The effectiveness of ion exchange resins in separating

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Ang, K.L., Li, D. and Nikoloski, A.N. (2017) The effectiveness of ion exchange resins in separating uranium and thorium from rare earth elements in acidic

aqueous sulfate media. Part 1. Anionic and cationic resins. Hydrometallurgy, 174. pp. 147-155.

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Accepted Manuscript

The effectiveness of ion exchange resins in separating uraniumand thorium from rare earth elements in acidic aqueous sulfatemedia. Part 1. Anionic and cationic resins

Kwang Loon Ang, Dan Li, Aleksandar N. Nikoloski

PII: S0304-386X(17)30483-8DOI: doi:10.1016/j.hydromet.2017.10.011Reference: HYDROM 4670

To appear in: Hydrometallurgy

Received date: 9 June 2017Revised date: 23 September 2017Accepted date: 4 October 2017

Please cite this article as: Kwang Loon Ang, Dan Li, Aleksandar N. Nikoloski , Theeffectiveness of ion exchange resins in separating uranium and thorium from rare earthelements in acidic aqueous sulfate media. Part 1. Anionic and cationic resins. The addressfor the corresponding author was captured as affiliation for all authors. Please check ifappropriate. Hydrom(2017), doi:10.1016/j.hydromet.2017.10.011

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The effectiveness of ion exchange resins in separating uranium

and thorium from rare earth elements in acidic aqueous sulfate

media. Part 1. Anionic and cationic resins

Kwang Loon Ang, Dan Li and Aleksandar N. Nikoloski*

Chemical and Metallurgical Engineering and Chemistry,

School of Engineering and Information Technology, Murdoch University, Australia

*Corresponding author. Telephone: +61 8 9360 2835; Fax: +61 8 9360 6343.

E-mail address: [email protected] (A. N. Nikoloski).

Postal address: School of Engineering and Information Technology, Murdoch University,

6150, Western Australia

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Abstract

Conventional ion exchange resins with different functional groups were evaluated for their

potential application in separating uranium U(VI) and thorium Th(IV) from rare earth

elements RE(III). The resins studied comprised strong- and weak-base anion exchange resins,

and strong- and weak-acid cation exchange resins. The selectivity of these resins to adsorb

U(VI) and Th(IV) in the presence of selected RE(III) was examined in sulfuric acid media of

varying concentrations. It was evident that the adsorption performance of the resins was acid

concentration-dependent. Most candidate resins had potentially feasible selective adsorption

at or below 0.1 mol/L H2SO4 (pH ≥ 0.7). Within the group of anion exchange resins, both the

strong- and weak-base resins exhibited a similar selectivity with U(VI) adsorbed in

preference to RE(III). The difference between them was their adsorption of Th(IV). The

weak-base resin with primary amine functional group demonstrated superior separation of

Th(IV) from RE(III). For this resin, 78% of U(VI) and 68% of Th(IV) were adsorbed while

RE(III) co-adsorption was less than 5% at 0.0005 mol/L H2SO4 (pH 3). In the case of the

strong-acid cation exchange resins, Th(IV) and RE(III) were adsorbed in preference to U(VI),

i.e., RE(III) > Th(IV) >> U(VI). The weak-acid cation exchange resins, on the other hand,

displayed limited adsorption of all elements.

Keywords:

Thorium; Uranium; Rare earth; Ion exchange; Separation; Sulfuric acid.

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1. Introduction

The separation of uranium and thorium from rare earths is one of the most important tasks in

the hydrometallurgical production of these elements. This is because U(VI) and Th(IV)

invariably coexist with the two most abundant rare earth minerals, i.e. monazite and

bastnaesite. The method of removing U(VI) and Th(IV) in conventional hydrometallurgical

processing of rare earths has problems pertaining to the disposal of solid and/or liquid waste,

in addition to substantial loss of rare earths to waste (Gui, et al., 2014; Zhu, et al., 2015). For

that reason, in the past decade there has been a resurgence of investigations into U(VI) and

Th(IV) separation involving the use of different methods (Cheng, et al., 2011; Deng, et al.,

2013; Fan, et al., 2011; Gao, et al., 2012; Nasab, et al., 2011; Song, et al., 2009; Zhang, et al.,

2012; Zhong & Wu, 2012; Zuo, et al., 2008).

A literature review into the separation of U(VI) and Th(IV) from rare earths shows that

extensive work has been conducted to separate U(VI) and Th(IV) using liquid or solvent

extractants. This is considered a convenient and efficient method to purify the rare earths

because of its simplicity and ease of operation (He, et al., 2013). However, one of its

drawbacks is the need to dispose of organic waste generated in the recovery process (Gui, et

al., 2014). In contrast, ion exchange (IX) is known to produce less liquid waste, whether

aqueous or organic in nature, and hence there are fewer waste disposal issues. IX also does

not have issues of phase separation, third phase formation, or solvent loss, and is particularly

advantageous for adsorption of metals present in low concentrations, e.g., from bastnaesite

ores with a low content of U(VI) and Th(IV). These merits of IX therefore justify research

into the technology to further develop its potential for separation of U(VI) and Th(IV) from

rare earths.

The objective of this study was to evaluate the adsorption affinity of various IX resins with

different physicochemical properties for U(VI) and Th(IV) in sulfuric acid media containing

selected light, medium, and heavy RE(III), i.e., lanthanum (III), cerium (III), gadolinium (III)

and ytterbium (III). Comparative adsorption data for the resins were presented to demonstrate

the ones that are most selective towards U(VI) and Th(IV) over RE(III). Rather than focusing

on a single element system, the impact that the presence of RE(III) have on the adsorption

ability of the resins to selectively adsorb U(VI) and Th(IV) was investigated. The scope of

this investigation was limited to readily available commercial IX resins rather than

chemically modified IX resins that might not be economically viable. The candidate resins

comprised anion and cation exchange resins, and chelating resins including two solvent-

impregnated resins containing extractants commonly used in SX processes for rare earths

separation, namely, di-(2-ethylhexyl) phosphoric acid (D2EHPA) and organophosphinic acid.

The first part of this study is reported in this present paper, and focuses on conventional anion

and cation exchange resins. It is interesting that the majority of IX studies on the extraction of

U(VI), Th(IV) and RE(III) only involve strong-acid cation and strong-base anion exchange

resins, with few studies on weak-acid cation exchange resins (Korkisch, 1989). A thorough

search of the literature yielded only one study involving weak-base anion exchange resin,

namely Amberlite CG-4B which is no longer manufactured (Kuroda, et al., 1972). Arguably,

Amberlite CG-4B is better classified as a polyamine chelating resin (Hubicki & Wójcik,

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2006). This paper presents, for the first time, the application of a weak-base anion exchange

resin with primary amine functional group to the separation of U(VI) and Th(IV) from

RE(III). The results from this paper will also be compared with the IX performance of

chelating resins reported elsewhere (Ang, et al., 2017b).

2. Experimental materials and methods

2.1. Candidate resins

The ion exchange resins used in this study were selected from commercially-available resins

supplied by resin manufacturing companies. Their physicochemical properties, as reported in

their product data sheets, are tabulated in Table 1.

Table 1. Physicochemical properties of candidate resins

Resin ID Functional group Structure Total capacity

min. eq/L

AnIXR 1 Primary amine Macroporous 2.2

AnIXR 2 92%-tertiary, 8%-quaternary amine Macroporous 1.6

AnIXR 3 80%-tertiary, 20%-quaternary amine Macroporous 1.6

AnIXR 4 Quaternary amine, type I Gel 1.4

AnIXR 5 Quaternary amine, type I Gel 1.6

AnIXR 6 Quaternary amine, type I Macroporous 1.15

AnIXR 7 Quaternary amine, type II Gel 1.2

AnIXR 8 Quaternary amine, type II Macroporous 1.1

AnIXR 9 Quaternary amine, type II Macroporous 1.0

CatIXR 1 Sulfonic Gel 2.0

CatIXR 2 Sulfonic Macroporous 1.8

CatIXR 3 Carboxylic Porous 4.5

CatIXR 4 Carboxylic Macroporous 4.3

Schematics of the functional groups belonging to the various ion exchange resins used in this

study are shown in Table 2.

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Table 2. Schematics of functional groups of various ion exchange resins

Types of resin Functional groups Chemical structures

Strong-base anion

exchange resin

Quaternary amine, type I R

N+

CH3

CH3

CH3

Quaternary amine, type II R

N+

CH3

CH3

CH2CH2OH

Weak-base anion

exchange resin

Tertiary amine R

N

CH3

CH3

Primary amine R

NH2

Strong-acid cation

exchange resin

Sulfonic acid

S

O

O

OHR

Weak-acid cation

exchange resin

Carboxylic acid

C

O

OHR

Before testing, pre-treatment was applied to ensure that all resins were converted to sodium

form for cation exchange resins and sulfate form for anion exchange resins. The pre-

treatment method was adapted from the work reported by Zagorodni (2007). Resins with a

wide bead size distribution were manually dry-sieved to the size range of 0.50–0.71 mm.

Monodispersed resins within the size range of 0.50–0.71 mm and resins with bead size

greater than 0.71 mm (i.e., AnIXR 6 and AnIXR 7) were not dry-sieved. All conditioned

resins were stored in sealed glass bottles.

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2.2. Synthetic solutions

Concentrated stock solution was prepared by dissolving appropriate amounts of

Th(SO4)2·8H2O, UO2SO4·3H2O, La2(SO4)3, Ce2(SO4)3, Gd2(SO4)3 and Yb2(SO4)3·8H2O in

distilled water. The synthetic solution was prepared fresh from the stock solution before each

test with a concentration of 2 mmol/L for each metal ion. Th(SO4)2·8H2O (99.9%) and

UO2SO4·3H2O (99.9%) were procured from International Bio-Analytical Industries, Inc.

(Florida, USA). La2(SO4)3 (99.99%), Ce2(SO4)3 (99.99%), Gd2(SO4)3 (99.99%) and

Yb2(SO4)3·8H2O (99.9%) were obtained locally from Sigma-Aldrich. All other chemicals

used were of analytical grade.

2.3. Adsorption procedure

The adsorption experiments were carried out in a batch setup to examine the adsorption of

U(VI), Th(IV) and RE(III) in H2SO4 media by different ion exchange resins. The following

steps were repeated for each candidate resin.

To investigate the effect of acidity, 100 mL of synthetic solution with varying concentrations

of H2SO4 (0.0005–2.0 mol/L) was added to a conical flask containing 1 g resin (dry, free-

rolling). The mixture was equilibrated in a Thermoline Scientific BT-350R refrigerated

shaking water bath machine at constant temperature of 20°C for 24 hours. The solution was

sampled 2 hours after the start of equilibration, and a second sample was extracted at the end

of the 24-hour test. The concentrations of metal ions in the sample were determined by

inductively coupled plasma mass spectrometry (ICP-MS iCAP Qc, Thermo Fisher Scientific,

Germany).

The adsorption percentage of each metal ion was calculated according to Equation 1:

% Adsorption =𝐶0 − 𝐶

𝐶0

× 100 (1)

where C0 and C are the initial and equilibrium concentrations, respectively, of metal ion in

aqueous solution (mg/L).

The distribution coefficient Kd for each metal ion was calculated according to Equation 2:

𝐾d =𝐶0 − 𝐶

𝐶∙

𝑉

𝑚 (2)

where V is the volume of the synthetic solution (L), and m is the weight of the resin (g).

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3. Results and discussion

3.1. Anion exchange resins

Nine anion exchange resins were tested for their adsorption affinity for U(VI) and Th(IV) in

acidic sulfate solution containing RE(III). The primary amine functionalised AnIXR 1 is a

weak-base anion exchange resin. AnIXR 2 and AnIXR 3, containing a mixture of tertiary and

quaternary amino groups, are classified as intermediate-base anion exchange resins. The

remaining resins with quaternary ammonium functionality are strong-base anion exchange

resins. Their adsorption of U(VI), Th(IV) and RE(III) after contact times of 2 hours and 24

hours are tabulated in Table 3 and Table 4 respectively. The distribution coefficient Kd values

of each resin for U(VI), Th(IV) and RE(III) are tabulated in Table 5.

Table 3. 2-hour IX adsorption of U(VI), Th(IV) and RE(III) in 0.05 mol/L H2SO4

Resin ID Adsorption (%)

U Th La Ce Gd Yb

AnIXR 1 37 28 7 7 4 2

AnIXR 2 45 15 5 5 4 1

AnIXR 3 48 16 4 5 4 3

AnIXR 4 47 21 6 7 7 3

AnIXR 5 47 20 4 4 8 8

AnIXR 6 58 22 7 7 5 4

AnIXR 7 48 16 6 5 7 5

AnIXR 8 54 12 4 3 3 3

AnIXR 9 50 18 7 10 9 8

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U Th La Ce Gd Yb

AnIXR 1 72 55 6 8 7 4

AnIXR 2 92 17 4 4 7 5

AnIXR 3 92 23 5 7 6 6

AnIXR 4 89 26 6 7 10 8

AnIXR 5 91 21 7 7 6 5

AnIXR 6 93 23 5 4 7 6

AnIXR 7 89 20 4 4 5 4

AnIXR 8 95 18 5 5 6 6

AnIXR 9 93 16 6 7 9 8

Table 5. Distribution coefficients for U(VI), Th(IV) and RE(III) in 0.05 mol/L H2SO4

Resin ID Distribution coefficient Kd (mL/g)

U Th La Ce Gd Yb

AnIXR 1 287.3 131.2 7.0 8.7 7.7 4.9

AnIXR 2 1,598.8 22.1 4.5 4.5 7.6 5.9

AnIXR 3 1,850.5 31.9 5.7 7.9 6.7 6.9

AnIXR 4 1,005.1 37.7 7.0 7.6 11.6 9.1

AnIXR 5 1,415.5 27.5 8.1 7.4 6.4 5.1

AnIXR 6 1,898.7 31.8 5.3 4.8 8.4 7.2

AnIXR 7 1,026.7 25.7 4.3 4.5 5.8 4.8

AnIXR 8 3,055.5 24.0 5.2 5.1 6.5 7.0

AnIXR 9 2,300.7 19.4 7.1 7.4 10.7 9.1

The anion exchange resins in Table 3 show appreciable adsorption of U(VI) and Th(IV) after

2 hours of contact in the acidic sulfate solution. It was observed that U(VI) was adsorbed to a

greater extent than Th(IV). The adsorption of RE(III) was less than 10%. After a contact time

of 24 hours, the percentage of U(VI) adsorbed doubled compared to after 2 hours. The

adsorption percentage of Th(IV) doubled in the case of AnIXR 1, while small increases in

adsorption were seen for the other anion exchange resins. The percentage of RE(III) adsorbed

was less than 10% for all resins. Based on the Kd values in Table 5Error! Reference source

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not found., it is concluded that anion exchange resins show high to moderate adsorption

affinity for U(VI), moderate to low for Th(IV), and negligible for RE(III) in acidic sulfate

media.

The results in Table 3 and Table 4 also highlight a marked difference in the adsorption

performance of the weak-base anion exchange resin AnIXR 1 and the rest of the anion

exchange resins. Table 4 shows that AnIXR 1 has the highest adsorption percentage of

Th(IV) among the anion exchange resins, more than twice the amount adsorbed by any of the

other resins. However, the adsorption of U(VI) by AnIXR 1 was less than 90%, whereas it

was approximately 90% or more for all the other anion exchange resins. This suggests that

weak-base anion exchange resin has higher affinity for Th(IV) and lower affinity for U(VI)

than intermediate- and strong-base anion exchange resins. This is likewise seen in

Figure 1 to Figure 9 for adsorption at different acidities.

It is also significant to point out that the strong-base anion exchange resins from different

manufacturers are made with certain different physicochemical properties. The results,

however, did not show any significant variation between strong-base anion exchange resins

with different physicochemical properties, but all strong-base anion exchange resins

exhibited similar adsorption patterns. Rather, significant variation was observed between

anion exchange resins with different functional groups, i.e., between weak-base anion

exchange resin that contains primary amine functional group and strong-base anion exchange

resins resin that contains quaternary amine functional group or alike. This highlights another

important finding that the key underlying factor behind the adsorption performance of the

resins is the functional groups.

There do not appear to be any report in the literature comparing the adsorption of U(VI) and

Th(IV) by weak- and strong-base anion exchange resins in H2SO4 media except for a study

by Kuroda, et al. (1972). This study reported that the weak-base anion exchange resin

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Amberlite CG-4B has a higher adsorption affinity for Th(IV) than the strong-base anion

exchange resin AG1X-8, which is consistent to what was reported in this present study. In the

case of U(VI), the weak-base anion exchange resin showed higher adsorption affinity than the

strong-base anion exchange resin, which is contrary to what was reported in this present

study. The difference is likely because Kuroda, et al. (1972) was reporting the individual IX

adsorption of U(VI) and Th(IV), whereas this present study involved the simultaneous IX

adsorption of both elements. Since U(VI) and Th(IV) were simultaneously adsorbed by the

weak-base anion exchange resin AnIXR 1, there could have been some sort of competing

effect from the anionic Th(IV) sulfato complexes adsorbed on the resin, which in turn results

in the lower adsorption of U(VI) by the resin.

In explaining the adsorption performance of the weak-base anion exchange resin, it should be

noted that its primary amino group can undergo coordination bonding with heavy metal ions

such as U(VI) due to the Lewis-base behaviour of the amino group (Zagorodni, 2007). This is

likewise the case for the tertiary amino groups of the intermediate-base anion exchange

resins. However, in the current experiment, it is not possible for complexation of metal ions

by weak- and intermediate-base anion exchange resins to occur. This is because the resins

were fully protonated under acidic conditions. In other words, these primary and tertiary

amino groups were converted to quaternary ammonium ions, deactivating their complexing

ability, which results in adsorption exclusively by IX. This explains the similar adsorption

performance of the intermediate-base anion exchange resins to the quaternary ammonium

functionalised strong-base anion exchange resins. The weak-base anion exchange resin

AnIXR 1 possibly shows higher Th(IV) adsorption because it has about twice the IX capacity

of other anion exchange resins, as is the case for Kuroda, et al. (1972).

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Figure 1. Adsorption of selected metal ions as a logarithmic function of [H+] in sulfuric acid

media for AnIXR 1 resin containing primary amine group. [UO22+

] = [Th4+

] = [La3+

] = [Ce3+

] =

[Gd3+

] = [Yb3+

] = 2 mmol/L. □ U; ◯ Th; △ La; ▽ Ce; ◁ Gd; ▷ Yb.


media for AnIXR 2 resin containing 92% tertiary, 8% quaternary amine group. [UO22+

] =

[Th4+

] = [La3+

] = [Ce3+

] = [Gd3+

] = [Yb3+


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media for AnIXR 3 resin containing 80% tertiary, 20% quaternary amine group. [UO22+

] =

[Th4+

] = [La3+

] = [Ce3+

] = [Gd3+

] = [Yb3+



media for AnIXR 4 resin containing type I quaternary amine group. [UO22+

] = [Th4+

] = [La3+

] =

[Ce3+

] = [Gd3+

] = [Yb3+


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] = [Th4+

] = [La3+

] =

[Ce3+

] = [Gd3+

] = [Yb3+




] = [Th4+

] = [La3+

] =

[Ce3+

] = [Gd3+

] = [Yb3+


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media for AnIXR 7 resin containing type II quaternary amine group. [UO22+

] = [Th4+

] = [La3+

] =

[Ce3+

] = [Gd3+

] = [Yb3+




] = [Th4+

] = [La3+

] =

[Ce3+

] = [Gd3+

] = [Yb3+


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] = [Th4+

] = [La3+

] =

[Ce3+

] = [Gd3+

] = [Yb3+


The effect of acidity on IX adsorption is illustrated in

Figure 1 to Figure 9. It is obvious that the adsorption of U(VI) and Th(IV) is dependent on

acid concentration, particularly that of U(VI). The resins show similar acid concentration-

dependent behaviour, having a sharp increase in the adsorption percentage of U(VI) as H2SO4

concentration decreases from 2 mol/L to 0.1 mol/L. This indicates that anion exchange resins

effectively adsorb U(VI) at 0.1 mol/L H2SO4 and below (or pH ≥ 0.7). Apart from the

hydrogen ions, the fact that U(VI) adsorption is strongly suppressed by increasing H2SO4

concentration is due to the formation of bisulfate ions rather than sulfate ions in H2SO4

solution (Jamrack, 1963; Preuss & Kunin, 1958; Zagorodnyaya, et al., 2013; Zagorodnyaya,

et al., 2015). The 2-step reaction below shows how H2SO4 undergoes stepwise dissociation to

form bisulfate and sulfate ions.

First step: H2SO4 (aq) → H+ (aq) + HSO4- (aq) (2)

Second step: HSO4- (aq) ⇌ H+ (aq) + SO4

2- (aq) (3)

The dissociation constants for the first and second steps are 1.0×103 and 1.2×10-2,

respectively (Spencer, et al., 2010). In other words, more bisulfate ions are present in H2SO4

solution than sulfate ions. The minor increase in sulfate ions also meant considerably less

U(VI) sulfato complexes are formed (Preuss & Kunin, 1958). Therefore, the hydrogen and

bisulfate ions effectively outcompete U(VI) for adsorption sites on the resins. In contrast, the

competing effects of the hydrogen and bisulfate ions diminish with decreasing H2SO4

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concentration, which in turn increases U(VI) adsorption. This is also most likely the

explanation behind the increasing Th(IV) adsorption with decreasing acidity.

Another observation of the results is that Th(IV) is consistently adsorbed to a much lesser

extent than U(VI) on anion exchange resins in H2SO4 solutions, even at very low acidity.

Other published works have also come to this conclusion (Bunney, et al., 1959; Kraus &

Nelson, 1958; Kuroda, et al., 1972; Strelow & Bothma, 1967). This suggests that the tested

anion exchange resins, apart from the weak-base anion exchange resin AnIXR 1, may be

more suitable for the separation of U(VI) from RE(III), than Th(IV) from RE(III), in H2SO4

media. The difference between the adsorption of U(VI) and Th(IV) by anion exchange resins

is explained in the latter section of this paper along with the results obtained for the cation

exchange resins. However, it may be of practical interest to report that intermediate- and

strong-base anion exchange resins could separate U(VI) from Th(IV), with the best

separation obtained at 0.1 mol/L H2SO4 in the current study.

None of the anion exchange resins exhibit any significant adsorption affinity for RE(III) at

any acid concentration. The results demonstrate that the adsorption affinity for RE(III)

remains low regardless of the acidity. This lack of adsorption of RE(III) on anion exchange

resins from pure aqueous H2SO4 solutions has been reported in many other studies (Banks, et

al., 1958; Hughes & Carswell, 1970; Jangida, et al., 1965; Korkisch, 1989; Kuroda, et al.,

1972; Nagle & Murthy, 1959; Strelow & Bothma, 1967). Kołodyńska and Hubicki (2012)

explained this by noting that RE(III) show little tendency to form anionic complexes with

simple inorganic ligands, e.g., sulfate ions. RE(III) are therefore weakly adsorbed onto anion

exchange resins in a pure aqueous H2SO4 system.

There is therefore a potential application of anion exchange resin in the separation of U(VI)

and Th(IV) from RE(III) in H2SO4 media. The weak-base anion exchange resin with a

primary amine functional group was the best performing resin among the anion exchange

resins. In the current experiment, feasible separation of U(VI) and Th(IV) from RE(III) was

attained at 0.1 mol/L H2SO4 and below, with the best separation at 0.0005 mol/L H2SO4 (pH

3). The separation factors for Th(IV)–RE(III) and U(VI)–RE(III) cannot be measured due to

the very low RE(III) adsorption by AnIXR 1. The intermediate- and strong-base anion

exchange resins, on the other hand, are only suitable for the separation of U(VI) from RE(III),

but not Th(IV) from RE(III), in H2SO4 media.

3.2. Cation exchange resins

Four cation exchange resins were tested for their adsorption affinity for U(VI) and Th(IV) in

acidic sulfate solution containing RE(III). The sulfonic acid functionalised CatIXR 1 and

CatIXR 2 are strong-acid cation exchange resins, while the carboxylic acid functionalised

CatIXR 3 and CatIXR 4 are weak-acid cation exchange resins. Their adsorption of U(VI),

Th(IV) and RE(III) at contact times of 2 hours and 24 hours are tabulated in Table 6 and

Table 7 respectively. The distribution coefficient Kd values of each resin for U(VI), Th(IV)

and RE(III) are tabulated in Table 8.


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U Th La Ce Gd Yb

CatIXR 1 30 43 49 49 48 47

CatIXR 2 28 37 45 46 46 44

CatIXR 3 7 5 8 8 8 4

CatIXR 4 4 4 5 5 4 3



U Th La Ce Gd Yb

CatIXR 1 25 80 96 95 95 94

CatIXR 2 27 80 94 93 91 89

CatIXR 3 4 4 4 4 4 4

CatIXR 4 6 3 3 2 5 2

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Table 8. Distribution coefficients for U(VI), Th(IV) and RE(III) in 0.05 mol/L H2SO4

Resin ID Distribution coefficient Kd (mL/g)

U Th La Ce Gd Yb

CatIXR 1 34.1 473.5 6,435.5 4,008.3 4,122.5 2,455.6

CatIXR 2 38.3 484.5 2,547.8 2,148.5 1,348.4 1,062.3

CatIXR 3 4.7 4.4 4.1 4.3 4.5 4.1

CatIXR 4 6.8 3.0 3.3 2.5 5.9 2.0

3.2.1. Cation exchange resins with sulfonic acid functionality

The strong-acid cation exchange resins in Table 6 show appreciable adsorption of U(VI),

Th(IV) and RE(III) after 2 hours of contact in the acidic sulfate solution. Among the elements

adsorbed, RE(III) had the highest adsorption percentage, followed by Th(IV) and U(VI).

After a contact time of 24 hours, the adsorption percentages of Th(IV) and RE(III) both

doubled compared to after 2 hours, whereas, for U(VI), the percentage adsorbed stayed nearly

the same as after 2 hours of contact. Based on the Kd values in Table 8, it is concluded that

strong-acid cation exchange resins show high adsorption affinity for RE(III), moderate for

Th(IV), and low for U(VI) in acidic sulfate media.

The low adsorption affinity that strong-acid cation exchange resins exhibit towards U(VI) is

because U(VI), in H2SO4 media, is predominantly anionic (Korkisch, 1989). This also

explains the high U(VI) uptake by the anion exchange resins. Nonetheless, the percentage of

U(VI) adsorbed by strong-acid cation exchange resins was fairly substantial, and more so in

dilute H2SO4 solution. For instance, in

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Figure 11, the percent U(VI) adsorbed was as much as 51% at 0.0005 mol/L H2SO4. Korkisch

(1989) attributed this to the increasing formation of U(VI) cationic species as H2SO4

concentration decreases. This most likely also explains the unexpected behaviour of anion

exchange resins whereby the adsorption percentage of U(VI) showed a marginal decrease

from 0.005 mol/L to 0.0005 mol/L H2SO4 (see Figure 2 to Figure 9). As for the ionic state of

Th(IV) in H2SO4 media, the results from both anion and cation exchange resins show that it

tends to be the opposite of that of U(VI), but similar to that of RE(III).

The observation that RE(III) is adsorbed in preference to Th(IV) on strong-acid cation

exchange resins in H2SO4 solution is consistent with what was previously reported (Chang, et

al., 1974; Marhol, 1982; Nietzel, et al., 1958; Strelow, 1961; Strelow, et al., 1965), but no

explanation has been provided as to why the resin exhibited this selectivity. Strong-acid

cation exchange resins show higher affinity for metal cations of higher charge (Hubicki &

Kołodyńska, 2012). This is because their adsorption affinity is determined mainly by the

valence of the metal cation (Page, et al., 2017). Unlike the weak-base anion exchange resins,

strong-acid cation exchange resins have no complexing ability (Zagorodni, 2007). It is thus

suggested that sulfate complexation of Th(IV) ions may have affected their adsorption

affinity for strong-acid cation exchange resins (Borai & Mady, 2002; Page, et al., 2017). This

is likely because Th(IV) ions, due to sulfate complexation, can exist as ThSO42+ ions in a

H2SO4 system (Kim & Osseo-Asare, 2012). Since ThSO42+ ions are of lower charge than

RE(III) ions, the resin would exhibit higher affinity for RE(III) than Th(IV).

In the case of metal cations with the same charge, the affinity of the strong-acid cation

exchange resins is towards the cation with the greater ionic radii (Hubicki & Kołodyńska,

2012). This is because larger cations have lower hydration energy as a result of their lower

charge density. The more strongly hydrated cations will have a greater tendency to migrate to

where there is more water, i.e., out of the resin and into the surrounding solution, as opposed

to weakly hydrated cations (Walton, 2011). This explains the selectivity trend observed for

the different RE(III), since their ionic radii decrease across the lanthanide series. If U(VI) was

present as UO22+ ions, the resin would prefer the larger ThSO4

2+ ions than the smaller UO22+

ions. Hence, this could explain the affinity series for strong-acid cation exchange resins

observed in this present study: La3+ > Ce3+ > Gd3+ > Yb3+ > ThSO42+ >> UO2

2+.

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media for CatIXR 1 resin containing sulfonic acid group. [UO22+

] = [Th4+

] = [La3+

] = [Ce3+

] =

[Gd3+

] = [Yb3+


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media for CatIXR 2 resin containing sulfonic acid group. [UO22+

] = [Th4+

] = [La3+

] = [Ce3+

] =

[Gd3+

] = [Yb3+


Based on these results, strong-acid cation exchange resins are not suitable for separating

U(VI) and Th(IV) from RE(III) in H2SO4 solution. Nevertheless, there seems to be a potential

use of the resin in separating U(VI) from Th(IV) and RE(III) at 0.1 mol/L H2SO4 and below.

The separation process in the current experiment was most efficient when the acid

concentration was between 0.05 mol/L and 0.1 mol/L H2SO4 (or 1 > pH > 0.7).

3.2.2. Cation exchange resins with carboxylic acid functionality

The weak-acid cation exchange resins in Table 6 show no appreciable adsorption of U(VI),

Th(IV) or RE(III) after 2 hours of contact in the acidic sulfate solution. After a contact time

of 24 hours, there is still no appreciable adsorption of U(VI), Th(IV) or RE(III). Based on the

Kd values in Table 8, it is concluded that weak-acid cation exchange resins do not adsorb

U(VI), Th(IV) and RE(III) in acidic sulfate media.

The results in Table 6 and Table 7 demonstrate that weak-acid cation exchange resins have

little or no adsorption capacity in an acidic solution. Unlike strong-acid cation exchange

resins, weak-acid cation exchange resins are not well dissociated over a wide pH range and

are known to suffer significant capacity loss in acidic solutions. This is because weak-acid

cation exchange resins exhibit a particularly high affinity for H+ ions (Hubicki &

Kołodyńska, 2012). The carboxyl functional group of the resins is thus prone to protonation

and losing its ionization under acidic conditions. In the current experiment, the weak-acid

cation exchange resins were fully protonated across the range of acidities tested (Durham,

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2006; Zaganiaris, 2011), as a result of which the non-ionized resins would lose their ability to

readily adsorb U(VI), Th(IV) and RE(III).

However, Figure 12 and Figure 13 show that the resins exhibited appreciable adsorption of

U(VI), Th(IV) and RE(III) at 0.005 mol/L H2SO4 and below. CatIXR 4, for instance, showed

an adsorption of 51% U(VI) and 79% Th(IV) at 0.0005 mol/L H2SO4. This is explained by

the fact that the carboxyl functional groups can form chelating complexes with metal cations

in addition to the ability for simple ion exchange interactions (Choppin, 1980; Pesavento, et

al., 1994; Pesavento, et al., 2003; Zaganiaris, 2016; Zagorodni, 2007). Since complexation

with metal cations is possible, the protons from the functional groups can be displaced by

certain metal cations to form complexes at pH values more or less acidic (Zaganiaris, 2016).

It is therefore deduced that U(VI), Th(IV) and RE(III) were adsorbed by the weak-acid cation

exchange resins through the displacing of protons from the carboxyl groups followed by the

formation of complexes with the functional groups.


media for CatIXR 3 resin containing carboxylic acid group. [UO22+

] = [Th4+

] = [La3+

] = [Ce3+

] =

[Gd3+

] = [Yb3+


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media for CatIXR 4 resin containing carboxylic acid group. [UO22+

] = [Th4+

] = [La3+

] = [Ce3+

] =

[Gd3+

] = [Yb3+


Figure 12 shows that the adsorption of U(VI) and Th(IV) by CatIXR 3 peaked at 0.005 mol/L

H2SO4 while its adsorption of RE(III) continued to increase with decreasing acidity. In the

case of CatIXR 4, there was no adsorption of RE(III) while its adsorption of U(VI) and

Th(IV) continued to increase with decreasing acidity. The discrepancy between these two

resins with the same functionality is due to their different physical structures, i.e., CatIXR 3

has a porous structure, while CatIXR 4 has a macroporous structure. A porous resin has a

smaller pore size than a macroporous resin which would impede the diffusion of larger

cations into the resin. This explains why the adsorption percentages of the larger cations

U(VI) and Th(IV) by CatIXR 3 were capped at 33% and 25% respectively, while the

adsorption of the smaller cation RE(III) continued to increase. With less competition from

U(VI) and Th(IV), the resin then became selective for RE(III).

On the other hand, a typical macroporous weak-acid cation exchange resin that allows free

diffusion of cations of all sizes would exhibit the following order of selectivity: Th(IV) >

U(VI) > RE(III). This is demonstrated by CatIXR 4 in Figure 13 which shows high

adsorption affinity for Th(IV), moderate for U(VI), but negligible for RE(III) at 0.005 mol/L

H2SO4 and below. The adsorption of each individual RE(III) across the range of acidities

tested was less than 5%. CatIXR 4 is therefore a potentially suitable resin for the separation

of U(VI) and Th(IV) from RE(III) at 0.005 mol/L H2SO4 and below (pH ≥ 2). The separation

factors for Th(IV)–RE(III) and U(VI)–RE(III) cannot be measured due to its very low RE(III)

adsorption.

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4. Conclusions

Conventional ion exchange resins with different physicochemical properties were evaluated

for their ability to separate U(VI) and Th(IV) from RE(III) in sulfuric acid media. The resins

are grouped into two broad categories according to their functional groups, namely, (i)

strong- and weak-base anion exchange resins, and (ii) strong- and weak-acid cation exchange

resins. Their adsorption of U(VI), Th(IV) in the presence of selected elements of RE(III) at

different acidities was investigated. It was evident from the results that the adsorption

performance of the resins was acid concentration-dependent. Most candidate resins showed

potentially feasible selective adsorption at or below 0.1 mol/L H2SO4 (pH ≥ 0.7).

Specifically, the strong-base anion exchange resins did not effectively separate Th(IV) from

RE(III) but were found to be effective in separating U(VI) from RE(III), and potentially also

U(VI) from Th(IV). Approximately 90% of U(VI) was adsorbed at 0.1 mol/L H2SO4 with the

adsorption of Th(IV) and RE(III) being less than 30%. The AnIXR 1 weak-base anion

exchange resin with primary amine functional group showed similar selectivity for U(VI)

over RE(III), but was able to also effectively separate Th(IV) from RE(III). At 0.0005 mol/L

H2SO4 (pH 3), the adsorptions of U(VI) and Th(IV) were 78% and 68% respectively, while

the adsorption of RE(III) was less than 5%.

On the other hand, both the strong- and the weak-acid cation exchange resins were ineffective

in separating U(VI) and Th(IV) from RE(III). In the case of the strong-acid cation exchange

resins, Th(IV) and RE(III) were adsorbed in preference to U(VI), i.e., RE(III) > Th(IV) >>

U(VI). The weak-acid cation exchange resins displayed limited adsorption capacity in an

acidic solution. The best performing cation exchange resin was CatIXR 4. At 0.0005 mol/L

H2SO4 (pH = 3), its adsorptions of U(VI) and Th(IV) were 51% and 79% respectively while

the adsorption of RE(III) was no more than 5%.

In summary, among these conventional ion exchange resins, the weak-base anion exchange

resin with primary amine functional group showed the best potential for effective application

in separating U(VI) and Th(IV) from RE(III) in acidic sulfate media. This highlights the

opportunity for the application of conventional ion exchange resins in the hydrometallurgical

processing of RE(III) to remove U(VI) and Th(IV) impurities from the rare earths liquors.

Acknowledgements

The authors are grateful for financial support by the Australian Government under the

Research Training Program Scholarship.

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Highlights

IX resins evaluated for separating U(VI) & Th(IV) from rare earth elements in sulfate

media

Studied were strong- and weak-base anion & strong- and weak-acid cation exchange

resins

The adsorption performance was acid concentration-dependent

Most resins potentially feasible for selective adsorption at pH ≥ 0.7

Primary amine resin separated 78% U(VI) & 68% Th(IV) with < 5% RE(III) adsorbed

at pH 3

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Documents

The effectiveness of ion exchange resins in separating