Review
Extraction and transport of metal ions and small organic compounds using polymer inclusion membranes (PIMs)

https://doi.org/10.1016/j.memsci.2006.03.035Get rights and content

Abstract

The stability of polymer inclusion membranes (PIMs) relative to other liquid membranes is amongst the major reasons for the recent rejuvenation of interest in carrier-mediated transport for selective separation and recovery of metal ions as well as numerous organic solutes. This is reflected by an increasing number of PIM investigations reported in the literature over the last two decades. Given the outstanding performance of PIMs compared to other types of liquid membranes particularly in terms of membrane lifetime, it has been predicted that practical industrial applications of PIMs will be realized in the near future. This review provides a comprehensive summary of the current knowledge relevant to PIMs for the extraction and transport of various metal ions and small organic solutes. PIM studies reported to date are systematically summarized and outlined accordingly to the type of carriers used, i.e. basic, acidic and chelating, neutral or solvating, and macrocyclic and macromolecular. The paper reviews the various factors that control the transport rate, selectivity and stability of PIMs. The transport phenomena observed by various authors are related to the membrane characteristics, physicochemical properties of the target solutes as well as the chemistry of the aqueous solutions making up the source and receiving phases. The results from these studies reveal an intricate relationship between the above factors. Furthermore, while the interfacial transport mechanisms in PIMs are thought to be similar to those in supported liquid membranes (SLMs), the bulk diffusion mechanisms in PIMs governing their permeability and selectivity require better understanding. This review also delineates two mathematical modeling approaches widely used in PIM literature: one uses a set of assumptions that allow the derivation of analytical solutions valid under steady-state conditions only; the other takes into account the accumulation of the target species in the membrane during the initial transport state and therefore can also be applied under non-steady-state conditions. The latter is essential when the interfacial complexation reaction kinetics is slow. It involves more complex mathematics and requires the application of numerical techniques. The studies included in this review highlight the potential of PIMs for various niche applications on a practical scale. The discussions provided, however, also emphasize the need for more fundamental research before any such practical applications of PIMs can be realized. This is specifically important for small organic compounds because to date scientific investigation involving the extraction and transport of these compounds remains limited. Transport mechanisms of small organic compounds are less well understood and are likely to be more complex than those observed with the transport of metal ions.

Introduction

In recent years, membrane-based processes have attracted considerable attention as a valuable technology for many industries. This significant gain in momentum is driven in part by spectacular advances in membrane development, wider acceptance of the technology as opposed to conventional separation processes, increased environmental awareness and most of all stricter environmental regulations and legislation. However, despite a recent market boom in all other membrane sectors including membrane filtration and electrodialysis, practical applications of liquid membranes remain largely limited. This includes bulk liquid membranes (BLMs), emulsion liquid membranes (ELMs) and supported liquid membrane (SLMs). BLMs have low interfacial surface areas and mass transfer rates while emulsion breakage is the main problem associated with ELMs. A major drawback associated with SLMs is poor stability. These factors have severely rendered liquid membranes mostly impractical for many large-scale applications [1], [2].

Nevertheless, given the essential need for metal ion recovery as well as for the extraction of numerous small organic compounds over the last two decades in hydrometallurgy, biotechnology and in the treatment of industrial wastewater, significant scientific effort has been expended to understand [3] and improve [4] the stability of liquid membranes. The number of scientific investigations devoted to this topic has been rising steadily [1]. Such dedicated works have resulted in a novel type of liquid membranes, commonly called polymer inclusion membranes (PIMs) [5], although a number of other names are also being used such as polymer liquid [6], [7], gelled liquid [8], polymeric plasticized [9], [10], [11], fixed-site carrier [12], [13], [14] or solvent polymeric [15], [16] membranes. PIMs are formed by casting a solution containing an extractant, a plasticizer and a base polymer such as cellulose triacetate (CTA) or poly(vinyl chloride) (PVC) to form a thin, flexible and stable film. The resulting self-supporting membrane can be used to selectively separate the solutes of interest in a similar fashion to that of SLMs. In several studies [8], [17], [18], PVC has been used to simply gel the liquid phase of an SLM to stabilize it within the pores of an inert support. In these cases, the PVC concentration of the membrane was much lower than that used for a self-supporting membrane.

PIMs retain most of the advantages of SLMs while exhibiting excellent stability and versatility. The lower diffusion coefficients often encountered in PIMs can be easily offset by creating a much thinner membrane in comparison to its traditional SLM counterpart. In several cases, PIMs with higher fluxes than those of SLMs have been reported [5], [19], [20]. In contrast to SLMs, it is possible to prepare a PIM with negligible carrier loss during the membrane extracting process [5], [11], [19], [20]. In addition, the amount of carrier reagent can be greatly reduced, hence creating the possibility of using more expensive extractants, which in the past could only be used for high value metals or organics. This will no doubt create a wider range of applications for PIMs. It is also noteworthy that the mechanical properties of PIMs are quite similar to those of filtration membranes. The technological advancements achieved with filtration membranes for manufacturing, module design and process configuration will be particularly useful for the large-scale practical realization of PIMs [21]. Consequently, this will enable PIM-based systems to exhibit many advantages such as ease of operation, minimum use of hazardous chemicals and flexibility in membrane composition to achieve the desired selectivity as well as separation efficiency.

It is interesting to note that PIMs have been used in chemical sensing for more than 30 years in the form of polymer membrane ion-selective electrodes (ISEs) [22]. In 1970 [23], it was demonstrated that the organic liquid of a calcium selective liquid membrane ISE could be immobilized into PVC to produce a polymer film with identical calcium sensing properties and selectivity as the organic liquid itself. Since that time there have been numerous PVC-based membranes developed for the potentiometric sensing of various cations and anions. Such membranes for use in potentiometry have also been termed “gelled liquid membranes” and “entangled liquid membranes” [22].

About the same time as this first reported use in ISEs, Bloch et al. [21] demonstrated that PVC-based membranes could also be used for metal ion separation although the requirements for the membrane characteristics were somewhat different for the two applications [21], [22]. In sensing, fast ion exchange or metal ion complexation is required at the sample solution/membrane interface to rapidly establish the interfacial electrical potential difference but there should be negligible transport of the metal containing species through the membrane within the timescale of the measurement. In separation, fast interfacial reactions are required but in this case, high diffusion coefficients of the metal containing species within the membrane are also desirable in order to achieve mass transport from the source to the receiving phase within a reasonable timeframe.

This review aims at providing a comprehensive summary of the current knowledge relevant to PIMs for the extraction of various metal ions and small organic solutes. Membrane stability, selectivity and transport rates are discussed in relation to the physicochemical properties of the base polymers, carriers and plasticizers as well as the characteristics of the target metal ions or the organic solutes. Transport mechanisms and their mathematical models are also delineated. It should be emphasized that all the research on PIMs carried out so far has been conducted on a laboratory scale and the transition of this research to a pilot or full-scale application presents a major challenge for the future.

Section snippets

Base polymers for membrane preparation

The base polymers play a crucial role in providing mechanical strength to the membranes. Despite a vast number of polymers currently used for many engineering purposes, it is surprising that PVC and CTA have been the only two major polymers used for most of the PIM investigations conducted so far. Although the feasibility of several cellulose derivatives (i.e. cellulose acetate propionate (CAP) and cellulose tributyrate (CTB)) as base polymers for PIMs has recently been studied [24], a large

Carriers

Transport in PIMs is accomplished by a carrier that is essentially a complexing agent or an ion-exchanger. The complex or ion-pair formed between the metal ion and the carrier is solubilized in the membrane and facilitates metal ion transport across the membrane. The well-known classes of solvent extraction reagents namely basic, acidic and chelating, neutral or solvating, and macrocyclic and macromolecular have all been studied in PIMs. The types of carriers used in PIM research as reported in

The role of plasticizers

The individual molecular chains in PIMs are held together by a combination of various types of attractive forces. Amongst them, van der Waals forces are abundant but are weak and non-specific, while polar interactions are much stronger but can only occur at polar centers of the molecule [44]. The latter often result in a rigid non-flexible thin film with a three-dimensional structure within its polymeric matrix [28], [44]. This three-dimensional structure rigidity is, however, unfavorable for a

Morphology

One important aspect of PIMs is the microstructure of the membrane materials, which determines the distribution of carriers in the polymer matrix and ultimately affects the membrane transport efficiency. Consequently, considerable research effort has been devoted to clarifying this issue. While a variety of surface characterization techniques have been employed in these studies, scanning electron microscopy (SEM) and atomic force microscopy (AFM) have been the most frequently used. Results

Extraction and transport studies in PIMs

To date, all research on PIMs has been conducted on a laboratory scale. For extraction studies, beaker experiments have been used where the PIM is immersed in a solution of the target species and samples of the solution are taken at various time intervals for analysis. Some extraction experiments have also been carried out in a two-compartment transport cell of a similar design to that used in a typical SLM experiment (Fig. 5). However, this type of cell is more commonly used in transport

Interfacial transport mechanisms

Both SLMs and PIMs involve the selective transport of a target solute from one aqueous solution to another via the membrane that separates them as can be seen in Fig. 5 [1]. This overall transport consists of two processes, namely the transfer of the target solute across the two interfaces and diffusion through the membrane. The former process is similar for both types of membranes, however, because PIMs are distinctively different from SLMs in their composition and morphology, the actual bulk

Mathematical modeling

The development of mathematical models to adequately describe the extraction and transport processes is fundamental for PIM investigations. Mathematical modeling is a vital tool for an in-depth understanding of the relevant physicochemical and transport processes, determining their thermodynamic and kinetic constants as well as optimizing the corresponding membrane separation systems (e.g. membrane and solution composition and system dimensions). Not surprisingly, a considerable number of PIM

The future of PIM research

One of the main goals of this review is to provide deeper insight into the factors that control the transport rate, selectivity and stability of PIMs by collating the transport phenomena observed by various authors and relating these to the membrane properties. As discussed in the various sections, a number of factors have been found to influence the performance of PIMs with the most important amongst them being: (1) the membrane composition, (2) the properties of the base polymers, the

Acknowledgements

The authors are grateful to the Australian Research Council for financial support under its Discovery Project scheme and to Professor Geoff Stevens from the Department of Chemical and Biomolecular Engineering of The University of Melbourne for stimulating discussions.

Glossary

2-NPOE
2-Nitrophenyl octyl ether
2-NPPE
2-Nitrophenyl pentyl ether
ACMs
Activated composite membranes
AFM
Atomic force microscopy
BLMs
Bulk liquid membranes
BMPP
4-Benzoyl-3-methyl-1-phenyl-5-pyrazolone
t-BuDC18C6
Di-tert-butylcyclohexano-18-crown-6
CAP
Cellulose acetate propionate
CMPO
Octyl(phenyl)-N,N-diisobutyl carbamoylmethyl phosphine oxide
CTA
Cellulose triacetate
CTB
Cellulose tributyrate
D2EHDTPA
Di(2-ethylhexyl) dithiophosphoric acid
D2EHPA
Di(2-ethylhexyl) phosphoric acid
DBBP
Dibutyl butyl phosphonate
DC18C6

References (115)

  • A. Gherrou et al.

    Fixed sites plasticized cellulose triacetate membranes containing crown ethers for silver(I), copper(II) and gold(III) ions transport

    J. Membr. Sci.

    (2004)
  • O. Arous et al.

    Comparison of carrier-facilitated silver (i) and copper (ii) ions transport mechanisms in a supported liquid membrane and in a plasticized cellulose triacetate membrane

    J. Membr. Sci.

    (2004)
  • M. Sugiura

    Coupled-ion transport through a solvent polymeric membrane

    J. Colloid Interf. Sci.

    (1981)
  • A. Legin et al.

    Solvent polymeric membranes based on tridodecylmethylammonium chloride studied by potentiometry and electrochemical impedance spectroscopy

    Anal. Chim. Acta

    (2004)
  • A.M. Neplenbroek et al.

    Supported liquid membranes: stabilization by gelation

    J. Membr. Sci.

    (1992)
  • A.M. Neplenbroek et al.

    The stability of supported liquid membranes

    Desalination

    (1990)
  • J.S. Kim et al.

    Selective transport of cesium ion in polymeric CTA membrane containing calixcrown ethers

    Talanta

    (2000)
  • J.S. Gardner et al.

    Permeability and durability effects of cellulose polymer variation in polymer inclusion membranes

    J. Membr. Sci.

    (2004)
  • J.C. Aguilar et al.

    Design, synthesis and evaluation of diazadibenzocrown ethers as Pb2+ extractants and carriers in plasticized cellulose triacetate membranes

    Talanta

    (2001)
  • G.W. Stevens et al.

    Metal ion extraction

    Curr. Opin. Colloid Interf. Sci.

    (1997)
  • J.C. Aguilar et al.

    Cd(II) and Pb(II) extraction and transport modeling in SLM and PIM systems using Kelex 100 as carrier

    J. Membr. Sci.

    (2001)
  • J. de Gyves et al.

    LIX(R)-loaded polymer inclusion membrane for copper(II) transport. 2. Optimization of the efficiency factors (permeability, selectivity, and stability) for LIX(R) 84-I

    J. Membr. Sci.

    (2006)
  • H. Matsuoka et al.

    Uphill transport of uranium across a liquid membrane

    J. Membr. Sci.

    (1980)
  • S.D. Kolev et al.

    Theoretical and experimental study of palladium(II) extraction into Aliquat 336/PVC membranes

    Anal. Chim. Acta

    (2000)
  • W.S. Gibbons et al.

    Effects of plasticizers on the mechanical properties of poly(vinyl chloride) membranes for electrodes and biosensors

    Polymer

    (1997)
  • W.S. Gibbons et al.

    Influence of plasticizer configurational changes on the mechanical properties of highly plasticized poly(vinyl chloride)

    Polymer

    (1998)
  • W.S. Gibbons et al.

    Influence of plasticizer configurational changes on the dielectric characteristics of highly plasticized poly(vinyl chloride)

    Polymer

    (1998)
  • S.P. Kusumocahyo et al.

    Development of polymer inclusion membranes based on cellulose triacetate: carrier-mediated transport of cerium(III)

    J. Membr. Sci.

    (2004)
  • Y.M. Scindia et al.

    Coupled-diffusion transport of Cr(VI) across anion-exchange membranes prepared by physical and chemical immobilization methods

    J. Membr. Sci.

    (2005)
  • M.E. Duffey et al.

    Simultaneous diffusion of ions and ion pairs across liquid membranes

    J. Membr. Sci.

    (1978)
  • C.A. Kozlowski et al.

    Applicability of liquid membranes in chromium(VI) transport with amines as ion carriers

    J. Membr. Sci.

    (2005)
  • S.D. Kolev et al.

    Mathematical modelling of membrane extraction of gold(III) from hydrochloric acid solutions

    J. Membr. Sci.

    (1997)
  • L. Wang et al.

    Chemical and morphological stability of Aliquat 336/PVC membranes in membrane extraction: a preliminary study

    Sep. Purif. Technol.

    (2005)
  • R. Tripathi et al.

    Backscattering spectrometry studies on metal ion distribution in polymer inclusion membranes

    Nucl. Instrum. Meth. Phys. Res., Sect. B

    (2003)
  • C.A. Kozlowski et al.

    Removal of chromium(VI) from aqueous solutions by polymer inclusion membranes

    Water Res.

    (2002)
  • P. Lacan et al.

    Facilitated transport of ions through fixed-site carrier membranes derived from hybrid organic–inorganic materials

    J. Membr. Sci.

    (1995)
  • J.D. Lamb et al.

    Lead(II) ion sorption and transport using polymer inclusion membranes containing tri-octylphosphine oxide

    J. Membr. Sci.

    (1997)
  • B. Wionczyk et al.

    Properties of 4-(1′-n-tridecyl)pyridine N-oxide in the extraction and polymer inclusion membrane transport of Cr(VI)

    Anal. Chim. Acta

    (2001)
  • K.C. Sole et al.

    Solvent extraction in southern Africa: an update of some recent hydrometallurgical developments

    Hydrometallurgy

    (2005)
  • I. Ivanov

    Increased current efficiency of zinc electrowinning in the presence of metal impurities by addition of organic inhibitors

    Hydrometallurgy

    (2004)
  • P.R. Danesi et al.

    Lifetime of supported liquid membranes: the influence of interfacial properties, chemical composition and water transport on the long-term stability of the membranes

    J. Membr. Sci.

    (1987)
  • M. Sugiura et al.

    Carrier-mediated transport of rare earth ions through cellulose triacetate membranes

    J. Membr. Sci.

    (1989)
  • C. Bourget et al.

    CYANEX(R) 301 binary extractant systems in cobalt/nickel recovery from acidic sulphate solutions

    Hydrometallurgy

    (2005)
  • R.-S. Juang et al.

    Extraction separation of Co(II)/Ni(II) from concentrated HCl solutions in rotating disc and hollow-fiber membrane contactors

    Sep. Purif. Technol.

    (2005)
  • M.S. El Sayed

    Uranium extraction from gattar sulfate leach liquor using Aliquat-336 in a liquid emulsion membrane process

    Hydrometallurgy

    (2003)
  • K.M. White et al.

    Mechanism of facilitated saccharide transport through plasticized cellulose triacetate membranes

    J. Membr. Sci.

    (2001)
  • I.L. Jenkins

    Solvent extraction chemistry in the atomic energy industry—a review

    Hydrometallurgy

    (1979)
  • M. Matsumoto et al.

    Separation of lactic acid using polymeric membrane containing a mobile carrier

    J. Ferment. Bioeng.

    (1998)
  • G. Salazar-Alvarez et al.

    Transport characterisation of a PIM system used for the extraction of Pb(II) using D2EHPA as carrier

    J. Membr. Sci.

    (2005)
  • M.-F. Paugam et al.

    Comparison of carrier-facilitated copper(II) ion transport mechanisms in a supported liquid membrane and in a plasticized cellulose triacetate membrane

    J. Membr. Sci.

    (1998)
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    Current address: School of Civil, Mining and Environmental Engineering, The University of Wollongong, NSW 2522, Australia.

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