Using model compounds to show how a change in thinking is required for regulation of brominated haloacetic acids in drinking water

Brominated disinfection by-products (Br-DBPs) are formed during the chlorination of drinking water where sufficient organic precursor molecules and bromide are present. They are of heightened concern due to their enhanced toxicity over the chlorinated analogues, yet despite this Br-DBPs are not as widely studied so their prevalence and capacity for formation is less well understood. Importantly, four brominated haloacetic acids (Br-HAAs) currently sit outside of regulatory standards. This raises the question of how thinking about DBPs will change if regulation expands to include all nine chlorinated and brominated HAAs (HAA9). The current work explored this question by utilising model compounds to establish the impacts associated with different groups of organic molecules and varying functional group on the ability to form Br-HAAs. This identified that high formation of unregulated HAAs was associated to precursors within all groups, with BDCAA (bromodichloroacetic acid) forming from the aromatic and amino acid-based groups, and BCAA (bromochloroacetic acid) forming from within the aliphatic, amino acid-based and lignin-derived groups. Critical structural insight was related to the balance of TXAA (tri-halogenated acetic acids) and DXAA (di-halogenated acetic acids) which was associated with stabilisation of the di-halogenated intermediates. Consequently, thinking about formation of the currently unregulated brominated HAAs needs to consider both hydrophobic and hydrophilic precursor types, appreciating that enhanced formation propensity can originate from small changes in molecular structure. Furthermore, variation in formation pathways and bromination preference between the THMs, DXAAs and TXAAs dictates that monitoring and management of the HAA9 will require divergence from current DBP strategies. • Unregulated brominated HAAs form from compounds within all the groups studied • Current proxy measures (DOC, UV) do not inform on potential for unregulated HAAs • Regulated DBPs (THM, HAA5) are poor predictors for unregulated HAAs • Aromatic precursors predominately form Br-TXAAs within the unregulated species • Aliphatic precursors form the most unregulated HAAs on a mass basis