1492Ch05

Manahan,StanleyE.PHASEINTERACTIONSEnvironmentalChemistryBocaRaton:CRCPressLLC,20005PHASEINTERACTIONS____________________________________________________5.1.CHEMICALINTERACTIONSINVOLVINGSOLIDS,GASES,ANDWATERHomogeneouschemicalreactionsoccurringentirelyinaqueoussolutionareratherrareinnaturalwatersandwastewaters.Instead,mostsignificantchemicalandbiochemicalphenomenainwaterinvolveinteractionsbetweenspeciesinwaterandanotherphase.SomeoftheseimportantinteractionsareillustratedinFigure5.1.Severalexamplesofphaseinteractionsinwaterillustratedbythefigurearethefol-4Figure5.1.Mostimportantenvironmentalchemicalprocessesinwaterinvolveinteractionsbetweenwaterandanotherphase.lowing:productionofsolidbiomassthroughthephotosyntheticactivityofalgaeoccurswithinasuspendedalgalcellandinvolvesexchangeofdissolvedsolidsand©2000CRCPressLLCgasesbetweenthesurroundingwaterandthecell.Similarexchangesoccurwhenbacteriadegradeorganicmatter(oftenintheformofsmallparticles)inwater.Chemicalreactionsoccurthatproducesolidsorgasesinwater.Ironandmanyimportanttrace-levelelementsaretransportedthroughaquaticsystemsascolloidalchemicalcompoundsoraresorbedtosolidparticles.Pollutanthydrocarbonsandsomepesticidesmaybepresentonthewatersurfaceasanimmiscibleliquidfilm.Sedimentcanbewashedphysicallyintoabodyofwater.Thischapterdiscussestheimportanceofinteractionsamongdifferentphasesinaquaticchemicalprocesses.Inageneralsense,inadditiontowater,thesephasesmaybedividedbetweensediments(bulksolids)andsuspendedcolloidalmaterial.Thewaysinwhichsedimentsareformedandthesignificanceofsedimentsasrepositoriesandsourcesofaquaticsolutesarediscussed.Mentionedinearlierchapters,solubilitiesofsolidsandgases(Henry’slaw)arecoveredhereinsomedetail.Muchofthischapterdealswiththebehaviorofcolloidalmaterial,whichconsistsofveryfineparticlesofsolids,gases,orimmiscibleliquidssuspendedinwater.Colloidalmaterialisinvolvedwithmanysignificantaquaticchemicalphe-nomena.Itisveryreactivebecauseofitshighsurface-area-to-volumeratio.5.2.IMPORTANCEANDFORMATIONOFSEDIMENTSSedimentsarethelayersofrelativelyfinelydividedmattercoveringthebottomsofrivers,streams,lakes,reservoirs,bays,estuaries,andoceans.Sedimentstypicallyconsistofmixturesoffine-,medium-,andcoarse-grainedminerals,includingclay,silt,andsand,mixedwithorganicmatter.Theymayvaryincompositionfrompuremineralmattertopredominantlyorganicmatter.Sedimentsarerepositoriesofavarietyofbiological,chemical,andpollutantdetritusinbodiesofwater.Ofparticularconcernisthetransferofchemicalspeciesfromsedimentsintoaquaticfoodchainsviaorganismsthatspendsignificantpartsoftheirlifecyclesincontactwithorlivinginsediments.Amongthesediment-dwellingorganismsarevariouskindsofshellfish(shrimp,crayfish,crab,clams)andavarietyofworms,insects,amphipods,bivalves,andothersmallerorganismsthatareofparticularconcernbecausetheyarelocatednearthebottomofthefoodchain.Althoughtheclassicpictureofpollutanttransferfromsedimentstoorganismsinvokesanintermediatestageinwatersolution,itisnowbelievedthatdirecttransferfromsedimentstoorganismsoccurstoalargeextent.Thisisprobablyparticularlyimportantforpoorly-water-solubleorganophilicpollutants,suchasorganohalidepesticides.Theportionofsubstancesheldinsedimentsthatisprobablymostavailabletoorganismsisthatcontainedinporewater,containedinmicroscopicporeswithinthesedimentmass.Porewateriscommonlyextractedfromsedimentsformeasurementsoftoxicitytoaquatictestorganisms.FormationofSedimentsPhysical,chemical,andbiologicalprocessesmayallresultinthedepositionof©2000CRCPressLLCsedimentsinthebottomregionsofbodiesofwater.Sedimentarymaterialmaybesimplycarriedintoabodyofwaterbyerosionorthroughsloughing(cavingin)oftheshore.Thus,clay,sand,organicmatter,andothermaterialsmaybewashedintoalakeandsettleoutaslayersofsediment.Sedimentsmaybeformedbysimpleprecipitationreactions,severalofwhicharediscussedbelow.Whenaphosphate-richwastewaterentersabodyofwatercontainingahighconcentrationofcalciumion,thefollowingreactionoccurstoproducesolidhydroxyapatite:5Ca2++H2O+3HPO42-→Ca5OH(PO4)3(s)+4H+(5.2.1)Calciumcarbonatesedimentmayformwhenwaterrichincarbondioxideandcontainingahighlevelofcalciumastemporaryhardness(seeSection3.5)losescarbondioxidetotheatmosphere,Ca2++2HCO3-→CaCO3(s)+CO2(g)+H2O(5.2.2)orwhenthepHisraisedbyaphotosyntheticreaction:Ca2++2HCO3-+hν→{CH2O}+CaCO3(s)+O2(g)(5.2.3)Oxidationofreducedformsofanelementcanresultinitstransformationtoaninsolublespecies,suchasoccurswheniron(II)isoxidizedtoiron(III)toproduceaprecipitateofinsolubleiron(III)hydroxide:4Fe2++10H2O+O2→4Fe(OH)3(s)+8H+(5.2.4)AdecreaseinpHcanresultintheproductionofaninsolublehumicacidsedimentfrombase-solubleorganichumicsubstancesinsolution(seeSection3.17).Biologicalactivityisresponsiblefortheformationofsomeaquaticsediments.Somebacterialspeciesproducelargequantitiesofiron(III)oxide(seeSection6.14)aspartoftheirenergy-extractingmediationoftheoxidationofiron(II)toiron(III).Inanaerobicbottomregionsofbodiesofwater,somebacteriausesulfateionasanelectronreceptor,SO42-→H2S(5.2.5)whereasotherbacteriareduceiron(III)toiron(II):Fe(OH)3(s)→Fe2+(5