Preparation of Polymeric Adsorbents Bearing Diglycolamic Acid

Jul 27, 2018 - We synthesized polymeric adsorbents modified with diglycolamic acid ligands for the recovery of rare earth elements. Styrene, divinylbe...
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Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 11424−11430

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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. 2018.57:11424-11430. Downloaded from pubs.acs.org by UNIV OF READING on 08/30/18. For personal use only.

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 S Supporting Information *

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 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. 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 © 2018 American Chemical Society

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 Received: Revised: Accepted: Published: 11424

April 25, 2018 July 9, 2018 July 27, 2018 July 27, 2018 DOI: 10.1021/acs.iecr.8b01797 Ind. Eng. Chem. Res. 2018, 57, 11424−11430

Article

Industrial & Engineering Chemistry Research Scheme 1. Preparation of Polymeric Adsorbents Modified with Diglycolamic Acid Ligands

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 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. 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 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

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 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 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′-azobis(isobutyronitrile) (AIBN), and poly(vinyl alcohol) (saponification degree 78− 82%, average polymerization degree >1500) (Wako Pure 11425

DOI: 10.1021/acs.iecr.8b01797 Ind. Eng. Chem. Res. 2018, 57, 11424−11430

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Industrial & Engineering Chemistry Research 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: amino group content (mmol/g) = 10(NR ‐ EDA /MN)

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

(1)

diglycolamic acid ligand content (mmol/g) = 1000(1 − NR ‐ DGA /NR ‐ EDA )/MDGA

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 %. 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. 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 RGMA, 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 peaks were absent in the spectrum of

(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 RDGA with a FT-IR spectrometer (PerkinElmer, 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 being shaken, 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(%) = (C0 − C)/C0 × 100

(3)

adsorption amount (mmol/g) = (C0 − C)V /w

(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. A stability experiment was performed in batches. R-DGA (600 mg) and a solution containing dysprosium ions (60 mL) were shaken at 180 rpm and 298 K in the adsorption process. After being shaken, RDGA 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 the desorption process. After being shaken, R-DGA was filtered and rinsed with distilled water. Adsorption and desorption processes were repeated for 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

Figure 2. ATR FT-IR spectra of R-GMA, R-EDA, and R-DGA that were synthesized at a heptane fraction of 50 vol %. 11426

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%. 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 3.3. Adsorption Behavior of R-DGA. Figure 3 shows the equilibrium pH dependence of the 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 RDGA 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. On the basis of these results, we concluded that synthesized polymeric adsorbents (R-DGAs) with a suitable particle size had high selectivity for rare earth elements.

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. Table 1 shows the content of amino groups in R-EDA and diglycolamic acid ligands in R-DGA as a function of the Table 1. Content of Amino Groups in R-EDA and Diglycolamic Acid Ligands in R-DGA heptane fraction (vol %)

amino group in R-EDA (mmol/ g-R-EDA)

diglycolamic acid ligand in R-DGA (mmol/g-R-DGA)

conversion ratio from amino group to diglycolamic acid ligand (%)

0 25 33 50 67 75 100

4.81 4.72 4.70 4.38 3.88 3.88 4.22

0 0.118 3.05 3.00 1.78 1.08 0.920

0 2.50 64.9 68.5 45.9 27.8 21.8

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 increased as the fraction of heptane increased from 0 to 33 vol

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. 11427

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Industrial & Engineering Chemistry Research Figure 4a shows a time series of dysprosium adsorption onto R-DGAs. R-DGA (0, 25, 75, and 100 vol %) adsorbed little

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 Adsorbentsa adsorbent

KL (L/mmol)

Qm (mmol/g)

R-DGA-33 R-DGA-50 R-DGA-67

2.81 2.25 0.303

0.113 0.0945 0.0579

a

Initial concentration of dysprosium ions, 0.2−10 mmol/L; initial pH, 1.0; solution volume, 5 mL; polymeric adsorbent, 50 mg.

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. Figure 6 shows the adsorption behavior for low concentrations of lanthanides (except for Pm) and high concen-

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.

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 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 RDGA-50 was the highest, which may be explained by its large specific surface area. 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 (eq 5): Q e = KLQ mCe/(1 + KLCe)

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; RDGA-50, 100 mg.

(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

trations 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 was much higher than that of rare earth ions. From these results, 11428

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we concluded that R-DGA is expected to be useful to separate dilute rare earth elements. A stability test was carried out to evaluate the durability of R-DGA. The results for five cycles of adsorption−desorption were shown in Figure 7. Although the adsorption amount of dysprosium ions slightly decreased, no significant change was observed. This result suggests that R-DGA can be used repeatedly.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +81-29-861-8481. E-mail: [email protected]. ORCID

Tomohiro Shinozaki: 0000-0003-0222-3251 Takeshi Ogata: 0000-0002-9860-8182 Notes

The authors declare no competing financial interest.

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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 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.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b01797. 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; effect of volume fraction of heptane on total pore volume and average pore size; pore size distribution of R-GMA (PDF) 11429

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DOI: 10.1021/acs.iecr.8b01797 Ind. Eng. Chem. Res. 2018, 57, 11424−11430