Photocatalytic ozonation of clopyralid, picloram and triclopyr. Kinetics, toxicity and influence of operational parameters

Abstract

BACKGROUND Photolysis, photocatalysis, ozonation and photocatalytic ozonation in the presence of titania have been applied to the elimination of three pyridine carboxylic acids herbicides (clopyralid, picloram and triclopyr) in water. RESULTS Photocatalytic ozonation led to the best results in terms of herbicide elimination rate and mineralization. The herbicides were relatively recalcitrant to direct ozonation (rate constants: 20 M−1 min−1 for clopyralid and triclopyr, and 105 M−1 min−1 in the case of picloram). Herbicides degradation proceeds by hydroxyl radicals attack (rate constants: 0.73, 3.80 and 1.73 × 109 M−1 s−1 for clopyralid, picloram and triclopyr, respectively). Monitoring of chloride and nitrates suggests dechlorination as the primary stage in the process. Further ring opening led to the accumulation of nitrates and the appearance of some low weight carboxylic acids. Toxicity decreased at the end of the process. CONCLUSION Tested herbicides slowly react with ozone. Photocatalytic ozonation shows different degrees of synergism, leading to 80% mineralization in less than 1 h. Small amounts of TiO2 (i.e. 0.005 g L−1) maintains the efficiency of the photocatalytic ozonation. Quantitative free chloride and nitrates rapidly appear in the O3/UVA/TiO2 system. First intermediates in the photocatalytic ozonation of clopyralid show higher toxicity than the parent compound. © 2014 Society of Chemical Industry

Photocatalytic degradation of the herbicide clopyralid: kinetics, degradation pathways and ecotoxicity evaluation

“…Thereafter, inhibition of bioluminescence was progressively decreased (34% at 360 min), implying that total detoxification would be achieved at prolonged irradiation times. A similar trend in toxicity evaluation was also observed by Solís et al , who studied the TPs of clopyralid after ozonation treatment …”

Photocatalytic ozonation of pyridine‐based herbicides by N‐doped titania

“…) confirms the recalcitrant nature of these compounds towards molecular ozone. In fact, in previous work the rate constants between ozone and the herbicides were tested and took values of 20 M −1 min −1 for clopyralid and triclopyr, and 105 M −1 min −1 in the case of picloram . TOC remains almost constant showing conversion values in the proximity of 10% after 3 h of ozonation.…”

“…In fact, in previous work the rate constants between ozone and the herbicides were tested and took values of 20 M −1 min −1 for clopyralid and triclopyr, and 105 M −1 min −1 in the case of picloram. 36 TOC remains almost constant showing conversion values in the proximity of 10% after 3 h of ozonation. Dissolved ozone and at the reactor outlet were immediately detected and remained unchanged throughout the process, corroborating the slow regime developed in this heterogeneous reaction.…”

Photoelectro-Fenton treatment of pesticide triclopyr at neutral pH using Fe(III)–EDDS under UVA light or sunlight

“…Despite being recalcitrant to photodegradation and oxidation, this pesticide has been efficiently degraded by several photo-assisted AOPs like photocatalysis with TiO2/UV (Qamar et al 2006) and ZnO/SnO2 (Yadav et al 2019), photocatalytic ozonation with Au/TiO2 (Maddila et al 2015;Solís et al 2016) and photo-assisted peroxonation (Pérez-Lucas et al 2020). The herbicide yields 3,5,6-trichloro-2-pyridinol as primary intermediate (Janíková-Bandžuchová et al 2015), further being converted into acetic acid (Qamar et al 2006) or oxalic acid (Lei et al 2018).…”