水文地球化学过程

水文地球化学过程第1页/共84页OutlineoftopicsWaterreactswithSoilsandrocksGroundwaterRegime&WaterQualityFlow&ReactionTimeOverviewofgroundwaterreactionsMostimportantindicatorparametersingroundwater(pH,EC,alkalinity,OrganicIndicators)Mostimportanttypesofreaction(acid/base,reduction/oxidation,solution/precipitation)Needtoconsiderreactionstoclassify,predictmigration,identifypersistenceandremediationywu@第2页/共84页WaterSoilsandRocksHydrogeochemistryHydrochemistryGeochemistryHydrogeologicalProcessWaterGroundwaterreactswithSoilsandrocksReactionywu@第3页/共84页GroundwaterRegime&WaterQualityywu@第4页/共84页Flow&ReactionTimeywu@第5页/共84页OverviewofgroundwaterreactionsSolidphaseinteractionsSolution-precipitationandvolatilizationAdsorptionincludingionexchange

EnrichmentNeutralisation-acidbasereactionOxidationReductionreactions(redox)

AdsorptionincludingIonexchange

ComplexationHydrolysisreactionsIsotopicreactionsDecarbonationDesulfationDiogenicagencyywu@第6页/共84页WhyshouldwestudyHydrochemicalReactions?ywu@第7页/共84页GroundwaterQualityDependentonrocktypesandlengthofreactiontimesUsuallylongenoughtoallowsignificantreactionHigherdissolvedsubstancesthansurfacewatersChemistryusuallyreflectscompositionofrocksinaquiferDolomites-Ca,Mg,HCO3Granites-Na,K,F,SandAquifer-lowTDS-relativelyinertrocks-rainfalldeterminingfactorywu@第8页/共84页Comparisonofreactionratesywu@第9页/共84页Oxidationofsulphideminerals,inthepresenceofoxygen,waterandbacteriaGenerallyinSouthAfricathemineralpyrite(FeS2)istheprimarycause.Pyriteoccursasanassociatedorganguemineralwithgold,thebasemetalsandalsocoalPyrite(Fool’sGold)3218BA1731:745mMalmesburyDissolveofPyriteywu@第10页/共84页Detailedreactions(1)FeS2+7/2O2+H2O=>Fe2++2SO42-+2H+

(2)Fe2++1/4O2+H+=>Fe3++1/2H2O(ratelimitingstep)(3)Fe3++3H2O=>Fe(OH)3(yellowboy)+3H+

(4)FeS2+14Fe3++8H2O=>15Fe2++2SO42-+16H+

BOTTOMLINE=Sulphides+water+oxygen(+bacteria)giveacidity(4H+)+sulphatesIfthisisnotneutralizeditcanresultinheavymetalmobilisationwhichcanhavedisastrousenvironmentalconsequencesywu@第11页/共84页Modelofpyriteoxidationywu@第12页/共84页aunreactedpyritebpyriteoxidation-oxygenconsumedcoxygendiffusestooxidationlayerdliquidphase-oxygenconcentrationinequilibriumwiththegasphaseeGasphasebetweenthewasterocks-oxygensupplybydiffussion andadvectionSulphideWaterfilmProductlayerMODELSOFPYRITEOXIDATION-microscaleywu@第13页/共84页Influenceofbacteriaywu@第14页/共84页InfluenceofpHonThiobacillusactivityywu@第15页/共84页Oxygendependenceofbacteriaywu@第16页/共84页Importanceofbacterialcatalysisonoxidationrateywu@第17页/共84页Bufferlevelofsomecommonmineralsywu@第18页/共84页BufferreactionsCaCO3+2H+

Ca2++H2O+CO2(g)H2O+CO2(g)H2CO3

H2CO3

H++HCO3-

H++CO32FeS2+2CaCO3+3,75O2+1,5H2OFe(OH)3+2SO42-+2Ca2++2CO2(opensystem)1moleofFeS2(64gsulphur)isneutralisedby2molesofCaCO3(200g)or1gsulphur:3.125gCaCO3

FeS2+2CaCO3+3,75O2+3,5H2OFe(OH)3+2SO42-+2Ca2++2H2CO3Closedsystem1moleofFeS2isneutralisedby4molesofCaCO3,whichresultsinamassratioof1gpyrite:6.25gcalcite

ywu@碳酸平衡第19页/共84页DistributionDiagramfortheCarbonateSystem

ywu@第20页/共84页OtherreactionsDolomiteCaMg(CO3)2+4H+=Ca2++Mg2++2H2O+2CO2AlbitedissolutionNaAlSi3O8(s)+H++9/2H2O=>Na++2H4SiO4+1/2Al2Si2O5(OH)4(s)AnorthitedissolutionCaAl2Si2O8(s)++H2O=>Ca2++Al2Si2O5(OH)4(s)K-feldspardissolutionKAlSi3O8(s)+H++9/2H2O=>K++2H4SiO04+1/2Al2Si2O5(OH)4(s)Ironoxy-hydroxidedissolutionFe(OH)3(s)+3H+=>Fe3++

ywu@第21页/共84页ImportanceofreactionkineticsAlthoughallthesereactionscanconsumeH+andthusbufferthesystem,thereactionratesofmostoftheminerals(apartfromthecarbonates)areslowerthanthesulphideoxidationratesThusunlesstheacidproductionrateisverylow(lowSmaterial)onlythecarbonatespreventacidity(generally)ywu@第22页/共84页OPENPITAtfieldscaletheneutralisingmineralscanbufferthesystemfromacidityPRECIPITATIONPRECIPITATIONPRECIPITATIONPRECIPITATIONWASTEPILERIVERVALLEYUNDERGROUNDMINEWORKINGSTAILINGSBASINFILLMATERIALSWASTEPILELinesofdefenceagainstacidityincludeNaturalwatersalkalinity-pHashighas8+Carbonateminerals(Calcite/Dolomite)–pHabove6Silicates-reacttooslowlytoactasbuffer-pHfallstolevelswhereheavymetalsaremobilisedetcywu@第23页/共84页GeneralresultsNotenoughneutralisingmineralsLowpHwaterHighsulphateHeavymetalsStaining(yellowboy)Neutralizedbyco-existingminerals(commonoccurrenceinSA)NeutralpHHighsulphateHighTDSStainingywu@第24页/共84页OutflowataSAcoalmineWhatkindofhydrochemicalprocessoccurredinthispicture?ywu@第25页/共84页.The10-90%concentrationrangeof23aciddrainagesfromcoalminesthroughouttheUnitedStatestakenfromtheEPAeffluentlimitationsdocument(U.S.EnvironmentalProtectionAgency,1982)..Medianof110drainagesfromcoalrefusedisposalsitesinSouthernIllinoisascompiledbyProudanetal.(1982)..RegionalestimatesfromCaruccio(1979)..AtypicalmetalminedrainagefromtheFrontRangeMineralBeltofColoradocollectedbyWildemanandLaudon(1989)..CompiledfromU.S.GovernmentPrintingOffice(1988).Forminedrainages,effluentlimitsinmilligrams/Literare:Fe,7.0dailymaximum,and3.5monthlyaverage;Mn,4.0dailymaximum,and2.0monthlyaverage;pHbetween6.0and9.0atalltimes.Fortheothersubstancesinthetable,therearenowrittenrestrictions(U.S.EnvironmentalProtectionAgency,1982)..AverageofU.S.coalcompiledbyValkovic(1983).ACIDMINEDRAINAGEinUSywu@第26页/共84页SummaryAMDiscausedbybacteriallymediatedoxidationofsulphidemineralsThe“essentialingredients”aresulphides,oxygen,waterandbacteriaSeveralmechanismsbufferthesystemThecarbonatesarethemostimportantoftheseduetoreactionrateconsiderations.ywu@第27页/共84页Groundwaterquality&It’sinfluencefactorsVegetationalsoaffectscompositionbyselectiveuptakeofions.IonssuchasCl,Na,MgcanaccumulateinsoilsandbeflushedintoGWbyrechargeInareasofhigherevapotranspirationgreateraccumulationofsaltsHigherrechargeareas-moredilutionofsaltsandclosercorrelationtorainfallOtherreactions:ionexchange,dissolution,formationwater,contaminationPrecipitationofover-saturatedmineralsseldomasignificantfactor-reductioninconcentrationsusuallyduetomixing/rechargeetcRainwaterchemistryimportantinGWstudiesandinfluenceneedstobeconsideredywu@第28页/共84页ywu@第29页/共84页AcidsandBasesBronsted-LowrytheoryAcidisaprotondonor.Baseisaprotonacceptorywu@第30页/共84页ywu@第31页/共84页StrongAcidsandBasesStrongacidswillcompletelydeionizeordissociatewhileweakacidswillonlypartlyionizeordissociateAtequilibrium,theweakeracidandtheweakerbasepredominateinconcentrationTheconjugatebaseofastrongacidisweakTheconjugateacidofastrongbaseisweakywu@第32页/共84页ConjugatePairsywu@第33页/共84页pHConceptsywu@第34页/共84页pH&pOHywu@第35页/共84页AnalysisChecksTotaldissolvedsolids-shouldbewithin20%ofsummationofdeterminedionsIonBalance(inmilliequivalents)Shouldbewithin5%(perhaps10%)DoesnotholdforveryhighandverylowpH-chelation,protonation,hydrolysisAtthesevaluesremembertobringH+orOH-intoaccountInverydilutewatersroundingoffcaninfluencebalanceywu@第36页/共84页EquilibriumVersusKineticDescriptionofReactionsInaclosedsystem,theequilibriumpointisapositionofmaximumthermodynamicstabilityAtequilibrium,thereisnochemicalenergytoaltertherelativedistributionofmassbetweenreactantsandproductsywu@第37页/共84页Theoreticalapproachesusedtomodelchemicalcompositionatequilibrium.Givenoinformationabouthowlongitwouldtaketoreachequilibriumnorthereactionpathwaysthatareinvolved.Kineticapproachisrequiredtoprovidethisinformationywu@第38页/共84页Groundwater=partialequilibriumsystem..Equilibriumreactionisfast/kineticreactionisslowinrelationtogroundwatermovement.ThesearerelativeconceptsEquilibriumrarelyreachedinrapidlyflowingandmixingsurfacewaters.Equilibriumtechniquesonlyprovideboundaryconditionsorbestorworstcasescenarios,showingthedirectionthatchangeswillmovetoward.Irreversiblereactionsproceedintheforwarddirectionuntilallthereactantsareusedup,suchreactionsarebestdescribedusingakineticapproach

ywu@第39页/共84页TypesofreactionsandtimesSolute-soluteandsolute-waterreactionsarefast&homogeneousreactions.Acidbaseandcomplexreactionsarehomogeneousandthereforefast.Dissolution-precipitation:heterogeneousandwidelyrangingtimes.Surfacereactions:relativelyfastRedoxveryslowbutcatalysedbymicroorganismOrganicreactionsaregenerallyveryslow.Biodegradationoforganiccompoundsisfaster.ywu@第40页/共84页Relativetimestoreachequilibriumywu@第41页/共84页Dissolution-PrecipitationReactionsEquilibriumModelsofReactionApplicabletoCarbonatesFeredoxreactionsIonexchangeHalitesCertainsulphatesetcywu@第42页/共84页GeneralFormulationMostarereversiblereactionsExpressedusingthegeneralformA+BC+DleChateleirsprincipleappliesAnychangewillforcethereactioninthedirectionthatminimizesthechangeywu@第43页/共84页MassactionlawsThelawofmassactionstatesthattherateofachemicalreactionisproportionaltotheconcentrationofthereactingsubstances.If A+2BC Rate[A][B]2or Rate=k[A][B]2Forreversiblereactions: aA+bBcC+dDForwardrate=k1[A]a[B]bBackwardrate=k2[C]c[D]dywu@第44页/共84页Atequilibriumtheforwardrate=backwardrateThus k1[A]a[B]b=

k2[C]c[D]dandthusywu@第45页/共84页TheKvalueConcentrationsofthereactantsatequilibriumexpressedintermsoftheequilibriumconstantK.ThevalueofKdependsontheunitsused(usuallymolarity)andontemperature.Formostsolidspeciesk2=1.Thusforasolidthatdissolvesinwatertheequilibriumconstantisequaltotheconcentrationoftheproducts:[C]c[D]d.ywu@第46页/共84页SolubilityProductpartlysolublesolids-assumeequilibriumThusKspisdefined(Solubilityproduct)KspisaconstantatequilibriumValuesaregivenintablesinliteratureywu@第47页/共84页TheSolubilityofcalciteanddolomiteareverysimilarinspiteoftheirKspvaluesbeingverydifferent10-8calciteand10-17dolomiteThisisduetothewaytheirmassactionlawsareexpressedKsp<10-4,compoundconsideredpartiallysolubleinnormalwater.SolubilitydependsonpH,temperature,ionicstrength,ductoftheconcentrationcalledionproductUsuallyuseIAP(activityratherthanconcentration)ywu@第48页/共84页SolubilityIndexIftheionproductis=Ksp,thesolutionissaturated.Iftheionproductis<Ksp,ionsareinsolution.Iftheionproductis>Ksp,precipitationoccurs.SolubiltyIndexywu@第49页/共84页SolubilityproductConstantsforSomeCommonMineralsHalite 101.54

Gypsum 10-4.58

Calcite 10-8.48

Dolomite 10-16.54

Fluorite 10-10.57

Quartz 10-3.98

Amorphoussilica 10-2.71

ywu@第50页/共84页

FeS2+2CaCO3+3,75O2+1,5H2OFe(OH)3+2SO42-+2Ca2++2CO2ywu@第51页/共84页GypsumvsSO4incoalmines

Over8000samplesConsideredSIofwatersPlotSIofgypsumvsSO4GypsumprecipitationformsanupperboundaryforSO4ywu@第52页/共84页Importantweakacidbasereactionsinnaturalwatersystemsywu@第53页/共84页CarbonatesystemCarbonatemineralsoftenpresent(calcite,dolomite)CO2occursfromatmosphere-enrichedinthesoilDissolvestoformcarbonicacidywu@第54页/共84页CarbonatesystemcontinuedCarbonicacidisaweakacidthatdissociatestobicarbonateH2CO3

H++HCO3-

BicarbonatealsodissociatestofromcarbonateinsolutionHCO3-

H++CO32-

Neutralisationreactioncanbewrittenasfollowsywu@第55页/共84页Carbonatesystem3equivalencepointswhichcanbedeterminebytitration(alkalinitytitration)ThreedistinctspecieswhicharedominantatdifferentpHvaluesywu@第56页/共84页pHdependenceofspeciesInacidconditionspH<6thepredominantspeciesisundissociatedcarbonicacidInthenormalgroundwaterpHrange6to8thepredominantcarbonatespeciesisbicarbonateInthealkalinerangepH>8carbonateionsaredominant.ywu@第57页/共84页ThereactionsgiveninthistableshowthatCO2dissolvedinwaterpartitionsbetweenH2CO3,HCO3-

andCO32IfthepHofthesolutionisfixedthenthemasslawequationsallowustocalculatetheconcentrationsoftheindividualspeciesywu@第58页/共84页NaturalwatershavepHintherange5to8.MinewatershavepH<3.Highlyalkalinewatersmaybeassociatedwithwastedisposalsites.Infieldmustdistinguishbetweenacid,neutralandalkalinewaters,todeterminewhatelementsshouldbeanalysedforywu@第59页/共84页SolubilityofcertainmetalswithpHywu@第60页/共84页ExamplesMost

waterinsolublemetalhydroxidesthatarebasicoramphoteric

dissolveinstrongacidsFe(OH)3(s)+3H3O+

Fe3++6H2O(l)Al(OH)3(s)+3H3O+

Al3+

+6H2O(l).AllinsolubleMetalcarbonates

dissolveinacidsolutions:MCO3(s)+2H3O+

M2++CO2(g)

+3H2O(l).Metalsulphides(e.g.CuFeS2or(Fe,Ni)9S8)withrelativelylargesolubilityproductsaresolubleinacid.ywu@第61页/共84页Fe3+

+3H2OFe(OH)3+3H+HydrolysisofmetalionstoformhydroxidesreleasesHpHCaCO3

Ca2++CO32-CO32-+H+

HCO3-HCO3-+H+

H2CO3ywu@第62页/共84页Manymetalhydroxidesareamphoteric

optimumpHfortheirremovalfromsolutioncanbefoundbyexperimentationMostmetalscanbeprecipitatedashydroxidesbyraisingthepHtobetween8and11Metalcarbonatescanbeprecipitatedbyaddingcalcite(expensiveoption)ordolomite

ywu@第63页/共84页CommonmetalsvspHywu@第64页/共84页Amphotericsolubilityywu@第65页/共84页Oxidation-ReductionReactionsredoxreactionsoftenmediatedbyorganismsWithoutthiscatalysisthesereactionswouldbeverysloworganismsuseredoxreactionsasasourceofenergy,theyarethereforeusuallyautotrophicbacteriaywu@第66页/共84页canbetreatedaselectrontransferreactionsfreeorsolvatedelectrons(e-)donotexistinaqueoussolutions.Oxidationisdefinedasane-

loss(increaseinoxidationstate).Reductionisdefinedasane-gain(decreaseinoxidationstate).(REG)Thesetwoalwayshappensimultaneouslybecausefreeelectrons(e-)donotexistinaqueoussolutionsywu@第67页/共84页Redoxreactionsarewrittenastwohalfreactionsasifthee-existTablesgiveninliteratureForexample: Fe3++e-

Fe2+and O2+4H++4e-

2H2Oywu@第68页/共84页EhisusuallyusedinplaceofE,becauseitismeasuredwithreferencetothehydrogenstandardelectrode,andisreferredtoastheredoxpotential

itisespeciallyusefulingeochemistry.uelectronactivity(pEorsometimespeisusedtoexpressitasnumbersywu@第69页/共84页VariationofOxidisingConditionsWithDepthBelowSurfaceywu@第70页/共84页NaturalgroundwatersituationGroundwatertendstoEh0(reducingconditions).Groundwaterisisolatedfromtheatmosphere,anyoxygenthatisconsumedbyhydrochemicalandbiochemicalreactionscannotbereplacedTheseoxygenconsumingreactionsusuallytakeplaceinthesoilwherethereisabundantorganicmatterfromdecayingplantdebrissothatO2isusedandCO2buildsupinthesoilywu@第71页/共84页OrganicOxidationinsoilAsimplecarbohydrate(CH2O)canbeusedtoillustrateatypicalreaction:CH2O+H2O+O2+4H++4e-=CO2(g)+4H++4e-+2H2O CH2O+O2=CO2+H2O CO2+H2O=H2CO3Mustbecombinedwithoxidation1/2reactionexamplecombinationwith: O2(g)+4H++4e-

=2H2Oyields: O2(g)+CH2O=CO2(g)+H2Oywu@第72页/共84页CommonsubsurfaceredoxreactionsProcess Equation

Sulphideoxidation 2O2+HS-=SO42-+H+

Ironoxidation O2+4Fe2++4H+=4Fe3++2H2O Nitrification 2O2+NH4+=NO3-+2H++H2O Manganeseoxidation O2+2Mn2++2H2O=2MnO2(s)+4H+

Ironsulphideoxidation 15/4O2+FeS2(s)+7/2H2O=Fe(OH)3(s)+2SO42-+H+

Thesereactionsconsumedissolvedoxygenywu@第73页/共84页NaturalRedoxZoningProcess Typicalreaction AerobicRespiration CH2O + O2(g)

= CO2

+ H2O

Denitrification CH2O + 4/5NO3

=

2/5N2(g) + HCO3- + 1/5H+ + 2/5H2O Manganese(IV)Reductio