Journal of Membrane Science 160 (1999) 225±233
Simultaneous recovery of EDTA and lead(II) from their chelated solutions using a cation exchange membrane Ruey-Shin Juang*, Shiow-Wen Wang, Li-Chun Lin Department of Chemical Engineering, Yuan Ze University, Chung-Li, Taoyuan 32026, Taiwan Received 7 December 1998; received in revised form 1 March 1999; accepted 3 March 1999
Abstract Factors affecting the ef®ciency of electrochemical recovery of EDTA and Pb(II) from their chelated solutions were systematically examined using a cation exchange membrane Neosepta CM-1. The catholyte contained an equimolar amount of Pb(II) and EDTA, and the anolyte contained 0.1 M NaNO3. The iridium oxide coated on titanium (IrO2/Ti) and stainless steel were used as anode and cathode, respectively. Experiments were carried out at different current densities (46.3±185 A/m2), initial catholyte pH values (1.47±6.02) and Pb(II) concentrations (0.005±0.03 M). An economically feasible current ef®ciency and recovery percentages of Pb(II) and EDTA could be achieved if the concentration of the chelated Pb(II) was suf®ciently high. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical process; Cation exchange membrane; Simultaneous recovery; Lead(II); EDTA
1. Introduction The streams containing strong chelating agents such as EDTA (ethylenediaminetetraacetic acid), NTA (nitrilotriacetic acid), citrate, and tartarate were often encountered in remediation washing of heavy metalcontaminated soils [1±4]. These constituents were also present in the process or waste ef¯uents of manufacturing printed circuit boards for electroless copper plating and of some metal ®nishing industries [5,6]. Although chemical precipitation methods (e.g., OHÿ, S2ÿ, etc.) are the most economic for the treatment of metal-bearing ef¯uents, the presence of strong chelat-
*Corresponding author. Tel.: +886-3-46-38-800; fax: +886-3-4559-373; e-mail:
[email protected]
ing agents may make the precipitation process ineffective, even with high metal levels [7]. In addition, the high buffer capacity provided by the chelating agents requires excessive amount of chemicals to neutralize alkalinity. These problems and restrictions led to search for effective recovery and/or removal processes, which depend greatly on the particular chelating agent and metals, and their concentrations. In the case of diluted solutions, resin ion exchange [5,8], adsorption on activated carbon [9,10], on inorganic materials [11,12], and on functional polymers [13] have been tried with varying success. Regardless of which concentrating method being used, the chelated metal cannot be destroyed and the ®nal process to recover metals should be aided via electrolysis such as electrowinning and/or electrore®ning [14±16]. During metal deposition onto a Pt
0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 9 9 ) 0 0 0 8 6 - 1
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anode in acidic sulfate solutions, Johnson et al. [17] reported that EDTA is anodically oxidized into many compounds including CO2, formaldehyde, and ethylenediamine. They obtained a series of reaction products indicating sequential removal of acetate group from the EDTA. Therefore, simultaneous recovery of metals and the chelating agents from their mixtures is inaccessible using conventional electrolysis cell. A two chamber electrolysis cell that was separated by a cation exchange membrane to prevent oxidation of EDTA at the anode during electrolysis was used to recover EDTA from its Cu(II) complexes [18]. The anode chamber was ®lled with Na2CO3 solution and the cathode chamber contained EDTA±chelated Cu(II) solutions. A graphite sheet was used as anode and a copper plate as cathode. Copper was deposited onto the cathode, so that the chelate was destroyed and EDTA was regenerated. The results gave an average yield of unchelated EDTA to be 94%. Etzel and Tseng [19] used a two chamber electro-membrane reactor to recover Cu(II) and Zn(II) from their EDTA complexes and used a dimensionally stable material designed for evolving O2 gas as the anode and a copper plate as the cathode. The deposition of Cu(II) and Zn(II) from their chelated solutions was achieved, but deposition of Ni(II) was impossible in the presence of EDTA, NTA, and citrate. Also, Allen and Chen [20] used a cell in which the anode and cathode chambers are separated by a cation exchange membrane to prevent electrochemical oxidation of EDTA during electrolysis. They indicated that over 95% of both Pb(II) and EDTA were recovered and that the Pb(II) deposited onto a copper anode was a mixture of hydrolysis products and salts. Although previous studies has shown that electrochemical membrane processes can be used to simultaneously recover metals such as Pb(II) and chelating agents such as EDTA from chelated solutions, several key points were not fully clari®ed. For example, at what feed concentration of the chelated Pb(II) an economically acceptable current ef®ciency can be obtained? How the electrolysis was affected by the catholyte pH? The aim of this work was to measure the current ef®ciency and recovery percentages of Pb(II) and EDTA. All experiments were carried out at an equimolar solution of Pb(II) and EDTA, and as a function of current density, initial catholyte pH and chelated Pb(II) concentration.
2. Experimental 2.1. Apparatus, membrane, and solutions The apparatus for electrolysis experiments was shown in Fig. 1. The unit was made of Pyrex glass. Two chambers were attached by inserting six screws on their ¯anges with a cation exchange membrane between. The membrane was cut as circle with an diameter of 34 mm and glued into the inner mounting wall of the two chambers with silicone rubber sealant to prevent leakage between the chambers. The electrodes were totally immersed in the solutions and were symmetrically separated about 65 mm from the membrane surface. An iridium oxide coated on titanium (IrO2/Ti) with dimensions of 50501.5 mm was used as the anode. The cathode was the stainless steel with the same dimensions as the anode. Power supply employed in electrolysis experiments was capable of supplying a potential up to 200 V and a DC up to 3 A. The cation exchange membrane used was the Neosepta CM-1 (Tokuyama). It is a homogeneous ®lm comprised of cross-linked sulfonated copolymer of vinyl compounds on synthetic reinforcing fabric. Table 1 lists its physical properties. Prior to use, it was washed with 0.1 M NaOH and 0.1 M HCl to remove inorganic impurities, and then with deionized water (Millipore
Fig. 1. Schematic of the electrolysis cell.
R.-S. Juang et al. / Journal of Membrane Science 160 (1999) 225±233 Table 1 Properties of the cation exchange membrane used in this study Property
Neosepta CM-1
Type Characteristics
Strongly acidic cation permeable Low electric resistance (Naform) Sulfonate 0.13±0.16 0.8±2.0 1.5±3.0
Ion exchange groups Thickness (mm) Electrical resistancea ( cm2) Burst strengthb (kgf/cm2) a b
Measured by equilibrating with a 0.5 M NaCl solution at 298 K. Measured by a Mullen Brust Strength Device.
Milli-Q) three times. The resulting membrane was immersed in 0.1 M NaNO3 before use. Analytical reagent grade EDTA and other inorganic chemicals were purchased from Merck. The catholyte was prepared by dissolving equimolar amount of Pb(NO3)2 and EDTA in deionized water and the pH was near 2.08. When the effect of initial catholyte pH was studied, it was adjusted to be in the range 1.47± 6.02 by adding a small amount of HNO3 or NaOH. The anolyte was 0.1 M NaNO3 and the initial pH was adjusted to 3. The initial Pb(II) concentration ranged from 0.005 to 0.03 M, which is about zero to one order of magnitude larger than the level of real washing streams of soil remediation [2±4]. 2.2. Experimental procedures and chemical analysis An equal volume (250 cm3) of the anolyte and catholyte were poured in the respective chamber after the cell was assembled. The cell voltage was allowed to vary so that the electrolysis was carried out at a constant current. When the metal concentration decreased in the cathode chamber, the voltage required to maintain a given current decreased to a minimum. Because the pH values in catholyte and anolyte changed during the electrolysis due to the production of OHÿ and H, respectively, they were monitored with a Horiba pH meter (Model F-23). It was expected that a more vigorous mixing reduces the thickness of double layer adjacent to the electrodes, enhancing mass transfer of the complexes toward the cathode. To prevent the formation of a serious vortex in the cell, however, a stirring speed of 700 rpm was selected.
227
Samples were taken at preset time intervals from the catholyte and the concentration of Pb(II) was analyzed using an atomic absorption spectrophotometer (GBC Model 932). In some cases, the concentration of Pb± unchelated EDTA in the catholyte was determined by potentiometric titration with Metrohm 702 SM Titrino using Pb-selective electrode (Herisau CH-9100) and Ag/AgCl reference electrode. The temperature was controlled at 298 K by immersing the whole cell in a water bath. Each experiment was duplicated at least under identical conditions. 3. Results and discussion 3.1. Reactions involved in the electrolysis cell EDTA, a polyprotic acid (H4L), can exist in many protonated forms in the aqueous solutions [21]. It readily forms stable complexes with most divalent metals in a 1:1 molar ratio. Basically, the distribution of species at different pH values can be obtained from a set of mass-balance equations considering the formation reactions [22]. For an equimolar diluted solution of Cu(II) and EDTA (0.01 M), it was indicated that the divalent species CuL2ÿ dominates at pH 3±12 and the univalent species CuHLÿ at pH<3 [8]. This is also the case for Pb(II) because they have equivalent overall formation constants (Kf). For example, the values of logKf (in molar unit) are 20.5 and 23.9 for CuL2ÿ and CuHLÿ, respectively, at 298 K and zero ionic strength, and are 19.8 and 23.0 for PbL2ÿ and PbHLÿ, respectively [22]. These ®ndings would be useful in data analysis. The cell voltage was allowed to vary so that the electrolysis experiments were carried out at a constant current. The catholyte pH increases during electrolysis because of the production of OHÿ at the cathode. In addition, H was generated at the anode. Thus, when a current is applied, the chelates are destroyed and Pb(II) is deposited onto the cathode. In addition, the negatively charged EDTA cannot pass through cation exchange membrane to prevent its electrochemical oxidation in the anode chamber during electrolysis. The following reactions may occur at the anode [20]: 2H2 O ! O2 4H 4eÿ
E0 ÿ1:229 V
(1)
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H2 O ! H OHÿ
E0 ÿ0:828 V
ÿ
2OHÿ ! H2 O2 2eÿ
(2)
0
(3)
E0 ÿ0:547 V
(4)
ÿ
(5)
4OH ! O2 2H2 O 4e H2 O2 ! O2 2H 2e
ÿ
E ÿ0:401 V 0
E ÿ0:682 V
whereas at the cathode: PbL2ÿ 2eÿ ! Pb
s L4ÿ O2 2H2 O 4eÿ ! 4OHÿ
ÿ
2H 2e ! H2
(6) E0 0:401 V
0
E 0
2H2 O 2eÿ ! H2 2OHÿ
(7) (8)
E0 ÿ0:828 V
(9)
Evidently, the evolutions of O2 and H2 gases are the main side reactions at the anode and cathode, respectively, which more or less reduce the current ef®ciency. 3.2. Effect of current density on electrolysis of chelated Pb(II) solutions The effect of current density on the recovery percentage of Pb(II), RPb, at different electrolysis times is shown in Fig. 2. Here, RPb is de®ned as
Fig. 2. Effect of electrolysis time on recovery percentage of Pb(II) from EDTA-chelated solutions at different current densities.
RPb
% 100 actual amount of Pb deposited onto the cathode initial amount of Pb in the catholyte (10)
result obtained previously [20]. However, RPb increases ®rst and then reaches a plateau at a current density more than about 140 A/m2. This ``limiting'' current density is considered to be economically optimal from the viewpoint of Pb(II) recovery, which is dominantly affected by the electrolysis time and solution environments of the catholyte such as pH, chelated metal concentration, etc. However, this is compensated with current ef®ciency; at this current density, equals around 34% only (Fig. 3). The lower at a higher current density is a result of the larger extent of side reactions such as the evolution of O2 and H2 gases. When a current is applied to the cell, the anolyte provides Na to carry the current through the membrane into the cathode chamber and to regenerate EDTA to its Na-form. To maintain
It is found that RPb increases when both electrolysis time and current density increase. In this work, the recovery percentages of Pb(II) and EDTA are almost consistent, indicating that the membrane is absolutely cation-selective permeable. The recovery percentage of EDTA is de®ned as the amount of unchelated EDTA remaining in the catholyte divided by the initial amount of EDTA in the catholyte. Fig. 3 re-plots the results at a given electrolysis time (50 min) and also shows the corresponding current ef®ciency (), which is commonly de®ned as
% 100
actual amount of Pb deposited onto the cathode amount of Pb deposited based on the Faraday0 s 2nd law of electrolysis
It is found that monotonously decreases with increasing current density, which agrees with the
(11)
electroneutrality in the anolyte, the loss of Na must be accompanied with a generation of other cations,
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229
Fig. 3. Effect of current density on current efficiency and recovery percentage of Pb(II) from EDTA-chelated solutions.
that is, H via the oxidation of water (Eqs. (1), (2) and (5)). At a higher current density, the more Na is transferred and therefore the more H is required, which reduces to a more extent. In fact, another type of cation exchange membrane Neosepta CM-2 was tested here. The CM-2 membrane has the same thickness and polymeric matrix as the CM-1 but has a twice electrical resistance (2.0± 3.5 cm2) more than CM-1 (Table 1). However, RPb with CM-1 is slightly larger than CM-2 only under comparable conditions (not shown). This indicates that the role of membrane resistance to the overall process is negligible and the catholyte has an extremely low conductivity. Most of the energy is consumed to facilitate side reactions, to migrate the chelated anions towards the anode, or to transfer Na across the membrane. An attempt was also made to compare the results using different cathodes including platinum plate, Pt/ Ti, copper plate, and stainless steel. It is seen that both and RPb drop in the above order at the early stage of electrolysis (10 min), but the differences are rather small. For example, in the electrolysis at 185 A/m2 and pHc,0 2.08, and RPb vary in the ranges 64±61%
and 55±50% only, respectively. On the other hand, there are no detectable differences in and RPb after 50 min electrolysis. That is, the role of cathode materials in the present process is also insigni®cant. 3.3. Effect of initial catholyte pH value Fig. 4 shows the effect of initial catholyte pH on RPb at different electrolysis times. In order to clearly show the role of catholyte pH, the results of Fig. 4 are replotted in Fig. 5 at several given electrolysis times. The trends of and the ®nal catholyte pH at a given electrolysis time of 50 min are also shown in Fig. 6. According to Eqs. (7) and (9), the catholyte pH would increase after electrolysis; however, it becomes nearly constant at pH 12.6 if the initial catholyte pH is more than about 2.08. Also, RPb and have a maximum at pH around 2.08. Satisfactory explanation concerning this critical pH cannot be given at this stage. One possible approach is that the pH trends of species distribution. As stated above, in equimolar diluted solutions of Pb(II) and EDTA, the species PbL2ÿ dominates at a wide range of pH. When electrolysis starts, Pb(II) is successively deposited
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onto the cathode and thus the dominant species varies due to the change of solution environments. For Pb(II)-free solutions, EDTA would exist as M5L (pH<2), M4L (pH 2), M3Lÿ (pH 2±3), M2L2ÿ (pH 3±6), ML3ÿ (pH 6±10), and L4ÿ (pH>10), where M denotes Na or H [21]. The slightly low RPb and at pH<2, compared to those at pH>2.08, may be explained by competitive deposition between M5L and Pb2 onto the cathode (Fig. 5). 3.4. Effect of initial chelated Pb(II) concentration in the catholyte
Fig. 4. Effect of electrolysis time on recovery percentage of Pb(II) from EDTA-chelated solutions at different initial catholyte pH values.
Fig. 5. Effect of the initial catholyte pH on recovery percentage of Pb(II) from EDTA-chelated solutions at different electrolysis times.
Figs. 7 and 8 show the typical effect of the EDTA± chelated Pb(II) concentration on the electrolysis at a given electrolysis time of 50 min. Increasing the chelate concentration increases the driving force for mass transfer and hence facilitate deposition of Pb(II) onto the cathode. This reduces the occurrence of side reactions and thus raises . It is noticed that the number of moles of Pb(II) deposited actually increases by increasing the chelated Pb(II) concentration although RPb drops (Fig. 7). Owing to the limited surface area of the cathode, no more increases by a continuous increase in chelated Pb(II) concentration. For example, reaches a plateau at a chelate concentration of about 0.02 M at 185 A/m2 (Fig. 8). Similar trends but showing a plateau at a lower chelate concentration are observed at a lower current density (not shown). In general, above such critical concentration white EDTA tends to precipitate, and deposits onto membrane surface or stays in the catholyte. In a summary, at a given current density the feed chelate concentration must be high enough to obtain an economically feasible current ef®ciency and metal recovery. At 185 A/m2, as shown in Fig. 8, it is 0.02 M when >35% and RPb>90%, which is easily achieved for 50 min electrolysis only. In the discussion of feasibility of electrolytic processes, it is accepted that the energy ef®ciency, de®ned as the energy consumed to produce an amount of product (kW h/kg), should be taken into account. In particular, it is of great importance for electrolysis in diluted electrolyte solutions and the operation using cells having membrane separator or diaphragm. Owing to the constant-current operation in this work, the energy ef®ciency is not easily presented because the cell voltage signi®cantly changes during electro-
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231
Fig. 6. Effect of the initial catholyte pH on current efficiency and final catholyte pH for electrolysis of Pb±EDTA solutions.
Fig. 7. Effect of the initial chelated Pb(II) concentration on recovery percentage of Pb(II) at 185 A/m2.
Fig. 8. Effect of the initial chelated Pb(II) concentration on current efficiency and final catholyte pH at 185 A/m2.
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lysis, especially at the initial stage. For example, at a current density of 185 A/m2, a catholyte pH of 2.08, and a chelate concentration of 0.02 M, the cell voltage changes from about 115 to 60 V when the electrolysis starts to 50 min. In a speci®c condition of 50 min operation, the energy ef®ciency is about 140 kW h/kg, which is signi®cantly higher than those obtained in commercial electrolytic processes (<60 kW h/kg) [15]. Because the anolyte provides Na to carry the current through the membrane into the cathode chamber, it is expected that increasing anolyte NaNO3 concentration leads to an increase in and RPb. This is not the case here. It has only little effect on and RPb by changing anolyte NaNO3 concentration from 0.001 to 0.2 M, but the ®nal catholyte pH does increase (not shown). However, the anolyte NaNO3 concentration must maintain a certain level (about 0.1 M) to raise the catholyte pH beyond 3.5 to avoid precipitation of Hform EDTA in the catholyte and onto the membrane surface. This is because the more Na is transferred at high NaNO3 concentration to regenerate EDTA to its Na-form. In a similar way, Etzel and Tseng [19] studied the effect of anolyte composition on the electrolysis. They found that EDTA is partially deposited, primarily on the membrane surface, in the cathode chamber when HCl or H2SO4 is used, reducing the conductivity of the membrane. This problem could be conquered if Na2CO3 was used instead of acid in the anode chamber. According to the present results, the concentration of Na2CO3 must still be high enough. 4. Conclusions Simultaneous recovery of Pb(II) and EDTA from their chelated solutions was studied using an electrochemical membrane process. The following results are obtained. 1. Increasing current density leads to a monotonous decrease in , whereas RPb increases ®rst and then reaches a plateau. The low at higher current density is a result of more serious side reactions (the evolution of O2 and H2 gases). 2. For the chelated Pb(II) solutions examined, RPb and reach a maximum at pH 2.08. The catholyte pH becomes nearly constant after electrolysis if its
initial pH is not less than 2.08. This may be related to different dominated species at each pH range. 3. A certain level of anolyte NaNO3 concentration is needed to avoid precipitation of the H-form of EDTA in the catholyte or onto membrane surface, say 0.1 M, although the effect of NaNO3 concentration (0.001±0.2 M) on and RPb is little. 4. The compensation effect of and RPb exists while changing both current density and the chelate concentration. At a given current density, an economically feasible and RPb can be obtained if the chelate concentration reaches a certain level. At 185 A/m2, for example, >35% and RPb>90% when the chelate concentration is about 0.02 M. 5. Nomenclature E0 H 4L i Kf R T t []
standard reduction or oxidation potential (V) EDTA current density (A/m2) overall formation constant (in molar unit) of the aqueous complexes recovery percentage of Pb(II) or EDTA (%) absolute temperature (K) electrolysis time (min) molar concentration of species in the bracket (M)
Greek Letter
current efficiency (%)
Subscripts a c f 0
anolyte catholyte final initial
Acknowledgements Support for this work by the ROC National Science Council under Grant no. NSC88-2214-E-155-004 is gratefully acknowledged.
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