Fe-based metal-organic frameworks as heterogeneous catalysts for highly efficient degradation of wastewater in plasma/Fenton-like systems

Article abstract of DOI:10.1039/d0ra07402k

Fe-based metal organic frameworks (Fe-MOFs) were successfully synthesized with the dielectric barrier discharge (DBD) plasma method and FeSO4·7H2O as the Fe precursor. Fe-MOFs were used as Fenton-like catalysts in DBD plasma/Fenton-like technology to trea

Full text of DOI:10.1039/d0ra07402k

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Fe-based metal–organic frameworks as
heterogeneous catalysts for highly efficient
degradation of wastewater in plasma/Fenton-like
systems†
Cite this: RSC Adv., 2020, 10, 36363
a
Xinjie Yuan,a Liang Huang,b Shuyong Shangc and Dongyan Xua
*
Xumei Tao,
Fe-based metal organic frameworks (Fe-MOFs) were successfully synthesized with the dielectric barrier
discharge (DBD) plasma method and FeSO4$7H2O as the Fe precursor. Fe-MOFs were used as Fenton-
like catalysts in DBD plasma/Fenton-like technology to treat wastewater, which addressed the issues
with iron solubility. Since the valence state of iron will affect the catalytic performance, the Fe precursor
FeSO4$7H2O was added to regulate the valence state and adjust the catalytic performance by improving
the availability of active sites. The influences of discharge voltage, catalyst addition amount, H2O2
addition amount and pH on the degradation efficiency of methyl orange (MO) were systematically
examined. Through free radical capture experiments, the reaction mechanism of the plasma/Fenton-like
catalytic degradation process was deduced primarily as the coordinated oxidation process of hydroxyl
radicals ($OH), photo-generated holes (h+) and superoxide radicals ($O2ꢀ). The reusability experiments
proved that the catalyst was stable and reusable. The possible degradation pathways were proposed
based on the identification of intermediate products generated in the degradation process by liquid
chromatography-mass spectrometry (LC-MS) analyses.
Received 28th August 2020
Accepted 14th September 2020
DOI: 10.1039/d0ra07402k
rsc.li/rsc-advances
which increases the amount of active substance, thereby
accelerating the degradation of organic matter.8 Plasma tech-
1 Introduction
nologies applied for the plasma/Fenton process include glow
discharge (GDP) plasma,9,10 high voltage pulsed discharge
plasma (PDP),11 gliding arc discharge (GAP) plasma12,13 and DBD
plasma.14 Hu et al.9 studied the synergistic effect of the GDP/
Fenton system and found that GDP can be signicantly
enhanced by the addition of iron ions as both Fenton catalysts
and occulants. Dai et al.11 studied high voltage PDP combined
with the Fenton reaction with both ferrous ion (Fe2+) salts and
a carbon steel grounded electrode as iron sources for the
degradation of bisphenol A (BPA). Plasma/Fenton treatment
induced an increased rate of BPA removal. Slamani et al.12
initiated the Fenton process by GAD plasma for the degradation
of paracetamol (PCM) in water. Combining Fenton with GAD
discharge induced a rapid degradation of PCM and gave a high
Fenton oxidation technology is considered a promising method
for the treatment of toxic organic pollutants in wastewater
because of its advantages such as strong oxidation potential,
mild reaction conditions, and easy operation.1,2 However, the
Fenton process possesses the disadvantages of a narrow pH
range, low H2O2 utilization rate, and sludge production, which
limits its large-scale application.3,4
Plasma, a novel technology, has attracted wide attention in
recent years.5,6 In order to degrade wastewater more effectively
and rapidly, researchers have studied the synergy of plasma
technology and Fenton technology. During the process of
plasma degradation of wastewater, active substances such as
H2O2 and $OH are generated.7 When an appropriate amount of
Fe2+-containing substance is added, a Fenton system is formed,
and a large amount of $OH is generated aer the reaction,
degree of mineralization. Markovic et al.14 studied the treatment
´
of ibuprofen with homogeneous catalyst FeSO4 in the DBD/
Fenton system, in which the degradation efficiency was
improved. The above results showed that the degradation effi-
ciency was promoted under the plasma/Fenton system, but the
degradation efficiency was improved at the expense of electrode
loss or iron ion erosion.15 DBD, with features of high safety,
uniform discharge and long electrode life,16 has not been
combined with Fenton technology using heterogeneous cata-
lysts to treat wastewater. Therefore, it is of great signicance to
aState Key Laboratory Base for Eco-Chemical Engineering, College of Chemical
Engineering, Qingdao University of Science and Technology, Qingdao 266042,
Shandong, China. E-mail: qiqitxm_2002@sina.com.cn
bCollege of Electromechanical Engineering, Qingdao University of Science and
Technology, Qingdao 266042, Shandong, China
cDepartment of Science, Technology and Discipline Construction, Chengdu Normal
University, Chengdu 611130, Sichuan, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra07402k
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 36363–36370 | 36363

View Article Online
RSC Advances
Paper
ꢁ
develop new heterogeneous catalysts with low cost, high several times for each. Then, it was dried at 160 C for 4 h to
activity, good stability and environment friendliness.
obtain the product, which was labelled as Fe-MOFs.
Fe-MOFs, which lose less iron ions in solution, have received
As shown in Fig. 1, a stainless steel mesh served as the high
increasing attention as Fenton-like catalysts. Fe-MOFs can voltage electrode while a stainless steel rod served as the low
provide iron active sites useful for the heterogeneous Fenton voltage electrode. A voltage generator (CTP-2000K, Nanjing
reaction with different iron environments, enabling the Suman Electronics Co, Ltd) was used to generate non-thermal
advantages of homogeneous iron complexes over a broader plasma. The voltage was measured by an oscilloscope
range of pH.17–19 More interestingly, Fe-MOFs prepared with (UTD2012CEX, UNI-T Co, Ltd). The solution was rst driven into
DBD plasma showed improved adsorption properties20 and the inner tube of the reactor through a peristaltic pump; aer
Fenton properties,21 as demonstrated in our previous work. lling the inner tube, the solution formed a wall-mounted
Also, the valence of an iron ion in Fe-MOFs affects the catalytic liquid lm on the outer surface of the inner tube and over-
performance. Li et al.22 synthesized a series of Fe-MOFs by owed to the outer tube. Aer discharging for a certain time, Fe-
controlling the composition of mixed valence Fe2+ and Fe3+ MOFs were synthesized.
precursors with dual functions, namely, high adsorption
capacity for toluene and good photocatalytic degradation
performance. In addition, Fe-MOFs could be recycled and
reused several times for the transformation without a signi-
cant degradation in catalytic activity.23
In this work, Fe-MOFs using FeSO4$7H2O as the iron
precursor were synthesized by DBD plasma and used as
heterogeneous catalysts to degrade wastewater in a DBD
plasma/Fenton-like system. The effects of different TA/Fe molar
ratios and reaction conditions on the catalytic degradation of
MO were studied. Through capture agent experiments and LC-
MS analysis, the reaction mechanism and possible degrada-
tion paths of the degradation process were deduced.
2.2 Materials characterization
X-ray diffraction (XRD) was carried out by a Japanese Rigaku D/
max-rA type X-ray diffractometer with Cu Ka radiation operated
at 40 kV and 150 mA. XRD patternsꢀw1ere scanned in the 2q range
of 3–80ꢁ at a scan rate of 10ꢁ min (step size 0.02ꢁ sꢀ1). Scan-
ning Electron Microscopy (SEM) was conducted with JSM-6700F
to characterize the surface morphology of the samples. Energy
Dispersive Spectroscopy (EDS) was used for the qualitative
detection of Fe and O in the Fe-MOFs. The transmission elec-
tron microscopy (TEM) images were acquired using a JEOL
(JEM-2100, Japan) operating at 200 kV. The BET specic surface
areas were measured by N2 adsorption at 196 ꢁC using an ASAP
2020 apparatus. The X-ray photoelectron spectra (XPS) were
collected with an ESCALAB 250xi spectrometer, monochromatic
radiation from the Al Ka radiation (150 W), and the Al Ka
monochromatic line (1486.6 eV) was adopted for the high-
resolution analysis. The intermediates and reaction products
of MO decontamination were analyzed using LC-MS (WATWES-
e2695 A0360F, America).
2 Material and methods
2.1 Preparation of Fe-MOFs by DBD plasma
The DBD plasma reaction device for preparing Fe-MOFs is
shown in Fig. 1. Briey, FeSO4$7H2O (2.87 g) and trimesic acid
(TA) (0.73 g) (TA/Fe source molar ratio of 1 : 3) were added into
150 mL of N,N-dimethylformamide (DMF) and 150 mL of
absolute ethanol, and stirred until completely dissolved. Then,
2.3 Plasma/Fenton-like process
the mixed solution was pumped into the DBD reactor and dis- The plasma/Fenton-like experimental device is shown in
charged for a certain time. Aer cooling naturally to room Fig. S1.† The reactor was similar to that for preparing Fe-MOFs.
temperature, the supernatant liquid was poured out and the Typically, MO and H2O2 were dissolved in water at room
dark orange solid product was washed with DMF and ethanol temperature. A certain amount of Fe-MOFs and H2O2 was added
Fig. 1 Fe-MOFs preparation device diagram.
36364 | RSC Adv., 2020, 10, 36363–36370
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
to 200 mL of MO solution (200 mg Lꢀ1) in a beaker. The air exposed; thus, the corresponding catalytic activity may be
pump played a role in preventing catalyst precipitation and higher.
participating effectively in the reaction. Magnetic stirring was
3.1.2 SEM results. SEM images of Fe-MOFs with different
set at 500 rpm. Samples were taken at set intervals using a 1 mL TA/Fe molar ratios are shown in Fig. 3. By observing pictures
pipette, then diluted and centrifuged at 3000 rpm for 5 min. with different magnication, the overall and local morphology
Then, the absorbance of the supernatant was measured with of the samples can be observed at a suitable distance. It can be
a UV-visible spectrophotometer at 463 nm. The degradation rate seen that the surface morphology of Fe-MOFs with different TA/
of the MO solution was dened as eqn (1-1).
Fe molar ratios was signicantly different. When the TA/Fe
molar ratio was 1 : 1, it showed a large area of aggregation
with irregular shapes and different particle sizes. With
increasing TA/Fe molar ratios, the morphology appeared
spherical and became more and more regular, and the particle
size also tended to be more uniform. When the TA/Fe molar
ratio was 1 : 3, the morphology was ower-like, and the ratio of
width to length was small, which provided a larger surface area
and increased number of active sites. SEM analyses revealed
that the TA/Fe molar ratio had a noticeable effect on the
morphology of the as-synthesized products.25
Degradation rate (%) ¼ (1 ꢀ C/C0) ꢂ 100%
(1-1)
where, C0 and C are the initial and concentration at a certain
time of MO at 463 nm, respectively. Each degradation experi-
ment was run in triplicate. The data were expressed as the
means ꢃ SD from three replicates.
Finally, in order to compare the synergistic degradation
effect, DBD alone and Fenton alone reactions were conducted
under the same conditions.
3.1.3 EDS results. EDS elemental analysis of Fe-MOFs with
different TA/Fe molar ratios is shown in Table S2.† The data
showed that with increasing TA/Fe molar ratios, the Fe content
relatively increased and the number of iron active sites
increased, which accelerated the progress of the Fenton reac-
tion. Furthermore, it was found that Fe-MOFs with a TA/Fe
molar ratio of 1 : 3 contained the most Fe. However, when the
TA/Fe molar ratio was 1 : 4, the percentage of Fe decreased. The
possible reason was that although the TA/Fe molar ratio
increased, the Fe source effectively participating in the reaction
was relatively reduced and the activity was reduced, which was
consistent with the law when studying the preparation
conditions.
During the plasma/Fenton-like degradation of MO, the
concentrations of Fe2+ and total Fe ions in the solution were
measured, as shown in Fig. S2.† The results showed that the
concentration of free Fe ions was 0.10–1.26 mg mLꢀ1, which only
accounted for 0.04–0.30% of the total amount of iron added. It
was veried that the material could effectively x Fe ions and
prevent Fe ion loss during the reaction.
3 Results and discussion
3.1 Characterization analysis of Fe-MOFs
3.1.1 XRD results. Fig. 2 shows the XRD patterns of Fe-MOF
samples with different TA/Fe molar ratios. The main diffraction
peaks in XRD analysis were 2q ¼ 10.3ꢁ, 14.1ꢁ, 18.8ꢁ and 24.1ꢁ.
The characteristic peak positions were nearly consistent with
those reported in the literature,24,25 and proved that MOFs were
successfully synthesized. By observing the XRD patterns, it can
be seen that the characteristics of the peaks were sharp, narrow
and symmetrical, and the baseline was stable. The sample with
a TA/Fe molar ratio of 1 : 1 had a small number of impurity
peaks, which might be caused by underutilization in the case of
excessive acid. With increasing TA/Fe molar ratios, the amount
of acid was relatively reduced, which could then be effectively
utilized and thus be more stable.
According to the XRD patterns, the average particle diame-
ters of Fe-MOFs with different TA/Fe molar ratios were calcu-
lated. As shown in Table S1,† the estimation error of the average
particle diameter was usually ꢃ 0.5 nm.26 It can be seen that the
particle size of the Fe-MOFs with a TA/Fe molar ratio of 1 : 3 was
the smallest. When the catalyst particle size is smaller, the
specic surface area may be larger and more active sites may be
3.1.4 TEM results. TEM images of Fe-MOFs with different
TA/Fe molar ratios are shown in Fig. 4. It can be seen that the
typical particle sizes of Fe-MOFs were mainly in the range of 20–
50 nm, and there were many irregular particles,24,27 which were
consistent with the XRD results. When the TA/Fe molar ratio
was 1 : 3, the width to length ratio of the sample was small,
which was consistent with the SEM results.
3.1.5 BET results. Nitrogen physical adsorption was the
method of choice to characterize the porous materials. This
method gave data on the specic surface area and the pore
diameters. The BJH method is based on the pore condensation
phenomena, which are applicable only for the mesoporous or
microporous regions.28 Fig. 5 shows the nitrogen adsorption–
desorption isotherms and pore size distribution diagrams of Fe-
MOFs with different TA/Fe molar ratios. According to the clas-
sication of the International Union of Pure and Applied
Chemistry (IUPAC), the isotherms of all samples were type IV. At
a relatively high pressure of 0.7 to 1.0, there was a hysteresis
loop in the isotherm and the type was H3 type.29 In addition, it
Fig. 2 XRD of Fe-MOFs with different TA/Fe molar ratios.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 36363–36370 | 36365

View Article Online
RSC Advances
Paper
Fig. 3 SEM images of Fe-MOFs with different TA/Fe molar ratios (a–d) 1 : 1; (e–h) 1 : 2; (i–l) 1 : 3; (m–p) 1 : 4.
can be seen from the pore size distribution diagram that Fe- TA/Fe molar ratio was 1 : 4, the specic surface area, average
MOFs were mainly mesoporous materials with a size of 2– pore size and pore volume of Fe-MOFs decreased, which may be
50 nm, which demonstrated the similar pore structures of Fe- caused by the differences in morphology and structure. This
MOFs and the predominance of the mesopores.25
was consistent with the SEM analysis results.
Table S3† shows the surface and pore properties of Fe-MOFs.
3.1.6 XPS results. X-ray photoelectron spectroscopy (XPS)
With increasing TA/Fe molar ratios, the values of specic analysis was performed to investigate the surface composition
surface area, average pore diameter, and pore volume of Fe- and chemical states of as-synthesized samples,30 as shown in
MOFs increased. When the TA/Fe molar ratio was 1 : 3, the Fig. 6. The XPS survey spectrum demonstrated that C, O, and Fe
maximum specic surface area was obtained, which was 32.021 are present in Fe-MOFs (Fig. 6(a)). The O peaks included O KL1
m2 gꢀ1. Due to the nature of the structure, the amount of MO and O 1s peaks, the C peaks were C 1s peaks, and the Fe peaks
molecules adsorbed on the inner surface increased, which was were Fe 2p. The binding energies of the peaks are shown in
convenient for adsorption and thus the catalytic reaction was Table S4.† In Fig. 6(b), four peaks at around 711 eV, 712 eV,
accelerated. Furthermore, a large specic surface area not only 724 eV and 726 eV can be assigned to Fe2+ 2p3/2, Fe3+ 2p3/2, Fe2+
results in more active sites, but also increases the contact area 2p1/2 and Fe3+ 2p1/2, respectively, indicating that the catalyst was
of the reaction, thereby accelerating the reaction rate. When the composed of Fe2+ and Fe3+ species. The peak at around 716 eV
Fig. 4 TEM images of Fe-MOFs with different TA/Fe molar ratios (a) 1 : 1; (b) 1 : 2; (c) 1 : 3; (d) 1 : 4.
36366 | RSC Adv., 2020, 10, 36363–36370
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Fig. 5 Nitrogen adsorption–desorption isotherms curves and pore size distribution of Fe-MOFs with different TA/Fe molar ratios.
was a satellite peak of iron, indicating that Fe-MOF samples
Table S5† presents the proportions of different valence iron
contained Fe2+ and Fe3+.25,31 The results from XPS further in Fe-MOFs. The ratio of Fe2+/Fe3+ and the relative proportion of
conrmed the successful synthesis of Fe-MOFs. The C 1s spec- oxygen vacancies in Fe-MOFs were estimated from the relative
trum in Fig. 6(c) could be deconvolved into three peaks centered area of the deconvolution peak. When the TA/Fe molar ratio was
at around 284 eV, 285 eV, and 288 eV. The peaks at around 1 : 3, the ratio of Fe2+/Fe3+ and [O]s was signicantly higher than
284 eV and 288 eV corresponded to the electron binding ener- that of the other samples, which presented more oxygen
gies in the phenyl and carboxyl groups, respectively. The peak at vacancies and was very benecial to the Fenton catalytic
around 285 eV was attributed to carbon on the surface of the degradation process. Fe-MOFs can drive the formation of more
sample.32,33 The O 1s spectrum could be t by two peaks at $OH and $OOH based on eqn (1) and (2). Then, MO is degraded
binding energies of arounꢀd 531 and 532 eV, which were attrib- by the abundant $OH and $OOH. eꢀ and h+ are produced under
uted to surface oxygen O2 and O2 in the O–Fe bond, respec- ultraviolet light (eqn (3)). The Fe3+ is easily reduced to Fe2+ by
ꢀ
tively.34 The results of XPS analysis further conrmed the absorbing eꢀ (eqn (4)), which enables the continuous formation
chemical bond morphology presented in the structure.
of $OH.22 In addition, surface oxygen vacancies are generated
Fig. 6 XPS spectra of Fe-MOFs with different TA/Fe molar ratios (a) survey spectrum; (b) Fe 2p; (c) C 1s; (d) O 1s.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 36363–36370 | 36367

View Article Online
RSC Advances
Paper
during the reduction of Fe3+ to Fe2+, which would also affect the reactant concentration of 12 g Lꢀ1, TA/Fe molar ratio of 1 : 3 and
catalytic performance. Fe-MOFs with a TA/Fe molar ratio of 1 : 3 discharge time of 100 min, which showed the best performance.
had the highest content of Fe2+/Fe3+ and oxygen vacancies. The samples still maintained more than 89% degradation efficiency
Furthermore, the content of Fe2+ and Fe3+ in Fe-MOFs with a TA/ for MO aer six cycles. A comparison of the degradation capacity
Fe molar ratio of 1 : 1 was higher, because the area formed by between the DBD plasma/Fenton system and other systems is
the satellite peak was small and the proportion was relatively shown in Table S6.† Fe-MOFs synthesized by DBD plasma had great
small, so the proportion of Fe2+ and Fe3+ was relatively large. advantages in the DBD plasma/Fenton treatment of MO.
This was consistent with the XRD and XPS results.
Fe2+ + H2O2 / Fe3+ + $OH + OHꢀ
Fe3+ + H2O2 / Fe2+ + $OOH + H+
hv / h+ + eꢀ
(1)
(2)
(3)
(4)
3.3 Reaction mechanism of plasma/Fenton-like process
In order to identify the role of various active species on MO
degradation in the plasma/Fenton-like process and to discuss
the reaction mechanism, tert-butanol (TBA), ammonium
oxalate (AO) and benzoquinone (BQ) were used as capture
agents of $OH, h+ and $O2ꢀ, respectively. As shown in Fig. S6,†
aer the addition of the capture agents, the reaction efficiency
was reduced because the capture agents consumed the active
species in the reaction system that contributed to the degra-
dation of MO. It proved that active species including $OH, h+
Fe3+ + eꢀ / Fe2+
3.2 Plasma/Fenton-like process of Fe-MOFs
Fig. S3† shows a comparison of the performance of plasma alone, and $O2ꢀ were generated during the reaction and played a vital
Fenton alone, and the plasma/Fenton-like system. When the role in the degradation process of MO.
discharge voltage was 14.5 kV, the addition amount of Fe-MOFs
For the Fenton process, Fe2+ catalyzed the decomposition of
with a TA/Fe molar ratio of 1 : 3 was 1 g Lꢀ1 and the addition H2O2, which resulted in the generation of $OH radicals.17 The
amount of H2O2 was 2.0 mL Lꢀ1, it can be found that the plasma/ generated Fe3+ could be reduced by reaction with excess H2O2 to
Fenton-like catalysis was better than plasma and Fenton alone.
form Fe2+ again and more radicals such as $O2ꢀ; it could also
Fig. S4† shows the effect of different reaction conditions on the attack organic contaminants.36 For the DBD plasma process,
performance of Fe-MOFs. From Fig. S4 (a),† it can be seen that energetic electrons exist that collide with other molecules such as
200 mg Lꢀ1 MO was completely degraded within 8 min, and O2 and H2O to produce various active species including $OH and
350 mg Lꢀ1 MO can reach 96% degradation within 12 min. With $O2ꢀ, in addition to some active molecules such as O3 and H2O2.
the increase of MO concentration, the degradation rate was Furthermore, h+ and photo-generated electrons (eꢀ) were gener-
correspondingly slower. It can be seen that Fe-MOFs could not ated on the surface of Fe-MOFs and the catalytic redox reaction
only process low-concentration MO solutions, but also have occurred to produce H2O2 and dissolved oxygen. This residual
a benecial effect on MO in higher concentrations. Solutions with H2O2 could be utilized through the Fenton reaction to produce
higher initial organic concentrations needed longer treatment more $OH,41 which further enhanced the degradation of MO.
times to achieve a given degradation degree.35 Fig. S4(b)† showed When the plasma reaction was combined with the Fenton reac-
that the degradation rate increased with the increase of discharge tion, the amount of active substances increased, thus the degra-
voltage. Nevertheless, energy consumption needs to be consid- dation of organic matter was accelerated. The degradation of MO
ered. It can be seen from Fig. S4(c)† that when the optimal was nally achieved by the redox reactions of the active species.
amount was added, the degradation efficiency was the fastest.36
The reaction intermediates of MO in the plasma/Fenton-like
The concentration of H2O2 played a crucial role in deciding the process formed during the rst 8 min were analyzed by LC-MS.
overall efficiency of the degradation process. However, the oper- The obtained mass spectra are provided in Fig. S7.† The
ating oxidant dosage must be carefully identied. The presence of detected intermediates with different mass-to-charge (m/z)
H2O2 was harmful to many of the organisms and would affect the ratios and molecular structures are summarized in Table S7.† A
overall degradation efficiency signicantly, hence the excess degradation mechanism (Scheme 1) was proposed based on the
amount was not recommended. Moreover, the negative effect of species detected in this study and previous literature. Before
H2O2 was the scavenging of generated $OH, which occurred at degradation, the peak of m/z ¼ 304.07 was the parent molecule
high quantities of H2O2.35 Therefore, the amount of H2O2 should of MO aer losing Na+ due to dissolution in aqueous solution.
be reduced as much as possible to ensure efficiency. According to The intermediate with a m/z of 290.06 was formed due to the
Fig. S4(e),† the pH of 3 was benecial for the Fenton reaction demethylation, followed by deamination to generate azo-
because pH signicantly affects the solubility and oxidation state benzesulfonic acid (m/z ¼ 261.03). Then, it was decomposed
of Fe.37 The kinetics of MO degradation in the experiments were into sulfanilic acid (m/z ¼ 172.00) and aniline (m/z ¼ 93.05)
studied.38–40 The kinetic behavior of MO degradation was t by (Path 1). Sulfanilic acid was methylated and deaminated to form
a pseudo-rst-order model, as highlighted in the inset panel of phenylmethylamine (m/z ¼ 107.07) and then converted to
Fig. S4.† It was found that the maximum pseudo-rst-order rate aniline. Azobenzesulfonic acid can also be decomposed to 4-
constant was 0.669 minꢀ1 (R2 ¼ 0.991).
diazenyl-benzene (m/z ¼ 106.05) and p-hydroxy-benzenesulfonic
Fig. S5† shows the recycling experiment results of Fe-MOFs acid (m/z ¼ 172.99) (Path 2). P-hydroxybenzenesulfonic acid was
prepared in the conditions of discharge voltage of 14.5 kV, oxidized to benzenesulfonic acid (m/z
¼
156.99) and
36368 | RSC Adv., 2020, 10, 36363–36370
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Scheme 1 Proposed degradation pathways of MO in the plasma/Fenton-like system.
nitrobenzene (m/z ¼ 138.0197), which was then oxidized to
Acknowledgements
phenol (m/z ¼ 110.03), while 4-diazenyl benzene was converted
to aniline. The further oxidation of hydroquinone and aniline
resulted in the aromatic ring opening and formation of short-
linear aliphatic carboxylic acids. Finally, the aliphatic carbox-
ylic acids can directly convert into CO2 and H2O.
This work was supported by the Talent Fund of Shandong
Collaborative Innovation Center of Eco-Chemical Engineering
from Qingdao University of Science and Technology
[XTCXQN23]. The authors would like to acknowledge the
support of Mengzhen Wang for useful discussions and help.
4 Conclusions
References
Fe-MOFs using FeSO4$7H2O as an iron source were successfully
synthesized by DBD plasma and applied for plasma/Fenton-like
technology as heterogeneous catalysts. The synergy of DBD
plasma and Fenton induced an increased rate of MO removal.
The results showed that Fe-MOFs had a stable crystal structure,
increased specic surface area and pore size, and thus, more
iron-based active sites. In addition, the valence of iron in the
synthesized Fe-MOFs affected the catalytic performance. At
a catalyst addition amount of 1 g Lꢀ1, H2O2 addition amount of
2.0 mL Lꢀ1, discharge voltage of 13.7 kV, and pH of 3, the
degradation rate of 200 mL (200 mg Lꢀ1) MO reached 96.4% at
8 min. Through free radical capture experiments, the reaction
mechanism of the plasma/Fenton-like catalytic degradation
process was deduced mainly as the coordinated oxidation
process of $OH, h+ and $O2ꢀ. Collaborative technology improves
the efficiency of wastewater treatment, and also provides new
ideas for the study of pollutant degradation. More importantly,
as Fenton-like catalysts, Fe-MOFs exhibited excellent durability
in six cycles of degradation, which proved that the catalyst was
stable and reusable. In addition, the degradation intermediates
of MO were identied using LC-MS, and possible trans-
formation pathways were proposed.
1 M. Malakootian, A. Shahesmaeili, M. Faraji, H. Amiri and
S. S. Martinez, Process Saf. Environ. Prot., 2020, 134, 292–
307, DOI: 10.1016/j.psep.2019.12.004.
2 M. H. Zhang, H. Dong, L. Zhao, D. X. Wang and D. Meng, Sci.
Total Environ., 2019, 670, 110–121, DOI: 10.1016/
j.scitotenv.2019.03.180.
3 L. J. Di, H. Yang, T. Xian, X. Q. Liu and X. J. Chen,
Nanomaterials, 2019, 9, 399, DOI: 10.3390/nano9030399.
4 H. Wang, M. Jing, Y. Wu, W. L. Chen and Y. Ran, J. Hazard.
Mater.,
2018,
353,
53–61,
DOI:
10.1016/
j.jhazmat.2018.03.053.
5 S. Ma, K. Kim, S. Chun, S. Y. Moon and Y. C. Hong,
Chemosphere, 2020, 243, 125377, DOI: 10.1016/
j.chemosphere.2019.125377.
6 G. S. Oehrlein and S. Hamaguchi, Plasma Sources Sci.
Technol., 2018, 27, 1–21, DOI: 10.1088/1361-6595/aaa86c.
7 A. Fahmy, J. Friedrich, F. Poncin-Epaillard and D. Debarnot,
Thin Solid Films, 2016, 616, 339–347, DOI: 10.1016/
j.tsf.2016.08.045.
8 F. Alaa, E. Z. Adham, A. M. Saeed, A. Z. Sayed, M. A. Ezz El-
Arab and H. A. Shehata, Chemosphere, 2018, 210, 102–109,
DOI: 10.1016/j.chemosphere.2018.06.176.
9 P. Hu, Y. K. Liu, B. Jiang, X. Zheng, J. T. Zheng and M. B. Wu,
Ind. Eng. Chem. Res., 2015, 54, 8277–8286, DOI: 10.1021/
acs.iecr.5b01012.
Conflicts of interest
The authors declare that they have no known competing 10 A. Khataee, S. Bozorg, S. Khorram, M. Mehrangiz Fathinia,
nancial interests or personal relationships that could have
appeared to inuence the work reported in this paper.
Y. Hanifehpour and S. W. Joo, Ind. Eng. Chem. Res., 2013,
52, 18225–18233, DOI: 10.1021/ie403283n.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 36363–36370 | 36369

View Article Online
RSC Advances
Paper
11 F. Dai, X. R. Fan, G. R. Stratton, C. L. Bellona, T. M. Holsen, 27 R. Wang, Y. Gong, H. J. Xu, S. Y. Wei and D. Y. Wu, Chin. J.
B. S. Crimmins, X. Y. Xia and S. M. Thagard, J. Hazard.
Mater., 2016, 308, 419–429, DOI: 10.1016/
j.jhazmat.2016.01.068.
12 S. Slamani, F. Abdelmalek, M. R. Redouane and A. Addou, J.
Inorg. Anal. Chem., 2018, 34, 906–916, DOI: 10.11862/
CJIC.2018.110.
28 M. Anbia, V. Hoseini and S. Sheykhi, J. Ind. Eng. Chem., 2012,
18, 1149–1152, DOI: 10.1016/j.jiec.2012.01.014.
´
Photochem. Photobiol., A, 2018, 359, 1–10, DOI: 10.1016/ 29 N. Torres, J. Galicia, Y. Plasencia, A. Cano, F. Echevarrıa,
j.jphotochem.2018.03.032.
13 C. M. Du, L. L. Zhang, J. Wang, C. R. Zhang, H. X. Li and
L. F. Desdin-Garcia and E. Reguera, Colloids Surf., A, 2018,
549, 138–146, DOI: 10.1016/j.colsurfa.2018.04.016.
Y. Xiong, Plasma Chem. Plasma Process., 2010, 30, 855–871, 30 Y. J. Li, G. S. Hou, J. Yang, J. Xie, X. L. Yuan, H. Yang and
DOI: 10.1007/s11090-010-9249-0.
M. M. Wang, RSC Adv., 2016, 6, 16395–16403, DOI:
´
´
´
ˇ
´
´
14 M. Markovic, M. Jovic, D. Stankovic, V. Kovacevic, G. Roglic,
10.1039/c5ra24915e.
´
´
´
G. Gordana Gojgic-Cvijovic and D. Manojlovic, Sci. Total 31 X. H. Li, W. L. Guo, Z. H. Liu, R. Q. Wang and H. Liu, Appl.
Environ.,
2015,
505,
1148–1155,
DOI:
10.1016/
Surf. Sci., 2016, 369, 130–136, DOI: 10.1016/
j.scitotenv.2014.11.017.
j.apsusc.2016.02.037.
15 J. L. Wang and L. J. Xu, Crit. Rev. Environ. Sci. Technol., 2012, 32 F. M. Zhang, Y. Jin, J. Shi, W. D. Zhu and M. S. El-Shall,
42, 251–325, DOI: 10.1080/10643389.2010.507698.
16 C. L. Chang and T. S. Lin, Plasma Chem. Plasma Process.,
2005, 25, 227–243, DOI: 10.1007/s11090-004-3034-x.
Chem. Eng. J., 2015, 269, 236–244, DOI: 10.1016/
j.cej.2015.01.092.
33 B. J. Zhu, X. Y. Yu, Y. Jia, F. M. Peng, B. Sun, M. Y. Zhang,
T. Luo, J. H. Liu and X. J. Huang, J. Phys. Chem. C, 2012,
116, 8601–8607, DOI: 10.1021/jp212514a.
¨
17 R. Thur, N. V. Velthovena, S. Slootmaekers, J. Diddena,
R. Verbekea, S. Smoldersa, M. Dickmannb, W. Eggerc,
D. D. Vosa and I. F. J. Vankelecom, J. Membr. Sci., 2019, 34 T. A. Vu, G. H. Le, C. D. Dao, L. Q. Dang, K. T. Nguyen,
576, 78–87, DOI: 10.1016/j.memsci.2019.01.016.
18 A. Shrivastav, S. Sundriyal, P. Goel, H. Kaur, S. K. Tuteja,
K. Vikrant, K. H. Kim, U. K. Tiwari and A. Deep, Coord.
K. Quang, Q. K. Nguyen, P. T. Dang, H. T. K. Tran,
Q. T. Duong, V. Tuyen, T. V. Nguyen and G. D. Lee, RSC
Adv., 2015, 5, 5261–5268, DOI: 10.1039/c4ra12326c.
Chem. Rev., 2019, 393, 48–78, DOI: 10.1016/j.ccr.2019.05.006. 35 L. Labiadh, M. A. Oturan, M. Panizza, N. B. Hamadi and
´
´
´
19 F. Martınez, P. Leo, G. Orcajo, M. Dıaz-Garcıab, M. Sanchez-
Sanchezb and G. Calleja, Catal. Today, 2018, 313, 6–11, DOI:
10.1016/j.cattod.2017.10.002.
S. Ammar, J. Hazard. Mater., 2015, 297, 34–41, DOI:
10.1016/j.jhazmat.2015.04.062.
36 A. Babuponnusami and K. Muthukumar, J. Ind. Eng. Chem.,
2014, 2, 557–572, DOI: 10.1016/j.jece.2013.10.011.
37 F. C. Moreira, R. A. R. Boaventura, E. Brillas and V. J. P. Vilar,
Appl. Catal., B, 2016, 202, 217–261, DOI: 10.1016/
j.apcatb.2016.08.037.
20 X. M. Tao, C. Sun, Y. Y. Han, L. Huang and D. Y. Xu,
CrystEngComm, 2019, 21, 1–24, DOI: 10.1039/C9CE00015A.
21 X. M. Tao, C. Sun, L. Huang, Y. Y. Han and D. Y. Xu, RSC
Adv., 2019, 9, 6379–6386, DOI: 10.1039/c8ra09211g.
22 P. Li, S. Kim, J. Jin, D. H. Chun and J. H. Park, Appl. Catal., B, 38 S. Ammar, M. A. Oturan, L. Labiadh, A. Guersalli,
2020, 263, 118284, DOI: 10.1016/j.apcatb.2019.118284.
23 T. D. Le, K. D. Nguyen, V. T. Nguyen, T. Truong and
R. Abdelhedi, N. Oturan and E. Brillas, Water Res., 2015,
74, 77–87, DOI: 10.1016/j.watres.2015.02.006.
N. T. S. Phan, J. Catal., 2016, 333, 94–101, DOI: 10.1016/ 39 S. Midassi, A. Bedoui and N. Bensalah, Chemosphere, 2020,
j.jcat.2015.10.026. 260, 127558, DOI: 10.1016/j.chemosphere.2020.127558.
24 G. Q. Song, Z. Q. Wang, L. Wang, G. R. Li, M. J. Huang and 40 Z. Q. Yang, H. L. Chen, J. H. Wang, Q. F. Yuan, F. Wang and
F. X. Yin, Chin. J. Catal., 2014, 35, 185–195, DOI: 10.1016/
B. H. Zhou, J. Environ. Chem. Eng., 2020, 8, 104057, DOI:
S1872-2067(12)60729-3.
10.1016/j.jece.2020.104057.
25 D. Z. Chen, S. S. Chen, Y. J. Jiang, S. S. Xie, H. Y. Quan, 41 X. L. Hao, M. H. Zhou, Y. Zhang and L. C. Lei, Plasma Chem.
L. Hua, X. B. Luo and L. Guo, RSC Adv., 2017, 7, 49024–
49030, DOI: 10.1039/c7ra09234b.
Plasma Process., 2006, 26, 455–468, DOI: 10.1007/s11090-006-
9028-0.
26 V. M. Kariuki, I. Y. azgan, A. Akgul, A. Kowal, A. Omowunmi,
M. Parlinska and O. A. Sadik, Environ. Sci.: Nano, 2015, 2,
518–527, DOI: 10.1039/C5EN00053J.
36370 | RSC Adv., 2020, 10, 36363–36370
This journal is © The Royal Society of Chemistry 2020