this case — it happened. A green chemistry approach to this problem would be to find
safer alternatives to cyanide so that there is no possibility of its release.
Excessive levels of ammoniacal nitrogen in the form of ammonium ion, NH 4
- , or
molecular ammonia, NH 3 , cause water-quality problems and may be harmful to aquatic
life. However, ammoniacal nitrogen at lower levels is a normal constituent of water
and is even added deliberately to drinking water so it can react with chlorine used for
disinfection to provide for residual disinfection in water distribution systems.
Hydrogen sulfide, H 2 S, is a toxic gas with a foul odor that is produced by anaerobic
bacteria acting upon inorganic sulfate (see Section 7.5 and Reaction 7.5.4), from
geothermal sources (hot springs) and as a pollutant from chemical plants, paper mills,
textile mills, and tanneries. Because of its bad odor and toxicity, it is an undesirable
pollutant in water.
Microbial degradation under ground may generate carbon dioxide, CO 2 , that exists
as free carbon dioxide in water (see Section 7.6). Excessive levels can be toxic to aquatic
organisms and can make water corrosive because of its acidity and tendency to dissolve
protective CaCO 3 coatings on pipe.
Nitrite ion, NO 2 - , can be generated by the action of bacteria on inorganic nitrogen
species and is added to some industrial process water as a corrosion inhibitor. Normally,
levels are low because of the narrow range of conditions under which nitrite is stable.
Nitrite is quite toxic, causing methemoglobinemia by converting the hemoglobin in
blood to methemoglobin, a form useless for transporting oxygen. Nitrate ion, NO 3 - , is
a more common water contaminant, but is of less concern because of the high tolerance
of adult humans for it. However, infants and multi-stomached ruminant animals (cattle,
sheep, goats, deer) have conditions in their stomachs that can result in the reduction of
nitrate to toxic nitrite. This has killed a large number of animals and has resulted in fatal
cases of methemoglobinemia in infants.
Acidity
Strong acid pollutants that cause water to have a low pH are very damaging to
organisms living in water. Although spills of acids can pollute water, the most common
acid pollutant comes from the bacterial mediated oxidation of iron pyrite, FeS 2 , to
produce sulfuric acid. The overall process is represented by the reaction,
4FeS 2 (s) + 2H 2 O + 15O 2 → 2H 2 SO 4 + 2Fe 2 (SO 4 ) 3 (7.12.2)which produces sulfuric acid, H 2 SO 4 , and Fe 2 (SO 4 ) 3 , which also acts as an acid. The
bacteria that carry out these reactions include Thiobacillus ferrooxidans, Thiobacillus
thiooxidans and Ferrobacillus ferrooxidans.
Another source of water pollutant acid is from acid rain. Hydrogen chloride, HCl,
emitted to the atmosphere forms hydrochloric acid, whereas nitric oxide, NO, and sulfur
dioxide, SO 2 , emitted to the atmosphere can be oxidized in the presence of atmospheric
water vapor to produce nitric acid, HNO 3 , and sulfuric acid, respectively. Falling from
the atmosphere as acid rain, these acids are especially damaging to life in lake water that