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wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 3 6 4e3 3 7 6
Available online at w
journal homepage: www.elsevier .com/locate /watres
An evaluation of a hybrid ion exchange electrodialysisprocess in the recovery of heavy metals from simulateddilute industrial wastewater
Akrama Mahmoud a,*, Andrew F.A. Hoadley b
a Laboratoire de Thermique Energetique et Procedes (EAD 1932), ENSGTI, rue Jules Ferry, BP 7511, 64075 Pau, FrancebDepartment of Chemical Engineering, Building 35, Clayton Campus, Monash University, Victoria 3800, Australia
a r t i c l e i n f o
Article history:
Received 10 August 2011
Received in revised form
25 December 2011
Accepted 20 March 2012
Available online 28 March 2012
Keywords:
Copper ions
Ion exchange
Electrodialysis
Electrodeionization
Electromigration
Wastewater
Treatment of dilute solutions
* Corresponding author. Tel.: þ33 0540175193E-mail addresses: akrama_mahmoud@ho
0043-1354/$ e see front matter ª 2012 Elsevdoi:10.1016/j.watres.2012.03.039
a b s t r a c t
Hybrid ion exchange electrodialysis, also called electrodeionization (IXED), is a technology in
whichaconventional ionexchange (IX) is combinedwithelectrodialysis (ED) to intensifymass
transfer and to increase the limiting current density and therefore to carry out the treatment
process more effectively. It allows the purification of metal-containing waters, as well as the
productionof concentratedmetal salt solutions,whichcouldbe recycled.Theobjectiveof this
paperwas to investigate the ability of the IXED technique for the treatment of acidified copper
sulphate solutions simulating rinsing water of copper plating lines. A single-stage IXED
process at lab-scale with a small bed of ion exchanger resin with a uniform composition was
evaluated, and the treatment performance of the process was thoroughly investigated. The
IXED stack was assembled as a bed layered with the ion exchanger resin (strong acid cation-
exchange Dowex�) and inert materials. The stack configuration was designed to prevent
a non-uniform distribution of the current in the bed and to allow faster establishment of
steady-state in the cell for IXED operation. The influence of operating conditions (e.g. ion
exchanger resin with a cross-linking degree from 2 to 8% DVB, and current density) on IXED
performancewas examined. A response surfacemethodology (RSM)was used to evaluate the
effects of the processing parameters of IXED on (i) the abatement yield of the metal cation,
which is a fundamental purification parameter and an excellent indicator of the extent of
IXED, (ii) the current yield or the efficiency of copper transport induced by the electrical field
and (iii) the energy consumption. The experimental results showed that the performance at
steady-state of the IXED operationwith a layered bed remainedmodest, because of the small
dimension of the bed and notably the current efficiency varied from 25 to 47% depending on
the conditions applied. The feasibility of using the IXED in operations for removal of heavy
metals frommoderately dilute rinsing waters was successfully demonstrated.
ª 2012 Elsevier Ltd. All rights reserved.
1. Introduction mainly due to more stringent legislations for the protection of
The recovery of heavy metals from industrial aqueous solu-
tions has received great attention in recent years. This is
; fax: þ33 0559407801.tmail.com, amahmoud@ier Ltd. All rights reserve
the environment. Most heavy metals are very toxic and cause
great environmental damage (JUttner et al., 2000; Janssen and
Koene, 2002).
gmx.fr (A. Mahmoud).d.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 3 6 4e3 3 7 6 3365
The conventional techniques for metal ions abatement,
such as hydroxide precipitation, or direct electro-reduction do
not provide sufficient removal efficiency and as a conse-
quence secondary treatment processes are required down-
stream. An important technique for secondary treatment
below ppm levels is ion exchange: the metal ions contained in
the waste solution are exchanged by the less toxic ions con-
tained in the fixed matrix of an ion exchange bed. Once the
bed has reached capacity, it must be regenerated with
a concentrated electrolyte such as a strong acid. This produces
a concentrated waste stream which has to be treated and
ultimately must be disposed of somewhere.
One alternative technique is electrodialysis, which is an
electrically driven process involving the use of ion-selective
membranes. This technique not only concentrates metals
from the rinse streams, but also helps to maintain the quality
of a plating bath. However, electrodialysis cannot be carried
out at ppm concentrations. Therefore, current research tends
to propose potential alternatives to enhance the treatment
ability of conventional processes. Different options have been
investigated to enhance the treatment of metal-containing
rinsing waters, such as electrodeionization, electro-
coagulation (Adhoum et al., 2004; Dermentzis et al., 2011a),
complexation/ultrafilration-electrolysis (Baticle et al., 2000),
and reverse osmosis/nanofiltration-flotation (Sudilovskiy
et al., 2008). Proper choice of treatment technologies for
a given system is essential, but a good choice can lead to
higher abatement yield, and lower energy and environmental
costs.
Hybrid systems combining ion exchange (IX) with electro-
dialysis (ED) have been suggested to combine the advantages
of the two individual techniques and in particular, to remove
heavy metals from dilute solutions in a continuous process.
The IXED process is not a new idea, but the practical
applications have been limited to treatment of radioactive
waste (Glueckauf, 1959), and production of ultrapure water,
with the removal of both anions and cations (Ganzi et al., 1987,
1997; Thate et al., 1999), or more simply for water purification
(Dejean et al., 1997; Souilah et al., 2000). The well-known
Millipore� single IXED cell for production of ultrapure water
is shown schematically in Fig. 1 as an example.
Since the initial commercialization in 1987, IXED systems
have found worldwide application in industries with themost
demanding high purity water requirements such as in the
manufacture of pharmaceuticals, semiconductors, and high
quality optics, in surface finishing of electronic components,
in the generation of electric power, and in food processing
(Ganzi and Parise, 1990; Soria et al., 1993; Gifford and Atnoor,
2000; Wood and Gifford, 2002).
More recently, the combined technique was considered for
treatment of metal-containing rinsing waters, where the IXED
process yields concentrated solutions of metal ions in the
cathode compartment and metal-free acidified water leaving
the cationic resin bed: the example of Ni salts was extensively
treated by Janssen’s group (Spoor et al., 2001, 2002) and by
Dzyazko and Belyakov (2004), Dzyazko (2006), Lu et al. (2007,
2010, 2011) and Priya et al. (2009). Other metals were also
investigated, including, zinc (Grebenyuk et al., 1998), cobalt
(Yeon et al., 2003; Song et al., 2004), copper (Mahmoud et al.,
2003; Guana and Wang, 2007; Feng et al., 2008; Arar et al.,
2011), chromium (Dzyazko et al., 2008a,b; Alvarado et al.,
2009; Xing et al., 2009a,b), cadmium (Dermentzis et al.,
2011b), arsenic (Basha et al., 2008) or mixtures of metals ions
(Souilah et al., 2000; Feng et al., 2007; Smara et al., 2007).
Interestingly, the possible competing adsorption of divalent
ions in the process water, e.g. Ca2þ and Mg2þ, was studied
(Spoor et al., 2001; Vasilyuk et al., 2004; Fu et al., 2009) and its
significance was shown to be reduced by using inorganic Zr-
containing sorbents (Vasilyuk et al., 2004). Most of these
studies are empirical and carried out in either lab-scale
devices or at pilot-scale. The equipment used for most IXED
studies comprised a number of test cells of plate and frame
design, spiral wound design, cylindrical and rectangular
shape and a variety of sizes. A number of these IXED cells,
with more chambers in the cell and with different ion-
exchange resin arrangements (mixed bed, clustered or
layered cation and anion-exchange resins, separate bed), have
been engineered by researchers in the field, and have reached
varying degrees of development (Kunz, 1987; Guiffrida et al.,
1990; Ganzi et al., 1997; Tessier et al., 1997; Gifford and
Atnoor, 2000; Grabowski et al., 2006).
The present paper investigates the effects of the processing
parameters (current density, resin nature) of the ion exchange
electrodialysis (IXED) process for Cu2þ removal from acidified
copper sulphate solutions, simulating the rinse water from
copper plating lines. The abatement yield of the copper cation
is a fundamental treatment parameter and an excellent indi-
cator of the extent of IXED. The current yield or the efficiency
of copper transport induced by the electrical field is also
analysed and is an indicator of the operating cost of the
process. A short bed of ion-exchange resin is used to prevent
a non-uniform distribution of the current in the bed and to
allow faster establishment of steady-state in the cell for IXED
operation. Finally, in order to design efficient processes, the
response surface methodology (RSM) is used to test for
statistical significance and determine the optimum treatment
performance.
2. Theoretical background
In this study, a hybrid system is obtained by inserting a bed of
ion-exchange resin initially in Hþ form into the central
compartment of an electrodialysis cell. This compartment is
located between two ion-selective membranes that essen-
tially divide the cell into three separate compartments. The
copper solution to be treated is injected continuously through
a packed-bed of ion exchange resins. During IXED, metal
cations are sorbed by the resins, and are transported to the
cathode compartment under the action of the applied elec-
trical field. Water electrolysis occurs in the two external
electrode compartments, and Hþ formed at the anode is
transferred through the membrane into the resin bed and
replace the metal ions which will then migrate through the
second membrane to the electrode compartment. This
phenomenon is known as electro-regeneration. In order to
explain more closely the mechanism of IXED, the role of the
ion-exchange resin bed in the presence of an electrical field is
first discussed. Following this, a theory for the ion exchange
equilibrium step and IXED efficiency is presented.
Fig. 1 e Schematic of the Millipore (Ionpure) single IXED configuration showing the mixed resins in the dilute compartment
[adapted from Ganzi et al. (1987); TSS Water Course; Mahmoud, 2004].
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 3 6 4e3 3 7 63366
2.1. Ion-exchange resin bed role
In a hybrid system such as IXED, the ion-exchange resin bed
plays a major role in the reduction of the high electrical
resistance in the dilute compartment, while the ion exchange
membranes allow depletion and concentration of solutions in
their dedicated compartments. Ion-exchange resins are
polymers that have fixed ionic sites prone to reactionwith free
ions of the opposite charge. The ionic groups of the resin
provide a location at which the dissolved ions can be
exchanged. Ion-exchange resins in electrolyte solutions are
electrically conductive, and counter ions can transfer across
the polymer under an electric field, allowing mass transfer
and associated current flow. The electrical conductivity of ion-
exchange resins varies with the mobility and affinity of the
counter ions with which the resins are in contact (Helfferich,
1962). In the case of a very low specific conductivity of the
interstitial solution, the specific conductivity of the bed is
enhanced by the presence of ion exchange resins. Therefore
the main advantage of using ion exchange resins in the dilute
compartment is the substantial reduction in electrical resis-
tance which is achieved when very low concentration solu-
tions are concerned. This result in an increase in the limiting
current density and, consequently, the treatment process
proceeds more effectively compared to electrodialysis alone.
Demkin et al. (1987), from their studies in which an ion-
exchange fibrous filler was used in an ED cell, observed that
the limiting current density was increased considerably from
4 to more than 24 A/m2 for the low and high concentration
cases, respectively.
2.2. Cu2þ/Hþ ion exchange isotherm
Themonodivalent ion exchange process betweenCu2þ andHþ
is represented by Equation (1):
2Hþresin þ Cu2þ
solution4Cu2þresin þ 2Hþ
solution (1)
The selectivity coefficient of the ion-exchange process can
be defined using the equivalent ionic fractions as follows:
KCu;H ¼�XCu2þ
XCu2þ
���XHþ
XHþ
�2
(2)
where Xi and Xi are the equivalent ionic fractions of species i
in the solution and in the resin phase, respectively. The
equivalent fractions of Cu2þ and Hþ are defined as:
XHþ ¼ ½Hþ�2½Cu2þ� þ ½Hþ� XCu2þ ¼ 2½Cu2þ�
2½Cu2þ� þ ½Hþ� (3)
XHþ ¼ ½Hþ�2½Cu2þ� þ ½Hþ�
XCu2þ ¼ 2½Cu2þ�2½Cu2þ� þ ½Hþ�
(4)
where ½Cu2þ� and ½Hþ� are the ion concentrations in the ion-
exchange resin, and ½Cu2þ� and ½Hþ� are the ion concentra-
tions in the solution phase.
2.3. IXED efficiency
The IXED performance can be evaluated in terms of (i)
abatement yield ðhCu2þ Þ in the metal cation and (ii) current
efficiency (or transport) yields (CE) of Cu2þ ions. The
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 3 6 4e3 3 7 6 3367
abatement allowed by IXED process was effectively assessed
by the difference between the copper concentration at the
inlet and the outlet divided by the copper concentration at the
inlet of cell, hCu2þ . On the other hand, the ratio of the quantity
of copper transferred to the cathode compartment and the
charge passed during electrodialysis taking into account
Faraday’s constant, expresses the efficiency of copper trans-
port induced by the electrical field:
CEcathodeCu2þ ¼ zCu � F� ncathode
Cu2þ
Q(5)
where ncathodeCu2þ is the number of moles Cu2þ species in the
cathode compartment and Q the electrical charge passed in
the circuit at time t. For the present case of constant current
operations, the electrical charge is given by: Q ¼ I� dt. A
similar yield can be defined at the anode:
CEanodeCu2þ ¼ zCu � F� nanode
Cu2þ
Q(6)
where nanodeCu2þ is the number of moles Cu2þ species in the
cathode compartment.
Moreover, a fraction of the copper cations transferred are
reduced into metal copper on the Pt-coated cathode. The
current yield of this copper deposited at the cathode can be
given by:
CEmetalCu ¼ zCu � F� nmetal
Cu
Q(7)
where nmetalCu is the mole amounts of copper deposited at the
cathode. Finally, the total current efficiency of the ion transfer
to the cathode side can be defined as follows:
CEcathodeCu ðtotalÞ ¼ CEcathode
Cu2þ þ CEmetalCu ¼ zCu � F� �
ncathodeCu2þ þ nmetal
Cu
�Q
(8)
This efficiency represents the transference number of
copper species through the membrane to the cathode
compartment.
Table 1 e Physical properties of the resins (Mahmoud,2004; Mahmoud et al., 2003, 2007).
Parameter Dowex 50WX (2%)
Dowex 50WX (4%)
DowexHCR-S (8%)
Granulometry (Mesh) 50e100 50e100 20e50
Capacity (eq./1) 1.81 1.17 0.76
Selectivity coefficient 9.5 � 2.8 10.1 � 3.9 13.7 � 3.9
k (Hþ-form) (mS/cm) 265 242 200
k (Cu2þ-form) (mS/cm) 29 20 10
k: The electrical conductivity of the solid phase.
3. Materials and methods
3.1. Experimental procedures
3.1.1. Feed solutionsThe experiments were carried out at room temperature with
mixed CuSO4 and H2SO4 solutions of various concentrations.
The solutionswere preparedwith reagents of analytical purity
(Merck) and conductimetry water. Concentration of copper, in
the form of copper sulphate was fixed at 1.57 mmol/1, corre-
sponding to 100 ppm Cu ions. The solutions were acidified
with sulphuric acid, whose concentration in the resulting
solutionswas 1mmol/l, corresponding to pH equal to 3.2� 0.1.
3.1.2. Ion-exchange resinsThree styrene-base cation exchangers Dowex� were used,
with a DVB cross-linking degree varying of 2, 4 and 8%. Prop-
erties of the resins were determined previously (Mahmoud,
2004; Mahmoud et al., 2003, 2007) are given in Table 1. In
particular, the copper capacity was measured at room
temperature with mixed CuSO4 and H2SO4 solutions of
various concentrations by fixing the total normality at 0.5 eq./l
Cu2þ solution. The composition of these solutions was calcu-
lated using a thermodynamic model of the mixed solution for
various proportions of copper salt over sulphuric acid, with
the total normality at 0.5 eq./l (Mahmoud et al., 2003). This
model derived from previous investigations of the
ZnSO4eH2SO4 system (Zouari and Lapicque, 1992) takes into
account the non ideal behaviour of the various species by
means of the Pitzer model for the various interactions. In
addition, the partial dissociation of copper sulphate was also
accounted for, as recommended in Wasylkiewicz (1990).
Fig. 2 shows the ion exchange isotherm of copper ions on
the resins investigated. The isotherm profile indicates that
copper is strongly preferred by those resins over protons, as
shown in the Fig. 2. For these three resins, the simple laws for
sorption isotherms fit well with the experimental data. For any
ionic composition, the separation factor equals the ratio of the
two rectangular areas of Hþ and Cu2þ touching one another in
the corresponding point on the isotherm. The diagonal dotted
line is the isothermof a fictitious ion exchange resinwhich has
no preference for either counter ion.
3.2. Experimental set-up
For the IXED experiments, a lab-scale three-compartment cell
was used (Mahmoud, 2004; Mahmoud et al., 2003, 2007;
Monzie et al., 2005). The electrode compartments and the
packed bed were 100 mm high and 10 mm wide, and the
membranes were Nafion� 117. The membrane gap, corre-
sponding to the bed thickness, was 15 mm. Both electrodes
were platinum-coated titanium plates which are dimension-
ally stable (not consumed coulometrically). For most of the
experiments, it was preferable to work with a smaller ion
exchange bed for the sake of its more uniform composition.
This bed was 15 mm high and was situated between two inert
resin beds of the same particle size, each of which was
42.5 mm high. The reduced active bed length allowed faster
transient loading of the resin upon percolation of the copper
solution, in addition to more uniform distributions of vari-
ables in the bed. The experimental set-up with the cell
geometry, including the beds inserted in the central
compartment, is illustrated in Fig. 3. A DC power supply
(HewlettePackard E3612A), operating under constant current
(or voltage), was used for the IXED operation. Two multi-
meters (ITT instruments MX20) were used to control the
current and to monitor voltage fluctuations in the IXED cell.
Fig. 2 e Ion-exchange isotherms of copper ion on the resins
investigated. Solid lines correspond to fitting with average
value of the selectivity coefficient.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 3 6 4e3 3 7 63368
Finally, to control the treated solution quality, the pH and the
electrical conductivity were measured by a pH/Ion meter
(pHM692, MeterLab) and a conductivity meter (CDM210,
MeterLab), respectively.
One litre of sulphuric acid solutions at 0.5 M were circu-
lated batchwise in the electrode compartments at 30 l h�1
using a centrifugal pump (Iwaki). The bed was continuously
percolated downwards by the prepared solutions at 0.60 l h�1
using a peristaltic pump (BVK, Ismatec). The resinwas initially
in the Hþ-form, and runs were carried out at fixed current at
ambient temperature (22 �C � 1 �C).
3.3. Measurements and analytical methods
The range of variables for the IXED conditions was based on
the preliminary studies conducted for the estimation of the
minimum current that would be required for complete
Cu2+
30 l/h
1 litre Tank
H2SO4
CuSO4
Solution
CM
-
42.5 mm
42.5 mm
15 mm
15
Treated
CM: Cation Membrane
H+-form ion-exchange bed
Inert bed
Fig. 3 e Schematic view of experimental set-up for IXED experi
compartment.
removal of the copper ion flux fed to the cell assuming a 100%
current efficiency (Mahmoud, 2004). Three constant current
densities 10, 13.3 and 20 mA/cm2 were used to investigate the
influence of the current density on the IXED treatment.
Experiments were carried out for 10 h with the three resins
considered.
The solutions of the electrode chambers and that leaving
the central compartment were collected at regular intervals.
The pH, conductivity, and copper concentration of these
samples were measured. The cell voltage was continuously
recorded. After the run, the bed was percolated with 2 M
H2SO4 solution for desorption of the copper ions, then rinsed
with deionized water. The cell was taken apart and the traces
of copper deposited on the Pt/Ti cathode were dissolved with
dilute nitric acid. The copper contents of the various liquid
fractions were determined by flame atomic absorption. At the
end of each run, a mass balance for Cu2þ was conducted,
taking into account the amount of metal ions sorbed on the
resin, the mole numbers transferred to both compartments
and the mole amount of metal deposited. Mass balances were
shown to hold within 7% in most cases.
4. Results and discussion
4.1. Effect of the resin nature in the dilute compartment
4.1.1. Measurements of the electrical resistance and thepower consumptionThe advantages of the IXED process in terms of voltage fluc-
tuation or electrical resistance and power consumption over
the ED process on its own are evaluated. The impact of the
resin nature was carried out at a constant current density of
20 mA/cm2 and for the 1.57 mmol/1 copper solution. Fig. 4
shows the transient behaviour of the electrical resistance
and power consumption for the three resins. During the
loading of the resin with copper ions, Period (I), there was
(0.6 l/h)
Ti-Pt
10 mm
100
mm
H2SO4
Solution
1 litre Tank
Electrode
30 l/h
CM
+
mm
Solution
ments with the beds inserted in the central (dilute)
0.0E+00
2.0E-04
4.0E-04
6.0E-04
8.0E-04
1.0E-03
1.2E-03
1.4E-03
1.6E-03
1.8E-03
2.0E-03
0 1 2 3 4 5 6 7 8 9 10
Time (h)
Pow
er c
onsu
mpt
ion
(kW
h)
0
200
400
600
800
1000
1200
1400
1600 ED
IXED with Dowex 2%
IXED with Dowex 4%
IXED with HCR-S 8%
Ele
ctri
cal r
esis
tanc
e (
Ohm
)
(I) (II)
58.4
%
67.8
%
70.7
%
56%
66%
69%
Fig. 4 e Electrical resistance and power consumption variation during ED and IXED runs for the three resins considered at
a constant current density of 20 mA/cm2. Period (I): Loading of the resin, Period (II): Steady-state.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 3 6 4e3 3 7 6 3369
a regular increase in cell voltage. In Period (II) steady values in
the range 11.9e14.5, 12e13.2, and 16.9e18.7 V were attained
for 2, 4 and 8% DVB, respectively, after 2 h or so. As shown in
Fig. 4, the corresponding electrical resistance was in range
395.7e482.7, 402e439, and 563e623 Ohm, respectively. High-
est cell voltages were recorded with 8% DVB resins, in agree-
ment with their lowest electrical conductivity (or highest
electrical resistance, see Table 1). Moreover, it was found that
the highest reduction in the electrical resistance (about 71%)
was offered by the 4% resin (with moderate degree of cross-
linking). This can be explained by its high capacity and its soft
structure. It was also found that the IXED technique requires
much less energy than ED technique alone. For example, this
technique ismore energy efficient requiring less than 32% and
44% of the ED energy for the moderate and high DVB cross-
linking degree, respectively. These findings seem to indicate
that the hybrid process seems to be an interesting technique
for improving the electrical efficiency of treating dilute
wastewater. An overview of the specific electric energy
consumptions based on copper removal (kW h/molremoved
copper) is discussed in Section 4.3.
4.1.2. Effect of the resin nature on the pH in the dilutecompartmentThe use of ion-exchange resins may also give rise to changes
in the pH caused by the production of hydrogen and hydroxide
ions, which may reduce problems typically associated with
concentration polarization near the membranes (Spoor et al.,
2002).
However, the experimental results show that the increase
in DVB cross-linking degree does not have a significant effect
on the pH values. As a matter of fact, the pH of the three
solutions sampled, changed very little during the batch
experiments. In particular, the pH of the sulphuric acid
solutions remained in the range 0.6e0.7, with a slight
increase at the cathode side due to production of OH� ions.
a
b
c
Cu
conc
entr
atio
n (m
mol
/l)
0.0
0.4
0.8
1.2
1.6
2.0
0 1 2 3 4 5 6 7 8 9 10
Outlet with Dowex 2%Outlet with Dowex 4%Outlet with HCR-S 8% Inlet
Time (h)
Cu2+
conc
entr
atio
n (m
mol
/l)
20 mA/cm
31%33%
41%
(I) (II)
0.0
0.4
0.8
1.2
1.6
2.0
0 1 2 3 4 5 6 7 8 9 10
Dowex 2% Dowex 4% HCR-S 8%
Time (h)
20 mA/cm
(I) (II)
0.0
0.1
0.2
0.3
0.4
0.5
0 1 2 3 4 5 6 7 8 9 10
Dowex 2% Dowex 4% HCR-S 8%
Time (h)
Cu
conc
entr
atio
n (m
mol
/l)
20 mA/cm
Fig. 5 e Treatment of the copper sulphate solution with the
small resin bed at a constant current density of 20 mA/cm2,
depending on the resin stiffness. (a): Copper ion
concentration at the outlet of the central compartment
versus time. (b): Copper ion concentration in the cathode
compartment versus time. (c): Copper ion concentration in
the anode compartment versus time. Period (I): Loading of
the resin, Period (II): Steady-state.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 3 6 4e3 3 7 63370
The solution leaving the central compartment had a pH
ranging from 2.6 to 2.8, due to the flux of Hþ transferred from
the anode side.
4.1.3. Effect of the resin nature in the dilute compartment onthe abatement yieldThe concentration of copper ions in the cathode liquid is
plotted against time in Fig. 5(b), whereas Fig. 5(a) gives the
outlet concentration of copper species. In the first period, the
increase in Cu2þ concentration in the cathode chamber was
rather low, due to the progressive loading of the resin; and as
a consequence the abatement of copper ions was complete.
After this, the accumulation in the cathode chamber pro-
gressed at a regular, higher pace, whereas the resin bed was
not sufficient for total removal of Cu2þ. After 2 h or so, the
concentration in the cathode chamber, levelled off to attain
a steady level at the end of the run. Both the accumulation rate
in the cathode chamber and the steady-state concentration at
the outlet depended on the resin stiffness: the resin with 4%
DVB cross-linking degree allowed the best water purification
performance and produced the most concentrated copper
sulphate solutions. As shown in Fig. 5(a), the abatement yield
ðhCu2þ Þ attained 41%with this resin, 33 and 31%with resins at 2
and 8% DVB cross-linking degree, respectively. The final cor-
responding value for copper concentration in the cathode
chamber was found at 1.79mmol/l to be compared to 1.72 and
1.35 mmol/l with resins at 2 and 8% DVB cross-linking degree,
respectively.
The best performance offered by the 4% resin can be
explained by its high capacity and its soft structure, which is
expressed indirectly by its electrical conductivity. The softest
grade gave the highest transport properties to the cations, but
its poor capacity limits the sorption flux. Finally, the HCR
grade with 8% DVB is far too rigid to allow efficient ion
transport, in spite of its high capacity. The poor ability of the
rigid resin was previously observed in electromigration
experiments (Mahmoud et al., 2003).
4.1.4. Effect of the resin nature in the dilute compartment onthe current yieldThe amount of copper ions transferred to the cathode
compartment is plotted against time in Fig. 5(b). Asmentioned
already, the amount of copper ions increased regularly with
time, depending on the resin grade.
On the other hand, copper ions were observed to be
transferred to the anode compartment, with the overall
transfer resulting from antagonistic migration and diffusion
fluxes. Concentration of copper ions in the anode chamber
increased regularly with time, with slightly higher rates in the
loading period of each experiment. Despite its high transport
property, the 2% resin gave the lowest side-transfer rates in
the anode chamber. As shown in Fig. 5(c), the 4% resin allowed
the highest side-transfer rates to the anode during the first 5 h
of testing. Thereafter the 8% resin gave the highest side-
transfer rates to the anode chamber. The corresponding final
concentration of copper ions in the anode chamber was
0.425 mmol/1, corresponding to 27 ppm.
Dismantling of the cell structure revealed a significant
copper deposit on the Ti/Pt surface of the cathode with
a mainly dendritic morphology. This was particularly evident
in the case of the 4% resin in spite of the supporting electrolyte
in the catholytic chamber flowing at 30 l h�1. The amount of
metal recovered at the cathode was comparable to the mole
amount of copper ions in the cathode chamber as shown in
Table 2. Moreover the deposition rate was likely to be
enhanced at the end of the runs by the increasing Cu2þ
concentrations in the cathode chamber. Corresponding
Faradic yields and values for the removal efficiency are given
in Table 2. The current efficiency for the copper ion transfer
through the membrane, CEcathodeCu ðtotalÞ, being the sum of the ion
Table 2 e Current efficiency of transport to the external compartments (CE); removal yield ðhCu2D Þ; the mole amount ofcopper ions in the anode and cathode chambers; and the mole amount of copper deposited at the cathode after 10 h ata constant current of 20 mA/cm2.
Resin nanodeCu2þ mmol nmetal
Cu mmol ncathodeCu2þ mmol CEanode
Cu2þ % CEmetalCu % CEcathode
Cu2þ % CEcathodeCu ðtotalÞ % hCu2þ %
50 WX (2%) 0.162 0.170 1.72 2.89 3.04 30.72 33.72 33
50 WX (4%) 0.339 0.114 1.79 6.06 2.03 31.94 33.97 41
HCR-S (8%) 0.426 0.0378 1.35 7.62 0.67 24.06 24.73 31
Copper sulphate injected into the bed is 9.63 mmol within 10 h.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 3 6 4e3 3 7 6 3371
and metal contributions, is also given. This efficiency corre-
sponds to the transference number through the bed and the
cation-sidemembrane and attained 33.97%with the 50WX 4%
resin (Table 2).
4.2. Effect of the current density
Table 3 summarizes the experimental results for the three
resins considered and for three processing current densities
10, 13.3 and 20mA/cm2. The amount of Cu2þ ions recovered in
the cathode compartment increased with the applied current,
but lower current efficiencies were obtained for the highest
current for the three resin grades. The moderately flexible 4%
resin appears to allow the best results. The amount of copper
deposited was generally far lower than the quantity of copper
ions, in particular at low current densities. The yield for
copper deposition increased stronglywith the current applied,
varying from 0.5% at 10 mA/cm2 to a few percents for the
highest current density (Table 3).
The removal yield was averaged over the period of steady
behaviour of the bed. This yield depended on the resin grade
and the current density (Table 3). The best results were also
obtained with the 4% grade. Moreover the efficiency of the
water treatment was enhanced with higher current densities;
however, for the 4% grade, the removal yield was observed to
be barely affected by the current density over 10 mA/cm2.
Fig. 6(a) shows theoutlet concentrationof copper species. In
thefirst period, the loadingof the resinparticles allowednearly
complete removal of copper ions from the feed solution. Then,
the copper concentration at the outlet increased rapidly to
attain a steady level after 2 h or so, as shown in Fig. 6(a).
Table 3 e Removal yield ðhCu2D Þ; current efficiency of transportcopper ions in the anode and cathode chambers; the mole am
Resin Current densitymA/cm2
nanodeCu2þ
mmolnmetalCu
mmolncathodeCu2þ
mmol
Dowex (2%) 10 0.532 0.0157 1.07
13.3 0.373 0.0375 1.45
20 0.162 0.0170 1.72
Dowex (4%) 10 0.469 0.0110 1.29
13.3 0.556 0.0894 1.50
20 0.339 0.0114 1.79
HCR-S (8%) 10 0.459 0.0118 1.161
20 0.427 0.0378 1.35
Copper sulphate injected to the bed is 9.63 mmol within the 10 h.
Concentrations of copper ions in the external compart-
ments followed a predictable trend over the course of the
experiment, as shown in Fig. 6(b), (c). In most cases the linear
variations indicate a constant ion flux, at least in the 10-h
experiments. Because of the moderate size of the resin bed
and the restricted periods of each experiment, Cu2þ concen-
trations in both compartments were below 2 mmol/l. Values
of the average flux to the external compartments were
deduced by linear regression of the data during the period
mentioned. The slight non-linearity in the copper concentra-
tion observed with the 8% resin reduced the accuracy of the
determination. Values for fluxes into the external compart-
ments were shown in Fig. 7.
In the cathode compartment, the accumulation fluxwas an
increasing function of the current density. As for the removal
efficiency, the best results were obtained with the 4% resin, as
shown in Fig. 7(a). The flux of copper ions transferred to the
anode compartment decreased with the current density, as
shown in Fig. 7(b). High current densities favour migration
from the anode towards the cathode through the resin bed,
hindering transport by diffusion from the central compart-
ment. This tendency observed with the three resins did not
allow accurate interpretation of the phenomenon. However,
extrapolation of the data to zero current led to estimates for
the diffusion flux to the anode compartment with values
ranging from 5.76 to 9 10�3 mmol/cm2 h. Finally, the current
efficiency of the electrolytic operation varied from 24 to 47%,
depending on the current density and the resin grade. The
best current efficiencies for the transfer of Cu2þ to the cathode
were obtained with the mid-range of current density, in
particular for the two softest resins. For all resins, the current
efficiency rapidly decreased for the high current densities.
to the external compartments (CE); the mole amount ofount of copper deposited at the cathode after 10 h.
CEanodeCu2þ
%CEmetal
Cu
%CEcathode
Cu2þ
%CEcathode
Cu ðtotalÞ%
hCu2þ
%
18.99 0.57 38.35 38.91 29
9.98 1.00 38.77 39.77 28
2.89 3.04 30.67 33.72 33
16.76 0.39 46.23 46.63 39.89
14.91 2.39 40.08 42.48 38.96
6.06 2.03 31.94 33.97 41
16.42 0.42 41.34 41.76 31
7.62 0.67 24.06 24.73 31
a
b
c
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0 1 2 3 4 5 6 7 8 9 10
InletOutlet at 10 Outlet at 13.3 Outlet at 20
Time (h)
Cu
conc
entr
atio
n (m
mol
/l)
40%
mA/cmmA/cm
mA/cm
Dowex 4%
0.00.20.40.60.81.01.21.41.61.82.0
0 1 2 3 4 5 6 7 8 9 10
10 mA/ 13.3 mA/ 20 mA/
(Dowex 4% ; Cathode compartment)
Time (h)
cm
cmcm
Cu
conc
entr
atio
n (m
mol
/l)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0 1 2 3 4 5 6 7 8 9 10
10 mA/ 13.3 mA/ 20 mA/
Time (h)
(Dowex 4% ; Anode compartment) cm
cmcm
Cu
conc
entr
atio
n (m
mol
/l)
Fig. 6 e Treatment of the copper sulphate solution, with (Dowex 50 WX 4%). (a): Effect of the current density on the
concentration at the outlet of the bed versus time. (b): Effect of the current density on the copper ion concentration in the
cathode compartment versus time. (c): Effect of the current density on the copper ion concentration in the anode
compartment versus time.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 3 6 4e3 3 7 63372
4.3. Electrical energy consumption
Fig. 8, gives an overviewof the different specific electric energy
consumptions based on copper removal, calculated from the
sum of the total number of mole of copper transferred to the
external compartments taking into account the moles of
copper deposited at the cathode. As shown in Fig. 8, it was
found that the specific electric energy consumptions increased
with the current density. This specific electric energy
consumption depended also on the resin grade. Themoderate
flexible 4% resin gave the best results for the highest current
density. The corresponding electric energy consumption at
20mA/cm2was 1.93 and 2.67 kWh/molremoved copper in the case
of 2%, and 8% resins, respectively. The electrical energy
consumption was only 1.54 kW h/molremoved copper in the case
of the 4% resin.
On the other hand, Fig. 8 also reveals that the IXED
technique required much less energy than the ED technique
alone. For example, this technique is much more energy
efficient requiring less than 32 and 44% of the ED energy for
the low and high current density cases, respectively. These
findings seem to indicate that the application of the IXED is
a promising technique for the purification of metal-
containing waters.
a
b
0.0000.0020.0040.0060.0080.0100.0120.0140.0160.0180.020
8 10 12 14 16 18 20 22
Dowex 2% Dowex 4%HCR-S 8%
Spec
ific
flu
x of
cop
per
ion
(mm
ol/c
m2 h
)
Cathode compartment
Current density (mA/cm2)
0.000
0.001
0.002
0.003
0.004
0.005
0.006
8 10 12 14 16 18 20 22
Dowex 2% Dowex 4%HCR-S 8%
Current density (mA/cm2)
Anode compartment
Spec
ific
flu
x of
cop
per
ion
(mm
ol/c
m2 h
)
Fig. 7 e Specific flux of copper ion recovered in the external
compartments. (a): Specific flux of copper ion transferred to
the cathode compartment. (b): Specific flux of copper ion
transferred to the anode compartment.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 3 6 4e3 3 7 6 3373
4.4. Analysis of the CCD design
4.4.1. Standardized Pareto ChartIn order to design efficient processes, the response surface
methodology (RSM) is used to establish an empirical model
thatmaps the response surface using data from a designed set
of experiments to identify the direction and parameter ranges
for the response optimization.
The objectives of the present study were (i) to evaluate the
effects of the processing parameters in a large parameter
space, (ii) to determine if the separation enhancement, if any,
results only from electrical effects or from coupled IXED
Current density (mA/cm2)
Ene
rgy
cons
umpt
ion
(kW
h/m
ol re
mov
ed c
oppe
r)
0
2
4
6
8
10
12
023.3101
Dowex 2%Dowex 4%HCR-S 8%ED
Fig. 8 e Energy consumption during IXED as a function of
both the current density and cross-linking DVB.
effects, (iii) to maximize the abatement yield ðhCu2þ Þ and the
current yield ðCEcathodeCu ðtotalÞÞ and (iv) to minimize the energy
consumption. As a consequence, RSM is a most suitable
method for this optimization.
A central composite design (CCD) is one of the most useful
approaches in determining optimum conditions of many
processes (Cochran and Cox, 1957; Khuri and Cornell, 1987;
Mahmoud et al., 2008). Therefore, in this study, CCD was used
to point out the relationship existing between the response
function, abatement yield ðhCu2þ Þ, current yield ðCEcathodeCu ðtotalÞÞ
and the energy consumption, process variables, current
density (X1) and DVB cross-linking degree (X2). The range of
the independent variables for the IXED process conditions
were the current density, X1 (8e20 mA/cm2), DVB cross-
linking degree X2 (2e8%).
The model calculation based on the standardized values
allows a comparison of the relative influence of the factors on
the response, by comparing the square roots of the sum of
squares of all the coefficients related to a factor. To determine
which factors have a significant impact on the response
variable, the Pareto Chart was used. The standardized Pareto
Chart contains a bar for each effect, classed from the most
significant to the least significant. The length of each bar is
proportional to the standardized effect. The length of each bar
indicates the effect of these factors and the level of their
effects on responses. A vertical line (reference line) is drawn at
the location of the 0.05 critical value. Any bars that extend to
the right of that line indicate effects that are statistically
significant at the 5% significance level. Fig. 9 shows the stan-
dardized Pareto Chart and depicts the main effect of the
independent variables on the (i) abatement yield, (ii) current
yield and (iii) the energy consumption calculated per the total
amount of copper transferred to the external compartments
and copper deposited. It can be inferred, as shown in Fig. 9(a),
that the factor X1 (current density), X1X1, X1X2 and X2X2
extending behind the reference line, have a significant effect
on the abatement yield.
As shown in Fig. 9(a), the quadratic effect of DVB (X2X2) has
a bigger impact on the response than both the linear and
quadratic effect of the current density (X1) and the interaction
effect of X1X2.
Note that the factor X1 (current density) has the most
significant main effect at the 95% confidence level on the
current yield, followed by the quadratic effect of DVB and the
interaction effect of X1X2, as can be seen from Fig. 9(b).
Moreover, the factor X1 (current density) has the most signif-
icant main effect at the 95% confidence level on the energy
consumption, followed by the interaction effect of X1X2, as
can be seen from Fig. 9(c).
4.4.2. Optimization stepIn order to optimize the IXED variables for the response Y, the
RSM was used. The response surfaces for the abatement yield
ðhCu2þ Þ, the current yield ðCEcathodeCu ðtotalÞÞ and the energy
consumption generated by STATGRAPHICS� Centurion XVI.I
software are displayed in Fig. 10.
Using the analysis options dialog box (energy consumption
STATGRAPHICS� Centurion XVI.I software) of the RSM, the
range of each factor allowing the optimum abatement yield
and current yield to be obtained is determined. After
a
b
c
0 2 4 6 8 10 12
+
-
0 2 4 6 8 10
+
-
0 2 4 6 8 10
+
-
Standardized effect
Standardized effect
Standardized effect
Reference line
Reference line
Reference line
X1
X2
X2X2
X1X2
X1X1
X1
X2
X2X2
X1X2
X1X1
X1
X2
X2X2
X1X2
X1X1
Fig. 9 e Standardized Pareto Chart showing the effects of
the independent variables X1 (current density) and X2 (DVB)
and their combined effects on the abatement yield (a), the
current yield (b) and the energy consumption (c).
Fig. 10 e Estimated response surface for the abatement
yield (a), the current yield (b) and the energy consumption
(c) as a simultaneous function of X1 (current density) and X2
(DVB).
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 3 6 4e3 3 7 63374
generating the polynomial equations relating the dependent
and independent variables, the process was optimized for the
response Y. The optimization was performed to obtain the
levels of X1 � X2 which maximize Y (abatement yield and
current yield).
As discussed previously, the energy consumption is very
important as well. Therefore, the best combination of process
variables for the energy consumption response function was
determined in order to limit the energetic cost of the process
and simultaneously obtain a satisfactory abatement yield and
current yield. The optimum processing conditions were
a current density of 9.6 mA/cm2 and DVB of 5%, which gave an
abatement yield, current yield and energy consumption of
36.63%, 45.13% and 0.531 kW h/molremoved copper, respectively.
5. Conclusions
The IXED was successfully used for the removal of heavy
metals from moderately acidified copper sulphate solutions
simulating rinsing water of copper plating lines, under
a variety of processing conditions ranging from 10 to 20 mA/
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 3 6 4e3 3 7 6 3375
cm2 and from 2 to 8% DVB. Importantly, it has been illustrated
that the IXED can be used to remove a significant proportion of
the copper ions from moderately dilute copper solutions, as
well as to produce a concentrated metal salt solution, which
could be recycled. It was also found that the IXED technique
requires less than 32 and 44% of the ED energy for the
moderate and high DVB cross-linking degree, respectively.
These findings seem to indicate that the hybrid process seems
to be a promising technique for improving the electrical effi-
ciency of treating dilute wastewater.
The analysis from the RMS appeared very well suited to
predict the optima set of operating variables in order to ach-
ieve a satisfactory treatment (abatement yield and current
yield) using the minimum amount of electricity.
Acknowledgement
This research was performed at Laboratoire des Sciences du
Genie Chimique, CNRS-ENSIC, Nancy, France. One of the
authors (A. Mahmoud) gratefully acknowledges F. Lapicque
and L. Muhr, my supervisors, for their kind help and
suggestions.
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