Radiocolloid behavior

Carrier addition is widely used, not only for yield determination, but because the added mass avoids problems of radiocolloidal behavior. Other advantages of added carrier are the ability to use precipitation for radionuclide separation and purification, and avoidance of unintended coprecipitation during scavenging. When carrier is not added because no stable isotope is available or the counting source must be very thin, radionuclide deposition on container walls or on suspended solids must be avoided by applying the techniques discussed in Section 4.4, notably use of nonisotopic carrier or low pH. 

Carriers are added to radionuchde solutions, as discussed in Section 4.7, to perform precipitation separations, avoid radiocolloidal behavior, and to eliminate the need for quantitative recovery by allowing the analyst to calculate the fractional chemical recovery ( yield ). A radioactive tracer (see Section 4.7.3) can be added to the sample in addition to or instead of the carrier to measure yield. Tracer addition may make yield determination easier than carrier addition, or may be necessary if no carrier is available. 

A common problem in water samples is the radiocolloidal behavior observed in extremely low concentration radionuclide samples, which represent the majority of all radionuclide samples. In brief, in deionized water, many radionuclides are sorbed on container walls and suspended material (see Section 4.4). This effect is reduced at higher salt concentration and increased acidity, hence the EPA requires acidification to pH 2 or less with HNO3 at sample collection for analyzing water from public supplies. 

It has been well established that plutonium hydrolysis products exhibit colloidal behavior (6, 22, 27, 31) and may sorb onto minerals and other surfaces as radiocolloids. It is often difficult to determine whether a radiocolloid is a true colloid or a pseudocolloid formed by sorption of the radioactive species onto other colloidal impurities in the solution (35). In some instances both types are present in the same solution (14). 

The term radiocolloidal was applied in the early days of radiochemistry to describe certain behavior patterns of radionuclides in aqueous solution that did not conform to expectations based on the normally higher concentrations of the chemical forms assigned to them. Because many of these deviations were related to failure to pass through filters or to remain uniformly distributed in solution, colloidal behavior was inferred. This behavior was observed to affect in different ways many of the steps in the radioanalytical chemistry process itemized above, with the potential of invalidating analytical results because the radioisotope did not consistently follow the known path of its stable isotopes. 

Samples submitted for analysis that contain radionuclides at extremely low concentration present a challenge to the analyst because these radioactive atoms unexpectedly can deposit on surfaces. Such behavior has been widely observed and was termed radiocolloidal (Hahn 1936). Radionuclides at trace levels are lost from solution by sorption on container walls (Korenman 1968), suspended solids (Wahl and Bonner 1951), immersed glass slides (Eichholz et al. 1965), metal foils (Belloni et al. 1959), and Alters (Granstrom and Kahn 1955). Filters of cellulose, cotton wool, and glass wool retain various radionuclides at trace levels. 

Investigations may be carried out on the tracer level, where solutions are handled in ordinary-sized laboratory equipment, but where the substance studied is present in extremely low concentrations. Concentrations of the radioactive species of the order of 10 m or much less are not unusual in tracer work with radioactive nuclides. A much larger amount of a suitably chosen non-radioactive host or carrier is subjected to chemical manipulation, and the behavior of the radioactive species (as monitored by its radioactivity) is determined relative to the carrier. Thus the solubility of an actinide compound can be judged by whether the radioactive ion is carried by a precipitate formed by the non-radioactive carrier. Interpretation of such studies is made difficult by the formation of radiocolloids, and by adsorption on glass surfaces or precipitates. Tracer studies provide information on the oxidation states of ions and complex-ion formation, and are used in the development of liquid-liquid solvent extraction and chromatographic separation procedures. Tracer techniques are not applicable to solid-state and spectroscopic studies. Despite the difficulties inherent in tracer experiments, these methods continue to be used with the heaviest actinide and transactinide elements, where only a few to a few score atoms may be available [11]. 

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