Civil Engineering Reference
In-Depth Information
Chlorine dioxide (ClO
2
) is a neutral compound with chlorine in the
IV oxidation
state. It disinfects by oxidation of the pathogens without chlorinating. It is a volatile
free radical. Even while in dilute aqueous solutions, chlorine dioxide forms a free
radical, making a highly energetic molecule. Chlorine dioxide is reduced to chlorite
via a single electron transfer mechanism (Equation 19-5). The oxidation-
(ClO )
2
reduction reaction for chlorine dioxide is:
ClO
e
ClO
E
0.954V
(19-5)
2(aq)
2
Other important half-reactions for the by-products of chlorine
(ClO )
2
and chlorate
(ClO )
3
are:
ClO
2H O
4
e
Cl
4OH
E
0.76V
(19-6)
2
2
ClO
HO
2
e
ClO
2OH
E
0.33V
(19-7)
3
2
2
ClO
2H
e
ClO
HO
E
1.152V
(19-8)
3
2
2
(ClO )
2
percent of the chlorine dioxide converted to chlorite. The remaining end products are
mainly
In drinking water
is the predominant reaction end product, with up to 70
ClO
3
and chloride (Cl
).
9
Chlorine Dioxide Generation
Chlorine dioxide gas cannot be compressed or stored
commercially. It is explosive under pressure. Therefore, it is always generated at the
point of use. Chlorine dioxide is also considered explosive at concentrations that ex-
ceed 10 percent by volume in air.
Most commercial generators use sodium chlorite (NaClO
2
) as the base chemical
from which chlorine dioxide is generated for drinking water application. Alternatively,
chlorine dioxide can be produced from sodium chlorate (NaClO
3
) with a mixture of
concentrated hydrogen peroxide (H
2
O
2
) and concentrated sulfuric acid (H
2
SO
4
).
Chlorate-based systems have traditionally been used in pulp and paper applications,
but are considered an emerging technology in the drinking water field. Other emergent
technologies include electrochemical systems and a solid chlorite inert matrix (flow-
through gaseous chlorine).
Chlorine dioxide can be formed by reacting sodium chlorite with any of the fol-
lowing: chlorine (Cl
2(g)
), hypochlorous acid (HOCl), or hydrochloric acid (HCl). Chlo-
rine dioxide is formed from these combinations by one of the following equations:
2NaClO
Cl
2ClO
2NaCl
(19-9)
2
2(g)
2(g)
2NaClO
HOCl
2ClO
NaCl
NaOH
(19-10)
2
2(g)
5NaClO
4HCl
4ClO
5NaCl
2H O
(19-11)
2
2(g)
2
Table 19-8 provides information on some types of available commercial generators.
The most undesirable by-product in chlorine di-
Chlorate By-Product Formation
oxide generation is the chlorate ion
(ClO ).
3
inefficient chlorine dioxide production. The overall reactions that describe chlorate ion
formation during chlorine dioxide generation are:
Chlorate production is representative of
