用STOKES定律解释尺寸分离效应

1/1用STOKES定律解释尺寸分离效应ANINVESTIGATIONINTOTHEEFFICIENCYOFPARTICLESIZE

SEPARATIONUSINGSTOKES'LAW

JULIANCLIFTON1*,PAULMCDONALD2,ANDREWPLATER3ANDFRANKOLDFIELD4

1

DepartmentofGeography,UniversityofPortsmouth,BuckinghamBuilding,LionTerrace,Portsmouth,PO13HE,UK2

EnvironmentalSciences,WestlakesScientificConsultingLtd,PrincessRoyalBuilding,MoorRow,Cumbria,CA243JZ,UK

3

DepartmentofGeography,UniversityofLiverpool,RoxbyBuilding,POBox147,L693BX,UK

4

PAGESProject,Barenplatz2,CH-3011Bern,Switzerland

Received7August1998;Revised16November1998;Accepted20January1999

ABSTRACT

Theaccuracyofgravimetricfractionationasameansofobtainingsizefractionsfrommarinesedimentshasbeenexplored.Analysisoftheparticlesizedistributionandsedimentpropertiesoffractionsobtainedusingthismethodwasundertaken.Thishashighlightedtheextenttowhichexperimentalartefactsratherthanvariationsinsedimentcharacteristicsmayadverselyaffecttheefficiencyofthefractionationprocess.Copyright#1999JohnWileylasergranulometry

INTRODUCTION

Tracemetalsandradionuclidesarepresentingreaterconcentrationsinassociationwithfine-grainedparticlesasaresultofthegreatersurfaceareaavailableforadsorption,thisrelationshipbeingwelldocumentedinbothmarineandterrestrialenvironments(Ackermann,1983;Assinderetal.,1993).Arangeoftechniqueshasthereforebeendevelopedtophysicallyseparatesedimentinthesiltandclayrange(b40;`63"m)onasize-specificbasisinordertostudythedistributionandfateofadsorbedcontaminants.Theseincludecentrifugation(DucaroirandLamy,1995),elutriation(HorowitzandElrick,1986;WallingandWood-ward,1993),heavy-liquidflotation(Cotter-Howells,1993),magneticseparation(Bulmanetal.,1984)andsieving(Mundschenk,1996).Gravimetricsettlingis,however,themostcommonlyemployedmethod;itinvolvesremovalofaspecificvolumeofmaterialsettlinginsuspensionwhich,accordingtoStokes'Law,shouldcontainparticlesfinerthanaspecificdiameter,therebyallowingremovaloffractionsatpredeterminedsizeclassintervals.Thismethodallowssizefractionstobequicklyobtainedandinvolvesminimalcapitaloutlay,leadingtoitswidespreaduseinstudiesoftraceelementdistribution(LivensandBaxter,1988;CundyandCroudace,1995;HeandWalling,1996).

However,theassumptionsimplicitwithinStokes'Law,thatsphericalgrainsofaknownuniformdensityaresettlingfreelyinnon-turbulentfluidofaconstanttemperature,willevidentlyseldombemetevenunderlaboratoryconditions,leadingtoalongstandingdebateontheefficiencyofthesettlingprocedure(Gibbs,1972).However,itisonlyrecentlythatadvancesinlasergranulometryasaparticlesizingtechniquehavepermittedtheaccurateanalysisofsizefractionsobtainedusingthesettlingtechnique(WaldenandSlattery,1993).Thepresentstudyaimstofollowupthisresearchthroughadetailedexaminationofsizefractionsobtainedfrommarinesedimentsandoftheextenttowhichpropertiessuchassedimentdensityandorganiccarboncontentmayinfluencetheobservedgrainsizedistribution.

EarthSurfaceProcessesandLandforms

Earthhttp://./doc/a65ec1444b73f242336c5f95.htmlndforms24,725±730(1999)

*Correspondenceto:DrJ.Clifton,DepartmentofGeography,UniversityofPortsmouth,BuckinghamBuilding,LionTerrace,Portsmouth,PO13HE,UK.E-mail:cliftonj@http://./doc/a65ec1444b73f242336c5f95.htmlContract/grantsponsor:BNFL

MATERIALSANDMETHODS

EightlocationsintheeasternIrishSea,representingadiversesuiteoffine-grainedsaltmarshandintertidalmudflatenvironments,werevisitedinMarch1995(Figure1).Approximately1kgofsurfacesedimentwassampledtoadepthof10mmateachsiteandfrozenwithin48hoursofcollection.

Representativesub-samplesweighingapproximately50gweretakenfromeachsampleandsuspendedin300mlofdistilledwater.Theseweredispersedthroughtheadditionof20mlaliquotsofCalgonsolution,followingwhichthesampleswereplacedinanultrasonicbathfor30minandwet-sievedat40.

Thedensityofthesedimentfinerthan40obtainedfromeachsamplewasdeterminedpriortofractionationinordertocalculatesettlingtimesreflectingvariationsinsedimentcomposition.TheSoilSurveymethod(AveryandBascomb,1974)wasusedforthispurpose,inwhichsedimentdensityiscalculatedusingthedifferenceinvolumebetweenaflaskfilledwithdistilledwatercontainingaknownmassofdrysedimentandthevolumeofthesameflaskcontainingdistilledwateralone.Theprecisionofthistechniquewasverifiedthroughduplicatemeasurementsofallsamples,whichyieldedresultswithin5percentoftheoriginaldata.Thesedimentfinerthan40fromeachsamplewasagaindispersedfollowingtheaboveprocedureanddecantedintoindividualglasssettlingtubes(140cm?15cm),maintainingaparticleconcentrationof

less

Figure1.SamplinglocationsintheeasternIrishSea

726

J.CLIFTONETAL.

PARTICLESIZESEPARATION727than1percentbyvolumeinordertominimizeparticlecollisionsduringthefractionationprocess(Galehouse,1970).

Immediatelypriortosizefractionation,eachtubewasshakenendoverendfor2mintofullydispersethesediment.Followingpredeterminedsettlingtimes,aperistalticpumpwasusedtoremovethetopmost10cmofsuspension;thistechniqueallowedminimaldisturbancetothesettlingsuspension.Eachtubewasthentoppedupto1000mlwithdistilledwaterandtheoperationrepeateduntilthesuspensionwithdrawnbecameclear,indicatingthatcompleteremovalofeachfractionhadbeenachieved.Thetemperatureofthesuspensioninallsettlingtubesrangedfrom18to21Cduringtheexperiment.

Sizefractionswithdrawnusingthisprocedurewereanalyseddirectlywithnofurtherpretreatmentinordertodeterminethegrainsizedistributionofeachfraction.Aminimumoffourmeasurementsofeachsizefractionwithdrawnwascarriedoutduringthesettlingprocessinordertodetectanytemporalchangesingrainsizedistribution.AMalvernMastersizerSlasergranulometerconfiguredtoanalysesedimentinthe0á1±140(900±0á05"m)rangeinonemeasurementwasusedforallparticlesizeanalyses.Whilstearliergenerationsofthisinstrumenthavebeencriticallyevaluated(Syvitskietal.,1991),nocomparativeassessmentofthismodelhasyetbeenmade.However,thedifferencebetweenthemeasuredandnominaldiameteroffivestandardreferencematerialsrangingfrom2á01"mto202"mwaslessthan5percent(Clifton,1998),indicatinganacceptabledegreeofinstrumentalaccuracy.Thelasergranulometercalculatesthepercentageofsedimentfoundtolieinsuccessivesizeclasses.Precisionofmeasurementwasthereforeensuredthroughundertakingduplicateanalysesofeachsampleuntiltheproportionofsedimentrecordedineachsizeclassdifferedbylessthan1percent.

Sizefractionsweredriedat40CandorganiccarboncontentmeasuredfollowingthemodifiedwetoxidationprocedureasdescribedbyLoringandRantala1992.Referencematerialswereutilizedtocalibrateindividualbatchesofsamples,allowingastandarddeviationof0á045percenttobeappliedtoallorganiccarbondata.

Statisticalanalyseswereundertakenusingthenon-parametricSpearmanrankcorrelationtestinordertoidentifyanyrelationshipsbetweenmeasuredsedimentpropertiesandsizedistributioncharacteristics.Thistestwasselectedastheassumptionsofnormalityimplicitintheuseofparametricstatisticaltestscouldnotbeensured.

RESULTSANDDISCUSSION

Density

Themeasureddensityvaluesrangedbetween1á66and2á99gcmà3,whichservestohighlighttheextenttowhichtheassumptionofaconstantsedimentdensitymayadverselyaffecttheaccuracyofgravimetricfractionation.Despitethewiderangeinorganiccarboncontentmeasuredpriortofractionation(0á95±5á47percent),nocorrelationwithsedimentdensitywasevident(rs=à0á1,&=b0á05).Mineralogicaldifferencesreflectingthediverseoriginofthesesamplesmay,therefore,exertthedominantinfluenceupondensity.

Monitoringofparticlesizedistributions

TableIindicatesthatthemeangrainsizeoffractionsobtainedfromallsamplesusingthisprocedurewaswithintherequiredsizerange.Furthermore,notemporaltrendinmeangrainsizecouldbeidentified.Thisimpliesthatthesizedistributionofsamplesobtainedviafractionationisnotdependentuponsedimentconcentration.Therefore,flocculationwasnotasignificantfactoraffectingfractionationinthisexperiment,asparticleaggregationathighersedimentconcentrationswouldbemanifestinanincreaseinthemeangrainsizeofsedimentremovedintheinitialstagesoffractionation.

Efficiencyofsizefractionationprocedure

TableIIsummarizestheefficiencyofthefractionationprocedureusingdatafromparticlesizeanalysesofallindividualfractions.Thisrevealsthattheproportionofsedimentlyinginthedesiredsizerangeineach

fractionvariedfrom26to65percentbyvolume,indicatingthattheefficiencyofsizefractionationusingthismethodisfarfromideal.Whilstthemagnitudeofthestandarddeviationvaluesindicatessomevariationbetweensamples,recoveryofsedimentinthe5±70rangeisleastefficient,withthepercentageofsedimentwithintherequiredsizerangebeinglessthanthatinthecoarseorfinetailsofthedistribution.Thefractionfinerthan90isassociatedwiththemaximumabundanceofsedimentwithintherequiredrangeasthereisnolowersizelimittothisfraction.

Figure2illustratesatypicalexampleofthegrainsizedistributionoffractionsobtainedbythegravimetricseparationprocedure.Thisdemonstratesthatallsizefractionsobtainedusingthismethodexhibitdistinctmodalpeakswhichareseparatedbyatleastonephiunit,indicatingthatthesettlingprocedureemployeddoesyieldqualitativelydifferentsizefractions.Thisalsoclarifiesthenatureofthecoarseandfinetailsofthesizefractions.Afinetailcomposedofsedimentfinerthan80isrecordedinsizefractionstheoreticallycontainingnomaterialfinerthan70.Furthermore,despitethepreliminarywet-sievingundertaken,materialcoarserthan40isevidentinmostsizefractionsfromallsamples,accountingforalmostone-thirdofthe4±50fractionintheexampleillustrated.

Despitethefactthatsedimentdensityisrecognizedasavariablepropertyinthisstudy,theefficiencyofthisproceduredoesnotrepresentasignificantimprovementuponpreviousfractionationexperimentsusingagriculturaltopsoilsandglacialsediments(WaldenandSlattery,1993).Thisimpliesthatcommondrawbacksmayexistingravimetricsettlingprocedureswhichmeritdiscussion.Thepresenceofafinetailextendingto120reflectsthefactthat,incommonwithpreviousworkers(OldfieldandYu,1994),completeremovalofthefinerfractionswasnotachievedinthisstudy,asthesuspensionbeingwithdrawnduringthefractionationproceduredidnotbecomeclearafteratotaloftenindividualwithdrawals.Itwasthereforeimpossibletoexcludesmallamountsoffiner-grainedmaterialinsuccessivesizefractions.Thishasparticularlysignificantimplicationswithregardtointerpretingpollutantdatafromanalysisofsizefractionsobtainedusingthismethod.

Discreteparticlesoforganicmatterhavebeenconsideredtogiverisetocoarsetailsinsizefractionsowingtothelowerdensityoforganicmatterand,hence,settlingvelocity(BarbantiandBothner,1993).However,despitethewiderangeoforganiccarboncontentinsizefractionsmeasuredinthisstudy(0á07±8á01percent),TableIIIindicatesthattheabundanceofcoarsetailsinsizefractionsisnotrelatedtoorganiccarboncontent.

TableI.Rangeofmeangrainsizesmeasuredinsizefractionsfromallsamples

Sizefraction

(phi)4±55±66±77±9b9("m)

32±6316±328±162±8`2Meangrainsize(0)

4á0±4á1

5á4±5á7

5á8±6á0

6á4±6á9

8á8±9á1

TableII.Averagepercentageofsedimentbyvolumecoarser,withinand?nerthanrequiredsizerangemeasuredinsize

fractionsfromallsamples

Sizefraction

(phi)4±55±66±77±9b9("m)32±6316±328±162±8`2%coarser28á424á034á234á235á0%?1'12á99á211á117á613á1%within45á131á326á340á765á0%?1'11á78á13á86á913á1%?ner26á544á939á625á1±%?1'

22á6

16á1

13á5

18á3

±

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J.CLIFTONETAL.

Thislackofcorrelationisinagreementwithevidencethatorganicmatterispredominantlypresentintheformofcoatingsonfine-grainedsedimentinthemarineenvironmentratherthanasindividualparticles(Mayer,1994).

ThecoarsetailsillustratedinFigure2maythereforereflectcertaininherentphysicaldrawbacksassociatedwiththismethod.Itwasnotedthatturbulencewithintheupper10cmofthesettlingtubepersistedforatleast20saftershakingthetubetoensureadequatesedimentdispersalpriortofractionation,therebydelayingtheonsetofverticalsettlingandgivingrisetothecoarsetailsobservedinFigure2.Furthermore,whilst

the

Figure2.Grainsizedistributionofsizefractionsobtainedfromsample1

TableIII.Rankcorrelationcoef?cientsbetweenpercentageofmaterialbyvolumecoarserthanrequiredsizerangeandorganiccarboncontentofsizefractionsfromallsamples.Noneare

signi?cantat5percentcon?dencelevel

Sizefraction

(phi)4±55±66±77±9b9("m)32±6316±328±162±8`2rscoef?cient

à0á64

à0á63

à0á19

à0á02

0á12

PARTICLESIZESEPARATION

729

730J.CLIFTONETAL.

accuracyoflasergranulometryisnotconsideredtobesignificantlyaffectedbyvariationsinparticleshape(Matthews,1991),non-streamlineddistortionsofparticleshapewillservetoreduceparticlesettlingvelocity,whichmayfurthercontributetothepresenceofcoarsetailsinsizefractionsobtainedusingthistechnique.Whilstthepresenceofcoarsetailsisundesirable,thesewillbeofrelativelylittleimportancewithregardtostudiesofcontaminantconcentrationsinparticlesizefractionsincomparisontothefinetailsnotedpreviously.Recentworkhasdemonstratedstrongcorrelationsbetweentheabundanceoffine-grainedsedimentandvariousmineralmagneticpropertiesinintertidalsediments(Clifton,1998;OldfieldandYu,1994).Giventheshortcomingsofgravimetricprocedureshighlightedinthepresentstudy,itisrecommendedthatnormalizationusingsedimentmagneticpropertiesisundertakeninstudieswhichutilizeanalysisofparticlesizefractionstoinferthegeochemicalbehaviouroftracemetalsorradionuclides.

ACKNOWLEDGEMENTS

TheauthorswouldliketoacknowledgethesupportofBNFLforthisproject.

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