Impact of UV-C irradiance and wavelength on the photodegradation of dibromoacetonitrile

Abstract

Ultraviolet-C (UV-C) irradiation is practiced at the point-of-use and point-of-entry as a last barrier disinfection strategy. Interaction between UV-C light and chlroinated drinking water can result in photo-induced transformation and remediation of disinfection by-products (DBPs). The study investigates how engineering design factors (such as the wavelength and irradiance of UV-C LEDs) and experimental parameters (such as solvent and reactor volume) affect the degradation kinetics of a photolyzable nitrogenous DBP, dibromoacetonitrile (DBAN). UV-C LEDs with characteristic peak wavelengths of 265, 275, and 280 nm and output power of 32–40 mW were studied to degrade DBAN, where acetone and methyl tert-butyl ether (MtBE) were used as the preparation solvents. Quantum yield fluence-based kinetic rate constants (k_f), and electrical energy per order (EEO) were calculated for different experimental conditions. EEO was inversely related to quantum yield and lowest for the 265 nm high-power UV-C LED at 80.43 kWh/m³. A significant finding is that incident irradiance greatly impacted the degradation kinetics even when normalized by fluence. The 265 nm high-power LED resulted in 2.3-times higher quantum yield and fluence-based degradation kinetics than the 265 low-power LED and a corresponding 3.5 times lower EEO despite the same wavelength of irradiance. Lastly, we demonstrate that the solvent selected significantly impacts kinetics, where the degradation of DBAN with acetone is 2.28-times greater than with MtBE at the 275 nm wavelength. Indirect photochemical reactions increase observed degradation kinetics; therefore, solvents should be carefully selected for photochemical studies targeting water treatment. This study provides key insights to engineers, as well as an understanding of the impact of UV-C-based POU treatment design for drinking water systems.