2-(1,3-dihydro-3-oxo-2H-indol-2-ylidene)-1,2-dihydro-3H-indol-3-one

PBT assessment

PBT assessment: overall result

PBT status:

the substance is not PBT / vPvB

Justification:

Indigo is not readily biodegradable and would therefore have to be considered being persistent. However, 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.

Tests for biodegradation with anaerobic bacteria in semi-static bioreactor show a degradation of up to 90% within 50 days.

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

The measured bioconcentration factor for aquatic bioaccumulation 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 bio-accumulative.

Indigo did not show any adverse effects in aquatic ecotoxicity studies at the limit of solubility or in terrestrial ecotoxicity and toxicity studies up to the highest dose tested and is therefore not toxic for organisms, plants, animals or humans.

Indigo does hence not meet the criteria for a PBT or vPvB substance.