Environmental Engineering Reference
In-Depth Information
The increase of fixed nitrogen impacts the environmental at both regional and global
scales. At a regional level, excess of fixed nitrogen produces acid precipitation, contamina-
tion of drinking water, alteration of ecosystems, loss of species diversity, and depletion of
calcium in soils (Socolow, 1999; Tillman and Lehman, 2001). The global impact is caused by
the release of nitrous oxide, a powerful greenhouse gas with a 100-year residence time, which
is 296 times more effective than carbon dioxide as a greenhouse gas (Environmental Protection
Agency [EPA], 2006a).
Acid precipitation is a term reserved exclusively for fog, rain, and snow with a high content
of nitrate and sulfate ions resulting from environmental pollution. Excesses of nitrogen
fertilization release nitrogen oxides, which get converted into nitrate ions in the atmosphere
and subsequently fall to the ground as dry deposition or as precipitation (Socolow, 1999). Acid
rain causes acidification of surface waters (i.e., lakes and streams), damage trees at high eleva-
tions, and affects man-made structures including buildings, automobile paint, statues, and
sculptures (EPA, 2007).
Contamination of drinking water with excess nitrate may result in a condition called methe-
moglobinemia or Blue Baby Syndrome that affects infants younger than age one and can
produce brain damage or death. Nitrate ions are converted in the gastrointestinal track to
nitrite, which subsequently binds to hemoglobin and makes it lose its carrying oxygen capa-
bility (Socolow, 1999).
A second consequence of high levels of nitrate in water is increased acidity, which promotes
the solubilization of metals that are toxic for humans and fish (EPA, 2002).
The runoff of nitrogen deposited as fertilizer affects the immediate surrounding as well
as distant aquatic ecosystems (EPA, 2002). Excesses of nitrogen in the soil produces
changes in species composition, reduction in forest growth, and alteration in soil chemistry
(EPA, 2002). The latter occurs because negatively charged nitrate sequesters positively
charged ions, such as calcium, and depletes soil from this element (Tillman and Lehman,
2001).
In freshwater environments, excess nitrogen is toxic to fish and other fauna. When excess
nitrogen reaches estuaries, the problem manifests as algal blooms (micro and macro) and
leads to deprivation of oxygen in the water (hypoxia), which eventually kills fish and other
aquatic animals and leads to the loss of habitats (EPA, 2002).
Other impacts of nitrogen fertilization
The most direct impact of nitrogen fertilization is
the production of the fertilizer itself. The preferred method to produce nitrogen fertilizer
is the Haber-Bosch process, which works by combining atmospheric nitrogen with a
source of hydrogen at high temperatures and pressures to produce anhydrous ammonia.
This is an energy-intensive process because high amounts of energy are required to break
the triple bond (N
≡
N) of the nitrogen molecule. Hydrogen for the reaction is mostly
obtained via steam methane reforming using methane contained in natural gas as the
starting material (Equation 3.1). The reaction takes place at temperatures between 700
and 1000°C and pressures between 0.3 to 2.5 MPa over a catalyst. A second reaction
(Equation 3.2), water-gas shift reaction, combines residual carbon monoxide with water
to produce more hydrogen (Department of Energy [DOE], 2008).
3H (endothermic)
[3.1]
CH
+→+
H O
CO
4
2
2
H (slightly exothermic)
[3.2]
CO
+→+
2
H O
CO
2
2
