Revisiting the Graphitized Nanodiamond-Mediated Activation of Peroxymonosulfate: Singlet Oxygenation versus Electron Transfer

Graphitized nanodiamonds (ND) exhibit outstanding capability in activating peroxymonosulfate (PMS) for the removal of aqueous organic micropollutants (OMPs). However, controversial observation and interpretation regarding the effect of graphitization degree on ND's activity and the role of singlet oxygen (O-1(2)) in OMP degradation need to be clarified. Herein, we investigated graphitized ND-mediated PMS activation. Experiments show that the activity of ND increases first and then decreases with the monotonically increased graphitization degree. Further experimental and theoretical studies unveil that the intensified surface graphitization alters the degradation mechanism from singlet oxygenation to an electron-transfer pathway. Moreover, for the first time, we applied a self-constructed, time-resolved phosphorescence detection system to provide direct evidence for O-1(2) production in the PMS-based system. This work not only elucidates the graphitization degree-dependent activation mechanism of PMS but also provides a reliable detection system for in situ analysis of O-1(2) in future studies.