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Cesium-adsorption capacity and hydraulic conductivity of sealing geomaterial made with marine clay, bentonite, and zeolite
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ScienceDirect Soils and Foundations 58 (2018) 1173–1186 www.elsevier.com/locate/sandf
Cesium-adsorption capacity and hydraulic conductivity of sealing geomaterial made with marine clay, bentonite, and zeolite Ooki Kurihara a, Takashi Tsuchida a,⇑, Genki Takahashi b, Gyeong-o Kang c, Hiroki Murakami d a
Department of Civil and Environmental Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi, Hiroshima 739-8527, Japan b Chuden Engineering Consultants Co., Ltd, 2-3-30, Deshio, Minami-ku, Hiroshima 734-8510, Japan c Honam Regional Infrastructure Technology Management Center, Chonnam National University, 50, Daehak-ro, Yeosu, Jeonnam 59626, Republic of Korea d Nippon Koei Co., Ltd, 1-14-6, Kudankita, Chiyoda-ku, Tokyo, Japan Received 19 April 2017; received in revised form 1 June 2018; accepted 6 June 2018 Available online 20 August 2018
Abstract Great amounts of soil and waste contaminated with radioactive cesium have been generated due to the decontamination work after the Fukushima Daiichi Nuclear Power Plant accident. The aim of this study is to develop a sealing geomaterial for use at the disposal facilities of the soil and waste constructed in the maritime environment. The geomaterial consists of marine clay, bentonite, and zeolite. The hydraulic conductivity and cesium-adsorption performance of the geomaterial were examined through laboratory tests with different proportions of bentonite and zeolite added to marine clay. It was concluded that the hydraulic conductivity could be reduced to the required level by increasing the amount of bentonite and that the cesium-adsorption capacity could be enhanced by increasing the amount of zeolite. Ó 2018 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.
Keywords: Marine clay; Bentonite; Zeolite; Radioactive cesium; Adsorption
1. Introduction Due to the severe accident at the Fukushima Daiichi Nuclear Power Plant, accompanying the Great East Japan Earthquake and tsunami on March 11th, 2011, a large amount of radioactive material was released into the environment in Fukushima Prefecture as well as adjacent prefectures. In order to reduce the radiation level to the level at which people can live safely, great efforts have been made in decontamination work by removing a large
Peer review under responsibility of The Japanese Geotechnical Society. ⇑ Corresponding author. E-mail addresses: [email protected] (O. Kurihara), ttuchida@ hiroshima-u.ac.jp (T. Tsuchida), [email protected] (G. Kang), a7926@ n-koei.co.jp (H. Murakami).
amount of surface soil from the affected areas. The contaminated soil removed from these areas has been packed in containers made of polyethylene or polypropylene, and has been piled up at temporary storage sites in various locations around the affected areas. According to the Ministry of the Environment (2016), the maximum estimated volume of contaminated soil generated by the decontamination work and contaminated waste in Fukushima Prefecture is about 22 million m3. In accordance with the guidelines of the Ministry of the Environment (2011), disaster waste having radioactivity of less than 8000 Bq/kg can be disposed of in general waste disposal facilities (ordinary controlled types of final disposal facilities). On the other hand, contaminated soil generated by the decontamination work and waste with radioactivity of more than 8000 Bq/kg should be disposed
https://doi.org/10.1016/j.sandf.2018.06.004 0038-0806/Ó 2018 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.
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of in specially designed interim storage facilities in Fukushima Prefecture or designated disposal facilities in other prefectures. Further, the Japanese government has decided to move all soil and waste from these interim storage facilities to final disposal facilities within 30 years in order to return the areas with the interim storage facilities back to the landowners. Consequently, the construction of final disposal facilities for the soil and waste presently stored in interim storage facilities will become an important social and engineering problem. Waste disposal facilities have been constructed in coastal areas of Japan (Bureau of Port and Harbor, Tokyo Metropolitan Government, 2015; Osaka Bay Regional Offshore Environmental Improvement Center, 2015). In order to assure environmental safety, the facilities have been constructed based on the technical criteria developed by the government (Waterfront Vitalization and Environment Research Foundation, 2008). For the sealing layer of general waste disposal facilities, the following technical conditions are required by the Waste Management and Public Cleansing Law (Ministry of the Environment, 2015).
(c) An impermeable sheet underlain by a layer of asphalt or concrete, 5 cm or more in thickness, with hydraulic conductivity equal to or less than 109 m/s (d) A protective layer of non-woven fabric to be covered with two independent impermeable sheets. According to the above technical criteria, when the seafloor is composed of marine clay that has low hydraulic conductivity and is more than 5 m in thickness, the clay layer can be considered as a bottom sealing layer. Most of the existing waste disposal facilities have been constructed on this type of clayey layer. On the other hand, when the seafloor of an available location is composed of sandy soil, it is necessary to construct a sealing layer on the seafloor. Studies on geotechnical material for sealing layers in a maritime environment have been carried out by several researchers (Watabe et al., 2003; Yamada et al., 2003; Ueno et al., 2008; Watabe et al., 2011). The first waste disposal facility using a sealing layer made of such geotechnical material was constructed on the sandy seafloor in the Seto Inland Sea of Japan (Kawasaki et al., 2009). Fig. 1 shows a full view and the cross-section (A-A0 ) of Sangawa Tobu Disposal Facility for industrial waste. A 2-m-thick sealing layer, made of a marine clay-bentonite mixture, was placed at the bottom of the disposal facility. Both sides of the facility were sealed by double-layer impermeable sheets and a pile of water cut-off sheets. The sealing material was placed directly on the seabed by tremie pipes from the special working vessel shown in Fig. 2. Two steel pipes, 8 in.
(a) Continuous soil layer, 5.0 m or more in thickness, with hydraulic conductivity of less than 107 m/s or a layer with a sealing effect equal to or greater than above (b) An impermeable sheet underlain by 50-cm-thick material, such as clay, with hydraulic conductivity equal to or less than 108 m/s
4m (high de)
219.5m
Sand fill (excavated seabed) Excavated area
Stones
0m
Level of original seabed Steel slag
-14m -17m
Water cut-off sheet pile
-19m
Double-layer impermeable Natural sand seabed sheets
Sand mat
Steel slag
Geotexle
Boom sealing layer by clay-bentonite mixture 119.0 m
Double-layer impermeable sheets
Natural sand seabed
Cross-secon of A-A’ line
500.0 m A
A̓
Sangawa disposal facility for industrial waste under construcon (2008) Fig. 1. Full view and cross-section of Sangawa Disposal Facility for industrial waste (Kawasaki et al., 2009).
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Working vessel 4m (high de)
Mixing plant
Sand fill (excavated seabed)
Transportaon pipe
Stones
Water cut-off sheet pile
Steel slag
Covering soil
Steel slag
Double-layer impermeable sheets
Double-layer impermeable sheets
Boom sealing layer by clay-bentonite mixture
Natural sand seabed
Guide pipe φ800 steel Tremie pipe 8 inch
500.0 m
GPS antenna
Working vessel for placing sealing material
Valve to control air pressure.
Guide pipe with tremie pipes
Lead
Control room
Transportaon pipe of sealing material
Sealing material made of marine clay Fig. 2. Placing of sealing material by tremie pipe in seawater (Kawasaki et al., 2009).
and the space between the vertical walls should be filled with sealing layers. These layers must have very low hydraulic conductivity and be flexible enough to withstand the deformations which may be caused by future strong earthquakes over the next hundreds of years (Watabe et al., 2003; Ueno et al., 2008; Kawasaki et al., 2009). To satisfy these conditions, a geotechnical sealing material, which is a mixture of marine clay, bentonite, and zeolite, has been examined. The purpose of this study, therefore, is to develop a geotechnical material for the sealing layer of final disposal facilities available for the soil and waste contaminated by radioactive materials. Fig. 3 shows a schematic view of the final disposal facility discussed in this study.
(20 cm) in inner diameter, were used as the tremie pipes attached to a guide pipe (an 800-mm steel pipe). At present, there are no technical criteria for waste disposal facilities in coastal areas for soil and waste that have been contaminated by radioactive material. However, considering the large amount of soil and waste for final disposal, the construction of such disposal facilities in coastal areas would be an effective option that ought to be examined from an engineering point of view. Moreover, if facilities are to be constructed in coastal areas, they should be located on a dense sand or gravel seabed to secure their structural safety against strong earthquakes. To maintain the safety of the maritime environment, a sealing layer must be placed at the bottom of the seabed,
Super structure
Tsunami impact reducon blocks
Double-steel pipe sheet pile
Covering soil Surface sealing layer
Backfill material
Foot-protecon works
Wave-dissipang blocks
Caisson
Backfill
stone
Sealing material
Soil and waste contaminated by radioacve cesium
Rubble mound Seabed sand
Sealing layer Geotexle
Protecve soil layer (counterweight)
Fig. 3. Schematic view of disposal facility in this study for soil and waste contaminated by radioactive cesium.
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In this study, mixtures of marine clay, bentonite, and zeolite were prepared at different proportions and tested by means of consolidation and seepage tests to assess their consolidation, hydraulic conductivity, and adsorption property. Bentonite was added to reduce the hydraulic conductivity of the sealing material to satisfy the requested level, and zeolite was added to the sealing material to create a higher adsorption capacity for cesium. As the adsorption capacity is an important factor for the safety of the sealing material, the adsorption capacity was increased by adding zeolite to sealing material made of marine clay. In this study, the hydraulic conductivity and the cesiumadsorption capacity of marine clay-bentonite-zeolite mixtures were studied through a series of laboratory tests. 2. Consideration of required properties of sealing material The performance characteristics of the coastal disposal facilities targeted in this study must be scaled up by a large margin compared to regular disposal facilities constructed under the existing technical criteria. The Ministry of the Environment published guidelines for the radioactivity concentration of waste. Waste which is not greater than 8000 Bq/kg in radioactivity can be disposed of in existing disposal facilities constructed under the present technical standards (Ministry of the Environment, 2011). However, according to the guidelines, waste which is greater than 8000 Bq/kg in radioactivity, but less than 100,000 Bq/kg, is defined as waste which must be kept safe until the radioactivity falls to a low level and its safety can be confirmed. Based on the above definition, this study attempts to develop a coastal disposal facility which can safely preserve the soil and waste whose radioactive concentration is less than 100,000 Bq/kg as of 2011. Although the required properties of the sealing material have been discussed in a previous study (Tsuchida et al., 2017), considering the decay of the radioactivity in the soil and waste, the required properties were reconsidered in this study. The government has decided that the final disposal must be carried out within 30 years from the start of the service of the interim storage facility. It is assumed here that the final disposal would start in 2035. This is because 2045 is 30 years from the start of the interim storage and it will take 10 years to complete the final disposal of the soil and waste in the interim storage facility. The radioactivity ratio of Cs-137 and Cs-134 at the accident was 1:1 (International Atomic Energy Agency, 2015). Assuming that the radioactivity concentration of Cs-137 and Cs-134 contained in the soil and waste as of 2011 is 50,000 Bq/kg, the radioactivity concentration of Cs-137 and Cs-134 contained in the soil and waste in 2035 will be 28,806 Bq/kg and 16 Bq/kg, respectively. The leaching rate of waste to pore water is assumed to be 5%, based on the results of leaching tests of polluted soil and incinerated ash with an added safety margin (Tsuchida et al., 2017). The void ratio and dry density of the soil and
waste are assumed to be 0.8 and 1700 kg/m3, respectively (Tsuchida et al., 2017). From these assumptions, the concentration of radioactivity in the pore water of contaminated soil and waste is calculated with Eq. (1): For Cs 137 : C w ¼ C s E ed ¼ 28;806 0:05 1;700 1000 ¼ 3;061 Bq=L 0:80
1000 ¼ 2 Bq=L For Cs 134 : C w ¼ C s E ed ¼ 16 0:05 1;700 0:80
ð1Þ
where Cw is the concentration of radioactivity in the pore water, Cs is the concentration of radioactivity in the soil and waste, E is the leaching rate of the contaminated soil, qd is the dry density of the soil and waste, and e is the void ratio of the soil and waste. The sealing material developed in this study was placed on a sea floor consisting of dense sand or gravel. It was assumed that the sealing layer was subjected to consolidation stress of 30 kPa, the overburden stress of the covering layer, before carrying the contaminated soil and waste. When the head difference between the inside of the disposal facility and the sea level is H m, the seepage flow from inside to outside of a coastal disposal facility will take place through the bottom sealing layer and the sand-gravel layer at the sea floor, as shown in Fig. 3. Moreover, seepage can take place through the vertical sealing layer. Therefore, the vertical sealing layer will be made as a double-steel sheet pile structure with sealing material between the steel sheet piles. Here the sealing of the bottom layer is discussed. For the sealing of hazardous material in the pore water of soil and waste, both diffusion and advection must be considered. In the case of radioactive cesium in soil and waste, it was found that advection dominates the movement of the contaminants in the pore water (Tsuchida et al., 2017). Accordingly, only the effect of advection, the movement of pore water, is investigated in this study. In the regulations for nuclear power plants, the level of radioactivity in discharged cooling water acceptable to the natural environment is determined with Eq. (2) (Science and Technology Agency, 2000). the concentration of Cs134 ðBq=LÞ 60 ðBq=LÞ þ
the concentration of Cs137 ðBq=LÞ 1 90 ðBq=LÞ
ð2Þ
The concentration of radioactivity in the leachate water is thought to satisfy Eq. (2). The concentration of 2 Bq/L of Cs-134 in Eq. (1) becomes extremely small until the concentration of 3,061 Bq/L of Cs-137 decays to 90 Bq/L or less. Therefore, considering only the Cs-137 from hereafter, the emission limit of Cs-137 in the leachate is regarded as 90 Bq/L. Since the half-life of radioactive cesium-137 is 30.1 years, the amount of time until the maximum elution amount of 3,061 Bq/L decreases to the environment limit of 90 Bq/L was calculated with Eq. (3). Tt 1 C ¼ C0 ð3Þ 2
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where C is the concentration of radioactivity at time t, C0 is the initial concentration of radioactivity, and T is the halfperiod of radioactive cesium. Therefore, the amount of time until the maximum concentration of elution 3061 Bq/L becomes 90 Bq/L, t90y, was calculated with Eq. (4), as follows: t90y ¼
T ln CC0 ln 12
¼
90 30:1 ln 3;061
ln 12
¼ 153 years
ð4Þ
The main performance of water sealing is to make the travel time of the sealing layer longer than t90y calculated by Eq. (4). The thickness of sealing layer H, which is large enough and feasible in construction practice, is considered to be 4 m. When the difference in design head between the inside and the outside of the disposal facility is h, the hydraulic conductivity of the sealing layer required to make the travel time of sealing layer t90y is calculated with Eq. (5). k req ¼
k Darcy H H2 ¼ ¼ t90y ðh=H Þ t90y h n
ð5Þ
When the maximum head difference h is assumed to be 2 m and n is the porosity, kreq is calculated as 1.65 109 m/s (1.65 107 cm/s). As discussed above, the required hydraulic conductivity for the sealing material is determined as 1.00 109 m/s in this study. A number of studies have been carried out on sealing geotechnical material made of soil-bentonite mixtures. However, most of the studies were made for unsaturated soil or soil-bentonite mixtures with low water contents, because the swelling property of bentonite in unsaturated soil-bentonite mixtures is expected to reduce the hydraulic conductivity of the compacted clay liners made for inland waste disposal sites (Haug and Wong, 1992; Kenney et al., 1992) or that of the barrier for the geological disposal of nuclear waste (Komine and Ogata, 1999; Japan Nuclear Cycle Development Institute, 1999a, 1999b; Komine, 2004, 2010). However, the sealing material for maritime disposal facilities is fully saturated by seawater and has a high water content. And the intensity of the radioactivity of the soil and waste generated by the decontamination work in Fukushima Prefecture is considerably lower than that of the waste to be placed at the geological disposal sites. Watabe et al. (2011) studied the hydraulic conductivity of marine clay-bentonite mixtures with different sand contents for the sealing layer of maritime disposal facilities, but they did not consider the use of a sealing layer for soil and waste contaminated by radioactive cesium. Although the required hydraulic conductivity of the sealing layer was calculated in order to have enough time to reduce the radioactivity of the cesium in the soil and waste to a safe level, multiple safety should be ensured to gain public acceptance. For the additional safety of the sealing layer, the cesium adsorption property should be taken into consideration. It has been shown that some clay minerals have strong
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cesium-adsorption properties (Wahlberg and Fishman, 1962; Sawhney, 1964; Mahara, 1993; Staunton and Roubaud, 1997; Wu et al., 2009). After the Fukushima Nuclear Plant accident, the cesium-adsorption properties of soil have been intensively studied in various fields of science and engineering (Fujita et al., 2013; Okumura et al., 2013a, 2013b; Mukai et al., 2016). However, only Tsuchida et al. (2017) have carried out a study on the cesium-adsorption properties of saturated marine clay and marine clay-bentonite mixtures. From a series of column tests, they found that marine clay and marine clay-bentonite mixtures with high water contents have a significant cesium-adsorption property, and that the pore water in marine clay-bentonite mixtures must be replaced 12 times by the flow of a cesium solution before the cesium can be measured in the outflow water. As a practical adsorption material for radioactive cesium, zeolites have been highlighted since the 2011 accident. Zeolites are porous crystalline aluminosilicates which include exchangeable cations at the sites of their threedimensional structure. Among them, mordenite, which has a relatively large Si/Al ratio and tunnel-shaped hole paths, clinoptilolite, and chabazite, which has lantern-like cavities, have a characteristic whereby they adsorb and capture Cs+ selectively (Mimura, 2014). A team from the Atomic Energy Society of Japan carried out the experiments to evaluate the adsorption properties of various adsorbents mainly on granular zeolite for Cs-137 from real seawater (Division of Nuclear Fuel Cycle and Environment, Atomic Energy Society of Japan, 2011; Yamagishi, 2011). According to Mimura et al. (2011), the distribution coefficient Kd of mordenite and chabazite, among some zeolites, was approx. 800 cm3/g (90% adsorption), and it has been confirmed that natural zeolites can selectively adsorb Cs in seawater. The adsorption property of cesium for remediation and decontamination in Fukushima Prefecture has been utilized in the in-situ replacement method of paddy fields where the contaminated surface soil layer was replaced by a deep layer (Fujimura et al., 2016; Fukushima Prefectural Government, Agriculture, Forestry and Fishery Department, 2014). The Ministry of the Environment (2013, 2016) has recommended placing a covering soil layer, mixed with zeolite, on the bottom sealing sheet for a temporary storage site. Ito et al. (2016) confirmed the Cs adsorption capacity of zeolite using leachate from an actual inland-controlled final disposal site. The hydraulic conductivity of zeolite-bentonite ¨ ren mixtures for compacted clay liners was studied by O et al. (2011, 2014). They showed that the hydraulic conductivity of compacted zeolite-bentonite mixtures was much larger than that of sand-bentonite mixtures. However, almost all the previous studies on the adsorption property of zeolite targeted the use of a sealing layer inland or covering soil and no studies have been carried out on marine clay-zeolite mixtures under saturated and high-water content conditions.
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3. Characteristics of hydraulic conductivity
Passing percentage by weight (%)
100
3.1. Sample preparation Marine clay dredged from Tokyo Bay, bentonite produced in Wyoming, and zeolite produced in Iwami of Shimane Prefecture, Japan, were used in this study. Table 1 illustrates the physical properties of Tokyo Bay clay, bentonite, and zeolite. Tokuyama Port clay was used for a comparison of the adsorption property with the samples tested in this study. Fig. 4 shows the gradation curve of the Iwami zeolite. As shown in this figure, the mean grain size of zeolite is 0.203 mm and the content of fines of less than 75 lm is 24.5%. From the Unified Soil Classification System, the zeolite was classified as silty sand (SM). The proportions of the samples of the sealing materials used in this study are shown in Table 2. When the samples were made, the bentonite was mixed with artificial seawater (1.03 g/cm3) to make the water content 200%. The composition of the artificial seawater is shown in Table 3. The added amounts of bentonite and zeolite were described as dry weights for 1 m3 of Tokyo Bay clay whose water content was 1.5 wL (wL: liquid limit). The zeolite additive rate in Table 2, zc, is the ratio of zeolite to the dry mass of Tokyo Bay clay. 3.2. Experiments on the performance of water-sealing properties The samples shown in Table 2 were analyzed with an incremental loading consolidation test using the apparatus shown in Fig. 5. The test was conducted according to the test method for the one-dimensional consolidation properties of soils using incremental loading (JIS A1217). The samples in the slurry states were filled up to the consolidation ring whose height was 20 mm and diameter was 60 mm. The eight stages of the incremental loading consolidation test were conducted using consolidation stresses from 4.9 kPa to 627.2 kPa with the stress increment ratio Dr0v / r0v of 1. In each stage, the end of primary consolidation p was determined by the t method. After the primary consolidation was over, the next stage was started. During consolidation, the immersion container was filled with distilled water. Using the theory of one-dimensional consolidation, the hydraulic conductivity due to Darcy’s law, kDarcy, was calculated from Eq. (6), where cv is the coefficient of consolp idation measured by the t method, mv is the coefficient of volume compressibility, and cw is the unit weight of water.
90 80 70 60 50 40 30 20 10 0
0.001
0.01
0.1 Sieve size (mm)
1
10
Fig. 4. Gradation curve of Iwami zeolite.
Table 2 Description of samples. Sample
Bentonite addition amount [kg/m3]
Zeolite addition amount [kg/m3]
Zeolite additive rate, zc (%)
TB TBz10 TBz20 TBz50 TBb50 TBb100 TBb50z10 TBb50z20 TBb100z10 TBb100z20
0 0 0 0 50 100 50 50 100 100
0 43 87 217 0 0 43 87 43 87
0 10 20 50 0 0 10 20 10 20
1
TB: Tokyo Bay clay, b: bentonite, z: zeolite. The bentonite addition amount is the value per 1 m3 of Tokyo Bay clay in 1.5 wL (wL: liquid limit). 3 The zeolite additive rate is the rate of the dry weight of soil per 1 m3 of Tokyo Bay clay in 1.5 wL. 2
Table 3 Composition of artificial seawater. Ions
Content [g/kg]
Resolved ratio [%]
Cl Na SO4 Mg K Ca HCO3 Br BO3 Total
19.13 10.75 1.89 1.37 0.38 0.32 0.08 0.08 0.04 34.02
56.22 31.58 5.53 4.03 1.13 0.94 0.25 0.19 0.11
Table 1 Physical properties. Property
Tokyo Bay clay
Tokuyama Port clay
Wyoming bentonite
Iwami zeolite
Density of soil particles qs [g/cm3] Liquid limit wL [%] Plastic limit wP (%) Plasticity index
2.79 133.9 60.38 73.52
2.616 110.6 43.43 67.17
2.898 187.0 (sea water) 58.7 (sea water) 128.3
2.38 – – –
O. Kurihara et al. / Soils and Foundations 58 (2018) 1173–1186
Consolidation pressure
Loading plate Guide ring Loading plate
Water holder
Sample
Compaction ring Height 20mm Diameter60mm
Porous stone Drain cock
Fig. 5. Consolidation test apparatus.
k Darcy ¼ cv mv w
ð6Þ
The hydraulic conductivity by Darcy’s law, kDarcy, was calculated from the flow velocity, vDarcy, through the soil samples assuming that conventionally the water flows all through the cross-section area of the soil sample. However, water flows only in the pores between the particles, and in order to calculate the travel time of the water flow, it is necessary to consider the actual flow velocity in the pore spaces, vpore. Assuming that the water flows all through the pores of the samples, vpore is calculated with vDarcy as follows: vpore ¼ vDarcy =n ¼ vDarcy ð1 þ eÞ=e
ð7Þ
where e is the void ratio and n is the porosity. Accordingly, the hydraulic conductivity for the flow through the pores, kpore, is calculated as follows: k pore ¼ k Darcy =n ¼ k Darcy ð1 þ eÞ=e
ð8Þ
3.3. Results and discussion on hydraulic conductivity In Eq. (7), vpore was obtained by using the porosity, but it was pointed out that the effective porosity is remarkably lower than the porosity due to the influence of the adsorbed water, etc., especially in the case of fine grained soil such as clay (Kamon et al., 2010). Therefore, a test was carried out to determine the effective porosity, and the assumption of the porosity was examined. The procedure up to consolidation was similar to that of the consolidation and seepage test to be described later. Matsushima Bay clay was used as the specimen. Soon after the primary consolidation was over, the seepage test was conducted. Artificial seawater, whose salinity concentration was 3%, was injected into the sample by applying 19.4 kPa of air pressure to the double-cylindrical burette until the salinity of the effluent reached 3%. Then, artificial seawater, whose salinity concentration was 6%, was injected into the sample. The effluent was collected every 5 mL and the electric conductivity was measured. The electrical conductivity was converted to the salinity concentration. The salinity
was normalized so that the salinity concentration level of 3% was 0 and that of 6% was 1, and normalized salinity concentration was obtained. Next, following the estimation method of the effective porosity (Japanese Geotechnical Society, 2008), the effective porosity was calculated. First, fitting was performed so as to minimize the sum of the squares of the residuals of the one-dimensional theoretical solution of the advection-dispersion equation and the measured values. Since nonadsorptive solute was used here, the delay coefficient was assumed to be R = 1. Delays due to dead zones, such as upstream and downstream porous stones, as well as downstream piping, were corrected. Then, the dispersion coefficient D of the theoretical solution and seepage velocity v were determined. Finally, the effective porosity, ne, was calculated from ne = u/v. The relationship between the normalized salinity concentration and time is shown in Fig. 6. Effective porosity ne, estimated by this experiment, was 0.703, and porosity n, obtained from void ratio e in the seepage test, was 0.724. The difference between the estimated effective porosity and the porosity was small. In this way, the method of using porosity instead of the effective porosity when calculating vpore in Eq. (7) was verified. Fig. 7 shows the relationship between void ratio e and effective consolidation pressure r0v for all the samples. The void ratios of the samples decreased with the increase in the zeolite additive rate, zc. At r0v = 20 kPa, the void ratios of the samples of zc = 10%, 20% and 50% were, respectively, 0.9 times, 0.8 times and 0.65 times that of zc = 0%. However, compression index Cc was almost unchanged by the zeolite addition, which means that the compression property was not affected much by the addition of zeolite. Comparing the e-log r0v relation of the same zeolite content, the void ratio became smaller with an increase in the bentonite addition, when the consolidation pressure was larger than 80 kPa. It is thought that the void ratio became smaller as the fine particles of the added bentonite filled the pores of the Tokyo Bay clay-zeolite mixture.
1 0.9
Normalized salinity concentraon, C /C 0
Displacement gauge
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0.8 measured value
0.7
theorecal soluon
0.6
Esmated D = 7.00 x 10 - 10 m 2 /s
0.5
Measured n = 0.724 Esmated n e = 0.703
0.4 0.3 0.2 0.1 0 0
0.5
1
1.5
2
2.5
Time (day)
Fig. 6. Normalized salinity concentration and time.
3
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O. Kurihara et al. / Soils and Foundations 58 (2018) 1173–1186 4.5
zc=0%
Void rao , e
4.0
zc=10%
TB TBz10 TBz20 TBz50 TBb50 TBb50z10 TBb50z20 TBb100 TBb100z10 TBb100z20
zc=20%
3.5
3.0
zc=50% 2.5
2.0
1.5
Hydraulic conducvity in pore , kpore (m/s)
1E-08
Tokyo Bay clay-bentonitezeolite mixture
Tokyo Bay clay-bentonitezeolite mixture
1E-09
bc=0 kg/m3 zc=0,10,20% TB TBz10 TBz20 TBz50 TBb50 TBb50z10 TBb50z20 TBb100 TBb100z10 TBb100z20
bc=50 kg/m3 zc=0,10,20%
bc=100 kg/m3 zc=0,10,20%
1E-10
1
10
100
1000
1
10
100
1000
Effecve consolidaon pressure, σv' (kPa)
Effecve consolidaon pressure, σv' (kPa) Fig. 7. e–logr0v curve.
Fig. 9. kpore and effective consolidation pressure r0v .
Fig. 8 illustrates the relationship between the hydraulic conductivity of Darcy’s law, kDarcy, and consolidation pressure r0v . As shown in Fig. 8, kDarcy was clearly reduced by the addition of bentonite. With the addition of zeolite, it seemed constant for samples with no bentonite addition. However, when bentonite was added, kDarcy was reduced by the increase in the additive rate of the zeolite. Fig. 9 shows kpore with effective consolidation pressure. From Eq. (8), kpore becomes larger than kDarcy. Fig. 9 shows that by the addition of bentonite, kpore is reduced, and among the samples under the same bentonite conditions of bc = 50 kg/m3 and 100 kg/m3, kpore was smaller as the content of zeolite became larger. As discussed above, the required hydraulic conductivity for the sealing layer is less than 1.00 109 m/s by considering the travel time of the flow of pore water. As seen in Fig. 8, the desired level
of hydraulic conductivity was achieved by the addition of zeolite and bentonite at consolidation pressures larger than 40 kPa. Fig. 10 shows the hydraulic conductivity ratio kpore/k*pore with the effective consolidation pressure, where k*pore is kpore of the Tokyo Bay clay without bentonite or zeolite at the same effective overburden pressure. As shown in this figure, by mixing in zeolite, the value of kpore/k*pore is the same or slightly increased. However, when both bentonite and zeolite are added to the clay, the value of kpore/ k*pore is reduced to 40–70% of that of the Tokyo Bay clay, and the reduction is larger when the content of zeolite is larger. In addition, the mixing of zeolite has no significant effect on the hydraulic conductivity. Watabe et al. (2011) carried out a series of incremental loading oedometer tests and microscopic observations in order to investigate the
TB TBz10 TBz20 TBz50 TBb50 TBb50z10 TBb50z20 TBb100 TBb100z10 TBb100z20
1E-09 bc=0 kg/m3 zc=0,10,20%
bc=50 kg/m3 zc=0,10,20%
Tokyo Bay clay-bentonitezeolite mixture kg/m3
bc=100 zc=0,10,20%
1E-10 1
10 100 Effecve consolidaon pressure, σv' (kPa)
1000
Fig. 8. Darcy’s hydraulic conductivity kDarcy and effective consolidation pressure r0v .
Hydraulic conducvity rao in pore , kpore /(kpore)*
Darcy's hydraulic conducvity, kDarcy (m/s)
1E-08 1.8 TBz10 TBb50 TBb100
1.6
TBz20 TBb50z10 TBb100z10
TBz50 TBb50z20 TBb100z20
1.4 1.2 1 bc=0kg/m3
0.8
bc=50kg/m3
0.6 0.4 0.2
Tokyo Bay clay-bentonite-zeolite mixture
bc=100kg/m3
0 1
10
100
1000
Effecve consolidaon pressure, σv' (kPa)
Fig. 10. Hydraulic conductivity ratio in pore kpore/(kpore)* and effective consolidation pressure r0v .
O. Kurihara et al. / Soils and Foundations 58 (2018) 1173–1186
6 TB TBz10 TBz20 TBz50 TBb50 TBb50z10 TBb50z20 TBb100 TBb100z10 TBb100z20
5.5 5
Void rao , e
4.5 4 3.5
Tokyo Bay clay-bentonitezeolite mixture
bc=100kg/m3 zc=0%
3 2.5 2 1.5
Covering sand
4.0 m
Sealing layer
Sandy seabed Fig. 12. Sealing layer and covering sand.
Tokyo Bay clay-bentonitezeolite mixture
bc=0kg/m3
100
bc=50kg/m3
bc=100kg/m3 TB TBz10
10
TBz20 TBz50 TBb50 TBb50z10 TBb50z20 TBb100 TBb100z10 TBb100z20
1
1
10
100
1000
Effecve consolidaon pressure, σv' (kPa) Fig. 13. cv–r0v curve.
layer, as shown in Fig. 1. The development of shear strength of the sealing layer, due to the consolidation by the selfweight and the weight of the covering layer, is an important factor for shortening the time for the construction of disposal facilities. The rate of the strength increase with consolidation is determined by the coefficient of consolidation, cv. Fig. 13 illustrates the relationship between cv and effective consolidation pressure r0v . The value of cv of the sealing material decreased with the addition of bentonite. At r0v = 40 kPa, cv decreased from 65 cm2/day to 38 cm2/day for bc = 50 kg/m3 and to 25 cm2/day for bc = 100 kg/m3. When zeolite and bentonite are mixed with clay, cv becomes slightly smaller with the increase in the zeolite content. This is due to the reduction of hydraulic conductivity by mixing in both zeolite and bentonite.
bc=0 zc=50%
bc=0 zc=20% bc=100kg/m3 bc=50kg/m3 zc=20% zc=20%
1 1E-08
Contaminated soil and waste
Coefficient of consolidaon, cv (cm2/day)
influence of sand/bentonite fractions on hydraulic conductivity. According to their results, an additive fraction of sand causes a decrease in compressibility, but does not affect the hydraulic conductivity, if the sand particles do not form a skeletal structure and each particle is independent in a clay matrix with a small sand fraction. In this study, as the gradation of zeolite was close to that of the sand and the content was not enough to form a skeletal structure, it was reasonable that the mixture of zeolite did not show any significant effect on the hydraulic conductivity. In Fig. 10, the values for the kpore in the mixtures of both zeolite and bentonite were 10–20% smaller than that of the mixture of bentonite alone. This fact cannot be explained by the previous studies on sand-bentonite mixtures, and further studies considering the microstructure of zeolite-bentonite mixtures will be necessary to find the reason. From the viewpoint of the purpose of this study, it is good that the zeolite addition will not cause any increase in the hydraulic conductivity of the mixture. Fig. 11 illustrates the relationship between hydraulic conductivity kpore and void ratio e. The kpore value became larger for the same void ratio as the addition of zeolite became larger, while the kpore value became smaller as the bentonite content became larger. For the design and construction of bottom sealing layers, the coefficient of consolidation is also important. To carry the covering layer on the sealing layer, the sealing layer has to have enough shear strength to support the overlaying materials. Further, an increase in strength accompanying consolidation will be necessary to fill the soil and waste on the covering layer. Fig. 12 shows the covering sand on the sealing layer. To prevent the disturbance of the sealing layer by the deposition of soil and waste, a covering layer must be placed on the sealing layer. In the case of the Sangawa Tobu Disposal Facility, a 2-meterthick covering layer was carefully placed on the soft sealing
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4. Adsorption property 1E-07
Hydraulic conducvity, kpore (m/sec) Fig. 11. Relationship between e and kpore.
1E-06
4.1. Experiments on the adsorption property In the previous study, the authors carried out an elution test on soil and waste which were contaminated by
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radioactive cesium (Tsuchida et al., 2017). It was found that the concentration of radioactive cesium, qCs137 (Bq/L), and the concentration of stable cesium, qCs133 (mg/L), were closely related and can be shown as follows: Cs133 ¼ 0:00014ðCs137 Þ1:12
ð9Þ
Based on their findings, the adsorption property of the sealing material was studied with a column seepage test with stable cesium Cs-133 instead of Cs-137. This test was conducted for the samples shown in Table 2, except for TBz50, TBb50z20 and TBb100z20. The results of the adsorption test were compared to the data on Tokuyama Port clay in the previous study (Tsuchida et al., 2017). Fig. 14 illustrates the consolidation and seepage test apparatus. The apparatus had a structure which could suffuse the cesium solution. A consolidation ring with a height of 20 mm was used for the general consolidation test, but a ring with a height of 10 mm was used in the adsorption study to shorten the time of the water flow. The samples in the slurry state were carefully put into the sample ring which had a height of 10 mm and a diameter of 60 mm. The three stages of the incremental loading in the consolidation test were conducted with consolidation stress levels of 4.9 kPa, 9.8 kPa and 19.6 kPa, with a stress increment ratio of Dr0v /r0v = 1. In each loading stage, the consolidation pressure was maintained until the samples achieved 100% consolidation before applying the next stress increment. In each stage, p the end of primary consolidation was determined by the t method. Soon after the primary consolidation was over, the seepage test was carried out for which
30 mg/L of CsCl aqueous solution (stable cesium, Cs-133) were used as the permeant. After consolidation at the consolidation stress of 19.6 kPa, the water that had drained out of the specimen remaining in the loading plate was replaced with the CsCl aqueous solution. The CsCl aqueous solution was flowed through the sample by giving 19.4 kPa of air pressure through the doublepipe burette. The effluent permeant was collected every 10 mL, and its concentration of cesium was measured using an atomic absorption analyzer. Due to the limitation of the test period, the seepage test was halted as soon as the trend of the breakthrough curve could be confirmed. As mentioned above, assuming that the concentration of radioactive cesium is 100,000 Bq/kg, the concentration of pore water was calculated to be 3061 Bq/L, as shown in Eq. (1). By using Eq. (9), the concentration of stable cesium can be evaluated as 1.14 mg/L. In the flame mode and the calibration line method of the atomic adsorption analyzer, the accuracy of the measurement fell when the measured concentration was out of the range of the calibration line. As the range of the calibration line in this study was from 1.0 mg/L to 5.0 mg/L and the seepage water was diluted by 100% in the treatment for the measurement, the reliability of the measured concentration of less than 2.0 mg/L was low. In the previous study (Tsuchida et al., 2017), the target of the sealing material on adsorption was to reduce the cesium concentration of seepage water to one-tenth of the initial concentration. To measure the cesium concentration around one-tenth of the initial concentration of cesium solution, the concentration of cesium solution in the seepage test was determined to be 30 mg/L.
Air pressure
Displacement gauge Double-pipe burette
Loading plate Guide ring
Compaction ring Sample Diameter 60 mm Height 10 mm Picking Fig. 14. Consolidation and seepage test apparatus.
O. Kurihara et al. / Soils and Foundations 58 (2018) 1173–1186
4.2. Results and discussion on adsorption property Fig. 15 shows the hydraulic conductivity kDarcy of Darcy’s law calculated from the seepage test, compared with the kDarcy calculated from the consolidation test. As shown in this figure, the values for kDarcy obtained in the seepage test were slightly smaller than those obtained from the consolidation test. Accordingly, the estimation of hydraulic conductivity by the consolidation test is made on the conservative side in order to consider the safety of the sealing material. Using the pore volume of specimen Vv and the volume of cumulative flowed water Vf, the pore volume of the flow (PVF) is defined as
Hydraulic conducvity kDarcy measured from seepage test (m/s)
PVF ¼ V f =V v
ð10Þ
4E-09
Tokyo Bay clay or Tokuyama Port clay -bentonite-zeolite mixture
When the PVF is 1, all the pore water in the specimen is replaced by the inflow and the PVF will become 1 when the travel time of the water flow passes. Accordingly, PVF means a safety factor of travel time and, for example, PVF = 3 means that the water flow is 3 times the pore volume and that 3 times the travel time passes. The results are shown in Fig. 16 for the adsorption test as the relationship between the effluent concentrations of cesium CCs and the values of PVF. As shown in this figure, the no concentration of cesium was measured in the meantime and after PVF reached a breakthrough point, the concentration of cesium was increased by the pore volume of the flow or the safety factor. In this study, the point where cesium is first detected in the effluent solution is defined as the breakthrough point. The CCs–PVF relationship of the Tokuyama Port clay in the previous study (Tsuchida et al., 2017) is also plotted in Fig. 16. Tokuyama Port clay is one of the typical marine clays found on the west coast of Japan and its index properties are shown in Table 1. From Fig. 16, the following were found on the breakthrough point of the cesium adsorption of the sealing material used in this study: The breakthrough point of adsorption is different in marine clay, bentonite and zeolite. The breakthrough point of Tokyo Bay clay (PVF 18) is much larger than that of Tokuyama Port clay (PVF 9). By adding bentonite to marine clay, the PVF value became smaller. Accordingly, the addition of bentonite was seen to be effective for reducing the hydraulic conductivity, but it also causes the adsorption property of cesium to decrease. By adding zeolite to marine clay, the PVF value increased drastically. Further, the increase rate of the cesium concentration after the PVF exceeded the breakthrough point also became much less than that of the samples without zeolite.
1E-09
4E-10 TB TBz20 TBb100z10 TBb100 TPb25 TPb150 1E-10 1E-10
4E-10
TBz10 TBb50z10 TBb50 TP TPb75
4E-09
1E-09
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Hydraulic conducvity kDarcy calculated from consolidaon test (m/s)
Fig. 15. Comparison of hydraulic conductivity measured in two tests.
Cesium concentraon of effluent, CCs (mg/L)
4 Tokuyama Port clay + bentonite
Tokyo Bay clay-bentonitezeolite mixture
bc=50kg/m3, zc=0%
3.5
TP TPb25 TPb75 TPb150 TB TBb50 TBb100 TBz10 TBz20 TBb50z10 TBb100z10
Tokyo Bay clay 3 bc=100kg/m3 zc=0%
2.5 2
bc=100kg/m3 zc=10%
1.5
bc=50kg/m3 zc=10%
bc=0kg/m3 zc=10%
1
bc=0kg/m3 zc=20%
0.5 0 0
10
20
30
40
50
Pore volume of flow, PVF Fig. 16. Cesium concentration of effluent CCs and PVF.
60
70
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O. Kurihara et al. / Soils and Foundations 58 (2018) 1173–1186
Cesium concentraon of effluent, CCs (mg/L)
4 bc=50kg/m3 zc=0%
3.5
Tokyo Bay clay-bentonitezeolite mixture
Tokyo Bay clay bc=100kg/m3 zc=0%
3
TB TBz10 TBz20 TBb50 TBb100 TBb50z10 TBb100z10
bc=100kg/m3 zc=10%
2.5
bc=50kg/m3 zc=10%
2
zc=10%
1.5
zc=20%
1 0.5 0 0
200
400
600
800
1000
Cumulave adsorbed cesium
1200
133Cs,
1400
1600
1800
ΣS (mg/kg)
Fig. 17. CCs and cumulative adsorbed cesium RS.
Table 4 Cation exchange capacity of samples. Sample
Tokyo Bay clay
Tokuyama Port clay
Wyoming bentonite
Iwami zeolite
Measured value (cmol/kga)
36.0
21.7
56.0
97.6
of bentonite. As the increase in adsorption capacity could not be explained by the CEC alone, further study is necessary on the adsorption capacity versus CEC relation. In Fig. 19, DCL, the increment in the cesium concentration of the effluent per unit travel time or pore volume of flow after the breakthrough point, is shown with the CEC.
b
Analysis method is the Schollenberger method. a Per dry weight of soil. TP TB TBz10 TBz20 TBb50 TBb100 TBb50z10 TBb100z10
50 40 30 20
Tokyo Bay clay PVF=2.45CEC-74.0 R2=0.529
zc=20%
zc=10%
Tokuyama Port clay
10
zc=0%
0 0
10
20
30
40
50
60
CEC (cmol/kg)
(a) 1400
Cumulave adsorbed cesium ΣS at breakthrough point (mg/kg)
In Fig. 17, the effluent concentration of cesium CCs is shown with the cumulative adsorbed cesium RS, which was calculated from the difference in the inflow and outflow concentrations of cesium. It was found that, when the cesium begins to flow out, the cumulative adsorbed cesium of the sample which mixed 10% of zeolite into Tokyo Bay clay is 1.5 times, the sample which mixed 20% of zeolite into Tokyo Bay clay is 2.0 times as much as that of the Tokyo Bay clay. It is known that the adsorption capacity of clay is related to the cation exchange capacity of soil (Kahr et al., 1995). Table 4 shows the cation exchange capacity (CEC) of Tokyo Bay clay, Tokuyama Port clay, Wyoming bentonite and Iwami zeolite. The cation exchange capacity of each tested sample was calculated by share pro rata in accordance with the volume of the four materials listed in Table 4. Fig. 18(a) shows the relationship between the CEC of each sample and the PVF at the breakthrough point. Further, in Fig. 18(b), the relationship between the CEC and the cumulative adsorbed cesium at the breakthrough point is shown. As shown in Fig. 18(a) and (b), the increase in CEC by the addition of zeolite seems to be related to the increase in the adsorption capacity of the samples. However, comparing the samples having the same zeolite additive rate, zc, the PVF and the cumulative adsorbed cesium at the breakthrough point are seen to decrease slightly with an increase in CEC by the addition
PVF at breakthrough point
60
TP
1200
TB TBz10
1000
zc=20%
TBz20
Tokyo Bay clay
TBb50
ΣS= 59.1CEC-1733 R2=0.627
TBb100
800
TBb50z10
zc=10%
TBb100z10
600 400
Tokuyama Port clay
zc=0%
200 0 0
10
20
30
40
50
60
CEC (cmol/kg)
(b) Fig. 18. (a) PVF at breakthrough point and CEC (cmol/kg) and (b) RS at breakthrough point (mg/kg) and CEC (cmol/kg).
ΔCL, increment in cesium concentraon of effluent per unit of travel me a er breakthrough point
O. Kurihara et al. / Soils and Foundations 58 (2018) 1173–1186 0.6 TP TB
0.5
zc=0%
Tokuyama Port clay
TBz10 TBz20
0.4
TBb50 TBb100
0.3
TBb50z10
zc=10%
TBb100z10
0.2
Tokyo Bay clay ΔCL=-0.0421CEC+2.04 R2=0.726
0.1
zc=20%
0 10
20
30
40
50
CEC (cmol/kg) Fig. 19. DCL, increment in cesium concentration of effluent per unit of travel time after breakthrough point and CEC (cmol/kg).
The relationship indicates that the addition of zeolite to marine clay could cause the increment in the cesium concentration of the effluent to be smaller after the breakthrough. From the results shown in Figs. 18 and 19, with respect to the adsorption property of cesium, it was confirmed that the mixing of zeolite into marine clay has an effect of suppressing the increase in the concentration of cesium in the effluent after the breakthrough as well as the effect of delaying the breakthrough.
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hydraulic conductivity decreased in proportion to the addition of bentonite. Further, when both zeolite and bentonite were added to marine clay, the hydraulic conductivity became smaller than that of marine clay mixed with bentonite alone. (3) The cesium-adsorption capacity of the sealing material was increased by adding zeolite, and the safety factor on the travel time of the pore water flow increased more than two times. However, the adsorption capacity was decreased by the addition of bentonite to marine clay. Hence, when a large amount of bentonite is mixed into marine clay, in order to reduce the hydraulic conductivity, the negative effect on the adsorption capacity should be taken into consideration. (4) The cation exchange capacity of zeolite was much higher than that of any other materials used in this study. Seeing the relationship between the adsorption capacity and the CEC, the increase in the CEC by the addition of zeolite seemed to be related to the increase in the adsorption capacity of the samples. However, the adsorption capacity decreased slightly with the increase in the CEC by the addition of bentonite. Further study will be necessary to gain a greater understanding of the adsorption capacity versus CEC relation.
5. Conclusions After the severe accident at the Fukushima Daiichi Nuclear Power Plant in 2011, about 22 million m3 of contaminated soils were generated by the decontamination work and temporarily stored at various sites in Fukushima Prefecture. In order to consider the coastal disposal facilities for soil and waste contaminated by radioactive cesium, geotechnical sealing material for the marine environment was studied. Using samples consisting of marine clay, bentonite and zeolite, the hydraulic conductivity and the cesium-adsorption capacity were investigated by a series of laboratory tests. The main conclusions of this study are listed below: (1) The required hydraulic conductivity was discussed considering the safe management of soil and waste contaminated by radioactive cesium whose radioactivity concentration is less than 100,000 Bq/kg. Based on the assumption that the travel time by the seepage of a 4-m-thick geotechnical sealing layer is enough to reduce the radioactivity, the required hydraulic conductivity was determined as 1.0 109 m/s. The hydraulic conductivity for the pore water flow of the sealing material in this study satisfied the condition by mixing bentonite and zeolite at an effective consolidation pressure levels of more than 40 kPa. (2) When zeolite was mixed into marine clay, the hydraulic conductivity for the pore flow was almost the same as marine clay without the addition of zeolite. The
Acknowledgments This study comprises part of the research being conducted under the project entitled ‘‘Development of Offshore Disposal Facility for Disaster Wastes and Decontamination Wastes Polluted by Radiation (3K122109), which was funded by the Ministry of the Environment, Japan in 2012-2013. References Bureau of Port and Harbor, Tokyo Metropolitan Government, 2015. Port History.. Division of Nuclear Fuel Cycle and Environment, Atomic Energy Society of Japan, 2011. Contaminated Liquid Water Treatment for Fukushima Daiichi NPS (CLWT). (in Japanese). Fujimura, S., Eguchi, T., Matsunami, H., Ota, T., Murakami, T., Ishikawa, T., Makino, T., Akahane, I., Kamiya, T., Aono, K., Naka, T., Okushima, S., 2016. Transfer factors of radiocesium to brown rice in the decontaminated paddy field. Jpn. J. Crop Sci. 85 (2), 211–217. https://doi.org/10.1626/jcs.85.211, in Japanese. Fujita, T., Wang, L.P., Yabui, K., Dodbiba, G., Okaya, K., Matsuo, S., Nomura, K., 2013. Adsorption of cesium ion on various clay minerals and remediation of cesium contaminated soil in Japan. Res. Proces. 60 (1), 13–17. https://doi.org/10.4144/rpsj.60.13. Fukushima Prefectural Government, Agriculture, Forestry and Fishery Department, (2014). ‘‘Inversion tillage work technical manuals for decontamination in the paddy field. p. 1–27. (in Japanese).
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O. Kurihara et al. / Soils and Foundations 58 (2018) 1173–1186
Haug, M.D., Wong, L.C., 1992. Impact of molding water content on hydraulic conductivity of compacted sand-bentonite. Can. Geotech. J. 29 (2), 253–262. https://doi.org/10.1139/t92-029. International Atomic Energy Agency, 2015. The Fukushima Daiichi Accident, vol. 4.. Ito, M., Norota, M., Ishikawa, N., Ito, A., Umita, T., 2016. Sorption of cesium in controlled landfill leachate by natural minerals: zeolite, illite, and vermiculite in Japanese. J. Jpn. Soc. Civil Eng., Ser. G (Environ. Res.) 72 (7), III_429–III_436. https://doi.org/10.2208/jscejer.72. III_429. Japan Nuclear Cycle Development Institute, 1999a. H12: Project to establish the scientific and technical basis for HLW disposal in Japan, project overview report. Japan Nuclear Cycle Development Institute, 1999b. H12: Project to establish the scientific and technical basis for HLW disposal in Japan, supporting report 2 repository design and engineering technology. Japanese Geotechnical Society, 2008. Sequel - Survey, Prediction and Countermeasure of Soil and Groundwater Contamination. (in Japanese). Kahr, G., Madsen, F.T., 1995. Determination of the cation exchange capacity and the surface area of bentonite, illite and kaolinite by methylene blue adsorption. Appl. Clay Sci. 9 (5), 327–336. https://doi. org/10.1016/0169-1317(94)00028-O. Kamon, M., Omine, K., Katsumi, T., 2010. Geo-Environmental Engineering. (in Japanese). Kawasaki, T., Yamada, K., Ueno, K., 2009. Case report of sealing geomaterials for waste disposal sites in coastal area. J. Chugoku Branch Jpn. Geotech. Soc. 27 (1), 187–194, in Japanese. Kenney, T.C., Veen, W.V., Swallow, M.A., Sungaila, M.A., 1992. Hydraulic conductivity of compacted bentonite–sand mixtures. Can. Geotech. J. 29 (3), 364–374. https://doi.org/10.1139/t92-042. Komine, H., 2004. Simplified evaluation on hydraulic conductivities of sand–bentonite mixture backfill. Appl. Clay Sci. 26 (1–4), 13–19. https://doi.org/10.1016/j.clay.2003.09.006. Komine, H., 2010. Predicting hydraulic conductivity of sand–bentonite mixture backfill before and after swelling deformation for underground disposal of radioactive wastes. Eng. Geol. 114 (3–4), 123–134. https://doi.org/10.1016/j.enggeo.2010.04.009. Komine, H., Ogata, N., 1999. Experimental study on swelling characteristics of sand-bentonite mixture for nuclear waste disposal. Soils Found. 39 (2), 83–97. https://doi.org/10.3208/sandf.39.2_83. Mahara, Y., 1993. Storage and migration of fallout strontium-90 and cesium-137 for over 40 years in the surface soil of Nagasaki. J. Environ. Qual. 22 (4), 722–730. https://doi.org/ 10.2134/jeq1993.00472425002200040013x. Mimura, H., 2014. Highly selective adsorbents and decontamination of highly contaminated water. J. Ion Exc. 25 (3), 45–51. https://doi.org/ 10.5182/jaie.25.45, in Japanese. Mimura, H., Sato, N., Kirishima, A., 2011. (1) Selective separation and solidification of radioactive nuclides by zeolites. J. Ion Exc. 22 (3), 96– 108. https://doi.org/10.5182/jaie.22.96, in Japanese. Ministry of the Environment, 2011. The Policy of Disposal of Disaster Wastes in Fukushima Prefecture. (in Japanese). Ministry of the Environment, 2013. Guideline for temporary storage of removed soil generated by decontamination work, May, 2013, revised in Sept. 2016, (in Japanese). Ministry of the Environment, 2015. Wastes disposal and public cleansing acts , June 2015. (in Japanese). Ministry of the Environment, 2016. Technology development strategy for volume reduction and recycling utilization of removal soil etc. for interim storage. (in Japanese).
Mukai, H., Hirose, A., Motai, S., Kikuchi, R., Tanoi, K., Nakanishi, T. M., Kogure, T., 2016. Cesium adsorption/desorption behavior of clay minerals considering actual contamination conditions in Fukushima. Sci. Rep. 6, 21543. https://doi.org/10.1038/srep21543. Okumura, M., Nakamura, H., Machida, M., 2013a. Cs adsorption in clay minerals and zeolites. First principle calculation studies toward understanding their microscopic mechanism. J. Surf. Sci. Soc. Jpn. 34 (3), 135–142. https://doi.org/10.1380/jsssj.34.135, in Japanese. Okumura, M., Nakamura, H., Machida, M., 2013b. Mechanism of strong affinity of clay minerals to radioactive cesium: first-principles calculation study for adsorption of cesium at frayed edge sites in muscovite. J. Phys. Soc. Jpn. 82 (3), 033802. https://doi.org/10.7566/ JPSJ.82.033802. ¨ ren, A.H., Durukan, S., Kayalar, A.Sˇ., 2014. Influence of compaction O water content on the hydraulic conductivity of sand-bentonite and zeolite-bentonite mixtures. Clay Miner. 49 (1), 109–121. https://doi. org/10.1180/claymin.2014.049.1.09. ¨ ren, A.H., Kaya, A., Kayalar, A.S., 2011. Hydraulic conductivity of O zeolite–bentonite mixtures in comparison with sand–bentonite mixtures. Can. Geotech. J. 48 (9), 1343–1353. https://doi.org/10.1139/t11042. Osaka Bay Regional Offshore Environmental Improvement Center, 2015.
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ScienceDirect Soils and Foundations 58 (2018) 1173–1186 www.elsevier.com/locate/sandf
Cesium-adsorption capacity and hydraulic conductivity of sealing geomaterial made with marine clay, bentonite, and zeolite Ooki Kurihara a, Takashi Tsuchida a,⇑, Genki Takahashi b, Gyeong-o Kang c, Hiroki Murakami d a
Department of Civil and Environmental Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi, Hiroshima 739-8527, Japan b Chuden Engineering Consultants Co., Ltd, 2-3-30, Deshio, Minami-ku, Hiroshima 734-8510, Japan c Honam Regional Infrastructure Technology Management Center, Chonnam National University, 50, Daehak-ro, Yeosu, Jeonnam 59626, Republic of Korea d Nippon Koei Co., Ltd, 1-14-6, Kudankita, Chiyoda-ku, Tokyo, Japan Received 19 April 2017; received in revised form 1 June 2018; accepted 6 June 2018 Available online 20 August 2018
Abstract Great amounts of soil and waste contaminated with radioactive cesium have been generated due to the decontamination work after the Fukushima Daiichi Nuclear Power Plant accident. The aim of this study is to develop a sealing geomaterial for use at the disposal facilities of the soil and waste constructed in the maritime environment. The geomaterial consists of marine clay, bentonite, and zeolite. The hydraulic conductivity and cesium-adsorption performance of the geomaterial were examined through laboratory tests with different proportions of bentonite and zeolite added to marine clay. It was concluded that the hydraulic conductivity could be reduced to the required level by increasing the amount of bentonite and that the cesium-adsorption capacity could be enhanced by increasing the amount of zeolite. Ó 2018 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.
Keywords: Marine clay; Bentonite; Zeolite; Radioactive cesium; Adsorption
1. Introduction Due to the severe accident at the Fukushima Daiichi Nuclear Power Plant, accompanying the Great East Japan Earthquake and tsunami on March 11th, 2011, a large amount of radioactive material was released into the environment in Fukushima Prefecture as well as adjacent prefectures. In order to reduce the radiation level to the level at which people can live safely, great efforts have been made in decontamination work by removing a large
Peer review under responsibility of The Japanese Geotechnical Society. ⇑ Corresponding author. E-mail addresses: [email protected] (O. Kurihara), ttuchida@ hiroshima-u.ac.jp (T. Tsuchida), [email protected] (G. Kang), a7926@ n-koei.co.jp (H. Murakami).
amount of surface soil from the affected areas. The contaminated soil removed from these areas has been packed in containers made of polyethylene or polypropylene, and has been piled up at temporary storage sites in various locations around the affected areas. According to the Ministry of the Environment (2016), the maximum estimated volume of contaminated soil generated by the decontamination work and contaminated waste in Fukushima Prefecture is about 22 million m3. In accordance with the guidelines of the Ministry of the Environment (2011), disaster waste having radioactivity of less than 8000 Bq/kg can be disposed of in general waste disposal facilities (ordinary controlled types of final disposal facilities). On the other hand, contaminated soil generated by the decontamination work and waste with radioactivity of more than 8000 Bq/kg should be disposed
https://doi.org/10.1016/j.sandf.2018.06.004 0038-0806/Ó 2018 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.
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of in specially designed interim storage facilities in Fukushima Prefecture or designated disposal facilities in other prefectures. Further, the Japanese government has decided to move all soil and waste from these interim storage facilities to final disposal facilities within 30 years in order to return the areas with the interim storage facilities back to the landowners. Consequently, the construction of final disposal facilities for the soil and waste presently stored in interim storage facilities will become an important social and engineering problem. Waste disposal facilities have been constructed in coastal areas of Japan (Bureau of Port and Harbor, Tokyo Metropolitan Government, 2015; Osaka Bay Regional Offshore Environmental Improvement Center, 2015). In order to assure environmental safety, the facilities have been constructed based on the technical criteria developed by the government (Waterfront Vitalization and Environment Research Foundation, 2008). For the sealing layer of general waste disposal facilities, the following technical conditions are required by the Waste Management and Public Cleansing Law (Ministry of the Environment, 2015).
(c) An impermeable sheet underlain by a layer of asphalt or concrete, 5 cm or more in thickness, with hydraulic conductivity equal to or less than 109 m/s (d) A protective layer of non-woven fabric to be covered with two independent impermeable sheets. According to the above technical criteria, when the seafloor is composed of marine clay that has low hydraulic conductivity and is more than 5 m in thickness, the clay layer can be considered as a bottom sealing layer. Most of the existing waste disposal facilities have been constructed on this type of clayey layer. On the other hand, when the seafloor of an available location is composed of sandy soil, it is necessary to construct a sealing layer on the seafloor. Studies on geotechnical material for sealing layers in a maritime environment have been carried out by several researchers (Watabe et al., 2003; Yamada et al., 2003; Ueno et al., 2008; Watabe et al., 2011). The first waste disposal facility using a sealing layer made of such geotechnical material was constructed on the sandy seafloor in the Seto Inland Sea of Japan (Kawasaki et al., 2009). Fig. 1 shows a full view and the cross-section (A-A0 ) of Sangawa Tobu Disposal Facility for industrial waste. A 2-m-thick sealing layer, made of a marine clay-bentonite mixture, was placed at the bottom of the disposal facility. Both sides of the facility were sealed by double-layer impermeable sheets and a pile of water cut-off sheets. The sealing material was placed directly on the seabed by tremie pipes from the special working vessel shown in Fig. 2. Two steel pipes, 8 in.
(a) Continuous soil layer, 5.0 m or more in thickness, with hydraulic conductivity of less than 107 m/s or a layer with a sealing effect equal to or greater than above (b) An impermeable sheet underlain by 50-cm-thick material, such as clay, with hydraulic conductivity equal to or less than 108 m/s
4m (high de)
219.5m
Sand fill (excavated seabed) Excavated area
Stones
0m
Level of original seabed Steel slag
-14m -17m
Water cut-off sheet pile
-19m
Double-layer impermeable Natural sand seabed sheets
Sand mat
Steel slag
Geotexle
Boom sealing layer by clay-bentonite mixture 119.0 m
Double-layer impermeable sheets
Natural sand seabed
Cross-secon of A-A’ line
500.0 m A
A̓
Sangawa disposal facility for industrial waste under construcon (2008) Fig. 1. Full view and cross-section of Sangawa Disposal Facility for industrial waste (Kawasaki et al., 2009).
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Working vessel 4m (high de)
Mixing plant
Sand fill (excavated seabed)
Transportaon pipe
Stones
Water cut-off sheet pile
Steel slag
Covering soil
Steel slag
Double-layer impermeable sheets
Double-layer impermeable sheets
Boom sealing layer by clay-bentonite mixture
Natural sand seabed
Guide pipe φ800 steel Tremie pipe 8 inch
500.0 m
GPS antenna
Working vessel for placing sealing material
Valve to control air pressure.
Guide pipe with tremie pipes
Lead
Control room
Transportaon pipe of sealing material
Sealing material made of marine clay Fig. 2. Placing of sealing material by tremie pipe in seawater (Kawasaki et al., 2009).
and the space between the vertical walls should be filled with sealing layers. These layers must have very low hydraulic conductivity and be flexible enough to withstand the deformations which may be caused by future strong earthquakes over the next hundreds of years (Watabe et al., 2003; Ueno et al., 2008; Kawasaki et al., 2009). To satisfy these conditions, a geotechnical sealing material, which is a mixture of marine clay, bentonite, and zeolite, has been examined. The purpose of this study, therefore, is to develop a geotechnical material for the sealing layer of final disposal facilities available for the soil and waste contaminated by radioactive materials. Fig. 3 shows a schematic view of the final disposal facility discussed in this study.
(20 cm) in inner diameter, were used as the tremie pipes attached to a guide pipe (an 800-mm steel pipe). At present, there are no technical criteria for waste disposal facilities in coastal areas for soil and waste that have been contaminated by radioactive material. However, considering the large amount of soil and waste for final disposal, the construction of such disposal facilities in coastal areas would be an effective option that ought to be examined from an engineering point of view. Moreover, if facilities are to be constructed in coastal areas, they should be located on a dense sand or gravel seabed to secure their structural safety against strong earthquakes. To maintain the safety of the maritime environment, a sealing layer must be placed at the bottom of the seabed,
Super structure
Tsunami impact reducon blocks
Double-steel pipe sheet pile
Covering soil Surface sealing layer
Backfill material
Foot-protecon works
Wave-dissipang blocks
Caisson
Backfill
stone
Sealing material
Soil and waste contaminated by radioacve cesium
Rubble mound Seabed sand
Sealing layer Geotexle
Protecve soil layer (counterweight)
Fig. 3. Schematic view of disposal facility in this study for soil and waste contaminated by radioactive cesium.
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In this study, mixtures of marine clay, bentonite, and zeolite were prepared at different proportions and tested by means of consolidation and seepage tests to assess their consolidation, hydraulic conductivity, and adsorption property. Bentonite was added to reduce the hydraulic conductivity of the sealing material to satisfy the requested level, and zeolite was added to the sealing material to create a higher adsorption capacity for cesium. As the adsorption capacity is an important factor for the safety of the sealing material, the adsorption capacity was increased by adding zeolite to sealing material made of marine clay. In this study, the hydraulic conductivity and the cesiumadsorption capacity of marine clay-bentonite-zeolite mixtures were studied through a series of laboratory tests. 2. Consideration of required properties of sealing material The performance characteristics of the coastal disposal facilities targeted in this study must be scaled up by a large margin compared to regular disposal facilities constructed under the existing technical criteria. The Ministry of the Environment published guidelines for the radioactivity concentration of waste. Waste which is not greater than 8000 Bq/kg in radioactivity can be disposed of in existing disposal facilities constructed under the present technical standards (Ministry of the Environment, 2011). However, according to the guidelines, waste which is greater than 8000 Bq/kg in radioactivity, but less than 100,000 Bq/kg, is defined as waste which must be kept safe until the radioactivity falls to a low level and its safety can be confirmed. Based on the above definition, this study attempts to develop a coastal disposal facility which can safely preserve the soil and waste whose radioactive concentration is less than 100,000 Bq/kg as of 2011. Although the required properties of the sealing material have been discussed in a previous study (Tsuchida et al., 2017), considering the decay of the radioactivity in the soil and waste, the required properties were reconsidered in this study. The government has decided that the final disposal must be carried out within 30 years from the start of the service of the interim storage facility. It is assumed here that the final disposal would start in 2035. This is because 2045 is 30 years from the start of the interim storage and it will take 10 years to complete the final disposal of the soil and waste in the interim storage facility. The radioactivity ratio of Cs-137 and Cs-134 at the accident was 1:1 (International Atomic Energy Agency, 2015). Assuming that the radioactivity concentration of Cs-137 and Cs-134 contained in the soil and waste as of 2011 is 50,000 Bq/kg, the radioactivity concentration of Cs-137 and Cs-134 contained in the soil and waste in 2035 will be 28,806 Bq/kg and 16 Bq/kg, respectively. The leaching rate of waste to pore water is assumed to be 5%, based on the results of leaching tests of polluted soil and incinerated ash with an added safety margin (Tsuchida et al., 2017). The void ratio and dry density of the soil and
waste are assumed to be 0.8 and 1700 kg/m3, respectively (Tsuchida et al., 2017). From these assumptions, the concentration of radioactivity in the pore water of contaminated soil and waste is calculated with Eq. (1): For Cs 137 : C w ¼ C s E ed ¼ 28;806 0:05 1;700 1000 ¼ 3;061 Bq=L 0:80
1000 ¼ 2 Bq=L For Cs 134 : C w ¼ C s E ed ¼ 16 0:05 1;700 0:80
ð1Þ
where Cw is the concentration of radioactivity in the pore water, Cs is the concentration of radioactivity in the soil and waste, E is the leaching rate of the contaminated soil, qd is the dry density of the soil and waste, and e is the void ratio of the soil and waste. The sealing material developed in this study was placed on a sea floor consisting of dense sand or gravel. It was assumed that the sealing layer was subjected to consolidation stress of 30 kPa, the overburden stress of the covering layer, before carrying the contaminated soil and waste. When the head difference between the inside of the disposal facility and the sea level is H m, the seepage flow from inside to outside of a coastal disposal facility will take place through the bottom sealing layer and the sand-gravel layer at the sea floor, as shown in Fig. 3. Moreover, seepage can take place through the vertical sealing layer. Therefore, the vertical sealing layer will be made as a double-steel sheet pile structure with sealing material between the steel sheet piles. Here the sealing of the bottom layer is discussed. For the sealing of hazardous material in the pore water of soil and waste, both diffusion and advection must be considered. In the case of radioactive cesium in soil and waste, it was found that advection dominates the movement of the contaminants in the pore water (Tsuchida et al., 2017). Accordingly, only the effect of advection, the movement of pore water, is investigated in this study. In the regulations for nuclear power plants, the level of radioactivity in discharged cooling water acceptable to the natural environment is determined with Eq. (2) (Science and Technology Agency, 2000). the concentration of Cs134 ðBq=LÞ 60 ðBq=LÞ þ
the concentration of Cs137 ðBq=LÞ 1 90 ðBq=LÞ
ð2Þ
The concentration of radioactivity in the leachate water is thought to satisfy Eq. (2). The concentration of 2 Bq/L of Cs-134 in Eq. (1) becomes extremely small until the concentration of 3,061 Bq/L of Cs-137 decays to 90 Bq/L or less. Therefore, considering only the Cs-137 from hereafter, the emission limit of Cs-137 in the leachate is regarded as 90 Bq/L. Since the half-life of radioactive cesium-137 is 30.1 years, the amount of time until the maximum elution amount of 3,061 Bq/L decreases to the environment limit of 90 Bq/L was calculated with Eq. (3). Tt 1 C ¼ C0 ð3Þ 2
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where C is the concentration of radioactivity at time t, C0 is the initial concentration of radioactivity, and T is the halfperiod of radioactive cesium. Therefore, the amount of time until the maximum concentration of elution 3061 Bq/L becomes 90 Bq/L, t90y, was calculated with Eq. (4), as follows: t90y ¼
T ln CC0 ln 12
¼
90 30:1 ln 3;061
ln 12
¼ 153 years
ð4Þ
The main performance of water sealing is to make the travel time of the sealing layer longer than t90y calculated by Eq. (4). The thickness of sealing layer H, which is large enough and feasible in construction practice, is considered to be 4 m. When the difference in design head between the inside and the outside of the disposal facility is h, the hydraulic conductivity of the sealing layer required to make the travel time of sealing layer t90y is calculated with Eq. (5). k req ¼
k Darcy H H2 ¼ ¼ t90y ðh=H Þ t90y h n
ð5Þ
When the maximum head difference h is assumed to be 2 m and n is the porosity, kreq is calculated as 1.65 109 m/s (1.65 107 cm/s). As discussed above, the required hydraulic conductivity for the sealing material is determined as 1.00 109 m/s in this study. A number of studies have been carried out on sealing geotechnical material made of soil-bentonite mixtures. However, most of the studies were made for unsaturated soil or soil-bentonite mixtures with low water contents, because the swelling property of bentonite in unsaturated soil-bentonite mixtures is expected to reduce the hydraulic conductivity of the compacted clay liners made for inland waste disposal sites (Haug and Wong, 1992; Kenney et al., 1992) or that of the barrier for the geological disposal of nuclear waste (Komine and Ogata, 1999; Japan Nuclear Cycle Development Institute, 1999a, 1999b; Komine, 2004, 2010). However, the sealing material for maritime disposal facilities is fully saturated by seawater and has a high water content. And the intensity of the radioactivity of the soil and waste generated by the decontamination work in Fukushima Prefecture is considerably lower than that of the waste to be placed at the geological disposal sites. Watabe et al. (2011) studied the hydraulic conductivity of marine clay-bentonite mixtures with different sand contents for the sealing layer of maritime disposal facilities, but they did not consider the use of a sealing layer for soil and waste contaminated by radioactive cesium. Although the required hydraulic conductivity of the sealing layer was calculated in order to have enough time to reduce the radioactivity of the cesium in the soil and waste to a safe level, multiple safety should be ensured to gain public acceptance. For the additional safety of the sealing layer, the cesium adsorption property should be taken into consideration. It has been shown that some clay minerals have strong
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cesium-adsorption properties (Wahlberg and Fishman, 1962; Sawhney, 1964; Mahara, 1993; Staunton and Roubaud, 1997; Wu et al., 2009). After the Fukushima Nuclear Plant accident, the cesium-adsorption properties of soil have been intensively studied in various fields of science and engineering (Fujita et al., 2013; Okumura et al., 2013a, 2013b; Mukai et al., 2016). However, only Tsuchida et al. (2017) have carried out a study on the cesium-adsorption properties of saturated marine clay and marine clay-bentonite mixtures. From a series of column tests, they found that marine clay and marine clay-bentonite mixtures with high water contents have a significant cesium-adsorption property, and that the pore water in marine clay-bentonite mixtures must be replaced 12 times by the flow of a cesium solution before the cesium can be measured in the outflow water. As a practical adsorption material for radioactive cesium, zeolites have been highlighted since the 2011 accident. Zeolites are porous crystalline aluminosilicates which include exchangeable cations at the sites of their threedimensional structure. Among them, mordenite, which has a relatively large Si/Al ratio and tunnel-shaped hole paths, clinoptilolite, and chabazite, which has lantern-like cavities, have a characteristic whereby they adsorb and capture Cs+ selectively (Mimura, 2014). A team from the Atomic Energy Society of Japan carried out the experiments to evaluate the adsorption properties of various adsorbents mainly on granular zeolite for Cs-137 from real seawater (Division of Nuclear Fuel Cycle and Environment, Atomic Energy Society of Japan, 2011; Yamagishi, 2011). According to Mimura et al. (2011), the distribution coefficient Kd of mordenite and chabazite, among some zeolites, was approx. 800 cm3/g (90% adsorption), and it has been confirmed that natural zeolites can selectively adsorb Cs in seawater. The adsorption property of cesium for remediation and decontamination in Fukushima Prefecture has been utilized in the in-situ replacement method of paddy fields where the contaminated surface soil layer was replaced by a deep layer (Fujimura et al., 2016; Fukushima Prefectural Government, Agriculture, Forestry and Fishery Department, 2014). The Ministry of the Environment (2013, 2016) has recommended placing a covering soil layer, mixed with zeolite, on the bottom sealing sheet for a temporary storage site. Ito et al. (2016) confirmed the Cs adsorption capacity of zeolite using leachate from an actual inland-controlled final disposal site. The hydraulic conductivity of zeolite-bentonite ¨ ren mixtures for compacted clay liners was studied by O et al. (2011, 2014). They showed that the hydraulic conductivity of compacted zeolite-bentonite mixtures was much larger than that of sand-bentonite mixtures. However, almost all the previous studies on the adsorption property of zeolite targeted the use of a sealing layer inland or covering soil and no studies have been carried out on marine clay-zeolite mixtures under saturated and high-water content conditions.
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3. Characteristics of hydraulic conductivity
Passing percentage by weight (%)
100
3.1. Sample preparation Marine clay dredged from Tokyo Bay, bentonite produced in Wyoming, and zeolite produced in Iwami of Shimane Prefecture, Japan, were used in this study. Table 1 illustrates the physical properties of Tokyo Bay clay, bentonite, and zeolite. Tokuyama Port clay was used for a comparison of the adsorption property with the samples tested in this study. Fig. 4 shows the gradation curve of the Iwami zeolite. As shown in this figure, the mean grain size of zeolite is 0.203 mm and the content of fines of less than 75 lm is 24.5%. From the Unified Soil Classification System, the zeolite was classified as silty sand (SM). The proportions of the samples of the sealing materials used in this study are shown in Table 2. When the samples were made, the bentonite was mixed with artificial seawater (1.03 g/cm3) to make the water content 200%. The composition of the artificial seawater is shown in Table 3. The added amounts of bentonite and zeolite were described as dry weights for 1 m3 of Tokyo Bay clay whose water content was 1.5 wL (wL: liquid limit). The zeolite additive rate in Table 2, zc, is the ratio of zeolite to the dry mass of Tokyo Bay clay. 3.2. Experiments on the performance of water-sealing properties The samples shown in Table 2 were analyzed with an incremental loading consolidation test using the apparatus shown in Fig. 5. The test was conducted according to the test method for the one-dimensional consolidation properties of soils using incremental loading (JIS A1217). The samples in the slurry states were filled up to the consolidation ring whose height was 20 mm and diameter was 60 mm. The eight stages of the incremental loading consolidation test were conducted using consolidation stresses from 4.9 kPa to 627.2 kPa with the stress increment ratio Dr0v / r0v of 1. In each stage, the end of primary consolidation p was determined by the t method. After the primary consolidation was over, the next stage was started. During consolidation, the immersion container was filled with distilled water. Using the theory of one-dimensional consolidation, the hydraulic conductivity due to Darcy’s law, kDarcy, was calculated from Eq. (6), where cv is the coefficient of consolp idation measured by the t method, mv is the coefficient of volume compressibility, and cw is the unit weight of water.
90 80 70 60 50 40 30 20 10 0
0.001
0.01
0.1 Sieve size (mm)
1
10
Fig. 4. Gradation curve of Iwami zeolite.
Table 2 Description of samples. Sample
Bentonite addition amount [kg/m3]
Zeolite addition amount [kg/m3]
Zeolite additive rate, zc (%)
TB TBz10 TBz20 TBz50 TBb50 TBb100 TBb50z10 TBb50z20 TBb100z10 TBb100z20
0 0 0 0 50 100 50 50 100 100
0 43 87 217 0 0 43 87 43 87
0 10 20 50 0 0 10 20 10 20
1
TB: Tokyo Bay clay, b: bentonite, z: zeolite. The bentonite addition amount is the value per 1 m3 of Tokyo Bay clay in 1.5 wL (wL: liquid limit). 3 The zeolite additive rate is the rate of the dry weight of soil per 1 m3 of Tokyo Bay clay in 1.5 wL. 2
Table 3 Composition of artificial seawater. Ions
Content [g/kg]
Resolved ratio [%]
Cl Na SO4 Mg K Ca HCO3 Br BO3 Total
19.13 10.75 1.89 1.37 0.38 0.32 0.08 0.08 0.04 34.02
56.22 31.58 5.53 4.03 1.13 0.94 0.25 0.19 0.11
Table 1 Physical properties. Property
Tokyo Bay clay
Tokuyama Port clay
Wyoming bentonite
Iwami zeolite
Density of soil particles qs [g/cm3] Liquid limit wL [%] Plastic limit wP (%) Plasticity index
2.79 133.9 60.38 73.52
2.616 110.6 43.43 67.17
2.898 187.0 (sea water) 58.7 (sea water) 128.3
2.38 – – –
O. Kurihara et al. / Soils and Foundations 58 (2018) 1173–1186
Consolidation pressure
Loading plate Guide ring Loading plate
Water holder
Sample
Compaction ring Height 20mm Diameter60mm
Porous stone Drain cock
Fig. 5. Consolidation test apparatus.
k Darcy ¼ cv mv w
ð6Þ
The hydraulic conductivity by Darcy’s law, kDarcy, was calculated from the flow velocity, vDarcy, through the soil samples assuming that conventionally the water flows all through the cross-section area of the soil sample. However, water flows only in the pores between the particles, and in order to calculate the travel time of the water flow, it is necessary to consider the actual flow velocity in the pore spaces, vpore. Assuming that the water flows all through the pores of the samples, vpore is calculated with vDarcy as follows: vpore ¼ vDarcy =n ¼ vDarcy ð1 þ eÞ=e
ð7Þ
where e is the void ratio and n is the porosity. Accordingly, the hydraulic conductivity for the flow through the pores, kpore, is calculated as follows: k pore ¼ k Darcy =n ¼ k Darcy ð1 þ eÞ=e
ð8Þ
3.3. Results and discussion on hydraulic conductivity In Eq. (7), vpore was obtained by using the porosity, but it was pointed out that the effective porosity is remarkably lower than the porosity due to the influence of the adsorbed water, etc., especially in the case of fine grained soil such as clay (Kamon et al., 2010). Therefore, a test was carried out to determine the effective porosity, and the assumption of the porosity was examined. The procedure up to consolidation was similar to that of the consolidation and seepage test to be described later. Matsushima Bay clay was used as the specimen. Soon after the primary consolidation was over, the seepage test was conducted. Artificial seawater, whose salinity concentration was 3%, was injected into the sample by applying 19.4 kPa of air pressure to the double-cylindrical burette until the salinity of the effluent reached 3%. Then, artificial seawater, whose salinity concentration was 6%, was injected into the sample. The effluent was collected every 5 mL and the electric conductivity was measured. The electrical conductivity was converted to the salinity concentration. The salinity
was normalized so that the salinity concentration level of 3% was 0 and that of 6% was 1, and normalized salinity concentration was obtained. Next, following the estimation method of the effective porosity (Japanese Geotechnical Society, 2008), the effective porosity was calculated. First, fitting was performed so as to minimize the sum of the squares of the residuals of the one-dimensional theoretical solution of the advection-dispersion equation and the measured values. Since nonadsorptive solute was used here, the delay coefficient was assumed to be R = 1. Delays due to dead zones, such as upstream and downstream porous stones, as well as downstream piping, were corrected. Then, the dispersion coefficient D of the theoretical solution and seepage velocity v were determined. Finally, the effective porosity, ne, was calculated from ne = u/v. The relationship between the normalized salinity concentration and time is shown in Fig. 6. Effective porosity ne, estimated by this experiment, was 0.703, and porosity n, obtained from void ratio e in the seepage test, was 0.724. The difference between the estimated effective porosity and the porosity was small. In this way, the method of using porosity instead of the effective porosity when calculating vpore in Eq. (7) was verified. Fig. 7 shows the relationship between void ratio e and effective consolidation pressure r0v for all the samples. The void ratios of the samples decreased with the increase in the zeolite additive rate, zc. At r0v = 20 kPa, the void ratios of the samples of zc = 10%, 20% and 50% were, respectively, 0.9 times, 0.8 times and 0.65 times that of zc = 0%. However, compression index Cc was almost unchanged by the zeolite addition, which means that the compression property was not affected much by the addition of zeolite. Comparing the e-log r0v relation of the same zeolite content, the void ratio became smaller with an increase in the bentonite addition, when the consolidation pressure was larger than 80 kPa. It is thought that the void ratio became smaller as the fine particles of the added bentonite filled the pores of the Tokyo Bay clay-zeolite mixture.
1 0.9
Normalized salinity concentraon, C /C 0
Displacement gauge
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0.8 measured value
0.7
theorecal soluon
0.6
Esmated D = 7.00 x 10 - 10 m 2 /s
0.5
Measured n = 0.724 Esmated n e = 0.703
0.4 0.3 0.2 0.1 0 0
0.5
1
1.5
2
2.5
Time (day)
Fig. 6. Normalized salinity concentration and time.
3
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O. Kurihara et al. / Soils and Foundations 58 (2018) 1173–1186 4.5
zc=0%
Void rao , e
4.0
zc=10%
TB TBz10 TBz20 TBz50 TBb50 TBb50z10 TBb50z20 TBb100 TBb100z10 TBb100z20
zc=20%
3.5
3.0
zc=50% 2.5
2.0
1.5
Hydraulic conducvity in pore , kpore (m/s)
1E-08
Tokyo Bay clay-bentonitezeolite mixture
Tokyo Bay clay-bentonitezeolite mixture
1E-09
bc=0 kg/m3 zc=0,10,20% TB TBz10 TBz20 TBz50 TBb50 TBb50z10 TBb50z20 TBb100 TBb100z10 TBb100z20
bc=50 kg/m3 zc=0,10,20%
bc=100 kg/m3 zc=0,10,20%
1E-10
1
10
100
1000
1
10
100
1000
Effecve consolidaon pressure, σv' (kPa)
Effecve consolidaon pressure, σv' (kPa) Fig. 7. e–logr0v curve.
Fig. 9. kpore and effective consolidation pressure r0v .
Fig. 8 illustrates the relationship between the hydraulic conductivity of Darcy’s law, kDarcy, and consolidation pressure r0v . As shown in Fig. 8, kDarcy was clearly reduced by the addition of bentonite. With the addition of zeolite, it seemed constant for samples with no bentonite addition. However, when bentonite was added, kDarcy was reduced by the increase in the additive rate of the zeolite. Fig. 9 shows kpore with effective consolidation pressure. From Eq. (8), kpore becomes larger than kDarcy. Fig. 9 shows that by the addition of bentonite, kpore is reduced, and among the samples under the same bentonite conditions of bc = 50 kg/m3 and 100 kg/m3, kpore was smaller as the content of zeolite became larger. As discussed above, the required hydraulic conductivity for the sealing layer is less than 1.00 109 m/s by considering the travel time of the flow of pore water. As seen in Fig. 8, the desired level
of hydraulic conductivity was achieved by the addition of zeolite and bentonite at consolidation pressures larger than 40 kPa. Fig. 10 shows the hydraulic conductivity ratio kpore/k*pore with the effective consolidation pressure, where k*pore is kpore of the Tokyo Bay clay without bentonite or zeolite at the same effective overburden pressure. As shown in this figure, by mixing in zeolite, the value of kpore/k*pore is the same or slightly increased. However, when both bentonite and zeolite are added to the clay, the value of kpore/ k*pore is reduced to 40–70% of that of the Tokyo Bay clay, and the reduction is larger when the content of zeolite is larger. In addition, the mixing of zeolite has no significant effect on the hydraulic conductivity. Watabe et al. (2011) carried out a series of incremental loading oedometer tests and microscopic observations in order to investigate the
TB TBz10 TBz20 TBz50 TBb50 TBb50z10 TBb50z20 TBb100 TBb100z10 TBb100z20
1E-09 bc=0 kg/m3 zc=0,10,20%
bc=50 kg/m3 zc=0,10,20%
Tokyo Bay clay-bentonitezeolite mixture kg/m3
bc=100 zc=0,10,20%
1E-10 1
10 100 Effecve consolidaon pressure, σv' (kPa)
1000
Fig. 8. Darcy’s hydraulic conductivity kDarcy and effective consolidation pressure r0v .
Hydraulic conducvity rao in pore , kpore /(kpore)*
Darcy's hydraulic conducvity, kDarcy (m/s)
1E-08 1.8 TBz10 TBb50 TBb100
1.6
TBz20 TBb50z10 TBb100z10
TBz50 TBb50z20 TBb100z20
1.4 1.2 1 bc=0kg/m3
0.8
bc=50kg/m3
0.6 0.4 0.2
Tokyo Bay clay-bentonite-zeolite mixture
bc=100kg/m3
0 1
10
100
1000
Effecve consolidaon pressure, σv' (kPa)
Fig. 10. Hydraulic conductivity ratio in pore kpore/(kpore)* and effective consolidation pressure r0v .
O. Kurihara et al. / Soils and Foundations 58 (2018) 1173–1186
6 TB TBz10 TBz20 TBz50 TBb50 TBb50z10 TBb50z20 TBb100 TBb100z10 TBb100z20
5.5 5
Void rao , e
4.5 4 3.5
Tokyo Bay clay-bentonitezeolite mixture
bc=100kg/m3 zc=0%
3 2.5 2 1.5
Covering sand
4.0 m
Sealing layer
Sandy seabed Fig. 12. Sealing layer and covering sand.
Tokyo Bay clay-bentonitezeolite mixture
bc=0kg/m3
100
bc=50kg/m3
bc=100kg/m3 TB TBz10
10
TBz20 TBz50 TBb50 TBb50z10 TBb50z20 TBb100 TBb100z10 TBb100z20
1
1
10
100
1000
Effecve consolidaon pressure, σv' (kPa) Fig. 13. cv–r0v curve.
layer, as shown in Fig. 1. The development of shear strength of the sealing layer, due to the consolidation by the selfweight and the weight of the covering layer, is an important factor for shortening the time for the construction of disposal facilities. The rate of the strength increase with consolidation is determined by the coefficient of consolidation, cv. Fig. 13 illustrates the relationship between cv and effective consolidation pressure r0v . The value of cv of the sealing material decreased with the addition of bentonite. At r0v = 40 kPa, cv decreased from 65 cm2/day to 38 cm2/day for bc = 50 kg/m3 and to 25 cm2/day for bc = 100 kg/m3. When zeolite and bentonite are mixed with clay, cv becomes slightly smaller with the increase in the zeolite content. This is due to the reduction of hydraulic conductivity by mixing in both zeolite and bentonite.
bc=0 zc=50%
bc=0 zc=20% bc=100kg/m3 bc=50kg/m3 zc=20% zc=20%
1 1E-08
Contaminated soil and waste
Coefficient of consolidaon, cv (cm2/day)
influence of sand/bentonite fractions on hydraulic conductivity. According to their results, an additive fraction of sand causes a decrease in compressibility, but does not affect the hydraulic conductivity, if the sand particles do not form a skeletal structure and each particle is independent in a clay matrix with a small sand fraction. In this study, as the gradation of zeolite was close to that of the sand and the content was not enough to form a skeletal structure, it was reasonable that the mixture of zeolite did not show any significant effect on the hydraulic conductivity. In Fig. 10, the values for the kpore in the mixtures of both zeolite and bentonite were 10–20% smaller than that of the mixture of bentonite alone. This fact cannot be explained by the previous studies on sand-bentonite mixtures, and further studies considering the microstructure of zeolite-bentonite mixtures will be necessary to find the reason. From the viewpoint of the purpose of this study, it is good that the zeolite addition will not cause any increase in the hydraulic conductivity of the mixture. Fig. 11 illustrates the relationship between hydraulic conductivity kpore and void ratio e. The kpore value became larger for the same void ratio as the addition of zeolite became larger, while the kpore value became smaller as the bentonite content became larger. For the design and construction of bottom sealing layers, the coefficient of consolidation is also important. To carry the covering layer on the sealing layer, the sealing layer has to have enough shear strength to support the overlaying materials. Further, an increase in strength accompanying consolidation will be necessary to fill the soil and waste on the covering layer. Fig. 12 shows the covering sand on the sealing layer. To prevent the disturbance of the sealing layer by the deposition of soil and waste, a covering layer must be placed on the sealing layer. In the case of the Sangawa Tobu Disposal Facility, a 2-meterthick covering layer was carefully placed on the soft sealing
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4. Adsorption property 1E-07
Hydraulic conducvity, kpore (m/sec) Fig. 11. Relationship between e and kpore.
1E-06
4.1. Experiments on the adsorption property In the previous study, the authors carried out an elution test on soil and waste which were contaminated by
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radioactive cesium (Tsuchida et al., 2017). It was found that the concentration of radioactive cesium, qCs137 (Bq/L), and the concentration of stable cesium, qCs133 (mg/L), were closely related and can be shown as follows: Cs133 ¼ 0:00014ðCs137 Þ1:12
ð9Þ
Based on their findings, the adsorption property of the sealing material was studied with a column seepage test with stable cesium Cs-133 instead of Cs-137. This test was conducted for the samples shown in Table 2, except for TBz50, TBb50z20 and TBb100z20. The results of the adsorption test were compared to the data on Tokuyama Port clay in the previous study (Tsuchida et al., 2017). Fig. 14 illustrates the consolidation and seepage test apparatus. The apparatus had a structure which could suffuse the cesium solution. A consolidation ring with a height of 20 mm was used for the general consolidation test, but a ring with a height of 10 mm was used in the adsorption study to shorten the time of the water flow. The samples in the slurry state were carefully put into the sample ring which had a height of 10 mm and a diameter of 60 mm. The three stages of the incremental loading in the consolidation test were conducted with consolidation stress levels of 4.9 kPa, 9.8 kPa and 19.6 kPa, with a stress increment ratio of Dr0v /r0v = 1. In each loading stage, the consolidation pressure was maintained until the samples achieved 100% consolidation before applying the next stress increment. In each stage, p the end of primary consolidation was determined by the t method. Soon after the primary consolidation was over, the seepage test was carried out for which
30 mg/L of CsCl aqueous solution (stable cesium, Cs-133) were used as the permeant. After consolidation at the consolidation stress of 19.6 kPa, the water that had drained out of the specimen remaining in the loading plate was replaced with the CsCl aqueous solution. The CsCl aqueous solution was flowed through the sample by giving 19.4 kPa of air pressure through the doublepipe burette. The effluent permeant was collected every 10 mL, and its concentration of cesium was measured using an atomic absorption analyzer. Due to the limitation of the test period, the seepage test was halted as soon as the trend of the breakthrough curve could be confirmed. As mentioned above, assuming that the concentration of radioactive cesium is 100,000 Bq/kg, the concentration of pore water was calculated to be 3061 Bq/L, as shown in Eq. (1). By using Eq. (9), the concentration of stable cesium can be evaluated as 1.14 mg/L. In the flame mode and the calibration line method of the atomic adsorption analyzer, the accuracy of the measurement fell when the measured concentration was out of the range of the calibration line. As the range of the calibration line in this study was from 1.0 mg/L to 5.0 mg/L and the seepage water was diluted by 100% in the treatment for the measurement, the reliability of the measured concentration of less than 2.0 mg/L was low. In the previous study (Tsuchida et al., 2017), the target of the sealing material on adsorption was to reduce the cesium concentration of seepage water to one-tenth of the initial concentration. To measure the cesium concentration around one-tenth of the initial concentration of cesium solution, the concentration of cesium solution in the seepage test was determined to be 30 mg/L.
Air pressure
Displacement gauge Double-pipe burette
Loading plate Guide ring
Compaction ring Sample Diameter 60 mm Height 10 mm Picking Fig. 14. Consolidation and seepage test apparatus.
O. Kurihara et al. / Soils and Foundations 58 (2018) 1173–1186
4.2. Results and discussion on adsorption property Fig. 15 shows the hydraulic conductivity kDarcy of Darcy’s law calculated from the seepage test, compared with the kDarcy calculated from the consolidation test. As shown in this figure, the values for kDarcy obtained in the seepage test were slightly smaller than those obtained from the consolidation test. Accordingly, the estimation of hydraulic conductivity by the consolidation test is made on the conservative side in order to consider the safety of the sealing material. Using the pore volume of specimen Vv and the volume of cumulative flowed water Vf, the pore volume of the flow (PVF) is defined as
Hydraulic conducvity kDarcy measured from seepage test (m/s)
PVF ¼ V f =V v
ð10Þ
4E-09
Tokyo Bay clay or Tokuyama Port clay -bentonite-zeolite mixture
When the PVF is 1, all the pore water in the specimen is replaced by the inflow and the PVF will become 1 when the travel time of the water flow passes. Accordingly, PVF means a safety factor of travel time and, for example, PVF = 3 means that the water flow is 3 times the pore volume and that 3 times the travel time passes. The results are shown in Fig. 16 for the adsorption test as the relationship between the effluent concentrations of cesium CCs and the values of PVF. As shown in this figure, the no concentration of cesium was measured in the meantime and after PVF reached a breakthrough point, the concentration of cesium was increased by the pore volume of the flow or the safety factor. In this study, the point where cesium is first detected in the effluent solution is defined as the breakthrough point. The CCs–PVF relationship of the Tokuyama Port clay in the previous study (Tsuchida et al., 2017) is also plotted in Fig. 16. Tokuyama Port clay is one of the typical marine clays found on the west coast of Japan and its index properties are shown in Table 1. From Fig. 16, the following were found on the breakthrough point of the cesium adsorption of the sealing material used in this study: The breakthrough point of adsorption is different in marine clay, bentonite and zeolite. The breakthrough point of Tokyo Bay clay (PVF 18) is much larger than that of Tokuyama Port clay (PVF 9). By adding bentonite to marine clay, the PVF value became smaller. Accordingly, the addition of bentonite was seen to be effective for reducing the hydraulic conductivity, but it also causes the adsorption property of cesium to decrease. By adding zeolite to marine clay, the PVF value increased drastically. Further, the increase rate of the cesium concentration after the PVF exceeded the breakthrough point also became much less than that of the samples without zeolite.
1E-09
4E-10 TB TBz20 TBb100z10 TBb100 TPb25 TPb150 1E-10 1E-10
4E-10
TBz10 TBb50z10 TBb50 TP TPb75
4E-09
1E-09
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Hydraulic conducvity kDarcy calculated from consolidaon test (m/s)
Fig. 15. Comparison of hydraulic conductivity measured in two tests.
Cesium concentraon of effluent, CCs (mg/L)
4 Tokuyama Port clay + bentonite
Tokyo Bay clay-bentonitezeolite mixture
bc=50kg/m3, zc=0%
3.5
TP TPb25 TPb75 TPb150 TB TBb50 TBb100 TBz10 TBz20 TBb50z10 TBb100z10
Tokyo Bay clay 3 bc=100kg/m3 zc=0%
2.5 2
bc=100kg/m3 zc=10%
1.5
bc=50kg/m3 zc=10%
bc=0kg/m3 zc=10%
1
bc=0kg/m3 zc=20%
0.5 0 0
10
20
30
40
50
Pore volume of flow, PVF Fig. 16. Cesium concentration of effluent CCs and PVF.
60
70
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Cesium concentraon of effluent, CCs (mg/L)
4 bc=50kg/m3 zc=0%
3.5
Tokyo Bay clay-bentonitezeolite mixture
Tokyo Bay clay bc=100kg/m3 zc=0%
3
TB TBz10 TBz20 TBb50 TBb100 TBb50z10 TBb100z10
bc=100kg/m3 zc=10%
2.5
bc=50kg/m3 zc=10%
2
zc=10%
1.5
zc=20%
1 0.5 0 0
200
400
600
800
1000
Cumulave adsorbed cesium
1200
133Cs,
1400
1600
1800
ΣS (mg/kg)
Fig. 17. CCs and cumulative adsorbed cesium RS.
Table 4 Cation exchange capacity of samples. Sample
Tokyo Bay clay
Tokuyama Port clay
Wyoming bentonite
Iwami zeolite
Measured value (cmol/kga)
36.0
21.7
56.0
97.6
of bentonite. As the increase in adsorption capacity could not be explained by the CEC alone, further study is necessary on the adsorption capacity versus CEC relation. In Fig. 19, DCL, the increment in the cesium concentration of the effluent per unit travel time or pore volume of flow after the breakthrough point, is shown with the CEC.
b
Analysis method is the Schollenberger method. a Per dry weight of soil. TP TB TBz10 TBz20 TBb50 TBb100 TBb50z10 TBb100z10
50 40 30 20
Tokyo Bay clay PVF=2.45CEC-74.0 R2=0.529
zc=20%
zc=10%
Tokuyama Port clay
10
zc=0%
0 0
10
20
30
40
50
60
CEC (cmol/kg)
(a) 1400
Cumulave adsorbed cesium ΣS at breakthrough point (mg/kg)
In Fig. 17, the effluent concentration of cesium CCs is shown with the cumulative adsorbed cesium RS, which was calculated from the difference in the inflow and outflow concentrations of cesium. It was found that, when the cesium begins to flow out, the cumulative adsorbed cesium of the sample which mixed 10% of zeolite into Tokyo Bay clay is 1.5 times, the sample which mixed 20% of zeolite into Tokyo Bay clay is 2.0 times as much as that of the Tokyo Bay clay. It is known that the adsorption capacity of clay is related to the cation exchange capacity of soil (Kahr et al., 1995). Table 4 shows the cation exchange capacity (CEC) of Tokyo Bay clay, Tokuyama Port clay, Wyoming bentonite and Iwami zeolite. The cation exchange capacity of each tested sample was calculated by share pro rata in accordance with the volume of the four materials listed in Table 4. Fig. 18(a) shows the relationship between the CEC of each sample and the PVF at the breakthrough point. Further, in Fig. 18(b), the relationship between the CEC and the cumulative adsorbed cesium at the breakthrough point is shown. As shown in Fig. 18(a) and (b), the increase in CEC by the addition of zeolite seems to be related to the increase in the adsorption capacity of the samples. However, comparing the samples having the same zeolite additive rate, zc, the PVF and the cumulative adsorbed cesium at the breakthrough point are seen to decrease slightly with an increase in CEC by the addition
PVF at breakthrough point
60
TP
1200
TB TBz10
1000
zc=20%
TBz20
Tokyo Bay clay
TBb50
ΣS= 59.1CEC-1733 R2=0.627
TBb100
800
TBb50z10
zc=10%
TBb100z10
600 400
Tokuyama Port clay
zc=0%
200 0 0
10
20
30
40
50
60
CEC (cmol/kg)
(b) Fig. 18. (a) PVF at breakthrough point and CEC (cmol/kg) and (b) RS at breakthrough point (mg/kg) and CEC (cmol/kg).
ΔCL, increment in cesium concentraon of effluent per unit of travel me a er breakthrough point
O. Kurihara et al. / Soils and Foundations 58 (2018) 1173–1186 0.6 TP TB
0.5
zc=0%
Tokuyama Port clay
TBz10 TBz20
0.4
TBb50 TBb100
0.3
TBb50z10
zc=10%
TBb100z10
0.2
Tokyo Bay clay ΔCL=-0.0421CEC+2.04 R2=0.726
0.1
zc=20%
0 10
20
30
40
50
CEC (cmol/kg) Fig. 19. DCL, increment in cesium concentration of effluent per unit of travel time after breakthrough point and CEC (cmol/kg).
The relationship indicates that the addition of zeolite to marine clay could cause the increment in the cesium concentration of the effluent to be smaller after the breakthrough. From the results shown in Figs. 18 and 19, with respect to the adsorption property of cesium, it was confirmed that the mixing of zeolite into marine clay has an effect of suppressing the increase in the concentration of cesium in the effluent after the breakthrough as well as the effect of delaying the breakthrough.
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hydraulic conductivity decreased in proportion to the addition of bentonite. Further, when both zeolite and bentonite were added to marine clay, the hydraulic conductivity became smaller than that of marine clay mixed with bentonite alone. (3) The cesium-adsorption capacity of the sealing material was increased by adding zeolite, and the safety factor on the travel time of the pore water flow increased more than two times. However, the adsorption capacity was decreased by the addition of bentonite to marine clay. Hence, when a large amount of bentonite is mixed into marine clay, in order to reduce the hydraulic conductivity, the negative effect on the adsorption capacity should be taken into consideration. (4) The cation exchange capacity of zeolite was much higher than that of any other materials used in this study. Seeing the relationship between the adsorption capacity and the CEC, the increase in the CEC by the addition of zeolite seemed to be related to the increase in the adsorption capacity of the samples. However, the adsorption capacity decreased slightly with the increase in the CEC by the addition of bentonite. Further study will be necessary to gain a greater understanding of the adsorption capacity versus CEC relation.
5. Conclusions After the severe accident at the Fukushima Daiichi Nuclear Power Plant in 2011, about 22 million m3 of contaminated soils were generated by the decontamination work and temporarily stored at various sites in Fukushima Prefecture. In order to consider the coastal disposal facilities for soil and waste contaminated by radioactive cesium, geotechnical sealing material for the marine environment was studied. Using samples consisting of marine clay, bentonite and zeolite, the hydraulic conductivity and the cesium-adsorption capacity were investigated by a series of laboratory tests. The main conclusions of this study are listed below: (1) The required hydraulic conductivity was discussed considering the safe management of soil and waste contaminated by radioactive cesium whose radioactivity concentration is less than 100,000 Bq/kg. Based on the assumption that the travel time by the seepage of a 4-m-thick geotechnical sealing layer is enough to reduce the radioactivity, the required hydraulic conductivity was determined as 1.0 109 m/s. The hydraulic conductivity for the pore water flow of the sealing material in this study satisfied the condition by mixing bentonite and zeolite at an effective consolidation pressure levels of more than 40 kPa. (2) When zeolite was mixed into marine clay, the hydraulic conductivity for the pore flow was almost the same as marine clay without the addition of zeolite. The
Acknowledgments This study comprises part of the research being conducted under the project entitled ‘‘Development of Offshore Disposal Facility for Disaster Wastes and Decontamination Wastes Polluted by Radiation (3K122109), which was funded by the Ministry of the Environment, Japan in 2012-2013. References Bureau of Port and Harbor, Tokyo Metropolitan Government, 2015. Port History.
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Haug, M.D., Wong, L.C., 1992. Impact of molding water content on hydraulic conductivity of compacted sand-bentonite. Can. Geotech. J. 29 (2), 253–262. https://doi.org/10.1139/t92-029. International Atomic Energy Agency, 2015. The Fukushima Daiichi Accident, vol. 4.
Mukai, H., Hirose, A., Motai, S., Kikuchi, R., Tanoi, K., Nakanishi, T. M., Kogure, T., 2016. Cesium adsorption/desorption behavior of clay minerals considering actual contamination conditions in Fukushima. Sci. Rep. 6, 21543. https://doi.org/10.1038/srep21543. Okumura, M., Nakamura, H., Machida, M., 2013a. Cs adsorption in clay minerals and zeolites. First principle calculation studies toward understanding their microscopic mechanism. J. Surf. Sci. Soc. Jpn. 34 (3), 135–142. https://doi.org/10.1380/jsssj.34.135, in Japanese. Okumura, M., Nakamura, H., Machida, M., 2013b. Mechanism of strong affinity of clay minerals to radioactive cesium: first-principles calculation study for adsorption of cesium at frayed edge sites in muscovite. J. Phys. Soc. Jpn. 82 (3), 033802. https://doi.org/10.7566/ JPSJ.82.033802. ¨ ren, A.H., Durukan, S., Kayalar, A.Sˇ., 2014. Influence of compaction O water content on the hydraulic conductivity of sand-bentonite and zeolite-bentonite mixtures. Clay Miner. 49 (1), 109–121. https://doi. org/10.1180/claymin.2014.049.1.09. ¨ ren, A.H., Kaya, A., Kayalar, A.S., 2011. Hydraulic conductivity of O zeolite–bentonite mixtures in comparison with sand–bentonite mixtures. Can. Geotech. J. 48 (9), 1343–1353. https://doi.org/10.1139/t11042. Osaka Bay Regional Offshore Environmental Improvement Center, 2015.