Am(III)/Nd(III) INTERACTIONS WITH BORATE: EXPERIMENTAL INVESTIGATIONS OF
Nd(OH)3(micro cr) SOLUBILITY IN NaCl SOLUTIONS IN EQUILIBRIUM WITH BORAX

Yongliang Xiong1)*, Leslie Kirkes1), Cassie Marrs1), Jandi Knox1)
1

Sandia National Laboratories (SNL), Carlsbad Programs Group, 4100 National Parks Highway, Carlsbad, NM
88220, USA

ABSTRACT In previous studies on Am(III)/Nd(III) interactions with borate, the borate concentrations
were kept at a relatively low level, up to ~0.16 mol•kg–1 as the highest. Such concentrations of borate are
not saturated, in terms of borate, with borate-bearing phases such as borax (Na2B4O7•10H2O) andtincalconite (Na2B4O7•5H2O) that could be present in geological repositories. For instance, tincalconite
has been observed as a corrosion product for boroslicate glass for HLW under the repository conditions in
China.
In this study, we examine the Nd(III) interactions with borate in equilibrium with borax in a wide
range of NaCl solutions at 25oC. In our experiments, the solubility-controlling phase for Nd(III) is
Nd(OH)3(micro cr). In synthesis of Nd(OH)3(micro cr), we followed the procedure from literature. Our
results indicate that high borate concentrations (~0.5 mol•kg–1) in 0.01 and 0.1 mol•kg–1 NaCl solutions
have an obvious effect on Nd(III). At such a high concentration of borate, borate can enhance thesolubility of Nd(OH)3(micro cr), or NdB9O13(OH)4(cr) may control the solubility.
repositories. Therefore, the interactions between
Am(III) and borate are important for performanceassessment (PA) for geological repositories fornuclear wastes. As an example, in the Waste
Isolation Pilot Plant (WIPP), a U.S. DOE
geologicalrepositoryfordefense-relatedtransuranic (TRU) waste in the bedded salt
formations in New Mexico, USA, the inventory ofAm(III) in waste was 143 kg for the WIPP
Compliance Application Re-Certification in 2009
(CRA-2009). The borate concentrations in the
two WIPP brines that are important for
performance assessment (PA), i.e., Generic Weep
Brine (GWB), and Energy Research andDevelopment Administration (WIPP Well) 6(ERDA-6),are
0.180
mol•kg–1
and-1
0.0692 mol•kg [10], respectively.
In the previous studies on Am(III)/Nd(III)interactions with borate, the borate concentrations
were kept at the relatively low level, and up to~0.16 mol•kg–1 as the highest (i.e., [4, 9]). Such
concentrations of borate are not saturated, in termsof borate, with borate-bearing phases such as
borax(Na2B4O7•10H2O)
andtincalconite(Na2B4O7•5H2O) that could be present ingeological repositories. For instance, tincalconite
has been observed as one of corrosion products forboroslicate glass for HLW under the repository
conditions in China [11].
The borate
concentrations saturated with borax and1. Introduction
Borate is important to geological processes,
although boron is the 41st abundant element in the
Earth’s Crust, lower than the abundances of rareearth elements such as Nd, which is the 28th
abundant element. In natural groundwaters suchas brines associated with salt formations, there
may be relatively high concentrations of borate [1].
In addition, when borosilicate glass for high level
nuclear waste (HLW) is corroded, borate is also
released into the groundwater [2-3]. Borate can
impact nuclear waste management in two aspects.
First of all, borate can form an aqueous complex
with Nd(III) and Eu(III) [4-6], analogs to actinides
in +III oxidation state, such as Am(III). Therefore,
borate could become a potential transport agent
for actinides if the Am(III)-borate complex
contributes significantly to the total Am(III)concentrations. In the second aspect, numerous
actinide borates have recently been successfully
synthesized [7-8], including a Pu(III) borate,
Pu2[B12O18(OH)4Br2(H2O)3]•0.5H2O, in addition to
the well known lanthanide borates such as
NdB9O13(OH)4 [9]. This implies that actinides inwaste could be transformed into, or besequestrated as, actinide borates, if their solubility
limits are reached.
Actinides in +III state represented by Am(III)are present in nuclear waste in geological1
6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japantincalconite are much higher than 0.16 mol•kg–1,
especially in solutions with low concentrations of
sodium. For example, the borate concentrations
are ~0.5 mol•kg–1 in 0.01 mol•kg–1, NaCl [12].
Therefore, the Am(III)/Nd(III) interactions withborate need to be futher studied, especially in highborate concentrations.

standard of Nd(OH)3(cr) from the International
Center of Diffraction Data (ICDD).
Of note, the deaerated DI water used in thesynthesis was prepared by vigorously bubblinghigh purity Ar or N2 gas through DI water for aminimum of 30 minutes in the glovebox,
following a similar procedure used by Xiong [14].
This deaeration process was intended to remove
any dissolved CO2 and therefore ensure thesynthesis process was not contaminated bycarbonate.

2. Methods and Results
In this section we describe the experimental
methodology used in this study.

2.2 Experimental Setup
Undersaturation experiments are conducted at298.15 ± 0.5 K. In our solubility experiments,
approximately 0.3 grams of Nd(OH)3(micro cr)
and about 10 grams of Na2B4O7•10H2O (borax)
were placed into serum bottles along with 100 mLof supporting solutions with the desired ionicstrength. The supporting solutions consisted of
0.01, 0.1, 1.0, 2.1, 3.2, 4.4 and 5.0 mol•kg–1 NaClsolutions where mol•kg–1 refers to concentration
on molal scale, i.e., moles per 1000 g of water.
All supporting solutions were prepared fromreagent grade chemicals from Fisher Scientific, or
its associated vendors, and deaerated DI water.
Solution samples were periodically withdrawn
from the experiments to determine if the system
had reached equilibrium. Before each sampling,
pH readings were taken for each experiment. In
each sampling, about 3 mL of solution samples
were taken from each experiment, and the solution
samples were filtered through a 0.2 m filter, andtransferred into pre-weighed 10 mL Grade A
volumetric flasks. After filtration, masses of eachsolution sample were determined with a balance
precise to the fourth decimal place. Samples werethen immediately acidified with 0.5 mL of the
Optima® Grade HNO3 from Fisher Scientific, anddiluted to 10 mL with DI water. Prior to chemical
analyses for Nd using the PerkinElmer NexION
300D ICP-MS, and for Na using the PerkinElmer
Optima 3300 Dual View (DV) ICP-AES, aliquots
from the afore-mentioned acidified samples were
further diluted to an appropriate ionic strength.
The pH readings were measured with an OrionRoss combination pH glass electrode, coupledwith an Orion Research EA 940 pH meter that wascalibrated with three pH buffers (pH 4, pH 7, andpH 10).
The measured pH readings wereconverted to negative logarithm of hydrogen ion
concentrations on molar scale (i.e., pcH) following

2.1 Synthesis of Starting Material
As detailed in Wood et al. [13], many othersynthesis
methods
sufferfromvariousshortcomings,
which
would
resultindifficulty/uncertainty interpreting solubility data.
Therefore, starting with well-defined high-purity
material is of fundamental importance to thesuccess of solubility experiments. In synthesis ofNd(OH)3(micro cr), high purity Nd2O3 was first
loaded into Paar® reaction vessels with deaerated
DI water, and then the reaction vessels were sealed
in a glovebox under a positive pressure of an inert
gas. The reaction vessels were then removed fromthe glovebox and placed into a muffle furnace.
Nd(OH)3(micro cr) was synthesized by reacting
the high purity Nd2O3 with the deaerated DI waterat 473.15 K in Paar® reaction vessels for a periodof two weeks. Following the synthesis step, thereaction vessels were then transferred back into
the glovebox, and were opened for drying in an
atmosphere of inert gas. This synthesis method
assures the complete conversion of Nd2O3 to
Nd(OH)3(micro cr), as demonstrated by XRD and
SEM-EDS characterizations.
The SEM-EDS
characterization shows that the size of crystals is
small, and the majority of crystals are aroundmicrometer or sub-micrometer in size, as indicatedby SEM-EDS images (Figure 1A and 1B). As
shown by the XRD analysis, the synthetic
neodymium has a well-defined XRD pattern(Figure 2) that is indicative of a crystalline phase.
Therefore, we term our synthetic neodymiumhydroxide as “micro crystalline neodymiumhydroxide”, Nd(OH)3(micro cr), throughout theentire paper. Figure 2 shows that the XRD patternof synthetic Nd(OH)3(micro cr) produced by thiswork has the exact match with the reference2
6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

the procedure detailed in Xiong [14] and Xiong
and Lord [10], and pcH’s were converted to pHm’s(negative
logarithm
ofhydrogenion
concentrations on molar scale) according to theequations in Xiong et al. [15].

by Figure 5, NdB9O13(OH)4(cr) could be converted
to the hydroxide form in high pHm region.

2.3 Experimental Results
Our experimental results are displayed in
Figures 3-5. In Figures 3-5, our experimental
results are compared with the predicted values forAm(OH)3(s) and AmB4O13(OH)4(cr) which were
calculated using EQ3/6 Version 8.0a, and thethermodynamic database data0.fm2. Note thedashed and dotted lines in Figures 3-5 represent
the EQ3/6 simulated concentrations, and thesymbols are the measured data.
They areproduced using EQ3/6 Version 8.0a [16-17] withthe database data0.fm2 [18-19]. EQ3/6 Version
8.0a has been extensively used in the previous
publications [e.g., 20-23].
The equilibrium
constant for AmB9O13(OH)4(cr) at 25oC is from
[24].
Figure 3 shows that there are relatively highconcentrations of Nd(III) in the experiments in
0.01 and 0.1 mol•kg–1 NaCl. Of note, the borate
concentrations saturated with borax in these two
experiments are 0.53 and 0.42 mol•kg–1,
respectively. The Nd(III) concentrations without
the species AmHB4O72+ are higher than thosepredicted for Am(OH)3(s), but are close to thosepredicted for AmB9O13(OH)4(cr).
This maysuggest that borate enhances the solubility ofAm(OH)3(s), or AmB9O13(OH)4(cr) controls thesolubility.
Figure 4 displays the results in 1.0 and 2.1
mol•kg–1 NaCl. In these two experiments, theborate concentrations are 0.22 and 0.18,
respectively. Again, the Nd(III) concentrations arehigher than those predicted for Am(OH)3(s), but
are close to those for AmB9O13(OH)4(cr).
The results in 3.2, 4.4, and 5.0 mol•kg–1 appear
in Figure 5. The Nd(III) concentrations are close
to those predicted for Am(OH)3(s). It implies thatthe role that borate plays to enhance the solubilityof Am(OH)3(s) is less at high NaCl concentrations.
The borate concentrations in these experiments are
0.20, 0.16 and 0.16 mol•kg–1, respectively.
It is of interest to note that the data from Hinz etal. [9] for NdB9O13(OH)4(cr) at pHm ~9 are close
to those predicted for Am(OH)3(s). As indicatedFig. 1A. SEM image of the starting material used inexperiments in this study.

Fig. 1B. SEM image of the starting material used inexperiments in this study.

Fig. 2. XRD pattern of Nd(OH)3(micro cr) used in solubility
experiments in this study.

3
6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan0.01 mol∙kg⁻¹ NaCl with 0.16 mol∙kg⁻¹ ΣB, AmB₉O₁₃(OH)₄(cr), model, This Work

1.0E+02

obvious effect on Nd(III). At such a highconcentration of borate, borate can enhance thesolubilityofNd(OH)3(micro
cr),or
NdB9O13(OH)4(cr) may control the solubility.

0.1 mol∙kg⁻¹ NaCl with 0.16 mol∙kg⁻¹ ΣB, AmB₉O₁₃(OH)₄(cr), model, This Work

1.0E+01

0.01 mol∙kg⁻¹ NaCl only, Am(OH)₃(s), WIPP model[Nd(III) or Am(III)]tot/mol•kg-1

1.0E+00

0.1 mol∙kg⁻¹ NaCl only, Am(OH)₃(s), WIPP model

1.0E-01
1.0E-02

0.01 mol∙kg⁻¹ NaCl, saturated with borax, Nd(OH)₃(micro cr), This Work

1.0E-03

0.1 mol∙kg⁻¹ NaCl, saturated with borax, Nd(OH)₃(micro cr), This Work

1.0E-04

0.1 mol∙dm⁻³ NaCl with 0.16 mol∙dm⁻³ ΣB, NdB₉O₁₃(OH)₄(cr), Hinz et al. (2015)

4. Acknowledgements

1.0E-05
1.0E-06
1.0E-07

Sandia National Laboratories is a multimission
laboratory operated by National Technology andEngineering Solutions of Sandia, LLC., a wholly
owned subsidiary of Honeywell International, Inc.,for the U.S. Department of Energy’s National
Nuclear Security Administration under contract
DE-NA-0003525. This research is funded by
WIPP programs administered by the Office ofEnvironmental Management (EM) of the U.S
Department of Energy. SAND2017-12040C

1.0E-08
1.0E-09
1.0E-101.0E-11
6

7

8

9

10

pHm

Fig. 3. A plot showing Nd(III)/Am(III) concentrations as afunction of pHm in low ionic strength range.
1.0 mol∙kg⁻¹ NaCl with 0.16 mol∙kg⁻¹ ΣB, AmB₉O₁₃(OH)₄(cr), model, This Work

1.0E+02

2.0 mol∙kg⁻¹ NaCl with 0.16 mol∙kg⁻¹ ΣB, AmB₉O₁₃(OH)₄(cr), model, This Work

1.0E+00

1.0 mol∙kg⁻¹ NaCl only, Am(OH)₃(s), WIPP model

1.0E-01

2.1 mol∙kg⁻¹ NaCl only, Am(OH)₃(s), WIPP model[Nd(III) or Am(III)]tot/mol•kg-1

1.0E+01

1.0E-02

1.0 mol∙kg⁻¹ NaCl, saturated with borax, Nd(OH)₃(micro cr), This Work

1.0E-03

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2.1 mol∙kg⁻¹ NaCl, saturated with borax, Nd(OH)₃(micro cr), This Work

1.0E-04

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1.0E-06
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1.0E-02

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1.0E-03

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1.0E-04

1.0E-05
1.0E-06
1.0E-07
1.0E-08
1.0E-09
1.0E-101.0E-11
6

7

8

9

10

pHm

Fig. 5. A plot showing Nd(III)/Am(III) concentrations as afunction of pHm in high ionic strength range.

3. Conclusions
We have conducted solubility measurements
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saturated with borax at 25oC. Our results indicatethat high borate concentrations (~0.5 mol•kg–1) in
0.01 and 0.1 mol•kg–1 NaCl solutions have an

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6th East Asia Forum on Radwaste Management Conference
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