Water Science Electroneutrality, pH, Alkalinity, Acidity Reversible Equations Carbonate Equilibrium System Examples Rain Ocean pH Acid Rain Acid Mine Drainage.

Water Science• Electroneutrality, pH, Alkalinity, Acidity• Reversible Equations• Carbonate Equilibrium System• Examples

• Rain• Ocean pH• Acid Rain• Acid Mine Drainage

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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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pH - reporting H+ concentration• pH = - log {H+}

• {} = activity, the effective concentration

• pH = - log [H+] for dilute solutions• [] = Molar

concentration (moles/Liter)

• [H+] = 10-pH

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Why do we care about pH?• Biological systems

• Extremes are disruptive

• Corrosivity

• Equilibrium relationships involving H+

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Good pH ranges• Freshwater...

• Marine ...

• Drinking...

• Soda?

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-

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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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

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

Reversible Reactions: Gas - Solution• At equilibrium

• Gaseous chemical dissolves • Dissolved chemical volatilizes• At equal rate

• Henry’s Law

• Example…

Gas

Dissolved

Reversible Reactions In Solution• At equilibrium

• Reactants become products • Products become reactants• At equal rate

• Example…

Reactants

Products

Reversible Reactions: Solution - Precipitate• At equilibrium

• Precipitate is formed • Precipitate is dissolved• At equal rates

• Example…

Reactants

Precipitate

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Equilibrium Equation

aA + bB ↔ cC + dD

[C]c [D]d

[A]a [B]b = k

K is constant at given temperature & relatively low concentration

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?...

pH of Pure Water @ 25 C?• Follow these steps

• Write down species...• Write electroneutrality eq...• Write equilibrium equation..

• Solve equations...

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

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?...

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),…

Main Sources• CO2 from atmosphere

• Byproduct of • biological or chemical reactions• combustion

• Solids containing carbonate• e.g., limestone

• Byproduct of sea life

CO2 g

CO2 aq

H2CO3

HCO3

CO3

Solid CO3,…

Atmosphere

Solution

Precipitate

CES Reactions

Reversible

• CO2 g ↔ CO2 aq

• CO2 aq + H2O ↔ H2CO3

• H2CO3 ↔ H+ + HCO3-

• HCO3- ↔ H+ + CO3

-2

• Ca+2 + CO3-2 ↔ CaCO3

Equilibrium (at 25C)

• 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

Carbonate System & pH• Typical Natural Water with 100 mg/L alkalinity as CaCO3

pH

100

mg

/L a

s C

aC

O3

6.5 118.5

50

CO2

CO3

HCO3

OH

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

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+

H+ Lowers pH

Atmosphere• Nitrogen - 0.781 atm• Oxygen - 0.209 atm• Argon - 0.0093 atm• CO2 - 0.00033 atm

• When this example was created. It’s now 0.00040 atm (398.78 ppm)

• Misc. - 0.0004 atm• TOTAL - 1.0 atm

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 = 55.56 mol/l

•

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]

•

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…

•

•

Solution: pH of Rain (4)• pH = - log [H+] = -log (2.13x10-6) =…

• Check assumptions? Use equilibrium equations:

• [OH-] = 10-14 M2 / [H+] = 10-14 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)

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]

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

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.

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

Precipitation pH downwind of Ohio Valleypower plants

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Sulfur Cap & Trade

Program

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

Acid Mine Drainage

http://www.ei.lehigh.edu/

Red Oak Mine Site

Mine Side ViewWater Infiltration

AnaerobicConditions

Low pHHigh AcidityHigh metals

FeS2 + O2 + H2O → Fe2+ + SO4

2- + H+ Fe2+ + O2 + H+ → Fe3+ + H2O

Fe3+ + H2O → Fe(OH)3 ↓ + H+

Presence of limestone will buffer the mine water, resisting pH change

Even lower pH

Oxidation of organic matter can lead to high concentrations of CO2 in mine headspace

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]

Red Oak Seep

seep discharge

Plan View

Not to scale

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.