[email protected]

Journal of Chemical Technology and Biotechnology
J Chem Technol Biotechnol 80:916–923 (2005)
DOI: 10.1002/jctb.1262
Electrochemical oxidation of textile dye indigo
Doǧan Doǧan1 and Haluk Türkdemir2∗
1 İnönü
2 İnönü
University, Faculty of Education, Department of Science, 44069 Malatya, Turkey
University, Science & Art Faculty, Chemistry Department, 44069 Malatya, Turkey
Abstract: Electrochemical methods are being used increasingly as an alternative treatment process for
the remediation of textile wastewaters. This study focused mainly on the colour removal and chemical
oxygen demand (COD) reduction of vat textile dye (CI Vat Blue 1: indigo) from its aqueous solution
by electrochemical oxidation. The process was carried out in a batch-type divided electrolytic cell
under constant potential using a Pt cage as anode and Pt foil as cathode. Operating variables such as
supporting electrolyte, pH, ultrasonification and treatment time were investigated to probe their effects
on the efficiency of the electrochemical treatment. Colour removal was estimated by monitoring the
disappearance of the absorbance peak at 681.5 nm. It was found that in acidic conditions the electrolysis
was more efficient. At pH 1, an NaCl concentration of 0.24 mol dm−3 , a dyeing solution concentration
of 0.1% (w/v) and a period of 90 min of electrolysis, there was almost 100% colour removal and 60%
reduction in COD. Voltammetric and IR investigations demonstrated that partial degradation of dye was
achieved. The experimental results indicate that this electrochemical method could effectively be used as
a pretreatment stage before conventional treatment.
 2005 Society of Chemical Industry
Keywords: electro-oxidation; dye treatment; indigo; colour removal; COD reduction
INTRODUCTION
The textile industry is one of the most polluting
industries in terms of the volume, colour and
complexity of its effluent discharge.1 These effluents
usually contain dyes which are discharged in large
quantities worldwide into natural water bodies.2 Dyes
are coloured substances, and resistant to fading on
exposure to light, water and many chemicals due
to their complex chemical structure and synthetic
origin,3 hence they persist in nature. The discharge of
dye-containing effluents into receiving waters without
appropriate treatment, actually limits aquatic plant
growth by creating anaerobic conditions.4,5 Therefore,
even the presence of very small amounts of dyes in the
effluent is highly undesirable.6
The treatment of textile dye wastewater has
been rather difficult due to both the presence of
strong colour and its high organic content.7 Various
traditional remediation methods have been used to
treat wastewater containing dyes to meet regulatory
discharge limits. Due to the drawbacks associated
with conventional treatment methods, there has been
a growing interest in the use of electrochemical
methods for the treatment of wastewaters as it
would not generate any pollutants and would give
complete degradation of the pollutants present in
the effluents.1,8 By an electrochemical treatment
of wastewaters or dyeing solutions either a partial
or a complete degradation of the pollutants can
be achieved. Electrochemical oxidation of organic
pollutants can be achieved using anodes having
high oxygen over-potential and corrosion stability
or indirectly using appropriate anodically-formed
oxidants such as chloride, hypochlorite, ozone and
Fenton’s reagent.9
World consumption of synthetic dyes for cellulosic
fibres is increasing gradually year by year. Vat
textile dyes, especially indigo, play an important
role in today’s dyeing industry due to the increasing
demand for denim production. The annual production
of synthetic indigo is estimated as 22 000 tons of
dyestuff.10 The textile dyeing process is based on
the chemical reduction of indigo in the presence of an
alkali into water-soluble leucoindigo, which has a high
affinity for the cellulose fibre and can be fixed on it by
re-oxidation in air, as shown in Scheme 1.11
The water-insoluble indigo dye is considered a recalcitrant substance that causes environmental concern12
and is treated primarily by chemical coagulation and
flocculation methods which generate large amounts
of sludge that pose handling and disposal problems.
There are some recent reports on indigo degradation using ligninolytic enzymes,13,14 white-rot fungal pellets,12,15,16 anaerobic mixed cultures17 and
photocatalysis.18 Decolourization of indigo is also possible by electrochemical reduction, yet reduction of
∗ Correspondence to: Haluk Türkdemir, İnönü University, Science & Art Faculty, Chemistry Department, 44069 Malatya, Turkey
E-mail: [email protected]
Contract/grant sponsor: Research Fund Unit of İnönü University; contract/grant number: APYB:2002/24
(Received 10 June 2004; revised version received 28 October 2004; accepted 21 December 2004)
Published online 14 March 2005
 2005 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2005/$30.00
916
Electrochemical oxidation of textile dye indigo
Scheme 1.
indigo is reversible in most cases and upon exposure
to the oxygen in the air, the dye molecule rapidly
oxidizes to its coloured insoluble form.19,20 Although
there is extensive literature available on electrochemical treatment of wastewater containing dyes,21 – 23 there
is, however, no report on electrochemical oxidation
of solutions of indigo dye or indigo dye-containing
wastewater. In the present study, therefore, we report
the investigation of the decolourization of aqueous
solutions of textile dye indigo by electrochemical oxidation and the effects of different operational parameters on colour removal and chemical oxygen demand
(COD) reduction.
EXPERIMENTAL
Chemicals
Indigo dye (CI Vat Blue 1), was obtained from GAP
Textile Co, Malatya, Turkey in pure form. This dye
is widely used in the cotton textile industry in Turkey.
The dyeing solution was prepared by mixing 100 g
powdered indigo, 80 g sodium dithionite, and 100 g
48 Bomé sodium hydroxide in a vessel making up to
a final volume of 50 dm3 with water at the industrial
scale. Since it was foreseen that the solution would be
diluted at least 100% in the dyeing unit of the plant;
considering this dilution ratio, each time 1 dm3 of
0.1% indigo solution by w/v was prepared freshly. The
average chemical oxygen demand and pH of these
solutions were measured to be 2299 mg O2 dm−3
and 12 respectively. The solutions were stored in
a coloured bottle at +4 ◦ C for instant use. In the
experiments, 150 cm3 of this solution was placed into
the electrolytic cell. All other chemicals used were
reagent grade.
Apparatus and instruments
In this study, electrochemical oxidation of the indigo
dyeing solution was carried out in a laboratory-scale
batch electrolytic cell with an approximate volume of
200 cm3 (Fig. 1). The cell was equipped with a magnetic stirrer (IKA RO5 model, Staufen, Germany) in
order to keep both the solutions in the compartments
well-mixed during electrochemical oxidation. A laboratory DC power supply (RAYSEL LPS 40-5 model,
Ankara, Turkey) with current–voltage monitoring was
employed to provide the electric power required for
electrolysis. In the electrolysis experiments, a cylindrical Pt cage electrode with a 1.7 cm diameter and 2 cm
height, and a Pt foil electrode with a 2 cm2 surface
area were used as anode and cathode respectively.
Solution pH values were measured using a digital
J Chem Technol Biotechnol 80:916–923 (2005)
Figure 1. Batch experimental set-up: schematic diagram. a, Divided
cell; b, Pt cage electrode; c, Pt counter electrode; d, electrical charge
counter; e, DC power supply; f, galvanometer; g, voltmeter; h, magnet
bar.
pH–mV meter (Orion 601A model, Cambridge, MA,
USA). Voltammetric measurements were taken by
using an electrochemical analyser system, BAS-100B
(Bio-analytic System, Lafayette, USA) and optical
absorbances of the solution were monitored using a
double beam UV-VIS spectrophotometer (Shimadzu1601 PC model, Rydolmere, MSW, Australia). IR
spectra were obtained using an FTIR spectrophotometer (UNICAM Mattson 1000 model, Danbury, CT,
USA). An ultrasonic bath (Branson 2200 model, UK)
was also used. All the measurements were performed
at 25 ± 2 ◦ C.
Methods
The electrochemical oxidation process was performed
under constant potential. During the preliminary
investigations different anode and cathode materials
such as graphite rod, Pt foil and Pt cage, and
different cell types such as divided with G3 porosity,
and undivided cells were tested. The divided cell
and Pt electrodes were selected as the working
917
D Doǧan, H Türkdemir
cell and electrodes respectively according to the
results of the pre-investigations. The applied potential,
taking colour removal time into consideration, was
systematically increased, starting from 5.0 V, to 40.0 V,
and it was found that the circuit current was at
a reasonable level at 40.0 V, which was selected as
the working potential in all the experiments. The
pH of the solution was adjusted using concentrated
sulfuric acid. As expected, pH changes were observed
in both compartments during electrolysis. Process
parameters were optimized in accordance with the
total electrical charge passed and electrolysis time
needed for the decolourization of the dyeing solution.
While decolourization was occurring by the electrooxidation process, changes in the COD of the
solution were monitored by withdrawing samples
from the anode compartment at determined time
intervals. Chemical oxygen demand measurements
were made titrimetrically according to the procedure
outlined in Ref 24. To estimate colour removal by
the electrochemical method, the disappearance of the
absorbance peaks of the solution was monitored.
Prior to each analysis, samples taken from anode
compartment were diluted appropriately with doubly
distilled water. To study the degradation mechanisms,
voltammetric and IR spectrometric investigations were
made. The electrochemical behaviour of the dyecontaining solution before and after electrochemical
treatment was investigated by cyclic voltammetry
(CV), at 100 mV s−1 , between +1.8 and −1.2 V versus
Ag/AgCl. CV measurements were performed in a
10 cm3 voltammetric cell using a Pt disk (2.54 mm2 )
working electrode, a Pt foil counter electrode and
an Ag/AgCl reference electrode. The IR spectra of
the dyeing solution were also analysed before and
after the electrochemical decolourization process. The
samples were dried at 104–105 ◦ C and pelleted with
KBr fine dusts in 5 mg/195 mg ratio. FTIR spectra
were obtained.
Table 1. Effects of supporting electrolyte types on the decolourization
of indigo dye
Electrolyte
NaCl
Na2 SO4
NaNO3
Na2 HPO4
No electrolyte
Electrolysis
time (min)
Colour
removed
Electrical
charge (C)
180
180
190
185
180
Yes
No
No
No
No
750
977
1056
262
255
It is clear from the results in Table 1 that indigo
dye is quite a recalcitrant substance, and its solution
colour can only be removed by the addition of sodium
chloride. Experiments showed that in the electrolysis
of different pollutants with chloride, an indirect electrochemical oxidation effect of chlorine/hypochlorite is
the main pathway for removal of pollutants.25 – 27 The
superior effect of chloride may be due to the indirect
oxidation effect of chlorine and the in situ generation
of hypochlorite. Electrolysis in the absence of chloride
was not effective in colour removal. This result can be
explained on the basis that the indigo dye was probably
in the molecular form at the operating pH and may
not undergo migration in the electrical field. The effect
of direct hypochlorite addition on colour removal was
also studied without electrolysing the dyeing solution.
When sodium hypochlorite was added to the solution
under well mixed conditions, the dark blue colour of
the indigo solution turned to brown, and finally to
light orange. This result implies that decolourization
of the solution is related to the formation of chloride
radicals during the anodic oxidation process.
Determination of optimum pH value
To examine the effect of pH on the decolourization
process, the dyeing solution was adjusted initially to
the desired pH for each experiment, using sulfuric
acid after the addition of 0.12 mol dm−3 NaCl. The
experiments have been carried out at 1.0, 2.0, 4.0,
7.0 and original pH (= 12) values sequentially. The
RESULTS AND DISCUSSION
The purpose of the study was to decolourize the
solution of indigo dye and to reduce the COD value
of the solution as much as possible. To achieve this,
the following studies were carried out sequentially to
establish the optimum operational conditions. Some
selected studies were repeated more than twice to be
sure of the repeatability of the results.
Determination of supporting electrolyte type
To investigate the effects of different types of supporting electrolytes on the decolourization process,
sodium chloride, nitrate and sulfate and disodium
hydrogen phosphate, each of which had a concentration of 0.12 mol dm−3 were used. The pH of the
solution was adjusted to 4.0, and the electrolysis was
run. The results obtained from the experiments are
summarized in Table 1.
918
Figure 2. Effect of pH on the colour removal time and total electrical
charge passed during electrolysis (operating conditions: applied cell
voltage = 40 V, NaCl concentration = 0.12 mol dm−3 ).
J Chem Technol Biotechnol 80:916–923 (2005)
Electrochemical oxidation of textile dye indigo
results were compared in terms of the time required
for colour removal and total electrical charge passed
during electrolysis. From Fig 2, it is evident that the
total electrical charge consumed remains nearly the
same with the variation in pH of the solution, but the
time for colour removal sharply decreases as the pH
changes from 2 to 1, which indicates that the effect
of pH value on the degradation of dye is remarkable
under highly acidic conditions. This result can be
explained by the basic structure of indigo dye and
hypochlorite ions, which act as better oxidizing agents
in acidic media. Therefore, pH 1 was selected as the
operating pH value.
Optimization of NaCl concentration
Using sodium chloride as the supporting electrolyte
at pH 1, colour removal time and total electrical
charge values were plotted against NaCl concentration
to determine the operating value. The electrolyte
concentration was maintained as low as possible to
avoid excess chloride pollution. The effect of changing
the electrolyte concentration (0.12, 0.24, 0.48, 1.00,
1.50 mol dm−3 ) on the total electrical charge and
colour removal time is illustrated in Fig 3. Although
the results indicate that the colour removal periods
decrease with increasing chloride concentration,
an electrolyte concentration of 0.24 mol dm−3 was
selected as the operating concentration owing to the
known toxic effects of excess chloride on aquatic life.28
Electrolyte concentrations higher than 0.24 mol dm−3
do not have a major effect on the total electrical charge
passed, which was slightly decreased depending on the
increasing current output.
Effect of sonoelectro-oxidation
As the remedial effect of ultrasonification has been
reported for many electrochemical processes,29 – 31
the effect of ultrasound on the electro-oxidation of
a solution of indigo dye was also studied under
the optimized experimental conditions. Unexpected
Figure 3. Effect of NaCl concentration on the colour removal time
and total electrical charge values (operating conditions: applied cell
voltage = 40 V, pH = 1).
J Chem Technol Biotechnol 80:916–923 (2005)
results, however, were obtained from the electrolysis
of indigo which showed that the sonification actually
reduced the effect of the electrochemical oxidation
process. Comparing these results with the results
obtained without using ultrasound showed that more
time was required for the process, and increased
electrical charge was required to achieve the same
degree of colour removal. These results may be related
to the degassing effect of ultrasound on chlorine, which
gives rise to reduced production of hypochlorite, hence
less is available for decolourization. Moreover, there
was no colour change in the control solution, which
was also prepared in optimized composition, but with
no electrochemical procedure applied.
Effect of treatment time on COD
COD reduction is one of the most important
parameters for evaluating the efficiency of wastewater
treatment methods. Therefore the electrochemical
oxidation experiments were performed to determine
the effect of treatment time on COD reduction and
the total electrical charge. Samples were taken from
the anode compartment to measure the COD value of
the solution without stopping the electrolysis. Each
time the experiments were started over again to
avoid solution level differences arising between the
compartments, and the related data are presented in
Table 2 and Fig 4. These results show as expected,
Table 2. Change of COD and electrical charge values with
treatment time
Time (min)
0
5
10
15
20
30
45
60
Electrical charge (C)
COD (mg O2 dm−3 )
0
30.72
56.80
89.76
122.56
178.32
306.08
393.68
2299
2232
2030
2053
2007
1860
1510
1306
Figure 4. Effect of treatment time on COD and electrical charge
values (operating conditions: applied cell voltage = 40 V, pH = 1,
NaCl concentration = 0.24 mol dm−3 ).
919
D Doǧan, H Türkdemir
that the COD of the solution is being remediated
gradually with increasing electrolysis time, whereas
total energy consumption increases with time.
Although it was reported that in remediation of
various textile wastewaters, a COD reduction ratio of
approximately 75% was achieved by using Fenton’s
reagent or other methods, a number of chemicals
were used in these studies, and they were applied
to wastewater treatment as a final process.32 – 34
In the present investigation, however, the dyeing
unit wastewater was modelled, and it was studied
directly using an indigo dyeing solution, 0.1% (w/v).
The results from Table 2 and Fig 4 show that the
COD of the solution was being remediated gradually
with time and almost 60% COD reduction was
obtained.
UV–VIS spectrophotometric investigations
The Changes in absorbance characteristics of indigo
dyeing solution were investigated over a large wavelength interval during the electrochemical decolourization process, and the results are shown in
Figure 5. UV-VIS spectra of treated solutions of indigo with a three-fold dilution (operating conditions: applied cell voltage = 40 V, pH = 1, NaCl
concentration = 0.24 mol dm−3 ). The times (in minutes) of the samples taken from the anode compartment are shown on the expanded curves.
Figure 6. The changes in the absorbance of anode compartment solutions at 681.5 nm with electrolysis time.
920
J Chem Technol Biotechnol 80:916–923 (2005)
Electrochemical oxidation of textile dye indigo
Figure 7. Changes of infrared absorption bands of indigo [Bold curve: before treatment, normal curve: after treatment, (operating conditions:
applied cell voltage = 40 V, pH = 1, NaCl concentration = 0.24 mol dm−3 )].
IR spectral studies
Figure 7 shows the IR spectra of the dried solution
residue before and after the electrochemical treatment.
It can be seen that some structural changes might have
occurred during the electrochemical process. After
electrolysis, there are peaks developing at about 3400
and 1800 cm−1 , while peaks at 1700 and 3200 cm−1
are reducing in intensity. The peaks at about 3400 and
1800 cm−1 are considered as expressions of oxime and
nitrogen–oxygen single bond stretching respectively.
The peak at about 1700 cm−1 is thought to belong to
carbon–oxygen double bonds, and the peak at about
3200 cm−1 is thought to belong to nitrogen–hydrogen
J Chem Technol Biotechnol 80:916–923 (2005)
single bonds. This variation in the IR spectra can
be explained by the production of nitrogen–oxygen
double bonds due to the radical attacks to the
nitrogen–hydrogen single bonds in the first oxidation
product of indigo, ie dehydroindigo.
Voltammetric studies
A cyclic voltammogram of aqueous solution of indigo
at its original pH value is shown in Fig 8. There is a
reduction peak at −384 mV, and when the potential
was scanned from −1.0 to +1.0 V, two oxidation peaks
appeared at a wide potential range and at +750 mV.
The latter corresponds probably to oxidation of indigo
through the –NH groups of the indol structures to
yield dehydroindigo.
+80.00
+64.00
N H
O
+48.00
H N
O
-2e- - 2H+
N
O
+2e + 2H+
N
O
+32.00
indigo
Current / µA
Fig 5. The spectra show that there is a maximum
absorbance at 681.5 nm in the visible region. This
peak disappears gradually during the electrochemical
oxidation process. There are also two peaks observed
at 290 and 350 nm respectively in the UV region which
become less intense as the electrolysis progresses.
The absorbance at 681.5 nm (Fig 6) was increased
in the first 5 min. This can be explained by the
←
leuco→indigo + 2e− equilibrium shifting in favour of
indigo at the oxidation potential of anode. Besides
this, sodium dithionite, a chemical used for preparing
the dyeing solution, may delay the electrochemical
degradation.
Colour removal is of prime importance for the textile industries. In removing the colour, 75–100 ppm
concentrations of dyeing solutions were used in technological and biotechnological studies.35,36 Manu and
Chaudhari have reported 85–90% decolourization
of indigo dye by using anaerobic mixed bacterial
cultures.17 It is clear from Figs 5 and 6 that the electrochemical oxidation process has effectively reduced the
colour of the indigo solution that has a concentration
of 0.1% (w/v), ie 1000 ppm. This result is comparable
to those of bacterial cultures.
dehydroindigo
+16.00
0.00
-16.00
-32.00
-48.00
-64.00
-80.00
+1.80 +1.40 +1.00 +0.60 +0.20 -0.20
Potential / V
-0.60
-1.00
Figure 8. CV voltammogram of dyeing solution of indigo 0.1% by w/v
between +1.8. V and −1.2 V versus Ag/AgCl. Scan rate 100 mV s−1 .
921
+1.000
+1.000
+0.750
+0.750
+0.500
+0.500
+0.250
+0.250
Current/mA
Current/mA
D Doǧan, H Türkdemir
0.000
0.000
-0.250
-0.250
-0.500
-0.500
-0.750
-0.750
-1.000
+1.80 +1.40 +1.00 +0.60 +0.20 -0.20
(a)
Potential/V
-1.000
-0.60
(b)
+1.80 +1.40 +1.00 +0.60 +0.20
Potential/V
-0.20
-0.60
Figure 9. CV voltammograms of dyeing solution of indigo 0.1% by w/v between 0.2 and 1.0V versus Ag/AgCl (operating conditions: scan rate
100 mV s−1 , pH = 1, NaCl concentration = 0.24 mol dm−3 ). a: Before colour removal, b: after colour removal.
Figure 9 shows that the cathodic regions of the cyclic
voltammograms become narrower depending on the
operating pH value (pH 1) and the reduction peak
of the indigo (Fig 8) disappears due to the hydrogen
evolution. It is interesting that there is no oxidation
peak of leucoindigo, and the anodic peak potential at
+750 mV (Fig 8) changes to +1350 mV (Fig 9), and
peak current increases approximately ten-fold. Some
current–potential curve characteristics were observed
after colour removal, but the oxidation peak at
+1350 mV (Fig 9(b)) increased two-fold, and a small
cathodic peak appeared at 1000 mV. These curves are
interesting in showing that any species which may
have different electroactivities are not formed after the
electrochemical degradation.
CONCLUSION
The electrochemical oxidation of aqueous solution of
indigo (1000 ppm) was carried out successfully in a
batch divided cell for simultaneous colour removal
and COD reduction. This solution was prepared
at the concentration anticipated in the dyeing unit
discharge. A COD reduction of 60% was obtained in
90 min, while decolourization of indigo was achieved
completely by electro-oxidation between platinum
electrodes.
The efficiency of constant potential process was
mainly influenced by chloride ions that can form
chlorine/hypochlorite species at the anodic potentials
and promote the progress of indirect oxidation.
Chloride concentration (0.24 mol dm−3 , ∼8.5 g
dm3 ), which may be seen as an environmental
problem, will decrease significantly during electrolysis,
and upon mixing with the plant’s general wastewater
after the electrochemical pretreatment, it will be
diluted at least 100-fold, and therefore will further
decrease below the discharge limits.
922
The current efficiency was not reparded as a problem as it may be related to oxygen evolution or further
decomposition of by-products and COD reduction.
Cyclic voltammetric and IR studies demonstrated that
partial degradation of the dye molecules was achieved.
The overall experimental results indicate that this
electrochemical method can be used effectively as a
pretreatment stage prior to conventional treatment.
ACKNOWLEDGEMENTS
This study is a part of the Master Thesis studies
of D Doǧan and it was financially supported by
the Research Fund Unit of İnönü University (Grant
no APYB:2002/24).
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