Decolorization of the azo dye Reactive Violet 5 by UV-A-Fenton and ultrasound-Fenton processes

The textile dye reactive violet 5 (RV5) is mutagenic, and teratogenic as well as carcinogenic and has to be degraded before the release of textile wastewater into the environment. Advanced oxidation processes (AOPs) can be applied, in which by catalytic and non-catalytic processes generates hydroxyl radicals (HO•) with high oxidation potential (2.80 V), which in turn degrades the azo dyes. Among the AOPs, Fenton based processes can be applied to degrade the RV5 from aqueous solution. These processes can be enhanced with application of UV-A and ultrasound radiation, which are economic and environmentally safer than UV-C radiation. The aim of this work was to test a UV-A LED and an ultrasound (US) reactor in RV5 degradation. Different AOPs were tested, and only UV-A-Fenton and Fenton processes showed the highest RV5 degradation with 95.5 and 86.6%, respectively. The UV-A-Fenton was optimized by variation of the pH (3.0 – 7.0), H₂O₂ (2.0 – 16.0 mM) and Fe²⁺ (0.05 – 0.20 mM) concentrations. The best operational conditions (pH = 3.0, [RV5] = 0.28 mM, [H₂O₂] = 4.0 mM, [Fe²⁺] = 0.15 mM) were applied to the US-Fenton process, achieving a RV5 removal of 95.7%. The lowest values of electric energy per order (E_EO) = 11 kWh m^-3 order^-1 and specific applied energy (E_SAE) = 38 kWh mol^-1 order^-1 were obtained with the treatment of RV5 aqueous solution by UV-A-Fenton process. This work shows that textile dyes can be treated by UV-A-Fenton and US-Fenton and that UV-A LED reactors can be economic.