Source-dependent photoreactivity of dissolved organic matter and its implications for specific disinfection byproduct formation.

Photochemical processing dictates the molecular evolution of dissolved organic matter (DOM) in sunlit waters and its subsequent reactivity during chlorination. However, how source-specific molecular signatures regulate the photochemical transformations of DOM and its associated disinfection by-product (DBP) formation potentials remains poorly understood. Here, DOM samples derived from algae (ADOM), soil (SDOM), litter (LDOM), and wildfire ash (BDOM) were exposed to 28 days of simulated solar irradiation to elucidate source-dependent molecular transformations and their impacts on specific DBP formation. DOM compositional changes were characterized using optical spectroscopy and Fourier transform-ion cyclotron resonance mass spectrometry, and specific DBP formation potentials were assessed via chlorination assays. Distinct molecular signatures differentiated DOM sources, with ADOM enriched in protein- and lipid-like compounds, whereas SDOM, LDOM, and BDOM contained greater proportions of aromatic, lignin-, and tannin-like molecules. Irradiation drove partial convergence of DOM toward a more oxidized state, lower molecular weight, and more aromatic structures via decarboxylation, dealkylation, and deamination. These transformations reduced specific carbonaceous DBP formation (up to 93 %) across most DOM sources, whereas LDOM showed a ∼9 % increase in specific trihalomethane and chloral hydrate formation, likely due to the persistence and transformation of lignin-like precursors. Notably, specific haloacetonitrile formation potentials substantially increased by 99-125 % across all DOM sources, associated with the generation of reactive nitrogenous precursors through photo-oxidation and depolymerization of macromolecules. These findings demonstrate that solar-driven DOM transformation governs distinct carbonaceous and nitrogenous DBP responses, highlighting the need for source-specific molecular insights to predict and manage DBP risks in watersheds.