OXIDATION-REDUCTION REACTIONS IN NATURAL WATERS

OXIDATION-REDUCTION REACTIONS IN NATURAL WATERSPRESENTATION BY WATER CHEMISTRY GROUP 4 MEMBERSLECTURER: DR. R. YUSUF (BENG, MENG, PHD)What is oxidation and reductionNatural watersRedox reactions in natural watersExamples and applications

OUTLINECVW 731 REDOX REACTIONS IN NATURAL WATERS2OXIDATION AND REDUCTIONgain of electronsloss of electronsgain in hydrogenloss of hydrogenloss of oxygengain in oxygenReductionOxidationCVW 731 REDOX REACTIONS IN NATURAL WATERS3OXIDATION AND REDUCTIONIn terms of Oxygen transfer:Oxidation: Gain of oxygen E.g. Mg is oxidized to MgO

Reduction: Loss of oxygenE.g. H2O is reduced to H2

CVW 731 REDOX REACTIONS IN NATURAL WATERS4OXIDATION AND REDUCTIONIn terms of Hydrogen:Oxidation: Loss of hydrogenE.g. H2O is oxidized to O2

Reduction: Gain of hydrogenE.g. O2 is reduced to H2O2

CVW 731 REDOX REACTIONS IN NATURAL WATERS5OXIDATION AND REDUCTIONIn terms of Electrons transferOxidation: Loss of electrons E.g. Mg is oxidized to MgO (Mg from 12 electrons to 10 electrons in Mg2+)

Reduction: Gain of electrons in a speciesE.g. O2 is reduced to H2O2 (O from 8 electrons to 9 electrons per O in O22-)CVW 731 REDOX REACTIONS IN NATURAL WATERS6OXIDIZING AND REDUCING AGENTOxidizing agents give oxygen to or remove hydrogen from a substanceReducing agents remove oxygen from or give hydrogen to a substanceCVW 731 REDOX REACTIONS IN NATURAL WATERS7LIST OF COMMON OXIDIZING AND REDUCING AGENTSRealize something?H2O2 is both an oxidizing and a reducing agent!If a stronger oxidizing agent is present, H2O2 is reducing

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NATURAL WATERS Water CycleSource: http://www.nasa.gov/vision/earth/environment/warm_wetworld.htmlCompartmentsAtmosphereLandGroundwaterRivers lakesOceans

CVW 731 REDOX REACTIONS IN NATURAL WATERS9NASA animation: http://www.nasa.gov/vision/earth/environment/warm_wetworld.htmlhttp://polaris.umuc.edu/cvu/envm/hydro/http://academics.rmu.edu/faculty/short/chem3500/chem3500-anim/hydrologic-cycle/

NATURAL WATERS Uses of WaterWorld70% irrigation (agriculture)20 % Industry10% residences

World Resources 1998-99CVW 731 REDOX REACTIONS IN NATURAL WATERS10Oxic watersSuboxic watersReducing watersREDOX CLASSIFICATION OF NATURAL WATERSCVW 731 REDOX REACTIONS IN NATURAL WATERS11Redox chemistry is part of our everyday livesPhotosynthesisNitrogen cycle, etcRedox conditions determine the oxidation states of many elements in natural watersREDOX REACTIONS IN NATURAL WATERSCVW 731 REDOX REACTIONS IN NATURAL WATERS12pE = (F/2.3RT)EhWhere: F = Faradays constant = 96500 (coulombs/mole) = 96.42 KJ/(voltmole)R= gas constant = (8.314 X 10-3 KJ/(degmole))T = absolute temperature = (K)pE = activity of the free electrons in water Eh = redox potential (volts) At 25C: pE = 16.9Eh Eh = 0.059pEThe pE indicates the tendency of a solution to donate or accept a proton.If pE is low - the solution is reducing.If pE is high - the solution is oxidizing.

REDOX EQUATIONSCVW 731 REDOX REACTIONS IN NATURAL WATERS13REDOX EQUATIONSCVW 731 REDOX REACTIONS IN NATURAL WATERS14Biological oxygen demand (BOD)Natural sequence of biological reduction 4NO3+5CH2O+4H+5CO2+2N2+7H2OPhotosynthesis CO2 + N + P + other = Corganic + O2Respiration (CH2O)+O2=CO2+H2ODenitrificationMuddy sediments at the bottom of a riverExamples of redox reactions in natural watersCVW 731 REDOX REACTIONS IN NATURAL WATERS15In the occurrence of nitrate in groundwaterDenitrification of NO3- and oxidation of bicarbonate produces N2 gas APPLICATION OF REDOX PROCESS IN GROUNDWATERCVW 731 REDOX REACTIONS IN NATURAL WATERS16