Noble metal-TiO2 supported catalysts for the catalytic ozonation of parabens mixtures

Parabens are endocrine disrupting compounds (EDCs) used as antimicrobial and preservatives and are reaching natural water courses with dangerous consequences. The efficiency of several catalysts in catalytic ozonation compared to single ozonation was tested for a mixture of five parabens degradation. On catalytic ozonation noble metals (Pd, Pt, Ag, Au) supported on TiO2 were used, and their activity was compared with TiO2. Single ozonation was able to totally oxidize the initial parabens mixture using a high transferred ozone dose (TOD) (around 170 mg/L). However, the COD reduction was very low (25%). The combination of ozonation with the catalysts tested improved the parabens degradation and COD removal using lower TOD values. For best catalysts, 0.5% Pt-TiO2 and 0.5% Pd-TiO2, total degradation of parabens was achieved for a TOD of 70 mg/L, which represents an ozone dose decrease by a factor of almost 3 in comparison to single ozonation. Moreover, these catalytic systems led to a COD and TOC removal of 36% and 18% respectively. The toxicity of the parabens solutions before and after treatments was evaluated using four different biological preparations. The initial parabens mixture was very toxic, but after treatment with all the conditions proposed the Vibrio fischeri luminescence inhibition and Asian clam mortality decreased while the germination index of Lepidium sativum increased. The studies made in a mammalian neuronal preparation, Wistar rat brain slices, indicate that the parabens mixture treated using single or catalytic ozonation had no effect on the measured neuronal ROS signals, unlike the raw mixture. Although total COD and TOC removal was not achieved, indicating the presence of refractory compounds, the toxicity analysis indicates that the transformation products are less environmentally harmful than parabens. These results show that, even using low catalyst loads, noble metals (especially Pt and Pd) can truly enhance ozone action over pollutants.