The efficient degradation of organic pollutants in an aqueous environment under visible light irradiation by persulfate catalytically activated with kaolin-Fe2O3

Article abstract of DOI:10.1039/c9ra09253f

In recent years, persulfate (PS) has been widely studied as a promising oxidant. In this work, a new K-Fe₂O₃ catalyst was synthesized via a facile impregnation method. K-Fe₂O₃ samples were utilized as heterogeneous photocatalysts for the degradation of aquatic organic pollutants (rhodamine, RhB, and ciprofloxacin, CIP). The catalysts showed excellent catalytic activity in the presence of PS under the irradiation of visible light, owing to the generation of SO₄^- and ·OH active radicals. The degradation ratio and COD removal ratio for RhB were 99.8% and 88.3%. More importantly, the system retained a high degradation activity for RhB within a wide operating pH range of 2.9-10. The results of cycling degradation experiments confirmed that the K-Fe₂O₃ catalyst was stable and recoverable. Large-scale experiments for treating dye wastewater under irradiation by natural sunlight were carried out, showing that this study can provide a new perspective for the treatment of wastewater.

Full text of DOI:10.1039/c9ra09253f

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The efficient degradation of organic pollutants in
an aqueous environment under visible light
irradiation by persulfate catalytically activated with
kaolin-Fe2O3
Cite this: RSC Adv., 2020, 10, 43
b
a
Qianqian He,a Chunsheng Xie,
Dexin Gan and Chun Xiaob
*
*
In recent years, persulfate (PS) has been widely studied as a promising oxidant. In this work, a new K-Fe2O3
catalyst was synthesized via facile impregnation method. K-Fe2O3 samples were utilized as
a
heterogeneous photocatalysts for the degradation of aquatic organic pollutants (rhodamine, RhB, and
ciprofloxacin, CIP). The catalysts showed excellent catalytic activity in the presence of PS under the
irradiation of visible light, owing to the generation of SO4cꢀ and $OH active radicals. The degradation
ratio and COD removal ratio for RhB were 99.8% and 88.3%. More importantly, the system retained
a high degradation activity for RhB within a wide operating pH range of 2.9–10. The results of cycling
degradation experiments confirmed that the K-Fe2O3 catalyst was stable and recoverable. Large-scale
experiments for treating dye wastewater under irradiation by natural sunlight were carried out, showing
that this study can provide a new perspective for the treatment of wastewater.
Received 7th November 2019
Accepted 9th December 2019
DOI: 10.1039/c9ra09253f
rsc.li/rsc-advances
SO4cꢀ + OHꢀ / SO4 + OHc
(3)
2ꢀ
1. Introduction
However, it is generally known that hydrogen peroxide needs
to be added continuously, and the pH requirements are strict
(the optimal pH range is only 2–4) for the traditional Fenton
system.14–16 As a comparison, PS is added to the system instead
of hydrogen peroxide, and PS can exist in a water environment
for a longer time.17 Generally speaking, the pH application
range is wider in the system of activation of persulfate. More-
As a new type of advanced oxidation process (AOP), the activa-
tion of persulfate (PS) has recently gained more interest, and it
has been used to treat dyes and toxic substances in wastewater
thanks to its easy operation, high reactivity and low cost.1–5
Heat,6 UV light irradiation,7 microwaves,8 and transition-metal
ions9 have been applied to enhance the production of
oxidizing species (sulfate radical, SO4cꢀ). Iron ions have widely
been used in the system of activation for persulfate due to their
non-toxicity and environmental friendliness. Therefore, the
addition of iron ions is most common during the activation
reaction, which is very similar to the Fenton reaction, so some
over, the reaction between S2O8 and Fe2+ is too rapid to
2ꢀ
control, which could lead to the result that excess ferrous ions
and SO4cꢀ in the solution react quickly within a short time.
Some of the SO4cꢀ radicals in the system are consumed, which
leads to a decrease in the utilization of SO4cꢀ (eqn (4)).18 Ferric
ions are prone to precipitation under alkaline conditions,
resulting in secondary pollution. In recent years, heterogeneous
catalysts have attracted the attention of researchers, and can
overcome the shortcomings mentioned above.19–21 Clays have
been used as catalyst carriers because of their high surface area
and rich reserves.22 A large number of studies have been carried
out to use modied clay as a photo-Fenton catalyst for the
degradation of pollutants.23,24 However, iron-pillared clay as
a heterogeneous catalyst activated S2O82ꢀ for the degradation of
organic dyes under visible light irradiation has hardly been
explored.
researchers also call it
a
Fenton-like reaction.10 Many
researchers have found that a system composed of ferric ions
and PS had high photocatalytic activity. The Fe2+/PS system can
catalyze PS to produce SO4cꢀ, and SO4cꢀ has a higher redox
potential than most other free radicals (E0 ¼ 2.5–3.1 V), as
shown in eqn (1).11 Then, OHc is formed aer further reaction
(eqn (2) and (3)).12 Therefore, the Fe2+/PS system exhibited
a capability for the oxidative removal of organic pollutants.13
2ꢀ
2ꢀ
S2O8 + Fe2+ / SO4 + SO4cꢀ + Fe3+
(1)
(2)
2ꢀ
SO4cꢀ + H2O / SO4 + OHc + H+
2ꢀ
SO4cꢀ + Fe2+ / SO4 + Fe3+
(4)
aSchool of Landscape Architecture and Art, Hunan Agricultural University, Hunan
410000, China. E-mail: dexingan@126.com; Tel: +86-13875991861
Herein, we successfully fabricated Fe2O3 pillared kaolin
photocatalysts, which displayed superior oxygen activation
bCollege of Environmental and Chemical Engineering, Zhaoqing University, Zhaoqing,
526061, China. E-mail: xiechsh@126.com; Tel: +86-13802754169
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ability for the degradation of rhodamine B (RhB) and antibiotic FTIR-8400, Japan) was used to detect the change in chemical
ciprooxacin (CIP) in the presence of PS. The optimal reaction bonds of the catalyst before and aer utilization.
conditions of the system were studied, including catalyst
dosage, PS concentration, RhB concentration, pH and inorganic
2.4. Degradation experiments
anions. The inhibition effects of inorganic anions on the system
The photocatalytic performance of the K-Fe2O3 catalyst was
were also observed. An environmental model application
evaluated by the degradation of RhB in water, using an LED
experiment using Fe2O3 pillared kaolin was carried out.
lamp (l ¼ 420 nm, 300 W) as light source. 0.5 g Lꢀ1 of the K-
Furthermore, a cycling experiment conrmed that the catalyst
Fe2O3 catalyst, 8 mM of K2O8S2 and 10 mg Lꢀ1 of RhB aqueous
has excellent stability. A reaction mechanism for the system was
solution (100 mL) were added to a 250 mL glass beaker under
proposed based on the experimental results of free radical
ꢁ
stirring at 25 C. 5 mL of the suspension was taken from the
capture.
tubes at predetermined time intervals and then centrifuged.
Lastly, the concentration of RhB was analyzed using a UV-vis
spectrophotometer (UV-1240, Shimadzu Corporation, Japan)
at the maximum absorption wavelength of 554 nm.
2. Experimental
2.1. Chemicals
Ferric nitrate (Fe(NO3)3$9H2O), sodium chloride (NaCl), sodium
3. Results and discussion
3.1. Characterization and properties of catalysts
carbonate (Na2CO3) hydrochloric acid (HCl), potassium per-
sulfate (K2O8S2), sodium hydroxide (NaOH), isopropanol (IPA,
C3H8O), methanol (MeOH, CH3OH), ethanol (C2H6O), p-ben-
zoquinone (BQ, C6H4O2), kaolin (Al2O3$2SiO2$2H2O), and
ciprooxacin (CIP, C17H19ClFN3O3) of analytical grade, were
purchased from Sinopharm Chemical Reagent Co. Ltd.
As shown in Fig. 1a, the phase and purity of the as-prepared
samples were investigated by XRD analysis. Most of character-
istic peaks agreed well with the standard card for kaolin (JCPDS:
25-0021). In addition, the diffraction peaks at 2q ¼ 33.1ꢁ and
Rhodamine B (RhB, C28H31ClN2O3), methyl orange (MO, C14
-
35.6ꢁ corresponded to the (104) and (110) planes of Fe2O3
(JCPDS: 33-0664), respectively. This implied that the introduc-
tion of Fe2O3 did not change the main structure of kaolin.
Moreover, the morphology of the catalyst can be clearly
observed by SEM analysis. Fig. 1b and d displays the SEM
images of kaolin and K-Fe2O3. We can see that all the catalyst
samples had a layered structure and a lot of amorphous Fe2O3
could be observed on the surface of the kaolin.25 Therefore, the
adsorption capacity of the modied catalyst has been greatly
improved.26 The results from XRD and SEM analyses of the
samples suggest that the K-Fe2O3 catalyst has been successfully
synthesized. Nitrogen adsorption–desorption isotherms of the
as-prepared K-Fe2O3 are shown in Fig. 1c. The inset of Fig. 1c
shows the pore size distribution of the samples. We can see that
the range of the main pore size distribution was 5–15 nm. The
BET surface areas of the kaolin and K-Fe2O3 catalyst were 11.818
and 27.715 m2 gꢀ1. Remarkably, the surface area substantially
increased aer pillaring, which was consistent with the result of
SEM.
H14N3NaO3S), and methylene blue (MB, C16H18N3S$Cl) were
obtained from the market. In all the experiments, all solutions
were prepared with deionized water.
2.2. Synthesis
In a typical procedure, rst, 8.05 g of Fe(NO3)3$9H2O and 1.06 g
of Na2CO3 respectively were added to 100 mL of water under
stirring, and the solutions were labelled A and B. Solution A was
added dropwise to solution B. Secondly, the mixed solution was
aged at room temperature for 24 h, and was labelled solution C
(brown). 100 mL of solution C was added dropwise to Na-
modied kaolin (a mixture of kaolin and 3 wt% NaCl) and
stirred continuously at 60 ꢁC for 2 h, and then allowed to stand
ꢁ
at 65 C for 12 h. Thirdly, the solid powder was washed with
water and ethanol three times each. Fourthly, the obtained dark
brown product was dried in an oven at 60 ꢁC for 6 hours. Finally,
the catalyst was further calcined at 400 ꢁC for 3 hours in air and
denoted as K-Fe2O3.
To further verify the chemical constitution of K-Fe2O3, the
EDX elemental mapping images and the HRTEM image of K-
Fe2O3 are shown in Fig. 2 and 3. As we know, there are only three
2.3. Characterization
The phase of the catalyst was determined by X-ray diffraction elements in kaolin: Si, Al and O (Al2O3$2SiO2). However,
(XRD, D8-Advance, Bruker, Germany) with a diffraction angle a homogeneous distribution of Fe, Si, Al and O can be found in
from 5ꢁ to 70ꢁ. The morphologies and microstructures of the the as-prepared samples. Moreover, many clear lattice fringes
samples were observed by scanning electron microscopy (SEM, are displayed in the HRTEM image of K-Fe2O3. The widths of
Zeiss Sigma) and high-resolution transmission electron these lattice fringe spacings were measured by using the cor-
microscopy (HRTEM, FEI Tecnai G2 f20 s-twin 200 kV). X-ray responding tools. Lattice fringe spacings of 0.25 nm correspond
photoelectron spectroscopy (XPS, K-Alpha model by Thermo to the (110) crystal plane of Fe2O3, which is consistent with the
Fisher Scientic) analysis was performed, and all binding result from XRD. These results indicate that the Fe2O3 particles
energies were calibrated with the binding energy of C 1s as were successfully incorporated into the inside of the kaolin.
reference. The specic surface areas of the samples were
XPS analysis was investigated to further study the chemical
measured with a gas sorption analyzer Autosorb-iQ instrument. composition of the catalyst. As presented in Fig. 4a, the XPS
Fourier transform infrared (FT-IR) spectroscopy (Shimadzu survey spectrum showed that O, Al, Si, and Fe elements were
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Fig. 1 (a) The XRD pattern of as-prepared K-Fe2O3, (c) N2 adsorption–desorption isotherms and the pore size distribution curve (inset) for K-
Fe2O3 and (b and d) SEM images of kaolin and K-Fe2O3.
present in K-Fe2O3 (the C element came from the carbon source appeared in the RhB aqueous solution aer extra addition of PS
in the instrument). Fig. 4b shows the XPS spectra of Fe 2p. The (curve VI). Moreover, in the RhB/K-Fe2O3/PS/visible light system
peaks appearing at 711 eV, 714.6, 724.6 and 727.5 eV correspond (curve VII), the degradation ratio of RhB increased to 99.8%.
to Fe 2p3/2 and Fe 2p1/2 in Fig. 4b, and are assignable to Fe3+
.
Next, in the RhB/K-Fe2O3/visible light system (curve IV), a 42.8%
The satellite peak at 718.8 eV is characteristic of Fe3+, suggest- degradation ratio of RhB was observed. The results of curve VII
ing that the Fe in the as-prepared samples is mostly Fe3+
.
and curve IV show that the addition of PS was crucial to
27
Moreover, the XPS spectrum of the Fe 2p peak for pure Fe2O3 is changing the photocatalytic activity of K-Fe2O3. Finally, as
shown in Fig. 4b, where it is found that the characteristic peaks shown in curve V, without the K-Fe2O3 catalyst, the degradation
of Fe 2p agree well with the K-Fe2O3 samples. The results of XPS ratio of RhB (54.7%) was slightly higher than that of curve IV.
2ꢀ
conrmed that Fe2O3 existed in the as-prepared samples.
This was because S2O8 could oxidize some RhB molecules
into degraded products under visible light irradiation. Fig. 5b
shows that the degradation curves of the different reaction
systems followed a rst-order kinetics model, and the reaction
kinetic constant (k) in curve VII was more than 3.5 times higher
than that of the other all curves. The results of the controlled
experiments showed that the catalyst, PS and visible light all
played an important role in the above system.
As shown in Fig. 5c, there was a signicant decrease in the
UV-vis absorption peak at 554 nm within 5 min aer turning on
the light. This result was in good agreement with degradation
activity curve VII. Interestingly, the maximum absorption
wavelength of RhB shied from 554 to 542 nm. Meanwhile, the
3.2. Degradation of organic pollutants by K-Fe2O3
3.2.1. Photo-Fenton degradation of RhB. The effects of
visible light and PS on the photocatalytic performance of K-
Fe2O3 under different comparative conditions were studied. The
results of the degradation of RhB are presented in Fig. 5a.
Firstly, in the RhB/visible light system (curve I), there was
almost no degradation effect. This shows that the photolysis
ability of RhB is very weak. In the RhB/K-Fe2O3/darkness system
(curve II), the degradation ratio of RhB was only 8% within
60 min, indicating that the K-Fe2O3 catalyst had a weak
adsorption capacity. More obvious discoloration phenomena
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Fig. 2 (a–e) EDX elemental mapping images of K-Fe2O3.
absorption wavelengths at 260, 302, and 352 nm disappeared visible light system is shown in Fig. 6a. When the dosages of the
aer the reaction. This might be because the dye molecule went catalyst were 0.1 g Lꢀ1, 0.25 g Lꢀ1, 0.5 g Lꢀ1 or 1.0 g Lꢀ1, the
through N-deethylation,28,29 indicating that the catalyst has degradation ratios of RhB were 83.2%, 88.8%, 99.5% and 100%
successfully destroyed the structure of the RhB molecules. In aer 60 min. An increase in the catalyst may increase the
order to further conrm the photocatalytic performance of the number of active sites in the reaction system by increasing the
K-Fe2O3 catalyst, we measured the change in chemical oxygen contact area between the catalyst and contaminant molecules.
demand (COD) value in aqueous solution during the degrada- This phenomenon was observed at the beginning of the
tion reaction, and the result is displayed in Fig. 5d. The value of degradation reaction. However, when the catalyst dosage
COD in the solution dropped from 69.8 mg Lꢀ1 to 8.2 mg Lꢀ1 exceeded a maximum value (0.5 g Lꢀ1), the degradation effect of
under optimal reaction conditions, and the degradation ratio RhB had not changed much, because the excessive catalyst
and COD removal ratio for RhB were 99.8% and 88.3%, blocked the visible light irradiation.30 Additionally, for recy-
respectively, exhibiting the excellent mineralization capacity of cling, the dosage of catalyst should not be too small. In this
Fe2O3.
3.2.2. Effect of PS concentration. The inuence of PS
study, the optimal catalyst dosage was 0.5 g Lꢀ1
3.2.4. Effect of RhB concentration. As we know, for the
.
concentration of the initial solution on the degradation of RhB same oxidation system, the degradation effect may vary greatly
in the K-Fe2O3/PS/visible light system is shown in Fig. 6b. It can with different concentrations of pollutant.31 The inuence of
obviously be observed from the gure that when the concen- the concentration of RhB in the initial solution on the degra-
tration of PS increased from 1 mmol Lꢀ1 to 4 mmol Lꢀ1, the dation of RhB in the K-Fe2O3/PS/visible light system is shown in
degradation ratio of RhB increased from 70.3% to 92.4% within Fig. 6c. We found that even if the concentration of the dye
60 min. But the initial PS concentration had a saturation value (10.0 mg Lꢀ1) was increased by 5 times (50.0 mg Lꢀ1), the
($8 mM), and although the concentration of PS continued to degradation ratio of RhB by the K-Fe2O3/PS/visible light system
increase, the photocatalytic activity of K-Fe2O3 remained almost was still over 98%. In contrast, other studies have reported
unchanged. In order to ensure a good degradation efficiency different experimental phenomena.32 With an increase in the
and save materials, in this study, the optimal initial PS concentration of RhB, the above system had a negative effect on
concentration was 1.0 mM.
the degradation of RhB. The results of this experiment
3.2.3. Effect of catalyst dosage. The inuence of the dosage demonstrate the strong oxidation performance of the K-Fe2O3
of K-Fe2O3 catalyst on the degradation of RhB in the K-Fe2O3/PS/ catalyst and the system could indiscriminately degrade
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degradation of RhB in the K-Fe2O3/PS/visible light system, as
shown in Fig. 6d, we adjusted the initial pH of RhB solution to
2.9–10.0 with NaOH/HCl. Interestingly, even when the initial pH
was 10.0, the degradation ratio of RhB still reached more than
95% within 60 min. This might be attributed to the electro-
negativity and strong adsorption of clay minerals.33 Without
adjusting the initial solution pH, the degradation ratio of RhB
was 98.8% (pH ¼ 6.3). At pH ¼ 2.9, the K-Fe2O3/PS/visible light
system showed the best degradation efficiency (100%). This was
because the Fe2+/PS system could produce active free radicals
under acidic, neutral or even alkaline conditions (eqn (2) and
(3)). The above results showed that this system could overcome
the shortcoming of pH value in dye wastewater treatment.34,35
3.2.6. Effect of inorganic anions. In practice, organic
wastewater contains a variety of complex components, which
can scavenge active radicals and reduce the degradation ratio of
pollutants.36 Typically, the effects of different inorganic anions
(Clꢀ, SO42ꢀ, CO32ꢀ, NO3ꢀ) on the degradation of RhB in the K-
Fe2O3/PS/visible light system were studied. From Fig. 7a, we can
see that 4 kinds of inorganic ions have certain negative effects
on the degradation reaction. Besides, it is shown that the
inhibition of RhB degradation by the K-Fe2O3/PS/visible light
2ꢀ
ꢀ
system followed the trend: SO4 < NO3 < Clꢀ < CO32ꢀ. The
main reason was that inorganic anions react with free radicals
(SO4ꢀc and OHc) to formꢀother products with weak oxidizing
properties (CO3ꢀc, Clc, Cl2 c, NO3c), as shown in eqn (5)–(9).37,38
2ꢀ
2ꢀ
SO4 and NO3 did not cause signicant inhibition of RhB
2ꢀ
degradation, which might be because SO4 did not react with
SO4cꢀ radicals and the reaction rate constant of eqn (9) was very
low.39 In order to further study the inuence of inorganic ions
on the degradation performance for the persulfate system, the
pH value of the solution aer adding different inorganic ions
was measured. We can see that the pH value of the other
systems was almost unchanged, except for the CO32ꢀ ion (pH ¼
9.4). Additionally, degradation experiments were carried out
under only PS and visible light irradiation, and the results are
shown in Fig. 7b. The degradation ratio of RhB in the system
Fig. 3 (a and b) TEM images and (c) a HRTEM image of K-Fe2O3.
different concentrations of RhB. This also showed that the
synthesized K-Fe2O3 catalyst has a good environmental appli-
cation performance under the activation of PS.
3.2.5. Effect of pH. Generally, the oxidation system did not
exhibit an excellent degradation effect in alkaline wastewater. In
order to explore the inuence of initial solution pH on the
Fig. 4 XPS spectra: (a) survey spectrum of K-Fe2O3 and (b) Fe 2p spectra of K-Fe2O3 and pure Fe2O3.
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Fig. 5 (a) Photocatalytic degradation activity curves for RhB under different conditions: (I) RhB/visible light; (II) RhB/K-Fe2O3/darkness; (III) RhB/
PS/darkness; (IV) RhB/K-Fe2O3/visible light; (V) RhB/PS/visible light; (VI) RhB/K-Fe2O3/PS/darkness, (VII) RhB/K-Fe2O3/PS/visible light, (b) kinetic
curves of RhB degradation under the above experimental conditions, (c) time-dependent UV-visible absorption spectra of RhB solution over K-
Fe2O3, and (d) the degradation ratio of RhB and the removal ratio of COD in the K-Fe2O3/PS/visible system. Initial conditions: CRhB ¼ 10.0 mg Lꢀ1
Wcatalyst ¼ 50 mg, CPS ¼ 8 mM, pH ¼ natural.
,
without any ions was the highest and the degradation trend of RhB in aqueous solution. At the same time, we wondered
the PS/visible light system is consistent with that of the K-Fe2O3/ whether the system could degrade other organic pollutants in
PS/visible light system, which further illustrated that SO4ꢀc the solution. The abuse of antibiotics has led to the increasingly
could react with inorganic ions. This result of this experiment serious pollution of the natural environment by antibiotics.40 In
can help environmental practitioners optimize organic this study, the degradation of a colorless antibiotic cipro-
pollutant removal efficiency by removing some of the anions.
oxacin (CIP) was carried out. Fig. 8a shows the UV-visible
absorption spectra changes of CIP solution (maximum
absorption wavelength ¼ 270 nm) over K-Fe2O3, indicating that
the degradation ratio of CIP reached 63% under optimal
experimental conditions within 30 min. In addition, we carried
out large-scale treatment experiments for dye wastewater
(location: 112.51ꢁ East, 23.11ꢁ North, experimental time: Sept.
12, 9:30 AM to 12:00 AM, 12:30 PM to 15:30 PM). As shown in
Fig. 8b, an obvious decolorization was observed in 20 L of RhB
dye wastewater aer 150 min. Furthermore, a kind of waste-
water with a complex composition, including three common
azo dyes Rhodamine B (RhB), methyl orange (MO) and methy-
lene blue (MB) was treated under the same conditions. K-Fe2O3
still exhibited high photocatalytic performance for a mixed
solution. These phenomena suggested that the K-Fe2O3/PS/
visible light system had general applicability for the degrada-
tion of organic pollutants.
SO4cꢀ + CO3 / SO4 + CO3ꢀc k ¼ 6.1 ꢂ 106 Mꢀ1
S
ꢀ1 (5)
2ꢀ
2ꢀ
OHc + CO3 / OHꢀ + CO3ꢀc k ¼ 3.9 ꢂ 108 Mꢀ1 Sꢀ1 (6)
2ꢀ
SO4cꢀ + Clꢀ / SO4 + Clc k ¼ 3.1 ꢂ 108 Mꢀ1 Sꢀ1
Clc + Clꢀ / Cl2ꢀc k ¼ 8.0 ꢂ 109 Mꢀ1 Sꢀ1
(7)
(8)
2ꢀ
ꢀ
2ꢀ
SO4cꢀ + NO3 / SO4 + NO3c k ¼ 2.0 ꢂ 102 Mꢀ1 Sꢀ1 (9)
3.3. Environmental application of kaolin-Fe2O3
Based on the above results, we found that the K-Fe2O3/PS/visible
light system exhibited an excellent performance in degrading
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Fig. 6 (a) The effect of catalyst dosage ([PS] ¼ 8 mM, pH ¼ 6.3, [RhB] ¼ 100 mL, 10 mg Lꢀ1), (b) the effect of PS concentration ([K-Fe2O3] ¼ 0.5 g
L
ꢀ1, pH ¼ 6.3, [RhB] ¼ 100 mL, 10 mg Lꢀ1), (c) the effect of RhB concentration ([K-Fe2O3] ¼ 0.5 g Lꢀ1, [PS] ¼ 8 mM, pH ¼ 6.3), and (d) the effect of
the initial pH on the degradation of RhB in the K-Fe2O3/PS/visible light system ([K-Fe2O3] ¼ 0.5 g Lꢀ1, [PS] ¼ 8 mM, [RhB] ¼ 100 mL, 10 mg Lꢀ1).
ratios of RhB were 99% on the rst run and 96% on the sixth
run, which showed that the K-Fe2O3 catalyst was stable during
the degradation process. The slight change in the degradation
3.4. Stability of K-Fe2O3
We explored the stability of the K-Fe2O3 through cycling
degradation experiments. As shown in Fig. 9a, the degradation
Fig. 7 (a) The effects of different inorganic anions on the degradation of RhB in the K-Fe2O3/PS/visible light system ([K-Fe2O3] ¼ 0.5 g Lꢀ1, [PS] ¼
8 mM, pH ¼ 6.3, [RhB] ¼ 100 mL, 10 mg Lꢀ1), and (b) the effects of different inorganic anions on the degradation of RhB in the PS/visible light
system ([PS] ¼ 8 mM, [RhB] ¼ 100 mL, 10 mg Lꢀ1).
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Fig. 8 (a) Time-dependent UV-visible absorption spectra of CIP solution over K-Fe2O3, and (b) a large-scale photodegradation system under
natural sunlight irradiation with K-Fe2O3 catalyst ([K-Fe2O3] ¼ 0.5 g Lꢀ1, [PS] ¼ 8 mM, [CIP] ¼ 100 mL, 10 mg Lꢀ1, pH ¼ natural).
ratio of RhB may be due to the loss of catalyst in the recovery patterns of fresh K-Fe2O3 and used K-Fe2O3 are displayed in
process.41 Additionally, in order to study the phase change of Fig. 9b. No marked shi in the main characteristic bands at
the catalyst during the degradation reaction process, the FT-IR 1057, 582, and 482 cmꢀ1 was observed. Besides, the
Fig. 9 (a) The recycling performance of the K-Fe2O3 catalyst for the degradation of RhB in aqueous solution, (b) the FT-IR patterns of pure RhB
powder and the catalyst after use for 0 min, 60 min, and 300 min, (c) the XPS pattern of the catalyst after use for 0 min and 300 min, and (d) the
XRD pattern of the catalyst after use for 0 min and 300 min.
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Fig. 11 The proposed mechanism for the degradation of organic
pollutants in the Fe2O3-pillared kaolin/PS/visible light system.
Fig. 11, rstly, the surface of the K-Fe2O3 catalysts produced
induced electrons and photo-induced holes under visible light
irradiation (eqn (10)). Secondly, the aggregated electrons reac-
Fig. 10 Photocatalytic activities of the K-Fe2O3 catalyst for the
degradation of RhB with different scavengers ([RhB] ¼ 10 mg Lꢀ1, pH ¼
initial value, [K-Fe2O3] ¼ 0.5 g Lꢀ1, [PS] ¼ 8 mM).
ted with S2O8 to produce SO4cꢀ and ferric ions in aqueous
2ꢀ
solution activated S2O8 to produce SO4cꢀ (eqn (11) and (12)).
2ꢀ
Thirdly, SO4cꢀ could react with H2O to produce OHc radicals
(eqn (13)) which further accelerate the degradation reaction rate
and improve the catalytic performance of the system. Finally,
the produced radicals with strong oxidizing properties would
attack organic pollutants to degrade it and then produce CO2
and H2O (eqn (14)).46
characteristic bands of pure RhB (3407 cmꢀ1) were not found in
the spectra of the used K-Fe2O3 catalyst, indicating that the dyes
were indeed degraded rather than adsorbed.42 Moreover, the
XPS and XRD patterns of the K-Fe2O3 catalyst aer using for
0 min and 300 min are displayed in Fig. 9c and d. The Fe 2p
peak of the used catalyst shied slightly toward the low binding
energy direction, indicating that part of the Fe species on the
surface of the K-Fe2O3 catalyst was converted to Fe(II) during the
degradation reaction. This further proved that the Fe species on
the K-Fe2O3 surface are the main active sites for the K-Fe2O3/PS/
visible light system.43 As shown in Fig. 9d, the main peaks of the
XRD pattern of the catalyst did not shi aer use. The above
results conrmed that the K-Fe2O3 catalyst has excellent
stability.
Photocatalyst + hn / eꢀ + h+
eꢀ + S2O8 / SO4 + SO4cꢀ
(10)
(11)
(12)
(13)
(14)
2ꢀ
2ꢀ
S2O8 + Fe2+ / SO4 + SO4cꢀ + Fe3+
2ꢀ
2ꢀ
2ꢀ
SO4cꢀ + H2O / SO4 + OHc + H+
$OH/SO4cꢀ + organic pollutants / CO2 + H2O
3.5. Reaction mechanism
As can be seen in Fig. 10, without using any free radical scav-
enger, a 99.8% degradation ratio was reached in the control
group. When IPA ($OH radical scavenger), BQ (O2cꢀ radical
4. Conclusions
scavenger) and MeOH (SO4cꢀ radical scavenger) were added into A K-Fe2O3 photocatalyst was prepared by a facile method. In a K-
the reaction solution during the RhB degradation, the degra- Fe2O3/PS/visible light system, a 99.8% degradation ratio of RhB
dation ratios of RhB dropped to 69.9%, 82.6% and 92.4% within was reached under optimal experimental conditions. The
60 min, respectively. On the one hand, the addition of MeOH effects of catalyst dosage, PS concentration, RhB concentration
greatly inhibited the degradation reaction, and this also showed and solution pH on the catalytic activity were researched. Most
that SO4cꢀ was the major active species in the K-Fe2O3/PS/visible interestingly, we found that K-Fe2O3 still showed high degra-
light system. On the other hand, the $OH radical also made dation activity for RhB over a wide range of pH (2.9–10). The
a certain contribution to degrading RhB in the solution. Similar practical application of the K-Fe2O3/PS/visible light system for
phenomena have been reported by other researchers, such as real dye wastewater remediation (MO, MB, RhB) was also
Guo et al.44 and Hu et al.45 The above results indicated that the shown. On the other hand, the inhibition effects of inorganic
K-Fe2O3/PS/visible light Fenton-like system can produce two anions on the system were observed. Furthermore, ve cyclic
active radicals, SO4cꢀ and $OH, and the organic pollutants were runs showed that the catalyst had good stability and its chem-
efficiently removed through the common oxidation of the two ical structure remained unchanged. Trapping experiments
radicals.
conrmed that SO4cꢀ and $OH were the main active radicals in
A possible degradation mechanism of organic pollutants in the organic pollutant photodegradation process. Finally, we
the K-Fe2O3/PS/visible light system is proposed. As displayed in proposed a schematic mechanism for the degradation of
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Paper
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This work was supported by Innovation Projects of Colleges and
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Zhaoqing City Science and Technology Innovation Guidance
Project (2017S002) and the Innovative Entrepreneurship Project
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