Photo-transformation of isoproturon under UV-A irradiation: The synergy of nitrite and natural organic matter

Isoproturon (IPU), a widely utilized phenylurea herbicide, is recognized as an emerging contaminant . Previous studies have predominantly attributed the degradation of IPU in natural waters to indirect photolysis by natural organic matter (NOM). Here, we demonstrate that nitrite (NO 2 − ) also serves as an important photosensitizer that induces the photo-degradation of IPU. Through radical quenching tests, we identify hydroxyl radicals (•OH) and nitrogen dioxide radicals (NO 2 • ) originating from NO 2 − photolysis as key players in IPU degradation, resulting in the generation of a series of hydroxylated and nitrated byproducts. Moreover, we demonstrate a synergistic effect on the photo-transformation of IPU when both NOM and NO 2 − are present in the reaction mixture. The observed rate constant ( k obs ) for IPU removal increases to 0.0179 ± 0.0002 min −1 in the co-presence of NO 2 − (50 μM) and NOM (2.5 mg C /L), surpassing the sum of those in the presence of each alone (0.0135 ± 0.0004 min −1 ). NOM exhibits multifaceted roles in the indirect photolysis of IPU. It can be excited by UV and transformed to excited triplet states ( 3 NOM*) which oxidize IPU to IPU •+ that undergoes further degradation. Simultaneously, NOM can mitigate the reaction by reducing the IPU •+ intermediate back to the parent IPU. However, the presence of NO 2 − alters this dynamic, as IPU •+ rapidly couples with NO 2 • , accelerating IPU degradation and augmenting the formation of mono-nitrated IPU. These findings provide in-depth understandings on the photochemical transformation of environmental contaminants , especially phenylurea herbicides, in natural waters where NOM and NO 2 − coexist.