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

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, Co PFC = 5–10 μg L−1, Co H2O2 = 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) × 109 M−1 s−1, (0.68 ± 0.05) × 109 M−1 s−1, (0.68 ± 0.05) × 109 M−1 s−1 and (0.53 ± 0.05) × 109 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.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (170 K)Download as PowerPoint slideHighlights► Bimolecular rate constants for photodegradation of PFCs have been determined. ► Structure affects hydroxyl radical initiated transformation of PFCs. ► Kinetic model predicts the transformation of N-EtFOSE to stable products. ► Model and experiment results show significant portion of unknown products. ► pH insignificantly influences phototransformation rate constants of PFCs.