Interim Report - Hanford Site

3.5.1 Interlayer Cation Release Rates
Contrary to uranium and phosphorus release rates, which were shown to increase as a function of pH
and also with increasing temperature (Figure 35), release of sodium is shown here to be independent of
pH and temperature. Release of sodium is ~9,800 times faster than uranium at the lower pH values of 7
and 8. As pH increases, the difference in the release rate of sodium relative to uranium is significantly
less; sodium release is only ~7 times greater than uranium at pH 10. Moreover, release rates for calcium
from GHR display similar characteristics to sodium release from Na-autunite, in that both are independent
of temperature and pH (Figure 37). Comparing release of calcium from GHR to sodium release from Naautunite
illustrates that the rates are identical within experimental error (Figure 37).
log 10 So dium Rate (g m -2 d -1 )
0
-1
-2
-3
Na-Autunite
0.01 M TRIS, 5 o C
-4 0.01 M TRIS, 23 o C
0.01 M TRIS, 40 o C
0.01 M TRIS, 70 o C
-5
5 6 7 8 9 10 11
pH(T)
log 10 Calcium Ra te (g m -2 d -1 )
0
-1
-2
-3
-4 5 o C
23 o C
-5
40 o C
70 o C
-6
6 7 8 9 10 11
pH(T)
GHR Autunite
-100 + 200 mesh
0.01 M TRIS Buffer
Figure 37. A) log 10 Sodium Release Rate as a Function of Temperature-Corre cted pH for Na-Autunite
in 0.01 M THAM solution, and B) log 10 Calcium Release Rate as a Function of
Temperature-Corrected pH for GHR in 0.01 M THAM Solution
Release of interlayer cations (i.e., Na + or Ca 2+ ) from minerals is generally subject to two separate
reactions: ma trix dissolution and alkali-hydrogen exchange. Based on the saturatio n state of the s ystem,
one or both of these mecha nisms may contribute to release of interlayer cations from the structure. For
exam ple, when the system is near saturation, the activities of dissolved species near and/or in contact with
the solid phase increase, thereby resulting in a decrease in the matrix diss olution rate. Concurrently, the
chemical potential difference between autunite and solution will be the driving force for cation diffusion.
Concentrations of both Na + and Ca 2+ appear to be constant
across the range of pH values, but this is
misleading. Dissolution of the autunite matrix will also contribute to the concentration of dissolved
cations in solution; therefore, two distinct mechanisms, ion exchange and matrix dissolution, account for
Na + or Ca 2+ release.
3.5.2 Structural Dissolution
SPFT experiments suggested the dissolution of autunite occurs via attack and removal of the uranium
polyhedra. To provide a more thorough understanding of the autunite dissolution mechanism, select
SPFT experiments were conducted at 40°C in D 2 O-based solutions. The test solution was a 0.1 M
3.37