Formation of Dichlorine Monoxide for Organic Pollutant Degradation by Free Chlorine in High Chloride-Containing Water
Chuanjing Lin, Li Ling, Chii Shang
Abstract
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 (Cl 2 O) was identified to be responsible for the high k CBZ ′. Cl 2 O degrades aromatic pollutants connected to more electron-donating functional groups faster when comparing k Cl 2 O 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 Cl 2 O formation mechanism, in which chloride transforms hypochlorous acid/hypochlorite (HOCl/ClO – ) to dichlorine (Cl 2 ) and polychloride monoanions (Cl n – ), which then react with HOCl/ClO – to form Cl 2 O. Decreasing pH from 8.60 to 4.33 increases Cl 2 concentrations and thus generates about 600× higher Cl 2 O concentrations. Weak acid anions, for example, phosphate and bicarbonate, are strong nucleophiles assisting the Cl 2 O hydrolysis, but their presence at the testing conditions does not much affect k CBZ ′. The proposed alternative Cl 2 O formation mechanism reveals the roles of chloride in shifting the equilibrium toward the Cl 2 O 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.