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

The radioactive fallout cesium (¹³⁷Cs) in the sewage sludge ashes (SSAs) produced in Japan after the Fukushima Daiichi Nuclear Accident was tested. Five samples of SSAs produced in 2011 and 2012 were tested. Two of the samples contained ¹³⁷Cs (23 and 9.6 kBq/kg, respectively) above the radioactivity criterion (8 kBq of radioactive Cs/kg of solid) for controlled landfill disposal in Japan. The mineral components of SSA are roughly divided into two groups: an HCl-soluble phase mainly composed of phosphates and oxides; and silicates, including quartz, feldspar, and clay. Both phases contained ¹³⁷Cs. The majority (up to 90%) of ¹³⁷Cs was contained in the HCl-soluble phase. Among the HCl-soluble subphases, Fe-bearing phases that were probably iron oxides were mainly responsible for ¹³⁷Cs retention. No positive evidence was obtained that showed that phosphate-bearing phases, which were included most in SSAs along with the silicate phase, retained ¹³⁷Cs. Pre-pulverizing SSAs and heating them at 95 °C in a 6 M or a concentrated aqueous HCl was the most effective method of dissolving the HCl-soluble phase. The radioactivity concentrations of ¹³⁷Cs in all the HCl-treatment residues were below the radioactivity criterion. This residue was mostly composed of silicates. After static leaching tests of the residue at 60 °C for 28 days, no ¹³⁷Cs was detected in simulated environmental water leachates (pure water and synthetic seawater), demonstrating that 137Cs in the residue is very stably immobilized in the silicates.