Chemical oxidation processes have been widely applied to water treatment and may serve as a tool to minimize the release of micropollutants (e g pharmaceuticals and endocrine disruptors) from municipal wastewater effluents into the aquatic environment The potential of several oxidants for the transformation of selected micropollutants such as atenolol, carbamazepine, 17 alpha-ethinylestradiol (EE2), ibuprofen, and sulfamethoxazole was assessed and compared The oxidants include chlorine, chlorine dioxide, ferrate(VI), and ozone as selective oxidants versus hydroxyl radicals as non-selective oxidant. Second-order rate constants (k) for the reaction of each oxidant show that the selective oxidants react only with some electron-rich organic moieties (ERMs), such as phenols, anilines, olefins, and deprotonated-amines in contrast, hydroxyl radicals show a nearly diffusion-controlled reactivity with almost all organic moieties (k > 10(9) M-1 s(-1)) Due to a competition for oxidants between a target micropollutant and wastewater matrix (i e effluent organic matter, EfOM), a higher reaction rate with a target micropollutant does not necessarily translate into more efficient transformation For example, transformation efficiencies of EE2, a phenolic micropollutant, in a selected wastewater effluent at pH 8 varied only within a factor of 7 among the selective oxidants, even though the corresponding k for the reaction of each selective oxidant with EE2 varied over four orders of magnitude in addition, for the selective oxidants, the competition disappears rapidly after the ERMs present in EfOM are consumed In contrast, for hydroxyl radicals, the competition remains practically the same during the entire oxidation Therefore, for a given oxidant dose, the selective oxidants were more efficient than hydroxyl radicals for transforming ERMs-containing micropollutants, while hydroxyl radicals are capable of transforming micropollutants even without ERMs Besides EfOM, ammonia, nitrite, and bromide were found to affect the micropollutant transformation efficiency during chlorine or ozone treatment. (C) 2009 Elsevier Ltd All rights reserved.