the carbonate systems in the Environment

Carbonate cycle

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 (H
2
CO
3
)
Can donate two protons (a weak acid)
Bicarbonate (HCO
3
-
)
Can donate or accept one proton (can be
either an acid or a base
Carbonate (CO3
2-
)
Can accept two protons (a base)

11. OPEN SYSTEM
Water is in equilibrium with the partial
pressure of CO
2 in the atmosphere
Useful for chemistry of lakes, etc
Carbonate equilibrium reactions are thus
appropriate

12. P
CO2 = 10
–3.5
yields pH = 5.66
»What is 10
–3.5
? 316 ppm CO
2
What is today’s P
CO2
? ~368 ppm = 10
-3.43
»pH = 5.63

13. Ocean pH and atmospheric CO2

14. NATURAL ACIDS
Produced from C, N, and S gases in the
atmosphere
H
2
CO
3
Carbonic Acid
HNO
3 Nitric Acid
H
2SO
4Sulfuric Acid
HClHydrochloric Acid

15. pH of Global Precipitation

16. http://www.motherjones.com/tom-philpott/2015/01/noaa-globes-coral-reefs-face-massive-bleaching-event-2015

17. OPEN SYSTEM
•Water is in equilibrium with the partial
pressure of CO
2 in the atmosphere
•Useful for chemistry of lakes, etc
•Carbonate equilibrium reactions are thus
appropriate

18. 
Carbonic acid forms when CO
2 dissolves
in and reacts with water:
CO
2(g)
+ H
2
O = H
2
CO
3
»
Most dissolved CO
2 occurs as “aqueous CO
2”
rather than H
2
CO
3
, 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:
H
2CO
3 = HCO
3
–
+ H
+

20. FIRST REACTION

21. »Second dissociation:
HCO
3
–
= CO
3
2–
+ 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. By measuring only two
parameters, such as alkalinity and pH, the
remaining parameters that define the
carbonate chemistry of the solution (P
CO2,
[HCO
3
–
], [CO
3
2–
], [H
2CO
3]) 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: HCO
3
–
, CO
3
2–
,
B(OH)
4
–
, H
3
SiO
4
–
, HS
–
, organic acids
(e.g., acetate CH
3
COO
–
, formate
HCOO
–
)

30. Carbonate alkalinity
Alkalinity ≈ (HCO
3
–
) + 2(CO
3
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 ≈ HCO
3
–

31. CLOSED CARBONATE
SYSTEM
•Carbon dioxide is not lost or gained to the
atmosphere
•Total carbonate species (C
T) 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 (C
T)

33. How does [CO
3
–2
] respond to changes in Alk or DIC?
 
C
T
= [H
2
CO
3
*] + [ HCO
3
–
] + [CO
3
–2
]
 
~ [ HCO
3
–
] + [CO
3
–2
] (an approximation)
 
Alk = [OH
–
] + [HCO
3
–
] + 2[CO
3
–2
] + [B(OH)
4
-
] – [H
+
]
 
~ [HCO
3
–
] + 2[CO
3
–2
] (a.k.a. “carbonate alkalinity”)
 
So (roughly):
 
[CO
3
–2
] ~ Alk – C
T

 
C
T
↑ , [CO
3
–2
] ↓ Alk ↑ , [CO
3
–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 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.

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:
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.

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, PO
4
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. 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. Mechanical Weathering
•Frost wedging
•Alternate heating and
cooling
•Decompression
causes jointing

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:
H
2
O +CO
2
= H
2
CO
3
(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 (SiO
2)

52. Hydrolysis
Feldspar + carbonic acid +H
2
O
= 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
Ca
2+
, Mg
2+
, Na
+
, K
+
Alkalinity = HCO
3
-
Clay = kaolinite (Al
2Si
2O
5(OH)
4)
Si = H
4SiO
4; no charge, dimer, trimer

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

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

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 CO
2
Slow chemical weathering of continental rocks balances
input of CO
2
to atmosphere
Chemical weathering reactions important
Hydrolysis and Dissolution

59. Atmospheric CO
2 Balance
Slow silicate rock weathering balances
long-term build-up of atmospheric CO
2
On the 1-100 million-year time scale
Rate of chemical hydrolysis balance rate of
volcanic emissions of CO
2
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

H
2
O is required for hydrolysis
Increased rainfall increases soil saturation

H
2O and CO
2 form carbonic acid
Vegetation

Respiration in soils produces CO
2

CO
2
in soils 100-1000x higher than atmospheric CO
2

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