Deep insight into methyl blue decolorization using non-thermal pulsed plasma: experimental and modeling approaches

Abstract

This study investigates the rapid decolorization of methyl blue (MB) dye in water using a novel atmospheric non-thermal pulsed plasma system with a multi-pin-to-plate electrode configuration. The system integrates optical emission spectroscopy, UV–visible absorption spectroscopy, and response surface methodology for experimental design. A global kinetic model simulating over 150 species and 3500 reactions was employed to elucidate degradation mechanisms. Under optimized conditions, 97% of MB (initial concentration: 5 ppm) was decolorized within 120 s of plasma treatment. Significant increases in reactive species concentrations (·OH, O₃) were observed, alongside changes in water chemistry (pH, ORP, EC). These findings demonstrate the efficiency and scalability of the proposed plasma system for water treatment, providing new mechanistic insights and a framework for optimizing plasma-based degradation of organic pollutants.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-16554-9.

Introduction

Dyes and pigments are widely used in various industries to color final products, and the output of dye products is released directly into the environment as wastewater. These dyes are toxic, biologically resistant, and apt to cause genetic mutations, and thus have to be removed before discharging into aquatic ecosystems^1–5. In the standard treatment, wastewater can be decontaminated using organic² or non-organic⁶ methods. These techniques often have issues like the duration and cost of treatment, the requirement for extensive facilities, and the existence of some pollutants that are not easily degraded.

To overcome these difficulties, advanced oxidation processes, particularly those utilizing ozone and cold atmospheric plasma discharges, have been explored for wastewater treatment^7–9. Plasma-based methods, including pulsed corona discharge, dielectric barrier discharge (DBD), and contact glow discharge electrolysis, generate highly reactive species capable of degrading organic contaminants^1,3. Previous studies have investigated the degradation of dyes such as methylene blue and methyl orange using various plasma reactor designs (e.g., needle-plate, DBD), reporting significant removal efficiencies^10–19. However, most of these studies focused on conventional reactor configurations, limited operational parameters, or relied on one-factor-at-a-time experimental approaches. While the roles of reactive species like hydroxyl radicals (·OH) and ozone (O₃) have been discussed, comprehensive and quantitative insights into the temporal evolution and interplay of these species during plasma-induced dye degradation, especially for methyl blue, remain limited. Additionally, the integration of advanced diagnostics, robust statistical experimental design, and detailed kinetic modeling is rarely addressed in a unified framework.

To address these gaps, this work introduces a novel multi-pin-to-plate non-thermal plasma system and combines real-time optical emission and absorption spectroscopy with response surface methodology (RSM) and a comprehensive global kinetic model. This integrated approach enables, for the first time, a detailed and quantitative analysis of both plasma and aqueous-phase chemistry during Methyl Blue decolorization. The main novel contributions of this study are:

• Demonstration of a new plasma reactor configuration that enhances reactive species generation and dye degradation efficiency;

• Simultaneous, time-resolved measurement of key plasma and water parameters using advanced spectroscopic techniques;

• Application of RSM for statistically robust experimental design and process optimization;

• Development and validation of a global kinetic model to simulate and elucidate the roles of multiple reactive species in the degradation process.

This work provides new mechanistic insights and quantitative data that advance the understanding of plasma-assisted dye removal, offering a pathway for optimizing and scaling up plasma-based water treatment technologies.

In this paper, a non-thermal plasma is applied for degrading the MB dye, an organic compound dissolved in water. The plasma system is formed in multi-pin-to-plate configurations to reach a non-uniform electric field in the discharge medium (air and water). The plasma parameters and water characteristics are dynamically calculated to find the decolorization processes in the treatment time of 0–120 s. In addition to the OES technique for identifying the produced reactive species and calculating the temperatures and electron density at the interface of air and water, UV–visible absorption spectroscopy is also used to specify the maximum absorption of MB dye dissolved in water during the treatment time. Next, RSM will be utilized to design the experiments for determining the degradation of MB dye, pH, electrical conductivity (EC), oxidation–reduction potential (ORP), total dissolved solids (TDS), salt, the concentration of ·OH radicals, and O₃ molecules during the treatment time. Then, four test strips are applied to measure the concentration of peroxide, ammonia, nitrite, and nitrate species produced in the water. Finally, the Global model, as a suitable method to calculate the temporal variations of the various species’ densities produced in the discharge gap, is used to simulate the behavior of the most important species affecting the degradation of MB dye in the gas and liquid phases.

Experimental setup

Figure 1 shows the experimental setup implemented in this work to study the effects of microsecond plasma discharge on the degradation of MB dye dissolved in water. A circular plate of aluminum which the pins with a length of 5 mm have been placed it was applied as a high-voltage electrode. The diameter of the plate was 5 cm, and the distance between the pins was about 5 mm. A circular electrode of aluminum with a diameter of 5 cm was placed on the water’s floor. The cylindrical container included a solution with 5 ppm of MB organic dye. The distance of the pins from the surface of the water was kept at 1 cm. A high-voltage pulse generator with a maximum output voltage of 10 kV, a pulse repetition frequency of 50 kHz was used in this work.

Fig. 1.

Schematic of experimental setup prepared for MB dye degradation.

A high-voltage pulse generator with a maximum output voltage of 22 kV, pulse repetition frequency of 50 kHz, a rise time of 1 ns, and a pulse duration of 1 μs was used in this work. A current probe (Tektronix, TCP202) and a high-voltage probe (Tektronix, P6015A) with an oscilloscope (HM1508) were used to record the waveforms of discharge current and voltage, respectively. Figure 2 shows the waveforms of the discharge voltage and current at a pulse duration.

Fig. 2.

Oscillogram of (a) voltage and (b) current waveforms.

In this work, the OES technique was used by two spectrometers, HR4000 Ocean Optic with a resolution of 0.8 nm at the wavelength of 300 nm, and Jobin Yvon, TRIAX550 with a resolution of 0.05 nm at 500 nm. The first spectrometer stores a complete spectrum at every millisecond with a wavelength range sensitivity of 200–1100 nm, which is applied for collecting the ·OH band. The second higher-resolution spectrometer can detect the hydrogen Balmer β line at the wavelength of 486.1 nm. A UV–Vis deuterium light source was also used to produce light for absorption spectroscopy. Moreover, two convex lenses with a diameter of 50 mm and a focal length of 30 mm were used to pass the light through the water for absorption spectroscopy, while one lens with a diameter of 50 mm and a focal length of 60 mm was applied to collect the light of plasma at the interface of water and air for OES measurements.

Moreover, water’s structural characteristics were achieved by measuring pH with a pH spear, ORP with ORP tester HI98201, EC, TDS, and salt with EC meter model 8200, ·OH species with a broad-band fluorescence method by two-hydroxyterephthalic acid, O₃ species with Ozone AccuVac Ampuls (0–1.5 mg/L). It must be noted that the test strips for measuring H₂O₂, , NH₃, and species were Mquant-Merck (0.5–25 mg/L), Macherey–Nagel (1–80 mg/L), Macherey–Nagel (0.5–6.0 mg/L), and Macherey–Nagel (10–500 mg/L), respectively.

To ensure the reliability and reproducibility of our experimental results, all key measurements, including MB degradation efficiency, plasma parameters, and water characteristics, were performed in triplicate under identical conditions. For each experimental condition, three independent experiments were conducted. The resulting data were averaged, and the standard deviation was calculated for each set of measurements. These values are used to represent the variability in our results.

Simulation method

This section will briefly introduce the Global model used to describe the plasma. More details of the Global model have been presented in²⁰.

Generally, the time evolution of the number density of different species generated in plasma can be studied by the Global Model with ZdPlasKin²¹. For species s, the time evolution of the density n_s is expressed by,

1

where j is the number of reactions influencing on n_s, α_i,s and β_i,s refer to the numbers of species s at the left- and right-hand sides of reaction i. Besides, R_i is the rate of reaction i, which can be defined as:

2

here n_z is the density of species z and k_i refers to the coefficient rate of reaction i affected by the cross-section given by:

3

where , v(), σ_i(v), f(), and E_th are respectively referred to the energy, electrons velocity, ith collisional cross-section, electron energy distribution function (EEDF), and threshold collision energy²¹.

In order to stabilize the modeling, the electron energy density equation should be solved together with the number density equation in the simulation procedure, which is described as²¹:

4

where P is the input power density, Q_elas and Q_inelas are the energy loss and net energy loss due to the elastic and inelastic processes, respectively.

In order to model the plasma discharge within a global scheme, the Boltzmann equilibrium condition has been applied. Additional modeling parameters are summarized in Table 1, while Table 2 presents the 150 species considered in the kinetic model, leading to approximately 3500 chemical and physical reactions—including electron impact, attachment, recombination, neutral–neutral, ion-heavy particle, and vibrational energy transfer processes—with cross-sections or rate coefficients available in the literature for most neutral and ionic interactions.

Table 1.

Simulation parameters.

Simulation parameters	Value
Max. voltage (kV)	10
Frequency (kHz)	50
Pressure (atm)	1
Total time (s)	120
Time step (s)	1 × 10−12
Density of charged species (m−3)	1 × 1011
Density of neutral species (m−3)	1 × 1020
Gas temperature (K)	300

Table 2.

The list of species considered in the model.

Neutrals	, , , , , , , , , ·, , , , , , , ,
Pos. ions	, , , , , , , , , , , , , , , ,
Neg. ions	, , , , , , , , ,
Elec. excited	, , , , , , , , , , , , , , [], [], [], , , , , , , , , , , , , [], , , [], [], , ,
Vib. excited	, , , , , ,

Results and discussion

Experimental results

Spectroscopic investigations

Figure S1 represents the entire emission spectrum of the plasma zone at a voltage of 10 kV and frequency of 50 kHz at different times. The dominant features are the NO γ bands, ·OH radicals, and atomic oxygen lines, which are known to play a central role in the oxidation and degradation of MB dye. The presence of strong ·OH and O emission indicates efficient generation of highly reactive species under our discharge conditions. Quantitative analysis reveals that the intensities of these bands increase with treatment time, correlating with the observed rapid decrease in MB concentration (see in “Simulation results” section). These results confirm that our plasma configuration efficiently produces the reactive species responsible for effective dye decolorization. Additional spectroscopic data and band assignments are provided in Supplementary Material Section S1.

Determination of rotational, vibrational, excitation temperatures, and electron density

The methodology for estimating rotational, vibrational, and excitation temperatures from the ·OH () emission band using OES and SPECAIR software is provided in the Supplementary Material (see Fig. S2). Briefly, this approach involves simulating spectra with instrumental resolution and collisional broadening considered, and fitting them to experimental data. Using the ro-vibrational spectrum of ·OH species at the wavelength of 309.1 nm ( transition), the rotational, vibrational, and excitation temperatures can be obtained³⁸. On the other hand, spectra with several emission systems enable estimating the electronic excitation temperature, T_exc, which corresponds to the Boltzmann distribution for the population of excited electronic states. The T_exc is assumed close to the electron temperature, T_e, in equilibrium conditions. The variations of rotational, vibrational, and excitation temperatures at the interface of air and water in the time interval of 0–120 s are illustrated in Fig. 3a. As can be seen, all temperatures are intensively reduced at the first 30 s, and then they remain almost constant during the treatment time after 30 s.

Fig. 3.

(a) Variations of rotational, vibrational, and excitation temperatures as a function of treatment time at the pulsed voltage of 10 kV and the frequency of 50 kHz. (b) Variations of electron density as a function of treatment time at the pulse voltage of 10 kV and a frequency of 50 kHz. Each data point represents the mean value from three independent experiments, and error bars indicate the standard deviation.

A detailed description of the methodology used to determine electron density via Stark broadening of the H_β line is provided in the Supplementary Material (Section S1.2). In brief, the electron density was extracted by fitting a Voigt profile to the H_β emission line (see Fig. S3), taking into account Doppler, resonance, and van der Waals broadening effects. The temporal trend of electron density is shown in Fig. 3b. The electron number density reaches its maximum value as soon as a voltage is applied to the pins. After the early times, the electron number density remains nearly constant during the treatment. In this case of plasma, a part of the energy applied to the discharge is used for the excitation and decomposition process at the interface of air and water. Thus, ionization and electron production are reduced when the time is increased.

Degradation of MB color and changes in water properties

In order to create RSM for taking the optimum number of experiments in this research, a Box–Behnken design (BBD) has been applied with four parameters (treatment time, volume of water, dye concentration, and electrode distance) and eight responses: dye degradation, pH, EC, ORP, TDS, salt, the concentration of ·OH radicals and O₃ molecules. The BBD optimization method can predict the model response by examining linear, quadratic, and interaction effects of the factors. Using a second-order polynomial as an efficient function to correlate the dependent and independent variables, the BBD needs the minimum number of experiments compared to other methods and is extensively applied for a desirability function (DF) model.

It must be noted that Design Expert has been used to create and evaluate the experiment’s findings. The second-order polynomial model applied to the responses is⁴⁴:

5

where y refers to the responses, β₀ denotes the intercept, and β_i, β_ii, and β_ij being the coefficients of factors for linear, squared, and interaction influences, respectively.

Moreover, the desirability function approach (DFA) described by Derringer’s desirability function has been applied to simultaneously obtain the optimized results for all responses⁴⁵. It is strongly advised to use the DFA to optimize the problems with multiple responses and to convert each response into an individual DF (d_i) with a value in an interval of [0–1]. When the value equals 1, the response corresponds to the target value and is the most desirable. Conversely, the value would be zero if the response is far away from the target and is the least desirable. To determine the total DF, each desirability value from the response can be calculated as follows⁴⁶:

6

where D, d_i, and w_i stand for the total DF, the individual response DF, and the response weight, respectively. The individual DF would be as follows, provided that the minimum number of responses is desired,

7

However, when the maximum number of responses is desired, the individual DF could be given as⁴⁶,

8

where s refers to the weight applied for computing the desirability scale, L_i, T_i, and U_i being the lower, target, and upper values, respectively.

It should be mentioned that the second-order equation that is applied to estimate the response functions can be defined as:

9

where t is the time of plasma treatment, d is the distance between the power and ground electrodes, V and n denote to the volume of solution and concentration of MB organic dye, respectively. It must be noted that the coefficients were predicted by the analysis of variance (ANOVA) method. To study the accuracy of the model, the coefficient of measurement (R²) was obtained, which was 0.97 for degradation, 0.98 for the pH, 0.96 for EC, 0.98 for ORP, 0.97 for TDS, 0.95 for salt, 0.98 for ·OH concentration, and 0.97 for O₃ concentration.

Figure 4a presents the variations of the degradation of the MB dye dissolved in the water and the pH value of the water as a function of the treatment time when a high voltage of 10 kV is applied to the pins. Clearly, the degradation increases with the treatment time, while the value of pH shows small changes after a significant initial reduction. When the plasma is formed between the electrodes (air and water), the reactive species in the gas phase penetrate into the water and are dissolved in it, and consequently, the hydrolytic reactions trigger in the water, resulting in the production of positive and negative ions in the liquid phase. The chemical reactions in the liquid phase result in degrading the MB dye dissolved in the water. In addition, the decrease of pH implies that the acidity property of water increases with plasma-induced ionization of H₂O_aq molecules and then forming species as⁴⁷:

10

11

Fig. 4.

(a) Variations of degradation of MB dye and pH value, (b) conductivity and ORP, (c) during the treatment time. Each data point deputes the mean value from three independent experiments, and error bars represent the standard deviation.

It must be noted that the pH value can be decreased by the dissociative ionization of species to the negative ions of and the positive ions of due to the pKa value of the chemical reaction of the positive ions as⁴⁵:

12

13

14

Moreover, the behavior of the EC and ORP responses of the solutions with 5 ppm of the MB dye during the treatment time is seen in Fig. 4b. ORP is a vital factor in degrading the MB dye because it shows the amount of reactive species in the water. The positive and negative values of ORP correspond to the oxidation and reduction of the potential, respectively. The excitation processes induced by the high-energy electrons in the plasma discharge make the oxidation potential⁴⁸. The value of ORP changes from 340 to 405 mV in this case during the treatment time, while its value equals about 300 mV for drinking water. It indicates that the oxidation potential is more than the reduction potential, and it is essential to remove the MB dye dissolved in the water. In this work, the value of EC varies from 175 to 460 μS/cm during the treatment time because of growing the reactive species in the water. The increase of EC along with the decline of pH could be explained by considering the contribution of , , , and species generated in the water as follows⁴⁷:

15

where […] is the concentration of the species mentioned above, which is multiplied by the specific conductance of positive and negative ions, and Λ₀ being the initial EC. Regarding the concentration value of these species in this case, nitrate plays the main role in increasing the EC of water, which agrees with reports in the literature^49,50.

The values of TDS and salt in the water linearly increased with growing treatment time (see Fig. 4c). As is known, there is some salt dissolved in the water that increases due to the plasma treatment.

All quantitative results presented in this section are based on the average values obtained from three independent experimental replicates for each condition. The error bars shown in Figs. 3 and 4 represent the standard deviation of the triplicate measurements. This approach provides a clear indication of the reproducibility and reliability of our findings. Furthermore, statistical analysis using one-way analysis of variance (ANOVA) was performed to assess the significance of differences observed between experimental groups. The ANOVA results confirmed that the observed trends in MB degradation efficiency across different treatment times are statistically significant (p < 0.05).

Figure 5 shows the UV–visible absorption spectra of the MB solution before and after being treated with plasma at different times in the range of 0–120 s. It should be mentioned that a solution with a dye concentration of 5 ppm was treated in these experiments. The maximum intensities of UV–visible spectra corresponding to the absorption are at wavelengths of 200, 320, 430, and 640 nm. It can be observed that these peaks are reduced with the increase of the treatment time, resulting in the removal of the color after 120 s of plasma treatment.

Fig. 5.

Absorption spectra of plasma discharge at different times.

The following provides a detailed description of the processes involved in the degradation of methyl blue (MB) at the plasma-liquid interface, as illustrated in Fig. 6. The processes are categorized into three main regions: gas phase, interface, and liquid phase. Each region contributes uniquely to the generation, transfer, and reaction of reactive species essential for dye removal.

Fig. 6.

Schematic illustration of plasma–liquid interface processes and their role in dye degradation.

1. Gas phase

In the gas phase, pulsed plasma discharge generates high-energy electrons that collide with atmospheric molecules, including nitrogen (), oxygen (), and water vapor (). These collisions result in ionization and excitation, producing various reactive species such as charged ions (e.g., , ) and radicals (e.g., ·O, ·H, ·OH, NO). The key reactions are as follows¹¹:

16

17

18

High-energy electrons are critical for generating these reactive species, which are subsequently transferred to the interface and liquid phase to facilitate MB degradation. The reactive species, including ·O and ·H, play a pivotal role in initiating dye degradation processes.

• (2)Interface

The plasma-liquid interface is a thin boundary layer that mediates the transfer of energy and reactive species between the gas and liquid phases. This region is crucial for facilitating chemical reactivity.

Species transfer Charged particles and radicals from the gas phase are transferred to the liquid phase through sputtering or diffusion, entering the plasma-liquid boundary layer.

UV radiation and localized electrolysis The plasma discharge generates UV radiation (200–300 nm) and induces localized electrolysis due to the pulsed electric field. These processes deliver heat and energy to the liquid phase, enhancing chemical reactivity. The interface thus serves as a critical bridge, enabling the transfer of reactive species to the liquid phase for MB degradation.

• (3)Liquid phase

In the liquid phase, reactive species transferred from the interface, along with those generated in situ, participate in chain reactions to produce aqueous oxidants, including , , , , and . These oxidants directly attack MB molecules, leading to their degradation. Key processes include:

: Generated from the decomposition of or reactions of with ¹⁵.

: Formed by the dissolution of gas-phase or in situ production by plasma.

: Produced by the recombination of two radicals or other reactions.

Strong oxidants, particularly react with MB, breaking it down into simpler compounds. The liquid phase is the primary site for dye degradation, driven by the direct interaction of oxidants with MB molecules¹¹.

Mechanism of dye removal by plasma-generated species

The degradation of Methyl Blue involves multiple chemical reactions facilitated by reactive species generated across the gas phase, interface, and liquid phase. The key mechanisms and reactions are outlined below:

1. Role of ·OH radicals

·OH radicals, transferred to the liquid phase, react with MB due to their high oxidation potential (2.8 V):

19

·OH is the primary agent for MB degradation, driving the breakdown of the dye into simpler compounds^11–15.

• (2)Role of ·OH radicals O₃

Gas-phase dissolves in water to form , which directly oxidizes MB and produces ·OH:

20

21

Ozone enhances degradation both directly and indirectly through ·OH production¹³.

• (3)Role of hydrogen peroxide ()

decomposes into ·OH under UV radiation, contributing to MB degradation⁷:

22

Although does not directly degrade MB, its decomposition into ·OH significantly enhances the degradation process¹⁰.

1. Role of nitrogen species (RNS)

Nitrogen species, such as , contribute indirectly to degradation by lowering the pH and producing additional oxidants:

23

These species support the overall chemical environment conducive to dye degradation^14,15.

• (5)Role of UV radiation

Plasma-generated UV radiation (200–300 nm) indirectly aids degradation by decomposing and into ·OH. While UV radiation has no direct effect on MB, it enhances the production of reactive species critical for degradation^12,13.

Test-strip results

By applying the high voltage to the electrodes, the variations in the deionized H₂O features are induced, which play an essential role in further increasing the chemical reactivity in the water treated by plasma. Additionally, these changes result in varying the pH and EC of the water, and therefore, the capability of the plasma in producing H₂O_2aq species in the water has been investigated. The combination of hydroxyl radicals is the top way to create the H₂O_2aq species in the plasma medium, i.e., air and water. Moreover, the ·OH species can effectively degrade many organic compounds due to their high reactivity in addition to their role in creating hydrogen peroxides^54–57. In the present work, ·OH molecules can penetrate the water, solvating from the gas phase and into the water, creating ·OH_aq radicals. A reaction between them and the third molecule can lead to hydrogen peroxide as⁵⁸:

24

The H₂O_2aq species is diffused in the bulk liquid; however, they did not play any role in the degradation process of the MB dye dissolved in the water. The ·OH radicals play the most critical role in the degradation process. Generating H₂O_2aq, NH_3aq, , and species in the considered plasma system have been measured at different treatment times. Figure 7a shows the amounts of H₂O_2aq and NH_3aq species generated into the water during the treatment interval of 0–120 s when a 10 kV voltage is applied to the pins. It is clear that producing H₂O_2aq species increases during the treatment and reaches up to 20 mg/L after 120 s. However, the amount of the NH_3aq species is zero until 30 s; then, it is produced, rising to 5 mg/L after 120 s (see Fig. 7a).

Fig. 7.

Concentrations of (a) hydrogen peroxide and ammonia, (b) nitrite and nitrate in water in terms of plasma treatment time.

Figure 7b represents the concentrations of and species produced in the water as a function of the treatment time in the range of 0–120 s by applying high voltage on the electrodes. As can be observed, the amount of nitrite is zero until 30 s, and then it increases to 1.82 mg/L after 120 s plasma treatment. The nitrate content in the water continuously increases and reaches 50 mg/L after a treating time of 120 s (see Fig. 7b).

Henry’s law at the first approximation can be used to describe the reactive species dissolutions from the gas phase to the liquid phase⁵⁹. Based on the Henry’s law, the solubility coefficient for H₂O₂ (k_H ≈ 10³ mol/m³ Pa) is almost 8 orders of magnitude higher than that for NO₂ (k_H ≈ 10⁻⁴ mol/m³ Pa), O₃ (k_H ≈ 10⁻⁴ mol/m³ Pa), and NO (k_H ≈ 2 × 10⁻⁵ mol/m³ Pa). Therefore, the dissolution of H₂O₂ gas in the water is faster than that of NO_x gas species. In addition, the chemical compounds of the reactive species in the gas and liquid phases are due to their solubility coefficients of Henry’s law.

According to the OES measurements, there are molecular and atomic forms of reactive oxygen and nitrogen species (RONS) in the plasma discharge, producing the nitrite and nitrate species in the water. In fact, RONS in both gas and liquid phases play an important role in the acidification of water by the chemical reactions with each other as follows⁶⁰:

25

26

27

28

The and species can be produced by the reactions between the nitrogen and oxygen atoms/molecules presented in the gas phase, solvating in liquid phase and then, the long-lived N_x created in the water reacts with the H_aq atoms, H₂O_aq and ·OH_aq molecules⁶¹, so generating acids HNO_xaq is as⁵⁹:

29

30

31

32

33

Both NO and NO₂ species are the main RONS in the gas phase that dissolve in the water, generating the , , and species and hence increasing the water acidity as⁵⁹:

34

35

36

Besides NO and NO₂ species, HNO₂ molecules are also generated in the air plasma due to the water vapors, enabling readily dissolve in the water and resulting in NO_2aq production⁶². The dissociation of species could lead to radicals as⁶³:

37

Moreover, species is able to form the positive ion of NO species by a protonation reaction of HNO₂ species as²⁸:

38

It must be noted that a pH-dependent reaction that triggers at pH < 3.5 can disproportionate the species in the acidic conditions as follows⁵⁷:

39

On the other hand, both nitrite and nitrate species are also produced by dissolving NO_2aq species in the water as⁶⁴:

40

41

In the acidic conditions, H₂O_2aq molecules reacting with the species can produce the neutral and negative ions of the peroxynitrous acid, i.e., HNO_3aq and as⁵⁷,

42

The rate constant of Eq. (59) is equal to k = 1.1 × 10⁻³ M⁻² s⁻¹ at the pH value of 3.3. Destruction of both H₂O_2aq and species depends on the pH and temperature values in the water, affecting the chemical kinetics by;

43

The initial concentrations of H₂O_2aq and species distinguishes which of them is dominant in the water. The HNO_3aq acid is one of the main factors in the degradation process of MB organic dye due to its dissociation to ·NO_2aq, ·OH_aq, and species⁶⁵.

Additionally, the nitrite and nitrate species are able to reduce the pH value (see Fig. 8a) of the water because of the negative value of pKa in the ionization reaction between nitrate and in the liquid phase as⁴⁷:

44

Fig. 8.

Concentrations of ·OH and O₃ concentrations into the water in terms of plasma treatment time.

However, the decrease of pH along with the increase of EC implies the disproportionation reduction of species into NO_aq and species with production as⁴⁷:

45

It must be noted that the species is formed in the water by dissolving the NO_x species and HNO₂ molecules (pKa = 3.4). Because of being the solubility coefficient of Henry’s law for the HNO₂ species is significantly larger than NO and NO₂ species, the presence of species in the water remarkably result from HNO₂ molecules rather than from dissolving NO and NO₂ species⁶².

The concentration of ·OH and O₃ species produced in the water can be observed during the treatment time (Fig. 8). Ozone is one of the most important species in the plasma discharge, contained in oxygen/air due to its instability and high chemical reactivity, enabling the degradation of organic dyes. The molecular, atomic, and excited species of oxygen (O₂, O, O^*) along with the UV radiation produced in the plasma discharge at the wavelength range of 200–300 nm contribute to form the ozone molecules in both gas and liquid phases which could be expressed as^51,52:

46

47

48

49

Also, ozone molecules are able to reabsorb a photon and then dissociate back into molecular and atomic oxygen species as⁵²:

50

Since the ability of ·OH_aq radicals to trigger the oxidation reactions is higher than that of O_3aq species, they are more useful to purify the MB dye molecules dissolved in the water. Besides, O_3aq molecules give rise to these decomposition reactions to produce ·OH_aq radicals as⁵³:

51

52

53

Simulation results

The most important species in the plasma discharges are electrons that start the reactions in the plasma region. Electrons can directly create reactive species through the ionization and attachment processes. Also, they indirectly generate the new neutral and charged species in the plasma through the reactions among the reactive species. In addition to the number density of the electrons, their temperature is another crucial parameter as a criterion of the energy required to make an effective collision between electrons and the neutral species. Figure 9 shows the variations in the number density and temperature of electrons in the region between the electrodes as a function of the treatment time. As seen, the electron number density increases to a maximum value in the early times due to the dominance of the ionization process. Then, it is reduced by starting the recombination process, which is predominant in the ionization process (see Fig. 9a). A master–slave behavior is observed between the ionization and recombination processes in the plasma over time. Furthermore, the electron temperature in the plasma rises to a high value (~ 79,000 K) due to not being in collision at the early times, but it immediately drops by transferring its energy to the other species due to starting the collisions. Then, it continues with an almost constant value (~ 32,000 K) during the time (see Fig. 9b).

Fig. 9.

Time evolution of electron (a) density and (b) temperature between the electrodes at pulsed high-voltage of 10 kV and a frequency of 50 kHz.

In this work, the discharge region consists of three parts: (1) air, (2) the interface of the air and water, and (3) water (see Fig. 10). Electrons play an essential role in the air and interface and cause the creation of other reactive species in these parts by collision with neutral species. The reactive oxygen and nitrogen species (RONS) in the plasma are not only produced by the free electrons but are also by-products of the reactions among the heavy reactive species. Some of the positive and negative ions produced by the electron impacts, such as , , , , , , and their clusters (see Fig. 11a,b) are sputtered into the liquid part when they reach the interface, and hence they react with the neutral species and generate the new reactive species, including O, H, ·OH, NO, , , , , , and so on (see Fig. 11c,d)^58,66. Therefore, all these species can penetrate the interface layer in addition to their production in this region through UV radiation and the electrolysis process to deliver heat to the water (see Fig. 10). So, not only is water part of the discharge medium, but these reactive species generated in the gas phase also transit to the liquid phase via the interface layer and get ready to transfer into the water.

Fig. 10.

Schematic diagram of some of the main species and processes for the presented plasma discharge, including air, interface, and water.

Fig. 11.

Time evolution of different species (a) , , , OH, NO, NH (b) , , , , , (c) , , , , , , , and (d) , , , , , in the plasma region (air and water) at the pulsed high-voltage of 10 kV and a frequency of 50 kHz.

It should be mentioned that the lifetime of the RONS is one of the most critical factors that directly affect the dye degradation process. From this point of view, they could be categorized into two classes: (1) Small lifetime; the species whose lifetime is almost from ns to ms, such as , ·O, ·H, ·OH, and ·NO, (2) high lifetime; RONS whose lifetime is about seconds to hours, including NO, H₂O₂, O₃, and NO₂⁶⁷. The detection of RONS in the first class is difficult, especially when they are dissolved in water. They are rapidly eliminated by turning the plasma off as well. The species in the second category can create the aqueous form of O₃, H₂O₂, , and , increasing the water acidity.

Besides, the hydroxyl radicals could be produced by O₃, H₂O₂, UV radiation (UVA-UVB), and reactions among the charged particles in the gas and liquid phases (see Fig. 10). It should be noted that the UV radiation coming from the plasma discharge does not directly affect the MB dye degradation. Still, it can highly produce reactive oxygen and nitrogen species (RONS) in the air and water by breaking the chemical bonds in compounds and molecules. Besides RONS, the electric field can be another influential factor in the dye degradation process by plasma because it is a vital agent at the plasma-induced biological influences^68–70, especially in the pulsed electric field that the increment of pulses numbers, pulse amplitude and length could lead to break the chemical bonds of the dye molecules dissolved in the water.

When a high voltage is applied to the electrodes, a plasma discharge is formed between the electrodes, including the gas phase, interface, and liquid phase. The produced species in the gas phase can enter the water and react with H₂O molecules, in addition to the species generated in the water. The new species are created in the bulk liquid, such as , , , , ·, , , and so on (see Fig. 11d). Besides, the changes in the pH value of the water show that the water acidity increases due to the presence of , , and species (see Fig. 7b). It can be seen that the and acid species are notably produced at the initial times and then faintly decrease (see Fig. 11b). These acids decrease the pH value of the water and can degrade the MB dye dissolved in the water because of having high reactivity. Their behaviors can be compared with the changes in pH value of the water, which significantly reduces at the initial time and then slightly decreases (see Fig. 4).

Conclusions

In this work, the effect of plasma discharge made by a new electrode configuration on the decolorization of MB organic dye dissolved in water was studied in the time interval of 0–120 s. To this aim, a non-thermal plasma was put on the solution with 5 ppm of MB dye during the treatment time. A pulsed high-voltage of 10 kV with a frequency of 50 kHz was applied to the plate of pins placed at the top of the water surface. The grounded electrode was put on the floor of the water. The non-uniform electric field in this structure made the plasma discharge between the electrodes, consisting of air (gas phase) and water (liquid phase).

In order to identify the effective plasma species on the MB dye degradation in the water, the OES technique was performed during the treatment time. This method was also used to calculate the electron density, rotational, vibrational, and excitation temperatures. The electrons are the main particles that trigger the plasma discharge and generate radicals, reactive species, and positive and negative ions through collisions. The maximum value of the electron number density (~ 4.7 × 10¹⁴ cm⁻³) was seen at the initial time and then slightly decreased and remained almost constant. On the other hand, the rotational temperature (~ 2400 K) is almost equal to the gas temperature, which can be a criterion of the plasma heat. The plasma heat may affect the decolorization of MB dye dissolved in the water by the electrolysis process. It should be mentioned that the excitation temperature (~ 22,000 K) is close to the electron temperature, which is a criterion of electron kinetic, leading to collisions in the plasma. Besides, the UV radiation of plasma could break the chemical bonds in the MB organic compounds, resulting in a fast reaction with other species and degradation.

Moreover, UV–visible absorption spectroscopy was applied in the treatment time interval of 0–120 s to study the light absorption by the MB dye dissolved in the water. It was observed that the maximum absorption is in the wavelengths of 200, 320, 430, and 640 nm, reducing during the treatment time.

The RSM was applied to design the experiments in the optimization conditions to study the degradation of the MB organic dye dissolved in water. The behavior of the degradation of MB dye was incremental during the time of plasma treatment and reached 95% after 120 s. As expected, the water characteristics were affected in the time interval of 0–120 s. The pH value decreased during the plasma treatment, while the other parameters increased. The value of EC changed from 175 to 460 μS/cm corresponding to an increase in the reactive species in the water. Moreover, the value of ORP varied from 340 to 405 mV during the plasma treatment time, showing that the oxidation potential is higher than the reduction potential, which is necessary for removing the MB dye dissolved in the water. Also, the TDS and salt values were increased with the growing treatment time. The concentration of ·OH radicals and O₃ molecules, two of the most important oxidizing factors in the plasma discharge contained oxygen and water, was increased with different mechanisms during the time of plasma treatment. The generation of , , , , and species raise the water acidity, which is the cause of the reduction of pH value and the increase of EC of the water. Despite a significant production of H₂O_2aq species in the water during the treatment time, it did not play a role in degrading the MB dye, and ·OH radicals and O₃ molecules were the main species in the decolorization process.

Four test strips were used to measure the concentrations of the peroxide, ammonia, nitrite, and nitrate species generated in the water during the time of plasma treatment. It was seen that H₂O_2aq and species are produced from the initial treatment time and reach 21 mg/L and 50 mg/L after 120 s, respectively. Additionally, the concentration of NH_3aq and species is zero at the early treatment time and then increases to 5 mg/L and 1.8 mg/L after 120 s, respectively. These results were confirmed by the results of pH value.

Furthermore, a global model was implemented to investigate the most critical species produced in the plasma discharge (including air and water) as a function of time. The number of 150 species and 4000 reactions in gas and liquid phases was considered in this model. The number density and temperature of electrons significantly increase at the initial time, then they are reduced and remain almost constant during the time because of the collisions acting as an energy loss sink. Also, this model obtained variations of the neutral and charged species generated in the plasma discharge, consisting of gas and liquid phases, over a time of 120 s.

The variations in the number density and temperature of electrons calculated by the experimental procedure were comparable to the results determined by the simulation during the time. In addition to the same behavior of experimental and simulation results, the maximum value of electron number density was equal to 4.7 × 10¹⁴ cm⁻³ and 1.6 × 10¹⁴ cm⁻³ at the initial time, respectively. There is a similar case in measuring the electron temperature by experiment and simulation processes over time. Besides, the behavior of number density of ·OH_aq, O_3aq, H₂O_2aq, NH_3aq, , and species determined by the simulation during the time could be compared with the changes in their concentration measured by the test strips. Moreover, the investigation of the and ·OH production in the simulation shows that they are significantly generated at the initial times and then slightly reduced. This behavior is comparable to the variations of pH value of the water, which remarkably decrease at the initial time and then marginally decrease.

In summary, this work demonstrates the effective decolorization of Methyl Blue dye using a novel atmospheric non-thermal pulsed plasma system with a multi-pin-to-plate electrode configuration. By integrating advanced spectroscopic diagnostics, RSM-based experimental design, and comprehensive global kinetic modeling, we have provided new insights into the mechanisms of plasma-induced dye degradation. Our findings reveal that the synergy between plasma-generated ·OH radicals, ozone, and nitrogen-based species plays a critical role in the rapid breakdown of MB molecules, as confirmed both experimentally and through simulation. The observed increase in water acidity and the identification of key reactive intermediates further distinguish this study from previous reports. These results not only advance the fundamental understanding of plasma-assisted water treatment but also highlight the potential for optimizing such systems for practical environmental applications.

Author contributions

All authors contributed equally.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.