Facile assembly of novel g-C3N4?expanded graphite and surface loading of nano zero-valent iron for enhanced synergistic degradation of tetracycline

Article abstract of DOI:10.1039/c9ra06620a

The two-stage removal process of tetracycline (TC) in aqueous solutions using a novel photocatalyst based on nano-zero-valent iron (NZVI), g-C₃N₄ and expanded graphite by carbon layer (EGC) is reported for the first time. The composite (NZVI/g-C₃N₄?EGC) exhibits remarkable adsorption, reduction ability and visible light activity over the reaction course. Compared with pristine g-C₃N₄ (25.9%) and pure NZVI (45.9%), NZVI/g-C₃N₄?EGC achieves high degradation efficiency of TC (98.5%) due to the formation of a heterogeneous photo-Fenton system. This study shows that synergistic effects are achieved in the reaction system, including maintaining the reduction ability of NZVI and enhancing the photocatalytic activity of g-C₃N₄ by facilitating the separation of photogenerated electrons (e^-) and holes (h⁺). TC removal involved a two-stage process of adsorption-reduction and photo-degradation. The quencher experiments determined that holes (h⁺) and superoxide radicals (O₂^-) are the major reactive species in the degradation of TC. The degradation pathways of TC were proposed based on the analysis of the intermediates. In addition, NZVI/g-C₃N₄?EGC revealed a high stability in a five-cycle test and good magnetic properties for facile separation from aqueous solutions. From an application viewpoint, NZVI/g-C₃N₄?EGC has favorable prospects in the direction of the photocatalytic degradation of antibiotic wastewater.

Full text of DOI:10.1039/c9ra06620a

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PAPER
Facile assembly of novel g-C N @expanded
3
4
graphite and surface loading of nano zero-valent
iron for enhanced synergistic degradation of
tetracycline†
Cite this: RSC Adv., 2019, 9, 34658
Xiangyu Wang, * Yu Xie, Jun Ma and Ping Ning *a
a
a
b
The two-stage removal process of tetracycline (TC) in aqueous solutions using a novel photocatalyst based
on nano-zero-valent iron (NZVI), g-C
first time. The composite (NZVI/g-C
light activity over the reaction course. Compared with pristine g-C
g-C @EGC achieves high degradation efficiency of TC (98.5%) due to the formation of a heterogeneous
photo-Fenton system. This study shows that synergistic effects are achieved in the reaction system,
including maintaining the reduction ability of NZVI and enhancing the photocatalytic activity of g-C
3
N
4
and expanded graphite by carbon layer (EGC) is reported for the
@EGC) exhibits remarkable adsorption, reduction ability and visible
(25.9%) and pure NZVI (45.9%), NZVI/
3
N
4
3 4
N
3 4
N
3 4
N
ꢀ
+
by facilitating the separation of photogenerated electrons (e ) and holes (h ). TC removal involved
a two-stage process of adsorption–reduction and photo-degradation. The quencher experiments
+
2ꢀ
determined that holes (h ) and superoxide radicals (cO ) are the major reactive species in the
degradation of TC. The degradation pathways of TC were proposed based on the analysis of the
Received 22nd August 2019
Accepted 9th October 2019
intermediates. In addition, NZVI/g-C
magnetic properties for facile separation from aqueous solutions. From an application viewpoint, NZVI/
g-C @EGC has favorable prospects in the direction of the photocatalytic degradation of antibiotic
wastewater.
3 4
N @EGC revealed a high stability in a five-cycle test and good
DOI: 10.1039/c9ra06620a
3 4
N
rsc.li/rsc-advances
2
compound. Therefore, the TC in the wastewater must be
removed before it can be discharged into water. Generally,
Introduction
Antibiotics are widely used in modern clinical medicine as methods for degrading TC include adsorption, biodegradation,
important drugs for the prevention and treatment of infectious advanced oxidation processes, electrochemical degradation and
3
diseases because of their inhibitory or killing effects on patho- photochemical degradation.
genic microorganisms. However, the abuse of antibiotics results
At present, the technology of nanoscale zero-valent iron in
in numerous adverse reactions, endangering human health. situ wastewater treatment has received widespread attention
Discharge into the environment causes the death of the because of the strong reduction ability and environmental
surrounding bacteria and disrupts the ecological balance. friendliness of iron ions. At the same time, heterogeneous
Tetracycline (TC) is a common antibiotic that is used mainly for Fenton technology involving iron-based materials as catalysts
4
–8
the treatment of respiratory, intestinal, and urinary tract has been proven to have potential application prospects.
infections. However, excessive use of TC alters the composition However, the defects of NZVI particles are severe aggregation in
of the human gastrointestinal ora, causing a range of gastro- aqueous solutions and the formation of a passivation layer on
1
intestinal and non-gastrointestinal diseases. Chee-Sanford their surfaces, which lead to decreased activity. Therefore, TC
et al. conrmed that aer tetracycline antibiotics enter the cannot be completely mineralized by pure NZVI particles. In
body, they are not readily absorbed by the stomach, and about order to avoid the generation of refractory intermediates, pho-
7
5% of the dose is discharged into sewage as a parent tocatalysis has been combined to achieve complete degradation
of TC. Photocatalytic degradation of pollutants is a highly
9
acclaimed method that converts light energy from nature into
a
Faculty of Environmental Science and Engineering, Kunming University of Science and
Technology, Kunming, 650500, China. E-mail: ningping58_2015@sina.com;
the energy required for chemical reactions to produce a catalytic
effect; the surrounding oxygen and water molecules are excited
State Key Laboratory of Urban Water Resources and Environment, Harbin Institute of to highly reactive species, and almost all environmental harm-
imusthlee2014@sina.com
b
Technology, Harbin, 150090, China
ful organic matter and some inorganic matter can be decom-
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra06620a
posed. g-C
3
N
4
is widely used as a photocatalyst due to its low
1
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band gap value (2.7 eV), which leads to highly efficient purchased from Aladdin Reagent Database Inc. (China). All the
10,11
absorption of visible light.
However, g-C N has the defects chemicals were of analytical grade and were used as received
3 4
of high recombination rate of photogenerated electron–hole without further purication.
pairs and low quantum efficiency, which greatly decrease its
12–14
utilization of visible light.
Also, the inconvenient recycling
3 4
Synthesis of NZVI/g-C N @EGC
and secondary pollution of powder photocatalysts in water
treatment applications is an ongoing problem that needs to be
solved.
The expanded graphite was obtained by the calcination of
expandable graphite in a muffle furnace at 900 C for 15 s. The
ꢁ
g-C N was prepared by calcining melamine in a tube furnace at
3
4
Theoretically, in order to overcome the abovementioned
defects, it is feasible to combine g-C N with graphene.
ꢁ ꢁ
50 C for 2 h at a heating rate of 5 C min . 50 mg of g-C N4
3
ꢀ1
5
3
4
powder was placed in 100 mL of water–methanol solution and
sonicated for 30 min to attain complete dispersion. 100 mg of
expanded graphite and 2 g of glucose were introduced into the
above suspension; then, the mixture was magnetically stirred
However, prior to the employment of graphene oxide, complex
pretreatment processes are required to transform it into
reduced graphene oxide. More importantly, if the pretreatment
process is not optimized, the graphene oxide may not be
15,16
for 24 h. At the end of this time, the formed mixture was dried at
completely transformed into reduced graphene oxide.
ꢁ
80
C overnight. Finally, the dried sample was calcined in
Therefore, in this study, an easily available and low-cost carbon
material, expanded graphite, was adopted. Expanded graphite
ꢁ
ꢀ1
a muffle furnace at a heating rate of 5 C min and maintained
ꢁ
at 550 C for 2 h, which formed a carbon layer to enhance the
(
EG) has excellent electrical conductivity, a high specic surface
17,18
mechanical strength of the EG. Aer cooling to room temper-
ature, it was used for further synthesis.
area, high porosity and outstanding adsorption capacity.
Due to these features, it is an ideal support material. However,
the mechanical strength of EG is weak, and it readily collapses
when subjected to external forces. According to Wang et al.,
coating a carbon layer on the EG surface is a good way to
increase its mechanical strength. Meanwhile, doping with
Ternary composites were prepared by adding g-C
to a FeSO $7H O solution (dissolving various amounts of
FeSO $7H O aqueous solution in a 1 : 3 volume ratio (etha-
3 4
N @EGC
4
2
4
2
nol : water) in ethanol solution) followed by slowly titrating the
ꢀ
19,20
KBH
4
solution with continuous stirring (excess BH
4
ensures
3 4
carbon enhances the photocatalytic activity of g-C N .
2
+
that the adsorbed Fe
on the g-C N @EGC surface is
3
4
Therefore, in order to maintain the stability of the EG structure,
expanded graphite by carbon layer (EGC) was developed. In
addition, the support of EGC effectively enhances the disper-
sion of NZVI particles, and the 1.2 V electrode potential differ-
ence between EGC and NZVI provides a high electrodynamic
completely reduced). The KBH4 reduction method proceeds
according to the following reaction:
2
+
Fe + 2BH + 6H
4ꢀ
0
2
O / Fe + 2B(OH)
3
+ 7H
2
[
(1)
21
driving force to remove TC.
The main objective of this study is to fabricate a novel
3 4
magnetic NZVI/g-C @EGC composite based on NZVI, g-C N
Aer the titration was complete, the mixture was stirred for
30 min at room temperature. Subsequently, the prepared
3
N
4
ternary composite material was separated from the liquid
solution by ltration. Finally, NZVI/g-C @EGC was rinsed
and EGC for the treatment of antibiotic wastewater; also, the
degradation mechanism of TC by NZVI/g-C N @EGC under
3 4
N
3
4
three times with deionized water, absolute ethanol and acetone,
alternately. The prepared sample was dried and stored under
vacuum conditions. All the above experimental steps were per-
formed under a nitrogen atmosphere. To attain the optimal
catalytic performance, the relevant products prepared with
different mass ratios (FeSO $7H O : g-C N @EGC ¼ 0.33, 0.50,
dark and visible light conditions was analyzed. The other
objectives are: (1) to characterize the newly prepared catalysts
using various techniques to study their surface morphologies,
elemental compositions, specic surface areas, crystal struc-
tures and optical properties; (2) to study the effects of different
reaction conditions on the degradation efficiency of tetracycline
by NZVI/g-C N @EGC; (3) to test the recyclability and stability of
4
2
3 4
1
.0, and 2.0, respectively) were abbreviated as Fe/CN@E-1, Fe/
CN@E-2, Fe/CN@E-3 and Fe/CN@E-4, respectively. Among
these, the mass ratios of doped NZVI and g-C @EGC were
calculated by eqn (1) to be 0.07, 0.1, 0.2 and 0.4, respectively.
The NZVI-deposited g-C composite was prepared by the
3
4
NZVI/g-C N @EGC; and (4) to analyze the intermediates in the
3
4
3 4
N
degradation process of tetracycline. The results show that NZVI/
g-C @EGC exhibits excellent adsorption, reduction and
3 4
N
3 4
N
photo-degradation abilities. The formation of synergistic effects
during the two-stage removal process led to high tetracycline
degradation efficiency.
above method and abbreviated as Fe–CN.
Characterization
The surface morphology of NZVI/g-C
eld emission scanning electron microscopy (FESEM, Tescan
MIRA3 Zeiss Merlin Compact) at an acceleration voltage of 20
3 4
N @EGC was observed by
Materials and methods
Materials

Expandable graphite (99%, grain size 50 mesh) was purchased kV and transmission electron microscopy (TEM, Tecnai G2 F20
from Qingdao Herita Graphite Products Co., Ltd., (China). JEOL-2100F). The elemental compositions of the ternary
Tetracycline (C H N O ), ferrous sulfate heptahydrate composites were examined by energy dispersive spectrometer
2
2
24 2 8
(
(
FeSO
4
$7H
2
O), potassium borohydride (KBH
4
), glucose (EDS, Oxford X-MAX) attached to the FESEM. The specic
@EGC
C
6
H
12
O
6
), melamine (C
3
N
3
(NH ) and other chemicals were surface area and pore size distribution of NZVI/g-C N
2
)
3
3 4
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were measured by BET N
2
adsorption analysis using a surface agglomeration of NZVI. Fig. 1f shows the EDS image of NZVI/g-
area analyzer (ASAP 2020 HD88Belsorp-max Microtrac). To C N @EGC, which displays the signals of elements C, N, Fe, and O
3
4
study the crystal structures of the composites, the XRD patterns in the composite, where the signal of O is observed due to the
were recorded using an X-ray diffractometer (XRD, D8 Advance presence of adsorbed water on the surface of the sample or
Davinci design) with Ni-ltered Cu/Ka radiation (l ¼ 0.15418 oxidation of a small amount of the NZVI surface; this further
nm) operating at generator voltage; the generator currents were proves the successful synthesis of NZVI/g-C
3
N
4
@EGC.
Fig. 2 shows the nitrogen adsorption–desorption isotherms of
recorded in the range from 10 to 80 . The surface chemical pristine g-C , g-C @EGC and NZVI/g-C @EGC. All the
structure and composition of NZVI/g-C @EGC were deter- samples show characteristic type IV isotherms with H3 hysteresis
mined by Fourier transform infrared spectroscopy (FT-IR, loops in accordance with the IUPAC classication, demonstrating
4
0 kV and 40 mA, respectively. The diffraction patterns were
ꢁ
ꢁ
N
3 4
N
3 4
3 4
N
3 4
N
22
Nicolet 6700 Bruker V70). X-ray photoelectron spectroscopy that a typical mesoporous structure exists in the composites.
XPS, Thermo Scientic Escalab 250Xi) measurements were The pore size distribution of the composite was obtained using
recorded to investigate the surface chemical states. The life- the BJH method. The specic surface areas and pore parameters
times of the photogenerated carriers were investigated by pho- of the samples are shown in Table 1. Because NZVI and g-C
toluminescence (PL) spectroscopy using an Edinburgh FS980 were successfully loaded onto the surface of EGC, the specic
uorescence spectrophotometer with an excitation wavelength surface area and total pore volume of NZVI/g-C @EGC are
of 325 nm. Also, the UV-vis diffuse reectance spectra of the higher than those of NZVI and g-C . In addition, the high
samples were obtained on a UV-vis spectrophotometer (Perki- specic surface area is benecial to provide more active sites,
(
3 4
N

3 4
N
3 4
N
23
nElmer Lambda 950). To study the magnetization of NZVI/g- which is benecial for the photocatalytic activity.
C N @EGC, a Quantum Design PPMS Dynacool measurement As shown in Fig. 3, the XRD patterns of g-C N and NZVI/g-
3
4
3 4
system was used to measure the magnetic moment per mass in
a magnetic eld between ꢂ2TG.
3 4
C N @EGC clearly show crystalline phase and crystal structure
ꢁ
characteristics. The diffraction peaks at the 2q values of 12.7 and
ꢁ
2
7.5 are characteristic of pristine g-C
N
3 4
and are attributed to the
Photocatalytic degradation of TC
(100) and (002) planes of the graphitic material, respectively. The
100) crystal plane is attributed to the periodic array associated
with tri-s-triazine, and the (002) crystal plane is attributed to
(
3 4
The photocatalytic performance of NZVI/g-C N @EGC was
evaluated by the degradation experiments of tetracycline under
a 300 W Xe lamp equipped with a 400 nm cutoff lter. The
initial concentration of TC was set to 30 mg L . In this pho-
tocatalytic degradation experiment, under magnetic stirring,
24
interlayer stacking of the graphite-like material structures. The
ꢁ ꢁ ꢁ
N @EGC sample shows peaks at 26.2 , 44.2 , and 54.6 ,
3 4
ꢀ
1
NZVI/g-C
which can be attributed to EGC, and the characteristic peak at
ꢁ
0
4
4.9 is a feature of Fe . Aer the modication of NZVI, the char-
3
0 mg of NZVI/g-C N @EGC was dispersed in 60 mL of the
3 4
acteristic peak of the EGC is still sharp, which conrms that the
target aqueous solution for 30 min to establish the adsorption–
desorption equilibrium in the dark. Aer that, the visible light
source was turned on to initiate the photocatalytic reaction. At
regular intervals, a 3 mL aliquot was collected and centrifuged.
The concentration of TC in the solution was analyzed by a UV
spectrophotometer (UV-2100, Rayleigh Analytical Instrument
Co. Ltd. Beijing, China) by monitoring the absorption
maximum band at 360 nm. The degradation efficiency of TC
was calculated using the equation:
25
NZVI loading does not destroy the crystal structure of the EGC.
Furthermore, the XRD patterns show that NZVI loading did not
lead to the crystal destruction of g-C N , which is consistent with
3
4
previous ndings. This indicates the successful synthesis of the
composite.
In order to analyze the chemical states and composition of
3 4
NZVI/g-C N @EGC, the X-ray photoelectron spectroscopy (XPS)
image is shown in Fig. 4a; comparison with the survey spectrum of
EG demonstrates the coexistence of Fe, C, N, and O elements in
freshly prepared NZVI/g-C
successful synthesis of the composite. As shown in Fig. 4b, the
3 4
N @EGC, which demonstrates the
DE (%) ¼ [(C
0
ꢀ C
t
)/C
0
] ꢃ 100%
(2)
where C is the initial concentration and C is the concentration main peaks of C1s appear at 284.4 eV and 288.1 eV, corresponding
0
t
26,27
during the photocatalytic process.
to C–C and N–C]N, respectively.
In Fig. 4c, the peaks of N1s
2
appear at 398.3 eV and 399.3 eV; these can be ascribed to the sp
hybridized aromatic N atoms bonded to carbon atoms (C]N–C),
Results and discussion
Characterization of photocatalysts
28
3 2
the tertiary nitrogen (N–C ) or H–N–C . In Fig. 4d, the peaks of
Fe2p appearing at 706.5 eV (Fe2p3/2) and 724.4 eV (Fe2p1/2) indi-
As shown in Fig. 1a, the SEM image of EG demonstrates an cate the presence of NZVI, and the peak at 711.2 eV (Fe2p3/2
)
irregular surface morphology and a large number of wrinkles on indicates the presence of ferric oxides; this is due to the oxidation
19
its surface, which is consistent with previously reported research.
As shown in Fig. 1b and c, the SEM images of NZVI/g-C
@EGC good agreement with the XRD analysis of NZVI/g-C
show that g-C nanosheets and NZVI particles are supported on
of a small amount of NZVI during storage. The above results are in
3 4
N
N @EGC.
3 4
3 4
N
the surface of expanded graphite. The TEM images of Fe/CN@E-3
are shown in Fig. 1d and e, with black spherical particles uniformly
Optical properties of the photocatalysts
loaded onto the g-C
surface; this proves that expanded graphite can hinder the
3
N
4
nanosheets and expanded graphite The optical properties and photocatalytic activities of pristine g-
, g-C @EGC and NZVI/g-C @EGC were investigated
C
3
N
4
3
N
4
3 4
N
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3 4 3 4 3 4
Fig. 1 SEM images of (a) EG, (b and c) NZVI/g-C N @EGC; TEM images of (d and e) NZVI/g-C N @EGC; (f) EDS spectrum of NZVI/g-C N @EGC.
visible wavelength range, and it is found that the absorption
intensity of g-C @EGC in the visible light region (l > 420 nm)
3 4
N
is signicantly improved. This may be due to the support of the
EGC; the multiscale pore distribution of the EGC may facilitate
the transfer of photo-induced electrons. However, aer the
support of EGC, g-C
3 4
N @EGC showed a decrease in the
absorption intensity in the range of wavelengths shorter than
29
420 nm, which is consistent with previously reported results.
Simultaneously, the absorption edge of NZVI/g-C @EGC
3 4
N
displays a dramatic red shi, and the absorption intensity of
NZVI/g-C N @EGC in the range of 250 to 800 nm was also found
3
4
to be signicantly enhanced compared with that of the pristine
g-C and g-C @EGC; this is probably because the existence
3
N
4
3 4
N
Fig. 2
and NZVI/g-C
N
2
adsorption–desorption isotherms of g-C
@EGC.
3 4 3 4
N , g-C N @EGC
3
N
4
by UV-vis diffuse reectance spectroscopy. The results are
shown in Fig. 5a, where the pristine g-C has an adsorption
edge at around 450 nm; compared with pristine g-C , g-
C N @EGC presents a red shi of the absorption edge in the
3 4
N
3 4
N
3
4
Table 1 The specific surface areas, pore volumes and pore sizes of the
various samples
2
ꢀ1
3
ꢀ1
Samples
Surface area (m g
)
Pore volume (cm g
)
g-C
g-C
NZVI/g-C
3
N
N
4
4
38.37
42.92
48.41
0.13
0.12
0.17
3
@EGC
@EGC
3
N
4
Fig. 3 XRD patterns of NZVI, EG, g-C
@EGC.
3 4 3 4
N , g-C N @EGC and NZVI/g-
3 4
C N
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3 4 3 4
Fig. 4 (a) XPS survey spectra of EG and NZVI/g-C N @EGC; (b) C 1s, (c) N 1s, and (d) Fe 2p XPS spectra of NZVI/g-C N @EGC.
of the iron energy level leads to a decrease in the energy gap of where a represents the absorption coefficient, h represents the
30
the electronic transition.
Planck constant, n represents the vibration frequency, k repre-
The energy band gaps of pristine g-C N , g-C N @EGC and sents the proportional constant and E represents the energy
3
4
3
4
g
3 4
NZVI/g-C N @EGC were calculated according to the Tauc band gap.
31
1/2
equation:
By plotting (ahn) and the corresponding photon energy
hn), the energy band gap was evaluated by extrapolating the
(
1
/2
(
ahn) ¼ k(hn ꢀ E
g
)
(3)
3 4 3 4 3 4 3 4 3 4 3 4
Fig. 5 (a) UV-vis-DRS of g-C N , g-C N @EGC and NZVI/g-C N @EGC; (b) Tauc plots of g-C N , g-C N @EGC and NZVI/g-C N @EGC.
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33,34
value to the X axis. As shown in Fig. 5b, the estimated band gap mechanism of NZVI is reduction and adsorption.
The low
energies of pristine g-C N , g-C N @EGC and NZVI/g- removal efficiency of TC is due to the fact that the defects of
3
4
3 4
C N @EGC were determined to be ꢄ2.7, ꢄ2.3 and ꢄ1.6 eV, facile agglomeration and oxidation of NZVI particles lead to
3
4
respectively. The UV-vis DRS results showed that all samples a decrease in the number of active sites. For Fe–CN, it degraded
have visible light absorption capacity, while NZVI/g-C @EGC 63.9% of TC aer 120 min, which is higher than the amounts
shows an enhanced ability to absorb and utilize visible light degraded by pure NZVI, pristine g-C and g-C @EGC; this
better than pristine g-C and g-C @EGC. is probably because the loading of NZVI provides reducing
Photoluminescence (PL) emission spectra are widely used to power to the composite and promotes the generation of synergy.
study the transfer behavior of charge carriers in prepared Meanwhile, compared with pristine g-C , NZVI, g-C @EGC
3 4
N
3
N
4
3 4
N
3
N
4
3 4
N
3
N
4
3 4
N
0
samples. Generally, a stronger PL signal represents a shorter and Fe–CN, Fe -doped ternary composites exhibit higher
lifetime of the photogenerated carriers excited under radia- degradation efficiency to TC and improved the removal effi-
32
tion. Fig. 6 presents the PL spectra of pristine g-C N , g- ciency of composites in TC under dark conditions. Aer 90 min
3
4
C
3
N
4
@EGC and NZVI/g-C
3
N
4
@EGC under an excitation wave- of visible light irradiation, Fe/CN@E-1, Fe/CN@E-2, Fe/CN@E-3
has and Fe/CN@E-4 degraded 68.8%, 85.9%, 98.5% and 93.7% of
length of 325 nm. It can be seen that pristine g-C
3 4
N
a higher PL emission peak at 455 nm, demonstrating the high the TC, respectively. Among the samples, Fe/CN@E-3 exhibits
recombination rate of photogenerated electron–hole pairs. the highest degradation efficiency for TC. The results show that
Conversely, g-C N @EGC showed a weaker PL peak, further in the photocatalytic reaction system, there are signicant
3 4
demonstrating that the presence of EGC can promote the synergistic effects of the three components, including adsorp-
transport of photogenerated electrons to achieve a highly effi- tion, reduction ability and photocatalytic activity. In addition,
cient charge separation process. In addition, the PL intensity of the photocatalytic activity of NZVI/g-C N @EGC was compared
3
4
35–42
NZVI/g-C
of g-C
tively promote the electron–hole migration ability. Due to the listed in Table 2. It can be seen from the comparison that NZVI/
introduction of graphene and nano-iron, the charge is g-C @EGC requires a lower addition dosage and less visible
frequently transferred between g-C , EGC and NZVI, which light irradiation time to remove higher concentrations of TC.
3
N
4
@EGC is obviously decreased compared with that with those of previously reported photocatalysts.
The
3 4
N @EGC, indicating that the doping of NZVI can effec- degradation efficiencies of tetracycline in different samples are
3 4
N
3 4
N
effectively promotes the photocatalytic activity.
The pseudo-rst-order kinetics model was applied to analyze
the photo-degradation of tetracycline by the as-prepared
samples under visible light illumination. As plotted in Fig. 7b,
Photocatalytic activity
2
the values of the correlation coefficients R were above 0.9,
Fig. 7a shows the degradation efficiency of TC in samples illustrating that the removal process of TC could be well
prepared under visible light irradiation; the results indicate that expressed with pseudo-rst-order kinetics. By observing the
TC was fairly stable in the absence of a catalyst, suggesting that linear correlation between ln(C /C) and the irradiation time, it
0
its self-degradation effects were negligible. The pristine g-C
and g-C N @EGC could degrade TC by 25.9% and 41.5% within tion rate. Especially, the reaction rate constant of Fe/CN@E-3 (k
o
3
N
4
3 4
can be seen that NZVI/g-C N @EGC exhibited the fastest reac-
3
4
ꢀ
1
9
0 min of visible light irradiation, possibly because the support ¼ 0.0269 min ) was approximately 15.8-fold, 12.8-fold, 8.7-fold
ꢀ1
of EGC provides considerable adsorption capacity and improves and 4.4-fold those of pristine g-C N (k ¼ 0.0017 min ), pure
3
4
o
ꢀ
1
ꢀ1
the absorption of visible light; as expected, g-C
3
N
4
@EGC has NZVI (k ¼ 0.0021 min ), g-C N @EGC (k ¼ 0.0031 min
)
o
3
4
o
ꢀ
1
a lower band gap value. For pure NZVI, its removal efficiency of and Fe–CN (k ¼ 0.0061 min ), respectively. This indicates that
o
TC is 45.9% within 120 min. However, aer 60 min, it shows the reduction ability provided by NZVI plays an important role.
a slower removal efficiency; this is because the main removal However, the reaction rate constant of Fe/CN@E-4 (k
¼
o
ꢀ1
0.0194 min ) is slightly lower than that of Fe/CN@E-3 because
the excess NZVI particles were deposited on the surface of g-
C N @EGC, which resulted in a decrease in the number of
3
4
active sites on the g-C N surface.
3
4
In order to detect changes in the concentration of TC and its
intermediates during the degradation process, the absorbance
variations of TC over NZVI/g-C N @EGC are shown in Fig. 7c.
3 4
The ultraviolet-visible light patterns show two characteristic
peaks of TC located at 275 nm and 360 nm. As the reaction
progresses, the absorbance of TC gradually decreases until it
completely disappears aer 120 min, thereby supporting the
43
complete degradation of tetracycline.
Meanwhile, the
appearance of a new peak at 240 nm indirectly proves that the
tetracycline molecules are mainly destroyed and form aromatic
44
degradation by-products with stable molecular structures.
Finally, the absorption peaks of TC and the degradation by-
3 4 3 4 3 4
Fig. 6 PL spectra of g-C N , g-C N @EGC and NZVI/g-C N @EGC.
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Fig. 7 (a) Comparison of the removal efficiency of tetracycline with different samples under the same conditions; (b) the pseudo-first-order
kinetic plots for the removal of tetracycline; (c) UV-vis spectral changes for tetracycline degradation with Fe/CN@E-3.
products of TC were found to disappear, indicating decompo- This is because as the initial concentration of tetracycline
45
sition into CO and H O.
increases, the active sites of Fe/CN@E-3 become more occupied,
2
2
In order to verify the feasibility of photocatalytic materials in resulting in a decrease in the degradation efficiency. At the
practical applications, it is necessary to study the removal effi- same time, the increase in concentration increases the turbidity
ciency of TC in different water environments. Fig. 8a shows the of the solution, which hinders the absorption of visible light by
effects of the initial TC concentration on the photocatalytic the photocatalyst.
performance of Fe/CN@E-3. When the initial concentrations of
Fig. 8b depicts the effects of reaction temperature on the
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
TC were 10 mg L , 20 mg L , 30 mg L , 50 mg L , and degradation efficiency of Fe/CN@E-3 towards TC. When the
ꢀ1
ꢁ ꢁ
7
0 mg L , the degradation efficiencies of Fe/CN@E-3 towards temperature is raised from 10 C to 50 C, the degradation
TC were 99.4%, 98.9%, 98.5%, 93.8%, and 81.8%, respectively. efficiency of Fe/CN@E-3 towards TC gradually increases from
Table 2 Comparison of different catalysts with removal efficiencies of TC and key operating parameters
ꢀ1
Catalyst
Dosage (g)
C
0
(mg L
)
Time (min)
Removal (%)
Radiation source
Ref.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
1
1
1
1
1
AgI/BiVO
4
0.03
0.08
0.05
0.025
0.01
0.05
0.05
0.03
0.03
100 mL, 20 mg L
100 mL, 20 mg L
100 mL, 10 mg L
100 mL, 20 mg L
100 mL, 10 mg L
50 mL, 20 mg L
50 mL, 20 mg L
100 mL, 20 mg L
60
120
60
420
140
240
120
60
95
90
75
61
85
67
70
94
91
UV-lter 300 W Xe lamp
UV-lter 300 W Xe lamp
UV-lter 300 W Xe lamp
UV-lter 500 W Xe lamp
UV-lter 250 W Xe lamp
UV-lter 300 W Xe lamp
UV-lter 300 W Xe lamp
Solar 300 W Xe lamp
35
36
37
38
39
40
41
MS@FCN
Ag PO /CuBiO
NaTaO @WO
SrTiO /Bi
3
4
4
3
3
3
2 3
O
ꢀ1
N-CNT/mpg-CN
GQDs/mpg-CN
ꢀ1
ꢀ1
ZnO/CeO
NZVI/g-C
2
/HNTs
@EGC
42
ꢀ1
—
3
N
4
60 mL, 30 mg L
30
UV-lter 300 W Xe lamp
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Fig. 8 Factorial effects on the photo-degradation of tetracycline over Fe/CN@E-3 under visible light: (a) initial tetracycline concentration, (b)
reaction temperature, (c) pH value.
66.8% to 99.4%. Obviously, as the reaction temperature behavior was studied by the hysteresis loop curve of Fe/CN@E-3
increases, the degradation efficiency of TC increases. This is at room temperature. Fig. S1† shows that Fe/CN@E-3 has ferro-
because higher temperatures cause the TC decomposition magnetic behavior with a saturation magnetization of 26.5 emu
ꢀ
1
products to desorb faster in water; as a result, new TC molecules
are more efficiently adsorbed onto the surface of Fe/CN@E-3, external magnetic eld. The reusability and stability of Fe/CN@E-
which facilitates the removal of TC. 3 for continuous tetracycline degradation reactions were veried
g ; thus, it could be readily collected from water by supplying an
The pH of the solution has an important inuence on water by a 5-cycle test. As can be seen from Fig. 9a, compared with the
treatment. As shown in Fig. 8c, when the pH values of the solu- freshly prepared catalyst, the photocatalytic performance of the
tion were 3, 5, 7 and 9, the degradation efficiencies of Fe/CN@E-3 regenerated catalyst did not deteriorate signicantly aer 5
towards tetracycline were 99.2%, 99.5%, 97.0%, 62.8% and successive cycling runs, which conrms that Fe/CN@E-3 has
43.6%, respectively. It can be seen that TC can be almost sufficient stability during the degradation of TC. In addition,
completely degraded in an acidic solution. This is because the aer water purication, as a magnetic composite, Fe/CN@E-3
corrosion rate of NZVI in the acidic solution was accelerated, can be easily separated from the target polluted water for reuse
a large amount of iron ions were generated in the solution and by using an external magnetic eld. Consequently, Fe/CN@E-3
the degradation of TC was further promoted by the homogeneous has good water treatment prospects due to its recyclability and
45
Fenton-like reaction process. However, the surface of NZVI was facile recycling characteristics.
easily passivated by iron hydroxide in an alkaline solution; the
formation of the hydrous ferric oxide passivation layer leads to
blockage of the reaction sites, which greatly decreased the pho-
tocatalytic performance of Fe/CN@E-3.
It is well known that the recyclability of catalytic materials is
important for practical applications. Hence, the magnetic
Possible degradation mechanism
To illustrate the photo-degradation mechanism of TC, a series
of free radical trap experiments were performed to examine the
major active species that cause TC degradation. Generally,
hydroxyl radicals (cOH), superoxide radicals (cO
ꢀ
2
)
and
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Fig. 9 (a) Recycling runs in the degradation of tetracycline over Fe/CN@E-3; (b) effects of different scavengers on TC degradation.
+
photogenerated holes (h ) are recognized reactive species in the
TC + NZVI/g-C N @EGC / TC–NZVI/g-C N @EGC (4)
3
4
3
4
46
photocatalytic degradation of organic pollutants. Hence, the
scavengers tert-butanol (t-BA), ammonium oxalate (AO) and p-
benzoquinone (BQ) were added to the reaction system to
0
Fe + O
+
2+
+ 2H / H
O
2
+ Fe
(5)
(6)
(7)
2
2
2
+
3+
ꢀ
+
ꢀ
47
2 2
Fe + H O / Fe + cOH + OH
remove cOH, h , and cO , respectively. The observed changes
2
in the degradation efficiency of TC are displayed in Fig. 9b.
Compared with the blank control group, the degradation effi-
ciency of TC decreased by about 14%, 52% and 73%, respec-
tively. Aer AO and BQ were respectively added to the reaction
system to capture h and cO
was signicantly decreased; however, the effect of the lack of
cOH was not obvious, which indicates that h and cO may be
the major reactive species and cOH serves as a minor reactive
species in the degradation reaction of TC. Furthermore, ESR
spectroscopy with the DMPO technique was used to reveal the
radical species in the reaction system in the dark and under
visible light irradiation; the results are shown in Fig. S2a and b.†
cOH + TC / intermediates
ꢀ
Under the irradiation of visible light, the electrons (e ) of g-
3 4
C N were promoted to the conduction band, leaving positive
+
ꢀ
2
, the degradation efficiency of TC
+
+
holes (h ) in the valence band; here, h could directly participate
+
ꢀ
2
ꢀ
in the degradation of TC, and e could be transferred in two
ways: (1) because of the excellent electrical conductivity of EGC,
48
ꢀ
e
was rapidly transferred to the surface of EGC and came in
ꢀ
contact with O , then generated superoxide radicals (cO ). (2)
2
2
ꢀ
0
e
was trapped by iron ions and inhibited further oxidation of
Fe , which accelerated the heterogeneous Fenton-like process
No signals were detected in the dark, indicating that the
0
50
and maintained the reduction ability of Fe . In addition, holes
ꢀ
amounts of cOH and cO
2
were insufficient. However, a strong
+
(
h ) could react with surface hydroxyl groups (–OH) and H
2
O
ꢀ
DMPO–cO2 signal and a weaker DMPO–cOH signal were
detected in the suspension of Fe/CN@E-3 aer 10 min of visible
light irradiation; these results indicate that cO2 is the main
adsorbed on the surface of the EGC, resulting in the formation
51
+
ꢀ
of cOH. With the formation of h , cO and the minor reactive
2
ꢀ
species cOH, the tetracycline molecules and intermediates were
reactive species in the degradation process of TC, while cOH
48
gradually mineralized into carbon CO and H O. The corre-
2
2
49
does not play a key role in the reaction system. The results of
the ESR-DMPO method are in good agreement with the results
of the abovementioned free radical trapping experiments.
Based on the above results, a possible mechanism for the
synergistic degradation of TC using Fe/CN@E-3 is schematically
shown in Fig. 10. Due to the adsorption capacity of Fe/CN@E-3,
a large amount of tetracycline was separated from the solution,
enhancing its contact with the active species. In addition, the
EGC support can effectively inhibit the aggregation of NZVI
particles and provide a large number of reaction sites to accel-
erate the reduction reaction during the degradation process. At
the same time, iron ions were formed in solution, which
promoted the generation of cOH by the Fenton-like reaction. At
this time, the adsorption and reduction reaction are the main
sponding reaction process can be expressed as follows (eqn. (8)–
13)):
(
ꢀ
+
NZVI/g-C
3
N
4
@EGC + hn / e + h
(8)
(9)
ꢀ
ꢀ
2
O + e / cO2
3
+
ꢀ
2+
Fe + e / Fe
(10)
(11)
(12)
(13)
+
ꢀ
h + OH / cOH
+
h + H
+
2
O / cOH + H
2ꢀ
+
cO /h /cOH + TC/intermediates / CO
2
2
+ H O
34
removal pathways of TC (eqn. (4)–(7)).
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3 4
Fig. 10 Proposed mechanism for the synergistic removal of TC on the NZVI/g-C N @EGC photocatalyst.
For further explanation and description of the photo- changes in the measured mass and the intermediate products
degradation process, the possible intermediates and degrada- of TC aer 30 min and 60 min of irradiation were analyzed, and
tion process were analyzed by HPLC-mass spectrometry. The the results are shown in Fig. 11a–c. As shown in Fig. 12, rst,
Fig. 11 EIC of m/z for degraded tetracycline: (a) the initial tetracycline solution, (b) degradation of tetracycline in 30 min by Fe/CN@E-3, (c)
degradation of tetracycline in 60 min by Fe/CN@E-3.
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3 4
Fig. 12 Proposed reaction pathways for TC degradation by NZVI/g-C N @EGC.
tetracycline A (m/z ¼ 445) was fragmented into B (m/z ¼ 403) and intermediate E was decomposed into H (m/z ¼ 291) by a rear-
C (m/z ¼ 360) by losing –CONH and –N(CH ) . With the attack rangement reaction. Subsequently, H was further decomposed
2
3 2
of the reactive species, B and C generated E (m/z ¼ 317) and D into G by an addition reaction and elimination reaction. Then,
(
m/z ¼ 306), respectively, by losing –N(CH
cleavage of the aromatic ring. As the reaction proceeded, D group. Finally, these intermediates could be further degraded to
formed F (m/z ¼ 274) by losing –OH and then lost –OH and ]O CO , H O and other small molecules.
groups to generate (m/z 241). Furthermore, the
3
)
2
and by direct G was decomposed into I (m/z ¼ 199) by the removal of the C]C
2
2
G
¼
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9
Z. H. Ai, N. Wu and L. Z. Zhang, Catal. Today, 2014, 224, 180–
Conclusions
187.
In this study, we rstly synthesized a magnetic NZVI/g- 10 X. Wang, M. Z. Hong, F. W. Zhang, Z. Y. Zhuang and Y. Yu,
@EGC composite. SEM, TEM, XRD, XPS and FTIR studies
ACS Sustainable Chem. Eng., 2016, 4, 4055–4063.
were used to analyze the structure and chemical composition of 11 Z. J. Xie, Y. P. Feng, F. L. Wang, D. N. Chen, Q. X. Zhang,
3 4
C N
NZVI/g-C
3
N
4
@EGC. The optical properties were analyzed by UV-
Y. Q. Zeng, W. Y. Lv and G. G. Liu, Appl. Catal., B, 2018,
229, 96–104.
vis-DRS and PL spectroscopy. The adsorption, reduction and
photocatalytic performance of NZVI/g-C N @EGC were used to 12 J. F. Chen, Q. Yang, J. B. Zhong, J. Z. Li, C. Hu, Z. Deng and
3
4
complete the degradation of TC; the removal efficiency reached
9
R. Duan, Mater. Chem. Phys., 2018, 217, 207–215.
8.5%. The high degradation efficiency of TC can be attributed 13 G. Z. Liao, D. Y. Zhu, L. S. Li and B. Y. Lan, J. Hazard. Mater.,
to the synergistic effect and the formation of a heterogeneous
2014, 280, 531–535.
photo-Fenton system. The results show that the doping of NZVI 14 J. Liu, Y. Liu, N. Y. Liu, Y. Z. Han, X. Zhang, H. Huang,
particles and the support of EGC promote the transfer of pho-
togenerated electrons while the photocatalytic performance of
Y. Lifshitz, S. T. Lee, J. Zhong and Z. H. Kang, Science,
2015, 347, 970–974.
g-C
removal process of TC experienced two stages; the rst stage
was adsorption and reduction, in which NZVI decomposed TC 16 D. Z. Lu, X. Y. Zhang, S. Wang, W. B. Peng, M. M. Wei,
3
N
4
and the reductive ability of NZVI are maintained. The 15 B. Y. Li, X. Yu, X. L. Yu, R. F. Du, L. Liu and Y. H. Zhang, Ind.
Eng. Chem. Res., 2018, 57, 4311–4319.
molecules into various intermediates. Then, the second stage
achieved complete mineralization of TC and its intermediates
D. Neena, H. Q. Fan and H. J. Hao, Appl. Surf. Sci., 2019,
470, 368–375.
by visible light-driven photo-degradation. Moreover, NZVI/g- 17 X. J. Yu, Y. H. Zhang and X. W. Cheng, Electrochim. Acta,
@EGC exhibited high stability and reusability in degrading
2014, 137, 668–675.
TC, and the magnetic composite was easily recycled from water. 18 X. Q. Yue, R. J. Zhang, F. C. Zhang and L. Q. Wang,
This work provides a potential environmentally benign material
Desalination, 2010, 252, 163–166.
for the treatment of antibiotic wastewater and offers new 19 X. Wang, X. J. Wang, J. F. Zhao, J. K. Song, L. J. Zhou,
3 4
C N
insight into improving the catalyst activity through the forma-
tion of synergistic effects.
J. Y. Wang, X. Tong and Y. S. Chen, Appl. Catal., B, 2017,
206, 479–489.
2
2
2
0 P. Xiao, D. L. Jiang, T. Liu, D. Li and M. Chen, Mater. Lett.,
2018, 212, 111–113.
Conflicts of interest
1 J. X. Zhang, Y. B. Zhang, X. Quan, Y. Li, S. Chen, H. M. Zhao
and D. Wang, Biochem. Eng. J., 2012, 63, 31–37.
2 X. Wang, W. Wang, X. J. Wang, J. Zhang, Z. L. Gu, L. J. Zhou
and J. F. Zhao, RSC Adv., 2015, 5, 41385–41392.
There are no conicts to declare.
Acknowledgements
23 Q. H. Liang, Z. Li, Z. H. Huang, F. Y. Kang and Q. H. Yang,
Adv. Funct. Mater., 2015, 25, 6885–6892.
This research was supported by the National Natural Science
Foundation of China (Grant No. 51968031) and the National Key
Research and Development Program of China (Grant No.
2
2
2
4 Z. W. Tong, D. Yang, J. F. Shi, Y. H. Nan, Y. Y. Sun and
Z. Y. Jiang, ACS Appl. Mater. Interfaces, 2015, 7, 25693–25701.
5 C. B. Xu, W. J. Yang, W. J. Liu, H. L. Sun, C. L. Jiao and
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6 S. Chella, P. Kollu, E. V. P. R. Komarala, S. Doshi, M. Saranya,
S. Felix, R. Ramachandran, P. Saravanan, V. L. Koneru,
V. Venugopal, S. K. Jeong and A. N. Grace, Appl. Surf. Sci.,
2015, 327, 27–36.
2018YFC1900301).
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