Batch elimination of cationic dye from aqueous solution by electrocoagulation process

Like other developing countries, Morocco has undergone a real industrial revolution that generates a huge amount of colored wastewater. Methylene blue (MB) could be considered as commonly model of cationic dyes that are widely used in industries. In the present work, the removal of MB was investigated in the presence of NaCl as electrolyte by electrocoagulation (EC) process. Parameters studied are the nature of electrodes, electrode spacing, initial dye concentration, agitation speed, applied current density, NaCl concentration, and the effect of the initial pH. Stainless steel electrodes have been used as cathode and anode in the electrochemical cell. The performance of the process was evaluated in terms of color removal, chemical oxygen demand (COD), total organic carbon (TOC), electrical power consumption, current efficiency, and the quantity of sludge formed. The results showed that the removal percent of 20 mg/L MB is 100 %, using stainless steel electrodes with an interspace of 1.5 cm and 20 mA/cm2 for 6 min in the presence of 0.1 M NaCl at natural pH. COD and TOC removals within 15 min were 45% and 51% respectively. The power consumption, in that case, was 5.84 KWh/m, while the amount of sludge formed was 1.64 Kg/m.


Introduction
The dyeing and finishing processes in the textile industries produce a considerable quantity of dyestuff 1 . Generally, this wastewater has low biodegradability and is known for its higher value of pH, temperature and COD. The effluents of textile industries have allergic, toxic, mutagenic or cancerogenic properties which create a severe problem in front of the environment protection 2, 3 . Industries produce about 7×10 8 Kg/year of dyes and about 5 to 10% among them are unacceptable as industrial wastes 4 . To remove dyes from wastewaters, a variety of treatments, such as electro-Fenton processes 5 , coagulation-flocculation 6 biological degradation, adsorption, photodegradation, ozonation [7][8][9] , membrane separation 10 have been reported. Some of those methods produce a large amount of sludge, and their efficiency is not sufficient.
On the other hand, some of them are limited by their high operation costs 11 . During the last few decades, the electrocoagulation (EC) process is observed to be a prominent technique for wastewater treatment. This technology has many advantages in being attractive for its versatility, ease of control, safety, selectivity, and being able for automation 12 . It involves the insitu generation of the coagulants by the application of direct current voltage to oxidize a sacrificial anode such as Fe 13 , Al 14 , and stainless steel 15 to form flocs.
Moreover, eliminate organic compounds from wastewater 3 . The process occurs under three successive stages; (i) electrolytic oxidation of sacrificial anode and production of coagulant, (ii) formation of flocs after the destabilization of organic compounds and particulate suspension, (iii) formation of sludge by the aggregation of destabilized particulate.
This process happens as follows: At the anode: M → M n+ + ne -(1) 2H2O → 4H + +O2+4e -(2) At the cathode: nH2O + ne -→ 2 H2 + nOH - M n+ + ne -→ M (4) In the bulk of solution: M n+ + nOH -→M(OH) n (5) Where 'M' is the anode material and 'n' is the number of electrons involved in the reaction 16,17 . There are four mechanisms of destabilization that could occur after the formation of the insoluble polymeric metal hydroxides. It may realize the surface complexation; when the dye acts as a ligand to link the polymers formed. The electrostatic attraction; where dyes neutralize the surface charge on the M(OH)n, compress the diffuse double layer due to the increase of the metal ion concentration in the bulk of the solution, and finally by the chain oxidation reaction in the presence of NaCl that may enhance the formation of the mono and the polynuclear hydroxide form of the metal 4,18 .
The anodically produced chlorine species enhance the indirect electrolysis that transforms pollutants into less harmful products. The main forms are active chlorine in form gaseous, hypochloruous acid, hypochlorite and Clions.
Methylene blue (MB) is a cationic dye, widely used for different dyeing processes for wool, cotton, silk, etc. In the event of exposure, it may lead to eye damage, mental confusion, nausea, vomiting and methamoglobinemia 33 . Many researchers have studied textile dyes treatment using EC process; however, very few studies on methylene blue dye removal using stainless steel electrodes is accessible till date. This paper aims to study the elimination of MB from water using stainless steel electrodes. The optimization of some parameters, such as current density, the initial pH, initial dye concentration, supporting electrolyte concentration, COD, TOC, color removal, power consumption, and the amount of the sludge formed, has been carried out during the electrocoagulation process and results, so obtained, are presented in this paper.

Materials and methods
The The elimination of methylene blue was carried out with 400 ml of the dye solution in a 600 ml Pyrex bulker. During the experiment, a magnetic stirrer was used to secure the homogeneity of the solution. The observed initial pH of the solution was 5.86 as measured with HANNA pH-meter. The pH effect on the color removal was investigated using 0.5 M H2SO4 or 0.5 M NaOH (from Prolabo) to adjust the initial pH of the solution 35 , and its value was not further controlled. A stabilized alimentation (digital DC ELC, model ALR3002M) was used for the electrolysis at 25 ± 2°C; the experimental set-up is presented in (Fig.1). The Chemical Oxygen Demand (COD) measurements were performed using Hach Lange cuvette test LCK314with a range of 0-150 mg O2/L and its value was determined using Hach DR6000 spectrophotometer. Total Organic Carbon (TOC) measurements were determined by injecting 50 µL sample through thermal catalytic oxidation at 680°C using TOC-L SHIMADZU analyzer. Before each analysis, samples were centrifuged at 4000 rpm using Rotafix 32 A(Hetttich Zentrifugen). COD removal, TOC removal, and the power consumption were calculated according to equations given below 34,[36][37][38] : where, CODo and TOCo correspond to initial COD and TOC, respectively, Ecell is the average of applied voltage on the cell (V), 'I'is the applied current intensity (A), 't' represents the electrolysis time (h), and Vs is the solution volume (L).
The chemical composition of the formed sludge was determined using scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) (SEM-EDAX TEAM apparatus). SEM micrographs were obtained with VEGA 3 TESCAN microscope at 10 KV.
To study the effect of the nature of electrodes, stainless steel (AISI 316) and Iron (99.5% purity) plates having dimensions 6.6 cm × 13.5 cm × 0.2 cm with 64 cm² submerged surface area were used to perform the experiments. Before each experiment, electrodes were polished on a fine emery paper (P1200), treated ultrasonically in a dilute NaOH solution for 30min to remove adsorbed substances on the electrode surface and then washed with distilled water.

Effect of electrodes' material on dye removal
The effect of electrode materials on the removal of MB has been investigated using two different kinds of electrodes, i.e. stainless steel (SS) and Iron. The experiment was carried out at 20 mA/cm² using 0.1 M NaCl at the solution with initial pH = 5.86. The color removal percentage vs time curve for the used electrodes is shown in (Fig. 2). From the figure, it is observed that MB elimination at stainless steel electrodes was found to be better as compared to Iron electrodes. The higher efficiency of SS may be because stainless steel promotes a higher potential of direct oxidation of the dye due to its high overpotential of oxygen evolution reaction compared to Iron 15 . Hence, further experiments have been performed by keeping SS as electrodes.

Effect of electrode spacing
The potential drop (ViR), developed between electrodes is a valuable property that affects the electrode kinetic parameters of the electrochemical system. The power consumption and solution resistance decrease as the inter-electrode gap is reduced 39 . The effect of inter-electrode spacing has been investigated for 20 mg/L of MB at a current density of 15 mA/cm² in 0.1 M of NaCl at natural pH. The observed data for MB removal is represented in (Fig.3a). The results show that as the distance between electrodes increases from 1.0 to 2.0 cm, the time for the total decolorization increases, accordingly. (Fig.3b) presents the power consumption computed at 6 min time electrolysis. The power consumption increases with increasing the inter-electrode distance due to the increase of electrical resistance of the solution between electrodes. Thus, the current passed through the electrodes decreases leading to low production of oxidizing species and polymeric metal hydroxides which give less decolorization efficiency 40 . Therefore, as the power consumption obtained using 1.5 cm inter-electrode space does not significantly increase, it is reasonable, in the current study, to adopt the later parameter for all further studies to avoid the accumulation of gas bubbles between electrodes in case of tiny gaps 41 . Aoudj et al. 42 reported that removal of Red 81 dye was achieved using a pair of aluminum electrodes with spacing 1.5 cm.

Effect of the initial dye concentration
The effect of the initial concentration of MB was studied at a current density of 20 mA/cm² using 0.1M of NaCl at pH = 5.86 (Fig.4). The removal efficiency decreases with increasing dye concentration. At higher concentration, the intermediate compounds compete with dye molecules and water on active sites of the electrode as reported by Modirshahla et al. . 43 . Adsorption on iron hydroxide is the main dye removal pathway 44 . According to Faraday's law, a constant amount of coagulation ions is liberated from the sacrificial anode for the same current density and electrolyzing time. Consequently, the same quantity of coagulants will be produced in the solution. Therefore, the formed flocs, at high pollutant concentrations, will not be sufficient to absorb all pollutant 45 .

Effect of agitation speed
The stirring magnetic effect was carried out using 20 mg/L MB, 0.1 M NaCl at natural pH, and a current density of 15 mA/cm². The removal efficiency increases with increasing agitation speed (Fig. 5). The agitation enhances mixing between gel and dye molecules and thus flocs formation, leading to easier precipitation and an increasing pollution removal efficiency 46 . The increase in color removal may be ascribed to the following: (i) stirring improves the mixing conditions between the coagulants [Fe(OH)3] and the dye molecules avoiding the formation of a gradient of concentration inside the reactor (ii) stirring eliminates concentration polarization at the anode with a consequent decrease in its passivation tendency 47 . Raising the stirring rate leads to intensifying the mobility of the electrogenerated ions, which have a positive effect on color removal.

Effect of current density on removal efficiency
The current density, which can be controlled directly, is an essential parameter for the electrocoagulation process. It determines the quantity of the coagulant dissolved in the solution, the size of the flocs formed and hydrogen bubble production rates. Higher current density leads to generate more coagulant agents 48,49 ; more H2 bubble is generated, thereby increasing the formation of metal hydroxide and treatment efficiency 40,50 . (Fig.6) represents the effect of current density for MB removal using 0.1 M NaCl with 1.5 cm interelectrode spacing for different electrolysis times. The results show that the efficiency of color removal is increased with increasing current density upto20 mA/cm², and then it remains constant. The treatment time required for MB removal decreases with increasing the current density. For instance, after 20 minutes of treatment, a current density of 10 mA/cm² is required to achieve about 99% removal of dye, while after 6 minutes of treatment 100% dye removal is obtained at the current density of 20 mA/cm². Therefore, for the study of the next parameters, 20 mA/cm² is taken as the optimum current density.

Effect of the concentration of the supporting electrolyte
The supporting electrolyte is used to increase the conductivity of the solution. The cell voltage is reduced as a result of increasing electrolyte concentration leading to decreasing in power consumption. Many textile factories use NaCl salt in their production process, and its presence influences the electrocoagulation. In the presence of NaCl, anodic discharge of chlorides leads to produce Cl2, HClO and/or OCl −, which are strong oxidants, capable to oxidize organic compounds presented in water 51 .
As reported by Senthilkumar et al. 52 , NaCl has a better effect on the removal of Reactive Red 120. The effect of NaCl concentration on MB was studied by applying EC technique with 20mA/cm² current density and at natural pH. Chloride ions reduce the contribution of other anions such as HCO3and SO4 2and contribute to a normal electrocoagulation 53 . (Fig.7) represents MB removal vs. NaCl concentration at different treatment times. It is observed that the efficiency is increased with increasing the ionic strength of the solution and reaches 100% removal in the presence of 0.1 M NaCl

Effect of the initial pH
The pH of the electrolytic solution strongly affected the efficiency of the electronic system 34,55,56 . It influences metal dissolution, nature of coagulants and metal hydroxides formation. This parameter contributes to adsorption, precipitation or destabilization and aggregation of particles 57 . The effect of initial pH for MB removal was carried out at the current density of 20 mA/cm² using 20 mg/L MB and 0.1 M of NaCl, (Fig.8a). The final pH is increased with time and stabilized at around pH =11 (Fig.8b); whatever its initial value. The formation of OHat the cathode rises the solution pH which compensates the acidic buffer species present in the solution 58,59 . The stabilization of pH can be explained by the use of NaCl that produces in situ an oxidizing agent in the solution that keeps the pH below 11. In this study, it is found that the best color removal was obtained at pH about 5.86. As it is known, at acidic pH the dissolution of Iron is found to be significant than alkaline pH because of the decrease in corrosion rate in the presence of oxygen leading to the passivation phenomena due to layer formed 60 . Mollah et al. 61 reported that the electrocoagulation was observed to be highly effective when the initial pH is slightly acidic to neutral (pH 3-7) where metal hydroxide has a significant effect on the elimination due to its large surface area. However, beyond the near-natural pH, less removal efficiency is observed due to the formation of Fe(OH)2 + , Fe(OH) 2+ , and Fe(OH)4 − species 40 . The same results were also reported by others [62][63][64][65][66][67][68] .

Kinetic study
In the present study, the pseudo-first-order model was employed to describe the adsorption kinetics of MB onto coagulant. The linear form of the model is presented by equation (5).
The presentation of ln (C/C0) against t (Fig. 9a) at different current densities yields a straight line (R² ≈ 1) having a slope of Kapp which increases with increasing current density as also shown in (Fig.9b), where, experimental data were extracted from section 3.5. The metal dissolution and chlorine species were increased with increasing current, which affect removal efficiency. For instance, in the case of 20 mA/cm² current density, the constant kinetic rate is around 1.21 min -1 .

COD and TOC removal
The chemical oxygen demand (COD) and total organic carbon (TOC) removal have been performed using 20 mg/L of MB at a current density of 20 mA/cm² using 0.1M of NaCl and at natural pH. Results are shown in (Fig. 10a) and (Fig.10b), for COD and TOC removal, respectively. (Fig.10a) showed that after 15 minutes of treatment, about 45% COD was achieved. The low value of COD removal may be due to the presence of by-products formed by direct oxidation at the anode surface. (Fig.10b) shows that the TOC removal is increased up to 51% with increasing time up to 15 min then it is decreased. the sludge surface. Therefore, 15 minutes time is considered as the maximum time for the removal of TOC. The TOC or COD can be further reduced by combining this treatment with another technique as anodic oxidation.

Power consumption
The power consumption for the feasibility of electrocoagulation process was calculated using (Eq.4) for 20 mg/L MB, 0.1 M NaCl at natural pH, and the result is represented in (Fig.11). The power consumption increases with applied current density up to 20mA/cm² and remains almost constant beyond this value. The obtained value at 20 mA/cm² was around 5.84KWh/m 3 over 15 minutes of electrolysis. As reported by Yuksel et al. 15 stainless steel has the lowest power consumption in comparison with Iron electrodes.

Sludge formation
According to Faraday's law, the higher current density increases both the dissolution of anode and sludge production. (Fig.12) shows the quantity of sludge formed at different current density over15 minutes of electrolysis using 20 mg/L MB, 0.1 M NaCl and at natural pH. Similar results were also observed by other workers 69 .

SEM and EDS analysis
The formed sludge using 20 mg/L MB, 0.1 M NaCl, at natural pH and 20 mA/cm² was characterized using SEM and EDS analysis. After being dehydrated at 105°C, it was annealed at 300°C for 1h. Figures 13 a  and

Conclusion
The removal of MB dye from aqueous solutions was performed in a batch-stirred reactor using EC process. The effect of some operating conditions like the nature of the electrodes, inter-electrode spacing, power consumption, the initial dye concentration, steering speed, the current density, initial pH, reaction time, the dye concentration, NaCl concentration, and parameters were tested and optimized. MB elimination at stainless steel electrodes was found to be better compared to Iron electrodes. The objective is to carry out the best way to remove MB from aqueous solution with low power consumption. The optimal conditions for 20 mg/L MB treatment were: 20 mA/cm² current density using stainless steel electrodes with an inter-distance of 1.5 cm and 850 rpm for the batch-stirred reactor in the presence of 0.1 M of NaCl, at natural pH. 100% of decolorization was achieved within 6 minutes of electrolysis time. Furthermore, 41% of COD and 51% of TOC were removed after 15 minutes of treatment. To increase the COD and TOC removals, combined treatment is needed. According to these parameters, the power consumption at 15 min of electrolysis time was around 5.84 KWh/m 3 . The MB removal follows the pseudo-first-order kinetic model with a constant kinetic rate of about 1.21 min -1 using 20 mA/cm². As a result of dye removal, a small amount of sludge around 1.6 Kg/m 3 is generated in the optimal experimental conditions. The sludge characterization shows the presence of different metallic elements such as Cr and Ni, reflecting the power of this process to transfer the pollution from water to the formed sludge. The work is in progress to characterize the effluent and the sludge for better reuse. framework of the European ERANET MED Water-13_043 project SETPROpER: (Sustainable treatment processes of effluents for reuse of water in agriculture) with the financial support of MOROCCAN MESRSFC.

Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.