SELECTIVE APPROACH TO SEPARATE AND CONCENTRATE RARE EARTH ELEMENTS

Abstract

Preparation and use of specialized nanoparticles containing tetrapods/graphene/metal organic frameworks, which are very effective at separating rare earth elements. Such methods and systems can be used for separating neodymium (Nd), Dysprosium (Dy), Praseodymium (Pr) and other REEs. Such metal organic frameworks may also be useful for separating other metals (e.g., so called critical metals). The metal organic framework (MOF) material is synthesized by solid phase reaction of metal oxide (e.g., ZnO) tetrapod or other nanostructured metal oxides, which are functionalized with nanoplatelet graphene, and a polyfunctional organic acid (e.g., an aromatic polycarboxylic acid). Such a resulting metallic organic framework exhibits high selectivity towards light REEs (e.g., Nd and Py), with lower selectively towards heavy REEs (e.g., Dy), allowing separation of such from one another.

Claims

1. A method for preparing a functionalized metal organic framework for use in concentrating and/or separating rare earth elements (REEs) or other metals, the method comprising: (a) providing a metal oxide tetrapod or other nanostructured metal oxide; (b) mixing the metal oxide tetrapod or other nanostructured metal oxide with a solution including nanoplatelet graphene or another 2D coating material; (c) burning the material resulting from (b); and (d) mixing the burned material from (c) with a polyfunctional organic acid, and allowing a solid-phase reaction therebetween to occur, to produce the functionalized metal organic framework.

2. The method of claim 1, wherein (c) occurs by burning in at least one of acetone or ethanol fire.

3. The method of claim 1, wherein the polyfunctional organic acid of (d) comprises an aromatic organic acid.

4. The method of claim 1, wherein the polyfunctional organic acid of (d) includes at least 2, or at least 3 carboxylic acid groups.

5. The method of claim 1, wherein the polyfunctional organic acid of (d) comprises trimesic acid.

6. The method of claim 1, wherein the solid-phase reaction of (d) occurs at a temperature of at least 80° C., at least 90° C., about 100° C., no greater than 300° C., no greater than 200° C., no greater than 150° C., or from 80° C. to 120° C.

7. The method of claim 1, wherein the solid-phase reaction of (d) occurs over a time period of at least 1 hour, at least 3 hours, at least 5 hours, about 24 hours, no greater than 72 hours, no greater than 48 hours, no greater than 36 hours, or from 10 hours to 30 hours.

8. The method of claim 1, wherein the metal oxide tetrapod or other nanostructured metal oxide is modified with one or more of graphene, reduced graphene oxide, a transition metal dichalcogenide, or black phosphorous.

9. The method of claim 8, wherein the metal oxide tetrapod or other nanostructured metal oxide is further modified with an aluminum silicate clay, perovskite material, or other material having a spinodal structure.

10. A functionalized metal organic framework for use in concentrating and/or separating rare earth elements (REEs) or other metals, comprising: (a) a metal oxide tetrapod or other nanostructured metal oxide; (b) wherein the metal oxide tetrapod or other nanostructured metal oxide is functionalized with nanoplatelet graphene or another 2D coating; and (c) wherein the metal oxide tetrapod or other nanostructured metal oxide functionalized with the 2D coating is further functionalized with a polyfunctional organic acid.

11. The functionalized metal organic framework of claim 10, wherein the metal oxide tetrapod or other nanostructured metal oxide comprises at least one of tetrapod ZnO, or ZnO in combination with another metal oxide.

12. The functionalized metal organic framework of claim 10, wherein the metal oxide tetrapod or other nanostructured metal oxide comprises tetrapod ZnO.

13. The functionalized metal organic framework of claim 10, wherein the metal oxide tetrapod or other nanostructured metal oxide is modified with one or more of graphene, reduced graphene oxide, a transition metal dichalcogenide, or black phosphorous.

14. A method for using a functionalized metal organic framework for selective adsorption of REEs or other metals, wherein the functionalized metal organic framework comprises: (a) a metal oxide tetrapod or other nanostructured metal oxide; (b) wherein the metal oxide tetrapod or other nanostructured metal oxide is functionalized with nanoplatelet graphene or another 2D coating; and (c) wherein the metal oxide tetrapod or other nanostructured metal oxide functionalized with the 2D coating is further functionalized with a polyfunctional organic acid; the method comprising contacting the functionalized metal organic framework with a composition including two or more REEs or other metals, the functionalized metal organic framework separating one of the REEs or other metals from another REE or other metal.

15. The method of claim 14, wherein the functionalized metal organic framework includes a tetrapod structure, wherein an arm of the tetrapod orients itself normal to a substrate on which separation of REEs occurs.

16. The method of claim 14, wherein the method includes selectively adsorbing metal ions of the REEs, or excluding such metal ions of the REEs from passing through a membrane material including the functionalized metal organic framework.

17. The method of claim 14, wherein the method is part of a chromatography application.

18. The method of claim 14, wherein the method includes manipulating one or more of pH, exchange ion concentration, or temperature to more effectively and selectively load or strip target metal ions of the REEs.

19. The method of claim 14, wherein the method is carried out in a modified adsorption matrix in a porous electrode structure to selectively adsorb and/or exclude target metal ions of the REEs relative to other metal ions of the REEs. The method of claim 14, wherein the method is carried out in a modified functionally graded or multi-layered adsorption matrix in a porous electrode structure to selectively adsorb and/or exclude metal ions of the REEs or other metals relative to other metal ions of the REEs or other metals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the drawings located in the specification. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings.

[0027] FIGS. 1A-1B show SEM images of ZnO tetrapods.

[0028] FIGS. 1C-1D show SEM images of ZnO-G (zinc oxide tetrapods treated with graphene) before REE adsorption.

[0029] FIGS. 1E-1F show ZnO-G after REE adsorption.

[0030] FIG. 2 shows Raman spectra of both ZnO tetrapods and ZnO-G before REE adsorption.

[0031] FIG. 3 shows EDS results for ZnO-G after REE adsorption.

[0032] FIG. 4 shows adsorption kinetics at a pH of 2, at ambient temperature (˜23° C.), and with a concentration of 0.00625 mol/L for each of the 3 REE salts.

[0033] FIG. 5 shows adsorption isotherm data for praseodymium, as compared to Langmuir and Freundlich models, at a pH of 2 and at ambient temperature (˜23° C.).

[0034] FIG. 6A shows the effect of pH on adsorption capacity Qe (mg/g).

[0035] FIG. 6B shows the effect of pH on percent adsorption at ambient temperature (˜23° C.) and with a concentration of 0.00625 mol/L for each of the 3 REE salts.

[0036] FIG. 6C shows the effect of pH on percent composition in the liquid phase, at ambient temperature (˜23° C.) and with a concentration of 0.00625 mol/L for each of the 3 REE salts.

[0037] FIG. 7A shows the effect of temperature on adsorption capacity Qe (mg/g) at a pH of 2 and with a concentration of 0.00625 mol/L for each of the 3 REE salts.

[0038] FIG. 7B shows the effect of temperature on percent adsorption at a pH of 2 and with a concentration of 0.00625 mol/L for each of the 3 REE salts.

[0039] FIG. 8 shows a predicted response surface plot for the combined effect of pH and temperature on adsorption capacity Qe (mg/g) for Pr.

[0040] FIG. 9 shows estimated stripping efficiency based on temperature.

DETAILED DESCRIPTION

I. Introduction

[0041] The present disclosure is directed to methods for preparing a functionalized metal organic framework for use in concentrating and/or separating rare earth elements (REEs) or other metals, as well as methods of such separation, and functionalized metal organic framework materials that can be used for such concentration and/or separation.

[0042] An exemplary method of preparation includes mixing a metal oxide tetrapod or other nanostructured metal oxide, e.g., formed in a simple flame transport synthesis step, with a solution including nanoplatelet graphene or a similar 2D coating material, and burning the material (e.g., in acetone and/or ethanol fire) resulting from such mixture. The burned material can then be mixed with a polyfunctional organic acid, where a reaction (e.g., solid-phase reaction) is allowed to occur, to produce the desired functionalized metal organic framework that can be used for REE separation and/or concentration. The metal or metal oxide tetrapod or other nano structured metal oxide base or backbone material can be formed in a simple flame transport synthesis step by combining the metal nanoparticles with a sacrificial polymer solvent such as poly (vinyl butyral) (PVB), ethyl cellulose, or similar sacrificial polymer solvent.

[0043] An exemplary functionalized metal organic framework includes a metal oxide tetrapod or similar nanostructured metal oxide material, where the tetrapod or similar nanostructured metal oxide material is functionalized with nanoplatelet graphene or similar 2D coating, where the tetrapod or other nanostructured metal oxide material is also functionalized with a polyfunctional organic acid.

[0044] Such materials may include more than one metal oxide (e.g., tetrapod ZnO mixed with another metal oxide) to provide particular functionality, and/or another material may be mixed therewith (e.g., aluminum silicate clays, perovskites, or other spinodal structured materials). Such variations may allow tuning of the selectivity of the produced MOF. Such MOFs can also be incorporated into another media material, to provide a robust structure, e.g., within a acrylic or other polymeric matrix, e.g., positioned within a filtering syringe, chromatography media, or the like. A solution or other composition including REEs or other metals to be separated can be passed through such a robust material, to separate and/or concentrate REEs or other metals from one another, as they pass through such a material.

[0045] Exemplary methods of use may include using such a modified tetrapod metal organic framework for selective absorption of REEs. Such methods of use may include selective adsorption of metal ions (e.g., REE metal ions), or exclusion of desired target metal ions (e.g., REE metal ions) from passing through a membrane material that includes such a modified tetrapod/nanostructured metal organic framework material. Another exemplary method of use may include use in a chromatography application. Another exemplary method of use may include use of such a material in a modified functionally graded or multilayered adsorption matrix of a porous electrode structure, to selectively adsorb and/or exclude target metal ions (e.g., REE metal ions) relative to other metal ions (e.g., other REE metal ions). Within any such methods, one or more of pH, exchange ion concentration, or temperature may be manipulated, in order to more effectively and selectively load or strip desired target metal ions.

II. Exemplary Methods and Materials

[0046] An exemplary method for preparing a functionalized metal organic framework for use in concentrating and/or separating REEs includes: (a) providing a metal oxide tetrapod or other nanostructured metal oxide; (b) mixing a metal oxide tetrapod (e.g., tetrapod ZnO) or other nanostructured metal oxide material with a solution including nanoplatelet graphene or another 2D coating (e.g., graphene, reduced graphene oxide, a transition metal dichalcogenide, or black phosphorus); (c) burning the material resulting from (b); and (d) mixing the burned material from (c) with a polyfunctional organic acid, and allowing a reaction (e.g., solid-phase reaction) therebetween to occur, to produce the functionalized metal organic framework.

[0047] The metal oxide tetrapod or other nanostructured material used as a base or backbone for the MOF may itself be formed in a simple single step flame transport synthesis process, where a sacrificial polymer solvent (e.g., a resin) such as poly (vinyl butyral) is mixed with metal microparticles. Another exemplary sacrificial polymer solvent may include ethyl cellulose. Others will be apparent to those of ordinary skill in the art, in light of the present disclosure, and are contemplated for use herein. Any of a variety of metal particles may be used, e.g., such as Zn, Fe, Sn, Bi, Al, and/or Si. Such particles may be micron sized, e.g., ranging in size from 3 μm to 45 μm. Zn is a particularly suitable material. In an embodiment where two or more such metal particles are used, at least one of the metal particles may comprise Zn (e.g., Zn+Fe, Zn+Sn, Zn+Bi, Zn+Al, or Zn+Si). The metal particles may be mixed with the sacrificial polymer solvent in any suitable ratio, such as from 1:5 to 5:1. In an exemplary embodiment, the ratio may include 2 parts PVB to 1 part zinc or other metal particles.

[0048] The furnace within which the flame transport synthesis process is carried out may be preheated to at least 400° C., at least 425° C., or at least 450° C. After preheating, a ceramic crucible with the PVB or other sacrificial polymer solvent and zinc or other metal particles may be placed within the furnace (e.g., a muffle-type furnace), which is then heated to a temperature of at least 800° C., at least 850° C., or at least 900° C., such as from 900° C. to 1100° C., or 900° C. to 1000° C., such as 900° C. or 950° C. The flame transport synthesis step may occur at such temperature over a period of time of at least 20 minutes, or at least 30 minutes, such as from 30 minutes to 3 hours, from 30 minutes to 2 hours, or from 30 minutes to 90 minutes. During such process, the PVB or other sacrificial polymer solvent begins to burn at the high furnace temperature, and the created flame carries the Zn or other metal microparticles upward, where they are transformed into ZnO or other metal oxide nano and/or micro-structures due to the high flame temperature. Shape of the resulting tetrapods or other micro or nanostructures depends on the temperature and other conditions within the furnace, as well as the particular metal employed. Metal oxide particles could alternatively be used, in place of metal particles, in an embodiment.

[0049] Such free powder tetrapod particles can be further treated, as will be seen, to eventually result in a self-assembled, interconnected macroscopic network of tetrapods (or other nanostructures). The specific resulting structure can depend on the conditions, and particular metal material employed. For example, at moderate temperatures, using Zn particles, ZnO tetrapods are formed. At more elevated temperatures, the same Zn particles may form a structure with additional needle-type arms (more than 4 as compared to a typical tetrapod). Use of Fe particles may result in an FeO structure including a core with spikes extending therefrom (e.g., resembling a sea-urchin). Use of Bi particles may result in a BiO structure that includes curved or bent spikes extending from a central portion of the nanostructure. Each of such differing structures may offer different selectivity characteristics when used to separate or concentrate REEs. Hybrid materials may be employed, formed using two or more such metal or metal oxide materials. For example, an exemplary hybrid may include both ZnO tetrapods and an FeO sea-urchin core/spike structure. Another exemplary hybrid may include both ZnO tetrapods and a BiO micro or nanostructure with bent or curved spikes extending from a central portion of the structure.

[0050] Once the desired metal oxide tetrapod or other micro or nanostructure is formed, this structure can then be functionalized with a nanoplatelet graphene or another 2D coating material (e.g., graphene, reduced graphene oxide, a transition metal dichalcogenide, or black phosphorus). This can be done by placing the nanocrystals formed in the flame transport synthesis step into a solution of the 2D coating material (e.g., nanoplatelet graphene). The materials can be mixed together for a suitable period of time e.g., at least 3 minutes, at least 5 minutes, or from 5 to 30 minutes, such as about 10 minutes. The material can then be filtered using filter paper, and the solids allowed to dry (e.g., overnight, at elevated temperature, such as at least 40° C., or at least 50° C., such as 55° C.). By way of example, the mass ratio of metal oxide nanocrystals to 2D coating material may be 1:1 to 50:1, such as 5:1 to 20:1, such as about 10:1. For example, 0.5 g of metal oxide nanocrystals may be placed in 10 mL of solution, including 5 g/L of nanoplatelet graphene (e.g., 0.5 g metal oxide nanocrystals and 0.05 g nanoplatelet graphene).

[0051] Once dried, the metal oxide nanostructure which has been infiltrated or coated with the 2D material is then burned one or more times, e.g., in ethanol and/or acetone fire.

[0052] Finally, the burned product can be mixed or ground with an aromatic carboxylic acid (e.g., trimesic acid), and allowed to react under a solid-phase reaction at elevated temperature (e.g., at least 80° C., at least 90° C., about 100° C., no greater than 300° C., no greater than 200° C., no greater than 150° C., or from 80° C. to 120° C.) for at least 1 hour, at least 3 hours, at least 5 hours, about 24 hours, no greater than 72 hours, no greater than 48 hours, no greater than 36 hours, or from 10 hours to 30 hours.

[0053] After reaction, the product can be washed with ethanol, and allowed to dry again.

[0054] The resulting material exhibits high selectivity relative to REEs, and depending on the particular selections made, with greater selectivity for some REEs over other REEs. Such selectively and adsorption can be dependent on pH, and temperature, such that these parameters can be selected or manipulated to achieve the desired separation between various REEs.

III. Examples and Experimental Data

[0055] An advantage that zinc oxide tetrapod structures as described, functionalized with graphene (ZnO-G) has over superficially similar appearing ZnO materials is its spatial distribution or orientation, which helps it self-orient on the surface of a substrate with one of its tetrapod arms positioned normal to the substrate surface.

[0056] Materials used for the adsorption experiments described herein are listed in Table 1.

TABLE-US-00001 TABLE 1 Material Purity Supplier Zinc Oxide tetrapods — — Nanoplatelets Graphene — Alfa Aeser Neodymiun (III) Nitrate Hexahydrate 99.9% Sigama-Aldrich Dysprosium (III) Nitrate Hydrate 99.9% Sigama-Aldrich Praseodymium (III) Nitrate Hexahydrate 99.9% Sigama-Aldrich Nitric Acid 69.4% Fisher Scientific

[0057] ZnO tetrapods can be formed using a flame transport synthesis approach. A 2:1 combination of PVB and zinc microparticles, sized from 3 μm to 45 μm are mixed together. A muffle-type furnace is preheated to 450° C., and a crucible containing the zinc microparticles and PVB is introduced into the furnace, which is then heated to 900° C. to 950° C., for 30-90 minutes, for flame transport synthesis of the ZnO tetrapod structures base or backbone structures.

[0058] After flame transport synthesis of ZnO tetrapods as described, further synthesis of the presently described product was done as follows. 0.5 g of ZnO tetrapods was placed in 10 mL of nanoplatelet graphene (NG) (5 g/L) slurry which was then mixed for 10 min. Next, the mixture was filtered using filter paper and the solids were left to dry overnight at 55° C. and treated. The final black synthesized product was characterized by SEM, and Raman Spectroscopy.

[0059] The adsorption test matrix as shown in Table 2 was used to determine the adsorption kinetics (examples A16-A20), the adsorption isotherm (examples A1-A5), the effect of pH on adsorption (examples A6-A10), and the effect of temperature on adsorption (examples A11-A16).

TABLE-US-00002 TABLE 2 Exam- Molar Concentration Time Temperature ple per REE Salt (mol/L) pH (min) (° C.) A1 0.00625 2 150 23 A2 0.01250 2 150 23 A3 0.01875 2 150 23 A4 0.02500 2 150 23 A5 0.03125 2 150 23 A6 0.00625 1.5 150 23 A7 0.00625 2.0 150 23 A8 0.00625 2.5 150 23 A9 0.00625 3.0 150 23 A10 0.00625 3.5 150 23 A11 0.00625 2.0 150 25 A12 0.00625 2.0 150 30 A13 0.00625 2.0 150 35 A14 0.00625 2.0 150 40 A15 0.00625 2.0 150 45 A16 0.00625 2 5 23 A17 0.00625 2 15 23 A18 0.00625 2 30 23 NNN A19 0.00625 2 60 23 A20 0.00625 2 150 23

[0060] The nitrate salts of dysprosium, neodymium, and praseodymium listed in 1 were used. Each test was performed with 5 mL of REEs nitrates solution, 50 mg of ZnO-G, the pH was adjusted using a 2 mol/L nitric acid solution. Each test was performed on a shaker. After each test, the solids were separated from the solution using filter paper, and all samples were analyzed by inductively coupled plasma mass spectroscopy.

[0061] The materials were characterized by SEM, Energy Dispersive Spectroscopy (EDS) and Raman Spectroscopy. FIGS. 1A-1B show the SEM images of tetrahedral ZnO where the structure of the ZnO is clearly depicted as a tetrahedral shape with four legs (e.g., 22 μm to 50 μm). The structure of the synthetized MOF or ZnO-G before adsorption is depicted in FIGS. 1C-1D, which shows evidence of chunky blocks (e.g., 90 to 130 μm) representing the treated nanographene mixed with the tetrahedral ZnO sitting normal to the block's surface due to its tetrahedral spatial legs configuration. The structure of the functionalized ZnO-G after adsorption has been modified during contact with acid media as shown in FIGS. 1E-1F.

[0062] Raman spectra shown in FIG. 2 were recorded in backscattering configuration. In those Raman measurements, the excitation wavelength was ˜785 nm and the laser power was ˜110 mW. The measurements were performed both for tetrahedral ZnO and tetrahedral ZnO treated with graphene (ZnO-G), before REEs adsorption. It can be seen that various peaks, e.g., ˜330 cm.sup.−1, 412 cm.sup.−11 , 432 cm.sup.−1, 458 cm.sup.−1, 638 cm.sup.−1 and 1130 cm.sup.−1 appear in the Raman spectrum of the tetrahedral ZnO. Other less intense peaks at 381 cm.sup.−1 (A1-TO) and 583 cm.sup.−1 (LO) mode can be seen in this spectrum. Both E2 (high) and E1(TO) modes are visible in this spectrum. Because of the number of atoms per unit cell in wurtzite-type ZnO, there are a total of 12 phonon modes, including one longitudinal-acoustic (LA), two transverse-acoustic (TA), three longitudinal-optical (LO), and six transverse-optical (TO) branches. Because of the macroscopic electronic fields associated with the LO phonons, both A1 and E1 and symmetries are polar and divide into LO and TO components with distinct frequencies. It can be seen that high frequency E2 mode (peaks at 330 and 432 cm.sup.−1) is clearly noticed in this spectrum. FIG. 3 shows the EDS analysis of ZnO-G after adsorption for example A12. The EDS confirms the presence of Dy, Nd, and Pr respectively as 1.9%, 4.8%, and 5.3% on a weight percentage basis. Based on these results, it is clear that this material exhibits high selectivity towards Nd and Pr versus Dy.

[0063] Kinetics experiments were used to determine optimum contact time or shaking time for adsorption. It is evident in FIG. 4 that the adsorption capacity increases with increasing contact time, up to a point. After 60 min of contact time, it is evident that over 80% of the total adsorption capacity is reached and beyond that it is gradually reaching a maximum. Thus, 150 min was chosen as the contact time for subsequent experiments.

[0064] In order to determine the adsorption (loading or saturation) capacity of ZnO-G, adsorption isotherm experiments were performed. The results of praseodymium loading are shown in FIG. 5 which shows a jump in the loading capacity with increasing concentration, and it reached a maximum of 129 mg/g at its peak. The saturation capacity or amount adsorbed was calculated as:

[00001]$\begin{array}{cc}{Q}_e=\frac{V\left(C_0-C_e\right)}{W}& \left(\mathrm{Eq}.1\right)\end{array}$

[0065] Where Q.sub.e (mg/g) is the amount adsorbed, C.sub.o mg/L) and C.sub.e (mg/L) are respectively the initial and final (equilibrium) concentrations. V and W are respectively the solution volume (L) and adsorbent mass (g). Subsequently, Langmuir and Freundlich isotherm models were used to fit the experimental data. The two models are expressed as equations (Eq. 2) and (Eq. 3):

[00002]$\begin{array}{cc}{Q}_e=\frac{K_L{Q}_m{C}_e}{1+K_L{C}_e}& \left(\mathrm{Eq}.2\right)\end{array}$$\begin{array}{cc}{Q}_e=K_F{C}_e^{1/n}& \left(\mathrm{Eq}.3\right)\end{array}$

where Q.sub.e (mg/g) is the amount adsorbed, Q.sub.m (mg/g) is the maximum amount adsorbed, and C.sub.e (mg/L) is the final (equilibrium) concentration. K.sub.L is a Langmuir constant and K.sub.F and n are both Freundlich constants.

[0066] The results show that pH has a beneficial effect on adsorption as depicted in FIG. 6A, where the adsorption capacity increases with higher pH values. It is noted that a sharp increase in the adsorption capacity of Nd, Dy, and Pr occurred between a pH of about pH 1.5 and 3, particularly 1.5 and 2 (with a further increase from pH of 2 to pH of 3). Above pH 3, the capacity converges to a steady maximum value exceeding 80 mg/g for Nd and Pr, and between about 40-50 mg/g for Dy. It is evident that at a pH of 1.5 adsorption is limited as it is believed that the active adsorption sites at such acidic pH are mostly occupied by H* ions. FIG. 6B shows the adsorption of each species increasing with increasing pH. At pH 1.5, substantially 0% of REEs are adsorbed, and at pH 3 to 3.5, approximately 98% of Nd and Pr are adsorbed whereas only about 50% of Dy is adsorbed. This means that 50% of the total Dy remains in solution, representing a 93.4% enrichment relative to the 3.1% Nd and 3.5% Pr that remain in solution at pH 3 as shown in FIG. 6C. Thus, a particularly beneficial pH for the contemplated adsorption is a pH of about 3 and future tests at this pH are expected to show improved results in adsorption kinetics, isotherm, and effect of temperature.

[0067] The results also show that increasing temperature has a negative effect on adsorption. FIG. 7A shows evidence of a significant drop in adsorption capacity at temperatures exceeding 35° C. Such may be due to REE ions being removed from the adsorbent surface as higher temperatures tend to weaken the attractive forces between the adsorbent and adsorbate. The adsorption data shown in FIG. 7B suggests that at temperatures exceeding 35° C. (e.g., 40° C. or 45° C.), the adsorption significantly decreases by almost a factor of 2. Thus, based on these results, lower temperatures, for example 20-30° C., such as 20° C., 25° C., or 30° C. may be optimal temperatures for loading since they are associated with the lowest equilibrium concentrations for Nd and Pr. In an embodiment, loading may be performed at ambient temperature (no heating required). The fact that good adsorption can be achieved without the necessity of adding heat is of course advantageous.

[0068] The combined effects of pH and temperature on adsorption were also evaluated using a quadratic two-factor surface response analysis. The results from the quadratic two-factor response surface analysis for praseodymium are shown in FIG. 8. While FIG. 8 may suggest maximum adsorption can be achieved at higher temperatures and higher pH, the actual data show optimum adsorption to be achieved at moderate temperatures, and pH values of greater than 2, greater than 2.5, or at least 3, such as from 3-4.

[0069] To determine the stripping efficiency for the fabricated material, a stripping experiment was performed on filtered solids of the 3.5 pH sample (example A10) with adsorbed REE ions in the presence of a 2 mol/L nitric acid solution. This was performed at ambient temperature (˜23° C.) and was shaken for 150 min. The average stripping efficiency of Dy, Nd, and Pr achieved was 68% based on the individual efficiency shown in Table 3 and can be improved by perhaps increasing the contact time or manipulating the temperature. Furthermore, the final solution concentration after stripping is also shown in Table 3 and shows similar adsorption behavior as seen previously. The values suggest that Nd and Pr were loaded and stripped at a factor of approximately 2:1 relative to Dy.

TABLE-US-00003 TABLE 3 Final stripping Stripping Element concentration (mg/L) Efficiency (%) Dy 330 65 Nd 642 75 Pr 529 63

[0070] In an embodiment, stripping or desorption can be achieved by manipulating temperature as shown by its effect on adsorption. Advantages of this approach include high efficiency with less acid consumption. In addition, such a process offers simple process control since the only variable is temperature. This substantially decreases operating costs, as power (to heat the solution) is cheaper and more environmentally tractable than use of concentrated acid.

[0071] The adsorption data from FIG. 7B was used to estimate future stripping efficiencies based on the expected ability of temperature to desorb REEs ions from the ZnO-G pores. FIG. 9 shows that an estimated stripping efficiency of approximately 50% can be achieved at 45° C., and even higher efficiencies may be achieved at even higher temperatures.

[0072] In order to determine the degree of separation of each dissolved species in the nitrate solution using ZnO-G, selectivity was assessed based on the results of the pH and temperature experiments discussed above. One way to assess the selectivity of ZnO-G toward a single dissolved species is by calculating its distribution coefficient. The distribution coefficient (K.sub.d, mL/g) of each species can be calculated as:

[00003]$\begin{array}{cc}{K}_d=\frac{\left(C_0-C_e\right)}{C_o}\times \frac{V}{m}& \left(\mathrm{Eq}.5\right)\end{array}$

[0073] where C.sub.O (mg/L) and C.sub.E (mg/L) are respectively the initial and final (equilibrium) concentrations. V (mL) and m (g) are respectively the solution volume and adsorbent mass. At pH 3, the K.sub.d values go up to 5142 mL/g for Pr(III) and 4695 mL/g for Nd(III) versus only 100 mL/g for Dy (III) which means that ZnO-G has a high affinity for Pr (III) and Nd (III) adsorption, since a higher K.sub.d value means higher selectivity. This allows use of the presently synthesized material for separation of Pr (III) and Nd (III) from Dy (III). Table 4 tabulates different K.sub.d values at various conditions. Since Nd and Pr are classified as light rare earth elements (LREEs) and Dy is classified as a heavy rare earth element (HREE) due to their difference in physical and chemical properties, these results show that the present ZnO-G material has a high selectivity for LREEs compared to HREEs or in other words, LREEs can be separated from HREEs using this material. As noted, alterations in selectivity can be achieved by adjusting the various parameters noted herein.

TABLE-US-00004 TABLE 4 K.sub.d Temperature Element (mL/g) pH (° C.) Dy 0 1.5 23 Dy 51 2 23 Dy 56 2.5 23 Dy 100 3 23 Dy 100 3.5 23 Dy 47 2 25 Dy 53 2 30 Dy 79 2 35 Dy 40 2 40 Dy — 2 45 Nd 0 1.5 23 Nd 325 2 23 Nd 712 2.5 23 Nd 4695 3 23 Nd 4695 3.5 23 Nd 651 2 25 Nd 957 2 30 Nd 935 2 35 Nd 471 2 40 Nd 107 2 45 Pr 0 1.5 23 Pr 440 2 23 Pr 930 2.5 23 Pr 5142 3 23 Pr 5142 3.5 23 Pr 947 2 25 Pr 1344 2 30 Pr 1205 2 35 Pr 529 2 40 Pr 97 2 45

[0074] The potential adsorption mechanism of the synthesized ZnO-G is believed to be due to the abundance of oxygen-rich groups in the treated nanographene which is used to functionalize the tetrahedral ZnO.

[0075] REE ions enter the pores of the ZnO-G and form a stable structure in the presence of such oxygen-rich functional groups. Furthermore, the selectivity between different types of REEs is believed to arise from different binding forces that such REEs form with the oxygen-rich functional groups present in tetrahedral ZnO-G. Additionally, it is also believed that the difference in atomic radii of different REEs plays an important role as far as selectivity is concerned since such a size difference may dictate which ions can enter the ZnO-G pore.

[0076] The inventive new mesoporous template prepared via a green synthesis between tetrapod ZnO, treated with nanoplatelet graphene can be used for selective separation of REE ions. The results suggest high selectivity towards LREEs (Nd, and Pr) versus HREEs (Dy). The adsorption capacity is shown to be pH and temperature dependent. Higher adsorption (>80 mg/g for Nd and Pr) was achieved at higher pH and low adsorption at relatively higher temperatures. Due to lower adsorption at higher temperatures, it is expected that stripping can be done by manipulating temperature only, without requiring (or at least limiting) use of concentrated acids.

[0077] Unless otherwise indicated, numbers expressing quantities, constituents, distances, or other measurements used in the specification and claims are to be understood as optionally being modified by the term “about” or its synonyms. When the terms “about,” “approximately,” “substantially,” or the like are used in conjunction with a stated amount, value, or condition, it may be taken to mean an amount, value or condition that deviates by less than 20%, less than 10%, less than 5%, less than 1%, less than 0.1%, or less than 0.01% of the stated amount, value, or condition.

[0078] As used herein, the term “between” includes any referenced endpoints. For example, “between 2 and 10” includes both 2 and 10.

[0079] Unless otherwise stated, amounts listed in percentage (“%'s”), as well as ratios are by mass.

[0080] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although a number of methods and materials similar or equivalent to those described herein can be used in the practice of the present invention, the preferred materials and methods are described herein.

[0081] Disclosure of certain features relative to a specific embodiment of the present disclosure should not be construed as limiting application or inclusion of said features to the specific embodiment. Rather, it will be appreciated that other embodiments can also include said features, members, elements, parts, and/or portions without necessarily departing from the scope of the present disclosure. Moreover, unless a feature is described as requiring another feature in combination therewith, any feature herein may be combined with any other feature of a same or different embodiment disclosed herein. Furthermore, various well-known aspects of illustrative systems, methods, apparatus, and the like are not described herein in particular detail in order to avoid obscuring aspects of the example embodiments. Such aspects are, however, also contemplated herein.

[0082] The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.