Formation of Targeted and Novel Disinfection Byproducts during Chlorine Photolysis in the Presence of Bromide

Chlorine photolysis is an advanced oxidation process
that relies
on the combination of direct chlorination by free available chlorine,
direct photolysis, and reactive oxidants to transform contaminants.
In waters that contain bromide, free available bromine and reactive
bromine species can also form. However, little is known about the
underlying mechanisms or formation potential of disinfection byproducts
(DBPs) under these conditions. We investigated reactive oxidant generation
and DBP formation under dark conditions, chlorine photolysis, and
radical-quenched chorine photolysis with variable chlorine (0–10
mg-Cl2/L) and bromide (0–2,000 μg/L) concentrations,
as well as with free available bromine. Probe loss rates and ozone
concentrations increase with chlorine concentration and are minimally
impacted by bromide. Radical-mediated processes partially contribute
to the formation targeted DBPs (i.e., trihalomethanes, haloacetic
acids, haloacetonitriles, chlorate, and bromate), which increase with
increasing chlorine concentration. Chlorinated novel DBPs detected
by high-resolution mass spectrometry are attributable to a combination
of dark chlorination, direct halogenation by reactive chlorine species,
and transformation of precursors, whereas novel brominated DBPs are
primarily attributable to dark bromination of electron-rich formulas.
The formation of targeted and novel DBPs during chlorine photolysis
in waters with elevated bromide may limit treatment applications.