Gd3+ Adsorption over Carboxylic- and Amino-Group Dual

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Separations 3+

Gd adsorption over carboxylic- and amino-group dual-functionalized UiO-66 Imteaz Ahmed, Yu-Ri Lee, Kwangsun Yu, Samiran Bhattacharjee, and Wha-Seung Ahn Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05220 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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Industrial & Engineering Chemistry Research

Gd3+ adsorption over carboxylic- and amino-group dualfunctionalized UiO-66 Imteaz Ahmed a, Yu-Ri Lee a, Kwangsun Yu a, Samiran Bhattacharjee b, Wha-Seung Ahn a*

a. Department of Chemistry and Chemical Engineering, Inha University, Incheon 402-751, Republic of Korea. E-mail: [email protected]; Fax: +82 328720959; Tel; +82 328607466 b. Centre for Advanced Research in Sciences (CARS), University of Dhaka, Dhaka 1000, Bangladesh. Email: [email protected]

ABSTRACT: Owing to the growing industrial demand worldwide and limited mineral deposit, the recovery of low-concentration rare earth elements (REEs) from waste sources is being considered, which also helps to reduce water pollution. In this work, the adsorption of gadolinium ions (Gd3+) in aqueous solutions over a functionalized metal-organic framework (MOF), UiO-66, was investigated. Initially, the MOF structure was synthesized solvothermally using a ligand mixture of terephthalic and trimellitic acids to produce coordination-free –COOH groups on the UiO-66 framework. Subsequently, the –COOH group was reacted with ethylenediamine to introduce additional –NH2 groups onto the MOF. The optimized product (denoted as UiO-66COOH-ED) showed an equilibrium adsorption capacity of 79 mg/g for Gd3+ compared with 16 mg/g by pristine UiO-66. This improvement in adsorption by a factor of 4.9 was a consequence of coordination of Gd3+ ions with the electron-abundant oxygen and nitrogen atoms of –COOH and –NH2 groups. A solution pH between 6.0 and 7.0 was found to be the best for Gd3+ capture, and a 1 ACS Paragon Plus Environment

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selectivity of 75% towards Gd3+ was observed against other competing alkali or transition metal ions coexisting in the solution. UiO-66-COOH-ED was reusable for at least five cycles without any noticeable deterioration in its adsorption capacity.

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1. INTRODUCTION Many modern industries including electronics, communications, and refinery are critically dependent on rare earth elements (REEs), i.e. yttrium, scandium, and other lanthanides (including gadolinium (Gd)).1,2 It is anticipated that the demand for REEs will increase by several folds within the next few years.3 However, the supply of REEs is hindered by the limited mineral reserves and controlled supply from China, the major producer of REEs.4 In this regard, recovering REEs from various sources including industrial wastes and post-consumer scraps becomes useful. Especially, suitable methods to recover them from aqua-environment for reuse need to be developed.5 Adsorption,6 biosorption,7 liquid-liquid extraction,8 ionic liquid solvent extraction,9-11 coprecipitation,12 ion-imprint,13 and ion-exchange14 have been used for recovery of REEs. Among these, adsorption has the advantages of low energy consumption, mild operating conditions, and simplicity.15 This method also allows the recovery from low-concentration sources of REEs.16 To this end, different kinds of adsorbents have been tested for separation of REEs, including porous silica-based materials,17,18 carbons,19,20 metal oxides,21,22 polymers23 and metal-organic frameworks (MOFs).24-26 Among these, MOFs comprised of metal or metal oxide clusters linked by organic ligands are promising for a broad range of adsorption applications, owing to their high porosity and tunable surface functionality.27-31 UiO-66 (Zr6O4(OH)4(CO2)12) is a robust MOF made of Zr (IV) oxide metal nodes connected by benzenedicarboxylate linkers,32-33 and the metal cluster of Zr6O4(OH)4 octahedra is 12-fold connected with adjacent cluster units through the linkers. This creates a tightly packed fcc structure, which makes this material among one of the most stable MOFs.30 The structure of UiO-66 contains two types of hollow cages (octahedral and tetrahedral, 11 and 8 Å in diameter, respectively), and has been tested for various applications in adsorption/separation and catalysis.34-41 3 ACS Paragon Plus Environment


Herein, we report a unique dual-functionalized UiO-66 with –COOH and –NH2 groups on the framework for the recovery of Gd3+ in water. The introduced –COOH and –NH2 groups markedly enhanced the adsorption capacity for Gd3+ compared to pristine UiO-66. After examining the adsorption equilibrium, kinetics, and selectivity against other ions, a possible adsorption mechanism was proposed for the improved uptake of Gd3+ ions by the functionalized MOF. Finally, the reusability of the adsorbent was examined.

2. EXPERIMENTAL 2.1. Chemicals and reagents. Zirconium chloride (ZrCl2, 99.0%), terephthalic acid (TPA, 98.0%), trimellitic acid (TMA, 99.0%), ethylenediamine (ED, 99.5%), N,N-dimethylformamide (DMF, 99.0%), toluene (99.0%), and hydrochloric acid (HCl, 36%, aq) were purchased from Sigma Aldrich. Metal salts of Gd(NO3)3·6H2O (99.5%), NaCl (99.9%), CaCl2·3H2O (99.9%), Mg(NO3)2·6H2O (99.9%), Al(NO3)3·9H2O (99.9%), Fe(NO3)·9H2O (99.9%), La(NO3)3·6H2O (≥99.0%), Nd(NO3)3·6H2O (99.9%), and Yb(NO3)3·5H2O (99.5%),·were also obtained from Sigma Aldrich. All the chemicals were used without further purification. 2.2. Synthesis of the materials Synthesis of UiO-66. UiO-66 was prepared by a previously reported method42 using TPA as a linker as shown in Scheme 1(a). The detailed synthesis was given in the Supporting Information (SI). Synthesis of UiO-66-COOH. The substrate mixture and the synthesis conditions are identical to the preparation of UiO-66. However, in this case, a mixture of TPA and TMA was used as linkers instead of TPA alone as shown in Scheme 1(b). Two coordination-free COOH-containing MOFs were prepared using different ligand combinations (10% TMA + 90% TPA and 25% TMA + 75% TPA). The products were designated as UiO-66-COOH(10) and UiO-66-COOH(25), 4 ACS Paragon Plus Environment

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respectively. Synthesis of UiO-66-COOH-ED. After –COOH functionalization, 500 mg of UiO-66COOH(10) or UiO-66-COOH(25) was added to 50 ml of toluene in a round bottom flask. Then, 0.54 g of ED was charged into the mixture and refluxed for 12 h. The product was filtered and washed with ethanol and dried overnight. The synthesis protocol is described in Scheme 1(c). The materials were designated as UiO-66-COOH(10)-ED and UiO-66-COOH(25)-ED, respectively. UiO-66-ED without –COOH functionalization was also prepared for comparison, using the same synthesis condition and precursor mixture but with pristine UiO-66 (without free –COOH groups in the linker) instead of UiO-66-COOH. 2.3. Characterization. Powder X-ray diffraction (XRD) patterns were obtained with a Rigaku X-ray diffractometer with Cu Kα radiation (λ = 1.54). The N2 adsorption isotherms were measured at 77 K by a BELsorp Mini (BEL Corporation, Japan) instrument. Prior to the measurements, the samples were heated at 150 °C for 12 h under vacuum to remove moisture and impurities. Fouriertransform infrared spectroscopy (FTIR) was done by a VERTEX 80 V FTIR spectrophotometer (Bruker, Germany) using a pellet prepared by mixing with potassium bromide (KBr). Determination of N content was carried out using an elemental analyzer (EA1112, Thermo Fisher, USA). Morphology of the materials was examined with a Hitachi-S-4300 scanning electron microscope (Japan). Surface charge of the MOFs was measured with an ELSZ-2000 zeta potential analyzer (Otsuka Electronics, Japan). 2.4. Adsorption experiments. The functionalized UiO-66 samples were tested for Gd3+ adsorption in aqueous environment, and the adsorption equilibrium isotherms, kinetics, effect of pH and temperature, selectivity against competing ions, and reusability were examined systematically. The detailed experimental procedure was given in SI. 5 ACS Paragon Plus Environment


3. RESULTS AND DISCUSSION 3.1. Characterization of the adsorbents. Figure 1(a) shows the powder XRD patterns of the prepared UiO-66 adsorbents. All the materials showed practically the same XRD patterns with comparable intensities, indicating that the original structure was maintained after introducing free –COOH groups on the pristine UiO-66 framework. Identical crystal shapes were also observed for UiO-66 and UiO-66-COOH(25)-ED in the SEM images (Supporting information, Figure S1). Figure 1(b) shows the N2 adsorption-desorption isotherms of the materials. The porosity decreased in the order of UiO-66 > UiO-66-COOH(10) > UiO-66-COOH(25) > UiO-66-COOH(10)-ED > UiO-66-COOH(25)-ED, which must be due to the partial blocking of the pores by the additional –COOH and –NH2 functional groups. Reduction in porosity for the MOFs after surface functionalization has been generally observed.43 Figure 1(c) shows the FTIR spectra of the adsorbents. C=O stretching vibration was observed in the range of 1720 to 1750 cm-1 for all the functionalized materials, while N-H wagging peaks were observed in for UiO-66-COOH(10)-ED and UiO-66-COOH(25)-ED at 794 cm-1,42,44 which supports the successful functionalization of – COOH and –NH2 groups on the UiO-66 surface. Furthermore, aliphatic C-N stretching for UiO66-COOH(10)-ED and UiO-66-COOH(25)-ED was detected at 1340 cm-1, confirming the presence of ED molecule.44 An aromatic C-N stretching vibration was also obtained for UiO-66COOH(10)-ED and UiO-66-COOH(25)-ED, indicating amide formation between –COOH groups from the MOF and –NH2 from ED. Scheme 1(c) is a schematic presentation of the attachment of ED to UiO-66-COOH through amide formation.31 This is also confirmed by EA, and the N content of UiO-66-COOH(10)-ED and UiO-66-COOH(25)-ED was found to be ca. 1.08% and 1.84%, respectively. The different N contents of the two materials indicate that the ED attachment depends on the amount of free –COOH groups on the UiO-66-COOH samples: practically all free –COOH 6 ACS Paragon Plus Environment

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groups available in UiO-66-COOH(10) was involved in the amine-functionalization, whereas approximately 89% of the free –COOH was reacted with amines in UiO-66-COOH(25). 3.2. Adsorption capacity and kinetics. Figure 2(a) shows the adsorption results of Gd3+ over five adsorbent materials after 6 h. The equilibrium adsorption capacity increased in the order of UiO-66 < UiO-66-COOH(10) < UiO-66-COOH(25) < UiO-66-COOH (10)-ED < UiO-66COOH(25)-ED. As expected, the higher total amounts of both –COOH and –NH2 were desirable for effective adsorption of Gd3+. The kinetics of Gd3+ adsorption over UiO-66-COOH(25) and UiO-66-COOH(25)-ED was examined to investigate the potential kinetic effect of the additional ED functionality. As shown in Figure 2(b), the adsorption practically approached equilibrium (ca. 90 %) after 2 and 3 h for UiO-66-COOH(25) and UiO-66-COOH(25)-ED, and complete equilibrium was attained after 4 and 6 h, respectively. Both pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetics model were tested for the adsorbents UiO-66-COOH(25) and UiO-66-COOH(25)-ED to fit the experimental data. The equilibrium constants (k1 and k2 for PFO and PSO model, respectively) and the correlation coefficients are tabulated in Table S1. The PSO model fitted the kinetics data more accurately for both adsorbents, which implied that these surface-functionalized adsorbents operate most likely in a chelate exchange process governed by secondary chemical reactions.45,46 The result also indicates that the adsorption is comparatively slower after the introduction of ED, because the decreased porosity hindered the diffusion of hydrated Gd3+ (which has a comparatively higher hydration or solvation number among REEs) through the microporous framework in UiO-66-COOH(25)-ED.47 Direct grafting of ED was also attempted on the pristine UiO-66 without having the uncoordinated –COOH groups in the linker. This material (denoted as UiO-66-ED) showed ca. 7 ACS Paragon Plus Environment


20.2 mg/g Gd3+ adsorption after 6 h, an improvement of merely around 5 mg/g from the pristine UiO-66. According to EA, only a very small amount of ED was found on UiO-66-ED (0.13% N). This result is in agreement with the other reports that UiO-66 has very weak coordination unsaturated metal sites and cannot accommodate ED molecules through grafting.32,48,49 Therefore, the –COOH functional sites are important for introducing ED moiety within the UiO-66 structure. 3.3. Adsorption isotherms. Figure 3(a) and 3(b) show the adsorption isotherms of Gd3+ over the adsorbent materials. The equilibrium adsorption data were fitted to both the Langmuir and Freundlich models to investigate the interaction between Gd3+ ions and adsorbents and to estimate the adsorption capacities. Both of these isotherm model equations are given in the Supporting Information (SI) and the corresponding isotherm linear plots for Langmuir and Freundlich models are shown in Figures S2 and S3, respectively. The parameters for the isotherms and corresponding correlation factors are given in Table S2. As shown by the figures and table, it was evident that adsorption results can be fitted to the Langmuir isotherm model significantly better than the Freundlich model, and the maximum adsorption capacities estimated from the Langmuir plots are listed in Table 1: the adsorption capacity exhibited by UiO-66 was 16 mg/g, while those by UiO66-COOH(10)-ED and UiO-66-COOH(25)-ED were 74 and 79 mg/g, respectively. The adsorption improved around 4.9 times from the pristine UiO-66 to UiO-66-COOH(25)-ED. When comparing the adsorption per unit surface area for the functionalized MOFs (Table 1), the adsorption improved around 7.9 times from the pristine UiO-66 to UiO-66-COOH(25)-ED. This indicates that the organic functionalization makes a much larger contribution than the surface area of a MOF for efficient removal of Gd3+ ions. A comparison of the adsorption performances for Gd3+ by different materials are given in Table S3. Although the adsorption capacity by UiO-66-COOH(25)ED is not among the highest, it is still reasonably competitive against many other reported 8 ACS Paragon Plus Environment

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adsorbents for Gd3+ adsorption. 3.4. Effect of pH and adsorption mechanism. The enhanced adsorption of Gd3+ can be explained by the surface –COOH and –NH2 functional groups on UiO-66. According to the FTIR spectra of UiO-66-COOH-NH2 before and after adsorption (Figure S4), the wagging or stretching nodes for –NH2 or –COOH decreased after adsorption, which indicates that these organic groups are responsible for binding of Gd3+ ions. Coordination bonding between Gd3+ and the electronrich O atom of –COOH or N atom of –NH2 can lead to strong interaction between the adsorbent and REE ions.50,51 The binding of Gd3+ by these functional groups was also strongly affected by the solution pH. Figure 4 shows the influence of solution pH on the adsorption of Gd3+ ions over UiO-66COOH(25)-ED. The adsorption was very low at pH of 2.0 and gradually increased with increasing pH to the highest value at around pH of 6.0. At low pH (pH < 2), a large number of the functional sites involved in adsorption are protonated, and as a consequence Gd3+ adsorption on them becomes difficult because of electrostatic repulsion. In the pH range of 2.0 to 7.0, the functional groups (–COOH and –NH2) become increasingly deprotonated and with growing negative charges, enhancing the adsorption of Gd3+ through coordination. This was confirmed by the zeta potential measurement of the adsorbent (Figure S5). The surface charge gradually became negative with the increase of pH and became the lowest at pH 7.0. When pH > 7.0, the Gd3+ of the solution forms an insoluble hydroxide (Gd(OH)3),52 and so it was not meaningful to study the adsorption of Gd3+ beyond pH 7.0. 3.5. Effect of temperature on adsorption equilibrium. The effect of temperature on the Gd3+ adsorption over UiO-66-COOH(25)-ED in the temperature range of 25 to 45 oC was examined. The equilibrium adsorption amounts (qe) at different solution temperatures are shown in Figure 9 ACS Paragon Plus Environment


5(a). The adsorbed amount increased with increases in temperature, and the corresponding thermodynamic parameters of the change in enthalpy (ΔHo) and entropy (ΔSo) were calculated using the van’t Hoff equation as follows53: ln Kd = ΔSo/R − ΔHo/RT,

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where, Kd is the distribution coefficient obtained from the qe values (discussed in SI), T is the absolute temperature of adsorption in Kelvin (K), and R is the gas constant (8.314 J/mol.K). A straight line was obtained by plotting lnKd vs 1/T as shown in Figure 5(b), and the slope and intercept of this line provided the ΔHo and ΔSo given in Table 2. The Gibbs free energy (ΔGo) at different temperatures were also given in Table 2 (ΔGo = ΔHo − T ΔSo). The positive value of ΔHo indicated an endothermic adsorption of Gd3+ ions on UiO-66-COOH(25)-ED, whereas the positive ΔSo values indicated that with the increase of temperature the disorder or the degree of freedom of the system also increased. This increased disorder helped to increase the adsorption amount due to the increased possibility of collision at higher temperature, and the resulting negative ΔGo indicated a spontaneous and favorable adsorption process for Gd3+ ions on the adsorbent surface. 3.6. Selectivity in adsorption. REEs including Gd3+ ions may co-exist with other alkali or transition metal ions in mining or industrial wastewater, and selective capture of REE ions is important for practical applications.25,54 Thus, the selectivity of the adsorbent for Gd3+ ions was tested using a solution containing Na+, Ca2+, Mg2+, Al3+, and Fe3+ in addition to Gd3+. The metal ions were chosen as common representatives of alkali, alkaline earth, and transition metal ions.55 The selectivity was calculated by measuring the corresponding distribution coefficients (Kd values) of the individual metal ions and comparing them in a percentage basis.17,25 The calculation for the Kd values were discussed in the SI. Results of the competitive adsorption, shown in Figure 6(a), indicate a selectivity of ca. 75% for the Gd3+ ions, meaning that the adsorbent can efficiently 10 ACS Paragon Plus Environment

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capture Gd3+ against other alkali and transition metal ions present in the solution. This selectivity is reasonably competitive, considering that other studies on the selective adsorption of REEs reported selectivities around 70%.55,56 The selectivity of Gd3+ ions can be attributed to the chelating effect of the ligand groups on the surface of adsorbent towards the comparatively bulky Gd3+.57 According to the Pearson’s hard and soft acids and bases (HSAB) theory,58 carboxyl, carbonyl, and amine groups of the adsorbent are hard bases and have stronger affinity towards the hard acid Gd3+.58,59 Na+, Ca2+, and Mg2+ were poorly adsorbed because of their lack of coordination capability, whereas Al3+ and Fe3+ were adsorbed to some extent due to their minor coordination capability. In general, higher-valent cations have stronger binding affinity than the lower-valent ones.52 It can be stated overall that a combined effect of higher coordination capability due to chelating effect, hard acid-hard base interaction, and higher valency are the contributing factors for selective adsorption of Gd3+ ions. Selectivity among the various REEs ions was also considered (shown in Figure 6(b)). The selectivity in adsorption increased in the sequence of Yb3+>Gd3+>Nd3+>La3+ with the decrease of the ionic radius among the REE ions as observed by early reports.17,25 This tendency can be explained by ionic contraction and a consequence of poor shielding of the nuclear charge by the 4f orbitals of lanthanides. This causes the electrons of 5s and 5p orbitals to experience a larger effective nuclear charge. Therefore, the affinity of the adsorbent enfolding the respective ion is increased with the decrease of ionic radius, resulting a stronger coordination.17 3.7. Reusability. To investigate the reusability of UiO-66-COOH(25)-ED, desorption of the adsorbed Gd3+ ions was conducted using 0.1 mol/L HCl as a stripping agent. Since the desorption of Gd3+ ions is promoted by the decreases in solution pH because of the resulting positive surface charge of the adsorbent as mentioned before, stripping of Gd3+ ions were performed at pH of 1.0. 11 ACS Paragon Plus Environment


As shown in Figure 7, the adsorbent retained more than 95% of its initial adsorption capacity during the second cycle and around 92% after the fifth cycle. The adsorption capacity remains nearly constant for the third, fourth, and fifth cycles and the XRD pattern remained unaltered after the fifth run (Figure S6) which supports that UiO-66-COOH(25)-ED is quite stable for commercial application.

4. CONCLUSIONS Adsorption of Gd3+ in aqueous solutions over –COOH and –NH2 dual-functionalized UiO-66 samples was investigated. After synthesizing UiO-66 with free –COOH groups on the surface, – NH2 groups were further introduced by reacting with ethylenediamine. The adsorption capacity increased in the sequence of UiO-66 < UiO-66-COOH < UiO-66-COOH-ED, and the equilibrium adsorption capacity of UiO-66-COOH-ED was about 4.7 times higher than pristine UiO-66. The improved adsorption could be explained by the coordination of Gd3+ ions with the –COOH and – NH2 groups. The adsorption process was diffusion-controlled, and it took close to 3 h to attain practical equilibrium due to the reduced porosity of the UiO-66-COOH-ED by the organic functional groups. The adsorption of Gd3+ was sufficiently selective against other co-existing metal ions in the solution. Finally, the adsorbent could be easily regenerated and reused for at least four times without any noticeable decrease in the adsorption capacity. Supporting Information Supporting Information (SI) is available for: (1) The synthesis and characterization of the adsorbent materials, (2) Experimental procedures for adsorption i.e. adsorption kinetics, effect of pH, effect of temperature, adsorption selectivity, and (3) Calculations related to adsorption equilibrium, distribution co-efficient, adsorption model 12 ACS Paragon Plus Environment

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calculations for kinetics and isotherm. Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant number: NRF2015R1A4A1042434) and also by a research funding supported by the Carbon mineralization flagship center in Korea (2017).



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Page 22 of 33

Table 1. Surface properties and adsorption capacities of the investigated adsorbents. Adsorbent

BET surface area

Total pore volume

Maximum adsorption capacity (Qo)

Qo per unit surface area

(cm3/g) (m2/g)

(mg/g)

(mg/m2)× 10-3

UiO-66

1305

0.521

16

12.4

UiO-66-COOH(10)

1110

0.476

46

41.6

UiO-66-COOH(25)

1010

0.456

56

55.6

UiO-66-COOH(10)-ED

890

0.432

74

83.2

UiO-66-COOH(25)-ED

835

0.416

79

94.5


Page 23 of 33 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


Table 2. Thermodynamic parameters for the adsorption of Gd3+ ions by UiO-66-COOH(25)-ED. ΔHo (kJ/mol)

ΔSo (J/mol)

R2

22.7

140

0.999

ΔGo (kJ/mol) 298 K

303 K

308 K

313 K

318 K

-18.9

-19.6

-20.3

-21.0

-21.7



Scheme 1. Preparation of the functionalized UiO-66 MOFs: (a) UiO-66 formed from a TPA linker, (b) UiO-66 with a free –COOH formed from a mixture of TPA/TMA linker, (c) UiO-66-COOH reacts with ED to from UiO-66-COOH-ED.

(a) UiO-66-COOH-ED(25)

Intensity (a.u.)

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

UiO-66-COOH(25)

UiO-66-COOH-ED(10) UiO-66-COOH(10)

24

ACS Paragon Plus Environment UiO-66

Page 24 of 33

(b)

400

3

Vads (cm /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


200

UiO-66 UiO-66-COOH(10) UiO-66-COOH(25) UiO-66-COOH(10)-ED UiO-66-COOH(25)-ED 0 0.0

0.2

0.4

0.6

0.8

1.0

P/Po

150

(c)

C=O

UiO-66-COOH(25)-ED

C-N

120

Transmittance (a.u.)

Page 25 of 33

UiO-66-COOH(10)-ED 90

C-N

N-H

C-N

UiO-66-COOH(25)

60

UiO-66-COOH(10) UiO-66

30

0 600

800

1000

1200

1400

1600

1800

2000

2200

-1

Wavenumber (cm )

Figure 1. Characterization of the adsorbents studied: (a) XRD patterns; (b) N2 adsorption isotherms; and (c) FTIR spectra.



80

1. UiO-66 2. UiO-66-COOH(10) 3. UiO-66-COOH(25) 4. UiO-66-COOH(10)-ED 5. UiO-66-COOH(25)-ED

80

(a)

(b)

60

40

40

qt

qt (ppm)

60

20

UiO-66-COOH(25) UiO-66-COOH(25)-ED 20

0 1

2

3

4

0

5

0

100

200

300

400

Time (min)

Adsorbent

Figure 2. (a) Amount of Gd3+ adsorbed after 6 h. (b) Effect of time for Gd3+ adsorption over UiO66-COOH(25) and UiO-66-COOH(25)-ED. 75

75

(a)

(a) UiO-66 UiO-66-COOH(10) UiO-66-COOH(10)-ED

50

qe (mg/g)

50

qe (mg/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

Page 26 of 33

25

0 0

30

60

90

120

150

UiO-66 UiO-66-COOH(25) UiO-66-COOH(25)-ED

25

0 0

Ce (ppm)

30

60

90

Ce (ppm)


120

150

Page 27 of 33

Figure 3. Adsorption isotherms of Gd3+ over the functionalized UiO-66 materials. 75

50

qe(mg/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


25

0 2.0

3.0

4.0

5.0

pH


6.0

7.0


Figure 4. Effect of solution pH for Gd3+ adsorption over UiO-66-COOH(25)-ED. 90

(a)

8.4

(b)

8.2

lnKd

75

qe (mg/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

Page 28 of 33

60

8.0

7.8

7.6 45 25

30

35

40

0.00315

45

o

Temperature ( C)

0.00320

0.00325 -1

1/T (K )


0.00330

0.00335

Page 29 of 33

Figure 5. (a) Effect of temperature and (b) Linear plot of lnKd versus 1/T for the adsorption of Gd3+ ions over UiO-66-COOH(25)-ED. 60

80

(a)

(b)

Selectivity (%)

60

Selectivity (%)

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


40

20

0

+

Na

2+

Ca

Mg

2+

3+

Al

3+

Fe

40

20

0

3+

Gd

3+

La

3+

Nd

Metal ions

Metal ions


3+

Gd

3+

Yb


Figure 6. (a) Adsorption selectivity of Gd3+ against other metal ions over UiO-66-COOH(25)-ED and (b) selectivity among various REE ions over UiO-66-COOH(25)-ED.

60

qe(mg/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

40

20

0

1

2

3

4

Number of cycle


5

Page 30 of 33

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Figure 7. Reusability of UiO-66-COOH(25)-ED for Gd3+.



Abstract Graphic


Page 32 of 33

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Abstract Graphic 230x183mm (150 x 150 DPI)

ACS Paragon Plus Environment

Gd3+ Adsorption over Carboxylic- and Amino-Group Dual

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