Chlorination amplifies source-dependent photochemical divergence between microplastic-derived and fulvic-acid dissolved organic matter.

Chlorination is widely applied before the discharge of wastewater effluent into receiving waters, yet the distinct response of artificial and non-plastic dissolved organic matter (DOM) to chlorination remains unclear. Here, fulvic-acid (FA), expanded polystyrene-derived DOM (EPS-DOM) and poly(butylene adipate-co-terephthalate)-derived DOM (PBAT-DOM) were compared to determine how chlorination-induced optical and molecular transformations regulate sulfamethazine (SMT) photodegradation. Chlorination caused pronounced source-dependent DOM restructuring. The E2/E3 ratio increased from 4.60 to 42.92 for EPS-DOM and from 3.09 to 41.50 for PBAT-DOM, but fluorescence intensities decreased by 81.0% and 97.2%, respectively. By comparison, FA showed positive responses after chlorination. FT-ICR-MS identified PBAT-DOM as the most extensively altered molecular structures. Additionally, chlorination increased the SMT photodegradation rate (kobs) from 0.0079 to 0.0111 h-1 for EPS-DOM and from 0.0073 to 0.0085 h-1 for PBAT-DOM; however, chlorinated PBAT-DOM displayed decreased kobs from 0.0089 to 0.0079 h-1 in river water, indicating greater matrix sensitivity. Mechanism analysis shows that chlorination affected the contributions of 3DOM*, 1O2, •OH, and O2•-, reduced electron-donating capacity, and altered the light-shielding effects of DOM, particularly MP-DOM. Overall, chlorination amplifies the photochemical divergence between MP-DOM and FA, making MP-DOM a more sensitive regulator of contaminant phototransformation in receiving water.