Several
ppm levels of free chlorine in high chloride-containing
water of no less than 0.20 M show remarkably high degradation rate
constants toward carbamazepine (k
CBZ′).
Dichlorine monoxide (Cl2O) was identified to be responsible
for the high k
CBZ′. Cl2O degrades aromatic pollutants connected to more electron-donating
functional groups faster when comparing k
Cl2O to 1,4-dimethoxybenzene, bisphenol A, benzoic acid,
and CBZ. However, its formation at low free chlorine levels in high
chloride-containing water cannot be explained by our current knowledge.
We proposed an alternative Cl2O formation mechanism, in
which chloride transforms hypochlorous acid/hypochlorite (HOCl/ClO–) to dichlorine (Cl2) and polychloride monoanions
(Cl
n
–), which then react
with HOCl/ClO– to form Cl2O. Decreasing
pH from 8.60 to 4.33 increases Cl2 concentrations and thus
generates about 600× higher Cl2O concentrations. Weak
acid anions, for example, phosphate and bicarbonate, are strong nucleophiles
assisting the Cl2O hydrolysis, but their presence at the
testing conditions does not much affect k
CBZ′. The proposed alternative Cl2O formation mechanism
reveals the roles of chloride in shifting the equilibrium toward the
Cl2O formation, allowing the simple chlorine addition strategy
to degrade recalcitrant organic pollutants in high chloride-containing
wastewater. It may also change our understanding of the speciation
of different free chlorine species and their impacts on the DBP formation
in high chloride-containing wastewater.
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