Pyridine-Induced Concerted Electron–Proton Transfer Drives Rapid Oxidation of Mn(II) by Free Chlorine under Near-Neutral Conditions

Abstract

In aqueous systems, Mn(II) oxidation by chlorine involves both electron and proton transfer. However, the slow proton transfer due to water molecule’s limited proton acceptance capacity leads to low Mn(II) oxidation efficiency under near-neutral conditions. Pyridine (pK_a = 5.25), a strong proton acceptor, was selected to investigate electron–proton transfer principles in Mn(II) oxidation. Kinetic studies show that pyridine enhances Mn(II) oxidation efficiency by chlorine under neutral to alkaline conditions (pH 7.0–9.5), with maximal enhancement observed when pH approaches pyridine’s pK_a. A high kinetic isotope effect (up to 6.9) and an E_1/2-pH slope near the Nernstian value in cyclic voltammetry experiments confirm that pyridine induces a concerted electron–proton transfer (CEPT) pathway. Density functional theory calculations reveal that the pyridine-induced CEPT pathway is thermodynamically more favorable than stepwise electron transfer followed by proton transfer (ETPT) and proton transfer followed by electron transfer (PTET) pathways, explaining the enhanced oxidation efficiency. Similarly, in pyridine-free systems, increasing pH from 7.0 to 9.5 promotes Mn(II) oxidation efficiency through an OH^–-mediated pathway switching from ETPT to PTET and CEPT pathways. This study demonstrates the controlling role of proton transfer in the Mn(II) oxidation process, providing theoretical guidance for the rational selection or design of Mn-removal catalysts.