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

P.27
Ca(P04)3(0H) + 4H + - 5Ca 2+ + 3HPO4 + H20 (P.4)
K 3 = 10- 12 . 1 7 x 107051.0[1/T-1/298]
Ca 5 (PO 4) 3 (OH) + 7H - 5Ca 2+ + 3H 2PO0 + H 20 (P.5)
K 4 = 10+9. 4 6 x 1 07698.0[1/T-1/298]
If we use E PO 4 = 10-6m (%0.1 ppm) and aF- = 1.6 x 10-5m (,0.3 ppm), typical values for
ground waters, we obtain the following values for the activity of calcium in equilibrium
with the apatites:
Fluor-apatite
aCa2+
pH/T 250C 75 0 C
6 1.04 x 10-5m 5.81 x 10 6 m
8 1.24 x 10-7m 7.95 x 10 8 m
Hydroxy-apatite
aCa 2+
pH/T 25 0 C 75 0 C
6 1.23 x 10- 3 m 2.24 x 10- 4 m
8 5.83 x 10- 6 m 1.22 x 10- 6 m
In ground waters, aCa 2 + is typically u10-3m (Rai and Lindsay 1975). Therefore in alka-
line environments we expect both apatites to be stable at temperatures from 25 0 C to 100 0 C.
However, in acid environments hydroxy-apatite will not be stable, while fluor-apatite will
be somewhat stable, more so at higher temperatures. Chien (1977) has also shown that the
carbonate substitution may increase the equilibrium dissolution of apatite.
P.2.4.2 Monazite-Xenotime Family--(Ce,La)PO4-YPO 4
This family is one of the most promising for the disposal of nuclear wastes. Both
monazite, (Ce,La)PO 4 and xenotime, YPO 4, as well as their hydrates, rhabdophanite,
(Ce,Ca)PO4.H 20, and churchite, YPO4 H 20, are simple orthophosphates. They are always crys-
talline even though they may contain significant amounts of U and Th. Monazite is isostruc-
tural with huttonite, ThSiO4, and xenotime is isostructural with zircon (ZrSiO4) and
coffinite (USiO 4 ). Monazite can acquire quite a high content of thorium (28%) by the sub-
stitution Th 4 + Si 4 + Ce 3 + p5+ (i.e., ThSiO 4-CePO 4 solid solution). Monazite is a
selective cerium mineral (i.e., large-radius rare earths). It has lesser amounts of uranium
(up to 4%) (Deer et al. 1962, Vlasov 1966). It is sparingly soluble in acids and is very
stable under weathering conditions, often collecting in placers formed from the disintegra-
tion of monazite-containing granites. It occurs as an accessory in granites and granitic
pegmatites and is abundant in metamorphic deposits (Vlasov 1966). It occurs as a detrital
mineral in sands from weathering of granites and gneisses.