ESM 202Water Quality:Additional conceptsAlkalinity3Alkalinity Alkalinity is a measure of the Acid Neutralizing Capacity (ANC) of an aqueous body (lake, ocean, stream, groundwater) Where does it come from: What does it mean?Carbonate System Carbonic acid is formed after CO2dissolves in water:CO2(g) ↔ CO2(aq)CO2(aq) + H2O ↔ H2CO3Carbonic acid can loose up to 2 H+H2CO3↔ HCO3-+ H+HCO3-↔ CO32-+ H+pK1= 6.3pK2= 10.3pKH= 1.5Carbonate System Minerals with carbonateLimestone/calcite: CaCO3Dolomite: MgCO3FeCO3Dissolved concentration of Ca2+, Mg2+and Fe2+is controlled by pH and [CO32-]↔ Ca2++ CO32-↔ Mg2++ CO32-↔ Fe2++ CO32-6Dissolved Carbon Dioxide: Closed System Start with a closed systemgroundwater situation where carbonate rocks (limestone, dolomite) are present, but there is no contact with atmosphere7Dissolved Carbon Dioxide: Closed System In a closed systemtotal concentration of carbonate is constantCtot= [H2CO3] + [HCO3-] + [CO32-] = DICA typical value is Ctot= 10-3mol/LpCtot= 3DIC = Dissolved Inorganic Carbon8Dissolved Carbon Dioxide: Closed SystempCpHCtotpK1pK2[H2CO3][HCO3-] [CO32-]9Dissolved Carbon Dioxide: Open System Concentration of CO2in the atmosphere~380 ppmv => PCO2= 10-3.5atmrelatively constant Since PCO2is ~ constant and KHis constant[H2CO3] is constantindependent of pH[H2CO3] = PCO2KH,CO2= 10 10 = 10 M-1.5-3.5 -5pH2CO3= 510Dissolved Carbon Dioxide: Open SystempCpHCtotpK1pK2[H2CO3][HCO3-][CO32-]Carbonate System Take home message:Type of inorganic carbon depends on pHLow pH => carbonic acidMid pH => bicarbonateHigh pH => carbonateDepends on whether system is open or closedHas major influence on alkalinity, buffering capacity and hardness12Alkalinity Alkalinitycapacity of water to accept H+sum of chemical species that accept H+in water:Alkalinity = [HCO3-] + 2[CO32-] + [OH-] - [H+] + [B(OH)4-] + [NH3] + [HS-] + ...In many cases we refer to only the carbonate components of alkalinity, since these are the major constituents:Alk = [HCO3-] + 2[CO32-] + [OH-] - [H+]14Effect of Photosynthesis A simplistic view of photosynthesisn CO2+ n H2O = (CH2O)n+ n O2 Photosynthesis is also accompanied by the assimilation of other ions, such as HPO42-, and NO3-or NH4+15Effect of Photosynthesis For example, the uptake of NH4+results in the release of H+, affecting alkalinity and pH106 CO2+ 16 NH4++ HPO42-+ 108 H2O= (CH2O)106(NH3)16PO4+ 107 O2+ 14 H+ If NO3-is used to produce algae, then H+are needed, increasing pH and affecting alkalinity106 CO2+ 16 NO3-+ HPO42-+ 122 H2O+ 18 H+ = (CH2O)106(NH3)16PO4+ 138 O2(algae)17Buffer Capacity An aqueous solution is “buffered” when the concentration of dissolved ions is relatively largeAddition of small amounts of strong acids or bases does not change the pH of solution significantlyHighest buffering near pK1and pK2Ocean is very well buffered18Oceanic carbon Oceanic carbon is present in four major forms: DIC = 37,500 Pg C ≈ 2.25 x 10-3mol/LDOC = Dissolved Organic Carbon = 1,000 Pg C ≈ 0.06 mMPOC = Particulate Organic Carbon = 30 Pg C ≈ 0.002 mMMarine biota (microorganisms, plants and animals)19Oceanic carbon Organic acids in DOC are considered as:H(DOC) = DOC-+ H+pKDOC~ 5.5I Marine biota are only ~ 3 Pg (0.0002 mM) I Large impact on cycling of carbon and nutrientsI Can have significant effect on alkalinityRedox PotentialRedox Conditions What are Redox Conditions?Determine whether local environment is Oxidative ReducingGradient of conditions Atmosphere is highly oxidative Deep sediments are highly reducing How do we measure redox conditions?Concentration of available electrons for transfers: pe = -log[e-]Redox Conditions Why do we care?Determines the form in which an element will be present Oxidized (e.g. CO2, CO, NO3-, SO42-, Fe3+) Reduced (e.g. CH4, NH3, H2S, Fe2+)Availability & toxicity of element depends on form Cr3+vs. Cr6+ Hg(0) vs. Hg+Energy stored in reduced forms23Quick Review of Oxidation States Only a few elements (C, N, O, S, Fe, Mn) participate significantly in natural redox processes As a rule, molecules of the element itself (e.g. N2, O2, H2, Fe, Pb) are in a “zero”oxidation state. Some elements have in general only one other oxidation state:H is always +1O is usually -2Halogens (Cl, Br, I, F) are usually -124Quick Review of Oxidation States Other elements have a range of oxidation states:CH4CH2=CH2CH2OCO2Example: Carbon (C) goes from -4 to +4-4 +4 -2 +2 -2 +2 0 +2 -2 +4 -4100% reduced <-------------> 100% oxidizedExample: Nitrogen (N) goes from -3 to +5NH3N2N2ONO2-3 +3 0 +1 -2 +4 -4NO3-+5 -6100% reduced <-------------> 100% oxidizedMinerals (Inorganic Ions) Common Anions:F-, Cl-, Br-, I-, OH-, NO2-, NO3-, SO42-, HS-, S2-, HCO3-, CO32-, PO43-, HPO42-Common Cations:NH4+,Ca2+, Mg2+, Fe2+, Fe3+, Na+, K+, H+Less common ions:Pb2+, Cd2+, Zn2+, Hg+, Hg2+, Cr3+26What are common redox conditions?27Redox Potential Examples of common oxidation/reduction reactions32chemical2OFe2O3Fe4 ⎯⎯⎯⎯→←+−+++⎯⎯⎯⎯→← eFeFe3bio/chem2OH2)g(CHe8H8)g(CO24bio2+⎯⎯→←++−+−+++⎯⎯→←+ e24H24)g(CO6OH6OHC2bio226 Energy is associated with these electron transfers28Redox Every oxidation reaction is coupled with a reduction reaction:OHeHO22244 =++−+reduction−+++=eFeFe44432oxidation++++=++32224244FeOHFeHO29Redox Oxidation of Organic Matter by SO42-OHgSHeHSO222421)(814581+=++−+−−+++=+eHgCOOHOCH)(414141222OHCOSHHOCHSO222224414181414181++=+++−Thermodynamic Sequence of ReductionReaction Eh (V) ∆ G (kcal/mol e-) Reduction of O2 O2 + 4 H+ + 4 e- = 2 H2O 0.812 -29.9 Reduction of NO3- NO3- + 2 H+ + 2 e- = NO2- + H2O 0.747 -28.4 Reduction of Mn4+ to Mn2+ MnO2 + 4 H+ + 2 e- = Mn2+ + 2 H2O 0.526 -23.3 Reduction of Fe3+ to Fe2+ Fe(OH)3 + 3 H+ + e- = Fe2+ + 3 H2O -0.047 -10.1 Reduction of SO42- to H2S SO42- + 10 H+ + 8 e- = H2S + 4 H2O -0.221 -5.9 Reduction of CO2 to CH4 CO2 + 8 H+ + 8 e- = CH4 + 2 H2O -0.244 -5.6 Assuming coupling to the oxidation of organic matter:−+++=+ eHgCOOHOCH )(41414122231Redox Potential Concentration gradient of O2: top layers oxic and the bottom anoxic Diffusion of O2from the surface of the water to the deeper layers is slow Many organisms make use of the available O2as it diffuses downward Reduction occurs in the