Toward Molecular Recognition of REEs: Comparative Analysis of

Oct 30, 2017 - A broad comparative study of single-crystal structures of the complexes between REEs and nongrafted complexonates at neutral and acidic...
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Article Cite This: Inorg. Chem. 2017, 56, 13938-13948

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Toward Molecular Recognition of REEs: Comparative Analysis of Hybrid Nanoadsorbents with the Different Complexonate Ligands EDTA, DTPA, and TTHA Elizabeth Polido Legaria,* Michail Samouhos, Vadim G. Kessler, and Gulaim A. Seisenbaeva* Department of Molecular Sciences, Biocenter, Swedish University of Agricultural Sciences, Box 7015, 75007 Uppsala, Sweden S Supporting Information *

ABSTRACT: Highly efficient tailored SiO2-based nanoadsorbents were synthesized for the selective extraction of rare-earth elements (REEs). Three different complexonates (EDTA, DTPA, and TTHA) were investigated in terms of uptake capacity and selectivity, showing capacities of up to 300 mg of RE3+/g and distinct preferential trends depending on the complexonate. EDTAfunctionalized nanoadsorbents showed higher uptake for Dy3+, DTPA-functionalized ones for Nd3+, and TTHA-functionalized ones for La3+. The selectivity was even more pronounced in desorption at pH 3, with separation factors of up to 76 in ternary mixtures. A broad comparative study of single-crystal structures of the complexes between REE and the nongrafted complexonates at different pHs led to a molecular understanding of their individual modes of action. EDTA-derived nanoadsorbents combine concerted action and chelation, whereas the latter is the preferential coordination mechanism for DTPA- and TTHA-derived nanoadsorbents. These different mechanisms result in quite specific REE affinities, which opens great possibilities toward molecular recognition of REEs and for tailoring nanoadsorbents for a particular REE or group of REEs in their production from minerals and in recycling. It also brings new insights into how REEs are adsorbed on nanomaterials applied in a broad variety of fields, including bioimaging and MRI.

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INTRODUCTION Rare-earth elements (REEs) have been increasingly gaining attention in the last few decades due to their unique electronic, magnetic, and catalytic properties among others, which make them crucial in a large variety of applications such as electronics, metallurgy, wind turbines, high-field-strength magnets for hybrid and electric vehicles, and numerous medical devices, applied in such areas as medical diagnostics (optical bioimaging and MRI).1−9 China is currently the absolutely dominating world producer of REEs, possessing the largest reserves of these elements.10 However, there are many other REE deposits available that have not been commercially exploited at the moment: namely, those in the central region of Brazil, which rank the country in second position in terms of REE abundance,11 or several deposits in Europe that are currently being explored and technically and economically assessed.12 The urge to develop sustainable REE industries that ensures autonomy from the current major producer has made REEs the focus point for many researchers around the world in various disciplines, especially after the European Commission and the U.S. Department of Energy declared them critical raw materials.13,14 For that, not only do REE deposits need to be investigated but also efficient technologies for extraction and separation of REEs in solution need to be established. The latter will even be crucial for the development of recycling technologies to make them commercially viable in the future. © 2017 American Chemical Society

Nanosized functional adsorbents are an attractive alternative to the current most used techniques, i.e. solvent extraction and ionic liquids, since those techniques often require high temperatures (resulting in high costs) or involve large amounts of harmful solvents and extractants.15−19 Nanoadsorbents offer an environmentally friendlier route for extraction and separation of REEs, and more specifically, nanosized SiO2-based adsorbents have been quite widely studied, given their excellent adsorption properties.20−23 Materials of this kind have even been proposed for use as contrast agents for MRI.24 The current, so far laboratory-scale results on the application of molecular recognition technology for REE extraction are encouraging. The method could be potentially successfully used for the processing of unexploited REE sources in the European territory, including (a) complex primary ores related with igneous rocks (in the first hand, carbonatites) and metamorphic carbonates and (b) secondary sources such as phosphogypsum and red mud, rendering the EU competitive in the sector of REE metallurgy. From an industrial point of view, centrifugation or filtration of the nanoparticles to separate them from solution may be an expensive and time-consuming process. Therefore, magnetite and SiO2 core−shell nanoparticles provide an ideal combination Received: August 10, 2017 Published: October 30, 2017 13938

DOI: 10.1021/acs.inorgchem.7b02056 Inorg. Chem. 2017, 56, 13938−13948

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Inorganic Chemistry

by centrifugation (10000 rpm, 10 min) and washed three times with H2O and twice with ethanol. Synthesis of Core−Shell SiO2-Coated γ-Fe2O3 Nanoparticles. First, magnetite (Fe3O4) nanoparticles were prepared by coprecipitation of iron(III) and iron(II) chlorides with ammonia under a nitrogen atmosphere.26,38 Afterward, core−shell SiO2-covered γ-Fe2O3 nanoparticles were synthesized via a modified Stöber method, leading to highly stable nanoparticles.39 The previously synthesized Fe3O4 nanoparticles (100 mg) were dispersed in Milli-Q water (32 mL) and sonicated for 20 min. Then, this dispersion was mixed with ethanol (160 mL) and NH4OH (4 mL, 25%) was slowly added. Finally, TEOS (1.6 mL) was added dropwise and the mixture was stirred at room temperature for 20 h. Magnetic nanoparticles were then separated from the solution with a magnet and washed three times with double-distilled H2O and twice with ethanol. Finally, the nanoparticles were dried under an N2(g) atmosphere. The characterization of magnetic properties for these particles has been reported earlier in detail.40 Synthesis of EDTA-, DTPA-, and TTHA-Grafted SiO2 and γFe2O3-SiO2 Nanoparticles. The SiO2 and γ-Fe2O3-SiO2 NPs described above were each surface-functionalized with EDTA, DTPA, and TTHA in two steps. First, the nanoparticles were surfacefunctionalized with aminopropyltriethoxysilane (APTES) and further reacted with the corresponding amino polycarboxylic acid. Functionalization with APTES. SiO2 or γ-Fe2O3-SiO2 nanoparticles (800 mg) were dispersed in dry toluene (20 mL). APTES (1 mL, 4.3 mmol) was added to the dispersion, and the reaction mixture was refluxed overnight. After reaction, the amino-functionalized nanoparticles were separated from the solution by centrifugation (10000 rpm, 10 min), washed with toluene and ethanol, and dried under an N2 atmosphere for further functionalization. Functionalization with Amino Polycarboylic Acids. Aminofunctionalized magnetic and nonmagnetic SiO2 nanoparticles (500 mg) were dispersed in dry toluene (20 mL), to which the corresponding amino polycarboxylic acid (EDTA, DTPA, or TTHA) (0.41 mmol) was added and the reaction was mixture refluxed overnight under an inert atmosphere. Finally, nanoparticles were separated from solution by centrifugation, washed with toluene and ethanol, and dried under an N2 atmosphere. Adsorption Isotherms with Rare Earth Metal (Dy3+, Nd3+, La3+) Ions. Stock solutions of REE(NO3)3 0.02 M, where REE = Dy, Nd, La, were prepared. For the REE isotherms, final concentrations were between 0 and 10 mM (typically the concentrations used were 0, 0.5, 1, 2, 3, 4, and 10 mM with some slight variations depending on the REE). Around 25 mg of functionalized magnetic or nonmagnetic SiO2 NPs was mixed with the amount of REE(NO3)3 to reach the desired concentration, 2 mL of NaNO3 1 M (to keep the ionic strength constant), and Milli-Q H2O to a final volume of 20 mL. After the mixture was shaken for 24 h in an orbital shaker, the particles were separated from solution by centrifugation and dried under an N2 atmosphere. All experiments were carried out in triplicate with different samples of the same adsorbent, and also for selected samples the experiments were repeated in adsorption−desorption cycles (at least three times and for some of the samples an even greater number of times). Selectivity Experiments with Ternary and Quinary Mixtures. EDTA- and DTPA-functionalized nanoadsorbents were used for selectivity studies. For this procedure, around 25 mg of the nanoadsorbents was mixed with equimolar ternary or quinary mixtures of REE (ternary, La3+, Nd3+ and Dy3+; quinary, La3+, Nd3+, Dy3+, Eu3+, and Y3+) for 24 h and, after separation and drying of the nanoparticles, the REE content of these was analyzed by energy-dispersive X-ray spectroscopy (EDS). Complexometric Titrations of REE in Mother Liquor over the Nanoparticles. Complexometric titrations of solutions after contact time with the nanoparticles were carried out with EDTA 5 mM using xylenol orange as an indicator. EDTA complexates REE metals in a 1:1 ratio; therefore, the adsorption capacity (qe) can be calculated by the formula

for the purpose: the magnetic core offers a simple way to separate the nanoadsorbents from solution via an external magnet and the SiO2 layer not only protects the magnetic core from leaching in acidic media but also brings all the beneficial properties of adsorption, opening possibilities for selective REE uptake via surface functionalization with specific organic ligands. Among others, amino polycarboxylic acid reagents have been rather intensely investigated and shown to possess high efficiency toward REEs.25−27 For example, diethylenetriaminepentaacetic acid (DTPA), an octadentate polycarboxylate, has been broadly studied in its complexation with REEs in aqueous solution,28−30 more specifically for the so-called TALSPEAK process (trivalent actinide lanthanide separation with phosphorus-reagent extraction from aqueous “komplexes”), a solvent extraction technique to separate actinides from lanthanides that makes use of the higher affinity of DTPA toward actinides in aqueous solution.31−33 However, fewer studies exist on the use of this type of ligand immobilized on solid adsorbents, particularly those focused on the different affinities within the REE series and toward different ligands. Zhang and co-workers studied the selective extraction of REEs by DTPA-functionalized magnetic nanosorbents34 and obtained excellent separation factors between heavy rare earth elements (HREEs) and light rare earth elements (LREEs). Almeida and co-workers35 studied the separation of Nd3+ and La3+ with the same type of adsorbent. In this work, we used the hybrid magnetic SiO2-based nanoadsorbent approach to make a compiled and comparative study of three different amino polycarboxylic acid reagents: ethylenediaminetetraacetic acid (EDTA), DTPA, and triethylenetetraminehexaacetic acid (TTHA) and their different affinities within the REE series. In comparison to previous studies, this work provides not only a much higher adsorption efficiency (up to 300 mg of REE/g of adsorbent) but also, even more importantly, an integral comprehensive investigation into the mode of REE adsorption on functionalized nanomaterials and in the selectivity of each ligand for specific REEs or group of REE from a molecular point of view. To the best of our knowledge, in spite of extensive work at lower and higher pH, no structural studies on the complexation of REE with TTHA at neutral pH had been done. Therefore, a molecular model with TTHA and Dy3+ was synthesized at neutral pH and its structure was solved by single-crystal X-ray crystallography; together with a review of the existing structural studies for REE complexes with EDTA and DTPA, this work gives molecular answers to the selective action of the studied aminopoly(carboxylic acid) as observed from complexometric titration and EDS analysis. It thus opens possibilities for the production of highly efficient tailored hybrid nanoadsorbents for a specific target REE to be used for their extraction and separation in hydrometallurgy and recycling. The obtained results can be used in further investigation of the coordination of adsorbed REEs by EXAFS spectroscopy (see ref 36) that will be addressed in our future studies.

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MATERIALS AND METHODS

Synthesis of SiO2 Nanoparticles. SiO2 nanoparticles were synthesized by the well-known Stöber method.37 The Stöber method leads to spherical and uniformly sized particles in which reaction conditions can be tuned in order to obtain the desired size. In our case, the method was optimized to obtain particles of around 80 nm in size. Typically, a mixture of 200 mL of ethanol, 35 mL of Milli-Q water, and 7.5 mL of NH4OH 25% was prepared in a reaction flask and set to 65 °C with a condenser. In the reaction flask was added 11.16 mL of tetraethyl orthosilicate (TEOS, 0.11 mol) dropwise, and the reaction mixture was stirred for 60 min. After this reaction time, the SiO2 NPs were separated

qe (mg/g) = 13939

(C0 − Ce) V m DOI: 10.1021/acs.inorgchem.7b02056 Inorg. Chem. 2017, 56, 13938−13948

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Inorganic Chemistry

Figure 1. Synthesis scheme for the surface functionalization of nanoadsorbents.

Figure 2. AFM, SEM, and TEM images for pure SiO2 matrix (the first row), derived hybrid adsorbent material loaded with REE (second row), DTPAderived magnetic hybrid adsorbent loaded with REE (third row), and TTHA-derived magnetic hybrid adsorbent loaded with REE (fourth row).

where C0 and Ce (mg/mL) are respectively the initial REE concentration and the REE concentration in solution after contact with the

nanoadsorbents and m (g) represents the mass of nanoadsorbent used and V (mL) the volume of solution. 13940

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Inorganic Chemistry Table 1. Summary of the FTIR Spectra of SiO2 and Functionalized SiO2 Nanoparticles sample SiO2 nanoparticles

SiO2-EDTA nanoparticles

SiO2-DTPA nanoparticles

SiO2-TTHA nanoparticles

absorption peak location (cm−1)

peak identification

1090 (saturated) 950 800 460 1634 1404 1250−1020 1636 1728 1397 1250−1020 1639 1734 1250−1020

νas(Si−O−Si) νas(Si−OH) ν(Si−O−Si) δ(Si−O−Si) ν(CO) (secondary amide) δ(O−H) (carboxylic acid) ν(C−N) (overlaps with saturated νas(Si−O−Si)) ν(CO) (secondary amide) ν(CO) (carboxylic acid) δ(O−H) (carboxylic acid) ν(C−N) (overlaps with saturated νas(Si−O−Si)) ν(CO) (secondary amide) ν(CO) (carboxylic acid) ν(C−N) (overlaps with saturated νas(Si−O−Si))

Synthesis of Molecular Model Compound with Dy3+ and TTHA. TTHA (100 mg, 0.2 mmol) was dissolved in 5 mL of Milli-Q water and heated until complete dissolution. Dy(NO3)3 (70.4 mg, 0.2 mmol) was then added. The solution was brought to neutral (pH 6.50) via dropwise addition of a NH4OH 5% solution and stirred overnight at room temperature. The resulting solution was left in open air for 2 weeks until crystals were formed. Characterization. Crystallographic data collection for a single crystal of the Dy3+-TTHA molecular model was carried out at room temperature using MoKα radiation (λ = 0.71073 Å) with a Bruker SMART Apex-II CCD diffractometer. A total of 5579 reflections (Rint = 0.0327) were collected by a series of ω scans with completeness of 99.6% to 2θ = 50.25°. Data for C18H45DyN8O19 are as follows: Mr = 840.10 Da, monoclinic, space group P21/c, a = 10.5791(15) Å, b = 12.0984(18) Å, c = 24.489(4) Å, β = 90.957(3)°, V = 3133.9(8) Å3, Z = 4. The structure was solved by direct methods. The positions of metal atoms were identified from the initial solution, and all other non-hydrogen atoms were located in difference Fourier syntheses. All non-hydrogen atoms were refined first in isotropic and then in anisotropic approximation. The hydrogen atoms of the ligand were added to the refinement in isotropic approximation with coordinates calculated geometrically. The hydrogen atoms of ammonium and oxonium cations were found in difference Fourier syntheses and added to the refinement in isotropic approximation, fixing their thermal parameters as 1.5 of the isotropic value of the non-hydrogen atoms they are bound to. The hydrogen atoms of interstitial water molecules could not be located unequivocally from the room-temperature experiment. Final discrepancy factors were R1 = 0.0311 and wR2 = 0.0788 for 5178 reflections with I > 2σ(I) and R1 = 0.0335 and wR2 = 0.0799 for all 5579 reflections. Full details of structure solution and refinement are available free of charge on request from the Cambridge Crystallographic Data Centre at www.ccdc.cam.ac.uk, citing the reference number CCDC 1568126. SEM-EDS analyses were performed using a Hitachi TM-1000-μ-DEX scanning electron microscope. For each sample, in EDS analyses at least five different areas were studied and the average value was calculated and given as the relative content of the elements. Fourier transform infrared (FTIR) spectra of the functionalized nanoadsorbents were recorded as KBr pellets on a PerkinElmer Spectrum 100 instrument. Thermogravimetric analysis (TGA) was carried out using a PerkinElmer Pyris 1 instrument in an air atmosphere at a heating rate of 5 °C/min in the 25−900 °C interval. pHs of the applied solutions were controlled by a pH meter (potentiometrically).

polycarboxylic acids were attached onto the surface of the nanoadsorbents, helped by a prior amination of the nanoadsorbents by aminopropyltriethoxysilane (APTES), as indicated in the reaction schemes in Figure 1. The figure pictures a scheme of the synthetic routes for SiO2 nanoparticles, given that the whole synthesis was analogous for magnetic SiO2 nanoparticles. Functionalization did not influence the morphology or size of the particles, as determined in the AFM studies (see Figure 2 and Figure FS1 in the Supporting Information). The FTIR spectra of the surface-functionalized nanoadsorbents are presented on the Figures FS3 and FS4 in the Supporting Information. Red spectra correspond to the amino polycarboxylic acid used in its pure form, whereas the black spectra represent the SiO2 nanoparticles functionalized with the corresponding ligand. In all three cases, the functionalized SiO2 NPs show the characteristic peaks of the SiO2 network, appearing at around 1090 (saturated in order to allow better resolution for less intense bands), 950, 800, and 460 cm−1, attributed to νas(Si− O−Si), νas(Si−OH), ν(Si−O−Si), and δ(Si−O−Si), respectively. The presence of several characteristic bands from the amino polycarboxylic acid in the functionalized nanoadsorbents gave evidence of their efficient grafting. In EDTA-functionalized nanoparticles (top), the characteristic ν(CO) band from the secondary amide group appears at 1634 cm−1 and the δ(O−H) band from carboxylic acid appears at 1404 cm−1, whereas the C− N stretching frequency can be observed in the pure EDTA spectrum at 1254 cm−1. DTPA-functionalized NPs (middle) display the ν(CO) band from the secondary amide at 1636 cm−1, and another ν(CO) band from carboxylic acid can be observed at 1728 cm−1; the δ(O−H) band from carboxylic acid appears in this case at 1397 cm−1. It should be noted that the band at 1819 cm−1, attributed to ν(CO) from anhydride, can be distinguished in the DTPA spectrum (as the product used was the dianhydride of the compound), but not in the SiO2 functionalized nanoparticles, as a result of the anhydride hydrolysis in the grafting reaction. TTHA-functionalized NPs show the ν(CO) band from the secondary amide at 1639 cm−1 and a less intense peak at 1734 cm−1, attributed to ν(CO) from carboxylic acid, while the δ(O−H) band from carboxylic acid can be observed at 1400 cm−1. In all three cases, the ν(C−N) band, which generally appears around 1250−1020 cm−1, cannot be easily distinguished in the functionalized SiO2 NPs since it overlaps with the saturated νas(Si−O−Si) peak from the silica network. Table 1 summarizes the most characteristic peaks that

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RESULTS AND DISCUSSION Preparation and Characterization of the Nanoadsorbent Materials. The nano carriers were synthesized by the modified Stöber method as described earlier in ref 26, producing uniform spherical silica particles with the size 80 ± 5 nm, which were separated and applied for further functionalization. Amino 13941

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Inorganic Chemistry can be observed and the functional groups that they are attributed to. Thermogravimetric analyses of the three functional nanoadsorbents were performed in the temperature range 25−900 °C. The thermal curves are shown in Figure FS2 in the Supporting Information. All of the samples revealed adsorbed water molecules, responsible for weight losses of around 2.75−6.3% up to 160 °C. Afterward, the organic molecules decomposed in the range 160−500 °C, followed by a last step of carbonization up to 900

Table 3. Average Maximum Adsorption Capacity (qe, in mg of REE/g of adsorbent) for Each Nanoadsorbent and REE and Standard Deviation of the Average from at Least Three Independent Measurements max qe (mg/g)

Table 2. Amounts of EDTA, DTPA, and TTHA Grafted (wt % and mmol/g) onto the Nanoadsorbents SiO2-EDTA SiO2-DTPA SiO2-TTHA

amt grafted (weight %)

amt grafted (mmol/g)

10.6 16.0 15.2

0.33 0.38 0.29

sample

La

Nd

Dy

SiO2-EDTA MNP-EDTA SiO2-DTPA MNP-DTPA SiO2-TTHA MNP-TTHA

132.8 ± 4.9 127.8 ± 5.5 171.8 ± 4.5 163.4 ± 5.9 222.5 ± 5.8 209.2 ± 6.7

213.9 ± 9.5 208.5 ± 8.6 299.9 ± 8.2 292.9 ± 7.5 219.9 ± 5.9 213.1 ± 5.8

300.4 ± 10.3 269.9 ± 9.5 168.6 ± 6.5 186.9 ± 5.9 173.3 ± 5.7 164.2 ± 5.2

investigated the coordination mechanism of iminodiacetic acid (IDA) with REEs and showed that it occurs only via concerted action, not chelation, and on that basis the observed selectivity toward HREEs could be explained. The ratio of REE to IDA was in that case rather exactly equal to 1:1. The higher ratio of REE of L on adsorption when more carboxylate groups are attached to the ligands also indicates a kind of monolayer formation, where the charge of the adsorbed cations is compensated apparently by anions, in this case nitrate, as clearly demonstrated by the IR spectra of the adsorbents loaded with REEs at neutral pH (with roughly 2 NO3− per each REE cation adsorbed). Very distinct bands at 1390 and 830 cm−1 typical for nitrate appear in the spectra (see Figure FS3 in the Supporting Information). Such adsorption indicates that no chelation can occur, and the uptake under these conditions is based on electrostatic interactions on the surface of the particles. The enhanced ratio of REE to L was also observed recently for a different kind of EDTA-derived ligand grafted on the different nanoadsorbents silica, titania, and iron oxide,25 showing that adsorption in a monolayer via concerted action (and not chelation) at neutral pH can constitute a general trend. The selectivity on adsorption is quite low. In contrast, on partial desorption of REE at pH 3−4, where the ratio of REE to L decreases to approximately 1:1, the selectivity strongly increases. The decrease in pH facilitates the exchange kinetics of the ligands in complexes and opens apparently for more specific binding. Considering the data, sorted by type of ligand grafted on the nanoadsorbents (upper row), some insights

°C. These results provided a clear evidence of the attachment of EDTA, DTPA, and TTHA onto the surface of the nanoadsorbents. Table 2 shows the amount of ligands grafted in wt % and in mmol/g. Adsorption Isotherms with REE (La3+, Nd3+, Dy3+) Ions: Influence of Different Ligands. Adsorption isotherms in the concentration range of REE from 0 to 10 mM were performed with three different REE cations (Dy3+, Nd3+, and La3+) in magnetic and nonmagnetic SiO2 nanoparticles functionalized with the three amino polycarboxylic acids (EDTA, DTPA, TTHA) studied. Figure 3 shows the obtained isotherms. To facilitate the discussion, results have been plotted in two different types of graphs, one sorted by REE cation adsorbed and the other sorted by ligand on the adsorbent, and Table 3 shows the maximum adsorption capacity (max qe) in mg/g for each nanoadsorbent and REE obtained by complexometric titration. The data show the high adsorption efficiency of the nanoadsorbents produced, with uptake values of up to 300 mg of REE/g. The amount of adsorbed REE is rather high and reaches the value of 1 REE per carboxylate group, thus achieving REE to EDTA ratios of about 3:1, REE of DTPA ratios of about 4:1, and REE to TTHA ratios of 5:1. In our previous work,26 we

Figure 3. REE adsorption isotherms on the six different functional nanoadsorbents tested. The top row (A) shows the data sorted by type of ligand grafted, and the bottom row (B) shows the data sorted by REE cation adsorbed. 13942

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Inorganic Chemistry Table 4. EDS Analysis of Functional SiO2 Nanoadsorbents with Ternary and Quinary Mixtures of REEa

a

sample

%Al

%Si

SiO2-EDTA (La3+, Nd3+, Dy3+) SiO2-EDTA (La3+, Nd3+, Dy3+, Sc3+, Eu3+) SiO2-DTPA (La3+, Nd3+, Dy3+) SiO2-DTPA (La3+, Nd3+, Dy3+, Sc3+, Eu3+) SiO2-DTPA (La3+, Nd3+, Dy3+, Y3+, Eu3+) SiO2-TTHA (La3+, Nd3+, Dy3+) SiO2-TTHA (La3+, Nd3+, Dy3+, Y3+, Eu3+)

0.43 0.24 0.42 0.3 0.58 0.52 0.44

70.7 96.4 61.2 91.9 69.3 69.3 71.8

%Sc

%Y

0.74 2.28 0.92 9.56

%La

%Nd

6.1 0.2 12.7 0.75 5.72 17.2 5.4

6.8 0.65 12.5 0.57 6.1 7.15 4.84

%Eu 0.41 1.35 8.38 4.23

%Dy 16 1.36 13.1 2.88 9 5.84 3.76

The mixture of REE used is specified in parentheses after each sample.

Table 5. Selectivity Trends in Desorption for the Three Nanoadsorbents under Different Conditions (pH 3 and pH 1), Expressed in Metal Molar Ratios adsorption SiO2-EDTA SiO2-DTPA SiO2-TTHA

desorption pH 3

desorption pH 1

total uptake capacity (mmol/g)

Dy:Nd

Dy:La

Dy:Nd

Dy:La

Dy:Nd

Dy:La

1.85 1.98 1.54

2.4:1 1.1:1 1:1.2

2.6:1 1.03:1 1:2.9

3.6:1 1:12 1:2.5

76:1 1:1.9 1:69

1.6:1 1:1.2 1:1.4

1.9:1 1:1.1 1:1.7

ternary mixtures and Y3+ in quinary mixtures. This agrees with the REE uptakes presented in the adsorption isotherms and with the structural studies which will be further displayed. The nanoadsorbents saturated with ternary mixtures of REEs were then subjected to desorption at pH 3 (selective desorption conditions) and pH 1 (total desorption conditions). The results are summarized in Table 5, given in metal molar ratios as obtained from EDS analysis. Selectivity in the desorption process at pH 3 becomes quite apparent here. For EDTA-functionalized nanoadsorbents, the separation factor is not very significant for the Dy:Nd ratio, but it is very pronounced for the Dy:La ratio, showing that the adsorbents have more affinity to Dy and therefore retain this metal in higher proportion, desorbing other metals. DTPAfunctionalized nanoadsorbents show a significant separation factor for Dy:Nd mixtures, and TTHA-functionalized nanoadsorbents display the trend opposite to the EDTA-functionalized nanoadsorbents, with an separation factor of 69 for La in the Dy:La ratio. At pH 1, the harsher conditions lead to total desorption and therefore no significant selectivity. Crystal Structure of the Synthesized Dy-TTHA Molecular Model Compound. The complex produced by crystallization from a solution of dysprosium nitrate and H6TTHA with pH 6.5 adjusted by dropwise addition of aqueous ammonia featured the formula (NH4)3[HDyTTHA](NO3)·4H2O (see Figure 4). Its structure bears apparent resemblance to those of heavier REE (Dy, Gd, Tb, Ho, Er) complexes with guanidinium as counterion reported by Ruloff et al. from the synthesis starting with REE carbonates and guanidinium carbonate at pH ∼8.44 The dysprosium atoms are nonacoordinated and bound to all nitrogen atoms in the ligand and to five single-bonded carboxylate oxygen atoms of the groups involved in complexation. The sixth group is unbound and protonated. This is a clear difference from the derivatives of light REEs, where the coordination number is 10 and all carboxylate groups are normally involved in complexation at neutral pH.45 It is interesting to note that in analogy with the Nd derivative reported in ref 41, which contained 2.5 equiv of NaClO4 for each complex anion, the structure here results from cocrystallization with a monovalent cation salt: i.e., 1 equiv of NH4NO3. The coordination of the central Dy atom can be described as a monocapped Archimedes antiprism, where the N(4) atom is

into selectivity (especially on desorption) can be clearly observed. EDTA-grafted nanoadsorbents show higher affinity for heavy (small) REE catios, the order of affinity thus being Dy > Nd > La. DTPA-grafted nanoadsorbents show higher affinity toward medium REE cations. This can be observed by the fact that the maximum uptake for Nd3+ in these types of adsorbents almost doubles the maximum uptake for La3+ and Dy3+. TTHAgrafted nanoadsorbents present higher affinity toward lighter (larger) REE (La3+) ions. The reasons for this observed selectivity (see below) lie in the differences in coordination of the three amino polycarboxylic acids with REE in a 1:1 ratio, when the chelation becomes supposedly a principal mode of interaction. In the case of EDTA, there is a combination of concerted action of the carboxylic groups and chelation,41 but the trend in selectivity toward HREEs remains. For DTPA, coordination occurs then only via chelation; the affinity is then guided by the stability constants and the adsorbents become more efficient toward medium REEs.42 Previous studies showed that the stability constant of the Ln-DTPA complexes increases from La3+ to Eu3+ and then remains more or less constant.28 Finally, in the case of TTHA, the coordination, also via chelation only, is pH- and element-dependent and the TTHA-grafted nanoadsorbents have clear preference for LREEs.43 Selectivity with Ternary and Quinary Mixtures: EDS Analysis. Energy dispersion spectroscopy (EDS) was used to analyze the composition of the different functional nanoadsorbents after adsorbing mixtures of three or five different REEs. Given the fact that some peaks for REE overlap with the peak corresponding to Fe and therefore would hinder the analysis, only nonmagnetic SiO2 based materials were used for this analysis. Table 4 shows the data obtained from at least five measurements in different areas of the sample and averaged. It is worth mentioning that EDS is able to provide only surface and local information; thus, although several measurements were taken and averaged for good statistics, the results are not as representative of the whole sample in comparison to those from complexometric titration, for example. Nevertheless, in some of the cases we can clearly observe selectivity toward a specific REE. For example, EDTA-functionalized nanoadsorbents present a distinctly higher affinity toward HREEs, particularly for Dy3+ in ternary mixtures, and TTHA-functionalized nanoadsorbents show higher affinity toward LREEs, particularly toward La3+ in 13943

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Inorganic Chemistry

Even for Ho the coordination number already increases to 9 and all the lighter REEs show the [M(EDTA)(H2O)3]− anionic units bound then into dimers via coordination to two alkali-metal cations (see Figure 5).46−48 The decrease in pH causes a very different structure to emerge: the absence of negative charge on the chelates leads to their aggregation and much denser packing (from 9 to 14 Å between REE atoms in anionic complexes to about 6.5 Å in the “acidic” complexes formed on interaction of REE2O3 with 1 equiv of EDTA under hydrothermal conditions).49 Part of the carboxylate groups become bridging between the chelating units. Water is no longer capable of competing with the carboxylate groups and is not incorporated into the inner coordination sphere of the cations. It is important to note that just as has recently been observed for the iminodiacetic acid (IDA) complexes26 and for anionic species with EDTA, also for these acidic or neutral forms there is apparent structural resolution with a lower coordination number of 8 for heavier and smaller REEs such as Dy and Sm,50 while lighter and larger REEs such as Nd51 and La52 feature coordination number 9. The distance of about 6.3−6.6 Å between the REE cations in these structures fits very well with the distances between the grafting sites of the ligands on the surface of sol−gel silica.26 It can be thus deduced that when an acidic EDTA function is grafted onto nano silica and the adsorption occurs under neutral conditions, the observed better capacity toward smaller and heavier REEs is very logical. A contribution from other ligands to coordination of each particular chelated center is compulsory, causing “concerted ligand action” derived selectivity, as it was described in ref 26 for the IDA-derived adsorbents. It is important to note that the lanthanide contraction is not always manifested in the difference in coordination numbers or coordination geometries. The discussion of REE complexes with complexonate ligands and especially EDTA in the literature very often assumes that the coordination is the same and the selectivity should be relatively low and be caused only by the change in the cation radii. This is correct for relatively basic conditions (pH ≥6.5). The situation is rather opposite, however, at lower pHs of below 6 and then down to 4.5, where the difference in coordination numbers and coordination geometries for REEs can be quite considerable, explaining the observed enhanced selectivity. The complexes of REE with DTPA have never been synthesized in relatively acidic medium, but rather at a pH of about 8 or a quite high pH of over 9. In our attempts to crystallize the compounds at pH 6 and below, we consequently obtained only the crystals of the DTPA acid itself. In basic media all REEs

Figure 4. Molecular structure of the complex anion in (NH4)3[HDyTTHA](NO3)·4H2O.

situated over the “upper” square plane defined by O(5), O(11), O(29), and N(3) atoms, resembling that reported for a Ho complex (NH4)3[Ho(TTHA)]·5H2O obtained in a basic medium (pH >8).46 Molecular Insights into Selective Action of Surface Complexonate Functions. Observed differences in capacity of adsorbents toward different REE cations and different selectivities related to them can be understood by exploiting the insight into the bonding of REE to different complexonate ligands on the surface. The latter can be obtained with some restrictions from the X-ray single-crystal structures of the complexes between REEs and nongrafted complexonate molecules. It is necessary, however, to keep in mind that grafting will further reduce the number of carboxylic groups in each ligand by 1, converting EDTA into diaminotricarboxylic acid, DTPA into triaminotetracarboxylic acid, and TTHA into tetraminopentacarboxylic acid moieties. The crystal structures of all complexonates distinctly change also with pH, which will have considerable consequences for bonding on the surface of the adsorbents. The REE complexes with EDTA feature at pH >8 single anionic chelate structures. The coordination number is in this case 8 for the smallest (and heaviest) REEs, starting with Er, which like ytterbium forms [M(EDTA)(H2O)2]− complexes.47

Figure 5. Crystal structures of Na[Dy(EDTA)(H2O)3]·5H2O (left) and (H3O)[Dy(EDTA)] H2O (right).45 13944

DOI: 10.1021/acs.inorgchem.7b02056 Inorg. Chem. 2017, 56, 13938−13948

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Inorganic Chemistry

Figure 6. Crystal structures of K2[Yb(DTPA)(H2O)]·7H2O53 (left) and (NH4)4[Dy2(DTPA)2]·8H2O54 (right).

Figure 7. Crystal structures of K3[Dy(TTHA)]·5H2O (right) and K4[Tb2(HTTHA)2]·14H2O (left).57

starting from Sm58 until Yb59 all display nonacoordination, as one carboxylate group is not attached to the central cation. The coordination polyhedron is in this case a monocapped Archimedes antiprism. For structures of complexes obtained at basic pH, this group is deprotonated and, bearing a negative charge, is coordinated to the counterion, while at neutral pH such as in the structure determined in this work it is protonated and uncharged. An interesting transformation of the structures occurs when they are forced to form at lower pH for REEs Nd− Yb.60,61 One of the nitrogen atoms becomes protonated and the coordination is completed to 9 with the help of two carboxylate oxygen atoms from the ligand chelated in a neighboring complex (see Figure 7). The coordination polyhedron transforms into a tricapped trigonal prism. It has to be mentioned that the structure with Nd requires a much more acidic medium (pH Nd > La, for DTPA-functionalized materials Nd > La > Dy, and for TTHA-functionalized materials La > Nd > Dy. The selectivity toward REEs was further studied with EDS analysis on ternary and quinary mixtures of REEs and further desorption of REE mixture loaded nanoadsorbents at pH 3 and pH 1. EDS analysis showed clear selectivity, especially toward Dy3+ in EDTA-

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02056. Additional data including illustrations of the details of AFM investigation, TGA studies, and FTIR characterization of both ligand-modified particles and the particles after uptake of rare-earth elements and a comparison of the FTIR spectra of free and grafted ligands (PDF) Accession Codes

CCDC 1568126 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via 13946

DOI: 10.1021/acs.inorgchem.7b02056 Inorg. Chem. 2017, 56, 13938−13948

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www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for E.P.L.: [email protected]. *E-mail for G.A.S.: [email protected]. ORCID

Elizabeth Polido Legaria: 0000-0002-8000-7290 Vadim G. Kessler: 0000-0001-7570-2814 Gulaim A. Seisenbaeva: 0000-0003-0072-6082 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the FP-7 EU Program Grant No. 309373 “Development of a Sustainable Exploitation Scheme for Europe’s Rare Earth Deposits” (EURARE). Project web site: www.eurare.eu. This publication reflects only the author’s view, exempting the Community from any liability. The authors are grateful to Arnaud Duperron for valuable help in the preparation and characterization of the hybrid nanoparticles and to the University of Clermont Auvergne for providing A.D. an internship for working at SLU.

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