AMD Armoring Limestone
Limestone is used because it reacts with acidic water to neutralize it. However, as it forms a gypsum shell and becomes coated in loose mineral waste such as iron it is unable to react, making remediation efforts less effective.
Acid is essentially loose hydrogen ions (H+). These show up in the watershed from the degradation of sulfates released as waste in the coal mining process. Limestone (CaCO3) reacts with the hydrogen according to the following equation:
CaCO3 + H+ → Ca2+ + HCO3-
so hydrogen ions aren’t floating around, making the water less acidic.
At the Silver Creek AMD remediation site AMD is flowed over a series small of limestone “waterfalls” gradually reducing the water’s acidity and causing heavy metals like iron to precipitate out of the water.
This technique is effective for a time, but the Ca2+ cation can react with any dissolved SO4 and water to form CaSO4·2H2O (gypsum). This gypsum coats the limestone so it can no longer touch the acidic water and neutralize it. The limestone also gets covered in iron precipitates—also called “yellow boy” (mostly goethite: Fe(OH)3) for its color. Which further prevents remediation over time.
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