Evaluation of Ion-Exchange Resins and Electrospun Membranes for Ac-225 Purification

Targeted alpha therapy (TAT) is a cancer treatment strategy taking advantage of radioactive decay products to attack tumors. Though promising, progression to clinical scales is hampered by scarcity of suitable radionuclides, especially actinium-225. Production of Ac-225 involves thorium irradiation with high-energy protons that also produce undesired byproducts like lanthanum, neodymium, promethium, and barium. Current standard purification methods include a series of acid-based extraction and cation-exchange chromatography using columns packed with ligand-functionalized resins, but diffusion transport limits performance to low flow rates and high pressures. Membrane chromatography is an alternative method, using the same ligand-metal adsorption chemistry as resin-based ion-exchange. A key advantage to membranes is relatively fast convection - meaning mass transport does not limit process performance. Membrane chromatography methods are still largely in development, but show potential to increase the scale of radionuclide production. Candidate materials for membranes must be evaluated for their physical (permeability, mechanical strength) and chemical (adsorption, solvent and reaction compatibility) properties for implementation at any scale.

In this work, blends of poly(vinylbenzyl chloride) and polystyrene in electrospun membranes are evaluated for stability in chemically relevant solvents, porosity, and pure-water flux. With these initial characterizations, commercial resins are evaluated for their adsorption performance to benchmark sorption in novel DGA membrane adsorbers. Performance at 4M and 6M nitric acid acid solutions provide valuable insight to design conditions for future process elution and retention steps. Equilibrium of adsorbed- and solution-phase metals is analyzed by ICP-OES and fit to a Langmuir isotherm model to determine maximum surface adsorption and association constant parameters. Novel membrane adsorbers show lower lanthanum capacity than standard resins, but slightly higher association. Work remains to explore ligand functionalization chemistry of membranes and to evaluate their performance regarding multi-metal and dynamic adsorption, post-functionalization permeability. With all the relevant chemistries characterized, a foundation may be laid to develop further materials and processes to bring Ac-225 to clinically practical scales.