A quantitative assessment of the production of OH and additional oxidants in the dark Fenton reaction: Fenton degradation of aromatic amines

Article abstract of DOI:10.1039/c3ra44585b

This paper reports the results of a kinetic study into the transformation of 2,4- and 3,4-dichloroaniline (2,4-DCA, 3,4-DCA) and of methyl yellow (MY) with the Fenton reagent in aqueous solution. All the substrates can be degraded in the presence of Fe(II) + H₂O₂, but the reaction between Fe(II) and H₂O₂ causes substrate degradation and Fe(II) oxidation within seconds under the adopted conditions. The HPLC, GC-MS and IC analyses only allow the monitoring of the reaction after all Fe(II) has been consumed, when degradation proceeds more slowly via Fe(III) reduction to Fe(II). Substrate degradation in the first part of the reaction was studied by stopped-flow spectrophotometry, using MY as substrate. The results are consistent with a reaction involving OH, where both Fe(II) and H ₂O₂ compete with MY for the hydroxyl radical. However, the experimental data indicate that OH is unlikely to be the only product of the reaction between Fe(II) and H₂O₂. Another species, possibly the ferryl ion (FeO²⁺), is formed as well but has a negligible role in MY degradation. The Fenton reaction would thus yield both OH (about 60% at pH 2) and ferryl (about 40%), and the 60:40 branching ratio between OH and the other species is compatible with additional data here reported concerning the degradation of 2,4-DCA and 3,4-DCA in the first ferrous step of the Fenton reaction. The reported findings will hopefully indicate a way out of a long-lasting controversy concerning the mechanism of the Fenton process, also suggesting an approach to quantitatively determine the formation yields of the reactive species as well as a strategy to identify the reactant that is actually involved in substrate transformation.

Full text of DOI:10.1039/c3ra44585b

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PAPER
A quantitative assessment of the production of _OH and
additional oxidants in the dark Fenton reaction: Fenton
degradation of aromatic amines
Cite this: RSC Adv., 2013, 3, 26443
Claudio Minero,* Mirco Lucchiari, Valter Maurino and Davide Vione*
This paper reports the results of a kinetic study into the transformation of 2,4- and 3,4-dichloroaniline
2,4-DCA, 3,4-DCA) and of methyl yellow (MY) with the Fenton reagent in aqueous solution. All the
(
2 2 2 2
substrates can be degraded in the presence of Fe(II) + H O , but the reaction between Fe(II) and H O
causes substrate degradation and Fe(II) oxidation within seconds under the adopted conditions. The
HPLC, GC-MS and IC analyses only allow the monitoring of the reaction after all Fe(II) has been
consumed, when degradation proceeds more slowly via Fe(III) reduction to Fe(II). Substrate degradation
in the first part of the reaction was studied by stopped-flow spectrophotometry, using MY as substrate.
2 2
The results are consistent with a reaction involving _OH, where both Fe(II) and H O compete with MY
for the hydroxyl radical. However, the experimental data indicate that _OH is unlikely to be the only
2+
2 2
product of the reaction between Fe(II) and H O . Another species, possibly the ferryl ion (FeO ), is
formed as well but has a negligible role in MY degradation. The Fenton reaction would thus yield both
OH (about 60% at pH 2) and ferryl (about 40%), and the 60 : 40 branching ratio between _OH and the
other species is compatible with additional data here reported concerning the degradation of 2,4-DCA
and 3,4-DCA in the first ferrous step of the Fenton reaction. The reported findings will hopefully
indicate a way out of a long-lasting controversy concerning the mechanism of the Fenton process, also
suggesting an approach to quantitatively determine the formation yields of the reactive species as well
as a strategy to identify the reactant that is actually involved in substrate transformation.
_
Received 22nd August 2013
Accepted 24th October 2013
DOI: 10.1039/c3ra44585b
www.rsc.org/advances
_
OH and produce interference even with otherwise selective
Introduction
6,7
techniques such as the EPR spin trapping.
The most common interpretation of the mechanism of the
A large amount of evidence has recently been provided in
8–14
Fenton reaction is traditionally based on the proposal by Haber favour of ferryl, which has also been isolated and identied,
1
and Weiss and further modications. According to that tradi- although its actual involvement in Fenton degradation is still
15
tional view, the key process is the generation of hydroxyl radi-
questioned. A reasonable interpretation of the literature nd-
ings is that both _O H and ferryl are formed by Fe(II) + H , with
variable yields depending on pH. In particular, the _O H yield
ing species, which questioned the role and sometimes even the would decrease with increasing pH and the opposite would
cals upon reaction between Fe(II) and H O . More recently a
2
2
2 2
O
controversy has appeared concerning the nature of the oxidis-
16
occurrence of _O H. Indeed, the Fenton reaction is sometimes happen with ferryl. On the other hand, extensive kinetic studies
2,3
different from the expected behaviour of homogeneous _OH.
and kinetic modelling have been carried out successfully based
The possible role of high-valence iron species has been on the classical interpretation of the Fenton process, considering
proposed (usually indicated as ferryl), in analogy with some not only the production of key oxidising species but also the many
1
7–19
Fenton-like processes that involve Fe(II) complexes with organic
subsequent reactions that take place in solution.
II
ligands and different peroxides than H O , such as Fe L
+
2
2
x
The Fenton reaction is frequently studied because it is one of
the most widely applied advanced oxidation processes for water
II
4,5
HOOH or Fe L + ROOH. The problem is certainly very diffi-
x
20
cult because, if additional oxidants are formed in the Fenton and wastewater decontamination, due to its effectiveness in
reaction, in aqueous solution they would mimic the reactivity of the degradation of many organic compounds.2
1–30
In the Fenton
treatment, Fe(II) can be added as such (dark process), or
photochemically generated upon photolysis of dissolved Fe(III)
31–33
34
(photo-Fenton reaction)
or suspended Fe(III) (hydr)oxides.
Dipartimento di Chimica, Universit a` di Torino, Via P. Giuria 5, 10125 Torino, Italy.
E-mail: claudio.minero@unito.it; davide.vione@unito.it; Web: http://www.
environmentalchemistry.unito.it; Fax: +39-011-6705242; Tel: +39-011-6705296
III
2+
+
Fe –L + hn / Fe + L _
(1)
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Other techniques (electro-Fenton, sono-Fenton) to produce
IC analyses were carried out with a Dionex DX 500 ion
Fe(II) from Fe(III) are also possible, although they are less widely chromatograph, equipped with Rheodyne injector (loop
35,36
applied than the photochemical variant.
volume 50 mL), LC 30 chromatography oven, GP 40 gradient
In this work we report on the Fenton degradation of aromatic pump, and ED 40 electrochemical detector (conductivity
amines, which allowed the quantication of the relative role of mode). Analyses for the anions made use of a Dionex Ion Pac
_
OH and other oxidants (e.g. ferryl) in the process. The fast AG9-HC 4 ꢀ 50 mm guard column, Ion Pac AS9-HC 4 ꢀ
2+
2 2
reaction between Fe and H O was studied by use of the 250 mm column, and Ion Pac ASRS-ULTRA 4 mm conductivity
stopped-ow technique and with an amine dye (methyl yellow) suppressor. Elution was carried out with a solution containing
ꢁ
3
ꢁ2
as substrate. Considerations suggested by the stopped-ow 4.75 ꢀ 10 M NaHCO
3
and 1.14 ꢀ 10 M K
2 3
CO . Cation
study were further conrmed by the degradation of dichlor- analyses made use of a Dionex Ion Pac CG12A 4 ꢀ 50 mm
oanilines, monitored by HPLC. This technique gave insight guard column, Ion Pac CS12A 4 ꢀ 250 mm column, and Ion
both into the substrate fraction that was degraded in the rst Pac CSRS-ULTRA 4 mm conductivity suppressor. Samples were
2
+
ꢁ2
reaction step (Fe + H
2
O
2
), and into the following steps that are eluted with a solution containing 2.5 ꢀ 10 M CH
3 3
SO H. In
ꢁ
1
dominated by the reduction of Fe(III).
both cases the ow rate was 1.0 mL min . Under the
respective conditions the retention times were [min]: chloride
[
5.60], nitrate [9.70], ammonium [4.15].
Aliquots for GC-MS analysis were treated with excess
Experimental
Chemicals
Na SO and extracted with dichloromethane. The extract was
2
4
dried with anhydrous Na SO and concentrated under a gentle
2
(
,4-Dichloroaniline (purity grade 99%), 3,4-dichloroaniline
98%), 2,4-dichlorophenol (99%), 3,4-dichlorophenol (99%),
methyl yellow, methanesulphonic acid (>99.5%), NaHCO
>99.7%), and NH Cl (98%) were purchased from Aldrich,
dichloromethane (for gas chromatography), 2-propanol
2
4
stream of high-purity nitrogen. Dichloromethane extracts
were injected into a Hewlett Packard 6890 GC-MS, equipped
with a phenylmethylsilicone capillary column (HP 5MS, length
3
(
4
3
0 m, i.d. 0.25 mm, lm thickness 0.25 mm) and a HP 5973
mass detector (EI-SCAN mode). The following conditions were
(
(
gradient grade for liquid chromatography), FeSO $7 H O
4 2
99.5%), NaH PO $H O (>99%), Na HPO $2 H O (>99.5%),
used: manual splitless injection (1 mL volume), gas carrier (He)
2
4
2
2
4
2
ꢁ
1
ꢂ

ow rate 1 mL min , injector temperature 300 C, column
K CO (>99.5%), H O (35%), H SO (96%), and NaCl (99.5%)
from VWR Int., acetonitrile (gradient grade) from Carlo Erba,
anhydrous Na SO (>99.5%) from ProLabo. FeSO $7 H O was
puried by recrystallisation, the other reagents were used as
received without further purication.
2
3
2
2
2
4
ꢂ
ꢂ
ꢂ
ꢁ1
temperature from 70 C (3 min) to 300 C at 10 C min
.
Under such conditions the retention times were [min]: 2,4-
DCA [15.10], 2,4-DCP [12.40], 2,4,6-trichloroaniline [16.00],
2
4
4
2
2
-chloro-1,4-benzoquinone [10.95].
The stopped-ow measures were carried out with a Hi-tech
stopped-ow spectrophotometer composed of a SF-3L support
unit, equipped with a temperature-control device (set at 25 C)
Instrumentation and procedures
ꢂ
The Fenton reaction was studied in aqueous solution. The and a SF-40C control unit. The spectrophotometer was equip-
degradation of methyl yellow (MY) at pH 1.8 by H SO
was ped with Hi-tech arc lamp and MG-10 grating monochromator.
assessed with the stopped-ow technique. The different Data acquisition was performed with an Agilent Inniium
2
4
ꢁ1
reactants (substrate + H
mixed by means of two syringes. The degradation of the
dichloroanilines at pH 2.0 by H SO (and of MY at pH 1.8 to double-beam UV-Vis spectrophotometer, using Hellma 104-QS
2
SO
4
+ FeSO
4
, H
2
O
2
+ H
2
SO
4
) were oscilloscope (500 MHz, 1 GSa s ).
UV-Vis spectra were obtained with a Varian CARY 100 Scan
2
4
detect the transformation intermediates) was carried out in quartz cuvettes (1.000 cm optical path length). The solution pH
00 mL beakers under magnetic stirring. The different was measured with a combined glass electrode connected to a
reagents were added from separate stock solutions and the Metrohm 713 pH-meter. Water used for the experiments was of
last addition was of H
to start the reaction. At selected time Milli-Q quality.
1
2 2
O
intervals, aliquots were withdrawn for successive analysis by
HPLC, IC or GC-MS.
HPLC analyses were carried out with a Merck-Hitachi chro-
matograph, equipped with a Rheodyne injector (loop volume The reaction between Fe and H O is relatively fast,
Results and discussion
2
+
37–39
thus
2
2
54 mL), model L-6200 and L-6000 pumps for high-pressure the related degradation of substrates cannot be properly
gradients, a 125 ꢀ 4 mm RP-C18 LiChroCART column (VWR) monitored by liquid chromatography unless very low concen-
packed with LiChrospher 100 RP-18 (particle diameter 5 mm), trations of the reactants are used. The stopped-ow technique is
and a model L-4200 UV-Vis detector. The injected samples were suitable to study fast processes but it cannot be coupled with
eluted with a 40/60 mixture of acetonitrile–aqueous phosphate separation devices. With our stopped-ow instrument the
2 4 2 4
buffer (NaH PO + Na HPO , total phosphate 0.050 M, pH 7.5) monitoring can be carried out spectrophotometrically, and the
ꢁ1
at 1.0 mL min ow rate, and detected at 240 nm. The reten- choice of a substrate absorbing visible radiation is needed to
tion times were [min]: 2,4-dichloroaniline (2,4-DCA) [11.00], 3,4- minimise spectral interference. Azo dyes such as MY are a
dichloroaniline (3,4-DCA) [8.80], 2,4-dichlorophenol (2,4-DCP) reasonable choice, because the –N]N– group is very reactive
[6.80], 3,4-dichlorophenol (3,4-DCP) [7.95].
and its cleavage or transformation causes a loss of conjugation
2
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and, therefore, the disappearance of the band that absorbs
40
visible radiation.
Degradation of methyl yellow
Experimental data. Methyl yellow (MY) is an azo dye (N,N-
dimethyl-4-(phenylazo)aniline) that undergoes acid–base equi-
librium. Its absorption spectrum at different pH values shows
the presence of an isosbestic point at 471 nm (see Fig. 1). At this
wavelength these is no spectral interference by the Fenton
reagent, thus we carried out the stopped-ow runs at 471 nm.
Experiments were carried out at pH 1.8, where practically all
MY is protonated as suggested by Fig. 1. A preliminary study of
the transformation intermediates was carried out with aqueous
saturated solutions of MY in the presence of 1 mM Fe(II) + 1 mM
Scheme 1 Early transformation intermediates of MY by the Fenton reagent
ꢂ
H
2
O
2
at pH 1.8 and 25 C. The reaction was quenched at
(
2 2
1 mM Fe(II) + 1 mM H O , pH 1.8, GC-MS analysis).
different time intervals with a phosphate buffer at pH 7.0 (total
phosphate concentration 0.10 M), which also deprotonated MY
and related compounds. Aer addition of Na SO , the solution
2
4
was extracted with dichloromethane and the organic phase was
analysed by GC-MS. The dichloromethane used for the extrac-
tion and CH Cl -extracted Milli-Q water were adopted as
2 2
analytical blanks. The main detected intermediates are deriva-
tives resulting from oxidative attack on the azo group (see
Scheme 1).
Oxidative cleavage of the –N]N– group is likely to be the
main reaction pathway, as conrmed by the detection of
many compounds clearly originating from such a cleavage
(
3
N,N-dimethyl-4-nitroaniline, N,N-dimethylaniline, aniline,
-(N,N-dimethylamino)phenol, 4-aminophenol, phenol, and 1,3-
dihydroxyphenol). The presence of many hydroxylated aromatics Fig. 2 Time trend of the 471 nm absorbance (decimal logarithm) for the
degradation of 10 mM MY in the presence of 1 mM Fe(II) in aqueous solution (the
(
phenolic compounds) is consistent with a relevant role played by
initial [H
2
O
2
] is reported on the figure). pH 1.8 by addition of H
2
SO
4
, reaction
_O H in the Fenton degradation of MY, though not constituting
ꢂ
temperature 25 C.
compelling evidence in its favour.
Fig. 2 reports the time evolution of 10 mM MY (decimal
logarithm of the absorbance as a function of time) in the
presence of 1 mM Fe(II), in aqueous solution at pH 1.8 adjusted
expected to be halved in a time varying from 0.5 s with 30 mM
. The disappearance of MY
H
2
O
2
to ꢃ20 s with 0.8 mM H
2 2
O
2 4 2 2
by addition of H SO , for different values of [H O ] (varied in the
down to ꢃ1/20 of the initial concentration requires a shorter
time (see Fig. 3). Therefore, MY degradation is mostly accoun-
ted for by the reaction between Fe(II) and H O , with a negligible
range of 0.8–30 mM). The disappearance of MY, monitored by
stopped-ow spectrophotometry, was practically complete
before the expected total consumption of Fe(II). Based on kinetic
2
2
17,18
role of the following ferric step (reactions (2)–(4)).
37–39
data reported in the literature,
the concentration of Fe(II) is
+
Fe(III) + H
2
O
2
/ Fe(II) + HO
_ / Fe(II) + O
2
_ + H
(2)
(3)
(4)
+
+ H
Fe(III) + HO
2
2
2ꢁ
2
Fe(III) + O _ / Fe(II) + O
Kinetic data treatment. A simplied kinetic model can be
elaborated to account for MY degradation by the Fenton reagent,
37
based on a free radical mechanism where the key oxidant is _O H
mechanism A, reactions (5), (6), (7a)–(10a)). Alternatively, these
(
38,39
reactions could be replaced by a non-radical mechanism
that
2
+
implies the participation of the ferryl ion (here indicated as FeO ;
mechanism B, reactions (5), (6), (7b)–(10b)). The formation of a
reactive complex between Fe(II) and H O is hypothesised,
2 2
Fig. 1 Absorption spectra (molar absorption coefficients 3) of MYat different pH
38,39,41
values in aqueous solution. The isosbestic point at 471 nm is highlighted.
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In the hypothesis that MY degradation takes place exclu-
2
+
sively upon reaction with either _O H (mechanism A) or FeO
mechanism B), the application of the steady-state approxima-
(
2
+
tion to the concentration of the reactive specie R ( _O H or FeO )
yields the following equation:
v½MYꢄ
ꢁ
¼ k
D
$½Rꢄsteady-state$½MYꢄ
(11)
(12)
vt
where
k
A
$½FeðIIÞꢄ$½H O
2
ꢄ
2
½
Rꢄsteady-state ¼ k
k1$k3
B
$½FeðIIÞꢄ þ k
C
$½H O
2
2
ꢄ þ k $½MYꢄ
D
and kA ¼
(ref. 38) is the observed rate constant for Fe(II)
Fig. 3 Reciprocal of the initial degradation rate of 10 mM MYas a function of the
k2 þ k3
reciprocal of [Fe(II)]
lines: predictions of eqn (13) with the data reported in Table 1, second column
MLR method).
o
(1–5 mM). [H
2
O
2
]
o
¼ 1–30 mM, pH 1.8 by H
2 4
SO . Dashed
consumption (k
3
¼ k3a or k3b). The choice of the rate constant
(
e.g. kDa or kDb for k ) depends on the actual mechanism (A or B)
D
(
that is operational. Within this approach, the two mechanisms
are considered as mutually exclusive.
2
+
The differential eqn (11) and (12) has no analytical solution if
[Fe(II)], [H O ] and [MY] are functions of time. To obtain a
which can evolve into _O H or FeO . Note that both oxidants could
2+
react with either Fe , H
2
O
2
or MY:
2 2
solution one can focus on the initial rates, for which [Fe(II)] z
(5) [Fe(II)] and [H ] z [H . As we monitored the time
evolution of MY, the analytical function [MY](t) cannot be
obtained unless the term k [MY] is neglected in the denomi-
2
+
2+
Fe + H O / {Fe –H O } [k ]
o
2
O
2
O ]
2 2 o
2
2
2
2
1
2
Fe –H
+
2+
{
2
O
2
} / Fe + H
2
O
2
[k
2
]
(6)
D
nator of eqn (12), in which case the degradation of MY follows a
pseudo-rst order kinetics. Under this approximation, the plots
of log10(A) vs. time should be linear. Fig. 2 indicates that the
pseudo-rst order approximation is valid only in the initial part
of the curves, thus some errors might derive from the time
interval chosen for the t. Numerical simulations suggested
that if [MY](t) varies by no more than 10% of its initial value, the
error associated to the pseudo rst-order approximation is
Mechanism A
2
+
+
3+
{
Fe –H O } + H / _O H + Fe +H O [k ]
(7a)
(8a)
(9a)
2
2
2
3a
2
+
+
3+
_OH + Fe + H / Fe + H
2
O [kBa]
_OH + H O / HO _ + H O [kCa]
2 2 2 2
_O H + MY / MYox [kDa
]
(10a) below 3–5%. This is comparable with data reproducibility
(
around 5%). Under the cited approximation the time evolution
ꢁ
obs
k t
Mechanism B
of MY absorbance is A
absorbance (0.27 for 10 mM MY), t the time in seconds, and k
the observed pseudo-rst order rate constant (s ). The initial
degradation rate of MY (r ) is given by r ¼ k ꢀ [MY] .
t
¼ A
o
e
o
, where A is the initial
obs
2
+
2+
{
Fe –H O } / FeO + H O [k ]
(7b)
(8b)
ꢁ1
2
2
2
3b
2
+
2+
+
3+
o
o
obs
o
FeO + Fe + 2H / 2Fe + H
2
O [kBb
]
o 2 2
Under the approximations [Fe(II)] z [Fe(II)] , [H O ] z
2
+
FeO + H
2+
/ Fe + O
[H
2 2 o o o
O ] and [MY] z [MY] , and considering the initial rate r ,
2
O
2
2
+ H
2
O [kCb
]
(9b)
eqn (11) and (12) can be linearised as follows:
2
+
FeO + MY / MYox [kDb
]
(10b)
k
B
k
C
ꢁ
1
r
o
¼ k
þ k
k
o
A
A
k
D
½H O
2
2
ꢄ ½MYꢄ
o
D
½FeðIIÞꢄ ½MYꢄo
o
The analogies between mechanism A and B only hold for the
fast step of the Fenton process, but there are different kinetic
implications for the following steps. Actually, in mechanism A
the Fe(III) would be reduced back to Fe(II), while the ferryl ion
1
þ k
(13)
A
½FeðIIÞꢄ ½H O
o
2
2
ꢄo
Based on eqn (13) and on stopped-ow experiments where
3
+
5+
could react with Fe to form FeOFe , for which the reduction [Fe(II)] was varied between 1 and 5 mM, it is possible to obtain
o
38
ꢁ1
ꢁ1
ꢁ1
ꢁ1
to Fe(II) is rather slow. However, these late processes would not the plots of r
affect the reaction studied by stopped ow.
plots are shown in Fig. 3 and their linearity is fully compatible
An additional issue is that pH was adjusted by addition of with eqn (13). Note that the dashed lines of Fig. 3 are not
SO and Fe(II) was added in the form of FeSO
, thus part of regression ones: their meaning will be explained later. The plots
OH formed in reaction (7a) would react with HSO
vs. [Fe(II)]
and r
vs. [H
O
]
. The former
o
o
o
2
2 o
H
_
2
4
4
ꢁ
ꢁ1
ꢁ1
(rate of r
o
vs. [H
2
O
2
]
o
(not shown) were linear as well.
(ref. 42)). However, considering the Eqn (13) is the starting point to obtain the values of the
4
6
ꢁ1 ꢁ1
constant 1.2 ꢀ 10 M
s
8
ꢁ1 ꢁ1
ꢁ1
ꢁ1
o
rate constants of reactions (8a) (4.0 ꢀ 10 M
s
(ref. 42)) kinetic constants. The plot of r
o
vs. [Fe(II)]
is a line with
oꢁ1
O ] } and intercept
2
7
ꢁ1 ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
and (9a) (2.7 ꢀ 10 M
s
(ref. 42)) and the concentrations of slope S
1
¼ {k
C
k
A
ꢁ1
k
D
[MY]
o
+ k
]
A
[H
2
ꢁ
ꢁ1
ꢁ1
Fe(II) and H
neglected.
2
O
2
, the reaction between _OH and HSO
4
can be
I
S
1
¼ {k
B
k
A
k
D
[MY]
o
[H
2
O
2 o
}. Furthermore, by plotting
2
2
oꢁ
1
1
0
A
ꢁ1
against [H
O
]
one obtains a line with slope S
¼ k
and
1
2
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0
Aꢁ1 Dꢁ1
oꢁ1
00
intercept I
1
¼ k
C
k
k
[MY] . Moreover, the plot of I
1
vs.
The application of the steady-state approximation to [R]
ꢁ1
ꢁ1
ꢁ1
ꢁ1
[
H O ]
is a line with slope S ¼ k k
kD [MY]o From yields:
2
2 o
1
B
A
0
0
00
0
ꢁ1
the values of S , I and S1 it is possible to obtain: k ¼ (S )
;
1
1
A
1
À Á
ꢁ1
k
E
ꢁ
1
0
0
ꢁ1
ꢁ1
00
0
ꢁ1
P
o
ꢁ1
o
r
¼ r
þ k
$½2-propanolꢄo
k k
¼ I (S ) [MY] , and k k
¼ S (S ) [MY] .
C
D
1
1
o
B
D
1
1
o
A
$kDa$½FeðIIÞꢄ $½H
o
2 2
O
ꢄo$½MYꢄ
o
ꢁ
1
ꢁ1
In a similar way the plot of r
o
vs. [H
2
O
2
]
o
is a line with
} and intercept
(15)
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
slope S
2
¼ {k
B
1
k
k
A
Dꢁ1
k
D
[MY]
o
+ k
A
[Fe(II)]
o
ꢁ
ꢁ1
oꢁ1
P
o
I
2
¼ {k
C
ꢁ
k
A
[MY]
o
[Fe(II)] }. By plotting S
2
against where r is the initial degradation rate measured in the presence
1
0
ꢁ1
[Fe(II)]
o
one obtains a line with slope S
2
¼ k
A
and intercept of 2-propanol and r that measured in its absence. The expression
o
0
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
I
(
2
¼ k
B
k
A
k
D
[MY]
o
. It is thus possible to obtain k
A
¼
for r
is reported in eqn (13). Eqn (15) implies that the plot of
o
0
ꢁ1
0
0
ꢁ1
P ꢁ1
S ) and k k
¼ I (S ) [MY] . Furthermore, by plotting (r ) vs. [2-propanol] is linear. Such a plot is reported in Fig. 4 in
2
B
ꢁ
D
2
2
o
o
o
1
00
ꢁ1
ꢁ1
ꢁ1
I vs. [Fe(II)]
one obtains a line with slope S ¼ k k
kD
2 2
the presence of 10 mM MY, 1 mM Fe(II) and 1.5 mM H O , varying
2
o
2
C
A
ꢁ
1
ꢁ1
00
0
[
MY]o . As a consequence, k kD ¼ S (S ) [MY] .
[2-propanol] in the range of 0.14–1.00 mM.
C
2
2
o
o
A
, k
B
k
Dꢁ
1
The plot of (r ) vs. [2-propanol] is a line with slope S ¼ (3.15
P ꢁ1
ꢁ2
o
3
The experimental data allow two assessments of k
ꢁ
1
8 4 ꢁ1
and k
C
k
D
, which agree within 10% and are reported as ꢅ 0.04) ꢀ 10 M s and intercept I ¼ (7.33 ꢅ 0.21) ꢀ 10 M s.
3
average values in Table 1, rst column (method ¼ slope/ From the slope it is possible to assess k ¼ k {S k [Fe(II)]
Da
E
3
A
o
ꢁ1
9
ꢁ1 ꢁ1
intercept, m ꢅ s). The initial degradation rates of MY as a [H O ] [MY] } . Using the value kE ¼ 1.9 ꢀ 10 M
s
for
2
2 o
o
42
function of [Fe(II)]
multiple linear regression (MLR) method using the Maple 0.1) ꢀ 10
soware, with eqn (13) as the regression function. This data This value is very similar to those reported for the reaction
o
and [H
2
O
2
]
o
were also processed with a reaction between 2-propanol and _OH, one obtains k ¼ (1.3 ꢅ
Da
10
ꢁ1 ꢁ1
M
s
as the reaction rate constant of MY with _O H.
ꢁ
1
10 ꢁ1 ꢁ1
treatment allows a further assessment of k , k kD and kC between_OH and other azo-dyes (Acid Blue 40, 1.2 ꢀ 10
M
s ;
A
B
ꢁ
1
10
ꢁ1 ꢁ1 42
k
D
and the relevant data are reported in Table 1, second Acid Blue 74, 1.8 ꢀ 10
M
s ). The data reported so far
column (method ¼ MLR). There is a very good agreement indicate that the kinetics of MY degradation by the Fenton reagent
between the data obtained with the two methods (MLR and is compatible with an _O H-initiated process (mechanism A).
slope/intercept), the average of which is reported in Table 1, Mechanism B (through the ferryl ion) could also be compatible,
2+
third column. The dashed lines in Fig. 3 were obtained from although unlikely, by assuming that FeO reacts with almost the
eqn (13) by introducing the data of k , kB kD and kC kD
ꢁ
1
ꢁ1
A
2 2
same rate constants of _OH with Fe(II), H O and 2-propanol.
calculated by MLR. The agreement between experimental and
predicted values is very good.
Quantication of_OH and ferryl generation. It is interesting to
ꢁ1
ꢁ1
observe that Table 1 reports a k value (30.4 ꢅ 2.2 M
A
s ) that is
B
k
Cꢁ
1
¼ 11 ꢅ 5 (m ꢅ s). lower than those reported in the literature for Fe + H O2
2+
2
From the data of Table 1 one obtains k
Under the hypothesis that the reactive species R is_OH, from the (ranging in the interval 50–65 M
ꢁ1 ꢁ1 17,37,39
s
).
Note that the liter-
8
ꢁ1 ꢁ1
7
literature values of kBa ¼ 4.0 ꢀ 10 M
s
and kCa ¼ 2.7 ꢀ 10
ature values have been obtained by measuring the disappearance
ꢁ1
ꢁ1
ꢁ1
M
s
(ref. 42) one gets kBa
k
Ca ¼ 14.8, which is inside the rate of either Fe(II) or H O , while we have monitored the trans-
2
2
interval we have obtained (11 ꢅ 5). Therefore, the experimental formation of MY and estimated as a consequence the generation
data are compatible with _O H as the reactive species for MY rate of the key reacting species R. The disagreement between the
degradation.
A
value of k calculated here and those reported in the literature
Reaction rate constant between MY and _O H. The value of suggests that Fe(II) disappears at a higher rate compared to
Da (bimolecular rate constant between MY and _O H in the formation of the key reacting species. This issue has already
aqueous solution) can be evaluated independently upon been described in the literature and might be due to the
k
17
addition of 2-propanol as _O H scavenger (with rate constant of formation of both ferryl and _O H in separate but parallel processes
9
ꢁ1 ꢁ1
ꢂ
1
.9 ꢀ 10 M
s
at 25 C (ref. 42)), under the assumption as depicted in reactions (5)–(10). This possibility is supported by
43
that the key reactant is the hydroxyl radical. In the presence molecular dynamic calculations, which showed that as H O is
2
2
of 2-propanol a reaction needs to be added to the kinetic
model:
2-Propanol + R / products [k
E
]
(14)
ꢁ1
ꢁ1
Dꢁ1
Dꢁ1
Table 1 Values of k
A
(units of M
s
), k
B
k
, and k
C
k
obtained with two
different methods (slope/intercept: see Fig. 3 and the relevant discussion; MLR:
multiple linear regression, with eqn (13) as the regression function). The data in
the third column are the averages of those in the first two columns. Errors ¼ ꢅs
Method
Slope/intercept
MLR
Average
k
k
k
A
B
C
30.5 ꢅ 2.5
30.4 ꢅ 2.0
30.4 ꢅ 2.2
Fig. 4 Reciprocal of the initial degradation rate of 10 mM MY as a function of [2-
propanol] . [Fe(II)]o ¼ 1 mM, [H O ] ¼ 1.5 mM, pH 1.8 by H SO , reaction
Dꢁ
1
ꢁ2
ꢁ3
ꢁ2
ꢁ3
ꢁ2
ꢁ3
k
k
(1.6 ꢅ 0.2) ꢀ 10
(1.6 ꢅ 0.4) ꢀ 10
(1.6 ꢅ 0.1) ꢀ 10
(1.3 ꢅ 0.4) ꢀ 10
(1.6 ꢅ 0.2) ꢀ 10
(1.4 ꢅ 0.4) ꢀ 10
o
2
2 o
2
4
Dꢁ
1
ꢂ
temperature 25 C. The regression line is also reported.
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coordinated to Fe(II) or is entering the coordination shell, between the reaction rate of Fe(II) and H
2
O
2
and the genera-
1
7
the oxygen–oxygen bond breaks to form Fe(III) (actually, tion rate of _O H.
III
2+
[
(H O) Fe OH] ) and a very short-lived _O H radical. The
2 5
Degradation of dichloroanilines with the Fenton reagent
hydroxyl radical could either react with a water ligand to form
IV
2+
[(H
2
O)
4
Fe (OH)
2
]
and a water molecule, producing the ferryl The Fenton degradation of dichloroanilines (2,4-DCA and 3,4-
ion in a second step by dehydration, or abstract hydrogen from DCA, hereaer both indicated as DCA, used at 0.8 mM initial
ꢁ
the OH ligand, directly producing ferryl. Moreover, other concentration) was studied in aqueous solution at pH 2.0 by
evidence has accumulated in favour of the contemporary forma- H SO . Under these conditions both substrates are proton-
2
4
44
8–14
tion of both species in the Fenton reaction.
ated. Fig. 5 reports that DCA degradation in the presence of
The application of the steady-state approximation to [_OH],
0
.2–0.6 mM Fe(II) and 1–3 mM H
2
O
2
has a very fast initial step
2
+
2+
2+
[
FeO ] and [{Fe –H O }], when both reaction pathways are (ferrous step, triggered by Fe + H O ) that goes to completion
2
2
2 2
included (in this case, mechanisms A and B are considered within 1 min reaction time. Aerwards the kinetic regime
together) yields an expression for the initial MY transformation changes to a slower ferric step, dominated by Fe(III) reduction.
2
+
rate (r
incompatible with the linearisation reported in Fig. 3.
The linearisation of 1/r vs. 1/[X] or Fe , is this step is due to the complete consumption of Fe(II).
, where X ¼ H
possible if only one MY degradation process (involving either
o
¼ kDa [_OH]
o
[MY]
o
+ kDb [FeO
o o
2 2
] [MY] , eqn (16)) that is Considering that we used [Fe(II)] < [H O ], based on the above
kinetic analysis of the ferrous step we assume that the ending of
2
+
o
o
2 2
O
If (as it is reasonable) the rate constant between DCA and _O H is
2
+
_
OH or FeO ) is operational, as already deduced. Since for other
of the same order of magnitude as that of MY, the initial
organic amines and azo dyes the reaction with _OH is not only concentration value of DCA (0.8 mM) implies that the competitive
possible, but also quite fast, we must conclude that MY reacts _O H reactions with Fe(II) and H O are negligible. Within this
2
2
preferentially with _OH and that the reaction with ferryl plays a context one obtains that the relative transformation of DCA at the
negligible role in MY degradation.
end of the ferrous step is (eqn (19)):
2
þ
2þ
1
ðk
2
þ k3a þ k3bÞðkBb½Fe ꢄ þ kCCb½H
2
O
2
ꢄ þ kDb½MYꢄ Þ ꢀ ðkBa½Fe ꢄ þ kCa½H
2
O
2
ꢄ þ kDa½MYꢄ Þ
o
o
2þ
o
o
o
o
¼
(16)
2þ
r
o
k ½H O ꢄ ½Fe ꢄ ½MYꢄ fðk k k þ k k k Þ½Fe ꢄo þ ðkDak k þ kDbk k Þ½H O ꢄ þ ðk þ k Þk kDa½MYꢄog
1
2
2 o
o
o
Da 3a Bb
Db 3b Ba
3a Cb
3b Ca
2
2 o
3a
3b Db
Therefore, assuming that r
expression almost identical to eqn (13) is obtained, with the
only difference that k assumes a different meaning:
o
o o
¼ kDa [_OH] [MY] , a rate
A
1
k $k3a
k
A
¼ k
(17)
2
þ k3a þ k3b
By so doing, the relevant kinetic model is based on reac-
tions ((5), (6), (7a), (8a), (9a), (10a), (7b), (8b), (9b)), excluding
reaction (10b). In the steady-state approximation, however,
reactions ((8b) and (9b)) have no inuence on the trans-
formation rate of MY. The model yields the following expres-
Fe
A
sion for the bimolecular rate constant k
between Fe(II) and
H
2
O
2
in reactions (5)–(7):
k
1
ðk3a þ k3bÞ
Fe
A
k
¼
(18)
k
2
þ k3a þ k3b
This result justies the observed difference between the
Fe
ꢁ1 ꢁ1
ꢁ1 ꢁ1
literature value for k
A
(52.4 M
s
(ref. 39), 53.0 M
s
ꢁ1
ꢁ1
(
ref. 37)) and our data (k ¼ 30.4 ꢅ 2.2 M
s
). The ratio of
A
Fe
A
Fe
ꢁ1 ꢁ1
k /k ¼ k /(k + k ) ¼ 0.58 (for k ¼ 52.8 M
s
) implies
A
3a
3a
3b
A
2
+
that ꢃ60% of the complex {Fe –H
2 2
O } would produce _O H
radicals, whilst the remaining ꢃ40% would form species
that do not react at an appreciable rate with MY under
Fe
Fig. 5 (A) Degradation of 0.8 mM 2,4-DCA with the Fenton reagent in aqueous
solution (the values of [Fe(II)] and [H are indicated on the figure). pH 2.0 by
addition of H SO . (B) Degradation of 0.8 mM 3,4-DCA with the Fenton reagent in
aqueous solution (the values of [Fe(II)] and [H are indicated on the figure).
SO
the adopted conditions. Because the k
dent on k
studied pH value (1.8) the kinetic pathways have the same
A A
/k ratio is indepen-
o
2 2 o
O ]
D
, it follows that for any Fenton system at the
2
4
o
2 2 o
O ]
Fe
A
k /k ratio. This nding helps explaining the disagreement pH 2.0 by addition of H
2
4
.
A
2
6448 | RSC Adv., 2013, 3, 26443–26450
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D½DCAꢄ
k
Fe
A
A
D½FeðIIÞꢄ
k
Fe
A
A
½FeðIIÞꢄ
½DCAꢄ
In this way, 2,4,6-TCA would constitute a reservoir of organic
chlorine and organic nitrogen, slowing down the generation of
both chloride and ammonium.
Furthermore, the Fenton degradation of 3,4-DCA yields 3,4-
DCP, ammonium, and chloride.
o
¼
b ¼ k
$
¼ k
$
(19)
½
DCAꢄ
½DCAꢄ
o
o
o
Fe
A
(
for k
A
and k
see eqn (17) and (18) and the relative discussion;
implies that Fe(II) is completely
consumed in the ferrous step).
Based on data reported in Fig. 5A/B, the plot of b vs. [Fe(II)]
the use of D[Fe(II)] ¼ [Fe(II)]
o
o
/
Fe
A
Conclusions
[DCA]
o
is a line with slope k
consistent with the value of 0.58 already found in the case of MY
and constitutes additional evidence that the reaction between
A
/k
¼ 0.66 (Fig. 6). This is fully
The dark Fenton process involving Fe(II) + H
2 2
O consists of
two regimes, a fast ferrous one that is triggered by the reac-
2
+
Fe(II) and H O yields both _OH (about 60% at pH 2) and FeO
2+
2
2
2 2
tion of Fe + H O and a slow ferric one that is dominated by
(
about 40%), with _O H alone contributing to substrate degra-
dation in the present case.
Among the intermediates of the Fenton degradation of 2,4-
DCA it was possible to detect the corresponding phenol
2,4-DCP), the 2-chloro-1,4-benzoquinone (CBQ) and a tri-
chloroaniline (2,4,6-TCA). Ammonium and chloride were also
formed. The formation of 2,4-DCP would occur upon deami-
nation of 2,4-DCA, which also yields ammonium, while no
nitrate could be detected in the system. A similar result has
been obtained by Bossmann et al. in the case of 2,4-dimethy-
the reduction of Fe(III). The ferrous regime was studied with
the stopped-ow technique, using methyl yellow (MY) as a
substrate. A kinetic model that is able to account for all the
observed results requires that the reaction between Fe(II) and
(
H
2
O
2
produces both _O H (ꢃ60% yield) and another species,
2+
possibly the ferryl ion (FeO ) (ꢃ40% yield) in parallel
processes, with ferryl reacting with MY at a negligible rate.
The experimental data are consistent with the degradation of
MY taking place upon reaction with _OH, with rate constant
10
ꢁ1 ꢁ1
ꢂ
kDa ¼ (1.3 ꢅ 0.1) ꢀ 10
M
s
at 25 C. The hypothesis of
3
laniline. The intermediate CBQ is most likely formed upon
2+
the formation of both _O H and FeO , with _O H alone being
reactive, also accounts for the degradation of dichloroani-
lines (2,4-DCA and 3,4-DCA) in the ferrous step. The kinetic
model we proposed might be a way out of a long-lasting
controversy concerning the Fenton reagent, and it also
allowed the quantication of the formation yields of the
reactive species.
oxidative dechlorination of 2,4-DCP, presumably initiated by
attack of _O H on the carbon atom bound to chlorine. In this way,
one of the two chlorine atoms initially present on the substrate
would be transformed into a chloride ion.
When 2,4-DCA disappears, about 70% of organic nitrogen is
transformed into ammonium and 35% of organic chlorine into
chloride. The low generation of chloride can be accounted for if
the transformation of organic chlorine into chloride is faster for
one of the two Cl atoms of 2,4-DCA. This would be consistent
with the detection of CBQ as intermediate. Another possible
contribution to the low chloride generation comes from the
formation of 2,4,6-TCA. This compound is likely to be formed
Notes and references
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ꢁ
_
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is a chlorinating agent. The chlorine-containing radicals can
be produced upon oxidation of chloride by _O H:
2
3
4
S. H. Bossmann, E. Oliveros, S. G ¨o b, S. Siegwart,
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7
5 F. Gozzo, J. Mol. Catal. A: Chem., 2001, 171, 1.
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¼ 1.6 ꢀ 10 ]
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Fig. 6 Plot of b ¼ D[DCA]/[DCA]
o o o
vs. [Fe(II)] /[DCA] , based on the data of Fig. 5.
Fe
A
Slope ¼ k
A
/k
¼ 0.66. Note that DCA ¼ 2,4-DCA or 3,4-DCA.
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6450 | RSC Adv., 2013, 3, 26443–26450
This journal is ª The Royal Society of Chemistry 2013