SAFE AND SELECTIVE MEMBRANE SYSTEMS FOR PERTRACTION
OF HARMFUL SUBSTANCES
Romuald Wódzki
Nicholas Copernicus University, Faculty of Chemistry
87-100 Toruń, 7 Gagarina St., Poland
E-mail: wodzki@chem.uni.torun.pl
1. INTRODUCTION
The pertraction process occurs when permeants from the liquid feed are transported
through a nonporous polymeric or liquid membrane, and then desorbed into another liquid
phase (stripping or receiving solution). This enables the combination of two different
processes of liquid and polymer pertraction to make the system more stabile and effective
while working with bulk or layered liquid membranes. The requirements for the quality of
membranes will depend on the composition of the feed and will differ in the case of
pertraction of ions (organic and inorganic) and organic compounds. The idea of such systems
is consistent with the idea of separation in a three-phase system (e.g. liquid membrane) with
the interfaces immobilized in the same way as in the membrane extraction. However, there is
a difference because the extraction process can be considered as an equilibrium phenomenon
whereas pertraction is controlled by diffusion and/or kinetics of interfacial phenomena
(sorption, reaction). Pertraction-based membrane processes can be related to the reactiondiffusion transport processes observed in natural cellular systems [1]. Thus, the biophysics of
membrane transport can be considered as a valuable source of knowledge for designing a
number of new membrane systems of practical importance. The main features of such
biomimetic pertractors are: coupling of different fluxes and transport mechanisms (i), specific
organization of a transport system through its compartmentalization (ii), and the use of
biomimetic components with no need for exact reconstruction of living organisms or their
components (iii). The idea of our studies is related to the fundamentals of transport process
occurring in a bacteria cell wall, the main component of which can be regarded as a
hydrophilic ion exchanging polymer membrane with phosphodiester and carboxylic acid
groups connected to a quasi-liquid cytoplasm membrane [2]. Consequently, two synthetic
membranes of different natures, i.e. at least a liquid membrane (LM), arranged in series with
an ion-exchange membrane, result in a system of the architecture parallel to a bacteria cell
envelope. The system can be additionally combined with other synthetic membranes in order
to construct various biomimetic pertractors, These are defined here as complex membrane
systems containing a liquid membrane the functions of which are associated and coupled with
processes occurring in polymer membranes of different composition and properties. A
number of pertractors of a similar structure (multimembrane hybrid systems, MHS) have been
developed in recent years, and evaluated as very practical membrane devices [2,3-7].
2. THE OBJECTIVES
Specific applications of membrane separation methods in fields such as analytical
chemistry, food technology, pharmaceutical industry, and environment protection need
efficient systems able to operate continuously with very dilute solutions of harmful substances
which should be removed (cleaning of the feed) or preconcentrated (analytical applications).
Consequently a kind of active transport should be generated in order to realize such a task. On
the other hand, such applications have to be safe, i.e. the components of the involved
membranes, frequently toxic, cannot contaminate the feed. Therefore, two directions of our
investigations can be distinguished, the first is to construct save membrane systems, and the
second one is the research for new, non-toxic, and easy to recycle membrane components.
This can be achieved after considering architecture of natural transport systems as the pattern
for artificial ones. In general, this principle is justified by the existence of various defence
mechanisms observed in microorganisms living in contaminated and/or rapidly changing
environment
The first task is herein illustrated by the application of the MHS in order to support the
biotechnological processes aimed at the production of carboxylic acids. The second one
concerns the properties of functionalised polymers when applied as macroionophores in the
liquid membrane systems.
3. THE APPLICATION OF MHS FOR PERTRACTION AND SEPARATION
OF CARBOXYLIC ACIDS [7,8]
Generally, liquid-liquid extraction remains a basic method for carboxylic acids
separation from aqueous media involving the fermentation broths. This method has been
recognized to be very effective but somewhat exhibiting many disadvantages because of
toxicity of solvents and other operating reagents in respect to bacteria, yeast, or fungi cultures.
The problem can be partly solved by various membrane methods such as: ultrafiltration,
reverse osmosis, reverse osmosis and nanofiltration, membrane electrodialysis, dialysis
through charged ionic membranes, membrane extraction, and liquid membrane pertraction.
On the other hand, a number of carboxylic acids, such as citric, lactic, acetic, and propionic
acid, are synthesized for the application in many industrial processes appearing in food or
pharmaceutical industry. These applications require healthy natural reagents, uncontaminated
by industrial reagents. This is possible when using carboxylic acids or other compounds
received after or during one of fermentation procedures. As an example, the fermentation of
sugars, in the presence of many Propionibacteria species, leads to a mixture of propionic acid
and acetic acid as valuable components and by-products for further usage. The recovery of
these acids and their separation can be further carried out using the MHS composed of three
membranes, i.e. two strongly basic polymer membranes (aminated polyelectrolyte) separated
by a hydrophobic liquid membrane operating according to the solution-diffusion mechanism.
Some typical results, presented in Fig.1, indicate that it is possible to separate PA from AA in
artificial systems with the effectivity dependent on solvent hydrophobicity, i.e. the
preferential transport of PA with selectivity coefficients ranging from 2.6 to 3.6 is observed.
The practical objective of this study is to develop an efficient extraction system for an
extractive fermentation process. Therefore, the MHS composed of the AFN-7 membranes and
either hexane or decane was used in order to test the pertraction of PA and AA from the
propionic fermentation broth. The purpose of this tentative experiment was to test the longtime operation of the MHS in contact with the fermentation mixture, and to measure fluxes
and separation coefficients as dependent on pH of the feed and the mode of preparation. Some
experimental results corresponding to native and pretreated broth are presented in Fig. 2. A
significant result deals with a long-time operation of the MHS in contact with the native
fermentation broth. The system worked for 600 h with unchanged selectivity and living
biomass indicating the pertraction of carboxylic acids with the use of the MHS to be a stable
process enabling a long time performance. The MHS components do not influence the ability
of microorganisms to produce carboxylic acids even in a very prolonged experiment
(provided the membrane system components are stable in time).
Fig.1. Dependence of fluxes on feed
concentration in the system composed
of the AFN-7 membranes and heptane,
octane, decane, cyclohexane, toluene
or octanol as a bulk liquid membrane:
() PA, () AA, [from ref.7]
Fig.2. Pertraction of propionic (, PA) and acetic acid
(, AA) from fermentation broths in a multimembrane
hybrid system: (A) feed: CPA=0.222 M, CAA=0.145 M,
pH=5.7; LM: hexane; stripping solution: 0.1 M NaOH, (B)
feed: CPA=0.114 M, CAA=0.060 M, pH=5.7; LM: decane;
stripping solution: water, (C) feed: CPA=0.09 M, CAA=0.05 M,
pH=4.0; LM: decane; stripping solution: water, (D) feed:
CPA=0.08 M, CAA=0.03 M, pH=2.2; LM: decane; stripping
solution: water. [from ref. 7]
Adding a selective carrier of the stronger acid can further modify the liquid membrane
properties. Thus, the MHS containing tri-n-butylphosphate (TBP) or tri-n-octylphosphine
oxide (TOPO) dissolved in the organic solvent of LM has been studied [8]. The reported
results show that in the presence of TOPO or TBP, the MHS selectivity towards acetic acid,
represented by the selectivity coefficient defined as the ratio of respective permeability
coefficients PAA /PPA increases with the carrier concentration.
4. NEW COMPONENTS [9-11]
Some frontier studies were carried out to evaluate soluble macromolecular compounds as
active components of liquid membranes. They are expected, and sometimes proved, to
transport metal cations, organic, and gaseous substances selectively. Among them,
poly(ethylene glycol)s and poly(propylene glycols) were used for preparing LMs and then
transporting various cations according to the co-transport mechanism. It is worth noticing that
poly(ethylene glycol)s exhibit an extraction ability towards many salts of uni- and divalent
metals comparable to crown ethers. On the other hand, such a liquid membrane can be
regarded as a dissolved polymer membrane. Thus, its hybrid nature is caused by the ability of
macroionophore to interact cooperatively with cations, or organic molecules similarly to
soluble polymers and to diffuse as low molecular carriers. However, the balance of
hydrophilic and hydrophobic components in the studied compounds and their amphiphilic
character causes or can cause water uptake by a liquid membrane. To show how to overcome
this problem, some model studies with simple poly(ethylene glycol), poly(propylene glycol),
their ionic derivatives, and star shaped polymers of similar composition as components of
liquid membrane phase were carried in the hybrid membrane system assisted by the
pervaporation process. The selected results concerning the pertraction and pervaporation with
the application of poly(propylene glycol) and its acidic poly phosphates are presented in
Fig.3. The separation curves reveal good separation properties of these macroionophores
against zinc or potassium cations.
5
separation coefficient
separation coefficient
4
3
A
2
1
0
4
3
B
2
1
0
0
10 20 30 40 50 60 70 80
0
20
40
Time [h]
6
80
100
120
60
70
7
separation coefficient
separation coefficient
60
Time [h]
5
4
3
C
2
1
0
6
5
4
D
3
2
1
0
0
10
20
30
40
50
60
0
10
20
Time [h]
30
40
50
Time [h]
Fig.3. Separation of cations in hybrid membrane system with macroionophores: () Zn, () Cu, () Ca, ()
Mg, () K, () Na:. A: HO-(CH2CH(CH3)O)n-OH (Mn 400 g/mol, n  7, B: HO-(CH2CH(CH3)O)n-OH (Mn
2000, n  34), C: -[(HO)P(O)-O-(CH2CH(CH3)O)n-]m (Mn 0000, n  7, m  20,8), D: -[(HO)P(O)-O(CH2CH(CH3)O)n-]m (Mn 10000, n  34, m  4,8).
5. CONCLUSION
Advanced membrane systems can be designed as an inspiration from the composition
of natural membrane systems in the framework of biomimetic membrane chemistry. Their
operation is based on coupling the specific mechanism of pertraction as independent of their
physical state (polymer, liquid). In this way also some new materials providing safe use of
membrane systems can be effectively exploited in contact with feeds requiring high purity
(foods, pharmaceutics, drinking water, etc.). These systems can be applied to remove harmful
substances from aqueous solutions or to be applied as technology supporting fermentation
processes by eliminating toxic effect of liquid membrane components on microorganisms.
6. REFERENCES
[1] Wódzki R., Polimery (Warsaw), 41, 426, (1996).
[2] Wódzki R., Szczepański P., Chem. Papers, 54, 430, (2000).
[3] Kedem O., Bromberg L., J. Membrane Sci., 78, 255, (1993).
[4] Isono Y., Fukushima K., Kawakatsu T., Nakajima M., J. Membrane Sci., 105, 293, (1995).
[5] Wódzki R., Sionkowski G., Sep. Sci. Technol., 30, 2763, (1995).
[6] Wódzki R., Szczepański P., J. Membrane Sci., 197, 297, (2002).
[7] Wódzki R., Nowaczyk J., Kujawski M., Sep. Purif. Technol., 21, 39, (2000).
[8] Wódzki R., Nowaczyk J., Sep. Purif. Technol., 26, 207, (2002).
[9] Wódzki R., Świątkowski M., Łapienis G., Macromol. Chem. Phys., 202, 145, (2001).
[10] Wódzki R, Świątkowski M., Kałużyński K., Pretula J., J. Appl.Polym. Sci., 84, 99, (2002).
[11] Wódzki R., Świątkowski M., Łapienis G., React. Funct. Polym., 52, 149, (2002).