By mediating chemical reactions, such as the one above, microorganisms, particularly
bacteria, largely determine the chemistry that occurs in water. As examples, bacteria in
the oxygen-deficient hypolimnion convert oxidized NO 3 - and SO 42 - to reduced NH 4 + and
H 2 S, whereas bacteria in the oxygen-rich epilimnion convert reduced NH 4 + and H 2 S to
oxidized NO 3 - and S 22 -.
A key species in determining the biological and chemical processes in water is
dissolved oxygen. Fish cannot live in water where the dissolved oxygen is too low.
Organic pollutants in the form of biodegradable organic matter may not be directly
toxic, but result in fish kills because of their high biochemical oxygen demand, BOD,
a measure of the amount of oxygen consumed in the biodegradation of the pollutant
biomass.
7.5. Chemical Processes in Water
Many chemical and biochemical reactions occur in water in the environment. These
are discussed here on the basis of their chemical classification. Several of these were
shown by example reactions in Figure 7.4
The photosynthesis reaction, which utilizes sunlight energy to produce biomass,
2HCO 3-
Sunlight
energy {CH 2 O} + O 2 + CO 32 -
(7.5.1)
is shown here for the conversion of inorganic carbon from dissolved HCO 3
-
ion to
organic carbon (biomass) abbreviated as {CH 2 O}. This reaction produces biomass that
can be acted upon biochemically by other organisms to form the basis of a number of
important biochemical processes in water.
The carbonate ion, CO 3
2 -
, generated by photosynthesis reacts with water,CO 32 -
+ H 2 O → HCO 3
-
+ OH
-
(7.5.2)
removing a hydrogen ion, H+, from the water molecule and producing OH
-
ion. Reactions
involving the exchange of H+ or the generation or consumption of OH
-
are acid-base
reactions. This reaction generates OH
-
ion, so it makes the water more basic.The carbonate ion generated by photosynthesis may become involved in another
kind of reaction as exemplified by its reaction with dissolved calcium ion, Ca
2+
, in
water,
Ca
2+
+ CO 32 -
→ CaCO 3 (s)
(7.5.3)to produce solid CaCO 3. This is a precipitation reaction. CaCO 3 is limestone, and it
is this kind of reaction, beginning with the CO 3
2 -
generated by photosynthesis, that is
responsible for large formations of limestone rock throughout the world.
Oxidation-reduction reactions (see Section 4.7), usually carried out by bacteria,
are common in natural waters. The bacterially-mediated reaction of sulfate ion, SO 4
2 -
,
acting as an oxidizing agent in the O 2 -deficient bottom regions of a body of water to
oxidize biodegradable organic matter, {CH 2 O},
Chap. 7. Water, The Ultimate Green Solvent 167