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Environmental fate & pathways

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Indigo is not readily biodegradable; however, 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); Poulin J (2007); 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. Tests for biodegradation with anaerobic bacteria in a semi-static bioreactor showed a degradation of up to 90% within 50 days [Manu et al. (2003)].

The OECD SIDS documents estimates a half-life of 40 days for photodegradation in water. A photocatalytic study with Indigo showed as the main degradation products isatin, 2-aminobenzoic acid (anthranilic acid) and isatoic anhydride [Vautier et al. (2001)]. The same degradation products were found on ancient textile samples dyed with Indigo [Sousa et al. (2008); Poulin J (2007)]. Isatin was identified as one of the products of the oxidation of Indigo by nitric acid and light. 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. 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 to CO2, NH4+and NO3-[Vautier et al. (2001)].