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Monroe L. Weber-Shirk S chool of Civil and Environmental Engineering Acid Rain Lake pH Probe Peristaltic Pump Acid Lake Remediation

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Where Are We Going? Source of Acid Rain Fate of strong acids in the environment Reactions Carbonate System Dissociation constants p notation Alpha notation Acid Neutralizing Capacity Defined Measured Gran Plot A conservative property!

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What is the Acid Source? Coal fired electric plants (and other fossil fuels) Gaseous emissions of sulfur oxides and nitrogen oxides + water + sunlight form sulfuric acid and nitric acid Tall stacks send pollutants into the troposphere Prevailing winds carry pollutants from Midwestern industrialized areas into New England and Canada. About half of the acidity in the atmosphere falls back to earth through dry deposition as gases and dry particles. The combination of acid rain plus dry deposited acid is called acid deposition.

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Acid Rain Formation Combustion product precursors to acid rain Reactions Strong acids Sulfuric acidNitric acid

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Where is Acid Rain Falling?

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Fate of Strong Acids in the Environment Strong acids completely dissociate in water If 0.1 M of nitric acid is added to 1 liter of pure water, what is the concentration of H + ? _________ What is the pH? [px = -log(x)] ________ What else can happen to the hydrogen ions if it isnt pure water? ________________ 1 reactions 0.1 M pK=-1 pK 1 = -3, pK 2 =1.9

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Fate of Strong Acids: Reactions Weak acids/bases can react with the added H + and reduce the final concentration of H + Examples of weak acids and bases in the environment: carbonates carbonate, bicarbonate, carbonic acid organic acids (A - ) acetic acid (pK = 4.7) When K A = [H + ] then [A - ] = [HA]

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Carbonate System species definition 6.3 10.3 Dissociation Constant

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Acid Neutralizing Capacity (ANC) The ability to neutralize (react with) acid ANC has units of _______________ or eq/L Possible reactants moles of protons/L 2

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Alpha Notation All species concentrations are related to the hydrogen ion concentration Total carbonate species 1

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Hydrogen Ion Concentration: The Master Variable (pH, pK 1, pK 2, C T )

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pH Diagram Add acid to a carbonate solution at pH 9. What happens?

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ANC Example Suppose we add 3 mM Ca(OH) 2 to distilled water. What is the ANC? What is the resulting pH if the system is closed to the atmosphere? 2.22 p(OH)= 14 - 2.22 = 11.78 pH= no carbonates [H + ] is small

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negative ANC: + or -! ANC = capacity to react with H + minus the concentration of H + ANC can be positive or __________ ANC is conservative Example: 10 liters of a solution with an ANC of 0.1 meq/L is mixed with 5 liters of a solution with an ANC of -1 meq/L. What is the final ANC?

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ANC Relationships At what pH is ANC=0? Which species dominate when ANC = 0? Which species dominate when ANC < 0?

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Where does ANC =0?

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More Complications: Open to the Atmosphere Natural waters exchange carbon dioxide with the atmosphere The total concentration of carbonate species is affected by this exchange Henrys constant Is ANC affected? _____ NO!

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ANC Example (continued) Suppose we aerate the 3 mM Ca(OH) 2 solution. What happens to the pH? All the alphas are functions of pH and it is not possible to solve explicitly for [H + ]. Solution techniques numerical methods - spreadsheets - goal seeking (pH=9, C T =0.0057M)spreadsheets Solve for pH rather than for [H+] to make it possible for goal seek to find a solution! graphical methods (CEE 653) Beware of precision!

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Open vs. Closed to the Atmosphere What is conserved in an open (volatile) system? _____________ What is conserved in a closed (nonvolatile) system? _____________ For conservative species we can use the Completely Mixed Flow Reactor equation for our well mixed lake ANC CTCT

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Completely Mixed Flow Reactor Q, C in Q, __ C Mass in Mass out = Increase of Mass in reactor V, C Hydraulic Residence Time Mass balance

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Completely Mixed Flow Reactor Equation applies to any conservative species. C 0 = time zero concentration in reactor C in = influent concentration C = concentration in the reactor as a function of time Set up integration and effluent! What is C when t is large? _______ C in

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Three Equations for C T ! CMFR for conservative species. (True if nonvolatile!) If in equilibrium with atmospheric CO 2... Can we measure C T ? Assuming no carbonates in influent

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What is the measured concentration of carbonates? Measured C T ? What is ANC?

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Spreadsheet Hints Use names to make your equations easier to understand Use Visual Basic for complex equations (see course website) Completely Mixed Flow Reactor (CMFR) alphas Function CMFR(Influent, t, theta, initial) CMFR = Influent * (1 - Exp(-t / theta)) + initial * (Exp(-t / theta)) End Function Function alpha0CO2(pH) alpha0CO2 = 1 / (1 + 10 ^ (-6.3) / invp(pH) + 10 ^ (-6.3) * 10 ^ (-10.3) / invp(pH) ^ 2) End Function Function invp(x) invp = 10 ^ (-x) End Function required!

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Function ANCopen(pH) ANCopen = ANCclosed(pH, invp(5) / alpha0CO2(pH)) End Function Function ANCclosed(pH, Ct) ANCclosed = Ct * (alpha1CO2(pH) + 2 * alpha2CO2(pH)) + 10 ^ (-14) / invp(pH) - invp(pH) End Function 10 -1.5 mol/(L atm) Visual Basic Functions for ANC ANC for a closed system ANC for an open system 10 -3.5 atm

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Results? If your results dont jive where do you look? Units of ANC? Units of C T ? How is t defined? ??

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Acid Rain Lab Report Checklist at website Spreadsheet report (will write a full combined report after measuring ANC next week) Strong recommendation: finish the lab by Monday night Use your time efficiently! What is the ANC of the acid rain? Assume pH = 3.0, but there is some uncertainty Note any differences between lab manual guidelines and what happened in the lab

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Acid Rain Lab Report Make sure you understand which assumptions might be incorrect Mass conservation No exchange with the atmosphere Equilibrium with the atmosphere High expectations for Well designed spreadsheets Thoughtful analysis Work done with pride Alternate analysis: solve for pH as f(t) based on both models and compare with measured pH

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Reflections Our lake was idealized and missing many of the interesting (and confounding) factors of real lakes What would happen if I Dumped 1000 lb scoops of NaHCO 3 into a lake? Used CaCO 3 instead of NaHCO 3 ?

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What Determines Lake Susceptibility to Acidification? Acidification = f(acid inputs, ANC) Acid inputs = f(power plants, cars, wind currents, mine tailings) Acid Neutralizing Capacity = f(?) Carbonates obtained from dissolution of minerals such as CaCO 3 (calcite or aragonite) MgCO 3 (magnesite) CaMgCO 3 (dolomite) ...

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Lake and/or Watershed Remediation Add a soluble mineral such as lime (CaO) or sodium bicarbonate (NaHCO 3 ) Application options ________________ meter into stream apply directly to lake spread on watershed

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Measuring ANC: Gran Titration The sample is titrated with a strong acid to "cancel" the sample ANC At the equivalence point the sample ANC is zero Further titration will result in an increase in the number of moles of H + equal to the number of moles of H + added. Use the fact that ANC is conservative... Why not titrate to target pH? f(C t )

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Conservation of ANC V e = ___________ volume = volume of titrant added so that ANC = 0 Need to find ANC T and V e T = titrant S = sample equivalent

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ANC of Titrant Why? ___________ N T = [H + ] ANC conservation This equation is always true, but when do we know what ANC is? _______________________________________ When pH is so low that no reactions are occurring. Could solve for Ve, but what is ANC?

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ANC of Titrated Sample When is this true? ____________ For pH

Gran Plot 0 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008 0.0009 0123456 Volume of Titrant (mL) First Gran Function VeVe Algorithm for choosing points to include? Minimum value of F 1 before it is worth attempting analysis? 3 points with F 1 >0.0001pH