Oxalic acid alters electron-transfer chain reactions of photoexcited humic acid and generates both oxidative/reductive radicals.

Humic acid (HA) as a vital component of dissolved organic matter can be photochemically driven to produce reactive oxygen species (ROS), which are essential for aquatic pollutant degradation. However, the role of organic acids (e.g., oxalic acid) in HA-induced photochemical ROS production remains poorly understood. Herein, we found that the addition of 100 μM oxalic acid to 16 mgC L-1 HA enhanced hydroxyl radical (•OH) formation rates by 6.5-fold. By combining electron paramagnetic resonance, photoelectrochemical measurements, laser flash photolysis, and density functional theory calculations, this study revealed that oxalic acid interacted with triplet-state HA to accelerate electron transfer, promoting the formation of carbon dioxide radical anion and HA-derived reducing intermediates (e.g., ketyl- and semiquinone-like radicals). These intermediates readily reacted with O2 to generate superoxide radical and hydrogen peroxide, which ultimately promoted •OH formation. Furthermore, a dual-pathway involving •OH and reductive free radicals induced both oxidative and reductive degradation of halogenated pollutants. Overall, these findings provide new insights into the inter-fractional interactions of natural organic matter under solar irradiation, and underscore the previously unappreciated contribution of reductive free radicals to ROS formation and pollutant transformation in acidic wastewaters.