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

Abstract

To prevent unexpected accidents at nuclear facilities caused by accumulated ammonium nitrate in an aqueous liquid waste containing ammonium salts and nitric acid, NH4+ in the liquid waste must be decomposed under mild reaction conditions. In this study, we investigated the oxidative decomposition of NH4+ with O3 at 333 K in the presence of a homogeneous Co2+ 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 NH4+ was obtained even in the absence of Co2+ and Cl−, and the main product was NO3−. However, Co2+ and Cl− in the solution greatly enhanced the decomposition rate of NH4+ in acidic to mild basic solutions (pH 1–8), while only low conversion of NH4+ was observed unless both Co2+ and Cl− were present. For the reaction with Co2+ and Cl− in the solutions, NH4+ was transformed mainly into chloramines (NHxCl3−x, x = 1–3) by the reaction with HClO, which was formed by the reaction of Cl− with O3 catalyzed by the homogeneous Co2+ catalyst, and led to the high decomposition rate of NH4+. Cl− suppressed the formation of the precipitate CoO(OH) during the reaction and consequently the Co2+ catalyst stably existed in the reaction solution, which was another reason for the high decomposition rate of NH4+ in the presence of Cl−. Owing to the swift decomposition of NH4+ under mild reaction conditions and small formation of secondary waste, the oxidative decomposition of NH4+ in the presence of the homogeneous Co2+ catalyst and Cl− is suitable and applicable for the treatment of the aqueous liquid waste containing ammonium salts and nitric acid.