ConspectusBecause
of the deterioration of global water
quality, the occurrence
of chemical and microbial contaminants in water raises serious concerns
for the health of the population. Identifying and developing effective
and environmentally friendly water treatment technologies are critical
to obtain clean water. Among the various technologies for the purification
of water, ultraviolet photolysis of chlorine (UV/chlorine), an emerging
advanced oxidation process (AOP), has multiple functions for the control
of contaminants via the production of hydroxyl radicals (HO·)
and reactive chlorine species (RCS), such as Cl·, ClO·,
and Cl2·–.This Account centers
around the radical chemistry of RCS and HO·
in different water matrices and their roles and mechanisms in the
abatement of contaminants. The concentrations of Cl·, ClO·,
and Cl2·– are comparable to or higher
than those of HO· (10–14 to 10–13 M). The reactivities of RCS are more selective than HO· with
a broader range of second-order rate constants (k). The k values of Cl· toward most aromatics
are higher or similar as compared to those of HO·, while those
of Cl2·– and ClO· are less
reactive but more selective toward aromatics containing electron-donating
functional groups. Their major reaction mechanisms with Cl· are
electron transfer and addition, while those with ClO· and Cl2·– primarily involve electron transfer.
As for aliphatics, their reactivities with both HO· and RCS are
much lower than those of aromatics. The reaction mechanisms for most
of them with Cl· and Cl2·– are
hydrogen abstraction, except for olefins, which are addition. In addition,
RCS greatly contribute to the inactivation of microbial contaminants.Toward future application, the UV/chlorine process has both pros
and cons. Compared with the traditional HO·-based AOP of UV/H2O2, UV/chlorine is more efficient and energy-saving
for oxidation and disinfection, and its efficiency is less affected
by water matrix components. However, the formation of toxic byproducts
in UV/chlorine limits its application scenarios. In dissolved organic
matter (DOM)-rich water, the formation of halogenated byproducts is
enhanced in UV/chlorine. In the presence of ammonia, reactive nitrogen
species (RNS) (e.g., ·NO and ·NO2) are involved,
and highly toxic nitro(so) products such as nitro(so)-phenolics and N-nitrosodimethylamine are generated. For a niche application,
the UV/chlorine process is recommended to be utilized in water with
low levels of DOM and ammonia.Strategies should be developed
to make full use of highly reactive
species (RCS and HO·) for the abatement of target contaminants
and to reduce the formation of toxic byproducts. For example, the
UV/chlorine process can be used in tandem with other treatments to
create multiple barriers for the production of safe water. In addition,
halogen radicals are very important in ecosystems as well as other
areas such as medical therapy and organic synthesis. UV/chlorine is
the most efficient homogeneous system to generate halogen radicals,
and thus it provides a perfect system to investigate the fates of
halogen radicals for interdisciplinary research.