11th International Seminar on Polymer Science and Technology
Iran Polymer and Petrochemical Institute, Tehran, Iran
6-9 October 2014
Bivalent to monovalent selectivity by surface modification of composite
cation exchange membrane
H. Farrokhzad, T.Van Gerven, B. Van der Bruggen
Chemical Engineering Department, KU Leuven, Willem de Croylaan 46, Heverlee, Belgium
Abstract
A novel hybrid cation exchange membrane was prepared by polymerization of polyaniline on a composite
membrane, made of polyvinylidene fluoride (PVDF) and sulfonated PVDF (S-PVDF). Polyaniline was doped
with hydrophobic dopants and the effect of this surface modification on total salt removal and selectivity of
bivalent to monovalent cations was evaluated.
Keywords: sulfonated PVDF, polyaniline, surface polymerization, cation selectivity, electrodialysis
Introduction
Electrodialysis (ED) is a separation technology for
ion removal using ion-exchange membranes (IEMs)
in a direct electrical field [1]. Since sulfonation as a
common method for synthesis of cation exchange
membranes (CEMs) has some drawbacks such as
dimensional instability, a novel composite CEM
based on sulfonated PVDF (S-PVDF) with excellent
physio-chemical properties was developed for ED
with bivalent cation selectivity [2].
ED has some important potential applications in
selective ion permeation such as NaCl concentration
for the chlor-alkali industry [3], removal of specific
cations from industrial waste water treatment and
water softening [4], etc. Thus, making a selective
membrane is very important approach in IEM
synthesis. In this work we used polyaniline (PANi)
for surface polymerization on S-PVDF/PVDF
composite membrane to prepare a membrane with
monovalent selectivity while keeping its salt
removal efficiency well.
Experimntal
Synthesis method
S-PVDF was synthesized by reaction of 10 g PVDF
powder with 50 ml chlorosulfonic acid at 80 °C for
45 min. Then it was washed by water, dried and
dissolved in N-methyl pyrrolidone (NMP).
Afterwards PVDF and S-PVDF were blended by codissolution method. Then a composite membrane
with 70% wt S-PVDF was prepared by evaporation
of the casted solution in vacuum oven at 60 °C. The
polymerization of aniline on the composite
membrane was performed by immersion of CEM in
aqueous anilinum sulfate solution and then in
aqueous ammonium peroxodisulfate solution. The
membrane immersed in 0.1 M ammonia for PANi
dedoping and then in appropriate 0.1 M of doping
agent solution (para-toluene sulfonic acid (p-TSA)
and
dodecylbenzenesulfonic
acid
(DBSA).
Composite and hybrid membranes are named S-70,
P-pTSA and P-DBSA, respectively.
Characterization
For measuring the ion exchange capacity (IEC), a
piece of membrane was immersed in 1 M HCl for 24
h, then washed, dried, weighted and immersed in 2
M NaCl. Then Na+ exchanged by H+ was estimated
by titration of remained solution according to eq. 1:
IEC = (CNaOH × VNaOH) / (Wdry)
(1)
For measuring the water uptake, the membranes
were dried in a vacuum oven at 80 °C for 24 h, then
immersed in DI water for 24 h; the uptake was
estimated by eq. 2:
Uptake = [(wwet-wdry)/ wdry] × 100%
(2)
For the ED experiments, a Berghof BEL-500 system
was used, which included two anion exchange
membranes (AEMs) and two CEMs. The initial
concentration in the diluate and concentrate
compartments was 0.01 M NaCl + 0.01 M MgCl2
and for rinsing circuit it was 0.1 M Na2SO4. The ED
performance was evaluated by conductivity
measurement (using a CDM 83 conductivity meter)
of the diluate each 30 min. The ion selectivity was
calculated as:
(3)
11th International Seminar on Polymer Science and Technology
Iran Polymer and Petrochemical Institute, Tehran, Iran
6-9 October 2014
where tMg and tNa are the transport numbers of the
cations in the membrane, and CMg and CNa are the
average concentrations of Mg2+ and Na+ during
electrodialysis. The concentrations were measured
by Atomic Absorption Spectroscopy (AAS) (Perkin
Elmer AAnalyst 100).
Results and Discussion
IEC & Water uptake
The IEC values shown in Table 1 are lower for
hybrid membranes compared to the composite
membrane. This indicates that the concentration of
fixed charges on the membrane matrix decreases
after PANi coating for different doping agents. Also
the water uptake decreased for both hybrid
membranes, because of barrier function of coating
layer and hydrophobic nature of doping agents.
As presented in Fig. 1, the S-70 membrane has a
better total salt removal than P-pTSA and P-DBSA,
which is attributed to the decrease in IEC after
surface modification. Furthermore, the removal of
each cation is given in Fig. 2. The selectivity of
Mg2+ to Na+ was quantified for different membranes
in Table 4. S-70 shows a good bivalent selectivity,
Mg
but PNa decreases for P-pTSA. The selectivity
achieved for PANi-pTSA was found to be the
opposite of the other membranes, i.e., it has a high
selectivity for the monovalent cation (i.e.
Na
PMg
≈1/0.14≈7.1).
According to diffusion theory of cations through a
CEM by Firdaous et al. [5], in order to pass a
solution/membrane interface cations need to
overcome the energy barrier caused by the necessity
of partial dehydration of ions. The effective
parameter is the ratio hydrated-ion-hop/ nothydrated-ion-hop, which is equal to 0.10 and 5.65
for Na+ and Mg2+, respectively. Thus, Na+ cations
intend to jump inside the membrane in their
dehydrated state. Therefore, after surface
modification of composite
membrane
by
hydrophobic PANi, the Na+ selectivity will increase
and is proportional to water uptake. The Na+
removal is high for P-pTSA, which indicates that it
is a good monovalent selective membrane.
Conclusion
Novel hybrid cation exchange membranes were
synthesized by surface polymerization of polyaniline
(PANi) on S-PVDF/PVDF composite membranes
with different doping agents. The results show that
S-70 is a bivalent selective membrane with high salt
extraction. After surface polymerization this
selectivity and total salt extraction decrease. PANipTSA has a very high selectivity for monovalent
cations with relatively high extraction of Na +. All
the results are attributed to the IEC and water
affinity of membranes.
Table 1. IEC and water uptake of composite and hybrid
membranes
Membrane
IEC (meq/g)
Water uptake (%)
S-70
0.64
13.7
P- DBSA
0.54
7.7
P- pTSA
0.55
4.3
Fig 1. Conductivity measurement of diluate for different
membranes by electrodialysis
Fig. 2. Cation removal percentage for different membranes
Table 2. Cation selectivity for composite and hybrid
membranes
Membrane
𝐌𝐠
𝐏𝐍𝐚
S-70
P- DBSA
P- pTSA
2.01
1.26
0.14
References
1. T. Xu, Ch. Huang, AIChE. J., 54, 3147–3159,
2008.
2. H. Farrokhzad, T. Van Gerven, B. Van der
Bruggen, Melpro conference, Czech, Prague, 2014.
3. S. Casas, C. Aladjem, J. L. Cortina, E. Larrotcha
and L. V. Cremades, Solvent Extr Ion Exc., 30, 322–
332, 2012.
4. G. Poźniak, 22nd International Symposium on
Physico-Chemical Methods of Separation - Ars
Separatoria, Poland, 2007.
5. L. Firdaous, J. P. Malériat, J. P. Schlumpf & F.
Quéméneur, Sep Sci and Tech., 42, 931–948, 2007.