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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
Acid Lake Remediation

pH Probe

Peristaltic Pump

“Acid Rain”

“Lake”

Lake Effluent

pH Meter

where are we going
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!
where is the acid coming from
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.
acid rain formation
Acid Rain Formation

Combustion product precursors to acid rain

Reactions

Strong acids

Sulfuric acid

Nitric acid

fate of strong acids in the environment
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

fate of strong acids reactions
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)
carbonate system
Carbonate System

species

definition

Dissociation

Constant

6.3

10.3

acid neutralizing capacity anc
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

alpha notation
Alpha Notation
  • All species concentrations are related to the hydrogen ion concentration

Total carbonate species

1

ph diagram
pH Diagram

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

anc example

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

slide14
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

anc relationships
ANC relationships
  • At what pH is ANC=0?
  • Which species dominate when ANC = 0?
  • Which species dominate when ANC < 0?
more complications open to the atmosphere
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

anc example continued
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!

open vs closed to the atmosphere
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

completely mixed flow reactor
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
three equations for anc
Three equations for ANC!
  • CMFR for conservative species. (True whether volatile or nonvolatile!)
  • If Nonvolatile...
    • What is CT?
  • If Volatile...
spreadsheet hints
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

visual basic functions for anc
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

measuring anc gran titration
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...
conservation of anc
Conservation of ANC

T = titrant

S = sample

equivalent

Ve= ___________ volume

= volume of titrant added so that ANC = 0

Need to find ANCT and Ve

anc of titrant
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.

anc of titrated sample
ANC of Titrated Sample

For pH << pK1

When is this true? ____________

Finally! An equation for equivalent volume!

gran function
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:
gran plot
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

gran plot using compumet
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+].

calculating anc
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
titration technique
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
fossil fuels to acid lakes
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
acid rain precursor sources
Acid Rain Precursor Sources

SO2

NOx

Utilities

Utilities

Transport

Ind/Mfg Process

Ind. Combustion

Other

Combustion

Other

Transport

Utilities

NAPAP Emissions Inventory, Nov. 1989.

acid rain precursor control
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
what are allowances
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)
acid rain experiment
Acid Rain Experiment

pH Probe

Peristaltic Pump

“Acid Rain”

“Lake”

Soil Column

Lake Effluent

pH Meter

sources of anc
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)
what determines lake susceptibility to acidification
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

lake and or watershed remediation
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