Photoreactivity of microplastic-derived dissolved organic matter governed by its oxygen-containing functional groups in hydrophobic/hydrophilic fractions rather than molecular weights

Dissolved organic matter (DOM) arising from global microplastic (MP) pollution exhibits strong photoreactivity, while decisive molecular structures governing photoreactivity remains unknown. This study revealed the relationship between structural features and photoreactivity of MP-DOM through bulk samples with different photoaging times (5-70 days) and fractional samples with distinct molecular weights (<1, 1-3 and >3 kDa), hydrophobic/hydrophilic properties (hydrophobic acid, hydrophobic neutral and hydrophilic fractions) and functional groups (carbonyl, hydroxyl and aromatic structures), as probed via sulfamethazine photodegradation. Results showed that bulk MP-DOM inhibited sulfamethazine photodegradation, with 1.83-2.23 folds reduction in observed rate constants, whereas the inhibition diminished under prolonged photoaging times. Multivariate fractionation experiments integrating PLS-SEM analysis revealed that the reversal phenomenon was primarily governed by oxygen-containing functional groups and hydrophilic/hydrophobic fractions within MP-DOM. The hydroxyl group in hydrophilic fractions contributed to strong light screening and quenching effects, while carbonyl group in hydrophobic fractions acted as photosensitizers facilitating reactive oxygen species generation (notably •OH), thereby critically modulating sulfamethazine photodegradation. Unexpectedly, molecular weights of MP-DOM exhibited negligible effects on the photoreactivity, due to the balanced quenching and photosensitization effects. This work firstly identifies the critical structures governing photoreactivity of MP-DOM, which addressed a long-standing enigma regarding previously reported divergent effects of MP-DOM.