Electrochemical dyeing

1. Electrochemical dyeing
Abstract
The textile industry uses the electrochemical techniques both in textile processes
(such as manufacturing fibers, dyeing processes, and decolorizing fabrics) and in
wastewaters treatments (color removal). Electrochemical reduction reactions are
mostly used in sulfur and vat dyeing, but in some cases, they are applied to
effluents discoloration. However, the main applications of electrochemical
treatments in the textile sector are based on oxidation reactions. Most of
electrochemical oxidation processes involve indirect reactions which imply the
generation of hypochlorite or hydroxyl radical in situ. These electro generated
species are able to bleach indigo-dyed denim fabrics and to degrade dyes in
wastewater in order to achieve the effluent color removal. The aim of this paper
is to review the electrochemical techniques applied to textile industry. In
particular, they are an efficient method to remove color of textile effluents. The
reuse of the discolored effluent is possible, which implies an important saving of
salt and water (i.e., by means of the “UVEC Cell”).
Electrochemical dyeing:
Condition required for different dyes:
Sufficient –ve reduction potential
450 mV to - 700 mV (for Sulphur and Sulphur vat dyes)
700 mV to - 800 mV (for Indigo dye)
750 mV to - 1000 mV (for all Vat dye)
Stable reduction condition in the dye bath.

2. The technique relates to a process for electrochemical
reduction of reducible dyes. Electrochemical reduction
of organic compounds in the presence of a cathode
comprising a support of an electrically conductive
material and an electrically conductive, catholically
polarized layer formed there on formed in-situ. Useful
metals include preferably all classic hydrogenation
metals, especially the metals of the 1ts, 2nd and 8th transition group of the
Periodic Table. Elements, especially Co, Ni, Fe, Ru, Rh, Re, Pd, Pt, Os, Ir, Ag, Cu, Zn,
Pb and Cd, Ni, Co, Ag, Fe and Cu are preferably. There are two methods by means
of which electrochemical dyeing can be carried out, direct electrochemical dyeing
and indirect electrochemical dyeing.
Fig. Electrochemical methods
Direct electrochemical dyeing:
In case of direct electrochemical dyeing technique, organic dyestuff has been
directly reduced by contact between dye and electrode. In order to start the
process, an initial amount of the leuco dye has to be generated by a conventional
reaction, i.e., by adding a small amount of a soluble reducing agent. Once the
reaction has set in, it is not needed anymore and further process is self-sustaining.
In such a system, a dyestuff particle must come into contact with the electrode
surface in order to get reduced.

3. Indirect electrochemical dyeing
In this system, the dye reduction does not take place due to direct contact of
dyestuff with the cathode, like in direct electrochemical reduction. The dye is not
directly reduced at the electrode rather, a reducing agent is added that reduces
the dye in the conventional manner, which in turn gets oxidized after dye
reduction. The oxidized reducing agent is subsequently reduced at the cathode
surface, which is then further available for dye reduction. This cycle is
continuously repeated during the dyeing operation. The agent, which undergoes
reduction and oxidation cycles, is known as a mediator.
Dyeing procedure
The electrolysis is carried out under galvanostatic condition by maintaining the
constant current. The pretreated fabric sample is introduced into the dye bath.
The dyeing is carried out by exhaustion method for 30 minutes with constant
stirring. Both electrolysis and electrochemical dyeing are carried out at 300 ± 2 K.
After completion of dyeing, the fabric sample was washed with cold water and
exposed to air, for oxidation/fixation of dye molecules. Then the fabric was
soaped at boil, rinsed with cold water, and air dried.
The electrochemical reduction process
The reducing agent in vat dyeing represents an electron source for the reduction
of dispersed dyestuff particles. In case of electrochemical dyestuff reduction
electrons are transferred from an electrode (cathode) to the dispersed dyestuff.
In some cases, this transfer can be achieved by immediate electron transfer from
the cathode to the dyestuff = direct electrochemical reduction, in case of
dispersed vat dyes a regenerable redox system (mediator) is required to transport
the electrons from the cathode to the dispersed dye = indirect electrochemical
reduction. In the Eureka Project E!2625 ECDVAT – Electrochemical Dyeing with
VAT dyes - package dyeing electrochemical dyeing was realised for package
dyeing where the added reducing agent is replaced by the cathodic electron

4. transfer which is performed in an electrochemical cell [2]. Figure 2 shows a flow
scheme the technical unit for package dyeing of 110 kg yarn (X-cones). The 500 A
cell is coupled to the dyeing apparatus and the dyebath is reduced
electrochemically. The oxidised dyestuff is added to the dyebath and reduced
therein by means of the mediator. The redox potential required for proper
dyestuff reduction is formed by cathodic reduction and permits the dyer to adjust
the redox potential in the dyebath by control of the cell current. At the point of
completed dyestuff reduction, the cell current is lowered to the minimum height
required to maintain the redox potential within well defined limits. In this phase
typical values of cell current are around 50 – 100 A. The dyebath and the
reductive rinse contain mediator and thus are collected in the regeneration tank
(used dyebath in Figure 2). Reuse of the mediator system respectively dyebath is
possible after removal of the oxidised dye by nanofiltration. dyestuff oxidized
dyestuff reduced reduction electron-transfer: reducing agent cathode/mediator
capacity of reducing agent to compensate oxidative loads equilibrium in dyestuff
exhaustion fibre finishing: oxidation soaping dyebath regeneration including
regenerable reducing agent dyed good fibre Basic chemical steps in vat dyeing on
molecular stage The dyeing process is completed with oxidation, soaping and
rinsing.
Figure 2. Scheme of the installation: Package dyeing apparatus, circulation
through cell, regeneration of used dyebath by Nano-filtration.

5. Figure 3. Installation of ECDVAT dyeing unit at Getzner Textil AG. 1 Storage tank,
2 cell current supply, 3 cell, 4 exhaust fan, 5 dyebath circulation, 6 dyeing
apparatus.
This installation included the following main elements (Figure 3): Electrolysis cell
for indirect electrochemical reduction of dispersed dyestuff. A new designed 500
A multi cathode electrolyser was developed for this application. Particularly the
relative low current densities required high cathode area. A special construction
had to be designet to run the electrolyser at the relative high dyebath
temperature of nearly 80°C. Minimum cell volume had to be realised to keep
liquor 1 2 3 4 6 5 1 2 3 4 6 5 ratio of dyeing as low as possible. At present the 500
A cell required a filling volume of 160 l for normal operaiton. Figure 4 shows front
view of the electrolyser and in Figure 5 a top view inside the specially designed
flow cell is shown. Relevant technical data of the installation are summarised in
Table 1.

6. Figure 4. Electrolyser (the arrow indicates the flow of the dyebath through the
cell)
Effect of dye concentration and material to liquor ratio

7. Electrocoagulation Methods
Electrocoagulation systems provide electrochemical aggregation of heavy metals,
organic and inorganic pollutants, to produce a coagulated residue to be separated
or removed from water.
This technique is an indirect electrochemical method which produces coagulant
agents (Fe3+
or Al3+
) from the electrode material (Fe or Al) in hydroxide medium.
These species, that is, Fe(OH)3, can remove dissolved dyes by precipitation or by
flotation [78, 79]. These complexed compounds are attached to the bubbles
of H2(gas) evolved at the cathode and transported to the top of solution. The
inconvenient of the electrocoagualtion in comparison to the other electrochemical
methods is that it produces secondary residues (the complex formed with pollutant
and hydroxide) which implies the use of tertiary treatments.
Electrochemical Reduction Methods
The electrochemical reduction method has been discussed in a restricted number
of papers because its yield in pollutants degradation is poor in comparison to direct
and indirect electro-oxidation methods. Bechtold et all consider that this method
is particularly suitable for the treatment of highly colored wastewaters such as the
residual pad-batch dyeing bath with reactive dyes. The dye reduction takes place
producing hydrazine (in the partial reduction) and its total reduction generates
amino compounds. They remark the importance of a divided cell in the case of dye
baths containing chlorides; this division is important to avoid the formation of
chlorine and chlorinated products.

8. Electrochemical Oxidation Methods
The electrochemical oxidation is a process based on pollutants removal by direct
anodic oxidation (which generally produces poor decontamination) or by chemical
reaction with electrogenerated species (hydroxyl radical M[OH•] or metal oxide
[MO], as it is showed in the follow reaction) [71]. The reactive dyes degradation can
be partial or total, according to the following mechanism:
Reaction: H2O+M→MOH+H++e−R+MOH→M+RO+H++e−
Many studies have shown that the total mineralization is possible with high
efficiencies depending on the anode material (SnO2 ,PbO2,BDD,
Ti/SnO2/SbO𝑥/RuO2, and Ti/TiO2 ). However, the dye solution is not decolorized
effectively using both glassy and reticulated vitreous carbon electrodes. The boron-
doped diamond (BDD) thin-film electrodes have physical characteristics as an inert
surface with low-adsorption properties, good corrosion stability, and a wide
potential window in aqueous medium. In spite of its high cost, the BDD electrode
has much greater O2-thovervoltage than de conventional anodes (Pt, PbO2, etc.).
Consequently, that produce generates more amount of [OH•] which implies a
faster oxidation of the pollutants.
In the same way, Martínez-Huitle and Brillas compared different kinds of electrodes
in two types of wastewaters (chloride-free dye wastewaters and effluents which
contain chloride). They supported that most of the anodes tested could destroy the
chromophore group (–N=N–) producing its discoloration efficiently, and when
chloride was present, the destruction of dyes was accelerated by active chlorine
species produced.
Photo-Assisted Methods
The photoassisted electrochemical methods are based on the exposure of the
effluent to a UV light source during the electrochemical treatment. In these
procedures, the intensity and the wavelength of the incident light plays an
important role on the mineralization rate.
The most studied photoassisted method is the photoFenton , which consists in the
simultaneous use of UV light and H2O2 (electrogenerated in situ with the presence
of Fe2+
); followed by the heterogeneous TiO2 photocatalysis method [106].
Although several photocatalysts (TiO2, WO3, SnO2, ZnO, CdS…) act via hydroxyl

9. radical and generate powerful oxidants, the TiO2 under UV radiation has been the
preferred catalyst, due to its low cost, nontoxicity, water insolubility and wide band
gap, which consequently implies a good stability and prevents photocorrosion .
Moreover, Carneiro et al noted that the use of photocatalysis with
Ti/TiO2 electrodes achieves efficiently discoloration with both electrolytes, NaCl
and Na2SO4. Their efficiency depend on the pH.
Xie and Li reported the coupling of electro-Fenton with electrocatalysis for the
removal of an azo dye. With respect to other electro-oxidation and photoassisted
methods, their results showed a better removal of the dye in the coupled system.
The major disadvantage of these methods was the excessive energy cost of the
artificial UV light used. However, this problem is easy to solve by using sunlight as
inexpensive energy source although it had less catalytic power.
Additionally, the combination between the indirect oxidation methods with the UV
irradiation has been the subject of recent investigations. According to Sala [116],
the energy consumption is around 5.7kW⋅h/m3 to achieve discoloration higher
than 90% when the photoelectrochemical treatment is applied to real industrial
effluents by means of a semi-industrial pilot. The discoloration process follows a
pseudo-first-order kinetic in the case of monochromies and a second order kinetics
in the case of trichromie, evaluated at the maximum absorbance wavelength of the
trichromie, which corresponds mainly to the contribution of two dyes (due to the
low absorbance of the third dye at the selected wavelength).
By another hand, the actual policies concerning water and energy consumption
conduce to recycling and reuse treatments. In this sense, recent studies [104]
demonstrate the possibility of reusing these discolored effluents for new dyeing
processes. The reuse of 70% of discolored dyebaths, after electrochemical
treatment assisted by UV irradiation, provides in most of cases, low color
differences (DECMC) with respect to the original dyeing with decalcified tap water.
This value increases from the first step until the 4th or 5th cycle of electrochemical
treatment and reuse, where DECMC become constant. In some cases, when the
bath is reused, an extra amount of dye must be added to obtain the required color.
Numerous studies can be found about the electrochemical discoloration of textile
wastewater, but some authors have advanced a further step: in the case of indirect
oxidation with active chlorine, the conditions for the effluents reutilization have
also been optimized [99, 100, 104]. In this sense, Gutierrez-Bouzan et al. in a recent
patent (ES201131159) claimed a process “UVEC Cell” for the discoloration and

10. reuse of reactive dyes effluents in a new dyeing process. Both prototype and
procedure are patented, based on an electrochemical cell combined with UV
source for the treatment and reuse of textile effluents saving more than 60% water
and electrolyte.
Effect of dye concentration and material to liquor ratio
Even dyeing was noticed for all MLR values, but depth
of the dyeing varied with increasing MLR. When
compared the better shade was obtained with MLR
value of 1:60. Thus, the liquor amount can be reduced
from 240 to 60 litters. The low MLR value saves the
consumption of chemicals, water, and cost of the
dyeing process.
Table 1: electrochemical dyeing process by using different ML
Exp. No.Concentration of Dye( mg)%age shadeMLRK/S valueL* a* b*
1. 100 0.8 1:80 2.27 34.13-43.101.87
2. 100 0.8 1:60 2.42 32.24-41.541.64
3. 100 0.8 1:30 2.65 30.15-40.251.75
4. 200 1.6 1:80 3.80 23.72-28.560.43
5. 300 2.5 1:80 6.39 19.52-16.95-0.44
6. 400 3.3 1:80 6.91 18.70-16.45-0.64
7. 500 4.2 1:80 6.42 19.15-17.780.77

11. Effect of electrode material
Both the conversion efficiency and the current efficiency values obtained for the
electro-generation of Fe(II)-oxalate-gluconate system employing different
cathode and anode materials. Both platinum and stainless steel may be employed
as anode material. Stainless steel is the material of choice as anode due to cost
consideration. Copper plate as well as wounded copper coil could be employed as
the cathode material with reasonable efficiency.
Table 2: electrochemical dyeing process by using different electrode
Experiment
No.
Anode
Material
Cathode
Material
Conversion
Efficiency (%)
Current
Efficiency (%)
1. Platinum Copper Plate 67.74 47.15
2. Stainless
Steel
Copper Plate 73.23 56.23
3. Stainless
Steel
Copper
Wounded
68.75 56.27
4. Stainless
Steel
Stainless Steel 46.76 34.41
5. Stainless
Steel
Mild steel 20.00 20.69
6. Stainless
Steel
Copper plate 33.33 25.85
Effect of electrolyte composition
Studies were carried out to determine the optimum concentration of ferric
sulfate, oxalic acid, and Ca-gluconate. The conversion efficiency increases when
the ligand concentrations are significantly higher than the Fe(III) concentrations.
When the ferric sulfate concentration increased beyond 25 mm by retaining the
oxalic acid concentration at 100 mm and calcium gluconate concentration at 50
mm, the conversion efficiency decreases probably due to the insufficiency of
ligands for complete complex formation.

12. Table 3: electrochemical dyeing process by using different electrolyte
Exp.
No.
Concentration of
mediator composition
Conversion
Efficiency (%)
Current
Efficiency (%)
K/S value
Fe2(SO4)3
(mol L-1
)
Oxalic acid
(mol L-1
)
Ca-gluconate
(mol L-1
)
1 0.015 0.100 0.050 61.05 35.46 2.08
2 0.020 0.100 0.050 73.23 56.23 2.27
3 0.025 0.100 0.050 58.07 46.22 1.35
4 0.075 0.100 0.050 55.50 30.66 1.13
5 0.020 0.0 0.050 30.54 26.74 1.52
6 0.020 0.025 0.050 47.31 29.06 1.57
7 0.020 0.075 0.050 50.25 37.66 2.05
8 0.020 0.150 0.050 54.70 34.38 2.02
9 0.020 0.100 0.005 38.10 13.71 1.37
10 0.020 0.100 0.025 46.66 24.55 1.49
11 0.020 0.100 0.075 58.55 37.99 1.47
Effect of complexing agents
The mediator system used in electrochemical dyeing is not stable under highly
alkaline condition. Industrial vat dyeing is preferably carried out above pH 12 and
very few Fe (III) complexes are stable in such alkaline conditions. The mediator
system gets precipitated out, so not works properly. In presence of excess
gluconate, the Fe (III)-oxalate complex does not lead to any hydrolysis or
precipitation. The pH of ferric-oxalate-gluconate system could be raised beyond
13.
Effect of the current intensity
More dye reduction can be achived at a faster rate by utilising higher current
values but heating effect and limited freedom for reaction. When operating with

13. a galvanostatic mode, the current intensity is a crucial factor. Indeed, low current
values will result in high selectivity of the electrochemical reaction but long lasting
electrolyses. High current values will produce short experiments but a bad
selectivity and a temperature increase due to joule effect.
Effect of pH
The reaction rate is enhanced with increasing pH. This effect is mainly based on
higher radical concentration, because it has been observed that the equilibrium
between radical, indigo and luceo indigo is shifted to radical side higher pH. The
non-ionic forms of indigo are poorly water-soluble substances.
Additional benefits of electrochemical dyeing
Liquor recycling possible
In electrochemical dyeing experiment the dye liquor recycling loop is repeated at
least 9 to 10 times. The same catholyte solution after dyeing is air oxidized and
filtered to remove the insoluble dye molecule (10 - 15% fresh electrolyte). The
conversion as well as current efficiencies remain reasonably same during each of
this recycling experiment. The dye intensity measured as K/S values were also
found to be reasonably stable in all the 10 dyeing recycles.
Stability of electrolyses
The pH is an important parameter influencing the performance of vat dyes
reduction. The Vat dyes are found in different form depending on the dye bath
pH. In conventional reduction, this parameter must be constantly controlled
because the decomposition of dithionite generates a decrease of the pH values,
which can cause some disturbances. Dyebath pH is more stable in the
electrochemical process than in the conventional one. Hence, there is no need for
the addition of alkali during electrolyses.

14. Saving
An estimation of chemical savings can be made on basis of the charge flow
required to reduce a certain amount of dyestuff. For sulphure dye the chemical
saving calculated as 0.644 kg dry SB1 (Sulphur Black 1) was reduced by a charge
flow of 57.5 Ah at a voltage of maximum 7.5 V. This corresponds to an amount of
3.33 mol reducing equivalents per 1 kg dry SB1 and to an energy consumption of
0.67 kWh/kg dry SB1 filter cake. When sulphide reducing agents are used instead
of cathodic reduction the amount of Na2S can be calculated from Equation:
According to Faraday’s law 3.33 moles equal an amount of 129 g Na2S, which will
be required to reduce 1 kg of dry SB1 filter cake.
An estimation of water and chemical saving in dyeing cotton with Vat dye is made
and the results are given in the table. It was found discharge in this technique was
approximately 15% of the liquor volume.
Online dye bath analysis
Controlling the bath composition is of great interest. ECD allows us to avoid a
useless prolongation of the reduction time and to prevent the phenomena of over
or partial reduction, which are frequent and difficult to control when reduction is
carried out by dithionite. And the final dyed fabric is of better quality because full
control over dyeing parameters is possible.

15. Electrochemical dyeing parameters
Determination of conversion efficiency and current efficiency
Conversion efficiency was calculated using initial and final concentration of Fe(III)
ion, which is determined by iodometric titration.
The current efficiency (f) was calculated using the following equation
Where, Z is the number of electrons involved the reaction, F is Faraday constant
(C mol-1
), V R is the volume of solution (L), C is the concentration of molecule (g L-
1
), M is molecular weight (g mol -1
), I is cell current (A), and it is time (s).
Reduction capacity of the mediators systems
In this theory as per Nernst equation, any desired reduction/oxidation potential
(upto the liberation of hydrogen gas) can be achieved in solution by combining an
appropriate proportion of reduced and oxidized species.
E = Eo + RT/nF . lnCox/Cred
Where,
Eo = Standard potential of the redox pair (under the experimental conditions) in
mV
E = Potential prevailing in the solution (mV)
R = Gas constant (8.314 J/Kmol)
F = Faraday constant (96,500°C)
T = Temperature (K)
n = Electrochemical valiancy
Cox = Concentration of the oxidized form of redox pair (mol/l)

16. Cred = Concentration of the reduced form of the redox pair (mol/l)
Conclusion
The electrochemical techniques have been proved to be efficient in different
oxidation or reduction steps of the textile processes such as: bleaching denim
fabrics or reduction of sulfur and vat dyes, where their applications are available in
both natural and synthetic fibers. They constitute a less harmful alternative than
the traditional processes. They also have been studied in new textile fields, such as
in the production of conductive polymers used as fibers which are applied in smart
textiles to produce fabrics with new functions.
In addition, the electrochemical treatments have been extensively applied to the
decontamination of wastewaters from the textile processes. They have been
mainly used in the removal of residual reactive dyes, but also in the discoloration
of acid and disperse dyes effluents. Taking into account the considerable amount
of salt contained in the reactive dyes residual dyebath, the best method for the
treatment of these effluents is the indirect oxidation with chlorine, because of the
following:(i)The degradation takes place in the same bath.(ii)The addition of
chemical reagents is not required (the residual salts act as electrolyte).(iii)The
modification of the pH is not necessary.(iv)No solid waste is generated.(v)The reuse
of the treated effluent for new dyeing is possible, which implies a saving of 70%
water and 60% salt.
The combination with UV and solar irradiation improved the discoloration kinetic
rates in different electrochemical techniques, and in some cases, the UV light
exposure also removes the undesirable compounds (such as chloroform) or avoids
their generation according to the patent ES201131159 (“UVEC Cell”).
The possibility of reusing dyeing effluents treated by electrochemical methods is
particularly interesting and it implies an important saving of water and salt. This
kind of studies are especially important in Mediterranean countries, where the
river flow rates are low and their salinity is nowadays an increasing environmental
problem.
The bases of electrochemistry are simple but, as showed in this review, it is possible
to find the application of these techniques in a wide range of textile processes.

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