Rubidium recovery

Abstract

The present disclosure broadly relates to materials and method for recovery and purification of rubidium using sorbent nanocomposites comprising Metal Organic Framework (MOF) and alkali metal cobalt hexacyanoferrate. The disclosure also encompasses methods for separating rubidium from a solution comprising exposing the solution to a sorbent, stripping rubidium from the sorbent using an ammonium solution, electro -dialysing the solution to produce ammonia and air-stripping to generate a rubidium solution. The nanocomposite material and rubidium extraction methods exhibit superior rubidium selectivity and uptake capacity, addressing the cunent limitations and paving the way for commercial application of rubidium separation from seawater.

Description

Rubidium recoveryField of the disclosure

[0001] The present disclosure broadly relates to materials and methods for recovery and purification of rubidium.Background of the disclosure

[0002] Any discussion of the prior art throughout this specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0003] Rubidium (Rb) is a highly-priced metal. The high price of rubidium is due to its rarity and the difficulty of separating it from land ores. Rubidium is an essential mineral for the defence and medical industry. For example it is used to develop high-precision rubidium lasers for submarines precise targeting, atomic quantum navigators for ships and navy vessels, and medical CT scan beams. Natural water sources such as salt lakes and seawater brine are considered promising alternative and renewable resources of rubidium. Typically rubidium is present in seawater at about 0.18 to 0.20 ppm on a w / v basis (i.e. mg / L).

[0004] The capacity of potassium cobalt hexacyanoferrate (KCoFC) nanomaterials for attaining selective rubidium recovery from complex saline seawater due to an exclusive Rb-K ion exchange mechanism within the KCoFC crystalline lattice has been demonstrated. Enhancing the selectivity of a KCoFC nanomaterial would significantly improve the economic feasibility of applying this material for extracting rubidium from seawater.Summary of the disclosure

[0005] In a first aspect, there is provided a sorbent comprising nanoparticles, each of said nanoparticles comprising a metal-organic framework (MOF) and an alkali metal cobalt hexacyanoferrate. The mass of the alkali metal cobalt hexacyanoferrate may be greater than 10% of the total mass of the nanoparticle.

[0006] The following options may be used in conjunction with the first aspect, either individually or in any suitable combination.

[0007] The mass of the alkali metal cobalt hexacyanoferrate may be greater than 50% of the total mass of the nanoparticle.

[0008] The nanoparticles may be core-shell nanoparticles. In this instance, the alkali metal cobalt hexacyanoferrate may form a shell surrounding a core of the MOF.

[0009] The MOF may be a zeolitic imidazolate framework (ZIF). It may be made by reacting 2-methylimidazole with a zinc salt.

[0010] The alkali metal cobalt hexacyanoferrate may be potassium cobalt hexacyanoferrate.

[0011] The sorbent may be encapsulated in an encapsulant. The encapsulant containing the sorbent may be in the form of beads. The beads may have mean diameter of from about 0.5 to about 3mm in diameter. The sorbent nanoparticles may be distributed evenly within the beads.

[0012] In a second aspect, there is provided a process for making a sorbent comprising combining a solution of an alkali metal hexacyanoferrate with a solution of a cobalt salt and adding a nanoparticulate MOF to the resulting solution to produce the sorbent.

[0013] The following options may be used in conjunction with the second aspect, either individually or in any suitable combination.

[0014] The MOF may be a ZIF.

[0015] The process may comprise the step of reacting 2-methylimidazole with a zinc solution so as to form the nanoparticulate MOF prior to the step of adding the nanoparticulate MOF.

[0016] The process may additionally comprise separating the sorbent from the solution, washing the sorbent and drying the sorbent.

[0017] The process may comprise encapsulating the sorbent in an encapsulant. The encapsulant may be a polymeric encapsulant.

[0018] The step of encapsulating may comprise extruding a mixture of the encapsulant and the sorbent so as to produce beads comprising the sorbent encapsulated within the encapsulant. The beads may be between about 0.5 to about 3 mm in diameter.

[0019] There is also provided a sorbent made by the process of the second aspect.

[0020] In a third aspect there is provided a method for separating rubidium from a solution thereof comprising exposing said solution to a sorbent according to the first aspect or to a sorbent made by the process of the second aspect.

[0021] The following options may be used in conjunction with the third aspect, either individually or in any suitable combination.

[0022] The exposing may comprise agitating the solution with the sorbent. It may comprise passing the solution through a bed of, or through a bed comprising, the sorbent.

[0023] The method may be conducted at between about 5 and about 30°C, or between about 15 and about 30°C

[0024] The method may comprise stripping the rubidium from the sorbent. The stripping may comprise exposing the sorbent with rubidium thereon to a solution of ammonium ions to produce a crude product solution containing rubidium and ammonium ions and separating the crude product solution from the sorbent. The method may comprise removing ammonium ions from the crude product solution to form a purified rubidium solution. The removing may comprise converting the ammonium ions to ammonia and passing a gas over and / or through the solution so as to remove the ammonia as ammonia gas. In this event, the ammonium ions may be converted to ammonia by electrodialysis of the crude product solution.

[0025] The method may comprise regenerating the sorbent. The regenerating may comprise exposing the sorbent to a solution of an alkali metal salt. The alkali metal may be potassium.

[0026] In a fourth aspect, there is provided a method for separating rubidium from a solution thereof comprising:• exposing said solution to a sorbent comprising nanoparticles, said nanoparticles comprising potassium cobalt hexacyanoferrate shell and a MOF;• stripping the rubidium from the sorbent using an ammonium solution so as to form a solution comprising the rubidium and ammonium;• electrodialysing the solution obtained in step b) so as to convert ammonium in the solution to ammonia;• air stripping the resulting solution so as to remove the ammonia therefrom so as to generate a rubidium solution containing negligible concentrations of ammonium and ammonia;• regenerating the sorbent by treatment with a solution of a potassium salt; and reusing the sorbent in step a).

[0027] The solution of rubidium may be seawater or saline water. The nanoparticles may be encapsulated in an encapsulant.Definitions

[0028] The following are some definitions that may be helpful in understanding the description of the present disclosure. These are intended as general definitions and should in no way limit the scope of the present disclosure to those terms alone, but are put forth for a better understanding of the following description.

[0029] Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. The term "consist of" and variations thereof will be understood to imply the inclusion of stated integers and the exclusion of any unstated integer. The term "consist essentially of" and variations thereof will be understood to imply the inclusion of stated integers, possibly together only with incidental other integers that have negligible effect on the operation of the invention. Where the term "comprise" or a variation thereof is used, this may also imply a variant of the statement in which either "consist of" or "consist essentially of" (or variants thereof) are used. For example, the statement "A comprises B" should be taken to imply both "A consists essentially of B" and "A consists of B".

[0030] The term "nanoparticle" as used herein refers to a solid particle having a diameter, or at least one dimension, between about 1 and about 500nm. A nanoparticle may be a nanosphere. A nanosphere is a nanoparticle that is approximately spherical.

[0031] Metals as referred to herein may refer to ions or free metals. For example "rubidium" may refer to metallic rubidium or to rubidium ions. By contrast however, "ammonia" refers specifically to NH3 whereas "ammonium" refers to NH4T

[0032] MOF is an abbreviation for "metal-organic framework" in which metal atoms or ions are connected by organic groups to form a 3-dimensional framework. A specific example of these is a ZIF, which is an abbreviation for "zeolitic imidazolate framework", in which the organic groups are imidazole derivatives. ZIF-L refers to a particular crystal structure of ZIF.

[0033] Where mention of a "solution" is made herein, this refers to an aqueous solution unless another solvent is explicitly specified.

[0034] The terms "a" and "an" are used herein to refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, "an element" means one element or more than one element.

[0035] In the context of this specification the term "about" is understood to refer to a range of numbers that a person of skill in the art would consider equivalent to the recited value in the context of achieving the same function or result. The term "about" may refer to ±10% of the recited value.

[0036] The term "may" should be understood to include both positive and negative recitations unless the context indicates otherwise. For example, the phrase "A may be B" should be taken to encompass both "A is B" and "A is not B".

[0037] Any numerical range recited herein is intended to include all sub-ranges of the same numerical precision subsumed within the recited range. For example, a range of 1.0 to 5.0 is intended to include all sub-ranges between (and including) the recited minimum value of 1.0 and the recited maximum value of 5.0, that is, having a minimum value equal to or greater than 1.0 and a maximum value equal to or less than 5.0, such as 2.1 to 4.5. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited herein is intended to include all higher numerical limitations subsumed therein.

[0038] The terminology A@B should not be taken to imply any particular structure, but merely indicates particles comprising A and B. Thus A@B may be a core of A surrounded by a shell of B, or a core of B surrounded by a shell of A, or an aggregate comprising A particles and B particles, or some other type of structure comprising both A and B.

[0039] Any description of prior art documents herein, or statements herein derived from or based on those documents, is not an admission that the documents or derived statements are part of the common general knowledge of the relevant art.

[0040] For the purposes of description, all documents referred to herein are hereby incorporated by reference in their entirety unless otherwise statedBrief Description of the Drawings

[0041] Figure 1. Schematic illustration of an in situ synthesis procedure for KCoFC@ZIF.

[0042] Figure 2(a) Schematic diagram of an electrodialysis and air stripping processes to recover high-purity Rb+; (b) Schematic diagram of a column design.

[0043] Figure 3. Rb+uptake performance of the KCoFC / ZIF composite adsorbents in contact with a 5 mg / L Rb+solution. Each solute ion was treated with adsorbent dose of 0.05 g / L, and the pH was maintained at 7.0 ± 0.5.

[0044] Figure 4. Effect of solution pH on the Rb+uptake performance of KCoFC and KCoFC@ZIF (B) nanomaterials (model Rb+solution = 5 mg / L; material dose = 0.05 g / L; initial pH = 2 - 12).

[0045] Figure 5. Comparison between KCoFC and KCoFC@ZIF (B) materials regarding Rb+uptake over time (model Rb solution = 5 mg / L; material dose = 0.05 g / L; pH = 7.0 ± 0.5).

[0046] Figure 6. Rb+uptake experimental data fitted to kinetic models of Pseudo-first order and Pseudo-second order for (a) KCoFC and (b) KCoFC@ZIF(B).

[0047] Figure 7. Rb uptake experimental data fitted to isotherm models of Langmuir and Freundlich for (a) KCoFC and (b) KCoFC@ZIF (B) (model Rb solution = 5 mg / L; material dose = 0 - 0.2 g / L; pH = 7.0 ± 0.5).

[0048] Figure 8. Comparison between three materials regarding selective Rb+uptake efficacy in seawater (seawater spiked with 5 mg / L Rb; material dose = 0.20 g / L; pH = 7.54 ± 0.5).

[0049] Figure 9. The regeneration performance capacity of KCoFC@ZIF (B) based on Rb uptake capacity with five reuse cycles of the materials (model Rb solution = 5 mg / L; material dose = 0.1 g / L; pH = 7.0 ± 0.5).

[0050] Figure 10. SEM image of in situ synthesized KCoFC@ZIF(B) particle.

[0051] Figure 11. EDX elemental map of KCoFC@ZIF(B).

[0052] Figure 12. Powder XRD patterns of ZIF-grafted KCoFC (KCoFC@ZIF(B)).

[0053] Figure 13. FTIR spectra of materials used herein.Detailed Description

[0054] Integrating metal-organic framework (MOF) structures into nanomaterials offers a way to modify the structure of a nanomaterial, which in turn can enhance their selectivity. MOF structures increase the surface area and porosity of composite materials due to their adjustable, diverse and adaptable morphology and can introduce additional functional groups based on the metal ions and organic linkers used. Combining zeolitic imidazole frameworks (ZIF) with KCoFC (KCoFC@ZIF) can enhance rubidium uptake capacity. The ZIF is thoughtto increase the surface area and maintain a suitable pore size, boosting rubidium uptake by up to 8 times compared to neat KCoFC. It is thought that, acting as a catalytic interface, the ZIF layer may promote the partial dehydration of rubidium ions, facilitating their transport and exchange with potassium (K) ions in the KCoFC lattice.

[0055] One aspect that determines well-defined selective structural features and functionality of M OF- in teg rated nanomaterials is the synthesis method. MOF structural integration with nanomaterials can be achieved through various methods, including ex situ methods, in situ methods and post-synthetic modification.

[0056] An ex situ synthesis of KCoFC@ZIF, while showing enhancement of rubidium uptake, is challenging to apply for practical bulk scale commercial production. This is because the ex situ synthesis approach produces the ion exchange nanomaterial (KCoFC) and ZIF separately and thereafter combines these two materials. This approach requires multiple steps and uses large quantities of reagent, resulting in high material losses and low material yield. This makes the synthesis process costly. Enhancing the synthesis process for practical application is important commercially.

[0057] The present invention provides an in situ method for the preparation of a KCoFC@ZIF nanomaterial. The in situ synthesis approach involves direct combination of ZIF crystals with KCoFC within a single reaction vessel, thereby minimizing reagent usage and reducing material losses. This method provides a more sustainable and scalable solution for producing KCoFC@ZIF nanomaterials, and enhances the functionality of the material for rubidium selectivity.

[0058] While selective diffusion and adsorption of rubidium from seawater can be efficiently achieved with KCoFC@ZIF, desorbing rubidium and isolating pure rubidium require further refinement. This is because KCI is typically used to recover rubidium from within the pores of KCoFC in line with the highly selective Rb-K exchange. Obtaining pure rubidium from a regenerated solution containing potassium and rubidium is a challenge due to the similar properties of these ions. Desorption with NH4+shows promise but requires additional chemicals for pH adjustment. To address this, for the first time, the inventors have used the potential of membrane electrodialysis for separating NH4+from rubidium. Typically in membrane electrolysis regulating the catalytic hydroxide reduction and oxidation of the solution will increase the solution pH without the need for additional chemicals. In turn, NH4+can be removed as NH3gas by air stripping following electrolytic pH adjustment, leaving a solution rich in Rb+.

[0059] The present invention represents a significant advancement in the field of critical metal recovery by developing an efficient and practical in situ synthesis method and a chemical-free separation process for rubidium recovery from seawater or other dilute rubidium solutions. The KCoFC@ZIF nanomaterials described herein exhibit superior rubidium selectivity and uptake capacity, addressing the current limitations and paving the way for commercial application of rubidium separation from seawater.

[0060] A schematic of an example of a process for synthesising sorbents according to the present invention is shown in Figure 1. Thus cobalt nitrate and potassium hexacyanoferrate were combined and ZIF nanoparticles added so as to form KCoFC@ZIF,. It should be noted that, whereas Figure 1 shows this product to have a core-shell structure, this may not be the actual structure and is shown merely for illustrative purposes.

[0061] Rubidium was then sorbed into these nanoparticles according to the following exchange equation:This may be conveniently conducted at room temperature, pH 7.5-8

[0062] Polymer encapsulation of the nanoparticles may be carried out by mixing the material with heated polymer (polyacrylonitrile (PAN)) and solvent (Tween 80), producing gel like solution. The solution may then be injected through an automated nozzle syringe pump, producing uniform particles, conveniently about 2 mm bead size material.

[0063] Figure 2a shows a diagram of a suitable electrodialysis and air stripping processes which may be used to recover high-purity Rb+. In an example of this method, after regeneration, the mixed solution containing NH4+and Rb+ions underwent a hybrid separation process to obtain a high-purity Rb+solution without NH4+. The regeneration solution (feed solution) was introduced into the electrolysis cathode cell, while an equal volume of 0.1 M Na2SO4served as the anolyte solution in the anode cell. An ultrafiltration (UF) membrane separated the two solutions. Electrolysis then produced a regenerated solution in which NH4+was converted to NH3gas. Subsequently, the regenerated solution was placed under a flow of air for 24 hours (air stripping process) to remove NH3from the solution. This process resulted in a final solution containing high-purity Rb+ions as RbCL

[0064] An example of a suitable exchange column is shown in Figure 2b.

[0065] The present invention relates in one aspect to a sorbent comprising nanoparticles. The nanoparticles comprise MOF and an alkali metal cobalt hexacyanoferrate. This may be in theform of a MOF core at least partially surrounded by a shell of the alkali metal cobalt hexacyanoferrate, for example potassium cobalt hexacyanoferrate (KCoFC). The alkali metal cobalt hexacyanoferrate is suitable for exchange with rubidium so as to scavenge rubidium from a solution. In the case of a core-shell structure, the core may be completely surrounded by the shell or it may be partially surrounded thereby. The shell may cover at least about 50% of the surface area of the core, or at least about 60, 70, 80, 90 or 95% thereof, or about 60, 65, 70, 75, 80, 85, 90, 95 or 100% thereof. These percentages may be an average over all of the nanoparticles in a sample of the sorbent.

[0066] The mass of the alkali metal cobalt hexacyanoferrate is preferably greater than 10% of the total mass of the nanoparticle, so that the sorption capacity of the nanoparticle per mass is suitable. The mass of the alkali metal cobalt hexacyanoferrate may represent at least about 15, 20, 25, 30, 35, 40, 450, 50, 55, 60, 65, 70, 75, 80, 85 or 90% of the total mass of the nanoparticle, or from about 10 to about 90% or about 10 to 80, 10 to 70, 10 to 60, 10 to 50, 10 to 30, 20 to 90, 50 to 90, 70 to 90, 30 to 80, 50 to 80 50 to 70 or 60 to 80%, e.g. about 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85 or 90% of the total mass of the nanoparticle. It will be understood that the greater the proportion of the nanoparticle is alkali metal cobalt hexacyanoferrate, the greater the sorbent capacity of a given mass of sorbent will be.

[0067] The nanoparticles may have a diameter of from about 20 to about 200 nm or from about 10 to 150, 20 to 100, 20 to 50, 50 to 200, 100 to 200, 50 to 100 or 100 to 150 nm, e.g. about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 20 nm. The nanoparticles may combine into agglomerates of multiple nanoparticles. These agglomerates may have a diameter, or at least one dimension of from about 10 to about 200 microns, or from about 10 to 150, 20 to 100, 20 to 50, 50 to 200, 100 to 200, 50 to 100 or 100 to 150 microns, e.g. about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 1 10, 120, 130, 140, 150, 160, 170, 180, 190 or 20 microns. These agglomerates may be irregular shaped or regular, e.g. roughly spherical.

[0068] For core-shell nanoparticles, the core may have a diameter of about 10 to about 100 nm, or about 10 to 50, 10 to 20, 20 to 100, 50 to 100 or 20 to 50 nm, e.g. about 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100 nm. The thickness of the shell may be from about 10 to about 100 nm, or about 10 to 50, 10 to 20, 20 to 100, 50 to 100 or 20 to 50nm, e.g. about 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100 nm.

[0069] The MOF may be a ZIF. It may be a ZIF-L or some other form of ZIF. It may be Zn(mim)2, or it may be Zn(mim)2Hmim)i / 2. (^0)3 / 2 (in which mim is 2-methylimidazolate and Hmim is 2-methylimidazole).

[0070] The nanoparticles may themselves be encapsulated. They may be encapsulated in a polymeric encapsulant. They may be encapsulated in a porous encapsulant. A suitable encapsulant is PAN however other materials that are porous and / or permeable to Rb may also be suitable. The beads of encapsulated nanoparticles may be from about 0.5 to about 3mm diameter, or from about 0.5 to 2, 0.5 to 1 , 1 to 3, 2 to 3 or 1 to 2mm, e.g. about 0.5, 1 , 1.5, 2, 2.5 or 3mm in diameter. They may be substantially spherical. They may be substantially homogeneous in diameter. They may vary from the mean diameter by less than about 20%, or by less than about 15, 10 or 5%. Encapsulation of the nanoparticles serves to generate larger particles, which thereby facilitates flow of liquid through a bed of the particles. Additionally, larger particles are less prone to material loss during use, particularly during regeneration, i.e. they provide durability. It should be noted that the encapsulant should be permeable to rubidium ions. It may be porous so as to allow the rubidium solution to access the active nanoparticles.

[0071] The sorbent described above may be made by exposing nanoparticulate MOFs to the alkali metal cobalt hexacyanoferrate precursors so as to form the nanoparticles comprising alkali metal cobalt hexacyanoferrate and MOF. In a representative method, nanoparticulate MOF is combined with a solution comprising hexacyanoferrate ions and cobalt ions.

[0072] The MOF nanoparticles may be made by combining an imidazole, e.g. 2- methylimidazole (Hmim), with a solution of a zinc salt. Suitable zinc salts include zinc nitrate, zinc halides and other water soluble zinc salts. Commonly the imidazole will be used in excess, for example about, or at least about, a 2-fold molar excess or about, or at least about, a 3-, 4- , 5- or 10-fold excess. The mixture should be agitated during the reaction to promote even mixing and relatively homogeneous particle size distribution. The resulting sorbent may be separated from the solution by suitable methods such as centrifugation. Residual reagents may be removed from the solid sorbent particles by washing with water and / or suitable organic solvents (ethanol, acetone, isopropanol etc.). The product may then be dried at a suitable temperature, suitably between about 50 and 100°C so as to be ready for use or for subsequent encapsulation as described below.

[0073] The nanoparticles may be encapsulated in an encapsulant. A suitable method for doing so is to combine the encapsulant with the nanoparticles, and optionally a surfactant and / or solvent and / or swelling agent and / or gelling agent. This may be heated so as to form a liquiddispersion or a gel dispersion. This dispersion may then be extruded. It may be extruded into air or into a liquid, e.g. water. As it cools, the dispersion solidifies so as to form beads of the encapsulated particles. The diameter of the beads may be adjusted by adjusting the diameter of the nozzle through which they are extruded. It will be understood that each bead may contain multiple nanoparticles. In this case, the nanoparticles may be distributed through the bead, optionally evenly distributed therethrough.

[0074] The sorbent described above is suitable for scavenging rubidium from a solution thereof. The solution may have a rubidium concentration of from about 0.1 to about 100 ppm w / v, or about 0.1 to 50, 0.1 to 20, 0.1 to 10, 0.1 to 5, 0.1 to 2, 0.1 to 1 , 0.1 to 0.5, 0.1 to 0.2, 0.2 to 100, 0.5 to 100, 1 to 100, 10 to 100, 50 to 100, 1 to 50, 1 to 10 or 10 to 50 ppm, e.g. about 0.1 , 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.6, 0.7, 0.8, 0.9, 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50,60, 70, 80, 90 or 100 ppm. In some instances it may even be lower than 0.1 ppm or even greater than 100 ppm.

[0075] The sorption may simply involve combining the sorbent, optionally of beads of encapsulated sorbent, with the rubidium solution. It may comprise agitating the sorbent, optionally of beads of encapsulated sorbent, in the rubidium solution, e.g. by stirring, swirling, shaking etc. It may comprise passing the rubidium solution through a bed of the sorbent, optionally of beads of encapsulated sorbent. The sorption may be conveniently conducted at ambient temperature or at some other suitable temperature. It may be conducted between about 15 and about 30°C, or between about 15 and 25, 15 and 20, 20 and 30, 25 and 30 or 20 and 25°C, e.g. at about 15, 20, 25 or 30°C. The rubidium solution may be neutral to mildly alkaline. It may have a pH of about 7 to about 8.5, or about 7 to 8, 7 to 7.5, 7.5 to 8.5, 8 to 8.5 or 7.5 to 8, e.g. about 7, 7.5, 8 or 8.5. It may be adjusted to a suitable pH prior to sorption of rubidium therefrom.

[0076] Following the sorption, the sorbent may be separated from the solution from which the rubidium has been stripped. The separation may be achieved by any suitable method, for example by simple sedimentation and decanting or by centrifugation. If the sorption was conducted using a bed of the sorbent, the separation may simply comprise allowing the stripped solution to drain from the bed or it may comprise washing the bed with a liquid having no rubidium therein or it may comprise applying a pressure of air or liquid to the bed. It may comprise a combination of any of these methods.

[0077] The sorbent having sorbed rubidium thereon and / or thereon may then be stripped so as to separate the sorbent from the rubidium. Whereas this may be achieved using a potassium solution, separation of rubidium from the resulting rubidium / potassium solution canbe problematic. The inventors have found that the separation may be conveniently conducted using an ammonium solution, resulting in a solution containing rubidium and ammonium. The ammonium solution may be any suitable soluble ammonium solution, e.g. an ammonium halide, conveniently ammonium chloride. The counterion of the ammonium solution should be such that its rubidium salt is also soluble.

[0078] Ammonium ions may be removed from the resulting solution by converting the ammonium ions to ammonia, followed by sparging to remove the ammonia from the solution. Conversion of ammonium to ammonia may be achieved by adding a base to the solution. However this method has a disadvantage of adding further ions to the solution which then need to be separated from the rubidium.

[0079] The inventors have found that a superior method is to electrolyse the solution. In a suitable method, the solution of rubidium and ammonium is loaded into the cathode half-cell and a salt, e.g. sodium sulfate, is introduced into the anode half-cell, the two half cells being separated by an ultrafiltration or dialysis membrane. Application of a potential across the two electrodes causes a rise in the pH of the rubidium solution and a drop in the pH of the solution in the anode half-cell. A suitable current density is from about 5 to about 15 mA / cm2, or about 5 to 10, 10 to 15, 7 to 12, 7 to 10 or 10 to 12 mA / cm2, e.g. about 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14 or 15 mA / cm2. The voltage may vary over time. It may be between about 3 and about 6V, or between about 3 and 5, 3 and 4, 4 and 6, 5 and 6 or 4 and 5V, e.g. about 3, 3.5, 4, 4.5, 5, 5.5 or 6V. The electrolysis may be conducted for sufficient time to reach a pH in the rubidium solution of at least about 10, or at least about 10.5, 1 1 , 1 1.5 or 12, or to reach a pH in the rubidium solution of about 10, 10.5, 11 , 11.5, 12, 12.1 , 12.2, 12.3, 12.4 or 12.5. The time may be from about 1 to about 5 hours, or about 1 to 3, 1 to 2, 2 to 5, 2 to 3 or 2 to 4 hours, e.g. about 1 , 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5 hours.

[0080] As will be well understood, at high pH, the majority of ammonium ions will be converted to ammonia. Due to the fact that an equilibrium exists between ammonium and ammonia (weighted towards ammonia at high pH), removal of ammonia disturbs the equilibrium, causing more ammonium to convert to ammonia. Thus continual removal of ammonia from the solution leads to removal of both ammonia and ammonium from the solution. Removal of ammonia can be promoted by simple evaporation from the solution. However due to the high solubility of ammonia in water, it is preferable to accelerate the removal. This may be achieved by bubbling a gas, for example air, through the solution and / or by applying a partial vacuum to the solution.

[0081] Following the removal of ammonia, a purified rubidium solution is obtained.

[0082] The used sorbent may conveniently be regenerated for reuse. This may be achieved by exposing the used sorbent to a regenerating solution. A suitable regenerating solution may be a potassium solution, e.g. potassium chloride. The regenerating solution may have a concentration of about 0.1 to about 1 M, or about 0.1 to 0.5, 0.1 to 0.2, 0.2 to 0.5, 0.5 to 1 , 0.5 to 0.8 or 0.4 to 0.6M, e.g. about 0.1 , 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1 M. The exposing may comprise soaking the used sorbent in the regenerating solution. It may comprise agitating the used sorbent in the regenerating solution, e.g. shaking, swirling or stirring. It may comprise passing the regenerating solution through a bed of the used sorbent. The exposing may be conducted at ambient temperature or any other suitable temperature. It may be conducted for example at between about 15 and about 30°C, or between about 15 and 25, 15 and 20, 20 and 30, 25 and 30 or 20 and 25°C, e.g. at about 15, 20, 25 or 30°C. It may be conducted for sufficient time to effectively regenerate the sorbent for rubidium sorption. It may be conducted for between about 1 and about 5 hours, or about 1 and 3, 3 and 5 or 2 and 4 hours, e.g. about 1 , 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5 hours. The regeneration may restore at least about 70% of the rubidium sorption capacity, or at least about 75, 80, 85 or 90% thereof. It will be understood that repeated regeneration can lead to gradual deterioration of sorption capacity, however each regeneration cycle may be as described above and may achieve the degree of regeneration set out above.

[0083] The rubidium solution obtained by the method described earlier may have a purity of at least about 90%, or at least about 95, 99, 99.5 or 99.9%. In this context, purity refers to the amount of rubidium in the solution as a molar percentage of all cations in solution.

[0084] The method of the present invention may remove, or may be capable of removing, at least about 80% of the rubidium in a solution, or at least about 85, 90, 95 or 99% thereof, e.g. about 80, 85, 90, 95, 96, 97, 98, 99, 99.5 or 99.9 thereof. It may be capable of removing the rubidium from a solution in which the rubidium is a minor portion of the total dissolved matter. The rubidium may represent less than 10% by weight of the total dissolved matter, or less than about 5, 2, 1 , 0.5, 0.1 , 0.05, 0.01 , 0.005 or 0.001 % thereof.Examples

[0085] The present disclosure is further described below by reference to the following nonlimiting examples.Example 1Material and methodsSynthesis of ZIP nanomaterial

[0086] The ZIF nanomaterial was prepared following the procedure outlined in D. Quyet Truong, Y. Choo, N. Akther, S. Roobavannan, A. Norouzi, V. Gupta, M. Blumenstein, T. Vinh Nguyen, G. Naidu, Selective rubidium recovery from seawater with metal-organic framework incorporated potassium cobalt hexacyanoferrate nanomaterial, Chemical Engineering Journal 454 (2023) 140107. .-..140107. Initially, 250 mLof 1.2 M 2-methylimidazole (HMIM) and 100 mL of 0.5 M zinc nitrate hexahydrate (Zn(NO3)2-6H2O) were mixed in a glass beaker and agitated with a magnetic stirrer for 1 hour. Subsequently, the mixture underwent centrifugation and was washed once with deionized (DI) water, followed by two washes with ethanol to remove any residual organic ligands. Finally, the resulting white powder was collected after drying at 85°C overnight in a convection oven.In situ synthesis of ZIF incorporated KCoFC

[0087] A neat potassium cobalt hexacyanoferrate adsorbent was prepared following the method reported in Quyet Truong et al. (see above). To prepare a KCoFC@ZIF composite adsorbent, ZIF material was integrated in situ during the KCoFC synthesis (Figure 1 ). Briefly, 23 mL of 0.5 M K4Fe(CN)6'3H2O solution was added to a glass beaker, followed by the addition of 48 mL of 0.3 M Co(N03)2-6H2Q solution. The mixture was stirred on a magnetic stirrer at 400 rpm for 5 minutes. Subsequently, 1.0 g of pre-synthesized dry ZIF nanoparticles was added to the mixture and stirred for 1 hour. The blend was then centrifuged and alternately washed with deionized (DI) water and ethanol to eliminate any impurities. The solid sample was dried at 85°C overnight to obtain the final product, denoted as KCoFC@ZIF(B). To investigate the effect of ZIF doping amount on the adsorption performance of the composited nanomaterial, additional samples of KCoFC@ZIF(B) were prepared with increased ZIF masses of 2.0, 4.0 and 6.0 g. These samples are denoted as KCoFC@ZIF(B1 ), KCoFC@ZIF(B2), and KCoFC@ZIF(B3), respectively. Detailed information about the composition of these products is presented in Table 1 .

[0088] The inventors also prepared encapsulated KCoFC@ZIF by incorporating the presynthesized KCoFC material during the ZIF synthesis. To enable direct comparison between the two synthesis methods, the composite materials denoted as KCoFC@ZIF(A) and KCoFC@ZIF(A1 ), were prepared with similar molar ratios. Table 1 summarizes the materials used in this study.Table 1. Summary of the nanomaterials used in this studyCompositionMaterial Precursors Synthesis Description Mass Ratio(wt%)K4Fe(CN)6andK4Fe(CN)6:K4Fe(CN)6, Co(N03)2were 100%KCoFC Co(N03)2Co(N03)2reacted at molar KCoFC= 1.4:1 ratio of 1 :1 .44HMIM and Zn(NO3)2HMIM, HMIM: Zn(NO3)2=ZIF were reacted at the 100% ZIF Zn(NO3)22.6:1 molar ratio of 6:1ZIF precursorsKCoFC,KCoFC@ZIF(A) Zn(NO3)2, were added to the Zn(NO3)2= 1 :7:2.7HMIMsynthesized KCoFCSynthesized ZIF wasK4Fe(CN)6, added into the 8% KCoFC,KCoFC: Z|FKCoFC@ZIF(A1 ) CO(N03)2, solution during the 92% ZIF 1 :1 1 -5 ZIF synthesis of KCoFCSynthesized ZIF wasK4Fe(CN)6, added into the 80% KCoFC,KCoFC@ZIF(B) CO(N03)2, KCoFC: ZIF = 4:1 solution during the 20% ZIFZIF synthesis of KCoFCSynthesized ZIF wasK4Fe(CN)6, added into the 67% KCoFC;KCoFC@ZIF(B1 ) CO(N03)2, KCoFC: ZIF = 2:1 solution during the 33% ZIF ZIF synthesis of KCoFCSynthesized ZIF wasK4Fe(CN)6, added into the 50% KCoFC;KCoFC@ZIF(B2) Co(N03)2, KCoFC: ZIF = 1 :1 solution during the 50% ZIF ZIF synthesis of KCoFCSynthesized ZIF wasK4Fe(CN)6, added into the 40% KCoFC;KCoFC@ZIF(B3) Co(N03)2, KCoFC: ZIF = 2:3 solution during the 60% ZIF ZIF synthesis of KCoFCCharacterization

[0089] A scanning electron microscope (SEM, Zeiss Supra 55VP) was utilized to examine the surface morphology of the adsorbents. The SEM was operated at an accelerating voltage of 10 kV and a working distance of 5 mm. Prior to SEM characterization, the powder samples were dried in a vacuum oven for 12 hours followed by sputter coating with a 5 nm thickness of iridium. Elemental mapping of the surface was performed using Energy-dispersive X-ray spectroscopy (EDS) with an X-ray detector (Oxford). Additionally, the crystalline structures of the nanomaterials were analysed using an X-ray diffractometer (XRD, Bruker D8 Discover) with Cu Ka radiation (20 = 5 - 50°) at a step size of 0.04°.

[0090] X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo Fisher) was implemented to determine the chemical composition of the synthesized materials comprehensively. Furthermore, the chemical bonds and functional groups of the nanomaterials were analyzed using a Fourier transform infrared instrument (FTIR, MIRacle 10, Shimadzu) in the wavenumber range between 4000 and 400 cm1. The BET specific area, pore volume, and pore diameter of the synthesized nanomaterial were analyzed based on the adsorption / desorption isotherm data determined by N2surface area analyzer (ASAP 2420, Micromeritics) at 77 K.Batch performance investigation of nanomaterials for Rb uptake

[0091] To ensure the accuracy and reliability of the experimental data, all tests were conducted following a consistent procedure. Initially, a specific amount of the material was placed in a series of conical flasks. Next, 100 mL of either synthetic solution (with an initial Rb concentration of 5 mg / L) or seawater sample was added to each flask. The mixtures were then agitated in a thermal shaker at 120 rpm and a temperature of 25 ± 1 °C for 24 h. Afterward, the solid phase was separated using a 0.45 pm filter, and the liquid phase was collected. Residual ion concentrations were analyzed using inductively coupled plasma mass spectrometry (ICP- MS, Agilent Technologies, Inc.). To ensure consistency, all tests were triplicated.Influence of solution pH

[0092] The dependence of Rb uptake performance on the solution pH was investigated in a wide pH range from 2 to 12. Initially, the pH of the solution was adjusted using two standard solutions: 0.1 M NaOH and 0.1 M HCL Next, a nanomaterial dosage of 0.5 g / L was loaded into each flask to conduct the adsorption process as described in the previous section. The remaining Rb concentration was measured by ICP-MS.Adsorption kinetics

[0093] The adsorption kinetics study was implemented with a 5 mg / L Rb+model solution loaded with 5 mg composite adsorbents mixed with the solution. The adsorption process was then carried out over 32 hours, and aliquots of solution were periodically collected for Rb+elemental analysis using ICP-MS. Eq. (1 ) was used to determine the amount of Rb+that adsorbed from the bulk aqueous solution to the adsorbent phase at time t:where qt (mg / g) represents the Rb+uptake capacity of the material, Co(mg / L) and Ct(mg / L) represent the Rb+concentration initially and at time t, respectively, V (L) is the volume of the model solution (0.1 L), and m (g) is the mass of the nanomaterial.

[0094] The collected data was then fitted to two empirical kinetics models: the pseudo-first- order (PFO) model and the pseudo-second-order (PSO) model. The mathematical equations of these models are presented below:PFO model: qt= qe(l - e~klt) (2)PSO model:

[0095] where qe(mg / g) is the Rb+uptake capacity at equilibrium, t (h) is the adsorption time, ki (1 / h) and k2(g / (mg-h)) are the PFO and PSO kinetics constant, respectively.Adsorption isotherm

[0096] The adsorption isotherm was investigated with the adsorbent loadings ranging from 0.01 to 0.2 g / L, while the initial Rb+concentration of the solution was fixed at 5 mg / L. The samples were shaken for 24 hours to ensure the equilibrium was reached. After that, the supernatant solutions were collected and filtered to detect Rb+ions. The isothermal equilibriumof the nanomaterial for Rb+sorption can be described using the Langmuir and Freundlich isotherms. The nonlinear expression of the Langmuir isotherm is demonstrated in Eq. (4):=QmaxkLCe(4)l+kLCeV’The corresponding equation of the Freundlich model is given by Eq. (5):where qmax (mg / g) is the saturated maximum Rb+adsorption amount, kL(L / mg) and kF((mg / g) / (mg / L)1 / n) are the Langmuir and Freundlich constants, respectively, Ceis the equilibrium concentration of the Rb+, and 1 / n is the Freundlich adsorption intensity of the nanomaterial.Selective Rb+uptake from seawater

[0097] To evaluate the Rb+uptake efficacy of the composite adsorbents in the presence of other competitive cations, an adsorption test was conducted using real seawater. The seawater sample, collected from Sydney Institute of Marine Science (Chowder Bay, New South Wales, Australia), was spiked with 5 mg / L of Rb+to allow for a straightforward comparison with previous experiments using a synthetic Rb+solution and to ensure reliable data from the ICP-MS characterization. The components of the seawater sample (after spiking) are presented in Table 2.Table 2. Composition of seawaterSpeciesNa+(mg / L) 12006.3Mg2+(mg / L) 1399.3K+ (mg / L) 450.9Ca2+(mg / L) 428.6Rb+ (mg / L) 5.4Total dissolved solids (mg / L) 41900.0 pH 7.54Regeneration and reusability of the material

[0098] The stability and reusability of the adsorbents were evaluated over five adsorption- regeneration-reactivation cycles. Initially, the Rb+ sorption test was performed using a 5 mg / L Rb+ model solution with an adsorbent loading of 0.1 g / L. Subsequently, the saturated material was treated with a 0.05 M NH4CI solution to facilitate ion exchange between Rb+sorbed in the composite adsorbent and NH4+in the bulk solution, regenerating the composite adsorbents. The NH4+ / Rb+mixture was preserved for Rb+separation studies described in the following section. The material was reactivated using 0.5 M KCI and was reused for the next adsorption cycle.Rubidium separation from the regenerated NH4+ / Rb+effluent using electrolysis and air stripping method

[0099] After regeneration, the mixed solution containing NH4+and Rb+ions underwent a hybrid separation process to obtain a high-purity Rb+solution without the presence of NH4+. Initially, electrolysis was conducted using a TiC>2 electrode as the cathode and an lrC>2 electrode as the anode. A 500 mL volume of the regeneration solution (feed solution) was introduced into the cathode cell, while an equal volume of 0.1 M Na2SO4served as the anolyte solution placed in the anode cell. An ultrafiltration (UF) membrane with a contact area of 100 cm2separated the two solutions. The electrolysis system operated under a constant current density of 8 mA / cm2for 2.5 hours, achieving a feed solution pH of approximately 12.3, conducive to converting NH4+to NH3gas. Subsequently, the feed solution was transferred to a 1000 mL beaker for a 24-hour air stripping process to remove NH3gas molecules from the aqueous phase. This process resulted in a final solution containing high-purity Rb+ions. Figure 2 schematic diagram summarizes the hybrid separation process employed in this study.Results and discussionProduct yield

[0100] The product yield was determined by weighing the final dry crystalline product obtained from the reaction and comparing it to the stoichiometric yield calculated from the total reactant used. Table 3 summarizes the product yields (%) of the synthesized adsorbents. Notably, the ex situ synthesized ZIF-decorated KCoFC composite, KCoFC@ZIF (A), exhibited a product yield of approximately 50%. In contrast, the in situ synthesized composite adsorbents, i.e., KCoFC (A1 ) and KCoFC@ZIF (B-B3), demonstrated significantly higher product yields, exceeding 97 % and approaching the theoretical stoichiometric yield.

[0101] Generally, the product yield in solvothermal synthesis of MOF composites significantly relies on various factors, including the concentration of reactants, reaction time, and solution pH. While specific data on the yield of KCoFC under different synthesis conditions is scarce, similar crystalline adsorbents, i.e., potassium manganese hexacyanoferrate, has shown a product yield of 74%.

[0102] In this study, the addition of pre-synthesized ZIF consistently doubled the product yield. Previous studies have reported that ZIF-composited microporous materials, such as zeolite@MOF, achieve higher yields compared to their parent materials, i.e., neat zeolite or neat MOFs. Although a systematic study is lacking, the inventors speculate that the highly porous nature of pre-synthesized ZIF particles enhanced the heterogeneous nucleation of KCoFC, thereby increasing the product yield. In contrast, when the pre-synthesized KCoFC was added to the ZIF precursors (ex situ synthesized composite, denoted as KCoFC@ZIF (A)), the surface morphology and surface functionality did not improve the catalytic activity for the nucleation of ZIF particles.Table 3. Product yield (%) of the synthesized nanomaterials.Material Product yield (%)KCoFC@ZIF (A) 50.56KCoFC@ZIF (A1 ) 97.48KCoFC@ZIF (B) 97.78KCoFC@ZIF (B1 ) 99.20KCoFC@ZIF (B2) 99.08KCoFC@ZIF (B3) 99.12Rb+uptake performance of the composite adsorbents

[0103] The Rb+sorption performance of various KCoFC / ZIF composite adsorbents was investigated. To compare the Rb+sorption performance directly, a preliminary test was conducted using these nanomaterials contacted with a 5 mg / L Rb+model solution, and the uptake performance is shown in Figure 4. The neat KCoFC exhibited a modest Rb+uptake performance, 77%, while the Rb+uptake performance of ZIF appears to be insignificant. The ZIF-decorated composite adsorbent, KCoFC@ZIF(A), demonstrated slightly superior Rb+uptake performance than that of neat KCoFC, achieving approximately 86%. Surprisingly, when the identical precursor concentration was used for in situ synthesis protocol (KCoFC@ZIF(A1 )), the Rb+uptake performance was negligible.

[0104] To better understand the effect of parental material composition on adsorption performance, the Rb+uptake for in situ synthesized KCoFC@ZIF was investigated. Among the series of in situ synthesized composites, denoted as KCoFC@ZIF(B-B3), the highest Rb+uptake was observed when a KCoFC to ZIF ratio of 4:1 was used (KCoFC@ZIF(B)), showing a 97.44% Rb+uptake rate. As the amount of added ZIF increased, the Rb+uptake performance sharply decreased, falling below that of neat KCoFC. When the amount of ZIF exceeded that of KCoFC (KCoFC@ZIF(B3)), the Rb+uptake reduced to approximately 40%.

[0105] Based on the empirical Rb+uptake data of the parent materials, i.e., KCoFC and ZIF, and the mass ratio of the composite adsorbents, the expected Rb+uptake rates were calculated as shown in Table 4. Notably, all of the in situ synthesized composite series investigated in this study showed superior Rb+uptake performance compared to the calculated Rb+uptake performance. This indicates that compositing KCoFC with ZIF using the in situ synthesis method effectively enhances the Rb+uptake capability of the adsorbents. Furthermore, KCoFC@ZIF(B) displays 35% enhancement in uptake, whereas in the KCoFC@ZIF(B3) case, the enhancement was only 5%. This result suggests that compositing KCoFC with ZIF is more effective when the amount of ZIF is low. Indeed, the composite adsorbent with the highest ZIF loading, KCoFC@ZIF(A1 ), where the KCoFC to ZIF ratio is 1 :1 1 .5, presents the lowest Rb+ uptake (4%), similar to that of the neat ZIF material. Hence, the further investigation was carried out with neat KCoFC and KCoFC@ZIF(B).Table 4. Rb+uptake performance of the in situ synthesized composite adsorbents and the calculated Rb+uptakeMaterial Rb+ uptake (%) Calculated Rb+ uptake (%)KCoFC@ZIF (B) 97.44 62.69KCoFC@ZIF (B1 ) 65.94 53.49KCoFC@ZIF (B2) 45.15 41.46KCoFC@ZIF (B3) 39.94 34.38Rb adsorption performance evaluationEffect of solution pH

[0106] Rb uptake performance was investigated across various initial pH values (Figure 4). For both neat KCoFC and KCoFC@ZIF(B), only marginal changes in Rb+uptake rates were observed as the solution initial pH increased from 2 to 10. This indicates that the solution pH does not significantly influence Rb+uptake performance. The average Rb+uptake of theKCoFC@ZIF(B) was 97%, compared to 77% for neat KCoFC. Additionally, KCoFC@ZIF(B) consistently demonstrated higher Rb+uptake than neat KCoFC across the studied pH range. The stable Rb+uptake window ranging from pH 2 to 10 also corroborates the chemical stability of the KCoFC@ZIF(B), regardless of the acidity or alkalinity of the environment.

[0107] At pH 12, a dramatic decrease in Rb+uptake was observed for both neat KCoFC and KCoFC@ZIF(B) to 42% and 29%, respectively. Prussian blue analogues (PBA), which typically have chemical compositions of metal hexacyanoferrate, exhibit good adsorption performance over a broad pH range. However, at high pH condition, the adsorption performance significantly drops due to the potential structural degradation of the PBAs, affecting their long-term stability and adsorption capacity. KCoFC, one of the PBA derivatives, shows reduced Rb+uptake at pH 12, similar to other PBAs. Intriguingly, KCoFC@ZIF(B) demonstrated similar pH-dependent Rb+uptake performance. This coupling of pH dependence indicates that the Rb uptake mechanism is predominantly governed by KCoFC domain in the composite adsorbent, and the contribution of the ZIF content in the composite is rather insignificant. Since the adsorption performance was independent of pH values between 2 and 10, an initial pH of 7.0 was selected for all further experiments.Kinetics study

[0108] To better understand the Rb+adsorption kinetics in neat KCoFC and KCoFC@ZIF(B) adsorbents, the time-dependent Rb+ uptake rate was measured and the results are shown in Figure 5. The Rb+uptake rate with the neat KCoFC adsorbent demonstrated a gradual increase over 32 hours. In contrast, the KCoFC@ZIF(B) displayed a rapid increase of in Rb+uptake rate within 5 hours and then equilibrated after 8 hours. These results indicate a significant difference in adsorption kinetics between KCoFC@ZIF(B) and neat KCoFC. It is thought that the highly nanoporous ZIF contents and their high surface area likely played a pivotal role in enhancing the adsorption kinetics.

[0109] The experimental data for Rb+uptake with time were analyzed using the pseudo-first order (RFC) and pseudo-second order (PSO) models (Figure 6, Table 5). The kinetic model fitting revealed that both neat KCoFC and KCoFC@ZIF(B) fit well to the PSO model, demonstrating R2values of 0.99, respectively. This observation implies that the adsorption modes for neat KCoFC and KCoFC@ZIF(B) follow chemisorption rather than diffusion-limited, which corroborates ion-exchange as the primary ion adsorption mechanism. Furthermore, this is in line with the observation that both neat KCoFC and KCoFC@ZIF(B) show a similar adsorption mechanism, confirming that the adsorption is predominantly dictated by the KCoFC phase rather than ZIF.[001 10] From the analysis results presented in Table 5, it was confirmed that the adsorption kinetic coefficient, k2, was 0.00032 for neat KCoFC, while that of KCoFC@ZIF(B) was 0.03983, two orders of magnitude higher. Along with the short equilibrium time, this significant difference in k2reveals that the in situ synthesized KCoFC@ZIF composite adsorbent has substantially superior adsorption kinetics.Table 5. Comparison between the modified KCoFC@ZIF(B) and neat KCoFC regarding parameters and correlation coefficient (R2) values of two applied kinetic modelsModel parameters Neat KCoFC KCoFC@ZIF (B)Pseudo-1stmodel parameters qe1(mg / g) 97.40 1 11 .92 k1 (1 / h) 0.06 2.88R2 0.99 0.95Pseudo-2ndmodel parameters qe2(mg / g) 143.01 1 17.21 k2(g / (mg.h)) 0.00032 0.03983R20.99 0.99Isotherm study[001 11 ] The ion sorption isotherm was investigated by varying adsorbent loadings from 0.01 to 0.2 g / L, while the initial Rb+concentration of the solution was fixed at 5 mg / L. The Rb+uptake capacity at equilibrium (qe) was measured, and the results are presented in Figure 7 and Table 6. The maximum Rb+uptake capacity for neat KCoFC was 137.83 mg / g, whereas that of KCoFC@ZIF(B) was 279.04 mg / g, effectively doubling the maximum Rb+uptake.[001 12] The sorption isotherm curves were fitted with both Langmuir and Freundlich models as shown in Figure 7. The correlation coefficients (R2) for both models exceeded 0.9 for both neat KCoFC and KCoFC@ZIF(B). However, the Langmuir model provided a better fit, with R2values of 0.95 and 0.97 for neat KCoFC and KCoFC@ZIF(B), respectively. This suggests that Rb+sorption mechanisms in neat KCoFC and KCoFC@ZIF(B) are similar. Moreover, the higher fidelity to the Langmuir model indicates that adsorption occurs via a single-layer deposition mechanism with homogeneous active sorption sites on the surface, corroborating the ion-exchange mechanism.Table 6. Comparison between neat KCoFC and KCoFC@ZIF (B) nanomaterials regarding parameters and correlation coefficient (R2) values of two applied isotherm models.Model Parameters Neat KCoFC KCoFC@ZIF(B)LangmuirKL(L / mg) 1 .80 7.31<7max (mg / g) 137.83 279.04R20.95 0.97FreundlichKf[(mg / g)(L / mg)1 / n] 79.36 214.55 n 2.59 3.84R20.90 0.96Selective rubidium separation from seawater solution[001 13] For practical utilization of Rb+recovery, achieving high Rb+selectivity in complex environments with competing ions present at high concentration is crucial. To evaluate this, seawater was used as the influent solution, and neat KCoFC, KCoFC@ZIF(A), and KCoFC@ZIF(B) were employed as adsorbents (Figure 8). The results indicated that the uptake rates of other competing cations, e.g., Na+, Mg2+, K+, and Ca2+, were insignificant, all below 5%. In contrast, the Rb+uptake rate exceeded 60% for all three adsorbents, demonstrating strong Rb+selectivity of the KCoFC and composite adsorbents over other cations. Notably, the K+uptake rate was significantly lower than that of Rb+, despite their similar ionic radii (K+: 1.38 A, Rb+: 1.64 A). The negative K+uptake rate observed for KCoFC@ZIF(B) is ascribed to the ion exchange that occurred during Rb+adsorption. Hence, the lack to K+release in KCoFC and KCoFC@ZIF(A) indirectly indicates that the ion exchange between K+and Rb+at the liquid-solid interface was not very active. Interestingly, KCoFC@ZIF(A) exhibited a slightly improved Rb+uptake rate compared to the neat KCoFC, while KCoFC@ZIF(B) achieved the highest Rb+uptake rate, exceeding 90%. Considering the high concentration of competing ions and the theoretical adsorption capacity of KCoFC imposed by K+occupied sites in its internal structure, the sorption of other competing ions is quenched by the saturation of the vacant sites in the adsorbent. However, the preferred adsorption of Rb+enabled to achieve high selectivity of the composite adsorbent. The results promise practical utilization of the in situ synthesized KCoFC@ZIF for Rb+recovery even from concentrated solutions.Regeneration and reusability of the material[001 14] Another critical feature expected for a practical adsorbent is regeneration and reusability. As mentioned above, the rubidium adsorption experiment was conducted with an adsorbent dosage of 0.1 g / L using an Rb+synthetic solution (5 mg / L). For regeneration, after adsorption, the adsorbent was collected and then soaked in a 0.05 M NH4CI solution to liberate the Rb+ions from solid into the liquid phase. Considering the ionic radius of 1.61 A, comparable to that of Rb+, NH4+ions can facilitate the liberation of Rb+effectively. The mixed fluid consisting of NH4+and Rb+was kept for the subsequent separation study. The used adsorbent was regenerated using a 0.5 M KCI solution and could be applicable for further adsorption cycles. Figure 9 shows the regeneration performance and adsorption uptake performance with the regeneration cycles. The Rb+uptake performance showed a gradual decrease from 95% to 80%, presumably due to incomplete regeneration and the reduction of vacant sites caused by the occupancy of other competing ions. Even after five consecutive cycles, the Rb+uptake performance remained above 80%, which is significantly higher compared to neat KCoFC, validating the applicability of the in situ synthesized KCoFC@ZIF composite adsorbent.Material analysis and selective mechanism propertiesSurface morphology[001 15] The Rb+adsorption studies revealed that the Rb+uptake performance and Rb+selectivity of the ZIF-composited KCoFC adsorbents are coupled with those of neat KCoFC. At the same time, while the Rb+uptake performance is governed by KCoFC contents, the inventors observed a significant improvement in adsorption kinetics, Rb+selectivity, and uptake capacity when it is in situ synthesized in the presence of ZIF particles. This suggests that the composite adsorbent material underwent structural changes influenced by the ZIF particles. To better understand its microscopic structure and its impact on sorption properties, the inventors conducted material characterizations, as detailed below.[001 16] The surface morphology of KCoFC@ZIF(B) particles was investigated with scanning electron microscopy (SEM), as shown in Figure 10. A rough surface morphology was observed, with globular particles aggregated on the surface. Energy-dispersive X-ray spectroscopy (EDX) data depicted in Figure 11 show that potassium, cobalt, and iron are uniformly distributed on the particle surface, indicating the homogeneous coverage of KCoFC material on the surface. On the contrary, zinc, the characteristic element of ZIF, was uniformly dispersed throughout the KCoFC@ZIF(B) particle, with some scattered high-intensity spots.This suggests that Zn was incorporated in the composite structure or present in the inner shell of the KCoFC, while unreacted ZIF particles were detected as high-intensity spots.X-ray diffraction[001 17] To determine the change of crystalline structure after in situ synthesis, powder X-ray diffraction (XRD) was performed on KCoFC@ZIF(B) (Figure 12). Strong diffraction peaks were observed at 20 = 17.4°, 24.8°, 35.5°, 39.8°, and 43.8°, corresponding closely to the (2 0 0), (2 2 0), (4 0 0), (2 2 2), and (4 2 2) crystal planes of neat KCoFC. While the close colocation of the characteristic peak positions between neat KCoFC and KCoFC@ZIF(B) suggests the crystal structure remained largely unaffected during the in situ synthesis, an apparent peak broadening in KCoFC@ZIF(B) was observed. Additionally, compared to neat KCoFC, where the (2 0 0) reflection typically appears at 17.7°, the peaks in KCoFC@ZIF(B), were shifted to lower angle by approximately 0.3°. These slight changes in peak positions and peak sharpness indicate marginal disordering of the KCoFC crystal structure with slight lattice expansion. Weak intensity peaks were located at the diffraction angles corresponding to ZIF- L (red dashed lines). Compared to the ex situ synthesized KCoFC@ZIF, where ZIF particles decorate the exterior surface of the KCoFC, the diffraction intensities of the ZIF phase appear significantly lower in KCoFC@ZIF(B). The lattice expansion, along with the reduced ZIF peak intensity, suggests that the zinc ions present in the ZIF particle may have introduced into the KCoFC phase, intercalating into the interstitial sites and possibly replacing K+ions. To further support this hypothesis, spectroscopic studies were conducted.Infrared Spectroscopy[001 18] FTIR spectra are shown in Figure 13.Rubidium separation from the regenerated NH4+ / Rb+effluent using electrolysis and air stripping methodTable 7. Changes in NH4+and Rb+concentrations during the electrolysis and air stripping processesNH4+removalTime (h) Description pH NH4+(mg / L) Rb+(mg / L)(%)0 NH4+ / Rb+effluent 6 0 1026.0 - 4502.5 After electrolysis 12.32 784.3 23.6 4.874 After air-stripping 12.30 56.6 94.5 5.68[001 19] After the regeneration, a significant amount of ammonium ion (NH4+) mixed with Rb+was present due to the excess NH4+used in the regeneration of KCoFC@ZIF(B). To separate Rb+and increase the purity of the Rb+, the inventors employed electrolysis followed by air stripping process (Table 7). The initial feed solution (effluent) contained 1026.0 mg / L of NH4+and 4.90 mg / L of Rb+. Following electrolysis, the pH increased from 6.50 to 12.32 due to the generation of hydroxide ions at the cathode. The increase of pH above 1 1 converted ammonium ions into ammonia, which was liberated as ammonia gas, resulting in a 23.6% removal of NH4+from the solution. After electrolysis, an air stripping process was conducted while maintaining the pH at 12.3. This process enabled the release of the remaining ammonium ions, thereby further reducing the NH4+concentration by 94.5%. During the electrolysis and air stripping processes, the concentration of Rb+remained unchanged (~ 5%).Conclusion

[0120] In this study, an in situ synthesized Rb+ion-exchange adsorbent, KCoFC@ZIF, was investigated for selective Rb+recovery from complex solutions like seawater. The in situ synthesis method increased product yield twofold compared to the ex situ synthesis method. Batch performance tests revealed superior selectivity for Rb+, while adsorption kinetics and adsorption isotherm studies confirmed that the composite adsorbent follows an ion-exchange adsorption pathway. Although KCoFC remained the dominant phase for Rb+adsorption within the composite, enhanced adsorption kinetics and capacity were observed. This improvement is thought to be due to zinc ion intercalation into the KCoFC lattice, replacing the interstitial K+ions and expanding the cell structure.

[0121] NH4+regeneration followed by membrane electrolysis and air stripping demonstrated that high purity Rb+could be recovered without chemical treatment. Desorption and regeneration studies showed that the adsorbent could be effectively reused for up to five cycles without significantly compromising its performance, offering a scalable and economically viable solution.

[0122] Those skilled in the art will appreciate that the disclosure described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the disclosure includes all such variations and modifications. The disclosure also includes all of the steps, features, compositions and compounds referred to or indicatedin this specification, individually or collectively, and any and all combinations of an two or more of said steps, features, compositions and compounds.

Claims

Claims:1 . A sorbent comprising nanoparticles, each of said nanoparticles comprising:• a metal-organic framework (MOF) and,• an alkali metal cobalt hexacyanoferrate, wherein the mass of the alkali metal cobalt hexacyanoferrate is greater than 10% of the total mass of the nanoparticle.

2. The sorbent of claim 1 wherein the mass of the alkali metal cobalt hexacyanoferrate is greater than 50% of the total mass of the nanoparticle.

3. The sorbent of claim 1 or claim 2 wherein the nanoparticles are core-shell nanoparticles.

4. The sorbent of claim 3 wherein the alkali metal cobalt hexacyanoferrate forms a shell surrounding a core of the MOF.

5. The sorbent of any one of claims 1 to 4 wherein the MOF is a zeolitic imidazolate framework (ZIF).

6. The sorbent of any one of claims 1 to 5 wherein the MOF is made by reacting 2- methylimidazole with a zinc salt.

7. The sorbent of any one of claims 1 to 6 wherein the alkali metal cobalt hexacyanoferrate is potassium cobalt hexacyanoferrate.

8. The sorbent of any one of claims 1 to 7 which is encapsulated in an encapsulant.

9. The sorbent of claim 8 wherein the encapsulant containing the sorbent is in the form of beads.

10. The sorbent of claim 9 wherein the beads have mean diameter of from about 0.5 to about 3mm in diameter.11 . A process for making a sorbent comprising: combining a solution of an alkali metal hexacyanoferrate with a solution of a cobalt salt; and• adding a nanoparticulate MOF to the resulting solution to produce the sorbent.

12. The process of claim 1 1 wherein the MOF is a ZIF.

13. The process of claim 11 or claim 12 wherein the alkali metal hexacyanoferrate is potassium hexacyanoferrate14. The process of any one of claims 11 to 13 comprising the step of reacting 2- methylimidazole with a zinc solution so as to form the nanoparticulate MOF prior to the step of adding the nanoparticulate MOF.

15. The process of any one of claims 11 to 14 additionally comprising:• separating the sorbent from the solution;• washing the sorbent; and• drying the sorbent.

16. The process of any one of claims 11 to 15 comprising encapsulating the sorbent in an encapsulant.

17. The process of claim 16 wherein the encapsulant is a polymeric encapsulant.

18. The process of claim 16 or claim 17 wherein the step of encapsulating comprises extruding a mixture of the encapsulant and the sorbent so as to produce beads of diameter between about 0.5 to about 3mm in diameter, said beads comprising the sorbent encapsulated within the encapsulant.

19. A method for separating rubidium from a solution thereof comprising exposing said solution to a sorbent according to any one of claims 1 to 10 or to a sorbent made by the process of any one of claims 11 to 18.

20. The method of claim 19 wherein said exposing comprises agitating the solution with the sorbent.21 . The method of claim 19 or claim 20 which is conducted at between about 15 and about 30°C.

22. The method of any one of claims 19 to 21 comprising stripping the rubidium from the sorbent.

23. The method of claim 22 wherein said stripping comprises:• exposing the sorbent with rubidium thereon to a solution of ammonium ions to produce a crude product solution containing rubidium and ammonium ions and• separating the crude product solution from the sorbent.

24. The method of claim 23 comprising removing ammonium ions from the crude product solution to form a purified rubidium solution.

25. The method of claim 24 wherein the removing comprises:• converting the ammonium ions to ammonia and• passing a gas over and / or through the solution so as to remove the ammonia as ammonia gas.

26. The method of claim 25 wherein the ammonium ions are converted to ammonia by electrodialysis of the crude product solution.

27. The method of any one of claims 19 to 26 comprising regenerating the sorbent.

28. The method of claim 27 wherein the regenerating comprises exposing the sorbent to a solution of an alkali metal salt.

29. The method of claim 28 wherein the alkali metal is potassium.

30. A method for separating rubidium from a solution thereof comprising: a) exposing said solution to a sorbent comprising nanoparticles, said nanoparticles comprising potassium cobalt hexacyanoferrate shell and a MOF; b) stripping the rubidium from the sorbent using an ammonium solution so as to form a solution comprising the rubidium and ammonium; c) electrodialysing the solution obtained in step b) so as to convert ammonium in the solution to ammonia; d) air stripping the resulting solution so as to remove the ammonia therefrom so as to generate a rubidium solution containing negligible concentrations ammonium and ammonia;e) regenerating the sorbent by treatment with a solution of a potassium salt; and f) reusing the sorbent in step a).

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