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Environmental fate & pathways

Phototransformation in water

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TiO2/UV-based photocatalysis was able to oxidize the dye with almost complete mineralization of carbon and of nitrogen heteroatoms into CO2, NH4+ and NO3-

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The photocatalytic degradation of Indigo has been successfully demonstrated when using UV-irradiated titania-based catalysts. In addition to a prompt removal of the colour, TiO2/UV-photocatalysis was simultaneously able to oxidize the dye, with an almost complete mineralization of carbon and of nitrogen heteroatoms into innocuous compounds.

The following degradation products of Indigo in water were identified.

Table Degradation products of Indigo in water

No.

Identity

1.              

Oxalic acid

2.              

Anthranilic acid

3.              

Malic acid

4.              

Pyruvic acid

5.              

Malonaldehydic acid/Malonic acid

6.              

Glycolic acid

7.              

Tartaric acid

8.              

Acrylic acid

9.              

2-Nitro-benzaldehyde and/or 2,3-dihydroxyindoline

10.           

Amino-fumaric acid

11.           

3-Amino propenoic acid

12.           

3-Amino; 2,3-dihydroxy- propanoic acid

13.           

Acetic acid

 

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.

A detailed degradation pathway, based on careful identification of degradation products was outlined. 

The irradiation of titania in the visible light produces a photoinduced decolourization of the dye but without final mineralisation, corresponding to a stoichiometric electron transfer from the dye, excited in the visible irradiation, to titania. The positive decolourization 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 also as a method for treatment of Indigo containing 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.