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Environmental fate & pathways

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It should be noted that the test substance is not considered as posing a hazard to the aquatic environment. Indigo is a natural dye which is gained fromIndigofera tinctoriaandIndigofera suffruticosaand is known to be not readily biodegradable.

As reported by Professor Seefelder in his publication “Indigo in culture, science and technology” [Seefelder M. (1994)] Indigo is a natural dye that already has been used globally since more than 5000 years. Neither remnants of dye on the ancient textiles nor the precursors (indoxyl) nor degradation products (anthranilic acid, isatin, isatoic anhydride) in nature so far to our best knowledge gave rise to any health or environmental concern. The substance Indigo as registered is identical to this colorant. Synthetic Indigo is a solid under all environmental conditions and is insoluble in water (< 0.05 mg/L) with an n-octanol/water partition coefficient of log Kow = 2.7. It has a low volatility based on a vapour pressure of < 3.5E-5 Pa at 100°C and sublimes at ca. 170°C before melting; thus compartmentalising into air compartment is not expected.

Based on the physico-chemical data of Indigo, it is expected to be found predominantly in soil with an estimated half-life of 75 days (PBT-profiler) – to a low extent in water and not in sediment. It is very efficiently adsorbed in wastewater treatment plants (88% to 97%) as shown in adsorption studies. However, with a log Koc of 2.85 it has a comparably low affinity for soil/sediment.

Possible sources for emissions of Indigo might be either in its production, its use in the textile industry or in its end-use (disposal of dyed articles or wash-off in household washing of a dyed article). Since colouration not only is affecting the aesthetic merit, wide ranges of technologies have been developed for the removal of dyes from water. These include physical methods, chemical methods and biological methods.


Physical methods using various filtration technologies for the dye removal from water are installed in production of Indigo to avoid any loss of synthesis product and thereby granting highest production yields.

Reduced, soluble Indigo (Leuco Indigo) in process water is oxidised to its insoluble form and completely separated by filtration. This Indigo is reduced again and re-circulated to the production process. The efficiency of this technique is 99.98%, which means that almost all of the Indigo is removed from the process water and re-used. This is analytically verified by quantitative spectrophotometric analysis of Leuco Indigo in the effluent after adding a reducing agent or by potentiometric titration of the Indigo with hydrosulphite.

By using these techniques, the process water of DyStar Indigo production passed to the effluent treatment plant of BASF/Ludwigshafen contains Indigo in concentrations below the limit of quantification.

For the production plant, no wastewater containing Indigo is released into the environment.

Exposure of soil and sediment can therefore be excluded.

Use in Textile Industry

More than 95% of Indigo is used in textile dyeing (95% cotton (denim), 5% wool (carpet)); all other uses of Indigo (e. g. pigment for artists; art print) are niche uses, often without resulting wastewater, and therefore not relevant for assessment.

Indigodyeing is a combination of a continuous and exhaustion dyeing process and the remaining dyeing bath after the dyeing process is not released into the environment but stored to be used in the next dyeing campaign.Indigo is only soluble in its reduced form at a strong alkaline pH. To avoid contact of the dye-bath with oxygen and hence oxidise the Leuco Indigo to the insoluble Indigo form, these dye-boxes are usually kept in a closed system with an inert gas (e.g. nitrogen) atmosphere. Dyeing is done at room temperature using dyeing liquors containing max. 0.6% Indigo.

Indigo in its reduced soluble form (Leuco Indigo, yellow) has a defined affinity to cotton and this property is used for this continuous process. During the dyeing process, Leuco Indigo migrates from the dye-bath to the fibre and in the same time, fresh Indigo has to be metered continuously into the dye‑bath to balance out the exhaustion. After this step, the yarn is conveyed to an airing passage where the Leuco Indigo is oxidised to its insoluble pigment form, which is blue.

This process has usually to be repeated between 3 to 12 times depending on the depth of shade the dyer has to achieve. Standard dyeing machines have 3 to 12 of these dye-boxes; each of them contains a dye-bath volume between 200 and 1500 litre. This leads to dye-bath volumes of 600 to 18000 litre, which are never drained after a dye set is finished. These dye-bathes go to stock tanks and wait for the next time the same or a similar shade has to be dyed.

Figure 1: Standard dyeing machine

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Because of the used technology there is no contamination of the effluent with Indigo sourced by the dye-bath itself.

To achieve the requested colourfastness properties of the finished textile it is necessary to rinse out the unfixed particles of insoluble Indigo from the surface of the yarn in the last wash boxes of a used dyeing machine. Usually, 2 to 5% of the consumed Indigo is washed off. Re-usage of this washed-off fraction is possible and recommended due to the economic impact. To regain this amount of Indigo which is re-useable DyStar offers to Downstream Users (DU) a filtration technology which has proven its efficiency in DyStar production sites. A pilot unit for trials is available and can be ordered for test runs. Below there is a systematic scheme of this patented technique.

Figure 2: Recycling Washing Water

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The efficiency of this technique is 99.98%, which means that almost all of the Indigo is removed from the process water and re-used. This is analytically verified by quantitative spectrophotometric analysis of Leuco Indigo in the effluent after adding a reducing agent or by potentiometric titration of the Indigo with hydrosulphite. Due to this technique, no considerable concentrations (below limit of quantification) of Indigo in the dye-house effluent to end in soil, sediment or water have to be expected.

Furthermore, according to information from the DUs gained in a very close technical exchange, there is standard technology used for the removal of Indigo from the effluent. In consistency to the methods described by Pereira et al. (2012)], this consists of aeration basins followed by addition of flocculation agents and sedimentation basins. The effluent sludge is filtered to be dried and disposed in accordance to regional legal requirements. To the best of our knowledge and based on the information from the DUs, the filtered sludge is usually combusted in an incineration plant for production of energy and not applied on agricultural soil.

Exposure of soil and sediment is therefore extremely unlikely.


Based on state of the art dyeing and finishing techniques for textile articles, considerably low fractions of Indigo will be released only in laundering processes of respectively dyed articles at consumer sites. The release is based more likely on abrasion than on solubilisation. The Indigo fraction is part of the household wastewater and accordingly treated in communal effluent treatment plants. Sewage treatment on household wastewater in the European Union includes successively various purification steps. Procedures are based on physical, chemical and biological methods.

Indigo is used for 95% on cotton (denim) articles and 5% in wool articles (carpets, etc.) all other uses are niche uses, often without resulting wastewater, and not relevant for environmental assessment. These denim articles are well known and a favourite due to their robustness and wear resistance. Disposal of dyed articles usually occurs only after – for textiles – a comparably long time period. At the end of the service life, the dyed textiles are subject to municipal waste and usually combusted in an incineration plant for production of energy.


Even if not readily biodegradable, Indigo has shown to be susceptible of degradation in various scientific studies either as a substance or on dyed textiles [e. g. Ajibola et al. (2005); Sousa et al. (2008); Vautier et al. (2001); Tian et al. (2013); Manu et al. (2003)]. Degradation can either be initiated physically, chemically or biochemically. Especially electro-oxidation, photo-oxidation, photocatalytic oxidation and microbial degradation either by microorganisms (e. g. bacteria, fungi, yeast) or by isolated enzymes (e. g. laccase) are of great interest due to their potential capacity in an efficient water treatment in general. Respective methods were reported to work efficiently on Indigo. As main degradation products isatin, 2-aminobenzoic acid (anthranilic acid) and isatoic anhydride were identified. Isatin was identified as one of the products of the oxidation of Indigo by nitric acid and light. This process may be involved in the fading of Indigo in museum collection. The same conversion can be realised by ozonolysis, acidic bromate or by a chemiluminescent autoxidation of Indigo. N-Methylisatin was also obtained in the photo-oxidation of N-methylindole-3-acetic acid [da Silva et al.(2001)]. The stable red coloured isatin results from oxidation of Indigo. This substance is a naturally occurring indole derivative, which is found in plants of the genusIsatis(Isatis tinctoria),Calanthe discolourorCouroupita guianensis. It has also been detected as a compound of the secretion from the parotid gland of Bufo frogs and in humans as it is a metabolic derivative of adrenaline [da Silva et al. (2001)].

Decarboxylation of Indigo leads to anthranilic acid. Degradation pathway via anthranilic acid has shown to be resulting in complete mineralisation [Vautier et al.,(2001)].

Figure3:Degradation pathway via anthranilic acid [Vautier et al. (2001)]

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This publication has elucidated the complete degradation process of synthetic Indigo and identified all possible metabolites that are expected to be released by the degradation of registered Indigo as well. All identified items are expected to be not persistent but fully degraded and mineralized to the elements. There is no indication of these chemicals to be of any harm to the environment.

Based on these considerations an impact on soil and sediment from Indigo in End-Use can be excluded.

The measured bioconcentration factor for aquatic bioaccumulation (log BCF) for Indigo is < 4.5 and does not exceed the bioconcentration criteria. Furthermore, there is no indication on bioaccumulation in mammals. Therefore, Indigo is not regarded as bioaccumulative.

Indigo did not show any adverse effects in ecotoxicity or toxicity studies and is therefore not toxic for organisms, plants, animals or humans.