Management of Commercially Generated Radioactive Waste - U.S. ...

P.3
One can usually establish only an upper limit to the concentration of a given element
by the use of thermodynamics. Assuming equilibrium between minerals and solution, the con-
centration of any particular nuclear waste element will then be governed by the solubility
of the minerals containing it.
Before discussing the thermodynamic approach to stability, a brief review of the gen-
eral qualitative work on weathering stability in the literature is presented. Soil geochem-
ists have set up a qualitative scale of the different inherent tendencies of minerals to
alter by weathering processes. The weathering rate depends on the structure and composition
of the minerals, as well as the weathering environment. Goldich (1938) formulated such a
weathering stability series for the major elements. He found that the major elements are
removed from rocks and minerals in the order:
+2 + ++ +
Ca > Na > Mg > K > SiO 2 > Fe203 > Al 20 3.
Loughnan (1969) gives a similar result (see Table P.1.1).
Much less is known about the relative mobilities of the trace elements (lanthanides,
actinides, and others). Jackson and his colleagues (Jackson et al. 1948, 1952, Jackson and
Sherman 1953) set up a weathering sequence of clay-size minerals in soils and sedimentary
deposits (see Table P.1.2). Pettijohn (1941) compared the frequency of occurrence of each
species in recent and older sediments and established an order of persistence, which is in
agreement with the Goldich series (see Table P.1.3).
TABLE P.1.1 Mobilities of the Common Cations
t 1. Ca++, Mg+, Na+--readily lost under leaching conditions.
~ 2. K --readily lost under leaching conditions but rate may be retarded through
o Es fixation in the illite structure.
S£ 3. Fe++--rate of loss dependent on the redox potential and degree of leaching.
>." 4+
a 4. Si --slowly lost under leaching conditions.
0 G 5. Ti4--may show limited mobility if released from the parent mineral as
i35 Ti(OH) 4 ; if TiO2 forms, immobile.
Ub 6. Fe 3 --immobile under oxidizing conditions.
S4- 7. A3+--immobile in the pH range of 4.9 to 9.5.
Although still poorly understood, structure must play an important part in the accessi-
bility of waters to the soluble cations. Thus, orthosilicates, e.g., olivine, weather much
faster than framework silicates, e.g., feldspars and quartz. However, zircon, also an
orthosilicate, is highly resistant to weathering, which indicates that resistance to weath-
ering cannot be based solely on such a simple structural division of the silicates.
The qualitative lists of minerals in Tables P.1.2 and P.1.3 should be quantitatively
understood in terms of both thermodynamics (i.e., solubility data) and kinetics (i.e.,
leaching rates). The solubility and hence the thermodynamic stability of a particular