Photodegradation of benzophenones sensitized by nitrite

Involvement of inorganic nitrogen species (NOX- (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/NOx- (x = 2, 3) and oxidant/NO2- processes are summarized for various contaminants. The pseudo-first-order rate constants (kpfo) 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).

Nitrate Enhanced Sulfamethoxazole Degradation by 222 nm Far-UVC Irradiation: Role of Reactive Nitrogen Species

The application of 222 nm far-UVC irradiation for degrading organic micropollutants in water shows promise. Nitrate (NO3-), found in nearly all water bodies, can significantly impact the performance of 222 nm far-UVC-driven systems. This work was the first to investigate the effect of NO3- on sulfamethoxazole (SMX) photodegradation at 222 nm, finding that NO3- significantly enhances SMX degradation in different dissociated forms. Besides the hydroxyl radical (•OH), reactive nitrogen species (RNS) also played important roles in SMX degradation. With increasing NO3- concentration, the RNS contribution to SMX degradation decreased from 25.7 to 8.6% at pH 3 but increased from 1.5 to 24.7% at pH 7, since the deprotonated SMX with electron-rich groups reacted more easily with RNS. The transformation mechanisms of SMX involving isomerization, bond cleavage, hydroxylation, nitrosation, and nitration processes were proposed. 15N isotope labeling experiments showed that the RNS-induced nitrated products even became the major products of SMX in the 222 nm far-UVC/NO3- system at pH 7 and exhibited a higher toxicity than SMX itself. Further research is necessary to avoid or eliminate these toxic byproducts. This study provides valuable insights for guiding the utilization of 222 nm far-UVC for treating antibiotics in NO3--containing water.

Exploration of a new approach for detection of nitrite with hydroxyl radical fluorescence probe in aqueous solutions

Nitrite (NO2-) has been widely recognized by the international community as an important substance affecting water quality safety and human health, and the detection of NO2- has always been a hot topic for researchers. Fluorescent probe method is an emerging and ideal way for detecting NO2-. Due to the high dependence of the reported reactive NO2- fluorescent probes on strong acidic systems, using the idea of photochemistry, a fluorescence analysis method for detecting NO2- was proposed in this work to change the necessity of strong acidic solutions in probe detection process. A 365 nm UV-LED lamp was used to irradiate NO2- in aqueous solution to convert it into hydroxyl radicals (HO·), and capture the photodegradation product of NO2- using coumarin-3-carboxylic acid as probe 3-CCA that can react with HO· to generate only one type of strong fluorescent substance. This probe has excellent photostability, selectivity, and anti-interference ability, and can realize the quantitative detection of NO2- (0-15 μM) in pure aqueous solution with pH of 7.4. In addition, its application in actual water samples is also satisfactory, with a recovery rate of (85.91 %-107.30 %). Importantly, we hope that this photolysis strategy can open up the novel thinking to develop suitable fluorescent probes for the analysis and detection of some hardly detected analytes.

Photo-transformation of isoproturon under UV-A irradiation: The synergy of nitrite and natural organic matter

Isoproturon (IPU), a widely utilized phenylurea herbicide, is recognized as an emerging contaminant. Previous studies have predominantly attributed the degradation of IPU in natural waters to indirect photolysis by natural organic matter (NOM). Here, we demonstrate that nitrite (NO2-) also serves as an important photosensitizer that induces the photo-degradation of IPU. Through radical quenching tests, we identify hydroxyl radicals (•OH) and nitrogen dioxide radicals (NO2•) originating from NO2- photolysis as key players in IPU degradation, resulting in the generation of a series of hydroxylated and nitrated byproducts. Moreover, we demonstrate a synergistic effect on the photo-transformation of IPU when both NOM and NO2- are present in the reaction mixture. The observed rate constant (kobs) for IPU removal increases to 0.0179 ± 0.0002 min-1 in the co-presence of NO2- (50 μM) and NOM (2.5 mgC/L), surpassing the sum of those in the presence of each alone (0.0135 ± 0.0004 min-1). NOM exhibits multifaceted roles in the indirect photolysis of IPU. It can be excited by UV and transformed to excited triplet states (3NOM*) which oxidize IPU to IPU•+ that undergoes further degradation. Simultaneously, NOM can mitigate the reaction by reducing the IPU•+ intermediate back to the parent IPU. However, the presence of NO2- alters this dynamic, as IPU•+ rapidly couples with NO2•, accelerating IPU degradation and augmenting the formation of mono-nitrated IPU. These findings provide in-depth understandings on the photochemical transformation of environmental contaminants, especially phenylurea herbicides, in natural waters where NOM and NO2- coexist.

What is the role of nitrate/nitrite in trace organic contaminants degradation and transformation during UV-based advanced oxidation processes?

The effectiveness of UV-based advanced oxidation processes (UV-AOPs) in degrading trace organic contaminants (TrOCs) can be significantly influenced by the ubiquitous presence of nitrate (NO3-) and nitrite (NO2-) in water and wastewater. Indeed, NO3-/NO2- can play multiple roles of NO3-/NO2- in UV-AOPs, leading to complexities and conflicting results observed in existing research. They can inhibit the degradation of TrOCs by scavenging reactive species and/or competitively absorbing UV light. Conversely, they can also enhance the elimination of TrOCs by generating additional •OH and reactive nitrogen species (RNS). Furthermore, the presence of NO3-/NO2- during UV-AOP treatment can affect the transformation pathways of TrOCs, potentially resulting in the nitration/nitrosation of TrOCs. The resulting nitro(so)-products are generally more toxic than the parent TrOCs and may become precursors of nitrogenous disinfection byproducts (N-DBPs) upon chlorination. Particularly, since the impact of NO3-/NO2- in UV-AOPs is largely due to the generation of RNS from NO3-/NO2- including NO•, NO2•, and peroxynitrite (ONOO-/ONOOH), this review covers the generation, properties, and detection methods of these RNS. From kinetic, mechanistic, and toxicologic perspectives, future research needs are proposed to advance the understanding of how NO3-/NO2- can be exploited to improve the performance of UV-AOPs treating TrOCs. This critical review provides a comprehensive framework outlining the multifaceted impact of NO3-/NO2- in UV-AOPs, contributing insights for basic research and practical applications of UV-AOPs containing NO3-/NO2-.

Photochemical reaction of glucocorticoids in aqueous solution: Influencing factors and photolysis products

Glucocorticoids (GCs), as endocrine disruptors, have attracted widespread attention due to their impacts on organisms' growth, development, and reproduction. In the current study, the photodegradation of budesonide (BD) and clobetasol propionate (CP), as targeted GCs, was investigated including the effects of initial concentrations and typical environmental factors (Cl^-, NO₂^-, Fe³⁺, and fulvic acid (FA)). The results showed that the degradation rate constants (k) were 0.0060 and 0.0039 min^-1 for BD and CP at concentration of 50 μg·L^-1, and increased with the initial concentrations. Under the addition of Cl^-, NO₂^-, and Fe³⁺ to the GCs/water system, the photodegradation rate was decreased with increasing Cl^-, NO₂^-, and Fe³⁺ concentrations, which were in contrast to the addition of FA. Electron resonance spectroscopy (EPR) analysis and the radical quenching experiments verified that GCs could transition to the triplet excited states of GCs (³GCs*) for direct photolysis under irradiation to undergo, while NO₂^-, Fe³⁺, and FA could generate ·OH to induce indirect photolysis. According to HPLC-Q-TOF MS analysis, the structures of the three photodegradation products of BD and CP were elucidated, respectively, and the phototransformation pathways were inferred based on the product structures. These findings help to grasp the fate of synthetic GCs in the environment and contribute to the understanding of their ecological risks.

Reactive Nitrogen Species Mediated Inactivation of Pathogenic Microorganisms during UVA Photolysis of Nitrite at Surface Water Levels

UVA photolysis of nitrite (NO₂^-) occurs in a number of natural and engineered aquatic systems. This study reports for the first time that pathogenic microorganisms can be effectively inactivated during the coexposure of UVA irradiation and NO₂^- under environmentally relevant conditions. The results demonstrated that more than 3 log inactivation of Escherichia coli K-12, Staphylococcus aureus, and Spingopyxis sp. BM1-1 was achieved by UVA photolysis of 2.0 mg-N L^-1 of NO₂^- in synthetic drinking water and real surface water. The inactivation was mainly attributed to the reactive species generated from UVA photolysis of NO₂^- rather than UVA irradiation or NO₂^- oxidation alone. The inactivation was predominantly contributed by the reactive nitrogen species (NO₂^• and ONOO^-/HOONO) instead of the reactive oxygen species (HO^• or O₂^•-). A kinetic model to simulate the reactive species generation from UVA photolysis of NO₂^- was established, validated, and used to predict the contributions of different reactive species to the inactivation under various environmental conditions. Several advanced tools (e.g., D₂O - labeling with Raman spectroscopy) were used to demonstrate that the inactivation by the UVA/NO₂^- treatment was attributed to the DNA destruction by the reactive nitrogen species, which completely suppressed the viable but nonculturable (VBNC) states and the reactivation of bacteria. This study highlights a novel process for the inactivation of pathogenic microorganisms in water and emphasizes the critical role of reactive nitrogen species in water disinfection and purification.

Emerging investigator series: contributions of reactive nitrogen species to transformations of organic compounds in water: a critical review

Reactive nitrogen species (RNS) pose a potential risk to drinking water quality because they react with organic compounds to form toxic byproducts. Since the discovery of RNS formation in sunlit surface waters, these reactive intermediates have been detected in numerous sunlit natural waters and engineered water treatment systems. This critical review summarizes what is known regarding RNS, including their formation, contributions to contaminant transformation, and products resulting from RNS reactions. Reaction mechanisms and rate constants have been described for nitrogen dioxide (˙NO₂) reacting with phenolic compounds. However, significant knowledge gaps remain regarding reactions of RNS with other types of organic compounds. Promising methods to quantify RNS concentrations and reaction rates include the use of selective quenchers and probe compounds as well as electron paramagnetic resonance spectroscopy. Additionally, high resolution mass spectrometry methods have enabled the identification of nitr(os)ated byproducts that form via RNS reactions in sunlit surface waters, UV-based treatment systems, treatment systems that employ chemical oxidants such as chlorine and ozone, and certain types of biological treatment processes. Recommendations are provided for future research to increase understanding of RNS reactions and products, and the implications for drinking water toxicity.

Stability and Removal of Benzophenone-Type UV Filters from Water Matrices by Advanced Oxidation Processes

Benzophenone (BP) type UV filters are common environmental contaminants that are posing a growing health concern due to their increasing presence in water. Different studies have evidenced the presence of benzophenones (BP, BP-1, BP-2, BP-3, BP-4, BP-9, HPB) in several environmental matrices, indicating that conventional technologies of water treatment are not able to remove them. It has also been reported that these compounds could be associated with endocrine-disrupting activities, genotoxicity, and reproductive toxicity. This review focuses on the degradation kinetics and mechanisms of benzophenone-type UV filters and their degradation products (DPs) under UV and solar irradiation and in UV-based advanced oxidation processes (AOPs) such as UV/H₂O₂, UV/persulfate, and the Fenton process. The effects of various operating parameters, such as UV irradiation including initial concentrations of H₂O₂, persulfate, and Fe²⁺, on the degradation of tested benzophenones from aqueous matrices, and conditions that allow higher degradation rates to be achieved are presented. Application of nanoparticles such as TiO₂, PbO/TiO₂, and Sb₂O₃/TiO₂ for the photocatalytic degradation of benzophenone-type UV filters was included in this review.