Spectrophotometric Oxidative Decolorization of Methyl Red with Chloramine-T and Bromamine-T: Comparative Kinetic Modeling and Mechanistic Study

Article abstract of DOI:10.1134/S0036024418130290

Abstract: Methyl Red (MR) is a synthetic mono-azo dye which is extensively used as a colorant in textile and paper printing industries. The widespread amount of MR dye in wastewater poses potential threat to ecosystem and human health. In this context, we have developed a simple and expeditious oxidative decolorization technique for the removal of MR dye by organic N-haloamines viz., chloramine-T (CAT) and bromamine-T (BAT) present in the wastewater. The reaction was focused to changes in: (i) concentration of p-toluenesulfonamide, (ii) ionic strength, (iii) dielectric permittivity, (iv) polymerization study, and (v) halide ions effects. The activation parameters were deduced. Oxidation products of MR dye were identified. The rate of oxidation of MR is about four times faster with BAT than with CAT. The chemical oxygen demand value of the dye was determined. Phytotoxicity and economic analysis were carried out for the proposed decolorization process. A plausible mechanism conforming the kinetic data, reaction stoichiometry and product analysis has been proposed. Further, the present redox system can be adopted for treating MR dye present in industrial effluents with suitable modifications to reduce the toxicity caused by MR dye in wastewater.

Full text of DOI:10.1134/S0036024418130290

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2018, Vol. 92, No. 13, pp. 2647–2655. © Pleiades Publishing, Ltd., 2018.
CHEMICAL KINETICS
AND CATALYSIS
Spectrophotometric Oxidative Decolorization of Methyl Red
with Chloramine-T and Bromamine-T: Comparative Kinetic
Modeling and Mechanistic Study1
Adalagere Somashekar Manjunathaa,*, Anu Sukhdevb, and Puttaswamyc
aDepartment of Chemistry, Donbosco Institute of Technology, Bengaluru, 560074 India
bDepartment of Chemistry, Presidency University, Bengaluru, 560089 India
cDepartment of Chemistry, Bangalore University, Bengaluru, 560001 India
*e-mail: madhumanjuas@gmail.com
Received January 2, 2018
Abstract—Methyl Red (MR) is a synthetic mono-azo dye which is extensively used as a colorant in textile and
paper printing industries. The widespread amount of MR dye in wastewater poses potential threat to ecosys-
tem and human health. In this context, we have developed a simple and expeditious oxidative decolorization
technique for the removal of MR dye by organic N-haloamines viz., chloramine-T (CAT) and bromamine-T
(BAT) present in the wastewater. The reaction was focused to changes in: (i) concentration of p-toluenesul-
fonamide, (ii) ionic strength, (iii) dielectric permittivity, (iv) polymerization study, and (v) halide ions effects.
The activation parameters were deduced. Oxidation products of MR dye were identified. The rate of oxida-
tion of MR is about four times faster with BAT than with CAT. The chemical oxygen demand value of the dye
was determined. Phytotoxicity and economic analysis were carried out for the proposed decolorization pro-
cess. A plausible mechanism conforming the kinetic data, reaction stoichiometry and product analysis has
been proposed. Further, the present redox system can be adopted for treating MR dye present in industrial
effluents with suitable modifications to reduce the toxicity caused by MR dye in wastewater.
Keywords: methyl red, chloramine-T, bromamine-T, oxidative-decolorization, kinetics-mechanism, waste-
water
DOI: 10.1134/S0036024418130290
INTRODUCTION
potential for the removal of MR dye without introduc-
ing any impurities, and also offer several advantages
such as its simplicity and cost-effective over the other
methods. From these reasons, we carried out the title
reaction.
Azo dyes are the components having the azo group
(N=N) and alkyl or aryl group (R–R') and hence the
functional group is R–N=N–R'. The color of an azo
dye is owing to the interaction between an azo group
and two aryl species. These dyes are classified as
mono-azo, di-azo, tri-azo dyes etc., depending on the
number of azo groups such as one, two, three, etc [1].
Azo dyes are most widely used colorants in textile
industries and also in paper printing, color photogra-
phy and food processing [2–4]. Methyl Red (MR) or
C.I. Acid Red 2 (4-dimethylaminoazobenzene-2′-car-
boxylic acid sodium salt) is a synthetic mono-azo dye
which is widely used in textile and paper industries [1,
2]. Wastewater released from these industries in which
MR dye used is highly colored and toxic and hence,
leads to environmental and health problems. In this
context, several physical, chemical, and biological
techniques have been developed for the removal of
MR dye from wastewater. Amongst all these methods,
oxidative decolorization technique has a enormous
N-Haloarenesulfonamidates, generally known as
organic N-haloamines, are mild oxidants containing a
strongly polarized N-bonded halogen in its +1 oxida-
tion state. Kinetics and mechanism of oxidation by
these reagents have fascinated the alertness of chem-
ists, particularly synthetic chemists and kineticists due
to their diverse property to act as halonium cations,
hypochlorites and N-anions [5]. They interact with a
wide range of functional groups in aqueous and non-
aqueous media in the presence of an acid or a base [6–
8]. As a result, these reagents have been used as mild
and selective oxidizing agents in synthetic and mecha-
nistic chemistry. The prominent member of this
group, sodium N-chloro-p-toluenesulfonamide com-
monly known as chloramine-T (CAT) is a well known
analytical reagent. The kinetic and mechanistic
aspects of many of its reactions have been well docu-
mented [6–9]. Bromamine-T (BAT), the bromine
1
The article is published in the original.
2647

2648
ADALAGERE SOMASHEKAR MANJUNATHA et al.
O
Br
H3C
S
N
Na
O
Bromamine-T
190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 ppm
13
Fig. 1. C FT-NMR spectrum of BAT.
analogue of CAT is giving much importance as a mild formed were filtered under suction, washed quickly
oxidant and has been found to be a better oxidizing with the minimum amount of ice-cold water, and
agent than the chloro compounds [5, 7, 10–13]. This dried over P2O5. The compound was recrystallized
reagent can be easily prepared by the bromination of
CAT. A glance of literature indicates that the reports
on the oxidation of dyes with these reagents are rela-
tively meager from its kinetic and mechanistic stand
point. Further, these oxidants are commercially avail-
able, cost-effective, water-tolerant, non-toxic and
easy to handle. Therefore, in the present article, we
have established the optimum conditions for the facile
oxidative decolorization of MR dye with CAT and
BAT in acid medium.
from hot water (50°C). The purity of BAT was checked
iodometrically through its active bromine content and
the compound was further characterized [15] by the
13C FT-NMR spectrum (Fig. 1) of BAB was obtained
in D2O, using tetramethyl silane (TMS) as the internal
standard. (recorded on a Bruker WH 400-MHz
nuclear magnetic resonance spectrometer), showed
13
signals C NMR spectrum (p.p.m. relative to TMS):
145.39 (C-1, carbon attached to S atom), 140.52 (C-
4), 131.52 (C-2,6), 129.36 (C-3,5), and 23.0 (methyl
carbon). Chloramine-T (Thomas Baker) was purified
by the method of Morris et al. [16]. An aqueous solu-
tion of these oxidants was prepared afresh whenever
required, standardized by iodometric method and
stored in brown bottles until further use to prevent its
any photochemical deterioration. The concentration
of stock solution was periodically determined iodo-
metrically. Methyl Red (Sigma) was used as received
and an aqueous solution of the desired strength of the
dye was prepared afresh each time. All other chemicals
used were of analytical grade. Doubly distilled water
was used throughout the experiment.
The main objectives of the present investigations
are to: (i) develop the optimum conditions for the oxi-
dative decolorization of MR dye by CAT and BAT in
acid medium, (ii) accumulate kinetic data, (iii) eluci-
date suitable mechanisms, (iv) deduce appropriate
rate laws, (v) ascertain the various reactive species, (vi)
identify the oxidation products, (vii) compute activa-
tion parameters, (viii) analysis of phytotoxicity and
economic analysis, and (ix) evaluate relative reactivity
and kinetic modelings of CAT and BAT toward oxida-
tive decolorization of MR dye in acid medium.
Detailed kinetic runs were performed under
pseudo first-order conditions of [oxidant]o ≫ [MR]o
at 303 K in presence of HClO4. Kinetic measurements
EXPERIMENTAL
Bromamine-T was prepared [14] using chlora-
mine-T. To a solution of CAT (20 g in 400 mL of were carried out using an UV–Vis spectrophotometer
water), about 4 mL of liquid bromine was added drop (Digital spectrophotometer 166, Systronics, India). In
wise with constant stirring at room temperature to the present study, the kinetic experiments were carried
yield dibromamine-T (DBT). The solid DBT was fil- out between 293 and 313 K. Raaga ultra cold chamber
tered under suction, washed thoroughly with ice-cold with digital temperature control (India) was used for
water until all the absorbed bromine had been kinetic experiments in which a constant desired tem-
removed, and then vacuum-dried for 24 h. About 20 g perature was maintained with an accuracy of 0.1°C.
of DBT thus obtained was dissolved in 30 mL of Reactions were carried out in glass stoppered pyrex
4 mol dm–3 NaOH with constant stirring at room tem-
perature, and the resultant aqueous solution was
cooled in ice. The pale yellow crystals of BAT that
boiling tubes whose outer surface was coated black to
eliminate any photochemical effects. The oxidants as
well as requisite amount of MR dye, HClO4 solutions
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SPECTROPHOTOMETRIC OXIDATIVE DECOLORIZATION
2649
and water (to keep the total volume constant for all the were used as model crop for this study [17]. The germi-
runs) were taken in separate tubes and were thermo- nation tests were carried out in three Petri dishes,
statted for about 30 min at 303 K. The reaction was where 20 maize seeds were sown per dish on a wet filter
initiated by the rapid addition of a measured amount paper among which one set was treated as control
of oxidant to the stirred reaction mixture. Immedi- using distilled water. Another two sets of dishes were
ately, 4 mL aliquot of the solution was pipetted into a treated 5 mL each with treated and untreated dye solu-
cuvette placed in the spectrophotometer. Absorbance tion at regular intervals. All the three petri plates were
measurements were made at 524 nm (λmax of the dye) incubated at room temperature and observed for radi-
cal emergence. The germination was recorded after
7 days.
for nearly three half-lives. The absorbance readings at
t = 0 and t are D0 and Dt. Plots of log D0/Dt versus time
were made to evaluate the pseudo first-order rate con-
stants (k', s–1). All kinetic runs were carried out twice
to check reproducibility with 3% error was found.
Regression analysis of the experimental data was car-
ried out on an fx-100 W scientific calculator to evalu-
ate the regression coefficient, R2.
RESULTS AND DISCUSSION
Varying ratios of oxidant to MR dye in the presence
of 4.0 × 10-3 mol dm-3 HClO4 were equilibrated at
303 K for 48 h. The iodometric titration of unreacted
The toxicity studies with respect to generation of oxidant in the reaction mixture showed that one mole
oxidative stress in plants are yet to get much significant of oxidant was consumed per mole of the dye accord-
attention. Due to this reason, maize seeds (Zea Mays L) ing to the Eq. (1):
O
X
N N
H C
3
S N
O
(CH3)2N
+
+ 2H2O
Na
CO2H
(Oxidant)
H+
(MR dye)
(1)
O
N
H
OH
N
(CH3)2N
+
+ TsNH2 + Na+ + X−
p-Toluene
CO2H
sulfonamide
4-Dimethylaminophenyl-
hydroxylamine
2-Nitrosobenzoic
acid
Here Ts = CH3C6H4SO2 for CAT and BAT, with observed structure. Further, it was noticed that there
X = Cl or Br.
was no reaction between 4-dimethylaminophenylhy-
droxylamine and 2-nitrosobenzoic acid with CAT
and BAT under the present set of experimental condi-
tions. Further, these compounds are largely used in
textile and tannery industries, and also in water treat-
ment and dye making plants [18]. The reduction prod-
uct of oxidant, p-toluenesulfonamide (PTS or
TsNH2), was extracted with ethyl acetate and detected
by paper chromatography [19]. Benzyl alcohol satu-
rated with water was used as the solvent with 0.5% van-
illin in 1% HCl solution in ethanol as spray reagent
(Rf = 0.905).
Preliminary kinetic runs revealed that CAT and
BAT (henceforth abbreviated as oxidant) decolorizes
MR dye completely within 45 and 20 min, respectively
under the present set of experimental conditions. The
kinetics of oxidative decolorization of MR dye with
oxidant was investigated at several initial concentra-
tions of the reactants in HClO4 medium at 303 K.
Under pseudo first-order conditions of [oxidant]o ≫
The reaction mixture in the stoichiometric ratio
under stirred condition was allowed to progress for
48 h at 303 K. After completion of the reaction (mon-
itored by TLC), the reaction products were neutral-
ized with NaOH and products were extracted twice
with diethyl ether. The organic products were sub-
jected to spot tests and chromatographic analysis
(TLC technique), which revealed the formation of
oxidation products, namely 4-dimethylaminophenyl-
hydroxylamine and 2-nitrosobenzoic acid. These oxi-
dation products were separated by column chromatog-
raphy and confirmed by GC-MS. The GC-MS data
were obtained on a 17A shimadzu gas chromatograph
with a QP-5050A Shimadzu mass spectrometer. The
mass spectra showed a molecular ion peak at 153 and
151 amu, clearly conforming 4-dimethylamino-
phenylhydroxylamine and 2-nitrosobenzoic acid,
respectively (Figs. 2 and 3). All other peaks observed
in GC-MS can be interpreted in accordance with the
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2650
ADALAGERE SOMASHEKAR MANJUNATHA et al.
100
80
153
60
49
134
40
107
152
63
20
0
35
25
50
75
100
125
150
175
m/z
Fig. 2. Mass spectrum of 4-dimethylaminophenylhydroxylamine with its parent molecular ion peak at 153 amu.
100
151
80
121
60
92
40
65
39
20
0
63
66
52
41
46
28
93
50
108
61
37
80
20
40
60
80
100
120
140
160
180
200
220
m/z
Fig. 3. Mass spectrum of 4-nitrosobenzoic acid with its parent molecular ion peak 151 amu.
[MR]o at constant [oxidant]o, [HClO4] and tempera- Under the same experimental conditions, an increase
in [oxidant]o increased the rate (Table 1). A plots of log
ture, plots of log (absorbance) versus time were linear
(R2 > 0.9910), indicating a first-order dependence of
rate on [MR]o. The values of pseudo first-order rate
constants (k', s–1) are reported in Table 1. The values
of k' remain unaffected with a change in [MR]o, con-
firming the first-order dependence of rate on [MR]o.
k' versus [oxidant] was linear (R2 > 0.9930) having a
slope equal to unity, indicating a first-order depen-
dence of the reaction rate on [oxidant]o. Further, a
plots of k' versus [oxidant], was linear (R2 > 0.9930)
passing through the origin, confirming the first-order
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SPECTROPHOTOMETRIC OXIDATIVE DECOLORIZATION
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Table 1. Effect of variation of [oxidant], [MR], and
dependence of rate on [oxidant]o and also showing
that the MR dye-oxidant complex has only transient
existence.
[HClO4] on the reaction rate at 303 K
103
103
104k' (s–1)
104 [MR]o
(mol dm–3)
At constant [oxidant]o, [MR]o, and the tempera-
ture, the reaction rate enhanced with increase in
[HClO4] (Table 1). A plots of log k' versus log [HClO4]
[oxidant]o [HClO4]
(mol dm–3) (mol dm–3)
CAT
BAT
was linear (R2 > 0.9891) with a slopes of 0.75 and 0.27
for CAT and BAT, indicating a fractional-order
dependence of rate on [HClO4]. Addition of p-tolue-
nesulfonamide (PTS or TsNH2), to the reaction mix-
0.5
2.0
4.0
6.0
8.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
4.0
4.0
4.0
4.0
4.0
1.0
2.0
4.0
8.0
12.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
2.0
4.0
6.0
8.0
10.0
5.90
5.97
5.92
6.02
5.96
1.50
2.92
5.97
12.0
15.8
3.37
5.97
7.48
7.68
26.45
26.47
26.42
26.50
26.48
7.38
12.79
26.47
51.17
55.76
21.54
26.47
28.11
30.56
33.37
ture in the concentration range of 2.0 × 10–3 to 8.0 ×
10–3 mol dm–3 did not influence the rate significantly.
It indicates that PTS is not involved in any step prior
to the rate-determining step (rds) in the proposed
reaction scheme. Similarly, addition of Cl– or Br– ions
in the form of their sodium salts in the concentration
range of 2.0 × 10–3 to 8.0 × 10–3 mol dm–3 had no sub-
stantial effect on the rate of reaction indicates that the
halide ions are not participate in the reaction
sequence.
Effect of ionic strength of the reaction system on
the rate of the reaction was studied by adding
0.2 mol dm–3 NaClO4 solution to the reaction mix-
ture. It was noticed that there was no notable change
on the rate of the reaction signifies the involvement of
at least one neutral molecule in the rate-determining
step. Hence, no attempt was made to keep the ionic
strength of the system constant for kinetic runs. The
dielectric constant (D) of the medium was varied by
adding MeOH (0–30 vol %) to the reaction mixture,
all other experimental conditions being held constant.
The rate increased with increase in MeOH content
(Table 2) and plots of log k' versus 1/D were linear
(Fig. 4; R2 > 0.9905) with positive slopes. It was fur-
ther noticed that no reaction of dielectric with the oxi-
dant occurred under the experimental conditions
employed. Values of D of MeOH–H2O mixtures were
taken from the literature [20] were employed.
11.0
Table 2. Effect of varying dielectric constant of the
medium on the rate of reaction at 303 K
104 k' (s–1)
MeOH
(vol %)
D
CAT
BAT
0
10
20
30
76.73
72.37
67.48
62.71
5.97
6.42
6.99
7.24
26.47
27.24
27.94
28.31
–3
–3
Experimental conditions: [oxidant] = 4.0 × 10 mol dm
;
.
o
–4
–3
–3
–3
[MR] = 2.0 × 10 mol dm ; [HClO ] = 4.0 × 10 mol dm
o
4
The reaction was studied at different temperatures
(293, 298, 303, 308, and 313 K), keeping other experi-
mental conditions constant. From the linear Arrhe-
nius plots of log k' versus 1/T (Fig. 5; R2 > 0.9907),
activation parameters (Ea, ∆H≠, ∆S≠, ∆G≠, and log A)
for the overall reaction were computed and these
results are summarized in Table 3. Addition of acryl-
amide to the reaction mixture did not initiate polym-
erization, showing the absence of free radical species
in the reaction sequence. The control experiments
were also performed under similar reaction conditions
without the oxidant.
CAT and BAT (TsNXNa) are acts as a mild oxidant
in both acidic and alkaline media. In general, oxidant
undergoes a two electron change in its reactions form-
ing the reduction products, PTS and NaCl [21]. The
oxidation potential of CAT-PTS redox couple is pH
dependent [22] and decreases with increase in pH of
the medium (the redox potential Eredox of CAT-PTS
couple is 1.138, 0.778, 0.614, and 0.50 V at pH 0.65,
7.0, 9.7, and 12, respectively). In view of the homoge-
neity in properties of CAT and BAT, similar redox
potential behavior can be expected for BAT as well.
The nature of the active oxidizing species and mecha-
nism depends on the nature of halogen atom, the
groups attached to the nitrogen, and the reaction con-
dition. The species responsible for such oxidizing
character may be different depending on the pH of the
medium. Aqueous solutions of CAT and BAT behave
as a strong electrolyte and depending on the pH of the
medium, it furnishes [15, 21–24] following equilibria
in solutions:
TsNXNa ꢀ TsNX− + Na+,
(2)
TsNX− + Na+ ꢀ TsNHX,
(3)
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2652
1.5
ADALAGERE SOMASHEKAR MANJUNATHA et al.
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
CAT
BAT
1.4
1.3
1.2
1.1
1.0
0.9
0.8
CAT
BAT
3.15
3.20
3.25
3.30
3.35
3.40
3.45
1.30 1.35 1.40 1.45 1.50 1.55 1.60
103/T
102/D
Fig. 4. Plots of log k' versus 1/D.
Fig. 5. Plots of log k' versus 1/T.
observations. If HOX is primarily involved, a first-
order retardation of rate by added p-toluenesulfon-
amide is expected (Eq. (5)). However, no such effect
was noticed and hence HOX can be feint out as the
oxidizing species. Hardy and Johnston [23] were made
detailed calculations on the concentration depen-
dence of various oxidizing species of CAT and BAT on
pH in aqueous oxidant solution and have shown that
TsNHX is the predominant oxidizing species in acid
medium.
(4)
2TsNHX ꢀ TsNH2 + TsNX2,
TsNHX + H2O ꢀ TsNH2 + HOX,
TsNX2 + H2O ꢀ TsNHX + HOX,
(5)
(6)
HOX ꢀ H+ + XO−,
HOX + H+ ꢀ H2O+X.
(7)
(8)
Therefore, the possible oxidizing species in acidified
CAT and BAT solutions are TsNHX, TsNX2, HOX,
and possibly H2O+X. If di-haloamine (TsNX2) were to
be the reactive species, then according to Eq. (4) the
rate law predicts a second-order dependence of rate on
[oxidant]o, which is converse to the experimental
In the present case, the fractional-order depen-
dence of rate on [H+] suggests that the protonation of
TsNHX results in the formation of TsN+H2X which is
likely to be the active oxidizing species in the oxidation
of MR dye in acid medium. In view of the foregoing
discussion and the experimental results, Scheme 1 is
Table 3. Temperature dependence and activation parame- formulated for the MR-oxidant redox system in acid
medium. In Scheme 1, an initial equilibrium H+ ion
ters for the oxidative decolorization of MR by oxidant in
acid medium
accelerating step (step (i)) involves protonation of the
104 k' (s–1)
conjugate acid TsNHX forming the active oxidizing
species TsN+H2X. In the next slow and rate-determin-
ing step (step (ii)), a lone pair of electrons on the nitro-
gen atom of the dye attacks the positive chlorine of the
protonated oxidant species to form an intermediate
complex (X) with the elimination of TsNH2. In the
next subsequent fast steps, this complex undergoes
hydrolysis followed by cleavage of N–N bond to yield
the ultimate products viz. 4-dimethylaminophenylhy-
droxylamine and 4-nitrosobenzoic acid. Based on
Scheme 1, the rate law can be formulated as shown
below.
CAT
2.60
4.43
5.97
7.93
9.03
45.6
43.0
88.7
BAT
Temperature (K)
293
298
303
308
313
12.9
18.4
26.4
37.3
51.3
40.2
37.7
90.5
Ea (kJ mol–1)
ΔH≠ (kJ mol–1)
ΔG≠ (kJ mol–1)
ΔS≠ (J K–1 mol–1)
log A
If [oxidant]t is the total effective concentration of
[oxidant], then
–174
9.01
–170
8.70
[oxidant] = [TsNHX] + [TsN+H2X].
(9)
–3
–3
Experimental conditions: [oxidant] = 4.0 × 10 mol dm
,
.
o
–4
–3
–3
–3
[MR] = 2.0 × 10 mol dm , [HClO ] = 4.0 × 10 mol dm
From step (i) of Scheme 1
o
4
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SPECTROPHOTOMETRIC OXIDATIVE DECOLORIZATION
2653
K1
(i) TsNHX + H+
TsN+H2X
(fast)
Cl
k2
+ TsN+H2
X
N
N
N
N
(CH3)2N
(CH3)2N
X
+ TsNH2
(ii)
(Slow
and rds)
+
(X)
CO2H
CO2H
(MR dye)
X
k3
CO2H
N N
(CH3)2N
N
N
(iii) (CH3)2N
(fast)
+
+O
+H+ −H+
X
H
H
(X)
CO2H
CO2H
H2O
N N
(CH3)2N
+
H
O
H
CO2H
X
O
N
+
H
H
(CH3)2N
N
+
CO2H
(2-Nitrosobenzoic acid)
−HX H2O
H
N
H
(CH3)2N
O+
H
−H+
N
(CH3)2N
H
OH
4-Dimethylaminophenylhydroxylamine
Scheme 1. A detailed mechanistic interpretation for the oxidative decolorization of MR dye with oxidant in HClO medium.
4
[TsN+H2X]
Most of the organic reactions are carried out in
(10)
[TsNHX] =
.
solution, and it is therefore important to realize the
effect of solvent on the course and rate of the reac-
tions. A change in solvent composition by varying
methanol content affects the reaction rate. Several
approaches [25–30] have been made to explain quan-
titatively the effect of the dielectric constant of the
medium on the rates of reactions in solutions. For the
limiting case of zero angle of approach between two
dipoles or an ion-dipole system, Amis [30] has shown
that a plot of log k' versus. 1/D gives a straight line with
a negative slope for the reaction between a negative ion
and a dipole or between two dipoles, while a positive
slope indicates a reaction between a positive ion and a
dipole. The positive dielectric effect observed in the
present studies (Table 2) clearly supports the involve-
ment of a positive ion and a dipole in the rate-deter-
mining step (step (ii)) of Scheme 1.
K1[H+]
By substituting for [TsNHX] from Eq. (11) into
Eq. (10), we get
K1[oxidant][H+]
[TsN+H2X] =
.
t
(11)
1 + K1[H+]
From slow and rds (step (ii)) of Scheme 1
Rate = k2[TsN+H2X][MR].
By substituting for [TsN+H2X] from Eq. (12) into
(12)
Eq. (13), we obtain
K1k2[oxidant][MR][H+]
t
(13)
Rate =
.
1 + K1[H+]
The rate law (13) is in complete agreement with the
experimentally observed results, wherein a first-order
dependence of rate each on [oxidant]o and [MR]o, and
fractional-order on [H+] was observed.
The rate constant of ionic reactions depends upon
the charges of reacting ions and also upon the ionic
strength of the solution. The study of the influence of
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 92 No. 13 2018

2654
ADALAGERE SOMASHEKAR MANJUNATHA et al.
ionic strength on the rate of the reaction gives a valu- chlorine has a higher value of 2.8. As the electronega-
able information about the type of the species involved tivity increases, the electropositive nature decreases.
in the rate-determining step, i.e., whether ionic or Because the halo cations are the reactive species in
non-ionic species are involved in the reaction. The these oxidation reactions, the electropositive nature is
effect of primary salt on the rate of the reaction is in the order Br > Cl. Therefore, the reactivity of BAT
explained by Bronsted and Bjerrum equation [28] is more, when compared with CAT. This tendency
may also be due to the moderate difference in the van
logk' = log ko + 1.018ZAZB . Here I is the ionic
I
der Waals’s radii of bromine and chlorine. A similar
behavior has been noted [7, 10–13, 31–33] in the oxi-
dation of several other substrates using CAT and BAT.
The details furnished in this research and the literature
reports, led to conclude that BAT is a stronger oxidant
compared with CAT.
strength of the reaction medium. k' and ko are the spe-
cific rate constants in the bulk of the reaction and at
infinite dilution; and ZA and ZB are the ionic charges
of the reactants A and B. The variation of k' with ionic
strength should depend on the sign of ZA and ZB. From
the above equation, a plot of log k' versus
would be
I
COD is a measure of oxidizable matter in dye stuff.
In the present research, an effort has been made to
determine the chemical oxygen demand (COD) for
MR dye. The COD of MR dye was determined using
the standard dichromate method. The procedure fol-
lowed to determine COD value was according to a lit-
erature procedure [34]. Under the prevailing experi-
mental conditions, COD of MR dye sample was found
to be 483.2 mg/L. Phytotoxicological analysis is used
for the discharge of treated industrial wastewater
before discharged into water bodies. They can be used
to estimate the function capability of aquatic biotops
and as an early warning system for the monitoring and
screening of surface water. The toxicity studies with
respect to generation of oxidative stress in plants are
yet to get much significant attention. Due to this rea-
son, maize seeds (Zea Mays L) were used as model
crop for this study [17]. The result of phytotoxicity
study showed that there is a significant effect on the
percentage germination of maize seeds: 38% germina-
tion was observed for untreated dye solution, 74% ger-
mination for treated dye solution, and 88% germina-
tion for control. The plumule and radicle lengths also
decreased in the case of untreated textile dye MR,
effluent and dye mixture samples. It shows that the
untreated dye solution is more toxic than treated solu-
tion which concludes that the decolorization of the
dye in wastewater is necessary.
linear yielding the slope equal to 1.018 ZAZB and an
intercept log ko. This equation predicts that: (i) if both
ions have same charges then, ZAZB is positive and so k'
increases with I; (ii) if ZA and ZB have opposite signs,
then ZAZB is negative and so k' decreases with I; and
(iii) if either ZA or ZB have neutral valence, then ZA ZB
is zero, so k' is independent of the ionic strength of the
solution. In the present investigations, variation of
ionic strength of the medium by adding NaClO4 solu-
tion (0.20 mol dm–3) does not alter the rate indicates
that one of the reacting species i.e., MR dye has neu-
tral valence in the rds as shown in Scheme 1 (step (ii)).
Therefore, the observed ionic strength effect is in
accordance with the Bronsted–Bjerrum concept and
proposed scheme.
The proposed mechanism is also supported by the
moderate values of energy of activation and other acti-
vation parameters. The positive values of ΔG≠ and
ΔH≠ indicate that the transition state is highly sol-
vated. The negative values of ΔS# suggest the forma-
tion of a rigid associative transition state with a few
degrees of freedom in both cases. Further, the compa-
rable value of ΔG# signifies the same type of the reac-
tion mechanism could be operative for the oxidation
of MR by CAT and BAT in acid medium. The values
of frequency factor (A) specify the frequency of colli-
sions and the orientation of reacting molecules. The
ineffectiveness of the additions of p-toluenesulfon-
amide and halide ions on the rate of reactions is in also
agreement with the proposed mechanism and the
derived rate law.
Cost plays an important role in any wastewater
remediation techniques. In this analysis, cost of the
materials were taken into account [35]. The operating
cost was calculated using an expression:
USD, m3 = aC + bC,
An evaluation of the rates of reaction of CAT and
BAT shows that the oxidation of MR is about four-
fold faster in BAT compared with that of CAT, under
identical set of experimental conditions. This is legiti-
mate by the relative magnitudes of activation energies
(Table 3). This trend may be due to the difference in
elctrophilicities of the halocations, Cl+ and Br+ ions,
involved in the oxidation processes and is also related
to the ease with which these species are generated in
reactions. In these oxidation reactions, the electro-
where a is cost of CAT/BAT, B is cost of perchloric
acid.
For treating 1 m3 of the synthesized aqueous dye
solution under present experimental conditions con-
sumed 0.18 kg of CAT (4.97 USD) and 1.4 L of per-
chloric acid (39.4 USD) which gives a total cost of
44.37 USD. This justifies the proposed decolorization
negativity values of Br+ and Cl+ play a crucial role. technique is an expeditious technique for the removal
Bromine has the electronegativity of 2.7, whereas of methyl red in wastewater effluents.
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SPECTROPHOTOMETRIC OXIDATIVE DECOLORIZATION
2655
10. Puttaswamy and R. V. Jagadeesh, in Electronic Encyclo-
pedia of Reagents for Organic Synthesis (Wiley, Chiches-
ter, 2013). https://onlinelibrary.wiley.com.
CONCLUSION
Optimum conditions for the facile oxidative decol-
orization of MR dye with CAT and BAT have been
developed. Under comparable experimental condi-
tions, reactions with both the oxidants follow identical
kinetics with a first-order dependence of rate each on
[oxidant]o and [MR]o, a fractional-order dependence
on [HClO4]. 4-Dimethylaminophenylhydroxylamine
and 2-nitrosobenzoic acid have been identified as the
oxidation products of MR dye. Thermodynamic
parameters were evaluated. TsNHX is the assumed to
be reactive oxidizing species in the present redox sys-
tem. Suitable mechanism and relevant rate law have
been worked out. COD of MR dye sample was found
to be 483.2 mg/L. Phytotoxicity and economic analy-
sis determined for the proposed decolorization pro-
cess. The rate of oxidation of MR is about four-fold
faster with bromamine-T when compared with chlora-
mine-T. This may be due to the van der Waal’s radii of
chlorine and bromine and also difference in electro-
philicites of Cl+ and Br+ ions. A plausible mechanism
conforming to kinetic data, reaction stoichiometry
and product analysis has been proposed. Accordingly,
this method will be a valuable addition to the existing
methods with more advantages.
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Author ASM thanks the Donbosco Institute of
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author (AS) thanks the Presidency University for their
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