O akci
Cold plasma - liquid interactions and their applications
Atmospheric air non-thermal (cold) plasma produces a cocktail of reactive oxygen and nitrogen species (RONS) with various lifetimes, reactivities, and multiple functions. The plasma chemistry is initiated by elementary processes of ionization, excitation, and dissociation; it leads to the formation of radicals and other RONS and induces their mutual reactions. Specific composition of plasma gaseous RONS depends on the plasma discharge type and its power and geometry, the feed gas, its flow rate, the presence of water, and other environmental parameters. Besides these long-lived gaseous RONS, plasma also creates short-lived ones (e.g. hydroxyl OH·, atomic oxygen O·, superoxide O2-· radicals, or molecular singlet delta oxygen 1O2) that strongly influence the plasma reactivity and its effects, although their diagnostics are challenging.
Once plasma interacts with a liquid, either by operating the discharge completely submerged in liquid, or at the gas-liquid boundary, or in hybrid gas-liquid aerosol or bubble systems, gaseous RONS are transported into the liquid and the evaporation strongly influences the plasma processes [2]. The transport of RONS to the liquid phase through the plasma–liquid interface can be significantly enhanced by increasing the interface area, e.g. by converting bulk water to electrosprayed aerosol microdroplets [3]. The solubility of various plasma RONS does not match with the equilibrium of Henry’s law. We verified the Henry’s law coefficients for plasma–liquid interaction with bulk water vs. charged electrosprayed vs. non-charged aerosols [4].
In the liquid, the plasma-formed, as well as the new ionic RONS diffuse and undergo further reactions. They are crucial when the plasma-treated (activated) liquid interacts e.g. with organic pollutants, biomolecules, live cells, tissues, or plant seeds.
We compare various air plasma discharges interacting with water solutions by analysing their gaseous and aqueous RONS. While low power streamers and DBDs in air produce dominantly ozone, higher power air discharges generate more OH·, hydrogen peroxide H2O2, NOx, and nitric/nitrous acids. However, due to various gas-liquid transport rates, even ozone-dominated plasma will not generate high liquid ozone concentrations but rather H2O2 and some NO3¯, which can be still efficient against microorganisms or cancer cells. On the contrary, NOx-dominated plasma makes RNS-dominated plasma-activated water, which can be strongly antimicrobial if both H2O2 and nitrites NO2¯ are present and form peroxynitrites. Or RNS-dominated PAW can be used as a NO3¯-rich fertilizer and plant growth promoter in sustainable agriculture.
The selection of the discharge regime and its operational environmental parameters strongly influence the properties of the plasma-activated water and determines its suitability for specific applications. This fundamental knowledge can lead to optimized designs of plasma–water interaction systems for multiple applications.
Prof. Zdenko Machala, D.Sc. (Faculty of Mathematics, Physics and Informatics of Comenius University Bratislava) has a background in plasma physics. His research is focused on various environmental, biomedical and agriculture applications of non-thermal (cold) atmospheric plasmas. He pioneers new research areas of plasma-liquid interactions, their optical diagnostics, transport of plasma active species into water, and fundamental mechanisms of physico-chemical and biomedical processes of cold plasma interaction with live cells and animal/plant organisms. He has served as a president of the International Society for Plasma Medicine, is a work group leader in COST Action CA 19110 Plasma for Sustainable Agriculture, and is responsible for Environmental Physics study program at his university.
