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An innovative membrane method for the separation of chromium ions from solutions containing obstructive copper ions
Desalination 274 (2011) 246–254
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
An innovative membrane method for the separation of chromium ions from solutions containing obstructive copper ions Mahshad Pazouki ⁎, Ahmad Moheb Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 8 September 2010 Received in revised form 8 February 2011 Accepted 9 February 2011 Available online 24 March 2011 Keywords: Electrodialysis Complexing agent Taguchi technique EDTA ANOVA
a b s t r a c t Chromium is one of the major pollutants of water and wastewater; this ion can be seen in different industrial effluents along with other metal ions such as Cu, As [E. Velizarova, A.B. Ribeiro, and L.M. Ottosen, A comparative study on Cu, Cr and As removal from CCA-treated wood waste by dialytic, Journal of Hazardous Material 94 (2) (2002)147–160]. In the current paper, experimental investigations have been done on the removal of the chromium ions from aqueous solutions which contain Cu2+ as obstructive ion. Electrodialysis is coupled with complexing to recommend a new technique for the elimination of chromium (III) ions. Ethylendiaminetetraacetate acid (EDTA) was used as the complexing agent. The effect of different operating conditions (feed flow rate, voltage or current density, the ratio of the chromium (III) concentration to the copper concentration, EDTA molar ratio to Cr (III) and pH of the feed) on the removal of chromium ions was investigated. The Taguchi method was implied to design the experiments. The optimum operating conditions were determined using the analysis of variance (ANOVA) method. The proposed method resulted in relatively high chromium removal (95% Cr (VI) and 87% Cr (III)) at the optimum operating conditions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Chromium is applied in different industries such as tanning factories, steel works, industrial electroplating, wood preservation, artificial fertilizers, paint and pigment manufacturing, corrosion control, textile and photography [1–4]. Chromium toxicity varies with the concentration and oxidation state. Hexavalent and trivalent chromium are two common oxidation states in nature. Although trivalent chromium is less toxic and similar to many other heavy metals, in small traces is necessary for life processes, the higher concentrations of it in the environment and consequently human intake cause many diseases [1,5,6]. Chromium (VI) is soluble in water over the entire pH range so it is mobile in nature. Because of its high mobility, Cr (VI) is able to easily permeate in to the biological membranes; therefore, it is highly toxic and known as a carcinogen material. Depending on the pH of the aqueous system, this ion is expected as anionic species such as 2− 2− HCr2O− and Cr2O2− 7 , HCrO4 , CrO4 7 . According to the Environment Protection Agent (EPA), release of 1 ppm of Cr (VI) and 1–5 ppm of Cr (III) in the industrial effluents is permitted [2,7,8]. Methods such as chemical precipitation, ion exchange resin, foam flotation, adsorption, and solvent extraction are used for the removal ⁎ Corresponding author at: Department of Chemical and Materials Engineering, University of Alberta, 7th Floor, Electrical & Computer Engineering Research Facility (ECERF), 9107-116 Street, Edmonton, Alberta, Canada T6G 2V4. Tel.: + 1 780 707 6824; fax: + 1 780 492 2881. E-mail address: [email protected] (M. Pazouki). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.02.017
of the chromium from industrial effluents. Membrane methods like ultrafiltration, nanofiltration, reverse osmosis, electrodialysis and liquid membrane may be suitable alternatives to the abovementioned methods [6,9–13]. The most widely applied method for the recovery of Cr (III) is precipitation with alkalis or sulfide; although this method is simple and inexpensive, it creates large loads of toxic waste sludge. The remaining sludge contains small suspended solids which have to be removed with filtration as a secondary method. The residue still contains trivalent chromium [5,8,14,15]. To remove Cr (VI), reduction is used prior to precipitation [16]. Another popular technique for the removal of chromium, especially chromium (VI), is ion exchange resins. This method has the advantage of treatment of huge quantities of wastewater. The drawback is the high cost of resins and their regeneration [15,17–19]. Electrodialysis (ED) is a proper alternative for the treatment of industrial effluents; it minimizes or prevents the production of the final sludge while still keeping high efficiency [20]. Many aspects of chromium removal from solutions using ED have already been studied; Rodrigues and his group investigated the separation of Cr+ 3 from Na+ in aqueous solutions using modified cation exchange membranes. They used a two-step process; in the first step, the separation of Cr+ 3 and Na+ was done by electrodialysis using a monovalent cation exchange membrane. In the second step, the optimum concentration of Cr (III) was achieved by electrodialysis using non modified membranes [21]. Cengeloglu et al. studied the transport of Cr (VI) ions in contact with different salt solutions from three different anion exchange membranes; their study showed that the transport of Cr (VI) ions is the highest when
M. Pazouki, A. Moheb / Desalination 274 (2011) 246–254
copper ions. The purpose of the EDTA addition is to engage the copper ions and prevent their immigration from cation exchange membrane in order to reach high removal of chromium ions.
Table 1 Physical and chemical characteristics of ion exchange membranes. Property
Membrane
Reinforcing fabric Thickness (mm) Specific weight (mg/cm2) Burst strength (kg/cm2) Water content (%) Capacity (meq/g dry membrane)
247
AR204SXR412 CR67, MK111
CR67, MK111
Acrylic 0.5 13.7 7 46 2.8
Acrylic 0.56–0.58 13.7 7 46 2.4
2. Material and methods 2.1. Experimental set up
there are no other salts available. For all three membranes, as the salt anion valence became higher the Cr (VI) anions separation was further reduced [22]. Tor et al. applied ED for the simultaneous recovery of Cr (III) and Cr (VI) ions from aqueous solution containing monovalent and divalent obstructive ions. Their investigation showed that the recovery of ions increased with the increase of current density and decreased in the presence of co-existent ions in the feed phase [23]. Although many original and modified methods for chromium ion removal are already recommended, there is still a need for innovating simpler and more efficient methods. Many researchers applied complexing agents along with the separation processes such as ultrafiltration and electrodialysis to increase the efficiency of the ion removal. Addition of a complexing agent resulted in higher selectivity and permeation fluxes, lower energy costs and high rejection coefficients [24–27]. EDTA as an organic chelating agent has high selectivity and creates stable complexes with most of the metal ions [25,28,34]. In the current study, with the complete attention to the advantages of EDTA as a complexing agent, complexing was combined with ED method to enhance the removal of chromium ions in co-existence with the
A 150 × 150 mm2 three-compartment ED cell made from Plexiglas was used. Plexiglas was chosen to be able to observe the inside of the cell and wash the membranes in the case of precipitation; Plexiglas also resisted against the corrosion in contact with acids. Because electrical resistance adversely changes with distance, thin Plexiglas frames were used (thickness of dilution chamber was 7 mm). Smaller compartments cause high turbulence of the feed flow; turbulence in the compartments prevents the formation of concentration polarization inside the cell. The ED cell was packed with a pair of ion exchange membranes (AR204SXR412 as anionic and CR67, MK111 as cationic both manufactured by Ionics Resources Conservation Co.). The effective area of membranes was 95 × 95 mm2. The membranes' physical and chemical properties are given in Table 1. The cathode was chosen from stainless steel and the anode was made of titanium coated with platinum. 0.05 M nitric acid and 0.05 M hydrochloric acid were used alternately, as the anode and cathode cleansing solutions. The electrodes were washed to avoid the accumulation of the produced gasses on them; the consequent result of gas accumulation is the increase of the cell electric resistance. All the experiments were done at room temperature (20–22 °C). To reach steady state, runs were continued for at least 20 min before sampling the dilute (discharge). A complete schematic of the experimental set up is illustrated in Fig. 1.
Rectifier R-DC ED Cell
TP
pH-I
P-2
+
-
TC
V-1
TA
Tee-1 P-3
TF
V-4
P-1
V-2
Fig. 1. Experimental set up.
F1
MS
248
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Concentrated cleansing solution
Dilute
Concentrated cleansing solution Cation exchange membrane
Anion exchange membrane
Cathode Anode
+
-
Cleansing solution
Cleansing solution
Filter Feed Fig. 2. Schematic of three-compartment ED cell.
2.2. Feed preparation
3. Experiments
To resemble the industrial effluents with various pollutant concentrations, feed with different concentrations of Cr (III) and Cr (VI) ions (100, 200, 300 and 400 ppm) was prepared. The initial concentration of Cu (II) ions was kept equal to 300 ppm for all the runs. Analytical grade salts (chromium nitrate, potassium dichromate, EDTA and copper sulfate) supplied by Aldrich and Merck were consumed. The pH was adjusted with the small addition of diluted sodium hydroxide and hydrochloric acid (supplied from Merck). To control the ion content, deionized water was consumed for preparing all the solutions; the conductivity of water was measured prior to preparation of the solutions to assure the purity of the water (HI8633, HANNA digital conductivity meter).
3.1. Electrodialysis (ED) ED is based on the selective separation of ions from an ionic solution with the help of two different or similar semi-selective membranes. Ions pass through the ion exchange 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 cation exchange membranes are arranged in an alternating pattern between an anode and a cathode to form individual cells. When a direct current (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-
2.3. Concentration measurements The Chromium (VI) concentration was measured with a photometric method. The procedure was based on the formation of colored complex with 1,5-dephenylcarbazide; the absorbent of the complex was then read with a UV spectrophotometer (Shimatzu-Model UV240) at wave length of 546 nm. The chromium (III) concentration was also measured with a similar photometric method, this time EDTA was used as the complexing agent and the absorbent was read at wave length of 540 nm [29–31]. The concentration of Cu (II) ions was measured by the atomic absorption Spectroscopy (VGP210, Buck Scientific).
HOOC
CH2
HOOC
CH2
N
CH2
CH2
CH2
COOH
CH2
COOH
N
Fig. 3. General ethylenediaminetetraacetic acid (EDTA) structure [33].
Fig. 4. Composition of EDTA solutions as function of pH [33].
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249
exchange membrane. On the other hand, negatively charged anions move toward the anode, pass through the positively charged anion exchange membranes but are retained by the negatively charged cation-exchange membranes. At the end, the ion concentration increases in the alternate compartments with simultaneous ion depletion in the feed compartment [32]. The schematic view of a three-compartment ED cell is presented in Fig. 2.
The constants refer to the equilibrium only involving the complex formation of the Y4− species with the metal ions. Although comparing the two given constants shows that Cr (III)– EDTA complex is more stable, the formation of this complex in ambient temperature takes at least 350 min and will be hastened with increasing the temperature. Cu–EDTA complex is created as soon as Cu+ 2 ions come in contact with the EDTA species at room temperature. Therefore, in a solution containing both Cu+ 2 and Cr+ 3 ions and EDTA species at room temperature (independent of pH), Cu+ 2 ions have a higher chance of being engaged with EDTA species and forming Cu–EDTA complexes [33–35]. Sadrzadeh et al. showed that the immigration of the cations through the cation exchange membrane decreased as the valence of the ion increased. This happens because of the larger hydrate radii and less diffusivity of the ions with higher valence [36,37]. The authors of this study benefited from the advantage of the fast complex formation of the Cu+ 2 ions compared to the Cr+ 3 ions (in room temperature) and tried to enhance the separation of the latter in the existence of the former. This occurred by engaging the copper ions with EDTA anionic species and as a result preventing the cations from passing through the cation exchange membrane.
3.2. EDTA as complexing agent
3.3. Design of experiments
The structure of EDTA is shown in Fig. 3; EDTA chemical structure is normally abbreviated as H4Y. Five EDTA-containing species (H4Y, H3Y−, H2Y2−, HY3−, and Y4−) can exist in an aqueous solution; the relative amount of each species depends on the pH of aqueous solution. α0, α1, α2, α3 and α4, are the mole fractions of the aforementioned species in the solution. The species mole fraction at each pH can be determined using the plot given in Fig. 4 [33]. It appears that H2Y2− is predominant species at pH = 3–6 and 3− HY seems to be the major species in the pH range of 6 to 10. Only at the pH values larger than 10, Y4− becomes the major component of the aqueous solution. The formation constants of Cu–EDTA and Cr (III)–EDTA complexes at the ambient temperature are as below:
The experiments were designed with the Taguchi technique. The Taguchi is applicable when the performance of a process is controlled with more than one factor. This technique dramatically decreases the number of required experiments while improving the quality of the results. Based on the Taguchi method, experiments are designed as orthogonal arrays and are done in parallel. Taguchi uses Signal/Noise (S/N) ratio to analyze the data. S/N is calculated according to Eq. (3), where MSD stands for mean square derivative [38].
Table 2 Operating factors and levels. Level Operating factor
1 2 3 4 a
Feed EDTA molar ratio Cation ratio [Cr+ 3]/ pH [EDTA]/[Cr+ 3] [Cu+ 2]a
Voltage Feed flow rate (V) (ml/min)
2.5 3.5 4 4
2 2.5 3 3.5
0.5 1 1.5 2
100/300 200/300 300/300 400/300
10 25 35 50
The unit for numerator and denominator is ppm.
+2 log Kf Cu − EDTA = 18:7
ð1Þ
+3 log Kf Cr −EDTA = 23:
ð2Þ
S=N = −10 log MSD:
ð3Þ
Taguchi also uses the ANOVA method to further analyze the experimental data; more information about this method is given in Section 4.2. The effect of the feed flow rate, voltage, the ratio of chromium (III) concentration to the copper concentration, EDTA molar ratio to Cr (III) and pH of the feed, each in 4 levels, on the removal of Cr (III), Cr (VI) and the relative removal of total chromium were investigated. The noise factors were considered as the voltage fluctuations, the change
Fig. 5. The effect of feed flow rate on the chromium ions removal.
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Fig. 6. The effect of voltage on the chromium ions removal.
of pH in the compartments during the runs and possible precipitation of Cr (OH)3 and Cu (OH)2 on the membranes. Based on the number of the operating factors and the levels, the Taguchi L16 array was chosen for the design of the experiments. Table 2 shows the operating factors and levels considered for this study. The preliminary experiments indicated a very low limiting density for the system; so to avoid precipitation on the cation exchange membrane; low voltages were applied between the cathodes [39].
2) Chromium (III) removal (C3R):
4. Results and discussion
h i h i Cri+6 + Cr0+6 +6 C6R = Cri
To analyze the collected experimental data, the target functions were defined as below: Removal of total chromium : 1) Relative removal of Chromium (RCR) = Removal of copper h i h i h i h i h i h i Cri+3 + Cri+6 − Cr0+3 + Cr0+6 = Cri+3 + Cri+6 +2 +2 +2 RCR = = Cui Cui − Cu0
ð4Þ
h i h i +3 +3 + Cr0 Cri +3 C3R = Cri
ð5Þ
3) Chromium (VI) removal (C6R):
where: 3 [Cr+ ]: i +6 [Cri ]: 3 [Cr+ 0 ]: +6 [Cr0 ]:
Feed chromium (III) concentration Feed chromium (VI) concentration Discharge chromium (III) concentration Discharge chromium (VI) concentration
Fig. 7. The effect of cation ration on the chromium ions removal.
ð6Þ
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251
Fig. 8. The effect of complex ratio on the chromium ions removal. 2 [Cu+ ]: i 2 [Cu+ 0 ]:
• On the other hand, applying high flow rate is a possible way for preventing the concentration polarization phenomenon. [40]
Feed copper (II) concentration Discharge copper (II) concentration
To assure the reliability of the collected data, all the runs were repeated at least two times and data were analyzed with both S/N and ANOVA analysis methods. 4.1. S/N analysis Reaching the maximum removal of the chromium ions was the target of this study; this means that high values of RCR, C3R and C6R were favorable. It is noteworthy that the increase of S/N was interpreted as the increase of the removal of the chromium ions and consequently the defined functions; this was proven by the researchers before applying. 4.1.1. The effect of flow rate The feed flow rate has two conflicting effects on the procedure of the ED cell: • On one hand, applying a low flow rate in the dilution compartment is needed to assure enough residence time for the ions to pass through the ion exchange membranes. [36]
Whereas, Sadrzadeh et al. showed that the increase of the flow rate reduces the removal of the cations, the system tested in the current study behaved differently. For the range of the current density Sadrzadeh et al. used, no sign of concentration polarization was reported. But concentration polarization may have been involved in the current system and this might have caused the unpredictable behavior of the system (Fig. 5). Increasing the flow rate from 10 to 25 ml/min for all the ions, decreased the polarization phenomenon but at the same time caused lower residence time for the ions, thus lower removal of all the ions was the integrated result of these two conflicting parameters. At higher flow rates (25 to 50 ml/min) though, the effect of polarization decreased more dramatically and caused the increase in the removal rates. 4.1.2. The effect of applied voltage Fig. 6 depicted the influence of applied voltage on the ion removal. For all three plots, the increase of ion removal was observed with the increase of voltage. The result was consistent with what other researchers had obtained. The enhancement of the ion transition
Fig. 9. The effect of feed pH on the chromium ions removal.
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M. Pazouki, A. Moheb / Desalination 274 (2011) 246–254 Table 4 ANOVA analysis for chromium (III) removal. Operating factor
SS
Variance
Feed flow rate (ml/min) Voltage (V) Cation ratio EDTA molar ratio Feed pH Error
205.94
68.646
(26.037) 85.131 68.198 151.697 26.037
– 28.377 22.732 50.565 8.679
F
P (%) 23.501
Pooled 5.922 4.361 5.009 –
– 11.004 7.851 23.4 24.244
(VI) anions succeeding in the immigration process. The decrease of RCR is as well a result of drop in chromium ions removal.
Fig. 10. Composition of Cr (VI) species as function of pH [40].
with voltage increase is a consequence of increasing driving force and decreasing electrical resistance of the feed solution [23,36].
4.1.4. The effect of EDTA concentration The effect of complex concentration in the feed on the chromium removal is illustrated in Fig. 8. Initially, high EDTA concentration provides a more favorable environment for forming more copper complexes; this consequently restrains the immigration of copper ions in favor of Cr (III) immigration. Adding more EDTA (ratio of 1 to 1.5) encourages the creation of more Cr (III)–EDTA complexes and hence lowers the separation of these ions [40]. But for EDTA molar ratios beyond 1.5, excess EDTA is available in the solution; this excessive amount of EDTA cannot react with any of the cations and thus the separation of Cr (III) remains constant. This is also confirmed for the C6R plot; when the EDTA species form complex with cations they do not interfere with the removal of Cr (VI) anionic species as much. The molar weight of the complexes is much higher than Cr (VI) species so they do not succeed in passing through the anion exchange membrane. However at ratios higher than 1.5, excess EDTA that appears as different anionic species (depending on the pH) compete with Cr (VI) anions for immigration through the anion exchange membrane and this causes the decrease of Cr (VI) removal [40]. The integration of these changes can be seen in the total chromium removal (RCR) as well.
4.1.3. The effect of cations concentration ratio The effect of Cr (III) to Cu (II) concentration ratio on the removal of chromium ions is presented in Fig. 7. As mentioned in Section 2.2, the initial concentration of copper ions remained constant in all the feeds (equal to 300 ppm). Although generally increase of ion concentration enhances the chance of it to pass through the membranes [36], in the case of this study a different behavior was observed. It appears that the increase of Cr (III) concentration resulted in the formation of Cr (III)–EDTA complex. At this stage, these complexes had to compete with more mobile Cu–EDTA complexes for transferring through the anion exchange membrane and did not succeed; hence the removal of Cr (III) decreased [40]. At higher concentration ratios, because of the absence of enough EDTA (the amount of complexing agent is constant) for the formation of Cr (III)–EDTA, larger number of Cr (III) ions remained in the form of cation. The free Cr (III) cations then won the competition of passing through the cation membrane and this resulted in the increase of Cr (III) removal at ratios larger than 1. In regard to the Cr (VI) species, it should be noted that the salt consumed for the preparation of the Cr+ 3 solution was chromium nitrate; so increase of this ion in the solution leads to the automatic increase of nitrate anions (NO−3 3 ). At the same time, because the concentration of both trivalent and hexavalent chromium in every solution was equal, the increase of trivalent chromium leads to the simultaneous enrichment of the dilution compartment with hexavalent chromium. Therefore an aqueous solution with higher anion ions exists in the dilution compartment. The nitrate ions which have smaller molar weight and higher mobility are more likely to transfer through the anion exchange membrane. The consequence result of the existing competition is the reduction of the number of chromium
Looking at Eqs. (7) and (8), it is concluded that the Cu–EDTA complexes species forming in this range of pH remain unchanged for the whole pH range. From Fig. 10, it is observed that for the entire experimented pH, the 2− feed practically contains about 25% HCrO− 4 and 75% Cr2O7 . It means that the competing anions did not change so the amount of removed Cr (VI) remained relatively constant.
Table 3 ANOVA analysis for the relative removal of total chromium.
Table 5 ANOVA analysis for chromium (VI) removal.
4.1.5. The effect of pH To investigate the influence of feed pH on the removal of chromium ions, we had better pay attention to how the composition of the species in the aqueous solution changes with pH for both EDTA and Cr (VI). According to Fig. 4, from pH 3 to 6, EDTA majorly constitutes H2Y2− and a small amount of HY3− exists. So the possible metal-complex formation reactions are: +2
+ H2 Y
+2
+ HY
Cu Cu
2−
3−
2−
↔CuY
2−
↔CuY
þ
ð7Þ
+ 2H3 O þ
ð8Þ
+ H3 O
Operating factor
SS
Variance
F
P (%)
Operating factor
SS
Variance
F
P (%)
Feed flow rate (ml/min) Voltage (V) Cation ratio EDTA molar ratio Feed pH Error
(8.254)
–
Pooled
–
309.515
103.171
10.19
44.788
44.282 48.887 35.995 41.345 8.254
14.76 16.295 11.999 13.781 2.751
5.364 5.922 4.361 5.009 –
20.153 22.729 15.52 18.51 23.088
Feed flow rate (ml/min) Voltage (V) Cation ratio EDTA molar ratio Feed pH Error
47.969 51.351 184.314 (30.372) 30.372
15.898 17.117 61.438 – 10.124
1.57 1.69 6.068 Pooled –
2.779 3.366 24.699 – 24.368
M. Pazouki, A. Moheb / Desalination 274 (2011) 246–254 Table 6 Comparison of ED technique with ED-complexing chromium (III) removal. Run
ED process
ED-complexing process
8.02 3.14 29.89 2.81
13.72 6.12 82.11 15.79
It is as well concluded that the mechanism of the cation transportation from membrane does not change, copper ions form complexes and Cr (III) remain as cations. The whole phenomena then lead to the relatively constant removal of the Cr (III) cations [33,41]. The plots related to these experiments are given in Fig. 9. 4.2. ANOVA analysis The Taguchi design method provides the opportunity to better analyze the data with ANOVA and determine the variation each operating factor contributes; it also determines the optimum experimental condition. This analysis offers more details such as sum of square (SS), mean square (variance), the ratio of factors variance to the error variance (F) and the contribution percentage of each factor on the response (P). The Taguchi design method also gives the chance to eliminate the factor(s) which do not have much influence on the response. To do this, the error associated with eliminating each parameter plus the experimental and instrumental errors (P%) are calculated. The factors with the lowest error are ignored or "Pooled" [38]. Tables 3–5 depict the ANOVA analysis for the relative removal of the total chromium (RCR), the removal of chromium (III) ions (C3R) and the removal of chromium (VI) ions (C6R); these tables are related to after pooling calculations. According to the tables flow rate, voltage and pH as operating factors can be ignored for the removal of total chromium, chromium (III) and chromium (VI), respectively. Using the ANOVA analysis, the optimum condition for the separation of each ion was determined. The removal of Cr (III) at the optimum condition (flow rate = 10 ml/min, voltage = 3 V, cation ratio = 1.33, EDTA ratio = 1 and feed pH = 4) reached to 87%, and 95% of Cr (VI) at the optimum condition of flow rate = 10 ml/min, voltage = 3.5 V, cation ratio = 0.33, EDTA ratio = 1 and pH = 4 was eliminated. 5. Comparing proposed ED-complexing method with ED To ascertain the superiority of the proposed ED-complexing method versus ED; some randomly chosen experimental runs were repeated with ED process. Table 6 illustrates the comparison between the two methods for the removal of chromium (III). The C3R values were calculated for both techniques. It was clearly shown that the combination of complexing with ED highly advances the chromium (III) removal. Similar results were obtained for the elimination of Cr
Table 7 Randomly chosen experimental runs. Run Operating factor
a
(VI) and the total chromium ions. Different operating parameters in which the experiments were done are revealed in Table 7.
C3R
6 9 11 14
6 9 11 14
253
Feed pH
EDTA molar ratio Cation ratio [Cr+ 3]/ [EDTA]/[Cr+ 3] [Cu+ 2]a
Voltage (v)
Feed flow rate (ml/min)
4 3.5 4 4
2 2 1 0.5
2.5 2 3 2.5
25 35 35 50
100/300 300/300 100/300 300/300
The unit for numerator and denominator is ppm.
6. Conclusion The application of electrodialysis with EDTA as the complexing agent significantly improves the removal of the total chromium, chromium (VI) and chromium (III) removal in the presence of copper ions. The optimum experimental condition for the maximum separation of each chromium ion (95% Cr (VI) and 87% Cr (III) removal) was determined. Once again it was confirmed that the increase of current density in ED cell enhances the separation of cations and anions. The effect of the flow rate on the chromium ions removal showed a different trend than what was observed before; so to come up with a conclusion, more investigation in this area seems to be essential. To better understand the effect of pH on the elimination of chromium ions from aqueous solutions, more experiments in different ranges of feed pH are suggested to be done. References [1] M. Tuzen, M. Soylak, Chromium speciation in environmental samples by solid phase extraction on chromosorb 108, Journal of Hazardous Material 129 (1–3) (2006) 266–273. [2] F. Gode, E. Pehlivan, Removal of Cr(VI) from aqueous solution by two Lewatitanion exchange resins, Journal of Hazardous Material 119 (1–3) (2005) 175–182. [3] The American water works association Inc., Water Quality and Treatment Handbook, 3rd Edition McGraw-Hill, 1971. [4] R. Karosi, V. Andruch, J. Posta, J. Balogh, Separation of chromium (VI) using complexation and its determination with GFAAS, Microchemical Journal 82 (2006) 61–65. [5] N.K. Chandra Buka, K. Asma, A. Raghupathi, R. Venba, R. Ramesh, S. Sadulla, Screening of leather auxiliaries for their role in toxic hexavalent chromium formation in leather — posing potential health hazards to the users, Journal of Cleaner Production 13 (12) (2005) 1189–1195. [6] F. Gode, E. Pehlivan, Removal of chromium (III) from aqueous solutions using Lawatit S100: the effect of pH, time, metal concentration and temperature, Journal of Hazardous Material 136 (1) (2006) 330–337. [7] OEHHA office of environmental health hazard assessment, Determination of nanocancer chronic reference exposure levels, Batch 2A 2000. [8] J. Roy, Environmental Contaminants Encyclopedia Chromium (III), IRWIN, National Park Service, 1997. [9] E. Velizarova, A.B. Ribeiro, L.M. Ottosen, A comparative study on Cu, Cr and As removal from CCA-treated wood waste by dialytic, Journal of Hazardous Material 94 (2) (2002) 147–160. [10] A. Hafez, S. EL-Manharawy, Design and performance of the two stage/two pass RO membrane system for chromium removal from tannery wastewater, Desalination 165 (2004) 141–151. [11] C.A. Kozlowski, W. Walkowiak, Applicability of liquid membranes in chromium (VI) transport with amines as ion carriers, Journal of membrane Science 266 (1–2) (2005) 143–150. [12] L. Lambert, M. Avila-Rodriguez, G. Durand, M. Rakib, Separation of sodium ions from trivalent chromium by electrodialysis using monovalent cation selective membranes, Journal of membrane science 280 (1–2) (2006) 219–225. [13] MWH, Water Treatment, Principles and Design, 2nd Edition John Wiley & Sons Inc, 2005. [14] B.R. James, Method to reduce hexavalent chromium in soils, sediments, industrial waste and other contaminated materials using ascorbic acid, US PATENT No. 5 951457. [15] P. Gao, Y. Chen, F. Shen, G. Chen, Removal of chromium(VI) from wastewater by combined electrocoagulation–electroflotation without a filter, Separation and Purification Technology 43 (2) (2005) 117–123. [16] S.J. Arceivala, Wastewater Treatment for Pollution Control, McGraw-Hill, 1991. [17] D.H.F. Liu, B.G. Liptak, Wastewater Treatment, Lewis Publication, 2000. [18] T. Emon, C. Lee, J. Kim, Ch. Lee, Development of an ion exchange system for plating wastewater treatment, Desalination 180 (2005) 163–172. [19] B. Galan, D. Castaneda, I. Ortiz, Removal and recovery of Cr (VI) from polluted ground waters, Water Research 39 (17) (2005) 4317–4324. [20] M.A. Rodriguez, R.F. Dallacosta, A.M. Bernardes, J.Z. Ferreira, Influence of ligand exchange on the treatment of trivalent chromium solutions by electrodialysis, Electrochimica 47 (5) (2001) 753–758. [21] J. Lamberte, M. Rakib, G. Durand, M.A. Rodriguez, Treatment of solutions containing trivalent chromium by electrodialysis, Desalination 191 (2006) 100–110. [22] Y. Cengeloglu, A. Tor, E. Kir, M. Ersoz, Transport of hexavalent chromium through anion exchange membranes, Desalination 154 (2003) 239–249. [23] A. Tor, Y. Cengeloglu, M. Ersoz, T. Buyukerkek, Simultaneous recovery of Cr(III) and Cr(VI) from the aqueous phase with ion-exchange membranes, Desalination 171 (2004) 233–241.
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[24] P. Canizares, A. Perez, R. Camarillo, Recovery of heavy metals by means of ultrafiltration with water-soluble polymers: calculation of design parameters, Desalination 144 (2002) 279–285. [25] C.R. Tavares, M. Vieira, J.C.C. Petrus, E.C. Bortoletto, F. Ceravollo, Ultrafiltration/ complexation process for metal removal from pulp and paper industry wastewater, Desalination 144 (2002) 261–265. [26] R.S. Juang, G.S. Yan, Enhanced flux and selectivity of metals through a dialysis membrane by addition of complexing agent to receiving phase, Journal of Membrane Science 186 (1) (2001) 53–61. [27] J.K. Wang, C.P. Chang, Selective transport of cupric–nickel–cobalt ion ternary system through cation-exchange membranes with a complexing agent by dialysis, Desalination 164 (2004) 269–280. [28] O. Gyliene, J. Aikaite, O. Nivinskiene, Recovery of EDTA from complex solution using Cu(II) as precipitant and Cu (II) subsequent removal by electrolysis, Journal of Hazardous Material B116 (2004) 119–124. [29] R.T. PFlaum, L.C. Howick, The chromium–diphenylcarbazide reaction, Journal of The American Chemical Society 78 (19) (1956) 4862–4866. [30] ASTM-Designation D1687-92, Standard test methods for chromium in water, 1996. [31] Z. Marczenko, Spectrophotometric Determination of Elements, Ellis Horwood Ltd, 1976. [32] P. Hillis, Membrane Technology in Water and Wastewater Treatment, R.S.C., 2000.
[33] D.A. Skoog, D.M. West, F.J. Holler, Fundamentals of Analytical Chemistry, Saunders College Publishing, 1988. [34] J.A. Dean, Analytical Chemistry Handbook, McGraw-Hill, 1995. [35] M. Grabarczyk, Catalytic adsorptive stripping Volta metric determination of Cr (VI) in EDTA extracts from solid samples, Electrochimica Acta 51 (11) (2006) 2333–2337. [36] R.K. Roy, Design of Experiments Using Taguchi Approach: 16 Steps to Process Improvement, John Wiley & Sons, 2001. [37] S.P. Sun, K.Y. Wang, D. Rajarathnam, T.A. Hatton, T.S. Chung, Polyamide-imide nanofiltration hollow fiber membranes with elongation-induced nano-pore evolution, American Institute of Chemical Engineers Journal 56 (2010) 1481. [38] R.K. Roy, Design of Experiments Using Taguchi Approach: 16 Steps to Process Improvement, John Wiley & Sons, 2001. [39] E. Drioli, M. Nakagaki, Membranes and Membrane Processes, Plenum Press, New York, 1986. [40] B. Baraj, M. Martinez, A. Sastre, M. Aguilar, Simultaneous determination of Cr (III), Fe (III), Cu (II) and Pb (II) as UV-absorbing EDTA complexes by capillary zone electrophoresis, Journal of Chromatography A 695 (1995) 103–111. [41] M.E. Vallejo, F. Persin, C. Innocent, P. Sistat, G. Pourcelly, Electrotransport of Cr(VI) through an anion exchange membrane, Separation and Purification Technology 21 (2000) 61–69.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
An innovative membrane method for the separation of chromium ions from solutions containing obstructive copper ions Mahshad Pazouki ⁎, Ahmad Moheb Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 8 September 2010 Received in revised form 8 February 2011 Accepted 9 February 2011 Available online 24 March 2011 Keywords: Electrodialysis Complexing agent Taguchi technique EDTA ANOVA
a b s t r a c t Chromium is one of the major pollutants of water and wastewater; this ion can be seen in different industrial effluents along with other metal ions such as Cu, As [E. Velizarova, A.B. Ribeiro, and L.M. Ottosen, A comparative study on Cu, Cr and As removal from CCA-treated wood waste by dialytic, Journal of Hazardous Material 94 (2) (2002)147–160]. In the current paper, experimental investigations have been done on the removal of the chromium ions from aqueous solutions which contain Cu2+ as obstructive ion. Electrodialysis is coupled with complexing to recommend a new technique for the elimination of chromium (III) ions. Ethylendiaminetetraacetate acid (EDTA) was used as the complexing agent. The effect of different operating conditions (feed flow rate, voltage or current density, the ratio of the chromium (III) concentration to the copper concentration, EDTA molar ratio to Cr (III) and pH of the feed) on the removal of chromium ions was investigated. The Taguchi method was implied to design the experiments. The optimum operating conditions were determined using the analysis of variance (ANOVA) method. The proposed method resulted in relatively high chromium removal (95% Cr (VI) and 87% Cr (III)) at the optimum operating conditions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Chromium is applied in different industries such as tanning factories, steel works, industrial electroplating, wood preservation, artificial fertilizers, paint and pigment manufacturing, corrosion control, textile and photography [1–4]. Chromium toxicity varies with the concentration and oxidation state. Hexavalent and trivalent chromium are two common oxidation states in nature. Although trivalent chromium is less toxic and similar to many other heavy metals, in small traces is necessary for life processes, the higher concentrations of it in the environment and consequently human intake cause many diseases [1,5,6]. Chromium (VI) is soluble in water over the entire pH range so it is mobile in nature. Because of its high mobility, Cr (VI) is able to easily permeate in to the biological membranes; therefore, it is highly toxic and known as a carcinogen material. Depending on the pH of the aqueous system, this ion is expected as anionic species such as 2− 2− HCr2O− and Cr2O2− 7 , HCrO4 , CrO4 7 . According to the Environment Protection Agent (EPA), release of 1 ppm of Cr (VI) and 1–5 ppm of Cr (III) in the industrial effluents is permitted [2,7,8]. Methods such as chemical precipitation, ion exchange resin, foam flotation, adsorption, and solvent extraction are used for the removal ⁎ Corresponding author at: Department of Chemical and Materials Engineering, University of Alberta, 7th Floor, Electrical & Computer Engineering Research Facility (ECERF), 9107-116 Street, Edmonton, Alberta, Canada T6G 2V4. Tel.: + 1 780 707 6824; fax: + 1 780 492 2881. E-mail address: [email protected] (M. Pazouki). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.02.017
of the chromium from industrial effluents. Membrane methods like ultrafiltration, nanofiltration, reverse osmosis, electrodialysis and liquid membrane may be suitable alternatives to the abovementioned methods [6,9–13]. The most widely applied method for the recovery of Cr (III) is precipitation with alkalis or sulfide; although this method is simple and inexpensive, it creates large loads of toxic waste sludge. The remaining sludge contains small suspended solids which have to be removed with filtration as a secondary method. The residue still contains trivalent chromium [5,8,14,15]. To remove Cr (VI), reduction is used prior to precipitation [16]. Another popular technique for the removal of chromium, especially chromium (VI), is ion exchange resins. This method has the advantage of treatment of huge quantities of wastewater. The drawback is the high cost of resins and their regeneration [15,17–19]. Electrodialysis (ED) is a proper alternative for the treatment of industrial effluents; it minimizes or prevents the production of the final sludge while still keeping high efficiency [20]. Many aspects of chromium removal from solutions using ED have already been studied; Rodrigues and his group investigated the separation of Cr+ 3 from Na+ in aqueous solutions using modified cation exchange membranes. They used a two-step process; in the first step, the separation of Cr+ 3 and Na+ was done by electrodialysis using a monovalent cation exchange membrane. In the second step, the optimum concentration of Cr (III) was achieved by electrodialysis using non modified membranes [21]. Cengeloglu et al. studied the transport of Cr (VI) ions in contact with different salt solutions from three different anion exchange membranes; their study showed that the transport of Cr (VI) ions is the highest when
M. Pazouki, A. Moheb / Desalination 274 (2011) 246–254
copper ions. The purpose of the EDTA addition is to engage the copper ions and prevent their immigration from cation exchange membrane in order to reach high removal of chromium ions.
Table 1 Physical and chemical characteristics of ion exchange membranes. Property
Membrane
Reinforcing fabric Thickness (mm) Specific weight (mg/cm2) Burst strength (kg/cm2) Water content (%) Capacity (meq/g dry membrane)
247
AR204SXR412 CR67, MK111
CR67, MK111
Acrylic 0.5 13.7 7 46 2.8
Acrylic 0.56–0.58 13.7 7 46 2.4
2. Material and methods 2.1. Experimental set up
there are no other salts available. For all three membranes, as the salt anion valence became higher the Cr (VI) anions separation was further reduced [22]. Tor et al. applied ED for the simultaneous recovery of Cr (III) and Cr (VI) ions from aqueous solution containing monovalent and divalent obstructive ions. Their investigation showed that the recovery of ions increased with the increase of current density and decreased in the presence of co-existent ions in the feed phase [23]. Although many original and modified methods for chromium ion removal are already recommended, there is still a need for innovating simpler and more efficient methods. Many researchers applied complexing agents along with the separation processes such as ultrafiltration and electrodialysis to increase the efficiency of the ion removal. Addition of a complexing agent resulted in higher selectivity and permeation fluxes, lower energy costs and high rejection coefficients [24–27]. EDTA as an organic chelating agent has high selectivity and creates stable complexes with most of the metal ions [25,28,34]. In the current study, with the complete attention to the advantages of EDTA as a complexing agent, complexing was combined with ED method to enhance the removal of chromium ions in co-existence with the
A 150 × 150 mm2 three-compartment ED cell made from Plexiglas was used. Plexiglas was chosen to be able to observe the inside of the cell and wash the membranes in the case of precipitation; Plexiglas also resisted against the corrosion in contact with acids. Because electrical resistance adversely changes with distance, thin Plexiglas frames were used (thickness of dilution chamber was 7 mm). Smaller compartments cause high turbulence of the feed flow; turbulence in the compartments prevents the formation of concentration polarization inside the cell. The ED cell was packed with a pair of ion exchange membranes (AR204SXR412 as anionic and CR67, MK111 as cationic both manufactured by Ionics Resources Conservation Co.). The effective area of membranes was 95 × 95 mm2. The membranes' physical and chemical properties are given in Table 1. The cathode was chosen from stainless steel and the anode was made of titanium coated with platinum. 0.05 M nitric acid and 0.05 M hydrochloric acid were used alternately, as the anode and cathode cleansing solutions. The electrodes were washed to avoid the accumulation of the produced gasses on them; the consequent result of gas accumulation is the increase of the cell electric resistance. All the experiments were done at room temperature (20–22 °C). To reach steady state, runs were continued for at least 20 min before sampling the dilute (discharge). A complete schematic of the experimental set up is illustrated in Fig. 1.
Rectifier R-DC ED Cell
TP
pH-I
P-2
+
-
TC
V-1
TA
Tee-1 P-3
TF
V-4
P-1
V-2
Fig. 1. Experimental set up.
F1
MS
248
M. Pazouki, A. Moheb / Desalination 274 (2011) 246–254
Concentrated cleansing solution
Dilute
Concentrated cleansing solution Cation exchange membrane
Anion exchange membrane
Cathode Anode
+
-
Cleansing solution
Cleansing solution
Filter Feed Fig. 2. Schematic of three-compartment ED cell.
2.2. Feed preparation
3. Experiments
To resemble the industrial effluents with various pollutant concentrations, feed with different concentrations of Cr (III) and Cr (VI) ions (100, 200, 300 and 400 ppm) was prepared. The initial concentration of Cu (II) ions was kept equal to 300 ppm for all the runs. Analytical grade salts (chromium nitrate, potassium dichromate, EDTA and copper sulfate) supplied by Aldrich and Merck were consumed. The pH was adjusted with the small addition of diluted sodium hydroxide and hydrochloric acid (supplied from Merck). To control the ion content, deionized water was consumed for preparing all the solutions; the conductivity of water was measured prior to preparation of the solutions to assure the purity of the water (HI8633, HANNA digital conductivity meter).
3.1. Electrodialysis (ED) ED is based on the selective separation of ions from an ionic solution with the help of two different or similar semi-selective membranes. Ions pass through the ion exchange 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 cation exchange membranes are arranged in an alternating pattern between an anode and a cathode to form individual cells. When a direct current (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-
2.3. Concentration measurements The Chromium (VI) concentration was measured with a photometric method. The procedure was based on the formation of colored complex with 1,5-dephenylcarbazide; the absorbent of the complex was then read with a UV spectrophotometer (Shimatzu-Model UV240) at wave length of 546 nm. The chromium (III) concentration was also measured with a similar photometric method, this time EDTA was used as the complexing agent and the absorbent was read at wave length of 540 nm [29–31]. The concentration of Cu (II) ions was measured by the atomic absorption Spectroscopy (VGP210, Buck Scientific).
HOOC
CH2
HOOC
CH2
N
CH2
CH2
CH2
COOH
CH2
COOH
N
Fig. 3. General ethylenediaminetetraacetic acid (EDTA) structure [33].
Fig. 4. Composition of EDTA solutions as function of pH [33].
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249
exchange membrane. On the other hand, negatively charged anions move toward the anode, pass through the positively charged anion exchange membranes but are retained by the negatively charged cation-exchange membranes. At the end, the ion concentration increases in the alternate compartments with simultaneous ion depletion in the feed compartment [32]. The schematic view of a three-compartment ED cell is presented in Fig. 2.
The constants refer to the equilibrium only involving the complex formation of the Y4− species with the metal ions. Although comparing the two given constants shows that Cr (III)– EDTA complex is more stable, the formation of this complex in ambient temperature takes at least 350 min and will be hastened with increasing the temperature. Cu–EDTA complex is created as soon as Cu+ 2 ions come in contact with the EDTA species at room temperature. Therefore, in a solution containing both Cu+ 2 and Cr+ 3 ions and EDTA species at room temperature (independent of pH), Cu+ 2 ions have a higher chance of being engaged with EDTA species and forming Cu–EDTA complexes [33–35]. Sadrzadeh et al. showed that the immigration of the cations through the cation exchange membrane decreased as the valence of the ion increased. This happens because of the larger hydrate radii and less diffusivity of the ions with higher valence [36,37]. The authors of this study benefited from the advantage of the fast complex formation of the Cu+ 2 ions compared to the Cr+ 3 ions (in room temperature) and tried to enhance the separation of the latter in the existence of the former. This occurred by engaging the copper ions with EDTA anionic species and as a result preventing the cations from passing through the cation exchange membrane.
3.2. EDTA as complexing agent
3.3. Design of experiments
The structure of EDTA is shown in Fig. 3; EDTA chemical structure is normally abbreviated as H4Y. Five EDTA-containing species (H4Y, H3Y−, H2Y2−, HY3−, and Y4−) can exist in an aqueous solution; the relative amount of each species depends on the pH of aqueous solution. α0, α1, α2, α3 and α4, are the mole fractions of the aforementioned species in the solution. The species mole fraction at each pH can be determined using the plot given in Fig. 4 [33]. It appears that H2Y2− is predominant species at pH = 3–6 and 3− HY seems to be the major species in the pH range of 6 to 10. Only at the pH values larger than 10, Y4− becomes the major component of the aqueous solution. The formation constants of Cu–EDTA and Cr (III)–EDTA complexes at the ambient temperature are as below:
The experiments were designed with the Taguchi technique. The Taguchi is applicable when the performance of a process is controlled with more than one factor. This technique dramatically decreases the number of required experiments while improving the quality of the results. Based on the Taguchi method, experiments are designed as orthogonal arrays and are done in parallel. Taguchi uses Signal/Noise (S/N) ratio to analyze the data. S/N is calculated according to Eq. (3), where MSD stands for mean square derivative [38].
Table 2 Operating factors and levels. Level Operating factor
1 2 3 4 a
Feed EDTA molar ratio Cation ratio [Cr+ 3]/ pH [EDTA]/[Cr+ 3] [Cu+ 2]a
Voltage Feed flow rate (V) (ml/min)
2.5 3.5 4 4
2 2.5 3 3.5
0.5 1 1.5 2
100/300 200/300 300/300 400/300
10 25 35 50
The unit for numerator and denominator is ppm.
+2 log Kf Cu − EDTA = 18:7
ð1Þ
+3 log Kf Cr −EDTA = 23:
ð2Þ
S=N = −10 log MSD:
ð3Þ
Taguchi also uses the ANOVA method to further analyze the experimental data; more information about this method is given in Section 4.2. The effect of the feed flow rate, voltage, the ratio of chromium (III) concentration to the copper concentration, EDTA molar ratio to Cr (III) and pH of the feed, each in 4 levels, on the removal of Cr (III), Cr (VI) and the relative removal of total chromium were investigated. The noise factors were considered as the voltage fluctuations, the change
Fig. 5. The effect of feed flow rate on the chromium ions removal.
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M. Pazouki, A. Moheb / Desalination 274 (2011) 246–254
Fig. 6. The effect of voltage on the chromium ions removal.
of pH in the compartments during the runs and possible precipitation of Cr (OH)3 and Cu (OH)2 on the membranes. Based on the number of the operating factors and the levels, the Taguchi L16 array was chosen for the design of the experiments. Table 2 shows the operating factors and levels considered for this study. The preliminary experiments indicated a very low limiting density for the system; so to avoid precipitation on the cation exchange membrane; low voltages were applied between the cathodes [39].
2) Chromium (III) removal (C3R):
4. Results and discussion
h i h i Cri+6 + Cr0+6 +6 C6R = Cri
To analyze the collected experimental data, the target functions were defined as below: Removal of total chromium : 1) Relative removal of Chromium (RCR) = Removal of copper h i h i h i h i h i h i Cri+3 + Cri+6 − Cr0+3 + Cr0+6 = Cri+3 + Cri+6 +2 +2 +2 RCR = = Cui Cui − Cu0
ð4Þ
h i h i +3 +3 + Cr0 Cri +3 C3R = Cri
ð5Þ
3) Chromium (VI) removal (C6R):
where: 3 [Cr+ ]: i +6 [Cri ]: 3 [Cr+ 0 ]: +6 [Cr0 ]:
Feed chromium (III) concentration Feed chromium (VI) concentration Discharge chromium (III) concentration Discharge chromium (VI) concentration
Fig. 7. The effect of cation ration on the chromium ions removal.
ð6Þ
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251
Fig. 8. The effect of complex ratio on the chromium ions removal. 2 [Cu+ ]: i 2 [Cu+ 0 ]:
• On the other hand, applying high flow rate is a possible way for preventing the concentration polarization phenomenon. [40]
Feed copper (II) concentration Discharge copper (II) concentration
To assure the reliability of the collected data, all the runs were repeated at least two times and data were analyzed with both S/N and ANOVA analysis methods. 4.1. S/N analysis Reaching the maximum removal of the chromium ions was the target of this study; this means that high values of RCR, C3R and C6R were favorable. It is noteworthy that the increase of S/N was interpreted as the increase of the removal of the chromium ions and consequently the defined functions; this was proven by the researchers before applying. 4.1.1. The effect of flow rate The feed flow rate has two conflicting effects on the procedure of the ED cell: • On one hand, applying a low flow rate in the dilution compartment is needed to assure enough residence time for the ions to pass through the ion exchange membranes. [36]
Whereas, Sadrzadeh et al. showed that the increase of the flow rate reduces the removal of the cations, the system tested in the current study behaved differently. For the range of the current density Sadrzadeh et al. used, no sign of concentration polarization was reported. But concentration polarization may have been involved in the current system and this might have caused the unpredictable behavior of the system (Fig. 5). Increasing the flow rate from 10 to 25 ml/min for all the ions, decreased the polarization phenomenon but at the same time caused lower residence time for the ions, thus lower removal of all the ions was the integrated result of these two conflicting parameters. At higher flow rates (25 to 50 ml/min) though, the effect of polarization decreased more dramatically and caused the increase in the removal rates. 4.1.2. The effect of applied voltage Fig. 6 depicted the influence of applied voltage on the ion removal. For all three plots, the increase of ion removal was observed with the increase of voltage. The result was consistent with what other researchers had obtained. The enhancement of the ion transition
Fig. 9. The effect of feed pH on the chromium ions removal.
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M. Pazouki, A. Moheb / Desalination 274 (2011) 246–254 Table 4 ANOVA analysis for chromium (III) removal. Operating factor
SS
Variance
Feed flow rate (ml/min) Voltage (V) Cation ratio EDTA molar ratio Feed pH Error
205.94
68.646
(26.037) 85.131 68.198 151.697 26.037
– 28.377 22.732 50.565 8.679
F
P (%) 23.501
Pooled 5.922 4.361 5.009 –
– 11.004 7.851 23.4 24.244
(VI) anions succeeding in the immigration process. The decrease of RCR is as well a result of drop in chromium ions removal.
Fig. 10. Composition of Cr (VI) species as function of pH [40].
with voltage increase is a consequence of increasing driving force and decreasing electrical resistance of the feed solution [23,36].
4.1.4. The effect of EDTA concentration The effect of complex concentration in the feed on the chromium removal is illustrated in Fig. 8. Initially, high EDTA concentration provides a more favorable environment for forming more copper complexes; this consequently restrains the immigration of copper ions in favor of Cr (III) immigration. Adding more EDTA (ratio of 1 to 1.5) encourages the creation of more Cr (III)–EDTA complexes and hence lowers the separation of these ions [40]. But for EDTA molar ratios beyond 1.5, excess EDTA is available in the solution; this excessive amount of EDTA cannot react with any of the cations and thus the separation of Cr (III) remains constant. This is also confirmed for the C6R plot; when the EDTA species form complex with cations they do not interfere with the removal of Cr (VI) anionic species as much. The molar weight of the complexes is much higher than Cr (VI) species so they do not succeed in passing through the anion exchange membrane. However at ratios higher than 1.5, excess EDTA that appears as different anionic species (depending on the pH) compete with Cr (VI) anions for immigration through the anion exchange membrane and this causes the decrease of Cr (VI) removal [40]. The integration of these changes can be seen in the total chromium removal (RCR) as well.
4.1.3. The effect of cations concentration ratio The effect of Cr (III) to Cu (II) concentration ratio on the removal of chromium ions is presented in Fig. 7. As mentioned in Section 2.2, the initial concentration of copper ions remained constant in all the feeds (equal to 300 ppm). Although generally increase of ion concentration enhances the chance of it to pass through the membranes [36], in the case of this study a different behavior was observed. It appears that the increase of Cr (III) concentration resulted in the formation of Cr (III)–EDTA complex. At this stage, these complexes had to compete with more mobile Cu–EDTA complexes for transferring through the anion exchange membrane and did not succeed; hence the removal of Cr (III) decreased [40]. At higher concentration ratios, because of the absence of enough EDTA (the amount of complexing agent is constant) for the formation of Cr (III)–EDTA, larger number of Cr (III) ions remained in the form of cation. The free Cr (III) cations then won the competition of passing through the cation membrane and this resulted in the increase of Cr (III) removal at ratios larger than 1. In regard to the Cr (VI) species, it should be noted that the salt consumed for the preparation of the Cr+ 3 solution was chromium nitrate; so increase of this ion in the solution leads to the automatic increase of nitrate anions (NO−3 3 ). At the same time, because the concentration of both trivalent and hexavalent chromium in every solution was equal, the increase of trivalent chromium leads to the simultaneous enrichment of the dilution compartment with hexavalent chromium. Therefore an aqueous solution with higher anion ions exists in the dilution compartment. The nitrate ions which have smaller molar weight and higher mobility are more likely to transfer through the anion exchange membrane. The consequence result of the existing competition is the reduction of the number of chromium
Looking at Eqs. (7) and (8), it is concluded that the Cu–EDTA complexes species forming in this range of pH remain unchanged for the whole pH range. From Fig. 10, it is observed that for the entire experimented pH, the 2− feed practically contains about 25% HCrO− 4 and 75% Cr2O7 . It means that the competing anions did not change so the amount of removed Cr (VI) remained relatively constant.
Table 3 ANOVA analysis for the relative removal of total chromium.
Table 5 ANOVA analysis for chromium (VI) removal.
4.1.5. The effect of pH To investigate the influence of feed pH on the removal of chromium ions, we had better pay attention to how the composition of the species in the aqueous solution changes with pH for both EDTA and Cr (VI). According to Fig. 4, from pH 3 to 6, EDTA majorly constitutes H2Y2− and a small amount of HY3− exists. So the possible metal-complex formation reactions are: +2
+ H2 Y
+2
+ HY
Cu Cu
2−
3−
2−
↔CuY
2−
↔CuY
þ
ð7Þ
+ 2H3 O þ
ð8Þ
+ H3 O
Operating factor
SS
Variance
F
P (%)
Operating factor
SS
Variance
F
P (%)
Feed flow rate (ml/min) Voltage (V) Cation ratio EDTA molar ratio Feed pH Error
(8.254)
–
Pooled
–
309.515
103.171
10.19
44.788
44.282 48.887 35.995 41.345 8.254
14.76 16.295 11.999 13.781 2.751
5.364 5.922 4.361 5.009 –
20.153 22.729 15.52 18.51 23.088
Feed flow rate (ml/min) Voltage (V) Cation ratio EDTA molar ratio Feed pH Error
47.969 51.351 184.314 (30.372) 30.372
15.898 17.117 61.438 – 10.124
1.57 1.69 6.068 Pooled –
2.779 3.366 24.699 – 24.368
M. Pazouki, A. Moheb / Desalination 274 (2011) 246–254 Table 6 Comparison of ED technique with ED-complexing chromium (III) removal. Run
ED process
ED-complexing process
8.02 3.14 29.89 2.81
13.72 6.12 82.11 15.79
It is as well concluded that the mechanism of the cation transportation from membrane does not change, copper ions form complexes and Cr (III) remain as cations. The whole phenomena then lead to the relatively constant removal of the Cr (III) cations [33,41]. The plots related to these experiments are given in Fig. 9. 4.2. ANOVA analysis The Taguchi design method provides the opportunity to better analyze the data with ANOVA and determine the variation each operating factor contributes; it also determines the optimum experimental condition. This analysis offers more details such as sum of square (SS), mean square (variance), the ratio of factors variance to the error variance (F) and the contribution percentage of each factor on the response (P). The Taguchi design method also gives the chance to eliminate the factor(s) which do not have much influence on the response. To do this, the error associated with eliminating each parameter plus the experimental and instrumental errors (P%) are calculated. The factors with the lowest error are ignored or "Pooled" [38]. Tables 3–5 depict the ANOVA analysis for the relative removal of the total chromium (RCR), the removal of chromium (III) ions (C3R) and the removal of chromium (VI) ions (C6R); these tables are related to after pooling calculations. According to the tables flow rate, voltage and pH as operating factors can be ignored for the removal of total chromium, chromium (III) and chromium (VI), respectively. Using the ANOVA analysis, the optimum condition for the separation of each ion was determined. The removal of Cr (III) at the optimum condition (flow rate = 10 ml/min, voltage = 3 V, cation ratio = 1.33, EDTA ratio = 1 and feed pH = 4) reached to 87%, and 95% of Cr (VI) at the optimum condition of flow rate = 10 ml/min, voltage = 3.5 V, cation ratio = 0.33, EDTA ratio = 1 and pH = 4 was eliminated. 5. Comparing proposed ED-complexing method with ED To ascertain the superiority of the proposed ED-complexing method versus ED; some randomly chosen experimental runs were repeated with ED process. Table 6 illustrates the comparison between the two methods for the removal of chromium (III). The C3R values were calculated for both techniques. It was clearly shown that the combination of complexing with ED highly advances the chromium (III) removal. Similar results were obtained for the elimination of Cr
Table 7 Randomly chosen experimental runs. Run Operating factor
a
(VI) and the total chromium ions. Different operating parameters in which the experiments were done are revealed in Table 7.
C3R
6 9 11 14
6 9 11 14
253
Feed pH
EDTA molar ratio Cation ratio [Cr+ 3]/ [EDTA]/[Cr+ 3] [Cu+ 2]a
Voltage (v)
Feed flow rate (ml/min)
4 3.5 4 4
2 2 1 0.5
2.5 2 3 2.5
25 35 35 50
100/300 300/300 100/300 300/300
The unit for numerator and denominator is ppm.
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