Pollution from Mining




An independent film "The Creek Runs Red" (on PBS) show the results of acid mine drainage on the small community of Picher, Oklahoma. More pictures and data are available on the Oklahoma USGS website.

Mining provides essential raw materials for an industrialized society. However, it causes serious pollution of surface and groundwater. Transferring minerals from deep below the surface to the surface promotes oxidation reactions. Oxidation of iron salts and of sulfur-containing materials in the presence of water leads to highly acidic solutions. Toxic substances are brought to the surface where they contaminate water and other toxic substances are used to extract valuable elements from ore. Organic pollutants results from human activity at the mine site.


Major contaminents

  1. suspended solids and sediment from runoff and processing operations
  2. acids from various processes
  3. acid mine drainage during and after site operation
  4. heavy metals leached from wastes and concentrates around the site
  5. sulfate, thiosulfate, polythionates etc. from acid drainage
  6. arsenic and other salts from oxidized mine waters
  7. mercury, if used in the process, or from ores
  8. cyanide if used in leaching processes
  9. oil and fuels from ancillary operations
  10. other processing chemicals as may be used on the site
  11. groundwater constituents that may be pumped or discharged off-site
  12. sewage from the site



Acid Drainage

Sub-surface mining frequently goes below the water table so water must be constantly pumped from the mine. Acid mine drainage results from the outflow of water from metal mines, coal mines, or other areas in which the ground has been disturbed. Metal salts, such as Fe+2, are oxidized upon contact with air to more highly acidic aquo species. Sulfur containing molecules are air oxidized to sulfuric acid. Certain bacteria, called acidophiles, promote the oxidation of iron sulfides in the presence of oxygen and water.

2FeS2(s) + 7O2(g) + 2H2O(l) 2Fe2+(aq) + 4SO42-(aq) + 4H+(aq)


Read more in the pdf file on acid mine drainage from the Environmental Mining Council of British Columbia, Canada.


Toxic Inorganic Pollutants

Mining operations release heavy metals that can be extracted by water. Depending on the mine and its location, Ni, V, Cr, Mn, Cu, Pb, Zn and As can be released into the hydrosphere.

Mercury

Mercury was widely used to in California gold mines to extract the precious metal from solid ore. Between the years 1850 and 1981, more than 220,000,000 lb of mercury was used in mining in that state.

Mercury and other metallic elements form an amalgam, or a solution of the solid metal in the liquid mercury metal. The dissolved material can be recovered by evaporating the mercury.

Mercury is still used this way in some mining operations outside the US. Rogue miners in the Amazon basin of South America are causing major problems with mercury contamination. In 2001, mercury levels in head hair were studied in residents of three fishing villages on the Tapajos River, an effluent of the Amazon, several hundred kilometres from the gold mining areas. Many had high mercury levels in addition to symptoms suggesting neurotoxic disease.

The mercury is highly toxic! It contaminates air, water, and soil where it has been used. Certain bacteria convert elemental mercury to the even more toxic methylmercury(I). Organomercury levels in fish exposed to mine drainage water can be at levels dangerous to human health.

Cyanide

Cyanide salts are widely used to extract gold from finely crushed ore because they combine with about 97% of the elemental gold. Typically, the NaCN solution is sprayed on heaps of ore and the liquid extract is collected. Of course, a great deal of the toxic cyanide salt remains on the mineral particals and is washed into ground and surface water in the rain.

4 Au + 8 NaCN + O2 + 2 H2O 4 NaAu(CN)2 + 4 NaOH

Once the gold has been extracted, zinc is used to recover solid gold from solution:

2 Au(CN) + Zn 2 Au + Zn(CN)4-2


Arsenic

Arsenic containing minerals are exposed to surface water during mining. Arsenite (H2AsO3-) is the more serious problem because of its high solubility in water.




Organic Pollutants from Mining

Water aqround mines is contaminated by waste oil, and biolgical waste from sewage. More toxic organic compounds are also present. Dioxin-like compounds including polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans and polychlorinated biphenyls (PCBs) are polyhalogenated aromatic hydrocarbons (PHAHs) are formed as a result of incomplete combustion during the smelting of metals.


Effluent Treatment

In the US, the maximum allowed pollutant levels from open pit mines (left) froth flotation processing (right) are shown below.



Various methods are used to remove pollutants from mine effluent. These include:
  • Neutralization: Bases, such as NaHCO3, moderate the acidity of acid mine drainage.

  • Chemical oxidation: Oxidation of cyanide produces carbon dioxide and ammonia, oxidation of highly soluble arsenic(III) salts produces the less soluble As(IV) materials.

  • Biological treatment: Bacteria can take up heavy metals and concentrate them in a sludge for separation. Microorganism can plants can take up organic compounds and oxidize some of these to carbon dioxide.

  • Co-precipitation: Trace metals are effectively removed from mining effluent by the addition of ferric salts. Through precipitation, arsenic is removed as either calcium or ferric arsenate.




Index

Professor Patricia Shapley, University of Illinois, 2009