Selective removal of cesium ions from wastewater using copper hexacyanoferrate nanofilms in an electrochemical system

Electrochimica Acta 87 (2013) 119–125

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Selective removal of cesium ions from wastewater using copper hexacyanoferrate nanofilms in an electrochemical system Rongzhi Chen a , Hisashi Tanaka a,∗ , Tohru Kawamoto a , Miyuki Asai a , Chikako Fukushima a , Haitao Na a , Masato Kurihara a,b , Masayuki Watanabe c , Makoto Arisaka c , Takuya Nankawa c a

Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8565, Japan Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, Yamagata 990-8560, Japan c Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, Tokai-mura 319-1195, Japan b

a r t i c l e

i n f o

Article history: Received 29 June 2012 Received in revised form 16 August 2012 Accepted 31 August 2012 Available online 7 September 2012 Keywords: Electrochemical adsorption Radioactive cesium Copper hexacyanoferrate Nanoparticles film Selective removal

a b s t r a c t A novel electrochemical adsorption system using a nanoparticle film of copper (II) hexacyanoferrate (III) was proposed for selectively removing cesium from wastewater. This system can be used for cesium separation without extra chemical reagents or any filtration treatment. Cesium uptake and elution can be simply controlled by switching the applied potentials between anodes and cathodes. Data from batch kinetic studies well fitted the intraparticle diffusion equation, reflecting a two-step process: a steepest ascent portion followed by a plateau extending to the equilibrium. The effective cesium removal with a high distribution coefficient (Kd > 5 × 105 mL/g) can be adopted in a large pH range from 0.3 to 9.2, and in the presence of several diverse coexisting alkaline cations, suggesting it can be taken as a promising technology for actual nuclear wastewater treatment. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction The earthquake and subsequent tsunami on March 11, 2011 caused huge damage to the nuclear plant, in Fukushima prefecture, Japan. This accident had widely spread radiation health effects including those caused by Cs137 that has a long half-life of 30.5 years. Since cesium is chemically similar to sodium and potassium, it is mobile in various environments and can be easily assimilated by many terrestrial and aquatic organisms [1]. Ingestion and accumulation of its radioisotopes results in its deposition in the soft tissues all over the body creating an internal hazard, especially to the reproductive system [2]. Several techniques such as chemical precipitation, ion exchange, and evaporation can be used for cesium removal. Among them, ion exchange appears to be the most effective technology to control radioactive wastewater [3]. Transition metal hexacyanoferrates (such as FeHCF, NiHCF or CuHCF) were preferred to be competitive Cs ion exchangers over other materials due to their selectivity and high capacity [4]. Since the FeHCF was only chemically stable in pH below 6 [5]; and the NiHCF was much more expensive, and toxic to human health [6], copper (II) hexacyanoferrate (III) (CuII HCFIII ) was often selected as

∗ Corresponding author. Tel.: +81 29 861 5141; fax: +81 29 861 5400. E-mail addresses: [email protected] (H. Tanaka), [email protected] (T. Kawamoto). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.08.124

the precipitant in practical analysis for cesium removal because it can be readily prepared, and chemically stable in a large pH range [7,8]. However, CuII HCFIII was available as fine powders or even colloidal precipitates, which was difficult to separate from liquid after adsorption completed. Though the CuII HCFIII was proved to be harmless to animal or human health [6], the filtration operation was still necessary to remove the produced sludge. Moreover, the life expectancy of powdery CuII HCFIII was short, when it was used in conventional packed columns. Since it was very difficult to elute, columns were normally used once and discarded. Instated of the conventional adsorption by using the powder, we synthesized and coated the CuII HCFIII nanoparticles on electrode substances, and then performed Cs removal in an electrochemical adsorption system (EAS). While as electroactive films were coated on an electrode substrate [9], uptake and elution can be highly reversible by modulating the potential of the films, and this predicted long life expectancy. In Fukushima case, the nuclear meltdowns had released several alkaline metals to the cooling water. In order to make the cooling water neutral, large amounts of acid was added to the reactor. Moreover, the nearby seawater was taken as cooling water, which contained plenty of cations, such as Li+ , Na+ , and K+ . To remove Cs under acid conditions and in the presence of these cations is an intractable problem need to be solved urgently [10]. The objective of this study was to check the feasibility of a nanoparticle film of

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CuII HCFIII for electrochemical removal of Cs, and to determine the effects of film thickness, solution pH, and coexisting cations.

2. Experimental 2.1. Reagents and apparatus All reagents were prepared from analytical grade chemicals. Nitric acid and sodium hydroxide solutions at various concentrations were used to adjust the solution pH. MilliQ water was used throughout all the experiments. The reagent CsNO3 was taken as a surrogate to dissolve in water, in order to mimic Cs radioactive wastewater. The chemical composition of the prepared CuII HCFIII was examined by energy dispersive X-ray spectroscopy (EDX, EMAX300, Horiba, Japan) that combined with Scanning Electron Microscope (SEM). Surface morphologies of CuII HCFIII film were observed by Field-Emission Scanning Electron Microscope (FE-SEM, S-4800 Hitachi, Japan). X-ray powder diffractometry (XRD, Ultima X, Rigaku, Japan) was carried out using a TF-XRD with Cu K␣ beam in order to determine the crystalline structures. Simultaneous cyclic voltammetry (CV) and microgravimetry using an electrochemical quartz crystal microbalance (EQCM, PAR model 263A, USA) was used for measuring the mass change on the CuII HCFIII film and recording the redox responses between the films and solutions. The quartz crystal (QC) was a 9-MHz AT-cut quartz crystal with gold electrodes (diameter: 5 mm).

2.4. Electrochemical adsorption system (EAS) Electrochemical adsorption system (EAS) introduced an advanced electrically switched ad/desorption technology for removing metal ions from wastewater. In this system, sorbent electrodes were incorporated into the electrodialysis stack, forming a film/electrode integrated structure. Comparing with the conventional sorption by using the CuII HCFIII powders or bulks in Cs solutions, it has several advantages: (1) No extra chemical addition and no sludge generation; (2) Reversible process controlled by switching the potential between anodes and cathodes, resulting in an easy regeneration; (3) Simple and rapid sorbent separation from solution. Electrochemical adsorption was performed in a three-electrode cell, in which an Hg/Hg2 Cl2 /KCl (saturated solution) was used as reference electrode, a platinum electrode (as counter electrode), and the CuII HCFIII film (2.5 cm × 2.0 cm) was used as working electrode. The electrochemical analyses were conducted using a potentiostat (ALS-711B, BAS Inc., Japan). The CuII HCFIII film was first pretreated in 1 ppm NaNO3 solution by the applied potentials +1.3 V (vs. Hg/Hg2 Cl2 ) for 30 min, in order to discharge the residual Na+ during the CuII HCFIII synthesis and surface treatment. The relevant reaction was shown as follows: Na4 [FeII (CN)6 ](sorbentelectrode) → 4Na+ + [FeIII (CN)6 ]3− + e− (2)

2.2. Preparation of water-dispersible CuII HCFIII ink Solutions of Cu(NO3 )2 ·2H2 O and K3 Fe(CN)6 were mixed into a conical tube for the CuII HCFIII precipitate formation. The chemical equation was shown as follows: 4Cu(NO3 )2 ·2H2 O + 3K3 [Fe(CN)6 ] → KCu4 [Fe(CN)6 ]3 + 8KNO3 + 8H2 O

(1)

KCu4 [Fe(CN)6 ]3 represents the CuII HCFIII film, a mixture of two compounds, Cu3 [Fe(CN)6 ]2 and KCu[Fe(CN)6 ]. The mixture solution was first agitated at a constant speed of 2000 rpm for 3 min using vibrator, followed by removing the supernatant using the centrifugal separation (4000 rpm for 15 min). The residual precipitation was washed with MilliQ water for 4 cycles to discharge the by-product of KNO3 . Then the precipitation of CuII HCFIII cores were mixed with Na4 [Fe(CN)6 ]·10H2 O to enhance the hydrophilicity for well dispersing in water [11]. The surface treatment was performed continuously for 2 weeks at an agitation speed of 2000 rpm. Finally 5% water-dispersible CuII HCFIII ink was obtained for film coating on electrodes.

After eliminating the residual Na+ , the CuII HCFIII film was transferred to a CsNO3 solution for Cs removal, and then to a NaNO3 solution for regeneration. Multi-potential step (MPS) technique was conducted by stepping to 0.0 V to load the film, and to +1.3 V to unload the film. This process can be simply controlled by switching the potentials of CuII HCFIII film between 0.0 and +1.3 V. The oxidation and reduction reaction, referring to Cs adsorption and desorption, was proposed as follows: Cu3 [FeIII (CN)6 ]2 (sorbentelectrode) + 2e− + 2Cs+ ↔ Cs2 Cu3 [FeII (CN)6 ]2

The adsorption and subsequent regeneration process was performed for 30 min, respectively and repeated 5 cycles. After each process, a sample was taken to measure Cs concentration by Inductively Coupled Plasma Mass Spectrometer (ICP-MS, NexIon300 D, PerkinElmer, USA). The measurement was run in Standard Mode with a detection limit of 0.01 ppb for Cs. The Cs removal results were given as removal efficiency (R%), uptake capacity (Qe ) and distribution coefficient (Kd ). The Kd inferring to selectivity for Cs, was measured as a function of contact time.

2.3. Preparation of CuII HCFIII coated electrodes The gold electrodes fabricated by sputtering 5 nm Ti and 200 nm Au on slide glass – Cut edges/Plain (As One, Corp., Japan) using vacuum vapor deposition (Eco Engineering, Ltd., Japan) were used for CuII HCFIII film coating. Firstly, acetone wash and plasma cleaning was performed to remove impurities and contaminants on the gold electrodes. Then, the CuII HCFIII film was coated on them using an ACT-300D II spin coater instrument (2000 rpm for 10 s following 2500 rpm for another 10 s). Afterwards, the CuII HCFIII coated electrodes were dried in an oven at 100 ◦ C for 2 h, and used for further characterization as well as Cs sorption studies.

(3)

R% =

100 × (C0 − Ct ) C0

(4)

Qe (mg/g) =

(C0 − Ce ) × V M

(5)

Kd (mL/g) =

(C0 − Ct )/C0 × V M

(6)

where C0 , Ct and Ce stand for the Cs concentration at initial stages, time t, and equilibrium stages, respectively. V is the solution volume and M is the mass of sorbents.

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Fig. 1. FE-SEM images of the CuII HCFIII films: (a) unused electrode ×110 K; (b) unused electrode ×300 K; (c) cross-section 60 K×; (d) film thickness from a Stylus-type surface profilometer.

Table 1 Chemical compositions on the surface of Cs load CuII HCFIII films. Element

Weight (%)

Atoms (%)

CK NK Si K Au M Cu K Fe K Cs L

25.63 9.56 1.00 48.40 6.77 3.86 4.78

64.47 20.63 1.08 7.42 3.22 2.09 1.09

3. Results and discussion

and after Cs adsorption. As shown in Fig. 2, the XRD patterns of CuII HCFIII film indicated two main crystal structures, the peaks at 2 of 38.29◦ , 44.51◦ and 64.92◦ represented Au signal, while peaks at 17.64◦ , 25.04◦ and 35.71◦ can be indexed as a cubic CuII HCFIII crystal. The XRD patterns also appeared a structure of copper hexacyanoferrate analogue, which might be from the surfactant of Na4 [Fe(CN)6 ]·10H2 O or the by-product of KCu[Fe(CN)6 ]. Obviously, the intensity of Au and CuII HCFIII signals became weak as Cs loaded. The mass of CuII HCFIII film was measured by EQCM equipments. According to the Sauerbrey equation, the mass change (mass) on the QC surface is proportional to the induced change of the resonance frequency freq [13]: −freq × A ×



q × q

3.1. Characterization of CuII HCFIII film

mass =

FE-SEM images of the CuII HCFIII film were shown in Fig. 1a, it was found that heterogeneous nanoparticles were well dispersed throughout the binding matrix, which predicted that the kinetic of sorption on the CuII HCFIII film might be fast. The enlarged image (Fig. 1b) indicated that the size of nanoparticle ranged from 20 to 80 nm. The cross-section image (Fig. 1c) indicated that the film thickness was about 220 nm, which was closed to the data estimated from a stylus-type surface profilometer (Fig. 1d, Alpha-step IQ; KLA Tencor Corp., USA). SEM-EDX technique was used for examining the elements distribution and chemical compositions of the CuII HCFIII film. As shown in Table 1, the EDX results indicated that the major element was Au, which was from the gold sputtering layer. The elements of Fe, Cu, C and N, can be named as CuII HCFIII . The atom ratio of Cu/Fe was 3.022:2.09, closed to the prepared Cu3 [Fe(CN)6 ]2 core (3:2). The Cs signals were also detected, referring to the Cs loading on the film surface. The maximum loading of CuII HCFIII film was 0.52 Cs/Fe which was much higher than the results from powdery CuII HCFIII (0.073) [12]. XRD technique was utilized to investigate the changes of CuII HCFIII structure by comparing the obtained XRD patterns before

Fig. 2. XRD pattern of CuII HCFIII films before and after Cs sorption in 1 ppm CsNO3 solution.

2 × Fq × F

(7)

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R. Chen et al. / Electrochimica Acta 87 (2013) 119–125 Table 2 Comparison results of Cs uptake and elution in conventional and electrochemical systems.a Sample description

Initial concentration (ppb) Cs residual concentration (ppb) 1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle

Electrochemical system

Conventional system

Uptake

Elution

Uptake

Elution

1007.5

0.0

1007.5

0.0

743.7 522.3 396.1 282.5 182.0

268.6 398.1 514.1 621.3 710.2

726.6 708.5 733.0 720.0 722.7

5.5 4.6 5.1 6.3 6.3

a Electrode area: 20 mm × 25 mm; solution pH: 0.3; room temperature: 22 ◦ C; agitation speed: 450 rpm.

Fig. 3. Cyclic voltammograms of CuII HCFIII films in various 0.1 M aqueous solutions of alkali metal nitrate (Li+ , Na+ , Sr2+ , K+ , Rb+ , Cs+ ). Scan rate = 5 mV/s.

q is the consist of AT-cut QC: 2.947 × 1011 g/cm/s2 , q is the density of AT-cut QC: 2.648 g/cm3 , Fq is the standard frequency 9.0 MHz, A is the working area of AT-cut QC: 0.196 cm2 . Through the calculation, Eq. (7) can be simplified as follows: mass = −freq × 1.068

current peaks. The obtained Ef values for CuII HCFIII film in various electrolytes were very close to the literature data [18,19], in which the Ef increased with increasing ionic radii of the cation: Li+ < Na+ < Sr2+ < K+ < Rb+ < Cs+ . This sequence may be explained by the interaction of CuII HCFIII film with hydrated alkaline cations which becomes stronger with the decrease of the ion radius in this series [19].

(8) 3.3. Batch-scale study

This means when 1 Hz resonance frequency decrease, 1.068 ng mass increases. In our case, the mass of CuII HCFIII film on the QC surface (0.196 cm2 ) was measured as 3.92 ␮g. As the area of the prepared sorbent electrode was 2.5 cm × 2.0 cm, the mass of coated CuII HCFIII film was approximately 100 ␮g.

M represents the investigated alkali metal cation. From such voltammograms, the formal potentials Ef were calculated as a half of the sum of potentials of anodic and cathodic

3.3.1. Electrochemical adsorption and desorption Table 2 compares the results of Cs adsorption and desorption in electrochemical and conventional systems. After 5 cycles adsorption, about 825.5 ppb Cs was adsorbed on the CuII HCFIII film due to the electrochemical reduction from Cu(II) CN Fe(III) to Cu(II) CN Fe(II). While the Cs concentration in final eluent was 710.2 ppb which indicated that only 13.97% Cs was remaining on the electrodes. This revealed that the used CuII HCFIII film can be successfully regenerated by the reversible electrochemical oxidation from Cu(II) CN Fe(II) to Cu(II) CN Fe(III). We also noticed that the Cs uptake gradually decreased from cycle to cycle, which might be due to the decreased electric charges from cycle to cycle, as shown in Fig. 4. The conventional experiments using the CuII HCFIII films were conducted by simply transferring the films in Cs solutions under the same conditions. As shown in Table 2, the magnitude of Cs adsorbed in conventional system was equal to that in EAS after the 1st cycle. However, almost no Cs can be removed from the 2nd cycle. This was because once the Cs loaded electrodes reached its saturation adsorption; it could not be regenerated in conventional system, and

Fig. 4. The charge generated during each cycle of Cs loading and unloading in an electrochemical adsorption system. (The change was converted from MPS data.)

Fig. 5. Variation of distribution coefficient of Cs with contact time by electrochemical and conventional system (electrode area 20 mm × 25 mm, pH 0.3, Cs concentration 1 ppm, room temperature, 2 h).

3.2. Electrochemical characterization of CuII HCFIII film The electrodes covered with CuII HCFIII films were transferred to 0.1 M solutions of LiNO3 , NaNO3 , KNO3 , RbNO3 , Sr(NO3 )2 or CsNO3 . The electrochemical response was dependent on the choice of electrolyte (Fig. 3). A single redox couple characterized all the CV curves, one cathodic and one anodic peak current were observed between 0.2 and 0.9 V (vs. Hg/Hg2 Cl2 ). This behavior is likely due to the electrode process which may formally be represented by the reaction [14–17]: Cu3 [FeIII (CN)6 ]2 + 2e− + 2M+ → M 2 Cu3 [FeII (CN)6]2

(9)

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Fig. 6. Intraparticle diffusion modeling (a) and effect of thickness (b) of Cs removal using CuII HCFIII films (electrode area 20 mm × 25 mm, pH 0.3, Cs concentration 1 ppm, room temperature, 2 h).

thus no Cs can be adsorbed. This indicated that the charges in EAS also took a significant role on Cs separation from CuII HCFIII film. 3.3.2. Kinetic consideration The influence of contact time between Cs and CuII HCFIII film on distribution coefficient for Cs was investigated. As shown in Fig. 5, the Cs adsorption was increased with time increase, until reaching equilibrium between two phases after 120 min. The Kd for Cs was found to be in excess of 5.55 × 105 mL/g, which was much higher than that of the conventional adsorption (3.81 × 105 mL/g). On the other hand, the data showed a continuous accumulation of Cs ions after 60 min comparing with the conventional adsorption, indicating that the Cs removal was dominated by the redox reactions. The magnitude of Kd using CuII HCFIII film in EAS was significantly higher than that of some other MHCF sorbents. Sangvanich et al. reported a copper ferrocyanide functionalized mesoporous silica and insoluble FeHCF for Cs removal with the Kd of 156,000 and 5400 mL/g, respectively [20]. Shakir et al. used the coprecipitate flotation (CPF) technique to remove Cs from low-level liquid radioactive waste; the Kd on ZnHCF for Cs was 10,061 mg/L [21]. The sorption of metal ions from the aqueous onto the solid phase is a multi-step process involving transport of metal ions from aqueous to the solid surface (bulk diffusion) and then, diffusion of metal ions via the boundary layer to the solid surface (film diffusion) followed by transport of metal ions from the solid particles surface to its interior pores (pore diffusion or intraparticle diffusion) [22], the later is likely to be a slow process and, therefore, it may be the rate-determining step. Electrochemical sorption of metal ions on the electrode surface could also occur through chemical processes such as ion-exchange, redox reaction and charge compensation. If the experiment is a batch system with rapid stirring, there is a possibility that intraparticle diffusion is the rate determining step [23]. The possibility of intraparticle diffusion resistance affecting the sorption was explored using the intraparticle diffusion model as follows [24]: qt = ki · t 0.5

(10)

where qt is the Cs adsorption amount at time t, ki is the intraparticle diffusion rate constant (mg/g/h−0.5 ). The plots are shown in Fig. 6a, it may be seen that there are two separate regions: the first region was attributed to the film diffusion and the second region to intraparticle diffusion [25]. This is in agreement with the plot of electrochemical adsorption in Fig. 5, which reflected a two-step process: a steepest ascent portion followed by a plateau extending to the equilibrium. The shape of Fig. 6a confirms that straight lines do not pass through the origin. The deviation of straight lines from the origin may be because of the difference between the rate of mass transfer in the initial and final steps of the sorption process.

Fig. 7. Effect of solution pH on Cs removal using CuII HCFIII films (electrode area 20 mm × 25 mm, film thickness 220 nm, Cs concentration 1 ppm, pH 0.3–11.5, room temperature, 2 h).

Further, such deviation of straight line from the origin indicates that the pore diffusion is not the sole rate-controlling step [26]. 3.3.3. Effect of film thickness The thickness of CuII HCFIII film, referring to adsorbent dose, is an important parameter in the determination of Cs uptake capacity. The effect of CuII HCFIII film thickness (Fig. 6b) was investigated by using the films prepared at various spin speeds. It was observed that the removal efficiency increased from 39.1% to 65.0% with an increase in film thickness from 145 to 323 nm. This can be attributed to the increase in the film surface area and availability of more redox sites. However, the further increase in film thickness did not affect the removal efficiency significantly. It is also observed that the removal capacity decreased from 285.8 to 182.7 mg/g as the film thickness increases from 145 to 323 nm. This might be due to the fact that some sorption sites remained unsaturated in thicker films resulting in decreased removal capacity. Consequently, CuII HCFIII film with a thickness of 223 nm was utilized in all the experiments, which was considered to be cost-effective for Cs removal. 3.3.4. Effect of solution pH The effect of pH on Cs removal was investigated by varying the initial pH from 0.3 to 11.5; results were shown in Fig. 7. The Cs removal efficiency (R%) showed no significant change in the pH range from 0.3 to 9.2, and then decreased dramatically with increasing pH to 11.5. The result was different from the powder or bulk sorbents researches [27,28], in which, at low pH competition with H+ for exchange sites or dissolution of a portion of

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Table 3 Effect of coexisting cations on the distribution coefficient for Cs on the CuII HCFIII films. Coexisting cations

Kd ( × 105 mL/g)

None Li+ Na+ K+ Rb+ Sr2+

5.88 5.89 5.55 5.18 5.09 5.52

Electrode area: 20 mm × 25 mm; solution pH: 0.3; coexisting cations concentration: 1 ppm; room temperature: 22 ◦ C; agitation speed: 450 rpm.

sorbent will result in a decrease of ions sorption. This was because the EAS produced more changes (especially under acidic conditions) and caused the reduction reaction from Cu(II) CN Fe(III) to Cu(II) CN Fe(II), accompany with the Cs loading on CuII HCFIII film. At pH greater than 9.2, the Cs hydroxides began to form and some of them was negatively charged Cs(OH)2 − which resulted in the decrease of Cs adsorption due to electrostatic repulsion [27,29]. A similar decreased Cs adsorption on hexacyanoferrates film in basic waste solutions has also been reported by Lilga et al. [30], in which hexacyanoferrates are soluble in caustic. 3.3.5. Effect of coexisting cations In order to investigate the effect of coexisting cations, the experiments were performed in the presence of several cations in the same group of the periodic table, such as, Li+ , Na+ , K+ , Rb+ , and the most common coexisting radionuclides Sr2+ . The distribution coefficient for Cs was determined in Table 3, an equal magnitude of Kd for Cs (>5 × 105 mL/g) was determined in each single test.

Comparing with the coexisting cations free experiment, such insignificant influence on Kd for Cs confirmed that the CuII HCFIII film can effectively adsorb Cs in the presence of the investigated cations. Fig. 8a compares the redox responses for CuII HCFIII films employed in a 0.1 M NaNO3 solution, a 0.1 M CsNO3 solution, and a mixture solution of 0.1 M CsNO3 and 0.1 M NaNO3 . In pure NaNO3 or CsNO3 solution, one cathodic and one anodic peak were observed on the CV response, relating to the electrode process of Cu3 [FeIII (CN)6 ]2 /Na2 Cu3 [FeII (CN)6 ]2 or Cu3 [FeIII (CN)6 ]2 /Cs2 Cu3 [FeII (CN)6 ]2 system [31]. However, a single redox couple was also observed in the mixture solution, its CV sharp was close to that in the solution containing CsNO3 only. Because Na+ has no electroactivity in the potential range from 0.6 to 1.2 V, the redox peaks corresponded to the Cs redox couple. This indicated the CuII HCFIII films appeared to have a prior selectivity for cesium. As shown in Fig. 3, the different redox peaks can also be found by other cations during the redox reactions. We assumed the removal selectivity of Cs can be improved, if the potential between anodes and cathodes was properly operated. An increase of mass in EQCM recorded (Fig. 8b) concurrently with the CV measurement indicated cations were adsorbed on the CuII HCFIII films. The ratio of mass was estimated at 5.67:1 for Cs/Na. It is close to their corresponding atomic mass ratio, 5.78:1 (Cs/Na), which suggested that CuII HCFIII film can absorb equal moles of the Cs and Na in each single test. However, when the CuII HCFIII film was transferred into the mixture solution, the mass change is equal to that in CsNO3 solution. This finding further confirmed that the CuII HCFIII films can selectively adsorb cesium. 4. Conclusion This study showed that the synthesized CuII HCFIII film with a crystalline structure and nanoparticle surface can selectively remove Cs in an EAS. Batch-scale experiments indicated that distribution coefficients for Cs were sufficient to allow the EAS to exceed the performance of conventional adsorption. Significant advantages also included the use of a regenerable film and the minimization of secondary waste. The effective Cs uptake under acidic conditions and in the presence of several alkaline cations, suggests a promising technology for actual nuclear wastewater treatment. In the future, we will coat the CuII HCFIII films on large surface electrodes (SUS sheets), and then use the rolled sheet electrodes in the columns for sequential removal of Cs. Acknowledgements A part of this study is the result of “Compact and reusable cesium recovery system by electrochemical adsorption/desorption” carried out under the Strategic Promotion Program for Basic Nuclear Research by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). References

Fig. 8. (a) Microgravimetric cyclic voltammograms (CV) of CuII HCFIII films employed in a 0.1 M NaNO3 solution, a 0.1 M CsNO3 solution, and a mixture solution of 0.1 M CsNO3 and 0.1 M NaNO3 . (b) The mass change in EQCM recorded concurrently with the CV of (a), scans rate = 5 mV/s.

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