1492Ch04

Manahan,StanleyE.OXIDATION-REDUCTIONEnvironmentalChemistryBocaRaton:CRCPressLLC,20004OXIDATION-REDUCTION____________________________________________________4.1.THESIGNIFICANCEOFOXIDATION-REDUCTIONOxidation-reduction(redox)reactionsarethoseinvolvingchangesofoxidationstatesofreactants.Suchreactionsareeasiesttovisualizeasthetransferofelectronsfromonespeciestoanother.Forexample,solublecadmiumion,Cd2+,isremovedfromwastewaterbyreactionwithmetalliciron.TheoverallreactionisCd2++Fe→Cd+Fe2+(4.1.1)Thisreactionisthesumoftwohalf-reactions,areductionhalf-reactioninwhichcadmiumionacceptstwoelectronsandisreduced,Cd2++2e-→Cd(4.1.2)andanoxidationhalf-reactioninwhichelementalironisoxidized:Fe→Fe2++2e-(4.1.3)Whenthesetwohalf-reactionsareaddedalgebraically,theelectronscancelonbothsidesandtheresultistheoverallreactiongiveninEquation4.1.1.Oxidation-reductionphenomenaarehighlysignificantintheenvironmentalchemistryofnaturalwatersandwastewaters.Inalake,forexample,thereductionofoxygen(O2)byorganicmatter(representedby{CH2O}),{CH2O}+O2→CO2+H2O(4.1.4)resultsinoxygendepletionwhichcanbefataltofish.Therateatwhichsewageisoxidizediscrucialtotheoperationofawastetreatmentplant.Reductionofinsolubleiron(III)tosolubleiron(II),©2000CRCPressLLCFe(OH)3(s)+3H++e-→Fe2++3H2O(4.1.5)inareservoircontaminatesthewaterwithdissolvediron,whichishardtoremoveinthewatertreatmentplant.OxidationofNH4+toNO3-inwater,NH4++2O2→NO3-+2H++H2O(4.1.6)convertsammoniumnitrogentonitrate,aformmoreassimilablebyalgaeinthewater.Manyotherexamplescanbecitedofthewaysinwhichthetypes,rates,andequilibriaofredoxreactionslargelydeterminethenatureofimportantsolutespeciesinwater.Thischapterdiscussesredoxprocessesandequilibriainwater.Insodoing,itemphasizestheconceptofpE,analogoustopHanddefinedasthenegativelogofelectronactivity.LowpEvaluesareindicativeofreducingconditionsandhighpEvaluesreflectoxidizingconditions.Twoimportantpointsshouldbestressedregardingredoxreactionsinnaturalwatersandwastewaters.First,asisdiscussedinChapter6,“AquaticMicrobialBiochemistry,”manyofthemostimportantredoxreactionsarecatalyzedbymicro-organisms.Bacteriaarethecatalystsbywhichmolecularoxygenreactswithorganicmatter,iron(III)isreducedtoiron(II),andammoniaisoxidizedtonitrateion.Thesecondimportantpointregardingredoxreactionsinthehydrosphereistheircloserelationshiptoacid-basereactions.WhereastheactivityoftheH+ionisusedtoexpresstheextenttowhichwaterisacidicorbasic,theactivityoftheelectron,e-,isusedtoexpressthedegreetowhichanaquaticmediumisoxidizingorreducing.Waterwithahighhydrogenionactivity,suchasrunofffrom“acidrain”,isacidic.Byanalogy,waterwithahighelectronactivity,suchasthatintheanaerobicdigesterofasewagetreatmentplant,issaidtobereducing.WaterwithalowH+ionactivity(highconcentrationofOH-)—suchaslandfillleachatecontaminatedwithwastesodiumhydroxide—isbasic,whereaswaterwithalowelectronactivity—highlychlorinatedwater,forexample—issaidtobeoxidizing.Actually,neitherfreeelectronsnorfreeH+ionsassucharefounddissolvedinaquaticsolution;theyarealwaysstronglyassociatedwithsolventorsolutespecies.However,theconceptofelectronactivity,likethatofhydrogenionactivity,remainsaveryusefulonetotheaquaticchemist.ManyspeciesinwaterundergoexchangeofbothelectronsandH+ions.Forexample,acidminewatercontainsthehydratediron(III)ion,Fe(H2O)63+,whichreadilylosesH+ionFe(H2O)63+←→Fe(H2O)5OH2++H+(4.1.7)tocontributeaciditytothemedium.ThesameionacceptsanelectronFe(H2O)63++e-←→Fe(H2O)62+(4.1.8)togiveiron(II).Generally,thetransferofelectronsinaredoxreactionisaccompaniedbyH+iontransfer,andthereisacloserelationshipbetweenredoxandacid-baseprocesses.For©2000CRCPressLLCexample,ifiron(II)losesanelectronatpH7,threehydrogenionsarealsolosttoformhighlyinsolubleiron(II)hydroxide,Fe(H2O)62+←→e-+Fe(OH)3(s)+3H2O+3H+(4.1.9)aninsoluble,gelatinoussolid.ThestratifiedbodyofwatershowninFigure4.1canbeusedtoillustrateredoxphenomenaandrelationshipsinanaquaticsystem.Theanaerobicsedimentlayerissoreducingthatcarboncanbereducedtoitslowestpossibleoxidationstate,-4inCH4.Ifthelakebecomesanaerobic,thehypolimnionmaycontainelementsintheirreducedstates:NH4+fornitrogen,H2Sforsulfur,andsolubleFe(H2O)62+foriron.Saturationwithatmosphericoxygenmakesthesurfacelayerarelativelyoxidizingmedium.Ifallowedtoreachthermodynamicequilibrium,itischaracterizedbythemoreoxidizedformsoftheelementspresent:CO2forcarbon,NO3-fornitrogen,ironasinsolubleFe(OH)3,andsulfurasSO42-.Substantialchangesinthedistri-butionofchemicalspeciesinwaterresultingfromredoxreactionsarevitallyimportanttoaquaticorganismsandhavetremendousinfluenceonwaterquality.VerylowO2CH4NH4H2SFe2+Mn2+OxidizingO2CO2NO3SO42-Fe(OH)3MnO2Reducing-+Figure4.1.Predominanceofvariouschemicalspeciesinastratifiedbodyofwaterthathasahighoxygenconcentration(oxidizing,highpE)nearthesurfaceandalowoxygenconcentration(reducing,lowpE)nearthebottom.Itshouldbepointedoutthatthesystemspresentedinthischapterareassumedtobeatequilibrium,astatealmostneverachievedinanyrealnaturalwaterorwaste-watersystem.Mostrealaquaticsystemsaredynamicsystemsthatmayapproachasteady-state,ratherthantruee