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Learning goals Know the carbon atom Where acid rain comes from
What is pH and how to calculate Carbonate equilibrium reactions
Why important Alkalinity Chemical weathering
Learning goals Climate controls on atmospheric CO2 Ocean acidification
What causes it Why important What does the future hold
CARBON Shells: 2,4 Minimum oxidation
number is –4 Maximum oxidation
number is +4
Carbon Isotopes C-12 C-13 C-14
Carbon forms Graphite Diamond Buckmisterfullerene Organic Matter DOC Particulate C
Types of carbon compounds Gas phase
CO2, methane, volitale organic compounds (VOCs)
Organic Amino acids, DNA, etc
Water Dissolved inorganic carbon (DIC) Dissolved organic carbon (DOC)
DOC in GROUNDWATER Less than 2 mg/L Microbial decomposition Adsorption Precipitation as solid > 100 mg/L in polluted ag systems Increases geochemical weathering
ORGANICS in WATER Solid phases (peat, anthracite, kerogen Liquid fuels (LNAPL), solvents (DNAPL) Gas phases Dissolved organics (polar and non-polar)
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
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)
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
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 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-coral-reefs-face-massive-bleaching-event-2015
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– = CO3
2– + H+
SECOND REACTION
Variables and Reactions Involved in Understanding the Carbonate System
Activity of Carbonate Species versus pH
CARBONATE SPECIES and pH
pH controls carbonate species 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.
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 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 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 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.
LITHOSPHERE 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: 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!
RockCycle
Sedimentary Processes1) 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
Chemical Weathering-
Decomposition of rock as the result of chemical attack. Chemical composition changes.
Mechanical Weathering -
Disintegration of rock without change in chemical composition
Mechanical Weathering
•Frost wedging•Alternate heating and cooling
•Decompression causes jointing
Chemical Weathering Processes 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 completelyDissolved by the water.Congruent weathering.
DEHYDRATION
Removal of water from a mineral.
HYDROLYSIS
H+ replaces an ion in the mineral.Generally incongruent weathering.
HYDROLYSIS 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 silicaClay is a soft, platy mineral, so the rock disintegrates
HYDROLYSIS Base cations are
Ca2+, Mg2+, Na+, K+
Alkalinity = HCO3-
Clay = kaolinite (Al2Si2O5(OH)4)
Si = H4SiO4; no charge, dimer, trimer
OXIDATION
Reaction of minerals with oxidation.An ion in the mineral is oxidized.
Oxidation can affect any iron bearing mineral, for example, ferromagnesian silicates which react to form hematite and limonite
Oxidation
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 Slow chemical weathering of continental rocks balances
input of CO2 to atmosphere Chemical weathering reactions important
Hydrolysis and Dissolution
Atmospheric CO2 Balance Slow silicate rock weathering balances
long-term build-up of atmospheric CO2
On the 1-100 million-year time scale Rate of chemical hydrolysis balance rate of
volcanic emissions of CO2
Neither rate was constant with time
Earth’s long term habitably requires only that the two are reasonably well balanced
What Controls Weathering Reactions? Chemical weathering influenced by
TemperatureWeathering rates double with 10°C rise
PrecipitationH2O is required for hydrolysis
Increased rainfall increases soil saturationH2O and CO2 form carbonic acid
VegetationRespiration in soils produces CO2
CO2 in soils 100-1000x higher than atmospheric CO2
Climate Controls Chemical Weathering
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? Chemical weathering can provide negative feedback that
reduces the intensity of climate warming
Chemical Weathering: Earth’s Thermostat? Chemical weathering can provide negative feedback that
reduces the intensity of climate cooling