the samples prepared with FeNPs of 1 wt% (Sample B-1-80-3) and 2 wt% (Sample B-2-80-3) had a similar value (~24% and ~25% for surface and inner interconnecting degrees, respectively), which were larger than those (16.1% and 17.0% for surface and inner interconnecting degrees, respectively) of B-4-80-3, because the excess and


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attached particles aggregate in B-4-80-328. The aggregation of particles caused thick particle layers around the dispersed droplets and very stable emulsion films which do not easily to be ruptured to form interconnected pores during the polymerization and post treatment28. Moreover, the aggregation of FeNPs caused an uneven distribution of particles in HIPEs and therefore broadened the distribution of both void and interconnected pore sizes (Figure 4). The mapping of Fe element analysis (Figure 3) also showed that there were many FeNPs successfully imbedded and survived in the surface of the void walls, which provided the material opportunity to be applied as catalyst of Fenton reaction. TGA results (Figure 5 and Figure S3 in Supporting Information) showed that FeNPs content in the final beads increased with increasing the amount of nanoparticles fed before emulsification, which slightly increased the bulk density of the resulting beads (Figure 5) and therefore slightly decreased the pore intrusion volume (10.3, 10.1, and 9.9 cm3/g for the Beads B-1-80-3, B-2-80-3, and B-4-80-3, respectively). It was also found that the more nanoparticles fed, the more nanoparticles lost during the preparation of beads (Figure 5). The loss of FeNPs may be caused by the post-treatment of the beads, such as purification of the beads with cyclohexane and deionized water after the polymerization. The higher FeNPs concentration in the beads, the more nanoparticles might be washed by the solvents. Due to the presence of FeNPs and their magnetism, these beads are magnetic (Figure 6). And their magnetic properties increased with an increase of the content of FeNPs (Figure 5 and 6). While, the beads prepared from Tween85-stabilized HIPE are non-magnetic (Figure 6).


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0.100

8 0.095 6

4

0.090

2

3

Fe3O4 content of polyHIPEs (wt%)

10

Density of the beads (g/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.085 1

2

3

4

The fed Fe3O4 content (wt%)

Figure 5. The ideal FeNPs content calculated from the fed amount of FeNPs (■), the FeNP content of the resulting polyHIPEs obtained from TGA (●), and the density of the beads (☉).

Figure 6. Left: A hysteresis loop measurement of porous beads with different FeNP concentration. Right: A photograph illustration of macroporous magnetic material absorbed on a magnet, the black beads were prepared with the HIPE stabilized by mixture of FeNPs (2 wt%, relative to the water of the aqueous phase) and Tween85 (3 wt%, relative to the water of the aqueous phase); and the white beads were prepared with the HIPE solely stabilized by Tween85 (10 wt%, relative to the water of the aqueous phase).


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Figure 7. SEM images of the surface and internal morphology of the polyHIPE beads with different feeding Tween85 concentration. The internal phase volume fraction was 80%, and the feeding FeNPs was 2 wt% relative to the continuous phase. The feeding Tween85 concentrations in B-2-80-1 and B-2-80-5 were 1 and 5 wt%, respectively.

In attempt to further tailor the morphology of the beads, samples based on the HIPE prepared with varied Tween85 concentration and internal phase volume fraction were prepared. As shown in Figure 7, when Tween85 of 1.0 wt% was introduced to the HIPE, it was difficult to identify either voids or interconnected pores on the surface of the resulting beads, although they could be observed clearly inside the beads. As Tween85 content increased from 1.0 to 3.0 wt%, welldefined void and interconnected pore structures appeared in both the surface and inside of the hydrogel beads (Figure 2 and Table 1). Increasing surfactant content in the HIPE stabilized by the mixture of nanoparticles and surfactant is considered helpful to form open-cell structure, because the surfactant tends to occupy the oil-water interface at the contact points of the adjacent dispersed droplets, and the interconnected pores may be formed at these points during the solidification of the continuous phase.31. Comparing with the beads prepared with Tween85 of 3 wt%, B-2-80-5 that was prepared with Tween85 of 5% had a much smaller average void size and


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interconnected pore size in both the surface and inner of the beads. Increasing surfactant concentration would decrease tension between the continuous phase and dispersed phase of the HIPE, and therefore decrease the void size of the resulting polyHIPEs15, which also caused an increase of the specific surface area of the beads (Table 1). However, although varying of the surfactant dramatically affected the morphologies of the resulting polyHIPE beads, the residual FeNPs content in the final beads kept constant (~3.9 wt%).

Figure 8. SEM images of the surface and inner morphology of the polyHIPE beads with varied internal phase volume fraction. The concentration of fed Tween85 was 3 wt% relative to the aqueous phase, and the concentration of fed FeNPs was 2 wt% relative to the continuous phase. The internal phase volume fractions of B-2-75-3 and B-2-90-3 were 75% and 90%, respectively.

As shown in Figure 8 and 2, when the internal phase volume fraction was increased from 75% to 80% and 90%, while Tween85 and FeNP contents were remained of 3.0 and 2.0 wt%, respectively, the voids of both the bead surface and inner increased dramatically (Table 1). As the internal phase volume fraction increased while the surfactant and nanoparticle concentration


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remained constant, less surfactant and nanoparticles could act as the stabilizer of HIPE, and therefore increased tension between the continuous phase and dispersed phase, and caused a larger average void size as well as a broader void size distribution. Meanwhile, the thickness of the film between adjacent dispersed phase droplets decreased, which helped the formation of interconnected pores during polymerization and consequently increased the interconnecting degree of the resulting porous polymers (Table 1).

Figure 9. Void diameter distribution of the bead surface (a) and inner (b). From front to back: Samples B-2-75-3, B-2-80-3, and B-2-90-3.

Figure 10. Photograph for methyl orange aqueous solution before (left) and after (right) underwent a 6-hour Fenton reaction within the porous beads filled chromatography column.


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Catalysis for Fenton reaction One of the aims of this work was to design and prepare a Fenton catalyst for potential application in decomposing water soluble organic pollutants. To test the catalysis efficiency, as shown in Scheme 1, 1.0 g beads were filled in a chromatography column with its inner diameter and length of 1.0 and 30 cm, respectively. Then 15 ml aqueous solution containing MO of 20 ppm was circulating in the column with the passing rate of about 2 ml/min. As shown in Figure 10, the orange MO solution became colorless, after the experiment was run for 5 h within the B-2-80-3 filled column.

MO residual amount (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 80 B-0-80-3 B-1-80-3 B-2-80-3 B-4-80-3

60 40 20 0 0

1

2

3

4

5

6

7

8

Time (h)

Figure 11. The residual amount of the MO dyes from the solution eliminated by porous beads with different FeNPs concentration.

To study the decomposition rate, the MO solution was monitored by an ultraviolet-visible spectrophotometer. As shown in Figure 11, the MO concentration decreased quickly at the first hour, followed by decreasing gradually in the next 5 hours. Eventually, the decomposition rate of MO kept a constant at 96.4%, 99.1% and 99.6%, when B-1-80-3, B-2-80-3 and B-4-80-3 were used respectively. In contrast, merely about 6% of MO was decomposed, when B-0-80-10 (without FeNPs) was used. This proved that the decomposition of MO was catalyzed by Fe3O4.


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And increasing the feeding FeNPs from 1 to 2 wt%, dramatically rose the final decomposition rate of MO from 96.4% to 99.1% (Figure 11). Nevertheless, further increasing the feeding FeNPs from 2 to 4 wt%, no distinct increase of the final decomposition rate of MO was observed (Figure 11). That may be caused by the difference FeNPs amount on the void surface of porous beads. When the feeding FeNPs content was increased from 1.0% to 2.0%, more FeNPs could be adsorbed on the interface. In comparison, when the concentration was further increased to 4%, the excess nanoparticles were more likely to stay in the aqueous phase, and subsequently were immersed in the polymer wall of the voids after polymerization. So the efficiency of the Fenton reaction catalyzed by Fe3O4 trended to be constant, when the feeding FeNPs were over 2%. In order to know whether the FeNPs were leached out from the beads after they were immersed in water during Fenton reaction, TGA of the nanocomposite beads B-2-80-3 after Fenton reaction was carried out. Before the analysis, the beads after Fenton reaction was washed with water, then was freezing-dried. The residue FeNP content in the nanocomposite beads after Fenton reaction was 3.88 wt%, which is similar to that (3.9 wt%) in the nanocomposite beads before Fenton reaction. This result means that no FeNPs were leached out from these beads during the reaction.

100

MO removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

60

1

2

3

4

5

6

Times

Figure 12. Repeatedly employing a same batch of porous bead to eliminate MO dyes aqueous.


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To investigate the reusability of the beads. Six batch of MO decomposition reaction were run within the column filled with the beads B-2-80-3. These beads were washed with deionized water for 24 h and dried under vacuum at 50 ºC to constant weight, before they were used for the next batch. As shown in Figure 12, the decomposition rate of MO ≥ 98.2% in these six processes, which also confirmed that no FeNPs were leached out from these beads during the reaction. All the results demonstrated that these FeNPs loaded porous hydrogel beads are reusable and efficient catalysis of Fenton reaction for decomposition of organic pollutants in water. CONCLUSION A magnetic open-cell macroporous hydrogel beads were designed and prepared as a catalyst of Fenton reaction using an O/W HIPE that was stabilized by the mixture of pristine FeNPs and Tween85 as templates. The structure of these porous beads could be tuned by simply varying the preparing parameters such as internal phase fraction, FeNPs content, and Tween85 concentration. Because of the presence of FeNPs at the void surface and their open-cell structure, these hydrogel beads were proved to be an excellent reusable catalyst of Fenton reaction for decomposition of MO in water, by flowing the MO aqueous solution through a column that was filled with the hydrogel beads. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Photograph of the porous polymeric beads prepared with a HIPE solely stabilized by FeNPs (Figure 1S a), SEM image of the inert of the porous polymeric bead prepared with a HIPE solely


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stabilized by FeNPs (Figure 1S b), Photograph for porous beads with varied FeNPs content (Figure 2S), and TGA of the beads performed in a nitrogen atmosphere (Figure S3). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S. Z.) ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of Shanghai (16ZR1407800), the National Natural Science Fund of China (51773059), and the Fundamental Research Funds for the Central Universities.


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Table of Contents


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Synthesis of Emulsion-Templated Magnetic Porous Hydrogel Beads

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