Electron shuttling catalytic effect of mellitic acid in zero-valent iron induced oxidative degradation

Article abstract of DOI:10.1016/j.cattod.2016.03.009

The enhanced oxidation capacity of zero-valent iron (ZVI) using mellitic acid (MA) as an electron shuttle catalyst was investigated using 4-chlorophenol (4-CP) as a model pollutant. In the presence of MA, enhanced electron transfer from ZVI surface to molecular oxygen resulted in higher production of hydrogen peroxide (H₂O₂), which subsequently increased the effective concentration of hydroxyl radical (HO[rad]) generated through the Fenton-type reaction. The possible role of MA as an efficient electron shuttle was supported by cyclic voltammetric estimation of MA reduction potential (E⁰ = ?0.184 V_NHE) and corroborated with photocurrent measurements in the ZVI suspension. Control experiments using Fe(II) ions instead of ZVI demonstrated that the presence of MA in the Fe(II)/H₂O₂ homogeneous system had no significant effect on the 4-CP oxidation efficiency. This indicates that the formation of a Fe(II)-MA complex does not contribute to the 4-CP oxidation pathway. The primary role of MA in the ZVI/O₂ system seems to mediate the electron transfer from the ZVI surface to dioxygen. It implies that organic species containing multiple carboxylic ligands (species like MA or its structural analogues) may function as an electron shuttle in the ZVI/O₂ catalytic system.

Full text of DOI:10.1016/j.cattod.2016.03.009

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Electron shuttling catalytic effect of mellitic acid in zero-valent iron
induced oxidative degradation
∗
Seung-Hee Kang, Alok D. Bokare, Yiseul Park, Chi Hun Choi, Wonyong Choi
School of Environmental Science and Engineering, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang
790-784, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The enhanced oxidation capacity of zero-valent iron (ZVI) using mellitic acid (MA) as an electron shut-
tle catalyst was investigated using 4-chlorophenol (4-CP) as a model pollutant. In the presence of MA,
enhanced electron transfer from ZVI surface to molecular oxygen resulted in higher production of hydro-
Received 15 December 2015
Received in revised form 28 February 2016
Accepted 3 March 2016
Available online xxx
•
gen peroxide (H2O2), which subsequently increased the effective concentration of hydroxyl radical (HO )
generated through the Fenton-type reaction. The possible role of MA as an efficient electron shuttle was
0
supported by cyclic voltammetric estimation of MA reduction potential (E = −0.184 VNHE) and corrob-
Keywords:
orated with photocurrent measurements in the ZVI suspension. Control experiments using Fe(II) ions
instead of ZVI demonstrated that the presence of MA in the Fe(II)/H2O2 homogeneous system had no
significant effect on the 4-CP oxidation efficiency. This indicates that the formation of a Fe(II)-MA com-
plex does not contribute to the 4-CP oxidation pathway. The primary role of MA in the ZVI/O2 system
seems to mediate the electron transfer from the ZVI surface to dioxygen. It implies that organic species
containing multiple carboxylic ligands (species like MA or its structural analogues) may function as an
electron shuttle in the ZVI/O2 catalytic system.
Zero-valent iron (ZVI)
Oxidation
Mellitic acid
Electron shuttle
Fenton
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
the iron corrosion process in the presence of molecular oxygen
12–17]. However, the addition of organic ligands like ethylenedi-
[
The generation of reactive oxygen species (ROS) by soluble iron
amine tetraacetic acid (EDTA), oxalate or nitrilotriacetic acid in the
species (Fe(II) in particular) in aerobic conditions is kinetically
unfavorable for electron transfer from the Fe(II)-aquo complex to
molecular oxygen [1]. However, by addition of organic acid lig-
ands capable of iron complexation, the reduction potential of the
Fe(II)/Fe(III) redox cycle is lowered, which allows efficient oxy-
gen reduction to form hydrogen peroxide (H O ) [2]. The higher
ZVI system exhibits the same effect as observed in the Fe(II)/H O2
2
homogeneous system. Cheng et al. reported increased oxidative
removal of pesticides by the ZVI/O2 system in the presence of
EDTA [14–16] and attributed this enhancement to complexation
of in situ generated Fe(II) by EDTA, which resulted in higher H O2
2
formation by Fe(II) complex-mediated oxygen reduction. Keenan
and Sedlak demonstrated that the presence of organic ligands in
the ZVI system exhibited enhanced oxidation efficiency over a
wide pH range, with the highest value obtained at neutral pH
[13]. This enhancement of Fenton reaction was explained in terms
of ligand-mediated (1) increase in oxygen reduction efficiency
through Fe(II) complexation, and (2) retardation in iron precipi-
tation due to higher solubility of the Fe(II)-ligand complex. Thus,
the complexation of Fe(II) with the organic ligand has always been
highlighted as a primary factor responsible for the enhanced oxi-
dation in ZVI/ligand/O2 system.
2
2
reactivity of the Fe(II)-ligand complex towards H O2 decomposi-
2
tion compared to the Fe(II)-aquo species [3–5] also enhances the
•
Fenton reaction and increases the effective hydroxyl radical (HO )
concentration for oxidation of aqueous organics [6–9]. The ligands
are also capable of reducing Fe(III) (formed during the Fenton reac-
tion) back to Fe(II) [5,7]. Thus, the dual role of organic ligand as
Fe(II) complexing agent and Fe(III) reductant prevents catalyst loss
through precipitation and extends the scope of the Fenton reaction
to neutral pH applications [10,11].
Compared to the homogeneous Fenton process, the zero-valent
iron (ZVI) system generates both Fe(II) and H O in situ during
On the other hand, we demonstrated that natural organic matter
2
2
(NOM), containing multiple groups capable of iron complexation,
can enhance the ZVI-mediated oxidation of organic pollutants
through an electron shuttling effect without any contribution from
the Fe(II)-complexation pathway [18,19]. Both fulvic and humic
∗ Corresponding author.
E-mail address: wchoi@postech.edu (W. Choi).
http://dx.doi.org/10.1016/j.cattod.2016.03.009
0
920-5861/© 2016 Elsevier B.V. All rights reserved.
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acids mediated electron transfer from the ZVI surface to O , which
2.3. Analysis
2
resulted in enhanced H O2 production and Fe(II) dissolution. This
2
•
subsequently increased the effective HO concentration for oxida-
Quantitative analysis of 4-CP and MA was done using high per-
formance liquid chromatography (HPLC, Agilent 1100) utilizing
an Agilent Zorbax 300SB C-18 column along with a diode-array
detector. The eluent solution consisted of (a) 0.1% phosphoric acid
and acetonitrile (70:30 v/v) for 4-CP and (b) 0.1% phosphoric acid
and methanol (95:5 v/v) for MA. Chloride ion production from 4-
CP degradation was quantified using ion chromatography (Dionex
DX-120) equipped with a Dionex IonPac AS-14 column and a con-
ductivity detector. The eluent composition was 3.5 mM Na CO and
tion of organic pollutants. Although the quinine moieties of NOM
were identified as the reactive sites responsible for the electron
transfer action using benzoquinone as a proxy substrate [18,19],
the effect of aromatic carboxyl groups present in the NOM matrix
on the electron shuttling pathway has not been investigated so
far. Although large variabilities in composition and functional
group distribution makes NOM chemistry highly complex, the car-
boxylate groups play an important role because of their higher
abundance, low pKa values, and metal complexation properties
2
3
1 mM NaHCO . The pH change during the course of the reaction was
3
[
5]. They dissociate much more easily than phenolic groups, which
monitored by pH meter (Orion, model 720A) equipped with Orion
8102BN electrode.
facilitates their participation in NOM-mediated redox transforma-
tions.
In this study, to understand the electron transfer capacity
of carboxylic ligands in ZVI-mediated oxidation, mellitic acid
Cyclic voltammetry (CV) analysis of MA was recorded using a
Reference 600 potentiostat (Gamry Instruments) in 0.91 M LiClO4
at pH 2 using a scan rate of 150 mV/s and initial potential fixed at 0 V
(vs. Ag/AgCl). The current was collected using a Pt electrode (work-
ing electrode), a graphite rod (counter electrode) and a Ag/AgCl
electrode (reference electrode) immersed in 1 M NaCl adjusted to
pH 2.5 in the presence of 0.2 g/L ZVI. Nitrogen gas (>99.9%) was
continuously purged through the suspension. For potentiostatic
current measurements, the potential of +0.4 V (vs. Ag/AgCl) was
applied to Pt working electrode using a potentiostat (EG&G Priceton
Applied Research, Model 263A). The zero-valent iron suspension
was magnetically stirred throughout the entire current measure-
ment.
(
benzenehexacarboxylic acid or MA) was chosen as a simplified
analogue of NOM [20–22]. Using 4-chlorophenol (4-CP) as a model
pollutant, this study reports the enhanced oxidation capacity of
the ZVI/O2 system in the presence of MA as a model electron shut-
tle. MA-mediated electron transfer between ZVI and O2 increased
the formation of H O for enhanced HO generation through the
Fenton-type reaction.
•
2
2
2
. Materials and methods
H O concentrations were measured spectrophotometrically
2
2
(absorbance measurement at 454 nm) using the 2,9-dimethyl-1,10-
2.1. Chemicals
phenanthroline (DMP) method [24]. Briefly, DMP (1 g in 100 mL
ethanol), copper(II) sulfate (0.01 M), and phosphate buffer (0.1 M,
pH 7) solutions were added to a 10-mL volumetric flask and mixed
together. 2 mL of the filtered sample aliquot was subsequently
added to the reagent mixture, and the remaining volume was
filled to 10 mL with distilled water. The absorbance at 454 nm
was then measured immediately. Blank samples were prepared in
the same manner except for replacing sample aliquot by distilled
Chemicals that were used as received in this study include:
-chlorophenol (4-CP, Sigma), mellitic acid (MA, Aldrich), 2,9-
4
dimethyl-1,10-phenanthroline (DMP, Aldrich), copper(II) sulfate
Kanto Chemicals), iron(II) perchlorate (Aldrich), lithium per-
chlorate (Aldrich), benzoic acid (Aldrich), 4-hydroxybenzoic acid
p-HBA, Aldrich), hydrogen peroxide (H O , Junsei), sodium chlo-
(
(
2
2
ride (Samchun), hydrochloric acid (Samchun), sulfuric acid (Junsei),
•
water. Hydroxyl radical (HO ) production was determined using
the oxidative conversion of benzoic acid (BA) to p-hydroxybenzoic
acid (p-HBA) as a probe reaction [25]. Cumulative OH radical yield
methanol (J.T. Baker), sodium phosphate (NaHPO , Aldrich) and
4
disodium phosphate (Na HPO , Aldrich). Iron powder (electrolytic
2
4
iron, 100 mesh) purchased from Fischer Scientific contains >99%
[
26,27] was then calculated by estimation of p-HBA generation in
2
iron content with a surface area of 0.3 m /g and a grain size of
ZVI slurry containing 10 mM BA in the presence or absence of MA.
The quantitative estimation of p-HBA was determined by HPLC
using 0.1% phosphoric acid and acetonitrile (85:15 v/v) as eluent,
flow rate of 1.0 mL/min and 255 nm as detection wavelength.
0
.15 mm [23]. For all experiments, it was pre-treated with 1 M HCl
for 5 min to remove surface oxide layer and the treated iron powder
was then suspended in water (typical concentration 0.2 g/L). This
aqueous slurry was then washed with degassed water (nitrogen
purged for 1 h) three times before use. Aqueous stock solutions of
MA were freshly prepared every week and stored in a refrigerator.
All solutions were prepared in distilled water (18 Mꢀ cm) prepared
by a Barnstead purification system.
3. Results and discussion
3.1. Effect of MA on ZVI-induced oxidation
To evaluate the electron transfer capacity of MA, the degrada-
2
.2. Experimental procedure
tion of 4-CP by ZVI was carried out in the presence and absence
of MA. As shown in Fig. 1, 4-CP degradation in air-equilibrated
ZVI aqueous suspension was enhanced in the presence of MA. 4-
CP degradation was not observed in the absence of ZVI, indicating
that there was no direct reaction between 4-CP and MA. Further,
the complete absence of 4-CP degradation in anaerobic conditions
The reactions were carried out in 120-mL glass bottles that were
continuously stirred on a rotary shaker (228 rpm). Using unbuffered
solutions, all experiments were carried out under open air condi-
tion to allow dissolved O equilibration in the reaction solution and
2
also to prevent its depletion during the reaction. Using stock solu-
tions of 4-CP (2 mM) and MA (5 mM), aliquots were diluted to make
the final concentration of 0.1 mM for both 4-CP and MA. The initial
pH of the suspension was then adjusted to 2.5 with 1 M HClO4.
Sample aliquots (1 mL) were withdrawn from the reactor at regu-
lar intervals, filtered through a 0.45-␮m PTFE filter (Millipore) and
injected into a 2-mL GC vial. All experiments were carried out in
triplicates for a given condition.
(
N -spurged solution) also confirmed that 4-CP was not removed
2
by direct reduction using ZVI. The concurrent generation of chloride
ions, which confirms the OH radical-mediated oxidative decompo-
sition of 4-CP [28], was also enhanced when MA was present in
the ZVI suspension (Fig. 1). Finally, when methanol (20 mM) was
added as a HO scavenger, 4-CP oxidation was completely inhibited,
which corroborates the primary role of HO in 4-CP oxidation using
•
•
the ZVI/MA system. All these results indicate that the enhanced
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Fig. 1. Degradation of 4-CP with a ZVI aqueous suspension containing MA under
different reaction conditions. Methanol was added as OH radical scavenger and
nitrogen purging was used to achieve anaerobic conditions. Experimental con-
ditions were: [ZVI] = 0.2 g/L, [4-CP]0 = 100 ␮M, [MA] = 100 ␮M, [methanol] = 20 mM
and pHi = 2.5.
•
HO production in the ZVI/MA system using molecular oxygen as a
precursor must be responsible for enhanced 4-CP oxidation.
Since carboxylate ligands form strong complexes with Fe(II) and
react more rapidly with H O2 than the Fe(II)-aquo species [5,29],
2
Fe(II) generated from ZVI corrosion can complex with MA and ini-
•
tiate O2 reduction to form H O2 and subsequently HO through
2
Fenton reaction (Eqs. (1)–(3)) [14].
•
−
[
[
[
Fe(II)-MA] + O → [Fe(III)-MA] + O2
(1)
(2)
(3)
Fig. 2. (a) Cyclic voltammetry analysis of MA aqueous solution in the presence of ZVI.
Experimental conditions were: [ZVI] = 0.2 g/L, [MA] = 1 mM, scan rate = 150 mV/s, ini-
tial potential = 0.0 V and electrolyte was 0.91 M LiClO4 at pH 2. (b) Photocurrent
measurements in the ZVI aqueous suspension in the presence of MA. Photocur-
rent was collected on a Pt electrode immersed in ZVI suspension and MA was
added into the solution after 10 min. Experimental conditions were: [ZVI] = 0.2 g/L,
2
Fe(II)-MA] + O2•− + 2H+ → [Fe(III)-MA] + H O2
Fe(II)-MA] + H O → [Fe(III)-MA] + HO + HO−
2
•
2
2
To investigate whether this complexation-mediated generation
[
MA]0 = 100 ␮M, initial potential = +0.4 V and pHi = 2.5. NaCl electrolyte solution
•
of HO can occur in the presence of MA, the oxidation of 4-CP was
tested in the solution of 2.5 mM Fe(II) and 100 ␮M MA in aero-
bic conditions. Unlike other Fe(II)-carboxylate complexes reported
in the literature [5,14,29], the oxidation of 4-CP was not observed
in the presence of Fe(II) and MA. Although this observation does
not clarify the presence or absence of a Fe(II)-MA complex, it sim-
ply implies that the Fe(II)-MA complex, if formed, is not capable of
oxygen reduction for oxidant generation. Since the oxygen activa-
tion pathway through Fe(II)-ligand complexation is more dominant
at pH > 5 [13], the use of acidic condition (pH = 2.5) in the present
system may explain the absence of the Fe(II)-MA complexation
pathway for 4-CP oxidation.
(1 M) was deoxygenated by continuous N -purging during the course of the exper-
2
iment.
shuttling effect, current measurements were performed using an
inert Pt electrode immersed in the ZVI slurry containing 1 M NaCl
electrolyte adjusted to pH 2.5 (Fig. 2(b)). In the absence of MA, the
2
low background current (∼1 ␮A/cm ) is attributed to the electron
release from ZVI oxidation accompanied by Fe(II) dissolution (Eq.
(4)). However, when MA was added in the ZVI suspension after
10 min, a sudden increase in current was observed, which indi-
cates that MA acts as an electron shuttle (electron transfer catalyst)
between Fe0 and the Pt working electrode (Eqs. (5)–(6)).
To ascertain whether 4-CP oxidation occurs via a hetero-
geneous pathway which involves the electron transfer from
ZVI surface to dioxygen mediated by MA acting as electron
shuttle, cyclic voltammetry was done to estimate the reduc-
tion potential of MA, which was determined to be −0.184 VNHE
Fe → Fe + 2e−
0
2+
(4)
(5)
(6)
−
MA + e → MAred
MAred → MA + Pt(e )
Since the solution was continuously purged with N2 to pre-
vent competitive electron scavenging by dioxygen, the observed
increase in current upon adding MA provided a good evidence for
MA-mediated electron transfer from the ZVI surface to the Pt elec-
trode.
−
(
see Fig. 2(a)). Thus, taking into account the redox potentials
0
2+
0
for ZVI corrosion [E (Fe /Fe ) = −0.44 VNHE] and oxygen reduc-
0
tion [E (O /H O ) = +0.695 VNHE] [17], H O production using
2
2
2
2
2
MA-mediated electron shuttling between ZVI and O2 is thermody-
namically feasible. To obtain a more direct evidence of the electron
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(
a)
Fig. 4. Time profiles of H2O2 generation in ZVI suspension in the presence or
absence of MA. 4-CP was not added in the solution. Experimental conditions were:
(
b)
[
ZVI] = 0.2 g/L, [MA]0 = 100 ␮M and pHi = 2.5.
Fenton reaction is marginally affected by this pH rise because the
increase in solubility of Fe(II) by ligand complexation reduces the
pH-dependence of the Fenton reaction [10,11,13]. It has been also
suggested that ligands which form bidentate-mono-nuclear com-
plexes with adjacent hydroxyl groups often enhance dissolution of
the metal surface oxides [31]. While there is no direct experimental
evidence to confirm the exact structure of the MA-oxide complex,
the observed enhancement of 4-CP degradation in the ZVI/MA sys-
tem indicates that the pH rise does not have a direct influence on
•
the HO generation mechanism.
3.2. Oxidative degradation mechanism in ZVI/MA system
The oxidation of 4-CP in the ZVI suspension containing MA as
an electron shuttle is initiated by the corrosion of ZVI in aerobic
conditions with concurrent reduction of oxygen to H O2 through
2
electron transfer mediated by MA (Eqs. (7)–(8)).
Fig. 3. (a) Effect of MA concentration and (b) Variation of solution pH during
-CP degradation with ZVI. Experimental conditions were: [ZVI] = 0.2 g/L, [4-
CP]0 = 100 ␮M, [MA]0 = 100 ␮M and pHi = 2.5
4
0
2+
Fe + MA → Fe + MAred
(7)
(8)
+
MAred + O + 2H → MA + H O2
2
2
Due to the strong tendency of MA to form surface complexes
with goethite (dominant iron oxide phase on acid-washed ZVI)
under acidic conditions [20,22], electron transfer is facilitated from
The effect of MA concentration on 4-CP oxidation in the ZVI/O2
system is shown in Fig. 3(a). When MA concentration increased
from 1 ␮M to 100 ␮M, 4-CP oxidation rate was also enhanced.
Although the increase in MA concentration may lead to higher cov-
erage of active sites on the ZVI surface, the observed increase in 4-CP
oxidation efficiently indicates that surface blocking by MA does not
have any negative impact on the electron transfer mechanism. At
the same time, the presence of MA in the ZVI/O2 system led to a
larger pH change (increase in pH) (Fig. 3(b)) during the course of
the reaction. This rapid rise in pH can be ascribed to enhanced H O
0
Fe pits on the iron particle to MA trapped on the nearby goethite
sites. The strong electrostatic interaction between the negatively
charged MA carboxyl groups and positively charged ZVI surface at
acidic pH also facilitates efficient electron transfer from Fe0 to MA
and subsequently from MA to dioxygen. The complexation of in situ
generated Fe(II) with MA then may promote the Fenton reaction for
subsequent OH radical production (Eq. (9)).
2
2
+
−
generation (O + 2H + 2 e → H O ) when more Fe(II) ions are dis-
Fe(II) − MA + H O → Fe(III) − MA + HO + OH−
•
2
2
2
(9)
2
2
soluted from ZVI corrosion (Eq. (4)) in the presence of MA acting as
an electron acceptor (Eq. (5)).
To confirm whether the increase in oxygen reduction efficiency
can be ascribed to MA-mediated electron transfer, the concentra-
However, the complexation of MA on goethite surface (major-
ity oxide surface of acid-washed ZVI [30]) may also contribute an
alternative mechanism for the higher pH rise in the ZVI/MA sys-
tem. During the complexation of MA on the goethite surface, the
release of hydroxide ions is directly responsible for the increase in
solution pH [20]. This pH increase then contributes to increased
passivation of ZVI surface through precipitation of iron hydrox-
tions of H O2 generated using ZVI were estimated in the presence
2
and absence of MA. As seen in Fig. 4, the concentration of H O2 is
2
higher in the presence of MA. Thus, in the presence of MA, both
H O formation and its subsequent decomposition are enhanced
2
2
in the initial reaction time, leading to an increase in 4-CP oxidation
within the first 1 h (Fig. 1). The observed increase in solution pH
within the same time period (Fig. 3(b)) confirms the higher con-
•
ides. However, direct HO generation through the ligand-mediated
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Fig. 5. 4-CP oxidation using Fe(II)-mediated homogeneous Fenton reaction in
Fig. 6. Time-dependent variation of MA concentration during 4-CP oxidation
the presence and absence of MA. Experimental conditions were: [Fe(II)] = 2.5 mM,
in the ZVI aqueous suspension. Experimental conditions were: [ZVI] = 0.2 g/L,
[H2O2] = 300 ␮M, [MA]0 = 100 ␮M and pHi = 2.5.
[MA]0 = 100 ␮M, [4-CP] = 100 ␮M and pH = 2.5.
i
sumption of H+ during the MA-mediated reduction of O2 to form
H O (Eq. (8)). The similar phenomenon was also observed in our
−
competitive electron acceptors (e.g., Cr(VI), NO3 ) or other pollu-
tants competing for HO•. Detailed effects of secondary pollutants,
2
2
previous study, wherein the oxidation capacity of ZVI increased in
the presence of NOM [19].
intermediates, and complex mixture will be required to understand
the overall environmental implications in further studies.
Since the Fenton reaction has been enhanced in the presence
of organic ligands due to higher reactivity of Fe(II)-ligand complex
with H O compared to the uncomplexed Fe(II) ions in previous
4. Conclusion
2
2
studies [3–5], the effect of MA acting as a complexing agent on
-CP oxidation was investigated using Fe(II) ions and H O2 in the
The present study demonstrates that mellitic acid, a NOM ana-
logue with multiple carboxylic groups, acts as an efficient electron
shuttling catalyst to enhance the oxidizing capacity of ZVI. In the
4
2
presence and absence of MA. As shown in Fig. 5, the presence of MA
in the Fe(II)/H O homogeneous system had no significant effect on
2
2
presence of MA, the increase in electron transfer efficiency from ZVI
the 4-CP oxidation efficiency. This indicates that the formation of a
Fe(II)-MA complex, if possible, does not contribute to the 4-CP oxi-
•
to dioxygen resulted in higher production of both H O and HO for
2
2
enhanced oxidation of 4-CP. Cyclic voltammetry and photocurrent
measurements support that MA can act as a good electron shuttle
dation pathway. Thus, the primary role of MA in the ZVI/O system
2
is to efficiently transfer electrons from the ZVI surface to dioxy-
gen, without any synergetic enhancement of the Fe(II)-mediated
with providing the facile dioxygen reduction and H O generation.
2
2
However, it should be noted that this study intends not to propose
the use of MA in real ZVI processes, but mainly to demonstrate a
possible role of the carboxylate moieties in NOMs in affecting the
ZVI redox chemistry.
•
decomposition of H O to HO .
2
2
To estimate the stability of MA in the ZVI-mediated 4-CP
oxidation process, the concentration of MA was also monitored
simultaneously during the 4-CP degradation reaction (Fig. 6).
The MA concentration slowly decreased initially, but was rapidly
removed after 1 h. During the initial reaction period, the decrease
of 4-CP (Fig. 1) was much more rapid compared to the decline in
MA concentration. Although this suggests that 4-CP is a more effi-
Acknowledgements
This work was supported by Global Research Laboratory pro-
gram (2014K1A1A2041044) funded by the Korea government
MSIP) through NRF.
•
•
cient scavenger of HO than MA, the degradation of MA by HO is
also possible and may result in the generation of secondary pol-
lutants. Detailed intermediate analysis and toxicity assays will be
required to confirm by-product toxicity but this is beyond the scope
of the present study. However, the rapid removal of MA after 1 h
(
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•
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-CP (Fig. 1) should still be more reactive towards HO . It appears
[
[
•
4
that MA was possibly removed from the solution as an insoluble
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[
[
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Please cite this article in press as: S.-H. Kang, et al., Electron shuttling catalytic effect of mellitic acid in zero-valent iron induced oxidative
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Please cite this article in press as: S.-H. Kang, et al., Electron shuttling catalytic effect of mellitic acid in zero-valent iron induced oxidative
degradation, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.03.009