heavymetal

1SOILQUALITY–URBANTECHNICALNOTENo.3HeavyMetalSoilContaminationIntroductionSoilisacrucialcomponentofruralandurbanenvironments,andinbothplaceslandmanagementisthekeytosoilquality.Thisseriesoftechnicalnotesexaminestheurbanactivitiesthatcausesoildegradation,andthemanagementpracticesthatprotectthefunctionsurbansocietiesdemandfromsoil.Thistechnicalnotefocusesonheavymetalsoilcontamination.MetalsinSoilMining,manufacturing,andtheuseofsyntheticproducts(e.g.pesticides,paints,batteries,industrialwaste,andlandapplicationofindustrialordomesticsludge)canresultinheavymetalcontaminationofurbanandagriculturalsoils.Heavymetalsalsooccurnaturally,butrarelyattoxiclevels.Potentiallycontaminatedsoilsmayoccuratoldlandfillsites(particularlythosethatacceptedindustrialwastes),oldorchardsthatusedinsecticidescontainingarsenicasanactiveingredient,fieldsthathadpastapplicationsofwastewaterormunicipalsludge,areasinoraroundminingwastepilesandtailings,industrialareaswherechemicalsmayhavebeendumpedontheground,orinareasdownwindfromindustrialsites.Excessheavymetalaccumulationinsoilsistoxictohumansandotheranimals.Exposuretoheavymetalsisnormallychronic(exposureoveralongerperiodoftime),duetofoodchaintransfer.Acute(immediate)poisoningfromheavymetalsisrarethroughingestionordermalcontact,butispossible.Chronicproblemsassociatedwithlong-termheavymetalexposuresare:!Lead–mentallapse.!Cadmium–affectskidney,liver,andGItract.!Arsenic–skinpoisoning,affectskidneysandcentralnervoussystem.Themostcommonproblemcausingcationicmetals(metallicelementswhoseformsinsoilarepositivelychargedcationse.g.,Pb2+)aremercury,cadmium,lead,nickel,copper,zinc,chromium,andmanganese.Themostcommonanioniccompounds(elementswhoseformsinsoilarecombinedwithoxygenandarenegativelychargede.g.,MoO42-)arearsenic,molybdenum,selenium,andboron.UnitedStatesDepartmentofAgricultureNaturalResourcesConservationServiceSoilQualityInstitute411S.DonahueDr.Auburn,AL36832334-844-4741X-177UrbanTechnicalNoteNo.3September,2000ThisisthethirdnoteinaseriesofSoilQuality-Urbantechnicalnotesontheeffectsoflandmanagementonsoilquality.2PreventionofHeavyMetalContaminationPreventingheavymetalpollutioniscriticalbecausecleaningcontaminatedsoilsisextremelyexpensiveanddifficult.ApplicatorsofindustrialwasteorsludgemustabidebytheregulatorylimitssetbytheU.S.EnvironmentalProtectionAgency(EPA)inTable1.Table1.Regulatorylimitsonheavymetalsappliedtosoils(AdaptedfromU.S.EPA,1993).AnnualpollutantloadingratesCumulativepollutantloadingratesHeavymetalMaximumconcentrationinsludge(mg/kgorppm)(kg/ha/yr)(lb/A/yr)(kg/ha)(lb/A)Arsenic7521.84136.6Cadmium851.91.73934.8Chromium300015013430002,679Copper4300756715001,340Lead4202114420375Mercury8401513.4300268Molybdenum570.850.801715Nickel750.900.801816Selenium1005410089Zinc750014012528002500Preventionisthebestmethodtoprotecttheenvironmentfromcontaminationbyheavymetals.Withtheabovetable,asimpleequationisusedtoshowthemaximumamountofsludgethatcanbeapplied.Forexample,supposecityofficialswanttoapplythemaximumamountofsludge(kg/ha)onsomeagriculturalland.Theannualpollutant-loadingrateforzincis140kg/ha/yr(fromTable1).Thelabanalysisofthesludgeshowsazincconcentrationof7500mg/kg(mg/kgisthesameaspartspermillion).Howmuchcantheapplicatorapply(tons/A)withoutexceedingthe140kg/ha/yr?Solution:(1)Convertmgtokg(1,000,000mg=1kg)soallunitsarethesame:7500mgX(1kg/1,000,000mg)=0.0075kg(2)Dividetheamountofzincthatcanbeappliedbytheconcentrationofzincinthesludge:(140kgZn/ha)/(0.0075kgZn/kgsludge)=18,667kgsludge/ha(3)Converttolb/A:18,667kg/haX0.893=16,669lbs/AConvertlbstotons:16,669lb/A/2,000lb/T=8.3Tsludgeperacre3TraditionalRemediationofContaminatedSoilOncemetalsareintroducedandcontaminatetheenvironment,theywillremain.Metalsdonotdegradelikecarbon-based(organic)molecules.Theonlyexceptionsaremercuryandselenium,whichcanbetransformedandvolatilizedbymicroorganisms.However,ingeneralitisverydifficulttoeliminatemetalsfromtheenvironment.Traditionaltreatmentsformetalcontaminationinsoilsareexpensiveandcostprohibitivewhenlargeareasofsoilarecontaminated.Treatmentscanbedoneinsitu(on-site),orexsitu(removedandtreatedoff-site).Bothareextremelyexpensive.Sometreatmentsthatareavailableinclude:1.Hightemperaturetreatments(produceavitrified,granular,non-leachablematerial).2.Solidifyingagents(producecement-likematerial).3.Washingprocess(leachesoutcontaminants).ManagementofContaminatedSoilSoilandcropmanagementmethodscanhelppreventuptakeofpollutantsbyplants,leavingtheminthesoil.Thesoilbecomesthesink,breakingthesoil-plant-animalorhumancyclethroughwhichthetoxinexertsitstoxiceffects(BradyandWeil,1999).Thefollowingmanagementpracticeswillnotremovetheheavymetalcontaminants,butwillhelptoimmobilizetheminthesoilandreducethepotentialforadverseeffectsfromthemetals–Notethatthekindofmetal(cationoranion)mustbeconsidered:1.IncreasingthesoilpHto6.5orhigher.CationicmetalsaremoresolubleatlowerpHlevels,soincreasingthepHmakesthemlessavailabletoplantsandthereforelesslikelytobeincorporatedintheirtissuesandingestedbyhumans.RaisingpHhastheoppositeeffectonanionicelements.2.Drain