Page 120 - Soil and water contamination, 2nd edition

Major dissolved phase constituents 107

5.10 INORGANIC CARBON
The inorganic carbon consists of dissolved carbon dioxide species, which include carbonic
2-
-
acid (H CO ), hydrogen carbonate or bicarbonate (HCO ), and carbonate (CO ).
2 3 3 3
These three species together are also often referred to as total inorganic carbon (TIC ).
Inorganic carbon is an essential nutrient for green plants that consume it to construct
organic molecules through photosynthesis . Terrestrial plants take up carbon dioxide from
the free atmosphere, but aquatic plants and phytoplankton (including algae , diatoms, and
photosynthetic bacteria) extract carbon dioxide species dissolved in the water to use in their
photosynthesis process. Foraminiferans take up dissolved carbon dioxide species to build
their calcareous shells. In animals and humans, dissolved inorganic carbon helps to maintain
the acid balance in the stomach and the intestines.
The principal sources of the dissolved carbon dioxide species in surface water or
groundwater are the CO in the free atmosphere and the CO produced from the respiration
2 2
and decomposition of organic matter in soil, sediment, or water. The CO content of the
2
atmosphere is about 0.03 volume percent. The air in the unsaturated zone of the soil can
be substantially enriched in CO due to respiration by plants and the oxidation reaction
2
in soil. In volcanic areas, CO can also be derived from degassing of the Earth’s mantle or
2
metamorphic decarbonation of carbonate rocks. Another important source of inorganic
carbon is the dissolution of carbonaceous rocks by acid compounds. The major direct
anthropogenic source of inorganic carbon is the application of lime (calcite (CaCO ) or
3
dolomite (Ca,MgCO )) fertilisers to increase the pH of agricultural or forest soils or lake
3
water. The rate at which the lime is applied to soils depends on the cation exchange capacity
and the soil pH, but the dissolved inorganic carbon concentrations in groundwater and
surface water may be greatly boosted by anthropogenic emissions of organic carbon or acidic
compounds. The organic compounds are decomposed to CO , which dissolves readily in
2
water. Immissions of acid compounds into soil or water via direct anthropogenic emissions
or through atmospheric deposition (‘acid rain ’) may lead to an increase in dissolved inorganic
carbon if these acids react with and dissolve carbonate solids. If these carbonates are not
available for reaction, a fall in the pH in response to acid immissions causes dissolved carbon
dioxide species to be removed from solution (see below).
Carbon dioxide species are important reactants that control the pH of natural waters.
When water is in equilibrium with a gas phase containing CO , such as the atmosphere,
2
carbon dioxide dissolves up to a specific solubility limit that depends on pressure and
temperature. At 1 atmosphere pressure and 25 °C, the following mass-law equation applies:
pCO
2 K 10 . 1 43 (5.6)
[H CO ] H
2 3
-1
where K = equilibrium (Henry’s law ) constant (atm ⋅ l mol ) for dissolution of CO ;
H 2
pCO = the partial pressure of carbon dioxide in the gas phase (i.e. the volume percent of
2
CO multiplied by the total pressure expressed in atmospheres and divided by 100) (atm);
2
-1
[H CO ] = the activity of carbonic acid (mol l ). Since this is a dilute solution, the activity
2 3
of H CO is virtually the same as the H CO concentration. The dissociation of dissolved
2 3 2 3
carbonic acid proceeds in two steps:
H 2 CO 3 H + + HCO 3 2H + + CO 3 2 (5.7)
1 2
Clearly, the equilibrium of these dissociation reactions is dependent on pH . With increasing
+
H concentrations (decreasing pH) the equilibrium shifts to the left. Figure 5.7 summarises
the relationships between the pH and the dissolved carbon dioxide species. In natural

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