Clay radionuclide sorption studies

Ion-exchange models are commonly used to describe radionuclide sorption onto the fixed-charged sites of materials like clays. Ion exchange will be strongly affected by competition with monovalent and divalent ions such as Na" " and Ca, whereas it will be less dependent on pH over the compositional ranges common for natural waters. Many studies of strontium and caesium sorption by aluminosilicates (e.g., Wahlberg and Fishman, 1962 Tamura, 1972) have been carried out within the framework of ion-exchange theory. Early mechanistic smdies... 

Studies of the sorption of natural radionuclides from groundwater have established that minor minerals, such as clays coating the fractures in crystalline rock and Fe- and Mn-oxyhydroxides, are important sinks for radionuclides. For example, investigations carried out at the Aspo facility (Smellie Karlsson 1999) showed qualitatively that rare earth elements (REEs), Sc, Th, U, Ra, and Ba have been scavenged by Fe-oxyhydroxide and calcite precipitates,... 

Radionuclide transport in natural waters is strongly dependent on sorption, desorption, dissolution, and precipitation processes. The first two sections discuss laboratory investigations of these processes. Descriptions of sorption and desorption behavior of important radionuclides under a wide range of environmental conditions are presented in the first section. Among the sorbents studied are basalt interbed solids, granites, clays, sediments, hydrous oxides, and pure minerals. Effects of redox conditions, groundwater composition and pH on sorption reactions are described. 

Another relevant general review summarizes the knowledge on the behavior of series radionuclides in soils and plants and is intended to provide a comprehensive source of information for environmental impact studies (Mitchell et al. 2013). The summary of the data on plant to soil concentration ratios that depends on the specific soil and type of plant and the distribution of uranium within the parts of the plant is especially important. The dependence of the sorption of dissolved uranium compounds on the type of soil (like the clay content) and the parameters mentioned earlier (pH, complex forming agents, anions, presence of iron, organic matter, etc.), based mainly on studies of the (distribution factor) of spiked soil samples, is discussed. It is noted that in general the uranium concentration in plants is several orders of magnitude lower than in soil, but some plants may efficiently absorb uranium and translocation within the plant is quite common (Mitchell et al. 2013). These features, and especially the soil-to-plant transfer factors, will be discussed in Section 3.4 that deals with the uranium content in plants and soil and the relation between them. 

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