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下水汚泥焼却灰中の$$^{137}$$Csの化学状態の解明及び放射能濃度低減方法

Chemical states of $$^{137}$$Cs in sewage sludge ash and radioactivity concentration reduction method

香西 直文; 鈴木 伸一; 青柳 登; 坂本 文徳; 大貫 敏彦

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

福島第一原子力発電所の事故により、大量の放射性物質が東日本の広い範囲に降下した結果、高濃度の放射性物質(主に$$^{137}$$Cs)が濃縮した下水汚泥焼却灰が各地で発生した。これらの焼却灰の安全な埋設処分に資するため、本研究では、焼却灰中の$$^{137}$$Csの化学状態を、$$^{137}$$Csの溶解実験及び静的浸出実験を行って検討した。焼却灰中の$$^{137}$$Csは、一部がアルカリ金属の塩に、大部分が酸化鉄に、残りがケイ酸塩(石英,長石)に含まれていた。$$^{137}$$Csの約80-90%がアルカリ金属の塩及び酸化鉄に含まれており、塩酸水溶液中で加熱することでそれらの固相の大部分が溶解した。焼却灰を微粉砕したのち塩酸水溶液中に入れ95$$^{circ}$$Cで加熱することにより、それらの塩酸溶解性成分を完全に溶解させることができ、ほぼケイ酸塩のみの残渣が得られた。この残渣には、焼却灰の約10-20%の$$^{137}$$Csが残留した。粉砕していない焼却灰では一部の$$^{137}$$Csが人工海水に溶出したが、微粉砕試料の塩酸溶解残渣からは$$^{137}$$Csは全く溶出しなかった。

Huge quantities of radionuclides were emitted by the accident at the Fukushima Daiichi Nuclear Power Plant and descended in wide areas of east Japan. A great amount of sewage sludge ashes (SSAs) containing fallout radionuclides (mainly $$^{137}$$Cs) at high concentration was generated and has been stored in sewage treatment plants until being disposed of. To contribute for the safety landfill disposal, we investigated the chemical states of $$^{137}$$Cs in SSAs by dissolution and static leach experiments. A fraction of $$^{137}$$Cs was contained in alkali metal salts, the major fraction of $$^{137}$$Cs was in iron oxides, and the rest was in silicates (quartz and feldspar). About 80-90% of $$^{137}$$Cs was contained in alkali metal salts and iron oxides and most of these solid phases were dissolved by heating in HCl aqueous solutions. These HCl-dissolvable phases were completely dissolved by pre-pulverizing SSAs and the subsequent heating in HCl aqueous solution at 95$$^{circ}$$C. The residues were mostly silicates and contained about 10-20% of $$^{137}$$Cs in the original SSAs. Several percent of $$^{137}$$Cs in the original SSA was leachable to synthetic seawater, while the $$^{137}$$Cs in the residue was non-leachable.

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