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1、Chapter11GeochemicalKineticsandTransportCarlI.Steefel11.1IntroductionThekineticsofgeochemicalandbiogeochemicalprocessescanbestudiediniso-lationinthelaboratory,butonlyrarelyisitpossibletoseparatetheseprocessesfromthoseoftransportwhenconsideringtheirimportanceinnaturalsystemsatthefield
2、scale.Thisisbecausethedrivingforceformostreactionsofinterestinwater–rockinteractionistransport.Themostintensegeochemicalandbiogeochem-icalactivityoccursattheinterfacebetweenglobalcompartmentsliketheoceans,theatmosphere,andtheEarth’scrustwhereelementalandnutrientfluxesprovidethemaximu
3、mdrivingforceforreactionstotakeplace.Theimportantroleoftrans-portinthesesettingsmakesitcriticaltoconsiderthesetime-dependentprocessesinconjunctionwiththoseprocesseswethinkofasmorepurelybiogeochemical.Inotherwords,theseglobalinterfacesareopensystems,wherebothmassandenergytransfersmus
4、tbeaccountedfor.Forthesakeofconvenience,geochemicalandbiogeochemicalkineticsareof-tenstudiedinclosedsystemsinthelaboratory,butitisimportanttokeepinmindhowdifferenttheclosedsystemscanbefromopensystems.Inaclosedsystem,thebestanalogueofwhichisthebatchreactorinthelaboratory(initssimples
5、tform,justalaboratorybeaker,Fig.11.1a),thesolutionbeginsoutofequilibriuminitiallyandevolvesovertimetowardseitherthermodynamicequilibrium,ortosomeotherendpointbeyondwhichthereactioncannotproceed.Eitherequilib-riumisachieved(wherenetreactionratesare,bydefinition,=0),orratesbe-comesoslo
6、wthatthesystemeffectivelynolongerchanges.Inanopensystem,agoodlaboratoryanalogueofwhichisthecontinuouslystirredtankreactor(CSTR)orwell-mixedflowthroughreactor(Fig.11.1b),thecontinuousinjectionofsolu-tionthatisoutofequilibrium(eitherwithitself,orwithasolidphaseinsidethereactor)causesth
7、esolutioncompositioncomingoutofthereactortoreachaLawrenceBerkeleyNationalLaboratory,EarthSciencesDivision,CISteefel@lbl.govAug7,2007545546CarlI.SteefelFig.11.1Contrastbetweenclosedandopensystembehavior:a.Inalaboratorybatchreactor,thesolutionevolvesfromsomeinitialstatetoafinalequilibr
8、iumstate,oronewhere
