Molecular-level insights into oxidant sensitivity of algal extracellular organic matter and disinfection by-product formation during chlorination/ozonation.

Algal blooms release substantial extracellular organic matter (EOM), challenging drinking water safety. Disinfection process inevitably transforms EOM, which is recognized as an important precursor of disinfection by-products (DBPs). However, the molecular-level mechanisms and oxidant-sensitive pathways underlying these transformations remain unresolved. Herein, we investigated dose-dependent reactions of chlorine or ozone with EOM to elucidate selective transformation pathways and associated DBPs formation. Both disinfectants decreased aromaticity, apparent molecular weight, total organic carbon, and electron donating capacity in a dose-dependent manner. In terms of molecular reactivity, as indicated by molecular formula analysis, chlorine preferentially targeted aliphatic and moderately oxidized N-containing compounds, whereas ozone selectively transformed electron-rich, less-oxidized aromatics, with hydroxyl radical non-selectively expanding the reactive molecular pool. Machine learning analysis further corroborated these oxidation preferences, revealing that disinfectant susceptibility was largely determined by molecular weight, unsaturation, and heteroatom content. This molecular sensitivity governed DBPs formation, which manifested as the production of large, aromatic, multi-chlorinated formulas under chlorination and small, more uniformly distributed formulas under ozonation. This study provides molecular-level insights into oxidative selectivity of disinfectants in EOM transformation, with implications for managing DBPs formation and drinking water safety.