Remediation of persistent organic pollutants in aqueous systems by electrochemical activation of persulfates: A review

Journal of Environmental Management 260 (2020) 110125

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Research article

Remediation of persistent organic pollutants in aqueous systems by electrochemical activation of persulfates: A review Dan Zhi 1, Yinghui Lin 1, Li Jiang 1, Yaoyu Zhou *, Anqi Huang , Jian Yang , Lin Luo College of Resources and Environment, Hunan Agricultural University, Changsha, 410128, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Electrochemical activation Persulfates Environmental remediation Mechanisms Influencing factors Practical application

Sulfate-radical-based advanced oxidation processes (SR-AOPs) have been widely applied in environmental remediation during the past decade, especially in the degradation of refractory organic contaminants. The electrochemical method, which is considered as one of the most efficient ways to generate sulfate radical, has been extensively investigated for the activation of persulfate recently. This work presented a thorough assess­ ment towards the performance of electrochemically activated persulfate for the removal of persistent organic pollutants (POPs) in aqueous systems. The mechanism and superiority of electrochemically activated persulfates were revealed accordingly. Some major factors (e.g., electrode material, pH, current density, and persulfate concentration) influencing the electrochemical activation of persulfates to remove POPs were also discussed. Considering the increasing quantity of publications on this subject, it is significant to broader guidelines such as the efficiency for practical application, quantization of organic by-products, and cost-effectiveness of the elec­ trochemical method to optimize active persulfate in the water treatment processes.

1. Introduction With the increasing numbers of persistent organic pollutants (POPs) discharged in water environment, relevant environmental regulations and laws become more stringent (Chen et al., 2019; Fuoco and Gian­ narelli, 2019). Accordingly, the development of eco-friendly ways for organic pollutants removal becomes an imperative task. Facing this environmental issue, many researches have focused on advanced oxidation processes (AOPs) to eliminate POPs which are tolerant to traditional treatment processes (Anjali and Shanthakumar, 2019; Babu et al., 2019; Kanakaraju et al., 2018; Mazivila et al., 2019). These techniques are based on the in-situ electro generation of strong reactive oxygen species (ROS) such as hydroxyl radicals (HO�), by using elec­ trochemical energy or other forms of energy input(Kanakaraju et al., 2018; Mazivila et al., 2019). These strong and highly-potent oxidizing radicals enable the efficient removal of a wide range of organic com­ pounds without selectivity (Liu et al., 2019; Wang et al., 2018). Noteworthy, the sulfate radical ( ) is nonselective for oxidizing organic compounds and has similar oxidation-reduction potential (E0 ¼ 2.5–3.1 V vs. NHE) with HO� (E0 ¼ 2.74 V vs. NHE) (Hao et al., 2014; Yu et al., 2016). In addition, the half-life of the with 30–40 μs is

has usually longer than that of the HO� with less than 1 μs, and thus better mass transfer performance and contact chance with the target pollutants. Furthermore, can efficiently react with the target compounds from pH 2.0 to 8.0 due to the pH-adjustment free merit. It has been reported that the formed free radicals (e.g., and HO�) from persulfates activation process can readily react with organic compounds resulting in complete or partial mineralization. Thus, the persulfate activation has been suggested as a more effective approach in the environmental remediation (Hou et al., 2019; Wang and Liang, 2014; Wang et al., 2014a, 2019). So far, persulfates (PS, e.g., perox­ ymonosulfate (PMS, HSO5 ) and peroxydisulfate (PDS, S2O82 )), have been widely employed in the reaction systems of organic pollutants oxidation (Chen and Huang, 2015; Fang et al., 2013; Wang et al., 2019). Generally, PS is usually activated by transition metal (Mnþ) such as copper, iron and zinc to produce (Paul et al., 2014; Wang et al., 2019). However, several disadvantages of the homogeneous activation processes may limit the development of the transition-metal-triggered activated PS processes (Liu et al., 2018; Zhang et al., 2014a). For instance, low-charge ions are difficult to regenerate or recover when they are converted to higher charged ions in reaction system, and thus a

* Corresponding author. E-mail address: [email protected] (Y. Zhou). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jenvman.2020.110125 Received 25 October 2019; Received in revised form 2 January 2020; Accepted 10 January 2020 0301-4797/© 2020 Elsevier Ltd. All rights reserved.

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lot of metal ions are required to activate PS, leading to the increased chemical input and a large amount of intractable excess sludge (Wang and Liang, 2014). In addition, the resultant metal sludge produced from the reaction system has an adverse effect on the environment and re­ quires further disposal (Wang and Liang, 2014). H2O2 has been reported to overcome its advantages to some extent by introducing electric cur­ rent in the electrolytic Fenton process (Liu et al., 2018). Accordingly, the method combining iron activated PS with electrochemical oxidation, namely electric/Fe (II, III)/PS method, has been proposed and success­ fully applied to the removal of toxic and bio-refractory organics (Wu et al., 2012, 2019; Song et al., 2018a, 2018b; Zhang et al., 2014a, 2018). The activation of PS by Eq. (1) or (2) is enhanced by cathodic reduction through continuous electrical regeneration of Fe2þ during electro/Fe (II, III)/PS process (Eq. (3)).

value further declines to below 7 (Farhat et al., 2015; Song et al., 2018a, 2018b). (7) (8) Though HO� can efficiently degrade a broad range of organic pol­ lutants into less-toxic intermediates (Cai et al., 2018; Chen and Huang, 2015), the chain reactions within (Eq. (9) and (10)) can produce more reactive intermediates which may lead to POPs degradation. Similarly, peroxymonosulfate (HSO5 ) might be activated to form owing to that the produced H2O2 may react with inorganic residue and generate other free radicals that react with HSO5 (Song et al., 2018a, 2018b). And these formed free radicals might even participate in subsequent reactions (Song et al., 2018a, 2018b).

(1)

(9)

(2) Fe3þþe →Fe2þ

2HO�→H2O2

(3)

Studies have investigated the mechanisms of electro-activated per­ sulfate reactions for POPs removal from aqueous systems (Cai et al., 2014; Farhat et al., 2015; Song et al., 2018a, 2018b; Matzek et al., 2018). As shown in Fig. 1, some studies indicated that was responsible for the removal of organic contaminants in Fe2þ-activated persulfate systems (Farhat et al., 2015; Song et al., 2018a). Meanwhile, HO� in organic activated persulfate reactions was also acknowledged and deemed to be more efficient than that of sulfate radical occasionally (Cai et al., 2014; Song et al., 2018b; Matzek et al., 2018). Other reactive species such as ROP (organo-peroxide) and R (organic) may originate from HO� and simultaneously, but they had limited potential for POPs removal due to their low concentration and half-life (Matzek et al., 2018). Overall, POPs degradation rates were reported to base on the coexistence of radical oxidation and nonradical oxidation (Cai et al., 2014; Farhat et al., 2015; Song et al., 2018a, 2018b). Normally, radical oxidation should be predominant in electrochemical activation of per­ sulfates (Farhat et al., 2015; Song et al., 2018a, 2018b). As shown in Fig. 2, the adsorbed HO� produce by water dissociation can oxidize most POPs because of its high redox potential (2.7 V vs. NHE) while the interaction between carbon surface functional groups and perox­ ydisulfate also play a significant role in the non-radical oxidation pathway. Additionally, electrochemical reactions are usually happened on electrode surface, and the produced HO� is always absorbed on

At the same time, in the electrochemical oxidation process, the heterogeneous hydroxyl radicals (M(HO�)) formed on the surface of the high oxygen overvoltage anodes (Eq. (4)) such as platinum (Pt), borondoped diamond (BDD) anode, and dimensionally stable anode (DSA), can also degrade target pollutants (Govindan et al., 2014). MþH2O→M(HO�)þHþþe

(10)

(4)

Currently, POPs such as orange 7 (Wu et al., 2012), orange II (Cai et al., 2014), bisphenol A (Lin et al., 2013), sulfamethoxazole (Song et al., 2017; Zhang et al., 2018), aniline (Chen and Huang, 2015), and polychlorinated biphenyls (Fang et al., 2013) have been effectively degraded by the electro-enhanced activated PS process. To the best of our knowledge, the data on the performance of electro-enhanced acti­ vating PS process could not be found in current reviews. Herein, we present the review on this topic, for the remediation of POPs by elec­ trochemically activated persulfates. Initially, an overview of electro­ chemically activated PS, with special emphasis on the mechanisms, was described to design the framework. Superiority of electrochemically activated PS system and some major factors influencing the degradation of POPs were carefully discussed. This review can be conducive to improving the future design of electrochemistry persulfate system, broadening the potential for interdisciplinary cooperation, and over­ coming the obstacles which might meet in future practical imple­ mentation of persulfate activation. 2. Mechanisms of electro-activated persulfate reactions Electrochemistry can achieve energy transfer towards persulfate anion, leading to the cleavage of peroxide bond and forming two (Eq. (5)) (Song et al., 2018a, 2018b; Wu et al., 2019; Zhang et al., 2018). Alternatively, persulfate can also accept one electron via the radiolysis of water or the transition metal (electrons donor) to realize the desired redox reaction, forming a single (Eq. (6)) (Song et al., 2018a; Wu et al., 2019; Zhang et al., 2018). (5) (6) The effects of activated persulfate on organic removal are reported to be improved by the secondary radicals triggered from the re­ actions (Cai et al., 2018; Farhat et al., 2015; Zhang et al., 2018). can readily react with water at a wide range of pH values, generating HO�(Eq. (7)), which is the major active substance under the alkaline condition (Eq. (8)) (Farhat et al., 2015; Song et al., 2018a, 2018b). At near neutral pH values, has similar effects on organic degradation compared to HO�, and is the main reactive specie when the pH

Fig. 1. Mechanism of degradation of pentachlorophenol by hydroxyl radicals and sulfate radicals using electrochemical activation of peroxomonosulfate, peroxodisulfate and hydrogen peroxide. 2

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Fig. 2. Possible mechanisms of electrochemical activated persulfate reaction.

electrode surface rather than dispersed in bulk solution, which is different from the conventional heterogeneous-catalysts-triggered per­ sulfate activation systems (Song et al., 2017).

many previous researches, PS in the sulfate electrolyte was prone to forming on the anode, and the as-formed persulfate (less than 1.0 mmol L 1) was thought to be the primary reactive matter for the removal of contaminants (Wang and Wang, 2017). It was reported that electro­ chemically activated persulfates might reduce the requirements of electrolyte (Bu et al., 2018), and this finding is propitious to the spread of electrochemically activated persulfates process in the drinking water treatment plants (Bu et al., 2018). In addition, researchers also observed that electrochemical activation of sulfate to , in which process the removal rates of some POPs was 10–15 times higher as compared to the HO� based oxidation (Bu et al., 2018; Farhat et al., 2017). Moreover, iron electrodes can be easily introduced into the contaminated area to conduct the in-situ activation of persulfates, thus reducing the potential loss during its transportation procedure or even the injection into the ground (Yuan et al., 2014). Therefore, the oxidation rate of POPs is expected to be promoted by the combination of the electrochemical processes and the persulfates injection (Govindan et al., 2014).

3. Superiority of electro-activated persulfates Myriads of catalytic methods have been applied to activate persul­ fates, such as iron (Rodriguez et al., 2014) or other mineral-based ac­ tivators (e.g., Cu (Duan et al., 2018), Co (Duan et al., 2018), Ti (Ji et al., 2017), Mn (Liu et al., 2016), V (Fang et al., 2017), etc.), heat (Wang et al., 2014b), adding bases (e.g., alkaline, phenols (Ahmad et al., 2013) and quinones (Fang et al., 2013)), UV light (Guo et al., 2014; Lin et al., 2014; Qi et al., 2014), etc. with the oxidation-reduction potential of 2.5–3.1 V vs NHE (similar to HO� 2.7 V vs NHE) is generated from the persulfate ions activated by the above technologies, which possesses the stronger oxidative ability than the PS with the E0 value of 2.1 V vs NHE. Moreover, has a stronger tendency towards the electron-abundant pollutants (e.g., phenols, antibiotics, dyes, etc.), while HO� is prone to participating in various reactions with equal preference. Therefore, for the degradation of many specified organic contaminants, gener­ ated from electrochemical activation is more selective than HO� which generated from electro-Fenton (EF) or other Fenton-based materials (Bu et al., 2016). Compared with other activation technologies, Fe2þ is the most widely applied transition metals because of its features of non-toxicity, abundance and low price (Han et al., 2015). However, the slow regen­ eration rate and the large amount of iron sludge deposition may impede the generation of the reactive oxyanions, thus leading to the limit of Fe2þ as an efficient activator. Heat and UV light are expensive methods and thus too restrictive for the field application (Matzek and Carter, 2016). Although the electrochemically activated persulfates still needs better exploration, more attentions have been paid owing to its effective, cheaper and nontoxic properties compared with other typical activators (Yu et al., 2016). The in-situ generation of the Fe2þ feature not only endows lower energy input, but also improves the utilization efficiency of PS and ferrous ion (Yuan et al., 2014). To the best of our knowledge, the desired concentrations of elec­ trolyte (e.g., NaCl or Na2SO4) in water treatment field are usually much higher than that found in the natural environment (Bu et al., 2018). In

4. Key factors influencing POPs removal by electro-activated persulfate The effects of various reaction conditions (e.g., electrode material, current density, pH and persulfate concentration) on the performance of electrochemically activated persulfates for the removal of POPs are shown in Fig. 3. Generally, the removal efficiencies of POPs can be improved with the increase of current density and persulfate concen­ tration, while the pH change exerts multiple influences within (Hu and Long, 2016). In this section, the influences of electrode material, current density, solution pH and persulfate concentration on POPs removal by electro-activated persulfate were analyzed in detail. 4.1. Effect of electrode material Electrochemical methods have been reported to achieve complete mineralization of various POPs which cannot be degraded by sole activated-persulfate (Antonin et al., 2015; Carter and Farrell, 2015). Combining electrochemically-activated persulfate and advanced elec­ trochemical oxidation can achieve higher synergistic degradation effi­ ciency, thereby reducing electricity usage and input. As shown in 3

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Fig. 3. Effect of key influencing factors on POPs removal by electro-activated persulfate.

Table 1, various electrode materials (e.g., Ti/RuO2-IrO2, Pt, Fe and BDD electrodes) have been applied to POPs removal by electrochemical activation of persulfate (Cai et al., 2014; Chen et al., 2018a, 2018b; Matzek and Carter, 2016; Yuan et al., 2014; Zhang et al., 2014a). Recently, electrochemical activation of persulfate with sacrificial iron electrode has attracted great attention (Cai et al., 2014), in which process iron electrode forms Fe2þ by anodic oxidation reactions to ac­ tivates PS. This anode not only demands lower energy, but also improves the utilization of persulfate and ferrous ion (Yuan et al., 2014). The excessive addition of iron can be avoided by the electrochemical PS activation with a platinum anode (Chen et al., 2014). BDD is also an efficient electrode for electrochemical activation of persulfate, at the surface of which the high reactivity HO� can be formed from water oxidation (Cai et al., 2018; Chen et al., 2018a, 2018b). Although satisfactory removal efficiencies of POPs can be achieved by electro-activated persulfate using the BDD electrode, the electro-activated persulfate reactions may still operate at relatively low current efficiency owing to the mass transfer limitation (Cai et al., 2018;

Chen et al., 2018a, 2018b; Song et al., 2018b). To overcome the mass transfer restrictions, increasing the formation of strong and stable oxy­ anions has been regarded as a better solution, like the surface-confined persulfates on the BDD anode (Serrano, 2014). Besides, additional per­ sulfate activation can be achieved with the regeneration of solid iron at the cathode (Matzek and Carter, 2016).Lin et al. (2018b) reported that immobilizing prussian blue particles onto graphene was effective for activating peroxymonosulfate as it could enhance catalytic activities and even facilitate recovery of acid red 27. With the increase of electrical conductivity and active sites of graphene doped with sulfur/nitrogen, the prussian blue immobilized composite and the sulfur/nitrogen doped graphene were enabled an enhanced activation capability of peroxymonosulfate. Different anode/cathode combinations can affect the electro­ chemical activation of persulfates. Matzek et al. (2018) investigated the persulfate behavior applying a single-cell rotating disk electrode system composed with three cathode/anode association: Pt/Gr, BDD/Pt and BDD/Gr, founding that the reaction rates of persulfate were independent

Table 1 Degradation of various organics by electrochemically activated persulfate. Organics

Anode

Cathode

Applied current

Removal efficiency

References

Acid Orange 7 Bisphenol A Orange II Orange II Landfill leachate Sulfamethoxazole Aniline Dinitrotoluenes Carbamazepine Pentachlorophenol Trichloroethene Sulfamethoxazole Diuron Ciprofloxacin Atrazine Dichlorophenol Carbamazepine Dichlorophonel Diatrizoate

Ti/RuO2/IrO2

Stainless steel

Ti/IrO2-RuO2-TiO2 Carbon Pt

Ti Stainless steel Pt

Fe

Fe Mixed metal oxide Graphite Fe Graphite Stainless steel Zirconium Stainless steel Ti Stainless steel

16.8 mA cm 2 16.8 mA cm 2 16.8 mA cm 2 8.4 mA cm 2 13.9 mA cm 2 100 A m 2 6V 6V 100 A m 2 90 mA cm 2 100 mA 18.4 mA cm 2 30 mA 1.33 mA cm 2 5 mA cm 2 20 mA cm 2 100 A m 2 30 mA cm 2 200 A m 2

>66% >99% 95.6% >90% 62.2% 98.1% 100% 98% >90% 38% 100% 86% >60% 84% 78.2% 100% >80% >75% >90%

Wu et al. (2012) Lin et al. (2013) Cai et al. (2014) Cai et al. (2014) Zhang et al. (2014a) Song et al. (2018a) Chen and Huang (2015) Chen et al. (2014) Song et al., 2017a Govindan et al. (2014) Yuan et al. (2014) Zhang et al. (2018) Yu et al. (2016) Matzek et al. (2018) Bu et al. (2018) Cai et al. (2018) Song et al. (2018b) Chen et al. (2018a), 2018b Farhat et al. (2015)

BBD

4

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of anode material and were higher applying Gr than Pt (Matzek et al., 2018).

of persulfates. 4.4. Effect of persulfate concentration

4.2. Effect of current density

, persulfate plays a significant role in the electroAs a source of activated persulfate process for POPs removal. Studies have shown that POPs removal efficiency can increase with the concentration of employed persulfate (Ji et al., 2015; Song et al., 2017; Yu et al., 2016; Zhang et al., 2014b; Zhao et al., 2014). POPs removal can be inhibited through the self-quenching of or the reaction between and its maternal persulfate molecule (Eq. (12)) when the employed persul­ fate dose is higher than the optimal concentration, thus decreasing the effectiveness of organic degradation (Drzewicz et al., 2012; Moghaddam et al., 2014).

Current density is considered to be a critical factor in the electroactivated persulfate systems due to its easy-controlled properties by operators (Matzek and Carter, 2016). When the electrochemical oxida­ tion reaction is not kinetically restricted by the mass transport of POPs onto the surface of the anode, the reaction rate can be primarily accel­ erated by increasing the current density (Chen et al., 2018a; Frontistis et al., 2018; Huang et al., 2017; Martínezhuitle et al., 2016; Song et al., 2017b). For example, studies have reported that the removal efficiencies of various POPs (e.g., peroxydisulfate, sulfamethoxazole and dini­ trotoluenes) obviously increased with the current density (Chen et al., 2014; Song et al., 2017, 2018a). Meanwhile, the increase of the current density can accelerate the evolution of some side reactions, which may subsequently result in an increase of energy cost and a decline in current efficiency (Cai et al., 2018; Martínezhuitle et al., 2016). Therefore, both current efficiency and removal rate need to be taken into consideration before the desired current density is set (Bu et al., 2018). Additionally, Cai et al. (2018) reported that the increase of current density led to HO� concentration increasing, which resulted in the faster increase of per­ oxodisulfuric acid and caused better PDS generation and activation ef­ ficiency. The current density increasing also resulted in the increase of temperature simultaneously, leading to an increased wastage of per­ sulfate (Cai et al., 2018). Thus, it is also significant to optimize the temperature and current density to stabilize the electro-generated per­ sulfate and avoid the decomposition of persulfate.

(12) Some studies have studied the effects of persulfate concentration on POPs removal by electro-activated persulfate (Song et al., 2017; Yu et al., 2016; Zhang et al., 2014b). Song et al. (2017) investigated car­ bamazepine removal by electrochemical activation of peroxydisulfate at Ti/Pt anode, indicating that increasing peroxydisulfate concentration from 1 mmol L 1 to 5 mmol L 1 improved the degradation of carba­ mazepine while peroxydisulfate alone could not effectively degrade carbamazepine. Yu et al. (2016) reported that diuron removal efficiency increased significantly with the persulfate concentration increasing when diuron was degraded by electro-activated persulfate, and the degradation rates under various persulfate dosages all followed pseudo-first-order kinetics. Zhang et al. (2014a) found that COD removal efficiency increased from 29.4% to 40.1% when perox­ ydisulfate concentration rose from 15.6 to 62.5 mmol L 1 when landfill leachate was treated by electro/Fe2þ/peroxydisulfate process. Matzek and Carter (2016) reported that the reaction rates of organic chemical degradation by activated persulfate actually followed second-order ki­ netics with respect to the analyte and persulfate concentrations while they were almost always reported as pseudo-first-order based on the analyte concentrations. Thus, establishing second-order kinetics and rate constants based on categories of POPs may help minimize the amount of persulfate needed to achieve desired results (Matzek and Carter, 2016).

4.3. Effect of pH The solution pH of the electro-activated persulfate systems may greatly affect the degradation of POPs, even in many situations where pH value is lower than the range desired for alkali activation (Matzek and Carter, 2016). Though degradation in acidic condition can frequently enhance the removal of the POPs by producing HO� (Eq. (11)), the improved POPs degradation can be found at neutral pH (6–8) and slightly basic pH (9–10) based on both the activators and the classes of the POPs (Moghaddam et al., 2014; Xie et al., 2012).

5. Applications of electro-activated persulfate to water treatment

(11) Various studies have investigated the effects of pH on POPs removal by electro-activated persulfate (Chen et al., 2018a, 2018b; Song et al., 2017; Wu et al., 2012; Yu et al., 2016; Zhang et al., 2014a). Wu et al. (2012) studied the degradation of acid orange 7 in aqueous solution by a novel electro/Fe2þ/peroxydisulfate process, and reported that the decolorization rates increased from 0.0070 to 0.0093 mmol L 1 min 1 and the decolorization efficiencies slightly increased from 71.8% to 77.1% when the initial pH value of orange II solution decreased from 9 to 3. Chen et al. (2018a, 2018b) investigated 2,4-dichlorophenol removal by electrochemical activation of sulfate over the BDD anode, and found that the formation of persulfate was higher in acid solution than in alkaline solution. Yu et al. (2016) investigated the diuron removal by electro-activated persulfate using ferrous ion as the homo­ geneous catalyst, indicating that the diuron removal rate decreased from 0.154 to 0.008 min 1 when the initial pH value increased from 3.0 to 11.0. The results might be ascribed to that FeOH2þ and Fe(OH)3 occu­ pied a dominant part instead of Fe2þ when the initial pH value was �2.5, and the generation of FeOH2þ and Fe(OH)3 reduced the concentration of free Fe2þ, thus decreasing the activation of . Similar results were also obtained in the study of Zhang et al. (2014a). They investigated the removal of chemical oxygen demand (COD) of landfill leachate by electro/Fe2þ/peroxydisulfate process, conducting that Fe2þ complexes were generated from the applied ferrous ion at the beginning of the reaction when the pH value was 6.0, which might hinder the activation

Electro-activated persulfates have been widely investigated to remove various POPs (e.g., pharmaceuticals, dyes and pesticides) from simulated water systems at the laboratory scale. In real water systems (e. g., wastewater and groundwater), electro-activated persulfates have also been proposed as a promising and satisfactory technique for POPs removal (Cai et al., 2014; Chen et al., 2014; Wu et al., 2012; Yu et al., 2016; Zhang et al., 2014a). For example, electro-activated persulfate was reported to remove 65.8% of acid orange 7 from wastewater in 60 min (Wu et al., 2012), while an electrochemical Fe-Co/SBA-15-catalyzed persulfate activation system achieved 95.6% removal of orange II in 60 min from the sewage (Cai et al., 2014). The electro-activated persulfate process for diuron removal from wastewater showed that more than 77% of diuron was removed under standard conditions (15 min reaction time, 30 mA applied current, 7.0 initial pH value and 0.5 mmol L 1 persulfate concentration), higher than those obtained by electrocoagulation alone (15%) and persulfate alone (diuron removal was almost negligible), indicating that the electro-activated persulfate process was a highly efficient, promising, novel and environmental-friendly way to degrade diuron compared with the other activation methods (Yu et al., 2016). Satisfactory COD removal efficiency (67.7%) of the landfill leachate was obtained by the electro-activated persulfate process under conditions of 240 min reac­ tion time, 13.89 mA cm 2 current density, 3.0 initial pH value, 62.5 5

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mmol L 1 persulfate concentration and 15.6 mmol L 1 Fe2þ concen­ tration, showing that the electro-activated persulfate process was effective to degrade organic pollutants in landfill leachate concentrate (Zhang et al., 2014a). High mineralization of dinitrotoluenes (>70%) in industrial wastewater and satisfactory trichloroethylene removal in groundwater (>80%) were also achieved by electro-activated persulfate oxidation (Cai et al., 2014). Given that the real water environments are very complicated, more studies are still needed for comprehensive in­ vestigations into the applications of electro-activated persulfates. Although satisfactory removal efficiencies of POPs can be achieved by electro-activated persulfates, the applicability of this technique may be limited to the generally high energy consumption of electrochemical oxidation (Ganiyu et al., 2016; Lin et al., 2018a, 2018b; Wang et al., 2018). Thus, the future design of electrochemistry persulfate system is still needed to improve the current efficiency of electro-activated per­ sulfates reactions. Studies have shown that efficient electrode materials and reaction modes can improve the current efficiency of electro­ chemical oxidation reactions, which may also improve the current ef­ ficiency of electro-activated persulfates reactions (Ganiyu et al., 2016; Lin et al., 2018a, 2018b; Nayak and Chaplin, 2018; Wang et al., 2018). For example, the Ti4O7 anode have been shown to possess similar electrochemical performance with the BDD anode but its deploying price is much cheaper, which may be applied in electrochemistry per­ sulfate systems (Ganiyu et al., 2016; Wang et al., 2018). The flow-through operating mode of reactive electrochemical membrane (membrane made by efficient electrode materials) has been concluded to overcome the diffusional limitation of traditional flow-by mode of flat plate electrodes, which can achieve advection-enhanced mass transfer rates and may be applied in electrochemistry persulfate systems (Lin et al., 2018a, 2018b; Nayak and Chaplin, 2018). More works on the future design of electrochemistry persulfate system still deserve further investigation.

additional cost of sulfate ion removal in order to prevent the possibility of removing POPs and their by-products. Though POPs removal is usually improved with the increase of persulfate dose, the “more is better” method will not only result in the elevated chemical input but the self-scavenging effect of reactive spe­ cies, limiting the removal of targeted POPs. Improved knowledge of secondary reaction rates concerning persulfate and POPs can help to achieve the optimization of persulfate utilization in some conditions. Usually, some POPs are converted into intermediate by-products rather than completely mineralized into CO2 and H2O. Therefore, it is neces­ sary to optimize the adding amount of persulfate, applied current, or initial pH value for a better mineralization rate. On the other hand, the new method of electrochemical persulfate activation can reduce the dose of iron ions and persulfate due to the regeneration merit, so that the iron sludge can be less than the other activation methods. Most impor­ tantly, it can still achieve favorable organic removal efficiency in the system because the Fe2þ ions formed on the iron electrode can efficiently activate persulfate to produce or HO�.Nevertheless, the imple­ mentation of this new method still demands subsequent researches to validate its effectiveness for promoting better activated persulfate removal and to optimize operative parameters for the practical largescale applications in future. Declaration of competing interest There is no conflict of interest associated with this work. CRediT authorship contribution statement Dan Zhi: Methodology, Validation, Data curation, Writing - original draft, Writing - review & editing. Yinghui Lin: Formal analysis, Inves­ tigation, Writing - review & editing. Li Jiang: Formal analysis, Inves­ tigation, Writing - review & editing. Yaoyu Zhou: Conceptualization, Funding acquisition. Anqi Huang: Resources, Investigation. Jian Yang: Supervision. Lin Luo: Writing - review & editing.

6. Outlook and conclusions A wide review of recent published researches confirms that the electrochemical activation of persulfate is a feasible way for the degra­ dation of POPs in water remediation. When applying optimal parame­ ters, complete removal can be readily achieved for some recalcitrant organics by electro-activated persulfate. However, there still remain challenges in optimizing the removal reaction conditions for more effective and cost-efficient pollutants removal in practical systems. Developing effective techniques to keep the concentration of acti­ vator and persulfate can favor the removal and mineralization of the target pollutants. Current density and persulfate concentration are deemed as critical factors in electrochemically activated persulfates due to that operators can control them directly. pH also affects the degra­ dation of some pollutants and the faster degradation can occur under acidic condition. The simultaneous effects of elevating zeta potential, reducing floc size and increasing dewaterability can contribute to the enhanced oxidation capability of reactive HO� formed in electro-Fenton (Chen et al., 2018b). To our knowledge, few studies have studied the influence of pH change on the zeta potentialduring electrochemical activation of persulfate, and more analysis in depth are urgently needed. In addition, the electrochemically activated persulfate processes are ineffective for the removal of hexachlorocyclohexanes isomers and more detailed investigations are needed as well. Researches concerning the benefits and costs of energy input PS removal results might have impact on the application of activated per­ sulfates. Persulfate is usually effective in degrading and removing POPs from the polluted sources. There are few researches on the combination of active persulfate reaction and following sulfate ion removal (e.g., filtration or ion exchange) since the implementation of active persulfate in some systems may require different coupling processes. Due to the fact that combining different processes might be needed for applying activated persulfate in some systems, it is necessary to assess the

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