Involvement of inorganic nitrogen species (NO_X^- (x = 2, 3)) in the degradation of organic contaminants in environmental waters via UV irradiation or chemical oxidation: A dual-edged approach

OH-mediated advanced oxidation processes (AOPs) are widely used in wastewater treatment and drinking water purification. Recently, an increasing number of studies have indicated that common inorganic nitrogen ions can efficiently generate •OH under UV irradiation, demonstrating strong performance in the degradation of various contaminants. Conversely, the presence of inorganic nitrogen ions in UV or other oxidation processes dramatically increases the yield of toxic nitro (so)-aromatic products and the formation potential of nitrogenous disinfection by-products with high genotoxicity and cytotoxicity. This suggests that the presence of inorganic nitrogen ions in water and wastewater treatment is a 'double-edged sword', offering both benefits and potential harms. Herein, we systematically review the dual roles of inorganic nitrogen ions in contaminant degradation and nitrogenous by-product formation. First, the degradation kinetics of the UV/NO_x^- (x = 2, 3) and oxidant/NO₂^- processes are summarized for various contaminants. The pseudo-first-order rate constants (k_pfo) of contaminant degradation in the UV/NO₃^- system range from 10^-3 to 10^-1min^-1, while those in the UV/NO₂^- and peracetic acid/NO₂^- system vary from 10^-3 to 10²min^-1 and 10^-2 to 10^-1min^-1, respectively. Moreover, the properties of the water matrix (i.e., pH and O₂) play a crucial role in the degradation kinetics by influencing the concentrations and distribution of reactive nitrogen species (RNS), as well as the morphology of the contaminants. Second, this review provides a general overview of the sources and properties of key RNS, including •NO₂, ONOO^-/ONOOH, and free nitrous acid (FNA), which are closely associated with the formation of nitrogenous by-products. Finally, the formation pathways of nitro (so)-aromatic products and nitrogenous disinfection by-products are discussed. These pathways are driven either by RNS alone or by the combination of RNS with reactive oxygen species (ROS).