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Remediation of Acid Mine Drainage – A Review

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Janakiraman1 , Sandeep Wasudeo Kowe2 , Neelam Phougat3

1-3Central Soil and Materials Research Station

Dept. of Water Resources, RD & GR Olof Plame Marg, New Delhi - 110016 ***

Abstract - Kopili hydroelectric project has suffered serious damage due to contamination of Kopili river by acid mine drainage from open cast coal mines and rat hole mines in Meghalaya. Acid mine drainage (AMD) forms when sulfide minerals deep in the earth are exposed to oxidizing conditions during mining, highway construction and other large-scale excavations. Upon exposure to water and oxygen, most sulfide minerals are oxidized to form sulfuric acid, metal ions and sulphate. Acid mine drainage enters surface or ground water and pollute it. Various biological, chemicaltreatment methods and AMD control methods are being used to reduce harmful effect of AMD. Comprehensive survey of the literature on acid mine drainage is done and various aspects related to the process of acid mine drainage formation, itshazardousimpact on the environment and living organisms, preventative and control measures are being discussed in this review.

Key Words: Acid mine drainage, Kopili River, Acidic Water, AMD control, Chemical treatment, Remediation, Sulphate Reducing Bacteria, Permeable Reactive Barrier, Bioremediation, Phytoremediation, Reverse Osmosis

1. INTRODUCTION

KopiliRiverflowsthroughthestates ofMeghalaya,Assam andisthelargesttributaryofBrahmaputrariver.Acidsfrom illegal rat-hole coal mines of neighbouring Meghalaya has pollutedtheKopiliriveranditstributaries.Theriversrun reddish due to a phenomenon called Acid Mine Drainage (AMD)whichiscausedbyactiveandabandonedminesand coalstoragesites[1-3].Kopilihydroelectricpowerprojectis an important project because Assam, Arunachal Pradesh, Manipur, Meghalaya Mizoram Nagaland and Tripura are benefitedfromthisproject.

2. ENVIRONMENTAL ISSUE

TheacidicwaterofKopiliriverhasbeenseriouslyaffecting theperformanceoftheKopiliHydroElectricProjectdueto severe corrosion and damage of the water pipe and underwaterpartsofthegeneratingunits.Kopilihydropower stationinAssamsufferedmajordisasteron7October2019 when it was submerged and flooded due to rupture in a pipeline.Thepipethatbursthadbeenrepairedjustayear ago. The pH level of the acidic water in the reservoir was 3.25.Burstpipewasdesignedtobringwaterattherateof 12,000 liters per second. The fountain of water from the burstpiperoseseveralhundredfeetandcontinuedtodoso

for hours. Huge loss of ₹6 billion due to this accident at Kopili hydropower station renews questions on rat-hole miningneardams.

ThoughwaterofKopiliriverisunfitforhumanconsumption butthousandsofvillagerslivingnearKopiliriverareusing thiswater.NationalGreenTribunal(NGT)bannedrat-hole mininginMeghalayaon17April2014butstillitcontinues illegally. The pH level of water collected from the river emanatingfromMeghalayawasconstantat3.4evenayear after the rat-hole coal mining was banned by the NGT because the acidic water emanating from these mines continuetocontaminateriverdownstream.Thereasonfor thisconstantlevelcouldbethattheratholesthroughwhich thewaterseepedinwereyettobesealedandwatergotinto theriversystem.Itcouldtakesomemoretimefortheacidic watertogetneutralized.

Waste streams rich with sulphate and metallic ions is generated when sulfide minerals deep in the earth are exposed to oxidizing conditions during mining, highway constructionandotherlarge-scaleexcavations[4,5].Upon exposure to water and oxygen, hydro-geochemical weathering of sulphide minerals generate acid mine drainagecontainingsulfuricacid,metalionsandsulphate. Pyriteandmarcasitearemostabundantlyavailablesuphide minerals. Acid mine drainage (AMD) is prominent in both operatingandabandonedmines.Composition ofAMDcan differfromoneregiontoanotherduetolocalgeologyand sourceofwater[6].DuetohighlyacidicnatureofAMD,vast arrayofmetalsandmineralsgetdissolvedinit.Highlyacidic and highly concentrated metallic streams are the characteristics of AMD. High concentrations of dissolved metalloidsandmetals,highlyacidicpHandlargeamountsof sulphate in the toxic flow of AMD contaminate rivers and othersurfacewaterbodiesthatcomeincontactwithit.Acid mine drainage pollutes groundwater on leaching into it. AMD causes corrosion and destruction of infrastructure. HighlyacidicnatureandheavymetalspresentinAMDare toxic to human health and plant physiology. Acid mine drainage(AMD)hashazardousimpactontheenvironment andlivingorganisms[7-9].Acidminedrainagecauseslossof biodiversity and deterioration of aquatic ecosystems [1012]. Even colour of water changes when AMD enters into waterbodies. WatercontaminatedwithAMDoftenbecomes unsuitable for domestic, agricultural and industrial uses. Industrial activitiessuchas flue-gasscrubbing, streams of pulp and paper milling, chemical manufacturing also

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produceeffluentswhichhavecharacteristicssimilartothat ofAMD[13,14].

The acidity level and metal composition of a given AMD sourcedependonthetypeandquantityofsuphideminerals present and acid-neutralizing minerals contained in the exposedrock.Thecarbonatecontentoftheareadetermines whether there is enough neutralization potential to counteracttheacidproducedfromsulphideoxidation.Ofthe manytypesofacid-neutralizingcompoundspresentinrocks, only carbonates occur in sufficient quantity to effectively neutralize acid-producing rocks. The ultimate acidity or alkalinity of AMD is determined by balance between the acid-producing potential and acid-neutralizing capacity of thedisturbedarea.Thereactionratesofthesematerialsare alsoimportant.Oxidationofsulphidestendstooccurquickly whiledissolutionoflimestoneisaslowerprocess.

3. AMD CONTROL METHODS

ProblemsrelatedtoAMDcanbesolvedbypreventativeand control measures. Remediation of AMD is done either by source control and mitigation techniques [15]. Working principleofsourcecontrolsinvolvestheremovalofoxygen andwatertoeliminateoxidationprocessofpyriticmaterials. Mitigation techniques are generally based on the neutralizationofpHandtheprecipitationofmetalsofAMD [16].Mitigationtechniquescanbeclassifiedasabioticand biologicaltreatmentwithsub-classesofactiveandpassive treatment.Passiveremediationsystemsareinexpensiveto operatebutrequirelargelandareaandprolongedtreatment period.Activeremediationsystemsofferquickertreatment timebutrequirehigherinvestmentcosts.

Acidminedrainagecanbechemicallycontrolled byactively orpassivelytreatingacid-producingrocktostoporretard the production of acid. Active treatment of acid mine drainageisdonebylandreclamation,alkalineamendment, installation of alkaline recharge trenches, remaining of abandoned areas, oxygen barriers, water covers, etc. and passivetreatmentofacidminedrainageisdonebyforming alkaline leach beds, open limestone channels, anoxic limestone drains, vertical flow wetlands, constructed wetlands, etc. [17-19]. Low maintenance is required for passivetreatmentsystemsimplementedonabandonedmine land and stream restoration projects. Over the long-term, maintenance of passive systems is necessary but it is not neededasfrequentlyasforactivetreatmentsystems.

Acid-formingmaterialcanbecompactedandcapped Caps andcoversstrategiesthatisolatepyriticmaterialsfromair and water are employed. Segregating and placing acidproducingmaterialsabovethewatertableinthebackfillis alsorecommended.Divertingsurfacewaterabovethemine sitetodecreasetheamountofwaterenteringtheminearea isalsopracticed.Thepitfloorormaterialunderthecoalis

generally rich in pyrite. Incoming groundwater can be movedawayfromthepitfloorbybuildinghighwalldrains orplacingimpermeablebarriersonthepitfloor.Diversion andreclamationreducethewaterflowfromseeps.Surface diversioninvolvingconstructionofdrainageditchestomove surface water quickly off reclaimed surface mines by providingimperviouschannelstoconveywateracrossthe disturbed area can also be done. Movement of water or oxygen into areas containing acid-producing rock can be retarded by oxygen barriers. These water control technologies include impervious membranes, dry seals, hydraulic mine seals and grout curtains/walls. Acidproducing rock and ground water can be separated by grouts. Ground water moving through backfills can be reducedbyinjectionofgroutbarriers.

Backfilling and revegetation together can be effective methodsofreducingacidloadsfromdisturbedlands.Other technique to control AMD include injection of alkaline materials into abandoned underground mines and into buried acid material in mine backfills. Abandoned surface minescomprisehugevolumesofcontaminantofunknown composition and hydrology. Re-handling and mixing alkalinity into an already reclaimed backfill is generally expensive.Fillingabandonedundergroundminevoidswith non-permeable materials is one of the best methods to prevent formation of AMD. Underground mine voids are extensivesofillmaterialandtheplacementmethodmustbe cheap,readilyavailableandshouldhavelowtransportation and pumping costs. Mixtures of class F fly ash and 3-5% Portlandcementareusedtocontrolsubsidenceinminein residentialareas.

Re-miningof abandoned surfaceorunderground mines is doneforfurthercoalremoval.Re-miningreducesacidloads bydecreasingrecharge,coveringacid-producingmaterials andremovingtheremainingcoalwhichisthesourceofmost ofthepyrite.

Disposalofsulphidetailingsunderawatercover,suchasin a lake is another way to prevent acid generation by excludingoxygenfromsulphides.Inpast,minewasteswere disposed into the ocean or lakes but now many countries havebannedthispractice.

Alkaline recharge trenches are surface ditches filled with alkalinematerialwhichcanminimizeoreliminateacidseeps throughanalkaline-loadingprocesswithinfiltratingwater. During reclamation alkaline recharge trenches can be installedonbackfillsorwastepilestoaddalkalinitytowater. Pumpingwaterintoalkalinetrenchescanmakeacidseepsto turnalkaline.

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4. CHEMCAL TREATMENT METHODS

4.1 Active Treatment of AMD

Chemical treatment system is commonly used if AMD problemsdevelopduringminingorafterreclamation.AMD fromacid-producingmaterialscanbecontrolledbycertain alkaline amendments. Blending acid-producing and acidneutralizingrocksinthereclamationprocesstodevelopa neutral material is done. If sufficient alkalinity is not availableatAMDsitethenexternalsourcesofalkalinitymust be imported. Alkaline chemicals such as Ca(OH)2, CaCO3, CaO,NaOH,Na2CO3, NH3 are addedtoacidicwaterexiting therockmass.ThepHmustberaisedtobetween6and9by adding chemicalsso that insolublemetal hydroxides form and settle out in treatment ponds. A simple chemical treatment system can consist of an inflow pipe, a storage tank holdingthetreatment chemical,avalvetocontrol its application rate, a settling pond to capture precipitated metalhydroxidesandadischargepoint. Flowrate,pHand concentrationsofmetalsintheAMDdeterminesizeandtype ofachemicaltreatmentsystem.Foreachspecificconditiona particular chemical is used to treat AMD adequately and cost-effectively.

Limestone (CaCO3) is very safe, easy to handle, cheap and readilyavailablesourceofalkalinity.Neutralizationpotential oflimestoneisbetween75-100%andithasbeenusedfor decadestoraisepHandprecipitatemetalsinAMD.Use of limestone in AMD treatment is limited due to its low solubility. Hydrated lime (Ca(OH)2) is useful and cost effective for treating high-flow and high-acidity AMD. Hydratedlimetreatmentplantwithlargestoragebinsanda mixer/aerator is employed to help dispense and mix the chemicalwiththewater[20].Quicklime(CaO)isusedwith limedosersandportablequicklimedispensingequipment machines. An average of 75% cost savings over NaOH systemsandabout20to40%savingsoverNH3 systemsis possibleifquicklime(CaO)isusedforremediationofAMD. Sodaash(Na2CO3)isoftenusedtotreatAMDwithlowflow andlowamountsofacidityandmetals.Causticsoda(NaOH) is often used in remote low-flow (<100 l min-1) and highaciditysituations.Thesystemcanbegravity-fedbydripping NaOHsolutiondirectlyintotheAMD.Causticsodaishighly soluble, disperses rapidly and raises the pH quickly. The majordrawbacksofusingNaOHsolution forAMDtreatment arehighcostanddangersinhandling.Ammoniacompounds areusedtotreatacidminedrainagebutanhydrousammonia isextremelyhazardous.Ammoniaissoluble,reactsrapidly andcan raisethe pH of water to9.2.Injection of NH3 into AMDisoneofthequickestwaystoraisepHofwater.NH3 is very cost effective and a cost reduction of 50% to 70% is generallyrealizedwhenNaOHissubstitutedwithNH3. For lowflowsof<750lmin-1,NaOH,Na2CO3andammoniahave thelowestcosts.Fluidizedbedcombustionashes,fluegas desulphurization products and kiln dust are also suitable alkalineamendmentmaterialsforremediationofAMD.

After chemical treatment, the treated water flows into settling ponds so that metals in the water can precipitate. Metal hydroxide sludge is formed on using these AMD treatmentchemicals.Sufficientresidencetimeofthewateris important for adequate metal precipitation or floc. Water quality and quantity determine the amount of metal floc generated. Ponds must be cleaned regularly. Flocs can be disposed by pumping onto land to get them age and dry because they become crystalline and behave like soil material.RemediationofAMDbyneutralizationisafastand simpleprocessbutitrequireslargeamountofneutralizing agents,generateslargeamountofsludgeandMo,Cr,andHg cannotbeinsolubilizedviapHcontrol.

4.2 Passive Treatment of AMD

Active chemical treatment of AMD is generally expensive. Passivetreatmentsystemshavebeendevelopedthatdonot require continuous chemical inputs and natural chemical and biological processes treat contaminated mine water. Constructedwetlands,anoxiclimestonedrains,verticalflow wetlands,openlimestonechannelsandalkalineleachbeds aresomeofthepassivetechnologies. Passivesystemscanbe reliably implemented as low costs and low maintenance solutionformanyAMDproblemsinlow-flowandlow-acidity situations.

AerobicconstructedwetlandsareusedforAMDtreatment whichareshallowponds(<30cm)withorwithoutwetland plants. Aerobic wetlands are being used to slow down influent water to allow metal oxidation, hydrolysis and particlesettling.Remediationefficiencyandsustainabilityof remediationofAMDbyconstructedwetlandsisdepending ontheclimateandvegetationenvironment Limestonecan be used to facilitate neutralization. Aerobic wetlands are typicallyusedasthefinaltreatmentstepbeforedischargeto receivingstreams.

Anoxiclimestonedrain(ALDs)treatsAMDbysendingthem throughanundergroundpassagethatisfilledwithcrushed limestone. ALD generate alkalinity when water passes throughalimestonebedinasemi-closedsystemwhichhave verylittledissolvedoxygen.CO2partialpressureisincreased when limestone is buried in a semi-closed system which leads to additional limestone dissolution and alkalinity leadingtoprecipitationofmetalspresentinAMD.ALDcan be operated effectively for more than 18 years without maintenance. Hydraulic head of AMD is forced down verticallythroughtheorganicsubstrateintoalimestonebed with pipes to convey treated water to the next treatment stageinverticalflowwetlands(VFW).

Inopen limestonechannels (OLC)whenAMDpasses over large (>30 cm) pieces of limestone in relatively steep channels alkalinity is generated. Treatment effectiveness hasbeenreportedtobebetween4to62%whichisavery good response from a low-tech, low-maintenance and inexpensivesystem.Alkalineleachbedsareoppositeofopen

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limestone channel. These aerobic treatment systems use pondsinsteadofchannelsandareconstructedwithsmaller piecesoflimestone(2-10cm).Limestonedissolvesslowly andcontinuallyaddalkalinityuntilthelimestonegetscoated withmetalhydroxidesandbecomeslesseffective.

5. AMD REMEDIATION BY STEEL SLAG

RemediationofAMDbyadsorptiononhigh-efficiency,lowcostandreadilyavailableadsorbentssuchassteelslag,fly ash,andkaolinforremovingheavymetalsfromwastewater of AMD is effective and flexible. These adsorbents have strongaffinityandhighloadingcapacityforpollutants.

Steel slag materials such as basic oxygen furnace slag, electric arc furnace slag and ladle furnace slag are the byproductsproducedinthesteelmakingprocess. Steelslag hasexcellentefficiencytodecontaminateAMD.Steelslagis capableofremovingpollutantsfromindustrialeffluentsdue to its excellent adsorption performance and strong alkali releasingcapacity.SteelslagremediatesAMDwithpH2.5,a sulphateconcentrationof5000ppmandironconcentration of 1000 ppm within 30 min by increasing its pH to 12.1, removing99.7%ofsolubleironand75%ofsuphate[11,12].

Dicalciumsilicate(C2S),Tricalciumsilicate(C3S)andcalcium oxide(CaO)insteel slaghaveexcellent alkalinitywhich is equivalent to 100–2000 ppm CaCO3 [21]. Neutralization abilityofsteelslaginwastewatertreatmentcanbeaslongas tenyears. Steelslagisaninexpensiveneutralizingadsorbent which can replace the traditional expensive alkaline chemicalstoremediateAMD.

Thetypeofsteelproducedandthequalityofrawmaterials determines the composition of steel slags. Composition of Steelslagvarygreatlyanditcontains30–60%CaO,2–35% FeO, 0–25% Al2O3, 1–15% MgO and 0–35% SiO2. Basic oxygen furnace slag has high CaO content (>35%) [22]. Crystalline phases of steel slag such as Gehlenite (Ca2Al2SiO7), Akermanite (Ca2MgSiO7), Bredigite (Ca5MgSi3O12) andMerwinite(Ca3Mg(SiO4)2) have vitalrole in the process of heavy metal ions sorption due to presenceofactivesitesappearingduringtheisomorphous substitution of the type Al3+ → Si4+ in the crystalline phases.Excellentadsorptiveperformanceofsteelslagisdue to its porous structure with a large specific surface area. Average pore diameter and specific surface area of steel slags are in the range of 17–27 nm and 0.8–1.9 m2/g, respectively.

The physicochemical and surface properties of steel slag determinetheperformanceandmechanismofsteelslagin removing heavy metal ions in AMD. Parameters such as initial pH, dosage and particle size of steel slag, the initial concentration of heavy metal ions in AMD affect the entrapment of metal ions in AMD by steel slag. AMD remediation by steel slag is done by physisorption and chemisorption. Physisorption relies on the van der waals force between steel slag and pollutants. Chemisorptions

plays a leading role in removing heavy metal ions which involvesthetransferofelectronsbetweentheadsorbateand adsorbent. Remediation by chemisorption involves combination of multiple mechanisms such as surface coordination,ionexchangeandchemicalprecipitation[23].

Removal of metal in AMD by steel slag is done due to the releaseofalkalinity. MgO,CaOandotheralkalinesubstances insteelslaghavehydrationreactionwhichafterhydrolysis releasealargeamountofalkalinitythusincreasingthepH value of acidic wastewater which leads to precipitation of metals.Precipitateformedisseparatedfromthewastewater to achieve the purpose of removing the heavy metals. Reduction of sulphate in AMD can be ascribed to the formationofgypsum(CaSO4·2H2O).

RemovalofheavymetalionspresentinAMDisalsoachieved bytheionexchangebetweentheCa2+ insteelslagandheavy metalionswhereCa2+ isreleasedfromsteelslagtocreate the active binding sites for metal ions. Most of the Ca2+ in steel slag is present in Ca2SiO4 and Ca2Fe2O5 crystalline phase. Ca2+ in the lattice of Ca2SiO4 and Ca2Fe2O5 can be replacedbytheheavymetalionspresentinAMD. Metalions get adsorbed to the surface of steel slag by electrostatic attraction[24].

Ion coordination of silicon, aluminum, calcium, iron etc. presentonthesurfaceofsteelslagisnotsaturated.These active sites available on the surface of steel slag get occupiedbyheavymetalionspresentinAMD,formsaltson thesurfaceofsteelslagandtherebygetfixedonsteelslag [25].Oncontactwithwater,steelslagform hydroxylatedM–OHgroupwhichreactwiththeheavymetalionspresentin AMD thus removing the heavy metal ions present in the wastewaterby surfacecoordinationreaction[26].

In comparison to other industrial wastes, Steel slag has uniqueadvantagesinthetreatmentofAMDbecauseitcan quicklyreleaselargeamountsofalkalinesubstanceswhich neutralizesthehydrogenionofAMDthus raisingthepHof AMDandhasexcellentremoval efficiencyforheavymetal ionsandsulphateofAMD[27,28].Activationtechnologiesto improve the adsorption performance of steel slagare employed. Removal of metal ion from AMD by steel slag modifiedwithaluminumhydroxideisenhancedsignificantly because active sites in the steel slag got increased, promotingthemetalionstodiffusetothesurfaceoftheslag. Bentonite-steelslagcompositeparticleswereemployedas an adsorbent to treat AMD containing Pb2+. Pb2+ was completelyremovedandacidicsubstancesfromAMDwere effectivelyneutralizeduetothestrongalkalireleasecapacity of steel slag and the excellent adsorption potential of bentoniteforheavymetalions[29].

RemovalefficiencyofmetalionsfromAMDisincreasedwith the increase in steel slag mass because there is positive correlationbetweenthedosageofsteelslagandadsorption sitesforremovalofheavymetalsfromAMD.Increasingthe dosageofsteelslagincreasesthenumberoftotalavailable

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active binding sites thereby increasing its adsorption efficiency. Optimum doses of steel slag are to be used to maketheremediationprocesscosteffectivebecausesizeof the reactor increases with increasing the amount of steel slag which significantly increases the overallprocessing costforwastewatertreatment[30].

EfficiencyforremovalofmetalfromAMDbysteelslaggot increased on increasing the pH of AMD. One objective of remediationofAMDistoincreasethepHlevelfromacidicto neutralorbasic.Variousalkalineindustrialwastesmaterials such as Fly ash, metallurgical slag, cement kiln dust can enhancethepHofAMDfrom2totherangeof8–12[31].

On increasing pH of solution, concentration of OH– ion is increased which is favorable for the adsorption or precipitation of metal ions. At higher pH, competition of H+ ions for free binding sites is attenuated and large amountsofpositivelycharged metalionshookupthefree binding sites. On increasing pH during AMD remediation, the numberofmetalionsadsorbedonthesurfaceofsteel slag getincreasedandtheremovalefficiencyof metalions bysteelslagexceeded 93%-99%atpH3ormore.

AMD generally contains multiple metal ions which have selectivitysequenceofmulti-ioncompetitiveadsorption. In AMD containing Cu2+, Ni2+, and Zn2+ , the adsorption sequencefollowedtheselectivitysequenceof Cu2+ >Zn2+ > Ni2+.InAMDconsistingPb2+,Zn2+,Cu2+ Cd2+,theselectivity sequence followed is Pb2+ > Cu2+ > Zn2+ > Cd2+. Higher preference for adsorption of Pb2+ can be correlated to its lowerhydrateionradius,lowerhydrationenergyandhigher electronegativity that contribute to more intense electrostatic interaction with the active binding sites availableonthesurfaceofslag.

6. BIOREMEDIATION

Compared to other chemical treatment methods for AMD remediation, biological treatments for AMD clean-up, also knownasbioremediation areverypromisingremediation technologies[32-34]duetolowoperationalcostsand easy tooperateforawiderangeofacidiceffluentswithvarying concentrations and better sulphate and metal recovery. Differentialremovalofmetalionsispossibleonremediation of AMD by microorganisms and does not results in secondarypollution

BioremediationtechniquesofAMDgenerallyengagegroup ofSulphateReducingBacteria(SRB)asmainworkingagents tocontrolpH.InbioremediationofAMD,SulphateReducing Bacteria (SRB) reduces the excessive sulphate present in AMDtohydrogensulphidewhichbindswiththemetalsin the AMD stream to precipitate as metal sulphides. Metal sulphidesareverystableandcanbe easilyrecoveredand recycled. SRB remove sulphate from AMD and facilitate recovery of metals to generate non-toxic streams. Bioremediation of AMD can be done in active or passive

mode.Activebio-remediationsystemisusuallyacontinuous process which requires constant resource input while passive bio-remediation systems require relatively low resource input. Hay, Manure, Woodchips, sludge and compostarebeingkeptin bioreactortosustaingrowthof SRB. SRBconvertssulphatetoadenosine-phosphosulphate whichismetabolicallyreducedtobiogenic-sulphiteandthen tobiogenic-sulphide.Thebiogenicsulphidereact withthe metalpresentintheAMDstreamtoprecipitatethemetalsas metallicsulphide.

Metalsulphideseparationthroughprecipitationtechniqueis well pronounced in bioremediation techniques of AMD wherepHofthesystemplaycriticalroleontheprecipitation ofmetal. SRBgenerallythriveinpHrangesof5.0to9.0and have reduced activity rates at other pH levels. At low temperaturesof1-80CSRBassumenormalactivityratesbut SRBgenerallydonottoleratehightemperatureconditions andattemperatureabove400Cbacterialactivitydecreases drastically.

6.1 Passive Biological Treatment

Passive biological treatment systems based on SRB have minimal operation and maintenance cost. Passive bioremediation of contaminated groundwater is done by enhancing microbial activity through organic substrate injection or permeable reactive barriers methods. In situ remediationofAMDisdonebyinjectingorganicsubstrates intoundergroundAMD.Highremovalefficienciesformetals and pH increment of mine water flowing through organic substrateenrichedareawereachievedbutduringhighflow rate seasons this technique is not effective. Permeable Reactive Barrier (PRB) is another in situ remediation technique in which a reactive barrier is installed perpendicular to the contaminated water body. NeutralizationofAMDandprecipitationofmetalstookplace on migration of AMD through the reactive barrier. Heavy metal precipitates produced during remediation get wrapped on the surface of the bacteria and hinder the remediation thus reducing the remediation efficiency of AMD by microorganisms. PRB allows water to flow easily andreactwithmetalcontaminantpresentingroundwater. PRB treatacidminedrainage consistoforganicmaterials, such as municipal compost, leaf manure and wood chips/sawdust.Additionsoforganicsubstratesencourages theproliferationofsuphatereducingbacteria[35].

Passive bioremediation of surface water contaminated by AMD is done by infiltration beds and modified wetland systems.Infiltrationbedsisanextensionoftheconceptof PRBwhichremediatesurfaceandshallowsub-surfaceAMD toadepthof50cm.Organicmaterialsarefilledintrenches forefficientperformanceof SRB.Anti-cloggingdesignand maintenance of constant hydraulic conductivity of the infiltration bed are essential for efficient remediation of AMD.

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Aerobicandanaerobicwetlandsarethemostcommonand cost-effectiveremediationtechniqueforAMD.Non-uniform waterlevelsofAMDthroughouttheperiodofremediation reduce the efficiency of remediation [36]. Substrates are indispensablepartofthewetlandsandplaysacrucialrolein theeffectivenessofthecontaminantsremoval. Substrates influence the biological reactions and the metabolic pathwaysforthecontaminantreduction[37].Contaminant removalefficiencyofthewetlandsisaffectedbylong-term performance and sustainability of the wetlands [38]. Substrateinfluencesthelong-termperformanceofwetlands while operational parameters such as hydraulic retention time,waterdepthanddesignaffectsthesustainabilityofthe wetlands[39].

Anaerobic wetlands are relatively deep (>30 cm) with substratesofcompost,sawdust,straw/manure,haybalesor otherorganiccarbon-based materialswhichare generally mixedwithlimestone.Waterbecomesanaerobicduetothe high biochemical oxygen demand on passing through the thick permeable organic subsurface layer. Anaerobic condition promotes bacterial reduction of sulphate to sulphides which then form insoluble metal precipitates. Reductionofsulphateincreasesalkalinityleadingtometal precipitationashydroxides.

6.2 Active Biological Treatment

Initiallyincomparisontopassivetreatmentsystems,capital and operating cost for active bioreactors was high but advancement in microbial technology has gradually made active bioreactors technology cost effective. Active bioremediation of AMD is done with biological reactors. Bioreactors for sulphate reduction treatment must have processdesignforconsistentand long-termperformance.

Biological sulphate reduction is being done using various reactor designssuchassequencingbatchreactors,gas lift reactors, anaerobic filters/packed bed reactors, fluidizedbed reactors, anaerobic sludge blanket reactors, continuously stirred tank reactors, anaerobic hybrid reactors, membrane bioreactors [40]. Biological Sulphate reductionreactionandmetalprecipitationreactionaretwo mainstepsofbiogenicH2S-basedprocessperformedbySRB for AMD treatment. Sulphate reduction processes utilize sulphate in AMD solution as electron acceptor during anaerobic digestion of organic matter done by SRB. SRB generatealkalinitythroughproductionofH2Swhichinturn precipitates metals. Excess sulphide is supplied to a separate oxidizing reactor to get elemental Sulphur. For AMD treatments, preliminary pH increment is required because highly acidic streams cannot be efficiently remediated. Alkaline materials are doped in organic substratestoprotectSRBcommunityagainstacidityandto maintain performance of bioreactors. Sulphide-containing effluent from biological sulphate reduction reactor can be recycledupstreamsothatSRBindownstreambioreactoris

protectedfromconcentratedsulphide waterbecausehigh dissolved sulphide content can be toxic to SRB, thus, reducing microbial activity. Functional groups such as carbonates,hydroxidesandphosphatespresentintheAMD causesmetalprecipitationinasulfatereducingbioreactor. Thesefunctionalgroupsareproducedduetothemetabolic reactionofthemicrobialagentpresentinthereactor.

BioremediationofAMDusingalgaeiscosteffectiveandhave highefficiencyofmetalandsulphatesremoval.Algalstrains suchasAnabaena,Chlamydomonas,Chlorella,Cladophora, Oscillatoria,Phaeodactylum,Scenedesmus,Spirulinasp.,etc. havebeenusedforthebioremediationofAMD.TheseAlgaes generatehighalkalinitywhichneutralizestheacidicnature ofAMDandfacilitateprecipitationofmetals[41].

7. PHYTOREMEDIATION

In phytoremediation, plants are being used for in situ remediation of AMD mainly by two main mechanisms: phytoextractionandphytostabilization.Inphytoextraction, theplantsuptakethecontaminantmetalspresentinthesoil and accumulate them as a part of their biomass Phytostabilizationinvolvestheimmobilizationofthemetals within the plant rhizosphere and thus reducing the metal availability[42].

8. MEMBRANE TECHNLOGY

Reverse osmosis (RO) and Nano-Filtration (NF) are the membrane based treatment being used for AMD remediation.Rejectionefficiencyof93%to98%isobserved whenNFmembraneswereappliedandrejectionefficiency ashighas99%isachievedonapplicationofROmembrane. Sulphatecontentaslowas10mg/l hasbeenachievedon remediation of AMD by membrane technology [43-44]. Rejection efficiency of the membrane based AMD remediationisdetermined bypH, Feedconcentrationand Temperature.

9. CONCLUSION

UntreatedAMDmayhavetoxiceffectsonorganismsinthe river and the acidic water of AMD can erode the soil. Effectivetreatmentmethodstoneutralizetheacidityofthe AMD and removal of heavy metals from AMD will help in conserving the ecosystem. Remediation of AMD convert pollutants to an acceptable form in which they have least negativeimpactonenvironment.MetalrecoveryfromAMD remediationmaybeperceivedasgenerationofrenewable rawmaterials[45,46].

AMD require site-specific treatment because each AMD is unique. Certainsystemsarebettersuitedtospecificwater qualityandcanprovidegoodtreatmentsuccess.Someofthe treatmentmethodsaresuitableforabandonedmineswhile othersarebestsuitedforactiveoperations.Othermethods canbeusedineithersetting.CostsfortreatingAMDaresite

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and case specific. Cost of AMD treatment methods are generally high. Cost-effective methods which prevent the formationofAMDatitssourcearepreferable[47].

There is no single reliable method for AMD treatment so efforts must be done to avoid the generation of AMD and prevention of contamination of water sources by existing AMD mustbedoneby avoiding surfacewaterandground water to come in contact with existing AMD Mining companiesmustpredictAMDbeforemining.Onsiteswhere apotentialexists,quickcoverageofacid-producingmaterials inthebackfillshouldbepracticed[48].

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