Title : Cobalt ion species in water as an active homogeneous catalyst for oxidative decomposition of ammonium ion with ozone
Treatment of accumulated radioactive liquid waste in laboratories is urgent task not only for the storage space but also for safety management of the facilities. Liquid wastes generated by R&D activities often contain various reactive reagents. Since the liquid waste has to be treated inside shielded devices such as concrete cell or glove box due to its radioactivity, easy operations for the treatment are preferred.
In order to develop new technologies for the treatment of radioactive liquid waste containing various reactive chemicals, Japan Atomic Energy Agency launched a project named STRAD (Systematic Treatment of RAdioactive liquid waste for Decommissioning). One of the targeted chemicals in this project is ammonium ion and now we are investigating oxidative decomposition with ozone as a promising method. Decomposition of ammonium ion under mild reaction conditions has been successfully achieved in the presence of Co3O4, which acts as a heterogeneous catalyst. In addition, we also have demonstrated that cobalt nitrate acted as a homogeneous catalyst for the reaction and chloride ion in water was indispensable for the catalyst to effective promote the reaction. Homogeneous catalyst is rather desirable than heterogeneous one for the operation in the shielded area due to its simple procedure and high efficiency for the reaction. Since treatment methodology of radioactive solution containing only nitric acid and metallic ions has already been established, the addition of nitrate salts to the solution are acceptable for the treatment. In the present study, chemical state of cobalt ion in the solution under the reaction conditions was evaluated through in-situ XAFS measurement on Co in order to gain insight into the reaction mechanism involving cobalt ion as a homogeneous catalyst for the decomposition of ammonium ion.
Test solutions were prepared by dissolving ammonium chloride and cobalt nitrate in water. pH of the solution was parametrically changed from 4 to 12. Mixture of ozone and oxygen (ozone concentration about 5%) was supplied into 80 mL of the solution with 100 mL/min. Co K edge XAFS measurement with fluorescence mode was performed at BL5S1 beamline of AichiSR, Japan. During the measurements, temperature of the solution was preciously controlled at 333 K. No energy shift in the XANES spectrum was observed during the measurement, demonstrating that cobalt ion was stably present as divalent cobalt. Whereas, intensity of the white line slightly increased as operation time with keeping intensity of the pre-edge. These results suggest that the divalent cobalt species coordinated with chloride ions was formed in the solution, which might be an active catalytic species for the reaction. EXAFS analysis on the X-ray absorption spectra is currently underway to give further information on the catalytic reactions.