ReviewExtraction and transport of metal ions and small organic compounds using polymer inclusion membranes (PIMs)
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
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Current address: School of Civil, Mining and Environmental Engineering, The University of Wollongong, NSW 2522, Australia.