3.1. Elemental Composition of Ash Samples
Figure 2a,b show the results of the ED-XRF analysis. The bottom ash (BA) samples contained high concentrations of Ca, Si, and Al, indicating that the main components were CaO, SiO
2, and Al
2O
3 [
11].
Conversely, the fly ash (FA) samples contained large amounts of Ca, Cl, and Na. The Ca in the FA, which is considered to exist as Ca(OH)
2, CaCl
2, CaO, and CaSO
4 [
12,
13], is derived from the slaked lime that is blown into the furnace as a measure to neutralize hydrogen chloride gas (
Figure 1).
The Na in the FA is likely to have been generated by the incineration of household food waste, and exists as NaCl [
8,
12,
13,
14]. Thus, the Cl content is higher in FA than in BA, suggesting the presence of large amounts of water-soluble chlorides.
Cs is known to be a volatile element [
15]. In addition, the Cs present in BA and FA exists in different forms [
11,
16,
17,
18]. This is because most r-Cs enters municipal solid waste (MSW) incineration facilities via vegetation and soil and is volatilized into a gaseous state during incineration, following which it migrates into the flue gas [
19].
The r-Cs cools near the baghouse filter, which is controlled at a temperature of approximately 200 °C. The r-Cs then solidifies and concentrates in the FA. Therefore, Cs may exist in FA in the form of highly water-soluble CsCl [
8,
17,
18], CsOH, and Cs
2SO
4 [
20,
21].
3.2. Moisture Absorption and Deliquescence of Ash Samples
Figure 3 shows the change in the weight of samples retained for 30 days at a temperature of 25 ± 5 °C and humidity of 75 ± 5%. The weight increase was calculated relative to the initial sample weight as follows:
where
WBefore and
WAfter represent the sample weights (g) before and after being placed in the thermo-hygrostat chamber, respectively. Although the BA and FA samples were maintained under the same conditions, the latter showed significantly higher moisture absorbency and a greater weight increase.
The FA from Facility A exhibited the greatest weight gain of 109% (2.09 times the original sample weight). Furthermore, all five FA samples showed deliquescence (
Figure 4). A comparison of these results with the ED-XRF results indicates that components such as CaCl
2, NaCl, and KCl influence the moisture absorption and deliquescence of incineration ash [
22,
23].
In Fukushima Prefecture, r-Cs-contaminated MSW incineration ash tends to be packed into flexible container bags during the recycling or landfill process and temporarily stored in the MSW incineration facility. In addition, MSW incineration ash that cannot be recycled or disposed of in landfills because of excess r-Cs concentration tends to be packed into flexible container bags and stored long-term in concrete boxes until its r-Cs concentration decreases (
Figure 5).
The flexible container bags block the flow of air and water to a certain extent; however, considering the results of this test, if MSW incineration ash is stored long-term in a humid location the liquid produced as a result of moisture absorption and deliquescence could leak from the flexible container bag and contaminate the storage area.
In addition, the increase in weight due to water absorbance could affect the durability of the flexible container bags and place a burden on operators when removing the containers. Therefore, in the case of long-term storage, it is advisable to regularly monitor the humidity and temperature around the storage area and check the flexible container bags for deterioration.
3.3. r-Cs Concentrations and Leaching Properties of Contaminated MSW Incineration Ash
Table 1 shows the r-CS concentrations and leaching test results of the ash samples. The r-Cs concentrations are expressed as the sum of the
134Cs and
137Cs concentrations. The leaching rate of r-Cs was calculated using Equation (3):
where
CSample and
CFiltrate represent the r-Cs concentrations (Bq/kg) of the sample and the filtrate used in the leaching test, respectively, while
WSample and
WFiltrate represent their respective weights (kg).
ρ represents the density of the solvent used in the leaching test (
ρ = 1.0 kg/L). If the r-Cs concentration of the filtrate was below the detection limit (below 0.1 Bq/L), it was calculated as 0 Bq/L, and shown as ‘ND’ in
Table 1. The water content of the ash samples, which was determined using an air oven drying method, is also shown in
Table 1.
Comparison of the r-Cs concentrations in
Table 1 demonstrates that there is substantial variation among incineration facilities. The reason for this wide variation may be that the type of waste differs between facilities owing to different industrial structures and regional characteristics, such as whether they are located in an urban or rural area.
For example, the amount of vegetation, leaf litter, and attached soil generated by agricultural work can differ depending on the incineration facility. This agrees with previous findings that the type of waste supplied to the incinerator influences the properties of the incineration ash [
24,
25].
Despite the differences among facilities, the FA samples had higher r-Cs concentrations than the BA samples at all facilities. Moreover, the r-Cs concentrations and leaching rates of the filtrates were higher for FA than BA.
Our results for MSW incineration ash disagree with previous findings that the r-Cs leaching rate from the incineration ash of decontamination waste and sewage sludge is typically not high [
8,
26]. The fact that r-Cs is present in different forms in the BA and FA could explain the difference in the r-Cs leaching rates between BA and FA [
11,
16,
17,
18].
The pH values and EC of the filtrates are also shown in
Table 1. All filtrates were strongly alkaline; however, there was no correlation between the pH and r-Cs leaching rate. In contrast, there was a certain correlation between the EC and r-Cs leaching rate, with the FA samples having higher EC values than the BA samples.
Considering the difficulty in preventing r-Cs leaching during recycling or landfill disposal using only chelating agents, additional measures are required to inhibit r-Cs leaching from FA, particularly for MSW incineration ash.
3.4. Inhibitory Effect of Acid Clay on r-Cs Leaching
Table 2 shows a summary of the r-Cs concentrations of samples prepared by adding 5 wt% acid clay to each FA sample and adjusting the water content to 30% using ultrapure water.
Table 3 shows a summary of the r-Cs concentrations of the filtrates obtained after leaching tests on each prepared sample.
For comparison, leaching tests were conducted under the same conditions on FA without adding acid clay (
Table 4).
Figure 6a–e summarize the r-Cs leaching rates of the samples as a function of the number of days of leaching.
Previous researchers have exploited the high r-Cs leaching rate from FA to develop a method for r-Cs removal by washing [
27]. However, this method requires the construction of washing facilities. Further, even if the r-Cs can be removed, other problems may arise, such as the safe management of contaminated water generated by the washing process. In addition, other methods of removing or inhibiting r-Cs leaching using chemicals are being considered [
28,
29].
However, chemical treatment may lead to restrictions on where the ash can be recycled, and other problems related to landfill disposal. Therefore, with the aim of inhibiting r-Cs leaching from FA, preferably using materials that exist in nature, this study examined the effect of added acid clay, which can be mined stably in areas of Japan on the Sea of Japan.
Some clay minerals such as zeolites are considered capable of capturing r-Cs [
30,
31]. However, few studies have analysed the inhibition of r-Cs leaching from r-Cs-contaminated MSW incineration ash by adding clay minerals that are thought to be effective for capturing r-Cs. Therefore, this study is the first to examine the extent to which r-Cs leaching can be inhibited by adding acid clay to FA.
According to the results in
Table 3 and
Table 4, adding 5 wt% of acid clay to FA reduced the r-Cs leaching rate from >80% to ≤30% in all FA samples, even after a maximum testing duration of 30 days.
This indicates that acid clay has a long-term inhibiting effect on r-Cs leaching, which will limit r-Cs leaching when FA is recycled or disposed of in landfills. Furthermore,
Table 1 shows that the r-Cs concentration of the filtrate differed depending on the r-Cs concentration of the FA; however, the r-Cs leaching rate was often higher than 80%. This can be used to approximate the r-Cs concentration of contaminated water produced by leaching.
Therefore, the amount of acid clay to be added could also be determined by estimating the r-Cs concentration of contaminated water produced by leaching from the r-Cs concentration of FA. For example, we estimated that the concentration of Cs in contaminated water generated by leaching was reduced to 10 Bq/L or less by adding acid clay. However, the excessive addition of acid clay could hinder recycling or lead to pressure on landfill sites. Therefore, we propose a maximum limit of 20 wt% acid clay.
3.5. Interference of Acid Clay with the Sorption Capacity
Table 5 and
Table 6 show the concentration of heavy metals contained in the filtrate after 6 h and 30 days leaching tests of FA, respectively, with and without the addition of 5 wt% acid clay. For comparison, leaching tests were also performed with only acid clay to confirm the heavy-metal concentration in the filtrate obtained from acid clay. If the heavy-metal concentration of the filtrate was below the limit of quantification (shown in the bottom row of
Table 5 and
Table 6), it was noted as ‘–’.
No significant change was observed in the sorption capacity with the addition of 5 wt% acid clay. Moreover, no unusual heavy-metal concentrations were observed in the leaching test results performed on only acid clay. Therefore, it is inferred that the addition of acid clay did not interfere with the sorption capacity, and that the inhibitory effect of chelating agents on heavy-metal leaching was maintained.
The Cl concentration tended to increase with increasing leaching test duration in all samples. Previous research suggests that Cs leaching is caused by an increase in the concentrations of K, Na, or Cl in the surrounding area (Cs leaching by ion exchange) [
32,
33,
34]. Therefore, the gradual increase in the r-Cs leaching rate with the test duration, as shown in
Figure 6, may be because the r-Cs captured by the acid clay was then leached out by ion exchange.