Kinetics and mechanism of the reaction of hydrogen peroxide with hypochlorous acid: Implication on electrochemical water treatment

Evaluation of the bactericidal activity of plasma-activated NaCl solution (PAN) against foodborne pathogens: Inactivation mechanism and application to mackerel

The objective of this study was to assess the bactericidal effect of plasma-activated NaCl solution (PAN) against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes and apply PAN as a brine salting solution for mackerel. To enhance the bactericidal effect of plasma-activated water (PAW), NaCl solutions (0, 3.5, 7, and 10%) were treated with plasma for 20 and 40 min to generate PAN. Plasma-activated glycerol solution (PAG) was also included to evaluate the influence of water activity on plasma activation and its effect on microbial activity. Physicochemical analysis revealed that elevating the NaCl concentration of PAN led to a decrease in pH, an increase in oxidation-reduction potential, and higher levels of reactive species such as H2O2 and HOCl. PAN showed greater antibacterial activity compared to PAW and PAG, except for L. monocytogenes, where 40 min activation time and treatment time exceeding 20 min was required for significantly higher reduction to occur. PAN exhibited greater antibacterial activity at higher NaCl concentrations, which was attributed to increased ionic strength and reactive chlorine species. Additionally, we evaluated the microbial mechanisms of PAN by assessing cellular damage and alterations. The common observation across the three pathogens was that PAN resulted in increased cell membrane damage, reduced intermembrane enzyme activity, higher intracellular ROS levels, and changes in zeta potential values, while DNA damage was observed only in PAN-treated L. monocytogenes. Furthermore, when PAW and PAN were stored for up to four weeks, PAN showed higher efficacy compared to PAW. 10% PAN was also effective against foodborne pathogens on mackerel, achieving log reductions of 3.62 for E. coli O157:H7, 4.62 for S. Typhimurium, and 3.18 for L. monocytogenes after a 20 min treatment without adversely affecting quality. Our results demonstrated the antibacterial activity and action mechanism of PAN, presenting its potential application in the seafood industry.

Inhibition of inorganic chlorinated byproducts formation during electrooxidation treatment of saline phenolic wastewater via synergistic cathodic generation of H2O2

The electrochemical treatment of saline wastewater is prone to the formation of inorganic chlorinated byproducts, being a significant challenge for this technology. In this study, we introduce an electrooxidation system utilizing a self-supporting nitrogen-doped carbon-based cathode embedded in carbon cloth (N@C-CC), designed to generate H₂O₂. This system aims to rapidly neutralize free chlorine produced at the anode, a precursor to inorganic chlorinated byproducts, thereby reducing their formation. Our results demonstrate that using the N@C-CC cathode in saline wastewater treatment yielded considerably lower concentrations of ClO₃⁻ and ClO₄⁻ (0.08 mM and 0.024 mM, respectively), which were only 20.5% and 22.7% of the levels produced using a Pt cathode without H₂O₂ generation. Moreover, the presence of cathodically generated H₂O₂ that quenches free chlorine did not significantly impact the degradation performance of phenol. Electron paramagnetic resonance tests and quenching experiments indicated that 1O₂ was primarily responsible for phenol removal. Validation with real wastewater demonstrated reductions of 68.6% and 56.3% in ClO3- and ClO4- concentrations, respectively, while effectively removing other pollutants. This study thus offers a compelling method for mitigating the formation of inorganic chlorinated byproducts during the electrooxidation of saline wastewater.

Environmental implications of superoxide radicals: From natural processes to engineering applications

The roles of superoxide radical (O2•-) in the domains of physiological, physical, and material chemistry are becoming increasingly recognized. Although extensive efforts have been directed to understand O2•- functions in diverse aquatic systems, there is a lack of systematic and in-depth review for its kinetics and mechanisms in various environmental scenarios. This review aims to bridge this gap through discussion of O2•- generation pathways under both natural and controlled conditions. The merits and limitations of the generation and detection methods under various conditions are compared, with emphasis on different approaches for the determination of O2•--triggered reaction kinetics. We summarize the reaction rate constants of O2•- with organic contaminants covering a wide diversity of structures and reactivity. The comparison indicates that O2•- exhibits weak reactivity with most contaminants and lacks selectivity towards compounds with different functional groups, except with quinones which exhibit higher reactivity compared to non-quinones. Further, the reaction mechanisms, namely single electron transfer, nucleophilic substitution, hydrogen atom abstraction, and radical-adduct formation, are critically evaluated. Various environmental implications of O2•- are highlighted including maintenance of biogeochemical iron cycle, synthesis of nanoparticles for antibacterial purposes, desorption of contaminants from heterogeneous interfaces, and synergetic degradation of contaminants.

Exploring the viability of peracetic acid-mediated antibiotic degradation in wastewater through activation with electrogenerated HClO

Electrochemical advanced oxidation processes (EAOPs) face challenging conditions in chloride media, owing to the co-generation of undesirable Cl-disinfection byproducts (Cl-DBPs). Herein, the synergistic activation between in-situ electrogenerated HClO and peracetic acid (PAA)-based reactive species in actual wastewater is discussed. A metal-free graphene-modified graphite felt (graphene/GF) cathode is used for the first time to achieve the electrochemically-mediated activation of PAA. The PAA/Cl- system allowed a near-complete sulfamethoxazole (SMX) degradation (kobs =0.49 min-1) in only 5 min in a model solution, inducing 32.7- and 8.2-fold rise in kobs as compared to single PAA and Cl- systems, respectively. Such enhancement is attributed to the occurrence of 1O2 (25.5 μmol L-1 after 5 min of electrolysis) from the thermodynamically favored reaction between HClO and PAA-based reactive species. The antibiotic degradation in a complex water matrix was further considered. The SMX removal is slightly susceptible to the coexisting natural organic matter, with both the acute cytotoxicity (ACT) and the yield of 12 DBPs decreasing by 29.4 % and 37.3 %, respectively. According to calculations, HClO accumulation and organic Cl-addition reactions are thermodynamically unfavored. This study provides a scenario-oriented paradigm for PAA-based electrochemical treatment technology, being particularly appealing for treating wastewater rich in Cl- ion, which may derive in toxic Cl-DBPs.

A Pyroptosis-Inducing Arsenic(III) Nanomicelle Platform for Synergistic Cancer Immunotherapy

Immunogenic cell death (ICD) could activate anti-tumor immune responses, which is highly attractive for improving cancer treatment effectiveness. Here, this work reports a multifunctional arsenic(III) allosteric inhibitor Mech02, which induces excessive accumulation of 1O2 through sensitized biocatalytic reactions, leading to cell pyroptosis and amplified ICD effect. After Mech02 is converted to Mech03, it could actualize stronger binding effects on the allosteric pocket of pyruvate kinase M2, further interfering with the anaerobic glycolysis pathway of tumors. The enhanced DNA damage triggered by Mech02 and the pyroptosis of cancer stem cells provide assurance for complete tumor clearance. In vivo experiments prove nanomicelle Mech02-HA NPs is able to activate immune memory effects and raise the persistence of anti-tumor immunity. In summary, this study for the first time to introduce the arsenic(III) pharmacophore as an enhanced ICD effect initiator into nitrogen mustard, providing insights for the development of efficient multimodal tumor therapy agents.

Evaluation of treatment of methicillin-resistant Staphylococcus aureus biofilms with intermittent electrochemically generated H2O2 or HOCl

Chronic wound infections can be difficult to treat and may lead to impaired healing and worsened patient outcomes. Novel treatment strategies are needed. This study evaluated the effects of intermittently produced hydrogen peroxide (H2O2) and hypochlorous acid (HOCl), generated via an electrochemical bandage (e-bandage), against methicillin-resistant Staphylococcus aureus biofilms in an agar membrane biofilm model. By changing the working electrode potential, the e-bandage generated either HOCl (1.5 VAg/AgCl) or H2O2 (-0.6 VAg/AgCl). The degree of biocidal activity of intermittent treatment with HOCl and H2O2 correlated with HOCl treatment time; HOCl treatment durations of 0, 1.5, 3, 4.5, and 6 hours (with the rest of the 6-hour total treatment time devoted to H2O2 generation) resulted in mean biofilm reductions of 1.36 ± 0.2, 2.22 ± 0.16, 3.46 ± 0.38, 4.63 ± 0.74, and 7.66 ± 0.5 log CFU/cm2, respectively, vs. non-polarized controls, respectively. However, application of H2O2 immediately after HOCl treatment was detrimental to biofilm removal. For example, 3 hours HOCl treatment followed by 3 hours H2O2 resulted in a 1.90 ± 0.84 log CFU/cm2 lower mean biofilm reduction than 3 hours HOCl treatment followed by 3 hours non-polarization. HOCl generated over 3 hours exhibited biocidal activity for at least 7.5 hours after e-bandage operation ceased; 3 hours of HOCl generation followed by 7.5 hours of non-polarization resulted in a biofilm cell reduction of 7.92 ± 0.12 log CFU/cm2 vs. non-polarized controls. Finally, intermittent treatment with HOCl (i.e., interspersed with periods of e-bandage non-polarization) for various intervals showed similar effects (approximately 6 log CFU/cm2 reduction vs. non-polarized control) to continuous treatment with HOCl for 3 hours, followed by 3 hours of non-polarization. These findings suggest that timing and sequencing of HOCl and H2O2 treatments are crucial for maximizing biofilm control when using an e-bandage strategy.

Evaluation of Treatment of Methicillin-Resistant Staphylococcus aureus Biofilms with Intermittent Electrochemically-Generated H2O2 or HOCl

Chronic wound infections can be difficult to treat and may lead to impaired healing and worsened patient outcomes. Novel treatment strategies are needed. This study evaluated effects of intermittently produced H2O2 and HOCl, generated via an electrochemical bandage (e-bandage), against methicillin-resistant Staphylococcus aureus biofilms in an agar membrane biofilm model. By changing the working electrode potential, the e-bandage generated either HOCl (1.5 VAg/AgCl) or H2O2 (-0.6 VAg/AgCl). The degree of biocidal activity of intermittent treatment with HOCl and H2O2 correlated with HOCl treatment time; HOCl treatment durations of 0, 1.5, 3, 4.5, and 6 hours (with the rest of the 6 hour total treatment time devoted to H2O2 generation) resulted in mean biofilm reductions of 1.36±0.2, 2.22±0.16, 3.46±0.38, 4.63±0.74 and 7.66±0.5 log CFU/cm2, respectively vs. non-polarized controls, respectively. However, application of H2O2 immediately after HOCl treatment was detrimental to biofilm removal. For example, 3-hours HOCl treatment followed by 3-hours H2O2 resulted in a 1.90±0.84 log CFU/cm2 lower mean biofilm reduction than 3-hours HOCl treatment followed by 3-hours non-polarization. HOCl generated over 3-hours exhibited biocidal activity for at least 7.5-hours after e-bandage operation ceased; 3-hours of HOCl generation followed by 7.5-hours of non-polarization resulted in a biofilm cell reduction of 7.92±0.12 log CFU/cm2 vs. non polarized controls. Finally, intermittent treatment with HOCl (i.e., interspersed with periods of e-bandage non-polarization) for various intervals showed similar effects (approximately 6 log CFU/cm2 reduction vs. non-polarized control) to continuous treatment with HOCl for 3-hours, followed by 3-hours of non-polarization. These findings suggest that timing and sequencing of HOCl and H2O2 treatments are crucial for maximizing biofilm control.

The synergistic effect of oxidant-peroxide coupling systems for water and wastewater treatments

With the growing complexity and severity of water pollution, it has become increasingly challenging to effectively remove contaminants or inactivate microorganisms just by traditional chemical oxidants such as O3, chlorine, Fe(VI) and Mn(VII). Up till now, numerous studies have indicated that these oxidants in combination with peroxides (i.e., hydrogen peroxide (H2O2), peroxymonosulfate (PMS), peracetic acid (PAA) and periodate (PI)) exhibited excellent synergistic oxidation. This paper provided a comprehensive review on the combination of aforementioned oxidant-peroxide applied in water and wastewater treatments. From one aspect, the paper thoroughly elucidated the synergy mechanism of each oxidant-peroxide combination in turn. Among these combinations, H2O2 or PMS generally performed as the activator of four traditional oxidants above to accelerate reactive species generation and therein various reaction mechanisms, including electron transfer, O atom abstraction and oxo ligand substitution, were involved. In addition, although neither PAA nor PI was able to directly activate Fe(VI) and Mn(VII), they could act as the stabilizer of intermediate reactive iron/manganese species to improve the latter utilization efficiency. From another aspect, this paper summarized the influence of water quality parameters, such as pH, inorganic ions and natural organic matter (NOM), on the oxidation performance of most combined systems. Finally, this paper highlighted knowledge gaps and identified areas that require further research.

Bimolecular versus Trimolecular Reaction Pathways for H2O2 with Hypochlorous Species and Implications for Wastewater Reclamation

The benchmark advanced oxidation technology (AOT) that uses UV/H2O2 integrated with hypochlorous species exhibits great potential in removing micropollutants and enhancing wastewater treatability for reclamation purposes. Although efforts have been made to study the reactions of H2O2 with hypochlorous species, there exist great discrepancies in the order of reaction kinetics, the rate constants, and the molecule-level mechanisms. This results in an excessive use of hypochlorous reagents and system underperformance during treatment processes. Herein, the titled reaction was investigated systematically through complementary experimental and theoretical approaches. Stopped-flow spectroscopic measurements revealed a combination of bi- and trimolecular reaction kinetics. The bimolecular pathway dominates at low H2O2 concentrations, while the trimolecular pathway dominates at high H2O2 concentrations. Both reactions were simulated using direct dynamics trajectories, and the pathways identified in the trajectories were further validated by high-level quantum chemistry calculations. The theoretical results not only supported the spectroscopic data but also elucidated the molecule-level mechanisms and helped to address the origin of the discrepancies. In addition, the impact of the environmental matrix was evaluated by using two waters with discrete characteristics, namely municipal wastewater and ammonium-rich wastewater. Municipal wastewater had a negligible matrix effect on the reaction kinetics of H2O2 and the hypochlorous species, making it a highly suitable candidate for this integration technique. The obtained in-depth reaction mechanistic insights will enable the development of a viable and economical technology for safe water reuse.

New insights into the mechanism of Fered-Fenton treatment of industrial wastewater with high chloride content: Role of multiple reactive species

Hydroxyl radical (OH) is considered the dominant reactive species in the electro-Fenton (EF) and Fered-Fenton (EF-Fere) processes for wastewater treatment. However, in chloride-rich media, this is arguable due to the obscure mechanisms for the oxidant speciation and pollutant degradation. Herein, the role of active chlorine and Fe(IV)-oxo species (Fe^IVO²⁺) as primary oxidizing agents in HClO-mediated Fered-Fenton (EF-Fere-HClO) process is discussed, along with the dependence of their contribution on the pollutant structure. HClO generated from anodic oxidation of Cl^- can be consumed by added H₂O₂ to form singlet oxygen (¹O₂), which is detrimental because this species is quickly deactivated by water. The reaction between HClO and Fe²⁺ was proved to generate Fe^IVO²⁺, rather than OH or Cl suggested in the literature. The yield of Fe^IVO²⁺ species was proportional to the Cl^- concentration and barely affected by solution pH. The long-lived HClO and Fe^IVO²⁺ can selectively react with electron-rich compounds, which occurs simultaneously to the non-selective attack of OH formed from Fenton's reaction. The Fe^IVO²⁺ and OH concentration profiles were successfully modelled. Although the accumulation of toxic chlorinated by-products from HClO-mediated oxidation might cause new environmental concerns, the toxicity of pesticide wastewater with 508 mM Cl^- was halved upon EF-Fere-HClO treatment.