Reuse of Salt-Containing Dyeing Effluents by Means …
7
Cl 2(aq) + H 2 O → HClO + Cl
−
+ H
+
(4)
HClO ↔ H
+
+ ClO
−
(5)
2. Degradation of dyes: The oxidant species generated are able to fragment the
dye molecule according to the reaction 6:
ClO
−
+ dye → dye fragments → CO 2 + H 2 O
( 6 )
It should be noted that colour removal is much faster than the degradation of the
dye, since the oxidising species attack first the chromophore group of the dye [29–31].
It was observed that colour removal can be enhanced by UV light irradiation. This
improvement is attributed to the production of highly efficient radicals (reactions 7
and 8):
ClO
−
+ hv → O
−•
+ Cl
•
(7)
O
−•
+ H 2 O → OH
−
+ OH
•
(8)
Figure 4 shows the steps involved in the indirect oxidation of dyes assisted by
UV, which is the base of the EC-UV system.
The EC-UV system has been already validated at industrial scale for the treatment
of textile effluents containing reactive dyes. The system not only solves the problem
of effluent colouration, but also enables the reuse of the treated water that still contains
salts. Therefore, the system allows to reduce the consumption of water and salts in
the dyeing processes.
Fig. 4 Steps involved in the indirect oxidation of dyes