Enhanced Photocatalytic Degradation of the Imidazolinone Herbicide Imazapyr upon UV/Vis Irradiation in the Presence of CaxMnOy-TiO2 Hetero-Nanostructures: Degradation Pathways and Reaction Intermediates

[Abstract] 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 CaᵪMnOᵧ-TiO₂, and on the identification of the corresponding degradation pathways and reaction intermediates. CaᵪMnOᵧ-TiO₂ was formed by mixing CaᵪMnOᵧ and TiO₂ by mechanical grinding followed by annealing at 500 °C. A complete structural characterization of CaᵪMnOᵧ-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, CaᵪMnOᵧ-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 CaᵪMnOᵧ-TiO₂ and 1.21 μM·h⁻¹ for TiO₂. In the CaᵪMnOᵧ-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 CaᵪMnOᵧ-TiO₂ for the complete degradation of the Imazapyr and intermediates is attributed to an increased adsorption of polar species on the surface of CaᵪMnOᵧ. 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 CaᵪMnOᵧ-TiO₂. Hydroxyl radicals were found to be main reactive oxygen species responsible for the photocatalytic degradation through radical scavenger investigations.This research received external funding from the British Council under the STREAM-MENA Institutional Links Scheme Grant number 278072873. This is a collaboration between Ulster University (UK), Technion Institute (Israel) and Rabat University (Morocco). MC acknowledges support from Ministerio de Economía y Competitividad (Spain) through project CTQ2015-71238-R (MINECO/FEDER). AS would like to acknowledge the financial support received from Ulster University (UK) through the VCRS scholarship. PS would like to acknowledge funding from Invest Northern Ireland for the BioDevices projectBritish Council; 27807287

Improving the Imazapyr Degradation by Photocatalytic Ozonation: A Comparative Study with Different Oxidative Chemical Processes

The degradation of imazapyr (C13H15N3O3), an active element in the aqueous solution of commercial herbicide, was investigated. This study was the first to evaluate in a comprehensive manner the efficiency of advanced oxidation processes for imazapyr degradation. Results showed that Imazapyr degradation is significantly affected by operational conditions such as TiO2 concentration, ozone concentration, initial concentration of imazapyr and pH. The kinetics of Imazapyr consumption was the first order with respect to Imazapyr concentration and zero order with respect to ozone concentration with a constant rate of 0.247 min−1 and 0.128 min−1 for photocatalytic ozonation and heterogeneous photocatalysis, while it was the first order with respect to Imazapyr and the first order with respect to ozone concentrations when only ozone was used with a constant rate of 0.053 mol L−1 min−1 at pH 7. The results revealed that more than 90 percent of the removal efficiency representing the elimination of imazapyr was held up to 7 μM. Further increase in the concentration of imazapyr leads to a drop in the removal efficiency, however the total imazapyr degradation was reached in 20 min utilizing photocatalytic ozonation for 5 μM of Imazapyr in the presence of 100 mg L−1 of TiO2, 10 mg L−1 of ozone at pH 7. Photocatalytic ozonation and heterogeneous photocatalysis utilizing TiO2 as a semiconductor process appeared possible and well suited for the treatment of organic contaminants such as imazapyr herbicides, although at certain dosages of pH and common time for wastewater treatment, imazapyr was not degraded with ozonation on its own. The association of two oxidation processes, ozonation and photocatalysis, has improved oxidation efficiencies for water treatment under optimal conditions, leading to the development of non-selective hydroxyl and more reactive radicals in the oxidation medium, as well as the resulting synergistic effects between photocatalysis and ozonation that react more rapidly with imazapyr herbicide

Factors Influencing Imazapyr Herbicide Removal from Wastewater Using Photocatalytic Ozonation

This study investigates the degradation of imazapyr herbicide from wastewater by photocatalytic ozonation using TiO2 as a semiconductor. Effects of operational parameters on imazapyr removal efficiency including TiO2 dosing, initial herbicide concentration and pH were also studied. Obtained results showed that more than 90% of removal efficiency representing the disappearance of imazapyr was maintained until 7 mu M in the presence of 200 mgL(-1) of UV100-TiO2. Otherwise, the degradation of imazapyr followed the first-order kinetics with a photocatalytic rate constant of 0.247 min(-1), and complete degradation was achieved within 20 min using photocatalytic ozonation for 5 mu M of Imazapyr at pH 7