Photodegradation of the radionuclide complexants

SSCI-VIDE+CARE+CGU:FDAInternational audienceThe degradation and mineralization of radionuclide complexants in nuclear liquid waste was studied by using different Advanced Oxidation Processes (AOPs). Three complexants of radionuclide were chosen for this study, tributylphosphate (TBP), dibutylphosphate (DBP) and ethylenediaminetetraacetic acid (EDTA). The efficiency of photocatalytic methods based on TiO2 (Evonik P-25) as means of degrading, mineralizing, TBP, DBP and EDTA were studied under UV-A and UV-C irradiation. In the case of EDTA, we also compared the efficiency of the photocatalytic process TiO2/UV-C and of the photolysis of H2O2 (H2O2 /UV-C) of EDTA alone and complexes with cobalt as radioelement and studied the recovery of the cobalt. In addition to HPLC, GC and TOC analysis allowing to determine the disappearance and the mineralization rates and the intermediate products formed, 31P NMR was used to follow the fate of phosphorus atom. Our results show that DBP is an intermediate product of the degradation of TBP and that the same types of carboxylic acids are formed during the degradation of TBP and DBP compounds. We also show that the kinetics of the mineralization process is described in both cases by the Langmuir-Hinshelwood model, with identical rate constants. The experiments realized with 59Co-EDTA allowed to highlight the efficiency of the UV/TiO2 and UV/H2O2 processes and to optimize them. This degradation is accompanied by an important mineralization of carbon and nitrogen initially present in EDTA and the intermediate formation of smaller molecules such as acetic acid and oxalic acids. Once separated from organic ligands, cobalt can afterwards be trapped in mineral materials in view of its storage. The efficiency of the UV/TiO2 and UV/H2O2 processes was found to be similar, provided H2O2 was added continuously during the UV/H2O2 process