Background: Phenylurea herbicides are one of the most important and widely used pesticides in the world. Due to its potential persistence and toxicity in the aquatic environment, it poses certain risks to the ecological environment and human health. Studying the photochemical degradation behavior of herbicides is important for understanding the degradation and transformation fate in the environment.
Results: This study evaluated the effectiveness of direct and indirect photo-degradation of the herbicides isopropiron (IUP) and methylamine (MN), investigating the influence of operational variables (initial herbicide concentration and light sources) and initial of induced nitrate concentration on these processes in aqueous solution. We also introduced a new technology of compound-specific isotope analysis (CSIA) to provide deeper information of the photochemical degradation mechanism. Results showed that the light source and the initial concentration have an important effect on the degradation of herbicides IUP and MN. The photolysis rate under the Hg lamp is higher than photolysis rate under Xe lamps. It is found that photolysis kinetics of herbicides were consistent with the quasi-first order model, and the photolysis rate decreases with the increasing of the initial concentration. In indirect photodegradation, the degradation rate increases with increasing NO 3 − concentration at low concentrations of pesticides (8 mg/L); while the degradation rate decreases with increasing NO 3 − concentration at high concentrations of pesticides (30 mg/L ). According to the isotope fractionation, photolysis of IUP exhibits normal carbon isotope fractionation with the degradation rate increases, and the stable isotope enrichment factors under different photolysis pathways are different. In the indirect photo-degradation process, no significant fractionation of nitrogen isotopes occurred, and stable nitrogen isotopes fractionation could not be fitted well in either of the two photodegradation pathways.
Conclusion: Therefore, the structure and chemical characteristics of the molecules of herbicides play a determinant role in their photodegradation. The CSIA is useful both for a mechanism-based evaluation of experimental results and as a valuable tool to explore transformation pathways for organic pollutants in different environmental systems.