Sodium Percarbonate Activation by Plasma-Generated Ozone for Catalytic Degradation of Dye Wastewater: Role of Active Species and Degradation Process

Abstract

In this paper, sodium percarbonate (SPC) was activated by ozone (O₃) from plasma for catalytic treatment of dye wastewater. Methyl blue (MB), a typical industrial dye, was selected as the target dye contaminant. Results showed that enhancing O₃ dosage and reducing MB concentration were beneficial to MB degradation. Compared to acid condition, a higher removal efficiency of MB was obtained in alkaline condition. With an increase of SPC dosage, the removal efficiency of MB first was raised, and then it declined. Under the optimal dosage of 50 mg/L, the removal efficiency of MB reached 85.7% with 30 min treatment time. The energy efficiency was improved from 5.21 g/kWh to 5.71 g/kWh. A synergetic effect can be established between O₃ and SPC. Radical capture experiments verified that ·OH, ·O₂⁻, ¹O₂, and ·CO₃⁻ played important parts in MB degradation. With increasing reaction time, the amount of total organic carbon (TOC) declined and the amount of ammonia nitrogen (NH₃-N) increased. The addition of SPC enhanced the solution’s pH value and conductivity. The degradation pathway was proposed based on density functional theory (DFT) analysis and relevant literatures. The toxicity of MB was alleviated after O₃/SPC treatment. The O₃/SPC process was also suitable for the treatment of other dyes and actual wastewater.

1. Introduction

The amount of hazardous substances has increased dramatically with the development of industrialization. According to statistics, 10–15% of all dyes are released into the environment during production and use [1]. A great majority of these dyes are very stable and do not easily degrade naturally after entering environmental waters. This can affect the normal life activities of plants and aquatic animals, and even disturb the ecological balance of water bodies [2]. More seriously, dyes are mostly comprised of toxic substances with carcinogenic and teratogenic effects, and their discharge into the environment constitutes a great threat to the health of humans [3]. Therefore, treatment of dye wastewater using efficient technology has become an urgent task.

Various methods have been adopted for the treatment of dye wastewater, such as adsorption [4,5], membrane separation [6,7], ion exchange [8,9], biological treatment [10,11], and advanced oxidation processes (AOPs) [12,13,14]. Although activated carbon and other physical adsorbents have good adsorption effects, most adsorbents are not ideal for the adsorption of soluble dyes [15]. They easily adsorb large molecular weight and non-polar dyes, but the cost is expensive, making it difficult to use them for the treatment of concentrated pollutants. Membrane separation technology was considered as a new high efficiency separation, concentration, and purification technology, having the advantages of high separation efficiency, simple process, convenient operation, easy control, and no pollution [16]. However, in practical application, the main disadvantages are high investment and operation costs, membrane clogging, advanced pretreatment, and regular chemical clean. Ion exchange mainly refers to the exchange of ions between wastewater and adsorbent. Although ion exchange can effectively remove dye, the cost of this method is higher, and the exchange membrane needs backwashing and regeneration [17]. Biological methods have been widely studied for dye wastewater treatment because of their low cost, simple operation, and environmental friendliness [18]. The traditional biological treatment technology has been mature, whereas it is difficult to achieve a satisfactory treatment effect since dye wastewater has the characteristic of high concentration and refractory biodegradation. Moreover, the relatively long treatment times and strict reaction conditions have also restricted their development [10].

Currently, AOPs have been deemed some of the most promising technologies for wastewater treatment, including Fenton [19], cavitation [20], ozonation [21], and photocatalytic oxidation [22]. Among these methods, ozonation is usually considered an eco-friendly and practical process for the elimination of dye contaminants. However, its ability to oxidize some dye molecules or their intermediates is very low, leading to limited mineralization efficiency. Moreover, the formation efficiency of free radicals from ozone was relatively low, especially in the acidic pH range. In recent years, the peroxone process has attracted increasing attention because it effectively improves the efficiency of ozone degradation and the mineralization of pollutants [23,24,25]. In the presence of H₂O₂, more active free radicals will be generated because of accelerated ozone decomposition, which can significantly accelerate pollutant oxidation. However, the rapid decomposition and explosion of liquid H₂O₂ in the process of transportation and storage will bring safety risks. Special facilities are required and relevant practitioners need to be trained before dealing with liquid H₂O₂, which leads to additional costs and limits the practical application [26]. Therefore, the search for alternative oxidants to replace the role of hydrogen peroxide in pollutant remediation has aroused great interest. As a promising in situ chemical oxidation oxidant, sodium percarbonate (2Na₂CO₃·3H₂O₂, SP), is considered a potential alternative to H₂O₂, and has a wider application prospect in contaminated water since its operation is safer and does not introduce any additional, potentially harmful, byproducts into the water matrix [27,28,29,30].

O₃ can be generated via various methods, such as the electrochemical method, the photochemical method, and the dielectric barrier discharge (DBD) plasma method [31]. Of those methods, the DBD plasma method is the most widely used to produce O₃ with lower energy consumption, a larger ozone output per unit, and higher market share [32]. Thus, we proposed the use of SP activated by plasma-generated O₃ for dye wastewater treatment. Methyl blue (MB), a typical industrial dye, can cause adverse symptoms, physiological reactions, and even physiological toxicity with excessive exposure to the environment. Therefore, MP was selected as the target contaminant [33]. The effects of ozone dosage, initial solution concentration, pH value, and SP dosage on dye removal were investigated first. Then, the role of the active species was inspected with various scavengers, including p-benzoquinone, isopropanol, triethylenediamine, and indole. Subsequently, the variation of pH, conductivity, total organic carbon (TOC), chemical oxygen demand (COD), and ammonia nitrogen was measured. The degradation pathways were proposed according to density functional theory (DFT) and previous literature. The toxicities of intermediates were estimated based on the US Environmental Protection Agency Toxicity Estimation Software Tool (US-EPA-TEST). Lastly, the application of O₃/SP for other dye degradation and actual wastewater treatment was evaluated.

2. Results

2.1. Effect of O₃ Concentration

O₃ concentration is a critical parameter for the ozonation degradation process. The effect of O₃ concentration on SMX removal was inspected first, and the results are shown in Figure 1a. MB concentration decreased quickly with increasing O₃ concentration, suggesting that higher O₃ concentration was conductive to MB elimination. When concentration enhanced from 2.0 mg/min to 8.0 mg/min, the removal efficiency increased from 55.9% to 81.7%. The kinetic fitting curve is shown as inset Figure 1a, exhibiting that MB degradation was expedited with higher O₃ concentration. When O₃ concentration was 2.0 mg/min, 4.0 mg/min, 6.0 mg/min, and 8.0 mg/min, the kinetic constant reached 0.030 min⁻¹, 0.049 min⁻¹, 0.055 min⁻¹ and 0.067 min⁻¹, respectively. Based on Henry’s law, increasing unit volume concentration could increase mass transfer driving force, thus improving O₃ mass transfer rate from gas phase to liquid phase [34]. As a result, the removal efficiency of MB was enhanced with higher O₃ concentration.

2.2. Effect of MB Concentration

The removal efficiency of various MB concentrations was shown in Figure 1b. It can be seen that higher MB concentration inhibits MB degradation. When the MB concentration was enhanced from 25 mg/L to 50 mg/L, 75 mg/L, and 100 mg/L, the removal efficiency declined from 85.9% to 75.9%, 69.5%, and 62.6%, respectively. Additionally, the removed quantity of MB increased with higher MB concentration, which reached 21.5 mg, 37.9 mg, 52.1 mg and 62.5 mg under MB concentration of 25 mg/L to 50 mg/L, 75 mg/L, and 100 mg/L, respectively. The inset Figure 1b presents the kinetic fitting curve under various MB concentrations, illustrating that the MB degradation rate was reduced by higher MB concentration. Under fixed ozone concentration, the produced active species in water was also definite. With an increase of MB concentration, the amount of pollutants and intermediates in the reaction system would be elevated, and a competitive relationship between MB molecules and their degradation intermediates would exist. However, by increasing the initial concentration, the collision probability between MB and active species was raised. Thus, the absolute removal quality of MB was improved. Therefore, the removal efficiency was reduced, but the absolute removal quality was enhanced with increasing MB concentration.

2.3. Effect of pH Value

The pH value is a crucial parameter for wastewater treatment by ozonation, which affects the existence of active substances in water. Figure 1c depicts the effect of solution pH in the range 2.70–10.68 on MB elimination. With an enhancement of pH value, MB elimination was accelerated, which indicated that the alkaline condition was conductive to MB elimination. The removal efficiency improved from 55.1% to 85.8% when pH value increased from 2.70 to 10.68, and the corresponding kinetic constant increased from 0.026 min⁻¹ to 0.076 min⁻¹ (Figure 1c). As for ozonation, it is widely recognized that direct ozone oxidation is the main process in the acid condition. As for dye decolorization, O₃ could attack the chromophoric group (-N=N-) of dye selectively. Under the alkaline condition, O₃ could be transformed into ·OH, which possesses a higher oxidation potential and a nonselective characteristic. It can open the ring structure of aromatic dyes, which is beneficial to MB elimination [35]. Thus, a better degradation performance was achieved in the alkaline condition.

2.4. Effect of SP Dosage

The effect of SP dosage on MB elimination is exhibited in Figure 1d. There was almost no removal efficiency for MB elimination with sole SP. Compared to the sole O₃ system, SP addition was conductive to MB degradation. With enhanced SP dosage, the removal efficiency of MB improved at first, and then decreased. The optimal SP dosage was verified as 50 mg/L. Under this condition, the removal efficiency of MB was raised from 79.3% to 85.7% with 30 min treatment time. The corresponding kinetic constants were improved from 0.061 min⁻¹ to 0.076 min⁻¹ (Figure 1d), respectively. SP could release H₂O₂ in an aqueous solution (Equation (1)).

2Na₂CO₃⋅3H₂O₂ → 2Na₂CO₃ + 3H₂O₂

(1)

In the O₃ system, when SP was added, the released H₂O₂ reacted with O₃ and generated a peroxygenation reaction, which accelerated the formation of ·OH [36]. In addition, an alkaline solution environment could be generated when SP was added in the O₃ system. It is well known that O₃ was conductive to transform into ·OH (Equations (2)–(7)), which could also promote MB elimination.

O₃ + H₂O₂ → 2⋅OH + 3O₂

(2)

O₃ + OH⁻ → HO₂⁻ + O₂

(3)

O₃ + HO₂⁻ → ⋅HO₂ + O₃⁻

(4)

⋅HO₂⁻ ↔ ⋅O₂⁻ + H⁺

(5)

⋅O₃⁻ + H⁺ → ⋅HO₃

(6)

⋅HO₃ → ⋅OH + O₂

(7)

However, excess SP in O₃ system would block the interaction between the MB molecule and the active substance. On the other side, excess SP would release a large amount of H₂O₂, and it would react with ·OH and consume ·OH (Equations (8)–(12)) [37]. As a result, excess SP dosage hindered MB degradation in the O₃ system.

H₂O₂ → O₂ + H₂O

(8)

H₂O₂ + ⋅OH → ⋅HO₂⁻ + H₂O

(9)

HO₂⁻ + ⋅OH → ⋅HO₂⁻ + OH⁻

(10)

HCO₃⁻ + ⋅OH → ⋅CO₃⁻ + H₂O

(11)

$${\mathrm{CO}}_3^{2-}+\mathbf{·}\mathrm{OH}\to \mathbf{·}{{\mathrm{CO}}_3}^{-}+{\mathrm{OH}}^{-}$$

(12)

Economic benefit is a significant index for evaluating a technology, and therefore energy efficiency under various SP dosages were calculated (

Table 1. Energy efficiency and synergetic factor.

SPC Addition (mg/L)	Energy Efficiency (g/kWh)						Synergetic Factor
	5 min	10 min	15 min	20 min	25 min	30 min
0	16.36	11.95	9.07	7.31	6.15	5.29	——
30	17.53	12.22	9.39	7.40	6.38	5.54	1.14
50	21.33	13.95	10.17	8.04	6.66	5.71	1.20
100	19.43	13.72	9.48	7.53	6.39	5.70	1.11
200	19.43	13.04	9.82	7.81	6.58	5.65	1.05

). It was noted that the energy efficiency in the O₃/SP system was higher than that in the sole O₃ system. When SP dosage was 50 mg/L, the energy efficiency reached 5.71 g/kWh with 30 min treatment time, which was higher than that (5.29 g/kWh) in the sole O₃ system. It was summarized that SP addition could lead to energy conservation for practical application. Synergy factor was adopted to assess the synergy efficiency between various systems [37]. Table 1 also exhibits a synergy effect between O₃ and SP. It can be seen that the highest synergy factor was obtained when SP dosage was selected as 50 mg/L, which could reach 1.20. Furthermore, it was worth noting that all synergy factors were greater than 1, demonstrating that a synergy effect could be formed between O₃ and SP.

2.5. Role of Active Species

For the free radical capture experiment, different concentrations of free radicals were added to the reaction solution, the effect of different concentrations of free radicals on the degradation efficiency was investigated, and the role of free radicals was given. The potential reactive oxygen species generated in the O₃/SP system were evaluated using various radical scavengers. Based on the peroxidation theory, ·OH formation could be promoted by O₃ and H₂O_2, thereby accelerating pollutant degradation. Therefore, isopropanol was selected to distinguish the role of ·OH due to the extreme reaction rate (10⁹ M⁻¹ S⁻¹) between isopropanol and ·OH [38]. The rate is higher than that of isopropanol and other radicals. The corresponding results are shown in Figure 2. It can be seen that MB degradation was greatly inhibited by the addition of isopropanol (Figure 2a). The removal efficiency of MB clearly declined with a higher concentration of isopropanol, declining from 88.5% to 66.0% with 1.00 mmol/L isopropanol (Figure 2b). The result indicated the involvement of ·OH in the degradation of MB.

It has been reported that liquid O₃ could transform into ·O₂⁻, which possesses higher oxidation capacity, allowing it to participate in contaminant decomposition [39]. Herein, a typical ·O₂⁻ scavenger, p-benzoquinone was adopted to inspect the role of ·O₂⁻, and the result is shown in Figure 3. It can be seen that, after p-benzoquinone addition, MB removal efficiency was diminished, and the inhibition was strengthened by enhancing the p-benzoquinone concentration (Figure 3a). The removal efficiency was significantly reduced from 88.5% to 30.2% with 3 mmol/L p-benzoquinone (Figure 3b), suggesting that ·O₂⁻ was definitely involved in the destruction of MB.

Recent literature has shown that ¹O₂ with higher oxidation potential could decompose organic compounds through electrophilic reactions [40]. In the O₃/SP system, ·OH could react with ·O₂⁻ and lead to the generation of ¹O₂ (Equations (13)–(15)). Therefore, in order to clarify the role of ¹O₂, triethylenediamine was chosen as the ¹O₂ scavenger. The effect of triethylenediamine addition on MB elimination is depicted in Figure 4. MB degradation was significantly impeded in the presence of triethylenediamine (Figure 4a). When the concentration of triethylenediamine was 1 mmol/L, 2 mmol/L, or 3 mmol/L, the removal efficiency of MB declined from 88.5% to 46.5%, 37.5%, and 30.2%, respectively (Figure 4b). Therefore, it can be confirmed that ¹O₂ acted as a critical radical responsible for MB degradation.

·O₂⁻ + ·OH → ¹O₂ + OH⁻

(13)

·O₂⁻ + ·HO₂ + H⁺ → ¹O₂ + H₂O₂

(14)

·HO₂ + ·HO₂ → ¹O₂ + H₂O₂

(15)

Na₂CO₃ and H₂O₂ could be released when SP was added in water. The generated ${\mathrm{CO}}_3^{2-}$ could react with ·OH, or HCO₄⁻ could self-decompose and form ·CO₃⁻ (Equations (16) and (17)) [29], which could contribute to organic contaminant degradation. To inspect the role of ·CO₃⁻, we conducted the experiment with indole (10⁶ M⁻¹ S⁻¹) present, a known quencher of ·CO₃⁻ [41]. The result is given in Figure 5. The addition of indole evidently decreased the removal efficiency of MB (Figure 5a). Approximately 88.6% removal efficiency could be achieved after 30 min treatment time without indole, which declined to 56.0% with 5.0 mmol/L indole (Figure 5b). These results suggested that ·CO₃⁻ might account for MB degradation in O₃/SP systems.

$${\mathrm{CO}}_3^{2-}+\mathbf{·}\mathrm{OH}\to \mathbf{·}{{\mathrm{CO}}_3}^{-}+{\mathrm{OH}}^{-}$$

(16)

HCO₄⁻ → ·CO₃⁻ + OH

(17)

2.6. UV-Vis Spectra, TOC Removal and Fluorescence Excitation-Emission Matrix Spectra

MB degradation processes at different regular time intervals were estimated by UV-Vis absorption spectra, as shown in Figure 6a. The main peak was located at 309 nm, with a gradual decrease in absorption intensity with an increase in reaction time. These phenomena suggested that MB molecules were decomposed efficiently. Additionally, it is worth noting that the intensity located at the period from 222–268 nm raised as time went on, compared to the original intensity, illustrating that degradation intermediates could be generated. In addition, in the process of degradation, a decolorization of the MB solution occurred, changing from dark blue to light blue to colorless, which meant that the concentration of MB decreased. Figure 6b presents the TOC value under various reaction times. With prolonging treatment time, TOC value was dropped. After 30 min treatment time, TOC value was reduced from 6.3 mg/L to 5.1 mg/L, indicating that MB molecules can be mineralized by O₃/SP, although many degradation intermediates were produced.

Fluorescence excitation-emission matrix spectroscopy was evaluated to investigate the decay of MB [42]. Figure 7a exhibits the spectra in the range of emission wavelength 350–600 nm and excitation wavelength 250–600 nm, the fluorescence peaks corresponded to the profile of a humic acid-like substance, suggesting that the structure of MB molecules was similar to that of humic acid [42]. With increasing the reaction time, the peak intensity declined, illustrating that the humic acid-like molecular structure can be destroyed (Figure 7b,c). After 30 min treatment, the fluorescence peaks shifted, possibly due to the interference of degradation intermediates (Figure 7d). An analogous phenomenon was also observed by Zhu, et al. [43]. Therefore, it can be concluded that MB molecules can be successfully decomposed by the O₃/SP process.

2.7. Variation of pH, Conductivity, and Ammonia Nitrogen

The changes in pH and conductivity under variation reaction systems were shown in Figure 8. Compared to the O₃ system, the O₃/SP system obtained a higher pH value. Meanwhile, the conductivity of the MB solution was enhanced after SP addition. These results are attributed to the released ${\mathrm{CO}}_3^{2-}$ and Na²⁺ in the MB solution, which could enhance the pH value and the conductivity. With increasing treatment time, the pH value decreased. After treatment for 30 min, the pH value declined from a range of 9.64 to 6.81 in the O₃/SP system. This was attributed to the generation of organic acid and inorganic acid during MB degradation [44]. Additionally, the conductivity was increased from 93.2 μS/cm to 144.8 μS/cm in the O₃/SP system after treatment for 30 min. During MB degradation, NH₄⁺, SO₄²⁻, and Na⁺ can be produced, which would enhance conductivity. From Figure 9, it was found that the amount of NH₃-N increased with the delay of processing time, which may be because the nitrogen atom that exists in MB molecules could be mineralized into the free form of NH₃-N.

2.8. Degradation Pathway

To explore the degradation pathway of MB, a DFT analysis with the CASTEP module and the Dmol 3 module was performed, illustrating the f⁰, population, and length of MB molecules. It is worth mentioning that the MB molecule was divided into three major modules, and two of the major modules were similar. The two major module divisions are shown in

Table 2. DFT analysis results of MB molecule with DMol3 module.

Molecular Formula and Atomic Number	Atom	f0	Atom	f0	Atom	f0
	C(1)	0.028	C(11)	0.016	H(21)	0.041
	C(2)	−0.009	C(12)	0.024	H(22)	0.032
	C(3)	0.018	C(13)	0.012	H(23)	0.052
	C(4)	0.025	C(14)	0.002	H(24)	0.051
	C(5)	0.022	S(15)	0.060	H(25)	0.043
	C(6)	0.016	O(16)	0.077	H(26)	0.013
	C(7)	0.085	O(17)	0.084	H(27)	0.045
	N(8)	0.022	O(18)	0.058	H(28)	0.029
	C(9)	0.020	H(19)	0.042	H(29)	0.054
	C(10)	0.024	H(20)	0.015

and

Table 3. DFT analysis results of MB molecule with CASTEP module.

	Bond	Population	Length (A)	Bond	Population	Length (A)
	H2-H8	−0.20	0.73	C11-C12	1.00	1.54
	H11-O3	0.45	1.11	C2-C4	0.93	1.54
	H7-C9	0.82	1.14	C4-C6	1.08	1.54
	H10-C12	0.82	1.14	C8-C10	0.99	1.54
	H5-C7	0.79	1.14	C5-C6	0.93	1.54
	H9-C11	0.82	1.14	C9-C10	1.04	1.54
	H4-C6	0.80	1.14	C1-C3	1.08	1.54
	H6-C7	0.79	1.14	C2-C7	0.99	1.54
	H1-C1	0.82	1.14	C1-C2	0.93	1.54
	H8-C10	1.10	1.14	C12-C13	0.99	1.54
	H3-C4	0.83	1.14	C8-C13	0.95	1.54
	H2-C3	1.09	1.14	O2-S1	0.43	1.78
	C5-N1	0.85	1.50	O3-S1	0.18	1.78
	C8-N1	0.81	1.51	O1-S1	0.44	1.78
	C3-C5	0.93	1.54	C13-S1	0.54	1.81
	C9-C11	0.96	1.54

. Based on calculated results and literature, we proposed the MB degradation pathway, which is shown in Figure 10. Firstly, the MB central aromatic ring was broken and then the side aromatic rings were ruptured, which produced P1 and P4 [45]. Then C=C was attacked by free radicals, leading to the transformation from P1 to P2. Table 2 shows that S (15) possesses relative large f⁰. Table 3 shows that C13–S1 have relatively small populations and large lengths. Therefore, P2 was converted into P3. Then, unstable C=N was attacked by free radicals, which induced the P3 change to P5. On the other hand, a conversion from P4 into P5 occurred, similar to the conversion from P2 to P3. Then, the N atom of P5 was apt to attack, resulting in the formation of P6. Then, the ring-opening reaction process of the intermediates occured, which further converted these intermediates to CO₂, H₂O, SO₄²⁻, NH₄⁺, Na⁺, and the final oxidation products [46,47].

2.9. Toxicity Evaluation

To further understand the toxicity of intermediates, a toxicity assessment of MB and its intermediates was performed using a quantitative structure–activity relationship (QSAR) program based on the QSAR method. In this process, evaluation indexes including oral rat LD₅₀, developmental toxicity, and mutagenicity were chosen. As shown in Figure 11a, aside from P3, the LD₅₀ content of all intermediates were located in the “Toxic” or “Very toxic” regions. It is worth noting that the LD₅₀ content of P4 cannot be predicted due to a limited database. Figure 11b depicts the developmental toxicity of MB and degradation intermediates. It can be noted that P1–P4 were situated at “developmental toxicant”, while P5 and P6 were situated at “developmental non-toxicant”. Figure 11c exhibits the mutagenicity of MB as “Mutagenicity positive”. Surprisingly, the mutagenicities of all intermediates were situated in the “Mutagenicity negative” range. By comprehensive analysis of toxicity evaluation results, it can be summarized that the toxicity of MB could be relieved after O₃/SP treatment.

2.10. For Other Dye Removal and Actual Wastewater Treatment

In order to figure out the feasibility of the technology for the degradation of other dyes, congo red, methyl orange, and rhodamine B were selected. Figure 12a shows that the removal efficiency of all dyes was higher than 90% after 30 min treatment time. Furthermore, the kinetic constant also exhibited a higher value. These results indicated that the current process was also suitable for the treatment of dye wastewater. In order to figure out the O₃/SP process for actual wastewater treatment, we selected underground wastewater from our university. Figure 12b illustrated that COD value decreased with an increase in reaction time. After 30 min treatment, COD declined from 243.3 mg/L to 205.7 mg/L. Therefore, it can be summarized that the O₃/SP process for actual wastewater treatment is feasible by further increasing the treatment time.

3. Materials and Methods

3.1. Chemicals

MB, SP, terephthalic acid, sodium hydroxide, and sodium indigo disulfonate (IDS) were obtained from Aladdin Reagent Co., Ltd. (Shanghai, China) Titanium oxysulfate, hydrogen peroxide 30% aqueous solution, isopropanol (IPA), p-benzoquinone, phosphoric acid, and indole were bought from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China) All aqueous solution was prepared with deionized water, which was prepared by high purity water machine (Biosafer-10R, East Hartford, CT, USA).

3.2. Degradation Analysis

O₃ was formed by a DBD plasma ozone generator (Changqing, CQ-802S, Nanjing, China). The reactor was as a cylindrical form style, and the diameter and height were 8 cm and 15 cm, respectively. The concentration was regulated by an atmospheric sampling instrument (Laobao, QC-350, Qingdao, China). Concentration of MB was detected by UV-Vis spectrograph (METASH, UV-5500PC, Shanghai, China) with a detection wavelength of 311 nm. The sampled time was set as every 5 min. The degradation efficiency was calculated as follows:

$$\mathsf{\eta }=\frac{C_0-C_t}{C_0}$$

(18)

where C₀ is the initial concentration of MB, mg/L; C_t is concentration of MB under t time; mg/L. The degradation kinetic was calculated according to pseudo-first order dynamic model, which is shown as follows [47]:

$$\ln (\frac{C_0}{C_t})=kt$$

(19)

where C₀ and C_t are defined as the same mean-like formula (18); k is the kinetic constant, min⁻¹; t is the sample time, min. The energy efficiency was calculated as follows [48]:

$$\mathrm{E}=\frac{(\mathsf{\eta }\times C_0)}{\mathrm{P}t}$$

(20)

where C₀, η and t are defined as the same mean-like formula (18); P is the power of the plasma generator.

The synergistic effect between O₃ and SP was calculated on the basis of the kinetic constant, shown as:

$$\mathrm{SF}=\frac{k_{{\mathrm{O}}_3+\mathrm{SP}}}{k_{{\mathrm{O}}_3}+k_{\mathrm{SP}}}$$

(21)

where SF is the synergistic factor; k_O3+SP is the kinetic constant for O₃/SP; k_O3 is the kinetic constant for sole O₃; k_SP is the kinetic constant for sole SP. During the experiment, SP was dispersed in MB solution evenly. The conductivity and pH were tested by conductivity meter (INESA, DDS-307A, Shanghai, China) and pH meter (INESA, PHS-3C), respectively. TOC detector (SHMADZU, TOC-V CPN, Kyoto, Japan) was adopted to measure TOC. The water quality detector (Lian hua, 5B-6C, Beijing, China) was utilized to test ammonia nitrogen and COD. The fluorescence characteristic of MB molecules was inspected by fluorescence spectrometer (F97Pro, Lingguang, Shanghai, China).

3.3. DFT Analysis and Toxicity Assays

In order to predict the degradation pathway of MB, f⁰, population and length of MB molecules were calculated based on DFT analysis, which was carried out on Material Studio (MS, version 4.2.1) [49]. The equipped CASTEP module was taken to compute the population, giving the result of population and length. During this calculation, a crystal with parameters of 15 × 15 × 15 A was established. The equipped Dmol 3 module was performed to present Fukui Function, giving the result of f⁰ [50,51]. The intermediates toxicity was assessed by US Environmental Protection Agency Toxicity Estimation Software Tool (US-EPA-TEST) based on oral rat LD₅₀, developmental toxicity and mutagenicity [52].

4. Conclusions

Herein, SP activated by plasma-generated O₃ for dye wastewater treatment was investigated. The effects of ozone dosage, initial MB concentration, pH value, and SP dosage on MB removal was first investigated. A higher removal efficiency of MB was achieved at a higher ozone dosage, lower MB concentration, and higher pH value. The addition of SP could accelerate MB degradation in an O₃ system. Energy efficiency can be enhanced with SP addition. Active species consisting of ·OH, ·O₂⁻, ¹O₂, and ·CO₃⁻ participated in MB degradation. UV-Vis spectra and fluorescence excitation–emission matrix spectra verified that MB molecules could be decomposed successfully. The pH value and conductivity increased after SP addition. The pH value decreased and conductivity increased as treatment time went on. Additionally, the amount of NH₃-N increased with prolonged reaction time. According to DFT analysis and literature, a degradation pathway of MB was proposed, which mainly involved the rupture of the central aromatic ring, C=C, and C=N. Toxicity indexes including oral rat LD₅₀, developmental toxicity, and mutagenicity were assessed. Treatment of other dyes and actual wastewater was feasible by the O₃/SP process.