Advanced oxidation processes of coumarins by hydroperoxyl radical: An experimental and theoretical study, and ecotoxicology assessment

Abstract Coumarin derivatives are an example of stable molecules with very complex biodegradation routes in wastewaters. The use of advanced oxidation processes offers a way to their chemical modification to less toxic products. In this study, the experimental reactivity of 4-hydroxycoumarin (1) and its two derivatives towards hydroperoxyl radical (HOO•) was proven by the ESR spectroscopy. The absence of measurable concentration of carbon- and oxygen-centered radicals, except for DEPMPO-HOO• adduct, showed that stable products were formed in the reaction. Two novel mechanisms, namely hydrogen atom abstraction - radical–radical coupling (HAT-RRC) and radical adduct formation-hydrogen atom abstraction (RAF-HAA) were suggested as possible reaction routes between HOO• and coumarin derivatives. Thermodynamic and kinetic parameters were investigated in detail to explain the plausibility of these two mechanisms with special emphasis on the determination of the most active positions. The theoretical calculations were accompanied by the Natural Bond Orbital Theory and Quantum Theory of Atoms in Molecules. The thermodynamically preferred mechanism of the reaction was RAF-HAA, while the highest reaction rate was obtained for the position C3 of 1. The preferred mechanism was further verified by the UV–Vis spectrophotometry and excited state optimization. The examined routes lead to the formation of stable products with lower toxicity towards aquatic organisms, as shown by the eco-toxicology assessment.