104937-201 METHODS FOR RECYCLING AND SEQUESTERING FLUORIDE SALT WASTE STREAMS GOVERNMENT LICENSE RIGHTS [0001] This invention was made with government support under award number DE- AR0001615, awarded by the Department of Energy. The government has certain rights to the invention. CROSS-REFERENCE TO RELATED APPLICATION [0002] This application claims priority to U.S. Provisional Patent Application Nos. 63/560,263, filed March 1, 2024, and 63/633,264, filed April 12, 2024, the disclosures of which are hereby incorporated by reference in their entireties. TECHNICAL FIELD [0003] This disclosure relates to methods of recycling and sequestering waste products from fluoride containing molten salt compositions used in molten salt nuclear fission reactors. BACKGROUND [0004] A molten salt reactor (MSR) is a nuclear fission reactor which uses molten salt as a coolant, and in some cases as a fuel source. The overall fuel and coolant chemistry depends on different MSR designs and types. Very complex waste streams may include volatile off-gas, salt-based waste components, separated salt streams, carbon- and metal- based waste streams, and wastes from operation and decommission. The salt wastes contain a wide range of waste elements across the periodic table, potentially including alkali- and alkaline-earth halides, fission products including Cs and Sr, lanthanides, and actinides. [0005] It is also desirable to recycle 7LiF from fluoride-based MSR salts. Lithium salts are used in MSR designs because they reduce the generation of tritium. Such lithium salts are typically ≥99.99% 7Li to reduce neutron absorption. Consequently, the recovery of 7Li from molten salt waste is very important to reduce the operation cost of MSRs and limit the mining stress by reducing the quantity of new products necessary to operate. SUMMARY [0006] According to some embodiments, a method is disclosed of treating a molten 1 30805769.1
104937-201 salt reactor waste comprising a mixture of water insoluble metal fluorides and water soluble metal fluorides, the method comprising: mixing the molten salt waste with water to form a first solid enriched in the water insoluble metal fluorides, and a first solution enriched in the water soluble metal fluorides, followed by separating the first solution from the first solid. According to some embodiments, the water insoluble metal fluorides include 7LiF. According to some embodiments, the water soluble metal fluorides include NaF and KF. According to some embodiments, the water soluble metal fluorides include NaF, KF, and CsF. [0007] For some embodiments, the water insoluble metal fluorides include 90SrF2, and the method further comprises dispersing the first solid in water to form a dispersion, the dispersion having a solid phase and a liquid phase, adding SrI2 to the dispersion, applying heat to the dispersion, stirring the dispersion for a period of time, so that SrFI dissolves in the liquid phase, thereby forming a solution of SrFI and leaving the solid phase depleted in SrF2, and separating the SrF2 depleted solid phase from the solution of SrFI. [0008] According to some embodiments, the first solution is further processed to form a metal halide perovskite. According to some such embodiments, the metal halide perovskite is formed by a process selected from the group consisting of: a) evaporating the first solution to form a water soluble metal fluoride solid, followed by mixing the water soluble metal fluoride solid with SnCl4 in a polar organic solvent, thereby sequestering the metal in a perovskite comprising metal salts of the hexafluorostannate anion; b) mixing the first solution with FeCl3 to form an insoluble metal salt of the hexafluoroferrate(III) anion and releasing a metal chloride salt in the supernatant, followed by mixing H2SiF6 with the supernatant to form an insoluble metal perovskite salt of the hexafluorosilicate anion and releasing HCl; and c) mixing the first solution with H2SiF6 in the presence of SiO2 to form an insoluble metal perovskite salt of the hexafluorosilicate anion. [0009] According to other such embodiments, the metal halide perovskite is formed by a process selected from the group consisting of: a) mixing the third solid with SnCl4 in a polar organic solvent, thereby sequestering the metal in a perovskite comprising metal salts of the hexafluorostannate anion; b) dissolving the third solid in water and mixing with FeCl3 to form an insoluble metal salt of the hexafluoroferrate(III) anion and releasing a metal chloride salt in the supernatant, followed by mixing H2SiF6 with the supernatant to form an insoluble metal perovskite salt of the hexafluorosilicate anion and releasing HCl; and 2 30805769.1
104937-201 c) dissolving the third solid in water and mixing with H2SiF6 in the presence of SiO2 to form an insoluble metal perovskite salt of the hexafluorosilicate anion. [0010] According to some such embodiments, the polar organic solvent is methanol. According to some such embodiments, the polar organic solvent has dissolved therein a cosolute, the cosolute enhancing the solubility of metal fluorides in the solvent. According to some such embodiments, the cosolute is a fatty acid. According to some such embodiments, the fatty acid is oleic acid. [0011] According to some embodiments, the water soluble metal fluorides include CsF, wherein the cesium in CsF is present as a radioactive isotope, and wherein the method further comprises evaporating the first solution to form a second solid, mixing the second solid with an organic solvent to form a solution of CsF and a third solid, depleted in CsF, sequestering the Cs in the solution of CsF as a cesium halide perovskite, and processing the third solid by a method selected from the group consisting of: a) directly recycling the third solid for use in a molten salt reactor, and b) forming a metal halide perovskite compound. According to some such embodiments, the organic solvent includes methanol. According to some embodiments, the organic solvent includes acetone. According to some embodiments, the organic solvent has dissolved therein a ligand, the ligand increasing the solubility of CsF in the organic solvent. According to some such embodiments, the organic solvent is acetone and the ligand is 18-crown-6. [0012] According to some embodiments, the cesium halide perovskite is formed by mixing the solution of CsF with SnCl4, thereby sequestering the metal in a perovskite comprising metal salts of the hexafluorostannate anion. According to some embodiments, the cesium halide perovskite is formed by mixing the solution of CsF with H2SiF6 to form an insoluble cesium perovskite salt of the hexafluorosilicate anion and releasing HF. According to some embodiments, the cesium halide perovskite is formed by mixing the solution of CsF with H2SiF6 to in the presence of SiO2 to form an insoluble cesium perovskite salt of the hexafluorosilicate anion. [0013] According to some embodiments, a method is disclosed of removing and sequestering an alkali metal M from a composition comprising a fluoride salt MF of the alkali metal, wherein MF is selected from the group consisting of NaF, KF, CsF, and combinations thereof, the method comprising: mixing the composition with a solvent to form a solution comprising dissolved MF, and adding H2SiF6 and SiO2 to the solution with mixing, thereby forming a precipitate sequestering the alkali metal as M2SiF6. For some such embodiments, the solvent is water. For some embodiments, the solvent is an organic solvent and MF is CsF. 3 30805769.1
104937-201 For some such embodiments, the organic solvent is selected from the group consisting of methanol, acetone, dimethylformamide, and combinations thereof. For some such embodiments, the organic solvent is methanol. BRIEF DESCRIPTION OF THE DRAWINGS [0014] Fig. 1 provides a flow chart of a process for separating, sequestering, and recycling radioactive wastes according to an embodiment of the present disclosure. [0015] Fig. 2 provides a flow chart of a process for separating LiF from NaF and KF, according to an embodiment of the present disclosure. [0016] Fig. 3 shows x-ray diffraction (XRD) results demonstrating the separation of LiF from NaF and KF as described herein. [0017] Fig. 4 provides a flow chart for the sequestration of NaF into the metal halide perovskite (MHP) Na2SnF6 according to an embodiment of this disclosure. [0018] Fig. 5 is a flow chart showing the separation of CsF from other fluoride salts and the sequestration of CsF into a Cs perovskite structure. [0019] Fig. 6A provides XRD results for a wet sample of NaF and KF from which CsF has been separated. [0020] Fig. 6B provides XRD results for the sample of NaF and KF from which CsF has been separated following drying of the sample. [0021] Fig. 7 provides XRD results for a dried solution of CsF after extraction of the CsF from a sample containing NaF and KF. [0022] Fig. 8 shows screening results for strontium salts based on their usefulness in separating SrF2 from LiF. DETAILED DESCRIPTION Definition: [0023] A “molten salt composition” is a mixture of salts capable of forming a molten salt at a suitable temperature. Separation and sequestration methods: [0024] Some embodiments of the present technology relate to recycling salts and sequestering molten salt reactor (MSR) waste material from fluoride salt based MSRs for which the fluoride molten salt serves both to provide the fuel, and to cool the reactor. Fluoride salts targeted for separation, recycling and sequestration include NaF, KF, LiF, CsF, 4 30805769.1
104937-201 and SrF2. Lithium fluoride salts are used in MSRs to reduce the generation of tritium, and such lithium salts have very high fractions of 7Li in order to reduce neutron absorption. Consequently, some embodiments relate to recycling 7Li from molten salt waste. [0025] 90Sr and 137Cs are both short half-life fission products targeted for separation and sequestration, in some embodiments. Removal of these two radionuclides has a substantial effect on total toxicity and heat generation. [0026] Fig. 1 provides an embodiment of a method according to this disclosure for separating, recycling, and sequestering waste material from a fluoride containing molten salt composition. In the first step, the fluoride molten salt composition 101 is mixed with water, so that soluble salts, including NaF, KF, and CsF, are dissolved in a supernatant phase 112 and insoluble salts 122, including LiF and SrF2, are left in solid form. [0027] In some embodiments, strontium ions can be extracted from the insoluble salts 122 in the form of soluble strontium salts by a process of reactive dissolution 123 involving mixing the insoluble salts 122 with a solution of the salt of a monovalent anion X having a soluble strontium salt for which the lithium salt is insoluble. According to some embodiments, such soluble strontium salts include the halide salts SrFX, where X is chosen from the group consisting of Cl, Br, and I, and the pentafluoroantimonate salt SrSbF5. According to one such embodiment, the soluble halide salt is SrFI, and the reactive dissolution occurs according to equation (1): ^^^^^^2(^^^^) + ^^^^^^2(^^) → 2^^^^^^^^(^^^^) (1) [0028] Since SrFI and SrI2 have relatively high solubility in water, but LiI is relatively insoluble in water, dispersing insoluble salts 122 of LiF and SrF2 in an aqueous solution of SrI2 selectively depletes the insoluble salts of SrF2, thereby effectively separating Sr solid waste from Li solid waste. [0029] The insoluble LiF, comprising 7Li, can then be directly recycled 134. Following solvent evaporation, radioactive solid SrFX waste product 155 remains, and can be further treated for recycling, in some embodiments. [0030] Following the first step of mixing with water, the supernatant phase 112 retains soluble salts of NaF, KF, and CsF. According to an embodiment, the soluble salts can be reactively precipitated to form metal halide perovskites 133. [0031] According to one such embodiment, water is evaporated from the supernatant phase 112 and the dried residue comprising NaF, KF, and CsF is mixed with tin(IV) chloride 5 30805769.1
104937-201 (SnCl4) in a polar organic solvent, resulting in a sequestration of the alkali metal M by reactive precipitation of M2SnF6 according to the equation: 6^^^^ + ^^^^^^^^4 → ^^2^^^^^^6 ↓ +4^^^^^^ (2) where M is selected from the group consisting of Na, K, Cs, and combinations thereof. [0032] According to another embodiment, reactive precipitation 133 of alkali metal M, where M is Na, K, and Cs, is effected by a two-step process in aqueous solution. According to the first step, the supernatant of soluble salts 112 is mixed with FeCl3 in water, leading to reaction and precipitation of M3FeF6, with residual MCl remaining in solution, according to the equation: 6^^^^ + ^^^^^^^^3 → ^^3^^^^^^6 ↓ +3^^^^^^ (3) [0033] In the second step, the residual MCl is then reacted with H2SiF6, leading to the formation and precipitation of M2SiF6, according to the equation: 2^^^^^^ + ^^2^^^^^^6 → ^^2^^^^^^6 ↓ +2^^^^^^ (4) thereby trapping Na, K, Cs, and potentially other components of radioactive waste in a solid, reduced volume form. [0034] According to yet another embodiment, the reactive precipitation 133 of MF in aqueous solution to form M2SiF6 can be performed in one step according to the reaction: 2^^^^ + ^^2^^^^^^6 → ^^2^^^^^^6 ↓ +2^^^^ (5) [0035] Alternatively, in a more environmentally benign process, M2SiF6 can be synthesized in a single step in the presence of SiO2. According to this alternative method, SiO2 is mixed with the supernatant of soluble salts 112 in water, followed by the addition of H2SiF6, to give a precipitate of M2SiF6, according to the equation: 6^^^^ + 2^^2^^^^^^6 + ^^^^^^2 → 3^^2^^^^^^6 ↓ +2^^2^^ (6) where M is selected from the group consisting of Na, K, Cs, and combinations thereof. Because this method does not release HF or HCl gas, it is more environmentally benign. [0036] According to another embodiment, the water in the supernatant phase 112 is evaporated away, leaving a solid comprising NaF, KF and CsF. CsF is separated from this 6 30805769.1
104937-201 supernatant for subsequent sequestration by means of a solubility separation 113 with an organic solvent chosen to selectively dissolve CsF in the solvent 124 and to leave insoluble NaF and KF solids 114. [0037] According to one such embodiment, the organic solvent is methanol. According to another such embodiment the organic solvent is acetone. According to one embodiment, the organic solvent includes a dissolved ligand capable of binding Cs+ and thereby selectively enhancing the solubility of CsF compared to NaF and KF in the solvent. According to some embodiments, the dissolved ligand is a crown ether. According to one embodiment, the solvent is acetone and the crown ether is 18-crown-6. [0038] As indicated by the upper dashed arrow, following this separation, in one embodiment the insoluble NaF and KF solids can be recycled for re-use as molten salts in a molten salt reactor 115. [0039] As indicated by the lower dashed arrow, according to an alternative embodiment, the NaF and KF solids 114 can be redissolved in water and sequestered by reactive precipitation 125 to form a metal halide perovskite (MHP), according to equations (2) – (6). [0040] According to one such embodiment, the NaF and KF solids are mixed with tin(IV) chloride (SnCl4) in a polar organic solvent, resulting in a sequestration of the alkali metal M by reactive precipitation of M2SnF6 according to equation (2), where M is one or both of Na and K. [0041] The addition of a cosolute can enhance the yield of this reaction. In some embodiments, the cosolute is a fatty acid. In some embodiments, the fatty acid is oleic acid. In some embodiments, the polar organic solvent is selected from the group consisting of methanol, acetone, dimethylformamide, and combinations thereof. In some embodiments, the polar organic solvent is methanol and the fatty acid is oleic acid. [0042] According to other embodiments, reactive precipitation 125 of alkali metal M, where M is Na and/or K is effected by a two-step process in aqueous solution according to equations (3) and (4), thereby trapping Na, K, and potentially other components of radioactive waste in a solid, reduced volume form. [0043] According to yet another embodiment, the reactive precipitation 125 of MF in water to form M2SiF6 can be performed in one step according to equations (5) or (6). [0044] Referring again to Fig. 1, the soluble CsF in organic solvent 124 can be processed to sequester cesium as radioactive waste 135 and to recycle the solvent 145. According to one embodiment, the processing involves adding H2SiF6 directly to the soluble 7 30805769.1
104937-201 CsF in the organic solvent and allowing Cs2SiF6 to precipitate out according to the equation: 2^^^^^^ + ^^2^^^^^^6 → ^^2^^^^^^6 ↓ +2^^^^ (7) [0045] An alternative solution method of cesium sequestration which avoids the release of highly corrosive HF involves the reaction of CsF with H2SiF6 and SiO2 according to the equation: 6^^^^^^ + 2^^2^^^^^^6 + ^^^^^^2 → 3^^^^2^^^^^^6 ↓ +2^^2^^ (8) [0046] The reaction of Equation (8) can be performed by adding H2SiF6 and SiO2 directly to the soluble CsF in organic solvent 124. Alternatively, the reaction can be performed in aqueous solution. [0047] According to yet another embodiment, cesium can be incorporated into MHP through a solid state reaction by high energy ball milling (HEBM) of CsF and SnF2 in a 1:1 molar ratio according to the equation: ^^^^^^ + ^^^^^^2 → ^^^^^^^^^^3 (9) Examples: [0048] The following examples provide experiments which demonstrate some embodiments of the methods described herein. [0049] FLiNaK simulant salt is used to demonstrate how to recycle the Li salts and separate CsF and SrF2 from the mixed salts via a low-temperature wet synthesis method. The properties of the component fluorides of the selected FLiNaK salt are given in Table 1: Table 1. The properties of the component fluorides of the FLiNaK salt. Chemical Weight ratio Molar ratio Molar mass Density Solubility Mass (wt.%) (at.%) (g/mol) (g/cm3) (g/100 mL) (g)
[0050] In further experiments (not shown) it has been demonstrated that the separation, recycling and sequestration methods described below can be scaled up to a large quantity of 500 grams total. A. LiF recycling from the simulant FLiNaK salt [0051] According to the distinguishable solubility of each fluoride salt, LiF can be 8 30805769.1
104937-201 separated and recycled from the simulant FLiNaK salt through filtration in water at room temperature (RT). A flow sheet of separation and recycling of LiF is given in Figure 2. A total of 10 g simulant FLiNaK salt-2.92 g LiF, 1.17 g NaF, and 5.91 g KF- was added into approximately 100 mL water 201, and mixed via a magnetic stir bar. Once well mixed, the aqueous solution was filtered through filter papers 212. The filtered LiF solid was dried in an oven at 70 °C overnight 213. The aqueous solution of NaF and KF can be recycled 223 or can be reacted to form Na and K perovskites 233, as discussed below. [0052] As can be seen in Fig. 3, the X-ray diffraction (XRD) pattern of the filtered solid 310 is identical to a control diffraction pattern of pure LiF 320, demonstrating the desired separation of LiF from the water soluble salts NaF and KF. In this system, the recycling efficiency REi can be calculated as ^^^^ ^^ ^^ = ^^^^^^,^^ ^^^^^^^^^^^^^^^^,^^ × 100% (10) where
mass of the chemical i in the recycled materials. [0053] Here, there was 2.92 g LiF in the 10 g simulant FLiNaK salt. After filtration and air drying, 2.833 g LiF with high purification (confirmed in Figure 2) was recycled, corresponding to a recycling efficiency of approximately 97.0 wt.%. Similar procedures have also been successfully completed on a larger scale using 500 g of simulant salt. B. Sequestration of NaF/KF into MHPs by low-temperature solution chemistry B.1 Sequestration into alkali metal salts of tin hexafluoride [0054] Figure 4 shows a flow sheet of the synthesis of the MHP Na2SnF6 in polar organic solvent with or without ligands. K2SnF6 is readily prepared by an analogous method. NaF and SnCl4 are mixed in the polar organic solvent 405. In some embodiments, a cosolute is added to increase the solubility of NaF. The mixture is allowed to react to form Na2SnF6 410. B.1.1 Without cosolute [0055] NaF (0.904 gram) and SnCl4.4.7H2O (1.1638 gram) were dissolved into 160 ml and 25 ml methanol, respectively. The stirring of both solutions in different glass beakers for 1 hr at 500 rpm at room temperature resulted in 100 % dissolution of the respective compounds. Both raw powders were weighed in a 6:1 molar ratio for the synthesis of 1 gram product of Na2SnF6. The glass beaker with NaF solution was kept stirring at 60 °C in a water 9 30805769.1
104937-201 bath to maintain uniform temperature. SnCl4.4.7H2O was added into NaF solution and instantly white precipitate formed. The mixed solution was kept stirring at 500 rpm for 1 hr. The finally-obtained solution was centrifuged at 9000 rpm for 30 min. The obtained damp product was washed two times with 45 ml methanol and then placed in an oven at 70 °C for 10 hrs. [0056] The synthesis of the MHP Na2SnF6 can also be performed at room temperature. For the room-temperature synthesis, NaF (0.226 g) was added into 50 ml methanol the solution was stirred for 5 hrs. An excessive amount of SnCl4·5H2O (0.377 g ~120%) was added into the solution, leading to immediate precipitation. Further reaction was performed for 1 hour with continuously mixing through magnetic stirrer at 300 rpm. After 1 hour, the precipitated solid was filtered out through centrifugation at 9000 rpm for 30 mins. The obtained damp solid was dried overnight in an oven at 70 oC. [0057] For both the elevated temperature and the room temperature syntheses, the reaction was verified by X-ray diffraction data (not shown). B.1.2 With cosolute [0058] NaF (0.904 gram) and SnCl4.4.7H2O (1.1638 gram) were dissolved into 160 ml and 25 ml methanol, respectively. The cosolute, oleic acid was added into the NaF solution to increase the solubility of NaF in methanol. The volume of OA was 10 ml. The stirring of both solutions in different glass beakers for 1 hr at 500 rpm at room temperature resulted in 100 % dissolution of the respective compounds. Both raw powders were weighed in a 6:1 molar ratio for the synthesis of 1 gram product of Na2SnF6. The glass beaker with NaF solution was kept stirring at 60 °C in a water bath to maintain uniform temperature. SnCl4.4.7H2O was added into NaF solution and instantly white precipitate formed. The mixed solution was kept stirring at 500 rpm for 1 hr. The finally obtained solution was centrifuged at 9000 rpm for 30 min. The obtained damp product was washed two times with 45 ml methanol and then placed in an oven at 70 °C for 10 hrs. [0059] The cosolute-assisted synthesis of the MHP Na2SnF6 can also be performed at room temperature. For the room-temperature synthesis, 50 ml methanol was mixed with 3.2 ml oleic acid in ratio 16:1 and after that, NaF (1.13 g) was added into the solution of methanol and oleic acid. The amount of NaF represents 4x higher than the baseline reaction without ligand. This allows us to explore the scale up potential and investigate how the drastically increased reactants affect the separation yield and efficiency. [0060] The mixed solution was kept stirred at 300 rpm at RT for 5 hrs. After that, an excessive SnCl4·5H2O (1.887 g~120%) was added into the solution and further reaction was 10 30805769.1
104937-201 performed for 2 hrs with continuously mixing through magnetic stirrer at 300 rpm. After 2 hours, the precipitated solid was filtered out through centrifugation at 9000 rpm for 30 mins. The obtained damp solid was also washed with 10 ml methanol three times to remove impurities. The obtained damp solid was dried overnight in an oven at 70 oC. B.2 Sequestration into alkali metal salts of iron hexafluoride and silicon hexafluoride [0061] In water, M3FeF6 and M2SiF6 salts were synthesized at room temperature to immobilize the Na+, K+ at the M site. As a first step, almost 100% fluorine F- was incorporated into the designed MHPs, e.g., Na3FeF6 and K3FeF6. However, only about 50% of the Na+ or K+ ions were incorporated into the MHPs, leaving the residual alkali ions in the form of NaCl and KCl salts dissolved in aqueous solution. The second step involved the immobilization of remaining Na+ or K+ ions into M2SiF6 by adding H2SiF6 with generation of hydrochloric acid. Multiple cycles were demonstrated in the step one to immobilize fluoride and then accumulate sufficiently high concentration of alkali ions in the solvent, and almost all of the accumulated alkali ions can be fully immobilized in the Na2SiF6 or K2SiF6 in step 2 with a high yield, according to: (Reaction 1): 6MF + FeCl3 ^ M3FeF6 ↓ + 3MCl (11) (Reaction 2): 2MCl + H2SiF6 ^ M2SiF6 ↓ + 2HCl (12) where M = Na or K. [0062] As an alternative approach, effective sequestration of sodium, potassium, and cesium alkali metal fluorides into M2SiF6, by one-step reaction was demonstrated. In particular, SiO2 was included as an additive in the reaction to assist the full immobilization of the waste fluorides into MHPs with a very high yield. This one-step reaction was also demonstrated for both aqueous and non-aqueous medium, representing exciting advancements for effective separation and waste elements into MHPs. One step reaction (reaction 3): 6MF + 2H2SiF6 + SiO2 -> 3M2SiF6 ↓ + 2H2O (13) where M = Na, K, or Cs. [0063] This reaction has been demonstrated in aqueous solution for NaF, KF, and CsF, and in organic solution for CsF. [0064] In one embodiment, NaF (0.06 moles~2.519 g) and SiO2 (0.01 moles~0.601 g) with 6:1 molar ratio were added into 70 ml of water at room temperature. The volume of the water added was only slightly higher than the solubility of NaF. After that, the mixed solution 11 30805769.1
104937-201 was stirred at 300 rpm at room temperature for a few minutes so that NaF was completely dissolved into DI water. Once NaF dissolved, a stoichiometric amount of H2SiF6 (6:2 molar ratio of NaF to H2SiF6) was poured into the NaF aqueous solution, and then a white precipitate was formed immediately. The measured amount of H2SiF6 was 0.02 moles~2.882 g which is equivalent of 8.234 g acidic solution of H2SiF6 (35 weight%). The reaction was performed for 45 minutes at RT and the precipitate was separated out from water through gravity filtration. The damped precipitate was dried at RT. [0065] Some embodiments of one-step SiO2-assisted reactions can have one or more of the following advantages: (1) a simplified process by one-step reaction to greatly accelerate the throughput of the synthesis process; (2) almost full immobilization of waste elements into the designed MHPs with a high yield without the needs for multiple cycles and multiple step reaction; (3) increased cost effectiveness with simplified processes and chemistry; (4) a more environmentally-benign process without the generation of byproduct HCl or HF. C. CsF separation from simulant salt mixture and immobilization into MHPs [0066] Fig. 5 shows the methods according to the present disclosure for separating CsF from a mixture of metal fluorides by use of an organic solvent separation. The method makes use of the preferential solubility of CsF in organic solvents compared to other metal fluorides, in particular other alkali metal fluorides. According to this method, CsF can be separated from solid mixtures of NaF and KF by mixing the solid mixture with a suitable organic solvent 500. In some embodiments, the organic solvent is selected from the group consisting of methanol, acetone, dimethylformamide, and combinations thereof. In some embodiments the organic solvent is methanol. In some embodiments, the organic solvent is acetone. In some embodiments, the partitioning of CsF into the organic solvent is facilitated by a ligand capable of binding cesium ion. In some embodiments, the ligand is a crown ether. In some embodiments, the ligand is 18-crown-6. In some embodiments, the ligand is 18- crown-6 and the organic solvent is acetone. [0067] After allowing sufficient time for partitioning, the soluble CsF in the organic solvent is separated from the insoluble NaF and KF solids. In the embodiment of Fig.5, the separation is by filtration 510. Following filtration 510, the Cs can be reactively precipitated to form a Cs perovskite 515. In some embodiments, the perovskite is Cs2SnF6. In other embodiments, the perovskite is Cs2SiF6. 12 30805769.1
104937-201 C.1 Separation by a solution of acetone and 18-crown-6 [0068] For the separation of CsF from NaF and KF by 18-crown-6 ether, NaF, KF and CsF with a 11.7: 59.1:10 weight ratio were added into the acetone and 18-crown-6 ether solution (0.1M 18-crown-6 ether concentration) mixture at room temperature. The mixture was stirred for more than 6 hours, after which the solids were filtered out. The obtained damp solid/precipitated was dried in an oven at 150 °C overnight. The XRD measurements shown in Figs.6A and 6B demonstrate that the wet filtered solid 605 was a mixture of KF and NaF, with only traces of CsF, and that the dried solid 610 was pure CsF, with no traces of CsF. [0069] The solution remaining after filtration of the solids was then dried to leave a residue. As shown in Fig.7, the XRD spectrum of the residue 620 shows the same peak structure as the XRD spectrum of pure CsF, providing further validation of the separation mechanism. C.2 Separation by methanol [0070] The separation of CsF from NaF and KF in methanol is based on the solubility difference of the targeted alkali metal fluorides in methanol which increases with increased cationic sizes as shown in Table 2. Large amounts of CsF can be dissolved in methanol and the majority of NaF-KF will remain undissolved because of a large difference in their solubilities in methanol, enabling effective separation of CsF (with the theoretical separation efficiency of CsF of 100%) from the salt mixture. Table 2. A list of solubilities in methanol and other physical properties of CsF, NaF, and KF. Chemical Weight Molar Molar Density Solubility Mass Separation 3 l)
C.3 Immobilization of CsF in Cs2SiF6 by SiO2 assisted room temperature solution chemistry [0071] The sequestration of CsF into Cs2SiF6 by means of the SiO2 assisted reaction approach has been demonstrated in aqueous as well as non-aqueous medium. 13 30805769.1
104937-201 C.3.1 Aqueous reaction [0072] In water, the chemical synthesis of Cs2SiF6 was demonstrated by mixing CsF (0.012 moles~1.823 g) and SiO2 (0.002 moles~0.1202 g) in a 6:1 molar ratio was added into an appropriate amount of DI water (6 ml) at RT. The volume of the water added was only slightly higher than that required to dissolve CsF. After that, the mixed solution was stirred at 300 rpm at RT for 15 minutes, and CsF was completely dissolved in the water. [0073] Following the dissolution of CsF, a stoichiometric amount of H2SiF6 (6:2 molar ratio of CsF to H2SiF6) was poured into the CsF aqueous solution, at which point a white precipitate formed immediately. The measured amount of H2SiF6 was 0.004 moles (~0.5764 g), which is equivalent to 1.6469 g acidic solution of H2SiF6 (with a concentration of 35 weight%). The synthesis followed equation (12), ^^^^^^^^ + ^^^^^^^^ + ^^^^^^^^^^^^^^ → ^^^^^^^^^^^^^^^^ ↓ +^^^^^^^^ (14) [0074] After 45 minutes, and the precipitate was separated out from water through gravity filtration. The damp precipitate was dried at room temperature. C.3.2 In non-aqueous solvent (Acetone) [0075] CsF (3 mmol~0.4557 g) and SiO2 (0.5 mmol~0.0301 g) with 6:1 molar ratio were mixed with 100 ml acetone at room temperature. The volume of the acetone added was slightly higher than that required to fully dissolve CsF. The mixed solution was stirred in a round bottom flask at 300 rpm at room temperature for 10 hours so that CsF was completely dissolved into acetone. After 10 hours, a stoichiometric amount of H2SiF6 (6:2 molar ratio of CsF to H2SiF6) was poured into the CsF non-aqueous solution, at which point a white precipitate formed immediately. The measured amount of H2SiF6 was 1 mmol ~0.1441 g which is equivalent of 0.4117 g acidic solution of H2SiF6 (35 weight%). After 1 hour the precipitate was separated out through gravity filtration. The damp precipitate was dried at room temperature. The formation of Cs2SiF6 was verified by XRD. C.3.2 In non-aqueous solvent (methanol) [0076] CsF was directly immobilized into Cs2SiF6 in methanol through SiO2 assisted RT solution chemistry. CsF (3 mmol~0.4557 g) and SiO2 (0.5 mmol~0.0301 g) with 6:1 molar ratio were added into an appropriate amount of methanol (5 ml~3.5 g) at room temperature. The volume of the methanol added was slightly higher than the solubility of CsF. After that, the mixed solution was stirred at 250 rpm at room temperature for only 1 min and CsF was completely dissolved into methanol. A stoichiometric amount of H2SiF6 (6:2 14 30805769.1
104937-201 molar ratio of CsF to H2SiF6) was poured into the CsF aqueous solution, at which point a white precipitate formed immediately. The measured amount of H2SiF6 was 1 mmol ~0.1441 g which is equivalent of 0.4117 g acidic solution of H2SiF6 (35 weight%). After 30 minutes the precipitate was separated out through gravity filtration. The damp precipitate was dried at room temperature. Formation of Cs2SiF2 was verified by XRD. D. Separation of SrF from LiF [0077] Management of fluoride salt wastes and recycling of Li from salt waste is important for the development of advanced nuclear fuel cycles and future nuclear technologies. Some radionuclides fission products targeted for separation are I, Tc, Cs, and Sr. Removal of the latter two significantly reduces the heat load of the residual conditioned wastes.90Sr and 137Cs, both short half-life fission products, have an influence on determining the total toxicity and heat generation of the fission product nuclides (<300 yrs). The efficient separation of SrF2 from LiF has been a long-term challenge for nuclear waste management and Li recycling under low temperatures, due to their similar solubility in water (with low and comparable solubility in water). To realize the separation of SrF from LiF, the potential of separating SrF into the SrFX (X: anions) compounds was explored. Herein, some embodiments provide selection principles/criterion for immobilizing SrF into water soluble SrFX compounds, enabling separation of SrF from water insoluble LiF. [0078] In order to separate insoluble SrF2 salts from LiF, strontium salts were screened for their ability to solvating Sr2+ but not Li+ in aqueous solution. [0079] The selection of appropriate salt with different chemical formulas and compositions began with the screening of stable SrFX structures (reported in Crystallography Open Database: http://www.crystallography.net/cod/search.html) with environmentally benign properties. The results of this screening are shown in Fig.8. Based on this screening, according to some embodiments, suitable soluble strontium salts include the halide salts SrFX, where X is chosen from the group consisting of Cl, Br, and I, and the pentafluoroantimonate salt SrSbF5. [0080] For the experiments described herein, SrFI was chosen as a target salt. To compare the solubility of SrFI with LiF and SrF2, a mixture of LiF, SrI2, and SrF2 was dispersed in 500 ml water and heated at 100°C for 12hrs. The precipitates were then redisbursed in 500 ml water. XRD revealed the formation of SrFI in the aqueous phase through this wet solution approach. Moreover, after filtration, the concentration of the SrFI phase is enhanced, which confirms the results that the solubility of SrFI is much higher than 15 30805769.1
104937-201 that of LiF and SrF2. [0081] As a consequence, in some embodiments, SrI2 can be used to solubilize the strontium in a SrF2 salt in water in the form of SFI, thereby providing a means to separate SrF2 from LiF in nuclear reactor waste products. [0082] The foregoing disclosure provides illustration and description but is not intended to be exhaustive or to limit the implementations to the precise form disclosed. Modifications may be made in light of the above disclosure or may be acquired from practice of the implementations. [0083] Although each dependent claim listed below may directly depend on only one claim, the disclosure of various implementations includes each dependent claim in combination with every other claim in the claim set. No element, act, or instruction used herein should be construed as critical or essential unless explicitly described as such. Also, as used herein, the articles “a” and “an” are intended to include one or more items and may be used interchangeably with “one or more.” Further, as used herein, the article “the” is intended to include one or more items referenced in connection with the article “the” and may be used interchangeably with “the one or more.” Furthermore, as used herein, the term “set” is intended to include one or more items (e.g., related items, unrelated items, a combination of related and unrelated items, and/or the like), and may be used interchangeably with “one or more.” Where only one item is intended, the phrase “only one” or similar language is used. Also, as used herein, the terms “has,” “have,” “having,” or the like are intended to be open- ended terms. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise. Also, as used herein, the term “or” is intended to be inclusive when used in a series and may be used interchangeably with “and/or,” unless explicitly stated otherwise (e.g., if used in combination with “either” or “only one of”). 16 30805769.1