Journal of Nuclear and Radiochemical Sciences, Vol. 20, pp. 32-42, 2020

ArticlesSolubility and solid phase of trivalent lanthanide hydroxides and oxides
Md. Moniruzzaman1,*, Taishi Kobayashi2,*, Takayuki Sasaki
Department of Nuclear Engineering, Kyoto University, Kyotodaigaku-katsura, Nishikyo-ku, Kyoto 615-8540,
Japan
Received March 10, 2020; Accepted June 10, 2020; Published online June 26, 2020
An undersaturation approach was adopted to reveal the solubility and solubility-limiting solid phase of La, Eu and
Tm in the sample solutions with the ionic strength (I) of 0.1 mol/L (M) and the hydrogen ion concentration range(pHc = –log [H+]) of 6 to 12 after aging at 25 to 90°C for certain periods up to 12 weeks. Solubility and solubilitylimiting solid phases were discussed with the aid of X-ray diffraction (XRD) patterns of the solid phases and temperature dependence of solubility products (log Ks°) values. The XRD patterns of the solid phases indicated thatthe initial solid phase of La2O3(cr) transformed to La(OH)3(cr) after aging at 25 °C and 90 °C. On the other hand,
Eu 2O3(cr) remained stable after aging at 25 °C, but after 90 °C, converted to Eu(OH)3(cr). It was observed thatTm2O3(cr) was not transformed after aging at both 25 °C or 90 °C. Thermodynamic properties obtained from thetemperature dependence of the solubility products agreed well with the observations in the XRD patterns.

1. Introduction
For the safety assessment of radioactive waste disposal, it isnecessary to predict the migration behavior of actinide elements under relevant geochemical conditions, as they areincluded in the waste as alpha-emitting radionuclides wit hlong half-lives. Actinide elements of thorium, uranium, neptunium, and plutonium can exist in a tetravalent oxidation state
under reducing geochemical conditions, deep undergroundand easily precipitate as a sparingly soluble amorphoushydroxide solid phase (An(IV)(OH) 4 (am)) under neutral toalkaline pH conditions of the waste repository systems [1-4].
The solubilities of An(IV)(OH)4(am), hence, play an importantrole in understanding their migration behavior. It is known
that a crystalline oxide solid phase as An(IV)O2(cr) is thermodynamically more stable and it has been reported that the crystallization of An(IV)(OH) 4 (am) towards An(IV)O 2 (cr)proceeded under certain solution conditions such as strong
alkaline pH or elevated temperatures [5-7]. The solubilities ofAn(IV)O2(cr) have been reported to several orders of magnitude lower than those of An(IV)(OH)4(am) [1-7].
Trivalent actinide elements of americium and curium also
exhibit a strong hydrolysis reactions under neutral to alkaline
pH conditions to precipitate the sparingly soluble amorphoushydroxide solid phase (An(III)(OH)3(am)) [8-11]. In contrast to
the tetravalent actinide elements, no crystalline oxide solid
phase (An(III)2O3(cr)) was observed in the solubility experiments [2,12]. A few literatures have observed crystallinehydroxide solid phase (An(III)(OH)3(cr)) from X-ray diffraction patterns instead of An(III)2O3(cr) and showed an order ofmagnitude lower solubility values than those of An(III)(OH)3(am) [13,14]. This can be explained by thermodynamic
data of An(III)2O3(cr). For example, the standard enthalpy(∆f Hm°) and entropy (Sm°) of Am2O3(cr) have been reported to be∆ f H m° = −1690.4±8.0 kJ/mol and S m° = 133.6±6.0 J/K/mol
resulted in the standard formation Gibbs energy of ∆ f G m° =−1605.449±8.284 [2]. Combined with the thermodynamic data
for Am 3+ and H 2O [2], the standard reaction Gibbs energy(∆ rGm°) for 1/2 Am 2O3(cr) + 3H+ Am3+ + 3/2 H2O was calculated to be ∆ rG m° = −151.59 kJ/mol, leading to the solubility*Corresponding author. Email: 1mohammad.moniruzzaman.23c@
st.kyoto-u.ac.jp, 2kobayashi@nucleng.kyoto-u.ac.jp

product (Ks°) of log Ks° = 26.56. This value is approximately 10
order s of mag n it ude h ig her t ha n t hose re por ted forAm(OH)3(am) and Am(OH)3(cr) [2,4], hinting the oxide solid
phase is less stable in aqueous systems. However, due toexperimental limitations for handling macro amounts of trivalent actinide elements, only few studies have investigated theAn(III) solubility with a definite solid phase characterization[8,11,13] and the stability of An(III)2O3(cr) in aqueous systems
has not been well experimentally clarified.
Trivalent lanthanide elements are often used as analogues oftrivalent actinide elements. A number of literatures haveinvestigated the hydrolysis behavior, solubilities and solid
phases of lighter to heavier lanthanide elements and reported
their thermodynamic data [15-24]. Several works occasionallysummarized the state of knowledge on the solubilities of trivalent lanthanide elements [25-28]. Most recently, Brown andEkberg [4] have compiled the literature data on the hydrolysisof lanthanide elements and selected the recommended values
for the solubility product of hydroxide solid phase (Ln(III)(OH)3(s)) and related thermodynamic data. It is noted that thesolubility product of amorphous hydroxide solid phase (Ln(III)(OH)3(am)) obtained after short aging periods was excluded in
the review due to poor identification of the solid phase [4].
Konings et al. [29] conducted a comprehensive review on thethermodynamic properties of lanthanide and actinide oxides,
where recommended values of ∆f Hm° and Sm° for the lanthanideoxide solid phase (Ln(III)2O3(cr)) were selected. For example,
the values of ∆ f Hm° and S m° for La 2O3(cr) were selected to be∆ f Hm° = −1791.6±2.0 kJ/mol and S m° = 127.32±0.84 J/K/mol,based on the reported results by solution calorimetry and heat
capacity measurements [29]. The calculated ∆f Gm° and subsequently ∆ rG m° for 1/2 La 2O3(cr) + 3H+ La 3+ + 3/2 H 2O was−192.33 kJ/mol, leading to log K s° = 33.70, which is muchlarger than the selected K s° value for La(OH)3(s) (log K s° =19.72) [4]. The transformation of Ln(III)2O3(cr) to Ln(III)(OH)3(cr) in aqueous systems was observed in a few literatures[30-32]. The formation of Ln(III)(OH)3(cr) was observed by
precipitating Ln(III) solutions with NaOH, after complete dissolution of initial Ln(III)2O3(cr) with a hot nitrate solution [31].
Neck et al. [32] observed that the initial material of Nd 2O3(cr)was converted to Nd(OH)3(cr) in a purified water at 25 °C aftera few months, prior to the their solubility experiment of© 2020 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/26/2020

33

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Nd(OH)3(cr). However, not many literatures have investigated
the solid phase transformation from lighter to heavier lanthanide elements with a combination of definite solid phase characterization and solubility measurement. It is evident that,clarifying the “solubility-limiting solid phase” is essential forthe reliable prediction of the solubility limits of trivalent radionuclides.
The present study focuses on the solubilities and solid
phases of trivalent lanthanum (La), europium (Eu) and thulium(Tm), in neutral to alkaline pH range to clarify the “solubilitylimiting solid phase.” The solubility experiments were conducted with an undersaturation approach by adding the solid
phases of cr ystalline oxides (La 2 O 3 (cr), Eu 2 O 3 (cr), andTm 2O3(cr)) into sample solutions, at given hydrogen ion concentration (pHc = –log [H+]) and ionic strength (I = 0.1 mol/L(M) by NaClO4)). The sample solutions were then aged undertemperature conditions of Tage = 25, 40, 50, 60, and 90 °C, forcertain periods up to 12 weeks. After aging at 25 to 90 °C, thesample solutions were set down at a certain measurement temperature (Tmeas) of 25, 40, 50, and 60 °C for the solubility measurement. The solubilities of La, Eu, and Tm were measured,and their solid phases were investigated by X-ray diffraction(XRD). The solubility-limiting solid phase for La, Eu, and Tm
was discussed alongside the XRD patterns of the solid phases
and the temperature dependence of the solubility products.
2. Experimental
2.1. Materials and sample preparation. Reagent gradesof sodium perchlorate monohydrate (NaClO4·H 2O, 98%), perchloric acid (HClO4, 60%), sodium hydroxide (NaOH, 97%),
we r e p u r c h a s e d f r o m WA KO P u r e C h e m . H E PE S
(C8H18N2O4S, 99.0%, Dojindo) was used to adjust pH of sample
solutions to the neutral pH region. Lanthanum oxide (La2O3,
99.99%), europium oxide (Eu 2O3, 99.9%), and thulium oxide(Tm 2 O 3 , 99.9%) were also purchased from WAKO Pure
Chem., and were used without any prior treatments. The
appearance of these initial oxides was investigated using adesktop scanning electron microscope (JCM-6000 NeoScope,
JEOL) and the particles size of these oxides were found to be
1-3 μm. Deionized purified water (Milli-Q, Millipore) wasused in the preparation of all solutions. The sample preparation was conducted in a CO2 free, argon gas filled glove box,
at 25 °C and 1 atm to prevent entering CO2 in the sample solutions. The pHc values of the samples were measured by a pH
meter (D-72, Horiba Ltd.) with a combined glass electrode(9615-10D, Horiba Ltd.). The reference electrode was filledwith 3.89 M NaCl and 0.41 M NaClO 4 (Wako Pure Chem.).
The electrode was calibrated against standard HClO 4 andNaOH solutions (pHc 1, 2, 3, 11, 12, and 13; Wako Pure Chem.)at I = 0.1 M using NaClO4 at 25, 40, 50, and 60 °C, to correct
the experimentally measured pHexp values, to the pHc values atdifferent temperatures.
Sample solutions were prepared by an undersaturation
approach. A certain amount of 0.1 M NaClO 4 solution was
prepared as an initial solution. Then, 0.1 M HClO4 or 0.1 M
NaOH was added to the initial solution to adjust the pHc, sothat the ionic strength (I) of the initial solution kept constant at
I = 0.1 M. Each 10 mL of aliquot was withdrawn at fixed pH
levels ranging from pHc 7 to 12 and put into polypropylene
tubes. For the sample solutions in the neutral pH region, an
appropriate amount of 0.1 M HEPES solution was added tostabilize the pHc. The concentration of HEPES was 10−3 M foreach sample solution. The La stock suspension was preparedby adding 16.3 g of La2O3 in 50 mL of pure water. If the La2O3
solid phase totally dissolved, the concentration would correspond to 2.0 M. An aliquot of 50 μL stock suspension wasthen added to each 10 mL of sample solution, so that the Laconcentration of the sample solution would be 0.01 M, if theMoniruzzaman & Kobayashi
added solid phase completely dissolved. The Eu and Tm stock
suspensions were prepared by adding 17.6 g and 19.3 g of
Eu 2O3 and Tm 2O3, respectively in 50 mL of pure waters andthe sample solutions containing Eu 2O3 and Tm 2O3 were prepared in the same method as those of La 2O3. For the solid
phase analysis, aliquots of 10 mL of the La stock suspension,containing approximately 3.3 g of La2O3, was added in 100 mLof sample solutions. The La concentration would be 0.2 M if
the added solid phase completely dissolved. Similarly, 100 mLof sample solutions containing 3.5 g of Eu 2O3 and 3.8 g ofTm 2O3 at certain pH conditions were prepared for the solid
phase analysis. After the preparation, caps of the sample tubes
were tightly closed, and the tubes were taken from the Ar
glove box. The sample tubes were placed and stored in temperature chambers controlled at the aging temperatures (Tage)at 25, 40, 50, 60, and 90 °C for up to 12 weeks (TAITEC
BIOShaker BR-43F2 for 25 °C and ETTAS E0450B for 40 to
90 °C). The sample tubes were shaken by a shaker equippedwith the temperature chamber during the aging at 25 °C, andshaken by hand occasionally for a few minutes during theaging at 40 to 90 °C.
2.2. Solid phase analysis. After aging at 25 and 90 °C for
given periods, selected sample tubes were taken from the temperature cambers and kept at room temperature for 1 day. The
pHc of the sample solution was measured and the solid phase
was separated by centrif uging at 5,000 r pm (H-103N,
Kokusan) for 5 min under atmospheric conditions, and driedin a vacuum desiccator for several hours. The diffraction patterns of the solid phase were then collected using XRD(Miniflex, RIGAKU), with Cu-Kα (λ = 0.154 nm) in the rangeof scattering angle 2θ = 10 to 60°, and a scan rate of 20 °/min.
The solid phase samples of La 2O3, Eu 2O3, and Tm 2O3 prior toadding into the sample solutions were measured as references
and compared to those after aging at 25 and 90 °C.
2.3. Solubility measurement. After aging the sample solutions at 25, 40, 50, 60, and 90 °C for certain weeks, the sample
tubes were taken out from the temperature chambers. Then,the sample tubes were placed in thermostatted baths maintained at the measurement temperature (Tmeas) of Tmeas = 25, 40,
50, and 60 °C, as described elsewhere [33,34]. The studied
conditions we re summarized in Table 1. After 1 day underrespective Tmeas conditions, the pH measurements and ultrafiltrations of the sample solutions at the desired temperatures
were performed. The observed pH values at each Tmeas werecorrected to their pHc values using temperature-specific calibration curves. The 0.5 mL of supernatants of the sample
solutions were filtrated through ultrafiltration membranes(Microcon, nominal molecular weight limit (NMWL) of 3,000(3 kDa), corresponding to pore sizes of approximately 2 nm,
Millipore). Filter units and membranes were equilibrated ateach Tmeas prior to the ultrafiltration, and the Tmeas values during the ultrafiltration were maintained by the thermostat
equipped with the centrifuge device (CF15R, Hitachi Koki Co.
Ltd.) run at 13,000 rpm for 20 minutes in the case of Tmeas = 25
or 40 °C. A desktop centrifuge (FB-4000, Kurabo) introduced
in a temperature chamber was used to filtrate the sample solutions at Tmeas in the case of Tmeas = 50 and 60 °C. After the pH
measurement and ultrafiltration at each Tmeas, the sample solutions were returned to the temperature chambers. A few of thesample tubes were kept at Tmeas longer than 1 day and pH measurement and ultrafiltration at the Tmeas were repeated to confirm the steady state was achieved at the Tmeas. The wholesolubility measurements were repeated several times for different aging times up to 12 weeks. After the ultrafiltration ateach Tmeas, 0.3 mL of filtrate was taken and acidified with 2.0
mL of 0.1 M HNO3. The La, Eu, and Tm concentrations werethen measured by ICP-MS (ELANDRC II, PerkinElmer). The

Solubility and solid phase of trivalent lanthanide hydroxides and oxides

3.1. Solid phases after aging at 25 and 90 °C
3.1.1. La solid phases after aging at 25 and 90 °C. Figure 1
shows the XRD patterns for La solid phases after aging of thesample solutions at 25 and 90 °C, in the neutral to alkaline pH
region for 4 weeks. The XRD pattern of La 2O3(cr) was measured at its initial state and shown in Fig. 1 for comparison.
After aging the sample solutions under the above mentioned
conditions, the XRD patterns showed that, the peaks corresponding to the initial La 2O3(cr) showing the hexagonal antiprismatic structure disappeared, whereas those corresponding
to La(OH)3(cr) as the UCl3-type structure (trigonal prismatic)(ICSD No. 31584) appeared. This indicates that the initial
La 2O3(s) transformed to La(OH)3(cr), by aging of the sample
solutions at 25 and 90 °C for 4 weeks. It was in fact notedfrom the XRD patterns, that the transformation of La2O3(cr) to
La(OH)3(cr) already took place after aging at 25 and 90 °C for
1 week, suggesting that the solid phase transformation has
rather fast kinetics under the investigative experimental conditions. This confirms that the solid phase of La2O3(cr) is unstable and it transforms to La(OH)3(cr) in aqueous solutions, as
has been pointed out in previous literature, where La 2O3(cr)was dissolved in a heated nitrate solution [31]. Similar transformation was observed for Nd 2O3(cr), where Nd 2O3(cr) converted to Nd(OH)3(cr) after aging in pure water at 25 °C for 3
months [32]. It can also be observed from Fig. 1, that there isno significant difference between the XRD patterns of thesolid phases, aged at different pHc conditions and aging temperatures.
3.1.2. Eu solid phases after aging at 25 and 90 °C. In Fig. 2,
the XRD patterns of Eu solid phases, derived after aging of thesample solutions at 25 and 90 °C in neutral to alkaline pH
region for 4 weeks, were compared with those of the initialstate of Eu 2O3(cr) which exhibited cubic structure (ICSD
No.27997). After aging at 25 °C, the XRD patterns corresponded to that of the initial Eu 2O3(cr) solid phase, indicatingthat no phase transformation occurred at 25 °C. On the other
hand, after aging at 90 °C, the peak positions of the samples
changed, and were identified as those of Eu(OH)3(cr), that had
UCl 3-type structure (ICSD No. 200488). This suggests that
Eu2O3(cr) transformed to Eu(OH)3(cr) during the aging at 90 °C,
whereas no solid phase transformation occurred at 25 °C. It isfurther noted, that the solid phase transformation from
Eu2O3(cr) to Eu(OH)3(cr) at 90 °C in fact occurs after aging forjust 1 week. This difference in trend between the solid phasetransformations for Eu2O3(cr) and La2O3(cr), Nd2O3(cr), may berelated to the difference in their crystal structures; La2O3(cr)and Nd 2O3(cr) have the hexagonal phase, whereas Eu 2O3(cr)has the cubic phase.
3.1.3. Tm solid phases after aging at 25 and 90 °C. In Fig. 3,
the XRD patterns of Tm solid phases, after aging at 25 and90 °C in the neutral to alkaline pHc region for 4 weeks, were
presented together with that of the initial Tm 2O3(cr) solid phaseexhibiting cubic structure. The XRD patterns did not change
after aging at both temperatures and remained stable as theinitial Tm 2O3(cr) (ICSD No. 33657). The same peak patterns
were obtained for samples aged at 25 °C and 90 °C for 1 week.

Relative Intensity

3. Results and discussion

34

25

60

pHc 12.13,
90°C
pHc 8.25,
90°C
pHc 7.00,
90°C
pHc 10.80,
25°C
pHc 9.30,
25°C
pHc 7.00,
25°C
La2O3(cr)

15

20

30

35

40

45

50

55


Figure 1. XRD patterns of the La solid phases after aging for 4
weeks at pHc 7.00, 9.30, and 10.80 at 25 °C and at pHc 7.00, 8.25, and12.13 at 90 °C, together with that of La 2O3(cr) as the initial solid
phase. Filled circles (●) and triangles (▼) represent the peaks fromICSD patterns of La 2O3 (No. 10278) and La(OH) 3 (No. 31584),
respectively.

pHc 11.73,
90°C
pHc 9.12,
90°C

Relative Intensity

detection limit was approximately 10 −9 M for each element.
The solubility measurement was repeated several times, toconfirm if the solubility reaction reached the steady state. The
standard error was within 10 % for each measurement by
ICP-MS, which resulted in ±0.1 in the log unit of La, Eu, andTm concentrations. It is note that taking three different supernatants from a certain sample tube resulted in a maximum
scattering of log [Ln] = ±0.5.

J. Nucl. Radiochem. Sci., Vol. 20, 2020

pHc 7.17,90°C
pHc 11.65,
25°C
pHc 8.65,
25°C
pHc 7.09,
25°C
Eu2O3(cr)

15

20

25

30

35

40 45


50

55

60

Figure 2. XRD patterns of the Eu solid phases after aging for 4
weeks at pHc 7.09, 8.65 and 11.65 at 25 °C and at pHc 7.17, 9.12, and11.73 at 90 °C, together with that of Eu 2O3(cr) as the initial solid
phase. Filled circles (●) and triangles (▼) represent the peaks fromICSD patterns of Eu 2O3 (No. 27997) and Eu(OH)3 (No. 200488),
respectively.

The results of the solid phase analysis are summarized inTable 1, together with the aging temperature conditions.
3.2. Solubilities measured at 25 °C after aging at 25 to 90 °C
3.2.1. La Solubilities measured at 25 °C after aging at 25
to 90 °C. The XRD patterns of the La solid phase indicatedthat the initial La2O3(cr) converted to La(OH)3(cr) after aging
of its sample solutions, at 25 and 90 °C for more than 1 week.
As described above, the reported values of ∆f Hm° = −1791.6±2.0
kJ/mol and S m° = 127.32±0.84 J/K/mol for La 2 O3 (cr) [29]resulted in ∆rGm° = −192.33 kJ/mol, and log Ks° = 33.70 at 25 °C
for 1/2 La 2O3(cr) + 3H+ La 3+ + 3/2 H 2O combined with the∆f Hm° and S m° for La 3+ and H 2O [2,4]. Larger log Ks° value for
La2O3(cr) possibly leads the transformation of initial La2O3(cr)35

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Moniruzzaman & Kobayashi
effect of the Tage on the La(OH)3(cr) solid phases below Tage =60 °C, as summarized in Table 1.
On the other hand, after aging at Tage = 90 °C, the obtainedsolubility values at pHc 8−9 measured at Tmeas = 25 °C were
found to be lower by about 2 orders of magnitude than thoseobtained after aging at 25 to 60 °C. As seen in Fig. 1, the
XRD patterns of the La(OH)3(cr) solid phase after aging at
90 °C, were similar to those obtained after aging at 25 °C.
This indicated that there is no significant difference between
the bulk structures of the solid phases aged at both temperatures. However, here we assumed a different solubility-limit-

to La(OH)3(cr) in the present study. Figure 4 shows the solubility of La(OH)3(cr) measured at Tmeas = 25 °C after aging at
Tage = 25, 40, 50, 60 and 90 °C for 8 to 12 weeks. Since no significant change in the solubility values was observed after different aging periods, it was considered that the solubility
reached a steady state after 8 weeks. It is noted that after
aging at 60 °C, the sample solutions kept at 25 °C for longer
days prior to the solubility measurement at 25 °C. No significant change was observed between different periods at 25 °C,
also suggesting that a steady state has been achieved at 25 °C
after the solid phase transformation at 60 °C. After aging at
25 °C, the La solubility at around pHc 7, was observed to benear 2 × 10 –1 M, which corresponded to the total dissolution of
the added La 2O3(cr) in the initial solid phase. The solubility
then rapidly decreased with increasing pHc and was almost
constant at 10 –9 M above pHc 10, which is close to the detection limit of ICP-MS. After aging at Tage = 40, 50, and 60 °C,
the solubility values measured at 25 °C were observed to be
similar to those aged at 25 °C, suggesting that no significant-1
La(OH)3(cr)This work

-2
-3
2+

-4 LaOH-5log [La]

pHc 12.52,
90°C
pHc 8.05,
90°C

Relative Intensity

25 °C
25 °C
25 °C
25 °C
25 °C

Tage

Tmeas

25 °C

25 °C

-7
-8

pHc 12.52,
25°C

La3+

-9

pHc 7.88,
25°C

-10

pHc 7.30,
25°C

6

7

8

9

10

11

12

13

pHc

Tm2O3(cr)

20

Tmeas

25 °C
40 °C
50 °C
60 °C
90 °C

Akselrud et al. [17]LaOH2+-6

pHc 7.53,
90°C

15

Tage

25

30

35

40 45


50

55

Figure 4. La solubilities measured at Tmeas = 25 °C after aging at
Tage = 25, 40, 50, 60, and 90 °C for 8 and 12 weeks. The ionicstrength was maintained at I = 0.1 M, and the solubilities wereobtained after filtration through 3 kDa membranes. The solid lines
with blue and red colors represent the calculated solubility of
La(OH)3(cr) and La(OH)3(cr,90), respectively. The broken lines withblue and red colors represent the contributions of La 3+ and firsthydrolysis species; LaOH2+. A gray field in the figure indicated thedetection limit level of the ICP-MS.

60

Figure 3. XRD patterns of the Tm solid phases after aging for 4
weeks at pHc 7.30, 7.88, and 12.52 at 25 °C and pHc 7.53, 8.05, and12.52 at 90 °C, together with that of Tm 2O3(cr) as the initial solid
phase. Filled circles (●) represent the peaks from ICSD pattern of
Tm 2O3 (No. 33657).

TABLE 1: Summary of the solid phases investigated in the present study
Aging Temp, Tage
25 °C

40 °C

50 °C

60 °C

90 °C

25 °C
La(OH)3(cr)aEu2O3(cr)aTm2O3(cr)a
La(OH)3(cr)b
Eu2O3(cr)b
Tm2O3(cr)b
La(OH)3(cr)b
Eu2O3(cr)b
Tm2O3(cr)b
La(OH)3(cr)b
Eu2O3(cr)b
Tm2O3(cr)b
La(OH)3(cr, 90 °C)a
Eu(OH)3(cr)aTm2O3(cr)aMeasurement temp, Tmeas
40 °C
50 °C
-

60 °C

-

-

-

-

La(OH)3(cr)b
Eu2O3(cr)b
Tm2O3(cr)b
La(OH)3(cr)b
Eu2O3(cr)b
Tm2O3(cr)b
La(OH)3(cr)b
Eu2O3(cr)b
Tm2O3(cr)b

La(OH)3(cr)b
Eu2O3(cr)b
Tm2O3(cr)b
La(OH)3(cr)b
Eu2O3(cr)b
Tm2O3(cr)b

La(OH)3(cr)b
Eu2O3(cr)b
Tm2O3(cr)b

-

-

-

a) The solid phase was confirmed by the XRD patterns.
b) The solid phase was assumed from the observed solubilities.

-

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Solubility and solid phase of trivalent lanthanide hydroxides and oxides

3.2.2. Eu Solubilities measured at 25 °C after aging at 25 to
90 °C. As confirmed by the XRD patterns, the solid phaseremained Eu 2O3(cr) after aging at 25 °C, while it converted toEu(OH)3(cr) after aging at 90 °C. The ∆f Hm° and Sm° values for
Eu 2O3(cr) were reported to be ∆f Hm° = −1650.4±4.0 kJ/mol and
Sm° = 143.5±0.5 J/K/mol [29]. Combined with the ∆f Hm° and Sm°
for Eu 3+ and H 2O [2,4], ∆ rG m° = −154.64 kJ/mol and log Ks° =27.09 were obtained at 25 °C for 1/2 Eu 2O3(cr) + 3H+ Eu3+ +3/2 H2O. Although the calculated Ks° was found to be orders ofmagnitude larger than those reported for Eu(OH)3(cr) (log Ks° =16.48) [4], the bulk structure remained Eu2O3(cr) after aging inthe sample solutions at 25 °C. In contrast to the case of La, thesolid phase transformation may be kinetically hindered whenstarted from Eu 2O3(cr) in aqueous solutions under the present
investigated conditions. Fig. 5 shows the Eu solubility values
measured at Tmeas = 25 °C after aging at Tage = 25, 40, 50, 60,
and 90 °C for up to 8 weeks. The obtained results showed there
was no significant change in solubility for the entire aging periods, suggesting that a steady state had been achieved. After
aging at 60 °C, no significant change was observed between
different cooling periods at Tmeas = 25 °C, also suggesting thata steady state has been achieved at 25 °C after the solid phasetransformation at 60 °C. The solubility of Eu 2O3(cr) obtained
after aging at 25 °C was around 10 −5 M at pHc 8, and thendecreased with the increasing pHc. As shown in Fig. 5, thesolubility values after aging at Tage = 40, 50, and 60 °C wereplotted within the range of the scattering data aged at Tage =25 °C. This suggested that the aging temperature up to 60 °C
had little impact on the solid phase transformation, as summarized in Table 1. On the other hand, the solubility of
Eu(OH)3(cr) obtained after aging at 90 °C was lower by about
one order of magnitude, than that of Eu 2O3(cr) at pHc 8. For
pHc > 9, the solubility values for both the solid phases were

-1

Eu2O3(cr)This work

Eu3+

-2

EuOH2+

Tage
25 °C
40 °C
50 °C
60 °C

-3
-4
Eu3+

log [Eu]

ing solid phase, La(OH)3(cr, 90 °C aged), after aging at 90 °C,
based on the observations in the solubility experiment. The
solubility values after aging at 90 °C at pHc < 8 were rather
scattered and approaching to those obtained after aging at 25°C, as shown in Fig. 4. After aging at 90 °C for given periods,
the sample solutions were cooled down to 25 °C before the pHc
and solubility measurements. During the cooling time for
25 °C, La(OH)3(cr, 90 °C aged) may begin transformation to
La(OH)3(cr) at 25 °C, especially in the lower pH region where
higher solubility is expected. In the region at pHc > 9, the solubility values after aging at 90 °C were near the detection limitof ICP-MS, similar to those obtained after aging at 25 °C.
In literatures, a number of studies have investigated the solubility of La hydroxides (La(OH)3(s)) at 25 °C or room temperatures under varying experimental conditions of preparation
method, medium, and aging durations [4, 17, 21-23, 25, 35-39].
Daikonov et al. [25, 26] have comprehensively reviewed thesolubility studies on lanthanide hydroxides and categorized the
La(OH)3(s) solid phases into two groups; freshly precipitatedand slightly aged La(OH)3(s) (within 1 day), and long term
aged La(OH)3(s). The experimental conditions of aging at
25 °C, for 8 and 12 weeks in the present study, are similar to
those for the long-term aging of La(OH) 3 (s), in studies
reviewed by Daikonov et al. [25, 26]. The solubility values
reported in Akselrud et al. plotted in Fig. 4 were obtained
after aging at 25 °C for 166 days by an oversaturation method[17]. The solubility values from their method [17] represented
the points of starting precipitation after adding NaOH solutions to La solution, while limiting the pH range to pHc 7−8,
whereas the solubility values in the present study wereobtained from an undersaturation approach at a wider pH
range. Since the pHc range of our experiment does not reallyoverlap with that in the studies by Akselrud et al. [17], the difference in the solubility values can be attributed to the difference in experimental conditions.

36

Tmeas
25 °C
25 °C
25 °C
25 °C

Eu(OH)3(cr)This work-5
EuOH2+

-6

Tage

Tmeas

90 °C

25 °C

Akselrud et al. [18]

-7

Tage

Tmeas

25 °C

25 °C

-8
-9
-10
6

7

8

9

10

11

12

13

pHc
Figure 5. Eu solubilities measured at Tmeas = 25 °C after aging at Tage
= 25, 40, 50, 60, and 90 °C for 4 and 8 weeks. The ionic strength wasmaintained at I = 0.1 M and the solubilities were obtained after filtration through 3 kDa membranes. The solid lines with green and blue
colors represent the calculated solubilit y of Eu 2 O 3 (cr) andEu(OH)3(cr), respectively. The broken lines with green and blue colors represent the contributions of Eu 3+ and first hydrolysis species;
EuOH2+. A gray field in the figure indicated the detection limit levelof the ICP-MS.

observed to be in the range of detection limit of ICP-MS.
The solubility of Eu(OH)3(s) has been investigated in several
literatures [4, 15, 16, 18, 24, 25, 39-42]. Akselrud andErmolenko [18] observed starting points of precipitation byadding NaOH solutions to Eu solution in certain concentrations. The pH of these sample solutions starting to precipitate
was measured several times for up to 150 days, to determine
the solubility product values [18]. Moeller et al. also investigated the solubility limit of Eu(OH)3(s) by measuring the starting points of precipitations [15,16]. The Eu concentrations
against pH of starting points for precipitation after 50 days
[18] were plotted in Fig. 5 for comparison. Although the pH
range of the present study and the literature do not overlap, theestimated trends from the measured solubility values of
Eu(OH)3(s), seem to be in agreement with those seen in previous studies.
3.2.3. Tm solubilities measured at 25 °C after aging at 25
to 90 °C. The XRD patterns of the Tm solid phases after
aging at 25 and 90 °C suggested that the solid phases remained
as Tm 2O3(cr) during the investigated periods. The reported
values of ∆f Hm° = −1889.3±5.7 kJ/mol and S m° = 139.7±0.4 J/K/
mol for Tm 2O3(cr) [29] resulted in ∆ rG m° = −129.39 kJ/mol,hence log K s° = 22.67 for 1/2 Tm 2O3(cr) + 3H+ Tm 3+ + 3/2
H2O, combined with the ∆f Hm° and S m° for Tm3+ and H2O [2,4].
Although the calculated K s° value was less than those for
Eu 2O3(cr) and La2O3(cr), the value was found to be still ordersof magnitude larger than those of reported log Ks° = 15.56 forTm(OH)3(cr) [4]. However, similarly to the case of Eu 2O3(cr),the solid phase transformation was thought to be kineticallyhindered under the experimental condition in the present
study. In Fig. 6, the Tm solubilities measured at Tmeas = 25 °C
after aging at Tage = 25, 40, 50, 60, and 90 °C up to 8 weeks
were plotted. No significant difference was observed after
aging different aging temperatures, indicating that the initial

37

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Moniruzzaman & Kobayashi
Tm 2O3(cr) was stable up to Tage = 90 °C. The solubility of
Tm2O3(cr) measured at 25 °C was approximately 10−5 M at pHc
7, after which it gradually decreased with an increase in pHc.
In the region where pHc > 10, solubility plots were observed
close to 10−9 M of the detection limit.
In literature, only one study has reported the experimental
solubility data for Tm(OH)3(s) [15]. Moeller and Kremers [15]measured the starting point for precipitation by adding NaOH
to Tm solutions, and assumed that the precipitate was
Tm(OH)3(s). On the other hand, the solubility experiment in
the present study started from Tm2O3(cr), by adding Tm2O3(cr)into the sample solutions as an initial solid phase. Further discussions on this are given in the following sections.

-1
Tm2O3(cr)This work

-2

Tage

-3

25 °C
40 °C
50 °C
60 °C
90 °C

TmOH2+log [Tm]

-4

Tmeas
25 °C
25 °C
25 °C
25 °C
25 °C

-5
-6
-7
-8

3.3. Solubilities measured at 25 to 60 °C. After aging theLa, Eu, and Tm sample solutions at Tage =25, 40, 50, and 60 °C
for 4 to 12 weeks, their solubilities were investigated under
different measurement temperature (Tmeas) conditions at 25, 40,
50, and 60 °C (See Table 1). As described above, the sample
tubes after aging at a certain temperature were kept in thermostatted baths at 25, 40, 50, and 60 °C for 1 day, then pHc wasmeasured and the supernatant of the sample solutions were
ultrafiltrated at a fixed Tmeas condition. The obtained La, Eu,
and Tm solubilities are presented in Fig. 7. Contrary to theeffect of aging temperature, the La, Eu, and Tm solubilities
decreased with increasing Tmeas at the same pH conditions. In
Fig. 7(a), the solubilities of La(OH)3(s) measured at 40, 60, 90,
and 150 °C by Deberdt et al. [23] were plotted for comparison.
The solubility values in the present study were in agreementwith the reported results at the same Tmeas condition. Since nosignificant difference by the aging temperatures up to 60 °C

Tm3+

-9
-10
6

7

8

9

10

11

12

13

pHc
Figure 6. Tm solubilities measured at Tmeas = 25 °C after aging at
Tage = 25, 40, 50, 60, and 90 °C for 4 and 8 weeks. The ionic strength
was maintained at I = 0.1 M, and the solubilities were obtained after
filtration through 3 kDa membranes. The solid line with green color
represents the calculated solubility of Tm 2O3(cr). The broken lines
with green color represent the contributions of Tm3+ and first hydrolysis species; TmOH2+. A gray field in the figure indicated the detection limit level of the ICP-MS.
-1

La(OH)3(cr)This work
Tage
Tmeas
25 °C
25 °C

(a)

-2
-3

40 °C
40 °C

40 °C
25 °C

50 °C
50 °C
50 °C

50 °C
40 °C
25 °C

60 °C
60 °C
60 °C
60 °C

60 °C
50 °C
40 °C
25 °C

-1
-2
-3

-6
-7
-8
-9

-10
6
-1

7

8

9

10

pHc

11

12

13

(c)

-2
-3

log [Tm]

-4
-5
-6
-7

Deberdt et al. [23]
Tage
Tmeas
40 °C
40 °C
60 °C
60 °C
90 °C
90 °C
150 °C 150 °C-4log [Eu]log [La]

-4
-5

(b)

-5
-6
-7

Tmeas
25 °C

40 °C
40 °C

40 °C
25 °C

50 °C
50 °C
50 °C

50 °C
40 °C
25 °C

60 °C
60 °C
60 °C
60 °C

60 °C
50 °C
40 °C
25 °C

Tmeas
25 °C

40 °C
40 °C

40 °C
25 °C

50 °C
50 °C
50 °C

50 °C
40 °C
25 °C

60 °C
60 °C
60 °C
60 °C

60 °C
50 °C
40 °C
25 °C

-8
-9
-10
6

Tm2O3(cr)This work
Tage
25 °C

Eu2O3(cr)This work
Tage
25 °C

7

8

9

10

pHc

11

12

13

-8
-9
-10
6

7

8

9

10

pHc

11

12

13

Figure 7. Solubilities of (a) La(OH)3(cr), (b) Eu 2O3(cr), and (c) Tm 2O3(cr) measured at Tmeas = 25, 40, 50, and 60 °C after aging at Tage = 25, 40, 50,
and 60 °C for 4 to 12 weeks. The ionic strength was maintained at I = 0.1 M, and the solubilities were obtained after filtration through 3 kDa
membranes. The solid lines with black, green, pink, and red colors represent the calculated solubilities of (a) La(OH)3(cr), (b) Eu 2O3(cr), and (c)Tm 2O3(cr) measured at Tmeas = 25, 40, 50, and 60 °C, respectively. A gray field in the figure indicated the detection limit level of the ICP-MS.

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Solubility and solid phase of trivalent lanthanide hydroxides and oxides

was observed for each element, we assumed that the solid
phases in Fig. 7 were La(OH)3(cr), Eu 2O3(cr), and Tm 2O3(cr),regardless of the aging temperatures. The solubilities at each
Tmeas are analyzed to determine the K s° values at respective
Tmeas to discuss their temperature dependences in the following
section.
3.4. Thermodynamic analysis of the solubility data.
3.4.1. Thermodynamic model for the solubility of Ln2O3(cr)and Ln(OH)3(cr). In the systems for La, Eu, and Tm solubilities in neutral to alkaline pH region after aging at Tage = 25 to
90 °C, oxide (Ln 2O3(cr)) or hydroxide (Ln(OH)3(cr)) (Ln = La,
Eu, Tm) solid phases were observed depending on the elements and aging temperatures. The dissolution and precipitation reactions of Ln 2O3(cr) and Ln(OH)3(cr) can be described
respectively by the following equations:
1
Ln2O3(cr)+3H+
2
Ks(Ln2O3(cr)) =Ln3+ +

3
H 2O
2[Ln3+][H+]3

Ln(OH)3(cr) + 3H+

Ks(Ln(OH)3(cr)) =

(1)

(2)

Ln3++ 3H2O
[Ln3+]
[H+]3

(3)

(4)

where Ks(Ln 2O3(cr)) and Ks(Ln(OH)3(cr)) represent the solubility products of Ln 2O3(cr) and Ln(OH)3(cr), respectively.
In the aqueous phase, the hydrolysis reactions and thehydrolysis constants (*βn; n = 1−3) were given as follows:
Ln3+ + nH2O*

βn =(3–n)+

Ln(OH) n+ nH+[Ln(OH)(3–n)+][H+]nn[Ln3+](5)(6)

The solubility as a total Ln concentration ([La] tot) can be
described as:[Ln]tot = [Ln3+] + [LnOH2+] + [Ln(OH)+2] + [Ln(OH)3(aq)]*= [Ln3+] 1 +

*
*
β1
β2
β3
+ +
+ 2 +
[H ] [H ][H+]3= Ks([H+]3 + *β1[H+]2 + *β2[H+] + *β3)(7)

According to the specific ion interaction theory (SIT)
method, the solubility product (Ks) and hydrolysis constants(*βn) can be corrected to I=0 by the following equations;
logKs = logK°s + 6D – Im· {ε(Ln3+, ClO4–) – 3ε(H+, ClO4–)}(8)log*βn = log*βn° + {(3–n)2 + n–9} D + nlog aH2O –Im· {ε(Ln(OH)n(3–n)+, ClO4–) + nε(H+, ClO4–) – ε(Ln3+, ClO4–)},(9)where Ks°, *βn°, Im, aH2O, and D are the solubility product, hydrolysis constant at I = 0, the ionic strength, activity of water, andthe Debye-Hückel term at 25 °C, respectively. ε(Ln(OH)(3–n)+,n
ClO4–), ε(Ln3+, ClO4–), and ε(H+, ClO4–) represent ion interaction
coefficients for each species.
Based on the integrated van’t Hoff equation, the Ks and *βn38

can be expressed as a function of Tmeas,
logKs(Tmeas) = logKs(T0) +ΔrHm(Ln2O3(cr) or Ln(OH)3(cr)) 1
1
–
R1n10
T0 Tmeas(10)log*βn(Tmeas) =log*βn(T0) +ΔrHm(Ln(OH)(3–n)+
) 1
1
n
–
R1n10
T0 Tmeas(11)

where R and T0 are the universal gas constant, and 298 K (25 °C),
respectively.

3.4.2. Solubility product of Ln2O3(cr) and Ln(OH)3(cr) at
Tmeas = 25 to 60 °C. The solubility data for La, Eu, and Tm in
0.1 M NaClO 4 solution at Tmeas = 25 to 60 °C were analyzed
based on Eq. (7) with a combination of the ionic strength correction by SIT shown in Eqs. (8) and (9) to determine the solubility product of Ln 2O3(cr) or Ln(OH)3(cr) at respective Tmeas.
Since the solubility values at pHc > 10 were close to the detection limit of ICP-MS, the solubility data used in the analysis
were limited at pHc < 10. Therefore, only Ln3+ and first hydrolysis species of LnOH 2+ in Eq. (7) was taken into account in
the analysis. It is note that the ion interaction coefficient values was fixed as ε(Ln(OH)(3–n)+, ClO4–) = 0.39 and ε(Ln3+, ClO4–)n= 0.47 [4] for La, Eu, and Tm, and ε(H+, ClO 4–) = 0.14 [2].
These values were assumed constant against Tmeas due to thelacking data on their temperature dependence, while the Dvalue was corrected based on the temperature correction in[2]. No polynuclear species was taken into account in theanalysis of the solubility data. The sample solutions were prepared by an undersaturation method, and the measured solubility values were mostly limited to lower than 10−4 M, wherecontribution of polynuclear species was considered to be negligible.
In the present analysis, the *β1 value was treated as a fixedparameter. The *β1 values for lanthanide elements have beenreported in many literatures [22,38,39,43-52] and summarizedin [4, 25-28]. Although some scatterings were found in the literatures, the reported or re-calculated *β1 values for 25 °C at I
= 0 were relatively in good agreement and Brown and Ekberg[4] have selected the recommended *β1 values for the lanthanide series. Hence, the *β1 value at 25 °C was taken from [4]and used as a fixed parameter in the present analysis. Several
literatures have also investigated the *β1 value at different temperatures [47, 49, 50]. For example, Klungness and Byrne [50]determined the *β1 values for yttrium and rare earth elements
in a combination of potentiometric and spectroscopic experiments at 25 to 55 °C. The log *β1 value for La 3+ + H 2O
LaOH 2+ + H+ increased with an increase of temperature andthe enthalpy value was obtained based on the analysis using
the van’t Hoff equation for lighter to heavier lanthanide elements [50]. In the present analysis, the *β1 values for La, Eu,
and Tm at 40, 50, and 60 °C were calculated from the enthalpy
values reported in [50] and used as fixed parameters after theionic strength correction.
The K s(La(OH)3(cr)), K s(Eu 2O3(cr)), and K s(Tm 2O3(cr)) at
Tmeas = 25, 40, 50, and 60 °C were determined by the leastsquare fitting analysis of the solubility data by treating the *β1
values as fixed parameters. The obtained values are summarized in Table 2 and the calculated solubility curves based ont he values i n Table 2 a re show n i n F i g s . 4 -7. T heKs(La(OH)3(cr)), Ks(Eu 2O3(cr)), and Ks(Tm 2O3(cr)) at Tmeas =25, 40, 50, and 60 °C were plotted as a function of the inverseof absolute temperature in Fig. 8, together with the literature
values. For La(OH)3(cr) aged at 25 to 60 °C, the log Ks valueJ. Nucl. Radiochem. Sci., Vol. 20, 2020

39

Moniruzzaman & Kobayashi

was determined to be log Ks° = 19.0±0.2 at 25 °C, which wasclose to the literature values for long term aged La(OH)3(s)reviewed by [25,26] and to the selected value of log K s° =19.72±0.34 for La(OH)3(s) in [4]. Diakonov et al. [25] calculated the solubility product based on the thermodynamic calculation from the enthalpy and entropy of formation of
La(OH)3(s). The calculated log K°s = 20.20 for La(OH)3(s) [25]was approximately one order of magnitude higher than thatobtained in the present study. The solubility product for
La(OH) 3 (cr, 90 °C aged) was determined to be log K s° =17.1±0.4, which was found to be lower by one orders of magnitude than that of La(OH)3(cr) aged at 25 to 60 °C. Although nosignificant difference was observed in the bulk solid phases
after aging at 25 °C and 90 °C as shown in their XRD pat-

TABLE 2: Solubility products and hydrolysis constants (I=0, Tmeas = 25 to 60 °C) for La, Eu, and Tm hydroxides and oxides
obtained and used in the present study

Tage
90 °C

La(OH)3(cr)Tmeas
log Ks°25 °C19.0±0.2
40 °C
18.3±0.3
50 °C
16.7±0.3
60 °C
17.0±1.1
La(OH)3(cr,90)
Tmeas
log Ks°25 °C17.1±0.4

Tage25~90 °C
40~60 °C
50~60 °C
60 °C

Tm2O3(cr)Tmeas
log Ks°25 °C16.9±0.2
40 °C
15.5±0.2
50 °C
14.3±0.2
60 °C
14.3±0.4

Tage
25~60 °C
40~60 °C
50~60 °C
60 °C

log *β1° a
−8.89
−8.54
−8.34
−8.11

Tage
25~60 °C
40~60 °C
50~60 °C
60 °C

log *β1° a
−8.89

Tage
90 °C

Eu2O3(cr)Tmeas
log Ks°25 °C18.4±0.2
40 °C
16.4±0.2
50 °C
14.8±0.4
60 °C
15.5±0.4
Eu(OH)3(cr)Tmeas
log Ks°25 °C17.2±0.3

log *β1° a
−7.66
−7.18
−6.90
−6.60
log *β1° a−7.66log *β1° a
−7.34
−6.78
−6.45
−6.10

a) The hydrolysis constants for La, Eu, and Tm at 25 °C were taken from [4]. The values at 40 to 60 °C were calculated fromthe temperature dependence of the hydrolysis constants observed in [50] using the van’t Hoff equation.

(a)

La(OH)3(s), La(OH)3(cr)This work
Deberdt [23]Spivakovskii [21]Akselrud [17]Diakonov [25]Chirkst [37]Kovalenko [36]Orhanovic [35]Kragsten [22]Lopez-Gonzalez [38]Ramirez-Garcia [39]Brown [4]20log K °s

19
18
17
16

La(OH)3(cr, 90 ºC)
This work

15
14
2.9

3.0

3.1

3.2

3.3

3.4 3.5×10-3

1/Tmeas (1/K)

Eu2O3(cr)This work
Eu(OH)3(s), Eu(OH)3(cr)This work
Diakonov [25]Yun [41]
Chirkst [42]Akselrud [18]Ramirez-Garcia [39]Jimenez-Reyes [40]Brown [4]Moeller [15]Moeller [16]Hummel [24]

19
18
17
16
15
14

2.8

2.9

3.0

3.1

3.2

3.3

1/Tmeas (1/K)

3.4 3.5×10-3

Tm2O3(cr)(c)

This work
Tm(OH)3(s)Moeller [15]Diakonov [25]Brown [4]1918

log K °s

(b)

13

2.8

20

20

log K °s

21

17
16
15
14
13
2.8

2.9

3.0

3.1

3.2

1/Tmeas (1/K)

3.3

3.4 3.5×10-3

Figure 8. Solubility product values of (a) La(OH)3(cr), (b) Eu 2O3(cr), Eu(OH)3(cr), and (c) Tm 2O3(cr) as a function of the inverse of measurement
temperature. The broken lines in the figures represent the fitted lines in this work based on the integrated van’t Hoff equation.

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Solubility and solid phase of trivalent lanthanide hydroxides and oxides

terns, surface conditions of the solid phases controlling thesolubility may be dependent on the aging temperature.
Further studies are needed to clarify the effect arising from thebulk and surface condition of the solid phases.
The obtained log Ks° values for La(OH)3(cr) at Tmeas = 25 to
60 °C are plotted in Fig. 8(a). The log K s° value linearly
increased with decreasing Tmeas and the ΔrHm(La(OH)3(cr)) was
determined to be −149.5±22.5 kJ/mol from the obtained slope
based on the integrated van’t Hoff equation of Eq. (10). A few
literature studies investigated the solubility of La(OH)3(s) at
different Tmeas from 25 °C [23, 38, 39]. Deberdt et al. [23] measured the solubility of La(OH)3(s) at 40, 60, 90, and 150 °C as
shown in Fig. 7 and determined the log K°s values at respective
temperatures. The log K°s values at the same Tmeas in both studies were found to be in an agreement as shown in Fig. 8(a). A
slight difference may arise from a difference in the analysis of
the solubility data. The first hydrolysis species, LaOH2+, wastaken into consideration in the present study, while the species
was not included in the analysis in [23]. The slope of log Ks°
against the 1/Tmeas, and reported ΔrHm(La(OH)3(cr)) = −134.570
kJ/mol [23] matched to those obtained in the present study
w i t h i n t h e m e a s u r e m e n t e r r o r. It i s n o t e t h a t t h e
ΔrHm(Ln(OH)3(cr)) values for several lanthanide elements wered e t e r m i n e d b y s o l u t i o n c a l o r i m e t r y [53, 5 4]. T h e
ΔrHm(La(OH)3(cr)) = −150.2 kJ/mol [53] also agreed well withthe obtained value in the present study. These results clearly
indicate that the initial La2O3(cr) transformed to La(OH)3(cr) at
25 °C, and control the La solubility as the solubility-limiting
solid phase.
For Eu, as suggested by the XRD patterns and the solubility
data, the solid phase after aging at 25 to 60 °C was considered
to be Eu2O3(cr), while the solid phase after aging at 90 °C was
Eu(OH)3(cr). The solubility products at Tmeas = 25 °C were
determined to be log K°s = 18.4±0.2 for Eu2O3(cr), and log K°s =17.2±0.3 for Eu(OH)3(cr). In literatures, a number of log Ks° for
Eu(OH)3(cr) has been reported [15, 16, 18, 25, 39-42], while nolog K s° value for Eu 2 O 3 (cr). The obtained log K s° for
Eu(OH)3(cr) in the present study was slightly higher than thereported values and the selected one of log K°s = 16.48±0.30 in[4].
In Fig. 8(b), the obtained log Ks values for Eu2O3(cr) at Tmeas= 25 to 60 °C are plotted as a function the inverse of Tmeas. The
log K s value i ncreased with decreasi ng Tme a s and theΔ r Hm(Eu 2O3(cr)) was determined to be −202.8±20.1 kJ/mol
from the its slope. The ΔrHm(Eu2O3(cr)) and ΔrHm(Eu(OH)3(cr))
has been reported to be −209.145 kJ/mol and −143.99 kJ/mol
determined by solution calorimetry of respective solid phases
as su m mar ized in [4, 25, 29]. Although the obtainedΔrHm(Eu2O3(cr)) in the present study much relied on the log K°s
value at each Tmeas, it is interesting that the obtained value wascloser to that reported value for Eu 2O3(cr). This may suggestthat Eu2O3(cr) remained stable at 25 °C, and control the Eu solubility as the solubility-limiting solid phase, in contrast to thecase of La.
For Tm, the solid phase was considered to be Tm 2O3(cr),regardless of the aging temperature up to 90 °C. The solubility product at Tmeas = 25 °C was determined to be log K s° =16.9±0.2. In literatures, only Moeller et al. performed the Tm
solubility experiment and reported the solubility product of log
K°s = 16.0 [15] for fresh Tm(OH)3(s) solid phase. The log K°s =16.8 was calculated from the ∆f Hm° and Sm° for Tm(OH)3(s) [25,26] was rather close to the value obtained in the present study
for Tm 2O3(cr). In Fig. 8(c), the obtained log K s values for
Tm 2O3(cr) at Tmeas = 25 to 60 °C are plotted as a function the
inverse of Tmeas. The ∆ r Hm(Tm 2O3(cr)) was determined to be−176.2±15.7 kJ/mol from the slope using Eq. (10). The∆ rHm(Tm 2O3(cr)) and ΔrHm(Tm(OH)3(cr)) has been reported tobe −189.295 kJ/mol and −141.59 kJ/mol determined by solution
calorimetry of respective solid phases [4, 25, 29]. Similarly to

40

the case of Eu, the obtained ΔrHm(Tm2O3(cr)) value was also inagreement with the reported value for Tm 2O3(cr), suggestingthat Tm2O3(cr) remained stable at 25 °C to control the Tm solubility as the solubility-limiting solid phase.
4. Conclusion
Solubility and solubility limiting solid phase of La, Eu andTm were investigated in the solutions of pHc 6 to 12 after aging
at Tage = 25 to 90 °C. Then, the La, Eu, and Tm solubilities
were measured at Tmeas = 25 to 60 °C to examine the temperature dependence of the solubility products (K s°). The XRD
patterns, the solubility values, and subsequently obtained theenthalpy of the reaction from the temperature dependence oflog Ks° suggested that La2O3(cr) transformed to La(OH)3(cr) tocontrol the La solubility. In the case of Eu, on the other hand,
the XRD patterns showed that Eu 2 O3 (cr) transformed toEu(OH)3(cr) only after the aging at 90 °C, whereas no solid
phase transformation was observed at 25 °C. The solubility
values and the enthalpy of the reaction suggested that
Eu2O3(cr) remained stable at 25 °C to control the Eu solubility.
For Tm, the XRD patterns, solubility values and the enthalpyof the reaction indicated that the Tm 2O3(cr) was stable up to
Tage = 90 °C. From lighter to heavier lanthanides (Ln), theoxides of Ln 2O3(cr) was considered to become more stablecomparing to the hydroxides of Ln(OH)3(cr) and play as thesolubility-limiting solid phase in the Ln-OH aqueous systems.
It is note that the present study focused on the steady state of
the solubility reaction after aging at certain temperatures andperiods and revealed the solubility-limiting solid phase andtheir thermodynamic data. The reaction kinetics of solid
phase transformation needs further investigations.
Acknowledgement
This work was suppor ted in par t by the Ministr y of
Education, Culture, Sports, Science and Technology, Japan(Grant-in-Aid for 16H02447).
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