- Email: [email protected]
Development of a water purifier for radioactive cesium removal from contaminated natural water by radiation-induced graft polymerization
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Development of a water purifier for radioactive cesium removal from contaminated natural water by radiation-induced graft polymerization ⁎
Noriaki Sekoa, , Hiroyuki Hoshinaa, Noboru Kasaia, Takuya Shibatab, Seiichi Saikia, Yuji Uekia a Department of Advanced Functional Materials Research, Takasaki Advanced Radiation Research Institute, Quantum Beam Science Research Directorate, National Institutes for Quantum and Radiological Science and Technology, 1233 Watanuki-machi, Takasaki, Gunma 370-1292, Japan b Instrumentation and Laser Application Laboratory, Utilization Technology Development Department, Naraha Remote Technology Development Center, Fukushima Research Infrastructural Creation Center, Sector of Fukushima Research and Development, Japan Atomic Energy Agency, 1-22 Aza-Nakamaru Yamadaoka, Naraha-machi, Futaba-gun, Fukushima 970-8026, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Radiation-induced graft polymerization γ-rays Cs removal Mass production Water purifier
Six years after the Fukushima-nuclear accident, the dissolved radioactive cesium (Cs) is now hardly detected in environmental natural waters. These natural waters are directly used as source of drinking and domestic waters in disaster-stricken areas in Fukushima. However, the possibility that some radioactive Cs adsorbed on soil or leaves will contaminate these natural waters during heavy rains or typhoon is always present. In order for the returning residents to live with peace of mind, it is important to demonstrate the safety of the domestic waters that they will use for their daily life. For this purpose, we have synthesized a material for selective removal of radioactive Cs by introducing ammonium 12-molybdophosphate (AMP) onto polyethylene nonwoven fabric through radiation-induced emulsion graft polymerization technique. Water purifiers filled with the grafted Cs adsorbent were installed in selected houses in Fukushima. The capability of the grafted adsorbent to remove Cs from domestic waters was evaluated for a whole year. The results showed that the tap water filtered through the developed water purifier contained no radioactive Cs, signifying the very effective adsorption performance of the developed grafted adsorbent. From several demonstrations, we have commercialized the water purifier named “KranCsair®”. Furthermore, we have also developed a method for the mass production of the grafted nonwoven fabric. Using a 30 L grafting reactor, it was possible to produce the grafted nonwoven fabric with a suitable range of degree of grafting. When an irradiated roll of nonwoven trunk fabric with a length of 10 m and a width of 30 cm was set in the reactor filled with glycidyl methacrylate (GMA), AMP, Tween 80 monomer emulsion solution at 40 °C for 1 h, the difference of Dgs in the length and the width on roll of fabrics was negligible.
1. Introduction The big tsunami caused by the severe earthquake in East Japan damaged the electricity supply and stopped the circulation of the cooling water of the nuclear reactor in the Fukushima Daiichi Nuclear Power Station. As a result of the melt-down of the nuclear reactor, radioactive materials were dispersed in the surroundings, including numerous water bodies. In order to remove the radioactive materials, it is important to focus on reducing volume of the radioactive wastes. Various decontamination methods have been proposed so far, and many types of adsorbents have been developed such as zeolite (Rajec and Domianova, 2008), Prussian blue (Fekry et al., 2003), potassium ferrocyanide (Haas, 1993), and ammonium 12-molybdophosphate (Todd et al., 2002; Iwanade et al., 2012). However, most of the synthesized
⁎
adsorbents were for decontaminating insoluble radioactive materials, i.e. none of these adsorbent materials is applicable in treatment of drinking water. In mountainous regions affected by the nuclear disaster, there are many areas that do not utilize water purification plants. Almost 90% of the residents in these areas used well and streaming waters for their daily life. Currently, it is known that most of radioactive cesium (Cs) are immobilized in clay minerals, etc. Mukai et al. (2014, 2016). In the disaster-stricken areas in Fukushima, however, water sources were drawn from highlands where decontamination has not been conducted, and radioactive Cs were likely to be mixed into the water source during heavy rain and snowfall. Therefore, it is an important task to develop an adsorbent for Cs removal to ensure the safety of drinking water in any case. In order to apply the developed materials in treatment of drinking
Corresponding author. E-mail address: [email protected] (N. Seko).
http://dx.doi.org/10.1016/j.radphyschem.2017.09.007 Received 24 April 2017; Accepted 11 September 2017 0969-806X/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Seko, N., Radiation Physics and Chemistry (2017), http://dx.doi.org/10.1016/j.radphyschem.2017.09.007
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
N. Seko et al.
Fig. 1. Preparation scheme of a fibrous grafted adsorbent for Cs removal.
crosslinking agent.
water, it was required that adsorption ligands do not release from the material. Almost all of the materials stated above have adsorption ligands that were desorbed during water treatment. Also, majority of these adsorbents are of granular resin type that was prepared by the copolymerization of styrene and divinylbenzene (Wachinski and Etzel, 1997). Such adsorbents were typically used at a flow rate less than 10 h−1 of space velocity which was normalized by dividing the flow rate of the solution by the volume of adsorbent packed in the column. If the synthesized adsorbent is to be used as a filter in the developed water purifier, it is necessary that it can treat the water at a flow rate similar to the normal operation of a faucet. To address the issues of ligand stability and water flow rate limitation, a fibrous adsorbent was synthesized by radiation-induced emulsion graft polymerization which could introduce a functional group having superior affinity for Cs ions. In our previous report, we have shown that the grafted Cs-adsorbent could work effectively for the dissolve Cs ions without any desorption of ligands (Iwanade et al., 2012; Shibata et al., 2016) and that the fibrous nature of the grafted materials could remove various metal ions even at high flow rates, e.g. passing with SV of 500 h−1 or more in the column treatment (Seko et al., 2004; Takeda et al., 2010; Sekine et al., 2010). An additional advantage of this polymeric grafted adsorbent, besides its capability to remove radioactive Cs, is the simplicity of its application because it directly adsorbed the toxic components. That is, sludge, which required a secondary treatment process, was not produced compared with the coagulation method using zeolite and polymeric flocculants. In the present study, a fibrous polymer adsorbent for the selective removal of radioactive Cs from contaminated waters have been developed by introducing ammonium 12-molybdophosphate (AMP) onto polyethylene nonwoven fabric through radiation-induced emulsion graft polymerization technique. To evaluate the efficiency of the AMP grafted fibrous adsorbent in removing Cs from contaminated waters, we conducted conventional batch and column experiments using real well water samples. Also, the performance of the water purifier containing the developed Cs-adsorbent was evaluated and monitored for one whole year at selected Fukushima households.
2.2. Emulsified monomer solution In order to uniformly disperse AMP, an inorganic substance, in DMSO solvent, a combination of emulsion graft polymerization, using a surfactant, and an immobilization method, using a crosslinking agent, was applied. The grafting approach used to synthesize the Cs-adsorbent is shown in Fig. 1. The radiation-induced emulsion graft polymerization method was implemented as previously reported (Seko et al., 2007, 2010). The emulsified monomer solution was prepared with the following composition: 9.8 (w/v)% of GMA, 0.5 (w/v)% of AMP, 0.9 (w/ v)% of TAIC, 0.8 (w/v)% of Tween80 in 88 (w/v)% of DMSO as a grafting solvent. All of these substances were mixed and then stirred for 10 min with a homogenizer (Yamato Scientific Co., Ltd.) to make them homogeneous. 2.3. Mass production by pre-irradiation grafting with γ-irradiation Because a large amount of Cs-adsorbent was necessary for field works to evaluate its performance to remove the Cs spread over the environment, we had to develop a mass production method for Cs-adsorbent synthesis. The prepared monomer solution was transferred to a 30 L reaction tank for graft polymerization. The monomer was deoxygenated using nitrogen gas bubbling to avoid contact between the created radicals and oxygen. The trunk nonwoven fabric (NF) of the Csadsorbent, which was cut into a roll shape with a width of 30 cm and a length of 10 m, was irradiated to 160 kGy with γ ray at a dose rate of 10 kGy/h. The irradiation was carried out under dry ice temperature. In order to reduce unevenness in absorbed dose, the NF was rotated by 180° after receiving half of the target absorbed dose, i.e. 80 kGy. Furthermore, a mesh-like net made of polyethylene was alternately wound up in a rolled NF in order to increase its contact efficiency with monomer solution during graft polymerization. To ascertain the distribution of degree of grafting in the total roll length, a small piece of NF was placed at equal intervals in the roll (Fig. 2). It was used as an index of the degree of grafting for the rolled NF. The degree of grafting (DG) of small pieces was calculated from the weight increment before and after grafting as follows:
2. Experiment 2.1. Chemicals and materials
Degree of Grafting(DG) (%) = 100(W1 − W0 )/W0
Polyethylene based nonwoven fabric (NF), with core-in-sheath shape and was composed of polyethylene coated polyporopylene, was kindly provided by Kurashiki Seni Kako Co. Okayama Japan and was used for as a trunk material of the radioactive cesium adsorbent (Csadsorbent). Glycidyl methacrylate (GMA) from Mitsubishi Gas Chemical Company, Inc., polyoxethylene sorbitan monooleate (Tween 80) and dimethyl sulfoxide (DMSO) from Kanto Chemical Co., Ltd. were used in the radiation-induced graft polymerization and chemical modification steps. Distilled water, for washing after the grafting and modification reactions, and ultrapure water, for diluting solutions, were obtained from a Millie-Q system from Merck Ltd. Methanol (MeOH) purchased from Taiyo Chemical Industry Co., Ltd. was used as washing solvent for the grafted NF. Ammonium 12-molybdophosphate (AMP) was purchased from Nippon Inorganic Color and Chemical Co., Ltd. In order to uniformly impart AMP on the grafted polymer chain, triallyl isocyanurate (TAIC, Nippon Kasei Chemical Co., Ltd.) was used as a
where, W0 and W1 are the weights of trunk NF and grafted NF, respectively. Grafting was performed at 40 °C for 1 h. The resulting grafted NF was washed with DMSO and water to remove non-reacted chemicals, and then dried at 40 °C.
(1)
2.4. Removal tests of radioactive Cs The well water samples, collected from areas where radioactive Cs was detected, were used to evaluate the developed Cs-adsorbent for drinking water. First, the insoluble Cs in well water was removed by filtration through 0.45 µm and 0.1 µm membrane filters. The filtrate was used in the adsorption tests. The soluble radioactive Cs concentration, which is the total number of Cs-134 and Cs-137, was measured by a germanium semiconductor detector (SEICO EG & G CO., LTD., Japan). Cs concentration was 56 Bq/L after filtration. In the batch removal test, a 0.18 g Cs adsorbent, with 35 mm diameter and about 2
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Fig. 2. Preparation of a rolled trunk NF for mass production method.
1 mm thickness, was divided into 4 pieces and placed in a polyethylene bottle containing 50 ml well water sample, as shown in Fig. 3. After stirring for 17 h, the Cs-adsorbent was taken out and the Cs concentration of the treated well water was measured with the germanium semiconductor detector. In the column mode evaluation, about 1.3 ml or approximately 0.35 g Cs-adsorbent was put in a syringe to make the column-packed adsorbent (Fig. 4). Then, 270 ml of well water sample was pumped through the column for 7 min. The flow rate was expressed as space velocity (SV h−1), which was calculated by dividing the flow rate of the well water by the adsorbent volume in the syringe.
main body part. The monitoring test was conducted for one year at Fukushima. The test was carried out by analyzing the spent cartridges that were replaced every 1 or 2 months. The Cs concentrations in the two layers of the spent cartridge were measured by the germanium semiconductor detector.
2.5. Monitoring test of a water purifier filled with the Cs-adsorbent
The grafting of the emulsified GMA/AMP proceeded efficiently. It has been found that the series of Tween's surfactant was a suitable reagent for stabilization of GMA in water (Sekine et al., 2010; Seko et al., 2010; Ma et al., 2012). When NF was used as a trunk polymer in emulsion grafting, Dg reached 300% or more after 1 h of grafting. To minimize homopolymer formation, the grafting of GMA/AMP was carried out by using pre-irradiation technique. The data from a preliminary laboratory scale experiment showed that the Dg increased with increasing reaction time, reaching 580% after grafting for 1 h. The grafting conditions, such as monomer composition and reaction time, for the large scale grafting were based from the above stated laboratory scale conditions. After transferring the prepared monomer solution into the graft reaction tank and confirming that the oxygen concentration in the monomer solution was zero (vol%), the irradiated NF was immersed into the tank. During graft polymerization, the reaction vessel was maintained at 50–60 °C and under nitrogen atmosphere by flowing nitrogen gas into the tank at 4 L/min. As shown in the Fig. 5, we succeeded in obtaining a uniform Dg and the Dgs in the length and the width on roll of fabrics was uniformly. FT-IR graphs of Cs-adsorbent, GMA grafted NF and low molecular AMP are shown in Fig. 6, exhibiting strong bands with peaks at 1065 cm−1, which was attributed to the stretching vibration of P˭O bonds, and at 925 cm−1, which was attributed to the stretching vibration of epoxy group. The presence of a
3. Results and discussion 3.1. Mass production of Cs adsorbent
In order to secure the drinking water for those who will return home from evacuation sites in the future, we developed a water purifier using a commercially available module. The water purifier could be directly attached to a faucet for family use. It is consisted of a main body part and a cartridge part filled with the developed Cs-adsorbent. The inside of the cartridge was made of two layers: one layer for the filtration of insoluble Cs and the other layer for the adsorption of soluble Cs components from water. The cartridge could be easily detached from the
Fig. 3. Experimental apparatus for batch adsorption test for Cs removal.
Fig. 4. Experimental apparatus for column (syringe) batch adsorption test for Cs removal.
Fig. 5. Monitoring of reaction temperature in the grafting tank.
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40 °C, but it reached 50–60 °C (Fig. 7) during the course of graft polymerization. As a result, we succeeded in obtaining about 1000 times of Cs adsorbent volume compared to the laboratory scale without any uneven distribution of Dg. 3.2. Removal tests of radioactive Cs A year after the nuclear accident at Fukushima, most of the detected radioactive Cs was not soluble in water media. It was found that almost of all Cs was immobilized in a clay etc. However, one year after the accident, soluble Cs was detected in a lot from the temporary storage area of decontamination wastes for plants. The wastes from plant parts became liquefied after fermentation in the flexible containers bags during hot summer days. When adapting the developed Cs-adsorbent to the wastes, it could succeeded in removing soluble Cs (Saiki et al., 2014). Such contaminated waters may unexpectedly mix into domestic waters that are used for the daily activities of the residents. In the disaster-stricken areas, drinking water was sourced from natural spring water and well water. Because the domestic waters were indeed contaminated with radioactive Cs, we applied the synthesized Cs adsorbent in the treatment of well water samples. A total Cs concentration of 56 Bq/L was dissolved in the well water that was used for the evaluation. Using the adsorbent, we tried to decrease it to 10 Bq/L or less, which was the regulation value for drinking water. The results presented in Table 1 showed that the Cs-adsorbent successfully removed the radioactive Cs to concentrations less than 10 Bq/L. Continuous removal of Cs was evaluated by passing the well water sample through the Cs-adsorbent, which was packed in a 7 mm inner diameter column (syringe), for 7 min. The well water, having a radioactive Cs concentration of 56 Bq/L, was pumped out by aspiration with hand force into the syringe at SV 1700 h−1. As shown in Table 1, the Cs was completely trapped in the Cs-adsorbent, and effluent concentration became less than 10 Bq/L. The results of the column (syringe) test implied that the developed Cs-adsorbent could be used as a water purifier.
Fig. 6. FT-IR charts of low molecular AMP, GMA grafted NF and GMA/AMP grafted fabric (Cs-adsorbent).
Fig. 7. Relationship between length from inner side and degree of grafting.
Table 1 Cs adsorption performance by batch and column test using the developed Cs-adsorbent.
3.3. Monitoring tests of water purifier
Well water at Fukushima Pre-filter (0.1 µm membrane) and Commercialized filters Grafted Cs-adsorbent (batch test) Grafted Cs-adsorbent (syringe test)
Cs-134
Cs-137
Total (Bq/ L)
32.5 (3) 19.0 (9.2) ND (4.5) ND (4.5)
55.5 (3) 37.0 (9.6) ND (4.4) ND (4.4)
88.0 56.0
Based on the evidences obtained from the preliminary evaluations, the water purifier filled with Cs-adsorbent was developed. The water purifier cartridge was consisted of two trapping layers. As shown in Fig. 8, one layer was for the filtration of insoluble components, while the other one was for adsorption of the soluble components. Evaluation of the water purifier was carried out in the one-year monitoring test. As shown in Fig. 8, the water purifier unit was attached to a faucet at home. The spent cartridge was replaced every two months for a total duration of one year. Table 2 shows the concentration of radioactive Cs in the actual untreated drinking water at each household. Cs was detected at only one house among 13 places but the concentration was below the regulation value for drinking water. In addition, the radioactive Cs was not detected after filtration of the water sample though a 0.1 µm filter membrane; hence, the detected Cs was considered to be insoluble Cs, probably attached to soil or dust. The Cs in cartridge was
ND ND
ND: Not Detectable, ( ): Detection Limit Value.
characteristic P˭O band originating from the phosphate group in AMP confirmed that it was successfully supported on the fabric. However, the resulting Dg's were quite higher than the laboratory results. The observed increase in Dg for large scale grafting was because of the elevation of the actual reaction temperature due to the significant contribution from the heat of graft reaction. The temperature was set at
Fig. 8. Specification of Cs water purifier.
4
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produce treated water with concentration less than 10 Bq/L, which was within the regulation value for drinking water. Furthermore, the monitoring tests were conducted for a period of one year, and the cartridge attached to the water purifier was replaced every one or two months to check how much radioactive Cs remains. As a results, Cs was detected from several cartridges and the detected Cs was in insoluble form. In either case, it was demonstrated that the developed water purifier with Cs-adsorbent could remove all forms of Cs. From these demonstrations, we have commercialized a water purifier named “KranCsair®”. Based from these results, even if radioactive Cs was mixed into the water resources for some reason, it was demonstrated that the radioactive Cs can be surely removed through the use of the developed water purifier.
Table 2 Monitoring test using the water supply filled with the Cs-adsorbent. Number of detection Raw Well/Stream Water Well/Stream Water filtered with 0.1 µm membrane Trapped Cs in cartridge Well/Stream Water treated with the developed water purifier
１ / 13 0 22 / 81 (6–1800 Bq/kg) 0
totally detected from 22 out of 81 pieces, and was 1800 Bq/kg at the maximum. The total amount of water passing through the cartridge between each collection period was unknown so it was difficult to calculate the actual Cs concentration. However, assuming that the maximum Cs amount trapped in the cartridge was 4.0 Bq, and if an average volume of 2 L of water passed through it daily for 2 months, then the average Cs concentration was 0.03 Bq/L (4.0 Bq/60 days × 2 L). This value was less than 10 Bq/L as the regulation value for drinking water. Finally, no cesium was detected from the water after being filtered through the developed water purifier.
References Fekry, A.E., El-Bieh, N.M., Elwan, K.M., Mangood, S.A., 2003. J. Radioanal. Nucl. Chem. 257 (1), 75–82. Haas, Paul A., 1993. Sep. Sci. Technol. 28, 2479–2506. Iwanade, A., Kasai, N., Hoshina, H., Ueki, Y., Saiki, S., Seko, N., 2012. J. Radioanal. Nucl. Chem. 293, 703–709. Ma, H., Yao, S., Li, J., Cao, C., Wang, M., 2012. Radiat. Phys. Chem. 81, 1393–1397. Mukai, H., Hatta, T., Kitazawa, H., Yamada, H., Yaita, T., Kogure, T., 2014. Environ. Sci. Technol. 48 (22), 13053–13059. Mukai, H., Hirose, A., Motai, S., Kikuchi, R., Tanoi, K., Nakanishi, T.M., Yaita, T., Kogure, T., 2016. Sci. Rep. 6, 21543. Rajec, P., Domianova, K., 2008. J. Radioanal. Nucl. Chem. 275 (3), 503–508. Saiki, S., Shibata, T., Hoshina, H., Ueki, Y., Kasai, N., Seko, N., 2014. J. Ion. Exch. 25, 170–175. Sekine, A., Seko, N., Tamada, M., Suzuki, Y., 2010. Radiat. Phys. Chem. 79, 16–21. Seko, N., Basuki, F., Tamada, M., Yoshii, F., 2004. React. Funct. Polym. 59, 235–241. Seko, N., Bang, L.T., Tamada, M., 2007. Nucl. Instr. Methods B 265, 146–149. Seko, N., Ninh, N.T.Y., Tamada, M., 2010. Radiat. Phys. Chem. 79, 22–26. Shibata, T., Seko, N., Amada, H., Kasai, N., Saiki, S., Hoshina, H., Ueki, Y., 2016. Radiat. Phys. Chem. 119, 247–252. Takeda, T., Tamada, M., Seko, N., Ueki, Y., 2010. Radiat. Phys. Chem. 79, 223–226. Todd, T.A., Mann, N.R., Tranter, T.R., Sebesta, F., John, J., Motl, A., 2002. J. Radioanal. Nucl. Chem. 254 (1), 47–52. Wachinski, A.M., Etzel, J.E., 1997. Environmental Ion Exchange. CRC Press, Boca Raton, FL (Chapter 4 laboratory-scale testing of ion exchange).
4. Conclusions To contribute in the Fukushima recovery, the novel adsorbent for Cs removal was developed by radiation-induced emulsion graft polymerization of GMA/AMP onto polyethylene nonwoven fabric. Especially, we focused on developing a water purifier that could be applied in drinking water for the safe consumption of the returning residents. In order to construct the water purifier, it was necessary to develop a mass production procedure of radiation graft polymerization. The grafting results indicate that it is possible to graft a NF with a length of 10 m and a width of 30 cm with even Dg distribution. In the batch and column (syringe) test using Cs contaminated well water samples, it has been demonstrated that the Cs could be trapped to
5
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Development of a water purifier for radioactive cesium removal from contaminated natural water by radiation-induced graft polymerization ⁎
Noriaki Sekoa, , Hiroyuki Hoshinaa, Noboru Kasaia, Takuya Shibatab, Seiichi Saikia, Yuji Uekia a Department of Advanced Functional Materials Research, Takasaki Advanced Radiation Research Institute, Quantum Beam Science Research Directorate, National Institutes for Quantum and Radiological Science and Technology, 1233 Watanuki-machi, Takasaki, Gunma 370-1292, Japan b Instrumentation and Laser Application Laboratory, Utilization Technology Development Department, Naraha Remote Technology Development Center, Fukushima Research Infrastructural Creation Center, Sector of Fukushima Research and Development, Japan Atomic Energy Agency, 1-22 Aza-Nakamaru Yamadaoka, Naraha-machi, Futaba-gun, Fukushima 970-8026, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Radiation-induced graft polymerization γ-rays Cs removal Mass production Water purifier
Six years after the Fukushima-nuclear accident, the dissolved radioactive cesium (Cs) is now hardly detected in environmental natural waters. These natural waters are directly used as source of drinking and domestic waters in disaster-stricken areas in Fukushima. However, the possibility that some radioactive Cs adsorbed on soil or leaves will contaminate these natural waters during heavy rains or typhoon is always present. In order for the returning residents to live with peace of mind, it is important to demonstrate the safety of the domestic waters that they will use for their daily life. For this purpose, we have synthesized a material for selective removal of radioactive Cs by introducing ammonium 12-molybdophosphate (AMP) onto polyethylene nonwoven fabric through radiation-induced emulsion graft polymerization technique. Water purifiers filled with the grafted Cs adsorbent were installed in selected houses in Fukushima. The capability of the grafted adsorbent to remove Cs from domestic waters was evaluated for a whole year. The results showed that the tap water filtered through the developed water purifier contained no radioactive Cs, signifying the very effective adsorption performance of the developed grafted adsorbent. From several demonstrations, we have commercialized the water purifier named “KranCsair®”. Furthermore, we have also developed a method for the mass production of the grafted nonwoven fabric. Using a 30 L grafting reactor, it was possible to produce the grafted nonwoven fabric with a suitable range of degree of grafting. When an irradiated roll of nonwoven trunk fabric with a length of 10 m and a width of 30 cm was set in the reactor filled with glycidyl methacrylate (GMA), AMP, Tween 80 monomer emulsion solution at 40 °C for 1 h, the difference of Dgs in the length and the width on roll of fabrics was negligible.
1. Introduction The big tsunami caused by the severe earthquake in East Japan damaged the electricity supply and stopped the circulation of the cooling water of the nuclear reactor in the Fukushima Daiichi Nuclear Power Station. As a result of the melt-down of the nuclear reactor, radioactive materials were dispersed in the surroundings, including numerous water bodies. In order to remove the radioactive materials, it is important to focus on reducing volume of the radioactive wastes. Various decontamination methods have been proposed so far, and many types of adsorbents have been developed such as zeolite (Rajec and Domianova, 2008), Prussian blue (Fekry et al., 2003), potassium ferrocyanide (Haas, 1993), and ammonium 12-molybdophosphate (Todd et al., 2002; Iwanade et al., 2012). However, most of the synthesized
⁎
adsorbents were for decontaminating insoluble radioactive materials, i.e. none of these adsorbent materials is applicable in treatment of drinking water. In mountainous regions affected by the nuclear disaster, there are many areas that do not utilize water purification plants. Almost 90% of the residents in these areas used well and streaming waters for their daily life. Currently, it is known that most of radioactive cesium (Cs) are immobilized in clay minerals, etc. Mukai et al. (2014, 2016). In the disaster-stricken areas in Fukushima, however, water sources were drawn from highlands where decontamination has not been conducted, and radioactive Cs were likely to be mixed into the water source during heavy rain and snowfall. Therefore, it is an important task to develop an adsorbent for Cs removal to ensure the safety of drinking water in any case. In order to apply the developed materials in treatment of drinking
Corresponding author. E-mail address: [email protected] (N. Seko).
http://dx.doi.org/10.1016/j.radphyschem.2017.09.007 Received 24 April 2017; Accepted 11 September 2017 0969-806X/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Seko, N., Radiation Physics and Chemistry (2017), http://dx.doi.org/10.1016/j.radphyschem.2017.09.007
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
N. Seko et al.
Fig. 1. Preparation scheme of a fibrous grafted adsorbent for Cs removal.
crosslinking agent.
water, it was required that adsorption ligands do not release from the material. Almost all of the materials stated above have adsorption ligands that were desorbed during water treatment. Also, majority of these adsorbents are of granular resin type that was prepared by the copolymerization of styrene and divinylbenzene (Wachinski and Etzel, 1997). Such adsorbents were typically used at a flow rate less than 10 h−1 of space velocity which was normalized by dividing the flow rate of the solution by the volume of adsorbent packed in the column. If the synthesized adsorbent is to be used as a filter in the developed water purifier, it is necessary that it can treat the water at a flow rate similar to the normal operation of a faucet. To address the issues of ligand stability and water flow rate limitation, a fibrous adsorbent was synthesized by radiation-induced emulsion graft polymerization which could introduce a functional group having superior affinity for Cs ions. In our previous report, we have shown that the grafted Cs-adsorbent could work effectively for the dissolve Cs ions without any desorption of ligands (Iwanade et al., 2012; Shibata et al., 2016) and that the fibrous nature of the grafted materials could remove various metal ions even at high flow rates, e.g. passing with SV of 500 h−1 or more in the column treatment (Seko et al., 2004; Takeda et al., 2010; Sekine et al., 2010). An additional advantage of this polymeric grafted adsorbent, besides its capability to remove radioactive Cs, is the simplicity of its application because it directly adsorbed the toxic components. That is, sludge, which required a secondary treatment process, was not produced compared with the coagulation method using zeolite and polymeric flocculants. In the present study, a fibrous polymer adsorbent for the selective removal of radioactive Cs from contaminated waters have been developed by introducing ammonium 12-molybdophosphate (AMP) onto polyethylene nonwoven fabric through radiation-induced emulsion graft polymerization technique. To evaluate the efficiency of the AMP grafted fibrous adsorbent in removing Cs from contaminated waters, we conducted conventional batch and column experiments using real well water samples. Also, the performance of the water purifier containing the developed Cs-adsorbent was evaluated and monitored for one whole year at selected Fukushima households.
2.2. Emulsified monomer solution In order to uniformly disperse AMP, an inorganic substance, in DMSO solvent, a combination of emulsion graft polymerization, using a surfactant, and an immobilization method, using a crosslinking agent, was applied. The grafting approach used to synthesize the Cs-adsorbent is shown in Fig. 1. The radiation-induced emulsion graft polymerization method was implemented as previously reported (Seko et al., 2007, 2010). The emulsified monomer solution was prepared with the following composition: 9.8 (w/v)% of GMA, 0.5 (w/v)% of AMP, 0.9 (w/ v)% of TAIC, 0.8 (w/v)% of Tween80 in 88 (w/v)% of DMSO as a grafting solvent. All of these substances were mixed and then stirred for 10 min with a homogenizer (Yamato Scientific Co., Ltd.) to make them homogeneous. 2.3. Mass production by pre-irradiation grafting with γ-irradiation Because a large amount of Cs-adsorbent was necessary for field works to evaluate its performance to remove the Cs spread over the environment, we had to develop a mass production method for Cs-adsorbent synthesis. The prepared monomer solution was transferred to a 30 L reaction tank for graft polymerization. The monomer was deoxygenated using nitrogen gas bubbling to avoid contact between the created radicals and oxygen. The trunk nonwoven fabric (NF) of the Csadsorbent, which was cut into a roll shape with a width of 30 cm and a length of 10 m, was irradiated to 160 kGy with γ ray at a dose rate of 10 kGy/h. The irradiation was carried out under dry ice temperature. In order to reduce unevenness in absorbed dose, the NF was rotated by 180° after receiving half of the target absorbed dose, i.e. 80 kGy. Furthermore, a mesh-like net made of polyethylene was alternately wound up in a rolled NF in order to increase its contact efficiency with monomer solution during graft polymerization. To ascertain the distribution of degree of grafting in the total roll length, a small piece of NF was placed at equal intervals in the roll (Fig. 2). It was used as an index of the degree of grafting for the rolled NF. The degree of grafting (DG) of small pieces was calculated from the weight increment before and after grafting as follows:
2. Experiment 2.1. Chemicals and materials
Degree of Grafting(DG) (%) = 100(W1 − W0 )/W0
Polyethylene based nonwoven fabric (NF), with core-in-sheath shape and was composed of polyethylene coated polyporopylene, was kindly provided by Kurashiki Seni Kako Co. Okayama Japan and was used for as a trunk material of the radioactive cesium adsorbent (Csadsorbent). Glycidyl methacrylate (GMA) from Mitsubishi Gas Chemical Company, Inc., polyoxethylene sorbitan monooleate (Tween 80) and dimethyl sulfoxide (DMSO) from Kanto Chemical Co., Ltd. were used in the radiation-induced graft polymerization and chemical modification steps. Distilled water, for washing after the grafting and modification reactions, and ultrapure water, for diluting solutions, were obtained from a Millie-Q system from Merck Ltd. Methanol (MeOH) purchased from Taiyo Chemical Industry Co., Ltd. was used as washing solvent for the grafted NF. Ammonium 12-molybdophosphate (AMP) was purchased from Nippon Inorganic Color and Chemical Co., Ltd. In order to uniformly impart AMP on the grafted polymer chain, triallyl isocyanurate (TAIC, Nippon Kasei Chemical Co., Ltd.) was used as a
where, W0 and W1 are the weights of trunk NF and grafted NF, respectively. Grafting was performed at 40 °C for 1 h. The resulting grafted NF was washed with DMSO and water to remove non-reacted chemicals, and then dried at 40 °C.
(1)
2.4. Removal tests of radioactive Cs The well water samples, collected from areas where radioactive Cs was detected, were used to evaluate the developed Cs-adsorbent for drinking water. First, the insoluble Cs in well water was removed by filtration through 0.45 µm and 0.1 µm membrane filters. The filtrate was used in the adsorption tests. The soluble radioactive Cs concentration, which is the total number of Cs-134 and Cs-137, was measured by a germanium semiconductor detector (SEICO EG & G CO., LTD., Japan). Cs concentration was 56 Bq/L after filtration. In the batch removal test, a 0.18 g Cs adsorbent, with 35 mm diameter and about 2
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Fig. 2. Preparation of a rolled trunk NF for mass production method.
1 mm thickness, was divided into 4 pieces and placed in a polyethylene bottle containing 50 ml well water sample, as shown in Fig. 3. After stirring for 17 h, the Cs-adsorbent was taken out and the Cs concentration of the treated well water was measured with the germanium semiconductor detector. In the column mode evaluation, about 1.3 ml or approximately 0.35 g Cs-adsorbent was put in a syringe to make the column-packed adsorbent (Fig. 4). Then, 270 ml of well water sample was pumped through the column for 7 min. The flow rate was expressed as space velocity (SV h−1), which was calculated by dividing the flow rate of the well water by the adsorbent volume in the syringe.
main body part. The monitoring test was conducted for one year at Fukushima. The test was carried out by analyzing the spent cartridges that were replaced every 1 or 2 months. The Cs concentrations in the two layers of the spent cartridge were measured by the germanium semiconductor detector.
2.5. Monitoring test of a water purifier filled with the Cs-adsorbent
The grafting of the emulsified GMA/AMP proceeded efficiently. It has been found that the series of Tween's surfactant was a suitable reagent for stabilization of GMA in water (Sekine et al., 2010; Seko et al., 2010; Ma et al., 2012). When NF was used as a trunk polymer in emulsion grafting, Dg reached 300% or more after 1 h of grafting. To minimize homopolymer formation, the grafting of GMA/AMP was carried out by using pre-irradiation technique. The data from a preliminary laboratory scale experiment showed that the Dg increased with increasing reaction time, reaching 580% after grafting for 1 h. The grafting conditions, such as monomer composition and reaction time, for the large scale grafting were based from the above stated laboratory scale conditions. After transferring the prepared monomer solution into the graft reaction tank and confirming that the oxygen concentration in the monomer solution was zero (vol%), the irradiated NF was immersed into the tank. During graft polymerization, the reaction vessel was maintained at 50–60 °C and under nitrogen atmosphere by flowing nitrogen gas into the tank at 4 L/min. As shown in the Fig. 5, we succeeded in obtaining a uniform Dg and the Dgs in the length and the width on roll of fabrics was uniformly. FT-IR graphs of Cs-adsorbent, GMA grafted NF and low molecular AMP are shown in Fig. 6, exhibiting strong bands with peaks at 1065 cm−1, which was attributed to the stretching vibration of P˭O bonds, and at 925 cm−1, which was attributed to the stretching vibration of epoxy group. The presence of a
3. Results and discussion 3.1. Mass production of Cs adsorbent
In order to secure the drinking water for those who will return home from evacuation sites in the future, we developed a water purifier using a commercially available module. The water purifier could be directly attached to a faucet for family use. It is consisted of a main body part and a cartridge part filled with the developed Cs-adsorbent. The inside of the cartridge was made of two layers: one layer for the filtration of insoluble Cs and the other layer for the adsorption of soluble Cs components from water. The cartridge could be easily detached from the
Fig. 3. Experimental apparatus for batch adsorption test for Cs removal.
Fig. 4. Experimental apparatus for column (syringe) batch adsorption test for Cs removal.
Fig. 5. Monitoring of reaction temperature in the grafting tank.
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40 °C, but it reached 50–60 °C (Fig. 7) during the course of graft polymerization. As a result, we succeeded in obtaining about 1000 times of Cs adsorbent volume compared to the laboratory scale without any uneven distribution of Dg. 3.2. Removal tests of radioactive Cs A year after the nuclear accident at Fukushima, most of the detected radioactive Cs was not soluble in water media. It was found that almost of all Cs was immobilized in a clay etc. However, one year after the accident, soluble Cs was detected in a lot from the temporary storage area of decontamination wastes for plants. The wastes from plant parts became liquefied after fermentation in the flexible containers bags during hot summer days. When adapting the developed Cs-adsorbent to the wastes, it could succeeded in removing soluble Cs (Saiki et al., 2014). Such contaminated waters may unexpectedly mix into domestic waters that are used for the daily activities of the residents. In the disaster-stricken areas, drinking water was sourced from natural spring water and well water. Because the domestic waters were indeed contaminated with radioactive Cs, we applied the synthesized Cs adsorbent in the treatment of well water samples. A total Cs concentration of 56 Bq/L was dissolved in the well water that was used for the evaluation. Using the adsorbent, we tried to decrease it to 10 Bq/L or less, which was the regulation value for drinking water. The results presented in Table 1 showed that the Cs-adsorbent successfully removed the radioactive Cs to concentrations less than 10 Bq/L. Continuous removal of Cs was evaluated by passing the well water sample through the Cs-adsorbent, which was packed in a 7 mm inner diameter column (syringe), for 7 min. The well water, having a radioactive Cs concentration of 56 Bq/L, was pumped out by aspiration with hand force into the syringe at SV 1700 h−1. As shown in Table 1, the Cs was completely trapped in the Cs-adsorbent, and effluent concentration became less than 10 Bq/L. The results of the column (syringe) test implied that the developed Cs-adsorbent could be used as a water purifier.
Fig. 6. FT-IR charts of low molecular AMP, GMA grafted NF and GMA/AMP grafted fabric (Cs-adsorbent).
Fig. 7. Relationship between length from inner side and degree of grafting.
Table 1 Cs adsorption performance by batch and column test using the developed Cs-adsorbent.
3.3. Monitoring tests of water purifier
Well water at Fukushima Pre-filter (0.1 µm membrane) and Commercialized filters Grafted Cs-adsorbent (batch test) Grafted Cs-adsorbent (syringe test)
Cs-134
Cs-137
Total (Bq/ L)
32.5 (3) 19.0 (9.2) ND (4.5) ND (4.5)
55.5 (3) 37.0 (9.6) ND (4.4) ND (4.4)
88.0 56.0
Based on the evidences obtained from the preliminary evaluations, the water purifier filled with Cs-adsorbent was developed. The water purifier cartridge was consisted of two trapping layers. As shown in Fig. 8, one layer was for the filtration of insoluble components, while the other one was for adsorption of the soluble components. Evaluation of the water purifier was carried out in the one-year monitoring test. As shown in Fig. 8, the water purifier unit was attached to a faucet at home. The spent cartridge was replaced every two months for a total duration of one year. Table 2 shows the concentration of radioactive Cs in the actual untreated drinking water at each household. Cs was detected at only one house among 13 places but the concentration was below the regulation value for drinking water. In addition, the radioactive Cs was not detected after filtration of the water sample though a 0.1 µm filter membrane; hence, the detected Cs was considered to be insoluble Cs, probably attached to soil or dust. The Cs in cartridge was
ND ND
ND: Not Detectable, ( ): Detection Limit Value.
characteristic P˭O band originating from the phosphate group in AMP confirmed that it was successfully supported on the fabric. However, the resulting Dg's were quite higher than the laboratory results. The observed increase in Dg for large scale grafting was because of the elevation of the actual reaction temperature due to the significant contribution from the heat of graft reaction. The temperature was set at
Fig. 8. Specification of Cs water purifier.
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produce treated water with concentration less than 10 Bq/L, which was within the regulation value for drinking water. Furthermore, the monitoring tests were conducted for a period of one year, and the cartridge attached to the water purifier was replaced every one or two months to check how much radioactive Cs remains. As a results, Cs was detected from several cartridges and the detected Cs was in insoluble form. In either case, it was demonstrated that the developed water purifier with Cs-adsorbent could remove all forms of Cs. From these demonstrations, we have commercialized a water purifier named “KranCsair®”. Based from these results, even if radioactive Cs was mixed into the water resources for some reason, it was demonstrated that the radioactive Cs can be surely removed through the use of the developed water purifier.
Table 2 Monitoring test using the water supply filled with the Cs-adsorbent. Number of detection Raw Well/Stream Water Well/Stream Water filtered with 0.1 µm membrane Trapped Cs in cartridge Well/Stream Water treated with the developed water purifier
１ / 13 0 22 / 81 (6–1800 Bq/kg) 0
totally detected from 22 out of 81 pieces, and was 1800 Bq/kg at the maximum. The total amount of water passing through the cartridge between each collection period was unknown so it was difficult to calculate the actual Cs concentration. However, assuming that the maximum Cs amount trapped in the cartridge was 4.0 Bq, and if an average volume of 2 L of water passed through it daily for 2 months, then the average Cs concentration was 0.03 Bq/L (4.0 Bq/60 days × 2 L). This value was less than 10 Bq/L as the regulation value for drinking water. Finally, no cesium was detected from the water after being filtered through the developed water purifier.
References Fekry, A.E., El-Bieh, N.M., Elwan, K.M., Mangood, S.A., 2003. J. Radioanal. Nucl. Chem. 257 (1), 75–82. Haas, Paul A., 1993. Sep. Sci. Technol. 28, 2479–2506. Iwanade, A., Kasai, N., Hoshina, H., Ueki, Y., Saiki, S., Seko, N., 2012. J. Radioanal. Nucl. Chem. 293, 703–709. Ma, H., Yao, S., Li, J., Cao, C., Wang, M., 2012. Radiat. Phys. Chem. 81, 1393–1397. Mukai, H., Hatta, T., Kitazawa, H., Yamada, H., Yaita, T., Kogure, T., 2014. Environ. Sci. Technol. 48 (22), 13053–13059. Mukai, H., Hirose, A., Motai, S., Kikuchi, R., Tanoi, K., Nakanishi, T.M., Yaita, T., Kogure, T., 2016. Sci. Rep. 6, 21543. Rajec, P., Domianova, K., 2008. J. Radioanal. Nucl. Chem. 275 (3), 503–508. Saiki, S., Shibata, T., Hoshina, H., Ueki, Y., Kasai, N., Seko, N., 2014. J. Ion. Exch. 25, 170–175. Sekine, A., Seko, N., Tamada, M., Suzuki, Y., 2010. Radiat. Phys. Chem. 79, 16–21. Seko, N., Basuki, F., Tamada, M., Yoshii, F., 2004. React. Funct. Polym. 59, 235–241. Seko, N., Bang, L.T., Tamada, M., 2007. Nucl. Instr. Methods B 265, 146–149. Seko, N., Ninh, N.T.Y., Tamada, M., 2010. Radiat. Phys. Chem. 79, 22–26. Shibata, T., Seko, N., Amada, H., Kasai, N., Saiki, S., Hoshina, H., Ueki, Y., 2016. Radiat. Phys. Chem. 119, 247–252. Takeda, T., Tamada, M., Seko, N., Ueki, Y., 2010. Radiat. Phys. Chem. 79, 223–226. Todd, T.A., Mann, N.R., Tranter, T.R., Sebesta, F., John, J., Motl, A., 2002. J. Radioanal. Nucl. Chem. 254 (1), 47–52. Wachinski, A.M., Etzel, J.E., 1997. Environmental Ion Exchange. CRC Press, Boca Raton, FL (Chapter 4 laboratory-scale testing of ion exchange).
4. Conclusions To contribute in the Fukushima recovery, the novel adsorbent for Cs removal was developed by radiation-induced emulsion graft polymerization of GMA/AMP onto polyethylene nonwoven fabric. Especially, we focused on developing a water purifier that could be applied in drinking water for the safe consumption of the returning residents. In order to construct the water purifier, it was necessary to develop a mass production procedure of radiation graft polymerization. The grafting results indicate that it is possible to graft a NF with a length of 10 m and a width of 30 cm with even Dg distribution. In the batch and column (syringe) test using Cs contaminated well water samples, it has been demonstrated that the Cs could be trapped to
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