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Oxidative decomposition of ammonium ion with ozone in the presence of cobalt and chloride ions for the treatment of radioactive liquid waste

放射性廃液処理のためのコバルトおよび塩化物イオン共存下でのアンモニウムイオンのオゾンによる酸化分解

粟飯原 はるか   ; 渡部 創  ; 柴田 淳広  ; Mahardiani, L.*; 大友 亮一*; 神谷 裕一*

Aihara, Haruka; Watanabe, So; Shibata, Atsuhiro; Mahardiani, L.*; Otomo, Ryoichi*; Kamiya, Yuichi*

To prevent unexpected accidents at nuclear facilities caused by accumulated ammonium nitrate in an aqueous liquid waste containing ammonium salts and nitric acid, NH $$_{4}$$ $$^{+}$$ in the liquid waste must be decomposed under mild reaction conditions. In this study, we investigated the oxidative decomposition of NH $$_{4}$$ $$^{+}$$ with O$$_{3}$$ at 333 K in the presence of a homogeneous Co$$^{2+}$$ catalyst and Cl$$^{-}$$ in the wide pH range of the test solution. The reaction behavior was greatly affected by pH of the test solution. In a basic solution at pH 12, high conversion of NH $$_{4}$$ $$^{+}$$ was obtained even in the absence of Co$$^{2+}$$ and Cl$$^{-}$$ and the main product was NO $$_{3}$$$$^{-}$$. However, Co$$^{2+}$$ and Cl$$^{-}$$ in the solution greatly enhanced the decomposition rate of NH $$_{4}$$ $$^{+}$$ in acidic to mild basic solutions (pH 1-8), while only low conversion of NH $$_{4}$$ $$^{+}$$ was observed unless both Co$$_{2+}$$ and Cl$$^{-}$$ were present. For the reaction with Co$$^{2+}$$ and Cl$$^{-}$$ in the solutions, NH$$_{4}$$ $$^{+}$$ was transformed mainly into chloramines (NH $$_{x}$$Cl $$_{3-x}$$, x = 1-3) by the reaction with HClO, which was formed by the reaction of Cl$$^{-}$$ with O$$_{3}$$ catalyzed by the homogeneous Co$$^{2+}$$ catalyst, and led to the high decomposition rate of NH$$_{4}$$ $$^{+}$$. Cl$$^{-}$$ suppressed the formation of the precipitate CoO(OH) during the reaction and consequently the Co$$^{2+}$$ catalyst stably existed in the reaction solution, which was another reason for the high decomposition rate of NH$$_{4}$$ $$^{+}$$ in the presence of Cl$$^{-}$$. Owing to the swift decomposition of NH$$_{4}$$ $$^{+}$$ under mild reaction conditions and small formation of secondary waste, the oxidative decomposition of NH$$_{4}$$ $$^{+}$$ in the presence of the homogeneous Co$$^{2+}$$ catalyst and Cl$$^{-}$$ is suitable and applicable for the treatment of the aqueous liquid waste containing ammonium salts and nitric acid.

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