UV Photolysis of Mono- and Dichloramine Using UV-LEDs as Radiation Sources: Photodecay Rates and Radical Concentrations

UV-LEDs with four characteristic wavelengths (255, 265, 285, and 300 nm) were used to investigate the wavelength-dependence of the photolysis of two inorganic chloramines (NH 2 Cl and NHCl 2 ) and their subsequent radical formation. The fluence-based photodecay rates of NH 2 Cl decreased with increasing wavelength from 255 to 300 nm, while NHCl 2 photodecay rates exhibited the opposite wavelength-dependence. The fluence-based photodecay rate of NH 2 Cl was comparable to that of NHCl 2 at 255 nm, but was lower than NHCl 2 at other tested wavelengths. The wavelength-dependence was more influenced by the molar absorption coefficient than the apparent/innate quantum yield and the lower photosensitivity was mainly attributed to the higher bond (N–Cl) dissociation energy (BDE) of NH 2 Cl than NHCl 2 . The steady-state concentrations of HO • and reactive chlorine species (e.g., Cl 2 •–, ClO •, and Cl • ) that were generated from the photolysis of NH 2 Cl and NHCl 2 at different wavelengths were determined experimentally and compared with the simulated results by a kinetic model. UV photolysis of NHCl 2 at 265, 285, and 300 nm generated higher concentrations of radicals (e.g., HO •, ClO •, Cl •, and Cl 2 –• ) than NH 2 Cl, while UV photolysis of NH 2 Cl at 255 nm generated higher concentrations of HO •, ClO •, and Cl • but not Cl 2 –• than NHCl 2 . The findings of this study provide fundamental information to be used in selecting specific wavelengths of UV radiation for enhancing/optimizing NH 2 Cl/NHCl 2 photodecay in swimming pools and radical generation for micropollutant abatement in drinking water treatment or potable water reuse.