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Acid Lake Remediation

Acid Lake Remediation. pH Probe. Peristaltic Pump. “Acid Rain”. “Lake”. Lake Effluent. pH Meter. 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

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Acid Lake Remediation

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  1. Acid Lake Remediation pH Probe Peristaltic Pump “Acid Rain” “Lake” Lake Effluent pH Meter

  2. 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!

  3. Where is the acid coming from? • 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.

  4. Acid Rain Formation Combustion product precursors to acid rain Reactions Strong acids Sulfuric acid Nitric acid

  5. Where is Acid Rain Falling?

  6. 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 when the hydrogen ion concentration changes? ________________ 0.1 M 1 reactions

  7. 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 • acetic acid (pK = 4.7)

  8. Carbonate System species definition Dissociation Constant 6.3 10.3

  9. 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

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

  11. Hydrogen Ion Concentration:The Master Variable (pH, pK1, pK2, CT)

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

  13. p(OH)= pH= 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 14 - 2.22 = 11.78

  14. ANC • 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? negative

  15. ANC relationships • At what pH is ANC=0? • Which species dominate when ANC = 0? • Which species dominate when ANC < 0?

  16. 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

  17. ANC example (continued) • Suppose we aerate the 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, CT=0.0057M) • graphical methods (CEE 653) Beware of precision!

  18. 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 _____ equation ANC CT ANC CMFR

  19. Completely Mixed Flow Reactor • Equation applies to any conservative species. • C0 = time zero concentration in reactor • Cin = influent concentration • C = concentration in the reactor as a function of time

  20. Three equations for ANC! • CMFR for conservative species. (True whether volatile or nonvolatile!) • If Nonvolatile... • What is CT? • If Volatile...

  21. Spreadsheet Hints • Use names to make your equations easier to understand • Use Visual Basic for complex equations • 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 invp(x) invp = 10 ^ (-x) End Function Function alpha0CO2(pH) alpha0CO2 = 1 / (1 + 10 ^ (-6.3) / invp(pH) + 10 ^ (-6.3) * 10 ^ (-10.3) / invp(pH) ^ 2) End Function

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

  23. Results?

  24. 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...

  25. Conservation of ANC T = titrant S = sample equivalent Ve= ___________ volume = volume of titrant added so that ANC = 0 Need to find ANCT and Ve

  26. ANC of Titrant NT = [H+] Why? ___________ 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.

  27. ANC of Titrated Sample For pH << pK1 When is this true? ____________ Finally! An equation for equivalent volume!

  28. Gran Function • A better measure of the equivalent volume can be obtained by rearranging the equation so that linear regression on multiple titrant volume - pH data pairs can be used. • Define F1 as:

  29. Gran Plot 0.0009 0.0008 0.0007 0.0006 0.0005 First Gran Function 0.0004 0.0003 0.0002 0.0001 0 0 1 2 3 4 5 6 Volume of Titrant (mL) Ve

  30. Gran Plot using Compumet slope = abscissa intercept of Ve F1 plotted as a function of Vt. The abscissa has units of mL of titrant and the ordinate is a Gran function with units of [H+].

  31. Calculating ANC • The ANC is obtained from the equivalent volume. • The ANC of the acid rain can be estimated from its pH. At low pH (< pK1) most of the carbonates will be carbonic acid and thus for pH below about 4.3 the ANC equation simplifies to

  32. Titration Technique • Titrate with digital pipette • Measure pH before first addition of titrant • Measure pH after each addition of titrant • After ANC is consumed Gran function will be linear • What should the incremental titrant volume be? • Techniques to speed up titration

  33. Fossil Fuels to Acid Lakes • Source of acid rain • Fate of strong acids in the environment • Carbonate species and reactions • Definition of acid neutralizing capacity • Equilibrium with atmospheric carbon dioxide • Lake susceptibility to acidification • Lake remediation • ANC measurements

  34. Acid Rain Precursor Sources SO2 NOx Utilities Utilities Transport Ind/Mfg Process Ind. Combustion Other Combustion Other Transport Utilities NAPAP Emissions Inventory, Nov. 1989.

  35. Acid Rain Precursor Control • Emission controls • neutralize acid at source (scrubbers) • Taller stacks • combustion products can travel 1000+ km • down wind regions suffer pollutant damage • sends pollutants further away, but does not mitigate problem • Allowances

  36. What Are Allowances? • An allowance authorizes a unit within a utility or industrial source to emit one ton of SO2 during a given year or any year thereafter. • Allowances are fully marketable commodities. Once allocated, allowances may be bought, sold, traded, or banked for use in future years. Allowances may not be used for compliance prior to the calendar year for which they are allocated. • 8.95 million tons of SO2 annually (250 Gmole of H+/year)

  37. Acid Rain Experiment pH Probe Peristaltic Pump “Acid Rain” “Lake” Soil Column Lake Effluent pH Meter

  38. Acid Precipitation and Remediation of Acid Lakes

  39. Sources of ANC • Carbonates obtained from dissolution of minerals such as • CaCO3 (calcite or aragonite) • MgCO3 (magnesite) • CaMgCO3 (dolomite) • ... • Minerals that are insoluble or of very limited solubility don’t contribute much to ANC • granite (very insoluble silicates) • quartz (very insoluble silicon dioxide)

  40. What determines lake susceptibility to acidification? • Acidification = f(acid inputs, ANC) • Acid inputs = f(power plants, wind currents...) • Acid Neutralizing Capacity = f(?) • Suppose only water input into lake is precipitation • only source of ANC is minerals on lake bottom • lake will soon have pH of acid rain • Suppose only water input is through groundwater • soluble minerals will neutralize acid • ANC=f(_______________________) minerals in watershed

  41. Lake and/or Watershed Remediation • Add a soluble mineral such as lime (CaO) or sodium bicarbonate (NaHCO3) • Application options • ________________ • ________________ • ________________ spread on watershed meter into stream apply directly to lake

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