Transformation of aminopyrine in the presence of free available chlorine: Kinetics, products, and reaction pathways.

Aminopyrine (AMP) has been frequently detected in the aquatic environment. In this study, the transformation mechanism of AMP by free available chlorine (FAC) oxidation was investigated. The results showed that FAC reacted with AMP rapidly, and a 74% elimination was achieved for 1.30 μM AMP after 2 min at 14.08 μM FAC dose. AMP chlorination was strongly pH-dependent, and its reaction included second- and third-order kinetic processes. Three active FAC species, including chlorine monoxide (Cl2 O), molecular chlorine (Cl2 ), and hypochlorous acid (HOCl), were observed to contribute to AMP degradation. The intrinsic rate constants of each FAC species with neutral (AMP0 ) and cation (AMP+ ) species were obtained by kinetic fitting. Cl2 O exhibited the highest reactivity with AMP0 (kAMP0, Cl2O  = (4.33 ± 1.4) × 109  M-1 s-1 ). In addition, Cl2 showed high reactivity (106 -107  M-1 s-1 ) in the presence of chloride, compared with HOCl (kAMP+, HOCl  = (5.73 ± 0.23) × 102  M-1 s-1 , kAMP0, HOCl  = (9.68 ± 0.96) × 102  M-1 s-1 ). At pH 6.15 and 14.08 μM FAC dose without chloride addition, the contribution of Cl2 O reached to the maximum (33.3%), but in the whole pH range, HOCl was the main contributor (>66.6%) for AMP degradation. The significance of Cl2 was noticeable in water containing chloride. Moreover, 11 transformation products were identified, and the main transformation pathways included pyrazole ring breakage, hydroxylation, dehydrogenation, and halogenation.