Sunlight transformation of sethoxydim-lithium in natural waters and effect of humic acids

Abstract

Photochemical behaviour of sethoxydim-lithium, a cyclohexanedione oxime herbicide, has been investigated in ultrapure and natural waters (mineral, well and river water). Photodegradation experiments were carried out under laboratory conditions using a solar silulator (Suntest equipment) and under natural solar irradiation in order to evaluate reaction kinetics of the active ingredient. Besides, the effect of humic acids (HA) on the degradation rate under simulated solar radiation was also studied. The photodegradation rates of sethoxydim-lithium in all the different waters and light sources studied followed first-order kinetics. Results showed that dark reactions were negligible. Experimental data showed that half-lives of the herbicide under simulated irradiation were higher in natural waters than in ultrapure, showing the lowest degradation rate in river water (t 1/2 = 135.5 ± 0.3 min). Under natural sunlight, sethoxydim-lithium photodegradation followed the same behaviour, with half-lives that range from 82 to 437 min for ultrapure and river water respectively, showing a strong dependence on the composition of water. However, rates of degradation of sethoxydim-lithium were faster under simulated radiation compared to natural sunlight (e.g. t 1/2 (ultrapure water) 59.8 ± 0.2 min vs. 82.1 ± 0.7 min). The presence of different concentrations of HA (1–20 mg L−1) retarded the photolytic degradation relative to ultrapure water with half-lives from 92 to 196 min.