Electrocoagulation of heavy metals containing model ...

Separation and Purification Technology 86 (2012) 248–254

Contents lists available at SciVerse ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Electrocoagulation of heavy metals containing model wastewater using monopolar iron electrodes Bassam Al Aji a, Yusuf Yavuz b,⇑, A. Savasß Koparal b a b

Damascus University, Civil Engineering Faculty, Dept. of Environmental Engineering, Syria Anadolu University, Dept. of Environmental Engineering, Turkey

a r t i c l e

i n f o

Article history: Received 13 July 2011 Received in revised form 28 October 2011 Accepted 10 November 2011 Available online 20 November 2011 Keywords: Electrocoagulation Monopolar electrode configuration Iron electrode Wastewater treatment Heavy metal removal

a b s t r a c t In this work, the performance of batch electrocoagulation (EC) using iron electrodes with monopolar configuration for simultaneous removal of copper (Cu), nickel (Ni), zinc (Zn) and manganese (Mn) from a model wastewater was investigated. The influences of current density (from 2 to 25 mA/cm2), initial metal concentration (from 50 to 250 mg/L) and initial pH (3, 5.68, 8.95) on removal efficiency were explored in a batch stirred cell to determine the best experimental conditions. The results indicated that EC was very efficient to remove heavy metals from the model wastewater having an initial concentration of 250 mg/L for each metal under the best experimental conditions. According to initial pH results, high pH values are more suitable for metal removal with EC treatment. At the current density of 25 mA/cm2 with a total energy consumption of 49 kWh/m3, more than 96% removal value was achieved for all studied metals except Mn which was 72.6%. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Industrial wastewaters with heavy metals are directly or indirectly discharged increasingly into the environment, especially with the rapid development of industries such as metal plating facilities, mining operations, fertilizer industries, tanneries, batteries, paper industries and pesticides, etc., which contain various kinds of toxic substances such as alkaline cleaning agents, degreasing solvents, oil, fat and heavy metals. Most of the metals such as copper, nickel, lead and zinc are harmful when they are discharged without treatment, because they are not biodegradable and tend to accumulate in living organisms, and many heavy metal ions are known to be toxic or carcinogenic [1]. Due to their high toxicity, industrial wastewaters are strictly regulated and have to be treated before being discharged [2]. There has been a serious environmental challenge for the produced heavy metal industry, due to pressures from public opinion and the numerous environmental regulations imposed. Various techniques have been employed for the treatment of heavy metals, such as precipitation, adsorption [3], ion-exchange [4], electrocoagulation [1,5–8] and ion exchange-assisted membrane separation [9,10]. Precipitation is most applicable among these techniques and considered to be the most economical. However, this technique produces a large amount of precipitate sludge that requires further treatment. Reverse osmosis and ion-exchange and other

⇑ Corresponding author. Tel.: +90 222 321 35 50; fax: +90 222 323 95 01. E-mail address: [email protected] (Y. Yavuz). 1383-5866/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2011.11.011

membrane separation techniques can effectively reduce metal ions, but their uses were limited due to a number of disadvantages such as high material and operational cost, and their operational problems [11]. This drawback, together with the need for low cost effective treatment, has opened new challenges for water treatment technologies. Innovative, cheap and effective methods of purifying water for human consumption as well as to clean the industrial wastewater effluents before discharging into any other water systems are needed [6]. The advantages of EC have encouraged many studies on the use of this technology for the treatment of several industrial effluents. According to Rajeshwar et al. [12] benefits from using electrochemical techniques include: environmental compatibility, versatility, energy efficiency, safety, selectivity, amenability to automation, and cost effectiveness. In addition to these, the following advantages can be added: electrochemical based systems allow controlled and rapid reactions, smaller systems become viable and instead of using chemicals and micro-organisms, the systems employ only electrons to facilitate water/wastewater treatment. Electrochemical techniques offer the possibility to be easily distributed, require minimum amount and number of chemicals. Robust and compact instrumentation is very easily achievable in electrochemical techniques, and hence, they will have the potential to replace sophisticated processes that require large volumes and/or number of chemicals, massive containers that are present in a typical water treatment plant, as it is also depicted in the study of Mollah et al. [13]. Of the known electrochemical techniques, there is much interest in using (EC) for treatment of wastewater containing heavy

B. Al Aji et al. / Separation and Purification Technology 86 (2012) 248–254

metals [7,14,15], phosphate [16], sulfide, sulfate and sulfite ions [17], boron [18,19], nitrate [20,21], fluorine [22], organic matter from landfill leachate [23], suspended particles [8], and humic acids [24]. Many approaches of EC are used for the treatment of industrial wastewater such as paper wastes [25], oil wastes [26,27], textile dyes wastes [28,29], polymeric wastes [30], chemical and mechanical polishing wastes [31], phenolic wastes [32]. EC has been found a promising technique in treating urban wastewater [33], treatment of restaurant wastewater [34,35], treatment of potable water [36,37], potato chips wastewater [38], treatment of laundry wastewater [39], olive mill wastewaters [40] and alcohol distillery wastewater [41]. Hence, the development of new treatment methods for effluents bearing heavy metals is an urgent issue. Among the methods that have recently been studied, EC proved very efficient in removing pollutants such as organic and inorganic matters from the industrial wastewaters [42]. In addition, a number of scientific works have indicated that heavy metals in the free form can be successfully removed by EC using aluminum, iron and cast iron electrodes [43,44]. There are few published studies using iron electrodes for the treatment of mixed pollutants. On the contrary, most of the papers in literature are dealt with the treatment of single metal. And some papers are investigating the effect of other pollutants on the removal of a main target pollutant. However, in this study, the mixed metal solution was used in all experiments The target of the present study was to examine the treatability of model wastewater containing heavy metals (Cu, Ni, Zn and Mn) by EC using monopolar iron electrodes and to explore the effects of varying operating parameters such as the applied current density, initial pH of solution, initial concentration and contact time on heavy metal removal.

2. Theory of EC The EC process involves many chemical and physical mechanisms [13]. Generally, aluminum or iron is dissolved by anodic dissolution. A range of coagulant species and hydroxides are formed which destabilize and coagulate the suspended particles or precipitate and adsorb dissolved contaminants [45]. It is generally accepted that the EC process involves three successive stages [13]: (i) Formation of coagulants by electrolytic oxidation of the electrode. The main reaction occurring at the metal anode is dissolution:

MðsÞ ! MðaqÞnþ þ ne

ð1Þ

249

(iii) Aggregation of the destabilized phases to form flocs. Anodic metal ions and hydroxide ions generated at the electrode surfaces react in the bulk wastewater to form various hydroxides and built up polymers. In addition, the following physicochemical reactions may also take place in the EC cell [45]: cathodic reduction of impurities present in wastewater, discharge and coagulation of colloidal particles, electrophoretic migration of the ions in solution, electroflotation of the coagulated particles by O2 and H2 bubbles produced at the electrodes, reduction of metal ions at the cathode, other electrochemical and chemical processes. If iron electrodes are used, the generated Fe(aq)3+ ions will immediately undergo further spontaneous reactions to produce corresponding hydroxides and/or polyhydroxides, and the hydrolysis products may form many monomeric ions Fe(OH)3 and/or polymeric hydroxy complexes depending on the pH of the aqueous medium and co-precipitate as [M,Fe](OH)3 [46]. According to Moreno-Casillas et al. [47] EC can be considered as an accelerated corrosion process that maybe explained by a Pourbaix diagram. Authors had studied electrocoagulation mechanism for COD removal using iron electrodes. And they had benefited from Fe-Pourbaix diagram to explain their results obtained at different pH values. Coagulation and EC generate sludge, and additionally EC generates scum. As an advantageous of EC, it is relatively a low sludge producing technique. Sludge formed by EC tends to be readily settable and easy to de-water, because it is composed of mainly metallic oxides/hydroxides [48,49]. According to Kushwaha et al. [50] sludge and scum generated in EC can be dried and used as a fuel in the boilers/incinerators, or can be used for the production of fuel-briquettes. On the other hand, the bottom ash obtained after incineration of scum and sludge may be blended with clay with higher ratio of clay to make fire bricks. Few studies have shown that addition of finely divided materials, such as silica, fly ash, etc. to clays and Portland cement not only increases heat resistance of these materials but also improves the microstructure and compressive strength of cement pastes [51]. Thus, sludge and scum generated by EC treatment of dairy wastewater can be disposed off through chemical and physical fixation [52]. Besides, some researchers have investigated the use of EC sludge for the treatment of other pollutants. For example, Golder et al. [53] have searched the removal of phosphate from aqueous solutions using calcined metal hydroxide sludge wastewater generated from EC. According to the study results [53], calcined electrocoagulated metal hydroxide sludge has been found as an efficient adsorbent for phosphate removal.

Additionally, water electrolysis occurs at the cathode and anode:

2H2 OðlÞ þ 2e ! H2 ðgÞ þ 2OH ðcathodic reactionÞ

ð2Þ

3. Materials and methods

2H2 OðlÞ ! 4Hþ ðaqÞ þ O2 ðgÞ þ 4e ðanodic reactionÞ

ð3Þ

To demonstrate the effect of initial metallic pollutants concentration and the time required for their quantitative removal, a stock solution of 1 L containing 1000 mg for each metal was prepared with sulfate salts of Cu, Ni, Zn and Mn. A set of the experiments were conducted with four different solutions containing the same concentrations of 50, 100, 150 and 250 mg/L of each metal ion respectively by diluting the stock solution. Experiments were performed in two stages. In the first stage, the effect of initial metal concentrations with different current densities on the system performance was investigated. Solutions, which were prepared from CuSO45H2O, NiSO47H2O, ZnSO47H2O, MnSO4H2O, were used in the EC setup at different concentrations (from 50 to 250 mg/L). Experiments were made by using a cylindrical glass cell of 500 mL on a magnetic stirrer (Falc Instruments F60 model, Italy). Six plates of iron, were installed vertically with a

(ii) Destabilization of the contaminants, particulate suspension, and breaking of emulsions. A direct electrochemical reduction of metal cations (Mn+) may occur at the cathode surface:

M nþ þ ne ! nM 0

ð4Þ

Furthermore, the hydroxide ions formed at the cathode increase the pH of the wastewater thereby inducing precipitation of metal ions as corresponding hydroxides and co-precipitation with hydroxides:

M nþ þ nOH ! MðOHÞn ðsÞ

ð5Þ

250


spacer to ensure fixed distance and immersed to a 4 cm depth, were used as electrodes in monopolar mode in the experiments. The total immersed area of both anode and cathode electrodes was 100 cm2. The inter-electrode distance was 0.3 cm. The experimental setup is shown in Fig. 1. The electrode surfaces were rinsed with diluted HNO3 and distilled water and then dried to eliminate the oxides and passivation layers between the experiments. A DCpower supply (Statron type 3262, Germany) characterized by the ranges 0–5 A for current and 0–80 V for voltage was used to apply different current densities i.e. 2, 5, 8, 15 and 25 mA/cm2. The polarity of the cell was reversed after each experiment to limit the formation of the passivation layers on the electrodes. To determine the system performance, samples of 5 mL were taken periodically from the reactor at predetermined time intervals (5 or 10 min), and then filtered using Whatman filter paper (Grade 40) to eliminate sludge formed during electrolysis. The residual heavy metal concentrations were determined by ICP-OES (Varian 720 ES model, Australia). In the second stage, the effect of initial pH on heavy metal removal was investigated with initial metal concentrations of 150 mg/L. The procedure in this stage was similar as the first stage. pH of the solutions was adjusted by adding either 0.5 N NaOH or 0.5 N H2SO4. pH and conductivity of solutions were measured by a pH meter (Thermo-Orion 420 A model, USA) and a conductivity meter (WTW Cond 720, Germany). All runs were performed at the laboratory temperature with a stirring speed of 200 rpm. Removal efficiency was calculated as follows:

Removal efficiency;% ¼

ðC 0 C t Þ 100 C0

ð6Þ

where C0 and Ct are heavy metal concentrations for each heavy metal at times 0 and t. 4. Results and discussions The EC process is affected by several operating parameters, such as initial concentrations, current density, initial pH and contact time. In this study, all these parameters were explored in order to evaluate a treatment technology for Cu, Ni, Zn and Mn removal from a model wastewater. In the first stage, experimental studies were carried out at the different current densities of 2, 5, 8, 15 and 25 mA/cm2, with different initial metal concentrations (50, 100, 150 and 250 mg/L) at the original pH (5.5–5.7) of the solutions. For the second stage of experiments, studies were carried out at different initial pHs of 3, 5.68 and 8.95 at the current density of 25 mA/cm2 and initial metal concentration of 150 mg/L.

Electrochemical reactor

Electrode connections

Multimeter

+

-

pH meter

Sacrificial iron electrodes

+-+-+ Power supply

Magnetic stirrer Fig. 1. Experimental setup used in the studies.

Model solution

4.1. Effect of current density In all electrochemical processes, current density is the most important parameter for controlling the reaction rate within the electrochemical reactor. It is well known that the current density determines the production rate of coagulant (amount of Fe2+ ions released by the anode), adjusts also bubble production, its size and distribution, and hence affects the growth of flocs Fe(OH)2 (s) or Fe(OH)3 (s) coagulate particles) in the EC reactor with different electrode configurations [54]. Fig. 2 shows the effect of current densities on the removal rate of heavy metals, and indicates that the removal rates of all studied metals increased, as expected, with increasing current density. Current density studies were accomplished at the original pH values (5.5–5.7) and the final pH values varied between 8.0 and 8.35. For the initial Cu concentration of 250 mg/L, the residual concentrations were 7.9, 2.9 and 2.15 mg/L at the current densities of 8, 15 and 25 mA/cm2, respectively after 40 min EC treatment as shown in Fig. 2(a). Fig. 2(b) shows that the residual Ni concentrations were 58.5, 17.25 and 11.00 mg/L after 40 min EC treatment at the current densities of 8, 15 and 25 mA/cm2, respectively, for the initial Ni concentration of 250 mg/L. As given in Fig. 2(c), initial Zn concentrations of 250 mg/L were reduced to 52.5, 10.55, and 6.4 mg/L after 40 min EC treatment at the current densities of 8, 15 and 25 mA/cm2, respectively. Similarly, initial Mn concentration of 250 mg/L were decreased to 104, 100.5, 82.5 mg/L after 40 min EC treatment at the current densities of 8, 15 and 25 mA/cm2, respectively as seen in Fig. 2(d). Variation of removal efficiency values with time for heavy metals and total energy consumption values as a function of current density are given in Fig. 3. Studies given in Fig. 3(a and b) were realized at the same initial concentration of 250 mg/L with the current densities of 15 and 25 mA/cm2, respectively. All heavy metals had similar trend for treatment. However, EC system was more successful in less time periods at the higher current density. For example, after 10 min EC treatment, removal efficiency values of 87.28, 41, 39.4 and 42.2% were obtained at the current density of 15 mA/cm2, whereas those were 90.8, 59.2, 63.6 and 66.4% at the current density of 25 mA/cm2 for Cu, Mn, Zn and Ni, respectively. According to Fig. 3(a and b) the removal rates of Cu were higher than other metals studied. Only after 20 min EC treatment 98.32% Cu removal was achieved at the current density of 25 mA/cm2 as shown in Fig. 3(b). It can be explained by additional partial removal of Cu by direct electroreduction at the cathode or by electroless deposition. Additionally, Ni and Zn exhibited very similar trend for all runs (Fig. 3(a and b)). These two metals had more times to reach high removal efficiency values in comparison to Cu. Although Mn removal was lower than the other metals studied, after 50 min EC treatment, removal efficiency value of 72.6% was reached at the current densities of 25 mA/cm2. Besides, energy consumption is directly related with the current density as also seen in Fig. 3(c). Because model wastewater is a mixture of four different heavy metals, energy consumption values represent total energy consumed to remove all metals simultaneously. Total energy consumption was 49 kWh/m3 to gain more than 96% removal value for Cu, Ni and Zn removals and 72.6% Mn removal at the current density of 25 mA/cm2. Using the current average electrical energy price in Turkey of €0.07 kWh1 the cost of energy consumed per m3 of treated water is €3.43. 4.2. Effect of initial concentration EC treatment for initial concentrations of 50, 100, 150 and 250 mg/L were tested. Cumulative removals of heavy metals from solution progress with EC. Initial concentrations from 50 to 250 mg/L of Cu, Zn, Ni and Mn did not influence the removal rates,

251


b

300

Residual Ni conc., mg/L

Residual Cu conc., mg/L

a

8 mA/cm2

250

15 mA/cm2

200

25 mA/cm2

150 100 50 0

300 8 mA/cm2

250

15 mA/cm2 200

25 mA/cm2

150 100 50 0

0

1

2

3

4

0

1

2

Charge, Ah/L

d

300

Residual Mn conc., mg/L

Residual Zn conc., mg/L

c

8 mA/cm2

250

15 mA/cm2

200

3

4

Charge, Ah/L

25 mA/cm2

150 100 50

300 8 mA/cm2

250

15 mA/cm2

200

25 mA/cm2

150 100 50

0

0 0

1

2 Charge, Ah/L

3

4

0

1

2 Charge, Ah/L

3

4

Fig. 2. Effect of current densities on the residual concentrations of heavy metals (C0 = 250 mg/L): (a) Cu, (b) Ni, (c) Zn, and (d) Mn.

b

100

Removal efficiency, %


a

80 60 40 Cu

Mn

20 Zn

Ni

100 80 60 40 Cu

Mn

Zn

Ni

20

0

0 0

10

20

30

40

0

10

c

Energy consumption, kWh/m 3

Time, min

20

30

40

50

Time, min

50 i= 15 mA/cm2

40

i= 25 mA/cm2 30 20 10 0 0

10

20

30

40

50

Time, min Fig. 3. Variation of removal efficiency values with time for heavy metals: (a) C0 = 250 mg/L, i = 15 mA/cm2, (b) C0 = 250 mg/L, i = 25 mA/cm2 and (c) total energy consumption values as a function of current density.

whereas higher initial concentrations caused higher removal rates of heavy metals. Fig. 4 shows the variations of Cu, Ni, Zn and Mn concentrations for different initial concentrations as a function of time at the current density of 8 mA/cm2. Initial concentration

studies were realized at the original pH values (5.5–5.7) and the final pH values varied between 8.09 and 8.34. All graphics from Fig. 4(a–d) for each metal exhibited a logical trend related with time depending on their initial metal concentrations. Certainly,

252


b

1

Ct/Co

0.8

0.8

100 mg/L 150 mg/L

0.6

1 50 mg/L

50 mg/L

Ct/Co

a

250 mg/L 0.4 0.2

100 mg/L 150 mg/L

0.6

250 mg/L

0.4 0.2

0

0 0

10

20

30

0

40

10

20

50 mg/L 150 mg/L

0.6

1 50 mg/L

0.8

100 mg/L Ct/Co

Ct/Co

d

1 0.8

40

Time. min

Time. min

c

30

250 mg/L

0.4

100 mg/L 150 mg/L

0.6

250 mg/L

0.4 0.2

0.2

0

0 0

10

20

30

0

40

10

20

30

40

Time. min

Time. min

Fig. 4. Effect of initial concentration on the removal efficiency of heavy metals (i = 8 mA/cm2): (a) Cu, (b) Ni, (c) Zn and (d) Mn.

for higher concentrations longer time for removal is needed, but higher initial concentrations were reduced significantly in relatively less time than lower concentrations. According to Fig. 4, no direct correlations exist between each metal concentration and removal efficiency. The EC treatment is more effective at the beginning of the process when the concentration is higher than at the end of the operation when the concentration is relatively lower. Fig. 4 shows that after 40 min EC treatment at the current density of 8 mA/cm2, 1.9, 2.55, 4 and 7.90 mg/L for Cu (Fig. 4(a)), 1.95, 7.53, 10.2 and 58.5 mg/L for Ni (Fig. 4(b)), 2.1, 5.18, 11.35 and 52.5 mg/L for Zn (Fig. 4(c)) and 13, 24.93, 52 and 104 mg/L for Mn (Fig. 4(d)) were achieved for the initial metal concentrations of 50, 100, 150 and 250 mg/L, respectively. Additionally, electrical conductivity of the solution was monitored during the EC treatment in one study performed at the initial concentration of 150 mg/L (for each metal), current density of 25 mA/cm2 and original pH. The initial electrical conductivity of 1467 lS/cm decreased to the final electrical conductivity of 520 lS/cm at the end of the experiment (40 min). Reduction in the electrical conductivity value generally complies with the initial concentration study results. First order kinetic model was applied to the results obtained. Cu, Ni and Zn removal kinetic fitted well; however, Mn removal did not fit with the kinetic model very much as also seen in Table 1. According to the Table 1, correlation coefficients (R2) for each metal studied increase with increasing initial metal concentration.

So, it can be said in general that the removal of metals fits well with the first-order kinetic model at high metal concentrations. For instance, correlation coefficients of Ni were calculated as 0.8878, 0.9296, 0.9524 and 0.9765 for initial Ni concentrations of 50, 100, 150 and 250 mg/L, respectively. On the other hand, when the first-order rate constants (k) of different heavy metals for a given concentration are investigated in horizontal order, it can be seen that k values are decreasing from Cu to Mn, also comply with the removal results. As an example, k values are 0.083, 0.0347, 0.0368 and 0.0308 min1 for Cu, Ni, Zn and Mn, respectively for initial metal concentration of 250 mg/L. These results, at the same time, mean that Cu can be removed 2.39, 2.25 and 2.69 times faster in comparison to Ni, Zn and Mn, respectively. 4.3. Effect of initial pH It has been established that the influent pH is an important operating factor influencing the performance of the EC process [55]. Generally, the pH of the medium changes during the process as also observed by other investigators and this variation depends on the type of electrode and initial pH. The final pH was 8.02, 8.30 and 11 for the initial pH of 3, 5.68 (original pH) and 8.95, respectively. In the current density, initial concentration and initial pH studies, pH was observed to increase during the reactive phase, then to stabilize at pH close to 8–11 depending on the initial pH value as also referred by Yavuz et al. [49]. This increase in pH

Table 1 First order rate constants for different initial concentration of heavy metals (i = 8 mA/cm2). Concent. (mg/L)

50 100 150 250

Cu

Ni

Zn

Mn

k (min1)

R2

k (min1)

R2

k (min1)

R2

k (min1)

R2

0.0943 0.1028 0.1024 0.083

0.8813 0.9304 0.9201 0.9753

0.0947 0.0734 0.0607 0.0347

0.8878 0.9296 0.9524 0.9765

0.0895 0.0772 0.0594 0.0368

0.9225 0.9925 0.9620 0.9821

0.0408 0.0423 0.0281 0.0308

0.5217 0.6398 0.8909 0.9079

253



100 80 60

120


can be explained by the occurrence of water electrolysis resulting in hydrogen evolution and production of OH ions. The relative stability of pH afterwards could be probably due to the formation of the insoluble Fe(OH)3 flocs and the rest metal hydroxides. To see the effect of initial pH on the removal efficiency, initial pH of 3, 5.68 and 8.95 was studied. As an example, variations of Cu removal efficiency values with initial pH are given in Fig. 5. According to this figure, best result was obtained at the original pH. Fig. 5 also shows that 15 min EC treatment is enough to have more than 97% Cu removal value, whereas it takes 25 min to reach the same removal value for the initial pH of 8.95. On the other hand, 94.7% Cu removal value was obtained after 45 min EC treatment for the initial pH of 3. Therefore, pH adjustment for better Cu removal seem does not necessary if solution contain only Cu. The removal efficiency values for all heavy metals studied as a function of the initial pH for 45 min EC treatment are demonstrated in Fig. 6. As shown in this figure, removal efficiencies increase generally at high pH values, because the majority of iron complexes (coagulants) are formed at these values. As aforementioned, natural pH of the solutions was initial pH, except studies that the effect of initial pH was investigated. Therefore, while initial pH was from 5.5 to 5.7, final pH varied between 8.0 and 8.35. For pH value of 4 < pH < 7, iron undergoes hydrolysis and Fe(III) hydroxide begins to precipitate as floc with yellowish color. For pH 6 < pH < 9, precipitation of Fe(III) hydroxide continues, and Fe(II) hydroxide precipitation also occurs presenting a dark green floc. The pH is in the range of 7–8 for minimum solubility of Fe(OH)n. EC floc is formed due to the polymerization of iron oxyhydroxides [47]. In this case, conditions throughout the electrochemical cell are not stable, and concentrations, species and pH vary as it may be illustrated using Fe-Pourbaix diagram. On the other hand, when the Pourbaix diagrams are investigated for the metals studied (Cu, Ni, Zn and Mn) in this work, followings can be mentioned: According to the Cu-Pourbaix diagram, cupper is solid in the form of Cu(OH)2 at pH approximately >4.5 until pH 14. Nickel is also solid in Ni(OH)2 form at pH approximately >6 according to its Pourbaix diagram. Zn-Pourbaix diagram shows that zinc is solid as ZnO at pH between 7 and 14. And finally, manganese is solid as Mn(OH)2 from pH 7.5 to 13 according to the Mn-Pourbaix diagram. Consequently, it can be concluded that all chemicals were desirable form for good EC performance with respect to pH values measured during the experimental studies and Pourbaix diagrams for working electrode and heavy metals. In some literature studies electrocoagulation of mixed metal solution [1,5] and electroplating wastewater [6] has been investigated using aluminum electrodes. According to Heidmann and Calmano [1], metals are 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 aluminium hydroxides. The authors have studied Zn, Cu, Ni, Ag and Cr metals

100 80 60 40 20 0 pH= 5,68

pH= 3 Cu

Zn

pH= 8,95 Ni

Mn

Fig. 6. Variation of removal efficiency with initial pH (initial concentration for each metal = 150 mg/L, i = 25 mA/cm2, t = 45 min).

alone and a mixed of these metals and found a linear decrease in metal concentrations with time except Cr. They have indicated that metal removal from mixed solution is harder than single metal solution for all metals studied. They have reached a total metal removal for Zn, Cu and Ni metals like us. However, we used iron electrodes different from their study. In another study performed by Hanay and Hasar [5], effect of sulfate and chloride anions on Cu, Zn and Mn removal has been examined. According to their results, electrocoagulation using aluminum electrode is more successful for specially Cu and Zn metals, whereas system shows a limited performance for Mn removal. Similar results for these three metals were obtained in the present work. Adhoum et al. [6] have studied a model and real wastewater and also found that Cu and Zn removal rate is faster than Cr. They indicated that 5 min is enough to reach admissible limit for Cu and Zn, whereas it is 20 min for Cr. 5. Conclusions In this work, EC treatment using iron sacrificial electrodes for a model wastewater containing Cu, Ni, Zn and Mn heavy metals were investigated. The effects of current density, initial metal concentration and initial pH on the removal efficiency were examined in a parallel plate electrochemical reactor. An evaluation using Pourbaix diagrams for working electrode and studied metals were also realized. In the study, at the current density of 25 mA/cm2 with a total energy consumption of 49 kWh/m3, more than 96% removal efficiency was obtained for Cu, Ni and Zn, whereas it was 72.6% for Mn. This means at the same time that the cost of energy consumed per m3 of treated water is €3.43 when the current average electrical energy price in Turkey of €0.07 kWh1 is taken into account. According to the results, EC using monopolar iron electrodes is a convenient route for effective removal of heavy metals from a model wastewater and achieves faster removal of pollutants in comparison to chemical coagulation, where several hours are needed and adsorption on activated carbon. Using both the same model solution and the electrochemical system in bipolar electrode configuration can be employed to comparison purposes for future studies.

40

References 20 0 0

15

30

45

Time, min Fig. 5. Variation of removal efficiency with initial pH for Cu (C0 = 150 mg/L, i = 25 mA/cm2).

[1] I. Heidmann, W. Calmano, Removal of Zn(II), Cu(II), Ni(II), Ag(I) and Cr(VI) present in aqueous solutions by aluminum electocoagulation, J. Hazard. Mater. 152 (2008) 934–941. [2] T.A. Kurniawan, G.Y.S. Chan, W.H. Lo, S. Babel, Physico-chemical treatment techniques of wastewater laden with heavy metals, Chem. Eng. J. 118 (2006) 83–98. [3] J.P. Chen, X. Wang, Removal of copper, zinc and lead ion by activated carbon in pretreated fixed bed columns, Sep. Purif. Technol. 19 (2000) 157–167.

254


[4] N. Sapari, A. Idris, N.H.Ab. Hamid, Total removal of heavy metal from mixed plating rinse wastewater, Desalination 106 (1996) 419–422. [5] Ö. Hanay, H. Hasar, Effect of anions on removing Cu2+, Mn2+ and Zn2+ in electrocoagulation process using aluminum electrodes, J. Hazard. Mater. 112 (2011) 572–576. [6] N. Adhoum, L. Monser, N. Bellakhal, J.E. Belgaied, Treatment of electroplating wastewater containing Cu2+, Zn2+ and Cr(VI) by electrocoagulation, J. Hazard. Mater. B112 (2004) 207–213. [7] I. Kabdasßlı, T. Arslan, T. Ölmez-Hancı, I. Arslan-Alaton, O. Tünay, Complexing agent and heavy metal removals from metal plating effluent by electrocoagulation with stainless steel electrodes, J. Hazard. Mater. 165 (2009) 838–845. [8] B. Merzouk, B. Gourich, A. Sekki, K. Madani, M. Chibane, Removal turbidity and separation of heavy metals using electrocoagulation–electroflotation technique, a case study, J. Hazard. Mater. 164 (2009) 215–222. [9] S.L. Vasilyuk, T.V. Maltseva, V.N. Belyakov, Influence of water hardness on removal of copper ions by ion-exchange-assisted electrodialysis, Desalination 162 (2004) 249–254. [10] Y.S. Dzyazko, V.N. Belyakov, Purification of a diluted nickel solution containing nickel by a process combining ion exchange and electrodialysis, Desalination 162 (2004) 179–189. [11] C.A. Basha, N.S. Bhadrinarayana, N. Anantharaman, K.M. Meera Sheriffa Begum, Heavy metal removal from copper smelting effluent using electrochemical cylindrical flow reactor, J. Hazard. Mater. 152 (2008) 71–78. [12] K. Rajeshwar, J. Ibanez, G.M. Swain, Electrochemistry and the environment, J. Appl. Electrochem. 24 (1994) 1077–1091. [13] M.Y.A. Mollah, P. Morkovsky, J.A.G. Gomes, M. Kesmez, J. Parga, D.L. Cocke, Fundamentals, present and future perspectives of electrocoagulation, J. Hazard. Mater. B114 (2004) 199–210. [14] Y.S. Jung, M. Pyo, Removal of heavy metal ions by electrocoagulation for continuous use of Fe2+/Fe3+-mediated electrochemical oxidation solutions, Bull. Korean Chem. Soc. 29 (5) (2008) 974–978. [15] P.R. Kumar, S. Chaudhari, K.C. Khilar, S.P. Mahajan, Removal of arsenic from water by electrocoagulation, Chemosphere 55 (2004) 1245–1252. _ [16] N. Bektasß, H. Akbulut, H. Inan, A. Dimoglo, Removal of phosphate from aqueous solutions by EC, J. Hazard. Mater. B 106 (2004) 101–105. [17] M. Murugananthan, G.B. Raju, S. Prabhakar, Removal of sulfide, sulphate and sulfite ions by EC, J. Hazard. Mater. B 109 (2004) 37–44. [18] A.E. Yilmaz, R. Boncukcuoglu, M.M. Kocakerim, B. Keskinler, The investigation of parameters affecting boron removal by electrocoagulation method, J. Hazard. Mater. B 125 (2005) 160–165. [19] G. Sayiner, F. Kandemirli, A. Dimoglo, Evaluation of boron removal by electrocoagulation using iron and aluminum electrodes, Desalination 230 (2008) 205–212. [20] N.S. Kumar, S. Goel, Factors influencing arsenic and nitrate removal from drinking water in a continuous flow electrocoagulation process, J. Hazard. Mater. 173 (2010) 528–533. [21] A.S. Koparal, U.B. Ogutveren, Removal of nitrate from water by electroreduction and electrocoagulation, J. Hazard. Mater. B 89 (2002) 83–94. [22] C. Hu, S. Lo, W. Kuan, Y. Lee, Treatment of high fluoride content wastewater by continuous electrocoagulation-flotation system with bipolar aluminum electrodes, Sep. Purif. Technol. 60 (2008) 1–5. [23] T. Tsai, S.T. Lin, Y.C. Shue, P.L. Su, Electrolysis of soluble organic matter in leachate from landfills, Water Res. 31 (1997) 3073–3081. [24] D. Ghernaout, B. Ghernaout, A. Saiba, A. Boucherit, A. Kellil, Removal of humic acids by continuous electromagnetic treatment followed by EC in batch using aluminium electrodes, Desalination 239 (2009) 295–308. [25] M. Zaieda, N. Bellakhal, Electrocoagulation treatment of black liquor from paper industry, J. Hazard. Mater. 163 (2009) 995–1000. [26] M. Asselin, P. Drogui, S.K. Brar, H. Benmoussa, J.F. Blais, Organics removal in oily bilge water by electrocoagulation process, J. Hazard. Mater. 151 (2008) 446–455. [27] U.T. Un, A.S. Koparal, U.B. Ogutveren, Electrocoagulation of vegetable oil refinery wastewater using aluminum electrodes, J. Environ. Manage. 90 (2009) 428–433. [28] A. Alinsafi, M. Khemis, M.N. Pons, J.P. Leclerc, A. Yaacoubi, A. Benhammou, A. Nejmeddine, EC of reactive textile dyes and textile wastewater, Chem. Eng. Process. 44 (2005) 461–470.

[29] G.B. Raju, M.T. Karuppiah, S.S. Latha, S. Parvathy, S. Prabhakar, Treatment of wastewater from synthetic textile industry by electrocoagulation– electrooxidation, Chem. Eng. J. 144 (2008) 51–58. [30] M. Panizza, C. Bocca, G. Cerisola, Electrochemical treatment of wastewater containing polyaromatic organic pollutants, Water Res. 34 (9) (2000) 2601– 2605. [31] B.M. Belongia, P.D. Haworth, J.C. Baygents, S. Raghvan, Treatment of alumina and silica chemical mechanical polishing waste by electrodecantation and electrocoagulation, J. Electrochem. Soc. 146 (1999) 4124–4130. [32] N. Adhoum, L. Monser, Decolourization and removal of phenolic compounds from olive mill wastewater by electrocoagulation, Chem. Eng. Process. 43 (2004) 1281–1287. [33] M.F. Pouet, A. Grasmick, Urban wastewater treatment by electrocoagulation and flotation, Water Sci. Technol. 31 (1995) 275–283. [34] X. Chen, G. Chen, P.L. Yue, Separation of pollutants from restaurant wastewater by electrocoagulation, Sep. Purif. Technol. 19 (2000) 65–76. [35] G. Chen, W. Chen, P.L. Yue, Electrocoagulation and electroflotation of restaurant wastewater, J. Environ. Eng-ASCE. 129 (2000) 858–863. [36] E.A. Vik, D.A. Carlson, A.S. Eikum, T. Gjessing, Electrocoagulation of potable water, Water Res. 18 (1984) 1355–1360. [37] G. Chen, Electrochemical technologies in wastewater treatment, Sep. Purif. Technol. 38 (2004) 11–41. [38] M. Kobya, H. Hiz, E. Senturk, C. Aydiner, E. Demirbas, Treatment of potato chips waste water by electrocoagulation, Desalination 190 (2006) 201–211. [39] J. Ge, J. Qu, P. Lei, H. Liu, New bipolar electrocoagulation–electroflotation process for laundry waste water, Sep. Purif. Technol. 36 (2004) 33–39. [40] U.T. Un, S. Ugur, A.S. Koparal, U.B. Ogutveren, Electrocoagulation of olive mill waste waters, Sep. Purif. Technol. 52 (2007) 136–141. [41] Y. Yavuz, EC and EF processes for the treatment of alcohol distillery wastewater, Sep. Purif. Technol. 53 (2007) 135–140. [42] D. Rajkumar, K. Palanivelu, Electrocoagulation treatment of industrial wastewater, J. Hazard. Mater. B113 (2004) 123–129. [43] A.K. Golder, A.N. Samanta, S. Ray, Removal of trivalent chromium by electrocoagulation, Sep. Purif. Technol. 53 (2007) 33–41. [44] A.K. Golder, V. Dhaneesh, A.N. Samanta, S. Ray, Removal of nickel and boron from plating rinse effluent by electrochemical and chemical techniques, Chem. Eng. Technol. 1 (2008) 143–148. [45] A.B. Paul, Proceedings of the 22nd WEDC Conference on Water Quality and Supply, New Delhi, India, 1996, p. 286. [46] P.N. Johnson, A. Amirtharajah, Ferric chloride and alum as single and dual coagulants, JAWWA 75 (1983) 232–239. [47] H.A. Moreno-Casillas, D.L. Cocke, J.A.G. Gomes, P. Morkovsky, J.R. Parga, E. Peterson, Electrocoagulation mechanism for COD removal, Sep. Purif. Technol. 56 (2007) 204–211. [48] M.Y.A. Mollah, R. Schennach, J.R. Parga, D.L. Cocke, Electrocoagulation (EC) – science and applications, J. Hazard. Mater. B84 (2001) 29–41. [49] Y. Yavuz, E. Öcal, A.S. Koparal, Ü.B. Ög˘ütveren, Treatment of dairy industry wastewater by EC and EF processes using hybrid Fe-Al plate electrodes, J. Chem. Technol. Biotechnol. 86 (2011) 964–969. [50] J.P. Kushwaha, V.C. Srivastava, I.D. Mall, Organics removal from dairy wastewater by electrochemical treatment residue disposal, Sep. Purif. Technol. 76 (2010) 198–205. [51] M. Heikal, Effect of temperature on the physico-mechanical and mineralogical properties of Homra pozzolanic cement pastes, Cement Concrete Res. 30 (2000) 1835–1839. [52] C. Thakur, V.C. Srivastava, I.D. Mall, Electrochemical treatment of a distillery wastewater: parametric and residue disposal study, Chem. Eng. J. 148 (2009) 496–505. [53] A.K. Golder, A.N. Samanta, S. Ray, Removal of phosphate from aqueous solutions using calcined metal hydroxide sludge wastewater generated from electrocoagulation, Sep. Purif. Technol. 52 (2006) 102–109. [54] M. Kobya, E. Demirbas, A. Dedeli, M.T. Sensoy, Treatment of rinse water from zinc phosphate coating by batch and continuous EC processes, J. Hazard. Mater. 173 (2010) 326–334. [55] M. Asselin, P. Drogui, H. Benmoussa, J.F. Blais, Effectiveness of electrocoagulation process in removing organic compounds from slaughterhouse wastewater using monopolar and bipolar electrolytic cells, Chemosphere 72 (2008) 1727–1733.