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1. SUSTAINABILITY OF VAT AND
SULPHUR DYEING
[Document subtitle]
By Group Members:
AFAQ AHMAD 17-NTU-0013
HASEEB AHMAD 17-NTU-0062
M. TALHA RASHID 17-NTU-0075
MUHAMMAD HASSAN 17-NTU-0113
WAQAR RAMZAN 17-NTU-0147
Submitted to: Dr. KASHIF IQBAL
MAY 10, 2020
NATIONAL TEXTILE UNIVERSITY, FAISALABAD

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Table of Contents
1 Abstract:.............................................................................................................................3
2 Introduction:.......................................................................................................................3
3 Problems of sustainability using Sulphur and Vat dyes: ...................................................4
3.1 Reducing agents containing Sulphur:.......................................................................4
3.2 Hazards of using the reducing agents containing Sulphur:.........................................5
4 Non-Sulphur reducing agents: ...........................................................................................5
4.1 Palm wine as a reducing agent:...................................................................................5
4.2 Hydrolyzed sugar as a reducing chemical:..................................................................7
4.3 Macaptoethanol as reducing agent:.............................................................................8
4.4 Reduction by indirect electrolysis:..............................................................................8
4.5 Stable reducing agents:................................................................................................8
5 Over coming the problems by optimizing the process: .....................................................9
5.1 Pad ox methods: ..........................................................................................................9
5.1.1 Pad-Ox Dyeing.....................................................................................................9
5.1.2 Pad-Dry-Ox Dyeing.............................................................................................9
5.1.3 Pad-Steam-Ox Dyeing .........................................................................................9
5.1.4 Effluent Quality in the dyeing solution after dyeing: ....................................10
5.1.5 Advantages of Pad ox methods over conventional pad steam methods: ...........10
6 Recycling of dyes.............................................................................................................10
6.1 Sustainable dyeing of denim:....................................................................................10
6.2 Recovery of vat dyes and Sulphur dyes. ...................................................................10
6.2.1 Direct reduction of vat dyes without use of chemical with electrochemical
method…..........................................................................................................................10
6.2.2 Employing a biochemical protecting group for a sustainable indigo dyeing
strategy.............................................................................................................................12
6.2.3 Electrochemical Degradation of C.I. Vat Orange 2 Dye on Carbon Electrode .13
7 Conclusion: ......................................................................................................................13
8 References:.......................................................................................................................14

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4. 3
SUSTAINABILITY OF VAT AND SULPHUR DYEING
1 Abstract:
Vat dye is very popular dye staff used for coloration of cotton, particularly when high fastness
is required. The importance of sulfur dyeing of cellulosic fibers, particularly cotton, is realized
economically throughout the dyeing industry. During vat dyeing a reducing agent is required
for vatting process. The reducing agents containing Sulphur are hazardous to the environment.
The most damaging of these is the reducing agents like sodium Sulphide or sodium
hydrosulphite (Na2S2O4) required to reduce the dye molecules to a water-soluble leuco form
to enable adsorption and diffusion into the fiber.
The conventional reducing agent sodium hydrosulphite (Na2S2O4) have replaced by the
material that have less environment side effects. Those materials include hydrolyzed sugar,
palm wine, Marcaptoethanol and stable reducing agents as the green method during dyeing of
vat dyes on cotton fabrics. Moreover, further methods like pad ox methods and electrochemical
methods of dye recycling are discussed. The study showed that the pad-ox processes produce
reduced oxygen demands of effluent with higher color yield and acceptable colorfastness.
Interestingly, a review on water and energy consumption showed that pad-ox dyeing is cheaper
than the conventional dyeing.
2 Introduction:
Textile dyeing is the process of coloration which adds color to the textile materials by using
different chemicals, dyestuffs and dyes by forming fiber-dye bond depending on the fabric and
process used. Sulphur and Vat dyes are traditionally used for the dyeing of cellulosic fibers in
varying hues. Sulphur dyes are economical, while Vat dyes show good color fastness. Sulphur
dyes are the most economical dyes ever introduce.
They are widely used in the textile industry for dyeing cotton to produce inexpensive black,
navy, brown, olive, and green shades in medium to heavy depth. Vat dyes have excellent wash
and light fastness and uses for the dyeing of cotton. But its cost is high when we compare with
others. Vat dyes are most important dyes for dyeing and printing on cotton fibers. They have
excellent colorfastness properties which includes-washing, light, perspiration and rubbing
fastness.

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The dye has the problem of insolubility in water. Hence the process of Vatting is needed to
reduce and solubilize the dye in water. The method of application is same for both dyes. In the
application of these dyes, the insoluble dyes are applied in the reduced form and are then
converted back to original pigment form by oxidation. Reducing agents used in these dyes
create major effluent problems in case of Sulphur and Vat dyes. Hence, there is a need to
develop ecofriendly reducing agents. There are various developments taken pace in the dyeing
of Sulphur and Vat dyes with ecofriendly chemicals and processes.
3 Problems of sustainability using Sulphur and Vat dyes:
There are various problems in using Sulphur and Vat dyes. Those problems are following.
➢ Reducing agents used in these dyes create major effluent problems.
➢ In case of Sulphur dyeing, the perspiration of dyes in the presence of Calcium, Magnesium
and Iron salts is problematic.
➢ This causes huge effluent load out of Sulphur dyes discharges.
➢ Wastage of water in the processes
➢ More use of hazardous chemicals
➢ Use of costly chemicals
3.1 Reducing agents containing Sulphur:
Some of the reducing agents containing Sulphur are
➢ Sodium Hydrosulphite (𝑁𝑎2 𝑆2 𝑂4) which is also known as Hydros
➢ Reducing agents of Sulphide types including
• Sodium Sulphide (𝑁𝑎2 𝑆)
• Sodium Hydrosulphide (𝑁𝑎𝑆𝐻)
• Sodium Polysulphide (𝑁𝑎2 𝑆 𝑥) ; where x varies from 1 to 6.

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3.2 Hazards of using the reducing agents containing Sulphur:
The disadvantages of Sulphide containing reducing agents are
➢ The discharge of thiosulphate formed by the atmospheric oxidation of Sulphides.
➢ This process is catalyzed by large quantities of Sulphur dyes in the effluent.
➢ The use of Sodium Sulphide as a reducing agent is toxic, bad smell and contamination for
sea and river life.
➢ The use of Sodium Hydrosulphite (Hydro) for Vat dye reduction is not environment
friendly.
➢ It decomposes to Sulphite, Sulphate, Thiosulphate and toxic Sulphur.
4 Non-Sulphur reducing agents:
There are various other reducing agents that can be used to replace the hazardous Sulphur
containing reducing agents. Those reducing agents involve
➢ Palm wine
➢ Hydrolyzed sugar
➢ Marcaptoethanol
➢ Reduction by indirect electrolysis
➢ Stable reducing agents
4.1 Palm wine as a reducing agent:
Using palm wine as reducing agent is eco-friendlier than hydrose. According to FT-IR for palm
wine, it can be suggested that there is the presence of hydroxyl (-OH), carbonyl groups (-C=O)
and (-C-O) in palm wine as palm wine shown showed strong band within these ranges. And
The H-NMR spectrum of palm wine shows that spectrum contains singlets at for the CH2
groups.
Due to following main advantages, use of palm wine as a reducing agent is more eco-friendly
➢ pH of dyed solution is lower when palm wine is used as compared to when hydrose is
used it shown in figure (1).

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Figure 1 pH of the solution after dyeing
➢ Total dissolved solid (TDS) value of residual dye bath is lower for solution when palm
wine is used rather than using hydrose.
➢ BOD values for the dye bath are lower when palm wine is used as a reducing agent. It is
shown in figure (2).
➢ It costs less treatment for effluent.
➢ COD values are lower when palm wine is used as a reducing agent.
➢ Dyed fabric using palm wine also shows better bursting strength than using hydrose.
Figure (2) BOD values for the dye bath

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4.2 Hydrolyzed sugar as a reducing chemical:
This is used for the reduction of Sulphur dyes.
Hydrolysis of sugar takes place in the following way. Firstly, sugar is dissolved in water then
heated to boiling. After this, the solution is cooled. And a dilute solution of hydrochloric acid
is used for hydrolysis of sugar.
The process of dyeing is done as follows
In this condition different samples are dyed in the same dyeing condition of Sulphur black.
Their difference is the waiting time of hydrolyzed sugar and batch of hydrolysis. The hydrolysis
of sugar is done in two batches in the same condition at room temperature. Then, the dyeing
from each batch is carried out at different waiting time intervals of the hydrolyzed sugar in the
same dyeing condition.
➢ For the 1st
batch Dyeing takes place after a wait of three days of hydrolysis and then for
40 days.
➢ For 2nd
batch, hydrolysis was done next to that of the first hydrolysis. And Dyeing is
takes place after one, two and three days of sugar hydrolysis consecutively in the same
dyeing conditions.
Waiting time means the time of glucose formation.
Generally using of hydrolyzed sugar as a reducing chemical has financial, environmental,
quality wise and formation of comfort conditions as main advantages.
Hydrolyzed sugar as a reducing chemical also has following advantages
➢ Fabrics dyed using hydrolyzed sugar as a reducing chemical show better shade depth as
compared to fabrics dyed under sodium Sulphide reduction.
➢ Bad smell and inhalation of Sulphur fumes have been removed and the substituted chemical
is much safer to handle.
➢ Quality of the dyed fabric is better in case of hydrolyzed sugar reduction.
➢ Formation of Sulphuric acid might damage the fabric while using Sulphide reducing agent.
Elimination of free sulfur from the fabric helps for the termination of formation Sulfuric.

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4.3 Macaptoethanol as reducing agent:
This reducing agent is used for Sulphur dyes. Solubilized Sulphur dyes can be applied by using
Marcaptoethanol and Caustic soda in both exhaust as well as one batch pad steam method.
Main advantages include
1) Sulphur free effluent discharge
2) Obnoxious smell emission
But it has disadvantage of high expenses and restricted use excluding ready to use chemical
4.4 Reduction by indirect electrolysis:
This method is used for the reduction of Vat dyes. In this method, the reducing potential of
cathode is transferred to the solution by soluble reversible redox system. The reversible redox
system is continuously regenerated at cathode so that the renewal of reducing gent is achieved.
This technique offers full use of dye bath including reuse of reducing agents.
The main features of this technique include
➢ Full use of reducing chemical
➢ Stable reducing conditions in the dye bath
➢ Reproducible dyeing
➢ Low concentration of chemicals employed
➢ Simple wastewater treatment
➢ Recycling of chemicals and wastewater
4.5 Stable reducing agents:
Sulphoxylate product resulting by the reaction of zinc and a molecule of bisulphite is found to
be more stable than hydros. The reaction takes place in the following way
𝑁𝑎2 𝑆2 𝑂4 + 2𝐻𝐶𝐻𝑂 + 𝐻2 𝑂 → 𝑂𝐻𝐶𝐻2 𝑆𝑂2 𝑁𝑎 + 𝑂𝐻𝐶𝐻2 𝑆𝑂3 𝑁𝑎
The reaction is followed by reduction in the presence of Zinc to produce Zinc Sulphatooxylate
Formaldehyde.
The addition of Sodium borohydride (NaBH4) as a stabilizer shown some good results. It has
ability to reduce Sodium hydrosulphite. It means that it can replenish hydrosulpite. Sodium
borohydride (NaBH4) is most active at pH close to 7.

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5 Over coming the problems by optimizing the process:
5.1 Pad ox methods:
Sulfur dyeing of cotton textiles is widely practiced in textile industry for producing inexpensive
black, navy, brown, olive, and green shades in medium to heavy depths. As a part of sulfur
dyeing process, intensive rinsing is carried out to remove unfixed dyes after dyeing. The
unfixed dyes produce high sulfide content and hence undesirable levels of oxygen demands to
the effluent. Clariant Ltd. introduced eco-sustainable processes for sulfur dyeing, generally
known as padox, using Diresul RDT sulfur dyes. As a part of Sulphur dyeing process, intensive
rinsing is carried out to remove unfixed dyes after dyeing. The unfixed dyes produce high
sulfide content and hence undesirable levels of oxygen demands to the effluent.
Clariant Ltd. introduced eco-sustainable processes for sulfur dyeing, generally known as
padox, using Diresul RDT sulfur dyes. This new process enables 100% fixation of the dyes to
substrate; hence, no dye is drained in rinsing.
There are different types of this method.
5.1.1 Pad-Ox Dyeing
The process route is as follows
➢ Fabric samples are padded at 60℃
➢ Then the fabrics are subjected to ambient airing for at least one minute
➢ After this, they are oxidized at 75 ℃ at a pH of 4–4.5 for at least one minute
➢ Then they are rinsed at 75 ℃ for at least three minutes
➢ hydro-extracted
➢ At last, the fabrics are dried (ambient air).
5.1.2 Pad-Dry-Ox Dyeing
It differs from the previous method in such a way that, the drying is done at 100℃ after
padding. The remaining process route is same as above.
5.1.3 Pad-Steam-Ox Dyeing
This method consists of padding the solution with dye liquor and then the process of steaming
is performed at a temperature of 102℃. After this, the oxidation is done. The remaining process
route is same as above.

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5.1.4 Effluent Quality in the dyeing solution after dyeing:
The new processes provide a 10–18% reduction in COD and 15–23% in BOD. The pad-dry-
ox gave better reduction in oxygen demands among the three new processes. These are
encouraging results for ecological benefit and may be for economic benefit in terms of costs of
effluent purification or load-based penalties on more polluted effluent.
5.1.5 Advantages of Pad ox methods over conventional pad steam methods:
The main advantages of this process over conventional pad steam method are
➢ The new pad-ox processes for dyeing cotton with Sulphur dyes are effective for reducing
effluent load and the cost.
➢ The processes produced higher color yields and acceptable colorfastness.
➢ 15–23% reduction in oxygen demands of the effluent
➢ 90% reduction in the amount of water use
➢ considerable reduction in the amount of energy consumption
➢ No use of soaping chemicals
6 Recycling of dyes
6.1 Sustainable dyeing of denim:
This is possible using indigo dye recovered with PVDF ultrafiltration membranes.
Indigo is one of the most consumed dyes in the textile sector, as it is widely used for the dyeing
of denim clothes. About 15% of indigo used in the dyeing process is discharged to the
wastewater treatment plants or sometimes into rivers, in countries where regulations are not
strictly applied. The studied membranes achieved up to 99% color removal and 80% chemical
oxygen demand (COD) decrease. Finally, the concentrates containing indigo dye are reused in
new dyeing processes. Indigo dye from wastewater can be successfully removed by means of
PVDF ultrafiltration membranes.
6.2 Recovery of vat dyes and Sulphur dyes.
6.2.1 Direct reduction of vat dyes without use of chemical with electrochemical
method
Until now, in most industrial vat dyeing processes, vat dyes are reduced mainly using sodium
dithionite. This process produces large amounts of sodium sulphate and Sulphite as by-products
which increase the costs for wastewater treatment. Hence, many attempts are being made to
replace the environmentally unfavorable sodium dithionite by ecologically more attractive

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alternatives, such as organic reducing agents or catalytic hydrogenation. In recent
investigations to improve the biocompatibility of the vatting process even further, various
electrochemical reducing methods have been described, such as indirect electrochemical
reduction employing a redox mediator, direct electrochemical reduction of indigo via the indigo
radical, electrocatalytic hydrogenation and direct electrochemical reduction of indigo itself on
graphite.
These methods offer tremendous environmental benefits, since they minimize the consumption
of chemicals as well as effluent load. However, most of these electrochemical processes are
still in the development stage. This gives an overview of the processes most commonly used
and the state of development of recent electro- chemical innovations. There are different
techniques:
➢ Vat process with an ultrasonic reactor
➢ Catalytic hydrogenation—pre-reduced dye
➢ Electrochemical techniques involving
• Mediator-enhanced electrochemical reduction
• Direct electrochemical reduction of indigo via the indigo radical
• Direct electrochemical reduction of indigo on graphite electrodes
• Stabilization or regeneration of dithionite by electrolysis
The mechanism of the direct electrochemical method is shown in Figure (3).
Figure (3) Mechanism of the direct electrochemical method

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Figure (4) Electrolytic hydrogenation of indigo
Figure (4) presents the electrolytic hydrogenation of indigo dye. This technique also makes use of
Electrochemical method for the recovery of Vat dyes.
6.2.2 Employing a biochemical protecting group for a sustainable indigo dyeing
strategy
Indigo is an ancient dye uniquely capable of producing the signature tonesin in blue denim;
however, the dyeing process requires chemical steps that are environmentally damaging. We
describe a sustainable dyeing strategy that not only circus vents the use of toxic reagents for
indigo chemical synthesis but also removes the need for a reducing agent for dye solubilization.
The strategy utilizes a glucose moiety as a biochemical protecting group to stabilize the reactive
indigo precursor indoxyl to indicane, preventing spontaneous oxidation to crystalline indigo
during microbial fermentation.
Application of a b-glucosidase removes the protecting group from indicane resulting in indigo
crystal formation in cotton fibers. We identified the gene coding for the glucosyltransferase
PtUGT1 from indigo plant Polygonum tinctorium and solved the structure of PtUGT1.
Heterologous expression of PtUGT1 in Escherichia coli supported high indicane conversion
and biosynthesized indicane was used to dye the cotton.

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6.2.3 Electrochemical Degradation of C.I. Vat Orange 2 Dye on Carbon Electrode
The electrochemical degradation of industrial wastewater has become an attractive method in
recent years. In this work simulated dye wastewater containing vat dye C.I. Vat Orange 2 is
degraded from electrochemical method using graphite carbon electrodes. The experimental
results indicated that initial pH, current density and supporting electrolytes were played an
important role in the degradation of dye.
Electrochemical behavior of dye has been studied with cyclic voltammetry in basic medium
using glassy carbon as working electrode. The potentials selected for the dye was in the range-
0.4 to-1.2 V. The UV-Vis and chemical oxygen demand (COD) studies were selected to
evaluate the degradation efficiency.
The maximum color removal efficiency of 99.24% and chemical oxygen demand (COD)
removal of 72.26% could be achieved for dye, at 25 g L-1 of NaCl concentration. The LC-MS
and FTIR studies revealed the degradation of dye and confirmed that aromatic rings were
destroyed. The results revealed the suitability of the present process for the effective
degradation of dye C.I. Vat Orange 2.
There are other many articles I have studied on the sustainability of vat dyeing some are related.
The above all data I have studied from journals, articles and books. I explain some articles
more because it crucial for explanation.
7 Conclusion:
The use of Sulphur containing reducing agents is the main problem for the Vat dyes to be
hazardous to the environment. So, the use of non-Sulphur reducing agents makes the
sustainability of Vat and Sulphur dyes possible.
Employment of reducing sugars in sulfur dyeing could provide a sustainable, nontoxic,
biodegradable, cost-effective alternative to sodium polysulfide and sodium hydrogen sulfide.

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The Green dyeing process of vat dyes with palm wine enhanced sustainability of Vat and
Sulphur dyes rather than hydrose by decreasing BOD, COD, TDS and pH. Furthermore, the
employment of Marcaptoethanol and Stable reducing agents also replaced Sulphur containing
reducing agents and creating a non-toxic environment resulting in the improvement in
sustainability.
The introduction of pad-ox processes produce reduced oxygen demands of effluent with higher
color yield and acceptable colorfastness. Interestingly, a review on water and energy
consumption showed that pad-ox dyeings are cheaper than the conventional dyeing.
The use of electrochemical methods provided incredible environmental advantages, since they
minimize the use of chemicals as well as effluent load. They cause the saving of water and
energy. Moreover, the recycling of dyes is possible using these methods.
8 References:
Khatri, A., Salam, A., Absarullah, F., & Anwar, R. (2011). Improved Sustainability of Cotton
Sulfur Dyeing by Pad-Ox Processes. Energy, Environment and Sustainable Development, 229–
235. doi:10.1007/978-3-7091-0109-4_23
Buscio, V., Crespi, M., & Gutiérrez-Bouzán, C. (2015). Sustainable dyeing of denim using
indigo dye recovered with polyvinylidene difluoride ultrafiltration membranes. Journal of
Cleaner Production, 91, 201–207. doi:10.1016/j.jclepro.2014.12.016
Hsu, T. M., Welner, D. H., Russ, Z. N., Cervantes, B., Prathuri, R. L., Adams, P. D., & Dueber,
J. E. (2018). Employing a biochemical protecting group for a sustainable indigo dyeing
strategy. Nature Chemical Biology, 14(3), 256–261. doi:10.1038/nchembio.2552
Roessler, A., & Jin, X. (2003). State of the art technologies and new electrochemical methods
for the reduction of vat dyes. Dyes and Pigments, 59(3), 223–235. doi:10.1016/s0143-
7208(03)00108-6