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Separation of copper ions by electrodialysis using Taguchi experimental design
DESALINATION Desalination 169 (2004) 21-31
ELSEVIER
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Separation of copper ions by electrodialysis using Taguchi experimental design Toraj Mohammadl"a*,Ahmad Moheb b, Mohtada Sadrzadeh b, Amir RazmP "Research Laboratory for Separation Processes, Faculty of Chemical Engineering, Iran University of Science and Technology, Narmak, Tehran, Iran Tel. +98 (21) 789-6621; Fax +98 (21) 789-6620; email: [email protected] bFaculty of Chemical Engineering, Isfahan University of Technology, Isfahan, Iran Received 1 September 2003; accepted 12 December 2003
Abstract
With the aid of atomic absorption, a systematical and analytical evaluation method called Taguchi's quality engineering has been applied for the separation of copper ions from a solution using a laboratory electrodialysis set-up to evaluate the optimal experimental conditions and hence to achieve the highest removal percentage and the best robustness of the quantitation from the least number of experimental runs. Four parameters at three levels were studied: concentration (100, 500, 1000 ppm), temperature (25, 40, 60°C), flow rate (0.07, 0.7, 1.2 mL/s) and voltage (10, 20, 30 V). Two types of different membranes with different ion-exchange capacities were used. The optimal levels thus determined for the four influential factors were: concentration 1000 ppm, temperature 60°C, flow rate 0.07 mL/s and voltage 30 V. It has also been found that using a membrane pair with higher ion-exchange capacity improves performance. The highest removal percentage was found to be 94.94% and 97.33% for the two types of membranes.
Keywords: Copper removal; Wastewater; Electrodialysis; Taguchi method; Experimental design
1. Introduction
Copper is a high-priority PBT chemical. PBT chemicals are persistent, bioaccumulative and toxic ehemicals that do not readily break down in the environment and are not easily metabolized. They may accumulate in human or ecological food chains through consumption or uptake and *Corresponding author.
may be hazardous to human health or the environment. Copper is a reddish metal that occurs naturally in rock, soil, water, sediment and air. Copper also occurs naturally in plants and animals. It is an essential element for all known living organisms. Copper does not degrade and is not destroyed by combustion. It cycles between the soil, the atmosphere, surface waters and ground waters. Large single or daily intakes of copper can harm
0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved doi: 10.1016/j.desal.2004.08.004
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T. Mohammadi et al. / Desalination 169 (2004) 21-31
human health. Long-term exposure to copper dust can irritate the nose, mouth and eyes and cause headaches, dizziness, nausea and diarrhea. Drinking water that contains higher than normal levels of copper may cause vomiting, diarrhea, stomach cramps and nausea. High levels of copper can cause liver and kidney damage. So copper usage and pollution should be reduced wherever possible [1]. Electrodialysis with ion-exchangemembranes represents one of the most important membrane methods. It deals with the problems of desalination of salted waters, wastewater minimization, ultra-pure water production, concentration of dilute solutions, separation of electrolytes and non-electrolytes and production of acids and alkalis from their salts. It is also applied for whey, organic acids and sugar demineralization, amino acid and blood treatments, concentration of mineral acids, preparation of isotonic solutions and wine stabilization. The deep ecological aspect of this method implies favorable forecast for its future. Electrodialysis is an electrochemical process for the separation of ions. They pass through ionexchange membranes from one solution to another under the influence of an electrical potential difference used as a driving force. In a typical electrodialysis cell, a series of anion- and cationexchange membranes are arranged in an alternating pattern between an anode and a cathode to form individual cells. When a DC potential is applied between two electrodes, positively charged cations move toward the cathode, pass through the negatively charged cation-exchange membranes and are retained by the positively charged anion-exchange membrane. On the other hand, negatively charged anions move toward the anode, pass through the positively charged anionexchange membranes and are retained by the negatively charged cation-exchange membranes. At the end, ion concentration increases in alternate compartments with simultaneous ion deple-
Dilute
Concentrate
l AEM CEMl ee.!
e" !ee
+ i
e
Feed
•
i T'T
Fig. 1. Schematic view of the electrodialysis stack.
tion in other compartments [2]. A schematic view of the electrodialysis stack is presented in Fig. 1. The effect of different operating conditions for the separation of copper ions from wastewater by an experimental electrodialysis cell was studied.
2. Experimental The electrodialysis plant consists of a feed tank where wastewater is stored, two pumps (RESUN submersible pump, P = 4 W, total head = 0.5 m) and a self-designed electrodialysis cell. The cell is packed with a pair of ion-exchange membranes (cation and anion) and a pair of platinum electrodes (anode and cathode) [3]. Membranes used in these experiments are" • Type 1: (AR204SXR412) and (CR67,MK 111) anion- and cation-exchange membranes (supplied by Arak Petrochemical Complex, Iran; manufactured by Ionics, Watertown, MA,
USA). • Type 2: AMV-CMV anion- and cationexchange membranes (Asahi Glass, Tokyo, Japan). The chemical and physical properties of the ion-exchange membranes are presented in Tables 1 and 2.
T. Mohammadi et al. / Desalination 169 (2004) 21-31
23
Table 1 Physical and chemical characteristics of type 1 membranes Property
Membrane
Reinforcing fabric Specific weight, mg/cmz Thickness, mm Burst strength, kg/cm2 Water content Capacity, meq/dry g resin Chemical stability, pH
AR204SXR412
CR67,MK111
Acrylic l 3.7 0.5 7.0 46% of wet resin only 2.8 1-10
Acrylic 13.7 0.56--0.58 7.0 46% of wet resin only 2.4 1-10
Table 2 Physical and chemical characteristics properties of type 2 membranes Property
Form Permeability Thickness, mm Burst strength, kg/cm2 Electrical resistance, ohm-cm2 Capacity, meq/dry g resin Chemical stability, pH
Membrane AMV
CMV
Cl-form Strongly basic anion-membrane 0.11-0.15 2-5 1.5-3.0 4.4 1-10
Na-form Strongly acidic anion-membrane 0.13~). 15 3-5 2.0-3.5 5.2 1-10
The effective area of each membrane is 60× 65 m m 2 while the thickness of the dilution cell (center) is 4 m m and the thickness of each concentrate cell (left and right) is 3 ram. Both electrodes are made of pure platinum [4]. The area of each electrode is 4.2×4.2 m m 2. A rectifier (RST Spastell TRFO) supplied required DC power at different voltages. Fig. 2 shows a simplified diagram of the pilot plant and Fig. 3 shows a diagram of the electrodialysis cell. An analytical-grade salt (copper sulfate, Merck) was used in all experiments to produce solutions with wastewater qualities. The purpose of these experiments was to study the
effect of voltage, flow rate, temperature and concentration on electrodialysis cell performance. Taguchi's procedure followed in this study is described as follows: (1) Select the major influential factors involved in this study from both theoretical and empirical viewpoints and set their respective levels (low, medium, and high) accordingly and appropriately. (2) Construct an appropriate orthogonal table. (3) Perform experiments in triplicate under the various conditions listed in the orthogonal table and calculate the relevant recoveries and means. (4) Justify the above influential factors using an F-test. (5) Recognize the optimal experimental conditions (levels)
24
T. Mohammadi et al. / Desalination 169 (2004) 21-31
Merubra~e Sta~k I- . . . . . . . . . "~ II Eket,~d~ I
.....
westewTrl" I HoldlngTank
~
P~r S~ ~
I
Supply (P~ct~kr)
i
L.---t
i
i /
Ci~-~tion Pump I DC Po~r
Water
I'
zi
Low P ~ T , ~
AC
Prodzt
'
'
i I
/
Coztc'ezttr~te Dnch=r~
[::::? ...........
Fig. 2. Simplified diagram of an electrodialysis pilot plant.
hence minimize the variance of separation data [5,6]. ¢
3. Results and discussion
c
Fig. 3. Diagram of an electrodialysis cell.
that achieve the optimum separation among those listed in the orthogonal table. (6) Further justify the above optimal levels by comparing the respective sums of "iso-level" separations. (7) Further justify the above optimal levels by comparing all the rl values resulting from various levels associated with the orthogonal table. [T! = S/N = 10 x log (1/02), where S/N is signal-tonoise ratio and o is the standard deviation. Efforts have been made to maximize the r I value and
Based on our previous experience in related works and those experimental conditions reported by other researchers for the separation of Cu [7], (a) temperature, (b) concentration, (c) flow rate and (d) voltage were chosen as the four factors to be investigated and three levels are the exclusory set for each of the four factors. Low, medium and high levels of the factors are as follows: • Temperature (T): 25, 40 and 60°C. A temperature of 60°C was chosen because at higher temperatures the cavitation effect is important. • Concentration (C): 100, 500 and 1000 ppm. • Flow rate (F): 0.07, 0.7 and 1.2 mL/s. • Voltage (V): 10, 20 and 30 V. It must be mentioned that in this study only copper and sulfate ions existed in the model solution, pH was a noise factor. Noise factors are those parameters which are either uncontrollable or are too expensive to control such as variation
25
T. Mohammadi et aL / Desalination 169 (2004) 21-31
of environmental operating conditions. Noise factors may have a negative impact on system performance or may not. The other noise factors of this study were room temperature, occurring electrolysis on both electrodes, variation of voltage, concentration polarization, etc. According to the literature, pH effect on the performance of an electrodialysis cell is negligible, especially at voltages greater than 10 V [8,9]. Since pH does not change significantly during electrodialysis in this experimental setup (4-6.5), the variation of pH through the separation process was not monitored. It was observed that precipitation of Cu(OH)2 occurs at the cation-exchange membranes. At lower flow rates, precipitation is more important. The precipitate was removed by cleaning in place (CIP) using distilled water after each run. According to the Taguchi parameter design methodology, one experimental design should be selected for the controllable factors. Table 3 is an L9 orthogonal array, a table of integers whose column elements (1, 2 and 3) represent the low, medium and high levels of the column factors. Each row of the orthogonal array represents a run, that is, a specific set of factor levels to be tested. The L9 orthogonal array accommodates four factors at three levels each in nine runs [10]. The three-level L9 (3 4) orthogonal table used for the optimization process for the type 1 membrane and the corresponding separation percenttage (retention) of copper with two replication (responses 1 and 2) obtained under the nine candidate conditions are displayed in Table 4. The last two columns of this table contain y'and SNL values for each run. As seen, entry No. 8 tops all the other eight entries with regard to the separation of Cu ions, thus leading to selection of entry No. 8 levels as optimal conditions for routine use. This manner of selection of optimal conditions and hence the achievement of optimum recovery has been also justified by preliminary processes.
Table 3 Taguchi L9 orthogonalarray [8] Run
Factors T
C
F
V
1
1
1
1
1
2 3 4 5 6 7 8 9
1 1 2 2 2 3 3 3
2 3 I 2 3 1 2 3
2 3 2 3 I 3 ! 2
2 3 3 1 2 2 3 1
The data obtained from the experiments may now be analyzed. Taguchi recommends analyzing the mean response for each run and also suggests analyzing variation using an appropriately chosen signal-to-noise ratio (SN). For the larger the better responses, the following relation is used for the SN calculation.
n i=1 Yi
(t)
Notice that the SN ratio is expressed in a decibel scale. In these experiments the system is optimized when the response is as large as possible, so we deal with the SNL and factor levels that maximizing the SNL ratio are optimal. The Taguchi method uses graphs of the marginal means of each factor, as shown in Figs. 4 and 5. The usual approach is to examine the graphs and pick the winner. In Figs. 4 and 5 the effects of controllable factors on mean response and S N L for the type 1 membrane are displayed, respectively. According to these figures, increasing of temperature, concentration and voltage increases
26
T. Mohammadi et al. / Desalination 169 (2004) 21-31
Table 4 and SNL values for the type 1 membrane
Run
Controllablefactors
1 2 3 4 5 6 7 8 9
Y
Response
T, °C
C, ppm
F, mL/s
V, V
1
2
25 25 25 40 40 40 60 60 60
100 500 1000 100 500 1000 100 500 1000
0.07 0.7 1.2 0.7 1.2 0.07 1.2 0.07 0.7
10 20 30 30 10 20 20 30 10
20.88 11.58 9.75 14.22 6.34 82.01 5.98 94.60 14.78
19.42 11.58 8.97 11.99 6.57 81.33 6.74 95.27 15.38
30
i
30
28
20
i
"~ 20
~ 24
10 i
10 / 25
40
T(oC)
60
45 25
SNL
100
500
C(pprr0
1000
20.15 11.58 9.36 13.10 6.45 81.67 6.36 94.94 15.08
J
22 25
40
T('c)
60
-- 30
5 0.07
0.7
"7" 1.2
100
500
1000
20
30
C (ppm)
24
221
Y 20
1
211 28t y
26.07 21.27 19.4 22.25 16.19 38.24 16.02 39.55 23.56
10 /
10 10
F(mUs)
20 V(V)
30
20
0.07
0.7
F (rd.Js)
1.2
10
V (V}
Fig. 4. Effect of controllable factors on y.
Fig. 5. Effect of controllable factors on SNL.
the S N L and mean response. Because the electrical resistance of feed solution decreases. Notice that there is almost no difference between S N L and mean values for C500ppm and C1000pp m. Mean response at 500 ppm is even greater than 1000 ppm because the concentration polarization phenomenon is more important at high concentrations. With the aid o f Duncan's multiple range test, which is a statistical manner o f comparison o f the means, it was observed that greater amounts o f concentration than 500 ppm have almost no effect on separation performance.
At higher flow rates, the S N L and mean values fall and separation performance decreases. Because a greater flow rate means a lower residence time, ions that are between the membranes do not have enough time to transfer through the membranes. In terms o f maximizing the SNL, Thigh (60°C), Chigh (1000 ppm), Flow (0.07 ml/s) and Vhigh(30 V) were selected. In terms o f maximizing the average response, Thigh (60°C), Cmedium (500 ppm), Flow(0.07 ml/s) and Vhigh(30 V) were chosen. For the type 2 membrane, the same results were obtained.
27
T. Mohammadi et al. / Desalination 169 (2004) 21-31
Table 5 SS and variance o f S N L data for the type 1 membrane
Table 6 SS and variance ofSN Ldata for the type 2 membrane
Factor
SS
DOF
Variance
Factor
Flow rate Concentration Voltage Temperature
480.04 51.39 40.8 28.72
2 2 2 2
240.02 25.69 20.19 14.36
Flow rate 379.34 Concentration 39.22 Voltage 37.44 Temperature 27.79
SS
DOF
Variance
2 2 2 2
189.67 19.61 18.87 13.89
Table 7 Statistical results based on ~ for the type I membrane Factor
SS
DOF
Variance
F
P
Flow rate Concentration Voltage Temperature Error
12,318.59 2,188.95 2,114.80 2,090.43 4.82
2 2 2 2 9
6,159.29 1,094.47 1,057.40 1,045.22 0.54
11,504.7 2,044.3 1,975.1 1,952.3 --
65.80 11.16 11.29 11.73
Table 8 Statistical results based on for .~ the type 2 membrane Factor
SS
DOF
Variance
F
P
Flow rate Voltage Concentration Temperature Error
12,185.27 2,127.92 2,122.59 2,098.70 5.3
2 2 2 2 9
6,092.64 1,063.96 1,061.29 1,049.35 0.59
10,336.8 1,805.1 1,800.6 1,780.3 --
65.71 11.47 11.31 11.49 --
Taguchi-oriented practitioners often use analysis o f variance (ANOVA) to determine the factors that influence the average response and the factors that influence the signal-to-noise ratio. Sum of squares (SS) and mean square (variance) based on S N L data are presented in Tables 5 and 6 for both types o f the membranes. Tables 7 and 8 present SS, variance, the ratio o f factor variance on error variance (F) and percent o f contribution of each factor on response (P) for both types o f membranes. According to these results, flow rate has the greatest effect on SN~. and mean response.
The F value o f all factors is greater than the extracted F value o f the table for 0: (risk) = 0.05 (F = 4.26) and c~ = 0.01 (F = 8.02). This means that the variance o f all factors is significant compared with the variance of error, and all & t h e m have a significant effect on the response. P values o f temperature, concentration and voltage are almost the same and are smaller than that o f flow rate (65.80% for the type 1 membrane and 65.71% for the type 2 membrane), which means that flow rate is the most influential factor on the response.
28
T. Mohammadi et al. / Desalination 169 (2004) 21-31
Separation peroent - Row rate
100.
m _ .~n.gn.lp=,+v
8o £
8.
C
= ~ 0
60
~
80
~,
60
C _o
lw O
= 40 E
OI 2o
Separation peroent- Row rate
lOO
olo • 20
Voltage=20 V • 40
,o
~ 2o
0.2
0.4
0.6
flow rate
0.8
1
1.2
Fig. 6. Separation percent at different flow rates and voltage.
0
0.2
0.4
0.6
Flow rate
0.8
1
1.2
Fig. 7. Separation percent at different flow rates and temperatures.
Separation percent - Row rate
Separation percent - Voltage
1OO
t
80
Temp=40 °C ° Voltage=20 V •
/
" ~
lo0 500
/ =emp =40 C 40 i Flow rate=0.7
i _
_
20]
o
• 500
mL/s
° loo
1
O/
0
0.2
0.4
0.6
Row rate
1
0.8
10
1.2
Fig. 8. Separation percent at different flow rates and concentrations.
25
30
Separation percent -Concentration
4, 25
50
Flow rate=0.7 mL/s • 40 Cone. =500 ppm
40
• 60
20
Voltage
Fig. 9, Separation percent at different voltages and concentrations.
Separation percent - Voltage 60
15
•
Flow rate=0.7 mL/s Voltage=20 V ~25
O. 30
i 2°
"j2o o
lo
is
2'0
Voltage
Zs
5o
Fig. 10. Separation percent at different voltages and temperatures.
o 0
250
500
Concentration
750
1000
Fig. 1 l. Separation percent at different concentrations and temperature.
29
T. Mohammadi et aL / Desalination 169 (2004) 21-3t
Table 9 and SNL values for both types of membranes Factor
Temperature, °C
Concentration, ppm
Flow rate, mL/s
Voltage, V
Level
25 40 60 100 500 1000 0.07 0.7 1.2 I0 20 30
Membrane type 2
Membrane type 1
13.69 33.74 38.79 13.20 37.66 35.37 65.58 13.25 7.39 13.89 33.20 39.13
Finally, using these findings and modeling significant effects by the Taguchi method, results for all combination o f levels could be predicted. Then these predictions should be confirmed by some experiments. The confidence interval (CIT) of the type 1 membrane for 5% and 1% risk is 1.65 and 2.37, respectively. So with spending less time and cost, acceptable results can be derived. Some o f these results are presented in Figs. 6-11. Table 9 shows S N L andy-values of both types of membranes at all levels of factors. Notice that Figs. 3 and 4 were plotted using this table. Comparing the amounts of SNL and mean response for the two types of membranes, it was found that the type 2 membrane's performance is better than type 1. The better performance of the type 2 membrane can be attributed to its higher ion-exchange capacity. (Ion-exchange capacities of the membranes are presented in Tables 1 and 2.) As ionexchange capacity increases, both ion conductivity and swelling increase, while ion-permselectivity decreases. The reason for reduction of ion-
SNL
~
SNL
22.25 25.56 26.38 21.45 25.67 27.07 34.62 22.36 17.20 21.94 25.18 27.07
15.88 35.67 40.96 15.56 39.84 37.11 67.51 15.10 9.91 15.92 35.07 41.52
23.66 26.97 27.70 23.22 27.04 28.08 35.03 23.53 19.78 23.39 26.62 28.33
permselectivity is facilitation of co-ion diffusion into the swollen polymer membrane network since the Donnan exclusion does no longer works efficiently. Reduction ofion-permselectivity and mechanical stability with increasing ion-exchange capacity can be avoided by crosslinking the membranes. This reduces the swelling [9,10]. Thus, the results show that desalination and ion removal rates enhance with increasing ionexchange capacity of membranes. Current density can be determined using applied voltage and total resistance of the membrane stack. Since total resistance of the membrane stack is not available, current density can be calculated using a general definition. The assumption of constant concentration in the cell compartments gives [13]: i - md I dA m
I A,,
(2)
where I can be measured in each run and Am is the effective membrane area. Increasing current
30
T. Mohammadi et al. / Desalination 169 (2004) 21-31
density leads to an increase in the number of ions transferred. In this research the constant voltage operation was studied. This means that current density varies with operating conditions and can not be regulated [ 14]. The current-voltage characteristic of the separation process for different feed concentrations is shown in Fig. 12. Electrodialysis was found to be very effective for copper removal from wastewater. Electrical energy requirements are of great importance in the design or operation of an electrodialysis process. Power consumption can be given by
[15]: P =IV
(3)
25
• C = 100 p p m . 20
-
• C=500ppm m . ~ . _ _ ___
_
v
.w
• C = 1000 ppm A
5
10
15 v (v)
•
20
25
30
Fig. 12. Current-voltagecharacteristics of the separation process for various feed concentrations. 2.2
1=~ er Consumption • Separation p e r c e n t
m
40
1,8
30
g 1.4 a.
1
20 ~.
Energy consumption and separation percent of the electrodialysis cell at 40°C, 0.7 mL/s, 500 ppm at different voltages is presented in Fig. 13. However, energy consumption should be taken into consideration along with separation efficiencies at commercial scale. Other studies confirm that electrodialysis is very competitive from an energy consumption point of view [ 16].
4. Conclusions
The effect of membrane type and operating conditions (temperature, concentration, flow rate and voltage) on performance of electrodialysis was studied with the Taguchi method. As a result, higher temperature, higher concentration (concentrations greater than 500 ppm have almost no effect on the performance), higher voltage and a lower flow rate are recommended as optimal operating conditions for the electrodialysis cell using platinum electrodes (CR67,MKlll)(AR204SXR412) and CMV-AMV as ion-exchange membranes. Optimum operating conditions for maximizing SNL are: concentration, 1000 ppm; temperature, 60°C; flow rate, 0.07 mL/s and voltage, 30 V. The same results were obtained for maximizing mean response with the exception of a concentration of 500 ppm having a greater mean response than a concentration of 1000 ppm. All factors had a significant effect on the response, but the effect of flow rate on the mean response was more significant than the other factors. The results confirm that membranes with higher ion-exchange capacity produced better results. Electrodialysis was found to be very effective for copper removal from wastewater.
0.6 t0
0.~
10
15
20
v (v)
25
30
Fig. 13. Power consumption and separation percentages as a function of voltage.
References
[l] State of Ohio Environmental Protection Agency, Persistent, bioaccumulation and toxic chemicals, Copper and Copper Compounds,2002.
T. Mohammadi et aL / Desalination 169 (2004) 21-31
[2] W.S. Winston Ho and K. Sirkar, Membrane Handbook, Champan & Hall, 1992. [3] M. Demircioglu and N. Kabay, Cost comparison and efficiency modeling in the electrodialysis of brine, Desalination, 136 (2001) 317-323. [4] V.A. Shaposhnik, N.N. Zubets and B.E. Mill, Demineralization of water by electrodialysis with ion-exchange membranes, grains and nets, Desalination, 133 (2001) 211-214. [5] S.M. Wang, Y.S. Giang and Y. Ling, Taguchi's method in optimizing the experimental conditions of simultaneous supercritieal fluid extraction and chemical derivatization for the gas chromatographicmass spectrometric determination of amphetamine and methamphetamine in aqueous matrix, Forensic Sci. J., 1 (2002) 47-53. [6] I. Masters, A.R. Khoei and D.T. Gethin, The application of Tagnchi methods to the aluminium recycling process, Proc. 4th ASM Intemat. Conf. Recycling of Metals, Vienna, 1999, pp. 115-124. [7] T. Mohammadi and A. Kaviani, Water shortage and seawater desalination by electrodialysis, Desalination, 158 (2003) 267-270. [8] R. Klischenko, B. Kornilovich, R. Chebotaryova and V. Linkov, Purification of galvanic sewage from metals by electrodialysis, Desalination, 126 (1999) 159-162. [9] N. Kabay, M. Ardab, L. Kurucaoval, P.E. Ersoza,
3t
H. Kahveci, M. Can and S. Dal, Effect of feed characteristics on the separation performances of monovalent and divalent salts by eleetrodialysis, Desalination, 158 (2003) 95-100. [ 10] D.C. Montgomery, Design and Analysis of Experiments, 3rd ed., Wiley, New York, 1991. [ 11] P. Zschocke and D. Quellmalz, Novel ion exchange membranes based on an aromatic polysulfone, J. Membr. Sci., 22 (1985) 325-332. [12] J. Kerres, W. Cui, R. Disson and W. Neubrand, Development and characterization of crosslinked ionomer membrane based upon sulfinated and solfonated PSU crosslinked PSU blend membranes by disproportionation of sulfinic acid group, J. Membr. Sci., 139 (1998) 211-225. [13] M. Demircioglu, N. Kabay, E. Ersoz, I. Kurucavali, C. Safek and N. Gizli, Cost comparison and efficiency modeling in the electrodialysis cell, Desalination, 136 (2001) 317-323. [14] S.J. Parulekar, Optimal current and voltage trajectories for minimum energy consumption in batch electrodialysis, J. Membr. Sci., 148 (1998) 91-103. [15] J.D. Seader and E.J. Henley; Separation Process Principles, Wiley, New York, 1998. [16] J.M. Blackburn, Electrodialysis application for pollution prevention in the chemical processing industry, J Air Waste Mgrnt. Assoc., 49 (1999) 934-942.
ELSEVIER
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Separation of copper ions by electrodialysis using Taguchi experimental design Toraj Mohammadl"a*,Ahmad Moheb b, Mohtada Sadrzadeh b, Amir RazmP "Research Laboratory for Separation Processes, Faculty of Chemical Engineering, Iran University of Science and Technology, Narmak, Tehran, Iran Tel. +98 (21) 789-6621; Fax +98 (21) 789-6620; email: [email protected] bFaculty of Chemical Engineering, Isfahan University of Technology, Isfahan, Iran Received 1 September 2003; accepted 12 December 2003
Abstract
With the aid of atomic absorption, a systematical and analytical evaluation method called Taguchi's quality engineering has been applied for the separation of copper ions from a solution using a laboratory electrodialysis set-up to evaluate the optimal experimental conditions and hence to achieve the highest removal percentage and the best robustness of the quantitation from the least number of experimental runs. Four parameters at three levels were studied: concentration (100, 500, 1000 ppm), temperature (25, 40, 60°C), flow rate (0.07, 0.7, 1.2 mL/s) and voltage (10, 20, 30 V). Two types of different membranes with different ion-exchange capacities were used. The optimal levels thus determined for the four influential factors were: concentration 1000 ppm, temperature 60°C, flow rate 0.07 mL/s and voltage 30 V. It has also been found that using a membrane pair with higher ion-exchange capacity improves performance. The highest removal percentage was found to be 94.94% and 97.33% for the two types of membranes.
Keywords: Copper removal; Wastewater; Electrodialysis; Taguchi method; Experimental design
1. Introduction
Copper is a high-priority PBT chemical. PBT chemicals are persistent, bioaccumulative and toxic ehemicals that do not readily break down in the environment and are not easily metabolized. They may accumulate in human or ecological food chains through consumption or uptake and *Corresponding author.
may be hazardous to human health or the environment. Copper is a reddish metal that occurs naturally in rock, soil, water, sediment and air. Copper also occurs naturally in plants and animals. It is an essential element for all known living organisms. Copper does not degrade and is not destroyed by combustion. It cycles between the soil, the atmosphere, surface waters and ground waters. Large single or daily intakes of copper can harm
0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved doi: 10.1016/j.desal.2004.08.004
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T. Mohammadi et al. / Desalination 169 (2004) 21-31
human health. Long-term exposure to copper dust can irritate the nose, mouth and eyes and cause headaches, dizziness, nausea and diarrhea. Drinking water that contains higher than normal levels of copper may cause vomiting, diarrhea, stomach cramps and nausea. High levels of copper can cause liver and kidney damage. So copper usage and pollution should be reduced wherever possible [1]. Electrodialysis with ion-exchangemembranes represents one of the most important membrane methods. It deals with the problems of desalination of salted waters, wastewater minimization, ultra-pure water production, concentration of dilute solutions, separation of electrolytes and non-electrolytes and production of acids and alkalis from their salts. It is also applied for whey, organic acids and sugar demineralization, amino acid and blood treatments, concentration of mineral acids, preparation of isotonic solutions and wine stabilization. The deep ecological aspect of this method implies favorable forecast for its future. Electrodialysis is an electrochemical process for the separation of ions. They pass through ionexchange membranes from one solution to another under the influence of an electrical potential difference used as a driving force. In a typical electrodialysis cell, a series of anion- and cationexchange membranes are arranged in an alternating pattern between an anode and a cathode to form individual cells. When a DC potential is applied between two electrodes, positively charged cations move toward the cathode, pass through the negatively charged cation-exchange membranes and are retained by the positively charged anion-exchange membrane. On the other hand, negatively charged anions move toward the anode, pass through the positively charged anionexchange membranes and are retained by the negatively charged cation-exchange membranes. At the end, ion concentration increases in alternate compartments with simultaneous ion deple-
Dilute
Concentrate
l AEM CEMl ee.!
e" !ee
+ i
e
Feed
•
i T'T
Fig. 1. Schematic view of the electrodialysis stack.
tion in other compartments [2]. A schematic view of the electrodialysis stack is presented in Fig. 1. The effect of different operating conditions for the separation of copper ions from wastewater by an experimental electrodialysis cell was studied.
2. Experimental The electrodialysis plant consists of a feed tank where wastewater is stored, two pumps (RESUN submersible pump, P = 4 W, total head = 0.5 m) and a self-designed electrodialysis cell. The cell is packed with a pair of ion-exchange membranes (cation and anion) and a pair of platinum electrodes (anode and cathode) [3]. Membranes used in these experiments are" • Type 1: (AR204SXR412) and (CR67,MK 111) anion- and cation-exchange membranes (supplied by Arak Petrochemical Complex, Iran; manufactured by Ionics, Watertown, MA,
USA). • Type 2: AMV-CMV anion- and cationexchange membranes (Asahi Glass, Tokyo, Japan). The chemical and physical properties of the ion-exchange membranes are presented in Tables 1 and 2.
T. Mohammadi et al. / Desalination 169 (2004) 21-31
23
Table 1 Physical and chemical characteristics of type 1 membranes Property
Membrane
Reinforcing fabric Specific weight, mg/cmz Thickness, mm Burst strength, kg/cm2 Water content Capacity, meq/dry g resin Chemical stability, pH
AR204SXR412
CR67,MK111
Acrylic l 3.7 0.5 7.0 46% of wet resin only 2.8 1-10
Acrylic 13.7 0.56--0.58 7.0 46% of wet resin only 2.4 1-10
Table 2 Physical and chemical characteristics properties of type 2 membranes Property
Form Permeability Thickness, mm Burst strength, kg/cm2 Electrical resistance, ohm-cm2 Capacity, meq/dry g resin Chemical stability, pH
Membrane AMV
CMV
Cl-form Strongly basic anion-membrane 0.11-0.15 2-5 1.5-3.0 4.4 1-10
Na-form Strongly acidic anion-membrane 0.13~). 15 3-5 2.0-3.5 5.2 1-10
The effective area of each membrane is 60× 65 m m 2 while the thickness of the dilution cell (center) is 4 m m and the thickness of each concentrate cell (left and right) is 3 ram. Both electrodes are made of pure platinum [4]. The area of each electrode is 4.2×4.2 m m 2. A rectifier (RST Spastell TRFO) supplied required DC power at different voltages. Fig. 2 shows a simplified diagram of the pilot plant and Fig. 3 shows a diagram of the electrodialysis cell. An analytical-grade salt (copper sulfate, Merck) was used in all experiments to produce solutions with wastewater qualities. The purpose of these experiments was to study the
effect of voltage, flow rate, temperature and concentration on electrodialysis cell performance. Taguchi's procedure followed in this study is described as follows: (1) Select the major influential factors involved in this study from both theoretical and empirical viewpoints and set their respective levels (low, medium, and high) accordingly and appropriately. (2) Construct an appropriate orthogonal table. (3) Perform experiments in triplicate under the various conditions listed in the orthogonal table and calculate the relevant recoveries and means. (4) Justify the above influential factors using an F-test. (5) Recognize the optimal experimental conditions (levels)
24
T. Mohammadi et al. / Desalination 169 (2004) 21-31
Merubra~e Sta~k I- . . . . . . . . . "~ II Eket,~d~ I
.....
westewTrl" I HoldlngTank
~
P~r S~ ~
I
Supply (P~ct~kr)
i
L.---t
i
i /
Ci~-~tion Pump I DC Po~r
Water
I'
zi
Low P ~ T , ~
AC
Prodzt
'
'
i I
/
Coztc'ezttr~te Dnch=r~
[::::? ...........
Fig. 2. Simplified diagram of an electrodialysis pilot plant.
hence minimize the variance of separation data [5,6]. ¢
3. Results and discussion
c
Fig. 3. Diagram of an electrodialysis cell.
that achieve the optimum separation among those listed in the orthogonal table. (6) Further justify the above optimal levels by comparing the respective sums of "iso-level" separations. (7) Further justify the above optimal levels by comparing all the rl values resulting from various levels associated with the orthogonal table. [T! = S/N = 10 x log (1/02), where S/N is signal-tonoise ratio and o is the standard deviation. Efforts have been made to maximize the r I value and
Based on our previous experience in related works and those experimental conditions reported by other researchers for the separation of Cu [7], (a) temperature, (b) concentration, (c) flow rate and (d) voltage were chosen as the four factors to be investigated and three levels are the exclusory set for each of the four factors. Low, medium and high levels of the factors are as follows: • Temperature (T): 25, 40 and 60°C. A temperature of 60°C was chosen because at higher temperatures the cavitation effect is important. • Concentration (C): 100, 500 and 1000 ppm. • Flow rate (F): 0.07, 0.7 and 1.2 mL/s. • Voltage (V): 10, 20 and 30 V. It must be mentioned that in this study only copper and sulfate ions existed in the model solution, pH was a noise factor. Noise factors are those parameters which are either uncontrollable or are too expensive to control such as variation
25
T. Mohammadi et aL / Desalination 169 (2004) 21-31
of environmental operating conditions. Noise factors may have a negative impact on system performance or may not. The other noise factors of this study were room temperature, occurring electrolysis on both electrodes, variation of voltage, concentration polarization, etc. According to the literature, pH effect on the performance of an electrodialysis cell is negligible, especially at voltages greater than 10 V [8,9]. Since pH does not change significantly during electrodialysis in this experimental setup (4-6.5), the variation of pH through the separation process was not monitored. It was observed that precipitation of Cu(OH)2 occurs at the cation-exchange membranes. At lower flow rates, precipitation is more important. The precipitate was removed by cleaning in place (CIP) using distilled water after each run. According to the Taguchi parameter design methodology, one experimental design should be selected for the controllable factors. Table 3 is an L9 orthogonal array, a table of integers whose column elements (1, 2 and 3) represent the low, medium and high levels of the column factors. Each row of the orthogonal array represents a run, that is, a specific set of factor levels to be tested. The L9 orthogonal array accommodates four factors at three levels each in nine runs [10]. The three-level L9 (3 4) orthogonal table used for the optimization process for the type 1 membrane and the corresponding separation percenttage (retention) of copper with two replication (responses 1 and 2) obtained under the nine candidate conditions are displayed in Table 4. The last two columns of this table contain y'and SNL values for each run. As seen, entry No. 8 tops all the other eight entries with regard to the separation of Cu ions, thus leading to selection of entry No. 8 levels as optimal conditions for routine use. This manner of selection of optimal conditions and hence the achievement of optimum recovery has been also justified by preliminary processes.
Table 3 Taguchi L9 orthogonalarray [8] Run
Factors T
C
F
V
1
1
1
1
1
2 3 4 5 6 7 8 9
1 1 2 2 2 3 3 3
2 3 I 2 3 1 2 3
2 3 2 3 I 3 ! 2
2 3 3 1 2 2 3 1
The data obtained from the experiments may now be analyzed. Taguchi recommends analyzing the mean response for each run and also suggests analyzing variation using an appropriately chosen signal-to-noise ratio (SN). For the larger the better responses, the following relation is used for the SN calculation.
n i=1 Yi
(t)
Notice that the SN ratio is expressed in a decibel scale. In these experiments the system is optimized when the response is as large as possible, so we deal with the SNL and factor levels that maximizing the SNL ratio are optimal. The Taguchi method uses graphs of the marginal means of each factor, as shown in Figs. 4 and 5. The usual approach is to examine the graphs and pick the winner. In Figs. 4 and 5 the effects of controllable factors on mean response and S N L for the type 1 membrane are displayed, respectively. According to these figures, increasing of temperature, concentration and voltage increases
26
T. Mohammadi et al. / Desalination 169 (2004) 21-31
Table 4 and SNL values for the type 1 membrane
Run
Controllablefactors
1 2 3 4 5 6 7 8 9
Y
Response
T, °C
C, ppm
F, mL/s
V, V
1
2
25 25 25 40 40 40 60 60 60
100 500 1000 100 500 1000 100 500 1000
0.07 0.7 1.2 0.7 1.2 0.07 1.2 0.07 0.7
10 20 30 30 10 20 20 30 10
20.88 11.58 9.75 14.22 6.34 82.01 5.98 94.60 14.78
19.42 11.58 8.97 11.99 6.57 81.33 6.74 95.27 15.38
30
i
30
28
20
i
"~ 20
~ 24
10 i
10 / 25
40
T(oC)
60
45 25
SNL
100
500
C(pprr0
1000
20.15 11.58 9.36 13.10 6.45 81.67 6.36 94.94 15.08
J
22 25
40
T('c)
60
-- 30
5 0.07
0.7
"7" 1.2
100
500
1000
20
30
C (ppm)
24
221
Y 20
1
211 28t y
26.07 21.27 19.4 22.25 16.19 38.24 16.02 39.55 23.56
10 /
10 10
F(mUs)
20 V(V)
30
20
0.07
0.7
F (rd.Js)
1.2
10
V (V}
Fig. 4. Effect of controllable factors on y.
Fig. 5. Effect of controllable factors on SNL.
the S N L and mean response. Because the electrical resistance of feed solution decreases. Notice that there is almost no difference between S N L and mean values for C500ppm and C1000pp m. Mean response at 500 ppm is even greater than 1000 ppm because the concentration polarization phenomenon is more important at high concentrations. With the aid o f Duncan's multiple range test, which is a statistical manner o f comparison o f the means, it was observed that greater amounts o f concentration than 500 ppm have almost no effect on separation performance.
At higher flow rates, the S N L and mean values fall and separation performance decreases. Because a greater flow rate means a lower residence time, ions that are between the membranes do not have enough time to transfer through the membranes. In terms o f maximizing the SNL, Thigh (60°C), Chigh (1000 ppm), Flow (0.07 ml/s) and Vhigh(30 V) were selected. In terms o f maximizing the average response, Thigh (60°C), Cmedium (500 ppm), Flow(0.07 ml/s) and Vhigh(30 V) were chosen. For the type 2 membrane, the same results were obtained.
27
T. Mohammadi et al. / Desalination 169 (2004) 21-31
Table 5 SS and variance o f S N L data for the type 1 membrane
Table 6 SS and variance ofSN Ldata for the type 2 membrane
Factor
SS
DOF
Variance
Factor
Flow rate Concentration Voltage Temperature
480.04 51.39 40.8 28.72
2 2 2 2
240.02 25.69 20.19 14.36
Flow rate 379.34 Concentration 39.22 Voltage 37.44 Temperature 27.79
SS
DOF
Variance
2 2 2 2
189.67 19.61 18.87 13.89
Table 7 Statistical results based on ~ for the type I membrane Factor
SS
DOF
Variance
F
P
Flow rate Concentration Voltage Temperature Error
12,318.59 2,188.95 2,114.80 2,090.43 4.82
2 2 2 2 9
6,159.29 1,094.47 1,057.40 1,045.22 0.54
11,504.7 2,044.3 1,975.1 1,952.3 --
65.80 11.16 11.29 11.73
Table 8 Statistical results based on for .~ the type 2 membrane Factor
SS
DOF
Variance
F
P
Flow rate Voltage Concentration Temperature Error
12,185.27 2,127.92 2,122.59 2,098.70 5.3
2 2 2 2 9
6,092.64 1,063.96 1,061.29 1,049.35 0.59
10,336.8 1,805.1 1,800.6 1,780.3 --
65.71 11.47 11.31 11.49 --
Taguchi-oriented practitioners often use analysis o f variance (ANOVA) to determine the factors that influence the average response and the factors that influence the signal-to-noise ratio. Sum of squares (SS) and mean square (variance) based on S N L data are presented in Tables 5 and 6 for both types o f the membranes. Tables 7 and 8 present SS, variance, the ratio o f factor variance on error variance (F) and percent o f contribution of each factor on response (P) for both types o f membranes. According to these results, flow rate has the greatest effect on SN~. and mean response.
The F value o f all factors is greater than the extracted F value o f the table for 0: (risk) = 0.05 (F = 4.26) and c~ = 0.01 (F = 8.02). This means that the variance o f all factors is significant compared with the variance of error, and all & t h e m have a significant effect on the response. P values o f temperature, concentration and voltage are almost the same and are smaller than that o f flow rate (65.80% for the type 1 membrane and 65.71% for the type 2 membrane), which means that flow rate is the most influential factor on the response.
28
T. Mohammadi et al. / Desalination 169 (2004) 21-31
Separation peroent - Row rate
100.
m _ .~n.gn.lp=,+v
8o £
8.
C
= ~ 0
60
~
80
~,
60
C _o
lw O
= 40 E
OI 2o
Separation peroent- Row rate
lOO
olo • 20
Voltage=20 V • 40
,o
~ 2o
0.2
0.4
0.6
flow rate
0.8
1
1.2
Fig. 6. Separation percent at different flow rates and voltage.
0
0.2
0.4
0.6
Flow rate
0.8
1
1.2
Fig. 7. Separation percent at different flow rates and temperatures.
Separation percent - Row rate
Separation percent - Voltage
1OO
t
80
Temp=40 °C ° Voltage=20 V •
/
" ~
lo0 500
/ =emp =40 C 40 i Flow rate=0.7
i _
_
20]
o
• 500
mL/s
° loo
1
O/
0
0.2
0.4
0.6
Row rate
1
0.8
10
1.2
Fig. 8. Separation percent at different flow rates and concentrations.
25
30
Separation percent -Concentration
4, 25
50
Flow rate=0.7 mL/s • 40 Cone. =500 ppm
40
• 60
20
Voltage
Fig. 9, Separation percent at different voltages and concentrations.
Separation percent - Voltage 60
15
•
Flow rate=0.7 mL/s Voltage=20 V ~25
O. 30
i 2°
"j2o o
lo
is
2'0
Voltage
Zs
5o
Fig. 10. Separation percent at different voltages and temperatures.
o 0
250
500
Concentration
750
1000
Fig. 1 l. Separation percent at different concentrations and temperature.
29
T. Mohammadi et aL / Desalination 169 (2004) 21-3t
Table 9 and SNL values for both types of membranes Factor
Temperature, °C
Concentration, ppm
Flow rate, mL/s
Voltage, V
Level
25 40 60 100 500 1000 0.07 0.7 1.2 I0 20 30
Membrane type 2
Membrane type 1
13.69 33.74 38.79 13.20 37.66 35.37 65.58 13.25 7.39 13.89 33.20 39.13
Finally, using these findings and modeling significant effects by the Taguchi method, results for all combination o f levels could be predicted. Then these predictions should be confirmed by some experiments. The confidence interval (CIT) of the type 1 membrane for 5% and 1% risk is 1.65 and 2.37, respectively. So with spending less time and cost, acceptable results can be derived. Some o f these results are presented in Figs. 6-11. Table 9 shows S N L andy-values of both types of membranes at all levels of factors. Notice that Figs. 3 and 4 were plotted using this table. Comparing the amounts of SNL and mean response for the two types of membranes, it was found that the type 2 membrane's performance is better than type 1. The better performance of the type 2 membrane can be attributed to its higher ion-exchange capacity. (Ion-exchange capacities of the membranes are presented in Tables 1 and 2.) As ionexchange capacity increases, both ion conductivity and swelling increase, while ion-permselectivity decreases. The reason for reduction of ion-
SNL
~
SNL
22.25 25.56 26.38 21.45 25.67 27.07 34.62 22.36 17.20 21.94 25.18 27.07
15.88 35.67 40.96 15.56 39.84 37.11 67.51 15.10 9.91 15.92 35.07 41.52
23.66 26.97 27.70 23.22 27.04 28.08 35.03 23.53 19.78 23.39 26.62 28.33
permselectivity is facilitation of co-ion diffusion into the swollen polymer membrane network since the Donnan exclusion does no longer works efficiently. Reduction ofion-permselectivity and mechanical stability with increasing ion-exchange capacity can be avoided by crosslinking the membranes. This reduces the swelling [9,10]. Thus, the results show that desalination and ion removal rates enhance with increasing ionexchange capacity of membranes. Current density can be determined using applied voltage and total resistance of the membrane stack. Since total resistance of the membrane stack is not available, current density can be calculated using a general definition. The assumption of constant concentration in the cell compartments gives [13]: i - md I dA m
I A,,
(2)
where I can be measured in each run and Am is the effective membrane area. Increasing current
30
T. Mohammadi et al. / Desalination 169 (2004) 21-31
density leads to an increase in the number of ions transferred. In this research the constant voltage operation was studied. This means that current density varies with operating conditions and can not be regulated [ 14]. The current-voltage characteristic of the separation process for different feed concentrations is shown in Fig. 12. Electrodialysis was found to be very effective for copper removal from wastewater. Electrical energy requirements are of great importance in the design or operation of an electrodialysis process. Power consumption can be given by
[15]: P =IV
(3)
25
• C = 100 p p m . 20
-
• C=500ppm m . ~ . _ _ ___
_
v
.w
• C = 1000 ppm A
5
10
15 v (v)
•
20
25
30
Fig. 12. Current-voltagecharacteristics of the separation process for various feed concentrations. 2.2
1=~ er Consumption • Separation p e r c e n t
m
40
1,8
30
g 1.4 a.
1
20 ~.
Energy consumption and separation percent of the electrodialysis cell at 40°C, 0.7 mL/s, 500 ppm at different voltages is presented in Fig. 13. However, energy consumption should be taken into consideration along with separation efficiencies at commercial scale. Other studies confirm that electrodialysis is very competitive from an energy consumption point of view [ 16].
4. Conclusions
The effect of membrane type and operating conditions (temperature, concentration, flow rate and voltage) on performance of electrodialysis was studied with the Taguchi method. As a result, higher temperature, higher concentration (concentrations greater than 500 ppm have almost no effect on the performance), higher voltage and a lower flow rate are recommended as optimal operating conditions for the electrodialysis cell using platinum electrodes (CR67,MKlll)(AR204SXR412) and CMV-AMV as ion-exchange membranes. Optimum operating conditions for maximizing SNL are: concentration, 1000 ppm; temperature, 60°C; flow rate, 0.07 mL/s and voltage, 30 V. The same results were obtained for maximizing mean response with the exception of a concentration of 500 ppm having a greater mean response than a concentration of 1000 ppm. All factors had a significant effect on the response, but the effect of flow rate on the mean response was more significant than the other factors. The results confirm that membranes with higher ion-exchange capacity produced better results. Electrodialysis was found to be very effective for copper removal from wastewater.
0.6 t0
0.~
10
15
20
v (v)
25
30
Fig. 13. Power consumption and separation percentages as a function of voltage.
References
[l] State of Ohio Environmental Protection Agency, Persistent, bioaccumulation and toxic chemicals, Copper and Copper Compounds,2002.
T. Mohammadi et aL / Desalination 169 (2004) 21-31
[2] W.S. Winston Ho and K. Sirkar, Membrane Handbook, Champan & Hall, 1992. [3] M. Demircioglu and N. Kabay, Cost comparison and efficiency modeling in the electrodialysis of brine, Desalination, 136 (2001) 317-323. [4] V.A. Shaposhnik, N.N. Zubets and B.E. Mill, Demineralization of water by electrodialysis with ion-exchange membranes, grains and nets, Desalination, 133 (2001) 211-214. [5] S.M. Wang, Y.S. Giang and Y. Ling, Taguchi's method in optimizing the experimental conditions of simultaneous supercritieal fluid extraction and chemical derivatization for the gas chromatographicmass spectrometric determination of amphetamine and methamphetamine in aqueous matrix, Forensic Sci. J., 1 (2002) 47-53. [6] I. Masters, A.R. Khoei and D.T. Gethin, The application of Tagnchi methods to the aluminium recycling process, Proc. 4th ASM Intemat. Conf. Recycling of Metals, Vienna, 1999, pp. 115-124. [7] T. Mohammadi and A. Kaviani, Water shortage and seawater desalination by electrodialysis, Desalination, 158 (2003) 267-270. [8] R. Klischenko, B. Kornilovich, R. Chebotaryova and V. Linkov, Purification of galvanic sewage from metals by electrodialysis, Desalination, 126 (1999) 159-162. [9] N. Kabay, M. Ardab, L. Kurucaoval, P.E. Ersoza,
3t
H. Kahveci, M. Can and S. Dal, Effect of feed characteristics on the separation performances of monovalent and divalent salts by eleetrodialysis, Desalination, 158 (2003) 95-100. [ 10] D.C. Montgomery, Design and Analysis of Experiments, 3rd ed., Wiley, New York, 1991. [ 11] P. Zschocke and D. Quellmalz, Novel ion exchange membranes based on an aromatic polysulfone, J. Membr. Sci., 22 (1985) 325-332. [12] J. Kerres, W. Cui, R. Disson and W. Neubrand, Development and characterization of crosslinked ionomer membrane based upon sulfinated and solfonated PSU crosslinked PSU blend membranes by disproportionation of sulfinic acid group, J. Membr. Sci., 139 (1998) 211-225. [13] M. Demircioglu, N. Kabay, E. Ersoz, I. Kurucavali, C. Safek and N. Gizli, Cost comparison and efficiency modeling in the electrodialysis cell, Desalination, 136 (2001) 317-323. [14] S.J. Parulekar, Optimal current and voltage trajectories for minimum energy consumption in batch electrodialysis, J. Membr. Sci., 148 (1998) 91-103. [15] J.D. Seader and E.J. Henley; Separation Process Principles, Wiley, New York, 1998. [16] J.M. Blackburn, Electrodialysis application for pollution prevention in the chemical processing industry, J Air Waste Mgrnt. Assoc., 49 (1999) 934-942.