402 



RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



however, is not based on ion exchange. A 1-cm-diameter by 25-cm col- 

 umn is packed with granular cellulose acetate carrying a carbon tetrachlo- 

 ride solution of dithizone. Metals that form dithizonates would be 

 expected to be retained from the water as it passes through the column. 

 Studies have shown that lead, zinc, manganese, cadmium, cobalt, and 

 copper are retained under the conditions described. Removal from the 

 column and partial separation of these elements were achieved by such 

 solvents as hydrochloric acid and ammonia. 



Separation of Materials. Early in the large-scale atomic-energy 

 developments (Plutonium Project) there was an urgent need for methods 

 of separation, concentration, and purification of the small but highly 

 radioactive masses of fission products. These substances included the 

 rare earths, notoriously difficult to separate. Ion-exchange methods 

 made an outstanding contribution to this problem, and references (2, 3, 5) 

 may be consulted for reviews of historical and procedural details. A 

 major factor was the use of organic acid complexing agents in such a way 

 as to enhance the differences in adsorbability normally existing among the 

 inorganic cations to be separated. Some of the complexing agents 

 employed were oxalic, citric, tartaric, and lactic acids. 



In principle, the fission-product mixture is percolated through a sulfonic 

 cation exchanger, and a water wash removes the anions. Treatment 

 with 0.05 M oxalic acid forms complexes with and removes selectively the 

 tetravalent ions, such as zirconium and niobium, which tend to form 

 radiocolloids. Elution with citrate solution of pH 3 removes the tri- 

 valent species (the rare-earth group). Elution with citrate solution of 

 pH 5 then removes the divalent and monovalent cations and the alkaline 

 earths and metals. The individual ions of each group can be subse- 

 quently separated by passage through another column and elution chro- 



Table 9-4. Summary of Separations of Alkaline Metals, Alkaline 



Earths, and Rare Earths 



Ions separated 



Na, K, Rb, Cs 



Ba, Sr 



Ra, Ba 



Pb, Ba 



Nd, Pr, Ce 



Ce, La 



Ac, La 



Nd, Sm, Pr 



Lu, Yb, Tm, Er, Ho, Y 



Resin 



Dowex 50 



Colloidal Dowex 50 

 Amberlite IR-1 

 Dowex 50 

 Dowex 50 



Dowox 50 



Elutriant 



0.15 A^ HCl; 0.3 N HCl 



Citrate, pH 5 



0.5 M citrate, pH 7.5 to 8 



5% citrate 



5% citrate, pH 2.9 



Citrate 



0.5% citrate, pH 3.9 



4.75% citrate, pH 3.2 to 3.4 



(Compiled from Frederick C. Nachod, ed., "Ion Exchange," Academic Press, Inc., 

 New York, 1949.) 



