Photolysis of Indigo Carmine solution by planar vacuum-ultraviolet (147 nm) light source

Advanced oxidation processes (AOPs) are an attractive method to decompose dye-containing wastewaters, because they avoid issues of secondary pollution. In particular, a vacuum-ultraviolet (VUV) process is the simplest method, because an oxidation accelerator or a catalyst is not required. Conventional VUV sources with wavelengths of 185 nm or 172 nm have been used. We predicted that a shorter wavelength VUV process would have a higher ability to decompose dyes in wastewater. We developed a new planar light source that could emit a resonance line at 147 nm and a broad molecular line at 172 nm. The irradiance was 8.7 mW/cm² at a distance of 20 mm from the emission surface to the UV power meter. We then conducted photocatalytic experiments of an Indigo Carmine solution at 3.33 × 10^-4 mol/L to confirm the decomposition abilities of the developed light source, an excimer lamp of 172 nm and a Hg lamp of 254 nm. From the HPLC results, changes in Indigo Carmine concentration with the developed light source were equivalent to those with the excimer lamp. However, the residual ratio of total organic carbon (TOC) with the developed light source was lower than those with the other lamps. Therefore, a wavelength of 147 nm is superior to the conventional wavelength of 172 nm for Indigo Carmine decomposition. In addition, the developed light source emits VUV from only one side, which is a flat emitting surface. Consequently, the developed light source would have reduced manufacturing and maintenance costs compared to current VUV processing equipment.