Temperature modulates the dissolved organic matter‑mediated triplet and singlet oxygen formation: Implications for...

The photoactivity of dissolved organic matter (DOM) plays a key role in pollutant transformation in aquatic systems. However, the combined effects of temperature and dissolved oxygen (DO) on DOM-mediated reactive species formation remain unclear. Here, we systematically investigated temperature-dependent formation of triplet excited states and singlet oxygen and their influence on ractopamine photodegradation. From 10-50 °C, triplet formation rates increased by up to 3.5-fold, accompanied by enhanced apparent quantum yields. In fulvic and humic acid systems, singlet oxygen yields increased from 0.12 to 0.26 as kinetic enhancement offset reduced DO solubility. In contrast, singlet oxygen production in natural organic matter decreased by 20-30% at elevated temperatures due to DO limitation. Direct photolysis of ractopamine was very limited, confirming the dominance of indirect pathways. The temperature dependence of indirect degradation exhibited strong correlations (R² = 0.991 for fulvic acid, 0.942 for humic acid, and 0.724 for natural organic matter). These results demonstrate that DOM photochemistry is governed by a competitive balance between kinetic acceleration and oxygen-dependent constraints, providing mechanistic insight into contaminant fate under warming conditions.