Selectivity and Adsorptivity of Various Anions in IonExchange Membranes
Do Hee Kim, Sang Youp Lee, and Jaeweon Cho
Dept. of Environ. Sci. and Eng., Kwang-Ju Institute of Sci, & Tech.(K-JIST), Kwang-ju, 500712, Korea
Introduction
Basic transport phenomena of ions are important for the application of ion-exchange
membranes (IEMs). Several studies on the development of mathematical models for predicting
the membrane transport have been performed. By using the principles of irreversible
thermodynamics, various transport equations have been derived, which described non-stationary
diffusion through a membrane separating pores.1, 3
Interactions between various anions (i. e., arsenic, borate, sulfate vs. nitrate) and
membrane were determined in terms of selectivity/adsorptivity and transmissibility of the
solutes in membranes. Relative selectivity of each anion for a membrane was determined by
adsorption tests and resulting isotherms, as determined with an adsorption cell. Transmissibility
of various anions through membrane pores was determined to obtain their diffusivities and mass
transfer coefficients. Affinity coefficients determined from selectivity tests and mass transfer
coefficients of the anions are anticipated to be used for membrane performance prediction.
Anions may exhibit different speculations in the natural system, depending on pH and
oxidation state. These different anions types are informative from the perspective of membrane.
The characteristics of anions were investigated rigorously to relate with their transport
phenomena near the membrane surface and treatability. Various types of IEMs were used for
this study.
The main objectives of this study are (1) to relate the chemical and transport
characters of anions with membrane properties, (2) to predict membrane performance for anions
removal, (3) to estimate fundamental properties of various anions of interest to membrane
process, and (4) to build up a robust prediction model for solute adsorption (or affinity) and
rejection.
Materials and Methods
Membranes and chemicals
Anion exchange membranes used in this study were AMX, AM1, ACM and AMV. All
of the tested membranes are chloride form. Table 1 shows the basic characteristics of these
IEMs.
Table 1. Characteristics of anion exchange membranes
AMX
AM1
ACM
AMV
Type
Cl-form
Cl-form
Cl-form
Cl-form
Thickness (mm)
0.14~0.18
0.12~0.16
0.10~0.13
0.11~0.15
2.0~3.5
1.3~2.0
3.5~5.5
1.5~3.0
Electric resistance
(ohm-cm2)
Apparatus and procedure
The ion exchange of ions equilibrium between solution and membrane phases was
measured using following procedures. A membrane with the chloride form was soaked in a 10 N
NaCl solution prior to experiment test ion was added in the solution. The solution was stirred
with a magnetic stirrer at a constant temperature of 25  1oC. After at least 24 hrs, a certain
amount of sample was taken to measure the ion concentration in solution phase. A 0.1 M NaOH
was added into the solution to extract chloride ions from membrane surface and the solution was
stirred for 24 hrs. A certain amount of solution was sampled out to measure chloride ion
concentration of membrane phase. The concentrations of chloride, sulfate, nitrate ions were
determined by Ion Chromatography (IC : Dionex), and the concentrations of other arsenic and
boric ions were measured by Induced Coupled Plasma Atomic Emission Spectrometry (ICP
AES :Thermo Jurrec Ash).
Adsorptivities of various anions on to the membrane surface were determined using a
diffusion cell. Figure 1 shows the schematic diagram of the diffusion cell. It consists of a
membrane, two compartments for two different solutions, and two magnetic stirrers. Only one
of compartment was filled with a solution in this study. The membrane located between the
compartments was fastened with a gasket and contain an effective area of 1.37  10-3 m2 and a
solution volume of 300 mL. Various anions concentrations were measured over time and anion
adsorption on the membrane surface was evaluated.
Fig. 1. Schematic diagram of diffusion cell.
Results and Discussions
Membrane concentration of various anions
0.02N solutions of arsenic, borate, sulfate, and nitrate were poured into a specially
designed test beaker for selectivity and adsorption tests in which different membranes (AMX,
ACM, AM1 and AMV) were soaked in it. Fig. 2 shows the concentration of each ion in
membrane phase.
Conc. in membrane (mol/L)
0.05
0.045
0.04
0.035
0.03
0.025
0.02
0.015
0.01
0.005
0
AM X
ACM
N itra te
AM 1
M e m b ra n e s
S u lfa te
A rs e n a te
AM V
B o ric a c id
Fig. 2. The concentration of ions in different membranes.
Nitrate ion represented the highest membrane concentration because it is mono-valent anion. On
the other hand, boric acid exhibited lowest membrane concentration because of its non-ionized
form and relatively low solubility in water.2
Anion selectivity
The selectivity coefficient of anions was calculated by the equation (1) :
~
K
C1 C k2
~k
C1 C 2
(1)
Where : 1 = Anion or Cation
2 = Cl- or Na+
k = valance of applied ion
superscript ~ = membrane phase
Table 2 shows the summary of selectivity coefficients of various anions for AMX membrane.
Table 2. Selectivity coefficient of various anions
Ion
Coefficient
NO3-
0.0425
SO42-
1.9510-5
H2AsO4-
0.0167
H3AsO3
0.0033
Nitrate showed the highest selectivity for the membrane and these results showed similarame
tendency of previous results.
Anion adsorption on the membrane surface
Fig. 3 shows nitrate adsorption on the AMX membrane surface.
25
Conc. of Nitrate
100
20
80
15
60
10
40
5
20
0
-1
Adsorbed Mass of Nitrate
120
0
4
9
14
19
24
100 ppm C onc.
Ti
e (hrs)
50m ppm
C onc.
25 ppm C onc.
25 ppm m ass
50 ppm m ass
100 ppm m ass
Fig. 3. Adsorption of various concentration of nitrate in AMX membrane.
Adsorption proceeded about 6 hours and above 70 % of nitrite was exchanged at every
concentration. Ratio of adsorbed mass with initial mass slightly declined with concentration
increment.
Acknowledgement
This study was supported by the Korea Institute of Science & Technology Evaluation and
Planning (KISTEP) through the
specific
Fundamental
Research
Program entitled,
"Minimization of Contamination in a Reactor Coolant System"
Reference
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J. of Membrane Sci., 81, 298~294 (1993)
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the ion-exchange membrane by means of batch wise donnan dialytic experiments, J. of
Membrane Sci., 53, 215~227 (1990)
3. Miyyoshi, Hirofumi, Diffusion coefficients of ions throygh ion exchange membrane in
Donnan dialysis using ions of different valence, J. of Membrane Sci., 141, 101~110
(1998)