Triplet-State Dissolved-Organic-Matter-Activated Oxalic Acid Triggers Contaminant Reductive Degradation: The Pivotal Role of the Carbon Dioxide Radical Anion.

While oxidative reactive species (RS) have been extensively studied for their critical role in trace organic contaminant (TrOC) degradation, the mechanistic contributions of reductive RS in aquatic environments remain poorly understood. In this study, we report a previously unrecognized photochemical pathway for generating a highly reductive carbon dioxide radical anion (CO2• -) through synergistic interactions between excited triplet-state dissolved organic matter (3DOM*) and ubiquitous low-molecular-weight carboxylic acids (LCAs) in surface waters. Using carbamazepine (CBZ) as a model compound, electron paramagnetic resonance confirmed photogenerated CO2• - in mixed oxalic acid and anthraquinone-2-sulfonate (AQ2S) systems, driving a 429% increase in the CBZ photodegradation rate with respect to AQ2S alone. The CO2• --driven enhancement was generalizable across diverse 3DOM* surrogates and LCAs as well as natural DOM samples. By combining advanced ab initio molecular dynamics simulations, quantum chemical calculations, and mass spectrometry analysis, we demonstrate that photogenerated CO2• - initiates ultrafast (within picoseconds) electron transfer reactions with CBZ, leading to the formation of degradation products. This study not only reveals a new source of reductive CO2• - in surface waters but also suggests the important role of reductive RS in the TrOC transformation in sunlit surface waters.