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

Abstract 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.