Solar-Light-Mediated Phototransformation of Herbicide Tribenuron-Methyl Initiated by Its Coexisting Nitrate Ion in Sunlit Agricultural Drainages

Understanding the environmental fate of chemical herbicides is crucial to sustainable agriculture. Due to their joint-use with nitrogen fertilizers, their residues often coexist with NO3– in agricultural drainages. In this study, tribenuron-methyl was used as a model to evaluate the role of NO3– in the phototransformation of chemical herbicides, which was characterized by a two-stage process. Initially, a slow hydrolysis occurs (kobs = 2.573 × 10–4 min–1), producing two hydrolysis products: methyl-2-(aminosulfonyl)-benzoate (MSB) and 2-methyl-4-methylamino-6-methoxy-1, 3, 5-triazine (MMT), which can be significantly accelerated by solar irradiation (kobs = 2.152 × 10–2 min–1). Subsequently, MSB undergoes a rapid NO3–-initiated photodegradation process (kobs = 2.251 × 10–2 min–1). MMT was identified as the refractory unit and undergoes a slow NO3–-initiated photodegradation process (kobs = 4.494 × 10–4 min–1). The underlying mechanisms were elucidated through electron paramagnetic resonance spectroscopy and reactive species quenching experiments. This study fills a knowledge gap on the interaction between NO3– and chemical herbicides, highlighting the pivotal role of NO3– in the phototransformation of chemical herbicides.