Ahmed Mehellou
email: mehellou-ahmed@univ-eloued.dz

Rachid Delimi

Lamia Allat

Ridha Djellabi
email: ridha.djellabi@urv.cat

Abdelkrim Rebiai
email: new.rebiai@gmail.com

Christophe Innocent
email: christophe.innocent@umontpellier.fr

Design of a synergistic combined electrodialysis/anodic oxidation system for simultaneous AMX membrane fouling mitigation and enhanced dye degradation

Keywords: Electrodialysis, Anodic oxidation, Pb/PbO2 anode, AMX membrane antifouling, Synergistic oxidation, Hybrid processes

Electrodialysis (ED) is a common process which is widely applied for water purification and desalination or/and chemical species recovery. Solving the fouling of ion-exchange membranes (IEMs) is the bottleneck of this technology for stable processing along with low maintenance at large scale. This work aims to design combined electrodialysis/anodic oxidation (ED-AO) using as-prepared Pb/PbO2 anode to boost the oxidation of organic dye (methyl orange, MO) and overcome the issue of AMX membrane fouling. Hybrid ED-AO process works synergistically to accumulate, by ion-exchange reaction, MO species in the anodic Pb/PbO2 oxidative compartment to be oxidized and mineralized. As compared to single ED, excellent oxidation rates and membrane anti-fouling activities were found in ED-AO process. The characterization of AMX membrane in single ED shows that the fouling was the main issue behind the low oxidation rate, causing as well a significant increase in the cell electrical resistance (CER), unlike in hybrid ED-AO process. Most of operating parameters including applied current density, initial concentration of MO, pH, supporting electrolyte (Na2SO4) concentration were investigated. The mechanistic pathways of MO oxidation and anti-fouling activity were discussed in depth. Through ten recycling tests, hybrid ED-AO process showed excellent stability and performance.

Introduction

The textile industry is considered as one of the major sources of dyes water pollution.

The amount of dyes discharged in industrial effluents was estimated at around 15% of total annual production [START_REF] Shi | Simultaneous decolorization and desalination of dye wastewater through electrochemical process[END_REF]. Many types of discharged organic dyes are nonbiodegradable which allows their accumulation in the environment, causing several toxic effects [START_REF] Mani | Textile wastewater dyes: toxicity profile and treatment approaches[END_REF]. Numerous conventional methods have been adopted for the treatment of dyes water pollution, such as adsorption [START_REF] Yagub | Dye and its removal from aqueous solution by adsorption: a review[END_REF], membrane filtration [START_REF] Muthu | Membrane Based Methods for Dye Containing Wastewater[END_REF], photocatalysis [START_REF] Saeed | Photocatalysis: An effective tool for photodegradation of dyes A review[END_REF], biological methods [START_REF] Zafar | Azo dyes degradation by microorganisms-An efficient and sustainable approach[END_REF] and other advanced oxidation processes [START_REF] Brillas | Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods. An updated review[END_REF][START_REF] Martínez-Sánchez | Review of recent developments in electrochemical advanced oxidation processes: application to remove dyes, pharmaceuticals, and pesticides[END_REF].

There processes have different pros and cons based on the type of application, nature and concentration of dye and so on. Physical removal of dyes from water by adsorption, coagulation or filtration leads to transfer simply the pollution from liquid to solid, which in turn needs further purification. These multi-processes usually are expensive and non-sustainable. The industrial community is seeking for single process to reduce the pollution without creating secondary environmental or technology issues. Synergism in processes is the result of cooperative effects that are higher than the sum of additive effects, and also synergism could solve some technological problem, allowing a continuous and effecting processing.

Conventional electrodialysis (ED) is ion-exchange electro-membrane based technology that has been widely used for ionized species removal or recovery under electric effect.

At large scale, it is largely used for seawater desalination [START_REF] Baker | Membrane technology and applications[END_REF][START_REF] Al-Amshawee | Electrodialysis desalination for water and wastewater: A review[END_REF], and the recovery of metals or for the separation in several industrial activities [START_REF] Schorr | Desalination: Trends and technologies[END_REF]. A lot of research groups are investigating the intensification of ED process at a laboratory scale for many applications such as the removal of metals and heavy metals [START_REF] Gering | Use of electrodialysis to remove heavy metals from water[END_REF][START_REF] Juve | Electrodialysis for metal removal and recovery: A review[END_REF], fluoride and nitrate from aqueous solutions [START_REF] Xu | Electrochemical removal of nitrate in industrial wastewater[END_REF][START_REF] Aliaskari | Nitrate, arsenic and fluoride removal by electrodialysis from brackish groundwater[END_REF], the production of organic acids [START_REF] Huang | Application of electrodialysis to the production of organic acids: State-of-the-art and recent developments[END_REF][START_REF] Wang | Integration of conventional electrodialysis and electrodialysis with bipolar membranes for production of organic acids[END_REF], whey proteins separation [START_REF] Casademont | Electrodialysis of model salt solution containing whey proteins: Enhancement by pulsed electric field and modified cell configuration[END_REF], and for the environmental protection [START_REF] Dumée | Purification of aqueous amine solvents used in post combustion CO2 capture: A review[END_REF][START_REF] Gurreri | Electrodialysis applications in wastewater treatment for environmental protection and resources recovery: A systematic review on progress and perspectives[END_REF].

ED technique has some considerable drawbacks limiting its effectiveness [START_REF] Al-Amshawee | Electrodialysis desalination for water and wastewater: A review[END_REF][START_REF] Juve | Electrodialysis for metal removal and recovery: A review[END_REF]. One of the main issues of ED is the membrane fouling during the process [START_REF] Oztekin | Wastewater treatment by electrodialysis system and fouling problems[END_REF][START_REF] Wang | Low-resistance anti-fouling ion exchange membranes fouled by organic foulants in electrodialysis[END_REF]. This phenomenon is generally due to the deposition of macromolecules or insoluble compounds on the surface and/or into the membrane. The membrane fouling consequently leads to a significant increase in its electrical resistance caused by the passivation of membrane active functional groups [START_REF] Wang | Low-resistance anti-fouling ion exchange membranes fouled by organic foulants in electrodialysis[END_REF][START_REF] Mikhaylin | Fouling on ion-exchange membranes: Classification, characterization and strategies of prevention and control[END_REF]. This can reduce the efficiency of the process by preventing the transfer of ionic species through the IEMs and also increases the energy consumption [START_REF] Banasiak | Removal of boron, fluoride and nitrate by electrodialysis in the presence of organic matter[END_REF][START_REF] Hansima | Fouling of ion exchange membranes used in the electrodialysis reversal advanced water treatment: A review[END_REF]. In general, the membrane fouling phenomenon can be classified in three types: (i) scaling by mineral precipitation, (ii) adsorption of organic molecules, proteins or colloidal matter and (iii) poisoning of membrane active groups by surfactants [START_REF] Hansima | Fouling of ion exchange membranes used in the electrodialysis reversal advanced water treatment: A review[END_REF][START_REF] Guo | A mini-review on membrane fouling[END_REF]. Under certain conditions, the fouling of the membranes is mostly irreversible phenomenon, which makes these membranes unusable and therefore must be cleaned or replaced. In the membrane and electromembrane processes, the membrane cleaning or replacement operation is extremely expensive and costs between 40 and 50 % of process cost [START_REF] Grebenyuk | Surface modification of anion-exchange electrodialysis membranes to enhance anti-fouling characteristics[END_REF][START_REF] Mallya | Removal of natural organic matter from surface water sources by nanofiltration and surface engineering membranes for fouling mitigation A review[END_REF]. For this reason, it is very essential to search reliable and guaranteed solutions to reduce the fouling phenomenon caused likely by organic matters.

Among the offered solutions, the coupling of ED processes with other nonconventional methods, such as electrochemical oxidation (ECO) [29 31], photocatalysis [START_REF] Aydin | Development of a UV-based photocatalytic electrodialysis reactor for ion separation and humic acid removal[END_REF], electrochlorination [START_REF] Badruzzaman | Innovative beneficial reuse of reverse osmosis concentrate using bipolar membrane electrodialysis and electrochlorination processes[END_REF][START_REF] Llanos | Novel electrodialysis electrochlorination integrated process for the reclamation of treated wastewaters[END_REF], electro-Fenton [START_REF] Leng | Degrade methyl orange by a reverse electrodialysis reactor coupled with electrochemical direct oxidation and electro-Fenton processes[END_REF], bio-electro-Fenton technology [START_REF] Li | Novel bio-electro-Fenton technology for azo dye wastewater treatment using microbial reverseelectrodialysis electrolysis cell[END_REF] and electro-peroxone process [START_REF] Wu | Simultaneous membrane fouling mitigation and emerging pollutant benzophenone-3 removal by electroperoxone process[END_REF]. ECO, which is also known as anodic oxidation (AO), is one of the most efficient, less expensive and environmentally-friendly advanced oxidation processes. The AO advantages make this method is most frequently used for the treatment of wastewaters [START_REF] Moreira | Electrochemical advanced oxidation processes: a review on their application to synthetic and real wastewaters[END_REF]. Furthermore, it has been proven that AO can effectively eliminate the majority of organic pollutants from wastewaters [ ].

Generally, the AO of organic or inorganic pollutants leads to the formation of simpler, non-toxic and biodegradable compounds [START_REF] Zaviska | Advanced oxidation processes for waters and wastewaters treatment : Application to degradation of refractory pollutants[END_REF]. The electrodes used in AO process are insoluble and active or non-active anodes such as boron-doped diamond (BDD), Pt, Gr, Ti/IrO2, Ti/SnO2, Ti/RuO2; PbO2, SnO2, etc [START_REF] Zaviska | Advanced oxidation processes for waters and wastewaters treatment : Application to degradation of refractory pollutants[END_REF][START_REF] Panizza | Applicability of electrochemical methods to carwash wastewaters for reuse. Part 1: Anodic oxidation with diamond and lead dioxide anodes[END_REF]. These electrodes allow a complete oxidation of the most organic molecules into CO2 and H2O. Several authors [START_REF] Hu | Anodic oxidation of organic pollutants: anode fabrication, process hybrid and environmental applications[END_REF][START_REF] Mcbeath | Application of borondoped diamond electrodes for the anodic oxidation of pesticide micropollutants in a water treatment process: a critical review[END_REF] have described the degradation mechanisms of organic compounds by AO which generally takes place into two stages:

1 st stage: Anodic discharge of water and production of hydroxyl radicals ( OH) on the metal oxide electrode surface (M):

(

2 nd stage: Oxidation of organic compounds (R) by OH radicals:

(2)

The different AO advantages make this method a good candidate for combination with conventional ED, in order to realize a simultaneous effective removal of organic pollutants and for mitigating the membrane fouling.

Some recent works have also shown the effectiveness of coupled ED and AO for different uses such as for the removal of antibiotics [START_REF] Shi | In situ electrochemical oxidation in electrodialysis for antibiotics removal during nutrient recovery from pig manure digestate[END_REF], degradation of 2,4dichlorophenoxyacetic acid [START_REF] Llanos | Exploring the applicability of a combined electrodialysis/electro-oxidation cell for the degradation of 2, 4-dichlorophenoxyacetic acid[END_REF][START_REF] Raschitor | Novel integrated electrodialysis/electro-oxidation process for the efficient degradation of 2, 4dichlorophenoxyacetic acid[END_REF], removal of synthetic dyes [START_REF] Shi | Simultaneous decolorization and desalination of dye wastewater through electrochemical process[END_REF] and for the oxidation of mature landfill leachate [START_REF] Bagastyo | Effect of chloride ions on the simultaneous electrodialysis and electrochemical oxidation of mature landfill leachate[END_REF]. For example, Raschitor et al. [START_REF] Raschitor | Novel integrated electrodialysis/electro-oxidation process for the efficient degradation of 2, 4dichlorophenoxyacetic acid[END_REF] tested a novel integrated ED/ECO process to concentrate and oxidize simultaneously the 2,4dichlorophenoxyacetic acid. In this process, the pollutant is transferred through an anionic membrane to an anodic compartment in which it is then concentrated and oxidized. Interestingly, they demonstrated that this combined process improves the degradation rate by more than twice compared to the conventional ECO. Additionally, in our previous work [START_REF] Allat | Mitigation of an anion exchange membrane fouling by coupling electrodialysis to anodic oxidation[END_REF], we studied the removal of sodium dodecyl benzene sulphonate (SDBS) using ED-AO combined system using Ti/SnO2-Sb2O3/PbO2 as oxidative anode. The results showed that the combination enhanced significantly the SDBS removal rate and also limits the membrane fouling.

The present study aims to investigate the integration of AO in ED to boost the oxidation of anionic azo-dye (methyl orange (MO)), and simultaneously overcome the membrane fouling. were carried out on MO solution to unravel the feasibility and effectiveness of ED-AO hybrid process. The influence of some operating parameters on the efficiency of ED-AO process was studied. Finally, the prepared Pb/PbO2 electrode reusability was also tested.

Materials and methods

Chemicals and Reagents

All chemicals used in this study have an analytical grade and all aqueous solutions were prepared by using double distilled water. MO (molecular formula:

C14H14N3O3SNa; molar mass: 327.33 g mol 1 ) obtained from Alfa Aesar was chosen as an organic pollutant. Sodium sulphate (Na2SO4), sodium carbonate (Na2CO3), trisodium orthophosphate (Na3PO4) and sodium chloride (NaCl) were from BIOCHEM-Chemopharma. Sulfuric acid (H2SO4) (96%), hydrochloric acid (HCl) (37%) and sodium hydroxide (NaOH) were purchased from Sigma-Aldrich. Nitric acid (HNO3) (69%) and oxalic acid (C2H2O4) (99.9%) were from Fluka. Hydrofluoric acid (HF) (48%) and Acetone (100%) was respectively obtained from Nice ® and VWR.

Other chemicals including potassium dichromate (K2Cr2O7), silver sulfate (Ag2SO4)

and mercuric sulfate (HgSO4) were purchased from Merck.

Ions-exchange membranes

The cation and anion exchange membranes (CEM and AEM) used in this study are Neosepta CMX and AMX (Tokuyama Soda Co., Japan) respectively. The ionexchange capacities are 1.4 1.7 meq g 1 and 1.5 1.8 meq g 1 for AMX and CMX membrane respectively. The both membranes have a homogenous hydrocarbonated structure (polystyrene-co-3 )

for the CMX and quaternary ammonium groups ( NR3 + ) for the AMX [START_REF] Xu | Ion exchange membranes: State of their development and perspective[END_REF][START_REF] Kariduraganavar | Ion-exchange membranes: preparative methods for electrodialysis and fuel cell applications[END_REF]. Before using, the membranes were pretreated with conventional procedure [START_REF] Mehellou | Affinity of cationexchange membranes towards metallic cations: application in continuous electropermutation[END_REF]. Firstly, the membranes are submerged in NaOH (1 M) under a slight stirring for 24 h, then, in HCl solution (1 M) for the same period. Finally, these membranes are carefully washed with double distilled water and saved in NaCl solution (0.5 M).

Pb/PbO2 electrode preparation

As mentioned before, the anodic oxidation process is carried out using Pb/PbO2

Overall reaction:

(

For this, a cylindrical Pb plate was pretreated according the procedure reported by Awad and Galwa [START_REF] Awad | Electrochemical degradation of Acid Blue and Basic Brown dyes on Pb/PbO2 electrode in the presence of different conductive electrolyte and effect of various operating factors[END_REF] to ensure good formation of PbO2 layer. Initially, the Pb surface was roughened by a mechanical abrasion using sand papers to increase the PbO2 layer adhesion. It was then cleaned with acetone to remove all solid particles and organic matters rested in the surface. Next, the metal surface was washed carefully with an alkali solution containing a mixture of Na2CO3 (20 g L 1 ), NaOH (50 g L 1 ), Na3PO4 (20 g L 1 ) and H2SO4 (2 g L 1 ). Then, it was cleaned by mixed acid solution (HNO3 (400 g L 1 ), HF (5 g L 1 )) for 2 min and boiled in oxalic acid solution (100 g L 1 ) for 5 min. Finally, in order to oxidize the Pb surface into PbO 2 , the pretreated Pb plate was placed as anode into electrolysis with H2SO4 solution (10 %) at a stable current density (i) of 50 mA cm 2 and duration of 1.5 h [START_REF] Quiroz | Electrocatalytic oxidation of p-nitrophenol from aqueous solutions at Pb/PbO2 anodes[END_REF][START_REF] Vazquez Gomez | Anodic oxidation as green alternative for removing diethyl phthalate from wastewater using Pb/PbO2 and Ti/SnO2 anodes[END_REF][START_REF] Wang | Electrocatalytic degradation of aspen lignin over Pb/PbO2 electrode in alkali solution[END_REF]. A stainless steel (SS) electrode was used as the cathode current feeder.

ED and ED-AO experimental set-up

A three-compartment cylindrical Plexiglas cell (Fig. 1(a)) was constructed in the laboratory. These compartments were separated by a pair of AMX and CMX membranes. Each separating membrane had an effective area of 9.61 cm 2 which equal to the area of the electrodes placed at the ends of E1 and E2 compartments. The diameter and the thickness of each compartment were 3.5 and 1.4 cm, respectively, where the volume is 13.45 cm 3 . In the case of ED process, two SS electrodes are used.

However, in the ED-AO process, the cathode is SS electrode, while the anode is made of prepared Pb/PbO2 electrode.

The new ED configuration applied in this study (Fig. 1(a)) is based in the circulation of feed solution (0.5 L containing MO and Na2SO4 in all experiments) firstly into the anodic compartment (E1), and it then circulates in the cathodic compartment (E2).

Finally, this solution is returned in the same tank by performing a closed circuit. The flow rate of feed solution is fixed at 40 mL min 1 . In the central compartment (C) (concentrate compartment) which delimited by CMX and AMX membranes circulates 0.25 L of H2SO4 (0.05 M). All solutions were agitated in their tanks and injected into the cell compartments by using peristaltic pumps (Masterflex L/S). A stabilized power supply (YD18303D DC Power Supplies) was used to apply an electric field between the electrodes. The current intensity and voltage are controlled by multimeters (Multimetrix DMM121). The duration of all experiments (ED and ED-AO) was fixed at 180 min and regular samples of 5 mL were taken from the feed solution. electrode was investigated. For this, we subjected it to ten cycles of use in the ED-AO coupling process.

Analytical methods

A morphological characterization of prepared Pb/PbO2 electrode was carried out to confirm the formation of PbO2 layer on the electrode surface. After its preparation, the Pb/PbO2 electrode surface was softly rinsed with double distilled water and dried at room temperature for a night and analyzed by Scanning electron microscopy (SEM)

and by energy dispersive X-ray spectroscopy (EDS) methods. To observe the AMX membrane fouling phenomenon by MO molecules deposition, the AMX pristine membrane and those used in the ED and ED-AO processes were analyzed by Fourier transform infrared (FTIR) and SEM-EDS methods. For this, the membranes samples were dried under vacuum at 50 °C for 4 h [START_REF] Choi | Structural change of ion-exchange membrane surfaces under high electric fields and its effects on membrane properties[END_REF] and were then analyzed. MEB-EDS and FTIR characterization was respectively done using a Phenom Pro X Scanning Electron

Microscope and an Agilent Technologies Cary 630 FTIR Spectrometer.

The membrane fouling phenomenon leads to the increase of its electrical resistance as well as increase in the cell electrical resistance (CER) [START_REF] Wang | Low-resistance anti-fouling ion exchange membranes fouled by organic foulants in electrodialysis[END_REF]. It is interesting to follow the CER variation during the processes. The CER variation was calculated by utilizing the voltage measured at regular time intervals during the processes and by Ohm s law application (Eq. ( 6)) :

Where U is the voltage measured during the processes and I is the applied current intensity.

For determinate the decolorization efficiency, the samples of MO feed solution were analyzed by using a UV-Vis spectrophotometer, SP-UV 300 SRB (Specterum Instruments GMBH) with maximum absorption seen at the wavelength of 464 nm [START_REF] Shi | Simultaneous decolorization and desalination of dye wastewater through electrochemical process[END_REF][START_REF] Adachi | Decolorization and degradation of methyl orange azo dye in aqueous solution by the electro fenton process: Application of optimization[END_REF]. The concentration of MO in its samples of aqueous solution was calculated using a calibration curve based on the known concentrations. Finally, the decolorization rate is calculated using Eq. ( 7).

Where: C0 and Ct are respectively, the MO concentration in the feed solution at initial and t time of the treatment.

In order to evaluate the quality of the treated water, the chemical oxygen demand (COD) removal which also represents global mineralization was determined for the sample of initial feed solution and that taken at the end of process. COD was determined by using the dichromate method followed by photometric measurement.

The COD removal rate was calculated as:

Where: COD0 and CODf are respectively, the initial and final COD of the feed solution.

Results and discussion

SEM-EDS characterization of prepared Pb/PbO2 electrode

Several authors have showed the efficiency of PbO2 layer deposition on lead plate through electrolysis in sulfuric acid solution [START_REF] Quiroz | Electrocatalytic oxidation of p-nitrophenol from aqueous solutions at Pb/PbO2 anodes[END_REF][START_REF] Awad | Electrochemical degradation of Acid Blue and Basic Brown dyes on Pb/PbO2 electrode in the presence of different conductive electrolyte and effect of various operating factors[END_REF][START_REF] Wang | Electrocatalytic degradation of aspen lignin over Pb/PbO2 electrode in alkali solution[END_REF][START_REF] Li | Electrodeposited lead dioxide coatings[END_REF][START_REF] Barbari | Photocatalytically-assisted electrooxidation of herbicide fenuron using a new bifunctional electrode PbO2/SnO2-Sb2O3/Ti//Ti/TiO2[END_REF]. This method is based on the oxidation of metallic lead and generation of dioxygen on anode surface [START_REF] Li | Electrodeposited lead dioxide coatings[END_REF]. Fig. 2 shows SEM images of Pb/PbO2 electrode surface before and after PbO2 layer formation. These images show that the original electrode (Fig. 2(a)) has a pseudo homogeneous and relatively uniform surface as well as the pores size was very fine. By contrast, after its electrolysis in sulfuric acid solution as anode for 1.5 h (Fig. 2(b)), the electrode surface is not uniform crystal size and this surface becomes rougher. This can be due to the production of dioxygen on the anode surface which changing the crystal structure during the electrochemical reaction process [START_REF] Wang | Electrocatalytic degradation of aspen lignin over Pb/PbO2 electrode in alkali solution[END_REF]. From Fig. 2(b) it can be also seen that the formed PbO2 layer covers the entire electrode surface, along with the formation of tightly packed and pyramid-shaped crystals. In addition, PbO2 crystal size is approximately uniform.

The results of elementary analysis by EDS of the electrode surface before and after electrolysis are presented in Fig. 2 and Table 1. Firstly, the obtained spectrogram (Fig.

2(c))

indicates the presence of C in electrode surface before electrolysis. This is probably due to the organic trace substances rested from the electrode preparation procedure [START_REF] Saaidia | Use of a PbO2 electrode of a lead-acid battery for the electrochemical degradation of methylene blue[END_REF]. Furthermore, this analysis shows that the O and Pb elements are present in electrodes samples before (Fig. 2(c)) and after (Fig. 2(d)) electrolysis. However, the atomic percentage (Table 1) of O is increased in 3 times after electrolysis. On the other hand, the proportion between Pb and O shifts from 1:0.3 (before electrolysis) to 1:1.53

(after electrolysis).

SEM-EDS and FTIR characterization of AMX membranes

In order to investigate the efficiency of AMX membrane fouling mitigation in the ED-AO process compared to single ED process, the membranes used in the both processes were analyzed by SEM-EDS and FTIR. These experiments were carried out at the same operating conditions. Fig. 3 shows the SEM images and EDS spectrograms of the AMX pristine membrane, and those used in the studied processes. Table 2 also illustrates the atomic percentage of chemical elements contained in the membrane surfaces. SEM images of pristine membrane (Fig. 3(a)) showed its plane surface. The comparison of this image with those of membranes used in ED (Fig. 3(b)) and in ED-AO (Fig. 3(c))

shows that the morphologies of membrane surface are changed. This difference is undoubtedly due to the formation of organic matters agglomeration. From the Fig. 3(b), it can be seen a large amount of MO agglomerates was deposited and covering an important part of the membrane. It was also observed that the formation of MO agglomerates at the AMX surface is less pronounced in the case of ED-AO process (Fig. 3(c)).

Indeed, this morphological analysis clearly shows the fouling of the membrane used in ED by MO deposition. However, the surface of membrane used in the ED-AO is less fouled with MO deposits. These results allow demonstrating the significant membrane fouling by MO deposition during the ED compared to ED-AO. Similar results are obtained in our previous work for the AMX membrane fouling by sodium dodecyl benzene sulphonate [START_REF] Allat | Mitigation of an anion exchange membrane fouling by coupling electrodialysis to anodic oxidation[END_REF].

From the EDS spectrograms (Fig. 3(d), 3(e) and 3(f)) and the atomic percentage of chemical elements (Table 2) we firstly note the presence of Cl on the pristine membrane surface. However, this chlorine will be disappeared in the cases of used membranes. The chlorine came from the membrane equilibration by HCl and NaCl solutions during its pretreatment procedure. Additionally, all spectrograms show the presence of C, O and N which constitute the AMX membrane structure [START_REF] Xu | Ion exchange membranes: State of their development and perspective[END_REF][START_REF] Kariduraganavar | Ion-exchange membranes: preparative methods for electrodialysis and fuel cell applications[END_REF].

However, only the membranes used in ED and ED-AO processes showed the presence of sulfur. This sulfur is due to the adsorption of MO on the membrane surface. The presence of sulfur and the increase of atomic percentage of oxygen (from 6.73 to 12.65 and 18.38 %) and nitrogen (from 5.16 to 11.64 and 16.14%) (Table 2) in the cases of used membranes confirm the presence of MO on membrane surface for both processes.

One can also see that the atomic percentages of oxygen and sulfur for the membrane of ED-AO (18.38 and 3.13 % respectively) are more important compared to those of membrane used in ED (12.65 and 2.37 % respectively), whereas it is the opposite for nitrogen. This it means that the amount of sulfate ions (SO ) in the membrane of ED-AO is more important, while the membrane of ED is richer into MO. Finally, these results demonstrated that the membrane fouling phenomenon by MO is more significant in the case of ED compared to ED-AO process.

investigated by FTIR. Fig. 4 shows the obtained spectra for the AMX pristine membrane, and for those used in ED and ED-AO processes. Several peaks observed on the pristine membrane spectrum are also present on the spectra of used membranes.

Moreover, the comparison of these spectra reveals the presence of new peaks for the used membranes characterizing the presence of MO on their surfaces. The first peak is observed at 1605.9 cm 1 , this peak corresponds MO structure. Two others peaks at 1519 and 1444 cm 1 confirmed with an intense peak appeared at 1359 cm 1 .

Two peaks are produced at 1035.9 and 946.1 cm 1 which are attributed to the ring vibration. Stretching of aromatic benzene ring was confirmed with a peak at 822 cm 1 .

The last peaks centered at 698.23, 624.94 and 570.93 cm 1 stretching vibrations.

On the other hand, it was noted that there is not a notable difference between both spectra of used membranes (in ED and in ED-AO), except different intensities in some peaks. This difference means that the membrane fouling is more significant in the case of ED compared with that in ED-AO. Finally, the results of SEM-EDS and FTIR characterization of the AMX membranes mean that the coupling of AO to ED process reduces the membrane fouling by MO deposition.

Efficiency of ED-AO coupling in MO removal and AMX membrane antifouling

To evaluate the performance of combined ED-AO on AMX membrane fouling mitigation and MO degradation, ED and ED-AO processes were compared. These experiments were carried out at the same operating conditions and during the same duration. The results are expressed by using the variation of CER (Fig. 5(a)) and decolorization rate (Fig. 5(b)) during the processes. COD removal and decolorization efficiencies (Fig. 5(c)) determined in the end of each experiment will be also discussed.

In electromembrane process, the CER control is a main factor for detecting the membrane fouling phenomenon, because the deposition of particles or/and organic molecules on the membrane surface leads to the increase of its electrical resistance and consequently causing the increase of CER [START_REF] Zhao | Electrochemical impedance spectroscopy and surface properties characterization of anion exchange membrane fouled by sodium dodecyl sulfate[END_REF]. Therefore, Fig. 5

As shown in Fig. 5(c), it was also inferred that the COD removal rate reflecting the mineralization of feed solution is the better in the case of ED-AO with a value of 68.36

%. However, it does not exceed 30.21 % in ED process. Indeed, the presence of Pb/PbO2 electrode in ED cell improved the decolorization and COD removal rates approximately 2.2 times after 3 hours of process. In addition, the comparison between the decolorization rate and COD removal rate values into the end of the both processes (Fig. 5(c)) reveals that there is not a notable difference between them. Indeed, the efficiency of Pb/PbO2 electrode into degradation of several organic pollutants was demonstrated in numerous previous studies [57,51,66 68], where, in most cases a pseudo-total degradation of these organic pollutants into CO2 and H2O was obtained.

These results showed clearly the benefit of the introduction of AO on a Pb/PbO2 electrode into ED process for attenuate the AMX membrane fouling and for enhance the MO azo-dye degradation. Generally, the efficiency of electrodes based on lead dioxide (PbO2) surfaces in electro-oxidation organics is could be explained by its good characteristics such as: the high oxygen overpotential and high electrical conductivity [START_REF] Yahiaoui | Electrochemical pre-treatment combined with biological treatment for the degradation of methylene blue dye: Pb/PbO2 electrode and modelingoptimization through central composite design[END_REF][START_REF] Awad | Electrochemical degradation of Acid Blue and Basic Brown dyes on Pb/PbO2 electrode in the presence of different conductive electrolyte and effect of various operating factors[END_REF].

Effect of operating parameters on ED-AO efficiency

The effect of some operating parameters such as: applied current density, initial MO concentration, initial pH of feed solution and concentration of supporting electrolyte were studied in order to improve the electro-oxidation of MO and to reduce simultaneously the AMX membrane fouling in ED-AO process. The results of this study are presented generally in term of variation of CER and decolorization rate during the process and in term of COD removal rate determined in the end of each experiment. In addition, the pseudo-first-order model was occasionally applied to discuss the kinetics of MO degradation.

Effect of applied current density

In an ED-AO coupling process, the role of applied electric field is double: transports the ionic species across the IEMs and generates the hydroxyl radicals ( OH) on the Pb/PbO2 electrode surface for degrading the MO molecules. Accordingly, the effect of various current densities (1 30 mA cm 2 ) on the ED-AO process was investigated. It was demonstrated that the current density applied in AO processes must be highest possible for achieving the maximum processing efficiency [START_REF] Niu | Electrochemical oxidation of perfluorinated compounds in water[END_REF][START_REF] Lin | Electrochemical degradation of perfluorooctanoic acid (PFOA) by Ti/SnO2 Sb, Ti/SnO2 Sb/PbO2 and Ti/SnO2 Sb/MnO2 anodes[END_REF][START_REF] Yu | Efficient Electrocatalytic Degradation of 4-Chlorophenol Using a Ti/RuO 2 SnO 2 TiO 2/PbO 2 CeO 2 Composite Electrode[END_REF]. However, in this study, it is worth noting that, we did not work at very high densities for avoid approaching to the value of limiting current density. Because in ED process, when this value is exceeded, the concentration polarization phenomenon in membranes surface is started which leads to a considerably increase of membranes electrical resistance [START_REF] Choi | Effects of electrolytes on the transport phenomena in a cation-exchange membrane[END_REF][START_REF] Lee | Determination of the limiting current density in electrodialysis desalination as an empirical function of linear velocity[END_REF].

From Fig. 6(a), it could be seen that for the two lowest current densities (1 and 7.5 mA cm 2 ) the CER increases drastically. The similar trend was observed previously in the case of ED (Section 3.3). The increase of CER at low densities is likely as a result of the continuous AMX membrane fouling by MO accumulation. However, the increase of the current density at more than 15 mA cm 2 makes a slightly variation of CER, especially for the two highest densities (25 and 30 mA cm 2 ). The increase in current density reinforces more the degradation of MO by electro-oxidation, and consequently attenuates its adsorption at AMX membrane surface.

Additionally, as shown in Fig. 6(b) and 6(c), the increase of current density boosts considerably the degradation of MO in terms of the decolorization and COD removal.

Particularly, the decolorization and COD removal efficiencies improved significantly (from 42 to 68 % and from 41 to 61 %, respectively) when current density increased from 7.5 to 15 mA cm 2 . By contrast, at low current densities, the decolorization and COD removal rates were the lowest. For example, at density of 1 mA cm 2 , no obvious degradation of MO was observed, because these results are extremely similar to those obtained previously for the ED process (Section 3.3), where there is not any electrooxidation. The decolorization and COD removal obtained at low current densities is mainly due to the adsorption of MO in the AMX membrane surface, but not to the electro-oxidation phenomenon.

On the other hand, the decolorization of MO solution by electro-oxidation on Pb/PbO2 electrode has been followed with pseudo-first-order kinetic by applying of Eq. [START_REF] Schorr | Desalination: Trends and technologies[END_REF].

From the obtained results (Fig. 6(d) and Table 3), it was observed that the rate constant (k) for densities 1, 7.5, 15, 20, 25 and 30 mA cm 2 were increased as follow, 0.24×10 2 , 0.31×10 2 , 0.66×10 2 , 0.73×10 2 , 0.99×10 2 and 1.04×10 2 min 1 , respectively. From which it was noted that, the increase in the applied current density obviously accelerated the MO degradation reactions.

(

) 11
Where, C0 and Ct are respectively, the MO concentration in the feed solution at initial and t time of the treatment, and k is the rate constant (min 1 ).

Indeed, it was inferred that the increase of applied current density simultaneously improving the MO degradation and reducing the membrane fouling which is reflected by the decrease of CER. The applied current contributes directly in the generation of hydroxyl radicals ( OH) (Eq. ( 9)) on the Pb/PbO2 electrode. For that reason, the higher current density led to the production of larger amount of OH and consequently improved the degradation efficiency of organic pollutant (e.g., MO) [START_REF] Niu | Electrochemical mineralization of pentachlorophenol (PCP) by Ti/SnO2 Sb electrodes[END_REF][START_REF] Maharana | Electrochemical oxidation of 2, 4, 5-trichlorophenoxyacetic acid by metal-oxide-coated Ti electrodes[END_REF][START_REF] Xu | Hydroxyl multiwalled carbon nanotube-modified nanocrystalline PbO2 anode for removal of pyridine from wastewater[END_REF].

On the other hand, it was also observed that the two densities 25 and 30 mA cm 2 gave approximately the similar results, although the current density increased by 5 mA cm 2 . This is probably explained by secondary reactions occurred at higher current density such as oxygen evolution (Eq. ( 12)), which compete with the MO degradation reaction (Eq. ( 10)) [START_REF] Duan | Fabrication of a hydrophobic SDBS-PbO2 anode for electrochemical degradation of nitrobenzene in aqueous solution[END_REF]. Based on these results, the density of 25 mA cm 2 was chosen for the rest of the work.

(12)

Effect of initial MO concentration

The effect of initial MO concentration (5 100 mg L 1 ) on the degradation efficiency and AMX membrane fouling in ED-AO process was investigated. The obtained results

were displayed in Fig. 7. From the Fig. 7(a), it can be observed that for all MO mg L 1 , there was no obvious effect of these concentrations on the CER. However, for concentrations equal to or higher than 50 mg L 1 , the increasing of MO concentration increases significantly the CER during the process. This evolution of CER in the cases of three highest MO concentration signifies that, the abundance of large amount of MO anion in close proximity to the membrane surface reinforces its fixation over the membrane active functional sites. The molecular size of MO ion and its high concentration in the solution makes it more difficult for this organic anion to pass through the AMX membrane or to desorb toward the solution. The adsorption of large amount of organic compound ions (e.g., MO ) in the membrane surface leads to its continuous fouling which consequently leads to the deterioration of its electrical proprieties [START_REF] Mikhaylin | Fouling on ion-exchange membranes: Classification, characterization and strategies of prevention and control[END_REF][START_REF] Suwal | Characterization of protein, peptide and amino acid fouling on ion-exchange and filtration membranes: Review of current and recently developed methods[END_REF]. For that reason, the CER increases considerably during the process when the organic compound concentration in the solution is very higher.

On the other hand, the increase of initial MO concentration from 5 to 100 mg L 1 decreased gradually the decolorization efficiencies (from 88.21 to 68.23%) and the COD removal rates (from 85.44 to 57.35%) (Fig. 7(b)). Several authors [START_REF] Shi | Simultaneous decolorization and desalination of dye wastewater through electrochemical process[END_REF][START_REF] Yahiaoui | Electrochemical pre-treatment combined with biological treatment for the degradation of methylene blue dye: Pb/PbO2 electrode and modelingoptimization through central composite design[END_REF][START_REF] Panizza | Degradation of Alizarin Red by electro-Fenton process using a graphite-felt cathode[END_REF] attributed the decreasing of decolorization efficiency for the higher initial dye concentration to the competition of generated intermediate by-products, with the original dye molecules to react with oxidizing species through the ECO process.

Moreover, at constant current density, the increasing of organic compound concentration (e.g., MO) leads to a decrease in the OH/MO molar ratio, which consequently decreases the probability of encounter between OH radicals and MO molecules [START_REF] Allat | Mitigation of an anion exchange membrane fouling by coupling electrodialysis to anodic oxidation[END_REF].

In addition, from the Fig. 7(b), it can be seen that the increase of the initial concentration from 5 to 100 mg L 1 leads to the increase of the difference between decolorization and COD removal rates. Meanly, there is not any consistency between them during the increase of initial concentration. The possible reason is that, with the increase of initial MO concentration, the most of generated OH radicals can be consumed during the oxidation of MO into by-products, but not by the mineralization into CO2 and H2O [START_REF] Duan | Fabrication of a hydrophobic SDBS-PbO2 anode for electrochemical degradation of nitrobenzene in aqueous solution[END_REF][START_REF] Comninellis | Anodic oxidation of phenol for waste water treatment[END_REF]. Finally, the concentration 25 mg L 1 was adopted in the rest of the work to avoid any obvious increase in CER and any strong decrease in process efficiency.

Effect of pH of feed solution

In ECO processes, the pH of pollutant solution affects the OH generated amounts, oxygen overpotential and stability of electrodes, which consequently affects the efficiency of the process [START_REF] Jiang | Anodic oxidation for the degradation of organic pollutants: anode materials, operating conditions and mechanisms. A mini review[END_REF][START_REF] Niu | Electrochemical oxidation of perfluorinated compounds in water[END_REF]. For this, some experiments of ED-AO on MO solution (25 mg L 1 ) were performed at different initial pH values varying in the range 1.5 11.5.

As shown in Fig. 8(a) and 8(b), the variation of pH value from 6.8 (natural pH) to 11.5 has no obvious effect on the decolorization and COD removal. However, the decolorization and COD removal rates were increased from 82.16 to 97.62 % and from 76.33 to 95.75%, respectively when pH was adjusted from 6.8 to 3. In addition, at strong acid pH (pH 1.5) the decolorization of MO solution was total (> 99 %) and the COD removal was quasi-total (> 98 %). Above results indicated that lower pH values were the benefit of the MO degradation and mineralization in ED-AO process. In addition, from the Fig. 8(a) it can be also seen that the decolorization rate reaches about 93 and 70 % after the first 60 min for the two acidic pHs 1.5 and 3, respectively, while within the same time it does not exceed 49 % for pHs equal to or higher than 6.8.

Accordingly, the results of kinetic analysis (Fig. 8(c) and Table 3) also showed that the rate constant (k) values in the cases of acid pH were the highest. Subsequently, k decreased with the increase of pH value indicating that, the lower the pH, the more rapidly the MO degradation reaction.

Many previous works [

] have reported that the AO of organic compounds is favored in acid pHs. A study by Ghalwa et al. [START_REF] Ghalwa | Electrochemical degradation of linuron in aqueous solution using Pb/PbO2 and C/PbO2 electrodes[END_REF] also found that the maximum removal of linuron and COD are obtained at pH 1.5 by using Pb/PbO2 electrode, which is in agreement with that obtained in this study for MO degradation. Maharana et al.

[74] and Zhao et al. [START_REF] Zhao | Degradation of 2, 4-dichlorophenol with a novel TiO2/Ti-Fe-graphite felt photoelectrocatalytic oxidation process[END_REF] have mentioned that the generation of OH radical is more favored at the low pH level. In contrast, the increase of pH value decreases the OH generated amount. Li et al. [START_REF] Li | Electrochemical oxidation of Reactive Blue 19 on boron-doped diamond anode with different supporting electrolyte[END_REF] have also ascribed the improvement of degradation of Reactive Blue 19 in acidic conditions and in presence of SO4 to the production of sulfate radicals (SO4 ) and peroxodisulfate ions (S2O8 ) by acid-catalyzed reactions according to Eqs. ( 13), ( 14), [START_REF] Aliaskari | Nitrate, arsenic and fluoride removal by electrodialysis from brackish groundwater[END_REF], and [START_REF] Huang | Application of electrodialysis to the production of organic acids: State-of-the-art and recent developments[END_REF]. These species present a high oxidation power and it could quickly attack any organic pollutant [84 86], increasing consequently the degradation rate. In addition, since its electrophilic characteristic, the SO4 radical is considered as stronger oxidant of organic matter (e.g., MO) when compared with OH. Therefore, a smaller amount of this oxidant is sufficient to ensure complete mineralization [START_REF] Ushani | Sulfate radicals-based advanced oxidation technology in various environmental remediation: A state-of-the art review[END_REF][START_REF] Tully | Kinetics of the reactions of hydroxyl radical with benzene and toluene[END_REF].

(

) (14) 13
On the other hand, the decrease of decolorization and COD removal rates at pHs especially for alkaline pHs (8.5 and 11.5) (Fig. 8(a) and 8(b)) compared to acidic pHs is probably due to the evolution of oxygen generation which is favored into alkaline medium [START_REF] Barbari | Photocatalytically-assisted electrooxidation of herbicide fenuron using a new bifunctional electrode PbO2/SnO2-Sb2O3/Ti//Ti/TiO2[END_REF][START_REF] Li | Fabrication of cerium-doped lead dioxide anode with improved electrocatalytic activity and its application for removal of Rhodamine B[END_REF]. Consequently, the evolution of oxygen (Eqs. ( 12) and ( 17)) reduces the current efficiency and it can also prevent the diffusion of organic anions towards the anode surface [START_REF] Song | Electrochemical degradation of azo dye CI Reactive Red 195 by anodic oxidation on Ti/SnO2 Sb/PbO2 electrodes[END_REF].

(

) 17
In this manner, the decolorization and mineralization of MO solution is favored in acidic pH. Therefore, in the ED-AO process, working in acidic conditions can accelerate significantly the degradation of organics contained in the solution, thus reduce the possibility of membrane fouling by these organics.

Effect of supporting electrolyte concentration

In ECO processes, the role of supporting electrolyte is normally to provide a conductive medium and to decrease the voltage drop. Sodium sulfate (Na2SO4) is usually used for its high solubility, its good conductivity and no parasitic reactions occurred with the use of this electrolyte [START_REF] Asim | Electrochemical treatment of 2, 4 dichlorophenol using a nanostructured 3D porous Ti/Sb SnO2 Gr anode: Reaction kinetics, mechanism, and continuous operation[END_REF][START_REF] Nidheesh | Trends in electro-Fenton process for water and wastewater treatment: an overview[END_REF]. Consequently, in order to investigate their effect on MO degradation by ED-AO, different concentrations (0.025 0.4 M) of Na2SO4 supporting electrolyte were applied.

The obtained results (Fig. 9(a) and 9(b)) show that the increase in Na2SO4 concentration into the range 0.025 0.1 M resulted a significant increase in both decolorization and COD removal rates where reached 98.67 and 96.17%, respectively. However, for the two highest concentrations (0.2 and 0.4 M), a total decolorization (>99%) with a slight regression in COD removal (from 96. 7 to 92. 7% and to 89.35%) were registered. It is also noticeable that the degradation of MO in the cases of three highest concentrations of Na2SO4 (0.1, 0.2 and 0.4 M) (Fig. 9(c) and Table 3) is very faster during the first 60 min (k= 3.39×10 2 , 4.34×10 2 and 5.21×10 2 min 1 , respectively), then it became relatively slower (k= 1.91×10 2 , 2.13×10 2 and 2.02×10 2 min 1 respectively) until the end of experiment. This tendency was not observed for the two others concentrations (0.025 and 0.05 M), where the degradation rate is slow from the starting of the experiment.

The greatly improvement in decolorization can possibly be attributed to the contribution of the mediated oxidation of MO by S2O8 and direct oxidation by SO4 , which are formed from the oxidation and OH attack of SO4 according to Eqs. ( 18), [START_REF] Dumée | Purification of aqueous amine solvents used in post combustion CO2 capture: A review[END_REF], and ( 20) [START_REF] Saaidia | Use of a PbO2 electrode of a lead-acid battery for the electrochemical degradation of methylene blue[END_REF][START_REF] Mora-Gómez | Influence of the reactor configuration and the supporting electrolyte concentration on the electrochemical oxidation of Atenolol using BDD and SnO2 ceramic electrodes[END_REF][START_REF] Ma | Electrochemical treatment of real wastewater. Part 1: Effluents with low conductivity[END_REF]. Probably, the same reason made the COD removal increased for the range 0.025 0.1 M of Na2SO4, although it has slightly regressed in the cases of the two others concentrations (0.2 and 0.4 M). Moreover, the increasing in supporting electrolyte concentration improves the medium conductivity, and consequently accelerates charge and mass transfer, thus benefiting the ECO reactions [START_REF] Nidheesh | Trends in electro-Fenton process for water and wastewater treatment: an overview[END_REF].

(

) (19) 18
Ma et al. [START_REF] Ma | Electrochemical treatment of real wastewater. Part 1: Effluents with low conductivity[END_REF] and Zhang et al. [START_REF] Zhang | The peculiar roles of sulfate electrolytes in BDD anode cells[END_REF] have inferred that the oxidation of SO4 into S2O8 is increasingly favored with the increase in its concentration. However, these authors have also found that at moderately high current density and with the increase of Na2SO4 concentration, a slight abatement in the degradation of targeted organics is registered. The same trend was observed in this study concerning the regression of COD removal for concentrations of Na2SO4 at more than 0.1 M (Fig. 9(b)). The negative effect of Na2SO4 at higher concentrations on the completely mineralization of organics, is possibly ascribed to the following reasons:

i) The consumption of generated OH by SO4 at anode surface for giving SO4 (Eq. ( 19)) which is retransformed into S2O8 (Eq. ( 20)). The S2O8 ion is relatively stable at room temperature [START_REF] Li | Electrochemical oxidation of Reactive Blue 19 on boron-doped diamond anode with different supporting electrolyte[END_REF], so it is majority in the system, this it makes the degradation of small molecules of by-products generated in the middle of degradation process is more difficult.

ii) The competition between SO4 oxidation (Eq. ( 18)) and OH generation (Eq. ( 9)) at the surface of anode. This can reduce the extent of the direct oxidation of organic compounds (e.g., generated by-products) by OH and consequently affects their completely mineralization [START_REF] Li | Electrochemical oxidation of Reactive Blue 19 on boron-doped diamond anode with different supporting electrolyte[END_REF][START_REF] Ma | Electrochemical treatment of real wastewater. Part 1: Effluents with low conductivity[END_REF].

On the other hand, a slightly decrease of CER after about 90 min of process was obtained when the Na2SO4 concentration equal to or higher than 0.1 M (Fig. 9 electrode for seven cycles makes the degradation of generated by-products is more difficult. This is can be explained by the change of electrode surface properties, which can reduce slightly the extent of the hydroxyl radicals generation. On the other hand, Fig. 10 shows that there is not obviously evolution in CER value, where it does not Mainly, there is not any significant fouling of the AMX membrane during these experiments.

Conclusions

A novel configuration of ED with AO integration was investigated in order to eliminate the MO anionic azo-dye, and to reduce simultaneously the AMX membrane fouling.

From this work, the following conclusions can be taken:

The characterization results showed that the PbO2 layer has been successfully and effectively deposited at Pb plate surface by using electrolysis in acidic solution. In addition, the membrane fouling by deposited MO was less significant in the case of ED-AO compared to ED process. Besides, the presence of sulfate ions in the membrane of ED-AO was the strongest.

The results of ED and ED-AO experiments demonstrated that the coupling of AO to ED process attenuated significantly the membrane fouling caused by MO deposition.

The presence of Pb/PbO2 electrode in ED cell also reduced the CER and improved the decolorization and COD removal rates approximately 2.2 times after 3 hours of process.

The study of operating parameters effect on ED-AO process showed that the increase of applied current density improving the MO degradation and reducing simultaneously the CER evolution. In the other hand, the increase of initial MO concentration decreased gradually the decolorization and COD removal efficiencies.

The decolorization and mineralization of MO solution is favored in acidic pH. It was also found that the increase in Na2SO4 concentration as supporting electrolyte led to a significant increase in MO degradation, where a pseudo-total and total decolorization was obtained when the concentration is high. However, a slight regression in the mineralization for the highest concentrations was observed. In all cases, the improvement of decolorization and mineralization of MO led to reducing the CER evolution and the AMX membrane fouling successively.

The study of Pb/PbO2 electrode reusability showed that the decolorization of MO slight regression in COD removal begins from the seventh cycle.

Finally, this work showed clearly the benefit of AO combined with ED process to control the AMX membrane fouling and for enhanced MO azo-dye degradation. ED-AO could be a useful synergistic process for enhanced oxidation of organic pollutants along with continuous processing which are very recommended factors at large scale.

CRediT authorship contribution statement

Highlights

-Electrodialysis membranes fouling leads to the increase of cell electrical resistance.

-ED-AO coupling reduces successfully the AMX membrane fouling by organics.

-ED-AO coupling intensifies the decolorization and COD removal.

-Hybrid ED-AO efficiency strongly depends on the current density, organic pollutant concentration, pH of feed solution and concentration of supporting electrolyte.

2. 5 .

 5 Principles and procedures of ED and ED-AO processes Schematic representation of the principles and procedures of ED and ED-AO processes is shown in Fig. 1(b) and 1(c). In the two processes, the feed solution containing MO as an organic pollutant and Na2SO4 as supporting electrolyte circulates successively into the side compartments (E1 and E2). An acidic solution (H2SO4, 0.05 M) as concentrate solution circulates in the central compartment (C). First of all, ED and ED-AO experiments at the same operating conditions were carried out to evaluate the feasibilities of simultaneous MO degradation and AMX membrane fouling mitigation in the ED-AO compared to single ED process. In these experiments, the initial concentrations of MO and Na2SO4 in feed solution were 25 mg L 1 and 0.05 M, respectively, with applied current density of 20 mA cm 2 the ED process (Fig. 1(b)), MO anionic molecules do not undergo any degradation and cannot cross the AMX membrane. However, as shown in Fig. 1(c), in the ED-AO process, the presence of Pb/PbO2 anode in E1 compartment can preliminary degrade the MO molecules. Then, in order to study the influence of operation factors affecting performance of the ED-AO process for treating the MO + Na2SO4 mixture solution, the main factors were studied, including applied current density (1, 7.5, 15, 20, 25 and 30 mA cm 2 ), initial MO concentration (5, 10, 25, 50, 75 and 100 mg L 1 ), initial pH of feed solution (1.5, 3, 6.8 (natural pH), 8.5 and 11.5) and concentration of supporting electrolyte (Na2SO4) (0.025, 0.05, 0.1, 0.2 and 0.4 M). The initial pH of the feed solution was adjusted by NaOH (1 M) and H2SO4 (1 M) solutions. All the experiments were conducted in three times. In the other hand, the reusability of prepared Pb/PbO2

  (a) shows a significant increase of CER during the ED process where it starts from 58 to reach at 130.46 ohm after 180 min of process. This notable increase of CER during the ED indicates the continuous fouling of the AMX membrane by MO anionic molecules accumulation.The molecular size of MO made it more difficult to pass through the AMX membrane which causing its fouling. While under the same conditions and for the same duration, the CER in the ED-AO process changes slightly between 46.02 and .53 ohm. From the Fig.5(a) it can be also seen that the CER has been reduced 2.2 times after 3 hours of process by the presence of AO. Accordingly, the coupling of AO to the ED process allows keeping a slight change of CER and consequently attenuates considerably the AMX membrane fouling. From the same figure (Fig.5(a)), it was also found that the CER at time 0 is slightly different in the two processes (58 and 46.02 ohm for ED and ED-AO respectively). This is probably due to the anode nature (into SS for ED and into Pb/PbO2 for ED-AO), because the Pb/PbO2 electrode has a good electrical conductivity compared to the several others types of electrodes[START_REF] Niu | Electrochemical oxidation of perfluorinated compounds in water[END_REF].From Fig.5(b), it was found that the ED-AO process presents a better decolorization effect on the MO molecules where the decolorization rate is reaching around 73.48 % after 180 min of treatment. By contrast, in the case of ED process the decolorization rate increases slightly during the first 60 min until 26.92 % and it could reach 32.98 % at the end of experiment. This 26.92 % of decolorization is possibly due to the MO anionic molecules adsorption in the AMX membrane surface in a significant way during the cited duration. However, in ED-AO there is an ECO process on Pb/PbO2 electrode which strongly contributes in MO degradation by following the reactions of Eqs. 9 and 10.[START_REF] Baker | Membrane technology and applications[END_REF]

3 . 5 .

 35 (d)), where it reaches in the end of experiments at 38.85, 32.53 and 29.25 ohm for concentrations 0.1, 0.2 and 0.4 M respectively. In contrast, a slightly increase of CER is observed in the cases of the two lowest concentrations (0.025 and 0.05 M) and values of 61.75 and 50.38 ohm were respectively registered at the end of experiments. In fact, the increase of supporting electrolyte concentration not only improves the MO degradation for attenuate the membrane fouling, but also increases significantly the feed solution conductivity. In view of this, the better concentration of Na2SO4 is 0.1 M because it gives a quasi-total decolorization (> 98%), the best COD removal (96.27%) and a CER approximately constant. Reusability of Pb/PbO2 electrode The reusability and continued effectiveness of electrodes is an important factor for a practical application, especially at industrial scale. Herein, ten experiments of ED-AO were performed at optimal conditions (i = 25 mA cm 2 , [MO]0 = 25 mg L 1 , natural pH 2SO4] = 0.1 M). The results of Pb/PbO2 electrode reusability are mainly discussed in term of decolorization and COD removal efficiencies after 3 hours of treatment. Besides, to confirm whether its reuse affects the membrane fouling mitigation, the CER values determined at the end of experiments are also discussed. As shown in Fig. 10, the decolorization rate remained virtually constant (balancing between 97.45 and 99.37 %) during 10 cycles. However, COD removal rate slightly decreases from the seventh cycle, where it has reached at 86 % in the tenth cycle. The regression in COD removal is probably due to the fact that the reuse of Pb/PbO2

A. Mehellou :

 Mehellou Writing, Experimental and characterization, R. Delimi: Visualization, Supervision and Methodology, L. Allat: Visualization and Methodology, R. Djellabi: Writing, Visualization, Validation and Revision, A. Rebiai: Writing and English revision, C. Innocent: Supervision and Methodology.

Fig. 1 .

 1 Fig. 1. (a) Experimental set-up; Procedures and principle of (b) ED and (c) ED-AO. (1): ED cell; (2): Power supply; (3) and (4): Multimeters; (5): Peristaltic pump; (6): SS electrode; (7): SS electrode in ED process or Pb/PbO2 electrode in ED-AO process; (8): Feed solution tank; (9): Concentrate solution tank; (10): Magnetic stirrer.

Fig. 2 .

 2 Fig. 2. SEM images of Pb/PbO2 electrode (a) before electrolysis (c) its EDS spectrogram, (b) after electrolysis (d) its EDS spectrogram.

Fig. 3 .

 3 Fig. 3. SEM images of AMX (a) pristine membrane (d) its EDS spectrogram, (b) membrane used in ED (e) its EDS spectrogram, (c) membrane used in ED-AO (f) its EDS spectrogram. (ED and ED-AO operating conditions: i = 20 mA cm 2 , [MO]0 = 25 mg L 1 2SO4] = 0.05 M).

Fig. 4 .

 4 Fig. 4. FTIR Spectra of AMX pristine membrane, and those used in ED and ED-AD processes.

Fig. 7 .

 7 Fig. 7. Representation of (a) CER variation, (b) COD removal and decolorization efficiencies for different initial MO concentrations. (ED-AO operating conditions: i = 25 mA cm 2 , natural 2SO4] = 0.05 M).

Fig. 8 .

 8 Fig. 8. Representation of (a) decolorization rate variation, (b) COD removal rate values and (c) pseudo-first-order kinetics for different pHs. (ED-AO operating conditions: i = 25 mA cm 2 , [MO]0 = 25 mg L 1 and [Na2SO4] = 0.05 M).

Table 1

 1 Chemical elements analysis results of Pb/PbO2 electrode surface by EDS before and after electrolysis.

	Pb/PbO2 electrode Atomic percentage (%)
		Pb	O	C
	Before electrolysis	67.30	20.67	12.03
	After electrolysis	39.45	60.55	0

Table 2

 2 Chemical elements analysis results of the membrane surface by EDS.

	AMX	Atomic percentage (%)
		C	O	Cl	N	S
	Pristine membrane	82.88	6.73	5.22	5.16	0
	Membrane used in ED	68.84	12.65	0	16.14	2.37
	Membrane used in ED-AO	66.85	18.38	0	11.64	3.13

Table 3

 3 Pseudo-first-order kinetic parameters.

	Parameter	Value		k (10 2 min 1 ) R 2
	i (mA cm 2 ) 1		0.24	0.9972
		7.5		0.31	0.9985
		15		0.66	0.9848
		20		0.73	0.999
		25		0.99	0.9973
		30		1.04	0.998
	pH	1.5		3.70	0.9786
		3		2.18	0.9887
		6.8		0.94	0.9956
		8.5		0.86	0.9994
		11.5		0.81	0.9984
	[Na2SO4]	0.025		0.85	0.9989
	(M)	0.05		0.94	0.9956
		0.1	Part 1 Part 2	3.39 1.91	0.9779 0.9655
		0.2	Part 1 Part 2	4.34 2.13	0.9974 0.9309
		0.4	Part 1 Part 2	5.21 2.02	0.991 0.9797

Acknowledgments

The authors sincerely thank the Ministry of Higher Education and Scientific Research of Algeria for the financially support for this work. The authors also would like to acknowledge the European Institute of Membranes (Montpellier, France) for their collaboration.

Data availability

No data was used for the research described in the article. Fig. 10. MO degradation efficiency (decolorization and COD removal) and CER value at the end of ED-AO experiment using Pb/PbO2 electrode for 10 cycles. (ED-AO operating conditions: i = 25 mA.cm 2 , [MO]0 = 25 mg L 1 [Na2SO4] = 0.1 M).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Dr. Ahmed Mehellou Laboratory of applied chemistry and environment (LCAE), Department of Chemistry, Faculty of Exact Sciences, University of El Oued, 39000, El Oued, Algeria Email: mehellou-ahmed@univ-eloued.dz

Statement of novelty and significance

Water pollution by organic dyes is considered one of the most dangerous pollution forms. Numerous methods have been investigated for the treatment of such pollution. However, very few works have been reported on the study treating this type of pollution by electrodialysis process. This is probably due to the problem of membranes fouling by dye molecules.

In this work, a new combination of electrodialysis/anodic oxidation (ED-AO) was achieved, in order to intensify the degradation of an organic dye and to solve the problem of membranes fouling by this pollutant. The results show an excellent performance of the hybrid ED-AO system.