Kinetic-based model and pathways of chlorinated-halonitromethanes formation from tyrosine and tryptophan during UV/NH2Cl disinfection

Abstract

Chlorinated-halonitromethanes (Cl-HNMs) are highly toxic nitrogen-containing disinfection by-products that are produced from amino acids (AAs) during UV/NH2Cl disinfection. This study aimed to establish a kinetic-based model to simulate Cl-HNMs formation from tyrosine (Tyr) and tryptophan (Trp) during UV/NH2Cl disinfection. Of note, once the degradation rates of AAs (kobs,AA) and Cl-HNMs (kd,HNM) and formation parameters (K) were obtained, the kinetic-based model could predict Cl-HNMs formation, which guided the selection of appropriate disinfection conditions. Firstly, the first-principles-based kinetic model of UV/NH2Cl system was established to describe kobs,AA in this study (sample deviation = 0.0776 for Tyr and 0.1240 for Trp), and that was also developed to obtain kd,HNM in our previous study. Then combined with kobs,AA and kd,HNM, the unknown values of K in the kinetic-based model of Cl-HNMs formation were estimated at different NH2Cl doses, pH, and alkalinity. The sample deviations of Cl-HNMs formation were 0.0257–0.3350. Compared with pH and alkalinity, increasing the NH2Cl dose significantly affected Cl-HNMs concentration and delayed the peak time of Cl-HNMs formation. Therefore, the correlations between K and the NH2Cl dose (K = f(NH2Cl)) were further explored to predict Cl-HNMs formation at different NH2Cl doses, and K = f(NH2Cl) was well described by elementary functions (R2 > 0.90). Combined with kobs,AA, kd,HNM and K = f(NH2Cl), the heatmaps for Cl-HNMs formation at different NH2Cl doses were determined and validated (R2 = 0.79 for Tyr and Trp) by the kinetic-based model of Cl-HNMs formation, which helped us choose appropriate UV and NH2Cl doses to control Cl-HNMs formation. Finally, the possible pathways of Cl-HNMs formation were proposed, including decarboxylation, (de)chlorination, hydroxylation, and nitration.