Numerous inorganic and organic micropollutants can undergo reactions with chlorine.
However, for certain compounds, the expected chlorine reactivity is low and only small
modifications in the parent compound’s structure are expected under typical water
treatment conditions. To better understand/predict chlorine reactions with micropollutants,
the kinetic and mechanistic information on chlorine reactivity available in literature
was critically reviewed. For most micropollutants, HOCl is the major reactive chlorine
species during chlorination processes. In the case of inorganic compounds, a fast reaction
of ammonia, halides (Br and I), SO32, CN, NO2
, As(III) and Fe(II) with HOCl is reported
(103–109M1 s1) whereas low chlorine reaction rates with Mn(II) were shown in
homogeneous systems. Chlorine reactivity usually results from an initial electrophilic
attack of HOCl on inorganic compounds. In the case of organic compounds, second-order
rate constants for chlorination vary over 10 orders of magnitude (i.e. o0.1–109M1 s1).
Oxidation, addition and electrophilic substitution reactions with organic compounds are
possible pathways. However, from a kinetic point of view, usually only electrophilic attack
is significant. Chlorine reactivity limited to particular sites (mainly amines, reduced sulfur
moieties or activated aromatic systems) is commonly observed during chlorination
processes and small modifications in the parent compound’s structure are expected for
the primary attack. Linear structure–activity relationships can be used to make predictions/
estimates of the reactivity of functional groups based on structural analogy. Furthermore,
comparison of chlorine to ozone reactivity towards aromatic compounds (electrophilic
attack) shows a good correlation, with chlorine rate constants being about four orders of
magnitude smaller than those for ozone.