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Effective removal of iodate by coprecipitation with barite; Behavior and mechanism

分配挙動とメカニズムの解明によるバライトへの共沈によるヨウ素酸の効果的な除去法の開発

徳永 紘平; 高橋 嘉夫*; 田中 万也; 香西 直文

Tokunaga, Kohei; Takahashi, Yoshio*; Tanaka, Kazuya; Kozai, Naofumi

本研究ではバライトへの共沈を利用することで、長い半減期と高い溶出性を持つ放射性ヨウ素の陰イオン(ヨウ素酸)を効果的な除去する手法を開発することを目的とする。本研究により、競合イオンの共存下でもヨウ素酸を効果的に除去する条件を見出すとともに、従来使用されていたハイドロタルサイトなどの層状鉱物に比べて2桁以上高い分配効率を達成した。また広域X線吸収微細構造法により、バライト構造内においてヨウ素酸は硫酸イオンと置換されて取り込まれており、電荷の過不足はバリウムイオンの周囲に存在するナトリウムイオンにより補われていることが示唆された。これらヨウ素酸のバライトに対する高い分配効率と強い結合性より、バライトは放射性ヨウ素の除去において有効な処理法であることが示された。

Radioactive iodine ($$^{129}$$I) is of great concern owing to its high mobility in the environment and long-term radiotoxicity, but there is a lack of effective techniques for removing iodate (IO$$_{3}$$$$^{-}$$) from aqueous solution. The aim of this study is to develop a new technique for removing radioactive iodate from contaminated solution by using barite (BaSO$$_{4}$$). In the present study, we examined the coprecipitation mechanism of iodate by barite at the molecular level for determining optimum conditions for iodate removal. The results showed that iodate was effectively removed from aqueous solution by coprecipitation, even in the presence of competitive anions in solution. Comparing our method with previous studies, iodate removal efficiency by barite was determined to be about two orders of magnitude greater than that by hydrotalcite-like layered double hydroxide at Cl$$^{-}$$ concentration of 10 mmol L$$^{-1}$$. Extended X-ray absorption fine structure (EXAFS) analysis indicated that incorporated iodate was strongly bound in the crystal lattice of barite by substituting the sulfate site in the structure when the iodine concentration was low. The charge compensation problem from the IO$$_{3}$$$$^{-}$$ substitution in SO$$_{4}$$$$^{2-}$$ site is achieved by the substitution of Na$$^{+}$$-IO$$_{3}$$$$^{-}$$ pairs at the nearest Ba$$^{2+}$$ site. Therefore, considering high removal efficiency and strong binding of iodate in barite, coprecipitation with barite is a promising material for removing radioactive iodate from various aqueous solutions contaminated with iodate.

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パーセンタイル:100

分野:Environmental Sciences

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