Modelling the occurrence and reactivity of hydroxyl radicals in surface waters: implications for the fate of selected pesticides

Abstract

This paper reports a simple model to describe the formation and reactivity of hydroxyl radicals in the whole column of surface freshwater systems. The model is based on empirical irradiation data and it is a function of the water chemical composition (the photochemically significant parameters Non-Purgeable Organic Carbon–NPOC–, nitrate, nitrite, carbonate and bicarbonate), the water body conformation, and the water absorption spectrum in a simplified Lambert-Beer approach. The purpose is to derive the lifetime of dissolved molecules, due to the reaction with •OH, on the basis of their second-order rate constants with the hydroxyl radical. It is also proposed a simplified (and approximated) approach to simulate the absorption spectrum of water when the latter is not available, based on the value of the NPOC. Such a simulation can be useful when the model is adopted to describe a degradation scenario for a certain compound, without a direct link to a definite ecosystem. The model was applied to the lifetime of various pesticides in surface water bodies, and it is suggested that the lifetime of a given compound can be very variable in different systems, even more than the lifetime of different compounds in the same water body. The variations of the chemical composition and of the depth of the water column are the main reasons for the reported finding.

Keywords:

• photochemistry
• photochemical fate
• sensitised photolysis
• pollutant photodegradation

Acknowledgements

Financial support by PNRA–Progetto Antartide, INCA Consortium (GLOB CHEM Working Group), and Università di Torino–Ricerca Locale is gratefully acknowledged. The work of RD in Torino was supported by MIUR–Progetto India and Compagnia di San Paolo, Torino, Italy.