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

Enhanced inactivation of Aspergillus niger biofilms by the combination of UV-LEDs with chlorine-based disinfectants

The presence of pathogenic fungal biofilms in drinking water distribution systems poses significant challenges in maintaining the safety of drinking water. This research delved into the formation of Aspergillus niger (A. niger) biofilms and evaluated their susceptibility to inactivation using combinations of ultraviolet light emitting diodes (UV-LEDs) with chlorine-based disinfectants, including UV-LEDs/chlorine (Cl2), UV-LEDs/chlorine dioxide (ClO2), and UV-LEDs/chloramine (NH2Cl) at 265 nm, 280 nm and 265/280 nm. Results indicated that A. niger biofilms reached initial maturity within 24 h, with matured three-dimensional filamentous structures and conidiospores by 96 h. UV-LEDs combined with chlorine-based disinfectants enhanced A. niger biofilm inactivation compared to UV-LEDs alone and low-pressure UV combined with chlorine-based disinfectants. At an UV fluence of 400 mJ/cm2, log reductions of UV265, UV280, and UV265/280 combined with chlorine-based disinfectants were 2.95-fold, 3.20-fold, and 2.38-fold higher than that of UV265, UV280, and UV265/280, respectively. During the inactivation, A. niger biofilm cells experienced increased membrane permeability and intracellular reactive oxygen species levels, resulting in cellular apoptosis. Extracellular polymeric substances contributed to the higher resistance of biofilms. Regarding electrical energy consumption, the order was: UV-LEDs/ClO2 > UV-LEDs/NH2Cl > UV-LEDs/Cl2. These findings provide insights into the effective utilization of UV-LEDs for fungal biofilm disinfection.

Reactive Nitrogen Species Generated from Far-UVC Photolysis of Nitrate Contribute to Pesticide Degradation and Nitrogenous Byproduct Formation

Climate change has resulted in increased use of pesticides and fertilizers in agriculture, leading to elevated pesticide and nitrate levels in aquatic ecosystems that receive agricultural runoff. In this study, we demonstrate that far-UVC (UV222) photolysis of nitrate rapidly degrades four pesticides in surface water, with a degradation rate constant 37.1-144.75 times higher than that achieved by UV254 photolysis of nitrate. The improved pesticide degradation is due not only to the enhanced direct photolysis by UV222 compared to UV254 but also to the increased generation of hydroxyl radicals (HO•) and reactive nitrogen species (e.g., NO2• and ONOO-) in the UV222/nitrate process. We determined the innate quantum yields of nitrate photolysis at 222 nm and incorporated these values into a kinetic model, allowing for the accurate prediction of nitrate photodecay and reactive species generation. While reactive nitrogen species predominantly contribute to pesticide degradation in the UV222/nitrate process, they also lead to the formation of nitration byproducts. Using stable isotope-labeled nitrate (15NO3-) combined with mass spectrometry, we confirmed that the nitration byproducts are formed from the reactive nitrogen species generated from nitrate photolysis. Additionally, we demonstrate that the UV222/nitrate process increases the formation potential of highly toxic nitrogenous chlorinated products (e.g., trichloronitromethane) during postchlorination in real surface water.

Global transcriptional response of Pseudomonas aeruginosa to UVA radiation

Ultraviolet A (UVA) radiation is the major fraction of UV radiation reaching the Earth's surface. Its harmful effects on microorganisms, due mainly to oxidative damage, have been exploited for development of natural solar and commercial UVA-based disinfection methods. In this work, the global transcriptional response of Pseudomonas aeruginosa exposed to ultraviolet A (UVA) radiation was analyzed. To conduct this study, we analyzed the whole transcriptome of the PAO1 strain grown to logarithmic phase under sublethal doses of UVA or in the dark. We found that a total of 298 genes responded to UVA with a change of at least two-fold (5.36% of the total P. aeruginosa genome), and showed equal amount of induced and repressed genes. An important fraction of the induced genes were involved in the response to DNA damage and included induction of SOS, prophage and pyocins genes. The results presented in this study suggest that one of the main UVA targets are proteins carrying [Fe-S] clusters since several genes involved in the processes of synthesis, trafficking and assembly of these structures were upregulated. The management of intracellular iron levels also seems to be a robust response to this stress factor. The strong induction of genes involved in denitrification suggest that this pathway and/or reactive nitrogen species such as nitric oxide could have a role in the response to this radiation. Regarding the down-regulated genes, we found many involved in the biosynthesis of PQS, a quorum-sensing signal molecule with a possible role as endogenous photosensitizer.

Copper single-atom catalysts for broad-spectrum antibiotic-resistant bacteria (ARBs) antimicrobial: Activation of peroxides and mechanism of ARBs inactivation

Antibiotic-resistant bacteria (ARBs) have been widely detected in wastewater and become a potential threat to human health. This work found that low-load single-atom copper (0.1 wt%) anchored on g-C3N4 (SA-Cu/g-C3N4) exhibited excellent ability to activate H2O2 and inactivate ARBs during the photo-Fenton process. The presence of SA-Cu/g-C3N4 (0.4 mg/mL) and H2O2 (0.1 mM) effectively inactivated ARBs. More than 99.9999 % (6-log) of methicillin-resistant Staphylococcus aureus (MRSA), and carbapenem-resistant Acinetobacter baumannii (CRAB) could be inactivated within 5 min. Extended-spectrum β-lactamase-producing pathogenic Escherichia coli (ESBL-E) and vancomycin-resistant Enterococcus faecium (VRE) were killed within 10 and 30 min, respectively. In addition, more than 5-log of these ARBs were killed within 60 min in real wastewater. Furthermore, D2O-labeling with Raman spectroscopy revealed that SA-Cu/g-C3N4 completely suppressed the viable but nonculturable (VBNC) state and reactivation of bacteria. Electron paramagnetic resonance spectroscopy results demonstrated that g-C3N4 mainly produced 1O2, while SA-Cu/g-C3N4 simultaneously produced both 1O2 and •OH. The •OH and 1O2 cause lipid peroxidation damage to the cell membrane, resulting in the death of the bacteria. These findings highlight that the SA-Cu/g-C3N4 catalyst is a promising photo-Fenton catalyst for the inactivation of ARBs in wastewater.

Regulatory Mechanisms of Quorum Sensing System of Bacteria in Response to Chlorine and Ozone Disinfection

Waterborne pathogens invariably present considerable threats to public health. The quorum sensing (QS) system is instrumental in coordinating bacterial growth and metabolisms. However, the responses and regulatory mechanisms of bacteria to various disinfection technologies through quorum sensing are still unclear. This study examines the inactivation effect of chlorination and ozonation on biofilms and planktonic cells of QS signaling-deficient mutants of Pseudomonas aeruginosa. Cell counting and viability assessment revealed that the combined disinfection of chlorine and ozone was the most effective for inactivating planktonic P. aeruginosa within 10 min of exposure. Additionally, microfluidic chip culture demonstrated that the secretion of quinolone signals escalated biofilms' disinfection resistance. Disinfection exposure significantly altered the gene expression of wild-type strains and QS signaling-deficient mutants. Moreover, the QS system triggered multilayered gene expression programs as a responsive protection to disinfectant exposure, including oxidative stress, ribosome synthesis, and the nutrient absorption of bacteria. These insights broaden our understanding of bacterial QS in response to disinfection, promising potential strategies toward efficient disinfection processes.

Dielectric barrier discharge plasma promotes disinfection-residual-bacteria inactivation via electric field and reactive species

Traditional disinfection processes face significant challenges such as health and ecological risks associated with disinfection-residual-bacteria due to their single mechanism of action. Development of new disinfection processes with composite mechanisms is therefore urgently needed. In this study, we employed liquid ground-electrode dielectric barrier discharge (lgDBD) to achieve synergistic sterilization through electric field electroporation and reactive species oxidation. At a voltage of 12 kV, Pseudomonas fluorescens (ultraviolet and ozone-resistant) and Bacillus subtilis (chlorine-resistant) were completely inactivated within 8 and 6 min, respectively, surpassing a 7.0-log reduction. The lgDBD process showed good disinfection performance across a wide range of pH values and different practical water samples. Staining experiments suggest that cellular membrane damage contributes to this inactivation. In addition, we used a two-dimensional parallel streamer solver with kinetics code to fashion a representative model of the basic discharge unit, and discovered the presence of a persistent electric field during the discharge process with a peak value of 2.86 × 106 V/m. Plasma discharge generates excited state species such as O(1D) and N2(C3Πu), and further forms reactive oxygen and nitrogen species at the gas-liquid interface. The physical process, which is driven by electric field-induced cell membrane electroporation, synergizes with the bactericidal effects of reactive oxygen and nitrogen species to provide effective disinfection. Adopting the lgDBD process enhances sterilization efficiency and adaptability, underscoring its potential to revolutionize physicochemical synergistic disinfection practices.

Degradation of carbamazepine in ice with bromate and nitrite: Role of reactive nitrogen species

Seasonal freezing of waters occurs during winter in cold regions. Bromate ( [Formula: see text] ) is a disinfection by-product generated during water treatment, its interaction with emerging contaminants may be affected by freezing. Nitrite ( [Formula: see text] ) is widely distributed in the environment, whereas its effect on the interaction of emerging contaminants and [Formula: see text] in ice may have been overlooked. Herein carbamazepine (CBZ) was selected as a model emerging contaminant to elucidate the role of reactive nitrogen species (RNS) in contaminant transformation during the reduction of [Formula: see text] by [Formula: see text] in ice. Results indicated that freezing significantly enhanced CBZ degradation by [Formula: see text] . The CBZ degradation by [Formula: see text] and [Formula: see text] in ice was 25.4 %-27.8 % higher than that by [Formula: see text] . Contributions of hydroxyl radical (•OH), bromine radical (•Br), and RNS to CBZ degradation in freezing/dark or sunlight systems were 8.1 % or 15.9 %, 25.4 % or 7.2 %, and 66.5 % or 76.9 %, respectively. Most CBZ was degraded by RNS generated during the reduction of [Formula: see text] by [Formula: see text] in ice, resulting in 16.4 % of transformation products being nitro-containing byproducts. Hybrid toxicity of CBZ/ [Formula: see text] / [Formula: see text] system was reduced effectively after the freezing-sunlight process. This study can provide new insights into the environmental fate of emerging contaminants, [Formula: see text] , and [Formula: see text] in cold regions.

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-.

Evaluating the Impact of Cl2•- Generation on Antibiotic-Resistance Contamination Removal via UV/Peroxydisulfate

The removal of antibiotic-resistant bacteria (ARB) and antibiotic-resistance genes (ARGs) using sulfate anion radical (SO4•-)-based advanced oxidation processes has gained considerable attention recently. However, immense uncertainties persist in technology transfer. Particularly, the impact of dichlorine radical (Cl2•-) generation during SO4•--mediated disinfection on ARB/ARGs removal remains unclear, despite the Cl2•- concentration reaching levels notably higher than those of SO4•- in certain SO4•--based procedures applied to secondary effluents, hospital wastewaters, and marine waters. The experimental results of this study reveal a detrimental effect on the disinfection efficiency of tetracycline-resistant Escherichia coli (Tc-ARB) during SO4•--mediated treatment owing to Cl2•- generation. Through a comparative investigation of the distinct inactivation mechanisms of Tc-ARB in the Cl2•-- and SO4•--mediated disinfection processes, encompassing various perspectives, we confirm that Cl2•- is less effective in inducing cellular structural damage, perturbing cellular metabolic activity, disrupting antioxidant enzyme system, damaging genetic material, and inducing the viable but nonculturable state. Consequently, this diminishes the disinfection efficiency of SO4•--mediated treatment owing to Cl2•- generation. Importantly, the results indicate that Cl2•- generation increases the potential risk associated with the dark reactivation of Tc-ARB and the vertical gene transfer process of tetracycline-resistant genes following SO4•--mediated disinfection. This study underscores the undesired role of Cl2•- for ARB/ARGs removal during the SO4•--mediated disinfection process.

Rapid Inactivation of Fungal Spores in Drinking Water by Far-UVC Photolysis of Free Chlorine

Effective and affordable disinfection technology is one key to achieving Sustainable Development Goal 6. In this work, we develop a process by integrating Far-UVC irradiation at 222 nm with free chlorine (UV222/chlorine) for rapid inactivation of the chlorine-resistant and opportunistic Aspergillus niger spores in drinking water. The UV222/chlorine process achieves a 5.0-log inactivation of the A. niger spores at a chlorine dosage of 3.0 mg L-1 and a UV fluence of 30 mJ cm-2 in deionized water, tap water, and surface water. The inactivation rate constant of the spores by the UV222/chlorine process is 0.55 min-1, which is 4.6-fold, 5.5-fold, and 1.8-fold, respectively, higher than those of the UV222 alone, chlorination alone, and the conventional UV254/chlorine process under comparable conditions. The more efficient inactivation by the UV222/chlorine process is mainly attributed to the enhanced generation of reactive chlorine species (e.g., 6.7 × 10-15 M of Cl•) instead of hydroxyl radicals from UV222 photolysis of chlorine, which is verified through both experiments and a kinetic model. We further demonstrate that UV222 photolysis damages the membrane integrity and benefits the penetration of chlorine and radicals into cells for inactivation. The merits of the UV222/chlorine process over the UV254/chlorine process also include the more effective inhibition of the photoreactivation of the spores after disinfection and the lower formation of chlorinated disinfection byproducts and toxicity.

Visible light-driven NH2Cl activation by g-C3N4 photocatalysis producing reactive nitrogen species to degrade bisphenol A

The photolysis of monochloramine (NH₂Cl), a widely used disinfectant, under UVC irradiation produces different radicals for the micropollutant degradation. For the first time, this study demonstrates the degradation of bisphenol A (BPA) via the NH₂Cl activation by graphitic carbon nitride (g-C₃N₄) photocatalysis using visible light-LEDs at 420 nm, termed as the Vis₄₂₀/g-C₃N₄/NH₂Cl process. The process produces •NH₂, •NH₂OO, •NO and •NO₂ via the e_CB^-- and O₂^•--induced activation pathways and •NHCl and NHClOO• via the h_VB⁺-induced activation pathway. The produced reactive nitrogen species (RNS) enhanced 100% of the BPA degradation compared with the Vis₄₂₀/g-C₃N₄. Density functional theory calculations confirmed the proposed NH₂Cl activation pathways and further demonstrated that e_CB^-/O₂^•- and h_VB⁺ induced the cleavage of N-Cl and N-H bonds in NH₂Cl, respectively. The process converted 73.5% of the decomposed NH₂Cl to nitrogen-containing gas, compared with that of approximately 20% in the UVC/NH₂Cl process, leaving much less ammonia, nitrite and nitrate in water. Among different operating conditions and water matrices tested, of particular significance is natural organic matter of 5 mgDOC/L only reduced 13.1% of the BPA degradation compared against that of at least 46% reduction in the UVC/NH₂Cl process. Only 0.017-0.161 µg/L of disinfection byproducts were produced, two orders of magnitudes lower than that in the UVC/chlorine and UVC/NH₂Cl processes. The combined use of visible light-LEDs, g-C₃N₄ and NH₂Cl significantly improves the micropollutant degradation and reduces the energy consumption and byproduct formation of the NH₂Cl-based AOP.

Uncovering novel disinfection mechanisms of solar light/periodate system: The dominance of singlet oxygen and metabolomic insights

Disinfection plays an essential role in waterborne pathogen control and disease prevention, especially during the COVID-19 pandemic. Catalyst-free solar light/periodate (PI) system has recently presented great potential in water disinfection, whereas the in-depth chemical and microbiological mechanisms for efficient bacterial inactivation remain unclear. Our work delineated firstly the critical role of singlet oxygen, instead of reported hydroxyl radicals and superoxide radicals, in dominating bacterial inactivation by the PI/simulated sunlight (SSL) system. Multi-evidence demonstrated the prominent disinfection performance of this system for Staphylococcus aureus in terms of culturability (> 6 logs CFU), cellular integrity, and metabolic activity. Particularly, the excellent intracellular DNA removal (> 95%) indicated that PI/SSL system may function as a selective disinfection strategy to diminish bacterial culturability without damaging the cell membrane. The PI/SSL system could also effectively inhibit bacterial regrowth for > 5 days and horizontal gene transfer between E. coli genera. Nontargeted metabolomic analysis suggested that PI/SSL system inactivated bacteria by triggering the accumulation of intracellular reactive oxygen species and the depletion of reduced glutathione. Additionally, the PI/SSL system could accomplish simultaneous micropollutant removal and bacterial inactivation, suggesting its versatility in water decontamination. Overall, this study deciphers more comprehensive antibacterial mechanisms of this environmentally friendly disinfection system, facilitating the technical development and application of the selective disinfection strategy in environmental pathogen control.