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

摘要

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_2Cl and NHCl_2) and their subsequent radical formation. The fluence-based photodecay rates of NH_2Cl 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_2Cl 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_2Cl 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_2Cl 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_2Cl, while UV photolysis of NH_2Cl 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_2Cl/NHCl_2 photodecay in swimming pools and radical generation for micropollutant abatement in drinking water treatment or potable water reuse.