1. 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

2. Learning goals Climate controls on atmospheric CO2 Ocean acidification
What causes it
Why important
What does the future hold

3. CARBON Shells: 2,4 Minimum oxidation number is –4
Maximum oxidation number is +4

4. Carbon Isotopes
C-12
C-13
C-14

5. Carbon forms Graphite Diamond Buckmisterfullerene Organic Matter DOC
Particulate C

6. 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)

7. DOC in GROUNDWATER Less than 2 mg/L Microbial decomposition Adsorption
Precipitation as solid
> 100 mg/L in polluted ag systems
Increases geochemical weathering

8. ORGANICS in WATER Solid phases (peat, anthracite, kerogen
Liquid fuels (LNAPL), solvents (DNAPL)
Gas phases
Dissolved organics (polar and non-polar)

9. 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

10. DIPROTIC ACID SYSTEM Carbonic Acid (H2CO3) Bicarbonate (HCO3-)
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)

11. 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

12. What is today’s PCO2? ~368 ppm = 10-3.43
PCO2 = 10–3.5 yields pH = 5.66
What is 10–3.5? 316 ppm CO2
What is today’s PCO2? ~368 ppm =
pH = 5.63

13. Ocean pH and atmospheric CO2

14. NATURAL ACIDS Produced from C, N, and S gases in the atmosphere
H2CO3 Carbonic Acid
HNO3 Nitric Acid
H2SO4 Sulfuric Acid
HCl Hydrochloric Acid

15. pH of Global Precipitation

16. http://www. motherjones

17. 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

18. 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

19. First dissociation:
H2CO3 = HCO3– + H+

20. FIRST REACTION

21. Second dissociation:
HCO3– = CO32– + H+

22. SECOND REACTION

23. Variables and Reactions Involved in Understanding the Carbonate System

24. Activity of Carbonate Species versus pH

25. CARBONATE SPECIES and pH

26. 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

27. 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.

28. Alkalinity is routinely measured in natural water samples
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.

29. 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–)

30. 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–

31. 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

32. TOTAL CARBONATE SPECIES (CT)

33. 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] ↑

34. Diurnal changes in DO and pH
What’s up?

35. 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 H CO2 + light energy —> C6H12O 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.

36. 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:
C6H12O O2 —> 6 H 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.

37. LITHOSPHERE Linkage between the atmosphere and the crust
Igneous rocks + acid volatiles = sedimentary rocks + salty oceans (eq 4.1)

38. 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

39. 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

40. IMPORTANCE OF ROCK WEATHERING
[3] Forms soil
[4] Regulates Earths climate
[5] Makes beach sand!

41. Rock
Cycle

42. Sedimentary Processes
1) Weathering & erosion
2) Transport & 3) deposition
4) Lithification

43. 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

44. Disintegration of rock without change in chemical composition
Weathering Processes
Mechanical Weathering -
Disintegration of rock without change in chemical composition
Chemical Weathering-
Decomposition of rock as the result of chemical attack. Chemical composition changes.

45. Mechanical Weathering
Decompression causes jointing
Frost wedging
Alternate heating and cooling

46. 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)

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

48. CARBONATE (DISSOLUTION)
All of the mineral is completely
Dissolved by the water.
Congruent weathering.

49. DEHYDRATION
Removal of water from a mineral.

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

51. HYDROLYSIS
Silicate rock + acid + water = base cations + alkalinity + clay + reactive silicate (SiO2)

52. 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

53. HYDROLYSIS Base cations are Alkalinity = HCO3-
Ca2+, Mg2+, Na+, K+
Alkalinity = HCO3-
Clay = kaolinite (Al2Si2O5(OH)4)
Si = H4SiO4; no charge, dimer, trimer

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

55. Oxidation
Oxidation can affect any iron bearing mineral, for example, ferromagnesian silicates which react to form hematite and limonite

56. 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. Products of weathering
Clay minerals further decompose to aluminum hydroxides and dissolved silica.

58. Removal of Atmospheric CO2
Slow chemical weathering of continental rocks balances input of CO2 to atmosphere
Chemical weathering reactions important
Hydrolysis and Dissolution

59. Atmospheric CO2 Balance
Slow silicate rock weathering balances long-term build-up of atmospheric CO2
On the 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

60. 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 x higher than atmospheric CO2

62. 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

63. Chemical Weathering: Earth’s Thermostat?
Chemical weathering can provide negative feedback that reduces the intensity of climate warming

64. Chemical Weathering: Earth’s Thermostat?
Chemical weathering can provide negative feedback that reduces the intensity of climate cooling