A Kinetic-Molecular Framework for Resolving Electron-Donating Moieties in Natural and Effluent Organic Matter

The redox activity of dissolved organic matter (DOM) plays a critical role in natural and engineering aquatic systems. Although phenolic moieties are typically recognized as the dominant electron-donating moieties (EDM) in naturally occurring DOM (NOM), the chemical nature and reactivity of EDM in wastewater-derived DOM (EfOM), remain poorly understood. In this study, a kinetic categorizing framework was developed for resolving the composition of EDM within DOM by using the second-order rate constants for the reactions of the radical cation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS•+) (kapp, ABTS•+) with DOM isolates and a suite of model electron-donating compounds. Our results reveal that NOM contains a larger fraction of rapidly reacting ABTS•+ reducible components with kapp, ABTS•+ > 1 × 103 M-1 s-1, accounting for more than 50% of the total EDM. In contrast, EDM in EfOM mainly consist of antioxidants with relatively low redox activity (kapp, ABTS•+ < 10 M-1 s-1), accounting for 70% of the total EDM. Combining kinetic categorization and Fourier transform ion cyclotron resonance mass spectrometry analysis, the results suggest that EDM in EfOM are associated with nitrogen- and/or sulfur-containing heteroatom compounds. This novel method has important implications for understanding the aquatic redox chemistry of DOM and developing source-adaptive water oxidative treatment strategies in engineering systems.