Transformation of methylparaben during water chlorination: Effects of bromide and dissolved organic matter on reaction kinetics and transformation pathways

Abstract

The reaction kinetics, products, and pathways of methylparaben (MeP) during water chlorination with and without bromide (Br−) were investigated to better understand the fate of parabens in chlorinated waters. During the chlorination of MeP-spiked waters without Br−, MeP was transformed into mono-Cl-MeP and di-Cl-MeP with apparent second-order rate constants (kapp) of 64M−1s−1 and 243M−1s−1 at pH7, respectively, while further chlorination of di-Cl-MeP was relatively slower (kapp=1.3M−1s−1 at pH7). With increasing Br− concentration, brominated MePs, such as mono-Br-MeP, Br-Cl-MeP, and di-Br-MeP, became major transformation products. The di-halogenated MePs (di-Cl-MeP, Br,Cl-MeP, and di-Br-MeP) showed relatively low reactivity to chlorine at pH7 (kapp=1.3–4.6M−1s−1) and bromine (kapp=32–71M−1s−1), which explains the observed high stability of di-halogenated MePs in chlorinated waters. With increasing pH from 7 to 8.5, the transformation of di-halogenated MePs was further slowed due to the decreasing reactivity of di-MePs to chlorine. The formation of the di-halogenated MePs and their further transformation become considerably faster at Br− concentrations higher than 0.5μM (40μg/L). Nonetheless, the accelerating effect of Br− diminishes in the presence of dissolved organic matter (DOM) extract (Suwannee River humic acid (SRHA)) due to a more rapid consumption of bromine by DOM than chlorine. The effect of Br− on the fate of MeP was less in the tested real water matrices, possibly due to a more rapid bromine consumption by the real water DOM compared to SRHA. A kinetic model was developed based on the determined species-specific second-order rate constants for chlorination/bromination of MeP and its chlorinated and brominated MePs and the transformation pathway information, which could reasonably simulate the transformation of MePs during the chlorination of water in the presence of Br− and selected DOM.