Materials Sciences and Applicatio ns, 2011, 2, 615-623
doi:10.4236/msa.2011.26083 Published Online June 2011 (http://www.SciRP.org/journal/msa)
Copyright © 2011 SciRes. MSA
615
Efficient Facilitated Transport of Lead and
Cadmium across a Plasticized Triacetate
Membrane Mediated by D2EHPA and TOPO
Omar Arous1,2*, Fairouz Saad Saoud1, Mourad Amara1,2, Hacène Kerdjoudj1
1Laboratory of Hydrometallurgy and Inorganic Molecular Chemistry, Faculty of Chemistry, University of Sciences and Technology
Houari Boumediene, Bab Ezzouar, Algeria; 2Center of Research in Physical and Chemical Analysis, Algiers, Algeria.
Email: omararous@yahoo.fr
Received February 9th, 2011; revised March 11th, 2011; accepted April 21st, 2011.
ABSTRACT
Cellulose triacetate membranes doped with organo-phosphoric carriers (2-ethylhexyl) phosphoric acid noted (D2EHPA)
or trioctyl phosphine oxide noted (TOPO) as fixed carriers and 2-nitro phenyl octyl ether noted (NPOE) or tris ethyl-
hexyl phosphate noted (TEHP) as a plasticizers have been prepared and applied for investigation to the facilitated
transport of Pb(II) and Cd(II) ions from aqueous nitrate source phase. The membranes Polymer - Plasticizer - Carrier
were characterised using chemical techniques as well as Fourier Transform Infra - Red (FTIR), X - ray Diffraction and
Scanning electron microscopy (SEM). A study of the transport across a polymer inclusion membrane has shown that the
lead or cadmium transport efficiency was increased using D2EHPA as carrier at pH 1-2.
Keywords: Cellulose Triacetate, NPOE, TEHP, D2EHPA, TOPO, Membrane
1. Introduction
Liquid membranes have received considerable attention
by many researchers because of their high selectivity
accomplished by carriers incorporated in the membranes.
Many efforts have been done to investigate the use of
liquid membranes for various separation and purification
processes such as separation of isomers [1,2], gases [3-5],
metal ions [6,7], etc.
The separation and removal of toxic metal cations and
neutral chemicals from water has frequently been ad-
dressed in membrane separation systems. Environmen-
tally damaging and toxic anions have received signifi-
cantly less attention primarily due to the challenging na-
ture of selectively binding anions. Recently, a novel type
of liquid membrane called polymer inclusion membrane
(PIM) has been developed which provides metal ion
transport with high selectivity, as well as easy setup and
operation [8]. Polymer inclusion membranes (PIM) show
great potential for industrial separations over other
membrane types. Bulk liquid membranes (BLM) are not
economically scalable to industrial levels, while the more
practical supported liquid membranes (SLM) tend to lose
solvent to the water phases. PIM consist of a polymer
support, which is commonly cellulose triacetate (CTA), a
plasticizer (solvent) and carrier molecules. The plasti-
cizer is an integral part of the membrane, acting as the
solvent in which the carrier diffuses. This makes the
membrane easy to handle and promotes membrane dura-
bility. In fact, the common plasticizer o-nitrophenyl octyl
ether (NPOE) suffers virtually no loss from the mem-
brane into adjacent water phases [9]. Also, PIM demon-
strate permeability of ionic and neutral species compara-
ble to SLM [10]. Any loss of transport in PIM in com-
parison to SLM is due to slower diffusion across the
membrane, however, the higher carrier capacity of CTA
membranes helps increase transport to counteract this
effect [11,12]. The PIM plasticizer can be changed to
optimize transport. NPOE, TEHP are common plasticiz-
ers in CTA-based PIM. The durability and relatively high
polarity of NPOE allow counter ions to be dissolved in
the membrane as free ions. NPOE has the further advan-
tages that it is non-volatile, high boiling and insoluble in
water. CTA-based PIM using NPOE as the plasticizer
retain macrocyclic carriers, such as calixarenes and pro-
vide a stable, durable membrane [13-17]. The fabrication
and characterization of new membranes is reported re-
cently [18,19].
Facilitated transport of metal ions through PIM carrier
Efficient Facilitated Transport of Lead and Cadmium Across a Plasticized
616
Triacetate Membrane Mediated by D2EHPA and TOPO
membranes has resulted in good selectivity in ions sepa-
rations with real improvements of the membrane stability
as compared to liquid membranes as well as polymer -
stabilized liquid membranes. This is reflected by an in-
creasing number of PIM investigations reported in the
literature over the last two decades [20,21]. Transport
studies through cellulose triacetate membranes as poly-
meric matrix with showing high flux and good stability
have been recently reported [22-34].
In this work, we have developed a novel class of plas-
ticized cellulose triacetate membranes modified by or-
gano-phosphoric carrier’s incorporation that are selec-
tively permeable to lead and cadmium cations. The mem-
brane CTA - Plasticizer - Carrier was characterised using
chemical techniques as well as Fourier Transform Infra -
Red (FTIR), X-Ray Diffraction, and Scanning electron
microscopy (SEM). We compared the behaviour of a Di
(2-ethylhexyl) Phosphoric acid noted D2EHPA (Ac-
id-type carrier) and a Trioctylphosphine oxide noted
TOPO (solvating-type carrier) towards a facilitated tran-
sport of lead and cadmium through a synthesized mem-
brane.
2. Experimental
2.1. Reagents
Analytical-grade inorganic chemicals: Pb(NO3)2, Cd(NO3)2,
were obtained from CARLO ERBA. Organic reagents:
cellulose triacetate (CTA), o-nitrophenyl octyl ether
(NPOE), chloroform were purchased from Fluka. Tris
ethylhexyl phosphate (TEHP) was purchased from
Merck. The ion carriers (2-ethylhexyl) phosphoric acid
(D2EHPA) and trioctyl phosphine oxide (TOPO) were
obtained from Aldrich. Doubly distilled water was used
for preparing all aqueous solutions.
2.2. Membrane Preparation
Polymer Inclusion Membranes were prepared using the
same procedure described by Sugiura et al. [34]. The
solution of cellulose triacetate (0.2 g) (Figure 1), ion
carrier (D2EHPA or TOPO) (Figure 2), and plasticizer
(o-nitrophenyl octyl ether) or tris ethylhexyl phosphate
(0.2 mL) (Figure 3) in chloroform (20 mL) was poured
into a Petri glass of 9.0 cm diameter. The solvent was
allowed to evaporate overnight and the resulting mem-
brane was separated from the Petri glass by immersion in
cold water.
2.3. Transport Studies
Transport experiments were carried out in a permeation
cell in which the membrane film was tightly clamped
between two cell compartments. Both the source and
receiving aqueous phases (50 mL each) were stirred at
R = H (Cellulose)
R = COCH3 (Acétate Cellulose).
Figure 1. Cellulose triacetate polymer.
P
OH
O
O
CH2
CH2
CH
C2H5
C3H7
CH
C2H5
C3H7
O
P
O
C8H17 C8H17
C8H17
(D2EHPA) (TOPO)
Figure 2. Organo-phosphoric carriers.
(TEHP) (NPOE)
Figure 3. Plasticizers used in the PIM.
800 rpm. Samples of the aqueous receiving phase were
removed periodically via a sampling port with a syringe
and after appropriate dilution with deionized water were
analyzed to determine the metal ion concentrations by the
Atomic Absorption Spectroscopy technique (AAS). Three
independent experiments were realized to determine the
lead and cadmium concentration. The experimental
standard deviation was determined to be ± 5%.
Transport of lead and cadmium ions obeys to a facili-
tated co-transport in the case of TOPO (solvating-type
carrier) and counter-transport in the case of D2EHPA
(Acid-type carrier).
The mechanism transport is represented in Figures 4(a)
and 4(b). The metal ion is complexed at the interface
feed-phase/membrane and the complex formed diffuses
through the membrane phase to the interface membrane/
strip-phase where the decomplexation of the metal ion is
realized [35]. The mass flux can be calculated by the
Copyright © 2011 SciRes. MSA
Efficient Facilitated Transport of Lead and Cadmium Across a Plasticized 617
Triacetate Membrane Mediated by D2EHPA and TOPO
equation n
JSt
where is the variation of the n
number of moles in the feed phase during the interval
time (s) and S is the membrane area (cm2). t
2.4. Analyses
The metal concentrations were determined by samplings
at different time intervals aliquots of 0.5 ml each from
the feed and strip solutions and analysed with an atomic
absorption spectrophotometer (Perkin Elmer 2380). Mass
flux J(mol·cm2·s–1) of the metal ions through the PIM
transferred from the feed side of the membrane to the
strip side was determined applying its definition: J =
n/St, where n represents the variation in mole num-
ber of metal ions in the receiving solution during the ref-
erence time t, and S is the membrane active area. IR
spectra were recorded on with Perkin Elmer spectropho-
tometer (Spectrum one). X-ray analyses were recorded
on a Bruker D8 Advance AXS diffractometer.
3. Results and Discussion
3.1. Physical and Chemical Characteristics of
Cellulose Triacetate Membranes
In Table 1, some of the characteristics of the membrane
made with the carriers have been listed in comparison
with those of the reference CTA membrane. As the carrier
(a)
(b)
Figure 4. (a) Facilitated co-transport; (b) Facilitated counter-
transport mechanisms.
Table 1. Chemical and physical characteristics of cellulose
triacetate membranes.
membrane thickness
(µm)
density
(mg/cm2) contact angle (˚)
CTA 10 4.88 46.4
CTA-NPOE 15 6.12 80.5
CTA-TEHP 12 5.84 75.8
CTA-NPOE-TOPO 32 7.42 79.2
CTA-TEHP-TOPO 28 6.91 76.3
CTA-NPOE-D2EHPA 27 6.87 78.8
CTA-TEHP-D2EHPA 25 6.33 76.1
molecules (TOPO and D2EHPA) and plasticizers (NPOE
and TEHP) are hydrophobic, the location of carrier
molecules at the surface of the CTA modified mem-
branes should modify the contact angle which is a pa-
rameter indicative of the wetting character of a material.
3.2. X. Ray Diffraction
Figures 5 (a-d) show the X - ray curves for cellulose
triacetate (CTA) + plasticizers + carriers membranes.
Based on this figure, we can observe the following:
The CTA membrane presents a single maximum lo-
cated at approximately 20˚ found in all polymers and
corresponds to the Van der Waals halo [36,37]. Thus,
this material presents basically amorphous character-
istics.
The systems constituted by the mixture of CTA +
NPOE + carrier and CTA + TEHP + carrier do not
give any diffraction. It can be due to the absence of
crystallisation within the membrane which permits us
to eliminate the mechanism of transfer of the ions by
successive jumps between carrier complexing sites in
a 3D assembled state.
3.3. Characterization by FTIR
The membranes CTA + Plasticizer + Carrier were char-
acterised using chemical techniques as well as Fourier
Transform Infra - Red (FTIR). IR spectra were recorded
on with Perkin - Elmer (Spectrum One) spectrophotome-
ter.
Figures 6 and 7 show the spectrums of the different
membranes synthesised.
Table 2 collects the peak values and the corresponding
radical of the reference CTA, TEHP, NPOE, CTA +
TEHP and CTA + NPOE membranes.
The obtained results showed that all the maximum
values extracted from the spectrum of the CTA reference
mebrane, i.e. without plasticizer and carrier, are present m
Copyright © 2011 SciRes. MSA
Efficient Facilitated Transport of Lead and Cadmium Across a Plasticized
Triacetate Membrane Mediated by D2EHPA and TOPO
Copyright © 2011 SciRes. MSA
618
(a) (b)
(c) (d)
Figure 5. X-ray curves for (CTA + NPOE + D2EHPA), (CTA + TEHP + D2EHPA), (CTA + NPOE + TOPO) and (CTA +
NPOE + TOPO) membranes.
in the modified membranes spectra in addition to those of
the carrier molecules. This result confirms the presence
of plasticizer and carrier in the polymer matrix.
3.4. Characterisation by Scanning Electron
Microscopy (SEM)
The morphologies of the CTA + Plasticizer + carrier
membranes (cross section) (Figure 8) show that the
CTA-plasticizer (NPOE or TEHP) and carrier (TOPO or
D2EHPA) membranes present a dense structure where
the pores of membrane have been filled by the NPOE,
TEHP TOPO and D2EHPA molecules yielding a thick
and less porous membrane.
3.5. Influence of the Plasticizer Nature
We examined the effect of a plasticizer nature. We used
two plasticizers (NPOE and TEHP).
Figure 9 shows the concentrations of lead in a strip
phase using TOPO as carrier and two plasticizers. It can
be perceived that tris (2-etheylhexyl) phosphate (TEHP,
viscosity, η = 10.2 cP, dielectric constant, εr = 4.8) and
2-nitrophenyl octyl ether (NPOE, η = 12.8 cP, εr = 23.1)
produces the highest PIM transport of ions.
3.6. Influence of the Carrier Nature
The transport has been achieved with two carriers
(TOPO and D2EHPA) using a same polymer and a same
plasticizer (NPOE).
Figure 10 represents the variation of the concentration
of lead ions in the strip phase versus time using TOPO
and D2EHPA carriers. The results show that the concen-
tration of lead in the strip phase obtained with D2EHPA
Efficient Facilitated Transport of Lead and Cadmium Across a Plasticized 619
Triacetate Membrane Mediated by D2EHPA and TOPO
Figure 6. FTIR spectrums of the CTA + TEHP + carriers membranes.
Figure 7. FTIR spectrums of the CTA + NPOE + carriers membranes.
Copyright © 2011 SciRes. MSA
Efficient Facilitated Transport of Lead and Cadmium Across a Plasticized
620
Triacetate Membrane Mediated by D2EHPA and TOPO
CTA + NPOE +TOPO (Cross-section) CTA + NPOE + D2EHPA (Cross-section)
CTA + TEHP +TOPO (Cross-section) CTA + TEHP + D2EHPA (Cross-section)
Figure 8. Scanning electronic microscopy of various membranes.
is slightly lower than that of TOPO. This can be also re-
lated to the difference in viscosity between the two or-
ganic phases of D2EHPA and TOPO, and probably, to the
formation of emulsions in the organic phase of TOPO
that would block the active surface of the membrane and
prevent the metallic ions to react with TOPO molecules
present in the membrane.
3.7. Influence of the pH of the Strip Phase
Extraction and transport of a metal cation by an acidic
carrier is governed by the exchange of the metal ion for
protons of the carrier. Consequently, counter-transport of
protons is the driving force and is achieved by maintain-
ing a suitable pH difference between the strip and feed
solutions. In addition, careful pH control in the source
solution can result in good selectivity as in the case of
solvent extraction systems using acidic reagents.
Figure 11 represents the variation of the concentration
of lead and cadmium ions in the strip phase versus pH
using D2EHPA carrier. We demonstrate that a maximum
permeability for Pb(II) and Cd(II) across a CTA + TEHP +
D2EHPA membrane from a source phase to a receiving
phase was obtained at pH 1 - 2.
4. Conclusions
A cellulose triacetate (CTA) membrane containing organo-
phosphoric carriers (2-ethylhexyl) phosphoric acid noted
(D2EHPA) or trioctyl phosphine oxide noted (TOPO) and
2 - nitrophenyloctyl ether noted (NPOE) or Tris ethyl-
hexyl phosphate (TEHP) noted (TEHP) as a plasticizer has
been synthesized. These CTA - plasticizer - Carrier
membranes were characterised using chemical techniques
as well as Fourier Transform Infra-Red (FTIR), X-ray
diffraction (DRX) and SEM analysis. The systems con-
stituted by the mixture of CTA + plasticizers + carriers do
not give any diffraction. The morphologies of the CTA +
Plasticizer + Carrier membranes (view of Cross-section)
show that these membranes present a dense structure. A
Copyright © 2011 SciRes. MSA
Efficient Facilitated Transport of Lead and Cadmium Across a Plasticized 621
Triacetate Membrane Mediated by D2EHPA and TOPO
Table 2. peak values and the corresponding radicals in
different membranes. m (CTA) = 0.2 g, v (TEHP) = v
(NPOE) = 0.2 mL.
Membrane Peak value (cm–1) Corresponding radical
3480 - 3550 O–H
2935 C–H
1755 C=O
1526 COO
1246 C–O–C asym
CTA
1054 C–O–C sym
2960 C–H
1464 CH2
1381 CH3
1285 P=O
TEHP
1020 P–O–C
1534 NO2 (NPOE )
1488 –CH3 (NPOE) NPOE
1325 CN (NPOE )
3477 O–H (TAC)
2913 C–H
1767 C=O (TAC)
1370 –CH3 (TAC)
1241 C–O–C asym
1054 C–O–C sym
TAC + TEHP
1034 P O C
3481 O–H (TAC)
1755 C=O (TAC)
1534 NO2 (NPOE )
1488 –CH3 (NPOE)
1325 C–N (NPOE)
1169 C–O–C (NPOE)
TAC + NPOE
1096 C–O–C (TAC)
Figure 9. Evolution of the concentration of lead in a strip
phase versus time using two plasticizer (NPOE) and (TEHP).
Figure 10. Evolution of the concentration of lead in a strip
phase versus time using (CT A + TOPO + NPOE) and (CTA +
D2EHPA + NPOE) membranes.
Figure 11. Evolution of the concentration of lead and cad-
mium in a strip phase versus the pH of a strip phase using
(CTA + D2EHPA + TEHP) membrane.
Copyright © 2011 SciRes. MSA
Efficient Facilitated Transport of Lead and Cadmium Across a Plasticized
622
Triacetate Membrane Mediated by D2EHPA and TOPO
study of the transport across a polymer inclusion mem-
brane has shown that the lead or cadmium transport effi-
ciency was increased using D2EHPA as carrier at pH 1 - 2.
The inclusion of a selective TOPO or D2EHPA in the
matrix of a polymer CTA gives rise to stable membranes
able to transport ions and to work for a long time. Our
results indicate that facilitated transport through plasti-
cized membranes is an attractive and effective way to
solve the enduring problem of membrane stability whilst
improving the permeability to metal ions.
Further efforts will be directed to the determination of
the nature of interactions between polymer and carrier by
use of other materials and analysis as well.
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