Optimization of Chromium reduction and Sludge production by bipolar Electrocoagulation using Response Surface Methodology

 

Hooshyar Hossini1, Abbas Rezaee*1, Hossein Masoumbeigi2

 

1 Department of Environmental Health, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran

2 Health Research Center and Environmental Health Engineering Department, Faculty of Health,

Baqiyatallah University of Medical Scienc, Tehran, Iran

 

 

Abstract

Introduction: Electrocoagulation (EC) is a high performance process for water and wastewater treatment. Recently, the process was proposed as an interest technology with various advantages.

Methods: In this study, chromium removal and sludge production by the bipolar electrocoagulation was investigated and the process optimized by response surface methodology (RSM). Different operating parameters, such as current density, running time, pH and initial chromium concentration during the process were examined.

Results: According to results of model, R2 for chromium removal and sludge production were obtained 0.9374 and 0.889, respectively. In the optimum condition, current density, running time, pH and initial concentration were 0.27 A, 70 min, pH 4.62 and 156 mg/l, respectively.

Conclusion:The results of this study showed that RSM is one of the suitable technique to optimize the best operating conditions to maximize the chromium removal. Chromium removal efficiency and sludge production in optimum condition were 73.13 and 174.02ml/l, respectively.

 

Key words: Electrocoagulation, Optimization, Chromium, Bipolar, Sludge production


Introduction

Chromium is one of the important heavy metal widely used in various industrials such as tanning, electroplating, metal finishing, paint industries, steel manufacturing [1]. It is found in rocks, soils and volcanic dusts. Chromium is a heavy metal with oxidation states that vary from −2 to +6; however, the +3 and +6 states are common in aqueous solutions. The Cr (VI) species may be in the form of dichromate, hydro chromate, or chromate in a solution depending on the pH values [2]. The hexavalent chromium is the most toxic of the other chromium species. It has been classified as a human carcinogen [3].Various techniques have been proposed to remove chromium from industrial effluents, include chemical precipitation, evaporation, reverse osmosis, adsorption, ion-exchange and membrane separation [4, 5]. Generally, chemical reduction�precipitation process is used to remove chromium from wastewater, but it has high operational costs due to the generation of high amount of sludge [6]. Recently,electrochemical treatment proposed as a considerable technology, because of high efficiency, its lower maintenance cost, easy to operation and time of operation [7, 8]. In electrocoagulation process, various metal hydroxide were produced as the sludge. The relation between the applied power and the amount of coagulants dissolved into the solution from the anode is described by Faraday�s law as below equation:

Where WA is the amount of dissolved anode material (g); I is current intensity (A); t is running time (s); m is specific molecular weight (g/mol); F is Faraday�s constant (96485 C/mol); and n is the number of electrons involved [9]. In this case, power consumption will be more than 10 times compare to the power of electrochemical reduction [10]. Generally, high sludge production in a general electrocoagulation is a problem that should be manage by high cost technologies. The conversion of the hexavalent chromium by a conventional electrocoagulation using iron as an anode electrode shown by following equations [11]:

In order to overcome the above problems, a bipolar electrocoagulation process was examined in this study. Platinum (Pt) electrode was used as an anode in the bipolar electrocoagulation process. Pt can catalyzes the electrolysis of water according to the follow reaction:

In the abow reaction, production of H2 or O2 have been depended to direction of current flow.

According to the standard electrode potential,Pt could be produced oxygen in this process. The oxidation have negative effect on the reduction of Cr (VI) to Cr (III).Therefore, we have investigated the chromium removal using bipolar electrocoagulation by Pt/Fe-Fe/Graphite electrodes. Chromium removal by conventional electrocoagulation techniques have been reported by several researchers. To the best of our knowledge and based on the literature, there is no previous report on the bipolar electrocoagulation using Pt as an anode. Also, few studies focused on the chromium removal and simultaneously optimization of the process using statistical and mathematical methods. To evaluate the bipolar electrocoagulation process, the chromium removal was studied as the function of various operational parameters, including current density, running time, pH and initial chromium concentration.

 

Materials and methods

Materials

All chemical used in this work were purchased from Merck. Stock solution of Cr (VI) was prepared by adding the specific values of potassium dichromate (K2Cr2O7) in deionized water. Desirable concentrations of Cr (VI) were prepared from the stock dilution.

Electrocoagulation experiments

Fig. 1 shows a schematic flow diagram of the experiments. The reactor was equipped with a Pt (4 cm wide, 6 cm high and 0.2 cm thickness) anode and graphite (6 cm wide, 15 cm high and 0.5 cm thickness) as cathode and iron (6 cm wide, 15 cm high and 0.2 cm thickness) as sacrificial electrodes. The outer electrodes were connected to a DC power supply (TEK-8051, 30V and 5A double). Two basic electrodes (Pt and graphite) are connected to positive and negative poles of the DC power supply. Sacrificial anode no connected to any poles. Therefore, the power to dissolve of iron electrodes is provided from the platinum. The distance between anode and cathode electrodes was fixed to1.5 cm. Before starting-up the process, the anode electrodes were cleaned with H2SO4(1 M )and rinsed in deionized water to eliminate impurities from the surface of the electrodes. The reactor was gently mixed with a magnetic stirrer (ATL-4200, Anytech Co., Korea). The pH was adjusted to the desired value with 1 M HCl and 1 M NaOH.

In bipolar process, because of higher surface area in the electrodes the intensity is higher (Ghosh et al., 2008). On other hand, generation of coagulant iron hydroxides is lower for bipolar electrode arrangement [12]. Hence, with higher current density in the process, the total removal efficiency increased.

Analysis

The residual chromium was determined via a colorimetric method using 1, 5 diphenylcarbazide [CO(NH.NHC6H5)2] regent by means of aUV/Visible spectrophotometer (Rayleigh UV 9200, China) set to 540 nm, according to standard methods for the analysis of water and wastewater [13]. The pH values were measured by a portable probe (Eutech, Singapore). All experiments were performed triplicate to determine the precision of the results.

Results and discussion

RSM is a statistical method that uses quantitative data from appropriate experiments to determine regression model equations and operating conditions. It is an important branch of experimental design and a critical methodology in developing new processes, optimizing their performance and improving design and formulation of new products [14].


 

Fig. 1.The schematic diagram of bipolar electrocoagulationreactor

 


The most extensive applications of RSM are in industrial research, particularly in situations where several input variables influence the process performance measure. This process performance measure is called the response and the input variables are called independent variables [15]. Also the behavior of the system is explained by the following quadratic equation:

In this study, the main operating factors, such as time, applied current, initial pH values and initial chromium concentration were evaluated. The experimental ranges of the variables concerning chromium removal are summarized in Table 1. In order to investigate the effect of operating factors in the bipolar electrocoagulation process, experiments were conducted for the parameters using statistically designed experiments. The current density ranges were between 0.1 and 0.5. The running time were between 30 and 180 min. The pH values were varied between 3 and 8 and initial concentration between 50 and 300mg/l. P-value for chromium removal was considered 0.05, and the analysis of variance obtained data shows high significant value for each variable (Table 2). The R-Squared, Lack of Fit and Mean Square of the model were achieved 532.48, 24.5 and < 0.0001, respectively. The P-value of model represents a significant technique, and confirm the study. Table 3 shows the experimental and predicated data for chromium removal efficiency and sludge produced during the bipolar electrocoagulation process. The results of� R-Squared and Adj R-Squared for chromium model shows a higher adjustdegree compare to sludge produced. The amonuts of the parameters for chromium and sludge model as following:

Cr model R2 (0.9374) > Cr model Adj R2(0.8991) > sludge model R2(0.889) > sludge model Adj R2(0.872)

The best fitting response for the chromium removal and sludge production models are concluced as following Equations:

 

Cr(VI) removal% model:

y = + 65.13 + 9.57 x1 + 9.88 x2 - 4.70 x3 - 2.49 x4 + 1.60 x1x3 - 3.08 x1 x4 - 1.83 x2 x3 + 2.56 x2 x4 + 1.78 x3 x4 + 1.23 x12 + 2.69 x42

 

Produced sludge model:

y = +165.57+41.42 x1 +39.75 x2+4.58 x3 -7.42 x4


 

Table 1. Ranges of the experimental parameters

Variable

Coded variables values

Low axial

(-α = -2)

factorial

High axial (+α =+2)

Low (-1)

High (+1)

Applied Current (A)

x1

0.1

0.20

0.40

0.5

Time (minute)

x2

30

67.50

142.50

180

pH

x3

3

4.25

6.75

8

Chromium Conc.(mg/l)

x4

50

112.50

237.50

300

 

Table 2. Analysis of variance (ANOVA) for the RSM model of chromium removal and sludge production.

Source

Chromium removal analysis

Sludge Generation analysis

Mean Square

F Value

p-value

Mean Square

F Value

p-value

Model

532.4795

24.49593

< 0.0001

1264.281

50.18792

< 0.0001

Lack of Fit

28.91871

9.431321

0.0110

30.70827

9.837138

0.0094

x1

2200.111

101.2129

< 0.0001

2573.01

102.1403

< 0.0001

x2

2344.486

107.8546

< 0.0001

2370.094

94.08514

< 0.0001

x3

530.6884

24.41354

0.0001

131.5104

5.250863

0.02740

x4

148.7053

6.840966

0.0175

82.51042

1.2754

0.1824

x1x3

140.7689

3.875513

0.0877

-

-

-

x1x4

152.0702

6.995767

0.0165

-

-

-

x2x3

153.57219

4.46451

0.0339

-

-

-

x2x4

104.7424

4.81852

0.0415

-

-

-

x3x4

150.74219

5.33432

0.01439

-

-

-

x12

143.17596

3.986246

0.0758

-

-

-

x42

206.3295

9.491885

0.0064

-

-

-

 


The optimum condition is provided for chromium removal and sludge generation using bipolar electrocoagulation process (Table 4).The Prediction values of models with 95% Confidence Interval (CI) and Pridition Interval (PI) is shown in Table 5. In the optimum conditions, chromium removal efficiency and sludge production were obtained 75.6% and 189 ml/l. The experimental results obtained from optimum conditions in contrast to CI and PI values reveal the precision of resulting data and models.Fig. 2 shows three-dimensional response surface plot which was indicated the effects of two significant variables on applied current and initial pH value. According to the results can be found that the higher removal efficiency occurred in lower pH and high applied current. Lower chromium removal can be obtain in higher pH value occurs by decrease requires H+ions for the reduction of chromium. Also, in the higher pH,reaction between Fe2+ and chromium occurs very slowly. The results showsthe removal function is more affected to applied current rather than pH value.With the increasing of running time in the bipolar electrocoagulation process, higher amount of coagulant is created for more reduction of chromium. According to results, higher removal chromium was obtained with increase of� running time (Fig.3).General reations of chromium removal using the Fe material given in following [16,17].

Response Model of chromium removal has offered optimum initial chromium concentration in 156 mg/l, and that is chear in plot. In optimum concentration higher removal persentage is provided in up 0.3 A. Increase of sludge in electrocoagulation process will increase efficiency and that done by higher coagulant dose. The effects of higher current by authors have been reported [18]. Running time is a important factor for all chemical and electrochemival ractions. It has a main effect on rate and compelection of reaction. Fig.5 shows the general sludge generated during electrochemical process at initial pH 3-8.


Table 3. Response for experimental and predicated data for chromium removal efficiency

and sludge producedaccording to central composite design.

Run

Experimental Removal%

Perediction Removal%

Experimental prduced sludge (mL/L)

Perediction prduced sludge (mL/L)

1

53.68

50.32

79

71.4

2

58

57.82

94

86.24

3

73.56

76.12

156

165.74

4

61.64

65.13

156

165.57

5

87.74

88.64

244

233.74

6

79.9

79.92

172

169.08

7

66.07

65.13

150

165.57

8

62.05

70.91

140

150.73

9

91.71

89.19

296

248.41

10

65.5

65.13

148

165.57

11

54.5

55.73

172

176.73

12

69.37

66.16

160

162.06

13

70.82

73.82

160

180.24

14

82.89

84.8

264

259.74

15

85.15

78.86

168

150.9

16

96.56

98.22

228

248.58

17

66.36

65.13

136

165.57

18

85

80.87

212

180.41

19

64.54

65.13

152

165.57

20

65.79

65.13

143.2

165.57

21

43.64

50.91

72

82.73

22

66.84

61.12

148

165.4

23

38.64

45.37

100

86.07

24

80.71

84.89

260

245.07

25

48.21

44.94

102

82.56

26

83.74

82.34

260

244.9

27

60.26

60.1

140

154.24

28

78

74.53

144

154.41

29

59.45

56.3

180

176.9

30

47.67

45.32

96

97.4

Table 4. Optimum values of the parameter for bipolar electrocoagulation.

i ( A)

t ( min)

pH

Cr ( mg/l)

0.27

70

4.62

156

Table 5.Prediction and Experimental values with 95% (CI) and (PI)

Response

Prediction

95% CI

95% PI

Experimental

low

high

low

high

Removal %

73.13

69.38

76.88

62.64

83.62

75.6

Sludge ml/l

174.02

161.86

186.17

130.92

217.12

189

Fig. 2. 3D surface plot of chromium removal efficiency at pH 3-8 and applied current 0.1-0.5A

Fig. 3. 3D surface polt of chromium removal efficiency at pH 3-8 and running Time 30-180.

direct electrochemical chromium reduction:

 

Fig. 4. 3D surface plotof chromium removal efficiency

at initial chromium concentration 50 300 mg/L and applied current 0.1-0.5 A.

Fig. 5. Response one factor plot of sludge production at pH 3-8.

 


Amount of sludge in lower pH is decrease and it may be due to dissolution of sludge is taken. In acidic pH reduction efficiency is higher for more available H+ and removal is occurred in short time and return decrease the amount of sludge for chromium removal.

The initial concentrations of chromium is an effective factor in the bipolar electrocoagulation (Fig. 4).

Conclusions

The present work demonstrated the applicability of bipolar electrocoagulation process using the Pt as an anode and graphite as a cathode for chromium removal. The results of this study showed that RSM is one of the suitable technique to optimize the best operating conditions to maximize the chromium removal. The RSM is successfully employed for experimental design and analysis of results. Satisfactory empirical model equations are developed for two parameters, the chromium removal efficiency and sludge production in solution using RSM to optimize the parameters. P-value with amounts of < 0.0001 for the two response model was significant. Chromium removal efficiency and sludge production in optimum condition were 73.13 and 174.02ml/l, respectively.

 

Acknowledgment

The authors wish to acknowledge the financial support of Tarbiat Modares University.

 

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