A facile heterogeneous system for persulfate activation by CuFe2O4 under LED light irradiation

Article abstract of DOI:10.1039/c9ra05574f

In this study, the removal performance for rhodamine B (RB) by persulfate (PS) activated by the CuFe₂O₄ catalyst in a heterogeneous catalytic system under LED light irradiation was investigated. The effect of vital experimental factors, including initial solution pH, CuFe₂O₄ dosage, PS concentration, co-existing anion and initial RB concentration on the removal of RB was systematically studied. The removal of RB was in accordance with the pseudo first-order reaction kinetics. Over 96% of 20 mg L^-1 RB was removed in 60 min using 0.5 g L^-1 CuFe₂O₄ catalyst and 0.2 mM PS at neutral pH. In addition, free radical quenching experiments and electron spin resonance (EPR) experiments were performed, which demonstrated the dominant role of sulfate radical, photogenerated holes and superoxide radical in the CuFe₂O₄/PS/LED system. The morphology and physicochemical properties of the catalyst were characterized by XRD, SEM-EDS, TEM, N₂ adsorption-desorption isotherm, UV-vis DRS, and XPS measurements. Moreover, 18.23% and 38.79% total organic carbon (TOC) removal efficiency was reached in 30 min and 60 min, respectively. The catalyst revealed good performance during the reusability experiments with limited iron and copper leaching. Eventually, the major intermediates in the reaction were detected by GC/MS, and the possible photocatalytic pathway for the degradation of RB in the CuFe₂O₄/PS/LED system was proposed. The results suggest that the CuFe₂O₄/PS/LED system has good application for further wastewater treatment.

Full text of DOI:10.1039/c9ra05574f

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A facile heterogeneous system for persulfate
activation by CuFe2O4 under LED light irradiation†
Cite this: RSC Adv., 2019, 9, 32328
*
Xin Zhong,
Xiao-Yu Ye, Di Wu, Kai-Xin Zhang and Wei Huang
In this study, the removal performance for rhodamine B (RB) by persulfate (PS) activated by the CuFe2O4
catalyst in a heterogeneous catalytic system under LED light irradiation was investigated. The effect of
vital experimental factors, including initial solution pH, CuFe2O4 dosage, PS concentration, co-existing
anion and initial RB concentration on the removal of RB was systematically studied. The removal of RB
was in accordance with the pseudo first-order reaction kinetics. Over 96% of 20 mg Lꢀ1 RB was
removed in 60 min using 0.5 g Lꢀ1 CuFe2O4 catalyst and 0.2 mM PS at neutral pH. In addition, free
radical quenching experiments and electron spin resonance (EPR) experiments were performed, which
demonstrated the dominant role of sulfate radical, photogenerated holes and superoxide radical in the
CuFe2O4/PS/LED system. The morphology and physicochemical properties of the catalyst were
characterized by XRD, SEM-EDS, TEM, N2 adsorption–desorption isotherm, UV-vis DRS, and XPS
measurements. Moreover, 18.23% and 38.79% total organic carbon (TOC) removal efficiency was
reached in 30 min and 60 min, respectively. The catalyst revealed good performance during the
reusability experiments with limited iron and copper leaching. Eventually, the major intermediates in the
reaction were detected by GC/MS, and the possible photocatalytic pathway for the degradation of RB in
the CuFe2O4/PS/LED system was proposed. The results suggest that the CuFe2O4/PS/LED system has
good application for further wastewater treatment.
Received 19th July 2019
Accepted 23rd September 2019
DOI: 10.1039/c9ra05574f
rsc.li/rsc-advances
Among the activation metal catalysts, iron-based catalysts
are widely utilized for the activation of persulfate due to the low
1. Introduction
cost, non-toxicity and natural existence of iron in the environ-
ment;20–22 metal spinel ferrite is one type of promising hetero-
geneous catalyst, which presents good activation of persulfate
and is easily separated from water due to its spinel structure
and magnetic properties. Moreover, it has been reported that
the application of metal spinel ferrite with the addition of
persulfate in heterogeneous systems under visible light signif-
icantly enhances the degradation efficiency of contami-
nants.23–25 Therefore, the development of powerful, economic
and environmentally friendly visible light-driven catalysts
remains important.
Copper ferrite (CuFe2O4) has attracted signicant attention
as a heterogeneous catalyst in the activation of persulfate due to
the transition of Cu(I)/Cu(II) and Fe(II)/Fe(III) on the catalyst
surface. On the other hand, copper ferrite is recognized as
a potential photocatalyst for the activation of persulfate, with
a narrow band gap ranging between 1.9 and 2.3 eV, which has
shown notable photocatalytic activities in catalyst/PS or catalyst/
H2O2 systems under light irradiation.26–28 In the presence of
light irradiation, the efficiency is improved via the reduction of
transition metals, photogenerated holes and electrons on the
CuFe2O4 surface, facilitating the formation of reactive free
radicals in the reaction. Consequently, it is reasonable to
assume that the degradation efficiency and rate constant in the
During the past few decades, advanced oxidation processes
(AOPs) have attracted signicant attention, and many efforts
have been made to enhance the degradation efficiency of
materials to eliminate bio-refractory organic pollutants.1
Notably, the use of sulfate radical ($SO4ꢀ)-based AOPs is an
emerging strategy that has obtained widespread application in
the mineralization of organic contaminants such as dyes,2–4
antibiotics,5–7 and phenolic compounds.8–10 Compared to
hydroxyl radicals, sulfate radicals possess higher standard
reduction potential and longer half-life time in solution, which
can result in more efficient degradation over a wide pH range
(2–11).11–13 Sulfate radicals can be simulated by the activation of
persulfate using various approaches, heat,14 UV light,15 and
transition metals.16–18 However, owing to the restricted pH range
(almost around 3.0) and secondary sludge pollution in homo-
geneous systems, heterogeneous catalysts have been developed
as alternatives, exhibiting high activation of persulfate and
minimizing the leaching of metal ions.19
Department of Environment Science and Engineering, Beijing Normal University,
Zhuhai, 519000, China. E-mail: zhongxin@whu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra05574f
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CuFe2O4/PS system will be accelerated under visible light
irradiation.
Powder X-ray diffraction (XRD) pattern analysis was per-
formed using a Quantachrome/NOVA 2000X-ray diffractometer
To date, the degradation mechanism of the heterogeneous coupled with graphite monochromatic Cu Ka radiation (l ¼
˚
persulfate system under visible light irradiation still needs 1.54 A) at an accelerating voltage of 40 kV and current of 30 mA
further elucidation. The combination of photocatalysis and in the 2q range of 10–80ꢁ. Photoelectron spectroscopy (XPS)
sulfate radical-based AOPs will not only effectively minimize the analysis was performed using an X-ray photoelectron (Thermo
recombination chance between photogenerated holes and Fisher ESCALAB250Xi) spectrometer with a monochromatized
electrons, but also increase the number of reactive oxidant Al–K X-ray source. UV-vis spectra were obtained on a Shimadzu
species, leading to a higher degradation efficiency. The work on UV-3600 Plus instrument. The morphology and chemical
CuFe2O4-activated persulfate under light emitting diode (LED) composition of the catalyst were characterized using a SIGMA
light is scarce, which can be considered a promising alternative 500/VP (ZEISS) scanning electron microscope (SEM) equipped
to the conventional UV light source due to its high conversion with an energy dispersive spectrometer (EDS). High-resolution
efficiency, narrow band emission and long usage time. More- transmission electron microscopy (HR-TEM) images were ob-
over, to the best of our knowledge, few works have focused on tained on a JEM-2100F (Japan) transmission electron micro-
the application of CuFe2O4 for the degradation of organic scope. N2 adsorption–desorption analysis and pore size
pollutants and activation of persulfate under LED light distribution were measured on a TriStar II 3020 instrument.
irradiation.
Magnetic curve measurements were performed with a SQUID-
In this work, visible LED light was introduced in the CuFe2O4/ VSM instrument at room temperature.
PS system to improve its catalytic activity for the removal of RB. RB
was chosen as the target compound due to its toxicity and wide
application in numerous industries.29 Firstly, it is important to
demonstrate that the CuFe2O4/PS/LED system is efficient and
environmentally friendly to remove RB in solution. The CuFe2O4
catalyst was synthesized through the co-precipitation method. The
key parameters for the removal of RB were investigated, such as
catalyst dosage, persulfate dosage, initial pH, co-existing anions,
and initial RB concentration. Secondly, as previous reported,
sulfate and hydroxyl radicals play a unique role in heterogeneous
sulfate-based systems.30–32 However, in photocatalytic reactions,
photogenerated holes and electrons also dominate the reaction.
Therefore, the possible catalytic mechanisms for the activation of
persulfate by the CuFe2O4 catalyst under LED irradiation were
proposed based on various characterization analysis, radical
quenching experiments and EPR tests. By using GC/MS tech-
nology to identify the intermediates in the reaction, the proposed
RB degradation pathway was investigated. This study provides
a fundamental understanding and support for the application of
CuFe2O4/PS/LED systems in wastewater treatment.
2.2 Photocatalytic activity experiments
Batch experiments were carried out in a 50 mL cylindrical
reactor at room temperature. An LED lamp (30 W, 460 nm, Xujia
Company, China, lamp spectrum showed in Fig. S2†) was used
in all experiments. The initial pH was adjusted using 0.1 M
H2SO4 or 0.1 M NaOH. In each stage, a certain amount of
catalyst was suspended in 50 mL RB (Sinopharm Chemical
Reagent Co., Ltd, China) solution for 30 min and stirred to
achieve absorption equilibrium in the dark. Then, a certain
amount of persulfate (DaMao Company, China) was added and
the LED lamp (the experimental set-up is shown in Fig. S1†) was
turned on, which was warmed up for 30 min to reach a steady
irradiation efficiency. 1 mL aliquots were regularly withdrawn
and ltered through a 0.22 mm membrane, which were
quenched with methanol before analysis. The experiments were
repeated three times and mean values were used, which had
a standard deviation of no more than 3%.
2.3 Analysis methods
2. Experimental
2.1 Synthesis and characterization of CuFe2O4 catalysts
The concentration of RB was measured by the absorbance in the
UV-visible spectrum at the characteristic wavelength of RB (lmax
¼ 554 nm) using a UV-vis spectrophotometer (UV3600 II,
Shanghai, China). The metal leaching measurements were per-
formed using an ICP-MS (Agilent 7000). The mineralization of RB
was performed using a total organic carbon (TOC) analyser
(Elementar Vario) aer the sample was quenched by methanol.
The pH was monitored with a pH meter (Shanghai LeiCi PHS-25)
equipped with a pH electrode. Electron spin resonance (ESR, JES
FA200, JEOL) was used to measure the intensity of free radicals in
methanol and deionized water. The degradation products were
detected by GC/MS, using an Agilent 7890B gas chromatograph
equipped with an Agilent 7000C mass spectroscopy instrument.
All chemicals and reagents were of analytic grade and used
without further purication. Magnetic CuFe2O4 was prepared
via the co-precipitation method.
A certain amount of
Cu(NO3)2$6H2O and Fe(NO3)3$9H2O (Sinopharm Chemical
Reagent Co., Ltd, China) were dissolved in 10 mL deionized
water, where the molar ratio of Cu to Fe reached to 1 : 2. The
mixed solution was added to the NaOH (4 M) solution under
vigorously stirred for 2 h. The dark mixture was transferredꢁto
a 50 mL Teon-lined stainless autoclave and heated at 160 C
for 24 h. Then the autoclave was cooled to room temperature,
and the suspension was washed with deionized water and
ethanol, respectively. Finally, the solid was dried at 60 ꢁC for
10 h. Simultaneously, CuO and Fe2O3 catalysts were also
Decolorization efficiency (%) ¼ (1ꢀC/C0) ꢂ 100%
(1)
synthesized via a similar procedure without the addition of iron where C represents the concentration of RB at time t and C0 the
nitrate or copper nitrate.
initial RB concentration.
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synthesized to evaluate the activating performance of PS. The
CuO and Fe2O3 catalysts coupled with PS under LED irradiation
slightly enhanced the degradation efficiency, yielding an RB
removal of 35.4% and 41.1%, respectively. During the process,
the persulfate oxidants could react with Fe and Cu repeatedly
due to the transition of Fe and Cu on the CuFe2O4 surface. The
kinetics for the removal of RB tted well with the pseudo-rst-
order reaction, and the rate constants were determined to be
0.0057, 0.007 and 0.0525 minꢀ1 in the CuFe2O4/PS, CuFe2O4/
LED and CuFe2O4/PS/LED systems, respectively. The addition of
LED light to the catalyst/PS systems enhanced the RB removal.
Consequently, the catalyst/PS combined with LED light showed
the best removal efficiency for RB under certain conditions.
3. Results and discussion
3.1 Removal of RB in different systems
Batch-controlled experiments were performed to evaluate the
removal efficiency of RB in different systems. As shown in Fig. 1,
more than 96% removal of RB was achieved in 60 min by the
CuFe2O4/PS/LED system. The adsorption of RB on the CuFe2O4
catalyst and use of LED light alone resulted in limited removal
efficiency under these experimental conditions, indicating the
RB molecule was stable under visible LED light irradiation. As
shown in Fig. S6,† the specic area of the synthesized CuFe2O4
was 12.1 m2 gꢀ1, with a pore diameter of 6.8 nm, which is
responsible for the limited adsorption ability. Negligible colour
removal was obtained using persulfate alone due to the limited
persulfate decomposition without activation. The combination
of persulfate and LED light resulted in 10.11% removal of RB,
which indicates persulfate can hardly be activated to produce
free radicals under LED light. However, the CuFe2O4/PS system
exhibited 29.17% removal of RB, implying the persulfate can be
activated by CuFe2O4 to obtain free radicals (eqn. (2) and (3)).
The oxidation of RB was not completed under the LED/CuFe2O4
system, which resulted in 34.16% removal efficiency owing to
the photogenerated holes and electrons produced in the reac-
tion with sequenced reactive oxidant free radicals (eqn. (4)–
(6)).33–35 The generated reactive oxidant species reacted with RB,
leading to the degradation process.
3.2 Effect of parameters on catalytic performance
To obtain the optimized reaction parameters, different opera-
tion parameters were carried out. The removal kinetics of RB
were demonstrated by the pseudo-rst-order reaction.
ln C/C0 ¼ ꢀk ꢂ t
(7)
where k is the pseudo-rst-order rate constant, C is the
concentration of RB at time t, and C0 is the initial RB
concentration.
3.2.1 The effect of catalyst dosage. The catalyst dosage was
investigated in the range of 0.1–1.0 g Lꢀ1 with the concentration
of RB xed at 20 mg Lꢀ1 and persulfate xed at 0.2 mM. The
results are depicted in Fig. 2(a). An increase in catalyst dosage
resulted in an increase in the removal efficiency to > 99% in
2ꢀ
ꢀ
2ꢀ
S2O8 + Fe(II) / $SO4 + SO4 + Fe(III)
(2)
(3)
(4)
(5)
(6)
2ꢀ
ꢀ
2ꢀ
S2O8 + Cu(I) / $SO4 + SO4 + Cu(II)
CuFe2O4 + LED light / h+ + eꢀ
h++ H2O / H+ + $OH
60 min. The rate constant k varied from 0.071 to 0.0747 minꢀ1
,
which can be attributed to (i) the increase in the amount of
reactive sites provided more surface area and (ii) the increase in
available active site generated more active free radicals.36
However, a negligible improvement in removal efficiency was
obtained when the catalyst dosage was higher than 0.7 g Lꢀ1
since the rate constant slightly decreased from 0.0747 to
0.0675 minꢀ1. With the overdosed catalyst, the number of free
radicals increased, and the extra free radicals could not be
efficiently utilized and tended to recombination (eqn. (8) and
(9)). Moreover, the excess catalyst would hinder light scattering,
leading to a reduction in photons.
ꢀ
eꢀ + O2 / $O2
As expected, an obvious improvement in the rate constant
was obtained upon the combination of CuFe2O4 with persulfate
under visible LED light irradiation. To evaluate the synergistic
effect of the CuFe2O4 catalyst, CuO and Fe2O3 catalysts were
ꢀ
ꢀ
2ꢀ
$SO4 + $SO4 / 2SO4
(8)
(9)
ꢀ
2ꢀ
$SO4 + Fe(II) + eꢀ / Fe(III) + SO4
Consequently, the high catalyst dosage decayed the removal
performance.37 Finally, 0.5 g Lꢀ1 was chosen as the optimal
dosage for the subsequent experiments.
3.2.2 Effect of initial pH. Since the solution pH is an
essential factor in aqueous reactions, the removal of RB under
different pH values in the CuFe2O4/PS/LED systems was inves-
tigated. Fig. 2(b) shows the RB decolorization when the initial
pH values were set at 3.0, 5.0, 7.0, 9.0 and 11.0. When the initial
pH decreased from pH ¼ 7.0 to acidic pH conditions (pH ¼ 3.0),
the decolorization varied slightly from 97.17% to 90.43% since
Fig. 1 Removal of RB in the different systems and the inset shows the
kinetic curves (C0 ¼ 20 mg Lꢀ1, [catalyst] ¼ 0.5 g Lꢀ1, [PS] ¼ 0.2 mM,
neutral pH).
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of the sulfate radicals, leading to a decrease in efficiency under
basic conditions. Meanwhile, the number of hydroxyl groups
(-OH) on the CuFe2O4 surface would decrease the oxidation
potential and catalytic activity at A high pH value. Thus, the
results demonstrate that the CuFe2O4 catalyst can be applied in
a wide range of pH values for the activation of persulfate under
LED light irradiation compared to the conventional sulfate
radical-based systems, which exhibit a better performance
under acidic conditions.
ꢀ
2ꢀ
$SO4 + OHꢀ / SO4 + $OH
(10)
(11)
(12)
ꢀ
ꢀ
$SO4 + $OH / HSO4 + 0.5O2
2ꢀ
ꢀ
S2O8 + H2O / 2HSO4 + 0.5O2
3.2.3 Effect of persulfate and RB concentration. Fig. 2(c)
shows the effect of persulfate concentration on the removal of
RB in the range of 0.1–1 mM. When the persulfate concentra-
tion increased from 0.1 mM to 0.2 mM, the removal efficiency
increased from 77.8% to 96.75% and the reaction rate constant
improved from 0.0238 to 0.0525 minꢀ1 during 60 min reaction.
The persulfate plays the role of an oxidant in this process, and
with an increase in oxidant, the generation of free reactive
radicals will be accelerated, leading to a higher RB removal
efficiency. However, a further increase in persulfate concentra-
tion to 1 mM did not result in a higher removal efficiency. This
was caused by the side reactions among the persulfate and free
radicals, which would be dominant when excess persulfate is
present. The free radicals would prefer to react with the extra PS
than RB, leading to the consumption of free radicals and
ꢀ
40
generation of weak reactive species, i.e.$S2O8
.
Finally, the
concentration of persulfate was chosen to be 0.2 mM for the
following experiments owing to the economical consideration.
2ꢀ
ꢀ
ꢀ
2ꢀ
S2O8 + $SO4 / $S2O8 + SO4
(13)
(14)
(15)
(16)
(17)
S2O8 + $OH / $S2O8 + OHꢀ
$OH + $OH / H2O2
2ꢀ
ꢀ
Fig. 2 Effect of the reaction conditions on the removal of RB: (a)
S2O8 + eꢀ / $SO4 + SO4
2ꢀ
ꢀ
2ꢀ
catalyst dosage, (b) initial pH, (c) persulfate concentration, and (d) initial
RB concentration ([catalyst] ¼ 0.5 g Lꢀ1, [PS] ¼ 0.2 mM, C0
¼
20 mg Lꢀ1).
Fe(III) + Cu(I) / Fe(II) + Cu(II)
The effect of initial RB concentration on the process
performance was studied in Fig. 2(d). It can be seen that with an
kobs changed from 0.0626 to 0.0337 minꢀ1. The decolorization
performance was hindered under basic conditions (pH ¼ 11.0),
which can be explained by two possible mechanisms. Persulfate
is considered be more stable under highly acidic conditions
rather than decomposed to sulfate radicals, thus delaying the
decolorization.38 On the other hand, persulfate may go through
side reactions and decompose to hydroxyl radicals instead of
sulfate radicals (eqn. (10)–(12)).39 The generated free radicals
are assumed to play a unique role in the removal of RB and
maintain a high removal rate. In addition, the oxidation
capacity of the generated hydroxyl radicals is weaker than that
increase in the initial RB concentration from 10 to 50 mg Lꢀ1
,
the RB removal decreased from 100% to 81.6% in 30 min, the
corresponding removal rate constant dropped from 0.119 to
0.03 minꢀ1. This may be because the number of generated free
radicals was constant with a xed CuFe2O4 catalyst and per-
sulfate concentration. As a result, the increase in RB concen-
tration resulted in a relative reduction in reactive species
towards RB molecules. Moreover, the high concentration of by-
products at a high RB concentration would compete with the RB
molecules, leading to a decrease in the removal efficiency.
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