Sometimes the water in the metal ion is displaced by other species. Such species
include organic chelating agents, which can bond to metal ions in 2 or more places.
Chelate formation is reversible, and a metal chelate can dissociate to reform the chelate
and the metal ion. Chelates tend to be particularly stable, and they are very important
in natural water systems. They are also involved in life systems; for example, blood
hemoglobin is a chelate that contains Fe2+ ion bonded simultaneously to 4 N atoms on
the hemoglobin protein molecule. This particular group binds with molecular O 2 and
enables hemoglobin to carry oxygen to all the organs of the body.
Water in nature may contain naturally-occurring chelating agents called humic
substances that are complex molecules of variable composition left over from the
biodegradation of plant material. Humic substances can be extracted from sediments
in bodies of water where plant material has undergone partial biodegradation leaving
humic materials as the residue from partial degradation of lignin in the plant material.
One of their most important effects in water is their ability to bind with Fe2+ ion, which is
normally present as insoluble species in water. Soluble humic substance chelates of iron
may be encountered in water as colored matter commonly called gelbstoffe (German
for “yellow stuff”). It is very difficult to remove such chelated iron by water treatment
processes.
Humic substances are of special concern because when they are present in water
during disinfection by treatment with chlorine, they give rise to the formation of
trihalomethanes, such as chloroform, HCCl 3. These pollutants are suspected of being
carcinogens (cancer-causing agents), and permissible levels are extremely low. Therefore,
before chlorination, water has to be treated carefully to remove humic substances.
7.10. Water Interactions with Other Phases
So far, the discussion of water chemistry has involved species dissolved in water. In
fact, most important processes that occur in natural waters and wastewaters and during
water purification involve other solid or gaseous phases. This is illustrated in Figure
7.6. Bodies of water rest on layers of sediments and are in contact with the atmosphere.
Materials are exchanged between water and sediments. Sediments are especially
susceptible to the effects of insoluble pollutants and tend to collect pollutants such as
heavy metals and dense organic liquids. Organisms living in contact with sediments may
accumulate high levels of such pollutants, which may be passed on to fish and birds that
feed on fish. Anaerobic bacteria that do not require molecular oxygen living in sediments
can generate gases such as methane, CH 4 , and hydrogen sulfide, H 2 S.
There is a constant exchange of carbon dioxide between water and the atmosphere.
The rapid transport of oxygen from air to water can be essential to fish in water. The
surface of water tends to be covered by a thin film of water-insoluble organic matter in
which compounds exposed to the sun may undergo photochemical degradation.
A particularly important phase associated with water consists of colloidal particles.
These are very small particles of around a micrometer (μ m, one millionth of a meter)
in size or less. Because of their very small sizes, colloidal particles stay suspended in
water and do not settle out. Such small particles have very large ratios of surface area to
Chap. 7. Water, The Ultimate Green Solvent 171