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Acid Mine DrainageAcid Mine Drainage (AMD) is produced when sulfide-bearing material is exposed to oxygen and water. The production of AMD usually but not exclusively occurs in iron sulfide-aggregated rocks.The sulfide minerals oxidize in the presence of water and oxygen to form acidic, sulfate-rich drainage.Releases of AMD have low pH, high specific conductivity, high concentrations of iron, aluminum, and manganese, and low concentrations of toxic heavy metals.

Acid generationPrimary ingredients are as follows: sulfide minerals; water or a humid atmosphere; and an oxidant, particularly oxygen from the atmosphere or from chemical sourcesIn the majority of cases, bacteria play a major role in accelerating the rate of acid generation like acidophiles. Acidithiobacillus ferrooxidans is a key contributor to pyrite oxidation.Dumps with high permeability have high oxygen ingress, which contributes to higher chemical reaction rates, hence, higher temperatures and increased oxygen ingress through convection.

Acid generation: The primary factors that determine the rate of acid generation are: pH; Temperature; Oxygen content of the gas phase, if saturation is less than 100%; Oxygen concentration in the water phase; Degree of saturation with water; Chemical activity of Fe3C; Surface area of exposed metal sulfide; Chemical activation energy required to initiate acid generation Bacterial activity

Chemical Reactions: Commonly accepted reactions:2FeS2+7O2+2H2O 2Fe+2 +4SO42 +4H+ (1)

4Fe+2 +O2 + 4H+ 4Fe+3 + 2 H20 (2)

4Fe+3 +12 H2O 4Fe(OH)3 + 12 H+ (3)

FeS2 + I4Fe+3 + 8H2O 15 Fe+2 + 2SO42 +16 H+ (4)

Sources of AMD

Source: Akcil, A., et al., (2006)

Primary sources Secondary sourcesMine rock dumpsTreatment sludge poundsTailings impoundmentRock cutsUnderground and open pit mine workingsConcentrated load-outPumped/nature discharged underground waterConcentrate spillsalong roadsDiffuse seeps from replaced overburden in rehabilitated areasEmergency pondsConstruction rock used in roads,dams, etc.Stockpiles

MitigationHigh concentrations of heavy metals and other toxic elements in AMD can severely contaminate surface and groundwater, as well as soilsAMD formation depend upon the availability of oxygen and water along with the mineralogy of the area so predicting the potential for AMD can be exceedingly challenging and costly No standardized methods for ranking, measuring and reducing the risk of AMDAvailable treatment technologies are either inefficient or economically not feasible so most of AMD is left without any treatment

Prevention/mitigation of AMD Three ways for prevention and mitigation of AMD: Chemical inhibition of acid generating reactions Inhibition of microbial activity in catalyzing formation of acidPhysical or geotechnical treatments to minimize water contact and leaching

Chemical methodsAlkaline Addition: A highly alkaline environment can be developed by loading trenches with alkaline, usually a combination of soluble sodium carbonate and crushed limestone is used

Phosphate:Phosphate can be used as a pyrite oxidation inhibitor. Iron combines with phosphates to form insoluble iron phosphate compounds and thus inhibits Fe+2 oxidation hence reduces acid generation

Coatings and sealants: Some studies research activities are focussed on the surface chemistry of pyrite and development of various types of sealers, coatings and inhibitors to halt acid production

Biological agents/bactericides Acidophiles survives at low pHThiobacillus ferooxidans catalyze the pyrite oxidation To inhibit the catalytic role of bacteria many compounds like bactericides and the anionic surfactants are applied Most reliable inhibitors are Sodium lauryl sulfate and alkyl benzene sulfonate Bactericides are generally water soluble and leaching time of bactericides from the spoil is uncertain Uncertainty about the reappearance of sulfur and iron oxidizing bacteria after the application of bactericides

Physical or geochemical treatments Controlled placement :Controlled placement (special handling) is a preventative measure involving the placement of pyritic or alkaline material during mining Two fold role:1. Inhibition of the acid-forming reactions by maintaining neutral to alkaline pH2. Neutralization of any acid formed

Encapsulation/physical barriers :Isolate or encapsulate pyritic material by using fly-ash, cements, bentonite, and other clays as sealants and flow barriers

Physical or geochemical treatments contd.Water management :Water management strategies both during and after mining are another option for reducing acid generation, which can be achieved by Diversions of surface drainage away from pyritic material or through alkaline materialRough grading of mine rejects to prevent ponding and subsequent infiltration Prompt removal of pit water can lessen the amount and severity of acid generatedIsolation of acidic water from non-contaminated sources to reduce the quantity of water requiring treatmentConstruction of drainage systems to route water away from contact with acid forming material

Control of AMD migration Water entry into the site of acid formation may be controlled by:1. Diversion of surface water flowing towards the site of pollution2. Prevention of groundwater infiltration into the pollution site3. Prevention of hydrological water seepage into the affected areas4. Controlled placement of acid-generating waste

Case studies:Hydrogeochemical characteristics of AMD and water pollution at Makum Coalfiled, India Management of AMD in Meghalaya

Makum Coalfield Sites:Two open cast mines Tikak an TirapThree underground Baragolai, Ledo and TipongCoal Mine Discharge:pH of direct mine discharge ranged from 2.3- 7.6 with an average 4.2EC varied from between 785-6760 S/cm (average 3227 S/cm)Mine discharge from Ledo and Baragolai is alkaline in nature while of Tirap, Tikak ad Tipong are highly acidic

Makum Coalfield Contd.Creek and river water:The pH of river and creek water ranged from 2.57.4 with an average of 5.1; whereas, EC varied from 1075228 S/cm (average 945 S/cm)The concentrations of Fe,Mn, Cd and Pb in the creeks carrying AMD are above their respective maximum permissible limits of USEPA; whereas concentration of Cr, Cu and Zn do not exceed their respective limitConcentrations of metals in AMD impacted creeks are much above in comparison to that of river samplesGround water:pH of ground water ranged from 4.27.8 (average 5.8); whereas, EC from 42542 S/cm (average 248 S/cm), Fe (0.0553.9 mg/L) and SO4 2 (8.394.5 mg/L). Concentrations of Mn, Fe and Pb are above the maximum permissible limits in most of the samples; however, Al, Ni, Zn, Cr and Cu are within limit

Management of AMD in Meghalaya

Physico-chemical analysis of freshly collected coal samples (from Bapung and Sutunga coal deposits) are shown in table:

The coals, coal fines, and mine rejects of Meghalaya coal mines are acid producing materials.

Proximate AnalysisMBMSMoisture 1.5 2.9 Ash 11.5 20.0 Volatile matter 40.5 35.6 Fixed Carbon 46.5 41.5 Total Sulphur 4.23 3.46 Pyretic Sulphur 4.23 0.15 Sulphate sulphur 0.43 0.29 Organic sulphur 3.54 2.90

ReferencesDiz HR. Chemical and biological treatment of acid mine drainage for the removal of heavy metals and acidity, Ph.D. thesis, Virginia Polytechnic Institute and State University, USA; 1997.Equeenuddin, S.M., et al., Hydrogeochemical characteristics of acid mine drainage and water pollution at Makum Coalfield, India, Journal of Geochemical Exploration, 105 (2010) 75-82Akcil, A., Koldas, S., Acid Mine Drainage (AMD): causes, treatment and case studies, Journal of Cleaner Production, 14 (2006) 1139-1145Baruah, B. P., Khare, P., Rao, P.G., Management of Acid Mine Drainage in Indian coal mines, Coal Chemistry Division, North-East Institute of Science and Technology, Jorhat- 785006, Assam