Molecular-level analysis of chemical transformation of algal extracellular organic matter during seawater ozonation: Dominant reaction pathways and impacts of halide ions

Abstract

Halide-rich matrices inevitably influence ozonation performance and transformation pathways, yet molecularlevel insights into how seawater composition governs organic matter conversion and byproduct formation remain limited. This study presents the application of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) to elucidate O3-driven transformation pathways of algae-derived extracellular organic matter (EOM) in a seawater matrix and to delineate the concomitant contribution of halogen-based oxidants to EOM compositional evolution. Bulk optical analyses showed that EOM enriched in highly O3-labile protein-like constituents progressively fragmented into aliphatic fractions via humified intermediates. Consistently, FT-ICRMS revealed a monotonic compositional shift toward aliphatic and highly unsaturated aromatic fractions, with minimal nitrogen and sulfur loss under competition between EOM and background halides for O3. Molecular descriptors and paired mass difference analyses indicated increasing oxidation state and saturation degree, dominated by oxygenation and dealkylation routes, whereas carbonyl functionalization coupled with dehydrogenation elevated double bond equivalents at high O3 dosages. In halide-laden seawater, ozonation preferentially promoted the formation of brominated disinfection byproducts (Br-DBPs) via electrophilic addition (+HOBr) and substitution (+Br-H) mediated by non-radical reactive bromine species selectively targeting lignin-like moieties, while halogen exchange reactions reduced the relative abundance of Cl-DBPs. I-DBPs likely originated not only from iodine-centered radical iodination but also from coupled iodination/non-iodination pathways (e.g., decarboxylation and dealkylation). From a practical perspective, molecular-level identification of EOM oxidation products enables assessment of O3-induced variations in membrane fouling propensity while concurrently unveiling potential toxicity risks associated with previously unrecognized X-DBPs that may be discharged into permeate or concentrate streams.