Non-thermal plasma is one of the most promising technologies used for the degradation of hazardous pollutants in wastewater. Recent studies evidenced that various operating parameters influence the yield of the Non-Thermal Plasma (NTP)-based processes. In particular, the presence of a catalyst, suitably placed in the NTP reactor, induces a significant increase in process performance with respect to NTP alone. For this purpose, several researchers have studied the ability of NTP coupled to catalysts for the removal of different kind of pollutants in aqueous solution. It is clear that it is still complicated to define an optimal condition that can be suitable for all types of contaminants as well as for the various types of catalysts used in this context. However, it was highlighted that the operational parameters play a fundamental role. However, it is often difficult to understand the effect that plasma can induce on the catalyst and on the production of the oxidizing species most responsible for the degradation of contaminants. For this reason, the aim of this review is to summarize catalytic formulations coupled with non-thermal plasma technology for water pollutants removal. In particular, the reactor configuration to be adopted when NTP was coupled with a catalyst was presented, as well as the position of the catalyst in the reactor and the role of the main oxidizing species. Furthermore, in this review, a comparison in terms of degradation and mineralization efficiency was made for the different cases studied.
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+.
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