Environmental Engineering Reference
In-Depth Information
Nitrogen Cycle
The nitrogen cycle (Fig. 13.6) can be represented in the format provided
in Chapter 12, divided into oxic and anoxic components. As with the carbon
cycle, anoxic habitat could be the hypolimnion of a stratified eutrophic lake;
sediments in a lake, stream, or wetland; or a decaying organic particle. This
generalized cycle illustrates some of the complexity of the nitrogen cycle re-
lated to varied redox potential of the different forms of inorganic nitrogen.
Understanding the complex fluxes in the nitrogen cycle is crucial be-
cause nitrogen can limit primary production. In addition, nitrate and ni-
trite can be toxic (Sidebar 13.1) and understanding the nitrogen fluxes and
factors controlling them may allow for mitigation of some problems.
SULFUR
The sulfur cycle is more complex than either the nitrogen or the car-
bon cycle. The complexity is of interest because it illustrates that organ-
isms have evolved to utilize most of the possible common compounds in
the biosphere that have potential energy.
The sulfur cycle is also important because
some water quality problems revolve around
sulfide contamination. Also, sulfur is tightly
coupled to the inorganic metal cycles such
as iron and manganese and, thus, indirectly
to phosphorus.
Sidebar 13.1.
Problems with Excessive Nitrate
Contamination
The United States and many other countries have
enacted limits on the amount of nitrate that can
be present in drinking water. In the United States,
the allowable level is 10 mg NO
3
-N liter
1
. The
primary reason for this limit is methemoglobine-
mia, a medical condition that occurs because ni-
trite binds more strongly to hemoglobin than O
2
.
This condition can result when water with large
amounts of nitrite or nitrate is consumed. In in-
fants, denitrifying bacteria produce nitrite. The
condition is most often fatal in infants that are
fed with formula made with high nitrate water.
Methemoglobinemia causes the infant's skin to
appear blue and it suffocates; this is also called
blue baby syndrome.
Nitrate is also converted to carcinogenic ni-
trosamines in the stomach. A relationship has
been documented between gastric cancer
rates and fertilizer use (Fig. 13.7). The relation-
ship is only a correlation; other factors such as
lifestyle, pesticide and herbicide use, and diet
may explain the correlation. However, the in-
terdependence of cancer and nitrate should
be investigated more thoroughly.
The problem of nitrate contamination is
widespread and worst in highly agricultural
areas. In France, 266 groundwater sources
Sulfur Forms
Inorganic sulfur can take a wide variety
of forms. Some of the forms that occur with
oxygen and hydrogen include
S
2
S
o
sulfide
elemental sulfur
S
2
O
3
2
SO
4
2
thiosulfate sulfate
In order of increasing redox these are S
2
,
S
0
, S
2
O
3
2
, and SO
4
2
. Hydrogen sulfide
can be dissolved in water as an ion (S
2
) un-
der basic conditions or as a gas (H
2
S) under
acidic conditions. Hydrogen sulfide gas is
toxic in high concentrations but can be de-
tected by odor in exceptionally small con-
centrations. Hydrogen sulfide is partially
responsible for the distinctive smell (an odor
reminiscent of rotten eggs) of anoxic sedi-
ments in lakes and wetlands.
Sulfur is found in many organic mole-
cules and is part of some amino acids that
are central to protein structure. Dimethyl-
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