Superiority of UV222 radiation by in situ aquatic electrode KrCl excimer in disinfecting waterborne pathogens: Mechanism and efficacy

Disinfection efficacy and safety of 222-nm UVC compared with 254-nm UVC: Systematic review and meta-analysis

BACKGROUND

Some studies have indicated disinfection with 222-nm Ultraviolet C (UVC) is more effective than that with 254-nm UVC. However, other studies reported the opposite findings. Moreover, additional studies have reported that 222-nm UVC exposure is safe for the skin and eyes. This study aimed to identify and quantitatively synthesize all studies evaluating the disinfection efficacy and safety of 222-nm UVC compared with 254-nm UVC.

METHODS

We conducted a systematic review and meta-analysis. We searched Web of Science, SCOPUS, Medline, Ovid Embase, and the Cochrane Library through November 2024 for studies that evaluated the disinfection efficacy and safety of 222-nm UVC compared with 254-nm UVC.

RESULTS

We identified 25 eligible publications including 15 publications providing data only on the efficacy, 7 only on the safety, and the remaining 3 on both efficacy and safety. The pooled odds ratio for studies comparing the efficacy of 222-nm UVC with that of 254-nm UVC was 1.382 (95% CI: 1.153-1.656, n=18 publications with 87 studies), indicating that 222-nm UVC is more effective for disinfection. The pooled risk difference for studies evaluating the safety of 222-nm UVC radiation was -0.211 (95% CI: -0.245,-0.177; n=10 publications with 29 studies), which indicates that the proportion of normal cells producing cyclobutane pyrimidine dimers via 222-nm UVC is 21.1% less than that via 254-nm.

CONCLUSION

Compared with 254-nm UVC, 222-nm UVC not only exhibits comparable or potentially superior efficacy in disinfecting diverse microorganisms but also causes less DNA damage to the mammalian cells.

Removal of disinfection residual bacteria in UV222, UV222/H2O2 and UV222/peroxymonosulfate systems: what is the safe usage for wastewater reclamation

Disinfection residual bacteria (DRB) are widely present in the reclaimed treatment effluents and can regrow during the downstream distribution and storage, posing a threat to the biosafety of reuse applications. Recently, far ultraviolet (UV222) have garnered augmented attention due to the highly efficient and energy-intensive oxidation, making them a potential approach for the deep inactivation of DRB. However, there remains a lack of quantitative analyses on how to monitor the disinfection intensity to mitigate the health risks associated with DRB. In this study, we used the UV222, UV222/H₂O₂ and UV222/peroxymonosulfate (PMS) systems to treat model DRB including Escherichia coli, Pseudomonas aeruginosa, and Bacillus subtilis, and developed a multiparameter model to accurately present the dose-culturability relationship. On this basis, we conducted the simulated disinfection, and detected the viability status and regrowth potential of DRB during the post-disinfection processes. It turned out that UV222 alone exhibited the superiority over UV254, especially for treating Pseudomonas aeruginosa. UV222/H2O2 and UV222/PMS systems further improved the inactivation rates. The practical UV doses for full-scale reclaimed disinfection (10-200 mJ/cm²) were sufficient for the UV222-based systems to inactivate DRB (initial 107 CFU/mL) to the safe level in effluent measured by culture methods. But substantial DRB still persisted in VBNC state, which necessitated higher doses of 200-450 mJ/cm2 to further inhibit the regrowth under accidental contamination and prolonged transport/storage culture. Fortunately, H2O2 provided residual disinfection for Bacillus subtilis, and PMS performed promising sustained disinfection for all the three DRB. This study provided valuable insights for the expanded application of UV222 disinfection and future updates of pathogen standards in reclaimed water treatment.

Wavelength-specific inactivation mechanisms and efficacies of germicidal UVC for airborne human coronavirus

Ultraviolet germicidal irradiation (UVGI) technology can inhibit the environmental transmission of airborne pathogens, but the dose-response behavior of airborne human coronavirus and wavelength-specific inactivation mechanisms are not well understood. This study investigated three competitive UVC sources for their inactivation efficacy and mechanisms against human coronavirus OC43 (HCoV-OC43). Results showed the following order of inactivation efficacy: 222-nm KrCl excimer lamp > 263-nm UV-LEDs > 254-nm low-pressure mercury lamp. The 222-nm KrCl excimer lamp achieved a 5-log inactivation of aerosolized HCoV-OC43 with a dose of less than 1 mJ/cm², while the 263-nm UV-LEDs had the highest genome damage rate constant at 7.08 ± 0.85 mJ/cm². Although 222-nm Far-UVC caused less genome damage, it affected viral proteins more significantly, specifically the nucleocapsid (N) and spike (S) proteins, which lead to compromising capsid integrity and binding ability to host cells. Capsid integrity RT-qPCR and binding assay RT-qPCR used in this study could better monitor infectivity of airborne coronavirus than standard RT-qPCR. Additionally, significant lipid oxidation of HCoV-OC43 was observed under 222-nm irradiation, potentially impacting overall inactivation efficacy. This study provides detailed evidence on the effects of different UVC wavelengths on airborne HCoV-OC43, contributing to the optimization of UVC irradiation for indoor bioaerosol disinfection.

The Antibacterial Efficacy of Far-UVC Light: A Combined-Method Study Exploring the Effects of Experimental and Bacterial Variables on Dose-Response

Far-ultraviolet C light, with a wavelength of 200-230 nm, has demonstrated broad-spectrum germicidal efficacy. However, due to increased interest in its use as an alternative antimicrobial, further knowledge about its fundamental bactericidal efficacy is required. This study had two objectives. Firstly, it investigated experimentally the Far-UVC dose-response of common bacteria suspended at various cell densities in transparent buffer, ensuring no influence from photosensitive suspending media. Increasing doses of Far-UVC were delivered to Enterococcus faecium, Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus in PBS at 101, 102, 103, 105 and 107 CFU·mL-1, with surviving colony-forming units enumerated (n ≥ 3). Secondly, through a systematised literature review, this work sought to explore the impact of genus/species, Gram type, cell form, cell density and irradiance on dose-response. The screening of 483 publications was performed with 25 included in the study. Data for 30 species were collated, analysed and compared with the experimental results. Overall, Gram-positive species showed greater resilience to Far-UVC than Gram-negative; some inter-species and inter-genera differences in resilience were identified; endospores were more resilient than vegetative cells; the results suggested that inactivation efficiency may decrease as cell density increases; and no significant correlation was identified between irradiance and bactericidal dose effect. In conclusion, this study has shown Far-UVC light to be an effective decontamination tool against a vast range of bacterial vegetative cells and endospores.

Far-UVC 222 nm Treatment: Effects of Nitrate/Nitrite on Disinfection Byproduct Formation Potential

Irradiation at far ultraviolet C (far-UVC) 222 nm by krypton chloride (KrCl*) excilamps can enhance microbial disinfection and micropollutant photolysis/oxidation. However, nitrate/nitrite, which absorbs strongly at 222 nm, may affect the formation of disinfection byproducts (DBPs). Herein, we evaluated model organic matter and real water samples and observed a substantial increase in the formation potential for trichloronitromethane (chloropicrin) (TCNM-FP), a nitrogenous DBP, by nitrate or nitrite after irradiation at 222 nm. At a disinfection dose of 100 mJ·cm-2, TCNM-FP of humic acids and fulvic acids increased from ∼0.4 to 25 and 43 μg·L-1, respectively, by the presence of 10 mg-N·L-1 nitrate. For the effect of nitrate concentration, the TCNM-FP peak was observed at 5-10 mg-N·L-1. Stronger fluence caused a greater increase of TCNM-FP. Similarly, the increase of TCNM-FP was also observed for wastewater and drinking water samples containing nitrate. Pretreatment using ozonation and coagulation, flocculation, and filtration or the addition of H2O2 can effectively control TCNM-FP. The formation potential of other DBPs was minorly affected by irradiation at 222 nm regardless of whether nitrate/nitrite was present. Overall, far-UVC 222 nm treatment poses the risk of increasing TCNM-FP of waters containing nitrate or nitrite at environmentally relevant concentrations and the mitigation strategies merit further research.

Accelerated mineralization of textile wastewater under 222 nm irradiation from Kr/Cl2 excilamp: an environmentally friendly and energy efficient approach

The textile dyeing and manufacturing industry is the major producer of significant amounts of wastewater that contain persistent substances such as azo dyes that require adequate remediation measures. Far ultraviolet at 222 nm light may provide an advantage for contaminants degradation as compared to conventional UV sources (254 nm). In this paper, the degradation of reactive black 5 (RB5) in artificial wastewater has been performed using a 222 nm Kr/Cl2 excimer source under direct photolysis and an advanced oxidation process using TiO2/H2O2. The solution pH, catalyst concentration, 222 nm intensity, initial concentration of dye, and addition of H2O2 influence the degradation rate constant. The molar absorption coefficient, quantum yield of RB5 at 222 nm and the electrical energy per order (EEO) from different treatment methods have been reported. RB5 shows 1.26 times higher molar absorption at 222 nm than at 254 nm. The EEO for excimer-222/H2O2 ( ∼ 13 kWh/m3) is five times lower than that of the excimer-222/TiO2 process, which makes the process energy efficient. The degradation of wastewater has been carried out at three distinct pH values (2, 6, and 10), and the pH level of 10 exhibited the highest degree of degradation. The degradation rate in the alkaline medium is 8.27 and 2.05 times higher than in the acidic or ambient medium. Since textile effluent is highly alkaline, this result is significant, as no neutralization of the wastewater is required, and direct treatment is possible. A possible degradation pathway has been established based on Fourier transform infrared spectroscopy (FTIR) and high resolution mass spectroscopy (HRMS) analysis. The phytotoxicity of the treated wastewater has also been evaluated for its suitability for reuse in agriculture. The study reveals that the excimer-222/H2O2 treated wastewater significantly enhanced the germination percentage of Raphanus sativus seed (97%) compared to dye wastewater-grown seeds (75%). This work offers crucial information for future studies on the direct and indirect photolysis of azo dyes, as well as insight into the process of RB5 degradation under Kr/Cl2 excimer radiation.

Suppression of photoreactivation of E. coli by excimer far-UV light (222 nm) via damage to multiple targets

Low-pressure mercury lamps emitting at 254 nm (UV254) are used widely for disinfection. However, subsequent exposure to visible light results in photoreactivation of treated bacteria. This study employed a krypton chloride excimer lamp emitting at 222 nm (UV222) to inactivate E. coli. UV222 and UV254 treatment had similar E. coli-inactivation kinetics. Upon subsequent irradiation with visible light, E. coli inactivated by UV254 was reactivated from 2.71-log to 4.75-log, whereas E. coli inactivated by UV222 showed negligible photoreactivation. UV222 treatment irreversibly broke DNA strands in the bacterium, whereas UV254 treatment primarily formed nucleobase dimers. Additionally, UV222 treatment caused cell membrane damage, resulting in wizened, pitted cells and permeability changes. The damage to the cell membrane was mainly due to the photolysis of proteins and lipids by UV222. Furthermore, the photolysis of proteins by UV222 destroyed enzymes, which blocked photoreactivation and dark repair. The multiple damages can be further evidenced by 4.0-61.1 times higher quantum yield in the photolysis of nucleobases and amino acids for UV222 than UV254. This study demonstrates that UV222 treatment damages multiple sites in bacteria, leading to their inactivation. Employing UV222 treatment as an alternative to UV254 could be viable for inhibiting microorganism photoreactivation in water and wastewater.

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.