Patent Application
20210060530
This document relates to solvent-free extractive separation of ions, and more particularly to particles, such as polymeric capsules, for extracting ions from mixed solutions. This document further includes methods and materials for making and using such polymeric capsules.
Selective separations of ions are often technically challenging, but important for various applications, including in the chemical and food industries, waste treatment, and hydrometallurgy. For example, metals play an essential role in modern society, but, when dissolved as ions in water, can be highly hazardous to human and environmental health (e.g., lead, mercury, radioactive species). Selective separations of metals are critical to efficient metal production and environmental management. Liquid-liquid extraction, also called solvent extraction (SX), is a common method for selective separations of aqueous metals. For example, SX is often the preferred choice in the cleanup of radioactive Cs+ from contaminated nuclear production sites. However, SX requires a large organic solvent inventory, is susceptible to loss of solvent and ligand into the aqueous phase, and is limited by the metal/ligand stoichiometry and the equilibrium established during extraction and stripping. Facilitated transport membranes have also received extensive research interest for selective separations of aqueous metals, however, their instability has severely limited industrial implementation.
Provided herein are particles, e.g., polymeric capsules, for extracting metal ions from mixed solutions. In some embodiments, the capsules can serve as a new format to enable stable, selective, and rapid facilitated transport of metal ions. The capsules can comprise a polymeric shell encasing an inner chamber and a lipophilic ligand within the polymeric shell. This document further includes methods for making such polymeric capsules, including, e.g., polymersomes. This document further includes methods for using polymeric capsules for extracting metal ions from mixed solutions.
In a first general aspect, this document provides polymeric capsules comprising: a polymeric shell encasing an inner chamber; and a lipophilic ligand within the polymeric shell.
In a second general aspect, this document provides a method of making a polymeric capsule comprising: forming, in a strip solution or a metal salt solution, a polymersome, from at least one amphiphilic block polymer having a hydrophobic block and a hydrophilic block; and exposing the capsule to a solution comprising a lipophilic ligand. In some embodiments, the capsule can be formed in a metal salt solution, and further comprising exposing the polymersome to a strip solution. In some embodiments, the metal salt solution can be MgCl2 or CaCl2.
In a third general aspect, this document provides a method of extracting target ions from a mixed solution comprising: exposing the mixed solution to polymeric capsules to produce a second solution, wherein the capsules comprise: a polymeric shell encasing an inner chamber; a lipophilic ligand within the polymeric shell; and a first strip solution contained within the inner chamber of the capsule. In some embodiments, the lipophilic ligand can specifically bind the target ion. In some embodiments, the method can further comprise removing the second solution from the capsules. In some embodiments, the method can further comprise exposing the capsules to a second strip solution to obtain a solution enriched in the target metal ion and regenerate the strip solution within the capsules; and collecting the target ions. In some embodiments, removing the second solution can comprise washing to remove non-target ions. In some embodiments, removing non-target ions can comprise a size-based separation technique or a packed bed technique. In some embodiments, the size-based separation technique can be filtration with porous membranes.
In a fourth general aspect, this document provides a method of making a polymeric capsule comprising: mixing, in a solvent, a lipophilic ligand with at least one amphiphilic block polymer having a hydrophobic block and a hydrophilic block; evaporating the solvent to produce a ligand-polymer mixture; adding an aqueous solution to the ligand-polymer mixture; and allowing the capsules to form.
In some embodiments, the polymeric capsules may optionally include one or more of the following features. The shell can comprise one or more amphiphilic block polymers. The one or more amphiphilic block polymers can comprise a hydrophilic block that is uncharged, cationic, zwitterionic, or anionic. The polymeric shell can comprise a water-insoluble polymer with a glass transition temperature below 20° C. The polymeric shell can comprise a plasticizer and a water-insoluble polymer with a glass transition temperature above 20° C. The capsule can be a polymersome. The polymeric shell can comprise at least one polymer selected from poly(isoprene), poly(chloroprene), poly(butadiene), poly(myrcene), poly(farnesene), poly(citrenellol), hydrogenated poly(isoprene), hydrogenated poly(butadiene), polyethylene, polypropylene, polydimethylsiloxane, polymers from styrenic derivatives, polymers from acrylic derivatives, polymers from acrylamide derivatives, and polymers from siloxane derivatives. The one or more amphiphilic block polymers can comprise a hydrophilic block comprising a monomer selected from ethylene oxide, allyl alcohol, methyl oxazoline, acrylamide and derivatives thereof, zwitterionic derivatives of styrenic monomers and vinylpyridine, and cationic derivatives of styrenic monomers and vinylpyridine, and combinations thereof. The hydrophilic block can comprise at least one of poly(ethylene oxide), poly(allyl alcohol), poly(2-methyl-2-oxazoline), and poly(acrylamide). The one or more amphiphilic block polymers can comprise a hydrophobic block comprising a monomer selected from isoprene, chloroprene, butadiene, myrcene, farnesene, citrenellol, styrenic derivatives, acrylic derivatives, acrylamide derivatives, and siloxane derivatives, and combinations thereof. The hydrophobic block can comprise at least one of poly(butadiene), poly(styrene), poly(isoprene), and poly(dimethyl siloxane). The lipophilic ligand can be selected from oxime derivatives, phosphoric acid derivatives, phosphinic acid derivatives, phosphonic acid derivatives, diketone derivatives, amine derivatives, ketone derivatives, polyether derivatives, crown ether derivatives, cryptand derivatives, calixarene derivatives, and combinations thereof. The capsule can further comprise a strip solution contained within the inner chamber of the capsule. The strip solution can be an acid solution, and optionally, wherein the acid is hydrochloric acid, nitric acid, or sulfuric acid. At least one dimension of the capsule can be from about 10 nm to about 5 mm. One dimension of the capsule can be from about 10 nm to about 10 μm. The shell can be from about 1 nm to about 20 μm thick. The lipophilic ligand can interact with metal ions. The lipophilic ligand can be a freely diffusing ligand. The lipophilic ligand can be covalently bonded to a polymer in the shell. The shell can be cross-linked. the polymeric shell can comprise, or can consist essentially of, a hydrophobic polymer. In some embodiments, the lipophilic ligand interacts with metal ions. In some embodiments, the lipophilic ligand is a freely diffusing ligand. In some embodiments, the lipophilic ligand is covalently bonded to a polymer in the shell. In some embodiments, the shell comprises a water-insoluble polymer and one or more surfactants. In some embodiments, the one or more surfactants comprise amphiphilic polymers. In some embodiments, the capsule comprises a multi-block copolymer comprising a hydrophilic segment, a hydrophobic segment, and an acidic segment.
In some embodiments, a method of extracting target ions from a mixed solution comprising: exposing the mixed solution to polymeric capsules to produce a second solution, wherein the capsules comprise: a polymeric shell encasing an inner chamber; a lipophilic ligand within the polymeric shell; and a first strip solution contained within the inner chamber of the capsule.
In some embodiments, the lipophilic ligand specifically binds the target ion. In some embodiments, the method comprising removing the second solution from the capsules. In some embodiments, the method further comprising: exposing the capsules to a second strip solution to obtain a solution enriched in the target metal ion and regenerate the first strip solution within the capsules; and collecting the target ions. In some embodiments, the removing the second solution comprises washing to remove non-target ions by using a size-based separation technique or a packed bed technique. In some embodiments, the lipophilic ligand is selected from oximes, phosphoric acid derivatives, phosphinic acid derivatives, phosphonic acid derivatives, diketone derivatives, amine derivatives, ketone derivatives, polyether derivatives, crown ether derivatives, cryptand derivatives, and calixarene derivatives. In some embodiments, the first strip solution, second strip solution, or combination thereof, is an acid solution, and optionally, wherein the acid is hydrochloric acid or sulfuric acid.
The polymeric capsules and methods of solvent extraction described herein provide several advantages. For example, the coupling of the extraction and stripping phases using polymeric capsules can allow for dramatic reductions (e.g., ˜104-fold) in ligand requirements. In some embodiments, polymeric capsules can serve as a general format for the selective separations of metals, without the need for organic solvents. In some embodiments, polymeric capsules may be useful for any application of SX, e.g., reversible extraction of a metal ion into a nonpolar phase using a selective ligand that is soluble in organic solvents and relatively insoluble in water. In some embodiments, the polymeric capsules can have high surface area, high dispersibility, and low wall-thickness, which can result in rapid kinetics, for example ˜99% metal removal in <2 min. In some embodiments, polymeric capsules described herein can have a selectivity at least as good as that of solvent extraction (SX) using the same ligand. Some embodiments of polymeric capsules described herein can be used for any application of SX. As another example of advantages, the low ligand requirements of some embodiments of the polymeric capsule provided herein can improve selectivity through the use of highly-selective, expensive ligands. The use of polymeric capsule in solvent extraction can have far-reaching implications for the efficient and rapid selective separations of aqueous metals, which could impact the fields of environmental remediation, metal production, and electronics recycling.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.
Provided herein are polymeric capsules, and methods for making and using polymeric capsules, for extracting metal ions from mixed solutions.
Metals provide structure and strength to buildings, transmit electricity through wires, and form the intricate circuitry of electronic devices. Human use of metals has grown increasingly complex. For example, a single mobile phone today can contain more than seventy different elements, most of which are metals [1]. This complexity can hinder responsible life-cycle management; many metals can have near-zero rates of recycling [2], while many others are not produced locally, which can put supply chains at risk.
Additionally, aqueous solutions of metal ions can be toxic (e.g., lead, mercury). Effective management methods should be employed at all stages (production, usage, and disposal) to ensure that dissolved metals do not contaminate the environment and endanger public health. One example of this dynamic is the nuclear energy sector. While the production of high-quality fuel remains important, management of the resulting waste streams and the contaminated legacy production sites can sometimes be more challenging. According to the Department of Energy's report on Basic Research Needs for Environmental Management [3], cleanup of the remaining contaminated sites and safe disposal of resulting radioactive waste is expected to cost over $300 billion. New technologies are needed to decrease this enormous cost [3].
Selective separations can allow efficient metal utilization, production, and recovery. In metal production, the decreasing quality of ore and desire to decrease the energetic demand of metal production have led to increased use of hydrometallurgy, in which metals are leached from ores into dilute acid, separated in solution, and reduced to form the pure metal. The solution-phase technique used for these separations is typically liquid-liquid extraction, also called solvent extraction (SX) [3]-[5]. In SX, a lipophilic ligand (also called a receptor or extractant) in an organic phase selectively and reversibly binds a target metal ion, extracting it from the aqueous solution into the organic phase. The metal can then be recovered in a stripping step, which can, in some cases, include contacting the metal-loaded organic phase with an acidic aqueous (strip) solution. Simple extractants such as organophosphates, -amines, and -oximes are often used in metal production, whereas more complex, and sometimes more efficient, extractants are typically used in high-value separations, such as the use of calixarenes in the caustic side solvent extraction (CSSX) process to remove radioactive cesium from the Savannah River Site [3]. Decades of research has resulted in highly selective ligands that can allow for exquisitely selective separations.
While SX is a mature and effective technology that builds on decades of research on metal ligand interactions, it can have several important limitations. First, it can require a large solvent inventory, which can pose a safety concern and risk for environmental discharge. Second, SX can suffer from ligand and solvent loss due to solvent entrainment in the aqueous phase (e.g., through the formation of emulsions). Such loss of ligand can be particularly critical for expensive ligands such as the calixarenes used in CSSX. To help recover ligand, extra steps are often necessary. For example, the CSSX process employs a coalescer and decanter to recover ligand-bearing organic phase that are present in the aqueous stream as small droplets <10 μm in diameter [6]. Third, SX is an equilibrium process (see, e.g.,
To address these issues, facilitated transport membranes were explored extensively in the 1980's and 1990's [4], [7]. These membranes place the ligand-containing organic phase between the aqueous feed and aqueous strip solutions as a thin barrier to couple the two extraction and stripping steps (see, e.g.,
Facilitated transport membranes thus far have predominantly taken two major forms. In supported liquid membranes, the liquid organic phase fills the pores of a hydrophobic membrane, which can retain the liquid phase by capillary forces. In emulsion liquid membranes, water-in-oil-in-water emulsions are created such that strip solution compartments are contained within small droplets of the organic phase, which can enable fast separation kinetics due to large surface areas.
Due to the chemical selectivity of metal-ligand interactions, facilitated transport membranes can be capable of exquisite separations (e.g., separating mixtures of divalent metal ions) that are not attainable using traditional membranes, which typically separate based on size or charge [7], [9]. Additionally, facilitated transport membranes use a chemical gradient (e.g., a proton gradient) to actively drive transport, allowing for the target metal to move against its concentration gradient (i.e., become concentrated in the strip solution). However, facilitated transport membranes can be unstable [7]. Supported liquid membranes can be susceptible to catastrophic pore wetting with the aqueous phase, caused by dissolution of the organic phase or osmotic pressure differences. Emulsion liquid membranes are complex to manage and can be susceptible to spontaneous break-up. Polymeric facilitated transport membranes have gained recent interest because of their enhanced stability, but have typically been flux-limited due to large thicknesses (˜30 μm) combined with increased diffusion resistance of the polymer matrix relative to a liquid organic phase [7], [10]. For facilitated transport membranes to successfully address the limitations of solvent extraction, fundamentally new designs are necessary.
As provided herein, polymeric capsules can provide a new format to enable stable, selective, and rapid facilitated transport of metal ions for metal separations (see, e.g.,
A polymeric capsule 30 according to one exemplary embodiment is shown in
In some embodiments, at least one dimension of the polymeric capsules can range from about 10 nm to about 5 mm, from about 10 nm to about 4 mm, from about 10 nm to about 3 mm, from about 10 nm to about 2.5 mm, from about 10 nm to about 2 mm, from about 10 nm to about 1.5 mm, from about 10 nm to about 1 mm, from about 10 nm to about 0.9 mm, from about 10 nm to about 0.8 mm, from about 10 nm to about 0.7 mm, from about 10 nm to about 0.6 mm, from about nm to about 0.5 mm, from about 10 nm to about 0.4 mm, from about 10 nm to about 0.3 mm, from about 10 nm to about 0.2 mm, from about 10 nm to about 0.1 mm, from about 0.5 mm to about 5 mm, from about 0.6 mm to about 5 mm, from about 0.7 mm to about 5 mm, from about 0.8 mm to about 5 mm, from about 0.9 mm to about 5 mm, from about 1 mm to about 5 mm, from about 0.5 mm to about 5 mm, from about 1.5 mm to about 5 mm, from about 2 mm to about 5 mm, from about 2.5 mm to about 5 mm, from about 3 mm to about 5 mm, from about 4 mm to about 5 mm, from about 10 nm to about 100 μm, from about 10 nm to about 90 μm, from about 10 nm to about 80 μm, from about 10 nm to about 70 μm, from about 10 nm to about 60 μm, from about 10 nm to about 50 μm, from about 10 nm to about 40 μm, from about 10 nm to about 30 μm, from about 10 nm to about 20 μm, from about 10 nm to about 10 μm, from about 10 nm to about 9 μm, from about 10 nm to about 8 μm, from about 10 nm to about 7 μm, from about 10 nm to about 6 μm, from about 10 nm to about 5 μm, from about 10 nm to about 4 μm, from about 10 nm to about 3 μm, from about 10 nm to about 2 μm, from about 10 nm to about 1 μm, from about 10 nm to about 900 nm, from about 10 nm to about 800 nm, from about 10 nm to about 700 nm, from about nm to about 600 nm, from about 10 nm to about 500 nm, from about 10 nm to about 400 nm, from about 10 nm to about 300 nm, from about 10 nm to about 200 nm, from about 10 nm to about 100 nm, from about 25 nm to about 700 nm, from about 25 nm to about 600 nm, from about 25 nm to about 500 nm, from about 25 nm to about 400 nm, from about 50 nm to about 600 nm, from about 50 nm to about 600 nm, from about 75 nm to about 550 nm, from about 100 nm to about 500 nm, from about 10 nm to about 5 μm, from about 20 nm to about 5 μm, from about 30 nm to about 5 μm, from about 40 nm to about 5 μm, from about 50 nm to about 5 μm, from about 60 nm to about 5 μm, from about 70 nm to about 5 μm, from about 80 nm to about 5 μm, from about 90 nm to about 5 μm, from about 100 nm to about 5 μm, from about 200 nm to about 5 μm, from about 300 nm to about 5 μm, from about 400 nm to about 5 μm, from about 500 nm to about 5 μm, from about 600 nm to about 5 μm, from about 700 nm to about 5 μm, from about 800 nm to about 5 μm, from about 900 nm to about 5 μm, or from about 1 μm to about 5 μm.
In some embodiments, the polymeric shell can have a thickness of from about 1 nm to about 20 μm, from about 1 nm to about 15 μm, from about 1 nm to about 10 μm, from about 1 nm to about 5 μm, from about 1 nm to about 2 μm, from about 1 nm to about 1 μm, from about 1 nm to about 0.8 μm, from about 1 nm to about 0.75 μm, from about 1 nm to about 0.5 μm, from about 1 nm to about 0.25 μm, from about 1 nm to about 0.1 μm, from about 1 nm to about 90 nm, from about 1 nm to about 80 nm, from about 1 nm to about 70 nm, from about 1 nm to about 60 nm, from about 1 nm to about 50 nm, from about 1 nm to about 40 nm, from about 1 nm to about 30 nm, from about 1 nm to about 20 nm, from about 1 nm to about 10 nm, from about 1 nm to about 5 nm, from about 1 nm to about 15 nm, from about 1 nm to about 25 nm, from about 5 nm to about 50 nm, from about 5 nm to about 40 nm, from about 5 nm to about 30 nm, from about 5 nm to about 25 nm, from about 5 nm to about 20 nm, from about 10 nm to about 40 nm, from about 10 nm to about 30 nm, from about 10 nm to about 25 nm, or from about 10 nm to about 20 nm.
In some embodiments, the polymeric capsule can contain an acid solution within the inner chamber 32 of the capsule. In some embodiments, the acid solution can be a hydrochloric acid, nitric acid, or sulfuric acid solution. In some embodiments, the pH of the acid solution can range from about −1 to about 6.5, from about −1 to about 6, from about −1 to about 5.5, from about −1 to about 5, from about −1 to about 4.5, from about −1 to about 4, from about −1 to about 3.5, from about −1 to about 3, from about 1 to about 4, from about 1 to about 3, from about 1 to about 2, or from about 2 to about 4.
In some embodiments, the polymeric capsule can contain free polyacids within the inner chamber (see
In some embodiments, the polymeric capsule can contain a strip solution within the inner chamber 32 of the capsule. Exemplary non-limiting strip solutions contained within the polymeric capsule can include pure water, acid solutions such as sulfuric acid and hydrochloric acid, and other solutions such as ammonia, ammonium hydroxide, ammonium carbonate, sodium hydroxide, and sodium bicarbonate. Strip solutions useful in solvent extraction methods are dependent on the target ions and the process being used, and can be readily determined by persons skilled in solvent extraction. Further exemplary strip solutions useful in solvent extraction can be found in Lo, T. C.; Baird, M. H. I.; Handson, C. Handbook of Solvent Extraction; Chapter 3B, Wiley: New York, 1983, incorporated herein by reference. In some embodiments, the polymeric capsule can contain a salt solution within the inner chamber 32 of the capsule. In some embodiments, the polymeric capsule can contain deionized water within the inner chamber 32 of the capsule. In some embodiments, the polymeric capsule can contain a solution having a high pH (e.g., a pH of from about 8 to about 14 or higher) within the inner chamber 32 of the capsule. For example, in some embodiments, the polymeric capsule can contain sodium hydroxide within the inner chamber 32 of the capsule.
In some embodiments, the polymeric capsule is a polymersome. Polymersomes, first developed in 1999 [11], are polymeric vesicles typically formed in aqueous solution via the self-assembly of amphiphilic block polymers (for example, polymers that contain discrete hydrophilic and hydrophobic segments or blocks), and have received tremendous interest in the fields of drug delivery, diagnostics, and controlled release [12]. The polymeric shell or vesicle membrane can contain a thin (e.g., 10-20 nm), dense nonpolar layer that is typically nearly impermeable to ions, formed by the hydrophobic block. In some embodiments, to enable metal transport, lipophilic metal carriers (e.g., ligands) can be dissolved within the nonpolar layer. In some embodiments, lipophilic ligands can be covalently bonded to the hydrophobic block of the amphiphilic block polymer. In such embodiments, upon formation of the polymersomes, the ligands can be sequestered within the nonpolar layer. Ligands that are dissolved, sequestered, or located within the polymeric shell or nonpolar layer can be partially or fully within the polymeric shell or nonpolar layer. In some embodiments, ligands dissolved, sequestered, or located within the polymeric shell or nonpolar layer can move freely back and forth between the exterior of the polymeric shell or nonpolar layer, where they complex one or more ions present outside the polymeric capsule and transport the complexed ions to the inner chamber 32 of the polymeric capsule, for example to a strip solution in the inner chamber 32.
A polymersome 40 according to one exemplary embodiment is shown in
In some embodiments, polymersomes can have a diameter of from about 10 nm to about 20 μm, from about 10 nm to about 10 μm, from about 25 nm to about 700 nm, or from about 100 nm to about 500 nm. In some embodiments, at least one dimension of the polymersomes (e.g., diameter, length, width, etc.) can range from about 10 nm to about 20 μm, from about 10 nm to about 10 μm, from about 10 nm to about 9 μm, from about 10 nm to about 8 μm, from about 10 nm to about 7 μm, from about 10 nm to about 6 μm, from about 10 nm to about 5 μm, from about 10 nm to about 4 μm, from about 10 nm to about 3 μm, from about 10 nm to about 2 μm, from about 10 nm to about 1 μm, from about 10 nm to about 900 nm, from about 10 nm to about 800 nm, from about 10 nm to about 700 nm, from about 10 nm to about 600 nm, from about 10 nm to about 500 nm, from about 10 nm to about 400 nm, from about 10 nm to about 300 nm, from about 10 nm to about 200 nm, from about 10 nm to about 100 nm, from about 25 nm to about 700 nm, from about 25 nm to about 600 nm, from about 25 nm to about 500 nm, from about 25 nm to about 400 nm, from about 50 nm to about 600 nm, from about 50 nm to about 600 nm, from about 75 nm to about 550 nm, or from about 100 nm to about 500 nm.
In some embodiments, the nonpolar layer of the polymersomes can have a thickness of from about 1 nm to about 50 nm, from about 1 nm to about 40 nm, from about 1 nm to about 30 nm, from about 1 nm to about 20 nm, from about 1 nm to about 10 nm, from about 1 nm to about 5 nm, from about 1 nm to about 15 nm, from about 1 nm to about 25 nm, from about 5 nm to about 50 nm, from about 5 nm to about 40 nm, from about 5 nm to about 30 nm, from about 5 nm to about 25 nm, from about 5 nm to about 20 nm, from about 10 nm to about 40 nm, from about 10 nm to about 30 nm, from about 10 nm to about 25 nm, or from about 10 nm to about 20 nm.
In some embodiments, polymeric capsules such as polymersomes can be produced by forming a polymersome from at least one amphiphilic block polymer having a hydrophilic block and a hydrophobic block. Polymersomes may be prepared from a number of methods, including direct assembly from bulk polymer in water [11], [17], rehydration of a thin polymer film [13], and rapid mixing with water of polymer dissolved in organic solvent [20], [36]. In some embodiments, the polymersomes can be formed (e.g., by self-assembly) in a solution. In some embodiments, the polymersome can be formed in an acid solution. Non-limiting examples of acid solutions useful in some embodiments of the method of making a polymeric capsule by forming a polymersome include hydrochloric acid, nitric acid, or sulfuric acid solutions. In some embodiments, the acid solution can have a molarity ranging from about 0.1 M to about 5 M, from about 0.1 M to about 4.5 M, from about 0.1 M to about 4 M, from about 0.1 M to about 3.5 M, from about 0.1 M to about 3 M, from about 0.1 M to about 2.5 M, from about 0.1 M to about 2 M, from about 0.1 M to about 1.5 M, from about 0.1 M to about 1 M, from about 0.1 M to about 0.5 M, from about 1 M to about 5 M, from about 1 M to about 4 M, from about 1 M to about 3 M, from about 1 M to about 2 M, from about 2 M to about 5 M, from about 2.5 M to about 5 M, from about 3 M to about 5 M, from about 3.5 M to about 5 M, from about 4 M to about 5 M, or from about 4.5 M to about 5 M. In some embodiments, the acid solution can have a pH ranging from about −2 to about 6.5, from about −2 to about 6, from about −2 to about 5.5, from about −2 to about 5, from about −2 to about 4.5, from about −2 to about 4, from about −2 to about 3.5, from about −2 to about 3, from about −2 to about 2, from about −1 to about 1, from about −1 to about 4, from about −1 to about 3, from about −1 to about 2, from about 2 to about 4, or from about −0.5 to about 0.5.
In some embodiments, an optimal preparation technique can be determined for each block polymer type. In some embodiments, extrusion through membranes of defined pore sizes can be used to control the vesicle diameter by transforming larger vesicles into several smaller vesicles, thus reducing vesicle size[13], [20].
In some embodiments, the polymersome can be formed in a metal salt solution. Non-limiting examples of metal salt solutions include metal chlorides, such as MgCl2, CaCl2, NaCl, and KCl, and metal sulfates, such as MgSO4, CaSO4, Na2SO4, and K2SO4. In order to encapsulate a desired strip solution, such as an acid solution, polymersomes formed in metal salt solution can be buffer exchanged using a desalting column [13] to remove residual metal ions (e.g., Ca2+ in cases where the polymersome is formed in CaCl2)). In some embodiments, after encapsulation of the desired strip solution, the lipophilic ligand can be added to the polymersome solution and allowed to dissolve into the polymersome walls. In some embodiments, the ligand can be directly dissolved during vesicle formation. However, it is believed that this may affect the aggregate morphology (i.e., vesicles or micelles). Micelles can be undesirable as they can have a hydrophobic central core and may not be able to encapsulate an aqueous (strip) solution.
In some embodiments, a lipophilic ligand can be added to the polymersome by exposing the polymersome to a solution comprising a lipophilic ligand. In some embodiments, the lipophilic ligand can be added to a polymersome formation solution. In some embodiments, the lipophilic ligand can be covalently bonded to the polymer that will form the polymersome.
In some embodiments, the polymersome can be formed directly in a strip solution, thus allowing direct encapsulation of the desired strip solution in the inner chamber of the polymersome. In some embodiments, the polymersome can be formed in any appropriate solution, which can later be exchanged, after formation of the polymersome, with a strip solution by exposing the formed polymersome to the desired strip solution. For example, in some embodiments wherein a polymersome is formed in a solution such as a metal salt solution, the method of making a polymeric capsule can further comprise exposing the formed polymersome to a strip solution such as an acid solution.
In some embodiments, the method of forming the polymersome can further comprise crosslinking the polymeric shell. In some embodiments, radical initiators can be used for crosslinking the polymeric shell. Exemplary non-limiting radical initiators include peroxides (e.g., benzoyl peroxide), azo-initiators, and redox initiators (e.g., persulfate/metabisulfite). In some embodiments, thiol-based crosslinkers can be used for crosslinking the polymeric shell. In some embodiments, after polymersomes of a desired size are formed, the hydrophobic block can be crosslinked using redox radical initiators or an aqueous photoinitiator [17]. In some embodiments, crosslinking can increase the stability and mechanical integrity of the polymersome or polymeric capsule. In some embodiments, such as where there is a large osmotic pressure difference (e.g., high salinity inside the vesicle and low salinity outside the vesicle), crosslinking can help prevent leakage of vesicle contents.
In some embodiments, the polymeric capsule is a microcapsule (see
In some embodiments, microcapsules can have a diameter of from about 1 μm to about 1,000 μm, from about 10 μm to about 100 μm, from about 25 μm to about 700 μm, or from about 100 μm to about 500 μm. In some embodiments, at least one dimension of the microcapsules (e.g., diameter, length, width, etc.) can range from about 10 μm to about 100 μm, from about 1 μm to about 10 μm, from about 1 μm to about 9 μm, from about 10 μm to about 8 μm, from about 1 μm to about 7 μm, from about 1 μm to about 6 μm, from about 1 μm to about 5 μm, from about 1 μm to about 4 μm, from about 1 μm to about 3 μm, from about 1 μm to about 2 μm, from about 1 μm to about 10 μm, from about 10 μm to about 900 μm, from about 10 μm to about 800 μm, from about 10 μm to about 700 μm, from about 10 μm to about 600 μm, from about 10 μm to about 500 μm, from about 10 μm to about 400 μm, from about 10 μm to about 300 μm, from about 10 μm to about 200 μm, from about 10 μm to about 100 μm, from about 25 μm to about 700 μm, from about 25 μm to about 600 μm, from about 25 μm to about 500 μm, from about 25 μm to about 400 μm, from about 50 μm to about 600 μm, from about 50 μm to about 600 μm, from about 75 μm to about 550 μm, or from about 100 μm to about 500 μm.
In some embodiments, the polymeric capsule (e.g., microcapsule) includes a block copolymer that serves as a microcapsule stabilizer, also referred to as a polymeric surfactant (see
In some embodiments, the shell of the polymeric capsule includes a water-insoluble polymer and one or more surfactants. The one or more surfactants can include amphiphilic polymers provided herein. In some embodiments, the one or more surfactants comprise small molecules with a low hydrophilic-lipophilic balance (<8), such as sorbitan monooleate (e.g., commercially available as Span 80), sorbitan trioleate (e.g., commercially available as Span 85), or lecithin. In some embodiments, the surfactants are polymeric surfactants. The polymeric surfactants can include amphiphilic polymers. Exemplary polymeric surfactants have segments comprising polyvinyl alcohol, poly(ethylene oxide), poly(propylene oxide), poly(dimethyl siloxane), poly(butadiene), poly(isoprene), or poly(styrene). Exemplary polymeric surfactants have acidic segments, such as poly(4-styrene sulfonic acid), poly(acrylic acid), and poly(methacrylic acid). In some embodiments, the polymeric capsule includes a multi-block (e.g., tri-block copolymer) that forms polyacid-containing micelles (see
In some embodiments of the polymeric capsules and methods described herein, the polymeric shell can comprise or be formed from one or more amphiphilic block polymers. In some embodiments, block polymers in metal selective capsules (e.g., polymersomes) are neutrally charged and non-interacting with ions. In some embodiments, block polymers in metal selective capsules (e.g., polymersomes) are highly stable. For example, in some embodiments, the block polymers in metal selective capsules can be more stable than liquid membrane extraction technology. In some embodiments, block polymers in metal selective capsules are crosslinkable to enhance stability. In some embodiments, the one or more amphiphilic block polymers can comprise a hydrophilic block that is uncharged, cationic, zwitterionic, or anionic. In some embodiments, the one or more amphiphilic block polymers can comprise a hydrophilic block that is uncharged, cationic, or zwitterionic. In some embodiments, uncharged, cationic, or zwitterionic hydrophilic blocks can provide high water solubility for the hydrophilic blocks of the polymers. In some embodiments, such as where a target species is an anion, the one or more amphiphilic block polymers can comprise a hydrophilic block that is anionic.
In some embodiments, the polymeric shell can comprise or be formed from a polymer with a glass transition temperature below 20° C. In some embodiments, where the glass transition temperature of the polymer is below 20° C., the polymeric capsule can exhibit high diffusivity of the ions during separation. In some embodiments, such as where the polymer is glassy or solid, e.g., where the glass transition temperature of the polymer is above 20° C., the polymeric capsule can exhibit decreased diffusivity of the ions during separation. In some embodiments, the polymeric shell comprises a polymer with a glass transition temperature above 20° C. and a plasticizer, which can, in some embodiments, allow the polymeric capsule to exhibit similar diffusivity as polymeric capsules comprising a polymer having a glass transition temperature below 20° C. In some embodiments, increasing glass transition temperature of the polymer, with or without the addition of plasticizer, can increase the stability of the polymeric capsule.
In some embodiments the one or more amphiphilic block polymers can comprise a hydrophilic block comprising a monomer selected from ethylene oxide, allyl alcohol, methyl oxazoline, acrylamide and derivatives thereof, zwitterionic derivatives of styrenic monomers and vinylpyridine, and cationic derivatives of styrenic monomers and vinylpyridine, and combinations thereof. In some embodiments, the hydrophilic block can comprise at least one of poly(ethylene oxide), poly(allyl alcohol), poly(methyl oxazoline), and poly(acrylic acid).
In some embodiments, the one or more amphiphilic block polymers can comprise a hydrophobic block comprising a monomer selected from isoprene, chloroprene, butadiene, myrcene, farnesene, citrenellol, styrenic derivatives, acrylic derivatives, acrylamide derivatives, and siloxane derivatives, and combinations thereof. In some embodiments, the hydrophobic block can comprise at least one of poly(butadiene), poly(styrene), poly(isoprene), and poly(dimethyl siloxane).
In some embodiments, the polymeric shell can comprise at least one polymer selected from poly(isoprene), poly(chloroprene), poly(butadiene), poly(myrcene), poly(farnesene), poly(citrenellol), hydrogenated poly(isoprene), hydrogenated poly(butadiene), polyethylene, polypropylene, polymers from styrenic derivatives, polymers from acrylic derivatives, polymers from acrylamide derivatives, polymers from siloxane derivatives, and polydimethylsiloxane.
In some embodiments, the polymeric shell can comprise poly(isoprene)-b-poly(allyl alcohol). A non-limiting example of synthesis of poly(isoprene)-b-poly(allyl alcohol) is provided according to Scheme I below:
In Scheme I, 2-(dodecylthiocarbonotbioylthio)-2-methylpropionic acid (DDMAT) and isoprene are reacted in the presence of an initiator and heat. Non-limiting exemplary initiators useful in this process include azobisisobutyronitrile, tert-butyl peroxide, and cumyl peroxide. Next, di-boc acrylamide (DBAm) is added in the presence of an initiator and heat or blue light. Non-limiting exemplary initiators useful in this process include azobisisobutyronitrile. Polymerization procedures generally follow existing procedures [27]-[30]. Reduction [27], with a reducing agent such as NaBHI, and deprotection of poly(DBArn) will yield acid-stable, neutrally charged, hydrophilic polymer blocks linked to poly(isoprene), which has a low glass-transition temperature (Tg˜−61° C.) and is crosslinkable. In some embodiments, poly(allyl alcohol) is the primary target polymer. In some embodiments, poly(acrylamide) is a backup target polymer. 1,4 addition of isoprene is the dominant form (˜90%) [28], [29].
Synthesis of an alternative polymer, poly(isoprene)-b-poly(acrylamide), is provided according to Scheme II below:
In some embodiments, the polymeric shell can comprise a polymer synthesized according to Scheme III:
In some embodiments, the isoprene blocks in Scheme III can be replaced any of the following polymer blocks:
In some embodiments, the polymeric shell can comprise a polymer, poly(vinylbenzyltrimethylammonium chloride)-poly(isoprene) (PVB3MA-PI), synthesized according to Scheme IV:
The polymeric capsules described herein can contain a lipophilic ligand within the polymeric shell. Any ligand that will dissolve in or covalently bond with the polymeric shell can be used. In some embodiments, the lipophilic ligand can be a freely diffusing ligand. In some embodiments, the lipophilic ligand can be a ligand that covalently bonds to the polymer of the polymeric shell. The specific ligand can be selected based on the desired target species for separation. For example, where the target species is a metal or metal ion, the lipophilic ligand can be a ligand that interacts with metal ions. In some embodiments, the ligand can specifically bind a target metal ion. In some embodiments, the lipophilic ligand can be selected from oximes, phosphoric acid derivatives, phosphinic acid derivatives, phosphonic acid derivatives, diketone derivatives, amine derivatives, ketone derivatives, polyether derivatives, crown ether derivatives, cryptand derivatives, calixarene derivatives, and combinations thereof.
In some embodiments, the lipophilic ligand can be di-(2-ethylhexyl)phosphoric acid (D2EHPA).
Also provided herein is a process for extracting target ions from a mixed solution. In some embodiments, the target ions are metal ions. The process can include exposing the mixed solution to polymeric capsules to produce a second solution. In some embodiments, the capsules can comprise a polymeric shell encasing an inner chamber and comprising an amphiphilic polymer having a hydrophobic block and a hydrophilic block; a lipophilic ligand within the polymeric shell; and a first strip solution (e.g., an acid solution) contained within the inner chamber of the capsule. In some embodiments, the capsules can comprise a polymeric shell encasing an inner chamber, with the shell comprising a hydrophobic polymer; a lipophilic ligand within the polymeric shell; and a first strip solution contained within the inner chamber of the capsule. In some embodiments, the capsule can be a polymersome. In some embodiments, the capsule can be a metal selective polymersome. In some embodiments, the lipophilic ligand can specifically bind the target metal ion, or other target ion. In some embodiments, the lipophilic ligand can bind a non-target ion in order to remove the non-target from the mixed solution.
In some embodiments, the method can further comprise removing the second solution from the capsules. In some embodiments, removing the second solution can comprise washing, e.g., washing the polymeric capsules, to remove non-target ions. In some embodiments, removing the second solution can comprise a size-based separation technique or a packed bed technique. Non-limiting exemplary separation techniques for removing the second solution can include membrane filtration, particle filtration using a packed bed, immobilization on particles within a packed bed, centrifugation, and the like. Non-limiting exemplary filtration techniques include filtration with one or more porous membranes and filtration using a packed bed of fine particles (e.g., sand). In some embodiments, the packed bed technique can comprise packing the capsules, clusters of the capsules, or capsules immobilized on granular supports within a column; and flowing aqueous solutions (e.g., the mixed solution, the second solution, or the like) through the column. In some embodiments, the method can further comprise exposing the capsules to a second strip solution (e.g., acid solution) to obtain a solution enriched in the target metal ion and regenerate the first strip solution within the capsules. In some embodiments, the method can further comprise collecting the target metal ions.
One exemplary embodiment of a method for extracting target ions from a mixed solution is shown in
Embodiment 1 is a capsule comprising a polymeric shell encasing an inner chamber; and a lipophilic ligand within the polymeric shell.
Embodiment 2 is the capsule of Embodiment 1, wherein the shell comprises one or more amphiphilic block polymers.
Embodiment 3 is the capsule of Embodiment 2, wherein the one or more amphiphilic block polymers comprises a hydrophilic block that is uncharged, cationic, zwitterionic, or anionic.
Embodiment 4 is the capsule of Embodiments 1-2, wherein the polymeric shell comprises a water-insoluble polymer with a glass transition temperature below 20° C.
Embodiment 5 is the capsule of Embodiments 1-2, wherein the polymeric shell comprises a plasticizer and a water-insoluble polymer with a glass transition temperature above 20° C.
Embodiment 6 is the capsule of Embodiments 1-4, wherein the capsule is a polymersome.
Embodiment 7 is the capsule of Embodiment 1, wherein the polymeric shell comprises at least one polymer selected from poly(isoprene), poly(chloroprene), poly(butadiene), poly(myrcene), poly(farnesene), poly(citrenellol), hydrogenated poly(isoprene), hydrogenated poly(butadiene), polyethylene, polypropylene, polymers from styrenic derivatives, polymers from acrylic derivatives, polymers from acrylamide derivatives, and polydimethylsiloxane.
Embodiment 8 is the capsule of Embodiment 2, wherein the one or more amphiphilic block polymers comprises a hydrophilic block comprising a monomer selected from ethylene oxide, allyl alcohol, methyl oxazoline, acrylamide and derivatives thereof, zwitterionic derivatives of styrenic monomers and vinylpyridine, and cationic derivatives of styrenic monomers and vinylpyridine, and combinations thereof.
Embodiment 9 is the capsule of Embodiment 8, wherein the hydrophilic block comprises at least one of poly(ethylene oxide), poly(allyl alcohol), poly(2-methyl-2-oxazoline), and poly(acrylamide).
Embodiment 10 is the capsule of Embodiments 2 or 8, wherein the one or more amphiphilic block polymers comprises a hydrophobic block comprising a monomer selected from isoprene, chloroprene, butadiene, myrcene, farnesene, citrenellol, styrenic derivatives, acrylic derivatives, acrylamide derivatives, and siloxane derivatives, and combinations thereof.
Embodiment 11 is the capsule of Embodiment 10, wherein the hydrophobic block comprises at least one of poly(butadiene), poly(styrene), poly(isoprene), and poly(dimethyl siloxane).
Embodiment 12 is the capsule of Embodiments 1-11, wherein the lipophilic ligand is selected from oxime derivatives, phosphoric acid derivatives, phosphinic acid derivatives, phosphonic acid derivatives, diketone derivatives, amine derivatives, ketone derivatives, polyether derivatives, crown ether derivatives, cryptand derivatives, calixarene derivatives, and combinations thereof.
Embodiment 13 is the capsule of Embodiments 1-12, further comprising a strip solution contained within the inner chamber of the capsule.
Embodiment 14 is the capsule of Embodiment 13 wherein the strip solution is an acid solution, and optionally, wherein the acid is hydrochloric acid, nitric acid, or sulfuric acid.
Embodiment 15 is the capsule of Embodiment 14, wherein the acid solution comprises a polymeric acid.
Embodiment 16 is the capsule of Embodiment 15, wherein the polymeric acid comprises a poly(styrene sulfonic acid), poly(acrylic acid), poly(methacrylic acid), or combinations thereof.
Embodiment 17 is the capsule of Embodiments 1-16, wherein at least one dimension of the capsule is from about 10 nm to about 5 mm.
Embodiment 18 is the capsule of Embodiment 17, wherein one dimension of the capsule is from about 10 nm to about 10 μm.
Embodiment 19 is the capsule of Embodiments 1-16, wherein the shell is from about 1 nm to about 20 μm thick.
Embodiment 20 is the capsule of Embodiment 17, wherein the shell is from about 1 nm to about nm thick.
Embodiment 21 is the capsule of Embodiments 1-18, wherein the lipophilic ligand interacts with metal ions.
Embodiment 22 is the capsule of any one of Embodiments 1-19, wherein the lipophilic ligand is a freely diffusing ligand.
Embodiment 23 is the capsule of any one of Embodiments 1-19, wherein the lipophilic ligand is covalently bonded to a polymer in the shell.
Embodiment 24 is the capsule of any one of Embodiments 1-21, wherein the shell is cross-linked.
Embodiment 25 is the capsule of Embodiment 1, wherein the shell comprises a water-insoluble polymer and one or more surfactants.
Embodiment 26 is the capsule of Embodiment 25, wherein the one or more surfactants comprise small molecules with a low hydrophilic-lipophilic balance (<8).
Embodiment 27 is the capsule of Embodiment 26, wherein the small-molecule surfactants comprise sorbitan monooleate, sorbitan trioleate, or lecithin.
Embodiment 28 is the capsule of any one of Embodiments 25-27, wherein the surfactants are polymeric surfactants.
Embodiment 29 is the capsule of any one of Embodiments 25-28, wherein the polymeric surfactants comprise amphiphilic polymers.
Embodiment 30 is the capsule of any one of Embodiments 25-29, wherein the polymeric surfactants have segments comprising polyvinyl alcohol, poly(ethylene oxide), poly(propylene oxide), poly(dimethyl siloxane), poly(butadiene), poly(isoprene), or poly(styrene).
Embodiment 31 is the capsule of any one of Embodiments 25-30, wherein the polymeric surfactants have acidic segments, such as poly(4-styrene sulfonic acid), poly(acrylic acid), and poly(methacrylic acid).
Embodiment 32 is the capsule of Embodiment 1, wherein the capsule comprises a multi-block copolymer.
Embodiment 33 is the capsule of Embodiment 32, wherein the multi-block copolymer comprises a hydrophilic segment, a hydrophobic segment, and an acidic segment.
Embodiment 34 is the capsule of Embodiment 33, wherein the hydrophobic segment forms a water-insoluble shell and the acidic segment faces the inner chamber.
Embodiment 35 is a method of making a polymeric capsule comprising:
forming, in a strip solution or a metal salt solution, a polymersome, from at least one amphiphilic block polymer having a hydrophobic block and a hydrophilic block; and
exposing the capsule to a solution comprising a lipophilic ligand.
Embodiment 36 is the method of any one of Embodiments 35 or 68-79, wherein the capsule is formed in a metal salt solution, and further comprising exposing the polymersome to a strip solution.
Embodiment 37 is the method of any one of Embodiments 35-36 or 68-73, wherein the one or more amphiphilic block polymers comprises a hydrophilic block comprising a monomer selected from ethylene oxide, allyl alcohol, methyl oxazoline, acrylamide and derivatives thereof, zwitterionic derivatives of styrenic monomers and vinylpyridine, and cationic derivatives of styrenic monomers and vinylpyridine, and combinations thereof.
Embodiment 38 is the method of Embodiment 35, wherein the hydrophilic block comprises at least one monomer selected from poly(ethylene oxide), poly(allyl alcohol), poly(2-methyl2-oxazoline), and poly(acrylamide).
Embodiment 39 is the method of any one of Embodiments 35-38 or 68-73, wherein the one or more amphiphilic block polymers comprises a hydrophobic block comprising a monomer selected from isoprene, chloroprene, butadiene, myrcene, farnesene, citrenellol, styrenic derivatives, acrylic derivatives, acrylamide derivatives, and siloxane derivatives, and combinations thereof.
Embodiment 40 is the method of Embodiment 39, wherein the hydrophobic block comprises at least one monomer selected from poly(butadiene), poly(styrene), poly(isoprene), and poly(dimethyl siloxane).
Embodiment 41 is the method of any one of Embodiments 35-40 or 68-73, wherein the lipophilic ligand is selected from oxime derivatives, phosphoric acid derivatives, phosphinic acid derivatives, phosphonic acid derivatives, diketone derivatives, amine derivatives, ketone derivatives, polyether derivatives, crown ether derivatives, cryptand derivatives, calixarene derivatives, and combinations thereof.
Embodiment 42 is the method of any one of Embodiments 35-36 or 69, wherein the strip solution is an acid solution, and optionally, wherein the acid solution comprises a hydrochloric acid, a nitric acid, a sulfuric acid solution, or combinations thereof.
Embodiment 43 is the method of any one of Embodiments 35-42 or 69-73, wherein the metal salt solution is MgCl2 or CaCl2).
Embodiment 44 is the method of any one of Embodiments 35-43 or 69-73, further comprising synthesizing the amphiphilic block polymer according to Scheme I, wherein Scheme I comprises:
Embodiment 45 is the method of any one of Embodiments 35-43 or 69-73, further comprising synthesizing the amphiphilic block polymer according to Scheme II:
Embodiment 46 is the method of any one of Embodiments 35-43 or 69-73, further comprising synthesizing the amphiphilic block polymer according to Scheme III:
Embodiment 47 is the method of any one of Embodiments 35-43 or 69-73, further comprising synthesizing the amphiphilic block polymer according to Scheme IV:
Embodiment 48 is a method of extracting target ions from a mixed solution comprising: exposing the mixed solution to polymeric capsules to produce a second solution, wherein the capsules comprise: a polymeric shell encasing an inner chamber; a lipophilic ligand within the polymeric shell; and a first strip solution contained within the inner chamber of the capsule.
Embodiment 49 is the method of any one of Embodiments 48 or 68-73, wherein the lipophilic ligand specifically binds the target ion.
Embodiment 50 is the method of any one of Embodiments 48-49 or 68-73, further comprising removing the second solution from the capsules.
Embodiment 51 is the method of any one of Embodiments 48-50, further comprising: exposing the capsules to a second strip solution to obtain a solution enriched in the target metal ion and regenerate the first strip solution within the capsules; and collecting the target ions.
Embodiment 52 is the method of any one of Embodiments 48-51, wherein removing the second solution comprises washing to remove non-target ions.
Embodiment 53 is the method of Embodiment 52, wherein removing non-target ions comprises a size-based separation technique or a packed bed technique.
Embodiment 54 is the method of Embodiment 53, wherein the size-based separation technique is filtration with porous membranes.
Embodiment 55 is the method of any one of Embodiments 48-54, wherein the shell comprises one or more amphiphilic block polymers, wherein the one or more amphiphilic block polymers comprises a hydrophilic block comprising a monomer selected from ethylene oxide, allyl alcohol, methyl oxazoline, acrylamide and its derivatives thereof, zwitterionic derivatives of styrenic monomers and vinylpyridine, and cationic derivatives of styrenic monomers and vinylpyridine, and combinations thereof.
Embodiment 56 is the method of Embodiment 55, wherein the hydrophilic block comprises at least one of poly(ethylene oxide), poly(allyl alcohol), poly(methyl oxazoline), and poly(acrylamide).
Embodiment 57 is the method of any one of Embodiments 55-56, wherein the one or more amphiphilic block polymers comprises a hydrophobic block comprising a monomer selected from isoprene, chloroprene, butadiene, myrcene, farnesene, citrenellol, styrenic derivatives, acrylic derivatives, acrylamide derivatives, and siloxane derivatives, and combinations thereof.
Embodiment 58 is the method of Embodiment 57, wherein the hydrophobic block comprises at least one of poly(butadiene), poly(styrene), poly(isoprene), and poly(dimethyl siloxane).
Embodiment 59 is the method of any one of Embodiments 48-58 or 68-73, wherein the lipophilic ligand is selected from oximes, phosphoric acid derivatives, phosphinic acid derivatives, phosphonic acid derivatives, diketone derivatives, amine derivatives, ketone derivatives, polyether derivatives, crown ether derivatives, cryptand derivatives, and calixarene derivatives.
Embodiment 60 is the method of any one of Embodiments 48-59 or 68-73, wherein the first strip solution, second strip solution, or combination thereof, is an acid solution, and optionally, wherein the acid is hydrochloric acid.
Embodiment 61 is the method of any one of Embodiments 48-60 or 68-73, wherein at least one dimension of the capsule is from about 10 nm to about 10 μm.
Embodiment 62 is the method of any one of Embodiments 48-61 or 68-73, wherein the shell is from about 1 nm to about 20 nm thick.
Embodiment 63 is the method of any one of Embodiments 48-62 or 68-73, wherein the lipophilic ligand is a freely diffusing ligand.
Embodiment 64 is the method of any one of Embodiments 48-63 or 68-73, wherein the lipophilic ligand is covalently bonded to a polymer in the shell.
Embodiment 65 is the method of any one of Embodiments 48-64 or 68-73, wherein the shell is cross-linked.
Embodiment 66 is the method of any one of Embodiments 48-65 or 68-73, wherein the extraction process is continuous or semi-continuous.
Embodiment 67 is the method of Embodiment 66, wherein removing non-target ions comprises a packed bed technique, and wherein the packed bed technique comprises:
Embodiment 68 is a method of making a polymeric capsule comprising:
Embodiment 69 is the method of Embodiment 68, wherein the aqueous solution is a strip solution or a metal salt solution.
Embodiment 70 is the method of any one of Embodiments 35 or 68-69, further comprising crosslinking the hydrophobic block of the polymer.
Embodiment 71 is the method of Embodiment 70, wherein the target ions are metal ions.
Embodiment 72 is the method of any one of Embodiments 1-34 or 48-67, wherein the polymeric shell comprises an amphiphilic polymer having a hydrophobic block and a hydrophilic block.
Embodiment 73 is the method of any one of Embodiments 1-34 or 48-67, wherein the polymeric shell comprises a hydrophobic polymer.
Embodiment 74 is a method of forming a polymeric capsule comprising:
Embodiment 75 is a method of forming a polymeric capsule comprising:
Embodiment 76 is a method of forming a polymeric capsule comprising:
Embodiment 77 is a method of forming a polymeric capsule comprising:
Embodiment 78 is a method of forming a polymeric capsule comprising:
Embodiment 79 is a method of forming a polymeric capsule comprising:
Unless noted otherwise, all reagents were obtained from commercial suppliers. Unless noted otherwise, appropriate laboratory and analytical procedures were employed.
Preliminary modeling was conducted to assess the expected performance of exemplary polymeric capsules such as metal selective polymersomes. Purely as an example, the models considered the extraction of cadmium ions (Cd2+) using the organophosphate ligand di-2-ethylhexylphosphoric acid (D2EHPA) [8]. This process involves cation exchange; during extraction each Cd2+ ion displaces two protons (H), which move into the aqueous phase. D2EHPA is a commonly used extractant in metal production (i.e., hydrometallurgy), typically exists as a dimer, and is well-characterized for the extraction of Cd2+ [8]. The results of this modeling, along with the underlying transport theory, illustrate the potential impact, expected characteristics, and practical requirements of exemplary metal selective polymersomes (MSPs).
In solvent extraction and facilitated transport membranes (including MSPs), the extraction and stripping of metal ions is governed by the reaction in Eqn. 1, shown for an aqueous divalent metal (M2+) [8].
M2++n(RH)2↔MR2(n−1)(RH)2+2H+ (Eqn. 1)
The forward reaction is extraction, while the reverse reaction is stripping. (RH)2 is the dimerized ligand and MR2(n−1)(RH)2 is the metal-ligand complex that includes both deprotonated and protonated forms of the ligand. Both the dimerized ligand and the complex are always in the organic phase. n is the number of ligand dimers that are needed to complex the metal ion, which for the Cd2+/D2EHPA system is approximately 2.5 (i.e., 5 equivalents of ligand) [8]. In other words, two D2EHPA molecules become anionic, losing protons to balance the charge of the divalent cation, and three D2EHPA molecules add to the complex as neutral species.
It is generally expected that the conplexation reaction be at equilibrium at the water/organic interface [4]. The concentrations can therefore be compared as follows, assuming that all activity coefficients are 1:
K is the equilibrium coefficient. Rearranging allows determination of the ratio of complexed metal in the organic phase compared to free metal in the aqueous phase:
Eqn. 3 can be used to calculate the expected performance of a SX stage at equilibrium, based on the initial concentrations of metal and ligand, and the relative volumes of the aqueous and organic phases.
In MSPs and other facilitated transport membranes, the membrane can couple the extraction and stripping steps. Eqn. 2 applies to both interfaces:
The subscripts “F” and “S” refer hereto the water/organic interfaces at the Feed solution and Strip solution, respectively. At equilibrium, the concentrations of free ligand and the metal/ligand complex are constant in the membrane, and the concentrations of aqueous metal and hydronium ions can be compared:
Neglecting the relatively small amount of metal that is complexed within the membrane, the removal of a divalent metal cation is driven by the square of the proton gradient, which is mostly dependent on the pH of the strip solution. For example, if the equilibrium strip and feed pH are 1 and 3, respectively, the metal concentration is expected to be 104-fold greater in the strip than in the feed.
The effect of this coupling of the feed and strip solutions is shown in
Because the amount of metal removed with MSPs can depend on the strip solution pH and not the ligand concentration, about 10,000-fold less ligand can be required to remove a given amount of metal using MSPs with a starting strip solution of 1 M HCl than in traditional SX. This presents a key advantage of the polymeric capsules and methods described herein. Furthermore, this modeling considered a relatively high ligand concentration of 10 wt %. Decreasing the ligand concentration to 1 wt % would result in ˜105-fold less required ligand in MSPs than in SX. The ligand concentration in MSPs is expected to mainly affect the kinetics.
The modeling also provides insight into the design requirements of MSPs. Because the amount of metal removed is related to the strip pH, MSPs should be able to tolerate very low pH levels (pH˜0). For example, for a feed of 100 ppm Cd2+, a strip solution of 0.04 M HCl (pH 1.4) would remove a maximum of 93 of the feed metal at 10% total vesicle volume, compared with 99.995% with a strip solution of 1 M HCl (see, e.g.,
Polymersomes were first developed as a fully synthetic, mechanically tough analog to lipid vesicles (or liposomes) [11]. Liposomes have been used in biochemistry for decades as model systems to study the properties of cell membranes [14] and have more recently been applied for use in drug delivery and diagnostics [15]. Polymersomes can enable a much broader range of properties than for lipids, as the chemical nature and polymer chain lengths can be independently tuned. The chain length is one basic difference, enabling the polymersome walls to reach 10-nm in thickness [16], as compared with the ˜4-nm thickness of lipid bilayers [14]. This enhanced thickness, even in the absence of crosslinking [17], can enable increased mechanical integrity [11]. The thickness of polymersome walls is still much thinner than the typical selective layer of industrial membranes (100-1000 nm) [9]. In terms of chemical structure, polymersomes have been formed from block polymers with rubbery and glassy hydrophobic blocks [18], [19]. Two of the most-studied block polymers with rubbery hydrophobic blocks—polymers with glass transition temperatures, Tg, below the operational temperature—are poly(ethylene oxide)-b-poly(1,2-butadiene) (PEO-PB; Tg,PB: −31° C.) and the ABA triblock polymer poly(2-methyl-2-oxazoline)-b-poly(dimethylsiloxane)-b-poly(2-methyl-2-oxazoline) (PMeOx-PDMS-PMeOx; Tg,PDMS: −123° C.) [13], [17], [20].
Molecular transport through the walls of liposomes and polymersomes typically follows the solution-diffusion model [13], [14], which is widely used for membrane transport [9], [21]. In this model, solutes dissolve from the aqueous phase into the membrane, diffuse across the membrane thickness, and desorb into the opposite aqueous phase. Transport in the solution-diffusion model is diffusive in nature, meaning that molecules move down their concentration gradients inside the membrane [9]:
Here, J is the solute flux in the x-direction, D is the solute diffusivity within the membrane, c is the concentration of solute, cm,0 and cm,δ are the solute concentrations within the membrane at x=0 and x=δ, respectively, and δ is the thickness of the membrane. For passive diffusive transport, a partition or sorption coefficient, Ks, can be included that relates the solute concentration in the membrane to that in the aqueous phase (i.e., Ks=cm,x/cx). Incorporating Ks into Eqn. 6 yields
where P is the diffusive permeability.
The permeability of PEO-PB and PMeOx-PDMS-PMeOx polymersomes was recently analyzed using the solution-diffusion model (Eqn. 7). Following similar work on liposomes [22], permeation behavior was analyzed by modeling the block polymer bilayer as a homogeneous slab of nonpolar phase, in other words, assuming that permeation was entirely dictated by the hydrophobic domain [13]. To test this model, the permeability of neutral organic solutes of varying polarity were compared with their hexadecane/water partition coefficients (Khdw), as shown in
In terms of ion transport, the dense, defect-free nonpolar phase results in extremely low ion permeability in the absence of an ion carrier or ion channel. In the study on polymersomes mentioned above [13], sodium (Na+) permeability was measured by entrapping 300 mM NaCl inside the polymersomes, buffer exchanging into an isosmotic MgCl2/MgSO4 solution, then measuring the extravesicular Na+ concentration over time. Over the course of 6 days, just ˜2% of the Na+ permeated the walls of the PEO-PB and PMeOx-PDMS-PMeOx polymersomes, corresponding to permeabilities of ˜10−15 m/s [13]. This extremely low permeability stems from the high Born solvation energies (˜57 kcal/mol) [15], [23] needed to bring ions from water (ε=78) into a medium of low dielectric constant such as a hydrocarbon or nonpolar phase (P=2.0 for dodecane).
Fast Extraction Kinetics of MSPs Will Depend on the Diffusivity and Carrier Concentration
Modeling was used to assess the expected performance of MSPs, this time looking at kinetics. In facilitated transport membranes, the flux of metal is diffusive in nature (similar to Eqn. 6), only in this case sorption is dependent on multiple factors (Eqn. 3) and the diffusing species is a metal/ligand complex. The flux can be written as [4]:
Flux can also be related to the overall metal concentration in the feed [M2+]F, via mass balance:
VF is the volume of the feed solution. Ap and Vp are the cumulative area and volume of MSPs, respectively. rp is the radius of the MSPs, which are assumed in the models to be monodisperse.
With respect to the impact of mass transfer limitations in the aqueous phase, incomplete mixing leads to boundary layers near the membrane surface, in which concentration gradients form due to the need for metal to diffuse to or from the surface. This phenomenon, termed concentration polarization, is typically modeled using film theory, in which the boundary layer is assumed to be a stagnant (unstirred) layer of thickness cui, as illustrated in
The effect of boundary layers on flux is shown in
With respect to estimating the kinetics of metal removal using MSPs, the modeling described herein assumed that the interfacial reactions are fast, meaning that local equilibrium is obtained, as has been observed in other facilitated transport membrane systems [4], [10]. Considering the ultra-thin thickness of the walls of MSPs, this assumption should be further tested experimentally. Under the assumption of interfacial equilibrium, metal removal was assessed for diffusivities within the membrane varying from 10−11 to 10−14 m2/s for 1 wt % D2EHPA (
Based on Eqn. 3, the ligand concentration helps determine the concentration of the metal/ligand complex, which affects the flux in Eqn. 8. As such, the ligand concentration may be tuned in MSPs to yield the desired kinetics while ensuring efficient use of possibly expensive ligands. Kinetics for D2EHPA concentrations of 1 wt % and 0.1 wt % are shown in
In some applications of extractive separations, the main goal may be the maximum removal of similarly binding metals, such as the removal and concentration of actinides in nuclear waste [4]. However, in most applications, success can be contingent on high selectivity between similar ions that are hard to separate by other methods (e.g., based on charge or size). As discussed earlier, the facilitated transport process in MSPs is extractive in nature. Selectivity is therefore similar in MSPs to the corresponding SX system. Because MSPs should experience minimal concentration polarization, the selectivity at any given point can be identical to solvent extraction at the same conditions (i.e., metal concentrations and pH) [4]. Metal/ligand complex diffusivities will likely be similar among different metals. Therefore, as in SX, the main driving force for selectivity can be the equilibrium coefficient (Eqn. 2), which is determined by the binding affinity of the ligands for the particular metal.
One large difference between SX and MSPs is that while SX reaches equilibrium in each stage, MSPs must be used in a non-equilibrium fashion for highly effective separations. Because of the pH gradient between the feed and strip solutions, the full equilibrium for a multi-component system would generally (at long times) be to remove the majority of all metals in solution. A key difference is their rate of removal or the relative flux, J1/J2, which is the measure of instantaneous selectivity of the system and, again, can be identical to an SX process at the corresponding conditions. The cumulative process selectivity at time t, (t), can be defined as the ratio of the normalized amount of removed metal:
where M12+ is the target metal for removal, M22+ is a non-target divalent metal, [M2+](t) is the metal concentration in the feed solution at time t, and [M2+]0 is the initial metal concentration in the feed solution.
The modeled separation of metals by MSPs based on their K values is shown in
For MSPs, unstirred layer thicknesses can be estimated by geometric approximations. For the strip solution, the maximum thickness is simply the internal radius of the polymersome. For the feed solution, it is first assumed that a system with n polymersomes of radius r is equally distributed such that the MSPs are arranged in a hexagonal close-packed (HCP) arrangement. Each polymersome is then at the center of a theoretical sphere of radius rhcp, where these theoretical spheres are close-packed. Assuming no mixing occurs in the system, the unstirred layer thickness becomes
δul=rhcp−r (Eqn. A1)
The total normalized volume occupied by spheres in an HCP system is π/(3✓2), or ˜0.74. Therefore, the volume of each sphere, Vhcp is
It is simpler to work with the total (outer) volume of polymersomes, Vp, which is equal to
where V is the volume of an individual polymersome. Substituting this in for n results in
From Eqn. A4, rhcp can be solved for:
Finally, plugging Eqn. A5 into Eqn. A1 results in:
For a Vp of 0.01, meaning 1% of the total system, δul=2.74rp. For a Vp of 0.1, δul=0.73rp. Based on a polymersome radius of 100-150 nm, δul can be estimated as ˜200 nm.
All modeling was completed in Python 3.6 (Python Software Foundation) with the NumPy and SciPy add-ons, using the Scientific Python Development Environment (Spyder Project Contributors). As discussed above, models considered the equilibrium extraction data established for the Cd2+/D2EHPA system in tetradecane [8]. Specifically, this data establishes in Eqn. 2 the equilibrium coefficient, K, as 0.0258 and the number of ligand dimers that complex each metal ion, n, as 2.5 (i.e., 5 ligand monomers per metal ion) [8]. All concentrations were taken as molarity, with the wt % of D2EHPA converted to molarity using the density of tetradecane (0.764 g/mL). Other important parameters—such as the MSP wall thickness and diameter, ligand concentration, initial feed concentration, and starting pH levels—are specified in the discussion of the relevant figures.
The general approach was to numerically find the roots of relevant equations (specifically, Eqns. 3, 5, 8 and 9) using built-in solvers in Python, specifically brentq for single nonlinear functions and fsolve for multivariate systems of nonlinear functions. The brentq function requires that bounds be specified, which were established given the parameters of the system. For example, if solving for the amount of ligand that was complexed, bounds were set at 0 and the concentration equivalent to all of the ligand being complexed. The fsolve function is sensitive to the initial guess, which were established by first solving for simpler situations (e.g., no concentration polarization).
The equilibrium models in
LT is the total ligand concentration. In Eqn. 3, [L] is given as [(RH)2]. The subscript 0 in Eqn. A7 refers to the initial condition. Eqns. A7, A8, and 3 were combined in order to determine [ML] and, subsequently, [M2+]F. For the solvent extraction curve in
For equilibrium with MSPs (
Eqns. A10-A12 were inserted into Eqns. 3 and 5 to solve for [ML] and [M2+]s, which were then used to solve for the rest of the concentrations.
For kinetic models (
Iterations started with initial conditions for the bulk concentrations of M2+ and H+. Three fluxes of metal occur simultaneously, one in the feed solution (JF), one within the membrane (Jm), and one within the strip solution (JS):
Eqn. A14 is equivalent to Eqn. 8 in the main text. DML is the diffusivity of the metal/ligand complex within the membrane, given as D in Eqn. 8. The fluxes should all be identical, yielding the root equations 0=JF−Jm and 0=JS−Jm. Simultaneously solving these two equations, plus Eqn. 3 at both interfaces, allows for determination of the interface concentrations [M2+]F,0, [M2+]s,δ, [ML]0, and [ML]δ. When two metals were considered, concentration polarization in the strip solution was ignored, resulting in only one root flux equation (0=JF−Jm) for each metal, which are combined with four equations describing the interfacial reactions for a total of six equations. These were solved simultaneously to yield values for [M12+]F,0, [M1L]0, [M1L]δ, [M22+]F,0, [M2L]0, and [M2L]δ. After solving for the interfacial concentrations, the fluxes and bulk concentrations could be determined. Fluxes were calculated from Eqns. A13-A15 and compared to ensure equality. Bulk concentrations for the next iteration (j+1) were then calculated from the flux at iteration j:
Ap is again the total area of MSPs, which is related to the (outer) volume of MSPs (Vp), and the radius. VS is the total strip volume, which is the total inner volume of MSPs. VF is the aqueous feed solution volume. Eqns. A16-A19 include mass balances accounting for the volumes of the system. Iterating using this procedure enabled the kinetic modeling shown in
Poly(isoprene)-b-poly(allyl alcohol) is chosen as a synthetic target. Poly(isoprene) is an aliphatic, crosslinkable polymer with a low Tg (−61° C.). Poly(allyl alcohol) is water-soluble and stable at low and high pH [27], and will likely be much more inert towards cations than PEO. Importantly, it can be produced via the mild reduction of a newly developed polymer: poly(di-boc acrylamide) or poly(DBAn) [27]. This allows the block polymer to first be made using poly(DBAm), which is soluble in most organic solvents, then reduced to form poly(allyl alcohol), which is soluble in water and polar organic solvents. Both poly(isoprene) and poly(DBAm) have been synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerizatin, a class of controlled radical polymerization, using the same chain transfer agent [27]-[30]. The proposed synthetic scheme (Scheme I) builds on these established reactions to form the block polymer.
In Scheme I, 2-(dodecylthiocarbonothioylthio)-2-methylpropiolic acid (DDMAT) and isoprene are reacted in the presence of an initiator and heat. Next, di-boc acrylamide (DBAm) is added in the presence of an initiator and heat or blue light. Polymerization procedures generally follow existing procedures [27]-[30]. Reduction [27], with NaBH4, and deprotection of poly(DBAm) will yield acid-stable, neutrally charged, hydrophilic polymer blocks linked to poly(isoprene).
For analysis of the effects of polymer chemistry on the performance of MSPs, other block polymers such as poly(isoprene)-b-poly(acrylamide) are synthesized. Poly(isoprene)-b-poly(acrylamide) is synthesized according to Scheme II below:
Poly(isoprene)-b-poly(acrylamide) can be formed from the same poly(isoprene)-b-poly(DBAm) precursor by removing the boc protecting groups using acid or ammonia (Scheme II). Scheme III shows an alternative synthetic scheme to form poly(isoprene)-b-poly(2-methyl-2-oxazoline) using anionic polymerization of isoprene [31] followed by cationic ring opening polymerization of 2-methyl-2-oxazoline [20].
Several alternative hydrophobic blocks for replacing the isoprene blocks in Scheme III are shown below, all of which can be produced through RAFT or anionic polymerization [32]-[35]:
These potential chemistries vary in a few important characteristics, including polarity and Tg, that could affect properties like ligand efficacy and diffusivity. Myrcene and farnesene are dimers and trimers, respectively, of isoprene. Poly(myrcene) and poly(farnesene) similarly have low glass transition temperatures of −40 and −75° C., respectively [32] [34]. Poly(styrene) will serve as a contrasting chemistry as it is glassy (Tg˜100° C.) and aromatic. Poly(alkylstyrene) allows for control of Tg based on the alkyl chain length [35]. The purity, chemical structure, and chain length of synthesized polymers are characterized using nuclear magnetic resonance (NM) spectroscopy, infrared spectroscopy, and size exclusion chromatography.
Di-(2-ethylhexyl)phosphoric acid (D2EHPA) will be used as the model ligand for initial MSP demonstrations. After ligand dissolution, HCl (e.g., 1 M HCl) will be added to exchange H+ for Ca2+, essentially stripping Ca2+ from the MSPs. Other metal salts besides CaCl2 could also be used, or ideally polymersomes could be formed directly in ICI solutions. Buffer exchanging again into deionized water will yield ligand-bearing MSPs loaded with strip solution.
The metal extraction process with MSPs will place the polymersomes in relatively extreme environments in terms of pH, ligand concentration, and osmotic pressure differences. Polymersomes used as a matrix for MSPs will need to be tested for chemical and physical stability at extreme pH levels (pH<0 and >14) and at elevated temperature, as elevated temperatures (e.g., 50° C.) [37] are sometimes used in SX to increase selectivity. Chemical stability is assessed using NMR and infrared spectroscopies. Physical stability is assessed by measuring the leakage of encapsulated ions (e.g., sodium) using an ion selective electrode or ion chromatography [13]. The ligands used, as well as any potential co-dissolved phase modifiers, are surfactant-like and could affect the self-assembly properties of polymersomes, even possibly causing the polymers to dissolve or adopt an alternative aggregate morphology, such as micelles [38]. The aggregate morphology of uncrosslinked and crosslinked polymersomes with increasing concentration of ligand is assessed using dynamic light scattering (DLS) and cryo-electron microscopy (cryo-EM).
The osmotic pressure differences will stem from the desire to have a concentrated strip solution (1-5 M HCl) within the MSPs, whereas the feed solution with the target metal could vary widely in composition and osmolarity. The osmotic pressure difference, A7, can be approximated using the van't Hoff equation [9], [21]:
Δp=Δπ≈(nscn−nFcF)RT (Eqn. 11)
Here, n is the number of ions per molecule (e.g., 2 for HCl), R is the gas constant, and T is the absolute temperature. Subscripts again refer to the feed and strip solutions. Δπ will cause swelling of the polymersomes by water influx to decrease the osmotic gradient. Polymersomes cannot swell indefinitely, and a pressure, Δp, will build within the vesicle. The maximum pressure, Δp max, that the polymersomes can withstand is defined by the Laplace equation [39]:
where τR is the rupture tension and rp is the polymersome radius. Rupture tension data is relatively limited for polymersomes. The most relevant study assessed the rupture tension of uncrosslinked and crosslinked PEO-PB vesicles, finding values of 20 and 1000 mN/m, respectively [17]. Inserting these values into Eqn. 12 yields
The organophosphate ligand D2EHPA is used for proof-of-principle evaluation of MSPs. D2EHPA has been found to extract divalent metals in the order Zn2+>Ca2+>Mn2+>Cu2+>Co2+>Ni2+>Mg2+, with the pH at which 50% of the metal is extracted varying from 1.5-4.5 [40]. This order can change depending on experimental conditions, such as the solvent used and the presence of any phase modifiers (i.e., cosolvents) [40]. The extraction of Cu2+ is characterized, with the concentration of Cu2+ being quantified by ion chromatography, inductively coupled plasma mass spectrometry (ICP-MS), or colorimetric assays.
Extraction using MSPs is assessed in batch experiments. Experiments will aim to produce corresponding results as the models assessing the equilibrium extraction of metal (
To complete the mass balance, MSPs are stripped using fresh strip solution (e.g., 1 M HCl) and the Cu2+ concentration assayed. Comparison with the quantity of MSPs, as determined based on the initial mass of MSPs and the average size as determined by DLS and cryo-EM [13], will enable estimation of the encapsulated Cu2+ concentration prior to stripping.
To better characterize the extraction kinetics with MSPs, parameters such as ligand concentration and strip solution pH are varied. Planar poly(isoprene) films are used as models for comparison. These planar films are first used for equilibrium extraction experiments, essentially testing the extraction behavior of D2EPA in poly(isoprene) as a solvent, enabling calculation of the equilibrium coefficient, K (Eqn. 2) [8]. In these experiments, the system is allowed to reach equilibrium, after which the Cu2+ and H+ concentrations are assessed. The planar films are also used to measure the diffusivity of Cu2+/D2EPA complexes within poly(isoprene). Diffusivity experiments will use simple diffusion cells [26] with the planar film essentially serving as a thick, low-permeability facilitated transport membrane. One cell will hold the Cu2+-rich feed solution, while the other cell will hold the initially copper-free strip solution. These experiments can enable determination of the diffusion coefficient, D. The K and D values determined using planar films are compared with the results of MSPs to determine the validity of the model and its assumptions.
Building off of Example 4, MSPs are used to separate mixtures of divalent ions, such as Cu2+ and Zn2+, Cu2+ and Mg2+, or Zn2+ and Mg2+. Metal pairs will be chosen to provide a range of intrinsic selectivities (i.e., K1/K2).The lab-scale membrane separation method (Method 2 in
As discussed above, due to the non-equilibrium separation with MSPs, it can be important to control the residence time of the system. Steps are taken to optimize the residence time using the lab-scale membrane setup. For example, parameters affecting the porous membrane can be varied to optimize the total flow rate relative to the solution volume (residence time in a continuous process is the volume divided by the flow rate). These parameters include the membrane type (pore size, membrane material), the transmembrane pressure, and the relative membrane area. The ligand concentration and total concentration of MSPs are also varied to allow for the kinetics to match an experimentally feasible residence time. Finally, an additional pump can be installed to allow for fresh feed solution to be added to the MSP solution, with the flow-rate likely matching the permeate flow. This additional pump can allow for operation similar to a continuous stirred-tank reactor, which is likely beneficial from a processing perspective, and can enable rapid switching between feed, wash, and strip solutions.
Example 6 is performed simultaneously with Examples 4 and 5. Extraction processes are generally dependent on characteristics of the organic phase, meaning the solvent and any added phase modifiers [40]. For MSPs, the “solvent” is essentially the nonpolar polymer block. For poly(isoprene), the extent of crosslinking is varied to assess the effect on mechanical robustness, the equilibrium coefficients for given metal ions, and the metal/ligand diffusivity. Different nonpolar polymer chemistries are also assessed, as described in Example 2. Styrene and alkylstyrene systems may be particularly interesting, as they differ from isoprene in terms of polarity and glass transition temperature. These systems can be more difficult to crosslink, but the high Tg of these systems may make them sufficiently robust. If needed, copolymerization of vinylbenzyl chloride can enable crosslinking chemistries. In addition to the base polymer, commonly used phase modifiers from SX are also assessed. For example, tributyl phosphate has been used as a phase modifier for SX with D2EHPA in kerosene [40].
The models discussed thus far for MSPs have largely ignored the hydrophilic block, assuming that any resistance for metal transport through the water-solubilized hydrophilic polymer layer is negligible. Rather, the hydrophilic block is assumed to solely aid with the self-assembly of the polymersome and the dispersibility (i.e., lack of aggregation) of the resulting polymersomes. However, the headgroups of lipids in lipid bilayers (analogous to the hydrophilic block in polymersomes) can in some instances play an important role in membrane permeation, largely by sterically obstructing solute penetration into the nonpolar layer [14], [15], [42]. The effect of the hydrophilic block is assessed in MSPs by varying the chemistry and chain length of the hydrophilic block. Deviations in MSP performance when compared with K and 1) values obtained using pure poly(isoprene) films are then putatively assigned to steric exclusion by the hydrophilic block. To test this directly, the relevant amphiphilic block polymers are mixed with the planar poly(isoprene) films, whereupon the block polymers will segregate to the water/poly(isoprene) surface to minimize the interfacial energy [43]. Crosslinking can stabilize the block polymers at the surface, which is confirmed using infrared spectroscopy, X-ray photoelectron spectroscopy, and water contact angle. These planar films with surface-bound hydrophilic polymer chains are then used to determine K and D, enabling direct comparison with values obtained using pure poly(isoprene).
Structural attributes of the polymersomes may also play an important role in mechanical robustness and the kinetics of separations. The thickness of the vesicle wall, in particular, is varied by changing the total polymer chain length and the relative fraction of the hydrophobic block and assessed [16]. Thickness factors directly into the flux (Eqn. 8), and therefore a direct comparison between wall thickness and flux is fundamentally important. Wall thickness is estimated using cryo-EM [44]. The vesicle diameter defines the ratio between MSP surface area and volume, which will inherently affect the kinetics. This effect is assessed, with control of the vesicle size obtained by extrusion through membranes of different pore size (e.g., 100 nm, 200 nm).
Examples 2-6 deal with facilitated transport membranes that are directly analogous to solvent extraction, with freely diffusing ligands, metal/ligand complexes, and cosolvents (phase modifiers). This format is experimentally accessible, easily tunable, and has tremendous potential to be practical on an industrial scale. However, MSPs with this design may still leach ligand into the aqueous solution over time—even if MSPs are fully retained by the porous membranes-due to the non-zero solubility of ligand in water. This potential problem also affects SX. In practice, it is likely that fresh addition of ligand to the MSP solution could allow for recovery of the desired performance. Such leaching of ligand and performance recovery with addition of fresh ligand is tested in this Example.
An alternative strategy that MSPs enable is to covalently link the ligand to the hydrophobic polymer backbone, eliminating any possibility of ligand loss into the aqueous phase [45]. In this type of system, the ligands are not able to diffuse between the interfaces, forcing the metal ion to “hop” between ligands [45]. Transport under this mechanism can be more complex, but can still e rapid. Use of extremely pensive ligands may incentivize covalent linkage of ligands. As a proof-of-concept, maleic anhydride is grafted onto the poly(isoprene) polymer block through the ene reaction [46], [47], after which hydrolysis using NaOH will yield a di-carboxylic acid as the polymer-bound ligand (Scheme V).
Scheme V (A) shows grafting of carboxyl groups to poly(isoprene) blocks using maleic anhydride [47]. The resulting carboxylic acids can serve as carriers that are tethered or covalently linked to the polymer backbone.
For comparison, freely diffusing di-carboxylic acid ligands will be synthesized in a similar fashion using decene or another long-chain alkene instead of a block polymer containing poly(isoprene). Scheme V (B) shows synthesis of a chemically similar freely-diffusing carrier using maleic anhydride and 1-decene, or a similar aliphatic alkene [46]. The metal removal efficiency and kinetics are characterized for the polymer-bound system and freely diffusing system in a similar fashion as laid out in Examples 4 and 5. In particular, MSPs and planar films are both used and results are compared with transport models [45].
In Example 8, an experiment was conducted to show extraction of copper using a solvent, a ketoxime solution Lix 84-I (supplied by BASF), containing 2-hydroxy-5-nonylacetophenone oxime (HNAPO) as the active agent that binds selectively to Cu2+. The following procedural steps were taken for this experiment:
As shown in
In Example 9, an experiment was conducted to observe facilitated transport characteristics associated with a control sample, a membrane containing a poly(styrene)-poly(butadiene)-poly(styrene) (SBS) commercial triblock polymer (D1157, 30 wt % polystyrene, supplied by Kraton) film with 30 wt % Lix 84-I. The membrane, with a 1.77 cm2 membrane area (˜5 mg Lix 84-I in active membrane area), was positioned between two solutions: (1) 10 mL of 15.7 mM CuSO4, Initial pH˜4.5; (2) 10 mL of 1 M H2SO4, as shown in
The results of this experiment are shown in
In Example 10, an experiment was conducted to observe copper ion uptake and retention using Span 85-stabilized capsules. The following procedural steps were taken for this experiment:
The above described formulation produced relatively stable W/O emulsions. PVA was used to stabilize oil droplet in water. Also, as a preconditioning step, the capsules were soaked in 1 M H2SO4 for 1-2 days to exchange Cu2+ with H+. Capsules went from dark green to white during this time, due to release of Cu2+.
Following preconditioning, the Span 85-stabilized capsules were mixed with 20 mL 15.7 mM CuSO4.
The results of this experiment are provided in
Copper uptake occurred on the order of hours, and exhibited clumped-up particles (see
It was observed that the H+ concentration exceeded copper uptake (was ˜1:1 in hexane experiment), which suggested some leakage of inner acid solution. Furthermore, it was observed that the copper released over time back into aqueous solution, possibly due to leakage of interior solution due to osmotic swelling of interior and/or surfactant instability.
In Example 11, experiments were conducted to observe copper ion uptake and retention using poly(vinylbenzyltrimethylammonium chloride)-poly(isoprene) (PVB3MA-PI)-stabilized capsules.
The following procedural steps were performed for the experiment:
The above described formulation produced relatively stable W/O emulsions. PVA was used to stabilize oil droplet in water. Also, as a preconditioning step, the capsules were soaked in 1 M H2SO4 to exchange Cu2+ with H+. After soaking in 1 M H2SO4 for 4 days, the particles were still observed as slightly green, signifying that the capsules still contain Cu2+, indicating that the Cu2+ release for PVB3MA-PI-stablised capsules was slower than that observed for Span 85-stabilized capsules.
After preconditioning, the PVB3MA-PI Stabilized capsules were mixed w/20 mL 15.7 mM CuSO4. The results of this experiment are provided in
Low total uptake and slow/incomplete stripping suggest that the cationic brush layer in its structure may hinder Cu2+ transport. The kinetics with the capsules were notably faster than the kinetics observed with the (control) planar membrane (see Example 9). However, the different Lix 84-I amounts used between the two studies may have contributed to the kinetics difference.
It was also observed that the PVB3MA-PI stabilized particles stayed well dispersed (see
Furthermore, the amount of [H+] was observed as being approximately equivalent to the amount of [Cu2+] removed, suggesting minimal leakage of the internal acid.
The equilibrium removal of [Cu2+] was similar to that expected by extraction using an equivalent amount of Lix 84-I (see Example 8). It was noted that during this experiment, some capsules were lost due to spillage. In the absence of any loss, the expected extraction would have been 5.8 mM.
In Example 11, experiments were conducted to show carboxyfluorescein (CF) encapsulation of the PVB3MA-PI-stabilized capsules.
The following procedural steps were performed for the experiment:
The above described formulation produced relatively stable W/O emulsions that formed yellow particles, which qualitatively demonstrates that CF encapsulation occurred.
A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, other synthetic methods can be used for making the polymeric capsules. Accordingly, other embodiments, aspects, advantages, and modifications are within the scope of the following claims.
This application is a utility application and claims the benefit of U.S. Application No. 62/891,854, filed Aug. 26, 2019. The disclosure of the foregoing application is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62891854 | Aug 2019 | US |
