Speaker
Description
Objective and method
Actinium-225 (225Ac) is a promising radionuclide for targeted radionuclide therapy (TRNT), but current production methods cannot meet market demand. Innovative selective transmutation pathways have been researched to increase the market supply, particularly those exploiting the 226Ra photonuclear reaction. A novel solvent extraction (SX) methodology based on intensified milliflow reactors has been developed to provide a continuous and reliable means of separating actinium from the radium target. Particularly, the scrubbing step in the process was studied in a single-stage milliflow reactor using $5\ M$ HNO3 solution as the scrubbing agent solution to remove the co-extracted radium. The loaded organic phase was prepared by testing two loading strategies for the short-lived 224Ra radionuclide, namely Loading-by-Extraction (LBE) with a high phase ratio extraction and Loading-by-Dissolution (LBD) achieved by contacting the solvent with dried radium nitrate. The aim is to maximize the loading of radium in the organic solution, in order to precisely measure the activity left in the scrubbed solvent. The scrubbing tests were carried in parallel for the two radium-loaded organic phases using a single-stage milliflow reactor setup, comprising of two syringe pumps, tubing with internal diameter of $1\ mm$ as the reactor and a 3D-printed in-line phase separator using Millipore$^{\circledR}$ membranes. The samples were collected at steady state after three residence times and analyzed by HPGe gammaspectrometry (i.e. High-Purity Germanium gamma spectrometry) to detect the activity (Bq) of radium in the solvent after one scrubbing stage, evaluated by means of the scrubbing efficiency ($\% E_{scrubbing}$ eq 1), where $\left[ A \right] _{loaded\ solvent}$ is the activity in the solvent after scrubbing and $\left[ A \right] _{extract}$ is the activity in the solvent before the scrubbing step.
Equation 1: $ \%E_{scrubbing}=\left( 1-\frac{\left[ A \right] _{loaded\ solvent}}{\left[ A \right]_{extract}} \right) 100 $
Results
The solvent was loaded with radium in both cases, the LBE strategy yielded a feed with $490 \pm 26$ Bq/mL, on the other hand LBS yield amounted to only $320 \pm 20$ Bq/mL. The speciation of the radium present in the loaded solvent could be affected by the loading technique, but there was no significant difference in scrubbing efficiency. Nonetheless, the LBE method was preferred since the mass transfer of radium occurred as liquid-liquid inter-phase complexation, opposite to a less representative and efficient direct organic dissolution. The best performance was achieved by setting the flowrate of each phase at $0.5\ mL/min$ and using a $3000\ mm$ reactor, resulting in a residence time of $170\ s$, for the LBE solvent yielding $ 96.6 \% \pm 7.4 \% $ scrubbing efficiency. A second experiment was performed with the longest residence time reactor repeated in a cross-flow three-stage scrubbing protocol, using LBE solvent and fresh scrubbing agent solution. The scrubbing efficiencies were $95.9 \% \pm 7.5 \%$, $79.2 \% \pm 27.5 \%$, and $4.3 \% \pm 8 \%$, for the first, second and third stages, respectively. The overall scrubbing efficiency of $99.20 \% \pm 7.6 \%$. Such evidence indicates that the three-stage scrubbing achieved close to quantitative radium removal from the solvent. Future work should focus on obtaining higher activities of 224Ra to reduce the uncertainty of the measurement.