Multifunctional magnetic oleic acid-coated MnFe2O4/polystyrene Janus particles for water treatment

Article abstract of DOI:10.1039/c6ta04010a

In this work, we have demonstrated the multifunctional magnetic oleic acid-coated MnFe₂O₄/polystyrene (MnFe₂O₄@OA/PS) Janus particles for eliminating multiple contaminants from water. The MnFe₂O₄@OA/PS Janus particles are simply prepared on a large scale by utilizing the phase separation of oleic acid coated MnFe₂O₄ nanoparticles (MnFe₂O₄@OA NPs) and polystyrene (PS) in evaporating n-hexane/chloroform droplets. The as-prepared MnFe₂O₄@OA/PS Janus particles can effectively integrate three intriguing properties, including the instinctive surface amphipathicity, excellent magnetic properties and high catalytic activity. Hence, they not only can act as particle surfactants to encapsulate and magnetically separate oil droplets from water, but also can efficiently degrade organic dyes in water by utilizing hydroxyl radicals generated from the active Fenton reaction between MnFe₂O₄ and H₂O₂. The integrated removal of oil and organic dyes by using the multifunctional MnFe₂O₄@OA/PS Janus particles exhibits obvious advantages including simple procedures, low cost and easy operation. The MnFe₂O₄@OA/PS Janus particles developed in this work may inspire novel strategies for the practical applications of Janus particles in the water treatment for multiple-contaminant removal due to their versatile components as well as the facile, large-scale fabrication approach.

Full text of DOI:10.1039/c6ta04010a

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DOI: 10.1039/C6TA04010A
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ARTICLE
Multifunctional Magnetic Oleic Acid-Coated MnFe2O4/Polystyrene
Janus Particles for Water Treatment
Deng Pan,a Fangzhi Mou,*a Xiaofeng Li,a Zhuoyi Deng,a Jing Sun,a Leilei Xua and Jianguo Guan*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
In this work, we have demonstrated the multifunctional magnetic oleic acid-coated MnFe2O4/polystyrene (MnFe2O4@OA/PS)
Janus particles for eliminating multiple contaminants from water. The MnFe2O4@OA/PS Janus particles are simply prepared
in a large scale by utilizing the phase separation of oleic acid coated MnFe2O4 nanoparticles (MnFe2O4@OA NPs) and
polystyrene (PS) in the evaporating n-hexane/chloroform droplets. The as-prepared MnFe2O4@OA/PS Janus particles can
effectively integrate three intriguing properties, including the instinctive surface amphipathicity, excellent magnetic
property and high catalytic activity. Hence, they not only can act as particle surfactants to encapsulate and magnetically
separate oil droplets from water, but also can efficiently degrade organic dyes in water by utilizing hydroxyl radicals
generated from the active Fenton’s reactions between MnFe2O4 and H2O2. The integrated removal of oil and organic dyes
by the multifunctional MnFe2O4@OA/PS Janus particles exhibits obvious advantages including simple procedures, low cost
and easy operation. The MnFe2O4@OA/PS Janus particles developed in this work may inspire novel strategies for the
practical applications of Janus particles in the water treatment for multiple-contaminant removal due to the versatile
components of them as well as the facile, large-scale fabrication approach.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Janus particles with biphasic geometry and distinct
compositions exhibit inherently anisotropic chemical and
physical properties in single particles.16, 17 They have shown a
Introduction
Water contamination has lately become a worldwide significant
issue of environmental concern.1 Contaminants in water
include the dissolved or dispersed inorganic or organic species,
such as heavy metal ions, microbes, organic dyes and oil spills
etc.2-5 Numerous technologies, including membrane separation,
disinfection, ion exchange, adsorption and catalysis, have been
great potential in diverse applications such as particle
surfactants,18, 19 catalysts,20, 21 and others.22-28 To obtain Janus
particles, various fabrication technologies have been
developed.16, 17, 29 Among them, the phase separation method
is straightforward, does not need any supporting facilities and
has advantages of low-cost, high-efficiency, large-scale and
controllable fabrication.30-34 It is firstly introduced to fabricate
polymer/polymer (e.g. polystyrene/polymethylmethacrylate,
PS/PMMA) Janus particles by Okubo and co-workers,30 and later
developed to eliminate the specific contaminant from water.6-
13
As polluted water generally
contains a variety of
contaminants, an ideal material for water treatment is expected
to have the capability of removing multiple contaminants, thus
enhancing the overall performance and avoiding excessive
redundancy.10 Magnetic nanoadsorbents and reactive
nanostructured membranes are promising for treating multiple
contaminants in water.14,
employed to construct metal oxide/polymer Janus particles,
34, 35
including Fe3O4/PS and SiO2/PS etc.,24,
which do not
simultaneously possess high catalytic activity and
amphipathicity.
15
However, the so far reported
In this work, by utilizing the phase separation of oleic acid
coated MnFe2O4 nanoparticles (MnFe2O4@OA NPs) and PS in
evaporating n-hexane/chiroform droplets, the multifunctional
MnFe2O4@OA/PS Janus particles have been successfully
prepared. Dexterously integrating surface amphipathicity,
excellent magnetic property and high catalytic activity for the
generation of hydroxyl radicals (·OH) from the active Fenton’s
reactions between MnFe2O4 and H2O2, the as-fabricated
MnFe2O4@OA/PS Janus particles not only can act as particle
surfactants to encapsulate and magnetically separate oil spills
from water, but also can efficiently degrade organic dyes in
water. Compared to the traditional technologies to realize the
removal of oil and organic dyes from water, which usually need
to use the combination of two or more of materials or
processes,6-9, 36 the integrated removal of these contaminants
magnetic nanoadsorbents target only for the absorbable
contaminants and demonstrate limitations in chemical
harmless conversion of contaminants, while the reactive
nanostructured membranes suffer from the drawbacks of
membrane fouling and high energy consumption.6 Hence, it is
of urgency to develop multifunctional materials for
economically treating various contaminants in water.
a. State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, 122 Luoshi road, Wuhan 430070, P.
R. China.
b. International School of Material Science and Engineering, Wuhan University of
Technology; 122 Luoshi road, Wuhan 430070, P. R. China.
*Corresponding Authors: Fangzhi Mou, moufz@whut.edu.cn; Jianguo Guan,
guanjg@whut.edu.cn.
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by the single multifunctional MnFe2O4@OA/PS Janus particles was placed at one side of the glass petri dish to VsieewpAarrtiaclteeOntlhinee
DOI: 10.1039/C6TA04010A
has obvious advantages of simple procedures, low cost and easy stabilized oil droplets from water. The separated oil droplets
operation.
with the MnFe2O4@OA/PS Janus particles were pipetted from
the treated water and stored in a centrifuge tube. To separate
oil from the MnFe2O4@OA/PS Janus particles, 0.1 ml water was
introduced into the pipetted oil phase to induce a phase
inversion. To better observe the phase inversion, 0.5 ml n-
octane was added into the above suspension. After the
stabilized oil droplets were pipetted, the MnFe2O4@OA/PS
Janus particles (150 mg L-1) were again added into the RhB
aqueous solution. The adsorption-desorption equilibrium
between RhB molecules and the Janus particles was performed
by continuously stirring in dark for 30 min. Then, 0.2 wt.% H2O2
was added into the above suspension. The catalytic degradation
of RhB is carried out at room temperature. After a specified time,
the MnFe2O4@OA/PS Janus particles were separated from the
solution by a permanent magnet, and the concentration of RhB
was analyzed by a UV-visible spectrophotometer (UV-2550,
Shimadzu, Japan).
Experimental Section
Preparation of MnFe2O4@OA/PS Janus particles.
MnFe2O4@OA NPs (Figure S1) and PS were firstly prepared
according to the reported methods.37, 38 For the preparation of
MnFe2O4@OA/PS Janus particles, an oil phase was prepared by
dispersing MnFe2O4@OA NPs and PS with a weight ratio of 5 : 4
in a mixture solvent containing chloroform and n-hexane (8 : 2
in volume) with a concentration of 90 g L-1. Then, 0.3 ml of the
oil phase was added into 45 ml water containing 3.0 mM CTAB
(water phase) and stirred at 10000 rpm for 25 s with a high-
speed homogenizer to obtain an aqueous suspension of oil
emulsion droplets. Then, the resulted suspension stood
statically at 30 oC for 90 min to gradually evaporate the volatile
oils. Finally, the resultant brown precipitates were isolated by
magnetic separation, washed twice with water, and then dried
in vacuum to obtain the MnFe2O4@OA/PS Janus particles. The
structure of the Janus particles can be modulated by changing
the weight ratio of MnFe2O4 into PS.
Results and discussion
MnFe2O4@OA/PS Janus particles as the particle surfactant
s
6.5 mg MnFe2O4@OA/PS Janus particles were firstly dispersed
in 0.25 ml deionized water. Then, 0.1 ml n-octane was added
into the above aqueous suspension. To make the Janus particles
encapsulate n-octane sufficiently, the mixture suspension was
vigorously shaked for 2 min. Then, the magnetic manipulation
of the Janus particle-stabilized emulsion droplets was tested by
applying a permanent magnet at different directions for
different time intervals.
Characterization
Scanning Electron Microscopy (SEM) and energy dispersive X-
ray (EDX) analysis were obtained using a Hitachi S-4800 Field-
emission SEM (Japan). Optical microscopy images and videos
were recorded through an optical microscope (Leica DMI
3000M, Germany). The X-ray diffraction (XRD) patterns of the
Figure 1. (A) SEM images of the as-prepared MnFe2O4@OA/PS Janus particles. (B) The
magnified SEM image of a typical Janus particle. (C) The line-scanning EDX analysis of C,
Fe and Mn over a typical Janus microsphere. (D) The bowl-shaped MnFe2O4 particle
obtained after the removal of the PS bead by calcination.
samples were recorded using
a
Rigaku D/Max-2000
The as-fabricated MnFe2O4@OA/PS Janus particles are shown
in Figure 1A, indicating that they have a spherical morphology
and an average size of 5 μm. Figure 1B further indicates that the
particle has an obvious binary heterostructure, which consists
of a hemispherical section with a rough surface and the rest
with a smooth surface. To confirm the asymmetric distribution
of MnFe2O4 and PS, the elemental line-scanning EDX analysis
over a typical Janus particle was conducted. As shown in Figure
1C, Mn and Fe are highly concentrated in the section with rough
surface, revealing that it consists of MnFe2O4@OA NPs, while
the other section is composed of PS, as confirmed by the C EDX
signals over the particle. It can be deduced that about 70%
surface of the particle is covered with MnFe2O4@OA NPs, and
the other is the exposed PS. To probe the interior structure of
the MnFe2O4@OA/PS Janus particles, the PS component is
diffractometer equipped with a Cu K radiation source
(λ=0.15418 nm). The magnetic properties of the samples were
investigated using a physical property measurement system
(PPMS, Model PPMS-9, Quantum Design, USA).
Water treatment
To evaluate the capability for the magnetic separation of oil
spills from water and the catalytic degradation of organic dyes
in water, a contaminated water sample was prepared by
dropping 0.2 ml n-octane into 2 ml RhB aqueous solution (10 mg
L-1) contained in a glass petri dish with a diameter of 30 mm.
Afterwards, MnFe2O4@OA/PS Janus particles (25 mg) were
added into the contaminated water sample and stirred for 2 min
to ensure that the n-octane droplets were encapsulated by the
MnFe2O4@OA/PS Janus particles. Then, a permanent magnet
o
selectively removed by calcination at 300 C for 1 h. The
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morphology of the residual is shown in Figure 1D, indicating that loss of the solvents, the oil phase (the hexane-rich solvent with
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the MnFe2O4 compartment is a hemispherical cap and the PS dispersed MnFe2O4@OA NPs) would be DdOeIw: 1e0t.1t0e3d9/bCy6TPAS04b0e1a0dA
compartment is a sphere-like bead in the Janus particle. Due to and rejected to one pole of the PS bead due to their
the strong hydrophobic interactions between the immiscibility and the volume shrinkage of the oil phase (24-41 s
MnFe2O4@OA NPs,37 the MnFe2O4@OA/PS Janus particles in Figure 2A). After the solvent exhausts completely,
exhibit a high structural stability in aqueous solutions. As shown MnFe2O4@OA NPs are self-assembled under hydrophobic
in Figure S2A-D, after a 24 h stirring, the Janus particles remains interactions to form a nanoparticle cluster37 at one pole of PS
their structure well and no MnFe2O4@OA NPs or nanoparticle bead, eventually generating the MnFe2O4@OA/PS Janus
aggregates are peeled off from the Janus particles. The XRD particles (Figure 1). Figure 2B represents the detailed formation
analysis (Figure S2E) also suggests that the long-term stirring mechanism of the MnFe2O4@OA/PS Janus particles through the
has a negligible influence on the composition of the Janus phase separation.
particles. In addition, the MnFe2O4@OA/PS Janus particles also According to the above formation mechanism, the existence of
have a good stability in the aqueous solution with pH ranging n-hexane in the oil droplet is essential for the formation of the
from 5 to 10, as shown in Figure S3.
MnFe2O4@OA/PS Janus particles. To determine this, a control
experiment was carried out, in which the chloroform/n-hexane
mixture oil was replaced by a pure chloroform oil phase. In this
condition, both MnFe2O4@OA NPs and PS are stably dispersed
in chloroform oil phase, no phase separation is initiated. As a
result, only MnFe2O4-PS composite particles are generated after
chloroform is completely exhausted, as shown in Figure 3A and
B. The irregular morphology of the obtained MnFe2O4-PS
composite particles can be explained by the fact that a solvent
vapor bubble forms within the droplet, inducing MnFe2O4@OA
NPs and PS to concentrate in a shell around the inner vapor
bubble.37 Then, the shell of the concentrated MnFe2O4@OA NPs
and PS is broken through by the inner growing bubble, and
finally collapses into an irregular microparticle due to its weak
rigidity.
Figure 2. (A) Time-lapse images indicating the transformation from an oil droplet into a
MnFe2O4@OA/PS Janus particle during the evaporation process of the volatile oil droplet
containing MnFe2O4@OA NPs and PS. (B) Schematic demonstration of the formation
mechanism of the MnFe2O4@OA/PS Janus particles by the phase separation.
The formation of the MnFe2O4@OA/PS Janus particles can be
attributed to the phase separation of MnFe2O4@OA NPs and PS,
which occurs in the evaporation process of n-hexane/chiroform
droplets, as evidenced by the time-lapse images of the volatile
oil droplet (Figure 2A and ESI-Video 1). MnFe2O4@OA NPs and
PS are both dissolved in the mixture solvent of n-
hexane/chiroform (8 : 2 in volume) at first. However, with the
heating process (30 °C ) going on, chloroform diffuses out of the
surfactant-stablized oil droplet through evaporation and
dissolution more quickly than hexane (0-1 s in Fugure 2A) as the
former has a lower boiling point (61.4 °C ) and higher solubility
(0.08 g L-1) in water than the latter (69 °C and 0.0014 g L-1). As a
result, the ratio of hexane/chloroform in the oil droplet
increases with prolonging time, and PS ultimately precipitates
to form a PS bead in the droplet due to the poor solubility of PS
in the n-hexane-rich solvent (5-9 s in Figure 2A). With further
Figure 3. SEM images of (A, B) the MnFe2O4-PS composite particles prepared by replacing
the chloroform/n-hexane mixture solvent by pure chloroform, and (C, D) the
MnFe2O4@OA/PS Janus particles prepared with different weight ratios of MnFe2O4@OA
NPs and PS of (C) 5:1 and(D) 2:5, respectively.
Furthermore, the above formation mechanism also implies that
the size of the two parts in the Janus particles, which may have
a significant influence on their properties and functionalities,16,
17
can be modulated by adjusting the weight ratio (λ) of
MnFe2O4@OA NPs into PS. When λ increases from 5 : 4 to 5 : 1,
their distinct boundary between two parts will shift to PS side,
and the surface area covered by MnFe2O4@OA NPs increases
from 70% (Figure 1) to 85% (Figure 3C). In contrast, when λ
decreases to 2 : 5, the surface area covered by MnFe2O4@OA
NPs decreases to about 50% (Figure 3D). In addition, it is
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Table 1. Estimated dispersive component (훾푠푑푣 ), polar component (훾푝 ), and the total
reasonable to speculate that the component pairs of the Janus
particles can be controlled by simply replacing MnFe2O4@OA
NPs and PS by other oleic acid coated nanoparticles (such as
TiO2@OA, CdS@OA or Pt@OA NPs ) and n-hexane immiscible
polymers, respectively. Compared to the reported metal
oxide/polymer Janus particles (such as Fe3O4/PS and SiO2/PS,
etc.24, 34, 35, 39, 40), the as-constructed MnFe2O4@OA/PS Janus
particles are expected to integrate the three intriguing
properties of the instinctive surface amphipathicity, excellent
magnetic property and high catalytic activity due to the
different affinity of the MnFe2O4@OA NPs and PS
compartments to oil or water, as well as the combined magnetic
property and high catalytic activity of MnFe2O4.37
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푠푣
DOI: 10.1039/C6TA04010A
surface energy (훾푠푣) for MnFe2O4@OA NPs (MF) and PS.
θ
θ
훾푠푑푣
[mN m-1]
훾푠푝푣
[mN m-1]
훾푠푣
[mN m-1]
(water)
(n-octane)
o
o
[
]
[
]
MF
PS
142
84
0
8
21.8
21.6
3.7
6.7
25.5
28.3
To estimate the solid surface energy from Equation 1 and 2, the
contact angles (θ) for n-octane and water on substrates with
spin-coated MnFe2O4@OA NPs and PS are measured (Figure S4).
The contact angles and the values of the estimated surface
energy of MnFe2O4@OA and PS are summarized in Table 1. It
can be seen that PS has a higher surface energy with a higher
polar component than the MnFe2O4@OA NPs, suggesting that
PS is relatively more hydrophilic compared to the MnFe2O4@OA
NPs. Even though the solid surface energy may vary with a pair
of nonpolar and polar liquid used in the contact angle
measurement,42 the relative degree of the hydrophilicity and
hydrophobicity of the MnFe2O4@OA NPs and PS can still be
estimated from the results in Table 1. This result not only
reveals the amphipathicity of the as-obtained MnFe2O4@OA/PS
Janus particles, but also provides a reliable evidence to
demonstrate the wetting behaviors of oil and water on the
surface of the MnFe2O4@OA and PS compartments of the Janus
particles.
Due to the amphipathicity of the MnFe2O4@OA/PS Janus
particles, they can serve as solid surfactants to stabilize
emulsions. As shown in Figure 4A, the MnFe2O4@OA/PS Janus
particles are firstly well dispersed in water and immiscible with
n-octane before shaking (Figure 4A, left). After shaking, a n-
octane-in-water emulsion forms (Figure 4A, right). The
emulsion droplets are 50−500 μm in diameter (Figure 4B), and
rather stable over 2 weeks. The magnified optical microscopy
image in the inset of Figure 4B indicates that MnFe2O4@OA/PS
Janus particles are adsorbed on the emulsion droplet to
constitute a monolayer shell for encapsulating n-octane. Due to
the small size (6 nm) of the contained MnFe2O4 NPs, the
MnFe2O4@OA/PS Janus particles show a superparamagnetic
property with a saturated magnetization (Ms) of 13.7 emu g -1,
as well as a negligible coercive force (Hc) and remanent
magnetization (Mr) at room temperature, as shown in Figure 4C.
Figure 4. (A) Digital images of the emulsification of n-octane and water with the
MnFe2O4@OA/PS Janus particles; left: immiscible mixture of n-octane (top) and water
with well-dispersed Janus particles (bottom); right: n-octane-in-water emulsion
stabilized with the Janus particles. (B) Optical microscopy images of the n-octane-in-
water emulsion. The inset in B shows a magnified optical microscopy image of an
emulsion droplet, which is not completely encapsulated by the Janus particles. (C) The
magnetic hysteresis loop of the MnFe2O4@OA/PS Janus particles. (D) Magnetic
manipulation of the emulsion droplets. (E) The magnetic separation of n-octane droplets
ecapsulated by the MnFe2O4@OA/PS Janus particles from water.
The amphipathicity of Janus particles can be used to stabilize
emulsions and subsequently determines their performance in
oil remediation. To quantify the relative hydrophilicity and
hydrophobicity of each compartment in the MnFe2O4@OA/PS
Janus particles, we have estimated the surface energy of
MnFe2O4@OA NPs and PS. The solid surface energy (훾푠푣) is the
sum of the contributions from both the intermolecular
dispersive forces (훾푠푑푣) and the polar forces (훾푠푝푣) at the surface,
which can be determined by Equation 1 and 2.41
Hence,
the
emulsion
droplets
encapsulated
by
MnFe2O4@OA/PS Janus particles can be magnetically
manipulated. Upon the application of a gradient magnetic field
(H) at different directions for different time intervals, the
emulsion droplets can be magnetically transported accordingly,
as shown in Figure 4D and ESI-Video 2. Thanks to the capabilities
for encapsulating and manipulating dispersed oil in water, the
MnFe2O4@OA/PS Janus particles may have a great potential in
water treatment for oil separation. To explore the performance
of the MnFe2O4@OA/PS Janus particles in oil separation, a
rhodamine B (RhB) aqueous solution with dispersed n-octane is
selected as the contaminated water sample. Upon adding the
MnFe2O4@OA/PS Janus particles into the contaminated water
and stirring for 2 min, the dispersed oil are stabilized and
encapsulated by the Janus particles (Figure 4E). Then the
2
1 + cos 휃
훾푠푑푣 = 훾푙푣
(
)
1
( )
2
2
1
훾′푙푣(1 + cos 휃)
훾푠푝푣
=
[
−
훾푠푑푣훾′푑푙푣
]
√
( )
2
훾′푝푙푣
2
Here
，훾푙푣 and 훾′푙푣 are the surface tension of a nonpolar liquid
and a polar liquid, respectively. In this work, n-octane (훾푙푣=21.8
mN m-1) and water (훾′푙푑푣=21.1 mN m-1 and 훾′푙푝푣=51.0 mN m-1) are
respectively used as the nonpolar and polar liquid. θ is the
equilibrium contact angle of the nonpolar and polar liquid on
the solid surface.
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encapsulated oil can be easily separated from water within 2 the long separation time (40 min) due to the weak magnetic
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47
DOI: 10.1039/C6TA04010A
min by applying a gradient magnetic field (Magnet in Figure 4E). response of the small-sized nanoparticles (5 nm). In contrast,
the amphiphilic MnFe2O4@OA/PS Janus particles not only can
effectively capture oil droplets, but also can separate oil from
water swiftly (within 2 min).
Figure 5. Schematic demonstration of the MnFe2O4@OA/PS Janus particles in the water
treatment for the (A, B) oil separation and (C) catalytic dye degradation.
The mechanism for MnFe2O4@OA/PS Janus particles to
encapsulate and separate oil from water is depicted in Figure
5A and B. At first, the oil phase was broken up into small
Figure 6. (A) UV-vis absorption spectra of the RhB solution at different reaction time after
the MnFe2O4@OA/PS Janus particles (150 mg L-1) and H2O2 (0.2 wt.%) are added. (B) The
normalized RhB concentration (Ct/C0) versus time (t) for the RhB aqueous solutions (10
mg L-1) with H2O2 (black square dots) or H2O2 and MnFe2O4@OA/PS Janus particles (blue
circle dots), respectively. The solid curves represent the fitting curves of the
experimental data by the first-order kinetics mode.
droplets under the action of fluid shear stress during the stirring
process. In the mean time, the MnFe2O4@OA/PS Janus particles
are transfered from the water phase onto the oil droplet, as
shown in Figure 5A, where the yellow arrows represent the
motions of the Janus particles in water under stirring. Once the
MnFe2O4@OA/PS Janus particles attach an oil droplet, they are
anchored in the oil/water interface, in which the PS
compartments are facing the water phase, while the
MnFe2O4@OA compartments point toward the oil phase
because PS is relatively more hydrophilic (or lipophobic) than
the MnFe2O4@OA compartments. Similar to molecular
surfactants, the MnFe2O4@OA/PS Janus particles would be self-
assembled into a monolayer particle shell on the interface of
the oil droplet to minimize the interfacial energy. Then the oil
droplets encapsulated with the Janus particles can be easily
separated from water by a gradient H due to their fine magnetic
response derived from the superparamagnetic property of the
MnFe2O4@OA/PS Janus particles, as shown in Figure 5B.
Besides the oil separation, the MnFe2O4@OA/PS Janus particles
can also be employed in the degradation of organic
contaminants due to the high catalytic activity of MnFe2O4 for
H2O2 decomposition to generate reactive hydroxyl radicals
(·OH).48, 49 To demonstrate this idea, after the separated oil
droplets (Figure 4E) are pipetted, the MnFe2O4@OA/PS Janus
particles (150 mg L-1) and H2O2 (0.2 wt.%) are added into the
RhB aqueous solution (10 mg L-1). The degradation of RhB was
monitored by the intensity changes of its characteristic
absorption peak (λ = 553 nm) at different time intervals, as
shown in Figure 6A. With extending the reaction time (t), the
peak intensity diminishes gradually. No new absorption band or
band shift appear in either the visible or ultraviolet regions,
indicating the effective degradation of RhB. Figure 6B shows the
normalized RhB concentration (Ct/C0) versus t. With prolonging
t, Ct/C0 has no obvious change for the RhB solution only with
H2O2, while it sharply decreases for that containing both H2O2
and the as-obtained MnFe2O4@OA/PS Janus particles. The
degradation of RhB follows the first-order kinetics mode (solid
fitting curves in Figure 6B).50 The first-order rate constant (k) for
the RhB solution only with H2O2 is 1.59×10-4 min-1, while that for
the RhB solution containing both H2O2 and MnFe2O4@OA/PS
Janus particles reaches 2.97×10-3 min-1, much larger than the
former. This indicates that the reaction between H2O2 and
MnFe2O4@OA/PS Janus particles plays a key role for the
degradation of RhB. MnFe2O4 has an excellent catalytic ability
because it can be reduced to a cation excess composite
MnFe2O4 and reoxidized to initial while keeping its spinel
structure, and thus can catalytically decompose H2O2 into ·OH
through Fenton’s reactions.48, 49 The generated ·OH is a highly
reactive and strongly oxidizing radical to degrade RhB molecules
in water.7 The detailed mechanism of the catalytic dye
degradation by the MnFe2O4@OA/PS Janus particles is given in
Figure 5C. Like the oil remediation process, the
MnFe2O4@OA/PS Janus particles can be easily separated from
the treated water by the magnetic separation after the catalytic
RhB degradation is accomplished.
The separated oil droplets with the MnFe2O4@OA/PS Janus
particles can be easily pipetted from the treated water.
Interestingly, a small amount of water may be included in the
pipetted oil phase, and the phase inversion occurs. As a result,
a water-in-n-octane emulsion forms, and the water droplets
with surface-adsorbed MnFe2O4@OA/PS Janus particles
precipitate to the bottom due to their high density (Figure S5A-
C). Hence, the removal of the adsorbed oil from the Janus
particles (Figure S5D) can be achieved by the phase inversion
method by introducing a small amount of water into the
pipetted oil phase containing amphiphilic MnFe2O4@OA/PS
Janus particles. The Janus particles are then recovered by an
o
evaporation process at 80 C for 4 h. The morphology of the
recovered Janus particles is shown in Figure S6, revealing that
the oil separation process has a negligible influence on their
Janus structures. Traditional materials for oil removal, including
clay, activated carbon and natural fibrous sorbent etc., usually
suffer from poor selectivity and low absorption capacity due to
their weak affinity to oils.43-45 On the other hand, oil filtration by
porous membranes exhibits serious drawbacks of membrane
46
fouling and high energy consumption.36,
Recently, the
polymer-coated magnetic nanoparticles show an excellent
capability to separate oil droplets from water, but suffer from
This journal is © The Royal Society of Chemistry 20xx
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Journal of Materials Chemistry A
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ARTICLE
Journal Name
3. Y. Gong, X. Zhao, Z. Cai, S. E. O’Reilly, X. Hao andVDie.wZAhraticole, OMnalinre.
Pollut. Bull., 2014, 79, 16-33.
As for a catalytic material, the stability is of high importance for
its practical applications. To determine the stability of the
MnFe2O4@OA/PS Janus particles in the catalytic reactions, we
have analyzed the morphology and XRD patterns of the Janus
particles before and after the catalytic RhB degradation for 10
h, as shown in Figure S7A-C. The similar SEM morphology and
XRD patterns of two samples suggest that the MnFe2O4@OA/PS
Janus particles have a good stability during the catalytic
reactions, in line with the previous report.49 As demonstrated in
Figure S7B, the degradation of RhB catalyzed by the
MnFe2O4@OA/PS Janus particles almost remains unchanged in
the three cycles, indicating that the MnFe2O4@OA/PS Janus
particles are capable of degrading RhB in water successively.
From the above results, it is concluded that the
MnFe2O4@OA/PS Janus particles can act as a multifunctional
material to remove both oil and organic dyes from water.
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In this work, by taking advantage of the phase separation of
MnFe2O4@OA NPs and PS in the evaporating n-
hexane/chiroform droplets, MnFe2O4@OA/PS Janus particles
have been successfully prepared. The as-obtained
MnFe2O4@OA/PS Janus particles exhibit an amphiphilic
property, of which the PS compartment is relatively hydrophilic
(or lipophobic) compared to the MnFe2O4@OA compartment.
Thus, the oil droplets in water can be effectively encapsulated
by the MnFe2O4@OA/PS Janus microparticles, and then easily
separated from water by the magnetic separation due to their
magnetic responses. Furthermore, the as-obtained
MnFe2O4@OA/PS Janus particles suspended in water can also
efficiently degrade organic dyes by utilizing the active Fenton’s
reactions between MnFe2O4 and H2O2. The MnFe2O4@OA/PS
Janus particles developed here may inspire novel design
strategies of the multifunctional Janus particles for their
practical applications in the water treatment for multiple-
contaminants removal due to the vast possibility of different
component materials as well as the facile, large-scale
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This work was supported by the National Natural Science
Foundation of China (21474078, 51303144 and 51521001), the
Top Talents Lead Cultivation Project and Natural Science
Foundation of Hubei Province (2015CFA003), the Fundamental
Research Funds for the Central Universities (WUT: 2015III060
and WUT: 2016III009), and the Yellow Crane talents plan of
Wuhan municipal government.
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