Efficiency comparison of advanced oxidation processes (AOPs) for the mineralization of azo-dyes in water

Efficiency comparison of advanced oxidation processes (AOPs) for the mineralization of azo-dyes in water F. Contea, C. Callonia, A. Tripodia, G. Ramisb and I. Rossettia a 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; b Dip. Ing. Chimica, Civile ed Ambientale, Università degli Studi di Genova and INSTM Unit Genova, via all’Opera Pia 15A, 16145 Genoa, 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. Different chemical, photochemical and photocatalytic processes have been compared to find the most suitable for this application, either in terms of technical feasibility and with a look to the safety of the treated wastewaters. 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 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 tune the light harvesting properties and activity. The various reaction parameters, such as pH, concentration of oxidant, quantity of Fe catalyst, type of light source (dark, LED, sunlight and UV) were changed and optimized to shorten the degradation time. The best results 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 was 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 feasibility assessment, 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. Since COD tests revealed that for most cases there was no residual organic carbon into the treated solution, the noxious effects were mainly attributed to the residues of hydrogen peroxide. This poses severe limits for the application of these technologies, at difference with heterogeneous photocatalytic processes, which may be slower, but by far safer, even when using nanostructured TiO2, which revealed no acute toxicity effect for the selected organisms.