Preparation of Polymeric Adsorbents Bearing Diglycolamic Acid

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Preparation of Polymeric Adsorbents Bearing Diglycolamic Acid Ligands for Rare Earth Elements Tomohiro Shinozaki, Takeshi Ogata, Ryo Kakinuma, Hirokazu Narita, Chiharu Tokoro, and Mikiya Tanaka Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01797 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 28, 2018

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Industrial & Engineering Chemistry Research

Preparation of Polymeric Adsorbents Bearing Diglycolamic Acid Ligands for Rare Earth Elements

Tomohiro Shinozaki,†,‡ Takeshi Ogata,*,† Ryo Kakinuma,†,‡ Hirokazu Narita,† Chiharu Tokoro,‡ and Mikiya Tanaka†

†

Environmental Management Research Institute, National Institute of Advanced Industrial Science

and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan ‡

School of Creative Science and Engineering, Department of Earth Sciences, Resources and

Environmental Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan

ABSTRACT: We synthesized polymeric adsorbents modified with diglycolamic acid ligands for the recovery of rare earth elements. Styrene, divinylbenzene, and glycidyl methacrylate were

copolymerized by suspension polymerization in the presence of diluent mixtures of heptane and

toluene. Varying the composition of the diluent mixtures changed the pore characteristics of the polymeric particles; the highest specific surface area (51.2 m2/g) was obtained with mixtures of

equal volumes of heptane and toluene. Polymeric adsorbents were prepared by functionalizing the

synthesized polymeric particles with amino groups and then diglycolamic acid ligands. The content

of the amino groups was almost constant, whereas that of the diglycolamic acid ligands depended on

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the specific surface area of the adsorbents. The synthesized polymeric adsorbents selectively

adsorbed rare earth elements from a solution containing rare earth and base metal ions. The high

adsorption rate of the adsorbents was due to their large specific surface area.

1. INTRODUCTION Rare earth elements have unique properties and are used in a wide variety of applications. Wind

turbines and electric vehicles rely heavily on permanent magnets containing dysprosium and

neodymium, so rare earth elements are essential in clean energy technologies. The demand for

permanent magnets is rising with the growing deployment of wind turbines and electric vehicles. However, there is a lack of diversity in the sources of supply of rare earth elements.1,2 One supplier country monopolizes the international supply market,3 which results in potential risks of shortages of

rare earth elements in the market. Nontraditional resources are therefore being investigated as a way to diversify the sources of supply and mitigate such potential risks. Apatite4,5 and deep-sea mud6,7

have attracted attention as new sources of rare earth elements. The recovery of rare earth elements

from nontraditional resources is considered to be technically difficult, because the concentrations of

rare earth ions in these sources are relatively low, and base metal ions coexist at high concentrations.

To overcome such technical difficulties, there is a need to separate rare earth ions from base metal

ions.

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Precipitation, solvent extraction, and adsorption are often used as separation techniques for

metal recovery. Among these methods, adsorption is known to be an effective separation technique

when the concentrations of the target metal ions are low. We have hypothesized that adsorption could

be a suitable method for the recovery of rare earth elements from nontraditional resources. Many

adsorbents have been studied for the recovery of rare earth elements. Over the last 10 years,

diglycolamide ligands were immobilized on resins, mesoporous silica, and mesoporous carbon, among other adsorbent supports.8-10 These adsorbents can be used for the recovery of rare earth

elements at pH 3–4. However, we aimed at selective recovery of rare earth elements from solutions

around pH 1, and it was considered that diglycolamic acid ligands were more suitable than

diglycolamide ligands in this pH region. Thus we have focused on diglycolamic acid ligands. In our previous work,11–16 we synthesized silica gel adsorbents modified with diglycolamic acid ligands

(EDASiDGA). We found that EDASiDGA adsorbed rare earth ions selectively from solutions

containing high concentrations of base metal ions and that adsorption and desorption rates of

EDASiDGA were high enough for practical use. We have concluded that the ether, amide, and

carboxylic acid oxygen atoms contained in the diglycolamic acid ligands allow EDASiDGA to selectively adsorb rare earth elements.16

Although EDASiDGA does an excellent job of adsorbing rare earth ions, there are two

disadvantages to its use that must be overcome for practical purposes: the formability and solubility

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of silica gel particles. A particle of EDASiDGA is approximately 100 µm in diameter,13 whereas the

preferable size of adsorbents with respect to their handling properties is approximately 500 µm for

practical use. However, it is difficult to enlarge a silica gel particle as an adsorbent support because a

large silica gel particle tends to break apart when it is immersed in water. Furthermore, although

silica gel is soluble in an alkaline solution, it is slightly soluble in an acidic solution after repeated

use, and the adsorption capacity of EDASiDGA therefore decreases. Due to these disadvantages

associated with silica gel supports, we used polymeric particles instead of silica gel particles as

adsorbent supports but did not change the diglycolamic acid ligands. Styrene-divinylbenzene

(STY-DVB) copolymers, which have been widely used as commercial adsorbent supports, were

selected because of their chemical and physical stability. In addition, the particle size and pore

characteristics of STY-DVB copolymers can be controlled by suspension polymerization in the presence of diluent mixtures.17,18

In this research, we synthesized polymeric adsorbents modified with diglycolamic acid ligands.

To introduce diglycolamic acid ligands onto the polymeric particles, we added glycidyl methacrylate (GMA) as a monomer; GMA contains epoxy groups that can be easily functionalized.19–22 We

copolymerized STY, DVB, and GMA by suspension polymerization in the presence of diluent

mixtures. The pore characteristics of the synthesized polymeric particles were investigated as a

function of the composition of the diluent mixtures. The polymeric adsorbents were prepared by

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functionalizing the synthesized polymeric particles with an amino group and then reacting the

obtained intermediate with diglycolic anhydride. Characteristics of the adsorption of rare earth ions

by polymeric adsorbents with different pore characteristics were evaluated in terms of selectivity

from base metal ions, adsorption rate, and adsorption capacity.

2. MATERIALS AND METHODS 2.1 Materials. STY (99%) and DVB (containing >50% mixtures of meta- and para-DVB) were purified with 10 wt % NaOH aqueous solutions, washed with distilled water, and then dried over

anhydrous sodium sulfate to remove an inhibitor. GMA, heptane, toluene, 2,2′-azobisisobutyronitrile (AIBN), and poly(vinyl alcohol) (saponification degree 78–82%, average polymerization degree

>1500) (Wako Pure Chemical Industries, Japan) were used for synthesis of the polymeric particles.

Ethylenediamine, tetrahydrofuran (Wako Pure Chemical Industries, Japan), and diglycolic anhydride

(Tokyo Chemical Industry, Japan) were used to introduce functional groups.

The stock solutions were prepared by dissolving neodymium(III) chloride hexahydrate,

dysprosium(III) chloride hexahydrate, iron(III) chloride hexahydrate, copper(II) chloride dihydrate,

and zinc(II) chloride (Wako Pure Chemical Industries, Japan).

2.2 Preparation of polymeric adsorbents. The polymeric adsorbents were synthesized in three steps (Scheme 1). The first step was to synthesize polymeric particles by suspension polymerization

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in the presence of diluent mixtures. The second step was to introduce amino groups onto the

synthesized polymeric particles. The final step was to immobilize diglycolamic acid ligands onto the

polymeric particles containing the amino groups.

Scheme 1. Preparation of polymeric adsorbents modified with diglycolamic acid ligands

In the first step, STY, DVB, and GMA were added as monomers to a 500-mL four-necked

separable flask. STY and GMA were added in equimolar amounts, and DVB was added at 12 wt %

of the total monomers. Heptane and toluene were added as diluents to the flask. The total volumes of

the monomers and diluent mixtures were both 59 mL. The volume percentages of heptane in the

diluent mixtures were 0, 25, 33, 50, 67, 75, and 100%. AIBN was used as an initiator (1.5 mol % of

the total monomers). The aqueous phase (150 mL) contained poly(vinyl alcohol) (0.15 g), which was

poured into the flask as a suspending agent. Suspension polymerization was carried out while

stirring at 250 rpm and 343 K for 6 h and then at 353 K for 2 h. Synthesized polymeric particles

were filtered through a Büchner funnel under reduced pressure and then washed with ethanol, hot

distilled water, and distilled water to remove unreacted substances. Obtained polymeric particles 6


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were dried under vacuum at 333 K for 3 h. After vacuum drying, the polymeric particles were sieved,

and particles in the 425–710 µm size range, which we designated R-GMA, were used in the

following steps.

In the second step, R-GMA (2.0 g) was functionalized with ethylenediamine (20 mL) at 323 K

for 24 h. After the reaction, the obtained particles were filtered and washed with ethanol and distilled

water. The resulting particles were dried under vacuum at 333 K for 3 h. We designated these

polymeric particles containing amino groups R-EDA.

In the final step, R-EDA (1.5 g) and diglycolic anhydride (3.5 g) were reacted in tetrahydrofuran

(35 mL) at 323 K for 24 h. After the reaction, the obtained particles were filtered and washed with

ethanol and distilled water. The resulting particles were dried under vacuum at 333 K for 3 h. We

designated these polymeric adsorbents immobilized with diglycolamic acid ligands R-DGA.

2.3 Characterization of the polymeric particles. The specific surface areas, total pore volume, and average pore size of R-GMA were determined by nitrogen adsorption (MicrotracBEL,

BELSORP-max) and evaluated with the Brunauer–Emmett–Teller method. Pore size distribution of

R-GMA was evaluated with Barrett-Joyner-Halenda method. Before the measurement, the samples

were degassed at 373 K for 6 h. The mass percentages of nitrogen in R-EDA and R-DGA were

measured with an elemental analyzer (Thermo Fisher Scientific, FLASH2000). The contents of

functional groups were roughly calculated using the following equations:

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Amino group content (mmol/g) = 10( ⁄ )

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(1)

Diglycolamic acid ligand content (mmol/g) = 1000 (1 − # ⁄ )⁄# (2) where NR-EDA and NR-DGA are the mass percentages of nitrogen in R-EDA and R-DGA, respectively; MN is the atomic weight of nitrogen; and MDGA is the molecular weight of diglycolic anhydride. Attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy was carried out to

determine the functional groups in R-GMA, R-EDA, and R-DGA with a FT-IR spectrometer (Perkin

Elmer, Spectrum 100). Before the measurement, the samples were pulverized into powder.

2.4 Procedure of adsorption experiments. Adsorption experiments were performed in batches. R-DGA (50 mg) and solutions containing each metal ion (5 mL) were added to a vial. The vial was

sealed and shaken at 180 rpm and 298 K. After shaking, R-DGA and the solutions were separated

using 0.2-µm membrane filters. The concentrations of each metal ion in the filtrates were measured

with an inductively coupled plasma spectrometer (Shimadzu, ICPE-9000). The adsorption ratio and

amount of adsorbed metal were calculated using the following equations: Adsorption ratio (%) = (&' − &)⁄&' × 100 Adsorption amount (mmol/g) = (&' − &))⁄*

(3)

(4)

where C0 is the initial concentration; C is the concentration of each metal ion in the filtrate; V is the solution volume; and w is the weight of R-DGA.

2.5 Stability experiment. Stability experiment was performed in batches. R-DGA (600 mg) and

8


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solution containing dysprosium ions (60 mL) were shaken at 180 rpm and 298 K in adsorption

process. After shaking, R-DGA was filtered and rinsed with distilled water. Subsequently, R-DGA

and 2 mol/L HCl solution (60 mL) were shaken at 180 rpm and 298 K in desorption process. After

shaking, R-DGA was filtered and rinsed with distilled water. Adsorption and desorption processes

were repeated five cycles.

3. RESULTS AND DISCUSSION 3.1 Characterization of polymeric particles. Spherical polymeric particles with the particle size of 500 µm were successfully prepared by suspension polymerization (Figure S1). Figure 1

shows the effect of the volume fraction of heptane on the specific surface area of R-GMA. For

comparison, Figure 1 also shows the specific surface areas of STY-DVB copolymers not containing

GMA. In STY-DVB copolymers, the specific surface area increased as the volume fraction of heptane increased. According to the work reported by Kangwansupamonkon et al.,18 this relationship

is due to the difference of phase separation between the polymer and diluents. If toluene is used as a

diluent, the phase separation occurs during gelation of STY and DVB. If heptane is used as a diluent,

the phase separation may occur before gelation. Thus, as the volume fraction of heptane increased,

greater microphase separation occurred, and the copolymers became more porous. In R-GMA, the

specific surface area increased with increasing volume fraction of heptane from 0 to 50 vol %.

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However, at heptane volume fractions exceeding 50 vol %, the specific surface area decreased with

increasing heptane fraction. We hypothesized that the addition of GMA to STY and DVB resulted in

different phase separations as the volume fraction of heptane increased. The highest specific surface

area was obtained at a heptane fraction of 50 vol %. These results indicated that the specific surface

area of R-GMA could be controlled by changing the composition of diluent mixtures during

suspension polymerization. The effect of the volume fraction of heptane on the total pore volume,

average pore size, and pore size distribution of R-GMA were shown in Figures S2–S4.

100 2

Specific surface area (m /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

R-GMA STY-DVB

80 60 40 20 0 0

25

50

75

100

Volume fraction of heptane (vol %)

Figure 1. Effect of volume fraction of heptane on specific surface areas.

3.2 Functionalization of the polymeric particles. To confirm that functional groups had been introduced onto polymeric particles, we measured the ATR FT-IR spectra of R-GMA, R-EDA, and

R-DGA that had been synthesized at a heptane fraction of 50 vol % (Figure 2). Peaks at approximately 915 and 840 cm–1 due to epoxy absorption were observed in R-GMA,23 but analogous 10


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peaks were absent in the spectrum of R-EDA. Finally, the distinct peak of amide appeared at approximately 1630 cm–1 in the spectrum of R-DGA.24 These results confirmed that diglycolamic

acid ligands were immobilized on R-DGA.

R-GMA

R-EDA

R-DGA

Transmittance (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


1600

1400

1200

1000

800

–1

Wavenumber (cm )

Figure 2. ATR FT-IR spectra of R-GMA, R-EDA, and R-DGA that were synthesized at a heptane fraction of 50 vol %.

Table 1 shows the content of amino groups in R-EDA and diglycolamic acid ligands in R-DGA

as a function of the volume fraction of heptane. The amounts of introduced amino groups were

almost constant, regardless of the specific surface area of R-GMA. The ATR FT-IR spectra (Figure

2) indicated that almost all the epoxy groups had probably been converted to amino groups. It was

hypothesized that epoxy groups other than those on the particle surface were also reacting with

ethylenediamine. The R-EDA contained more than twice the number of amino groups as commercial amino-silica gel (Tokyo Chemical Industry, Japan, 1.4–2.0 mmol/g)12 and probably introduced large

amounts of diglycolamic acid ligands. The amounts of introduced diglycolamic acid ligands

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increased as the fraction of heptane increased from 0 to 33 vol %. The amount of diglycolamic acid

ligands on the polymeric particles became saturated at approximately 3 mmol/g, because the

optimum conversion ratio from amino groups to diglycolamic acid ligands is approximately 65% in the case of silica gel support.12 In contrast, at heptane fractions greater than 50 vol %, the content of

diglycolamic acid ligands decreased as the fraction of heptane increased. The dependence of the

diglycolamic acid ligand content on the fraction of heptane was consistent with the analogous

dependence of the specific surface area. This pattern was probably due to the reaction of diglycolic

anhydride with the amino groups introduced onto the particle surface. The largest amount of

diglycolamic acid ligands was achieved when the fraction of heptane was 33 vol %; that amount (3.05 mmol/g) was larger than the amount of silica gel adsorbent (1.04 mmol/g).12

Table 1. Content of amino groups in R-EDA and diglycolamic acid ligands in R-DGA Heptane

Amino group

Diglycolamic acid ligand

Conversion ratio from amino group

fraction

in R-EDA

in R-DGA

to diglycolamic acid ligand

(vol %)

(mmol/g-R-EDA)

(mmol/g-R-DGA)

(%)

0

4.81

0

0

25

4.72

0.118

2.50

33

4.70

3.05

64.9

50

4.38

3.00

68.5

67

3.88

1.78

45.9

75

3.88

1.08

27.8

100

4.22

0.920

21.8

3.3 Adsorption behavior of R-DGA. Figure 3 shows the equilibrium pH dependence of the

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adsorption ratio of each metal ion adsorbed onto R-DGAs. The initial solution contained 1 mmol/L

each of dysprosium(III) and neodymium(III) as rare earth ions and iron(III), copper(II), and zinc(II)

as base metal ions. The equilibrium pH increased from the initial pH due to the protonation of amino

group residues when the amounts of diglycolamic acid ligands in R-DGA were small (0, 25, 75, and

100 vol %). Under these conditions, the R-DGAs adsorbed small amounts of rare earth elements. In

contrast, when the introduced amounts of diglycolamic acid ligands were relatively large (33, 50,

and 67 vol %), the increase in pH from the initial pH was small, and rare earth ions, especially

dysprosium ions, were selectively adsorbed at low pH values. Based on these results, we concluded

that synthesized polymeric adsorbents (R-DGAs) with a suitable particle size had high selectivity for

rare earth elements.

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(a)

Adsorption ratio (%)

100

(b)

100

80

80

60

60

40

40

20

20

(c)

(d)

0

0 0

4

8

0

4

pHeq

8

0

1

pHeq

(e)

100


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(f)

100

2

3

0

1

2

3

pHeq

pHeq

(g)

100

Cu(II) 80

80

80

60

60

60

Fe(III)

40

40

40

Nd(III)

20

20

20

Dy(III)

Zn(II) 0

0 0

1

2

pHeq

3

0 0

2

4

pHeq

0

2

4

pHeq

Figure 3. Equilibrium pH dependence of metal ion adsorption ratio on polymeric adsorbent synthesized at heptane volume fractions of (a) 0, (b) 25, (c) 33, (d) 50, (e) 67, (f) 75, and (g) 100. Initial concentration of each metal ion, 1 mmol/L; pH adjusted with HCl; solution volume, 5 mL; polymeric adsorbent, 50 mg.

Figure 4a shows a time series of dysprosium adsorption onto R-DGAs. R-DGA (0, 25, 75, and

100 vol %) adsorbed little dysprosium, whereas dysprosium was adsorbed by R-DGA-33,

R-DGA-50, and R-DGA-67, which were derived from polymeric particles synthesized at heptane

fractions of 33, 50, and 67 vol %, respectively. The amount of dysprosium adsorbed onto R-DGA-33

was larger than that adsorbed onto R-DGA-50 and R-DGA-67. To compare adsorption rates among

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the R-DGAs, the amounts of adsorbed dysprosium were normalized to the amount adsorbed at 72 h

for each heptane fraction (Figure 4b). The adsorption rate onto R-DGA-50 was the highest, which

may be explained by its large specific surface area.

Adsorption of Dy (mmol/g)

(a) 0.08 Heptane fraction (vol %) 0

0.06

25 33

0.04

50 67

0.02

75 100

0 0

20

40

60

80

Time (h)

Normalized adsorption of Dy (%)

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) 100 Heptane fraction (vol %) 33

80

50 60

67

40 20 0 0

1

2

3

4

Time (h) Figure 4. (a) Time series of dysprosium adsorption onto polymeric adsorbent and (b) adsorption of dysprosium normalized to adsorption of dysprosium at 72 h. Initial concentration of dysprosium ions, 1 mmol/L; initial pH, 1.0; solution volume, 5 mL; polymeric adsorbent, 50 mg.

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Figure 5 shows the isotherms for adsorption of dysprosium ions onto R-DGAs. The adsorption

isotherms of R-DGA-33, R-DGA-50, and R-DGA-67 agreed well with the Langmuir equation

(Equation 5): +, = -. +/&,⁄(1 + -. &,)

(5)

where Qe is the amount of adsorbed dysprosium ions at equilibrium; Qm is the adsorption capacity of dysprosium ions; KL is the Langmuir constant; and Ce is the equilibrium concentration of dysprosium ions. Table 2 shows the Langmuir constants and the adsorption capacities of the

polymeric adsorbents. The adsorption capacity of R-DGA-50 was smaller than that of R-DGA-33,

although the amount of diglycolamic acid ligand introduced onto R-DGA-50 was almost the same as

that introduced onto R-DGA-33 (Table 1). This result suggests that the usage ratio of diglycolamic

acid ligands decreased as the fraction of heptane increased. Observations of the polymeric

adsorbents during the adsorption experiments revealed that they tended to float on the surface of the

solution as the volume fraction of heptane increased. Thus, the polymeric adsorbents might have

become more hydrophobic as the fraction of heptane increased, and then, the usage ratio of

diglycolamic acid ligands decreased.

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0.15 Heptane fraction (vol %) 0

Qe (mmol/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

0.1

33 50 0.05

67 75 100

0 0

2

4

6

8

10

Ce (mmol/L) Figure 5. Isotherms for adsorption of dysprosium on polymeric adsorbent at 333 K. The solid curves represent the fitted values. Initial pH, 1.0; pH adjusted with HCl; solution volume, 5 mL; polymeric adsorbent, 50 mg.

Table 2. Langmuir constants (KL) and adsorption capacities (Qm) for the polymeric adsorbents. Initial concentration of dysprosium ions, 0.2–10 mmol/L; initial pH, 1.0; solution volume, 5 mL; polymeric adsorbent, 50 mg Adsorbent

KL (L/mmol)

Qm (mmol/g)

R-DGA-33

2.81

0.113

R-DGA-50

2.25

0.0945

R-DGA-67

0.303

0.0579

Figure 6 shows the adsorption behavior for low concentrations of lanthanides (except for Pm)

and high concentrations of base metal ions. We found that the adsorption ratios of heavy rare earth

ions were larger than those of light rare earth ions. In addition, it was found that the adsorption ratios

of base metal ions were much smaller than those of rare earth ions even though the concentration of

base metal ions were much higher than that of rare earth ions. From these results, we concluded that

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R-DGA is expected to be useful to separate dilute rare earth elements.

50 Adsorption ratio (%)

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 30 20 10 0

La Pr Sm Gd Dy Er Yb Al Fe Zn Ce Nd Eu Tb Ho Tm Lu Ca Cu

Figure 6. Adsorption ratios of each rare earth ion and base metal ion onto R-DGA-50. Initial concentration of each rare earth ion, 0.1 mmol/L; Initial concentration of each base metal ion, 100

mmol/L; Initial pH, 1.0; pH adjusted with HCl; solution volume, 5 mL; R-DGA-50, 100 mg.

Stability test was carried out to evaluate durability of R-DGA. The result for five cycles of

adsorption–desorption was shown in Figure 7. Although the adsorption amount of dysprosium ions

slightly decreased, no significant change has been observed. This result suggests that R-DGA can be

used repeatedly.

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Adsorption of Dy (mmol/g)

Page 19 of 25

0.07 0.06 0.05 0.04 0.03 0.02 0.01 0

1

2

3

4

5

Cycle Figure 7. Stability experiment for R-DGA-50. Adsorption conditions: Initial concentration of dysprosium ions, 3 mmol/L; Initial pH, 1.0; pH adjusted with HCl; solution volume, 60 mL;

R-DGA-50, 600 mg. Desorption conditions: Concentration of HCl, 2 mol/L; solution volume, 60

mL; R-DGA-50, 600 mg.

4. CONCLUSIONS In this study, we synthesized polymeric adsorbents modified with diglycolamic acid ligands. Their

adsorption behavior was investigated in terms of selectivity for rare earth elements, adsorption rate,

and adsorption capacity.

1. Styrene, divinylbenzene, and glycidyl methacrylate were copolymerized by suspension

polymerization in the presence of diluent mixtures. Copolymers with different pore characteristics

were obtained by changing the composition of the diluent mixtures. These results indicated that the

19



pore characteristics of the polymeric particles were controllable.

2. The amount of introduced amino groups was almost constant, regardless of the pore

characteristics, whereas that of diglycolamic acid ligands depended on the pore characteristics. The

amount of diglycolamic acid ligands was larger than that of silica gel adsorbents.

3. The polymeric adsorbents selectively adsorbed rare earth elements at low pH values. The fact

that the adsorption rate was closely related to the specific surface area suggested the possibility of

improving the adsorption rate by controlling the specific surface area. The adsorption isotherm was a

Langmuir-type isotherm, and the adsorption capacity was 0.113 mmol/g.

AUTHOR INFORMATION

Corresponding author *T. Ogata. Tel. and fax: +81-29-861-8481; E-mail: [email protected]

Supporting Information Figure S1. Microscope images of R-GMA synthesized at heptane volume fractions of (a) 0, (b) 25, (c) 33, (d) 50, (e) 67, (f) 75, and (g) 100.

Figure S2. Effect of volume fraction of heptane on total pore volume.

Figure S3. Effect of volume fraction of heptane on average pore size.

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Figure S4. Pore size distribution of R-GMA.

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Table of Contents Contents

Adsorption 50

Diglycolamic acid ligands O

O O

HN

OH

O N O OH STYDVB

O OH


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

Selective recovery of REEs

30 20 10 0

Fe

Polymeric particles

Cu

Zn

Nd

Dy

REEs

25


Preparation of Polymeric Adsorbents Bearing Diglycolamic Acid

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