Oxide drainage

Air or biological oxidation of pyrite leads to sulfate formation and dilute sulfuric acid in the mine drainage. This pollutes streams and the water supphes into which the mine water is drained. Means of controlling this problem are under study. 

The estimation of flow functions from an actual experiment is reported next. A multi-rate primary drainage experiment was conducted on a Texas Cream limestone sample. Hexadecane was used as the oleic phase and deuterium oxide (D20) was used as the aqueous phase. Protons are imaged, so only the oil phase is observed. The pressure drop data, production data and saturation data are shown in Figures 4.1.11-... 

Sorption can significantly diminish the mobility of certain dissolved components in solution, especially those present in minor amounts. Sorption, for example, may retard the spread of radionuclides near a radioactive waste repository or the migration of contaminants away from a polluting landfill (see Chapters 21 and 32). In acid mine drainages, ferric oxide sorbs heavy metals from surface water, helping limit their downstream movement (see Chapter 31). A geochemical model useful in investigating such cases must provide an accurate assessment of the effects of surface reactions. 

Finally, whereas most laboratory experiments have been conducted in largely abiotic environments, the action of bacteria may control reaction rates in nature (e.g., Chapelle, 2001). In the production of acid drainage (see Chapter 31), for example, bacteria such as Thiobacillus ferrooxidans control the rate at which pyrite (FeS2) oxidizes (Taylor et al., 1984 Okereke and Stevens 1991). Laboratory ob-... 

Acid drainage is a persistent environmental problem in many mineralized areas. The problem is especially pronounced in areas that host or have hosted mining activity (e.g., Lind and Hem, 1993), but it also occurs naturally in unmined areas. The acid drainage results from weathering of sulfide minerals that oxidize to produce hydrogen ions and contribute dissolved metals to solution (e.g., Blowes et al., 2005). 

Acid drainage results from the reaction of sulfide minerals with oxygen in the presence of water. As we show in this section, water in the absence of a supply of oxygen gas becomes saturated with respect to a sulfide mineral after only a small amount of the mineral has dissolved. The dissolution reaction in this case (when oxygen gas is not available) causes little change in the water s pH or composition. In a separate effect, it is likely that atmospheric oxygen further promotes acid drainage because of its role in the metabolism of bacteria that catalyze both the dissolution of sulfide minerals and the oxidation of dissolved iron (Nordstrom, 1982). 

Whereas pyrite oxidation in the absence of calcite produces H-Fe-S04 drainage, the reaction in the presence of calcite yields a Ca-HC03-SC>4 drainage. 

As pH rises, the metal content of drainage water tends to decrease. Some metals precipitate directly from solution to form oxide, hydroxide, and oxy-hydroxide phases. Iron and aluminum are notable is this regard. They initially form colloidal and suspended phases known as hydrous ferric oxide (hfo, FeOOH n O) and hydrous aluminum oxide (HAO, AlOOH nH.2O), both of which are highly soluble under acidic conditions but nearly insoluble at near-neutral pH. 

The concentrations of other metals attenuate when the metals sorb onto the surfaces of precipitating minerals (see Chapter 10). Hydrous ferric oxide, the behavior of which is well studied (Dzombak and Morel, 1990), has a large specific surface area and is capable of sorbing metals from solution in considerable amounts, especially at moderate to high pH HAO may behave similarly. The process by which hfo or HAO form and then adsorb metals from solution, known as coprecipitation, represents an important control on the mobility of heavy metals in acid drainages (e.g., Chapman etal., 1983 Johnson, 1986 Davis etal., 1991 Smith et ai, 1992). 

Taylor, B.E., M. C. Wheeler and D.K. Nordstrom, 1984, Isotope composition of sulphate in acid mine drainage as measure of bacterial oxidation. Nature 308, 538-541. 

Fig. 3. Extrapolated oxidation-neutralization curve for the long-term prediction of acid mine drainage of the tree samples (Benzaazoua Al 2001)...

Site 2 is situated on the south shore of Moore s pit (Figs. 1 2b). This site consists of i) a pyritic waste rock pile, covered with a thin veneer of Fe-oxide tailings and ii) a down stream acid drainage affected area that is characterized by a Fe oxide + sulfur-rich hard pan surface layer that overlies a stratified unit comprised of intensely altered (Fe-oxide stained) and unaltered Quaternary glaciolacustrine deposits. A small creek (product of beaver activities) flows along the eastern margin of the site into Moore s pond. 

Hedin, R. 2003. Recovery of marketable iron oxide from mine drainage in the USA. Land Contamination Reclamation, 11, 93-97. 

Effluents emerging from sulfide-rich waste-dumps have special characteristics, such as very low pH (< 4), high metal solubility and presence of iron colloids, which provokes water turbidity and precipitation of ochre-products. These effluents are generically named acid mine drainage (AMD), since they result, primarily, from mineral-water interactions involving some sulfide minerals that typically produce acidity upon oxidative dissolution. 

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