Enhanced Decontamination in Mn(II)/Periodate Systems with EDTA: Mechanistic Insights into Self-Accelerating Degradation of Pollutants

Research promoting the use of aminocarboxylate ligands to enhance metal ion oxidation of pollutants in advanced oxidation processes (AOPs) has gained increasing interest in recent years. In this work, the most common ligand, ethylenediaminetetraacetic acid (EDTA), was used to enhance Mn(II) activation of periodate (PI, IO₄^-) for the degradation of the sulfisoxazole (SIZ). Chromatographic analysis of EDTA showed that EDTA, like SIZ, also exhibits self-accelerated degradation in this system. This suggests that the accelerated degradation of pollutants may be closely related to the structural changes of Mn(II)-EDTA. Oxidative pretreatment of Mn(II)-EDTA showed that the decay of Mn(II)-EDTA was accompanied by the disappearance of the self-accelerated degradation characteristic of the pollutants. Mass spectrometry analysis suggested that the decarboxymethylation of Mn(II)-EDTA to Mn(II)-ED3A is an important pathway for its structural transformation. The standard ED3A exhibited superior assistance to the Mn(II)/PI system compared to EDTA, without showing the self-accelerated degradation trend. Density functional theory (DFT) calculations indicated that Mn(II)-ED3A more readily reacts with PI, and the corresponding oxygen transfer product (O-Mn(IV)-ED3A)^- possesses stronger oxidative properties. A series of mechanistic investigations confirmed the formation of transient Mn-oxo species, accompanied by the generation of hydroxyl radicals (^•OH) as secondary radicals in the system. The degradation of SIZ primarily involves processes such as hydrolysis, hydroxylation, and nitration, accompanied by the attenuation of toxicity. This study provides mechanistic insights into the evolution of ligands in the Mn(II)/EDTA/PI system and highlights the need for greater attention to structural changes of organic ligands in catalytic oxidation systems.