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 best expressed as the average depth, 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 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.

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The photocatalytic process is a possible way for obtaining value-added chemicals from glycerol (Gly). The kinetic and mechanism of transformation of glycerol are explored as a function of several operating parameters (type of the catalyst, pH, substrate concentration, fluorination of the catalyst surface). Different transformation rates are observed for glycerol on pristine P25 and Merck powders. By use of Degussa P25, the phototransformation rate shows a sharp maximum at low glycerol concentration, thus deviating from Langmuir-Hinshelwood behavior. These results are qualitatively rationalized with a kinetic model that includes substrate mediated carrier recombination (back reaction) and invokes two reactive sites on P25 TiO

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