Uptake and Electrochemical Ejection of Cesium Ion by a Prussian Blue-modified ElectrodeTetsu Tatsuma,* Yoshinori Kuroiwa, Kazuyuki Ishii, Kazuaki Kudo, and Akiyoshi Sakoda
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505(E-mail: tatsuma@iis.u-tokyo.ac.jp)

Electrochemically deposited Prussian blue
was loaded with cesium ion in a diluted
aqueous CsCl solution followed by reduction or oxidation. Although reductionof Prussian blue does not increase thecapacity of cesium uptake, oxidation ofPrussian blue results in release of entrapped cesium ion.

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Vol.43 No.8

2014 p.1281–1283
CMLTAG
August 5, 2014

The Chemical Society of Japan

Received: April 7, 2014 | Accepted: April 29, 2014 | Web Released: May 10, 2014

CL-140338

Uptake and Electrochemical Ejection of Cesium Ion by a Prussian Blue-modified ElectrodeTetsu Tatsuma,* Yoshinori Kuroiwa, Kazuyuki Ishii, Kazuaki Kudo, and Akiyoshi Sakoda
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505(E-mail: tatsuma@iis.u-tokyo.ac.jp)
Prussian blue (FeIII4[FeII(CN)6]3) electrochemically deposited on an indium tin oxide electrode was loaded with cesium
ion in a diluted aqueous CsCl solution (ca. 40 ppb) followedby reduction or oxidation. Although reduction of Prussian blue
does not increase the capacity of cesium uptake, oxidation ofPrussian blue releases the entrapped cesium ion.
Prussian blue (FeIII4[FeII(CN)6]3, PB)1 has been practicallyused as a sorbent for radioactive Cs+ (134Cs+ and 137Cs+)
released to the environment in the atomic weapon tests in the1960s and the Chernobyl nuclear plant accident in 1986.2­4 It is,therefore, expected to be eﬀective for the decontamination of
the fallout due to the Fukushima nuclear accident in 2011.5­7 PB
has also been known and used as an electrochromic material,
because its color depends on the redox state.8­11 Its blue color isbleached to colorless by reduction to Prussian white (PW, orEveritt’s salt) (eq 1) and is changed to pale yellow or greenby oxidation to Prussian yellow (PY, or Berlin green) in thepresence of K+ (eq 2).11
FeIII 4 ½FeII ðCNÞ6 3 þ 4Kþ þ 4e  K4 FeII 4 ½FeII ðCNÞ6 3IIIFeII

4 ½Fe

ðCNÞ6 3 þ 3A  FeIII

4 ½FeIIIðCNÞ6 A3 þ 3eð1Þ
ð2Þ

where A¹ is an anion in the solution. Reduction10,12 andoxidation12 of PB are also possible in the presence of Cs+. On
the basis of an analogy from those equations, one may expectthat reduction of PB will improve the Cs+-sorption capacity andthat oxidation of PB will result in the uptake of A¹. However,neutral PB containing water molecules is known to absorb Cs+
before reduction13,14 because of an exchange with H+ (eq 3).15
FeIII 4 ½FeII ðCNÞ6 ¢xH2 O þ yCsþ FeIII 4 ½FeII ðCNÞ6 3 ¢yCsþ ¢yOH ¢ðx  yÞH2 O þ yHþ ð3Þ
Therefore, the above-mentioned expectation might nothold for the Cs+-containing PB. Nevertheless, precise ionexchange behavior of the Cs+-containing PB upon its reduction
and oxidation has not yet been studied to the best of ourknowledge, particularly at low Cs+ concentrations. In the caseof nickel(II)16,17 and copper(II)18,19 hexacyanoferrate(II), it isreported that absorbed Cs+ is released upon oxidation tohexacyanoferrate(III). Similar release of Cs+ may be expected
for the oxidation of PB to PY, as well as for the release of H+
and Fe2+/3+ and uptake of OH¹ and other coexisting anions. In
the present work, we prepared a PB ﬁlm on a transparent indium
tin oxide (ITO) electrode and loaded it with Cs+ at ca. 40 ppm.
Then, we polarized the electrode to reduce or oxidize the ﬁlm
and examined the retention of the preabsorbed Cs+. As a result,
we found that Cs+ is retained in the ﬁlm when it is reduced andthat Cs+ is released when the ﬁlm is oxidized.
PB was electrodeposited onto a clean ITO-coated glass plate(Kinoene Kogaku) (electrode area: 2 cm2) from an aqueoussolution containing 0.02 M FeCl3, 0.02 M K3[Fe(CN)6], andChem. Lett. 2014, 43, 1281–1283 | doi:10.1246/cl.140338

Figure 1. (a) Cyclic voltammograms of the PB ﬁlm in 0.01 M
HCl solution containing KCl and CsCl ([KCl] + [CsCl] =1.0 M) at 50 mV s¹1 and (b) that in 0.01 M CsCl at 10 mV s¹1with (c) absorbance changes at 700 nm.
0.01 M HCl at a constant cathodic current of 20 ¯A cm¹2 for
240 s.9,10 The as-deposited blue ﬁlm exhibits a broad absorption
peak at ca. 700 nm, which is ascribed to charge transfer between[FeII(CN)6]4¹ and FeIII.20 The average thickness of the ﬁlm
observed by scanning electron microscopy is ca. 100 nm, whichcorresponds to ca. 30 nmol of PB deposited on the electrode. Thedeposited PB was dissolved in 0.1 M aqueous ammonia, andthe amount of Fe in the solution was determined by inductively
coupled plasma mass spectrometry (ICP-MS, by using SPQ9000, SII) to be 210 nmol, which corresponds to 30 nmol of PB.
The Coulomb eﬃciency of the electrodeposition was ca. 90%.
The PB-coated electrode was characterized by cyclic
voltammetry at 50 mV s¹1 in a 0.01 M aqueous HCl solution
containing 1 M KCl by using a Pt counter electrode and aAg«AgCl reference electrode (Figure 1a, black curve). The
initial open circuit potential was ca. +0.5 V vs. standardhydrogen electrode (SHE). It is obvious that the PB ﬁlm is
reduced at ca. +0.4 V and oxidized at ca. +1.2 V. These redox
reactions were repeatable. After the electrode was poised at+0.1 V for 2 min, the blue color was almost bleached; PB was© 2014 The Chemical Society of Japan | 1281

reduced to PW. On the other hand, the color was changed to palegreen after polarization at +1.6 V for 2 min; PB was oxidized toPY. Thus, the electrochemical behavior of PB was essentially thesame as that reported previously.9­12
The electrochemistry of PB was also examined in a 0.01 M
aqueous HCl solution containing KCl and/or CsCl (Figure 1a).
The total concentration of K+ and Cs+ was 1.0 M. In ourexperiments, we used nonradiative Cs+ because its size isessentially the same as those of radioactive 134Cs+ and 137Cs+.
The redox peaks were suppressed as the Cs+ content
increased, in particular for the PB/PY redox reactions, asreported previously.12 However, the absorbance at 700 nm
decreased obviously in a cyclic voltammetry in 0.01 M CsCl,
at more negative potentials than +0.7 V and more positive
potentials than +0.9 V (Figures 1b and 1c). These redox
reactions were repeatable with reversible absorbance changes.
We, therefore, conclude that the PB ﬁlm can be reduced andoxidized, at least in part, even in the presence of Cs+. In
addition, the anodic current at more positive potentials than
+1.3 V was enhanced in the presence of Cs+ at the PB-coatedelectrode, whereas no obvious current was observed at a bare
ITO electrode. The charges passed in this potential region in one
cycle (6­9 mC cm¹2) are larger than that required for the PB ﬁlm
deposition (4.8 mC cm¹2). Therefore, the current might reﬂectthat the PB ﬁlm has higher electrocatalytic activity for theoxidation of water and/or Cl¹, particularly in the presence ofCs+.
The distribution coeﬃcient Kd for the Cs+ sorption into thePB ﬁlm was also examined. An as-prepared PB ﬁlm was soaked
in a CsCl solution (1­100 ppb, 20 mL, pH 5.5, ca. 20 °C) forat least 15 h, and the amount of Cs+ left in the solution wasdetermined by ICP-MS. The distribution coeﬃcient Kd wasevaluated from the relationship between the amount of Cs+ inthe ﬁlm and the Cs+ concentration in the solution at equilibrium
to be 8.7 © 105 mL g¹1. This value is in the range of the reported
values (3 © 105­3 © 106 mL g¹1).21­23
Next, the retention of Cs+ was examined after the reduction
and oxidation of PB. An as-prepared electrode with the PB
coating was soaked in a 100-ppb CsCl solution (20 mL;
15.0 nmol of Cs+ is contained) for at least 12 h. A 2-mL aliquotof the solution was sampled, and the amount of Cs+ absorbed
by PB was determined by ICP-MS to be 9.05 « 0.8 nmol
(average « standard deviation, n = 12). Then, the electrode waspolarized at a certain potential (varying from +0.4 to +1.3 V)
for 1 h to reduce or oxidize the PB ﬁlm. The Cs+ concentrationof the solution was measured again, so that %retention of Cs+
in the ﬁlm was determined and plotted as a function of theelectrode potential in Figure 2.
When the PB ﬁlm is completely reduced to PW, ca.
120 nmol of electrons must be injected into the ﬁlm. At +0.4 V,
20 « 1 nmol (n = 3) of electrons was consumed for thereduction, suggesting that ¯17% of the PB ﬁlm was reduced.
Injection of ca. 20 nmol electrons into the ﬁlm must be
accompanied by the injection of the same amount of cations
or ejection of anions to maintain charge neutrality. In a solutionof a potassium salt, K+ is injected into the ﬁlm (eq 1).11 In thepresent case, however, no further uptake of Cs+ was observed.
This might suggest that the negative charges generated in thereduced ﬁlm is compensated by H+ uptake or OH¹ release,which gives rise to only a slight pH shift of <0.2 in the solution.
1282 | Chem. Lett. 2014, 43, 1281–1283 | doi:10.1246/cl.140338

Figure 2. Amount of Cs+ retained in the PB ﬁlm after
potential application of 1 h.
On the other hand, ca. 90 nmol of electrons must beremoved from the ﬁlm for total oxidation of the PB ﬁlm to PY.
It should be accompanied by ejection of the same amount ofcations or injection of anions. In our experiment at +1.3 V,
96 « 4 nmol (n = 3) of electrons was taken out of the ﬁlm, andthe ﬁlm ejected most of the preloaded Cs+ (ca. 9 nmol). Because
PY may catalyze the oxidation of water or Cl¹ as mentioned
above, >93 nmol of electrons might be consumed for thecatalytic oxidation. The oxidation resulted in the loss of ca. 20%
PB, which might be responsible for the Cs+ release in part,
possibly due to mechanical stress generated by the ejection ofbulky Cs+.
In conclusion, reduction of Cs+-containing PB does notcause further uptake of Cs+, whereas oxidation results in therelease of Cs+. When PB is used as a sorbent for radioactiveCs+, extensive oxidation of PB should be avoided not todeteriorate the sorption ability and to prevent the ﬁlm fromunintended release of Cs+.
This work was supported in part by The Environment
Research and Technology Development Fund (No. 5ZB-1203)
from the Ministry of Environment, Japan.
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