1. CARBONATE SYSTEM
Know the carbon atom
Where acid rain comes from
What is pH and how to calculate
Carbonate equilibrium reactions
Why important
Alkalinity
Chemical weathering
6. DOC in GROUNDWATER
Less than 2 mg/L
Microbial decomposition
Adsorption
Precipitation as solid
> 100 mg/L in polluted ag systems
Increases geochemical weathering
7. ORGANICS in WATER
Solid phases (peat, anthracite, kerogen
Liquid fuels (LNAPL), solvents (DNAPL)
Gas phases
Dissolved organics (polar and non-polar)
8. 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
9. 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)
10. 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
11. 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
13. NATURAL ACIDS
Produced from C, N, and S gases in the
atmosphere
H2CO3 Carbonic Acid
HNO3 Nitric Acid
H2SO4 Sulfuric Acid
HCl Hydrochloric Acid
16. 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
17. 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
22. Variables and Reactions Involved in Understanding
the Carbonate System
Gas
equilibria
Dissociation of
carbonic acid
Dissociation
of water
Cations Measure-
ments
PCO2 [H2CO3] [H+
] [Ca2+
] DIC
[HCO3
–
] [OH–
] Alkalinity
[CO3
2–
]
25. 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
26. 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.
27. 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
–], [CO3
2–], [H2CO3]) can be
determined.
28. 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
–, CO3
2–,
B(OH)4
–, H3SiO4
–, HS–, organic
acids (e.g., acetate CH3COO–,
formate HCOO–)
29. 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
–
30. 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
34. 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.
35. 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.
36. LITHOSPHERE
Linkage between the atmosphere and the
crust
Igneous rocks + acid volatiles =
sedimentary rocks + salty oceans (eq 4.1)
37. 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
38. 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
42. 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
43. 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
45. 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)
46. CARBONIC ACID
Carbonic acid is produced in rainwater by
Reaction of the water with carbon dioxide
Gas in the atmosphere.
51. 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
52. HYDROLYSIS
Base cations are
Ca2+, Mg2+, Na+, K+
Alkalinity = HCO3
-
Clay = kaolinite (Al2Si2O5(OH)4)
Si = H4SiO4; no charge, dimer, trimer
54. Oxidation can affect any
iron bearing mineral, for
example, ferromagnesian
silicates which react to
form hematite and limonite
Oxidation
55. 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)
57. Removal of Atmospheric CO2
Slow chemical weathering of continental rocks balances
input of CO2 to atmosphere
Chemical weathering reactions important
Hydrolysis and Dissolution
58. 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
59. What Controls Weathering
Reactions?
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
60.
61. 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
62. Chemical Weathering: Earth’s Thermostat?
Chemical weathering can provide negative feedback that
reduces the intensity of climate warming