Enhanced degradation of gaseous benzene by a Fenton reaction

Article abstract of DOI:10.1039/c6ra26016k

A wet scrubbing process coupled with advanced oxidation processes (AOP) has raised great interest for the abatement of volatile organic compounds (VOCs) owing to its strong oxidation capacity and few byproducts. Fenton as an AOP has been widely used in wastewater treatment. However, the dosing method of Fenton reagents is not suitable for the treatment of continuous-flow VOCs waste-gas since H₂O₂, a major reagent in the Fenton reaction, will be rapidly eliminated. In this study, the Fenton process was used to enhance benzene degradation in an aeration container with intermittent dosing of H₂O₂. With the same total dosage of H₂O₂, the benzene removal efficiency reached 85% by intermittent dosing while it was quickly decreased to 0% by dosing in a lump. The generated CO₂ by intermittent dosing of H₂O₂ is nearly 3 times that in the latter. In addition, no by-product was generated in the gas phase. The possible pathways and mechanism of benzene degradation by the Fenton process were proposed according to the detected intermediates in the solution. This study provides a simple and efficient way to significantly enhance VOCs degradation by a Fenton reaction.

Full text of DOI:10.1039/c6ra26016k

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Enhanced degradation of gaseous benzene by
a Fenton reaction†
Cite this: RSC Adv., 2017, 7, 71
*
Gaoyuan Liu, Haibao Huang, Ruijie Xie, Qiuyu Feng, Ruimei Fang, Yajie Shu,
Yujie Zhan, Xinguo Ye and Cheng Zhong
A wet scrubbing process coupled with advanced oxidation processes (AOP) has raised great interest
for the abatement of volatile organic compounds (VOCs) owing to its strong oxidation capacity
and few byproducts. Fenton as an AOP has been widely used in wastewater treatment. However,
the dosing method of Fenton reagents is not suitable for the treatment of continuous-flow VOCs
waste-gas since H2O2, a major reagent in the Fenton reaction, will be rapidly eliminated. In this study,
the Fenton process was used to enhance benzene degradation in an aeration container with
intermittent dosing of H2O2. With the same total dosage of H2O2, the benzene removal efficiency
reached 85% by intermittent dosing while it was quickly decreased to 0% by dosing in a lump. The
generated CO2 by intermittent dosing of H2O2 is nearly 3 times that in the latter. In addition, no by-
product was generated in the gas phase. The possible pathways and mechanism of benzene
degradation by the Fenton process were proposed according to the detected intermediates in the
solution. This study provides a simple and efficient way to significantly enhance VOCs degradation
by a Fenton reaction.
Received 29th October 2016
Accepted 9th December 2016
DOI: 10.1039/c6ra26016k
www.rsc.org/advances
conditions.9,10 Fenton reagent, consisting of both H2O2 and
ferrous sulfate, as an AOP has been widely used for wastewater
Introduction
treatment since it can generate highly active, non-specically
but short-lived $OH.11–14 The overall mechanism of the Fenton
reaction is generally known by reaction (1).15,16
Many volatile organic compounds (VOCs) are harmful to human
health and detrimental to the atmospheric environment. Some
of them have potential toxicity and carcinogenicity.1 They
are key precursors for the formation of atmospheric secondary
organic aerosols (SOA) and O3. Many attempts have been
made to remove VOCs pollution using physical, chemical and
biological methods,2 including absorption,3 catalytic combus-
tion,4 photocatalytic oxidation5 and biolters.6 However, these
methods still cannot meet with the increasingly stringent
discharge standards. For example, the process of adsorption
just transfers VOCs from waste gas to the adsorbent but does
not destroy them. Catalytic combustion needs expensive cata-
lysts and excessive energy consumption for low-concentration
VOCs. Photocatalytic oxidation has some disadvantages such
as catalytic deactivation and low efficiency.7 Worst of all, these
removal methods may produce intermediates, resulting in
secondary gaseous pollution.8
Fe2+ + H2O2 / Fe3+ + $OH + OHꢀ
(1)
As for wastewater treatment, the pollutants are generally well
mixed with Fenton reagent. Fenton reagents, including H2O2,
could be dosed in a lump, generating $OH to rapidly degrade
contaminants in short time. However, such way of dosing H2O2
is not suitable for the treatment of waste-gas in continuous-ow
since the oxidizing capacity was quickly decreased with the
consumption of H2O2. Recently, photo-Fenton has been used
to enhance VOCs degradation2,17 while it requires additional
UV energy and still become inefficient with the consumption of
H2O2. Therefore, it is of great interest to take simple and effi-
cient steps to enhance oxidation capacity of Fenton reaction for
VOCs degradation. In addition, as an emerging method for
VOCs degradation, the mechanism of Fenton reaction is still
not clear.
In this study, we rstly report the enhanced degradation of
VOCs via the Fenton process by intermittent dosing of H2O2.
Toxic and refractory benzene was chosen as a representative
VOC. The intermediates from benzene degradation were
detected and the possible pathways and mechanism of Fenton
process were proposed.
As an alternative technology, wet scrubbing process coupled
with advanced oxidation processes (AOP) has raised great
interest in VOCs degradation owing to its merits such as
simplicity, strong oxidation capacity and mild reaction
School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou,
China. E-mail: seabao8@gmail.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra26016k
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automatic gas sampling valve. The intermediates from benzene
degradation in the liquid and gas phase were extracted with
dichloromethane and then analyzed by GC-MS (DSQII, Thermo
Fisher). The acid substances, which cannot be extracted with
dichloromethane from the liquid phase, were detected by the
ion chromatography (DX-600, Dionex). The concentrations of
H2O2,19 ferrous ion and total Fe ion in the solution were
detected by a spectrophotometer (UV2550, Shimadzu). The
concentrations of total carbon (TC), total organic carbon (TOC)
and inorganic carbon (IC) were analyzed by a TOC meter (TOC-V
CSH, Shimadzu).
Experimental methods
Chemicals and reagents
In the present study the H2O2 (30% solution, reagent grade) was
obtained from Yong Da Chemical Reagents (Tianjin) while
ferrous sulfate was provided from Sinopharm Chemical
Reagent. Benzene vapor, generated from passing an air stream
into a benzene solution, was used as an indicative VOC. The
benzene solution was saved in the refrigerator for a stable
supply of VOC. The intermediates, generated from the benzene
degradation, were extracted by dichloromethane. Both the
liquid benzene (analytical grade) and dichloromethane (chro-
matographic grade) were obtained from Tianjin Fuyu Fine
Chemical. Sulfuric acid, which was used to adjust the pH value
of Fenton reagent, was acquired from Guangzhou Chemical
Reagent Factory. All the reagents were used without any further
purication.
Results and discussion
Benzene degradation by Fenton
The performance of Fenton reaction for benzene degradation
was compared with that of a reference solution made up with
pure water. As shown in Fig. 2, without Fenton reagent, benzene
removal in the initial stage is mainly ascribed to physical
absorption by the water and no CO2 product was observed. In
case of pure water, H2O2 and Fe2+ alone, benzene concentration
at the outlet was quickly increased to about 22 ppm in 30 min
due to the saturation of benzene absorption in the aqueous
solution. As for Fenton reagent, benzene concentration sharply
dropped to below 2.3 ppm and remained at about 5 ppm for
60 min, indicating that benzene was removed continuously by
the Fenton reaction. The optimum [H2O2]/[Fe2+] ratio for Fenton
reaction is approximately 4 : 1 (shown in Fig. S1†). $OH gener-
ated by the Fenton reagents rapidly attacked the organic
molecules. However, with the consumption of the Fenton
reagents and the amount of $OH reduced, leading to a decrease
in benzene removal efficiency aer 90 min's reaction (shown
in Fig. 3). Therefore, it is highly necessary to enhance the
oxidizing capacity of the Fenton reaction.
Experimental setup
Schematic of the experimental setup is shown in Fig. 1. A
continuous-ow aeration container, made of Pyrex glass with
a diameter of 65 mm, height of 290 mm and effective volume of
0.5 L, was adopted for generating a stream of air containing the
target pollutant. The temperature of the solution in the aeration
container was kept at 25 ꢁC by heat collection-constant
temperature type magnetic stirrer (DF-101S, Gongyi Yu Hua
Instrument). Air, free of CO2, was generated from a zero air
generator (HGA-5L, Beijing Huilong). The air was used to
bubble through a benzene solution to generate an air stream
containing high concentration of benzene vapor. The benzene
vapor was diluted in the buffer and then fed to the bottom of the
reactor through an aerator. Flow rate of the simulated gas was
set at 1 L minꢀ1 and the initial concentration (C0) of benzene
was xed at 25 ppm. Fenton reagents, with 4 mmol of H2O2 and
1 mmol of ferrous ion, were added into the aeration container.
The initial pH value of the solution was adjusted to 3.0 which is
the optimal value for Fenton reactions.18 The concentrations of
benzene (Ct), and CO2 (generated from benzene oxidation) in
gas phase were measured by a gas chromatograph (GC) (9790II,
Fuli) equipped with two ame-ionization detectors (FID). One
FID was connected with a Rt-Q-BOND PLOT column (30 m ꢂ
0.25 mm id, lm thickness 10 mm) and used for benzene anal-
ysis. The other FID equipped with a packed column (TDX-01,
3 m ꢂ 3 mm) followed by a methanizer which was used to
determine the concentrations of CO2. Gas samples from the
reaction system were dried and fed to the GC on-line by an
Effect of intermittent dosing of H2O2
In the test of intermittent dosing, 4 mmol H2O2 was dosed into
the 0.5 L aeration container every 2 h for 4 rounds while
16 mmol H2O2 was dosed once at a time as for the test of H2O2
Fig. 2 Comparison of Fenton reaction and reference solution for
Fig. 1 Schematic of experimental setup.
72 | RSC Adv., 2017, 7, 71–76
benzene degradation.
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dosing in a lump. Their performance for benzene degradation is time of H2O2 dosing would further improve the oxidizing
shown in Fig. 3. Benzene removal efficiency [¼(C0 ꢀ Ct)/C0 ꢂ ability.
100%] in gas phase was gradually declined with the consump-
Fig. 5 shows the concentration of CO2 generated from
tion of H2O2 in both cases. Benzene removal efficiency was kept benzene degradation by Fenton with the intermittent and the
at about 80% for nearly 90 min in the test of H2O2 dosing in lump dosing cases. CO2 was quickly generated due to the
a lump. Then, it dropped to near 0% aer 150 min's reaction generation of $OH through the addition of H2O2 but its
owing to the depletion of H2O2 as shown in Fig. 4. In the test concentration dropped as the H2O2 was gradually consumed.
of intermittent dosing of H2O2, the oxidizing capacity was kept The CO2 concentration reached 45 ppm aer 30 min's reaction
for much longer time. The proles of benzene degradation in the case of H2O2 dosing in a lump and dropped to nearly
are similar in the 4 rounds of H2O2 dosing with benzene 0 ppm aer 150 min's reaction. This is not completely consis-
removal efficiency reached 85% in each run. Aer the benzene tent with the result of benzene removal efficiency probably due
removal efficiency dropped to near 0%, the oxidizing capacity to the absorption of benzene and CO2 in the solution. As for
of Fenton was quickly recovered with the addition of H2O2. It is intermittent dosing of H2O2, the CO2 concentration also
clear that benzene degradation via Fenton was greatly enhanced reached about 45 ppm for the rst round while it gradually
by intermittent dosing of H2O2. As for H2O2 dosing in a lump, increased to about 70 ppm in the fourth round. $OH can be
H2O2 was soon consumed and abundant $OH were generated continuously generated in the case of intermittent dosing of
while only part of the $OH was used for benzene degradation H2O2. CO2 was formed not only from benzene degradation but
due to its short-lived life. However, $OH can be continuously also from the degradation of the intermediates in the solution.
generated and used for the degradation of continuous-ow For the whole process, the CO2 yield with intermittent dosing of
benzene waste gas in the case of intermittent dosing of H2O2. H2O2 is nearly 3 times of that of H2O2 dosing in a lump. It
It is expected that continuous supply of H2O2 or shorter interval conrms that intermittent dosing of H2O2 can greatly enhance
VOCs and its intermediates.
Theoretically, nearly 150 ppm of CO2 can be produced if all
the removed benzene was completely oxidized into CO2.
However, the measured concentration of CO2 at outlet is much
lower than the theoretical value. ꢃ25% of CO2 (shown in
Fig. S2†) was dissolved in solution, which was conrmed by IC.
The residual removal benzene was oxidized into intermediates
in gas-phase or liquid phase. However, no gaseous intermedi-
ates were detected by the GC-MS, showing that no by-product
was discharged in the effluent during benzene degradation by
Fenton reaction. The aqueous products of benzene degradation
by intermittent dosing of H2O2 were analyzed by a TOC analyzer.
The concentrations of TC, TOC and IC in the solution are shown
in Fig. 6. Both TOC and IC were detected, and the TOC
concentration was much larger than the IC concentration. IC
mainly comes from mineralized products from benzene degra-
dation and TOC mainly comes from intermediates or benzene
Fig. 3 Profiles of intermittent and lump dosing of H2O2 for benzene
dissolved in the solution. Both the IC and TOC concentration
degradation.
Fig. 4 The concentration change of H2O2 with intermittent and lump Fig. 5 Profiles of CO2 concentration with intermittent and lump
dosing.
dosing of H2O2.
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Fig. 8 Profiles of the concentration of Fe ions in liquid with inter-
Fig. 6 Profiles of carbon in liquid with intermittent dosing of H2O2.
mittent dosing of H2O2.
increased, from 1.5 mg Lꢀ1 to 4.6 mg Lꢀ1 and from 2.6 mg Lꢀ1 to
14.8 mg Lꢀ1, respectively. This indicated that the intermediates
formed are soluble and could be easily trapped in the solution.
The TOC concentration decreased aer each dosing of H2O2,
indicating that part of the intermediates were further oxidized
by the Fenton reaction. This agrees well with the observed
results of CO2 concentration variation prole shown in Fig. 5.
Fe3+ + H2O2 / Fe2+ + HO2$ + H2O
Fe3+ + HO2$ / Fe2+ + O2 + H+
(2)
(3)
As shown in Fig. 8, most of them were converted into Fe3+
which seems difficult to be reduced back to Fe2+ through H2O2,
whose reaction rate constant is only 0.02 Mꢀ1 sꢀ1
.
Future
22–24
Mechanism of benzene degradation
work should be focused on the efficient transfer path of Fe3+
back to Fe2+.
Benzene degradation by Fenton reaction in an aeration
container involves gas–liquid mass transfer of gaseous benzene
and the following oxidation of dissolved benzene in the liquid,
as shown in Fig. 7. Gaseous benzene was transferred into
the liquid phase through the bubbling and the transfer rate
of absorption is proportional to the driving force.16 As shown
in Fig. 2, water has poor capacity for benzene absorption.
However, the quick degradation of dissolved benzene in the
solution can promote continuous benzene absorption.
Fenton reaction involves electron transfer, leading to the
generation of $OH with strong oxidation capacity. The genera-
tion of $OH is characterized by the reaction (1).
The gaseous and aqueous intermediates from benzene
degradation were identied by the GC-MS and the ion-
chromatograph. No intermediates from benzene degradation
were detected in the gas phase, which can prevent the discharge
of gaseous by-products.17 As listed in Table 1, the intermediates
Table 1 Intermediates of benzene degradation by Fenton process in
the liquid phase
No.
1
Intermediates
Phenol
Structure
Fe2+ initiates the decomposition of H2O2 to generate $OH.
Fig. 8 shows the proles of the concentration of Fe ions in the
solution with intermittent dosing of H2O2. In the rst round,
the concentration of Fe2+ rapidly dropped from 101.0 mg Lꢀ1 to
2.1 mg Lꢀ1 as H2O2 was added into the solution and slightly
increased to 7 mg Lꢀ1, indicating that partial Fe3+ can also
be recovered to Fe2+, according to Fenton-like reactions (2)
and (3).20,21
2
Pyrocatechol
3
4
P-Benzoquinone
Formic acid
5
6
Acetic acid
Oxalic acid
Fig. 7 Schematic of benzene degradation by Fenton reaction in an
aeration container.
74 | RSC Adv., 2017, 7, 71–76
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demonstrates that Fenton reaction is a promising process for
the degradation of low-concentration VOCs with few gaseous
byproducts under mild reaction conditions.
Acknowledgements
The authors gratefully acknowledge the nancial supports from
National Key Research and Development Program (No.
2016YFC0204800), NSFC-RGC (No. 51561165015, No.
N_HKU718/15), Guangdong Applied Science and Technology
Research & Development Fund (2015B020236004), Guangdong
Special Fund for Science and Technology Development (Hong
Kong Technology Cooperation Funding Scheme) (No.
2016A050503022), Science and Technology Project in Guangz-
hou (No. 2014Y2-00094&2060404), and the Key Fundamental
Research Fund for the Central Universities (15lgjc07).
Fig. 9 Mechanism and pathways of benzene degradation by Fenton
process.
in the solution are mainly phenolic, p-benzoquinone and acid
substances dissolved. Phenol, observed by the GC-MS at m/z 94,
is the main by-product in liquid for the Fenton process. The
intensity of phenol peaks was gradually increased with inter-
mittent dosing of H2O2 (shown in Fig. S3†), which accounts for
the increased TOC concentration (Fig. 6). The residual phenol
in solution can be easily removed by dosing more Fenton
regent25–27 or treated in a separate sewage treatment process.
Thus, Fenton also can be used as pretreatment method for the
degradation of low-concentration VOCs.
Based on the intermediates above, the possible pathways of
benzene degradation were composed of benzene ring excitation
and pyrolysis. $OH is highly active and rapidly attack benzene
and its degradation intermediates, generating CO2 and H2O.28–30
It was reported that benzene could be dehydrogenated and
cleaved by $OH, forming unsaturated hydrocarbon and radi-
cals.31 The unsaturated hydrocarbon can react with active
hydrogen and oxygen, generating aldehydes.32 The aldehydes
were further oxidized into acid substances such as formic acid,
acetic acid and oxalic acid.33 The excited benzene ring is rstly
oxidized by $OH to form phenol, which is further oxidized into
pyrocatechol and p-benzoquinone.34,35 According to the detected
products and by-products, the probable pathways of benzene
degradation by the Fenton process were proposed as shown
in Fig. 9.
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