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

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P.25<br />

complex carbonates <strong>of</strong> the alkali metals, the alkaline earth metals, and the transition metals.<br />

Most carbonates are sensitive to pH and dissolve easily in low pH solutions.<br />

Exceptions to the general instability <strong>of</strong> carbonate minerals are the fluorocarbonate<br />

compounds <strong>of</strong> the rare earths. These are:<br />

Bastnaesite (Ce,La)CO 3F<br />

Parisite Ca(Ce,La) 2 (CO 3 ) 3F 2<br />

Cordylite Ba(Ce,La) 2 (CO3) 3F 2<br />

Synchisite Ca(Ce,La)(CO3) 2F.<br />

Bastnaesite and parisite are relatively insoluble even in low pH solutions at ambient tem-<br />

peratures. None are insoluble in hot, low pH solutions. The rare-earth fluorocarbonates<br />

could act as hosts for rare-earth elements in neutral or alkaline repository rocks.<br />

P.2.3.2 Sulfate Minerals<br />

The number <strong>of</strong> sulfate minerals numbers several hundred but nearly all are soluble in<br />

water or are otherwise unstable. Two exceptions <strong>of</strong> interest are barite, BaSO 4 , and celes-<br />

tine, SrSO 4 . The solubility <strong>of</strong> barite in cold water is only 2.2 ppm while the solubility <strong>of</strong><br />

celestine is 113 ppm. There is a complete solid solution between barite and celestine<br />

although intermediate compositions are not found in nature.<br />

Use <strong>of</strong> barite and celestine as hosts for 9Sr would be <strong>of</strong> value in a bedded anhydrite<br />

repository (anhydrite = CaSO 4 ) because <strong>of</strong> the chemical compatiblity.<br />

P.2.4 Phosphate Minerals<br />

Natural phosphate minerals are all orthophosphates, the major one being fluorapatite.<br />

The phosphate-containing minerals include a subset, that seems particularly suited to the<br />

disposal 6f nuclear waste elements: the apatite family and the monazite-xenotime family.<br />

Since in nature phosphorus will exist in only one valence state (+5) (for example,<br />

H2PO3 > H 2PO only when f0 2 < 10 -10 1 at 250 0 C), the distribution and stability <strong>of</strong> its<br />

species in solution will be Eh-independent. On the other hand, the dominant phosphorus species<br />

in solution will be strongly dependent on pH and on possible complexing cations, since<br />

PO, HPO4 and H 2PO form strong complexes [e.g. with uranium (Langmuir 1978)]. The reaction<br />

H 2PO4 - HPO4 + H +<br />

has a AG° = 9.83 kcal/mole-and a AH 0 = +0.99 kcal/mole at 250 2 1<br />

C, which yields a K1 = 107.<br />

r r<br />

at 25°C. Hence, for pH

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