Water being purified for municipal use is usually aerated by contacting it with air.
This treatment blows out odor-causing impurities. When, as is often the case, dissolved
iron is present as soluble iron(II) ion, Fe2+, oxygen from the air oxidizes it,
4Fe2+ + O 2 + 10H 2 O → 4Fe(OH) 3 (s) + 8H+ (7.17.1)forming a gelatinous solid of Fe(OH) 3. As it settles, this solid entrains and carries with
it very small colloidal solid impurities suspended in the water, such as mud, a process
called coagulation.
Water often contains excessive levels of dissolved calcium along with bicarbonate
ion, HCO 3
-
. This temporary hardness can be removed from water (the water softened)
by adding lime, Ca(OH) 2 :
Ca
2+
+ 2HCO 3 - + Ca(OH) 2 → 2CaCO 3 (s) + 2H 2 O (7.17.2)The CaCO 3 solid formed settles in a settling basin. The process can be somewhat slow and
incomplete. So to avoid deposition of solid CaCO 3 precipitation in the water distribution
system, which can clog pipes, CO 2 gas is added to the water.
If residual organic matter, such as humic substances (Section 7.9) is present in the
water, it may be treated by running it over granular activated carbon. This is a form of
carbon that has been reacted with steam at a high temperature to cause the formation
of an enormous number of microscopic pores in the carbon, giving it a huge internal
surface area of up to 2000 square meters per gram! Carbon so treated is very effective in
removing dissolved organic matter from water.
As a final step in water purification, water is disinfected. This may be done by
adding elemental chlorine to the water, which reacts with water as follows:
Cl 2 + H 2 O → H
+
+ Cl- + HOCl (7.17.3)Hypochlorous acid, HOCl, is a good disinfecting agent that kills virus and bacteria in
the water. Salts of HOCl including NaOCl and Ca(OCl) 2 can also be used for water
disinfection.
When used as a disinfectant, chlorine may form undesirable organic compounds
called trihalomethanes. To avoid this problem chlorine dioxide, ClO 2 , an effective
water disinfectant that does not produce trihalomethanes, is now commonly used. This
compound is a dangerously reactive gas that is unsafe to move from a manufacturing site
to where it is needed, so it is made by reacting sodium chlorite with elemental chlorine:
2NaClO 2 (s) + Cl 2 (g) → 2ClO 2 (g) + 2NaCl(s) (7.17.4)This process of making potentially dangerous chlorine dioxide only in the quantities
needed, where needed, when needed is in keeping with the best practice of green
chemistry.
186 Green Chemistry, 2nd ed