High absorbance of nitrate leads to surprising effects on hydroxyl radical formation during 222 nm UV treatment

Interest in krypton chloride excimer lamps, which emit primarily at 222 nm (UV222), for applications to water treatment has been growing rapidly in the last few years, due to the greater contaminant degradation and pathogen inactivation enabled by this lower wavelength. Nitrate absorbs UV very strongly at 222 nm (ε=2747 M^-1 cm^-1) and is thus of particular interest in KrCl* water treatment. While the ability of nitrate to promote hydroxyl radical formation under UV irradiation from other UV sources has been well-demonstrated, the effect of nitrate on UV222/H₂O₂ has not been previously investigated. When nitrate is present at 5 mg-N/L or greater, addition of hydrogen peroxide, a common radical promoter in UV advanced oxidation processes, leads to a 7.3 to 20.8 % decrease in degradation of para-chlorobenzoic acid (pCBA), a probe compound for hydroxyl radical formation. This effect is attributed to 1) H₂O₂ acting solely as a scavenger, rather than source, of hydroxyl radicals due to light screening by nitrate during 222 nm UV and 2) increased formation of nitrite from nitrate photolysis when peroxide is present, leading to more hydroxyl radical scavenging. Nitrite was found to exceed the maximum contaminant level of 1 mg-N/L when nitrate was present at 7.5 mg-N/L, presenting a possible challenge for applications of UV222. However, it was also found that nitrite may act as a source of hydroxyl radicals due to its high absorbance and quantum yield at 222 nm, which can compensate for the increased hydroxyl scavenging by photo-produced nitrite. Lastly, the impact of irradiation path length, an important experimental design parameter, was investigated for the UV₂₂₂/nitrate process and found to significantly influence chemical degradation results (k_pCBA varied by 1.4-1.9 times as a function of path length), due to the high light absorption of nitrate violating several key assumptions in the standard methods for calculating UV fluence. In particular, this work challenges the inclusion of the water factor in calculating UV fluence in 222 nm studies, as the water factor corrects for photon attenuation by the background water matrix but leads to erroneous results when nitrate is present due to both nitrate's impact on the water factor by photon absorption and its role as the primary source of hydroxyl radicals during 222 nm irradiation. This work demonstrates the significant, and unexpected, impact of nitrate on UV222 advanced oxidation, identifying key issues that researchers of this technology should consider.