セシウムフリー鉱化法を用いた土壌除染における硝酸塩添加剤の効果

Effects of nitrate additives in soil decontamination using Cs-free mineralization

下山 巖   

Shimoyama, Iwao

Csフリー鉱化法は化学反応により粘土鉱物を異なる鉱物に変換して土壌から放射性Csを除去する除染法であり、その効果は添加剤に大きく依存する。本研究では処理温度低減化を目的として硝酸塩試薬を添加剤として用いた。非放射性Csを収着させた風化黒雲母(WB)を模擬土壌とし、硝酸塩試薬を添加して2時間熱処理を行った試料に対し蛍光X線分析とX線回折による分析を行った。硝酸カルシウム添加の場合、450CのAr置換熱処理でCs除去率が100%となり、塩化物添加時と比べて大幅な処理温度の低減化を達成できた。NaClとKClの場合は700Cでイオン交換によるCs除去過程が主であったが、NaとKの硝酸塩では634CでWBの相変態が誘起され、Cs除去過程が試薬中のアニオンに依存することを明らかにした。硝酸カルシウムの場合もWBの相変態温度は500C付近まで低下する効果が見られたが、450CではWBの構造が保たれており、この温度での主なCs除去過程はイオン交換によるものと推測される。

Cs-free mineralization is a decontamination method to remove radiocesium from soil by converting clay minerals to different minerals through chemical reaction with additives, and its efficiency considerably depends on reagents. In this work, nitrate reagents were used to decrease processing temperatures. Weathered biotite (WB) which sorbed non-radioactive Cs was used as model soil. WB was heated with an additive for two hours and analyzed using X-ray fluorescence and X-ray diffraction after rinsing with water and drying. When calcium nitrate was added, Cs removal ratio reached to 100%at 450C in Ar atmosphere, and this achieves significant decrease of processing temperature as compared with the cases of chloride reagents. In the case of NaCl or KCl addition, ion exchange was the major Cs removal process at 700C, whereas in the case of NaNO or KNO addition, phase transformations were induced at 634C under vacuum, which suggests that Cs removal process depends on the anion in the reagent. In the case of calcium nitrate, the phase transformation temperature decreased to approximately 500C, however, ion it is presumed that ion exchange was the major Cs removal process at 450C because WB was preserved at this temperature.