Separation of Adjacent Light Rare Earth Elements Using Silica Gel Modified with Diglycolamic Acid

The separation of adjacent rare earth elements (REEs) is a challenging issue due to their chemical similarity. We have investigated the separation of adjacent REEs using four types of adsorbents consisting of silica gel modified with diglycolamic acid with different functional groups at the amide position. For all the adsorbents, the adsorption ratio of REEs increased with the increase in atomic number from La to Sm and then became constant for heavy REEs. Among them, EDASiDGA, an adsorbent containing secondary and tertiary amides, showed a high separation factor for Nd/Pr of 2.8. The EDASiDGA-packed column was tested for individual recovery of Pr, Nd, and Sm. After the adsorption of these REEs from 0.10 M HCl, desorption tests were performed with 0.32 and 1.0 M HCl. As a result, Pr and Nd were eluted separately with 0.32 M HCl, and Sm was recovered with 1.0 M HCl. Since the EDASiDGA-packed column showed excellent separation of Pr/Nd/Sm without any chelating agent, it is promising for practical use.


Introduction
Rare earth elements (REEs) are a group of 17 elements consisting of Sc, Y, and the lanthanides, and they are traditionally divided into two groups: the light rare earth elements (LREEs: La, Ce, Pr, Nd, Sm, and Eu) and the heavy rare earth elements (HREEs: Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y).REEs are critical elements in a carbon-neutral society due to their use in clean energy technologies, such as wind turbines and electric vehicles; therefore, demand for rare earth magnets whose main components are LREEs such as Nd is expected to grow in the future [1][2][3].
REEs are mostly present as mixtures in primary resources, and therefore, it is necessary to separate each REE from the others for industrial application.Solvent extraction is one of the most common techniques for the separation of REEs because of its excellent selectivity, continuity, and large loading capacity.However, it is difficult to recover REEs individually due to their similar physicochemical properties.For example, acidic organophosphorus extractants such as bis (2-ethylhexyl) hydrogen phosphate (D2EHPA) and mono-2-ethylhexyl (2-ethylhexyl)phosphonate (PC-88A) are typical industrial extractants for the separation of adjacent REEs, which have a large separation factor (SF) for REEs, e.g., SFs for Nd/Pr by D2EHPA and PC-88A are 1.38 and 1.55, respectively [4].However, this value of SF would not necessarily be cost-effective to obtain high-purity Pr and Nd, respectively.Therefore, the development of separation reagents/separation methods with larger SF than conventional ones is completely imperative to make REE refining more efficient.
For the recycling of REEs from secondary resources, it is important to have a technology that allows the separation of small amounts of REEs on demand rather than continuous separation on a large scale as in the case of separation of REEs from primary resources; the adsorption method is suitable for this purpose.The industrial separation method for REEs by adsorption is generally based on the use of ion exchange resins and complexing agents [5][6][7].However, the presence of chelating agents requires a further purification process that is very costly.Therefore, an adsorbent with a high SF can be used to achieve separation with a simple operation.In the basic research phase, many adsorbents have been studied and many reports are available.Examples include polymer-based adsorbents (ion-imprinted polymer-based adsorbents, impregnation of functional groups into adsorbents, etc.), carbon-based adsorbents (graphene oxide, carbon nanotubes, etc.), silica-based adsorbents (silica beads, zeolites, etc.), metal-organic frameworks (MOFs), clay minerals (kaolinite, halloysite, illite, expanded vermiculite, etc.), and biosorbents (cellulose-based adsorbents, chitosan-based adsorbents, etc.) [6][7][8][9][10][11].Most of these adsorbents are designed to separate only REEs or REEs from other impurities such as Fe or Al.Focusing on the individual separation of REEs, there are several reports on the separation of HREE and LREE, such as the separation of Dy and Nd, and separation is possible [12][13][14][15].However, there are no reports of effective adsorbents for adjacent REEs, such as Nd and Pr, and classical industrial methods must be used to separate individual REEs by adsorption.We have developed a diglycolamic acid-type adsorbent for the recovery of REEs from unutilized resources [16][17][18], which can selectively recover REEs in the presence of base metals with concentrations more than 1000 times higher [19].Since this adsorbent shows that the adsorption capacity for LREEs increases linearly with increasing atomic number, it could be applied to the separation of LREEs.
In this study, four types of diglycolamic acid adsorbents were prepared and their separation ability for adjacent REEs was evaluated.In particular, we focused on the separation of Pr, Nd, and Sm, which are used in rare earth magnets.

Preparation of Adsorbents
The adsorbents, which were silica gel-modified with diglycolamic acid, were prepared as described in our previous paper [16,20].Briefly, diglycolic anhydride and amino-silica gels were added to dichloromethane and allowed to react at 298 K for 3 days, after which the particles were filtered off and washed with dichloromethane, ethanol, and water.The washed particles were dried under vacuum.The adsorbents made from these amino-silica gels are referred to as ASiDGA, MeASiDGA, BuASiDGA, and EDASiDGA (Figure 1), respectively.The results of attenuated total reflection Fourier transform infrared spectroscopy of the prepared adsorbents are shown in the Supplementary Materials.

Preparation of Adsorbents
The adsorbents, which were silica gel-modified with diglycolamic acid, were prepared as described in our previous paper [16,20].Briefly, diglycolic anhydride and aminosilica gels were added to dichloromethane and allowed to react at 298 K for 3 days, after which the particles were filtered off and washed with dichloromethane, ethanol, and water.The washed particles were dried under vacuum.The adsorbents made from these amino-silica gels are referred to as ASiDGA, MeASiDGA, BuASiDGA, and EDASiDGA (Figure 1), respectively.The results of a enuated total reflection Fourier transform infrared spectroscopy of the prepared adsorbents are shown in the Supplementary Materials.

Procedure of Batch Adsorption Tests
In the batch adsorption experiments, the adsorbent (50 or 100 mg) was added to 5 or 10 mL of a solution containing RE ions (0.1 or 1.0 mM), and the mixture was shaken at 298 K for 3 days.After filtration, the concentration of each ion in the filtrate was determined with an inductively coupled plasma atomic emission spectrometer (ICPE-9000, Shimadzu, Kyoto, Japan) or an inductively coupled plasma mass spectrometer (ICPMS-2030, Shimadzu, Kyoto, Japan), and the adsorbed amount of each ion was calculated.Adsorption ratio, adsorption amount (Q), distribution ratio (Kd), and SF were obtained from the following formulas.The estimated uncertainty was <5%.
Adsorption ratio [%] = (C0 − C)/C0 × 100 (1) In the above equations, 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 the adsorbents.

Procedure of Column Separation
The column test was performed as follows: the adsorbent, EDASiDGA, was packed into a glass column (5 mm inner diameter, 100 mm height).The volume of the adsorbent bed and the dry weight of the adsorbent in the column were 1.96 cm 3 and 1.06 g, respectively.The column was conditioned with 0.1 M HCl.The adsorption solution ([Pr], [Nd], and [Sm]; 0.1 mM, pH 1.0, 5.9 mL), wash solution (HCl, pH 1.0, 5.9 mL), and desorption solution 1 (HCl, pH 0.50, 29.4 mL) were sequentially passed through the conditioned column at a constant flow rate (space velocity: 3 h −1 ), and finally, the desorption solution 2 (HCl, pH 0.50, 14.7 mL) was passed through the column at a space velocity of 5 h −1 , and

Procedure of Batch Adsorption Tests
In the batch adsorption experiments, the adsorbent (50 or 100 mg) was added to 5 or 10 mL of a solution containing RE ions (0.1 or 1.0 mM), and the mixture was shaken at 298 K for 3 days.After filtration, the concentration of each ion in the filtrate was determined with an inductively coupled plasma atomic emission spectrometer (ICPE-9000, Shimadzu, Kyoto, Japan) or an inductively coupled plasma mass spectrometer (ICPMS-2030, Shimadzu, Kyoto, Japan), and the adsorbed amount of each ion was calculated.Adsorption ratio, adsorption amount (Q), distribution ratio (K d ), and SF were obtained from the following formulas.The estimated uncertainty was <5%.

Adsorption ratio
In the above equations, C 0 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 the adsorbents.

Procedure of Column Separation
The column test was performed as follows: the adsorbent, EDASiDGA, was packed into a glass column (5 mm inner diameter, 100 mm height).The volume of the adsorbent bed and the dry weight of the adsorbent in the column were 1.96 cm 3 and 1.06 g, respectively.The column was conditioned with 0.1 M HCl.The adsorption solution ([Pr], [Nd], and [Sm]; 0.1 mM, pH 1.0, 5.9 mL), wash solution (HCl, pH 1.0, 5.9 mL), and desorption solution 1 (HCl, pH 0.50, 29.4 mL) were sequentially passed through the conditioned column at a constant flow rate (space velocity: 3 h −1 ), and finally, the desorption solution 2 (HCl, pH 0.50, 14.7 mL) was passed through the column at a space velocity of 5 h −1 , and the effluent was collected.The concentration of each element in the aqueous solution was measured for each fraction using the ICP atomic emission spectrometer.The column experiment was performed at room temperature (291 ± 2 K).

Batch Adsorption Tests of REEs
Figure 2 shows the results of adsorption tests of mixtures containing 16 REEs by the four prepared adsorbents.The adsorption trends were similar for four types of diglycolamic acid adsorbents: for the LREEs, the higher the atomic number, the higher the adsorption ratio; for the HREEs and Sc, each ion had a uniformly high adsorption ratio; and the adsorption ratio of Y was similar to that of Sm.These results for lanthanides show the same trend as those obtained by solvent extraction with diglycolamic acid extractant [21,22].The La 3+ complex with N-butyldiglycolamic acid has three diglycolamic acid ligands that coordinate to the La 3+ ion in a tridentate fashion through the amide, ether, and carboxyl oxygen atoms [22].Accordingly, it is assumed that the adsorption of REEs by diglycolamic acid adsorbents has a similar coordination structure.The magnitude of the adsorption ratios of the REEs showed a difference among the adsorbents.The lowest magnitude was observed for ASiDGA and BuASiDGA.For ASiDGA, this was due to the low coordination ability of the secondary amides when forming a hydrogen bond between the oxygen atom and the N-H in the amide group, since the RE ion can coordinate with three oxygen donor atoms in the carboxyl, ether, and amide groups [20].MeASiDGA, a tertiary amide, had the highest adsorption ratios for REEs, in contrast to BuASiDGA, also a tertiary amide.The low adsorption ratio of BuASiDGA was probably due to the hydrophobic butyl side chain of the amide group, which reduced the contact efficiency between the adsorbent site and the aqueous solution.For EDASiDGA, large differences in adsorption ratios between adjacent REEs were observed for LREEs.

Batch Adsorption Tests of REEs
Figure 2 shows the results of adsorption tests of mixtures containing 16 REEs by the four prepared adsorbents.The adsorption trends were similar for four types of diglycolamic acid adsorbents: for the LREEs, the higher the atomic number, the higher the adsorption ratio; for the HREEs and Sc, each ion had a uniformly high adsorption ratio; and the adsorption ratio of Y was similar to that of Sm.These results for lanthanides show the same trend as those obtained by solvent extraction with diglycolamic acid extractant [21,22].The La 3+ complex with N-butyldiglycolamic acid has three diglycolamic acid ligands that coordinate to the La 3+ ion in a tridentate fashion through the amide, ether, and carboxyl oxygen atoms [22].Accordingly, it is assumed that the adsorption of REEs by diglycolamic acid adsorbents has a similar coordination structure.The magnitude of the adsorption ratios of the REEs showed a difference among the adsorbents.The lowest magnitude was observed for ASiDGA and BuASiDGA.For ASiDGA, this was due to the low coordination ability of the secondary amides when forming a hydrogen bond between the oxygen atom and the N-H in the amide group, since the RE ion can coordinate with three oxygen donor atoms in the carboxyl, ether, and amide groups [20].MeASiDGA, a tertiary amide, had the highest adsorption ratios for REEs, in contrast to BuASiDGA, also a tertiary amide.The low adsorption ratio of BuASiDGA was probably due to the hydrophobic butyl side chain of the amide group, which reduced the contact efficiency between the adsorbent site and the aqueous solution.For EDASiDGA, large differences in adsorption ratios between adjacent REEs were observed for LREEs.Figure 3 shows the pH dependence of the adsorption ratios using single-component systems of Pr, Nd, and Sm, respectively.The adsorption ratio increased with increasing pH for each adsorbent.This result confirms that the deprotonation of the carboxylic acid group in diglycolamic acid occurs during the REE adsorption.BuASiDGA showed lower adsorption ratios than the other adsorbents at all pHs tested.ASiDGA adsorbed LREEs at pH 1 or higher.On the other hand, MeASiDGA adsorbed LREEs at a low pH.It was found that the difference in the adsorption ratios of Pr, Nd, and Sm on EDASiDGA becomes large at pH 0.50-1.5.These results indicate that EDASiDGA has a high potential for Pr/Nd/Sm separation.Figure 3 shows the pH dependence of the adsorption ratios using single-component systems of Pr, Nd, and Sm, respectively.The adsorption ratio increased with increasing pH for each adsorbent.This result confirms that the deprotonation of the carboxylic acid group in diglycolamic acid occurs during the REE adsorption.BuASiDGA showed lower adsorption ratios than the other adsorbents at all pHs tested.ASiDGA adsorbed LREEs at pH 1 or higher.On the other hand, MeASiDGA adsorbed LREEs at a low pH.It was found that the difference in the adsorption ratios of Pr, Nd, and Sm on EDASiDGA becomes large at pH 0.50-1.5.These results indicate that EDASiDGA has a high potential for Pr/Nd/Sm separation.
The SF for Nd/Pr, Sm/Nd, and Sm/Pr were investigated by using a mixed solution of Pr, Nd, and Sm.We performed adsorption tests on MeASiDGA and EDASiDGA, which are expected to have good separation of LREEs.The adsorption results are shown in Figure 4, and the SFs for Nd/Pr, Sm/Nd, and Sm/Pr are summarized in Table 1.As with the singlecomponent systems, the K d of Pr, Nd, and Sm increased with increasing pH of the mixed solution.It was confirmed that at pH 1.5, the K d of Pr increased more than that of Nd and Sm, and as a result, the SFs for Nd/Pr and Sm/Pr became smaller.Therefore, the pH range of 0.59 to 1.1 is considered the optimal condition where high separation performance can be expected.Table 2 shows the SFs for Nd/Pr, Sm/Nd, and Sm/Pr values obtained by a well-known industrial adsorption method using ion exchange resin with aminocarboxylic acid chelating agents; here, the SFs for Nd/Pr, Sm/Nd, and Sm/Pr were calculated based on the stability constants of REEs and the aminocarboxylic acid chelating agents because there is almost no difference in the stability constants of REEs in the REE ion exchange resin system [5][6][7].A comparison of the SFs showed that the prepared adsorbents were superior in almost all SF values in the pH range of 0.59 to 1.1.Especially for Nd and Pr, EDASiDGA showed a maximum SF Nd/Pr of 2.8.The SF for Nd/Pr, Sm/Nd, and Sm/Pr were investigated by using a mixed solution of Pr, Nd, and Sm.We performed adsorption tests on MeASiDGA and EDASiDGA, which are expected to have good separation of LREEs.The adsorption results are shown in Figure 4, and the SFs for Nd/Pr, Sm/Nd, and Sm/Pr are summarized in Table 1.As with the single-component systems, the Kd of Pr, Nd, and Sm increased with increasing pH of the mixed solution.It was confirmed that at pH 1.5, the Kd of Pr increased more than that of Nd and Sm, and as a result, the SFs for Nd/Pr and Sm/Pr became smaller.Therefore, the pH range of 0.59 to 1.1 is considered the optimal condition where high separation performance can be expected.Table 2 shows the SFs for Nd/Pr, Sm/Nd, and Sm/Pr values obtained by a well-known industrial adsorption method using ion exchange resin with aminocarboxylic acid chelating agents; here, the SFs for Nd/Pr, Sm/Nd, and Sm/Pr were calculated based on the stability constants of REEs and the aminocarboxylic acid chelating agents because there is almost no difference in the stability constants of REEs in the REE ion exchange resin system [5][6][7].A comparison of the SFs showed that the prepared adsorbents were superior in almost all SF values in the pH range of 0.59 to 1.1.Especially for Nd and Pr, EDASiDGA showed a maximum SFNd/Pr of 2.8.

Column Tests with EDASiDGA
A separation test of LREEs was performed using a column packed with EDASiDGA, which has a high SF value for LREEs.The adsorption solution (pH 1.0 solution containing Pr, Nd, and Sm), wash solution (HCl, pH 1.0), desorption solution 1 (HCl, pH 0.50), and desorption solution 2 (HCl, 1.0 M) were sequentially passed through the column.Figure 5 shows the ratio of the concentration of each ion in the effluent (C) to its initial concentration (C0) versus the bed volume (BV).When the adsorption test solution containing Pr, Nd, and Sm was passed through the EDASiDGA-packed column, all LREEs were cap-

Column Tests with EDASiDGA
A separation test of LREEs was performed using a column packed with EDASiDGA, which has a high SF value for LREEs.The adsorption solution (pH 1.0 solution containing Pr, Nd, and Sm), wash solution (HCl, pH 1.0), desorption solution 1 (HCl, pH 0.50), and desorption solution 2 (HCl, 1.0 M) were sequentially passed through the column.Figure 5 shows the ratio of the concentration of each ion in the effluent (C) to its initial concentration (C 0 ) versus the bed volume (BV).When the adsorption test solution containing Pr, Nd, and Sm was passed through the EDASiDGA-packed column, all LREEs were captured on the column, i.e., C/C 0 = 0.The adsorbed LREEs were not eluted by the wash solution.Using the desorption solution 1, Pr was desorbed first, followed by Nd.Then, the adsorbed Sm was eluted out by the desorption solution 2. Accordingly, Pr, Nd, and Sm were recovered individually by using the EDASiDGA-packed column.Unlike the conventional adsorption system using a large amount of chelating agent, this column can precisely separate Pr, Nd, and Sm from each other simply by varying the HCl concentration in a narrow range of 0.10-1.0M without any chelating agent.This is due to the high SF between adjacent REEs by EDASiDGA, an adsorbent, and the easy desorption of the adsorbed REEs with about 1.0 M HCl.These properties support that the EDASiDGA-packed column is promising for practical separation of Pr, Nd, and Sm.

Conclusions
We prepared four types of adsorbents with different groups a ached to the nitrogen atom of the amide bond of diglycolamic acid and investigated the separation of adjacent REEs by batch and column adsorption tests.Regarding the relationship between the atomic number and adsorption ratio of REE, all the adsorbents displayed a similar trend; the adsorption ratio increased from La to Sm and then became almost constant for HREEs.

Table 1 .
Separation factors for Nd/Pr, Sm/Nd, and Sm/Pr by MeASiDGA and EDASiDGA.

Table 2 .
Separation factors for Nd/Pr, Sm/Nd, and Sm/Pr calculated from the stability constants (K1) of REEs and aminocarboxylic acid chelating agents.

Table 1 .
Separation factors for Nd/Pr, Sm/Nd, and Sm/Pr by MeASiDGA and EDASiDGA.

Table 2 .
Separation factors for Nd/Pr, Sm/Nd, and Sm/Pr calculated from the stability constants (K 1 ) of REEs and aminocarboxylic acid chelating agents.