Acrylic acid functionalized metal-organic frameworks (MOFs) for Sc(III

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Functional Nanostructured Materials (including low-D carbon)

Acrylic acid functionalized metal-organic frameworks (MOFs) for Sc(III) selective adsorption Zhenning Lou, Xin Xiao, Mengnan Huang, Yuejiao Wang, Zhiqiang Xing, and Ying Xiong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00476 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 16, 2019

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ACS Applied Materials & Interfaces

Acrylic acid functionalized metal-organic frameworks (MOFs) for Sc(III) selective adsorption Zhenning Lou *, Xin Xiao, Mengnan Huang, Yuejiao Wang, Zhiqiang Xing, Ying Xiong * College of Chemistry, Liaoning University, Shenyang, 110036, China *Corresponding author. Tel.: +86-24-62207873; fax: +86-24-62202380. E-mail: [email protected] E-mail: [email protected] (Y. Xiong)

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ABSTRACT:

The increasing demand for Rare Earth Elements (REEs) due to their extensive hightech applications has encouraged the development of new sustainable approaches for REEs recovery and separation. In this work, a series of acrylic acid functionalized metal-organic framework materials (named as y-AA-x@MIL-101s) were prepared and used for selective adsorption of Sc(III). The adsorbent was characterized by scanning electron microscope (SEM), fourier transform infrared (FT-IR) spectroscopy, x-ray diffraction (XRD), nitrogen adsorption, x-ray photoelectron spectroscopy (XPS), zeta potential and surface functional group titration. The adsorption isotherm and kinetics data were accurately described by the Langmuir and Pseudo-second-order models. The adsorption capacity of material for Sc(III), Nd(III), Gd(III) and Er(III) was 90.21, 104.59, 58.29 and 74.94 mg.g-1, respectively. Importantly, the adsorbent was better for selective recovery Sc(III) not only from 16 REEs mixed system but also Cu(II), Zn(II), Mn(II), Co(II) and Al(III) coexistence solution. Except for Sc(III), the material displayed high affinity for Nd(III) in the light rare earth mixture and for Gd(III) in the middle rare earth mixture. All in all, this study provides a new method for separation and recovery of rare earth elements, which makes this work highly significant in separation and enrichment.

Keywords: Sc(III); metal-organic frameworks; acrylic acid; adsorption; selectivity. 2


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1.

INTRODUCTION

Rare earth elements (REEs), as industrial vitamin, have been employed in advanced materials and futuristic industries such as supermagnets, electrical car engines, wind turbines and fluorescent lamps.1-3 Due to the increasing demand and limited resources of REEs, the prices of REEs soared in rencent years. Moreover, the REEs are usually by-products of mining operations and only 1% of REEs were estimated to be recycled.4 Therefore, it is indispensable to explore an environmentally friendly approach for the separation and recovery of worthwhile REEs.

REEs, as oxyphilic elements, can form stable complexes with many oxygen-containing ligands such as carboxylic acids, crown ethers, diketones, oxygen-containing phosphorus extractants. Now, several technologies of recovery REEs from solution were electrochemical,5 solvent extraction,6,7 ion exchange8,9 and chemical precipitation.10,11 However, most of them had evident disadvantages such as high costs, inefficient at low concentrations of REEs, and environmental pollution. Adsorption method has been recognized as a cost-efficient and eco-friendly way for separation and adsorption of metal ions and organic pollutants from solution.12-18

Metal-organic frameworks (MOFs) are constructed from metal cations and metal clusters with organic ligands via strong covalent bonds forming an infinite network structure.19 MOFs have been considered as promising materials and used in catalysis,20,21 gas storage,22,23 drug delivery,24,25 and metal ion recovery26-30 because of large pore volume, highly ordered pore structure, high density of active sites, great 3



specific surface area, better chemical and thermal stability. Yang et al31 reported a MOF membranes on cloth (CMC-MOF/Cloth) for efficient removing Pb(II), the maximum uptake of which for Pb(II) was 862.44 mg.g-1. Samuel et al32 synthesized a nanocomposite adsorbent material (GO-CS@MOF) using chitosan, graphene oxide and metal organic framework Zn(BDC) for adsorption Cr(VI). GO-CS@MOF exhibited an excellent adsorption capacity of 144.92 mg.g-1 for Cr(VI) at pH 3. Lin et al33 fabricated Zr-based MOFs of UiO-66 and UiO-66-NH2 for separation of precious metal ions from strongly acidic solutions. These MOF materials were exhibited rapid and high adsorption capability. Thus, it can be seen that metal organic framework materials have been widely applied in metal ion separation and adsorption. However, metal-organic frameworks have rarely been used as an adsorbent for REEs so far. Among the massive metal-organic framework materials, MIL-101 is one of the substances with large specific surface area (4620 m3.g-1) and displays highly porous nature and remarkable stability in water. These characteristics make it most suitable for recovery of metal ions in the aqueous solution.

Therefore, on the basis of these considerations, we developed acrylic acid functionalized metal-organic frameworks for separation and adsorption rare earth elements. The effect of addition of acrylic acid on the adsorption performance was investigated. The adsorption selectivities of adsorbent in light, middle, heavy rare earth solution and 16 REEs mixed system were investigated. And the effect of coexisting ions on the adsorption properties of Sc(III) was also explored. It was found that the adsorbent exhibited higher adsorption selectivity for Sc(III) compared with other REEs. Furthermore, the adsorption kinetics, isotherms, adsorption mechanism and the stability of adsorbent were also explored. 4


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2. EXPERIMENTAL

2.1. Reagents and instruments.

All chemicals were obtained from Sinopharm Chemicals Reagent Co. Ltd. (China) and were used without additional purification. A stock solution of REEs was prepared by dissolving rare earth oxide in 6 mol.L-1 hydrochloric acid.

The pH of the solution was determinated by S-3C model pH meter. Before and after adsorption, the REEs concentration in the aqueous was determined by a UV spectrophotometry (UV-2600, Shimadzu, Japan). The concentrations of REEs mixture and Cu(II), Zn(II), Mn(II), Co(II) and Al(III) were measured by inductively coupled plasma-optical emission spectrometry (ICP-OES) (PE-8000 Pekin-Elmer, America). SEM images were recorded using a scanning electron microscope (S-4800, Hitachi, Japan). FT-IR spectra were measured on a spectrum one FT-IR spectrometer (IR-Prestigr, Shimadzu, Japan). Zeta potential of material at different pH was measured by potential particle size analyzer (Zeta-Plus4, Brookhaven, America). The X-ray diffraction was recorded on a Bruker D8 diffractometer using a Cu Ka X-ray radiation with 45 kV voltages and 40 mA current, and the patterns were collected over 5 ~ 30º. Nitrogen adsorption isotherms were measured at 77 K using Tristar 3020 volumetric adsorption analyzers. (Quantachrome, America) and the specfic surface area of samples were calculated by Brunauer-Emmett-Teller (BET). XPS spectra were measured by a thermo ESCALAB 250 X-ray photoelectron spectrometer with Al Kα Xray source. 5



2.2. Preparation of adsorbent materials.

2.2.1. Synthesis of AA-x@MIL-101s.

Generally, 3.20 g Cr(NO3)3.9H2O (8 mmol), 1.32 g terephthalic acid (8 mmol), 2.20 mL glacial acetic acid were blended in 40 mL water. The stirring was maintained for 30 min at room temperature, after which a certain amount of acrylic acid (AA) was added. The suspension was placed in a Teflon-lined autoclave bomb and kept in an oven at 473 K for 9 h without stirring. Then, the solution had been cooled to room temperature in air and the light green solid was washed with DMF, ethyl alcohol and water, respectively. The product was dried in an oven at 423 K for 12 h, which were abbreviated as AA-x@MIL-101s (x is the mass of added AA, x = 0.058, 0.072, 0.094 and 0.144 g).

2.2.2. Synthesis of y-AA-0.072@MIL-101s.

According to 2.2.1, it was found that the best adsorption capacity was achieved when the addition of AA was 0.072 g (x = 0.072). Therefore, the AA-0.072@MIL-101 was used to conduct the next synthesis. AA-0.072@MIL-101 of 0.1 g was dispersed into 40 mL water. An amount of extra acrylic acid was added to the solution, followed by stirring at 333 K for 20 min. Then, a certain amount K2S2O8 was added and the mixture was heated at 353 K for 1 h under a nitrogen atmosphere to complete the polymerization. The obtained products were cooled to room temperature and washed with water to remove non-reacted chemicals. Finally, the products were oven-dried at 323 K 6


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overnight. The adsorbents were named as y-AA-0.072@MIL-101s. (y denotes for the mass of added AA during the polymerization process, y = 0.025, 0.05, 0.075 and 0.1 g). The reaction steps are presented in Scheme 1.

2.3. REEs uptake experiments.

Due to the adsorbents have good adsorption selectivity for Sc(III), Sc(III) was taken as an example to investigate the adsorption properties of adsorbents. The adsorption experiments were conducted by mixing 10 mg of the adsorbent with 10 mL of Sc(III) solution. The effect of pH was investigated by varying Sc(III) solution pH from 1.0 to 4.5 with initial concentration of 50 - 80 mg.L-1. 0.5 mol.L-1 HCl and 0.5 mol.L-1 NaOH were used for pH adjustment. The kinetics was explored with initial Sc(III) concentration of 45 mg.L-1 for contact time ranging from 5 min to 24 h. The adsorption isotherms were conducted by adding 10 mg adsorbent into 10 mL REEs solution at 303 K. The amount of REEs adsorbed (qe) onto the functionalized MOFs were calculated as followed:

(1)

(2) where qe (mg.g-1) is the amount of adsorbed metal ions at equilibrium, Ci and Ce (mg.L-1) are the initial and equilibrium concentrations of REEs, respectively. W (mg) and V (mL) are the weight of the adsorbent and the volume of the solution, A% was the adsorption efficiency, respectively. The competitive adsorption of REEs onto the adsorbent was investigated by preparing the mixture of light rare earth, middle rare earth, heavy rare 7



earth and 16 REEs mixture at pH from 1.0 to 4.5, respectively, and the initial concentration of each rare earth ion was 20 mg.L-1.

2.4. Desorption and regeneration experiments.

For regeneration evaluation, the optimum adsorbent was first contacted with 20 mg.L-1 Sc(III) for 5 h at pH 4.5. Then, the adsorbent loaded with Sc(III) was immersed into 0.3 mol.L-1 HCl and shaken at 303 K for 3 h. After that, adsorbent was rinsed with distilled water to remove any residual solution and dried at 323 K overnight, repeating the adsorption-desorption procedure to achieve 5 cycles. Subsequently, the concentrations of Sc(III) ions were determined by UV spectrophotometry.

3. RESULTS AND DISCUSSION

3.1. Properties and characterization of y-AA-x@MIL-101s.

To investigate the effect of addition of AA on the properties of AA-x@MIL-101s, four AA-x@MIL-101s were fabricated by changing the mass of AA (x = 0.058 g, 0.072 g, 0.094 g and 0.144 g). As shown in Fig 1a, the adsorption capacity of MIL-101 was only 18.84 mg.g-1 for Sc(III) at pH 4.5. However, the adsorption capacity of AA-x@MIL101s significantly increased. When x = 0.072 g, the maximum adsorption capacity of Sc(III) was 36.82 mg.g-1. Therefore, the AA-0.072@MIL-101 was used to implement the next synthesis.

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The y-AA-0.072@MIL-101s were prepared by polymerization reaction between AA0.072@MIL-101 and AA. After polymerization, great amounts of carboxyl groups were introduced onto AA-0.072@MIL-101, which could increase the amount of adsorption for REEs. Meanwhile, the effect of addition of AA in the process of polymerization on the properties of the adsorbent was also studied. The obtained acrylic acid

functionalized

y-AA-0.072@MIL-101s

displayed

different

adsorption

performance. As shown in Fig 1b, the absorption capacity of Sc(III) gradually increased. At pH 4.5, the Sc(III) uptake increased from 48.08 mg.g-1 (0.025-AA-0.072@MIL-101) to 63.04 mg.g-1 (0.1-AA-0.072@MIL-101). When y = 0.075 g and 0.1 g, there was no significant difference in adsorption capacity, which was in good agreement with the content of carboxylic acid of surface functional group titrations (shown in Table 1). It was found that the carboxylic acid content of y-AA-0.072@MIL-101s increases from 5.13 to 5.79 mmol.g-1. Moreover, there was not much difference between 0.075-AA0.072@MIL-101 (5.77 mmol.g-1) and 0.1-AA-0.072@MIL-101 (5.79 mmol.g-1). According to the adsorption capacity of Sc(III) and the surface functional group titrations data, the 0.075-AA-0.072@MIL-101 was selected as optimum adsorbent to characterize and implement the next experiment.

The SEM images of MIL-101, AA-0.072@MIL-101 and 0.075-AA-0.072@MIL-101 were shown in Fig 2. It can be observed from Fig 2a that the octahedral morphology of MIL-101 sample was obtained. However, after adding AA into the matrix, the octahedral morphology of MIL-101 became irregular (Fig 2b). As shown in Fig 2c, those irregular octahedral morphology were aggregated together. Indeed, these aggregations were formed by polymerization process of AA.

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The FT-IR spectra of materials were shown in Fig 3a. Notably, due to polymerization of acrylic acid, 0.075-AA-0.072@MIL-101 exhibited more intense absorption peaks than MIL-101 and AA-0.072@MIL-101 at 1712 and 3430 cm-1, which assigned to C=O and -OH of carboxylic acid group. More importantly, a new band at 2935 cm-1 on the 0.075-AA-0.072@MIL-101 appeared, which corresponded to C-H stretching vibration of alkanes. Thus, all these confirmed the successful introduction of carboxylic acid groups on AA-0.072@MIL-101 during polymerization.

The XRD patterns for MIL-101, AA-0.072@MIL-101 and 0.075-AA-0.072@MIL-101 were shown in Fig 3b. It can be seen clearly that the XRD patterns of 0.075-AA0.072@MIL-101 and AA-0.072@MIL-101 matched for MIL-101 with no additional notable characteristic peaks, indicating the crystal structure of the MIL-101 is well preserved during the functionalized process with acrylic acid.

The N2 isotherms of the prepared materials in this work was shown in Fig 3c. MIL-101 and (0.025-0.1)-AA-0.072@MIL-101s presented typical I-type profiles, which demonstrated that microporous existed in the materials.34 The BET specific surface area was listed in Table 1. It could be seen that the specific surface area of MIL-101 and AA-0.072@MIL-101s were 2451.87 and 2336.67 m2.g-1, respectively. The specific surface area of (0.025-0.1)-AA-0.072@MIL-101s became smaller gradually from 1497.72 to 1083.46 m2.g-1 with the increase of acrylic acid contents. It can be inferred that acrylic acid was polymerized on the surface of AA-0.072@MIL-101, which resulted in specific surface area decreasing.

3.2. Effect of pH on Sc(III) adsorption. 10


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It is generally known that pH is a critical parameter influencing the adsorption processes. It affects the protonation degree of functional group on the surface of the adsorbents as well as the speciation of the REEs. In order to avoid the precipitation of Sc(III), the pH over 4.5 was not studied.35 The predominated species of REEs was trivalent positive ions in solution at pH < 7.36,37 As shown in Fig 1b, the Sc(III) adsorption capacity of 0.075-AA-0.072@MIL-101 increased gradually at pH from 1.0 to 4.5. To analyze the surface charge of material at different pH, zeta potential was measured, and the result was shown in Fig 3d. The isoelectric point of 0.075-AA-0.072@MIL-101 was approximately pH 2.3. At low pH (pH ≤ 2.3), the adsorption capacity was low due to the electrostatic repulsion between 0.075-AA-0.072@MIL-101 and Sc(III) cation ions. As the pH > 2.3, the negative charges on the surface of the 0.075-AA-0.072@MIL-101 increased with pH increased. Electrostatic attraction occured between the 0.075-AA0.072@MIL-101 and Sc(III) cation ions at pH > 2.3, which led to the adsorption capacity increasing. The maximum adsorption capacity (62.45 mg.g-1) for Sc(III) was achieved at pH 4.5.

3.3. Stability of 0.075-AA-0.072@MIL-101.

In order to study the stability of adsorption material, the x-ray diffraction (XRD) and the residual weight of 0.075-AA-0.072@MIL-101 were conducted by immersing it at different acidity solution for 24 h, which were shown in Fig 4. It was found that the residual weight of 0.075-AA-0.072@MIL-101 in 0.3 mol.L-1 HCl could still remain more than 87%, shown in Fig 4a. The XRD patterns of 0.075-AA-0.072@MIL-101 at

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different acidity solution were still well preserved (Fig 4b). All of these showed that 0.075-AA-0.072@MIL-101 had better stability within the range of acidity studied.

3.4. Adsorption kinetics, isotherms and thermodynamics.

The adsorption behavior of the 0.075-AA-0.072@MIL-101 towards Sc(III) at pH 4.5 was investigated. The adsorption time curves of 0.075-AA-0.072@MIL-101 were shown in Fig 5a. It can be seen that the adsorption of Sc(III) was initially rapid. The uptake could be more than 63% within 5 min, and the adsorption equilibrium were all accessed after 5 h at three different temperatures. The fast adsorption performance could be attributed to the large amount of functional groups (-COOH) on the surface of 0.075-AA-0.072@MIL-101, and the adsorption process was completed in a short time. Furthermore, in order to better understand the controlling mechanism during the adsorption, the Pseudo first-order,38 Pseudo second-order,38 Elovich39 and Intraparticle diffusion kinetic models40 also were undertaken, according to Eqs. 3 ~ 6. k log(q  q )  log q  ( 1 )t e t e 2.303

(3)

t 1 t   2 qt k2 qe qe

(4)

qt 

1



ln( ) 

1



lnt

(5)

qt  k p t 0.5  C

(6)

where qt and qe (mg.g-1) stand for the adsorption capacities at time and at equilibrium, respectively. The rate constant of Pseudo first-order and Pseudo-second-order model are k1 (h-1) and k2 (g.mg-1.h-1) respectively. The rate constant of initial adsorption is α 12


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(mg.g-1.h-1), and β (g.mg-1) represents the activation energy constant. The particle diffusion rate constant is kp (mg.g-1.h-1/2), and C is a constant.

It could be observed that Pseudo-second-order kinetic model gave a better interpret for the adsorption data of Sc(III) with correlation coefficients of 1 at three different temperatures, shown in Table 2. These suggested that the chemical process may well be the rate-limiting step of the adsorption.

The adsorption characteristics of 0.075-AA-0.072@MIL-101 for REEs were investigated, such as adsorption capacity and interactive behaviors between the REEs and carboxylic acid sorption sites. The experimental equilibrium data were fitted by using three typical isotherms (Langmuir, Frenundich and Temkin), shown in Table 3. It was found that the experiment data of Sc(III), Nd(III), Gd(III) and Er(III) fitted better to the Langmuir model (0.97 - 0.99), which was significantly better than the Frenundich model and Temkin model, suggesting that the monolayer adsorption process of REEs on the adsorbent. From the results of fitting parameter based on the Langmuir model, 0.075-AA-0.072@MIL-101 exhibits the potential to recovery Sc(III), Nd(III), Gd(III) and Er(III) from aqueous solutions with a maximum adsorption capacity of 90.21, 104.59, 58.29 and 74.94 mg.g-1 and the adsorption isotherms were shown in Fig S1. Meanwhile, in order to evaluate the advantages of the 0.075-AA-0.072@MIL-101, the adsorption capacities of 0.075-AA-0.072@MIL-101 and some adsorbent materials for REEs were shown in Table 4. It was observed that acrylic acid functionalized metalorganic framework (0.075-AA-0.072@MIL-101) displayed much higher adsorption capacity for REEs than other adsorbents.41-46 Interestingly, diethylenetriamine functionalized MIL-10147 also exhibited high adsorption capacity for rare earth 13



elements. Thus, it can be seen that MIL-101 is favorable matrix material for REEs. It is easy to find that 0.075-AA-0.072@MIL-101 showed much higher adsorption capacity toward Sc(III), Nd(III), Gd(III) and Er(III) in comparison with the diethylenetriamine functionalized MIL-101, indicating that AA is a good functional agent. By polymerizing of AA, more carboxylic acid groups were introduced into the MIL-101 to increase the adsorption capacity of rare earth elements. These results showed that the AA functionalized MIL-101 could be used as an efficient adsorbent for REEs.

In addition, the separation factor RL, can be obtained from Eq.7, C0 is the initial REEs ion concentration, KL is constant of Langmuir. 1

(7)

𝑅𝐿 = 1 + 𝐾𝐿 ⋅ 𝐶0

The smaller value of RL (0 < RL < 1) indicates that 0.075-AA-0.072@MIL-101 is a favorable middle for the adsorption. It can be seen that RL values decreased from 0.22 to 0.02 for Sc(III), from 0.75 to 0.05 for Nd(III), from 0.48 to 0.026 for Gd(III) and 0.8 to 0.058 for Er(III) in Fig S2.

The effect of temperature was studied from 303 K to 323 K, shown in Fig 5a. It can be shown that the Sc(III) adsorption efficiency decreased as the temperature increased, suggesting that Sc(III) adsorption on this materials was an exothermic process (∆Hθ = -11.39 kJ.mol-1). Moreover, the negative values of ΔGθ (-5.94 to -5.51 kJ.mol-1) confirmed the process of adsorption was spontaneous in nature, shown in Table 5. The 303 K was used for further test.

3.5. Adsorption mechanism.

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To determine the interaction between the sorption sites and Sc(III), the FT-IR spectra of the 0.075-AA-0.072@MIL-101 before and after Sc(III) adsorption were shown in Fig 6a. The peak at 1712 cm-1 in the 0.075-AA-0.072@MIL-101 assigned to C=O from carboxylic acid groups, which was significantly reduced after adsorption, suggesting that C=O was involved in the Sc(III) adsorption. Furthermore, the peak at 3430 cm-1, which belongs to characteristic peak of -OH from the carboxylic acid groups, weakened after loading Sc(III). It indicated that -OH groups of carboxylic acid was an important functional group for the adsorption of rare earth elements. These observations revealed that the carboxylic acid groups of AA participated in the adsorption of Sc(III). The adsorption mechanism for Sc(III) on the 0.075-AA-0.072@MIL-101 was described in Scheme 2.

For further investigating the adsorption mechanism, XPS survey spectra of the 0.075AA-0.072@MIL-101 before and after Sc(III) adsorption were obtained, and the results were shown in Fig 6(b-d). As shown in Fig 6b, the Sc 2p peak was detected at 403.26 eV, providing evidence of Sc(III) adsorbed successfully on the 0.075-AA-0.072@MIL101. After Sc(III) adsorption, the O 1s peaks at 533.51 and 531.94 eV (Fig 6c) assigned to C=O and -OH were shifted to 533.56 and 532.11 eV, respectively (Fig 6d). In addition, after adsorption of Sc(III), the area ratio of C=O reduced from 24.30 % to 10.89 % and from 22.54 to 16.10 % for -OH. Moreover, a new peak (O-Sc) was observed at 531.06 eV and the area ratio of O-Sc was 22.59 % in the O 1s spectra after Sc(III) adsorption. The reason for the changes of the peak area and the binding energy of C=O and O-H before and after Sc(III) adsorption were that O-Sc bond was formed by chelation effect of C=O and Sc(III), and O-Sc bond was formed by cation exchange between -OH and Sc(III). The change of pH before and after adsorption Sc(III) was 15



also determined, shown in Fig S3. It can be seen that the pH of the solution decreased after adsorption Sc(III), which further proved the cation exchange occured between OH and Sc(III). All those proved that the adsorption mechanism for Sc(III) on the 0.075-AA-0.072@MIL-101 were chelation and ion exchange.

3.6. Adsorption in light, middle, heavy REEs and 16 REEs mixture.

REEs have same chemical properties, making them difficult to separate each other. In order to explore the selective adsorption for REEs onto 0.075-AA-0.072@MIL-101, selective adsorption experiments were conducted in light, middle, heavy REEs and 16 REEs mixture containing the same concentrations (20 mg.L-1), respectively. From Fig 7(a,b), it can be seen that the 0.075-AA-0.072@MIL-101 showed higher affinity for Nd(III) and Gd(III) in light and middle REEs at pH ≥ 2. However, the adsorbent had no obvious preferential adsorption trend for heavy REEs (Fig 7c). It is important that the adsorbent can selectively separate Sc(III) from 16 REEs mixed system, shown in Fig 7d. The selectivity of 0.075-AA-0.072@MIL-101 toward Sc(III) was investigated and selectivity coefficient can be calculated by Eq. 8. Sel𝑆𝑐/𝑋 = log

(𝑞𝑒/𝐶𝑒)𝑆𝑐

(8)

(𝑞𝑒/𝐶𝑒)𝑋

Where qe (mg.g-1) is the adsorption amount of REEs, Ce (mg.L-1) is the residual REEs ion concentration after adsorption, X stands for others REEs, except for Sc(III) and SelSc/X represents adsorption selectivity for Sc(III).

The selectivity coefficients were listed in Table S1. It can be seen that SelSc/X were far greater than 1.5 at pH ≥ 2 in 16 REEs mixture, which indicated that 0.075-AA0.072@MIL-101 possessed higher affinity toward Sc(III) than other REEs. Selective 16


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adsorption for Sc(III) could be related to strong interaction between Sc(III) and the chemical groups (-OH and C=O of carboxyl ) in the 0.075-AA-0.072@MIL-101. According to HSAB principle, -COOH group was hard Lewis base and REEs were taken as hard Lewis acids. Compared with other rare earth elements, the radius of Sc(III) is the smallest, so Sc(III) is the hardest Lewis acids in REEs, which could form more stable complex with adsorbent. Moreover, the small ion radius of Sc(III) makes it get closer to the adsorbent, which was listed in Fig S4.

3.7. Effects of coexisting ions.

The rare earth ions can be coexisted with many metal ions in real leachate liquor. To evaluate adsorbent applicability, the effects of Cu(II), Zn(II), Mn(II), Co(II) and Al(III) on Sc(III) adsorption were investigated at concentration ratio of 1:1 and 2:1. As shown in Fig 5b, as the concentration ratio of coexisting ions to Sc(III) was 2:1, the adsorption percentage of Sc(III) decreased to 62 %, which indicated that high concentration coexisting ions inhibited the adsorption of Sc(III). The adsorption capacity followed the order: Cu(II) < Co(II) < Zn(II) < Mn(II) < Al(III) < Sc(III), which indicated that the higher valent cations were more easily combined with active sites of 0.075-AA0.072@MIL-101 than the lower valent cations. In short, the 0.075-AA-0.072@MIL101 still has good selectivity for Sc(III) even in high concentrations of coexisting ions solution, suggesting that 0.075-AA-0.072@MIL-101 has great potential for Sc(III) recovery and separation.

3.8. Stripping and reusability tests.

17



Efficient regeneration of the adsorbent was an important factor in measuring its practical application. In this study, Sc(III) was desorbed from 0.075-AA-0.072@MIL101 using different concentrations of CH3COOH, HCl, H2SO4 and HNO3. It could be easily seen that 0.3 mol.L-1 HCl was an efficient eluent, shown in Table S2. As observed in the Fig 8, the adsorption efficiency for Sc(III) still remained above 80% after five adsorption-desorption cycles by using 0.3 mol.L-1 HCl, and XRD patterns of 0.075-AA0.072@MIL-101 were still well preserved. Those revealed that the 0.075-AA0.072@MIL-101 has an excellent reusability. Moreover, the presence of acrylic acid in the solution of Sc(III) adsorption was detected by HPLC, shown in Fig S5. The retention time of AA is obtained at 3.36 min. However, the AA was not detected in the solution of Sc(III) adsorption. All those suggested that 0.075-AA-0.072@MIL-101 has great potential in separation and uptake of Sc(III) from solutions.

4. CONCLUSIONS

In a word, a series of MIL-101 based adsorbents were prepared by using acrylic acid as functional agent. Importantly, the acrylic acid polymerization introduced abundant carboxyl, which is the most efficient functional group for REEs adsorption in this paper. The recovery of Sc(III) increased with the amount of acrylic acid in materials increasing inferred from surface functional group titration. The 0.075-AA-0.072@MIL-101 was considered as the optimum adsorbent for REEs adsorption at pH 4.5. The 0.075-AA0.072@MIL-101 showed high uptake capacity toward Sc(III), Nd(III), Gd(III) and Er(III), and the maximum adsorption capacity reached 90.21, 104.59, 58.29 and 74.94 mg.g-1, respectively. More importantly, the 0.075-AA-0.072@MIL-101 can selectively adsorb Sc(III) from 16 REEs mixed systems as well as coexisting ions of Cu(II), Zn(II), 18


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Mn(II), Co(II) and Al(III) mixture solution. Compared with other light and middle rare earths, the adsorbent for Nd(III) and Gd(III) showed higher affinity, but it was not a favourable material for heavy rare earths selective separation. The adsorption mechanism were cation exchange and coordination reaction inferred by FT-IR, XPS and the pH change before and after adsorption Sc(III). This study showed that the material fabricated with MIL-101 by using acrylic acid as a functional reagent was a promising adsorbent, which can be used as the separation and recovery of REEs, and it also developed MOFs application.

Supporting Information

Adsorption isotherms of Sc(III), Nd(III), Gd(III) and Er(III) onto 0.075-AA0.072@MIL-101; RL for Sc(III), Nd(III), Gd(III) and Er(III); the pH before and after adsorption Sc(III); the ion radius of the REEs(III); the chromatogram of pure acrylic acid and after adsorption of Sc(III), selectivity coefficient of the adsorbent for Sc(III) and desorption of loaded Sc(III) adsorption on the 0.075-AA-0.072@MIL-101.

ACKNOWLEDGEMENTS This project is supported by National Natural Science Foundation of China (21201094， 21171080, 51674131), Scientific Research Found of Liaoning Provincial Education Department (L2014004), Program for Liaoning Excellent Talents in University (LR2015026), Liaoning Provincial Department of Education Innovation Team Projects (LT2015012) and Project supported National Science Technology Ministry (2015BAB02B03). 19



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The table of contents (TOC) graphic

27



Caption Scheme 1. Preparation of adsorbent y-AA-x@MIL-101s.

Scheme 2. The mechanism of REEs(III) adsorbed on the 0.075-AA-0.072@MIL-101.

Fig 1. Effect of pH on Sc(III) recovery by (a) MIL-101 and AA-(0.058-0.144)@MIL101s, (b) (0.025-0.1)-AA-0.072@MIL-101s. (t = 24 h; T = 303 K).

Fig 2. SEM images of (a) MIL-101, (b) AA-0.072@MIL-101 and (c) 0.075-AA0.072@MIL-101.

Fig 3. (a) FT-IR spectra, (b) XRD patterns of MIL-101, AA-0.072@MIL-101 and 0.075-AA-0.072@MIL-101, (c) N2 isotherms of MIL-101, AA-0.072@MIL-101 and (0.025-0.1)-AA-0.072@MIL-101s, (d) Zeta potential of 0.075-AA-0.072@MIL-101.

Fig 4. (a) The residual weight and (b) XRD patterns of 0.075-AA-0.072@MIL-101 were soaked in different acidity aqueous solution for 24 h.

Fig 5. Effect of (a) contact time and (b) coexisting ions concentrations on the adsorption of Sc(III) onto 0.075-AA-0.072@MIL-101. (solid-to-liquid ratio = 1 g : 1 L; pH 4.5; t = 24 h)

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Fig 6. (a) FT-IR spectra and (b) XPS spectra of 0.075-AA-0.072@MIL-101, O1s XPS spectra before (c) and (d) after adsorption of Sc(III).

Fig 7. Selective adsorption of 0.075-AA-0.072@MIL-101 at pH 1 - 4.5 in (a) light rare earth, (b) middle rare earth, (c) heavy rare earth and (d) 16 REEs rare earth mixture. (C0 = 20 mg.L-1 for each REEs; solid-to-liquid ratio = 1 g : 1 L; T = 303 K).

Fig 8. (a) Adsorption performance and (b) XRD patterns of the 0.075-AA-0.072@MIL101 in consecutive 5 cycles.

Table 1. Surface functional group titration, BET and adsorption capacity of Sc(III) of MIL-101, AA-0.072@MIL-101and (0.025-0.1)-AA-0.072@MIL-101s.

Table 2. Kinetic parameters of different temperatures for Sc(III) adsorption on the 0.075-AA-0.072@MIL-101.

Table 3. Isotherm parameters for Sc(III), Nd(III), Gd(III) and Er(III) by using Langmuir, Freundlich and Temkin models.

Table 4. Comparison of REEs(III) adsorption capacity with some adsorbent materials in the references.

Table 5. Thermodynamic parameters of Sc(III) adsorption on the 0.075-AA0.072@MIL-101.

29



Scheme 1. Preparation of adsorbent y-AA-x@MIL-101s.

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Scheme 2. The mechanism of Sc(III) adsorbed on the 0.075-AA-0.072@MIL-101.

31



a 40

b

MIL-101 AA-0.058@MIL-101 AA-0.072@MIL-101 AA-0.094@MIL-101 AA-0.144@MIL-101

30

20

40

20

10

0

MIL-101 AA-0.072@MIL-101 0.025-AA-0.072@MIL-101 0.05-AA-0.072@MIL-101 0.075-AA-0.072@MIL-101 0.1-AA-0.072@MIL-101

60

qe(mgg-1)

qe(mgg-1)

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

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1

2

3

pH

4

0

4.5

1

2

3

4

4.5

pH

Fig 1. Effect of pH on Sc(III) recovery by (a) MIL-101 and AA-(0.058-0.144)@MIL101s, (b) (0.025-0.1)-AA-0.072@MIL-101s. (t = 24 h; T = 303 K).

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Fig 2. SEM images of (a) MIL-101, (b) AA-0.072@MIL-101 and (c) 0.075-AA0.072@MIL-101.

33



b

a 0.075-AA-0.072@MIL-101 3430

0.075-AA-0.072@MIL-101 AA-0.072@MIL-101 2935

1712

AA-0.072@MIL-101 MIL-101

MIL-101

c 1200 1000 800

3500

3000

2500

2000

1500

Wavenumbers (cm-1)

1000

15

20

25

2 theta degree

d 10


600 400 200 0.0

10

500

Zeta Potential (mv)

4000

Volume (cm3g-1STP)

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

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0 -10 -20 -30 -40 -50

0.2

0.4

0.6

0.8

Relative Pressure (P/P0)

1.0

1

2

3

4

pH

5

6

7

8


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a

b

0.3 mol.L-1 HCl

0.3mol.L-1 HCl

pH 1

pH 1

pH 2 pH 2

pH 3

pH 3

pH 4

pH 4

pH 4.5

pH 4.5

water

0

water

20

40

60

Residual weight (wt%)

80

6

100

9

12

15

18


21

24


35



a 95

b 100

303 K

90

313 K

Adsorption (%)

Adsorption (%)

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

323 K

85 80 75 70 65

80

CSc/x was 1:1 (Sc(III): 20mgL-1) CSc/x was 1:2 (Sc(III): 20mgL-1)

60 40 20

60

0

4

8

12

t (h)

16

20

0

24

Cu(II) Zn(II) Mn(II) Co(II) Al(III) Sc(III)


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a

b

0.075-AA-0.072@MIL-101 loaded Sc(III)

0.075-AA-0.072@MIL-101 loaded Sc(III) C1s

O1s

1712

2935

403.26 eV 3430

Sc 2p

0.075-AA-0.072@MIL-101

0.075-AA-0.072@MIL-101

4000

3500

3000

2500

2000

1500

Wavenumbers (cm

c

538

536

534

1000

500

0

200

531.94

532

530

528

600

Raw 532.76 Sum Background O=C (10.89%) O-C (26.64%) O-H (16.10%) O-Cr (23.78%) O-Sc (22.59%) 533.56

531.28

Binding Energy (eV)

400

526

524

540

800

Binding Energy (eV)

)

d

Raw 532.71 Sum Background O=C (24.30%) O-C (26.76%) O-H (22.54%) O-Cr (26.40%) 533.51

540

-1

538

536

534

1000

1200

532.11 531.60

531.06

532

530

528

526

524

Binding Energy (eV)


37



b 60 La(III) Ce(III) Pr(III) Nd(III)

60

Adsorption (%)

Adsorption (%)

a 80

40

20

Sm(III) Eu(III) Gd(III) Tb(III) Dy(III)

50 40 30 20 10 0

0 1

2

pH

3

1

4

2

pH

3

4

d

c 30

Ho(III) Er(III) Tm(III) Yb(III) Lu(III)

20

La(III) Ce(III) Pr(III) Nd(III) Sm(III) Eu(III) Gd(III) Tb(III) Dy(III) Ho(III) Er(III) Tm(III) Yb(III) Lu(III) Y(III) Sc(III)

100

Adsorption (%)

Adsorption (%)

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

Page 38 of 56

10

80 60 40 20 0

0 1

2

pH

3

1

4

2

3

pH

4

Fig 7. Selective adsorption of 0.075-AA-0.072@MIL-101 at pH 1-4.5 for (a) light rare earth, (b) middle rare earth, (c) heavy rare earth and (d) 16 REEs rare earth mixture. (C0 = 20 mg.L-1 for each REEs; solid-to-liquid ratio = 1 g : 1 L; T = 303 K).

38


Page 39 of 56

a 100 Adsorption (%)

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


b 0.075-AA-0.072@MIL-101

80

cycle 1

60

cycle 2

40

cycle 3 cycle 4

20

cycle 5

0

1

2

3

cycles

4

5

6

9

12


15

18

Fig 8. (a) Adsorption performance and (b) XRD patterns of the 0.075-AA-0.072@MIL101 in consecutive 5 cycles.

39


ACS Applied Materials & Interfaces 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

Page 40 of 56

Table 1. Surface functional group titration, BET and adsorption capacity of Sc(III) of MIL-101, AA-0.072@MIL-101 and (0.025-0.1)-AA-0.072@MIL101s.


Adsorption capacity of Sc(III) at pH 4.5 (mg.g-1)

Carboxyl (mmol.g-1)

18.84 36.82 48.08 52.32 62.45 63.04

4.25 4.75 5.13 5.50 5.77 5.79

Phenolic hydroxyl (mmol.g-1) 4.38 5.44 5.75 5.83 6.63 6.38

40


Lactone (mmol.g-1) 0 0 0 0 0 0

Total acid (mmol.g-1) 8.63 10.19 10.88 11.33 12.40 12.17

Specific surface Area (m2.g-1) 2451.87 2336.67 1497.72 1286.85 1230.87 1083.46

Page 41 of 56 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


Table 2. Kinetic parameters of different temperatures for Sc(III) adsorption on the 0.075-AA-0.072@MIL-101. T (K)

qe (mg.g-1)

Pseudo first-order equation qe k1 R2 (mg.g-1) (h-1)

Pseudo second-order equation qe k2 R2 (mg.g-1) (g.mg-1.h-1)

Elovich equation β .mg-1) . -1. -1 (g (mg g h ) α105

R2

Intraparticle diffusion kp R2 (mg.g-1.h-1/2)

303

40.19

11.13

-0.39

0.88

40.50

6.67

1

4.1

0.37

0.94

2.95

0.77

313

39.66

1.04

-0.36

0.92

40.01

5.72

1

3.5

0.38

0.93

2.99

0.79

323

38.99

0.97

-0.24

0.89

39.11

5.16

1

11

0.43

0.95

2.68

0.82

41


ACS Applied Materials & Interfaces 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

Page 42 of 56

Table 3. Isotherm parameters for Sc(III), Nd(III), Gd(III) and Er(III) by using Langmuir, Freundlich and Temkin models. Isotherms Langmuir

Freundlich Temkin

Constants qmax (mg.g-1) KL (L.mg-1) R2 KF (L1/n.mg(1-1/n).g-1) n R2 A (L.mg-1) b R2

42


Sc(III) 90.21 0.17 0.97 30.13 4.58 0.89 4.64 183.93 0.96

Nd(III) 104.59 0.03 0.98 15.86 3.19 0.85 0.77 150 0.90

Gd(III) 58.29 0.12 0.97 15.90 4.03 0.89 3.58 288 0.94

Er(III) 74.94 0.03 0.99 12.09 3.24 0.92 0.67 195.92 0.98

Page 43 of 56 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


Table 4. Comparison of REEs(III) adsorption capacity with some adsorbent materials in the references. Adsorption capacity (mg.g-1 )

No.

Adsorbent

1 2 3 4 5 6 7 8

Gum Arabic grafted polyacrylamide based silica nanocomposite EDTA functionalized Chitosan-silica 1-(2-pyridylazo)-2-napththol mobilized gels Acetylacetone modified silica gel using microcolumn Imprinted mesoporous film Cellulose based silica nanocomposite Diethylenetriamine functionalized Cr-MIL-101 0.075-AA-0.072@MIL-101

pH 6 4 5 6 3 6 5.5 4.5

Sc(III) 11.05 42.41 25.65 23.76 90.21

Nd(III) 12.24 60.00 23.77 34.94 70.90 104.59

43


Gd(III) 25.58 90.00 58.29

Er(III) 16.05 74.94

Ref. 41 42 43 44 45 46 47 This paper


Page 44 of 56

Table 5. Thermodynamic parameters of Sc(III) adsorption on the 0.075-AA-0.072@MIL-101. T (K) ∆Gθ (kJ.mol-1) ∆Hθ (kJ.mol-1) ∆Sθ (J.mol-1.K-1)

303 K -5.94

313 K -5.76 -11.39 -18

44


323 K -5.51

Page 45 of 56 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


Caption Scheme 1. Preparation of adsorbent y-AA-x@MIL-101s.

Scheme 2. The mechanism of REEs(III) adsorbed on the 0.075-AA-0.072@MIL-101.

Fig

1.

Effect

of

pH

on

Sc(III)

recovery

by

(a)

MIL-101

and

AA-(0.058-0.144)@MIL-101s, (b) (0.025-0.1)-AA-0.072@MIL-101s. (t = 24 h; T = 303 K).

Fig

2.

SEM

images

of

(a)

MIL-101,

(b)

AA-0.072@MIL-101

and

(c)

0.075-AA-0.072@MIL-101.






Page 46 of 56


Fig 7. Selective adsorption of 0.075-AA-0.072@MIL-101 at pH 1 - 4.5 in (a) light rare earth, (b) middle rare earth, (c) heavy rare earth and (d) 16 REEs rare earth mixture. (C0 = 20 mg.L-1 for each REEs; solid-to-liquid ratio = 1 g : 1 L; T = 303 K).

Fig

8.

(a)

Adsorption

performance

and

(b)

0.075-AA-0.072@MIL-101 in consecutive 5 cycles.


XRD

patterns

of

the

Page 47 of 56 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


Scheme 1. Preparation of adsorbent y-AA-x@MIL-101s.



Scheme 2. The mechanism of Sc(III) adsorbed on the 0.075-AA-0.072@MIL-101.


Page 48 of 56

Page 49 of 56

a 40

b

MIL-101 AA-0.058@MIL-101 AA-0.072@MIL-101 AA-0.094@MIL-101 AA-0.144@MIL-101

30

20

40

20

10

0


60

qe(mgg-1)

qe(mgg-1)

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


1

3

2

4.5

4

0

1

2

pH

Fig

1.

Effect

of

3

4

4.5

pH

pH

on

Sc(III)

recovery

by

(a)

MIL-101

and

AA-(0.058-0.144)@MIL-101s, (b) (0.025-0.1)-AA-0.072@MIL-101s. (t = 24 h; T = 303 K).



Fig

2.

SEM

images

of

(a)

MIL-101,

(b)

AA-0.072@MIL-101

0.075-AA-0.072@MIL-101.


Page 50 of 56

and

(c)

Page 51 of 56

a

b 0.075-AA-0.072@MIL-101 3430 0.075-AA-0.072@MIL-101

AA-0.072@MIL-101 2935

1712 AA-0.072@MIL-101

MIL-101

MIL-101

4000

3500

3000

2500

2000

1500

1000

10

500

15

1000 800 600 400 200 0.0

25

d 10


Zeta Potential (mv)

c 1200

20

2 theta (degree)

Wavenumbers (cm-1)

Volume (cm3g-1STP)

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


0 -10 -20 -30 -40 -50

0.2

0.4

0.6

0.8

1.0

1

2

Relative Pressure (P/P0)

3

4

5

6

7

8

pH




Page 52 of 56

b

a

0.3 mol.L-1 HCl

0.3mol.L-1 HCl

pH 1

pH 1

pH 2

0

pH 2

pH 3

pH 3

pH 4

pH 4

pH 4.5

pH 4.5

water

water

20

40

60

80

100

6

9

Residual weight (wt%)

12

15

18

2 theta (degree)

21

24



Page 53 of 56

b 100

a 95 303 K

CSc/x was 1:1 (Sc(III): 20mgL-1)

313 K

CSc/x was 1:2 (Sc(III): 20mgL-1)

Adsorption (%)

90

Adsorption (%)

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


323 K

85 80 75 70 65

80 60 40 20

60

0

4

8

12

t (h)

16

20

24

0

Cu(II) Zn(II) Mn(II) Co(II) Al(III) Sc(III)




a

b



1712

2935

Page 54 of 56

C1s O1s 403.26 eV

3430

Sc 2p

0.075-AA-0.072@MIL-101

0.075-AA-0.072@MIL-101

4000

3500

3000

2500

2000

1500

1000

500

Wavenumbers (cm-1)

c

538

536

534

200

400

531.94

Raw 532.76 Sum Background O=C (10.89%) O-C (26.64%) O-H (16.10%) O-Cr (23.78%) O-Sc (22.59%) 533.56

531.28

532

530

600

528

800

1000

1200

Binding Energy (eV)

d

Raw 532.71 Sum Background O=C (24.30%) O-C (26.76%) O-H (22.54%) O-Cr (26.40%) 533.51

540

0

526

524

540

Binding Energy (eV)

538

536

534

532.11 531.60

531.06

532

530

528

526

524

Binding Energy (eV)



Page 55 of 56

b 60

La(III) Ce(III) Pr(III) Nd(III)

60

Sm(III)

Adsorption (%)

Adsorption (%)

a 80

40

20

Eu(III) Gd(III) Tb(III) Dy(III)

50 40 30 20 10 0

0 1

2

3

4

1

2

pH

3

4

pH

d

c 30

Ho(III) Er(III) Tm(III) Yb(III) Lu(III)

20

La(III) Ce(III) Pr(III) Nd(III) Sm(III) Eu(III) Gd(III) Tb(III) Dy(III) Ho(III) Er(III) Tm(III) Yb(III) Lu(III) Y(III) Sc(III)

100

Adsorption (%)

Adsorption (%)

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


10

80 60 40 20 0

0 1

2

pH

3

4

1

2

3

4

pH

Fig 7. Selective adsorption of 0.075-AA-0.072@MIL-101 at pH 1-4.5 for (a) light rare earth, (b) middle rare earth, (c) heavy rare earth and (d) 16 REEs rare earth mixture. (C0 = 20 mg.L-1 for each REEs; solid-to-liquid ratio = 1 g : 1 L; T = 303 K).



a 100 Adsorption (%)

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

Page 56 of 56

b 0.075-AA-0.072@MIL-101

80

cycle 1

60 cycle 2

40

cycle 3 cycle 4

20

cycle 5

0

1

2

3

4

5

6

cycles

Fig

8.

(a)

Adsorption

performance

and

9

12

2 theta (degree)

(b)

0.075-AA-0.072@MIL-101 in consecutive 5 cycles.


XRD

15

patterns

18

of

the

Acrylic acid functionalized metal-organic frameworks (MOFs) for Sc(III

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