Recovery of Radioactive Cesium from Sewage Sludge Ashes—Paving the Way for Disposal of Sewage Sludge Ashes Containing RadioactiveCesium
N. Kozai1), S. Suzuki2), N. Aoyagi3), F. Sakamoto1), T. Ohnuki1)
1)
Research Group for Bioactinide Chemistry, JAEA
2)
Research Group for Actinide Coordination Chemistry, JAEA
3)
Research Group for Radiochemistry Gr., JAEA

After the accident of the Fukushima Daiichi Nuclear PowerPlant, the radioactive Cs was dispersed across large areas ofeastern Japan. Some of the radioactive Cs has been discharged
into sewage system. At sewage treatment plants, sewage isbiologically treated and converted to sewage sludge. Sewage
sludge is finally incinerated at 800–850 °C. The end product ofsewage treatment is called sewage sludge ash (SSA). Since theFukushima Daiichi accident, highly concentrated radioactive Cs
has been detected in the SSAs produced at many sewage
treatment plants in Japan. Most of the contaminated SSAs havebeen stored at those sewage treatment plants (as of October 2012,
120,000 t [1]).
Based on the experiences of decontaminating soils, someattempts were made to reduce radioactive Cs from SSA by
dissolving SSA in organic acid or nitric acid. However, those
attempts only evaluated the total amount of Cs dissolved underpredetermined conditions without being accompanied byscientific grounds of the dissolution behavior of Cs.
Under these circumstances, we collected SSA samples
contaminated with radioactive Cs fallout and investigatedrelations among elements such as radioactive Cs and irondissolved from SSAs in acid solution to elucidate the chemical
states of Cs, i.e., the solid phase retaining Cs [2]. We found thatradioactive Cs is mainly contained in iron oxides, and a part ofthe iron oxides is not easily dissolved in acid solution because itis present in dense sintered aggregates of numerous
nanoparticles with silicate-framework. These aggregates aresupposed to be formed during incineration of sewage sludge.
Accordingly, we pulverized SSA down to several-hundred
nanometer (one nanometer: one billionth of one meter), and thendissolved in hydrochloric acid. Almost all of the iron oxides
were dissolved by these treatments, and as a result, wesucceeded in recovering more than 90% of radioactive Cs,
while the maximum recovery had been only 70% withoutpulverization. By applying pulverization and subsequentdissolution in hydrochloric acid, the original radioactivity
concentration of an SSA sample, 23k Bq/kg, was reduced to
4.8k Bq/kg, which is much lower than the standard value ofradioactive Cs for the designated waste, 8k Bq/kg.
The dissolution residue is composed of three kinds ofsilicates: naturally-occurring quartz and feldspar, andunidentified silicate (not clay) probably formed by theincineration of sewage sludge. We confirmed that no radioactiveCs is dissolved into solution even when the dissolution
residues were immersed in pure water or sea water at 60oC,
indicating that Cs is tightly fixed in the silicate. On the other
hand, several percentages of radioactive Cs is dissolved fromthe original SSA. Therefore, the dissolution residue has anadvantage over the original SSA for landfill disposal.
The present results were obtained in the laboratory. There still
remain many problems to be solved, such as the development ofmethods to reuse solution for dissolving more efficiently.

%0  1%0%   Fig. 1 Fractions of 137Cs in sewage sludge ashes. The
majority of 137Cs is contained in iron oxides. Fig. 2 SEM image of a typical particle of SSA. Thisparticle looks sintered aggregate ofnumerous
nanoparticles. In SSA, iron is contained in particles likethis. References[1] 地方共同法人日本下水事業団資料
www.jswa.go.jp/gesuidouten/2013/pdf/h3.pdf[2] N. Kozai et al., Water. Res. 68, 616 (2015).