Photolytic and photocatalytic degradation of quinclorac in ultrapure and paddy field water: Identification of transformation products and pathways

Quinclorac (QNC) is an effective but rather persistent herbicide commonly used in rice production. This herbicide presents a mean persistence in the environment so its residues are considered of environmental relevance. However, few studies have been conducted to investigate its environmental behavior and degradation. In the present work, direct photolysis and TiO2 photocatalysis of the target compound in ultrapure and paddy field water were investigated. After 10 h photolysis in ultrapure water, the concentration of QNC declined 26% and 54% at 250 and 700 W m−2, respectively. However, the amount of quinclorac in paddy field water remained almost constant under the same irradiation conditions. QNC dissipated completely after 40 min of TiO2 photocatalysis in ultrapure water, whereas 130 min were necessary to degrade 98% of the initial concentration in paddy field water.Possible QNC photolytic and photocatalytic degradation pathways are proposed after structure elucidation of the main transformation products, through liquid chromatography–electrospray ionization–quadrupole time-of-flight mass spectrometry and exact mass measurements. Pyridine ring hydroxylation at C-9 followed by ring opening and/or oxidative dechlorination were the key steps of QNC degradation.

► The degradation of QNC under photolysis and TiO2 photocatalysis was studied.
► QNC degradation was only relevant under TiO2 photocatalysis.
► Fourteen TPs were found and a transformation pathway was proposed.
► Common key TPs were identified in both types of water under photolytic processes.