Bacterial inactivation by dichloride radical in water: Molecular-level reaction kinetics and disinfection mechanism

Water disinfection via sulfate radical (SO₄^•−)-based oxidation has attracted considerable attention for its high efficacy. In real water matrices, however, secondary radicals such as dichloride radical (Cl₂^•−) can form at concentrations several orders of magnitude higher than SO₄^•−, thereby dominating the disinfection process. Despite this, the molecular-level kinetics and mechanism governing Cl₂^•−-mediated bacterial inactivation remain largely unexplored. This study demonstrates that Cl₂^•−can target both the extracellular and intracellular barriers of two representative bacterial species, E. coli and S. aureus. The second-order reaction rate constants of Cl₂^•− with over 30 key biomolecules constituting these barriers are quantified using laser flash photolysis. The results reveal that Cl₂^•− primarily inactivates bacteria by targeting electron-rich amino acids containing sulfur or aromatic structures, most of which exhibit rate constants ≥ 10⁸ M⁻¹ s⁻¹. The results of quantitative structure-activity relationship analysis, transient spectroscopy measurement, and quantum chemical calculation collectively provide a molecular-level disinfection mechanism of Cl₂^•−. This work offers a molecular-level insight into the reaction kinetics and disinfection mechanism underlying the bacterial inactivation by Cl₂^•−, a key secondary radical generated during practical SO₄^•−-based water disinfection.