Carbon Cycle

The carbon cycle is the series of interconnected changes by which carbon is being continuously circulated among the natural compartments of atmosphere, biosphere, hydrosphere, geosphere and pedosphere. There are four major reservoirs involved in the carbon cycle. These are the plants, the terrestrial biosphere (fresh water systems and non- living organic material), the oceans (where we have dissolved inorganic carbon and living and non-living marine biota), and the sediments (including fossil fuels).

Living organisms are mainly composed of water and various carbon compounds hence the cycling of carbon is of prime importance to the support of life. The concentration of carbon in living matter (18per cent) is about 100 times greater than its concentration in the earth (0.19per cent). Carbon exists as carbon dioxide (CO₂) in the atmosphere, bicarbonate ion ( HCO ₃
) in water and calcium carbonate (CaCO₃) in carbonate rocks (limestone, chalk, coral). It is the major part of hydrocarbon molecules in petroleum and natural gas; and the main constituent of coal and dead organic matters.

In the Earth’s atmosphere, carbon exists as carbon dioxide in 0.03per cent level by volume. This level is currently being shifted towards excess due to imbalance of anthropogenic activities (human-induced) such as excessive combustion of fossil fuels and deforestation. In 1850, atmospheric CO_{2 was} about 280 ppm and by 2007, it had increased to about 383 ppm.

Ways by which CO₂ is released into the Atmosphere

Some of the ways by which CO_{2 is} released into the atmosphere are:

Respiration of plants and animals: This is an exothermic reaction involving the breaking down of organic molecules, e.g. glucose, into carbon dioxide and water

C ₆ H₁₂ O₆ + 6O₂

6CO_{2 +} 6H₂O + energy.
Decay of plants and animals: Fungi and bacteria breakdown the carbon compounds i.e. carbohydrates, proteins and lipids in dead plants and animals, and convert the carbon to carbon dioxide in the presence of oxygen or carbon dioxide and methane (CH₄) in the absence of oxygen e.g.

Methanogens

C6H12O6

3CO<span style="font-size:8pt">2 (g) </span>+3CH<span style="font-size:8pt">4 (g)</span>

Fermentation of carbohydrates: The enzymatic decomposition of carbohydrates produces CO₂ as a by-product

C6H12O6 (aq) Zymase 2C2H5OH (aq) + 2CO2 (g)

Burning of fossil and agro fuels: Combustion of fossil fuels like petroleum products, coal, natural gas and agro fuels releases CO₂ (and water vapor) into the atmosphere.

C₅H₁₂ + 😯₂

6H₂O + 5CO₂
Thermal decomposition of carbonate rocks or limestone: When limestone soils are heated up or during the production of cement, CO_{2 is} released into the atmosphere.
Heat CaCO3(s)

CaO(s) + CO 2 (g)

Warming of surface waters: This leads to the releasing of dissolved CO_{2 back} into the atmosphere.

Volcanic eruptions: During volcanic eruptions, the volcanic gases released into the atmosphere include water vapor, CO_{2 and} SO₂.

Ways by which CO₂ is removed from the Atmosphere

Photosynthesis: Primarily, photoautotrophs (plants and algae) use light energy to convert CO_{2 and} water to organic molecules like glucose and other carbohydrates. To a less extent, chemoautotrophs (bacteria and archaea) convert CO_{2 and} water to organic matter using energy derived from the oxidation of molecules of their substrates.

6CO2 + 6H2O

C6H12O6 + 6O2
Formation of carbonic acid: Carbon dioxide dissolves in rain water and droplets pass through the atmosphere. Also, at the surface of the oceans towards the poles where sea water becomes cooler, carbon dioxide dissolves in water to from carbonic acid. Carbonic acid reacts with weathered silicate rocks to produce bicarbonate ions which are used to make marine carbonates.
Conversion of carbon to tissues and shells: Organisms in upper ocean areas of high biological productivity convert reduced carbon to tissues or shells.
In the oceans, the major carbon reservoir is the inorganic carbon: When CO₂ dissolves in water, a hydrated CO₂ molecule is produced which then forms an equilibrium mixture containing bicarbonate ( HCO₃

) and carbonate ( CO₃²
) ions. At pH_{s lower} than those found in sea water, carbonic acid (H₂CO₃) will also be present. This can be summarized as:

CO2 (g) = CO2 (aq)

H₂O (^)
+ CO₂ (aq) = H₂CO_{3 (aq)}

H2CO3 (aq) =
H (+
aq)
+
HCO 3(aq)

HC O 3(aq)
= H (+
aq)
+
CO 32 (aq)

However, due to the reactions
HCO 3(

aq ) = CO
2( aq ) +
OH ( aq
) and C O 23( aq
) + H2O ()= HCO 3( aq ) +
OH ( aq
)

Most ocean waters have a pH in the range 8 to 8.3 as they contain more 0H^{– ions} than H^{+ ions}.

The overall reaction that takes place when CO_{2 dissolves} in sea water can be summarized as:

CO2 (aq) +H2O ( 
) + C O3(2 aq ) = 2HC O3 .

The carbon cycle shown below summarizes all the processes so far itemized.

The Carbon Cycle

Source: O.Y. Ababio, 2002 (modified).

Source/Reference

CHM134 Environmental Chemistry

Published By:

National Open University of Nigeria

First Printed 2010

ISBN: 978-058-776-4