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Suchasimulationmightnotbeabsolutelyexactbecauseof nonlineareffects.XANESfitsusinglinearcombinationsdonot accountsufficientlyfornonlinearabsorptioninmixtureswhen theend-memberscomponentshaveverydifferentmassabsorp- tions(differentdensitiesandabsorptioncharacteristics).O’Day etal. 46 havepreciselystudiedironspeciationinnaturalmixtures, andtheyhaveshownthattherelativeproportionsofeach componentofthetotalspectrumdependonthelocalbonding environment(s)aroundtheelementofinterest,onthetypeand numberofcomponentsinthemixture,andonthecomposition ofthematrix.Theynotedthattheobserveddifferencesbetweentrueandapparentfractionsdependonthetotalfluorescenceyield oftheironinthecompound,whichisproportionaltothemass fractionofironinthecompounds,andonmatrixabsorption andfluorescence.Sofluorescenceintensitiesofcomplex mixturescanbeinfluencedsignificantlybysampleandmatrix absorptionthatdoesnotvaryinasimplelinearfashionasa functionofconcentration.Thisstudysuggeststhatcalibration curvesshouldbeconstructedonacase-by-casebasisif componentfractionsofcomplexmixtureshavetobedetermined quantitativelyfromXANESfitsanditpointsouttheimportance ofahigh-quality,experimentallyconsistentwithstandard compoundslibrary.Inourstudy,thegoodagreementbetween trueandapparentfractionsmaybeduetoalowmatrixeffects. Indeed,thelinearcombinationswerecalculatedwithspectra ofstandardcompoundsdilutedinBNfortransmissionmode. TheBNdilutioncertainlymitigatesthematrixeffects. ACaseStudy:VanadiumOxidationStateandSymmetry withinAlteredSteelSlag.Analysisofthepre-edgefeatures ofstandardcompoundsshowedthatitispossibletoderive informationonthevanadiumspeciation(oxidationstateand symmetry)fromthecentroidpositionandthetotalareaofthe pre-edgepeak.Whenstudyingvanadiumspeciationinunknown sampleassteelslag,variationsinthesymmetryandinthe oxidationstatecanbeexpectedduetothedifferentalteredareas. Thismakestheanalysisofthepre-edgemuchmorechallenging. Averyhighsignal-to-noiseratio,thebestexperimentalresolu- tionpossible,andacarefulpre-edgepeakexaminationwiththe appropriateinterpretativemethodsarerequired. Theoxidationstateandsymmetryofvanadiuminleached steelslagweredeterminedusingthemethodpresentedprevi-ouslybasedonpre-edgeanalysis(Figure4)andcorrelations betweencentroidpositionandnormalizedarea.Thealteredsteel slagisaheterogeneousmatrixatthemicrometerscale,interm ofvanadiumvalenceandsymmetrydistribution.Bulk-XANES measurementsaretheninadequatetoassessaccuratelythe variationoftheoxidationstateandsymmetry.Wehaverecorded micro-XANESspectratoelucidatethemicrodistributionof vanadiuminsteelslagfromtheunalteredtothealteredlayer. SeveralROIwereselectedtorecordmicro-XANESspectrawith thefocusedbeam(Figures6and7).Becauseofahighvanadium releaseduringlaboratory-scaleleachingtests, 27 theoxidation ofvanadiumwassuspected.MoreoversinceV5 exhibitssome toxicity, 47 itiscrucialtoevaluatetheoxidationstateofvanadium withinalteredsteelslagintermsofenvironmentalimpacts especiallyifsteelslagisreusedasanaggregateinroad constructions.Theresults(Figure8)showthatvanadiumis mainlypresentinunalteredregionasoctahedralV3 (ROI4 6). Asignificantincreaseofthepre-edgepeakareaisobservedas wellasaslightpositiveshiftofitscentroidpositionforROI1, 2,and9.Itreflectsthelossofsymmetryofthevanadiumsite anditsoxidationduringleaching.Vanadiumispresentinalteredregioninthepentavalentformwithtetrahedralsymmetry.Then Figure8showsthatvanadiumispresentinlittlealteredregion (ROI3,7,and8)asamixtureofvariousoxidationstateand symmetrybutdoesnotallowarigorousdiscriminationofthe natureofthemixture.However,thecorrespondencewiththe mixingcurvecoulsonite/vanadinitesuggeststhatvanadiumis presentasamixtureofoctahedralV3 andtetrahedralV5 (around50:50mixture),showingthecomplexityofvanadium oxidationstatedistributioninleachedsteelslag.Butthe calibrationcurve(opentrianglesinFigure4)isthennotwell- adaptedbecauseofmatrixvariationbetweenXANESspectra ofstandardcompoundsandmicro-XANESspectracollectedin fluorescencemode. 46 Sothisobservationcannotbequantita- tivelyconfirmed.Micro-XANESresults(Figure8)showsthedistributionof vanadiumformsatthesub-micrometerlevelinalteredsteelslagandtheirevolutionduringleaching.Theseresultssuggestthat thevanadiumspeciationisdifferentinalteredorunaltered region.Theyprovideessentialinformationtoexplainvanadium releaseandtoidentifythemechanismsassociated.Wehave chosenvariousstandardcompoundsinthetypeofcoordinating atom(oxides,minerals).Thechoiceofstandardcompounds shouldbealwaysdonewithcare,andstandardcompoundswith similarchemicalbondingarepreferred.However,thenatureof standardcompoundsappearslesscrucialwiththisselected interpretationmethod.Thismethodiswelladaptedtothe assessmentofvanadiumspeciationinaheterogeneousunknown sample,suchassteelslag,evenwithstandardcompoundswith differentchemicalbonding. |
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