The role and synergistic effect of the light irradiation and H2O2 in photocatalytic inactivation of Escherichia coli

Identifying the mediators of intracellular E. coli inactivation under UVA light: The (photo) Fenton process and singlet oxygen

Solar disinfection (SODIS) was probed for its underlying mechanism. When Escherichia coli was exposed to UVA irradiation, the dominant solar fraction acting in SODIS process, cells exhibited a shoulder before death ensued. This profile resembles cell killing by hydrogen peroxide (H2O2). Indeed, the use of specialized strains revealed that UVA exposure triggers intracellular H2O2 formation. The resultant H2O2 stress was especially impactful because UVA also inactivated the processes that degrade H2O2-peroxidases through the suppression of metabolism, and catalases through direct enzyme damage. Cell killing was enhanced when water was replaced with D2O, suggesting that singlet oxygen plays a role, possibly as a precursor to H2O2 and/or as the mediator of catalase damage. UVA was especially toxic to mutants lacking miniferritin (dps) or recombinational DNA repair (recA) enzymes, indicating that reactions between ferrous iron and UVA-generated H2O2 lead to lethal DNA damage. Importantly, experiments showed that the intracellular accumulation of H2O2 alone is insufficient to kill cells; therefore, UVA must do something more to enable death. A possibility is that UVA stimulates the reduction of intracellular ferric iron to its ferrous form, either by stimulating O2•- formation or by generating photoexcited electron donors. These observations and methods open the door to follow-up experiments that can probe the mechanisms of H2O2 formation, catalase inactivation, and iron reduction. Of immediate utility, the data highlight the intracellular pathways formed under UVA light during SODIS, and that the presence of micromolar iron accelerates the rate at which radiation disinfects water.

Roles of reactive oxygen species in antibiotic resistant bacteria inactivation and micropollutant degradation in Fenton and photo-Fenton processes

Reactive oxygen species play a critical role in degrading chemical or biological contaminants in advanced oxidation processes. However, it is still not clear whether conventional Fenton and photo-Fenton processes generate different reactive oxygen species, respectively. This study revealed the roles of reactive oxygen species (ROS) for simultaneous removal of antibiotic resistant bacteria (ARB) and recalcitrant micropollutant using three processes, i.e., conventional Fenton, photo-Fenton, and ethylenediamine-N, N'-disuccinic acid (EDDS) modified photo-Fenton. Both chemical scavengers and electron paramagnetic resonance spectroscopy confirmed the generation of various ROS and their contribution towards bacterial inactivation and micropollutant degradation. Results showed ARB and carbamazepine (CBZ) elimination efficiency in the order: EDDS modified photo-Fenton process > photo-Fenton process > Fenton process. The ARB detection limit (6-log ARB) was observed within 10 min at lower doses of 0.1 mM Fe³⁺, 0.2 mM EDDS, and 0.5 mM hydrogen peroxide (H₂O₂). With the same dose, it took longer (60 min) to remove CBZ, while 2.5 times higher H₂O₂ dose (1.25 mM) removed around 99% of CBZ within 10 min treatment. The present study highlighted that the hydroxyl radical (HO^•) plays a dominant role, while singlet oxygen (¹O₂) and superoxide radical anion (O₂^•-) exhibit moderate effects to remove the hazards. Our findings provide mechanistic insights into the role of various reactive oxygen species on degrading micropollutants and inactivating ARB.

Increased sanitization potency of hydrogen peroxide with synergistic O3 and intense pulsed light for non-woven polypropylene

Supplies of respiratory masks have recently become a concern due to the onset of the SARS-CoV-2 pandemic. Sanitization and reuse of masks can alleviate high mask consumption and production stresses. In the present work, improved sanitization potency of vaporous hydrogen peroxide (VHP) treatment of resilient bacterial spores while retaining polymeric filter performance was explored. A batch fumigation chamber with hydrogen peroxide (H₂O₂) vapor and ozone (O₃) is featured, followed by intense pulsed light (IPL) flash treatments. A resilient bacterial indicator, Geobacillus stearothermophilus (G. stearothermophilus), was utilized to compare the efficacy of various H₂O₂ concentrations in combination with O₃ and IPL. It was found that exposure to 30 minutes of 4.01 L min⁻¹ 0.03% H₂O₂ aqueous vapor and 3 g h⁻¹ O₃ followed by 10 IPL flashes per side completely inactivated G. stearothermophilus. The xenon sourced IPL irradiation was found to synergistically enhance radical production and strengthen the complementary biocidal interaction of H₂O₂ with O₃. Due to the synergistic effects, H₂O₂ was able to sanitize at a diluted concentration of 0.03% H₂O₂. The physical properties, such as surface potential, tensile strength, hydrophobicity, and filtration efficiency of >300 nm saline water aerosol of fibrous polypropylene (PP) sheets, were maintained. In addition, no residue of sanitizers was detected, thus confirming the biosafety and applicability of this method to disposable masks. Performance was benchmarked and compared with commercially available processes. The synergistic regime was found to achieve sterilization of G. stearothermophilus at drastically reduced H₂O₂ concentrations and in ambient conditions relative to commercial methods.

By introducing synergistic elements to the VHP processes, potent sanitization of polymeric filters is achieved at low H₂O₂ concentrations.

A novel application of In2S3 for visible-light-driven photocatalytic inactivation of bacteria: Kinetics, stability, toxicity and mechanism

Photocatalytic bacterial inactivation under visible light emerges as a new alternative to control microbial contamination by utilizing free and renewable sunlight. However, the exploration of highly effective and safe visible-light-driven (VLD) photocatalysts remains an important step toward accessing this new technology. Herein, an eco-friendly photocatalyst, namely Indium Sulfide (In₂S₃), was fabricated through a facile hydrothermal method for VLD photocatalytic inactivation of bacteria. The energy band gap of the as-prepared In₂S₃ was measured as 2.25 eV. As expected, the obtained In₂S₃ photocatalyst showed remarkable inactivation efficiency toward E. coli under fluorescent tubes irradiation. The photocatalytic inactivation kinetic was perfectly fitted by a mathematical model for bacteria inactivation. In addition, In₂S₃ exhibited high stability and could be reused. The leakage of In³⁺ was not significant and showed no toxic effect to the bacteria. Based on the results of scavenger study and ESR technology, the dominant reactive species causing In₂S₃ VLD photocatalytic bacterial inactivation were proposed as O₂^-, h⁺, H₂O₂ and e^-, rather than OH. The SEM study suggested that the damages to the intracellular components occurred prior to the destruction of cell wall. This study provides novel application of In₂S₃ for VLD photocatalytic inactivation of bacteria as well as comprehensive insight into the inactivation mechanism.

Catalyst-free activation of persulfate by visible light for water disinfection: Efficiency and mechanisms

The development of cost-effective water disinfection methods is highly desired to address the problems caused by outbreak of harmful microorganisms. Sulfate radical (•SO₄^-)-based advanced oxidation technology has attracted increasing attention. However, various catalysts or UV irradiation are usually used to activate persulfate (PS), which is high-cost and the recovery of nano-sized catalysts is also challenging. This work demonstrates a new method of catalyst-free activation of persulfate by visible light (VL) for bacterial inactivation. The 6-log of E. coli cells can be inactivated within 40 min and 7-log of E. coli cells could be inactivated within 120 min by the VL/PS system. The major responsive wavelength is 420 nm, and no heat activation of PS is found during VL irradiation. A synergistic effect with synergy factor of 51.2% is found when combining the VL irradiation with heating at 50 °C. The acidic pH is benefit for the VL/PS-triggered bacterial inactivation, while bicarbonate inhibits the E. coli inactivation at the range of 0.1-20 mg/L. Mechanism study indicates the main reactive species are •SO₄^-, •O₂^- and •OH, in which •SO₄^- plays the most important role. The bacterial inactivation process shows to begin from outer membrane to intracellular components. Subsequently, the antioxidant enzyme (i.e. SOD, CAT) is induced, followed by damaging to the genomic DNA leading to fatal death of the cells. In addition, the VL/PS system is also applicable for the inactivation of other pathogenic bacteria, including Staphylococcus aureus and Pseudomonas aeruginosa, showing universality for water disinfection applications. This work not only provides a new cost-effective disinfection method without a catalyst, but also sheds light on understanding the bacterial inactivation mechanism by •SO₄^--based AOPs.

Influence of industrial contamination in municipal secondary effluent disinfection by UV/H2O2

Advanced oxidation processes, including UV/H₂O₂, are methods able to remove diverse classes of organic contaminants and disinfect water and wastewater. However, the variation in the matrix composition can influence the inactivation of microorganisms due to the presence of competing reactive material, which consumes the available oxidants. This problem can lead to the use of inadequate oxidant/radiation dose and disturb a correct treatment. The aim of this study was to assess the efficiency of UV/H₂O₂ to inactivate microbiological indicators in secondary effluents in the presence of high concentration of carbonate, nitrate, metals, and industrial organic contaminants. Metals had a positive influence on inactivation acting as catalysts. Zn, Fe, and all metals simultaneously presented toxic effects to the indicator organisms in the higher concentrations before the treatment. Even in metals presence, the negative effect of carbonate and the industrial organic contaminants on indicators inactivation was very important. Bacteria regrowth after 72 h was also affected by the same inhibiting substances, but the metals acted positively inhibiting it. The disinfection indicators had different sensibilities to the spiked substances. Escherichia coli inactivation was more affected than total coliforms by the presence of the industrial contamination, which can lead to different interpretation of inhibition degree depending of the used disinfection indicator.

Analogies and differences among bacterial and viral disinfection by the photo-Fenton process at neutral pH: a mini review

Over the last years, the photo-Fenton process has been established as an effective, green alternative to chemical disinfection of waters and wastewaters. Microorganisms' inactivation is the latest success story in the application of this process at near-neutral pH, albeit without clearly elucidated inactivation mechanisms. In this review, the main pathways of the combined photo-Fenton process against the most frequent pathogen models (Escherichia coli for bacteria and MS2 bacteriophage for viruses) are analyzed. Firstly, the action of solar light is described and the specific inactivation mechanisms in bacteria (internal photo-Fenton) and viruses (genome damage) are presented. The contribution of the external pathways due to the potential presence of organic matter in generating reactive oxygen species (ROS) and their effects on microorganism inactivation are discussed. Afterwards, the effects of the gradual addition of Fe and H₂O₂ are assessed and the differences among bacterial and viral inactivation are highlighted. As a final step, the simultaneous addition of both reagents induces the photo-Fenton in the bulk, focusing on the differences induced by the homogeneous or heterogeneous fraction of the process and the variation among the two respective targets. This work exploits the accumulated evidence on the mechanisms of bacterial inactivation and the scarce ones towards viral targets, aiming to bridge this knowledge gap and make possible the further application of the photo-Fenton process in the field of water/wastewater treatment.

Simultaneous bacterial inactivation and degradation of an emerging pollutant under visible light by ZnFe2O4co-modified with Ag and rGO

Simultaneous photo-inactivation ofE. coliand degradation of EE2 was achieved in the presence of ZnFe₂O₄-Ag/rGO. H₂O₂was mainly responsible for bacterial inactivation whereas, OH˙ was found to have more influence in EE2 degradation.

Synergistic photocatalytic inactivation mechanisms of bacteria by graphene sheets grafted plasmonic AgAgX (X = Cl, Br, I) composite photocatalyst under visible light irradiation

By coupling graphene sheet and plasmonic photocatalysis technologies, a series of AgAgX/RGOs (X = Cl, Br, I; RGO = reduced graphene oxide) composites were prepared and found to be efficient antimicrobial agents for water disinfection upon visible light. Attributed to the efficient charge transfer by RGO sheets, the optimum AgAgBr/0.5% RGO could completely inactivate 2 × 10(7) cfu mL(-1) of Escherichia coli within 8 min, much faster than bare AgAgBr within 35 min. The synergistic antimicrobial mechanism of AgAgBr/0.5% RGO was studied by Ag(+) ions release evaluation, radical scavengers study, and radical determination. The enhanced photocatalytic activity of irradiated AgAgBr/0.5% RGO originated from the synergistic activities of its three components including Ag, AgBr and RGO, and the proposed mechanisms contained enhanced attraction by RGO followed by two pathways: primary oxidative stress caused by plasma induced reactive species like H2O2 and bactericidal effect of released Ag(+) ions. Furthermore, characterization of E. coli cells using SEM, fluorescent microscopy, and cytoplasmic substance leakage illustrated that VL irradiated AgAgBr/0.5% RGO could not only cause metabolic dysfunction but also destroy the cell envelope and biomolecular, while irradiated Ag(+) ions play a differential bactericidal action with a limited metabolic injury and no cell-membrane damage. The present work provides an efficient water disinfection technology and also opens a new idea in studying the antimicrobial mechanism of plasmonic photocatalyst.