ESM202_acid mine drainage_08_2

ESM202_acid mine drainage_08_2 - 1 1 ESM 202 More S, Fe...

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Unformatted text preview: 1 1 ESM 202 More S, Fe Cycling & Acid Mine Drainage 2 More on S 2 5 Acid Mine Drainage 6 Acid Mine Drainage AMD = very acidic, metal-rich drainage from mines In acidic environments, ferrous iron oxidation is by Thiobacillus ferrooxidans Overall Reaction: 4 FeS 2 + 15 O 2 + 14 H 2 O 4 Fe(OH) 3 + 8 H 2 SO 4 Pyrite + Oxygen + Water "Yellowboy" + Sulfuric Acid 7 First Reaction 2 FeS 2 + 7 O 2 + 2 H 2 O 2 Fe 2+ + 4 SO 4 2- + 4 H + Pyrite + Oxygen + Water Ferrous Iron + Sulfate + Acidity Weathering of pyrite (oxidation of exposed pyrite by oxygen) Sulfur is oxidized to sulfate and ferrous iron (Fe 2+ ) is released Generates two moles of acidity for each mole of pyrite oxidized 8 Pyrite and Marcasite 3 9 Pyrite is exposed in coal mines. 10 Pyrite is also exposed in copper mines. 11 Second Reaction 4 Fe 2+ + O 2 + 4 H + 4 Fe 3+ + 2 H 2 O Ferrous Iron + Oxygen + Acidity Ferric Iron + Water Acidity consumed in the reaction Thiobacillus ferrooxidans bacteria catalyze the oxidation of ferrous to ferric iron They get energy from Fe 2+ , reduced (ferrous) iron They also can oxidize reduced S as an energy source They need oxygen The reaction rate is pH dependant reaction is slow under acidic conditions (pH 2-3) several orders of magnitude faster at pH values near 5 This reaction is referred to as the "rate determining step" in the overall acid-generating sequence. 12 Thiobacillus ferrooxidans T. ferrooxidans is generally assumed to live aerobically T. ferrooxidans plays an important role in the iron and sulfur cycles in acidic environments Source: Colorado School of Mines 4 Iron oxidizing bacteria Thiobacillus ferrooxidans oxidizes ferrous iron under acidic conditions. If no bacteria are present, ferrous iron is stable at low pH. 14 Third Reaction 4 Fe 3+ + 12 H 2 O 4 Fe(OH) 3 + 12 H + Ferric Iron + Water Ferric Hydroxide (yellowboy) + Acidity Formation of ferric hydroxide precipitate (solid) is pH dependant Solids form if the pH is above about 3.5 Below pH 3.5 few solids will precipitate....
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This note was uploaded on 08/06/2008 for the course ESM 202 taught by Professor Keller during the Winter '08 term at UCSB.

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ESM202_acid mine drainage_08_2 - 1 1 ESM 202 More S, Fe...

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