Catalytic wet air oxidation of phenol with functionalized carbon

Transcription

Catalytic wet air oxidation of phenol with functionalized carbon
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Catalytic wet air oxidation of phenol with functionalized carbon materials as catalysts: Reaction
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Catalytic wet air oxidation of phenol with functionalized
carbon materials as catalysts: Reaction mechanism
and pathway
Jianbing Wang1,⁎, Wantao Fu1 , Xuwen He1 , Shaoxia Yang2 , Wanpeng Zhu2
1. School of Chemical and Environmental Engineering, Beijing Campus, China University of Mining and Technology, Beijing 100083,
China. E-mail: [email protected]
2. School of Environment, Tsinghua University, Beijing 100084, China
AR TIC LE I N FO
ABS TR ACT
Article history:
The development of highly active carbon material catalysts in catalytic wet air oxidation (CWAO)
Received 5 September 2013
has attracted a great deal of attention. In this study different carbon material catalysts
Revised 22 December 2013
(multi-walled carbon nanotubes, carbon fibers and graphite) were developed to enhance the
Accepted 28 January 2014
CWAO of phenol in aqueous solution. The functionalized carbon materials exhibited excellent
Available online 23 June 2014
catalytic activity in the CWAO of phenol. After 60 min reaction, the removal of phenol was nearly
100% over the functionalized multi-walled carbon, while it was only 14% over the purified
Keywords:
multi-walled carbon under the same reaction conditions. Carboxylic acid groups introduced on
Catalytic wet air oxidation
the surface of the functionalized carbon materials play an important role in the catalytic activity
Phenol
in CWAO. They can promote the production of free radicals, which act as strong oxidants in
Carbon materials
CWAO. Based on the analysis of the intermediates produced in the CWAO reactions, a new
Radical
reaction pathway for the CWAO of phenol was proposed in this study. There are some
Reaction pathway
differences between the proposed reaction pathway and that reported in the literature. First,
maleic acid is transformed directly into malonic acid. Second, acetic acid is oxidized into an
unknown intermediate, which is then oxidized into CO2 and H2O. Finally, formic acid and oxalic
acid can mutually interconvert when conditions are favorable.
© 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.
Published by Elsevier B.V.
Introduction
Wet air oxidation (WAO) is one of the most effective technologies
for degrading hazardous, toxic and highly concentrated organic
contaminants in aqueous solutions. In WAO, organic compounds
can be ultimately oxidized to CO2 and H2O and some other
innocuous end products without the emission of HCl, NOx, and
SOx, dioxins and fly ash (Luck, 1999; Yang et al., 2005).
Although WAO is a promising technology, its application in
industrial wastewater treatment is limited because it is usually
performed at high temperature (125–320°C) and pressure (0.5–
20 MPa). Catalytic wet air oxidation (CWAO) is more promising
than WAO because it can be performed under much milder
conditions. In recent years many noble metal catalysts and
transition metal oxide catalysts have been proposed for CWAO
(Yang et al., 2007a, 2010; Li et al., 2007; Fortuny et al., 1999; Liu et
al., 2006). Although these catalysts have presented high activity in
CWAO, the deactivation of catalytic activity usually occurs during
the reaction. The catalytic activity deactivation mainly results
from the leaching of active components from the catalysts under
the severe operating conditions (Fortuny et al., 1999; Kim et al.,
2007). Therefore, we sought to develop effective catalytic systems
using readily available and stable materials.
Since carbon materials including graphite, activated carbon
(AC), multi-walled carbon nanotubes (MWCNTs) and nanofibers
are stable under both acidic and basic reaction conditions, the use
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⁎ Corresponding author.
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http://dx.doi.org/10.1016/j.jes.2014.06.015
1001-0742/© 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
J O U R NA L OF EN V I RO N M EN T A L S CI EN C ES 2 6 ( 20 1 4 ) 1 7 41–1 7 4 9
5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) was purchased from
Sigma-Aldrich Chemical Company (St. Louis, MO, USA). A stock
solution of DMPO (100 mmol/L) was prepared and stored in the
dark at −20°C. Commercial graphite and CFs were obtained from
Tianjing Chemical Reagent Company in China (Tianjing, China).
MWCNTs from Tsinghua Nafine Nano-powder Commercialization Engineering Centre (Beijing, China) were prepared by a
chemical vapor deposition (CVD) method (Yang et al., 2007b). The
other reagents were purchased from Beijing Chemical Company
in China (Beijing, China) and used without further purification.
1.2. Catalyst preparation
Before use, the graphite and CFs were immersed in a HNO3
solution (5%) for 24 hr while the MWCNTs were immersed in
an HCl solution (37%) for 30 min. Then each sample was
divided into two parts. One part was washed several times
with deionized water, dried at 80°C in air overnight and then
crushed. The resulting samples were referred to as purified
carbon materials. The other part was functionalized by
wet-chemical treatment with strong acids (HNO3–H2SO4) to
introduce surface functional groups onto the surface of the
carbon materials as follows: (1) The graphite was impregnated
The surface areas and pore structure of the carbon materials
were determined by N2 adsorption at 77 K in an automated gas
sorption system (Autosorb, Quantachrome, Boynton Beach, FL,
USA). Before each measurement, the samples were outgassed at
573 K for 3 hr. SEM measurement was carried out on a scanning
electron microscope (HITACHI S-4500, Tokyo, Japan) at an
accelerating voltage of 15 kV. The FT-IR analysis was carried
out on a spectrophotometer (Magna-IR 750, Bruker, Karlsruhe,
Baden-Württemberg, Germany). A spectrum was recorded in the
range of 500–4000 cm−1 (100 scans) at a resolution of 4 cm−1.
Boehm titration was conducted to determine the number of
oxygen-containing functional groups on the surface of carbon
materials according to the procedures described by Boehm
(2002). One gram of carbon sample was added into 50 mL of
NaHCO3 solution (0.05 mol/L) and stirred at room temperature
for 24 hr. The resultant slurry was then filtered and titrated with
HCl to determine the number of carboxylic acid groups.
1.4. Catalytic test and analysis
CWAO of phenol was carried out in a 1-L autoclave reactor,
which was specifically described in our previous study (Zhu
et al., 2002). First 500 mL of 2000 mg/L phenol solution and
0.4 g of carbon materials (if necessary) were introduced to the
reactor. The reactor was then sealed. After purging with
nitrogen gas for 10 min, the reactor was heated to the reaction
temperature of 160°C. As soon as the temperature was
reached, pure oxygen was introduced into the reactor until
the total pressure was equal to 2.5 MPa. This time was set as
“zero” reaction time. Afterwards, the reaction was performed
for 120 min. During the reaction, water samples were periodically taken from the reactor. The used carbon materials were
washed with deionized water, dried about 80°C overnight, and
then analyzed to characterize their structure.
In order to determine the adsorption capacity of the carbon
materials for phenol, adsorption experiments were conducted
with the carbon materials at room temperature for 24 hr. The
dosage of the carbon materials and initial phenol concentration were the same as in the experiments for the CWAO of
phenol.
The experiments for the CWAO of carboxylic acids were
conducted according to the procedures mentioned above. The
tested carboxylic acids included maleic acid, acrylic acid,
malonic acid, oxalic acid, acetic acid and formic acid. The used
catalyst was the functionalized MWCNTs.
The catalytic activity of the carbon materials in CWAO was
assessed by the removal of phenol. The concentration of phenol
was determined using an HPLC method with an ODS-3 column
and a UV detector (LC-10AD, Shimadzu, Kyoto, Japan). The
mobile phase was a mixture of methanol/water (0.1% acetic
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1.1. Reagents and materials
1.3. Characterization
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1. Materials and methods
in a mixture of 67% HNO3 and 98% H2SO4 (1:3, in volume),
sonicated for 20 min and kept overnight at room temperature;
(2) The CFs were refluxed at 85°C for 2 hr in a mixture of 67%
HNO3 and 98% H2SO4 (1:3, in volume); and (3) The MWCNTs
were sonicated for 20 min in a mixed solution of 67% HNO3
and 98% H2SO4 (1:3, in volume), and then refluxed at 50°C for
4 hr (Yang et al., 2007b).
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of them as catalyst supports arouses great interest (Serp et al.,
2003; Liu et al., 2008; Gallezot et al., 1997; Huang et al., 2009.
Carbon materials can be also used as catalysts directly (Serp et al.,
2003; Liu et al., 2011, 2013; Pyun, 2011). AC with high surface
area was investigated first as a catalyst in CWAO and there
have been many studies on its activity and stability (Stuber et al.,
2005; Suarez-Ojeda et al., 2005, 2007; Santiago et al., 2005;
Morales-Torres et al., 2010). Later the use of MWCNTs and
commercial nanofibers (CNFs) as catalysts was also studied in
CWAO (Serp et al., 2003; Yang et al., 2007b, 2008; Soria-Sánchez et
al., 2011). Soria-Sánchez et al. (2011) studied the CWAO of phenol
over CNFs, CNTs and graphite. They thought that the surface
functional groups on the materials might be favorable for the
activity of the catalysts. In our previous work, we also found that
the surface functional groups on MWCNTs have a great effect on
catalyst activity (Yang et al., 2007b, 2008). However, the mechanism whereby the surface functional groups influence catalytic
activity remains unclear (Barbier et al., 1998; Rivas et al., 1998).
Moreover, which types of surface functional groups play a main
role in catalyst activity is also unknown. Finally, the reported
reaction pathway for the CWAO of phenol over carbon materials
includes many assumptions, and deserves more study.
In this study, we developed carbon material catalysts, including
MWCNTs, carbon fibers (CFs) and graphite, with different chemical
treatment methods. The catalysts were characterized by scanning
electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). In addition, we studied the effect of surface
functional groups on the catalyst activity of the carbon materials in
the CWAO of phenol, detected the free radicals produced in the
reaction with electron paramagnetic spin resonance (ESR), and
analyzed the intermediates produced in the oxidation of various
compounds. Based on the results from these experiments we
investigated the mechanism through which surface functional
groups influence the catalyst activity of carbon materials and
explored the reaction pathway for the CWAO of phenols over the
carbon materials.
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JO U R N A L OF EN V I RO N M EN T A L S CI E NC ES 2 6 ( 20 1 4 ) 1 7 4 1–1 7 4 9
300 μm
1 μm
a
5 μm
b
c
Fig. 1 – SEM images of the functionalized MWCNTs (a), CFs (b), and graphite (c).
acid was added) with a ratio of 60/40 (V/V). The detection
wavelength was 254 nm. The TOC (total organic carbon) of the
water samples was analyzed with a TOC analyzer (TOC-Vwp,
Shimadzu, Kyoto, Japan).
To identify the intermediates produced in the various
CWAO experiments, water samples were also analyzed with
the same HPLC system. The mobile phase was a mixture of
methanol/water (0.1% phosphoric acid was added) with a ratio
of 10/90 (V/V). The detection wavelength was 210 nm.
ESR spectra were recorded on an EPR 300E spectrometer
(Bruker, Karlsruhe, Baden-Württemberg, Germany). DMPO was
used as a trap. The liquid samples in the reactor were sprayed
into a vessel containing the DMPO solution (100 mmol/L), which
was immediately frozen in liquid nitrogen to avoid the quenching
of radicals. The irradiation source (λ = 532 nm) was a Quanta-Ray
Nd:YAG pulsed laser (Bruker, Karlsruhe, Baden-Württemberg,
Germany). The measurement was performed under the following
conditions: center field, 3470.00 G; sweep width, 100.0 G; microwave frequency, 9.77 GHz; power, 5.05 mW; time constant,
20 ms; and scan time, 200 sec.
2. Results and discussion
2.1. Characterization of carbon materials
Fig. 1 shows the SEM images of the functionalized carbon
materials. The functionalized MWCNTs appear as interlacing
a
b
1672
1029
1553
1258
1452
Graphite
CFs
1727
MWCNTs
1107
1156
1568
1033
802
Graphite
724
1727
1727
CFs
tortuous filaments with diameters in the range of 10–15 nm
and lengths up to several tens of micrometers (Fig. 1a).
The functionalized CFs show a rod-type structure with
diameters in the range of 5–30 μm and lengths up to 400 μm
(Fig. 1b). The functionalized graphite shows a regular
lamellar structure at the micrometer scale, with diameters
in the range of 3–10 mm and stacking thickness of sheets
greater than 1 mm (Fig. 1c).
The FT-IR analysis was conducted to obtain detailed
information on the functional groups on the surface of the
carbon materials. The recorded spectra are shown in Fig. 2. No
significant band was observed in the range of 2000–4000 cm−1
in our study. The bands at ca. 1580 and 1160 cm−1, observed
for all samples, are attributed to the aromatic C_C stretching
vibrations involving sp2- and sp3-hybridized carbons, respectively (Dandekar et al., 1998; Kim et al., 2005; Kouklin et al.,
2004). The bands in the 1000–1450 cm−1 range, which are also
detected for all carbon materials, are commonly assigned to
the C-O vibrations originating from different surface species
(Dandekar et al., 1998; Kim et al., 2005). The functionalized
carbon materials have a band at ca. 1720 cm− 1, which is
assigned to the C_O stretching vibrations in carboxylic acid
groups (Dandekar et al., 1998; Kim et al., 2005), while the
purified carbon materials do not have this band (Fig. 2a, b).
The observation clearly indicates that the functionalization
treatments effectively introduce oxygen-containing functional groups, such as carboxylic acid groups, on the surface of the
functionalized MWCNT, graphite and CFs. The content of
c
1002
1157
1568
1246
1520
Graphite
1582 1160 1038
1571
1746
1027
CFs
1563
1142
1734
1458
1542
1180
1137
1458
1087
1038
MWCNTs
1714
1176
1554
1026
1458
MWCNTs
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2500
2000 1500 1000
Wavenumber (cm-1)
500
3000
2500
2000 1500 1000
Wavenumber (cm-1)
500
3000
2500
2000 1500 1000
Wavenumber (cm-1)
500
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Fig. 2 – FT-IR spectra of the fresh purified (a) and functionalized ( b) carbon materials and the used functionalized carbon
materials (c).
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Table 1 – Main characteristic of the functionalized carbon
materials.
Samples
Surface
area (m2/g)
Fresh Used
MWCNTs
CFs
Graphite
195
19
21
Adsorption
capacity a
(mgphenl/gcat)
Content of
carboxylic acid
groups (mmol/g)
83
13
51
1.45
0.79
0.21
183
19
18
a
Derived from the difference between the initial phenol concentration and final phenol concentration.
WAO
MWCNTs
CFs
Graphite
15
10
5
0
0
30
60
90
Reaction time (min)
120
Fig. 3 – Phenol removal in the CWAO of phenol over the
purified carbon materials.
carboxylic acid groups on the surface of the different
functionalized carbon materials ranks as follows: MWCNTs >
CFs > graphite (Table 1).
2.2. CWAO of phenol over carbon materials
In the WAO of phenol (without catalyst), the removal of phenol
was only 10% after 120 min reaction (Fig. 3). In the CWAO of
phenol with the purified carbon materials, the removals of
80
a
b
70
MWNTs
CFs
Graphite
60
80
60
WAO
MWCNTs
CFs
Graphite
40
20
50
40
30
20
0
20
40
60
80
Reaction time (min)
100
120
0
0
20
40
60
80
Reaction time (min)
100
120
sc
Fig. 4 – Phenol (a) and TOC ( b) removals in the CWAO of phenol over the functionalized carbon materials.
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Phenol conversion (%)
100
TOC removal (%)
Phenol conversion (%)
20
phenol were no more than 25% after 120 min reaction (Fig. 3). If
the contributions of WAO and adsorption were deducted from
the overall phenol removal, the remaining phenol removals
attributable to catalytic oxidation with the purified carbon would
be less than 10%. Thus the purified carbon materials do not show
excellent activity in the CWAO of phenol.
The functionalized carbon materials significantly improved the removal of phenol in the CWAO of phenol, and
the removals of phenol over them were almost 100% after
120 min reaction (Fig. 4). It should be pointed out that the
removal rates of phenol in the presence of the functionalized
carbon materials were much higher than those in the
presence of the purified carbon materials. For example, the
nearly complete removal of phenol was achieved after 60 min
reaction over the functionalized MWNTs (Fig. 4a), while the
removal of phenol was only 14% over the purified MWNTs
under the same operating conditions (Fig. 3). The adsorption
capacities of the functionalized carbon materials for phenol
are shown in Table 1. The functionalized MWNTs, with the
highest surface area, displayed the highest adsorption capacity for phenol. The adsorption capacities of the functionalized
carbon materials suggest that the adsorption of phenol on the
functionalized carbon materials resulted in low phenol
removal (less than 15%) in CWAO of phenol. Most of the
phenol removals can be attributed to catalytic oxidation. Thus
the functionalized carbon materials show excellent activity in
the CWAO of phenol. The order of the catalytic activity of the
functionalized carbon materials is as follows: MWCNTs >
CFs > graphite (Fig. 4).
Usually in catalytic oxidation a catalyst with larger surface
area exhibits higher activity. However, this was not observed
in our study. The phenol removal rate in the reaction with the
functionalized CFs is faster than that with the functionalized
graphite, while the surface area of the functionalized CFs is
lower than that of the functionalized graphite (Table 1). This
indicates that the surface area was not the determining factor
for the catalytic activity of the functionalized carbon materials in the CWAO of phenol.
It should be noticed that the order of the activity for the
functionalized carbon materials is in good agreement with
that of the content of carboxylic acid groups on the surface of
these materials. Namely, the higher the content of surface
1745
JO U R N A L OF EN V I RO N M EN T A L S CI E NC ES 2 6 ( 20 1 4 ) 1 7 4 1–1 7 4 9
2.3. Active species produced in the CWAO of phenol
b
a
3420
3440
3460
3480
Magnetic field (G)
3500
3520
Fig. 5 – ESR spectra of DMPO-added phenol aqueous solution in
the WAO (a) and CWAO with the functionalized MWCNTs ( b).
carboxylic acid groups the carbon materials have, the higher
the activity they display in the CWAO of phenol. SoriaSánchez et al. (2011) have drawn the similar conclusion
through XPS (X-ray photoelectron spectroscopy) analysis of
CNFs, CNTs and graphite. In our previous study we also found
that MWCNTs with higher content of surface functional
groups exhibit higher activity (Yang et al., 2007a, 2007b; Yang
et al., 2008). As both the XPS and FI-IR analysis show this
correlation between catalyst activity and the content of the
surface functional groups for the carbon materials, we
conclude that carboxylic acid groups on the surface of the
carbon materials play an important role in their activities in
the CWAO of phenol.
After the CWAO of phenol, the carbon materials were
taken from the reactor and studied to obtain information
about any possible structural changes. There was little
difference in surface area between the fresh and used carbon
materials. Fig. 2 shows that the bands for carboxylic acid
surface groups (ca. 1720 cm−1) appearing on the fresh carbon
materials slightly shift to ca. 1735 cm− 1 for the used samples.
This can be attributed to the adsorption of organic compounds on the surface of the carbon materials during the
reaction (Yang et al., 2008). This shows that these carbon
materials have good chemical stability in the CWAO reaction.
It has been commonly accepted that the mechanism of the
CWAO of organic compounds involves radical chain reactions
U
with active species such as OU−
2 , OOH, OH and OR. These reactive
species greatly enhance the oxidation of organic compounds. In
our study the ESR spin-trapping technique was used to determine
whether radicals were produced in the reactions. In the absence
of catalysts, no observable ESR signal was detected (Fig. 5 line a),
indicating that no radicals or very low concentration of radicals
are formed in the WAO experiments. However, clear ESR signals
were detected when the functionalized MWCNTs were added
into the reaction system. These signals could be characteristic of
the 4-oxo-TEMPOL adduct according to the literature (Yamakoshi
et al., 2003; Xu et al., 2012). The signals of DMPO-OOH and
DMPO-OOH were not observed, perhaps due to the presence of
4-oxo-TEMPOL. According to the ESR results, we propose that
U
OOH/OU−
2 species could be formed in the CWAO of phenol. Thus
the functionalized carbon materials can effectively promote the
production of the active radicals which enhanced the CWAO of
phenol.
2.4. Reaction pathway for the CWAO of phenol
In this study complete mineralization of organic compounds was not achieved after 120 min reaction in the
CWAO of phenol (Fig. 4), indicating that some refractory
intermediates were formed during the reaction. In order to
investigate the reaction pathway for the CWAO of phenol,
we used the HPLC method to analyze the reaction solutions
withdrawn from the reactor. The identification of the peaks
was carried out by direct comparison of retention times
with the corresponding standards. It should be pointed out
that there was no 100% correspondence between real peaks
and injected standards because of the complexity of the
intermediates.
In this study the selected standards and identified intermediates are listed in Table 2. Though Santos et al. (2002)
reported that phenol oxidation starts by hydroxylation of the
molecule leading to hydroquinone or catechol, catechol was
not detected in the reaction solutions in our study. This might
be due to the difference of the catalyst used between the two
studies.
Maleic acid is the main intermediate in the CWAO of
phenol according to references (Santos et al., 2002). We also
Table 2 – Selected standards and identified intermediates for the analysis of reaction pathway.
Malonic acid, oxalic acid, acetic acid and formic acid
Acetic acid and formic acid
Formic acid
Unknown compound
Oxalic acid
Malonic acid, oxalic acid, acetic acid and formic acid
.cn
Malonic acid
Oxalic acid
Acetic acid
Formic acid
Acrylic acid
Hydroquinone, p-benzoquinone, maleic acid,
fumaric acid, cis butenedioic anhydride, malonic
acid, oxalic acid, acetic acid and formic acid
.ac
Maleic acid
Hydroquinone, catechol, p-benzoquinone, maleic acid,
fumaric acid, cis butenedioic anhydride, butyric acid,
succinic acid, propionic acid, acrylic acid, malonic acid,
oxalic acid, acetic acid and formic acid
Butyric acid, succinic acid, propionic acid, acrylic acid,
malonic acid, oxalic acid, acetic acid and formic acid
Oxalic acid, acetic acid and formic acid
Acetic acid, formic acid
Oxalic acid, formic acid
Oxalic acid and acetic acid
Malonic acid, oxalic acid, acetic acid and formic acid
Identified intermediates
sc
Phenol
Selected standards
je
Test compound
1746
J O U R NA L OF EN V I RO N M EN T A L S CI EN C ES 2 6 ( 20 1 4 ) 1 7 41–1 7 4 9
detected it in the reaction solutions. Moreover, we detected
fumaric acid (the structural isomer of maleic acid) and cis
butenedioic anhydride (the acid anhydride of maleic acid) in
the reaction solutions with very low concentration. No studies
have reported that these intermediates were observed in the
CWAO of phenol.
In order to further elucidate the oxidation pathway of
phenol, we conducted the CWAO experiments with maleic
acid, malonic acid, oxalic acid, acetic acid and formic acid. The
reaction solutions were also analyzed with the HPLC method
to investigate the organic compounds produced in the
reactions. The selected standards and identified intermediates are shown in Table 2. The evolution of the concentrations
of the parent compounds and intermediates is shown in Fig. 6.
In the experiments for the CWAO of maleic acid, the
identified intermediates include malonic acid, oxalic acid,
acetic acid and formic acid (Fig. 6a). Acetic acid was not
measured in the first 30 min of reaction. As acetic acid is
2000
Maleic acid
Gormic acid
Oxalic acid
Malonic acid
Acetic acid
a
Concentration (mg/L)
1600
400
200
700
b
600
Concentration (mg/L)
1800
500
400
Formic acid
Malonic acid
Acetci acid
300
200
100
1.0
20
60
80
Time (min)
100
0
0
120
c
0.8
0.4
Unknown
0.2
70
60
50
40
30
20
10
0
25000
20
40
60
80
100
120
Acetic acid
Concentration (mg/L)
0.6
0.0
0
2100
2080
2060
2040
2020
2000
0
20
140
120
100
80
60
40
20
0
0
2000
60
80
Time (min)
100
120
d
Oxalic acid
20
40
60
80
100
120
Formic acid
1500
1000
500
20
40
60
80
Time (min)
100
0
0
120
20
40
60
80
Time (min)
100
120
1000
e
900
Formic acid
Oxalic acid
Malonic acid
Acetic acid
Acrylic acid
f
800
Formic acid
20
40
60
80
100
120
Oxalic acid
2000
1500
700
600
500
400
300
1000
200
500
100
0
0
20
40
60
80
Time (min)
100
120
0
20
40
60
80
Time (min)
100
120
.cn
0
40
.ac
Concentration (mg/L)
40
Concentration (mg/L)
Concentration (mg/L) Normalized concentration (C/C )
max
0
0
je
sc
Fig. 6 – Evolution of the concentration of parent compound and the intermediates in the CWAO of (a) maleic acid, ( b) malonic
acid, (c) acetic acid, (d) formic acid, (e) oxalic acid, and (f ) acrylic acid.
1747
JO U R N A L OF EN V I RO N M EN T A L S CI E NC ES 2 6 ( 20 1 4 ) 1 7 4 1–1 7 4 9
1400
Calculated TOC
Measured TOC
TOC (mg/L)
1200
1000
800
600
400
0
20
40
60
80
Time (min)
100
120
Fig. 7 – Variation of the measured and calculated TOC in the CWAO of phenol.
OH
O
O
O
O
OH
OH
O
OH
HO
O
HOOCCH 2COOH
CH 3COOH
O
HCOOH
O
O2
HOOCCOOH
CO 2+H 2O
CH 2O 2
Fig. 8 – Proposed reaction pathway for the CWAO of phenol.
.cn
O
.ac
OH
sc
OH
intermediate for the oxidation of acetic acid, although the
detection of dioxirane was not reported in their study. This
unknown peak might be attributed to dioxirane. However, as
we were not able to obtain dioxirane with high enough purity
as a standard, it is hard for us to determine whether the peak
is attributable to dioxirane.
Oxalic acid was identified as an intermediate of the oxidation
of formic acid (Fig. 6d). Oxalic acid formation from the oxidation
of formic acid can be explained as a termination path in
free-radical oxidation of formic acid. In this process a hydroxyl
radical attacks the aliphatic compound to remove a hydrogen
atom bonded to a carbon atom, and the free-radicals (UCOOH)
thus form oxalic acid. It is surprising that formic acid was also
identified as the intermediate for the oxidation of oxalic acid
(Fig. 6e). The explanation for this is that oxalic acid is thermally
decomposed into formic acid under the conditions of CWAO. The
concentrations of the intermediates produced in the oxidation of
oxalic acid and formic acid were no more than 100 mg/L. This
suggests that most of the oxalic acid and formic acid were
oxidized directly into carbon dioxide and water. In the CWAO of
oxalic acid, the concentration of formic acid still increased after
the concentration of oxalic acid decreased to zero. This indicates
that there are other intermediates that were not identified in the
CWAO of oxalic acid.
Santos et al. (2002) reported that acrylic acid is an
intermediate in the oxidation of phenol. They also reported
that acrylic acid comes from the oxidation of maleic acid and
will be oxidized into malonic acid in the CWAO of phenol.
However, acrylic acid was not identified in the reaction
solutions from the CWAO of phenol and maleic acid in our
study. As acrylic acid is relatively stable compared to oxalic
acid and formic acid (Fig. 6f), we conclude that acrylic acid is
not an intermediate in the CWAO of phenol with the
functionalized MWCNTs. We also did not identify butyric
acid, succinic acid or propionic acid as intermediates in the
CWAO of phenol with the functionalized MWCNTs.
The theoretical TOC can be calculated from the concentrations of intermediates. Fig. 7 shows the variation of the
measured TOC determined from the carbon analyzer and the
theoretical TOC calculated from the concentration of the
intermediates. As observed, there is little difference between
je
relatively stable in CWAO (Barbier et al., 1998), we conclude
that acetic acid is not the intermediate directly produced from
the oxidation of maleic acid. It might be produced from the
oxidation of malonic acid and oxalic acid.
In the CWAO of malonic acid, its concentration decreased
quickly (Fig. 6b), and both acetic acid and formic acid were
soon detected in the reaction solutions. This indicates that
acetic acid and formic acid are intermediates in the direct
oxidation of malonic acid.
In our study, there are two peaks in the HPLC spectrum of
the reaction solutions from the CWAO of acetic acid (Fig. 6c).
One is attributed to acetic acid, and the other is unknown.
Most references reported that acetic acid is directly oxidized
into carbon dioxide and water (Luck, 1999; Barbier et al., 1998;
Santos et al., 2002). Duprez et al. (1996) proposed a mechanism
for the CWAO of acetic acid, in which dioxirane is an
J O U R NA L OF EN V I RO N M EN T A L S CI EN C ES 2 6 ( 20 1 4 ) 1 7 41–1 7 4 9
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (No. 51078143) and the Fundamental
Research Funds for the Central Universities of China.
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Shandong University, China
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The Ohio State University, USA
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Jae-Seong Lee
Sungkyunkwan University, South Korea
Christopher Rensing
University of Copenhagen, Denmark
Bojan Sedmak
National Institute of Biology, Slovenia
Lirong Song
Institute of Hydrobiology,
Chinese Academy of Sciences, China
Chunxia Wang
National Natural Science Foundation of China
Gehong Wei
Northwest A & F University, China
Daqiang Yin
Tongji University, China
Zhongtang Yu
The Ohio State University, USA
Environmental toxicology and health
Jingwen Chen
Dalian University of Technology, China
Jianying Hu
Peking University, China
Guibin Jiang
Research Center for Eco-Environmental Sciences,
Chinese Academy of Sciences, China
Sijin Liu
Research Center for Eco-Environmental Sciences,
Chinese Academy of Sciences, China
Tsuyoshi Nakanishi
Gifu Pharmaceutical University, Japan
Willie Peijnenburg
University of Leiden, The Netherlands
Bingsheng Zhou
Institute of Hydrobiology,
Chinese Academy of Sciences, China
Environmental catalysis and materials
Hong He
Research Center for Eco-Environmental Sciences,
Chinese Academy of Sciences, China
Junhua Li
Tsinghua University, China
Wenfeng Shangguan
Shanghai Jiao Tong University, China
Yasutake Teraoka
Kyushu University, Japan
Ralph T. Yang
University of Michigan, USA
Environmental analysis and method
Zongwei Cai
Hong Kong Baptist University,
Hong Kong, China
Jiping Chen
Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, China
Minghui Zheng
Research Center for Eco-Environmental Sciences,
Chinese Academy of Sciences, China
Municipal solid waste and green chemistry
Pinjing He
Tongji University, China
Environmental ecology
Rusong Wang
Research Center for Eco-Environmental Sciences,
Chinese Academy of Sciences, China
Editorial office staff
Managing editor
Editors
English editor
Qingcai Feng
Zixuan Wang
Suqin Liu
Catherine Rice (USA)
Zhengang Mao
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