Synergistic Recovery of Dysprosium(III) from Water via an Emulsion Liquid Membrane at Low Concentrations of Cyanex 272–D2EHPA: Impact of Process Factors and Water Sources

Abstract

This study reports an investigation of the synergistic extraction of dysprosium (Dy(III)) from aqueous media using a low-concentration, binary carrier mixture of Cyanex 272 and D2EHPA within an emulsion liquid membrane (ELM). Within the tested formulations, the one containing 0.42% (w/w) Cyanex 272 and 0.28% (w/w) D2EHPA yielded the best results. The impact of process factors that maximize recovery efficiency and minimize emulsion breakdown was also examined. A Span 80 loading of 0.75% (w/w) achieved 97.5% extraction with minimal breakage (less than 2.1%). An external phase pH of 5.8 achieves an optimal balance of high-throughput Dy(III) recovery and membrane stability; 0.2 N HNO₃ as the stripping phase strikes the optimal balance, providing strong initial uptake with minimal emulsion degradation. As the initial Dy(III) loading increases, extraction efficiency decreases. Increasing the temperature from 15 to 45 °C accelerates mass transfer, achieving near-complete extraction in under 15 min. However, above 45 °C, emulsion breakage spikes, causing a collapse in efficiency. Similarly, increasing NaCl levels suppresses Dy(III) uptake and promotes coalescence. This reduces recovery from seawater to just over 70%. Nevertheless, the balanced mineral content of Zamzam water preserves emulsion integrity and enables 100% extraction. The activation energy was found to be 26.16 kJ/mol, suggesting that mass transfer, rather than the chemical reaction at the interface, controls the process. The results of this study highlight the synergistic efficiency advantage of the ELM system at lower carrier concentrations, even in complex water sources.

1. Introduction

The soaring demand for high-performance permanent magnets in renewable energy and advanced electronics has put dysprosium (Dy) in the spotlight as a critical material for modern, sustainable technology [1]. However, the low natural abundance of dysprosium poses significant challenges to conventional mining and separation processes. These issues lead to supply vulnerabilities and a considerable environmental footprint. These challenges highlight the urgent need for innovative recovery strategies that can extract Dy(III) from various aqueous environments while minimizing chemical usage and secondary waste.

Classical recovery techniques often fail when attempting to extract Dy(III) from complex matrices. Solvent extraction requires multiple stages and large volumes of organic solvents, increasing capital and operating expenses [2,3,4]. Precipitation and coprecipitation approaches lack specificity, producing mixed hydroxide sludges that require additional purification [5,6]. Ion exchange suffers from resin fouling and finite capacity, necessitating frequent replacements and increasing costs [7,8,9]. Adsorption methods typically have low uptake efficiencies at low concentration levels, and they face desorption challenges that complicate sorbent regeneration and disposal [10,11,12]. These drawbacks collectively highlight the necessity of a more efficient, selective, and low-waste process.

Emulsion liquid membrane (ELM) extraction is a compelling solution that combines extraction and stripping within a single, self-contained droplet system. The internal aqueous (stripping) phase is stabilized by a surfactant and dispersed into very fine droplets, ranging from 1 to 3 µm, via high-speed agitation [13]. These droplets are then encapsulated within the organic membrane phase, forming a water-in-oil (W/O) emulsion. The resulting emulsion disperses into the external aqueous feed phase as globules ranging from 100 to 2000 µm in diameter [13]. The dispersed emulsion’s high interfacial surface area dramatically accelerates mass transfer, enabling rapid uptake of solutes without requiring large external tanks or multistage contactors. Compared to thermal or crystallization methods, energy demands are reduced, and the compact, modular configuration of ELM facilitates scaling up and on-site deployment [14,15]. Tailoring the membrane composition allows operators to achieve remarkable selectivity for solutes. The closed-loop nature of the process reduces solvent inventory and minimizes reagent losses.

Some studies have examined the recovery of Dy(III) from aqueous media via ELM. Raji et al. [16] investigated the extraction of 50 mg/L Dy(III) from a nitric acid medium using an ELM containing 0.05 M D2EHPA and 2.5% Span 85/Span 80 in kerosene at pH 4, with a 1 M HNO₃ internal phase. Under optimal conditions, they achieved 99.6% Dy(III) extraction within 10 min. Additionally, they developed an ELM system that uses 1.25 M Cyanex 572, an internal phase at pH 1.4, 2.1% (v/v) Span 80, and kerosene to selectively extract 50 mg/L Dy(III) from mixtures containing Nd(III) [17]. Under optimized conditions, the system achieved 98.99% Dy(III) extraction. Sadehlari et al. [18] examined the continuous extraction of 50 mg/L Dy(III) at pH 4.0. They used an ELM (1.0 M D2EHPA and 2.5% Span 80 in kerosene) with an internal solution concentration of 1.0 M HNO₃ in a vertical, pulsed, packed column. They achieved 99.7% extraction in a single stage. Karmakar et al. [19] developed an ELM system for selectively extracting Dy(III) at 0.05 and 0.1 mM from a lanthanoid mixture to extract about 90% of the Dy(III) using mustard oil as the organic phase and aniline yellow as the complexing agent at pH 3. However, even though these studies were efficient, most of them used high concentrations of extractants.

By customizing the membrane composition, the exceptional capability of an ELM process may be achieved. To preserve extraction efficiency while reducing extractant concentrations, a diversified strategy is necessary. One policy is to use extractant pairings that work well together. These interactions reduce the overall load inside the liquid organic phase, thereby increasing extraction efficiency. Additionally, evenly distributing extractants throughout the liquid membrane using optimal phase ratios ensures that all molecules participate in transport and prevents redundant dosage.

This work was focused on synergistically recovering Dy(III) from aqueous media via the ELM technique at low extractant concentrations. The extraction performance of Cyanex 272 and D2EHPA was examined, both individually and in combination, at low loadings. The impact of process factors that maximize Dy(III) recovery efficiency and minimize emulsion breakdown was also investigated. The effect of temperature was investigated to determine the activation energy and identify whether mass transfer or the chemical reaction at the interface is the controlling step. Additionally, the impact of salt on the ELM process’s ability to remove Dy(III) from aqueous media was studied. Finally, the efficiency of the ELM system at lower carrier concentrations was tested in complex water sources.

2. Materials, Methods, and Procedures

2.1. Reagents

Primary solutions of Dy(III) were prepared by dissolving dysprosium acetate hydrate ((CH₃CO₂)₃Dy·4H₂O, CAS No. 15280-55-4; 99.9% purity) obtained from Sigma-Aldrich in deionized water. Employed solutions of Dy(III) were obtained by diluting measured volumes of the primary solution with deionized water to achieve the desired concentration.

Diisooctylphosphinic acid (Cyanex 272) and bis(2-ethylhexyl) hydrogen phosphate (D2EHPA) were used as analytical-grade reagents. Both chemicals were acquired from Fluka and utilized without further purification.

Sorbitan monooleate (Span 80), a non-ionic surfactant acquired from Sigma-Aldrich, served as the emulsifying agent in this study.

All additional reagents employed were of analytical purity and sourced from Sigma-Aldrich.

2.2. Methods

The process of extracting Dy(III) using an ELM technique involves three primary stages. First, the formation of the emulsion. Second, the pertraction of Dy(III) from the external solution through contact with the emulsion. Third, the separation of the emulsion from the aqueous feed solution via gravitational settling.

Figure 1 illustrates a schematic representation of the overall ELM procedure.

• Emulsion preparation

To prepare the internal stripping phase, the necessary amount of acid was dissolved in deionized water. For the liquid membrane, a specific quantity of the emulsifier Span 80 and the desired concentration of extractant were dissolved in a suitable diluent under gentle stirring with a magnetic agitator.

The emulsion was formed by combining the internal stripping phase with the liquid membrane using a homogenizer (Ultra-Turrax IKA T18) for a defined stirring duration.

• Dy(III) extraction

A measured volume of the resulting water-in-oil (W/O) emulsion was subsequently introduced into a predetermined volume of the feed aqueous phase containing Dy(III), inside a temperature-controlled cylindrical vessel equipped with a mechanical stirrer. The vessel was agitated using a 45° pitched, four-blade, impeller (5 cm in diameter) to disperse the W/O emulsion throughout the feed aqueous phase, producing a water-in-oil-in-water (W/O/W) double emulsion system.

Samples were periodically collected from the feed solution to monitor the extraction technique. The Dy(III) concentration was measured using the Arsenazo-III spectrophotometric technique with a Biochrom WPA Lightwave II instrument. Each experimental run was performed in triplicate to ensure reproducibility, with a maximum standard deviation observed between 2% and 3%.

Several process factors were investigated for their impact on Dy(III) extraction efficiency. These included surfactant concentration (0.25–9% w/w), carrier concentration (0–1.3% w/w), feed solution pH (1–5.8), emulsifying duration (1–9 min), internal stripping acid concentration (0.05–1 N HNO₃), type of acid in the internal solution (HClO₄, HCl, H₂SO₄, or HNO₃), agitation speed (100–400 rpm), internal/membrane volume ratio (1/2 to 2/1), treatment ratio (5/200 to 60/200), diluent type (trichloroethylene, dichloromethane, xylene mixture isomers, heptane, hexane, or kerosene), initial Dy(III) concentration (25–400 mg/L), temperature (15–65 °C), and NaCl loading in the external solution (0.5–40 g/L).

• Emulsion stability

The water-in-oil (W/O) emulsion stability was assessed in precise conditions. For the experiment, a cylindrical vessel was used that was kept at a consistent temperature and equipped with an overhead mechanical stirrer. In this setup, 200 mL of deionized water (the external solution) was combined with a specified volume of prepared W/O emulsion. To monitor changes in the external phase over time, the pH of the liquid was monitored in real time. A decrease in pH due to hydrogen ion (H⁺) migration from the internal phase to the external solution served as an indicator of emulsion destabilization. The extent of emulsion breakage was quantified as the percentage of the internal aqueous solution that leaked into the external aqueous solution relative to its original volume.

• Statistical analysis

To ensure reproducibility, all experiments were conducted in triplicate. The main values are reported throughout the manuscript and were usually the average of the three trials. The differences observed in the data of this work were evaluated for statistical significance using Student’s t-test, and results with p-values below 0.03 were deemed statistically significant.

3. Results and Discussion

3.1. Synergistic Carrier Combination

In an ELM system, the type and concentration of the carrier are critical factors affecting the extraction effectiveness and the membrane stability [20]. Selecting the right carrier is key to improving the selective pertraction of Dy(III) from the aqueous phase. These carriers act as mobile binding agents that promote the transfer of Dy(III) cations through the membrane by forming reversible complexes. The extraction behavior of Cyanex 272 and D2EHPA, individually and in combination, for Dy(III) from aqueous solutions was examined. The total extractant concentration was maintained at 0.7% (w/w), which will be further investigated later, while the mass fractions of each component were systematically varied. As shown in Figure 2a, the results indicate that the extraction efficiency steadily increases until equilibrium is reached. The extraction kinetics of six extractant formulations, Cyanex 272 and D2EHPA at 0.70% (w/w) and four binary mixtures with a total loading of 0.70% (w/w), span three performance tiers. Pure D2EHPA (0.70% w/w) performed the worst, plateauing at 58.8% removal after 50 min. Pure Cyanex 272 (0.70% w/w) accelerates early uptake, reaching 86.7% after 50 min. Thanks to its phosphinic acid group, Cyanex 272 outperforms D2EHPA by forming stronger complexes with Dy(III). Its faster kinetics and stronger chemical affinity for Dy(III) are evident in the steeper rise and higher plateau of its extraction efficiency curve. Adding a second extractant significantly improves both aspects. A mixture of 0.56% (w/w) Cyanex 272 and 0.14% (w/w) D2EHPA achieves 96.9% removal within 50 min. In contrast, a mixture of 0.14% (w/w) Cyanex 272 and 0.56% (w/w) D2EHPA only achieves 85.3% removal within the same time frame. A mixture of 0.28% (w/w) Cyanex 272 and 0.42% (w/w) D2EHPA reaches 90.9% after 50 min, demonstrating diminishing returns when D2EHPA dominates. A mixture of 0.42% (w/w) Cyanex 272 and 0.28% (w/w) D2EHPA quantitatively extracts 97.5%, achieving optimal efficiency. At a loading of 0.28% (w/w) D2EHPA alone, the extraction kinetics plateaued at 14.1% after 50 min. In contrast, 0.42% Cyanex 272 alone reached a maximum of approximately 75% within the same contact time. These results confirm the synergistic effect between 0.42% (w/w) Cyanex 2720 and 0.28% (w/w) D2EHPA. The synergistic effect observed in this formulation is especially insightful because the extraction efficiency exceeds what would normally be expected from the combined effects of each extractant alone. It seems that this improvement arises from the complementary roles of the extractants. The synergy stems from the formation of a mixed-ligand complex at the droplet interface via a specific process. The phosphinic acid sites of Cyanex 272 rapidly chelate Dy(III), reducing interfacial tension and enabling fast flux [21,22]. Then, the phosphoric acid groups of D2EHPA strengthen metal extraction, limit back-extraction, and stabilize the interfacial aggregate [21,22].

Mixed-extractants in the optimal liquid membrane containing 0.42% (w/w) Cyanex 272 and 0.28% (w/w) D2EHPA disperse the molecules of both compounds into microdomains. This disrupts their tight packing and reduces film viscosity and interfacial tension more effectively than either acid alone [23]. The looser monolayer shortens diffusion paths for Dy(III)–carrier complexes and continually exposes fresh extractant sites to the aqueous boundary, sustaining high instantaneous partitioning. A secondary synergy stems from the different acid strengths and molecular structures of Cyanex 272 and D2EHPA. These differences mechanistically reduce aggregation and stabilize the Dy(III)–extractant complexes. This decreases the free energy required for extraction and enables more efficient transfer through the membrane [21]. This elucidates why the mixture 0.42% (w/w) Cyanex 272 and 0.28% (w/w) D2EHPA dramatically outperforms all others. This formulation is ideal for practical deployment because it reduces residence times and the amount of extractant required compared to single-component systems.

3.2. Carrier Concentration Effect

The carrier concentration significantly impacts both the extraction capacity and the membrane stability. In previous experiments, the total extractant concentration was held at 0.7% (w/w). Therefore, optimizing the carrier dosage is essential to ensuring high separation efficiency while maintaining the membrane’s structural integrity. As demonstrated in the previous section, the optimal mixture for removal consists of 0.42% (w/w) Cyanex 272 and 0.28% (w/w) D2EHPA. Thus, the proportions of each carrier in the mixture are 60% Cyanex 272 and 40% D2EHPA. These percentages were maintained at a constant level when the total concentration of the mixture was changed from 0.1% to 1.3% (w/w). The impact of this concentration range on removing Dy(III) from aqueous solutions was examined (Figure 2b). As shown in this figure, both the extraction rate and the maximum achievable extraction improve significantly as the carrier concentration increases. Without a carrier, extraction remains minimal, which highlights the essential role of the carrier in facilitating extraction. Insufficient carrier concentrations (0.1–0.3% (w/w)) result in limited Dy(III) uptake due to reduced complexation capacity. As the carrier concentration increases, the efficiency curves evolve from a gradual incline at low concentrations to a rapid, near-sigmoidal ascent at moderate to high concentrations, ultimately approaching a plateau. Notably, the data indicate an optimal content of 0.7% (w/w). Beyond this threshold, additional increases yield almost no change in extraction efficiency. While augmenting the extractant loading in the liquid membrane increases extraction capacity, emulsion stability decreases above a threshold of 0.7% (w/w). Excessive carrier loading can lead to altered interfacial tension and increased viscosity, compromising the membrane’s stability. Therefore, maintaining a balanced carrier system increases Dy(III) recovery rates and ensures sustained membrane performance by controlling transport kinetics and minimizing emulsion breakage. Thus, 0.7% (w/w) was selected as the ideal extractant loading.

3.3. Surfactant Loading Effect

In an ELM system, surfactant loading impacts membrane stability and solute transfer. Therefore, studying the impact of surfactant concentration on Dy(III) extraction is crucial. To this end, experiments were conducted in which the Span 80 loading was changed from 0.25% to 3% (w/w). Figure 3a shows the extraction efficiency over time at various Span 80 concentrations. At low concentrations (0.25% and 0.5% w/w), the extraction efficiency is modest, reaching only 41.3% at the lowest concentration. This occurs because there is a deficient amount of surfactant to surround the entire membrane interface. With the lowest loading (0.25% w/w), the interfacial tension remains high, resulting in frequent droplet coalescence. Consequently, the accessible interfacial area for Dy(III) transport is limited. Therefore, the extraction efficiency reaches 50.5% after 15 min and decreases to 41.3% after 50 min. Increasing the surfactant concentration to 0.5% (w/w) increases extraction efficiency to 88.8%. Span 80 primarily functions by diminishing the interfacial tension between the aqueous and organic phases, thereby stabilizing the emulsion. Excellent Dy(III) ions extraction of 97.5% is achieved at a 0.75% (w/w) Span 80 concentration. At this loading, the emulsion droplets formed are more uniform and resistant to coalescence. This enhanced stabilization minimizes leakage of the internal solution and ensures a sustained driving force for solute extraction across the membrane. Conversely, a slight decrease in Dy(III) extraction yield was observed for loadings varying from 0.75% to 3% (w/w). Increasing the surfactant loading diminishes the membrane’s surface tension, resulting in the formation of small globules that offer a higher contact surface area. The decrease in extraction yield at a Span 80 loading higher than 0.75% (w/w) is due to an increase in membrane thickness and resistance to Dy(III) transfer. The emulsion breakage data highlights the importance of achieving a balance between stability and efficiency. Breakage decreases from 41.6% at 0.25% Span 80 (w/w) to less than 2.1% at 0.75% Span 80 (w/w). This transition reflects the formation of robust surfactant monolayers that resist film thinning and coalescence under shear. Above 0.75% (w/w) Span 80, slight increases in breakage suggest interface overpacking.

Figure 3b shows that higher concentrations of Span 80 lead to quicker attainment of the steady state. The initial steep ascent of the extraction curve at these concentrations indicates reduced mass transport resistance, likely due to the formation of smaller, more stable emulsion droplets. In contrast, incomplete emulsification at lower surfactant levels results in larger droplet sizes. This limits the effective interfacial area for mass transport, thereby slowing the extraction process. Excessive amounts of surfactant can augment the liquid membrane viscosity, which complicates the subsequent demulsification process necessary for recovering the internal phase. Additionally, the economic implications of using high surfactant levels must be considered, as the increased cost must be justified by improved extraction efficiency. Thus, optimizing ELM systems often involves finding a balance between achieving high extraction rates and maintaining manageability and cost-effectiveness. For this reason, the surfactant concentration in the liquid membrane was kept at 0.75% (w/w) for all subsequent experiments.

3.4. Emulsification Time Effect

Figure 3b provides a comparison of the extraction efficiency of the developed ELM process over time for emulsification durations ranging from 1 to 9 min. The curves illustrate how extraction efficiency changes over time. There is a quick initial increase, succeeded by an eventual approach to a plateau after approximately 20 min. This behavior indicates initial dominance of mass transfer, followed by an equilibrium state as the available interfacial area for mass exchange is fully utilized. The differences among the curves emphasize the importance of emulsification time in determining the overall performance of the ELM technique. Increasing the emulsification duration from 1 to 5 min augmented extraction efficiency from 91.5% to 97.5%. Longer emulsification durations result in finer, more uniformly distributed droplets, as seen with the 5 min curve. This increased uniformity enhances the interfacial area between the dispersed internal phase and the continuous feed solution, thereby accelerating mass transfer. Such refined droplet formation minimizes coalescence and leakage, ensuring the internal solution remains well encapsulated. This is critical for achieving high extraction efficiency. From a kinetic standpoint, the 5 min emulsification duration initially benefits from the large concentration gradients and high interfacial area generated by effective emulsification. During the initial stages, the process is dominated by rapid diffusion across the feed/membrane boundary layer. However, as emulsification continues and the droplets reach their optimal size, the extraction efficiency curve begins to level off. This reflects a transition to equilibrium conditions, wherein the rate of mass transfer is no longer the limiting factor. Minimal improvements in extraction yield and kinetics were observed with emulsification times of 7 and 9 min. Although increased emulsification time improves extraction, there is a practical limit beyond which further agitation does not significantly improve results and may destabilize the emulsion.

Emulsion breakage data mirror the trend in extraction performance, revealing a subtle stability optimum. With only 1 min of emulsification, weak interfacial films fail under shear, resulting in the leakage of over 11% of the internal aqueous phase. Increasing the emulsification time to 3 min produces a robust film and causes breakage to plummet to less than 2%. This suggests that the surfactant and carrier molecules have had sufficient time to align and pack optimally at the interface. Further agitation for 5 to 7 min maintains low breakage at around 2%. However, emulsifying for 9 min results in only a modest additional decrease in breakage (to approximately 1.6%). While this shows slight improvements in film uniformity, it also raises concerns about surfactant micelle formation in the organic phase.

The higher efficiency achieved with 5 min of emulsification indicates that this duration provides an optimal balance of droplet size reduction and emulsion stability under the studied conditions. This balance prevents excessive energy input and droplet breakup. Therefore, 5 min was selected as the optimal emulsification duration for preparing stable emulsions and extracting Dy(III) ions.

3.5. External Phase pH Effect

Figure 4a clearly illustrates that the pH of the external phase is crucial for achieving optimal Dy(III) extraction efficiency with the developed ELM system. At the highest pH condition (5.8, often referred to as the natural pH), the extraction efficiency quickly reaches 97.5%. This emphasizes the influence of the external phase’s chemical environment on the kinetics and equilibrium of Dy(III)–carrier complex formation. Higher pH conditions favor the deprotonation of the carrier species (D2EHPA’s is 3.90, and Cyanex 272’s pKa is 6.37) and facilitate Dy(III) ion complexation. This enhances mass transfer across the liquid membrane. In contrast, efficiency drops dramatically at lower pH values, particularly at pH 1.0, where extraction is less than 4.1%, even after extended periods. Efficiency and kinetics increase at pH 2; however, yield decreases within the pH range of 2 to 4, depending on chemical equilibria and emulsion stability. Efficiency increases further at pH 5.0, reaching about 71.5% after 50 min. Under highly acidic conditions, the dominant species in the external phase is free dysprosium (Dy³⁺). However, the carriers remain heavily protonated (D2EHPA’s pKa is 3.90, and Cyanex 272’s pKa is 6.37). Proton competition at the membrane interface suppresses the formation of Dy(III)–carrier complexes, drastically reducing the driving force for mass transfer into the organic membrane film. As the pH increases from 2.0 to 4.0, the progressive deprotonation of the carrier species creates binding sites for Dy(III), thereby enhancing the interfacial complexation reaction. This shift increases the partition coefficient of the Dy(III)–carrier complex and strengthens the concentration gradient that drives diffusion. Thus, the mid-pH region sees a rapid rise in the extraction rate. Above pH 4.0, D2EHPA is mostly deprotonated, enabling nearly all binding sites to engage with Dy(III). From pH 5.0 to 5.8, the extraction rate accelerates dramatically as the concentration gradient through the membrane peaks and the residence time required for high recovery decreases. Under these conditions, the rate-limiting step shifts from the chemical reaction at the interface to the diffusion of the Dy(III)–extractant complex through the organic phase. However, maintaining the external phase at a moderately high pH level requires vigilance to prevent the precipitation of metal hydroxides. Dysprosium begins forming colloids of Dy(OH)₃ at a pH level above 6.5. Data suggest that operating at pH 5.8 maximizes complexation-driven transport without triggering precipitation.

Additionally, the external phase pH influences the physical characteristics of the emulsion. The stability, size distribution, and morphology of the dispersed droplets depend on the pH. Under optimal pH conditions, the emulsion remains stable and maintains a high interfacial area, which is essential for efficient mass transfer. However, when the pH is too low, excessive protonation hinders chemical extraction and exacerbates the loss in efficiency caused by the physical breakage of the emulsion. This dual chemical and physical influence explains why the system is highly sensitive to pH variations.

Ultimately, the results offer a compelling indication of the critical role of external solution pH in Dy(III) extraction via the ELM process. Higher pH conditions facilitate optimal deprotonation of the carrier and maintain emulsion integrity. These conditions accelerate the extraction process, minimize acid consumption costs, and increase efficiency. Thus, an external solution pH of 5.8 was selected for all subsequent experiments.

3.6. Stirring Speed Effect

Figure 4b vividly illustrates the direct relationship between stirring speed (100 to 400 rpm) and Dy(III) pertraction effectiveness in the ELM technique. The ELM technique creates stable, finely dispersed globules that enable mass transfer at the interface between the feed solution and the internal stripping phase. Lower stirring speeds (100 and 150 rpm) result in a comparatively slow augmentation in pertraction efficiency. For example, at 100 rpm, the extraction effectiveness was 27.9% after 50 min. Gentle mixing produces larger globule sizes and a thicker stagnant layer at the interface, limiting the available interfacial area for diffusion of the Dy(III) ions. Increasing the speed to 150–200 rpm sharply steepens the curve. At 150 rpm, extraction surges to 40% within 10 min and levels off at around 58%. At 200 rpm, extraction surpasses 64% within the same time frame and approaches 86% in the long term. At intermediate stirring speeds (250–300 rpm), the balance between intensity and stability shifts toward greater efficiency. Enhanced agitation at these speeds produces smaller globules, which augments the effective contact area between phases and accelerates interfacial kinetics. At the highest stirring speeds (350–400 rpm), the extraction efficiency nearly reaches complete recovery in a much shorter amount of time. However, this effect diminishes over time. This rapid improvement underscores the benefits of vigorous agitation, which maximizes the interfacial area available for Dy(III) transfer. However, aggressive stirring can threaten emulsion stability by promoting faster kinetics. When the energy input surpasses the threshold necessary to maintain droplet stability, phenomena such as globule coalescence or breakage may occur. This can lead to phase separation and potentially undermine the overall extraction process.

Results on emulsion stability show that minimal leakage occurs at 150 rpm (0.94%), less than at 100 rpm (1.24%). This suggests that mild shear disperses the droplets and enables the formation of robust surfactant films without overstressing them. Beyond 150 rpm, breakage increases steadily: 1.43% at 200 rpm, 2.08% at 250 rpm, and 4.16% at 400 rpm. This increase reflects the onset of emulsion rupture under excessive shear, where the viscoelastic limit of the interfacial film is exceeded. This results in significant coalescence and internal leakage. Thus, while high speeds maximize the extraction rate, they also compromise emulsion integrity.

From a mechanistic perspective, the balance between globule size distribution and interfacial film strength depends on stirring speed. Moderate agitation produces fine, uniform globules that maximize the surface area available for Dy(III)–carrier complexation. At this speed, there is enough time for the surfactant and carrier molecules to organize into an elastic barrier that resists coalescence. However, speeds exceeding 300 rpm generate compressive and extensional stresses caused by micro-turbulence, which deform or puncture the globules before the film can recover.

These results underscore the importance of achieving a delicate operational balance. While increasing the stirring speed enhances mass transfer and accelerates extraction kinetics, the speed must be moderated to maintain emulsion stability. Future explorations will be conducted at 250 rpm, the optimal stirring speed for Dy(III) extraction.

3.7. Internal Phase Concentration Effect

Although an ELM technique can be used to extract Dy(III), it is important to strike a balance between mass transfer efficiency and emulsion stability. The stripping phase, an aqueous solution of nitric acid, provides the force needed to extract the ions from the external phase. As exposed in Figure 5a, augmenting the HNO₃ concentration intensifies the concentration gradient through the membrane, resulting in the rapid uptake of Dy(III) during the initial stages of extraction. However, while a higher acid concentration enhances mass transfer, it also endangers emulsion stability, as shown in Figure 5b. This figure depicts emulsion breakage and shows how the emulsion behaves over time at different HNO₃ concentrations. As the nitric acid dosage augmented from 0.05 to 0.1 N, the emulsion breakdown diminished from 8.15% to 0.73%. As the acid dosage augmented from 0.1 N to 1 N, the increase in breakdown became more pronounced, passing from 0.73% to 11.51%. At lower concentrations, the emulsion globules remain relatively stable for a longer duration, ensuring that the stripping phase retains its integrity. In contrast, elevated HNO₃ levels induce higher osmotic pressure across the droplet interface, promoting droplet swelling and subsequent coalescence. This ultimately leads to accelerated breakage. This observation highlights a critical trade-off: increasing the internal phase concentration enhances extraction kinetics, but it can also compromise the emulsion’s structural integrity over time.

Integrating insights from Figure 5a,b, it is evident that the driving force for extraction and the emulsion stability are interlinked variables influenced by internal phase composition. The destabilization mechanism is attributed to the intensified osmotic imbalance created by high acid concentrations. As the droplets swell, the protective surfactant layer that maintains their stability can be compromised, leading to coalescence and irreversible breakage. This behavior is especially critical in applications like Dy(III) extraction, where the retention of the stripping agent is essential for both recovery efficiency and process continuity. The obtained results demonstrate that beyond a specific acid concentration threshold (0.2 N HNO₃), the benefits of increased driving force may be negated by the rapid deterioration of the emulsion structure.

Consequently, optimizing the internal phase concentration in an ELM system is not solely about maximizing the transfer of Dy(III) ions but also about preserving the emulsion’s structural integrity. The ideal operational concentration requires a careful balance; providing sufficient acid concentration to ensure a strong concentration gradient, yet remaining below the level at which emulsion breakage becomes significant. Based on membrane stability and extraction yield, the nitric acid concentration will be 0.2 N for further investigation.

3.8. Type of Internal Solution Effect

Selecting the right internal aqueous stripping solution is essential for achieving optimal efficiency and maximum yield when extracting Dy(III) ions via the ELM method. In an ELM system, the internal solution removes the recovered species from the organic membrane and regenerates the carrier. This maintains a driving force for mass transfer. Figure 6a compares the performance of four mineral acid species at 0.2 N in the internal phase on extraction efficiency over time. This detailed kinetic profile reveals how acid type affects stripping capability and, consequently, overall Dy(III) recovery. The extraction behavior indicates that nitric acid is more effective than perchloric acid. Perchloric acid, in turn, is more effective than sulfuric and hydrochloric acids, which have similar effects.

It was observed from Figure 6a that all internal phase types exhibit a rapid initial uptake of Dy(III) within the first 10 min, which can be attributed to the high concentration gradient between the aqueous and organic phases. However, the rate of extraction gradually decelerates as the system approaches equilibrium. The identity of the anion in the internal phase affects how easily the carrier–Dy(III) complex releases the metal cation and regenerates the extractant at the water–membrane interface. Nitric and perchloric acids produce the fastest stripping. Their monovalent anions do not form stable inner-sphere complexes, so the equilibrium strongly favors metal release. In contrast, sulfuric and hydrochloric acids, despite having equal proton concentrations, show slower kinetics and lower ultimate recovery. This demonstrates that proton activity alone does not determine the stripping rate. Nitrate and perchlorate ions facilitate nearly unhindered Dy(III)–carrier dissociation because they exist as free, weakly coordinating anions within the droplet core. Once the Dy(III)–carrier complexes reach the interface, the high local proton concentration instantly reprotonates the carrier, ejecting the Dy(III) ion into the aqueous droplet. During the first 8 min, perchlorate slightly outperforms nitrate because its larger ionic radius and low hydration sheath favor faster diffusional exchange of H⁺ and the freed carrier across the membrane film. This compresses the stripping cycle. These features explain why HClO₄ sometimes yields marginally sharper early-time kinetics, even though both acids achieve comparable recoveries within 7–10 min. Complexes formed in the presence of sulfuric acid are resistant to displacement by protonated carriers. This decreases the pool of free Dy(III) ions and reduces the stripping flux. Additionally, the double charge on sulfate increases the osmotic pressure of droplets more quickly than monovalent anions do. To balance osmotic gradients, water influx causes the surfactant film to swell and thin slightly. This further retard mass transfer by stretching the interfacial barrier and slightly raising interfacial tension under shear. Hydrochloric acid has slower stripping kinetics and a similar extraction capacity to sulfuric acid. Chloride anions form tightly bound chloro-complex species with Dy(III), retarding the stripping reaction. Additionally, high chloride levels decrease the electrostatic repulsion between surfactant headgroups at the droplet boundary [24]. This makes the interfacial film slightly less elastic and more susceptible to thinning under agitation. Together, these two factors slow down the regeneration of the carrier.

From a mechanistic perspective, the performance differences among these acids arise from several intertwined factors. First, the acid strength and its dissociation behavior under the conditions of the ELM process determine the effective concentration of hydrogen ions available for the stripping reaction. Consequently, nitric acid was designated as the optimal internal stripping solution.

3.9. Internal/Membrane Volume Ratio Effect

Figure 6b shows the multifaceted relationship between the volume ratios of the internal and membrane phases (V_int/V_org) and how they affect the pertraction efficiency of Dy(III) in the implemented ELM technique. Dy(III) pertraction profiles measured at V_int/V_org ratios ranging from 1/2 to 2/1 reveal a pronounced maximum at an intermediate volume ratio rather than at the extremes. At the lowest tested ratio of 1/2, extraction steadily increases and plateaus at around 95% after 50 min. The highest extraction efficiency occurs at a 1/1 ratio, which indicates that an equal volume of the two phases produces uniformly stabilized emulsion droplets and enhances interfacial mass transfer. The best extraction kinetics and efficiency occur at a 1/1 ratio. However, for ratios greater than 1/1, the kinetics slow down, and the extraction efficiency decreases considerably. Deviating from this balance by increasing or decreasing the internal solution volume relative to the liquid membrane appears to disrupt stability and diminish the contact area between the phases. This can lead to larger or irregular droplets, slowing the diffusion of Dy(III) cations from the aqueous feed phase into the internal stripping phase. For instance, the extraction effectiveness is 93% at a 3/2 ratio and decreases to 78.45% at a 2/1 ratio. This nonlinear trend illustrates the trade-off between sufficient stripping capacity to prevent early saturation and adequate organic film area for rapid mass transfer. Augmenting the volume portion of the internal solution enlarges the internal droplets, making the W/O emulsion highly viscous. Consequently, an augmentation in the diameter of the emulsion globules diminishes the interfacial area between the external phase and the emulsion, thereby diminishing Dy(III) extraction effectiveness. These large globules cannot disperse in the phase being treated, slowing down extraction. The appropriate internal phase volume governs both droplet size and the intrinsic driving force for mass transfer. An optimal ratio creates a high concentration gradient and maintains an effective concentration of the stripping reagent within the droplets. This directly affects extraction kinetics. The optimal internal/membrane volume ratio is 1/1.

3.10. Treatment Ratio Effect

Figure 6c offers a compelling insight into how the treatment ratio directly influences the recovery effectiveness of Dy(III) via the ELM technique. In this system, the emulsion acts as the medium that transports Dy(III) ions from the aqueous external phase into the internal stripping phase. As the volume ratio increases from 5/200 to 60/200, there is a proportional enhancement in available interfacial area between the emulsion globules and the feed solution, which in turn accelerates the mass transfer. Notably, at the highest ratio examined (60/200), the extraction efficiency rapidly climbs to 100% within 5 min, whereas a low ratio such as 5/200 only reaches about 19.1% even after 50 min. This strongly suggests that a higher emulsion volume in relation to the external phase provides more interfacial contact area, thereby reducing diffusion distances and driving a more efficient uptake of Dy(III). Diving deeper into the mechanism, the extraction across an ELM is typically governed by a series of resistances: the external aqueous boundary layer, the liquid membrane, and the internal stripping phase. Increasing the treatment ratio effectively multiplies the number of micro-droplets present, each acting as a micro-reactor for Dy(III) extraction. This multiplication not only increases the aggregate interfacial area available for the ion transport but also minimizes the path length that Dy(III) ions must traverse. Consequently, the residence time for the ions in the external solution decreases, and their diffusion into the interior of each droplet is greatly facilitated. Additionally, the steep kinetic profiles at higher volume ratios suggest that mass transfer is primarily controlled by interfacial phenomena rather than by bulk diffusion limitations.

From an economic perspective, determining the optimal treatment ratio involves balancing enhanced extraction effectiveness against the cost implications of increased reagent consumption and process complexity. A higher emulsion-to-external phase ratio accelerates mass transfer, minimizing the residence time of Dy(III) ions in the external phase and improving throughput, thereby potentially reducing equipment size and processing time. Yet, beyond a certain point, the marginal gain in extraction performance may not justify the extra cost. In contrast, operating at a lower ratio might preserve financial resources on chemicals and reduce energy expenditure, but could lead to longer extraction times and potentially lower throughput, which would ultimately affect productivity and increase operational costs over extended runs. The optimal economic ratio is, therefore, a trade-off point where enhanced extraction performance and reduced processing time adequately balance the added capital and operating costs incurred by using a higher volume of the emulsion phase. Consequently, a 20/200 ratio was designated as the optimal processing ratio.

3.11. Diluent Type Effect

Figure 7a illustrates the significant impact of selecting the right diluent on the recovery efficiency of Dy(III) in an ELM technique. Figure 7b shows the temporal variation in emulsion breakage for the tested diluents. In these experiments, the extraction efficiency of several diluents, including dichloromethane, heptane, hexane, trichloroethylene, and xylene mixed isomers, was compared to that of kerosene over 50 min. The first set of diluents notably facilitated a rapid extraction process, achieving an efficiency greater than 86% within the first 10 min. In contrast, dichloromethane severely underperformed, remaining below an efficiency of 18.6%. Closer examination of the underlying mechanisms reveals that the high extraction efficiencies observed with xylene mixed isomers, heptane, hexane, trichloroethylene, and kerosene are primarily due to the favorable physicochemical properties of these compounds. These diluents typically have lower viscosities and interfacial tensions, which are essential for the fast diffusion of Dy(III) ions from the feed phase into the organic membrane. Lower interfacial tension supports the generation of a stable emulsion and enhances the partitioning of the Dy(III)–carrier complex into the organic phase. Conversely, dichloromethane’s higher polarity results in higher interfacial tension, hindering efficient mass transfer and reducing its overall extraction efficiency.

Xylene mixed isomers are an interesting diluent during the first 10 min. This is due to their combination of properties inherent to aromatic solvents, including moderate viscosity, suitable interfacial tension, and a molecular structure that promotes favorable interactions with carrier molecules. Xylene mixed isomers’ rapid performance is due to its aromatic rings, which facilitate the solubilization of the Dy(III)–carrier complex in the organic phase. This enhances the overall mass transfer rate. However, the benefits of fast extraction come with challenges. The physicochemical properties that underpin xylene mixed isomers’ excellent extraction performance may also contribute to moderate emulsion instability (Figure 7b). The molecular arrangement of xylene mixed isomers can foster interfacial phenomena that lead to a higher propensity for droplet coalescence under the influence of shear forces and pulsation.

When comparing kerosene, heptane, hexane, and trichloroethylene, it is important to consider their extraction efficiency and emulsion stability. Although heptane, hexane, and trichloroethylene are excellent for rapid, complete extraction, they result in higher emulsion breakage, which could affect the entire operation. Kerosene offers a better balance, providing high extraction efficiency and greater stability compared to heptane, hexane, and trichloroethylene. For this reason, it was chosen as the optimal diluent for Dy(III) extraction.

3.12. Dysprosium Initial Concentration Effect

Figure 7c illustrates the impact of initial Dy(III) concentration on extraction efficiency in the developed ELM process. The extraction curves at various initial concentrations clearly demonstrate an inverse relationship between feed loading and removal efficiency. At lower concentrations (e.g., 25–50 mg/L), the efficiency of extraction increases almost instantaneously. Complete extraction was achieved in 3 min at 25 mg/L and in 7 min at 50 mg/L. This is due to the abundant availability of active carrier sites and a steep concentration gradient that drives rapid mass transfer in the internal solution. Upon augmenting the concentration to 150–200 mg/L, the kinetics of the initial period slow down slightly. However, as the initial Dy(III) concentration increases (e.g., 300–400 mg/L), the initial rapid uptake is followed by a slower approach to equilibrium. After 50 min, 85–87% removal occurs, and clear plateaus emerge well below quantitative recovery. This suggests that the internal phase becomes increasingly saturated, which impedes the diffusion of ions across the liquid membrane due to concentration polarization and viscous sublayer effects. This behavior is consistent with previous studies on ELM systems that found optimal extraction conditions occur at moderate feed concentrations [15], balancing the driving force for mass transfer with the membrane phase’s finite capacity. Beyond this optimum, further increases in concentration tend to limit the overall extraction efficiency due to diffusional constraints and interfacial phenomena.

3.13. Temperature Effect

Figure 8a,b illustrate the impact of temperature on the effectiveness of extracting Dy(III) and the stability of the membrane. The extraction profiles at different temperatures demonstrate the sensitivity of Dy(III) transport to thermal conditions. Between 15 and 45 °C, the uptake curves accelerate significantly. At 15 °C, the extraction plateaus at around 97% after 30 min. Between 25 and 45 °C, extraction nearly reaches 100% in under 15 min. However, beyond this optimal range, performance deteriorates. At 55 °C, maximum extraction decreases to 78.5%. At 65 °C, extraction stagnates at around 77.1%. After prolonged contact, extraction declines sharply, reaching only 7.8%. This non-monotonic trend suggests that moderate heating improves mass transfer, but excessive temperatures undermine the driving forces and cause instability. Temperature affects the system in two ways: thermodynamically and kinematically. Higher temperatures decrease the viscosity of the aqueous and organic phases, while increasing the diffusivity of the Dy(III)–carrier complex. This is an example of an Arrhenius-type enhancement of mass transfer coefficients. Within the temperature range of 15 to 45 °C, droplets deform more readily under shear. The organic film thins and, according to Fick’s law, the concentration gradients across the membrane become steeper. These factors underpin the rapid, nearly quantitative extraction observed in this regime. However, when temperatures exceed 45 °C, the solubility of water in the organic diluent increases. The interfacial tension decreases too rapidly, which can cause an unfavorable shift in the carrier’s partition coefficient. This reduces the net transfer driving force. However, Figure 8b shows that there is a critical threshold beyond which the membrane’s stability is compromised. Specifically, the results demonstrate that extraction performance improves in the lower temperature range with incremental yield increases and that membrane breakage remains acceptable (e.g., below 3%) until temperatures exceed 35 °C. Beyond this point, the rate of membrane rupture escalates dramatically (8.1% at 35 °C, 20.9% at 45 °C, 37.2% at 55 °C, and 58.9% at 65 °C), indicating the thermal stress imposed on the surfactant-stabilized emulsion structure. The thermal softening of the surfactant film reduces its viscoelastic modulus and accelerates the desorption of Span 80 molecules. This makes the droplets susceptible to coalescence under shear. Furthermore, increased thermal energy exacerbates internal-phase swelling. Elevated water solubility in the organic phase drives osmotic influx, stretches the surfactant shell, and causes micro-ruptures, which become particularly acute above 45 °C. These results demonstrate that while higher temperatures boost kinetic processes and enhance ion transfer across the membrane, they also trigger instability and degradation of the emulsion structure. Therefore, optimizing the temperature is crucial for balancing maximizing extraction efficiency and preserving membrane integrity.

The Arrhenius equation illustrates the dependency of the pseudo-first-order kinetic constant on temperature. Across the temperature span of 15–45 °C, this relationship can be used to compute the activation energy required for extracting Dy(III) using the implemented ELM process. The activation energy is determined by finding the slope of the lnk versus 1/T plot. The slope yielded an activation energy value of 26.16 kJ/mol. Activation energies less than 40 kJ/mol typically indicate that the process is governed by mass transfer. This means that diffusion across the phases, rather than the chemical reaction at the interface, determines the overall kinetics. Consequently, thermally increased diffusion across the liquid membrane and the aqueous phases is consistent with the noticeable acceleration at higher temperatures (up to 45 °C).

3.14. NaCl and Natural Water Matrices Effect

Figure 9a shows that increasing the concentration of NaCl negatively affects the recovery efficiency of Dy(III). As the NaCl concentration augments, the extraction effectiveness declines. This outcome is due to two interrelated mechanisms. First, a higher chloride ion concentration intensifies the competition between Dy(III) and the carrier extractant for binding sites, hindering the formation of the critical Dy(III)–extractant complexes. Second, increased ionic strength appears to destabilize the emulsion by promoting droplet coalescence and reducing the effective interfacial area. Both factors are essential for facilitating mass transfer in the ELM process. This nuanced understanding underscores the importance of precise control over ionic conditions, especially when adapting ELM technologies for use in saline or other complex aqueous environments.

Figure 9b shows the varying extraction efficiencies of Dy(III) from two water sources, seawater and Zamzam water, using the developed ELM process. The 97.5% extraction efficiency in bidistilled water indicates favorable mass transfer conditions because there are no interfering ions present, and the interfacial conditions are optimized for the carrier extractant. In contrast, the extraction efficiency decreases in seawater. It peaks at approximately 72.1% after 15 min due to the high ionic strength of chloride ions and competing cations, which interfere with the carrier’s ability to form complexes with Dy(III) ions. After 15 min, the extraction efficiency in seawater decreases slightly and reaches 69.8% after 50 min. Meanwhile, Zamzam water exhibits the highest performance, reaching 100% efficiency after 20 min, likely due to its favorable composition. Zamzam water contains a specific balance of minerals that enhances emulsion stability, facilitating remarkably higher extraction kinetics and efficiency. This balanced ionic environment minimizes unwanted interactions at the membrane interface while favoring the formation of stable Dy(III)–carrier complexes.

4. Conclusions

This study demonstrates the exceptional performance of an optimized ELM system that uses low concentrations of extractants to recover Dy(III) from aqueous solutions. A binary carrier mixture of Cyanex 272 and D2EHPA produced a true synergistic effect. Specifically, a mixture containing 0.42% (w/w) Cyanex 272 and 0.28% (w/w) D2EHPA removed 100 mg/L of Dy(III) quantitatively in 50 min, outperforming the individual extractants. This synergy arises from complementary interfacial film properties and dynamic proton shuttling, which sustain rapid cycles of complexation and decomplexation. Systematic variation of key process parameters revealed the optimal conditions for achieving high extraction efficiency, robust emulsion stability, and practical operability: total carrier concentration: 0.7% (w/w) (0.42% (w/w) Cyanex 272 and 0.28% (w/w) D2EHPA), surfactant (Span 80) loading: 0.75% (w/w), emulsification time: 5 min, internal (stripping) phase: 0.2 N HNO₃, external (feed) phase pH: 5.8 (natural), stirring speed: 250 rpm, internal / membrane volume ratio: 1/1 (v/v), treatment ratio: 20/200 (v/v), diluent: kerosene, and temperature: 15 °C. Under these conditions, emulsion breakage was minimized to less than 2%, and high mass-transfer rates were maintained. Deviations above or below the prescribed parameter windows reduced Dy(III) recovery. For instance, excess carrier loading (more than 0.7%) increased viscosity and caused emulsion rupture. High temperatures (above 45 °C) accelerated droplet swelling and coalescence. Extreme pH or salt concentrations compromised interfacial complexation and stability.

Beyond tests with synthetic feed, the optimized ELM exhibited strong resilience in complex matrices. Performance was influenced by saline conditions (up to 40 g/L NaCl) and real waters (seawater and Zamzam water) due to the effects of ionic strength. However, the balanced mineral content of Zamzam water unexpectedly enhanced emulsion robustness and kinetics, achieving full Dy(III) recovery in 20 min. This versatility underscores the process’s adaptability to real-world applications.

In summary, this work provides a blueprint for practical Dy(III) recovery via a low-carrier-concentration ELM. The identified synergistic carrier blend and finely tuned operating window deliver high selectivity, rapid kinetics, and stable emulsions. Future efforts will focus on pilot-scale validation, continuous-flow operation design, and the recovery of other critical rare earth elements using similar frameworks.