Study on the degradation mechanism and pathway of benzene dye intermediate 4-methoxy-2-nitroaniline: Via multiple methods in Fenton oxidation process

Article abstract of DOI:10.1039/c8ra00627j

Benzene dye intermediate (BDI) 4-methoxy-2-nitroaniline (4M2NA) wastewater has caused significant environmental concern due to its strong toxicity and potential carcinogenic effects. Reports concerning the degradation of 4M2NA by advanced oxidation process are limited. In this study, 4M2NA degradation by Fenton oxidation has been studied to obtain more insights into the reaction mechanism involved in the oxidation of 4M2NA. Results showed that when the 4M2NA (100 mg L^-1) was completely decomposed, the TOC removal efficiency was only 30.70-31.54%, suggesting that some by-products highly recalcitrant to the Fenton oxidation were produced. UV-Vis spectra analysis based on Gauss peak fitting, HPLC analysis combined with two-dimensional correlation spectroscopy and GC-MS detection were carried out to clarify the degradation mechanism and pathway of 4M2NA. A total of nineteen reaction intermediates were identified and two possible degradation pathways were illustrated. Theoretical TOC calculated based on the concentration of oxalic acid, acetic acid, formic acid, and 4M2NA in the degradation process was nearly 94.41-97.11% of the measured TOC, indicating that the oxalic acid, acetic acid and formic acid were the main products. Finally, the predominant degradation pathway was proposed. These results could provide significant information to better understand the degradation mechanism of 4M2NA.

Full text of DOI:10.1039/c8ra00627j

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PAPER
Study on the degradation mechanism and pathway
of benzene dye intermediate 4-methoxy-2-
nitroaniline via multiple methods in Fenton
oxidation process
Cite this: RSC Adv., 2018, 8, 10764
Ying Guo,a Qiang Xue,
a Kangping Cui,b Jia Zhang,a Hui Wang,a Huanzhen Zhang,a
*
a
a
*
Fang Yuan and Honghan Chen
Benzene dye intermediate (BDI) 4-methoxy-2-nitroaniline (4M2NA) wastewater has caused significant
environmental concern due to its strong toxicity and potential carcinogenic effects. Reports concerning
the degradation of 4M2NA by advanced oxidation process are limited. In this study, 4M2NA degradation
by Fenton oxidation has been studied to obtain more insights into the reaction mechanism involved in
the oxidation of 4M2NA. Results showed that when the 4M2NA (100 mg Lꢀ1) was completely
decomposed, the TOC removal efficiency was only 30.70–31.54%, suggesting that some by-products
highly recalcitrant to the Fenton oxidation were produced. UV-Vis spectra analysis based on Gauss peak
fitting, HPLC analysis combined with two-dimensional correlation spectroscopy and GC-MS detection
were carried out to clarify the degradation mechanism and pathway of 4M2NA. A total of nineteen
reaction intermediates were identified and two possible degradation pathways were illustrated.
Theoretical TOC calculated based on the concentration of oxalic acid, acetic acid, formic acid, and
4M2NA in the degradation process was nearly 94.41–97.11% of the measured TOC, indicating that the
oxalic acid, acetic acid and formic acid were the main products. Finally, the predominant degradation
pathway was proposed. These results could provide significant information to better understand the
degradation mechanism of 4M2NA.
Received 21st January 2018
Accepted 11th March 2018
DOI: 10.1039/c8ra00627j
rsc.li/rsc-advances
threatened groundwater safety seriously. Therefore, it is greatly
urgent to control the BDIs pollution in our environment.13,14
In recent years, many methods, such as extraction,15,16
1. Introduction
Benzene dye intermediates (BDIs) like 4-methoxy-2-nitroaniline
(4M2NA) are important industrial chemicals and have a wide
range of applications in the dye and pharmaceutical industries.
Their production wastewater is oen highly toxic, high in color,
high in both substrate and salt concentrations, and does not
degrade easily.1–3 Because of its highly toxic, carcinogenic,
teratogenic and mutagenic hazards,4–6 BDI 4M2NA has been
listed as a priority pollutant in many countries.7 In recent years,
due to the illegal discharge and leakage in the production
process,8 BDIs have been detected in many industrial parks and
surrounding surface and underground water.9–11 According to
the Pollution Events of Tengger Desert in China,12 a wide range
of BDIs were detected with a concentration of up to 100–
120 mg Lꢀ1 in both surface and underground water, which have
adsorption17 and biodegradation,18,19 have been used to
decompose the wastewater of BDIs. Because of its stable struc-
ture and strong toxicity, BDIs wastewater is difficult to be
effectively treated by conventional physicochemical and
biochemical effects. In addition, these technologies have some
disadvantages. For example, the extraction chemicals are also
very toxic and expensive. Adsorption method is not cost effec-
tive, not easy to be regenerated and only move the pollutants
from the water phase to absorbents, which is not a complete
treatment method.6,20 Biodegradation is usually slower and with
less shock resistant.21 Therefore, it is necessary to nd a more
effective way to treat the wastewater of BDIs.
Due to the properties of anti-biodegradation, anti-oxidation
and anti-photolysis of BDIs,22 advanced oxidation technology
(AOT) seems a suitable and promising treatment method.23,24
This is based on their strong oxidation performance and a wide
range of applications of AOT.25–29 For AOT, electrochemical
incineration,30 microwave radiation,5 photocatalysis,31,32 and
other Fenton methods2,4 have been also used to treat BDIs
wastewater effectively. However, the above methods have some
aBeijing Key Laboratory of Water Resources and Environmental Engineering, School of
Water Resources and Environment, China University of Geosciences, Beijing 100083,
China. E-mail: xueqiang@cugb.edu.cn; chenhh@cugb.edu.cn; Tel: +86-10-82322281;
+86-10-82323345
bSchool of Resources and Environmental Engineering, Hefei University of Technology,
Hefei 230009, China
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limitations in practical application. Such as photocatalysis, the cause liver poisoning and hemolytic anemia.44 It has been listed
absorption range of commonly used ultraviolet light is narrow. as a priority pollutant in several countries.7
Some suspended matter and deep chroma in printing and
However, the researches at home and abroad are mostly
dyeing wastewater will affect the photocatalytic effect, and the focused on the synthesis, manufacture, physical properties43,45
electron–hole pair is easy to be recombined and inactivated.33 In and toxicity tests44 of 4M2NA, but the studies mainly on the
contrast, Fenton advanced oxidation method has a great prac- oxidative degradation by Fenton method have seldom been
tical potential, due to its high efficiency, simple equipment, reported.
short reaction time, low additional energy and easy to operate,
the Fenton oxidation process has been selected for the treat- nology have shied from the complete elimination of pollutants
ment of BDIs wastewater.34,35
to the partial degradation of pollutants aiming at reducing
In recent years, the implementation goals of Fenton tech-
During the Fenton reaction, the chain reaction between Fe2+ toxicity of pollutants and improving the biodegradability of
and H2O2 catalyzes the formation of hydroxyl radicals (cOH) wastewater. Therefore, monitoring the reaction intermediates is
with strong oxidizing ability,36 which can oxidize many kinds of also essential for understanding and predicting the biocom-
organic matters non-selectively and is especially good for the patibility of treated wastewater by Fenton method.46,47 Hence, it
treatment of organic wastewater that is recalcitrant to biological is of great signicance to understand the characteristics and
treatment or general chemical oxidation.37,38
mechanism of the degradation of 4M2NA by Fenton method.
In this study, 4M2NA was selected as the research target.
Therefore, in recent years, researchers have paid more
attention to the use of various Fenton technologies for the Firstly, the effect of various factors (initial pH, initial H2O2
treatment of BDIs, and study the mechanism and identify the concentration, and initial Fe2+ concentration) on Fenton
degradation products. El-Ghenymy et al.4 have studied the use oxidative degradation of 4M2NA was analyzed. Then the
of electro-Fenton and solar photoelectro-Fenton technology for mechanism and possible pathway of the 4M2NA degradation
the treatment of sulfanilic acid solutions, optimized the pro- were predicted using UV-Vis spectra analysis based on Gauss
cessing parameters, and identied the intermediate products. peak tting, HPLC measurement combined with two-
Masomboon et al. have compared the characteristics and dimensional correlation spectroscopy and GC-MS analysis.
kinetics of oxidation degradation of 2,6-dimethylaniline by Finally, based on the theoretical TOC calculation between
Fenton,39 electro-Fenton40 and photoelectro-Fenton process,41 4M2NA and main reaction intermediates, the predominant
and discussed the reaction mechanism and possible degrada- degradation pathway was also proposed.
tion intermediates. Shen J. et al.42 studied the Fenton tech-
nology combined with zero valent iron reduction for the
pretreatment 2,4-dinitroanisole and discussed the change of
intermediate products in each reaction stage.
2. Materials and methods
2.1 Chemicals and materials
Although several studies have been conducted on the
oxidation of Fenton to some of the BDIs, however, with the
development of dye intermediates industry, more stable BDIs
have been developed and applied. 4M2NA, as an important new
benzene dye intermediate, has been widely used in the manu-
facture and synthesis of many organic dyes, drugs and sensitive
materials in recent years.43 Compared to other BDIs, 4M2NA has
electron donating group (–NH2), electron withdrawing groups
(–NO2) and neutral groups (–OCH3), and its structure is more
complex. In addition, 4M2NA, a highly toxic substance, has
potential mutagenic and carcinogenic effects and even can
The 4M2NA (CAS: 96-96-8), with the purity of 99%, was
purchased from J&K Scientic Ltd., The NaOH, anhydrous
Na2SO4, FeSO4$7H2O, H2O2 (30%), methanol, phosphoric acid,
KH2PO4, n-hexane, formic acid, acetic acid and oxalic acid
were received from Sinopharm Chemical Reagent Co., Ltd. The
physical and chemical properties of 4M2NA are shown in
Table 1.
2.2 Experimental setup
The 4M2NA wastewater sample (500 mL) was put in a beaker (1 L)
with a magnetic stirrer. The initial solution pH was adjusted to
Table 1 Physical and chemical properties of 4M2NA
Molecular
weight
Melting
point
Boiling
point
Flash
point
Vapor
pressure
Chemical formula
Density
Characters and solubility
Orange red powder,
soluble in water, ethanol,
ether, slightly soluble in benzene
168.15
1.318 g cmꢀ3
124.0 ꢁ
C
338.3 ꢁ
C
158.4 ꢁ
C
9.93 ꢂ 10ꢀ5 mmHg
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the desired value by using 0.1 M H2SO4 or 0.1 M NaOH solution. A
measured amount of catalytic FeSO4$7H2O was added, followed
by the addition of H2O2 into the solution to start the reaction. At
selected time intervals (1, 5, 10, 30, 60, 90, 120, 180 min), 5 mL
aliquots of the reaction mixture were taken and immediately
injected with 1.0 or 0.1 M NaOH (or methanol, for the identi-
cation of organic products) to terminate the reaction. The mixed
solution was centrifuged for 10 min at 12 000 rpm, and the
supernatant was withdrawn for the subsequent analysis.
All measurements were performed in triplicate, and the
results displayed in tables were the average value of at least
three measurements with an accuracy of ꢃ5%.
x~j(v) ¼ x~(v,tj) ¼ x(v,tj) ꢀ xꢀ(v), j ¼ 1, 2,., m,
~
The variable v is the spectral index of HPLC. x(v,tj) is the
dynamic spectra measured at m equally spaced points in
perturbation t between t1 and tm, and the average spectrum
m
X
xðvÞ ¼
xðv; tjÞ=m has been subtracted from raw data x(v,tj).
j¼1
The synchronous correlation intensity of two spectral indices
can be directly calculated from the following:
m
X
1
Fðv1; v2Þ ¼
x~jðv1Þx~jðv2Þ
m ꢀ 1
j¼1
2.3 Chemical analysis
Synchronous correlation spectrum F(v1,v2) represents
synchronous changes of two spectral intensities measured at v1
and v2. A positive F(v1,v2) indicates that the spectral intensities
at v1 and v2 are either increasing or decreasing simultaneously.
A negative value indicates that one of the spectral intensities is
increasing while the other is decreasing.51 The two-dimensional
correlation spectroscopy analysis was produced using 2Dshige
soware (Kwansei-Gakuin University, Japan).
The concentration of 4M2NA was measured by high perfor-
mance liquid chromatography (HPLC, LC-20AT, SHIMADZU),
equipped with a C18 analytical column (5 mm, 150 mm ꢂ 4.6
mm) at 30 ꢁC. The mobile phase was methanol at a ow rate of
0.5 mL minꢀ1, and the detection wavelength was set at 254 nm.
The injection volume was 10 mL. TOC was measured using
a TOC analyzer (TOC-VWP, SHIMADZU). The residence time
was 12 min. pH was determined by glass electrode method.
The oxidation products of 4M2NA were identied by GC-MS
(Agilent 6890N/5975C) measurement. At selected time inter-
vals, 15 mL reaction solution (with 5 mL methanol) was
extracted with 10 mL hexane in triplicate. The organic phases
3. Results and discussion
3.1 Degradation of 4M2NA
were combined and dried by nitrogen gas at a gentle stream, In this study, experiments were carried out to investigate the pH
and then dissolved in 1 mL hexane for GC-MS analysis. and operational variables that could affect the 4M2NA degra-
Organic phase was injected into the GC-MS for analysis aer dation efficiency for the preparation of suitable reaction
desiccation with anhydrous Na2SO4. The oxidation products conditions.52
were analyzed using the GC-MS with a HP-5MS chromato-
3.1.1 Effect of initial pH. The effect of pH on the 4M2NA
graphic column (60 m ꢂ 0.25 mm ꢂ 0.25 mm). The tempera- degradation is illustrated in Fig. 1. As can be seen from Fig. 1(a),
ture programmed was that the initial temꢁperature of 40 ꢁC at the initial one min of the reaction, the degradation efficiency
maintained for 7 min and post run at 150 C maintained for of 4M2NA in each group reached about 40%, but there were still
4 min. The ow rate of the carrier gas (He) was 1.0 mL minꢀ1 differences under different pH conditions. When pH was 2.0,
and the split ratio was 10 : 1. The temperature of the ion the overall reaction rate was lower than that of the other groups,
source was 230 C, respectively. The energy of the ion source and the apparent rate constant was only 0.0068 sꢀ1. However,
ꢁ
was 70 eV. The full scan spectrum (m/z 25–500) was acquired when pH was in the range of 3.0–7.0, the rate of reaction
and the solvent delay was set to 4 min. Chromatograms were decreased slightly with an increase of pH. The rate constant
compared between the control and experimental groups to (0.0276 sꢀ1) was highest at pH of 3.0. And when the reaction was
detect possible products.
carried out at 180 min, the removal efficiency of 4M2NA could
Low molecular weight organic acids (LMWOAs) were reach more than 98%.
measured by (HPLC, LC-20AT, SHIMADZU), equipped with an
On the other hand, as shown in Fig. 1(b), when pH was 2.0,
inert sustain swi-C18 analytical column (5 mm, 250 mm ꢂ 4.6 the TOC removal efficiency was only 7.66%, but the TOC
mm) at 30 ꢁC. The mobile phase was KH2PO4 solution (with 2% removal efficiency (16.88%) was the highest at pH of 3.0. When
methanol) methanol at a ow rate of 0.8 mL minꢀ1, and the pH was increased from 3.0 to 7.0, the removal efficiency of TOC
detection wavelength was set at 210 nm. The injection volume was slightly decreased, but they were basically maintained at
was 10 mL.
The UV-Vis spectras of 4M2NA samples were analyzed by Fenton oxidation can be carried out at a wide pH range.
using Gauss multi-peak tting.48 The method is based on the
around 13.0–16.0%. In summary, the degradation of 4M2NA by
For analysis, when the solution pH was lower (pH ¼ 2.0), the
description of each spectrum by a series of Gaussian peaks.49 high concentration of H+ reacted with cOH and reduced the
The HPLC spectra of a series of 4M2NA samples were analyzed amount of cOH production, thus affecting the overall efficiency
by the method of two-dimensional correlation spectroscopy of the reaction.53 Besides, the formation of the complex species
analysis described by Noda and Ozaki.50 The dynamic spectra [Fe(H2O)6]2+ slows down the oxidation reaction due to its slower
that represent the variation of spectral intensity compared with reaction with H2O2 compared to that of [Fe(OH)(H2O)5]+. The
the reference spectrum can be calculated from the following:
formation of the stable oxonium ion [H3O2]+ in the presence of
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Fig. 1 Effect of initial pH on (a) degradation of 4M2NA and (b) TOC removal and kobs ([4M2NA]0 ¼ 100 mg Lꢀ1, [H2O2]0 ¼ 2 mM, [Fe2+]0 ¼ 0.4 mM,
and temperature ¼ 25 ꢁC; the pH without adjustment was about 6.12 ꢃ 0.01).
high concentrations of H+ also makes the peroxide electrophilic due to the formation of large amount of oxygen bubbles,
enhancing its stability and presumably hindering the reaction thereby wasting a large number of cOH. Therefore, it is very
activity between H2O2 and Fe2+
.
On the contrary, generally important to determine the most suitable dosage of H2O2. The
54,55
under the conditions of near neutral pH, the production of cOH effect of initial H2O2 concentration on the 4M2NA degradation
is inhibited by ferric hydroxo complexes. Moreover, Fe2+ or Fe3+ is illustrated in Fig. 2.
tends to generate a variety of amorphous complex precipita-
In this study, the degradation effect of 4M2NA by Fenton
tion,56 which signicantly decreases the oxidation capability of method was gradually increased with an increase of initial H2O2
the system. However, the degradation efficiency was generally concentration. Besides, it could be seen from Fig. 2(a) that the
good, when the pH was in the range of 3.0–7.0 in our current degradation efficiency of 4M2NA did not decrease signicantly
study. It was probably due to the formation of LMWOAs in the when the concentration of H2O2 was too high. However, from
reaction process,57 so that the solution pH can be reduced Fig. 2(b), the reaction rate and overall TOC removal efficiency
rapidly to about 3.0 when the initial pH near neutral condition, increased greatly with an increase the initial H2O2 concentra-
and the treatment efficiency is not greatly affected.
tion (from 1.0 to 4.0 mM), and the increase rate became slow
Based on the above results, the degradation of 4M2NA by when the initial concentration of H2O2 increased from 4.0 to
Fenton method was feasible and efficient at an initial pH of near 10.0 mM. The apparent rate constant kobs showed a similar
neutral condition. Therefore, subsequent experiments were trend. However, if according to the ratio of the TOC removal and
performed under unadjusted pH (6.12 ꢃ 0.01) condition.
initial H2O2 concentration to calculate effective utilization
3.1.2 Effect of initial H2O2 concentration. Generally, it has coefficient of H2O2, it could be found that with an increase of
been accepted that the degradation efficiency of an organic initial H2O2 concentration, the effective utilization coefficient of
contaminant increases with an increase of the concentration of H2O2 was stable from 1.0 to 4.0 mM but decreased substantially
H2O2. However, in the actual wastewater treatment process, from 4.0 to 10.0 mM. The maximum effective utilization coef-
large initial H2O2 dosage may affect the treatment efficiency, cient of 0.3785 was observed at the initial H2O2 concentration
Fig. 2 Effect of initial H2O2 concentration on (a) degradation of 4M2NA and (b) TOC removal and kobs ([4M2NA]0 ¼ 100 mg Lꢀ1, [Fe2+
0.4 mM, and temperature ¼ 25 ꢁC; pH ¼ 6.12 ꢃ 0.01).
]
¼
0
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of 4.0 mM. This is because too much H2O2 generated large substances highly recalcitrant to the Fenton process had been
amounts of oxygen, which reduce the efficiency of effective formed58 (the analysis of intermediates formed was discussed in
utilization of H2O2.39
detail below).
Therefore, considering the above factors, the most suitable
initial concentration of H2O2 was 4.0 mM, simultaneously the
TOC removal efficiency and the apparent rate constant were
30.70% and 0.0512 sꢀ1, respectively.
3.2 Oxidation products and proposed pathways
The oxidative degradation of 4M2NA using Fenton reagent by
producing cOH was a complex chemical reaction process. The
cOH increases or terminates the reaction chain by means of
hydrogen abstraction, addition, fragmentation, etc. Because of
the high activity of cOH, the reaction is usually fast with low
activation energy, and several reaction pathways can occur
simultaneously, thus producing complex chemical products. In
this study, we used UV-Vis, HPLC, and GC-MS methods to
characterize the variation of the products during the degrada-
tion of 4M2NA. Based on the results in 3.1 Section, the reaction
conditions were set as follows: [4M2NA]0 ¼ 100 mg Lꢀ1, [H2O2]0
¼ 4.0 mM, [Fe2+]0 ¼ 0.4 mM, pH ¼ 6.12 ꢃ 0.01, temperature ¼
3.1.3 Effect of initial Fe2+ concentration. Experiments were
performed as the Fe2+ initial concentration ranged from 0.1 to
1.0 mM. The results are shown in Fig. 3.
The results showed that the degradation of 4M2NA by Fen-
ton oxidation was enhanced signicantly with an increase of
initial concentration of Fe2+. The apparent rate constant
increased exponentially from 0.0009 to 0.1674 sꢀ1 when the
initial Fe2+ concentration ranged from 0.1 to 1.0 mM. And when
the initial Fe2+ concentration was larger than or equal to
0.4 mM, the removal efficiency of 4M2NA could reach 100%
aer 180 min. But the TOC removal efficiency initially increased
then decreased. When the initial concentration of Fe2+
increased from 0.1 to 0.4 mM, the TOC removal efficiency
increased from 3.26% to 31.54%, reaching the maximum value.
However, when the initial concentration of Fe2+ continued to
increase up to 1.0 mM, the TOC removal efficiency gradually
decreased to 22.95%.
ꢁ
25 C.
3.2.1 UV-Vis. In order to explain the structural character-
istics of intermediates, UV-Vis analysis was carried out for
samples taken at different time periods. And the UV-Vis spectra
obtained at different reaction time (0, 1, 30, 90 and 180 min)
were analyzed by Gauss peak tting. The possible structures and
their changes were also analyzed, as shown in Fig. 4.
Fe2+ is the catalyst in Fenton oxidation reaction. With an
increase of initial Fe2+ concentration, H2O2 could be catalyzed
to produce more cOH, which could degrade 4M2NA effectively.
However, when the initial concentration of Fe2+ increased to
a certain level, the TOC removal efficiency was still minor
although the removal of 4M2NA was quite signicant, which
was probably because 4M2NA produced more refractory inter-
mediates. On the other hand, excessive Fe2+ react with cOH to
cause the depletion of cOH (cOH + Fe2+ / Fe3+ + OHꢀ), thereby
reducing the degradation efficiency of organic constituents in
the whole system. Therefore, a suitable initial concentration of
Fe2+ was 0.4 mM.
Fig. 4(a) showed full wavelength UV-Vis absorption spectra of
4M2NA proceeded Fenton reaction. Wiff chromophore theory
can be used to explain the relationship between the chromo-
spheres behavior of most dyes and their intermediates and
chromospheres.59 For the dye intermediate 4M2NA, the chro-
mosphere is benzene ring, the chromophore is –NO2, and the
chromophore bodies are –NH2 and –OCH3. Through the Gauss
peak tting (Fig. 4(b)), it could be seen clearly that the absorp-
tion peaks of the ultraviolet region at 230, 258 and 285 nm were
characteristic peaks of benzene ring, –NH2 and –OCH3,
respectively.6 The absorption peak at 450 nm corresponded to
the –NO2 chromophore, produced by n / p* transition,
namely, the visible chromophore.
The above results revealed that when the 4M2NA degrada-
tion was quite complete, it seemed the apparent removal for
total TOC did not occur, which was a sign that some organic
The experimental results showed that with an increase of
processing time, the peak intensities at 450, 230, 258 and
Fig. 3 Effect of initial Fe2+ concentration on (a) degradation of 4M2NA and (b) TOC removal and kobs ([4M2NA]0 ¼ 100 mg Lꢀ1, [H2O2]0 ¼ 4 mM,
and temperature ¼ 25 ꢁC; pH ¼ 6.12 ꢃ 0.01).
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Fig. 4 Evolution of UV-Vis spectrum of 4M2NA during Fenton process (Gauss peak fitting).
285 nm in the visible region were gradually weakened, i.e., the degradation process more accurately, more analytic methods
–NO2, benzene ring, –NH2 and –OCH3 were gradually destroyed. should be carried out for further understanding of the reaction
However, when the reaction time was 1 min, a new absorption mechanism.
peak appeared at 326 nm in the ultraviolet region. The
3.2.2 HPLC. In order to further clarify the degradation
absorption peak was mainly expressed as C]O or C]N bond, mechanism of 4M2NA in Fenton process, the analysis of HPLC
indicating the possibility of producing ketones or acids.6 When and two-dimensional correlation spectroscopy of retained
the reaction time increased from 0 to 30 min, the intensity of peaks was carried out, as shown in Fig. 5.
absorption peak gradually increased, then it decreased step by
step when the reaction proceeded from 30 to 180 min.
Fig. 5(a) showed the retention peaks of solution in the
chromatographic column at different reaction times aer Fen-
The TOC removal efficiency was approximately 30% aer the ton oxidation. The X axis indicated the retention time, and the Y
reaction, indicating that there were still a great amount of axis indicated response sensitivity. The peak at retention time of
intermediate products in the solution. In order to illustrate the 9.623 min was the characteristic peak of 4M2NA, and the peak
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Fig. 5 HPLC spectrum (a) and two-dimensional correlation spectroscopy (b) in the process of degradation of 4M2NA by Fenton oxidation.
area gradually decreased with the reaction time prolonged, chain carboxylic acid produced by the opening of the benzene
indicating that the 4M2NA molecule was fully degraded in ring. Because such organic acids were linked to hydroxyl
a relatively short time (less than 1 h), which was consistent with groups, carboxyl groups and other polar functional groups, and
the result achieved by UV-Vis spectrum measurement. Among the retention time in the chromatographic column always
them, the characteristic peaks at retention time of 5.302 and short.59
7.102 min indicated the trace impurities in the original sample,
It should be noted that this HPLC operating conditions were
namely, 5-methoxybenzofurazan (CAS: 4413-48-3) and 3-nitro- only suitable for relatively weaker polar organic molecules. For
anisole (CAS: 555-03-3), which were also decayed as the reaction the main components of TOC in the later stage of the reaction,
proceeded. When the reaction was carried out at 1 min, three further analysis should be carried out by targeted methods such
new characteristic peaks appeared at 1.502, 1.801 and as GC-MS.
1.921 min, and the peak intensity at 1.502 min was higher than
3.2.3 GC-MS. The 4M2NA produced a variety of interme-
those at both 1.801 and 1.921 min. This was probably because diate products during the Fenton oxidation process. In order to
4M2NA was rapidly oxidized by cOH to produce phenols and the identify the structures of intermediates, GC-MS analyses of
cOH capture hydrogen from phenol, causing it open the ring to samples at different time intervals (0, 1, 5, 30, 60, 90 and 180
form the short chain carboxylic acids. As the reaction min) were carried out. The mass spectra corresponding to the
continued, the area of the peak at 1.502 min initially increased larger peaks at the total ion ow were analyzed, and their
and then decreased. While, the peak areas at both 1.801 and similarity was evaluated by computer. Finally, the possible
1.921 min decreased gradually.
composition structure was obtained as shown in Table 2.
The two-dimensional correlation spectroscopy method was
According to the total ion ow diagram of the standard dye
used to further analyze the spectrum data of HPLC. The sample at 0 min, the peak at retention time of 17.455 min indi-
synchronous maps generated from the HPLC spectra during the cated 4M2NA, and the remaining peaks were dominated by
whole oxidation process are shown in Fig. 5(b). The sequential column loss and indicated some trace impurities such as
relationship of the main peaks was obtained through the 5-methoxy-2,1,3-benzoxadiazole (CAS: 4413-48-3) and 1-methoxy-
comparison of the signs (positive or negative) from synchro- 3-nitrobenzene (CAS: 555-03-3). As the reaction proceeded, more
nous maps (upper-le corner). As mentioned above, a positive small peaks appeared aer 1 min reaction, mainly including
sign F(v1,v2) indicates that the spectral intensities at v1 and v2 newly generated 5-methoxy-2,1,3-benzoxadiazole (CAS: 4413-48-
are either increasing or decreasing simultaneously. A negative 3), 1-methoxy-3-nitrobenzene (CAS: 555-03-3) and 4-methoxy-2-
value indicates that one of the spectral intensities is increasing, nitrophenol (CAS: 1568-70-3). At the same time, some new
while the other is decreasing.51 As can be seen, the comparison peaks were formed, mainly including 4-methoxy-1,2-benzenediol
of the signs at upper-le corner of Fig. 5(b) was negative. (CAS: 3934-97-2), 4-methoxy-1,2-benzoquinone (CAS: 69818-23-1),
Therefore, the change of 4M2NA peak area showed the opposite toluene (CAS: 108-88-3). When the reaction proceeded to 5 min,
trend with that of the three unknown peaks, which indicated the above substances were still present and other new peaks were
that some other products were produced during the process of formed including (2E,4Z)3-methoxy-2,4-hexadienedinitrile (CAS:
the 4M2NA oxidative degradation.
1789-46-4), 3-butenamide (CAS: 432491-67-3), methacrylamide
Since the degradation process of BDIs by Fenton method was (CAS: 79-39-0), acrylamide (CAS: 79-06-1), 3-hexanone (CAS: 589-
complex, it was difficult to infer the detailed degradation 38-8), 2-methyl-3-pentanone (CAS: 565-69-5), ethyl cyanate (CAS:
products of 4M2NA only on the basis of the retention peaks of 627-48-5), cyanoacetic acid (CAS: 372-09-8), acetic acid (CAS: 64-
HPLC.
19-7), acetaldehyde (CAS: 75-07-0) and hydrogen azide (CAS: 7782-
According to previous studies, it was speculated that the 79-8). Aer 30 min of reaction, the peaks above gradually dis-
peaks at retention time of 1.502 min were probably the short appeared. However, there were still several small peaks. Further
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Table 2 Intermediate products detected from TIC by GC-MS
Compound name
Retention time/min
CAS number
565-69-5
Molecular structure
2-Methyl-3-pentanone
4.119
4.140
Methacrylamide
79-39-0
Toluene
4.381
4.449
108-88-3
627-48-5
Ethyl cyanate
Acrylamide
4.574
4.580
79-06-1
Hydrogen azide
7782-79-8
3-Hexanone
4.638
5.173
589-38-8
4-Methoxy-1,2-benzenediol
3934-97-2
4-Methoxy-1,2-benzoquinone
7.645
69818-23-1
Acetaldehyde
Acetic acid
9.671
75-07-0
64-19-7
10.553
5-Methoxy-2,1,3-benzoxadiazole
1-Methoxy-3-nitrobenzene
13.408
13.441
4413-48-3
555-03-3
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Table 2 (Contd.)
Compound name
Retention time/min
13.563
CAS number
1789-46-4
372-09-8
Molecular structure
(2E,4Z)3-Methoxy-2,4-hexadienedinitrile
Cyanoacetic acid
15.549
4-Methoxy-2-nitrophenol
17.193
1568-70-3
4-Methoxy-2-nitroaniline
3-Butenamide
17.455
18.360
96-96-8
432491-67-3
identication by HPLC showed that the nal products were and then the substitution reaction took place to form poly-
mainly oxalic acid (CAS: 144-62-7), formic acid (CAS: 64-18-6) and phenols60 such as 4-methoxy-1,2-benzenediol, which might
other substances.
react with Fe3+ to produce Fe2+ and 4-methoxy-1,2-benzoqui-
3.2.4 Proposed degradation pathways of 4M2NA. Accord- none.61 However, 4-methoxy-1,2-benzoquinone was unstable.
ing to the identication results of intermediate products by UV- When it was further attacked by cOH, the benzene ring was
Vis, HPLC and GC-MS measurements and the information in oxidized to fracture to form oxalic acid, acetaldehyde and other
Table 2, the possible degradation pathway of 4M2NA in Fenton substances,62 which continued to be oxidized to form acetic
process could be shown in Fig. 6. There were two possible acid, formic acid, etc., and eventually were mineralized to
pathways for the degradation of 4M2NA by Fenton oxidation.
From Fig. 6, for the major pathway (red box), at the begin-
carbon dioxide, water and so on.
On the other hand, there may be another way. 5-Methoxy-
ning of the reaction, 4M2NA was oxidized to 2-nitro-4- 2,1,3-benzoxadiazole and 1-methoxy-3-nitrobenzene were
methoxyphenol. Because of the highest density of electron detected at the early stage of reaction. Although these impuri-
clouds near the –NH2 of the three substituents on 4M2NA, cOH ties contained in the original 4M2NA sample, the peak area
attacked –NH2 rst, and –NH2 was oxidized to form hydroxyl varied with the reaction. So it was possible to combine –NO2
(–OH). At the same time, 4M2NA might also undergo deami- with –NH2 on the 4M2NA molecule to produce 5-methoxy-2,1,3-
nation to produce 1-methoxy-3-nitrobenzene, which was then benzoxadiazole in the early intense oxidizing atmosphere.
attacked by cOH and a hydroxyl substitution reaction occurred Moreover, 4M2NA is usually used as an raw material for the
to produce 2-nitro-4-methoxyphenol. Because if there is an manufacture of 5-methoxy-2,1,3-benzoxadiazole. Therefore, the
ortho- and para-position orienting group (–OCH3) and an inter- result was reasonable. As the reaction continued, (2E,4Z)3-
position orienting group (–NO2) on the benzene ring, the posi- methoxy-2,4-hexadienedinitrile, ethyl cyanate and hydrogen
tion of the third substituent is determined mainly by the ortho- azide were identied mainly due to the attack by cOH on the
and para-position orienting group, namely –OCH3. Therefore, C]N bond and the benzene ring, resulting in that the ring of 5-
the generation of 2-nitro-4-methoxyphenol from 1-methoxy-3- methoxy-2,1,3-benzoxadiazole was broken. Meanwhile, the
nitrobenzene was possible and reasonable. The presence of detected cyanoacetic acid, ethyl cyanate, hydrogen azide, 3-
–OH in benzene ring enhanced its activity. As the reaction butenamide, methacrylamide, acrylamide might be the
continued, the –NO2 on the benzene ring was attacked by cOH,
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Fig. 6 Proposed degradation pathway of 4M2NA by Fenton method.
products of (2E,4Z)3-methoxy-2,4-hexadienedinitrile aer cOH the major components of the remaining TOC in the system.59,63
attack. They might eventually continue to be fully mineralized. The LMWOAs identied in this study mainly included oxalic
In summary, since the Fenton reaction is rapid and complex, acid, acetic acid and formic acid, and the variation of their
the above reactions may occur simultaneously. Subsequent concentrations with time were detected by HPLC method.
oxidation of these intermediate by-products yielded the Besides, to further conrm the major degradation pathway, the
formation of short-chain carboxylic acids like oxalic acid, acetic measured TOC at various time intervals during the degradation
acid and formic acid, which were nally mineralized into CO2 process of 4M2NA was compared with the theoretical TOC
and H2O aꢀs well as the formation of other inorganic ions like calculated by the actual concentration of 4M2NA, oxalic acid,
+
ꢀ
NH4 , NO3 and NO2
.
acetic acid and formic acid, as shown in Fig. 7.
In general, LMWOAs are the end products of the degradation
The concentration of acetic acid increased rapidly with
process of Fenton systems, such as acetic acid, which was reaction time from 1 to 60 min, and reached a peak value of
difficult to be attacked by cOH. Hence, LMWOAs are likely to be 68.88 mg Lꢀ1 at 60 min. Then, as the reaction continued, the
concentration of acetic acid decreased gradually. Aer 120 min,
the decrease rate slowed down, and the concentration dropped
to 29.95 mg Lꢀ1 at 180 min. The concentration of oxalic acid
increased gradually with the reaction proceeding and remained
at about 8.42 mg Lꢀ1 at the later stage of the reaction. Oxalic
acid and iron ions could form quite stable organic complexes,
which were not susceptible to degradation attacked by free
radical.59 The concentration of formic acid showed a gradually
increasing trend, and the nal concentration was about
85.37 mg Lꢀ1 at 180 min. On the other hand, it was clear that the
theoretical TOC was nearly 94.41–97.11% of the measured TOC,
which indicated that the oxalic acid, acetic acid and formic acid
were the main end products and the pathway marked by red box
was the major degradation way. These results provide signi-
cant information to deep understand the degradation mecha-
nism of 4M2NA.
ꢀ
ꢀ
Besides, NH4+, NO3 and trace amounts of NO2 were
detected at concentrations of 44.78, 12.71 and 0.36 mg Lꢀ1
respectively at the end of the reaction, which indicated that the
Fig. 7 Evolution of the major small molecular acid and measured and
theoretical TOC.
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removal of –NO2 and –NH2 by cOH during Fenton oxidation Water Pollution Control and Treatment Science and Technology
process was partly mediated by ionization.
Major Project (2015ZX07406005-001).
4. Conclusions
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