Cl• and Cl2 •– radicals contribute to the degradation of trace organic contaminants (TrOCs) such as pharmaceutical and personal care products and endocrine-disrupting chemicals. However, little is known about their reaction rate constants and mechanisms. In this study, the reaction rate constants of Cl• and Cl2 •– with 88 target compounds were determined using laser flash photolysis. Decay kinetics, product buildup kinetics, and competition kinetics were applied to track the changes in their transient spectra. Cl• exhibited quite high reactivity toward TrOCs with reaction rate constants ranging from 3.10 × 109 to 4.08 × 1010 M–1 s–1. Cl2 •– was less reactive but more selective, with reaction rate constants varying from <1 × 106 to 2.78 × 109 M–1 s–1. Three QSAR models were developed, which were capable of predicting the reaction rate constants of Cl2 •– with TrOCs bearing phenol, alkoxy benzene, and aniline groups. The detection of Cl•-adducts of many TrOCs suggested that Cl• addition was an important reaction mechanism. Single electron transfer (SET) predominated in reactions of Cl• with TrOCs bearing electron-rich moieties (e.g., sulfonamides), and their cation radicals were observed. Cl• might also abstract hydrogen atoms from phenolic compounds to generate phenoxyl radicals. Moreover, Cl• could react with TrOCs through multiple pathways since more than one transient intermediate was detected simultaneously. SET was the major reaction mechanism of Cl2 •– reactions with TrOCs bearing phenols, alkoxy benzenes, and anilines groups. Cl2 •– was found to play an important role in TrOC degradation, though it has been often neglected in previous studies. The results improve the understanding of halogen radical-involved chemistry in TrOC degradation.

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Chlorine radicals, including Cl• and Cl2 •–, can be produced in sunlight waters (rivers, oceans, and lakes) or water treatment processes (e.g., electrochemical and advanced oxidation processes). Dissolved organic matter (DOM) is a major reactant with, or a scavenger of, Cl• and Cl2 •– in water, but limited quantitative information exists regarding the influence of DOM structure on its reactivity with Cl• and Cl2 •–. This study aimed at quantifying the reaction rates and the formation of chlorinated organic byproducts produced from Cl• and Cl2 •– reactions with DOM. Laser flash photolysis experiments were conducted to quantify the second-order reaction rate constants of 19 DOM isolates with Cl• (k DOM–Cl•) and Cl2 •– (k DOM–Cl2•–), and compare those with the hydroxyl radical rate constants (k DOM–•OH). The values for k DOM–Cl• ((3.71 ± 0.34) × 108 to (1.52 ± 1.56) × 109 MC –1 s–1) were orders of magnitude greater than the k DOM–Cl2•– values ((4.60 ± 0.90) × 106 to (3.57 ± 0.53) × 107 MC –1 s–1). k DOM–Cl• negatively correlated with the weight-averaged molecular weight (M W) due to the diffusion-controlled reactions. DOM with high aromaticity and total antioxidant capacity tended to react faster with Cl2 •–. During the same experiments, we also monitored the formation of chlorinated byproducts through the evolution of total organic chlorine (TOCl) as a function of chlorine radical oxidant exposure (CT value). Maximum TOCl occurred at a CT of 4–8 × 10–12 M·s for Cl• and 1.1–2.2 × 10–10 M·s for Cl2 •–. These results signify the importance of DOM in scavenging chlorine radicals and the potential risks of producing chlorinated byproducts of unknown toxicity.

Bromine radicals can pose great impacts on the photochemical transformation of trace organic contaminants in natural and engineered waters. However, the reaction kinetics and mechanisms involved are barely known. In this work, second-order reaction rate constants with Br• and Br2 •– were determined for 70 common trace organic contaminants and for 17 model compounds using laser flash photolysis and steady-state competition kinetics. The k Br• values ranged from <108 to (2.86 ± 0.31) × 1010 M–1 s–1 and the k Br2 •– values from <105 to (1.18 ± 0.09) × 109 M–1 s–1 at pH 7.0. Six quantitative structure–activity relationships were developed, which allow predicting additional unknown k Br• and k Br2 •– values. Single-electron transfer was shown to be a favored pathway for the reactions of Br• and Br2 •– with trace organic contaminants, and this was supported by transient spectroscopy and quantum chemical calculations. This study is essential in advancing the scientific understanding of halogen radical-involved chemistry in contaminant transformation.

Advanced oxidation processes (AOPs) can degrade a wide range of trace organic contaminants (TrOCs) to improve the quality of potable water or discharged wastewater effluents. Their effectiveness is impacted, however, by the dissolved organic matter (DOM) that is ubiquitous in all water sources. During the application of an AOP, DOM can scavenge radicals and/or block light penetration, therefore impacting their effectiveness toward contaminant transformation. The multiple ways in which different types or sources of DOM can impact oxidative water purification processes are critically reviewed. DOM can inhibit the degradation of TrOCs, but it can also enhance the formation and reactivity of useful radicals for contaminants elimination and alter the transformation pathways of contaminants. An indepth analysis highlights the inhibitory effect of DOM on the degradation efficiency of TrOCs based on DOM's structure and optical properties and its reactivity toward oxidants as well as the synergistic contribution of DOM to the transformation of TrOCs from the analysis of DOM's redox properties and DOM's transient intermediates. AOPs can alter DOM structure properties as well as and influence types, mechanisms, and extent of oxidation byproducts formation. Research needs are proposed to advance practical understanding of how DOM can be exploited to improve oxidative water purification.