UV photolysis of nitrate has been shown to promote UV advanced oxidation by generating hydroxyl and other radical species capable of degrading organic micropollutants. New advances in UV source technology hold the potential to further capitalize on nitrate’s photosensitizing behavior. KrCl* excimer lamps emit UV light uniquely at 222 nm, which coincides with the highly absorptive range of nitrate. Because nitrate absorbs light strongly at 222 nm and has been shown to produce hydroxyl radicals at modest quantum yields in the UVC range (9-21%), we hypothesized that a UV advanced oxidation process (UV/AOP) which uses a KrCl* excimer lamp as the UV source and nitrate as the radical promoter could perform well for radical production and contaminant degradation. We compared hydroxyl radical concentration in three UV/AOPs: KrCl* with 5 ppm nitrate as the radical promoter (KrCl*/NO3-); KrCl* with 10 ppm hydrogen peroxide as the radical promoter (KrCl*/H2O2); and conventional low-pressure UV at 254 nm and 10 ppm hydrogen peroxide (LPUV/H2O2). We found that in ultrapure water KrCl*/NO3 performed 1.4 times better at hydroxyl radical production than KrCl*/H2O2 and 13.1 times better than LPUV/H2O2. Moderate nitrate levels of several mg/L are frequently present in wastewater effluent and contaminated groundwater, presenting the possibility of using native nitrate as the radical promoter in diverse water treatment applications. This would present an advantage over current UV advanced oxidation in which the radical promoter (typically hydrogen peroxide) must be added to the water, often accounting for a significant fraction of the total operating costs. One potential drawback of a nitrate-promoted UV/AOP is that nitrite is a significant product of nitrate photolysis. Nitrite is stringently regulated with a USEPA maximum contaminant level (MCL) of 1 mg-N/L, so a UV/AOP which relies on nitrate photolysis to produce radicals runs the risk of also producing elevated nitrite concentrations. We are investigating this concern by measuring nitrite production in water with varying levels of nitrate, TOC, alkalinity, pH and UV fluence. These results will be used to determine the quantum yield of nitrite from nitrate photolysis at 222 nm and to develop a model which describes the impact of water quality and UV fluence on nitrite production. This research, is an important step in establishing the viability of KrCl*/NO3- as a novel UV/AOP.

Nitrate-promoted UV advanced oxidation using 222 nm emission from KrCl* excimer lamps

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