Transformation of dissolved organic matter and its model compounds by dichlorine monoxide and the corresponding generation of chloro-organic byproducts

Abstract

Dichlorine monoxide (Cl₂O) is an overlooked but highly reactive free chlorine species capable of degrading recalcitrant organic pollutants in water. A recently proposed strategy introduces free chlorine (mainly HOCl/OCl⁻) directly in high chloride-containing wastewater to facilitate the in situ formation of Cl₂O to degrade recalcitrant organic pollutants. In this study, the transformation of dissolved organic matter (DOM) and representative model compounds by Cl₂O was systematically investigated, along with the corresponding generation of chloro-organic byproducts and the associated genotoxicity. Cl₂O showed limited reactivity toward 3-hydroxybutyric acid and methoxyacetic acid, two aliphatic model compounds of DOM, while remaining highly reactive with resorcinol, 3-hydroxybenzoic acid, uracil, L-tyrosine, and sorbic acid, with second-order rate constants towards Cl₂O (k_Cl2O) approximately 1–6 orders of magnitudes higher than those toward Cl₂ (k_Cl2), HOCl (k_HOCl) and ClO⁻ (k_ClO⁻). Among these five model compounds reactive with Cl₂O, the formation of known chloro-organic byproducts was confirmed for resorcinol, 3-hydroxybenzoic acid, and uracil, whereas Cl₂O contributed little when reacting with L-tyrosine and sorbic acid. Mechanistic investigation using non-targeted screening of transformation products by ultra-high-performance liquid chromatography with quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS) revealed that Cl₂O is more likely to react through chlorination pathways with resorcinol, while reacting through non-chlorination pathways (e.g., oxidation) with L-tyrosine. Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS)-based non-targeted analysis was applied to track molecular-level changes of DOM and the formation of unknown chloro-organic byproducts in reactions with Cl₂O. Compared to the scenario without Cl₂O, the presence of Cl₂O resulted in a significant decrease in the number and total intensity of unknown chloro-organic byproducts (8.76-fold and 12.54-fold decreases, respectively, for landfill leachate DOM, and 2.09-fold and 1.77-fold decreases, respectively, for saline sewage DOM). Cl₂O was found to preferentially react with DOM moieties exhibiting high aromaticity, producing compounds with a higher degree of oxidation. Cl₂O also facilitated ring cleavage of nitrogen-containing aromatic moieties, as evidenced by the formation of compounds with reduced aromaticity. In addition, the presence of Cl₂O reduced the genotoxicity of landfill leachate DOM by approximately 45% and maintained comparable genotoxicity levels when reacting with saline sewage DOM relative to the scenario without Cl₂O. The findings of this study highlight the merits of using Cl₂O to effectively degrade organic pollutants, suppress the formation of chloro-organic byproducts arising from reactions with DOM, and reduce overall toxicity in high chloride-containing wastewaters. In addition, the study advances the fundamental understanding of Cl₂O-mediated transformation mechanisms of organic compounds.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Chii SHANG (Supervisor)