Selective Adsorption of Rare Earth Elements over Functionalized Cr

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Selective adsorption of rare earth elements over functionalized Cr-MIL-101 Yu-Ri Lee, Kwangsun Yu, Seenu Ravi, and Wha-Seung Ahn ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07130 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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ACS Applied Materials & Interfaces

Selective adsorption of rare earth elements over functionalized Cr-MIL-101 Yu-Ri Lee, Kwangsun Yu, Seenu Ravi, and Wha-Seung Ahn* Department of Chemistry and Chemical Engineering, Inha University, Incheon, Republic of Korea

*Corresponding author: [email protected]

ABSTRACT: Efficient rare earth elements (REEs) separation and recovery are crucial to meet the ever-increasing demand for REEs extensively used in various high technology devices. Herein, we synthesized a highly stable chromium-based metal-organic framework (MOF) structure, Cr-MIL-101, and its derivatives with different organic functional groups: MIL-101-NH2, MIL-101-ED (ED: ethylenediamine), MIL-101-DETA (DETA: diethylenetriamine), and MIL-101-PMIDA (PMIDA: N(phosphonomethyl) iminodiacetic acid), and explored their effectiveness in the separation and recovery of La3+, Ce3+, Nd3+, Sm3+, and Gd3+ in aqueous solutions. The prepared materials were characterized using various analytical instrumentation. These MOFs showed increasing REE adsorption capacities in the sequence MIL-101 < MIL-101-NH2 < MIL-101-ED < MIL-101-DETA < MIL-101-PMIDA. MIL-101-PMIDA showed superior REE adsorption capacities compared to other MOFs, with Gd3+ being the element most efficiently adsorbed by the material. The adsorption of Gd3+ onto MIL-101-PMIDA was examined in detail as a function of the solution pH, initial REE concentration, and contact time. The obtained adsorption equilibrium data were well represented by the Langmuir model and the kinetics were treated with a pseudo-second-order model. A plausible mechanism for the adsorption of Gd3+ on MIL-101-PMIDA was proposed by considering the surface complexation and electrostatic interaction between the functional groups and Gd3+ ions under different pH conditions. Finally, recycling tests were carried out, and demonstrated the higher structural stability of MIL-101-PMIDA during the five adsorption–regeneration runs.

KEYWORDS: Rare earth elements; Metal-organic frameworks; Organic-functionalization; Adsorption equilibrium; Adsorption kinetics; Structural stability.

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1. INTRODUCTION Rare earth elements (REEs) are core components in wind turbines, batteries, electric car engines, and communication devices, and their industrial demand is rising sharply.1 However, the limited deposits of REE-containing minerals, and the control of their supply by China, their major producer, make the separation and recycling of REEs present in industrial and urban wastes increasingly important.2 Various methods have been employed industrially for the separation and recovery of REEs from aqueous solutions, such as solvent extraction,3 electrochemical extraction,4 ionexchange,5 and chemical precipitation.6 However, most of these methods require high chemical and energy consumption, making them costly, environmentally hazardous, and inefficient for handling low concentrations of REEs.7 The commonly used solvent extraction method, for example, requires a large volume of organic solvent for multiple sequential extraction steps, which produces a significant amount of undesired and radioactive wastes. Therefore, the development of cost-effective and environmentally friendly alternatives for the recovery of REEs is desirable. Adsorption has been recognized as a green alternative for REE recovery owing to its simple operation, low cost, reusability, high efficiency, and relatively low secondary waste generation.8,9 Various adsorbents such as porous carbon,10 silica-based materials,11-13 metal oxide,14,15 and ionic imprinted film16 have been used. Since REEs are present typically at ppm levels in acidic aqueous solutions containing many other impurities,17 adsorbents for REEs should have adequate stability under aqueous acidic conditions and should show high selectivity for REEs over other competing transition metal ions. These adsorbents usually consist of a porous inorganic or organic support supplemented by surface functionalization with organic groups that interact with the specific REE ions. Since REEs are Lewis acids, 2


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oxygen- and phosphorous-based functionalities such as phosphonic acid, amides, carbonyl groups, and nitrogen containing ligands (Lewis bases) have been used as efficient chelating ligands with a high affinity toward REEs.18,19 Furthermore, it has been reported that different donor species coexisting within the same ligand may generate a synergistic effect on the ligand for selective binding of REEs.20,21 The stability of the adsorbent thus depends on not only the stability of the support material itself, but also that of the linkage between the support and the functionalized active species that interacts with the REE ions in order to prevent leaching and structural collapse during REE adsorption experiments.22 Metal organic frameworks (MOFs) are a relatively new class of organic-inorganic hybrid materials comprised of metal clusters interconnected by a rigid organic linker. MOFs have been tested for a variety of applications including gas storage and separation, heterogeneous catalysis, drug delivery, and chemical sensing.23-25 Recently, there has also been growing interest in applying MOFs as adsorbents in aqueous environments.26-29 Compared to traditional adsorbents, which show low REE adsorption capacities because of their nonuniform pore size and low surface areas, MOFs have several advantages, including high surface area, adjustable pore size, and the ability to introduce diverse functionalities by simple post-synthesis modifications. Several MOFs, including MIL-101, UiO-66, and MOF76, also exhibit high stability under long-term exposure to acidic aqueous conditions.30-35 MIL-101 has been reported to possess a large surface area with two types of mesopores (ca. 2.9 and 3.4 nm) with free openings of ca. 1.2 and 1.6 nm, respectively.33 These openings are sufficiently large to promote the transportation of metal ions into the pores and to enable the incorporation of additional organic ligands to bind the target metal ions via postfunctionalization methods.36 After removing the terminal solvent molecules on the metal nodes by heating in a vacuum, the resulting coordinatively unsaturated metal sites (CUS) can

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also offer accessible sites for further functionalization. Despite these promising properties of MOFs for adsorption, only two cases of the adsorption of REEs by MOFs have been reported. Decker et al. prepared a carbamoylmethylphosphine oxide-functionalized MIL-101 via a complex multi-step preparation method, and tested it for the adsorption of several REE ions;22 this MOF was found to be most efficient for the adsorption of U(VI) ions.37-39 Jiang et al. reported the adsorption of La3+ ions using ZIF-8 and ZIF-90.40 However, both these studies reported relatively low adsorption capacities of the MOF materials towards REEs, along with sluggish kinetic behavior. Further improvement is necessary to explore the potential of MOFs for REE separation by employing a more effective organic functional group on the MOFs via a simple preparation scheme. In this work, MIL-101 and a series of derivatives functionalized with different organic groups (MIL101-NH2, MIL101-ED, MIL-101-DETA, and MIL-101-PMIDA) were prepared by both direct and post-synthetic modification methods, and were systematically investigated for their adsorption behavior towards REEs (La3+, Ce3+, Nd3+, Sm3+, and Gd3+). A series of Nbased ligands and N-(phosphonomethyl) iminodiacetic acid (PMIDA), which incorporates several Lewis bases, were tested for REE separation. The adsorption equilibrium and kinetics were determined by varying the solution pH, contact time, and initial concentration of the REEs. The adsorption selectivity of the MOFs in the presence of competing ions and the reusability of the adsorbents were also investigated.

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2. EXPERIMENTAL SECTION

Chemicals and reagents. Chromium(III) nitrate (Cr(NO3)3·9H2O, 99%), 1,4benzenedicarboxylic acid (H2BDC, 98%), 2-aminoterephthalic acid (H2BDC-NH2, 99%), ethylenediamine (ED, 99.5%), diethylenetriamine (DETA, 99%), N-(phosphonomethyl) iminodiacetic

acid

(PMIDA,

95%),

N,N’-dicyclohexylcarbodiimide

(DCC,

99%),

hydrofluoric acid (HF, 47 % aq.), ammonium fluoride (NH4F, 99.9%), N,Ndimethylformamide (DMF, 99%), and toluene (anhydrous, 99.8%) were purchased from Sigma-Aldrich. Aqueous rare-earth stock solutions for the adsorption experiments were prepared from the corresponding REE salts: La(NO3)3·6H2O (99.9%), Ce(NO3)3·6H2O (99.9%), Nd(NO3)3·6H2O (99.9%), Sm(NO3)3·6H2O (99.9%), and Gd(NO3)3·6H2O (99.9%). The transition metal precursors, Ni(NO3)2·6H2O (99.9%), Co(NO3)2·6H2O (99.9%), Zn(NO3)2·6H2O (98%), Al(NO3)2·9H2O (99.9%), and Fe(NO3)3·9H2O (99.9%), were supplied by Sigma-Aldrich. All chemicals were used as-received without further purification. Preparation of adsorbent materials Synthesis of MIL-101. MIL-101 was prepared using the ligand H2BDC (Fig. 1(a)) following a previously reported procedure.41 The synthesis details are provided in the supplementary SI-1. Preparation of MIL-101-NH2. MIL-101 with an amine-functionalized ligand was prepared directly using the ligand H2BDC-NH2 (see Fig. 1 (b)). The synthesis details are given in SI-1. Preparation of MIL-101-ED and MIL-101-DETA. The functional groups ED- and DETA- were grafted to the chromium sites in MIL-101 post-synthetically (Fig. 1 (c) and (d)). Details are provided in SI-1. 5



Preparation of MIL-101-PMIDA. The synthetic sequence for MIL-101-PMIDA is shown in Scheme 1. Based on the amount of nitrogen in the MIL-101-NH2 after activation at 353 K for 12 h, a MIL-101-NH2/PMIDA/DCC mixture with a molar ratio of 1:1.5:2 was prepared in 80 mL of DMF in a round-bottom flask. DCC was used as the catalyst for the reaction between the grafted NH2 in MIL-101 and the PMIDA groups. The reaction mixture was stirred under reflux at 150 °C for 48 h. The product was filtered and washed with toluene and methanol, and dried at 373 K for 12 h. Characterization of materials. The experimental details of the characterization work carried out for the prepared MOFs are given in SI-2. REE adsorption experiments. A series of adsorption experiments were carried out to examine the adsorption equilibrium and its pH-dependence, adsorption kinetics, ion selectivity, and reusability at 25 °C in batch mode. Adsorption equilibrium isotherms. Stock solutions of the REEs (La3+, Ce3+, Nd3+, Sm3+, and Gd3+) were prepared and diluted to a final concentration of 100 ppm for each element. Initially, 15 mg each of the five adsorbents (MIL-101, MIL-101-NH2, MIL-101-ED, MIL-101-DETA, and MIL-101-PMIDA) was added to 15 mL of the stock solutions for each element, and the solutions were stirred at 150 rpm for 6 h to examine the general affinity of the adsorbents for the REEs. After each adsorption run, the adsorbents were separated from the solution using a 0.45 µm PET syringe filter, and the concentrations of initial and filtered solutions were measured by ICP–OES. The adsorption capacities (qe, mg/g) were calculated using equation (1):42

qe = (Ci − Ce ) ×V m ……………… (1) where Ci and Ce are the initial and equilibrium metal concentrations (mg/L) in the solution,

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respectively, V is the volume of the solution (L), and m is the mass of the adsorbent (g). The effect of the pH on the adsorption equilibrium was then examined using Gd3+ (which showed the highest affinity in the initial screening described above) at a concentration of 100 mg/L over the pH range of 2–6 at 25 °C. The pH of the Gd3+ solution was adjusted to the desired value using either dilute 0.1 M HCl or NaOH solution. Alkaline conditions were avoided because of the formation of REE hydroxides.43 To obtain the adsorption isotherms, 15 mg of MIL-101-PMIDA or MIL-101-DETA (that showed the higher adsorption capacities than others) was added to 15 mL of Gd3+ solutions with concentrations ranging from 20–150 ppm at the optimized pH of 5.5, and stirred for 6 h to reach equilibrium. The concentrations of the initial and filtered solutions were measured by ICP–OES. The affinities of the adsorbents towards a given metal ion were estimated by the distribution coefficient value (Kd, mL/g) calculated using equation (2):

K d = ((Ci − C f ) C f ) × (V m) ……………… (2) where Ci is the initial metal ion concentration in the aqueous solution (mg/L), Cf is the final metal ion concentration in the aqueous solution (mg/L), V is the volume of the solution (mL), and m is the mass of the adsorbent (g). Adsorption kinetics. The absorption kinetics were only measured for Gd3+. The kinetics measurements were carried out using 15 mL of 100 ppm Gd3+ solution over 15 mg of MIL101-PMIDA with contact times of 10–240 min at pH 5.5. Adsorption selectivity. 15 mL of a solution containing a mixture of Gd3+ and transition metal ions (Al3+, Fe3+, Ni2+, Zn2+, and Co2+) at a concentration of 50 ppm each, was tested over 15 mg of MIL-101-PMIDA for 6 h at pH 5.5.

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Reusability tests. Reusability runs were conducted by initially saturating the adsorbents (MIL-101-DETA and MIL-101-PMIDA) with a 100 ppm Gd3+ solution at pH 5.5. After adsorption, the adsorbents were filtered from the suspension and dried under vacuum, and the Gd3+ concentration of the filtrate was analyzed. Regeneration of the adsorbent was then carried out by shaking it in 0.1 M HCl for 2 h.

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3. RESULTS AND DISCUSSION Materials characterization. The series of amine-grafted MIL-101 derivatives shown in Fig. 1 were prepared via two different synthetic procedures: the direct modification of the aromatic rings in MIL-101 using the amine-incorporated BDC ligand for MIL-101-NH2, or utilizing the CUS as anchoring sites for the amine ligands in MIL-101-ED and MIL-101DETA (see Supplementary Information). Scheme 1 illustrates the synthetic procedure for MIL-101-PMIDA by the post-synthesis modification of MIL-101-NH2; the amine group in MIL-101-NH2 was modified with PMIDA using DCC as a dehydrating agent, resulting in the formation of a C–N bond to obtain MIL101-PMIDA. As shown in Fig. 2, the XRD patterns of the as-prepared MIL-101, MIL-101-NH2, MIL-101-ED, and MIL101-DETA were similar to those reported previously.33 The XRD pattern of MIL-101-PMIDA was almost the same as that of the pristine MIL-101-NH2, indicating that the grafting of the PMIDA group to MIL101-NH2 caused no apparent loss of crystallinity. Fig. 3 shows the N2 adsorption–desorption isotherms, and the corresponding physicochemical properties of the obtained materials are given in Table 1. Compared to the pristine MIL-101, the functional-group grafted materials showed significantly decreased N2 adsorption, and both the BET surface area and the pore volumes of the samples decreased after grafting, from 3680 m2/g and 2.08 cm3/g for MIL-101 to 989 m2/g and 0.65 cm3/g for MIL-101-PMIDA. Although the functionalization resulted in reduction of the total pore volume, the average pore sizes of the functionalized materials were sufficient (ranging from 1.2 to 2.2 nm) to be accessed by the REE ions investigated. Fig. 4 shows the FT-IR spectra of the functionalized materials. For the amine-grafted materials, the asymmetric and symmetric N–H stretching vibrations were observed in the range of 3250–3500 cm-1,44 indicating the presence of amino groups. For MIL-101-NH2, the C–N (aromatic) stretching vibration was located at 1260 cm-1 and the split peak at ~1400 cm-

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1

indicated intermolecular hydrogen bonding between the grafted amino and carboxyl groups

in the MOF structure.18 The additional peaks in the ranges 1088–920 cm-1 and 1620–1550 cm-1 after functionalization with PMIDA were ascribed to phosphonic (–PO3H2) and carboxyl (–COOH) stretching modes, respectively, and the peaks at 2455 and 1665 cm-1 were ascribed to the P-OH and carbonyl (–C=O) stretching mode. For MIL-101-ED and MIL-101-DETA, the absorption bands attributed to the C–H (aliphatic) and C–N (aliphatic) stretching modes were observed at 2950 and 1050 cm-1, respectively, as a result of the alkylamines attached to the chromium(III) sites in the MIL-101 structure.38 The morphology and chemical composition of the materials were examined by SEM and SEM–EDS as shown in Fig. 5 and Fig. S1, respectively. All the materials showed discrete octahedral morphologies, and little changes were detected after grafting of the different functional groups. The particle sizes of MIL-101-NH2 and MIL-101-PMIDA (ca. 0.5 µm) were smaller than those of other amine-grafted materials (ca. 1 µm). The N contents of the amine-grafted materials obtained from SEM-EDS analysis were very close to those by EA (Fig. S1 and Table 1). From the EA analysis of the N and P contents in MIL-101-NH2 and MIL-101-PMIDA, it can be estimated that approximately 74 % of PMIDA group was functionalized on MIL-101-NH2 with 0.90 mmol/g of free -NH2 groups (see Table 1). Both the –NH2 and PMIDA groups are expected to contribute to the adsorption of REEs. The FTIR results also indicated that some free NH2 groups remained on MIL-101-PMIDA (see Fig. 4). REE adsorption by the prepared materials. Initially, the REE adsorption behaviors (La3+, Ce3+, Nd3+, Sm3+, and Gd3+) of MIL-101 and its functionalized derivatives were determined using an initial concentration of 100 ppm for each metal ion. As summarized in Table 2, the adsorption capacities for all the REEs increased substantially after

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functionalization of MIL-101, while pristine MIL-101 showed negligible REE adsorption. The adsorption capacities of the functionalized materials increased in the sequence MIL-101PMIDA > MIL-101-DETA > MIL-101-ED > MIL-101-NH2. Thus, we selected MIL-101PMIDA, which showed high adsorption capacity for all the REE ions tested, especially for Gd3+ (Fig. 6), as the standard adsorbent, and investigated its adsorption of Gd3+ at various pH values, initial ion concentrations, and contact times, as well as its selectivity against competing transition metal ions. Effect of solution pH on REE adsorption. Solution pH is an key parameter in metal ion adsorption, because it influences the surface charge and binding sites of the adsorbents as well as the ionization of the adsorbates.37 To analyze the surface charge states of MIL-101PMIDA under different pH conditions, their zeta potential values were measured in the pH range of 2-7, and the result is shown in Fig. S2. At pH >3, high negative charge on the surface of the MIL-101-PMIDA was detected and the negative charges increased as the solution pH value increases because of deprotonation of the carboxylic-, phosphonic-, and carbonyl groups in PMIDA and free amine groups. At lower pH values (pH < 3), on the other hand, bulk of the functional groups are protonated, which make the Gd3+ adsorption difficult. Gd3+ can be hydroxylated or formed as insoluble precipitate Gd(OH)3 under basic conditions in water (pH >7).45 To avoid the precipitation of Gd3+ at higher pH values, the pH range investigated was limited to 2−6. As shown in Fig. 7, the Gd3+ adsorption capacity of MIL-101-PMIDA increased rapidly from 10.3 to 87.6 mg/g between pH 3–5.5 at 25 °C, which is similar to the reported result of the functionalized MIL-101 materials in the absorption of U(VI).32-34 The highest adsorption capacity towards Gd3+ was achieved at a pH value of approximately 5.5. Since the adsorption proceeds by forming a coordination bonding between the Gd3+ ions and the Lewis basic sites in PMIDA, deprotonation of the phosphonic-,

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carboxyl-, carbonyl- and free amine groups in MIL-101-PMIDA at increased solution pH was expected to enhance their binding abilities for adsorption. In addition, Gd3+ ions can also interact with -NH2-OH- sites formed on the surface of MIL-101-PMIDA electrostatically at high pH.46 As expected, little adsorption took place below pH 3 due to the electrostatic repulsion between the highly protonated free amines and functional groups in PMIDA and the Gd3+ ion. Therefore, the pH value of approximately 5.5 was selected for the subsequent adsorption studies, and adsorbent regeneration was conducted at pH 3. Adsorption isotherms. The adsorption equilibrium data were fitted to either Langmuir or Freundlich models to understand the interaction between the REE ions and the adsorption sites of MIL-101-PMIDA and to estimate its adsorption capacity. The detailed description of the two isotherm models is given in section SI-3, and the fitted plots are presented in Fig. S3. The REE adsorption isotherms of MIL-101-PMIDA were fitted to the two models, and the obtained isotherm constants are summarized in Table 3. As shown in Fig. 8, MIL-101PMIDA displays typical Langmuir behavior, corresponding to monolayer adsorption on independent binding sites. This behavior is similar to previous reports of functionalized MIL101.38 The fit of the experimental data was significantly better using the Langmuir model, with values of 0.990–0.996 for the correlation coefficient R2, than that obtained using the Freundlich isotherm model (Table 3, Fig. 8, and Fig. S3), which resulted in low R2 values. The equilibrium adsorption capacities (qm) for La3+, Ce3+, Nd3+, Sm3+, and Gd3+ on MIL-101PMIDA calculated using the Langmuir model were 37.4, 49.0, 70.9, 72.7, and 90.0 mg/g, respectively (Table 3), which matched well with the experimental data (see Fig. 8). The adsorption capacities for Gd3+ on MIL-101-PMIDA and other previously reported adsorbents are summarized in Table 4. The measured capacity of MIL-101-PMIDA for Gd3+ in this work (90.0 mg/g) is among the highest of the reported adsorbents.

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Adsorption kinetics. To measure the kinetics of Gd3+ adsorption over MIL-101-PMIDA, the adsorption level was monitored at different contact times from 0 to 6 h. As shown in Fig. 9, Gd3+ adsorption equilibrium was attained within 120 min. During the adsorption process, the exterior surface first reached saturation, and the ions then diffused into the pore interior of the adsorbent to achieve equilibrium.47 The equilibrium time for MIL-101-PMIDA was in agreement with that of the reported MIL-101 material for the U(IV) ion 33, and was close to those of mesoporous silica materials.13,48 Additionally, it was much faster than the 6 h required for Fe3O4@SiO2(TMS-EDTA) nanoparticles,49 or the 24 h for ZIFs40 or nanoporous carbon.50 This relatively fast adsorption could largely be attributed to the abundant adsorption sites easily available for Gd3+ on the surface of MIL-101-PMIDA (2.66 mmol g-1 –COOH, 0.90 mmol g-1 –NH2, and 2.66 mmol g-1 –PO3H2), the mesoporous structure of MIL-101 with sufficient pore size (see Table 1), and the relatively small particle size (0.5 µm). To analyze the adsorption process of Gd3+ on MIL-101-PMIDA, pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models were used to fit the experimental kinetics data. The details of the two models and the fitted results are given in section SI-4 and Fig. S4, respectively. The parameters of the PFO and PSO models and the correlation coefficients estimated using the two models, are given in Table 5. The slope and intercept of log (qe-q) vs t and t/q vs t, respectively, were used to determine the rate constants. Based on the good fitting results with a high correlation coefficient of R2 > 0.994 and the good agreement between the experimental and estimated qe values (Fig. S4 and Table 5), the PSO model matched the adsorption kinetics of MIL-101-PMIDA toward Gd3+ quite well, which indicates that chemisorption is the rate-determining step in the adsorption process.51 MOF adsorbents with highly porous structures and functional groups on the surface probably operate in a chelate exchange process, possibly controlled by secondary chemical reactions.38

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Selectivity tests. Considering that the REE ions are commonly present with other transition metal ions in environmental and mining wastes, high selectivity of an adsorbent towards REE ions is desired for its practical applications.29 The effect of transition metal ions (Co2+, Ni2+, Zn2+, Al3+, and Fe3+) on the adsorption of Gd3+ by MIL-101-PMIDA was investigated at pH 5.5; the concentration of all the aforementioned transition metals and of the Gd3+ ions was 50 ppm each. As shown in Fig. 10, the selectivity toward Gd3+ was approximately 90 %, while those of the other transition metal ions were all less than 10 %. The selectivity to Gd3+ ions was estimated by the amount of the adsorbed Gd3+ ions divided by the amount of the total adsorbed ions. The trivalent cations Al3+ and Fe3+ were adsorbed more than the bivalent cations Co2+, Ni2+, and Zn2+. A similar trend was reported by Zhao et al.,43 who explained this result as the higher valent cations having more favorable binding with active sites in comparison to the lower valent cations. According to the HSAB (hard and soft acids and bases) theory, the phosphonic-, carboxyl-, and carbonyl groups in the PMIDA ligand belong to the hard base group,13,52 which can show high selectivity for the coordination of REE3+ ions (typical hard acids) over other competing trivalent ions. There are also residual -NH2 groups of Lewis basic character on the surface of MIL-101-PMIDA. Thus, the highly selective adsorption towards Gd3+ must be owing to a synergistic effect generated by the different donor species coexisting within the same ligand (PMIDA).20,21 Additional electrostatic interaction between -NH2-OH- and Gd3+ ions at high pH is also a contributing factor.46 Furthermore, hydroxylated species of Al predominate above pH 5.53 Thus Gd3+ showed higher adsorption than other trivalent transition metals under the conditions of this work conducted at pH 5.5. Adsorption mechanism. To examine the interaction between the adsorption sites of MIL-101-PMIDA and Gd3+, FT-IR spectra of MIL-101-PMIDA before and after Gd3+

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adsorption at pH 5.5 were measured as shown in Fig. S5. After Gd3+ adsorption on MIL-101PMIDA, the intensity of the peaks at 2445 cm-1 and in the ranges of 1088–920 cm-1, and 1620–1550 cm-1, assignable respectively to phosphonic (–PO3H2) and carboxyl (–COOH) stretching modes, were significantly reduced, suggesting that these functional groups in PMIDA were involved with the Gd3+ adsorption. In addition, their adsorption bands of the coordinated functional groups shifted slightly toward the low wave number as a result of the interaction between the functional groups in PMIDA and Gd3+ ion.35,46 Additionally, the band at 3100-3550 cm-1, attributed to the stretching vibration of free amine (-NH2) groups, was also significantly weakened after adsorption of Gd3+, indicating the interaction between Gd3+ and NH2 groups.5 The SEM-EDS elemental distribution mapping of MIL-101-PMIDA after Gd3+ adsorption at pH 5.5 is shown in Fig. S6. Gd3+ ions were well-distributed on the surface of MIL-101-PMIDA, indicating the successful adsorption of Gd3+ on the material. Importantly, it can be noticed that the distribution of Gd3+ ions closely matched with the signal spots of the active adsorption sites in PMIDA functional group containing phosphorous, nitrogen, and oxygen species. Based on the results of the zeta potential measurements (Fig. S2), FT-IR spectra of MIL101-PMIDA before and after Gd3+ adsorption (Fig. S5), and the trend of Gd3+ adsorption at different pH values (Fig. 7), a plausible mechanism for the adsorption of the Gd3+ by MIL101-PMIDA is shown in Scheme 2. At higher pH value, the adsorption of Gd3+ is mainly accomplished by surface complexation of the deprotonated functional groups of MIL-101PMIDA, in which carboxylate (–COO-) and phosphonic (–PO32-) have strong coordinative affinity towards Gd3+ ion. In addition, Gd3+ can interact with both the free –NH2 group having lone pair of electrons via coordination bonding between the N atoms and Gd3+ ions, and –NH2-OH- sites formed on the surface of MIL-101-PMIDA by electrostatic interactions.46

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At low pH values, protonation of the functional groups in MIL-101-PMIDA results in electrostatic repulsion of Gd3+ ions. Adsorbent reusability. Reusability of an adsorbent is necessary for determining the economics and its practical applications.54-56 Stability of the adsorbents was analyzed from the recycle experiment as shown in Fig. 11. Five adsorption/desorption cycles were carried out using 0.1 M HCl as a stripping agent. MIL-101-PMIDA retained almost all its initial efficiency during the recycling runs, which implied that no loss of organic functional groups occurred during the adsorbent regeneration. Additionally, the structural stability of MIL-101PMIDA was confirmed by FT-IR (Fig. S7 (a)); no distinct changes in the bonding between the PMIDA groups and the MIL-101 structure were observed after five cycles. This suggests that MIL-101-PMIDA has good chemical stability. For comparison, the structural stability of MIL-101-DETA, which showed the next-best performance in Gd3+ adsorption, was also examined by FT-IR (Fig. S5 (b)) after the recycling runs. The spectrum showed a significant decline in the intensity of the DETA signals after the third cycle. It is believed that DETA was protonated during the acid regeneration treatment, and that the bonding between the Cr(III) sites in MIL-101 and DETA was disrupted. Therefore, organic functionalization on the CUS in MOFs for REE recovery should be considered with care.

4. CONCLUSIONS In summary, Cr-MIL-101 and its derivatives with organic functional groups were successfully prepared using two post-synthetic modification methods, fully characterized, and explored for their effectiveness in the separation and recovery of REEs (La3+, Ce3+, Nd3+, Sm3+, and Gd3+) from aqueous solutions. These MOFs showed increasing REE adsorption

16


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capacities in the order MIL-101 < MIL-101-NH2 < MIL-101-ED < MIL-101-DETA < MIL101-PMIDA. MIL-101-PMIDA showed superior adsorption capacities for REE ions, with Gd3+ being the element most efficiently adsorbed by the material. The REE adsorption equilibrium was fitted satisfactorily to the Langmuir model, and the maximum adsorption capacity for Gd3+ was 90.0 mg/g at the optimum pH of 5.5. The selectivity toward Gd3+ was ca. 90 % against competitive transition metal ions, including both trivalent and bivalent cations. In addition, the adsorption equilibrium was attained in less than 2 h, and could be described by a pseudo-second-order kinetic model. The adsorption capacity of the recycled MIL-101-PMIDA was remained relatively constant during five consecutive cycles, and the structural stability of MIL-101-PMIDA was confirmed. The adsorption of Gd3+ on MIL-101PMIDA involved the surface complexation (between COO-, PO32- , free NH2 and Gd3+) and electrostatic attraction (between -NH2OH- and Gd3+) at high pH conditions.

17



Supporting Information Preparation of adsorbent materials; Characterization of materials; Adsorption data fitting by isotherm models; Adsorption data fitting by kinetic models; SEM-EDS analysis of MIL-101NH2 (a), MIL-101-ED (b), and MIL-101-DETA (c); Zeta potential of MIL-101-PMIDA at different pH; Langmuir isotherm (a) and Freundlich isotherm model (b) plots for REE adsorption on MIL-101-PMIDA; The pseudo-first-order (a) and pseudo-second-order kinetics model (b) plots for Gd3+ adsorption on MIL-101-PMIDA; FT-IR spectra of MIL-101-PMIDA before (black) and after (red) Gd3+ adsorption at pH 5.5; SEM and EDS elemental distribution mapping analysis of MIL-101-PMIDA after Gd3+ adsorption at pH5.5; FT-IR spectra of the fresh (black) and reused (red) adsorbents: MIL-101-PMIDA (a) and MIL-101-DETA (b).

ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (Grant number: NRF-2015R1A4A1042434) and the National Strategic Project−Carbon Upcycling project of the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (MSIT), the Ministry of Environment (ME), and the Ministry of Trade, Industry, and Energy (MOTIE) (NRF-2017M3D8A2086050).

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Page 24 of 42

Table 1. Physicochemical properties of the functionalized MIL-101 materials Material

BET surface area

Pore volume

Ave. pore diameter

(m2/g)

(cm3/g)

(nm)

MIL-101

3680

2.01

MIL-101-NH2

1870

1.19

N contenta

P contentb

(wt. %)

(wt.%)

1.71/2.31

-

-

1.56/ 2.15

4.92

-

(3.51 mmol/g) MIL-101-PMIDA

MIL-101-ED

989

2340

0.65

1.31

1.22/1.98

8.71

8.21

(6.22 mmol/g)

(2.66 mmol/g)

7.63

-

1.63/ 2.20

(5.45 mmol/g) MIL-101-DETA

2112

1.23

1.42/ 2.07

9.80 (6.99 mmol/g)

a

Obtained from EA, b Obtained from ICP-OES.

24


-

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Table 2. Adsorption capacities of the REEs on the obtained samples qe(mg/g) Materials

La3+

Ce3+

Nd3+

Sm3+

Gd3+

MIL-101

14.6

11.3

18.5

17.9

16.5

MIL-101-NH2

21.6

23.7

28.4

32.1

29.3

MIL-101-PMIDA

37.2

48.3

63.9

69.1

87.7

MIL-101-ED

30.3

36.5

54.0

58.9

66.0

MIL-101-DETA

39.7

52.8

67.5

68.9

73.6

25



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Table 3. Adsorption equilibrium constants for the adsorption of REEs on MIL-101-PMIDA Langmuir isotherm equation

Freundlich isotherm equation

Kd

REEs

qm (mg g-1)

KL (L mg-1)

R2

n

KF (mg g-1)

R2

La3+

37.4

0.20

0.990

4.12

12.05

0.954

612

Ce3+

49.0

0.31

0.993

4.25

17.61

0.895

960

Nd3+

70.9

0.41

0.990

4.21

26.42

0.796

2125

Sm3+

72.7

0.17

0.991

3.33

20.40

0.864

2593

Gd3+

90.0

0.36

0.996

4.40

33.56

0.878

7129

26


(mL/g)

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Table 4. Comparison of the organic ligand and supports used for Gd3+ recovery Experimental conditions Functional groupa

Support

Performance

Optimal pH

Time (h)

Competitive ion

Kd (ml/g)

qmb Recycle (mg/g) runs Ref.

PMIDA

MIL101PMIDA

5.5

0-6

Al,Fe,Ni,Zn,Co

7129

90.0

5

This work

MAH

Mesoporous Silica NP

4

0-4

Al,Fe,Sm,Eu,Tb,Dy,Ho,Er

2673

85.4

5

13

TMSEDTA

Fe3O4

6.5

0-24

La,Pr,Y,Dy,Ho,Er,Lu

-

113

-

14

ATMP

Metal oxide (ZrTi)

0-3 M HNO3

0-24

n.a.

10145

0.8

n.a.

15

CA Cys

Fe3O4 NPs

7

0-2

Mg, Ca, Ni

~28 ~140

52 98

-

mIIP-CS/CNT

7.2

0.057

La, Ce

1814

88

46 5

57

EDTA and Ionic 6.7 0-24 n.a. 24.5 58 DTPA imprinted derivatives resins a PMIDA: N-(phosphonomethyl) iminodiacetic acid hydrate, MAH: maleic anhydride, TMS-DETA: N-[(3trimethoxysilyl)propyl] ethylenediamine tracetic acid, ATMP: Tris-methylenephosphonic acid, CA: citric acid, and Cys: L-cysteine, mIIP-CS/CNT: magnetically retrievable ion imprinted polymer-chitosan/carbon nanotube, EDTA: ethylenediamine tracetic acid, DTPA: diethylenetriaminepentaacetic acid. All of the organic group names mentioned here were from the corresponding literatures. bqm (mg/g) : maximum adsorption capacity.

27



Page 28 of 42

3+

Table 5. The kinetics parameters for Gd adsorption on MIL-101-PMIDA Pseudo-first-order kinetics model

Pseudo-second-order kinetics model

qe,cal.

K1

qe,cal.

K2

(mg g-1)

(min-1)

R2

(mg/g)

(g mg-1 min-1)

86.6

0.046

0.953

89.3

0.002

28


R2

0.994

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Scheme 1 Synthesis procedure for MIL-101-PMIDA by post-synthetic modification of MIL101-NH2

with

N-(phosphonomethyl)

iminodiacetic

acid

dicyclohexylcarbodiimide) was used as a catalyst.

29


(PMIDA):

DCC

(N,Nʹ-


Scheme 2 Schematic representations of the possible binding sites and proposed mechanisms for the adsorption of Gd3+ on the surface of MIL-101-PMIDA.

30


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Figure 1. MIL-101 (a) and its functionalized derivatives: MIL-101-NH2 (b); MIL-101-ED (c); and MIL-101-DETA (d).

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Figure 2. XRD patterns of MIL-101 (a) and its functionalized derivatives: MIL-101-NH2 (b); MIL-101-PMIDA (c); MIL-101-ED (d); and MIL-101-DETA (e).

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Figure 3. N2 adsorption–desorption isotherms of the prepared samples.

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Figure 4. FT-IR spectra of the prepared samples.

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Figure 5. SEM images of MIL-101 (a), MIL-101-NH2 (b), MIL-101-ED (c), MIL-101-DETA (d), MIL-101-PMIDA (e), and SEM-EDS analysis (f).

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Figure 6. Gd3+ adsorption equilibria over the prepared samples.

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100 80

q e (mg Gd 3+/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40 20 0 2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

pH Figure 7. Effect of pH on Gd3+ adsorption on MIL101-PMIDA at 25 °C.

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6.0


Figure 8. Adsorption isotherms and modeling of REE adsorption on MIL101-PMIDA at pH 5.5: Langmuir isotherm (line) and Freundlich adsorption isotherm (dots).

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100

80

qe (mg Gd3+/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0 0

50

100

150

200

250

300

350

time (min) Figure 9. Effect of contact time on Gd3+ adsorption on MIL101-PMIDA at pH 5.5.

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Figure 10. Adsorption selectivity for Gd3+ over co-existing transition metal ions on MIL101PMIDA at pH 5.5.

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100 3+

Adsorption capacity (mg Gd /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MIL101-PMIDA MIL101-DETA

80

60

40

20

0

1

2

3

Cycle

4

5

Figure 11. Adsorption–desorption recycling runs of MIL101-PMIDA and MIL101-DETA at an initial Gd3+ concentration of 100 mg/L at pH 5.5.

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Highly stable metal organic framework MIL-101 was functionalized with amino-, carboxyl-, and phosphonic acid groups (MIL-101-PMIDA), which demonstrated effective recovery of rare-earth elements (Gd3+, Sm3+, Nd3+, Ce3+, and La3+) with high reusability. 238x182mm (96 x 96 DPI)


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Selective Adsorption of Rare Earth Elements over Functionalized Cr

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