Radioactive characteristics and leaching behavior of Ra and Th isotopes on ishikawaite Ryoken Shiobara, Kenta Komatsubara, Yuichi Kurihara, Kiyoshi Nomura & Yuya Koike Journal of Radioanalytical and Nuclear Chemistry An International Journal Dealing with All Aspects and Applications of Nuclear Chemistry ISSN 0236-5731 J Radioanal Nucl Chem DOI 10.1007/s10967-017-5325-8
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Author's personal copy J Radioanal Nucl Chem DOI 10.1007/s10967-017-5325-8
Radioactive characteristics and leaching behavior of Ra and Th isotopes on ishikawaite Ryoken Shiobara1 • Kenta Komatsubara1 • Yuichi Kurihara2 Kiyoshi Nomura3 • Yuya Koike3
•
Received: 19 February 2017 Ó Akade´miai Kiado´, Budapest, Hungary 2017
Abstract Radioactive ishikawaite was characterized and the leaching behavior of U and Th series nuclides from ishikawaite was studied. With the pH values of dilute HCl solutions, the activity ratios of 226Ra/228Ra and 230Th/232Th in the leachate decreased, whereas the activity ratios of 224 Ra/228Ra and 228Th/232Th increased. The leaching behavior of annealed ishikawaite was different from that of original ishikawaite. The leaching behavior of U and Th series nuclides is considered due to the different crystallinity and dominant U in ishikawaite as well as due to mineral damages by a-particles and recoiled atoms. Keywords Ishikawaite U mineral Leaching of Ra and Th isotopes Radioactive disequilibrium
Introduction A radioactive disequilibrium for different isotopic pairs in U and Th decay series has been studied by many researchers using various geological samples. Chu and Wang [1] reported on radioactive disequilibrium between 228 Th and 232Th in river and hot spring water. Kigoshi [2] & Kiyoshi Nomura
[email protected] 1
Applied Chemistry Course, Graduate School of Science and Technology, Meiji University, 1-1-1 Higashi-mita, Tama-ku, Kawasaki, Kanagawa 214-8571, Japan
2
Organization for the Strategic Coordination of Research and Intellectual Properties, Meiji University, 1-1-1 Higashi-mita, Tama-ku, Kawasaki, Kanagawa 214-8571, Japan
3
Department of Applied Chemistry, School of Science and Technology, Meiji University, 1-1-1 Higashi-mita, Tama-ku, Kawasaki, Kanagawa 214-8571, Japan
reported that amount of dissolved 234U in the aqueous phase from zircon is larger than the parent nuclide, 238U. It seems to be caused by a-recoiled 234Th nuclide from the surface of a solid silicate. Similar results were obtained by leaching experiments on aquifer rocks [3]. Kobashi et al. [4] observed that 228Th and 224Ra nuclides are leached more efficiently than 232Th and 228Ra, respectively. In addition to a-recoil ejection from a solid surface, it was suggested from the result of euxenite that the mobility of recoiled 228Th and 224Ra nuclides is larger than that of their parent nuclides, 232Th and 228Ra, respectively. The mobility of recoiled atoms is also one of the leaching factors. Kumar et al. [5] reported recently that activity ratio of 234U/238U in alluvial aquifers of groundwater is not equal to 1. Thus, leaching experiments have been carried out in order to investigate radioactive disequilibrium in natural radionuclides. Two leaching mechanisms have already been proposed by using Ra nuclides released into a liquid phase from a rock [6]. One is due to the chemical dissolution of Ra from the surface of a rock and another is to the damage of a rock by a-decay process from Th nuclides on the surface. It has been reported that leaching behaviors of nuclides are different among some minerals as follows: the activity ratio of 230Th/232Th in leachate from thorianite is smaller than the unity of pristine rock, but the activity ratio in leachate from uraninite is larger than the unity [7]. This suggests that the activity ratio depends on the kinds of Th and U enriched rocks. The other radioactive minerals such as monazite and euxenite have been studied by leaching experiments. The leaching behaviors of 226Ra and 230Th as decay products from 238U, and of 228Ra, 224Ra and 228Th as decay products from 232Th are different, depending on pH values (0 ^ pH ^3) of acidic solutions in contact [8, 9]. Granite, which
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contains a small amount of U and Th, has also been studied [10]. Granite shows the similar behavior as monazite. When euxenite is annealed, the activity ratio of Ra and Th isotopes in the leachate is close to that in the pristine mineral sample [9]. This fact suggests that damages due to the a-recoil nuclei in the crystal lattice are recovered by thermally annealing. The leaching behavior of 226Ra and 230 Th from euxenite was different from that of monazite and granite. In order to confirm the difference, leaching experiments with dilute HCl solutions were carried out using uranium mineral, ishikawaite, which is one of samarskite group. Ishikawaite contains a relatively large amount of U. The radioactive characteristics and leaching behavior of ishikawaite are reported here.
Table 1 analysis
Compositions of ishikawaite rock by X-ray fluorescent
Analytical valuea/mass %
a
Fe2O3
6.82 (3)
Na2O
Y2O3
2.23 (1)
MgO
0.11 (7)
ZrO2
1.31 (2)
A12O3
3.00 ( (2) 6.49 (7)
0.61 (8)
Nb2O5
22.1(6)
SiO2
Dy2O3
0.59 (8)
P2O5
0.08 (2)
Ta2O5
4.87 (5)
K2O
0.78 (4)
WO3
1.48 (5)
CaO
0.40 (2)
PbO
0.21 (3)
MnO
0.95 (3)
ThO2
0.72 (2)
The others
35.2 (16)
U3O8
12.1 (2)
Values before separation of black and white parts
Experimental Sample description Ishikawaite, one of samarskite (A3?B5?O4) group, was found in Ishikawa, Fukushima Pref., Japan. Ishikawaite was basically described as YNbO4 in 1922 by Shibata and Kimura [11]. It was redefined by Hanson et al. [12, 13] as the empirical formula of (U, Y, FeII, FeIII)(Nb, Ta, Ti)O4. The name ishikawaite is used when U and Th are dominant in samarskite. The picture and photo-stimulated luminescence image (PLI) of ishikawaite used are shown in Fig. 1a, b, respectively. The PLI was obtained using a storage film coated with photo-stimulated phosphor (BaFBr:Eu2?). The brown black color of the picture is corresponding to the black area on the contact surface of PLI plate as shown in Fig. 1. The black color is intensified due to the strong b- and c-rays so that the areas contain a lot of radioactive materials. The ishikawaite rock without separation of black and white parts was measured by a X-ray fluorescent spectrometer. The results are listed in Table 1. The composition ratios were obtained as each element oxide. It is found that the major elements are Nb, Ta, U, Fe and Y, and that the oxides are similar to the empirical formula of (U,Y, Fe) (Nb, Ta, Ti) O4 [12] although Ti is not so much included in this mineral. This
mineral contains relatively a large amount of U and a small amount of Th. The powder of pristine ishikawaite, pulverized to a grain size less than 53 lm, was measured by 57Fe Mo¨ssbauer spectrometry. It was found that paramagnetic Fe3? and Fe2? species with the content ratio of 1:3 were included. X-ray powder diffraction (XRD) pattern of the ishikawaite showed almost halo peaks and some sharp peaks as shown in Fig. 2a. After the black and white parts were separated physically, the impurities of white parts were removed by a separation method using a heavy liquid solution of sodium poly-tungstate (3Na2WO49WO3H2O) with density of 3.06 g cm-3 [14]. When the ishikawaite sample thus refined was observed by a scanning electron microscope (SEM), the average grain size found was around 28.9 lm. XRD pattern of the ishikawaite sample after removing the light impurity in a heavy liquid solution is shown in Fig. 2b. The XRD pattern showed only so remarkable halo peaks that ishikawaite can be classified as amorphous or metamict state. Radioactive characteristics of ishikawaite Ishikawaite sample was dissolved with the acid mixture of 2 mL of 23 mol L-1 HF, 6 mL of 6 mol L-1 HClO4 and
Fig. 1 a Picture and b photo-stimulated luminescence image (PLI) of ishikawaite sample
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: Before heavy liquid separation : After heavy liquid separation 300 cps Ab 2-60
Ab 130 Ab 1-1-2, 2-2-1
Zeo 020
Ab 020
Intensity / cps
Ab 040
J Radioanal Nucl Chem
(a)
(b) 5
45
85
2θ / degree Fig. 2 XRD patterns of ishikawaite samples a before and b after separation in a heavy liquid. Zeo zeolite: hydrated aluminosilicate minerals, Ab albite: NaAlSi3O8
8 mL of 16 mol L-1 HNO3 for 1 g of sample in Teflon dishes [15]. The solution was dried up and dissolved again in 20 mL of 2% H3BO3 and 20 mL of HCl. The specific activities and activity ratios of U and trace Th isotopes in the ishikawaite sample were determined by isotope dilution analysis, using 232U tracer for chemical yield determination. Determination of radioactivity on Th and U isotopes Alpha sources were measured using a passivated implanted planer Si detector (PIPS) with the active area of 900 mm2. Radioactivity concentrations of 230Th and 232Th in ishikawaite, spike standard solution and spiked ishikawaite samples are defined as a, b and (a ? b), respectively. Each activity ratio, A, B and C, of 230Th/232Th in ishikawaite, spike standard solution and spiked ishikawaite samples was obtained by measuring the peak area intensity of 230Th and 232 Th in each alpha energy spectrum. Each activity of 230 Th in ishikawaite, spike solution and spiked ishikawaite samples is expressed as aA, bB, and (a ? b)C. Therefore, aA ? bB = (a ? b)C. The unknown concentration, a, of 232Th in ishikawaite sample was determined as the following: a ¼ bðC BÞ=ðA CÞ
ð1Þ 234
238
Radioactivity concentrations of U and U were determined using a specific radioactivity of non-natural nuclide, 232U, as spiked standard solution. Determination of radioactivity on Ra isotopes Radium isotopes were measured by gamma spectrometry using a high purity germanium semiconductor (HpGe) detector as shown in Fig. 3.
Gamma photon count rate, Ca, of radionuclide a in sample is expressed as Ca ¼ ea Ica Aa : Therefore Aa ¼ Ca =ðea Ica Þ
ð2Þ
where Aa is radioactivity of nuclide a in sample, Ica is gamma photon emission rate at a certain energy (E) of nuclide a, and ea is detection efficiency of gamma photon with a certain energy. The relationship between energy (E; keV) and detection efficiency (e) in the range of 200–1500 keV was determined by measuring gamma photons of standard sealed source, 152Eu. The detection efficiency of Ge semiconductor used was calibrated by using gamma photon efficiency of 306.78 keV of 176Lu, emitting from the same amount of Lu2O3 as sample; e ¼ 2:7732E1:068 :
ð3Þ
About 4 g of the ishikawaite sample was packed into a canister of tin-can (34 mm/, 5 mmH). The canister was completely sealed with epoxy resin (AralditeÒ). The sample for c-ray measurement thus prepared was stored for 3 weeks so that 228Ac, 214Pb, and 212Pb reached the equilibrium with 228Ra, 226Ra, and 224Ra within the canister, respectively. The prepared sample was measured at the position of 6 cm above the center of the head of HpGe because the radioactivity of ishikawaite was strong. The gamma photon efficiency depends on the sample activity and is calibrated using 176Lu standard sample. The radioactivity concentrations of 228Ra, 226Ra and 224 Ra were determined by measuring the following gamma photons from daughter nuclides in the radioactive equilibrium: 911.2 keV for 228Ac (e: 0.19%), 351.9 keV for 214 Pb (e: 0.53%) and 238.6 keV for 212Pb (e: 0.80%), respectively. The determined specific activities and activity ratios of U, Th, and Ra isotopes in ishikawaite sample used are listed in Table 2. The activity ratios of 234U/238U, 228 Th/232Th and 224Ra/228Ra in the sample were constant within the limit of experimental error, indicating that their 234 U, 228Th, and 224Ra in the sample are in radioactive equilibrium with the parent nuclides, 238U, 232Th and 228 Ra, respectively. Leaching procedure of Ra isotopes Each pH value of HCl solutions was adjusted using a pH meter. The different amounts of ishikawaite sample and HCl solution were used for each pH value solution because the amounts of Ra isotopes in leachate from sample were smaller in the higher pH value solution. The solid–liquid ratio in each sample mixed solution was constant as follows. The ishikawaite sample (0.4 g) was put in 40 mL of
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Author's personal copy J Radioanal Nucl Chem Fig. 3 c-ray energy spectrum of ishikawaite sample. ANN annihilation radiation peak (511 keV). Measurement time: 10,000 s
214Bi
214Bi 214Bi 214Bi
214Bi
214Bi
214Bi 214Bi
214Bi 214 214Bi Bi 214Bi 214Bi
214Bi
214Bi 214Bi
228Ac
214 214Bi Bi
214Bi
214Bi
10-1
214Bi
100
ANN
Counting rate / cps
101
226Ra 212Pb 214Pb 214Pb
214Pb
102
10-2 10-3
0
1000
2000
Energy / keV
Table 2 Activity ratios and specific activities of uranium, thorium and radium isotopes in ishikawaite sample Elements
Activity ratioa
Specific activity/Bq g-1
Uraniuma
234
234
U
(2.94 ± 0.10) 9 103
238
U
(2.94 ± 0.10) 9 103
1.02 ± 0.02
228
Th
(6.02 ± 0.30) 9 10
50.4 ± 0.8
230
Th
(3.04 ± 0.16) 9 103
232
Th
(5.88 ± 0.29) 9 10
228
Ra
1.02 ± 0.4
224
Ra
(5.93 ± 0.10) 9 10
Ra/228Ra
50.9 ± 2.6
226
Ra
(2.96 ± 0.10) 9 103
228
Ra
(5.82 ± 0.23) 9 10
a
Thorium
U/238U
228
232
Th/
Th
230
Th/232Th
Radium
224
Ra/
226
1.00 ± 0.01
a
Activity ratios of U and Th isotopes were calculated from the area intensity of a spectral lines
dilute HCl solution with pH 0 and 1. The ishikawaite sample (1.5 g) was put in 150 mL of HCl solution with pH 2. The ishikawaite sample (2.0 g) was put in 200 mL of HCl solution with pH 3. The above mixed solutions were sometimes stirred during 15 days at 25 °C and then filtered. 25 mL aliquots of the dissolved solutions with pH 0 and 1, 50 mL of the dissolved solution with pH 2, and 100 mL of the dissolved solution with pH 3 were taken into Teflon dish, respectively. Each aliquot was concentrated on a hot plate. The concentrated solution was put into a canister of tin-can (34 mm/, 12 mmH) [17], in which 1.0 g of glucomannan (PROPOLÒA), 2.5 mL of water and 0.5 mL of 2 mol L-1 NaOH solution were added. The mixture in the canister was solidified while stirring. The canister was completely sealed with epoxy resin (AralditeÒ). Samples sealed were stored for 2 days for the enough growth of 212Pb from 224Ra. After 224Ra activity was
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measured by a gamma spectrometer. Samples were further stored for 3 weeks in order to reach the radioactive equilibrium of 214Pb/226Ra and 228Ac/228Ra, respectively. The samples were measured again by a gamma spectrometer. Leaching procedure of Th isotopes Two aliquots with 4 and 4 mL of the filtered solution (pH 0), with 2 and 4 mL of the filtered solution (pH 1), and with 20 and 40 mL of the filtered solution (pH 2) were prepared, respectively. The aliquots of the filtered solution (pH 3) could not be determined because amount of Th was very small. Each one of two aliquots was dried up and dissolved in 20 mL of 8 mol L-1 HNO3. The other Th spiked solution (88.45 ± 0.25 Bq mL-1, 230Th/232Th = 0.155 ± 0.008, 228 Th/232Th = 1.040 ± 0.026), which was determined by gravimetric analysis [10], was dried up, and dissolved in 20 mL of 8 mol L-1 HNO3. The two solutions were loaded onto 2 mL column (UTEVA Spec. resin), respectively, because only Th isotopes were eluted with 30 mL of 5 mol L-1 HCl. The Th solution was dried up. A small amount of resin included in the solution was decomposed by adding several drops of H2O2 and several mL of conc. HNO3. U remained in the decomposed solution. The remained U was further dissolved in 20 mL of 2 mol L-1 HNO3 solution. The solution was loaded into 2 mL column (TEVA Spec. resin). Th isotopes were completely eluted with 30 mL of 5 mol L-1 HCl. The solution was dried up, and dissolved with an electroplating solution [18]. Thorium isotopes electrodeposited on a stainless steel planchet were measured by an alpha spectrometer. The above leaching procedures of Ra and Th isotopes are shown in Fig. 4.
Author's personal copy J Radioanal Nucl Chem Fig. 4 Schematic drawing of leaching procedures for measurement of Ra and Th isotopes
Th isotope
Ra isotope Leaching
in HCl with pH = 0-3
Gelation of the leachate
Sealing of canister
Storing time
Leaching
glucomannan + H2O + NaOH
in HCl with pH = 0-3
UTEVA and TEVA specific resin
Separating of U isotopes
epoxy resin
Decomposition of resin
H2O2 + HNO3
2 days or 3weeks
Electrodepositing
on a stainless steel planchet
HpGe
Alpha spectrometry
PIPS
Gamma spectrometry
Results and discussion
Activity ratios of leachate to mineral itself
The ratios between the activity found in the leachate with several pH values and the activity found in the original mineral (224Ra, 228Ra, 228Th, and 232Th) are shown in Fig. 5. Each ratio decreased with the pH values in the acidic solutions. It is found that especially 224Ra is the nuclide most easily eluted from ishikawaite. Activity ratios of 224Ra/228Ra and 228Th/232Th in leachate with different pH values are shown in Fig. 6. Activity ratios of 224Ra/228Ra and 228Th/232Th in Th series nuclides are depending on pH values in HCl solutions. The observed activity ratios of 224Ra/228Ra and 228 Th/232Th in the leachate was larger than those of 2.0
10-3
1.5
10-3
1.0
10-3
0.5
10-3
Activity ratios
Activity ratios of Th series nuclides in leachate to ishikawaite
0
1
2
3
pH Fig. 6 Activity ratios of 224Ra/228Ra and 228Th/232Th in leachate from ishikawaite sample in HCl solutions with different pH values. Filled circle 224Ra/228Ra, Filled triangle 228Th/232Th, continuous line activity ratio of 224Ra/228Ra and 228Th/232Th in the ishikawaite sample (1.0)
0 0
1
2
3
pH Fig. 5 Activity ratios of Th series nuclides in leachate (pH 0–3) to ishikawaite itself. open circle 224Ra, filled circle 228Ra, open triangle 228 Th, filled triangle 232Th
ishikawaite itself. Activity ratios increased with the pH values of the acidic solutions, although 224Ra and 228Th were in radioactive equilibrium with 228Ra and 232Th in the ishikawaite mineral, respectively. Radium-224 is formed through two a-decays from 232Th, whereas 228Ra is formed through a single a-decay from 232Th. Thorium-228 is also the daughter formed through a single a-decay process from 232 Th. The nuclides formed through a-decay processes may give damages on the crystal lattice of ishikawaite by recoil process together with emitted a-particles [19, 20]. Damages in the crystal lattice with a-particle emitters may be affected easily by dilute acidic solution as compared with the lattice containing no a-particle emitters. Plural number of a-decay process may give wide damages on the lattice
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site surrounding the decay product by a single decay process [21]. It has been considered that the leaching behavior of Ra isotopes and 228Th may depend on the number of decay event which produces the respective isotopes. Thus it is easily understood that in the same elements of Th series, the daughters are easily eluted due to the number of adecay process. These ratios of 224Ra/228Ra and 228Th/232Th increased gradually with the pH values. It may be due to the prominent damage by a-decay process. Activity ratios of U series nuclides in leachate to ishikawaite Each activity ratio of U series (226Ra, and 230Th) in leachate to ishikawaite mineral is shown in Fig. 7. Radium226 and 230Th nuclei are produced through three a-decays and two a-decays from 238U nucleus, respectively. The activity in leachate was less than around 0.3 9 10-3 of the activity of ishikawaite mineral. It is found from the pH dependency of activity ratios that 226Ra nucleus is more easily eluted than 230Th nucleus. Variation in the activity ratios of 226Ra/228Ra and 230 Th/232Th in leachate with different pH values is shown in Fig. 8. The leached activity of 226Ra and 230Th (U series) from ishikawaite were larger than that of 228Ra and 232Th (Th series). The observed activity ratios of 226Ra/228Ra and 230 Th/232Th in the leachate were smaller than that of ishikawaite itself. Activity ratios decreased with the pH values of the solution. U series nuclides in the ishikawaite were exhibited as the same leaching behavior as monazite, euxenite and granite [8–10].
Fig. 8 Activity ratios of 226Ra/228Ra and 230Th/232Th in leachate from ishikawaite sample. Filled circle 226Ra, Filled triangle 230Th, continuous line activity ratio of 226Ra/228Ra and 230Th/232Th in ishikawaite mineral sample: 51
In U enriched mineral, U should be located in the original crystal lattice of minerals although the mineral became metamict with aging. Activity ratio of 230Th/232Th in leachate was smaller than that of ishikawaite mineral sample: 51. Thorium series nuclides located out of ishikawaite mineral were leached more easily than U series nuclides. It is considered because a large amount of U series nuclides are more strongly bound within ishikawaite than a low level of Th series. Amount of daughter nuclides by a-recoil ejection Range of a-recoil In a-decay processes, daughter nuclides are ejected from the initial positions of their parents by the a-recoil energy. The theoretical projected range Rp in lg/cm2 units of Uand Th- series nuclides can be estimated by using the LSSequation [19] as follows; Rp ðlg/cm2 Þ¼C1 ðlÞ A2
2=3 2=3 2=3 Zl þZ2 Er =ðZ1 Z2 Þ ð4Þ
where Z1, A1, Z2, A2 are atomic and mass numbers of recoil atoms and the constituents of the minerals, respectively, and Er is recoil energy (keV), resulting from the energy partition between recoil atom and the correlated a-particles. As the function C1(l) varies with A2/A1 (=l), the following function of C1(l) was introduced for the comprehensive evaluation [19]. Fig. 7 Activity ratios of U series nuclides in leachate (pH 0–3) to ishikawaite mineral itself. Filled circle 226Ra, Filled triangle 230Th
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0:092l1=3 ð1 þ 0:9lÞ for 0:556 [ l; and 0:018 log ðlÞ þ 0:163 for l [ 0:556:
Author's personal copy J Radioanal Nucl Chem Table 3 a-recoil range of Uand Th-series nuclides in ishikawaite
Parent nuclide
Daughter nuclide
Recoil energy/keV
Recoil range/nm
228
224
95.1
15.67
230
226
81.5
14.14
232
228
69.2
12.69
Th
Ra
Th
Ra
Th
Ra
Table 4 Comparison of calculated and measured activity concentrations of Ra isotopes in leachate from ishikawaite by a-recoil Activity concentration/lBq mL-1 Calculated value 224
Ra
459
Measured value pH 0
pH 1 3
(34.8 ± 0.8) 9 10
(34.6 ± 0.5) 9 103
226
0.391
(377 ± 2) 9 10
(287 ± 2) 9 103
228
1.91
(10.2 ± 0.1) 9 103
(8.96 ± 0.60) 9 103
Ra Ra
3
For the multicomponent minerals, the predicted ranges (Rpm) are calculated from the following equation; X xi =ðRp Þi ð5Þ 1=Rpm ¼ where xi and (Rp)i are the fraction of the total mass and the projected range of the i-th component, respectively. The a-recoil ranges were calculated by dividing Rpm with the density of ishikawaite as the composition of (U, Y, Fe, Ca)(Nb, Ta, Ti)O4 [12]. The a-recoil ranges of U- and Thseries nuclides in ishikawaite are listed in Table 3. The ranges of daughter nuclides within ishikawaite are 12.0–16.0 nm. The ratio of the recoil range is 1.1 for 226Ra to 228 Ra and 1.2 for 224Ra to 228Ra. Each ratio is consistent with the ratio of Ra isotopes in monazite and euxenite. Estimation of Ra isotopes in leachate from ishikawaite by a-recoil The concentration of Ra isotopes ejected into the leachate by a-recoil process was shown by the following calculation
Fig. 10 Activity ratios of Ra isotopes in HCl solution of pH 0 from ishikawaite sample before and after annealing. Filled square 224 Ra/228Ra, continuous line 1.2, Half Filled square 226Ra/228Ra, continuous line 51, (continuous line activity ratios of Ra isotopes in the untreated sample)
method of Kigoshi [3]: For example, the rate of supply Q of Th atoms from the solid surface to the solution is given by
234
Q ¼ ð1=4ÞLSu8 qk8
ð6Þ
where L is the a-recoil range of 234Th in the ishikawaite, S and q are the surface area and the density of the sample, respectively, u8 is the number of 238U atoms in 1 g of sample, and c8 is the decay constant of 238U. The
Fig. 9 X-ray diffraction pattern of annealed sample. UTO UNb3O10, INO NbFeO4, UYT UYO4
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(a)
(b)
(c)
(d)
Fig. 11 pH dependency on activity ratios of Ra and Th isotopes in leachate from radioactive samples. Rm: activity ratio in the radioactive mineral. Data of euxenite, monazite, and granite were cited from the references [8–10], respectively
accumulated amount A4 of 234Th in the solution after time t is given by A4 ¼ ð1=4ÞLSu8 qk8 ð1 expðk4 tÞÞ=k4 234
ð7Þ
where k4 is the decay constant of Th. According the above, the concentrations of 224Ra, 226Ra and 228Ra in 40 mL of leachate ejected only by a-recoil from a 0.4 g of the ishikawaite sample dipped during 15 days were calculated, and are compared with the specific activity of Ra isotopes leached into HCl solutions with pH 0 and 1 from the ishikawaite sample as listed in Table 4. The calculated concentrations of 224Ra, 226Ra and 228Ra in leachate from ishikawaite sample by a-recoil alone were extremely smaller than those observed in the leachate from the ishikawaite sample. The results indicate that the amount of Ra isotopes emitted directly from ishikawaite sample by a-recoil is negligibly small for the observed specific activity of Ra isotopes in the leachate from the ishikawaite sample.
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Activity ratios of 224Ra/228Ra and 226Ra/228Ra in leachate from annealed sample The black sample after separation in heavy solution was annealed at 1000 °C for 5 days in air atmosphere. XRD pattern of the annealed sample is shown in Fig. 9. The XRD pattern didn’t show so halo peaks that the annealed sample became from a metamict state to the crystal phase. However, the annealed sample is composed of each crystal of UNb3O10, NbFeO4 and UYO4, not to recover perfectly into ishikawaite like samarskite structure. Sugitani et al. [23] has obtained the recrystallized sample by heating to a temperature of 1100 °C for 12 h in a reducing atmosphere mixture of 95% Ar and 5% H2. The difference is due to the heating temperature and atmosphere. The annealed sample in this experiment is considered to be the intermediate compounds of samarskite. Variation in the activity ratios of 224Ra/228Ra and 226 Ra/228Ra in leachate from the annealed sample is shown in Fig. 10. Activity ratios of 224Ra/228Ra and 226Ra/228Ra
Author's personal copy J Radioanal Nucl Chem
in the leachate were not so close to those of the Ra isotopes in the un-annealed sample, respectively. Both activity ratios of 224Ra/228Ra and 226Ra/228Ra in leachate decreased after annealing as compared with the ratio in mineral although these ratios increased before annealing. This result shows that the leaching behavior depends on the crystallinity of minerals. The leaching behavior of the radioactive mineral is considered to be largely due to the formation of different phases and crystallinity although a-decay of a radioactive mineral can contribute to the leaching behavior because the recoil atoms and a-particle emitters give the damage of the crystal structure. Comparison of the leaching behavior of ishikawaite with other minerals The leaching behaviors of Ra and Th isotopes from euxenite, monazite, granite, and ishikawaite samples are summarized in Fig. 11. It is found from Fig. 11a 224 Ra/228Ra, b 228Th/232Th that daughter nuclides, 224Ra and 228Th, in all minerals are more easily eluted than parent nuclides of Ra and Th by numerical decay process. As shown in Fig. 11c, the leaching behavior from ishikawaite is similar to that from euxenite in activity ratio of 228 Ra/226Ra. The ratio of 228Ra/226Ra was 0.02 and 0.10 for ishikawaite and euxenite minerals, respectively. Th series nuclides are easily eluted from ishikawaite and euxenite. But Th series are not easily leached from monazite and granite. The ratio of 230Th/232Th in leachate became smaller with the pH values than for ishikawaite and euxenite. Thus the activity ratio of daughter to parent nuclides in Th series shows a little different behavior among the kinds of minerals, the crystallinity and metamict states, and rich U contents.
Conclusions Ishikawaite is metamict state, caused by a-decay process of rich U and poor Th included. Any kind of nuclides in leachate from ishikawaite decreased with the pH values in acidic solutions. However, activity ratios of leached 224Ra and 228Ra in Th series increased with the pH values in acidic solutions although activity ratios of 226Ra and 228Ra in U/Th series decreased. It is noteworthy that leaching behavior of Ra and Th isotopes from ishikawaite is similar to that from euxenite, and largely different from that of monazite and granite. Activity ratios of 228Th and 232Th in Th series increased with the pH values of acidic solutions, and radioactivity concentrations of 230Th and 232Th in U and Th series decreased. These results suggest that solubility of U series
nuclides is so smaller than that in Th series nuclides in ishikawaite. It is considered that U series nuclides are strong coupling states as mineral compounds although the crystal lattices are largely damaged because U compounds be dominant in ishikawaite. Acknowledgements Authors express thanks to Mrs. Takako Mimori, who was a former teacher in Ishikawa High School, for donation of ishikawaite mineral, to the Emeritus Professor Toshihiro Nakamura for supporting the above study, and to Dr. Kohta Nagai, Nuclear Material Control Center, Tokai Safeguards Center, for agreement for usage of the previous data [8–10].
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