Alkalinity. Water Quality: Additional concepts ESM 202. Alkalinity. Carbonate System. Carbonate System

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1 ESM 202 Water Quality: Additional concepts Alkalinity Alkalinity Alkalinity is a measure of the Acid Neutralizing Capacity (ANC) of an aqueous body (lake, ocean, stream, groundwater) Where does it come from: Carbonate System Carbonic acid is formed after CO 2 dissolves in water: CO 2 (g) CO 2 (aq) CO 2 (aq) + H 2 O H 2 CO 3 pk H = 1.5 Carbonic acid can loose up to 2 H + What does it mean? H 2 CO 3 HCO H + pk 1 = HCO 3- CO 3 + H + pk 2 = 10.3 Carbonate System Minerals with carbonate Limestone/calcite: CaCO 3 Ca 2+ + CO 3 Dolomite: MgCO 3 Mg 2+ + CO 3 FeCO 3 Fe 2+ + CO 3 Dissolved concentration of Ca 2+, Mg 2+ and Fe 2+ is controlled by ph and [CO 3 ] Dissolved Carbon Dioxide: Closed System Start with a closed system groundwater situation where carbonate rocks (limestone, dolomite) are present, but there is no contact with atmosphere 6

2 Dissolved Carbon Dioxide: Closed System In a closed system total concentration of carbonate is constant C tot = [H 2 CO 3 ] + [HCO 3- ] + [CO 3 ] = DIC Dissolved Carbon Dioxide: Closed System ph pk 1 pk 2 [H 2 CO 3 ] [HCO 3- ] [CO 3 ] C tot DIC = Dissolved Inorganic Carbon pc A typical value is C tot = 10-3 mol/l pc tot = Dissolved Carbon Dioxide: Open System Dissolved Carbon Dioxide: Open System Concentration of CO 2 in the atmosphere ~380 ppmv => P CO2 = atm relatively constant pk 1 ph pk 2 Since P CO2 is ~ constant and K H is constant [H 2 CO 3 ] is constant independent of ph pc [H 2 CO 3 ] [HCO 3- ] C tot [H 2 CO 3 ] = P CO2 K H,CO2 = = 10 M ph 2 CO 3 = 5 [CO 3 ] 9 10 Carbonate System Take home message: Type of inorganic carbon depends on ph Low ph => carbonic acid Mid ph => bicarbonate High ph => carbonate Depends on whether system is open or closed Has major influence on alkalinity, buffering capacity and hardness Alkalinity Alkalinity capacity of water to accept H + sum of chemical species that accept H + in water: Alkalinity = [HCO 3- ] + 2[CO 3 ] + [OH - ] - [H + ] + [B(OH) 4- ] + [NH 3 ] + [HS - ] +... In many cases we refer to only the carbonate components of alkalinity, since these are the major constituents: Alk = [HCO 3- ] + 2[CO 3 ] + [OH - ] - [H + ] 12

3 Effect of Photosynthesis A simplistic view of photosynthesis n CO 2 + n H 2 O = (CH 2 O) n + n O 2 Photosynthesis is also accompanied by the assimilation of other ions, such as HPO 4, and NO 3- or NH Effect of Photosynthesis For example, the uptake of NH 4+ results in the release of H +, affecting alkalinity and ph 106 CO NH 4+ + HPO H 2 O = (CH 2 O) 106 (NH 3 ) 16 PO O H + (algae) If NO 3- is used to produce algae, then H + are needed, increasing ph and affecting alkalinity 106 CO NO 3- + HPO H 2 O + 18 H + = (CH 2 O) 106 (NH 3 ) 16 PO O 2 15 Buffer Capacity An aqueous solution is buffered when the concentration of dissolved ions is relatively large Addition of small amounts of strong acids or bases does not change the ph of solution significantly Highest buffering near pk 1 and pk 2 Ocean is very well buffered 17 Oceanic carbon Oceanic carbon is present in four major forms: DIC = 37,500 Pg C 2.25 x 10-3 mol/l DOC = Dissolved Organic Carbon = 1,000 Pg C 0.06 mm POC = Particulate Organic Carbon = 30 Pg C mm Marine biota (microorganisms, plants and animals) 18

4 Oceanic carbon Organic acids in DOC are considered as: H(DOC) = DOC - + H + pk DOC ~ 5.5 Marine biota are only ~ 3 Pg ( mm) Large impact on cycling of carbon and nutrients Can have significant effect on alkalinity Potential 19 Conditions What are Conditions? Determine whether local environment is Oxidative Reducing Gradient 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 - ] Conditions Why do we care? Determines the form in which an element will be present Oxidized (e.g. CO 2, CO, NO 3-, SO 4, Fe 3+ ) Reduced (e.g. CH 4, NH 3, H 2 S, Fe 2+ ) Availability & toxicity of element depends on form Cr 3+ vs. Cr 6+ Hg (0) vs. Hg + Energy stored in reduced forms Quick Review of Oxidation States Quick 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. N 2, O 2, H 2, Fe, Pb) are in a zero oxidation state. Some elements have in general only one other oxidation state: H is always +1 O is usually -2 Halogens (Cl, Br, I, F) are usually Other elements have a range of oxidation states: Example: Carbon (C) goes from -4 to +4 CH 4 CH 2 =CH 2 CH 2 O CO % reduced < > 100% oxidized Example: Nitrogen (N) goes from -3 to +5 NH 3 N 2 N 2 O NO 2 NO % reduced < > 100% oxidized 24

5 Minerals (Inorganic Ions) What are common redox conditions? Common Anions: F -, Cl -, Br -, I -, OH -, NO, NO 3-, SO 4, HS -, S, HCO 3-, CO 3, PO 3-4, HPO 4 Common Cations: NH 4+,Ca 2+, Mg 2+, Fe 2+, Fe 3+, Na +, K +, H + Less common ions: Pb 2+, Cd 2+, Zn 2+, Hg +, Hg 2+, Cr Potential Examples of common oxidation/reduction reactions chemical 4Fe+ 3O 2 2Fe2 O3 2+ chem/ bio 3+ Fe Fe + e + bio CO2 ( g ) + 8H + 8e CH 4( g ) + 2H 2O bio C6 H2O + 6H2O 6CO 2( g) 24H 24e O Every oxidation reaction is coupled with a reduction reaction: Fe = 4 Fe + 4 e O H + 4 e = 2 H 2 O H + 4 Fe = 2H 2O oxidation + reduction 4 Fe 3 + Energy is associated with these electron transfers Thermodynamic Sequence of Reduction Oxidation of Organic Matter by SO CH + 2 O + H 2 O = CO 2 ( g ) + H + e SO4 + H + e = H2S ( g ) + H2O SO 4 + CH2O + H = H2S + CO2 + 4 H2O Reaction Eh (V) G (kcal/mol e-) Reduction of O 2 O H e - = 2 H 2O Reduction of NO 3 NO H e - = NO H 2O Reduction of Mn 4+ to Mn 2+ MnO H e - = Mn H 2O Reduction of Fe 3+ to Fe 2+ Fe(OH) H + + e - = Fe H 2O Reduction of SO 4 to H 2S SO H e - = H 2S + 4 H 2O Reduction of CO 2 to CH 4 CO H e - = CH H 2O Assuming coupling to the oxidation of organic matter: CH O + H O = CO ( g) + H e 4 4 4

6 Potential Concentration gradient of O 2 : top layers oxic and the bottom anoxic Diffusion of O 2 from the surface of the water to the deeper layers is slow Many organisms make use of the available O 2 as it diffuses downward Reduction occurs in the anoxic environment (low redox potential) CO 2 Sequence of Conditions Aerobic Aerobic, O 2 based Facultative Anaerobic, O 2 or NO - 3 Anaerobic, Mn 4+ or Fe 3+ Anaerobic, SO 4 H 2 S, HS - Atmosphere Surface water NH + 4 Mn 2+ or Fe 2+ Anaerobic, CO 2 CH 4 31 Reducing (low pe) Oxidizing (high pe) 33 36

7 Rate of Reaction Kinetics: Rate of Reaction Favorable (thermodynamically) chemical reaction may proceed very fast (picoseconds) very slowly (geologic timescales) Rate of a reaction depends on energy barrier to bring together reactants must be overcome to proceed to equilibrium Catalysts and biota (via enzymes) can reduce energy barrier provide specific sites for reaction to occur 38 Rate of Reaction Overall rate of reaction May be controlled by mixing Availability for reacting In some cases, rate of reaction depends only on concentration of reactants: d [ A] = A + B = C + D d [ B ] = k[ A][ B ] d [ C ] = = d [ D ] Rate of Reaction Example: oxidation of CH 4 in the atmosphere atmos CH 4 + OH CH 3 + H 2 O d [ CH 4 ] = k [ CH4][ OH ] Rate of Reaction Aquatic Chemistry In other conditions, one of the reactants is abundant, so the reaction is independent of it: d[ CH4] = k[ CH4] This is the case for methanothropic conversion of CH 4 to CH 3 OH by bacteria Other reactions can be quite a bit more complex: 2+ d[ Fe ] 2+ 2 = k[ Fe ][ OH ] P Thermodynamics: determine how various chemical components in environment will behave under a given set of conditions (e.g. ph, pe, T) Kinetics understand rates at which these processes (reactions) can occur understand how can they be accelerated by biotic activity 42

8 Key Points on Water Quality Diversity of indicators of WQ Dissolution of gases: controlled by Henry s Law (K H ) Dissociation of acids & bases: controlled by K A or K B ph controls whether chemical is present as acid, base or dissociated Important systems: Carbonate, Nitrate, Ammonia and Phosphate Alkalinity: capacity to neutralize acid : oxidizing vs. reducing conditions Controlled by pe Availability of oxygen Kinetics: equilibrium can take time