Development and comparison of advanced oxidation processes (aops) for the mineralization of azo-dyes from wastewaters

The Dystar s Levafix Brilliant Red E-6BA dye was tested as model molecule and treated through three different Advanced Oxidation Processes (APOs): H2O2/UV, Fenton and Photo-Fenton reactions. Amount of oxidant, quantity of Fe catalyst, type of light source were evaluated and optimized to complete the degradation in the shortest time. The best performances were observed when using a low-power UV lamp directly immersed into the solution. The time required to degrade 100 ppm solution of dye (pH 7, 25 °C, 36 mg/L of catalyst, 1 equivalent of oxidant) was ca. 10 minutes for both Photo-Fenton and UV/H2O2 processes, compared with 160 minutes required to complete the degradation in dark conditions. The reaction time almost doubled when employing an external UV lamp, while both visible LED and solar light sources were comparable in terms of results (ca. 50 min), but the latter strictly depended on the weather conditions. To check the possible formation of harmful intermediates in vivo acute toxicity tests of treated samples were carried out using Daphnia magna specimens. Results of toxicity test showed the importance to minimize the residual amount of H2O2 into the treated solution, as it was individuated as the main cause of toxicity.