Kinetic stability and transformation pathway of halogenated nitrophenols in drinking water

Halogenated nitrophenols (HNPs) have raised significant concerns due to their extensive detection and potent toxicity. Understanding how these compounds persist and degrade over time is crucial for predicting their potential accumulation and assessing the long-term exposure risks. Kinetic studies showed that the degradation of HNPs through hydrolysis and chloramination over realistic hydraulic retention times is relatively low. The species-specific second-order rate constants for the reaction of hypochlorous acid and the acid-catalyzed reaction of hypochlorous acid with anionic HNP species, which accounted for HNP degradation during chlorination, range from 3.84 ± 1.09-363.25 ± 50.44 M⁻¹min⁻¹ and from (1.50 ± 0.091) × 10⁶ to (1.34 ± 0.36) × 10⁸ M⁻²min⁻¹ , respectively. The reactivity order of reactive species in hydrolysis, chlorination, and chloramination with HNP species has also been verified through computational modeling using density functional theory. The introduction of nitro group onto HNPs significantly diminishes the electron density of the benzene ring, thereby reducing their inherent reactivity compared with other aromatic DBPs. The hydrolysis and chlorination of HNPs primarily result in the formation of halogenated hydroquinones and various polyhaloquinones and polyhalophenols, respectively. These results suggest that HNPs and their transformation products exhibit relatively high persistence and exposure risks.