1、Acid Mine Drainage: From Formation to Remediation,CE 367 - Aquatic Chemistry Julie Giardina Dominike Merle,Introduction: What is Acid Mine Drainage (AMD)?,Highly acidic water with elevated levels of dissolved metals. Drainage from surface or deep coal or metal mines and coal refuse piles. An importa
2、nt environmental issue in many areas where mining has taken place.,Sources of Acid Mine Drainage,Mining of gold, silver, copper, iron, zinc, lead (or combined metals), and coal Past and present During exploration, operation, and closure of mine, from the mines: dewatering system tailings disposal fa
3、cilities waste heaps Water table rebound after pumping equipment is removed.,Process of Acid Mine Drainage,Geochemical and microbial reactions during weathering of sulfide minerals (pyrite) in coal, refuse, or mine overburden Oxidation of sulfide minerals in the presence of air, water, and bacteria
4、Formation of sulfuric acid and increase in acidity Solubilization of metals due to low pH,A Side Note: Acid Rock Drainage,Formation of acidic waters Occurs naturally due to weathering of sulfide minerals in rocks Occurs at a much slower rate,Effects of Acid Mine Drainage,Water resources Increased ac
5、idity Depleted oxygen Increased weathering of minerals release of heavy metals/toxic elements into stream Precipitation of Fe(OH)3 bright orange color of water and rocks,Effects of AMD (contd),Biological resources Low pH and oxygen content water unsuitable for aquatic life Precipitation of Fe(OH)3 I
6、ncreased turbidity and decreased photosynthesis Gill-clogging, smothering of bottom dwellers and food supply, and direct toxicity (benthic algae, invertebrates, and fish) Clogging of interstitial pore space in coars aquatic substrate habitat,Effects of AMD (contd),Biological resources Elimination of
7、 aquatic plants change in channel hydraulics Stress on other biota associated with aquatic habitats Human resources Corrosion of pipes, pumps, bridges, etc. Degradation of drinking water supplies Harm to fisheries,Chemistry of Acid Mine Drainage,Reaction 1 2FeS2 + 7O2 + 2H2O 4Fe 2+ + 4SO4 + 4H+ weat
8、hering of pyrite in the presence of oxygen and water to produce iron(II), sulfate, and hydrogen ions Reaction 2 4Fe2+ + 7O2 + 2H2O 4Fe3+ + 2H2O oxidation of Fe(II) to Fe(III) rate determining step,Chemistry of AMD (contd),Reaction 3 2Fe3+ + 12H2O 4Fe(OH)3 + 12H+ hydrolysis of Fe(III) precipitation o
9、f iron(III) hydroxide if pH 3.5 Reaction 4 FeS2 + 14Fe3+ + 8H2O 15Fe2+ + 2SO42- + 16H+ oxidation of additional pyrite (from steps 1 and 2) by Fe(III) - here iron is the oxidizing agent, not oxygen cyclic and self-propagating step,Chemistry of AMD (contd),Overall Reaction 4FeS2 + 15O2 + 14H2O 4Fe(OH)
10、3 + 8H2SO4,Typical Case: Manila Creek, VA,Iron content:567 mg/L, pH:3.5, flow from mine of 42GPM.Wetlands were used to increase pH.pH increased to 5.1, iron contents reduced to 67 mg/L.,Extreme Case: Iron Mountain, Ca,Extreme pH measurements from 1.51 to 3.6 over a temperature range of 29-47oC. Tota
11、l iron from 2.67 to 141 g/L. SO4: 14-50 g/L Zn: 0.058-23 g/L. Regulatory actions initiate to increase pH and reduce metal concentrations.,Remediation,Use of acid generating rocks to segregate/blend waste. Bacteria Desulfovibrio and Desulfotomaculum SO4-2 + 2 CH2O = H2S + 2 HCO3- Alkaline Materials (
12、CaCO3, NaOH, NaHCO3, anhydrous ammonia). CaCO3 + H+ = Ca+2 + HCO3 Soil, clay, synthetic covers. Chemical additives,Remediation Procedures,Future/Ongoing Research,Prediction of acid generation Acidbase accounting Weathering tests Computer models Prevention/Mitigation Rock phosphate to inhibit pyrite
13、oxidation. Coatings and sealant to inhibit acid production. Improve time for bactericide leaching. Encapsulation of pyrite material.,Conclusions,AMD is an environmental problem results from the oxidation of pyrite by bacteria air, and water. Oxidation of pyrite decrease pH and increase concentration
14、s of dissolve metals in water. The latter results in the pollution of water, which can be harmful for the environment and living species. Several methods such as wetlands have been done to increase pH and decrease metal concentrations in water. AMD research continues in order to find better ways to mitigate pollution and reduce the overall effects in the environment such as global warming.,
