Enhanced Photocatalytic Degradation of the Imidazolinone Herbicide Imazapyr upon UV/Vis Irradiation in the Presence of CaMnO-TiO Hetero-Nanostructures: Degradation Pathways and Reaction Intermediates.

The determination of reaction pathways and identification of products of pollutants degradation is central to photocatalytic environmental remediation. This work focuses on the photocatalytic degradation of the herbicide Imazapyr (2-(4-methyl-5-oxo-4-propan-2-yl-1H-imidazol-2-yl) pyridine-3-carboxylic acid) under UV-Vis and visible-only irradiation of aqueous suspensions of CaMnO-TiO, and on the identification of the corresponding degradation pathways and reaction intermediates. CaMnO-TiO was formed by mixing CaMnO and TiO by mechanical grinding followed by annealing at 500 °C. A complete structural characterization of CaMnO-TiO was carried out. The photocatalytic activity of the hetero-nanostructures was determined using phenol and Imazapyr herbicide as model pollutants in a stirred tank reactor under UV-Vis and visible-only irradiation. Using equivalent loadings, CaMnO-TiO showed a higher rate (10.6 μM·h) as compared to unmodified TiO (7.4 μM·h) for Imazapyr degradation under UV-Vis irradiation. The mineralization rate was 4.07 µM·h for CaMnO-TiO and 1.21 μM·h for TiO. In the CaMnO-TiO system, the concentration of intermediate products reached a maximum at 180 min of irradiation that then decreased to a half in 120 min. For unmodified TiO, the intermediates continuously increased with irradiation time with no decrease observed in their concentration. The enhanced efficiency of the CaMnO-TiO for the complete degradation of the Imazapyr and intermediates is attributed to an increased adsorption of polar species on the surface of CaMnO. Based on LC-MS, photocatalytic degradation pathways for Imazapyr under UV-Vis irradiation have been proposed. Some photocatalytic degradation was obtained under visible-only irradiation for CaMnO-TiO. Hydroxyl radicals were found to be main reactive oxygen species responsible for the photocatalytic degradation through radical scavenger investigations.