Enhancement of the advanced Fenton process by weak magnetic field for the degradation of 4-nitrophenol

Article abstract of DOI:10.1039/c4ra16318d

A weak magnetic field (WMF) was employed to enhance the degradation of 4-nitrophenol (4-NP) by the advanced Fenton process (Fe⁰/H₂O₂) in this study. Although the oxidation rates of 4-NP by Fe⁰/H₂O₂ and WMF-Fe⁰/H₂O₂ dropped sharply upon increasing the initial pH (pH_ini), the introduction of WMF could remarkably improve the 4-NP degradation by Fe⁰/H₂O₂ at pH_ini ranging from 3.0 to 6.0. The quenching and electron paramagnetic resonance experiments verified that the hydroxyl radical was the primary oxidant responsible for the 4-NP degradation at pH_ini 4.0 and the cumulative concentration of HO in the WMF-Fe⁰/H₂O₂ system was about 3-fold that in Fe⁰/H₂O₂ system. The superimposed WMF increased the generation of HO in the Fe⁰/H₂O₂ process by accelerating the Fe⁰ corrosion and Fe^II generation, which was the limiting step of the Fe⁰/H₂O₂ process. The application of WMF largely enhanced the mineralization of 4-NP but it did not change the 4-NP degradation pathways, which were proposed based on the degradation products detected with LC-MS/MS. The optimum intensity of the magnetic field for 4-NP oxidation by WMF-Fe⁰/H₂O₂ was determined to be 20 mT. Response surface methodology (RSM) was applied to analyze the experimental variables and it was found that lower pH and higher Fe⁰ and H₂O₂ dosages were beneficial for 4-NP degradation by WMF-Fe⁰/H₂O₂. Among the three factors (pH_ini, Fe⁰ dosage, and H₂O₂ dosage) investigated, pH_ini was the most important factor affecting the performance of the WMF-Fe⁰/H₂O₂ process. The WMF-Fe⁰/H₂O₂ technology provides a new alternative for scientists working in the field of water treatment. This journal is

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Enhancement of the advanced Fenton process by
weak magnetic field for the degradation of 4-
nitrophenol†
Cite this: RSC Adv., 2015, 5, 13357
Xinmei Xiong,ab Yuankui Sun,a Bo Sun,a Weihua Song,c Jingyi Sun,a Naiyun Gao,a
a
a
*
Junlian Qiao and Xiaohong Guan
A weak magnetic field (WMF) was employed to enhance the degradation of 4-nitrophenol (4-NP) by the
advanced Fenton process (Fe0/H2O2) in this study. Although the oxidation rates of 4-NP by Fe0/H2O2 and
WMF–Fe0/H2O2 dropped sharply upon increasing the initial pH (pHini), the introduction of WMF could
remarkably improve the 4-NP degradation by Fe0/H2O2 at pHini ranging from 3.0 to 6.0. The quenching and
electron paramagnetic resonance experiments verified that the hydroxyl radical was the primary oxidant
responsible for the 4-NP degradation at pHini 4.0 and the cumulative concentration of HOc in the WMF–
Fe0/H2O2 system was about 3-fold that in Fe0/H2O2 system. The superimposed WMF increased the
generation of HOc in the Fe0/H2O2 process by accelerating the Fe0 corrosion and FeII generation, which was
the limiting step of the Fe0/H2O2 process. The application of WMF largely enhanced the mineralization of 4-
NP but it did not change the 4-NP degradation pathways, which were proposed based on the degradation
products detected with LC-MS/MS. The optimum intensity of the magnetic field for 4-NP oxidation by
WMF–Fe0/H2O2 was determined to be 20 mT. Response surface methodology (RSM) was applied to analyze
the experimental variables and it was found that lower pH and higher Fe0 and H2O2 dosages were beneficial
for 4-NP degradation by WMF–Fe0/H2O2. Among the three factors (pHini, Fe0 dosage, and H2O2 dosage)
investigated, pHini was the most important factor affecting the performance of the WMF–Fe0/H2O2 process.
The WMF–Fe0/H2O2 technology provides a new alternative for scientists working in the field of water treatment.
Received 13th December 2014
Accepted 19th January 2015
DOI: 10.1039/c4ra16318d
www.rsc.org/advances
FeOH2+ + HO2c / FeII + O2 + H+ k3 < 2 ꢁ 103 Mꢀ1 sꢀ1 (3)
1. Introduction
The classic Fenton process involves aqueous ferrous ions (FeII)
and H2O2 that react together to form the highly reactive HOc
under acidic conditions, as shown in eqn (1) (in acidic solution,
FeII is usually present as Fe2+ and FeIII may be present as
FeOH2+).1,2
Reaction (1) is the fast step of the Fenton process, while the
conversion of FeIII to FeII (eqn (2) and (3)) is considerably slower.
Therefore, the FeII concentration in classic Fenton process
decreases sharply and a very fast rst step followed by a
considerable slowing down of the reaction is oen observed.3
The main shortcomings associated with this technology are
related with the narrow effective pH range (2.5–3.0) with the
optimum pH for Fenton at 2.8,4 the requirement of high
amount of the homogeneous catalyst (ferrous iron salts), and
generation of large amount of iron containing sludge which has
to be separated and disposed.5
FeII + H2O2 / FeIII + OHꢀ + HOc k1 ¼ 63.0 Mꢀ1 sꢀ1 (1)
The formed FeIII can be transformed to FeII following eqn (2)
and (3).1,2
FeOH2+ / FeII + HO2c k2 ¼ 2.7 ꢁ 10ꢀ3 sꢀ1
(2)
An improvement in the Fenton process is the advanced
Fenton process (AFP), which uses zero-valent iron (Fe0) to
replace ferrous iron salts.2,6 Initially, Fe0 is oxidized by protons
via a two electron transfer following eqn (4) and FeII is gener-
ated.7 The FeII reacts rapidly with H2O2 to produce hydroxyl
radicals via eqn (1), and in the meantime generate FeIII, which is
then reduced to FeII by further interaction with the Fe0 surface
following eqn (5) at a faster rate compared to the homogeneous
process.8,9
aState Key Laboratory of Pollution Control and Resources Reuse, College of
Environmental Science and Engineering, Tongji University, Shanghai 200092, P. R.
China. E-mail: guanxh@tongji.edu.cn; Tel: +86-21-65980956
bDepartment of Civil Engineering, Jiujiang University, Jiujiang 332005, Jiangxi, P. R.
China
cDepartment of Environmental Science & Engineering, Fudan University, Shanghai,
P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra16318d
Fe0 + 2H+ / FeII + H2
(4)
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chemicals were purchased from J&K Chemical Co. Fe0 powder
($98% pure, and BET surface area 0.87 m2 gꢀ1) was obtained
from Shanghai Jinshan smelter (Shanghai, China). The Fe0
particles were agglomerated with D50 of ꢂ24.9 mm, as shown in
Fig. S1.†H2O2 (30%) and other chemicals were obtained from
the Sinopharm Group Chemical Reagent Co., Ltd (Shanghai,
China). All solutions were prepared with high-purity water
obtained from aꢃ Millipore Milli-Q system with resistivity
>18 MU cm at 25 C.
Fe0 + 2FeOH2+ / 3FeII + 2OHꢀ
(5)
Compared to the classic Fenton process, AFP avoids the addi-
tion of counteranions (Clꢀ or SO4 ) to the treated system10 and the
2ꢀ
amount of iron-containing precipitates generated in AFP is signif-
icantly lower than that in the classical Fenton process.5,8 However,
the performance of AFP is limited by the amount of FeII available to
catalyze H2O2 and additional assistants such as UV8 or visible light
irradiation11 and ultrasound12 have been proposed to enhance
contaminants removal by AFP. Nanoscale Fe0 has also been
proposed to replace the microscale Fe0 to catalyze H2O2 so as to
improve the performance of AFP.7,13 Despite much effort has been
made to enhance the AFP, much room still remains for improving
the cost-effectiveness and easy operation of this technology.
It has been generally believed that only magnetic eld (MF)
with high intensity (>2 T) affects chemical reactions14 and this
viewpoint prevents the application of MF in water and waste-
water treatment. Recently, it was found in our lab that the
application of an inhomogeneous weak magnetic eld (WMF)
(Bmax < 20 mT) could signicantly enhance Se(IV) removal by both
pristine Fe0 and aged Fe0 (ref. 15 and 16) and greatly improve
As(V) and As(III) removal by Fe0 at pHini 3.0–9.0.17 The accelerated
Se(IV), As(III) and As(V) removal by Fe0 was mainly ascribed to the
improved Fe0 corrosion and FeII generation. Consequently,
WMF and Fe0 were employed to activate persulfate (PS) syner-
gistically and it was found that the applied WMF induced a
signicant enhancement in the removal rates of organic
contaminants by Fe0/PS.10 Therefore, a hazardous and refractory
aromatic compound 4-nitrophenol (4-NP) was selected as the
model compound to explore the possibility of employing WMF
to enhance 4-NP degradation by AFP in this work.
The response surface methodology (RSM), which had been
widely employed for the optimization of the Fenton process as
well as in other catalytic studies,18 was employed to evaluate the
relative signicance of several independent factors and predict
the optimum operating conditions for desirable responses in
this study. The Box–Behnken experimental design (BBD), a
modied central composite experimental design with excellent
predictability,19 was employed to investigate the effect of initial
pH (pHini), H2O2 dosage and Fe0 dosage on the removal effi-
ciency of 4-NP by WMF assisted AFP (WMF–Fe0/H2O2).
Therefore, the objectives of this study were to (1) investigate
the feasibility of applying WMF to enhance 4-NP degradation by
Fe0/H2O2 process at various pHini and MF intensities; (2) explore
the mechanism of enhanced 4-NP degradation in WMF–Fe0/
H2O2 process; (3) determine the key parameters (pH, Fe0
dosage, H2O2 dosage) affecting the WMF–Fe0/H2O2 process by
employing the RSM with BBD; (4) analyze the possible degra-
dation pathways and mineralization of 4-NP in both Fe0/H2O2
and WMF–Fe0/H2O2 systems.
2.2. Magnetic elds (MFs)
Two different forms of MFs, one was uniform and the other was
nonuniform, were adopted in this study. The nonuniform MF
was generated by positioning two thin cylindrical neodymium–
iron–boron permanent magnets under the water bath, as
illustrated in Fig. S2.† The intensity of the MF was determined
with a Teslameter (HT201, Shanghai Hengtong Magnetic &
Electric Technology Co., Ltd) to be 10–40 mT at the bottom of
the reactor. The uniform MF was offered by an electromagnetic
eld generator (EM5-C, East Changing Technology Co., Ltd,
China) with MF intensity range <1 T. To investigate the inu-
ence of MF intensity on 4-NP degradation in WMF–Fe0/H2O2
process, the uniform MF was employed. Otherwise, the
nonuniform MF was applied.
2.3. Experimental procedures
All experiments were performed open to the air in a series of
borosilicate glass jars under constant stirring rate (400 rpm)
with a mechanical stirrer (D2004W, Shanghai Sile Instrument
Co., Ltd). With this stirring intensity, Fe0 could be evenly
distributed in the solution and no aggregation of Fe0 was
observed at the bottom of the reactor in the MF. The inuence
of MF intensity on 4-NP degradation by Fe0/H2O2 process was
determined at room temperature. All the other experiments
were carried out at 25 ꢄ 1 ꢃC, which was controlled with a water
bath.
Each 500 mL unbuffered reaction solution with desired
concentrations of 4-NP (0.02 mM) and H2O2 (0.1–1.0 mM) was
prepared and adjusted to the pre-determined pHini with sulfuric
acid and sodium hydroxide. Experiments were initiated imme-
diately once Fe0 (0.1–1.0 mM) powder was dosed into the
reactor. Samples were withdrawn at predetermined time inter-
vals and quenched by methanol (for 4-NP analysis) or sodium
sulte (for TOC analysis). The samples were ltered through a
0.22 mm membrane lter (PES) before analysis. Electron para-
magnetic resonance (EPR) experiments were carried out at room
temperature on an EPR spectrometer (Bruker A200 ESP 300E
instrument at 300 K) and the details are presented in Text S1.†
All experiments were run in duplicate, batch mode and the data
were reported as the mean of the two replicates with error bars.
2. Experimental
2.4. Chemical analysis
2.1. Chemicals
The concentrations of 4-NP and p-HBA were analyzed by UPLC
4-NP, benzoic acid (BA), p-hydroxybenzoic acid (p-HBA), and 5,5- (Waters) with a Symmetry C18 column (2.1 ꢁ 100 mm, 1.7 mm)
dimethyl-1-pyrroline-1-oxide (DMPO) were reagent grade while and UV-visible detector. 4-NP was detected at a wavelength of
methanol, acetonitrile, and formic acid were HPLC grade. These 318 nm with an isocratic method (H2O : MeOH ¼ 60 : 40) at
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tR ¼ 3.09 min, while p-HBA was detected at a wavelength of 255
nm with an isocratic method (H2O : acetonitrile ¼ 10 : 90) at tR
¼ 1.57 min. The concentration of H2O2 was determined by the
potassium titanium oxalate method (detection limit: 0.1 mg
Lꢀ1) using a UV-Vis spectrophotometer at 400 nm (TU1902,
Universal Analysis, Beijing, China).20 The concentrations of
ferrous and ferric ion (aer reduction to FeII with hydroxyl-
amine hydrochloride) were determined on UV-Vis spectropho-
tometer at 510 nm aer complexing with 1,10-phenanthroline
(detection limit: 0.03 mg Lꢀ1).
3. Results and discussion
3.1. Effect of WMF on 4-NP removal by Fe0/H2O2 at different
pHini levels
Only ꢂ3.0% of 4-NP could be removed by H2O2 alone or Fe0
alone at pHini 4.0 in 2 h without WMF, as shown in Fig. S3(a).†
The application of WMF had no inuence on 4-NP degradation
by H2O2 but slightly enhanced 4-NP sequestration by Fe0 from
ꢂ3.0% to ꢂ8.0% at pHini 4.0 in 2 h, as illustrated in Fig. S3.†
The slight improvement in 4-NP removal by Fe0 due to the
introduction of WMF should be mainly ascribed to the
enhanced Fe0 corrosion with WMF.15,16 The simultaneous
application of 0.5 mM Fe0 and 0.5 mM H2O2 could remove
ꢂ60.0% 4-NP at pHini 4.0 within 60 min even without WMF,
indicating the high catalytic ability of Fe0 to H2O2 activation, as
demonstrated in Fig. S4(b).† Surprisingly, 4-NP was completely
removed within 60 min in WMF–Fe0/H2O2 process at pHini 4.0,
implying the feasibility of employing WMF to improve the
performance of Fe0/H2O2 process.
It is well known that pH plays a key role in the performance
of the Fenton process because it affects the solubility of FeII/
FeIII, and ultimately controls the production of hydroxyl radi-
cals. Thus, the kinetics of 4-NP degradation at pHini ranging
from 3.0 to 6.0 in both Fe0/H2O2 and WMF–Fe0/H2O2 systems
were determined and demonstrated in Fig. S4.† During the
reaction process, the change of solution pH value was less than
ꢄ0.3 (data were not shown). The rates of 4-NP degradation
drastically decreased with the increase of pHini in both systems,
consistent with the phenomena reported in literatures.5,22
However, the drop in 4-NP degradation rates in Fe0/H2O2
process with elevating pH was more considerable than those in
WMF–Fe0/H2O2 process. Negligible 4-NP was removed by Fe0/
H2O2 at pHini 6.0 in 3 h while 36.8% of 4-NP could be decom-
posed by its counterpart with WMF in 3 h. Moreover, more 4-NP
was removed by WMF–Fe0/H2O2 at pHini 6.0 than by Fe0/H2O2 at
pHini 5.0 in 3 h, indicating that the WMF–Fe0/H2O2 process had
a stronger oxidation activity and a wider effective pH range
compared to the Fe0/H2O2 process. This was of great signi-
cance in real practice since less pH adjustment was necessary to
achieve a similar removal efficiency of organic contaminant by
WMF–Fe0/H2O2 process than by Fe0/H2O2 process.
TOC was monitored using a TOC analyzer (L-CPH CN200,
Shimadzu). In order to ensure the accurate measurement of
TOC, the initial concentration of 4-NP was increased to 100 mM
and the dosages of both Fe0 and H2O2 were correspondingly
elevated to 2.5 mM. UPLC together with Electrospray-Ionization
Quadruple Time-of-Flight Tandem Mass Spectrometry (UPLC-
ESI-QTOF MS), Waters Acquity UPLC-Xevo G2 QTOF, was used
to detect the intermediates of 4-NP degradation. In this study,
the mass spectrometer was operated in the m/z range of 50–300.
The eluent was delivered at 0.4 mL minꢀ1 by a gradient system
(Table S1†) with a C18 column 2.1 mm ꢁ 100 mm, 1.7 mm,
ꢃ
45 C.
The strength and gradient of MF induced by Fe0 particles
were characterized using an nite element calculation soware,
assuming that a pure Fe0 sphere with diameter of 10 mm was
exposed to an external uniform MF with ux density of 5, 10, or
20 mT and the relative magnetic permeability of the Fe0 sphere
is 1700.
2.5. Experimental design
RSM based on BBD was applied to investigate the effects of the
three independent variables on the response function. The
independent variables were pH (A), Fe0 dosage (B) and H2O2
dosage (C). The low, center and high levels of each variable were
designated as ꢀ1, 0 and +1, respectively, as illustrated in Table
S2,† which were selected based on available resources and
preliminary experiments. The square-root of kobs (the pseudo
rst-order rate constants of 4-NP degradation) was chosen for
the response factor (Y) in order to ensure that the predicted kobs
values were greater than zero.
The loss of 4-NP in both Fe0/H2O2 and WMF–Fe0/H2O2
systems could be simulated with the pseudo-rst order kinetics
(eqn (6)). The inuence of WMF on kobs of 4-NP oxidation by Fe0/
H2O2 over the pHini range of 3.0–6.0 is shown in Fig. 1(a). The
rate constants of 4-NP oxidation by WMF–Fe0/H2O2 at pHini 3.0–
5.0 were 1.6–7.9 folds of those by Fe0/H2O2. Furthermore, the
rate constant of 4-NP degradation by WMF–Fe0/H2O2 at pHini
6.0 was 2.2 fold of that by Fe0/H2O2 at pHini 5.0, further con-
rming the application of WMF could widen the working pH
range of Fe0/H2O2.
ln C/C0 ¼ ꢀkobs
t
(6)
The mathematical relationship between the response func-
tion (Y) and the independent variables (A, B, C) can be approx-
imated by a quadratic polynomial equation as follows:
Y ¼ b0 + b1A + b2B + b3C + b12AB + b13AC + b23BC + b11A2
+ b22B2 + b33C2
(7)
where Y is the response and A, B, C, AB, AC, BC, A2, B2, and C2 are
the independent variables' effects, square effects and interac-
tion effects; bi, bij and bii are the linear coefficients, interaction
coefficients and squared coefficients, respectively; b0 is the
intercept parameter.21 The soware design expert 8.0.6 was used
for experimental design, determination of the coefficients and
data analysis.
The inuence of WMF on the mineralization of 4-NP by Fe0/
H2O2 was assessed by measuring the drop in TOC with an initial
4-NP concentration of 0.10 mM or 7.2 mg Lꢀ1 TOC at pHini 4.0.
At the end of 3 h, 41% of 4-NP was mineralized by Fe0/H2O2, as
shown in Fig. 1(b). However, it took only 1 h to achieve a
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Fig. 1 (a) Influence of WMF on the pseudo first order rate constants of 4-NP degradation by Fe0/H2O2 system at different pHini levels. Reaction
conditions: [4-NP]0 ¼ 0.02 mM, [H2O2]0 ¼ 0.5 mM, [Fe0]0 ¼ 0.5 mM, T ¼ 25 ꢃC; (b) influence of WMF on mineralization of 4-NP by Fe0/H2O2.
Reaction conditions: [4-NP]0 ¼ 0.1 mM, [H2O2]0 ¼ 2.5 mM, [Fe0]0 ¼ 2.5 mM, pHini ¼ 4.0, T ¼ 25 ꢃC.
mineralization rate of 41% in the WMF–Fe0/H2O2 process. that HOc was the dominant active species responsible for the
Therefore, the application of WMF accelerated not only the 4-NP oxidation of 4-NP in the Fe0/H2O2 system, regardless of the
degradation but also its mineralization. Even in the presence of presence or absence of WMF. The minor removal of 4-NP in the
WMF, about 50% of TOC could not be removed by Fe0/H2O2 presence of excessive TBA at pHini 4.0 indicated that 4-NP may
process at the end of reaction, indicating that some degradation be removed in another way besides HOc oxidation, which was
products of 4-NP were very refractory.
veried by analyzing the reaction intermediates. However, the
degradation of 4-NP at pHini 6.0 was only partially inhibited by
TBA, as illustrated in Fig. S5,† which indicated that both HOc
and Fe(IV) were active oxidative species at near neutral pH.24
To verify the formation of HOc at pHini 4.0 in both Fe0/H2O2
and WMF–Fe0/H2O2 systems, EPR tests with DMPO were per-
formed to detect HOc by measuring the intensity of the
DMPO–OH adducts signal.25 As shown in Fig. 2(b), the specic
3.2. Role of WMF in the WMF–Fe0/H2O2 process
Tert-Butyl alcohol (TBA) is an efficient scavenger of HOc (k ¼ 6.0
ꢁ 108 Mꢀ1 sꢀ1) but is believed to be less reactive toward high-
valent oxoiron complexes like Fe(IV).23 Fig. 2(a) shows that only
ꢂ4.5% 4-NP was degraded at pHini 4.0 in both Fe0/H2O2 and
WMF–Fe0/H2O2 systems aer dosing excessive TBA, implying
Fig. 2 (a) Effect of radical quenching agent on 4-NP removal in Fe0/H2O2 and WMF–Fe0/H2O2 systems; (b) comparison of the intensity of
DMPO–OH adducts signals in Fe0/H2O2 and WMF–Fe0/H2O2 systems after 1 min. Reaction conditions: [DMPO]0 ¼ 100 mM, [H2O2]0 ¼ 30 mM,
[Fe0]0 ¼ 15 mM, pHini ¼ 4.0; (c) cumulative hydroxyl radical formation and p-HBA concentration over time; (d) influence of WMF on dissolved
ferric iron generation and H2O2 consumption in the Fe0/H2O2 system. Reaction conditions for (a), (c) and (d): [TBA]0 ¼ 0.1 M, [BA]0 ¼ 5 mM,
[4-NP]0 ¼ 0.02 mM, [H2O2]0 ¼ 0.5 mM, [Fe0]0 ¼ 0.5 mM, pHini ¼ 4.0, T ¼ 25 ꢃC.
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spectra characteristic of DMPO–OH adduct (quartet lines with our experiments, namely, a superimposed WMF could signi-
peak height ratio of 1 : 2 : 2 : 1) were detected in both Fe0/H2O2 cantly improve the oxidative ability of Fe0/H2O2 process toward
and WMF–Fe0/H2O2 systems. However, the intensity of DMPO– 4-NP and widen the applicable pH range of Fe0/H2O2 process.
OH adduct signal in WMF–Fe0/H2O2 process was much stronger
than that in Fe0/H2O2 process, indicating that applying a WMF
could greatly enhance the generation of HOc. This result was
3.3. Effect of the MF intensity on 4-NP degradation in WMF–
Fe0/H2O2 process
further veried by estimating the cumulative HOc production in
both systems. BA could be transformed into three isomers of Since the intensity of superimposed MF will inuence the
hydroxybenzoic acid by HOc reaction. The three isomers of intensity of induced MF and the MF gradient around the iron
hydroxybenzoic acid account for 90 ꢄ 5% of the products with spheres, the inuence of the MF intensity on 4-NP degradation by
the ratio of o-HBA, m-HBA, and p-HBA products reported to be WMF–Fe0/H2O2 was investigated in a uniform MF and the results
1.7 : 2.3 : 1.2.26 For the oxidation of BA by solution-phase HOc, are presented in Fig. 3. The inset in Fig. 3 shows the change of
the concentration of p-HBA can be used to estimate the cumu- pseudo rst-order degradation rate constants of 4-NP with MF
lative HOc production using eqn (8):27
intensity. The observed rate constants of 4-NP removal increased
progressively from 0.0127 to 0.1018 minꢀ1 by increasing the MF
intensity from 0 to 20 mT, which may be ascribed to the larger FL
and FB at higher MF intensity. To verify this point, the MF
strength distributions of the plane parallel to the applied
homogeneous MF and through the center of a Fe0 sphere and the
MF gradients around a Fe0 sphere when the ux densities of
applied MF are 5 mT, 10 mT and 20 mT, respectively, were
calculated by numerical simulations and presented in Fig. S6.†
Obviously, as the intensity of the applied uniform MF increased
from 5 mT to 20 mT, the maximum MF intensity and gradient
increased proportionally with increasing the intensity of applied
MF and they appeared close to the Fe0 particle surface. Conse-
quently, the FL acting on charged species and the FB acting on
paramagnetic ions would increase accordingly with increasing
the intensity of applied MF and thus strengthen their inuence
on mass transport and the uneven distribution of paramagnetic
ions (FeII) around Fe0 particles.16 Consequently, the accelerating
effect of MF on 4-NP degradation by Fe0-Fenton was increased
with the intensity of MF. Nevertheless, a further increase in MF
intensity from 20 mT to 40 mT had negligible inuence on the
rate constants of 4-NP degradation by Fe0/H2O2. In addition, the
rate constant of 4-NP oxidation by Fe0/H2O2 was remarkably
Cumulative HOc produced ¼ [p-HBA] ꢁ 5.87
(8)
As shown in Fig. 2(c), the cumulative concentration of HOc in
WMF–Fe0/H2O2 system was about 3-fold of that in the Fe0/H2O2
system in 60 min. The enhanced HOc generation was accom-
panied with the accelerated decomposition of H2O2 and
generation of FeIII, as illustrated in Fig. 2(d). A close inspection
of the data revealed that the concentration of FeIII detected in
the WMF–Fe0/H2O2 system was always ꢂ2 times of that in Fe0/
H2O2 system. In addition, the amount of decomposed H2O2 in
the presence of WMF was ꢂ2 fold of that in the absence of
WMF. No FeII was detected through the whole experiments in
both Fe0/H2O2 and WMF–Fe0/H2O2 systems, implying that FeII
was immediately oxidized by H2O2 aer it was released from
Fe0. However, the consumption rate of H2O2 does not equal to
the generation rate of HOc because H2O2 can be decomposed to
water and oxygen via the non-radical-producing pathway. The
utilization efficiency of H2O2 (the molar ratio of generated HOc
to consumed H2O2) was calculated to be 79.5% in WMF–Fe0/
H2O2 process, whereas it was only 65.9% in Fe0/H2O2 process.
Therefore, the superimposed WMF also improved the utiliza-
tion efficiency of H2O2 in Fe0/H2O2 process.
The above results conrmed that Fe0 was the source of FeII,
which catalyzed H2O2 in the Fe0/H2O2 system to produce HOc
following eqn (1), and releasing of FeII from Fe0 was the limiting
step in the Fe0/H2O2 system. An external WMF could enhance the
corrosion of Fe0, accelerating the generation of FeII and thus
leading to an increase in HOc concentration, consistent with the
observations reported in our previous studies.15–17 Due to its
ferromagnetic property, Fe0 is magnetized in a superimposed
WMF and generates an induced inhomogeneous MF, which is
stronger than the superimposed WMF.16 The Lorentz force, FL,
acting on the charged ions can increase the mass transport28 and
the magnetic eld gradient force, FB, tends to move paramagnetic
FeII along the higher eld gradient at the Fe0 particle surface.29
The uneven distribution of FeII will result in localized corrosion
and thus corrosion is accelerated in the presence of WMF.16
Moreover, pH at the Fe0 particle surface in the presence of WMF
should be lower than that in the absence of WMF, due to the
increased mass transport of H+ towards the Fe0 particle surface
caused by the additional convection induced by the FL.30 These
speculations can reasonably explain the phenomena observed in
Fig. 3 Influence of intensity of uniform MF on 4-NP degradation by
WMF–Fe0/H2O2 system. The inset shows the change of first-order
degradation rate constants with MF intensity. Reaction conditions:
[4-NP]0 ¼ 0.02 mM, [H2O2]0 ¼ 0.5 mM, [Fe0]0 ¼ 0.5 mM, pHini ¼ 4.0.
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dropped from 0.1039 to 0.051 minꢀ1 as the intensity of MF was response surface plots are presented in Fig. S9.† Obviously, the
elevated from 40 mT to 50 mT, which should be associated with variation of the solution pHini remarkably affected the process
the aggregation of Fe0 particles in the uniform MF with intensity efficiency, while the variations of Fe0 and H2O2 dosages were
greater than 20 mT (as shown in Fig. S7†). Therefore, the less important. Lower pHini and larger dosages of Fe0 and H2O2
optimum intensity of MF for 4-NP degradation in WMF–Fe0/H2O2 were benecial for 4-NP removal by WMF–Fe0/H2O2 process
process was determined to be 20 mT in this study.
within the range of variable chosen.
3.4. BBD and data analysis
Since it is very difficult to offer an uniform MF in practical
application, the nonuniform WMF was employed in the Box–
Behnken experimental design. Table 1 shows the design matrix
applied and the actual experimental results (Yexp) and data
obtained from the BBD (Ycalc) for the response (Y) corresponding
to the square root of kobs of 4-NP degradation in WMF–Fe0/H2O2
system. The coefficients of the response function (eqn (7)) were
obtained using experimental data and presented in eqn (9).
Y ¼ 0.7650 ꢀ 0.3127A + 0.8128B + 0.5322C ꢀ 0.1281AB ꢀ
0.1107AC ꢀ 0.0002BC + 0.0344A2 ꢀ 0.0742B2 + 0.0343C2 (9)
This model explains perfectly the results in the experimental
range studied (R2 adjusted ¼ 0.93). Moreover, the model
adequacy and signicance was further evaluated by ANOVA, as
shown in Table S3.† The F-value of 24.62 and its p-value of 0.0002
(less than 0.05) implied the high signicance of this model.
Furthermore, the plot of experimental rate constants versus the
predicted ones (Fig. S8†) shows satisfactory correlation (R2
¼
0.96). Therefore, this is a suitable model for predicting the
removal rate constant under the investigated reaction conditions.
The contour plots of the quadratic model with one variable
kept at its central levels and the other two variables varying
within the experimental ranges are shown in Fig. 4 and
Table 1 Experimental data points used in the Box–Behnken designa
Variable levels
Run pH Fe0 (mM) H2O2 (mM) kobs (minꢀ1
)
R2
Yexp
Ypred
1
2
3
4
5
6
7
8
2.0 0.10
4.0 1.00
2.0 1.00
4.0 0.55
6.0 1.00
2.0 0.55
2.0 0.55
4.0 1.00
6.0 0.55
4.0 0.10
4.0 0.55
6.0 0.55
6.0 0.10
4.0 0.55
4.0 0.10
4.0 0.55
4.0 0.55
0.55
1.00
0.55
0.55
0.55
1.00
0.10
0.10
1.00
1.00
0.55
0.10
0.55
0.55
0.10
0.55
0.55
0.2108
0.1055
0.9630
0.0806
0.0083
0.9155
0.3094
0.0881
0.0048
0.0494
0.0992
0.0045
0.0009
0.0650
0.0378
0.0544
0.0631
0.96 0.4591 0.5085
0.97 0.3248 0.4058
0.97 0.9813 0.9361
0.94 0.2839 0.2621
0.93 0.0911 0.0302
0.99 0.9568 0.8972
0.98 0.5562 0.5914
0.95 0.2968 0.2992
0.95 0.0693 0.0226
0.95 0.2223 0.2087
0.96 0.3150 0.2621
0.96 0.0671 0.1153
0.92 0.0300 0.0638
0.93 0.2550 0.2621
0.98 0.1944 0.1023
0.93 0.2332 0.2621
0.95 0.2512 0.2621
9
10
11
12
13
14
15
16
17
a
Fig. 4 Contour plots of the rate constant of 4-NP for the three most
important pair of factors.
Note: Y ¼ sqrt(kobs) (l ¼ 0.5).
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To test the reliability of the response functions predictions,
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As described in the previous section, HOc was identied to be
three experiments different from BBD points were performed. It the major reactive species in both Fe0/H2O2 and WMF–Fe0/H2O2
was found that the response function predictions (calculated by processes at pHini 4.0 and thus the main reaction pathway was
eqn (9)) were in good agreement with the experimental results 4-NP oxidation by HOc. It is well known that the reaction of HOc
(as listed in Table S4†), which conrmed the adequacy and with aromatic groups occurs via electrophilic addition.31,33,35
validity of the model simulating the degradation rate of 4-NP in HOc has a strong electrophilic character, and the attack of
WMF–Fe0/H2O2 system.
electrophilic HOc preferentially occurs at the ortho-position of
the –OH group to form the corresponding OH-adduct, 4-nitro-
catechol, as demonstrated in Fig. 5. On the subsequent HOc
attack, the 4-nitrocatechol was converted into p-nitropyrogallol
and o-nitrobenzoquinone, which were subjected to further
attack by HOc, leading to the formation of nitro o-benzoqui-
none.33 Alternatively, 4-nitrocatechol could be transformed to 4-
nitropyrogallol by HOc oxidation.36 The further oxidation of
nitro o-benzoquinone and 4-nitropyrogallol by HOc resulted in
the aromatic ring opening, formation of aliphatic acids, and
eventual generation of mineralization products.36,37
Besides the major oxidative degradation pathway, 4-NP
could be degraded by reduction since 4-nitrosophenol at low
concentration was detected in the process of 4-NP removal by
Fe0/H2O2 or WMF–Fe0/H2O2. This degradation intermediate
was also observed in 4-NP removal by Fe0 with ultrasonic irra-
diation and its appearance should be ascribed to the nascent
FeII ions from the corrosion reaction of Fe0.34 But the nascent
reductant was not enough to further reduce 4-nitrosophenol to
4-aminophenol in the Fe0/H2O2 and WMF–Fe0/H2O2 processes
since 4-aminophenol was not detected. The minor removal of 4-
NP (ꢂ4.5%) at pHini 4.0 in the presence of excessive TBA should
be ascribed to the side reaction pathway.
3.5. Possible degradation pathways of 4-NP
There have been some studies on identifying the degradation
intermediates of 4-NP by advanced oxidation processes, which
have been summarized in Table S5.† Because different analytical
methods, including GC-MS, LC-MS, and HPLC, had been
employed in identifying the reaction intermediates and the
mechanisms of 4-NP degradation in different AOPs were different,
the amount and species of reaction intermediates detected in 4-NP
degradation in different AOPs were very different.31,32 No one had
studied the mechanisms of 4-NP degradation by Fe0/H2O2 up to
date and more importantly, the inuence of WMF on degradation
pathways of 4-NP by Fe0/H2O2 needs clarication. Therefore,
LC-MS/MS was employed to analyze the reaction intermediates of
4-NP in both Fe0/H2O2 and WMF–Fe0/H2O2 systems at pHini 4.0,
based on which the 4-NP degradation pathways were proposed.
It was found that the application of WMF had no inuence
on the detected intermediate products, indicating that the
application of WMF accelerated the removal of 4-NP while did
not change the 4-NP degradation pathways. Six reaction inter-
mediates were detected in the process of 4-NP oxidation besides
the peak of 4-NP at m/z 139, as summarized in Table S5.†
Relying on the intermediates specied in this study and the
reaction pathways of 4-NP in other AOPs,32–34 the possible
3.6. Practical application prospect
pathways of 4-NP degradation by Fe0/H2O2 were proposed, as In real practice, it's less likely to provide a magnetic eld around a
illustrated in Fig. 5. Two alternative pathways existed for the treatment unit by applying electromagnetic eld generator due to
degradation of 4-NP in the Fe0/H2O2 or WMF–Fe0/H2O2 process. its high cost and energy use. Through our further studies,
Fig. 5 Possible degradation pathways of 4-NP in Fe0/H2O2 and WMF–Fe0/H2O2 systems.
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pretreatment of Fe0 powder, which is ferromagnetic, in a
magnetic eld and then taking advantage of its residual magne-
tism may be a feasible method. As shown in Fig. S10,† pre-
magnetizing Fe0 in a static and uniform MF with the intensity
of 100 or 300 mT remarkably improved the degradation rate of 4-
NP by Fe0/H2O2 process. Surprisingly, 4-NP was degraded by Fe0/
H2O2 process, which employed Fe0 pre-magnetized in MF of 300
mT, at a similar rate as that by WMF–Fe0/H2O2 process. Therefore,
in real practice, the pre-magnetized Fe0 may replace the pristine
Fe0 to enhance the performance of Fe0/H2O2 process toward
organic pollutants degradation, which is very easily applied.
Further studies are still necessary to gure out the suitful and best
working conditions to employ pre-magnetized Fe0. In our lab,
efforts are also being made on designing a continuous ow
reactor similar to the folded-plate occulating tank with perma-
nent magnets (low cost) to generate MF, in case the Fe0/H2O2
process with premagnetized Fe0 could not decompose organic
contaminant effectively.
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rates of 4-NP by Fe0/H2O2 and the enhancement was greater at
higher pHini. HOc was identied to be the primary oxidant
responsible for the 4-NP degradation by either Fe0/H2O2 or
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min, which may be associated with the accelerated Fe0 corrosion
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presence of WMF. The application of WMF enhanced the
mineralization of 4-NP by Fe0/H2O2. Six reaction intermediates
were detected in the process of 4-NP oxidation by Fe0/H2O2 or
WMF–Fe0/H2O2 and the degradation pathways of 4-NP were
proposed. The optimum intensity of MF for 4-NP oxidation by
WMF–Fe0/H2O2 was determined to be 20 mT and the MF with
higher intensity would result in the aggregation of Fe0 particles,
which would deteriorate the 4-NP oxidation by WMF–Fe0/H2O2.
Lower pH and higher Fe0 and H2O2 dosages were benecial for 4-
NP degradation by WMF–Fe0/H2O2 and the derived RSM model
could reasonably predict the rate constants of 4-NP oxidation in
WMF–Fe0/H2O2 system under the investigated reaction condi-
tions. Hence, applying WMF to enhance the production of
hydroxyl radical and broaden the working pH range of Fe0/H2O2
is efficient, energy-saving, chemical-free, and environmental
friendly. The WMF–Fe0/H2O2 technology will provide a new
alternative to scientists working in the eld of water treatment.
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This work was supported the Specialized Research Fund for the
Doctoral Program of Higher Education (20130072110026), the
National Natural Science Foundation of China (21277095,
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