Oxidation to Treat Water
Strong oxidants chemically transform contaminants to more benign forms and
can kill bacteria and viruses.
In order of strength these are
potent oxidizers:
- Hydroxyl
Radical (OH0)
- Ozone (O3)
- Hydrogen Peroxide (H2O2)
- Chlorine (OCl) chlorine gas; Sodium
Hypochlorite (NaOCl) bleach; there are several other forms of chlorine
- Air (Oxygen) O2
The Science
Oxidation within water is the
loss of electrons from the item being oxidized (reducing agent) to oxygen.
Oxidation reduces the number of electrons orbiting an element causing the
element to bond with oxygen, which has an attraction for those electrons. Since
electrons carry negative charges, oxidation results in an increase of positive
valence.
Oxidation
Oxidation within water is
the loss of electrons from the item being oxidized (reducing agent) to
oxygen. Oxidation reduces the number of electrons orbiting an
element causing the element to bond with oxygen, which has an attraction for
those electrons. Since electrons carry negative charges, oxidation results in an
increase of positive valence.
An ionic bond is formed when
electrons are transferred from one atom to the other.
A covalent bond is formed when
electrons are shared between atoms. Often, covalent bonds form with a partial
transfer (unequal sharing of electrons), resulting in a polar covalent bond.
Iron is most commonly found in
its soluble state as ferrous
bicarbonate, Fe (HCO3)2. Ferrous iron has a positive
two valence. As ferrous iron is oxidized, the number
of electrons is reduced and the iron develops a valence of positive three,
ferric hydroxide, Fe(OH)3.
Fe(HCO3)2
+ |
1/2
O2
+ |
H2O
-> |
Fe(OH)3
+ |
4CO2 |
Ferrous
Bicarbonate |
Oxygen |
Water |
Ferric
Hydroxide |
Carbon
Dioxide |
Oxygen
changes iron to the ferric state which reacts with alkalinity in the water and
forms an insoluble brown ferric hydroxide precipitate. The dissolution of the
carbon dioxide in water also forms carbonic acid. The presence of carbonic acid
lowers the pH, and in low alkalinity water (2-3 gpg total solids), this may
cause corrosion problems within the system.
Bleach, Sodium
Hypochlorite (NaOCl) is a clear slightly yellow liquid solution and has an
available chlorine content of approx. 5.25 percent.
When the highly alkaline bleach
is added to water the alkalinity is reduced, releasing the chlorine. The chemical equation of adding
NaOCl to water is:
NaOCl
+ |
H2O
-> |
HOCl
+ |
NaOH |
Sodium
Hypochlorite |
Water |
Hypochlorous
Acid |
Sodium
Hydroxide |
The potent bactericide is the Hypochlorous
Acid (HOCl). The
chlorine in hypochlorous acid is an active oxidizing agent combining readily
with other substances. As hypochlorous acid combines with substances
it is used up. The amount of chlorine required to maintain the target
residual is called chlorine demand.
As this acid ionizes as HOCl
--> H+ + OCl- its efficiency is decreased. pH strongly
affects the degree of
ionization.
pH |
Hypochlorous
Acid (HOCl) |
Hypochlorite
Ion (OCl-) |
|
(active
oxidizing agent) |
(Inactive
agent) |
|
|
|
7.0 |
78
% |
22% |
7.5 |
50% |
50% |
8.0 |
21% |
79% |
9.0 |
1% |
99% |
Manganese
The simplified reaction of
manganese oxidation is as follows:
2Mn
+ |
O2
+ |
2H2O
-> |
2MnO2
+ |
4H |
Manganese
Ion |
Oxygen |
Water |
Manganese
Dioxide |
Hydrogen
Ions |
The resultant Manganese Dioxide
is a black insoluble material. The hydrogen ions will cause the water to
become more alkaline.
|