Photolysis characteristics and influencing factors of the pesticide pyrimethanil in natural waters

Environmental context The widespread use of pesticides leads to a large amount of residues in the natural water where photochemical degradation is the main removal process. The influencing factors on the photodegradation of pyrimethanil were investigated in different aqueous matrix, and the chlorinated product was found in saline water, indicating a higher ecologic risk of residual pyrimethanil in estuarine and nearshore water than in fresh water. A better understanding of the natural degradation mechanisms for pesticides will inform their safer use in agriculture. Rationale The photodegradation behaviour of pesticides in natural environments significantly affects their ecological safety. Pyrimethanil (PYR) is a widely used fungicide and is classified as a possible human carcinogen. The influencing factors for the photodegradation of PYR in different aqueous matrixes were studied, and the degradation pathway was evaluated. Methodology The photodegradation behaviour of PYR was investigated in fresh water and seawater, and the effects of the main photosensitive components including dissolved organic matter (DOM), chloride, Fe3+, NO3− and HCO3− were evaluated in aqueous solutions, and the degradation products determined by gas chromatography–mass spectrometry (GC-MS). Results The photolysis of PYR was much faster in fresh water than in seawater, where DOM and chloride played a role. As DOM model substances, humic acid (HA) and Suwannee River natural organic matter (SRNOM) both promoted PYR degradation, whereas chloride inhibited PYR’s degradation in the presence of SRNOM and resulted in a chlorinated byproduct. Ferric, nitrate and bicarbonate ions promoted PYR degradation. Discussion PYR is a refractory pesticide for photodegradation, especially in saline water environments where a chlorinated product was formed. The result indicates a high ecological risk of residual PYR in estuarine and nearshore water.