Advanced oxidation process of coumarins by hydroxyl radical: Towards the new mechanism leading to less toxic products

Abstract

The excessive use of coumarins and their derivatives is becoming an ecological concern due to their toxicity towards different organisms. One of the possible ways of their removal is the advanced oxidation processes. The oxidative breakdown of 4-hydroxycoumarin and its two derivatives, induced by a very powerful oxidizer hydroxyl radical (HO•), was investigated experimentally and theoretically. The new mechanism, namely radical adduct formation followed by hydrogen atom abstraction (RAF-HAA), was proposed. The thermodynamic parameters and rate constants, calculated by the Transition State Theory for several active positions indicated a possible reaction in which less toxic products were obtained, as confirmed by the ecotoxicity assessment. The mechanism included the reaction with two HO• and the introduction of an additional OH group to the structure. These results were compared to the more common mechanism that includes HAA between the OH group of 4-hydroxycoumarin and HO•. Based on the Quantum Theory of Atoms in Molecules and Natural Bond Orbital theory the exo-coupled electron transfer (PCET) mechanism was pointed out as a dominant pathway. Radical addiction with another HO•, followed by keto-enol tautomerism, led to the formation of the stable final product, identical to one obtained in the RAF-HAA mechanism. The reaction rates for RAF-HAA were higher then those for pure HAA and reactions were more spontaneous, therefore leading to the conclusion that the newly proposed mechanism could be a dominant pathway for the aromatic molecules’ breakdown in the advanced oxidation processes.