Transcript CARBONATE SYSTEM - University of Colorado Boulder

Learning goals
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Know the carbon atom
Where acid rain comes from
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Carbonate equilibrium reactions
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What is pH and how to calculate
Why important
Alkalinity
Chemical weathering
Learning goals
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Climate controls on atmospheric CO2
Ocean acidification
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What causes it
Why important
What does the future hold
CARBON
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Shells: 2,4
Minimum oxidation
number is –4
Maximum oxidation
number is +4
Carbon Isotopes
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C-12
C-13
C-14
Carbon forms
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Graphite
Diamond
Buckmisterfullerene
Organic Matter
DOC
Particulate C
Types of carbon compounds
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Gas phase
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Organic
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CO2, methane, volitale organic compounds
(VOCs)
Amino acids, DNA, etc
Water
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Dissolved inorganic carbon (DIC)
Dissolved organic carbon (DOC)
DOC in GROUNDWATER
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Less than 2 mg/L
Microbial decomposition
Adsorption
Precipitation as solid
> 100 mg/L in polluted ag systems
Increases geochemical weathering
ORGANICS in WATER
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Solid phases (peat, anthracite, kerogen
Liquid fuels (LNAPL), solvents (DNAPL)
Gas phases
Dissolved organics (polar and non-polar)
CARBONATE SYSTEM
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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
DIPROTIC ACID SYSTEM
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Carbonic Acid (H2CO3)
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Bicarbonate (HCO3-)
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Can donate two protons (a weak acid)
Can donate or accept one proton (can be
either an acid or a base
Carbonate (CO32-)
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Can accept two protons (a base)
OPEN SYSTEM
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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
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
Ocean pH and atmospheric CO2
NATURAL ACIDS
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Produced from C, N, and S gases in the
atmosphere
H2CO3 Carbonic Acid
HNO3 Nitric Acid
H2SO4 Sulfuric Acid
HCl
Hydrochloric Acid
pH of Global Precipitation
http://www.motherjones.com/tom-philpott/2015/01/noaa-globes-co
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
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:
»Note we have a gas in the reaction and use partial
» pressure rather than activity
»First dissociation:
H2CO3 = HCO3– + H+
FIRST REACTION
»Second dissociation:
HCO3– = CO32– + H+
SECOND REACTION
Variables and Reactions Involved in Understanding
the Carbonate System
Gas
Dissociation of Dissociation Cations
equilibria carbonic acid
of water
PCO2
[H2CO3]
[H+]
[Ca2+]
[HCO3–]
[CO32–]
[OH–]
Measurements
DIC
Alkalinity
Activity of Carbonate Species versus pH
CARBONATE SPECIES and pH
pH controls carbonate species
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Increased CO2 (aq) increases H+ and
decreases carbonate ion
Thus increasing atmospheric CO2
increases CO2 (aq) and causes the water
system to become more acidic
However, natural waters have protecting,
buffering or alkalinity
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.
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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.
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
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Alkalinity ≈ (HCO3–) + 2(CO32–)
Reason is that in most natural waters,
ionized silicic acid and organic acids are
present in only small concentrations
If pH around 7, then
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Alkalinity ≈ HCO3–
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
TOTAL CARBONATE
SPECIES (CT)
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 pH
What’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.
LITHOSPHERE
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Linkage between the atmosphere and the
crust
Igneous rocks + acid volatiles =
sedimentary rocks + salty oceans (eq 4.1)
IMPORTANCE OF ROCK
WEATHERING
[1] Bioavailability of nutrients that have no
gaseous form:
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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!
Rock
Cycle
Sedimentary Processes
1) Weathering & erosion
2) Transport &
3) deposition
4) Lithification
Weathering:
decomposition and
disintegration of rock
Product of weathering
is regolith or soil
Regolith or soil that is
transported is called
sediment
Movement of
sediment is called
erosion
Weathering Processes
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Mechanical Weathering Disintegration of rock without change in
chemical composition
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Chemical WeatheringDecomposition of rock as the result of
chemical attack. Chemical composition
changes.
Mechanical Weathering
•Frost wedging
•Alternate heating and cooling
•Decompression
causes jointing
Chemical Weathering Processes
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Hydrolysis - reaction with water (new minerals
form)
Oxidation - reaction with oxygen (rock rusts)
Dissolution - rock is completely dissolved
Most chemical weathering processes are
promoted by carbonic acid:
H2O +CO2 = H2CO3 (carbonic acid)
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 completely
Dissolved by the water.
Congruent weathering.
DEHYDRATION
Removal of water from a mineral.
HYDROLYSIS
H+ replaces an ion in the mineral.
Generally incongruent weathering.
HYDROLYSIS
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Silicate rock + acid + water = base cations
+ alkalinity + clay + reactive silicate (SiO2)
Hydrolysis
Feldspar + carbonic acid +H2O
= kaolinite (clay)
+ dissolved K (potassium) ion
+ dissolved bicarbonate ion
+ dissolved silica
Clay is a soft,
platy mineral, so
the rock
disintegrates
HYDROLYSIS
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Base cations are
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Ca2+, Mg2+, Na+, K+
Alkalinity = HCO3Clay = kaolinite (Al2Si2O5(OH)4)
Si = H4SiO4; no charge, dimer, trimer
OXIDATION
Reaction of minerals with oxidation.
An ion in the mineral is oxidized.
Oxidation
Oxidation can affect any
iron bearing mineral, for
example, ferromagnesian
silicates which react to
form hematite and limonite
Oxidation of pyrite and other sulfide minerals forms
sulfuric acid which acidifies surface water and rain
Pyrite + oxygen + water = sulfuric acid + goethite
(iron sulfide)
(iron oxide)
Products of weathering
Clay minerals further decompose to aluminum hydroxides
and dissolved silica.
Removal of Atmospheric CO2
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Slow chemical weathering of continental rocks balances
input of CO2 to atmosphere
Chemical weathering reactions important
 Hydrolysis and Dissolution
Atmospheric CO2 Balance
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Slow silicate rock weathering balances
long-term build-up of atmospheric CO2
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On the 1-100 million-year time scale
Rate of chemical hydrolysis balance rate of
volcanic emissions of CO2
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Neither rate was constant with time
Earth’s long term habitably requires only that
the two are reasonably well balanced
What Controls Weathering
Reactions?
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Chemical weathering influenced by
 Temperature
 Weathering rates double with 10°C rise
 Precipitation
 H2O is required for hydrolysis
 Increased rainfall increases soil
saturation
 H2O and CO2 form carbonic acid
 Vegetation
 Respiration in soils produces CO2
 CO2 in soils 100-1000x higher than
atmospheric CO
Climate Controls Chemical Weathering
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Precipitation closely linked with
temperature
 Warm air holds more water
than cold air
Vegetation closely linked with
precipitation and temperature
 Plants need water
 Rates of photosynthesis
correlated with temperature
Chemical Weathering: Earth’s Thermostat?
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Chemical weathering can provide negative feedback that
reduces the intensity of climate warming
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Chemical
Weathering:
Chemical
weathering
can provideEarth’s
negative feedback that
reduces
the intensity of climate cooling
Thermostat?