Factors affecting vacuum ultraviolet (185 nm) emission intensities in low-pressure mercury lamp photoreactors for photolytic destruction of water contaminants

Abstract

Direct photolysis by 185 nm vacuum ultraviolet (VUV) photons emitted by low-pressure mercury (LP-Hg) lamps enables destructive treatment of water contaminants that resist degradation by conventional advanced oxidation processes. The treatment challenges associated with poly-/perfluoroalkyl substances, in particular, have renewed interest VUV photolysis, which offers a relatively simple and effective means of PFAS degradation. Since LP-Hg lamps have traditionally been used more widely in ultraviolet-C applications, output data and operational guidelines specific to VUV applications are more difficult to access or measure. Herein, we assessed perfluorobutanoic acid (PFBA) photodegradation efficiency using LP-Hg lamp systems from various manufacturers to gauge the variability in 185 nm output and isolate key design and operational aspects. Results indicated a wide variation in VUV output efficiency, with over a 90 % reduction in electrical energy per order destruction (EE/O) between the least and most effective combinations of lamp, ballast, and quartz sleeves, as a result of their physical design aspects. Furthermore, we found that output efficiency was significantly affected by the steady-state temperature achieved when submerged in the photoreactor, relative to the intended operating temperature of the lamp. Consequently, VUV intensity comparisons performed in an N₂-purged collimated beam with VUV radiometer were not useful predictors of relative performance when submerged in a photoreactor. The results also suggest that many past laboratory studies of VUV photolytic treatment approaches likely used sub-optimal lamp system configurations, resulting in misleading results with respect to the energy efficiency of VUV treatment.