Polychromatic UV photon irradiance measurements using chemical actinometers based on NO₃^- and H₂O₂ Excitation: Applications for industrial photoreactors

Ultraviolet light (UV) is a proven technology for disinfecting and decontaminating drinking water. For this purpose, both low-pressure monochromatic and medium-pressure polychromatic mercury arc lamps are commonly being used. Methods for photon irradiance measurements of monochromatic light are not necessarily appropriate for polychromatic light, and there is a need for the latter especially when large volumes of water are involved. The present manuscript reports a comparative study of polychromatic UV photolysis of aqueous N0₃^- containing RH (RH = HC0₂^-, CH₃OH, or C₂H₅OH) and H₂0 ₂ containing CH₃OH, all in aerated buffered aqueous solutions. The UV photolysis of NO₃^- generates ONOO ^- OH, and NO₂ intermediates and the stable NO ₂^- ion. The maximum yield of NO₂^- is obtained in the presence of RH. The UV photolysis of H₂O₂ generates OH radicals, which in the presence of CH₃OH form formaldehyde. The H₂O₂/CH₃OH actinometer is limited to low and moderate light intensities because the reaction mechanism involves competition between second-and first-order processes. Therefore, the NO₃^- actinometer is preferable at high photon irradiance despite the relatively low quantum yield of N0₂^- and its dependence on the excitation wavelength. The two actinometers are compared to radiometry and to iodide/iodate actinometer. The latter is limited to the 200-280 nm range due to the absorption of the photoproduct I₃ ^-. The NO₃^-/C₂H₅OH actinometer is particularly useful for large volumes of water in industrial high-intensity UV photoreactors as the actinometer solution can be safely disposed.