Improving the efficiency of Fenton reactions and their application in the degradation of benzimidazole in wastewater

Article abstract of DOI:10.1039/c8ra00240a

Reducing the quantity of sludge produced in Fenton reactions can be partly achieved by improving their efficiency. This paper firstly studies the effect of uniform deceleration feeding (ferrous iron and hydrogen peroxide) on the efficiency of a Fenton reaction by measuring the yield of hydroxyl radicals (OH) and chemical oxygen demand (COD) removal rate. The dynamic behavior of OH was also investigated. The results indicated that uniform deceleration feeding was the best feeding method compared with one-time feeding and uniform feeding methods when the same amount of Fenton reagents and the same reaction times were used. Besides, it was found the COD removal rate reached 79.3% when this method was applied to degrade 2-(a-hydroxyethyl)benzimidazole (HEBZ); this COD removal rate is larger than those when the other two modes were used (they reached 60.7% and 72.1%, respectively). The degradation pathway of HEBZ was determined using PL, UV-vis, FTIR, HPLC and GC-MS. Ultimately, HEBZ was decomposed into three small molecules (2-hydroxypropylamine, oxalic acid, and 2-hydroxypropamide). This research is of great significance for the application of Fenton reactions in wastewater treatment.

Full text of DOI:10.1039/c8ra00240a

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PAPER
Improving the efficiency of Fenton reactions and
their application in the degradation of
benzimidazole in wastewater
Cite this: RSC Adv., 2018, 8, 9741
*
Kun Qian, Jinxu Qi, Chenru Li, Chen Yao, Wei Song and Yihong Wang
Qinyao Liu,
Reducing the quantity of sludge produced in Fenton reactions can be partly achieved by improving their
efficiency. This paper firstly studies the effect of uniform deceleration feeding (ferrous iron and hydrogen
peroxide) on the efficiency of a Fenton reaction by measuring the yield of hydroxyl radicals (cOH) and
chemical oxygen demand (COD) removal rate. The dynamic behavior of cOH was also investigated. The
results indicated that uniform deceleration feeding was the best feeding method compared with one-
time feeding and uniform feeding methods when the same amount of Fenton reagents and the same
reaction times were used. Besides, it was found the COD removal rate reached 79.3% when this method
was applied to degrade 2-(a-hydroxyethyl)benzimidazole (HEBZ); this COD removal rate is larger than
those when the other two modes were used (they reached 60.7% and 72.1%, respectively). The
degradation pathway of HEBZ was determined using PL, UV-vis, FTIR, HPLC and GC-MS. Ultimately,
HEBZ was decomposed into three small molecules (2-hydroxypropylamine, oxalic acid, and 2-
hydroxypropamide). This research is of great significance for the application of Fenton reactions in
wastewater treatment.
Received 9th January 2018
Accepted 19th February 2018
DOI: 10.1039/c8ra00240a
rsc.li/rsc-advances
in a raceway pond reactor, both achieving relatively high
chemical oxygen demand (COD) removal rates. However, all
1. Introduction
The Fenton1,2 reaction, as one of the most successful advanced
oxidation processes (AOPs),3–9 has attracted great interest in
recent years due to its advantages of high performance, proce-
dural simplicity, low cost and low reagent toxicity.10 The reac-
tion is based on the in situ production of hydroxyl radicals
(cOH), which are one of the most reactive free radicals and
these studies suggested that a long reaction time, low initial
COD and high clarity of water were necessary. Recently, Fenton-
like processes have attracted far-ranging attention and have
been studied extensively. Wang et al.27 used iron(III) and
hydrogen peroxide (H2O2) to treat acid black 1 wastewater, and
the results indicated that the decolorization rate of the waste-
water was similar to that in Fenton processes. Souza’s group28
studied the removal of diclofenac from aqueous solutions at
near neutral pH with ferric oxalate complexes, and the results
exhibit non-selective reactivity, with reactive rate constants
commonly in the order of 107–1010 Mꢀ1 Sꢀ1
,
and oxidizing
11
potentials of 2.8 V, second only to uorine gas.12–14 The Fenton
reaction can be applied to degrade a wide range of recalcitrant
and toxic pollutants found in pharmaceutical and dye waste-
waters,15,16 especially aromatic compounds with electron-
donating groups.17 Its efficiency is determined by the amount
of cOH produced in the process.18
2ꢀ
showed that the introduction of C2O4 can promote the recy-
cling of Fe2+ and Fe3+ and broaden the pH range. Yongkoo and
Iraj29 investigated the oxidation of chlorinated ethylenes in soil
solution systems using a citric acid-modied Fenton reaction,
and proved that this process was benecial for the degradation
of pollutants. However, these Fenton-like processes had some
grave drawbacks, such as being time-consuming due to the low
reaction rate, increasing the value of COD for the addition of
complexes, and the difficulty of removing the iron complexes, as
well as the lack of occulation of Fe(OH)3, which greatly reduces
the COD value etc. All the above drawbacks limited the appli-
cation of Fenton-like processes in wastewater treatment.
Therefore, it is still of great signicance to improve the effi-
ciency of Fenton reactions.
The problem with the Fenton reaction is the large quantity of
sludge generated in the process. To overcome this issue, electro-
Fenton, photo-Fenton, Fenton-like etc. technologies have been
developed. Most electro-Fenton processes were used for dye
wastewater treatment,9,19–21 but the process was cumbersome
and high-cost, and wasted power. The photo-Fenton process
was investigated for its high degradation efficiency,22–24 such as
by Carra’s group,25 who dealt with agricultural effluent using the
technology, and by Cabrera et al.26 who degraded pyrimethanil
In recent years, some groups have made efforts to improve
the efficiency of Fenton reactions. Hui Zhang’s group30 and
Yoo et al.31 disposed of landll leachate using batch feedings
School of Chemistry and Chemical Engineering, Southeast University, Nanjing,
211189, P. R. China. E-mail: yihongwang@seu.edu.cn
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2. Materials and methods
2.1. Materials
2-(a-Hydroxyethyl)benzimidazole (HEBZ, 19018-24-7, 98%),
FeSO4$7H2O (AR, 99.0 ꢁ 101.0%), H2O2 (AR, 30%, w/v), meth-
ylene blue (BS), and titanium oxalate potassium (AR, 98%) were
purchased from Aladdin. The pH value of the wastewater in
each test was adjusted using H2SO4 (Adamas, AR, 98%) solu-
tions. The initial pH and COD value of the simulated wastewater
were 3 and 3555 mg Lꢀ1, respectively.
Fig. 1 The structure of 2-(a-hydroxyethyl)benzimidazole.
of Fenton’s reagent. Likewise, Yang Deng et al.32 treated
leachate with stepwise addition of H2O2, when Fe2+ was
added at once. All these studies proved that the feeding
method can affect the removal rate of organic matter (RH).33
In our previous work, a ME (micro-electrolysis)-Fenton
process34 was used to treat high concentration pesticide
wastewater when only H2O2 was added at a slow and uniform
rate. The results suggested that the COD removal rate was
well improved.
2-(a-Hydroxyethyl)benzimidazole (HEBZ), with an electron-
rich benzene ring and a heterocycle coupling structure as an
intermediate, is widely found in pharmaceutical and dye
wastewater (Fig. 1).35 Since it is difficult to degrade due to its
very stable chemical structure, it has brought some negative
effects on the environment and human health. Fenton reac-
tions are ideally suitable for degrading HEBZ in wastewater,
based on its structural properties. Herein, we rst utilized
a deceleration feeding mode in the Fenton process and
researched its effect on the degradation efficiency of HEBZ in
wastewater, by measuring the yield of hydroxyl radicals (cOH)
and the COD removal rate, and comparing these with those
from one-time feeding and uniform feeding methods. The
dynamic behaviour and total amount of cOH, as well as the
Fe2+ and H2O2 concentrations over time, were investigated for
these feeding modes. At the same time, UV-vis absorption
spectroscopy (UV-vis), photoluminescence spectroscopy (PL),
Fourier transform infrared spectroscopy (FTIR), high perfor-
mance liquid phase chromatography (HPLC), and gas chro-
matography mass spectrometry (GC-MS) were used to explore
the degradation pathway of HEBZ in the optimal feeding
mode.
2.2. Experimental set-up
The Fenton process was performed in a magnetically stirred
Fenton reactor (1.5 L). The simulated wastewater was prepared
using HEBZ solid, to give a concentration of 2.5 g Lꢀ1. The pH of
the wastewater was adjusted to 3.0 using sulfuric acid (H2SO4)
and maintained at this value throughout the experiment (this is
the optimal pH of our previous experiment). The theoretical
H2O2 concentration required for the degradation of HEBZ was
calculated using the COD, as shown in eqn (1).36,37
[H2O2] ¼ 34/16(CODHEBZ
)
(1)
The addition of FeSO4$7H2O was based on a molar ratio of
Fe2+ to H2O2 of 1 : 3 (1 : 3 is the best molar ratio as determined
in the previous experiment). For the three feeding methods the
total concentrations of Fe2+ and H2O2 were 3.73 g Lꢀ1 and
21.60 mL Lꢀ1, respectively. In the rst method of feeding, 3.73 g
Fe2+ and 21.6 mL H2O2 were added to 1 L HEBZ wastewater at
one time; this is denoted one-time feeding. In the second
method of feeding, FeSO4$7H2O and H2O2 were added at
0.187 g Lꢀ1 minꢀ1 and 1.080 mL Lꢀ1 minꢀ1 for 20 min using
a uniform auto sampler. In the uniform deceleration feeding
method, FeSO4$7H2O and H2O2 were added using the decom-
pression device depicted in Fig. 2 (the initial speed of uniform
deceleration feeding and uniform feeding was the same). All the
above experiments were performed in triplicate. In addition, PL,
UV-vis, FTIR, HPLC and GC-MS were employed to qualitatively
characterize the wastewater before and aer the Fenton
reactions.
Fig. 2 A diagram of the uniform deceleration feeding Fenton reaction device.
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2.3. Analytical methods
The samples were taken out and immediately added to a sample
tube containing manganese dioxide, which was employed to
stop the reaction as it reacted with H2O2. In order to show the
change in COD removal rate over time for the three feeding
methods, the COD values at different reaction times during the
Fenton processes were compared. The COD values were ob-
tained via standard methods for the examination of waste-
water.38 The H2O2 concentrations were measured via a titanium
potassium oxalate colorimetric method using a Hitachi U2910
UV-vis spectrometer at a wavelength of 388 nm.39 The Fe2+
concentration was determined via photometric measurement
with 1,10-phenanthroline according to ISO 6332 using a UV-vis
spectrophotometer (Zuzi 4418PC).40 Quantication of cOH was
carried out using methylene blue spectrophotometry at
666 nm.41–43 The HEBZ concentration was determined using
HPLC (Agilent, Palo Alto, CA) at 240 nm. The mobile phase was
a mixture of acetonitrile and water (5/95, v/v) at a ow rate of 1.0
mL minꢀ1. The photoluminescence spectra (PL) were obtained
using a FluoroMax-4 uorescence spectrophotometer (Horiba,
Fig. 3 The COD removal rate over the reaction time for the different
feeding methods (reaction time ¼ 40 min, [H2O2] ¼ 24.0 g Lꢀ1, Fe2+
/
H2O2 (mol molꢀ1) ¼ 1 : 3, CODinitial ¼ 3555 mg Lꢀ1).
USA). The UV-vis absorption spectra were recorded using a Shi- deceleration feeding was the best feeding mode, which may be
madzu UV-2450 spectrophotometer (Tokyo, Japan). The FTIR related to the non-reactive oxidation of Fe2+, H2O2 self-decom-
spectra were obtained using a Nicolet 5700 (USA) IR spectrom- position,31,44 and the quenching of cOH.
eter in the range 400–4000 cmꢀ1. Analysis of the target
compounds was performed using a GC-MS (Agilent, 7890B GC,
5977A Quadrupole MS) with a HP-5MS analytical column.
3.2. Study of the hydroxyl radicals in the three feeding
methods in the Fenton reaction
3.2.1 The dynamic behavior of cOH in the three feeding
methods. In order to illustrate the effect of the feeding mode on
3. Results and discussion
3.1. COD removal rate using three different feeding methods
the COD removal rate, the dynamic behavior of cOH and the
concentrations of Fe2+ and H2O2 in the solution were investi-
in the Fenton reaction
gated. Fig. 4a shows the evolution of the cOH concentration over
The COD removal rates using the three feeding modes (one- the reaction time for the three feeding methods of Fe2+ and
time feeding, uniform feeding, and uniform deceleration H2O2. It can be seen that, in the one-time feeding mode (Line I),
feeding) under the same operating conditions (reaction time ¼ the concentration of cOH sharply grew to a maximum (12.9 mM)
40 min, [H2O2] ¼ 24.0 g Lꢀ1, Fe2+/H2O2 (mol molꢀ1) ¼ 1 : 3, in 10 min and the COD removal rate rapidly increased in the
CODinitial ¼ 3555 mg Lꢀ1) are shown in Fig. 3. From Fig. 3 we corresponding time. Then it continuously decreased to 0 mM
can see that, in the one-time feeding mode (Line I), the COD and the COD degradation rate was basically unchanged (as
removal rate increased rapidly in the rst 10 min and then shown in Fig. 3). In the uniform feeding mode (Line II), the
became steady during the next 10 min. Aer 20 min, little concentration of cOH increased slowly over 1–20 min reaching
change took place and the total COD removal rate of HEBZ was a maximum (9.1 mM) at 20 min and the COD removal rate was
60.7% at 40 min. In the uniform feeding mode (Line II), when growing equably during this time. Then the degradation rate of
both Fe2+ and H2O2 were added equably at a low speed, the COD COD became steady while the cOH concentration quickly
removal rate raised evenly in the rst 20 min and then more reduced to 0 mM. Remarkably, the concentration of cOH excee-
slowly during the next 10 min. What was noteworthy was that ded that in the one-time feeding mode at 15 min (as shown in
the COD removal rate exceeded that of the one-time feeding Fig. 3). In the uniform deceleration feeding mode (Line III), the
mode at 15 min. Aer 30 min, the COD removal rate became cOH concentration generated was almost at over 1–30 min,
stable and eventually reached 72.1%, which is 11.4% higher reaching a maximum (7.9 mM) at 30 min, which indicated that
than that of the one-time feeding mode. In the uniform decel- the generation and consumption of cOH was approximately
eration feeding mode (Line III), when both Fe2+ and H2O2 were equal during the feeding. Then the concentration of cOH
introduced at a decreasing rate, the COD removal rate rose gradually reduced to 0 mM and the COD removal rate of the
persistently in the initial 30 min and then the growth became whole process grew steadily and rapidly (as shown in Fig. 4a).
smooth. Notably, the COD removal rate surpassed that of the The above results revealed that the degradation rate of COD was
uniform feeding mode in 23 min and ultimately reached 79.3%, consistent with the concentration of cOH, and the cOH gener-
which is 18.6% higher than that of the one-time feeding mode. ated rapidly via the reaction in eqn (2)29,45 was not consumed
The above results suggested that the feeding method had a big immediately and accumulated to a maximum. In addition, in
effect on the degradation of HEBZ and that uniform the uniform and uniform deceleration feeding modes, the cOH
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Fig. 4 (a) The change of cOH concentration with the reaction time and (b) the total concentration of cOH and the COD removal rate within
40 min for the three feeding methods of Fe2+ and H2O2 (reaction time ¼ 40 min, [H2O2] ¼ 24.0 g Lꢀ1, Fe2+/H2O2 (mol molꢀ1) ¼ 1 : 3, CODinitial
¼
3555 mg Lꢀ1).
concentration would drop promptly once the feeding was
terminated because its consumption was greater than the yield.
It should be noted that, compared with the other two feeding
modes, cOH was accumulated very smoothly in the uniform
deceleration feeding mode, proving that this method can better
control the generation and consumption of cOH, thus
improving its utilization efficiency. Therefore, reducing excess
Fe2+ and H2O2 properly via uniform deceleration feeding can
actually improve the efficiency of the Fenton response.
Fe2+ + H2O2 / Fe3+ + cOH + OHꢀ
(2)
Fig. 5 Variation of the H2O2 and Fe2+ concentrations during the
reaction time in HEBZ wastewater for the three feeding modes
(reaction time ¼ 40 min, [H2O2] ¼ 24.0 g Lꢀ1, Fe2+/H2O2 (mol molꢀ1) ¼
1 : 3, CODinitial ¼ 3555 mg Lꢀ1).
3.2.2 The total concentration of cOH in the three feeding
methods. Improving the efficiency of the Fenton reaction can
also be studied by monitoring the total amount of cOH in the
solution. As depicted in Fig. 4b, the total concentration of cOH
in each feeding method increased in the order of one-time
feeding, uniform feeding and uniform deceleration feeding
(the total concentration of cOH was obtained from the integral
area of the polyline in Fig. 4a), and was 187.52 mM, 221.52 mM
and 247.77 mM, respectively. The corresponding COD removal
rates were 60.7%, 72.1% and 79.3%, respectively. This further
veried that the uniform deceleration feeding according to the
reaction mechanism may make the best use of cOH. This may be
attributed to the fact that non-reactive oxidation of Fe2+ and
H2O2 self-decomposition34,44 were vastly reduced in this feeding
method. In addition, the quenching effects of Fe2+ and H2O2 on
cOH (through reactions (3) and (4))29,45 and even those between
free radicals (through reaction (5)) were effectively weakened.35
the efficiency of the Fenton reaction, since H2O2 is catalyzed by
Fe2+ to produce cOH, which oxidizes many pollutants.45 The
residual H2O2 and Fe2+ content was investigated in the three
feeding modes (Fig. 5). In the one-time feeding mode (Line I),
the concentrations of Fe2+ and H2O2 were reduced drastically in
the rst 10 minutes, therefore, the production of cOH was high
and the COD removal rate grew faster during this period (as can
be seen in Fig. 3 and 4a). Aer 10 min, the Fe2+ and H2O2 were
basically consumed, so the cOH concentration was very small
and the COD degradation rate was mostly unchanged. In the
uniform feeding mode (Line II), the Fe2+ and H2O2 were
uniformly consumed in the rst 20 min. This indicated that the
concentration of cOH and the removal rate of COD increased
cOH + H2O2 / HO2c + H2O
cOH + Fe2+ / Fe3+ + OHꢀ
2HO2c / H2O2 + O2
(3)
(4)
(5)
Table 1 The efficiency of H2O2 usage
Feeding
mode
One-time
feeding
Uniform
feeding
Uniform deceleration
feeding
3.2.3 The effect of Fe2+ and H2O2 concentration on cOH. As
is known, the Fe2+ and H2O2 content has a signicant effect on
h/%
78.9
96.2
99.1
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steadily (as seen in Fig. 3 and 4a). With increasing reaction time, the remaining Fe2+ and H2O2 content decreased very uniformly
the cOH concentration decreased quickly and the COD removal within 30 min. Simultaneously, the concentration of cOH and
rate was essentially unchanged. This suggested that slowing the the corresponding COD removal rate were growing evenly
addition rate of Fe2+ and H2O2 would enhance the utilization during the process, indicating that Fe2+ and H2O2 were more
efficiency of Fe2+ and H2O2, which may be benecial to the effectively used compared to in the uniform feeding mode. Of
production of cOH and the augmentation of the COD degrada- the two reagents, H2O2 was more critical because it directly
tion rate. In the uniform deceleration feeding mode (Line III),
Fig. 6 UV-vis spectra (a), PL spectra (b) and FTIR spectra (c) of HEBZ Fig. 7 (a) The variation of HEBZ concentration over time, (b) the HPLC
wastewater before and after Fenton reaction with the uniform profiles of HEBZ wastewater before and after Fenton reaction, (c) the
deceleration feeding mode.
MS profile of HEBZ.
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Fig. 8 GC-MS profile of wastewater containing HEBZ after the Fenton reaction with the uniform deceleration feeding mode, (a) TIC profile, (b)
MS profile of 2-hydroxypropylamine, (c) MS profile of oxalic acid, and (d) MS profile of the 2-hydroxypropamide.
affected the theoretical maximum mass of cOH generated. peak at 200 nm belongs to carboxylic acid substances, so it can
Generally, eqn (6) was used to evaluate the efficiency of H2O2 be inferred that the degradation products of HEBZ may contain
usage in the Fenton processes.35,46
small molecule carboxylic acids.
Additionally, the successful degradation of HEBZ was
conrmed by FTIR analysis (Fig. 6c). An absorption peak at
3418 cmꢀ1 appeared and can be attributed to –NH2, implying
the fracture of the heterocyclic bond in HEBZ and the produc-
tion of amides.15 The two bands at 1450 cmꢀ1 and 1600 cmꢀ1
corresponding to the skeleton vibrations of benzene dis-
appeared, further indicating the breakage of the benzene ring
h ¼ 2.12CODoxi/[H2O2]
(6)
where h represents the efficiency of H2O2 usage, CODoxi is the
COD removed by oxidation, and [H2O2] is the dosage of added
peroxide. Table 1 shows that the utilization efficiency of H2O2
was highest in the uniform deceleration feeding mode, and was
99.1% in this mode. This revealed that the self-decomposition
of H2O2 could be ignored in this feeding mode. The above
analysis proves that the uniform deceleration feeding mode was
the best feeding method when a Fenton reaction was used to
degrade HEBZ, and immensely enhanced the efficiency of the
Fenton reaction. Therefore, the addition rate of the reagents
was a very crucial operating parameter in practice and needed to
be carefully controlled.
3.3. The degradation pathway of HEBZ
In order to investigate the HEBZ degradation pathway in the
optimal feeding mode for the Fenton reaction, UV-vis, PL, FTIR,
HPLC and GC-MS were performed. Fig. 6a and b show the UV-
vis absorption and PL spectra of HEBZ before and aer the
Fenton reaction with the uniform deceleration feeding mode.
The strong and broad absorption peak of HEBZ was mainly in
the region 190–290 nm and its corresponding emission peak
was located at 370 nm when excited at 300 nm, due to the fused
structure of the benzene ring and imidazole heterocyclic ring in
HEBZ. The absorption range dramatically shied to 190–
230 nm and the PL emission peak completely disappeared aer
Fenton treatment. This difference may be attributed to the
Fig. 9 The proposed degradation pathway of HEBZ in the Fenton
structural decomposition of HEBZ. Moreover, the absorption system.
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skeleton. Furthermore, two new bands at 1040 cmꢀ1 and
1670 cmꢀ1 appeared aer the Fenton reaction and were
ascribed to the characteristic absorptions of –OH and –C]O,
respectively, which suggested alcohols or carboxylic acids might
be produced. These results corroborated that the benzene ring
and heterocyclic structure of HEBZ were destroyed aer the
Fenton reaction. In addition, hydramines, amides or carboxylic
acids may be generated.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China under grant number 81571812, and
a project funded by the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions (PAPD) under
grant number 1107047002.
To further probe the nal degradation products of HEBZ, the
HPLC pattern and GC-MS prole before and aer the Fenton
reaction were investigated (Fig. 7 and 8). As shown in Fig. 7a, the
HEBZ concentration varied from 2.5 g Lꢀ1 at the beginning, to
0 g Lꢀ1 when the Fenton reaction reached 40 min. Fig. 7b also
reveals that HEBZ (m/z ¼ 163.1) disappeared completely and
could be degraded into three new substances aer the Fenton
process. From the GC-MS prole in Fig. 8, it could be inferred
that the three new substances were 2-hydroxypropylamine (m/
z ¼ 75.1), oxalic acid (m/z ¼ 90.03) and 2-hydroxypropamide (m/
z ¼ 89.1), which was consistent with the results of UV-vis, PL
and FTIR. Moreover, according to the above results, the pathway
of a possible degradation mechanism of HEBZ is proposed in
Fig. 9. Briey, when cOH attacks the benzene ring of a HEBZ
molecule, the bonds may break down to form oxalic acid (the
bonds were broken from (1) (2) or (1) (6)). When the imidazole
heterocycle of HEBZ was attacked by cOH, 2-hydroxypropyl-
amine and 2-hydroxypropamide may be produced (the bonds
were broken from (3) (4) and (3) (5)). These small molecules may
be easily degraded via biological methods.
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Conflicts of interest
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