A Novel Method for the Highly Effective Removal of Binary Dyes from Colored Dyeing Wastewater by Periodic Reversal/Direct Current-Activated Persulfate

Abstract

In recent years, electrochemical synergistic activation of persulfate (PDS) degradation technology has demonstrated significant potential in wastewater treatment applications. Given the challenges posed by the complex water quality, high COD content, and recalcitrant degradation of dyeing wastewater, this study aimed to evaluate the efficacy of iron/aluminum dual-electrode electrochemical activation of PDS for degrading simulated dyeing wastewater. The results showed that under optimal conditions, utilizing both periodic reversal and direct current electrochemical activation of PDS achieved removal rates of 99.2% and 98.3% for Reactive Black 5 (RB5) and Reactive Red X-3B (RRX-3B), respectively, demonstrating promising removal efficiency. Notably, the removal efficiency of RB5 surpassed that of RRX-3B, suggesting a dependence on initial concentration influencing reaction kinetics. Furthermore, full-spectrum scanning and quenching experiments revealed that RB5 and RRX-3B were primarily degraded through the potent oxidation action of SO₄⁻· and ·OH, with a small number of intermediates present in the solution. Periodic reversal proved effective in mitigating electrode passivation and enhancing electrode longevity. This study provides a highly effective removal method of binary dyes from dyeing wastewater by periodic reversal Fe-Al dual-electrode electrochemical activation of PDS technology, offering valuable insights for sustainable treatment of dyeing wastewater with binary components.

1. Introduction

Azo dyes, with a wide array of varieties, account for more than half of total dye production, serving as a main source of dyeing wastewater [1]. Among them, Reactive Black 5 (RB5) and Reactive Red X-3B (RRX-3B) are typical azo dyes extensively utilized in the printing and dyeing industry [2,3]. However, due to their intricate molecular structures, including benzene and naphthalene rings, these dyes present considerable challenges for complete degradation in water. Their persistence poses potential carcinogenic risks, environmental harm, and adverse effects on both human health and ecosystems [4]. Effective treatment of dyeing wastewater containing RB5 and RRX-3B remains a daunting challenge, hindering the progress of azo dyes’ sustainable application in the printing and dyeing industry. Consequently, innovative and cost-effective treatment methods are imperative to address dyeing wastewater efficiently and responsibly.

Some traditional wastewater treatment technologies, such as adsorption, electro-flocculation, etc., are prone to produce secondary pollution. Compared with them, electrochemical methods appear to use cleaner and more sustainable technology. In light of the issues concerning high energy consumption and electrode passivation, the authors proposed employing the synergistic effect of iron and aluminum cations for treating printing and dyeing or pharmaceutical wastewater, which is a promising treatment technology. After 30 min of reaction, the decolorization rate and COD removal rate of RB5-containing dyeing wastewater can reach 99% [5], and for berberine-containing wastewater, they can reach 99% and 95%, respectively [6]. Even after multiple cycles of the polar plate, the degradation efficacy remains satisfactory. The azo bond undergoes decolorization during the reaction with RB5, leaving some intermediates in the solution, while ionic berberine is thoroughly removed via flocculation. Additionally, hydroxyl radicals can be detected in the reaction process, albeit in limited quantities, resulting in modest organic matter degradation [6]. This electrochemical method demonstrates high decolorization efficacy for organic wastewater, alleviation of plate passivation, and reduction of energy consumption, but it requires a prolonged treatment time and is difficult to achieve complete degradation.

Advanced oxidation technologies (AOPs) have gained substantial attention for their exceptional ability to degrade recalcitrant organic pollutants. With the development of AOPs, their definition has expanded to encompass not only hydroxyl radical (OH·) but also superoxide radical (O₂·⁻) and sulfate radicals (SO₄⁻·) in the oxidation process [7]. AOPs based on activated persulfate can generate both SO₄⁻· and OH· simultaneously for efficient organic pollutant degradation. Compared to OH·, SO₄⁻· possesses obvious advantages, such as a limited pH influence range, strong selectivity, prolonged half-life, and stable initial state, and is capable of efficiently degrading certain organic pollutants [8]. Persulfate activation can induce –O–O– bond rupture, generating sulfate radical (SO₄⁻·) via thermal [9,10], UV [11,12], ultrasonic [13,14], and transition metal ion pathways [15,16]. This straightforward process effectively degrades organic pollutants, avoiding secondary pollution and boasting high efficiency, making it a prominent water treatment process in recent years. At present, the most common persulfate activation method involves transition metal ions [17]. Fe²⁺ activation of persulfate has been proven effective in removing organic matter from aqueous solutions, with its catalytic effect continuously validated [18]. However, optimizing the synergistic effect of Fe²⁺ and persulfate to enhance efficacy, as well as reducing agent usage and costs, remains a hot topic among researchers.

With the advantages of electrochemical technology and the deepening research on persulfate activation, electrochemical synergistic persulfate catalytic degradation technology has emerged as a potent solution for organic wastewater treatment. Combined electrochemistry and persulfate in organic wastewater treatment enables the oxidative degradation of organic pollutants into smaller molecules through electrochemical reactions or even complete oxidation into CO₂ and H₂O, etc. Concurrently, strong oxidative radicals produced by activated persulfate aid in organic pollutant removal within the system. Therefore, we can control the release of Fe²⁺ by controlling the electrochemical reaction conditions to solve the drug waste caused by one-time or excessive dosage of Fe²⁺ so as to optimize the degradation process. Moreover, research suggests that Fe³⁺ can be continuously reduced to Fe²⁺ under electric field action, expediting Fe³⁺ regeneration and overcoming Fe²⁺ instability, thus reducing Fe²⁺ dosage and consequent iron sludge production [19]. Rahmani et al. [20] investigated the degradation of Acid Blue 113 via electrochemically activated persulfate, noting a 17% dye removal rate within 2 min with persulfate only, which increased to 31% under electrochemical conditions. Another study employing electrochemical activation of persulfate to degrade Reactive Brilliant Blue (RBB) demonstrated significantly improved removal efficiency. The intermediate metabolites were identified using high-performance liquid chromatography–mass spectrometry during RBB degradation, concluding with mineralization into CO₂ and H₂O [21]. Electrochemical synergic persulfate has attracted much attention for its environmentally friendly and cost-effective properties. However, there are still some deficiencies in the technique, such as limited efficiency and large energy consumption. For example, Li et al. [22] found that the removal rate of lime can reach only 81.7% by electrochemical activation of persulfate with IrO₂ anode. Although the removal rate of basic violet 16 (BV16) can reach 95% via the electrochemical/persulfate process, the electrolysis voltage has to be set at up to 11.43 V [23]. Moreover, most of the research on the electrochemical activation of persulfate focuses on single dyes, but the reports about the treatments of mixed dyeing wastewater with this method are relatively limited. Therefore, a novel high-efficiency and low-energy treatment method is urgently necessary, especially for the treatment of mixed-dying wastewater.

The objectives of this study were to (1) develop a novel high-efficiency and low-energy method for the treatment of binary dyeing wastewater containing RB5 and RRX-3B, based on electrochemically assisted PDS, and (2) explore the removal mechanisms of binary dyes with this method.

2. Materials and Methods

2.1. Chemicals and Reagents

RB5 (C₁₆H₁₀N₂Na₂O₇S₂) and RRX-3B (C₂₇H₂₀N₂Na₂O₇S₂) were purchased from Shanghai McLean Biochemical Technology Co., Ltd. (Shanghai, China) Sodium persulfate (Na₂S₂O₈) and anhydrous sodium sulfate (Na₂SO₄) were purchased from Tianjin Damao Chemical Reagent Factory. Tert-butanol (C₄H₁₀O) came from Shenyang Xinxi Reagent Factory. Methanol (CH₃OH) came from Shenyang Xinhua Reagent Factory. Baking soda (NaHCO₃) was purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd. (Tianjin, China). Potassium iodide (KI) came from Liaoning Quanrui Reagent Co., Ltd. (Jinzhou, China).

2.2. Degradation Experiment

Degradation of RB5 and RRX-3B from binary dye wastewater (100 mg/L RB5, 100 mg/L RRX-3B) was performed in a 500 mL volume beaker with continuous stirring at 400 rpm using a magnetic agitator (Baita Jinchang experimental instrument Co., Ltd., CJ-200, Jintan, China). Aluminum and iron plates served as reaction electrodes and were assembled and secured with insulated rods with a pad spacing of 10 mm. The aluminum electrode was pretreated by polishing with sandpaper, soaking in NaOH and HCl until bubbling occurred, followed by rinsing with distilled water and storing in anhydrous ethanol. The iron electrode was pretreated by polishing with sandpaper, soaking in dilute HCl solution until bubbles formed, rinsing with distilled water, and storing in anhydrous ethanol [18]. The DC power supply was provided by the DC-regulated power supply (Baita Jinchang experimental instrument Co., Ltd., NHWY40-5, Jintan, China), while the periodically reversing power supply was constructed using a clock relay (Shanghai Oufu Electric Co., Ltd., DH48S-S, Shanghai, China), relay (Zhejiang Chint Electric Co., Ltd., RS-NXJ-2Z/C1, Leqing, China) and DC-regulated power supply.

The degradation experiments were conducted using 400 mL of 8 mM sodium persulfate (PDS) as a supporting electrolyte. The initial pH is adjusted by adding 1.0 M sulfuric acid and 0.1 M sodium hydroxide. At specific time intervals (0.5 min, 1 min, 2 min, 3 min, 4 min, 5 min, 8 min, 10 min, 15 min), 5 mL samples were extracted from the reaction system, filtered through a 0.45 µm polyether sulfone membrane, and then the absorbance at 600 nm and 540 nm was measured. Equations (3) and (4) were employed to calculate the effluent concentration of dyeing wastewater with binary components, and subsequently, the removal rates of RB5 and RRX-3B were determined, respectively.

2.3. Quenching Experiment

The electrochemically assisted PDS may produce free radicals, such as SO₄⁻· and ·OH, during degradation of organic pollutants [24]. To assess the impact of SO₄⁻· and ·OH in the PREC/EC-assisted PDS degradation of RB5 and RR-X3B, radical quenching agents were introduced into the reaction system to quantify their effect. Methanol (MeOH) is a highly efficient quenching agent for SO₄⁻· and ·OH [25], while tert-butanol (TBA) effectively quenches ·OH but has a weak effect on SO₄⁻· [26]. Methanol (MeOH) quickly reacts with SO₄⁻· and ·OH, thus serving to quench both radicals. TBA, on the other hand, quenches ·OH to terminate the oxidation reaction. Based on an alcohol-to-persulfate of 25:1, 16.2 mL of MeOH and 38.3 mL of TBA were added. To maintain volume consistency, MeOH and TBA were added as solvents during preparation of dyeing wastewater.

Following the degradation experiment protocol outlined in Section 2.2, a specific quantity of quencher (MeOH and TBA) was added to the water sample to treat for 15 min. Removal effects of RB5 and RRX-3B were measured by sampling at fixed intervals. The removal effects were compared with those without adding quencher, and the types of free radicals involved in the degradation process were analyzed.

2.4. Quantitative Analysis of RB5 and RRX-3B

Simulated dyeing wastewater containing 100 mg/L of single RB5 or RRX-3B, as well as dyeing wastewater with binary components ([RB5]1 + [RRX-3B]2 = 100 mg/L + 100 mg/L) were prepared. Full spectrum scanning (190–800 nm) was performed with UV-Vis photometer on the three simulated dyeing wastewater samples. The result is shown in Figure 1. The results indicated that the λ_max of RB5 and RRX-3B were 600 nm and 540 nm, respectively. At 600 nm, the absorbance of dyeing wastewater with binary components was only affected by RB5 absorbance, while at 540 nm, the absorbance was influenced by both RB5 and RRX-3B. Therefore, the concentration of RB5 was calculated using Formula (1) at 600 nm, and based on the principle of absorbance additivity, RB5 concentration was determined using the wavelength partial overlap Formula (2) at 540 nm.

$$\mathrm{At} 600 \mathrm{nm}: {A_{600}}^{x+y}=0.0209C_x-0.0001$$

(1)

$$\mathrm{At} 540 \mathrm{nm}: {A_{540}}^{x+y}={A_{540}}^x+{A_{540}}^y={{\epsilon }_{540}}^x{c}_x+{{\epsilon }_{540}}^y{c}_y$$

(2)

Among them, ${A_{540}}^{x+y}$ is the absorbance at 540 nm, $A_{600}^{x+y}$ is the absorbance at 600 nm, ${{\epsilon }_{540}}^x$ is the absorption coefficient of RB5 at 540 nm, and ${{\epsilon }_{540}}^y$ is the absorption coefficient of RRX-3B at 540 nm.

Within the absorbance range of 0.2–0.7, the solution is in line with Beer’s law. k is the absorption coefficient.

So, ${\mathsf{\epsilon }}_{\mathrm{x}}^{540}$ = 0.0126, ${\mathsf{\epsilon }}_{\mathrm{x}}^{600}$ = 0.0209, and ${\mathsf{\epsilon }}_{\mathrm{y}}^{540}$ = 0.0122.

According to the absorption coefficient ε of the two components at 540 nm and 600 nm and the absorbance A₅₄₀^x+y of the mixed solution, the concentration of RRX-3B can be calculated, as shown in Equation (2). When the liquid layer thickness b is 1 cm, the concentration of the two components can be calculated by solving equations, as shown in Equations (3) and (4).

$$c_x=\frac{A_{600}^{x+y}+0.0001}{0.0209}$$

(3)

$$c_y=\frac{{A_{540}}^{x+y}-{{\epsilon }_{540}}^{y}b{C}_x}{{{\epsilon }_{540}}^{y}b}$$

(4)

Among them, $c_x$ is the concentration of RB5, and $c_y$ is the concentration of RRX-3B.

2.5. Evaluation of Removal Effect

The removal rates of RB5 and RRX-3B were used as the evaluation index. And the efficiency of the periodically reversing/direct current-assisted PDS to degrade dyeing wastewater in the binary system was investigated. The removal rate (RR) was calculated following Equation (5).

$$\mathrm{R}\mathrm{R}=\frac{C_0-C_1}{C_0}\times 100\%$$

(5)

2.6. Determination of Sodium Persulfate Content

Sodium persulfate in the reaction system was quantitatively analyzed by iodometry. PDS mother liquor with a concentration of 1 mmol/L was prepared, and 1 mL, 2 mL, 3 mL, 3.5 mL, 4 mL, 4.5 mL, and 5.0 mL were transferred into 100 mL volumetric bottles, respectively. Add 0.2 g NaHCO₃ and 4 g KI successively and mix evenly with distilled water. After reaction for 15 min, the absorbance was measured at the wavelength of 352 nm, and the standard concentration–absorbance curve was drawn, y = 20.435x + 0.0867, R² was 0.999, which was used as the basis for subsequent experimental analysis. Under the optimal reaction conditions, Fe-Al double-electrode periodically reversing/direct current-assisted PDS degradation of dyeing wastewater with binary components was carried out. Samples were taken at 1 min, 3 min, 5 min, 8 min, 10 min, and 15 min, respectively, and the remaining sodium persulfate in the water was determined by iodometry. The relationship between the removal efficiencies of RB5 and RRX-3B was analyzed.

2.7. Analysis of Reaction Product

Under optimized activation conditions, Fe-Al double-electrode-assisted PDS degradation of dye wastewater in binary system was carried out. Samples were taken and filtered at 0, 0.5, 1, 2, 3, 4, 5, 8, 10, and 15 min, followed by UV-VIS spectral scanning from 190 to 800 nm to examine spectral variations. Additionally, TOC analysis was performed on the effluent after 15 min of reaction to assess the extent of degradation.

3. Results and Discussion

3.1. System Parameter Optimization

3.1.1. The Influence of Reaction Voltage

The impact of reaction voltage on periodically reversing/DC electrochemically assisted PDS degradation of binary dyes in wastewater is depicted in Figure 2. The effect of the reaction voltage on the removal rate of RB5 and RRX-3B was basically the same under the periodically reversing and DC energizing modes; that is, it increased rapidly at first and then leveled off as the reaction time increased. Mainly in the initial stage of reaction (0~5 min), the concentration of dyes and PDS in the binary system was large, and the electrochemical activation of PDS produced a large number of free radicals such as SO₄⁻· which rapidly interacted with the dye molecules in the system and destroyed their color rendering group, so the removal rate of RB5 and RRX-3B was fast. However, as the reaction progressed beyond 5 min, PDS depletion led to reduced radical generation, weakening the oxidation process and achieving near-complete decolorization of most dyes in the binary system. At this point, continuing the reaction had little effect on the removal efficiency, so the growth of the removal rates of RB5 and RRX-3B entered a flat stage.

The removal efficiencies of RB5 and RRX-3B varied with the increase in reaction voltage in the process of DC-assisted PDS degradation of wastewater. For RB5, the degradation rate of RB5 peaked at U = 1 V within the first 5 min of reaction, reaching 99.2% after 10 min. RRX-3B removal rates displayed an initial increase followed by a decline with rising voltage, achieving a maximum removal rate of 98% at U = 1 V. Due to the suitable increase in reaction voltage, the production of Fe²⁺ increased, and the number of free radicals such as SO₄⁻· generated by activated PDS increased, which enhanced the oxidation capacity of the reaction system, while the H₂ and [H] produced by the cathode increased [27]. In the initial binary system, the dyeing wastewater concentration was high, and the azo bonds in the molecular structure were quickly decolorized by hydrogenation, so the removal efficiencies of RB5 and RRX-3B increased. However, as the reaction voltage continues to increase, the degradation effect decreases. The main reason was that the higher the reaction voltage, the faster the Fe²⁺ production rate and the more SO₄⁻· produced by activated PDS. However, when SO₄⁻· was excessive, quenching might occur and reduce the degradation ability of the activated system.

However, the removal efficiencies of RB5 and RRX-3B remained relatively stable with the increase in reaction voltage in the process of cyclic commutation-assisted PDS degradation. This suggested that the effect of reaction voltage on the periodically reversing electrochemical-assisted persulfate degradation was less than that of direct current. RB5 exhibited superior removal efficiency compared to RRX-3B overall, attributed to differences in their chemical structures. RB5’s simpler molecular structure and lower REDOX potential facilitated decolorization through REDOX reactions compared to the complex molecular structure of RRX-3B, which exhibited lower reaction rates and degradation efficiencies.

In summary, both periodically reversing and direct current-assisted PDS degradation of RB5 and RRX-3B in dyeing wastewater achieved good treatment efficiency, but the periodic commutation mode had a better adaptability to reaction voltage fluctuations. Considering degradation efficacy and energy consumption, an optimal reaction voltage of 1 V was determined for subsequent investigations into other factors.

3.1.2. The Influence of PDS Dosage

The effect of PDS dosage (2 mM, 5 mM, 8 mM, and 10 mM) on the degradation of dyeing wastewater with binary components using periodically reversing/direct current electrochemically assisted PDS is illustrated in Figure 3. The analysis showed that when the dosage of PDS increased from 2 mM to 10 mM, the removal efficiencies of RB5 and RRX-3B showed a similar trend with the reaction time, showing a rapid increase at first (0–3 min) and then a steady trend at 5–15 min. However, at different reaction times, the removal efficiencies of RB5 and RRX-3B increased with the increase in PDS dosage, and the degradation rate also accelerated. This phenomenon can be attributed to the elevated PDS concentration facilitating faster mass transfer to the electrode surface, thereby expediting the electrochemical activation process and the generation of free radicals [28]. When the dosage of PDS was 8 mM and the reaction time was 15 min, the removal efficiencies of RB5 and RRX-3B reached 99% and 98%, respectively. When the dosage of PDS was 10 mM, and the reaction was 15 min, the removal efficiencies of RB5 and RRX-3B were not significantly improved. This was mainly because the increase in the dosage of PDS might lead to the excessive production of SO₄⁻· isoquenching, resulting in a lower utilization rate of PDS [29]. Therefore, the optimal dosage of PDS was determined to be 8 mM based on the principle of efficient treatment and avoiding reagent waste, and it was used for further investigation of other factors.

3.1.3. The Influence of Plate Spacing

When the distance between plates is small, the electron migration distance decreases, and when the same voltage is applied, the current between plates increases as the distance between plates decreases, which facilitates the generation of Fe²⁺, enhancing the activation ability of the degradation system, and improving the degradation effect of dyes in wastewater with binary components. Therefore, in practical engineering, it is always desirable to reduce the distance between plates to improve the degradation effect and reduce energy consumption. However, the reduction of plate spacing is also limited. When the plate spacing is too small, if the solution is doped with solid substances, it is easy to cause a short circuit between electrodes. Therefore, the appropriate plate spacing should be selected based on various factors. Figure 4 shows the influence of pad spacing (0.5 cm, 1.0 cm, 1.5 cm) on the degradation of dyes in wastewater with binary components with periodic commutation/DC-assisted PDS. As the plate spacing increased from 0.5 cm to 1.5 cm, the removal rates of RB5 and RRX-3B showed a similar trend with the reaction time, with rapid initial increases (0–3 min) followed by stabilization (5–15 min).

When the plate spacing was 1.0 cm, the degradation rates of RB5 and RRX-3B were the highest. With the assistance of periodically reversing, the removal efficiencies of RB5 and RRX-3B reached 98.5% and 96.2%, respectively, which were slightly higher than those of RB5 and RRX-3B degraded by direct current-activated PDS (97.8% and 95.7%).

3.1.4. The Influence of Initial pH

Figure 5 shows the effect of the initial pH value (6, 8, 10) on the periodically reversing/DC electrochemically assisted PDS degradation of dyes in wastewater with binary components. It can be observed that when the pH value changes in the range of 6~10, RB5 and RRX-3B achieved good removal efficiencies after 10 min of reaction. When the pH value was 6, the removal efficiencies of RB5 and RRX-3B by DC-assisted PDS reached 99.9% and 99.2%, respectively, which were slightly higher than the removal efficiencies of RB5 and RRX-3B degraded by cycle commutation-assisted PDS (99.8% and 98.8%). When the pH value continued to increase to 8 and 10, the removal rate of RB5 and RRX-3B assisted PDS degradation decreased to a certain extent. This indicated that the acidic environment was conducive to the activation system to degrade the dyes in wastewater with binary components. The main reason was that SO₄⁻·free radicals existed under low pH conditions, and their activities decreased gradually as the pH increased, the activity of SO₄⁻· will gradually decrease, resulting in lower degradation efficiency of organic pollutants. Based on the comprehensive analysis, 6.0 was determined as the optimal pH value, which was used for subsequent research.

Moreover, the actual dye wastewater contains a variety of pollutants, which may have a potential impact on the experimental results and deserve further study.

3.2. Mechanism Analysis

3.2.1. Sodium Persulfate Allowance

Figure 6 shows the change of PDS residual concentration over time in the process of periodically reversing/direct current-assisted PDS degradation of dyes in wastewater with binary components. In the direct current-assisted PDS degradation system, PDS rapidly decreased to 0.092 mM after 1 min of reaction, followed by a slower decline in residual PDS concentration. The results indicated that SO₄⁻·, which was quickly activated by PDS under the activation of Fe²⁺, was used to decolorize the dye molecules, which was the reason for the rapid increase in the removal rate of dyes in wastewater with binary components at the initial stage of the reaction. Although the amount of SO₄⁻· produced by PDS consumption was small after 1 min, the generated SO₄⁻· had a continuous removal effect due to the long residence time of SO₄⁻· and strong oxidation capacity and the removal efficiency continued to increase. However, PDS rapidly dropped to 0.055 mM when the cyclic commutation-assisted PDS degradation system reacted for 1 min. Different from the direct current-assisted PDS degradation system, the residual concentration of PDS continuously decreased with the extension of reaction time. The results showed that the auxiliary mode of periodic commutation had more persistent activation of PDS, which was the reason why its degradation effect on dye wastewater of binary system was better than that of direct current, which was consistent with the analysis results in Section 3.1.1.

3.2.2. Identification of Free Radicals

Figure 7 shows the effect of quenching agents (MeOH and TBA) on the degradation of RB5 and RRX-3B by cyclic commutation/DC-assisted PDS. As shown in the figure, the inhibitory effect of MeOH on the degradation of RB5 and RR-X3B in the reaction system was significantly higher than that of TBA. The quantified data of inhibition effects of MeOH and TBA on the degradation of RB5 and RRX-3B in the system are presented in

Table 1. Identification and semi-quantitative analysis of SO₄⁻· and ·OH (measured by reaction 15 min).

Mode of Electrify		Periodically Reversing		Direct Current
Type of Dye		RB5	RRX-3B	RB5	RRX-3B
➀	Blank sample	98.8	96.92	98.27	96.19
➁	TBA	96.78	89.66	97.55	92.92
➂	MeOH	61.83	49.94	68.09	35.88
(➀–➁)	The role of ·OH	2.02	7.26	0.72	6.31
(➁–➂)	The role of SO4−·	34.95	39.72	29.46	57.04

, based on the reaction of 15 min.

Therefore, it can be inferred that the role of SO₄⁻· in the degradation of RB5 and RRX-3B by periodically reversing/DC electric-assisted PDS is much greater than that of ·OH.

In the electrochemically assisted PDS process, the dioxo bond of sodium persulfate can be broken by the input of external electrical energy (Equation (6)), while persulfate generates SO₄⁻· by electron transfer at the cathode (Equation (7)) [30].

$${{{\mathrm{S}}_2\mathrm{O}}_8}^{2-}+\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\to {2{\mathrm{S}\mathrm{O}}_4}^{-}·$$

(6)

$${{{\mathrm{S}}_2\mathrm{O}}_8}^{2-}+e^{-}\to {{\mathrm{S}\mathrm{O}}_4}^{2-}+{{\mathrm{S}\mathrm{O}}_4}^{-}·$$

(7)

The advanced oxidation technology of activated persulfate is mainly attributed to SO₄⁻·, but in the process of electrochemical activation of persulfate, ·OH will be generated due to the difference in electrode material and pH [31]. Its main sources are divided into two parts: first, the anode with high oxygen extraction potential will electrolysis water to produce ·OH, and second, under neutral or alkaline conditions, SO₄⁻· will react with water to generate ·OH (see Equations (8) and (9)) [32].

$${{\mathrm{S}\mathrm{O}}_4}^{-}·+{\mathrm{H}}_2\mathrm{O}\to {{\mathrm{S}\mathrm{O}}_4}^{2-}+{\mathrm{H}}^{+}+·\mathrm{O}\mathrm{H}$$

(8)

$${{\mathrm{S}\mathrm{O}}_4}^{-}·+{\mathrm{O}\mathrm{H}}^{-}\to {{\mathrm{S}\mathrm{O}}_4}^{2-}+·\mathrm{O}\mathrm{H}$$

(9)

3.2.3. Reaction Product Analysis

Figure 8 shows the changes in UV-VIS spectral scanning patterns at different reaction times during the periodically reversing/DC-assisted PDS degradation of dyes in wastewater with binary components. It can be observed that as the reaction time increased, the characteristic absorption peaks at 600 nm and 540 nm gradually decreased until they disappeared. This indicated that the –N=N– structure of the chromogenic group of RB5 and RRX-3B was destroyed in the degradation process of the electrochemical-activated PDS system, thus realizing the decolorization of the dye wastewater [33]. Meanwhile, the absorption peak slightly varied in the wavelength range of 230~330 nm, suggesting that some intermediate products were formed in the degradation process. Measured by reaction 15 min, the specific values of TOC, RB5, and RRX-3B removal rates in the effluent of dye wastewater of the binary system are shown in

Table 2. Changes in the removal rates of TOC, RB5, and RR-X3B (measured by reaction 15 min).

Mode of Electrify		Periodically Reverse (T = 10S)	Direct Current (T = 0S)
removal rates	TOC	68.07	71.53
	RB5	99.04	98.27
	RRX-3B	96.92	96.19

. The analysis revealed that the removal efficiency of TOC was much lower than that of RB5 and RRX-3B. This showed that the removal rate of RB5 and RR-X3B in the dye wastewater of the binary system was higher because of the destruction of the –N=N– color group, but intermediate products were generated and not fully degraded.

4. Conclusions

In this paper, the single-factor method was applied to optimize the process conditions of periodically reversing/direct current-assisted persulfate degradation of dyes in wastewater with binary components. At a reaction voltage (U) of 1 V, PDS dosage (m) of 8 mmol/L, stirring speed (v) of 400 r/min, electrode plate spacing (d) of 1.0 cm, pH of approximately 6.0, an initial concentration (C₀) of 200 mg/L, and a volume (V) of 400 mL, after 15 min of reaction, the removal rates of RB5 and RRX-3B by periodic reversal and direct current electrochemical activation of PDS were 99.2%, 98.3%, 98.3%, and 96.2%, respectively, and the removal efficiencies of RB5 and RRX-3B were ideal and stable. Compared with aluminum/iron double-electrode periodically reversing electrochemical technology, the periodically reversing/DC-assisted persulfate degradation of dyes in wastewater was more rapid and efficient, indicating the feasibility of this method for the treatment of wastewater with binary components. In comparison, the periodic commutation-assisted PDS method had a slightly better degradation effect than the direct current method. By quantifying the effects of SO₄⁻· and ·OH, it was inferred that the SO₄⁻· played a much greater role than ·OH in the degradation of RB5 and RRX-3B by the periodically reversing/DC electrochemical method by PDS.