Electrochemically Generating High-Spin MnIII for the Targeted Production of 1O2 from O2 Mediated by Dissolved Organic Matter

Abstract Dissolved organic matter (DOM), ubiquitously present in aquatic systems, exerts a significant impact on advanced oxidation processes (AOPs), typically acting as an inhibitor. However, our investigation in this work reveals that DOM can function as an activator of molecular oxygen (O2) for selective singlet oxygen (1O2) production through an innovative electrochemical regulation strategy. A positive electric field was utilized to trigger the electrochemical oxidation of MnII in MnO, generating high-spin MnIII that targets the transformation of ortho-hydroquinone moieties in DOM to ortho-semiquinone radical (ortho-SQ•–). The metal–ligand cooperation stabilized ortho-SQ•– on the MnO surface as MnII-ortho-SQ•– complexes, which effectively activated dissolved O2 into 1O2 with •O2– as an intermediate. Elementary electron-transfer steps among high-spin MnIII, ortho-hydroquinone, and O2 were systematically elucidated, based on which a quantitative relationship between the structural characteristics of ortho-hydroquinone and 1O2 production was established. Electron-donating groups on ortho-hydroquinone showed an enhancing effect on 1O2 production by promoting the formation of surface MnII-ortho-SQ•– complexes as well as their reactions with O2. By integrating the DOM-mediated O2 activation pathway into an electrochemical oxidation system, we achieved a remarkable improvement in phenol degradation efficiency, with a twofold acceleration of the degradation rate and over 50% reduction in energy consumption. This work establishes a mechanistic framework describing how DOM synergizes with electrochemical oxidation reactions to in situ activate dissolved O2 under an electric field drive, paving a new avenue for developing innovative water remediation processes by exploiting DOM-O2 interactions in aquatic systems.