Molecular insights into the reactivity of aquatic natural organic matter towards hydroxyl (•OH) and sulfate (SO4•-) radicals using FT-ICR MS

The higher scavenging capacity of natural organic matter (NOM) to hydroxyl radical ((OH)-O-center dot) than sulfate radical (SO4 center dot-) has been long-acknowledged. However, the difference in reactivity and the influence of initial characteristics, especially at the molecular-level, remain unaddressed. In this study, the reactivities of different NOM isolates to (OH)-O-center dot and SO4 center dot- were compared based on the determined second-order rate constants following the depletion of UV254-absorbing moieties. Three NOM isolates with varying characteristics were selected to investigate the influence of initial characteristics on their reactivities. With the identified reactive molecules using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), the distinct reactivity between the radicals and the influence of the initial characteristics were illustrated. The reactivity towards SO4 center dot- was dominated by the electron density of the molecules (i.e., double bond equivalent (DBE)), while that of (OH)-O-center dot was also shaped by molecular size (i.e., m/z) and composition (i.e., N- or S-incorporation). The examination on the exclusively reactive molecules (accounting for 10-20%) reflected a preferred H-abstraction by (OH)-O-center dot and decarboxylation by SO4 center dot-. Moreover, the analysis on the shared reactive molecules (80-90%) based on the UV254 versus electron-donating capacity (EDC) dependency revealed a prevalent (OH)-O-center dot addition while single electron transfer to SO4 center dot-. The different reaction rates associated with the proposed transformation pathways supported the observed higher reactivity of NOM to (OH)-O-center dot than SO4 center dot-.

Automated summary

This study compared the reactivities of different NOM isolates to hydroxyl radical and sulfate radical, finding that the reaction was influenced by factors such as electron density and molecular size. Certain reactive molecules tended to prefer hydrogen abstraction by hydroxyl radical and decarboxylation by sulfate radical.