Abstract:
The disclosure relates to the design and synthesis of selected ligands, dendrimers, polymers and other solid phase substrates for selective chelation of rare earth elements (i.e. lanthanides), and use of those selective ligands for synthesis of resins, polymers and other types of solid supports for separation and recovery of lanthanides from aqueous media. Recovery of critical elements from aqueous media occurs in a simple two-step process: pre-concentration of REE on the adsorbent and recovery by acid elution. The present invention can be used for design of selective ligands immobilized on solid substrates for extraction of various constituents, such as lanthanides, actinides, radionuclides, trace metals, etc., from aqueous media.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. §119 of Provisional Application Ser. No. 62/284,916, filed Oct. 13, 2015, Provisional Application Ser. No. 62/386,712, filed Dec. 10, 2015, and Provisional Application Ser. No. 62/494,656, filed Aug. 16, 2016, each of which is incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    This invention was made with government support under Grant No. DE-EE0006749 awarded by the Department of Energy. The government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    This invention relates generally to the recovery of rare earth elements from aqueous media. More specifically, this invention relates to the design and synthesis of solid phase substrates that can be used to recover specific rare earth elements from aqueous media containing various components. 
         [0004]    Rare Earth Elements (REE) constitute a group of chemically similar metals in the lanthanide series plus Yttrium and Scandium. REE are a critical part of modern energy technologies and electronics. However, REE are highly dispersed in the earth&#39;s crust, making it costly and difficult to extract and concentrate them for industrial use. The high demand of the use of REE in advanced energy technologies makes urgent the need for alternative approaches for REE separation and recovery from different matrices compared to traditional mining and extraction operations. Aqueous media such as natural waters, sea water, brines, and waste waters produced from conventional oil/gas and shale gas extraction or thermal energy operations, offer a new opportunity for recovery of REE. 
         [0005]    Separation techniques for lanthanides and actinides from water have been employed in various industries, such as nuclear power generation. However, most of those strategies target the separation of actinides from lanthanides but they do not address the challenges of separating individual lanthanide elements and do not attempt to separate REE present at low concentration in hypersaline water, such as brines. In other words, recovering REEs from complex aqueous media is challenging because existing separation technologies for REE are not adequately selective or involve many chemical steps which increase the overall cost of the process. 
         [0006]    Ionic Imprinted Polymer (IIP) is an effective way to separate one REE element from other REE, such as selectively absorbing Gd from La. IIP utilizes metal ions as a template and imprinting the size and/or shape of the ions into the polymer by the process of polymerization. A properly formed polymer will selectively form bonds with the target ions rather than interfering ions due, thus separating target ions from other ions with similar properties. However, the various methods of IIP synthesis suffer from drawbacks, especially when used in complex or low-concentration solutions. For example, using a chemical immobilization method of synthesis, the loading rate is around 10% or less because the ligand can be blocked by the bulk structure of the polymer and the template ion cannot be leached out during the acid washing, making the ligand unavailable for ions in solution. 
         [0007]    Although IIPs can be developed for most REE, the effectiveness can vary across depending on the element being targeted. For example, Er-IIP has a mediocre selectivity and the separation factor is around 5 for Y, Dy, Ho, Tb, Tm. In addition, the IIPs for Gd, Tm, and Y have the highest selectivity not to the template element, but another one. For example, Gd-IIP has the highest selectivity to Eu instead of Gd. In addition, many IIPs are designed to attract medium or heavy REEs, except Nd-IIP. It is a challenge to develop an IIP for light elements such as La. 
         [0008]    Ligands have also been used to bind REE. Use of ligands has typically focused on simple systems containing low concentrations of background electrolytes and high sorbate concentrations. However, extraction from complex solutions, such as brines, that contain low levels of REE requires selective, high-capacity adsorbents in order to be effective and economical. 
         [0009]    Rare earth elements are abundant in Earth&#39;s crust but are highly dispersed. Currently the global production of rare earth elements is based on mining ore deposits and processing and refining of the mining extracts. However, different types of water (for example, seawater, groundwater, tailings, etc.) contain levels of rare earth elements. Traditional mining techniques would be unable to recover these elements. It would therefore be advantageous to develop materials that can concentrate and extract REE from aqueous media. 
       BRIEF SUMMARY 
       [0010]    According to one embodiment of the present invention is a functionalized adsorbent used for extracting rare earth elements from aqueous solutions. The adsorbent comprises a solid substrate with a rare earth element (REE)-attractive material disposed on the surface of the substrate. Solid-phase extraction (SPE) of REEs from aqueous matrices has desirable advantages relative to other separation techniques, such as liquid-liquid separation or co-precipitation, because consumption of reagents is limited, the separations can be performed rapidly with few process steps, and SPE can be scaled-up more easily. In one embodiment, ion imprinted polymers produce consistent, selective recovery of REEs from a range of aqueous matrices. In alternative embodiments, ligands are adhered to the surface of a substrate. 
         [0011]    Further disclosed herein is the synthesis of selected solid phase substrates for the selective chelation of rare earth elements. The solid phase substrates are used in a two-step process to recover certain REEs: first, REEs are pre-concentrated on a surface of the solid phase substrate; next, the REEs are recovered by acid elution, which separates the REE from the substrate. After acid elution, the adsorbent can be used again. The adsorbents can recover lanthanides from seawater, brines, groundwater, tailings, and other aqueous media. With the proper selection of the attractive material, they can also be used for the extraction of various other materials such as actinides, radionuclides, and trace metals. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0012]      FIG. 1  illustrates a processing strategy for preparation of high surface-area adsorbents based on an IIP-coated substrate, according to one embodiment. 
           [0013]      FIG. 2  is a flow diagram of the synthesis of the monomer. 
           [0014]      FIG. 3  shows the flow diagram of the polymerization of IIP, according to one embodiment. 
           [0015]      FIG. 4  is a graph showing recovery of various rare earth elements. 
           [0016]      FIG. 5  depicts the structure of various ligands. 
           [0017]      FIG. 6  is a representation of functionalization methods, according to various embodiments. 
           [0018]      FIG. 7  is a graph showing acid-base titration curves for adsorbents according to embodiments of the invention. 
           [0019]      FIG. 8  depicts characterization of the absorbent according to one embodiment. 
           [0020]      FIG. 9  depicts characterization of the absorbent by an alternative method. 
           [0021]      FIG. 10  depicts characterization of the absorbent by an alternative method. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    In one embodiment, ion imprinted polymers (IIP)  110  are tethered to surfaces of solid substrates  101 , creating a functionalized adsorbent  100 . In alternative embodiments, rare earth element (REE)-reactive ligands  120  are attached to the substrate  101 , which can include silica and sulfonated polystyrene, for example. By attaching the REE-attractive material  110 ,  120  to the surface, a high-surface area adsorbent  100  is created that has specific binding sites for REE. 
         [0023]    According to one embodiment, a solvent exchange process is optimized for coating the IIP  110  onto the substrate  101 , as shown schematically in  FIG. 1 . The IIP  110 , which initially contains template REE ions  111 , is dissolved in toluene, methylene chloride, or another suitable solvent for the IIP  110 . After mixing the IIP  110  solution with the substrate  101 , a second solvent, such as diethyl ether, in which the IIP  110  is insoluble, is slowly added, causing a controlled deposition of the IIP  110  onto the surface of the substrate  101 . After coating is completed, the substrate  101  is isolated and the REE template ions  111  are extracted, resulting in a high surface-area solid substrate  101  capable of binding REEs in solution with high specificity. 
         [0024]    In one embodiment, silica beads are used as the substrate  101  because of their ease of functionalization with the IIP  110  or other REE-attractive material. Surface hydroxyl groups are used for the attachment of organo-silanes. The most common silane, because it allows for facile attachment of amino-polycarboxylic acids, is aminopropyl triethoxysilane (APTES; 22). Additionally, silica gels are resistant to dissolution under acidic conditions, limiting the potential for degradation of the adsorbent with repeated uptake and elution cycles. In many instances, silica beads are commercially available with reactive carboxylic acid and amine groups, for example. In an alternative embodiment, sulfonated polystyrene is readily available with carboxylic acid groups present in the form of imidoacetate moieties and can be readily aminated by diamine functionalization using simple modification chemistries. With both materials (silica and polystyrene), the substrate  101  is chemically robust under a broad range of conditions and can be regenerated repeatedly. 
         [0025]    To create an IIP  110  suitable for deposition on the substrate  101 , the following process steps, according to one example embodiment, are completed: 
         [0026]    1. Synthesis of monomer (Dimethylformamide (DMF) based synthesis) (See  FIG. 2 ) 
         [0027]    a) To avoid oxidation, a small amount (−50 mg) of ascorbic acid sodium salt (VC) is added into a round bottom flask (250 mL) along with 0.37 g of DTPA-bisanhydride. 
         [0028]    b) 30 mL of DMF is added into the flask by pipette to dissolve the solids and then the mixture is heated to 60° C. and stirred (475 rpm) until the solids are completely dissolved. 
         [0029]    c) 0.297 g of 4-Ethynylaniline (4-VA) is added to the solution dropwise by syringe and the mixture is heated and stirred for an extended period at 60° C. During this period, the round bottom flask is sealed with rubber cap and covered with aluminum foil to avoid oxygen and exposure to light, which could degrade the materials. 
         [0030]    d) After about 24 hours of reaction, the monomer solution with DMF is condensed using Rotavap (P: 15 mbar and T: 60° C.) for about 30 minutes until it becomes viscous/sticky. 
         [0031]    e) At this stage, the concentrated monomer is re-dissolved in 6 mL of methanol. The flask is heated at 50° C. for about half an hour with stirring until all of the crude products are dissolved in the methanol again. 
         [0032]    f) To precipitate the monomer, re-dissolved monomer solution is extracted and added into several clean centrifuge tubes. 7.5 mL of diethyl ether is added gently into each of the tubes. The solution is left sitting to allow precipitation. 
         [0033]    g) The precipitates are separated by vacuum filtration and washed with diethyl ether. 
         [0034]    2. Polymerization of IIP (as shown in  FIG. 3 ) 
         [0035]    a) 0.2 g of the monomer and 0.16 g of Gd(NO 3 ) 3  re added into a tube and 2 mL of methanol is added to dissolve. 
         [0036]    b) 5 mL Divinylbenzene (DVB) and 5 mL 1 mol/L NaOH solution is added into a tube, causing the liquid to separate. The supernatant layer is a colorless transparent liquid containing inhibitor-free DVB. 
         [0037]    c) 0.78 mL of inhibitor-free DVB and 10 mg of 2,2-Azobisisobutyronitrile (AIBN) are added to the solution, which causes the mixture to begin to polymerize. 
         [0038]    d) After several days of polymerization, the crude polymer is extracted by vacuum filtration and washed by methanol. 
         [0039]    3. Cleaning polymers: Two different methods are presented to clean the polymer  110 . In a first method, the polymer powder is dried in an oven at 65° C. for 30 minutes, then cooled to room temperature. To remove the chelated Gd ions from the IIP  110 , 1 mol/L HCl is used for acid rinsing. In a second method, the IIP  110  is subjected to acid rinsing first. After rinsing, the polymers  110  are washed with deionized water until the pH of its filtrate was adjusted to 5.10 (or above). The IIP  110  is then left to dry. 
         [0040]    After polymer synthesis, the IIP  110  is deposited onto the substrate  101  according to the solvent exchange method previously described. In addition, once the IIP  110  is fabricated, testing can corroborate proper functionalization. An example testing protocol, according to one embodiment, comprises preparing a stock REE solution, mixing the IIP  110  in the solution, and measuring the amount of REE bound to the IIP  110 . Pursuant to this testing procedure, Gd-IIP and its filtrates were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) in high energy Helium mode (HEHe). The results of the testing are shown in the following table, with the results also shown in  FIG. 4   
         [0000]    
       
         
               
             
               
               
             
               
             
               
               
             
           
               
                   
               
             
             
               
                 Sample 1 
               
             
          
           
               
                 87.74% 
                 Total Gd Recovered. 
               
               
                 83.91% 
                 Recovery (1st elution) 
               
               
                 3.18% 
                 Recovery (2nd elution) 
               
               
                 0.65% 
                 Recovery (3rd elution) 
               
             
          
           
               
                 Sample 2 
               
             
          
           
               
                 96.00% 
                 Total Gd Recovered 
               
               
                 86.65% 
                 Recovery (1st elution) 
               
               
                 6.29% 
                 Recovery (2nd elution) 
               
               
                 3.06% 
                 Recovery (3rd elution) 
               
               
                   
               
             
          
         
       
     
         [0041]    While the following examples have been described in specific quantities of materials, a person having skill in the art will appreciate that the basic procedural steps can be replicated with different amount of materials, particularly if the process is scaled for commercial production. 
         [0042]    In an alternative method, silica gel substrates  101  are functionalized with REE-reactive ligands  120 , including diethylenetriaminepentaacetic acid (DTPA), diethylenetriaminepentaacetic dianhydride (DTPADA), phosphonoacetic acid (PAA), and N,N-bisphosphono(methyl)glycine (BPG), for example. The structures of the ligands  120  are illustrated in  FIG. 5 . 
         [0043]    In these alternative embodiments, the substrates  101  are functionalized using either 3-aminopropyl silica gel (d: 75-ISO m) or high-purity silica gel (d: 150-250 m). In one embodiment, ligands  120  are grafted to the surface of the substrate  101  through the formation of an amide bond between a free carboxyl group and the surface amines. The related ligands  120  DTPADA and DTPA were chosen to compare the effects of forming a targeted amide tether to the surface (i.e. on the lone carboxyl of the DTPADA) to the non-specific coupling with DTPA. The use of propylphosphonic anhydride (T3P) and N,N′-dicyclohexylcarbodiimide (DCC) for amide formation promotes the reaction at the free carboxyl over the anhydride. Upon introduction to an aqueous environment, the anhydride groups hydrolyze to leave four carboxyl groups free in solution. 
         [0044]    Two schemes for functionalization of the substrate  101  can be employed. In one embodiment, pre-aminated silica gel substrates  101  are functionalized via a “bottom-up” scheme, building functional moieties piece by piece from the surface. In an alternative embodiment, a “top-down” scheme is employed by first forming a ligand-functionalized silane, which is subsequently attached to a silica gel substrate  101 . The procedure for each scheme is illustrated schematically in  FIG. 6 , with the bottom-up approach shown in the top of  FIG. 6 . 
         [0045]    The following disclosure describes synthesis via the bottom-up functionalization process, according to one example embodiment. In this embodiment, a solution of the desired ligand  120  (128 mM), 4-dimethylaminopyridine (4-DMAP; 154 mM), 3-aminopropyl functionalized silica (25.7 mM amine), and T3P (77.1 mM) in dimethylformamide (DMF; 35 ml, total volume) is stirred at room temperature (˜15-25° C.) for a period of several hours. The suspension is then transferred to a centrifuge tube and centrifuged for 15 minutes at 25° C. The supernatant is removed, leaving a pellet comprising the adsorbent  100 . Optionally, the pellet is resuspended in 25 ml of DMF and centrifuged for 10 minutes. After this second centrifuge stage, the supernatant removed and the pellet can be resuspended for additional centrifuge separations in DMF. 
         [0046]    In yet another alternative embodiment, the adsorbent  100  is created in a top-down scheme. An example of the top-down process is described as follows. First, diethylenetriaminepentaacetic dianhydride (2,5 g, 7.0 mmol) and DCC (1.6 g, 7.7 mmol) are added to a flask. The flask is then filled with dichloromethane (25 mL) and (3-Aminopropyl)triethoxysilane (APTES) (1.8 mL, 7.7 mmol) and stirred for a period of several hours. The reaction solution is then filtered and concentrated under vacuum. This product was then added to 1.62 g of substrate  101  in dry toluene (25 ml) and stirred for a period of several hours. The final product is washed multiple times with toluene, multiple times with tetrahydrofuran, and multiple times with warm water and dried in a vacuum oven (65° C.). 
         [0047]    As with the adsorbent  100  created with IIP  110 , the adsorbent  100  utilizing ligands  120  can be characterized to determine if it was adequately functionalized by the various ligands. For example, the formation of amide bonds between the surface amines and the desired ligand  120  results in a shift of surface acid-base chemistry from highly basic (amine pK a ˜ 9-10) to acidic (ligand pK a1 ˜2). This shift can be investigated by rapid acid base titrations of particle suspensions. This shift can also be tested by inferring surface charge from electrophoretic mobility measurements. Additionally, the presence of the desired amide tether can be confirmed by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). 
         [0048]    Further, adsorbent  100  REE reuptake can be quantified through a series of measurements. The measurements determine the reactivity of the functionalized adsorbents  100  under a variety of conditions and probe a variety of adsorbent properties including: uptake kinetics, pH dependance, and affinity. 
         [0049]    As an example, acid-base titrations of an adsorbent  100  according to one embodiment demonstrated a significant shift from highly basic (dominated by amines) to highly acidic (dominated by carboxyl groups (see  FIG. 7 ). In addition to starting as a more acidic suspension, a DTPA-based adsorbent  100  showed approximately four times greater base neutralizing capacity (from pH ˜4-8) than did a DTPADA-based adsorbent  100 , caused by a greater grafting density of the DTPA, owing to its flexibility of attachment site. The rigid conformation of the DTPADA attachment may create steric hindrances to greater grafting densities. 
         [0050]    By way of further example, infrared spectroscopy can be used, via ATR-FTIR, to investigate the chemistry of the surface functionalization of the adsorbent  100 .  FIG. 8  shows the presence of the expected amide bond in all samples studied (V c=0, amide ≈1650 cm −1 ). These spectra, along with the previously presented characterization data, indicate that the functionalization was successful via the proposed attachment scheme. 
         [0051]    Referring again to the figures,  FIG. 9  shows the rapid uptake (within 5 minutes) of the REE by both the DTPADA-based and PAA-based adsorbents  100 . A pseudo equilibrium was reached in each of these tests in under one hour, indicating that the desirable, short contact times allowed with these materials. The mono-dentate PAA adsorbent  100  had notably more rapid kinetics than the more complex, and multi-dentate DTPADA adsorbent  100 , reaching a steady removal after just 5 minutes of mixing. 
         [0052]    By way of further example of adsorbent  100  characterization, adsorption isotherms were used to quantify the affinity of the surface-attached ligands for the REE under a range of conditions. The performance of the adsorbents  100  with four different ligands are compared in  FIG. 10  on the uptake of Nd, Gd, and Ho from 0.5 m NaCl. In  FIG. 10 , a more reactive adsorbent  100  plots further to the left. 
         [0053]    As can be concluded from the adsorption edge data, it is apparent that the ability to attach the DTPA at a single carboxyl group (by performing the synthesis with the dianhydride form) offers benefits over a functionalization scheme using the acid-form. This likely results from two factors. First, the high affinity for DTPA towards the REE in solution is based on the ability of the ligand to form a highly-coordinated “cage” around the metal. This mechanism utilizes all five of the carboxyl groups in solution to maximally coordinate the ion. The attachment of the ligand  120  to a surface at any of the carboxyl groups will alter the ability to coordinate the REE ions. 
         [0054]    This “penalty” is limited by attaching at the lone carboxyl group, emanating from the central, tertiary amine of the DTPA molecule. Stated differently, the potential for the attachment of the acid-form DTPA at any of its carboxyl groups results in surface groups that are sterically hindered from forming the desired coordination geometry, and thus relies primarily on electro-static interactions with the ion. Alternatively, because there are no protected carboxyl groups in the acid-form molecule, there is a potential for multiple carboxyl groups from the same molecule to attach to the surface via amide bonds. 
         [0055]    Thus, the characterization validates the affinity of surface-attached ligands  120  for REE using a substrate  101  and various attachment schemes. Silica gel substrates  100  functionalized with REE-reactive ligands  120  can be synthesized and characterizated by a suite of techniques to confirm successful synthesis. Batch adsorption testing determines the performance of the adsorbents  100  under environmentally relevant sorbate concentrations. Multi-dentate ligands, DTPADA and BPG yielded the most reactive functionalized adsorbents  100  (average Kd=2413 and 3829 mL/g for DTPADA and BPG respectively), were chemically and physically robust (no loss of performance after aggressive acid and base washes followed by heated dessication), and regenerateable (&gt;80 elution of REE from DTPADA in a single step). However, amino-polycarboxylic acid adsorbents showed limited uptake at mid-range pH and low-sorbate concentrations. As a potential remedy to this observation an alternative functionalization scheme was employed to limit excess surface amines and an improvement is observed with respect to mid-ranged pH uptake. The characterization highlights the complexity of inter-molecular interactions between even moderately sized reactive sites when developing high-capacity, high-selectivity adsorbents  100 . 
         [0056]    While the disclosure has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modification can be made therein without departing from the spirit and scope of the embodiments. Thus, it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.