Detoxification of parabens using UV-A enhanced by noble metals-TiO2 supported catalysts

The photocatalytic oxidation with UV-A irradiation was tested for the treatment of a mixture of five parabens (methylparaben, ethylparaben, propylparaben, butylparaben and benzylparaben) that may be present industrial wastewater. A wide variety of noble metals such as palladium (Pd), platinum (Pt), silver (Ag) and gold (Au), supported on titanium dioxide (TiO2) were used, and their activity was compared with that of pure TiO2 and photolysis. The degradation rate of single parabens followed a pseudo-first order kinetics. Besides, the photocatalytic reaction rate constant based on fluence was also determined. The kinetic constant increased with the number of carbons on the chain linked to the aromatic ring. Direct photolysis led to only 4% of parabens degradation whilst the presence of catalysts improved parabens depletion as well as TOC and COD removal. For the best catalysts, Ag and Pd on TiO2, a reduction higher than 50% of the initial parabens concentration was achieved after 180 min of reaction. The highest degradation rate was attained for BeP with Pd-TiO2 (75%). These catalysts (Ag and Pd) led to a COD removal of 17 and 19%, whereas TOC removal reached 22 and 25%, respectively. The toxicity of the parabens solutions before and after all treatments was evaluated using three different species. For Vibrio fischeri the toxicity after treatment remains high. The germination index of Lepidium sativum increased in the presence of treated effluents while Corbicula fluminea mortality decreased for the solutions treated by photocatalysis using Pd-TiO2 and Ag-TiO2. The fact that total COD removal and mineralization were not achieved, indicates the presence of some final refractory compounds. The toxicity analysis points out that, for the best catalysts, the treated solutions are less environmentally harmful than the initial mixture, since the parabens concentration could be substantially reduced. These results reveal that, even using low catalyst loads, Ag-TiO2 and Pd-TiO2 are interesting catalytic materials for photocatalytic degradation of industrial wastewater containing parabens.