13.01.2013 Views

Dames & Moore, 1999 - USDA Forest Service

Dames & Moore, 1999 - USDA Forest Service

Dames & Moore, 1999 - USDA Forest Service

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

63.1.1 Sulfide Mineral Oxidation<br />

a,<br />

The dominant source process controlling release of inorganic constituents at the Holden Mine is expected<br />

to be the oxidation of iron sulfide minerals, which releases oxidized iron and acidity (Figure 6.3-1).<br />

Oxygen is the typical oxidizing agent due to its abundance in the atmosphere. However, it is also a very<br />

significant control on the rate of weathering of sulfide minerals. It can limit the rate of weathering if it<br />

cannot be re-supplied at a greater rate than the maximum rate of oxidation. The role of oxygen supply is<br />

discussed further in Subsection 6.5.1 under the description of the chemical processes in the individual<br />

sources. I<br />

The overall process is summarized by Equation (6-1) above, but it is usually described as occurring in<br />

three steps:<br />

a Oxidation of supde to sulfote. The first step results in the formation of soluble iron(I1)<br />

sulfate weathering products on the surfaces of the iron sulfide mineral (Figure 6.3-1).<br />

The types of mineral formed depend on composition of groundwaters in contact with the<br />

mineral and the overall humidity. The reaction is catalyzed by sulfur-oxidizing bacteria<br />

(for example, thiobacilli). Acidity formed may be entrained in the initial oxidation<br />

product or dissolved in groundwaters in contact with the sulfide minerals. In some cases,<br />

oxidation of sulfur to sulfate may be incomplete resulting in formation of elemental<br />

sulfur. The overall reaction can be summarized as:<br />

• Oxidation of iron(Z0 to iron(ZZ9. The second stage is catalyzed by iron-oxidizing<br />

bacteria (e.g., Ilriobaci1lu.s ferrooxidans), and is the rate-determining step for the<br />

complete oxidation of iron sulfide. The oxidation may occur in minerals containing<br />

iron(I1) or by oxidation of groundwaters containing iron(I1). The process consumes<br />

acidity:<br />

Fe2' + !44 + H' + Fe3' + W20 (6-5)<br />

• Hydrolysis of iron(IZr). The final stage involves formation of ferric hydroxide. The<br />

reaction is rapid and produces further acidity:<br />

The final step only occurs at pH>3. Under stronger acidic conditions (pHO), femc hydroxide does not<br />

precipitate and iron(II1) remains in solution. The resulting strongly acidic oxidized solution is capable of<br />

oxidizing pyrite directly without oxygen:<br />

This reaction is capable of sustaining very low pHs (much less than 3) and does not require bacteria as a<br />

catalyst. However, the pH must be less than 3 to allow ~ e to ~ be ' available as an oxidant.<br />

\\DM-SMI\VOLI\COMMOMWR~W)~\hok-2\mw.doc<br />

6- 1 0<br />

I769UMU)I Wuly 27, <strong>1999</strong>;4:11 PM;DUFT FINAL RI REPORT

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!