1 Polymers as membranes
Membrane techniques are successfully used in the separation in the liquid and gas systems for many years. They are used in scientific researches, but also have numerous applications. One of the criteria for the success of the applied membrane technology is to choose a suitable membrane.
Generally, membranes vary according to the mechanism of separation (solubility–diffusion and sieving), chemical composition (organic and inorganic) and physical structure (symmetric and asymmetric). Depending on the mechanism by which the separation is achieved, membranes are divided into two groups: dense or porous. Operating porous membranes separation is done mechanically, like in the conventional filtration processes. In the process with dense membrane, the selectivity depends on interactions between components of feed solution and membrane material. Membranes, of course, should be stable at the operating temperatures of the process and resist chemical attack by the solution components.
Membranes can be also classified into charged and uncharged type. Ion-exchange membranes have ionic permselectivity and are classified into cation-exchange membranes (CEMs) and anion-exchange membranes (AEMs). In the AEMs, positive-charged groups are fixed; therefore cations are rejected by the positive charge and cannot permeate through the AEM. This is because AEMs are only permeable by anions. The CEMs perform the opposite way compared to anion ones. A similar behaviour, as in ion-exchange membrane, happens in ion-exchange resin. The ion-exchange resins are in the granular form and perform as adsorptive exchange of ions. During the process, the adsorptive capacity of resin is consumed and after certain time period it requires regeneration. In the process with the ion-exchange membrane, the regeneration is unnecessary and allows for continuous use for an extended period of time.
As it was mentioned above, due to the material used, membranes are divided into inorganic membrane (ceramic, glass, metal, zeolite, carbon molecular sieve) or organic membranes (polymer or liquid). The polymeric and ceramic membranes are used most commonly, wherein much of the research works and applications are based on polymer ones. In Table 1, the most important advantages and disadvantages of both types of membranes are presented [1, 2].
Among all of the advantages and disadvantages of both types of membranes, using polymeric membranes is supported mainly by economic facts (lower cost of producing) and ease of modification of their surface, which make them suitable in different separation processes. Commercial polymer membranes are formed by different kind of materials  – from fully hydrophilic polymers such as cellulose acetate (CA) to fully hydrophobic polymers such as polypropylene (PP) and polyethylene (PE) and some examples of medium properties – polyvinylidene fluoride (PVDF), polyacrylontrile (PAN) and polysulfone (PS)/polyethersulfone (PES) family.
There are some solutions that help to improve the properties of polymeric membranes. Membranes can be modified by using of additives, either as copolymers or by post-treatment, i. e. the incorporation of nanoparticles (inclusion of silver, iron, zirconium, silica, aluminium, titanium, and magnesium) into membranes. Addition of nanoparticles affects the permeability, selectivity, hydrophilicity, conductivity, mechanical strength, thermal stability, antiviral and antibacterial properties of the polymeric membranes . Moreover, surface of membrane can be treated, for example by water-soluble solvents such as acids and alcohols or with ammonia or alkyl compounds (e. g. ethylenediamene and ethanolamine) or be modified by various techniques, such as radical-, photochemical-, radiation-, redox-, and plasma-induced grafting .
One of the examples of using polymeric membranes is recovery of metal from aqueous solutions. Several methods are proposed including reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), diffusion and Donnan dialysis, electrodialysis (ED) and membrane extraction system, which have been described in detail in the following chapters. These processes are an alternative to conventional metal removal from inorganic effluent processes, such as chemical precipitation, ion exchange, solvent extraction and electrochemical removal.
1.2 Reverse osmosis
RO, besides NF, UF and microfiltration, is an example of membrane separation process where driving force is generated by pressure. Depending on the transmembrane pressure (TMP) applied and the type of membranes used in the process, particles of varying size can be separated (Figure 1).
RO is the process for removing solvent from a solution using dense membranes. In description of the transport across the membrane in RO system, the solution–diffusion model is proposed with three steps: (i) sorption from the feed solution to the membrane surface, (ii) diffusion through membrane, (iii) desorption from membrane to permeate solution. Separation in RO occurs due to the difference in solubility and diffusivity rate of dissolved species in solvent. Thus, the selection of the appropriate membrane is very important. Membranes, which are used in RO process, should be freely permeable to water and impermeable to solutes (such as salt ions). Moreover, they should be chemical and high pressure resistant and tolerant to wide ranges of pH and temperature. Generally, in RO, three major types of membranes are used: CA, polyamide and thin- film composite (TFC) with three layers – structural support (e. g. polyester, 120–150 μm thick), a microporous interlayer (e. g. polysulfonic polymer, 40 μm thick) and an ultra-thin barrier layer on the upper surface (e. g. polyamide, 0.2 μm thick).
RO is commonly used in water desalination and ultrapure water production [5, 6]. Moreover, it is proposed to the treatment of industrial wastewater from metal ions. Application of RO to separation of metal ions is shown in Table 2.
Results presented in Table 2indicate that RO sufficiently removes the metal ions from the aqueous solutions. The retention depends on the process conditions (TMP, concentration, pH, addition of chelating agent).
NF is applied to concentrate multivalent salt solutions and to fractionate salts due to the different charge densities and hydrated sizes of the ions. The mechanism of separation in NF is described both by sieving and by solution–diffusion model. Moreover, during fractionation of dilute salt mixtures by NF, the effect of Donnan force is observed . Additional phenomena can also affect membrane performance, i. e. specific adsorption, reduced dielectric permittivity and hydration . In NF separation of large and uncharged molecules larger than 200 Da, e. g. sugar, sieving effect is observed, while in ion separation – electrostatic force between membrane material and particles should be taken into account. The majority of commercially NF membranes are made of either cellulosic or aromatic polyamide material, like RO membranes. In contrast to RO process, NF membranes are porous and charged, mainly negative. The modification of polymers, in NF membranes, occurs by attachment of some ionic groups to the polymer or by coating a thin layer of e. g. sulfonated polyphenylene oxide on the surface of a porous membrane (support). The most characteristics of NF membranes are low rejection of monovalent ions, high rejection of divalent ions and higher flux compared to RO membranes.
NF is proposed for the removal of multivalent ions in water softening operations, treatment of wastewater from mineral processing, nuclear, metal finishing and metal recycling industries because of its high rejection of multivalent ions, among others metal cations  (examples in Table 3). Moreover, it can be used to separate large molecules such as dye, sugars, amino acids and their oligomers.
Based on the results presented in Table 3, it can be concluded that polymer NF membranes are capable of removing a high percentage of metal ions.
The separation mechanism of UF is sieving effect; thus porous membranes are used. This technique is applied for the removal of dissolved and colloidal material, mainly in food technology for milk concentration, recovery of proteins from cheese whey, recovery of starch, concentration of egg products and clarification of fruit juice and beer. Moreover, UF processes are used in pharmaceutical, chemical and pulp and paper industries. The most widely used materials for UF membranes are: PS and PES (chemical, thermal and mechanical stability), CA (more hydrophilic – tend to foul more difficult) and polyacrylonitrile (PAN).
In the classical UF, effectiveness of removing of the metal ions is unsatisfactory. When the pore sizes of membranes are larger than dissolved metal ions, these ions would pass easily through UF membranes. However, some results of UF application for metal separation are presented in the literature. Values of retention of Cd(II) ions in UF process with PS membrane are equal to 11 %  or 10 % and 15 % for CA and PVDF membrane, respectively . Similarly, weak retention is obtained for Cr(III) ions – 3–5 %, 15–20 % and 15–22 % for CA, PVDF and PES membranes, respectively , and for other metal ions, as: Cs(I), Sr(II), Mn(II), Co(II), Cu(II), Zn(II), Cr(III) lower than 20 % (mostly near 10 %) using polyamide (PA, TFC) and PES membranes . The effect of rejection of metal ions in classical UF could be explained by the electrostatic repulsion and the effect of steric exclusion described by Ferry’s law . It means that the membrane surface charge significantly influences the separation properties. The surface charge density of a porous membrane is related to the zeta potential of the membrane. The surface charge of all membranes changed from a positive charge to a negative charge as the pH was increased, possibly causing an increase of the membrane repulsive electrostatic force, thereby making it more influential during the membrane process. For example, the comparison of three membranes – PES, PVDF and CA – indicates that their zeta potential is different. PES membranes are neutrally charged or slightly negatively charged (zeta potential slightly below zero), PVDF membrane is more negatively charged (zeta potential from –2 to –10 mV)  and CA membrane is the most negative surface charge (zeta potential from –5 to –20 mV) . Transport of positive ions of metal through membrane with negative surface charge is easy as a result of electrostatic attraction, and consequently the retention of these ions is low. This effect was observed in the case of retention of Cr(III)  and Cd(II)  ions. The value of retention is the lowest for CA membrane, in which the transport of metal through membrane is the easiest one. On the other hand, retention of As(V) ions in the form of anions H2AsO4–, HAsO42–, AsO43– is higher for regenerated cellulose (RC) membrane (52 %) in comparison to PES one (5 %) . Much higher value for the retention of arsenic using cellulose membrane could be explained by the negatively charged surface of membrane in the experimental conditions.
Despite these examples of UF of aqueous solutions of metal ions, classical UF is found to be an ineffective method for metal recovery. To obtain high removal efficiency of metal ions, the modification methods of UF – micellar-enhanced ultrafiltration (MEUF) and polymer-enhanced ultrafiltration (PEUF) – were proposed. These two complexation-UF techniques are promising methods for the removal of metal ions and small organic compounds from the aqueous solution.
1.4.1 Micellar-enhanced ultrafiltration process
In the MEUF process, the surfactant solution at a concentration higher than the critical micelle concentration (CMC) is added to the solution containing the separated compounds, such as metal ions or low molecular compound. In these conditions, the micelles of surfactants are formed and metal ions and soluble organic compounds can be “captured” by the micelles, with a strong ability to attract through the surface of the micelles or solubilization inside the micelles. Micelles have hydrodynamic diameter significantly larger than the pore diameter of UF membrane, and therefore they are rejected by the membrane together with solubilized/bound contaminants. Permeate contains small amount of unsolubilized contaminants and monomeric form of surfactant. After the process, the surfactant can be recovered by precipitation with one or multivalent counterions .
The results presented in Table 4show that MEUF is an effective separation technique to remove metal ions from wastewater. Metal removal efficiency by MEUF depends on the type and concentrations of the metals and surfactants, pH, ionic strength, type of membrane and operation parameters (TMP, flux, temperature). To obtain the highest retentions of metal ions, surfactants of electric charge opposite to that of the ions to be removed should be used. Thus, to reject metal ions in the form of cations, the anionic surfactants, e. g. sodium dodecyl sulfate (SDS), are added. Because anionic surfactants generally have got high critical micelle concentration, in MEUF process also the mixture of anionic and nonionic surfactant (e. g. Rofam 10, Brij 35, Triton X-100, OP-10) is proposed. The introduction of nonionic surfactant to anionic one usually results in a decrease of CMC  and as a consequence less surfactants are necessary to add.
1.4.2 Polymer-enhanced UF process
In the PEUF process, the water-soluble metal-binding polymers are used. The ionic forms of metals are complexed by a macroligand and form a macromolecule, to increase their molecular weight. These macromolecules, with a size larger than the pores of the UF membrane, can be rejected by the membrane during UF . Thus, the principles of this process are the same like in MEUF. The only difference is the complexing agent – polymer instead of surfactant. A very important issue in PEUF is proper selection of water-soluble macroligands. Several polymers are proposed: biopolymer e. g. chitosan [36, 37], synthetic macroligands such as carboxyl methyl cellulose [38, 39], diethylaminoethyl cellulose , polyvinylethylenimine , polyvinyl alcohol  and poly(acrylic acid) [43, 44] for removal of metal ions from aqueous solutions. The investigations show that water-soluble polymeric ligands are powerful substances to remove metal ions from aqueous solutions and industrial wastewater in PEUF processes. Some results of metal ion separation by PEUF are presented in Table 5.
Depending on the polymer used and the type of separated metal ion, different complexes are formed in the system. For example, the Cr(III) or Ni(II) bond with carboxymethyl cellulose (CMC) by etheroxygen of the hydroxyl group (octahedral geometry), while in Cu(II) complexes, the binding sites are the oxygen of ethoxyl groups and the primary alcoholic O atom of glucopyranose rings (square planar configuration) .
Dialysis is a separation process in which a semipermeable membrane separates a feed solution and a receiving solution (dialysate, mainly water). The dialysis membrane should be permeable to selected components in the feed which should be removed and impermeable to remaining components. The driving force for dialysis is difference in the concentration of the solution across the membrane. The best known application of dialysis is its use in the artificial kidney (hemodialysis), where membrane made of cellophane-like material is applied. The typical membrane in dialysis is uncharged and the selectivity of the process is based on the size of the diffusing components. In such process, there is no possibility to separate the metal ions. However, there are specialized dialysis processes where charged membranes are used: diffusion dialysis, Donnan dialysis and ED. These membrane technologies are utilized in the recovery of the metal ions and are described in detail in the following chapters.
1.5.1 Diffusion dialysis
As was mentioned above, diffusion dialysis is a subset of dialysis in which the ion-exchange membranes are utilized. This technique is applied to separate acids from salts and bases from salts, depending on the type of membrane: AEM and CEM, respectively. Anions are transported across the AEMs, while membranes are impermeable to cations. However, hydrogen and hydroxyl ions permeate through both types of membrane – AEM and CEM. Their migration occurs by both diffusion and transfer from one water molecule to another, according to the Grotthus mechanism . The principle of separation of acid from the salts (e. g. H2SO4 and ZnSO4) is presented in Figure 2. There are H+, Zn2+ and SO42– ions in the feed solution. To maximize the driving force in the process, generally water is used as a dialysate. Sulfate anions can pass through the AEM, and zinc cations are blocked. Because the radius of H+ ion is very small and its migration velocity is fast , it can be transported across the membrane together with SO42–.
Dialysis diffusion is proposed mainly for recovery of acids and alkalis from the discharges from steel production, metal refining, electroplating, non-ferrous metal and tungsten ore smelting, aluminum etching and cation-exchange resin regeneration . Due to these applications, more attention has been placed on AEMs compared to cation-exchange ones. The properties, including structure, charged groups and thickness of the membrane, can affect the diffusion dialysis performances. Typical applications of diffusion dialysis for the separation of the metal ions from acid solutions are presented in Table 6.
1.5.2 Donnan dialysis
Donnan dialysis is a method of ionic separation using an ion-exchange membrane, in which ions move across the membrane based on Donnan equilibrium . It means that the transport of a counter-ion from the feed solution to the dialysate solution is accompanied by the transport of an equivalent amount on another counter-ion in the opposite direction until reaching Donnan equilibrium (Figure 3). The feed solution contains ions that should be removed, and the receiving solution contains the electrolyte with a relatively high concentration.
Generally, this process is used in the separation of inorganic ions, rarely of organic compounds . In contrast to the diffusion dialysis process, in the Donnan dialysis CEMs are used for the recovery of metal (examples presented in Table 7). Donnan dialysis with the AEM is applied mainly for the removal of troublesome anions from drinking water. The AEMs (SB-6407 and Neosepta® AMX) are proposed, for example, for the removal of nitrate from the aqueous phase .
In Donnan dialysis, the thickness of the membrane is not a key issue. The research study  shows that under typical hydrodynamic conditions in Donnan dialysis, the boundary layer of the dilute solution poses the major resistance to transport rather than the membrane itself. Thus, even thicker commercial ion-exchange membranes could be applied in this process. However, electrical properties of the Donann diaysis’ membrane have got impact on mass transport during the process. Membrane with low electrical resistance has low resistance to ion diffusion, and that with low electroosmotic water transport has low osmotic water transport .
ED is a membrane process where driving force is generated by an electric potential. Ionized species in the solution are transported through ion-exchange membrane, under the influence of an electric potential. Similar to diffusion dialysis and Donnnan dialysis, the AEMs and CEMs are applied in ED; here however both types of membranes are used simultaneously. The anions migrate toward the anode and the cations toward the cathode, crossing the AEMs and CEMs, respectively. Because the membranes are in the alternate stock, the ions can be separated from the solution. In some chambers, increase of concentration of ions will take place and in other chambers ions will be removed. In Figure 4, the principles of ED are presented. A special type of ED process is ED with bipolar membranes (EDBMs). Bipolar membranes consist of a CEM, an AEM and a catalytic intermediate layer to accelerate the splitting of the water into protons and hydroxide ions. This process is proposed for the separation and treatment of organic acids from fermentation broths .
ED is used for the production of drinking and process water from brackish water and seawater, treatment of industrial effluents, desalination of organic substances, recovery of useful materials from effluents and salt production . The examples of using ED in metal recovery are presented in Table 8.
1.6 Membrane extraction
Membrane extraction is an alternative process to the classical solvent extraction. In both processes, the same extractants are used. A listing of solvent extraction reagents is given in Table 9, together with the proposed mechanism of extraction .
In the liquid–liquid extraction process, organic solvents are used, which are often toxic, flammable or otherwise hazardous. As an alternative separation technology, the membrane processes are proposed: bulk liquid membranes (BLMs), emulsion liquid membranes (ELMs), supported liquid membranes (SLMs) and polymer inclusion membranes (PIMs). In the processes with SLMs, the major drawback is their poor stability; in ELMs there is a problem with emulsion breakage. BLMs find application only in theoretical studies because of the low mass transfer rates . The analysis of literature shows that the most popular are PIMs, unfortunately only for laboratory scale, and SLMs, using mainly hollow-fibre membranes, and are described in detail in the following chapters.
1.6.1 Polymer inclusion membranes
PIMs have good stability in comparison to other types of the liquid membranes. They are formed by casting solution containing a carrier (extractant, see Table 9), a plasticizer, such as o-nitrophenyl octyl ether (NPOE), and a base polymer, such as cellulose triacetate (CTA) or poly(vinyl chloride) (PVC), to form thin, flexible and stable film. The base polymer provides mechanical strength to the membrane. Plasticizer is a solvent for an ion carrier (extractant) and improves the flexibility and transport properties of the membrane. Thus, the proper choice of the plasticizer is very important. PIMs are proposed for the removal of organic species and metal ions . Examples of application of PIMs in recovery of metals ions from aqueous solutions are presented in Table 10.
1.6.2 Supported polymer membranes
SLM process has been designed to apply for the removal of valuable metal ions from various multielement liquid resources. It is one of the promising technologies for possessing the attractive features such as high selectivity and combine extraction and stripping into one single stage, however, this separation system is efficient for aqueous feed with concentration much lower than that can be used in a classical extraction process. SLM acts on nonequilibrium mass-transfer characteristics where the separation is not limited by the conditions of equilibrium (the mass-transfer reaction involves extraction of metal ions at aqueous feed–membrane interface, diffusion of the metal–ligand complex inside the membrane pores and dissociation of the metal–ligand complex at membrane–strip interface). However, the choice of the extractant has an important role on metal distribution coefficient value, membrane diffusion coefficient value and membrane thickness.
Flat sheet SLM and hollow-fiber SLMs are two commonly used SLM configurations, but only the second one has been designed for industrial application. Hollow-fiber-supported liquid membrane (HF-SLM) has advantages in a large surface area, which enable rapid feed transportation. Moreover, receiving phases are easy to recover. In this separation system, a HF module is used for extraction of metal ions. Inside the shell, there are countless thin fibres running lengthwise whose pores are impregnated with the organic liquid membrane phase. The fiber is a single nonporous material through which the solution present inside cannot be transported. The flow of all streams through the HF module is presented in Figure 5.
HF-SLM system can work not only as a single module, but also as a two-module hollow-fibre system, wherein feed phase is piped through one shell and the receiving phase through another. Possibilities of industrial applications of HF-SLM system to separate or recovery of metals ions are shown in Table 11.
An even more innovative version of HF-SLM is a pseudo-emulsion-based hollow-fiber strip dispersion (PEHFSD), wherein the stripping phase is dispersed in the organic membrane phase and a pseudo-emulsion being formed before injection into hollow-fiber module. During this separation, extraction and stripping occur simultaneously in a single hollow-fiber contactor. In PEHFSD, the solute is transported from the feed to the membrane and then to the stripping phase simultaneously. Using this method, the recovery of metal ions even from concentrated solution can proceed efficiently; however, the selectivity of such processes is low.
Table 12lists selected examples of PEHFSD applications.
2 Polymer resins
2.1 Ion-exchange resin
An ion-exchange resin, similar to ion-exchange membrane, is an insoluble polymer containing positively or negatively charged groups which reacts with the counter-ions of recovered metal. The material has a highly developed structure of pores on the surface, where the ions are trapped or released. Commercially, the most common cation exchangers are the acid-type groups, such as sulfonate ((−SO3–), strong acidic group) and carboxylate groups ((-C=O)O–, weak acidic group). Other type of cation-exchange functional units having wide diversity matching different requirements is phosphonic (-PO3H2), phosphinic (-PO2H), arsonic (-AsO3–2) and selenonic (-SeO3–) acid groups. In case of the anion-exchange resins functionality groups containing proton-accepting nitrogen and the commonly used functionality are amino groups e. g. (−N+(CH3)3) or (−N+(CH3)2CH2 CH2OH) and much softer amine (−N(CH3)2). The anion-exchange resin, besides the proton-accepting nitrogen functionality groups, can also have a strong base quaternary phosphonium (−P+R3) and tertiary sulphonium anion-exchange group (−S+R2). A limitation of these materials is the selectivity of metal separation especially in case of trace metals in the presence of significant amounts of alkali metals . Most of the selective resins are of the chelating type. This type of material contains mostly as functional groups iminodiacetic (-N(CH2COO–)2), dithiocarbamate (-R2NCS2−2), thiol (-SH), aminophosphoric (-NH2CH2PO3–2) and bis-picolylamine (Table 13). Chelating resins bind the heavy metals selectively towards the alkali and alkaline earth metals and are efficient, quantitative preconcentrating agents .
Available commercially resins are mainly formed via suspension polymerization reaction of styrene in the presence of 1,4-divinylbenzene as a cross-linked agent. As was presented in Table 13, the two types of resins can be used: gel and macroporous type. Resins as gel are a cross-linked polymers exhibiting microporosity with pore volumes typically up to 10 or 15 Å. This type of resin is dedicated mainly to sorption of organic substances from water. Macroporous ion-exchange resins are highly cross-linked and have greater porosity and surface area than those of the gel type resins. Unfortunately, because of the high cross-linkage in the matrix, the macroporous resins have greater exposure to potential oxidants than gel resins.
The incorporation of functional group to a polymer matrix requires specific synthesis conditions ; e. g. sulfonic ion is incorporated via a reaction of a polystyrene resin with a concentrated sulfuric acid, while incorporation of amine or ammonium group is combined mainly with nucleophilic substitution between chloromethyl polystyrene-divinylbenzene copolymer and corresponding amine, nucleophilic substitution between poly(4-vinylpyridine) with halogenated compounds (e. g. benzyl chloride) or by radical polymerization (e. g. synthesis of poly(4-vinylbenzyl)trimethylammonium chloride). Incorporation of phosphonic functionalizing group is prepared by an Arbuzov-type reaction between chloromethyl polystyrene-divinylbenzene copolymers and triethylphosphite, yielding the phosphonate ester, which after hydrolysis gives a phosphonic acid copolymer. Functionalization of acrylic copolymer can be carried out, for example, through an aminolysis of the ester groups in the polymer matrix.
The individual ions present in the sample are retained in varying degrees depending on their different affinity for the resin phase. In the form of a cations, metal ions such as Co(II), Cr(III) and Ni(II) can be removed either with a cation exchanger or with a chelating resin [94, 95, 96, 97, 98, 99], but in case of the anionic metal species as in case of Cr(VI) the anionic exchanger is used . The consequence of this phenomenon is the separation of desired metals ions; however, the effectiveness of the process depends on the nature and characteristics of the resin. Ion-exchange process is particularly suitable for purification of metal ions, but the total concentration of dissolved salts in solution must generally be low (<1 g/L). At high concentrations, the exchange potentials for ions diminish and there is more competition for the available exchange sites. Other limitations are their limited radiation and thermal stabilities. Under exposure of both impacts, most organic resins will degrade even at macroscopic level in Table 14.
The ion-exchange resins are suitable for the removal of metal anions or cation but mostly with too low selectivity and to improve their separation potential (especially towards alkali and alkaline earth cations), as well as the sorption capacities, metal- or metal oxide-loaded exchange resins have been studied (Table 15). In this hybrid system, metal cation (usually Fe(III), Zr(III)) or metal oxide (Fe3O4, MnO2, TiO2, ZrO2) have been immobilized on polymer matrices and subsequently studied for heavy metal ion recovery . The metal oxides have been exploited as adsorbents with inner-sphere complexation. Such interaction enables the metal ion complexation with high capacity and specific adsorption towards heavy metals such as As(III) and As(V)  in the case of metal-loaded ion-exchange resin, and metal cations have been immobilized on polymer support and subsequently studied for metal ion sorption (As(V), Cr(VI)). The requirement of a polymer matrix for loading with cations is the presence of exchangeable functional group, which enables the coordination of extractable metal ion with loaded metal [104, 105, 106]
2.2 Solvent impregnated resin
The application of reactive extraction for processing dilute aqueous solutions is rather uneconomic. More efficient technique seems to be adsorption on chelating, ion-exchange or enantioselective resins, but those materials are very expensive becuase the preparation is time consuming and difficult. Ideal seem to be linking of the solvent extraction supported a selective extractant with ion-exchange process suitable for processing of dilute solutions. The concept includes the incorporation of the extractant into hydrophobic resin, which was first proposed by Warshawsky  for metal recovery in hydrometallurgical process. In this study, a hydroxyoxime solvent-impregnated resins (SIRs) was proposed for selective copper removal. This concept of SIRs is based on the physical impregnation of a selected extractant in a porous adsorbent, but the impregnated extractant should behave as in classical reactive extraction process and simultaneously maintain a strong affinity for the polymer matrix.
2.2.1 Impregnation procedure
SIR, in which the extractant is incorporated into non-ionic macroreticular polymer structure, can be prepared according to a few types of procedures:
I. “Dry” method has been developed for extractant having strong hydrophilic centres (oximes, ethers, ketones). According to this impregnation procedure, a liquid extractant or as a mixture in polar diluent (acetone, acetonitryle, methanol) is contacted with the polymer in batch mode for few hours; however the period of time is adjusted individually to both reagents and should enable to obtain maximum incorporation of the extractant within the polymer pores. Next the diluent is removed via vacuum evaporation resulting in a polymer–extractant material. Using this method, an adsorption of the extractant on the polymeric can be also observed and that requires additional procedures of washing process.
II. “Wet” impregnation procedure has been developed for strong hydrophobic extractant. According to this procedure, an extractant together with the diluent is incorporated into the polymeric pores resulting in a polymer–extractant–diluent material. During this procedure of impregnation, the polymer support is shaken with extractant–diluent mixture. The next stage was the removal of diluent with remaining extractant.
III. Procedure with a modifier addition. According to this procedure, a diluted extractant in the presence of phase modifier is incorporated into the polymer pores. Goal of the modifier is to promote a water wettability and a simultaneously reduction of too high hydrophobicity of impregnated resin. The added modifier is typically more polar than the extractant and the most commonly used mixtures are acetone, methanol or ethanol with water.
IV. Dynamic column method, according to which the impregnated polymer is placed in a column and fully swelled by the diluent. Then, the extractant solution is fed into the column until the extractant concentration in the outlet is constant. The resulting SIR is finally washed with water. This procedure is directed to materials with analytical application. The choosing of the methods depends mainly on the chemical properties of the extractant, hydrophobicity of polymer support as well as the interaction of extractant–polymer. Both in the industrial and in laboratory scale the most often used polymer supports are a macroreticular polymer resins, mainly series Amberlite, Dovex and Lewatit. The selection of the polymer support is adjusted to the extractant properties to give the optimal balance of hydrophobic versus hydrophilic properties. Moreover, very important is to minimize losses of the extractant to the aqueous phase (e. g. by maintaining enough hydrophilicity or additional coating) and to maximize mass transfer of the solute into the resin pores and between phases, which depends on the complexing properties of the impregnated extractant. The incorporated in a macroreticular polymer structure extractant has a strong affinity towards the polymeric support, but it still behaves as extractant and the extracting properties are as in the liquid state. The immobilization of the extractant on the internal surface of the macroreticular polymer is mainly combined with forces which are responsible to “catching of extractant”–van der Waals forces, π-π interactions and capillary forces [113, 114, 115, 116]. Unfortunately, so soft polymer–extractant interactions can cause much higher loss of the extractant than that of classical reactive solvent extraction process. The stability of the resin can be improved by carrying out several impregnation cycles, longer (several hours) or addition wet drying procedure and a post-impregnation encapsulation techniques [117, 118, 119, 120]. Using the post-impregnation encapsulation process as loss extractant prevented method enables achieving a much stable material than using other stabilizing procedures. However, supplying a permeable coat around each bead of the resin is difficult and the procedure requires adjusting of the condition and type of coating materials to an impregnated resin.
More stable and selective resins can be obtained by impregnation of ion-exchange materials. For that system, charged or polar functional groups mainly provide hydrophilic character as well as interact with the extractant mainly by covalent binding. So multiple interactions can cause the possibility of strong associations between metal, impregnated extractant and resin functional groups .
The impregnated resins can also be obtained by an extractant entrapping directly in polymerization process. The procedure is more difficult especially because of the polymerization procedure, which varies from a type of the extractant, its acidity, viscosity and water-organic diluent activity, as well as, it depends on the initial concentration ratio of monomer and cross-linking agent to initiator. The synthesized beads have gained much attention presently in the metal separation technology due to, in comparison with classical SIRs, a high adsorption capacity, faster kinetics and ease of regeneration. Furthermore, the incorporation of extractant during a polymerization process is a commonly encountered method in an industrial production of SIRs such as Lewatit TP272 .
2.2.2 Metals recovery with solvent impregnated resin
Mechanism of metal ions recovery with SIR is partially identical as in case of corresponding solvent extraction systems and partially as in case of ion-exchange resin. Kamio et al. . have developed a theoretical model to describe a metal recovery with a microcapsule containing an acidic extractant. The determined kinetic constants for the complex formation reaction are almost the same as that determined for the liquid–liquid extraction system and is supported by Fick’s diffusion laws [124, 125]. It was shown that the extraction mechanism of metal ions into a microcapsule progresses through the three processes (Figure 6): I. diffusion of metal ion across the aqueous liquid film exists near the surface of a microcapsule; II. intermediated and final extracted complex formation on the surface of a microcapsule, and the last process III. diffusion of extracted complex through resin pores. The extracted complex gradually accumulates at the surface of the resin. The surface of the microcapsule then is saturated with the extracted complex.
2.2.3 Application of SIRs in metal recovery
The SIRs show to be effective sorbents for the selective recovery of rare and valuable metals ions from aqueous solutions or separation of toxic element; however, those extraction properties depend mainly on the specific properties of the incorporated extractant (see the mechanisms of extraction presented in Table 9). Commercial SIRs being cross-linked polystyrene with an extractant integrated into the resin beads during polymerization process have been evaluated mainly for the processes originally designed for the extractant e. g. separation of uranium and plutonium from mixed actinide residues [126, 127] (Table 16).
The SIRs have been shown to be effective sorbents for the selective recovery of metal ions from aqueous solutions; however, the industry applications of commercial resins are limited to the few examples. In case of synthetic SIRs prepared usually by the wet impregnation process, the studies are conducted on sorption equilibria of a wide group of transition metals, some metalloids, lanthanides and actinides. So far, the results obtained have led to new technological developments from which the most interesting examples are listed in Table 17and Table 18.
This research was supported with 03/32/DS-PB/0700 and 03/32/DS-PB/0701 grant.
This article is also available in: Tylkowski, Polymer Engineering. De Gruyter (2017), isbn 978-3-11-046828-1.
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