Environmental Engineering Reference
In-Depth Information
Finally, nitrogen dioxide combined with deionized water in an absorption tower produces
nitric acid:
[3.9]
3NO
+
H O
→
2 HNO
+
NO
2
2
3
Ammonia is then reacted with nitric acid to create ammonium nitrate in an acid-base
exothermic reaction:
HNO (aqueous)
+
NH (gas)
→
NH NO (aqueous)
[3.10]
3
3
4
3
Next, after water evaporation the remaining wet solid is converted into granules or prills by
drying the residual water. Ammonium nitrate is also sold as an aqueous solution in some
countries (EPA, 1995). In addition, ammonia is combined with sulfate and phosphate ions
produce ammonium sulfate and ammonium phosphate (mono and di, respectively).
All these additional steps that transform ammonia into derivative fertilizer forms come
with the cost of additional environmental impacts. At every step, burning fossil fuels to pro-
duce steam and electricity to run the process generate emissions of greenhouse gases and the
consumption of nonrenewable resources.
Phosphorus and potassium fertilization
An advantage of nitrogen fertilizers over other
fertilizers is that they can be prepared from air and a source of hydrogen with the only
requirement of having inexpensive energy. So if these fertilizers can be produced with
renewable energy and hydrogen obtained from renewable resources, it could be said that
nitrogen fertilizer is a renewable resource.
However, fertilizers coming from ore mining operations, such as phosphorus and potas-
sium, are not renewable. Once they are gone, they are gone for good. Phosphorus and potas-
sium will not disappear from the face of the earth; however, their recovery from rivers, lakes,
and oceans that are the ultimate repository of these two nutrients would be impractical and
highly energy intensive with current technologies.
Phosphorus is a limiting nutrient in most soils, so agriculture is dependant on phosphorus
fertilization. The only economical source of phosphorus for manufacturing of fertilizers and
chemicals is phosphate rock that after extraction is treated with sulfuric acid to transform the
insoluble phosphorus into phosphoric acid. Unfortunately, there are no substitutes for
phosphorus in agriculture (United States Geological Survey [USGS], 1999).
Table 3.2 shows the world reserves of phosphorus and the world production in 2008.
The world most available reserves are estimated to be 15
10
9
metric tons. A mining rate of
167 million metric tons a year (estimate for 2008) would indicate an availability of phosphate
rock for another 90 years. Using the reserve base instead, which includes marginally economic
and subeconomic reserves, the projection of phosphate rock availability would climb to about
280 years. These estimates assume a constant demand of phosphate rock, which is not realistic
because of the projected population growth and the potential new crops such as bioenergy
crops that would require extra fertilization.
Phosphorus applied as fertilizer is partially absorbed by plants, a fraction remains in the
soil, and the rest is loss to runoff, mainly in overfertilized fields. An important fraction of
phosphorus absorbed by plants ends up in the edible part of fruits and vegetables; so, when
plant foods are ingested, phosphorus is partially absorbed and used for metabolic functions in
the body and the rest is excreted mainly via urine. In addition, urine is responsible for the
elimination of other important nutrients including potassium, calcium, urea, magnesium, and
sulphate (Udert et al., 2006). All these nutrients end up in sewer systems that carry them to
×
