This document examines the use of complexing agents like EDTA, NTA, IDA, and citric acid to selectively separate Co2+ and Ni2+ ions via cation exchange resins. The study found that: 1) Adding complexing agents can enhance the selectivity of Co2+ over Ni2+ by complexing the Ni2+ ions and preventing their exchange onto the resin. 2) The selectivity is strongly dependent on solution pH and the type and amount of complexing agent used. Higher pH and stronger complexing agents like EDTA and NTA provide better selectivity. 3) Optimal separation of Co2+ from Ni2+ can be achieved at pH 2-3 with ED

1. Water Research 37 (2003) 845–852
Use of complexing agents for effective ion-exchange separation
of Co(II)/Ni(II) from aqueous solutions
Ruey-Shin Juang*, Yi-Chieh Wang
Department of Chemical Engineering, Yuan Ze University, Chung-Li 320, Taiwan
Received 10 May 2002; accepted 29 August 2002
Abstract
Cation-exchange separation of Co2+/Ni2+ from aqueous solutions using water-soluble complexing agents of
ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), iminodiacetic acid (IDA), and citrate was
experimentally studied at 298 K. Experiments were carried out as a function of initial aqueous pH (1.0–6.0),
concentration of total metals (1.5–45.0 mol/m3), the concentration ratio of two metals (0.1–10) and of complexing agent
to the total metals (0–1). It was shown that the exchange selectivity strongly depended on solution pH and was not
completely related to the affinity of any metal with the complexing agents. When a certain level of complexing agent was
present, highly effective separation could be achieved at an appropriate pH range (for an equimolar metal solution, e.g.,
pH 2–3 with EDTA and NTA as well as pH>3 with IDA and citrate). The application potential of this method was
highlighted for the separation of Co2+ from binary mixtures in the presence of trace amount of Ni2+ due to its high
selectivity and the smaller amount of the complexing agents needed.
r 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Co/Ni separation; Cation exchange; EDTA; NTA; IDA; Citrate
1. Introduction crystallization, it is difficult to separate in a simple and
economical way.
Process effluents containing Co2+ and Ni2+ are often In the past years, successful development of Co2+/
encountered in chemical process industries. The major Ni2+ separation by solvent extraction has been received
source for production of Co2+ and Ni2+ appears to be much attention using organophosphorus acids from
from certain raw materials such as oxide and sulfide weakly acidic and neutral solutions (pH>5) [4–6] as well
ores/concentrates, spent lithium secondary batteries, as using amines at a high chloride concentration [7].
and sludge/wastes/scrap/dust of spent catalysts [1–3]. Nevertheless, the separation of Co2+/Ni2+ is still
Hydrometallurgical processes of leaching and dissolu- difficult particularly in more acidic sulfate solutions
tion of such raw materials under pressure or atmo- (e.g., pHo3), which are often obtained in hydrometal-
spheric conditions using HCl or H2SO4 result in leach lurgical operations [4]. In addition, conventional solvent
liquors containing Co2+ and Ni2+ along with some extraction processes are operated in devices such as
impurities. The similar physicochemical properties of packed towers, mixer-settlers, etc., which seek to
Co2+ and Ni2+ make their separation from aqueous maximize the contact area of the two phases for mass
solutions a challenging task. Using conventional meth- transfer [7]. The intimate mixing that occurs in these
ods such as chemical precipitation, oxidation and devices can lead to the formation of stable emulsions,
thereby inhibiting phase separation and product recov-
*Corresponding author. Tel.: +886-3-4638-800; fax: +886- ery. The solvent extraction systems avoid using liquids
3-4559-373. having similar densities, a situation which appears to
E-mail address: cejuang@ce.yzu.edu.tw (R.-S. Juang). promote this problem. Additional limitations present in
0043-1354/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 3 - 1 3 5 4 ( 0 2 ) 0 0 4 2 3 - 2

2. 846 R.-S. Juang, Y.-C. Wang / Water Research 37 (2003) 845–852
Nomenclature V volume of the solution (m3)
W weight of dry resins (kg)
Ce aqueous-phase metal concentration at equili-
brium (mol/m3) Greek letters
C0 initial aqueous-phase metal concentration a molar concentration ratio of the complexing
(mol/m3) agent to total metals
Hx L complexing agent (x ¼ 4 for EDTA, 3 for bCo=Ni exchange selectivity of Co2+/Ni2+ defined in
NTA, IDA and citric acid) Eq. (2)
Kf overall formation constant of metal and
anionic ligand in the unit of mol/dm3 Subscripts
qe amount of metal exchange at equilibrium 0 initial
(mol/kg) tot total
packed towers include loading requirements and flood- complex Ni2+ to allow effective separation of trace
ing restrictions. An alternative way is thus highly amount of Co2+ from Ni2+ in emulsion liquid
needed. membranes containing di(2-ethylhexyl)phosphoric acid
Ion exchange has been also widely used for recovery (D2EHPA) as mobile carriers.
and concentration of metals from aqueous streams in To our knowledge, little attention has been paid to
chemical process and water treatment industries, due to solid ion-exchange resin systems. In this work, the
its simplicity to build-up, well-understood operating complexing agents EDTA, NTA, IDA, and citrate were
principles, and considerable performance data [8]. selected because their 1:1 overall formation constants
However, selective recovery of one or more metals from with Co2+ and Ni2+ are high enough, but a certain
a multicomponent mixture using common organic difference still exists among these constants [18,19].
cation-exchange resins is generally not feasible, espe- Experiments were carried out at different initial aqueous
cially for those metals having the same valence. pH values (1.0–6.0), concentrations of total metals (1.5–
Although the resins with specific chelating groups (for 45.0 mol/m3), the concentration ratios of two metals
example, the iminodiacetic acid (IDA) in Chelex-100 (0.1–10) and of complexing agent to total metals (0–1).
resin) can preferentially exchange some metals (e.g., The suitable conditions of the proposed method were
Cu2+), the high production cost and specific affinity of finally suggested for highly selective separation of Co2+
such resins make them rather limited for practical uses. and Ni2+ from binary solutions.
An attempt was thus made in this work to selectively
separate Co2+/Ni2+ using common strong-acid cation-
exchange resins by adding water-soluble complexing 2. Materials and methods
agents as masking reagents for Ni2+.
In practice, such a complexation-enhanced separation 2.1. Resins and solutions
concept has been applied to similar systems such as
cation-exchange membrane dialysis and electrodialysis Strong-acid cation-exchange resins Purolite NRW100
[9–14], solvent extraction [15] and liquid membranes (with -SO3H group) were used in this work. They were
[16,17]. Previous studies have shown that selective made of poly(styrene) and cross-linked with divinylben-
transport of metals is achieved by adding anionic zene. Physical and chemical properties of the resin are
ligands as masking reagents to the feed phase in listed in Table 1. Prior to use, the resin was washed with
membrane dialysis [10–12]. The anionic ligands com- NaOH (1 mol/dm3), HCl (1 mol/dm3), and n-hexane to
monly used include FÀ, SCNÀ, PO3À, ethylenediamine-
4 remove possible organic and inorganic impurities, and
tetraacetic acid (EDTA), nitrilotriacetic acid (NTA), was finally washed with deionized water (Millipore
citrate, oxalate and glycine. This can differentiate the Milli-Q) three times. The resins were converted to Na+
equilibrium uptake of metals on the membrane so as to form by column flushing with 1 mol/dm3 NaCl for 12 h.
increase the membrane selectivity for metals. However, They were finally washed with deionized water and dried
the enhancement of selectivity is inevitably at the in vacuum oven at 333 K.
expense of a decrease of mass transfer rate due to a Analytical reagent grade of EDTA, NTA, IDA, and
reduction of driving force, the concentration gradient. citric acid were purchased from Merck Co. The aqueous
On the other hand, Azis et al. [15] studied the enhanced phase was prepared by dissolving CoSO4 and NiSO4
separation of rare-earth metals by solvent extraction (Merck) and different amounts of complexing agents in
from aqueous solutions containing diethylenetriamine- deionized water. The initial aqueous pH was adjusted to
pentaacetic acid. Li et al. [16] also employed EDTA to be in the range 1.0–6.0 by adding a small amount of

3. R.-S. Juang, Y.-C. Wang / Water Research 37 (2003) 845–852 847
Table 1
Physical and chemical properties of the resin Purolite NRW100
Properties Purolite NRW100
Matrix Polystyrene DVB, gel type
Functional group Sulfonic acid—SO3À
Ionic form H+ (>99.9%)
Exchange capacity (equiv/ 4.5
kg of dry resin)
Moisture content (%) 50–55
Particle size (mm) 0.425–1.2
Specific gravity 1.20
pH limit None
0.1 mol/dm3 HCl or NaOH. The initial concentration of
total metals varied from 1.5 to 45.0 mol/m3. The
concentration ratios of the two metals (0.1–10) and of
complexing agent to total metals (0–1) in aqueous phase
were prepared.
2.2. Experimental procedures
In exchange experiments, an aliquot of dry resins Fig. 1. Effect of added EDTA on the amount of exchange of
Co2+ and Ni2+.
(0.3 g) and 100 cm3 of aqueous phase were placed in a
125 cm3 glass-stoppered flask and shaken at 120 rpm for
12 h using a shaker (Firstek Model B603, Taiwan). The with a stronger complexing affinity (that is, Ni2+ in this
temperature of the water bath was controlled at 298 K. case as shown in Table 2) significantly decreases, to even
Preliminary runs had shown that the exchange reactions zero, with increasing a or pH values. In this regard, a
studied were complete after 4 h. After equilibrium, the higher solution pH favors the separation of Co2+ from
aqueous-phase concentrations of metals were deter- Ni2+; however, this is not necessarily the case using
mined using an atomic absorption spectrophotometer stronger complexing agents such as EDTA and NTA at
(Varian Model 220FS, USA). The solution pH was high enough a values (>0.75). As also shown in Figs. 1
measured with a pH meter (Horiba Model F22, Japan). and 2 only, the difference of qe values between Co2+ and
The resin-phase concentration of each metal at equili- Ni2+ increases first and then decreases with increasing
brium, qe (mol/kg), was calculated according to the amounts of added EDTA and NTA (a). At low pH
(o2) and low a values (o0.75), the decrease in qe value
ðC0 À Ce ÞV
qe ¼ ; ð1Þ for Co2+ with decreasing the pH is a result of the
W
competition of exchange of H+ with free Co2+.
where C0 and Ce are the initial and equilibrium aqueous- Korngold et al. [20] have studied the removal of
phase concentrations of metal ions, respectively (mol/ Cu2+, Ni2+, Co2+ and Cd2+ from tap water containing
m3), and W =V is the dosage of dry resins (3 kg/m3). small amounts of salts of carboxylic acid by cation-
Each experiment was duplicated under identical condi- exchange resins having IDA group, Purolite S930. From
tions. Reproducibility of the measurements was within the column tests, they also indicated that the presence of
4%. one carboxylic group salts such as acetate has little effect
on amount of exchange, but the presence of salts with
two or four carboxylic groups such as tartrate and
3. Results and discussion EDTA dramatically decreases the exchange efficiency to
even zero. However, the effect of such carboxylic acid
3.1. Exchange equilibria in binary systems salts on separation performance of metals was not
discussed in their study.
Figs. 1–4 show the effects of equilibrium pH (pHeq) As shown in Figs. 3 and 4, the effect of metal
on amounts of exchange (qe ) in binary systems by complexation on the amount of exchange is weaker at
adding different amounts of EDTA, NTA, IDA and low pH (o2) using weaker complexing agents IDA and
citrate, respectively. In these figures, a is defined as the citrate. This behavior can be explained as follows. It is
molar concentration ratio of complexing agent to the known that EDTA, NTA, IDA, and citric acid exist in a
total metals. It is found that the qe value of the metal number of protonated forms in aqueous solutions. They

4. 848 R.-S. Juang, Y.-C. Wang / Water Research 37 (2003) 845–852
Fig. 2. Effect of added NTA on the amount of exchange of Fig. 4. Effect of added citrate on the amount of exchange of
Co2+ and Ni2+. Co2+ and Ni2+.
single Co2+ and Ni2+ systems are unavailable and those
of Cu2+ are thus referred to because Cu2+ have
comparable log Kf values of the 1:1 complexes (20.5
for EDTA, 14.2 for NTA, 11.5 for IDA and 7.2 for
citrate) with Co2+ and Ni2+ [19].
In an equimolar solutions of Cu2+ and EDTA (H4L),
it was found that the anions CuL2À dominates at
pH>3.2 and the anions CuHLÀ at pHo3.2 [21]. The
exchangeable Cu2+ ions are absent in the pH range
tested. In the case of citric acid (H3L), the dominant
species are the free Cu2+ ions at pHo4.0 and are CuLÀ
at pH>6.8. Within pH 4.0 and 6.8, the neutral species
CuHL dominate [21]. On the other hand, the anions
CuLÀ dominate at pH 2.0–11.0 and the free Cu2+ ions
at pHo2.0 in NTA (H3L) system [22]. Results of the
Cu-IDA system are essentially similar to those of the
Cu-NTA system except that each dominating pH range
is somewhat different.
3.2. Exchange selectivity in binary systems
Fig. 3. Effect of added IDA on the amount of exchange of In order to quantitatively know the effect of
Co2+ and Ni2+. equilibrium pH and a values on separation factor, the
exchange selectivity of Co2+ over Ni2+, bCo=Ni ; is
defined as
form stable complexes with most metals in a 1:1 molar
ðqCo;e =qNi;e Þ
ratio [19]. Table 2 lists the related overall formation bCo=Ni ¼ : ð2Þ
constants Kf at zero ionic strength and 298 K. The pH ðCCo;0 =CNi;0 Þ
diagrams of species distribution can be calculated from a Figs. 5 and 6 show the results of binary equimolar
set of mass-balance equations. Literature results of solutions. As expected, the effective separation of Co2+/

5. R.-S. Juang, Y.-C. Wang / Water Research 37 (2003) 845–852 849
Table 2
The overall formation constants (log Kf ) for the complexes of metals and anionic ligands at 298 K and zero ionic strength [19]a
Ion L=OHÀ L=SO2À
4 L=citrate3À L=IDA3À L=NTA3À L=EDTA4À
H+ HL (14.0) HL (1.99) HL (6.40) HL (9.73) HL (10.33) HL (11.12)
H2L (11.16) H2L (12.63) H2L (13.27) H2L (17.80)
H3L (14.29) H3L (14.51) H3L (14.92) H3L (21.04)
H4L (16.02) H4L (23.76)
H5L (24.76)
Co2+ CoL (4.3) CoL (2.4) CoL (6.3) CoL (7.9) CoL (11.7) CoL (18.1)
CoL2 (9.2) CoHL (10.3) CoL2 (13.2) CoL2 (15.0) CoHL (21.5)
CoL3 (10.5) CoH2L (12.9) CoOHL (14.5)
Ni2+ NiL (4.1) NiL (2.3) NiL (6.7) NiL (9.1) NiL (12.8) NiL (20.4)
NiL2 (9.0) NiHL (10.5) NiL2 (15.7) NiL2 (17.0) NiHL (24.0)
NiL3 (12.0) NiH2L (12.9) NiOHL (15.5) NiOHL (21.8)
a
Kf is in the unit of mol/dm3.
Ni2+ by organic cation-exchange resins is impossible at
a ¼ 0 under the pH ranges studied.
Two types of figures are observed. In the presence of
strong complexing agent such as EDTA and NTA
(Fig. 5), the selectivity is acceptable (>5, for example)
only if a > 0:5: A maximum selectivity is obtained at
a ¼ 1 to be 70.6 at pH 2 with EDTA and to be 10.9 at
pH 3 with NTA. For the weaker complexing agents such
as IDA and citrate (Fig. 6), the exchange separation is
impossible at pHo3 regardless of the amounts of the
complexing agents added. At pH>3, the selectivity
increases with increasing solution pH and a values under
the conditions studied; and a highest selectivity of 15.1
and 4.4 is obtained at pH 5.7 and a ¼ 1 using IDA and
citrate, respectively.
It should be noted that the exchange selectivity at a
given pH (e.g., pH 4) does not fully follow the affinity of
the complexing agents, i.e., EDTA>NTA>IDA>ci-
trate. This can be understood because not only the
absolute values of Kf ’s but also their differences play an
important role in exchange selectivity. If we take the Kf
values based on the formation of 1:1 complexes ML,
except for MHL in citrate systems, the difference in Kf Fig. 5. Effect of added EDTA and NTA on exchange
values of Co2+ and Ni2+ with EDTA, NTA, IDA and selectivity of Co2+/Ni2+.
citrate are 2.3, 1.1, 1.2 and 0.2, respectively.
As indicated above, separation of Co2+/Ni2+ by
solvent extraction has been of practical interest [1–6]. A
selectivity of 36.4 was reported when the loaded organic the selectivity to be 2.2, 21 and 380, respectively.
phase (20 vol% D2EHPA+20% LIX860+kerosene) Recently, the sodium salts of organophosphorus acids
containing 1.26 g/dm3 Ni(II) and 0.25 g/dm3 Co(II) such as D2EHPA, PC88A and Cyanex 272 have been
was stripped with 0.3 mol/dm3 H2SO4 [4]. A selectivity proposed as highly effective extractants for separation of
of 30–70 was obtained at an organic loading ratio of 0.6 Co2+/Ni2+ [1,2]. However, the stripping of the loaded
using 25.7 vol% 2-ethylhexylphosphonic acid mono-2- organic phase and the regeneration of extractants for
ethylhexyl ester (PC88A) in 90% xylene and 10% 2- continuous operations appear to be somewhat difficult.
ethylhexyl alcohol [5]. Danesi et al. [6] applied toluene Compared to the use of anionic ligands in different
solutions of D2EHPA, PC88A, and di(2,4,4-trimethyl- processes, the selectivity obtained in this work is
pentyl)phospninic acid (Cyanex 272) to separate trace acceptable. For example, Li et al. [16] added EDTA in
amounts of Co2+/Ni2+ (10À3–1 mol/m3). They found the external phase to complex Ni2+ to separate trace

6. 850 R.-S. Juang, Y.-C. Wang / Water Research 37 (2003) 845–852
Fig. 7. Effect of total metal concentration on exchange
selectivity of Co2+/Ni2+ in the presence of EDTA.
concentration (not shown). The amount of Co2+
exchange is larger than that of Ni2+ using EDTA, even
Fig. 6. Effect of added IDA and citrate on exchange selectivity that Ni2+ is initially in large excess of Co2+ in the
of Co2+/Ni2+.
aqueous phase. This is not the case with NTA because of
its weaker complexing ability than EDTA.
amount of Co2+ from Ni2+ in an emulsion liquid Figs. 8 and 9 show the corresponding exchange
membrane containing D2EHPA as carriers. The selec- selectivity of Co2+/Ni2+. It is found that the selectivity
tivity of Co2+ to Ni2+ was found to be very high (up to increases with increasing the concentration ratio of
200). This may be due to the synergistic effect of EDTA- Co2+ and Ni2+. When Co2+ is initially in excess of
masking for Ni2+ and the preferential D2EHPA- Ni2+, the selectivity increases first with increasing
complexation for Co2+. Similar to solvent extraction, equilibrium pH and reaches a plateau at pH beyond
the repeated emulsion formation/breaking and phase 3–5. Under the conditions examined, the exchange
separation makes emulsion liquid membranes more selectivity is about 70 with EDTA and 20 with NTA
complicated than ion exchange. at pH>4 and CCo,0/CNi,0=10/1. These results indicate
The effect of total metal concentration Ctot on the present method to be especially promising for
exchange selectivity is shown in Fig. 7 in the presence of selective separation of Co2+ from a binary mixture
EDTA (a ¼ 0:5). A high initial solution pH favors containing trace amount of Ni2+, because of its high-
exchange separation and the selectivity increases first exchange selectivity and the less consumption of
but then decreases with increasing Ctot : Because the complexing agents. The used complexing agents could
solution contains equimolar Co2+ and Ni2+, most of the be likely regenerated by an electrolytic membrane
EDTA will complex with Ni2+. Increasing Ctot means an process [23,24]. Metal complexes remaining in the
increase in amount of free Co2+ in solution, thus aqueous solutions are dissociated in an electrolytic cell
increasing amount of exchange. When the amount of equipped with a cation-exchange membrane, which
free metals in solution exceeds the saturated exchange allows the metals to be deposited on the cathode and
capacity of the resins (pH-dependent); however, Co2+ prevents the complexing agents such as EDTA from
exchange is limited by equilibrium relationship and Ni2+ oxidation at the anode.
will compete with Co2+ for cation exchange. This leads
to a reduction of selectivity. Such trends are essentially
affected by the dosage of the resins and the a value.
4. Conclusions
3.3. Effect of concentration ratio of two metals on The effects of adding EDTA, NTA, IDA, and citrate
exchange selectivity on separation of Co2+/Ni2+ by cation exchange using
strong-acid Purolite NRW100 resins were studied at
The effect of concentration ratio of two metals on 298 K. In the absence of complexing agents, effective
amount of exchange is also studied at a fixed total metal separation was impossible under the ranges examined.

7. R.-S. Juang, Y.-C. Wang / Water Research 37 (2003) 845–852 851
3–5 when Co2+ was initially in excess of Ni2+ in the
aqueous phase. Under the conditions studied, the
selectivity was 70 with EDTA and 20 with NTA at
pH>4 and CCo,0/CNi,0=10/1. The present method was
proven to be promising for selective separation of Co2+
from binary mixtures containing trace amount of Ni2+,
because of its high selectivity and the smaller amount of
complexing agents needed.
Acknowledgements
Support for this work by the ROC National Science
Council under Grant NSC90-2214-E-155-001 is grate-
Fig. 8. Effect of concentration ratio of Co2+ and Ni2+ on fully appreciated.
exchange selectivity of Co2+/Ni2+ in the presence of EDTA.
References
[1] Devi NB, Nathsarma KC, Chakravortty V. Separation
and recovery of cobalt(II) and nickel(II) from sulfate
solutions using sodium salts of D2EHPA, PC88A and
Cyanex 272. Hydrometallurgy 1998;49:47–61.
[2] Sarangi K, Reddy BR, Das R. Separation of Co(II)/Ni(II)
by the sodium salts of D2EHPA, PC88A, and Cyanex 272
and their mixtures. Hydrometallurgy 1999;52:253–65.
[3] Zhang PW, Yokoyama T, Itabashi O, Suzuki TM, Inoue
K. Hydrometallurgical process for recovery of metal values
from spent lithium-ion secondary batteries. Hydrometal-
lurgy 1998;47:259–71.
[4] Zhang PW, Yokoyama T, Suzuki TM, Inoue K. The
synergistic extraction of nickel and cobalt with a mixture
Fig. 9. Effect of concentration ratio of Co2+ and Ni2+ on of di(2-ethylhexyl)phosphoric acid and 5-dodecylsalicyl-
exchange selectivity of Co2+/Ni2+ in the presence of NTA. aldoxime. Hydrometallurgy 2001;61:223–7.
[5] Komasawa I, Otake T. Practical study of a solvent
extraction process for separation of cobalt and nickel with
2-ethylhexylphosphonic acid mono-2-ethylhexyl ester. J
The amount of exchange of Ni2+, the metal with a Chem Eng Jpn 1984;17:417–23.
stronger complexing affinity, considerably decreased at [6] Danesi PR, Reichley-Yinger L, Mason G, Kaplan L,
high pH values by the addition of complexing agents. Horwitz EP, Diamond H. Selectivity-structure trends in
the extraction of Co(II) and Ni(II) by dialkylphosphoric,
For treatment of binary equimolar mixtures, the
alkyl alkylphosphonic, and dialkylphosphinic acids. Sol-
exchange selectivity was acceptable (>5) only if a > 0:5
vent Exch Ion Exch 1985;3:435–52.
when strong complexing agents such as EDTA and [7] Ritcey GM, Ashbrook AW. In: Solvent extraction.
NTA were present. A maximum selectivity was obtained principles and applications to process metallurgy, Vol. II.
to be 70.6 at a ¼ 1 with EDTA at pH 2 and to be 10.9 Amsterdam, The Netherlands: Elsevier, 1984. p. 279–361.
with NTA at pH 3. In the presence of weaker agents [8] Bolto BA, Pawlowski L. Wastewater treatment by ion
such as IDA and citrate, no separation was found at exchange. New York: E & FN Spon Ltd., 1987.
pHo3 regardless of the amount of complexing agent [9] Cherif AT, Elmidaoui A, Gavach C. Separation of Ag+,
added, and the selectivity increased with increasing pH Zn2+ and Cu2+ ions by electrodialysis with a monovalent
and a values at pH>3. A selectivity of 15.1 and 4.4 was cation specific membrane and EDTA. J Membr Sci
1993;76:39–49.
obtained at pH 5.7 and a ¼ 1 using IDA and citrate,
[10] Huang TC, Lin YK, Chen CY. Selective separation of
respectively.
nickel and copper from a complexing solution by a cation
The exchange selectivity of Co2+/Ni2+ increased with exchange membrane. J Membr Sci 1987;37:131–44.
increasing the concentration ratio of Co2+ and Ni2+. In [11] Huang TC, Wang JK. Selective transport of metals
addition, the selectivity increased first with increasing through cation exchange membrane in the presence of
the equilibrium pH and reached a plateau at pH beyond complexing agents. Ind Eng Chem Res 1993;32:133–9.

8. 852 R.-S. Juang, Y.-C. Wang / Water Research 37 (2003) 845–852
[12] Matsuyama H, Fujii K, Teramoto M. Selective separation [18] Conway M, Holoman S, Jones L, Leenhouts R, William-
of rare earth metals by Donnan dialysis in the presence of son G. Selecting and using chelating agents. Chem Eng
water-soluble complexing agent. J Chem Eng Jpn 1999;106(3):86–90.
1991;24:253–5. [19] Morel FMM, Hering JG. In: Principles and applica-
[13] Labbe M, Fenyo JC, Selegny E. Separation of Ni2+ and tions of aquatic chemistry. New York: Wiley, 1993.
Co2+ by electrodialysis by ion exchange membranes in the p. 332–43.
presence of EDTA. Sep Sci 1975;10:307–22. [20] Korngold E, Belayev N, Aronov L, Titelman S. Influence
[14] Kubal M, Machula T, Strnadova N. Separation of calcium of complexing agents on removal of metals from water by
and cadmium by electrodialysis in the presence of ethyle- a cation exchanger. Desalination 2001;133:83–8.
nediaminetetraacetic acid. Sep Sci Technol 1998;33:1969–80. [21] Juang RS, Ju HS. Effect of added complexing agents on
[15] Azis A, Matsuyama H, Teramoto M. Equilibrium and extraction of Cu(II) from sulfate solutions by di(2-
non-equilibrium extraction separation of rare earth metals ethylhexyl)phosphoric acid. Sep Sci Technol 2001;36:
in the presence of diethylenetriaminepentaacetic acid in 2499–514.
aqueous phase. J Chem Eng Jpn 1995;28:601–8. [22] Juang RS, Huang IP. Extraction of copper(II)-NTA
[16] Li LQ, Wang C, Li YD. Separation of cobalt and nickel by chelated anions from water with Aliquat 336. Sep Sci
emulsion liquid membranes using EDTA as masking Technol 1999;34:2407–20.
reagent. J Membr Sci 1997;135:173–7. [23] Allen HE, Chen PH. Remediation of metal contaminated
[17] Matsuyama H, Komori K, Teramoto M. Selectivity soil by EDTA incorporating electrochemical recovery of
enhancement in the permeation of rare earth metals metal and EDTA. Environ Prog 1993;12:284–93.
through supported liquid membranes by the addition of [24] Juang RS, Wang SW. Electrolytic recovery of binary
diethylene-triaminepentaacetic acid to aqueous phase. J metals and EDTA from strong complexed solutions.
Membr Sci 1989;47:217–28. Water Res 2000;34:3179–85.