Synergistic Extraction of Samarium(III) from Water via Emulsion Liquid Membrane Using a Low-Concentration D2EHPA–TOPO System: Operational Parameters and Salt Effects

Abstract

The synergistic effect of using D2EHPA and TOPO together to enhance the extraction of samarium(III) from aqueous media via emulsion liquid membrane (ELM) technology was explored. D2EHPA in binary mixtures with TBP and in ternary mixtures with TOPO and TBP was also tested. Among the tested extractants, a binary mixture of 0.1% (w/w) D2EHPA and 0.025% (w/w) TOPO achieved 100% samarium(III) extraction at a low loading. This mixture outperformed D2EHPA-TBP and other systems because D2EHPA strongly binds to Sm(III) ions, while TOPO increases the solubility and transport efficiency of metal complexes. Additionally, process factors that optimize performance and minimize emulsion breakage were examined. Key insights for successfully implementing the process include the following: 5 min emulsification with 0.75% Span 80 in kerosene at pH 6.7 (natural), 250 rpm stirring, a 1:1 internal/membrane phase volume ratio, a 20:200 treatment ratio, and a 0.2 N HNO₃ stripping agent. These insights produced stable, fine droplets, enabling complete recovery and rapid carrier regeneration without emulsion breakdown. Extraction kinetics accelerate with temperature up to 35 °C but declined above this limit due to emulsion rupture. The activation energy was calculated to be 33.13 kJ/mol using pseudo-first-order rate constants. This suggests that the process is diffusion-controlled rather than chemically controlled. Performance decreases with Sm(III) feed concentrations greater than 200 mg/L and in high-salt matrices (Na₂SO₄ > NaCl > KNO₃). Integrating these parameters yields a scalable, low-loading ELM framework capable of achieving complete Sm(III) separation with minimal breakage.

1. Introduction

The ever-growing demand for high-performance materials in clean energy, electronics, and defense technologies has put rare earth elements (REEs) in the spotlight due to their unique electronic and magnetic properties [1,2,3]. Among them, samarium(III) stands out for its pivotal role in high-strength permanent magnets, neutron capture in nuclear reactors, and a variety of pioneering electronic applications [4]. However, efficiently recovering Sm(III) from aqueous solutions is challenging due to the limitations of conventional solvent extraction methods, which require high reagent concentrations and produce secondary waste [5,6].

Membrane processes are widely valued for their inherent advantages, including their high efficiency and simple operation. They exhibit excellent selectivity and permeability when separating targeted components, making them ideal for specialized applications. These systems are also compatible with one another, enabling integration into multi-step processes. Other benefits include low energy consumption, stability under various operational conditions, and minimal environmental impact. Their ease of control, scalability, and adaptable operating parameters further enhance their suitability for a wide range of industrial and environmental applications [7,8].

Emulsion liquid membrane (ELM) technology offers an effective solution in the form of an integrated extraction-stripping platform that achieves high selectivity and throughput under mild operational conditions. ELMs encapsulate an internal stripping phase within fine droplets suspended in an oil-based membrane. The combination of liquid–liquid extraction and membrane separation provides several advantages. These include rapid mass transfer, a low solvent inventory, and potentially higher selectivity. However, many ELM systems struggle to maintain membrane integrity and performance due to the nature and interaction of the extractants at the oil/water interface.

Based on a thorough review of the literature, few studies have examined Sm(III) extraction via ELM [9]. Additionally, the performance of ELM systems depends on physicochemical factors, including extractants, surfactants, and membrane stability. Maintaining extraction efficiency while minimizing extractant and surfactant concentrations requires a multifaceted approach. One effective strategy is to use synergistic extractant pairs. These pairs enhance metal ion coordination at lower concentrations when used together. This interaction increases extraction efficiency and decreases the total chemical load within the membrane phase. Additionally, evenly distributing extractants across the organic phase through optimized phase ratios and gentle mixing techniques ensures that each molecule participates in transport, thus avoiding redundant dosing.

Organophosphorus extractants, particularly D2EHPA, strongly bind to REE ions via a cation-exchange mechanism [10,11]. Meanwhile, neutral donors such as TOPO and TBP enhance the solvation of REE-D2EHPA complexes, thereby increasing extraction efficiency and selectivity [12,13]. While both reagents have been extensively studied in solvent extraction, their combined role with D2EHPA in an ELM framework, particularly under conditions that minimize reagent concentrations, remains unexplored. The predominance of organophosphorus extractants within the 5–20 vol% range [14,15] suggests that testing lower extractant concentrations is feasible without sacrificing extraction efficiency or compromising membrane stability. However, emulsion instability and operational challenges remain key issues.

The present study examines the complementary effects of D2EHPA and TOPO on Sm(III) recovery from aqueous media via the ELM technique. TBP is another neutral donor that can modulate the lipophilicity and interfacial behavior of the extraction complex. Its performance in binary and ternary mixtures containing D2EHPA and TOPO was examined. The objective was to minimize extractant and surfactant dosages while maintaining membrane stability and operational efficiency. Key process variables were systematically optimized to improve recovery performance and membrane stability. Additionally, the impact of salts (e.g., NaCl, Na₂SO₄, and KNO₃) on Sm(III) extraction efficiency via ELM process was studied.

2. Material and Methods

2.1. Material

Standard solutions of Sm(III) were produced via dissolution of samarium nitrate (Sm(NO₃)₃·6H₂O, CAS No. 13759-83-6; 99.9% purity) sourced from Sigma-Aldrich (St. Louis, MI, USA) in bidistilled water. Operational solutions of Sm(III) were subsequently produced through dilution of specific volumes of the standard solution with bidistilled water to reach the required concentrations.

Bis(2-ethylhexyl) hydrogen phosphate (D2EHPA), trioctylphosphine oxide (TOPO), and tributyl phosphate (TBP) were employed as analytical-grade reagents. D2EHPA was procured from Fluka, and TOPO and TBP were procured from Sigma-Aldrich and employed as received. Sorbitan monooleate (Span 80), a non-ionic surface-active agent obtained from Sigma-Aldrich, was utilized as the emulsifying agent.

All remaining reagents utilized were purchased from Sigma-Aldrich in analytical-grade quality.

2.2. Experimental Protocols

The separation of Sm(III) via the ELM method comprises three main stages. Initially, the emulsion was prepared, followed by the recovery of Sm(III) from the feed phase through contact with the emulsion, and, finally, the decantation of the emulsion phase from the feed phase.

To formulate the internal stripping liquid, a specified amount of acid was dissolved in bidistilled water. The organic liquid membrane was formulated via dissolution of a precise concentration of Span 80 and the carrier in an appropriate diluent, with mild stirring.

Emulsification was achieved by blending the internal stripping liquid with the liquid membrane utilizing a homogenizer-disperser (IKA Ultra-Turrax T18, IKA Group, Breisgau, Germany) for a preestablished period. A known volume of the produced water-in-oil (W/O) emulsion was then contacted with a designated volume of the Sm(III)-containing external aqueous phase inside a temperature-controlled cylindrical cell equipped with an overhead mechanical stirrer. A 5 cm, four-blade impeller angled at 45° and designed for down-pumping was employed to achieve dispersion of the W/O emulsion uniformly, thus producing a water-in-oil-in-water (W/O/W) double emulsion system.

To evaluate the progress of Sm(III) extraction, sample aliquots were collected from the feed phase at regular intervals. The concentration of Sm(III) was determined using the Arsenazo-III spectrophotometric technique with a spectrophotometer (WPA Lightwave II, Biochrom Ltd., Cambridge, UK). Each experiment was conducted in triplicate to ensure data reliability, with the standard deviation across replicates ranging between 2% and 3%.

A range of process parameters were systematically examined to determine their influence on extraction performance. These included surfactant concentration (0.65–0.95% w/w), carrier loading (0.075–0.15% w/w), feed phase pH (1–7), emulsification duration (1–9 min), strippant concentration (0.05–1 N HNO₃), type of acid used in the internal phase (HNO₃, H₂SO₄, HCl, or HClO₄), mixing speed (100–400 rpm), internal-to-liquid membrane volume ratios (1:2 to 2:1), treatment ratios (5:200 to 60:200), diluent type (trichloroethylene, dichloromethane, heptane, mixed xylene isomers, hexane, or kerosene), initial Sm(III) concentration (25–400 mg/L), temperature (15–65 °C), and salt dosage in the external phase (0.5–30 g/L).

The W/O emulsion stability was assessed under controlled experimental conditions. In this assessment, a constant-temperature cylindrical vessel equipped with an overhead stirrer was used. A volume of 200 mL bidistilled water (external solution) was contacted with a specified volume of the W/O emulsion. The external liquid pH was monitored over time to detect any migration of hydrogen ions (H⁺) from the internal solution, which would indicate emulsion destabilization. The degree of emulsion breakdown was quantified as the percentage of internal liquid solution that had leaked into the external solution, relative to its starting volume.

To demonstrate the robustness and potential applicability of the process in different operational contexts, three representative salts were selected for evaluation: Na₂SO₄, NaCl, and KNO₃. These salts differ in ionic strength, anion type, and kosmotropic/chaotropic character. This enables evaluation of the process under various physicochemical conditions. Na₂SO₄ was chosen due to its strongly kosmotropic nature, which promotes salting-out effects and enhances phase separation. NaCl was included because it is the most common and widely available electrolyte in industrial and environmental systems. It was selected to reflect realistic application scenarios and test performances under ubiquitous saline conditions. KNO₃ was chosen due to its weakly hydrated nitrate anion, which enables examination of the influence of chaotropic species on extraction efficiency and selectivity. Together, these salts provide a broad spectrum of ionic environments, demonstrating the process’s ability to adapt to different feed compositions and its potential for large-scale industrial and environmental applications.

To ensure reproducibility, all experiments were performed in triplicate. Unless specified otherwise, the values presented in the manuscript are the means of three independent runs. We assessed the statistical significance of the observed differences using a Student’s t-test. p-values less than 0.05 were considered significant.

3. Results and Discussion

3.1. Synergistic Extractant Selection

The extraction of Sm(III) from aqueous media via an ELM process is a complex phenomenon governed by the interaction between metal ions and extractants in the organic phase. Experimental tests were performed to investigate the impact of extractant composition on ELM system accomplishment. These experiments used various combinations of organophosphorus extractants, including D2EHPA, TBP, and TOPO. All extractions were performed with identical process factors and 100 mg/L of Sm(III) in the aqueous feed solution. Figure 1a–c show how extractant composition affects Sm(III) extraction efficiency over time. The caption of these figures provides details on the operational factors used. Each extractant contributes in a distinctive way to the permeation mechanism, influencing the kinetics and equilibrium of the separation process. D2EHPA is a primary extractant due to its strong acidity and ability to form chelating complexes with REEs [16].

Figure 1a illustrates the impact of combining D2EHPA with various concentrations of TOPO. The results reveal a pronounced synergistic effect between the two extractants. The synergy achieved in this formulation is particularly significant, given that its extraction efficiency outperforms the combined efficiencies of the extractants when used separately. TOPO acts as a co-extractant, modifying the coordination sphere of the metal–D2EHPA complex [17]. By bonding with the Sm(III) ion, TOPO increases the solubility of the formed complex in the liquid membrane [18]. This promotes faster, more efficient transfer across the membrane interface. Compared to using 0.1% (w/w) D2EHPA alone, Figure 1a shows that adding incremental amounts of TOPO (e.g., from 0.0125% to 0.05% (w/w)) results in notably higher extraction efficiencies. Mixtures containing 0.1% D2EHPA and either 0.025% or 0.05% TOPO achieved nearly 100% extraction within 30 min. This significantly outperforms the extraction efficiency of D2EHPA alone (approximately 85%) and any individual concentration of TOPO, which exhibited negligible extraction. TOPO acts as a neutral donor that stabilizes the Sm(III)-D2EHPA complex, thereby enhancing mass transfer across the membrane interface and explaining this behavior.

In Figure 1b, the focus shifts to the influence of TBP when combined with a constant D2EHPA concentration of 0.1% (w/w). The highest efficiency (~99%) was obtained with 0.1% D2EHPA and 0.2% TBP, indicating an optimal molar ratio for synergistic interaction. D2EHPA alone achieved reasonable extraction efficiency, while TBP alone exhibited minimal activity (less than 4%), regardless of concentration. Additionally, the data suggest that including TBP in the organic phase enhances extraction efficiency and accelerates equilibrium attainment. The enhanced performance observed in binary D2EHPA–TBP systems is likely due to TBP’s ability to reduce the liquid membrane viscosity, improve solubility, and alter the interfacial tension between the aqueous and organic membrane liquids [19], thereby facilitating a faster transfer of the Sm(III) complexes.

To further optimize extraction kinetics and efficiency, ternary extractant systems were explored, as illustrated in Figure 1c. D2EHPA initiates complexation with Sm(III); TBP modulates the membrane’s transport properties; and TOPO stabilizes the formed complex [17]. The optimal composition, consisting of 0.075% (w/w) D2EHPA, 0.05% (w/w) TBP, and 0.025% (w/w) TOPO, exhibited superior performance. It achieved 69.8% extraction within 20 min while remaining stable throughout the process. This results in enhanced extraction efficiency that appears to surpass what can be achieved with any one carrier used independently. The experimental curves demonstrate rapid initial uptake followed by steady-state behavior, suggesting a system with minimized mass transfer resistance. This enhanced efficiency is attributed to a triple synergistic mechanism. First, D2EHPA acts as the primary extractant by forming coordination bonds with Sm(III). Meanwhile, TBP and TOPO act as co-extractants, stabilizing the Sm(III)-D2EHPA complex and facilitating its rapid transport through the membrane phase.

Figure 1a–c show that using synergistic extractant combinations is important for improving the recovery of Sm(III) ions from aqueous media. The data highlight the importance of determining the optimal extractant ratios to achieve the desired interplay between complexation, interfacial mass transfer, and membrane stability. The role of D2EHPA as the primary complexing agent is indispensable, while neutral co-extractants such as TOPO and TBP serve to stabilize and mobilize the metal–ligand complex. The improved extraction kinetics observed in some systems are indicative of facilitated transport and better interfacial activity, resulting in shorter equilibrium times and higher extraction capacities.

The results clearly demonstrate that the combination of D2EHPA and TOPO provides a markedly enhanced synergistic effect compared to both the binary system (D2EHPA-TBP) and the ternary mixture (D2EHPA, TOPO, and TBP), as well as the individual extractant systems tested. Consequently, it was recommended to use the D2EHPA-TOPO mixture for the remainder of the study at low loadings of 0.1% (w/w) and 0.025% (w/w), respectively. These findings are significant for the design of efficient ELM systems for rare-earth recovery, particularly under low-extractant loading conditions. The use of optimized mixtures of extractant systems offers a promising pathway for low-cost separation of Sm(III) from aqueous media.

3.2. D2EHPA–TOPO Loading Impact

The binary extractant system for synergistic Sm(III) extraction via the ELM method consists of 0.1% (w/w) D2EHPA and 0.025% (w/w) TOPO (i.e., 80% D2EHPA-20% TOPO), resulting in a total concentration of 0.125% (w/w). In this system, D2EHPA acts as a proton donor while TOPO serves as a neutral solvating agent, since TOPO can stabilize the metal–extractant complex and boost loading capacity. To evaluate the impact of extractant concentration, the ratio was kept constant (80% D2EHPA-20% TOPO) while the overall concentration varied from 0.075% to 0.15% (w/w). Figure 2a clearly shows that extracting Sm(III) becomes much more efficient as the concentration of the binary extractant increases from 0.075% to 0.15% (w/w). With extractant loadings of 0.125% and 0.15% (w/w), 93–95% extraction is achieved in 15–20 min and 100% in 50 min. In contrast, extraction efficiency increases more gradually at lower concentrations and never reaches the same level within the experimental timeframe. This enhancement in kinetics occurs because extractant molecules are more available at the aqueous–organic interface. This allows for faster complexation and diffusion of Sm(III) ions into the membrane phase.

Figure 2b shows that emulsion stability decreases as the amount of extractant increases. At a loading of 0.075% (w/w), emulsion breakage remains low, staying below 7.3% throughout the experimental period. However, at a loading of 0.15% (w/w), breakage rises to approximately 12.4% within 50 min. This increase in emulsion breakage is typically caused by changes in interfacial properties resulting from increased extractant loading [20]. Excess extractant increases the oil–water interfacial tension and viscosity. Additionally, higher extractant loadings can alter the interactions between the extractant and the surfactant at the oil–water interface, causing emulsion breakage. These interactions modify the interfacial tension and mechanical properties of the interfacial film. At moderate extractant concentrations, surfactant molecules dominate the interface. This lowers the interfacial tension and forms an elastic, cohesive barrier that resists droplet coalescence. However, as the extractant concentration increases, it can compete with or displace the surfactants at the interface. This disrupts the optimal packing of the interfacial layers and alters the interfacial rheology. These changes can lead to reduced elasticity and heterogeneous film coverage, making the film more susceptible to rupture under shear or during droplet collisions [21]. Furthermore, extractant aggregation at high concentrations can induce local interfacial tension gradients, accelerating film drainage and coalescence. Competitive adsorption and structural rearrangements at the interface significantly impact emulsion stability [22]. Large loadings of D2EHPA-TOPO cause the globules to swell and grow. This results in the carrier displacing the surfactant at the interface. In short, each additional increment of carrier hastens Sm(III) transport, but it also destabilizes the emulsion, leading to faster breakage.

Thus, the experimental trend is clear. Extraction efficiency increases with higher D2EHPA-TOPO concentrations. However, emulsion breakage also increases with greater extractant loading. At a carrier loading of 0.075% (w/w), the membrane remains stable, but Sm(III) removal is relatively slow and incomplete. With carrier concentrations of 0.125% and 0.15% (w/w), Sm(III) is completely extracted within minutes. However, the membrane begins to fail more quickly. This trade-off reflects the carrier’s dual role of driving mass transfer and altering emulsion properties. In practice, the optimal carrier concentration is chosen to maximize Sm(III) uptake while keeping breakage acceptably low. Consequently, the optimal concentration of the extractant for achieving maximum extraction efficiency and membrane stability is 0.125% (w/w).

Figure 3 illustrates a schematic representation of the Sm(III)-D2EHPA-TOPO complex and transport pathway in the ELM system.

3.3. Surfactant Concentration Impact

Figure 4a illustrates the impact of varying concentrations of Span 80 in the liquid membrane on Sm(III) extraction efficiency over time using an ELM technique. At the lowest concentration (0.65% w/w), the extraction process is quick and efficient. However, insufficient surfactant can cause the emulsion to become unstable or leak, as illustrated in Figure 4b. Conversely, a slightly higher concentration of 0.75% (w/w) appears optimal. Emulsion stability is maintained, and viscosity remains low. 85.7% extraction is achieved in about 15 min, and complete removal (100%) is attained after 50 min. However, as the Span 80 loading augments from 0.75% to 0.85% to 0.95% (w/w), the extraction rate slows markedly. Even after 50 min, the extraction does not reach maximum efficiency, indicating increased resistance to mass transfer caused by the high surfactant concentrations [23]. Nevertheless, stability is not disrupted (Figure 4b), confirming the increased resistance caused by the surfactant viscous film formed at these concentrations.

The results in Figure 4a,b align with well-established observations in the ELM literature [24]. Span 80 reduces interfacial tension and stabilizes droplets in W/O systems. However, beyond the optimal concentration, the surfactant significantly increases the liquid membrane viscosity. This thickening slows the diffusion of Sm(III)-carrier complexes across the organic liquid membrane, thereby impairing extraction kinetics. The relationship between droplet stability and mass transfer is clear. With insufficient surfactant, the droplets coalesce and break, reducing the active interfacial area. Conversely, if there is too much surfactant, the droplets become too viscous, impeding solute transport. In addition to the effects of viscosity, higher surfactant concentrations can increase the effective membrane thickness by forming rigid, structured interfacial films. This thickened barrier increases the diffusion path length for Sm(III)–carrier complexes, resulting in greater mass transfer resistance and slower extraction rates. The data curve representing 0.75% (w/w) Span 80 reflects the optimal balance between stabilization and diffusion. This concentration is used in subsequent experiments.

3.4. Emulsification Duration Impact

Figure 5 illustrates the intricate relationship between the emulsification period and the extraction efficiency of Sm(III) via the developed ELM process. The data demonstrates that extraction efficiency increases as emulsification time extends, reaching a plateau where further increases in the emulsification period produce diminishing yields. In particular, shorter emulsification times (1–3 min) appear to rapidly achieve high extraction efficiencies, while longer durations (7–9 min) require more time to reach similar performance levels. Extended mixing can result in over-processing of the emulsion, leading to the destabilization or uneven breaking of droplets due to excessive shear stress. This phenomenon may explain why curves representing longer emulsification times show a slower approach to the efficiency plateau. A destabilized emulsion may experience droplet coalescence or phase separation, which hinders the optimal transfer of ions. These findings indicate that the kinetics of droplet production and stabilization in the membrane phase are essential for maximizing the transport of Sm(III) ions from the feed solution to the organic membrane. For a 5 min emulsification period, the data curve reflects an optimal balance of factors governing the extraction process, such as droplet size, interfacial area, and stability [23].

Optimal extraction occurs after 5 min of emulsification. At this point, a finer and more uniform dispersion of droplets is formed. The small size of these droplets provides a large cumulative interfacial area, promoting rapid and effective ion exchange. A narrow droplet size distribution minimizes coalescence and fosters a stable emulsion, thereby enhancing extraction kinetics. The mechanical energy imparted during emulsification produces droplets that maximize contact with the aqueous phase, ensuring efficient transport of Sm(III) ions.

3.5. Strippant (HNO₃) Concentration Impact

Figure 6a illustrates the effect of strippant (HNO₃) concentration on the efficiency and kinetics of Sm(III) extraction. Lower acid concentrations (0.05 and 0.1 N) result in slower kinetics and lower overall efficiency. In contrast, a moderate concentration of 0.2 N achieves rapid, complete (100%) extraction. This trend reflects an optimal balance wherein the proton activity provided by the acid is sufficient to drive the stripping reaction without compromising the emulsion’s stability (Figure 6b). Mechanistically, HNO₃ plays a dual role in this scenario. First, it creates a strong proton gradient, which enhances the transfer of Sm(III) ions from the feed solution to the internal stripping phase across the membrane. This increased driving force accelerates the kinetics of the ion exchange reaction occurring at the emulsion droplet interface. Second, HNO₃ concentration influences the structural integrity of the emulsion itself. At concentrations exceeding 0.2 N, the membrane system experiences increased osmotic pressure, causing the emulsion droplets to swell or coalesce. This reduces extraction kinetics and efficiency. Additionally, the concentration of HNO₃ in the stripping phase has mixed effects. On the one hand, an increased acid concentration enhances the driving force for Sm(III) ion transport. However, excessive acid can cause osmotic imbalances and disruptions in interfacial tension, resulting in emulsion breakage. As shown in Figure 6b, the emulsion rapidly destabilizes when the acid concentration exceeds 0.2 N. Emulsion breakage occurs when the interfacial film stabilized by the surfactant is overwhelmed by a proton-rich environment. When the HNO₃ concentration surpasses the optimal level, the surfactant molecules become less effective at the organic/water interface. Increased proton activity alters the surface charge and molecular arrangement of the surfactant [25]. This reduces the interfacial tension barrier that typically prevents droplet merging. In this state, external forces, such as shear from mixing or osmotic pressure differences, dominate and lead to the emulsion’s rapid coalescence and rupture.

The integrated analysis of both Figure 6a,b reveals that the efficiency of Sm(III) extraction is governed by a delicate interplay between chemical driving forces and the physical stability of the emulsion. An optimal concentration of HNO₃ is necessary to preserve the physical stability of emulsion droplets and provide a chemical driving force. However, an ideal emulsion structure cannot compensate for suboptimal chemical conditions. Properly tuning the acid concentration and controlling the emulsification process ensures a high mass transfer rate, minimal interfacial resistance, and sustained membrane stability. These factors maximize the Sm(III) extraction yield. Consequently, a strippant concentration of 0.2 N HNO₃ was selected for further investigation.

3.6. Strippant Type Impact

In the developed ELM process, the stripping phase relies on the internal aqueous phase’s ability to rapidly protonate and release Sm(III) from its carrier complex. This regenerates the extractant and drives mass transfer [26]. However, the choice of stripping agent is critical because it directly affects the kinetics and equilibrium of recovering the substrate from the aqueous phase. Figure 7 provides a comparative analysis of four types of acids (0.2 N) used in the stripping phase of an ELM process for Sm(III) extraction. Significant differences in kinetics and ultimate recovery emerge when comparing the four 0.2 N mineral acids. These differences stem from factors such as the dissociation characteristics of each acid, its tendency to form complexes with Sm(III), osmotic effects on emulsion stability, and physical properties, such as viscosity and ionic strength. Nitric acid was the most effective stripping agent, achieving complete (100%) recovery of Sm(III) within a relatively short time frame. As a strong monoprotic acid, it dissociates completely at 0.2 N, generating a high concentration of free protons that efficiently outcompete the carrier for coordination sites on Sm(III). Additionally, nitrate ions have little affinity for forming inner-sphere complexes with rare earth elements [27]. This ensures that, once released, Sm(III) remains in the internal liquid solution rather than rebinding to the liquid membrane. Furthermore, nitric acid’s lower tendency to induce secondary reactions in the liquid membrane preserves the emulsion’s integrety, which is crucial for consistent extraction performance. The low viscosity and high ionic mobility of nitric acid solution accelerates proton diffusion, thus enhancing the rate and extent of stripping. In contrast, the extraction efficiencies using HCl and HClO₄ as strippants were moderate. Perchloric acid is a strong acid that does not form significant complexes with metals. HClO₄ is monoprotic and fully dissociates at 0.2 N, providing high proton activity. However, the larger perchlorate anion and its kosmotropic character can slightly increase solution viscosity and ionic strength, thereby slowing diffusion rates. While the complete dissociation of HCl provides an abundance of protons for carrier regeneration, the formation of weak chloro-complexes between Sm(III) and chloride anions temporarily stabilizes Sm(III), slowing its transfer into the aqueous phase. However, other factors, such as the acid’s diffusivity and its interaction with carrier molecules, also play a significant role. Among the four acids, sulfuric acid performs the worst, exhibiting slower kinetics and lower overall Sm(III) recovery. As a diprotic acid, only the first proton dissociates completely at a concentration of 0.2 N, resulting in a significant amount of bisulfate ions remaining behind. These ions increase viscosity and decrease proton mobility compared to monoprotic acids. Furthermore, sulfate anions readily form inner-sphere complexes with Sm(III), which anchors the metal in the organic phase and opposes its stripping. Additionally, the physical properties of sulfuric acid, such as its higher viscosity and density compared to other acids, may hinder the diffusion of Sm(III) ions through the aqueous and membrane phases.

On the whole, the results underscore the significant impact of acid selection on the efficiency of Sm(III) extraction. The stark differences observed, from a rapid, complete extraction with nitric acid to a sluggish performance with sulfuric acid, suggest that optimizing the stripping phase requires balancing chemical reactivity, complexation behavior, and physical characteristics, such as diffusivity and viscosity. Therefore, this work will continue with HNO₃ as a strippant at a concentration of 0.2 N.

3.7. Mixing Rate Impact

Figure 8a shows how extraction efficiency changes with contact time at stirring speeds of 100–400 rpm. At lower speeds (100 and 150 rpm), the extraction process is less efficient and occurs gradually. After 50 min, the values reach 25% and 70%, respectively. In contrast, complete extraction is rapidly achieved within 50 min at higher speeds (300–400 rpm). At a moderate speed of 250 rpm, sufficient turbulence is generated to reduce the emulsion globules size. This increases the interfacial area between the aqueous and organic phases. The enhanced interface enables Sm(III) ions to rapidly diffuse into the membrane, significantly improving extraction efficiency to 100%.

Figure 8b shows that as the mixing rate augments from 100 to 400 rpm, the percentage of emulsion breakage increases as well. At lower speeds, breakage remains minimal. However, the emulsion’s extraction efficiency was low due to the membrane’s thickness and the droplets’ larger size. At these speeds, the emulsion consisted primarily of large, nearly spherical (>50 µm) droplets with thick membrane layers and smooth interfaces. Though breakage events were infrequent, the limited external interfacial area restricted mass transfer, resulting in low extraction efficiency. Increasing the speed to 250 rpm resulted in finer dispersion and a broader droplet size distribution ranging from 10 to 40 µm. This process also yielded thinner membrane films. Conversely, at higher speeds, particularly beyond 300 rpm, the increased hydrodynamic shear forces cause the droplets to coalesce and eventually rupture. The morphology shifted toward polydisperse, irregularly shaped droplets. Many of these droplets had deformed or partially ruptured membranes. This phenomenon is important because, although smaller droplets initially favor efficient extraction, excessive breakage leads to the loss of the liquid membrane. Therefore, balancing the enhancement of extraction kinetics with the maintenance of emulsion stability is a key operational challenge.

From a mechanistic standpoint, stirring plays a dual role. First, it reduces droplet size, thereby improving contact between phases. However, there is a subsequent risk of structural failure due to mechanical stress. The enhanced mass transfer observed at higher stirring rates is due to the thinner boundary layers around the droplets. Once emulsions begin to break, though, the loss of droplet integrity can lead to the premature release of the internal phase into the external solution. Kinetically, a high stirring rate accelerates the overall reaction, rapidly reaching near-equilibrium extraction conditions. Nevertheless, this kinetic advantage may be offset by the adverse effects of emulsion breakage over longer operational cycles. In essence, vigorous stirring speeds increase Sm(III) uptake but jeopardize the system’s stability over time. To optimize efficiency, the stirring speed must be carefully adjusted to maximize the benefits of enhanced mass transfer without reaching the point where emulsion breakage becomes detrimental to process integrity.

In summary, increasing the stirring rate improves mass transfer and speeds up the kinetics of the Sm(III) extraction. However, it also increases the risk of emulsion breakage. The challenge lies in identifying and maintaining an optimal stirring regime that leverages the benefits of finer droplet dispersion while preserving emulsion stability. Thus, 250 rpm was designated as the optimum mixing speed.

3.8. Internal/Membrane Phase Volume Ratio Impact

Figure 9 illustrates the impact of the internal phase to the liquid membrane volume ratio on Sm(III) separation via the developed ELM technique. The extraction efficiency curves for various ratios reveal important trends. A 1:1 ratio achieves 100% extraction in 50 min. Lower (1:2 and 3:4) and higher (3:2) ratios fall short. This can be attributed to the delicate interplay between mass transfer and emulsion stability. The internal phase is responsible for stripping the Sm(III) ions once they have traversed the membrane, so it must be present in optimal quantities. From a mass transfer perspective, the optimal 1:1 ratio likely promotes the formation of finely dispersed, stable emulsion droplets with a large interfacial area. This enhanced interfacial area optimizes the diffusion of Sm(III) ions from the external feed solution across the liquid membrane into the internal stripping solution. An insufficient volume, as seen with the 1:2 and 3:4 ratios, limits stripping capacity and reduces the driving force for further extraction. Conversely, an excess of the internal phase (as with a 3:2 ratio) can lead to emulsion instability, coalescence, or disruption of membrane phase continuity. This hinders diffusion processes and results in lower overall extraction efficiency.

This work also studied the extraction of Sm(III) with a volume ratio of 2:1. However, it should be noticed that after a few minutes, the W/O/W emulsion undergoes inversion to form a W/O emulsion with this ratio.

Additionally, as Figure 9 shows, the observable time profile indicates that the extraction rate depends on the phase ratio and the interaction between diffusion and reaction kinetics. At the optimal 1:1 ratio, the combination of adequate stripping capacity and a stable, high-surface-area interface enables rapid initial uptake of Sm(III) ions. Higher ratios can result in thicker or more viscous internal droplets that hinder ion diffusion through the membrane. This explains the lower efficiency observed at a 3:2 ratio. Conversely, if the internal phase volume is too low, limited stripping capacity causes premature internal phase saturation and a weakened concentration gradient, slowing the extraction process.

The optimal 1:1 ratio exemplifies the necessity of balancing physical and chemical processes, such as diffusion, emulsion stability, and interfacial reactions, to achieve high extraction efficiencies.

3.9. Treatment Ratio Impact

Figure 10 illustrates the impact of the treatment ratio, ranging from 5:200 to 60:200, on Sm(III) separation efficiency from aqueous media. Notably, extraction efficiency increases dramatically with higher ratios. For example, the lowest ratio (5:200) achieves only 44.5% efficiency after 50 min, while the highest ratio (60:200) nearly achieves full extraction in less time. Increasing the treatment ratio naturally results in more globules and a greater interfacial surface area per volume of external solution [24], thereby accelerating Sm(III) extraction. From a mechanistic perspective, the impact of the volume ratio extends beyond merely increasing the interfacial area. The size distribution and stability of the emulsion globules also play pivotal roles. Higher emulsion volumes tend to produce smaller, more uniformly dispersed globules that create a larger effective interfacial surface area and augment the mechanical stability of the emulsion system. This stability is essential for maintaining efficient extraction over time because coalescence of droplets or phase separation can lead to reduced mass transfer efficiency. While increasing emulsion volume generally improves extraction efficiency, striking the right balance is crucial. Excessively high emulsion volumes could introduce challenges such as increased viscosity or limited diffusion within the emulsion phase itself. These challenges could counteract the benefits of an increased interfacial area. From a processing perspective, using a smaller emulsion volume during extraction is advantageous because it maximizes the enrichment of the external phase. Moreover, from an economic standpoint, a 20:200 emulsion-to-feed volume ratio proves optimal, ensuring effective dispersion of the emulsion in the external feed solution and enhancing the Sm(III) loading in the striping solution.

3.10. Diluent Type Impact

Figure 11a illustrates the substantial impact of diluent type on Sm(III) uptake. Kerosene achieves 100% extraction in just 50 min, outperforming heptane, hexane, trichloroethylene, and xylene-mixed isomers. Kerosene’s superior performance is attributed to its optimal viscosity and nonpolar nature. These properties enhance the solvation and transport of the Sm(III)–carrier complex from the aqueous solution to the organic liquid membrane. Nonpolar diluents favor the formation and stabilization of Sm(III)–carrier complexes by reducing interfacial tension and promoting effective mass transfer kinetics. Conversely, more polar diluents, i.e., xylene-mixed isomers, tend to lower extraction efficiency due to increased phase miscibility and diminished interfacial stability. The properties of a diluent, particularly its polarity, viscosity, and solubility parameters, perform essential functions in regulating the equilibrium and kinetic aspects of extraction.

Figure 11b illustrates the change in emulsion breakage over time (0–50 min) for each diluent. Breakage increases in all cases, but by different amounts. Kerosene produces the minimum breakage (9.1% after 50 min), while the xylene-mixed isomers produces the most (36.9%). Hexane and heptane produce intermediate amounts (27.3–32.1%), and trichloroethylene produces the least amount (22.6%).

The differences in extraction behavior and membrane stability can be traced back to physicochemical properties. For example, viscosity governs droplet rigidity. Kerosene is significantly more viscous than hexane and heptane. Kerosene has a viscosity of 1.64 mPa·s, whereas hexane and heptane have viscosities of 0.297 and 0.376 mPa·s, respectively [28]. A higher-viscosity organic phase produces thicker membrane layers around internal droplets, reducing turbulent mixing. This slows solute diffusion but makes droplets more resistant to shear. Conversely, solvents with very low viscosity allow for rapid solute diffusion and finer dispersions. However, the thin organic film cannot withstand prolonged shear and ruptures quickly. Thus, high-viscosity diluents yield stable emulsions, whereas low-viscosity diluents break easily under agitation. As shown in Figure 11b, hexane and heptane, which have the lowest viscosities, demonstrate the fastest increase in breakage, whereas viscous kerosene remains intact. Therefore, the viscosity ranking (kerosene > xylene mixed isomers ≈ trichloroethylene > heptane > hexane) helps explain the order of breakage.

The polarity, or dielectric constant, and chemical nature of each diluent significantly impact membrane stability and extraction. Kerosene and aliphatic alkanes, such as hexane and heptane, are highly nonpolar with a dielectric constant of about 1.80–1.92 and negligible water solubility [29]. These characteristics favor stable water-in-oil emulsions by minimizing water uptake and maintaining a cohesive organic phase. In contrast, xylene-mixed isomers (aromatic) have a higher dielectric constant (approximately 2.3) and significantly greater water solubility [30]. Aromatic solvents dissolve more water and interact differently with the surfactant layer. Trichloroethylene is polarizable, with a dielectric constant of about 3.4, and is moderately soluble in water at approximately 1.3 g/L [31]. Its polarity can disrupt the Span 80 surfactant film, allowing more water to enter the organic phase and weakening the membrane. Indeed, polar or halogenated solvents often reduce membrane stability. However, trichloroethylene’s high density (approximately 1.46 g/cm³), compared to water, may suppress buoyancy-driven droplet instabilities to some extent. Our data show that emulsion with trichloroethylene breaks down at a rate of 22.6% after 50 min, which is slower than xylene-mixed isomers but faster than kerosene. This reflects the balance of effects.

3.11. Feed Phase pH Impact

Figure 12 demonstrates that Sm(III) extraction efficiency depends critically on the pH level of the external feed solution. When the external phase has a very low pH (1–2), the extraction performance is essentially nonexistent. At these highly acidic pH levels, H⁺ ions and Sm(III) ions compete for the same active sites on carrier molecules. The extractant, which is designed to bind to Sm(III) through a delicate balance of protonation states, becomes excessively protonated at these pH levels. This hinders its ability to form the necessary metal–extractant complexes. This competitive inhibition results in lower extraction yields because the carrier’s capacity to selectively extract Sm(III) is compromised by the overwhelming presence of H⁺ ions. However, as the pH shifts toward the range of 3–6 from these extremely acidic conditions, the scenario changes considerably. After 50 min of mixing, the recovery effectiveness augmented from 61.3% at pH 3 to 99.8% at pH 6. At the natural pH of 6.7, the proton concentration decreased to a level that is no longer dominant in competing for the carrier’s binding sites. This allows for the complete (100%) removal of Sm(III) from the feed phase. Reduced interference enables extractant molecules to interact effectively with Sm(III) ions and form stable complexes. Furthermore, Sm(III) may exist in a more extractable form at a natural pH, boosting extraction efficiency even further. The result is a significant improvement in extraction yield, with the highest efficiency observed under near-neutral pH conditions.

In addition to extraction kinetics, the external feed solution pH is critical for the physical stability of the ELM system. At pH 1, the excessive acidity can destabilize emulsion droplets by disrupting the interfacial tension and effectiveness of surfactant. This can lead to issues such as droplet coalescence and phase leakage, hindering the transport of Sm(III) ions through the organic membrane. Conversely, the emulsion is more stable at a natural pH level. This stability promotes efficient mass transfer and enhances the robustness of the extraction process. Achieving high extraction yields requires balancing reducing competitive protonation with maintaining optimal carrier activity and emulsion stability. Although extremely acidic conditions can be advantageous in other situations, a natural or slightly neutral pH is ideal for Sm(III) extraction via the ELM method.

3.12. Temperature Impact

Figure 13 illustrates the change in Sm(III) extraction efficiency over time at various temperatures (5, 15, 25, and 35 °C). The data indicate that higher temperatures considerably accelerate the extraction process. For instance, after 10 min, only 35.4% of Sm(III) had been extracted at 5 °C, whereas 87.1% had been extracted at 35 °C. Likewise, equilibrium is reached much faster at higher temperatures. At 15–35 °C, the curves reach a plateau of 100% extraction. However, at 5 °C, the maximum attained is lower (87.6% after 50 min) and would likely rise slowly. This strong temperature dependence implies a substantial activation energy for the process. Increasing the temperature raises the diffusion and reaction rates at the interface via Arrhenius behavior. Thus, the steeper uptake curves observed at 25–35 °C suggest that transport is diffusion- or reaction-limited.

This study also examined the impact of temperature on Sm(III) recovery at temperatures ranging from 35 to 65 °C. However, extraction becomes impossible once the temperature exceeds 35 °C because the emulsion separates into two phases (rupture).

Physically, a rise in temperature decreases the liquid membrane viscosity and aqueous phases and increases diffusion coefficients. Increased molecular motion enables Sm(III) ions to cross the boundary layers and organic film more quickly. Conversely, the slow diffusion rate at low temperatures can cause the membrane to become the rate-limiting barrier. Consequently, less Sm(III) will reach the internal phase within a given time. However, very high temperatures can also undermine the ELM. Known stability issues, such as internal-phase coalescence, membrane swelling, and rupture, can worsen if the surfactant layer is stressed. In this study, membrane breakdown occurred when the temperature exceeded 35 °C.

The activation energy of Sm(III) extraction by ELM can be estimated using the Arrhenius relation, which establishes the reliance of the pseudo-first-order kinetic constant (k) on temperature. The relationship is shown below (Equation (1)):

$$\mathrm{l}\mathrm{n}\mathrm{k}=\mathrm{l}\mathrm{n}\mathrm{A}-\frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{R}}·\frac{1}{\mathrm{T}}$$

(1)

where k is the pseudo-first-order kinetic constant of extraction (1/min), A is the Arrhenius constant (1/min), E_a is the activation energy (kJ/mol), R is the gas constant (8.314 J/mol K), and T is the temperature (K).

Determining the slope of lnk as a function of 1/T allows calculation of the activation energy. The slope yielded an E_a value of 33.13 kJ/mol, suggesting a diffusion-controlled mechanism rather than a chemically controlled process. Activation energies below 40 kJ/mol typically indicate that the membrane process is typically mass-transfer controlled, meaning the overall rate is determined by diffusion through the phases rather than the chemical reaction at the interface. Therefore, the pronounced acceleration at higher temperatures is consistent with thermally enhanced diffusion through the aqueous and organic films.

3.13. Sm(III) Initial Loading Impact

Figure 14 plots the extraction efficiency against time for various initial Sm(III) concentrations ranging from 25 to 400 mg/L. Extraction efficiency shows a clear inverse relationship with the initial metal concentration. For lower feed loadings (e.g., 25 and 50 mg/L), the extraction efficiency approaches 100% within the first 10 min. This is driven by the high concentration gradient, which facilitates the rapid diffusion of Sm(III) toward the unsaturated droplets of the stripping phase. As the initial loading augments to 100–150 mg/L, the extraction percentage remains high (>90%), but the process takes longer. This reflects the gradual depletion of the driving force and the beginning of internal phase saturation. Conversely, as the initial concentration increases beyond 200 mg/L, the extraction rate slows intensely. Even after 50 min, the efficiency remains lower attaining 36.4%, 49.7% and 72.2% at 400, 300 and 200 mg/L, respectively. These results suggest that the ELM system is highly effective with dilute solutions but performs poorly with higher solute loads. This concentration-dependent behavior arises from the sequential steps that govern ELM transport. These steps include diffusion of Sm(III) across the feed boundary layer, Sm(III)–carrier complexation at the interfacial region, diffusion of the Sm(III)–carrier complex through the liquid membrane, and stripping of the complex into the internal phase. When Sm(III) levels are low, the chemical reaction at the interface controls the overall kinetics. The carrier remains in excess, and mass transfer through the membrane is fast. However, at higher feed concentrations, the carrier sites become progressively saturated. This shifts the rate-determining step toward membrane diffusion and carrier regeneration. The diminished concentration gradient across the membrane further slows diffusive flux. Additionally, these observations stem from the stripping phase’s droplets reaching saturation. At low concentrations, the abundance of internal phase droplets enables Sm(III) ions to quickly strip and diffuse, allowing for efficient and rapid extraction. In contrast, a high concentration results in a competitive regime where the limited number of internal phase droplets must cope with an oversupply of ions, slowing the transfer rate.

3.14. Salts Impact

Figure 15a–c illustrate the impact of various salt concentrations (KNO₃, NaCl, and Na₂SO₄) on Sm(III) separation effectiveness from aqueous solutions. The figures illustrate the removal of Sm(III) over a 50 min period at salt concentrations ranging from 0.5 to 30 g/L. A consistent trend emerges across all three salts: the presence of salt inhibits the extraction process to varying degrees, and extraction efficiency decreases as salt concentration increases. In the absence of salt (0 g/L), the extraction efficiency was 100% after 50 min. This indicates that the emulsion liquid membrane system achieves optimal mass transfer and minimal resistance in the absence of salt. However, the presence of salts negatively affects performance. This can be attributed to several factors, including salting-out effects, the influence of ionic strength on speciation, and competition between salt cations and Sm(III) ions.

The extraction efficiency decreases gradually with the addition of KNO₃. As the concentration increases from 0.5 to 30 g/L, the kinetics flatten. The nitrate anion is a weakly coordinating ion. The observed effect is likely due to increased ionic strength, which compresses the electrical double layer. This reduces the diffusion of Sm(III) toward the emulsion interface by increasing membrane viscosity and decreasing diffusivity. At equivalent concentrations, NaCl has a more pronounced negative impact than KNO₃. Chloride ions weakly bind to Sm(III), and their higher charge density relative to nitrate ions may increase electrostatic competition at the interface. Additionally, sodium ions may compete with Sm(III) for interaction with carrier molecules. Na₂SO₄ exhibits the most significant suppression of Sm(III) extraction efficiency, especially at higher concentrations. As a divalent anion and strong salting-out agent, sulfate increases ionic strength and reduces the mobility of Sm(III) toward the interface. Additionally, sulfate forms complexes with lanthanides in solution, which alters the chemical speciation of Sm(III) and reduces the concentration of free ions available for transport into the membrane.

The observed decline in extraction efficiency with increasing salt concentrations can be explained by several mechanisms. Higher ionic strength reduces activity coefficients and decreases metal ion mobility, as supported by the Debye–Hückel theory. Sulfate and chloride anions may form complexes with Sm(III) in an aqueous solution, thereby reducing the amount of free ions available for extraction. Increased salt content may destabilize the emulsion, increase osmotic pressure differences, or alter membrane phase viscosity. These changes can lead to coalescence and reduced interfacial area.

This study emphasizes the important roles that salt type and concentration play in the efficiency of Sm(III) extraction. Among the examined salts, Na₂SO₄ exhibited the strongest inhibitory effect, followed by NaCl and KNO₃. This hierarchy correlates with the salts’ ability to cause salting out and the behavior of the anions when forming complexes.

4. Conclusions

An ELM system containing low concentrations of D2EHPA (0.1% w/w) and TOPO (0.025% w/w) for the complete extraction of Sm(III) was optimized. The D2EHPA-TOPO combination was more effective than the D2EHPA-TBP pair or any ternary mixture. TOPO enhances complex stability, solubility in the membrane, and mass transfer. An overall carrier loading of 0.125% (w/w) achieved an optimal balance of extraction efficiency and emulsion stability. Using Span 80 at a concentration of 0.75% (w/w) preserved droplet integrity, maintained low viscosity, and enabled full extraction. Five minutes of emulsification produced fine, uniform droplets that maximized the interfacial area. The optimal operating conditions were an internal phase of 0.2 N HNO₃, a stirring speed of 250 rpm, and a 1:1 internal-to-membrane phase volume ratio. These conditions enhanced stripping capacity and droplet stability. A 20:200 treatment ratio yielded mechanically stable, well-dispersed droplets without excessive viscosity. Kerosene outperformed other diluents, achieving 100% removal in 50 min with only 9.1% breakage. A near-neutral pH (~6.7) ensured stable operation. Increasing the temperature from 5 °C to 35 °C accelerated uptake. The activation energy (33.13 kJ/mol) confirmed diffusion-controlled transport. Temperatures above 35 °C caused rupture. Salt inhibition followed the order Na₂SO₄ > NaCl > KNO₃. These results demonstrate a robust, scalable ELM protocol for high-efficiency rare-earth separation with minimal reagent use.

In addition to demonstrating the high extraction efficiency of Sm(III) using a low-concentration D2EHPA-TOPO system, it is important to acknowledge that the emulsion’s limited long-term stability and reusability could prevent its large-scale use. This study focused on extraction performance and emulsion breakage under controlled laboratory conditions. Future work should systematically evaluate the emulsion’s operational lifespan, resistance to repeated extraction–stripping cycles, and potential performance decline due to surfactant or extractant loss. Such investigations would provide critical insight into the economic and operational feasibility of this system in industrial settings. This would help ensure that promising laboratory-scale results can be translated into robust, sustainable, and cost-effective processes for rare-earth element recovery.