Degradation of hazardous organic dyes with solar-driven advanced oxidation process catalyzed by the mixed metal–organic frameworks

Article abstract of DOI:10.1002/aoc.4928

phosphonate-based bimetallic metal- organic frameworks, namely STA-12(M₁, M₂) (M₁, M₂?=?Mn, Fe, Co), show photocatalytic activity for the degradation of Rhodamine B (RhB) and Methylene blue (MB) from aqueous solution under natural sunlight irradiation. The degradation of the dyes, appears to be faster with STA-12(Fe, Mn) than other synthesized MOFs. Thus, photo-Fenton oxidative discoloration of dyes has been studied by H₂O₂ catalyzed with the STA-12(Fe, Mn). The process is first order with respect to dyes and the synergistic index in the STA-12(Fe, Mn)/sunlight/H₂O₂system reached as high as 472%. Mineralization of dyes was discussed by spectroscopic and TOC measurement. Besides, the efficiency of STA-12(Fe, Mn) used in photocatalytic process was attentively investigated through the characterization of reactive radicals, the stability and reusability of the photocatalyst, also the effect of operational parameters such as H₂O₂ dosage, solution pH and initial dye concentration. This work demonstrates the first example of facilitating photo-Fenton-like excitation of H₂O₂ via phosphonate based mixed metal organic frameworks as photocatalysts and explained a new opportunity for solar-induced AOP environmental remediation and protection.

Full text of DOI:10.1002/aoc.4928

Received: 1 November 2018
DOI: 10.1002/aoc.4928
Revised: 29 January 2019
Accepted: 11 March 2019
F U L L P A P E R
Degradation of hazardous organic dyes with solar‐driven
advanced oxidation process catalyzed by the mixed metal–
organic frameworks
Alireza Farrokhi
| Fahimeh Feizpour | Maryam Asaadzadeh
Department of Chemistry, Faculty of
Science, University of Birjand, Birjand
phosphonate‐based bimetallic metal‐ organic frameworks, namely STA‐12(M1,
97179‐414, Iran
M ) (M , M = Mn, Fe, Co), show photocatalytic activity for the degradation of
2
1
2
Rhodamine B (RhB) and Methylene blue (MB) from aqueous solution under
natural sunlight irradiation. The degradation of the dyes, appears to be faster
with STA‐12(Fe, Mn) than other synthesized MOFs. Thus, photo‐Fenton oxida-
tive discoloration of dyes has been studied by H O catalyzed with the STA‐
Correspondence
Alireza Farrokhi, Department of
Chemistry, Faculty of Science, University
of Birjand, Birjand, 97179‐414 Iran.
Email: afarrokhi@birjand.ac.ir
2
2
12(Fe, Mn). The process is first order with respect to dyes and the synergistic
index in the STA‐12(Fe, Mn)/sunlight/H O system reached as high as 472%.
2
2
Mineralization of dyes was discussed by spectroscopic and TOC measurement.
Besides, the efficiency of STA‐12(Fe, Mn) used in photocatalytic process was
attentively investigated through the characterization of reactive radicals, the
stability and reusability of the photocatalyst, also the effect of operational
parameters such as H O dosage, solution pH and initial dye concentration.
2
2
This work demonstrates the first example of facilitating photo‐Fenton‐like
excitation of H O via phosphonate based mixed metal organic frameworks
2
2
as photocatalysts and explained a new opportunity for solar‐induced AOP envi-
ronmental remediation and protection.
KEYWORDS
dye degradation, hydrogen peroxide, natural sunlight, phosphonate based MOFs, Photocatalysis
1
| INTRODUCTION
Generally, the AOP generates highly reactive oxidizing
agents to degrade stable hazardous organic substances into
Increasing worldwide contamination of water resources
and decrease of water quality has become a drastic prob-
lem in the human societies. The global increasing request
for clean water has promote extensive research to achieve
the modern methods for water treatment. Among vari-
ous biological, chemical, and physical technologies in con-
tamination control, the advanced oxidation processes
less harmful molecules, even eventually mineralization of
[
3,4]
them to CO and water.
Particularly, the application of
2
a green oxidant, hydrogen peroxide (H O ), to produce
2
2
highly reactive hydroxyl radical has triggered great atten-
tion. Essentially, UV light irradiation, metal and nonmetal
containing catalyst and ultrasonic irradiation could easily
[1]
•
[5–9]
decompose H O to OH radicals.
But, most of these
2
2
(
AOPs), including ozonation, photocatalysis, sonolysis,
methods require long runtimes, hard operating pH range
and high energy consumption. Therefore, extensive
research effort has been devoted to progress a heteroge-
neous catalytic method activating H O to overcome the
Fenton reaction and their combination, have significant
practical importance owing to their superior efficiency,
easy handling, good reproducibility, and simplicity.
[
2]
2
2
Appl Organometal Chem. 2019;e4928.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 11
https://doi.org/10.1002/aoc.4928

2
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FARROKHI ET AL.
serious problems of above homogeneous catalytic systems.
Also, recent studies have displayed that H O could be
2.2 | Synthesis of ligand (H L)
4
2
2
•
decompose to OH radicals by the semiconductor
photocatalysts, utilizing to increase efficiency of the visible
The 1,4‐piperazinediylbis (methylene) phosphonic acid,
H O P‐CH ‐NC H N‐CH ‐PO H , was prepared follow-
2
3
2
4
8
2
[25]
3
2
[10–12]
1
light induced photocatalytic procedures.
Accord-
ing the previous report
NMR, P NMR, C NMR (Figures S1‐S4) and chemical
and characterized by IR, H
31
13
ingly, design and construction of the efficient heteroge-
neous catalytic system for H O2 activation is an
analysis (CHN).
2
important subject to realize the required long‐term stabil-
ity and activity. The present metal‐oxide photocatalysts
have low quantum yield and inability for utilize the solar
light or daylight. Therefore, the researchers have
prompted to develop next descendant solar‐responsive
2
.3 | Preparation of MOFs
STA‐12(Fe, Mn), STA‐12(Fe, Co) and STA‐12(Mn, Co)
were synthesized according to the published procedures
with slight modifications (supporting information).
[13]
photocatalysts.
Studies have shown that metal–organic
[26]
frameworks (MOFs) could be revealed as favorable
[14–17]
replacements for conventional photocatalysts.
Recently, the MOF‐involved AOPs has been applied
2.4 | Characterization
for dye degradation in the presence of H O , have shown
2
2
enhanced photocatalytic efficiency compared to those
Elemental analyses were performed with a Heraeus
CHN‐O‐RAPID elemental analyzer and inductively
coupled plasma atomic emission analyses (ICP‐AES,
Perkin‐Elmer Optima 7300DV). The IR spectra were
recorded on a Shimadzu model 800 FT‐IR spectrometer.
Powder X‐ray diffraction (PXRD) data were collected on
a X'Pert PRO MPD diffractometer using Cu‐Kα radiation.
Diffuse reflectance UV/Vis spectra were recorded using
an Avantes spectrometer (Avaspec‐2048‐TEC model).
[18–22]
using merely MOFs.
Meanwhile, reports on a natu-
ral sunlight utilizing MOF for photodegradation of water
[23,24]
pollutants are very limited.
Therefore, the sensible
synthesis of novel photo‐active MOFs and the study of
solar‐driven advanced oxidation process for dyes destruc-
tion are currently required and significant.
Herein, we report the rational synthesis and capability
of a series of mixed‐metal MOFs (STA‐12 family) for the
activation of H O to attain high competency in daylight
1
13
31
2
2
H, C, and P NMR experiments were carried out on
photocatalysis. The photo‐degradation rates of
Rhodamine B (RhB) and methylene blue (MB) over
STA‐12(Fe, Mn), STA‐12(Fe, Co), STA‐12(Mn, Co) were
increased obviously in the presence of H O . The Fenton
a Bruker (AvanceDRS)500 spectrometer. The porosity
and textural properties of catalysts were measured using
an Belsorp Mini II instrument. The samples were
degassed in vacuum at 150 °C for 6 hr, the nitrogen
adsorption isotherms were measured at 77 K. Thermal
stability was measured by TGA (NETZSCH TG 209 F1
Iris) from 25 to 900 °C with heating rate of 10 °C/min.
2
2
and photo‐Fenton mechanism of process was inspected
and suggested according to experimental recognition of
hydroxyl radicals which approve the enhanced catalytic
activity maybe arisen from the synergetic effects of
STA‐12 and H O under daylight irradiation.
2
2
2.5 | Photocatalytic degradation
experiments
2
2
| EXPERIMENTAL
.1 | Materials
The photocatalytic activities of STA‐12(Fe, Mn), STA‐
12(Fe, Co) and STA‐12(Mn, Co) were investigated by
the degradation of MB and RhB under natural sunlight
conditions (1 atm and 30 °C). The photo‐degradation
experiments were accomplished in open spaces where it
was feasible to gain direct solar irradiation and in succes-
sive sunny days in fall months of October to November
All solvents and reagents used in this research are com-
mercially available and consumed without purification.
Manganese (II) acetate tetrahydrate (Mn (OAc) .4H O),
Ferrous chloride tetrahydrate (FeCl .4H O), Cobalt (II)
nitrate hexahydrate (Co (NO ) .6H O), terephthalic acid
2
2
[
27]
between 10.00 AM and 2.00 PM.
Aqueous solutions
2
2
−
1
−1
of 0.1 molL HCl or 0.1 molL NaOH were added for
adjusting the pH of dye solution. Desired amount of as‐
prepared MOFs was added into 50 ml of dyes aqueous
solution (10 mgL ) in an 80 ml jacketed cylindrical
Pyrex reaction vessel. Before exposed to sunlight, the mix-
ture was stirred in the dark for 3 hr to gain the
3
2
2
(H BDC), Piperazine, Formaldehyde, Rhodamine B
2
(RhB), Methylene blue (MB), H O2 (30%, v/v), HCl,
2
−
1
KOH and NaOH were purchased from Merck Company
and used without further purification. All the solutions
were prepared with deionized water.

FARROKHI ET AL.
3 of 11
adsorption/desorption equilibrium, desired volume of
known concentration of H O was then added and the
2
2
reaction vessel placed in direct solar radiation. The mix-
ture of the suspension was stirred evenly during the dyes
photodegradation reaction and total degradation reaction
time was 1 hr. 2 ml of samples were collected at ordered
intervals (10 mins) with centrifugation to separate solids
for measure.
Determination of the dye concentration in the reaction
mixture was carried out at maximum absorption wave-
length for RhB (560 nm) and MB (664 nm) by using a
Analytik Jena, SPECORD PLUS UV–vis spectrophotome-
ter. Terephthalic acid was used as a probe agent for detec-
•
tion of the hydroxyl radicals ( OH) generated in the
[28]
interaction of MOF catalyst and H O .
The processes
2
2
were similar to the measurement of catalytic experiments
in the presence of MOF and H O under sunlight irradia-
2
2
tion however the solution of dye was replaced by a solu-
tion of 0.01 mM sodium terephthalate (NaTA). Sampling
was performed with the start of photocatalytic reaction,
and the analysis was carried out at 315 nm of excitation
FIGURE 1 PXRD patterns of as‐prepared STA‐12(M
samples: simulated XRD pattern of STA‐12 [29] (a), (Fe, Mn) (b),
Mn, Co) (c), (Fe, Co) (d)
1 2
, M )
(
for 2‐hydroxy terephthalic acid wavelength by
RF‐5301PC Shimadzu fluorescence spectrophotometer.
a
3
3
| RESULTS AND DISCUSSION
.1 | Characterization of the metal–
organic frameworks
The solid state structures of the as‐synthesized samples
were inspected by powder X‐ray diffraction (PXRD).
Figure 1 shows PXRD patterns of the MOFs. Obviously,
the distinct and sharp peaks in the diffraction patterns
exhibited the well‐defined crystallinity of the MOFs and
are in good compliance with the simulated pattern of
[29]
STA‐12 MOFs.
The IR spectra of MOFs are quite similar and the
results were shown in Figure 2.
FIGURE 2 FTIR spectra of as‐prepared STA‐12(M
Fe, Mn) (a), (Mn, Co) (b), (Fe, Co) (c)
1 2
, M ) samples:
(
These spectra display a number of strong peaks
−
1
between 1126 and 970 cm correspond to the stretching
analysis and FTIR spectra evidently approve the forma-
tion of STA‐12 structures.
Thermogravimetery analysis of MOFs show water
loss below 150 °C, interpreted as due to physically and
chemically absorbed water (Figure S6). The total water
loss calculations for synthesized materials corresponding
vibrations of the phosphonate PO groups which are in
agreement with reported data for STA‐12 MOFs.
The existence of lattice water molecules interacting by
hydrogen bonding was confirmed via broad bands
3
[
30]
−
1
around 3400 cm
produced by the O‐H stretching
vibrations. Bending vibration of water molecules appear
to
chemical
formulas
of
FeMnL.4.14H O,
2
−
1
2−
at 1631 cm . δ (PO ) bands are found in MOFs at
FeCoL.3.77H O and MnCoL.5.67H O. The curves have
3
2
2
−
1
5
72 cm . These samples also exhibit C‐H stretching
a plateau in the 150–320 °C range before a progressive
weight loss due to structure decomposition. Combustion
of the ligand starts at 330 °C and continues up to
800 °C. At 800 °C the weight loss trace for MOFs has
vibrations and bending vibrations of CH attached to
2
1
−
nitrogen atoms, near 2960 and 1457 cm , respectively.
Therefore, the results of powder X‐ray diffraction

4
of 11
FARROKHI ET AL.
a slight weight rise, which can be attributed to the
formation of metal pyrophosphate (M P O ) via
3.2 | Photocatalytic properties of the
MOFs
2
2
7
incorporation of oxygen from the flowing air in the
[31]
experiment.
It is desirable to develop photocatalysts that would work
in the presence of natural sunlight as there are not many
The adsorptive properties of as‐prepared MOFs have
[
34]
been studied with N at 77 K. The samples in Figure S7
such catalysts.
Photodegradation of MB and RhB was
2
exhibited type IV isotherms, with H3 typical hysteresis
loop. The isotherms at high relative pressure (0.3 < P/
P0 < 1.0) demonstrated a high adsorption, indicating of
the occurrence of stacking macropores/mesopores in
carried out under irradiation by natural sunlight to eval-
uate the catalytic performance of synthesized MOFs. In
the presence of sunlight, the degradation of RhB appears
to be faster with STA‐12(Fe, Mn) than other synthesized
MOFs (Figure 3a). So, a number of experiments were
intended to elucidate the nature of photocatalytic degra-
dation reaction of RhB over STA‐12(Fe, Mn) (Figure 3b).
The adsorption experiment indicates that the dye
molecule could not be absorbed by STA‐12(Fe, Mn) and
the dye is rather stable under sunlight irradiation.
[32]
synthesized MOFs.
The corresponding textural param-
eters of the photocatalysts are presented in Table S2.
The diffuse reflectance UV–vis spectroscopy of the
MOFs samples was recorded (Figure S8). In the visible
region, catalysts exhibit many absorption bands, due to
ligand field transitions of M (II) ions, which are in agree-
[31]
ment with reported data for STA‐12.
absorption peaks were observed in the range of 200–
00 nm which can be attributed to charge transfer (LMCT)
The characteristic
Whereas the attendance of H O2 could increase the
2
degradation yield of RhB up to 13% by producing reactive
4
hydroxyl free radical owing to the possible H
O photoly-
2 2
of O (II) → M (II). Clearly, the beginning of the highest
sis prompted by incident light (H O + sunlight
2
2
•
¯ [35]
absorption edges of catalysts were located between 310–
→ OH + OH ). With STA‐12(Fe, Mn) alone under sun-
light irradiation, about 30% RhB removal was achieved,
signifying that the synthesized STA‐12(Fe, Mn) have a
moderate photocatalytic activity. By the addition of
[
33]
5
07 nm. According to the relation Eg = 1240/λ,
the
optical bandgaps of MOFs are calculated (Figure S9).
Obviously, the outstanding spectral properties in UV–vis
region of STA‐12(Fe, Mn) could provide an effective
photo‐activity.
H
O
2
, the degradation efficiency of dye over STA‐12(Fe,
2
Mn) under solar irradiation was significantly increased.
1 2
FIGURE 3 (a) Degradation of RhB with STA‐12(M , M ) MOFs. (b) Degradation of RhB under various conditions. (c) The UV–vis spectral
changes of RhB under degradation with STA‐12(Fe, Mn). (d) Comparison of the apparent reaction rate constants of the degradation of RhB
−
1
−1
2 2
under different Situations. Conditions: RhB, 10 mgL ; H O , 10.2 mM; Catalyst, 0.06 gL ; and initial pH = 3

FARROKHI ET AL.
5 of 11
After the addition of H O , the Fenton process could also
catalytic method. The enhanced photocatalytic activity of
STA‐12(Fe, Mn) system can be evidently verified by the
variation of the UV–vis spectra of the reaction mixture
during the dye degradation (Figure 3c). Also, the removal
of RhB approximately followed pseudo‐first‐order kinet-
2
2
promote the decomposition of RhB. So, we performed the
control experiment in the dark by mixture of H O and
2
2
STA‐12(Fe, Mn). As displayed in Figure 3b, about 6% of
RhB were degraded within 40 min, which indicates that
STA‐12(Fe, Mn) revealed ability to the activation of
H O and driving the Fenton degradation of RhB. Never-
ics, as demonstrated by the linear plot of ln(C /C) vs. pro-
0
cess time t (Figure 3d). The degradation reaction of RhB
over STA‐12(Fe, Mn) was more rapid with the existence
of both H O and sunlight (i.e., the STA‐12(Fe, Mn)/
2
2
theless, the efficiency in Fenton catalytic route was much
lower than photo‐Fenton STA‐12(Fe, Mn)/sunlight/H O
2
2
2 2
−
1
2 2
FIGURE 4 Effect of pH on the degradation of RhB and MB with STA‐12(Fe, Mn). Conditions: dye, 10 mgL ; H O , 10.2 mM and Catalyst,
−
.1 gL
1
0
−
1
2 2
FIGURE 5 Effect of H O concentration on the degradation of RhB and MB with STA‐12(Fe, Mn). Conditions: dye, 10 mgL ; pH, 3 and
−
1
Catalyst, 0.1 gL
2 2
FIGURE 6 Effect of initial dye concentration on the degradation of RhB and MB with STA‐12(Fe, Mn). Conditions: H O , 10 mM; pH, 3
−1
and Catalyst, 0.1 gL

6
of 11
FARROKHI ET AL.
sunlight/H O system), according to the pseudo‐first‐order
The influence of H O concentration on RhB and MB
2
2
2 2
−
2
−1
rate constants (kMSH = 6.29× 10 min for STA‐12(Fe,
degradations over STA‐12(Fe, Mn) /sunlight/H O system
2
2
−
3
−1
Mn) /sunlight/H O ; k
1
= 9.5× 10 min for STA‐
was examined and the results are revealed in Figure 5.
2
2
MS
−
3
−1
2(Fe, Mn) /sunlight; kMH = 1.54× 10 min for STA‐
1
2(Fe, Mn) /H O ). Besides, the RhB removal rate constant
2
2
in the sum of STA‐12(Fe, Mn) /sunlight and STA‐12(Fe,
−
2
−1
Mn) /H O catalytic systems (1.1× 10 min ) was much
2
2
lower than that of STA‐12(Fe, Mn) /sunlight/H O system
2
2
−
2
−1
(
6.29× 10 min ), which confirms that the synergetic
effect of the STA‐12(Fe, Mn) catalyst, H O and sunlight
2
2
in the photocatalytic procedure could provide collabora-
tively to the catalytic activity for the RhB decomposition.
Also, we calculate the synergetic index (SI): SI = kMSH
kMH kMS for the STA‐12(Fe, Mn)/sunlight/
H O system. The magnitude of SI was attained to be
/
(
+
)
2
2
5
.72, suggesting this synergistic effect could increase the
catalytic efficiency of STA‐12(Fe, Mn) by 472%.
For practical application goals, we investigated the
effect of various operating parameters such as H O2
2
dosage, initial pH, dyes concentration and the amount
of catalyst on the photocatalytic performance of STA‐
1
2(Fe, Mn)/sunlight/H O system for the degradation of
2 2
RhB and MB.
The effect of solution pH on the removal efficiency of
dyes over STA‐12(Fe, Mn)/sunlight/H O2 system was
2
examined at fixed amount of the catalyst as well as
constant concentrations of the dyes and H O at 30 °C.
2
2
As presented in Figure 4, the catalyst could work very
efficiently over acidic solution and the degradation rate of
dyes diminished with the increase of pH and a further
increase of pH until 10 stopped the reaction. This results
are similar to the Fenton reaction usually performed at
acidic condition. One probable reason is that for
photocatalysts the potential of the valence‐band hole
[36]
decreases when pH increases,
thus the oxidizing
ability of a hole became weaker, causing in the reduce
of the production rate of hydroxyl radical. Another cause
could be related to the instability and decomposition of
FIGURE 8 The photocatalytic degradation of RhB (a) and MB
(b) during four consecutive runs over STA‐12(Fe, Mn)/sunlight/
H O system
2 2
[37]
H O in basic solution to O and H O.
2
2
2
2
−
1
2 2
FIGURE 7 Effect of catalyst dosage on the degradation of RhB and MB with STA‐12(Fe, Mn). Conditions: dye, 10 mgL ; H O , 10 mM; pH, 3

FARROKHI ET AL.
7 of 11
When H O dosage increased from 6 to 10 mM, the
Figure 6, the catalytic degradation processes of RhB and
were more proficient at a lower dyes concentration. The
assumed reason is that with the increase in the dye con-
centration, more and more dye ions are covered the active
sites of catalyst, as a result, the generation of free radicals
2
2
degradation yields improved. This augmentation in
decomposition rate is owing to an enhancement in
hydroxyl radicals generated by photocatalytic interaction
•
of H O with the MOF. Meanwhile, OH radicals are very
2
2
[
40]
reactive and oxidizing agent, they can attack to the dye
molecules and cause the degradation of the original
solution. However, the rate of degradation reduced with
an increase in H O concentration from 10 to 14 mM.
on the surface is reduced.
Another reason maybe that the large molar extinction
coefficients of RhB and MB dyes produce more imperme-
able solution to irradiation, which decrease efficiencies of
light exploitation and results in the low rate of photocat-
alytic processes.
The influence of the amount of catalyst on the decom-
position of dyes was investigated while all other effective
parameters fixed. The results showed that the degrada-
tion rate enhanced with increasing the dosage of catalyst
(Figure 7). This may be understood in terms of exposing
the dye molecules and hydrogen peroxide to more
catalytic active sites encountered form STA‐12(Fe, Mn)
sample.
2
2
One explain can be that excess H O molecules act as
2
2
collector of hydroxyl radicals to produce less reactive
[38]
hydroperoxyl radicals, (Equation 1).
•
•
7
−1 −1
OH þ H2O2→H2O þ HO2 ; k1 ¼ 2:7 × 10 M S
(1)
This reaction decreased the possibility of attack of dye
•
molecules by OH radicals, and subsequently the
degradation rate of dye to drop. Also hydroxyl radicals
produced at higher concentration can be dimerize to
[39]
H O , (Equation 2).
2
2
2•
OH→H2O2; k2 ¼ 5:5 × 10 M S
9
−1 −1
(2)
3.3 | Recyclability and stability of STA‐12
(
Fe, Mn)
As a result, there is a critical hydrogen peroxide concen-
tration for the removal of dyes.
The Reusability of STA‐12(Fe, Mn) was tested by succes-
The effect of original dyes concentration on the
removal rates were also evaluated. As displayed in
sive reactions in the catalytic degradation of RhB and
MB over STA‐12(Fe, Mn)/sunlight/H O2 system. As
2
FIGURE 9 FTIR spectra (A) and PXRD
patterns (B) of STA‐12(Fe, Mn) before (a)
and after (b) catalytic reaction

8
of 11
FARROKHI ET AL.
shown in Figure 8, the catalytic degradation efficiency of
the catalyst remained unchanged after four reaction runs
for the degradation of dyes. It confirms that the catalyst is
highly stable and can be used for several degradation
reactions.
Furthermore, STA‐12(Fe, Mn) almost retained its
primary color after four catalytic cycles. Also, the XRD
pattern and FTIR spectrum of the reused catalyst were
found to be very similar to that of as‐prepared sample
(Figure 9), demonstrating that, the crystal and chemical
structure of the catalyst remain unchanged after four
repeated reactions.
To show the competency of this photocatalytic system,
the results obtained for degradation of MB in the pres-
ence of STA‐12(Fe, Mn) were compared with those
reported by other MOFs under sunlight irradiation
(Table 1). All parameters advocate that this photocatalytic
system is an efficient method for degradation of organic
dyes in comparison over others include high removal per-
centage, short reaction time, using gratis light source,
lower catalyst loading and conditions used.
3.4 | DETERMINATION OF HYDROXYL
RADICAL
To identify the reactive oxidizing species produced in the
catalyst STA‐12(Fe, Mn)/sunlight/H O system, sodium
2
2
terephthalic acid salt (NaTA) was used. Sodium
terephthalate could react with hydroxyl radical to give
2
‐hydroxy terephthalic acid (HTA), which shows a bril-
[41]
liant stable fluorescence.
by other reactive species such as H O , O and HO ,
This reaction is not affected
•¯
•
2
2
2
2
therefore it could be used as a selective probe in sensing
•
[42]
OH radicals.
As shown in Figure 10a, in the fluores-
FIGURE 10 •OH‐trapping PL spectra of STA‐12(Fe, Mn)/
sunlight/H O catalytic system, (a) in the absence of dye; (b) in
2 2
cence spectra of the solution containing the catalytic
STA‐12(Fe, Mn)/sunlight/H O system and NaTA, peak
2
2
the presence of dye and (c) effect of methanol inhibitor on the
degradation of RhB
intensity increases sharply to 930 within 10 min,
TABLE 1 The Comparison of photocatalytic activity of STA‐12(Fe, Mn) with other reported MOFs under sunlight irradiation
Dye/MOF
Light/Time for
MOFs
(AgL)(CF
H[Cu (μ ‐OH)
L (CrMo
[K(H O)
Conc. (mg/l)
degradation (min)
Removal (%)
Ref.
[23]
[
3
SO
3
)]n
‐
NS/7980
SS/780
SS/780
SS/150
SS/80
90
87
83
88
82
91
93
1
[48]
[48]
2
2
2
L (CrMo
6
(OH)
18)(H O)
[Cu (pzca)(H O)
6
O
18)]·4H
]·4H
(TeMo
2
O
‐
2
[
Cu
2
6
(OH)
6
O
2
4
2
O
‐
[
[
49]
50]
H
2
2
5
]
2
2
3
]
2
6
O
24
)
‐
UiO‐66(1.25Ti)
10 and 1000
3.7 and 100
10 and 140
.
[24]
[
(CH
3
)
2
NH
2
] [Cu
2
(CN)
3
]
NS/1440
STA‐12(Fe, Mn)
NS/40
This work
1
2
L = 2,3‐bis(6′‐methyl‐2,2′‐bipyridin‐6‐yl) pyrazine; L = N,N′‐bis(3‐pyridinecarboxamide)‐piperazine; L = N,N′‐bis(4‐pyridinecarboxamide)‐piperazine);
pzca = 2‐pyrazinecarboxylic acid; NS = natural sunlight; SS = simulated sunlight.

FARROKHI ET AL.
9 of 11
•
indicating that OH radicals were truly produced in the
system. However, when the dye and NaTA were concur-
rently added into the reaction, the fluorescence intensity
only increasing to 75 within 10 min (Figure 10b). An
Mn) can accelerate the decay of H O to generate hydroxyl
radicals by the Fenton‐like process (Equations 3–5).
2
2
[45]
•
−
MðIIÞ þ H2O2→MðIIIÞþ OH þ OH
(3)
(4)
(5)
•
explanation may be that part of OH radicals reacted
quickly with dyes, as the dye degradation was detected
during fluorescence measurement. Furthermore, the
inhibitory effect of methanol, which is an effective trap-
ping material for hydroxyl radicals,
Figure 10c shows the effect of high concentrations of
methanol in inhibition of the photo‐degradation reaction,
indicating that OH radicals are the main oxidizing
species during the procedure.
Generally, the metal nodes of MOFs can be regarded
as isolated semiconductor quantum dots, which can be
excited directly upon light irradiation or activated by
the organic linkers acting as the light absorption
MðIIIÞ þ H O →MðIIÞ þ HO þ Hþ
•
2
2
2
[41]
was measured.
•
þ
MðIIIÞ þ HO2 →MðIIÞ þ O2 þ H
•
From the other hand, H O could absorb the photoex-
2
2
cited electrons in the conduct band of STA‐12(Fe, Mn) to
produce hydroxyl radicals (Equations 6–8).
þ
−
MOF þ hν→h þ e
(6)
[43]
antenna.
Since the linker in STA‐12(Fe, Mn) don't
absorb any visible light, the fused M‐NO clusters in this
MOF are responsible directly for the sunlight absorption
5
−
•
−
H O þ e
2
2
→ OH þ OH
(7)
½MOFꢀ
and the related photocatalytic activity, as observed in
[
44]
reported iron‐based metal–organic frameworks.
According to this consideration, we suggested the
þ
•
þ
H O þ h
2
→ OH þ H
(8)
probable mechanism for the activation of H O2 with
½MOFꢀ
2
STA‐12(Fe, Mn) under solar irradiation. As shown in
Figure 11, the STA‐12(Fe, Mn) structure is constructed
from bisphosphonate anions and chains of M (II) octa-
hedra sharing oxygen atoms of phosphonate groups to
As a result, all of the above reactions could participate
collaboratively to the activation of H O and generate
more and more amounts of hydroxyl radicals, hence sig-
nificantly increasing the decomposition efficiency of dyes.
2
2
[25]
create a 3D porous framework.
II
Under natural sunlight irradiation, the M ‐centered
cluster in STA‐12(Fe, Mn) adsorb received photons and
responsible for the visible light photocatalytic activity. Sig-
nificantly, in the presence of H O , the positive synergistic
3.5 | Analyses of degradation products
The materials in solution after complete degradation of
2
2
properties could facilitate the catalytic efficiency of STA‐
2(Fe, Mn). From the one hand, M (II) of STA‐12(Fe,
RhB by the STA‐12(Fe, Mn) and H O were examined by
FTIR spectra and UV–vis spectroscopy. As shown in
2
2
1
FIGURE 11 Proposed mechanism for
the Fenton and photo‐Fenton activation of
H
2
O
2
by STA‐12(Fe, Mn) under natural
sunlight irradiation

10 of 11
FARROKHI ET AL.
FIGURE 12 IR spectra (a) of the
original RhB and (b) the degraded
products with STA‐12(Fe, Mn)/sunlight/
H O catalytic system
2 2
Figure 12 the FT‐IR spectra of RhB before and after degra-
dation are totally different. Before degradation, RhB
displayed the bands of the typical C=C aromatic ring
irradiation. The catalytic behavior was strongly depen-
dent on the various parameters, such as H O dosage,
2
2
pH of solution and initial dye concentration. In the pres-
ence of sunlight, the degradation of the dyes, RhB and
MB, appears to be faster with STA‐12(Fe, Mn) than other
synthesized MOFs. Considerably, degradation reaction of
RhB proceeded about 6.6 and 41.9‐fold faster in the STA‐
12(Fe, Mn)/sunlight/H O system relative to the STA‐
−
1
stretching vibrations at 1590 cm , C‐C stretching vibra-
−
1
tions at 1551 and 1481 cm , ring stretching vibrations at
−
1
−1
1
412 cm , C ‐N stretching vibrations at 1343 cm , and
Ar
−
[46]
N‐CH3 stretching vibrations at 1250 and 1180 cm 1.
After degradation process, the loss of the above individ-
ual bands identified complete decomposition of dye aro-
matic rings besides demethylation. At the same time, two
2
2
12(Fe, Mn)/sunlight and STA‐12(Fe, Mn)/H O systems,
2
2
respectively. This rapid catalytic reaction was considered
to be arise from the synergic effect of STA‐12(Fe, Mn)
and H O under solar radiation, which was confirmed
−
1
new multiple bands at 1612 and 1353 cm appeared,
which might be due to the stretching vibrations of COO¯
2
2
[46]
•
and NO ¯, respectively,
indicating that the CO and
by detection of hydroxyl radicals ( OH) in the degrada-
tion reaction. Additionally, STA‐12(Fe, Mn) demonstrates
excellent reusability and stability after cyclic usage. It is
3
2
some acids are the main final products. The complete oxi-
dative degradation of the dye led to the conversion of
organic carbon into gaseous CO which was trapped by
expected that this study could present
a
new
2
Ca (OH) solution and the precipitated CaCO was deter-
photo‐Fenton catalytic AOP process for environmental
protection and expands the application scope of MOFs
in photocatalysis, sunlight‐driven water remediation.
2
3
mined gravimetrically. Moreover, the TOC measurement
shows that about 93% of the dye was degraded into its min-
eral constituents after about 40 min. UV–vis spectrum of
RhB (Figure 3c) shows one absorbance band at 560 nm
and other band located at 520 nm. The conjugation system
in the structure of rhodamine B is responsible for the
absorption at 560 nm and shoulder at 520 nm is related
ACKNOWLEDGMENTS
The authors appreciate the financial support from
Research Council of the University of Birjand.
[47]
to the dye dimmer.
It was observed that the above
mentioned peaks disappeared very fast (Figure 3c). This
suggested a rapid destruction of the aromatic system and
consequently degradation of RhB.
CONFLICT OF INTEREST
The authors declare no conflict of interest.
ORCID
4
| CONCLUSIONS
Alireza Farrokhi https://orcid.org/0000-0003-2015-7841
A series of bimetallic phosphonate‐based MOFs, namely
STA‐12(M , M ) has been prepared successfully by a
straight hydrothermal method and utilized for activation
1
2
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