PHOSPHATE MINERAL MATRICE MATERIALS DEVELOPMENT
FOR ALKALINE-EARTH AND REE RADIONUCLIDES DISPOSAL
ANDREY M. KOVALSKII, VALERIA A. SUVOROVA, ALEXEY R. KOTELNIKOV,
GALINA M. AKHMEDSHANOVA, VERA I. TIKHOMIROVA
Institute of experimental mineralogy, Russian academy of sciences, Chernogolovka, 142432 Russia;
[email protected]
Резюме. Предложен метод синтеза матричных материалов, основанный на реакциях метасоматического
замещения по схеме „мокрого процесса”. Метод позволяет проводить иммобилизацию радионуклидов в
фосфат-содержащих минеральных композициях при комнатной температуре и атмосферном давлении в
„мокрых условиях”. В работе приведены результаты синтеза минеральных матриц, содержащих как простые
фосфаты стронция и церия, так и двойные (Sr, Na)- и (Ce, Na)- фосфаты. Продукты экспериментов
исследованы методом рентгеновской дифракции, для определения химического состава синтезированных
твердых фаз применялся метод рентгеноспектрального микроанализа.
Показано, что дефицит растворов Sr- и Ce- нитратов способствует формированию минералов, подобных
природным ольгиту NaSrPO4 или витуситу Na3Ce(PO4)2. Простые ортофосфаты Sr и Ce были синтезированы
с применением избыточных количеств растворов нитратов. Для последующей оптимизации процесса
синтеза минеральной матрицы – снижения температуры синтеза и увеличения плотности матрицы, была
применена пропитка минеральной композиции силикатом Na в виде жидкого стекла. В результате
экспериментов были синтезированы полиминеральные керамики, сложенные полевыми шпатами, кварцем и
фосфатами стронция и церия.
Abstract. Method of synthesis of matrice materials based on metasomatic replacement reactions using the scheme
of „wet process” is presented. This method allows spending process of radionuclides incorporating on phosphatecontaining mineral compositions at room temperature and atmospheric pressure under wet conditions. Synthesis of
phosphates of strontium and cerium were carried out, as well as synthesis of mineral matrices on the basis of (Sr,
Na) - and (Ce, Na) – phosphates. Products of experiments were investigated by the X-ray method; for an estimation
of compositions of synthesized solid phases the microbeam method was applied.
Deficiency of solutions of Sr- or Ce- nitrates leads formation of minerals similar to natural minerals olgite NaSrPO4
or vitusite Na3Ce(PO4)2. Simple orthophosphates of Sr and Ce were synthesized using of super quantities of nitrates
solutions. For the further optimization of mineral matrice synthesis - decrease in temperature and increase of matrice
density, the addition of Na- silicate in the form of liquid glass to solid mineral composition was used. As a result of
experiments the polymineral ceramics consisting of feldspars, quartz and phosphates of strontium and cerium have
been received.
INTRODUCTION
The IAEA guidelines for the safe storage, transportation, and disposal of radioactive waste
(RAW) include their obligatory conversion into a solidified form [1]. In order to solidify liquid
RAW, glass (aluminophosphate and borosilicate) matrices are currently used in Russia and other
countries. A general limitation of glasses is their relatively high solubility in water at high
temperatures and rapid crystallization [2–4]. Therefore, glass matrices are inappropriate for
disposal in geologic structures with unfavorable hydrogeologic conditions. The most resistant to
leaching hydrothermal fluids for the moment is the polymineral composite Synroc, a ceramic
matrice based on zirconates and titanates [5]. However, it has not been widely used because of
the high cost and complex production. In addition to Synroc, matrice materials based on
zirconium phosphates have been actively developed during the last decade [6]. These matrices
show a higher resistance to Cs, Sr, and P leaching compared with Synroc [7], but their
production is also rather complex and requires specific synthesis conditions.
The previously developed [8] concept of phase and chemical correspondence (PCC) in the
matrix – solution – enclosing rock system allowed us to conclude that optimal matrix materials
for the immobilization of radionuclides are solid solutions of rock-forming and accessory
minerals.
We demonstrated [9] that rock-forming minerals are most appropriate for the
immobilization of radionuclides of the alkaline and alkaline-earth element groups, whereas
accessory minerals (phosphates, titanates, and titanozirconates) can be recommended for the
immobilization of rare earth and transuranic radionuclides. Kotelnikov et al. [10] showed the
55
high stability of natural calcium phosphate (apatite) to leaching processes. In turn, synthetic
phosphates and ceramic materials on their basis show properties necessary for the reliable and
prolonged disposal of radionuclides [11].
SYNTHESIS OF MINERAL MATRICES ON THE BASIS OF (Sr,Na) AND (Ce,Na)
PHOSPHATES
Ceramic materials based on titanates, zirconates, and complex phosphates are produced for
the immobilization of lanthanide and actinide radionuclides by the methods of hot pressing and
sintering [5, 12]. Highly resistant matrix materials (for example Synroc) can be produced by
these techniques, but the procedures of their synthesis are rather complex. Thus an important
stage in the manufacturing of these ceramics is the production of a very homogeneous mixture,
which is attained by the intensive mixing under dry conditions of finely ground calcinated RAW.
The problem of material dispersion remains to be solved. The method of matrix material
synthesis on the basis of the ion exchange sorption of elements from aqueous solutions on
zeolites was described in detail in [13]. This paper presents the method of matrix material
synthesis on the basis of metasomatic replacement reactions [14]. Process of replacement can be
carried out at room temperature and atmospheric pressure. Strontium and cerium phosphates
were synthesized using the following reactions:
2Na3PO4(s) + 3Sr(NO3)2(aq) = Sr3(PO4)2(s)↓ + 6NaNO3(aq)
(1)
Na3PO4(s) + Ce(NO3)3(aq) = CePO 4(s)↓ + 3NaNO3(aq)
(2)
The essence of these reactions is the replacement of the crystalline phase of sodium
phosphate by slightly soluble strontium (cerium) phosphate and the removal of a soluble
compound (sodium nitrate) into aqueous solution. In order to carry out the replacement reaction,
strontium (cerium) nitrate solutions were filtered through columns filled with a model or natural
granite and grains of crystalline sodium orthophosphate.
Starting materials
Anhydrous sodium orthophosphate was obtained by drying and annealing at 1100°C of
Na3PO4·12H2O reagent in a corundum crucible until a constant weight was attained, which
resulted in the sintering of individual crystallites into polycrystalline aggregates. The model
granite was prepared by mixing of albite, quartz and microcline grains of a certain size;
sometimes river sand was used instead of the granite mixture. The compositions of starting albite
and microcline are given in Table 1. In some later experiments, samples of fine-grained granite
from the Spokoininskoe deposit (Eastern Transbaikalia) were used as a matrix. The compositions
of rock-forming minerals from this granite are also shown in Table 1.
Table 1. Compositions of starting materials according to the data of electron microprobe analysis; oxides
are in wt % normalized to totals of 100 wt %.
Starting material
Model granite
Model granite
Granite
(Spokoininskoe
deposit)
Mineral
albite
microcline
muscovite
Na2O
11.82(51)
0.62(38)
0.45(51)
MgO
0.12(23)
Al2O3
19.33(47)
18.27(15)
36.77(47)
SiO2
67.85(20)
64.89(26)
64.89(26)
K2O
0.23(43)
15.95(12)
11.03(43)
feldspar
0.45(38)
-
18.79(15)
65.16(26)
15.60(12)
FeO
3.02(34)
-
Numbers in parentheses are uncertainties in concentrations corresponding to the 95% confidence interval and
expressed in the units of the last decimal signs.
56
The grain-size composition of mixtures is important for the successful fulfillment of
metasomatic replacement reactions. The choice of grain sizes for the experimental minerals was
carried out by the example of the synthesis of strontium phosphate in a granite matrix. The
following grain size fractions were used: 0.32÷0.40 mm, 0.40÷0.63 mm, and 0.63÷1.00 mm. In
the experiments with the 0.32÷0.40 mm fraction of the starting mixture, solution infiltration
stopped before the stoichiometrically necessary amount of strontium was introduced into the
matrix. Perhaps, solution infiltration was significantly impeded due to the high bulk density of
the mixture in the column. When the 0.63÷1.00 mm fraction was used, the rate of solution
infiltration was so high that the replacement reaction practically did not occur. In columns with
the 0.40÷0.63 mm fraction of the starting mixture the rate of solution percolation provided the
occurrence of exchange reactions in phosphates. This fraction was used for the investigation of
metasomatic replacement reactions.
Experimental techniques and methods
The following procedure was used for the synthesis of mineral matrices on the basis of
(Sr,Na) and (Ce,Na) phosphates: sodium orthophosphate granules and mixtures of mineral grains
(feldspar + quartz) or granite in the 1 : 3 mass ratio were loaded into quartz glass tubes, 8 mm
inner diameter. Necessary amounts of 1 M Sr(NO3)2 or Ce(NO3)3 water solutions were passed
through the filled columns for the complete substitution for sodium in the phosphate in
accordance with reactions (1) or (2). In order to accelerate solution filtration, a low pressure
(~10-1 atm) was maintained at the output from the column using a water-jet pump. The filtrate
was collected in flasks and analyzed for strontium (cerium) and sodium. The appearance of more
than 0.08 mg/ml of Sr or Ce was a criterion for the completion of the metasomatic replacement
reaction. After the experiment column was washed with distilled water until Na content in
scoring water will be less than 0.03 mg/ml. The matrices bringing from the columns were
annealed in a laboratory furnace at temperatures of 1100–1250°C for two hours. This resulted in
the complete dehydration of aqueous phosphate and sintering of materials into massive
aggregates.
Synthesis of compact ceramic compositions
containing phosphate of strontium and Sr- containing glass
Method of synthesis of matrix material with addition of industrial liquid glass in
composition was developed for optimization of process of ceramics synthesis (decrease in
temperature of sintering and increase of matrix density). Strontium phosphate is synthesized at
stoichiometric quantity of 0.2M Sr(NO3)2 solution with using the same technique. Na- silicate in
the form of liquid glass with silica ratio equal 2 was added into a column after metasomatic
replacement reaction. The increasing of speed of percolation of liquid glass through leicogranite
grains was caused by assisted depression on an exit from a column. Further drying of ceramic
composition without it’s unloading from a column was carried out at 110ºС within 24 hours. The
cylindrical fragments of a column received as a result of a cutting were exposed by hot pressing
at 850ºС and axial pressure of 680 kg/sm2. Density of a final ceramics was about 2.2 g/sm3.
On the data of microprobe analysis as a result of this run series ceramic materials
composed of feldspar, quartz, strontium phosphate and Sr-containing glass were synthesized.
Analytical methods
Products of experiments on the synthesis of matrice materials were investigated by X-ray
diffraction using an HGZ-4 diffractometer operating in a continuous scanning mode. Silicon
(cubic symmetry, a = 5.4307 Å) was used as an internal standard. The setting angles of
reflections were refined by the polygonal correction method using the SPECTR program
57
designed in the Institute of experimental mineralogy of Russian academy of sciences. The unitcell parameters (UCP) were calculated on the basis of 12–17 reflections within the angle range
5÷40 Θ using the LCC, PUDI [15] and REFLAT [16] programs.
The contents of Na, K, Ca, Mg, Sr, Ce, Al, P and Si in the synthesized solid phases were
measured by electron microprobe analysis using a CamScan MV2300 (VEGA TS 5130MM)
scanning electron microscope equipped with YAG detectors of secondary and back-scattered
electrons and an energy-dispersive X-ray microanalyzer with a Link INCA Energy Si(Li)
semiconductor detector. The results of microanalysis were processed by the INCA Energy
program and subsequently recalculated using programs designed in the Institute of experimental
mineralogy of Russian academy of sciences.
Strontium in filtrates was analyzed by atomic absorption spectroscopy using AAS-1N
instrument, cerium was analyzed by photocolorimetry with arsenazo III [17] using a Spekol-11
instrument.
RESULTS AND DISCUSSION
The experiments showed that method of metasomatic replacement reactions can be used to
synthesize orthophosphates of alkaline-earth and rare earth elements. We used the previously
developed [18, 19] procedure of a „wet process” which can solve a problem of material
dispersion [5, 12]:
Separation of
RAW on
Aqueous
groups of
solution 
elements
of RAW
Sorption from
solutions,
replacement

reactions,
precipitation
reactions
Mineral
matrice
Phase
for
 transformation 
disposal
in crustal
rocks
According to this scheme, it is necessary, firstly, to perform an initial fixation of elements
on a sorbent or a hydrous mineral by methods of ion exchange sorption or reactions of
metasomatic replacement or precipitation. These reactions occur at low (~25°C) temperatures in
aqueous solutions. Then the obtained compound is subjected to a phase transformation by hightemperature annealing for its conversion into a stable mineral matrix.
In the early experimental series with the model granite and quartz sand as starting material
only up to 50 wt % of Sr(NO3)2 or Ce(NO3)3 water solutions were passed through the columns.
Then the columns were blocked up and the solution flow was completely passed off. After
drying at 400°C and annealing at 1100°C, binary orthophosphates of Na and Sr or Na and Ce
were formed in columns. The density of the matrices in the annealed columns was near 60% of
the theoretical value. The following reactions occurred in these experiments:
Na3PO4(s) + Sr(NO3)2(aq) = NaSr(PO4)2(s)↓ + 2NaNO3(aq)
2Na3PO4(s) + Ce(NO3)3(aq) = Na3CePO4(s)↓ + 3NaNO3(aq)
Thus, the deficit of Sr and Ce nitrate solutions resulted in formation of phases similar in
composition to the natural minerals olgite NaSrPO4 (Fig. 1), and vitusite Na3Ce[PO4]2) (Fig. 2)
in the column.
58
Fig. 1. Polymineral matrice for Sr immobilization
in phosphate form on the basis of model granite.
The small light grains are mainly the Sr phosphate
olgite (NaSrPO4) and also Sr3(PO4)2 and SrSiO3.
Fig. 2. Polymineral matrice for Ce immobilization
on the basis of quartz sand. The small light grains
are Ce- phosphate (vitusite-Ce, Na3Ce[PO4]2).
These minerals were found in the hydrothermally altered rocks of Lovozerskiy and
Khibinskiy alkaline massifs [20, 21] and showed a high resistance to leaching, because their
formation age is more than 360 Ma [22]. Therefore, these minerals can be used as matrix
materials, although the contents of strontium and cerium in them are significantly lower than in
simple orthophosphates. Khomyakov [20] noted that there are olgite group of minerals
containing both barium and strontium, Na(Ba,Sr)PO4, in Lovozerskiy and Khibinskiy alkaline
massifs, and the maximum strontium content is not higher than 0.5 apfu. Author concluded that
pure strontium analogues of olgite cannot exist because of structural limitations. However,
strontium olgite was synthesized in our experiments. The composition of synthetic olgite is as
follows: 12.82 wt % Na2O, 51.51 wt % SrO, and 33.39 wt % P2O5. Vitusite is occured in the
hydrothermal veins of Lovozerskiy massif [20] and the arfvedsonite lujavrites of Ilimaussaq
massif [21]. Vitusite in Lovozerskiy massif was found in the natrolite zone of pegmatite [22]; i.e.
it associates with such a relatively low-temperature mineral as natrolite. Vitusite was synthesized
at much higher temperatures (>1000°C) in our experiments. The obtained data suggest a wide
temperature range of vitusite existence and correspondingly its stability in natural systems.
Vitusite synthesized in our experiments contained 21.64 wt % Na2O, 40.51 wt % Ce2O3, and
35.63 wt % P2O5.
Simple strontium and cerium orthophosphates were synthesized with excess amount of
nitrate solutions. In order to decrease the temperature of matrix sintering and increase its density
the leucogranite of the Spokoininskoe deposit of eastern Transbaikalia was used as a starting
material instead of separate feldspar and quartz grains. Synthesis of mineral matrices was carried
out by the method described above under the same optimal conditions. In these experiments,
compulsory depression was created at the output of the column, which accelerated the solution
flow and provided the completeness of its infiltration trough the leucogranite – Na phosphate
mixture.
The experiments on metasomatic replacement with subsequent drying and annealing
produced polymineral ceramic composites consisting of feldspar, quartz and strontium phosphate
in two experiments (Fig. 3) or cerium phosphate in one experiment (Fig. 4).
59
Fig. 3. Mineral matrice for Sr immobilization based
on granite from the Spokoininskoe deposit
containing strontium α-phosphate, Sr3(PO4)2 (Srphosphate).
Fig. 4. Mineral matrice for Ce immobilization based
on granite from the Spokoininskoe deposit containing
Ce-monazite (monazite-Ce).
Due to the data of the microprobe analysis in the products of experiments on the method of
synthesis of previously exsiccated mineral compositions with addition of Na- silicate in the form
of liquid glass (Fig. 5) the ceramics consists of feldspar, quartz, strontium phosphate (small
white phases on fig. 5) and Sr-containing glass (small whitish-gray phases on fig. 5) were
obtained.
Fig. 5. Matrice for Sr immobilization in a mineral composition with addition of of liquid glass (Nasilicate).
Compositions of minerals in products of experiments on synthesis of mineral matrices
due to the data of the electron microprobe analysis are presented in table 2. Crystal chemical
formulaes of minerals are given in Table 3.
60
Table 2. Compositions of minerals from experiments 5869, 5870, and 5871 according to the electron
microprobe analysis, normalized to 100 wt %.
Run
no.
Mineral \
Oxides
5869
Feldspar
Sr phosphate
Feldspar
Sr phosphate
Feldspar
Ce
phosphate
5870
5871
Compositions of minerals, wt %
SiO2
K2O
SrO
Na2O
CaO
Al2O3
6.92(47)*
1.21(11)
7.05(45)
1.03(12)
6.76(43)
2.30(13)
-
18.09(51)
1.21(12)
21.76(49)
0.34(11)
19.78(48)
66.59(21)
3.84(13)
63.10(22)
1.59(15)
66.01(23)
4.67(23)
0.27(19)
4.76(21)
0.12(11)
4.92(18)
0.40(14)
0.85(11)
0.13(12)
-
-
Ce2O3
P2O5
2.41(18)
59.90(42)
2.54(19)
62.03(39)
-
-
30.99(32)
31.16(34)
-
-
69.75(46)
28.63(30)
* Numbers in parentheses are uncertainties in concentrations corresponding to the 95% confidence interval and
expressed in the units of the last decimal signs.
Table 3. Crystal chemical formulaes of minerals from the granite matrix obtained in the experiments
with natural granite from the Spokoininskoe deposit, eastern Transbaikalia.
Mineral
Feldspar
5869
Na0.60K0.30Sr0.06Al0.99Si2.98O8
Run no.
5870
Na0.62K0.27Sr0.07Al1.16Si2.85O8
5871
Na0.56K0.30Al1.03Si2.97O8
Sr phosphate
Sr2.77 Na0.16 Ca0.17P1.86O8
Sr2.60Na0.14Ca0.12 P1.91O8
-
Ce
phosphate
-
-
Ce2.013P1.910O8
Based on the results of X-ray investigation, the unit-cell parameters of the synthesized
minerals were refined (Table 4). The angle positions of the most intense reflections of the
synthesized strontium phosphate exactly correspond to those of strontium phosphate, αSr3(PO4)2 (PDF no. 24-1008 [23]). The angle positions of the most intense reflections of the
synthesized cerium phosphate coincide with those of monazite-(Ce) (PDF no. 83-0650 [24]).
Unit-cell parameters of feldspar correspond to low microcline, although the results of
microprobe analysis showed that the feldspar contains more than 0.5 apfu of sodium. Feldspar
from the Spokoininskoe deposit (used in experimental starting materials) is relatively poor in
sodium (0.04 apfu). Most probably a considerable portion of initial feldspar did not change
and the refinement of the unit-cell parameters corresponded to those of the initial feldspar.
Table 4. Unit-cell parameters of minerals from the experiments with the natural granite of the
Spokoininskoe deposit, eastern Transbaikalia.
Mineral
Fsp 5869
Fsp 5870
Fsp 5871
Sr phosphate
Ce phosphate
Quartz
A(Ǻ)
8.561(2)
8.582(1)
8.566(2)
5.387(2)
6.790(1)
4.914(1)
b(Ǻ)
12.958(2)
12.956(2)
12.962(3)
5.3871(2)
7.0203(6)
4.914(1)
c(Ǻ)
7.213(1)
7.225(1)
7.219(2)
19.780(1)
6.467(7)
5.405(1)
α()
90.06(1)
90.65(2)
90.72(1)
90.00
90.00
90.00
β()
115.82(1)
115.97(1)
115.86(2)
90.00
103.380(1)
90.00
γ()
90.01(1)
87.68(2)
87.53(2)
120.00
90.00
90.00
V(Ǻ)3
720.2(3)
721.6(2)
720.6(4)
497.13
299.93
113.03
Thus polymineral matrices containing strontium and cerium orthophosphates were
synthesized. These minerals are highly resistant to leaching: strontium orthophosphate is an
analogue of apatite, which high stability was demonstrated in [10], and cerium phosphate
(mineral monazite) is a rather widespread accessory mineral in different rocks, its high
resistance to leaching was established in [5].
61
CONCLUSIONS
(1) Method of metasomatic replacement reactions was developed for the synthesis of
matrices for immobilization of the radionuclides of alkaline-earth and REE elements. The
method can be used under „wet conditions” to minimize the dispersion of the material.
(2) Stable phosphates were synthesized in a granite matrix, which serves as an
additional barrier for the minimization of leaching of matrice by hydrothermal solutions.
(3) Optimization of a method of synthesis of ceramic matrices was carried out. As a
result of experiments on introduction of liquid glass in a granite - phosphate composition after
metasomatic replacement and the subsequent hot pressing, the polymineral ceramic
compositions consisting of feldspar, quartz, orthophosphate strontium and Sr-containing glass
were received.
Acknowledgments
This study was financially supported by Program № 9 of the Department of earth
sciences of Russian academy of sciences and Russian president program, grant № MK4888.2009.5.
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