Wavelength-dependent photo-dechlorination: mechanistic insights into the transformation of phenicol pharmaceuticals under UV222 and UV254.

Photo-dechlorination is a critical yet often overlooked aspect of UV treatment for chlorinated organic contaminants. Although the 222 nm KrCl* excimer lamp has emerged as a promising alternative to conventional 254 nm low-pressure mercury lamps, its effectiveness in promoting structural dechlorination remains unclear. Here, chloramphenicol (CAP) was employed as a model chlorinated organic contaminant to investigate wavelength-dependent photo-dechlorination behavior under direct photolysis. Comparative experiments evaluated parent degradation, chloride release, degradation-dechlorination coupling, and transformation pathways at 222 and 254 nm. At equal UV photon dose, 222 nm irradiation resulted in more closely coupled CAP degradation and chloride release, indicating efficient direct photo-dechlorination. In contrast, 254 nm photolysis without N2 purging led to delayed chloride release and accumulation of chlorinated transformation products. N2 purging significantly enhanced dechlorination at 254 nm, indicating that dissolved oxygen suppresses reductive C-Cl bond cleavage. Mechanistic analyses revealed distinct photo-dechlorination pathways: 222 nm promoted excited-state electron redistribution toward chlorine-containing moieties, facilitating direct C-Cl bond activation, while 254 nm excitation was mainly localized within chromophoric moieties and was less favorable for direct dechlorination. These findings reveal the unique advantage of 222 nm irradiation for direct photo-dechlorination and provide a wavelength-dependent mechanistic framework for UV treatment of chlorinated organic contaminants.