Superoxide radicals (O₂^•-) have been suggested as an important chain carrier in the radical chain reaction that promotes ozone (O₃) decomposition to hydroxyl radicals (^•OH) during ozonation. However, due to the difficulty in measuring transient O₂^•- concentrations, this hypothesis has not been verified under realistic ozonation conditions during water treatment. In this study, a probe compound was used in combination with kinetic modeling to evaluate the role of O₂^•- for O₃ decomposition during ozonation of synthetic solutions with model promotors and inhibitors (methanol and acetate or tert-butanol) and natural waters (one groundwater and two surface waters). By measurement of the abatement of spiked tetrachloromethane (as a O₂^•- probe), the O₂^•- exposure during ozonation was determined. Based on the measured O₂^•- exposures, the relative contribution of O₂^•- to O₃ decomposition, in comparison to OH^-, ^•OH, and dissolved organic matter (DOM), was quantitatively evaluated using kinetic modeling. The results show that water compositions (e.g., the concentration of promotors and inhibitors, and the O₃ reactivity of DOM) have a considerable effect on the extent of the O₂^•--promoted radical chain reaction during ozonation. In general, the reaction with O₂^•- accounted for ∼59-70% and ∼45-52% of the overall O₃ decomposition during ozonation of the selected synthetic solutions and natural waters, respectively. This confirms that O₂^•- plays a critical role in promoting O₃ decomposition to ^•OH. Overall, this study provides new insights on the controlling factors for ozone stability during ozonation processes.

Recent studies suggested that long-lived photooxidants (LLPO), which are reactive intermediates formed during irradiation of dissolved organic matter (DOM), may consist of phenoxyl radicals derived from phenolic moieties of the DOM. Besides the well-studied excited triplet states of chromophoric DOM (³CDOM*), LLPO presumably are important photooxidants for the transformation of electron-rich contaminants in surface waters. The main objective of this study was to further test the potential role of phenoxyl radical as LLPO. Suwannee River fulvic acid (SRFA) as a model DOM was pre-oxidised using the phenol-reactive oxidants chlorine and ozone, followed by its characterization by the specific UV absorption at 254 nm (SUVA₂₅₄), the ratio of absorbance at λ = 254 nm and λ = 365 nm (E2:E3), and the electron donating capacity (EDC). Subsequently, the photoreactivity of pre-oxidized SRFA was tested using 3,4-dimethoxyphenol (DMOP) as a LLPO probe compound at two initial concentrations ([DMOP]₀ = 0.1 and 5.0 μM). Linear inter-correlations were observed for the relative changes in SUVA₂₅₄, E2:E3, and EDC for increasing oxidant doses. Pseudo-first-order transformation rate constants normalized to the changing SRFA absorption rate (i.e., and for 0.1 and 5.0 µM, respectively) exhibited the following distinct trends: The LLPO-dominated ratio decreased with increasing oxidant dose and with decreasing SUVA₂₅₄ and EDC, while the ³CDOM*-dominated ratio positively correlated with E2:E3. Finally, it was concluded that precursors of ³CDOM* and LLPO are chemically modified differently by pre-oxidation of DOM, and LLPO precursors likely consist of phenolic moieties of DOM, suggesting phenoxyl radicals as LLPO.

Amides are common constituents in natural organic matter and synthetic chemicals. In this study, we investigated kinetics and mechanisms of the reactions of chlorine with seven amides, including acetamide, N-methylformamide, N-methylacetamide, benzamide, N-methylbenzamide, N-propylbenzamide, and N-(benzoylglycyl)glycine amide. Apparent second-order rate constants for the reactions of the amides with chlorine at pH 8 are in the range of 5.8 × 10^-3 - 1.8 M^-1s^-1 and activation energies in the range of 62-88 kJ/mol. The second-order rate constants for the reactions of chlorine with different amides decrease with increasing electron donor character of the substituents on the amide-N and N-carbonyl-C in the amide structures. Hypochlorite (^-OCl) dominates the reactions of chlorine with amides yielding N-chloramides with species-specific second-order rate constants in the range of 7.3 × 10^-3 - 2.3 M^-1s^-1. Kinetic model simulations suggest that N-chlorinated primary amides further react with HOCl with second-order rate constants in the order of 10 M^-1s^-1. The chlorination products of amides, N-chloramides are reactive towards phenolic compounds, forming chlorinated phenols via electrophilic aromatic substitution (phenol and resorcinol) and quinone via electron transfer (hydroquinone). Meanwhile, N-chloramides were recycled to the parent amides. At neutral pH, apparent second-order rate constants for the reactions between phenols and N-chloramides are in the order of 10^-4-0.1 M^-1s^-1, comparable to those with chloramine. The findings of this study improve the understanding of the fate of amides and chlorine during chlorination processes.