173

Insolubilization of Cesium Contained in Fly Ash by Co-heatingwith Potassium Feldspar
Yasumasa TOJO
Graduate School of Engineering, Hokkaido University
N13 W8, Kita-ku, Sapporo, Hokkaido, 060-8628, Japan*E-mail: tojo@eng.hokudai.ac.jp

Abstract
At present, reducing the volume of decontamination wastes/soils is an indispensable task in Japanbecause to secure final disposal sites with vast capacity is considered difficult. One promising option isthermal treatment to separate radioactive cesium (Cs) from wastes/soils. This, however, generates Csconcentrates in which Cs is thought to have high solubility. From a long-term viewpoint on ensuring safety,
the solubility of Cs itself in the concentrates should be reduced even though it will be stored in containers. In
this study, insolubilization of Cs by co-heating with potassium feldspar (hereafter, K-feldspar) wasinvestigated. This process can insolubilize almost 100% of Cs when only Cs carbonate and K-feldspar are coheated. However, concentrates in actuality consist mainly of fly ash and contain various elements. Thus theobjective of this study was to elucidate the influence of the presence of fly ash. In particular, elements that
inhibit Cs capture were focused on. Various co-heating experiments on mixtures of CsCl, K-feldspar, fly ash
and alkali metal compounds were conducted. As a result, the following findings were obtained: 1) the
efficiency of Cs insolubilization decreased in the presence of fly ash, 2) Ca in fly ash inhibited Cs
insolubilization. Notably, when Ca was in chloride form, this inhibition was at a maximum, 3) converting
CaCl2 in fly ash to CaCO3 by use of Na2CO3, improved the efficiency of Cs insolubilization. When the ratio of
fly ash to K-feldspar was increased, however, the efficiency showed a gradual decrease.
Key words :	cesium, decontamination waste, fly ash, insolubilization, K-feldspar

1. Introduction
After the nuclear accident in 2011, various
decontamination activities were carried out in the region
affected by radioactive fallout. As a consequence, hugeamounts of contaminated wastes and other substanceswere generated, the majority of which were incineration
ash from combustible wastes and soil removed duringdecontamination of environments. It is estimated thatthe amount of wastes and soils generated throughdecontamination activities was about 14 million m3(Ministry of the Environment, Japan, 2019). There areplans to store these wastes at interim storage sites for
30 years hereafter and then they are to be disposed of
at a final disposal site. It is thought, however, that to findand secure a suitable site with vast capacity will be
extremely difficult because of opposition from residents.
Therefore, the government is planning to reduce thevolume/mass of these wastes/soils. First, soils of low
radioactivity will be utilized as construction material. The
remainder, 380 thousand tons that cannot be thus utilized,
will be subjected to thermal treatment to separate out
radioactive cesium (Cs). After treatment, soils/wastes
Global Environmental Research
24/2020: 173-180
printed in Japan

from which Cs has been removed are to be used as
construction materials at interim storage sites. On theother hand, however, residues with highly concentrated
Cs will be generated simultaneously as a byproduct. The
residues (mainly composed of fly ash) are planned to be
stored in rigid containers with sufficient durability forlong-term storage. From various previous studies, it has
been reported that Cs in fly ash generated from manymunicipal solid waste (MSW) incinerators after theaccident is soluble because most of it is in chloride form
(National Institute of Environmental Studies, 2012). Cs
concentrates produced through thermal treatment of
decontamination wastes are thought to have the samecharacteristics, since some volatilization acceleration
agents (such as CaCl2) are added during the heatingprocess for Cs separation (Kamata, 2015; Abe, 2015).
Thus, from a long-term viewpoint on securing safety
related to leakage of Cs from stabilized fly ash bodies afterthermal treatment, it would be better to insolubilize Cs.
Past studies (Saffarzadeh, 2014; Dote, 2014; Tojo,2016) have clarified that Cs in bottom ash is hardly
soluble at all unlike Cs in fly ash. The authors
investigated why Cs in bottom ash is insoluble and©2020 AIRES

174

Y. TOJOrevealed that Cs in bottom ash is captured in an
amorphous phase formed around a specific oxide mineralcomposed of K, Al and Si. That mineral was identified as
potassium feldspar (KAlSi3O8) (hereafter, K-feldspar)(Tamura, 2016). The surface of K-feldspar particles,
when heated at 700˚C to 900˚C in the presence of Cs,
melts and captures Cs in the amorphous melt phase,making the Cs insoluble. In order to reproduce thiscapturing mechanism, the authors mixed K-feldspar with
Cs2CO3 and heated the mixture at 900˚C. As a result,nearly 100% of the Cs remained in the sample after
heating in an insoluble state, and particles with Cs
concentrated in the peripheral region (white region aroundthe particle in back-scattered electron images (BEI) or
bright part in Cs mapping images in energy dispersive
spectrographs) were found as shown in Fig. 1 (Sakamoto,2017). When the Cs compound was changed to CsCl,
however, the efficiency of insolubilization drasticallydecreased. (Hereafter, the term “insolubilization
efficiency” is used to express the ratio of insolubilized Cs,
which cannot be extracted by water after co-heating, tothe initial amount of Cs.) Thus, the effects of pretreatmentof K-feldspar and optimum heating temperature wasfurther examined. Grinding K-feldspar as a pretreatment,
resulting in amorphization of K-feldspar by grinding itover long periods, significantly increased insolubilization
efficiency even if the Cs was in CsCl form. Additionally,a 700˚C heating temperature produced the highest
insolubilization efficiency.
As mentioned above, the previous studies had been
conducted mainly under pure conditions (only feldspar
and Cs compounds present). However, the actual Cs
concentrates generated from thermal processing of soils/
wastes from decontamination activities, are deemed tocontain many other elements. Therefore, it is crucial toconfirm whether Cs insolubilization by K-feldspar works
when fly ash containing various other elements is present.
If any element interferes with Cs insolubilization, thereason should be elucidated. In addition, measures toprevent such interference must be developed. Therefore,
in this study, the following objectives were set: 1) toconfirm the influence of fly ash on the insolubilization

efficiency of Cs, 2) to identify substances contained in fly
ash that interfere with Cs insolubilization and 3) todevelop a method to prevent such interference.
Concretely, the following experiments were performed.
First, co-heating experiments using mixtures of fly
ash, CsCl and K-feldspar were conducted to confirm theinfluence of fly ash on Cs capture. From the co-heating
experiments above, increasing the ratio of fly ash to
K-feldspar resulted in decreased Cs insolubilization
efficiency. To elucidate the reason, a pure system free of
fly ash was set up and the effects of alkali elements on Cs
insolubilization were examined. The reason alkali
elements were focused on was that this Cs capturing (andinsolubilization) is thought to be related to the chargebalancing ability of cations in K-feldspar’s matrix. In+
K-feldspar (KAlSi3O8), part of the Si4 in the silicate (SiO4
tetrahedral matrix) is substituted with Al3+. Potassium ispresent to compensate for the positive charge deficiency.
In albite (NaAlSi3O8), Na plays this role and in anorthite(CaAl2Si2O8), Ca plays it. Cs is also a monovalent alkali
element. In this study, the capture was considered tooccur mainly through replacement of the “K” in
K-feldspar by Cs. Thus, what may interfere with thisreplacement was thought to be alkali elements which canbehave similarly. Thus, Na, K, and Ca were selected. An
additional reason was that they are contained in large
amounts in fly ash.
From co-heating experiments using certain alkalisalts, it was found that the proportion of insoluble Cs wasparticularly low when using CaCl2. Therefore, subsequent
experiments were conducted to confirm whether
specifically only CaCl2 interferes with Cs insolubilizationor the presence of Ca itself interferes, by trying Ca(OH)2and CaCO3. The reason these two compounds wereselected as the Ca compounds was that fly ash generally
contains slaked lime (Ca(OH)2) blown in for hydrogenchloride removal, and CaCl2 is produced as aneutralization product. Furthermore, Ca(OH)2 reacts with
CO2 in the air relatively easily in the presence of water,converting to CaCO3.
From the above experiments, it was found that only
CaCl2 causes a decrease in Cs insolubilization efficiency.
Therefore, it was thought that by replacing the CaCl2 in
fly ash with other Ca compounds interference in Cs
insolubilization could be avoided. Therefore, a few
pretreatment methods to change the form of Ca were
examined.

2. Materials and Methods

Back-scattered electron images (BEI)

Cs mapping images

Fig. 1 SEM-EDS images of a K-feldspar particle after co-heatingwith Cs2CO3 (Sakamoto, 2017).

2.1 Materials
2.1.1 Fly Ash
MSW fly ash was obtained from an MSW incinerator(stoker type). If it had been possible, it would have been
better to use fly ash generated from an actual thermal
treatment process for decontamination soils/wastes, but

Insolubilization of Cs in fly ash by co-heating with K-feldspar

175

Table 1 Elemental composition of the MSW (Municipal Solid Waste) fly ash used in this study.
Element

Ca

Cl

Na

K

Zn

Si

S

OthersContent (wt%)

40.1

32.3

10.3

8.1

2.3

2.3

2.0

2.6

Measurement conditions: device: Horiba MESA-500, method: fundamental parameter method, qualitative: automatic, voltage: automatic (15kV, 50kV onceeach), current: automatic adjustment, display of results: element)

no full-scale plants were in operation yet, so its
characteristics were not known. Thus, MSW fly ash wasused as an alternative. Table 1 gives the elemental
composition of the fly ash as determined by X-ray
fluorescence (XRF) analysis (Horiba MESA-500). Since
the Cs concentration in the MSW fly ash was too low toanalyze after co-heating experimentation, the Cs content
of the fly ash was artificially increased by adding CsCl
(hereafter, this fly ash is termed as “Cs-fly ash”). For Cs,
stable Cs was used.
2.1.2 K-feldspar
For K-feldspar, Indian feldspar (Quantum®, ceramic
grade potassium feldspar CAS# 68476-25-5) purchasedfrom a ceramics raw material supplier (Nittorengenryo
Co., Ltd) was used. According to an XRD (X-ray
Diffraction) analysis and phase identification by Match!
Version 2 (Crystal Impact GbR.), the major components
were 68.6% microcline (KAlSi3O8) and 31.4% albite(NaAlSi3O8). As a pretreatment, the feldspar was groundfor 1,000 hours in a ball-mill to break down its crystalline
structure and make it amorphous. Grinding was
performed by placing 50 g of the K-feldspar and 1,000 gof alumina balls with a diameter of 1.8 cm in a ceramic
pot. Confirmation of amorphization of this K-feldspar
was made by XRD, though it isn’t shown here.
2.1.3 Chemical ReagentsThe following chemical reagents were used:
Cesium chloride (99%, FUJIFILM Wako Pure
Chemical Corporation), sodium chloride (99.5+%,
FUJIFILM Wako Pure Chemical Corporation), potassium
chloride (99.5+%, FUJIFILM Wako Pure Chemical
Corporation), calcium chloride (95.0+%, FUJIFILM
Wako Pure Chemical Corporation), calcium hydroxide(96.0+%, FUJIFILM Wako Pure Chemical Corporation)and calcium carbonate (99.5+%, FUJIFILM Wako Pure
Chemical Corporation).
2.2 Methods
2.2.1 Confirmation of the Influence of Fly Ash on Cs
Insolubilization by K-feldspar
Three grams of pretreated K-feldspar were added to
Cs-fly ash. The amount of CsCl in the Cs-fly ash was
fixed at 0.6 g and the amount of fly ash was varied from0 to 2.0g. In other words, the influence of fly ash underratios of fly ash to K-feldspar of from 0 to 0.67 was
examined. A sample without K-feldspar was also
prepared. Then, the mixture was put into a ceramic
combustion boat and heated in a tubular electric furnacefor 2 h. The temperature was set at 700ºC.

The sample after heating was treated in the order ofwater washing, aqua regia digestion, and hydrofluoric
acid digestion to fractionate soluble Cs and insoluble Cs.
The fraction obtained by water washing was regarded assoluble and fractions obtained by the two acid digestions
were regarded as insoluble. The liquid solutions obtained
from these three processes were analyzed by atomic
absorption spectrometry (Shimadzu AA-6800).
2.2.2 Identification of Substances in Fly Ash that
Interfere with Cs Insolubilization
Five grams of pretreated K-feldspar were added to
1g of CsCl reagent and 1g of powdered reagent ofeither NaCl, KCl or CaCl2. Then they were mixed wellusing a mortar and pestle. The mixed sample was placedin a crucible and heated in a muffle furnace for 2 h.
The heating temperature was set at 700°C.
After heating, the sample was removed after thetemperature in the furnace became sufficiently low, andtransferred to a desiccator to cool further. The cooled
sample was treated in the order of water washing andhydrofluoric acid digestion to quantify soluble Cs andinsoluble Cs in the sample. Among the added Cs, theportion which could not be detected in the solutions
obtained by either water washing or acid digestion was
regarded as volatilized Cs.
2.2.3 Interference by Ca in Cs Insolubilization fromDifferent Ca Compound Forms
Five grams of pretreated K-feldspar were added to
1 g of CsCl reagent and 1 g of powdered reagent of Ca
compound (i.e., Ca(OH)2 or CaCO3). Then they weremixed well using a mortar and pestle. The procedures ofheating and subsequent chemical analysis were the sameas described above in Sub-section 2.2.2. In thisexperiment, analysis of Ca was also performed on thesolution obtained by washing and acid digestion toconfirm the form of Ca after heating. Thermogravimetricanalysis was also carried out to elucidate the behavior ofeach Ca compound in the heating temperature range ofthis experiment. For TG-DTA analysis, a Thermo plus
EVO-II (Rigaku) was used.
2.2.4 Pretreatment of Fly Ash to Change the Form of
Ca to Prevent Interference
To make Cs-fly ash, the amount of fly ash was set at0.75 g, 1.12 g and 2.24 g. To each of these three fly ashes,
1g of CsCl reagent was added. Thus three types of Cs-fly
ash were prepared. For the co-heating experiment,
pretreated K-feldspar was used.
Three pretreatment methods were examined:• Adding water to the Cs-fly ash to dissolve the CaCl2,

Y. TOJO• Moistening the Cs-Fly ash and exposing it to a CO2
gas environment for 48 hours to form CaCO3• Adding Na2CO3 to the Cs-fly ash with water,
dissociating Ca from CaCl2 to form CaCO3. This
carbonation reaction can be expressed as [CaCl2 +Na2CO3 = CaCO3 + 2NaCl].
The water-added sample was subjected directly toheating with 5 g of pretreated K-feldspar. For Cs-fly ash
subjected to carbonation with CO2 gas or carbonation
with Na2CO3, the sample was dried after treatment andthen mixed with 5 g of pretreated K-feldspar. Then thesemixtures were heated in a furnace at 700°C for 2 h. After
heating, the sample was subjected to water washing andacid digestion to determine the ratio of soluble andinsoluble Cs by the same methodology as describedabove in Sub-section 2.2.2. In addition, the residue after
washing was examined by SEM-EDS (JSM-6360LA(JEOL)) to confirm the presence of insoluble Cs.

3. Results and Discussion
3.1 
Influence of Fly Ash on Cs Insolubilization by
K-feldspar
Figure 2 shows the ratios of insoluble Cs andsoluble Cs after co-heating of K-feldspar with Cs-fly
ash. The horizontal axis shows the amount of fly ash
added to the 3 g of K-feldspar. Although more than 90%of the Cs became insoluble when no fly ash was present,
the ratio of insoluble Cs decreased drastically as theamount of fly ash was increased. When the amount of
fly ash was almost half that of the feldspar (i.e., amountof fly ash of 2.0 on the horizontal axis), the ratio of
insoluble Cs decreased to less than 40%.To secure morethan 80% insoluble Cs, the mass of the fly ash must be
only 10% that of the feldspar. From this result, it can besaid that the presence of fly ash reduces the ratio of
insolubilized Cs.

3.2 
Substances in Fly Ash that Interfere with Cs
Capture and Insolubilization
Figure 3 shows the results of the experiment inwhich three types of alkali salts were used to confirm
their influence on Cs insolubilization by co-heating withfeldspar. The vertical axis indicates the ratio of soluble,
insoluble and volatilized Cs to the total amount of Cs
added. When NaCl, KCl and CaCl2 were mixed and coheated, the ratio of insoluble Cs decreased compared tothe case in which no alkali salt was added. In particular,
the ratio of insoluble Cs decreased significantly when
CaCl2 was added. Therefore, it was thought that thedecrease in the ratio of insoluble Cs confirmed in the coheating experiment conducted in Section 3.1 above wasmainly attributable to Ca in the fly ash.
3.3 Identification of Ca Compounds that Inhibit Cs
Insolubilization
Figure 4 shows the ratio of Cs in the sample after
heating when CaCO3 or Ca(OH)2 was added as a Cacompound. For comparison, the results with CaCl2 andwithout any Ca compound are also shown. Compared to100
Ratio of Cs form (%)

176

Insoluble Cs

Soluble Cs

Volatilized Cs

80
60
40
20
0

None

KCl

NaCl

CaCl2

Alkali salts added in co-heating
Fig. 3 Ratios of Cs forms after co-heating with CsCl, K-feldspar
and each alkali salt.

100%

40%
20%
0%

Without
0.0 0.33 0.53 0.75 1.29 2.0 K-feldspar
Amount of fly ash (g) added to 3 g of feldspar and 0.6 g of CsCl

Fig. 2 
Changes in the ratio of insolubilized Cs resulting fromaddition of fly ash in co-heating with K-feldspar.

100
Ratio of Cs form (%)

Soluble60%

Insoluble

Ratio of Cs form

80%

Insoluble Cs

Soluble Cs

Volatilized Cs

80
60
40
20
0

None

CaCl2

Ca(OH)2

CaCO3

Ca compound added in co-heating
Fig. 4 Ratios of Cs forms after co-heating with CsCl, K-feldspar
and each Ca compound.

Insolubilization of Cs in fly ash by co-heating with K-feldspar

Insoluble Ca

80
60
40
20
0

CaCl2
Ca(OH)2 CaCO3
Calcium compounds added in co-heating
Fig. 5 Forms of calcium present in the same experiment as shownin Fig. 4.

(a) Ca(OH) 2
1220

TG (mg)

10

-20
8
6

-40

0

200

400
600
Temp. (ºC)800(b) CaCO3
15

TG

-60
1000

30
20

DTA (μ V)

10

TG (mg)0

DTA (μV)

DTA

TG

10

DTA

5
0

0

200

400
600
Temp. (ºC)0800(c) CaCl2
18

TG

16

-10
1000

20

DTA

0

DTA (μ V)

Ratio of Ca form (%)

100

Soluble Ca

captured in the glassy amorphous phase created at thesurface of the feldspar particle as indicated in Fig. 1.
If this phenomenon is inhibited by Ca, there is apossibility that the Ca in CaCl2 is also likely to movewhen heated at 700°C, similar to Cs and K. Figure 6
shows the results of a thermogravimetric analysis of the
Ca(OH)2, CaCO3, and CaCl2 used in this study.
In the case of Ca(OH)2 in Fig. 6(a), a significant
decrease in TG and an endothermic peak in the DTA canbe confirmed above 400°C. This is because Ca(OH)2 isthermally decomposed as it approaches 580°C andbecomes CaO. The reaction of CaO with KAlSi3O8 to

TG (mg)the ratio obtained in the case of adding CaCl2, no
significant decrease in the ratio of insoluble Cs was
confirmed with the addition of CaCO3 or Ca(OH)2. Thatis, it is thought that the decrease in insoluble Cs was
mainly due to CaCl2 in the fly ash. It is inferred that thisis related to the state of the Ca compound at the heating
temperature used in this study (i.e., 700°C).
The ratio of Ca forms after the co-heating
experiments is shown in Fig 5. The ratio was calculated
based on recovered amounts, i.e., the sum of the soluble
fraction and insoluble fraction was regarded as 100%.
When CaCl2 was used as the Ca source, the ratio of
insoluble Ca was low, and the majority of the added Ca
was soluble. On the other hand, when CaCO3 or Ca(OH)2
was used as the Ca source, most was in an insoluble form
and there was almost no soluble Ca. That is, when CaCl2
was added and co-heated with K-feldspar and CsCl, mostof both the Cs and Ca existed in a soluble form in thesample after heating. Thus, it is thought that the fact thatboth Ca and Cs remained in the soluble state even whenheated at 700°C was related to inhibition of Cs
insolubilization.
The phenomenon of Cs capture and insolubilization
by K-feldspar applied in this study is most efficientaround 700°C. The reason for this is thought to be as
follows: the melting point of CsCl is 645°C. Based on
Bowen’s Reaction Series, which describes formation
steps of minerals from magma, K-feldspar is formed ataround 750° to 800°C (Earle, 2019). This means that at
around this temperature range, K-feldspar has both asolid phase and melt phase. In a state in which both CsCland K-feldspar melt and both K and Cs can become
readily mobile, substitution of Cs and K is thought tooccur. Since Cs has a higher charge balancing ability than
K (Hess, 1991), the K in K-feldspar is replaced by Cs,
and Cs is as consequence captured in the melt phase andinsolubilized. There is a big difference in their ion
radiuses, however. Cs cannot enter into the crystal matrixof K-feldspar. The author thinks this is why Cs is

177

14

-20

12

-40

10

0

200

400
600
Temp. (ºC)

800

-60
1000

Fig. 6 Results of thermogravimetric analysis of the Ca compounds
used in this study.

178

Y. TOJO

form compounds such as CaAl2Si2O8 is said to be a
general reaction in the cement manufacturing process(Arai, 1990). That is, the mobility of Ca is low because
Ca(OH)2 has already been converted to other compounds
in the heating temperature range used in this study. Thus,
it is thought not to be involved in the Cs capture reaction.
The thermal decomposition temperature(decarbonation temperature) of CaCO3 is said to be about
890°C. In Fig. 6(b), the decrease in TG is confirmed fromaround 700°C, and the DTA shows a large endotherm ataround 800°C. It is reported that CaCO3 reacts with Al2O3,
SiO2, etc. to form compounds even below the thermal
decomposition temperature in the cement clinker
formation process (Arai, 1990). Thus, since CaCO3 also
participates in such reactions, CaCO3 is thought not to
inhibit the Cs capture reaction.
In the case of CaCl2 as shown in Fig. 6(c), there is adecrease in TG and an endotherm before 200°C, which isconsidered to be due to dehydration of absorbed moisture.
The melting point of CaCl2 is 772°C. In the figure, agradual decrease from around 700°C in TG and an
endotherm at around 770°C are observed. This indicatesthat CaCl2 starts melting and the mobility of Ca becomeshigh in the heating temperature range adopted in thisstudy. That is, in the case of CaCl2, it only melts around
700°C, and the mobility of Ca becomes high. However,
because conversion to other compounds does not occur, itis thought to be involved in the Cs capture reaction.
3.4 Effect of Pre-treatment of Fly Ash on Interferenceof CaCl2
Figure 7 shows the ratio of Cs that became insoluble
after co-heating of Cs-fly ash with K-feldspar. The ratio
of insoluble Cs was 74% when the amount of fly ash was
0.75 g and no pretreatment of the Cs-fly ash was done.
This, however, improved to 100% when a carbonation

Ratio of insoluble Cs (%)

100

Carbonation(by Na2 CO3 )80
No pretreatment

60
40
20
0
0.5

Water addition
Carbonation (by CO2 gas)

1.0

1.5

2.0

2.5

Amount of fly ash added to 5 g offeldspar and 1 g of CsCl.
Fig. 7 
Ratio of insoluble Cs after co-heating of Cs-fly ash
pretreated by each method and K-feldspar.

pretreatment for the Cs-fly ash using Na2CO3 wasconducted. On the other hand, in the case of ionization
and CO2 gas exposure, the efficacy was slightly reduced.
Though the results are not shown, from XRD analysis of
the samples after pretreatment, a clear increase in the
CaCO3 peak was observed for carbonation with Na2CO3,
but not for the sample after CO2 gas exposure. In addition,
when a theoretical calculation using PHREEQC(Parkhurst & Appelo, 2013) was performed, no CaCO3
precipitate was formed through contact of the CaCl2
solution with CO2 gas. These results corroborate theobservation that only in the case of Na2CO3 could an
increase in the ratio of insoluble Cs after co-heating be
confirmed. When the amount of fly ash was small relativeto K-feldspar (K-feldspar 5 g: fly ash 0.75 g), carbonation
with Na2CO3 was sufficiently effective.
In any pretreatment, however, the ratio of insoluble
Cs decreases as the added amount of fly ash is increased.
In the carbonation pretreatment with Na2CO3, although
the amount of Na2CO3 was increased according to any
increase in the amount of fly ash, there is a possibility
that sufficient carbonation of the Ca did not proceed.
Alternatively, since inhibition of Na and K on Cs
insolubilization was not zero according to theexperiments done in 3.2 (as indicated in Fig. 3), theincreased amount of Na may also have had an effect.
Especially in carbonation by Na2CO3, although Ca isconverted to carbonate, NaCl is generated in the same
reaction. That is, it is thought that increased Na in thesystem may have contributed to the inhibition of Cs
insolubilization.
SEM-EDS images of the particles after water
washing of the sample obtained by co-heating of the Csfly ash pretreated by carbonation using Na2CO3 andpretreated K-feldspar are shown in Fig. 8.
Particles having a high concentration of Cs have thefollowing characteristics; in BEI (Backscattered electron
images), the particles have a gray core at the center, awhite area surrounding it, and a shell-like gray surface
existing outside it. The center is considered to be
K-feldspar (KAlSi3O8) because it is rich in Al, Si and K,
and also no Cl, Ca, and Na are confirmed in the elemental
mapping image. The structure in which Cs concentrates
around K-feldspar is the same as the concentrating
mechanism of Cs identified so far (Fig. 1) (Tojo, 2014;
Tamura, 2016; Sakamoto, 2017). That is, Cs is trappedwhere the feldspar surface melts and becomes glassy andamorphous. A further characteristic newly observed isthat the outer surface is rich in Ca or Na. The positions
where Cs and Ca or Na concentrate are reciprocal andthey are located so as to avoid each other.

4. Conclusions
In this study, aiming to facilitate long-term safe
management of Cs concentrate generated from thermalInsolubilization of Cs in fly ash by co-heating with K-feldspar

179

Fig. 8 SEM-EDS image of particles obtained from co-heating of Cs-fly ash pretreated with Na2CO3 together with pretreated K-feldspar.
The sample contained only insoluble Cs because the particles were obtained after water washing.

treatment of decontamination waste, a method of
capturing and insolubilizing Cs using K-feldspar wasinvestigated. The insolubilization efficiency of Cs by
K-feldspar decreased as the amount of fly ash present
was increased. To boost the ratio of insoluble Cs to 80%
or more, it was necessary to reduce the amount of fly ash
to 10% of the amount of K-feldspar. The salt in fly ashthat inhibited Cs insolubilization by K-feldspar was
CaCl2. Inhibition by NaCl and KCl was found to be slightcompared to that by CaCl2. Besides, no inhibition was
identified when CaCO3 or Ca(OH)2 were present. The
reason was thought to be the state of CaCl2. The heating
temperature applied in this study was 700°C. At thistemperature, both K-feldspar and CsCl melt, so
replacement of Cs and K, which are both monovalentcations, was likely to occur. Since CaCl2 is also in a
molten state at that temperature, the high mobility of Ca
is thought to interfere with the substitution reaction of Cs
and K. By converting the CaCl2 in fly ash to CaCO3 by
pretreatment with Na2CO3, it was possible to avoid
inhibition from Ca. When the amount of fly ash wasincreased, however, the total amount of alkali elements
increased and as a result, the Cs insolubilization

efficiency gradually decreased. The most significant issue
in the management of decontamination waste is to reduce
its mass/volume because to secure a final disposal site isextremely difficult. Thus, it will still be necessary todevelop a method that realizes a higher insolubilization
efficiency of Cs using minimal amounts of K-feldspar
even if the total amount of fly ash is increased.
Acknowledgements
This research was supported by the Environment
Research and Technology Development Fund (3K153015
and 3-1803 [JPMEERF20183003]) of the Environmental
Restoration and Conservation Agency of Japan.
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Yasumasa TOJO
Yasumasa Tojo is an associate professor at the
Solid Waste Disposal Laboratory, Hokkaido
University. His main fields of interest are thelong-term behavior of substances in landfills,
landfill stabilization, insolubilization of hazardous
elements, heavy metal leaching from wastes andrecycled materials, long-term management of
cesium, and landfill management under extreme conditions.

(Received 26 August 2020, Accepted 22 December 2020)