Electrokinetic Removal of Na-24 and Tc-99m Scintillation Liquid Content Absorbed in Soil Type Phaeozem
Authors: Valdovinos Viridiana , Bustos Erika , Monroy Fabiola 
Published: February 2020

The application of radioisotopes in medicine and research generates radioactive waste. A large part of these wastes are composed by scintillation liquid (mixtures of solvents -toluene, xylene-, fluorescent materials and surfactants) contaminated with radioisotopes such as H-3 (12.3 y), C-14 (5730 y), U-238 (4.468x109 y), Th-232 (1.41x1010 y), Tl-204(3.7 y) or Na-22 (2.6 y), etc.. In the 80's, these wastes were absorbed on soil to covert it to a less hazardous form during interim storage. However this treatment was only a temporary solidification strategy and these wastes have to re-treated and conditioned. Therefore, the objective of this work is to assess the electrokinetic removal of Na-24 and Tc-99m contained in liquid scintillation absorbed on soil types Phaeozem.

To define the experimental conditions of the electrokinetic treatment for the radioactive wastes previously described, control samples of liquid scintillation-soil mixtures were prepared with different scintillation liquid (INSTALL GEL ® XF, ULTIMA GOLD ABTM and Ultima Gold XR™) as support electrolyte and constructed the polarization curves, to select the potential with the highest mass transfer. Liquid phase was characterized by Gas Chromatography and Infrared (IR) Spectrometry, solids by IR and electrodes by Scanning Electronic Microscopy (SEM), before and after the application of potential. From polarization curves, the experimental conditions following were chosen in the sodium and technetium removal: working electrodes, meshes and rod of titanium, anode and cathode, Ultima Gold scintillation liquid XRTM:Water (1:1), step current 1 mA for 6 h. Subsequently radioactive control samples were prepared with soil-scintillation liquid Ultima Gold XRTM:Water (1:1), labeled with Na-24 or Tc-99m to determine the removal efficiency of these radioisotopes. Na-24 was chosen, instead of Na-22, by its short half-life (15.03 h) to facilitate handling of samples and reduce the radiation hazards involved in using Na-22 (2.6 y). In the case of Tc-99m, it is not adsorbed in the medium analyzed, and was used as a standard. The Na-24 and Tc-99m were quantified on a Gamma Spectrometer constituted by a Germanium Hyperpure detector connected to a multichannel card, in 140.5 keV and 1368.6 keV for Tc-99m and Na-24 respectively, at a fixed geometry., The liquid and solid phases were analyzed, as well as the electrodes, by the same analytical techniques mentioned previously, once both radiotracers decayed.

From six supports electrolytes studied, three have not a diffusion current stable and three show a diffusion current plateau in 0.4 mA, 0.6 mA and 1 mA. It was chosen the support electrolyte with the higher current passage (1mA): scintillation liquid Ultima Gold XRTM:Water (1:1). According to IR analysis, soils not present any deterioration and in the liquid phase the amount of water increases, as the applied potential increases. The liquid phase chromatograms indicate that some compounds were modified in 5-7 V. The kinetic study of Tc-99m shows that 15% of Tc-99m is removed each hour near the anode, this value increasing to 20% in 4 hours. In the case of Na-24, a treatment for 4 h produces a diminution around 7% of the functional group -C=N- in the liquid phase, which corresponds to 2,5-Diphenyl-Oxazole and 2-Amino-5-choropyrimidine.