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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 NO3- containing RH (RH = HCO2-, CH3OH, or C2H5OH) and H2O2 containing CH3OH, all in aerated buffered aqueous solutions. The UV photolysis of NO3- generates ONOO-, *OH, and *NO2 intermediates and the stable NO2- 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 NO2- 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.