The present study focuses on the characterization of a hyperbolic vortex plasma reactor through the comparison of various plasma-atmospheric regimes for the production efficiency of reactive nitrogen (RNS) and reactive oxygen (ROS) species. The research also explores effectiveness in the removal of micropollutants, including pharmaceuticals and per- and polyfluoroalkyl substances (PFAS). The technology includes several degradation mechanisms, such as advanced oxidation, ultraviolet photolysis, ozonation, electrolysis, and shockwave water purification, without the need for additional chemicals. Our results indicate that the plasma of bipolar or ‘flashover’ mode is notably more effective and efficient than both positive or negative polarity. Through the testing of various energy levels, it has been demonstrated that higher energy plasma yields lower efficiency but necessitates shorter treatment times compared to lower energy treatment. When plasma is produced under ambient atmosphere, water chemical properties change significantly in comparison to treatment under argon (Ar) or nitrogen (N₂) due to the presence of both oxygen and N₂ molecules. In a N₂ atmosphere, the predominant formation is of RNS due to the chemical reactivity of N₂ exited states, whereas under Ar atmosphere, predominantly ROS are generated. Notable advantages of this technology are its scalability and its low energy requirements. The scalability of the technology involves increasing the size of the reactor, the power and electrode count.

Experimental investigations into the characterization of vortices in hyperbolic funnels have shown efficient aeration properties. Certain regimes of vortices have been observed to exhibit high gas dissolution rates. This phenomenon has prompted inquiries into the underlying physical mechanisms at both micro and macroscopic scales. The present study employs computational fluid dynamics to numerically analyze the flow field organization inside these vortices, aiming to elucidate the observed high gas transfer rates. Transient simulations are performed on a three-dimensional radially structured hexahedral mesh, utilizing a multiphase Euler–Euler approach-based volume of fluid method for modeling, along with shear stress transport turbulence modeling based on k−ω equations with curvature correction. The evaluation of the two vortex regimes was conducted in terms of hydraulic retention time, water volume in the reactor, air–water interfacial area, and bulk mixing. Instabilities resembling Taylor vortices observed in Taylor–Couette flow systems emerge in the secondary flow field of these vortical structures, facilitating turbulent mixing. A qualitative analysis of the strength of these instabilities in terms of average vorticity per unit mass of water explains the high gas transfer efficiency. Despite high gas transfer rates, water exiting the funnel remains undersaturated under given operating conditions due to the short hydraulic retention time.

This paper presents an experimental study comparing the aeration efficiencies of hyperbolic funnels and a cylindrical reactor, focusing on key parameters such as dissolved oxygen (DO) concentration, standard oxygen transfer rate (SOTR₂₀), and standard aeration efficiency (SAE). The unique geometry of the hyperbolic funnel induces a helical water flow, which expands the gas–liquid interfacial area within the water vortex, thereby enhancing aeration efficiency via vortex dynamics. The cylindrical reactor forms larger water “umbrellas” at its outlet due to increased internal water pressure, specifically optimizing the umbrella-driven aeration. The study also evaluated a three-funnel cascade system, demonstrating that a single funnel operating in the umbrella regime is more aeration-efficient than multiple funnels in cascade, as additional funnels reduce the SAE, due to the increased pumping height required. Further experiments using 3D-printed funnels investigated the influence of outlet diameter on flow rates and aeration efficiency. Our results indicated that larger outlet diameters allowed higher flow rates and umbrella sizes, yielding a superior aeration efficiency and outperforming all other reactors tested. The study also highlights the importance of funnel positioning relative to the water reservoir, which significantly influences both the SOTR₂₀ and SAE. For the reactor investigated, a height of 75 cm was optimal for balancing both parameters. Whereas the SOTR₂₀ values of the lab reactors were lower than those of commercial systems, due to the lower flow rates, the SAE values were notably high, surpassing those of mechanical aeration systems. Our findings suggest that hyperbolic funnels are a promising and highly efficient alternative for wastewater and groundwater aeration, with a strong potential for scalability.

This study investigates the removal of per- and polyfluoroalkyl substances (PFAS) using a hyperbolic vortex plasma discharge under various plasma-atmospheric conditions demonstrating varied degradation times for PFAS of different chain lengths. Experiments with spiked long-chain perfluorooctane sulfonic acid in deionized (DI) water show that bipolar ‘flashover’ plasma polarity achieves more effective degradation compared to monopolar positive or negative polarity. For a spiked PFAS matrix of varying chain lengths in DI water, results indicate that the production of reactive species in the gas phase by plasma discharge, and their subsequent dissolution in water through the water vortex, enables the degradation of short-chain perfluorobutanoic acid in the bulk liquid. In contrast, the degradation of long-chain PFAS primarily occurs at the gas-water interface, likely due to direct interactions between the plasma and the PFAS molecules. The addition of the Hyamine 1622 surfactant during treatment significantly enhances the degradation of both short- and long-chain PFAS in DI water, groundwater, and industrial effluent. The results of groundwater treatment indicate that the presence of high concentrations of other substances, particularly anions, slows down the degradation of PFAS, especially short-chain PFAS with carboxylic acid groups. Depending on the conditions and the type of PFAS, degradation can be achieved up to 99% after 75 min of treatment, with typical energy input around 7.2 kJ L⁻¹ or 2 kWh m⁻³.

The contamination of ground and surface waters with per- and polyfluoroalkyl substances (PFASs) is of major concern due to their potential adverse effects on human health. The carbon–fluorine bond makes these compounds extremely stable and hardly degradable by natural processes. Therefore, methods for PFAS removal from water are desperately needed. In this context, plasma treatment of water has been proposed as an effective method with reported removal rates exceeding 90%. However, the high reactivity of plasma discharge results in the formation of many reactive species, like radicals, ozone, or even solvated electrons, which lead to a complex reaction cascade and, consequently, to the generation of a wide variety of different chemical products. The toxicological properties of these PFAS breakdown products are largely unknown. The present study focuses on a toxicological assessment of PFAS-containing plasma-treated water samples. Aqueous solutions of long-chain perfluorooctanesulfonate (PFOS) were treated with various plasma-atmospheric regimes. Subsequently, plasma-treated water samples were subjected to in vitro bioassays. Cytotoxicity and genotoxicity were assessed with the MTS assay using human liver cells (HepG2) and the Ames MPF^TM assay using Salmonella Typhimurium strains. Our results demonstrate varying cyto- and genotoxic properties of water containing PFAS breakdown products depending on the atmosphere present during plasma treatment. Based on the results of this study, the atmosphere used during plasma treatment affects the toxicological properties of the treated sample. Further studies are therefore needed to uncover the toxicological implications of the different treatment parameters, including the PFAS starting compound, the atmosphere during treatment, as well as the quantity of plasma energy applied.

The presence of micropollutants in water is an increasingly pressing environmental concern. While some micropollutants are readily biodegradable, others, such as per- and polyfluoroalkyl substances (PFAS), are extremely persistent and resistant to conventional water treatment technologies. Plasma-based treatment has been investigated for water and wastewater decontamination for decades, with recent studies demonstrating its high efficacy in degrading both short- and long-chain PFAS. Here, plasma-based waste treatment is combined with a free surface hyperbolic water vortex, which has an oxygen volumetric mass transfer coefficient exceeding that of similar systems. Various types of plasma discharges can be employed for such applications, each requiring specific power supply configurations and operational strategies. The use of pulsed signals, in particular, presents unique engineering challenges. This study explores the generation and characterization of six different plasma discharge types within a Hyperbolic Vortex Plasma Reactor: pulsed monopolar (negative and positive), pulsed bipolar "flashover," AC and DC arc, and glow discharge. The pulse characteristics of monopolar and bipolar pulsed discharges were analyzed, and their efficiency in PFAS degradation was evaluated. Among the tested configurations, the bipolar flashover discharge exhibited the highest degradation efficiency in a Hyperbolic Vortex Plasma Reactor. However, its practical implementation poses significant engineering challenges, making its utilization challenging on a larger scale.