Dames & Moore, 1999 - USDA Forest Service
Dames & Moore, 1999 - USDA Forest Service Dames & Moore, 1999 - USDA Forest Service
63.1.1 Sulfide Mineral Oxidation The dominant source process controlling release of inorganic constituents at the Holden Mine is expected to be the oxidation of iron sulfide minerals, which releases oxidized iron and acidity (Figure 6.3-1). Oxygen is the typical oxidizing agent due to its abundance in the atmosphere. However, it is also a very significant control on the rate of weathering of sulfide minerals. It can limit the rate of weathering if it cannot be re-supplied at a greater rate than the maximum rate of oxidation. The role of oxygen supply is discussed further in Subsection 6.5.1 under the description of the chemical processes in the individual sources. The overall process is summarized by Equation (6-1) above, but it is usually described as occurring in three steps: Oxidation of sulfde to sulfate. The first step results in the formation of soluble iron(I1) sulfate weathering products on the surfaces of the iron sulfide mineral (Figure 6.3-1). The types of mineral formed depend on composition of groundwaters in contact with the mineral and the overall humidity. The reaction is catalyzed by sulfur-oxidizing bacteria (for example, thiobacill~. Acidity formed may be entrained in the initial oxidation product or dissolved in groundwaters in contact with the sulfide minerals; In some cases, oxidation of sulfur to sulfate may be incomplete resulting in, form'ation of elemental sulfur. The overall reaction can be summarized as: Oxidation of iron(I4 to iron(ZZ4. The second stage is catalyzed by iron-oxidizing bacteria (e.g., Thiobacillw ferrooxidans), and is the rate-determining step for the complete oxidation of iron sulfide. The oxidation may occur in minerals containing iron@) or by oxidation of groundwaters containing iron(I1). The process consumes acidity: Hydrolysis of iron(I.4. The final stage involves formation of ferric hydroxide. The reaction is rapid and produces further acidity: The final step only occurs at pH>3. Under stronger acidic conditions (pH
Another iron sulfide mineral at the Holden Mine is pyrrhotite @el-$). This mineral is commonly more reactive than pyrite (MEND 1991) but the reaction products are similar. Sulfur may be incompletely oxidized to elemental sulfur rather than sulfate. The dominant copper mineral at the Holden Mine is chalcopyrite. It oxidizes by similar processes, releasing acid, sulfate and copper to solution: Chalcopyrite is less readily oxidized than pyrrhotite or pyrite (MEND 1991). Like iron, copper will precipitate due to hydrolysis: This reaction occurs most at a higher pH (4.0) than iron and produces a precipitate. 63.1.2 Oxidative Dissolution Oxidative dissolution refers to the breakdown of other minerals by oxidizing their individual components (Figure 6.3-2). It is an important mechanism for release of heavy metals from other types of sulfide minerals (at the Holden Mine, primarily sphalerite and chalcopyrite). These minerals may not oxidize rapidly when exposed to atmospheric oxygen alone but when exposed to the strongly acidic solutions . generated by oxidation of pyrite can be oxidized by dissolved iron(III): Sphaleiite can also be oxidized directly by air: ZnS + 202 + zn2+ + SO? (6-1 1) A third mechanism of oxidative dissolution occurs when two different minerals with different rest potentials are in contact and form an electrical cell with one mineral acting as the cathode and the other as the anode (Kwong, 1995). If sphalerite (Rest Potential -240 mV) is on .contact with pyrite (630 mV), oxidation of sphalerite occurs preferentially, and pyrite is protected from oxidation. Sphalerite acts as the anode undergoing oxidation: ZnS + zn2' + So + 2e- (6- 12) This process only occurs in sulfide deposits where there is a strong electrical connection between sulfide grains. 63.13 Acid-Consuming Minerals The water produced by weathering of sulfide minerals is typically acidic (pH
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63.1.1 Sulfide Mineral Oxidation<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.<br />
The overall process is summarized by Equation (6-1) above, but it is usually described as occurring in<br />
three steps:<br />
Oxidation of sulfde to sulfate. 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, thiobacill~. 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, form'ation of elemental<br />
sulfur. The overall reaction can be summarized as:<br />
Oxidation of iron(I4 to iron(ZZ4. The second stage is catalyzed by iron-oxidizing<br />
bacteria (e.g., Thiobacillw ferrooxidans), and is the rate-determining step for the<br />
complete oxidation of iron sulfide. The oxidation may occur in minerals containing<br />
iron@) or by oxidation of groundwaters containing iron(I1). The process consumes<br />
acidity:<br />
Hydrolysis of iron(I.4. 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 (pH
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