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A General Method for Detecting Nitrosamide Formation in the In Vitro Metabolism of Nitrosamines by Cytochrome P450s
Published on: September 25, 2017
Breakpoint Chlorination Chemistry in a Chlorine-Cyanurate System and Trade-Offs between Nitrosamine Formation and
Yi-Hsueh Chuang1, Chia-Shun Chou1, Yi-Lin Chu1
1Institute of Environmental Engineering, National Yang Ming Chiao Tung University No. 1001, University Rd., Hsinchu city 30010, Taiwan.
Abstract:
Breakpoint chlorination occurs in swimming pools and generates •OH and nitrosating agents. While •OH facilitates micropollutant removal, nitrosating agents promote nitrosamine formation. Cyanuric acid, a common chlorine stabilizer, reacts reversibly with free chlorine to form chlorinated cyanurates, effectively "locking" free chlorine and altering breakpoint chemistry. Accurate estimation of free chlorine is essential for evaluating these impacts and depends on the hydrolytic dissociation constants of chlorinated cyanurates, yet reported values vary widely. This study re-evaluates these constants and examines how cyanuric acid influences breakpoint reactions, focusing on the trade-off between •OH-driven micropollutant degradation and nitrosamine formation. Using phenolic probes, we show that electrochemically determined hydrolytic dissociation constants more accurately predict free chlorine concentrations under pool-relevant conditions than spectrophotometric values. Kinetic experiments reveal that chlorinated cyanurates participate in breakpoint reactions, chlorinating NH2Cl and NHCl2 with rate constants approximately half of those of HOCl. Cyanuric acid also catalyzes NHCl2 formation from NH2Cl. Under simulated pool conditions, cyanuric acid enhanced micropollutant removal, suppressed NCl3 formation, but promoted nitrosamine formation. A refined kinetic model captured these trends and provided mechanistic insights. Cyanuric acid, while mitigating scavenging of •OH and nitrosating agents by oxidants, prolongs NCl3-NHCl2 interactions, thereby increasing •OH and nitrosating agent yields while lowering residual NCl3.
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