Ozonation of azo dye Acid Black 1 under the suppression effect by chloride ion

Article abstract of DOI:10.1590/S0103-50532010000300009

The aim of this study is to determine the kinetics of the Acid Black 1 dye by oxidizing action of ozone and to evaluate the influence of chloride ion contamination on the dye degradation. Pseudo-first order kinetics was observed for both decolorization (620 nm) and aromatic structure (321 nm) degradations. A complete color removal was verified in 25 min ([dye]0 = 1.8 × 10 ^-5 mol L^-1), while aromatic structures degraded at slower rates. The presence of chloride ion, the principal impurity of the solid dye reagents, influences significantly the ozonation rate (-42percent), even at low chloride concentration (5.6 × 10^-4 mol L^-1). The most probable mechanism for the chloride suppression effect involves a direct reaction between O₃ and Cl^- generating HOCl, among other by-products. In despite of that, ozonation could be a competitive degradation process for this dye.

Full text of DOI:10.1590/S0103-50532010000300009

J. Braz. Chem. Soc., Vol. 21, No. 3, 452-460, 2010.
Printed in Brazil - ©2010 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Ozonation of Azo Dye Acid Black 1 under the Suppression Effect by Chloride Ion
Alexandre Paprocki, Heldiane S. dos Santos, Marta E. Hammerschitt,
Marçal Pires and Carla M. N. Azevedo*
Faculdade de Química, Pontifícia Universidade Católica do Rio Grande do Sul, Av. Ipiranga, 6681,
90619-900 Porto Alegre-RS, Brazil
O objetivo deste estudo é determinar a cinética degradação do corante Acid Black 1 por
ação oxidante do ozônio e avaliar a influência da contaminação de íons cloreto na degradação
deste corante. Foram observadas cinéticas de pseudo-primeira ordem, tanto para a degradação
das estruturas cromóforas (620 nm), quanto das aromáticas (321 nm). A completa remoção da
coloração foi verificada em 25 min ([corante]0 = 1,8 × 10-5 mol L-1), enquanto que as estruturas
aromáticas foram degradadas com menor velocidade.A presença de íon cloreto, principal impureza
do reagente sólido, influencia significantemente a velocidade de ozonização (-42%), mesmo em
baixa concentração (5,6 × 10-4 mol L-1). O mecanismo mais provável para o efeito supressor do íon
cloreto envolve a direta reação entre O3 e Cl- gerando HOCl, dentre outros subprodutos. Apesar
deste aspecto, a ozonização pode ser um processo de degradação competitivo para este corante.
The aim of this study is to determine the kinetics of theAcid Black 1 dye by oxidizing action of
ozone and to evaluate the influence of chloride ion contamination on the dye degradation. Pseudo-
first order kinetics was observed for both decolorization (620 nm) and aromatic structure (321 nm)
degradations. A complete color removal was verified in 25 min ([dye]0 = 1.8 × 10-5 mol L-1), while
aromatic structures degraded at slower rates. The presence of chloride ion, the principal impurity
of the solid dye reagents, influences significantly the ozonation rate (-42%), even at low chloride
concentration (5.6 × 10-4 mol L-1). The most probable mechanism for the chloride suppression
effect involves a direct reaction between O3 and Cl- generating HOCl, among other by-products.
In despite of that, ozonation could be a competitive degradation process for this dye.
Keywords: azo dye, ozonation, salt additives, chloride ion, suppression effect
Introduction
coagulation, or by alternative technologies including
electrochemical treatment,3,4 absorption using new materials
and advanced oxidative processes (AOPs).5 Despite the fact
that ozonation in acid medium is not an AOP, O3 proved
to yield excellent results on dye effluents treatment.6-9
Some studies have been reported about destruction ofAcid
Black 1 by ozonation.10-13 However, none of these studies
concerned the by-products formation during ozone attack.
The possible interferences on ozonation efficiency
by the presence of additives have been reported for the
treatment of the dyes,9,11,14,15 as well as for other class
of compounds.16,17 However, the available information
concerning the role of these dye additives on the kinetics
and by-product formation is scarce.16 The inorganic salts
are the most common additives used, as exhausting and
retarding agents, during textiles dyeing processes and its
quantity varies from 5 to 60 g L-1.18 Gahr et al.19 mentioned
the effect of salt during ozone decoloration of reactive dye
The Acid Black 1 (C.I. 20470) is an important azo dye
widely used in the textile industry. This compound is used
not only to dye or to print on synthetic and natural fibers,
but also as intermediate in the synthesis of other azo dyes.
Another application of the Acid Black 1 is as indicator to
assist in the protein separation for chromatography and
electrophoresis analyses.1
Textile effluents, when not correctly treated, can cause
high impact to the environment. Azo dyes are considered as
harmful compounds due to their mutagenic and carcinogenic
characteristics. The toxicity of theses dyes is often related to
the amines employed or formed in their syntheses.2
Dye wastewaters can be treated by conventional
processes, such as activated carbon absorption or
*e-mail: cazevedo@pucrs.br

Vol. 21, No. 3, 2010
Paprocki et al.
453
effluent and reported that, at higher levels, the presence of
salt lowers the decomposition of reactive dyes.
and Sigma (> 80%). In Figure 1 the chemical structure of
the dye, which has a molecular formula C22H14N6Na2O9S2
(616.48 g mol-1) and solubility less than 0.1 g L-1 in water,
is shown. The reagents were used as received andAcros dye
reagent was also purified to verify the possible influence
of impurities on degradation process.22 The aqueous
solutions containing nominal concentrations of the 5, 10
and 20 mg L-1 on dye reagent were used in kinetic tests.All
solutions were prepared with ultra pure water (MilliQPlus,
Millipore; < 18 MW cm). Chloride ion work solutions were
prepared using NaCl (Merck, 99.5%) or HCl (Merck 37%).
All other reagents used were of analytical grade.
Muthukumar et al.11 observed that the ozonation
kinetics of the Acid Black 1 is affected by the presence
of some sodium salts (carbonate, chloride and sulfate).
These authors explained the influence of added salts by
side-reaction with O3, which becomes less available for
dye degradation. However, with respect to chloride ion,
Gunten20 stated that inorganic chlorine-derived by-products
are only formed during ozonation if Cl- aqueous solution
is pretreated with chlorine or chlorine dioxide.
The influence of the impurities, commonly encountered
in commercial dyes, is less studied. According to Lyon,21
these impurities present in dyes are: (i) diluents such
as inorganic salts, starch and dispersing agents; (ii)
by-products formed during manufacture and (iii) dyes
of different constitution and color that are added for
shading. Zhang and co-workers14 investigated the effects
of impurities on the ozonation degradation of the azo dye
C.I. Reactive Red 120. These authors compared the results
obtained for unpurified (75%) and purified (90%) dyes, and
concluded that the presence of impurities affects mostly the
biodegradability of the dye. Zhang et al.14 also suggested
that dyes need to be purified before ozonation treatment
if detailed information on the oxidative processes and
by-production formation are required.
In this context, special attention has been given to
chloride ion that is additive as well as impurity for textile
dyes.9,11,14,15,18,19,22 When present as additive, at relatively
high concentration (0.5-5 g L-1), Cl- seems to play an
important role on dyes degradation processes.9,11,15,18
The possible influence of the low chloride concentration
(< 50 mg L-1, as an impurity) on dye ozonation has not been
reported in literature. Despite the importance of chloride in
this system, the mechanism of its action on dye ozonation
is not completely understood.
Figure 1. Chemical structure and essential properties of Acid Black 1.
Dye purification was performed following Lankin
procedure,22 which consists of the several sequential
washings with sodium chloride and aqueous isopropanol
solutions. In this way, 5 g of dye was weighed into
centrifuge bottle and 10 mL of 6% sodium chloride
solution was added and shaken vigorously for 20 min and
centrifuged (2,000 rpm) for 10 min. The supernatant was
separated and the treatment was repeated one more time
with 6% sodium chloride solution and twice with aqueous
isopropanol (100:20, isopropanol:water, v/v). For these
washings the period of stirring was 10 min and then the dye
was filtered using a Buchner funnel and dried in an oven
at 110 oC. Next the dye content on solid was estimated by
its absorbance (lmax 620 nm) on aqueous solutions using
1%
Lankin absorption coefficient (E 921).
1cm
Experimental set-up
The aim of this study is to determine the kinetics and
by-products formation on the Acid Black 1 ozonation and
to evaluate the influence of sodium chloride impurity on
dye degradation. The reaction between O3 and Cl- was
also investigated for a better understanding of the chloride
suppression effect. Finally, the literature data about Acid
Black 1 degradation processes as well as the chloride
suppression effect on dye ozonization are discussed.
The experimental set-up consisted of a prototype corona
discharge generator (OZ Engenharia, Brazil) which uses
dried atmospheric air to generated ozone, driven by air
pump with flow rate of 1.32 0.04 L min-1. Production
rate was 35 mg O3 h-1, determined by iodometric and
spectrophotometric methods.6,23 The on-line O3 absorbance
was measured by UV-visible spectrophotometer (Micronal,
B382)usingaquartzflowcell(pathlength10mm)at254nm.
Dye solutions (0.15 L) were ozonized in a cylindrical glass
reactor (volume 0.2 L) by bubbling ozone/air mixture into
the solution through a sinterized glass filter (pore size
50-80 mm). Due to the small reactor volume, 0.15 L fresh
dye solution was used for each reaction time period studied
(1 up to 25 min). Excess ozone leaving the reactor was
trapped by two sequential bubblers containing KI aqueous
Experimental
Materials
TheAcid Black 1 reagent (C.I. 20470) was obtained as
a commercial dye from two suppliersAcros (purity 63.2%)

454
Ozonation of Azo Dye Acid Black 1 under the Suppression Effect by Chloride Ion
J. Braz. Chem. Soc.
solution (2%).After ozonation the samples were purged for
5 min with N2 to remove residual ozone. All experiments
were conducted at room temperature (295 1 K).
The purity information about dye reagents were
obtained from supplier catalog and in the certificates of
analysis. ForAcros reagent a purity of 63.2% is mentioned
while a higher content is indicated to Sigma reagent (> 80%)
both on dry basis. However, the measured dye contents for
unpurified reagents indicated different values, 61.2 0.5%
(moisture 12.1 0.3%) forAcros and 86.6 0.8% (moisture
8.7 0.6%) for Sigma on dry basis. The higher purification
level was obtained for Acros reagent with dye purity
increasing from 61.2 to 86.3%. Same tests were performed
using the purified reagent (Acros ca. 86%).
In Figure 2 the spectra of Acid Black 1 solutions from
unpurified Acros and Sigma reagents and purified Acros
reagent, prepared in the same conditions (20 mg L-1, natural
pH) is shown. The three spectra presented in this figure
are similar in all wavelength range studied (190-800 nm).
The obtained data is also in good agreement with literature
data.1 These results indicate that the presence of possible
contaminants does not interfere on dye absorption.
Analyses
The optical absorption of dye solutions was determined
and recorded by a UV-Vis spectrophotometer (Hewlett
Packard 8453) through a 10 mm path length. In order to
calculate the kinetics, the concentrations of the dye in
solution were determined by its absorbance at maximum
absorption wavelengths in the visible (620 nm) and UV
(321 nm) regions following standard procedure.24
Organic and inorganic anions and inorganic cations
were determined by ion chromatography (Dionex DX500)
operated in autosuppression recycle mode and equipped
with a conductivity detector, detailed methods are described
elsewhere.6 Ion chromatography technique (IC) allowed
separation and quantification of most important low
molecular carboxylic acids (formic, acetic and oxalic),
inorganic anions (fluoride, chloride, nitrite, bromide,
nitrate, phosphate and sulfate) and cations (sodium,
potassium, calcium and magnesium). The detection limits,
for all compounds, were ≤ 5 mg L-1. Chloride ion present
as impurity on dye solutions was also determined by Ion
Selective Electrode (ISE) using sensing (Orion, Model 96-
17B) and double-junction reference (Orion Model 90-02)
electrodes following standard procedure.20 Conductivity
(Digimed DM-41) and pH (Digimed DM-21) were also
determined in some tests.
Due to the possible chloride interference on dye
ozonation reaction,9,11,14,15 some experiments were
performed with O3 and Cl- solutions, without the presence
ofAcid Black 1. For theses tests, chloride ion was supplied
from either NaCl or HCl solutions (2.8×10-4 mol L-1) and its
concentration was monitored by IC and ISE techniques.6,25
Chloride solutions were ozonized, following the procedure
described previously, and the UV-Vis spectra were obtained
at different reaction times, using 10 and 100 mm path
length cells. For comparison, the individual spectra of the
sodium hypochlorite and ozone (in 0.25 mol L-1 H2SO4)
were obtained in same conditions.
Figure 2. Spectra of the aqueous solutions (nominal concentration
20 mg L-1) prepared using two commercial gradesAcid Black 1 unpurified
reagents, from Acros and Sigma, and purified Acros reagent.
The IC and ISE analyses (not shown) indicated that
chloride ion is the most important impurity identified on
dye reagents (not ozonized). Unpurified Acros and Sigma
reagents presented chloride contaminations of 6.8 and
2.0% (m/m on dry basis), respectively. Phosphate ion was
also identified at average level ca. 0.5% in both unpurified
dye reagents, while other anions (potential identifiable by
IC) were not detected. The most abundant cations were
found in the following order: Na+ >> Ca2+ >> K+ > Mg2+
for unpurified dyes. These results were expected because
the probable chloride counter ion is sodium.22 On the other
hand, purifiedAcros dye presents a significant decrease on
Ca2+ (-98%) and Na+ (-42%) contents, while K+ and Mg2+
levels did not change with the purification process. The
inorganic impurities on theAcros reagent correspond to ca.
30% of the dye contamination while organic species, such
as starch or other dye synthesis by-products, are responsible
for 70% of the impurities.2
Results and Discussion
Dye purification
Since the efficiency of ozonation might be affected by
salt additives and/or dye reagent impurities, preliminary
tests were performed to assess dye purity and the presence
of possible contaminants on purified and unpurified reagents.

Vol. 21, No. 3, 2010
Paprocki et al.
455
Ozonation kinetics
and ozone, and k0 is the corresponding kinetic rate constant.
In addition, ozone concentration in the solution can be
assumed to be constant through the continuous ozone
supply thus the term k0[O3] is constant, therefore the
equation (1) can be simplified to,
Preliminary tests were performed using the Acros
unpurified reagent (61%), for comparison with literature
data. In Figure 3 the absorption spectra of Acid Black 1
solution (1.8×10-5 mol L-1) during the ozonation process are
shown.A quick decrease in absorption band at 620 nm was
observed, resulting in a decolorized solution, suggesting a
rapid destruction of the dye chromophore structure (-N=N-)
directly by ozone. Under these conditions decolorization
was completed in 25 min reaction time. On the other hand,
the absorbance at 321 nm, wavelength generally attributed
to aromatic structures, presented a less intense diminution.
These results indicated that chromophore group seems to
be less recalcitrant to ozone attack compared to aromatic
dye moiety.
d[dye]
–––––– = –k[dye]
(2)
td
where k is the overall pseudo-first-order rate constant.
Finally, as dye concentration is proportional to absorbance
(at specific l), the equation (2) becomes:
[Abs]t = [Abs]0e-tk
(3)
In Figure 4 the calculated logarithmic absorption ratio
(at lmax 620 nm) for ozonation at three different initial dye
concentrations is shown. These results confirmed pseudo-
first order kinetics. Several authors have also reported
pseudo first-order kinetics for reactions between different
dyes and ozone.7,8,13,14,22 The similar result was observed
at 321 nm (not shown) which suggests the same kinetics
behavior, but at lower ozonation rate (four times slower)
for aromatic degradation. A summary of the kinetics data,
pH and conductivity measurements, obtained during Acid
Black 1 ozonation, is presented in Table 1. The inverse
relation between the dye concentration and the ozonation
rate was observed, in accordance with literature.4,5,8,20 The
decrease of pH was expected, due to the formation of
inorganic and organic acids during ozonation, confirmed
by a significant conductivity increase, more pronounced
for high dye concentrations.
Figure 3. Absorption spectra of the Acid Black 1 (1.8×10-5 mol L-1)
solutions prepared with unpurified reagent (Acros 61%) at different
ozonation times (35 mg O3 h-1).
Similar results were observed for other dye
concentrations studied, prepared with the same dye reagent
(Acros 61%). Complete color removal (lmax 620 nm,
A < 0.001) was observed after reaction times of 6 and
11 min for initial dye concentration of the 4.5×10-6 and
8.9×10-6 mol L-1, respectively.
Kinetic parameters were obtained following dye
degradation at their maxima visible and UV absorption
bands (620 and 321 nm). The reproducibility of these
calculations was evaluated by replicate tests and errors
were in the range of 4.3-5.0%. According to previous
studies the ozonation in a continuous purging mode could
be formulated by equation (1), considering only direct
oxidation by molecular ozone.10,13,24 The indirect oxidation
by hydroxyl free radicals can be neglected because the low
hydroxide ion concentration in solution at pH used (< 5.3).
Figure 4. First-order ozonation degradation of unpurified Acid Black 1
solutions at different dye concentrations, measured by absorbance
(620 nm) at 295 K.
By-products formation
To achieve detailed information on the dye degradation
by O3, complementary studies about ionic by-products
generation were performed for ozonized solutions of the
unpurifiedAcros dye reagent. The data presented in Table 2
d[dye]
–––––– = –k0[O3][dye]
(1)
td
where [dye] and [O3] are the concentrations ofAcid Black 1

456
Ozonation of Azo Dye Acid Black 1 under the Suppression Effect by Chloride Ion
J. Braz. Chem. Soc.
Table 1. Rate constants and reaction half-times of the ozonation of the Acid Black 1 solutions (reagent Sigma 61%), measured at lmax. 620 and 321 nm at
ambient temperature (295 1 K)
620 nm
321 nm
pH
conductivity / (µS cm-1)
Initial/Final
[reagent]o /
(mg L-1)
[dye]o /
(mol L-1)
k / (min-1)
0.80
t1/2 /(min)
0.9
k / (min-1)
0.19
t1/2 / (min)
3.6
5
4.5 × 10-6
8.9 × 10-6
1.8 × 10-5
5.22/4.95
5.27/4.54
4.96/4.04
5.56/13.43
6.60/11.31
11.08/58.90
10
20
0.50
1.4
0.12
5.8
0.26
2.7
0.10
6.9
shows that sulfate and carboxylic acids (formic, oxalic and
acetic) were the main oxidation by-products (chloride is
also present but as a dye impurity). These species were not
observed as dye contaminants (initial concentrations < LD)
but were actually formed during dye ozonization in all
concentrationconditionsstudied(4.5×10-6 to1.8×10-5 mol L-1).
Nitrite and phosphate (not shown) were also detected but
at lower concentrations (< 1 mmol L-1). Formic acid is
the principal organic compound formed, corresponding
40-49% of total identified short chain carboxylic acids.
Oxalic acid was the second most abundant acid (34 up to
40%), with acetic acid presenting lower concentrations
(0.8-4.5 µmol L-1). These organic acids correspond to 8-10%
of the initial dye concentration expressed in carbon basis.
remaining nitrogen atoms could still be in amino form or be
converted into nitro-compounds, more recalcitrant to ozone
attack. On the other hand, sulfonic groups seem to be easily
converted to sulfate with high yields (up to 60%). It is also
observed a raise of sulfur conversion with increase of the dye
concentration (from 47 to 60%). These preliminary results
and the presence of the small organic fragments (carboxylic
acids) indicated that the dye structure was not completely
mineralized under the prevailing oxidation conditions.
Comparison with other degradation processes
In Table 3 a summary of the literature data about Acid
Black 1 degradation processes is shown.10,11,13,22-29 Only
three works concerning Acid Black 1 ozonation kinetics
were found and unfortunately in some studies important
experimental conditions were not cited (pH, temperature,
O3 concentration etc). Shu and co-workers10,13 also
obtained pseudo first-order kinetics for unpurified Acid
Black 1 ozonation. A rate constant of the 0.45 min-1 was
calculated for a reagent dye concentration of 20 mg L-1. In
these conditions a t1/2 =1.6 min was obtained, indicating a
faster kinetics compare to our system (t1/2 ca. 3 min). This
behavior is probably due to higher ozone doses used by
these authors. It is difficult to draw a comparison of these
data with results obtained by Muthukumar et al.11 due to
different experimental conditions used (basic medium with
higher O3 and dye concentrations).
Only two degradation processes (Table 3) present
faster kinetics than ozonation: UV/H2O2 (power density
700 W L-1) and Fenton with t1/2 ca. 0.6 min.26,27 All other
processes have similar or lower kinetic rates compared to
O3 treatment. However, generic conclusion must be made
with caution due to the different experimental conditions
used in some studies.
Table 2. By-products generated from ozonized solutions at differentAcid
Black 1 concentrations
[dye]0 (mol L-1)
Ozonation time (min)
Organic Acids
Formic
4.5 × 10-6
6
8.9 × 10-6
11
1.8 × 10-5
25
mmol L-1
%
49
mmol L-1
%
40
mmol L-1
%
40
2.4
0.8
1.7
4.9
4.6
2.5
9.6
4.5
Acetic
17
22
20
Oxalic
34
4.4
39
9.6
40
Sub-total
100
11.5
100
23.6
100
Carbon Conversion (%)
Inorganic Anions
Nitrite
8
9
10
mmol L-1
0.7
%
40
mmol L-1
0.7
%
27
mmol L-1
0.9
%
8
Nitrate
1.0
60
1.8
73
9.7
92
100
Sub-total
1.6
100
2.4
100
10.5
21.2
Sulfate
4.2
9.4
Nitrogen Conversion (%)
Sulfur Conversion (%)
6
5
10
60
47
53
The molecule ofAcid Black 1 also contains six nitrogen
atoms in the form of azo (4), amino (1) and nitro (1) groups
and two sulfur atoms in the form of sulphonic groups
(Figure 1). These heteroatoms react in different ways with
ozone due to their specific reactivity and process conditions
(O3 dose, pH etc), being converted in several intermediates
and end-products.22 A partial transformation (5-10%) of
nitrogen atoms into nitrate and nitrite were observed. The
Another important aspect is the stated initialAcid Black
1 concentrations in Table 3, which in fact represent the
dye reagent concentration. As shown in the present study,
very different purities of the Acid Black 1 reagents could
be found, 61 and 87% which rate constants of the 0.26
and 0.21 min-1, respectively. As consequence, significant
differences on actual dye concentration are obtained

Vol. 21, No. 3, 2010
Paprocki et al.
457
Table 3. Kinetic data of aqueous solutions of Acid Black 1 in different degradation systems
Systems
[reagent]0 /
Reactants
(mol L-1)
Power density /
(W L-1)
pH
T /
(°C)
n.m.
n.m.
20
k /
(min-1)
0.44
0.26
0.26a
0.21b
0.45
0.5
t1/2
/
References
(mg L-1)
(min)
2.6
2.7
2.9
3.3
1.5
1.4
15
O3
O3
O3
20
O3
O3
7.2×10-5
n.m.
9
13
11
c
310
20
2 g h-1
c
O3
35 mg h-1
5
This work
20
O3
35 mg h-1 c
n.m.
5
20
O3
20
O3
5.3
5.3
5.3
n.m.
n.m.
n.m.
10
UV/O3
UV/H2O2
UV/H2O2
Fenton
20
O3
n.m.
50
50
20
H2O2
H2O2
H2O2
Fe2+
H2O2
Fe3+
Fe2+
H2O2
Fe2+
H2O2
Fe-Clay
TiO2
7.08 × 10-3
2.12 × 10-2
5.4 × 10-4
3.4 × 10-5
5.4 × 10-4
3.4 × 10-5
2.5 × 10-5
8.0 × 10-3
2.5 × 10-5
6.4 × 10-3
0.5 g L-1 d
0.05
1.10
1.25
17
700
0.6
0.6
26
27
46
6.3
6.3
30
30
Fenton
46
0.03
22.3
Fenton/US
50
62
10
50
3
3
4
20
30
1.5
3.6
30
Photo-Fenton
TiO2/Photoc
8 W
0.194
n.m.
28
29
180 W
n.m.
a Solid unpurified reagents Acros 61% or b Sigma 87%; c ozone production of the O3 generators; d Fe-Acetylacetonate+bentonite loading 0.5 g L-1;
n.m.: means not measured.
by using the same reagent concentration from different
suppliers. For instance, a reagent concentration of 20 mg L-1
resulting in dye concentrations of the 1.8×10-5 mol L-1
(unpurifiedAcros 61%) and 2.4×10-5 mol L-1 (for unpurified
Sigma 87%). If a correct dye concentration is not taken
into account, kinetics data could be misinterpreA further
problem is the role that those reagent impurities (for
instance: 13 up to 39% in the dye reagents used in this
work) could play on the dye degradation processes.
Special attention has been given to the inorganic salts,
normally present on dyestuff wastes from dye impurities
and/or salt added during textile processing.15,19 Among
these impurities, sodium chloride is a key species due to
its wide use on dye synthesis and purification processes.19
Thus, the dye impurities, especially chloride ions, could be
responsible for some discrepancies observed in literature
data presented in Table 3.
rate significantly decreased (6 up to 42%) with chloride
addition. This effect was observed for both purified and
unpurified reagent (Figure 5), but in different ways. The
most important influence was observed for Acros 86%
(-42%) at 5.6×10-4 mol L-1 of the added chloride. A less
intense, but significant decrease (-35%) on rate variation,
was observed for unpurified Acros dye at highest chloride
concentration tested (2.8×10-2 mol L-1).
Influence of chloride ion on dye ozonation
Figure 5. Influence of added chloride (as NaCl) on kinetic constants of
Acid Black 1 ozonization.
In Table 4 the kinetics data of Acid Black 1 ozonation
in presence of the different added chloride concentrations
(1.4×10-6 up to 2.8×10-2 mol L-1) is shown. The dye
decay always followed pseudo-first-order kinetics no
matter the NaCl was involved or not. However, reaction
These results were not expected because other
authors have reported a chloride suppression effect at
higher Cl- concentrations.9,11,15,16 Muthukumar et al.11,15

458
Ozonation of Azo Dye Acid Black 1 under the Suppression Effect by Chloride Ion
J. Braz. Chem. Soc.
Table 4. Influence of the chloride added (as NaCl) to the Acid Black 1
on ozonation
O3 faster then Cl- generating brominated compounds.
However, in the conditions tested the bromide ion was
neither identified in dye solutions (LOD < 2 mg L-1) nor in
the NaCl solid reagent (< 0.02%) used. Thus, the influence
of chloride on Acid Black 1 degradation should take place
from different mechanism, neither related to the presence
of bromide nor due to a salting effect.
[dye]0 /
(mol L-1) purity
Supplier
[Cl-]added
/
[Cl-]total
/
k /
var. /
(%)
t1/2 /
(mmol L-1) (mmol L-1) (min-1)
(min)
2.7
2.8
3.0
3.1
4.1
2.9
4.0
4.2
5.1
3.3
3.8
4.0
4.3
0
141
282
563
28,169
0
38a
179
320
601
28,207
22a
0.26
0.25
0.23
0.22
0.17
0.24
0.18
0.17
0.14
0.21
0.18
0.17
0.16
0
Acros 61%
1.8×10-5 (not
purified)
-6
-11
-15
-35
0
Reaction between chloride ion and ozone
The possible mechanism explaining the Cl- suppression
effect could be a direct reaction between O3 and chloride.
Yeatts and Taube30 firstly indicated that chlorine can
be formed in ozone/chloride solutions in acid media as
indicated in reaction A (Table 5). More recent works
confirm the second order rate constant for this reaction as
well as the catalytic effect of the H+.31-33 Levanov et al.32,33
proposed the mechanism indicated by reactions B-D for
this system. Siddiqui suggested that hypochlorite could
react with ozone generating chloride and chlorite ions. In
addition, chlorite ion could also react with O3 and produces
chlorate ion by pathway shown in reactions E to G.34
Acros 86%
(purified)
2.4×10-5
141
282
563
0
163
304
585
11a
-26
-30
-42
0
Sigma 87%
2.4×10-5 (not
141
282
563
152
293
575
-14
-19
-23
purified)
a: chloride contaminations originally present in the reagents dye (see text).
working with concentrated chloride solutions (1.4×10-1
to 2.6×10-1 mol L-1) observed an increasing of ca. 60%
on reaction time for complete color removal. Soares et
al.9 also observed a negative effect in decolorisation of
Acid Blue 113 by O3 in the presence of NaCl (1.4×10-2
to 1.4×10-1 mol L-1) and verified a non linear behavior on
color removal by ozone with the addition of higher chloride
concentration.
The observed chloride suppression on dye ozonation
rate constant is complex and indicates a possible
inhibition effect at high Cl- levels. In Figure 5, logarithmic
correlations between k and [Cl-] are obtained for three
studied conditions (Acros 61 and 86%, Sigma 87%). These
results sugested that chloride suppression is not a linear
effect in our system. A more detailed discussion about this
behavior is presented in the next section.
Table 5. Main reactions involved on chloride ozonation mechanism
Equilibrium or
Kinetic Constants
Number
Reaction
A
B
C
D
E
F
O3 + 2 Cl− + 2 H+ → Cl2 + H2O + O2
O3 + Cl− → ClO− + O2
k = 1.5×10-3 L mol-1 s-1
pKa = 7.5
HOCl
OCl− + H+
HOCl + H+ + Cl− ↔ Cl2(g) + H2O
ClO− + O3 → 2 O2 + Cl−
K = 2.2×104
k = 110 L mol-1 s-1
k = 30 L mol-1 s-1
k = 4×106 L mol-1 s-1
ClO− + O3 → O2 + ClO2−
ClO2− + O3 → O2 + ClO3−
G
The influence of added salt on ozonation dye kinetics
could be related to a simple decreasing of O3 solubility
(salting effect). To test this hypothesis an empirical relation
was used to account for ionic strength influence on ozone
partition between gas and water phases (Henry constant).22
For higher chloride concentration (2.6×10-1 mol L-1),
an important reduction on O3 solubility (16%) was
calculated. This fact could in part explain the decrease
on dye degradation rate observed by Muthukumar et
al.11 On the other hand, for low chloride concentration
(< 2.8×10-2 mol L-1) an insignificant decrease on ozone
solubility was estimated (< 2%). These calculations
indicated that salting effects seem to remain marginal in
our conditions.
Levanov et al.32,33 suggested that Cl2 formation and its
release into a gas phase result from a rapidly established
equilibrium. Unfortunately it is very difficult to identify
chlorine gas in the presence of the ozone, especially at
low reactants concentration used. On the other hand,
a decrease in the chloride level in ozonized solutions,
with a concomitant hypochlorite generation, could be an
alternative form to confirm the proposed mechanism.
Our experimental results indicated a significant
decrease on Cl- concentration (-20%) for ozonized
hydrochloric acid solutions (2.8×10-4 mol L-1 in Cl-,
pH 1) without the presence of dye. As expected, this effect
was not observed in sodium chloride solution (2.8×10-
4 mol L-1 in Cl-, pH 6) probably due to low [H+]. In order
to verify the possible formation of the hypochlorous acid
(HOCl), UV spectra of these solutions were recorded.
Azevedo et al.17 suggested that the bromide ion,
normally present as a contaminant of NaCl, reacts with

Vol. 21, No. 3, 2010
Paprocki et al.
459
For dilute HCl concentration (≤ 0.1 mol L-1) only the
peaks of ozone (254 nm) and chloride (ca. 200 nm) were
detected. However, on increasing HCl concentration up to
0.5 mol L-1, a new peak around 230 nm appears, while ozone
peak (254 nm) is no longer observed (Figure 6). This new
peak is close to the HOCl lmax (230 nm) one, as could be
observed in the spectrum obtained from a NaOCl acidic
solution (in 0.25 mol L-1 H2SO4) also shown in Figure 6.
the aromatic structures are also attacked but at slower
rates. Pseudo first-order rate kinetics were obtained, with
an inverse relation between initial dye concentration and
rate constant.
The ozonized unpurifed dye solutions showed the
presence of short chains carboxylic acids (formic, oxalic
and acetic) and inorganic anions (sulfate, nitrate and nitrite)
as degradation by-products. The calculated conversions on
molar ratios suggest that dye structure was not completely
mineralized under used conditions.
The dye purity is a very important parameter not just
to state the real dye concentration but also to indicate the
presence of impurities which could be reactive towards
O3. The main identified contaminant (chloride ion) affects
significantly the dye ozonation kinetics. Interference
studies indicated that rate constants decrease up to
42% in the presence of 5.6×10-4 mol L-1 added chloride.
However, the presence of other impurities could also
interfere in the dye degradation. The mechanism of the
chloride suppression effect seems to involve a direct
reaction between O3 and Cl- generating HOCl, among
other by-products.
Figure 6. Spectra UV-Vis of the aqueous solutions: 0.5 mol L-1 HCl
(ozonized and not ozonized) and HOCl (2×10-3 mol L-1 in 0.25 mol L-1
H2SO4) and ozone (1.2×10-4 mol L-1 in 0.25 mol L-1 H2SO4).
Similar behavior was observed by Vinals et al.35
studying the influence of chloride ion on the ozone assisted
leaching of gold and palladium from metallic scraps. These
authors verified the presence of a peak around 230 nm
and attributed to the presence of HOCl, for chloride
concentration up to 0.5 mol L-1.
Acknowledgments
The authors thank FAPERGS and CNPq for financial
support.
It is possible that hypochlorous acid formation could
also take place in slight acid solutions (pH 4) and at low
O3 and chloride concentrations, such as the ones used
in this study. If true, the direct reaction between O3 and
chloride could compete with dye ozonation and explain,
at least in part, the observed Cl- suppression effect onAcid
Black 1 degradation. However, the global mechanism is
probably more complex than that considered in this work.
The presence of dye could directly influence the chloride
reaction with ozone and their by-products also could
interact among them, generating different products. Thus,
for a best elucidation of the involved mechanism and to a
precise estimation of chloride suppression effect further
tests must be performed using more sensitive analytical
techniques, free of interference.
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