Radioactive fallout cesium in sewage sludge ash produced after the Fukushima Daiichi nuclear accident

Kozai, Naofumi   ; Suzuki, Shinichi; Aoyagi, Noboru   ; Sakamoto, Fuminori  ; Onuki, Toshihiko

The radioactive fallout cesium (Cs) in the sewage sludge ashes (SSAs) produced after the Fukushima Daiichi Nuclear Accident was tested. Two of tested five SSAs contained Cs above the radioactivity criterion for controlled landfill disposal in Japan. The minerals of SSAs are divided into two groups: an HCl-soluble phase mainly composed of phosphates and metal oxides; and silicates. The majority of Cs was contained in the HCl-soluble phase. Among the HCl-soluble subphases, Fe-bearing phases, probably iron oxides, were mainly responsible for Cs retention. Pre-pulverizing SSAs and heating them in an aqueous HCl was the most effective method of dissolving the HCl-soluble phase. The radioactivity concentrations of Cs in all the HCl-treatment residues were below the criterion. This residue was mostly composed of silicates. Static leaching tests of the residue revealed that Cs is very stably immobilized in the silicates.