Polymers in separation processes

Karolina Wieszczycka; Katarzyna Staszak

doi:10.1515/psr-2016-0127

Article Publicly Available

Polymers in separation processes

Karolina Wieszczycka and Katarzyna Staszak

Published/Copyright: May 16, 2017

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From the journal Physical Sciences Reviews Volume 2 Issue 5

Abstract

Application of polymer materials as membranes and ion-exchange resins was presented with a focus on their use for the recovery of metal ions from aqueous solutions. Several membrane techniques were described including reverse osmosis, nanofiltration, ultrafiltration, diffusion and Donnan dialysis, electrodialysis and membrane extraction system (polymer inclusion and supported membranes). Moreover, the examples of using ion-exchange resins in metal recovery were presented. The possibility of modification of the resin was discussed, including hybrid system with metal cation or metal oxide immobilized on polymer matrices or solvent impregnated resin.

Keywords: membrane techniques; ion-exchange resins; metal ion; separation

1 Polymers as membranes

1.1 Introduction

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].

Table 1:

Comparison of polymeric and ceramic membranes.

	Polymeric membrane	Ceramic membrane
Advantages	– Easy control of the pore forming mechanism;	– Chemical, mechanical and thermal stability;
	– High flexibility;	– Ability of steam sterilization and back flushing; dry storage after cleaning;
	– Low costs;	– High abrasion resistance;
	– Easy to up-scaling and down-scaling of membrane;	– High fluxes and durability;
	– Easy forming properties;	– Bacteria resistance;
	– Selectively transfer of chemical species.	– Possibility of regeneration;
Disadvantages	– Usually high hydrophobicity;	– High costs;
	– Exposure to biofouling;	– Relatively poor control in pore size distribution;
	– Low mechanical strength.	– Low flow rate.

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 [3] – 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 [1]. 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 [4].

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).

Figure 1

Comparison of process where driving force is pressure

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.

Table 2:

Examples of metal ion removal by RO.

Metal	Membrane (name, material, supplier)	Results	Ref.
CuSO₄ Cu(NO₃)₂	SE RO, TFC, Sterlitech Co	– Retention is higher than 95 %.	[7]
CuSO₄ Cu(NO₃)₂	SE RO, TFC, Sterlitech Co	– Results independent of the TMP and the kind of salt.	[7]
CuSO₄	TW30-1812-50, polyamide TFC, Filmtec	– Metal concentration has a significant influence on the membrane separation.	[8]
Cu(NO₃)₂
Cu(NO₃)₂		– The influence of the anions and TMP is observed.
Ni(NO₃)₂		– Lack of retention results.
ZnSO₄		– Lack of retention results.
CuCl₂	Multilayer TFC aromatic polyamide (ES20), Nitto ULPROM, Denko	– Retention is higher than 95 % and increases with increasing TMP and pH of feed solution.	[9]
NiCl₂		– Increasing concentration of other ions slightly decreased the rejection of metals.
Cr(NO₃)₃
wastewater from the electroplating
CuSO₄ NiSO₄	RE2012-100, polyamide TFC, CSM	– Retention is about 98.5 %, slightly higher for Cu(II).	[10]
CuSO₄ NiSO₄	RE2012-100, polyamide TFC, CSM	– Rejection increases by adding chelating agent (Na₂EDTA) up to 99.5 %.	[10]
CuSO₄ AsO₄^3–, AsO₃^3–	Polyamide TFC	– Retention is equal to 91–99 % for As(V) and 20–55 % for As(III),	[11]
CuSO₄ AsO₄^3–, AsO₃^3–	Polyamide TFC	– Rejection depends on the initial concentration and pH.	[11]
ZnCl₂ NiCl₂	AG4021FF, TFC, GE Osmonics	– Retention is equal to 99.3 % for Zn(II) and 98.9 % for Ni(II).	[12]
ZnCl₂ NiCl₂	AG4021FF, TFC, GE Osmonics	– Rejection increases by adding EDTA to 99.7 % and 99.6 %, for Zn(II) and Ni(II), respectively.	[12]
CuSO₄ CdSO₄	Polyamide	– Retention is equal to 97 % for Cu(II) and 98.5 % for Cd(II).	[13]
CuSO₄ CdSO₄	Polyamide	– The explanation of higher cadmium retention – size of the Cd²⁺ is larger than that of Cu²⁺ (the same like in [10]).	[13]

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).

1.3 Nanofiltration

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 [14]. Additional phenomena can also affect membrane performance, i. e. specific adsorption, reduced dielectric permittivity and hydration [15]. 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 [7] (examples in Table 3). Moreover, it can be used to separate large molecules such as dye, sugars, amino acids and their oligomers.

Table 3:

Examples of metal ion removal by NF.

Metal	Membrane (name, material, supplier)	Results	Ref.
CuSO₄	MPF-44, proprietary composite, Koch	– Retention higher than 95 %, independent of TMP.	[7]
Cu(NO₃)₂	MPF-44, proprietary composite, Koch	– Retention higher than 95 %, independent of TMP.	[7]
CuSO₄	Polyamide	– Retention of Cu(II) is equal to 84–96 % and that of Cd(II) is equal to 82–97 %.	[13]
CdSO₄	Polyamide		[13]
CuSO₄	GE Osmonics: Desal KH, TFC and Desal-5DK, TFC, polyamide with polysulfone support;	– Retention is equal to 96–98 %, small changes with different H₂SO₄ concentration for NF45, NF270 and Desal-5DK membranes are observed.	[14]
	DowFilmtec: NF45 and NF270, polyamide;	– MPF-34 and Desal KH, which have been described as more open NF membranes, retain less copper.
	Koch: MPF-34, proprietary composite	– Retention is equal to 69–83 % for MPF-34.
		– Retention is equal to 87–90 % for Desal KH.
ZnCl₂	Nanomax 50, polyamide arylene on a polysulfone support layer, Millipore Co.	– Retention is equal to 96–99 %, depending on the initial concentration, pH and TMP.	[15]
CuSO₄ with 0-2 M H₂SO₄	DK, TFC, GE Osmonics MPF-34, proprietary composite, Koch	– Retention up to 42 % and 80 % (feed solution with 2 M H₂SO₄) for MPF-34 and DK, respectively.	[16]
		– With increasing of acid concentration, retention of Cu(II) decreases (electromigration effects).
		– DK has a better copper retention and a higher permeate flux than MPF-34. However, MPF-34 has better chemical resistance than DK.
		– Retention of acid lower than 25 % for both membranes (good selectivity between acid and copper).
CuCl₂	Nanomax 50, polyamide arylene on a polysulfone support layer, Millipore Co	– Retention up to 55 %, depending on the initial concentration and TMP.	[17]
Pb²⁺, Cd²⁺, Cu²⁺, Cr⁶⁺ from waters containing sulfate	NF membrane, no data	– Maximum retention of Pb, Cd, Cu and Cr for synthetic samples is equal to 91, 97, 98 and 95 %, and those for real samples were 72, 53, 87 and 99 %, respectively.	[18]
Pb²⁺, Cd²⁺, Cu²⁺, Cr⁶⁺ from waters containing sulfate	NF membrane, no data	– Retention of Pb, Cd and Cu decreases with increasing pH and initial metal concentration.	[18]

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.

1.4 Ultrafiltration

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 % [19] or 10 % and 15 % for CA and PVDF membrane, respectively [20]. Similarly, weak retention is obtained for Cr(III) ions – 3–5 %, 15–20 % and 15–22 % for CA, PVDF and PES membranes, respectively [21], 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 [22]. 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 [23]. 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) [24] and CA membrane is the most negative surface charge (zeta potential from –5 to –20 mV) [25]. 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) [21] and Cd(II) [20] 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^–, HAsO₄^2–, AsO₄^3– is higher for regenerated cellulose (RC) membrane (52 %) in comparison to PES one (5 %) [26]. 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 [27].

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 [28] and as a consequence less surfactants are necessary to add.

Table 4:

Examples of metal ion removal by MEUF.

Metal surfactant	Membrane (name, material, supplier)	Results	Ref.
CdCl₂ Rofam 10 (non-ionic) SDS (anionic)	CA, PVDF, GE Osmonics	– Retention is equal to 13 % and 15 % for CA, PVDF membrane, respectively in system with Rofam 10.	[20]
		– Retention is equal to 83% and 89 % for CA, PVDF membrane, respectively in system with SDS.
		– Retention is equal to 88% and 90 % for CA, PVDF membrane, respectively in system with SDS/Rofam 10.
Cd(NO₃)₂ Triton X-100 Brij 35 SDS	PES, Tianjin Motian Membrane Engineering Technology Company	– Retention is equal to 40–70 % for SDS/Brij 35 and 50–70 % for SDS/Triton X-100, depending on the molar ratio of surfactants.	[28]
Cd(NO₃)₂ Triton X-100 Brij 35 SDS	PES, Tianjin Motian Membrane Engineering Technology Company	– The rejection of Cd(II) in MEUF is enhanced with the higher SDS concentration and moderate Cd(II) concentration.	[28]
Cd(NO₃)₂ Zn(NO₃)₂ SDS	PES, Tianjin Motian Membrane Engineering Technology Company	– Retention is equal to 92–98 %, depending on the metal and surfactant.	[29]
CdCl₂ ZnCl₂ SDS	Amicon, regenerated cellulose (RC), PL series, Millipore	– Retention is equal to 98 % for Zn(II) and 99 % for Cd(II).	[30]
CdCl₂ ZnCl₂ SDS	Amicon, regenerated cellulose (RC), PL series, Millipore	– 84 % of the initial SDS is recovered by precipitation.	[30]
Cd(NO₃)₂, Cu(NO₃)₂ Pb(NO₃)₂, Zn(NO₃)₂ cetylpyridinium chloride (CPC)	RC	– Retention for all tested metals is equal to 92–95 %.	[31]
Ni(II) SDS OP-10, nonionic monoalkylphenol polyetoxilate	OPMN-K, polyamide UPM-10, polysulfonamide UPM-20, PS, Vladipor	– Retention is equal to almost 100, 88 and 99 % for OPMN-K, UPM-20 and UPM-10, respectively, in system with SDS.	[32]
Ni(II) SDS OP-10, nonionic monoalkylphenol polyetoxilate		– Retention is equal to 20–40 % and 88–96 % for UPM-20 in system with OP-10 and SDS/OP-10.	[32]
Ni(SO₄)₂, Cu(NO₃)₂ Cd(NO₃)₂, Zn(NO₃)₂ SDS, TWEEN 80, polyoxyethylene sorbitanmonooleate	PES, Hydronautics Nitto Denko Company	– Retention is equal to 83 %, 83 %, 94 %, 98 % for Ni, Cu, Zn, Cd, respectively (total surfactant concentration=4 mM).	[33]
	PES, Hydronautics Nitto Denko Company	– Retention increases to almost 100 % with increasing of surfactant concentration.	[33]
NiCl₂, Ni(NO₃)₂, CuSO₄, CuCl₂, Mg(NO₃)₂, MgCl₂, Zn(NO₃)₂, ZnCl₂, Cd(CH₃CO₂)₂,CdCl₂, FeCl₂ RO90, nonaoxyethylene oleylether carboxylic acid	PLCC, RC, Millipore RC70PP, RC, Alfa Laval UC030 and UC100, RC, Microdyn Nadir UP010, PES, Microdyn Nadir GR81PP, PS, Alfa Laval	– Retention of metal ions is >95 %.	[34]
		– The simultaneous removal results in the following order: Fe²⁺~Cu²⁺>Cd²⁺>Zn²⁺>Ni²⁺>Mg²⁺, which is a result of the different complex binding constants.	[34]

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 [35]. 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 [40], polyvinylethylenimine [41], polyvinyl alcohol [42] 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.

Table 5:

Examples of metal ion removal by PEUF.

Metal	Membrane (name, material, supplier)	Polymer	Recovery	Ref.
Cu(II)Ni(II) Cr(III)	FUS 0181, PES, MOLSEP® Hohlfasermodule	Carboxy methyl cellulose (CMC)	– Retention is equal to 97.6, 99.5 and 99.1 % for Cu(II), Cr(III), and Ni(II) ions, respectively at c=10 mg/L.	[38]
			– Retention of Cu(II) and Cr(III) ions is high on a wide range of concentration up to 100 mg/L, while Ni(II) ions’ retention decreases to 57 %.
			– Abilities to rejection in the same accordance as complexing abilities of CMC: Cu(II)>Cr(III)≫Ni(II).
CdCl₂ CuCl₂ ZnCl₂	PTGC OMS 10, PES, Millipore	poly(acrylic acid) (PAA)	– Retention is equal to 75, 80 and 77 % for Cd(II), Cu(II) and Zn(II), respectively (TMP=3 bars) and increases with the increase of TMP and PAA concentrations.	[44]
			– The correction of pH to 5 allows to obtain retention around 80, 93 and 70 % for Cd(II), Cu(II) and Zn(II), respectively.
			– In the mixture solution, Cu(II) retention is higher than other metal ions.
Hg(NO₃)₂	HGO1, PS, GE Osmonics	Polyethyleneimine (PEI)	– Retention is equal to 98 %.	[45]
Co(NO₃)₂ Ni(NO₃)₂Cu(NO₃)₂Zn(NO₃)₂Cd(NO₃)₂ Pb(NO₃)₂	Biomax PBGC, PES, Amicon Bioseparations-Millipore Co.	Poly(vinyl sulfonic acid) (PVSA)	– Retention is equal to 20–70 % depending on pH and cleaned or fouled membrane.	[46]
CdCl₂	PTGC OMS 10, PS, Millipore	Poly(ammonium acrylate)	– Retention increases with polymer concentration and reaches 99 % values at twofold cadmium concentrations.	[47]
HgCl₂	PES, Sepro Membranes Inc.	Polyvinylamine (PVAm)	– Retention is equal to 99 %.	[48]
HgCl₂	PES, Sepro Membranes Inc.	Polyvinylamine (PVAm)	– PVAm dosage does not affect the Hg(II) rejection considerably.	[48]
CoSO₄ CuSO₄ NiCl₂ FeCl₃ Pb(NO₃)₂CdCl₂ ZnCl₂ MnCl₂	PES-10, PES, Sepro Membranes.	Polyvinylamine (PVAm)	– Retention is equal to 99 , 97 and 99 % for Pb(II), Cu(II) and Fe(III), respectively.	[49]
			– The temperature and TMP have little effects on metal rejection.
			– Metal rejection in PEUF is highly correlated to the coordination properties between the polymer and the metals.
Ni(NO₃)₂ Cu(NO₃)₂ Cd(NO₃)₂	RC, Millipore	Poly(N-vinylpyrrolidone-co-2-acrylamido-2-methylpropanesulfonate sodium) (PAMPS) poly(VP-co-AMPS) poly(vinylpyrrolidone) (PVP)	– Retention of all metals is higher for PAMPS.	[50]
			– The selectivity for Ni(II) for PVP is obtained.
			– For poly(VP-co-AMPS), selectivity for nickel ions is obtained, and the retention of Cu(II) and Cd(II) increases compared to PVP.

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) [38].

1.5 Dialysis

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 [51]. The principle of separation of acid from the salts (e. g. H₂SO₄ and ZnSO₄) is presented in Figure 2. There are H⁺, Zn²⁺ and SO₄^2– 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 [52], it can be transported across the membrane together with SO₄^2–.

Figure 2

Mechanism of separation acids from salts by diffusion dialysis

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 [53]. 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.

Table 6:

Examples of metal ion separation by diffusion dialysis.

Feed solution	Membrane (name, type, material, supplier)	Results	Ref.
Acid: H₂SO₄; ions: As^–, F^–, Cu²⁺, Zn²⁺, Fe³⁺	Neosepta^® AFX, AEM type, polystyrene-co-divinylbenzene, aminated; Astom Co S203, AEM type, chloromethylized polysulfone membrane followed by quaternary amination	– Most of the metal cation can be removed effectively, but the separation of As^– and F^– is comparatively unsatisfactory.	[52]
Acid: H₂SO₄; ions: As^–, F^–, Cu²⁺, Zn²⁺, Fe³⁺		– The dialysis of mass transfer coefficient of H₂SO₄ for AFX membrane is higher than for S203 one. However, separation of ions is better for S203 membrane.	[52]
Acids: HCl, H₂SO₄, HF/HNO₃; ions: Fe³⁺, Zn²⁺, Cu²⁺, Ni²⁺, Cr³⁺	Neosepta^® AFN, AEM type, polystyrene crosslinking and amination; Tokuyama Co	– Recovery of HCl, HNO₃ and H₂SO₄ is equal to 90, 90, and 70 %, respectively.	[54]
		– Fe, Ni, Cr and Cu in the feed waste are rejected effectively, while Zn in HCl solution leaked through the membrane.	[54]
Acid: H₂SO₄; ions: Ni²⁺, Fe³⁺	Selemion CI, AEM type, DSV Asahi Glass Co	– Recovery of H₂SO₄ increases with the concentration of acid and temperature up to 80 %.	[55]
Acid: H₂SO₄; ions: Ni²⁺, Fe³⁺	Selemion CI, AEM type, DSV Asahi Glass Co	– Rejection of Ni²⁺ and Fe³⁺ is equal to 96 % and 99 %, respectively.	[55]
Acid: H₂SO₄; ions: Cu²⁺	Neosepta^® AFN, AEM type, polystyrene crosslinking and amination; Tokuyama Co	– H₂SO₄ permeates well through the membrane used, while CuSO₄ is efficiently rejected.	[56]
		– Process is very effective at high acid concentrations and low concentrations of CuSO₄.
		– Even at the highest concentration of CuSO₄ the rejection is higher than 96 %.

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 [57]. 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.

Figure 3

Donnan dialysis process with a cation exchange membrane

Generally, this process is used in the separation of inorganic ions, rarely of organic compounds [58]. 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 [59].

Table 7:

Examples of metal ion separation by Donnan dialysis.

Feed solution	Receiver solution	Membrane (name, type, material, supplier)	Results	Ref.
Calcite (CaCO₃) gypsum (CaSO₄·2H₂O), or lime softening sludge (Ca²⁺, Mg²⁺,Na⁺, SO₄^2–)	HCl, H₂SO₄, HNO₃ or water	Nafion^® 117, CEM type, chemically stabilized perfluorosulfonic acid/PTFE copolymer in the acid (H⁺) form, Dupont Co	– Over 99 % of the hydrogen ions are exchanged in the recovery of up to 20 and 50 % of the Ca and Mg from the lime softening sludge.	[62]
	HCl, H₂SO₄, HNO₃ or water		– Recovery of Ca from synthetic feed suspensions comprised of gypsum or calcite depends on equilibration time and sweep side solution composition.	[62]
Red mud (bauxite wastes of alumina manufacture) containing Al₂O₃, Fe₂O₃, TiO₂, Na₂O, CaO, SiO₂	HCl	Strongly acidic cation-exchange polysulfone with polyester suport, CEM type	– Transport of metals is influenced by the acid concentration.	[63]
	HCl		– Flux of ions increases with acid concentration in the receiver solution.	[63]
CaCl₂, MgCl₂	NaCl	Neosepta CMX, CEM type, homogenous membranes containing sulfonic acid groups as fixed charges, Tokuyama Co Selemion CMV, CEM type, homogenous sulfonated styrene divinyl benzene copolymer membrane (PS/DVB) on a PVC backing, Asahi Glass	– Removal of the Ca²⁺ and Mg²⁺ is similar for both membranes and equal to 80–88 %, depending on stream volume ratio and NaCl concentration.	[64]
CaCl₂, MgCl₂	NaCl		– Ionic fluxes for the Selemion CMV membrane are higher by about 30 % than those for the Neosepta CMX membrane. This result significantly reduces the time of the process.	[64]
Cr(III)/Cr(VI) mixture solutions	NaCl	PP membranes incorporating poly[(ar-vinylbenzyl) trimethylammonium chloride], P(ClVBTA), poly[2-(acryloyloxy)ethyl]trimethylammonium chloride] P(ClAETA), poly(acrylic acid) P(AA), poly(2-acrylamidoglycolic acid) P(AGA), poly(glycidylmethacrylate-N-methyl-D-glucamine) P(GMA-NMG), poly(2-acrylamido-2-methyl-1-propane sulfonic acid) P(AMPS) and poly[sodium (styrene sulfonate)], P(SSNa) are modified via an ‘‘in situ’’ radical polymerization.	– The separation of Cr(III) and Cr(VI) ion mixture by the selective transport properties is obtained.	[65]
			– The Cr(III) ions are selectively separated using MP(SSNa)6 (42 %) respect to 2.5 % for Cr(VI) ions.
			– The Cr(VI) ions are selectively separated using MP(ClVBTA)6 (61 %) respect to 5.8 % for Cr(III) ions.
			– Extraction percentage is equal to 1.6–3.1 % and 2.3–2.6 % for Cr (VI) and Cr(III), respectively, using PP membrane.

In Donnan dialysis, the thickness of the membrane is not a key issue. The research study [60] 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 [61].

1.5.3 Electrodialysis

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 [66].

Figure 4

Electrodialysis process

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 [67]. The examples of using ED in metal recovery are presented in Table 8.

Table 8:

Examples of metal ion separation by electodialysis.

Feed solution	AEM (name, material, supplier)	CEM (name, material, supplier)	Results	Ref.
Pb(NO₃)₂	AR204SXR412 Arak Petrochemical Complex, Selemion AMV, poly-styrene-co-divinylbenzene, Asahi Glass	CR67,MK111, Ionics Watertown, Selemion CMV, homogenous sulfonated styrene divinyl benzene copolymer membrane on a PVC backing, Asahi Glass	– The separation percentage is equal up to 95 %.	[68]
			– With increasing of temperature and voltage, cell performance is improved.
			– The separation percentage decreased with an increasing flow rate.
CuSO₄ ZnSO₄ Pb(NO₃)₂ Cr(NO₃)₃	AR204SXR412 Arak Petrochemical Complex, Selemion AMV, poly-styrene-co-divinylbenzene, Asahi Glass	CR67,MK111, Ionics Watertown, Selemion CMV, homogenous sulfonated styrene divinyl benzene copolymer membrane on a PVC backing, Asahi Glass	– Performance of an ED cell is almost independent of the type of ions and only depends on the operating conditions (flow rate, temperature, voltage).	[69]
CuSO₄ ZnSO₄ Pb(NO₃)₂ Cr(NO₃)₃			– Maximum separation percents are equal to 66, 68, 69, 70, 61 % for Cu²⁺, Zn²⁺, Pb²⁺ and Cr³⁺.	[69]
Gold mine effluent, containing Na⁺, Cu²⁺, Zn²⁺, K⁺, Fe³⁺, Al³⁺, Au⁺, CN^–, Cl^–	No-name, Qianqiu Environmental Protection & Water Treatment Co	No-name, Qianqiu Environmental Protection & Water Treatment Co	– The highest removal is equal to 99.41 % and 99.83 % for copper and cyanide, respectively.	[70]
			– The results depend on the flow rate, initial concentration and voltage.	[70]
NiSO₄ CoSO₄	Neosepta^® AHA, poly-styrene-co-divinylbenzene, aminated; Astom Co	Nafion® 117, sulfonated perfluoropolymer, DuPont SPVDF 4Sb, sulfonated polyvinyldifluoride, which contains 22 % of styrene.	– The performance of the SPVDF 4Sb membrane is slightly better than commercial Nafion® 117.	[71]
			– A significant improvement with the use of corrugated membranes on the amounts of metal extracted is observed.
			– The separation of Ni²⁺ and Co²⁺ with complexing agent (EDTA) by ED is achieved.

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 [72].

Table 9:

Type of extractants and mechanism of complexation.

Class of extractant	Type	Examples	Mechanism
Acid extractants	Alkyl phosphoric acids	Dialkyl phosphoric acids and sulphur analogs (e. g. D2EHPA)	Acids extract metal ions by a cation-exchange mechanism viz.: M_aqⁿ⁺+(n+x)(RH)_org↔ (MR_n·xRH)_org+nH_aq⁺
	Alkylphosphonic acids	2-Ethylhexyl phosphonic acid 2-ethylhexyl ester and sulphur analogs (e. g. PC-88A, Ionquest 801),
	Alkyl phosphinic acids	Dialkyl phosphinic acids and sulphur analogs (e. g. CYANEX 272, 302 and 301)
Acid chelating extractants	Hydroxyoximes	Alpha alkaryl hydroxyoximes, β alkaryl hydroxyoximes (Acorga P50, Lix 860)	Me²⁺_aq + HL_org ↔ MeL⁺_org + H⁺_aq
			MeL⁺_org + HL_org ↔ MeL_2,org+ H⁺_aq
			MeL_2,org+ 2HL_org↔ MeL_2,org+ HL_org + H⁺_aq
	β-diketone	Lix 54	MeL_2,org+ 2HL_org↔ MeL_2,org+ HL_org + H⁺_aq
Basic extractants	Primary amines	Primene JMT, Primene 81R	The extraction is represented by an equation: RNH_2org+ H⁺ A^–_aq ↔ [RNH₃⁺A^–]_org
	Secondary amines	LA-1, LA-2.
	Tertiary amines	Various Alamines, in particular Alamine 336.
	Tertiary amines	Various Alamines, in particular Alamine 336.	The acid was extracted by amine to form salt: [RNH₃⁺A^–]_org+ B^–_aq ↔ [RNH₃⁺B^–]_org+ A_aq^–
	Quaternary amines	Aliquat 336,
Solvating extractants	Phosphoric, phosphonic and phosphinic acid esters and thio analogs	Trialkyl phosphine oxides and sulphide (TOPO, CYANEX 921, CYANEX 923) TBP, DBBP, CYANEX 471X.	Trialkyl phosphine oxides can extract both acids and metal complexes while trialkyl phosphine sulphides can extract metal complexes. Thus, their extraction chemistry is as shown, first for acids and second for metal complexes: m(HX)_aq+ n(S)_org+ xH₂O ↔ ((HX)_mS_n(H₂O)_x)_org
			MX_n + y(S)_org ↔ (MX_nS_y)_org
			HMX_n+1 + x(S)_org ↔ (HS_x)⁺(MX_n+1)_org

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 [73]. 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 [74]. Examples of application of PIMs in recovery of metals ions from aqueous solutions are presented in Table 10.

Table 10:

Examples of metal ion separation by PIMs.

Metal	Composition of PIM	Results	Ref.
CdCl₂ in saline or acidic media	Base polymer: CTA	– The transport of Cd is effective in both highly saline and highly acidic media; removal near 80 %.	[74]
	Plasticizers: NPOE, Tris-(2-ethylhexyl)phosphate (TEHP), dibutyl sebacate (99 %) (DBS)	– Because of the absence of plasticizer and low carrier content the transport of the metal is low, but at high amount of carrier, the transport of Cd is possible.
	Carrier: Aliquat 336
FeCl₂, FeCl₃	Base polymer: CTA	– Recovery factor of Fe(II) does not exceed 40 %, for	[75]
	Plasticizer: NPOE	– Fe(III) – from 60 % to almost 100 %.
	Carrier: Cyphos^®IL101, Cyphos^®IL104	– The highest initial flux of Fe(III) and permeability coefficient are obtained for the membrane containing 40 mass % Cyphos® IL 101.
CdCl₂, CuCl₂	Base polymer: CTA	– Extraction of Cd(II) is very fast and efficient (over 99 %).	[76]
	Plasticizer: NPOE
	Carrier: Cyphos^®IL101, Cyphos^®IL104	– The presence of HCl decreases the selectivity coefficient for Cd(II) over Cu(II).
K[Ag(CN)₂]Zn(NO₃)₂	Base polymer: CTA	– Silver cyanide complex can be efficiently transported from the feed aqueous solutions to the stripping phase (73 %).	[77]
	Plasticizer: Bis (2-ethylhexyl) sebacate (BEHS)
	Carrier: Aliquat 336	– Selectivity of separation Zn and Ag is obtained.

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.

Figure 5

Scheme of HF-SLM/PEHFSD system

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.

Table 11:

HF-SLM systems with their industrial applications.

Metal	Membrane (name, material, supplier)	Carrier	Results	Ref.
Hg(II)	PP, Liqui-Cel Extra-Flow module, Hoechst Celanese, USA	Trioctylamine (TOA) in toluene (2 % (v/v))	– Selective recovery of Hg(II) over As(III).	[78]
Hg(II)	PP, Liqui-Cel Extra-Flow module, Hoechst Celanese, USA	Trioctylamine (TOA) in toluene (2 % (v/v))	– Stripping solution: NaOH solution; Feed phase: HCl solution	[78]
As(III)	PP, Celgard^®x-30 240, Hoechst Celanese, USA	0.75 M (35 %, v/v) Aliquat 336 in kerosene	– As(III) reduction in produced water from the gas separation plant in the Gulf of Thailand.	[79]
As(III)	PP, Celgard^®x-30 240, Hoechst Celanese, USA	0.75 M (35 %, v/v) Aliquat 336 in kerosene	– Stripping solution: 0.5 M NaOH	[79]
U(VI)	PP, Liqui- CelX50: 2.5×8, Hoechst Celanese, USA	1 M di(2-ethylhexyl) isobutyramide in dodecane	– Selective recovery of U(VI) over Th(IV) from THOREX feed.	[80]
U(VI)	PP, Liqui- CelX50: 2.5×8, Hoechst Celanese, USA	1 M di(2-ethylhexyl) isobutyramide in dodecane	– Stripping solution: distilled water.	[80]
Au(I)	PP, Liqui-Cel 8 cm × 28 cm 5PCG-259, Hoechst Celanese, USA	Mixture of LIX79+TOPO in heptane	– Selective Au(I) recovery from cyanide solution containing also Fe(II), Cu(I), Ni(II), Ag(I) and Zn(II) (pH range of 9–10.5).	[81]
Au(I)	PP, Liqui-Cel 8 cm × 28 cm 5PCG-259, Hoechst Celanese, USA	Mixture of LIX79+TOPO in heptane	– Stripping phase: NaOH solution (pH 12–13).	[81]
V(V)	Liqui-Cel 2.5 × 8 Extra-Flow Membrana- Hoechst Celanese, USA	LIX 84I (2-hydroxy-5-nonylacetophenoneoxime)in kereosene	– V(V) recovery from a multi-metal solution containing V(V), Cu(II), Ni(II), Co(II), Zn(II), Fe(III) and Mg(II).	[82]
Pu(IV)	Liqui-Cel 2.5 × 8 Extra-Flow Membrana- Hoechst Celanese, USA	TBP in dodecane	– Separation and recovery of Pu(VI) from nuclear waste.	[83]
Pu(IV)		TBP in dodecane	– Stripping phase: 0.1 M NH₂OH·HCl in 0.3 M HNO₃.	[83]
Cr(VI)	PP, Celgard X10, Celgard USA	30 % Aliquat 336 and 30 % isodecanol in kerosene	– Cr(VI) recovery from aqueous solutions. Initial Cr(VI) = 2.88 mmol/L, reduction to 71.5 μmol/L	[84]
Cr(VI)	PP, Celgard X10, Celgard USA	30 % Aliquat 336 and 30 % isodecanol in kerosene	– Stripping phase: 1 M NaCl.	[84]

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.

Table 12:

Pseudo-emulsion based hollow-fiber strip dispersion (PEHFSD) applications in metal separation processes.

Metal	Membrane (name, material, supplier)	Carrier	Results	Ref.
Zn(II)	PP,Liqui-Cel® Extra-Flow 2.5 x 8, Celgard (USA)	Ionquest 801 (phosphonic acid (2-ethylhexyl)-mono(2-ethylhexyl)ester) in ShellSol D70a	– Zn(II) recovery over Ca(II) from sulphate solution at pH of 3. Initial Zn(II) concentration 0.1–1.0 g/L.	[85]
Zn(II)	PP,Liqui-Cel® Extra-Flow 2.5 x 8, Celgard (USA)		– Pseudo-emulsion: 10 % Ionquest 801 in ShellSol D70 and 150 g/L H₂SO₄ or 90 g/L Zn(II) + 150 g/L H₂SO₄.	[85]
Zn(II)	PP,Liqui-Cel® Extra-Flow 2.5 x 8, Celgard (USA)	TBP in ShellSol D70 or 3-pyridine ketoxime in toluene with decanol (9:1 v/v)	– Zn(II) recovery from chloride solution (HCl or NaCl).	[86]
Zn(II)	PP,Liqui-Cel® Extra-Flow 2.5 x 8, Celgard (USA)		– Pseudo-emulsion: TBP in ShellSol D70 and water or 0.1 M 3PC10 in toluene-decanol (9:1 v/v) and 5 % Na₂SO₄ solution.	[86]
Cr(VI)	PP, Liqui-Cel, 8 × 28 cm 5PCG-259, Hoechst Celanese, Charlotte (USA)	0.36 M Cyanex 923 in keroseneb	– Cr(VI) recovery from HCl solution with simultaneously reduction to Cr(III).	[87]
Cr(VI)		0.36 M Cyanex 923 in keroseneb	– Stripping solution: hydrazine sulphate.	[87]
Co(II)	PP, Liqui-Cel 2.5x8 5PCG-354, Hoechst Celanese, Charlotte (USA)	0.64 M DP-8R (di(2-ethylhexyl) phosphoric acid) in Exxsol D100c	– Efficient recovery of Co(II) from sulphate solution at pH of 5. Selectivity over Li ions is around 25.	[88]
Co(II)			– Pseudo-emulsion: 0.64 M DP-8R in Exxsol D100 and 0.1 M H₂SO₄.	[88]
Au(I)	PP, Liqui-cel, 8×28 cm 5PCG-259, Hoechst Celanese, Charlotte (USA)	5–10 % LIX79 in heptane	– Selective Au(I) recovery from cyanide solution. Separation over Fe(II), Cu(I), Ni(II), Ag(I) and Zn(II).	[89]
Au(I)		5–10 % LIX79 in heptane	– Pseudo-emulsion: 12 %LIX-79/heptane and 0.2 M NaOH.	[89]
Cu(II)	PP,Liqui-Cel® Extra-Flow 2.5x8, Celgard (USA)	5–10 % Acorga M5640 in ShellSol D70	– Recovery of Cu(II) from sulphate medium. Initial Cu(II) concentration 0.1 or 1.0 g/L. pH =1.4.	[90]
Cu(II)	PP,Liqui-Cel® Extra-Flow 2.5x8, Celgard (USA)	5–10 % Acorga M5640 in ShellSol D70	– Pseudo-emulsion: 2.5–10 % (v/v) Acorga M5640 and 180 g/L H₂SO₄	[90]

Notes: ShellSol D70 consists predominantly of C11–C14 paraffins and naphthenes;
Kerosene is a mixture of paraffins and naphthenes (fractional distillation 150–275 °C);
Exxsol D100 consists predominantly of C12–C15 paraffins, naphthenes and below 2 % aromatic compounds

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 [91]. 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 [92].

Table 13:

Physicochemical properties of commercial ion-exchange resins.

Resin	Structure	Polymer	Functional group	Ionic form	Bead size [mm]	Total capacity [meq/g]
Cation-exchange resins
Lewatit™ S 100	G	Polystyrene	-SO₃^–	Na	0.6–0.7	2.0
Amberlite™ IRN77	G	Polystyrene	-SO₃^–	H	0.6–0.7	1.9
Dowex™ 88	M	Polystyrene	-SO₃^–	Na	0.4–1.2	1.8
Amberlite™ IRP-64	M	Polyacrylic	(-C=O)O^–	H	0.04–0.15	10
Amberlite™ IRC-50	M	Polyacrylic	(-C=O)O^–	H	0.3–1.2	10
Anion-exchange resins
Dowex™	G	Polystyrene	–N⁺(CH₃)₃	Cl	0.4–0.6	4.0
Marathon™ A	G	Polystyrene	–N⁺(CH₃)₃	Cl	0.4–0.6	4.0
Amberlite™ IRA900	M	Polystyrene	– N⁺(CH₃)₃	Cl	0.7–0.8	4.2
Lewatit M™ 610	G	Polystyrene	–N⁺(CH₃)₂CH₂CH₂OH	Cl	0.6–0.7	1.3
Dowex™ 22	M	Polystyrene	–N⁺(CH₃)₂CH₂CH₂OH	Cl	0.4–0.8	-
Lewatit™ MP 62	M	Polystyrene	–N(CH₃)₂	-	0.3–1.3	1.7
Chelating resins
Dowex™ M4195	M	Polystyrene	bis-picolylamine	-	0.30–0.84	35–42*
Amberlite™ IRA743	M	Polystyrene	N-methylglucamine	-	0.5–0.7	0.7
Amberlite™ IRC 748	M	Polystyrene	-NH(CH₂COONa)₂	Na	0.5–0.7	1.4
Purolite™ S957	M	Polystyrene	-SO₃^–	H	<0.425	18*
Purolite™ S957	M	Polystyrene	-PO₃^–2	H	<0.425	18*
Duolite™ GT-73	M	Polystyrene	-SH	H	0.4–0.8	3.9
Diphonix™	G	Polystyrene	-COOH	H	0.3–0.15	5.3
Diphonix™	G	Polystyrene	-PO₃H₂	H	0.3–0.15	5.3
Duolite™ C467	M	Polystyrene	-NH₂CH₂PO₃^–2	Na	0.4–1.0	3.5
Purolite™ S920	M	Polystyrene	-SC(NH₂)₂	H	0.3–1.2	200*

G = gel (or microporous); M = macroporous (or macroreticular); * g/L

g of metal/L

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 [93]; 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 [100]. 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.

Table 14:

Currently marketed ion-exchange resins with examples of their application.

Product name	Polymer	Functional group	Application	Ref.
Amberlite™ IRN77	Polystyrene	-SO₃H	Co(II), Cr(III), Ni(II) recovery from synthetic nuclear power plant coolant water.	[94]
Amberlite™ IRN77	Polystyrene	-SO₃H	Maximum adsorption capacities at pH 2.75: 1 g/g (Co(II)), 4 g/g (Cr(III)) and 15 g/g (Ni(II)).	[94]
Lewatit™ S 100 Chelex-100	Polystyrene	-SO₃Na -NH(CH₂COONa)₂	Cr(III) recovery from aqueous solutions. Maximum ion-exchange capacities at pH of 3.5 and 4.5: 0.39 mmol/g (Lewatit S 100) and 0.29 mmol/g (Chelex-100).	[95]
Amberlite™ IRC 748	Polystyrene	-NH(CH₂COONa)₂	Ni(II) recovery from laterite leach tailings (after Co(II) extraction). Maximum ion-exchange capacities: 97.6 mg Ni/g at pH of 4 with low recovery Mg, Na and Fe(III).	[96]
Amberlite™ IRC 748	Polystyrene	-NH(CH₂COONa)₂	Ca(II) and Mg(II) recovery from potassium chromate solution. Maximum ion-exchange capacities: 7.21 mg/g (Ca(II)) 27.70 mg/g (Mg(II)).	[97]
Diphonix®	Polystyrene	-COOH -PO₃H₂	Separation Zr(III) and Hf(III) from acidic solution. Separation condition: 0.5 M H₂SO₄ at 22 °C with a linear flow rate < 7.5 cm/h.	[98]
Duolite™ GT-73	Polystyrene	-SH	Hg(II) removal from waste water generated at The Integrated Defense Melter System (IDMS), located at the Savannah River Site. Initial Hg concentration 0.2–50 μg/L other ions (traces) AI, B, Ca, Li, Mg, Mn, Mo, P, Si, W and Zn. Maximum ion-exchange capacities: 0.1 Mg/g.	[99]
Lewatit™ MP 62 Lewatit™ M 610	Polystyrene	–N(CH₃)₂ –N⁺(CH₃)₂CH₂CH₂OH	Cr(VI) recovery from aqueous solutions. Maximum ion-exchange capacities at pH of 5: 27 mg/g (Lewatit™M 610) 17.1 mg/g (Lewatit™MP 62).	[100]
Dowex™ M-4195	Polystyrene	- bis-picolylamine	Cu(II), Co(II), Ni(II) and Pb(II) preconcen-tration from chloride solution. Maximum adsorption capacities at pH 2 were Cu(II) 0.043 mol/L, Ni(II) 0.045 mol/L, Co(II) 0.045 mol/L, Pb(II) 0.010 mol/L desorption condition: solution at pH of 1.4.	[101]

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 [102]. 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) [103] 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]

Table 15:

Examples of metal ion recovery using hybrid ion-exchange resins.

Product name	Polymer	Functional group	Application	Ref.
Lewatit™ FO 36	Polystyrene	FeO(OH)	Pb(II) recovery from aqueous phase at different pH. Maximum adsorption capacities: 62.5 mmol/g at pH 7.	[107]
ArsenXnp	Polystyrene	-N⁺(CH₃)₃ Fe₂O₃	As(III) and As(V) removal from sulfate, fluoride, bicarbonate, and chloride solution.	[108]
ArsenXnp	Polystyrene	-N⁺(CH₃)₃ Fe₂O₃	Maximum adsorption capacities: 6.9 mg/g of regenerated resin. Desorption using a warm caustic solution.	[108]
D-001 (HFO-001)	Polystyrene	-SO₃Na Fe₂O₃	Efficient removal of Cd(II), Pb(II) and Cu(II) metals from water and wastewater. Maximum adsorption capacities: 1.6 mmol Pb(II)/g and 1.4 mmol Cu(II) and Cd(II)/g resin. Desorption solution: HCl–NaCl solution (pH 3).	[109]
D-001 (ZrP-001)	Polystyrene	-SO₃Na ZrP (zirconium phosphate)	Selective removal of Pb(II) in the presence of Ca(II). Maximum adsorption capacities: 0.4 g Pb(II)/g resin (total capacity: 0.79 meq/g). Desorption solution: 2 % HNO₃	[110]
Dowex 1×8–400	Polystyrene	-N⁺(CH₃)₃ MnO₂	As(III) and As(V) removal from sulfate, fluoride, bicarbonate, and chloride solution.	[111]
Dowex 1×8–400	Polystyrene	-N⁺(CH₃)₃ MnO₂	Maximum adsorption capacities: 6.9 mg/g of regenerated resin. Desorption using a warm caustic solution.	[111]

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 [112] 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 [121].

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 [122].

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. [123]. 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.

Figure 6

Extraction mechanism of metal ions into a microcapsule

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).

Table 16:

Commercial resins being cross-linked polystyrene containing extractant incorporated into resin beads during polymerization process.

Product name	Matrix	Extractant	Application	Ref.
Lewatit™ OC 1023	Polystyrene	TBP	U(VI) separation from a plutonium–uranium solution (5 M HNO₃ -0.1 M Fe(SO₃NH₂)₂) residual plutonium was removed by washing with 0.35- HNO₃ -0.1 M Fe(SO₃NH₂)₂ solution. Extraction capacities 10 g/g U 25 g/L Pu. Uranium stripping solution: 0.1 M Fe(SO₃NH₂)₂.	[127]
Lewatit™ VP OC 1026	Polystyrene	D2EHPA	Zn(II) separation from a nickel electrolyte	[128, 129]
			prior to electrowinning (Vale’s Port Colbourne (Canada) refinery). Metal concentration 10–50 mg/ml Zn, 50–70 mg/L Ni and 0.1–0.3 g/L Co, Selectivity at pH below 3. Extraction capacities 17 mg/g (Zn). 1 M H₂SO₄
			Zn(II) separation from electroplating effluent containing Cr(VI). Initial metals concentration 0.52 mg/ml Zn and 0.017 mg/ml Cr, Selectivity at pH of 6.2. Extraction capacities 58 mg/g. Stripping solution: 5 % H₂SO₄ solution.
Lewatit™ TP 272 Lewatit™ TP807ʹ84	Polystyrene	Cyanex 272	Co(II) selectively extraction over Ni(II) from sulfate media. Initial Co(II) concentration 59 mg/L, Selectivity at pH range of 4.8–6.0. Extraction capacities 18 mg/g. Stripping solution: H₂SO₄ solution (pH below 2).	[130, 131]
TVEX-TBP	Polystyrene	TBP	Sc(III) extraction from hydrochloric solution. Initial composition 0.08–0.1 Sc, 8 mol/L HCl. Extraction capacities 0.8 mg/g (mol/L TBP). Stripping solution: water.	[132]
TVEX-TBP	Polystyrene	TBP	Zr(III) and Hf(III) selective extraction from HNO₃ solution. Extraction capacities 0.7 mol Zr/L TBP and 0.3 mol HF/L of TBP. Resin composition TVEX-65 % TBP. Stripping solution: 30 g/L HNO₃.	[132]
TVEX-TBPHDEHP	Polystyrene	TBP:HDEHP (1:1)	Separation of Bk(IV), Cf(III) and Eu(III) from Am(III) and Cm(III). 95 % of Am separation over Cm (below 1 %) from 0.2 M HNO₃ during one cycle.	[133]
TVEX-DIOMP	Polystyrene	DIOMP (bis-isooctyl methyl phosphonate)	Scandium recovery from solutions containing above 4 M HCl. Extraction capacities: 1.4 mol/L of DIOMP (8 M HCl). Stripping solution: diluted HCl.	[134]

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.

Table 17:

Synthetic resins being cross-linked polystyrene containing acidic extractant incorporated into resin beads during impregnation process.

Commercial name of polymer support	Matrix	Extractant	Application	Ref.
Amberlite™ XAD4	Polystyrene	D2EHPA	Selective Separation of Pb(II) and Cu(II). Resin composition: 30 wt. % of D2EHPA. Initial metals concentration: 0.01 mol/L, pH=2 Metals were separated in 6 batches of contacts.	[135]
Amberlite™ XAD4	Polystyrene	D2EHPA	Separation of Zn(II) and Cu(II) from aqueous sulfate Solutions Resin composition: 40 wt. % of D2EHPA. Initial metal concentration: 1.58 mol/L, pH ranged 1.05–3.34, SO₄^2–concentration equal to 0.5 mol/L. Separation can be obtained in either batch or column operation.	[136]
Amberlite™ XAD-1180	Polystyrene	D2EHPA	Extraction of Bi(III) from nitrate medium. Resin composition: 90 wt. % of D2EHPA. Extraction capacities 490.7 mg/g. Stripping solution: concentrated HCl.	[137]
Amberlite™ XAD7	Polyacrylate	D2EHPA-Fe	Extraction of As(V) from aqueous solution. Resin composition: 30 wt.% of D2EHPA-Fe. pH= 9. Extraction capacities 19 μg/g.	[138]
Amberlite™ XAD2, XAD4	Polystyrene	PC-88A	Extraction of La(III), Sm(III), Tb(III) and Yb(III) from chloride solution. Resin composition: 20 or 50 wt.% of PC-88A. Initial metals concentration: 0.5 mmol/L, pH= 3.5.	[139]
Amberlite™ XAD7	Polyacrylate	PC-88A		[139]
Amberlite™ XAD7	Polyacrylate	Cyanex 301	Extraction of Cd(II) from phosphoric acid (diluted and concentrated). Initial Cd(II) concentration 10 mg/L. Resin composition: 5–14 wt.%. Extraction capacities 11 mg/g. Stripping solution: 5 M HCl.	[140]
Amberlite™ XAD7	Polyacrylate	Cyanex 302	Extraction of Cd(II) and Cu(II) from 40 % phosphoric acid.	[141]
Amberlite™ XAD7	Polyacrylate	Cyanex 302	Initial metals concentration 50 mg/L. Resin composition: 50 wt.% of Cyanex 302. Extraction capacities 27–34 mg/g. Stripping solution: 4 M HCl.	[141]
Amberlite™ XAD2	Polystyrene	Cyanex 272 and Cyanex 302	Two fixed-bed columns (XAD-2 – Cyanex 272 (50 wt.%) and XAD-2 Cyanex 302 (15 wt.%)) were successfully used for separation and recovery of from Fe(III), Cd(II) and Ni(II) multi-component solutions. Initial metals concentration: 10 mg/L, pH= 2 and 8. Stripping solution: 3 M HCl.	[142]

Table 18:

Synthetic resins being cross-linked polystyrene containing basic extractant incorporated into resin beads during impregnation process.

Commercial name of polymer support	Matrix	Extractant	Application	Ref.
Amberlite™ XAD4	Polystyrene	Trioctylamine (TOA)	Extraction of Au(III), Pt(II) and Pd(II) from 1 M HCl. Initial metal concentration 0.05–0.5 mg/L. Resin composition: 3.5 wt.% of TOA. Extraction capacities 50 μg/g. Stripping solution: 11.9 g/l of N-(dithiocarboxyl) sarcosine diammonium salt (DTCS) in 50 % ethanol.	[143]
Amberlite™ XAD4	Polystyrene	Aliquat 336	Separation of Re(III) from Rd(III) in HNO₃ solution.	[144]
Amberlite™ XAD4	Polystyrene	Aliquat 336	Initial metal concentration 0.1 g/L. Resin composition: 40 wt.% of Aliquat 336. Extraction capacities of Re 2.0 meq/g.	[144]
Diaion™ HP-20 Diaion™ HP-2 MG.	Polystyrene Polyacrylate	Aliquat 336	Cr(III) extraction from aqueous solution at pH 3 and higher.	[145]
Diaion™ HP-20 Diaion™ HP-2 MG.	Polystyrene Polyacrylate	Aliquat 336	Initial metal concentration 10 and 20 mg/L. Resin composition: 33 and 50 wt.% of Aliquat 336. Extraction capacities 38(HP-20) ad 40(HP-2 MG) mg/g. Stripping solution: 0.1 M NaOH–0.1 M NaCl.	[145]
Amberlite™ XAD2	Polystyrene	Aliquat 336	Extraction of Pd(II) from HCl solution. Initial metal concentration 0.47 mmol/L. Resin composition: 30–60 wt.% of Aliquat 336. Extraction capacities 64 mg/g (60 % resin, 1MHCl).	[146]
Amberlite™ XAD7	Polyacrylate	Cyphos IL 101	Extraction of Zn(II) from HCl solution (optimum 2-4MHCl). Initial metal concentration 20 mg/L. Resin composition: above 30 wt.% of Cyphos IL 101 made process efficient. Extraction capacities 20 mg/g (40 % resin). Stripping solution: water or 0.1 M mineral acid.	[147]
Amberlite™ XAD7	Polyacrylate	Cyphos IL 101	Extraction of Cd(II) from HCl solution (optimum 2-4MHCl). Initial metal concentration 60–100 mg/L. Resin composition: 10–50 wt.% of Cyphos IL 101. Extraction capacities 40–50 mg/g (depended on Cd(II) initial concentration). Stripping solution: water or 0.1 M mineral acid.	[148]

Acknowledgments

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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Published Online: 2017-5-16

Articles in the same Issue

https://doi.org/10.1515/psr-2016-0127

Keywords for this article

membrane techniques; ion-exchange resins; metal ion; separation