Irradiation of nitrate (NO3−) with UVC light below 240 nm generates photo-sensitized oxidants, such as hydroxyl radicals (˙OH) and reactive nitrogen species (RNS). Hence, the UV/NO3− combination can be regarded as an advanced oxidation treatment of wastewater and groundwater, using indigenous NO3− to promote radicals and degrade contaminants. The present study demonstrates UV/NO3− degradation of important groundwater contaminants, using a polychromatic medium pressure Hg lamp. Compounds were divided into groups, based on their UV/NO3− degradation kinetics and photochemical parameters: photo-reactivity and photo-stability, and slow and fast reaction with radicals. Two metrics were proposed to determine the photosensitivity of a contaminant: fluence-based rate constants (kUV, cm2 mJ−1) and the product of the molar absorption coefficient around 223 nm and photolysis quantum yield (ε223 × Φ), with thresholds separating low and high values of 2 × 10−4 cm2 mJ−1 and 4 L cm−1 E−1, respectively. Radical reactivity was determined using kOH,C, with 1 × 109 M−1 s−1 as the cutoff between slow and fast reacting contaminants. NO3− at concentrations ≤5 mg L−1 N enhanced UV degradation of photo-stable compounds with fast ˙OH reaction, due to the dominant role of NO3− as the radicals' promoter. At higher NO3− concentrations, degradation rate stabilized or even decreased, due to the formation of NO2−, an ˙OH scavenger. For compounds with low ˙OH reaction, the presence of NO3− (up to 15 mg L−1 N) either slowed their degradation rate or did not affect their UV degradation. Only contaminants with a high range of reactivity will be significantly degraded by UV/NO3−, without generating levels of NO2− above regulatory thresholds. These include contaminants with k˙OH,C > 8 × 109 M−1 s−1 and contaminants with k˙OH,C > 1 × 109 M−1 s−1 and kUV > 5 × 10−4 cm2 mJ−1 or ε223 × Φ > 10 L cm−1 E−1. A simplified decision tree was proposed to predict the degradability of a contaminant during UV/NO3− groundwater treatment.
