Enhanced Photocatalytic Degradation of the Imidazolinone Herbicide Imazapyr upon UV/Vis Irradiation in the Presence of Ca<i>_x</i>MnO<i>_y</i>-TiO₂ Hetero-Nanostructures: Degradation Pathways and Reaction Intermediates

• Main Authors: Salma Bougarrani, Preetam K. Sharma, Jeremy W. J. Hamilton, Anukriti Singh, Moisés Canle, Mohammed El Azzouzi, John Anthony Byrne
• Format: Article
• Language: English
• Published: MDPI AG 2020-05-01
• Series: Nanomaterials
• Subjects: persistent organic pollutants; photocatalysis; TiO₂; birnessite; water remediation; hydroxyl radicals
• Online Access: https://www.mdpi.com/2079-4991/10/5/896
• ISSN: 2079-4991

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 Ca<i>_x</i>MnO<i>_y</i>-TiO₂, and on the identification of the corresponding degradation pathways and reaction intermediates. Ca<i>_x</i>MnO<i>_y</i>-TiO₂ was formed by mixing Ca<i>_x</i>MnO<i>_y</i> and TiO₂ by mechanical grinding followed by annealing at 500 °C. A complete structural characterization of Ca<i>_x</i>MnO<i>_y</i>-TiO₂ 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, Ca<i>_x</i>MnO<i>_y</i>-TiO₂ showed a higher rate (10.6 μM·h⁻¹) as compared to unmodified TiO₂ (7.4 μM·h⁻¹) for Imazapyr degradation under UV-Vis irradiation. The mineralization rate was 4.07 µM·h⁻¹ for Ca<i>_x</i>MnO<i>_y</i>-TiO₂ and 1.21 μM·h⁻¹ for TiO₂. In the Ca<i>_x</i>MnO<i>_y</i>-TiO₂ 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 TiO₂, the intermediates continuously increased with irradiation time with no decrease observed in their concentration. The enhanced efficiency of the Ca<i>_x</i>MnO<i>_y</i>-TiO₂ for the complete degradation of the Imazapyr and intermediates is attributed to an increased adsorption of polar species on the surface of Ca<i>_x</i>MnO<i>_y</i>. 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 Ca<i>_x</i>MnO<i>_y</i>-TiO₂. Hydroxyl radicals were found to be main reactive oxygen species responsible for the photocatalytic degradation through radical scavenger investigations.