Similar to that of the atmosphere and hydrosphere, the radioactivity of the litho­sphere originates from both natural and artificial sources. The main sources of natu­ral radioactivity are rocks; their radioactivity determines the radioactivity of the soils formed on the rocks. The radioactivity of rocks depends on their mineral and chemical composition and can be quite different. As a result, the radioactivity also depends on the geographic position. The mean radioactivity is higher in the Northern Hemisphere than the Southern one, and it is also higher in American continent than in Europe.

The most important natural radionuclides in rocks and soils are 40K and the members of the radioactive decay series. Thorium is accumulated in monazite because it has similar chemical properties as the lantanoid elements, which are present in significant quantities in monazite. The mean radioactivities of several isotopes present in rocks are listed in Table 13.2. The standard deviations are rather high due to the wide variety of rocks, which were used to measure the activities, and the varying activities of which lead to high uncertainty in the mean values.

As seen in Table 13.2, the radioactivities of 214Bi and 214Po, as well as 212Bi,

Pb, and Tl, are approximately the same, showing that they are in radioactive

214 214 222

equilibrium. Bi and Po are the daughter nuclides of Rn, which is the daughter nuclide of 226Ra (see Figure 4.4). However, the radioactivity of 226Ra is much higher, proving the emission of the intermediate member, 222Rn, into the atmosphere. The activities of the daughter nuclides of 232Th are approximately the same. In this case, a radioactive equilibrium exists because the half-life of 220Rn (55 s) is too short to escape from the soil.

It is important to note that besides the radioactive isotopes listed in Table 13.2, Pb and Po, the members of the U series, are also important because of the long half-life of 210Pb (21.6 years).

As seen in Section 9.3.2.2, the migration of the radioactive isotopes in the geo­logical formations (as porous solids), including the isotopes present in nuclear waste, is determined by hydrological processes. The migration rate of water pro­vides the upper limit for the migration rate of the water-soluble radioactive nuclide. This actual rate may be significantly lower when the radioactive isotopes can be sorbed on the surfaces of rocks and soils. The sorption is mostly influenced by the chemical species (mainly the charge) of the radioactive isotopes. On the basis of the chemical forms characteristic in geological systems, the radioactive isotopes can be classified as follows:

1. Cations (e. g., 134’137Cs+, 41Ca21, 90Sr21, 54Mn21, 55Fe31, 58’60Co2+, and 59’63Ni2+).

2. Uranium and transuranium elements (U, Np, Pu, and Am isotopes), basically (complex) cations or anions (e. g., 99mTc isotopes as pertechnetate TcO^, 14C isotope as carbonate CO|2, 36Cl", and 129I").

3. Neutral species (e. g., 3H isotope as water H2O, metallic 110mAg).

Migration takes place in the following geological formations: clay rocks (espe­cially bentonite), granitic rocks, soils, oxides, and other minerals (carbonates, sul­fates, etc.). Since the surface charge of the rocks and soil is usually negative under usual geological conditions (pH, redox conditions), cations usually adsorb on the geological formations, while anions do not. Cesium, then, can occupy a space in the crystal lattices; thus, the sorption becomes irreversible. Other cations adsorb reversibly. In the case of cations of transition metals and transuranium elements, the adsorption is affected by their hydrolytic products. Transuranium elements can form colloids. Cations, except for cesium, readily form stable complexes that increase migration rate. In addition, precipitation, redox processes, and microbial activity can also influence the sorption and, as a result, the migration rate.

Of course, the different migration rate of cations and anions cannot result in the unbalancing of the electric charges. The faster migration of anion is followed by the migration of inactive cations dissolved from the geological formations.

The neutral species are very different behavior. Two extreme cases are tritiated water migrating with natural water (the isotope effect, described in Chapter 3, can be ignored) and Ag-110m reduced to metallic silver, the migration rate of which is practically zero.

Under equilibrium conditions, the sorption of the radioactive isotopes can be characterized by the distribution coefficient. This is the ratio of the sorbed quantity (mol/g) and the equilibrium concentration of the solution (mol/dm3). In Table 13.3,

Table 13.3 Distribution Coefficients of Radioactive Ions on Rocks or Minerals (g/dm3) Rock/mineral 137Cs 45Ca 85Sr 226Ra 60Co 14C 99mTc 131I

the distribution coeffecients of radioactive ions on different rocks are listed. The higher values of the distribution coefficients mean the stronger sorption of the iso­tope on the given rock. The data in Table 13.3 illustrate well the differences in the sorption of cationic and anionic radioactive isotopes.