アルカリ塩化物を用いたセシウムフリー鉱化法による土壌除染; 反応剤のカチオン価数に依存したCs除去過程

Soil decontamination using cesium-free mineralization with alkaline chloride; Cs removal processes depending on valence of cations in reactants

下山 巖   ; 小暮 敏博*; 奥村 大河*; 馬場 祐治  

Shimoyama, Iwao; Kogure, Toshihiro*; Okumura, Taiga*; Baba, Yuji

熱処理による土壌除染の最適化のため、大気及び低圧条件で4種類のアルカリ塩化物試薬を添加した場合における土壌の除染率と構造変化について調べた。約1万Bq/kgの汚染土壌に重量比1:1で試薬を添加し、2時間の加熱処理後の放射能変化と試料中の鉱物相を調べた。CaClの場合は圧力条件による除染率の差は小さく、790Cで約95%の除染率が得られた。一方MgClの場合は695Cの熱処理に対し大気条件では除染率40%だったのに対し、低圧下では90%に達した。CaCl添加時は大気・低圧条件どちらにおいても土壌中の黒雲母の分解と相変態が生じたのに対してMgCl添加時は低圧下で黒雲母の相変態が促進されることがわかった。KClとNaClの場合も低圧下で除染が促進され、NaClの場合は790Cでの除染率は大気圧下15%に対して低圧下94%であった。しかし大気・低圧条件どちらにおいても土壌中の黒雲母が熱処理後も明瞭に観測されたことから、KClとNaClについてはイオン交換によるCs除去過程が主であると結論した。

We study decontamination ratio and structural change of soil after heat treatments with four kinds of alkaline chloride reagents in air and under low-pressure conditions to optimize soil decontamination method. Each reagent was added to contaminated soil with the weight ratio of 1/1, and we measured radioactivity and analyzed crystal phases of the samples after the heat treatments for 2 hours. In the case of CaCl, only small differences in decontamination ratio were observed in air and low-pressure conditions, and decontamination ratio reached to about 95% at 790C. After the processing, biotite in soil decomposed and products depended on in air and under low-pressure conditions. Whereas, in the case of MgCl, decontamination ratios were 40 and 90% after the heat treatments at 695C in air and under low-pressure conditions, respectively. We found that decomposition and phase transformation of biotite were more facilitated under low-pressure conditions than in air. Also, in the cases of NaCl and KCl, soil decontamination was more facilitated under low-pressure conditions; decontamination ratios were 15 and 94% after the heat treatments with NaCl at 790C in air and under low-pressure conditions, respectively. However, biotite remained after the heat treatments both in air and under low-pressure conditions. Based on this result, we conclude that ion exchange was the dominant Cs removal process for NaCl and KCl.