Virus inactivation effect of hypochlorite solution containing nitrate ions

Hypochlorous acid (HClO) is widely used as a disinfectant due to its strong oxidizing power, but its efficacy is highly dependent on pH and decreases under neutral to alkaline conditions. In addition, its high concentration poses safety and environmental concerns. In this study, we investigated the enhancement of virucidal activity at lower HClO concentrations through the formation of monochloramine (NH₂Cl) by supplementing with ammonium-form nitrogen (NH₄⁺-N) or nitrate-form nitrogen (NO₃^--N) under different pH conditions. Using bacteriophage φ6 as an enveloped virus model, plaque assays revealed that virus inactivation was most effective at pH 8 when 15-20 ppm of NH₄⁺-N was added to 30 ppm HClO solution, with over 90% reduction in infectivity. Under acidic conditions (pH 6), the formation of NH₂Cl was limited due to the rapid consumption of free chlorine and insufficient generation of NH₃, resulting in reduced virucidal efficacy; however, increasing the amount of NH₄⁺-N led to improved inactivation effects. In contrast, at pH 8, NH₂Cl formation was enhanced, contributing to improved disinfection efficiency. Similar effects were observed when NO₃^--N was used, likely due to its reduction to NH₄⁺ in the presence of organic matter. These results demonstrate that optimizing pH and nitrogen sources can improve the efficacy of low-concentration HClO solutions by generating NH₂Cl in situ, offering a safer and more sustainable approach to viral disinfection.