Sulfide-mineral weathering; Sulfide-ore oxidation
Sulfide mineral. A metal-sulfide compound, such as pyrite (FeS2), which forms at high temperature (>50°C) in well-crystallized veins or masses and at low temperatures (<50°C) in poorly crystalline and fine-grained particles.
Oxidation. The chemical process of reacting with oxygen. More generally, the chemical process of removing electrons from an atom or group of atoms.
Metal-sulfide minerals are valuable as ores for metals that have a wide variety of uses from jewelry to components in vehicles and electronic equipment. They are found primarily in hydrothermal mineral deposits that occur in numerous geologic environments. The most common sulfide mineral is pyrite; other important sulfide ore minerals include chalcopyrite (copper ore), molybdenite (molybdenum ore), sphalerite (zinc ore), galena (lead ore), and cinnabar (mercury ore). When these minerals are exposed to weathering at the Earth’s surface, either through natural processes or as a result of mining activities, they react with oxygen in air in the presence of water to form drainage that is frequently acidic and detrimental to the environment because of the high metal concentrations. The general term for this water is acid-rock drainage. When it originates from mining activities, it is called acid-mine drainage (Nordstrom and Alpers, 1999). Acid-rock drainage is produced by numerous chemical and microbiological processes within a complex hydrogeological environment. Extensive research has been employed to understand these processes, so that mine-site remediation can be more cost-effective and long-lasting. Weathering of sulfide minerals leads to natural enrichment in copper, gold, and silver in a process known as supergene enrichment. Copper dissolves in the infiltrating water and precipitates when it reaches unweathered sulfides downgradient, enriching the original sulfides. Gold and silver are highly insoluble and are enriched in the residual iron oxide cap or gossan that forms above the unweathered ore deposit. Sulfide-mineral oxidation from both mining and natural weathering is a major source of both dissolved sulfate and dissolved metals in natural waters.
Metal extraction of primary ores and of waste piles from previous mining activities is also accomplished by hydrometallurgical techniques that have optimized sulfide-mineral oxidation by increasing the temperature of leaching, adding catalytic reagents, and bioleaching by adding microorganisms that are specially suited for increasing the rate of sulfide-mineral oxidation.
Acid-mine drainage has been known since the early days of mining by ancient civilizations and was documented by early Greek and Roman writers (Nordstrom, 2009). Oxidation of sulfide minerals by microorganisms, however, has a more recent history. Winogradsky (1888) recognized that microbes could actually derive metabolic energy from the oxidation of inorganic compounds. These microbes gain energy from the oxidation (or reduction) of inorganic compounds and acquire their carbon needs from the carbon dioxide in the air (autotrophy). Hence, many of them are chemolithoautotrophs or chemoautotrophs for short. During the first quarter of the twentieth century, it was suspected that microbes affected the oxidation of sulfide minerals, but it was not until Rudolfs and Helbronner (1922) demonstrated that microbes could oxidize zinc sulfide, and Colmer and Hinkle (1947) isolated Thiobacillus ferrooxidans, now Acidithiobacillus ferrooxidans, from acid-mine drainage that microbes were recognized to be important catalysts in the production of acid-mine drainage from sulfide-mineral oxidation.
The geochemistry of sulfide-mineral oxidation and its effect on surface-water quality can vary substantially depending on the composition and grain size of the sulfide minerals. Common sulfide minerals are usually either disulfides like pyrite (FeS2) and molybdenite (MoS2) or monosulfides like sphalerite (ZnS) and galena (PbS). Molybdenite is hydrophobic and rather insoluble so that it oxidizes and weathers very slowly whereas sphalerite is more soluble and readily reacts with acid water to produce hydrogen-sulfide gas. The grain size of these minerals can greatly affect their oxidation and dissolution rate. The smaller the grain size, the faster they react. Hence, fine-grained crystals of pyrite can react more quickly than large crystals of sphalerite even though sphalerite would dissolve more quickly if it were the same grain size as pyrite. Marcasite has the same composition as pyrite but it reacts more rapidly because it has a different crystal structure that is more amenable to oxidative weathering.
As acid-mine drainage mixes with surface waters, the dissolved ferrous iron oxidizes and precipitates to hydrous ferric oxide (HFO) which is an excellent sorbent for trace metals. Consequently, some of the dissolved copper, lead, zinc, arsenic, and other metals will be sorbed onto colloidal particles of HFO and decrease the potential toxicity, one of several processes known as natural attenuation. This process is enhanced by the growth of masses of bacteria and algae which also sorb some of the metals, removing them from solution and decreasing the concentration transported further downstream.
Environmental consequences of sulfide mineral oxidation
Innumerable ideas have been suggested for alleviating the harmful effects of sulfide-mineral oxidation and acid-mine drainage production. These suggestions include passive wetlands treatment, bioreactors, electrochemical treatment, exchange resins, reverse osmosis, and the use of various biosolids. However, the most commonly used, safe, and cost-effective treatments today are lime–limestone neutralization and precipitation of the metal-rich sludge in settling ponds combined with water-management techniques. An important engineering aspect is the construction of containment structures for hazardous materials that can withstand intense rainstorms and floods and that will prevent releases into surface-water and groundwater supplies.
Sulfide-mineral oxidation is a complex hydrobiogeochemical process involving the oxidation of metal-sulfide minerals, catalyzed by numerous bacterial and archaeal microbial species, and producing acid-rock drainage. Mining activities enhance the rate of this process, causing the production of acidic metal-rich drainage that make receiving streams, lakes, and rivers unfit for general usage and injurious to aquatic and terrestrial biota. Major spills of mine wastes into receiving waters have severely damaged valuable aquatic resources, farmland, community water supplies, and the economic stability of those who depend on farming and fishing. Much effort has gone into the remediation of old inactive mines and current mining activities to improve water quality for beneficial uses but the scale and complexity of the problem continues to challenge engineers and regulators.