W-Doped ZnO Photocatalyst for the Degradation of Glyphosate in Aqueous Solution

Abstract

In this paper, the photocatalytic degradation of glyphosate by zinc oxide (ZnO) photocatalysts doped with tungsten (W) was investigated under solar simulated light. The photocatalysts were successfully synthesized through a simple precipitation method and subsequently characterized by different techniques: Raman spectroscopy, UV–Vis, N2 adsorption at −196 °C, X-ray diffraction, and SEM analysis. In particular, all the prepared catalysts were characterized by a crystallite size of about 28 nm and a hexagonal wurtzite structure. After the W doping, the bandgap energy decreased from 3.22 of pure ZnO to 3.19 for doped ZnO. This allowed us to obtain good results in terms of glyphosate degradation and simultaneous mineralization under solar simulated lamps, making the process environmentally friendly and with almost zero energy costs. In particular, the best photocatalytic performance was obtained with 100 W-ZnO (prepared with 1.5 mol% of W). With this catalyst, after 180 min of exposure to solar simulated light, the glyphosate degradation and mineralization was equal to 74% and 30%, respectively. Furthermore, it has been shown that the best catalyst dosage was equal to 1.5 g/L. The study on the influence of pH evidenced that the best photocatalytic performances are obtained at spontaneous (neutral) pH conditions. Finally, to determine the main reactive species in the glyphosate oxidation, the effects of different radical scavengers were tested. The results evidenced that the glyphosate oxidation mechanism seems to be related mainly to the O2•− generated under simulated solar light irradiation, but also in minor part to h+.