Construct dichlorine monoxide equilibrium at high chloride concentrations to reveal its overlooked reactivity and...

Adding hypochlorous acid/hypochlorite (HOCl/ClO^-) directly to high chloride-containing wastewater was proposed and confirmed to form dichlorine monoxide (Cl₂O) in situ to degrade recalcitrant micropollutants. The previously developed Cl₂O formation equilibrium failed to predict the unprecedented degradation of recalcitrant micropollutants when adding HOCl/ClO^- in high chloride-containing water. In this study, we construct a new Cl₂O formation equilibrium model in high chloride-containing wastewaters at different concentrations of chloride, free chlorine, and pHs. The calculated concentrations of Cl₂O using the new model are at least 832 times higher, and up to several thousand times higher under certain conditions than those using the previous one. Based on this model, a new speciation diagram of HOCl, ClO^-, Cl₂, and Cl₂O was reconstructed. By coupling experimental and computational methods, we determined the previously overlooked second-order rate constants of Cl₂O with five micropollutants. These constants revealed that Cl₂O reacts faster with compounds possessing higher-energy highest occupied molecular orbitals (HOMOs). Cl₂O attacks the nitrogen atom in the triazine ring, similar to that of singlet oxygen, and attacks the oxygen atom in amide groups, which is different from the mechanisms of HOCl and Cl₂ that attack primary amides. Thermodynamics of Cl₂O formation and its reactivity and site-selectivity provide insights into leveraging high concentrations of chloride to form Cl₂O in situ to treat high chloride-containing wastewaters.