Rate laws and kinetic modeling of N-ethyl perfluorooctane sulfonamidoethanol (N-EtFOSE) transformation by hydroxyl radical in aqueous solution

摘要

The degradation of perfluorochemicals (PFCs) by hydroxyl radical (`OH) follows complex pathways resulting in stable products. Kinetic models are needed to predict the product distribution of ?OH-initiated PFC degradation under environmental and treatment conditions. The bimolecular rate constants were measured in water for the reaction of `OH and N-ethyl perfluorooctane sulfonamidoethanol (N-EtFOSE), and intermediates, N-ethyl perfluorooctane sulfonamidoacetate (N-EtFOSAA), N-ethyl perfluorooctane sulfonamide (N-EtFOSA) and perfluorooctane sulfonamidoacetate (FOSAA). Under standard conditions (pH = 6, 25 ± 2 ℃, C_o PFC = 5-10 μg L~(-1), C_o H_2O_2 = 10 mM, irradiation intensity = 765 W m~(-2)), the measured constants for N-EtFOSE, N-EtFOSAA, N-EtFOSA and FOSAA were (1.05 ± 0.12) × 10~9 M~(-1)s~(-1), (0.68 ± 0.05) × 10~9 M~(-1) s~(-1) (0.68 ± 0.05)× 10~9 M~(-1) s~(-1) and (0.53 ± 0.05)× 10~9 M~(-1)s~(-1) respectively. Constants in the pH range from 1 to 10 varied within a factor of 2-4 for most compounds. Over a period of 2-days, N-EtFOSE reacted directly (without forming long-lived intermediates) to perfluorooctane sulfonamide (FOSA) (18.8%) and perfluorooctanoic acid (PFOA) (39.1%). N-EtFOSE reacted via oxidation of the ethanolic hydroxyl group to N-EtFOSAA (12.4%) and N-dealkylation to N-EtFOSA (13.3%) and FOSAA (0.2%) and unknown intermediates. In sunlit surface waters, the `OH-induced transformation of N-substituted sulfonamide compounds to photostable products occurs on a time scale of days to weeks by model prediction.