Redox-Active Moieties in Dissolved Organic Matter Accelerate the Degradation of Nitroimidazoles in SO4•–-Based Oxidation

The presence of dissolved organic matter (DOM) is known to inhibit the degradation of trace organic contaminants (TrOCs) in SO₄^•--based advanced oxidation processes (AOPs) due to filtering of the photochemically active light and radical scavenging effects. This study revealed an unexpected contribution for DOM in the degradation of nitroimidazoles (NZs) in the UV/persulfate AOP. The apparent second-order rate constants of NZs with SO₄^•- increased by 2.05 to 4.77 times in the presence of different DOMs. The increments were linearly related to the total electron capacity of DOM. Quinone and polyphenol moieties were found to play a dominant role. The reactive species generated from SO₄^•-'s oxidation of DOM, including semiquinone radical (SQ^•-) and superoxide (O₂^•-), were found to react with NZs via Michael addition and O₂^•- addition. The second-order rate constants of tinidazole with SQ^•- is determined to be (5.69 ± 0.59) × 10⁶ M^-1 s^-1 by laser flash photolysis. Reactive species potentially generated from DOM may be considered in designing processes for the abatement of different types of TrOCs.