Reactions of dissolved organic matter with hydroxyl radicals: effects on DOM molecular properties and modulation by Cu(ii)

Abstract

The reaction of hydroxyl radical (˙OH) with dissolved organic matter (DOM) is a key process in engineered and natural aquatic systems. In this study, the second reaction rate constants between ˙OH and three kinds of DOM (k_DOM,˙OH) were measured and the changes of molecular properties of DOM induced by ˙OH were characterized. The room-temperature k_DOM,˙OH value ranged from 3.26 to 7.38 × 10⁹ M_C⁻¹ s⁻¹. Oxidation by ˙OH led to mineralization, bleaching of chromophoric groups, and alterations in fluorescent moieties, as revealed by UV-vis and fluorescence spectroscopy. The presence of cupric ion (Cu²⁺) decreased k_DOM,˙OH values for all three kinds of DOM to varying extents. The interaction mechanism between DOM and Cu²⁺ indicated the aggregation of the DOM fragment for the cation bridge effect and charge transfer from ligand to metal for the electron shuttle effect of Cu²⁺ were the dominant reasons for the decrease of k_DOM,˙OH. Spectroscopic indices, hydrodynamic size, and XPS analyses indicate that this inhibition is mainly attributable to Cu²⁺-induced aggregation of DOM via cation bridging and coordination with electron-donating functional groups, which reduces the accessibility of reactive sites to ˙OH. In addition, ligand-to-metal charge transfer and the associated Cu(II)/Cu(I) electron-shuttle behavior likely provide a supplementary pathway that further suppresses DOM oxidation. Overall, these results show that DOM significantly reduces the effective utilization of ˙OH in advanced oxidation processes and can inhibit ˙OH-driven degradation of organic pollutants, while the observed mineralization and optical changes provide insight into the fate and transformation of DOM in natural and engineered waters.