Picolinic acid-mediated Mn(II) activated periodate for ultrafast and selective degradation of emerging contaminants: Key role of high-valent Mn-oxo species

The utilization of periodate (PI, IO4-) in metal-based advanced oxidation processes (AOPs) for the elimination of emerging contaminants (ECs) have garnered significant attention. However, the commonly used homogeneous metal catalyst Mn(II) performs inadequately in activating PI. Herein, we exploited a novel AOP technology by employing the complex of Mn(II) with the biodegradable picolinic acid (PICA) to activate PI for the degradation of electron-rich pollutants. The performance of the Mn(II)-PICA complex surpassed that of ligand-free Mn(II) and other Mn(II) complexes with common aminopolycarboxylate ligands. Through scavenger, sulfoxide-probe transformation, and O-18 isotope-labeling experiments, we confirmed that the dominant reactive oxidant generated in the Mn(II)-PICA/PI system was high-valent manganese-oxo species (Mn(V)=O). Due to its reliance on Mn(V)=O, the Mn(II)-PICA/PI process exhibited remarkable selectivity and strong anti-interference during EC oxidation in complex water matrices. Nine structurally diverse pollutants were selected for evaluation, and their lnk(obs) values in the Mn(II)-PICA/PI system correlated well with their electrophilic/nucleophilic indexes, E-HOMO, and vertical IP (R-2 = 0.79-0.94). Additionally, IO4- was converted into non-toxic iodate (IO3-) without producing undesired iodine species such as HOI, I-2, and I-3(-). This study provides a novel protocol for metal-based AOPs using PI in combination with chelating agents and high-valent metal-oxo species formation during water purification.