Optimization of Cation Exchange for the Separation of Actinium-225 from Radioactive Thorium, Radium-223 and Other Metals

Actinium-225 (225Ac) can be produced with a linear accelerator by proton irradiation of a thorium (Th) target, but the Th also underdoes fission and produces 400 other radioisotopes. No research exists on optimization of the cation step for the purification. The research herein examines the optimization of the cation exchange step for the purification of 225Ac. The following variables were tested: pH of load solution (1.5–4.6); rinse steps with various concentrations of HCl, HNO3, H2SO4, and combinations of HCl and HNO3; various thorium chelators to block retention; MP50 and AG50 resins; and retention of 20–45 elements with different rinse sequences. The research indicated that HCl removes more isotopes earlier than HNO3, but that some elements, such as barium and radium, could be eluted with ≥2.5 M HNO3. The optimal pH of the load solution was 1.5–2.0, and the optimized rinse sequence was five bed volumes (BV) of 1 M citric acid pH 2.0, 3 BV of water, 3 BV of 2 M HNO3, 6 BV of 2.5 M HNO3 and 20 BV of 6 M HNO3. The sequence recovered >90% of 225Ac with minimal 223Ra and thorium present.


Citric acid studies
Purpose: Determine optial pH of Load solution. Figure s1. Retention of La on cation exchange resin at different pH values in citric acid.
• Results: Studies with La only solutions indicate a pH from 1.5-2.5 is optimal to retain La, and Load pH values greater than or equal to 3 result in less retention on AG50 resin.
Purpose: Determine optial pH of Load solution with mixed metals and examine elution profile with HCl.  • Results: The pH of the load solution was best at pH values of 1.5 and 2, and pH 2.5 a small quantity; 2.3% of La was eluted. Many elements were eluted in the 1.5 M HCl rinse, and La was present in the 4 M HCl rinse.
Purpose: Evaluate various rinse solutions. Examined the retention of different elements with varying concentrations of HCl, HNO3, and H2SO4 in the rinse steps. A load solution of citric acid at pH 2 was used for subsequent studies.

Tartaric acid testing
Similar testing was performed with tartaric acid. The following graphs summarize the data. • Results: Studies with La only solutions indicate a pH from 1.5-2.5 is optimal to retain La, and Load pH values greater than or equal to 3 result in less retention on AG50 resin.                      • Results: The MP50 resin performed differently then the AG50 resin. Further testing will use AG50.

M HNO3
MP50 Tartaric acid AG50 Tartaric acid AG50 trial 2 Tartaric acid AG50 Citric acid AG50 Citric acid trial 2    • Results: The elution profile of 2 & 3 M HCl followed by 2.5 M HNO3 was able to remove many of the elements that were used in the study while La was retained on the AG50 resin. Performance of Citric and Tartaric acid were similar. The water rinse steps added nothing to the elution profile and can be eliminated. Subsequent studies with 6 M HNO3 were performed and >10 bed volumes were needed to elute La. Studies examining the elution profile 2.5 M HNO3 followed by 2 & 3 M HCl have been performed and the data is be graphed.

Citric acid combination nitric then HCl rinse sequence
Comparison of rinse sequence for total removal of metals   • Results: The elution profile of 2.5 M HNO3 followed by 3 M HCl were able to remove many of the elements that were used in the study while La was retained on the AG50 resin. The water rinse step did elute additional metals, but in less than 20%. The HCl rinses eluted more metals in higher amounts than the nitric rinse step.
Comparison of rinse sequence for total removal of metals

combinations or rinses
HCl then HNO3 HCl then HNO3 trial 2 HNO3 then HCl • Results: The elution profile of 2 & 3 M HCl followed by 2.5 M HNO3 and 2.5 M HNO3 followed by 3 M HCl were able to remove many of the elements that were used in the study while La was retained on the AG50 resin. Use the elution sequence 2 & 3 M HCl followed by 2.5 M HNO3 eluted more Lu and Ba then the elution sequence 2.5 M HNO3 followed by 3 M HCl. The water rinse steps added nothing to the elution profile and will be eliminated.

Studies of Rh and La on cation ion exchange resin
To a vial was added 50 mL of 1 M citric acid pH 2, 50 μg of La the pH was adjusted to 2 with ammonium hydroxide and loaded onto a one mL AG 50W-X8 column. The column was rinsed with 3 bed volumes of 2 M HCl, 3 BV of 3 M HCl, 3BV of 18MΩ water, 3 BV of 2.5 M HNO3, 3 BV of 18 MΩ water, and 10 BV of 10 M HNO3. The study was repeated with 50 μg Rh and La and the elution profile was changed to 3 BV of 2 M HCl, 3BV of 3 M HCl, 3 BV of 18 MΩ water, 3 BV of 2.5 M HNO3, 10 BV of 3.5 M HCl, 10 BV of 4 M HCl, 10 BV of 5 M HNO3, and 10 BV of 6 M HNO3.                                                                      Table S1. Percent of various elements in the various steps of the separation using the rinse sequence: 3 ml of 1 M citric acid pH 2, 3 ml of water, 2X 3 ml of 2.5 M HCl, 3 ml of water, 2x 3 ml of 2.5 M nitric acid, 2x10 ml of 6 M nitric acid. Bolded values are the highest value for the elution of the element, and empty boxes represent samples that were below 1% or not detected. NM not measured. Elemental Analysis Methods