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), cm(2) mJ(-1)) and the product of the molar absorption coefficient around 223 nm and photolysis quantum yield (& epsilon;(223) x & phi;), with thresholds separating low and high values of 2 x 10(-4) cm(2) mJ(-1) and 4 L cm(-1) E-1, respectively. Radical reactivity was determined using k(OH,C), with 1 x 10(9) M-1 s(-1) as the cutoff between slow and fast reacting contaminants. NO3- at concentrations & LE;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 x 10(9) M-1 s(-1) and contaminants with k(OH,C) > 1 x 10(9) M-1 s(-1) and k(UV) > 5 x 10(-4) cm(2) mJ(-1) or & epsilon;(223) x & phi; > 10 L cm(-1) E-1. A simplified decision tree was proposed to predict the degradability of a contaminant during UV/NO3- groundwater treatment.