Transitions - Sustainable Design: The Science of Sustainability and Green Engineering - page 97

that are soluble in water under environmental conditions: the cation (posi-
tively charged ion) ammonium (NH 4 + ) and the anions (negatively charged
ions) nitrate (NO 3 − ) and nitrite (NO 2 − ). Nitrates and nitrites combine with
various organic and inorganic compounds. Once taken into the body, NO 3 −
is converted to NO 2 . Since NO 3 − is soluble and readily available as a nitrogen
source for plants (e.g., to form plant tissue compounds such as amino acids and
proteins), farmers are the biggest users of NO 3 − compounds—in commercial
fertilizers (although even manure can contains high levels of NO 3 − ).

N 2 N 2 NH 3 N O NO

2

Air

Nonsymbiotic Symbiotic

Fixation of nitrogen

Plant uptake

Soil

Nitrification
(aerobic
processes)

Organic matter in
detritis and dead organisms

Mineralization

+

NH 3 /NH 4 NH 2 OH NO 2 NO 3

N 2 NO NO 2

Dentrification (anaerobic processes)

N 2

Figure B3.1 Biochemical nitrogen cycle.

Ingesting high concentrations of nitrates (e.g., in drinking water) can cause
serious short-term illness and even death. A serious illness in infants, known
as methemoglobinemia, is due to the body's conversion of nitrate to nitrite,
which can interfere with the oxygen-carrying capacity of the blood. Especially
in small children, when nitrates compete successfully against molecular oxy-
gen, the blood carries methemoglobin (as opposed to healthy hemoglobin),
giving rise to clinical symptoms. At 15 to 20% methemoglobin, children can
experience shortness of breath and blueness of the skin (i.e., clinical cyanosis).
At 20 to 40% methemoglobin, hypoxia will result. This acute condition can
cause a child's health to deteriorate rapidly over a period of days, especially
if the water source continues to be used. Long-term elevated exposure to
nitrates and nitrites can cause an increase in the kidneys' production of urine