Core-Double-Shell Magnetic Microspheres for Cr(VI) Reduction and Immobilization

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Cr(VI) Reduction and Immobilization by Core-Double-Shell Structured
Magnetic Polydopamine@Zeolitic Idazolate Frameworks-8 Microspheres
Article in ACS Sustainable Chemistry & Engineering · July 2017
DOI: 10.1021/acssuschemeng.7b01036
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Cr(VI) Reduction and Immobilization by Core-Double-Shell
Structured Magnetic Polydopamine@Zeolitic Idazolate
Frameworks‑8 Microspheres
Kairuo Zhu,†,‡
Changlun Chen,*,†,‡,§
Huan Xu,†
Yang Gao,†
Xiaoli Tan,†
Ahmed Alsaedi,§
and Tasawar Hayat§,∥
†
Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, People’s Republic of China
‡
Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou
215123, People’s Republic of China
§
NAAM Research Group, King Abdulaziz University, Jeddah 21589, Saudi Arabia
∥
Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan
*
S Supporting Information
ABSTRACT: A well-defined core-double-shell structured magnetic
polydopamine@zeolitic idazolate frameworks-8 (MP@ZIF-8) hydrid
microsphere consisting of the core of magnetic Fe3O4 nanoparticles,
the inner shell of a polydopamine layer, and the outer shell of a porous
ZIF-8 nanocrystal was prepared through a facile and green approach to
achieve synergistic reduction and adsorptive removal of Cr(VI). The
microsphere property was characterized methodically. The batch
adsorption experiments showed that the MP@ZIF-8 exhibited high
efficiency in the Cr(VI) removal from aqueous solutions, affording
Cr(VI) removal capacity of 136.56 mg g−1
, surpassing pristine MP
(92.27 mg g−1
). The pseudo-second-order model fitted the Cr(VI)
removal kinetics well. Cr(VI) removal on the MP@ZIF-8 relied highly
on pH values. More significantly, with the reduction of nitrogen atom
group on ZIF-8 and PDA, Cr(VI) was easily converted into low toxicity
Cr(III) and then immobilized on the MP@ZIF-8. Thus, the hybrid microspheres provided excellent adsorptive activity in treating
Cr-contaminated wastewater.
KEYWORDS: MP@ZIF-8 hydrid microsphere, Cr(VI), Adsorption, Reduction
■ INTRODUCTION
Water pollutions from heavy metal ions are of growing
worldwide concern for their high detrimental and toxic
effects.1−4
Among major heavy metal ions, chromium is
considered the second most common inorganic contaminant.5
In general, there are two steady oxidation states of Cr(III) and
Cr(VI) for chromium in aquatic environment.6−8
As compared
to Cr(III) species, which can readily precipitate in the
formation of Cr(OH)3, Cr(VI) is more poisonous to biological
systems due to its carcinogenicity and mutagenicity. Thus,
converting Cr(VI) into Cr(III) is a crucial motivation for
Cr(VI) contaminant elimination. Recently, different purifica-
tion methods have been implemented to eliminate Cr(VI)
contamination, among which the adsorption-reduction method
holds promising significance due to its considerable advantages
including being fast, efficient, and eco-friendly.9−12
However,
investigation on synthesizing advanced adsorbent with
relatively simple fabrication procedure and high activity for
Cr(VI) adsorption-reduction is still underway.
Innovation in the structure and morphology of adsorbent is
one key of the enhancement of its adsorption areas and
adsorption capacity as well as density of reaction sites. As a
novel subclass of complex nanostructures, magnetic core-
double-shell structured materials have aroused significant
concern.13−15
On the one hand, the magnetic core can provide
a facile adsorbent recovery approach after reaction;16
on the
other hand, it can improve the stability and dispersity of
adsorbent materials.17
What is more, this nanostructure
suggests great potential for wider applications such as being a
nanoreactor because its permeable shell can prevent the core
from aggregation while allowing small active molecules to
diffuse in and out of the interior microenvironment.18
Magnetic polydopamine hybrid materials used as templates
for convenient fabrication of core−shell structured composites
have drawn significant attention for removing heavy metal ions.
Received: April 5, 2017
Revised: June 27, 2017
Published: July 5, 2017
Research Article
pubs.acs.org/journal/ascecg
© XXXX American Chemical Society A DOI: 10.1021/acssuschemeng.7b01036
ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Furthermore, strong magnetic property of the hybrid material
brings new prospects for the treatment of environmental
pollutants due to its easy separation and fast uptake rate. For
example, Li et al.19
found that magnetic polydopamine (PDA)-
LDH assemblies can simultaneously remove toxic metals and
anionic dyes well. Xie et al.20
synthesized Fe3O4@polydop-
amine (PDA)-Ag core−shell microspheres for MB fast
adsorption. Liu et al.21
reported the core−shell Fe3O4/PDA
nanoparticles were used as adsorbent, catalyst support, and
drug carrier. Magnetic PDA hybrid material possesses the
following characteristics in adsorption: (1) PDA shell can
protect magnetic particles from corrosion in acid media, and,
meanwhile, provide magnetic cores with excellent colloidal
stability. (2) Particularly, the redox reactivity of PDA can enable
the in situ reduction of adsorbed heavy metal ions. Zeolitic
imidazolate frameworks-8 (ZIF-8), a subclass of metal−organic
frameworks (MOFs), has been widely investigated in removing
inorganic and organic pollutants with excellent capacities due to
its porous property and high surface area.22−24
More recently,
ZIF-8 has been proved as high efficient material for Cr(VI)
adsorptive removal, accompanying the partial reduction
process.25
In particular, the highly uniform distribution of
pores is ideal for trapping guest molecules and forcing them to
participate in chemical coordination in some cases, which
endows ZIF-8 with attractive versatility for adsorption related
applications. For instance, one-dimensional β-MnO2@ZIF-8
nanostructures possess simultaneous oxidation and adsorptive
removal of As(III).26
The Fe3O4@PDA@Zr-MOF can be used
as a novel immobilized metal ion affinity platform for
phosphoproteome study.27
Motivated by all of these studies,
an adsorbent combining the moving core, the redox reactivity
of PDA, and the porous of ZIF-8 is expected with high removal
capacity and efficiency.
Here, a well-defined core-double-shell microsphere com-
posed of a magnetic core, PDA inner shell, and a ZIF-8 outer
shell (designated as MP@ZIF-8) was fabricated via a facile and
green approach. These synthesized materials can serve as a
high-efficient adsorbent to eliminate Cr(VI) from water. In this
investigation, the physical and chemical characterization of the
synthesized MP@ZIF-8 microspheres was conducted. The
adsorption kinetics, isotherms, and reuse, as well as the effect of
pH values on their removal capability, were investigated
systematically. Moreover, the interaction mechanisms between
MP@ZIF-8 and Cr(VI) were proposed by using Fourier
transformed infrared spectroscopy (FT-IR) and X-ray photo-
electron spectroscopy (XPS). This work may provide a
potential exploration of MP@ZIF-8 microspheres in adsorbing
Cr(VI).
■ EXPERIMENTAL SECTION
Materials. All solvents and chemicals were obtained from
Sinoreagent without further purification. All water in this experiment
is Milli-Q (Milli-pore, Billerica, MA). K2Cr2O7 was dissolved in Milli-
Q water to prepared for the 60 mg L−1
of Cr(VI) stock solution. The
MP@ZIF-8 microspheres were prepared via three-step routes (Scheme
1). First, the magnetic Fe3O4 microspheres were prepared through a
simple solvothermal method.28
Next, 0.3 g of the as-synthesized Fe3O4
microspheres was dispersed in 150 mL of dopamine-tris solution (2
mg mL−1
, pH 8.5, 10 mM tris buffer), and the mixture was stirred by
mechanical agitator for 24 h at room temperature. Fe3O4 microspheres
were embedded into the PDA polymer to prepare MP composites.
Finally, the MP nanohybrid was washed and freeze-dried at −60 °C in
the vacuum condition for subsequent use. The MP@ZIF-8 micro-
spheres were prepared according to the former reports.29
Briefly, 0.1 g
of MP hybrid material was added to 150 mL of methanol containing
0.225 g of Zn(NO3)2·6H2O with mechanical stirring, and then 0.622 g
of 2-methylimidazole (Hmim) was added, and the reaction was
continued for 3 h at room temperature for ZIF-8 shell growth. The
obtained MP@ZIF-8 microspheres were separated with a magnet and
rinsed with Milli-Q water, then freeze-dried at −60 °C under vacuum
for 12 h. In addition, ZIF-8 was synthesized through a similar process
without MP.
Cr(VI) Removal Experiments. The adsorption measurements
were carried out in the polycarbonate tubes with 1.2 g L−1
MP@ZIF-8
and 60 mg L−1
Cr(VI) solutions. The initial solution pH was adjusted
by using 0.1−0.01 mol L−1
NaOH or HCl solutions. The reaction
system achieved uptake equilibrium under oscillation for 24 h at T =
293 K, and then was centrifuged at 8000 rpm for 20 min. The
concentration of Cr(VI) was determined with 1,5-diphenyl carbazide
method at 540 nm with a detection limit of 0.005 mg L−1
in a UV−vis
spectrophotometer (UV-2550, Shimadzu).30
Removal capacities were
then calculated as follows:
=
− ·
Q
C C V
m
( )
e
o e
(1)
Qe is the equilibrium removal capacity (mg g−1
), Co and Ce are the
initial and equilibrium Cr(VI) concentrations (mg L−1
), respectively, V
is the solution volume (mL), and m is the adsorbent mass (mg).
Characterization, sample preparation after adsorption, and model of
adsorption isotherm and adsorption kinetics are described in the
Supporting Information.
■ RESULTS AND DISCUSSION
Characterization of MP and MP@ZIF-8 Microspheres.
Figure 1 displays the SEM/TEM images of the as-prepared
nanoparticles. Monodisperse pristine Fe3O4 nanoparticles have
uniform spherical shape with a rough surface (Figure 1A and
B), and their average diameters are approximately 400 nm
(Figure S1A). The representative core−shell structure is
observed for MP microspheres with a smooth surface, and
the thickness of PDA shell is approximately 29 nm (Figures
1C,D and S1B). In Figure 1E and F, the boundary between the
ZIF-8 shell and the PDA shell is indistinct, which can be
assigned to the slight mass difference of the two components.
Of note, ZIF-8 shell is apparent to the PDA shell in
morphology in that it is crystalline-like. Moreover, after being
coated with ZIF-8, the diameter of the microspheres increases,
and the average size of MP@ZIF-8 is about 503 nm (Figure
S1C). Time-dependent TEM was used to study how the ZIF-8
shell was formed. As compared to the pristine MP (Figure 1C),
tiny ZIF-8 nanocrystals adhered to the surface of the MP
microspheres after reacting for 0.5 h (Figure S2B). With the
rise in reaction time up to 1.5 h, more nanocrystals are
deposited to generate an integrated ZIF-8 shell (Figure S2C).
After reacting for 3 h, MP@ZIF-8 with thick ZIF-8 shell
thickness (Figure S2D) was successfully synthesized.
The EDS spectrum in Figure 2A demonstrates the existence
of C, O, N, Fe, and Zn elements in MP@ZIF-8. EDS line
distributions are taken across the radius of MP@ZIF-8
microspheres in Figure 2B. Fe element is mainly distributed
Scheme 1. Schematic Illustration of the Formation
Procedure of the Core-Double-Shell Structured MP@ZIF-8
Microspheres
ACS Sustainable Chemistry & Engineering Research Article
B

in the core part, and O is primarily distributed on the core and
partially distributed in the inner shell, while Zn element
primarily enriches in the outer shell layer (Figure 2B), which is
consistent with the consequence of the elemental mapping
image analysis (Figure S3). It is of note that Zn also existed in
the core of MP, which can be ascribe to some Zn ions adsorbed
into MP during the ZIF-8 growth. By combining the
morphology with the component of MP@ZIF-8 through
TEM image and XRD pattern analysis, it is sure that ZIF-8 is
able to be formed on the surface of MP@ZIF-8. The above
results further confirm the successful synthesis of core-double-
shell spheres.
Figure 2C shows the XRD patterns of the Fe3O4, MP, and
MP@ZIF-8. All of the peaks in the black and red spectra are
indexed to the face-centered cubic Fe3O4 (JCPDF card no. 74-
0748). The XRD patterns of the MP and Fe3O4 exhibit similar
features, meaning that the amorphous PDA shell does not affect
the Fe3O4 crystalline phase. For MP@ZIF-8, the new peaks
marked by quadrate come from the cubic phase of ZIF-8, which
should be accordingly assigned to the (011), (002), (112),
(013), (233), and (134) lattice planes of the ZIF-8 phase.31
Figure 2D displays the FT-IR spectra of Fe3O4, MP, and
MP@ZIF-8. In the spectrum of Fe3O4, the absorption peak at
1427 cm−1
is corresponding to the vibration of OCO
groups modified on Fe3O4. The broad peak at 1636 cm−1
is
corresponding to the vibration of overlapping O−H and O
CO groups of Fe3O4.32
The bands at 3440 and 592 cm−1
are
associated with the O−H stretching vibration and Fe−O−Fe
vibration, respectively.26,33
Plentiful new peaks could be
observed in the MP spectrum. The peak of OCO group
is covered and the bands at 1615 and 1497 cm−1
appear, which
are attributed to the CC stretching vibrations of aromatic
ring.34
A representative peak at 1302 cm−1
might be the
primary amine vibration of the PDA shell.35
In the MP@ZIF-8
spectrum, the peak intensities of aromatic ring reduce, notably,
the peaks at 1145 and 994 cm−1
are assigned as the plane
bending of imidazole ring, and the band at 421 cm−1
is
attributed to the Zn−N stretch mode.23
FT-IR was also used to
characterize the structure variation of MP@ZIF-8 after Cr(VI)
adsorption. Figure 2E shows that the band at 870 cm−1
in the
chromium-adsorbed MP@ZIF-8 spectrum corresponds to the
stretching vibrations of the CrO4 tetrahedron.36
The character-
istic peaks at 1497, 1427, and 1302 cm−1
related to the aromatic
ring stretch, N−H vibration, and stretching vibration of
phenolic C−O groups of the PDA either reduced or
disappeared after Cr(VI) removal, implying that the Cr(VI)
species are bonded to both phenolic hydroxyl and amino
groups of PDA and certain chemical bonds are formed.37
Figure 2F shows the TG analysis of Fe3O4, MP, and MP@
ZIF-8. The observed curve of Fe3O4 represents two steps of
mass loss. The first mass loss stage (<150 °C) is attributable to
absorbed water elimination. Of note, one slight mass increases
from 150 to 350 °C due to the transformation of Fe3O4 to
Fe2O3.38
Furthermore, the second mass loss step (<600 °C) is
attributed to the decomposition of impurities. For MP and
MP@ZIF-8, the mass increase disappears, but an obvious mass
loss appears from 220 to 500 °C. Additionally, the mass loss is
relatively strong in MP@ZIF-8, assigned to a synergistic effect
of the removal of PDA polymers and the collapse of ZIF-8.39
The TG curves validate that the mass loss values of the Fe3O4,
MP, and MP@ZIF-8 are ca. 2.3, 20.7, and 21.6 wt %,
Figure 1. SEM, TEM images of pristine Fe3O4 (A and B), MP (C and
D), and MP@ZIF-8 microspheres (E and F).
Figure 2. EDS image (A), EDS line scanning of O, Fe, and Zn
elements (B) of MP@ZIF-8, XRD patterns (▼, Fe3O4; blue ■, ZIF-8)
(C), FTIR spectra (D), TGA curves (F) of Fe3O4, MP, and MP@ZIF-
8, and (E) FTIR spectra of MP@ZIF-8 before and after Cr(VI)
removal.
C

respectively. The results above demonstrate that ZIF-8 has
been successfully attached onto the MP.
The N2 adsorption−desorption isotherms of MP, ZIF-8, and
MP@ZIF-8 are displayed in Figures S4 and S5. The curve of
ZIF-8 is a typical type I nitrogen adsorption−desorption
isotherm,40
which fits well with the microporous frameworks of
ZIF-8. The initial stage of the isotherm could be attributed to
adsorption by micropores. Table S1 lists the pore structure
parameters (specific surface area, pore volume, and average
pore diameter) of MP ZIF-8 and MP@ZIF-8. The surface area
of the MP@ZIF-8 (93.10 m2
g−1
) is greatly lower than that of
pure ZIF-8 (1465.21 m2
g−1
). It is postulated that the
nonporous MP makes no contribution to the mesoporous
structures. Also, the presence of micropores (0−20 Å) in MP@
ZIF-8 also indicates that the ZIF-8 is located on the surface of
MP (Figure S6). These pores in MP@ZIF-8 microspheres
provide more contact area between the adsorbent and target
ions and offer more sites to adsorb metal ions.
The magnetization hysteresis loops at room temperature
were also characterized. Because of the addition of non-
magnetic PDA and MOF components, the resulting MP@ZIF-
8 double-core-shell microspheres have a decreased magnet-
ization saturation value of 59.4 emu g−1
in comparison with
those of Fe3O4 (84.4 emu g−1
) and MP (65.0 emu g−1
) (Figure
S7), which is in accordance with the TGA data analysis.
Furthermore, the inset digital picture indicates the convenient
separation after Cr(VI) adsorption because of the superior
magnetic property.
Adsorption Kinetics. The adsorption kinetics of Cr(VI)
onto MP@ZIF-8 was investigated using an initial Cr(VI)
concentration of 30 mg L−1
. Figure 3A shows the Cr(VI)
adsorption promptly increases at the first shaking time of 4 h,
hereafter slowly increases, and finally attains uptake equilibrium
after 16 h. The adsorption kinetics of Cr(VI) was simulated by
the pseudo-first-order and pseudo-second-order models (eqs S1
and S2). The kinetic parameters and the correlation coefficients
(R2
) are calculated by line regression (Figure 3B,C) and are
listed in Table S2. The plots of the pseudo-second-order model
show quite good linearity with R2
values of 0.991 (pseudo-first-
order: 0.930), which confirms that the adsorption kinetics of
Cr(VI) follows the pseudo-second-order model well, suggesting
a chemical reaction process.41
Effect of Solution pH. Figure 4A presents the pH effects
on Cr(VI) adsorption. The adsorption of Cr(VI) decreases
with the rise in the original solution pH, consistent with the
consequence of the earlier reports.8
Approximately 143.24 and
42.59 mg g−1
of Cr(VI) is removed by MP@ZIF-8 at pH 2.72
and 9.05, respectively. It is noteworthy that Cr(VI) removal
curves can be explained by the surface properties of MP@ZIF-8
and relative distribution of Cr(VI) species under a wide variety
of pH values.42
Surface charge values of MP and MP@ZIF-8
were measured and shown in Figure 4B. The surface of MP is
negatively charged within the pH range from 2.0 to 7.0, the
surface of ZIF-8 is positively charged at pH < 10.1, and the
MP@ZIF-8 surface is negatively charged ranging from pH 3.0
to 7.0 because ZIF-8 is anchored onto MP. Thus, the zero
charge potential of MP@ZIF-8 is 3.0. Also, as shown in Figure
4C, Cr(VI) dominatingly exists in anionic species over the pH
range, such as HCrO4
−
(pH 3.0−7.0) and CrO4
2−
species (pH
7.0−10.0). Thus, the lower adsorption of Cr(VI) on MP@ZIF-
8 with the increase in pH values can be ascribed to the intensive
electrostatic repulsion between anionic species and negatively
charged MP@ZIF-8. The consequence is in agreement with the
removal of Cr(VI) by ZIF-8.43
Adsorption Isotherms. The adsorption isotherm data of
Cr(VI) were regressively fitted with the Langmuir and
Freundlich models (eqs S3 and S4), and are displayed in
Figures 4D and S8. MP@ZIF-8 shows a more intensive
adsorption capability of Cr(VI) than pristine MP. This result
reveals that MP@ZIF-8 possesses obvious superiority in
adsorption capacity, and implies that ZIF-8 anchored onto
MP is beneficial for Cr(VI) removal. Table S3 presents the
Qemax values of Cr(VI) removal onto other adsorbents. The
maximum adsorption capability of MP@ZIF-8 (136.56 mg g−1
)
is higher as compared to the former reported adsorbents (such
as polyaniline nanowires,44
ZIF-8,25
and AMH,45
etc.). Table
S4 lists the model parameters. Results show that the Langmuir
model fits the adsorption isotherm data of Cr(VI) better than
the Freundlich model.
Recycling Performance of MP@ZIF-8. The recycling of
MP@ZIF-8 was investigated to evaluate the cost-effectiveness.
In brief, the samples after Cr(VI) adsorption were immersed in
NaOH solution (6 mL and 0.05 mol L−1
) and agitated for 12 h,
and then centrifuged and washed several times with Milli-Q
water. Finally, they were freeze-dried at −60 °C under vacuum
and reused. As shown in Figure S9, clearly, after five cycles,
Cr(VI) removal efficiency decreased slightly; however, almost
72% of the initial uptake still remained. The slight decreases in
Figure 3. (A) Time profile, (B) pseudo-first-order, and (C) pseudo-
second-order kinetic plots of Cr(VI) removal on MP@ZIF-8 (pH =
5.0 ± 0.1, m/V = 0.2 g L−1
, and Co = 30 mg L−1
).
Figure 4. (A) The effects of pH on Cr(VI) adsorption capacity (m/V
= 0.2 g L−1
and Co = 30 mg L−1
), (B) surface charge of ZIF-8, MP, and
MP@ZIF-8, (C) speciation distribution of Cr(VI) as a function of pH,
and (D) adsorption isotherms of Cr(VI) on MP and MP@ZIF-8 (pH
= 5.0, m/V = 0.2 g L−1
).
D

adsorption amount come from the incomplete adsorbate
desorption from the surface of adsorbent.46
Thus, MP@ZIF-8
shows a good reusability.
Cr(VI) Adsorption-Reduction Mechanisms. XPS was
conducted to measure the elemental species to confirm the
reduction of Cr(VI) on MP@ZIF-8 microspheres. As shown in
Figure 5A of the scan spectra, there were obviously peaks
corresponding to C 1s, O 1s, N 1s, Zn 2p, and Cr 2p. The Cr
2p peak in the XPS spectrum of chromium-adsorbed MP@ZIF-
8 clearly confirmed the adsorption of Cr(VI). The spectra of
the Cr 2p showed two major peaks with binding energies at
577.2 and 586.5 eV, corresponding to Cr 2p3/2 and Cr 2p1/2,
respectively (Figure 5B). After XPS-peak-differentiating anal-
ysis, the Cr 2p spectrum was resolved into five peaks, which
corresponded to Cr(VI) and Cr(III) species. The presence of
Cr(III) indicated the reduction of Cr(VI) during the adsorption
process.47
Moreover, the deconvolution of the Cr 2p3/2 line
peak into two peaks showed that Cr2O3 or Cr(OH)3
precipitates took shape. Considering the aperture pore of
ZIF-8 crystals (3.4 Å), the hydrated diameter of Cr(VI) (7.5 Å)
is too large to diffuse.31,48
However, gap defects between the
ZIF-8 crystals have been recognized to exist;49−51
hence,
Cr(VI) successfully penetrates into the interior and simulta-
neously achieves chemical interaction onto PDA.
From Figure 5C, the O 1s peak strength decreases
considerably after Cr(VI) adsorption, and this result indicates
the involvement of the hydroxyl group and chemical interaction
of Cr(VI) onto MP@ZIF-8.52
The O 1s spectra of virgin MP@
ZIF-8 produced H2O, Zn−OH, and C−OH at 532.8, 531.8,
and 529.9 eV, respectively. However, a new Cr−O peak
appeared with a binding energy (BE) of 531.8 eV after
adsorption, primarily due to Cr(VI) anions or hydroxylated
Cr(III). Therefore, it can be confirmed that the external active
sites (i.e., hydroxyl groups bonding with zinc) produced during
the adsorption of dissociative water molecules may further
interact with Cr(VI) species. Moreover, the C−OH peak
disappeared after the adsorption process, which showed the
phenolic hydroxyl groups on PDA surfaces might chelate with
Cr(VI) ions.
As displayed in Figure 5D, the BE at 400.2 eV of N 1s was
upward shifted to the BE of 399.2 eV after adsorption,
indicating the decreased electron density of nitrogen species,
and revealing that a transformation of the regional bonding
environments occurred near to nitrogen species. For MP@ZIF-
8, N 1s peak was deconvoluted into two peaks, −NH− at 399.6
eV and −N at 399.1 eV. The peak area of −NH− and −N
group decreased after Cr(VI) adsorption. Meanwhile, a new
peak appeared at 401.2 eV for the protonated −N+
. −N+
was produced due to the doping of H+
on −N, through
protonated reaction. Moreover, protonated N atoms group (
Figure 5. XPS spectra of MP@ZIF-8 before and after Cr(VI) removal:
wide scan (A) and high-resolution spectra of (B) Cr 2p, (C) O 1s, and
(D) N 1s.
Figure 6. XPS spectra of ZIF-8 (A), MP (D) before and after Cr(VI) removal, Cr 2p spectra of Cr(VI) loaded ZIF-8 (B) and Cr(VI) loaded MP
(E), and N 1s spectra of ZIF-8 (C), MP (F) before and after Cr(VI) removal.
E

N−+
) might also be created due to the in situ doping of positive
Cr(III), generated from the reduction of Cr(VI). The presence
of −N+
was favorable for electrostatic interaction with
negative Cr(VI) species. These results implied that nitrogen
species of MP@ZIF-8 were related to the Cr(VI) reduction.
To further investigate the reduction mechanisms, the XPS
spectra of ZIF-8, MP before and after Cr(VI) removal were
recorded in Figure 6. As shown in Figure 6A and D, Cr species
successfully were loaded on ZIF-8 and MP. According to the
above Cr 2p spectrum analysis, the presence of Cr 2p3/2 (Figure
6B and E) verified that parts of the adsorbed Cr(VI) were
reduced to less toxic Cr(III), meaning that reduction existed in
the ZIF-8-Cr(VI) and MP-Cr(VI) system. The N 1s spectra of
ZIF-8, MP before and after Cr(VI) adsorption were compared
in Figure 6C and F. From Figure 6C, the BE at 398.8 and 399.3
eV was attributed to the N− and −NH− groups of
imidazolate ligands, respectively. After Cr(VI) adsorbed on
the ZIF-8, a new peak at 400.5 eV appeared, which was
attributed to the protonated nitrogen atom coordinate with Cr
species. As shown in Figure 6F, the two fitted peaks of pristine
MP were assigned to the groups of N− and −NH− on the
quinone structure of PDA. For MP after Cr(VI) adsorption, 
N−+
also appeared to chelate with Cr species. Moreover, the
shift degree of the N 1s peak for chromium-adsorbed MP@
ZIF-8 was more than the shift degree of N 1s peak for
chromium-adsorbed MP and ZIF-8. The above results implied
that reduction on chromium-adsorbed MP@ZIF-8 is the
synergistic promoting reaction mechanism.
In summary, reduction occurred along with the adsorption
processes, and the reducing agent was nitrogen atom groups on
ZIF-8 and PDA. After reduction, the generating Cr(III) species
were immobilized onto the adsorbent. It is worth noting that
the Cr adsorption mechanism is not an ordinary and single
process but includes multisteps. The Cr(VI) adsorption
involves (i) the water molecules adsorbed on ZIF-8 in aqueous
solution at pH = 5.0, generating activated sites such as Zn−OH
and the protonated N atoms groups, (ii) Cr(VI) adsorbed at
active sites, partially reduced to Cr(III) by nitrogen atom
groups on ZIF-8, and (iii) the correspondingly retarded
diffusion of Cr(VI) to PDA and transformation of Cr(VI)
into less toxic Cr(III) by the reduction of amine group, then
Cr(III) chelated onto imino groups, as well as Cr(VI)
interacted with hydroxyl groups on PDA through chemical
bonding.
■ CONCLUSIONS
In this study, well-defined core-double-shell MP@ZIF-8
microspheres were fabricated successfully and applied to
eliminate Cr(VI) from water. Superb Cr(VI) adsorption
properties and interaction mechanisms were investigated.
According to the results of the XPS analysis, Cr(VI) was partly
reduced to Cr(III) along with the adsorption process. The
results also showed that through reduction property of the
nitrogen atom group on ZIF-8 and PDA, MP@ZIF-8
microspheres could make function well in reducing and
immobilizing the toxic Cr(VI). On the basis of this
investigation, MP@ZIF-8 microspheres are proposed as high-
efficiency adsorbent materials for Cr(VI)-containing wastewater
cleanup.
■ ASSOCIATED CONTENT
*
S Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acssusche-
meng.7b01036.
Characterization, preparation of IR, XPS samples after
adsorption, adsorption kinetics, and adsorption isotherm
description, size distributions of Fe3O4, MP, and MP@
ZIF-8 particles obtained from the SEM images, SEM
image of ZIF-8, TEM image of MP@ZIF-8 (ZIF-8
growth times of 0.5, 1.5, and 3 h), elemental mapping
images of MP@ZIF-8 microspheres, linear plots of the
Langmuir isotherm model and the Freundlich isotherm
model for Cr(VI) removal on MP and MP@ZIF-8, and
Cr(VI) adsorption capacity in five recycle times (PDF)
■ AUTHOR INFORMATION
Corresponding Author
*Phone: +86-551-65592788. Fax: 86-551-65591310. E-mail:
clchen@ipp.ac.cn.
ORCID
Changlun Chen: 0000-0002-7986-8077
Xiaoli Tan: 0000-0003-4427-3396
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (21477133, 21377132, and U1607102),
the Jiangsu Provincial Key Laboratory of Radiation Medicine
and Protection, and the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions.
■ REFERENCES
(1) Li, J.; Zhang, S. W.; Chen, C. L.; Zhao, G. X.; Yang, X.; Li, J. X.;
Wang, X. K. Removal of Cu(II) and fulvic acid by graphene oxide
nanosheets decorated with Fe3O4 nanoparticles. ACS Appl. Mater.
Interfaces 2012, 4, 4991−5000.
(2) Li, J.; Chen, C. L.; Zhang, R.; Wang, X. K. Reductive
immobilization of Re(VII) by graphene modified nanoscale zero-
valent iron particles using a plasma technique. Sci. China: Chem. 2016,
59, 150−158.
(3) Li, J.; Zhang, S. W.; Chen, C. L.; Zhao, G. X.; Yang, X.; Li, J. X.;
Wang, X. K. Removal of Cu(II) and fulvic acid by graphene oxide
nanosheets decorated with Fe3O4 nanoparticles. ACS Appl. Mater.
Interfaces 2012, 4, 4991−5000.
(4) Yu, S.j.; Wang, X. X.; Yao, W.; Wang, J.; Ji, Y. F.; Ai, Y. J.; Alsaedi,
A.; Hayat, T.; Wang, X. K. Macroscopic, spectroscopic, and theoretical
investigation for the interaction of phenol and naphthol on reduced
graphene oxide. Environ. Sci. Technol. 2017, 51, 3278−3286.
(5) Klonowska, A.; Clark, M. E.; Thieman, S. B.; Giles, B. J.; Wall, J.
D.; Fields, M. W. Hexavalent chromium reduction in desulfovibrio
vulgaris hildenborough causes transitory inhibition of sulfate reduction
and cell growth. Appl. Microbiol. Biotechnol. 2008, 78, 007−1016.
(6) Zhao, D. L.; Gao, X.; Feng, S. J.; Wu, C. N.; Xie, R.; Chen, C. L.
Facile preparation of amino functionalized graphene oxide decorated
with Fe3O4 nanoparticles for the immobilization of Cr(VI). Appl. Surf.
Sci. 2016, 348, 1−9.
(7) Wen, T.; Fan, Q. H.; Tan, X. L.; Chen, Y. T.; Chen, C. L.; Xu, A.
W.; Wang, X. K. Core-shell structure of polyaniline coated protonic
titanate nanobelt composites for both Cr(VI) and humic acid removal.
Polym. Chem. 2016, 7, 785−794.
F

(8) Gao, Y.; Chen, C. L.; Tan, X. L.; Xu, H.; Zhu, K. R. Polyaniline
modified 3D-flowerlike molybdenum disulfide composite for Cr(VI)
removal. J. Colloid Interface Sci. 2016, 46, 62−70.
(9) Zhao, Y.; Zhao, D. L.; Chen, C. L.; Wang, X. K. Enhanced photo-
reduction and removal of Cr(VI) on reduced graphene oxide
decorated with TiO2 nanoparticles. J. Colloid Interface Sci. 2013, 405,
211−217.
(10) Janssen, L. J. J.; Koene, L. The role of electrochemistry and
electrochemical technology in environmental protection. Chem. Eng. J.
2002, 85, 137−146.
(11) Kurniawan, T. A.; Chan, G. Y. S.; Lo, W. H.; Babel, S. Physico-
chemical treatment techniques for wastewater laden with heavy metals.
Chem. Eng. J. 2006, 118, 83−98.
(12) Zou, Y. D.; Wang, X. X.; Khan, A.; Wang, P. Y.; Liu, Y. H.;
Alsaedi, A.; Hayat, T.; Wang, X. K. Environmental remediation and
application of nanoscale zerovalent iron and its composites for the
removal of heavy metal ions: a review. Environ. Sci. Technol. 2016, 50,
7290−7304.
(13) An, Q.; Li, Z. F.; Graff, R.; Guo, J.; Gao, H. F.; Wang, C. C.
Core-double-shell Fe3O4@carbon@poly(InIII-carboxylate) micro-
spheres: cycloaddition of CO2 and epoxides on coordination polymer
shells constituted by imidazolium-derived AlIII-Salen bifunctional
catalysts. ACS Appl. Mater. Interfaces 2015, 7, 4969−4978.
(14) Hou, J. Y.; Luan, Y.; Yu, J.; Qi, Y.; Wang, G.; Lu, Y. F.
Fabrication of hierarchical composite microspheres of copper-doped
Fe3O4@P4VP@ZIF-8 and their application in aerobic oxidation. New
J. Chem. 2016, 40, 10127−10135.
(15) Zhu, J. H.; Wei, S. Y.; Gu, H. B.; Rapole, S. B.; Wang, Q.; Luo,
Z. P.; Haldolaarachchige, N.; Young, D. P.; Guo, Z. H. One-pot
synthesis of magnetic graphene nanocomposites decorated with core@
double-shell nanoparticles for fast chromium removal. Environ. Sci.
Technol. 2012, 46, 977−985.
(16) Dehghan, M.; Motaharinejad, A.; Saadat, M.; Ahdenov, R.;
Babazadeh, M.; Khanmiri, R. H. Novel approach to synthesizing
polymer-functionalized Fe3O4/SiO2-NH2 via an ultrasound-assisted
method for catalytic selective oxidation of alcohols to aldehydes and
ketones in a DMSO/water mixture. RSC Adv. 2015, 5, 92335−92343.
(17) Chaudhuri, R. G.; Paria, S. Core/shell nanoparticles: classes,
properties, synthesis mechanisms, characterization, and applications.
Chem. Rev. 2012, 112, 2373−2433.
(18) Zhou, J. J.; Wang, P.; Wang, C. X.; Goh, Y. T.; Fang, Z.;
Messersmith, P. B.; Duan, H. W. Versatile core-shell nanoparticle@
metal-organic framework nanohybrids: exploiting mussel-inspired
polydopamine for tailored structural integration. ACS Nano 2015, 9,
6951−6960.
(19) Li, J.; Fan, Q. H.; Wu, Y. J.; Wang, X. X.; Chen, C. L.; Tang, Z.
Y.; Wang, X. K. Magnetic polydopamine decorated with Mg-Al LDH
nanoflakes as a novel bio-based adsorbent for simultaneous removal of
potentially toxic metals and anionic dyes. J. Mater. Chem. A 2016, 4,
1737−1746.
(20) Xie, Y. J.; Yan, B.; Xu, H. L.; Chen, J.; Liu, Q. X.; Deng, Y. H.;
Zeng, H. B. Highly regenerable mussel-inspired Fe3O4@polydop-
amine-Ag core-shell microspheres as catalyst and adsorbent for
methylene blue removal. ACS Appl. Mater. Interfaces 2014, 6, 8845−
8852.
(21) Liu, R.; Guo, Y. L.; Odusote, G.; Qu, F. L.; Priestley, R. D. Core-
shell Fe3O4 polydopamine nanoparticles serve multipurpose as drug
carrier, catalyst support and carbon adsorbent. ACS Appl. Mater.
Interfaces 2013, 5, 9167−9171.
(22) Zhang, Y. J.; Xie, Z. Q.; Wang, Z. Q.; Feng, X. H.; Wang, Y.;
Wu, A. G. Unveiling the adsorption mechanism of zeolitic imidazolate
framework-8 with high efficiency for removal of copper ions from
aqueous solutions. Dalton Trans. 2016, 45, 12653−12660.
(23) Feng, Y.; Li, Y.; Xu, M. Y.; Liu, S. C.; Yao, J. F. Fast adsorption
of methyl blue on zeolitic imidazolate framework-8 and its adsorption
mechanism. RSC Adv. 2016, 6, 109608−109612.
(24) Wu, Y. N.; Zhou, M. M.; Zhang, B. R.; Wu, B. Z.; Li, J.; Qiao, J.
L.; Guan, X. H.; Li, F. T. Amino acid assisted templating synthesis of
hierarchical zeolitic imidazolate framework-8 for efficient arsenate
removal. Nanoscale 2014, 6, 1105−1112.
(25) Ding, Y.; Xu, Y. F.; Ding, B.; Li, Z.; Xie, F. Z.; Zhang, F. J.;
Wang, H. T.; Liu, J.; Wang, X. B. Structure induced selective
adsorption performance of ZIF-8 nanocrystals in water. Colloids Surf.,
A 2017, 520, 661−667.
(26) Jian, M. P.; Wang, H.; Liu, R. P.; Qu, J. H.; Wang, H. T.; Zhang,
X. W. Self-assembled one-dimensional MnO2@zeolitic imidazolate
framework-8 nanostructures for highly efficient arsenite removal.
Environ. Sci.: Nano 2016, 3, 1186−1194.
(27) Zhao, M.; Deng, C. H.; Zhang, X. M. The design and synthesis
of a hydrophilic core-shell-shell structured magnetic metal-organic
framework as a novel immobilized metal ion affinity platform for
phosphoproteome research. Chem. Commun. 2014, 50, 6228−6231.
(28) Wang, Y. Q.; Zou, B. F.; Gao, T.; Wu, X. P.; Lou, S. Y.; Zhou, S.
M. Synthesis of orange-like Fe3O4/PPy composite microspheres and
their excellent Cr(VI) ion removal properties. J. Mater. Chem. 2012,
22, 9034−9040.
(29) Wang, F. F.; Huang, Y. J.; Chai, Z. G.; Zeng, M.; Li, Q.; Wang,
Y.; Xu, D. S. Photothermal-enhanced catalysis in core-shell plasmonic
hierarchical Cu7S4 microsphere@zeolitic imidazole framework-8.
Chem. Sci. 2016, 7, 6887−6893.
(30) Zhang, Y. C.; Li, J.; Zhang, M.; D. Dionysiou, D. Size-tunable
hydrothermal synthesis of SnS2 nanocrystals with high performance in
visible light-driven photocatalytic reduction of aqueous Cr(VI).
Environ. Sci. Technol. 2011, 45, 9324−9331.
(31) Kuo, C. H.; Tang, Y.; Chou, L. Y.; Sneed, B. T.; Brodsky, C. N.;
Zhao, Z. P.; Tsung, C. K. Yolk-shell nanocrystal@ZIF-8 nanostruc-
tures for gas-phase heterogeneous catalysis with selectivity control. J.
Am. Chem. Soc. 2012, 134, 14345−14348.
(32) Zeng, T.; Zhang, X. L.; Niu, H. Y.; Ma, Y. R.; Li, W. H.; Cai, Y.
Q. In situ growth of gold nanoparticles onto polydopamine-
encapsulated magnetic microspheres for catalytic reduction of
nitrobenzene. Appl. Catal., B 2013, 134−135, 26−33.
(33) Zou, Y. D.; Wang, X. X.; Ai, Y. J.; Liu, Y. H.; Li, J. X.; Ji, Y. F.;
Wang, X. K. Coagulation behavior of graphene oxide on nano-
crystallined Mg/Al layered double hydroxides: batch experimental and
theoretical calculation study. Environ. Sci. Technol. 2016, 50, 3658−
3667.
(34) Zhao, G. X.; Li, J. X.; Ren, X. M.; Chen, C. L.; Wang, X. K. Few-
layered graphene oxide nanosheets as superior sorbents for heavy
metal ion pollution management. Environ. Sci. Technol. 2011, 45,
10454−10462.
(35) Zhao, Z. W.; Li, J. X.; Wen, T.; Shen, C. C.; Wang, X. K.; Xu, A.
W. Surface functionalization graphene oxide by polydopamine for high
affinity of radionuclides. Colloids Surf., A 2015, 482, 258−266.
(36) Stammreich, H.; Bassi, D.; Sala, O.; Siebert, H. The vibrational
spectrum of the dichromate ion. Spectrochim. Acta 1958, 13, 192−196.
(37) Wang, J.; Zhao, L.; Duan, W. J.; Han, L. L.; Chen, Y. F.
Adsorption of aqueous Cr(VI) by novel fibrous adsorbent with amino
and quaternary ammonium groups. Ind. Eng. Chem. Res. 2012, 51,
3655−13662.
(38) Wang, T.; Zhang, L. Y.; Li, C. F.; Yang, W. C.; Song, T. T.;
Tang, C. J.; Meng, Y.; Dai, S.; Wang, H. Y.; Chai, L. Y.; Luo, J.
Synthesis of core-shell magnetic Fe3O4@poly(m-Phenylenediamine)
particles for chromium reduction and adsorption. Environ. Sci. Technol.
2015, 49, 5654−5662.
(39) Xiong, S. Q.; Wang, Y.; Yu, J. R.; Chen, L.; Zhu, J.; Hu, Z. M.
Polydopamine particles for next-generation multifunctional biocom-
posites. J. Mater. Chem. A 2014, 2, 7578−7587.
(40) Wang, X. B.; Liu, J.; Leong, S. W.; Lin, X. C.; Wei, J.; Kong, B.;
Xu, Y. F.; Low, Z. X.; Yao, J. F.; Wang, H. T. Rapid construction of
ZnO@ZIF-8 heterostructures with size-selective photocatalysis prop-
erties. ACS Appl. Mater. Interfaces 2016, 8, 9080−9087.
(41) Wang, R. X.; Wen, T.; Wu, X. L.; Xu, A. W. Highly efficient
removal of humic acid from aqueous solutions by Mg/Al layered
double hydroxides-Fe3O4 nanocomposites. RSC Adv. 2014, 4, 21802−
21809.
G

(42) Liu, M. C.; Wen, T.; Wu, X. L.; Chen, C. L.; Hu, J.; Li, J.; Wang,
X. K. Synthesis of porous Fe3O4 hollow microspheres/graphene oxide
composite for Cr(VI) removal. Dalton Trans. 2013, 42, 14710−14717.
(43) Shahrak, M. N.; Ghahramaninezhad, M.; Eydifarash, M. Zeolitic
imidazolate framework-8 for efficient adsorption and removal of Cr
(VI) ions from aqueous solution. Environ. Sci. Pollut. Res. 2017, 24,
9624−9634.
(44) Guo, X.; Fei, G. T.; Su, H.; Zhang, L. D. High-performance and
reproducible polyaniline nanowire/tubes for removal of Cr(VI) in
aqueous solution. J. Phys. Chem. C 2011, 115, 1608−1613.
(45) Li, Y. J.; Gao, B. Y.; Wu, T.; Sun, D. J.; Li, X.; Wang, B.; Lu, F. J.
Hexavalent chromium removal from aqueous solution by adsorption
on aluminum magnesium mixed hydroxide. Water Res. 2009, 43,
3067−3075.
(46) Zhu, K. R.; Lu, S. H.; Gao, Y.; Zhang, R.; Tan, X. L.; Chen, C. L.
Fabrication of hierarchical core-shell polydopamine@MgAl-LDHs
composites for the efficient enrichment of radionuclides. Appl. Surf.
Sci. 2017, 396, 1726−1735.
(47) Bhaumik, M.; Maity, A.; Srinivasu, V. V.; Onyango, M. S.
Removal of hexavalent chromium from aqueous solution using
polypyrrole-polyaniline nanofibers. Chem. Eng. J. 2012, 181−182,
323−333.
(48) Nightingale, E. R., Jr Phenomenological theory of ion salvation.
Effective radii of hydrated ions. J. Phys. Chem. 1959, 63, 1381−1387.
(49) Shamsaei, E.; Low, Z. X.; Lin, X. C.; Mayahi, A.; Liu, H. Y.;
Zhang, X. W.; Wang, H. T. Rapid synthesis of ultrathin, defect-free
ZIF-8 membranes via chemical vapour modification of a polymeric
support. Chem. Commun. 2015, 51, 11474−11477.
(50) Yao, J. F.; Wang, H. T. Zeolitic imidazolate framework
composite membranes and thin films: synthesis and applications.
Chem. Soc. Rev. 2014, 43, 4470−4493.
(51) Xie, Z.; Yang, J. H.; Wang, J. Q.; Bai, J.; Yin, H. M.; Yuan, B.; Lu,
J. M.; Zhang; Zhou, Y. L.; Duan, C. Y. Deposition of chemically
modified α-Al2O3 particles for high performance ZIF-8 membrane on a
macroporous tube. Chem. Commun. 2012, 48, 5977−5979.
(52) Li, J.; Wu, Y. N.; Li, Z. H.; Zhang, B. R.; Zhu, M.; Hu, X.;
Zhang, Y. M.; Li, F. T. Zeolitic imidazolate framework-8 with high
efficiency in trace arsenate adsorption and removal from water. J. Phys.
Chem. C 2014, 118, 27382−27387.
H
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