Environmental fate of methomyl pesticide in river water: Kinetic study, mathematical simulations and risk assessment

Methomyl (MET) is a pesticide widely used as an acaricide with toxic effects on aquatic organisms and humans. The environmental persistence of pollutants can be determined by different routes, including photochemical transformations driven by sunlight and photoproduced reactive intermediates such as HO center dot, 3CDOM* , and 1O2. Photolytic experiments were carried out under simulated solar radiation, revealing a low value of the direct photolysis quantum yield, phi MET = (2.01 +/- 0.62) x 10 4 mol Einstein 1. The second-order kinetic rate constants of the reactions between MET and HO center dot, 3CDOM* , and 1O2 were also measured using the competition kinetics approach; the values obtained were kMET, HO center dot = (1.42 +/- 0.04) x 109 L mol 1 s 1, kMET, 3CBBP* = (3.49 +/- 0.87) x 108 L mol 1 s 1 and kMET, 1O2 = (3.36 +/- 1.09) x 104 L mol 1 s 1, respectively. Kinetic mathematical simulations were used to predict the half-life and self-purification distance of MET in a river, considering phenomena as photodegradation, adsorption, biodegradation, geographical location, and water chemistry. The half-life of MET was estimated to be 82 h, with biodegradation making the largest contribution to the drop in MET concentration. The relative contribution of photochemical pathways to MET degradation in sunlit water will depend greatly on the irradiance conditions, water depth and the environmental characteristics of the water body, including the concentrations of organic matter, nitrate, nitrate and carbonate. Finally, the predicted risk coefficients, also based on QSAR models, confirmed that MET and its metabolite S-methyl-N-hydroxythioacetamidate pose a high risk to aquatic species.