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salinity,alkalinity,pH,levelofsulphides,thepresenceofinorganicandor-
ganiccomplexingagents(SoumyaandWood,2004).
Animportantroleinmetalspeciationisplayedbyorganicmatter.Tey
arerepresentedbyheterogeneousmolecules;namelydissolved,colloidalor
particulatecompounds.Inaquaticecosystems,organicmaterialgenerally
takestheformofparticulatematter,whichformssedimentsormaybesus-
pendedinwater(Lepaneetal.,2007).Organicmatterrangesfrommini-
maltoover20%ofsediments.Televelofsuspendedorganicmaterialis
lessstable.Generally,itsconcentrationsharplyincreasesinturbidwatersor
duringheavyrainfallswhenparticlesofsoilsarewashedintotheriversor
lakes(Mülleratal.,1997).Bothdissolvedandparticulateorganicmaterial
formsolubleorinsolublecomplexeswithpolyvalentcations,modifyingthe
bioavailabilityofdiferentmetalions.Teimpactoforganiccompoundson
bioavailabilityandtoxicityofheavymetalsisveryimportant.Itdepends
onthemultitudeoffactors,suchasthestrengthandstabilityoforgano-
metallicbindings,molecularweightofspecificorganiccompoundsbound
tometalions,theirafnitytometalions,degreeofsaturation,numberof
bindingsitesonparticulatematter,andthepHofwater,i.e.competition
ofthemetalionswithhydrogenprotonsforthebindingsitesonorganic
molecules(ChrzanandMarko-Worłowska,2006;Appelbladetal.,1999).
Animportantroleincadmiumspeciationisplayedbyinorganiccom-
pounds.Inaquaticenvironmentcadmiumformssolubleinorganiccom-
poundssuchas[Cd(OH)]+,[CdCO
3(aq)]0,[CdSO
4(aq)],[CdCl]+.Athigh
pHvalue[Cd(OH)
3]-and[Cd(OH)
4]2-occur(BjerregaardandAndersen,
2007).Inorganicinsolublecompoundsplayanimportantroleincadmium
sedimentation.Cadmiumisoftenboundtocalciumcarbonateaswellas
manganeseandironoxides.Tesemoleculestypicallyexhibitlargesurfaces
withnumeroussurfacesitestowhichcadmiummaybebound(Mülleret
al.,1997).Cadmiummaybealsoabsorbedonclaymineralssuchasillite,
kaolinite,chloriteandsmectite.Ontheotherhand,illiteandkaolinitehave
relativelylowcationexchangecapacity,somostofthecadmiumionsedi-
mentsforminsolublesulphidesandchloridesandalsoco-precipitatewith
carbonatesandoxides(Nascimentoetal.,2006).
Metalpollutantsenteringaquaticecosystemsaccumulateinthebottom
sediments,fromwheretheycanberemobilisedinfavourableconditions,
forinstanceatlowpHofwaterandinturbidwatersbringingmoreoxygen
tothebottom.Concentrationofdissolvedcadmiumincreasesgreatlywhen
reducingsystemsareoxidized.Tismostoftenleadstocadmiummobi-
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