Effects of bromide on the formation and transformation of disinfection by-products during chlorination and chloramination

Abstract

The presence of bromide ion (Br−) complicates the formation of disinfection by-products (DBPs) during chlorination and chloramination greatly. To better illustrate the role of Br−, Br− was introduced at different time intervals, i.e., 0min, 5min, 30min, and 24h, after dosing with chlorine (Cl2) or chloramine (NH2Cl), and the formation of trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles, and haloacetones was investigated during these two disinfection scenarios. Ammonia rapidly reacts with chlorine and forms low-reactivity NH2Cl, and this effect inhibits the formation of these DBPs greatly. Br− promotes the formation of THMs, HAAs, and dichloroacetone (DCP) during chlorination, and the later bromide is introduced, i.e., the higher TCl2→Br− is, the more significant the formation of THMs and HAAs observed. Bromide incorporation factors (BIF) increase upon the introduction of Br−, and lower TCl2→Br− is related to higher BIF values. Additionally, Br− inhibits the formation of dichloroacetonitrile (DCAN) and trichloroacetone (TCP), owing to its catalytic degradation effect towards them. In the chloramination process, Br− shows similar effects towards the formation of THMs and HAAs, except that higher TNH2Cl→Br− inhibits their formation. Br− greatly inhibits the formation of DCP, TCP, and DCAN, and the formed haloacetones rapidly degrade upon the introduction of Br−. The results of UV and EEM spectral analysis indicate that the reducing Br− may improve rather than inhibit the oxidation of both the reactive components (DOC1) and the slowly reactive sites (DOC2) within HA, possibly owing to its buffering effect towards chlorine. In chlorination of source water with Br− present, Br− promotes the formation of most DBPs and enhances the incorporation of Br atoms therein, and in this case, DBP formation may be remarkably decreased by dosing with ammonia to transform chlorination to chloramination.