Acid Mine Drainage From Formation to Remediation CE

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 important 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 mine’s: • dewatering system • tailings disposal facilities • 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 – Formation of sulfuric acid and increase in acidity – Solubilization of metals due to low p. H

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 acidity – 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 (cont’d) • Biological resources – Low p. H and oxygen content water unsuitable for aquatic life – Precipitation of Fe(OH)3 • Increased 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 (cont’d) • Biological resources – Elimination of 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 2 Fe. S 2 + 7 O 2 + 2 H 2 O 4 Fe 2+ + 4 SO 4 + 4 H+ • weathering of pyrite in the presence of oxygen and water to produce iron(II), sulfate, and hydrogen ions Reaction 2 4 Fe 2+ + 7 O 2 + 2 H 2 O 4 Fe 3+ + 2 H 2 O • oxidation of Fe(II) to Fe(III) • rate determining step

Chemistry of AMD (cont’d) Reaction 3 2 Fe 3+ + 12 H 2 O 4 Fe(OH)3 + 12 H+ • hydrolysis of Fe(III) • precipitation of iron(III) hydroxide if p. H > 3. 5 Reaction 4 Fe. S 2 + 14 Fe 3+ + 8 H 2 O 15 Fe 2+ + 2 SO 42 - + 16 H+ • 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 (cont’d) Overall Reaction 4 Fe. S 2 + 15 O 2 + 14 H 2 O 4 Fe(OH)3 + 8 H 2 SO 4

Typical Case: Manila Creek, VA • Iron content: 567 mg/L, p. H: 3. 5, flow from mine of 42 GPM. • Wetlands were used to increase p. H. • p. H increased to 5. 1, iron contents reduced to 67 mg/L.

Extreme Case: Iron Mountain, Ca • Extreme p. H measurements from 1. 51 to – 3. 6 over a temperature range of 29 -47 o. C. • Total iron from 2. 67 to 141 g/L. • SO 4: 14 -50 g/L • Zn: 0. 058 -23 g/L. • Regulatory actions initiate to increase p. H and reduce metal concentrations.

Remediation • Use of acid generating rocks to segregate/blend waste. • Bacteria Desulfovibrio and Desulfotomaculum – SO 4 -2 + 2 CH 2 O = H 2 S + 2 HCO 3 - • Alkaline Materials (Ca. CO 3, Na. OH, Na. HCO 3, anhydrous ammonia). – Ca. CO 3 + H+ = Ca+2 + HCO 3 • 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 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 p. H and increase concentrations 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 p. H 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.