Composition and photodegradation transformation of Dissolved Organic Matter from microplastics versus natural sources: impacts on copper (Cu) and tetracycline (TC) binding behaviors.

Dissolved organic matter (DOM) regulates pollutant fate in aquatic ecosystems, but its behavior depends strongly on sources and photodegradation. Microplastic-derived DOM (MPDOM) is increasingly abundant and may differ from natural DOM, yet its photodegradation and pollutant complexation mechanisms remain unclear. This study compared photodegradation characteristics and Cu/tetracycline (TC) binding of MPDOM from petroleum-based (HDPE) and biodegradable (PLA) microplastics with natural DOM from leaf litter and Potamogeton crispus using EEM-PARAFAC and 2D-COS. The results showed that a unique tyrosine-like component (C1) accumulated during MPDOM photodegradation, and aromaticity decreased more in MPDOM (a: 12.10-46.94%) than in natural DOM (a: 6.31-19.64%). In terms of pollutant binding, natural DOM exhibited stronger Cu binding affinity (LogK: 4.46-5.69) than MPDOM (LogK: 3.95-4.69), primarily driven by tryptophan-like components, while initial TC binding affinity was comparable but dominated by different protein-like components (tryptophan-like in natural DOM vs. tyrosine-like in MPDOM). Photo-activated amide groups were identified as the key binding sites for MPDOM with Cu and TC. Notably, TC binding affinity of MPDOM weakened after photodegradation, opposite to the trend observed for natural DOM. This weakening may increase the free TC fraction and potentially promote antibiotic-resistance risks in microplastic-contaminated waters. These findings provide mechanistic insights into the dynamic interactions between MPDOM and contaminants in irradiated waters.