Environmental Engineering Reference
In-Depth Information
oxidant used for disinfection and organic compound removal from water. Ozone has
a large aqueous solubility (8.9 mg/L) and a high reactivity (
G
f
Δ
=+
163 kJ/mol). Its
redox potential is high, making it a powerful oxidant.
E
H
−
2.1V
2H
+
+
2
e
−
O
3
+
O
2
+
H
2
O.
(6.106)
Wastewater treatment using ozone requires an ozone generator, a contactor, and
off-gas treatment devices. Ozone is generated using an electric arc in an atmosphere of
oxygen. The ozone contactor system is a submerged diffuser where ozone is bubbled
into the water column immediately upon generation. The typical depth of the water
column is 20 ft (Ferguson, Gramith, and McGuire, 1991). With contact times as small
as 10 min in most water treatment plants, ozone can perform microbial destruction,
but can only partially oxidize organic compounds. Hence, it is used in combination
with H
2
O
2
and UV oxidation processes (Masten and Davies, 1994). Series (cascade)
reactors are required to complete the oxidation process.
The reaction of ozone in “pure” water is a radical chain reaction comprising the
formation and dissipation of the powerful hydroxyl radical (OH
•
). The sequence
begins with the base-catalyzed dissociation of ozone
OH
−
→
HO
2
+
O
•
2
O
3
+
,
(6.107)
HO
2
→
H
+
+
O
•
2
.
The above acid-base equilibrium has a p
K
a
of 4.8. Since, per reaction two radicals
are produced, the rate constant
k
1
=
70 L/mol/s. The subsequent reactions lead
to the regeneration of the catalyst OH
−
and the formation of O
2
and HO
•
. Staehelin
and Hoigne (1985) published a concise summary of the various reactions involved in
the overall scheme (Figure 6.21). The reaction involving the consumption of ozone by
the superoxide anion (O
2
) has a rate constant
k
2
=
2
×
10
9
L/mol/s. In the presence
1.5
×
of an acid, the last reaction proceeds in two steps:
k
3
=
5
×
10
1
0 L
/
mol
/
s
−−−−−−−−−−−→
O
•
3
H
+
HO
3
,
+
(6.108)
10
5
L
/
mol
/
s
k
4
=
1.4
×
HO
3
−−−−−−−−−−−−→
HO
•
+
O
2
.
The hydroxyl radical forms an adduct with ozone, which gives rise to HO
2
and O
2
via decomposition, thus propagating the chain. The HO
2
also reacts with
the adduct forming O
3
,O
2
, and water with a rate constant
k
6
of approximately
10
10
L/mol/s.
RadicalscavengersthatreactwithOH
•
canefficientlyterminatethischainreaction.
In freshwater that is slightly alkaline (due to carbonate and bicarbonate species), the
hydroxyl radical is efficiently scavenged by inorganic carbonate species. Hence the
decay rate of ozone is reduced. The presence of organic solutes (such as alcohols,
DOC) and other species (such as phosphates) in water impacts the radical chain
reaction in different ways.The following discussion is based on the scheme suggested
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