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Environmental fate & pathways

Mode of degradation in actual use

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Administrative data

Endpoint:
mode of degradation in actual use
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Vat Indigo Potassium Salt reacts spontaneously with water and/or air to form indigo and potassium hydroxide.
Cross-reference
Reason / purpose:
reference to same study

Data source

Reference
Reference Type:
publication
Title:
Photocatalytic Degradation of Dyes in Water: Case Study of Indigo and of Indigo Carmine
Author:
Vautier M, Guillard C, Herrmann JM
Year:
2001
Bibliographic source:
Journal of Catalysis 2001;201(1):46-59

Materials and methods

Principles of method if other than guideline:
Photocatalytic tests were performed in a batch microphotoreactor of 100 mL made of Pyrex (transmittance: λ >290 nm)
GLP compliance:
not specified
Type of study / information:
Materials
Indigo was supplied by a textile firm and used as received. The photocatalyst was titania Degussa P-25 (anatase/rutile=3.1; surface area=50m2/g, nonporous particles). The slurry, which is not colloidal, consists of large fractal aggregates of individual particles, each one having a mean diameter, d, of ca. 30 nm, in agreement
(i) with the value calculated from the formula (given as a function of the volumic mass ρ and the specific surface area S for homodispersed nonporous particles), d = 6/ρS;
(ii) with transmission electron microscopy performed on sonicated samples.

Photoreactor and Light Source
Photocatalytic tests were performed in a batch microphotoreactor of 100 mL made of Pyrex (transmittance: λ > 290 nm. It was equipped with a
high-pressure mercury lamp (Philips HPK 125 W). For visible irradiation, the same lamp was used but the light beams were filtered by a Corning 3-71 optical filter (λ > 440 nm) incorporated in the water cell used to remove IR beams from the lamp and to avoid heating. The UV-irradiation enters
the reactor through the bottom optical window made of Pyrex, which plays the role of a UV-filter with transmittance for λ > 290 nm.

Test material

Reference
Name:
Unnamed
Type:
Constituent
Details on test material:
Indigo was supplied by a textile firm and used as received.

Results and discussion

Any other information on results incl. tables

The kinetics of formation of intermediate acids during the degradation of indigo:

The first acids formed after 20 min of UV irradiation are oxalic, anthranilic, and malic acids, in line with the complete bleaching of the dye in 8 min. The shorter carboxylic acids appear subsequently, with a maximum amount obtained after ca. 30 min of UV irradiation. All the intermediate products, except acetic acid, are degraded within 1 h of UV-irradiation, in agreement with the time of formation of CO2 (1 h). Acetic acid requires a longer time for mineralization, as generally observed (see attachment)

Degradation products of indigo in water

No. Identifier Identity
#1 common name Oxalic acid
#2 common name Anthranilic acid
#3 common name Malic acid
#4 common name Pyruvic acid
#5 common name Malonaldehydic acid/Malonic acid
#6 common name Glycolic acid
#7 common name Tartaric acid
#8 common name Acrylic acid
#9 common name 2-Nitro-benzaldehyde and/or 2,3-dihydroxyindoline
#10 common name Amino-fumaric acid
#11 common name 3-Amino propenoic acid
#12 common name 3-Amino; 2,3-dihydroxy- propanoic acid
#13 common name Acetic acid

Degradation products of solid indigo

No. Identifier Identity
#1 common name Oxalic acid
#2 common name 2-Nitrobenzoic acid
#3 common name Nitrobenzene
#4 common name Malic acid
#5 common name Fumaric acid
#6 common name Dihydroxyfumaric acid
#7 common name Glycolic acid
#8 common name Acetic acid

Applicant's summary and conclusion

Conclusions:
The photocatalytic degradation of indigo has been successfully demonstrated when using UV-irradiated titania-based catalysts. In addition to a prompt removal of the color, photocatalysis was simultaneously able to oxidize the dye, with an almost complete mineralization of carbon and of nitrogen and sulfur heteroatoms into innocuous compounds. A detailed degradation pathway, based on careful identification of intermediate products, is proposed.
The irradiation of titania in the visible light produces a photoinduced decolorization of the dye but without any degradation, corresponding to a stoichiometric electron transfer from the dye, excited in the visible irradiation, to titania. The positive decolorization and degradation of solid indigo, mechanically mixed, constitutes an encouraging result for self-cleaning titania-coated objects (glasses, steel, walls, etc.) fouled by solid dirt particles.
The ensemble of these results clearly suggest that TiO2/UV photocatalysis may be envisaged as a method for treatment of diluted colored waste waters in textile industries, especially in sunny semi-arid countries where water has to be recycled. Large solar pilot experiments are programmed for this purpose.
Executive summary:

The TiO2/UV photocatalytic degradations of indigo has been investigated both in aqueous heterogeneous

suspensions and in the solid state. In addition to prompt removal of the color, TiO2/UV-based photocatalysis was simultaneously able

to oxidize the dye, with almost complete mineralization of carbon and of nitrogen heteroatoms into CO2, NH4+and NO3-, respectively. A detailed degradation pathway has been determined by careful identification of intermediate products, in

particular, carboxylic acids, whose decarboxylation by photo-Kolbe reactions constitutes the main source of CO2 evolution. The only

persistent organic compound was acetic acid, whose degradation required a longer period of time. These results suggest that TiO2/UV

photocatalysis may be envisaged as a method for treatment of diluted wastewaters in textile industries. The irradiation of titania

with visible light did produce a photoinduced decolorization of the dye, probably induced by the breaking of the double-bond conjugation system of the chromophoric group. However, this decolorization was not accompanied by final mineralisation since no loss of total organic carbon (TOC) nor release of inorganic ions were observed. This corresponded to a stoichiometric reaction of an electron transfer from the dye molecule excited in visible irradiation to titania. Because indigo is very poorly soluble (< 2 ppm), it

was tentatively degraded in its solid state, mixed with titania in a photocatalytic solid-solid-type reaction. Observation of the decolorization and of the degradation of solid indigo constitutes a surprising and encouraging result for the development of self-cleaning

titania-coated objects (glasses, steel, aluminium, metals, walls, etc.) fouled by solid dirt particles.