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Water Pollution: Acid Mine Drainage: Remediation
Image of a mine drainage treatment facility.Remediation is the process by which something is corrected. The remediation of acid mine drainage (AMD) refers to the process by which heavy metals are removed from the AMD-polluted water before it is discharged into a stream. Treatment of AMD commonly consists of adding strong bases, such as caustic soda (NaOH, sodium hydroxide), soda ash (Na2CO3, sodium carbonate), or lime (CaO, calcium oxide; or Ca(OH)2, calcium dihydroxide) to the polluted water. The bases neutralize the acidity of the AMD. Because they are strong bases, they ionize and dissolve in water. The resulting hydroxide (OH-) and carbonate (CO32-) ions combine with hydrogen (H+) ions in the AMD, taking them out of solution. The products of this reaction are water (H+ + OH- H2O) and carbonic acid (H2CO3). Carbonic acid is relatively harmless because it is a weak acid. It can be dissociated as water and carbon dioxide, as described in the bicarbonate buffering system. Photo:Mine drainage treatment facility. Photo courtesy of Dr. Ben Stout.

Note the color change in the treatment pond pictured above. The water is red where mine effluent enters the pond. The red color is the iron hydroxide precipitating out of the water. Mechanical aeration (mixing the water) speeds the process by making more oxygen available. (See reactions 1 and 3 in "AMD Chemistry"). The iron hydroxide precipitate eventually settles to the bottom of the pond. This is the area where the red water turns green. The green color is a result of sulfate (SO42-), sodium (Na+), and calcium (Ca2+) ions remaining in the water after acid neutralization reactions occur.

 

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