A new UV source activates ozone for water treatment: Wavelength-dependent ultraviolet light-emitting diode (UV-LED)

Abstract

The newly emerged ultraviolet light-emitting diode (UV-LED) as a new UV source may replace the conventional UV lamps for water treatment in the foreseeable future. In this study, UV-LED with four wavelengths (i.e., 255, 265, 280, and 310 nm) was used to investigate the wavelength-dependency of UV-LED/O3 process on the radical/non-radical formation and hexazinone degradation. The degradation of hexazinone in UV-LED/O3 process with different wavelengths (31.1–97.4%) was higher than that of O3 oxidation process alone (20.0%), and the hexazinone removal rate at 280 nm was the highest (97.4%). The fluence-based rate constant of hexazinone degradation in UV-LED/O3 process (0.19–2.97 × 10−5 m2·J−1) was higher than that in low-pressure UV/O3 (LPUV/O3) process (0.15 × 10−5 m2·J−1). Hydroxyl radical (·OH) and superoxide radicals (O2−) were the dominant active species in UV-LED/O3 process, and the single oxygen (1O2) was also beneficial to the degradation of hexazinone. The possible degradation pathways were proposed based on high resolution mass spectrum (MS). The reaction sites of free radicals were further predicted by density functional theory (DFT) calculations. Besides, the Toxicity Estimation Software Tool (T.E.S.T.) and Vibrio fischeri were used to estimate the potential toxicity of hexazinone and intermediates and it was suggested that alkaline environment and sufficient oxidation time were conductive to reducing the toxicity of wastewater.