Acta Physica Universitatis Comenianae
Volume LIII (2016) 101–109
Chromatographic Separation Methods
for Preparation of Iodine-129 and
Plutonium Targets in AMS Measurements*
J. Kaizer1, M. Ješkovský1, J. Qiao2, L. Y. Zhang2,
X. L. Hou2, P. P. Povinec1
1
2
Centre for Nuclear and Accelerator Technologies (CENTA), Department of Nuclear Physics and Biophysics,
Faculty of Mathematics, Physics and Informatics, Comenius University, 84248 Bratislava, Slovakia
Center for Nuclear Technologies, Technical University of Denmark, DTU Risoe Campus,
DK-4000Roskilde, Denmark
Abstract: One of the most frequently exploited methods in radiochemistry is definitely chromato129
239,240
Pu
graphic separation due to its high performance. Long-lived radionuclides, such as I and
which natural levels are mostly analyzed by accelerator mass spectrometry (AMS), are often separated
by ion exchange and/or extraction chromatography using resins, e.g. AG-1, DOWEX or TEVA. Here we
125
studied the elution profile of inorganic iodine exploiting anion exchange chromatography and I as a
tracer. Results clearly show a starting point of the elution, and suggest that the volume of the eluting
solution, generally much greater than theoretical amount, is indeed adequate and necessary. On top of
that, we have successfully developed two procedures based on the extraction and anion exchange
chromatography for separation of plutonium isotopes deposited on stainless steel discs. Both techniques,
more or less comparable in the overall complexity, have been already applied in the treatment of the
Fukushima samples. A good quality of the prepared target material was confirmed by mass scanning.
Basic principles of the chromatographic methods are briefly summarized as well.
129
Keywords: Anion exchange chromatography, Elution characteristics, Extraction chromatography, I,
Plutonium isotopes, Stainless steel disc, AMS.
1. Introduction
Since the dawn of the nuclear era in the middle of the 20th century the amounts of released anthropogenic radionuclides to the environment have been so enormous that many
radionuclides, such as 129I and plutonium isotopes, have been frequently used as tracers in
studies of environmental processes. The 129I (T1/2 = 15.7 My) is naturally produced in the
atmosphere by interactions of cosmic rays with xenon isotopes, and terrestrially by the
neutron activation of tellurium. On top of that, it is also formed in the Earth´s crust by
spontaneous fission of 238U (yield of 0.027 %), and by thermal neutron-induced fission of
235
U (yield of 0.54 %). It has been estimated that a steady state inventory of natural 129I is
about 250 kg in total, and that the value of the isotopic ratio of 129I/127I in the hydrosphere
and in the lithosphere, unbiased by human activities, should be around 3 ´ 10 -12 to 3 ´ 10 -13
and 10 -14 to 10 -15 , respectively [1, 2]. However, due to the release of 129I during nuclear
weapon tests, from nuclear accidents (mainly Chernobyl), and especially from nuclear
fuel reprocessing facilities, the 129I/127I ratio has increased in some places up to10 -5 . From
*) Dedicated to Prof. V. Martišovitš 75-th anniversary
102
J. KAIZER, M. JEŠKOVSKÝ, J. QIAO et al.
the beginning of the nuclear bomb testing in 1950s approx. 370 GBq (64 kg) of 129I has accumulated in the Northern Hemisphere [3–5]. While the accident in the Chernobyl nuclear power plant contributed to the total inventory of anthropogenic 129I only marginally
(1.3–6 kg [6, 7]), nuclear reprocessing facilities (in Europe) has become the main sources
of human produced 129I since the start of the their operation. About 3800 kg of 129I from
the reprocessing plant at La Hague in France, and 1400 kg of 129I from the facility at
Sellafield in the UK were discharged until 2007; the atmospheric releases were 1–2 orders
of magnitude lower. In 2014, both plants cumulatively released around 294 kg of radioiodine via pipelines into the sea [8, 9].
Excluding isotope effects, the radioiodine after entering the environment follows the
fate of the stable isotope, 127I. Despite the fact that the 129I is formed in the atmosphere and
lithosphere, the vast majority of this radionuclide is stored in the ocean. In water, iodine
primarily exists in the form of iodide (I–) and iodate (IO3-), plus, it can also be bound to organic molecules. There are several parameters, such as pH, Eh and temperature which influence speciation of iodine in water. While in oxic water (e.g. ocean water) the dominant
species of iodine is iodate, in anoxic conditions iodine prevails in the form of iodide. Iodine can evaporate from water to the atmosphere, either as molecular iodine I2, which is
produced from iodide photochemically or through reaction with ozone, or as CH3I from
dying seaweed and plankton [10]. The iodine cycle is driven by the interchange of iodine
between ocean water and the above atmosphere [11]. The transfer to land and biosphere,
which decreases with the distance from the ocean is brought about by dry and wet deposition and uptake of gasses by plants. In soils, iodine can react with polyphenols and tyrosine residues of humic substances [12].
Plutonium isotopes (239Pu, T1/2 = 24.1 ky, and 240Pu, T1/2 = 6.56 ky) represent another
group of radionuclides of interest, which have been, because of their low massic activities,
accessed to environmental radioactivity studies by the development of advanced mass
spectrometric techniques, such as inductively coupled plasma mass spectrometry
(ICP-MS) or accelerator mass spectrometry (AMS) [13]. Even though plutonium is
generated naturally from uranium, the amount is so small compared to the nuclear
industry production, that one can say that all plutonium in the environment has
anthropogenic origin. Beta decay of 239Np, formed in the nuclear reactor or during the
nuclear bomb explosion via neutron capture of 238U, leads to the generation of 239Pu and,
consequently, by capturing of another neutron, to 240Pu. During the nuclear weapon
testing in the previous century 7.4 PBq of 239Pu and 5.2 PBq of 240Pu became a part of the
global fallout. These amounts are much higher than activities released due to the
Chernobyl accident, accidents of aircrafts carrying nuclear weapons in late 1960s, or
operation of the reprocessing plants in Sellafield and La Hague [14]. Nowadays, spent
nuclear (uranium) fuel and radioactive waste produced during its reprocessing are
considered to be the main sources of plutonium.
The aqueous chemistry of plutonium is rather complex, which is well-documented by
the fact that plutonium can occur in compounds in all oxidation states between III and VII
[14, 15]. While oxidation state VII is quite rare, others are common. Furthermore, depending on the conditions, plutonium can co-exist in several different oxidation states in
the same solution. In strongly acidic solutions the most stable plutonium ions are PuIII and
PuIV, whereas higher oxidation states V and VI are more stable in neutral and alkaline media, respectively. Because of their large positive charge cations PuV and PuVI tend to hy-
CHROMATOGRAPHIC SEPARATION METHODS ...
103
drolyze in aqueous solutions almost immediately, resulting in the formation of dioxo
cations, PuO2+ and PuO22+; the PuIV ion appears without its hydroxide complexes only in
strongly acidic solutions. In the absence of ligands and in acidic solution, the PuIV undergoes disproportionation. On the other hand, if ligands (e.g. fluorides, nitrates, dihydrogen
phosphates) are present, it forms strong, high-stable complexes.
As mentioned above, one of the options for measuring long-lived plutonium isotopes
is AMS, which is also exploitable for determination of 129I. A disadvantage of the AMS
technique is that it requires an extensive use of chemistry; before concentration of the
radionuclide in the sample can be measured, sample itself needs to be pre-treated,
processed, and at the end transformed into a suitable form as a target material. There have
been developed several methods for separation of iodine and plutonium from different
matrices, some of these are based on the principles of the column chromatography
[16–19]. Here we present results from the study of the elution characteristics of iodine
from the chromatographic column, and from the development of procedures for
preparation of 239,240Pu targets from stainless steel discs, used for alfa spectrometry
measurements. Pros and cons of the studied methods are discussed as well.
2. Study of elution characteristics of inorganic iodine
The essential part of the ion exchange (ionex) chromatography is reversible ion exchange between a mobile phase (sample) and a stationary phase (resin) which contains
corresponding function groups; their charges are compensated by ions with the opposite
charge; these ions, known also as counterions, are exchanged with the ions of interest. Ion
exchange is a stepped process and its rate is driven by diffusion of ions towards the function groups of the resin. Both cations and anions can be exchanged, but as was discussed
earlier, inorganic iodine is naturally present in the environment mainly as iodide and
iodate, so it is clear that we should focus on the anion exchange [14]. There are few commercially available resins, usable for the purpose of iodine separation, which are more or
less similar, e.g. DOWEX 1 (The Dow Chemical Company, USA) and AG-1 (Bio-Rad
Laboratories, USA) resins, both made of divinylbenzene copomolymer, have granular
form and quaternary amines as function groups.
Theoretically, anion exchange can be done either in the batch setup or column setup,
though the latter is much more convenient. In the case of column chromatography, a glass
or plastic column with an appropriate length to diameter ratio is fully packed with an
anion exchange resin. After preconditioning of the resin with a pH buffer, the sample is
poured into the column in small portions of its volume so that the resin bed is not
disturbed. Anions with high values of affinity (selectivity) to bind to the exchanger, e.g.
iodides, are strongly sorbed to the resin, while those with a low affinity (selectivity) are
not extracted, and pass through the column without any interaction. Anions (radionuclides) concentrated in the resin are then eluted out, which is usually achieved by
washing the column with a small amount of the solution of an anion with higher selectivity
than the sample ion, or with a larger volume of the counterion with lower affinity. This
means that the elution characteristics depend on the concentration and volume of the
eluting solution. In our study we tried to examine the elution profile of iodine in its
104
J. KAIZER, M. JEŠKOVSKÝ, J. QIAO et al.
3000
2500
12 5
A( I) [imp/min]
2000
1500
1000
500
0
1.
3.
5.
7.
9.
11.
13.
15.
Portion of the eluate
Fig. 1. Elution characteristics of inorganic iodine from the chromatography column. The activity of 125I
was corrected for the blank sample. One portion of the eluate represents 10 mL of its volume.
separation from water in order to understand basic parameters (volume and concentration
of the eluting solution), influencing the outcome of anion exchange chromatography.
The methodology for the determination of the elution characteristics of iodine followed to some extent general anion exchange methods used for total inorganic 129I separation or its speciation [20, 21]. First, iodine carrier (127I) and 125I tracer were added to the
sample (tap water). To ensure that all iodine in the sample is present in the form of iodide,
small amount of 1 M NaHSO3 was also put into the solution which was then acidified with
HNO3 to pH ~2 and intensively stirred. A chromatographic column was packed with the
AG-1´4 (100–200 mesh, Cl– form) resin which was then converted to NO3- form by passing 1 M NaNO3 through it. After almost all sample solution passed through the column,
0.2 M NaNO3 solution is added to the column to remove any unbound or even weakly
bound anions.
The elution of iodide strongly adsorbed to the resin was done with the 2 M NaNO3 solution in such a way that every 10 mL of the eluate was collected separately. All portions
of the eluate, fifteen in total, were then one by one measured by g-spectrometry (using
NaI(Tl) detector) to track eluted 125I. Next, the whole eluate was processed by the standard
procedure of solvent extraction [21], which is only briefly described here, in order to prepare a sample for mass spectrometry (MS) measurements, which results can be found
elsewhere [20]. After acidification of the eluate with nitric acid, iodine was oxidized to its
elementary form by addition of NaNO2 and extracted in a separation funnel to the CHCl3
phase. From the organic phase iodine, reduced to the iodide form with HSO3–, was
back-extracted into the water and precipitated as AgI which at the end was used for the
preparation of the MS target.
The result of the study of the elution characteristics of iodine expressed as a function
of the measured g-activity of 125I (corrected for the blank sample) in the eluted volume is
shown in Fig. 1; the integral signal of the activity was gathered from the channels
3.3–58.5 keV of the NaI(Tl) detector. As it can be clearly seen from the graph, first
portions of the eluate, which were taken and measured in the ascending order, already
CHROMATOGRAPHIC SEPARATION METHODS ...
105
contained minor amounts of 125I, meaning that probably not all of the iodine was
successfully extracted by the resin. The elution of the sorbed iodide started after passing
some 55 mL of the eluting solution of sodium nitrate through the column. It is interesting
to point out that in the eleventh portion of the eluate, a non-negligible signal of 125I was
still detected. The residual activity accumulated in the last portions, suggests that for the
complete elution of iodine even 150 mL of 2 M NaNO3 might not be sufficient; a loss of
the analyte caused by incompleteness of the process has been estimated to be max. 3.5 %.
If we compare the total added and measured activity of 125I we can conclude that the
overall efficiency of the anion exchange chromatography in iodine separation using our
setup was 92–93 %. The chemical yield of the consequent solvent extraction was very
similar (95–96 %).
3. Development of chromatographic methods
for plutonium separation
There are three radiochemical analytical methods generally used for separation of
plutonium isotopes from other actinides. Among solvent extraction and anion exchange
chromatography, which was shortly introduced in the previous chapter, one can also
exploit extraction chromatography. The last technique combined solvent extraction and
column chromatography, and its principle is the same as the principle of anion exchange
chromatography. In the extraction chromatography, a stationary phase which is made of
liquid-liquid extraction reagent impregnated into the some kind of porous inert organic
polymer or silica gel. An analyte (radionuclide) is sorbed on the resin from the mobile
phase passing through the column. Its elution is achieved by addition of the right solution
with the appropriate concentration or complexing agents, or by changing of the valence
state [14]. We have worked with DOWEX 1 (mentioned above) and TEVA (Triskem
International, France) resins. Although both resins contain aliphatic quaternary amines as
function groups, a slight difference between them is that in the TEVA resin these amines
are in the form of free molecules, not bound to the polymeric structure.
Methodology for preparation of plutonium AMS targets from Pu deposits on stainless
steel discs exploiting anion exchange chromatography and extraction chromatography is
illustrated in the diagram (Fig. 2). To dissolve material electrodeposited on a discs, the
disc is soaked in 2 M HCl for 10–15 min and washed with 0.2 M HCl to ensure that the
whole sample ends in the acidic solution, which is then diluted with deionized/distilled
water to the HCl concentration of 0.5 M. Approx. 100 mg of iron (as Fe3+), which serves
as a carrier and catalyst of the plutonium reduction reaction, together with K2S2O5 or
NaHSO3 are added to the sample solution. By its stirring, all forms of plutonium are reduced to the oxidation state III; the end of the reaction is indicated by the color change as a
result of the reduction of iron to Fe2+ state. After that, pH of the sample is adjusted to 8–9
with conc. NH4OH, leading to precipitation of ferrous hydroxide and scavenging of plutonium and other actinides. Dark green precipitate is centrifuged and dissolved in a small
volume of conc. HNO3; the dissolution of the precipitate can be supported by short boiling. The activity of the nitrites in the acid causes oxidation of PuIII to PuIV. Before the
column separation step, the concentration of nitric acid in the sample solution is lowered
to 8 M for the anion exchange, or to 1 M for the extraction chromatography.
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J. KAIZER, M. JEŠKOVSKÝ, J. QIAO et al.
Fig. 2. Schematic diagram of two chromatographic methods used for separation of plutonim from
stainless steel discs.
Since only a small amount of the resin is necessary for the extraction chromatography, a column of adequate dimensions (0.7´5 cm) is packed with 2 mL of TEVA
(100–150 µm). However, this would not be enough in the case of anion exchange chromatography where a 15 cm long column, full of the DOWEX 1´8 (100–200 mesh, NO3–
form) is necessary. Preceding the loading both resins need to be preconditioned with
HNO3 solution, with the same concentration as the sample solution. Because of the fact
that americium nor uranium form nitrate complexes, plutonium, adsorbed in the TEVA or
DOWEX resin, is easily separated from these actinides; trace amounts of the elements are
washed out from TEVA and DOWEX with 1 and 8 M HNO3, respectively. On the other
hand, thorium retained in the column as Th(NO3)62– is converted to ThIV and eventually
eluted with 6–9 M HCl. Independent of the method, the chromatographic separation sequence ends by stripping plutonium from the resin with 0.1–0.2 M NH2OH·HCl–2 M HCl
which selectively reduces PuIV to PuIII. Collected eluate is slowly evaporated to dryness.
The residue is dissolved in few milliliters of conc. HNO3, and heated to boil to remove
the remained hydroxylammonium chloride. The sample is transferred to a centrifuge tube
CHROMATOGRAPHIC SEPARATION METHODS ...
107
with 0.5 M HNO3. Then, exactly 1 mg of iron (again as Fe3+) is added to the tube followed
by pH raise to 9–10 with conc. NH4OH to precipitate ferric hydroxide, which quantitatively scavenges plutonium. The precipitate is centrifuged, washed with distilled/
deionized water, dried in the oven at 90° C and calcinated at 500° C for 20 h in air to transform hydroxide to oxide. Finally, a target for the AMS analysis is prepared by mixing of
the Fe oxide (with deposited plutonium) with aluminum powder (min. purity 99.9 %) in a
weight ratio of 1:3, and pressed into an Al holder. More detailed information on the procedures was presented elsewhere [20].
For evaluation of the separation efficiency of anion exchange and extraction chromatography methods (described above) one would need to know a chemical yield. Plutonium
has no stable isotopes, therefore, the use of a tracer is the only possibility how to quantify
the yield which is determined from the difference of the activity before and after the separation. There are several candidates that could be used for this purpose (236Pu, 242Pu,
244
Pu), however, none of them was available for us during the methodology development.
Nevertheless, this should not hinder an intended option to measure the 240Pu/239Pu mass
ratio with AMS in the samples, and thus to obtain valuable data.
More than twenty stainless steel discs with electrodeposited material were altogether
processed by anion exchange and extraction chromatography. All of these samples are
expected to contain plutonium signal from the Fukushima accident; determination of their
240
Pu/239Pu ratio by AMS, which is planned in a near future, could possibly confirm or
disprove the assumption. To add more, we were able to gather some interesting empirical
results during the development, which helped to better understand the methods. Even
though anion exchange and extraction chromatography are very similar, the latter seems
more viable, at least from the practical point of view. Volumes and concentrations of
washing solutions, used to remove interfering actinides, as well as the amount of resin, are
much lower in the case of the extraction chromatography, which leads to non-negligible
save of chemical reagents. As the porosity of the TEVA and DOWEX 1 resins is identical,
the rate of mobile phase in the column should be significantly higher for extraction
chromatography where much less amount of the resin is applied. The elution speed we
observed was, however, on the same level (0.7–1 mL/min) for both resins and columns.
The reason for this may be hidden in the difference in the structures of the TEVA and
DOWEX resins, discussed earlier. Another disadvantage of the anion exchange chromatography is the necessity to use greater amount of the eluting solution compared to anion
exchange, resulting in considerable shortening of the time separation.
In order to evaluate the quality of the prepared targets, some of them were chosen for
mass scanning by the use of the injection part of the tandem system at the Centre for Nuclear and Accelerator Technologies (CENTA) of the Comenius University in Bratislava
[20, 22–24]. As the CENTA facility is not yet capable of the determination of heavy
radionuclides at ultra-low levels, the purpose of these measurements was not to check plutonium presence in the samples, but to see if the targets were contaminated. A cut of the
mass spectra (from 10 to 80 amu), typically obtained in these measurements is illustrated
in Fig. 3. A presence of all identified ions was anticipated, due to chemical reagents and
methods used for the target preparation. Despite the fact that iron has four stable isotopes,
heavier ones than the most abundant 56Fe (91.72 %) did not show in the spectra, neither in
the monoatomic form nor as oxides; lighter ion 54Fe– increased the intensity of the 27Al2–
peak.
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J. KAIZER, M. JEŠKOVSKÝ, J. QIAO et al.
1E-6
16
O-
27
Al16O-
27
Al-2 + 54Fe56
16
-
Fe O
1E-7
16 1
-
OH
56
-
FC current[A]
Fe
27
-
Al
1E-8
1E-9
1E-10
20
30
40
50
60
70
80
Mass [amu]
Fig. 3. Cut of the typical mass scan of the prepared plutonium target material measured at the CENTA
facility.
4. Conclusions
Chromatographic techniques have been for many years routinely used for separation
of long-lived radionuclides, such as 129I or plutonium isotopes, though, with development
of the accelerator mass spectrometry (AMS), which is fundamentally different when
compared to conventional radiometric methods, an interest in their investigations and
further developments has been recently renewed. To even better comprehend the
mechanics of sorption of inorganic iodine on the anion exchange resin we studied the
elution characteristics in a modelled experiment. Our results clearly showed the starting
point of the elution, and also that the volume of the eluting solution, sufficient for removal
of all iodine, should be carefully considered.
Regarding target preparation for AMS measurements of plutonium isotopes deposited
on stainless steel discs, we successfully developed and established two procedures based
on the principles of anion exchange and extraction chromatography. According to our experience, the latter method appeared slightly more advantageous, although both are more
or less comparable. Altogether, twenty two samples, in which the 240Pu/239Pu signal from
the Fukushima accident is supposed to be detected, were reprocessed. Preliminary mass
scanning confirmed the anticipated composition and high quality of the prepared targets.
Acknowledgments
The work was supported by the EU Research and Development Research Program
funded by ERDF (projects No. 26240120012, 26240120026 and 26240220004), and the
Technical Cooperation Program of the International Atomic Energy Agency (project
No. SLR/0/008).
CHROMATOGRAPHIC SEPARATION METHODS ...
109
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