Ammonia-Mediated Bromate Inhibition during Ozonation Promotes the Toxicity Due to Organic Byproduct Transformation

Ammonia (NH₄⁺) and hydrogen peroxide (H₂O₂) have been widely used to inhibit bromate formation during ozonation. However, organic byproducts can also pose a risk under these conditions. During bromate inhibition, the influence of NH₄⁺ and H₂O₂ on organic byproducts and their toxicity should be elucidated. Our study found that NH₄⁺ suppressed organic bromine, but might result in increased toxicity. Adding 0.5 mg/L of NH₄⁺-N substantially increased both the formation of cytotoxicity and genotoxicity (DNA double-strand breaks) of organic byproducts from 0.6 to 1.6 mg-phenol/L, and from 0.3 to 0.8 μg-4-NQO/L (0.5 mg/L Br^-, 5 mg/L O₃). NH₄⁺ decreased bromate, but increased the overall toxicity of the integrated byproducts (organic byproducts and bromate). Organic nitrogen measurements and ¹⁵N isotope analysis showed enhanced incorporation of nitrogen into organic matter when NH₄⁺ and Br^- coexisted during ozonation. NH₄⁺ decreased the formation of brominated acetonitriles, but enhanced the formation of brominated nitromethanes and brominated acetamides. These brominated nitrogenous byproducts were partially responsible for this increase in toxicity. Different from ammonia, H₂O₂ could reduce both bromate and the toxicity of organic byproducts. In the presence of 0.5 mg/L Br^- and 10 mg/L O₃, adding H₂O₂ (0.5 mM) substantially suppressed bromate, cytotoxicity formation and genotoxicity formation by 88%, 63% and 67%. This study highlights that focusing on bromate control with NH₄⁺ addition might result in higher toxicity. Efforts are needed to effectively control the toxicities of bromate and organic byproducts simultaneously.