Self-Regulated Ion Permeation through Extraction Membranes

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Self-regulated Ion Permeation through Extraction Membranes Jean Duhamet, Helmuth Möhwald, Maximilian Pleines, and Thomas N Zemb Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02256 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on September 2, 2017

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Self-Regulated Ion Permeation through Extraction Membranes Jean Duhamet†, Helmuth Möhwald‡, Maximilian Pleines§, Thomas Zemb*§ †

CEA, DEN, Research Department on Mining and Fuel Recycling Processes, BP 17171, 30207 Bagnols-sur-Cèze, France ‡

Max Planck Institute of Colloids and Interfaces, Department of Interfaces, 14476, Potsdam, Germany §

Institut de Chimie Séparative de Marcoule, UMR 5257 (CEA/CNRS/UM2/ENSCM), BP. 17171, 30207 Bagnols-sur-Cèze, France

KEYWORDS: phase transfer, ion separation, pertraction, complex fluids

ABSTRACT Separation of rare earth compounds from water into an organic phase in practical cases requires the use of specific ion binding ligands in high concentrations. These tend to form complex liquid crystalline phases preferentially at ion rich locations inside a pertraction membrane. They form a blocking layer above an ion concentration threshold, which is experimentally characterized. It is shown to limit the flux through the membrane, which is studied for the application of rare earth recycling, an example being the phase transfer of Nd from water

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into organic phase. This feedback leads to a stationary membrane permeation rate that can be modelled without any free parameters in very good agreement with experiment. The ion specific formation and dissolution of the blocking layer, a feature found also in nature, and its control suggest further studies to enhance permeation as well as its selectivity control..

1. INTRODUCTION The recovery of metal ions is an important task in many areas.1 Toxic heavy metal ions, and in particular nuclear waste materials, should be removed efficiently from water and valuable ions should be recovered and recycled. The focus of this study is the recovery of precious ions, notably rare earth ions, that are used in many electronic and electromagnetic devices, and that should be recovered from waste of these devices.2 In a typical separation process the ion to be recovered is dissolved in small quantity in water, and should be complexed into an organic phase by an oil soluble ligand. In order for this to occur, the ion must transfer across an oil/water interface, and to achieve this in practical quantities there are conceptually two approaches: (i) Prepare an emulsion with a high specific interface and later break this emulsion.3,4 However, the mixing in traditional liquid-liquid extractors can lead to the formation of emulsions that can be difficult to separate in order to recover the product. It is also well known that a density difference between the phases is needed to ensure correct operation in continuous counter-current devices, which are nevertheless subject to flooding and load limits. In addition, operating and maintenance costs of the equipment required for this process can be high. As the concentration of metal in the aqueous phase is low, it is important to obtain a higher concentration in the organic phase. This can be done, if the distribution coefficient is high, by using a small organic/aqueous flow ratio. 5


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Such a small flow ratio is not easy to use in conventional liquid-liquid extractors as it leads to poor interfacial area. (ii) Set up a pertraction membrane system with water and ions flowing on one side, and oil and the complexing agent flowing on the other side and in the membrane. 4 In this case the ions should pass into the oil phase without any additional separation, but to have sufficient amount separated one would need many separation columns in parallel. No density difference between phases is required. This is the process chosen for this study, which concentrates on the understanding of the transfer within one column. The extractants are typically asymmetric organic molecules with ion complexing groups at one end, and as they are normally used in high concentration, they tend to form a complex liquid crystalline phase.6 Moreover, the extractants used in all extraction processes are typically surface active.2 They exhibit specific complexing properties, mainly via « soft » or « hard » negatively charged active functions.7 Therefore, they can self-assemble into weak aggregates comprising a curved 2D self-assembled film containing water/oil hexagonal tubes effectively filled with a 1D ion containing liquid.8 The amphiphilicity is necessary to allow aggregation nucleated by ions of matching affinity.9,10 It has been demonstrated that the degree of nucleation of the aggregates and thus the threshold of concentration for appearance of the liquid crystalline phase depends on the chaotropic character or equivalently the free energy of hydration of the ions involved.11 If the ion to extractant molar ratio grows above a certain value, so-called “third phases” form. With single or short chain amphiphiles this third phase is a concentrated microemulsion phase, while longer chains always favor w/o liquid crystalline phases. Inside the membrane, such a phase, comprising hexagonally packed w/o cylinders with protruding chains 12,13, may form . As one expects concentration gradients to exist over typical membrane dimensions of some 100 µm, these


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complex phases could extend over some 10µm and influence the separation process. Here, for a typical extractant bis(2-ethylhexyl) phosphate (HDEHP), rare earth ion (Nd) and oil (Isane® IP175) in a pertraction membrane it is shown that such a phase can exist and that it drastically affects the separation process. The transfer kinetics for specific membrane parameters, diffusion coefficients and flows applied are modelled, with good agreement with measurements performed with optical ion detection and quantification. It is shown that the third phase changes the phase transfer kinetics, by in this specific case slowing the process down. A mechanism is proposed that is similar to a feedback loop, where the ion transfer kinetics is controlled by the ion concentration and its variation in the membrane.

2. EXPERIMENTAL SECTION 2.1 Materials Aqueous phases are prepared with dilutions of Rectapur nitric acid from VWR Chemicals and neodymium(III) nitrate hexahydrate, 99.9% from Aldrich. The organic phase is prepared with bis(2-ethylhexyl) phosphate (HDEHP) 97% from Sigma-Aldrich and Isane® IP 175 from TOTAL Special Fluids. The ligand and solvent concentration is typically given in Molarity, the Nd concentration in weight concentration. The atomic weight of Nd is 144 Da. 2.2 Set-up to measure pertraction The setup designed for the experiments is shown schematically in Figure 1. An oil phase (red) is cycled in a cylindrical tube with length 270 mm and inner diameter 1.8 mm. The cylindrical membrane (Microdyn Nadir) made of polypropylene (hydrophobic) has pores of about 200 nm and a thickness of 400 µm. The porosity is 72% (measured by mercury porosimetry) and the tortuosity of the membrane, the square of the ratio of the average distance between two points


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along the paths defined by the pores over the shortest distance between these two points, is around 2.14 The oil channel is connected to an optical absorption spectrophotometer USB 400 from Ocean Optics to measure the Nd concentration. Whereas the oil flows in a closed loop, the water outside (blue) flows in an open loop in an outer cylindrical tube of length 270 mm and 4 mm inner diameter. The outer tube housing the water is made of glass, and this is in turn embedded in a tube (grey), that enables the treatment by ultrasound. The ultrasonic device (Hielscher GDmini2) is commercialized for emulsification and is adapted here to potentially accelerate liquid flow through the membrane. This can be achieved, but in this study measurements were carried out without ultrasound. 2.3 X-ray scattering and rheology For analysis by small-angle X-ray scattering and rheology the third phase was prepared by contacting 1 M HDEHP in Isane IP 175 with an aqueous solution of 2.4 g L-1 Nd(NO3)3 in a 1:20 volume ratio. The resulting phases were centrifuged and separated. The white and viscous third phase was transferred into a 2 mm capillary. X-ray scattering experiments were carried out using a bench built by Xenocs and a molybdenum source from GENIX, delivering a 1 mm circular beam of energy 17.4 keV. Monochromatic radiation was provided by a Fox-2D multi-shell mirror. The diffraction pattern is recorded via a MAR345 image plate detector. Absolute intensities were obtained by using a 2.38 mm thick high-density polyethylene sample as a calibration standard. Data pre-analysis was performed using the software FIT2D and by taking into account the electronic background of the detector, empty cell subtraction and transmission measurements. The acquisition time of the samples was 3600 s. For rheology measurements, an Anton Paar MCR 301 rheometer was used. A plate-plate geometry with a diameter 24.981 mm was chosen to measure shear viscosities under thermostatic control from shear rates of 0.01 to


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500 1/s. Shear rates were increased with 10 measurement points per decade. The measurement duration per point was set to 10 s. 2.4 Modelling of time dependent pertraction Due to the axisymmetry of the system, the model is written in 2 dimensions: the axis of the cylindrical fiber z and the radius r. Typical values of flow rate in the fiber and the shell side lead to low Reynolds numbers and the flows can be considered as laminar. Variation of velocity of the flows is only a function of the radius and can be derived from Hagen-Poiseuille law (parabolic shape inside the cylindrical membrane and quasi parabolic shape, with a logarithmic term, in the annular zone outside the membrane). The general transport equation in both phases can be written as: 𝜕𝐶𝑖 (𝑟, 𝑧, 𝑡) 𝜕𝐶𝑖 (𝑟, 𝑧, 𝑡) 𝜕 𝜕𝐶𝑖 (𝑟, 𝑧, 𝑡) 𝜕 𝜕𝐶𝑖 (𝑟, 𝑧, 𝑡) 1 𝜕𝐶𝑖 (𝑟, 𝑧, 𝑡) = −𝑢(𝑟) + (𝐷𝑖 ) + (𝐷𝑖 ) + 𝐷𝑖 𝜕𝑡 𝜕𝑧 𝜕𝑧 𝜕𝑧 𝜕𝑟 𝜕𝑟 𝑟 𝜕𝑧 Where 𝐶𝑖 represents the neodymium concentration in the aqueous phase (i=aq), in the organic phase (i=org). In the same way 𝐷𝑖 represents the molecular diffusion in the aqueous or in the organic phase. Due to the hydrophobicity of the membrane, it is filled with the organic phase. There is no convection in the membrane and the transport of the ions is provided by the molecular diffusion of the organic phase in a porous medium characterized by its porosity ε, measuring the fraction of pore volume, and its tortuosity τ, measuring the extension of paths through the membrane compared to normal. In this case, neglecting inner wall interactions, the diffusion coefficient that must be taken into account is

𝐷𝑜𝑟𝑔 𝜀 𝜏

.


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For a given value of z, the flux of transfer of neodymium at the interphase (radius 𝑟𝑖𝑛𝑡𝑒𝑟𝑓), located at the outer diameter of the membrane, can be written as:

∅(𝑧, 𝑡) = 𝐷𝑎𝑞

𝜕𝐶𝑎𝑞 (𝑟𝑖𝑛𝑡𝑒𝑟𝑓 , 𝑧, 𝑡) 𝐷𝑜𝑟𝑔 𝜀 𝜕𝐶𝑜𝑟𝑔 (𝑟𝑖𝑛𝑡𝑒𝑟𝑓 , 𝑧, 𝑡) 𝐶𝑜𝑟𝑔 (𝑟𝑖𝑛𝑡𝑒𝑟𝑓 , 𝑧, 𝑡) = = 𝜀𝑘𝑣 (𝐶𝑎𝑞 (ℎ, 𝑧, 𝑡) − ) 𝜕𝑟 𝜏 𝜕𝑟 𝐾𝑑

with 𝑘𝑣 the interfacial transfer velocity at the interphase (m.s-1) and 𝐾𝑑 the distribution coefficient (ratio of organic concentration versus aqueous concentration of neodymium). 𝐾𝑑 is not constant and can be expressed as a function of neodymium concentration in the aqueous phase (figure 7). In order to represent the blocking layer formed in the membrane near the aqueous phase, we assume that above a threshold concentration of neodymium in the organic phase, the diffusion coefficient of this phase tends to zero as presented in figure 5. The system of equations has been solved numerically with a finite difference method using Scilab software. The fineness of the geometric mesh has been optimized for results independent of the size of the mesh while maintaining a quick calculation time. For the resolution in time, an implicit first order scheme has been used to increase the stability. However, when the concentration threshold is reached in the membrane, numerical instabilities occur if the time step is not small enough. Typically, this time step must not exceed one second in our case. 3. RESULTS AND DISCUSSION A typical representative measurement of the Nd concentration in the oil phase as a function of time after loading together with two scenarios for simulations is presented in Figure 2. There is an initial lag time before the liquid reaches the detector, followed by an increase in Nd concentration, as expected for a diffusion limited exchange. Given a time-independent diffusion


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coefficient, the data would be expected to follow the blue line on Figure 2. However, there is a well-defined time where the data deviates from this line and a slower linear increase is observed. This deviation can also be modelled (red curve), as discussed in the following section. Obviously the permeation rate decreases with time, which is likely to indicate clogging in the membrane.Normally there are many reasons for membrane fouling, but it is suggested here that this is a typical type of clogging related with the specific nature of the extractant system used in this study.It is related to the phase diagram of the ligand/ion/solvent system, exhibiting a viscous phase at high ligand and ion concentration. Understanding the nature of the clogging will enable greater control over the extraction process, which may in the future enable ion specific permeation. To simulate the evolution of the Nd concentration in the device in both phases the axisymmetric model described above considers pure laminar flow inside and outside of the tube (Poiseuille flow) and molecular diffusion in both phases. At the interface, a distribution coefficient of Nd is taken into account to represent the extraction of Nd in the oil phase.

To model the permeation versus time it is first assumed that the third phase does not exist. In this case the input parameters are the diffusion coefficients Daq and Dorg in water and oil, respectively. The diffusion Dm through the membrane is then given by the tortuosity t and porosity ε, according to Dm= ε∙Dorg/t . The diffusion coefficient Daq is known from literature (Daq = 5.9∙10−10 m2 sec−1), while Dorg is derived from extrapolation of data obtained for 1M Nd in HDEHP.14 The latter is necessary, as D org is expected to depend on the HDEHP concentration within the organic phase, which also alters the viscosity. From viscosity data the error in this extrapolation is presumably around 20%, however it is still possible to establish the influence of


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varying the diffusion coefficients, which is assessed by the simulations in Figure 3. Reducing D reduces the extraction rate, but the curve retains the same shape in all cases. Likewise, changing the Nd concentration in the feed has little effect on the curve shape. The shape with permeation rate decreasing with time can easily be understood, as the rate basically depends on the concentration gradient across the membrane. This gradient decreases with time at lower Nd concentration, and hence this also holds for the permeation rate. Qualitatively different results are observed if a blocking layer is assumed, that depends on ion concentration. Here, based on experiments presented below, this is established for an ion concentration of 13 g L-1 near the membrane. A transition to a linear increase in Nd concentration with time is then observed in the modeled data (Figure 3), which now has only a slight dependence on Daq, but is strongly dependent on Dorg. In addition, the slope of the linear region does not vary with feed concentration above 1 g L-1 and the linear range is only reached at earlier times for higher ion concentration (Figure 4). For lower Nd concentration in the feed, the slope varies as the conditions required to form a third phase in the membrane are not met. The variation of the slope can be correlated with the increase of the concentration gradient in the membrane. For the simulations, a specific blocking layer is assumed that is explained and justified as follows. Firstly, a uniform blocking layer is assumed to form as a third phase between HDEHP, Nd and lsane IP 175. This can be observed above a threshold ion and ligand concentration (Figure 5). For the HDEHP concentration 0.5 M used here the threshold ion concentration must be 10.5 g L-1 to fit with experimental data. This Nd concentration can be reached in the organic phase adjacent to the water phase with a Nd concentration around 1 g L-1. Hence after starting the pertraction, the phase transfer of Nd first enriches it in the organic phase, and if the concentration there exceeds 10.5 g L-1, the third phase forms. If the third phase blocks permeation, the phase


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transfer of the ion is stopped. However, on the organic side of the blocking third phase layer (Figure 6) Nd can diffuse into the bulk organic phase. This reduces the Nd concentration in the blocking layer, which therefore shrinks, becomes permeable, and Nd can again be transferred from water. Therefore, after the threshold Nd concentration in the membrane/water interfacial layer is reached, a stationary flow is established, and the permeation rate is determined by the flow through the organic phase. This explains the increase of the linear slope with increasing D org in Figure 3 (bottom left) and the independence from feed concentration (Figure 3, bottom right). This model of a feedback loop is supported by the observation of a third phase (Figure 7), and by its relative simplicity, as the only free parameters are those describing the permeation through the third phase. These require only the selection of a threshold concentration, which has been determined experimentally. Therefore, the simulations agree with experiments without assuming any free parameters on the bulk liquids. It could be argued that fouling of a membrane is typically caused by irregular deposits in the pores that reduced permeation.15,16 However, if that were the case a normally-shaped dependence of permeation on time, with no linear region, would be expected, in contradiction to the experimental results. The new model obviously points to the need for many further supporting experiments to control pertraction, which will be subject of future studies. Yet it is pertinent to consider how the process might be accelerated, and for this an ultrasonic transducer was integrated into the setup. With 26 kHz ultrasound an acceleration is observed, but the shape of the time dependence is unaffected. The process can be modeled with a higher Dorg, which indicates that the effect is an increase of the diffusion in the organic phase. Detailed quantification of this is subject of a separate publication. Experimental data of the distribution coefficient have been measured by varying the volume of a nonacid aqueous phase initially loaded at 2.4 g L-1 Nd and contacted with a constant volume of


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0.5 M HDEHP in Isane IP 175 (Figure 7). At the higher Nd concentration (0.55 g L-1) a third phase is observed. A mass balance calculation leads to an overall Nd concentration in organic phases of 13 g L-1, not far from the 10.5 g L-1 used in the simulation. The third phase was characterized by shear-viscosity measurements and small-angle X-ray scattering (SAXS). The viscosity is rather high and decreases with increasing shear-flow (Figure 8). This shear-thinning behavior is typical for a hexagonal phase due to alignment of its cylinders perpendicular to the shear flow.17 Furthermore, the SAXS spectra also indicate the presence of a hexagonal symmetry between water-rich connected channels. An increase at low q decaying with q−3 can be observed as well as strong crystalline peaks: demonstrating that the sample is microphase separated.18 The micro-structure of the sample is a hexagonal phase, dispersed as small (sub)micrometer size domains of hexagonal phase, dispersed by patches of a solvent-rich phase, most likely to be mainly solvent. SAXS analysis confirms the presence of this hexagonal arrangement of cylinders. Bragg peaks at 3.92 nm-1 and their allowed reflections √3 × 3.92 nm−1 = 6.74 nm−1, 2 × 3.92 nm−1 = 7.83 nm−1, √7 × 3.92 nm−1 = 10.36 nm−1 and 3 × 3.92 nm−1 = 11.75 nm−1 are all observed, indicating that the hexagonal phase must be correlated over more than ten periods, i.e. ≥50 nm.19 The higher reflections are extremely weak, confirming the size of hexagonal crystallites of the highly symmetric local microstructure present. The size of the hexagonal unit cell is obtained according to 2/√3 * 2π/q = 1.86 nm. The presence of the third phase was further confirmed by investigations of Ellis et al.20 They reported hexagonally-packed “bottlebrush” cylinders, comprising polar cores covered with protruding chains persisted up to 70 °C. The fact that the wide-angle diffraction lines are very narrow, indicates that the hydrophobic moieties are also crystalline. This explains why the permeation of unipolar species through a film of this phase is strongly reduced.


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4. CONCLUSIONS The high concentration of complexing ligands and ions near the oil/water interface in a pertraction membrane can cause a third phase to form, which blocks further phase transfer. This blocking layer forms due to transfer from the water phase and is removed by ion transfer into the bulk oil phase. The interplay between the processes of transfer to the membrane forming a permeation barrier and transfer into the oil removing the barrier may cause oscillations or a stationary state. The latter is distinguished by a constant rate of Nd exchange into the oil phase, which is determined by transport through the oily part of the membrane. These experimental findings can be simply modeled without assuming any free parameters on the bulk phases, which points to the need for further studies focusing on the quantitative control and optimization of this process of high relevance for rare earth recycling. Key parameters will be membrane structure, thickness and wettability, the type of oil and ligand, as well as relevant concentrations still keeping in mind the practical feasibility of the process. In industrial ion extraction this third phase should be avoided, but the ligand concentration should be kept as high as possible. For optimization on one hand the ternary phase diagram should to be studied for different ligands and ions, on the other hand it may be possible to constantly destroy this phase either mechanically or by increasing the ion flux, thus decreasing its concentration. This can be done by external fields, and in a forthcoming publication we will show how ultrasound can be of help. 5. OUTLOOK Beyond the relevance for rare earth separation it is important to note that this is a system that exhibits self-regulation of transfer across a membrane. Such systems are ubiquitous in nature,


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and are typically used to limit the concentration of a chemical species in a specific compartment. One example is the maintenance of the water evaporation through the skin. This is surprisingly constant over a broad range of relative humidity21,22, which is obviously important for a function largely independent from the environment. There is ample evidence, that the skin structure causes this regulation by establishing additional permeation barriers in response to humidity. 23 A simple model system with a similar response has been proposed that makes use of lipid assembly dependent on humidity.24 It has also recently been shown that a barrier film between a water reservoir and air can be formed by phospholipid assembly, and this causes a flux through the membrane over a broad range independent of humidity.25 This basically is due to the fact that there is a barrier layer formed, that reduces permeation at lower humidity and therefore protects the skin from drying. In this case the negative feedback is a reduction of permeation with lower permeant concentration, in our case it follows increasing permeant concentration. Also, for technical systems the protection against evaporation is needed, and an example on this would be protection of wet foams against drying.26 On a more general level, nature also tries to drive communication by self-regulated systems, often in oscillatory manner. A classical example is the Belusov Zhabotinski reaction, that recently has been coupled to phospholipid membranes to affect the information transfer between intracellular compartments.27 In conclusion, the system studied here reflects a widespread tendency of nature to control fluxes and thereby concentrations, which while unwanted in the specific application of ion separation may become relevant in many other disciplines.


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FIGURES


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Figure 1. Schematics of the setup used for the pertraction measurement

Figure 2. Nd concentration in the oil phase as a function of time after loading the outside of the membrane by a solution of 0.05 M HNO3 and 1 g L−1 Nd. The organic phase (Isane IP 175) contains 0.5 M HDEHP. The flow rates are 100 mL h−1 for the water phase and 10 mL h−1 for the organic phase.


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Figure 3. Top: Simulation of the ion concentration in the organic phase in absence of a blocking layer for different assumed diffusion coefficients(left) and ion concentration in water(right). Bottom: Simulations with a blocking layer, varying the parameters as in top row. The units of diffusion coefficients in the figures are m2/sec.


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Figure 4. Nd concentration in the oil phase as a function of time after loading the outside of the membrane by a solution of 0.05 M HNO3 and for different concentrations of Nd. The organic phase (Isane IP 175) contains 0.5 M HDEHP. The flow rates are 100 mL h−1 for the water phase and 10 mL h−1 for the organic phase. The increase rate of the Nd concentration in the oil phase (28 mL in closed loop) is reported for each initial Nd concentration in the aqueous phase.


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Figure 5. Left: Photograph of a vial with a third phase (milky) between the oil(top) and the water(bottom) phase forming at high Nd concentration. Right: Assumed dependence of D org as a function of Nd concentration. For the simulation a step function cannot be used, but a drastic decay above a threshold concentration is found. Consequently the details of the exponent and the thickness of the blocking layer are therefore less relevant.


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Figure 6. Top: schematics of the membrane with pores filled by oil and ligand (red) adjacent to the water phase (blue) and a thin blocking layer (black) close to the interface towards water. Bottom: schematics of the ion concentration from the organic phase with complexing ligand towards the water phase. The partition coefficient between an ion in water and in the organic phase determines the concentration increase at the membrane/water interface, which is very important for the formation of a third phase discussed in the text. The concentration at the interface towards the organic phase corresponds to the minimum concentration at which the third phase is stable.


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The linear concentration changes within one region are only schematics, and would be expected in a steady state.

Figure 7. Evolution of the distribution coefficient of Nd for 0.5 M HDEHP in Isane contacted with aqueous phase 0.05-0.04 M HNO3.


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Figure 8. Left: SAXS spectra of the third phase at 25, 40 and 70 °C. Right: Shear viscosity of third phase derived from an organic phase of 1 M HDEHP in Isane IP 175 and contacted with an aqueous solution of Nd.


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AUTHOR INFORMATION Corresponding Author *Email: [email protected]; Tel. +33 4 66 33 92 79 Author Contributionss The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGMENT We acknowledge support from the European Research Council under the ERC Grant Agreement Nr. 320915 “REE-CYCLE”: Rare Earth Element reCYCling with Low harmful Emissions. We would also thank Olivier Miollan and Orlane Sagi for their technical assistance in the realization of the extraction tests by pertraction at CEA-Marcoule. We acknowledge critical reading of the manuscript and improving the English by Martin Hollamby, Keele University, UK.

; REFERENCES (1) Rydberg, J.; Cox, M.; Musikas, C. Solvent Extraction Principles and Practice; Marcel Dekker Inc: Hoboken, 2004.


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(2) Zemb, T.; Bauer, C.; Bauduin, P.; Belloni, L.; Déjugnat, C.; Diat, O.; Dubois, V.; Dufrêche, J.-F.; Dourdain, S.; Duvail, M. et al. Recycling metals by controlled transfer of ionic species between complex fluids: En route to “ienaics”. Colloid Polym Sci 2015, 293, 1–22. (3) Osseo-Asare, K. Aggregation, reversed micelles, and microemulsions in liquid-liquid extraction: The tri-n-butyl phosphatediluent-water-electrolyte system. Advances in Colloid and Interface Science 1991, 37, 123–173. (4) Handbook of solvent extraction; Lo, T. C.; Baird, M. H. I.; Hansen, C., Eds.; A Wileyinterscience publication; Wiley: New York, 1983. (5) Porhel, S. Extraction liquide-liquide sur matériaux poreux. Mise en œuvre et recherche de paramètres influents. PhD Thesis, University of Aix Marseille, Aix-Marseille, 2013. (6) Gabelman, A.; Hwang, S.-T. Hollow fiber membrane contactors. Journal of Membrane Science 1999, 159, 61–106. (7) Guilbaud, P.; Zemb, T. Solute-induced microstructural transition from weak aggregates towards a curved film of surface-active extractants. Chemphyschem 2012, 13, 687–691. (8) Ferru, G.; Gomes Rodrigues, D.; Berthon, L.; Diat, O.; Bauduin, P.; Guilbaud, P. Elucidation of the structure of organic solutions in solvent extraction by combining molecular dynamics and X-ray scattering. Angewandte Chemie 2014, 53, 5346–5350. (9) Vlachy, N.; Jagoda-Cwiklik, B.; Vacha, R.; Touraud, D.; Jungwirth, P.; Kunz, W. Hofmeister series and specific interactions of charged headgroups with aqueous ions. Advances in Colloid and Interface Science 2009, 146, 42–47. (10) Jungwirth, P.; Cremer, P. S. Beyond Hofmeister. Nature chemistry 2014, 6, 261–263.


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Page 24 of 27

(11) Dourdain, S.; Déjugnat, C.; Berthon, L.; Dubois, V.; Pellet-Rostaing, S.; Dufrêche, J.-F.; Zemb, T. Liquid-Liquid Extraction of Acids by a Malonamide: II-Anion Specific Effects in the Aggregate-Enhanced Extraction Isotherms. Solvent Extraction and Ion Exchange 2014, 32, 620– 636. (12) Ellis, R. J.; Meridiano, Y.; Muller, J.; Berthon, L.; Guilbaud, P.; Zorz, N.; Antonio, M. R.; Demars, T.; Zemb, T. Complexation-induced supramolecular assembly drives metal-ion extraction. Chemistry 2014, 20, 12796–12807. (13) Qiao, B.; Ferru, G.; La Olvera de Cruz, M.; Ellis, R. J. Molecular Origins of Mesoscale Ordering in a Metalloamphiphile Phase. ACS central science 2015, 1, 493–503. (14) Touré, M. Etude des conditions de mise en œuvre de la pertraction pour l’extraction et la purification des métaux d’intérêt. PhD Thesis, University of Montpellier, 2015. (15) Kobayashi, T. Ultrasound enhanced cross-flow membrane filtration. Separation and Purification Technology 1999, 17, 31–40. (16) Muthukumaran, S.; Kentish, S. E.; Stevens, G. W.; Ashokkumar, M. Application of ultrasound in membrane separation processes: a review. Reviews in Chemical Engineering 2006, 22. (17) Montalvo, G.; Valiente, M.; Rodenas, E. Rheological Properties of the L Phase and the Hexagonal, Lamellar, and Cubic Liquid Crystals of the CTAB/Benzyl Alcohol/Water System. Langmuir 1996, 12, 5202–5208.


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(18) Morvan, M.; Espinat, D.; Lambard, J.; Zemb, T. Ultrasmall- and small-angle X-ray scattering of smectite clay suspensions. Colloids and Surfaces A: Physicochemical and Engineering Aspects 1994, 82, 193–203. (19) Hyde, S. T. Identification of Lyotropic Liquid Crystalline Mesophases. In Handbook of applied surface and colloid chemistry; Holmberg, K., Ed.; Wiley: Chichester, 2002. (20) Ellis, R. J.; Demars, T.; Liu, G.; Niklas, J.; Poluektov, O. G.; Shkrob, I. A. In the Bottlebrush Garden: The Structural Aspects of Coordination Polymer Phases formed in Lanthanide Extraction with Alkyl Phosphoric Acids. The journal of physical chemistry. B 2015, 119, 11910–11927. (21) Cravello, B.; Ferri, A. Relationships between skin properties and environmental parameters. Skin research and technology 2008, 14, 180–186. (22) Blank, I. H.; Moloney, J.; Emslie, A. G.; Simon, I.; Apt, C. The Diffusion of Water Across the Stratum Corneum As a Function of Its Water Content. Journal of Investigative Dermatology 1984, 82, 188–194. (23) Alonso, A.; Meirelles, N. C.; Yushmanov, V. E.; Tabak, M. Water Increases the Fluidity of Intercellular Membranes of Stratum Corneum: Correlation with Water Permeability, Elastic, and Electrical Resistance Properties. Journal of Investigative Dermatology 1996, 106, 1058–1063. (24) Åberg, C.; Sparr, E.; Wennerström, H. Lipid phase behaviour under steady state conditions. Faraday Discuss 2013, 161, 151–166. (25) Roger,K.;Liebi,M.;Heimdal,J.; Pham,Q.D.;Sparr,E.; Controlling water evaporation through self-assembly. Proc.Nat.Acad.Sci.2016,113,10275-10280


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(26) Hurcom, J.; Paul, A.; Heenan, R. K.; Davies, A.; Woodman, N.; Schweins, R.; Griffiths, P. C. The interfacial structure of polymeric surfactant stabilised air-in-water foams. Soft matter 2014, 10, 3003–3008. (27) Torbensen, K.; Rossi, F.; Pantani, O. L.; Ristori, S.; Abou-Hassan, A. Interaction of the Belousov-Zhabotinsky Reaction with Phospholipid Engineered Membranes. The journal of physical chemistry. B 2015, 119, 10224–10230.


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Table of Content

Extraction at liquid-liquid interface Organic phase

Aqueous phase


Self-Regulated Ion Permeation through Extraction Membranes

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