Removal of Mn2+ ions from synthetic wastewater by electrocoagulation process

st i oces avy ns as i on ove e ha solu duc he wer 1. Introduction metal minin anufact d sho e seco the hu e. Man cells, many Desalination 260 (2010) 23–28 Contents lists available at ScienceDirect Desalin j ourna l homepage: www.e l uncontrollable laughter, sexual excitement and impotence [2]. It is also reported to cause ‘manganese pneumonia’. Like most of the metallic species, excessive supply of Mn to plants has detrimental effects [5]. Therefore, before the disposal of Mn containing industrial wastes, their manganese content should be eliminated. There are several ways for the removal of manganese from water and wastewater [6–13]. process for specific problems [15]. The EC process is based on the in-situ production of a coagulant by anodic dissolution and, subsequently, produces aluminium or iron hydroxides in the contaminated water. In addition, cathodic reactions allow for the pollutant removal either by deposition on cathode electrode or by flotation (evolution of hydrogen at the cathode) [16,17]. It has been shown that EC is able to eliminate a variety of pollutants from water and wastewaters, such as suspended particles ⁎ Corresponding author. Tel.: +98 21 64543200; fax: E-mail addresses: [email protected] (A. Shafaei), re (M. Rezayee), [email protected] (M. Arami), nikazar@aut. 1 Tel.: +98 21 64543200; fax: +98 21 66405847. 2 Tel.: +98 21 64542614; fax: +98 21 66405847. 3 Tel.: +98 21 64543192; fax: +98 21 66405847. 0011-9164/$ – see front matter © 2010 Elsevier B.V. Al doi:10.1016/j.desal.2010.05.006 alloys. Intake of higher nganese psychosis, an It is characterized by century with limited success and popularity. In the last decade, this technology has been increasingly used in the world for treating the industrial wastewater and many studies conducted to optimize this concentrations of manganese causes ma irreversible neurological disorder [2–4]. Wastewaters containing heavy different industrial activities such as facilities and electrical equipment m are toxic and non-biodegradable an wastewaters [1]. Manganese (Mn) is th in nature. It is an essential metal for enzymes are activated by manganes applications in ceramics, dry battery manganese is an alloying element of pollutants come from g, power plants, plating uring. All heavy metals uld be separated from ndmost abundant metal man system and many ganese has a variety of and electrical coils and The most widely used method for the treatment of metal polluted wastewater is precipitation with alkaline solution and coagulation with Fe2(SO4)3 or Al2(SO4)3 with subsequent time-consuming sedimentation. Although precipitation is shown to be quite efficient in treating industrial effluents, the chemical coagulation may induce secondary pollution caused by the added chemical substances. These disadvantages encouraged many studies to use electrocoagulation for the treatment of several industrial effluents [14]. Electrocoagulation (EC) is an electrochemical wastewater treatment technology. Treat- ment of wastewater by EC has been practiced for most of the 20th [18], industrial w organic pollutan removal [23], d emulsions [27], [14,29–33]. The electroly monomeric spec species and hydr +98 21 66405847. [email protected] ac.ir (M. Nikazar). l rights reserved. Removal of Mn2+ ions from synthetic wa Ashraf Shafaei a,⁎, Maryam Rezayee a,1, Mokhtar Aram a Chemical Engineering Department, Amirkabir University, Hafez Ave, Tehran, Iran b Textile Engineering Department, Amirkabir University, Hafez Ave, Tehran, Iran a b s t r a c ta r t i c l e i n f o Article history: Received 20 January 2010 Received in revised form 3 May 2010 Accepted 4 May 2010 Available online 2 June 2010 Keywords: Heavy metal Manganese ion Wastewater Electrocoagulation process Electrocoagulation (EC) pr contaminants, especially he of Mn2+ ions from solutio influential parameters such initial metal concentration optimum initial pH to rem density and electrolysis tim was not influenced by the increase in the solution con decreased with increasing t electrocoagulation process ewater by electrocoagulation process b,2, Manouchehr Nikazar a,3 s has been used as an effective method for the removal of a variety of metal ions, from water and wastewater effluents. In this study, the removal has been studied by EC process with aluminium electrodes. The effect of nitial pH, applied current density, electrolysis time, solution conductivity and the performance of EC process has been investigated. It was found that the Mn2+ ions was 7.0. Also, the results indicated that increasing the current s a positive effect on the Mn2+ removal efficiency. The removal of Mn2+ ions tion conductivity but the electrical energy consumption decreased with an tivity. In addition, the results of our study revealed that Mn2+ removal rate initial concentration of the contaminant. The by-products obtained from the e analyzed and explained using SEM, EDAX and XRD. © 2010 Elsevier B.V. All rights reserved. ation sev ie r.com/ locate /desa l astewater [19], phenolic wastewater [20], refractory ts including surfactants [21], COD removal [22], NOM yes [24,25], restaurant wastewater [26], oil–water fluoride [28] and heavy metal-containing solution tic dissolution of the anode produces the cationic ies such as Al3+ and Fe3+. A range of coagulant oxides is formed which destabilize and coagulate the of 0.5 nm. 3. Results and discussion The data of different conditions were obtained and analyzed using the parameter Re, as the C removal fraction (or efficiency in Re %) as: Re = C0 � C C0 � � × 100 ð8Þ where C0 and C are concentrations of Mn2+ ions at initial and time t in mg/L, respectively. 3.1. Effect of pH One of the important parameters in the electrochemical process is the initial pH of the solution. It has been established that pH has a considerable influence on the performance of EC process [16,20]. To evaluate its effect, the pH of the samples was adjusted to the desired value by sodium hydroxide or sulfuric acid solution. The current density was fixed at 6.25 mA/cm2 and the Mn2+ concentration was 100 mg/L. The Mn2+ removal efficiency after 30 min of electrolysis, as 24 A. Shafaei et al. / Desalination 260 (2010) 23–28 suspended particles or precipitate and adsorb the dissolved con- taminants. It is well known that, in EC process, the main reactions occurring at the aluminium electrodes during electrolysis are [17]: (a): Anodic reactions: Al→Al3þ þ 3e− ð1Þ 2H2O→O2ðgasÞ þ 4HþðaqÞ þ 4e− ð2Þ (b): Cathodic reactions: 2H2O þ 2e−→H2ðgasÞ þ 2ðOHÞ−ðaqÞ: ð3Þ Likewise, direct electrochemical reduction of metal cations (Mn+) may occur at the cathode surface [14]: M nþ þ ne−→nM0: ð4Þ Furthermore, the hydroxide ions formed at the cathode increase the pH of the wastewater thereby inducing the precipitation of metal ions as corresponding hydroxides and co-precipitation with alumin- ium hydroxides [14]: M nþ þ nOH−→MðOHÞn↓ ð5Þ In addition, the electrolytic dissolution of the aluminium anode produces the cationic monomeric species such as Al3+ and Al(OH)2+ at low pH, which, at appropriate pH values, are transformed initially into Al(OH)3 and finally polymerized to Aln(OH)3n according to the following reactions [15]: AL 3þ ðaqÞ þ 3H2O→AlðOHÞ3 þ 3HþðaqÞ ð6Þ nAlðOHÞ3→AlnðOHÞ3n: ð7Þ However, depending on the pH of the aqueous medium, other ionic species such as dissolved Al(OH)2+, Al2(OH)4+ and Al(OH)4− hydroxide complexes may also be present in the system. The suspended aluminium hydroxides can remove pollutants from the solution by sorption, co-precipitation or electrostatic attraction, followed by coagulation [14]. The aim of this research is to study the removal of Mn2+ ions by the electrocoagulation process. The effect of important parameters such as time, initial pH, initial solution conductivity and applied current density was examined. By-products obtained from the EC bath were characterized by SEM, EDAX and XRD analysis. 2. Experiment 2.1. Materials and methods Manganese sulfate monohydrate from Riedl-de Haën Company was used to prepare the synthetic wastewater solutions. Sodium chloride salt (Merck, 99.9% purity) was used to adjust the initial solution conductivities. Initial pH of solutions was adjusted by NaOH and HCl solutions 0.1 molar (Merck). 2.2. EC cell Experiments were carried out in a double-walled Pyrex batch cylindrical EC cell with 250 mL capacity of aqueous solution. Water was circulated in jacket to keep the internal temperature of reactor solution in the range of 25±2 °C. The cell lidwas providedwith ports for the inlet and outlet of gases and sampling port. The solution was continuously aerated during the EC process. The experimental set-up is shown in Fig. 1. Two aluminium (Arak Alopars Co. with 98% purity) rectangular shape electrodes were installed vertically in the cell. Dimensions of electrodeswere 50 mm×30 mm×2mmand the distance between two electrodes in EC cell was 10 mm in all experiments. The electrodes were directly connected to a DC power supply (ESCORT, 3060 TD Dual Tracking, Taiwan) providing 0–30 V (0–6 A) with galvanostatic operational options for controlling the current density. 2.3. Procedure Before each experiment, the electrodes were abraded with sand paper, then dipped in HCL solution, cleaned with water, dipped in acetone for 5 min and finally put in oven in 110 °C for 30 min. 250 mL of the samples containing Mn2+ ion was transferred to the EC cell. A steady temperature of 25 °C wasmaintained for all test runs. The air speed was found to be sufficient to provide good mixing in the electrolytic cell and yet not strong enough to break up the flocs formed during the treatment process. The operation started when the current density was adjusted to a desired value. The samples were taken at regular time intervals, then filtered through 0.2 µm syringe filter paper and the solution concentration was analyzed using atomic absorption spectrophotometer (model Varian CP3800). Absorbance was measured at the wavelength of 279.5 nm and spectral bandwidth Fig. 1. A schematic of EC cell. a function of pH, is shown in Fig. 2. It can be seen that the initial pH has 25A. Shafaei et al. / Desalination 260 (2010) 23–28 a significant effect on the removal efficiency especially at low pH. As illustrated in Fig. 2, the removal efficiencies (Re) of manganese reached the value as high as 70% when pH exceeded 4 but removal yield lower than 13% was achieved at pH=2. When the initial pH is increased above 4, an insignificant change is seen in Re values. The decrease of Re at a pH less than 4 and higher than 8 was observed by many investigators and was attributed to an amphoteric behavior of Al(OH)3 which led to soluble Al3+ cations when the initial pH is low, and to monomeric anions Al(OH)4− when the initial pH is high. These soluble species have insignificant effect on the EC performance. When the initial pH was kept in the range of 4–8, all aluminium cations produced at the anode formed polymeric species Al13O4(OH)247+ and precipitated Al(OH)3 leading to a more effective treatment [18,33]. The Mn2+ removal efficiency exceeds higher than 99.5% at pH N7.0, the results of which are not shown in Fig. 2. This phenomenon might be ascribed to the precipitation of Mn hydroxide at the cathode, which started at pH ∼7–8. Fig. 2. Effect of initial pH on the Mn2+ removal efficiency: CD=6.25 mA/cm2; tEC=30 min; C0=100 mg/L. In Table 1, the final pH of the treated samples after 30 min is reported. As observed by other investigators [33], the treatment induced an increase in pH when the initial pH was low. This might be explained by the excess of hydroxyl ions produced at the cathode in sufficiently acidic conditions and by the liberation of OH− due to the occurrence of a partial exchange of Cl− with OH− in Al(OH)3 [26]. 3.2. Effect of current density In electrochemical processes, current density determines the coagulant production rate and the size and rate of the bubble production and hence affects the growth of flocs on the EC [25]. The Faraday's law explains the relationship between the amounts of Table 1 pH variation after EC (tEC=30 min). Initial pH Final pH 2.3 5.21 4.0 6.47 5.0 6.61 6.0 7.1 7.0 6.97 aluminium and hydroxide ions generated at a given time, within the electrocoagulation cell and the current flow: m = ItM zF ð9Þ where I is the current intensity, t is the time,M is themolecular weight of aluminium or hydroxide ion (g mol−1), z is the number of electrons transferred in the reaction (3 for aluminium and 1 for hydroxide) and F is the Faraday's constant (96,486 Cmol−1). To study the effect of current density on the Mn2+ removal efficiency, EC process was carried out using various current densities in the range of 1.5–9.4 mA/ cm2. Fig. 3 shows the Re against current density applied to the aluminium electrodes in the EC process. The Mn2+ removal efficiency increased from 35.8% at 1.5 mA/cm2 to 87.9% at 9.4 mA/cm2. According to Faraday's law, the amount of Al3+ species formed by the dissolution of the anode increases with the current density in the EC cell. Therefore, higher current density will generate significant 2+ Fig. 3. Effect of current density on the Mn2+ removal efficiency: tEC=30 min; pH0=7.0; C0=100 mg/L. amount of flocs, which in turn will trap the Mn ions and enhance the EC removal efficiency. The effect of current density on the removal efficiency of different contaminants was studied by other researchers and the similar results were observed [19–33]. 3.3. Effect of operation time As mentioned, the efficiency of EC process depends directly on the concentration of aluminium ions, which is produced by the electrode dissolution. In addition, the production rate of Al3+ ions is related to the time and current flow in the EC cell, according to Faraday's law (Eq. (9)). Fig. 4 shows the effect of electrolysis time on the Mn2+ removal efficiency. The results indicated that the efficiency of Mn2+ removal increases from 34.1% after 10 min to 94.4% after 60 min. Because when the operation time increases, an increase occurs in concentration of Al3+ ions and their hydroxide flocs so the EC process yield is enhanced [25]. 3.4. Effect of solution conductivity The effect of solution conductivity on the removal efficiency ofMn2+ has been investigated using NaCl salt as supporting electrolyte. The initial conductivity of solution was adjusted in the range 2.5–7 mS/cm by adding NaCl. As seen in Fig. 5, the solution conductivity had little positive effect on treatment efficiencies. The increasingRe can be related to the increase of chloride ions in the solution. Chloride ions not only increase the solution conductivity, but also decrease the passivity of the electrodes by removing the passivating oxide layer form on electrode Fig. 4. Effect of operation time on the Mn2+ removal efficiency: CD=6.25 mA/cm2; pH0=7.0; C0=100 mg/L. Fig. 6. Effect of solution conductivity on the cell voltage. 26 A. Shafaei et al. / Desalination 260 (2010) 23–28 surface [30] due to its catalytic action. It increases the availability of aluminium hydroxide in solution. Therefore, it is expected that in the presence of higher Cl− ions, the concentration removal of Mn2+ ions should be more. In addition, it should be noted that the conductivity of the solution affects the cell voltage and electrical energy consumption in EC cells. Figs. 6 and 7 show the effect of solution conductivity on the cell voltage and electrical energy consumption, respectively. It is clear that the cell voltage decreased with an increase in the conductivity at constant current density. When the conductivity of solution increases, IR-drop decreases, so the necessary voltage to reach on optimum current density will be diminished, and consequently the consumed electrical energy is decreased [25]. Fig. 5. Effect of solution conductivity on the Mn2+ removal efficiency: CD=6.25 mA/ cm2; tEC=30 min; pH0=7.0; C0=100 mg/L. 3.5. Effect of initial Mn2+ ion concentration The removal efficiencies of Mn2+ ions after 30 min from the solutions ofmanganese sulfate at different initial concentrations by EC are shown in Fig. 8. It shows that after 30 min of EC with initial concentrations, 25, 50, 100, 150 and 400 mg/L about 98.1, 94.8, 78.2, 73.8 and 36.2% of Mn2+ ions are removed from aqueous solution. Therefore, the Mn2+ removal efficiency is reduced by increasing the initial Mn2+ concentration. In addition, decreasing the efficiency of EC is considerable in higher initial concentration of Mn2+ (400 mg/L). Even though the removal efficiency of process has decreased, the amount of Mn2+ removed has increased when the initial concentration of solutions increased. In otherwords, after 30 min of EC under identical operating conditions, 24.5, 47.3, 78.2, 110.7 and 144.8 mg of Mn2+ ions are removed from the solutionswhen initialMn2+ loadingswere 25, 50, 100, 150 and 400 mg, respectively. This can be explained by the fact that the amount of aluminium electrodes dissolution is not affected by the initial Mn2+ concentration and at higher initial Mn2+ loading, number density of Mn2+ ions is higher and consequently these ions have higher chances of being trapped and removed (sweep coagulation) from the solution by the hydroxidematrices. This is a probable cause for a slightly 2+ higher removal of Mn in case of higher initial concentration. In the Fig. 7. Effect of solution conductivity on the electrical energy consumption. surface, as it was shown in Fig. 9. As can be seen from this figure the Fig. 10. SEM image of EC by-product. 27A. Shafaei et al. / Desalination 260 (2010) 23–28 study of Golder et al. on the removal of Cr3+ ions with EC, the similar results on the effect of initial Cr3+ concentration were observed [30]. 3.6. Characterization of the cathode surface and the EC by-products The cathode surface was characterized by EDAX after EC process. The precipitate (solid phase) that formed during EC process was separated by filter paper at the end of the process. It was dried in an oven and then ground to a fine powder. The by-product obtained from the EC cell was characterized by SEM, EDAX and XRD analyses. Microscopic observation of dried solid by-productwas carried out by a scanning electron microscope (Philips XL30, Holland). The elemental composition of the cathode surface and EC by-products was identified by energy dispersive X-ray analysis (EDAX) that was available in the scanning electron microscope (SEM). The crystalline phase of the EC by-product was examined by XRD analysis. Powder XRD patterns were recorded by the use of an automatic powder X-ray diffractometer (Philips, PC-APAD, Diffraction software) using Cu Kα (λ=1.5418 A°) radiation. The XRD scans were Fig. 8. Effect of initial concentration on the Mn2+ removal efficiency: CD=6.25 mA/ cm2; pH0=7.0; tEC=30 min.. recorded from 5° to 90° 2θ, with 0.08° step-width. 3.6.1. Characterization of the cathode surface The objective of performing EDAX analysis on the cathode surface is to investigate the possibility of direct deposition of Mn on cathode Fig. 9. EDAX analysis graph of the cathode surface after EC process. amount of deposited Mn on cathode surface is too low. 3.6.2. Characterization of the EC by-product A gray precipitate was formed at the end of the EC process. To illustrate the presence and the structure of the Mn particles which were removed from the aqueous solution, the dried by-product was characterized by SEM, EDAX and XRD analyses. The SEM image and EDAX analysis graph of by-product are shown in Figs. 10 and 11. The results show that the surface of this by-product was coated with a layer of Mn species. Also, it was seen that the by-product was composed of elements like Al, Cl and Mn and the amount of aluminium hydroxides was much higher than manganese element present in the EC by-product. The EC by-product was analyzed by XRD to investigate the structure of manganese and aluminium components which was removed from the aqueous solution. Fig. 12 shows the XRD pattern of EC by-product. The results revealed that no peaks related to the manganese components have been observed in this spectrum. Also, the peaks related to the aluminium component are broad and diffuse. Therefore, the XRD results indicated that the manganese and aluminium components which were formed in the EC flocs are amorphous [20,21]. 4. Conclusion In this study, the removal of Mn2+ ions from aqueous solution by electrocoagulation was investigated using aluminium electrodes. The Fig. 11. EDAX analysis graph of EC by-product. results showed that the EC is a feasible process for removing heavy metal ions like manganese from aqueous solutions. The Mn2+ ion is removed by direct reduction at the cathode surface, as hydroxides by the hydroxyl ions formed at the cathode via water electrolysis and by co-precipitation with the aluminium hydroxides. At elevated pH, decreased by increasing the initial conductivity. The experimental this project. The authors gratefully acknowledge Dr. M. Mirzazadeh for the kind help on editing this manuscript. References [1] A. Shafaei, F. Zokaee Ashtiani, T. Kaghazchi, Chem. Eng. J. 133 (2007) 311–316. [2] Y.C. Sharma, Uma, S.N. Singh, Paras, F. Gode, Chem. Eng. J. 132 (2007) 319–323. [3] A. Takeda, Brain Res. Rev. 41 (2003) 79–87. [4] J. Donaldson, Neuro. Toxicol. 8 (1987) 451–462. [5] J.O. Nriagu, Environ. Pollut. 50 (1988) 139–161. [6] G. Newcombe, D. Dixon (Eds.), Interface Science in Drinking Water Treatment: Theory and Applications, Elsevier Ltd, Queensland, 2006, pp.179–192. [7] R. Buamah, B. Petrusevski, J.C. Schippers, J. Water Supply Res. Technol. 57 (1) (2008) 1–11. [8] T. Suzuki, Y. Watanabe, G. Ozawa, S. Ikeda, Desalination 117 (1998) 119–130. [9] K.H. Choo, S.C. Han, S.J. Choi, J.H. Jung, D. Chang, J.H. Ahn, M.M. Benjamin, J. Ind. Eng. Chem. 13 (2) (2007) 163–169. [10] E. Okoniewska, J. Lach, M. Kacprzak, E. Neczaj, Desalination 206 (2007) 251–258. [11] B. Guan, W. Ni, Z. Wu, Y. Lai, Sep. Purif. Technol. 65 (2009) 269–274. [12] Z. Li, S. Imaizumi, T. Katsumi, T. Inui, X. Tang, Q. Tang, J. Hazard. Mater. 177 (2010) 501–507. [13] J.C. Xu, G. Chen, X.F. Huang, G.M. Li, J. Liu, N. Yang, S.N. Gao, J. Hazard. Mater. 169 (2009) 309–317. [14] I. Heidmann, W. Calmano, J. Hazard. Mater. 152 (2008) 934–941. [15] M.Y.A. Mollah, R. Schennach, J.R. Parga, D.L. Cocke, J. Hazard. Mater. B. 84 (2001) 29–41. [16] G. Chen, J. Sep. Purif. Technol. 38 (2004) 11–41. [17] M.Y.A. Mollah, P. Morkovsky, J.A.G. Gomes, M. Kesmez, J. Parga, D.L. Cocke, J. Hazard. Mater. 114 (1–3) (2004) 199–210. [18] P.K. Holt, G.W. Barton, M. Wark, C.A. Mitchell, J. Colloid Sur., A: Physicochem. Eng. Asp. 211 (2002) 233–248. Fig. 12. XRD patterns of EC by-product. 28 A. Shafaei et al. / Desalination 260 (2010) 23–28 results revealed that the high initial concentration of Mn2+ solution had a decreasing effect on the EC removal efficiency. However, the higher amount of Mn2+ was removed in the case of higher initial concentration. The solid by-product obtained from EC process was characterized by SEM, EDAX and XRD analyses. Acknowledgements The authors would like to express their gratitude to the Iranian National Petrochemical Company (NPC) for their financial support of higher Mn2+ removal has been achieved, so the best studied pH was 7.0. The Mn2+ removal efficiency by EC enhanced when the applied current density and the operating time increased. It was found that the solution conductivity had little increasing effect on the Mn2+ removal but the cell voltage and the electrical energy consumption [19] M. Khemis, J.P. Leclerc, G. Tanguy, G. Valentin, F. Lapicque, Chem. Eng. J. 61 (2006) 3602–3609. [20] N. Adhoum, L. Monser, J. Chem. Eng. Process. 43 (2004) 1281–1287. [21] G.A. Ciorba, C. Radovan, I. Vlaicu, L. Pitulice, J. Electrochim. Acta 46 (2000) 297–303. [22] H.A. Moreno-Casillas, D.L. Cocke, J.A.G. Gomes, P. Morkovsky, J.R. Parga, E. Peterson, J. Sep. Purif. Technol. 56 (2007) 204–211. [23] A. Bagga, S. Chellam, D.A. Clifford, J. Membr. Sci. 309 (2008) 82–93. [24] T.H. Kim, C. Park, E.B. Shin, S. Kim, Desalination 150 (2002) 165–175. [25] N. Daneshvar, A.R. Khataee, N. Djafarzadeh, J. Hazard. Mater. B 137 (2006) 1788–1795. [26] X. Chen, G. Chen, P.L. Yue, J. Sep. Purif. Technol. 19 (2000) 65–76. [27] U.B. Ogutveren, S. Koparal, J. Environ. Sci. Health A 32 (9–10) (1997) 2507–2520. [28] J.R. Parga, J.L. Cocke, J.A. Gomes, J. Hazard. Mater. B 124 (2005) 247–254. [29] D. Ghosh, C.R. Medhi, M.K. Purkait, Chemosphere 73 (2008) 1393–1400. [30] A.K. Golder, A.N. Samanta, S. Ray, J. Sep. Purif. Technol. 53 (2007) 33–41. [31] I. Heidmann, W. Calmano, J. Sep. Purif. Technol. 61 (2008) 15–21. [32] N. Meunier, P. Drogui, C. Montan´e, R. Hausler, G. Mercier, J. Hazard. Mater. B 137 (2006) 581–590. [33] N. Adhoum, L. Monser, N. Bellakhal, J.E. Belgaied, J. Hazard Mater. B 112 (2004) 207–213. Removal of Mn2+ ions from synthetic wastewater by electrocoagulation process Introduction Experiment Materials and methods EC cell Procedure Results and discussion Effect of pH Effect of current density Effect of operation time Effect of solution conductivity Effect of initial Mn2+ ion concentration Characterization of the cathode surface and the EC by-products Characterization of the cathode surface Characterization of the EC by-product Conclusion Acknowledgements References