  ARTICLES

Sci. Rep. Kanazawa Univ.
Vol. 63, pp. 59–66, 2019

Rapid separation method for protactinium fromuranium using manganese oxide filter cake
Hiroki NAKADA1
1

and

Akihiko YOKOYAMA2,*

Kanazawa University Senior High School, School of Teacher Education, College of Human and Social
Sciences, Kanazawa University, Heiwa, Kanazawa 921-8105, Japan
2Institute of Science and Engineering, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan(Received April 11, 2019, Revised May 23, 2019, and accepted June 3, 2019)

Abstract

Adsorption and separation performance of protactinium (Pa) on a manganese oxide (MnO2)
filter cake was studied by measurements of α-, β-, and γ-rays and the results compared with those ofsolid-phase extraction using MnO2 and TK400 resins. The MnO2 filter cake method demonstrated
excellent recovery. Radiochemical purity was comparable with that achieved by other newly developed
methods, with only small amounts of contamination by Bi, Ra, Ac, Th, and U. The method, originallydeveloped for recovering Pa from uranium solutions, has the merit of rapidity in chemical separation. It
may be able to be applied to other purposes, such as investigation of the chemical properties of dubnium,
a superheavy element.

Keywords protactinium, manganese oxide filter cake, MnO2 resin, TK400 resin, chemical separation

1

Introduction

Adsorption behaviors of several cations on manganese oxide (MnO2) filter cakes, prepared as layered
manganese dioxide, have been investigated for several decades [1-4], and developed for practicalapplications such as collection of radium (Ra) ions [5,6]. With application to the study of actinides, theMnO2 filter cake method has been used in a demonstration experiment [7] for the isolation ofprotactinium (Pa) from uranium (U) for students at Kanazawa University, Japan, because it can be
performed using very simple and rapid procedures that yield a valuable method to measure even shortlived nuclides.
Solid-phase extraction methods, similar to the MnO2 filter cake method, have recently been developed
and have become very popular for separation of many elements, including actinides. Several methods
utilize chromatographic materials, such as the TRU [8] and TEVA [9] resins, because of their excellent
selectivity; however, the MnO2 filter cake method still has advantages of rapidity, simplicity, and cost
performance for particular elements.
Protactinium, the third actinide element, is a pseudo-Group V element. It has several oxidation states
and thus complicated chemical behavior. The element is also notable for having properties that can becompared with those of dubnium (Db), a superheavy element. Such comparison is important to clarify
the relativity effect that may cause deviation from the periodicity of the elements [10].

*Corresponding author

E-mail: yokoyama@se.kanazawa-u.ac.jp

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Rapid separation method for protactinium

In the present study, we performed an experiment to isolate Pa from uranium, including parent nuclides
that decayed into Pa atoms and other daughter nuclides of the uranium series, as shown in Figure 1. It
makes one able to obtain the Pa solution by a milking method, which is useful for the research work forchemical properties of Pa as well as a demonstration experiment for students learning the radioactive
equilibrium of parent-daughter nuclides. We evaluated and confirmed the performance of the MnO2 filter
cake method with respect to recovery and radiochemical purity of Pa. We prepared a uranium solution
from a uranyl nitrate reagent purchased several decades ago and which thus included many daughter
nuclides of uranium.
TK400 [11] and MnO2 [12] resins are commercially available. The former is used for isolation of Pa
from other elements; the latter is modified with MnO2 and used for Ra ion collection from environmental
waters. We compared the resin performances with that of the MnO2 filter cake using a revised procedurefor Pa separation. We evaluated the radiochemical purity of each method from the relative radioactivities
of nuclides, although the observed radionuclides depended on the emitted radiation, i.e., α-, β-, and γrays. We performed α- and γ-spectrometry in addition to βmeasurements to identify and measure thenuclides contaminating the MnO2 filter cake.

2

Experimental

2.1 Reagents and apparatus
The reagents used for the chemical procedures included: uranyl nitrate solution (UO2(NO3)2; 70–100 mg
U/mL in 0.25 M nitric acid); thorium nitrate solution (Th(NO3)4; 40 mg Th/mL in 0.25 M nitric acid); 16
M HNO3; manganese chloride solution (MnCl2; 15 mg Mn/mL); potassium permanganate solution(KMnO4; 5 mg Mn/mL); and wash liquids of 2 M nitric acid, H2O, and acetone. The U sample used in
the present study is not analyzed in detail on isotopic abundance but classified into depleted uranium,
more enriched than natural uranium of 99.3% in the 238U content. The Th sample is ordinarily available
sample of 232Th (100%).
The apparatus included a glass filter of 25 mm internal diameter, a suction bottle, aspirator, and a glassfiber filter of 24 mm in diameter.


2.2 Chemical procedures
In any chemical procedure for separation of Pa from U solution that includes daughter nuclides of thesame radioactivities due to radioactive equilibrium, contamination by 234Th, a daughter nuclide of 238U,
should be used to purify the sample of Pa. Thus, a solution of Th as a retention carrier was prepared for
addition to the U solution. A MnO2 filter cake was prepared prior to the separation of Pa.
The basic procedures used in this study followed the method given in the textbook used forradiochemistry training courses at Kanazawa University, entitled “Experiments in Basic Chemistry B”[7]. Details of the chemical procedures are described below. The conditions are described as Methods 1
to 4 in Table 1. Methods 1 and 2 used MnO2 filter cake, Methods 3 and 4 did MnO2 resin, and Methods
5, TK400 resin. Methods 2 and 4 are performed with purified Th carrier, while Methods 1, 3 and 5 aredone with Th carrier by the original procedure [7]. Preparation for the Th carriers is described in 2.2.1 inmore detail.

2.2.1

Preparation of retention carrier of Th and MnO2 filter cake

For preparation of the Th retention carrier, 1 mL of Th(NO3)4 solution, 2 mL H2O, 1 mL of 16 M HNO3,
and 2 mL of MnCl2 solution were mixed in a beaker before heating in a hot-water bath and adding
approximately 20 drops of KMnO4 solution to precipitate MnO2. The Th-containing solution wasHiroki NAKADA and Akihiko YOKOYAMA

61

separated from the precipitate through a glass-fiber filter and stored as retention carrier solution for the
Pa separation. A suction bottle connected to aspirator was used for this filtration, as well as for the otherfiltrations in this study. The remaining filtrate contained radioactive nuclides with chemical properties
similar to those of Pa. After washing with a small amount of water and drying, it was subjected tomeasurements using a Geiger Müller (GM) counter. The solution was purified with another MnO2 filter
cake for Methods 2 and 4 to reduce contaminants other than Th. This was the only revised process that
had an effect on the results in this study.
For preparation of the MnO2 filter cake, 1 mL of 16 M HNO3 and 2 mL of MnCl2 solution were mixed
in a beaker before heating in a hot-water bath and adding approximately 6 drops of KMnO4 solution toprecipitate MnO2. A MnO2 filter cake comprising the precipitate was thus uniformly prepared on a glassfiber filter.

2.2.2 Isolation of Pa with MnO2 filter cake
Five milliliters of UO2(NO3)2 solution and 3 mL of the Th retention carrier, prepared as described in
Section 2.2.1, were mixed in a beaker and allowed to stand for a while before the solution was slowly
passed through the MnO2 filter cake, prepared as described in Section 2.2.1, by suction for
approximately for 30 s, i.e., using a flow rate of 16 mL/min. This filtration time is important because it
affects the recovery. We determined the time by repeating the procedures so that as little radioactivityof Pa-234m as possible was lost during the measurement. The cake was then sequentially washed undersuction with 1 mL of 2 M HNO3, 1 mL H2O, and 1 mL acetone until it was fairly dry. The filter cakewas then placed in a sample dish for radioactivity measurements. For the methods used in this study,
possible contaminants originated from the nuclides in the uranium and thorium series, except for the Pa
isotopes shown in Figure 1.








Figure 1 Decay chains and half-lives of nuclides in the uranium and thorium series [13,14].

2.2.3 Isolation of Pa with MnO2 and TK400 resins
The MnO2 and TK400 resins were used for comparison with the MnO2 filter cake with respect toperformance of the Pa separation. The MnO2 resin (Eichrom Technologies, Inc., USA) [12] wasmodified with MnO2. We prepared the same mass of resin in a suspension instead of as a cake under
the same conditions and procedures as described in Section 2.2.2. The TK400 chromatographic resin(TrisKem International, France) [11] was modified with octanol for the separation of Pa. A resin cakewas similarly prepared using a strong acid solution of HCl, which provided conditions for efficient

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Rapid separation method for protactinium

adsorption by the resin.


2.3 Measurement of radioactivities of Pa and contaminants
We measured the radioactivities of the samples using α-, β-, and γ-rays. Alphaspectrometry was
performed using a silicon solid-state detector of 400 mm2 active area, attached to an Octête PC alpha
spectrometer (EG&G Ortec, USA). The data were passed through an analog-to-digital converter (ADC)with a 1k channel conversion gain that included energies up to 10 MeV. The measurement of β-rays was
carried out using an Aloka GP-101GM counter (Hitachi, Japan) for analysis of the radioactive decay of
nuclides. Gamma spectrometry was carried out using a BE2020 Ge semiconductor detector with 20%
relative efficiency (Canberra, USA). The data were processed through an ADC with 4k channel
conversion gain, covering energies up to 2000 keV.

3

Results and Discussion

Figure 2 shows a plot of counts per minute after subtraction of background counts as a function of elapsed
time for the β measurements of MnO2 filter cakes after elution of the sample solution. Decay curves
derived from the plots demonstrated that the main β-radioactivity occurring immediately after separation
was due to Pa-234m (1.159 min) except for longer components. Two-component analysis suggested thatthe latter components had half-lives of 10 to 47 min or more than 10 days. The data with lower purity(i.e., Method 1 in Table 1) were not reproduced well by calculated curves, possibly because of theinvolvement of more components or another component derived from a parent nuclide. Such analyses
enabled determination of the recovery and radiochemical purity of Pa just after filtration through the
MnO2 filter cake, as listed in Table 1, where uncertainties of the data are from statistics in counting.
Recovery values may be shifted to some extent due to uncertainties in the decay correction for the shortlived product.
These analyses demonstrated that Method 2, with a MnO2 filter cake, had the best recovery of themethods tested. Suction time was crucial: about 30 s was needed. Less suction time resulted in loss ofadsorption and more time resulted in loss of radioactivity due to decay of short-lived nuclides. We needed
1 to 2 min after filtration to start the β-measurement. The data in Table 1 were therefore corrected becauseof decay for this duration. Within the uncertainties, Method 2 and that using TK400 showed the best
purity. These measurements only provided information concerning the half-lives of the isolated nuclides:
we used γ- and α-spectrometry to identify the nuclides.
Table 1 Recoveries and radiochemical purities of Pa achieved for the three separation methods.
Adsorbent

Pa recovery (%)

Radiochemical purity (%)

1

MethodMnO2 filter cake> 100

96.4 ± 1.5

2

MnO2 filter cake> 100

98.9 ± 1.0

3

MnO2 resin> 100

96.6 ± 1.4

4

MnO2 resin

84.5 ± 0.9

97.3 ± 1.55TK400 resin

92.6 ± 1.0

98.3 ± 1.5

RemarkTh carrier purified
Th carrier purified

63

Hiroki NAKADA and Akihiko YOKOYAMA

100000Net Counts /min

㽢0.1
10000

Method 2

1000

Method 1

100
0

500

1000

1500

2000

Time /s
Figure 2 Decay curves of MnO2 filter cake methods, as determined from Geiger Müller countermeasurements.
Figure 3 shows a γ-spectrum as an example of the absorption of Pa by the MnO2 resin. This was quite
similar to the spectrum of the MnO2 filter cake, but of better statistical quality. It enabled identificationof the photopeaks of 234Pa (6.70 h), 228Ac (6.15 h), 212Bi (60.55 min), and 212Pb (10.64 h). In reference tothe γ-ray data, 208Tl (3.053 min) originated from 224Ra (3.6319 d) [15]. Several peaks of 234Pa found in
the spectrum demonstrated an excellent yield and purity of Pa, considering that radioactivity of thenuclide amounted to only 0.16% of that of 238U. It is noteworthy that observed nuclides were not always
adsorbed on the cake in their original form, but adsorbed parent nuclides may have decayed into these
nuclides. All contaminants except for 234Pa were nuclides of the Th series, from the Th carrier solution.
A small amount of 234mPa was considered to originate from the decay of 238U contaminant because theinitial 234mPa component was lost before starting the γ-counting.
The separation factor, SF, of the U concentration relative to the Pa concentration was obtained from
Equation (1) according to the definition of Pa/U radioactivity ratios before and after a separation process,(Pa/U)pre and (Pa/U)post, respectively, and the reasonable assumption that (Pa/U)pre was nearly unity
because of its radioactive equilibrium. Thus, SF was estimated from the radioactivity ratio of 234mPa, adaughter nuclide of contaminant 238U, and 234Pa, which represented the separated Pa component after theprocess, as SF ~ 130.
 
(1)

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Rapid separation method for protactinium

Figure 3 γ spectrum of absorption of Pa by MnO2 resin, measured for 155 973 s after preparation by
Method 4.
Figure 4 shows α-spectra of (a) Pa absorption and (b) Th daughters obtained in the preparation of theretention carrier solution of Th described in Section 2.2.1, both adsorbed by a MnO2 filter cake. The
spectra suggested that most of the nuclides observed in (a) were contaminants, because few nuclides ofPa are α-emitters, although a peak of 231Pa, a daughter nuclide of 235U, appeared in the spectrum. Nuclides
from the Th series were assigned by comparison with (b), except for the U nuclides present atoverwhelming material abundances. The nuclides of 232Th, 228Th, 224Ra, and 212Bi were concluded to beadsorbed on the MnO2 filter cake, except for short-lived nuclides produced from parent nuclides andcontaminants of the Pa separation using the MnO2 filter cake.

Figure 4 Alpha spectra for (a) Pa and (b) daughter nuclides in the thorium series adsorbed by MnO2
filter cakes.

Hiroki NAKADA and Akihiko YOKOYAMA

65

In short, the elements adsorbed on the MnO2 filter cakes were deduced from these results to be Pb (Bi),Ra, Ac, and Th, present in small amounts of one order-of-magnitude less than the U contamination. The
MnO2 filter cake is understood to be a cation adsorbent, depending on the crystal structure of MnO2 [2,3].
For this reason, it was interesting to note which species of Pa were adsorbed by the filter cake. It wasreported that Pa and some non-metal elements adsorb on MnO2 [1] and that Pa can be separated as PaO2+
ion with a cation-exchange resin in the same nitric acid system [16] as used in this study. The crystal
structure of MnO2 in the study is unclear although an XRD spectrum was taken for the MnO2 filter cakemade in the same procedure as the present study [4]. Therefore, the chemical species of Pa relevant for
adsorption are not clarified in the study.

4

Conclusions

From E-measurement results, absorption using a MnO2 filter cake showed excellent performance inrecovery and radiochemical purity of Pa in comparison with the other methods studied. This may besomewhat unexpected because this is a conventional method that was developed many years ago. Itshould be noted, however, that many conditions for absorption by the TK400 and MnO2 resins were not
optimized in this study: it is possible that the newer methods would show better performance than the
MnO2 filter cake method under other conditions or for other purposes. Despite this, it is noteworthy thatthe conventional method using a MnO2 filter cake has excellent performance in recovery, purity, andrapidity, and costs much less than the recently developed methods discussed in this study. The MnO2
filter cake technique was originally developed for isolation of Pa-231, Pa-234g, and Pa-234m from U,
although it may be also used for nuclide production of Pa from a Th target induced by a proton beam andspallation of a U target [17]. Pa atoms adsorbed to Mn oxides can be easily collected using an anionexchange resin with 8 M HNO3 eluant [18] after dissolving the oxide with hydrogen peroxide solution.
This method can be easily scaled up because no target material should be adsorbed during the chemical
procedures.
The method is useful not only for Pa separation, but also for the determination of 238U determination
in solution, where radioactive equilibrium between 238U and 234mPa is achieved in a short time (less than
10 min) and 234mPa recovery is almost quantitative. For instance, the determination can be made using βcounting with a calibrated GM counter just after separation with a MnO2 filter cake and decay analysisof the components at about 1.2 min, as done in this study. The deduced radioactivity of 234mPa at the
separation is the same as that of 238U, which can easily be converted to a material mass, because adaughter nuclide has the same radioactivity as its parent, according to the secular equilibrium law.
It is also worth emphasizing the possibility of utilizing this chemical system for research on superheavy
elements, which need rapid chemistry because of their short half-lives. For the example of Db, a pseudohomologue of Pa, the half-lives of the 262Db (33.8 s) and 263Db (27 s) isotopes often used for such research
are comparable with that of 234mPa in this study. Furthermore, an α-source sample of a thin disc shape iseasily prepared just after the separation process, which is convenient for superheavy element experiments
mainly performed by α-spectrometry. For this reason, separation from possible alpha emitters, such as
Po, Ra, Th, and U, is important: this capability was demonstrated in this study. Homologues of Db, such
as V, Nb, and Ta, in addition to Pa, are reported to adsorb on MnO2 in weak nitric-acid solution [1], so
this method is suitable for experiments using superheavy elements in extremely low concentrations dueto the absence of radiocolloidal adsorption. Further examination, including use of MnO2 resin, isnecessary to construct an experimental system suitable for such research.

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Rapid separation method for protactinium

Acknowledgements
The authors would like to express their gratitude to the Japan Science and Technology Agency for its
financial support of this study through the Global Science Campus Program. We thank Kathryn Sole,
PhD, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

References[1] C. Bigliocca, F. Girardi, J. Pauly, E. Sabbioni, S. Meloni and A. Provasoli, Anal. Chem. 39, 1634
(1967).
[2] H.S. Posselt, F. J. Anderson and W. J. Weber, Jr., Curr. Res. 2, 1087 (1968).
[3] M. J. Grey, Effluent Water Treat. J. 21, 201 (1981).
[4] S. Hinami, Master thesis of Grad. Sch. Natur. Sci. Tech., Kanazawa Univ. (in Japanese) (2011).
[5] W. S. Moor, Deep-See Res. 23, 647 (1976).
[6] D. S. Moon, W. C. Burnett, S. Nour, P. Horwitz and A. Bond, Appl. Radiat. Isot. 59, 255 (2003).
[7] Text for Radiochem. Training Course of Expts. in Basic Chem. B, Kanazawa Univ. (in Japanese)(2012).
[8] Eichrom Technologies website, http://www.eichrom.com/eichrom/products/tru-resin/ (2018).
[9] Eichrom Technologies website, http://www.eichrom.com/eichrom/products/teva-resin/ (2018).
[10] M. Sch¦del ed., The Chemistry of Superheavy Elements, Kluwer Acad., Dordrecht (2003).
[11] Triskem international website, http://www.triskem-international.com/catalog/products/
resins-and-accessories/tk400-resin/bl,product,416,0.
[12] Eichrom Technologies website, http://www.eichrom.com/eichrom/products/mno2-resin/ (2018).
[13] J. Magill, R. Dreher and Zs. Sόti, Chart of the nuclides 10th ed. (2018).
[14] Japan Radioisotope Assoc., Radioisotope Pocket Data Book 11th ed.(2011).
[15] National Nuclear Data Center, Brookhaven National Lab., http://www.nndc.bnl.gov/ (2018).
[16] K. Kimura, U. Yokoyama, H. Sano and W. Mabuchi, Japan Analyst, 6, 637 (in Japanese) (1957).
[17] Y. Y. Chu and M. L. Zhou, IEEE Trans. Nucl. Sci., 30 (2), 1153 (1983).
[18] F. Ichikawa, S. Uruno and H. Imai, Bull. Chem. Soc. Japan, 34(7), 952 (1961).