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Application of Prussian blue nanoparticles for the radioactive Cs decontamination in Fukushima region
Journal of Environmental Radioactivity 151 (2016) 233e237
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Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Short communication
Application of Prussian blue nanoparticles for the radioactive Cs decontamination in Fukushima region Durga Parajuli a, *, Akiko Kitajima b, Akira Takahashi a, Hisashi Tanaka a, Hiroshi Ogawa a, Yukiya Hakuta a, Kazunori Yoshino c, Takayuki Funahashi d, Masaki Yamaguchi d, Mitsuo Osada d, Tohru Kawamoto a, ** a
Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, 305-8565, Japan Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, 305-8565, Japan Kanto Chemical Company Inc., 7-1-1, Inari, Soka, Saitama, 340-0003, Japan d Tokyo Power Technology Ltd., 5-5-13, Toyosu, Koto-ku, Tokyo, 135-0061, Japan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 July 2015 Received in revised form 14 October 2015 Accepted 16 October 2015 Available online xxx
Cs decontamination efficiencies of the composites of iron hexacyanoferrate nanoparticles were investigated in comparison with commercial Prussian blue and natural zeolite. In pure water solution, the adsorption rate varied with sizes. In ash extract, where Cs adsorbing ability of zeolite was sharply dropped due to its poor selectivity, the impact of coexisting ions was negligible for FeHCF. FeHCF-n11, having the finest primary and secondary particle size, resulted the highest distribution coefficient, which was comparable to the high efficiency analogues, CoHCF or NiHCF. This observation suggested the possibility of preparing the high performance FeHCF by particle size and composition adjustment. FeHCF nanoparticle in bead form was tested for the removal of radioactive Cs in pilot scale. Due to larger secondary particle size, pronounced effect of solution temperature on the Cs adsorption kinetics on FeHCF bead was observed. Adjusting the mass of the adsorbent for the given solution temperature is recommended for achieving high decontamination rate. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Iron hexacyanoferrate Particle size Selectivity Kinetics Pilot scale decontamination
1. Introduction 137 Cs comprises 6e7% of the fission products (IAEA, 2008). Studies on the separation of Cs was focused on the nuclear waste until its release to the environment after the impactful Chernobyl accident (IAEA, 2006) and recently the Fukushima accident (Safety of Nuclear Power Reactors (2015)). It is not clear whether the fallout Cs is in readily soluble form (Niimura et al., 2015). However, because of its mobility via ground water, transfer to the flora and gradual fixation with clay minerals, environmental decontamination of Cs is tough (Kogure et al., 2012; Kozai et al., 2012). The concentration of radioactive Cs (r-Cs) in the environment is negligible when compared with other alkali metals. Cs in soil, plants, water, and in the air, requires different decontamination approaches (Parajuli et al., 2014; Vandebroek et al., 2012; Wendling
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D. Parajuli), tohru.kawamoto@aist. go.jp (T. Kawamoto). http://dx.doi.org/10.1016/j.jenvrad.2015.10.014 0265-931X/© 2015 Elsevier Ltd. All rights reserved.
et al., 2005). In any cases, bringing Cs to solution phase followed by enrichment to smallest possible volume is aimed (Parajuli et al., 2014, 2013). Therefore, for tremendously higher concentration of other elements, the Cs-selectivity factor becomes additionally important. Two groups of materials, in particular, are known for Cs decontamination: zeolites and metal hexacyanoferrates (MHCFs) (Barton et al., 1958; Borai et al., 2009; Clearfield, 1982; Nielsen et al., 1987; Tananaev, 1958; Torad et al., 2012; Tusa et al., 2007). These materials are extensively studied and used in masses (Cash et al., 1997; Hepworth et al., 1957). The best features of zeolite are, naturally availability, wide working pH, and substantial adsorption capacity (Misaelides, 2011). Similarly, MHCFs can be easily prepared, and are known for their unique Cs-selectivity. Their in-situ synthesis for the removal of Cs has long been practiced (Barton et al., 1958). On the other hand, these materials are equally debated for their demerits. Zeolites are described as less selective and MHCFs are questioned for their stability. However, when the primary purpose is selective removal for the volume reduction of environmental wastes, MHCFs with unbeatable selectivity satisfy
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the requirement. Among the MHCFs, cobalt hexacyanoferrate (CoHCF) and nickel hexacyanoferrate (NiHCF) are considered highly efficient (Haas, 1993). However, for huge area decontamination, the production cost of CoHCF and NiHCF can be problematic. Therefore, high efficiency FeHCF prepared with controlled composition and/or the primary particle size can be a sustainable choice (Ishizaki et al., 2013). In this paper, in particular, we discuss on the particle size optimization as an effective approach for increasing the performance. At first two kinds of FeHCFs were compared for their Cs removing behavior in pure water and wood ash-extracted solution systems. One FeHCF, FeHCF-n, is nanoparticles synthesized with the downsizing of primary particle and the optimization of the chemical composition for the Cs adsorption. The other is the commercial pigment without the optimization. Zeolite were also tested for comparison. For understanding the performance in real decontamination systems, FeHCF beads prepared using FeHCF-n were tested for the removal of r-Cs from ash washed solution. The decontamination efficiencies of these materials are discussed mainly based on the selectivity in pure water and wood ash extract. In addition, the effect of particle size of FeHCF and temperature on the decontamination of r-Cs is discussed based on the data obtained in pilot plant experiment. 2. Materials and methods 2.1. Adsorbent materials Iron hexacyanoferrate (FeHCF), (Fe4(Fe(CN)6)3$nH2O) nanoparticles, FeHCF-n, were prepared, where the composition ratio was set for higher Cs-adsorption performance in accordance with Ishizaki et al. (Ishizaki et al., 2013). The product obtained in slurry form contained FeHCF-n particles of average 10 nm (Fig. S1). Commercial FeHCF, FeHCF-co MILORI BLUE 905 by Dainichiseika Color & Chemicals, with the composition of (NH4)0.70Fe1.10[Fe(CN)6]$1.7H2O (Ishizaki et al., 2013), and zeolite Ayasi (0.5 mm, natural mordenite by Shin-Tohoku Chemicals) were used as obtained. The crystallite sizes of FeHCF-co and FeHCF-n are 8.8 nm and 36 nm, respectively (Ishizaki et al., 2013). The slurry of FeHCF-n was spray-dried to obtain powders with average 11 mm and 60 mm secondary sizes, which are named as FeHCF-n11 and FeHCF-n60, respectively. For the r-Cs decontamination in plantscale, FeHCF-n in bead form, FeHCF-nb, with the composition of 80% FeHCF-n and 20% calcium alginate was used (Chen et al., 2015). The physical properties of these FeHCF's are shown in Table 1. 2.2. Cs solutions Stock solution of Cs in pure water was prepared by dissolving aliquot amount of analytical grade CsNO3. It was diluted with pure water or with ash extract for obtaining the solution of Cs in water or the solution of Cs in ash extract, respectively. Fly ashes with and without r-Cs generated during the pilot-scale study for the decontamination of burnable wastes in Fukushima area was used for preparing the extract solutions. For this, ash samples
were stirred with pure water in 1:10 solid to solvent ratio, at 40 C for 1 h. The extracts were neutralized using concentrated sulfuric acid to about pH 6.5 and the precipitate was separated using 0.45 mm membrane filter. Concentrations of some major elements in the neutralized solutions are listed in Table S1. The extract diluted to 1:100 ratio was used for the adsorption test. In case of ash extract free of r-Cs, the experiment was performed with added Cs (stable 133Cs) for the ease of measurement and distinct comparison. 2.3. Adsorption experiment Preliminary tests were performed taking Cs in pure water and the variation was compared with ash extract. For this, FeHCF-n11, FeHCF-n60, FeHCF-co, and natural zeolite, were mixed with 1 mg/ L Cs solution in respective media at varying solvent-to-solid (v/m) ratios by changing the mass of the adsorbent. After mixing for 100 min at 600 rpm and 30 C, solution phase was collected for analysis by filtration through 0.45 mm membrane filter. Also, FeHCF-n60 and zeolite were compared in flow experiment taking ash extract solution with 100 mg/L added Cs. 1 g each of the adsorbents were packed in 1 cm diameter column and the solution was passed to the respective columns at the rate of 2.5 ml/min. The effluent was collected in 20 ml fractions for the analysis of residual Cs. In addition, Cs adsorption kinetics of FeHCF-nb was studied at 10, 30, and 50 C and was compared with the kinetics of Cs adsorption on FeHCF-n60 at 30 C. For this, temperature of the adsorbent and the solution were first adjusted to the required temperature and then mixed at 1000 v/m ratios. Sampling was performed between 5 and 60 min. The concentration of Cs was analyzed using Perkin Elmer NEXION 300D ICP-MS and r-Cs was analyzed using a gamma-ray spectrometer equipped with an ORTEC model GEM25P4-70 Ge-detector. 2.4. Pilot test for r-Cs decontamination For the decontamination of burnable wastes, a pilot scale plat equipped with safe incineration system designed for trapping the vaporized Cs was established in Fukushima area (Parajuli et al., 2013). For the removal of extracted Cs, which contained both stable and radioactive isotopes, FeHCF-nb packed in 1.9 cm stainless column was tested. Pre-test was performed in the lab using 100 g FeHCF-nb packed in the same column. With 10 cm bed height and 0.1 L/min flow rate, the retention time was 1.13 min. 30 L of 1 mg/L Cs solution in ash extract was passed at lab temperature, 20e22 C. The effluent was collected in 500 mL fraction. The decontamination rate was tracked based on the residual Cs solution in the effluent solution. For the r-Cs decontamination experiment, two columns of same size of lab-test packed with 135 g each of FeHCF-nb (bed height 13.5 cm) were connected in a line (Fig. S2). Ash extract containing 12,850 Bq/kg r-Cs was passed to the line at the same rate as in the lab test, 0.1 L/min, which gives 1.526 min retention time. On the day of experiment, the solution temperature was about 10.5 C. The effluent solution was collected in 1 L fraction, in a manner as illustrated in Fig. S1.
Table 1 Physical properties of FeHCFs. Chemical composition
FeHCF-n11
FeHCF-n60
FeHCF-nb
Fe4(Fe(CN)6)3 Shape Crystallite size Secondary particle size
powder 8.8 nm 11 mm
FeHCF-co (NH4)0.70Fe1.10[Fe(CN)6]
powder 8.8 nm 60 mm
beads 8.8 nm 1.0 mm
powder 36 nm 9 mm
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% Adsorption
80
60 40
20 0 0
1000
2000 3000 V/M (L/kg)
FeHCF-n11-ash FeHCF-n60-ash FeHCF-co-ash Zeolite-ash
4000
5000
FeHCF-n11-mq FeHCF-n60-mq FeHCF-co-mq Zeolite-mq
Fig. 1. Difference in Cs adsorption behaviors of FeHCF in various forms and natural zeolite Ayashi from pure water and an ash extract solution. [Cs] ¼ 1 mg/L 100 min mixing at 30 C, 600 rpm.
3. Results and discussion 3.1. The role of size and selectivity Cs adsorption behavior of FeHCF adsorbents and zeolite were compared at various solid to solvent (v/m) ratios. For the solution in pure water containing 1 mg/L Cs in pure water and 3 h mixing at 30 C, nearly complete adsorption was observed at 1:500 solid to solvent ratio, Fig. 1. However, with the increase in the solvent volume, the difference among the materials becomes apparent. For FeHCF-n11, the adsorption was found to be almost independent of the v/m ratio. However, for the other adsorbents, the adsorption dropped with the increasing value of v/m in the order as: FeHCFn11>>FeHCF-n60> FeHCF-co > Zeolite, demonstrating a trend of faster adsorption on finer sized adsorbents, in both primary and secondary sizes, which offer larger available outer surface. For FeHCF, the primary size is between about 10 and 40 nm. Primary size of FeHCF-n11 and FeHCF-n60 is the same, however difference in the sizes of dried-composites is making a reasonable difference in the Cs adsorption performance. Primary particle size of FeHCFco, however, is relatively larger, 36 nm, which can be considered one factor for its slower adsorption kinetics, together with possible effect from its chemical composition. The distribution coefficient (Kd) was calculated as:
C Ceq V Kd ¼ 0 $ m Ceq where, V is the solution volume, m is the mass of the adsorbent, C0 and Ceq are the concentrations of Cs before and after adsorption, respectively. Kd of the FeHCF-n11 at the v/m ¼ 5000 L/kg is 1.6 106 mL/g, which is 5e20 times larger than that of previously report value for FeHCF, (Sangvanich et al., 2010) and is comparable with its highest Kd analogues CoHCF or NiHCF (Haas, 1993). Because the primary size of FeHCF-n11 is almost same as of micromixer synthesized nanosized CuHCF (Takahashi et al., 2015), high Kd with FeHCF could also be achieved by size downsizing and the optimization of chemical composition. The experiment was repeated in ash solution. In contrary to
nearly unchanged adsorption on FeHCF, a sharp drop in the Cs adsorption efficiency of zeolite was observed. Here, the variation among the FeHCFs is similar to that in pure water. The difference in adsorption kinetics owing to the size factor is obvious. In such cases, the adsorption rate can be improved by increasing the mixing time. However, the problem with zeolite is its poor selectivity for Cs (NIES, 2013). A comparative experiment in dynamic mode taking the zeolite sample and FeHCF-n60 was performed. For this, solution of 100 mg/ l added Cs in ash-extract (1:100 ratio) was passed through a column packed with 1 g adsorbent, zeolite-Ayashi (0.5 mm) or FeHCF-n60 (60 mm average). As shown in Fig. 2, because of substantially higher concentration of competing alkali metal ions (Table 1), the Cs decontamination efficiency of zeolite was very low. Compared to immediate breakthrough observed for the zeolite, as expected from the results of batch test, FeHCF-n60 column showed 100% removal (for Cs detection limit of the ICP-MS: 0.1 ppt in pure water) until 1000 bed volume. Almost all of the studies on Cs removal are related to the decontamination of its radioisotopes from the environment and from the nuclear fuel dissolved solutions. The choice of adsorbent can be concentration dependent as well. It is well known that the concentration of Cs in the spent nuclear fuel solutions is comparatively much higher than the concentration of Cs in the environment. Also, comparing the concentrations among the alkali metals, Cs in the environment can be considered in negligible level. In Japanese river water, for example, where the concentration of Na is 0.76e13.6 mg/l and that of K is 0.14e3.43 mg/l, the concentration of Cs lies between 0.002 and 0.33 mg/L (NIRS, 2006). The difference in seawater is incomparably higher, as for 0.3 mg/L Cs (Bolter et al., 1964), the concentrations of Na and K are 10560 mg/l and 380 mg/l, respectively (Sverdrup et al., 1942). Therefore, when the target is environmental Cs, the selectivity of the adsorbent material should be the foremost requirement, mainly for two important reasons. The first reason is, decontamination of Cs from the environment is almost always done for the removal of its radioactive isotopes. This means, the purpose of adsorptive recovery is for the enrichment of Cs to highest possible concentration so that the radioisotopes can be confined to the smallest possible volume. Second and equally crucial reason is, during the environmental decontamination, as all the effluent cannot be stored, it should be
1.0 Cs in effluent/Cs in feed
100
235
0.8 0.6 0.4 FeHCF-n60
0.2
Zeolite
0.0
0
500 1000 Bed Volume
1500
Fig. 2. Comparison of the Cs selectivity of FeHCF-n60 against zeolite (Ayashi- 0.5 mm). 1 g adsorbents packed in a column of 1 cm diameter. 100 mg/L Cs in ash extract, pH 7.2, neutralized with HCl to neutral range. Space velocity ¼ 2.293 min1. Bed volume is the ratio of the volume of solution passed to the volume of the adsorbent packed in the column.
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decontaminated below the free release limit. For this, the concentration of radioisotopes should be under the detection limit 5e10 Bq/L (IAEA, 1989). Therefore, in order for achieving these two requirements in the system with relatively very low concentration Cs, material with nearly absolute selectivity should be the primary choice. In this sense, MHCF can be highly effective Cs decontaminants (Tusa et al., 2007). Along with selectivity, the cost factor also becomes crucial when the area to be decontaminated is extremely large. Fukushima nuclear accident, for example, contaminated the Tohoku and Kanto area of Japan, in addition to the bordering pacific coast. In such case, the area in need of decontamination range from fresh water to saline water, forest, productive land, and so on. In this sense, selecting the most cost effective MHCF becomes necessary. So, despite their high efficiency Cs decontamination performances, CoHCF and NiHCF should be replaced by a cheaper analogue, FeHCF. Therefore, improvement of the performance of FeHCF, for example, which contains a “common metal”, by particle downsizing and the optimization of the chemical composition to the same level as CoHCF and NiHCF can be a realistic solution for the environmental decontamination.
3.2. Adsorption kinetics in bead form Although the adsorption kinetics is faster with the finer adsorbents, it would be practically difficult using FeHCF-n11 or FeHCF-n60 for mass scale decontamination. For such purpose FeHCF-nb was fabricated. Aiming its application in large scale decontamination, Cs adsorption kinetics of FeHCF-nb at 10 C, 30 C, and 50 C was studied. For this, higher concentration Cs solution was used so that enough Cs is remained in the solution phase until the equilibrium is reached. As shown in Fig. 3, between the sampling times of 5 min to 1 h, at respective temperatures, the residual concentration of Cs is decreasing with time, following the zero order rate law. The rates of adsorption were calculated as: 0.124, 0.216, and 0.231 mmol h1, at 10 C, 30 C, and 50 C, respectively. The intercept of each plots, which should be, in principle, 1, ranged between 0.93 and 0.99. Adsorption on FeHCFn60 was much faster than onto the beads. For it, a straight line was obtained with the intercept of 0.74, suggesting a distinct deviation from the zero order rate law. This difference is expected to be due to the involvement of the diffusion factor in the case of beads. It is
obvious that the role of diffusion become pronounced for larger particles. In this sense, the deviation at 50 C can be considered as the result of accelerated diffusion at higher temperature. Overall, the results observed for FeHCF-nb reveal that, in real applications, Cs uptake rate varies with the field temperature. Therefore, depending upon the temperature of the water to be decontaminated, either the amount of adsorbent or the flow rate should be adjusted. 3.3. Pilot scale decontamination of radioactive Cs Based on the result of the high selectivity adsorption of Cs on FeHCF powder, Fig. 1, and the temperature dependent adsorption profile, Fig. 3, FeHCF-nb was studied for its application in decontamination plants. Pre-test was carried out in the lab taking 0.0075 mM Cs solution prepared in wood ash extract (1:20). The solution was passed to 100 g of FeHCF-nb packed column in a rate of 0.1 L/min. As shown in Fig. 4, nearly 100% removal of Cs was achieved. Following this successful outcome, radioactive Cs decontamination test was carried out in the pilot-scale plant constructed in a hillside at the Kawauchi village, Fukushima. Different from the lab temperature of 22e24 C, the solution temperature on the day of pilot test was 10.5 C, which, based on Fig. 3, means the amount of the adsorbent used in the lab test may not be sufficient for achieving effective decontamination. Therefore, a line containing two similar columns packed with 135 g each of FeHCF-nb was established for the plant test. The extract of pH 6.5, 12,850 Bq/kg r-Cs concentration and stable Cs concentration of 0.332 mg/L was passed to the line at the rate of 0.1 L/min. Column-1 (through the first 135 g adsorbent) was sampled for every 6 L and column-2 (through total 270 g adsorbent) was sampled for every 5 L. Despite containing larger mass of the adsorbent and lower total Cs concentration than in the lab, at the same flow rate, the decontamination was lower. Samples from column-1 were analyzed for both stable and r-Cs. As given in Fig. 4, the results were closer enough. The effluent from Column-2, which was passed through total 270 g FeHCF-nb, high adsorption rate was observed. Although the result was satisfying under the given circumstances, it was not as promising as the results of several preliminary tests performed in the lab, which is most probably due to the difference in the solution temperature. Results shown in Fig. 1 explain the role of particle size. The secondary particle sizes (primary particle size being the size of FeHCF in
0.95
100
R² = 0.9988
0.90
% Adsorption
Concentration Ratio (final/initial)
1.00
R² = 0.9979 0.85 R² = 0.9638 0.80 0.75
10ºC 30ºC 50ºC FeHCF-n60 30ºC
0.70 0.65 0
10
r-Cs, Col-1
80
R² = 0.970
20 30 40 Mixing Time (min)
90
r-Cs, Col-2 Cs, Col-1 Cs, lab
70 50
60
Fig. 3. Relationship between the residual Cs concentration and mixing time showing the temperature dependent adsorption of Cs onto FeHCF-nb. Adsorption on FeHCF-n60 is faster. Initial Cs concentration ¼ 1.41 mM; mixing at 600 rpm; 1000 v/m ratio.
0
10
20
30
Cumulative Volume (L) Fig. 4. Comparison of Cs removing performance of FeHCF-nb in flow at different conditions. Lab: 100 g FeHCF-nb packed in 1.9 cm radius stainless column, 20-22 C. Plant: two columns of same size packed with 135 g each FeHCF-nb in a line, 10-12 C.
D. Parajuli et al. / Journal of Environmental Radioactivity 151 (2016) 233e237
the dispersion solution) of FeHCF-n11 and FeHCF-nb are 0.011 mm and 1 mm, respectively. Therefore, FeHCF-nb is nearly 91 times larger than FeHCF-n11 and double the size of zeolite-Ayashi. However, the v/m ratio in the case of FeHCF-nb column experiment is much smaller. Also, the results observed in the lab show nearly complete removal of Cs with only 100 g FeHCF-nb packed column. Summarizing the results observed in Figs. 1,3 and 4, besides the selectivity, the role of temperature together with the particle size should be specifically considered for achieving high efficiency decontamination. 4. Conclusion Among the FeHCF adsorbents and the zeolite, the variation in adsorption rate was mainly due to the size factor. However, in ash extract, different from the FeHCFs, zeolite showed pronounced impact of coexisting ions as its Cs adsorbing ability was sharply dropped. FeHCF-n11, with the finest primary particle size and with the secondary one, resulted the highest distribution coefficient, which was comparable to the high efficiency analogues, CoHCF or NiHCF. This observation suggested the possibility of preparing the high performance FeHCF by particle size and composition adjustment. In addition, high decontamination rate observed with FeHCFn60 in ash extract is noteworthy. FeHCF in bead form maintained its Cs selectivity despite the presence of calcium alginate. Because of slow adsorption rate, Cs adsorption kinetics on FeHCF-nb was striking temperature dependent. It became obvious with the results observed during the removal of radioactive Cs in pilot scale. Therefore, although the selectivity of FeHCF remains nearly unchanged even in highly saline solutions, adjusting the mass of the adsorbent or the flow rate with respect to the solution temperature is required for achieving high decontamination rate. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvrad.2015.10.014. References Barton, G.B., Hepworth, J.L., McClanahan, E.D., Moore, R.L., Tuyl, H.H.V., 1958. Chemical processing wastes. recovering fission products. Ind. Eng. Chem. 50, 212e216. http://dx.doi.org/10.1021/ie50578a039. Bolter, E., Turekian, K.K., Schutz, D.F., 1964. The distribution of rubidium, cesium and barium in the oceans. Geochim. Cosmochim. Acta 28, 1459e1466. http:// dx.doi.org/10.1016/0016-7037(64)90161-9. Borai, E.H., Harjula, R., Malinen, L., Paajanen, A., 2009. Efficient removal of cesium from low-level radioactive liquid waste using natural and impregnated zeolite minerals. J. Hazard. Mater 172, 416e422. http://dx.doi.org/10.1016/ j.jhazmat.2009.07.033. Cash, R.J., Meacham, J.E., Babad, H., 1997. Resolution of the Hanford Site Ferrocyanide Safety Issue, in: WM 1997. WM Symposia. Chen, G.-R., Chang, Y.-R., Liu, X., Kawamoto, T., Tanaka, H., Kitajima, A., Parajuli, D., Takasaki, M., Yoshino, K., Chen, M.-L., Lo, Y.-K., Lei, Z., Lee, D.-J., 2015. Prussian blue (PB) granules for cesium (Cs) removal from drinking water. Sep. Purif. Technol. 143, 146e151. http://dx.doi.org/10.1016/j.seppur.2015.01.040. Clearfield, A., 1982. Inorganic Ion Exchange Materials. CRC Press Inc., Florida. Haas, P.A., 1993. A review of information on ferrocyanide solids for removal of cesium from solutions. Sep. Sci. Technol. 28, 2479e2506. http://dx.doi.org/ 10.1080/01496399308017493. Hepworth, J.L., McClanahan Jr., E.D., Moore, R.L. (Eds.), 1957. Hepworth 5 7 Hanford. Cesium Packaging Studies Conversion of Cesium Zinc Ferrocyanide to a Cesium Chloride Product.
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IAEA, 1989. Measurement of Radionuclides in Food and the Environment Technical Report 295. IAEA, 2006. Radiological Assessment Reports Series: Environmental Consequences of the Chernobyl Accident and Their Remediation Twenty Years of Experience. Vienna. IAEA, 2008. Fission Product Yield Data for the Transmutation of Minor Actinide Nuclear Waste. Ishizaki, M., Akiba, S., Ohtani, A., Hoshi, Y., Ono, K., Matsuba, M., Togashi, T., Kananizuka, K., Sakamoto, M., Takahashi, A., Kawamoto, T., Tanaka, H., Watanabe, M., Arisaka, M., Nankawa, T., Kurihara, M., 2013. Proton-exchange mechanism of specific Csþ adsorption via lattice defect sites of Prussian blue filled with coordination and crystallization water molecules. Dalton Trans. 42, 16049. http://dx.doi.org/10.1039/c3dt51637g. Kogure, T., Morimoto, K., Tamura, K., Sato, H., Yamagishi, A., 2012. XRD and HRTEM evidence for fixation of cesium ions in vermiculite clay. Chem. Lett. 41, 380e382. http://dx.doi.org/10.1246/cl.2012.380. Kozai, N., Ohnuki, T., Arisaka, M., Watanabe, M., Sakamoto, F., Yamasaki, S., Jiang, M., 2012. Chemical states of fallout radioactive Cs in the soils deposited at Fukushima Daiichi nuclear power plant accident. J. Nucl. Sci. Technol. 49, 473e478. http://dx.doi.org/10.1080/00223131.2012.677131. Misaelides, P., 2011. Application of natural zeolites in environmental remediation: a short review. Microporous Mesoporous Mater 144, 15e18. http://dx.doi.org/ 10.1016/j.micromeso.2011.03.024. Nielsen, P., Dresow, B., Heinrich, H.C., 1987. In vitro study of 137Cs Sorption by Hexacyanoferrates(II). Z. Fur Naturforsch. Sect. B a J. Chem. Sci. 42b, 1451e1460. NIES, 2013 (NIES Annual Report). http://www.nies.go.jp/kanko/annual/ae19.pdf. Niimura, N., Kikuchi, K., Tuyen, N.D., Komatsuzaki, M., Motohashi, Y., 2015. Physical properties, structure, and shape of radioactive Cs from the Fukushima Daiichi nuclear power plant accident derived from soil, bamboo and shiitake mushroom measurements. J. Environ. Radioact. 139, 234e239. http://dx.doi.org/ 10.1016/j.jenvrad.2013.12.020. NIRS, 2006. Elemental Concentrations in JapanEse Rivers 2002e2006. Japan. Parajuli, D., Takahashi, A., Tanaka, H., Sato, M., Fukuda, S., Kamimura, R., Kawamoto, T., 2014. Variation in available cesium concentration with parameters during temperature induced extraction of cesium from soil. J. Environ. Radioact. 140C, 78e83. http://dx.doi.org/10.1016/j.jenvrad.2014.11.005. Parajuli, D., Tanaka, H., Hakuta, Y., Minami, K., Fukuda, S., Umeoka, K., Kamimura, R., Hayashi, Y., Ouchi, M., Kawamoto, T., 2013. Dealing with the aftermath of Fukushima Daiichi nuclear accident: decontamination of radioactive cesium enriched ash. Environ. Sci. Technol. 47, 3800e3806. http://dx.doi.org/10.1021/ es303467n. Safety of Nuclear Power Reactors, 2015 ([WWW Document]). http://www.worldnuclear.org/info/Safety-and-Security/Safety-of-Plants/Safety-of-Nuclear-PowerReactors/. Sangvanich, T., Sukwarotwat, V., Wiacek, R.J., Grudzien, R.M., Fryxell, G.E., Addleman, R.S., Timchalk, C., Yantasee, W., 2010. Selective capture of cesium and thallium from natural waters and simulated wastes with copper ferrocyanide functionalized mesoporous silica. J. Hazard. Mater 182, 225e231. http:// dx.doi.org/10.1016/j.jhazmat.2010.06.019. Sverdrup, H.U., Johnson, M.W., Fleming, R.H., 1942. Chemistry of Sea Water, in: the Oceans Their Physics, Chemistry, and General Biology. Prentice-Hall, New York, pp. 165e227. Takahashi, A., Minami, N., Tanaka, H., Sue, K., Minami, K., Parajuli, D., Lee, K.M., Ohkoshi, S., Kurihara, M., Kawamoto, T., 2015. Efficient synthesis of sizecontrolled open-framework nanoparticles fabricated with a micro-mixer: route to the improvement of cs-adsorption performance. Green Chem. http:// dx.doi.org/10.1039/c5gc00757g. Tananaev, I.V., 1958. New data on the chemistry of some of the rare elements. Russ. Chem. Bull. 6, 1447e1453. http://dx.doi.org/10.1007/bf01169748. Torad, N.L., Hu, M., Imura, M., Naito, M., Yamauchi, Y., 2012. Large Cs adsorption capability of nanostructured Prussian Blue particles with high accessible surface areas. J. Mater. Chem. 22 http://dx.doi.org/10.1039/c2jm32805d. Tusa, E., Harjula, R., Yarnell, P., 2007. Fifteen Years of Operation with Inorganic Highly Selective ion Exchange Materials. WM2007. Vandebroek, L., Hees, M.V., Delvaux, B., Spaargaren, O., Thiry, Y., 2012. Relevance of Radiocaesium Interception Potential (RIP) on a worldwide scale to assess soil vulnerability to 137Cs contamination. J. Environ. Radioact. 104, 87e93. http:// dx.doi.org/10.1016/j.jenvrad.2011.09.002. Wendling, L.A., Harsh, J.B., Ward, T.E., Palmer, C.D., Hamilton, M.A., Boyle, J.S., Flury, M., 2005. Cesium desorption from illite as affected by exudates from rhizosphere bacteria. Environ. Sci. Technol. 39, 4505e4512. http://dx.doi.org/ 10.1021/es048809p.
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Short communication
Application of Prussian blue nanoparticles for the radioactive Cs decontamination in Fukushima region Durga Parajuli a, *, Akiko Kitajima b, Akira Takahashi a, Hisashi Tanaka a, Hiroshi Ogawa a, Yukiya Hakuta a, Kazunori Yoshino c, Takayuki Funahashi d, Masaki Yamaguchi d, Mitsuo Osada d, Tohru Kawamoto a, ** a
Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, 305-8565, Japan Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, 305-8565, Japan Kanto Chemical Company Inc., 7-1-1, Inari, Soka, Saitama, 340-0003, Japan d Tokyo Power Technology Ltd., 5-5-13, Toyosu, Koto-ku, Tokyo, 135-0061, Japan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 July 2015 Received in revised form 14 October 2015 Accepted 16 October 2015 Available online xxx
Cs decontamination efficiencies of the composites of iron hexacyanoferrate nanoparticles were investigated in comparison with commercial Prussian blue and natural zeolite. In pure water solution, the adsorption rate varied with sizes. In ash extract, where Cs adsorbing ability of zeolite was sharply dropped due to its poor selectivity, the impact of coexisting ions was negligible for FeHCF. FeHCF-n11, having the finest primary and secondary particle size, resulted the highest distribution coefficient, which was comparable to the high efficiency analogues, CoHCF or NiHCF. This observation suggested the possibility of preparing the high performance FeHCF by particle size and composition adjustment. FeHCF nanoparticle in bead form was tested for the removal of radioactive Cs in pilot scale. Due to larger secondary particle size, pronounced effect of solution temperature on the Cs adsorption kinetics on FeHCF bead was observed. Adjusting the mass of the adsorbent for the given solution temperature is recommended for achieving high decontamination rate. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Iron hexacyanoferrate Particle size Selectivity Kinetics Pilot scale decontamination
1. Introduction 137 Cs comprises 6e7% of the fission products (IAEA, 2008). Studies on the separation of Cs was focused on the nuclear waste until its release to the environment after the impactful Chernobyl accident (IAEA, 2006) and recently the Fukushima accident (Safety of Nuclear Power Reactors (2015)). It is not clear whether the fallout Cs is in readily soluble form (Niimura et al., 2015). However, because of its mobility via ground water, transfer to the flora and gradual fixation with clay minerals, environmental decontamination of Cs is tough (Kogure et al., 2012; Kozai et al., 2012). The concentration of radioactive Cs (r-Cs) in the environment is negligible when compared with other alkali metals. Cs in soil, plants, water, and in the air, requires different decontamination approaches (Parajuli et al., 2014; Vandebroek et al., 2012; Wendling
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D. Parajuli), tohru.kawamoto@aist. go.jp (T. Kawamoto). http://dx.doi.org/10.1016/j.jenvrad.2015.10.014 0265-931X/© 2015 Elsevier Ltd. All rights reserved.
et al., 2005). In any cases, bringing Cs to solution phase followed by enrichment to smallest possible volume is aimed (Parajuli et al., 2014, 2013). Therefore, for tremendously higher concentration of other elements, the Cs-selectivity factor becomes additionally important. Two groups of materials, in particular, are known for Cs decontamination: zeolites and metal hexacyanoferrates (MHCFs) (Barton et al., 1958; Borai et al., 2009; Clearfield, 1982; Nielsen et al., 1987; Tananaev, 1958; Torad et al., 2012; Tusa et al., 2007). These materials are extensively studied and used in masses (Cash et al., 1997; Hepworth et al., 1957). The best features of zeolite are, naturally availability, wide working pH, and substantial adsorption capacity (Misaelides, 2011). Similarly, MHCFs can be easily prepared, and are known for their unique Cs-selectivity. Their in-situ synthesis for the removal of Cs has long been practiced (Barton et al., 1958). On the other hand, these materials are equally debated for their demerits. Zeolites are described as less selective and MHCFs are questioned for their stability. However, when the primary purpose is selective removal for the volume reduction of environmental wastes, MHCFs with unbeatable selectivity satisfy
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the requirement. Among the MHCFs, cobalt hexacyanoferrate (CoHCF) and nickel hexacyanoferrate (NiHCF) are considered highly efficient (Haas, 1993). However, for huge area decontamination, the production cost of CoHCF and NiHCF can be problematic. Therefore, high efficiency FeHCF prepared with controlled composition and/or the primary particle size can be a sustainable choice (Ishizaki et al., 2013). In this paper, in particular, we discuss on the particle size optimization as an effective approach for increasing the performance. At first two kinds of FeHCFs were compared for their Cs removing behavior in pure water and wood ash-extracted solution systems. One FeHCF, FeHCF-n, is nanoparticles synthesized with the downsizing of primary particle and the optimization of the chemical composition for the Cs adsorption. The other is the commercial pigment without the optimization. Zeolite were also tested for comparison. For understanding the performance in real decontamination systems, FeHCF beads prepared using FeHCF-n were tested for the removal of r-Cs from ash washed solution. The decontamination efficiencies of these materials are discussed mainly based on the selectivity in pure water and wood ash extract. In addition, the effect of particle size of FeHCF and temperature on the decontamination of r-Cs is discussed based on the data obtained in pilot plant experiment. 2. Materials and methods 2.1. Adsorbent materials Iron hexacyanoferrate (FeHCF), (Fe4(Fe(CN)6)3$nH2O) nanoparticles, FeHCF-n, were prepared, where the composition ratio was set for higher Cs-adsorption performance in accordance with Ishizaki et al. (Ishizaki et al., 2013). The product obtained in slurry form contained FeHCF-n particles of average 10 nm (Fig. S1). Commercial FeHCF, FeHCF-co MILORI BLUE 905 by Dainichiseika Color & Chemicals, with the composition of (NH4)0.70Fe1.10[Fe(CN)6]$1.7H2O (Ishizaki et al., 2013), and zeolite Ayasi (0.5 mm, natural mordenite by Shin-Tohoku Chemicals) were used as obtained. The crystallite sizes of FeHCF-co and FeHCF-n are 8.8 nm and 36 nm, respectively (Ishizaki et al., 2013). The slurry of FeHCF-n was spray-dried to obtain powders with average 11 mm and 60 mm secondary sizes, which are named as FeHCF-n11 and FeHCF-n60, respectively. For the r-Cs decontamination in plantscale, FeHCF-n in bead form, FeHCF-nb, with the composition of 80% FeHCF-n and 20% calcium alginate was used (Chen et al., 2015). The physical properties of these FeHCF's are shown in Table 1. 2.2. Cs solutions Stock solution of Cs in pure water was prepared by dissolving aliquot amount of analytical grade CsNO3. It was diluted with pure water or with ash extract for obtaining the solution of Cs in water or the solution of Cs in ash extract, respectively. Fly ashes with and without r-Cs generated during the pilot-scale study for the decontamination of burnable wastes in Fukushima area was used for preparing the extract solutions. For this, ash samples
were stirred with pure water in 1:10 solid to solvent ratio, at 40 C for 1 h. The extracts were neutralized using concentrated sulfuric acid to about pH 6.5 and the precipitate was separated using 0.45 mm membrane filter. Concentrations of some major elements in the neutralized solutions are listed in Table S1. The extract diluted to 1:100 ratio was used for the adsorption test. In case of ash extract free of r-Cs, the experiment was performed with added Cs (stable 133Cs) for the ease of measurement and distinct comparison. 2.3. Adsorption experiment Preliminary tests were performed taking Cs in pure water and the variation was compared with ash extract. For this, FeHCF-n11, FeHCF-n60, FeHCF-co, and natural zeolite, were mixed with 1 mg/ L Cs solution in respective media at varying solvent-to-solid (v/m) ratios by changing the mass of the adsorbent. After mixing for 100 min at 600 rpm and 30 C, solution phase was collected for analysis by filtration through 0.45 mm membrane filter. Also, FeHCF-n60 and zeolite were compared in flow experiment taking ash extract solution with 100 mg/L added Cs. 1 g each of the adsorbents were packed in 1 cm diameter column and the solution was passed to the respective columns at the rate of 2.5 ml/min. The effluent was collected in 20 ml fractions for the analysis of residual Cs. In addition, Cs adsorption kinetics of FeHCF-nb was studied at 10, 30, and 50 C and was compared with the kinetics of Cs adsorption on FeHCF-n60 at 30 C. For this, temperature of the adsorbent and the solution were first adjusted to the required temperature and then mixed at 1000 v/m ratios. Sampling was performed between 5 and 60 min. The concentration of Cs was analyzed using Perkin Elmer NEXION 300D ICP-MS and r-Cs was analyzed using a gamma-ray spectrometer equipped with an ORTEC model GEM25P4-70 Ge-detector. 2.4. Pilot test for r-Cs decontamination For the decontamination of burnable wastes, a pilot scale plat equipped with safe incineration system designed for trapping the vaporized Cs was established in Fukushima area (Parajuli et al., 2013). For the removal of extracted Cs, which contained both stable and radioactive isotopes, FeHCF-nb packed in 1.9 cm stainless column was tested. Pre-test was performed in the lab using 100 g FeHCF-nb packed in the same column. With 10 cm bed height and 0.1 L/min flow rate, the retention time was 1.13 min. 30 L of 1 mg/L Cs solution in ash extract was passed at lab temperature, 20e22 C. The effluent was collected in 500 mL fraction. The decontamination rate was tracked based on the residual Cs solution in the effluent solution. For the r-Cs decontamination experiment, two columns of same size of lab-test packed with 135 g each of FeHCF-nb (bed height 13.5 cm) were connected in a line (Fig. S2). Ash extract containing 12,850 Bq/kg r-Cs was passed to the line at the same rate as in the lab test, 0.1 L/min, which gives 1.526 min retention time. On the day of experiment, the solution temperature was about 10.5 C. The effluent solution was collected in 1 L fraction, in a manner as illustrated in Fig. S1.
Table 1 Physical properties of FeHCFs. Chemical composition
FeHCF-n11
FeHCF-n60
FeHCF-nb
Fe4(Fe(CN)6)3 Shape Crystallite size Secondary particle size
powder 8.8 nm 11 mm
FeHCF-co (NH4)0.70Fe1.10[Fe(CN)6]
powder 8.8 nm 60 mm
beads 8.8 nm 1.0 mm
powder 36 nm 9 mm
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% Adsorption
80
60 40
20 0 0
1000
2000 3000 V/M (L/kg)
FeHCF-n11-ash FeHCF-n60-ash FeHCF-co-ash Zeolite-ash
4000
5000
FeHCF-n11-mq FeHCF-n60-mq FeHCF-co-mq Zeolite-mq
Fig. 1. Difference in Cs adsorption behaviors of FeHCF in various forms and natural zeolite Ayashi from pure water and an ash extract solution. [Cs] ¼ 1 mg/L 100 min mixing at 30 C, 600 rpm.
3. Results and discussion 3.1. The role of size and selectivity Cs adsorption behavior of FeHCF adsorbents and zeolite were compared at various solid to solvent (v/m) ratios. For the solution in pure water containing 1 mg/L Cs in pure water and 3 h mixing at 30 C, nearly complete adsorption was observed at 1:500 solid to solvent ratio, Fig. 1. However, with the increase in the solvent volume, the difference among the materials becomes apparent. For FeHCF-n11, the adsorption was found to be almost independent of the v/m ratio. However, for the other adsorbents, the adsorption dropped with the increasing value of v/m in the order as: FeHCFn11>>FeHCF-n60> FeHCF-co > Zeolite, demonstrating a trend of faster adsorption on finer sized adsorbents, in both primary and secondary sizes, which offer larger available outer surface. For FeHCF, the primary size is between about 10 and 40 nm. Primary size of FeHCF-n11 and FeHCF-n60 is the same, however difference in the sizes of dried-composites is making a reasonable difference in the Cs adsorption performance. Primary particle size of FeHCFco, however, is relatively larger, 36 nm, which can be considered one factor for its slower adsorption kinetics, together with possible effect from its chemical composition. The distribution coefficient (Kd) was calculated as:
C Ceq V Kd ¼ 0 $ m Ceq where, V is the solution volume, m is the mass of the adsorbent, C0 and Ceq are the concentrations of Cs before and after adsorption, respectively. Kd of the FeHCF-n11 at the v/m ¼ 5000 L/kg is 1.6 106 mL/g, which is 5e20 times larger than that of previously report value for FeHCF, (Sangvanich et al., 2010) and is comparable with its highest Kd analogues CoHCF or NiHCF (Haas, 1993). Because the primary size of FeHCF-n11 is almost same as of micromixer synthesized nanosized CuHCF (Takahashi et al., 2015), high Kd with FeHCF could also be achieved by size downsizing and the optimization of chemical composition. The experiment was repeated in ash solution. In contrary to
nearly unchanged adsorption on FeHCF, a sharp drop in the Cs adsorption efficiency of zeolite was observed. Here, the variation among the FeHCFs is similar to that in pure water. The difference in adsorption kinetics owing to the size factor is obvious. In such cases, the adsorption rate can be improved by increasing the mixing time. However, the problem with zeolite is its poor selectivity for Cs (NIES, 2013). A comparative experiment in dynamic mode taking the zeolite sample and FeHCF-n60 was performed. For this, solution of 100 mg/ l added Cs in ash-extract (1:100 ratio) was passed through a column packed with 1 g adsorbent, zeolite-Ayashi (0.5 mm) or FeHCF-n60 (60 mm average). As shown in Fig. 2, because of substantially higher concentration of competing alkali metal ions (Table 1), the Cs decontamination efficiency of zeolite was very low. Compared to immediate breakthrough observed for the zeolite, as expected from the results of batch test, FeHCF-n60 column showed 100% removal (for Cs detection limit of the ICP-MS: 0.1 ppt in pure water) until 1000 bed volume. Almost all of the studies on Cs removal are related to the decontamination of its radioisotopes from the environment and from the nuclear fuel dissolved solutions. The choice of adsorbent can be concentration dependent as well. It is well known that the concentration of Cs in the spent nuclear fuel solutions is comparatively much higher than the concentration of Cs in the environment. Also, comparing the concentrations among the alkali metals, Cs in the environment can be considered in negligible level. In Japanese river water, for example, where the concentration of Na is 0.76e13.6 mg/l and that of K is 0.14e3.43 mg/l, the concentration of Cs lies between 0.002 and 0.33 mg/L (NIRS, 2006). The difference in seawater is incomparably higher, as for 0.3 mg/L Cs (Bolter et al., 1964), the concentrations of Na and K are 10560 mg/l and 380 mg/l, respectively (Sverdrup et al., 1942). Therefore, when the target is environmental Cs, the selectivity of the adsorbent material should be the foremost requirement, mainly for two important reasons. The first reason is, decontamination of Cs from the environment is almost always done for the removal of its radioactive isotopes. This means, the purpose of adsorptive recovery is for the enrichment of Cs to highest possible concentration so that the radioisotopes can be confined to the smallest possible volume. Second and equally crucial reason is, during the environmental decontamination, as all the effluent cannot be stored, it should be
1.0 Cs in effluent/Cs in feed
100
235
0.8 0.6 0.4 FeHCF-n60
0.2
Zeolite
0.0
0
500 1000 Bed Volume
1500
Fig. 2. Comparison of the Cs selectivity of FeHCF-n60 against zeolite (Ayashi- 0.5 mm). 1 g adsorbents packed in a column of 1 cm diameter. 100 mg/L Cs in ash extract, pH 7.2, neutralized with HCl to neutral range. Space velocity ¼ 2.293 min1. Bed volume is the ratio of the volume of solution passed to the volume of the adsorbent packed in the column.
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decontaminated below the free release limit. For this, the concentration of radioisotopes should be under the detection limit 5e10 Bq/L (IAEA, 1989). Therefore, in order for achieving these two requirements in the system with relatively very low concentration Cs, material with nearly absolute selectivity should be the primary choice. In this sense, MHCF can be highly effective Cs decontaminants (Tusa et al., 2007). Along with selectivity, the cost factor also becomes crucial when the area to be decontaminated is extremely large. Fukushima nuclear accident, for example, contaminated the Tohoku and Kanto area of Japan, in addition to the bordering pacific coast. In such case, the area in need of decontamination range from fresh water to saline water, forest, productive land, and so on. In this sense, selecting the most cost effective MHCF becomes necessary. So, despite their high efficiency Cs decontamination performances, CoHCF and NiHCF should be replaced by a cheaper analogue, FeHCF. Therefore, improvement of the performance of FeHCF, for example, which contains a “common metal”, by particle downsizing and the optimization of the chemical composition to the same level as CoHCF and NiHCF can be a realistic solution for the environmental decontamination.
3.2. Adsorption kinetics in bead form Although the adsorption kinetics is faster with the finer adsorbents, it would be practically difficult using FeHCF-n11 or FeHCF-n60 for mass scale decontamination. For such purpose FeHCF-nb was fabricated. Aiming its application in large scale decontamination, Cs adsorption kinetics of FeHCF-nb at 10 C, 30 C, and 50 C was studied. For this, higher concentration Cs solution was used so that enough Cs is remained in the solution phase until the equilibrium is reached. As shown in Fig. 3, between the sampling times of 5 min to 1 h, at respective temperatures, the residual concentration of Cs is decreasing with time, following the zero order rate law. The rates of adsorption were calculated as: 0.124, 0.216, and 0.231 mmol h1, at 10 C, 30 C, and 50 C, respectively. The intercept of each plots, which should be, in principle, 1, ranged between 0.93 and 0.99. Adsorption on FeHCFn60 was much faster than onto the beads. For it, a straight line was obtained with the intercept of 0.74, suggesting a distinct deviation from the zero order rate law. This difference is expected to be due to the involvement of the diffusion factor in the case of beads. It is
obvious that the role of diffusion become pronounced for larger particles. In this sense, the deviation at 50 C can be considered as the result of accelerated diffusion at higher temperature. Overall, the results observed for FeHCF-nb reveal that, in real applications, Cs uptake rate varies with the field temperature. Therefore, depending upon the temperature of the water to be decontaminated, either the amount of adsorbent or the flow rate should be adjusted. 3.3. Pilot scale decontamination of radioactive Cs Based on the result of the high selectivity adsorption of Cs on FeHCF powder, Fig. 1, and the temperature dependent adsorption profile, Fig. 3, FeHCF-nb was studied for its application in decontamination plants. Pre-test was carried out in the lab taking 0.0075 mM Cs solution prepared in wood ash extract (1:20). The solution was passed to 100 g of FeHCF-nb packed column in a rate of 0.1 L/min. As shown in Fig. 4, nearly 100% removal of Cs was achieved. Following this successful outcome, radioactive Cs decontamination test was carried out in the pilot-scale plant constructed in a hillside at the Kawauchi village, Fukushima. Different from the lab temperature of 22e24 C, the solution temperature on the day of pilot test was 10.5 C, which, based on Fig. 3, means the amount of the adsorbent used in the lab test may not be sufficient for achieving effective decontamination. Therefore, a line containing two similar columns packed with 135 g each of FeHCF-nb was established for the plant test. The extract of pH 6.5, 12,850 Bq/kg r-Cs concentration and stable Cs concentration of 0.332 mg/L was passed to the line at the rate of 0.1 L/min. Column-1 (through the first 135 g adsorbent) was sampled for every 6 L and column-2 (through total 270 g adsorbent) was sampled for every 5 L. Despite containing larger mass of the adsorbent and lower total Cs concentration than in the lab, at the same flow rate, the decontamination was lower. Samples from column-1 were analyzed for both stable and r-Cs. As given in Fig. 4, the results were closer enough. The effluent from Column-2, which was passed through total 270 g FeHCF-nb, high adsorption rate was observed. Although the result was satisfying under the given circumstances, it was not as promising as the results of several preliminary tests performed in the lab, which is most probably due to the difference in the solution temperature. Results shown in Fig. 1 explain the role of particle size. The secondary particle sizes (primary particle size being the size of FeHCF in
0.95
100
R² = 0.9988
0.90
% Adsorption
Concentration Ratio (final/initial)
1.00
R² = 0.9979 0.85 R² = 0.9638 0.80 0.75
10ºC 30ºC 50ºC FeHCF-n60 30ºC
0.70 0.65 0
10
r-Cs, Col-1
80
R² = 0.970
20 30 40 Mixing Time (min)
90
r-Cs, Col-2 Cs, Col-1 Cs, lab
70 50
60
Fig. 3. Relationship between the residual Cs concentration and mixing time showing the temperature dependent adsorption of Cs onto FeHCF-nb. Adsorption on FeHCF-n60 is faster. Initial Cs concentration ¼ 1.41 mM; mixing at 600 rpm; 1000 v/m ratio.
0
10
20
30
Cumulative Volume (L) Fig. 4. Comparison of Cs removing performance of FeHCF-nb in flow at different conditions. Lab: 100 g FeHCF-nb packed in 1.9 cm radius stainless column, 20-22 C. Plant: two columns of same size packed with 135 g each FeHCF-nb in a line, 10-12 C.
D. Parajuli et al. / Journal of Environmental Radioactivity 151 (2016) 233e237
the dispersion solution) of FeHCF-n11 and FeHCF-nb are 0.011 mm and 1 mm, respectively. Therefore, FeHCF-nb is nearly 91 times larger than FeHCF-n11 and double the size of zeolite-Ayashi. However, the v/m ratio in the case of FeHCF-nb column experiment is much smaller. Also, the results observed in the lab show nearly complete removal of Cs with only 100 g FeHCF-nb packed column. Summarizing the results observed in Figs. 1,3 and 4, besides the selectivity, the role of temperature together with the particle size should be specifically considered for achieving high efficiency decontamination. 4. Conclusion Among the FeHCF adsorbents and the zeolite, the variation in adsorption rate was mainly due to the size factor. However, in ash extract, different from the FeHCFs, zeolite showed pronounced impact of coexisting ions as its Cs adsorbing ability was sharply dropped. FeHCF-n11, with the finest primary particle size and with the secondary one, resulted the highest distribution coefficient, which was comparable to the high efficiency analogues, CoHCF or NiHCF. This observation suggested the possibility of preparing the high performance FeHCF by particle size and composition adjustment. In addition, high decontamination rate observed with FeHCFn60 in ash extract is noteworthy. FeHCF in bead form maintained its Cs selectivity despite the presence of calcium alginate. Because of slow adsorption rate, Cs adsorption kinetics on FeHCF-nb was striking temperature dependent. It became obvious with the results observed during the removal of radioactive Cs in pilot scale. Therefore, although the selectivity of FeHCF remains nearly unchanged even in highly saline solutions, adjusting the mass of the adsorbent or the flow rate with respect to the solution temperature is required for achieving high decontamination rate. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvrad.2015.10.014. References Barton, G.B., Hepworth, J.L., McClanahan, E.D., Moore, R.L., Tuyl, H.H.V., 1958. Chemical processing wastes. recovering fission products. Ind. Eng. Chem. 50, 212e216. http://dx.doi.org/10.1021/ie50578a039. Bolter, E., Turekian, K.K., Schutz, D.F., 1964. The distribution of rubidium, cesium and barium in the oceans. Geochim. Cosmochim. Acta 28, 1459e1466. http:// dx.doi.org/10.1016/0016-7037(64)90161-9. Borai, E.H., Harjula, R., Malinen, L., Paajanen, A., 2009. Efficient removal of cesium from low-level radioactive liquid waste using natural and impregnated zeolite minerals. J. Hazard. Mater 172, 416e422. http://dx.doi.org/10.1016/ j.jhazmat.2009.07.033. Cash, R.J., Meacham, J.E., Babad, H., 1997. Resolution of the Hanford Site Ferrocyanide Safety Issue, in: WM 1997. WM Symposia. Chen, G.-R., Chang, Y.-R., Liu, X., Kawamoto, T., Tanaka, H., Kitajima, A., Parajuli, D., Takasaki, M., Yoshino, K., Chen, M.-L., Lo, Y.-K., Lei, Z., Lee, D.-J., 2015. Prussian blue (PB) granules for cesium (Cs) removal from drinking water. Sep. Purif. Technol. 143, 146e151. http://dx.doi.org/10.1016/j.seppur.2015.01.040. Clearfield, A., 1982. Inorganic Ion Exchange Materials. CRC Press Inc., Florida. Haas, P.A., 1993. A review of information on ferrocyanide solids for removal of cesium from solutions. Sep. Sci. Technol. 28, 2479e2506. http://dx.doi.org/ 10.1080/01496399308017493. Hepworth, J.L., McClanahan Jr., E.D., Moore, R.L. (Eds.), 1957. Hepworth 5 7 Hanford. Cesium Packaging Studies Conversion of Cesium Zinc Ferrocyanide to a Cesium Chloride Product.
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