Dames & Moore, 1999 - USDA Forest Service

pH Control on Precipitation/Dissolntion
Change in pH is probably the most important mechanism controlling the initial fate of metals in mine
drainage. As described in Section 6.2.1, mine drainage often has a pH of less than 4 due to oxidation of
sulfide minerals. As the acidic leachates contact buffering sources (minerals) and mix with alkaline
waters, the pH increases. The stability of minerals is defined in part by pH. For example, the stability of
aluminum hydroxide in terms of pH can be defined based on the reaction:
The equilibrium constant for the reaction is:
This defines the relationship between pH (-log aH+) and aluminum activity (am+) which in turn can be
used to calculate part of the pH-activity diagram shown in Figure 6-3.4. Note that activities (a) are
determined from concentrations adjusted using activity coefficients (y), for example:
Other reactions define other parts of the curve:
The outcome of this diagram is that as an acidic solution evolves in pH by contact with aluminum-
containing minerals, pH increases and aluminum also increases (see hypothetical trajectory). Eventually,
the stability line for aluminum hydroxide is reached and aluminum hydroxide precipitates. As long as
aluminum hydroxide is in contact with the solution, the trajectory then follows the curve down as pH
increases further (for example, if the water mixes with another alkaline water). Aluminum concentrations
decrease as aluminum hydroxide precipitates. This is a very common effect at mine sites as demonstrated
by the occurrence of white precipitates within the 1 500-level main portal drainage (Figure 6.1 - 1 a). Figure
6.3-4(a) shows aluminum concentrations on a log scale to illustrate the curve. Figure 6.3-4(b) shows the
same diagram on an arithmetic scale to illustrate that once the curve is intersected, and pH continues to
increase, aluminum concentrations decrease very rapidly, becoming "undetectable" by pH 5.
Similar curves can be drawn for other common precipitates such as red ferric hydroxide (Figure 6.3-5)
and green basic copper carbonate. When carbonates form, the partial pressure of carbon dioxide also
affects the pH and the type of mineral that form. For example, under atmospheric conditions, the stable
basic copper carbonate is malachite, but at higher carbon dioxide pore pressures, azurite will form.
The pH changes therefore control the fate. of dissolved potential contaminants by causing them to
precipitate. Reversals in pH also cause dissolution by the same processes.
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1769MO5019Uuly 21,1999.4:ll PMDRAFT FINAL RI REPORT