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Separation and Purication Technology 22-23 (2001) 637 – 642
www.elsevier.com:locate:seppur
A comparison between streaming potential and membrane
potential measured across single charged and bipolar
membranes
J. Benavente
a,
*, G. Jonsson
b
a
Departamento de Fsica Aplicada
,
Facultad de Ciencias
,
Uni
6
ersidad de M laga
,
E
-29071
Malaga
,
Spain
b
Chemical Engineering Department
,
Technical Uni
6
ersity of Denmark
,
DK
-2800
Lyngby
,
Denmark
Abstract
Pressure-induced (streaming) potential and membrane potential across two ion-exchanger membranes (one posi-
tively charged and another negatively charged) have been measured with the membranes in contact with NaCl
solutions at different salt concentrations. Membrane permselectivity was calculated from membrane potential and
lower values were obtained for the cation-exchanger membrane. For comparison, measurements with a bipolar
membrane obtained by a series association of both single-charged membranes has also been made. The inuence of
electrical asymmetry of bipolar membranes in both electrical potentials was studied by reversing the direction of
pressure and concentration gradients. Results show that the streaming potential coefcient depends on salt
concentration for the bipolar membrane, while for both ion-exchanger membranes, it is practically constant within the
range of concentration studied (5 10
4
M
5
C
5
10
1
M). Membrane potential values for the bipolar membrane
show a higher inuence of the anion exchange sublayer. In both types of measurements, differences due to the
opposite directions of the external gradients across the bipolar membrane were obtained. © 2001 Elsevier Science
B.V. All rights reserved.
Keywords
: Ion-exchanger membranes; Bipolar membranes; Streaming potential; Membrane potential
1. Introduction
sublayers), while bipolar membranes used for
bases and acids production or for the treatment of
efuents also consist of anion-exchange and
cation-exchange layers [1 – 3]. The inuence of
each sublayer in the performance of composite
membranes depends on the structure and trans-
port characteristic of each layer and the external
conditions [4,5]. Particularly, orientation-depen-
dence for membrane potential measured with
both composite reverse osmosis and bipolar mem-
branes have been reported in the literature [4,6].
Membranes consisting of two or more layers
are commonly used in different separation pro-
cesses. For instance, reverse osmosis or nanoltra-
tion membranes for desalting purposes basically
consist of two different layers (active and support
* Corresponding author. Tel.: 34-952-131929; fax: 34-
952-132000.
E
-
mail address
: j-benavente@uma.es (J. Benavente).
1383-5866:01:$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S1383-5866(00)00158-1
638
J
.
Bena
6
ente
,
G
.
Jonsson
:
Separation
:
Purification Technology
22-23 (2001) 637–642
However, standard procedures for in situ mem-
brane characterization, this means, with the mem-
branes in contact with solutions, can not be used
for composite membranes without different extra
assumptions, which usually suppose to neglect:
minimize the contribution of one sublayer [4].
In this paper, pressure-induced (streaming) and
membrane potentials (
D
E
and
D
0.2)°C.
The experimental system used for both stream-
ing and membrane potential measurements is sim-
ilar to that indicated elsewhere [7] and it basically
consists of a test cell made of PVC, with two holes
in the center of each half-cell to place the Ag:
AgCl electrodes, which were connected to a high
impedance voltmeter.
Pressure-induced (streaming) potential mea-
surements were made under dialysis conditions,
this means, xing and having an external control
of feed and product concentrations. Measure-
ments were carried out for a concentration rang-
ing between 5 10
4
and 5 10
1
M. Pressure
difference ranged between 0.4 and 9 atm, the
speed of the circulating solution at the high pres-
sure side was 115 cm:s approximately, while the
pump output at the low pressure side was 136
cm
3
:min.
Membrane potentials were measured keeping
the NaCl concentration at one side of the mem-
brane constant (
C
c
), and gradually changing the
concentration at the other side (
C
v
). Measure-
ments were carried out at three different NaCl
constant concentrations (
C
c
(M) 5 10
3
,5
10
2
and 5 10
1
) while the concentration
C
v
ranged between 10
3
9
P
), the ion transport num-
bers and the membrane permselectivity for both
single-charged membranes were determined from
experimental data. A bipolar membrane was ob-
tained joining both oppositely charged ion-ex-
changer membranes. In order to correlate the
behavior of bipolar and single-layer membranes, a
comparison of results obtained under similar ex-
ternal conditions is made. The electrical asymme-
try presented by the bipolar membrane on ionic
uxes under pressure and concentration gradients
was also studied by changing the direction of both
gradients. The inuence of salt concentration on
these parameters was considered by carrying out
the
g
(
D
E
:
D
measurements
at
different
NaCl
concentrations.
2. Experimental
Two commercial polyethylene – styrene graft
membranes by American Machine and Foundry
Co., a cation-exchanger AMF ion C-60 and an
anion-exchanger AMF ion A-60, and a bipolar
and 10
1
M. Membrane
ø
m
) were obtained from experimental
values by subtracting the electrode potential con-
tribution:
D
D
ø
elec
(
RT
:
z
–
F
)ln(
C
1
:
C
2
).
Table 1
Characteristic parameters for cation-exchanger C-60 and anion-exchanger A-60 membranes
a
Membrane
Capacity (meq:g)
a
Thickness (
m
m)
Electrical Resistance (
V
cm
2
)
b
C-60
1.6
300
5 (K
)
A-60
2.0
300
7(Cl
)
a
meq:g dry membrane.
b
Ionic form.
membrane (BP) obtained by a series association
of these two oppositely charged membranes were
used. Some characteristic parameters for both
ion-exchanger membranes are indicated in Table
1. Measurements were carried out with NaCl so-
lutions at different concentrations, neutral pH
and constant temperature
t
(25.0
ø
m
, respectively)
for two commercial single-layered oppositely
charged membranes, a cation-exchanger and an
anion-exchanger, have been measured with the
membranes in contact with NaCl solutions at
different concentrations. The streaming potential
coefcient,
potential (
J
.
Bena
6
ente
,
G
.
Jonsson
:
Separation
:
Purification Technology
22-23 (2001) 637–642
639
external salt concentration for the different mem-
branes and external conditions studied. For both
ion-exchange membranes
g
10
1
), while for the BP
membrane it strongly increases when the concen-
tration increases, independently of the pressure
gradient direction. It is worth indicating that for
the BP membrane signicant differences in
C
(M)
5
val-
ues were obtained at low concentrations for both
opposite pressure gradients.
Fig. 3 shows the membrane potential versus
ln(
C
c
:
C
v
) for three different values of the constant
concentration,
C
c
5 10
3
,5 10
2
and 5
10
1
M. For C-60 and A-60 ion-exchanger mem-
branes, ion transport numbers,
t
i
, were obtained
from the slopes of the corresponding straight
lines. Membrane permselectivity,
P
S
(
i
), which is a
measure of the membrane selectivity of the coun-
terions over the co-ions, can be obtained from
transport numbers [8]:
g
Fig. 1. Pressure-induced potential vs. applied pressure differ-
ence at
C
10
2
M. (A) single ion-exchanger membranes:
P
S
(
i
) (
t
i
t
i
0
):(1
t
i
0
)
i
, )
where
t
i
0
is the transport number of ions in solu-
tion. Variation of membrane permselectivity with
salt concentration is shown in Fig. 4. For both
ion-exchanger membranes a decrease of
t
i
and
P
S
(
i
) values when salt concentration increases
was obtained, which is due to the increase of
co-ions inside the membrane. However, as can be
observed in Fig. 4, the exclusion of co-ions for the
cation-exchanger membrane is signicantly lower
than that presented by the anion-exchanger
membrane.
Comparison of membrane potential values for
both single-layered charged membranes and the
bipolar membrane shows signicant differences
depending on the constant concentration level or
on the charged layer in contact with the constant
concentration (external concentration gradient
direction):
(
) C-60 membrane; (
) A-60 membrane. (B) Bipolar mem-
brane: (
) high pressure in contact with the negative charged
layer; (
) high pressure in contact with the positively charged
layer.
3. Results and discussion
E
values comparing both single-
layered membranes and the bipolar membrane,
but for the latter, differences depending on the
direction of the pressure gradient were also ob-
tained. Although other possible potentials must
be included in
D
E
values [5], it can be assumed
that they are not pressure dependent, because of
which the streaming potential coefcient,
D
Membrane potential values obtained with the
BP membrane at the lowest constant concen-
tration (
C
c
5 10
3
M) under both opposite
external gradients are practically symmetric.
Only small differences were found at the
highest
C
v
values, when concentration ratio is
rather high (
C
v
:
g
,was
obtained from the slopes of the
D
E
versus
D
P
straight lines.
Fig. 2 shows
g
values as a function of the
4
C
c
). The slopes of the
is practically con-
stant for the whole range of concentration (5
10
4
5
Pressure-induced potentials versus applied pres-
sure difference for C-60, A-60 and BP membranes
at a given NaCl concentration (
C
10
2
M) are
shown in Fig. 1(A, B). Important differences can
be observed in
640
J
.
Bena
6
ente
,
G
.
Jonsson
:
Separation
:
Purification Technology
22-23 (2001) 637–642
straight lines obtained for both opposite con-
centration gradients are practically the same as
those corresponding to each single-layered
charged membrane. This result indicates that
the BP membrane behaves as an anionic ex-
change membrane when
C
c
is in contact with
the positively charged sublayer (membrane A-
60), but it behaves as a cationic exchanger
membrane for the opposite situation, when
C
c
is in contact with the negatively charged sub-
layer (membrane C-60).
sponds to a concentration ration
r
mx
C
c
:
C
mx
,
where
C
mx
represents the concentration of the
external solution when the slope changes.
Variation of
r
mx
with external salt concentra-
tion is also shown in Fig. 4. From this picture, a
strong decrease of C-60 membrane permselectivity
and
r
mx
when the concentration increases can be
observed, while the permselectivity of negatively
charged A-60 membrane hardly depends on salt
concentration (relative decrease of 2% for A-60
membrane and higher than 10% for C-60 one).
Differences obtained in membrane potential val-
ues for the BP membrane and both opposite
concentration gradients could be related to this
fact, a relatively important decrease in the selec-
tivity of the negatively charged sublayer at high
concentrations and, as a result of this, a higher
inuence of the positively charged sublayer in the
behavior of the bipolar membrane.
When the constant concentration increases, the
D
ø
m
ln(
C
c
:
C
v
) relationships change depend-
ing on the external concentration gradient. The
same type of tendency, an increase in
ø
m
values when
C
v
increases, was always obtained
when
C
c
was in contact with the positively
charged layer. However, the
D
ø
m
ln(
C
c
:
C
v
)
relationships drastically change from positive
slope (negatively charged membrane) to nega-
tive slope (positively charged membrane) when
the constant concentration solution was in con-
tact with the negative charged sublayer. Under
these conditions a maximum value for the
membrane potential is reached, which corre-
D
4. Conclusions
Measurements of streaming and membrane po-
tentials were carried out with two oppositely
Fig. 2. Concentration dependence for the streaming potential coefcient. (
) C-60 membrane; (
) A-60 membrane; bipolar
membrane: (
) high pressure in contact with the negative charged layer; (
) high pressure in contact with the positively charged
layer.
J
.
Bena
6
ente
,
G
.
Jonsson
:
Separation
:
Purification Technology
22-23 (2001) 637–642
641
with those corresponding to each charged
sublayer.
Streaming potential results show that for both
ion-exchanger membranes, the streaming poten-
tial coefcient is practically independent of the
salt concentration within the range of concentra-
tion studied (although it depends on the mem-
brane charge), while for the bipolar membrane
its value increases when the concentration in-
creases.
A slight decrease of counterion transport num-
ber and membrane permselectivity when the con-
centration increases was obtained for A-60
membrane (around 2%), however, a more
important reduction in membrane permselectivity
was found for C-60 membrane (higher than
10%).
The electrical asymmetry inherent to bipolar
membranes clearly affects the transport of ions
under both concentration and pressure gradients,
as is clearly shown by differences in membrane
and streaming potential values obtained for simi-
lar but opposite external conditions. Results show
a higher inuence of the positively charged sub-
layer at high concentrations, while at low concen-
trations the inuence of the negative charged
sublayer also presents an important contribution
to membrane potential values.
Fig. 3. Membrane potential versus ln(
C
c
:
C
v
) for different
constant concentrations (
C
c
). A,
C
c
0.005 M; B,
C
c
0.05
M; C,
C
c
0.5M. (
) C-60 membrane; (
) A-60 membrane;
bipolar membrane: (
)
C
c
in contact with the negative
charged layer; (
)
C
c
in contact with the positively charged
layer.
charged single-layered membranes and a bipolar
membrane (BP) obtained by joining together both
charged membranes. This allows the correlation
of the results obtained for the bipolar membrane
Fig. 4. Concentration dependence for membrane permselectiv-
ity,
P
S
, of single-charged membranes and concentration ration,
r
mx
, for the BP membrane. Left hand side: permselectivity; (
)
)
P
S
( ) C-60 membrane. Right
hand side: concentration ratio
r
mx
(
x
).
P
S
( ) A-60 membrane; (
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