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1. EnSE 6254: Atmospheric Science Assignment 2 : Aqueous-phase chemistry and bactericidal effects from an air discharge plasma in contact with water: evidence for the formation of peroxynitrite through a pseudo-second-order post-discharge reaction of H2O2 and HNO2 Fredrick P. Girenga ( M028/T23) MSc. Environmental Science and Engineering Course instructor: Dr. Nelson Mpumi 1

2. Contents 1. Abstract 2. Introduction 3. Experiment 4. Discussion and Results 5 . Conclusion 2

3. 1. Abstract The study explores the formation of transient species like hydroxyl radicals, nitrogen dioxide radicals, and nitric oxide radicals, as well as long-lived chemical products like ozone, hydrogen peroxide, nitrate, and nitrite, produced by gas discharge plasma at the gas-liquid interface and directly in the liquid. Formations depend on the gas atmosphere and the pH of the plasma-treated water. It evaluates the aqueous-phase chemistry and their contributions to the chemical and biocidal effects of air discharge plasma in water. It demonstrates the antibacterial properties of plasma-activated water (PAW) mediated by peroxynitrite . 3

4. 2. Introduction It explores the chemical effects of gas-phase discharges in contact with liquid, focusing on plasma-chemical reactions at the gas-liquid interface. Key reactive species like hydroxyl radicals, ozone, and hydrogen peroxide are identified as crucial for the biocidal effects of non-thermal atmospheric-pressure plasma systems, particularly plasma-activated water (PAW). 4

5. Introduction (1) 5 Figure 1. Scheme of experimental apparatus for generating the gas-phase pulsed discharge in contact with the water surface. Figure 2. Photo of the gas-phase discharge filaments propagating along the water surface.

6. 3. Experiment: Discharge apparatus The experimental setup involved a gas-phase discharge apparatus where discharge channels propagated along the gas-liquid interface, influenced by the conductivity of the solution. The system maintained a fixed discharge gap and utilized a pulse high voltage to generate plasma, with controlled pH levels achieved through phosphate buffer solutions. Continuous gas flow through the reactor ensured consistent conditions for plasma generation, allowing for systematic investigation of the chemical processes involved. 6

7. Cont..: Chemical analysis The study used HPLC and ion chromatography to quantify reactive species like hydrogen peroxide, nitrites, and nitrates in plasma-treated water. A method for hydrogen peroxide detection involved forming a stable complex with titanyl ions. The analysis of phenol degradation products provided insights into ROS and RNS reactions . NO2 − +H2O2 +H+ → NO3− +H2O+H+ ( 1) 3N3 − +NO2− +4H+ → 5N2 +2H2O. ( 2) 7

8. Cont.. NO2 − +H+ ↔ HNO2 (6) 2HNO2 → NO· +NO2· +H2O (7) HNO2 +H+ → H2NO2+ → NO+ +H2O (8) 2NO2· +H2O → NO3− +NO2− +2H+ (9) 4NO· +O2 +2H2O → 4NO2− +4H+ (10) 4NO2· +O2 +2H2O → 4NO3− +4H+. (11 8 Fig Products of plasma-chemical degradation of phenol by the gas-phase discharge plasma generated in contact with the water surface , analyzed in this work.

9. Cont …. Bacterial analysis Study aimed to assess E. coli viability using freeze-dried bacteria suspensions treated with plasma-activated water. The antibacterial efficacy of plasma-treated solutions was determined by counting colony-forming units (CFUs). Its also correlated bacterial inactivation rates with reactive species concentrations. 9

10. Graghs at different pH 10 fig. 3 Effect of solution pH (3.3, 6.9 and 10.1) on the inactivation of bacteria E. coli treated by the air discharge plasma. Data for NaH2PO4 solution (initial pH 5.1) show E. coli inactivation in the non-buffered solution. Fig 4 : Effect of solution pH (3.3, 6.9 and 10.1) on the inactivation of bacteria E. coli treated by the Ar /O2 discharge plasma. Data for NaH2PO4 solution (initial pH 5.1) show E. coli inactivation in the non-buffered solution.

11. Discussion and results It reveals that acidic conditions enhance bactericidal effects, but not the sole antibacterial agents. The combined action of reactive oxygen species (ROS) and reactive nitrogen species significantly influences antibacterial properties . The study highlights the complexity of plasma's biocidal mechanisms and the importance of post-discharge reactions in PAW. 11

12. Cont.. Its explores the formation of peroxynitrite in plasma-treated water through a reaction between hydrogen peroxide and nitrite ions . It found a pseudo-second-order rate constant, indicating its significant role in PAW's antibacterial effects. It highlights the complexity of peroxynitrite chemistry and suggests its formation and subsequent reactions contribute to the overall antimicrobial efficacy of plasma-treated solutions. 12

13. Cont.. The study found that ozone generation from plasma discharge in PAW is negligible in air plasma, and its direct impact on bacterial inactivation is limited. The primary mechanisms of action involve reactive nitrogen and oxygen species, rather than ozone itself, and its presence in plasma-treated water was inferred from phenol degradation products. 13

14. Conclusion The study confirms the formation of peroxynitrite , which plays a role in the antibacterial activity of plasma-activated water. It also highlights the importance of pH and reactive species concentrations in determining plasma treatment efficacy. The research acknowledges that other reactive species may also contribute to the overall antibacterial effects observed in plasma-treated solutions. 14

15. 7.References [1] Lukes P, Locke B R and Brisset J L 2012 Aqueous-phase chemistry of electrical discharge plasma in water and ingas –liquid environments Plasma Chemistry and Catalysis in Gases and Liquids edVIParvulescu et al ( Weinheim:Wiley-VCH ) pp 241–307 [2] Lukes P, Brisset J L and Locke B R 2012 Biological effects of electrical discharge plasma in water and in gas–liquid environments Plasma Chemistry and Catalysis in Gases and Liquids edVIParvulescu et al ( Weinheim:Wiley-VCH ) pp 309–52 [3] Graves D B 2012 J. Phys. D: Appl. Phys. 45 263001 [4] Stalder K R, McMillen D F and Woloszko J 2005 J. Phys. D: Appl . Phys. 38 1728–38 [5] Fridman G, Friedman G, Gutsol A, Shekhter A B, Vasilets V N and Fridman A 2008 Plasma Process. Polym . 5 503–33 [6] Kong M G, Kroesen G, Morfill G, Nosenko T, Shimizu T,van Dijk J and Zimmermann J L 2009 New J. Phys.11 115012 [7] Laroussi M 2005 Plasma Process. Polym . 2 391–400 [8] Dobrynin D, Fridman G, Friedman G and Fridman A 2009 New J. Phys. 11 115020 [9] vanGilsCAJ,Hofmann S, Boekema B, Brandenburg R and Bruggeman P J 2013 J. Phys. D: Appl. Phys. 46 175203 [10] Oehmigen K, Hahnel M, Brandenburg R, Wilke C,Weltmann K D and von Woedtke T 2010 Plasma Process. Polym . 7 250–7 15