Oxidation of sulfide minerals

Patrick, 1977). In addition, the bioavailability of trace elements in arid soils contaminated with mine tailings increased with decreasing soil pH due to oxidation of sulfide minerals. 

Dry air is an inefficient oxidant of sulfide minerals. In the presence of water, however, oxidants (such as O2 and Fe(III) compounds) and strong reductants (such as H2S) are important in determining the stability of... 

Temperature, humidity, precipitation, and evaporation are important factors that contribute to the oxidation of sulfide minerals. In warm and wet climates, excessive precipitation may produce persistently high water tables and extensive biological activity that may create reducing conditions in the shallow subsurface and hinder sulfide oxidation (Seal et al., 2002, 208). At the surface, high humidity and temperatures would promote the oxidation of sulfide minerals (Williams, 2001, 274). Frequent precipitation would also suppress evaporation and the formation of arsenic salt deposits (Seal et al., 2002, 208). Furthermore, precipitation and groundwater, which are controlled by climate, are the major sources of water for the production of arsenic-contaminated runoff from sulfide-bearing rock outcrops. 

Bacteria may catalyze and considerably enhance the oxidation of pyrite and Fe(II) in water, especially under acidic conditions (Welch et al., 2000, 597). Many microbial species actually oxidize only specific elements in sulfides. With pyrite, Acidithiobacillus thiooxidans is important in the oxidation of sulfur, whereas Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans (formerly Thiobacillus fer-rooxidans) oxidize Fe(II) (Gleisner and Herbert, 2002, 140). Acidithiobacillus ferrooxidans obtain energy through Reaction 3.45 (Gleisner and Herbert, 2002, 140). The bacteria are most active at about 30 °C and pH 2-3 (Savage, Bird and Ashley, 2000, 407). Acidithiobacillus sp. and Leptospirillum ferrooxidans have the ability to increase the oxidation of sulfide minerals by about five orders of magnitude (Welch et al., 2000, 597). 

In some circumstances, the oxidation of sulfide minerals to sulfate (oxy)(hydr)oxides involves one or more intermediate steps that are related to the properties of the field location. For example, realgar and orpiment in mining wastes at the Kusa mine in Sarawak, Malaysia, initially weather to arsenolite. The arsenolite readily dissolves in sulfate-rich waters in open pits. As the water evaporates, arsenic-rich jarosite precipitates (Williams, 2001, 274). 

Weathering of geologic materials Oxidation of sulfide minerals... 

Sulfur in groundwater is primarily in the oxidized sulfate species, SO . Sulfate in ground-water can have several sources. These include (i) dissolution of evaporite sulfate minerals such as gypsum and anhydrite, (u) oxidation of sulfide minerals, (rii) atmospheric deposition, and (iv) mineralization of organic matter. 

In certain environments, localized anomalously low concentrations of soil O2 have been used by exploration geologists to indicate the presence of a large body of chemically reduced metal sulfides in the subsurface. Oxidation of sulfide minerals during weathering and soil formation draws down soil gas po below regional average. Oxidation of sulfide minerals generates solid and aqueous-phase oxidation products (i.e., sulfate... 

Oxidation of sulfide minerals can occur naturally or as a result of mining activity. Arsenic-rich minerals around mines may, therefore, produce arsenic-rich drainage locally, but this tends to be attenuated rapidly as a result of adsorption of various arsenic species by secondary minerals. Some of the best-documented cases of arsenic contamination occur in areas of sulfide mineralization, particularly those associated with gold deposits. 

The oxidation of sulfide minerals appears to have been promoted by groundwater abstraction which has led to the lowering of the piezometric surface at a rate of —0.6 m yr since the 1950s, leading to partial dewatering of the confined aquifer. The high arsenic concentrations occur where the piezometric surface intersects, or lies close to, the sulfide cement horizon (Schreiber et al., 2000). 

Since selenium substimtes for sulfur in the stmcture of sulfide minerals, drainage from mineralized and mined areas may have high dissolved selenium concentrations. Acid seeps derived from oxidation of sulfide minerals draining the Moreno Shale in the Coast Ranges, USA, have selenium concentrations up to 420pgL with concentrations of aluminum, manganese, zinc, and nickel in the mg L range (Presser, 1994). 

A principal environmental concern associated with mine wastes results from the oxidation of sulfide minerals within the waste materials and mine workings, and the transport and release of oxidation products. The principal sulfide minerals in mine wastes are pyrite and pyrrhotite, but others are susceptible to oxidation, releasing elements such as aluminum, arsenic, cadmium, cobalt, copper, mercury, nickel, lead, and zinc to the water flowing through the mine waste. 

The microbially mediated oxidation of sulfide minerals within mine-tailings impoundments generates acidic conditions and releases high concentrations of dissolved metals. Mill tailings... 

The exposure of sulfide minerals contained in mine wastes to atmospheric oxygen results in the oxidation of these minerals. The oxidation reactions are accelerated by the catalytic effects of iron hydrolysis and sulfide-oxidizing bacteria. The oxidation of sulfide minerals results in the depletion of minerals in the mine waste, and the release of H, SO4, Fe(II), and other metals to the water flowing through the wastes. The most abundant solid-phase products of the reactions are typically ferric oxyhydroxide or hydroxysulfate minerals. Other secondary metal sulfate, hydroxide, hydroxy sulfate, carbonate, arsenate, and phosphate precipitates also form. These secondary phases limit the concentrations of dissolved metals released from mine wastes. 

The microbial oxidation of sulfide minerals in field situations. 388... 

The mechanisms by which the solid, insoluble mineral sulfides are biologically oxidized have been less intensively studied and the details of the interactions are incompletely understood. As will be discussed in greater detail in later sections, not only are mechanisms facilitating the oxidation of sulfur operative but an important role is played by iron oxidation reactions. The reactions of greatest significance in the biological oxidation of sulfide minerals are summarized in Table 6.3.3. 

Various ways in which microorganisms might facilitate the rate of oxidation of sulfide minerals have been summarized and discussed by Vanselow (1976). The principal modes suggested by a number of investigators are ... 

As mentioned earlier (see p. 374), organisms other than T. ferrooxidans have been demonstrated to catalyse the oxidation of ferrous iron at low pH. The Sulfolobus-like organism isolated by Brierley and Brierley (1973) had a temperature optimum of 70 C and oxidized both sulfur and ferrous iron the rates of oxidation are considerably lower than those recorded for T. ferrooxidans. The isolate, however, was able to oxidize molybdenite, M0S2, at 60°C and the rate was increased by addition of ferrous sulfate. The organism showed a unique tolerance to molybdenum (2 g l ) (Brierley and Murr, 1073). Whether organisms of this type play a significant role in the oxidation of sulfide minerals under the conditions of elevated temperature known to exist in leaching heaps, remains to be demonstrated. 

THE MICROBIAL OXIDATION OF SULFIDE MINERALS IN FIELD SITUATIONS... 

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