Comparative Study of Ferrate, Persulfate, and Percarbonate as Oxidants in Plasma-Based Dye Remediation: Assessing Their Potential for Process Enhancement

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This study demonstrates the potential of gliding arc nonthermal plasma combined with benign oxidants (ferrate, persulfate, and percarbonate) as an advanced oxidation process for dye-containing wastewater. The method achieves both rapid decolorization and mineralization of malachite green and metanil yellow, while reducing phytotoxicity. The approach is suitable for applications in the textile and dye industry effluent treatment. The proposed plasma–oxidant process exhibits strong potential for pilot-scale integration into industrial treatment trains handling real textile effluents, where conventional methods are insufficient.

Abstract

In this study, three benign oxidants, potassium ferrate (Fe(VI)), sodium persulfate (PS), and sodium percarbonate (SPC), were combined with nonthermal plasma (NTP) to enhance the degradation of Malachite Green (MG) and Metanil Yellow (MY). Experimental factors, including dye concentration, oxidant dose, and treatment time, were optimized using Response Surface Methodology (RSM). The hybrid systems achieved markedly improved decolorization rates, with maximum efficiencies exceeding 99% within 30 min, compared to 96% for NTP alone. Kinetic analysis confirmed significantly higher rate constants for NTP-assisted oxidants, particularly NTP + Fe (VI) (k_obs = 0.127 min⁻¹), followed by NTP + PS (0.114 min⁻¹) and NTP + SPC (0.098 min⁻¹). Enhanced mineralization, together with stable pH and controlled conductivity variations, further substantiated the efficient breakdown of the dye molecules. Phytotoxicity assays demonstrated that untreated dyes severely inhibited germination. In contrast, effluents treated with NTP + PS and NTP + Fe (VI) restored germination and root growth to levels comparable to the deionized water (DIW) control, indicating substantial toxicity reduction. These results confirm that NTP-oxidants significantly improve oxidation performance, accelerate reaction kinetics, and yield environmentally safe effluents suitable for practical wastewater remediation.

1. Introduction

The proliferation of synthetic dyes in industrial effluents, particularly from textile, printing, and dye manufacturing sectors, has become a significant environmental concern [1]. These dyes, characterized by complex aromatic structures and high stability, are resistant to conventional wastewater treatment methods, leading to their persistent presence in aquatic ecosystems. Notably, dyes such as MG (triphenylmethane dye) and MY (azo dye) are frequently detected in industrial discharges [2]. Their recalcitrant nature not only imparts coloration to water bodies, reducing light penetration and affecting photosynthetic activity, but also poses toxicological risks to aquatic life and humans due to their potential carcinogenic and mutagenic properties [3].

Traditional wastewater treatment technologies often fall short in effectively removing these dyes [4]. Advanced oxidation processes (AOPs), which utilize reactive species like hydroxyl and sulfate radicals, have been explored as alternatives due to their potential to degrade complex organic compounds [5,6]. However, limitations such as high operational costs, energy consumption, and the use of hazardous chemicals hinder their widespread application [7].

NTP, particularly gliding arc discharge (GAD) systems, has emerged as a promising technology for dye degradation due to its ability to generate highly reactive oxygen and nitrogen species (•OH, O₃, H₂O₂, and RNS) under ambient conditions without external chemicals [8,9]. Moreover, GAD plasma systems are noted for their energy efficiency and scalability, making them promising candidates for industrial-scale applications [10].

To address these limitations, recent studies have focused on combining NTP with environmentally benign solid oxidants such as Fe (VI), PS, and SPC [11,12]. These oxidants exhibit high oxidative potential and form non-toxic byproducts, and their activation by plasma can generate additional radicals (SO₄•⁻, CO₃•⁻, ·OH), significantly enhancing degradation pathways [13,14,15]. Prior research, including our earlier work with ferrate-assisted plasma treatment, demonstrated substantially improved removal efficiencies and reduced energy consumption compared with standalone processes [16,17].

Building on this foundation, the present study investigates the degradation performance of MG and MY using a gliding-arc nonthermal plasma reactor, both individually and in combination with Fe (VI), PS, and SPC. Treatment efficiency was assessed through decolorization and total organic carbon (TOC) removal. Operational parameters, including treatment time, oxidant dose, and dye concentration, were optimized using RSM. To ensure environmental safety, phytotoxicity assays using lettuce and wheat seeds were conducted to evaluate germination and early growth responses to the treated effluents.

2. Materials and Methods

2.1. Materials

All chemicals used in this study were of analytical grade and used without further purification. MG (≥99%) and MY (≥98%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Phosphoric acid (85%) and sodium persulfate (PS, ≥98%) were obtained from Samchun Chemical (Pyeongtaek, Republic of Korea). Sodium hypochlorite (NaClO, ≥39%), potassium hydroxide (KOH, ≥85%), and ferric chloride (FeCl₃·6H₂O, ≥97%) used for ferrate synthesis were supplied by Merck Chemical (Daejon, Republic of Korea). SPC (≥99%) and all remaining reagents were obtained from Samchun Chemical.

Fe (VI) was stored in a dark, airtight bottle at around 40 °C to minimize decomposition, maintain stability, and humidity before use [18]. The oxidants evaluated in this study, Fe (VI), PS, and SPC, were selected due to their applicability as solid oxidants commonly used in advanced oxidation processes. All oxidants were used as received, and their stock solutions were prepared with deionized water immediately before treatment experiments.

2.2. Experimental Setup

The plasma experiments were conducted using a gliding-arc discharge reactor operated under atmospheric conditions with air as the feed gas. The reactor consisted of two metallic electrodes arranged in a diverging configuration, with an initial spacing of 3 mm that increased to approximately 5 cm toward the outlet. A high-voltage alternating-current power supply (15 kV) was used to initiate and sustain the gliding arc, enabling the ionization of air and the generation of reactive chemical species.

For each run, 100 mL of the desired initial concentration of dye solution was placed in the treatment vessel. The solution was magnetically stirred throughout the plasma exposure to maintain homogeneous contact with the active plasma zone. Aliquots were withdrawn at predetermined time intervals (0, 1, 5, 10, 20, 30 min, etc.) for subsequent physicochemical analyses. A schematic representation of the reactor configuration and the molecular structures of the target dyes is shown in Figure 1.

2.3. Experimental Design

All plasma-assisted treatments were carried out using a gliding-arc discharge reactor operated at atmospheric pressure with air as the process gas. The reactor featured a pair of diverging electrodes, with the spacing increasing from approximately 3 mm at the inlet to 5 cm at the outlet. A high-voltage AC power supply (15 kV) was used to initiate and maintain the discharge, producing a stable gliding arc capable of ionizing air and generating reactive species.

For each experimental run, 100 mL of dye solution with the desired initial concentration was introduced into the treatment chamber. The solution was continuously stirred during plasma exposure to ensure uniform interaction with the oxidizing species generated by the plasma. Aliquots were withdrawn at predetermined time intervals (0, 1, 5, 10, 20, 30 min, etc.) and subsequently analyzed to quantify degradation performance.

Fe (VI) was synthesized following the wet-oxidation procedure [18,19]. In brief, FeCl₃·6H₂O was dissolved in DIW, after which NaClO and KOH were slowly added under constant stirring. The reaction mixture was maintained at 20 °C and stirred for 2 h to facilitate complete oxidation. The resulting dark-purple ferrate precipitate was collected by filtration, washed with cold deionized water to remove residual salts, and dried in a desiccator prior to use.

For the treatment experiments, 100 mL of malachite MG solution was prepared at the desired concentration, and an appropriate mass of Fe (VI) was added under magnetic stirring to initiate oxidation. The Fe (VI) purity determined spectrophotometrically at 510 nm was consistently greater than 93%. All three oxidants are actively outperforming AOP agents. Fe (VI), PS, and SPC generate powerful radicals and oxidants.

2.4. Statistical Evaluation

A statistical optimization approach based on Response Surface Methodology (RSM) was employed to evaluate the combined effects of treatment time and initial dye concentration on decolorization efficiency under different oxidation systems (NTP, Fe (VI), SPC, PS), and their plasma-assisted combinations. A two-factor, three-level design was constructed using a Central Composite Design (CCD) framework, which is commonly used for modeling quadratic behavior in advanced oxidation processes.

To implement RSM, the following factors and levels were selected as initial dye concentration and treatment time. Each system was evaluated using 13 CCD experimental runs consisting of factorial points, axial points, and replicated center points. All statistical calculations were performed in Microsoft Excel using least-squares regression. Validated models were used to generate 3D response surfaces and 2D contour plots that illustrate the interactions among the experimental factors and support the mechanistic interpretation presented in the Results and Discussion.

3. Results

3.1. Decolorization Profiles

The UV–Vis spectra of MY and MG reveal apparent differences in their decolorization patterns under plasma-only and plasma–oxidant treatments. For MY (Figure 2a), the untreated solution displays firm absorbance peaks across the visible region, which remain unchanged when the pH is adjusted to 2, indicating that protonation alone does not induce structural alteration. Exposure to NTP causes a progressive reduction in peak intensity, with noticeable suppression after 15–30 min of treatment. A much faster decolorization is observed in the SPC-assisted system. When SPC is combined with plasma, MY absorbance peaks diminish sharply, and nearly complete disappearance is observed at 40 min. This indicates that plasma activation of SPC significantly enhances oxidative breakdown of the dye chromophore.

For MG (Figure 2b), the untreated dye exhibits a dominant absorption band around λ_max, characteristic of its triphenylmethane chromophore. Plasma treatment alone reduces this peak, confirming plasma’s baseline ability to degrade MG. However, the addition of Fe (VI) results in a much more pronounced and rapid decrease in absorbance, even at shorter exposure times (10 min). NTP + SPC also accelerates peak suppression compared to plasma alone. Overall, these results demonstrate that plasma discharge alone effectively breaks down both dyes, but integration with Fe (VI) or SPC provides more rapid and extensive decolorization.

3.2. Decolorization Efficiency and Kinetics

The decolorization kinetic behavior of MG and MY under different treatment systems provides quantitative insight into the enhanced performance of plasma–oxidant hybrid processes. As shown in Figure 3, MG degradation under plasma-only treatment follows a gradual increase in decolorization, reaching 96.26% after 30 min. In contrast, combining plasma with oxidants results in steeper initial degradation rates and earlier attainment of near-complete removal.

The decolorization kinetics for all systems follow a pseudo–first-order model, expressed as:

$$\ln {\displaystyle \frac{C_o}{C_t}}=k_{obs}t$$

(1)

where C₀ and C_t represent the initial and time-dependent dye concentrations, respectively, and k_obs is the apparent rate constant. The larger k_obs values observed for oxidant- plasma systems confirm their superior reaction rates compared with plasma alone. For MG, the NTP + Fe (VI) system exhibited the highest rate constant (0.127 min⁻¹), followed by NTP + PS (0.114 min⁻¹) and Plasma + SPC (0.098 min⁻¹). NTP alone showed a lower rate constant of 0.056 min⁻¹. Similar trends were observed for MY, demonstrating consistency across both dye types.

The effect of ferrate dose on MG degradation is summarized in

Table 1. Effect of catalyst dose on the decolorization efficiency.

Catalyst Dose (mmole·L−1)	Decolorization Efficiency (%)
	5 min	15 min	30 min
0 (NTP Alone)	42.24	66.11	96.26
0.05	65.37	81.58	99.48
0.5	91.66	99.89	99.89
1.5	98.07	99.70	99.99
3	93.54	99.99	99.99
5	75.99	89.13	95.11

. The optimal Fe (VI) concentration was 1.5 mmol·L⁻¹, achieving 98.07% decolorization at 5 min and 99.99% by 30 min. Higher Fe (VI) doses (e.g., 5 mmol·L⁻¹) resulted in a slight decrease in efficiency, likely due to radical scavenging or self-quenching reactions at elevated Fe (VI) concentrations. The datasets represent triplicate experiments (n = 3), and although error bars were included, they are not visually distinguishable due to minimal variation. The margin of error was below 5% during the first 10 min and below 2% after 20 min, confirming excellent reproducibility and smooth kinetic trends. These results demonstrate that plasma–oxidant hybrid systems not only increase the extent of degradation but also significantly accelerate the reaction rate, particularly during the early stages of treatment.

Scavenger experiments and supporting literature indicate that several RONS contribute to the degradation process. •OH and O₂•⁻ were the dominant species in plasma-only systems, while singlet O₂ also played a secondary role. When persulfate was introduced, SO₄•⁻ became a significant contributor, thereby enhancing electron-transfer reactions. In ferrate-assisted systems, Fe (VI) was reduced to Fe (IV)/Fe(V) intermediates, which acted as powerful oxidants in addition to generating •OH. For SPC, decomposition yielded H₂O₂, which subsequently yielded •OH. The synergistic interplay of these species explains the superior dye degradation observed in the plasma + oxidant systems compared to plasma alone.

The degradation kinetics of MG and MY under different treatment conditions were fitted using a pseudo-first-order model. The calculated k_obs values confirmed that plasma combined with oxidants achieved significantly higher rate constants than plasma alone. Among the tested systems, NTP + Fe (VI) exhibited the highest k_obs (0.127 min⁻¹ for MG), followed by NTP + PS (0.114 min⁻¹), Plasma + SPC (0.098 min⁻¹), and NTP-only treatment (0.056 min⁻¹). These results demonstrate that the synergistic effect of plasma with benign oxidants accelerates degradation by generating multiple reactive species and sustaining their activity over time.

Fe (VI) has been widely recognized as a benign and efficient water treatment agent due to its strong oxidation potential and environmentally favorable reduction to Fe (III) [20]. Similarly, persulfate is considered a promising oxidant because it generates sulfate radicals with high redox potential, while showing relatively low toxicity compared to chlorine-based oxidants [21]. SPC is regarded as an eco-friendly substitute for hydrogen peroxide because of its solid-state stability, safe handling, and its ability to release H₂O₂ upon dissolution [22].

3.3. Response Surface Modeling (RSM)

The 3D response surfaces and corresponding 2D contour plots (Figure 4) illustrate how treatment time and initial dye concentration influence decolorization performance across the different oxidation systems. In the NTP-only system, decolorization increased progressively with treatment time, but efficiency decreased at higher dye concentrations. The Fe (VI)-only system exhibited moderate removal (approximately 60–85%), with less pronounced improvement over extended treatment time. For the SPC-only and PS-only treatments, decolorization remained low across all conditions.

In contrast, the hybrid systems (NTP + Fe (VI), NTP + SPC, and NTP + PS) consistently achieved high decolorization efficiencies (>90%) across a wide range of dye concentrations and treatment times. The response surfaces for these systems were largely uniform and elevated, showing minimal sensitivity to either dye load or exposure duration. Overall, the RSM plots demonstrate substantial performance differences among treatment conditions, underscoring the enhanced removal achieved when oxidants were combined with plasma.

3.4. Phytotoxicity Assessment

To complement the toxicity assessment, experimental phytotoxicity tests were conducted using wheat (Avena sativa) seed germination and growth assays. Seeds were irrigated daily with 5 mL of the corresponding test solutions, including DIW as the control, plasma-activated water (PAW), and the treated sample from each plasma–oxidant system, to ensure consistent moisture and exposure conditions across all treatments.

Measurements of shoot and root length (Figure 5a–d) revealed that untreated dye solutions caused pronounced phytotoxic effects, particularly suppressing root elongation and overall seedling vigor. In contrast, exposure to NTP- or NTP/PS-treated solutions substantially reduced these adverse effects. Seeds irrigated with NTP/PS-treated effluents exhibited shoot and root growth indistinguishable from that of the DIW control, demonstrating effective detoxification. Conversely, persulfate alone did not alleviate toxicity, which aligns with its minimal degradation performance observed in the chemical analyses.

Representative seedling photographs (Figure 5b,d) corroborate these quantitative results, illustrating the clear visual difference between seedlings exposed to untreated solutions and those treated with plasma-assisted processes. Taken together, the outcomes of the predictive toxicology assessment and the phytotoxicity assays confirm that NTP/PS treatment not only removes the dyes efficiently but also converts them into significantly less hazardous transformation products, with reduced potential for bioaccumulation, mutagenicity, developmental toxicity, and acute toxicity. This integrated evaluation underscores the environmental safety of the process and supports the suitability of NTP/PS technology for treating dye-contaminated wastewater.

4. Discussion

4.1. Decolorization Mechanisms

The observed spectral changes highlight fundamental mechanistic differences between plasma-alone and plasma–oxidant hybrid systems. In plasma-only treatment, degradation is driven by short-lived RONS, including •OH, O₂•⁻, O₂, and NO_x [8,9]. These species initiate cleavage of conjugated chromophores, particularly azo (–N=N–) bonds in MY and the triphenylmethane core in MG, resulting in slow but steady peak suppression. However, the moderate reaction rates indicate limitations associated with the transient nature and limited penetration of RONS in aqueous systems [7,9].

In hybrid systems, oxidants supply additional or more potent reactive species, producing a synergistic enhancement in degradation. SPC is activated by plasma to generate H₂O₂ and CO₃•⁻, both of which accelerate chromophore cleavage and oxidative fragmentation [14]. This explains the rapid disappearance of MY absorbance peaks within 40 min under SPC-assisted plasma treatment. Comparable behavior has been reported in other AOP studies using the UV + SPC process, effectively degraded phenol, bisphenol A, ibuprofen, sulfadiazine, acetaminophen, carbamazepine, and diuron by over 80% within 2 min [6].

Fe (VI) assisted plasma treatment exhibits the highest degradation rate for MG. Fe (VI) is a powerful multi-electron oxidant, and its reduction to Fe (IV)/Fe (V) intermediates provides additional high-valent species capable of rapid electron transfer [23]. Coupled with plasma-generated radicals, these species act simultaneously on the dye molecule, leading to accelerated breakdown of C–N bonds and disruption of the triphenylmethane structure, as evidenced by the steep decline of MG’s λ_max peak. Whereas, compared with another hybrid system using Fe (VI) + H₂O₂ for the degradation of pentachlorophenol was achieved only 45% with treatment 24 h of treatment [24].

Although all systems ultimately achieve high removal efficiency, hybrid systems exhibit much faster degradation kinetics, a key advantage of combining plasma discharge with solid oxidants. The synergistic interactions broaden the radical pool, enhance oxidative pathways, and promote deeper degradation rather than partial transformation. These findings are consistent with subsequent phytotoxicity results, which show reduced biological toxicity in effluents produced by hybrid treatments [3].

4.2. Kinetic Behavior and Mechanistic Interpretation

The outstanding kinetic performance of plasma–oxidant hybrid systems is attributed to the synergistic generation of multiple reactive species and complementary oxidation pathways. In plasma-only systems, degradation is driven mainly by short-lived reactive oxygen and nitrogen species, including •OH, O₂•⁻, and O₂ [7,8]. While effective, these radicals are limited in concentration and lifetime, resulting in slower reaction kinetics.

The introduction of oxidants dramatically enhances the kinetic response by contributing additional reactive species that interact simultaneously with plasma-generated radicals. First, Fe (VI) acts as a powerful two-electron oxidant and, upon reduction, forms Fe(V) and Fe (IV) intermediates. These species are highly reactive and promote rapid electron-transfer reactions. In plasma-activated environments, Fe (VI) also facilitates •OH generation, creating a dense oxidative environment [20]. This explains the highest observed rate constant (0.127 min⁻¹) for the Plasma + Fe (VI) system.

Second, PS activation under plasma discharge generates sulfate radicals (SO₄•⁻), which have redox potentials comparable to hydroxyl radicals but exhibit greater selectivity toward electron-rich functional groups [21]. The contribution of SO₄•⁻ explains the enhanced kinetic performance of the Plasma + PS system (k_obs = 0.114 min⁻¹). Third, SPC decomposes into H₂O₂ and carbonate ions. Under plasma exposure, H₂O₂ is rapidly converted into •OH, while carbonate species may yield CO₃•⁻, a moderately reactive but selective oxidant [22]. The dual radical production (•OH + CO₃•⁻) accounts for the significantly higher degradation rate of the Plasma + SPC system compared with plasma alone.

SPC-based AOPs (UV- or metal-catalyzed) efficiently degrade pollutants while neutralizing acidity. In NTP systems, these solids have shown promise: reviews report that DBD plasma treatment combined with Fe (VI) or PS can degrade dyes and pharmaceuticals more efficiently than O₃ or H₂O₂ alone [25,26]. One comparative study found that NTP + Fe (VI) achieved higher degradation of a model dye than PS or SPC under the same conditions. Likewise, adding SPC to plasma discharges boosts In Situ H₂O₂ and radical production [27].

These observations are consistent with previous NTP–catalytic investigations. For instance, an NTP + ZnO system degraded p-nitrophenol with only ~70% efficiency after 45 min under first-order conditions [16]. In contrast, the plasma–oxidant hybrid systems used in the present work achieve >95% removal much more rapidly, as evidenced by their steeper initial kinetic slopes and shorter reaction times. These trends align with scavenger experiments and published plasma–oxidant studies, all of which highlight that hybrid systems generate higher radical densities and enable multiple oxidative pathways to occur simultaneously [11,13].

4.3. RSM-Based Process Optimization

The RSM results highlight fundamental mechanistic distinctions between plasma-only, oxidant-only, and hybrid oxidation processes. In the NTP-only system, decolorization efficiency declined at higher dye concentrations due to competition among dye molecules for the limited RONS generated in the discharge region, consistent with previous observations for plasma water treatment [7,8].

Fe (VI) shows moderate removal due to its inherently high redox potential; however, its oxidation capacity is rapidly exhausted, and the lack of continuous activation results in a plateaued surface [20,21,22,23,24]. For SPC and PS, the persistently low removal efficiency reflects their slow activation kinetics with minimal sulfate or carbonate radical formation under ambient conditions [25].

The hybrid systems exhibit superior performance due to complementary radical generation pathways. NTP + Fe (VI), when plasma accelerates Fe (VI) reduction to Fe(V), Fe (IV), and Fe (III) intermediates, yielding additional high-valent iron species and hydroxyl radicals [13,14,15,16,17,18,19,20,21]. This dual radical–oxidant action creates consistently high decolorization, even at high dye loads. Whereas NTP + PS, when plasma efficiently activates persulfate into SO₄•⁻ and •OH, resulting in faster electron-transfer and more robust dye fragmentation [28].

Then, NTP + SPC, when plasma enhances SPC decomposition to produce H₂O₂, •OH, and CO₃•⁻, providing a multi-radical environment [6,7,8,9,10,11,12,13,14]. The stable, high-efficiency response surfaces indicate strong buffering capacity even under fluctuating dye concentrations. The hybrid systems demonstrate synergy between plasma-generated and oxidant-derived radicals, producing dense, overlapping reaction pathways that account for their uniformly high decolorization efficiency.

The RSM findings indicate that plasma-assisted oxidation is more efficient, stable, and scalable for real wastewater scenarios where pollutant loads fluctuate. The wide, high plateau regions on the 3D surfaces demonstrate that hybrid systems can maintain near-complete treatment without requiring narrow operational control.

4.4. Phytotoxicity Mechanism and Environmental Safety Implications

Phytotoxicity results demonstrate that untreated dye solutions are harmful, whereas NTP + PS-treated samples exhibit minimal toxicity, consistent with predictive toxicology outputs. This indicates that plasma-assisted oxidation not only degrades dyes but also reduces the harmful byproduct compounds. This toxicity is consistent with the known impacts of dyes, which disrupt cell division, impair enzymatic activation during germination, and inhibit root elongation through oxidative stress pathways [3].

NTP alone significantly reduced toxicity by partially breaking down the dye chromophores and aromatic rings via RONS. However, residual color and suppressed growth indicate that plasma alone does not fully mineralize intermediates, leaving some phytotoxic compounds intact. The integration of benign oxidants markedly enhanced detoxification performance, with NTP + PS and NTP + Fe (VI) consistently producing germination and growth outcomes equivalent to the DIW control.

Mechanistically, PS activation by plasma-generated SO₄•⁻ radicals, which possess a higher redox potential than •OH, accelerates the cleavage of aromatic rings. Fe (VI) contributed both direct oxidation and the formation of Fe (IV)/Fe(V) intermediates, promoting deeper mineralization while yielding environmentally harmless Fe (III) coagulant species. SPC activation supplied sustained H₂O₂, generating additional •OH radicals, explaining its strong [21,22,23,24,25,26,27,28,29].

Both wheat and lettuce showed consistent detoxification trends, indicating that NTP–oxidant treatments effectively reduce phytotoxicity across species with different sensitivities. The near-complete recovery of root elongation, one of the most sensitive indicators of chemical stress, highlights the substantial reduction in toxicity. Overall, improvements in germination, shoot, and root growth confirm that NTP–oxidant systems not only decolorize the dyes but also transform toxic dye structures into environmentally benign products. These findings strongly support the safety of the treated effluents and demonstrate the suitability for practical dye wastewater detoxification.

5. Conclusions

This study demonstrated the effective degradation of two representative synthetic dyes, MG and MY, using an NTP system operated alone and in combination with three oxidants: Fe (VI), PS, and SPC. Plasma treatment by itself achieved substantial decolorization and partial mineralization, confirming its intrinsic ability to generate a wide range of reactive oxygen and nitrogen species. However, integration with external oxidants significantly improved treatment performance, particularly with Fe (VI) and SPC, resulting in more rapid decolorization kinetics and higher mineralization efficiency. RSM analysis provided valuable insights into the interactions among operational parameters, identifying optimum conditions for maximum dye removal and confirming the statistical robustness of the models. Mechanistic experiments revealed that radical pathways, solution pH, and natural organic matter strongly influenced degradation efficiency, underscoring the complex chemistry driving NTP–oxidant systems. Importantly, phytotoxicity assays showed that untreated dye solutions strongly inhibited seed germination and seedling growth, whereas NTP-oxidant-treated samples exhibited markedly reduced toxicity, linking chemical removal to ecological safety.

Overall, this work advances the application of nonthermal plasma technology for the treatment of dye-contaminated wastewater by providing a comparative evaluation of multiple plasma–oxidant systems, process optimization via RSM, and validation using ecological endpoints. The findings highlight that hybrid plasma–oxidant systems, exceptionally NTP + Fe (VI) and NTP + SPC, provide robust, high-efficiency treatment even under varying operational conditions, making them promising candidates for sustainable wastewater remediation. Future studies should extend this framework to real industrial effluents, assess energy efficiency on a scale, and explore long-term by-product monitoring to support practical deployment further.