# Characterization of a Plasma Source for Water Treatment Using Electrical, Optical, and Chemical Diagnostics

Maria Saba, Cristina Muja, Philippe Guillot, Thomas Maho

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Maria Saba, Cristina Muja, Philippe Guillot, Thomas Maho. Characterization of a Plasma Source for Water Treatment Using Electrical, Optical, and Chemical Diagnostics. 2025. hal-05375995

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# Characterization of a Plasma Source for Water Treatment Using Electrical, Optical, and Chemical Diagnostics

Maria Saba, Cristina Muja, Philippe Guillot and Thomas Maho

November 21, 2025

## Abstract

Hospital and urban wastewaters constitute major reservoirs of multidrug-resistant bacteria (MRB), antibiotic resistance genes (ARGs), and persistent pharmaceutical contaminants, which are only partially removed by conventional treatment plants. As a result, these pollutants are frequently detected in treated effluents destined for environmental discharge or reuse, raising serious public health concerns—especially for agricultural irrigation. This study explores a novel immersed dielectric barrier discharge (DBD) plasma source engineered for point-of-use decontamination of wastewater at emission sites. The plasma device was thoroughly characterized by electrical, optical, and chemical diagnostics to elucidate its operating regimes and capacity to generate reactive oxygen and nitrogen species (RONS) at the plasma-liquid interface. Findings demonstrate that the composition of the carrier gas critically shapes the production and transfer of short- and long-lived oxidants into the liquid phase. These results provide quantitative insights that support the ongoing optimization of plasma-based advanced oxidation systems for decentralized water treatment. They further illustrate how the careful control of plasma and gas composition can enhance the formation and transfer of oxidants, contributing to the broader field of plasma-liquid applications for the mitigation of emerging contaminants.

## 1 Introduction

Hospital, laboratory, and urban wastewater represent major vectors for both microbiological and chemical contamination. These effluents contain a wide range of microorganisms, including pathogens, together with pharmaceutical residues and antibiotics. Such compounds exert a selective pressure on microbial populations, promoting the emergence and dissemination of multidrug-resistant bacteria (MRB) and antibiotic resistance genes (ARGs). Their increasing presence in the environment now constitutes a significant public health concern [21].

Conventional wastewater treatment plants (WWTPs) efficiently reduce organic matter and nutrient loads but are not specifically designed to eliminate MRB or degrade persistent pharmaceutical compounds [35]. Several studies have shown that the biological and chemical conditions prevailing in these facilities—high microbial density, biofilm development, and sub-inhibitory antibiotic concentrations—can, conversely, facilitate the horizontal transfer of resistance genes [22]. Consequently, treated effluents released into receiving environments often still contain resistant microorganisms and micropollutants that can contaminate soils and surface waters. This issue is further aggravated by the increasing adoption of the REUSE concept, which promotes the reclamation of treated wastewater for agricultural irrigation or industrial processes in response to water scarcity and climate change [38]. While this practice offers ecological and economic advantages, numerous studies have reported the persistence of pathogenic microorganisms and MRB in reclaimed water, raising concerns regarding soil, plant, and food-chain contamination [24] [5]. Managing this challenge requires the development of decentralized or complementary treatments capable of reducing both microbial and chemical loads upstream of existing sanitation systems [31].

In this context, cold atmospheric plasma (CAP) technology has attracted attention as a promising advanced oxidation and antimicrobial process [8]. CAP generates *in situ* a rich mixture of reactive oxygen and nitrogen species (RONS)—including O, OH, NO, NO₂, O₃, and H₂O₂—known for their strong oxidative and biocidal properties [19]. Through simultaneous physical effects (electron impacts, UV radiation, electric
fields) and chemical reactions (radical formation, oxidation cascades), plasma-liquid systems can achieve synergistic inactivation of microorganisms and degradation of organic contaminants [12] [37].

Among the various plasma configurations investigated, Dielectric Barrier Discharges (DBDs) are among the most prominent for producing non-thermal plasma at atmospheric pressure owing to their stability, controllability, and suitability for liquid-phase applications [15]. In water treatment, different plasma-liquid coupling modes have been developed according to the location of the discharge relative to the liquid [45] [4]. In the indirect mode, the plasma is generated *above the liquid surface*, and reactive species formed in the gas phase diffuse and dissolve into the aqueous phase. This approach allows better control of gas composition and discharge uniformity but is limited by the diffusion of RONS across the gas-liquid interface [7]. In contrast, the direct or immersed mode involves plasma formation *within the liquid*, either through microdischarges or within gas bubbles injected into the medium [17]. This setup enhances plasma-liquid coupling, leading to higher production and transfer rates of short- and long-lived reactive species, improving both oxidation and microbial inactivation efficiency. Hybrid designs combining both modes have also been reported, maximizing interfacial area and plasma exposure to better address complex aqueous matrices [12].

Building on this context, the present study was designed to develop and thoroughly characterize an immersed bubble dielectric barrier discharge (DBD) plasma source, with a specific focus on its optimization for local wastewater treatment, particularly near emission points such as hospital sinks or laboratory drains. The research aimed first to achieve a rigorous electrical characterization of the plasma source by quantifying discharge power, energy transfer efficiency, and their dependence on key operating parameters. Next, the study sought to elucidate the spatiotemporal dynamics of the discharge using fast optical diagnostics, providing insight into the ignition, propagation, and extinction processes occurring within the reactor and at the plasma-liquid interface. A further objective was to identify and quantify the reactive species generated in both the plasma phase and the liquid, employing optical emission spectroscopy and chemical assays to reveal how gas composition modulates interfacial plasma chemistry. Finally, the work integrated electrical, imaging, and chemical diagnostic data to establish the principal mechanisms by which reactive oxygen and nitrogen species (RONS) are formed and transferred into the aqueous phase. This multidisciplinary approach advances both the fundamental understanding and the practical control of plasma-liquid interactions, supporting the development of robust and scalable plasma-based technologies for on-site wastewater decontamination.

# 2 Materials and Methods

## 2.1 Experimentat set-up

The plasma reactor used in this study belongs to the class of bubble discharge dielectric barrier discharge (DBD) reactors, specifically designed to produce plasma directly within the liquid phase through gas injection. The system consists of two main components, both made of 1.6 mm-thick borosilicate glass: the liquid container and the reactor head.

The container, which holds the liquid to be treated, is cylindrical, with a diameter of 3.3 cm and a height of 7.4 cm. In this study, 47 mL of ultra-pure water was used as the working liquid. The head consists of a 15 cm-long tube with an inner diameter of 4 mm, embedded in an arched structure containing two parallel gas outlet ports.

One extremity of the tube is immersed into the liquid, while the arched structure acts as the reactor lid and mechanical support. The feed gas is introduced through an inlet located at the upper part of the arched section, flows through the internal tube, and exits from its open, submerged end. This configuration produces a continuous stream of gas bubbles in the liquid, enabling the *in situ* formation of microdischarges at the gas-liquid interface.

Four gas compositions were employed to investigate the effect of gas chemistry on discharge behavior and reactive species formation: He, He-0.2% O₂, He-0.2% N₂, and He-0.2% air. The gas flow rate was maintained at 2 L.min⁻¹ using a mass-flow controller (EL-FLOW Select, model F-201CV, Bronkhorst B.V.).

The high-voltage electrode is a metallic rod, 1 mm thick, inserted into the tube until its tip reaches the end. The grounded electrode is wrapped spirally around the container. The figure 1A shows the schematic diagram of the plasma source.
<figure><img src="image_2.png" /><figcaption>Figure 1: (A) Schematic diagram of the plasma source. (B) Photograph of the plasma source using pure helium as the carrier gas.</figcaption></figure>

Both electrodes are connected to a pulsed power supply developed at the GREMI Laboratory (Orléans, France), which delivers fixed pulses of 4 µs duration. The applied voltage can be varied from 0 to 20 kV, with a frequency adjustable between 1 and 30 kHz. For this study, the operational frequency was set at 20 kHz.

When the high voltage is applied, microdischarges ignite inside the glass tube and within the gas cavity formed at its outlet, establishing an active plasma region that interacts directly with the surrounding liquid medium. This configuration combines the advantages of dielectric confinement with direct plasma-liquid contact, enabling efficient generation and transfer of reactive oxygen and nitrogen species essential for water decontamination.

## 2.2 Electrical diagnostics

The electrical characterization of the plasma reactor was performed using the equivalent electrical circuit (EEC) approach, which models the plasma source as an electrical circuit that relects its electrical properties. This method, widely applied to dielectric barrier discharge (DBD) systems [25], [23], [14], [16] and other plasma configurations [6], [30], provides a simplified yet accurate framework for determining internal discharge parameters that cannot be directly measured.

In the EEC model, the dielectric barriers and the gas gap are represented by capacitors, while the plasma channel is described as a resistor connected in parallel with the gas-gap capacitance. The resistor accounts for the conductive phase of the discharge, during which microdischarges locally bridge the electrodes. By solving the circuit equations using experimental voltage and current waveforms, it is possible to extract key internal quantities such as the discharge current and the voltage drop across the gas gap. These values were subsequently used to calculate the power dissipated in the plasma and to analyze the energy coupling efficiency of the reactor.

The applied voltage (V) was recorded using a high-voltage probe (Tektronix P6015A), and the total current (I) was measured with a Pearson Current Monitor (Model 6585). Both signals were simultaneously captured with a digital oscilloscope (Rohde & Schwarz RTE1204). (see figure 2).

To determine the capacitances of the reactor components, a Q-V (Lissajous) method was employed [26]. The plasma reactor was connected in series with a known measurement capacitor (Jennings UCSL-1000-3S, set to $C_m$ = 100 pF) (see Figure 3A). A sinusoidal voltage was applied to the system using a high-voltage AC supply (Spitzenberger & Spies EV-1200/FS) operating at 20 kHz (see Figure 3B). The applied voltage and
<figure><img src="image_3.png" /><figcaption>Figure 2: Experimental setup for electrical measurements</figcaption></figure>

the voltage across the measurement capacitor were monitored using two identical Tektronix P6015A probes and recorded with the same oscilloscope. The charge transferred through the circuit, $Q_m = C_m \times V_m$, was plotted as a function of the applied voltage $V(t)$, generating a Q–V plot characteristic of the reactor. The slope of each linear segment provided the equivalent capacitances associated with the dielectric and gas regions.

This analysis allowed the determination of the intrinsic electrical parameters of the plasma source and provided a quantitative basis for correlating discharge behavior with operating conditions, a methodology commonly used for the characterization of microdischarge-driven DBD systems.

## 2.3 Optical diagnostics

### 2.3.1 Camera imaging

The spatio-temporal evolution of the discharge inside the reactor during a single high-voltage pulse was investigated using an intensified charge-coupled device (ICCD) camera (Princeton Instruments PI-MAX-1). The emitted light was collected through a UV-grade objective (Nikon Nikkor 80–400 mm) equipped with an extension ring to ensure optimal focusing on the discharge region. The ICCD camera was synchronized with the applied voltage pulse through an external trigger to capture time-resolved images. Each image was acquired with a gate width of 10 ns, corresponding to a temporal resolution sufficient to resolve individual stages of the microdischarge development. To improve the signal-to-noise ratio, each frame represents the cumulative average of 1000 gated exposures.

### 2.3.2 Optical Emission Spectroscopy

Plasma spectral characterization was performed by optical emission spectroscopy (OES) to identify reactive species generated under different gas compositions (He, He-0.2% O₂, He-0.2% N₂, and He-0.2% air). Emission spectra were recorded using a spectrometer (Princeton Instruments SpectraPro HRS-750) coupled to an ICCD detector (Princeton Instruments PI-MAX-4). Plasma emission was transmitted to the spectrometer via a 600 µm-core, 2 m-long optical fiber (Ocean Optics). Spectra were acquired over a wavelength range of 200–900 nm with an integration time of 1 ms. Wavelength calibration was performed using the IntelliCal system provided with the spectrometer, and relative intensity calibration was performed using a deuterium halogen reference lamp (Ocean Optics DH2000).
<figure><img src="image_4.png" /><figcaption>Figure 3: (A) Experimental setup for plotting the Lissajous curve. V(t) is the applied voltage, I(t) is the total current, C<sub>m</sub> is the capacitance of the measurement capacitor, and V<sub>m</sub>(t) is the voltage across the measurement capacitor. (B) Sinusoidal applied voltage waveform used for the Lissajous plot, with an RMS value of 1200 V and a frequency of 20 kHz.</figcaption></figure>

Two distinct configurations were employed for each gas mixture (Figure 4). In the first configuration, the optical fiber was placed at position 1 to collect light from the plasma formed inside the tube. In the second configuration (Position 2), the fiber was positioned below the reactor, facing the gas cavity at the tube outlet, to capture the emission from the plasma-liquid interface. During this second measurement, the tube was covered with black tape to suppress any optical contribution from the plasma inside the tube, ensuring that only light from the interfacial region was recorded.

Each emission spectrum was acquired in triplicate to ensure reproducibility, and the final spectra reported in this work correspond to the averaged results. This combined imaging and spectroscopic analysis provided complementary insights into the discharge propagation dynamics and the formation pathways of reactive species within the plasma and at the plasma-liquid interface, critical for understanding the mechanisms of plasma-assisted water treatment.

## 2.4 Chemical diagnostics

Preliminary chemical diagnostics were performed using the potassium iodide-starch (KI-starch) indicator method, a qualitative and semi-quantitative technique commonly applied to visualize reactive oxygen species (ROS) generated in plasma-treated water [33], [20]. This method enables the indirect detection of strong oxidants such as ozone ($O_3$), hydroxyl radical (OH) and hydrogen peroxide ($H_2O_2$), based on their ability to oxidize iodide ions.

In this system, iodide ions ($I^-$) are oxidized by reactive species with an oxidation potential higher than that of molecular iodine ($E°= 0.54V$), leading to the formation of iodine ($I_2$). The produced $I_2$ then reacts with excess iodide to form triiodide ($I_3^-$), which further forms a stable blue-black complex with starch [3]. The intensity of this complex, characterized by a strong absorption peak near 600 nm, reflects the abundance of oxidizing species in the treated solution.

A 500 mL KI-starch stock solution was prepared by dissolving 5 g of soluble starch and 3 g of potassium iodide (KI) in ultrapure water under gentle heating to ensure complete dissolution. Following plasma treatment, equal volumes of the treated water and KI-starch solution were mixed at a 1:1 ratio and allowed to react for 40 minutes at ambient temperature to ensure reaction completeness. The mixture was then transferred into well plates for spectrophotometric analysis, and the absorbance was measured at 600 nm using a multimode microplate reader (BioTek Synergy H1). The measured absorbance serves as an indicator of the total concentration of RONS accumulated in the plasma-treated water.

This diagnostic was conducted for the four carrier gas compositions—He, He-0.2% $O_2$, He-0.2% $N_2$, and
He-0.2% air—to assess how gas composition influences the oxidative strength of the plasma. Each measurement was performed in triplicate, and the results were averaged to ensure reproducibility and minimize systematic variability.

<figure><img src="image_5.png" /><figcaption>Figure 4: Schematic showing the position of the optical fiber for the two OES measurements.</figcaption></figure>

# 3 Results and Discussion

## 3.1 Power dissipated in the plasma discharge

Although the geometric configuration of the plasma reactor is relatively complex, a cross-sectional view taken through the center of the container (see Figure 5A) shows that the system can be approximated as a coaxial dielectric barrier discharge (DBD). In this representation, the glass tube, the water layer, and the outer glass container jointly constitute the dielectric barriers surrounding the plasma region.

<figure><img src="image_6.png" /><figcaption>Figure 5: (A) Cross section of the plasma reactor. (B) Equivalent electrical circuit of the plasma source.</figcaption></figure>

Ultrapure water, characterized by its very low electrical conductivity (0.055 µS.cm⁻¹ at 25 °C) and high relative permittivity ($ε_r$ ≈ 80 at room temperature), behaves as an effective dielectric under these conditions.
However, when exposed to plasma, chemical reactions at the plasma-liquid interface generate reactive oxygen and nitrogen species (RONS), which progressively increase the ionic conductivity of the liquid. Accordingly, all electrical measurements presented in this section were conducted at the initial stage of plasma operation, when only minimal concentrations of reactive species had accumulated. Under these conditions, the water can still be reasonably treated as a dielectric layer in the analytical model [41].

The equivalent electrical circuit (EEC) used to describe the reactor behavior is presented in figure 5B. The model follows those typically applied to conventional coaxial DBD systems. The capacitive elements $C_g$, $C_{d1}$, $C_w$ and $C_{d2}$ represent the equivalent capacitances of the gas gap (inside the tube), the inner dielectric barrier (tube wall), the water layer, and the external container wall, respectively.

The applied voltage is denoted by $V(t)$, and the total current through the circuit by $I(t)$. The discharge current $I_{dis}(t)$ represents conduction through the microdischarges, while $I_g(t)$ corresponds to the displacement current across the gas gap. The voltages across the gas region and dielectric barriers are represented by $V_g(t)$ and $V_D(t)$, respectively.

The total capacitance of the dielectric barrier $C_D$ is given by the following equation:

$$C_D = \left[ \frac{1}{C_{d1}} + \frac{1}{C_w} + \frac{1}{C_{d2}} \right]^{-1} \qquad (1)$$

The total capacitance of the reactor $C_R$ is expressed as:

$$C_R = \left[ \frac{1}{C_D} + \frac{1}{C_g} \right]^{-1} \qquad (2)$$

By applying Kirchhoff's law to the equivalent electrical circuit, the instantaneous voltage across the gas gap and the discharge current can be expressed as functions of the applied voltage $V(t)$ and the total measured current $I(t)$:

$$V_g(t) = V(t) - \frac{1}{C_D} \int_0^T I(t) dt \qquad (3)$$

$$I_{dis}(t) = I(t) - C_g \frac{dV_g(t)}{dt} \qquad (4)$$

The total power delivered by the power supply, $P_{tot}$, and the power dissipated in the discharge, $P_{dis}$, are given by the following equations:

$$P_{tot}(t) = I(t) \times V(t) \qquad (5)$$

$$P_{dis}(t) = I_{dis}(t) \times V_g(t) \qquad (6)$$

The instantaneous total energy delivered by the power supply, $E_{tot}$, and the energy dissipated in the discharge, $E_{dis}$, are given by the following equations:

$$E_{tot}(\tau) = \int_{0}^{\tau} P_{tot}(t) dt \qquad (7)$$

$$E_{dis}(\tau) = \int_{0}^{\tau} P_{dis}(t) dt \qquad (8)$$

To solve equations (3) and (4), the capacitances $C_D$ and $C_g$ were first determined experimentally. A typical Q-V (Lissajous) plot for a sinusoidally driven DBD exhibits a parallelogram shape 6A. The slope of the ascending (blue) branch corresponds to the total capacitance of the reactor, whereas the slope of the descending (red) branch represents the capacitance of the dielectric barrier. The Q-V plot obtained for our reactor under sinusoidal excitation, shown in Figure 6B, displays a similar quasi-parallelogram geometry, confirming that the plasma reactor can be suitably approximated as a coaxial DBD.

Since the experimental figure was slightly distorted, the parallelogram was graphically reconstructed, and the slopes of the completed sides were taken to compute the capacitances. From this analysis, the equivalent
<figure><img src="image_7.png" /><figcaption>Figure 6: (A) Typical Q-V plot for a sinusoidally operated DBD. (B) Q-V plot of our plasma reactor operating sinusoidally</figcaption></figure>

capacitances were determined as $C_R = (4.08 \pm 0.19)$ pF, $C_D = (21.43 \pm 0.40)$ pF, and $C_g = (4.71 \pm 0.27)$ pF. The value of $C_g$ was obtained by rearranging Equation (2).

For comparison, the theoretical capacitances were calculated using the expression for a coaxial dielectric layer:

$$C = \frac{2\pi\epsilon_0\epsilon L}{ln(r_1/r_2)} \quad (9)$$

where $\epsilon_0$ is the vacuum permittivity, $\epsilon_r$ the relative permittivity of the dielectric material, L the axial length of the dielectric layer, r₁ and r₂ the inner and outer radius, respectively.

Applying equation (9) to the reactor geometry yields the following theoretical values: $C_g=6.5$ pF, $C_{d1}=34.7$ pF, $C_w=140$ pF and $C_{d2}=239.2$ pF. According to equations (1) and (2), the derived composite capacitances are $C_D=25.63$ pF and $C_R=5.18$ pF. The experimental and theoretical results agree within approximately 15-30%, consistent with expected deviations arising from the simplified coaxial approximation of the reactor geometry.

Figure 7A presents the measured waveforms of the applied voltage V(t) and total current I(t). The carrier gas used was helium, and the reactor contained 47 mL of ultrapure water. Although the power supply was set to 15 kV at 20 kHz, the actual voltage amplitude applied to the reactor was 6.7 kV. The voltage waveform consists of a symmetrical main pulse, with both the rising and falling edges lasting 2 µs. Following the main pulse, the voltage slightly changes polarity of about 1.2 kV and then exhibits damped oscillations for approximately 20 µs before returning to zero.

The current waveform shows an initial discharge of approximately 0.1 A during the rising edge of the voltage pulse, followed by a second, negative discharge of about -0.07 A during the falling edge. Slightly before returning to zero, the current undergoes a rapid oscillation and then returns slowly to zero. This coincides with the polarity reversal of the voltage.

$I_{dis}(t)$ and $V_g(t)$ were derived by solving equations (3) and (4) using the experimental data obtained during V(t) and I(t) measurements. Figure 7B shows the resulting waveforms. The discharge current exhibits a waveform similar to the total current but with a slightly lower amplitude of approximately 0.096 A, due to the removal of the displacement current from the total current. $V_g(t)$ starts to rise with the total voltage, but at 1.6 kV it starts to drop. In fact, the first discharge charges the dielectric capacitance $C_D$ through accumulating charges on the surface of the dielectric barrier. This creates an electric field that opposes the externally applied electric field, which causes the voltage drop across the gas gap [ref]. Then $V_g(t)$ rises again with opposite polarity, which causes the second discharge.

Figure 7C presents both the instantaneous total power delivered by the power supply and the power dissipated in the plasma, calculated using equations (5) and (6). These curves reveal two distinct discharge
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

Figure 7: Electrical diagnostics of the plasma source operated with pure helium as the carrier gas (2 L·min⁻¹) and 47 mL of ultrapure water inside the container. The voltage indicated on the power supply was 15 kV at 20 kHz. (A) In blue, the measured waveform of the applied voltage; in red, the waveform of the total current. (B) In green, the voltage across the gas gap; in magenta, the discharge current, both obtained from the equivalent electrical circuit model. (C) In black, the instantaneous total power delivered by the power supply; in blue, the instantaneous power dissipated in the plasma. (D) In black, the instantaneous total energy delivered by the power supply; in blue, the instantaneous energy dissipated in the plasma. The total energy dissipated in the plasma during one pulse is 448.1 µJ

events. During the first event, the power delivered by the supply ($P_{tot}(t)$) is positive, indicating active energy injection into the system, while the plasma dissipates a portion of this power ($P_{dis}(t)$). In the second discharge, the ($P_{tot}(t)$) curve becomes negative, reflecting a return of energy from the system to the power supply as the residual energy stored in the dielectric is released and used to sustain the discharge without further input.

Figure 7D shows the time evolution of the total energy delivered by the power supply and the energy dissipated in the plasma, calculated using equations (7) and (10). The first rise of the ($E_{dis}(t)$) curve gives the energy dissipated in the first discharge, which is equal to 144 µJ, while the second rise corresponds to the energy dissipated in the subsequent discharge, 83.8 µJ - found by subtracting the first event from the total energy dissipated during the pulse. The ($E_{tot}(t)$) curve, representing the cumulative energy delivered by the supply, exhibits a single rise during the first discharge, measuring 448.1 µJ. Part of this energy powers the first discharge, while another part is temporally stored in the dielectric barrier through charge accumulation ([28]). The stored charge is later released and powers the second discharge without continued supply input [27].
At the end of the excitation, ($E_{dis}(t)$) and ($E_{tot}(t)$) converge around 227.8 µJ. This outcome reflects the boundary of the model, which incorporate only plasma dissipation and neglects losses due to the dielectric or electromagnetic radiation. Thus, the final value depicts the net energy transferred to the plasma after both discharge events are completed.

### 3.1.1 Spatiotemporal plasma evolution

Figure 8 presents time-resolved ICCD images acquired at successive delay times during a single voltage pulse. The experiments were performed with pure helium as the carrier gas at a flow rate of 2 L·min⁻¹, using 47 mL of ultrapure water as the working liquid. The applied voltage was set to 15 kV with a pulse frequency of 20 kHz. These images provide insight into the spatiotemporal evolution of the plasma inside the reactor during a voltage pulse. The positions of the tube and container walls, not directly visible in the raw images, are outlined with dashed white lines for reference.

<figure><img src="image_12.png" /></figure>

Figure 8: Spatiotemporal evolution of the plasma inside the reactor. The carrier gas is pure helium, flowing at 2 L·min⁻¹. The voltage indicated on the power supply is 15 kV at 20 kHz. The container holds 47 mL of ultrapure water. The tube and container boundaries are marked by white dashed lines. The time shown in the top left of each image corresponds to the time elapsed since the beginning of the pulse.

At the very beginning of the pulse (0 µs), no emission is detected. The first visible plasma emission appears at 0.52 µs, localized at the tip of the high-voltage electrode. This observation suggests that field enhancement at the electrode tip leads to initial breakdown, which is typical for DBD reactors with a pin-to-liquid or pin-to-dielectric geometry [1]. Shortly after (0.58 - 0.64 µs), a second ionization front emerges from the upper region of the tube and propagates rapidly downward along the glass wall. By 0.74 µs, the plasma column extends nearly the full length of the tube, indicating fast streamer propagation supported by the applied voltage and the helium atmosphere. This phase is characterized by intense emission, with the maximum brightness reached in the central region. From 0.94 to 1.44 µs, the plasma fills almost the entire tube tube. The nearly uniform emission profile suggests that the discharge has transitioned from initial streamer channels to a more diffuse glow discharge [46]. The propagation halts just before the tube's outlet, likely due to increased local humidity and vapor-phase water content, which raise the breakdown threshold and locally suppress plasmas. Between 1.74 and 2.64 µs, the brightness progressively decreases, indicating
plasma decay as the applied voltage drops and less energy is supplied to the discharge. A secondary, fainter emission inside the tube appears at 2.64 µs, possibly representing residual or memory discharges initiated by charge accumulation on the dielectric surfaces discussed in the previous section. Complete plasma extinction is observed after approximately 4.04 µs consistent with the trailing edge of the voltage pulse and dissipation of residual surface or space charges.

Simultaneously, starting at around 0.74 µs, a luminous region appears at the tube outlet and within the gas cavity formed in the water. This region reaches peak intensity at 1.44 µs, in phase with glow discharge development inside the tube, then declines and vanishes by 2 µs. This secondary discharge is attributed to field focusing at the interface and the enhanced conductivity of the vapor cavity, which facilitates charge transfer from the plasma in the tube to the water or bubble interface [34]. In some images, faint light is observed outside the tube, within the water container volume. These emission do not originate from discharges occurring inside the gas bubbles but are due to optical reflections and scattering of the intense plasma emission produced inside the tube. The presence and intensity of this scattered light are influenced by the distribution and dynamics of the bubbles in the liquid phase.

### 3.1.2 Reactive species generated in the gas phase

Figure 9A shows the optical emission spectrum (OES) measured inside the discharge tube (position 1), while Figure 9B presents the OES at the gas-liquid interface (position 2). In both cases, the carrier gas is pure helium (2 L·min⁻¹), the applied voltage is 15 kV at 20 kHz, and the liquid phase consists of 47 mL ultrapure water.

<figure><img src="image_13.png" /></figure>

Figure 9: The carrier gas is pure helium flowing at 2 L·min⁻¹. The voltage indicated on the power supply is 15 kV at 20 kHz. The container holds 47 mL of ultrapure water. (A) Calibrated spectrum taken at position 1, corresponding to the plasma inside the tube. (B) Calibrated spectrum taken at position 2, corresponding to the plasma-water interface.

For both measurement positions, the emitted photons must traverse the same optical path—through water and glass—before being collected by the optical fiber. The overall transmission below 280 nm is thus similarly affected by absorption for both positions. Yet, a key distinction emerges: at position 2 (plasma-liquid interface), a clear NO(A-X)(0-0) emission band is observed at 247 nm, whereas it is absent at position 1 (inside the tube). This result demonstrates that the appearance of NO emission at the interface is not a consequence of differential absorption, but reflects a true chemical difference: the formation of NO occurs much more efficiently at the plasma-liquid boundary than within the dry plasma core [13] [10]. The absence of this signature in the tube supposes that the generation pathway for reactive nitrogen species is closely tied to plasma-liquid interaction processes—likely involving direct reactions between plasma-generated N/O fragments and water molecules or dissolved gases at the interface [43].

At position 1, the emission spectrum is dominated by He lines, and also displays pronounced signals from OH(A-X) (282-308 nm) and Hα (656 nm), both originating from the dissociation of trace water molecules present in the plasma channel [44]. At position 2, the emission from OH and H grows markedly, reflecting
intensified production of these radicals at the interface—where direct plasma contact with liquid promotes enhanced dissociation and radical formation.

Similar trends were observed for all tested gas mixtures (He-0.2% O₂, He-0.2% N₂, and He-0.2% air): the plasma-liquid region is systematically characterized by enriched NO, OH, and H emissions, underscoring the efficient chemistry enabled by plasma-liquid interaction (not shown).

To investigate the effect of the carrier gas composition on plasma-generated at the plasma-liquid interface, Figure 10 presents a comparative histogram of the integrated intensities of major emission lines and bands detected at position 2. The selected features - NO(A-X)(0-0) (247 nm), OH(A-X)(0-0) (308 nm), N₂(C-B)(0-0) (337 nm), N₂⁺(B-X)(0-0) (391nm), Hα (656 nm), He (706 nm), O (777 nm), and O (844nm) - represent the main oxidizing and energetic species involved in plasma-induced aqueous chemistry. The error bars reflect substantial intensity fluctuations, primarily attributable to the dynamic nature of bubble formation and gas flow at the tube outlet, which modulate local plasma stability and optical collection.

NO and H emissions remain relatively unchanged regardless of the carrier gas composition, suggesting that NO formation at the interface is robust across all tested conditions and that its production is governed predominantly by interfacial plasma-liquid interactions (rather than bulk N₂ content), in line with previous reports [47], [32]. The persistent Hα intensity similarly indicates consistent water dissociation independent of the small variations in gas admixture [36].

In contrast, N₂ (C-B) and N₂⁺ (B-X) bands are markedly enhanced for He-0.2% N₂ and He-0.2% air with the strongest emissions for the He-0.2% N₂ mixture. This is expected given the direct supply of molecular nitrogen, and highlights efficient electron-impact excitation and ionization of N₂ under these conditions. The observation that N₂⁺ emission is higher for pure He than for He-0.2% O₂ likely arises from residual N₂ impurities in helium and possible quenching by oxygen in the latter mixture [11].

Atomic oxygen (O) emissions at 777 nm and 844 nm are most intense for the He-0.2% O₂ mixture, as O₂ provides a direct source of oxygen atoms through electron-impact dissociation. This confirms that O₂ admixtures effectively boost the yield of reactive atomic oxygen, an important oxydant in plasma-activated solutions [29].

The OH(A-X) emission demonstrates the following trend: He-0.2% N₂ > He ≈ He-0.2% air > He-0.2% O₂. The enhancement in the presence of N₂ may reflect synergistic effects between nitrogen and water vapor at the interface, triggering more efficient production of hydroxyl radicals via excitation transfer or secondary chemistry [42], [44]. The relatively lower OH emission with O₂ may be explained by quenching or competition with O atom formation pathways.

In summary, the data highlight the specific of plasma-liquid interfacial chemistry. The addition of N₂ or air strongly promotes N₂ and N₂⁺-related emissions, and maximizes OH formation. The addition of O₂ increases atomic oxygen, but tends to reduce OH intensity, possibly via secondary reactions. NO and H emissions are comparatively insensitive to the gas admixture, emphasizing their formation at the plasma-liquid frontier. These findings underscore the critical influence of minor carrier gas modifications on the generation of short-lived oxidizing and nitrogen species, which directly affect the chemical reactivity of plasma-treated water.

### 3.1.3 Reactive species generated in the liquid phase

The chemical composition of the plasma phase, influenced by the carrier gas mixture, determine the nature and quantities of reactive oxygen and nitrogen species (RONS) produced at the plasma-liquid interface and subsequently transferred to the treated water. To quantify this effect, 47mL of ultrapure water was exposed for 15 min to plasma generated using various carrier gas compositions (He, He-0.2% N₂, He-0.2% air, He-0.2% O₂) at 2 L·min⁻¹, 15 kV and 20 kHz. The presence of reactive oxygen species (ROS) in the water were then assessed using the potassium iodide KI-starch method, with absorbance at 600 nm serving as proxy for overall oxidant formation (see Figure 11).

The measured absorbance follows the trend:

$$ \text{He} + 0.2\% \text{N}_2 > \text{He} + 0.2\% \text{air} > \text{He} > \text{He} + 0.2\% \text{O}_2 $$

The reactive oxygen species (ROS) created in water originate from the dissociation of water molecules and, when O₂ is present, from the dissociation of O₂ at the water-plasma interface. When Helium is used as the carrier gas, ROS - such as OH and H₂O₂ - primarily originate via direct electron impact dissociation of water molecule at the plasma-liquid interface, a mechanism well documented in the literature [42]:
<figure><img src="image_14.png" /><figcaption>Figure 10: Histogram comparing the intensities of the main spectral features obtained at the plasma-water interface for different carrier gases. The gas flow rate was fixed at 2 L·min⁻¹, with an applied voltage of 15 kV at 20 kHz. The container held 47 mL of ultrapure water.</figcaption></figure>

$$e^{-} + H_{2}O \rightarrow H + OH + e^{-} \qquad (10)$$

$$OH + OH \rightarrow H_{2}O_{2} \qquad (11)$$

Addind a small fraction of nitrogen (He-0.2% N₂) or air to the carrier gas increases the ROS concentration in the liquid. This enhancement is attributed to the formation of excited N₂ molecules by electron impact in the plasma, which can subsequently transfer their energy efficiently to water molecules, promoting OH and H production via secondary reactions [40], [39]. In OES measurements (Figure 10), this effect is directly evidenced by the growth of N₂(C-V) and N₂⁺ (B-X) emissions for these mixtures.

$$e^{-} + N_{2} \rightarrow N_{2}^{*} + e^{-} \qquad (12)$$

$$N_{2}^{*} + H_{2}O \rightarrow N_{2} + OH + H \qquad (13)$$

Somewhat counterintuitively, the introduction of O₂ (He-0.2% O₂) does not increase but rather decreases the final ROS content. The electronegativity of O₂, which efficiently captures plasma electrons, leads to a reduction in the overall electron density and therefore diminishes the efficiency of both direct water dissociation and the secondary energy transfer from excited species [48]. Additionally, the presence of O₂ favors quenching processes, in particular the rapid removal of excited N₂⁺ according to reaction below, which both decreases the available N₂⁺ for water activation and increases O₂ atoms in the plasma [2] [9]. This in turn explains the lower N₂ emission intensity (see Figure 10) and the relatively low ROS yield detected in the plasma-treated water for O₂-containing mixtures (see Figure 11).

$$N_{2}^{*} + O_{2} \rightarrow 2O + N_{2} \qquad (14)$$

In summary, the experimental data and their interpretation are in excellent agreement with current plasma-liquid literature: the presence of N₂ (pure or in air) in the carrier gas introduces additional energy transfer channels that synergistically boost ROS formation [47], [39], whereas O₂ tends to reduce overall ROS yields via electron scavenging and quenching of energetic intermediates [18], [21].
<figure><img src="image_15.png" /><figcaption>Figure 11: Absorbance at 600 nm of the mixture of KI-starch solution and plasma-treated water. A total of 47 mL of ultrapure water was treated for 15 min using a plasma device with a gas flow rate of 2 L·min⁻¹, an applied voltage of 15 kV, and an operating frequency of 20 kHz. The control sample was prepared by purging ultrapure water with helium flowing at 2 L·min⁻¹ for 15 min without plasma discharge.</figcaption></figure>

# 4 Conclusion

This study demonstrates the potential of an immersed dielectric barrier discharge (DBD) plasma source for the generation of reactive oxygen and nitrogen species (RONS) and the chemical modification of water at the plasma-liquid interface. Through a comprehensive suite of electrical, optical, and chemical diagnostics, we systematically characterized the discharge behavior, energy coupling, spatiotemporal propagation, and species formation under varying carrier gas compositions. Detailed optical emission spectroscopy revealed that plasma-liquid interactions uniquely favor the generation of short-lived reactive species, such as OH, O, NO, and H, with their yields strongly modulated by the gas-phase composition and the nature of the plasma-liquid interaction.

Key findings include the strong enhancement of OH and $N_2/N_2^+$ emissions in the presence of nitrogen, and the increased of atomic oxygen with $O_2$ admixture - each reflecting distinct underlaying reaction pathways. Chemical analysis using the KI-starch method confirmed that the oxidant yield in plasma-treated water is maximized with small additions of $N_2$ or air to the helium carrier. In contrast, introducing $O_2$, despite providing a direct source of atomic oxygen, leads to a decrease in overall ROS content, as a result of electron attachment and quenching of excited nitrogen species - emphasize the balance required between plasma reactivity and gas composition.

While these insights provide a useful basis for optimizing plasma water treatment in our experimental conditions, further studies are needed to comprehensively quantify the main oxidizing and nitrogenous species ($O_3$, OH, $H_2O_2$, $NO_2^-$, $NO_3^-$) in solution under realistic conditions, and to correlate their formation with the plasma's physicochemical environment and intended decontamination targets. Such future investigations will be essential to refine mechanistic understanding and guide the practical implementation of DBD plasma technologies for local and decentralized water treatment applications.
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