Characterization of soluble and insoluble radioactive cesium in municipal solid waste incineration fly ash
Introduction
Considerable amounts of radioactive cesium having 11.8–18 PBq (134Cs) and 13–62.5 (137Cs) PBq (Steinhauser et al., 2014) were released into environment by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. After the FDNPP accident, the concentration of radioactive cesium in soil near Kawasaki, located 300 km from the FDNPP, increased by a factor of ˃100 (Ochi et al., 2017). Moreover, the radioactive cesium concentration in waste collected at Kyusyu, located approximately 1200 km from FDNPP, was 5 times higher than the levels measured before the accident (Iwahana et al., 2013). Eight years after the FDNPP accident, radioactive cesium can be detected in high concentrations due to the long half-lives of 134Cs and 137Cs (T1/2 = 2.06 and 30.07 years, respectively).
The deposited radioactive cesium on the ground via dry and wet deposition were incinerated at an incineration plant to reduce the waste volume. After incineration, municipal solid waste incineration (MSWI) fly ash and bottom ash were produced, and both ashes were buried in landfills after stabilization treatment to prevent the elution of toxic components. The Ministry of the Environment Government of Japan has reported radioactive cesium in contaminated materials is contained in the fly and bottom ashes to fairly high concentrations, and a higher concentration of radioactive cesium in MSWI fly ash compared to that in MSWI bottom ash (Ministry of the Ministry of Environment, 2011). The limit for radioactivity set by Japanese Ministry of the Environment is ˂8000 Bq kg−1 in waste materials including fly ash. Radioactive waste is incinerated as usual waste material, and the generated ashes are subsequently stored in landfills. Waste materials with radioactivities of ˃8000 Bq kg−1 are treated to control the radioactivity of the landfill site by enclosing around with soils of a water resistant layer to prevent contact with water, and putting an absorption layer of radioactive materials at underside (Ministry of the Ministry of the Environment Government of Japan, 2015). In contrast, fly ash with ˂8000 Bq kg−1 can be recycled for uses in cement (Aubert et al., 2006; Saikia et al., 2007), ceramics (Qian et al., 2006), stone (Nishida et al., 2001), and zeolite (Fan et al., 2008), similar to regular MSWI fly ash.
The physicochemical properties of fly ash were investigated by conducting leaching tests of heavy metals (Harada et al., 2011; Okada and Matsuto, 2009). Lead in fly ash was specified by sequential chemical extraction (Sukandar et al., 2009), leaching of Cu, Cr, and Pb were suppressed by ball milling (Chen et al., 2016). Moreover, enrichment and distribution of heavy metals were investigated (Raclavská et al., 2017). However, the physicochemical properties of radioactive nuclides are required for further comprehensive understanding of fly ash characteristics. Studies regarding the speciation analyses of radioactive cesium in fly ash have been reported after the FDNPP accident (Saffarzadeh et al., 2014; Tojo et al., 2014; Shiota et al., 2015; Iwahana et al., 2017). Iwahana (Iwahana et al., 2017) and co-workers reported the chemical speciation of 210Pb using the sequential extraction method proposed by Tessier (Tessier et al., 1979), and the chemical form differed from that of stable Pb. An insoluble radioactive cesium in bottom ash was contained to the amorphous matter (Tojo et al., 2014). The chemical form of radioactive cesium could be separated into water-soluble and insoluble forms using the Japanese leaching test No. 13 (JLT-13) defined by the Ministry of the Environment Government of Japan. The JLT-13 can be applied to MSWI fly ash to determine the leaching properties of metals (Sakai et al., 1995; Pariatamby et al., 2006). The physicochemical properties of radioactive cesium in fly ash were also studied using the JLT-13 (Ohbuchi et al., 2016; Fujii et al., 2018). Approximately 60% of radioactive cesium was present as water-soluble forms in fly ash (Ohbuchi et al., 2016). Moreover, the elution ratio of radioactive cesium from fly ash increased with decrease in the particle size and was influenced by the presence of chloride compounds such as sodium chloride (NaCl) and potassium chloride (KCl). Therefore, radioactive cesium in fly ash is likely contained as chloride compounds such as CsCl (Fujii et al., 2018). Estimation of the chemical forms of radioactive cesium in fly ash is useful for the safe recycling and control of landfilled fly ash.
Herein, characteristic of radioactive cesium in MSWI fly ash contaminated by the FDNPP accident was evaluated. Especially, estimation of the chemical form of insoluble radioactive cesium in the amorphous phase was performed, including quantitative analysis of the amorphous phase by Rietveld refinement (Rietveld, 1969) which was applied to various materials such as cement (Guirado et al., 2000; Taylor et al., 2000), and waste materials (Singh and Subramaniam, 2016; Ohbuchi et al., 2019). The sequential extraction method developed by Tessier was used to further evaluate the form of soluble radioactive cesium. Moreover, the chemical speciation of insoluble radioactive cesium was evaluated considering the relationship between the concentrations of radioactive cesium and amorphous phase by chemical and heat treatments.
Section snippets
Sample preparation
MSWI fly ash samples, which was same sample with previous study (Ohbuchi et al., 2016), were collected from a municipal waste incineration plant in Fukushima Prefecture, Japan, on January 2013 and dried at 105 °C for 24 h. The combustion capacity of plant is 300-ton day−1 with two stoker furnaces in the plant at temperature between 800 and 1000 °C where municipal solid waste (MSW) of general component is combusted. The bottom ash was composed of cinder cooled with water. The fly ash was
Crystal morphology in sequentially extracted MSWI fly ash
XRD analysis was conducted to investigate the matrix and crystalline phase of the MSWI fly ash since crystalline in the MSWI fly ash and varied by waste composition and incineration conditions used in the incineration facility. Also, speciation of soluble radioactive cesium was estimated as chloride compound by crystalline components in MSWI fly ash (Fujii et al., 2018). Therefore, crystalline phases contained in MSWI fly ash were investigated via XRD combined with the sequential extraction
Conclusion
Radioactive cesium in MSWI fly ash contaminated by the FDNPP accident was analyzed by XRD and gamma-ray spectrometry. A total of 12 crystalline phases were identified in the diffraction pattern of raw MSWI fly ash, and the chloride-containing compounds of halite and sylvite were dissolved via IE extraction using the sequential extraction method. In addition, approximately 60% of the radioactive cesium was dissolved in the IE fraction and was considered water-soluble radioactive cesium because
CRediT authorship contribution statement
Atsushi Ohbuchi: Writing - original draft, Writing - review & editing, Data curation. Kengo Fujii: Formal analysis. Miki Kasari: Formal analysis. Yuya Koike: Conceptualization, Supervision, Writing - review & editing.
Acknowledgments
The authors would like to thank Dr. Kiyoshi Nomura, Meiji University, for their continuing guidance and encouragement throughout this work.
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