Suppression mechanism contributing to the low leaching rate of cesium from incineration bottom ash

Abstract

After the Fukushima Daiichi nuclear accident, municipal solid waste (MSW) contaminated with radiocesium was generated. In Japan, approximately 80% of MSW by weight has been incinerated. As consequence, radiocesium was retained in incineration residue after the accident. Among the incineration residues, fly ash with high radioactivity was transported to an interim storage facility and special treatment has been carried out. While, bottom ash with radioactivity levels of <8000 Bq kg⁻¹ wet has been directly deposited in conventional MSW landfills. The radiocesium leaching from bottom ash is low, but the reason for this low leaching remains unclear. In this study, leaching tests and microscopic observations of bottom ash containing stable Cs were conducted to investigate the mechanism making Cs leaching low. It is noted that the ash used was simulated ash created from combusting refuse derived fuel (RDF) to which stable Cs was added, not real radioactive ash. Based on pH dependence testing, the Cs leaching increased as the pH decreased. The amount of Cs leached in the neutral range was 2–4% mass of the total content. Electron probe microanalysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy confirmed that certain particles in bottom ash contain concentrated Cs. These particles were found to comprise aluminum, silicon, potassium, and oxygen at their core, surrounded by concentrated Cs. Raman microscopy suggested that these particles are microcline. Co-heating of microcline with Cs carbonate led to the condensation of Cs in a manner similar to particles observed in bottom ash, and Cs was captured in a glassy substance formed on the microcline surface, which reduced the leaching of Cs.