Generation of hydroxyl radicals via activation of Cr(VI) by UVA-LED for rapid decontamination: The important role of Cr(V).

Hexavalent chromium (Cr(VI)) was activated by ultraviolet-A light-emitting diode (UVA-LED), resulting in efficient removal of various pollutants, including dye, pharmaceuticals, and pesticides, with pseudo-first-order rate constants of 0.0610-0.159 min-1. Comparatively, UVA-LED or Cr(VI) alone barely degraded selected pollutants. Both HO• and Cr(V) were produced in the UVA-LED/Cr(VI) system based on scavenging and probing experiments, UV-visible and electron spin resonance spectra analysis. HO• was demonstrated to be the dominant reactive species via stepwise regeneration of Cr(V) to Cr(VI). The quantum yield of HO• was determined to be 7.79 × 10-4 mol Es-1 at a Cr(VI) dosage of 0.5 mM and pH of 6.0. Additionally, the degradation efficiency of sulfamethoxazole (SMX) as a model compound decreased linearly as UVA-LED wavelengths increased from 365 to 405 nm, while SMX was barely degraded at visible light irradiation wavelength ranges (449-505 nm). SMX degradation efficiency increased from 71.0 % to 97.5 % as Cr(VI) dosage increased from 0.05 to 0.7 mM. pH displayed a negative impact on SMX degradation with its removal efficiency decreasing from 99.4 % to 13.3 % as pH increased from 3.0 to 9.0. This study first reported that HO• was generated via activation of Cr(VI) by UVA-LED, which is instructive for the removal of pollutants co-existed in chromium-containing wastewater.