Transformation reactivity of parent polycyclic aromatic hydrocarbons and the formation trend of halogenated polycyclic aromatic hydrocarbons in the presence of bromide ion during chlorination

Abstract

The chlorination of parent polycyclic aromatic hydrocarbons (PAHs) has become a recognized fact during chlorination disinfection. However, bromide ion (Br−), which exists in natural water, may fundamentally affect the reaction mechanism. This study investigated the transformation mechanism of PAHs and the formation trend of halogenated PAHs (HPAHs) in the presence of Br− during chlorination. Twelve PAHs were divided into two groups, active and inactive, containing only hypochlorous acid (HOCl). However, the presence of Br− accelerated the reaction process of active PAHs. At a high Br− concentration, inactive PAHs transformed to become “false active”. Two reaction types, including electrophilic substitution and single electron transfer reactions, occurred in PAHs during chlorination. We screened the transformation products using comprehensive two-dimensional gas chromatography-time of flight-mass spectrometry (GC × GC-TOF MS) and identified the occurrence of two reaction types. The presence of Br− affected the species of transformation products. In general, the number of species of chlorinated PAHs decreased and those of brominated PAHs increased with an increase in the Br− concentration. In addition, we first observed the occurrence of 2 types of brominated and chlorinated acenaphthenes, predicted the formation pathways, and inferred the most probable structure using a quantum chemical computation method. Therefore, the formation mechanism of HPAHs in drinking water during the chlorination treatment process was evaluated. The presence of HPAHs in drinking water requires more attention.