1. Introduction
Industrial pollutants have increased since the last century as a result of the increased need to produce many industrial products, which led to an increase in pollutants, especially heavy metals. Humans and the surrounding environment are threatened by many pollutants, the most important of which are heavy metals, as a result of the increased presence of these elements in the sewage water for many industries that humans needed, including the mining industry, rayon industry, paint industry, electroplating, batteries, pesticides, metal rinsing operations, tanning industry, bio-reactors for the fluidized layer and textile industry. Metal smelting, petrochemicals, paper making, and electrolysis applications. Heavy metals are considered nonbiodegradable pollutants [1] and could be destructive [2–6]; thus, Health problems that humans and microorganisms may face if such elements are present in water in significant quantities.
As a result, more research and survey are needed to find and develop efficient methods and more environmentally friendly to remove these pollutants, and among these methods to do that treatment is liquid membrane extraction. A good solution that has already been provided to some of the major problems of our modern industrialized society is membrane engineering. Membrane processes are considered as a kind of process that meets the requirements of process intensification (PI) because it can technically replace energy-intensive technologies, can transfer some compounds and elements effectively, and is also considered a factor that plays a good role in improving reactive processes. Potable water, energy generation, tissue repair, pharmaceutical production, food packaging, and the separations needed for the manufacture of chemicals, and electronics are examples of a range of applications of using membrane techniques [7].
Membranes act as barriers, effectively partitioning two fluid phases while enabling the discerning permeation of solutes exclusively from one side to the other [8]. Membranes extend beyond solids as viable materials, allowing for the exploration of liquids as potential membrane mediums. Liquid membranes manifest in diverse manifestations within our daily experiences. For instance, an oil layer atop a water surface represents a classic example of an organic liquid membrane formed by immiscible liquid phases. Other instances include beer froth, foam on detergent or surfactant solutions, and oil films expertly applied to metal surfaces for rust protection and lubrication purposes. These familiar liquid films aptly serve as separators, segregating two distinct phases [9].
A solid or liquid membrane (LM) acts as a separator between the treated solution and the extract/solvent from each other in the membrane extraction [10–14]. A long time is required to reach equilibrium because the passive diffusion is the base of the liquid membrane extraction (LME), so electromembrane extraction (EME) was introduced to improve extraction kinetics in 2006 [15]. EME is a selective extraction technique in which the extraction process takes place by applying an electrical potential across a liquid membrane [15, 16]. EME is a three-phase system where the transfer of the extracted pollutants takes place from the sample solution to the aqueous receiver through the SLM. The effective electrical potential is the driving force for the extraction process via this technology. As it is possible to switch in the direction of transfer of positive and negative charges through the membrane depending on several factors, including the pH of the receiving solution and the sample, as well as on the type of organic solvent used, thus it depends on the chemical properties of the solutions. Therefore, EME is a sample preparation process with highly selective extraction potential [16].
Supported Liquid Membrane (SLM), which is made of a polymeric material after soaking in a solvent, can be used in the EME process, which is the most familiar phase in that process [15, 17]. The electroextraction process can be applied to various types of analytics, whether polar or non-polar, basics or acidic, cations, anions, peptides, and amino acids [18]. This study investigates the possibility of using EME, taking into account the types of liquid membranes that can be used and what are the parameters affecting the progress of this process.
2. Liquid Membrane Configurations
Liquid membranes can be categorized into three fundamental configurations: bulk, supported, and emulsion liquid membranes. In the case of a bulk liquid membrane (BLM), a relatively dense layer of immiscible liquid acts as a separator between the source and receiving phases. For a thinner liquid membrane, the liquid can be introduced into the pore structure of a microporous solid, such as a polymeric medium, resulting in what is known as an immobilized or supported liquid membrane (ILM or SLM). The third configuration, which offers expansive interfacial areas, involves emulsifying the receiving phase within an immiscible liquid membrane [19]. The feed solution encompasses the dispersed liquid surfactant membrane, also known as an emulsion liquid membrane (ELM), facilitating mass transfer from the feed phase to the internal receiving phase. The supported liquid membrane, owing to its stability and practicality, has emerged as a crucial component within this domain, garnering significant importance [20]. Polymer inclusion membranes (PIMs) have emerged in recent years as an attractive alternative to SLMs due to their better stability [21, 22]
2.1. Bulk Liquid Membrane (BLM)
The simplest form for LMs is known as BLM [23]. To separate the bulk sample and receiving phase a bulk immiscible organic layer can be used [24]. The separation may be accomplished with porous support as shown in Fig. 1 or without as shown in Fig. 2 [24]. The BLM structure with solid support uses cylindrical or rectangular vessels and H and U-tube vessels are used for BLM composition without solid support [25].
There are several factors affecting the transduction process that can be studied during laboratory analysis. It is also possible to test multiple types of carriers during the extraction and separation process [25, 26]. The main inferred advantages of BLM are the high degree of phase separation, small solvent volume, and superb membrane stability [23, 27]. Since the liquid membrane is dense and the area per unit volume is rather small, in addition to the membrane’s resistance to movement, it led to a decrease in the flow through it [23, 27, 28]. The resistance of the membrane has factors that affect it, including viscosity, stirring speed, as well as operating temperature [23].
2.2. Emulsion Liquid Membrane (ELM)
Li feigned the emulsion liquid membrane (ELM) in 1968 [24, 27]. An ELM is established by emulsifying the accepter phase in an immiscible organic liquid and interspersing this water-in-oil (W/O) emulsion into the surrounding donor solution [24, 27, 29, 30]. It consists of a three-phase dispersion system as shown in Fig. 3 [29]. Surfactants, carriers, and organic diluents all act homogeneously to act ELM [31, 32].
The extraction and stripping of the analyte occurred in one step in the ELM process [31]. The diluent, carrier, concentration of surfactant, and agitation level are important factors affecting on ELM system [30]. The emulsions are thermodynamically unstable which causes creaming, flocculation, coalescence, cracking, and swelling of emulsion [29, 32].
2.3. Supported Liquid Membrane (SLM)
During the last decade, the use of supported liquid membranes has increased very rapidly, especially the combination of micro-extraction and extraction using membranes, which has helped to manufacture modern forms in terms of producing clean samples with the least amount of organic liquid [33, 34]. Other advantages involve Low cost, automatic operation, and the ability to work on small samples [35].
In liquid membranes, a layer of an inert material suitable for this purpose is saturated with an organic liquid (containing usually dissolved reagents), and all of them form the supported liquid membrane (SLM), which is placed as a separating part between the donor and the acceptor solutions [36–39].
SLM is a liquid membrane of non-diffusing type, as the polymeric layer that makes up the membrane has an inactive role in the extraction process, but only works as a support layer through very microporous hydrophobic polymers that are injected with the organic liquid substance (which is the active part of this process) and form a supporting structure for it [40].
The solid support in the liquid membrane has different properties according to the type of use, as it is a symmetric or asymmetric polymer structure and may be hydrophilic or hydrophobic, and it can also be neutral or charge-carrying, whether positive or liquid. In general, the supporting material must be of the hydrophobic type to prevent the spread of the organic liquid and retain it inside it, in addition to the need for it to be thermally and chemically stable during the extraction process and its exposure to sample or acceptor phases [41].
Some of the common examples used in the supported liquid membrane are PTFE, polypropylene, and polysulphones 0.1–10 μm is the range of solution flow through which the film material has an optimal balance between film stability, which is provided by these polymeric films [42]. Sometimes a composite film is used, which is laminated to a coarse material or one of the polymers [43].
Several processes are applied to manufacture polypropylene as a material in the backed liquid membrane, including extrusion and annealing, and to obtain elongated pores, it is stretched to induce its formation. This type was applied to remove heavy metals from industrial wastewater [44].
Polysulfone membranes are produced through a phase inversion process where the membrane is mixed with a solvent to form a solution. To form a hollow type of fiber, polysulfone is cast or spun, it is worth noting that the fibers resulting from this manufacturing are of an asymmetric type with very fine pores. Several operations can be followed to get rid of the liquid inside the membrane, and these include evaporation, softening, and sedimentation processes [45].
Within this membrane variant, the extractant liquid is introduced into the polymeric membrane layer, permeating its pores and achieving saturation. This impregnated supported liquid membrane (SLM) acts as a mediator, facilitating the connection and interaction between the donor and acceptor phases. It encompasses the entire process of extraction, stripping, and regeneration of the extract, enabling seamless progression of these essential steps [42, 46, 47]. Compared to traditional extraction processes, the liquid membrane system is a new trend for it. In general, the extraction process is applied through SLM in several steps as shown in Fig. 4 [48].
The migration of metal ions takes place from the donor phase toward the inner surface of the membrane. Simultaneously, hydrogen ions migrate from the inner surface of the membrane to the feed phase. Upon reaching the interface between the membrane and the donor phase, the metal ions are transformed into their divalent form. This overall chemical reaction can be represented by Eq. (1).
Here, Mn+ denotes an n-valent cation, RH represents a monobasic acid, and the bar denotes organic or membrane phase species.
The formed complex is capable of transportation from the inner to the outer surface of the membrane phase. Once it reaches the interface between the membrane and the acceptor phase, a strip reaction occurs, resulting in the regeneration of the carrier and the release of the metal ions.
The formed complex is capable of transportation from the inner to the outer surface of the membrane phase. Once it reaches the interface between the membrane and the acceptor phase, a strip reaction occurs, resulting in the regeneration of the carrier and the release of the metal ions.
The regenerated carrier subsequently disperses back to the inner surface of the membrane. Simultaneously, the free metal ions disperse from the outer surface of the membrane into the bulk phase of the acceptor. Additionally, hydrogen ions disperse from the bulk phase of the acceptor to the outer surface of the membrane phase.
Flat sheet (FS) and hollow fiber in cylindrical type (HF) are the types on which the supporting liquid membrane can be formed, as they are distinguished among themselves in terms of size, shape, surface area, and applications, and based on that the classification is [48].
2.3.1. Flat sheet supported liquid membrane (FSSLM)
The flat sheet supported liquid membrane is a basic form of supported liquid membrane. It consists of a solid layer with microporous properties that allow easy penetration of liquid. In this configuration, the solid layer is impregnated with a liquid extractant, creating two separate cells within a system. Fig. 5 [48] illustrates this setup, with mechanical stirrers present in each cell.
A study by [49] focused on investigating flat sheet supported liquid membranes, specifically using Alamine 300 as an extractant for the extraction and separation of cobalt from acidic solutions. Under optimal conditions, with feed solutions containing 400 mg/l cobalt and 400 mg/l nickel, a remarkable cobalt extraction efficiency of 98.4% was achieved within 8 hours. The separation coefficient of cobalt on nickel was measured at 234.4. When a nonequimolar saturation feed solution was used, such as 500 mg/l of cobalt plus 1000 mg/l of nickel, the extraction efficiency increased to 99.9% and the separation factor for cobalt reached 506 within the same time period.
A recent investigation was dedicated to the concurrent extraction of lead(II) and cadmium(II) heavy metals from a mixed feed using the supported liquid membrane (SLM) technique. The method employed the introduction of a carrier solvent complex, comprising a 4% (w/w) sodium salt of Di-2-ethylhexylphosphoric acid (D2EHPA) within environmentally friendly coconut oil, into the pores of a solid polyvinylidene fluoride (PVDF) support polymer. In this process, the receiving phase, represented by sodium carbonate (Na2CO3), played a crucial role in facilitating the formation of lead(II) and cadmium(II) carbonate salts, thereby aiding their removal. Sedimentation was observed to positively influence the transport of solutes, with lead(II) benefitting from its favorable electronic configuration. This study exemplifies an effective approach for converting liquid waste containing heavy metals into solid waste, presenting a promising solution for the simultaneous removal of lead(II) and cadmium(II) from mixed feed [50].
2.3.2. Hollow fiber supported liquid membrane (HFSLM)
The liquid supported membrane of this type utilizes a collection of hollow fibers arranged in an orderly fashion. Each fiber is composed of a single non-porous material, preventing the transfer of the solution contained within and forming a hollow fiber liquid membrane structure [51]. Fig. 6 [52] illustrates the setup, where the feed phase passes through the fibers while the receiving phase flows through the shell side with the assistance of pumps.
In a specific investigation led by [53], the utilization of hollow fiber supported liquid membranes (HFSLM) was explored to extract arsenic ions from industrial wastewater. The study’s findings exhibited the remarkable efficacy of this technique, with optimized operating conditions yielding extraction and removal ratios of 99.9% and 98%, respectively. This successful outcome underscores the potential of HFSLM as a viable approach for the efficient extraction of arsenic ions from industrial wastewater.
2.4. Polymer Inclusion Membrane (PIM)
An alternative extraction technique to commonly used methods involve the utilization of a polymer that serves as both an extractor and a plasticizer within liquid films. This type of membrane is known as Polymer Inclusion Membrane (PIM) [54]. PIMs find applications in various fields such as wastewater treatment [55, 56], hydrometallurgy [57], and the reuse and recycling of secondary materials.
Cellulose triacetate (CTA) or polyvinyl chloride (PVC) are commonly employed to provide mechanical strength in PIMs. These polymers work in conjunction with a carrier responsible for facilitating the transfer of target chemical elements across the membrane. In some cases, a plasticizer is added to enhance the membrane flexibility and solubility of the extracted material [22, 57]. PIMs are typically characterized by their lamina structure, elasticity, and stability [58, 59]. They are easy to manufacture, possess favorable mechanical properties such as durability and elasticity, and demonstrate versatility in extracting and transporting a wide range of target chemical species [57].
PIMs have found application in the extraction and separation of minerals, including Au(III), As(V), Cd(II), and Co(II), owing to their aforementioned advantages that drive their utilization and development [60–62]. An illustrative investigation was undertaken to explore the application of a polymeric inclusion membrane (PIM) comprising 55% D2EHPA (by weight) and 45% PVDF-HFP (by weight) to extract and separate Zn(II) from a zinc sulfate solution. This study provides valuable insights into the potential of utilizing the aforementioned PIM composition for efficient Zn(II) extraction and separation from zinc sulfate solutions [63].
3. Electromembrane Extraction (EME)
In the modern environment of analytical chemistry, for complex samples, despite the improvement of sensitivity and selectivity significantly due to the development of direct analytical tools, it is necessary to increase research in preparing samples in new and improved ways. Extraction methods took a significant part in this trend. The need for environmentally friendly methods is what prompts more desire to search for such methods, as it reduces the quantity of organic solvents in return for good efficiency. Electromembrane extraction (EME) [15]. It is an example of a technique that requires very little organic solvent, which is estimated at a quantity of a few microliters, from which a small sample size is prepared. Either a supported liquid membrane (SLM) is used as a support for the solvent or it is free using a free liquid membrane (FLM) [64].
The main part of the EME system is the supported liquid membrane (SLM), a layer of hollow, porous fibers made of polypropylene that holds the organic solvent and is fixed inside the system. The charged target analyte is separated by SLM from the aqueous sample solution, which is located in a sample tank placed on a stirrer to the wand stage located in a hollow fiber cavity. The two electrodes of the electric field (cathode and anode) are fixed in the donor and receiver chambers, and each is coupled to an electrical power supply source as shown in Fig. 7. The force that causes the movement of charged ions from the donor solution to the receiving solution is the electric potential difference between the two electrodes [15, 17, 65, 66].
When using the EME system to extract positively charged materials, the cathode is fixed in the recipient chamber. To ensure the protein state (positively charged), the pH of both the donor and recipient chambers must be acidic or nearly neutral as shown in Fig. 8a. In the case of the ions to be extracted negatively charged, the anode electrode is the one that is placed in the recipient chamber, and concerning the pH, it is basic to ensure the negative charge as shown in Fig. 8b [65–67].
A recent addition to the liquid membrane family in the EME (emulsion-based extraction) system is the Free Liquid Membrane (FLM), which is considered a variant of the Supported Liquid Membrane (SLM)[68]. FLM involves the formation of a thin film of organic solvent positioned between the feed solution and the acceptor solution. It serves a similar purpose as SLM in comparable extraction processes. However, several characteristics set FLM apart from SLM. These include the smaller size and thinner thickness of the FLM, as well as the adjustable contact area between the membrane and both the donor and receiving solutions. To enable continuous monitoring of the required preparations and any changes during operation, the device used in FLM is assumed to be visible. This allows for real-time observation of factors such as colored compound presence or bubble formation in the acceptor solution. Monitoring tasks also involve tracking the transport of analytes across the membranes [68].
Fluid liquid membranes (FLM) find utility in both two-phase and three-phase emulsion liquid membrane (ELM) systems. In the three-stage ELM approach, charged compounds undergo transfer from the donor solution to the FLM and subsequently to the receiving solution. In the biphasic scenario, the compounds move directly from the donor solution to the organic acceptor solvent. The applications of FLM-based EME include estimating basic compounds in biological samples, extraction and pre-concentration of anionic and cationic dyes from aqueous samples and investigating the effects of electrolysis on the EME process. These applications highlight the diverse functionalities and potential applications of FLM-type EME techniques. [64]
Another application of the EME system is micro-EME (μ-EME), in this variant, the horizontal arrangement is adopted in a sandwich approach, in which the organic layer mediates between the donor and recipient solutions, which are respectively inserted into transparent polymeric tubes [64]. In this type, the organic solvent is significantly smaller in volume compared to the vertical arrangement (or the usual conventional arrangement of EME), and it is possible to make adjustments to the liquid phase both in terms of shape and size. Also, what distinguishes this system is the required sample size. To deal with volumes less than 1 microliter, μ-EME is used among all EME formats, as it is capable of such sizes, because the samples used do not need dilution before applying the system to them, and thus μ-EME may be of benefit for use with biological fluids that are available in limited volumes. Finally, since all μ-EME phases are formed as free liquids, all operational solutions can be analyzed after extractions, and the fundamentals of EME can be verified experimentally [69].
4. Variables Influence EME
There are two distinct categories of parameters involved in the process: one group relates to the setup, while the other pertains to the extraction procedure and mechanism. Here is a brief explanation of each category of variables [70].
4.1. Exploration of Setup Parameters
4.1.1. Analysis of electrode materials
To prevent electrolysis from occurring on the surface of the electrodes during the EME process, it is essential to utilize inert materials such as platinum. Platinum electrodes are a suitable choice for this purpose [70]. For example, Asl et al. used platinum wires as an anode and stainless steel wire as a cathode for extracting diclofenac and mefenamic acid from biological fluids [71]. However, using sacrificial electrodes have been applied for example Hansen et al. reported the use of sacrificial electrodes such as silver wire with a layer of electroplated silver chloride in the acceptor phase during EME. The electrode effectively prevented the electrolysis which made it possible to apply high currents without suffering from gas formation or pH changes from the electrolysis of the water [72].
4.1.2. Electrode distance
The effectiveness of the EME technique relies significantly on the spacing between the electrodes. The electric field strength (E) can be calculated using the Eq. (2) [73]:
The equation for determining the electric field strength (E) in the EME process is as follows: E = V/d, where d represents the inter-electrode distance and V denotes the applied voltage. When electric charges do not traverse the organic membrane, the electrodes and phase interfaces function similarly to a capacitor. In this scenario, the polarized Supported Liquid Membrane (SLM) acts as a dielectric capacitor [74]. Eq. (2) is widely recognized for its utility in calculating the breakdown voltage of capacitors, as well as determining the electric current passing through the insulator of a capacitor or an organic liquid membrane present in an electromembrane system.
Effective control of electric current levels plays a pivotal role in electromembrane processes, ensuring the smooth phase transfer and electrokinetic migration of analytes while preventing undesired electrochemical reactions. As per Eq. (2), increasing the distance between electrodes results in a reduction in the electric field strength and current level, thus limiting the phase transfer of analytes. Conversely, decreasing the inter-electrode distance intensifies the electric current and lowers the breakdown voltage, potentially causing the dielectric material to become conductive. This conductivity raises the risk of sparking, which can lead to electrode damage. Furthermore, when electrodes are positioned closely together, ion migration through the Supported Liquid Membrane (SLM) becomes more pronounced, leading to enhanced Joule heating. This phenomenon can result in system instability, elevated temperatures, and even puncturing of the SLM. It is therefore imperative to strike a balance in inter-electrode distance to ensure controlled electric current levels, minimize the risk of sparking or damage, and maintain stability within the electromembrane system [75].
The configuration of the EME setup can play a significant role in determining the internal distances between the electrodes. Consequently, the optimal electrode distance may vary depending on the specific EME settings employed. While the distance between electrodes is recognized as a crucial factor in most EME studies, its impact has primarily been explored at a theoretical level, lacking supporting documentation or laboratory experiments. The influence of electrode distance has been theoretically established through the principles outlined in Eq. (2). It is important to note that the distance between electrodes should be within limits that do not exceed the inner diameter of the sample container. This consideration must be taken into account when preparing the EME setup, ensuring that the setup is capable of maintaining a constant inter-electrode distance [70].
4.1.3. Container characteristics
In an EME system, mass transfer occurs through two mechanisms: convection and electrical migration. When the chamber size of the donor and recipient phases is reduced, the transfer of analytes is enhanced under the influence of the electric field [76]. Gjelstad et al. found an inverse relationship between the electric field and the volume of the container [76]. Additionally, the need for stirring is directly influenced by the volume of the container, as illustrated in Fig. 2–8 and Fig. S1 in the supplementary materials. Interestingly, stirring proved to be effective even when dealing with smaller volumes or in the presence of small components.
4.2. Extraction Procedure and Mechanism Parameters
4.2.1. Organic liquid membrane composition
The properties of the organic liquid employed within a membrane hold significant importance in the realm of EME technology. The nature of the supported liquid membrane (SLM) plays a vital role in shaping the efficiency of the analyte extraction process. Previous studies have revealed that utilizing organic solvents with lower viscosity yields enhanced extract recovery, thereby underscoring the positive correlation between organic solvent viscosity and the effectiveness of the extraction process [77]. The organic solvent used should have a similar polarity to that of the polypropylene fibers, facilitating its penetration into the pores of the supporting membrane fibers. It is important to note that the solvent should be immiscible with water. To ensure efficient operation with minimal current, the organic solvent should possess suitable electrical resistance. Furthermore, the organic solvent must possess favorable chemical properties that facilitate the transfer and migration of analytes from the donor phase to the recipient phase [78]. The selection of an appropriate organic solvent with these desired properties is critical for the success of EME.
The primary challenge in the development and implementation of the new system lies in selecting an appropriate organic solvent for the supported liquid membrane (SLM). This selection is crucial due to the specific properties required in the solvent. These properties include immiscibility with water, stability within the pores of the hollow membrane fibers, a high boiling point to prevent evaporation, and the ability to effectively dissolve charged analytes to enable their movement across the SLM membrane. Initially, among several solvent options, nitrogen-compensated organic solvent (NPOE) was chosen, resulting in an extraction rate of approximately 70% for all the analyzed substances. Subsequently, green peppermint oil solvent was selected as an alternative option, yielding promising results. While considering the environmental impact of chemical waste, it was deemed secondary in this context due to the minimal volume of solvent used, which does not exceed 20 μl. Thus, efforts focused on identifying an organic solvent that fulfills the necessary criteria, balancing its functional properties with minimal environmental consequences [15].
1-octanol is an excellent solvent for heavy metal cations extraction. It has high immiscibility with water hence avoiding its leakage from pores of the membrane followed by its dissolution in the doner or acceptor phase due to the stirring.
4.2.2. Carrier type
Although the EME system was optimized for best performance concerning the selection of organic solvent and donor and acceptor solutions, the total recovery of heavy metal cations was not high. It has been recently reported that several carriers can be used in organic solvents for better extraction of heavy metal cations, which improve the general transport of the analytes through an SLM into an acceptor solution. The most probable transport mechanism for heavy metal cations is proton drive, which is based on proton/analyte ion exchange between the carrier and the donor solution, and subsequently analyte/proton ion exchange between the carrier and the acceptor solution [79].
4.2.3. Ion balance (X)
The concentration gradient of ions between the donor phase and the receiving phase holds a pivotal role in optimizing the efficiency of the extraction process. A greater disparity in ion concentration results in heightened extraction efficiency, facilitating the desired separation. Conversely, a lower concentration gradient impedes system performance, limiting the effectiveness of the extraction process. Thus, maintaining a substantial concentration gradient between the donor and receiving phases is essential for achieving optimal extraction outcomes [76]. The pH level plays a crucial role in influencing ion concentration within the phases and is therefore a significant factor in successful extraction processes. Establishing a pH gradient becomes essential to facilitate the effective extraction of target analytes. The sample solution’s pH should be appropriately adjusted to promote ionization of the analytes and facilitate their migration in the electric field. When the acceptor phase has a low pH, analytes are directly released into the acceptor solution. The importance of a pH gradient becomes even more pronounced when an ion-pairing reagent, such as DEHP, is present in the supported liquid membrane (SLM). In such cases, a high concentration of H+ ions in the acceptor phase are required to interact with the anionic carrier (DEHP) and release the cationic analytes. Notably, variations in pH within the acceptor phase have a significant impact on extraction efficiency, while the pH of the donor phase has negligible effects. Thus, careful control of pH, particularly in the acceptor phase, is critical for optimizing the extraction process in electro-membrane extraction (EME) techniques [78]. Therefore, controlling and optimizing the pH gradient, particularly in the acceptor phase, is crucial for achieving efficient ion extraction in EME.
4.2.4. Effect of applied voltage
Within the electro-membrane extraction (EME) process, the migration of analytes from the donor phase to the receiving phase takes place within the liquid membrane under the influence of an applied electric field. The strength of this field is directly governed by the applied voltage, rendering voltage a crucial factor that influences the migration of analytes across the membrane. By adjusting the applied voltage, the efficiency and extent of analyte migration can be controlled, highlighting the significance of voltage in regulating the electrokinetic transport of analytes during EME [76]. While it is expected that increasing the applied voltage would enhance extraction recovery, there is a limit to consider. Exceeding this limit can have a negative effect, leading to a decrease in recovery. This is primarily due to mass transfer resistance caused by the accumulation of ion boundary layers at the interfaces on both sides of the liquid membrane or the saturation of analytes in the receiving phase. Moreover, in the case of extracting basic analytes, increasing the voltage can raise the pH in the receiving solution, resulting in electrolysis and the back-extraction of analytes into the donor phase. This back-extraction reduces the efficiency of the extraction process [76]. Hence, while voltage plays a crucial role in analyte migration, it must be carefully controlled to avoid diminishing extraction efficiency. Consequently, the extractability is affected by the strength of the electric field inversely [73, 78, 80–83]. Kjelsen et al. revealed that by enhancing the sample-to-SLM apportionment ratio, the EME could be performed at potential differences (5–10 V) attainable by ordinary batteries [84]. They also discovered that the nature of the SLM determines the limits of applied voltage and that the limits may be increased by using a liquid membrane reinforced with electrical resistance. Thus, for some organic liquid films, the applied voltage does not exceed 100 V for long-chain alcohols, while it can be as high as 300 V for NPOE. It is worth noting that the increase in current caused by the increase in voltage causes instability in the system. The direct relationship between current and voltage in a stationary system can be proved by Ohm’s law.
4.2.5. Extraction time
Extraction time as the key parameter determines the total amount of transferred ions from the donor phase to the acceptor phase. For method optimization, it is important to establish the extraction time profiles of target analytes to configure optimal extraction time. First, the extraction increased with time then after a period come to have to decrease. It may be due to organic solvent evaporation and dissolution of the organic phase in the sample solution because of Joule heating and high stirring rate, respectively [85].
It can be postulated that prolonged extraction time leads to the dissolution and discharge of a portion of the selective liquid membrane (SLM) into the aqueous solutions on both sides of the cell. This results in a decrease in the thickness of the liquid membrane, which may hinder the complete separation of the two aqueous phases across the membrane leading to the back transfer of ions from the acceptor phase to the donor phase [79].
4.2.6. Effect of initial concentration of the donor
At higher concentrations, the driving force is high leading to the transfer of a higher number of ions from the donor phase to the acceptor phase. However, the removal efficiency has an adverse effect where it was decreased as high concentration was used [86]. Assessing the consequence of varying Initial-stage concentrations on the electromembrane extraction (EME) process is valuable and should be considered to demonstrate if system behavior departs from the mass transfer regime.
5. Recent Papers on The Application of Electromembrane Extraction
The application of electro-membrane extraction (EME) to biological fluids has received much attention with extensive papers have been published in this field [87–90] while the application of EME to the extraction of heavy metals is relatively limited in the available literature. However, there have been a few published papers focusing on this area. However, there have been a few published papers focusing on this area listed in Table. S1 in the supplementary materials.
In a dedicated study performed by Davarani et al. on uranium extraction, comprehensive investigations were carried out to explore various factors influencing the extraction process [85]. These factors included the composition of the supported liquid membrane (SLM), applied voltage, and extraction time. The primary objective of the research was to determine the optimal conditions for achieving efficient uranium extraction. The obtained results unveiled that the most favorable conditions for uranium extraction involved employing a supported liquid membrane (SLM) composed of 1% di-2-ethyl hexyl phosphonic acid in nitrophenyl octyl ether (NPOE), applying an applied voltage of 80 volts, and conducting the extraction process for a duration of 14 minutes. Under these optimized conditions, precise estimation of U+6 with exceptional sensitivity and accuracy was achieved. Specifically, the linear range for uranium estimation encompassed the concentration range of 1 to 1000 ng ml−1, while the limit of detection (LOD) was determined to be 0.1 ng ml−1. These significant findings underscore the substantial potential of the proposed approach in facilitating effective uranium analysis. Moreover, they demonstrate the applicability of this approach in environmental and analytical studies that entail uranium detection, showcasing its versatility and relevance in the field [85]
In a separate investigation, Davarani et al. focused on the extraction of four other heavy metals (Co2+, Cu2+, Cd2+, Zn2+) using optimized conditions [79]. These conditions included an applied voltage of 60 V, 1-octanol with 0.5% v/v bis(2-ethylhexyl) phosphate, and 0.5% v/v tris(2-ethylhexyl)phosphate as the most suitable solvent for the supported liquid membrane (SLM), an extraction time of 10 minutes, stirring rate set at 1000 rpm, acceptor phase pH of 1.2 (using 65 mM HCl), donor phase pH of 10, and a total of 2 extraction cycles. The obtained results demonstrated that over 71.2% of the free ions present in the original aqueous samples were successfully extracted. This study provides valuable insights into the effective extraction of heavy metals, specifically cadmium, along with other metal ions, using the optimized parameters mentioned above. The findings highlight the potential application of this extraction method for the removal of heavy metal contaminants from aqueous solutions, emphasizing the importance of appropriate solvent selection and optimized operating conditions for achieving high extraction efficiency [79].
Meng, et al. investigated the transport behavior of Cr(VI) from the aqueous phase through a polymer inclusion membrane (PO-PIM) containing 1-octanol (OCT) as the carrier and polyvinyl chloride (PVC) as support at a low voltage drive (0–30 V) [91]. Results show that the voltage drive effectively solves the residue problem of Cr(VI) in the membrane phase, and the permeability coefficient (P) of PO-PIM to Cr(VI) increases with the voltage. The P of PO-PIM to Cr(VI) reaches 43.38 μm·s−1 at 30 V when feed and stripping phases are pH 2.0 HCl solution and 0.1 mol·L−1 NaOH solution environments, respectively [91].
In a separate research investigation, Meng et al. utilized a polymer inclusion membrane (PD-PIM) based on polyvinyl chloride, with bis(2-ethylhexyl) phosphate serving as a carrier, in electro-membrane extraction (EME) to study the behavior of Cd(II) [92]. The EME process was carried out by varying the pH of the feed phase within the range of 3 to 8, while dilute acid was employed for the stripping phase. The applied voltage ranged from 0 to 80 V. The study outcomes revealed the feasibility of Cd(II) extraction using PD-PIM within the specified pH range. The application of an electric field played a vital role in enhancing the mass transfer rate of Cd(II). By utilizing PD-PIM, the electric field effectively reduced the mass transfer activation energy associated with Cd(II) and alleviated the mass transfer interference caused by background materials present in the donor phase. Significantly, operating at pH 5 and applying a voltage of 60 V for a duration of 120 hours resulted in a notable reduction in Cd(II) concentration within a 1 L solution. Specifically, the Cd(II) concentration decreased from 15 mg/L to 0.08 mg/L, indicating an enrichment factor of 9.79. Successful extraction was achieved using kerosene-stabilized PD-PIM, demonstrating the potential of this approach for efficient Cd(II) extraction from solution. The PD-PIM showed high enrichment capabilities and achieved a significant reduction in Cd(II) concentration, highlighting its suitability for Cd(II) removal applications [92].
An extremely effective EME method was developed by Khan et al. for the selective extraction of Cu(II) followed by Red-Green-Blue (RGB) detection [93]. The effective parameters optimized for the extraction efficiency of EME include applied voltage, extraction time, supported liquid membrane (SLM) composition, pH of acceptor/donor phases, and stirring rate. Under optimized conditions, Cu(II) was extracted from a 3 mL aqueous donor phase to 8 μL of 100 mM HCl acceptor solution through 1-octanol SLM using an applied voltage of 50 V for 15min. Finally, the developed technique was applied to different environmental water samples for monitoring environmental pollution. Obtained relative recoveries were within the range of 93–106%. The relative standard deviation (RSD) and enhancement factor (EF) were found to be ≤4.8% and 100 respectively. this method can be introduced for the quantitative determination of Cu(II) as a fast, simple, portable, inexpensive, effective, and precise procedure [93].
In a distinct investigation, Meng et al. employed the combination of a polymer inclusion membrane (PIM) and electro-membrane extraction (EME) to eliminate Li+ ions [94]. D2EHPA was utilized as the carrier within the PIM. Comparative analysis of the obtained results with previous studies confirmed a noteworthy enhancement in the mass transfer coefficient of Li+ ions, improving it by at least one order of magnitude. Furthermore, it was observed that the mass transfer rate of the PIM system for Li+ ions exhibited a substantial increase as the applied electric field voltage was raised. These observations emphasize the potential of the PIM-EME approach for efficient extraction and removal of Li+ ions. The findings highlight the significant influence of the applied electric field voltage on the mass transfer process, underscoring its importance in optimizing the extraction efficiency of Li+ ions using the PIM-EME system. This study contributes to the understanding of the enhanced extraction capabilities of the PIM-EME method for targeted ion removal and underscores its relevance in various applications involving Li+ ion separation and recovery [94].
The influence of the electrostatic properties, derived from the electrical field, on the ionic transport rate and extraction recovery, in flat sheet supported liquid membrane (FSLM) and electro flat sheet supported liquid membrane (EFSLM) were numerically investigated by Monesi et al. [95]. Both FSLM and EFSLM modes of operation, in terms of implementing electrostatic, were considered. Through adopting a numerical approach, Poisson-Nernst-Planck, and Navier–Stokes equations were solved at unsteady-state conditions by considering different values of permittivity, diffusivity, and viscosity for the presence of electrical force and stirrer, respectively. The most important result of this study is that under similar conditions, by increasing the applied voltage, the extraction recovery increased. For instance, in EFSLM mode, by increasing the applied voltage from 10 to 30V, the extraction recovery increased from 53 to 98%. Furthermore, it was also observed that the presence of nanoparticles has significant effects on the performance of the SLM system [95].
Conclusions
Different types of liquid membranes were reviewed in this study, and they consider effective contaminant removal separation methods that can be used in industrial pollutant disposal. The mechanism of these membranes and the difference between their types in terms of installation methods were presented.
Developing new and effective materials removal techniques is a challenging task, particularly in light of increasingly severe environmental constraints. In addition, the need to use low-cost technologies has encouraged the search for innovative tailored methodologies. The need accompanying the development of the industry to find new methods that are more efficient in terms of energy saving, more economical, and easier to use made researchers find other methods. The method of using the electric field was resorted to and included in the work of extracting liquid membranes.
In this paper, the working principle of this type of field and the types that researchers used in their research are discussed. It is considered an effective method because the extraction is faster than without it, and it is more economical and energy-saving. Also, this method can be used to remove many industrial pollutants, including heavy metals, which are among the most important pollutants that hinder sustainable industrial development.
From the previous studies, it was found that the best applied potential difference through the electric field ranges between 1–80 V, and the time required for extraction is approximately 15 minutes. In addition, the gradient in the pH is an influential factor in the extraction process that must be controlled, as the acid PH value increases the extraction of ions Positively and vice versa.
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