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

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P.5<br />

* pH. Most minerals are leached faster and have higher solubilities in acid environments.<br />

The natural range <strong>of</strong> possible weathering solutions is pH = 4 to 10. One <strong>of</strong> the<br />

earliest steps in the chemical weathering <strong>of</strong> a mineral is the exchange <strong>of</strong> the small and<br />

mobile H ion for a cation on the mineral surface, with subsequent disruptions <strong>of</strong><br />

the structure (Loughnan 1969). Obviously, low pH .solutions can accomplish this<br />

more effectively.<br />

* Eh. For ions that can exist in several valence states (e.g., U +4 and U+6) Eh is<br />

very important in determining their solubility. The Eh <strong>of</strong> natural solutions in<br />

contact with the atmosphere is 6600 mv. Subsurface solutions can have an Eh range<br />

<strong>of</strong> -400 my to +400 mv, with the more reducing (low Eh) conditions found in alka-<br />

line environments (Garrels and Christ 1965). For example, a mineral with very low<br />

solubility, such as uraninite (U0 2 ), requires a low Eh for stability to weather-<br />

ing (i.e., Eh

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