Aqueous chlorination of ephedrine: Kinetic, reaction mechanism and toxicity assessment

Abstract

Ephedrine (EPH) is widely detected in the water environment, because it is the major ingredient in drugs treating influenza, asthma or hypotension, and is also a highly sought-after chemical precursor in the illicit manufacture of methamphetamine. In this study, transformation of EPH during the chlorination process was investigated for the first time, and the impact of water parameters including pH, different cations and anions on EPH transformation was evaluated as well. The degradation of EPH in the presence of NaClO fit the second order reaction kinetics, with a rate constant of 7.43 × 102 ((mol·L−1)−1·min−1). Increasing the dosage of NaClO increased the observed pseudo first order rate constant for EPH degradation (kobs). Degradation rate of EPH decreased with the increasing pH from 2.0 to 10.0, due to the formation of a chlorammonium intermediate that reacted with NaClO. Low concentration of Br− and I− did not exert significant influence on the degradation of EPH, while at high concentrations a promotive effect was observed. Other ions including Fe3+, Cu2+, NO3−, SO42−, Mg2+ and Ca2+ exerted negative effects even at relatively low concentrations. Based on the degradation products/intermediates identified by UPLC-MS/MS, the EPH degradation pathways were proposed. The reaction mechanism involved in the EPH degradation included dehydration, hydroxylation, deamination and demethylation. Toxicity assays by V. qinghaiensis sp. nov proved that the EPH transformation products were much more toxic than the parent compound. Results indicated that chlorination is an effective approach for the elimination of EPH in the aquatic environment, however, attention should be paid to its toxicity involvement during the chlorination process.