Incorporation of iodates into hydroxyapatites
Danielle Laurencin, Rémy Delorme, Lionel Campayo and Agnès Grandjean.
Institut Charles Gerhardt de Montpellier, UMR 5253 CNRS-UM2-ENSCM-UM1, Place E. Bataillon, 34095
Montpellier cedex 5, France ; CEA, DEN, DTCD, SECM, Laboratoire d’étude et de Développement de Matrices de
Conditionnement, Centre de Marcoule, 30207 Bagnols sur Cèze, France ; Institut de Chimie Séparative de Marcoule,
UMR5257 CEA-CNRS-UM2-ENSCM, 30207 Bagnols sur Cèze, France
RECEIVED DATE (automatically inserted by publisher); Add Author E-mail Address Here
ABSTRACT. A new strategy for the confinement of iodine
has been developed: it consists in incorporating iodate
anions (IO3-) into hydroxyapatite (Ca10(PO4)6(OH)2). The
materials were synthesized by precipitation in water, and
characterized by powder X-ray diffraction, 1H and 31P solid
state NMR, as well as IR and Raman spectroscopies. It was
found that the iodate enters the hydroxyapatite lattice, that it
mainly substituted for the hydroxyl group, and that fairly
high levels of iodine (up to ~14 wt%) can be incorporated.
The thermal stability of the materials and their resistance to
leaching were evaluated, revealing promising properties in
view of radioactive iodine confinement applications.
Apatites are a family of compounds of general formula
M10(XO4)6Y2 (M = Ca2+, Sr2+, Pb2+, Na+…; XO4 = PO43-, VO43,
CO32-… ; Y = OH-, F-…),[A,B] which are widely spread in
nature, calcium hydroxyapatite (Ca10(PO4)6(OH)2, Ca-HA) being
for example the main inorganic component of bone tissue and
teeth. Because apatites are able to incorporate many different ions,
they can display a broad range of properties, and apatite-based
materials have thus been widely exploited over the last 30 years
for the development of bone implants,[C] catalysts,[D] fast ion
conductors,[E] and confinement matrices for toxic elements like
plutonium.[F]
Iodine-129 is a product of nuclear fission which is radiotoxic
for wildlife and especially for humans, as it accumulates in the
thyroid. Several methods have thus been proposed to try to ensure
the safe disposal of this radioactive isotope, and notably the
development of confinement materials.[G] It has been found that
iodine can be particularly difficult to incorporate inside solid
matrices, because of its tendency to transform into volatile I2
under heat treatment, and because of its low incorporation rate in
classical confinement matrices like glasses and cements. In this
context, apatites have been shown to be good candidates for the
efficient trapping of iodine under the form of iodide (I-) ions:[H,I]
a solid state reaction between Pb3(VO4)1.6(PO4)0.4 and PbI2 leads
to the formation of an iodine-bearing apatite of formula
Pb10(VO4)4.8(PO4)1.2I2. However, this phase presents several
drawbacks:, (i) its synthesis implies the use of polluting reactants
(e.g., lead (II) oxide and vanadium (V) oxide) and (ii) its
industrial processing would require using non-routine techniques
like Spark Plasma Sintering. So far, in the literature, attempts to
incorporate iodine in apatite phases have only been made using
iodide ions.[H-J] Here, we describe a new and more sustainable
approach to introduce this element in hydroxyapatites, using
iodate anions (IO3-),[K] and we demonstrate its suitability for the
confinement of radioactive iodine.
Iodate substituted Ca-HA phases were prepared by
precipitation in water.[L] This synthetic route was privileged
because (i) Ca-HA forms at high pH (pH~10 to 12), i.e. in
conditions in which IO3- is a stable form of iodine in water; (ii) no
strong heating is necessary, in contrast with other synthetic
procedures described so far in the literature such as solid state
reactions[M]; (iii) it is fast and straightforward to carry out, and
thus potentially transferable for industrial applications.
The syntheses of substituted Ca-HA were performed using
different amounts of iodate (see Table 1); the samples are referred
to as CaI-HA1 and CaI-HA2 in the rest of the article.
Concentrated NH4OH was used to maintain the pH at pH~10.0
throughout the precipitation, and thereby ensure the formation of
a HA phase. As shown in Figure 1a, the X-ray diffraction (XRD)
powder patterns of these phases are very similar to those of nonsubstituted Ca-HA, and no extra crystalline phase was detected.
The lattice parameters were determined assuming a P63/m space
group, as it is the case for hexagonal Ca-HA:[A] the a parameter
appears to increase with the amount of iodate initially used in the
reaction, while the c parameter does not seem to follow any clear
trend (see Table 1). The small change in the a parameter suggests
that iodine has entered the HA lattice. This is further confirmed
by 31P solid state NMR. Indeed, the linewidth of the 31P NMR
signal of is broader for CaI-HA1 and CaI-HA2 than for Ca-HA,
thereby proving that there is a larger variety of local environments
for the phosphates, because of the presence of substituents in the
HA lattice (see Figure S1 in Supplementary Materials).
Ca-HA
CaI-HA1
CaI-HA2
Elemental composition
%I
%Ca
%P
0
34.5
16.0
7
34.5
16.0
12
33.0
15.0
XRD lattice parameters
a
c
9.40
6.87
9.48
6.74
9.63
6.88
Table 1. Composition and XRD lattice parameters for the CaI-HA phases
Raman, IR, and I L3-edge X-ray Absorption Near Edge
Structure (XANES) spectroscopies show that iodine has entered
the Ca-HA lattice under the form of an iodate anion. Indeed, on
the one hand, the Raman and IR spectra display a broad band
centered at ~793 cm-1, which is characteristic of the I-O stretching
vibration in iodates (see Figures 1b and 1c), and which increases
with the iodine content.[N] On the other hand, the overall aspect
of the I L3-edge XANES spectrum of an iodate substituted Ca-HA
phase is very similar to that of NaIO3, meaning that the oxidation
state of the iodine is the same in both cases (see Figure S2).[O]
There are two possible sites of incorporation for IO3- inside
the HA structure: they can substitute for hydroxyl groups (OH-) or
a-
(I-O)
CaI-HA2
CaI-HA2
CaI-HA1
CaI-HA1
Ca-HA
Ca-HA
400
20
30
40
50
2Q( )
60
70
c-
600
800
1000
Raman shift (cm-1)
d(I-O)
L(O-H)
H2O
OH
CaI-HA2
CaI-HA2
CaI-HA1
Ca-HA
CaI-HA1
Ca-HA
1800
1400
1000
σ (cm-1)
600
1.6
1.2
0.8
0.4
0.0
0
5
10
15
time (days)
Figure 2. Normalized weight loss of CaI-HA1 in a leach test
b-
10
weight loss (10-2 g.m-2)
for phosphates (PO43-). In the latter case, different charge
balancing schemes can occur, such as the creation of ionic
vacancies.[A,B] Both substitution sites can a priori be proposed
for the incorporation of iodates, notably because their charge is
the same as for the hydroxyls, but their size is more similar to that
of phosphates. IR and 1H solid state NMR provide information on
the substitution mechanism. Indeed, as shown in Figure 1c, a
decrease of the intensity of the O-H IR vibration band at 628
cm-1[M] is observed when the iodate content increases. Similarly,
on the 1H NMR spectra (Figure 1d), a decrease in intensity of the
OH peak is observed. Thus, it appears that the iodate mainly
enters the hydroxyl site. It is noteworthy that the 1H hydroxyl
signal is also broader for the iodate substituted HA than for the
non-substituted phase, because of the increase of local
environments after incorporation of iodates.
12 10 8
6
4 2 0 -2 -4 -6 -8
1H chemical shift (ppm)
Figure 1. a/XRD, b/Raman, c/ IR and d/ 1H solid state NMR data of CaI-HA
Elemental analyses were carried out to determine the exact
composition of the samples (Table 1). It is worth noting that the
actual Ca, P and I contents of these substituted HA phases are
different compared to what could have been expected from the
relative amounts of precursors initially introduced in solution
(Table S1). Nevertheless, it appears that the iodine content in CaIHA1 and CaI-HA2 is ~9.9 and 13.1 wt %, which is comparable to
that had been obtained previously for Pb10(VO4)4.8(PO4)1.2I2,
taking into account the difference in densities. Another big
advantage of CaI-HA1 and CaI-HA2, in view of confinement
applications, is that apart from the iodates, they are composed of
environmentally friendly ions, i.e. calcium and phosphates.
For the samples to be potentially used for confinement
applications, their stability in temperature and their leaching
resistance must be evaluated. Thermo Gravimetric Analyses
(TGA) were carried out, showing that the samples remain intact
up to ~550°C, temperature at which the iodine progressively starts
to react to be released under the form of volatile I2 (see Figure
S3). On the other hand, the long-term behavior of CaI-HA1 was
evaluated using a simple leaching test, in which the sample is
exposed to pure water at 90°C, and the amount of iodine released
titrated using an iodine selective electrode (see Supplementary
Materials for details). Figure 2 shows the normalized weight loss
of the solid during the leaching test. The initial rate of release of
iodine from this material is ~10-3 g.m-2.day-1, and it rapidly
decreases to ~10-4 g.m-2.day-1 after a few days. This leaching
experiment is not normative due to the large specific surface area
of CaI-HA1 (92 m².g-1), which leads to a larger S/V ratio than that
used for the determination of initial leaching rates in confinement
matrices (cf MCC-1 leach test). Nevertheless, the leaching rates
measured here are qualitatively comparable to those found in the
literature for other iodine-confinement materials. Indeed, dynamic
leaching tests performed on phases like Pb10(VO4)4.8(PO4)1.2I2 at
90°C and using pure water, also showed an initial leaching rate
~10-3 g.m-2day-1 on the basis of iodine release.[H] In these results,
S/V ratio ranges between 30 and 80 cm-1 by comparison with a
value of 460 cm-1 used in this work. These differences could lead
to an underestimation of the real initial leaching rate. Although
additional experiments on CaI-HA dense pellets would be
necessary to fully evaluate their iodine confinement capacity,
these preliminary leaching tests appear particularly promising for
future applications of these materials.
In conclusion, a new method of incorporating iodine in HA has
been developed: for the first time, iodate-substituted apatite
phases have been synthesized. Their thermal stability up to
~550°C and the slow release rate of the iodate under leaching
conditions make them interesting candidates for the confinement
of radioactive 129I. Additionally, iodate substituted Ca-HA phases
could also find applications in medicine: indeed, studying these
phases could help learn more about iodine related diseases like
goiter, and could also lead the way to the development of new
cures.
Acknowledgements. CEA and CNRS for their financial
support. We acknowledge the French synchrotron SOLEIL for
provision of synchrotron radiation facilities (project 20090572),
and Dr Delphine Vantelon for her assistance in the XANES
experiments.
Supporting Information Available. Experimental details on the
synthesis of CaI-HA phases, on the leach test, and on the analytical
techniques used to characterize the materials. 31P MAS NMR spectra,
I L3 XANES data of CaI-HA. TGA of CaI-HA1.
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