ADSORPTION AND COLLOIDAL PROPERTIES OF RADIOACTIVE ELEMENTS IN TRACE CONCENTRATIONS F.KEPAK The Nuclear Research Institute, Czechoslovak Academy of Sciences, lie2 b y Prague, Czechoslovakia Received September 22, 1970 (Revised Manuscript Received March 11, 1971)
Contents I. Introduction 11. Methods for the Study of Radioactive Elements in Solution A. Centrifugation, Ultracentrifugation, Filtration, and Ultrafiltration B. Ion Exchange and Sorption on Sorbents C. Adsorption on Surfaces D. Self-Diffusion E. Dialysis, Electrophoresis, and Other Methods F. Separation and Application of Radiocolloids 111. Analysis and Evaluation of Results
1.
are used, e.g., ultracentrifugation, ultrafiltration, dialysis, electrophoresis, and self-diffusion. Radioactive sols have been treated in general in the monographs by Whal and Bonner,s Hai~sinsky,~ and Starik.6 In the present review the results of studies dealing with the radionuclide behavior in trace concentrations are reviewed according to the methods used with regard to their adsorption, physical-chemical properties, and methods of their separation. The results of papers are summarized in tables according to applied methods. The individual authors have, however, worked under different conditions : dissimilar electrolytes, ionic strength, another experimental arrangement. The obtained results were not presented in the same way, and therefore their values are not always mutually comparable. The values are given in the tables under conditions when the studied phenomenon became maximally evident, Le., when the formation of the radiocolloid or the adsorption reached the maximum. When the concentrations of trace elements are presented, the fact must be taken into consideration in most cases that some of these radionuclides are always present in extremely low concentrations as nonradioactive ones, as they are introduced into the solutions from chemicals or by means of their elution from the glass walls of the vessels. In the most papers, the results have been presented in peroentage, and for this reason, the tables are arranged from these values. Those results expressed in a different way are discussed in the text. Most authors have plotted their results in graphs, and therefore the majority of the presented values were read from them.
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357 357 359 361 364 365 361 368
introduction
Radioactive elements in aqueous solutions are present in trace or very low concentrations. It is well known that under these conditions phenomena often occur, which do not occur in solutions of electrolytes at common concentrations. For example, the radioactive component disappears from the solution owing to adsorption on walls, or its chemical properties are changed because of conversion from the ionic to the colloidal state. This can happen when the solubility product has been exceeded or as a result of adsorption on foreign particles, or by coprecipitation with other ions, where the radioactive element can acquire properties of the other prevailing microcomponent. These effects are important for the physical-chemical characterization of elements in trace concentration as well as for their separation from solution. It will become clear from the present review that by adjusting the conditions leading to the formation of radiocolloids, better separation can result, and, on the other hand, by neglecting colloid formation a decreasing separation effect of individual methods may result. Even biological processes are affected by the formation of radiocolloids, l z 2 because the rate of diffusion of colloidal particles is lower than that of ions, resulting in a decrease in the rates of reactions. The topic of trace concentrations is connected with adsorption and colloid formation, and hence the study of chemistry of trace radioelements in solution deals mainly with their adsorption on the surfaces of various materials and with the conditions of their transition into the colloidal dispersion. For this purpose some methods current in colloid chemistry
/I.
Methods for the Study of Radioactive Elements in Solution
A. CENTRIFUGATION, ULTRACENTRIFUGATION, FILTRATION, AND ULTRAFILTRATION Results obtained by these methods are given in Table I. The influence of various factors such as ageing time of solutions, change of pH, and the presence of nonaqueous solvents and electrolytes on the formation of radiocolloids was studied (3) A. C. Wahl and N. A. Bonner, “Ispolzovanie radioaktivnosti pir khimicheskikh issledovaniakh” (Russian translation), Izd. Inostr. Lit., Moscow. 1954, pp 116, 118. (4) M. Haiss;msky,“La chimie nucldaire et ses applications,” Masson, Paris.. 1957. D 3.~4. (5) I. E. Starik, “Osnovy radiokhimii,” 2nd ed, Izd. Nauka. Leningrad, 1969, pp 39, 150, 244.
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(1) J. Schubert and E. E. Conn, Nucleonics, 4 (6). 2 (1949). (2) G. K. Schweitzer and M. Jackson,J. Chem. Educ., 29,513 (1952).
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F. Kspllk Table I
Separation of Radionuclides by Centrifugation, Ultracentrifugation, Filtration, and Ultrafiltration Concn, Ma
Radioelement
Trace Trace ~Po, 5 ageing also had no influence on the filtrable fraction. l7 With regard to the number of molecuks, it is possible to form particles of colloidal dimensions from trace elements. Formation of radiocolloids of Ru(IV), Ru"'NO, Ce(III), Pm(III), Eu(III), and others was detected by the self-diffusion method, but it cannot be proved whether the difl'using colloid is formed solely of radioactive microcomponent or of adsorption colloid with other nonradio4,95 active micro~omponents.~ Colloidal properties of Y are explained by its ion-exchange adsorption with particles suspended in solution,7a whereas the formation of colloidal Hgg7 and C O ( I I ) ~is~ explained by adsorption or coprecipitation with foregin particles. Radiotracer zirconium formed colloids in nitric acid with precipitant phenylarsonic acid. 16*The tendency toward colloid formation is related to hydrolyzability and low solubility. 169,170 Radioactive materials are adsorbed on impurities which have no defined nature, and hence the behavior of the radiocolloid tends to be irreproducible.20 With respect to the fact that the presence of various ions and colloidal particles in the solution cannot be excluded, the formation of a radiocolloid is more likely due to heterogeneous nucleation with submicroscopic particles, which pass into the solution from the walls of vessles and from chemicals, and by changing the pH at which precipitation took place. Because of differences in the chemical nature and mutual nonadsorbability, besides colloidal impurities there may exist also radiocolloids formed of radioactive matter only. Pseudocolloids, also can be formed from radionuclides, the hydroxide of which is soluble and does not have colloidal pr0perties.~6 There exist often in parallel more forms of radiocolloids, which result in a more difficult separability, as each kind of such a radiocolloid has its own distribution c o e f f i ~ i e n tso ~ ~ that the attainment of their most effective separation is hindered. (168) M. KyrH, P. Seluck?, and P. Pibtkk, Radiochim. Acta, 6,72 (1966). (169) V . LSpitsyn, R. N. Bernovskaya, and N. I. Popov, Dokl. Akud. Nauk S S S R , 182, 855 (1968). (170) J. Takagi and H. Shimojima,J. Znorg. Nucl. Chem., 27,405 (1965).
