Oxidative Degradation of Azo Dyes in Aqueous Solution by Water-Soluble Iron Porphyrin Catalyst

Article abstract of DOI:10.1002/ejic.202100373

Textile and food industries produce a significant volume of effluents containing azo dyes and other pollutants. These effluents are serious environmental threats, and new methods for their treatment and for the degradation of azo dyes are thus attracting much attention. The current study deals with the oxidative degradation of azo dyes by meso-tetrakis(1-methylpyridinium-4-yl)prophyrinatoiron(III), [Fe^III(tmpyp)], and meta-chloroperoxy benzoic acid (m-CPBA) in aqueous solution at room temperature. The catalytic degradation of azo dyes was investigated by using rapid-scan stopped-flow spectrophotometry as a function of solution pH, [catalyst], [m-CPBA], [dye] and [surfactants]. To obtain mechanistic insight, the reaction between [Fe^III(tmpyp)] and m-CPBA was also studied in aqueous solution in absence of azo dyes. Spectral analyses and kinetic data show that [Fe^III(tmpyp)] is transformed into the transient intermediate [Fe^IV(O)(tmpyp)]^.+ (a compound I analog) within 20–30 ms followed by the formation of relatively stable [Fe^IV(O)(tmpyp)] (a compound II analog). Batch experiments reveal that the dye degradation rate is influenced by the solution pH and the concentrations of [Fe^III(tmpyp)], m-CPBA, dye, and surfactants. On the basis of the kinetic and spectroscopic data, a mechanistic scheme for the dye degradation reaction and a steady-state rate equation are proposed. The products resulting from oxidative degradation of the azo dye amaranth have been analyzed by HPLC-UV-HRMS.

Full text of DOI:10.1002/ejic.202100373

European Journal of Inorganic Chemistry
Accepted Article
Title: Oxidative Degradation of Azo Dyes in Aqueous Solution by
Water-Soluble Iron Porphyrin Catalyst
Authors: Tapan Kumar Saha, Holm Frauendorf, and Franc Meyer
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100373
Link to VoR: https://doi.org/10.1002/ejic.202100373
0
1/2020

European Journal of Inorganic Chemistry
10.1002/ejic.202100373
Oxidative Degradation of Azo Dyes in Aqueous Solution by
Water-Soluble Iron Porphyrin Catalyst
Tapan Kumar Saha[a,b],* Holm Frauendorf and Franc Meyer
[c]
[b],*
[
a] Prof. Dr. Tapan Kumar Saha, Department of Chemistry, Jahangirnagar University, Savar,
Dhaka 1342 (Bangladesh)
Fax: (+880) 2-7791052
Email: tksaha_ju@yahoo.com
Homepage: https://juniv.edu/teachers/tksaha_ju
[
b] Prof. Dr. Franc Meyer, Universität Göttingen, Institut für Anorganische Chemie,
Tammannstrasse 4, D-37077 Göttingen (Germany)
Email: franc.meyer@chemie.uni-goettingen.de
Homepage: https://uni-goettingen.de/de/611271.html
Twitter: @franc_meyer
[
c] Dr. Holm Frauendorf, Universität Göttingen, Institut für Organische und Biomolekulare
Chemie, Tammannstrasse 2, D-37077 Göttingen (Germany)
Abstract: Textile and food industries produce a significant volume of effluents containing
azo dyes and other pollutants. These effluents are serious environmental threats, and
new methods for their treatment and for the degradation of azo dyes are thus attracting
much attention. The current study deals with the oxidative degradation of azo dyes by
III
meso-tetrakis(1-methylpyridinium-4-yl)prophyrinatoiron(III), [Fe (tmpyp)], and meta-
chloroperoxy benzoic acid (m-CPBA) in aqueous solution at room temperature. The
catalytic degradation of azo dyes was investigated by using rapid-scan stopped-flow
spectrophotometry as a function of solution pH, [catalyst], [m-CPBA], [dye] and
III
[
surfactants]. To obtain mechanistic insight, the reaction between [Fe (tmpyp)] and m-
CPBA was also studied in aqueous solution in absence of azo dyes. Spectral analyses
III
and kinetic data show that [Fe (tmpyp)] is transformed into the transient intermediate
IV
•+
[
Fe (O)(tmpyp)] (a compound I analog) within 20-30 ms followed by the formation of
IV
relatively stable [Fe (O)(tmpyp)] (a compound II analog). Batch experiments reveal that
the dye degradation rate is influenced by the solution pH and the concentrations of
III
[
Fe (tmpyp)], m-CPBA, dye, and surfactants. On the basis of the kinetic and
spectroscopic data, a mechanistic scheme for the dye degradation reaction and a steady-
state rate equation are proposed. The products resulting from oxidative degradation of
the azo dye amaranth have been analyzed by HPLC-UV-HRMS.
Introduction
Azo dyes are the most common dyes used in dyestuff, textile, leather, paper, food and plastic
industries to color a broad variety of final products. The widespread use of azo dyes is due to
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
their bright colors, color fastness and ease of application.[1,2] However, the presence of azo
dyes in industrial aqueous effluents causes serious environmental pollution, because most of
them are recalcitrant, toxic, carcinogenic, genotoxic and highly resistant to decomposition.
These dyes remaining in waste effluents undergo chemical changes, consume dissolved
oxygen, and destroy aquatic creatures. Therefore, the efficient treatment of azo dye containing
industrial wastewaters is currently in real demand.
Various research works have been aiming at the development of efficient processes for the
recovery of organic contaminants from the effluents of textile industries.[3-6] Traditional
methods such as precipitation, coagulation and flocculation have been used in wastewater
treatment although these techniques are not very effective for eliminating several common
dyes, especially from dilute solutions.[3] Photo-oxidation has also been proposed for the
treatment of dye–containing effluents,[7-10] however, this process is relatively expensive and
not appropriate for the treatment of large flows. Recently, biological degradation has been
cited as an alternative process for the decoloration of azo dyes in wastewater.[11-13]
On the other hand, adsorption processes remain the most common and useful techniques for
the decontamination of the effluents of textile and dyeing industries. Many studies have
exploited potential adsorbents such as chitosan,[14-18] mineral sorbents,[19] activated
carbons,[20-22] peat,[23,24] chitin,[25-28] soy meal hull,[29] and agro wastes.[30-32] However, the
adsorption capacity of those materials generally is not very large; to improve the adsorption
performance new types of adsorbents are still under development.
A number of reports have appeared in which the catalytic effect of metalloporphyrin complexes
on the peroxidative degradation of azo dyes in aqueous solution has been examined.[33-41]
Nango et al. investigated the manganese porphyrin catalyzed decoloration of azo dyes using
as an oxidant in the presence of imidazole in aqueous solution[33,34] and in nonaqueous
solvents.[42] We recently studied the oxidative degradation of azo dyes by meso-tetrakis(1-
2
H O
2
III
[35]
methylpyridinium-4-yl)porphyrinatomanganese(III), [Mn (tmpyp)], in aqueous solution.
Such manganese porphyrin complexes are achieving broader recognition as efficient catalysts
for the oxidation of various substrates, and high-oxidation state oxomanganese(IV) and
oxomanganese(V) intermediates have been suggested to play a crucial role in those
reactions.[35,43-47] However, a literature survey reveals few studies in which iron porphyrin
complexes, serving as cytochrome P450 models, have been exploited for the oxidative
degradation of dyes in aqueous solution.[38-42] It has been proposed that an oxoferryl porphyrin
•+
IV
π-cation ( PFe =O), generated from heterolytic cleavage of the O-O bond in H
2
O catalyzed
2
by hemin, may play a major role in dye degradation.[41] Furthermore it has been mentioned
that ionic substituents on the porphyrin ring have a beneficial effect on the dye degradation
catalysis.[38] Therefore, we set out to study in detail the catalytic activity of a water soluble
ferric porphyrin for the peroxidative degradation of azo dyes in aqueous solution, and to
2
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
identify the high-valent oxoiron species involved in those reactions. Our findings provide
valuable insight into the mechanistic scenario of the catalytic process, which could hold great
potential for future applications in wastewater treatment.
For these mechanistic investigations, rapid-scan stopped-flow spectrophotometry was applied
in the present study. This technique was previously proven to be very useful for the
identification of high-valent oxoiron(IV) porphyrin species that form upon oxidation of ferric
porphyrins with various oxidants.[48-50] It enables the detection as well as the characterization
of key intermediates in such catalytic systems.[48,49] Here we have investigated the kinetics
and reaction mechanism of peroxidative degradation of a selected azo dye, amaranth (Figure
1, right) by meta-chloroperoxy benzoic acid (m-CPBA) in aqueous solution, using [meso-
III
tetrakis(1-methylpyridinium-4-yl)porphyrinato]-iron(III) pentaperchlorate, [Fe (tmpyp)] (Figure
1
left) as the catalyst. This cationic iron complex has previously been reported to serve as a
highly reactive P450 model compound capable of cleaving strong C-H bonds.[49] The dye
degradation reaction was investigated as a function of solution pH and other parameters that
affect the reaction kinetics, and the products resulting from catalytic degradation of amaranth
were analyzed by HPLC-UV-HRMS.
III
Figure 1. Structure of [Fe (tmpyp)] (left) and amaranth (right).
III
The reaction between [Fe (tmpyp)] and m-CPBA in the absence of amaranth was also
examined in aqueous solution by rapid-scan stopped-flow spectrophotometry, and the
possible involvement of high-valent oxoiron(IV)porphyrin species ([Fe (O)(tmpyp)]•+ and
IV
IV
[
Fe (O)tmpyp)]) in the dye degradation reaction was probed. A plausible dye degradation
mechanism and a steady-state rate law of the reactions are proposed.
Results and discussion
III
[
Fe (tmpyp)]-catalyzed peroxidative degradation of azo dyes
Initially, some screening experiments were performed which revealed that the aqueous
solutions of various azo dyes, such as amaranth, reactive orange 13 (RO13) and reactive red
3
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
III
2
39 (RR239) were effectively decolorized upon treatment with [Fe (tmpyp)] and m-CPBA.
Amaranth was then selected as a representative azo dye substrate for more detailed studies.
III
In the absence of [Fe (tmpyp)], when an aqueous solution of amaranth was mixed with m-
CPBA, no change in color was observed over 12 h. Spectral measurements confirmed that
III
the presence of both [Fe (tmpyp)] and m-CPBA is essential for the oxidative degradation of
amaranth. The reaction of an amaranth solution (40 µM) containing catalytic amounts of
III
[
Fe (tmpyp)] (1 µM) with m-CPBA (400 µM) in 50 mM acetate buffer (pH 3.7) was examined
by rapid scan stopped-flow spectrophotometry at 25 °C. A typical set of time-resolved
absorption spectra is shown in Figure 2.
III
Figure 2. Typical time-resolved spectral changes observed during [Fe (tmpyp)]-catalyzed oxidative
III
degradation of amaranth by m-CPBA. Experimental conditions: [Fe (tmpyp)] = 1 µM, [amaranth] = 40
µM, [m-CPBA] = 400 µM, pH 3.7 (50 mM acetate buffer), T = 25 °C.
It is evident that the characteristic absorption peak of amaranth (λmax 523 nm) has completely
disappeared within 0.36 s while a new peak appeared at 392 nm. Kinetic traces for the
characteristic absorption peak (λmax 523 nm) of amaranth are shown in Figure 3a. In the
III
presence of both [Fe (tmpyp)] and m-CPBA in aqueous solution (pH 3.7) the characteristic
peak decays according to a single exponential function.
Kinetic experiments were conducted under pseudo first-order condition with respect to
amaranth, and the kinetic trace was used to calculate an observed pseudo first-order rate
III
constant (kobs). The first-order kinetic plot at constant [amaranth], [Fe (tmpyp)], [m-CPBA],
and temperature, respectively, is presented in Figure 3b; kobs was determined from the slope
2
of the straight line (R = 0.9947) according to the equation lnA
t
= lnA
0
- kobs.t, where A is the
t
absorbance (λmax 523 nm) at time t and A
0
is the absorbance at time 0.
4
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
Figure 3. (a) Kinetic traces for the characteristic absorption peak (λmax 523 nm) of amaranth during the
III
reaction observed in 50 mM acetate buffer (pH 3.7) at 25 °C: (i) 40 µM amaranth + 1 µM [Fe (tmpyp)];
III
(
(
ii) 40 µM amaranth + 400 µM m-CPBA; (iii) 40 µM amaranth + 1 µM [Fe (tmpyp)] + 400 µM m-CPBA.
III
b) First-order kinetic plot of [Fe (tmpyp)]-catalyzed oxidative degradation of amaranth by m-CPBA.
Experimental conditions: [Fe (tmpyp)] = 1 µM, [amaranth] = 40 µM, [m-CPBA] = 400 µM, pH 3.7 (50
mM acetate buffer), T = 25 °C.
III
Effect of pH
The initial pH of a dye solution is an important parameter affecting the degradation process
because pH influences the chemistry of both catalyst and dye molecules in aqueous solution.
III
The effect of pH on the overall rate of the [Fe (tmpyp)]-catalyzed oxidative degradation of
amaranth by m-CPBA was studied in acetate buffer (50 mM) in the pH range of 3 – 6 and in
phosphate buffer (50 mM) in the pH range of 5.5 – 12.8. A few drops of NaOH (1 M) were
added to achieve higher pH values.
The pseudo first-order rate constants (kobs) obtained from the dye degradation reactions as a
function of solution pH are shown in Figure 4. Considering the entire pH range studied, it is
noticed that the dye degradation rate is very high in acidic (pH 3.0 – 4.2) and basic (pH 10.2
–11.2) solutions. The values of kobs were found to reach maxima at pH 3.7 and 10.6,
III
respectively. UV-visible spectra of [Fe (tmpyp)] in dilute HClO
4
at pH 3.7 and in 50 mM acetate
III
buffer at pH 3.7 are similar (Figure S1a). Likewise, UV-vis spectra of [Fe (tmpyp)] in NaOH at
pH 10.6 and in 50 mM phosphate buffer at pH 10.6 are similar (Figure S1b). Hence we
5
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
consider binding of acetate or phosphate to the catalyst unlikely under these conditions (at pH
.7 or 10.6, respectively). The kinetic results may be explained in terms of the pK values of
Fe (tmpyp)]. Earlier some of us reported that the pKa1 and pKa2 values of [Fe (tmpyp)] are
3
a
III
III
[
estimated to be 4.8 and 12.25, respectively,[50] in reasonable agreement with other
reports.[51,52] We thus assume that the dominant species are [Fe (tmpyp)(H
III
O) ] and
2
2
III
[
Fe (tmpyp)(OH)(H
2
O)] in aqueous solutions at pH 3.7 and 10.6, respectively, as reported by
Kurihara et al..[51a] Both of these species may undergo rapid ligand exchange of the axial water
ligands. The values of kobs are significantly lower in the range of pH 5.2 – 8, which may be due
to the coordination of acetate (pH 5 – 6) and phosphate (pH 5.5 – 8) anions, thus blocking the
III
active site of [Fe (tmpyp)] that is needed for the binding and activation of m-CPBA. Differences
III
were indeed observed in the UV-vis spectra of [Fe (tmpyp)] in HClO
4
at pH 5 (perchlorate
being non-coordinating) and in 50 mM acetate buffer at pH 5 (Figure S2a). UV-vis spectra of
III
[
Fe (tmpyp)] in NaOH at pH 8 and in 50 mM phosphate buffer at pH 8 were also different
(
Figure S2b), indicating the formation of a phosphate complex under the latter conditions.
On the other hand, the pK value of m-CPBA is 7.57 and, hence, the increase in the activity
above pH 7.5 (Figure 4) may reflect the pH dependence of the formation of the active
a
IV
•+
intermediate [Fe (O)(tmpyp)] through O–O bond heterolysis concomitant with the formation
of m-chlorobenzoate. It has been reported that the pK
of amaranth is 10.4[53] and, hence, the
a
III
deprotonated amaranth may coordinate to [Fe (tmpyp)], thus occupying the active site that is
needed for m-CPBA activation. We therefore assume that the decreasing activity at pH >10.6
is, at least in part, caused by the coordination of deprotonated amaranth to the metal ion of
the catalyst.[35] In view of the pH profile (Figure 4), all further kinetic studies were performed at
III
these two pH values, 3.7 and pH 10.6. The present findings for [Fe (tmpyp)]/m-CPBA clearly
differ from the reported optimal pH range for the oxidative amaranth degradation by
III
, which showed a maximum value of kobs at pH 10.4 in aqueous solution.[35]
[
Mn (tmpyp)]/H
2
O
2
III
Figure 4. Effects of pH on kobs for the [Fe (tmpyp)]-catalyzed oxidative degradation of amaranth by m-
III
CPBA. Experimental conditions: [Fe (tmpyp)] = 1 µM, [Amaranth] = 40 µM, [m-CPBA] = 400 µM, T =
25 °C. The pH in the range 3-6 was adjusted with 50 mM acetate buffer and in the range 5.5-12.8 it was
adjusted with 50 mM phosphate buffer; a few drops of NaOH (1 M) were added to attain higher pH
values.
6
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
Detection of high-valent oxoiron(IV) porphyrin intermediates
Previously, some of us reported that at least two active oxoiron(IV) species, a short-lived and
III
a long-lived, are responsible for [Fe (tmpyp)]/H
2
O -induced biphasic luminol emission in
2
aqueous solution (pH 11).[48] The transformation of [Fe (tmpyp)] into [Fe (O)(tmpyp)] (a
compound II-type intermediate) was completed within 1 s and then persisted as a dominant
product over a period of 1 min. The time-resolved absorption spectra at the initial 0.5 s was
III
IV
III
not well fitted by a simple combination of the two spectra of [Fe (tmpyp)] and
Fe (O)(tmpyp)]. More recently, Bell and Groves[49] reported that the transient intermediate
IV
[48]
[
IV
•+
[
Fe (O)(tmpyp)] (a compound-I type intermediate) was formed in the reaction between
III
[
Fe (tmpyp)] and m-CPBA in aqueous solution (pH 4.7) with a second-order rate constant (k
1
7
-1 -1
IV
•+
=
(1.59 ± 0.06) ×10 M s ). The transient intermediate [Fe (O)(tmpyp)] consequently
IV
converted into [Fe (O)(tmpyp)] (λmax 427 nm) by one-electron reduction with a first-order rate
-1 [49]
constant (k
2
= 8.8 ± 0.1 s ). In order to elucidate the potential contribution of those reactive
III
oxoiron(IV) species in the present [Fe (tmpyp)]-induced peroxidative degradation of
amaranth, rapid-scan stopped-flow experiments were carried out under various conditions.
III
Typical changes in time-resolved absorption spectra observed upon reaction of [Fe (tmpyp)]
(
7 µM) with m-CPBA (10 µM) in aqueous solution at pH 3.7 in the absence of amaranth are
III
shown in Figure 5. The starting [Fe (tmpyp)] spectrum (λmax 421 nm) is completely
IV
•+
transformed within 20-30 ms into a new transient intermediate, [Fe (O)(tmpyp)] , which has
a weak blue-shifted Soret band at 402 nm and an additional absorbance at 673 nm, typical of
a porphyrin π-radical cation.[49] Isobestic points between the spectra of [Fe (O)(tmpyp)] and
IV
•+
IV
[
Fe (O)tmpyp)] were found at 413 and 570 nm.
III
Figure 5. Typical time-resolved spectral changes observed in the reaction between [Fe (tmpyp)] and
III
m-CPBA in aqueous solution (pH 3.7). Experimental conditions: [Fe (tmpyp)] = 7 µM, [m-CPBA] = 10
µM, pH 3.7 (50 mM acetate buffer), T = 25 °C.
7
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
III
IV
•+
The kinetic traces for [Fe (tmpyp)] (λmax 421 nm; 500 nm), [Fe (O)(tmpyp)] (λmax 402 nm)
IV
and [Fe (O)tmpyp)] (λmax 427 nm) observed in the above reaction are shown in Figure 6. The
III
initial sharp decay was observed in the kinetic traces of [Fe (tmpyp)] recorded at λmax 421 nm
IV
•+
and 500 nm, indicating the formation of transient intermediate [Fe (O)(tmpyp)] (λmax 402 nm).
The initial rapidly decaying part of the collected kinetic trace at λmax 500 nm was fitted with a
single exponential equation to obtain an apparent first-order rate constant (kobs) which was
utilized to determine the second-order rate constant (k
1
) for the above reaction. The values of
III
k
obs for the above reaction are varying with initial concentrations of [Fe (tmpyp)] and m-CPBA
as presented in Figure 7. It is clear from the plots that kobs for the above reaction is linear in
III
2
2
[
Fe (tmpyp)]
0
(R = 0.9899) and [m-CPBA]
0
(R = 0.9960), respectively. The slope of the linear
) for the formation
of [Fe (O)(tmpyp)] in the reaction between [Fe (tmpyp)] and m-CPBA in aqueous solution.
response was used to calculate the apparent second-order rate constant (k
1
IV
•+
III
5
-1 -1
At pH 3.7 an average value k
1
= 4.54 × 10 M s was derived, which is lower than the second-
III
order rate constant reported for the reaction between [Fe (tmpyp)] and m-CPBA at pH 4.7,
7
-1 -1 [49]
(
1.59 ± 0.06) × 10 M s , reflecting a pronounced pH dependence. The reactive transient
IV
•+
IV
intermediate [Fe (O)(tmpyp)] accordingly converts into [Fe (O)(tmpyp)] (λmax 427 nm) by
-1
one-electron reduction with a first-order rate constant k
determined from the kinetic trace of the characteristic peak of [Fe (O)(tmpyp)] at 402 nm
Figure 6).
1
' = 3.38 ± 0.29 s . The value of k
1
' was
IV
•+
(
III
IV
•+
Figure 6. Kinetic traces for [Fe (tmpyp)] (λmax 421 and 500 nm), [Fe (O)(tmpyp)] (λmax 402 nm) and
IV
III
[
Fe (O)(tmpyp)] (λmax 427 nm) observed in the reaction between [Fe (tmpyp)] and m-CPBA in
aqueous solution (pH 3.7). Experimental conditions: [Fe (tmpyp)] = 7 µM, [m-CPBA] = 10 µM, pH 3.7
50 mM acetate buffer), T = 25 °C.
III
(
8
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
III
Figure 7. Observed rate constant kobs at 500 nm versus [Fe (tmpyp)]
0
(a) and [m-CPBA] (b) for the
0
III
reaction between [Fe (tmpyp)] and m-CPBA in absence of amaranth in 50 mM acetate buffer at pH 3.7
and 25 °C.
III
The reaction between [Fe (tmpyp)] (7 µM) and m-CPBA (10 µM) was also examined in 50
mM phosphate buffer solution at pH 10.6 in the absence of amaranth. The time-resolved
III
spectral changes are shown in Figure 8. The transformation of [Fe (tmpyp)] into
IV
[
[
Fe (O)(tmpyp)] is more rapid at pH 10.6 than at lower pH, and the transient intermediate
IV
•+
Fe (O)(tmpyp)] could not be detected spectroscopically. This is in good agreement with
previous results from some of us[48] and the results reported by Bell and Groves.[49] The kinetic
III
IV
traces for [Fe (tmpyp)] (λmax: 600 nm) and [Fe (O)(tmpyp)] (λmax: 433 nm) obtained for the
above reaction are shown in Figure 9.
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
III
Figure 8. Typical time-resolved spectral changes observed upon reaction of [Fe (tmpyp)] with m-CPBA
III
in aqueous solution. Experimental conditions: [Fe (tmpyp)] = 7 µM, [m-CPBA] = 10 µM, pH 10.6 (50
mM phosphate buffer), T = 25 °C.
III
IV
Figure 9. Kinetic traces for [Fe (tmpyp)] (λmax: 600 nm) and [Fe (O)(tmpyp)] (λmax: 433 nm) observed
III
during the reaction between [Fe (tmpyp)] and m-CPBA in 50 mM phosphate buffer solution (pH 10.6).
III
Experimental conditions: [Fe (tmpyp)] = 7 µM, [m-CPBA] = 10 µM, pH 10.6 (50 mM phosphate buffer),
T = 25 °C.
The kinetic trace for the initial rapid decay of the band at λmax 600 nm was fitted with a single
exponential equation to obtain an apparent first-order rate constant (kobs) which was utilized
to determine the second-order rate constant (k
1
) for the above reaction at pH 10.6. These
III
values of kobs for the above reaction are varying with the initial concentrations of [Fe (tmpyp)]
and m-CPBA as shown in Figure 10. It is clear from the plots that kobs for the above reaction
III
2
2
is linear in [Fe (tmpyp)]
0
(R = 0.9944) and [m-CPBA] (R = 0.9937), respectively. The slope
0
of the linear response was then used to calculate the apparent second-order rate constant (k
1
)
IV
•+
III
for the formation of [Fe (O)(tmpyp)] during the reaction of [Fe (tmpyp)] with m-CPBA in
6
-1 -1
aqueous solution (pH 10.6). The average value of k
1
was estimated to be 1.11 × 10 M s at
7
-1 -1
pH 10.6, and a second-order rate constant of (1.59 ± 0.06) × 10 M s could be derived for
III
[49]
the reaction between [Fe (tmpyp)] and m-CPBA.
1
0
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
Figure 10. Dependence of the observed rate constant kobs for the decay of the band at 600 nm on
III
III
[
Fe (tmpyp)]
0
(a) and [m-CPBA] (b) for the reaction between [Fe (tmpyp)] and m-CPBA in the absence
0
of amaranth in 50 mM phosphate buffer at pH 10.6 and 25 °C.
Effect of catalyst concentration
The effects of catalyst concentration on the peroxidative degradation of amaranth were studied
in aqueous solution at both pH 3.7 and 10.6 with a fixed [m-CPBA] and [amaranth]. Time-
resolved UV/Vis spectra were recorded every 0.012 s for a total of 6 s. A typical set of time-
III
resolved absorption spectra recorded for the [Fe (tmpyp)]-catalyzed peroxidative degradation
of amaranth in 50 mM phosphate buffer solution (pH 10.6) is shown in Figure 11. The
characteristic absorption peak of amaranth (λmax 523 nm) has completely disappeared within
0.36 s.
1
1
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
III
Figure 11. Typical time-resolved spectral changes observed for the [Fe (tmpyp)]-catalyzed oxidative
III
degradation of amaranth by m-CPBA. Experimental conditions: [Fe (tmpyp)] = 1 µM, [amaranth] = 40
µM, [m-CPBA] = 400 µM, pH 10.6 (50 mM phophate buffer), T = 25 °C.
III
Stopped-flow kinetic traces recorded at λmax 523 nm for various [Fe (tmpyp)] concentrations
with fixed concentrations [m-CPBA] and [amaranth] are shown in Figure 12a (at pH 3.7) and
Figure 12b (at pH 10.6).
III
Figure 12. Absorbance versus time traces recorded at λmax 523 nm and 25 °C for the [Fe (tmpyp)]-
catalyzed oxidative degradation of amaranth by m-CPBA in aqueous solutions (a) at pH 3.7 and (b) at
III
pH 10.6. The concentrations of [Fe (tmpyp)] were i) 0, ii) 0.5, iii) 1.0, iv) 2.0, v) 3.0, vi) 4.0 and vii) 5.0
III
µM, respectively. Experimental conditions: [amaranth] = 40 µM, [m-CPBA] = 400 µM, [Fe (tmpyp)] =
0–5 µM, pH 3.7 (50 mM acetate buffer), pH 10.6 (50 mM phosphate buffer), T = 25 °C.
1
2
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
It is evident that the dye degradation rate increases at higher catalyst concentrations, both at
III
pH 3.7 and pH 10.6. These results corroborate that a catalytic amount of [Fe (tmpyp)] is
needed for the peroxidative degradation of amaranth by m-CPBA. The dependence of the
observed pseudo first-order rate constant (kobs) on catalyst concentration is shown in Figure
III
IV
•+
1
3. The transformation of [Fe (tmpyp)] into [Fe (O)(tmpyp)] in aqueous solution occurs
III
within 20-30 ms. Hence, the initial concentration of [Fe (tmpyp)] denotes the actual
IV
•+
concentration of [Fe (O)(tmpyp)] , and the second-order rate constant (k
2
) for the reaction
IV
•+
between [Fe (O)(tmpyp)] and amaranth as shown in Scheme 1 can be determined from the
2
2
slope of the straight lines R = 0.9994 (at pH 3.7), and R = 0.9959 (at pH 10.6) in Figure 13.
6
-1 -1
6
-1 -1
The values of k
2
thus obtained are 6.39 × 10 M s at pH 3.7 and 7.07 × 10 M s at pH
10.6, respectively. These values are two orders of magnitude higher than second-order rate
V
constant (k
2
) for the previously reported reaction between [Mn (O) (tmpyp)] (a compound I
2
4
-1 -1
[35]
analog) and amaranth, 7.18×10 M s at pH 10.4.
III
III
Figure 13. Effect of [Fe (tmpyp)] on kobs for the [Fe (tmpyp)]-catalyzed oxidative degradation of
III
amaranth by m-CPBA. Experimental conditions: [m-CPBA] = 400 µM, [amaranth] = 40 µM, [Fe (tmpyp)]
=
0.5–5 µM, ○:pH 3.7 (50 mM acetate buffer); ●:pH 10.6 (50 mM phosphate buffer), T = 25 °C.
Effect of m-CPBA concentration
The kinetics of dye degradation was further examined at various initial concentrations of m-
III
CPBA at fixed concentrations of [Fe (tmpyp)] and amaranth in aqueous solutions at pH 3.7
and 10.6; the values of kobs as a function of m-CPBA concentrations are shown in Figure 14.
Initially, the values of kobs linearly increase with increasing [m-CPBA], but then they reach
saturation at higher m-CPBA concentration at both pHs. The initial slope of the curves was
4
-1 -1
2
4
-1 -1
2
estimated to be 2.05 × 10 M s at pH 3.7 (R = 0.9980) and 6.55 × 10 M s at pH 10.6 (R
0.9765). These values are significantly lower than the estimated values of k for the formation
=
1
1
3
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
IV
•+
III
of [Fe (O)(tmpyp)] upon reaction of [Fe (tmpyp)] with m-CPBA in aqueous solution at pH
3.7 and 10.6 as described above in absence of amaranth.
III
Figure 14. Effects of [m-CPBA] on kobs for the [Fe (tmpyp)]-catalyzed oxidative degradation of
III
amaranth by m-CPBA. Experimental conditions: [Fe (tmpyp)] = 1 µM, [Amaranth] = 40 µM, [m-CPBA]
o
=
10 – 700 µM, ○: pH 3.7 (50 mM acetate buffer), ●: pH 10.6 (50 mM phosphate buffer), T = 25 C. The
solid lines are numerically generated based on the (simulated rate of dye degradation /[Amaranth]) as
a function of [m-CPBA] at respective pH.
Therefore, the steady-state reaction rates of the dye degradation reaction were measured at
various concentrations of m-CPBA and at constant concentrations of amaranth and
III
[
Fe (tmpyp)]. As shown in Figure 15a, the rate of dye degradation shows a first-order
dependence on [m-CPBA] at lower concentrations of m-CPBA, whereas the order decreased
III
at higher concentrations of m-CPBA at both pHs. A Lineweaver-Burk plot (2[Fe (tmpyp)]
0
/ʋ
versus 1/[m-CPBA]) of the dye degradation reaction rate is shown in Figure 15b. The straight
2
2
lines (R = 0.9988; pH 3.7) and (R = 0.9974; pH 10.6) obtained from this presentation are in
III
agreement with the proposed mechanism of the [Fe (tmpyp)]-catalyzed oxidative degradation
of amaranth by m-CPBA (Scheme 1). From the slope of the straight lines the values of k are
1
5
-1 -1
6
-1 -1
estimated to be 4.32 × 10 M s at pH 3.7 and 1.86 × 10 M s at pH 10.6, respectively.
1
4
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
III
Figure 15. (a) Effects of [m-CPBA] on the rate (υ) of the [Fe (tmpyp)]-catalyzed oxidative degradation
III
of amaranth by m-CPBA. Experimental conditions: [Fe (tmpyp)] = 1 µM, [Amaranth] = 40 µM, [m-CPBA]
o
=
10 – 700 µM, ○: pH 3.7 (50 mM acetate buffer), ●: pH 10.6 (50 mM phosphate buffer), T = 25 C. The
solid lines are numerically generated based on equation (4) and (1/k
1
), ((k
2
+ k
3
)/k
2
k )(1/[amaranth]),
3
III
and [m-CPBA] values. (b) Plot of 2[Fe (tmpyp)]
0
/ʋ versus 1/[m-CPBA]; data were taken from Figure
15a.
These values of k
1
are very similar to those derived above from data shown in Figures 7 and
5
-1 -1
6
-1 -1
1
0, respectively (4.54 × 10 M s at pH 3.7 and 1.11 × 10 M s at pH 10.6). From the
6
-1 -
intercept of the straight lines (Fig. 15b), the values of k
3
are estimated to be 63.93 × 10 M s
1
6
-1 -1
at pH 3.7 and 10.59 × 10 M s at pH 10.6, respectively.
Effect of amaranth concentration
The effect of amaranth concentration on its degradation rate was studied in aqueous solution
III
(
pH 3.7 and 10.6) at fixed concentrations of [Fe (tmpyp)] and m-CPBA. The rate of dye
degradation (υ) as a function of amaranth concentration is shown in Figure 16a. It can be seen
that the dye degradation rate follows a first-order kinetics at lower amaranth concentrations
while the reaction order decreases at higher amaranth concentrations at both pHs. The initial
2
2
slopes (R = 0.9612 at pH 3.7 and R = 0.9905 at pH 10.6) of the curves in Figure 16a are
-1
-1
estimated to be 8.0955 s at pH 3.7 and 9.7628 s at pH 10.6, respectively. The slope is
1
5
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
IV
•+
described by k
2
[Fe (O)(tmpyp)] , where k
2
is the second-order rate constant for the reaction
IV
•+
between amaranth and [Fe (O)(tmpyp)] as shown in Scheme 1. Because the transformation
III
IV
•+
of [Fe (tmpyp)] into [Fe (O)(tmpyp)] was completed within 20-30 ms, the initial concentration
III
IV
•+
of [Fe (tmpyp)] (1 µM) can be considered as a concentration of [Fe (O)(tmpyp)] . Therefore,
6
-1 -1
6
-1 -1
the values of k
2
were determined to be 8.10 × 10 M s at pH 3.7 and 9.76 × 10 M s at
pH 10.6, respectively. These values are in good agreement with k
2
derived from the slopes of
6
-1 -1
6
-1 -1
the lines in Figure 13 (6.39 × 10 M s at pH 3.7 and 7.07 × 10 M s at pH 10.6).
III
Additionally, a Lineweaver-Burk plot (2[Fe (tmpyp)]
0
/ʋ versus 1/[amaranth]) of the reaction
2
2
rate as shown in Figure 16b produces straight lines (R = 0.9932; pH 3.7) and (R = 0.9957;
5
-1 -1
pH 10.6) at both pHs. The value of k
1
was estimated to be 4.10 × 10 M s at pH 3.7 and
.90 × 10 M s at pH 10.6 from y-intercepts of the straight lines at the respective pHs. These
values of k are comparable with those derived from the slopes of the straight lines shown in
5
-1 -1
6
1
5
-1 -1
6
-1 -1
Figure 15b (4.27 × 10 M s at pH 3.7 and 1.85 × 10 M s at pH 10.6).
III
Figure 16. (a) Effect of [amaranth] on the rate (υ) of the [Fe (tmpyp)]-catalyzed oxidative degradation
III
of amaranth by m-CPBA. Experimental conditions: [Fe (tmpyp)] = 1 µM, [amaranth] = 5 – 70 µM, [m-
o
CPBA] = 400 µM, ○: pH 3.7 (50 mM acetate buffer), ●: pH 10.6 (50 mM phosphate buffer), T = 25 C.
The solid lines are numerically generated based on equation (4) and (1/k
1
)(1/[m-CPBA]), ((k
2
+ k
3
)/k
2
k ),
3
III
and [amaranth] values. (b) Plot of 2[Fe (tmpyp)]
0
/ʋ versus 1/[amaranth]; data were taken from Figure
16a.
1
6
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
III
Proposed mechanism for the [Fe (tmpyp)]-catalyzed oxidative degradation of amaranth
by m-CPBA
Based on the kinetic data obtained from the various experiments described above, the
III
mechanism for the [Fe (tmpyp)]-catalyzed oxidative degradation of amaranth by m-CPBA
III
shown in Scheme 1 is proposed. It is assumed that the reaction between [Fe (tmpyp)] and m-
CPBA produces a transient intermediate [Fe (O)(tmpyp)]•+ (a compound I analog). This
IV
IV
intermediate rapidly interacts with amaranth to form [Fe (O)(tmpyp)] (a compound II analog),
III
which then reacts with another amaranth molecule to regenerate [Fe (tmpyp)] in the rate-
determining step. A thorough kinetic analysis of the catalytic cycle shown in Scheme 1 gives
the steady-state rate equation (4) and its linearized form, equation (5) (see the Supporting
Information for further details).
푘ꢁ
III
IV
[
[
[
Fe (tmpyp)] + m-CPBA
[Fe (O)(tmpyp)]•+
(1)
(2)
(3)
푘ꢅ
IV
•+
IV
Fe (O)(tmpyp)] + amaranth
[Fe (O)(tmpyp)] + degradation products of amaranth
푘3
IV
III
Fe (O)tmpyp)] + amaranth
[Fe (tmpyp)] + degradation products of amaranth
III
Scheme 1. Proposed catalytic cycle for the degradation of amaranth by [Fe (tmpyp]/m-CPBA
III
2
[Fe (tmpyp)]0
푘 + 푘3
휐 =
(4)
1
1
1
ꢀ
ꢂ ꢃ
푘ꢁ [m − CPBA]
ꢄ + ꢀ ꢅ
ꢂ ꢃ
ꢄ
푘 푘
[amaranth]
ꢅ
3
III
2
[Fe (tmpyp)]0
휐
1
1
푘ꢅ + 푘3
푘 푘
1
=
ꢃ
ꢄ ꢃ
ꢄ + ꢃ
ꢄ ꢃ
ꢄ
(5)
푘ꢁ [푚 − CPBA]
[amaranth]
ꢅ
3
III
Therefore, a plot of 2[Fe (tmpyp)]
0
/ʋ versus 1/[m-CPBA] gives a straight line with the slope
)(1/[amaranth]) as shown in Figure 15b. Similarly, the plot
+k )/k ) and the
1/k
1
and the y-intercept ((k
2 3
+k )/k
k
2 3
III
of 2[Fe (tmpyp)]
0
/ʋ versus 1/[amaranth] gives a straight line with a slope ((k
2
3
k
2 3
y-intercept (1/k
1
)(1/[m-CPBA]) as shown in Figure 16b.
Effect of surfactants
Surfactants play an important role for dyeing processes in the textile industries, ideally in
aqueous media by wetting the fiber surface quickly and uniformly. Therefore, textile effluents
1
7
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
are containing a significant amount of surfactants. To examine the effects of anionic and
cationic surfactants on the catalytic activity of the present system, the rate of dye degradation
III
was measured as a function of surfactant concentration at constant [Fe (tmpyp)], m-CPBA
and amaranth concentrations in aqueous solutions (pH 3.7 and pH 10.6); commonly used
sodium dodecylsulfate (SDS) and cetyltrimethylammonium bromide (CTAB) were chosen as
representative surfactants. The characteristic absorption peak (λmax 523 nm) of amaranth did
not change in presence of SDS, even at high SDS concentration (400 µM). This reflects that
both amaranth and SDS have anionic functional groups. The values of kobs obtained from dye
degradation reactions in presence of various concentrations of SDS are shown in Figure S3a.
The dye degradation rate was found to decrease with an increase in SDS concentration. This
can be tentatively ascribed to (i) the blocking of the axial active sites of the positively charged
iron porphyrin catalyst by forming aggregates with the anionic SDS,[35] which thus prevents
III
the formation of active species from m-CPBA and [Fe (tmpyp)], or (ii) an increase in the
viscosity of the reaction medium because of the formation of micelles, which can decrease the
diffusion rate and the efficiency of the reactants. In fact, it has previously been shown that
[
Fe (tmpyp)] interacts with SDS even at submicellar surfactant concentrations.[54] At SDS
III
concentrations above 10 µM positively charged bulk colloidal particles are formed, causing a
decrease of the dissolved metalloporphyrin concentration. This then leads to aggregation and
precipitation above 100 µM, and it has been suggested that the precipitate is mainly a
III
[54]
III
[
Fe (tmpyp)]-SDS complex.
Eventually, these interactions of [Fe (tmpyp)] with SDS
drastically decrease the catalytic rate of amaranth degradation.
On the other hand, the characteristic absorption peak (λmax 523 nm) of amaranth shifts to lower
energy (λmax 545 nm) in the presence of the cationic surfactant CTAB. The intensity of the new
peak increases at higher CTAB concentrations at the expense of the decreasing band at 523
nm. It is likely that the appearance of the new band reflects a dye–surfactant association
process, that is, the formation of encapsulated dye–surfactant species, which is mainly
governed by electrostatic long-range and dispersive short-range attractive interactions. Similar
observations have been reported for the association of acid orange dye and cetylpyridinium
chloride.[55] As shown in Figure S3b, the values of kobs at pH 3.7 initially increase with
increasing CTAB concentration (5 – 40 µM), but then the degradation rate decreases at even
higher CTAB concentrations (80 – 400 µM). Similarly, at pH 10.6 the values of kobs increase
slightly at low CTAB concentration (5 – 10 µM) but then decrease significantly upon further
increase in [CTAB], which reflects an inhibitory effect of CTAB on the dye degradation reaction
rate.
Identification of degradation products
1
8
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
The products obtained from the oxidative degradation of amaranth catalyzed by
III
[
Fe (tmpyp)]/m-CPBA were identified by HPLC equipped with a photodiode UV detector and
high-resolution mass spectrometry (HRMS). The observed chromatograms are shown in
Figure 17. It is noticed that ten peaks with different retention times were detected in the MS-
(
Figure 17a) and UV-based (Figure 17b) chromatograms. Suggested elemental compositions
with observed fragmentation patterns and molecular weights of the detected compounds are
compiled in Table 1 (see the Supporting Information Figures S4-S24 for further details).
Figure 17. HPLC-UV-HRMS traces of the product mixtures obtained from oxidative degradation of
III
amaranth catalyzed by [Fe (tmpyp)]/m-CPBA. (a): MS base peak chromatogram and (b): UV
chromatogram.
III
Table 1. Products obtained from oxidative degradation of amaranth catalyzed by [Fe (tmpyp)]/m-
CPBA. Numbers refer to peak labelling in Figure 17.
1
9
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
No.
Retention
[M-H]-
m/z
Elemental
composition
Fragmentation
proposed
Molecular weight
time t
R
1
2
2.04
244.9764
270.9923
C
8
H
5
O
7
S
201 -CO
57 -CO
227 -CO
2
2
2
2
M = 246
M = 272
1
-CO
2
3.18
C
10
H
7
O
7
S
163 -CO
-SO
2
119 -2xCO
2
-SO
2
C
8
H O
7
3
6.45
9.49
251.0023
C
11
H
7
O
5
S
207 -CO
2
M = 252
187 -SO
2
143 -CO
2
2
-SO
2
4
5
165.0197
207.0124
C
8
H
5
O
4
121 -CO
M = 166
Phthalic acid
M = 208
Naphthalene
sulfonic acid
M = 362
11.80
18.58
C
10
H
7
O
3
S
143 -SO
2
6
360.9949
C
17
H
10
O
5
SCl
297 -SO
2
2
2
2
261 -SO
253 -SO
217 -SO
-HCl
-CO
-CO
2
2
-HCl
7
8
19.57
21.11
360.9949
360.9953
C
C
17
H
H
10
O
O
5
SCl
SCl
M = 362
M = 362
17
10
5
317 -CO
2
3
2
2
2
15 -HCOOH
97 -SO
81 -CO
51 -SO
2
2
-HCl
-HCOOH
2
9
19.51
20.00
379.0290
154.9907
C
20
H
11
O
6
S
351 -CO
M = 380
271 -CO -SO
3
10
C
7
H
4
O
2
Cl
111 -CO
2
M = 156
m-chlorobenzoic
acid
Amaranth was not detected in any of the analyzed samples after treatment with
III
[
Fe (tmpyp)]/m-CPBA, reflecting efficient catalytic oxidative degradation. Besides m-
chlorobenzoic acid (10) that originates from m-CPBA, various aromatic sulfonic acids and
carboxylic acids were identified as degradation products of amaranth; major products are
-
shown in Figure 18. Furthermore, three species (6, 7, 8) showing a [M-H] ion at m/z 361 with
elemental composition of C17
H
10
O SCl were detected, which might be formed as a reaction
5
product of naphthalene sulfonic acid (5) and m-chlorobenzoic acid (10). While compound 9
-
featuring a [M-H] ion at m/z 379 and elemental composition C20
H
11
O S as well as naphthalene
6
sulfonic acid 5 were also observed as products after oxidative amaranth degradation catalyzed
III
O
,[35] dicarboxylic acids 1 and 4 were not. The predominance of
by [Mn (tmpyp)]/H
2
2
naphthalene derivatives among the amaranth degradation products in case of
III
[
Mn (tmpyp)]/H
2
O compared to the occurrence of carboxylic acid products such as 1 and 4
2
2
0
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
III
V
in case of [Fe (tmpyp)]/m-CPBA suggests that active intermediate [Mn (O)
2
(tmpyp)] (and
IV
[
Mn (O)(tmpyp)]) are not capable of oxidatively breaking down the naphthalene core, in
IV
•+
IV
contrast to [Fe (O)(tmpyp)] (or [Fe (O)(tmpyp)]) identified in the present study. In contrast,
in a previous report on the oxidative degradation of smaller azo dyes lacking sulfonate groups
III
it was concluded that the combination of ferric porphyrin catalysts, including [Fe (tmpyp)], and
tert-butyl hydroperoxide (TBHP) in an acetonitrile/water mixture mainly leads to oxidation of
the substrates’ terminal amine groups while the azo bond is left intact.[38]
Figure 18. Major products resulting from oxidative degradation of amaranth catalyzed by
III
[
Fe (tmpyp)]/m-CPBA.
Conclusions
III
In this study, the water soluble ferric porphyrin complex [Fe (tmpyp)] in combination with m-
CPBA has been used for the oxidative degradation of a representative azo dye, amaranth,
and a mechanistic picture has been derived from extensive kinetic investigations using rapid-
scan stopped-flow measurements. The experimental data confirm that the reaction between
III
[
Fe (tmpyp)] and m-CPBA in aqueous solution gives the transient intermediate
IV
•+
5
-1 -1
[
Fe (O)(tmpyp)] with a second-order rate constant k
1
= 4.54 × 10 M s (at pH 3.7) or 1.11
6
-1 -1
IV
×
10 M s (at pH 10.6), prior to the formation of a relative stable [Fe (O)(tmpyp)] species.
III
Batch kinetic studies further show that the [Fe (tmpyp)]-catalyzed oxidative degradation of
amaranth by m-CPBA is greatly influenced by the solution pH as well as by the concentrations
of catalyst, m-CPBA and amaranth dye substrate. Within the studied pH range from 3 to 12.8,
reaction rates were found particularly high at both pH 3.7 and 10.6. In contrast,
III
[
Mn (tmpyp)]/H
2
O showed high catalytic rates for the oxidative degradation of amaranth only
2
under basic conditions (maximum at pH 10.4), but negligible activity at lower pH.[35]
A
III
mechanistic scenario for the oxidative degradation of amaranth catalyzed by [Fe (tmpyp)] as
shown in Scheme 1 has been derived, and a steady-state rate equation (4) has been also
proposed. The main degradation products of amaranth were identified by using HPLC–UV–
HRMS measurements. Interestingly, the product pattern resulting from the catalytic oxidative
III
[35]
III
degradation using either [Mn (tmpyp)]/H
2
O
2
or [Fe (tmpyp)]/m-CPBA differ, suggesting that
2
1
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
IV
•+
IV
[
Fe (O)(tmpyp)] (or [Fe (O)(tmpyp)]) are capable of oxidatively breaking down the
V
IV
naphthalene core, while [Mn (O) (tmpyp)] (and [Mn (O)(tmpyp)]) are not. Both anionic and
2
cationic surfactants show some inhibitory effects on the dye degradation process using
III
[
[
Fe (tmpyp)]/m-CPBA, but only at higher concentrations. These results demonstrate that
III
Fe (tmpyp)], which is readily prepared, in combination with suitable oxidants such as m-
CPBA can be used as an efficient and robust catalyst for the peroxidative treatment of azo
dyes containing wastewater.
Experimental Section
Materials
The tosylate salt of the ligand meso-tetrakis(1-methylpyridinium-4-yl)-porphyrin
(
2
[H (tmpyp)](OTs)
4
) was obtained from Frontier Scientific Inc. (Logan, UT, USA). Ferrous
chloride tetrahydrate (FeCl
2
·4H O, Sigma–Aldrich, Germany), sodium perchlorate
2
monohydrate (J.T.Baker, Holland), and 70–72% perchloric acid (J.T. Baker, Holland) were
used as provided. Amaranth (trisodium 2-hydroxy-1,1’-azonaphthalene-3,4’,6-trisulfonate)
(
Figure 1, right), reactive orange 13 (RO13), reactive red 239 (RR239) and remazol brilliant
violet (RBV) were obtained from Sigma–Aldrich, Germany and used as supplied. meta-
Chloroperoxy benzoic acid (m-CPBA) (Sigma–Aldrich, Germany), sodium dodecylsulfate
(
SDS) and cetyltrimethylammonium bromide (CTAB) were used as received. Sodium chloride
was provided by Grussing GmbH (Filsum, Germany). Water used in all experiments was
distilled and deionized. All other reagents and solvents were of analytical grade of purity;
hence, they were used without further purification.
III
Synthesis of [Fe(tmpyp)(H
2 2
O) ](ClO
)
4 5
([Fe (tmpyp)])
The pentaperchlorate salt of the iron(III) complex of [H
2
(tmpyp)]4+ – viz. the meso-tetrakis(1-
methylpyridinium-4-yl)porphyrinatoiron(III) pentacation bearing two axial water ligands,
III
III
[
Fe (tmpyp)(H
2
O)
2
](ClO
4
)
5
, in the following abbreviated [Fe (tmpyp)] – was prepared
III
according to the method described previously for the preparation of [Mn (tmpyp)] with slight
modifications.[35] [H
(0.970 g, 0.711 mmol) and FeCl
·4H O (2.827g, 14.22
2
(tmpyp)](OTs)
4
2
2
mmol) were heated to reflux in water (100 mL) for 6 h. A precipitate formed initially, and then
an intensely red solution was obtained. The solution was cooled and filtered from a trace
amount of insoluble material, and NaClO (10 g) was added, which produced a dark
4
precipitate. The mixture was cooled in an ice bath for 2 h, and the solid was isolated by filtration
and washed with dilute (2%) perchloric acid (100 mL) to remove excess metal ions. Finally,
the solid product was dried under reduced pressure at room temperature for 3 days, which
2
2
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
yielded 0.778 g (86 %) of a deep red-brown solid. Calcd (%) for C44
H36Cl
5
N
8
FeO20·2H O: C
2
41.81, H 3.19, N 8.87; found: C 41.78, H 3.18, N 8.85.
Absorption spectra and kinetic experiments
All the absorption spectra were measured by using a spectrophotometer (Shimadzu UV-1800)
in the wavelength range 200–800 nm and with a single mixing stopped-flow photodiode array
spectrophotometer (Bio-Logic Science Instrument mSFM-20 and J & M Analytik AG TIDAS
spectrometer). Time-resolved absorption spectra were recorded on the above-mentioned
photodiode array spectrophotometer after mixing two injected solutions in 1:1 ratio (by
volume). The spectral resolution was approximately 1 nm. Temperature was regulated by
using a Huber CC90 cryostat, and all time-resolved absorption spectra were measured at (25
±
0.1) °C unless otherwise noted. Oxidation reactions were performed by injecting a solution
III
of [Fe (tmpyp)] (either alone or containing amaranth) and the solution of m-CPBA into the
III
mixing chamber (observation cell). [Fe (tmpyp)], amaranth, m-CPBA, and other solutions
were freshly prepared in acetate or in phosphate buffer (50 mM) at a desired pH value. Time-
resolved absorption spectra were measured immediately after the mixing by using a
photomultiplier by an optical fiber mounted with the observation cell. Reaction kinetic profiles
III
were deconvoluted with the SPECFIT/32 global analysis software. Amaranth, [Fe (tmpyp)],
IV
•+
IV
[
Fe (O)(tmpyp)] , and [Fe (O)tmpyp)] were monitored in aqueous solution (pH 3.7) at λ =
523, 421, 500, 402, and 427 nm, respectively. The pH measurements were performed with a
780 pH meter (Ω-Metrohm, Herisau, Switzerland) and the pH meter was calibrated by standard
(
pH 4.00 and 7.00) calibration solutions at 25 °C. The same reactions were also performed in
the presence of surfactants (SDS and CTAB) in 50 mM acetate buffer (pH 3.7) and in 50 mM
phosphate buffer (pH 10.6). In the kinetic experiments, the surfactant was supplied with the
III
solution of [Fe (tmpyp)] containing amaranth.
HPLC-UV-HRMS analysis
HPLC-HRMS analyses of the reaction products were performed using a Synergi Hydro-RP
column (150 mm length, 2 mm inner diameter, 4 µm partical size, Phenomenex). The aqueous
reaction solution was mixed with methanol (1/1 v/v) and 10 µL sample solution were injected.
Water with 0.05 % formic acid (A) and methanol with 0.05 % formic acid (B) were used as
eluents with a flow of 0.2 mL/min applying the following gradient elution: 0-2 min 90/10 A/B,
2-15 min to 50/50 A/B, 15-20 min to 10/90 A/B, 20-25 min 10/90 A/B. The oven temperature
was set to 45 °C. UV spectra were acquired in the range of 200 - 390 nm. Mass spectra were
recorded on a high-resolution LTQ Orbitrap XL mass spectrometer (Thermo, R ~ 60,000)
applying electrospray ionization (ESI) with a spray voltage of 5.0 kV, sheath gas flow of 50
2
3
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European Journal of Inorganic Chemistry
10.1002/ejic.202100373
(
arb units) and a capillary temperature of 275 °C in the m/z range from 100 to 1,000. MS/MS
spectra were acquired applying dependent scan mode.
Acknowledgements
We thankfully acknowledge support by the Alexander von Humboldt Foundation (Georg
Forster research fellowship to T.K.S.), the University of Göttingen and the Ministry of Science
and Technology, Government of the People’s Republic of Bangladesh.
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Table of Contents Entry
The efficient degradation of a common azo dye, known to be a critical pollutant of textile
industry effluents, by a soluble iron porphyrin catalyst and meta-chloroperoxy benzoic acid
is demonstrated, and a mechanistic scheme involving high-valent oxoiron intermediates has
been developed using rapid-scan stopped-flow spectrophotometry.
2
7
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