Mini review on the roles of nitrate/nitrite in advanced oxidation processes: Radicals transformation and products formation

Advanced oxidation processes (AOPs) are ideal alternative to remove contaminants of emerging concern (CECs). Nitrate and nitrite commonly co-exist with CECs in surface water, groundwater, and agricultural runoffs, which impact the performance of AOPs. Interests in investigation on the impacts of nitrate/nitrite on CEC degradation in AOPs have grown exponentially, due to the participation of reactive nitrogen species (RNS). RNS as a daughter radical in AOPs, are generally formed from photolysis of nitrate/nitrite or reactions of nitrate/nitrite with hydroxyl radical. It was documented that nitrate/nitrite in photochemical AOPs could accelerate CEC degradation, while that in non-photochemical AOPs usually play an inhibitory role. Except the performance of AOPs, nitrate/nitrite present in AOPs also have significant impact on the degradation pathway of CECs, of which the typical one is the nitration/nitrosation of CECs. Formation of nitrated/nitrosated CEC products in AOPs is almost inevitable when nitrite is present in water, because RNS are typical nitrating/nitrosating agents. The nitrated/nitrosated products not only show higher toxicity, but have higher formation potential of nitro/nitroso disinfection byproducts. Therefore, nitrate/nitrite can be used as a source of hydroxyl radical and RNS in photochemical AOPs (especially at λ > 280 nm), and should be avoided in non-photochemical AOPs. Besides, using pretreatment to turn nitrite to nitrate is a useful method to reduce the formation of nitrated/nitrosated CECs. Overall, this critical review aims to summarize the all-round impacts of nitrate/nitrite on AOPs, and provide practical implications to better utilize the naturally present nitrate/nitrite.