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

Development and comparison of advanced oxidation processes (AOPs) for the mineralization of azo-dyes from wastewaters G. Ramisa, F. Conteb, C. Callonib, A. Tripodib and I. Rossettib a Dip. Ing. Chimica, Civile ed Ambientale, Università degli Studi di Genova and INSTM Unit Genova, via all’Opera Pia 15A, 16145 Genoa, Italy b Chemical Plants and Industrial Chemistry Group, Dip. Chimica, Università degli Studi di Milano, CNR-ISTM and INSTM Unit Milano-Università, via C. Golgi 19, 20133 Milan, Italy The aim of this work was to find out a sustainable and scalable process in order to treat dye-rich wastewaters from textile industries and fully degrade these organic pollutants to non-harmful substances. The Dystar’s Levafix Brilliant Red E-6BA dye was used as a model molecule and treated through different advanced oxidation processes (APOs): H2O2/UV, Fenton and Photo-Fenton reactions, defined as homogeneous phase reactions, to be further compared in part 2 with heterogeneous photocatalyzed processes. The use of heterogeneous titania may be advantageous since the catalyst can be separated and re-used after the treatment, in contrast with the iron salt used for Fenton reaction which forms sludges. Moreover, metallic co-catalysts can be deposed over TiO2 nanoparticles in order to improve the light harvesting properties and activity. The reaction parameters, such as pH, concentration of oxidant, quantity of Fe catalyst, type of light source (dark, LED, sunlight and UV) 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, as 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 (20 min) 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. The treatment with 50 ppm of titania P25 is very effective when using an UV lamp directly immersed into the solution (irradiance = 260 W/m2), indeed more than 95% of the pollutant was degraded in ca. 40 min and we observed even better performance when adding hydrogen peroxide to the reactor (4 eq. H2O2, 8 min). To conclude, the 48-LC50 values of the treated samples were determined performing the acute toxicity test using Daphnia magna to check the toxicity of the final products. The treated solutions were characterized by acute toxicity, even higher than the original dye when H2O2 was used, however COD tests revealed that for most cases there was not organic carbon into the solution and the noxious effects were mainly attributed to the residues of hydrogen peroxide.