Fotoreattività dell'erbicida propanil in acqua di risaia

The photoreactivity of propanil, a post-emergence contact herbicide that is widely used in rice cultivation, has been studied experimentally in paddy-field water. Particularly, the second-order reaction rate constants have been measured for the reactions between propanil and the most photochemically relevant transients: hydroxyl radicals (HO¿), carbonate radicals (CO3¿-), excited triplet states of chromophoric dissolved organic matter (3CDOM*) and singlet oxygen (1O2). Furthermore, direct photolysis quantum yield and propanil dark degradation extent at different pH values have been assessed. For some of these propanil phototransformation pathways, the formation of photoproducts has been studied. To evaluate the effective propanil photochemistry in paddy-field waters, water samples from rice fields sampled near Vercelli (Piedmont, Italy) have been photochemically characterized. Experimental results have been used to model the propanil photoreactivity in the studied paddy-water samples using the APEX software ('Aqueous Photochemistry of Environmentally occurring Xenobiotics'). Overall, the results suggest that propanil photoreacts mainly by direct photolysis and reactions with HO¿ and CO3¿, although dark degradation (possibly microbiological) is not negligible. Moreover, the dark reaction generates 3,4-dichloroaniline (3,4-DCA, a transformation intermediate of considerable concern) with practically quantitative yield, while the photochemical reactions do not or do it to a minor extent. In paddy-field water samples under laboratory irradiation the main photochemical pathways are direct photolysis and reactions with HO¿/CO3¿, while dark reactions seem to be light-inhibited. Modeling shows that photochemistry has an important role in the total propanil degradation in rice field waters with a high content of nitrate and/or nitrite. If other degradation processes dominate over photochemistry, including most notably the biological degradation, the production of 3,4-DCA can be of relevant environmental concern.