Purification of Actinium-225 from Thorium via Selective Precipitation

Numerous promising cancer treatments currently in clinical trials rely on the production and purification of actinium-225 (225Ac), an actinide with alpha emissions that can kill cancer cells via targeted alpha therapy. To enable ongoing and future studies, and to support anticipated future demand, it is necessary to increase the supply of 225Ac. High-energy proton irradiation of thorium metal (Th0(s)) is one of the leading production methods of 225Ac. This process requires the chemical separation of microscopic amounts (μg) of 225Ac from large quantities (>10 g) of thorium. Current methods to accomplish this thorium removal step can be slow, tedious, generate large quantities of radioactive liquid waste, and require very strict control of the processing conditions. To improve this separation, we investigated the ability of four nitrate salts (NH4NO3, KNO3, RbNO3, and CsNO3) to act as selective Th4+ (aq) precipitation agents in the presence of 225Ac3+(aq) in aqueous nitric acid to allow for their separation through a simple filtration. First, we used an automated separations platform to screen the ability of these nitrate salts to precipitate Th4+. We found the Th4+ precipitation yields and amount of precipitating agent needed to maximize this yield were dependent on the identity of the precipitating agent cation. Separation studies with 225Ac3+(aq) and subsequent down-selection of the most promising Th4+ precipitating agents and conditions enabled us to develop its effective selective precipitation. We demonstrated that the separation was compatible with Th0(s) quantities that can produce medically relevant amounts of 225Ac. We observed 99.9% of Th4+(aq) could be removed via precipitation with KNO3(s) in less than two hours in the presence of co-produced isotopes. Meanwhile, other experiments demonstrated that the 225Ac3+(aq) recovery was > 97% at 1–10 g Th0(s) scale.