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Diffusion behavior of selenite in purified bentonite
Progress in Nuclear Energy xxx (2015) 1e7
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Diffusion behavior of selenite in purified bentonite Kazuya Idemitsu*, Hikaru Kozaki, Masaru Yuhara, Tatsumi Arima, Yaohiro Inagaki Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka, Japan, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 December 2014 Received in revised form 20 July 2015 Accepted 16 August 2015 Available online xxx
Selenium (Se) is an important element for assessing the safety of high-level waste disposal. In this study, the diffusion coefficients of selenite in purified bentonite were investigated. A Japanese purified sodium bentonite, Kunipia-F, which contains approximately 99 wt% montmorillonite, was used in this experiment. Bentonite powder was compacted into cylinders (diameter, 10 mm; height, 10 mm; dry density, 0.8 e1.6 Mg m3). Each compacted bentonite specimen was inserted into an acrylic resin column and saturated with 0.01 Me1.0 M of NaCl (aq.) for 30 days. One side of the bentonite was spiked with 10 mL of tracer solution containing 0.13 M Na2SeO3 and it was kept at 10, 25, 40, or 55 C for 1e29 days. The apparent diffusion coefficients of selenite in compacted bentonite were 2.5 1011 to 1.9 1013 m2 s1 and decreased with the increase in dry density. The ionic strength showed no significant effect on the apparent diffusion coefficient of selenite for bentonite densities higher than 1.4 Mg m3. However, the apparent diffusion coefficients for 1 M NaCl were several times larger than those in 0.1 or 0.01 M NaCl for densities lower than 1.2 Mg m3. This increase in diffusion coefficient would be caused by a change in anion accessible porosity. The apparent diffusion coefficient would be explained by Archie's law and is proportional to exponentiation of porosity. It is desired to measure exact anion accessible porosity. The activation energy of selenite in purified bentonite was 20 ± 4 kJ mol1 and was similar to the activation energies of oxo-anions in free water. This also indicates that selenite diffuses in free pore water in the bentonite. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Purified bentonite Diffusion Selenite Ionic strength Activation energy
1. Introduction Bentonite clay has been selected as potential buffer material in disposal plans in many countries. Therefore, the diffusion of radioactive ions in bentonite is very important for the safety assessment of disposal systems. The bentonite currently used in each country has various smectite contents; Kunigel-V1 in Japan: 46e49 wt% (Ito et al., 1994), MX-80: 80e84 wt% (Karnland, 2010), GMZ bentonite: 75.4 wt% (Wu et al., 2014), Opalinus Clay: a few wt % but 40e80 wt% of clay minerals (Wenk et al., 2008), Slovak bentonite: ca. 60 wt% with Fe rich minerals (Galambos et al., 2011) so on. Radionuclides diffuse through pore water in smectite region. Thus it is necessary to observe the diffusion behavior in smectite. Because typical bentonites such as Kunigel-V1, MX-80 have impurities such as quartz, feldspar, calcite, illite, chlorite, pyrite so on, it is necessary to take into consideration the influence of the impurities contained in them. In this paper in order to minimize the
* Corresponding author. Tel.: þ81 92 802 3492; fax: þ81 92 802 3494. E-mail address: [email protected] (K. Idemitsu).
influence of the impurities, research focused on the diffusion phenomena in purified bentonite. Selenium-79 (79Se) is one of the key radionuclides for examining the safety of high-level waste disposal. Selenium has also chemical toxicity as known selenosis at higher intake than 0.4 mg d1 (NIH, 2009). Selenium-79 has a long half-life of 2.95 105 years and is produced as a fission product which fission yields are 1.6 108 for 235U and 3.0 107 for 239Pu, respectively (National Nuclear Data Center). The inventory of 79Se is mostly proportional to burn-up(BU), and its density reaches approximately 6 g t1 in the spent fuel of 45 GW d t1 BU (Okumura et al., 2011). In aqueous solution, Se is present in mainly its anionic form and thus is expected to be less affected by sorption than cations in the natural and engineered barriers. Selenium is redox-sensitive, and its oxidation state varies from 2 to þ6 depending on the redox conditions and the pH of the solution. The pore water in Kunigel-V1 had a pH of 8e9 (Taniguchi et al., 1998). In this pH range, selenate (SeO2 4 ) is predominant under strongly oxidizing conditions, hydroselenide (HSe) dominates under reducing conditions, and selenite (SeO2 3 ) is just predominant at the boundary between both redox conditions (Brokkins, 1988). The thermodynamic relations of
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these chemical species are as follows (Olin et al., 2005; Doi et al., 2010; Benedicto et al., 2013), 2 þ SeO2 3 þ H2 O⇔SeO4 þ 2H þ 2e
logK ¼ 28:04±0:55 (1)
þ SeðcrÞ þ 3H2 O⇔SeO2 3 þ 6H þ 4e
logK ¼ 61:15±0:33 (2)
HSe ⇔SeðcrÞ þ Hþ þ 2e 2 þ HSeO2 3 ⇔SeO3 þ H
logK ¼ 7:61±0:35
logK ¼ 8:36±0:51
(3) (4)
2 The boundary of SeO2 3 /SeO4 at pH 9 can be estimated around 300 mV from Eq. (1). The boundary of Se(cr)/SeO2 3 and HSe /Se(cr) are also estimated around 0 mV and 250 mV at pH 9 in the similar way with assumption of selenium concentration as 108 mol dm3 2 respectively. The boundary of HSeO2 3 /SeO3 is pH 8.36. However additional relation with iron should be take into consideration because ferrous would be derived from Fe overpack corrosion and precipitation of FeSe2(cr) would be predicted as most thermodynamically stable solid phase in HLW disposal in Japan (JNC, 2000). Ferrous would be also provided from pyrite as an impurity in bentonite. Thus, deep underground is under reducing environment and is especially strong educing environment by ferrous in the vicinity of repository. Because ferrous would retain selenium strongly by et al., 2001; Idemitsu et al., 2014), it would be difficult to (Se obtain diffusion coefficient under existence of ferrous in a comparatively short period. In this study, the diffusion of selenite was investigated in purified bentonite to avoid the influence of impurities. In order to cover broad conditions and to acquire the knowledge of diffusion mechanism, the salt concentration dependence and temperature dependency of diffusion were also investigated.
2. Experimental 2.1. Materials A Japanese purified sodium bentonite (Kunipia-F; Kunimine Industries Co. Ltd.) was used in this study. Kunipia-F is purified from Kunigel-V1 by elutriation and contains approximately 99 wt% of montmorillonite. Quartz was only identified as impurity. The chemical composition of Kunipia-F is shown in Table 1. The chemical formula of Kunipia-F is estimated to be (Na0.484Ca0.110K0.028) (Al3.258Mg0.532Fe0.236) (Al0.064Si7.936)O20(OH)4, with molecular weight MW ¼ 0.7433 kg mol1. The crystal density, rmm, is estimated 2.847 Mg m3 assuming monoclinic unit cell as a b c* ¼ 0.516 0.898 0.94 (nm3), where c* ¼ c sin (95 ) is the orthogonally projected c-axis, d001 is XRD (Madsen, 1998). Sodium selenite powder (Na2SeO3; Soekawa Rikagaku Co. Ltd.) was dissolved in pure water and the solution of 0.13 M was adjusted in the ambient condition. The pH and Eh of the stock solution were approximately 9 and 330 mV, respectively. Some of the stock solution was saved in a globe box that filled with Arþ5%H2.
Table 1 Chemical composition of Kunipia-F. Components
wt%
SiO2 Al2O3 Fe2O3 TiO2 MgO CaO Na2O K2O MnO P2O5 SrO CuO ZnO Cr2O3 Ignition lossa Cl SO3
62.5 22.2 2.47 0.12 2.81 0.81 1.97 0.17 0.02 0.01 0.02 0.01 0.01 0.02 6.22 0.03 0.56
a Weight loss with heating up to 800e900 C. The dehydration of constitution water occurred at 530 C in the 2-layer and 800 C in the 3-layer clay mineral (Tomita et al., 1992).
the bentonite of 0.8 103 kg m3 contacted with the NaCl solution of 1.0 M did not fully swell, the other bentonites swelled and were saturated by each contact solution after 30 days. Some specimens were saturated with 0.01 M of NaCl solution in a globe box that filled with Arþ5%H2 for comparison. The pH and Eh of the contact solutions were around 7 and 500 mV in the ambient condition and 8 to 9 and around 100 mV in globe box condition, respectively. Iron concentrations in some contact solutions were measured by atomic absorption spectrometry (AAS: Shimazu; AA-6300). Iron concentrations in all measured contact solution were less than 1 ppm. After saturation of each bentonite one side of the bentonite was spiked with 10 mL of tracer solution containing 0.13 M Na2SeO3 and it was kept at 10, 25, 40, or 55 C for 1e29 days for diffusion (Fig. 1). Most of the experiments were conducted in the ambient condition. Some experiments were also carried out in a globe box that filled with Ar þ 5%H2 from the saturation procedure for comparison. The experimental conditions are listed in Table 2. After a set time, the bentonite specimens were extracted from the columns and cut into slices 0.5e2 mm thick. The concentration profiles of Se in bentonite were determined from the measurement
2.2. Diffusion experiments Bentonite powder was compacted into cylinders with a diameter of 10 mm and a height of 10 mm with a dry density of around 0.8 to 1.6 103 kg m3. Each cylinder was inserted into an acrylic resin column and saturated with a NaCl solution with a concentration of 0.01e1.0 M for 30 days in the ambient condition. Though
Fig. 1. Schematic of apparatus for diffusion experiments.
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Table 2 Summary of Da values for selenite obtained and experimental conditions. NaCl conc. (M)
Dry density (mg m3)
εtot
0.01
0.80 0.82 1.00 1.03 1.23 1.25 1.41 1.39 1.47 0.81 0.81 1.04 1.03 1.24 1.39 1.46 1.03 1.03 1.02 1.23 1.46 1.60 1.02 1.00 1.01 1.01 1.02 1.04 1.03 1.01 1.03 1.02 1.04 1.04 1.05 1.04 1.05 1.03 1.03 1.02 1.05 1.07
0.72 0.71 0.65 0.64 0.57 0.56 0.50 0.51 0.48 0.72 0.72 0.63 0.64 0.56 0.51 0.49 0.64 0.64 0.64 0.57 0.49 0.44 0.64 0.65 0.65 0.65 0.64 0.63 0.64 0.65 0.64 0.64 0.63 0.63 0.63 0.63 0.63 0.64 0.64 0.64 0.63 0.62
0.1
1.0
0.01
0.1
0.5
1.0
a b c
a
()
εaa
b
()
0.43 0.42 0.29 0.27 0.13 0.11 0.06 0.07 0.06 0.43 0.43 0.26 0.27 0.12 0.07 0.06 0.39 0.39 0.40 0.28 0.14 0.06 e e e e e e e e e e e e e e e e e e e e
Temp. ( C)
Diffusion period (d)
Apparent diffusion coefficient (m2 s1)
25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 10 25 40 55 10 25 25 40 55 10 25 40 50 55 10 25 25 25 40 50
1 1 7 7 14 14 20 21 29 1 1 7 14 14 21 29 2 3 7 14 20 28 10 7 5 5 10 7 14 5 5 7 7 2 1.5 1.5 5 2 3 7 1 1.5
(1.8 (1.6 (4.2 (3.6 (1.3 (1.1 (4.0 (4.0 (3.3 (2.5 (1.4 (4.6 (4.6 (1.1 (4.5 (3.8 (1.6 (1.5 (1.3 (2.2 (3.5 (1.9 (3.7 (4.2 (6.9 (1.2 (3.4 (4.6 (4.6 (6.9 (1.1 (3.4 (9.1 (1.1 (2.1 (1.4 (7.1 (1.3 (1.6 (1.5 (2.4 (2.7
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.1) 0.1) 0.2) 0.2) 0.1) 0.1) 0.2) 0.2) 0.2) 0.2) 0.1) 0.2) 0.4) 0.1) 0.2) 0.2) 0.1) 0.1) 0.2) 0.2) 0.5) 0.1) 0.3) 0.2) 0.3) 0.1) 0.3) 0.2) 0.4) 0.3) 0.1) 0.2) 0.7) 0.1) 0.1) 0.1) 0.5) 0.1) 0.1) 0.1) 0.2) 0.1)
1011 1011c 1012 1012c 1012 1012c 1013 1013c 1013 1011 1011 1012 1012 1012 1013 1013 1011 1011 1011 1012 1013 1013 1012 1012 1012 1011 1012 1012 1012 1012 1011 1012 1012 1011 1011 1011 1012 1011 1011 1011 1011 1011
Total porosities were calculated from dry densities and theoretical density as shown in text. Anion accessible porosities were estimated by anion-free interlayer model. Experiments were carried out in glove box filled with Arþ5%H2.
of the quantities in each slice and its weight. Each slice had weight measured then was submerged in 1 N HNO3 solution (Ultra Pure, Kanto Kagaku Co. Ltd.) to extract Se. After the liquid phase was separated by centrifugation (2000 g, 5 min), the supernatant was collected to measure the concentrations of Se by inductively coupled plasma-mass spectrometry (ICP-MS; Agilent, 7500C). The recovery rate of the Se obtained by the preliminary examination by this method was 90% or more.
version 2.5 (Rigaku). Two samples were prepared for XANES spectra measurement. Approximately 50 mg of Kunipia-F was mixed with 10 mL of tracer solution containing 0.13 M Na2SeO3 and was sealed in vinyl plastic in the ambient condition two weeks before the measurement. The other sample was prepared as same procedure but in a globe box that filled with Arþ5%H2. FeSe, elemental Se, SeO2, H2SeO3, Na2SeO3, and H2SeO4 powders were also prepared as reference samples.
2.3. X-ray absorption near-edge structure measurements (XANES) 3. Results and discussion XANES measurements of Se were carried out at the BL-11 beamline of SAGA Light Source in order to confirm oxidation state of Se. A Si(111) double-crystal monochromator was used, and the beam was focused by using bent conical mirrors coated with Rh. At the measurement position, the beam had a width of 1e5 mm and a height of 1 mm. XANES spectra were acquired in fluorescence mode by using a silicon drift detector (SDD). In this mode, the electrical signal of Se Ka X-rays (11,222.4 eV) from the SDD was selected by means of a single-channel analyzer at the K-edge. Data analysis, including background subtraction, normalization, and linear combination fitting of XANES spectra, was performed with REX2000
3.1. Oxidation state of Se measured The results of the Se K-edge (12,652 eV) XANES measurements for FeSe, Se, SeO2, H2SeO3, Na2SeO3, H2SeO4, and the samples are shown in Fig. 2. The background was subtracted from the original spectra by extrapolation of the linear absorption or curve (as defined by the Victoreen equation) from the pre-edge region. From Fig. 2, the peaks of the samples prepared in the ambient condition and in the globe box are in close agreement with the peaks of SeO2, H2SeO3 and Na2SeO3. It means that selenium
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as following equation;
M x2 : Cðx; tÞ ¼ pffiffiffiffiffiffiffiffiffiffiffi exp 4Da t pDa t
Fig. 2. Se K-Edge XANES spectra of Se, FeSe, SeO2, H2SeO3, Na2SeO3, H2SeO4, and the mixtures of Kunipia-F and Na2SeO3 in the ambient condition and in the globe box filled with Ar-5%H2. Oxidation state of selenium used in this study was shown as selenite.
remains tetravalent under these experimental conditions. If the chemical equilibrium is reached, the oxidation states of selenium in the samples should follow the Eqs. (1)e(3) or Se(VI) for ambient condition and Se(0) for in the globe box respectively. However, the valence of selenium was still kept in the state of Se(IV) in each condition. It could be considered that oxidationereduction reaction did not occur during the experimental period because of the late oxidationereduction reaction of selenium. 3.2. Apparent diffusion coefficients for selenite in purified bentonite One-dimensional non-steady diffusion is derived from Fick's second law by the following equation;
ε
vC v2 C vC ¼ De 2 rd Kd : vt vt vx
(5)
In the equation t (s) is the time, C (mol m3) the tracer concentration in the mobile phase, ε () the connected porosity of the bentonite, rd (Mg m3) the dry density of the bentonite, Kd (m3 Mg1) the linear sorption equilibrium distribution coefficient, De (m2 s1) the effective diffusion coefficient and x (m) the position respectively. The left side of the equation means time dependence of the tracer concentration in liquid phase and the first and second terms of the right side mean mass balance of tracer by diffusion and removal of tracer from the liquid phase to solid phase by sorption. This equation is rewritten as follows;
ðε þ rd Kd Þ
vC vC v2 C ¼a ¼ De 2 ; vt vt vx
vC De v2 C v2 C ¼ ¼ Da 2 2 vt a vx vx
(6)
(7)
where, Da (m2 s1) is the apparent diffusion coefficient and a () is capacity factor respectively. Supposing one-dimensional diffusion with the instantaneous plane source and a limited amount of diffusing substance in a cylinder of infinite length, the analytical solution of Eq. (7) is written
(8)
where M (mol) is the total amount of tracer and x is the distance from source respectively. The concentration (mmol g1 of bentonite, aC/1000rd) profiles of the selenite diffused into the bentonite specimens are shown as a function of x2/t (m2 s1) in Fig. 3. The profiles obtained in the diffusion experiments were in close agreement with the solution. The profiles for the 0.01 and 0.1 M NaCl contact solutions had almost the same slopes at the same dry density. However, the slopes were smaller for dry densities lower than 1.2 Mg m3 for the 1.0 M contact solution. Fitting the measured profiles yielded the apparent diffusion coefficient as shown in Table 2. The apparent diffusion coefficients of selenite in compacted bentonite at 25 C were 2.5 1011 to 1.9 1013 m2 s1 and are plotted as a function of dry density in Fig. 4. The difference was not observed by the experimental result performed in the ambient condition and glove box as shown in Table 2. The apparent diffusion coefficients of selenite decreased with increased dry density. The effect of the ionic strength of the contact solution on the apparent diffusion coefficient of selenite was negligible for densities higher than 1.4 Mg m3. However, the apparent diffusion coefficients for 1.0 M NaCl were 2 or 3 times larger than those for 0.1 or 0.01 M NaCl for densities lower than 1.2 Mg m3. These apparent diffusion coefficients are 1e2 orders of magnitude smaller than data in the literature such as 4.6 1010 to 3.2 1011 in 0.4e1.8 Mg m3 of Kunigel-V1 by in-diffusion method (Sato et al., 1995) and 7.8 1011 to 7.0 1012 in 1.3e1.8 Mg m3 of GMZ bentonite by through-diffusion method (Wu et al., 2014). It seems that this difference is based on the difference in the content of montmorillonite. The apparent diffusion coefficients at the same dry density decrease with the increase in the content of montmorillonite. This is because the increase in the content of montmorillonite decreases porosity for diffusion. Furthermore, effective porosity for diffusion becomes small by anion exclusion of the negatively charged montmorillonite (e.g. Drever, 1988). Uneven distribution of montmorillonite might also influence the diffusion coefficient of anions when specimen contains impurities other than montmorillonite. 3.3. Dependence of apparent diffusion coefficients for selenite in bentonite on dry density and ionic strength The microstructure of clay affects the diffusion behavior. The apparent diffusion coefficient is expressed as follows (e.g. Sato et al., 1995),
Da ¼
De ε d ¼ D a t2 0 a
(9)
Where, d () is constrictivity, t () is tortuosity and D0 (m2 s1) is the ionic diffusion coefficient of selenite in free water. Capacity factor, a, is nearly equal porosity because distribution coefficient, Kd, for selenite on montmorillonite is negligibly small. Though distribution coefficients reported were 0.14e0.18 (Mg m3) for GMZ bentonite (Wu et al., 2014) and 1 to 2 (Mg m3) for Kunigel-V1 (Shibutani et al., 1994), selenite might sorb not in montmorillonite but impurities such as pyrite. The second fractional number of the equation, d/t2, is formation factor and means constriction and detour of the diffusion path. If the formation factor is unity, pore can be denoted by cylindrical geometry parallel to a diffusion direction. The pore in actual montmorillonite is having complicated structure, and the formation factor is much smaller than 1.
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Fig. 4. Apparent diffusion coefficients of selenite at 25 C in compacted bentonite as a function of dry density. The interlayer of montmorillonite consists of 3-water layer (d001 ¼ 1.88 nm) and 2-water layer (d001 ¼ 1.56 nm) with transition occurring between 1.3 and 1.6 Mg m3 of dry density in contact with distilled water (Kozaki et al., 1998; Kozaki, 2003). In the higher ionic strength than 0.5 M NaCl 2-water layer appeared even in lower dry density than 1.3 Mg m3 (Kozaki et al., 2008).
clay (Iversen and Jørgensen, 1993). Porosities of the specimens used in this study are listed in Table 2. The total porosities were calculated by the following equation,
ε¼1
rd ; rmm
(11)
where rmm (Mg m3) is theoretical density of montmorillonite, 2.847 (Mg m3). The apparent diffusion coefficients of selenite obtained in this study are plotted as a function of total porosity in Fig. 5. The ionic diffusion coefficient in free water is also plotted at porosity of 1 in Fig. 5. The ionic diffusion coefficient in free water of selenite, D0, was 9.8 1010 (m2 s1) calculated from equivalent conductance at infinite dilution, l0; 73.9 104 (S m2 eq1) (Valaev and Georgieva, 2004) by Nernst expression (e.g. Robinson and Stokes, 1959),
Fig. 3. Concentration profiles of Se infiltrated in bentonite specimens. Lines in the figures represent the results of the fitting by Eq. (8) and slopes of the lines mean reciprocal of apparent diffusion coefficients.
The relation between t and ε was proposed as follows (Archie, 1942), 2
t ¼ε
1n
;
(10)
where, n is an adjustable parameter and in the range of 2.5e5.4 for
Fig. 5. Apparent diffusion coefficients of selenite at 25 C in compacted bentonite as a function of total porosity, εtot, or anion accessible porosity, εaa. The ionic diffusion coefficient of selenite in free water, 9.8 1010 m2 s1, is also plotted at porosity of 1. Porosities were calculated from dry densities, theoretical density and interlayer water ratio as shown in text. “n” shows adjustable parameter of Archie's law.
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D0 ¼
RTl0 ; F 2 jzj
(12)
where R is gas constant, F is Faraday constant and jzj is the absolute value of the valence of the ion. The dashed line in the Fig. 5 is the result of fitting the apparent diffusion coefficients obtained in Kunipia-F contact with the ionic strength 0.01 M and 0.1 M of NaCl and the ionic diffusion coefficient in free water, D0, by Eq. (10), assuming d to be 1. The adjustable parameter, n, is obtained as 12.2 ± 0.2 and much larger than that for clay in the range of 2.5e5.4 (Iversen and Jørgensen, 1993). The obtained n value of 12.2 is based on the assumption that capacity factor, a, equal porosity and constrictivity, d, is unity. Though distribution coefficient of selenite on montmorillonite is negligible, retardation could occur by dead end pore that increases with the rise of density. The effect of dead end pore might be included in the n value. Concerning constrictivity, it is necessary to take into consideration anion exclusion. Water in pore space is classified into 3 types as free pore water, electrostatic double layer (EDL) water and interlayer water (Bourge, 2004, Bradbury and Baeyens, 2003, Kozaki et al., 2008). Anions would exist in free pore water and be excluded almost fully from interlayer and partially from EDL by negatively charged montmorillonite surface, so-called anion exclusion. There are some models for anion exclusion (Van Loon et al., 2007; Tournassat and Appelo, 2011) and anion exclusion in EDL are calculated as a function of dry density and ionic strength. In this study anion accessible porosity, εaa, was estimated by taking into account the interlayer water but omitting EDL because there are negligible difference between the results in 0.01 M and 0.1 M of ionic strength. The interlayer porosity was calculated assuming 3-water layer (d001 ¼ 1.88 nm) and 2-water layer (d001 ¼ 1.56 nm) with transition occurring between 1.3 and 1.6 Mg m3 of dry density in contact with distilled water (Kozaki et al., 1998; Kozaki, 2003). According the model water ratio for 3and 2-water layer, Wi, are 0.357 and 0.238 Mg of H2O/Mg of Montmorillonite respectively (Pusch et al., 1990). Anion accessible porosity, εaa, was calculated as follows,
εaa ¼ εtot
X
xi Wi rd ;
(13)
x3 þ x2 ¼ 1:
x3 ¼ 1;
x2 ¼
3.4. Activation energy of the apparent diffusion coefficient of selenite in compacted bentonite The activation energy of the diffusion was derived from the temperature dependence of the apparent diffusion coefficient measured at different salinities. Arrhenius law are shown as bellows,
E ; D0 ¼ Di exp RT
(14)
where Di is intrinsic diffusion coefficient, E is activation energy respectively. The temperature dependence of the apparent diffusion coefficients are shown in Fig. 6 as Arrhenius plots at a dry density of 1.0 Mg m3 in contact solutions of each ionic strength. The activation energy for the diffusion was calculated by the least squares method. The activation energies obtained were similar values of 20 ± 4 kJ mol1. Therefore, the activation energy was not affected by the ionic strength of the contact solution. The activation energies of other chemical species are shown in Table 3 for comparison. The activation energy of the selenite (SeO2 3 ) obtained in this study is similar to the activation energies of 2 diffusion of oxo-anions, such as selenate (SeO2 4 ), sulphate (SO4 ), and iodate (IO ) in free water, and differ a little from that of chlo3 ride (Cl) and iodide (I) anions, and the cesium cation (Csþ). 4. Conclusion The apparent diffusion coefficient of selenite in purified bentonite, Kunipia-F, was found to be in the range of 2.5 1011 to 1.9 1013 m2 s1 in 0.8e1.6 Mg m3 of dry density and decreased with increasing bentonite density. The effect of the ionic strength on the apparent diffusion coefficient of selenite was negligible for densities higher than 1.4 Mg m3. However, the apparent diffusion coefficients in 1.0 M NaCl were several times larger than the coefficients in 0.1 or 0.01 M NaCl for densities lower than 1.2 Mg m3. This increase in diffusion coefficient would be caused by a change in anion accessible porosity. The apparent diffusion coefficient would be explained by Archie's law and is proportional to exponentiation of porosity. It is desired to measure exact anion accessible porosity. The activation energy of selenite in purified bentonite was 20 ± 4 kJ mol1 and was similar to the activation energies of oxo-
x2 ¼ 0 rd 1:3 Mg m3 :
rd 1:3 1:3 rd 1:6 Mg m3 0:3
where subscript i represents 3- or 2-water layer, xi is fraction of each layer. In the higher ionic strength than 0.5 M NaCl 2-water layer appeared even in lower dry density than 1.3 Mg m3 (Kozaki et al., 2008), so x2 is set as 1. Calculated anion accessible porosities are listed in Table 2. The apparent diffusion coefficients of selenite obtained in this study are also plotted as a function of anion accessible porosity in Fig. 5. The data in high ionic strength shows good coincidence with other data in low dry density or large anion accessible porosity. The solid line in the Fig. 5 is the result of fitting the apparent diffusion coefficient by Eq. (10). The adjustable parameter, n, is obtained as 5.7 ± 0.1 and close to those for clay in the range of 2.5e5.4 (Iversen and Jørgensen, 1993). However the fitting is not good agreement in low anion accessible porosity. It is desired to measure exact anion accessible porosity.
Fig. 6. Temperature dependence of the apparent diffusion coefficients of selenite in compacted bentonite at a dry density of 1.0 Mg m3 for contact solutions of various ionic strengths. E means activation energy of diffusion.
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7
Table 3 Activation energy of diffusion coefficients of various anions in free water and compacted bentonite. Chemical species
Activation energy kJ mol1
Remark
SeO2 3 SeO2 4 2 SO4 IO3 I Cl Csþ
20 ± 4 23.2 20.7 20.1 18.2 17.4 16.2
In montmorillonite (This paper) Li and Gregory, 1974 Calculated from ionic conductivity (Marcus, 1997) Ibid. Ibid. (Parsons, 1959) Ibid.
anions in free water. The selenite could diffuse mainly through the free pore water in bentonite. Acknowledgments This research is partly financed by a Grant-in-Aid for Scientific Research (grant number S24226021). Authors are indebted to Dr. Midori Watanabe, the Center of Advanced Instrumental Analysis, Kyushu University for the measurement of selenium concentration with ICP-MS. XANES measurements were performed with the approval of SAGA Light Source (Proposal No. 1304023R). References Archie, G.E., 1942. The electrical resistivity log as an aid in determining some reservoir characteristics. Trans. Am. Mech. Eng. 146, 54e67. Benedicto, A., Missana, T., Degueldre, C., 2013. Predictions of TiO2-driven migration of Se(IV) based on an integrated study of TiO2 colloid stability and Se(IV) surface adsorption. Sci. Total Environ. 449, 214e222. Brokkins, D.G., 1988. Eh-pH Diagrams for Geochemistry. Springer-Verlag, Berlin. Bourge, I.C., 2004. Tracer Diffusion of Water and Inorganic Ions in Compacted Saturated Sodium Bentonite. University of California, Berkeley. Bradbury, M.H., Baeyens, B., 2003. Power water chemistry in compacted resaturated MX-80 bentonite. J. Contam. Hydrol. 61, 329e338. Doi, R., Tachikawa, H., Yui, M., 2010. Transformation of selenium solid phase in the presence of iron under reducing conditions. J. Nucl. Sci. Tech. 47 (3), 278e285. Drever, J.I., 1988. The Geochemistry of Natural Waters, second ed. Prentice Hall, Englewood Cliffs. Ch4. , O., Kuf kova , J., Rajec, P., 2011. Utilization of Slovak Galambos, M., Rosskopfova ca bentonites in deposition of high-level radioactive waste and spent nuclear fuel. J. Radioanal. Nucl. Chem. 288 (3), 765e777. Idemitsu, K., Kozaki, H., Akiyama, D., Kishimoto, M., Yuhara, M., Maeda, N., Inagaki, Y., Arima, T., 2014. Migration behavior of selenium in the presence of iron in bentonite. Sci. Basis Nucl. Waste Manag. XXXVII (Mater. Res. Soc. Proc. 1665) 157e163. Ito, M., Okamoto, M., Shibuta, M., Sasaki, Y., 1994. Mineral Composition Analysis of Bentonite. PNC Tech. Rep. PNC, Tokai (in Japanese). PNC TN8430 93e003. Iversen, N., Jørgensen, B.B., 1993. Diffusion coefficients of sulfate and methane in marine sediments: influence of porosity. Geochim. Cosmochim. Acta 57, 571e578. Japan Nuclear Cycle Development Institute (JNC), 2000. Second Progress Report on Research and Development for the Geological Disposal of HLW in Japan, H12: Project to Establish the Scientific and Technical Basis Foe HLW Disposal in Japan, JNC Tech. Rep. JNC TN1410 2000e001. Karnland, O., 2010. Chemical and Mineralogical Characterization of the Bentonite Buffer for the Acceptance Control Procedure in a KBS-3 Repository. SKB TR10e60. Kozaki, T., Saito, N., Fujishima, A., Sato, S., Ohashi, H., 1998. Activation energy for diffusion of chloride ions in compacted sodium montmorillonite. J. Contam. Hydrol. 35, 67e75. Kozaki, T., 2003. Effects of Microstructure of Clay on Diffusion Behavior of Radionuclides in Buffer Materials. JNC TJ8400 2002-053, pp. 74e80 (in Japanese). Kozaki, T., Liu, J.H., Sato, S., 2008. Diffusion mechanism of sodium ions in compacted montmorillonite under different NaCl concentration. Phys. Chem. Earth 33,
957e961. Li, Y.-H., Gregory, S., 1974. 1974. Diffusion of ions in sea water and in deep-sea sediments. Geochim. Cosmochim. Acta 38, 703e714. Madsen, F.T., 1998. Clay mineralogical investigations related to nuclear waste disposal. Clay Min. 33, 109e129. Marcus, Y., 1997. Ion Properties. Marcus Dekker Inc, p. 81. Ch.3. National Institutes of Health, 2009. Dietary Supplement Fact Sheet: Selenium. Retrieved 2009-01-05. National Nuclear Data Center, Brookhaven national laboratory. Information extracted from the NuDat 2.6 database, http://www.nndc.bnl.gov/nudat2/ chartNuc.jsp. Okumura, K., Asai, S., Hanzawa, Y., Suzuki, H., Toshimitsu, M., Inagawa, J., Okamoto, T., Shinohara, N., Kaneko, S., Suzuki, K., 2011. Analyses of assay data of LWR spent nuclear fuels with a continuous-energy Monte Carlo Code MVP and JENDL-4.0 for inventory estimation of 79Se, 99Tc, 126Sn and 135Cs. Prog. Nucl. Sci. Technol. 2, 369e374. € €ng, B., Ohman, n, E., 2005. Chemical TherOlin, A., Nola L.-O., Osadchii, E.G., Rose modynamics of Selenium. OECD Nuclear Energy Agency, ELSEVIER, The Netherlands. Parsons, R., 1959. Handbook of Electrochemical Constants. Butterworths Sci. Publ., London, p. 79. €kmark, H., 1990. GMM e a General Microstructural Model Pusch, R., Karnland, O., Ho for Qualitative and Quantitative Studies of Smectite Clays. SKB Technical Report 90e43. Swedish Nuclear Fuel and Waste Management Corporation, Stockholm, Sweden. Robinson, R.A., Stokes, R.H., 1959. In: Electrolyte Solutions, the Measurement and Interpretation of Conductance, Chemical Potential and Diffusion in Solutions of Simple Electrolytes. London Butter Worths. Sato, H., Yui, M., Yoshikawa, H., 1995. Diffusion behavior for Se and Zr in sodiumbentonite. Sci. Basis Nucl. Waste Manag. XVIII (Mater. Res. Soc. Proc. 353) 269e276. Shibutani, T., Yui, M., Yoshikawa, H., 1994. Sorption mechanism of Pu, Am and Se on sodium-bentonite. Sci. Basis Nucl. Waste Manag. XVII (Mater. Res. Soc. Proc. 333) 725e730. by, F., Point-Gautier, M., Giffaut, E., Borge, G., Donard, O.F.X., 2001. A critical reSe view of thermodynamic data for selenium species at 25 C. Chem. Geol. 171, 173e194. Taniguchi, N., Honda, A., Ishikawa, H., 1998. Experimental investigation of passivation behavior and corrosion rate of carbon steel in compacted bentonite. Sci. Basis Nucl. Waste Manag. XXI (Mater. Res. Soc. Proc. 506) 495e501. Tomita, T., Tanabe, K., Yamamoto, K., 1992. Determination of clay minerals by ignition loss methods with a muffle furnace. Dobokugakkai ronnbunnsyuu 445/ III-18, 55e63 (in Japanese). Tournassat, C., Appelo, C.A.J., 2011. Modelling approaches for anion-exclusion in in compacted Na-bentonite. Geochim. Cosmochim. Acta 75, 3698e3710. Valaev, L.T., Georgieva, V.G., 2004. Activation energy for electroconduction of aqueous solutions of sulfuric and selenic acids and potassium tellurate. Russ. J. Electrochem. 40 (6), 674e678. Van Loon, L.R., Glaus, M.A., Müller, W., 2007. Anion exclusion effects in compacted bentonites: towards a better understanding of anion diffusion. Appl. Geochem. 22, 2536e2552. Wenk, H.-R., Voltolini, M., Mazurek, M., Van Loon, L.R., Vinsot, A., 2008. Preferred orientation and anisotropy in shales:Callovo-oxfordian shake(France) and Opalinus clay(Switzerland). Clay Clay Minerals 56 (3), 285e306. Wu, T., Wang, H., Zheng, Q., Zhao, Y.L., Van Loon, L.R., 2014. Diffusion behavior of Se(IV) and Re(VII) in GMZ bentonite. Appl. Clay Sci. 101, 136e140.
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Diffusion behavior of selenite in purified bentonite Kazuya Idemitsu*, Hikaru Kozaki, Masaru Yuhara, Tatsumi Arima, Yaohiro Inagaki Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka, Japan, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 December 2014 Received in revised form 20 July 2015 Accepted 16 August 2015 Available online xxx
Selenium (Se) is an important element for assessing the safety of high-level waste disposal. In this study, the diffusion coefficients of selenite in purified bentonite were investigated. A Japanese purified sodium bentonite, Kunipia-F, which contains approximately 99 wt% montmorillonite, was used in this experiment. Bentonite powder was compacted into cylinders (diameter, 10 mm; height, 10 mm; dry density, 0.8 e1.6 Mg m3). Each compacted bentonite specimen was inserted into an acrylic resin column and saturated with 0.01 Me1.0 M of NaCl (aq.) for 30 days. One side of the bentonite was spiked with 10 mL of tracer solution containing 0.13 M Na2SeO3 and it was kept at 10, 25, 40, or 55 C for 1e29 days. The apparent diffusion coefficients of selenite in compacted bentonite were 2.5 1011 to 1.9 1013 m2 s1 and decreased with the increase in dry density. The ionic strength showed no significant effect on the apparent diffusion coefficient of selenite for bentonite densities higher than 1.4 Mg m3. However, the apparent diffusion coefficients for 1 M NaCl were several times larger than those in 0.1 or 0.01 M NaCl for densities lower than 1.2 Mg m3. This increase in diffusion coefficient would be caused by a change in anion accessible porosity. The apparent diffusion coefficient would be explained by Archie's law and is proportional to exponentiation of porosity. It is desired to measure exact anion accessible porosity. The activation energy of selenite in purified bentonite was 20 ± 4 kJ mol1 and was similar to the activation energies of oxo-anions in free water. This also indicates that selenite diffuses in free pore water in the bentonite. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Purified bentonite Diffusion Selenite Ionic strength Activation energy
1. Introduction Bentonite clay has been selected as potential buffer material in disposal plans in many countries. Therefore, the diffusion of radioactive ions in bentonite is very important for the safety assessment of disposal systems. The bentonite currently used in each country has various smectite contents; Kunigel-V1 in Japan: 46e49 wt% (Ito et al., 1994), MX-80: 80e84 wt% (Karnland, 2010), GMZ bentonite: 75.4 wt% (Wu et al., 2014), Opalinus Clay: a few wt % but 40e80 wt% of clay minerals (Wenk et al., 2008), Slovak bentonite: ca. 60 wt% with Fe rich minerals (Galambos et al., 2011) so on. Radionuclides diffuse through pore water in smectite region. Thus it is necessary to observe the diffusion behavior in smectite. Because typical bentonites such as Kunigel-V1, MX-80 have impurities such as quartz, feldspar, calcite, illite, chlorite, pyrite so on, it is necessary to take into consideration the influence of the impurities contained in them. In this paper in order to minimize the
* Corresponding author. Tel.: þ81 92 802 3492; fax: þ81 92 802 3494. E-mail address: [email protected] (K. Idemitsu).
influence of the impurities, research focused on the diffusion phenomena in purified bentonite. Selenium-79 (79Se) is one of the key radionuclides for examining the safety of high-level waste disposal. Selenium has also chemical toxicity as known selenosis at higher intake than 0.4 mg d1 (NIH, 2009). Selenium-79 has a long half-life of 2.95 105 years and is produced as a fission product which fission yields are 1.6 108 for 235U and 3.0 107 for 239Pu, respectively (National Nuclear Data Center). The inventory of 79Se is mostly proportional to burn-up(BU), and its density reaches approximately 6 g t1 in the spent fuel of 45 GW d t1 BU (Okumura et al., 2011). In aqueous solution, Se is present in mainly its anionic form and thus is expected to be less affected by sorption than cations in the natural and engineered barriers. Selenium is redox-sensitive, and its oxidation state varies from 2 to þ6 depending on the redox conditions and the pH of the solution. The pore water in Kunigel-V1 had a pH of 8e9 (Taniguchi et al., 1998). In this pH range, selenate (SeO2 4 ) is predominant under strongly oxidizing conditions, hydroselenide (HSe) dominates under reducing conditions, and selenite (SeO2 3 ) is just predominant at the boundary between both redox conditions (Brokkins, 1988). The thermodynamic relations of
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K. Idemitsu et al. / Progress in Nuclear Energy xxx (2015) 1e7
these chemical species are as follows (Olin et al., 2005; Doi et al., 2010; Benedicto et al., 2013), 2 þ SeO2 3 þ H2 O⇔SeO4 þ 2H þ 2e
logK ¼ 28:04±0:55 (1)
þ SeðcrÞ þ 3H2 O⇔SeO2 3 þ 6H þ 4e
logK ¼ 61:15±0:33 (2)
HSe ⇔SeðcrÞ þ Hþ þ 2e 2 þ HSeO2 3 ⇔SeO3 þ H
logK ¼ 7:61±0:35
logK ¼ 8:36±0:51
(3) (4)
2 The boundary of SeO2 3 /SeO4 at pH 9 can be estimated around 300 mV from Eq. (1). The boundary of Se(cr)/SeO2 3 and HSe /Se(cr) are also estimated around 0 mV and 250 mV at pH 9 in the similar way with assumption of selenium concentration as 108 mol dm3 2 respectively. The boundary of HSeO2 3 /SeO3 is pH 8.36. However additional relation with iron should be take into consideration because ferrous would be derived from Fe overpack corrosion and precipitation of FeSe2(cr) would be predicted as most thermodynamically stable solid phase in HLW disposal in Japan (JNC, 2000). Ferrous would be also provided from pyrite as an impurity in bentonite. Thus, deep underground is under reducing environment and is especially strong educing environment by ferrous in the vicinity of repository. Because ferrous would retain selenium strongly by et al., 2001; Idemitsu et al., 2014), it would be difficult to (Se obtain diffusion coefficient under existence of ferrous in a comparatively short period. In this study, the diffusion of selenite was investigated in purified bentonite to avoid the influence of impurities. In order to cover broad conditions and to acquire the knowledge of diffusion mechanism, the salt concentration dependence and temperature dependency of diffusion were also investigated.
2. Experimental 2.1. Materials A Japanese purified sodium bentonite (Kunipia-F; Kunimine Industries Co. Ltd.) was used in this study. Kunipia-F is purified from Kunigel-V1 by elutriation and contains approximately 99 wt% of montmorillonite. Quartz was only identified as impurity. The chemical composition of Kunipia-F is shown in Table 1. The chemical formula of Kunipia-F is estimated to be (Na0.484Ca0.110K0.028) (Al3.258Mg0.532Fe0.236) (Al0.064Si7.936)O20(OH)4, with molecular weight MW ¼ 0.7433 kg mol1. The crystal density, rmm, is estimated 2.847 Mg m3 assuming monoclinic unit cell as a b c* ¼ 0.516 0.898 0.94 (nm3), where c* ¼ c sin (95 ) is the orthogonally projected c-axis, d001 is XRD (Madsen, 1998). Sodium selenite powder (Na2SeO3; Soekawa Rikagaku Co. Ltd.) was dissolved in pure water and the solution of 0.13 M was adjusted in the ambient condition. The pH and Eh of the stock solution were approximately 9 and 330 mV, respectively. Some of the stock solution was saved in a globe box that filled with Arþ5%H2.
Table 1 Chemical composition of Kunipia-F. Components
wt%
SiO2 Al2O3 Fe2O3 TiO2 MgO CaO Na2O K2O MnO P2O5 SrO CuO ZnO Cr2O3 Ignition lossa Cl SO3
62.5 22.2 2.47 0.12 2.81 0.81 1.97 0.17 0.02 0.01 0.02 0.01 0.01 0.02 6.22 0.03 0.56
a Weight loss with heating up to 800e900 C. The dehydration of constitution water occurred at 530 C in the 2-layer and 800 C in the 3-layer clay mineral (Tomita et al., 1992).
the bentonite of 0.8 103 kg m3 contacted with the NaCl solution of 1.0 M did not fully swell, the other bentonites swelled and were saturated by each contact solution after 30 days. Some specimens were saturated with 0.01 M of NaCl solution in a globe box that filled with Arþ5%H2 for comparison. The pH and Eh of the contact solutions were around 7 and 500 mV in the ambient condition and 8 to 9 and around 100 mV in globe box condition, respectively. Iron concentrations in some contact solutions were measured by atomic absorption spectrometry (AAS: Shimazu; AA-6300). Iron concentrations in all measured contact solution were less than 1 ppm. After saturation of each bentonite one side of the bentonite was spiked with 10 mL of tracer solution containing 0.13 M Na2SeO3 and it was kept at 10, 25, 40, or 55 C for 1e29 days for diffusion (Fig. 1). Most of the experiments were conducted in the ambient condition. Some experiments were also carried out in a globe box that filled with Ar þ 5%H2 from the saturation procedure for comparison. The experimental conditions are listed in Table 2. After a set time, the bentonite specimens were extracted from the columns and cut into slices 0.5e2 mm thick. The concentration profiles of Se in bentonite were determined from the measurement
2.2. Diffusion experiments Bentonite powder was compacted into cylinders with a diameter of 10 mm and a height of 10 mm with a dry density of around 0.8 to 1.6 103 kg m3. Each cylinder was inserted into an acrylic resin column and saturated with a NaCl solution with a concentration of 0.01e1.0 M for 30 days in the ambient condition. Though
Fig. 1. Schematic of apparatus for diffusion experiments.
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3
Table 2 Summary of Da values for selenite obtained and experimental conditions. NaCl conc. (M)
Dry density (mg m3)
εtot
0.01
0.80 0.82 1.00 1.03 1.23 1.25 1.41 1.39 1.47 0.81 0.81 1.04 1.03 1.24 1.39 1.46 1.03 1.03 1.02 1.23 1.46 1.60 1.02 1.00 1.01 1.01 1.02 1.04 1.03 1.01 1.03 1.02 1.04 1.04 1.05 1.04 1.05 1.03 1.03 1.02 1.05 1.07
0.72 0.71 0.65 0.64 0.57 0.56 0.50 0.51 0.48 0.72 0.72 0.63 0.64 0.56 0.51 0.49 0.64 0.64 0.64 0.57 0.49 0.44 0.64 0.65 0.65 0.65 0.64 0.63 0.64 0.65 0.64 0.64 0.63 0.63 0.63 0.63 0.63 0.64 0.64 0.64 0.63 0.62
0.1
1.0
0.01
0.1
0.5
1.0
a b c
a
()
εaa
b
()
0.43 0.42 0.29 0.27 0.13 0.11 0.06 0.07 0.06 0.43 0.43 0.26 0.27 0.12 0.07 0.06 0.39 0.39 0.40 0.28 0.14 0.06 e e e e e e e e e e e e e e e e e e e e
Temp. ( C)
Diffusion period (d)
Apparent diffusion coefficient (m2 s1)
25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 10 25 40 55 10 25 25 40 55 10 25 40 50 55 10 25 25 25 40 50
1 1 7 7 14 14 20 21 29 1 1 7 14 14 21 29 2 3 7 14 20 28 10 7 5 5 10 7 14 5 5 7 7 2 1.5 1.5 5 2 3 7 1 1.5
(1.8 (1.6 (4.2 (3.6 (1.3 (1.1 (4.0 (4.0 (3.3 (2.5 (1.4 (4.6 (4.6 (1.1 (4.5 (3.8 (1.6 (1.5 (1.3 (2.2 (3.5 (1.9 (3.7 (4.2 (6.9 (1.2 (3.4 (4.6 (4.6 (6.9 (1.1 (3.4 (9.1 (1.1 (2.1 (1.4 (7.1 (1.3 (1.6 (1.5 (2.4 (2.7
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.1) 0.1) 0.2) 0.2) 0.1) 0.1) 0.2) 0.2) 0.2) 0.2) 0.1) 0.2) 0.4) 0.1) 0.2) 0.2) 0.1) 0.1) 0.2) 0.2) 0.5) 0.1) 0.3) 0.2) 0.3) 0.1) 0.3) 0.2) 0.4) 0.3) 0.1) 0.2) 0.7) 0.1) 0.1) 0.1) 0.5) 0.1) 0.1) 0.1) 0.2) 0.1)
1011 1011c 1012 1012c 1012 1012c 1013 1013c 1013 1011 1011 1012 1012 1012 1013 1013 1011 1011 1011 1012 1013 1013 1012 1012 1012 1011 1012 1012 1012 1012 1011 1012 1012 1011 1011 1011 1012 1011 1011 1011 1011 1011
Total porosities were calculated from dry densities and theoretical density as shown in text. Anion accessible porosities were estimated by anion-free interlayer model. Experiments were carried out in glove box filled with Arþ5%H2.
of the quantities in each slice and its weight. Each slice had weight measured then was submerged in 1 N HNO3 solution (Ultra Pure, Kanto Kagaku Co. Ltd.) to extract Se. After the liquid phase was separated by centrifugation (2000 g, 5 min), the supernatant was collected to measure the concentrations of Se by inductively coupled plasma-mass spectrometry (ICP-MS; Agilent, 7500C). The recovery rate of the Se obtained by the preliminary examination by this method was 90% or more.
version 2.5 (Rigaku). Two samples were prepared for XANES spectra measurement. Approximately 50 mg of Kunipia-F was mixed with 10 mL of tracer solution containing 0.13 M Na2SeO3 and was sealed in vinyl plastic in the ambient condition two weeks before the measurement. The other sample was prepared as same procedure but in a globe box that filled with Arþ5%H2. FeSe, elemental Se, SeO2, H2SeO3, Na2SeO3, and H2SeO4 powders were also prepared as reference samples.
2.3. X-ray absorption near-edge structure measurements (XANES) 3. Results and discussion XANES measurements of Se were carried out at the BL-11 beamline of SAGA Light Source in order to confirm oxidation state of Se. A Si(111) double-crystal monochromator was used, and the beam was focused by using bent conical mirrors coated with Rh. At the measurement position, the beam had a width of 1e5 mm and a height of 1 mm. XANES spectra were acquired in fluorescence mode by using a silicon drift detector (SDD). In this mode, the electrical signal of Se Ka X-rays (11,222.4 eV) from the SDD was selected by means of a single-channel analyzer at the K-edge. Data analysis, including background subtraction, normalization, and linear combination fitting of XANES spectra, was performed with REX2000
3.1. Oxidation state of Se measured The results of the Se K-edge (12,652 eV) XANES measurements for FeSe, Se, SeO2, H2SeO3, Na2SeO3, H2SeO4, and the samples are shown in Fig. 2. The background was subtracted from the original spectra by extrapolation of the linear absorption or curve (as defined by the Victoreen equation) from the pre-edge region. From Fig. 2, the peaks of the samples prepared in the ambient condition and in the globe box are in close agreement with the peaks of SeO2, H2SeO3 and Na2SeO3. It means that selenium
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as following equation;
M x2 : Cðx; tÞ ¼ pffiffiffiffiffiffiffiffiffiffiffi exp 4Da t pDa t
Fig. 2. Se K-Edge XANES spectra of Se, FeSe, SeO2, H2SeO3, Na2SeO3, H2SeO4, and the mixtures of Kunipia-F and Na2SeO3 in the ambient condition and in the globe box filled with Ar-5%H2. Oxidation state of selenium used in this study was shown as selenite.
remains tetravalent under these experimental conditions. If the chemical equilibrium is reached, the oxidation states of selenium in the samples should follow the Eqs. (1)e(3) or Se(VI) for ambient condition and Se(0) for in the globe box respectively. However, the valence of selenium was still kept in the state of Se(IV) in each condition. It could be considered that oxidationereduction reaction did not occur during the experimental period because of the late oxidationereduction reaction of selenium. 3.2. Apparent diffusion coefficients for selenite in purified bentonite One-dimensional non-steady diffusion is derived from Fick's second law by the following equation;
ε
vC v2 C vC ¼ De 2 rd Kd : vt vt vx
(5)
In the equation t (s) is the time, C (mol m3) the tracer concentration in the mobile phase, ε () the connected porosity of the bentonite, rd (Mg m3) the dry density of the bentonite, Kd (m3 Mg1) the linear sorption equilibrium distribution coefficient, De (m2 s1) the effective diffusion coefficient and x (m) the position respectively. The left side of the equation means time dependence of the tracer concentration in liquid phase and the first and second terms of the right side mean mass balance of tracer by diffusion and removal of tracer from the liquid phase to solid phase by sorption. This equation is rewritten as follows;
ðε þ rd Kd Þ
vC vC v2 C ¼a ¼ De 2 ; vt vt vx
vC De v2 C v2 C ¼ ¼ Da 2 2 vt a vx vx
(6)
(7)
where, Da (m2 s1) is the apparent diffusion coefficient and a () is capacity factor respectively. Supposing one-dimensional diffusion with the instantaneous plane source and a limited amount of diffusing substance in a cylinder of infinite length, the analytical solution of Eq. (7) is written
(8)
where M (mol) is the total amount of tracer and x is the distance from source respectively. The concentration (mmol g1 of bentonite, aC/1000rd) profiles of the selenite diffused into the bentonite specimens are shown as a function of x2/t (m2 s1) in Fig. 3. The profiles obtained in the diffusion experiments were in close agreement with the solution. The profiles for the 0.01 and 0.1 M NaCl contact solutions had almost the same slopes at the same dry density. However, the slopes were smaller for dry densities lower than 1.2 Mg m3 for the 1.0 M contact solution. Fitting the measured profiles yielded the apparent diffusion coefficient as shown in Table 2. The apparent diffusion coefficients of selenite in compacted bentonite at 25 C were 2.5 1011 to 1.9 1013 m2 s1 and are plotted as a function of dry density in Fig. 4. The difference was not observed by the experimental result performed in the ambient condition and glove box as shown in Table 2. The apparent diffusion coefficients of selenite decreased with increased dry density. The effect of the ionic strength of the contact solution on the apparent diffusion coefficient of selenite was negligible for densities higher than 1.4 Mg m3. However, the apparent diffusion coefficients for 1.0 M NaCl were 2 or 3 times larger than those for 0.1 or 0.01 M NaCl for densities lower than 1.2 Mg m3. These apparent diffusion coefficients are 1e2 orders of magnitude smaller than data in the literature such as 4.6 1010 to 3.2 1011 in 0.4e1.8 Mg m3 of Kunigel-V1 by in-diffusion method (Sato et al., 1995) and 7.8 1011 to 7.0 1012 in 1.3e1.8 Mg m3 of GMZ bentonite by through-diffusion method (Wu et al., 2014). It seems that this difference is based on the difference in the content of montmorillonite. The apparent diffusion coefficients at the same dry density decrease with the increase in the content of montmorillonite. This is because the increase in the content of montmorillonite decreases porosity for diffusion. Furthermore, effective porosity for diffusion becomes small by anion exclusion of the negatively charged montmorillonite (e.g. Drever, 1988). Uneven distribution of montmorillonite might also influence the diffusion coefficient of anions when specimen contains impurities other than montmorillonite. 3.3. Dependence of apparent diffusion coefficients for selenite in bentonite on dry density and ionic strength The microstructure of clay affects the diffusion behavior. The apparent diffusion coefficient is expressed as follows (e.g. Sato et al., 1995),
Da ¼
De ε d ¼ D a t2 0 a
(9)
Where, d () is constrictivity, t () is tortuosity and D0 (m2 s1) is the ionic diffusion coefficient of selenite in free water. Capacity factor, a, is nearly equal porosity because distribution coefficient, Kd, for selenite on montmorillonite is negligibly small. Though distribution coefficients reported were 0.14e0.18 (Mg m3) for GMZ bentonite (Wu et al., 2014) and 1 to 2 (Mg m3) for Kunigel-V1 (Shibutani et al., 1994), selenite might sorb not in montmorillonite but impurities such as pyrite. The second fractional number of the equation, d/t2, is formation factor and means constriction and detour of the diffusion path. If the formation factor is unity, pore can be denoted by cylindrical geometry parallel to a diffusion direction. The pore in actual montmorillonite is having complicated structure, and the formation factor is much smaller than 1.
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5
Fig. 4. Apparent diffusion coefficients of selenite at 25 C in compacted bentonite as a function of dry density. The interlayer of montmorillonite consists of 3-water layer (d001 ¼ 1.88 nm) and 2-water layer (d001 ¼ 1.56 nm) with transition occurring between 1.3 and 1.6 Mg m3 of dry density in contact with distilled water (Kozaki et al., 1998; Kozaki, 2003). In the higher ionic strength than 0.5 M NaCl 2-water layer appeared even in lower dry density than 1.3 Mg m3 (Kozaki et al., 2008).
clay (Iversen and Jørgensen, 1993). Porosities of the specimens used in this study are listed in Table 2. The total porosities were calculated by the following equation,
ε¼1
rd ; rmm
(11)
where rmm (Mg m3) is theoretical density of montmorillonite, 2.847 (Mg m3). The apparent diffusion coefficients of selenite obtained in this study are plotted as a function of total porosity in Fig. 5. The ionic diffusion coefficient in free water is also plotted at porosity of 1 in Fig. 5. The ionic diffusion coefficient in free water of selenite, D0, was 9.8 1010 (m2 s1) calculated from equivalent conductance at infinite dilution, l0; 73.9 104 (S m2 eq1) (Valaev and Georgieva, 2004) by Nernst expression (e.g. Robinson and Stokes, 1959),
Fig. 3. Concentration profiles of Se infiltrated in bentonite specimens. Lines in the figures represent the results of the fitting by Eq. (8) and slopes of the lines mean reciprocal of apparent diffusion coefficients.
The relation between t and ε was proposed as follows (Archie, 1942), 2
t ¼ε
1n
;
(10)
where, n is an adjustable parameter and in the range of 2.5e5.4 for
Fig. 5. Apparent diffusion coefficients of selenite at 25 C in compacted bentonite as a function of total porosity, εtot, or anion accessible porosity, εaa. The ionic diffusion coefficient of selenite in free water, 9.8 1010 m2 s1, is also plotted at porosity of 1. Porosities were calculated from dry densities, theoretical density and interlayer water ratio as shown in text. “n” shows adjustable parameter of Archie's law.
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K. Idemitsu et al. / Progress in Nuclear Energy xxx (2015) 1e7
D0 ¼
RTl0 ; F 2 jzj
(12)
where R is gas constant, F is Faraday constant and jzj is the absolute value of the valence of the ion. The dashed line in the Fig. 5 is the result of fitting the apparent diffusion coefficients obtained in Kunipia-F contact with the ionic strength 0.01 M and 0.1 M of NaCl and the ionic diffusion coefficient in free water, D0, by Eq. (10), assuming d to be 1. The adjustable parameter, n, is obtained as 12.2 ± 0.2 and much larger than that for clay in the range of 2.5e5.4 (Iversen and Jørgensen, 1993). The obtained n value of 12.2 is based on the assumption that capacity factor, a, equal porosity and constrictivity, d, is unity. Though distribution coefficient of selenite on montmorillonite is negligible, retardation could occur by dead end pore that increases with the rise of density. The effect of dead end pore might be included in the n value. Concerning constrictivity, it is necessary to take into consideration anion exclusion. Water in pore space is classified into 3 types as free pore water, electrostatic double layer (EDL) water and interlayer water (Bourge, 2004, Bradbury and Baeyens, 2003, Kozaki et al., 2008). Anions would exist in free pore water and be excluded almost fully from interlayer and partially from EDL by negatively charged montmorillonite surface, so-called anion exclusion. There are some models for anion exclusion (Van Loon et al., 2007; Tournassat and Appelo, 2011) and anion exclusion in EDL are calculated as a function of dry density and ionic strength. In this study anion accessible porosity, εaa, was estimated by taking into account the interlayer water but omitting EDL because there are negligible difference between the results in 0.01 M and 0.1 M of ionic strength. The interlayer porosity was calculated assuming 3-water layer (d001 ¼ 1.88 nm) and 2-water layer (d001 ¼ 1.56 nm) with transition occurring between 1.3 and 1.6 Mg m3 of dry density in contact with distilled water (Kozaki et al., 1998; Kozaki, 2003). According the model water ratio for 3and 2-water layer, Wi, are 0.357 and 0.238 Mg of H2O/Mg of Montmorillonite respectively (Pusch et al., 1990). Anion accessible porosity, εaa, was calculated as follows,
εaa ¼ εtot
X
xi Wi rd ;
(13)
x3 þ x2 ¼ 1:
x3 ¼ 1;
x2 ¼
3.4. Activation energy of the apparent diffusion coefficient of selenite in compacted bentonite The activation energy of the diffusion was derived from the temperature dependence of the apparent diffusion coefficient measured at different salinities. Arrhenius law are shown as bellows,
E ; D0 ¼ Di exp RT
(14)
where Di is intrinsic diffusion coefficient, E is activation energy respectively. The temperature dependence of the apparent diffusion coefficients are shown in Fig. 6 as Arrhenius plots at a dry density of 1.0 Mg m3 in contact solutions of each ionic strength. The activation energy for the diffusion was calculated by the least squares method. The activation energies obtained were similar values of 20 ± 4 kJ mol1. Therefore, the activation energy was not affected by the ionic strength of the contact solution. The activation energies of other chemical species are shown in Table 3 for comparison. The activation energy of the selenite (SeO2 3 ) obtained in this study is similar to the activation energies of 2 diffusion of oxo-anions, such as selenate (SeO2 4 ), sulphate (SO4 ), and iodate (IO ) in free water, and differ a little from that of chlo3 ride (Cl) and iodide (I) anions, and the cesium cation (Csþ). 4. Conclusion The apparent diffusion coefficient of selenite in purified bentonite, Kunipia-F, was found to be in the range of 2.5 1011 to 1.9 1013 m2 s1 in 0.8e1.6 Mg m3 of dry density and decreased with increasing bentonite density. The effect of the ionic strength on the apparent diffusion coefficient of selenite was negligible for densities higher than 1.4 Mg m3. However, the apparent diffusion coefficients in 1.0 M NaCl were several times larger than the coefficients in 0.1 or 0.01 M NaCl for densities lower than 1.2 Mg m3. This increase in diffusion coefficient would be caused by a change in anion accessible porosity. The apparent diffusion coefficient would be explained by Archie's law and is proportional to exponentiation of porosity. It is desired to measure exact anion accessible porosity. The activation energy of selenite in purified bentonite was 20 ± 4 kJ mol1 and was similar to the activation energies of oxo-
x2 ¼ 0 rd 1:3 Mg m3 :
rd 1:3 1:3 rd 1:6 Mg m3 0:3
where subscript i represents 3- or 2-water layer, xi is fraction of each layer. In the higher ionic strength than 0.5 M NaCl 2-water layer appeared even in lower dry density than 1.3 Mg m3 (Kozaki et al., 2008), so x2 is set as 1. Calculated anion accessible porosities are listed in Table 2. The apparent diffusion coefficients of selenite obtained in this study are also plotted as a function of anion accessible porosity in Fig. 5. The data in high ionic strength shows good coincidence with other data in low dry density or large anion accessible porosity. The solid line in the Fig. 5 is the result of fitting the apparent diffusion coefficient by Eq. (10). The adjustable parameter, n, is obtained as 5.7 ± 0.1 and close to those for clay in the range of 2.5e5.4 (Iversen and Jørgensen, 1993). However the fitting is not good agreement in low anion accessible porosity. It is desired to measure exact anion accessible porosity.
Fig. 6. Temperature dependence of the apparent diffusion coefficients of selenite in compacted bentonite at a dry density of 1.0 Mg m3 for contact solutions of various ionic strengths. E means activation energy of diffusion.
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Table 3 Activation energy of diffusion coefficients of various anions in free water and compacted bentonite. Chemical species
Activation energy kJ mol1
Remark
SeO2 3 SeO2 4 2 SO4 IO3 I Cl Csþ
20 ± 4 23.2 20.7 20.1 18.2 17.4 16.2
In montmorillonite (This paper) Li and Gregory, 1974 Calculated from ionic conductivity (Marcus, 1997) Ibid. Ibid. (Parsons, 1959) Ibid.
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