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The definitive version is available at:
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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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Table 4. 24-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 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.
Figure 2. Adsorption of selected metal ions as a logarithmic function of [H+] in sulfuric acid
media for AnIXR 2 resin containing 92% tertiary, 8% quaternary amine group. [UO22+
] =
[Th4+
] = [La3+
] = [Ce3+
] = [Gd3+
] = [Yb3+
] = 2 mmol/L. □ U; ◯ Th; △ La; ▽ Ce; ◁ Gd; ▷ Yb.
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Figure 3. Adsorption of selected metal ions as a logarithmic function of [H+] in sulfuric acid
media for AnIXR 3 resin containing 80% tertiary, 20% quaternary amine group. [UO22+
] =
[Th4+
] = [La3+
] = [Ce3+
] = [Gd3+
] = [Yb3+
] = 2 mmol/L. □ U; ◯ Th; △ La; ▽ Ce; ◁ Gd; ▷ Yb.
Figure 4. Adsorption of selected metal ions as a logarithmic function of [H+] in sulfuric acid
media for AnIXR 4 resin containing type I quaternary amine group. [UO22+
] = [Th4+
] = [La3+
] =
[Ce3+
] = [Gd3+
] = [Yb3+
] = 2 mmol/L. □ U; ◯ Th; △ La; ▽ Ce; ◁ Gd; ▷ Yb.
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Figure 5. Adsorption of selected metal ions as a logarithmic function of [H+] in sulfuric acid
media for AnIXR 5 resin containing type I quaternary amine group. [UO22+
] = [Th4+
] = [La3+
] =
[Ce3+
] = [Gd3+
] = [Yb3+
] = 2 mmol/L. □ U; ◯ Th; △ La; ▽ Ce; ◁ Gd; ▷ Yb.
Figure 6. Adsorption of selected metal ions as a logarithmic function of [H+] in sulfuric acid
media for AnIXR 6 resin containing type I quaternary amine group. [UO22+
] = [Th4+
] = [La3+
] =
[Ce3+
] = [Gd3+
] = [Yb3+
] = 2 mmol/L. □ U; ◯ Th; △ La; ▽ Ce; ◁ Gd; ▷ Yb.
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Figure 7. Adsorption of selected metal ions as a logarithmic function of [H+] in sulfuric acid
media for AnIXR 7 resin containing type II quaternary amine group. [UO22+
] = [Th4+
] = [La3+
] =
[Ce3+
] = [Gd3+
] = [Yb3+
] = 2 mmol/L. □ U; ◯ Th; △ La; ▽ Ce; ◁ Gd; ▷ Yb.
Figure 8. Adsorption of selected metal ions as a logarithmic function of [H+] in sulfuric acid
media for AnIXR 8 resin containing type II quaternary amine group. [UO22+
] = [Th4+
] = [La3+
] =
[Ce3+
] = [Gd3+
] = [Yb3+
] = 2 mmol/L. □ U; ◯ Th; △ La; ▽ Ce; ◁ Gd; ▷ Yb.
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Figure 9. Adsorption of selected metal ions as a logarithmic function of [H+] in sulfuric acid
media for AnIXR 9 resin containing type II quaternary amine group. [UO22+
] = [Th4+
] = [La3+
] =
[Ce3+
] = [Gd3+
] = [Yb3+
] = 2 mmol/L. □ U; ◯ Th; △ La; ▽ Ce; ◁ Gd; ▷ Yb.
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.
Table 6. 2-hour IX adsorption of U(VI), Th(IV) and RE(III) in 0.05 mol/L H2SO4
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Resin ID Adsorption (%)
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
Table 7. 24-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
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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Figure 10. Adsorption of selected metal ions as a logarithmic function of [H+] in sulfuric acid
media for CatIXR 1 resin containing sulfonic acid group. [UO22+
] = [Th4+
] = [La3+
] = [Ce3+
] =
[Gd3+
] = [Yb3+
] = 2 mmol/L. □ U; ◯ Th; △ La; ▽ Ce; ◁ Gd; ▷ Yb.
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Figure 11. Adsorption of selected metal ions as a logarithmic function of [H+] in sulfuric acid
media for CatIXR 2 resin containing sulfonic acid group. [UO22+
] = [Th4+
] = [La3+
] = [Ce3+
] =
[Gd3+
] = [Yb3+
] = 2 mmol/L. □ U; ◯ Th; △ La; ▽ Ce; ◁ Gd; ▷ Yb.
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.
Figure 12. Adsorption of selected metal ions as a logarithmic function of [H+] in sulfuric acid
media for CatIXR 3 resin containing carboxylic acid group. [UO22+
] = [Th4+
] = [La3+
] = [Ce3+
] =
[Gd3+
] = [Yb3+
] = 2 mmol/L. □ U; ◯ Th; △ La; ▽ Ce; ◁ Gd; ▷ Yb.
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Figure 13. Adsorption of selected metal ions as a logarithmic function of [H+] in sulfuric acid
media for CatIXR 4 resin containing carboxylic acid group. [UO22+
] = [Th4+
] = [La3+
] = [Ce3+
] =
[Gd3+
] = [Yb3+
] = 2 mmol/L. □ U; ◯ Th; △ La; ▽ Ce; ◁ Gd; ▷ Yb.
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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