Enhanced photocatalytic oxidation of a phenoxyacetic acid herbicide using TiO2-FeOOH/Fe2O3 assisted with sulfate radicals

This work investigated the photocatalytic degradation of the 4-chloro-2-methylphenoxyacetic acid herbicide and its primary intermediate (4-chloro-2-methyl phenol) using the coupled catalyst TiO2 modified with the iron oxide species, FeOOH and Fe2O3 (FeOx, 0.5 wt.%), under UV-Vis radiation assisted with persulfate or peroxymonosulfate. The FeOx-TiO2 composite prepared by microwave-assisted sol-gel method was characterized by X-Ray Diffraction, Nitrogen Physisorption, UV-Vis Diffuse Reflectance Spectroscopy, Fourier Transform Infrared Spectroscopy, Atomic Absorption Spectroscopy, Photoluminescence, X-Ray Photoelectron Spectroscopy, Thermogravimetric Analysis, Scanning Electron Microscopy, and Transmission Electron Microscopy. The anatase phase was conserved in the FeOx-TiO2 material. The coupling of FeOx in TiO2 decreased the crystallite size and enhanced the specific surface area and visible light response. A response surface methodology based on Central Composite Design was used to obtain the optimal oxidant agent (persulfate or peroxymonosulfate), oxidant dosage, and solution pH conditions to enhance the 4-chloro-2-methylphenoxyacetic acid mineralization. The 87.04% of 4-chloro-2-methylphenoxyacetic acid mineralization was achieved by FeOx-TiO2 composite with persulfate (5 mM) under UV-Vis radiation at pH 7 in 150 min showing a synergic effect. This result allowed the complete elimination of 4-chloro-2-methylphenoxyacetic acid and the by-product 4-chloro-2-methyl phenol generated during the reaction compared to 61.6 and 27.9% mineralization obtained with FeOx-TiO2 + h upsilon and persulfate oxidation + h upsilon, respectively. The photogenerated electrons in FeOx-TiO2 improved mineralization efficiency that allows the persulfate activation. Besides, the coupling between FeOx and TiO2 suppresses the electron/hole (e(-)/h(+)) pair recombination of FeOx-TiO2 photocatalyst.

Automated summary

This study investigated the photocatalytic degradation of the herbicide 4-chloro-2-methylphenoxyacetic acid using a composite catalyst FeOx-TiO2 under UV-Vis radiation and with the assistance of persulfate or peroxymonosulfate. The FeOx-TiO2 composite showed enhanced mineralization efficiency compared to other oxidation methods, resulting in complete elimination of the herbicide and its by-product 4-chloro-2-methyl phenol. The coupling of FeOx in TiO2 suppressed the recombination of electron/hole pairs and improved the visible light response.