Batch processing of overlapping molecular spectra as a tool for spatio-temporal diagnostics of power modulated microwave plasma jet

Enhanced dye degradation using plasma bubbles for the sustainable environmental remediation

This study proposes a novel and eco-friendly approach for wastewater treatment using plasma jet technology under bubble condition. This method allows for the controlled production of highly reactive hydroxyl radicals () while minimizing unwanted interactions with nitrogen in the air. The presence of bubbles in liquid significantly boosts the diffusion of within the wastewater, leading to a two-fold increase in degradation rate compared to normal condition. The effectiveness of the treatment was confirmed through ultraviolet-visible spectroscopy, which showed a significant decrease in rhodamine B and methyl orange absorbance peaks . Raman spectroscopy further revealed structural changes in both pollutants, indicating successful degradation. Additionally, plasma characteristics like power, electron temperature, and density were monitored to gain deeper insights into the underlying mechanism. Importantly, the process minimizes the formation of harmful secondary pollutants such as ozone and nitrogen oxides. These pollutants were found under concentration of 0.14 mg m-3 which is below established safety thresholds, adhering to World Health Organization guidelines. This research demonstrates that plasma jet treatment in bubble condition not only enhances the degradation efficiency of pollutants in wastewater but also minimizes the formation of harmful byproducts. This represents a significant breakthrough in developing sustainable wastewater treatment technologies.

Different Radial Modification Profiles Observed on APPJ-Treated Polypropylene Surfaces according to the Distance between Plasma Outlet and Target

The plasma jet transfer technique relies on a conductive wire at floating potential, which, upon entering in contact with a primary discharge, is capable of igniting a small plasma plume at the distal end of a long flexible plastic tube. In this work, two different long tube configurations were employed for the surface modification of polypropylene (PP) samples using argon as the working gas. One of the jet configurations has a thin copper (Cu) wire, which was installed inside the long tube. In the other configuration, the floating electrode is a metallic mesh placed between two plastic tubes in a coaxial arrangement. In the first case, the tip of the Cu wire is in direct contact with the working gas at the plasma outlet, whereas, in the second, the inner plastic tube provides an additional dielectric barrier that prevents the conductor from being in contact with the gas. Water contact angle (WCA) measurements on treated PP samples revealed that different surface modification radial profiles are formed when the distance (d) between the plasma outlet and target is changed. Moreover, it was found that the highest WCA reduction does not always occur at the point where the plasma impinges the surface of the material, especially when the d value is small. Through X-ray photoelectron spectroscopy (XPS) analysis, it was confirmed that the WCA values are directly linked to the oxygen-functional groups formed on the PP surfaces after the plasma treatment. An analysis of the WCA measurements along the surface, as well as their temporal evolution, together with the XPS data, suggest that, when the treatment is performed at small d values, the plasma jet removes some functional groups at the point where the plasma hits the surface, thus leading to peculiar WCA profiles.

Low Temperature Plasma Strategies for Xylella fastidiosa Inactivation

The quarantine bacterium Xylella fastidiosa was first detected in Salento (Apulia, Italy) in 2013 and caused severe symptoms in olives, leading to plant death. The disease, named Olive Quick Decline Syndrome (OQDS), is caused by the strain “De Donno” ST53 of the subspecies pauca of this bacterium (XfDD), which is spread by the insect Philaenus spumarius. The epidemic poses a serious threat to the agricultural economy and the landscape, as X. fastidiosa infects several plant species and there is yet no recognized solution. Research on OQDS is focused on finding strategies to control its spread or mitigate its symptoms. As a perspective solution, we investigated the efficacy of the low-temperature plasma and plasma-activated water to kill bacterial cells. Experiments were conducted in vitro to test the biocidal effect of the direct application of a Surface Dielectric Barrier Discharge (SDBD) plasma on bacteria cells and Plasma Activated Water (PAW). PAW activity was tested as a possible biocidal agent that can move freely in the xylem network paving the way to test the strategy on infected plants. The results showed a high decontamination rate even for cells of XfDD embedded in biofilms grown on solid media and complete inactivation in liquid culture medium.

Microwave plasma-based high temperature dehydrogenation of hydrocarbons and alcohols as a single route to highly efficient gas phase synthesis of freestanding graphene

Understanding underlying processes behind the simple and easily scalable graphene synthesis methods enables their large-scale deployment in the emerging energy storage and printable device applications. Microwave plasma decomposition of organic precursors forms a high-temperature environment, above 3000 K, where the process of catalyst-free dehydrogenation and consequent formation of C₂molecules leads to nucleation and growth of high-quality few-layer graphene (FLG). In this work, we show experimental evidence that a high-temperature environment with a gas mixture of H₂and acetylene, C₂H₂, leads to a transition from amorphous to highly crystalline material proving the suggested dehydrogenation mechanism. The overall conversion efficiency of carbon to FLG reached up to 47%, three times as much as for methane or ethanol, and increased with increasing microwave power (i.e. with the size of the high-temperature zone) and hydrocarbon flow rate. The yield decreased with decreasing C:H ratio while the best quality FLG (low D/G-0.5 and high 2D/G-1.5 Raman band ratio) was achieved for C:H ratio of 1:3. The structures contained less than 1 at% of oxygen. No additional hydrogen was necessary for the synthesis of FLG from higher alcohols having the same stoichiometry, 1-propanol and isopropanol, but the yield was lower, 15%, and dependent on the atom arrangement of the precursor. The prepared FLG nanopowder was analyzed by scanning electron microscopy, Raman, x-ray photoelectron spectroscopy, and thermogravimetry. Microwave plasma was monitored by optical emission spectroscopy.

Effects of O2 Addition on the Discharge Parameters and Production of Reactive Species of a Transferred Atmospheric Pressure Plasma Jet

The therapeutic effects of atmospheric pressure plasma jets (APPJs) have been associated with the presence of reactive species, mainly the reactive oxygen and nitrogen ones, generated in this kind of plasmas. Due to that, many studies attempting to enhance the production of reactive species in APPJs have been performed. The employment of gas admixtures, usually mixing a noble gas with oxygen (O2) or water vapor, is one of the most common methods to achieve such goal. This work presents a study of how the addition of small amounts of O2 affects the electrical parameters and the production of reactive species in a transferred APPJ produced at the tip of a long and flexible plastic tube. The study was carried out employing helium (He) as the working gas and applying a high voltage (HV) in the form of amplitude-modulated sine waveform (burst mode). With this configuration it was possible to verify that the O2 addition reduces the discharge power and effective current, as a result of late ignition and shorter discharge duration. It was also found that the addition of O2 to a certain content in the gas admixture makes the light emission from oxygen atoms increase, indicating an increment in oxygen related reactive species in the plasma jet. However, at the same time the light emitted from hydroxyl (OH) and nitric oxide (NO) exhibits the opposite behavior, i.e., decrease, indicating a reduction of such species in the APPJ. For these reasons, the addition of O2 to the working gas seems to be useful for increasing the effectiveness of the plasma treatment only when the target modification effect is directly dependent on the content of atomic oxygen.

Statistical physics of two-temperature rotational energy distributions in stationary plasmas

Two-temperature rotational energy distributions from rarefied diatomic molecules are very often observed in laboratory plasmas. There has been much debate over the years about the physical meaning of this kind of rotational energy distributions and the associated statistical physics. We show here that under certain reasonable assumptions and constraints the condition of Shannon-Jaynes entropy maximization may produce a two-temperature distribution. This may happen, for instance, when a system is simultaneously coupled to different thermal baths. In plasmas this is possible because rarefied molecular species may be immersed in a medium where electrons and the dominant atomic species are quasidecoupled, each of them acting as distinct thermal baths. Considering that molecular species may interact both with electrons and heavy neutral species, we may ask what should be the resulting molecular energy distribution. We answer this question in this paper and give some examples on how this can be used to interpret experimental molecular distribution from partially ionized plasmas.

Potential of N2/O2 atmospheric pressure needle-water DC microplasmas for nitrogen fixation: nitrite-free synthesis of nitrates

A needle-water DC microplasma system working at atmospheric pressure in N₂/O₂ gas mixtures is used to study the fundamental mechanisms of nitrate/nitrite synthesis in highly complex and yet little-known plasma-water systems. Plasma properties are investigated by means of optical emission spectroscopy while the activated water is analyzed following the treatment using ionic chromatography and UV-Vis absorbance spectroscopy. Experiments highlight that the energy efficiency and selectivity of the process are influenced by the oxygen content and the plasma-induced water heating, with strong differences when the water surface is the anode or the cathode electrode. Nitrates are successfully synthesized without residual nitrites in the solution with a comparatively higher energy efficiency when the water is the cathode. The possible reactions involved in the gas phase and aqueous phase chemistry are presented and future scope for the optimization of the system is discussed.

Mass Production of Plasma Activated Water: Case Studies of Its Biocidal Effect on Algae and Cyanobacteria

Efficient treatment of contaminated water in industrially viable volumes is still a challenging task. The hydrodynamic cavitation plasma jet (HCPJ) is a promising plasma source for industrial-scale generation of biologically active environments at high flow rates of several m3/h. The combined effect of a hydro-mechanical phenomenon consisting of hydrodynamic cavitation and electrical discharge in cavitation voids was found to be highly efficient for large-volume generation of reactive oxygen species, ultraviolet (UV) radiation, and electro-mechanical stress in a liquid environment. Here, the persistence of biocidal properties of HCPJ-activated water (i.e., plasma-activated water (PAW)) was tested by the study of algae and cyanobacteria inactivation. Algae and cyanobacteria cultivated in media containing PAW (1:1) were completely inactivated after 72 h from first exposure. The test was performed at a total power input of up to 0.5 kWh/m3 at the treated liquid flow rate of 1 m3/h. A beneficial modification of our previous HCPJ design is described and thoroughly characterized with respect to the changes of hydrodynamic flow conditions as well as discharge performance and its optical characteristics. The modification proved to provide high biocidal activity of the resulting PAW, which confirms a strong potential for further design optimization of this promising water (liquid) plasma source.

Deducing rotational quantum-state distributions from overlapping molecular spectra

A novel method for fast and robust calculation of Boltzmann plots from molecular spectra is presented. Its use is demonstrated on the OH(A-X) spectrum near 310 nm. A limitation of the method is identified: for overlapping spectra of the OH(A-X) and N₂(C-B, Δv = 1) band sequence, the calculation may often fail due to insufficient number of measured points. This is solved by introducing experimentally determined bounds for the N₂(C) rotational distribution. Three cases are presented: (i) with undisturbed OH(A-X) emission, (ii) with strong emission of N₂(C-B) in the said spectral range, and (iii) with weak but not negligible nitrogen emission. In case (ii), the data in the spectral range 306-320 nm are sufficient for the analysis. In case (iii), information from another spectral range with undisturbed N₂(C-B) emission is necessary. These illustrate all relevant cases often encountered in laboratory plasmas. The calculated Boltzmann plots are not further analyzed in this article but can be used for development and validation of kinetic models with rotational resolution. The implementation of the reported method using the massiveOES software package written in the Python language is available in the supplementary material.