Control of emerging organic contaminants (EOCs) and degradation of antibiotic resistance
genes (ARGs) are two emerging issues in protecting the aquatic environment and supplying
safe drinking water. The breakpoint chlorination process, which generates reactive species such
as HNO, ONOOH and HO
• in ammonia-containing water, is a promising process tackling in
such emerging issues because the reactive species can effectively degrade chlorine-refractory
EOCs and may serve as an additional barrier to ARG dissemination. However, there are two
critical challenges in applying the breakpoint chlorination process: the complexity of the nature
of reactive species generated through breakpoint chlorination reactions and the formation of
undesirable disinfection byproducts (DBPs) from natural organic...[
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Control of emerging organic contaminants (EOCs) and degradation of antibiotic resistance
genes (ARGs) are two emerging issues in protecting the aquatic environment and supplying
safe drinking water. The breakpoint chlorination process, which generates reactive species such
as HNO, ONOOH and HO
• in ammonia-containing water, is a promising process tackling in
such emerging issues because the reactive species can effectively degrade chlorine-refractory
EOCs and may serve as an additional barrier to ARG dissemination. However, there are two
critical challenges in applying the breakpoint chlorination process: the complexity of the nature
of reactive species generated through breakpoint chlorination reactions and the formation of
undesirable disinfection byproducts (DBPs) from natural organic matter. This research was
undertaken to provide a thorough investigation of the generation of reactive species and their
roles in EOC degradation in the breakpoint chlorination process and to establish a novel multi-step
breakpoint chlorination process to balance the risks posed by EOCs, ARGs, and DBPs.
The pathway of HO
• generation in the breakpoint chlorination process was investigated. The
decomposition of ONOOH only contributed approximately 7% to the formation of HO
•,
suggesting the dominant role of other pathways, such as the decomposition of dimerized HNO
(i.e., HO-N=N-OH) in HO
• formation in the breakpoint chlorination. The generation of reactive
chlorine species (RCS, i.e., Cl
•, ClO
• and Cl
2•‒
) and reactive nitrogen species (RNS, e.g., HNO
and ONOOH) in breakpoint chlorination was demonstrated and their contributions to the
degradation of EOCs were quantified. In addition, the chlorine to nitrogen (Cl/N) ratio and pH
greatly affected the reactive species generation and EOC degradation. At pH 7.2, an optimum
Cl/N ratio for HO
•, Cl
2•‒, and RNS generation occurred at 9, while the radical speciation shifted
towards ClO
• due to the scavenging effect of excessive free chlorine at a Cl/N ratio above 9.
Increasing the pH from 7.2 to 9.5 decreased the degradation of EOCs by HO
• and RNS but
significantly increased that by RCS, especially ClO
•, due to the strong scavenging of HO
• by
OCl
‒.
The impacts of three operational parameters, the number of chlorine dosing steps (3-step, 5-step, and continuous dosing), the Cl/N ratio of the initial dosing step (starting from an initial
Cl/N mass ratio of 5.0 and dosing evenly), and the time interval between each dosing (3, 5, 7,
and 9 min) on EOC degradation and reactive species generation were evaluated. The optimum
chlorine dosing procedure to degrade five representative EOCs was determined to be 5-step
chlorine dosing starting from an initial Cl/N mass ratio of 5 with a dosing time interval of 5
min. Compared to single-step breakpoint chlorination, the optimized multi-step breakpoint
chlorination fostered HO
• generation and enhanced the EOC degradation. Although the single-step
and multi-step breakpoint chlorination achieved a comparable level of ARG degradation,
the latter scenario increased the relative contribution of the reactive species, rather than the
chlor(am)ine species, to ARG reduction and retained a higher level of residual chlorine. In addition, a significantly decreased formation of carbonaceous and nitrogenous DBPs was
observed by using the multi-step chlorine dosing approach.
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