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The current mini-review focuses on the use of liquid membranes in the platinum group metal (PGM/PGMs) extraction from various types of wastewaters to prevent environmental pollution; and for the metal recovery to address the scarcity of the PGMs in the industrial cycles. The bulk liquid membranes have been used to the extracted PGMs from the (acidic) aqueous media with recoveries of up to 96.3 ± 2.5% of the original PGM amount. The extraction time generally ranges from 2 to 24 hours. The bulk membrane liquid in the PGM extraction will depend on the covalent structure of the extractant, the feed phase PGM concentration and the complex of the PGM in question that is actually extracted from the aqueous environment. The advantages of this type of liquid membrane include its operational simplicity, but the disadvantages include limited possibility to improve the extraction performance of the system. Literature data are encouraging as they indicate that extraction of PGMs from mining and metal-refinery side-streams does not suffer from interference from metal contaminants that are commonly found in the mining and metal refinery side-streams, e.g. iron. Thus further research should focus on the application of ELM to extraction of PGMs from said wastewaters and major research drive should focus on the use of the Taylor-vortex column and the non-Newtonian ELMs. With the supported liquid membranes, 78-82% of the original PGM content could be recovered from model side-streams. The selectivity of the extraction for individual PGMs can be controlled by the extractant used.
 

PGMs; Demulsification; Non-Newtonian liquids; BLM; ELM; SLM; Diluent
 

A membrane is a thin, film-like structure separating two adjacent phases [1]. If the rates of mass transfer through the membrane are different among the various components of a mixture, then such a membrane can be deemed semi-permeable [1]. It allows some chemical molecules to pass through but remains impermeable to others [2]. Membranes are used to separate mixtures of various components in two different phases based on variations in the mass transfer of mixture components through the membrane [3]. Advantages of using membranes in industrial chemical processes include their conceptual simplicity, cost-effective operation and their low energy consumption [1]. If the membrane-based systems are used in liquid-liquid extraction and the semipermeable membrane is liquid in nature or can be classified as an emulsion, then this gives rise to the concept of liquid membranes (LM/LMs) [1]. The LMs can be divided into the bulk liquid membranes (BLM/BLMs) [4], the emulsion liquid membranes (ELM/ELMs) [5] and the supported liquid membranes (SLM/SLMs) [6]. Different LM configurations allow for different LM application in processes such as the platinum-group metal (PGM/PGMs) recovery from ore leachates and aqueous waste side-streams [7].

The LM systems for wastewater treatment have three main components. The first one is the source/treated wastewater (so-called the feed phase or external phase). The second one is the LM itself which is made up of an organic diluent, in the case of metal extraction contains a ligand, i.e. extractant or carrier, a surfactant and sometimes a modifier [3]. Finally, the LM systems contain the receiving (stripping) phase in which the extracted metal is recovered [1,3]. The particular LM should be immiscible with the feed phase and the extractant should have low aqueous solubility [3]. The LM technology has been used in the recovery of heavy metals [1] and PGMs [4]. Thus extensive literature exists on the subject of solvents metal extraction using the LM technology. However, to date the extraction of PGMs has not been systematically reviewed and critically evaluated.

Palladium is the most studied PGM with respect to extraction from wastewater using the LM technology. Extractants that have been used in this regard include di-2-ethylhexyl-thiophosphoric acid [8], 8-quinolone derivatives [9] and Cyanex 471 [10]. Various arrangements of the LM have been used, e.g. the ELM with LIX 984N-C as the extractant [11]. The SLMs have been used to extract various PGMs from wastewater using the following extractants: Aliquat 336 for Pt (IV) [12], Pd (II) [13] and Rh (III) [14]; and MSP-8 for Pd(II) [15]. Quantitative extraction is generally achieved within 2 to 24 hours [16]. Stripping phases generally contain HCl or HNO₃ and sometimes this is supplemented by thiourea [17]. The current mini-review focuses on the use of LMs in the PGM extraction from various types of wastewaters. The article will cover the metal removal from the mining and metal-refinery side-streams, wastewaters from battery production and other potential sources of environmental pollution; and metal recovery to address the scarcity of the PGMs in the industrial cycles.
 

The LM technology has been reported as a tool in the preparation of the PGM particles [18]. The role of surfactants in LM was also investigated in the PGM extraction from aqueous matrices [19]. Recoveries of up to 97% of Pd (II) were reported from model solutions with the initial metal concentration around 100 mg/L [20]. Carriers such as N2-substituted N1-phenylbenzamidines and N'-di-substituted N-benzoyl thiourea derivatives with thiocyanates and thiourea in the stripping agents have been applied in 1994 [6]. The thiocyanate anion has been shown to stabilise the extracted complex composition of Pd (II) in aqueous environments [21]. The extent of the effect is, however, dependent on the extractant which is used as demonstrated through the experimental results with triphenylphosphine sulphide and triisobutylphosphine sulphide [21]. The BLMs are the simplest type of LMs [1,3]. Generally, a diluent with extractant is placed between the feed phase and the stripping phase and the mass transfer through the BLM takes place by diffusion and/or ion-pairing [1,3]. After extraction, the loaded metal is stripped from the stripping phase in a separate step. The BLM applications in the extraction of PGMs from aqueous solutions are outlined below.

The BLM schematic representation can be found in Figure 1. The treated wastewater, i.e. the feed phase is layered above or below the BLM as function of the density ratio. The BLM is a solution of the extractant in the diluent. The extracted metal diffuses through the feed phase to the feed phase/BLM interface where it reacts with the extractant and a complex is formed. This complex then diffuses through the BLM to the BLM/stripping interface where the metal is released from the complex with the extractant and sequestered in the stripping. Sequestration leads to the maintenance of the metal activity in the stripping phase near zero, thus maintaining the metal gradient between the feed phase and the stripping phase. This in turn drives the extraction to completion.

Palladium followed first-order extraction kinetics during extraction using a BLM with bis(2-ethylhexyl)monothiophosphoric acid dissolved in EXXSOL D-80 at 303 K [22]. The extractant formed a dimer in the diluent and Pd (II) was extracted as a 1:2 complex with the extractant [22]. Chloroform was used as the diluent in a BLM containing thioridazine hydrochloride and oleic acid as extractants [23]. The extracted metal was Pd (II) which formed a stable complex with the thioridazine hydrochloride and oleic acid; and the BLM extracted the metal into the stripping phase (a NaNO₂ aqueous solution) [23]. After a 120 minute extraction 96.3 ± 2.5% of the initial Pd (II) amount was recovered in the stripping phase and this value was unaffected by the presence of the following cations in the source phase extractants [23]: K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Pb²⁺, Cd²⁺, Fe³⁺, Cr³⁺, and Al³⁺.

Some authors have proposed and tested the use of surfactants as extractants and emulsifiers at the same time [24]. These bi-functional surfactants have been reported to lower the required concentration of surfactant in the liquid membrane [24], thus increasing the rates of mass transfer of the metal due to decrease in the LM viscosity [3]. The metal transport through the BLM is faster if a more hydrophobic surfactant is used [24]. The BLMs in the extraction of PGMs have seen reports on the use of extractants such as crown ethers [25] and the pyrimidium surfactants have strong affinity for the metal-halide complexes of PGMs [26,27]. Trioctyl amine has been used in the extraction of molybdenum [28] and other extractants such as LiX84-I for palladium [17] from wastewaters.

Alizadeh et al. [29] reported on the use of a BLM containing hexadecyl pyrimidium bromide (HDPB) in the extraction of Pd (II) from a 0.1 M HCl solution. The BLM consisted of a 0.0002 M solution of HDPB and the stripping phase was 10 ml of 4 M HBr with 0.0002 M addition of thiourea [29]. Extraction of Pd (II) was performed from the feed phase, i.e. 0.0005 M Pd (II) solution, with varying addition of NaBr [29]. The transported species of Pd was the [Pd(Br)₄]^2- complex and the maximum rate of extraction was obtained 0.05 M NaBr concentration in the feed phase [29]. The Pd recovery in the stripping phase was a function of the HDPB concentration in the BLM with the optimum value equal to 0.0002 M of HDPB [29]. The maximum recovery ranged from 93-95% of the initial Pd(II) amount in the feed phase, if the bromide concentration in feed phase was 0.05 M or higher [29].

If the BLM concentration of HDPB increased above 0.0002 M, then aggregation of the extractant in the chloroform BLM took place. This resulted in the decrease of the number of HDPB molecules available to reaction with [Pd(Br)₄]^2- and which in turn led to a drop in the recovery of Pd (II) in the stripping phase [29]. The stripping phase had to contain 4 M HBr to maintain stability of the [Pd(Br)₄]^2- complex and the thiourea concentration had to be kept at 0.0004 M [29]. The maximum recovery of Pd (II) was achieved after continuous extraction for 110 minutes. During these experiments, the maximum recovery on average was equal to 97.9 ± 1.1% [29]. The rate-limiting step of the extraction was the release of the Pd from the carrier complex and the diffusion of said complex through the BLM; or the complexation of Pd (II) with thiourea in the stripping phase [29].

Low hydration of Pd (II) in acidic and chloride-rich media has been listed in the literature as an important factor in the extraction of the metal [30]. Khayatian and Shamsipur [31] studied the use of a crown ether, namely using NH₄⁺-dibenzyldiaza-18-crown-6 as carrier, for the extraction of [Pd(Cl)₄]^2- from 0.000088 M solution of palladium with 0.2 M NH₄Cl [31]. The stripping phase was 1M KSCN in water and the BLM was made up of 20 ml of CHCl₃ containing 0.01 M of the crown ether [31]. Presence of NH₄⁺ cation is required to facilitate the fit of [Pd(Cl)₄]^2- into the cavity of the crown ether, i.e. it increases the stability constant of the extracted palladium species [31]. The best extraction efficiency was reached for Pd if the concentration of NH₄⁺ was equal to 0.2 M [31]. The SCN-anion was the best stripping agent for Pd and the extraction efficiency was detrimentally affected by exchange of the isothiocyanate for EDTA and other stripping agents [31].

The extraction did not take place in the absence of the NH4⁺-dibenzyldiaza-18-crown-6 from the BLM [31]. The presence of dibenzyl substituents in the structure of the crown ether increased the lipophilicity of the compound and facilitated higher Pd extraction efficiencies. The extraction reached a maximum recovery of Pd in the stripping phase after 100 minutes and the average recovery of 92.2 ± 1.4% of the initial Pd amount in the feed phase [31]. Only significant interference of the [Pd(Cl)₄]^2- ion was observed with Hg²⁺ which forms similar chloro complexes as Pd under the conditions of the feed phase [31]. This will have significant influence on the extraction of PGMs from waste materials containing trace amounts of mercury, either through material origin or follow-up/environmental contamination.

Reddy et al. [16] stated that application of surfactants as extractants can be used to perform the metal extraction independently on the pH adjustment of the feed phase. A mixed-micelle BLM was prepared from trioctyl methyl ammonium chloride (TOAC) TX-100 in trichloroethylene (TCE) as diluent to perform Pt and Pd extraction from spent catalytic converters [16]. The concentrations of the components of BLM were as follows: 0.05% (w/v) TOAC and 1% (w/v) of TX-100 in TCE [16]. Solutions of Pt and Pd were prepared by dissolution of the pure metals in 10% HCl and aqua regia [16]. Extraction of metals was performed using the BLM out of the Cu/Ni matte and a spent catalytic converter with 200 mg of the solids mixed with 4 ml of aqua regia [16].

The suspensions were micro-waved for two cycles and the resulting solution was diluted to 50 ml with deionised water [16]. Five millilitres of the resulting solution was then spiked with KI (final weight fraction 0.4% w/w), 5.0% HCl and 2.5% of HNO₃ (both concentrations in w/v) and finally 0.25% of TX-100 [16]. Chemical additives were dosed into the aqueous phase to stabilise the dominant metal complexes and thus speed up extraction. The final solution was then extracted with 20 ml of 0.05% solution of TOAC in TCE [16]. The extraction efficiency was comparable to the other BLM type studies on PGMs and palladium recovery. The practical problem with the application of this system would the dependence of stability of the aqueous Pd (II) solutions depending on the pH and chemical composition of the feed phase [7].

One of the practical problems in the solvent extraction of PGMs is its slow kinetics due to limited by the interfacial area, but the BLM technology has found use in testing the potential and extraction specificity of newly synthesized extractants [4,32]. Covalent structure of the calix [4] arene group of ligands facilitates the structural arrangement that allow for selectivity of the ligand to form complexes with high stability constants with a wide variety of metals [33,34]. Akin et al. [4] examined the efficiency of three calix [4] arenes as extractants for Pd (II) from model wastewaters. During the treatment of the model treated wastewater, 10 mL of 0.001 M solution of Pd (II) with 1 M KCl [4]. On the other hand, the stripping phase consisted of 10 mL of 1 M HCl and the BLM was a solution of calix [4] arene with a concentration of 0.001 M in CH₂Cl₂ [4]. Optimum feed phase pH for extraction is 2.0, or inside the range 1.0-4.0 [4]. The optimum concentration of calixarenes inside the BLM is 0.001 M [4]. Extraction percentage increased with extraction time and reached a maximum of 82% after 24 hours [4]. Mechanism of extraction is based on the ion pair formation and complexation [4].

The facts mentioned in section 2 demonstrate the use potential of the BLM technology in the extraction of the PGMs. The efficiency will depend on the covalent structure of the extractant, the feed phase PGM concentration and the complex of the PGM in question that is actually extracted from the aqueous environment. Advantages of the BLM in PGM extraction include the operational simplicity, in line with the general properties of the LM systems. The limited nature for making changes to the design is, however, the major problem with the BLM in the PGM metal/solvent in extraction. This is addressed by the use of the ELMs and the SLMs which are described in the next two sections of this mini-review.
 

An emulsion is a dispersion consisting of at least two immiscible phases, one of which is finely subdivided and uniformly dispersed as globules (dispersed phase) throughout the other continuous liquid phase [35]. This is achieved using mechanical agitation of the two immiscible liquids. Usually one liquid in an emulsion is polar/aqueous, while the other is relatively non-polar (e.g. an organic solvent phase otherwise known as diluent). The liquid usually used is water to which water-immiscible organic solvents are added. Emulsions are thermodynamically unstable and thus a third component, the emulsifying agent (surfactant), is required to stabilize the emulsion [35,36]. In the oil-in-water emulsion (o/w), the oil (organic) phase/diluent is dispersed as globules throughout an aqueous continuous phase, while in the water in oil (w/o) emulsions; the oil/diluent serves as the continuous phase with water being the disperse phase. An ELM results from combining the concepts of the LMs with that of emulsions.

The ELMs separation technique was invented by Li [37-39]. An ELM is a three-phase dispersion system [40], with the general composition of a LM [38]. The schematic representation of the ELM globule immersed in the respective feed phase and can be modeled by the shrinking core model depicted in figure 2 [41,42]. The extracted metal moves through the feed phase towards the interface with the ELM, where it becomes complexed with the extractant on the diluent side of the feed/ELM interface. The required number of H⁺ is released from the ELM extractant into the feed phase to maintain electroneutrality of the system [1].

The metal/extractant complex then diffuses through the diluent towards the stripping phase microdroplets (see space between the circumference of the ELM globule and the dashed line, seconds from the outside of the globule of the ELM in Figure 2) [41,42]. At the diluent/stripping phase interface, the metal ion is released into the inner droplet, and the required number of H+ protons is bound to the extractant molecule to maintain electroneutrality of the diluent pat of the ELM. The closer the particular stripping phase microdroplet is towards the diluent/feed phase interface, the more likely and the faster the extracted PGM will get sequestered in it. There is always a reactive front (see the dark globules adjacent to the dashed line in Figure 2). Once the layer closest to the feed phase/diluent interface becomes depleted of protons, no more metal ions can be extracted from the complex. The complex molecule diffuses further inside the ELM globules as depicted by the open globules adjacent to the dashed line in Figure 2.

In comparison with BLM and SLM, the ELM membrane phase contains the extractant and the surfactant to stabilize the emulsion and stripping phase droplets dispersed in it [40]. During the extraction process, an ELM is then dispersed in the external/feed phase by (mechanical) agitation. ELMs have been applied in the fields of environmental and pharmaceutical engineering, hydrometallurgy and the food industry [39]. The ELM technique has been successfully applied to extraction of heavy metals, organic acids, amines and phenols from industrial, mining and metal refinery wastewaters [37,38,40]. Matrices and metals that have been treated and recovered using the ELMs technology include radioactive wastes and the rare earth metals [43]. On the other hand, up to 98% of lanthanides was recovered from model wastewater after 5 minutes of extraction, if the volume ratio of ELM to wastewater was equal to 1 to 32 [44].

For palladium extraction, an ELM with the N7301 extractant (a mixture of tertiary amines) with the alkyl chains ranging from 8 to 12 carbon atoms with the average molecular weight 400 g/mol was prepared [45]. SPAN 80, i.e. sorbitan monooleate, was used as the surfactant and kerosene as the diluent, while an acidic aqueous solution of EDTA was used as the stripping phase [45]. The system provided a 96% recovery of palladium in the stripping phase, with interference from the presence of Zn²⁺, Cu²⁺, Cd²⁺, Pb²⁺, Ni²⁺ and Fe²⁺ [45]. This observation could be explained by the complex chemistry of palladium in aqueous environments, i.e. presence in the form of negatively charged chloro-complexes [7]. Thus the extraction of Pd using the ELM technology is likely to be affected by the similar chloro-complex-forming metals, i.e. the other members of the PGMs group [7]. However, the results are encouraging for practical applications of the ELM extraction on the large scale as the efficiency of the process is unaffected by common contaminant in the metal refinery side-streams such as iron. Thus practical applications of the ELM will be dependent on the selectivity of the actual extraction protocol used (see below).

The ELM literature for the extraction of PGMs is limited to date. In comparison with BLMs and SLMs, this type of LM can be applied to existing contactors already installed and routinely used in the solvent extraction of metals in the PGM refineries. Thus in the next part of this section, the general properties of the ELMs are reviewed in detail and possibilities for extension of their practical application in PGM extraction and processing are outlined. At first, the general properties and types of ELMs are reviewed and this is followed by the individual steps in the extraction processes. Some of the outstanding research questions and less commonly reviewed types of extraction systems are presented, together with the existing and possible future applications in the PGM extraction and recovery from waste side-streams in the metal-refinery and mining operations.
 

Types of ELM systems

The ELM extraction process

The proposed solutions of membrane instability

In the SLM, the solution of extractant in the diluent is impregnated onto a solid support. The transport takes place through the pore of the support using an analogical principle as with BLM and ELMs [1]. Schematic representation of the possible SLM arrangement can be seen in Figure 4. The SLM have been applied in various studies to the extraction of PGMs from mining and metal-refinery side-streams. Chaturabul et al. [111] reported on 78-82% recovery and extraction efficiency for Pd by an SLM from a feed phase of pH=2.0 and the stripping into a 0.03 M NaNO₂ solution. The SLM contained 0.005 M thioridazine and 0.05 M oleic acid as extractants and the palladium extraction process followed the first-order kinetics [111].

At the flow rate of 100 mL/min, the extraction rate constant of the same PGM was equal to 0.0140/s, while the stripping rate constant was equal to 0.0248/s [111]. Zaghbani et al. [112] used thiacalix [4] arene as a carrier in an SLM to separate Pd and Au. The diluent was 1-(2-nitrophenoxy) octane and the stripping phase was 0.5 M solution of NaSCN with pH adjusted to 2.0 [112]. Selective extraction of PGMs from acidic aqueous solutions can be achieved by using selective extractants as demonstrated for Pd over Pt and Rh in an SLM, if benzyl (2-methoxy-3-diphenylphosphino) propyl ether is used as the extractant and decalin as the diluent [113] and by N-benzoyl-N',N'-diethylthiourea [114].

Fontas et al. [115] examined the influence of the SLM diluent on the membrane stability. Previous studies have indicated that an ideal diluent should have the following properties [116]: low aqueous solubility, high boiling point and interfacial tension at the diluent/water interface. For aromatic diluents, the literature data seems to indicate that SLMs suffer from lower stability in these solvents in comparison to their aliphatic counterparts [117,118]. The extractant used [119] and the preparation procedures of the LM in question [120] have also been shown to influence its stability. Stability of the SLMs is also a function of the pore size of the solid support with the stability of the SLM increasing with decreasing pore diameter [115]. Fontas et al. [115] studied the influence of the diluent solvent on the stability of SLMs. Cumene and 3-chlorotoluene as diluents led to higher permeability of Pd (II) through a Cyanex 471 impregnated SLM [115]. On the other hand, decaline improved the membrane stability at 24 hours of extraction/exposure [115].

Wu et al. [121] studied the transport of Pd²⁺ in the dispersion SLM which contained N, N-di (1-methylheptyl) acetamide as extractant and kerosene as diluent. These membrane components were impregnated into the polyvinylidene fluoride membrane [121]. A solution of potassium thiocyanate was used as the stripping phase [121]. The Pd²⁺ transport and extraction characteristics were studied as function of the feed-phase HCl molarity, the volumetric ratio of the diluent phase to the stripping phase and finally the stripping phase salt molarity [121]. The optimised conditions for Pd²⁺ were achieved under the following conditions: 0.10 mol/L of HCl in the feed phase, the diluent phase-to-the stripping phase ratio is 1:1 and the stripping phase is a 1.60 M potassium thiocyanate solution [121]. At the initial feed phase concentration of Pd²⁺ of 0.00005 M, 79.8% of the initial amount of palladium was recovered in the stripping phase after 130 minutes of extraction [121].

Ruhela et al. [122] reported a study on the extraction efficiency for palladium with a SLM that consisted as 0.05 M N,N,N',N'-tetra-(2-ethylhexyl) thiodiglycolamide as the extractant and n-dodecane as the diluent. The metal was dissolved in 3.0 M HNO₃ solution and the stripping phase was a solution of 0.01 M thiourea in 2.0 M nitric acid [122]. The palladium mass transfer through the SLM was examined as function of the pH of the feed phase, the SLM concentration of the extractant and the pore size distribution of the membrane support/complete SLM [122]. The diffusion coefficient of Pd²⁺ through the SLM was equal to 3.56×10^-5 cm²/s [122]. The extractant was 1000 more selective for palladium than for the interfering impurities and the loss of extractant into the feed phase limited [122]. Similar observations were reported by Panja et al. [123]. More studies are needed on the separation of the individual PGMs from their mixtures as described for the extraction of Ph, Pd and Pt from automotive catalytic converters was examined [124].
 

Advantages of the BLM in the PGM extraction originate from the operational simplicity, but they suffer from the limited nature for adaptation of the given system design in the particular PGM metal/solvent extraction. In contrast to BLMs, the ELM extraction can be conducted in the “classical” solvent contactors with cost savings of up to 40%, compared with the simple metal-solvent extraction. The up-to-date data of the ELM application in the PGM does not suffer from interference caused by the common metal contaminants found in the mining and metal refinery side-streams, e.g. iron. Thus further research should focus on the application of ELM to extraction of PGMs from said wastewaters using selective extractants such as crown ethers, which provide selectivity for a given PGM and eliminate the majority of potential interferences. At the same time, more research should be focused on the use of the Taylor-vortex column and the non-Newtonian ELMs that improve the ELM stability and extraction kinetics. For aromatic diluents, the literature data seems to indicate that SLMs suffer from lower stability in these solvents in comparison to their aliphatic counterparts. The SLM application in the PGM extraction from mining wastewater and side-streams should be further investigated. However, based on the currently available data ELMs seem to be the most practical liquid membranes for the practical application in the PGM extraction from mining-related wastewaters. The main focus should be on the optimisation of the process and the competition studies with all six PGMs.
 

The authors are grateful to the Rhodes Research Council Fund, Institute of Water Research, and the Water Commission of South Africa (Grant number K5/2011/3) for financial support and the Faculty of Pharmacy at Rhodes University and the laboratory colleagues from the Environmental Health and Biotechnology Research Group in the Faculty of Pharmacy of Rhodes University, South Africa.