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 (NH₂Cl and NHCl₂) and their subsequent radical formation. The fluence-based photodecay rates of NH₂Cl decreased with increasing wavelength from 255 to 300 nm, while NHCl₂ photodecay rates exhibited the opposite wavelength-dependence. The fluence-based photodecay rate of NH₂Cl was comparable to that of NHCl₂ at 255 nm, but was lower than NHCl₂ 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 NH₂Cl than NHCl₂. The steady-state concentrations of HO^• and reactive chlorine species (e.g., Cl₂^•–, ClO^•, and Cl^•) that were generated from the photolysis of NH₂Cl and NHCl₂ at different wavelengths were determined experimentally and compared with the simulated results by a kinetic model. UV photolysis of NHCl₂ at 265, 285, and 300 nm generated higher concentrations of radicals (e.g., HO^•, ClO^•, Cl^•, and Cl₂^–•) than NH₂Cl, while UV photolysis of NH₂Cl at 255 nm generated higher concentrations of HO^•, ClO^•, and Cl^• but not Cl₂^–• than NHCl₂. The findings of this study provide fundamental information to be used in selecting specific wavelengths of UV radiation for enhancing/optimizing NH₂Cl/NHCl₂ photodecay in swimming pools and radical generation for micropollutant abatement in drinking water treatment or potable water reuse.