Prediction of organic groundwater contaminant degradation during medium pressure UV/NO₃⁻ treatment

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 (k_UV, 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 k_OH,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 k_UV > 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.