Photolysis of free chlorine and production of reactive radicals in the UV/chlorine system using polychromatic spectrum LEDs as UV sources

Abstract

Recently, ultraviolet light-emitting diodes (UV-LEDs) and chlorine combined system has been employed as an emerging advanced oxidation process. However, UV-LEDs were commonly considered as monochromatic UV sources. In this study, the obvious quantum yields of chlorine photolysis under 265 nm and 280 nm LEDs irradiations were investigated with treating LEDs as polychromatic UV sources. Particularly, Φobs-poly of HOCl and OCl⁻ for 265 nm LED were found to be 1.50 and 0.70 mol E−1, respectively, whereas Φobs-poly of HOCl and OCl⁻ for 280 nm LED were 1.28 and 0.64 mol E−1, respectively. It was identified that Φobs-poly were 5.66–14.63 % lower than Φobs-mono. This suggests that obvious quantum yield using peak emission wavelength would overestimate the true quantum yield. The production of radical species in LED UV/chlorine systems were determined by the degradation of BA, and illustrated by a mathematical model. Different trends were observed for 265 nm and 280 nm LED UV/chlorine systems as pH increased from 5.0 to 10.0. As pH increased, the formation of OH continuously decreased in both 265 nm and 280 nm LED systems. The formation of Cl increased at neutral pH and more Cl and OH were formed due to the higher molar absorbance coefficient at 280 nm. The chlorine dose-dependent effects on radical productions at pH of 5.0, 7.5 and 10.0 were also assessed. At pH of 5.0, OH was the main radical product and had linear correlation with chlorine dose. At pH of 7.5, the productions of OH and Cl showed similar profiles that increased rapidly at low chlorine dosage and then slowed down.