Excited Triplet Humic Acid Boosts UVA-Activated Periodate (PI) Oxidation Process for Enhanced Micropollutant Degradation

Activation of periodate (PI) by ultraviolet radiation at the UVA range (UVA/PI) is promising for the oxidative treatment of emerging contaminants in aqueous environments. This study is the first to uncover the inherent mechanisms of the enhancing effects of humic acid (HA) on the UVA/PI process. Accelerated degradation of the model compound naproxen (NAP) and six other structurally diverse compounds was observed, with enhancement factors of first-order rate constants varying from 1.2 to 5.5 in the presence of 2 mgC/L HA. The excited triplet of HA (³HA*), generated from HA photolysis, was identified as the dominant intermediate, and it reacts rapidly with PI at a diffusion-controlled second-order rate constant of about 1.19 × 10⁹ M^-1s^-1. This interaction led to the generation of the hydroxyl radical (•OH) and ozone (O₃), with •OH as the predominant oxidant responsible for over 70% of NAP degradation. Aromatic ketone moieties within HA were potential triplet candidates, as suggested by the model triplet sensitizer. Additionally, Suwannee River natural organic matter (SRNOM) was found to similarly enhance the performance of the UVA/PI process, indicating that this effect may extend to other natural organic matter sources. This study offers a novel perspective on PI-based advanced oxidation processes, revealing that HA can serve as an in situ enhancer to drive pollutant degradation with reduced reliance on chemical activators. This work not only advances the understanding of UVA/PI processes but also proposes a sustainable approach for optimizing water treatment systems.