Effective oxidation decomplexation of Cu-EDTA and Cu2+ electrodeposition from PCB manufacturing wastewater by persulfate-based electrochemical oxidation: Performance and mechanisms

Complex wastewater matrices such as printed circuit board (PCB) manufacturing wastewater present a major environmental concern. In this work, simultaneous decomplexation of metal complex Cu-EDTA and reduction/electrodeposition of Cu 2+ was conducted in a persulfate-based electrochemical oxidation system. Oxidizing/reductive species were simultaneously produced in this system, which realized 99.8% of Cu-EDTA decomplexation, 94.5% of Cu 2+ reduction/electrodeposition and within 3 h treatment under the original solution pH 3.2. Almost complete mineralization (74.1% total organic carbon removal) of the solution was obtained after 3 h of treatment. • OH and SO 4•– drove the Cu-EDTA decomplexation, destroying the chelating sites and �nally it was effectively mineralized to CO 2 , H 2 O and Cu 2+ . The mechanisms of copper electrodeposition on the stainless steel cathode and persulfate activation by the BDD anode were proposed based on the electrochemical measurements. The electrodes exhibited excellent reusability and low metal (total iron and Ni 2+ ) leaching during 20 cycles of application. This study provide an effective and sustainable method for the application of the electro-persulfate process in treating complex wastewater matrices.


Introduction
Printed circuit boards (PCB) are essential basic components in the electronic industry, supporting various components.In recent years, the output value of China's PCB industry has grown by about 20% annually.
In 2006, it was ranked as the largest PCB producer in the world with an output value of nearly 13 billion US dollars (USD).It is expected that the total output value of PCB will reach 40.6 billion USD by 2023 (Huang et al., 2022).The PCB industry is a major water user and also a large wastewater producer.Especially with the development and progress of PCB technology, the industry continues to pursue the miniaturization, high-density, blind and buried holes of PCB.The manufacturing process of these highend PCB is complex, with a variety of chemicals used, resulting in high-strength wastewater with complex composition and great di culty in treatment (Xu et al., 2020;Yaashikaa et al., 2022).PCB manufacturing wastewater contains various organic pollutants with high concentrations, such as surfactants, inks, organic dispersants, metal complexants, etc.At the same time, PCB manufacturing wastewater also contains a large amount of heavy metal ions such as Cu 2+ , Ni 2+ , etc. (Min et al., 2018).These pollutants have brought enormous di culties and high cost to the treatment of PCB manufacturing wastewater.
Complexants, such as ethylenediaminetetraacetic acid (EDTA), have been extensively employed in the PCB industry.The resulting metal complexes have high water solubility and stability at a wide pH range, making them di cult to eliminate from the aqueous phase (Hu et al., 2022).In addition, heavy metalorganic complexes present high toxicity to aquatic organisms (Frei, 2020).Conventional methods, including precipitation, coagulation, ion exchange and adsorption, exhibit inadequate e ciency in eliminating heavy metal complexes (Hu et al., 2022).Thereupon, it is imminent to establish new methods to remove heavy metal-organic complexes in PCB manufacturing wastewater.
Advanced oxidation technologies (AOTs) are a group of chemical treatment approaches that differs from traditional activated sludge treatment technology.It can e ciently treat highly toxic and recalcitrant pollutants and has good development potential and broad application prospects (Miklos et al., 2018).
The free radicals generated by AOTs include hydroxyl radicals ( • OH), sulfate radical SO 4 •-, singlet oxygen ( 1 O 2 ), etc.Among them, • OH and SO 4 •-have high redox potentials.The half-life of SO 4 •-is 3-4 × 10 − 5 s, which is much higher than that of • OH (2 × 10 − 8 s), ensuring that it can effectively react with organic pollutants (Yang et al., 2023).Moreover, the persulfate activation methods for producing SO 4 •-are more extensive than • OH, such as UV activation, ultrasonic activation, metal and metal oxide activation, carbon material activation, etc. (Chanikya et al., 2021).Furthermore, the conditions for activating persulfate to generate SO 4 •-are mild and less affected by the pH and background components in the reaction system (Lee et al., 2020).Therefore, the persulfate-based AOTs have been widely studied due to its wide applicability under various conditions.Among various SO 4 •-production processes, the electro-activated persulfate (EAP) process was extensively studied and considered the most e cient approach for SO  The total volume of Cu-containing wastewater generated by the PCB industry accounts for 24%-58% of the total volume of PCB manufacturing wastewater.If the ionic and complex copper in PCB manufacturing wastewater can be recovered and the organic matter in the complex can be removed simultaneously through some methods, it will reduce the treatment cost of PCB manufacturing wastewater.Among various processes, electrodeposition is recognized as one of the most effective methods for recovering precious metals (Gwak et al., 2018).However, Cu element in the form of metal complexes in PCB manufacturing wastewater can not be effectively recovered by the ordinary electrodeposition method.
For this reason, this study constructed an electrochemical AOTs system to treat an actual PCB manufacturing wastewater, wherein a boron-doped diamond (BDD) electrode was used as the anode to oxidize organic compounds and to achieve decomplexation, while a stainless steel electrode was employed as the cathode to recover Cu by electrodeposition.In addition, sodium persulfate (PDS) was added to the system to form an EAP system to produce SO

2.2.Experimental setup
The schematic diagram of the experimental apparatus used in this work is plotted in Fig. 1.A 500-mL beaker was employed as the electrolysis cell.A commercial BDD electrode (5 cm × 5 cm× 0.5 cm) was used as the anode, which was produced by Hunan Xinfeng Technology Co., Ltd., China.A 304 stainless steel plate (5 cm × 5 cm × 0.5 cm) acted as the cathode for Cu electrodeposition.The anode and cathode were xed with titanium clips to the hard foam pad above the beaker to ensure the complete immersion of electrodes in the reaction solution.The electrolysis cell was powered by a direct current power supply.
Solution was continuously stirred by a magnetic stirrer.The volume of reaction solution was 400 mL.Solution pH was adjusted using 1.0 M NaOH or H 2 SO 4 solution.In uences of key parameters (i.e., electrode distance, initial solution pH, PDS dosage and current density) on Cu-EDTA degradation and Cu 2+ electrodeposition were investigated by batch tests.At pre-set time intervals, solution samples were taken, followed by quick addition of excess Na 2 SO 3 solution (0.1 M) and methanol to extinguish the radical reactions.The solutions were then passed through 0.45 µm micro lters before analysis.Samples that were not measured in a timely manner were stored at 4°C and the measurement was completed within 24 h.

2.3.Analytical methods
TOC was measured by using a TOC Analyzer (Shimazu, TOC-Vcph, Japan).The concentration of Cu 2+ ions were measured using an inductively coupled plasma optical emission spectrometry (ICP-OES) (model ICAP 7000, Thermo Fisher Scienti c Instruments, PA, USA).Cu-EDTA was detected by using an Agilent 1100 high performance liquid chromatography system (HPLC, Agilent Technologies, Wilmington, DE, USA); based on the changes of its concentration, decomplexation ratio was calculated.All electrochemical measurements were conducted in a three electrode cell assembled using an electrochemical workstation (CHI760E, Chenhua Co., Ltd., Shanghai, China).Radical quenching experiments were conducted to differentiate the formed reactive oxidant species (ROS) in the solution by adding 10 mM of quenchers including methanol, tert-butanol or benzoquinone (Yang et al., 2023).

2.4.Calculation of current e ciency and energy consumption
The performance of Cu 2+ electrodeposition on the cathode was evaluated using current e ciency according to Eq. 1 (Farinos and Ruotolo, 2017).
where ŋ is current e ciency, z is the number of charges carried by Cu 2+ ions, F is the Faraday constant (96500 C/mol), n is the moles of electrodeposited Cu, I is current (A), and t is electrolysis time (s).
where E is cell voltage (V), I is the applied current (A), t is reaction time (s), △TOC is TOC elimination (mg/L), and V is solution volume (L).

Results and discussion
3.1.In uence factors

Electrode distance
The electrode distance may has a signi cant impact on the rate of electrochemical reactions, determining the potential difference of electrolytic reactions.Excessive or insu cient electrode distance can have adverse effects on the reactions.In this work, due to limitations in the sizes of the reactor and electrode clamps, electrode distance was varied from 2 to 4 cm to evaluate its effect on the degradation e ciency of Cu-EDTA.The experimental conditions were: initial solution pH = 3.2, [PDS] 0 = 5mM, current density = 12 mA/cm 2 .
As shown in Fig. 2a, electrode distance has no signi cant effect on the removal of Cu-EDTA; the degradation ratio of Cu-EDTA reaches over 99% after 150 min at various electrode distances.The degradation ratio at 3-cm electrode distance is slightly higher than that at 2 cm.From Fig. 2b, it can be seen that at different electrode distances, the removal ratio of TOC differs by less than 2 percentage points after 180 min of reaction, further demonstrating that Cu-EDTA degradation is slightly affected by electrode distance.As shown in Fig. 2c, the electrodeposition e ciency of Cu 2+ is the highest at 3-cm electrode distance with insigni cant differences.After 180 min, the electrodeposition ratio of Cu 2+ is basically the same at various electrode distances.

Solution pH
The solution pH can affect the stability of Cu-EDTA complex, and the conditional stability constant of Cu-EDTA increases with the increase of pH, which is very stable under alkaline conditions (Tang et al., 2020).
In addition, as is well known, pH has a signi cant impact on the pollutant removal e ciency of AOTs.pH not only in uences the distribution type of organic pollutants in the solution, but also affects the generation of free radicals in the AOTs (Darsinou et al., 2015).The effect of initial solution pH on Cu-EDTA degradation and Cu 2+ electrodeposition were investigated under the conditions of electrode distance = 3 cm, [PDS] 0 = 5 mM, current density = 12 mA/cm 2 .
As illustrated in Fig. 3a, the degradation rate of Cu-EDTA is highest at pH 3. It can be observed from Fig. 3c that the trend of Cu 2+ electrodeposition is similar to that of Cu-EDTA degradation, namely the electrodeposition ratio of Cu 2+ gradually declines with increasing solution pH.As displayed in Fig. 3d, solution pH does not exert a signi cant impact on the removal ratio of TOC.This is because the initial solution pH only in uences the degradation rate of Cu-EDTA during the initial stage.
When the reaction lasts for 180 min, Cu-EDTA was almost completely removed under the conditions of various initial solution pH.

PDS dosage
In the process of electro-activated persulfate oxidation, persulfate can generate SO 4 •-to oxidize organic compounds through electron acception, and can also form non-radical ROS (Wang et al., 2022).The effect of PDS dosage on Cu-EDTA decomplexation and Cu 2+ electrodeposition were evaluated under the conditions of electrode distance = 3 cm, initial solution pH = 3.2, current density = 12 mA/cm 2 .
Increasing PDS dosage signi cantly enhances Cu-EDTA decomplexation (Fig. 4a).The Cu-EDTA degradation ratio reaches 68.2% within 180 min in the absence of PDS, whereas it dramatically increases to 99.8% when PDS dosage is 5 mM (Fig. 4a).This result indicates that PDS-derived ROS participate in the decomposition of Cu-EDTA.Cu-EDTA decomplexation was mainly initiated by ROS, and increasing PDS dosage may be conducive to the formation of these oxidative species, and thus promoting Cu-EDTA degradation (Wang et al., 2022).As illustrated in Fig. 4b, Cu 2+ electrodeposition e ciency increased from 44.3-94.5% with increasing PDS dosage from 0 to 5 mM after 180 min.Nevertheless, when PDS dosage is further increased, the e ciency of both Cu-EDTA decomplexation and Cu 2+ electrodeposition are not signi cantly improved.Compared to Cu-EDTA decomposition, Cu 2+ is not completely removed by electrodeposition.After 180 min, the highest electrodeposition ratio is 94.5%, which is lower than the Cu-EDTA removal e ciency.This may be due to that the organic intermediates produced through EDTA oxidation complex with Cu 2+ , resulting in the residue of Cu 2+ in the solution (Huang et al., 2016).As shown in Fig. 4c, the removal ratio of TOC increases to 74.1% with the increase of PDS dosage up to 5 mM after 180 min.When PDS dosage is higher than 5 mM, TOC removal changes insigni cantly and remains at around 80%.This is consistent with the situation of Cu 2+ electrodeposition demonstrated above, which proves that Cu 2 + may form complex with the intermediate product of EDTA degradation, resulting in incomplete electrodeposition of Cu 2+ .

Current density
In electrochemical oxidation processes, current is the driving force behind the loss of electrons in the anode to generate free radicals, and the reduction of electrons in the cathode for metal electrodeposition and persulfate activation.Thus, the effect of current density on Cu-EDTA decomplexation and Cu 2+ electrodeposition were studied under the conditions of electrode distance = 3 cm, initial solution pH = 3.2, [PDS] 0 = 5 mM.
As shown in Fig. 5a, Cu-EDTA degradation e ciency gradually increases with increasing current density.
In addition, Cu 2+ electrodeposition e ciency in 180 min increases with raising current density from 4 mA/cm 2 (51.6%) to 12 mA/cm 2 (94.5%) (Fig. 5b).When current density is further increased, the treatment performance including TOC removal (Fig. 5c) is not signi cantly improved after 180 min.The increase in current density can accelerate the migration and charge transfer process of Cu-EDTA to the electrode surface.Moreover, current density can in uence the generation of • OH (Eq. 1) and SO 4 •-.Nevertheless, excessively high current density may cause side reactions (Eq. 3) and thus lower current e ciency (Lu, 2021).
As shown in Fig. 5d, the cathode current e ciency is the highest at 4 mA/cm 2 , and it gradually decreases with increasing current density.At lower current density, the cathode potential is lower than the hydrogen evolution potential, thus no side reaction that affects the cathodic reduction process.Meanwhile, at 4 mA/cm 2 current density, the reaction system has poor ability to oxidize and degrade Cu-EDTA complex, the main cathode reaction is the reduction of Cu 2+ ions in the complex, so the current e ciency is only 25.8%.At higher current e ciency, the hydrogen evolution reaction may impair the e ciency of cathode copper reduction.Nevertheless, Cu 2+ electrodeposition e ciency reaches 94.5% at 12 mA/cm 2 (Fig. 5b).This indicates that the increment in current density promoted the activation of PDS in BDD anode, accelerated the degradation of Cu-EDTA, and subsequently more Cu 2+ ions were released for cathodic reduction, resulting in a higher removal rate of copper.
Energy consumption re ects the amount of electrical energy consumed in the electrochemical process, and is one of the main parameters for applying electrochemistry to engineering practice.As displayed in Fig. 5e, as the current density increases, the energy consumption continues to increase.When current density increases to 20 mA/cm 2 , energy consumption in the reaction system reaches 0.46 kWh/g TOC, which is 2.6 and 5.7 times that of 12 and 4 mA/cm 2 , respectively.This is because that the high current density may cause the BDD anode to shift towards the indirect oxidation of organic compounds by electrolyzing water to produce • OH, while the direct oxidation through electron transfer decreases.The electron transfer e ciency will decline during indirect oxidation, leading to increased energy consumption.From Fig. 5a-c, it can be seen that the increase in energy consumption promotes the removal of Cu-EDTA and TOC as well as Cu 2+ electrodeposition.Therefore, it is necessary to balance the relationship between current density, energy consumption, and pollutant removal performance to ensure the goal of low energy consumption and high removal e ciency (Babu and Nidheesh, 2022; Nidheesh et al., 2023).Thus, 12 mA/cm 2 current density is deemed as the optimum in this work.
Therefore, based on the above results, the optimum operating conditions are as follows: electrode distance = 3 cm, initial solution pH = 3.2, [PDS] 0 = 5 mM, current density = 12 mA/cm 2 , reaction time = 180 min.It should be noted that the pH value of raw wastewater is 3.2, namely there is no need to adjust the wastewater pH before treatment, which will save time and operating costs for wastewater treatment practice.

Identi cation of ROS
Previous demonstrated that some ROS such as SO 4  •-in the system.All in all, • OH is the dominant ROS responsible for Cu-EDTA in the system, followed by SO 4 •-.

Analysis coating composition
The purity of copper produced by electrodeposition affects the value of the recovered copper.During the electrochemical treatment of Cu-EDTA, the coating layer may contain different valence states of copper, such as Cu + and Cu 2+ precipitates (Song et al., 2019).To determine the purity of the recovered copper, the coating layer was stripped off from the stainless steel cathode, dried under vacuum, soaked in 0.5 M HCl solution for 2 h, taken out and dried again under vacuum, resulting in only 0.85% mass loss.
Subsequently, the coating layer was digested with a mixture of HNO 3 :HClO 4 (3:1, v/v) at 145°C for 2 h, then the digestion solution was subjected to ICP-OES analysis.The result shows that the copper content in the coating layer is about 99.1%.Thus, it can be safely concluded that high-purity copper can be obtained by the electrodeposition process in this work, which is conducive to copper recovery from the wastewater.increases with the increase of potential, which can be due to that the oxygen evolution potential of the BDD electrode had been reached, and water began to be electrolyzed to produce oxygen.In the case of PDS/water, the changing trend of the curve is similar to that of water (Fig. 7a).Additionally, for the cases of water and PDS/water, oxidation peaks appear at around 1.6 V, and oxygen starts to evolve at around 2.1 V.These results indicate that adding PDS has no inhibitory effect on the oxygen evolution process of the BDD electrode, and PDS is only activated without competition with water electrolysis.It can be observed that the current response value slightly declines when PDS is added into the wastewater (Fig. 7a), which suggests the activation of PDS by the BDD electrode, leading to competition between PDS and Cu-EDTA on the electrode surface, and thus a decrease in the response current.The oxygen evolution potential of wastewater (around 1.8 V) is lower than that of PDS/water (around 2.0 V) (Fig. 7a), indicating that the addition of PDS may inhibit the side reactions of the BDD electrode (namely the oxygen evolution reaction), which is bene cial for the degradation of pollutants.
Considering the in uence of PDS concentration on the electro-activation process and pollutant degradation, linear voltammetry scanning was performed at an initial PDS concentration of 3, 5 and 7 mM, respectively, while other conditions remained unchanged.As shown in Fig. 7b, the increase of PDS concentration shows a negligible impact on the current response value, and the oxygen evolution potential at three different PDS concentrations is 2.0 V during the electro-activation process.These results demonstrate that the increase in PDS concentration has little effect on the electrolysis of water by the BDD electrode to produce • OH, which also indirectly indicates that the BDD electrode cannot directly activate PDS to produce SO 4 •-.

Chronocurrent measurement
The chronocurrent curve shows the variation of current with time at a constant electrode potential, which can further re ect the changes in current during the electrolysis of different reactants.According to Fig. 7a, the oxygen evolution potential is around 2.1 V (vs Ag/AgCl), thus the BDD electrode potential was set at 1.5 and 2.5 V to explore current changes in the absence and presence of the oxygen evolution reaction, respectively.The experimental conditions are: electrode distance = 3 cm, initial solution pH = 3.2, [PDS] 0 = 5 mM, current density = 12 mA/cm 2 , electrolysis time = 5 min.As displayed in Fig. 8a, the current response is basically the same in the case of of water and PDS/water at 1.5 V, and the current value is very low, indicating no direct oxidation of PDS during this process.In contrast, the current response values of wastewater and PDS/wastewater are signi cantly higher than that of water and PDS/water (Fig. 8a).
When the applied potential reaches 2.5 V, the current response value undergoes signi cant increment under different reaction conditions (Fig. 8b), which is related to the occurrence of oxygen evolution reaction, indicating that this side reaction has an impact on the electrolysis of Cu-EDTA complex.Nevertheless, the current response values of water and PDS/water are very close, and the difference in the current response value between wastewater and PDS/wastewater is enlarged but the overall trend remains consistent with that of 1.5 V (Fig. 8b).These results indicate that the oxygen evolution side reaction affects the oxidation reaction processes to some extent.
On the basis of above analysis, it can be concluded that during the treatment of the PCB manufacturing wastewater by the persulfate electro-activation process at the BDD anode, the activation method of the BDD electrode for PDS is not directly to generate SO  or CuSO 4 dosage = 1-5 mM.As shown in Fig. 9a, when different concentrations of PDS are dosed to the reaction system, all three curves show reduction peaks at − 0.5 V (vs Ag/AgCl), indicating the occurrence of PDS activation on the cathode through accepting electrons; moreover, as the PDS concentration increases, the current response value becomes more negative.The three curves also show reduction peaks at − 0.5 V under different concentrations of Cu-EDTA, indicating that the cathode undergoes Cu-EDTA reduction reactions (Fig. 9b).The current response value is very close at 4 and 6 mM Cu-EDTA, but the negative shift of the response current is signi cant when Cu-EDTA concentration is increased to 8 mM (Fig. 9b).When different concentrations of CuSO 4 are dosed to the reaction solution, the three curves show reduction peaks at − 0.25 V, indicating the occurrence of cathodic electrodeposition of Cu 2+ ions (Fig. 9c).The reduction potential of Cu 2+ (+ 0.34 V) is more positive than H + ions, thus Cu 2 + ions will be reduced and precipitated before the hydrogen evolution takes place.Low concentrations of CuSO 4 may affect the liquid-phase mass transfer process during the electrodeposition, resulting in smaller reduction current (Fig. 9c).
Figure 9d shows the changes of cathodic reduction current in the case of equal concentration dosing (5 mM) of PDS, Cu-EDTA or CuSO 4 .It can be observed that the potential of the current peak generated by three different substances at the cathode is − 0.5, − 0.5 and − 0.25 V (vs Ag/AgCl), respectively.The reduction potential of CuSO 4 is greater than that of Cu-EDTA, suggesting that the formation of Cu-EDTA complex causes a negative shift in the equilibrium potential of Cu 2+ ions and thus makes the reduction of Cu 2+ ions more di cult.Thereupon, conventional electrochemical processes cannot achieve good results for metal complexes.The reduction potential of Cu-EDTA complex is identical to that of PDS, indicating the possible competition of them on the cathode surface in the co-existence of both Cu-EDTA and PDS in the reaction system.As negatively charged anions, S 2 O 8 2-ions may induce electrostatic repulsion towards the cathode, whereas the stainless steel cathode cannot adsorb and activate S 2 O 8 2-like carbon material cathodes (Bu et al., 2019).Meanwhile, it is known from Figs. 2-5 that Cu 2+ was continuously reduced and electrodeposited from the beginning of the treatment.Therefore, in the reaction system of this experiment, the cathode mainly underwent the electrodeposition of Cu 2+ ions in the Cu-EDTA complex and free Cu 2+ ions released by the decomplexation of Cu-EDTA, without the occurrence of cathodic PDS activation.

Chronocurrent measurement
To further investigate the cathodic reduction mechanism in actual situations, chronocurrent measurement was conducted on the stainless steel cathode under the conditions of electrode distance = 3 cm, initial solution pH = 3.2, [PDS] 0 = 5 mM, current density = 12 mA/cm 2 , electrolysis time = 5 min, [Cu-EDTA] 0 = 3.78 mM, cathode potential = − 0.6 V (vs Ag/AgCl).As plotted in Fig. 10, the current response is observed in the case of both wastewater and PDS/wastewater with little differences, whereas no current response occurs in the case of water and PDS.These results suggest that the stainless steel cathode has the reduction ability for Cu-EDTA but not for PDS, and there is no competition in the reduction of PDS and Cu-EDTA.From this, it can be inferred that in the reaction system of the present work, PDS was activated by the BDD anode and free Cu 2+ ions, while the stainless steel cathode only undergoes the reduction electrodeposition of complex and free copper without activating PDS.

Evaluation of electrode recycling
The stability and reusability of electrodes are one of the key factors affecting the long-term operational e ciency and the cost of electrochemical reaction systems (Wang et al., 2023).In the present work, the wastewater has strong acidity (pH 3.2), thus the electrodes may be corroded due to long-term acidic conditions during the reaction process, and impurities may be produced and mixed in the electrodeposited copper because of the corrosion of the cathode.Meanwhile, the activity of electrodes may decline during during the repeated use of electrodes.
Thereupon, the BDD anode and the stainless steel cathode were repeatedly used to treat the PCB manufacturing wastewater under the above optimized experimental conditions: electrode distance = 3 cm, initial solution pH = 3.2, [PDS] 0 = 5 mM, current density = 12 mA/cm 2 , reaction time = 180 min.As listed in Table 1, after 20 cycles of use, the removal e ciency of Cu-EDTA and Cu 2 + remains above 95% and 91%, respectively.In addition, the leached concentration of total iron and Ni 2+ is below 0.4 and 0.2 mg/L, respectively (Table 1).These results indicates that the electrodes have excellent stability and can be used for long-term treatment of the PCB manufacturing wastewater.

2 .
As pH increases, the degradation e ciency gradually decreases, which may be related to the stability of Cu-EDTA under alkaline conditions.At the same time, solution pH was monitored in real-time during the experiments.It can be found that solution pH declines to around 4 within 30 min under different initial pH conditions, and then solution pH remains acidic until the end of the experiments (Fig.3b).This is ascribed to the generation of H + ions through the electrolysis of H 2 O by the BDD electrode (Eq. 1) and the activation of PDS (Eq.2), resulting in a gradual decrease in the solution pH(Murugananthan et al., 2008;Lu et al., 2023).

3. 4 .
Mechanism PDS activation by the BDD anode 3.4.1 Linear sweep voltammetry The linear sweep voltammetry was used to study the changes in the current response of the BDD electrode in various reaction systems under the conditions: electrode distance = 3 cm, initial solution pH = 3.2, [PDS] 0 = 5 mM, current density = 12 mA/cm 2 , [Cu-EDTA] 0 = 3.78 mM (namely the wastewater).Figure 7a displays the linear sweep voltammetry curves of the BDD electrode in different reaction media.As shown, at around 2.1 V (vs Ag/AgCl) during water electrolysis, the current response value gradually

3. 5 .
Mechanism of copper electrodeposition on the stainless steel cathode3.5.1 Linear sweep voltammetryThe linear sweep voltammetry was employed to analyze the current response values of the stainless steel cathode in different reduction conditions: electrode distance = 3 cm, initial solution pH = 3.2, current density = 12 mA/cm 2 ; PDS dosage = 3-7 mM, [Cu-EDTA] 0 = 4-8 mM (adding Cu-EDTA to the wastewater), 4 2 (Li et al., 2021))ct with PDS to form a certain transition state substance.This substance can promote the generation of • OH at the BDD electrode, accelerating the removal of pollutants.In addition, the reaction process is also accompanied by electron transfer.Song et al. (2018)reported a similar conclusion, namely persulfate was activated on the surface of the BDD electrode to form a special transition state molecule (PDS*), which can degrade organic compounds, i.e. the nonradical oxidation process of PDS; meanwhile, PDS* can also promote the electrolysis of water at the BDD electrode to produce • OH.Moreover, free Cu 2 + ions released by Cu-EDTA degradation can also catalyze PDS before the electrodeposition to the cathode, since copper is a widely used transition metal for persulfate activation(Li et al., 2021).Nevertheless, due to the di culty in distinguishing free and complex Cu 2 + ions in the solution, it is di cult to investigate the contribution of free Cu 2 + ions released by Cu-EDTA decomplexation to PDS activation, which will be further studies in future work.

Table 1
Effect of cycle number of electrodes for pollutant removal and metal leaching.
In this work, simultaneous Cu-EDTA decomplexation and Cu 2+ reduction/electrodeposition were successfully realized in the electrochemical AOTs system from the PCB manufacturing wastewater.High Cu-EDTA decomplexation and Cu 2+ electrodeposition performances were achieved at mild reaction conditions, due to PDS activation.Radical quenching experiments show that • OH and SO 4•-are largely