Drastic enhancement on Fenton oxidation of organic contaminants by accelerating Fe(III)/Fe(II) cycle with l-cysteine

Article abstract of DOI:10.1039/c6ra07091d

The development of a highly efficient and pH-tolerant Fenton system has been one of the most important and challenging goals in water remediation. Herein, a green natural organic ligand, l-cysteine (Cys), was innovatively introduced into Fenton's reagent to construct an excellent catalytic oxidation system. The introduction of Cys into the Fenton system expanded the effective pH range up to 6.5 and achieved a superior oxidation efficiency, representing about 70% higher removal ratio and 12 times higher reaction rate constant with methylene blue dye as the probe compound. The Cys-driven Fenton reaction presented an outstanding pH adaptability and oxidative activity compared with other common organic ligands or reducing agent-modified Fenton reactions. An investigation of the reaction mechanism indicated that the addition of Cys into the system accelerated the Fe(iii)/Fe(ii) cycle, and led to a relatively steady Fe(ii) recovery, which enhances the generation of hydroxyl radicals (OH). The presence of Cys in the Fenton system remarkably reduced the apparent activation energy from 95.90 to 47.93 kJ mol^-1. The findings from this study provide a feasible approach for a highly efficient and pH-tolerant wastewater treatment process with environmentally benign characteristics, and initiates an inspiring research domain of amino acids in the environmental catalysis field.

Full text of DOI:10.1039/c6ra07091d

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Drastic enhancement on Fenton oxidation of
organic contaminants by accelerating Fe(III)/Fe(II)
Cite this: RSC Adv., 2016, 6, 47661
cycle with L-cysteine†
a
a
Lianshun Luo, Yuyuan Yao, Fei Gong,a Zhenfu Huang,a Wangyang Lu,a
*
a
ab
*
Wenxing Chen and Li Zhang
The development of a highly efficient and pH-tolerant Fenton system has been one of the most important
and challenging goals in water remediation. Herein, a green natural organic ligand, L-cysteine (Cys), was
innovatively introduced into Fenton's reagent to construct an excellent catalytic oxidation system. The
introduction of Cys into the Fenton system expanded the effective pH range up to 6.5 and achieved
a superior oxidation efficiency, representing about 70% higher removal ratio and 12 times higher reaction
rate constant with methylene blue dye as the probe compound. The Cys-driven Fenton reaction
presented an outstanding pH adaptability and oxidative activity compared with other common organic
ligands or reducing agent-modified Fenton reactions. An investigation of the reaction mechanism
indicated that the addition of Cys into the system accelerated the Fe(III)/Fe(II) cycle, and led to a relatively
steady Fe(II) recovery, which enhances the generation of hydroxyl radicals (cOH). The presence of Cys in
Received 17th March 2016
Accepted 8th May 2016
the Fenton system remarkably reduced the apparent activation energy from 95.90 to 47.93 kJ molꢀ1
.
The findings from this study provide a feasible approach for a highly efficient and pH-tolerant
wastewater treatment process with environmentally benign characteristics, and initiates an inspiring
research domain of amino acids in the environmental catalysis field.
DOI: 10.1039/c6ra07091d
www.rsc.org/advances
treatment of wastewater because of the generation of highly
reactive radicals which can decompose organic pollutants into
non-toxic small molecule substances.6–9
1. Introduction
In recent decades, environmental deterioration issues such as
sandy desertication, soil erosion, and water contamination
have become increasingly severe, which have resulted in notably
widespread changes in natural ecosystems and climatic
disturbances. Excessive hazardous pollutants in industrial
wastewater have exceeded the natural ecosystem biodegrad-
ability. To remedy biodegradation weaknesses, two alternative
strategies were formulated and investigated. These are the
physical adsorption and chemical catalytic oxidation of
contaminants.1–4 Although physical adsorption methods
present remarkable performance for phase disengagement,
adsorbed pollutants cannot be destroyed completely, and the
subsequent disposal may bear the risk of secondary pollution.5
Advanced oxidation processes (AOPs) have received increasing
attention as promising and attractive technologies for the
Among the AOPs, the Fenton system has been studied
extensively because of its fast reaction rate, simple operation
and environmental friendliness.10,11 In the Fenton system,
ferrous ions serve as a catalyst to activate H2O2 to generate
highly active cOH and trigger free radical chain reactions, which
can oxidize most organic contaminants rapidly and non-selec-
tively.12,13 However, the Fenton system has some limitations
during the reaction: (i) the reaction rate of Fe(III) and H2O2 (eqn
(2)) is much lower than that of Fe(II) and H2O2 (eqn (1)), which
may hinder the Fe(III)/Fe(II) cycle and result in a decline in
reaction rates and (ii) Fe(III) will precipitate as iron hydroxides in
insoluble form, which slows down or blocks the cOH generation
rate, and thus acidic conditions (pH # 3.0) are required to
maintain Fe(III) in soluble form.14,15 Obviously, Fe(III) precipita-
tion must be reduced and a more effective transformation from
Fe(III) to Fe(II) is required to improve the oxidation efficiency of
the Fenton reaction.
aKey Laboratory of Advanced Textile Materials and Manufacturing Technology,
Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, PR China.
E-mail: yyy0571@126.com; Fax: +86 571 86843255; Tel: +86 571 86843810
bThe School of Material Science and Chemical Engineering, Ningbo University, Ningbo
325211, China. E-mail: zhangli2@nbu.edu.cn; Fax: +86 15869369738; Tel: +86 574
87609986
Fe(II) + H2O2 / cOH + OHꢀ + Fe(III), k ¼ 63 Mꢀ1 sꢀ1 (1)
Fe(III) + H2O2 / Fe(II) + H+ + HOOc, k ¼ 0.01–0.02 Mꢀ1 sꢀ1
(2)
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra07091d
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To overcome these problems, polycarboxylates (e.g., citrate,
malonate, and oxalate) and aminopolycarboxylic acids (e.g.,
ethylenediaminetetraacetic acid (EDTA) and ethylenediamine-
N,N0-disuccinic acid (EDDS)) were used as organic ligands to
solubilize Fe(III) by complexation.16–18 These ligands could form
stable complexes with Fe(III) in solution and reduce or eliminate
Fe(III) precipitation, which could expand the effective pH and
increase the reaction rates. However, they could not enhance
the reduction of Fe(III) to Fe(II) because of the lack of reducing
capacity, which limited further improvements in oxidation
efficiency. In addition, aminopolycarboxylic acids may also
suffer from high environmental risks for practical application
in view of its poor biodegradability.19–22 Consequently,
researchers have developed some feasible measures such as
adding additional reductants to remedy limitations of these
organic ligands. For example, Wan et al. demonstrated that
a signicant improvement is achieved in the oxidative decol-
orization of orange G in the Fe(II)/EDDS/persulfate process with
hydroxylamine introduction. In the Fe(II)/EDDS/persulfate/
hydroxylamine system, EDDS could alleviate the formation of
iron sludge by complexing, and hydroxylamine was combined
to accelerate Fe(II) recovery to improve persulfate activation.23
Nevertheless, these extra reductants may burden the ecological
environment because of their potential toxicity.24 Therefore, an
eco-friendly alternative strategy should be developed that could
reduce Fe(III) accumulation and accelerate the transformation
from Fe(III) to Fe(II) simultaneously.
Compared with the organic ligands mentioned above, Cys
possesses more remarkable properties. It has a unique structure
with a reducing sulydryl group (–SH), which is capable of
reducing Fe(III) to Fe(II) directly.25,26 Cys can form complexes
with Fe(III), which reduces the precipitation of Fe(III). On the
other hand, Fe(III)–Cys complexes may present convenient
channels for the transformation from Fe(III) to Fe(II) by an
internal redox triggered by protonation.27 In addition, Cys is
a type of green natural amino acid that exists in organisms, and
will not exert a negative impact on the environment. Based on
these considerations, Cys may be an ideal candidate that can act
as a complexing and reducing agent simultaneously to maintain
iron in soluble form and enhance the Fe(III) to Fe(II) trans-
2. Experimental
2.1. Materials and chemicals
Ferric chloride hexahydrate (FeCl3$6H2O), hydrogen peroxide
(H2O2) and Cys were analytical grade reagents (Mike Chemical
Instrument Co., Ltd., Hangzhou, China). The spin trapping
agent 5,5-dimethyl-pyrroline-N-oxide (DMPO) was supplied by
Tokyo Chemical Industry Co. Ltd., Tokyo, Japan. MB dye was
obtained commercially and was used without further purica-
tion. All other chemical reagents used were also provided by
Hangzhou Mike Chemical Instrument Co., Ltd., China. Doubly
distilled water was used throughout this study.
2.2. Experimental procedures
Batch experiments on the catalytic oxidation of the dyes were
carried out in 100 mL glass beakers by shaking in a constant
temperature shaker water bath (DSHZ-300A, Taicang, Jiangsu)
that was adjusted to the required temperature. A reaction
volume of 50 mL was used in all experiments. Desired concen-
trations of MB, Fe(III), H2O2 and Cys were added simultaneously
at the beginning of each experiment. At predetermined intervals
(3 min), samples were withdrawn from the ask for analysis
using an ultraviolet/visible (UV/Vis) spectrophotometer (Hitachi
U-3010). The initial solution pH was adjusted with NaOH or
HClO4 prior to Fe(III) and H2O2 addition. Alcohol quenching
experiments with tert-butyl alcohol (TBA) were performed by
adding desired alcohols into the reaction solution before the
addition of H2O2.
2.3. Analytical methods
The decoloration efficiency of the dyes was determined using
the UV/Vis spectrophotometer. The kinetics of decoloration of
the dyes can be described by a pseudo-rst-order equation (eqn
(3) and (4)):
dC/dt ¼ ꢀkobs
C
(3)
(4)
Ct ¼ C0 exp(ꢀkobst)
formation for more efficient Fenton oxidation. To the best of where Ct is the concentration of the dye solution at time t, C0 is
our knowledge, researches on the ability of Cys to promote the initial concentration of the dyes and kobs is the pseudo-rst-
Fenton reactions are almost blank. In this work, we introduced order rate constant (minꢀ1). kobs values were obtained from the
Cys into the Fenton system and investigated the effect of Cys on slopes of the regression lines for plots of ꢀln(Ct/C0) versus time
the oxidation efficiency of the Fenton system with methylene (t).
blue (MB) as probe compound. The role of Cys during the
EPR spectra of radicals trapped by DMPO were examined
reaction process was revealed and electron paramagnetic reso- using a Bruker A300 spectrometer at ambient temperature.
nance (EPR) technology combined with radical scavengers was Settings for the EPR spectrometer were: center eld, 3520 G;
employed to investigate the mechanism of the Cys-driven Fen- sweep width, 100 G; microwave frequency, 9.77 GHz; modula-
ton reaction. The main inuencing factors, such as Cys dosage, tion frequency, 100 kHz and power, 12.72 mW. To quantify cOH,
catalyst and oxidant concentration, initial pH, reaction coumarin was used as a chemical probe. 7-Hydroxycoumarin,
temperature were examined. This research not only provides a product of reaction between coumarin and cOH, was
new insight into the development of a highly efficient and pH- measured by monitoring its uorescence emission at 445 nm
tolerant Fenton oxidation process for wastewater treatment using a spectrouorometer (F-7000).28 The excitation wave-
with environmentally benign characteristics, but also initiates length was 332 nm. The Fe(II) concentration was obtained from
an up-to-date research domain of the application of amino acids the difference between the total Fe and the measured Fe(III)
in environmental catalysis.
concentration, and the concentration of Fe(III) was monitored
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by spectrophotometric detection of phenanthroline.29 The
concentration of H2O2 was measured by titanium potassium
oxalate colorimetry.30 For more accurate measurement, the
concentration of H2O2, Fe3+ and Cys increased by 25 times
simultaneously. Chemical oxygen demand (COD) was deter-
mined by a COD instrument (HACH DR1010, Shanghai), and
the concentrations of MB, H2O2, Fe(III) and Cys increased by 5
times simultaneously.
3. Results and discussion
3.1. Oxidative removal of MB
MB, a representative cationic dye, is used widely as a stain in
many industries. In this study, MB was selected as the probe
compound to compare the oxidation efficiency of different
systems. As shown in Fig. 1, with H2O2 alone, the removal of MB
could be neglected. In the Fenton system, less than 25% of MB
was removed in 36 min. Surprisingly, more than 96% of MB was
removed in 36 min in the Cys modied Fenton system. UV/Vis
spectral changes of MB in the Fenton–Cys system are also
shown in Fig. S1.† The experimental results suggested that the
introduction of Cys into the Fenton system enhanced the
removal of MB signicantly.
Fig. 2 Comparison of apparent pseudo-first-order rate constants
(conditions: [H2O2] ¼ 0.4 mM, [Fe(III)] ¼ 7.5 mM, [Cys] ¼ 20 mM, [MB] ¼
10 mM, initial pH 3.0, T ¼ 20 ꢁC) and oxidant consumption indices
(conditions: [H2O2] ¼ 10 mM, [Fe(III)] ¼ 187.5 mM, [Cys] ¼ 500 mM, initial
pH 3.0, T ¼ 20 ꢁC) for the Fenton and Fenton–Cys systems.
increased the utilization efficiency of H2O2 and enhanced the
oxidation efficiency remarkably.
3.2. Effect of initial pH
In order to further investigate the inuence of Cys, a general
pseudo-rst-order kinetic approach (ln(Ct/C0) ¼ ꢀkobst) was
employed. As shown in Fig. 2, the Fenton–Cys system gave an
almost twelve-fold higher reaction rate constant (0.08838
minꢀ1) than the Fenton system (0.00736 minꢀ1). Furthermore,
the utilization efficiency of H2O2 was also compared for the
Fenton and Fenton–Cys system, which was quantied by the
oxidant consumption index (X), dened as the number of moles
of oxidant consumed per mole of MB removed. Hence, a lower
value of X indicates a higher utilization efficiency of H2O2. As
can be seen in Fig. 2, the Fenton–Cys system (18.0) showed
a superior utilization efficiency of H2O2 to the Fenton system
(31.2). Together, these experimental results above demon-
strated that the introduction of Cys into the Fenton system
The Fenton system is strongly pH dependent and the pH
inuences the Fenton oxidation reaction signicantly.31,32
A
series of experiments were carried out in which the MB solution
pH was adjusted to 2.0, 3.0, 4.0, 5.0, 5.5, 6.0, 6.5 and 7.0 to
investigate the effect of pH on MB removal in the Fenton and
Fenton–Cys systems. As shown in Table 1, the Fenton–Cys
system was superior to the traditional Fenton system in decol-
orizing MB at the same pH value. The removal of MB in the
Fenton–Cys system at pH 6.5 was higher than that achieved in
the Fenton system at the optimal pH 3.0. Meanwhile, kobs of the
Fenton–Cys system was consistently higher than that of the
Fenton system at the same initial pH. Hence, the Fenton–Cys
system showed a more satisfactory pH adaptability compared
with the Fenton system.
MB removal in the Fenton system decreased sharply with
increasing pH from 3.0 to 7.0, which occurs primarily because
of Fe(III) precipitation as ferric oxyhydroxides,33,34 However, MB
removal in the Fenton–Cys system was affected slightly by an
increase in initial pH from 3.0 to 5.0, which could be interpreted
by the complexation between Cys with Fe(III).27 As shown in
Fig. S2,† Cys could form colored complex with Fe(III) to maintain
Fe(III) in solution, which alleviated Fe(III) precipitation and
expended the effective pH range to a certain degree.
The experimental results presented above indicate that the
introduction of Cys into a Fenton system broadened the effec-
tive pH range up to 6.5 but also enhanced the oxidation effi-
ciency. It has been reported that the addition of other common
and intensively investigated organic ligands, such as EDTA,
citrate and oxalate to the Fenton system could also extend the
pH range.35–37 Therefore, we compare Cys with other organic
ligands in the performance of expanding pH. As shown in Fig. 3,
the addition of these complexants into the Fenton system
Fig.
1 The concentration changes of MB in different systems.
Conditions: [H2O2] ¼ 0.4 mM, [Fe(III)] ¼ 7.5 mM, [Cys] ¼ 20 mM, [MB] ¼
10 mM, initial pH 3.0, T ¼ 20 ꢁC.
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Table 1 Effect of initial pH on MB removal rate and kobs in Fenton and Fenton–Cys systems. Conditions: [H2O2] ¼ 0.4 mM, [Fe(III)] ¼ 7.5 mM, [Cys]
¼ 20 mM, [MB] ¼ 10 mM, T ¼ 20 ꢁC, reaction time ¼ 36 min
Fenton system
Fenton–Cys system
Initial pH
Removal rate (%)
kobs (minꢀ1
)
R2
Removal rate (%)
kobs (minꢀ1
)
R2
2.0
3.0
4.0
5.0
5.5
6.0
6.5
7.0
4.5
24.9
22.4
7.4
4.7
2.8
0.00119
0.00736
0.00663
0.00237
0.00084
0.00028
0.00051
0.00043
0.993
0.997
0.994
0.985
0.982
0.970
0.970
0.967
31.6
96.3
96.6
91.8
76.5
46.6
31.6
8.3
0.01023
0.08838
0.10158
0.07681
0.04239
0.01999
0.00614
0.00072
0.998
0.990
0.996
0.993
0.994
0.978
0.956
0.959
2.7
2.0
improves the removal of MB to some extent. However, MB
removal in EDTA, citrate and oxalate-modied Fenton reactions
was inferior to that achieved in the Cys-driven Fenton reaction,
which indicates that the oxidation efficiency of the Cys-driven
Fenton reaction was superior to the ligand-driven Fenton
reactions.
3.3. Analysis of catalytic oxidation mechanism
In the traditional Fenton reaction, the generation of cOH is
a predominant catalytic oxidation mechanism.40–42 Based on the
results presented above, we speculated that the introduction of
Cys into the Fenton system promoted the generation of cOH,
and thus improved the oxidation efficiency. To verify our
hypotheses, a series of radical scavenging experiments was
carried out. TBA was used as a probe compound for the total
ux of cOH because of its high-rate constant reaction with cOH
(kcOH/TBA ¼ (3.8–7.6) ꢂ 108 Mꢀ1 sꢀ1).43,44 As shown in Fig. 4, the
MB removal reaction slowed down and the kobs decreased
signicantly with increasing concentration of TBA in the Fenton
and Fenton–Cys systems, which indicated that cOH plays an
important role in MB removal in the Fenton–Cys system. When
same concentration of TBA was added to the two systems, the
inhibitory effect on MB removal in the Fenton system was much
more obvious than that in the Fenton–Cys system, which could
be ascribed to the more generation of cOH in the Fenton–Cys
system.
Some researchers have reported that the introduction of
reducing agents into the Fenton system could improve the
organic contaminant removal. Among these reducing agents,
hydroxylamine (HA) is commonly considered to be one of the
most promising reductive chemicals because of its strong
reducing property,38 and HA has been introduced into the
Fenton process to enhance benzoic acid degradation.39 We
compared the removal of MB in the Fenton–HA and Fenton–Cys
systems, as shown in Fig. S3,† the addition of HA to the Fenton
system enhanced the oxidative removal of MB. However, the
removal rate (83%) of MB and kobs in the Fenton–HA system
were lower than that in the Fenton–Cys system (96.3%). The
experimental results indicated that the introduction of Cys into
the Fenton system achieved a more superior oxidation efficiency
compared with HA. Therefore, it is necessary to reveal the role of
Cys in the Fenton–Cys system and investigate the mechanism of
the Cys-driven Fenton reaction.
An EPR experiment was carried out to conrm the role of
cOH by measuring the intensity of the DMPO–cOH adduct signal
(Fig. 5(A)).45 A typical four-peak spectrum of DMPO–cOH
Fig. 3 Removal rate of MB in different systems at different pH. Fig. 4 Effect of TBA on MB removal in the Fenton and Fenton–Cys
Conditions: [H2O2] ¼ 0.4 mM, [Fe(III)] ¼ 7.5 mM, [Cys] ¼ [EDTA] ¼ system. Conditions: [H2O2] ¼ 0.4 mM, [Fe(III)] ¼ 7.5 mM, [Cys] ¼ 20 mM,
[citrate] ¼ [oxalate] ¼ 20 mM, [MB] ¼ 10 mM, T ¼ 20 ꢁC.
[MB] ¼ 10 mM, initial pH 3.0, T ¼ 20 ꢁC, reaction time ¼ 36 min.
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adducts with an intensity ratio of 1 : 2 : 2 : 1 was detected in the accelerated the transformation from Fe(III) to Fe(II) and thus
Fenton and Fenton–Cys system, which suggests that cOH was generated more cOH and improved the Fenton oxidation effi-
also formed in the Cys-driven Fenton reaction. The intensity of ciency. To verify our hypothesis, the variation of Fe(II) concen-
the DMPO–cOH signal of the Fenton–Cys system was at least tration with time in the Fe(II)/H2O2, Fe(III)/H2O2 and Fe(III)/Cys/
three times stronger than that in the Fenton system, which H2O2 systems was detected. As can be seen in Fig. 6, in Fe(II)/H2O2
indicates that the Fenton–Cys system could generate a higher system, the concentration of Fe(II) sharply declined with reaction
concentration of cOH compared with the conventional Fenton time, and the Fe(II) concentration was less than 0.1 mM aer 12
system. Furthermore, coumarin was added as a probe molecule min. In Fe(III)/H2O2 system, the Fe(II) concentration was less than
to quantify cOH. The reaction between cOH and coumarin could 0.1 mM throughout the reaction. This phenomenon could be
generate the 7-hydroxycoumarin with uorescence, which could interpreted by the rapid reaction between Fe(II) and H2O2 and the
be monitored at 445 nm using a spectrouorometer to achieve slow recovery of Fe(II) from Fe(III). Conversely, in the Fenton–Cys
the quantication of cOH. As shown in Fig. 5(B), a markedly system, the Fe(II) concentration was more than four times higher
enhanced production of 7-hydroxycoumarin was observed in than that in the Fenton system, and the Fe(II) concentration was
the Fenton system with the addition of Cys whereas the Fenton relatively steady during the reaction process (the inset of Fig. 4).
system, Cys/H2O2 system and H2O2 alone did not generate These results indicated that the introduction of Cys into the
a signicant level of the hydroxylated product (spectral change Fenton system accelerated the Fe(III)/Fe(II) cycle, lead to a rela-
shown in ESI Fig. S4†). These experimental results demon- tively steady Fe(II) recovery and generated more cOH.
strated that the introduction of Cys into the Fenton system
promoted the generation of cOH drastically throughout the agent and reducing agent simultaneously during the reaction
reaction and thus showed a much higher oxidation activity. process. In addition, it has been demonstrated that cOH was the
As discussed above, Cys played a major role as complexing
Based on the aforementioned data and analysis, we assumed primary reactive oxidant in the Cys-driven Fenton reaction. By
that the introduction of Cys into the Fenton system signicantly combining the above experimental results with relevant litera-
ture, we propose a mechanism for the more efficient removal of
MB in the Fenton–Cys system. First, Cys formed complex with
Fe(III) to maintain iron in soluble form, which relieved the Fe(III)
precipitation and expanded the effective pH range. Then, the
presence of Cys in the Fenton system accelerated the Fe(III)/Fe(II)
cycle through the redox reaction, and the redox process between
Cys and Fe(III) resulted in the more rapid transformation from
Fe(III) to Fe(II), accompanying with the oxidation of Cys to
cysteine (eqn (5)).26 This process promoted the generation of
cOH and improved the oxidation efficiency.
(5)
Fig. 5 (A) EPR spectra of DMPO–cOH adducts in aqueous solution in
Fenton and Fenton–Cys system, (B) variations of fluorescence emis- Fig. 6 Concentration changes of Fe(II) in different systems. And the
sion intensity of the solution with 0.05 mM coumarin during the inset shows more clearly the variation of Fe(II) concentration in the
reaction for different systems. Conditions: [H2O2] ¼ 0.4 mM, [Fe(III)] ¼ Fenton and Fenton–Cys systems. Conditions: [H2O2] ¼ 0.4 mM, [Fe(III)]
7.5 mM, [Cys] ¼ 20 mM, initial pH 3.0, T ¼ 20 ꢁC.
¼ 7.5 mM, [Fe(II)] ¼ 7.5 mM, [Cys] ¼ 20 mM, initial pH 3.0, T ¼ 20 ꢁC.
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concentration increased from 0 to 50 mM, and then the increase
in Cys concentration resulted in a decrease in kobs. A Cys
concentration of 20 mM was selected to obtain the highest
decolorization efficiency and economic benets.
3.4.2. Effect of oxidation and catalyst concentration on MB
removal. The oxidative removal of dyes is regulated directly by
oxidant and catalyst dosage, therefore, it is essential to inves-
tigate the effect of H2O2 and Fe(III) concentration on MB
removal. As shown in Table 2, the removal rate of MB in the
Fenton system improved with increase in H2O2 concentration
from 0.1 to 2.0 mM. In contrast, in the Fenton–Cys system, the
MB removal rate increased rapidly from 60.2% to 98.7% when
the H2O2 concentration increased from 0.1 to 2.0 mM. Mean-
while, an obvious and continual increase in kobs was observed
with increase in H2O2 concentration in the Fenton–Cys system,
and the kobs of the Fenton–Cys system was consistently higher
than that of the Fenton system. This indicated that the intro-
duction of Cys promoted the oxidative removal of MB in the
Fenton system signicantly at different H2O2 concentrations.
The inuence of Fe(III) concentration on MB removal in the
Fenton and Fenton–Cys system was also investigated. As shown
in Table 3, MB removal in the Fenton system increased slowly
from 10.5% to 33.7% with increase in Fe(III) concentration from
2.5 to 15 mM. However, in the Fenton–Cys system, MB removal
increased from 69.0% to 97.8% with increasing Fe(III) concen-
tration from 2.5 to 15 mM. In particular, MB removal in the
Fig. 7 (A) Effect of Cys concentration on MB removal in Fenton–Cys
system, (B) effect of Cys concentration on kobs of MB removal in Fenton–Cys system was much higher than that achieved in the
Fenton–Cys system. Conditions: [H2O2] ¼ 0.4 mM, [Fe(III)] ¼ 7.5 mM,
Fenton system at the same Fe(III) concentration. Furthermore,
the variation trend of kobs with Fe(III) concentration presented in
Table 3 was similar to that for the MB removal rate with Fe(III)
concentration. The experimental results above suggested that
the introduction of Cys impelled the Fenton system to achieve
a higher oxidation efficiency at the tested H2O2 and Fe(III)
concentrations.
3.4.3. Effect of temperature on MB removal. Reaction
temperature is a critical operating parameter for the Fenton
reaction. Therefore, the inuence of reaction temperature on
MB removal was studied. As shown in Fig. 8(A) and (B), an
increase in temperature favors MB removal in the Fenton and
Fenton–Cys systems. To further investigate the relationship,
kinetic rate constants are presented in Fig. 8(C). kobs in the two
systems obviously increased with increasing reaction tempera-
ture. Moreover, the kobs for the Fenton–Cys system were much
higher than those for the Fenton system at same temperature,
[MB] ¼ 10 mM, initial pH 3.0, T ¼ 20 ꢁC.
3.4. Effect of other factors on MB removal
3.4.1. Effect of Cys concentration on MB removal. The
experimental results and analysis reveal the role of Cys in the
Fenton–Cys system for the oxidative removal of MB. The effect
of Cys concentration on MB removal should be investigated in
the Fenton–Cys system. As shown in Fig. 7(A), the MB removal
increased with increase in Cys concentration from 0 to 50 mM.
However, when the Cys concentration exceeded 50 mM, the MB
removal decreased with further increase in Cys concentration,
which may be ascribed to the consumption of cOH by reaction
with excess Cys. kobs presented in Fig. 7(B) exhibited a similar
variation trend with removal rate. The kobs of MB removal
increased from 0.00736 to 0.08863 minꢀ1 when the Cys
Table 2 Effect of H2O2 concentration on MB removal rate and kobs in Fenton and Fenton–Cys systems. Conditions: [Fe(III)] ¼ 7.5 mM, [Cys] ¼ 20
mM, [MB] ¼ 10 mM, initial pH 3.0, T ¼ 20 ꢁC, reaction time ¼ 36 min
Fenton system
Fenton–Cys system
H2O2 concentration
(mM)
Removal rate
(%)
kobs
Removal rate
(%)
kobs
(minꢀ1
)
R2
(minꢀ1
)
R2
0.1
0.2
0.4
0.8
2.0
17.5
19.2
24.9
20.1
25.0
0.00496
0.00581
0.00736
0.00635
0.00790
0.993
0.999
0.997
0.997
0.999
60.2
94.0
96.3
96.0
98.7
0.02213
0.07449
0.08838
0.09296
0.12584
0.965
0.986
0.990
0.998
0.999
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Table 3 Effect of Fe(III) concentration on MB removal rate and kobs in Fenton and Fenton–Cys systems. Conditions: [H2O2] ¼ 0.4 mM, [Cys] ¼ 20
mM, [MB] ¼ 10 mM, initial pH 3.0, T ¼ 20 ꢁC, reaction time ¼ 36 min
Fenton system
Fenton–Cys system
Fe(III)
concentration (mM)
Removal rate
(%)
kobs
Removal rate
(%)
kobs
(minꢀ1
)
R2
(minꢀ1
)
R2
2.5
5.0
7.5
10
10.5
16.8
24.9
26.0
33.7
0.00314
0.00529
0.00736
0.00802
0.01037
0.999
0.997
0.997
0.998
0.991
69.0
87.2
96.3
97.8
97.8
0.02985
0.05367
0.08838
0.09553
0.13210
0.985
0.987
0.990
0.990
0.975
15
Fig. 8 (A) Effect of temperatures on MB removal in Fenton system, (B) effect of temperatures on MB removal in Fenton–Cys system, (C) effect of
temperature on kobs of MB removal in Fenton and Fenton–Cys systems, (D) Arrhenius plots for MB removal at different temperatures in Fenton
and Fenton–Cys systems. Conditions: [H2O2] ¼ 0.4 mM, [Fe(III)] ¼ 7.5 mM, [Cys] ¼ 20 mM, [MB] ¼ 10 mM, initial pH 3.0.
which indicated that the Fenton oxidation process for dye conditions without special temperature and pressure require-
removal was more rapid with introduction of Cys at the tested ments, MB can be removed to a signicant extent by the Fen-
temperature. It can be inferred that Cys play a benecial role in ton–Cys system.
enhancing Fenton oxidative activity. Further discuss described
by an Arrhenius plot (ln K vs. 1/T), as shown in Fig. 8(D), from
which the values of apparent activation energy (Ea) for MB
4. Conclusions
removal were determined as 95.90 and 47.93 kJ molꢀ1 for the In this study, the highly efficient and pH-tolerant Cys-driven
Fenton and Fenton–Cys systems, respectively. This implies that Fenton oxidation reaction was offered as a promising water
the removal of MB in the Fenton–Cys system could proceed remediation method. Compared with the traditional Fenton
easily at a relatively low energy. Indeed, even under mild system, the Fenton–Cys system displayed a superior pH-tolerant
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47668 | RSC Adv., 2016, 6, 47661–47668
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