Synergistic degradation of PNP via coupling H2O2 with persulfate catalyzed by nano zero valent iron

Article abstract of DOI:10.1039/c9ra02901j

H₂O₂ and persulfate (PDS) activated by iron are attracting much attention due to their strong oxidation capacity for the effective degradation of organic pollutants. However, they face problems such as requiring an acidic reaction pH and difficulty of Fe²⁺ regeneration. In this study, the simultaneous activation of H₂O₂ and persulfate by nanoscaled zero valent iron (nZVI) was investigated for the degradation of p-nitrophenol (PNP). The nZVI/H₂O₂/PDS oxidation system exhibited significantly higher reactivity toward PNP degradation than the systems with a single oxidant. A synergistic effect was explored between H₂O₂ and PDS during nZVI-mediated activation, and the molar ratio of H₂O₂/PDS was a key parameter in optimizing the performance of PNP degradation. The nZVI/H₂O₂/PDS system could function well in a wide pH range, and even 95% PNP was removed at an initial pH 10, thus markedly alleviating the pH limitations of Fenton-like processes. Both hydroxyl radicals and sulfate radicals could be identified during H₂O₂/PDS activation, in which H⁺ produced during PDS decomposition promoted H₂O₂ activation. The increase of oxidant concentration could significantly enhance the PNP degradation, while the presence of HCO₃^- and HPO₄^2- exerted great inhibition. Furthermore, five degradation intermediates of PNP were detected and its degradation pathways in the nZVI/H₂O₂/PDS system were presented. This study reveals that the simultaneous activation of H₂O₂ and PDS by nZVI is a promising advanced oxidation tool as an alternative to typical Fenton processes for recalcitrant pollutant removal.

Full text of DOI:10.1039/c9ra02901j

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Synergistic degradation of PNP via coupling H2O2
with persulfate catalyzed by nano zero valent iron†
Cite this: RSC Adv., 2019, 9, 20323
a
a
Jiangkun Du,
and Jianguo Bao
Yang Wang,a Faheem,
Tiantian Xu,a Han Zhengb
a
*
H2O2 and persulfate (PDS) activated by iron are attracting much attention due to their strong oxidation
capacity for the effective degradation of organic pollutants. However, they face problems such as
requiring an acidic reaction pH and difficulty of Fe2+ regeneration. In this study, the simultaneous
activation of H2O2 and persulfate by nanoscaled zero valent iron (nZVI) was investigated for the
degradation of p-nitrophenol (PNP). The nZVI/H2O2/PDS oxidation system exhibited significantly higher
reactivity toward PNP degradation than the systems with a single oxidant. A synergistic effect was
explored between H2O2 and PDS during nZVI-mediated activation, and the molar ratio of H2O2/PDS was
a key parameter in optimizing the performance of PNP degradation. The nZVI/H2O2/PDS system could
function well in a wide pH range, and even 95% PNP was removed at an initial pH 10, thus markedly
alleviating the pH limitations of Fenton-like processes. Both hydroxyl radicals and sulfate radicals could
be identified during H2O2/PDS activation, in which H+ produced during PDS decomposition promoted
H2O2 activation. The increase of oxidant concentration could significantly enhance the PNP degradation,
Received 17th April 2019
Accepted 10th June 2019
ꢀ
2ꢀ
while the presence of HCO3 and HPO4
exerted great inhibition. Furthermore, five degradation
intermediates of PNP were detected and its degradation pathways in the nZVI/H2O2/PDS system were
presented. This study reveals that the simultaneous activation of H2O2 and PDS by nZVI is a promising
advanced oxidation tool as an alternative to typical Fenton processes for recalcitrant pollutant removal.
DOI: 10.1039/c9ra02901j
rsc.li/rsc-advances
nature.5 Typically, ZVI can provide Fe2+ ions to activate H2O2
generating hydroxyl radicals ($OH) through the routes pre-
1. Introduction
sented by eqn (1)–(4). However, in case of micrometric ZVI
powder applied,6 the reactions represented by eqn (1) and (2)
are slow in nature because of the low material reactivity.7 Thus,
to alleviate the limitation of ZVI/H2O2 system, a number of
studies were carried out with incorporating additional setup
such as ultrasound and heat8,9 to speed up the electron transfer
process. Besides, nano-scaled ZVI (nZVI) was also frequently
studied as an alternative Fenton-like activator because of its
much higher reactivity with versatile functions for contami-
nants removal.10 However, no matter micro- or nano-ZVI was
applied, the activation of H2O2 should still be initiated under
acidic conditions.
Rapid industrial development produces huge amounts of
wastewater containing lots of hazardous non-biodegradable
pollutants. As a widely used raw chemical material, p-nitro-
phenol (PNP) is of high toxicity and difficult to biodegrade,
meaning that it can remain in the environment for a long time.1
Untreated emissions of PNP will denitely cause water pollution
and bring serious risk to the ecosystem and human health.2
Therefore, it is crucial to develop effective techniques for rapidly
and efficiently eradicating PNP present in contaminated envi-
ronments. Therein, Fenton and Fenton-like processes (AOPs)
have been considered as effective pathways for the degradation
of a variety of hazardous and bio-refractory organics during the
last decades.3,4
2Fe0 + O2 + 2H2O / 2Fe2+ + 4OHꢀ
Fe0 + 2H+ / Fe2+ + H2
(1)
(2)
(3)
(4)
Zero valent iron (ZVI) is a well-known Fenton-like catalyst for
the treatment of variety of refractory organics due to several
benets, such as cheap price and benign environmental
Fe2+ + H2O2 / Fe3+ + $OH + OHꢀ
Fe3+ + H2O2 / Fe2+ + $OOH + H+
aSchool of Environmental Studies, China University of Geosciences, Wuhan 430074,
People's Republic of China. E-mail: bjianguo@cug.edu.cn; Fax: +86-2-9585447; Tel:
+86-2-9585426
bHubei Provincial Key Laboratory of Mining Area Environmental Pollution Control and
Remediation, Hubei Polytechnic University, Huangshi, People's Republic of China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra02901j
Alternatively, the activation of persulfate (PDS) with
production of sulfate radicals (SO4cꢀ) has gained wide atten-
tions in recent years. In comparison with H2O2-based Fenton-
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like processes, PDS can be effectively activated in a wide pH PNP as well as the mechanism involved in nZVI/H2O2/PDS
range as represented by eqn (5) and (6), and SO4cꢀ possesses combined system.
a better selectivity on pollutants oxidation. However, unlike the
ZVI/H2O2 system that the formed Fe3+ can get reduced to Fe2+ by
2. Materials and experimental
procedure
H2O2 (eqn (4)), the Fe3+ generated in eqn (3) is quite more stable
in ZVI/PDS system, which becomes the main cause of ceasing of
2.1 Chemicals and reagents
activation reaction as well as the generation of more iron
sludge.11,12 Recently, some studies reported the simultaneous
activation of H2O2 and PDS, which might exert synergistic effect
to alleviate the faced drawbacks as they used alone.
Monteagudo et al. studied the activation of persulfate in assis-
tant of UV–Fe2+–H2O2, and found that the PDS activated by UV/
Fe2+/H2O2 presented signicantly higher efficiency for carba-
mazepine degradation compared to other systems including
PDS/Fe2+, PDS/UV and H2O2/PDS.13 However, without the
assistant of UV irradiation, the synergistic role between H2O2
and PDS would be greatly hindered, as reported by Dulova
et al.,14,15 that traditional Fenton process showed higher efficacy
for the antibiotic degradation than the combined Fe2+/H2O2/
PDS system. The possible radicals quenching might be induced
by an excessive addition of ferrous ions. As an alternative, zero
valent iron can act as an activator with capacity of controllable
ferrous release. Li et al. investigated the coupling of micro-
metric Fe0 with H2O2/PDS for the degradation of p-nitrophenol,
and their results supported the signicantly higher efficacy of
the combined Fe0/H2O2/PDS system than that obtained with
single oxidant.16 As concluded, the promoted iron corrosion and
also alleviated Fe0 passivation both contributed to the syner-
gistic effect in the Fe0/H2O2/PDS process. However, the effect of
solution pH and the role of PDS on radical generation was not
fully evaluated. Since Fenton-like reactions were pH-dependent
processes, the interplay between H2O2 and PDS with various
ratio would be remarkably affected under different pH condi-
tions. In addition, the catalytic performance of nano-scaled ZVI
for simultaneous activation of H2O2 and PDS is necessary to be
explored given that nZVI is extensively applied for water reme-
diation and acts very different reactive feature compared to
micrometric ZVI and ferrous ions.
NaBH4, FeSO4$7H2O and p-nitrophenol were bought from
Aladdin company. 5,5-Dimethylpyrroline-N-oxide (DMPO),
sodium peroxydisulfate (PDS, Na2S2O8) were supplied by Sigma-
Aldrich. Methanol (MeOH) and acetic acid in HPLC-grade were
obtained from Fisher Scientic, and other chemicals were
provided by Sinopharm with analytical grade. All reagents were
prepared by ultrapure water (18.2 MU cmꢀ1). The nano-scaled
zero valent iron (nZVI) particles were fabricated by reducing
FeSO4$7H2O with NaBH under nitrogen protection based on
our previous report.17
2.2 Degradation experiments
To carry out the degradation experiments, a certain amount of
PDS and H2O2 stock solution was spiked into p-nitrophenol (100
mL, 20 ppm, equal to 0.1438 mmol Lꢀ1) in 250 mL glass conical
asks. The initial solution pH was adjusted using NaOH or
H2SO4 to reach desired condition. The reactions were initiated
by adding predetermined dosage of nZVI particles. The mixed
reactants were stirred using a mechanical stirrer in xed speed
to maintain a good mixture. The effects of water matrices were
investigated with addition of designated level of inorganic salts.
At scheduled time intervals, 1 mL liquid samples were with-
drawn from the reaction system, and well-mixed with 1 mL of
methanol to stop the further p-nitrophenol oxidation initiated
by reactive radicals. Then the mixed samples were immediately
ltered through 0.45 mm syringe lters for HPLC measurement.
To evaluate the production of hydroxyl radicals ($OH) during
the reaction, benzoic acid (BA) was adopted as the substrate
instead of PNP, and the generated p-hydroxybenzoic acid (p-
HBA) was recorded as the $OH indicator. As to monitor the
aqueous concentration of iron ions, PDS and H2O2, samples
were taken, ltered and analyzed immediately. Each batch test
was carried out at least duplicate runs, while the average results
and the standard deviations were calculated and presented in
the gures.
2ꢀ
2ꢀ
S2O8 + Fe2+ / SO4cꢀ + Fe3+ + SO4
(5)
(6)
2S2O8 + Fe0 / 2SO4cꢀ + Fe2+ + 2SO4
2ꢀ
2ꢀ
In order to overcome the variety of limitations encountered
both in nZVI/H2O2 and nZVI/PDS system, joint venture of nZVI/
H2O2/PDS might be a possible solution providing synergetic
degradation of organic by combining nZVI, H2O2 and persul-
fate. Furthermore, the role and catalytic mechanism of H2O2
and persulfate during simultaneous activation in the combined
system should be further specied. Therefore, the objective of
this study was to (1) explore and compare the performance
evaluation of PNP degradation in nZVI/H2O2, nZVI/PDS, and the
nZVI/H2O2/PDS systems; (2) to optimize key parameters
involved in combined nZVI/H2O2/PDS process; (3) to reveal the
synergistic effects existed in case of the combined nZVI/H2O2/
PDS system; (4) to propose the possible degradation pathway of
2.3 Analytical procedures
The concentration of residual PNP was tested using RIGOL L-
3000 HPLC system installed with a Compass C18 column (5
mm, 4.6 ꢁ 250 mm). Mobile phase was a mixture of methanol
and 1% acetic acid at a ratio of 42 : 58 (v/v). The ow rate of
mobile phase was xed at 1.0 mL minꢀ1, and the lmax of PNP
was determined at 317 nm. The formed p-HBA was measured by
HPLC at 270 nm applying a combination of 0.65% TFA and
acetonitrile (65 : 35 v/v) as the mobile phase. In addition, the
amounts of leached Fe ions were detected by atomic adsorption
spectrometer. The concentration of H2O2 was analyzed at
405 nm on a spectrophotometer aer chromogenic reaction
with titanium sulfate.18 The total concentration of H2O2/PDS in
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the dual oxidants system or the concentration of PDS in single PDS, nZVI/H2O2 and nZVI/H2O2/PDS, were investigated during
oxidant system was monitored using iodometric titration the batch experiments. It was noticed that negligible amount of
method at a detection wavelength of 350 nm because both PDS PNP was degraded when H2O2/persulfate were applied alone
and H2O2 can oxidize iodide to iodine.19 Thus the residual without nZVI, demonstrating the poor oxidation ability of non-
amount of PDS in the nZVI/H2O2/PDS system was quantied as activated H2O2 or PDS.20 In case of nZVI alone, almost 51% of
the difference value between H2O2/PDS and H2O2. As to deter- total PNP removal was noticed which was attributed to reductive
mine reactive radicals generated by interaction between nZVI capability of as-prepared nZVI nanoparticles. The nitryl group of
and oxidants, aqueous samples were immediately withdrawn PNP was reduced to amino with generation of p-aminophenol.21
and ltered from the reaction system, mixed with DMPO, and However, it was noted that the PNP removal rate achieved by
analyzed on an EPR (JES-FA200, Hitachi) spectrometer. The nZVI alone was even higher than that obtained by nZVI/H2O2. A
identication and quantitation of the PNP intermediates were sudden removal of 24% of PNP by nZVI/H2O2 was observed in
performed on an LC tandem mass spectrometry Q Exactive™ the rst minute followed by rarely slow degradation within the
Hybrid Quadrupole-Orbitrap™ (LC-HRMS, Thermo Scientic, rest reaction times. Since it is well-known that the activation of
Bremen, Germany) equipped with an ESI (electrospray ioniza- H2O2 by Fe2+ or Fe0 suffers the limitation of acidic reaction
tion) mode. The mobile phase started with 5/95 (v/v) condition, the initial neutral condition in the present study may
acetonitrile/water and a ow rate of 0.2 mL minꢀ1. The mass not be suitable to motivate the oxidative capability of the nZVI/
spectra were recorded in electron-impact (EI) mode in a range of H2O2.
60–750 m/z for qualitative analysis.
In comparison, relatively higher PNP degradation (83% in 60
min) was monitored in case of nZVI/PDS than nZVI/H2O2. This
observation can be described by rapid formation of ROSs from
PDS activation and sudden generation of ferric species (Fe3+) as
prescribed by equations scheme given by eqn (5). Similar nd-
ings were reported in literatures which conrmed the higher
effectiveness of PDS than H2O2 during ferrous ion-activated
degradation of levooxacin.22 On contrary, rapid and complete
PNP degradation was accomplished in the reaction system
combining nZVI with H2O2/PDS dual oxidants. These results
supplied the evidence of the synergistic effect between H2O2
and PDS as they were activated by nZVI particles.
The residual concentration of oxidants applied during Fen-
ton process was monitored for all nZVI/PDS, nZVI/H2O2 and
nZVI/H2O2/PDS systems. As depicted in Fig. 2, signicant
amount of H2O2 was not consumed and remained in solution of
the nZVI/H2O2 system. On the one hand, fewer of Fe2+ ions were
produced on nZVI surface in neutral condition resulting in
slower H2O2 decomposition mediated by ferrous.23 On the other
hand, since PNP was not effectively degraded by nZVI/H2O2
3. Results and discussion
3.1 Characterization of nZVI particles
The morphology of as-prepared nZVI shows that the nZVI
particles have rough spherical shape and are present in chain-
like aggregated form in nanoscaled size range (Fig. S1†). The
XRD technique was also applied in order to evaluate the degree
of crystallinity as well as the speciation for as-prepared nZVI
particles. The two dominant diffraction peaks (Fig. S2†) at 2q
position of 44.68ꢂ (110) and 65.03ꢂ (200) were assigned to the
typical orientation of a-Fe, supporting the higher purity and
crystalline structure of nZVI particles.
3.2 Application of nZVI for catalytic degradation of PNP
As presented in Fig. 1, the PNP degradation performance in the
various systems, including H2O2/PDS alone, nZVI alone, nZVI/
Fig. 1 Degradation of PNP in different reaction systems. Reaction
condition: [PNP]0 ¼ 20 mg Lꢀ1, [H2O2]0 ¼ 2 mmol Lꢀ1, [PDS]0
¼
Fig. 2 Oxidants consumption in nZVI/H2O2, nZVI/PDS and nZVI/
2 mmol Lꢀ1, [H2O2/PDS]0 ¼ 2 mmol Lꢀ1, [nZVI]0 ¼ 0.2 g Lꢀ1, initial pH ¼ H2O2/PDS systems. Reaction condition: [nZVI]0 ¼ 0.2 g Lꢀ1, [PNP]0
¼
7. Note: in the nZVI/H2O2/PDS system, the molar ratio of H2O2/PDS 20 mg Lꢀ1, [PDS]0 ¼ [H2O2]0 ¼ 2 mM, [H2O2/PDS (1 : 1)]0 ¼ 2 mmol Lꢀ1
,
was 1.
initial pH ¼ 7.
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under the neutral conditions, H2O2 was only decomposed on oxidation system was not monitored before adding nZVI cata-
the nZVI surface without generating reactive radical spices. In lyst (Fig. S3†), indicating the essential role of nZVI for the
contrast, the oxidants detected in nZVI/PDS and nZVI/H2O2/PDS production of reactive radicals. The DMPOX generated and
systems were consumed very fast and negligible amount of remained strong during the rst several minutes, but vanished
residual oxidants remained in solution aer 10–15 min reac- aer 10 min reaction. This was consistent with the variation of
tion. Additionally, it was noticed that the maximum consump- oxidants concentration as shown in Fig. 2 that most of H2O2/
tion of oxidants occurred in presence of PDS which would PDS were consumed aer 10 min reaction. The formation of
decompose and generate H+ protons to accelerate nZVI corro- DMPOX supported the involvement of SO4cꢀ radicals generated
sion and in turn promote H2O2 activation. This conrmed that as a result of PDS activation for PNP degradation route. The
both nZVI/PDS and nZVI/H2O2/PDS were more applicable over presence of $OH in the nZVI/H2O2/PDS system was further evi-
nZVI/H2O2 Fenton-like system under neutral conditions.
denced using benzoic acid (BA) as the probe, which would
In order to distinguish different radicals generated in the transform to p-hydroxybenzoic acid (p-HBA) by reaction with
nZVI/H2O2, nZVI/PDS and nZVI/H2O2/PDS systems, the EPR $OH. As displayed in Fig. S4,† the concentration of generated p-
spectra were monitored by using DMPO as spin trap during the HBA increased notably along the time prole, implying the
reactions. As given in Fig. 3(a), the formation of $OH radicals signicant production of $OH during simultaneous H2O2/PDS
was supported by strong DMPO-OH signal in nZVI/H2O2 system. activation.
However, for both nZVI/PDS and nZVI/H2O2/PDS systems, seven
To further ensure the role of $OH and SO4cꢀ towards PNP
peaks referred to DMPOX adduct were detected, and signi- degradation, quenching tests with chemical probes were further
cantly stronger spectrum intensity was observed in the dual performed. In specic, tert-butyl alcohol (TBA) without a-
oxidation system. DMPOX was the nal adduct obtained as hydrogen was added as a scavenger for $OH radicals due to its
a result of SO4cꢀ-assisted oxidation of DMPO or DMPO-OH, tendency to react faster with $OH than SO4cꢀ radicals. In
because DMPO/SO4cꢀ was highly unstable and directly con- addition, methanol (MeOH) with a-hydrogen is commonly
verted to DMPOX. In addition, this DMPOX pattern in the dual utilized as a quenching reagent for both SO4cꢀ and $OH radi-
cals. As shown in Fig. 3(b), the great decrease in PNP degrada-
tion was noticed when 1000 mmol Lꢀ1 alcohols (alcohol : PDS ¼
500 : 1) were added into the nZVI/H2O2/PDS system. MeOH
exhibited a slightly higher inhibition rate on PNP degradation
compared to TBA, indicating that both SO4cꢀ and $OH were
predominant radicals in the nZVI/H2O2/PDS system.
3.3 Effect of pH
The effect of initial pH on the removal efficiency of PNP was
investigated based on different reaction systems, including
nZVI/H2O2, nZVI/PDS and nZVI/H2O2/PDS with different molar
ratio between H2O2 and PDS. As shown in Fig. 4, the pH range of
3–10 was adopted to evaluate the performance for all Fenton-
like systems. The system of nZVI/H2O2 exhibited an extremely
high efficiency for PNP degradation at initial pH 3 that complete
PNP removal was achieved within 5 min reaction. However, its
performance at pH 5, pH 7 and pH 10 was unsatised due to its
intrinsic pH limitation.24 On contrary, the PNP degradation by
nZVI/PDS was slightly affected by the initial solution pH. Overall
80% of PNP could be removed within 60 min reaction in the
nZVI/PDS system while changing initial pH from 3 to 10. These
results were consistent with some early studies that persulfate
oxidation of PNP was not sensitive to the variation of solution
pH.25,26 According to the above results, nZVI/H2O2 possessed
a stronger oxidation ability towards PNP than that obtained by
nZVI/PDS under optimized pH conditions (pH ¼ 3). This might
be due to the generation of different ROSs from nZVI/H2O2 and
nZVI/PDS, resulting in dissimilar reactivity to PNP compound.
For the combined system of nZVI/H2O2/PDS, PNP could be
Fig. 3 (a) EPR spectra obtained for nZVI/H2O2, nZVI/PDS and nZVI/
PDS/H2O2 systems; (b) degradation of PNP in the nZVI/PDS/H2O2
rapidly and completely degraded within the rst 10 min in both
acidic and neutral conditions, while this process became
signicantly slower in alkaline environment at pH 10. In addi-
tion, the maximum PNP removal was noticed when 1 : 1 ratio of
system with addition of MeOH and TBA. Reaction condition: [nZVI]0
¼
0.2 g Lꢀ1, [PNP]0 ¼ 20 mg Lꢀ1, [PDS]0 ¼ [H2O2]0 ¼ 2 mM, [H2O2/PDS
(1 : 1)]0 ¼ 2 mmol Lꢀ1, [MeOH or TBA]0 ¼ 1000 mmol Lꢀ1
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Fig. 4 PNP degradation in different oxidation systems with different initial pH (a) pH ¼ 3, (b) pH ¼ 5, (c) pH ¼ 7, (d) pH ¼ 10. Reaction condition:
[nZVI]0 ¼ 0.2 g Lꢀ1, [PNP]0 ¼ 20 mg Lꢀ1, [H2O2]0 ¼ 2 mmol Lꢀ1, [PDS]0 ¼ 2 mmol Lꢀ1, [H2O2/PDS]0 ¼ 2 mmol Lꢀ1
.
H2O2–PDS was employed than others H2O2–PDS ratios such as systems with dual oxidants, as high as 71.8 mg Lꢀ1 aqueous Fe
1 : 3 and 3 : 1 when different oxidants ratios were adopted. ions were detected in terms of higher PDS proportion
These results indicated that the simultaneous activation of (PDS : H2O2 ¼ 3 : 1), but about 20 mg Lꢀ1 Fe ions released as the
H2O2 and PDS by nZVI exhibited better application potential for PDS/H2O2 ratios was 1 : 1 and 1 : 3. This was attributed to fast
pH variation and synergistic effect on PNP degradation conversion of generated Fe3+ into Fe2+ in the presence of H2O2.
compared to the systems with single oxidant.
In fact, the formation of inactive iron species including ferric
Table 1 presents the value of solution pH before and aer the ions as well as oxyhydroxides not only lead to nZVI passivation
reactions. It was noted that the pH value kept stable for the but also result in scavenge of the formed $OH and SO4cꢀ.
system of nZVI/H2O2, but decreased remarkably to the range of However, the introduction of H2O2 into PDS-based oxidation
3–4 for the system of nZVI/PDS no matter the initial pH was 3 or systems could alleviate this drawback to some extent. As
10. This pH transformation was likely to promote the H2O2- a summary, for all pH values including pH 3, the combination
based Fenton-like degradation of PNP in the nZVI/H2O2/PDS of H2O2 with PDS system depicted higher PNP removal than
dual oxidants system. In neutral or alkaline conditions, PDS nZVI/H2O2 and nZVI/PDS systems, conrming the synergistic
would be rst activated by nZVI along with the decrease of effect of the dual oxidant due to more feasible pH variation and
solution pH to 3–4 at which H2O2 activation was sequentially iron release.
initiated for further PNP oxidation. Therefore, the optimum
ratio between H2O2 and PDS was a key parameter for PNP
degradation in view of efficiency, economy as well as environ-
3.4 Effect of oxidant dosage
In order to investigate the effect of combined use of oxidants
mental safety point since replacement of PDS by H2O2 would
2ꢀ
H2O2 and PDS for the degradation of PNP, the different dosages
of dual oxidants were taken from 0.5 mM to 1.0 mM by keeping
the ratio constant at 1 : 1. It was noticed that the increase of
H2O2/PDS dosage accelerated the degradation of PNP, and
complete PNP removal could be observed as the total H2O2/PDS
concentration increased over than 1.5 mM (Fig. 6). Even though
it was reported that H2O2 was able to enhance PDS oxidation of
pollutants, in which the combined persulfate/H2O2 based
advanced oxidation process was implemented for the treatment
reduce SO4 discharge to receiving water bodies.
The activation of H2O2 and persulfate on nZVI would also
result in iron release in some extent. Thus the release of Fe ions
from nZVI in different reaction systems at initial pH 7 was
further investigated. As depicted in Fig. 5, less Fe ions were
released in the ZVI/H2O2 system, while over 90 mg Lꢀ1 Fe irons
escaped in the ZVI/PDS system. The acidic nature of PDS would
promote iron corrosion and lead to high Fe ions release. For the
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Table 1 Results summarized for PNP degradation at different initial pH in various reaction systemsa
pH (average value)
PDS concentration
Removal rate
of PNP
Reaction systems
H2O2 concentration (mmol Lꢀ1
)
(mmol Lꢀ1
)
Initial
Final
nZVI/H2O2
nZVI/PDS
nZVI/H2O2/PDS (3 : 1)
nZVI/H2O2/PDS (1 : 1)
nZVI/H2O2/PDS (1 : 3)
nZVI/H2O2
2.0
—
—
3.0
3.0
3.0
3.0
4.9
100%
84.5%
100%
100%
99.5%
39.1%
84.3%
100%
100%
99.6%
29.4%
82.8%
100%
100%
99.9%
16.9%
76.7%
23.5%
94.9%
61.9%
2.0
0.5
1.0
1.5
—
2.0
0.5
1.0
1.5
—
2.0
0.5
1.0
1.5
—
2.0
0.5
1.0
1.5
3.22
3.89
3.74
3.48
6.38
3.17
3.97
3.43
3.24
6.94
2.88
4.44
3.59
3.02
8.47
3.34
6.1
1.5
1.0
0.5
2.0
—
1.5
1.0
0.5
2.0
—
1.5
1.0
0.5
2.0
—
1.5
1.0
0.5
3.0
5.07
5.07
5.07
5.07
5.07
7.01
7.01
7.01
7.01
7.01
9.96
9.96
9.96
9.96
9.96
nZVI/PDS
nZVI/H2O2/PDS (3 : 1)
nZVI/H2O2/PDS (1 : 1)
nZVI/H2O2/PDS (1 : 3)
nZVI/H2O2
nZVI/PDS
nZVI/H2O2/PDS (3 : 1)
nZVI/H2O2/PDS (1 : 1)
nZVI/H2O2/PDS (1 : 3)
nZVI/H2O2
nZVI/PDS
nZVI/H2O2/PDS (3 : 1)
nZVI/H2O2/PDS (1 : 1)
nZVI/H2O2/PDS (1 : 3)
3.45
3.52
a
The initial concentration of nZVI and PNP was 0.2 g Lꢀ1 and 20 mg Lꢀ1 (equal to 0.1438 mmol Lꢀ1), respectively.
Fig. 6 PNP degradation in the nZVI/H2O2/PDS system with different
Fig. 5 Concentration of leached iron in nZVI/H2O2, nZVI/PDS and
oxidant dosage. Reaction condition: [nZVI]0 ¼ 0.2 g Lꢀ1, [PNP]0
¼
nZVI/H2O2/PDS systems. Reaction condition: [nZVI]0 ¼ 0.2 g Lꢀ1
,
20 mg Lꢀ1, [H2O2]0/[PDS]0 ¼ 1 : 1, initial pH ¼ 7.
[PNP]0 ¼ 20 mg Lꢀ1, [H2O2]0 ¼ [PDS]0 ¼ 2 mM, [H2O2/PDS (1 : 1)]0
2 mmol Lꢀ1, initial pH ¼ 7.
¼
system of nZVI/PDS, the degradation of PNP was enhanced
when the PDS dosage increased from 0.5 mM to 1.0 mM, but
remained similar as further increase to 1.5 mM and 2.0 mM. It
could be due to the limited ferrous and Fe0 active site to
generate optimized or enough SO4cꢀ which boost up the PNP
removal to some extent.
of landll leachate.27 However, the PDS/H2O2 couple hardly
degraded PNP in the present study. Therefore, the increase of
oxidants induce more ROSs for PNP degradation because more
PDS/H2O2 was catalytically activated on nZVI.
In comparison, different performances of nZVI/H2O2 and
ZVI/PDS oxidation systems were observed with increasing
oxidant dosage (Fig. S5†). Because of unavailable activation of
H2O2 in neutral conditions, no improvement but decrease in
PNP was monitored with the increase of H2O2 dosage in nZVI/
H2O2 Fenton-like system. The maximum PNP removal was
achieved when the H2O2 concentration was 0.5 mM. For the
3.5 Effect of water matrices
The effect of inorganic ions normally available in real waste-
water on PNP removal performance was investigated. The Clꢀ,
2ꢀ
HCO3ꢀ, NO3ꢀ, HPO4 anions which were considered as most
common water matrices were selected as probe ions. The
20328 | RSC Adv., 2019, 9, 20323–20331
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experimental evidences revealed that HCO3 and HPO4
RSC Advances
ꢀ
2ꢀ
anions were dominant water matrix affecting the PNP removal
performance when compared with others anions encountered
at optimized pH value 7. As shown in Fig. 7, unobvious change
was noticed for PNP removal with the addition of Clꢀ ions,
indicating that Clꢀ could not quench $OH and SO4cꢀ species.
Moreover, tꢀhe slight decrease in PNP degradation was examined
when NO3 was incorporated in the nZVI/H2O2/PDS dual
ꢀ
oxidation system. Since NO3 ions was unable to form
complexes with Fe2+/Fe3+ and also showed non-reactive nature
for $OH radicals.28 This phenomenon could be assigned to the
scavenging effect of NO3ꢀ on SO4cꢀ species given that they could
react at a rate of 5.0 ꢁ 104 Mꢀ1 Sꢀ1
.
In case of HCO3ꢀ, the
29,30
profound inhibitory effect on PNP removal efficiency was
monitored. This was attributed to scavenging reaction between
$OH/SO4cꢀ and HCO3ꢀ, and also the nZVI passivation caused by
2ꢀ
precipitation with Fe2+/Fe3+. Similarly, for HPO4
ions the
remarkable inhibitory effect on PNP degradation was depicted
2ꢀ
because HPO4 could complex with reactive Fe2+, therefore,
competing the interaction with H2O2/PDS oxidants.31
3.6 Degradation pathways of PNP
Six intermediates were identied by LC-HRMS and their
molecular structures were preliminarily speculated to further
analyze the pathway of PNP decomposition. It can be seen from
Table S1 and Fig. S6† that ve corresponding byproducts were
captured from the PNP oxidation via nZVI catalyzed H2O2/PDS.
Fumaric acid, oxalic acid, 5-nitro-1,2,3-benzenetriol, p-nitro-
sophenol, 4-nitrocatechol, and 2,4-dinitrophenol were detected
at the residence time of 2.02 min, 2.14 min, 3.70 min, 4.50 min
and 8.33 min, respectively. Based on the above intermediates,
the possible pathways of PNP degradation in the nZVI/H2O2/
PDS system were proposed in Fig. 8. Firstly, hydroxyl radicals
and sulfate radicals preferred to attack the nitro substituent on
the benzene ring on account of high reaction rate and non-
selectivity. It was postulated that PNP was denitrated with the
production of phenol. However, phenol was not detected in the
Fig. 8 Possible reaction pathways of PNP degradation in the nZVI/
H2O2/PDS dual oxidation system.
product, probably because the oxidation reaction was so fast
that the formed phenols immediately attacked by residual free
radicals in the system and converted to other products. Deni-
tration reaction was accompanied by the generation of a large
amount of nitro radicals, which further attacked the meta
position of hydroxyl radicals on the benzene ring to form 2,4-
dinitrophenol.32,33 Secondly, in view of the strong electron-
pushing ability and also the electrophilicity of hydroxyl radi-
cals, it was more likely to undergo an additional hydroxylation
reaction at the meta-position of the benzene ring, thereby
producing 4-nitrocatechol. In addition, 4-nitrocatechol could
further undergo electrophilic oxidation to form 5-nitro-1,2,3-
benzenetriol. Furthermore, because of the strong reducing
ability of nZVI, a part of the PNP in the reaction system could
also be directly reduced on nZVI to form p-nitrosophenol.
Finally, benzene rings of intermediates were opened and then
oxidized to form small molecular organic acids, such as fumaric
acid and oxalic acid, and nally mineralized into CO2 and H2O.
3.7 Implications of the synergistic effect between H2O2 and
PDS
The instructive implications of the present study will be bene-
cial for the improvement and optimization of typical H2O2-
based or PDS-based advanced oxidation processes as elucidated
in Fig. 9. As for the nZVI/H2O2 Fenton-like reaction, the oxida-
tive degradation of recalcitrant pollutants by $OH should be
Fig. 7 The effect of water matrices on PNP degradation in the nZVI/
H2O2/PDS system. Reaction condition: [nZVI]0 ¼ 0.2 g Lꢀ1, [PNP]0
¼
20 mg Lꢀ1, [H2O2/PDS (1 : 1)]0 ¼ 2 mM.
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Compared to nZVI, nZVI/H2O2 and nZVI/PDS systems, the nZVI/
H2O2/PDS oxidation system exhibited signicantly higher effi-
ciency towards PNP degradation proving the synergistic effect
between H2O2 and PDS. Effect of pH on PNP degradation
indicated that the nZVI/H2O2 process could only successfully
run at pH 3, while the nZVI/PDS system was not sensitive to pH
variation. Conversely, the join of PDS accelerated the activation
of H2O2 by producing H+, and enable the nZVI/H2O2/PDS system
to proceed efficient in a wide initial pH range from 3 to 10. The
best performance of nZVI/H2O2/PDS was achieved when the
H2O2/PDS molar ratio was 1 : 1, and even 95% of PNP could be
degraded within 60 min reaction even at pH 10. It was depicted
that the activation of PDS would decrease solution pH and
alleviate nZVI passivation, thus creating a favorable condition
for subsequent H2O2 activation.
Water anions were signicant interferential factors for the
nZVI/H2O2/PDS system. The presence of carbonate and phos-
phate anions would remarkably inhibit PNP degradation and
nitrate would exert slight inhibition. The EPR characterization
and quenching tests suggested that both SO4cꢀ and $OH played
dominant role for the degradation of PNP. The identied
intermediates behind PNP degradation in the nZVI/H2O2/PDS
system suggested that nitro-reduction by nZVI, hydroxylation
and denitration induced by radicals oxidation were main
transformation routes. As a summary, the proposed nZVI/H2O2/
PDS synergistic system should serve as an improved and effi-
cient Fenton-like process with multi-radicals generation for
recalcitrant organic pollutants removal.
Fig. 9 Proposed mechanisms for simultaneous activation of H2O2/
PDS with nZVI as the activator.
typically carried out in narrow pH conditions,34 especially at pH
3. The reaction between nZVI and H2O2 under improper pH
condition would result in invalid H2O2 decomposition. In
contrast, the activation of PDS by nZVI with generation of SO4cꢀ
could be conducted in a wide pH range, but producing too
much H+ protons thus remarkably decrease the solution pH.
Therefore, their distinct features comprised H2O2 and PDS as
a complementary group.
At pH 3, both H2O2 and PDS could be easily activated by nZVI
in the nZVI/H2O2/PDS system for the degradation of PNP.
However, in neutral or alkaline conditions, nZVI/H2O2 could
hardly run so that only PDS was activated during the initial
stage. The H+ protons produced from catalytic PDS activation
would decrease solution pH to acidic condition (pH 3–4) and
facilitate further activation of H2O2 with $OH formation.
Meanwhile, the corrosion of nZVI induced by PDS oxidation was
Conflicts of interest
There are no conicts to declare.
Acknowledgements
accompanied by the concomitant production of a mass of Fe3+
.
The study was supported by the Natural Science Foundation of
Hubei Province (2018CFB262), Fundamental Research Funds
for the Central Universities of the China University of Geo-
sciences (Wuhan) (No. CUG170646) and the Open Funds of
Hubei Key Laboratory for Mine Environmental Pollution
Control and Remediation (2014103).
However, signicant amount of these Fe3+ could be reduced to
Fe2+ by H2O2 as evidenced in Fig. 5, in turn promoted the PDS or
H2O2 activation.14
In addition, even though both $OH and SO4cꢀ possess high
oxidizing capacity, SO4cꢀ is more likely to degrade organic
pollutants via electron transfer while $OH prefers to nucleo-
philic attack by hydroxylation and H abstraction. For example,
the degradation of PNP by $OH mainly generated intermediates
including hydroquinone, benzoquinone and multi-hydroxy
benzene as reported previously.3 However, it was demon-
strated that the processes of denitration and renitration were
more frequently occurred during the SO4cꢀ-mediated degrada-
tion of nitrobenzene.35 Therefore, the coexistence of compa-
rable amount of SO4cꢀ and $OH from the simultaneous
activation of H2O2 and PDS would initiate a multi-radical attack
and resulted in a more efficient or complete PNP degradation.
References
1 S. A. Dos, M. F. Viante, D. J. Pochapski, A. J. Downs and
A. Cap, J. Hazard. Mater., 2018, 355, 136–144.
2 Z. I. Bhatti, H. Toda and K. Furukawa, Water Res., 2002, 36,
1135–1142.
3 M. A. Oturan, J. Peiroten, P. Chartrin and A. J. Acher, Environ.
Sci. Technol., 2000, 34, 3474–3479.
4 A. Mirzaei, Z. Chen, F. Haghighat and L. Yerushalmi,
Chemosphere, 2017, 174, 665–688.
5 X. Guan, Y. Sun, H. Qin, J. Li, I. M. Lo, D. He and H. Dong,
Water Res., 2015, 75, 224–248.
6 L. Xu and J. Wang, J. Hazard. Mater., 2011, 186, 256–264.
7 D. H. Bremner, A. E. Burgess, D. Houllemare and
K.-C. Namkung, Appl. Catal., B, 2006, 63, 15–19.
4. Conclusion
This study investigated the simultaneous activation of H2O2 and
PDS using nZVI as the activator for the degradation of PNP.
20330 | RSC Adv., 2019, 9, 20323–20331
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
8 N. Zhang, G. Xian, X. Li, P. Zhang, G. Zhang and J. Zhu, Front. 22 I. Epold and N. Dulova, J. Environ. Chem. Eng., 2015, 3, 1207–
Chem., 2018, 6, 12. 1214.
9 N. Liu, F. Ding, C.-H. Weng, C.-C. Hwang and Y.-T. Lin, J. 23 J. Liu, C. Ou, W. Han, J. Shen, H. Bi, X. Sun, J. Li and
Environ. Manage., 2018, 206, 565–576. L. Wang, RSC Adv., 2015, 5, 57444–57452.
10 W. Wang, M. Zhou, Q. Mao, J. Yue and X. Wang, Catal. 24 J. Du, W. Guo, D. Che and N. Ren, Chem. Eng. J., 2018, 351,
Commun., 2010, 11, 937–941. 532–539.
11 C. Liang, C.-P. Liang and C.-C. Chen, J. Contam. Hydrol., 25 J. Li, Y. Ren, F. Ji and B. Lai, Chem. Eng. J., 2017, 324, 63–73.
2009, 106, 173–182.
12 G. P. Anipsitakis and D. D. Dionysiou, Environ. Sci. Technol.,
2004, 38, 3705–3712.
13 J. M. Monteagudo, A. Duran, R. Gonzalez and A. J. Exposito,
Appl. Catal., B, 2015, 176–177, 120–129.
14 I. Epold, M. Trapido and N. Dulova, Chem. Eng. J., 2015, 279,
452–462.
15 N. Dulova, E. Kattel and M. Trapido, Chem. Eng. J., 2017, 318,
254–263.
26 X. Chen, M. Murugananthan and Y. Zhang, Chem. Eng. J.,
2016, 283, 1357–1365.
27 A. H. Hilles, S. S. A. Amr, R. A. Hussein, O. D. El-Sebaie and
A. I. Arafa, J. Environ. Manage., 2016, 166, 493–498.
28 J. De Laat, G. T. Le and B. Legube, Chemosphere, 2004, 55,
715–723.
29 T. Umschlag, R. Zellner and H. Herrmann, Phys. Chem.
Chem. Phys., 2002, 4, 2975–2982.
30 Y. Xu, Z. Lin and H. Zhang, Chem. Eng. J., 2016, 285, 392–401.
´
´
´
16 J. Li, Q. Ji, B. Lai and D. Yuan, J. Taiwan Inst. Chem. Eng., 31 D. Whebi, H. Hafez, M. El Masri and M. El Jamal, J. Univ.
2017, 80, 686–694. Chem. Technol. Metall., 2010, 45, 303–312.
17 J. Du, J. Bao, C. Lu and D. Werner, Water Res., 2016, 102, 73– 32 C. Cheng, Y. Han, G. Jing, L. Zhou and Y. Lan, J. Taiwan Inst.
81. Chem. Eng., 2018, 88, 169–176.
18 G. M. Eisenberg, Ind. Eng. Chem., Anal. Ed., 1943, 15, 327– 33 Z. Mi, X. Chen, Z. He, M. Murugananthan and Y. Zhang,
328. Chem. Eng. J., 2015, 264, 39–47.
19 C. Liang, C.-F. Huang, N. Mohanty and R. M. Kurakalva, 34 J. Deng, H. Dong, C. Zhang, Z. Jiang, Y. Cheng, K. Hou,
Chemosphere, 2008, 73, 1540–1543. L. Zhang and C. Fan, Sep. Purif. Technol., 2018, 202, 130–137.
20 T. Olmez-Hanci, I. Arslan-Alaton and B. Genc, J. Hazard. 35 Y. Ji, Y. Shi, L. Wang and J. Lu, Chem. Eng. J., 2017, 315, 591–
Mater., 2013, 263(part 2), 283–290.
597.
21 Y. Ren, J. Li, L. Lai and B. Lai, Chemosphere, 2018, 194, 634–
643.
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