Cl^–/NO₂^– Ratio Governs Phenolic Compound Degradation Kinetics and Mechanism in SO₄^•– Oxidation Processes

The presence of individual chloride (Cl–) or nitrite (NO2–) has been shown to influence the efficiency of sulfate radical (SO4•–) oxidation and induce the formation of toxic byproducts, warranting considerable attention. Due to their comparable reactivity with SO4•– and phenolic compounds, their coexistence may dramatically alter reaction kinetics and pathways. Despite its critical implications, this interaction is poorly understood. In this study, we revealed that the Cl–/NO2– ratio (at environmental concentrations) played a pivotal role in modulating these processes during phenol degradation by a combined experimental and statistical approach. At a ratio of 100, it played a critical role in determining the promotion or inhibition of kinetics. Variation in the Cl–/NO2– ratio triggered distinct electrophilic substitution reactions before ring-opening, leading to a significant shift in the diversity of aromatic products within the 10–50 range. This further exerted influence on the yield of nitrophenols (NPs), chlorophenols (CPs), and the final products of haloacetic acids (HAAs) (inhibition from 4.0% to 95.2%), which were observed and innovatively certified via experimental and density functional theory (DFT) evidence. Our findings underscore the imperative to consider the interaction effects among coexisting ions during SR-AOPs.