Introduction

Metal sulfides are ubiquitous in the environment. For example,

(a) They are the main components of ore rocks and are present in abandoned mining sites.

(b) Large coastal areas are covered with marine sulfide-rich sediments.

(c) Oceanic black smokers contain sizeable amounts of sulfides.

When sulfide minerals are exposed to air, they become oxidized. Microorganisms make this oxidation some 105-106 times faster. After coming in contact with water (mainly rainwater), these minerals form aqueous solutions that are notably acidic and are called acid mine drainage {AMD) or acid rock drainage (ARD). A wider classification of waters associated with mining projects is that of mining-influenced waters (MIW), as summarized in Table 1.

Because Fe is often the main metal present, AMD can be represented by the oxidation of pyrite:

The Fe24" thus produced oxidizes to Fe34" in air, albeit very slowly at low pH (with t\/i values on the order of years):

Then, Fe34" can either hydrolyze and form insoluble Fe(III) hydroxide, or act as an additional oxidant for FeS2 to produce Fe2+, as predicted from the corresponding Pourbaix diagram (see McNeil and Little, 1999):

The residues obtained after the main mineral or metal has been extracted are called mine tailings. They normally appear in a substantially divided form (as a result of crushing and grinding) and therefore have high surface areas that favor reaction 1 upon contact with air. The ensuing production of protons and sulfate ions yields sulfuric acid capable of dissolving minerals, thereby increasing metal and non-metal ion content and acidity in nearby surface waters, ground waters, and in the corresponding receiving water bodies. When the water table is near the surface, capillary rise and evaporation may also contaminate the upper soil layers. Amazingly, tailing dumps can be oxidized at depths of even several meters.

Table 1 Mining-Influenced Waters

Type

Abbrev.

PH

Primary treatment problem

Acid rock drainage

ARD

Acidic

Elimination of acidity

(or acid mine drainage)

(AMD)

Mineral processing water

MPW

Basic

Elimination of CN~, As043", Se042

Marginal waters

MW

Circumneutral

Removal of small conc. of contaminants in high flows of water

Residual waters

RW

Basic

Removal of high levels of TDS

Source: Wildeman and Schmiermund, 2004.

Source: Wildeman and Schmiermund, 2004.

Schemes devised to ameliorate AMD production include the use of coatings to encapsulate the pyritic surface and the sequestration of Fe3+ with chelating agents, which reduces the effective concentration of this oxidant and can reduce its standard potential as well. Addition of sacrificial reducing agents (e.g., CaSOs) may also prove helpful.

AMD processes typically occur in long time scales, and as such this subject is hardly fit for school laboratory experiments. The handling of the necessary bacteria would introduce additional difficulties. Nonetheless, because these phenomena are important from the perspective of environmental education, we present here suitable conditions to simulate and study the (non-biological) production of AMD in a laboratory session using simple equipment and materials.

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