1. Water Science Electroneutrality, pH, Alkalinity, Acidity
Reversible Equations
Carbonate Equilibrium System
Examples
Rain
Ocean pH
Acid Rain
Acid Mine Drainage
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2. Water Electroneutrality
Balanced: ∑ Cations = ∑ Anions
∑ CC x eC = ∑ CA x eA
CC = moles / liter of cation c (one mole = 6.02x1023)
eC = number of charges per molecule of cation C
A subscript is for anions
Pure water: [H+] = [OH-]
Natural water: [H+] = [OH-] + [HCO3-] + 2[CO3-2]
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3. pH - reporting H+ concentration
pH = - log {H+}
{} = activity, the effective concentration
pH = - log [H+] for dilute solutions
[] = Molar concentration (moles/Liter)
[H+] = 10-pH

4. Why do we care about pH? Biological systems Corrosivity
Extremes are disruptive
Corrosivity
Equilibrium relationships involving H+

5. Good pH ranges
Freshwater...
Marine ...
Drinking...
Soda?

6. Alkalinity Capacity of water sample to neutralize an...
Capacity of a water sample to take H+ without significant change in...
Common ions that give water alkalinity
HCO3-, CO32- & OH-

7. Acidity Capacity of water to take OH- without significant pH change
Opposite of Alkalinity
Capacity of water sample to neutralize a base
Common ion: H+
For Acid Mine Drainage
Acidity =~f(Fe2+, Fe3+, Al, Mn, and H+)
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8. pH, Alkalinity & Acidity
Waters with same pH can have different amounts of Alkalinity or Acidity
Chemicals may be present that can react with or release H+ or OH-
The lower the alkalinity (or acidity), the easier it is to change pH

9. Reversible Reactions aA + bB ↔ cC + dD A & B can react to form C and D
C & D can react to form A and B
After sufficient time has passed, equilibrium is reached
Equilibrium can be perturbed by adding more reactant or product

10. Reversible Reactions: Gas - Solution
At equilibrium
Gaseous chemical dissolves
Dissolved chemical volatilizes
At equal rate
Henry’s Law
Example…
Gas
Dissolved

11. Reversible Reactions In Solution
At equilibrium
Reactants become products
Products become reactants
At equal rate
Example…

12. Reversible Reactions: Solution - Precipitate
At equilibrium
Precipitate is formed
Precipitate is dissolved
At equal rates
Example…

13. K is constant at given temperature & relatively low concentration
Equilibrium Equation
aA + bB ↔ cC + dD
K is constant at given temperature & relatively low concentration

14. Example - Pure Water What is water? ... Reversible equation?...
Equilibrium equation?...
At 25 C, Kw = ...
Even if other sources of H+ (acids) or OH- (bases) are present
Which substance predominates?...

15. pH of Pure Water @ 25 C? Follow these steps Write down species...
Write electroneutrality eq...
Write equilibrium equation..
Solve equations...

16. Now, add… OH- ? (add a strong base)
Systems finds new equilbrium, with less H+ and more OH-
pH goes up
H+ ? (add a strong acid, precipitate iron,…)
Systems finds new equilbrium, with more H+ and less OH-
pH goes down.
Either way, [H+] [OH-] = Kw still holds

17. Precipitation of Limestone
CaCO3-2 ↔ Ca+2 + CO3-2
[Ca+2] [CO3-2] = Ksp
Ksp = Solubility product
Do not include precipitate in equilibrium equation
[Ca+2] [CO3-2] < Ksp?...
[Ca+2] [CO3-2] > Ksp?...

19. Carbonate Equilibrium System (CES)
Keeps natural waters in good pH range
Common Chemicals
Carbon Dioxide, CO2
Carbonic Acid, H2CO3
Bicarbonate, HCO3-
Carbonate, CO3-2
Solids containing Bicarbonate, Carbonate, Hydroxide, and (though rarely) certain Oxides (CaO),…

20. Main Sources CO2 from atmosphere Solids containing carbonate CO2 g
CO2 aq
H2CO3
HCO3
CO3
Solid CO3,…
Atmosphere
Solution
Precipitate
CO2 from atmosphere
Byproduct of
biological or chemical reactions
combustion
Solids containing carbonate
e.g., limestone
Byproduct of sea life

21. CES Reactions Reversible Equilibrium (at 25C) CO2 g ↔ CO2 aq
CO2 aq + H2O ↔ H2CO3
H2CO3 ↔ H+ + HCO3-
HCO3- ↔ H+ + CO3-2
Ca+2 + CO3-2 ↔ CaCO3
CO2 aq = CO2 g / 1637 atm
[H2CO3] / [CO2 aq] = 1.58 x 10-3
[H+] [HCO3-] / [H2CO3*] = 4.47x10-7 M
[H+] [CO3-2] / [HCO3-] = 4.68x10-11 M
[Ca+2] [CO3-2] = 3.80 x 10-9 M2
Where CO2 g is in atm & CO2 aq is in mole fraction

22. Carbonate System & pH HCO3 CO3 mg/L as CaCO3 CO2 OH pH
Typical Natural Water with 100 mg/L alkalinity as CaCO3
100
mg/L as CaCO3
6.5 11
8.5 50
CO2
CO3
HCO3 OH pH

23. Carbonate System Scenarios
Open / Closed
Open - in equilibrium with atmosphere
open container, streams & shallow lakes, upper ocean
Closed - not in equilibrium
Closed tank, quick reactions, deep regions of water bodies, soil gases
Homogeneous / Heterogeneous
Homogeneous – in equilibrium with CO3 solids
Heterogeneous– not in equilibrium with CO3 solids
Closed & heterogeneous
bottom of stratified lake with floor of limestone
water treatment unit with soda ash / acid or base / CO2 & some CaCO3 precipitates

24. Example - Natural pH of Rain
CO2 in atmosphere naturally “shifts” pH of rain from 7 to ?
Open system
No solid source of CO3
CO2 g
CO2 aq
H2CO3
HCO3-
CO3- H+
Lowers pH

25. Atmosphere Nitrogen - 0.781 atm Oxygen - 0.209 atm Argon - 0.0093 atm
CO atm
When this example was created. It’s now atm ( ppm)
Misc atm
TOTAL atm

26. Solution: pH of Rain (1)
Atmosphere to raindrop - Henry’s Law: CO2 aq = CO2 g / 1637 atm
Convert to M: [CO2 aq] = CO2 aq x Mw
Mw = molar density of water = mol/l

27. Solution: pH of Rain (2)
Reaction with water forms carbonic acid - Equilibrium Equation: [H2CO3 aq] = 1.58x10-3 M • [CO2 aq]
[H2CO3* aq] = [CO2 aq] + [H2CO2 aq]

28. Solution: pH of Rain (3)
Dissociation of carbonic acid - Equilibrium Equation: [H+] [HCO3-] = 4.47 x 10-7 M • [H2CO3*]
Two unknowns? Use electroneutrality: [H+] = [OH-] + [HCO3-] + 2[CO3-2]
Rain water is acidic, so [OH-] & [CO3-2] will be small giving [H+] ≈ [HCO3-], leading to…

29. Solution: pH of Rain (4) pH = - log [H+] = -log (2.13x10-6) =…
Check assumptions? Use equilibrium equations:
[OH-] = M2 / [H+] = M2 / 2.13x10-6 M = 4.68x10-9 M (small, OK))
[CO3-2] = 4.68x10-11 M • [H+] / [HCO3-] = 4.68x10-11 M • 2.13x10-6 M / 2.13x10-6 M = 4.68x10-11 M (small, OK)

30. pH of Rain CO2 in atmosphere goes up?...
Use this method for different conditions?
Need to check assumptions about relative concentration of [OH-] & [CO3-2]

31. Atmospheric CO2 & Ocean pH
Oceans can be CO2 sink or source
Currently: sink, taking ~ 30% of anthropocentric CO2
Primary mechanisms
Carbonate equilibrium system (Henry’s Law,…)
Biological Pump
Living creatures take up carbon, some are trapped in sea bed
Too much CO2?
pH drop of oceans could effect sea organisms
From 1751 to 1994 surface ocean pH estimated to have dropped from ~ 8.18 to 8.10

32. Ocean pH Change
~half Anthropogenic CO2 has been absorbed by ocean so far
Currently absorbing ~1/3
22 M tons / day
Could drop to 7.6
Drastic effect on shell-forming organisms
National Geographic (2014) “Ocean Acidification”, ocean.nationalgeographic.com.

33. Acid Rain Rain with pH below 5.7 Most prevalent cause:
SOx produced from burning coal
SOx reacts with water to form sulfuric acid
Sulfuric acid dissolves into atmospheric water and dissociates into H- and SO4-2

34. Precipitation pH downwind of Ohio Valley power plants

35. Sulfur Cap & Trade Program

36. CES, Alkalinity, Acid Rain’s Effect
Carbonate solids
Common source of alkalinity in natural waters (as they dissolve)
Lakes and streams with alkalinity can accept acid rain without big pH changes
Alkalinity buffers acid rain
Acid rain can damage water bodies that don’t have alkalinity

37. Acid Mine Drainage

38. Red Oak Mine Site

39. Mine Side View Water Infiltration
pH – 4.4 (low) [drops to ~2 after seep]
Acidity 434 ppm CaCO3 (high)
Partial Pressure CO2 in mine headspace - ~6% [200x higher than atmosphere]
FeS2 + O2 + H2O →
Fe2+ + SO42- + H+
Fe2+ + O2 + H+ → Fe3+ + H2O
Fe3+ + H2O → Fe(OH)3 ↓ + H+
Even lower pH
Anaerobic
Conditions
Low pH
High Acidity
High metals
Oxidation of organic matter can lead to high concentrations of CO2 in mine headspace
Presence of limestone will buffer the mine water, resisting pH change

40. Red Oak Seep
seep discharge

41. Plan View
Not to scale

42. Red Oak Remediation
Post injection, seep characteristics were pH - 6.3, Alkalinity - 150 mg/L as CaCO3, Fe- 120 mg/l; however, this level of treatment lasted only 15 months.