WATER QUALITY IN STREAMS ANDRIVERS IS THE END PRODUCT OF ALL

PROCESSES IN THE BASIN

WATERSHEDS ARE THEKIDNEYS OF AN ECOSYSTEM

KIDNEY ANALOGOUSTO A WATERSHED

NITRATE EXAMPLE

Fingerprint water

Isotopes

Geochemical content

Nutrients

Rock Weathering

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LITHOSPHERE

• Linkage between the atmosphere and the crust. Weathering results in:

• Igneous rocks + acid volatiles = sedimentary rocks + salty oceans

IMPORTANCE OF ROCK WEATHERING

[1] Bioavailability of nutrients that have no gaseous form:– P, Ca, K, Fe

• Forms the basis of biological diversity, soil fertility, and agricultural productivity

• The quality and quantity of lifeforms and food is dependent on these nutrients

IMPORTANCE OF ROCK WEATHERING

[2] Buffering of aquatic systems-Maintains pH levels

-regulates availability of Al, Fe, PO4

Example: human blood.-pH highly buffered-similar to oceans

IMPORTANCE OF ROCK WEATHERING

[3] Forms soil

[4] Regulates Earths climate

[5] Makes beach sand!

NATURAL ACIDS thatWEATHER ROCK

• Produced from C, N, and S gases in the atmosphere

• H2CO3 Carbonic Acid

• HNO3 Nitric Acid

• H2SO4 Sulfuric Acid

• HCl Hydrochloric Acid

Stoichiometry

Stoichiometry is the accounting, or math, behind chemistry. Given enough information, one can use stoichiometry to calculate masses, moles, and percents within a chemical equation.

Keep track of atoms, molecules, and charge

Calcite dissolutionCaCO3 + CO2 + H2O Ca2+ + 2 HCO3

-

reactants products

TYPES OF CHEMICAL WEATHERING

• Carbonate weathering

• Dehydration

• Oxidation

• Hydrolysis

CARBONIC ACID

Carbonic acid is produced in rainwater by Reaction of the water with carbon dioxide Gas in the atmosphere.

CARBONATE (DISSOLUTION)

All of the mineral is completelyDissolved by the water.Congruent weathering.

DEHYDRATION

Removal of water from a mineral.

OXIDATION

Reaction of minerals with oxidation.An ion in the mineral is oxidized.

OXIDATION: REDOX REACTIONS

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Loss of electrons = Oxidation

Gain of electrons = Reduction

For example, rusting

* Oxidation of elemental iron to iron(III) oxide by oxygen

4 Fe + 3 O2 → 2 Fe2O3

Occurs in nature as the mineral hematite, and is the principal ore for iron

HYDROLYSIS

H+ replaces an ion in the mineral.Generally incongruent weathering.

HYDROLYSIS

• Silicate rock + acid + water = base cations + alkalinity + clay + reactive silicate (SiO2)

HYDROLYSIS

• Base cations are– Ca2+, Mg2+, Na+, K+

• Alkalinity = HCO3-

• Clay = kaolinite (Al2Si2O5(OH)4)

• Si = H4SiO4; no charge, dimer, trimer

Mineral Solubility

• Solubility - relative capability of being dissolved

• Salt dissolution - solids break down in solution to yield ions

• Example: Barium chloride BaCl2BaCl2 (s) = Ba2+ + 2 Cl–

– Define K using the Law of Mass Action (“activity” in brackets):

K =[Ba2+]⋅[Cl−]2

BaCl2(s)

•Inside the [] are the measured concentrations •Multiply [] by number of atoms

Solubility constant Ksp

– Because the activity of the solid is 1, the equation becomes

Ksp = [Ba2+] · [Cl–]2

– The equilibrium constant for the dissolution reaction is called the solubility product constant or Ksp.

Measurements of Disequilibrium

• It can be important to know whether a solution is saturated or undersaturated with respect to a mineral

• Consider: AaBb = aA + bB• At equilibrium: Ksp = [A]a [B]b

• How do we know the solution is in equilibrium with the mineral? Measure [A] and [B] in solution (activity product or ion activity product) and compare to Ksp

Degree of saturation

Ω =[A]a[B]b

Ksp

»where [A] and [B] are for the solution, »which may or may not be in equilibrium with the mineral» > 1 Supersaturated» = 1 Saturated» < 1 Undersaturated

Problem: What is the degree of saturation of anhydrite in College Station tap water?

• (Ca2+) = 3 mg/L = 0.003 g·L-1/40 g Ca·mol-1 = 0.000075 M

• (SO42-) = 10 mg/L = 0.010 g·L-1/96 g SO4

2-·mol-1 = 0.00010 M

• T = 25°C• Assume ideal behavior ( = 1)• Write the reaction in terms of dissolution and make

use of Ksp values

CaSO4 = Ca2+ + SO42-

• We calculate the ion activity product in solution:

IAP = [Ca2+][SO42-] = 0.000075 · 0.00010 = 7.5 x

10–9 = 10–8.1

• Degree of saturation

Ω =IAP

K sp,anh

=10−8.1

10−4.36 =10−3.74

Water is undersaturated with respect to annhydrite

Ksp,calcite=[Ca2+][CO32– ]Calcite dissolution:

CaCO3 = Ca2+ + CO32–

Is water undersaturated or oversaturated with respect to calcite?

Get stalagmites/stalagtites?Or dissolve them?

Tea pots: where does mineral deposits come from?

But ions don’t behave ideally . . .

• Concentration related to activity using the activity coefficient , where [z] = z (z)

• The value of depends on:– Concentration of ions and charge in the solution– Charge of the ion– Diameter of the ion

• Ionic strength I = concentration of ions and charge in solution

I = 1/2 mizi2

– where mi = concentration of each ion in moles per kg, zi = charge of ion

Activity and Concentration

• Activity – “effective concentration”• Ion-ion and ion-H2O interactions (hydration

shell) cause number of ions available to react chemically ("free" ions) to be less than the number present

• Concentration can be related to activity using the activity coefficient , where [z] = z (z)

• Activity coefficient z 1 as concentrations 0 and tend to be <1 except for brines

Carbonate Chemistry

The Carbonate System

• pH of most natural waters controlled by reactions involving the carbonate system

• Groundwater and seawater chemistries are often poised near calcite equilibrium, with pH buffered by calcite dissolution and precipitation

• Applications– Fate of CO2 from fossil fuels and other CO2

sources on the atmosphere – Effect of acid rain on lakes– Effect of acid mine drainage on rivers

Carbonate System• Carbonate species are necessary for

all biological systems• Aquatic photosynthesis is affected by

the presence of dissolved carbonate species.

• Neutralization of strong acids and bases

• Effects chemistry of many reactions• Effects global carbon dioxide content

PCO2 = 10–3.5 yields pH = 5.66

»What is 10–3.5? 316 ppm CO2

What is today’s PCO2? ~368 ppm = 10-3.43

»pH = 5.63

pH of Global Precipitation

DIPROTIC ACID SYSTEM• Carbonic Acid (H2CO3)

– Can donate two protons (a weak acid)

• Bicarbonate (HCO3-)

– Can donate or accept one proton (can be either an acid or a base

• Carbonate (CO32-)

– Can accept two protons (a base)

TOTAL CARBONATE SPECIES (CT)

OPEN SYSTEM• Water is in equilibrium with the partial

pressure of CO2 in the atmosphere

• Useful for chemistry of lakes, etc

• Carbonate equilibrium reactions are thus appropriate

First the CO2 dissolves according to:

(1) CO2 (g) ⇔ CO2 (l)

According to Henry’s Law, solubility increases as water temperature decreases

(2) CO2 (l) + H2O (l) ⇔ H2CO3 (l)

Equilibrium is established between the dissolved CO2 and H2CO3, carbonic acid.

(3) H2CO3 + H2O H3O+ + HCO3-

Carbonic acid is a weak acid that dissociates in two steps.

pKa1 (25 C) = 6.37

(4) HCO3- + H2O H3O+ + CO3

2-

pKa2 (25 C) = 10.25

The above presented more schematically:

+ H2O + H2O + H2O + Ca2+ CO2(g) Û CO2 (l) Û H2CO3 Û HCO3

- Û CO32- Û CaCO3 i

+ H3O+ + H3O

+

Note that the reverse is also true and that the scheme represents the solubility of CaCO3 in an acidic solution resulting in the liberation of CO2 in the atmosphere.

Activity of Carbonate Species versus pH

CARBONATE SPECIES AND pH

Carbonate Buffering: Humans

We can describe the formation and dissociation of carbonic acid through the following chemical and equilibrium equations

Carbonic acid forms when CO2

dissolves in and reacts with water:

CO2(g) + H2O = H2CO3

»Most dissolved CO2 occurs as “aqueous CO2” rather than H2CO3, but we write it as carbonic acid for convenience»The equilibrium constant for the reaction is:KCO2 =

[H2CO3]PCO2 [H2O]

=[H2CO3]

PCO2

»Note we have a gas in the reaction and use partial» pressure rather than activity

K1 =[H+][HCO3

– ][H2CO3]

»First dissociation: H2CO3 = HCO3

– + H+

FIRST REACTION

»Second dissociation:

HCO3– = CO3

2– +

H+

K2 =[H+][CO3

2– ][HCO3

– ]

SECOND REACTION

Variables and Reactions Involved in Understanding the Carbonate System

Gasequilibria

Dissociation ofcarbonic acid

Dissociationof water

Cations Measure-ments

PCO2 [H2CO3] [H+] [Ca2+] DIC

[HCO3–] [OH–] Alkalinity

[CO32–]

ALKALINITY refers to water's ability, or inability, to neutralize acids.

The terms alkalinity and total alkalinity are often used to define the same thing.

Total alkalinity - sum of the bases in equivalents that are titratable with strong acid (the ability of a solution to neutralize strong acids)

Bases which can neutralize acids in natural waters: HCO3

–, CO32–, B(OH)4

–, H3SiO4

–, HS–, organic acids (e.g., acetate CH3COO–, formate HCOO–)

Carbonate alkalinity

• Alkalinity ≈ (HCO3–) + 2(CO3

2–)

• Reason is that in most natural waters, ionized silicic acid and organic acids are present in only small concentrations

• If pH around 7, then– Alkalinity ≈ HCO3

–

Alk = OH– + HCO3– + 2CO3

–2 + B(OH)4- - H+

Bicarbonate dominates alkalinity of sea water

Gran Titration for Acid Neutralizing Capacity (total

alkalinity)• This method determines ANC by

titration with 0.1 N Hydrochloric Acid between the pH range of 4.5 and 3.5 at which the contributions of organic acids, carbonate and bicarbonate are neutralized.

• Explicitly accounts for most organic acids

Charge Balance

• Fundamental principle of solution chemistry is that solutions are electronically neutral

• Sum of positive charges must equal the sum of negative charges in any sample

+ = -

+ > -, there is an unmeasured anion• Equivalents: moles/L x valence

ION Percent DifferenceQuality Control

+ - - / + + -

• Normalizes the charge balance

• NADP guidelines for allowable error

• Error should be random and equal zero

• If error always positive, means there is an unmeasured anion (negative charge)

• Alkalinity is routinely measured in natural water samples. By measuring only two parameters, such as alkalinity and pH, the remaining parameters that define the carbonate chemistry of the solution (PCO2, [HCO3

–], [CO32–], [H2CO3])

can be determined.

Calcium (Ca++) and Magnesium (Mg++) are primarily responsible for hardness. However, in most waters, alkalinity and hardness have similar values because the carbonates and bicarbonates responsible for total alkalinity are usually in the form of Calcium carbonate or Magnesium carbonate. However, waters with high total alkalinity are not always hard, since the carbonates can be in the form of Sodium or Potassium carbonate.

Alkalinity and hardness.

CLOSED CARBONATE SYSTEM

• Carbon dioxide is not lost or gained to the atmosphere

• Total carbonate species (CT) is constant regardless of the pH of the system

• Occurs when acid-base reactions much faster than gas dissolution reactions

• Equilibrium with atmosphere ignored

How does [CO3–2] respond to changes in Alk or DIC?

 CT = [H2CO3*] + [ HCO3

–] + [CO3–2]

 ~ [ HCO3

–] + [CO3–2] (an approximation)

 Alk = [OH–] + [HCO3

–] + 2[CO3–2] + [B(OH)4

-] – [H+]

 ~ [HCO3

–] + 2[CO3–2] (a.k.a. “carbonate alkalinity”)

 So (roughly): 

[CO3–2] ~ Alk – CT

  CT ↑ , [CO3

–2] ↓ Alk ↑ , [CO3–2] ↑

 

Diurnal changes in DO and pHWhat’s up?

Photosynthesis is the biochemical process in which plants and algae harness the energy of sunlight to produce food. Photosynthesis of aquatic plants and algae in the water occurs when sunlight acts on the chlorophyll in the plants. Here is the general equation:

6 H20 + 6 CO2 + light energy —> C6H12O6 + 6 O2

Note that photosynthesis consumes dissolved CO2 and produces dissolved oxygen (DO). we can see that a decrease in dissolved CO2 results in a lower concentration of carbonic acid (H2CO3), according to:

CO2 + H20 <=> H2CO3 (carbonic acid)

As the concentration of H2CO3 decreases so does the concentration of H+, and thus the pH increases.

Cellular Respiration

Cellular respiration is the process in which organisms, including plants, convert the chemical bonds of energy-rich molecules such as glucose into energy usable for life processes.

The equation for the oxidation of glucose is:

C6H12O6 + 6 O2 —> 6 H20 + 6 CO2 + energy

As CO2 increases, so does H+, and pH decreases.

Cellular respiration occurs in plants and algae during the day and night, whereas photosynthesis occurs only during daylight.

Rock Cycle

Carbonate weathering

Hydrolysis

%A R. M. Garrels%A F. T. MacKenzie%T Origin of the chemical composition of some springsand lakes%B Equilibrium Concepts in Natural Water Systems%E R. G. Gould%S Am. Chem. Soc. Adv. Chem. Ser.%V 67%P 222-242%D 1967

Classic reference on geochemical weathering