Dames & Moore, 1999 - USDA Forest Service

Eh Control on Preeipitation/Dissolntion
Eh describes the oxidation state of a constituent. Unlike pH which refers to the actual activity of
hydrogen ions in a water, Eh does not refer to concentration of any particular ion. The oxidation state is
defined by the balance between oxidation/reduction couples (in mine waters most commonly ~ e~'l~e~3:
where:
e- is an electron.
The equilibrium constant is:
Fez+ = Fe3+ + e-
Since k is constant, the ratio of activities of the iron ions defines a, which is proportional to Eh.
Like pH, the stability of minerals can be defined in terms of Eh. Eh-pH diagrams are often constructed to
show the stability of minerals and ions. If a mineral and dissolved ion are in contact and in equilibrium,
the pH and Eh is constrained to the line shown on the pH-Eh diagram. In Figwe 6.3-6, the line between
Fe2' and FeOOH is defined by the half reaction:
Fez' + 2H20 + FeOOH + 3H' + e- (6-25)
The e- indicates that an electron has been lost due to oxidation of FqI) to Fe(1II). The reaction shows
that the slope on the Eh-pH diagram is negative as shown in Figure 6.3-6 (assuming a fixed total iron
concentration).
The electron can be taken up by any oxidizing species, for example oxygen (02):
These diagrams can be used to explain the evolution of tailings groundwater. At the surface of the
tailings, the sulfide minerals oxidize to produce acidic groundwater containing dissolved iron. At the
surface, the presence of oxygen could allow Fe(I1) and Fe(II1) to be in equilibrium. As the water
infiltrates into the tailings mass, only ~ e is ~ stable. ' If neutralizing minerals are present, the pH will
increase while Eh is decreasing. Alumino-silicate buffering constrains pH to between 4 and 5. If the
porewater mixes with oxygenated groundwater (e-g., water in contact with a fast-flowing surface stream),
the Eh increases, and eventually the solution reaches the line defining the stability between Fez' and iron
oxyhydmxide. This causes iron oxyhydroxide to precipitate, removing iron from solution. As Eh further
increases, pH will decrease due to the release of H' during precipitation. Eventually, the solution could
be expected to enter the field in which only iron oxyhydroxide is stable.
Efflorescence
Efflorescence refers to the precipitation of minerals due to the evaporation of water. The process is very
commonly observed along the edge of seeps, and mine drainage collection pools and streams during the
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17693005619Vuly 27. 1999;4:11 PM;DRAFT FINAL RI REPORT